Patent Application
20230382727
A method for using a catalyst system to convert a methane composition to produce H2 and a carbon co-product is disclosed, which utilizes operational parameters and catalyst system tuning to control yields and morphologies of H2 and/or carbon co-product formation. Also disclosed is a method to make the catalyst system used in method aspects herein.
This invention was made under a CRADA (CRADA 401) between C4-MCP, LLC and Pacific Northwest National Laboratory operated for the United States Department of Energy.
Thermocatalytic decomposition (TCD) of methane (CH4) offers a path to generating H2 without incurring the production of any CO2. Catalyst systems conventionally used in such methods often experience deactivation at the high temperatures typically desired for good yields using TCD. A need exists in the art for a method to generate and use a catalyst system that is stable and active at relatively high operation temperature to enable broader application of TCD. Commercialization of TCD process also would benefit from being able to recover and sell carbon by-products generated by TCD methods; therefore, there also exists a need in the art for a method that can produce suitable carbon by-products having high quality and purity.
Disclosed herein are aspects of a method, comprising: contacting a methane composition with a catalyst system at a reaction temperature ranging from 500° C. to 700° C. to produce H2 and a carbon co-product. In some aspects, the catalyst system comprises (i) a Ni—Cu alloy catalyst comprising Ni and Cu, and (ii) a support, wherein the Ni and Cu are present at a Ni:Cu mass ratio ranging from greater than zero to 4.5.
Also disclosed are aspects of a method for making a catalyst system, comprising: i) contacting a solution comprising a first metal with a support material to impregnate the support material with the first metal, thereby forming an impregnated support; ii) heating the impregnated support using a ramping temperature protocol to provide a pre-catalyst system, wherein the ramping temperature protocol comprises increasing a temperature to which the impregnated support is exposed by 5° C. per minute until a final temperature of 350° C. is reached; iii) contacting the pre-catalyst system with a second metal to form a bimetallic impregnated support; and (iv) heating the bimetallic impregnated support using the ramping temperature protocol to provide the catalyst system. In some aspects, the first metal and the second metal are different from each other and independently are selected from Ni and Cu and wherein the first metal and the second metal provide a Ni:Cu mass ratio ranging from greater than zero to 4.5.
The foregoing and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
is 5Ni-0.5Cu-CNT, Δ is 10 Ni-1Cu-CNT, □ is 20 Ni-2Cu-CNT, and ◯ is 40Ni-4Cu/CNT) and 100 vol. % CH4 (wherein
is 5Ni-0.5Cu-CNT, ▴ is 10 Ni-1Cu-CNT, ▪ is 20 Ni-2Cu-CNT, and ● is 40Ni-4Cu/CNT).
The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.
Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.
The disclosure of numerical ranges should be understood as referring to each discrete point within the range, inclusive of endpoints, unless otherwise noted. Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those of ordinary skill in the art. When directly and explicitly distinguishing aspects from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.
Also, the following description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the present disclosure. Various changes to the described embodiment may be made in the function and arrangement of the elements described herein without departing from the scope of the preset disclosure. Further, descriptions and disclosures provided in association with one particular embodiment are not limited to that embodiment and may be applied to any embodiment disclosed. Further, the terms “coupled” and “associated” generally mean fluidly, electrically, and/or physically (e.g., mechanically or chemically) coupled or linked and does not exclude the presence of intermediate elements between the coupled or associated items absent specific contrary language.
Although the operations of exemplary aspects of the disclosed method and/or system aspects may be described in a particular, sequential order for convenient presentation, it should be understood that disclosed aspects of the disclosure can encompass an order of operations other than the particular, sequential order disclosed, unless the context dictates otherwise. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Further, descriptions and disclosures provided in association with one particular embodiment are not limited to that embodiment and may be applied to any disclosed embodiment.
To facilitate review of the various aspects of the disclosure, the following explanations of specific terms are provided.
Carbon co-product: A solid carbonaceous product produced by thermocatalytic decomposition (TCD) of methane (CH4) using a method according to the present disclosure.
CH4 conversion rate: CH4 conversion, XCH
where FIn is the flow rate of the feed gas before reaction starts; [CH4]In is the concentration of CH4 in the feed gas; Fout is the flow rate of the outlet gas; [CH4]Out is the concentration of CH4 in the outlet gas.
Carbon yield: Carbon yield, YC(t), is calculated as the accumulated weight of carbon produced per mass of the catalyst based on the CH4 conversion.
Carbon deposition rate: Carbon deposition rate is calculated as the accumulated weight of carbon produced per mass of the catalyst based on the CH4 conversion over a specific time period.
Carbon nanotube (CNT): Carbon nanotubes are cylindrical structures made of carbon atoms. Carbon nanotubes typically have diameters measured in nanometers. As used herein, this term includes single-wall CNT (SWCNT), double-wall CNT (DWCNT), multi-wall CNT (MWCNT), and other forms of CNTs.
Carbon Nanomaterial: A carbon-based material having at least one dimension on the nanometer scale in size (e.g., from 1 nm to 1000 nm). Carbon nanomaterials can include, but are not limited to, nanoparticles, fullerenes, carbon filaments, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and various graphene-based materials.
COx-Free: A method as described herein that does not produce measurable amounts of carbon dioxide (wherein x=2), carbon monoxide (wherein x=1), or related compounds as by-products. In some aspects, the processes and methods disclosed herein are COx-free, CO2-free, or both. In a further aspect, COx-free and CO2-free processes are environmentally-sound as they do not release excess greenhouse gases into the atmosphere.
Nominal Amount: An amount of a metal (e.g., Cu or Ni) typically present in an alloy catalyst disclosed herein. In some aspects, the nominal amount of a metal in an alloy catalyst may be different from a measured amount of that alloy catalyst, but typically not by an amount that deleteriously affects the properties of the alloy catalyst. The mass ratio values of the present disclosure are based on nominal amounts, unless otherwise indicated.
Sequential impregnation: A process in which different metal species are applied to a support material (e.g., a surface of the support material) in a sequential order. The process comprises sequentially depositing one or more layers of a first metal species onto the support material followed by depositing one or more layers of a second metal species. As used herein, sequential impregnation is different from a solvothermal method (ST).
ID/IG ratio: The ratio of the intensity of the D band (ID) (1340 cm−1) to the intensity of the G band (IG) (1580 cm−1) as determined using Raman spectroscopy.
IG′/IG ratio: The ratio of the intensity of the G′ band (IG′) (2700 cm−1) to the intensity of the G band (IG) (1580 cm−1) as determined using Raman spectroscopy.
Thermocatalytic decomposition (TCD) of methane (CH4) produces hydrogen (H2) and forms solid carbon as by-product. TCD offers a path to generating H2 without incurring the production of CO2. Thus, it converts a fossil fuel (methane) into H2 that could be employed without increasing emissions of greenhouse gases. Catalyst systems have been explored for TCD reactions; however, catalyst system deactivation is still one of the challenges for methane TCD as conventional catalyst systems often exhibit deactivation at temperatures desirable for optimal yields in TCD (e.g., temperatures above 600° C.). For example, conventional nickel-based catalyst systems suffer catalyst system deactivation due to plugging of active sites with undesirable graphitic carbon. While bimetallic catalyst systems like NiPd have been used for TCD, the expensive nature of the Pd component prohibits their use in commercial/large-scale operations. Also, conventional TCD methods have not been able to produce by-products suitable for other applications, such as carbon-based by-products that might be used as materials for methods other than the TCD process.
The present disclosure is directed to a method using a tailored catalyst system for TCD that enables continuous production of CO2-free H2 and further produces a carbon co-product with tunable properties. The catalyst system used in the disclosed methods comprises Ni—Cu bimetallic catalyst that is specially designed to have suitable Ni:Cu ratios that provide enhanced TCD yields at high temperatures. The disclosed method further provides the ability to harness the catalyst system and operational parameters for controlling the generation and morphology of a carbon co-product that is produced during TCD and that can be isolated. In addition, the method described herein utilize controlled metal particle sizes and/or operating temperatures to influence TCD yields and/or properties of the carbon co-product.
In particular aspects of the present disclosure, relationships between catalyst system deactivation and metal particle sintering, increased Ni:Cu ratio, and/or choice of operating temperature are described and can be used to improve the output of the method (e.g., increasing H2 and/or carbon co-product yields and/or controlling/tuning the morphology and/or identity of the carbon co-product). For example, a monometallic Ni/CNT catalyst system quickly deactivates at operating temperatures >550° C.; however, the inventors of the present disclosure determined that increasing the amount of Cu addition in the Ni catalyst systems results in increasing catalyst system stability. Additionally, catalyst system stability at increased operating temperatures (e.g., >650° C.) is facilitated by Ni catalyst systems with Cu loadings described herein.
Also, as described herein the carbon co-product quality can be tuned by selecting particular Ni:Cu ratios and/or operating temperatures. The carbon co-product is mainly composed of multiwalled carbon nanotube (MWCNTs). The present inventors have determined how to control parameters like Cu addition amounts and operational temperatures to positively impact carbon co-product quality and catalyst system stability.
Disclosed herein is a method for using a catalyst system to convert a methane composition to produce H2 and a carbon co-product, wherein operational parameters and/or the catalyst system composition is used to tune product yields and/or carbon co-product morphology/quality. The method comprises contacting a methane composition with a catalyst system at a reaction temperature as described herein to produce H2 and a carbon co-product. In some aspects, the catalyst system comprises an alloy catalyst and a support. In further aspects, the methane composition is converted to H2 at a particular CH4 conversion rate, and in such aspects a carbon co-product is made. In particular aspects, the catalyst system comprises a Ni—Cu alloy catalyst comprising Ni, Cu and a support, wherein the Ni and Cu are present at a Ni:Cu mass ratio that is controlled so as to improve product yields and/or carbon co-product morphology/quality. In yet additional aspects, the reaction temperature may range from 500° C. to 700° C.
In some aspects, the catalyst system comprises a bimetallic catalyst. In certain aspects, the catalyst system comprises Ni and Cu. In yet additional aspects, the catalyst system further comprises a support. In particular aspects, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio ranging from greater than zero to 25, such as greater than zero to 24.5, or greater than zero to 20, or greater than zero to 15, or greater than zero to 10, or greater than zero to 6, or greater than zero to less than 5, or greater than zero to 4.5, or greater than zero to 2, or greater than zero to 1, or 0.01 to 1, or 0.05 to 1, or 0.1 to 1, or 0.2 to 1, or 0.3 to 1, or 0.4 to 1, or 0.5 to 1. In one aspect, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio of 2:3 (Ni:Cu). In an independent embodiment, the Ni and Cu are not present in amounts that provide a Ni:Cu mass ratio of 5 (or 5:1, Ni:Cu), 10 (or 10:1, Ni:Cu), or 15 (or 15:1, Ni:Cu).
