Patent Application
20240360576
The present disclosure is drawn to electrocatalysts, and specifically, to nickel-enhanced transition metal oxide electrocatalysts for use in reducing CO2.
As society's demand for energy continues to increase, the development of renewable and energy-efficient alternatives has become critical. The conversion of CO2 to organic feedstocks and energy-dense liquid fuels would help mitigate this issue. One very appealing target product is 1-butanol due to comparable energetics to gasoline as well as its compatibility with current internal combustion engines. Since 1-butanol is a liquid at STP, it has considerable volumetric energy density advantages over a gaseous fuel like hydrogen.
From an industrial standpoint, a minimum of three processes are necessary for the conversion of CO2 into liquid fuels: capture of CO2, a two-electron reduction to carbon monoxide (CO) or formate (HCO2−), and transformation of the primary product into multicarbon products (C2+). Catalysts that perform two or more of these steps in tandem with high selectivity and yield are exceedingly rare. An ideal catalyst for the functionalization of CO2 is reusable, is easily separable from the products, and operates under mild conditions.
In various aspects, a system for reducing carbon dioxide to produce a desired reaction product may be provided. The system may include a container, an aqueous solution within the container, and a plurality of electrodes operably coupled to the aqueous solution and separated from each other, and circuitry configured to provide a voltage to the plurality of electrodes. The aqueous solution may include an inorganic salt and carbon dioxide. The electrodes may include a first electrode. The first electrode may include a metal enhanced transition metal oxide, wherein the metal enhanced transition metal oxide is free of copper. In some embodiments, the system may be free of a salt bridge. In some embodiments, at least two of the electrodes may be separated by a salt bridge.
The metal enhanced transition metal oxide may include nickel. The metal enhanced transition metal oxide may include nickel enhanced (M12O3)x(M22O3)4-x, where M1 and M2 are different metal oxides, and 0<x<4. M1 may be Cr and M2 may be Ga.
The aqueous solution may include a pH adjusting agent for adjusting the pH of the aqueous solution to fall within a target pH range. The target pH may be 0-7. The target pH range may be 3.8-5.5. The voltage applied may be 0V-10V.
In various aspects, a method for reducing carbon dioxide to produce a desired compound may be provided. The method may include passing a predetermined electrical voltage across a plurality of electrodes at least partially within an aqueous solution. The plurality of electrodes may include a first electrode comprising a metal enhanced transition metal oxide, wherein the metal enhanced transition metal oxide is free of copper. The aqueous solution may include carbon dioxide. The method may include collecting at least some of the reaction product after a predetermined period of time, the reaction product comprising the desired compound.
The metal enhanced transition metal oxide may include nickel. The metal enhanced transition metal oxide may include nickel enhanced (M12O3)x(M22O3)4-x, where M1 and M2 are different metal oxides, and 0<x<4. M1 may be Cr and M2 may be Ga.
The aqueous solution may include a pH adjusting agent for adjusting the pH of the aqueous solution to fall within a target pH range. The target pH may be 0-7. The target pH range may be 3.8-5.5.
The voltage applied may be 0V-10V. The voltage may be configured to vary over time. The voltage may be configured to remain substantially constant over time.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.
It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.
The following description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be only for illustrative purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, “or,” as used herein, refers to a non-exclusive or, unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. Those skilled in the art and informed by the teachings herein will realize that the invention is also applicable to various other technical areas or embodiments.
An electrocatalyst composed of (Cr2O3)3(Ga2O3) had previously been reported. Unfortunately, as disclosed herein, the observed product distribution is very sensitive to metal impurities. This sensitivity to metal impurities makes it difficult to repeatedly generate oxalate in high yield using this system.
However, as disclosed herein, by incorporating a small amount of nickel into the transition metal oxide systems, using a variety of synthetic approaches, generating 1-butanol with high faradaic efficiencies can be achieved.
