Patent Application
20170128881
Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”. In addition to particulate matter, internal combustion engines including diesel engines produce a number of combustion products including hydrocarbons (“HC”), carbon monoxide (“CO”), nitrogen oxides (“NOx”), and sulfur oxides (“SOx”). Aftertreatment systems may be utilized to reduce or eliminate emissions of these and other combustion products.
A number of catalysts are used to reduce emissions in diesel aftertreatment systems.
Once the exhaust has flowed through DOC 96, the DPF 100 is utilized to capture unwanted diesel particulate matter from the flow of exhaust gas exiting engine 20, by flowing exhaust across the walls of DPF channels. The diesel particulate matter includes sub-micron sized solid and liquid particles found in diesel exhaust. The DPF 100 can be manufactured from a variety of materials including but not limited to cordierite, silicon carbide, and/or other high temperature oxide ceramics.
The treated exhaust gases can then proceed through diesel exhaust fluid doser 102 for the introduction of a reductant, such as ammonia or a urea solution. The exhaust gases then flow to a selective catalytic reduction (SCR) system 104, which can include a catalytic core having a selective catalytic reduction catalyst (SCR catalyst) loaded thereon.
System 24 can include one or more sensors (not illustrated) associated with components of the system 24, such as one or more temperature sensors, NOx sensor, oxygen sensor, mass flow sensor, and a pressure sensor.
As discussed above, diesel oxidation catalysts (DOCs) are used in the processing of engine exhaust for pollution control, as a key component of the engine aftertreatment system (EAS). There are three key roles for the DOC as an oxidation catalyst:
Current industry trends are geared towards cutting costs, decreasing size and improving fuel economy, while simultaneously meeting the increasingly stringent pollution regulations. However, data from vehicle testing demonstrate that even higher Pt loading is required in the DOC when SCR catalysts are combined with diesel filters in a SCR on filter (SCRF). Indeed, Pt loading levels in the range of 40% higher than in conventional EAS configurations have been reported. The primary cause for the substantial increase in Pt loading is the increased need for NO2-make in SCRF technology; based upon the competition between soot oxidation and the fast SCR reaction for optimal levels of NO2.
Therefore, effective DOC compositions that utilize minimal amounts of platinum (Pt) and palladium (Pd) are highly desirable. Of Pt and Pd, both platinum group (PGM) metals, Pt is the most costly. But at the same time, of Pt and Pd, Pt is important for the oxidative processes and thus is the most abundant in DOC applications. To decrease costs and enhance efficiency, there is a need for catalyst formulations with low amounts of Pt and Pd. To be practical, the catalyst formulation should have good hydrothermal durability. The catalyst formulation should also be able to initiate a lightoff (or oxidation) process at a relatively lower temperature than Pt can achieve alone. The present disclosure seeks to fulfill these needs and provides further related advantages.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
In one aspect, this disclosure features a diesel oxidation catalyst, including a metal oxide that includes a metal element on a metal oxide surface, and less than 10 g/ft3 by weight of Pt or Pd, wherein the diesel oxidation catalyst oxidizes carbon monoxide and hydrocarbons of a diesel exhaust to carbon dioxide and water.
In another aspect, this disclosure features a substrate including a coating of a diesel oxidation catalyst as described above, wherein the substrate is a catalytic converter support, a diesel particulate filter, or a combined catalytic converter support and a diesel particulate filter.
In yet another aspect, this disclosure features a method of oxidizing NO in diesel engine exhaust in a selective catalytic oxidation system, including exposing a NO-containing diesel engine exhaust to a diesel oxidation catalyst as described above, and oxidizing the NO to NO2. The diesel oxidation catalyst is disposed on or within a catalyst support structure.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
The present disclosure describes a diesel oxidation catalyst, including a metal oxide including a metal on a metal oxide surface, and less than 10 g/ft3 by weight of Pt or Pd, wherein the diesel oxidation catalyst oxidizes carbon monoxide and hydrocarbons of a diesel exhaust to carbon dioxide and water. The metal oxide can be in the form of a particle.
In some embodiments, the diesel oxidation catalyst includes a metal oxide with high oxidative power including a metal on the metal oxide surface, minimal amounts of Pd (e.g., 0.001 g/ft3) to ignite lightoff, and optionally minor amounts (e.g., 0.001 g/ft3) of Pt to enhance oxidation.
In some embodiments, the metal oxide is surface-modified with one or more metal elements, such as Nb, Ca, Sc, Ta, Ti, V, Cr, Mn, Mo, Al, Si, Ge, Ir, Os, Fe, Co, Ni, Cu, Y, Zr, Re, Ru, Rh, Pd, Pt, Ag, Ba, W, La, and/or Ce, each of which can be independently positively charged and/or uncharged. As used herein, “metal elements” include both uncharged metal elements and metal cations. The one or more metal elements can form an intimate layer with an underlying metal oxide. In some embodiments, the one or more metal elements are covalently bonded to the underlying metal oxide surface, where the one or more metal elements can occupy a location in the crystal lattice of the metal oxide in the form of a metal ion surrounded by the requisite number of oxide counter ions to achieve overall electrical neutrality.