In exemplary aspects, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio ranging from greater than zero to 2, such as from 0.01 to 1, or 0.1 to 0.15, or 0.2 to 0.3, or 0.6 to 0.7. In certain aspects, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio ranging from 0.1 to 2.
A total metal weight loading may range from greater than 0 wt. % to 95 wt. %, such as greater than 0 wt. % to 90 wt. %, or greater than 0 wt. % to 80 wt. %, or greater than 0 wt. % to 70 wt. %, or greater than 0 wt. % to 60 wt. %.
In some aspects, the Ni and Cu are present at a Ni:Cu mass ratio of 2 and the total metal weight loading may range from 1 wt. % to 60 wt. %, with representative amounts including, but not limited to, 1.5 wt. %, 3 wt. %, 7.5 wt. %, 15 wt. %, 16.5 wt. %, 18 wt. %, 24 wt. %, 30 wt. %, 45 wt. %, or 60 wt. %. In certain aspects, the Ni and Cu are present as 1 wt. % Ni and 0.5 wt. % Cu, 2 wt. % Ni and 1 wt. % Cu, 5 wt. % Ni and 2.5 wt. % Cu, 10 wt. % Ni and 5 wt. % Cu, 11 wt. % Ni and 5.5 wt. % Cu, 12 wt. % Ni and 6 wt. % Cu, 16 wt. % Ni and 8 wt. % Cu, 20 wt. % Ni and 10 wt. % Cu, 30 wt. % Ni and 15 wt. % Cu, 40 wt. % Ni and 20 wt. % Cu. In one aspect, the Ni and Cu are present as 10 wt. % Ni and 5 wt. % Cu.
In some aspects, the Ni and Cu are present at a Ni:Cu mass ratio of 1 and a total metal weight loading may range from 1 wt. % to 60 wt. %, with representative amounts including, but not limited to, 2 wt. %, 10 wt. %, 20 wt. %, 30 wt. %, 40 wt. %, 50 wt. %, or 60 wt. %. In certain aspects, the Ni and Cu are present as 1 wt. % Ni and 1 wt. % Cu, 5 wt. % Ni and 5 wt. % Cu, 10 wt. % Ni and 10 wt. % Cu, 15 wt. % Ni and 15 wt. % Cu, 20 wt. % Ni and 20 wt. % Cu, 25 wt. % Ni and 25 wt. % Cu, 30 wt. % Ni and 30 wt. % Cu. In one aspect, the Ni and Cu are present as 10 wt. % Ni and 10 wt. % Cu.
In some aspects, the Ni and Cu are present at a Ni:Cu mass ratio of 0.67 and a total metal weight loading may range from 1 wt. % to 60 wt. %, with representative amounts including, but not limited to, 2.5 wt. %, 5 wt. %, 7.5 wt. %, 10 wt. %, 12.5 wt. %, 15 wt. %, 17.5 wt. %, 20 wt. %, 22.5 wt. %, 25 wt. %, 30 wt. %, 40 wt. %, 50 wt. %, or 60 wt. %. In certain aspects, the Ni and Cu are present as 1 wt. % Ni and 1.5 wt. % Cu, 2 wt. % Ni and 3 wt. % Cu, 3 wt. % Ni and 4.5 wt. % Cu, 4 wt. % Ni and 6 wt. % Cu, 5 wt. % Ni and 7.5 wt. % Cu, 6 wt. % Ni and 9 wt. % Cu, 7 wt. % Ni and 10.5 wt. % Cu, 8 wt. % Ni and 12 wt. % Cu, 9 wt. % Ni and 13.5 wt. % Cu, 10 wt. % Ni and 15 wt. % Cu, 12 wt. % Ni and 18 wt. % Cu, 16 wt. % Ni and 24 wt. % Cu, 20 wt. % Ni and 30 wt. % Cu, 24 wt. % Ni and 36 wt. % Cu. In one aspect, the Ni and Cu are present as 10 wt. % Ni and 15 wt. % Cu.
In an independent aspect, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio of 10 and a total metal weight loading may range from 1 wt. % to 60 wt. %, such as 5.5 wt. %, 11 wt. %, 22 wt. %, or 44 wt. %. In certain aspects, the Ni and Cu are present as 5 wt. % Ni and 0.5 wt. % Cu, 10 wt. % Ni and 1 wt. % Cu, 20 wt. % Ni and 2 wt. % Cu, or 40 wt. % Ni and 4 wt. % Cu. In one aspect, the Ni and Cu are present as 10 wt. % Ni and 1 wt. % Cu.
In an independent aspect, the Ni and Cu are present in amounts that provide a Ni:Cu mass ratio of 5. A total metal weight loading may range from 1 wt. % to 60 wt. %, such as 1.2 wt. %, 6 wt. %, 12 wt. %, 18 wt. %, 24 wt. %, 30 wt. %, 36 wt. %, 42 wt. %, 48 wt. %, 54 wt. %, 60 wt. %. In certain aspects, the Ni and Cu are present as 1 wt. % Ni and 0.2 wt. % Cu, 5 wt. % Ni and 1 wt. % Cu, 10 wt. % Ni and 2 wt. % Cu, 15 wt. % Ni and 3 wt. % Cu, 20 wt. % Ni and 4 wt. % Cu, 25 wt. % Ni and 5 wt. % Cu, 30 wt. % Ni and 6 wt. % Cu, 35 wt. % Ni and 7 wt. % Cu, 40 wt. % Ni and 8 wt. % Cu, 45 wt. % Ni and 9 wt. % Cu, 50 wt. % Ni and 10 wt. % Cu. In one aspect, the Ni and Cu are present as 10 wt. % Ni and 2 wt. % Cu.
In some aspects, the Ni of the Ni—Cu alloy catalyst is provided by using a nickel-containing precursor, such as Ni(NO3)2·6H2O, Ni(NO3)2·6H2O, NiCl2, NiCl2·6H2O, NiBr2, NiF2, NiBr2·xH2O, NiBr2·3H2O. In certain aspects, the nickel-containing precursor is Ni(NO3)2·6H2O. In some aspects, the Cu of the Ni—Cu alloy catalyst is provided by using a copper-containing precursor, such as Cu(NO3)2·2.5H2O, CuSO4, CuCl2, Cu(NO3)2, Cu(NO3)2·3H2O, CuO, Cu(CH3COO)2, Cu3(PO4)2, Cu(ClO4)2, CuO2, Cu(hfac)2, CuO3Si, Cu(CO2CH3), Cu(NH3)4, Cu(SCN)2, Cu(NH3)4SO4·H2O, Cu(OH)2, CuBr2. In certain aspects, the copper-containing precursor is Cu(NO3)2·2.5H2O.
In some aspects, the catalyst system comprises Ni—Cu alloy catalysts that comprise nanoparticles having an average particle size ranging from greater than 0 nm to 10 nm before reaction in TCD. Such catalyst system aspects can be referred to herein as “fresh” catalyst system because they have not yet been exposed to a TCD cycle. In certain aspects, a “fresh” catalyst system (as referred to herein in certain examples and/or figures) is a catalyst system that has not been heated at a reaction temperature above 450° C. In some aspects, a “spent” catalyst system is a catalyst system that has undergone at least one TCD cycle. In certain aspects, a “spent” catalyst system is a catalyst system that has been heated at a reaction temperature above 450° C.
In certain aspects, the fresh Ni—Cu alloy catalysts comprise nanoparticles having an average particle size ranging from greater than 0 nm to 10 nm, such as from 1 nm to 9 nm, or 2 nm to 9 nm, or 3 nm to 9 nm, or 4 nm to 9 nm, or 5 nm to 9 nm, or 6 nm to 9 nm, or 7 nm to 9 nm, or 7 nm to 8 nm, or 8 nm to 9 nm. In one aspect, the Ni—Cu alloy catalysts comprise nanoparticles having an average particle size ranging from 7.3 nm to 8.6 nm.
In certain aspects, the fresh catalyst system comprises Ni—Cu alloy catalysts having a Ni:Cu mass ratio ranging from greater than zero to less than 5, and such Ni—Cu alloy catalysts comprise nanoparticles having an average particle size ranging from 1 nm to 10 nm before reaction in TCD. In one aspect, the catalyst system comprises Ni—Cu alloy catalysts having a Ni:Cu mass ratio ranging from greater than zero to 2, and such Ni—Cu alloy catalysts comprise nanoparticles having an average particle size ranging from 7 nm to 9 nm.
In some aspects, the Ni—Cu alloy catalysts comprise nanoparticles that exhibit a change in average particle size after being used in a TCD process, particularly after being exposed to temperatures of 600° C., or higher. In some such aspects, the nanoparticles may exhibit a 5% to 150% average particle size increase after reaction, such as from 5% to 120%, or 6% to 110%, or 40% to 110%.
In some particular aspects, the Ni—Cu alloy catalysts comprise Ni and Cu in amounts that provide a mass ratio ranging from 0.8 to 1.2, and can exhibit a 5% to 10% average particle size increase after reaction at 600° C., or higher. In certain aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio of 1, the reaction temperature is 600° C., and the nanoparticles have a size change ranging from 6% to 7% after reaction.
In some other aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio ranging from 1.5 to 2.5, and can exhibit a 40% to 50% average particle size increase after reaction at 600° C., or higher. In certain aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio ranging from 1.9 to 2.1, the reaction temperature is 600° C., and the nanoparticles have a size change ranging from 45% to 46% after reaction.
In yet other aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 0.75, and can exhibit an 80% to 120% average particle size increase after reaction at 600° C., or higher. In certain aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio ranging from 0.65 to 0.7, the reaction temperature is 600° C., and the nanoparticles have a size change ranging from 95% to 110% after reaction.
In some aspects, the Ni—Cu alloy catalysts comprises Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 0.75, the nanoparticles have similar particle sizes after reacting at a reaction temperature of 550° C., 600° C., 650° C., or higher. In certain aspects, the nanoparticles can exhibit a size change of less than 30% after reacting at a reaction temperature of 550° C., 600° C., 650° C., or higher. In one aspect, the nanoparticles can exhibit a size change of less than 25% after reacting at a reaction temperature of 550° C., 600° C., 650° C., or higher.
As described herein, the catalyst system can further comprise a support. Support components for use in the catalyst system are described herein.