As disclosed herein, various synthetic methodologies can be used for introducing nickel into transition metal oxide catalysts. Also disclosed is characterization and optimization of a Ni-enhanced catalyst for the reduction of CO2 to 1-butanol with a maximum faradaic efficiency (ξmax) of 42%. This occurs at an overpotential of 900 mV (−1.48 V vs Ag/AgCl), with an onset overpotential of 320 mV. The product selectivity is potential-dependent. Other C2+ products include 3-hydroxybutanal, acetic acid, acetone, and acetaldehyde. All four products are proposed to be generated from formic acid. The overall faradaic efficiency of the nickel-enhanced system for total CO2RR is 63%, which is greater than six times higher than that seen for the nickel-free system.
In various aspects, a system for reducing carbon dioxide to produce a desired reaction product may be provided. Referring to
The system may include an aqueous solution (120) within the container (e.g., within the internal volume of space). The container may utilize glass or other appropriate materials for containing the aqueous solution.
The aqueous solution may include an inorganic salt. Non-limiting examples of inorganic salts include, e.g., sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl). In some embodiments, the salt is potassium chloride.
The salt may be present at any appropriate concentration. The inorganic salt and water may form a solution having a concentration Csalt of 0 M<Csalt≤1 M. In some embodiments, the concentration may be Csalt>1 M. In some embodiments, the concentration may be Csalt≤0.75 M. In some embodiments, the concentration may be Csalt≤0.5 M. In some embodiments, the concentration may be Csalt≤0.25 M. In some embodiments, the concentration may be Csalt≤0.1 M.
The aqueous solution may include carbon dioxide. In some embodiments, the inorganic salt/water solutions may be sparged and saturated with CO2 via, e.g., bubbling of the solution for a period of time (e.g., 15 minutes, 30 minutes, 60 minutes, etc. as appropriate).
The aqueous solution may include a pH adjusting agent for adjusting the pH of the aqueous solution to fall within a target pH range. Any appropriate pH adjusting agent may be utilized. For example, NaHC03, NaOH, etc., may be used. The pH adjusting agent may be provided as a pH adjusting aqueous solution. The pH adjusting aqueous solution may have any appropriate concentration; as will be understood in the art, the concentration may be no more than 1 M, no more than 0.75 M, no more than 0.5 M, no more 0.25 M, or no more than 0.1 M. The target pH may be 0-7. In some embodiments, the target pH range may be 3.8-5.5.
The system may include a plurality of electrodes (here, first electrode (130) and second electrode (131) are shown). The plurality of electrodes are intended to be at least partially within the aqueous solution and physically separated from each other by at least a small distance (e.g., a distance>0 cm).
The first electrode may include a metal enhanced transition metal oxide electrocatalyst, wherein the metal enhanced transition metal oxide is free of copper. Preferably, the metal enhanced transition metal oxide may include nickel. The metal enhanced transition metal oxide may include nickel enhanced (M12O3)x(M22O3)4-x (sometimes written as (M12O3)x(M22O3)4-x:[Ni]), where M1 and M2 are different metal oxides, and 0<x<4. In some embodiments, M1 may be a group 4-10 transitional metal. In some embodiments, M1 may be a group 4-9 transitional metal. In some embodiments, M1 may be a group 4-8 transitional metal. In some embodiments, M1 may be a group 5-7 transitional metal. In some embodiments, M1 may be a group 6 transitional metal. In some embodiments, M1 may be Cr. In some embodiments, M2 may be a group 13 or 14 post-transition metal. In some embodiments, M2 may be a group 13 post-transition metal. In some embodiments, M2 may be Ga.
In some embodiments, the nickel may be present in the electrocatalyst in a concentration CNi greater than 0 and no more than 100 ppm. In some embodiments, 0<CNi≤50 ppm. In some embodiments, 0<CNi≤25 ppm. In some embodiments, 0<CNi≤20 ppm. In some embodiments, 0<CNi≤20 ppm.
Referring again to
The system may include circuitry (140) configured to provide a voltage to the plurality of electrodes. The circuitry may be a power source, or may be coupled to a power source (not shown). Such circuitry is well known in the art. In some embodiments, the voltage applied may be 0V-10V. The voltage may be configured to vary over time. In some embodiments, the voltage may include at least one step change in voltage over time. In some embodiments, the voltage may include a ramp of voltage from a first voltage to a second voltage. The voltage may be configured to remain substantially constant over time. As used herein, the term “substantially constant” voltage refers to a voltage that varies by less than ±0.25 V over a period of time.