In some embodiments, treatment of the metal oxide with organometallic compounds in organic solvents, followed by reduction of the organometallic compounds (e.g., with sodium formate reductant), creates a zone of amalgamation between the metal oxide surface and the metal of the organometallic compounds to create a catalyst with enhanced hydrothermal durability. The treatment of the metal oxide with organometallic compounds in organic solvents can provide control over the distribution and aging characteristics of catalytic centers at the surface of the catalyst.
In some embodiments, the metal oxide is cerium oxide (e.g., CeO2), titanium oxide (e.g., TiO2), zirconium oxide (e.g., ZrO2), aluminum oxide (Al2O3), silicon oxide (SiO2), hafnium oxide (e.g., HfO2), vanadium oxide (e.g., V2O5, V2O3, VO2), niobium oxide (e.g., Nb2O5, NbO), tantalum oxide (e.g., Ta2O5, Ta2O), chromium oxide (e.g., Cr2O3), molybdenum oxide (e.g., MoO2), tungsten oxide (e.g., WO3), ruthenium oxide (e.g., RuO2), rhodium oxide (e.g., Rh2O3), iridium oxide (e.g., IrO2), nickel oxide (e.g., NiO), barium oxide (e.g., BaO), yttrium oxide (e.g., Y2O3), scandium oxide (e.g., Sc2O3), calcium oxide (e.g., CaO), manganese oxide (e.g., MgO), lanthanum oxide (e.g., La2O3), strontium oxide (e.g., SrO), cobalt oxide (e.g., CoO, Co2O3, Co3O4), and any combination thereof. In some embodiments, the metal oxide is titanium oxide, zirconium oxide, and/or cerium oxide. In certain embodiments, the metal oxide is zirconium oxide and/or cerium oxide.
In some embodiments, the metal oxide in the catalyst composition has high thermal stability combined with electrical conductivity, ionic conductivity, or magnetic properties (e.g., paramagnetism, ferromagnetism, etc.). In some embodiments, the metal oxide is present in the form of a vanadate, niobiate, molybdate, borate, manganate, etc.
In some embodiments, the catalyst composition includes a metal oxide having a mixture of cationic dopants, such as (BaTiO3)(SrTiO3).
In some embodiments, the metal oxide in the catalyst composition is one or more of yttria-stabilized zirconia, yttria-stabilized ceria, or yttria-stabilized ceria-zirconia mixed oxide; barium zirconate (e.g., BaZrO3), and/or yttria-doped barium zirconium oxide (a spinel oxide with high proton conducting properties, such as BaZr0.8 Y0.2O3).
Examples of vanadates include ziesite (a copper vanadate mineral with formula β-Cu2V2O7), bismuth copper vanadate (e.g., BiCu2VO6); lithium nickel vanadate (e.g., LixNiVO4, where x=0.8, 1.0, or 1.2, such as Li0.8NiVO4 or Li1.2NiVO4); iron vanadate (e.g., Fe4(VO4)4.5H2O); ferric vanadate (e.g., FeVO4); nickel vanadate (e.g., Ni(VO3)2); nickel vanadium oxide (e.g., NiV2O6); zirconium vanadate (e.g., ZrV2O7, Zr(OH)2(HOV4)2.2H2O); cerium vanadate (e.g., ortho-Ce2O3.V2O5, pyro-2Ce2O3.3V2O5, meta-Ce2O3.3V2O5, CeVO4, or CeV2O10); barium vanadate (Ba3(VO4)2); and/or manganese vanadate (MnV2O6).
In some embodiments, the metal oxide is barium strontium titanate (Ba0.6Sr0.4TiO3, a semiconducting perovskite oxide); and/or lanthanum strontium cobalt oxide (e.g., La0.6Sr0.4CoO3, a semiconducting perovskite oxide).
The metal oxide includes a cationic dopant. The cationic dopant can be Sr2+, Ru4+, Rh3+, Mg2+, Cu2+, Cu3+, Ni2+, Ti4+, V4+, Nb4+, Ta5+, Cr3+, Mo3+, W6+, W3+, Mn2+, Fe3+, Zn2+, Ga3+, Al3+, In3+, Ge4+, Si4+, Co2+, Ni2+, Ba2+, La3+, Ce4+, Nb5+, Y3+, Sc3+, and Ca2+. In some embodiments, the dopant includes a rare-earth metal (e.g., Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or Lu), at any positive oxidation state. In some embodiments, the dopant is Ru, Rh, or Cu. For example, the cationic dopant can be Y3+, Sc3+, and/or Ca2+. In some embodiments, the cationic dopant is Y3+. In certain embodiments, the cationic dopant is Sc3+. In some embodiments, the cationic dopant is Ca2+.