The methane composition used in the method described herein can be obtained from industry sources that produce methane as a by-product, such as fossil fuel-generating (coal, oil, and/or natural gas industries), and/or industries that produce food waste and/or green waste. In some aspects, the methane composition comprises CH4 in an amount ranging from 1 vol % to 100 vol %, such as from 5 vol % to 90 vol %, 10 vol % to 80 vol %, 15 vol % to 70 vol %, 20 vol % to 60 vol %, 25 vol % to 50 vol %, 25 vol % to 40 vol %, or 25 vol % to 35 vol %. The methane composition can further comprise an inert gas, such as an inert gas selected from N2, Ar, and the like. In certain aspects, the methane composition comprises at least 30 vol % CH4 in N2. In some other aspects, the methane composition comprises 100 vol % CH4. The methane composition can be utilized at a suitable flow rate. In some aspects, the flow rate can range from 5 cm3/min to 120 cm3/min, such as 10 cm3/min to 100 cm3/min, or 15 cm3/min to 100 cm3/min, or 20 cm3/min to 100 cm3/min, or 30 cm3/min to 100 cm3/min. In some aspects, the flow rate is at least 30 cm3/min. In one aspect, the methane composition comprises 30 vol % CH4, which is utilized in the method at a flow rate of 30 cm3/min. In further aspects, the methane composition comprises 30 vol % CH4 in N2, which is utilized in the method at a flow rate of 30 cm3/min to maintain a constant space velocity of 9,000 cm3/g/h. In certain aspects, the methane composition comprises 100 vol % CH4 which is used at a flow rate ranging from 30 cm3/min to 120 cm3/min, such as at 30 cm3/min, 60 cm3/min or 120 cm3/min.
In some aspects, the reaction temperature used for the disclosed method ranges from 400° C. to 900° C., such as at least 500° C. to 800° C., or 500° C. to 700° C., or 550° C. to 700° C., or 550° C. to 700° C., or 600° C. to 700° C., or 650° C. to 700° C. In certain aspects, the reaction temperature ranges from 600° C. to 650° C., or 640° C. to 660° C., or 670° C. to 700° C. In particular aspects, the reaction temperature may be 550° C., 600° C., 650° C., or 700° C. In one aspect, the reaction temperature is 600° C. or 650° C. In some aspects, a reactor, in which at least the methane composition-catalyst system contacting step is carried out, can be heated by any practical means such as, for example, an electric tube furnace.
In some aspects, the method is performed for a reaction time period ranging from greater than 0 hours to several days, such as from 1 hour to 5 days, or 1 hour to 4 days, or 1 hour to 3 days, or 1 hour to 48 hours, or 1 hour to 24 hours, or 1 hour to 20 hours, or 1 hour to 14 hours, or 1 hour to 10 hours, from 1 hour to 8 hours, from 1 hour to 7 hours, or 1 hour to 6 hours, or 1 hour to 5 hours, or 1 hour to 4 hours. In certain aspects, the method is performed for a reaction time period ranging from 3 hours to 5 hours. In one aspect, the reaction time period is 4 hours. At least certain steps of the method can be carried out in a fixed-bed reactor, a continuous flow reactor, or other suitable reactor capable of being operated under batch and/or continuous flow conductions. In some aspects, the reactor is any reactor suitable for bench-scale work and evaluation and/or for industrial-scale processes.
Catalyst systems described herein can exhibit good stability under TCD conditions, even at high temperatures (e.g., temperatures above 500° C., such as above 550° C. or above 600° C.). In some aspects, catalyst systems include selected nickel and copper amounts that provide mass ratios that facilitate catalyst stability such that the catalyst system can remain stable over multiple TCD cycles. In some aspects, the catalyst system is stable for at least 1.5 hours, such as at least 2 hours, at least 3 hours, or at least 4 hours. In some aspects, the catalyst system comprises a Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to less than 5, or greater than zero to 4.5 and that remain stable for at least 1.5 hours, such as at least 2 hours, at least 3 hours, or at least 4 hours when exposed to reaction temperatures of at least 500° C., such as from 500° C. to 700° C., or 550° C. to 700° C., or 550° C. to 600° C., or 600° C. to 650° C.; or 650° C. to 700° C., or 600° C. to 700° C., or 650° C. to 700° C.
In particular aspects, the catalyst system comprises a Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5 and that remain stable for at least 4 hours when exposed to reaction temperatures ranging from 600° C. to 650° C. In certain aspects, the Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1 and that remain stable for at least 4 hours when exposed to reaction temperatures ranging from 600° C. to 650° C.
In some other aspects, the catalyst system comprises a Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5 and that remain stable for at least 2 hours when exposed to reaction temperatures ranging from 660° C. to 700° C. In certain aspects, the Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1 and that remain stable for at least 2 hours when exposed to the reaction temperatures ranging from 660° C. to 700° C. In one aspect, the Ni—Cu alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1 and that remain stable for at least 1.5 hours when exposed to reaction temperatures ranging from 690° C. to 710° C.
In some aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5, the reaction temperature is at least 550° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours. In certain aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 2, the reaction temperature ranges from 550° C. to 600° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours. In one aspect, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1, the reaction temperature ranges from 550° C. to 600° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours.
In some aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5, the reaction temperature is at least 600° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours. In certain aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 2, the reaction temperature ranges from 600° C. to 650° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours. In one aspect, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1, the reaction temperature ranges from 600° C. to 650° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 30% for at least 4 hours.
In some aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5, the reaction temperature is at least 640° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 40% for at least 4 hours. In certain aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 2, the reaction temperature ranges from 640° C. to 690° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 45% for at least 4 hours. In one aspect, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1, the reaction temperature ranges from 640° C. to 660° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 40% for at least 4 hours.
In yet other aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5, the reaction temperature is at least 670° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 10% for at least 1.5 hours. In certain aspects, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 2, the reaction temperature ranges from 670° C. to 710° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 10% for at least 1.5 hours. In one aspect, the catalyst system comprises an alloy catalyst having Ni and Cu in amounts that provide a mass ratio ranging from 0.5 to 1, the reaction temperature ranges from 670° C. to 700° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 10% for at least 1.5 hours.
In some representative aspects of the disclosure, the reaction temperature is at least 500° C., the methane composition comprises CH4 in an amount ranging from 10 vol % to 50 vol %, and H2 is produced at a rate ranging from 0.5 g H2/(g metal·h) to 15 g H2/(g metal·h). In certain other aspects, the reaction temperature ranges from 550° C. to 700° C., the methane composition comprises CH4 in an amount ranging from 20 vol % to 40 vol %, and the H2 is produced at a rate ranging from 0.5 g H2/(g metal·h) to 15 g H2/(g metal·h). In one aspect, the reaction temperature is 600° C., the methane composition comprises 30 vol % CH4, and the H2 is produced at a rate ranges from 0.5 g H2/(g metal·h) to 4 g H2/(g metal·h). The any or all of the above-mentioned aspects, the catalyst may comprise Ni and Cu in amounts sufficient to provide a Ni:Cu mass ratio ranging from greater than zero to 4.5, such as 0.67, 1, 2, or 4.5 or less.
Also described herein are carbon co-products generated by the method of the present disclosure. The carbon co-products disclosed herein can be controlled in terms of morphology and yield utilizing operational parameters described herein and/or by varying metal content and/or ratios in the catalyst system. In some aspects, the carbon co-products comprise a carbon nanomaterial, such as carbon nanotubes (including single-wall CNTs, double-wall CNTs, and multi-wall CNTs). In some aspects, the carbon co-product can be controlled such that it is provided in the form of the carbon nanomaterial, or in the form of graphene or graphite. In particular aspects, operational and/or catalyst system parameters are modified to provide different forms of the carbon nanomaterial, such as to provide SWCNTs, DWCNTs, or MWCNTs. In some aspects, the carbon co-product can comprise 50% to 100% multi-wall CNT, such as from 60% to 100%, or 70 to 100%, or 80 to 100%.
In some aspects, the carbon co-product can be MWCNTs with diameters ranging from 3 nm to 40 nm, such as from 4 nm to 35 nm, or 5 nm to 30 nm, or 10 nm to 25 nm, or 15 nm to 30 nm. In certain aspects, the carbon co-product comprises MWCNTs with diameters ranging from 20 nm to 30 nm. In some particular aspects, the carbon co-product can be MWCNTs with outer diameters ranging from 3 nm to 40 nm, such as from 4 nm to 35 nm, or 5 nm to 30 nm, or 10 nm to 25 nm, or 15 nm to 30 nm. In certain aspects, the carbon co-product comprises MWCNTs with outer diameters ranging from 20 nm to 30 nm. In some such embodiments, the support and the carbon co-product can be the same material.
In some aspects, Raman spectroscopy is used to assess the morphology, diameter, and/or identity of the carbon co-product. In some such aspects, three main bands within the Raman spectrum of a sample are analyzed to determine an amount of “structured” versus “unstructured” carbon co-product. These three bands include: i) the D-band (located at wavenumbers ranging from 1300 cm−1 to 1400 cm−1, such as 1340 cm−1), which is associated with defects in the graphitic lattice; ii) the G-band (located at wavenumbers ranging from 1500 cm−1 to 1600 cm−1, such as 1580 cm−1), which is associated with ordered carbon; and iii) the G′-band (or 2D band) (located at wavenumbers ranging from 2600 cm−1 to 2800 cm−1, such as 2700 cm−1), which is associated with interactions between stacked graphene layers and that can be used to distinguish between SWCNTs or MWCNTs. The ID/IG ratio, as discussed herein, represents the ratio of the intensity of the D band (ID) relative to the intensity of the G band (IG). The IG′/IG ratio, as discussed herein, represents the ratio of the intensity of the G′ band (IG′) to the intensity of the G band (IG). In some aspects, the ID/IG ratio can range from 1 to 2, such as 1.1 to 1.9, or 1.1 to 1.6; and the IG′/IG ratio can range from 0.3 to 1.3, such as 0.4 to 1.2, or 0.5 to 1. In some aspects wherein the catalyst comprises Ni and Cu in an amount that provides a Ni:Cu mass ratio ranging from greater than zero to 4.5, the ID/IG ratio can range from 1.1 to 1.81, the IG′/IG ratio can range from 0.407 to 1.11. In yet other aspects, the ID/IG ratio can range from 1.1 to 1.6, and the IG′/IG ratio can range from 0.55 to 0.95.
In some particular aspects, the reaction temperature ranges from 500° C. to 700° C., such as 550° C., 600° C., 650° C., or 700° C., the catalyst comprises Ni and Cu in an amount that provides a Ni:Cu mass ratio ranging from greater than zero to 4.5, and the carbon co-product has an ID/IG ratio ranging from 1 to 2, such as from 1.2 to 2, or 1.4 to 2, or 1.6 to 2, or 1.8 to 2; and an IG′/IG ratio lower than 0.90. In one aspect, the reaction temperature is 600° C., the catalyst comprises Ni and Cu in amounts that provide a mass ratio of greater than zero to 4.5, and the carbon co-product has an ID/IG ratio ranging from 1.5 to 2 and an IG′/IG ratio of lower than 0.70.