Referring to
In various aspects, a method for reducing carbon dioxide to produce a desired compound may be provided. The method may include passing a predetermined electrical voltage across a plurality of electrodes at least partially within an aqueous solution. The plurality of electrodes may include a first electrode comprising a metal enhanced transition metal oxide, wherein the metal enhanced transition metal oxide is free of copper. The aqueous solution may include carbon dioxide. The method may include collecting at least some of the reaction product after a predetermined period of time, the reaction product comprising the desired compound.
The metal enhanced transition metal oxide may include nickel. The metal enhanced transition metal oxide may include nickel enhanced (M12O3)x(M22O3)4-x, where M1 and M2 are different metal oxides, and 0<x<4. M1 may be Cr and M2 may be Ga.
The aqueous solution may include a pH adjusting agent for adjusting the pH of the aqueous solution to fall within a target pH range. The target pH may be 0-7. The target pH range may be 3.8-5.5.
The voltage applied may be 0V-10V. The voltage may be configured to vary over time. The voltage may be configured to remain substantially constant over time.
In various aspects, a method for reducing carbon dioxide to produce a desired compound may be provided. Referring to
To avoid potential contaminations that arise from previous electrode designs, an electrode akin to that shown in
The electrodes were cleaned by first soaking in HCl overnight and then MILLI-Q® lab water for at least 4 h, before being rinsed and mechanically abraded using 600 grit sandpaper followed by 1 μm alumina slurry to a glassy finish. The glassy carbon rods were then sonicated in MILLI-Q® lab water for 15 min cycles until the electrodes appeared black and the sonication solvent was no longer cloudy. These electrodes were dried at 80° C. for at least 1 h. After the use of the electrode, the electrode was sanded with 240, 400, 600, 1200, and 2000 grit sandpapers and then polished using an alumina slurry until a mirror-like finish was achieved. These were all done using an Allied High Tech products autopolishing station.
(Cr2O3)3(Ga2O3) was then synthesized, using a modified procedure first reported by Paris and Bocarsly. The electrodes were cleaned using the method described above. Once dry, 30 μL of 3:1 nitrate salts of Cr/Ga (chromium(III) nitrate nonahydrate and gallium (III) nitrate hydrate) (0.025 M) was drop-casted on a hot plate held at 170° C. in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used on the second day.
Six different synthesis routes were utilized.
(Cr2O3)3(Ga2O3) electrodes were synthesized as described above and into the electrode as described above. The electrodes were then immersed in a solution that contained 0.1 M KCl in MILLI-Q® lab water, which was saturated with CO2 and buffered to a pH of 4.1 with sodium bicarbonate. To this solution, 10 μL of 0.1 M NiCl2 was added for every 10 mL of solution. This cell was held at a potential of −1.48 V vs Ag/AgCl for 20 h. The electrode was then removed from the shrink tubing and cleaned as described above. Once dry, 30 μL of 3:1 Cr:Ga solution (as described above) was drop-casted on a hot plate held at 170° C. in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace using the heating procedure mentioned previously. The electrodes were allowed to cool for 1 day and used on the second day.
Before synthesis, electrodes were cleaned as described above. For this solution, a NiCl2, NiSO4, and boric acid Watts bath was made using standard literature procedures. A gas-tight, four-necked reaction cell was utilized, where three of the four necks were occupied by the working electrode, a Pt mesh, and an Ag/AgCl reference electrode to serve as the working, counter, and reference electrodes, respectively. The counter electrode was housed in a fritted glass tube to keep its contents separate from the bulk solution, preventing the reoxidation of products formed. The fourth neck was fitted with an open-top septa cap to allow for syringe removal of headspace samples for gas chromatography (GC) analysis of gaseous products. A clean electrode was placed in a 20 mL Watts bath with a clean nickel wire as a counter electrode and Ag/AgCl as a reference electrode. A CPC experiment was done at −1.1 V vs Ag/AgCl for 100 s. The electrodes were heated on a hot plate held at 170° C., and 30 μL of the (Cr2O3)3(Ga2O3) drop-cast solution was added in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used after at least 30 h of resting.