In some embodiments, the metal oxide can include 0.001 mol % or more (e.g., 0.01 mol % or more, 0.1 mol % or more, 0.5 mol % or more, 1 mol % or more, 2 mol % or more, 5 mol % or more, 7 mol % or more, 10 mol % or more, 15 mol % or more, 20 mol % or more, 25 mol % or more, 30 mol % or more, 35 mol % or more) and/or 40 mol % or less (e.g., 35 mol % or less, 30 mol % or less, 25 mol % or less, 20 mol % or less, 15 mol % or less, 10 mol % or less, 7 mol % or less, 5 mol % or less, 2 mol % or less, 1 mol % or less, 0.5 mol % or less, 0.1 mol % or less, or 0.01 mol % or less) of the cationic dopant, relative to the total metal oxide composition (i.e., the metal oxide and any cationic dopants). For example, the metal oxide can include between 0.1 mol % and 25 mol % (e.g., between 0.1 mol % and 15 mol %, between 0.1 mol % and 10 mol %, between 5 and 10 mol %, or between 5 and 15 mol %) of the cationic dopant, relative to the total metal oxide composition. In some embodiments, the metal oxide includes about 3 mol %, about 8 mol %, or about 20 mol % of the cationic dopant, relative to the total metal oxide composition. As used herein, the term “about” indicates that the subject value can be modified by plus or minus 5% and still fall within the described and/or claimed embodiment.
In some embodiments, when the cationic dopant is Y3+, Sc3+, and/or Ca2+, the metal oxide can include 0.1 mol % or more (e.g., 0.5 mol % or more, 1 mol % or more, 2 mol % or more, 5 mol % or more, 7 mol % or more, 10 mol % or more, 15 mol % or more, 20 mol % or more, 25 mol % or more, 30 mol % or more, 35 mol % or more) and/or 40 mol % or less (e.g., 35 mol % or less, 30 mol % or less, 25 mol % or less, 20 mol % or less, 15 mol % or less, 10 mol % or less, 7 mol % or less, 5 mol % or less, 2 mol % or less, 1 mol % or less, or 0.5 mol % or less) of the cationic dopant, relative to the total metal oxide composition. For example, the metal oxide can include between 0.1 mol % and 25 mol % (e.g., between 0.1 mol % and 15 mol %, between 0.1 mol % and 10 mol %, between 5 and 10 mol %, or between 5 and 15 mol %) of Y3+, Sc3+, and/or Ca2+. In some embodiments, the metal oxide includes about 3 mol %, about 8 mol %, or about 20 mol % of Y3+, Sc3+, and/or Ca2+.
In some embodiments, the metal oxide is yttria-doped zirconia (i.e., yttria-stabilized zirconia). In some embodiments, the metal oxide is yttria-doped ceria (i.e., yttria-stabilized ceria). In some embodiments, the metal oxide is yttria-doped mixed zirconia and ceria. The yttrium can be present in an amount of about 3 mol %, about 8 mol %, or about 20 mol % relative to the total metal oxide composition. In some embodiments, the yttrium is present in an amount of about 8 mol %, relative to the total metal oxide composition. In some embodiments, the metal oxide is scandia-doped zirconia and/or ceria (i.e., scandia-stabilized zirconia and/or ceria). The scandium can be present in an amount of about 3 mol %, about 10 mol %, or about 20 mol %, relative to the total metal oxide composition. In some embodiments, the scandium is present in an amount of about 10 mol %, relative to the total metal oxide composition. In some embodiments, the metal oxide is calcium-doped zirconia and/or ceria (i.e., calcium-stabilized zirconia and/or ceria). The calcium can be present in an amount of about 5 mol %, about 10 mol %, about 16 mol %, or about 20 mol % relative to the total metal oxide composition. In some embodiments, the calcium is present in an amount of about 16 mol %, relative to the total metal oxide composition.
In some embodiments, the metal oxide is surface-modified with one or more metal elements, such as Nb (e.g., Nb5+, Nb4+), Ca (e.g., Ca2+), Sc (e.g., Sc3+), Ta (e.g., Ta5+), Ti (e.g., Ti4+), V (e.g., V4+), Cr (e.g., Cr3+), Mn (e.g., Mn2+), Mo (e.g., Mo3+), Al (e.g., Al3+), Si (e.g., Si4+), Ge (e.g., Ge4+), Ir (e.g., Ir4+), Os (e.g., Os4+), Fe (e.g., Fe3+), Co (e.g., Co2+), Ni (e.g., Ni2+), Cu (e.g., Cu+), Y (e.g., Y3+), Zr (e.g., Zr4+), Ru (e.g., Ru4+), Rh (e.g., Rh3+), Pd (e.g., Pd2+), Pt (e.g., Pt2+), Ag (e.g., Ag+), Ba (e.g., Ba2+), W (e.g., W6+, W3+), La (e.g., La3+), Re, Ce (e.g., Ce4+), each of which can be independently positively charged. As used herein, “metal elements” include both uncharged metal elements and metal cations. The one or more metal elements can form an intimate layer with an underlying metal oxide surface. In some embodiments, the one or more metal elements are covalently bonded to the underlying metal oxide surface, where, the one or more metal elements can occupy a location in the crystal lattice of the metal oxide in the form of a metal ion surrounded by the requisite number of oxide counter ions to achieve overall electrical neutrality. When the metal oxide is surface-modified, the metal oxide can further catalyze the conversion of NO to NO2 and facilitate the NOx conversion to N2 and H2O, and/or the conversion of hydrocarbons to CO2 and H2O.