In some aspects, the catalyst comprises Ni and Cu in a Ni to Cu mass ratio ranging from greater than zero to 1, such as from 0.1 to 0.9, or greater than 0.5 to 0.8, or greater than 0.6 to 0.7, the reaction temperature ranges from 500° C. to 700° C., such as 550° C., 600° C., 650° C., or 700° C., and the carbon co-product has an ID/IG ratio ranging from 1 to 2 and an IG′/IG ratio lower than 1.2. In one aspect, the reaction temperature is 550° C., and the carbon co-product has an ID/IG ratio ranging from 1.7 to 1.8, and an IG′/IG ratio ranging from 0.6 to 0.7. In another aspect, the reaction temperature is 600° C., and the carbon co-product has an ID/IG ratio ranging from 1.8 to 1.9 and an IG′/IG ratio ranging from 0.4 to 0.5. In yet other aspects, the reaction temperature is 650° C., and the carbon co-product has an ID/IG ratio ranging from 1.7 to 1.8 and an IG′/IG ratio ranging from 0.7 to 0.8. In another aspect, the reaction temperature is 700° C., and the carbon co-product has an ID/IG ratio ranging from 1.0 to 1.1 and an IG′/IG ratio ranging from 1.1 to 1.2.
With the method described herein, CH4 conversion, XCH
where FIn is the flow rate of the feed gas before reaction starts, [CH4]In is the concentration of CH4 in the feed gas determined by GC; Fout is the flow rate of the outlet gas; [CH4]Out is the concentration of CH4 in the outlet gas determined by GC.
Carbon yield, YC(t), and the rate of deposition of carbon are calculated as the accumulated weight of carbon per mass of the catalyst based on the CH4 conversion. In some aspects, the method is performed for a reaction time sufficient to provide a total carbon yield of at least 3.5 gcarbon/gcatalyst, or until at least 80% of the carbon present in a support used with the catalyst system (e.g., 80% of the amount of carbon present in a spent catalyst system) is equivalent to the amount of carbon co-product generated from the method. In such aspects, such an accumulation rate facilitates the ability to use the spent catalyst system as a metric to determine the amount of carbon co-product produced, rather than the starting carbon amount present in a fresh catalyst system.
In some aspects, the mole balance ranges between 95 and 100% and can be calculated using Equation 2:
To approximately quantify the deactivation of the catalysts, first, it is assumed that the carbon accumulated with time on stream, θ, raised to a small power, viz. C∝θ0.5, as postulated for catalytic cracking. Second, it is assumed that the catalyst deactivation could be represented through a poisoning factor, ϕ, that would multiply the initial rate of conversion, X0:
X(θ)=X0×Φ(C(θ)) (3)
It is assumed that ϕ(C) is a decaying exponential that depended on the accumulated carbon:
Φ(C)=exp(−kC) (4)
The CH4 conversion is fit as a function of time on stream using a functional form with two coefficients, the initial conversion, X0, and a deactivation rate parameter, k.
X(θ)=X0×exp(−kθ0.5) (5)
The mass of carbon at any time on stream, C(θ), is estimated by integrating equation 5 multiplied by the inlet hourly mass flow rate of carbon, Cfeed and the initial, fitted conversion, X0
In some aspects, the predicted amount of carbon co-product accumulated (carbon yield) is related to the actual amount of carbon measured. The projected carbon yield extrapolated the accumulation of carbon co-product to θ=∞ normalized by the weight of catalysts:
wherein Cf is carbon feeding rate, X0 is initial conversion, K is deactivation constant, θ is reaction time.
As confirmed with Equation 8, the disclosed method can be expected to produce high carbon yields. For example, in some aspects comprising (i) a catalyst system that comprises Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5 and (ii) a reaction temperature ranging from 550° C. to 650° C., a projected carbon yield ranging from 0 g carbon/g catalyst to 6150 g carbon/g catalyst can be obtained. In yet further aspects comprising (i) a catalyst system that comprises Ni and Cu in amounts that provide a mass ratio of 0.67 and (ii) a reaction temperature of 650° C., a projected carbon yield ranging from 0 g carbon/g catalyst to 6150 g carbon/g catalyst can be obtained.
In some aspects comprising (i) a catalyst system, (ii) a reaction temperature ranging from 550° C. to 700° C., and (iii) a methane composition comprises 100 vol % CH4, a carbon co-product can be produced at a carbon deposition rate ranging from 5 to 50 g carbon/(g metal·h).
In some aspects comprising (i) a catalyst system that comprises Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5 and (ii) a reaction temperature ranging from 550° C. to 700° C., a carbon co-product can be produced at carbon deposition rate ranging from 1 g carbon/(g metal·h) to 4 g carbon/(g metal·h). In one aspect, the reaction temperature is 650° C., the carbon co-product can be produced at carbon deposition rate ranging from 2 g carbon/(g metal·h) to 3 g carbon/(g metal·h). In another aspect, the reaction temperature is 600° C., the carbon co-product can be produced at carbon deposition rate ranging from 1 g carbon/(g metal·h) to 2 g carbon/(g metal·h). In yet another aspect, the reaction temperature is 550° C., the carbon co-product can be produced at carbon deposition rate ranging from 1 g carbon/(g metal·h) to 2 g carbon/(g metal·h).
In some aspects comprising (i) a catalyst system that comprises Ni and Cu in amounts that provide a mass ratio ranging from greater than zero to 4.5 and (ii) a reaction temperature ranging from 550° C. to 700° C., a carbon yield of at least 5 g carbon/g catalyst after contacting the methane composition and the catalyst system for 5 hours can be obtained. In one aspect, the reaction temperature is 550° C., a carbon yield ranging from 5 g carbon/g catalyst to 7 g carbon/g catalyst after contacting the methane composition and the catalyst system for 5 hours can be obtained. In another aspect, the reaction temperature is 600° C., a carbon yield ranging from 7 g carbon/g catalyst to 10 g carbon/g catalyst after contacting the methane composition and the catalyst system for 5 hours can be obtained. In yet another aspect, the reaction temperature is 650° C., a carbon yield ranging from 10 g carbon/g catalyst to 15 g carbon/g catalyst after contacting the methane composition and the catalyst system for 5 hours can be obtained.
In some aspects, the disclosed method further comprises separating the catalyst system from the carbon co-product. In certain aspects, separating the catalyst system from the carbon co-product comprises contacting the catalyst system and the carbon co-product with an acid to produce a suspension, wherein the suspension comprises (i) the carbon co-product (in the form of a solid) and (ii) a liquid solution; and separating the carbon co-product from the liquid solution. In some aspects, the acid can be selected from nitric acid (HNO3) or other suitable acids. In certain aspects, the liquid solution comprises metal salts, such as metal nitrates (e.g., as Ni(NO3)2 and/or Cu(NO3)2). In some aspects, the carbon co-product is separated from the liquid solution by using filtration.
In aspects where the carbon co-product is separated from the catalyst system, it can be reused in the method. In such aspects of the disclosure, the method can further comprise a regeneration step, which involves using the carbon co-product as the support component of the catalyst system. In some such aspects, the TCD reaction of the method can be repeated for multiple cycles, wherein one or more reaction cycle comprises separating the carbon co-product from the catalyst system. In certain aspects, repeating the reaction for multiple cycles comprises repeating the cycle for from four times to a hundred times, such as two times, three times, four times, five times, or six times.
In some aspects, the method for using the catalyst system comprises at least two reaction cycles, such as two reaction cycles, three reaction cycles, four reaction cycles, or five reaction cycles. In one aspect, the method for using the catalyst system comprises performing at least four reaction cycles. A reaction cycle typically includes (i) contacting the methane composition with the catalyst system; (ii) treating the catalyst system (after reaction with the methane) with an acid treatment; (iii) separating the carbon co-product from the liquid solution comprising the metal salts; (iv) regenerating the metal precursors by concentrating and/or crystallizing the metal salts; and (vi) combining the metal precursors with at least a portion of the isolated carbon co-product.
In some aspects, repeating the reaction for multiple cycles comprises using the carbon co-product generated from a previous reaction cycle as the support in a subsequent reaction cycle. In such aspects, steps of the disclosed method are repeated over at least two cycles, wherein the methane composition is contacted with a catalyst system comprising a Ni—Cu alloy catalyst and a support at a reaction temperature to produce H2 and a carbon co-product; separating the catalyst system from the carbon co-product; regenerating the metal precursors for use in a regenerated catalyst system that is used in a subsequent cycle; isolating the carbon co-product; contacting the methane composition with the regenerated catalyst system to produce H2 and additional amounts of a carbon co-product, wherein the support used with the regenerated catalyst system is the carbon co-product generated and isolated from the previous reaction cycle. In further aspects, repeating the reaction for multiple cycles comprises repeating the cycle for at least two times, such as two times, three times, four times, five times, or six times.
The carbon co-product may be mixed with the spent catalyst which comprises metal nanoparticles. In some aspects, the carbon co-product oxidizes at a temperature of at least 300° C. in the presence of metal nanoparticles. In certain aspects, the carbon co-product is CNT, and the carbon co-product oxidizes at a temperature of from 300° C. to 500° C. In one aspect, the carbon co-product is CNT, and the carbon co-product oxidizes at a temperature of from 400° C. to 450° C.
Also disclosed herein is a method for making the catalyst system described herein. In some aspects, the method can comprise a sequential impregnation (SI) technique, a solvothermal method (ST) technique, an incipient wetness (IW) technique, or a co-impregnation (CI) technique. In particular aspects, the method comprises an SI technique.
Disclosed herein are methods to make a catalyst system by sequential impregnation, in which substances are applied to a support material or surface of the support material in a sequential order. The process comprises deposition of different layers of the substances onto the support material. Each layer is applied one after another in a sequential order.
In some aspects, the method comprises i) contacting a solution containing a first metal with a support material to impregnate the support material with the first metal, thereby forming an impregnated support; ii) heating the impregnated support using a ramping temperature protocol to provide a pre-catalyst system; (iii) contacting the pre-catalyst system with a second metal to form a bimetallic impregnated support; and (iv) heating the bimetallic impregnated support using the ramping temperature protocol to provide the catalyst system. In some aspects, the method can further comprise performing a preliminary heating step before performing the ramping temperature protocol.
In some aspects, the support material may be a non-carbonaceous support, such as a silica support. In an independent embodiment, the support material and/or the catalyst system does not comprise alumina. In some other aspects, the support material is a carbonaceous support. In certain aspects, the support material comprises a carbon material, such as a carbon nanomaterial (e.g., single-wall CNT, double-wall CNT, multi-wall CNT, and the like). In one aspect, the support material comprises multi-wall CNTs. The support material may be generated carbon co-product from one or more previous reaction cycles.
The support may be treated with acid to functionalize its surface. In some aspects, the support is treated with an acid to generate an acid-washed support prior to combing the acid-washed support with the alloy catalyst. In certain aspects, the acid comprises nitric acid (HNO3). In some aspects, the support is carbon co-product generated from previous reactions, and the acid-washed support is acid-washed generated carbon co-product. In certain aspects, the support is CNT, and the acid-washed support is acid-washed CNT. In one aspect, the support is CNT, the acid is nitric acid (HNO3), and the acid-washed support is an acid-washed CNT (HCNT).
In some aspects, the ramping temperature protocol comprises increasing a temperature to which the impregnated support is exposed by 2° C./min to 10° C. per minute, such as from 3° C. per minute to 9° C. per minute, or 4° C. per minute to 8° C. per minute, or 4° C. per minute to 7° C. per minute, or 4° C. per minute to 6° C. per minute. In certain aspects, the temperature is increased by 5° C. per minute.
In some aspects, the final temperature of the ramping temperature protocol ranges from 300° C. to 400° C., such as from 310° C. to 390° C., or 320° C. to 380° C., or 330° C. to 370° C., or 340° C. to 360° C. In certain aspects, the final temperature ranges from 345° C. to 355° C. In one aspect, the final temperature is 350° C.
In some aspects, the first metal and the second metal are Ni and Cu. In one aspect, the first metal is Ni, the second metal is Cu. In another aspect, the first metal is Cu, and second metal is Ni.
In further aspects, a preliminary heating step is performed before performing the ramping temperature protocol. In such aspects, the preliminary heating step comprises heating the impregnated support at a temperature ranging from 130° C. to 200° C. for a time period ranging from 6 hours to 10 hours. In some aspects, the temperature in the preliminary step ranges from 100° C. to 200° C., such as from 110° C. to 170° C., from 120° C. to 160° C., from 120° C. to 150° C., and from 130° C. to 150° C. In certain aspects, the temperature ranges from 130° C. to 150° C. In one aspect, the temperature is 140° C.
In some aspects, the time in the preliminary step ranges from 6 hours to 10 hours, such as from 7 hours to 10 hours, from 7 hours to 9 hours. In certain aspects, the time ranges from 7.5 hours to 8.5 hours. In one aspect, the time is 8 hours. In certain aspects, the temperature in the preliminary step is 140° C., and the time in the preliminary step is 8 hours.
In some independent aspects, the catalyst system is prepared by ST deposition. In this process, catalyst precursors are dissolved in a solvent to react and form the catalyst. In certain aspects, the catalyst system comprises Ni and Cu, and the Ni and Cu precursors are dissolved in a solvent to form a solution, the support material is added to the solution to create a mixture, and the mixture is heated and dried to create the catalyst system.
In some independent aspects, the catalyst system is prepared by IW impregnation. In certain aspects, the catalyst system comprises Ni and Cu, and the catalyst synthesized by incipient wetness impregnation (IW) may be prepared by slowly adding a concentrated aqueous solution of Ni and Cu to the support material to just wet the support and create a slurry. Then, the slurry may be dried and heated to create the catalyst system.
In some independent aspects, the catalyst system is prepared by wet CI. In certain aspects, the catalyst system comprises Ni and Cu, and the catalyst synthesized by wet CI were prepared by mixing an aqueous solution containing Ni and Cu with the support material to create a mixture. The mixture may be dried and heated to create the catalyst system.
Disclosed herein are aspects of a method, comprising: contacting a methane composition with a catalyst system at a reaction temperature ranging from 500° C. to 700° C. to produce H2 and a carbon co-product; wherein the catalyst system comprises (i) a Ni—Cu alloy catalyst comprising Ni and Cu, and (ii) a support, wherein the Ni and Cu are present at a Ni:Cu mass ratio ranging from greater than zero to 4.5.
In any or all of the above aspects, the method further comprises separating the catalyst system from the carbon co-product.
In any or all of the above aspects, separating the catalyst system from the carbon co-product comprises: contacting the catalyst system and the carbon co-product with an acid to produce a suspension comprising (i) the carbon co-product and (ii) a liquid solution; and separating the carbon co-product from the liquid solution.
In any or all of the above aspects, the carbon co-product is used as the support in the Ni—Cu alloy catalyst.
In any or all of the above aspects, the carbon co-product is treated with an acid prior to combining the carbon co-product with the Ni and the Cu.
In any or all of the above aspects, the reaction temperature is 600° C., and the carbon co-product has an ID/IG ratio ranging from 1 to 2, and/or an IG′/IG ratio lower than 0.70.
In any or all of the above aspects, the reaction temperature ranges from 550° C. to 700° C., the methane composition comprises 30 vol % CH4, and the H2 is produced at a rate ranging from 0.5 to 15 g H2/(g metal·h).
In any or all of the above aspects, the reaction temperature ranges from 550° C. to 700° C., and the carbon co-product is produced at carbon deposition rate ranging from 1 to 4 g carbon/(g metal·h).
In any or all of the above aspects, the reaction temperature ranges from 600° C. to 650° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 25% for at least 4 hours.
In any or all of the above aspects, the reaction temperature ranges from 670° C. to 700° C., and the methane composition is converted to H2 at a CH4 conversion rate of at least 10% for at least 1.5 hours.
In any or all of the above aspects, the Ni—Cu alloy catalyst comprises nanoparticles having an average particle size ranging from greater than 0 nm to 10 nm before the Ni—Cu alloy catalyst is contacted with the methane composition.
In any or all of the above aspects, the nanoparticles exhibit a particle size change after being contacted with the methane composition at a reaction temperature of 600° C., and the Ni—Cu alloy catalyst comprises nanoparticles that exhibit a size change ranging from 40% to 110% after reaction.
In any or all of the above aspects, the Ni and Cu are present at a Ni:Cu mass ratio ranging from greater than zero to 2.
In any or all of the above aspects, the Ni and Cu are present at a Ni:Cu mass ratio ranging from or 0.6 to 0.7.
In any or all of the above aspects, the Ni and Cu are present at a Ni:Cu mass ratio ranging from 0.1 to 2; and the reaction temperature ranges from 550° C. to 700° C.
Also disclosed are aspects of a method, comprising: contacting a methane composition with a catalyst system at a reaction temperature ranging from 600° C. to 650° C. to produce H2 and a carbon nanotube; wherein the catalyst system comprises (i) a Ni—Cu alloy catalyst comprising Ni and Cu, and (ii) a carbonaceous support, wherein the Ni and Cu are present at a Ni:Cu mass ratio ranging from 0.6 to 0.7.
Also disclosed are aspects of a method for making a catalyst system, comprising: i) contacting a solution comprising a first metal with a support material to impregnate the support material with the first metal, thereby forming an impregnated support; ii) heating the impregnated support using a ramping temperature protocol to provide a pre-catalyst system, wherein the ramping temperature protocol comprises increasing a temperature to which the impregnated support is exposed by 5° C. per minute until a final temperature of 350° C. is reached; iii) contacting the pre-catalyst system with a second metal to form a bimetallic impregnated support; and (iv) heating the bimetallic impregnated support using the ramping temperature protocol to provide the catalyst system; wherein the first metal and the second metal are different from each other and independently are selected from Ni and Cu and wherein the first metal and the second metal provide a Ni:Cu mass ratio ranging from greater than zero to 4.5.
In any or all of the above aspects, the method further comprises performing a preliminary heating step before performing the ramping temperature protocol, wherein the preliminary heating step comprises heating the impregnated support at a temperature ranging from 130° C. to 200° C. for a time period ranging from 6 hours to 10 hours.
Materials and Catalyst Synthesis
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), copper nitrate hemi-pentahydrate (Cu(NO3)2·2.5H2O), acetone and concentrated HNO3 were purchased from Sigma Aldrich (St. Louis, MO). Multiwalled CNTs with outer diameter 20-30 nm were purchased from Cheap Tubes (catalog number 030104) (Grafton, VT).
Preparation of Ni—Cu/CNT Catalyst Systems Via the Solvothermal (ST) Method
A series of Ni—Cu/CNT catalyst systems were prepared with different total metal weight loadings (nominally 5.5 wt. %, 11 wt. %, 22 wt. %, and 44 wt. %) following the ST method. The mass ratio between Ni and Cu was kept constant at 10. In the typical ST synthesis of a 10 wt. % Ni-1 wt. % Cu on CNT [10Ni-1Cu/CNT (ST)], 0.347 g of Ni(NO3)2·6H2O and 0.0154 g of Cu(NO3)2·2.5H2O first were dissolved in 60 mL acetone and sonicated for 30 min. As-received CNT (0.626 g) was added to the acetone solution and sonicated for additional 30 min. The mixture was then transferred to a 100 mL Teflon-lined Parr reactor, sealed, and stirred for 30 min. The Parr reactor was heated to 120° C. for 1 hour and maintained at this temperature for 12 hours under static conditions. After cooling to room temperature, the solution was retrieved from the Parr reactor, placed in a glass container, and allowed to evaporate overnight at room temperature and atmospheric pressure in a hood. The dry solids were placed in a furnace with stagnant air at 80° C. overnight. The dry solids then were crushed and sieved (>100 mesh) and stored in a glass vial.
Preparation of Ni—Cu/CNT Catalyst System Via Incipient Wetness (IW) Impregnation
A series of Ni—Cu/CNT catalyst systems were prepared with different total metal weight loadings (11 wt. %, 22 wt. %, and 44 wt. %) following the IW impregnation method. The mass ratio between Ni and Cu was kept constant at 10. In the typical IW synthesis of a 10 wt. % Ni-1 wt. % Cu on CNT [10Ni-1Cu/CNT (IW)], an aqueous solution (0.635 mL) of Ni(NO3)2·6H2O (0.595 g) and Cu(NO3)2·2.5H2O (0.0439 g) was slowly added to as-received CNT (1.068 g) using the amount of liquid that was previously determined to just wet the support (0.595 mL/g). Then, the slurry was dried in a furnace under stagnant air at 80° C. overnight. Once dry, the solids were heated in air at 140° C. for 8 hours followed by heating a 500 mg aliquot in flowing N2 (30 cm3/min) at 350° C. for 3 hours. The temperature ramp rate was 5° C./min. The solids were cooled to room temperature and stored in a glass vial.
The catalyst systems were characterized before reaction (fresh) and after reaction (spent) to determine their stability and to determine relationships between activity, stability, and surface properties. Fresh catalyst systems (500 mg) were reduced at 400° C. for 4 hours under 30 cm3/min of 5 vol. % H2 in N2 followed by heating to reaction temperature (typically 600° C.) in 30 cm3/min N2. The samples were cooled to room temperature and then passivated by flowing (30 cm3/min) of 1.0 vol. % O2 in N2 overnight. The spent catalyst systems were characterized as retrieved from the TCD reactor.
Nitrogen (N2) physisorption at 77 K by the fresh and spent catalyst systems was measured using a Quadrasorb EVO/SI Gas Sorption System from Quantachrome Instruments. Samples were degassed at 150° C. under vacuum for 12 hours. Surface areas were determined using the five-point Brunauer-Emmett-Teller method from the adsorption data in the relative pressure range of 0.05-0.3. Metal loadings (Table 1) were determined by inductively coupled plasma optical emission spectrometry.