The metal element can be present in or on a metal oxide in an amount of 0.001 wt % or more (e.g., 0.01 wt % or more, 0.1 wt % or more, 1 wt % or more, 5 wt % or more, 10 wt % or more, 15 wt % or more, 20 wt % or more, 25 wt % or more, 30 wt % or more, or 35 wt % or more) and/or 40 wt % or less (e.g., 35 wt % or less, 30 wt % or less, 25 wt % or less, 20 wt % or less, 15 wt % or less, 10 wt % or less, 5 wt % or less, 1 wt % or less, 0.1 wt % or less, or 0.01 wt % or less), relative to the total metal oxide composition (i.e., the metal oxide including any cationic dopants and metal elements). In some embodiments, the metal element is present in or on a metal oxide in an amount of about 0.001 wt %, relative to the total metal oxide composition. In some embodiments, the metal element is present in or on a metal oxide in an amount of about 0.1 wt %, relative to the total metal oxide composition. In some embodiments, the metal element is present in or on a metal oxide in an amount of about 5 wt %, relative to the total metal oxide composition. In some embodiments, the metal element is present in or on a metal oxide in an amount of about 15 wt %, relative to the total metal oxide composition. In some embodiments, the metal element is present in or on a metal oxide in an amount of about 25 wt %, relative to the total metal oxide composition. In some embodiments, the metal element is present in or on a metal oxide in an amount of about 40 wt %, relative to the total metal oxide composition.
In some embodiments, when the metal element is Pd or Pt, the Pd or Pt can each independently be present in the metal oxide in an amount of less than 10 g/ft3 (e.g., less than 8 g/ft3, less than 5 g/ft3, less than 3 g/ft3, less than 1 g/ft3, less than 0.1 g/ft3, less than 0.01 g/ft3, or less than 0.001 g/ft3). In some embodiments, the Pd or Pt can each independently be present in the metal oxide in an amount of 0.001 g/ft3 or more (e.g., 0.01 g/ft3 or more, 0.1 g/ft3 or more, 1 g/ft3 or more, 5 g/ft3 or more, or 8 g/ft3 or more) and/or 10 g/ft3 or less (e.g., 8 g/ft3 or less, 5 g/ft3 or less, 1 g/ft3 or less, 0.1 g/ft3 or less, or 0.01 g/ft3 or less). In some embodiments, Pd is absent in the metal oxide. In some embodiments, Pt is absent in the metal oxide. In certain embodiments, both Pd and Pt are absent in the metal oxide.
In some embodiments, the metal element is in the form of a layer having a thickness of from 0.001 nm (e.g., from 0.01 nm, from 0.1 nm, or from 0.5 nm) to 1 nm (e.g., to 0.5 nm, to 0.1 nm, to 0.01 nm). The layer can have a variety of morphologies, such as complex mosaic of functionalities (i.e., micro-environmental domains of different compositions of various mixed oxides) or a uniformly transformed surface layer. Without wishing to be bound by theory, it is believed that as the organometallic reagent penetrates the metal oxide surface layers and chemically reacts with the metal oxide, a range of different stoichiometry of the resulting amalgam can occur as a function of depth of penetration and as a function of access to the particle surface. It is believed that these stoichiometric differences can result in the existence of catalytically active species in two or more valency states, thereby enhancing the catalytic redox properties of the resulting catalyst. The surface modification methods described herein can anchor active metal element moieties into the upper layers and enable catalytic sites to grow from the anchor sites. The catalytic sites can be more stable and less likely to migrate (i.e., sinter or cluster), and less likely to lose activity over time under high temperature conditions, thereby affording more robust and more durable catalysts. In some embodiments, the surface-modified catalysts can enhance specific surface area for desirable reactions to occur on the surface of the catalyst.
In some embodiments, the surface-modified metal oxide can serve as storage for NOx and O2. For example, the surface-modified metal oxide can participate in redox reactions in the DOC system.