X-ray diffraction (XRD) patterns were collected using a Rigaku SmartLab SE Bragg-Brentano diffractometer, equipped with a fixed Cu anode operated at 40 kV and 44 mA and a D/Tex Ultra 250 one-dimensional detector. Patterns were collected with a variable divergence slit between 2°<2θ<100° at intervals of 0.01°. The composition, lattice parameters, and crystallite sizes of the crystalline components were determined by Rietveld fitting between 30°<2θ<100° using Topas v6 (Bruker AXS) as discussed elsewhere. Because of the presence of Ni—Cu alloys containing a range of compositions, it is acknowledged that this method could underestimate the crystallite size. The compositions of the metallic phases (Table 2) were estimated from their refined cubic lattice parameters by linear interpolation between Ni (a=3.5238 Å) and Cu (a=3.615 Å). The XRD patterns are shown in
A Micromeritics AutoChem 2920 instrument was used to conduct temperature programmed oxidation (TPO). Samples (ca. 50 mg) were first loaded and pretreated at 120° C. for 120 min under helium, and then heated to 800° C. at a ramp rate of 5° C.·min−1 in 5 vol. % O2 in helium flowing at 30 cm3/min.
Raman spectra were recorded on a Renishaw InVia Raman microscope with excitation from a 10 mW laser (532 nm) Each spectrum was averaged over three scans to characterize the solid carbon co-produced by TCD of CH4.
Morphology of solid carbon co-products, metal particle size, and element distribution before and after reaction were determined using a FEI Titan 80-300 high-resolution transmission electron microscope operated at 300 kV. The microscope was equipped with a CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, and an energy-dispersive X-ray spectroscopy detector. Metal particle size and composition distributions were calculated from the images by sampling an average of 100 particles.
A JEOL 7001F field emission gun scanning electron microscope (SEM) equipped with a dual Bruker X-Flash|60 EDS detector was also used to assess the morphology of the carbon co-product at 2 kV accelerating voltage. Imaging and X-ray spectroscopy detection were performed in high vacuum mode at 15 kV accelerating voltage.
A Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70) was used to obtain the infrared absorption spectra of samples embedded in potassium bromide (KBr) pellets. KBr pellets are prepared by mixing and grinding 0.3 mg of sample with 300 mg KBr, using a mortar and pestle. Then, the mixture was compressed for 3 min (10,000 lb/in2) using a hydraulic press and a 12 mm diameter die. Each FTIR result was obtained by accumulating 128 scans with a resolution of 2 cm−1.
In this example, a fixed-bed, continuous-flow, vertical stainless-steel reactor was used for the TCD reaction at ambient pressure. As-synthesized catalyst systems (0.2 g, density ≃0.33 g/cm3) were loaded between two plugs of quartz wool. N2 gas was used as a carrier gas and as an internal standard for product analysis (on-line gas chromatography). Prior to each test, the 0.2 g catalyst system samples were reduced in situ at 400° C. for 4 hours under 70 cm3/min 10 vol. % H2 in N2 at a ramp rate of 3° C./min. Subsequently, the reactor was heated to the reaction temperature (e.g., 550-700° C.) under 70 cm3/min of N2. Before reaction, the H2 had been completely purged from the system (monitored by on-line gas chromatograph). Then, the feed was switched to 30 cm3/min of 30 vol. % CH4 in N2 to maintain a constant space velocity of 9,000 cm3/g/h (≃3000 h−1 at the assumed density of the bed). The outlet gas flow rate was measured by a digital flow meter (DryCal). Composition of the outlet gas was analyzed by a two-channel Inficon Micro GC Fusion equipped with molecular sieve 5A and PLOT U columns and a TCD detector. When the test concluded, the reactor system was cooled to room temperature under 30 cm3/min of N2, and the spent catalyst systems (containing solid carbon co-product) were retrieved from the reactor for analysis. Hydrogen was the only gaseous reaction product; CO2 and carbon monoxide were not detected.
Conversion of the CH4, carbon yield, and the rate of deposition of carbon were monitored as a function of time-on-stream (TOS). Typically, the reactions were run for over 14 hours to make sure it achieved a minimum total carbon yield of at least 3.56 gcarbon/gcatalyst (e.g., ≃80% of the carbon in the spent catalyst systems was freshly accumulated carbon co-product), which typically took between 7 and 8 hours for the 10Ni-1Cu/CNT catalyst systems (see
After retrieving the spent catalyst system from the reactor, they were crushed and sieved to >100 mesh. Crushed solids were mixed with a solution of 5 M HNO3 (in deionized water) using a volume of acid solution-to-crushed solid mass of 50. In a typical treatment, 5 g of spent catalyst systems were treated using 250 mL of solution in a 500 mL round-bottomed flask. The flask was submerged in an oil bath connected to a condenser cooled to 5° C. The top of the condenser was connected to house N2 at atmospheric pressure to mitigate the loss of solution. The oil bath was heated to 120° C. and maintained at that temperature for 24 hours. The suspension was cooled to room temperature and then sieved using 100, 200, and 480 mesh sieves to size select and retain the agglomerates. Each trance was rinsed with deionized water until the pH of the water passing through the carbon was 5.5-6. The collected solution was initially black because of the presence of small carbon particles that were not collected by the mesh. Once the carbon particles settled, the color of the solution became slightly green because of the presence of dissolved Cu. The solids collected by the mesh were placed in a drying oven under stagnant air at 80° C. for 12 hours. The dry solids then were cooled to room temperature, weighed, and stored in a glass vial. The total mass of metal originally present in the 5 g of spent catalysts was 122 mg (e.g., 1.11 g fresh catalyst system with 11 wt. % metal). The dry collected solids lost 250 to 450 mg during the acid wash step, suggesting that between 90.8.5% and 94.9% of the carbon co-product were recovered (=(5,000 mg−122 mg−450 mg)/(5,000 mg−122 mg)).
In this example, a series of Ni—Cu catalyst systems prepared via the ST technique, with an eightfold range of metal loadings but a constant Ni:Cu weight ratio of 10:1, was tested at 600° C. under 30 cm3/min of 30 vol. % CH4 in N2 and 30-120 cm3/min of 100 vol. % to study the effects of CH4 concentration and CH4 residence time on catalyst system performance. The results, CH4 conversion, carbon deposition rate, and H2 production rate, are summarized in
Among catalyst systems tested under the same reaction conditions, there is a <20% variation in the average initial CH4 conversion during the initial 20 min of TOS irrespective of total metal loading (
Table 2 summarizes the XRD analysis of the fresh and spent catalyst systems and reveals that there was a difference in metal crystallite size between the fresh and spent catalyst systems. The ST catalyst systems with more stable TCD performances (20Ni-2Cu and 40Ni-4Cu) had the largest crystallite sizes (18.8 and 26.9 nm, respectively) while the less stable catalyst systems (5Ni-0.5Cu and 10Ni-1Cu) had a smaller crystallite sizes (11.4 and 11 nm, respectively) for the fresh catalyst systems. A change was observed in the crystallite site and Ni:Cu compositions during the reaction experiment. After reaction, the most stable ST catalyst systems (20Ni-2Cu and 40Ni-4Cu) had larger metal crystallite sizes (23.7 and 29.2 nm, respectively) compared to the less stable catalyst systems (5Ni-0.5Cu and 10Ni-1Cu) that had a smaller crystallite size (11.8 and 9.4 nm, respectively). A decrease was observed in the Ni:Cu ratio (e.g., loss of Ni) of the metal crystallites for the all the spent catalyst system regardless of the metal loading, suggesting that Ni is being selectively lost during the reaction. The metal particle size distribution was evaluated via SEM images shown in
Hence, in this example, a continuous loss of Ni from the active metal particle during reaction might form Ni deficient particles and cause catalyst system deactivation. The particle size effect on catalyst systems synthesized via IW was explored by changing the metal weight loading (and a constant Ni:Cu to Cu mass ratio of 10). As shown in
The fresh and spent catalyst systems via XRD was evaluated and it was observed that the average crystallite sizes (e.g., particle size) of the IW catalyst system were consistently smaller for the fresh (7.9 nm-9.31 nm) and spent catalyst systems (9.60 nm-14.5 nm) compared to those obtained via ST (Table 2). In this example, the smaller crystallite size was associated with faster catalytic deactivation. The metal particle size of the spent catalyst systems was evaluated via SEM and it was observed that the 10Ni-1Cu/CNT (IW) was primarily composed of <20 nm particles (
In this example, the TCD performance (at a constant Ni:Cu ratio and temperature) was also evaluated by changing CH4 composition and space velocity (
In this example, the spent catalyst systems were analyzed via XRD, TPO, SEM, and Raman spectroscopy to characterize the property and morphologies of the carbon co-product. The XRD analysis (
As shown in
In this example, the quality of the carbon co-product can be assessed using Raman spectroscopy by monitoring the D-band, G-band, and G′-band.
is 5Ni-0.5Cu-CNT, Δ is 10 Ni-1Cu-CNT, □ is 20 Ni-2Cu-CNT, and ∘ is 40Ni-4Cu/CNT. The reactions conditions were 600° C. under 30 vol. % CH4 in N2 (hollow symbols) and 100 vol. % CH4 (solid symbols) The Raman spectra can be found in
Raman spectroscopy was performed on the spent catalyst systems synthesized by IW and observed similar trends to that of the ST catalyst systems (
The 10Ni-1Cu/CNT formulation was used to demonstrate the MWCNT harvesting and catalyst system regeneration cycle as it had the highest carbon deposition rate, carbon yield, and longevity out of all the ST and IW catalyst systems evaluated by this example. The reactivity tests were performed using a larger catalyst bed (800 mg) to generate a sufficient quantity of spent catalyst system (3.5-4.0 g) to permit harvesting and regenerating cycles and product characterization. The reproducibility of the experimental performance is shown in
The effect of cycling (specially the acid wash step) on carbon quality and presence of metal was evaluated. As shown in
Raman spectroscopy showed small changes in the carbon co-product MWCNT as a function of cycling (see
FTIR measurements were performed on the acid-washed MWCNT co-products generated after each cycle and compared it to the commercial (raw) MWCNT material used as a support. As shown in
To assess the commercial viability of the TCD process, Aspen Plus V10 was used to develop a process model of the process for the production of carbon co-product and CO2-free H2 and then conducted a preliminary techno-economic analysis (TEA).
In the preliminary TEA analysis, In the main reaction system and in the TCD reaction, natural gas (NG) was converted into H2 and carbon co-product on heterogeneous catalyst systems (i.e., 10Ni-1 Pd/CNT and 10Ni-1Cu/CNT) at 600° C., 2.5 bar. The gas-phase product was compressed and subjected to pressure swing absorption (PSA) to separate H2 from unconverted natural gas. Part of the off-gas from PSA, which mainly is unconverted NG, is recycled back to the main reactor, while the remainder is used to supply the heat required in the TCD reactor and other unit operations. The solid-phase product from the main reactor is sent to the acid wash and carbon recovery section where the metal catalyst is dissolved in concentrated HNO3 solution at 120° C. Carbon co-product then was separated from the solution by filtration and dried. The metal nitrate solutions (Ni(NO3)2/Pd(NO3)2 or Ni(NO3)2/Cu(NO3)2) solution along with part of the carbon co-product from the acid wash section were sent to the catalyst system regeneration section to generate fresh catalyst system by impregnation, calcination, and reduction. A small amount of H2 produced in the main reactor was used for catalyst system reduction. Considerable amounts of nitrogen oxides were generated during acid wash and calcination, which was mixed with air, compressed and sent to the HNO3 recovery unit to be converted into HNO3 by water absorption at 11 bar—a technology developed as a part of the Ostwald process.
In this analysis, the single-pass NG conversion in the TCD reactor was assumed to be 43% based on reaction equilibrium calculated by minimizing Gibbs free energy, as shown in
To demonstrate a distributed hydrogen refueling system, a TEA was conducted for large (100,000 kgH
The CO2 emitted from the TCD process are 85% lower than that from the conventional SMR process (1.67 and 9.6-11.5 kgCO
For the proposed CH4 TCD process, four cases were evaluated for two catalyst system compositions (Ni—Pd and Ni—Cu) and two different metal losses to account for the metal unrecovered for the carbon co-product. Cases NiPd0.1L and NiCu0.1L assumed a metal loss of 0.1% at the acid wash section. Case NiPd5L and NiCu5L assumed a metal loss of 5%. The disclosed method has the potential to produce H2 economically at both small and large scales, provided sufficient value can be obtained from the carbon co-product (e.g., MWCNT). The MCSP and MH2SP can be significantly reduced when replacing the expensive Ni—Pd catalyst with the cheaper Ni—Cu catalyst. Both MCSP and MH2SP are sensitive to the metal loss rate if using the expensive Ni—Pd catalyst due to the high cost of Pd.
Compared to SMR and SMR+CCS, the method described herein represents a more environmentally friendly approach for H2 production from NG as it has lower overall CO2 emissions relative to SMR and SMR+CCS, by 85% and 45%, respectively. The proposed technology has the potential to produce H2 economically at both small and large scales, provided sufficient value can be obtained from the carbon co-product.
Examples 1-8 shows that the cyclic CH4 TCD process to produce H2 with low-to-zero CO2 emission and recoverable carbon nanomaterials using CNT-supported Ni—Cu alloy catalysts were explored in this example. In this example, the stability of the Ni—Cu catalyst systems depends on the metal particle size, which can be tuned with metal weight loading and catalyst system synthesis methods. In this example, the catalyst systems with an average metal particle size >45 nm show stable TCD performance regardless of the overall metal loading and synthesis method, either ST or IW. In this example, process performance (e.g., carbon deposition rate and H2 production rate) was improved by nearly on order of magnitude by optimizing reaction conditions and catalyst system composition. In this example, characterization of the carbon co-product via SEM, Raman, and FTIR analyses revealed that the properties of the generated carbon co-product is representative of MWCNTs. In this example, the properties of the MWCNTs did not change substantially with catalyst system composition or catalyst system synthesis method. In this example, the catalyst system cycling process was demonstrated by harvesting the carbon co-product and resynthesizing the catalyst systems for four different cycles and the resynthesized catalyst system performance remained similar as well as the properties of the MWCNT generated after each cycle. In this example, characterization of the carbon co-product at spent catalyst systems and after acid washing for four different cycles showed that the properties of the produced MWCNTs do not change as a function of cycle and is similar to that of commercially available MWCNTs.
Materials and Catalyst System Synthesis
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), copper nitrate hemi-pentahydrate (Cu(NO3)2·2.5H2O), acetone and concentrated nitric acid (HNO3) were purchased from Sigma Aldrich. Multiwalled CNTs with outer diameter 20-30 nm were provided by Cheap Tubes (www.cheaptubes.com, catalog number 030104).
Preparation of NiCux/CNT Catalyst Systems Via the Solvothermal (ST) Method
A series of NiCux/CNT catalyst systems were prepared, where x represents the targeted weight loading of Cu, following the solvothermal (ST) method. The Ni loading was kept constant at approximately 10 wt % and the Cu wt % loading was varied to target a loading of 0 to 15 wt %. In the typical ST synthesis of a 10 wt % Ni-1 wt % Cu on CNT (NiCu1/CNT (ST)), 0.347 g of Ni(NO3)2·6H2O and 0.0154 g of Cu(NO3)2·2.5H2O were first dissolved in 60 mL acetone and sonicated for 30 min. As-received CNT (0.626 g) was added to the acetone solution and sonicated for additional 30 min. The mixture was then transferred into a 100 mL Teflon-lined Parr reactor, sealed, and stirred for 30 min. The Parr reactor was heated to 120° C. and maintained at this temperature for 12 hours under static condition. After cooling to room temperature, the solution was retrieved from the Parr reactor, placed in a glass container, and allowed to evaporate overnight at room temperature and atmospheric pressure in a hood. The dry solids were placed in a furnace with stagnant air at 80° C. overnight. The dry solids were then crushed and sieved (100 mesh) and stored in a glass vial.
Preparation of Acid-Treated CNT (HCNT) as Support Material
The CNT was acid treated to functionalize its surface and allow for a better distribution of the metal during impregnation. In a typical acid treatment, 3 g of as-received CNT was suspended in 150 mL 10 M HNO3 solution and sonicated for 30 min. The solution was then placed in a reflux apparatus heated with an oil bath set at 100° C. for 14 hours. Once the system cooled to room temperature, the solution was filtered to recover the solid, which was washed with copious distilled water until the pH of the filtrate was around 7. The acid-washed products were dried at 60° C. in a furnace with stagnant air for 12 hours. The dried, acid-washed CNT (HCNT) were cooled and stored in a glass vial.
Preparation of Catalyst System Via Co-Impregnation (CI) and Sequential Impregnation (SI)
The catalyst system synthesized by co-impregnation were prepared by mixing a 46 mL aqueous solution containing Ni(NO3)2·6H2O (0.7331 g) and Cu(NO3)2·2.5H2O (0.0570 g) with 1.3 g HCNT. The mixture was sonicated for 30 min and stirred for 2 hours at room temperature. Then, the solvent was allowed to evaporate at ambient conditions in a ventilated hood. Once dry, the solids were heat treated: first dried in air at 140° C. for 8 hours and then 1.72 g were heated to 350° C. at a ramp rate of 5° C./min in a N2 and held at 350° C. for 3 hours. Once cooled to room temperature, the treated solids were stored in a capped glass vial. The catalyst system synthesized by this method are denoted as NiCu1/HCNT (CI).
The sequential-impregnated catalyst systems followed the same protocol as the co-impregnated catalyst systems (0.771 g Ni(NO3)2·6H2O, 0.0570 g Cu(NO3)2·2.5H2O), but only impregnating one metal at the time and heat treating after each impregnation. The catalyst system was first impregnated with Ni followed by Cu impregnation and is denoted as Cu1Ni/HCNT (SI).
Preparation of Incipient Wetness Catalyst Systems NiCu1/HCNT(IW)
The catalyst system synthesized by incipient wetness impregnation (IW) were prepared by slowly adding a concentrated aqueous solution of Ni(NO3)2·6H2O (0.595 g) and Cu(NO3)2·2.5H2O (0.0439 g) to HCNT (1.068 g) using the amount of liquid that was previously determined to just wet the support (0.595 mL/g). Then, the slurry was dried in a furnace under stagnant air at 80° C. overnight. Once dry, the solids were then heated in air at 140° C. for 8 hours followed by heating 500 mg in flowing N2 (30 cm3/min) of at 350° C. for 3 hours. The temperature ramp rate was 5° C./min. Once cooled to room temperature, the solids were stored in a capped glass vial. The nominally 10 wt % Ni and 1 wt % Cu catalyst system prepared by IW is denoted as NiCu1/HCNT (IW).
The catalyst systems were characterized before (fresh) and after reaction (spent) to determine their stability and to devise relationships between activity, stability, and surface properties. The 500 mg of fresh catalyst systems were reduced at 400° C. for 4 hours under 30 cm3/min of 5 vol % H2 in N2 followed by heating to reaction temperature (typically 600° C.) in 30 cm3/min N2. After cooling to room temperature, the samples were passivated by flowing (30 cm3/min) 1 vol % O2 in N2 overnight. The spent catalyst systems were characterized as retrieved from the TCD reactor.
Nitrogen (N2) physisorption of the fresh and spent catalyst systems was conducted on a Quadrasorb EVO/SI Gas Sorption System from Quantachrome Instruments at 77 K. Samples were degassed at 150° C. under vacuum for 12 hours. Surface areas were determined using the 5-point Brunauer-Emmett-Teller (BET) method from the adsorption data in the relative pressure range of 0.05-0.3. Metal loadings were determined by inductively coupled plasma optical emission spectrometry (ICP-OES).
XRD patterns were collected using a Rigaku SmartLab SE Bragg-Brentano diffractometer, equipped with a fixed Cu anode operated at 40 kV and 44 mA and a D/Tex Ultra 250 1-dimensional detector. Patterns were collected with a variable divergence slit between 2 and 1000 (20) at intervals of 0.01°. The composition present, lattice parameters, and crystallite sizes of the crystalline components were determined by Rietveld fitting between 30 and 1000 (20) using Topas v6 (Bruker AXS) as discussed elsewhere. Because of the presence of NiCu alloys containing a range of compositions, it is possible that this method could underestimate the crystallite size. The compositions of the metallic phases were estimated from their refined cubic lattice parameters by linear interpolation between Ni (a=3.5238 Å) and Cu (a=3.615 Å).
A Micromeritics AutoChem 2920 instrument was used to conduct temperature programmed oxidation (TPO). The samples were first loaded and pretreated at 120° C. for 120 min under He, and then heated to 800° C. at a ramp rate of 5° C./min under 5 vol % O2 in He.
Raman spectra were recorded on a Renishaw InVia Raman microscope with a 532 nm excitation wavelength at 10 mW laser power. Each spectrum was averaged over three scans to characterize the solid carbon co-produced by CH4 TCD.
A FEI Titan 80-300 High-resolution transmission electron microscope (HRTEM) microscope operated at 300 kV and equipped with a CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, energy-dispersive X-ray (EDX) spectroscopy detector was used to determine the morphology of solid carbon co-products, metal particle size, and element distribution before and after reaction. Metal particle size and composition distributions were calculated from HRTEM images by sampling an average of 100 particles.