In some embodiments, one or more metal oxides and one or more surface-modified metal oxides are mixed in any combination to form a catalyst composition. For example, the catalyst composition can include from 2 wt % (e.g., from 5 wt %, from 10 wt %, from 15 wt %, from 20 wt %, from 30 wt %, from 40 wt %) to 50 wt % (e.g., to 40 wt %, to 30 wt %, to 20 wt %, to 15 wt %, to 10 wt %, to 5 wt %) of the surface-modified metal oxide, so long as the sum of the total amount of components of the catalyst composition is 100%. When viewed microscopically, the catalyst composition can appear as a mixture of metal oxide particles and surface-modified metal oxide particles. If a metal element is present, the metal element can form an intimately mixed layer with the surface of the metal oxide. The metal element can form a continuous or discontinuous coating on the surface of the metal oxide. The coating can have a thickness of from 0.001 nm (e.g., from 0.01 nm, from 0.1 nm, from 1 nm, from 10 nm, from 100 nm, or from 500 nm) to 1,000 nm (e.g., to 500 nm, to 100 nm, to 10 nm, to 1 nm, to 0.1 nm, or to 0.01 nm). In some embodiments, when viewed microscopically, the catalyst composition is in the form of a particle or a coating.
Without wishing to be bound by theory, it is believed that in the catalyst composition, the surface-modified metal oxide can have increased hydrothermal stability compared to the non-surface modified metal oxide.
The surface-modified metal oxide can be made using the following synthetic methods. Synthetic methods are also described and exemplified in U.S. Ser. No. 14/934,955 (attorney docket no. PCCR154479), entitled “Surface-Modified Catalyst Precursors for Diesel Engine Aftertreatment Applications,” filed concurrently with the present application and herein incorporated by reference in its entirety.
In some embodiments, the method includes providing a solution including an organic solvent and an organometallic compound; mixing the solution with a metal oxide to provide a mixture; drying the mixture; and calcining the mixture to provide a surface-modified metal oxide catalyst. The organometallic compound can be, for example, a metal alkoxide, a metal carboxylate, a metal acetylacetonate, and/or a metal organic acid ester.
In some embodiments, the method further includes exposing the surface-modified metal oxide catalyst to a solution including metal salts, such as a solution including nickel ions and/or copper ions. The method can further include calcining the surface-modified metal oxide catalyst after exposing the surface-modified metal oxide catalyst to a solution including metal salts (e.g., nickel ions, copper ions).
In some embodiments, mixing the solution with a metal oxide is done by milling and/or stirring, so long as the mixing ensures that the mixture is homogeneous, i.e., with minimal aggregation and/or clumping of the mixture.
In some embodiments, drying the mixture is done by air drying, and/or by heating at a temperature of from 20° C. (e.g., from 40° C., from 60° C., from 80° C., or from 100° C.) to 110° C. (e.g., to 100° C., to 80° C., to 60° C., or to 40° C.) to remove residual solvent in the mixture.
In some embodiments, calcining includes heating the mixture to a temperature of from 450° C. (e.g., from 475° C., from 500° C., from 525° C.) to 550° C. (e.g., to 525° C., to 500° C., to 475° C.) for a duration of from 0.5 to 5 hours (e.g., from 1 to 5 hours, from 2 to 5 hours, from 3 to 5 hours, from 4 to 5 hours, from 1 to 2 hours, from 1.5 to 2 hours, from 2 to 3 hours). Calcining the mixture removes organic materials from the mixture, such as from the organometallic compounds. When performed in the presence of oxygen, calcining can form metal oxides from the organometallic compounds, and/or can cause the organometallic compounds to react with a substrate, such as a metal oxide to form covalent bonds between the metal in the organometallic compounds and the substrate.
Without wishing to be bound by theory, it is believed that surface modification of a suitable substrate material (e.g., a substrate such as a metal oxide) enables the easy synthesis of catalysts with desired properties. In some embodiments, the surface-modified substrate material may then be incorporated into washcoat formulations and applied to suitable supports (e.g., cordierite, silicon carbide, metallic supports, etc.).
In some embodiments, rather than mixing a solution including an organic solvent and an organometallic compound with a metal oxide to provide a mixture; drying the mixture; and calcining the mixture to provide a surface-modified metal oxide catalyst; the catalyst is made by mixing the catalyst precursor components and the reagents in a slurry, applying the slurry to a substrate in an engine aftertreatment system, and calcining the slurry to obtain a surface-modified catalyst composition on the engine aftertreatment system substrate.
As discussed above, the method can include providing a solution including an organic solvent and an organometallic compound.
In some embodiments, the organometallic compound used for modifying a surface of a given substrate material (e.g., a metal oxide) is insoluble in water, sparingly soluble, or readily decomposes in water, but is soluble in organic solvents including alcohols (e.g., propanol, isopropanol, pentanol, butanol, octanol, decanol), ethers (e.g., diethyl ether, ethyl propyl ether, dipropyl ether, butyl propyl ether, pentyl propyl ether), and esters (e.g., ethyl acetate, methyl methanoate, propyl propanoate, ethyl propanoate, ethyl benzoate). Without wishing to be bound by theory, it is believed that a solvent having an optimal process temperature can allow the amalgamation of the metal element into the lattice structure of the upper atomic layers of substrate material, and/or achieve a coating of the metal element.
The organometallic reagent and the solvent can have the following characteristics:
The metal elements on the surface of the metal oxide can be readily formed from organometallic compounds. The organometallic compounds can react with water to precipitate the surface modifiers on a surface of a washcoat precursor material. In some embodiments, the organometallic compounds are soluble in organic solvents such as alcohols and ethers and the like, which are also miscible with water.
As an example, the organometallic compounds can be metal alkoxides; metal carboxylates, metal acetyl acetonates, and/or metal organic esters. Examples of metal alkoxides include metal ethoxides (e.g., titanium(IV) ethoxide; Ti(OC2H5)4), metal propoxides (e.g., titanium(IV) isopropoxide; Ti[OCH(CH3)2]4); metal butoxides (e.g., titanium(IV) butoxide (Ti(OCH2CH2CH2CH3)4), barium tert-butoxide (C8H18BaO2), etc.); metal pentoxides, methoxyethoxides such as yttrium 2-methoxyethoxide; Y(OEtOMe)3), niobium (III) chloride 1,2-dimethoxyethane complex; NbCl3.CH3OCH2CH2OCH3, niobium ethoxide, polynuclear and heterometallic alkoxides such as Re4O6-y(OCH3)12+y, Re4-xMoxO6-y(OCH3)12+y, Re4-xWxO6-y(OCH3)12+y, titanium isopropoxide, titanium ethoxide, zirconium ethoxide, tetraethyl orthosilicate, aluminum isopropoxide, niobium ethoxide, tantalum ethoxide, potassium tert-butoxide, [CrAl(OPri)4]3, Mn[Al(OPri)4]2, [Fe {Al(OPri)4}2or3], Co[Al(OPri)4]2, Ni[Al(OPri)4]2, Ni[Ga(OPri)4]2, Ni[Nb(OPri)6]2, Ni[Ta[OPri]6]2, Ni[Zr2(OPri)9]2, and Cu[Al(OPri)4]2. As used herein, “Pri” indicates an isopropyl group.
Examples of metal carboxylates include zirconium acetato-propionate; Zr(acac)4; dicalcium barium propionate, Ca2Ba(C2H5COO)6; zirconium propionate; Zr(CH3CH2COO)4; lanthanum propionate; metal with chelating agents such as ethyl diamine and poly(ethyldiamine), phthalimide, where the metal is Zr, Ba, Ti, La, Sr, Ce, Nb, etc. In some embodiments, the metal chelate is
Examples of metal acetyl acetonates include titanium diisopropoxide bis(acetylacetonate) (CH3)2CHO]2Ti(C5H7O2)2); zirconium (IV) acetylacetonate; Zr(C5H7O2)4; palladium(II) acetylacetonate, C10H14O4Pd; platinum(II) acetylacetonate, Pt(C5H7O)2; titanium bis(acetylacetonate)dichloride; vanadyl acetylacetonate; chromium acetylacetonate; manganese(III) acetylacetonate; iron acetylacetonates; ruthenium acetylacetonates; cobalt acetylacetonates; iridium acetylacetonates; nickel(II) acetylacetonate; copper acetylacetonate; and/or zinc acetylacetonate.
In some embodiments, the solution that includes an organic solvent and an organometallic compound further includes oligomers or low molecular weight polymers (e.g., less than 5,000 molecular weight), such as poly(propylene glycol), poly(ethylene glycol), and copolymers thereof. In certain embodiments, the low molecular weight polymer is poly (propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) and/or H(OCH2CH2)nOH.
As an example, Table 1 lists the properties of vanadia and niobia, which can be taken into consideration when selecting niobia as a comparatively preferred surface modifier. As shown in Table 1, niobium ethoxide (a metal alkoxide) is an example of a suitable organometallic reagent as it readily reacts (i.e., decomposes) in water, and melts at a low temperature (5° C.) such that when it is calcined, it affords niobia, which is a highly stable compound that has a melting point of 1512° C. and that is insoluble in water.
Without wishing to be bound by theory, it is believed that niobium is suitable as a surface modifier because of its ability to migrate to grain boundaries of metal alloys where it can effectively bind the grains together, thereby markedly improving the density and overall strength of a given alloy.
The structure of niobium ethoxide is shown below in Scheme 1. It is believed that the strong tendency to form covalent bonds can be exploited to enable amalgamation of the Nb surface modifier with the selected substrate. Likewise, this property may help to bind grains of washcoat particles together and provide enhanced durability for a catalyst coating.
Other surface modifying metal oxides than Nb can be derived from organometallic reagents containing: Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Ru, Rh, Pd, Ag, Ba, W, La, Ce, Ta, Mo, Al, Si, Ge, Ir, Os, Re, and/or Pt.