A fixed-bed, continuous-flow, vertical stainless-steel reactor was used for CH4 TCD reaction at ambient pressure. As-synthesized catalyst systems (0.2 g, assumed density=0.33 g/cm3) were loaded between two plugs of quartz wool. N2 gas was used as a carrier gas and an internal standard for product analysis using on-line gas chromatography (GC). Prior to each test, the 0.2 g catalyst system samples were reduced in situ at 400° C. for 4 hours under 70 cm3/min 10 vol % H2 in N2 at a ramp rate of 3° C./min. Subsequently, the reactor was heated to the reaction temperature (e.g., 550 to 700° C.) under 70 cm3/min of N2. Before reaction, the H2 had been completely purged from the system (monitored by on-line GC). Then, the feed was switched to 30 cm3/min of 30 vol % CH4 in N2 to maintain a constant space velocity of 9,000 cm3/g/h (≈3000 h−1 at the assumed density of the bed). The outlet gas flow rate was measured by a digital flow meter (DryCal). Composition of the outlet gas was analyzed by a four-channel Agilent Micro GC equipped with Molecular Sieve 5A, PLOT U, alumina, and OV-1 columns and a TCD detector for each column. When the test concluded, the reactor system was cooled to room temperature under 30 cm3/min of N2, and the spent catalyst systems (containing solid carbon co-product) were retrieved from the reactor for analysis. H2 was the only gaseous reaction product, CO2 and CO were not detected.
CH4 conversion, XCH
a Results derived from ICP
In this example, series of NiCu catalyst systems synthesized by the solvothermal method (ST) with a constant 10 wt % Ni loading were tested at 600° C. under 30 cm3/min of 30 vol % CH4 in N2. Cu loading was changed 0 wt % to 15 wt % to probe the effect of Cu on the TCD activity and morphology of carbon co-product. The catalyst systems properties such as BET and ICP-derived metal content are summarized in Table 6. The addition of Cu to Ni affects the initial TCD activity and stability (
The approximate quantifications can be performed using equations (3) to (5) as described herein.
Curves corresponding to the fitting parameters presented in Table 7 were superposed on the data shown in
The properties of the Ni and Cu metal in freshly reduced catalyst systems were investigated with XRD (
The spent catalyst systems were also analyzed via XRD to elucidate the role of Cu on catalyst system stability and the results are sown in Table 8. Overall, there were changes in both crystallite size and re-distribution of metal compositions. All but one of the catalyst systems showed a loss of Ni on the alloy (e.g., Ni:Cu ratio decreased), most likely resulting from the selective Ni migration from the metal particles to the carbon co-product. The segregation of Cu and Ni from the different Ni:Cu alloy nanoparticles at the reaction temperatures is consistent with the solubility gap regions of the NiCu phase diagram. Out of the 7 different Ni:Cu catalyst systems evaluated, only NiCu1 underwent particle fragmentation as evidenced by the 15% decrease in average crystallite size. The other six catalyst systems underwent metal sintering as evidence by the average crystallite size increase between 7% and 100%, Table 8. Hence, it was speculated that the changes in TCD catalytic performance depicted in
In this example, catalysts with the same nominal weight loadings of 10 wt % Ni and 1 wt % Cu were prepared using different synthesis methods to investigate the effect on metal particle size, Ni:Cu composition, and catalytic performance; solvothermal deposition (ST), incipient wetness impregnation (IW), wet co-impregnation (CI), and wet sequential impregnation (SI). The synthesis method affected the CH4 conversion and catalyst system stability (
XRD analysis of the spent catalyst systems revealed an even larger variation in Ni:Cu ratios (19 to 49). The catalyst systems synthesized by ST and SI methods had the lowest Ni:Cu ratios of 19 (each), while the catalyst systems prepared by IW and CI had significantly higher Ni:Cu ratios of 32 and 49 (Table 9). The ST and SI catalyst systems also had the lowest deactivation rate constants of 0.42 and 0.08. In contrast, the catalyst systems prepared by IW and CI had higher deactivation rate constants of 0.76 and 1.2 (Table 7). Thus, catalyst systems composed of bimetallic NiCu metal nanoparticles with significantly higher Cu contents (smaller Ni:Cu ratios) were more stable. Further, while XRD analysis showed that metal particle size increased after reaction for three of the four catalyst systems, the catalyst system prepared by SI had the largest particle size (15.1 nm) and also exhibited the largest increase in growth (from 8.4 nm). This combination of large particle size and low Ni:Cu ratio, compared to the other catalyst systems, could explain its superior stability. Taken together, the XRD analysis of the spent catalyst systems reveals that the particle size and Ni:Cu ratio changed with the synthesis method, and this can directly explain differences in catalytic stability.
aResults derived from ICP
bResults derived from XRD analysis
In this example, the performance of three catalyst systems were evaluated [Ni/CNT (ST), NiCu1/CNT (ST), and NiCu15/CNT (ST)] as a function of reaction temperature (550° C.-700° C.).
Given the assumed functional form for the deactivation function, the mass of carbon at any time on stream, C(θ), can be estimated using Equations 5-7 as described herein.
The predicted amount of carbon co-product accumulated (e.g., carbon yield) closely tracked the actual amount of carbon measured in the reactor (
Those values exhibit a maximum that depends on both the operating temperature and catalyst system composition, represented by the mol fraction of Cu (
The spent catalyst systems were analyzed via XRD to elucidate the role of Cu on catalyst system stability. The results show that metal particle restructuring might have caused catalyst system deactivation at the different reaction temperatures. For example, as shown in Table 9 the metal particles in Ni/CNT remained small at 9.2 nm when operating at 550° C. but sintered to larger metal particle sizes (14.6 nm to 19.4 nm) when the reaction temperature increased, suggesting that a cause of deactivation was metal sintering. NiCu1/CNT maintained small metal particle sizes and TCD activity at 550° C. and 600° C., which is consistent with the small change in metal particle size and Ni:Cu ratio observed with respect to the fresh material. However, NiCu1 deactivated at 650° C. in less than 1 hour while the metal particles sintered and segregated Cu into a secondary Cu-rich alloy. At 700° C., NiCu1 was not active for TCD and the composition and particle size remained similar to that of the fresh catalyst system, suggesting that the catalyst system deactivated before metal restructuring occurred.
It was speculated that the deactivation at higher temperatures might be caused by selective formation of graphitic carbon and subsequent plugging of the active site. Interestingly, NiCu15/CNT catalyst system had similar metal particle size (17.2 to 21.4 nm) at 550° C., 600° C., and 650° C. for which the catalyst system was active and stable; however, the Ni:Cu ratio of the metal particles changed with reaction condition. These results suggest there was a preferential segregation of Ni out of the metal particle at 550° C. by the change in Ni:Cu ratio with respect of the fresh catalyst system (0.133 and 0.790 respectively). At 600° C. and 650° C., the Ni:Cu ratio increased (0.254 and 0.418) suggesting that the segregation of Ni was less at higher reaction temperatures. At 700° C., the Ni:Cu ratio of the metal particles was similar to that of fresh catalyst system (0.676 and 0.790), further corroborating that the Ni segregation is less at higher temperatures; however, the metal particle size only stabilized to 10.4 nm (from 8.60 nm on the fresh catalyst system as opposed to the larger metal particle size observed at the lowest temperatures. It was speculated that the slow deactivation at 700° C. was caused by the poisoning of active sites before they could stabilize to the preferential particle morphology (e.g., <0.254 Ni:Cu ratio and >17 nm). In this example, these results suggest that the role of Cu is to stabilize large metal particles (>17 nm); however, increasing the reaction temperatures cause metal migration and changes the Ni:Cu ratio below the required to stabilize the large metal particles. The properties of the carbon co-product form might also play a role on the catalyst system stability, which is explored in the following section.
As shown in
The addition of 1 wt % Cu (e.g., NiCu1/CNT) produced multiwall CNT carbon co-product, which had a wall diameter ≈5 nm. Further increasing the Cu loading resulted in the formation of larger Ni:Cu metal particles, which generated large CNT with wall thickness >10 nm.
Raman spectroscopy can be used to assess the carbon quality using the three main bands: a) D-band (1340 cm−1) associated with defects in the graphitic lattice, 2) G-band (1580 cm−1) associated with ordered carbon, and 3) G′-band (or 2D band, 2700 cm−1) associated with interactions between stacked graphene layers that can be used to distinguish between single-wall CNTs (SWCNTs) or multi-wall CNTs (MWCNTs).
The amount of Cu addition to Ni can therefore be directly correlated to both catalyst system stability and quality of the carbon as evidenced by Raman spectroscopy.
As shown in
aBelow Detection Limit: The features of the metal were too dilute due to carbon formation and potential metal fragmentation and crystallite properties could not be determined.s
As shown in Table 11 the crystallite size (10.4 nm and 8.6 nm) and Ni:Cu molar ratio (0.676 and 0.790) of the spent NiCu15/CNT remained similar to that of the fresh catalyst system suggesting that a fraction of the metal nanoparticles deactivated before restructuring into the larger crystallite needed to catalyze the stable TCD reaction. HAADF-STEM images of the NiCu15/CNT catalyst system run at 700° C. confirms that the spent catalyst system still had sections of intact ≈10 nm NiCu metal particle (
Examples 9-16 investigated the role of Cu in the TCD performance of Ni—Cu/CNT catalyst systems. The examples showed that crystallite size, Ni:Cu ratio, and operating temperature were key factors for TCD activity, stability, and carbon coproduct morphology. NiCu catalyst system synthesized with different Ni:Cu ratios offered different benefits as a function of reaction temperature. At 550° C., there was a detrimental TCD activity effect with addition of Cu; however, the resulting carbon co-product had different properties. At 600° C. the addition of Cu benefitted the TCD activity and stability and modified the properties of the carbon co-product. At >650° C., only the catalyst system with high Cu loading remained active and stable for TCD.
Characterization of the catalyst systems after reaction revealed that the addition of Cu to Ni results in the stabilization of larger metal nanoparticles, which are more stable for TCD at higher reaction temperatures and more selective towards CNT growth. In the absence of Cu, bamboo-shaped CNT was the main morphology observed and the addition of Cu resulted in a change in CNT morphology to multiwall CNT. In all cases, the active site restructuring by the preferential segregation of Cu out of the NiCu alloy (e.g., the Ni:Cu ratio increases) and change in metal particle size. At low Cu loadings (e.g., high Ni:Cu ratio) the catalyst system deactivated at reaction temperatures >600° C. due to the encapsulation of the metal active sites before restructuring and loss of Cu. At high Cu loadings (e.g., low Ni:Cu ratio), reaction temperatures between 550 and 650° C. caused metal particle restructuring into <17 nm particles with higher Ni:Cu loading compared to the fresh catalyst system, which resulted in the preferential multiwall CNT formation. Operation at 700° C. caused encapsulation of metal particles before being able to restructure, resulting in catalyst system deactivation. These examples highlight how catalyst system composition and operation conditions can be used to optimize catalyst system stability and yield different carbon co-product morphologies generated during methane TCD.
In view of the many possible aspects to which the principles of the present disclosure may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the present disclosure. Rather, the scope is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.
This application claims the benefit of and priority to the earlier filing date of U.S. Provisional Application 63/345,603, filed May 25, 2022, which is incorporated by reference in its entirety herein.
This invention was made with government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63345603 | May 2022 | US |