The diesel oxidation catalysts of the present disclosure have numerous advantages. For example, the diesel oxides catalysts can include a metal oxide with the combined properties of relatively high oxygen and NOx storage capacity, where both properties can be modulated by surface modification of catalyst precursors using organometallic compounds. In some embodiments, the diesel oxidation catalysts are hydrothermally stable when heated for 40 hours (e.g., 40 hours or more) at 650° C. (e.g., at 650° C. or more). In some embodiments, the diesel oxidation catalysts have a 50% hydrocarbon to CO2 and H2O conversion temperature of about 300° C. or less (e.g., 275° C. or less, or 250° C. or less). In some embodiments, the diesel oxidation catalysts have a 50% NO to NO2 and H2O conversion temperature of about 300° C. or less (e.g., 275° C. or less, or 250° C. or less).
The diesel oxidation catalysts of the present disclosure can be coated onto a substrate. For example, the substrate can be a catalytic converter support, a diesel particulate filter, or a combined catalytic converter support and a diesel particulate filter. In some embodiments, the substrate is a cordierite catalytic converter support, a cordierite wall flow filter, a silicon carbide wall flow filter, a ceramic fiber filter, and a metal fiber flow-through filter. The diesel oxidation catalysts can be disposed on or within a catalyst support structure, such as a ceramic monolith or a metallic substrate.
The diesel oxidation catalysts can be used for oxidizing NO in diesel engine exhaust in a selective catalytic oxidation system. For example, when a NO-containing diesel engine exhaust is exposed to a diesel oxidation catalyst of the present disclosure, which can be disposed on or within a catalyst support structure, NO is oxidized to NO2. The diesel oxidation catalysts can further be used to oxidize a hydrocarbon in a hydrocarbon-containing diesel engine exhaust to CO2 and H2O.
In some embodiments, the surface-modified metal oxide has a smaller BET surface area compared to a metal oxide without surface modification. The reduced BET surface area is a quick an easy way to determine that the surface modification has taken place.
In some embodiments, the surface-modified metal oxide catalyst can modulate the oxidative power of the underlying metal oxide (e.g., a metal oxide such as ceria), and can facilitate selective catalytic oxidation (SCO) and the in situ formation of NO2 reaction intermediates in environments where NO2 concentrations are below the desired levels (i.e., NO2/NOx=0.5). In some embodiments, the surface-modified catalyst compositions (e.g., barium oxide (BaO2) surface-modified metal oxides) modulate NOx storage compared to non-surface modified compositions (e.g., zirconia-based metal oxides and ceria-based metal oxides), to meet low temperature emissions standards. As discussed above, the surface-modified catalyst compositions can enable low cost and compact platinum group metal-free diesel oxidation catalyst development.
The surface-modified catalyst or a slurry containing non-surface modified metal oxide and organometallic reagents (i.e., catalyst precursors), can be applied in a variety of locations within an engine aftertreatment system. As described above, the slurry can be calcined to provide a surface-modified catalyst composition in an engine aftertreatment system. For example, referring to
The following examples are included for the purpose of illustrating, not limiting, the described embodiments.
Example 1 describes the screening and selection of NO2-make catalysts. Example 2 describes the evaluation of DOC catalysts on a high porosity silicon carbide wall filter in an SCRF application. Example 3 describes the synthesis and characterization of niobia surface-modified metal oxides. Example 4 provides an example of a platinum-free DOC composition.
YSZ: yttria-stabilized zirconia
YSC: yttria-stabilized ceria
PEG: poly(ethylene glycol)
PPG: poly(propylene glycol)
PEO: polyethylene oxide
DI water: deionized water
DOC: diesel oxidation catalyst
GHSV: gas hourly spatial velocity
NRE: NOx reduction efficiency
PGM: platinum-group metal
The following washcoat composition was dip coated onto a cordierite (5/300) substrate (available from NGK Automotive Ceramics, U.S.A., Inc.), in the form of 1″×1″ core samples at 30° C., with a vacuum applied to pull excess washcoat through the channel and assist in drying: 25.6% YSZ-8 (MEL Chemicals); 8.3% YSC-10 (Sigma-Aldrich); 19.9% Nyacol® (Nyacol Nano Technologies); 3.4% PEG/PPG (Sigma-Aldrich); 0.3% PEO (Sigma-Aldrich); and 42.2% DI water. The washcoat was dried at 105° C. in air and calcined at 450° C. for 1 hr.
A synthetic gas test bench for testing catalyst core samples was employed to evaluate various catalyst washcoats for their ability to activate undesirable oxidative side reactions. Their potential ability to oxidize the NH3 (produced from DEF dosing) was also investigated.
Catalyst coated core samples were evaluated in a DOC lightoff experiment. A fresh core sample from a commercial DOC catalyst was used as a reference.
The gas mixture used at 60,000 GHSV to simulate a diesel exhaust was as follows: 600 ppm NO; 75 ppm C2H4; 300 ppm CO; 10% O2; 5.6% CO2; 6% H2O; balance N2; at 60,000 GHSV.
A reverse lightoff test procedure was employed, where the temperature was increased from 160° C. to the setpoint of 600° C., and it was allowed to stabilize. Heating was then discontinued and both the inlet temperature and the reactor outlet gas concentration were monitored.
From the results, conversion efficiencies were computed and plotted to obtain the temperature at which 50% of the total conversion efficiency was achieved for CO conversion to CO2 (T50CO); NO conversion to NO2 (T50NO); and C2H4 conversion to CO2 and H2O (T50C2H4).
The results are shown in Table 2, where all of the tested catalysts could potentially be employed for NO2-make catalyst, because they did not activate oxidative lightoff reactions below 500° C. in the absence of PGM catalytic species.
The NO2-make catalyst of the disclosure can be selected from metal oxides which exhibit the least oxidative power, and as such will be the least likely to oxidize NH3, while enabling the reaction and stabilization of NO2 to facilitate high NOx reduction in NO2-depleted exhaust streams. Therefore, taking Table 2 as an example, the screened NO2-make catalysts are preferred in the order: YSZ>YSC>>CeO2—ZrO2.
Table 3 contains a summary of the composition of (1″×3″) core samples of high porosity SiC filter material that were coated with catalyst washcoat and tested for SCRF functionality (i.e., NOx reduction (i.e. conversion) efficiency to evaluate SCR function with and without soot, ΔP (i.e., change in simulated exhaust flow) as a function of soot loading, and soot lightoff temperature).
SiC DPF core samples were dip coated with the vacuum aided technique described in Example 1, from the downstream side of the filter only. Washcoat compositions were as follows:
Optionally apply washcoat: 5.8% yttria stabilized ceria (10 mol %), 12.2% PEG-PPG (Mn ˜2,500), both from Sigma Aldrich, with 8.8% NYACOL ZR 10/15 (Nano Technologies Inc.), and 73.2% DI water, then dried at 105° C. for 1 hr.
Optionally treat with 1M (CuSO4, FeCl3, AgNO3, or NiSO4) or, dilute PdCl2 or PtCl2 solutions at 0.1M, followed 0.3 M Na formate, and dried at 105° C. for 1 hr and calcining at 450° C. for 1 hr.
Apply washcoat: 27% CuZSM-5 (from ACS as nanoZSM-5), 1.8% PEG-PPG, 10.8% NYACOL ZR 10/15, 0.8% PEG (300,000) and 58.9% DI water. Drying as conducted at 105° C. and calcining at 450° C. for 1 hr.
Standard pretesting (degreening) procedure for all samples: 4 hr @ 600° C. with 10% H2O vapor.
1. Clean ΔP—with no soot in the filter
2. Clean NOx reduction efficiency (NRE): NO2/NOx=0.5, NII3/NOx, 35,000 GHSV, 500 ppm NOx; NH3/NOx=1.
3. ΔP vs. collected soot
4. NRE with soot (same conditions as step 2)
5. Temperature programmed oxidation (TPO) of soot; i.e. soot lightoff temperature
The following is the ΔP after the initial phase of soot loading for the SCRF core samples tested:
The data demonstrated the ability of low PGM DOC catalysts of the present disclosure to accomplish optimal catalytic performance with reduced ΔP compared to commercially available catalysts.
Niobium pentoxide (Nb2O5) surface modifier was applied to redox metal oxide catalyst precursors by reaction with niobium ethoxide dissolved in isopropanol (IPA) with:
Yttria stabilized zirconia was obtained from MEL Chemicals, while all other reagents were obtained from Sigma-Aldrich.
10% Niobium ethoxide was prepared in isopropanol and was added to the amount of two component redox metal oxides; in an amount sufficient to obtain a composition after calcining to form (Nb2O5) equivalent to:
The mixture was milled in a SPEX SAMPLEPREP MIXER MILL with methyl methacrylate balls for 20-30 minutes. Air drying in a fume hood was conducted, and over drying at 105° C. completed the drying process. Calcining was conducted by increasing the temperature to 500° C. at a rate of 15° C./minute. 500° C. was maintained for 5 hours and cooling was accomplished at a rate of 15° C./minute, down to 30° C.
The catalysts were characterized by X-ray diffraction, BET (Brunauer, Emmett, and Teller surface area analysis), SEM/EDAX (scanning electron microscopy and energy dispersive x-ray analysis), and Raman analytical techniques, by comparison with the corresponding untreated metal oxide.
The effect of N2O5 surface modification on the surface area of metal oxide catalyst precursors is shown in Table 7. SEM/EDAX analysis of the surface-modified precursors is shown in
Washcoat compositions shown in Table 8 were applied to cordierite core samples as described in Example 1, and treated with either nickel acetate in methanol, or a mixture of nickel and palladium acetates in methanol, to obtain containing 0.01 g absolute amount of Ni and ≦0.01 g absolute amount of Pd. These metal species were reduced with sodium formate, as described for the core samples prepared in Example 2. These core samples are examples of Pt-free DOC catalyst compositions according to this invention.
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