Before synthesis, electrodes were cleaned as described above. For this solution, a NiCl2, NiSO4, and boric acid Watts bath was made using standard literature procedures. In a four-necked reaction cell as described above, a clean electrode was placed in a 20 mL Watts bath, with a platinum wire encased in a glass frit as a counter electrode and Ag/AgCl as a reference electrode. A CPC experiment was done at −1.1 V vs Ag/AgCl for 100 s. The resulting electrode was soaked in HCl for 16 h, then sanded, and polished. The electrodes were then sonicated until no alumina was detected. The electrodes were heated on a hot plate held at 170° C., and 30 μL of the (Cr2O3)3(Ga2O3) drop-cast solution, vide supra, was added in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used after at least 30 h of resting.
Before synthesis, electrodes were cleaned as described above. A 0.05 wt % Ni nanopowder suspension was prepared. This suspension was sonicated for at least 30 min prior to drop-casting. The electrodes were heated on a hot plate held at 170° C. Two microliters of the Ni suspension was placed onto the electrode and allowed to dry, followed by 30 μL of the Cr: Ga drop-cast solution (as described above) in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used on the second day.
Before synthesis, electrodes were cleaned as described above. A 0.05 wt % Ni nanopowder suspension was prepared. Forty microliters of this suspension was then mixed with 600 μL of (Cr2O3)3(Ga2O3) drop-cast solution (as described above). This suspension was sonicated for at least 30 min prior to drop-casting. The electrodes were heated on a hot plate held at 170° C. Thirty-two microliters of this suspension was drop-casted in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used on the second day.
Before synthesis, electrodes were cleaned as described above. The nickel surface for route 2 was obtained by drop-casting 60 μL of 1.0 M Ni(NO3)2 on a hot plate held at 170° C. in 10 μL increments. The electrode was then reduced in the tube furnace, generating a nickel surface. The electrode was then drop-casted with 30 μL of 3:1 Cr:Ga solution (as described above) on a hot plate held at 170° C. in 10 μL increments. After 30 min of drying, these cylinders were placed in a tube furnace. The electrodes were allowed to cool for 1 day and used on the second day.
As will be understood, these synthesis techniques are readily adaptable to other transition metal oxides.
All electrolytes for these experiments were made by producing a 0.1 M KCl solution in MILLI-Q® lab water. All solutions were sparged and saturated with CO2 through bubbling of the solution for at least 30 min. The pH of the electrolyte (3.8) was then buffered to the desired pH using 0.1 M NaHCO3 in MILLI-Q® lab water. Approximately 25 mL of this solution was then transferred to a custom, gas-tight, four-necked cell. The volume of solution was determined by mass of solution added, since 0.1 M KCl has a density of 1.0 g/mL. The resulting headspace of the cell was approximately 8-10 mL. The total volume of the cell was determined by the mass of water. Three of the four necks were occupied by the working electrode, a Pt mesh, and an Ag/AgCl reference electrode to serve as the working, counter, and reference electrodes, respectively. The counter electrode was housed in a fritted glass tube to keep its contents separate from the bulk solution, preventing the reoxidation of products formed. The fourth neck was fitted with an open-top septa cap to allow for syringe removal of headspace samples for gas chromatography (GC) analysis of gaseous products. CPCs were performed by holding at the desired potential for the desired length of time. All reductive LSV measurements were scanned from 0 to −1.6 V vs Ag/AgCl at a scan rate of 100 mV/s.
Using the electrocatalyst of route 1, headspace analysis showed that hydrogen (H2) was the only gaseous product. Notably, CO was not detected. Bulk electrolysis data obtained using a nickel-free (Cr2O3)3(Ga2O3) electrode indicated the formation of formic acid, methanol, acetic acid, and acetone as the electrosynthesized liquid products. Similarly, 1H NMR analysis of the catholyte obtained using a Ni-enhanced (Cr2O3)3(Ga2O3) electrocatalyst also indicated the presence of methanol, formate, acetone, and acetic acid. However, an additional set of four signals was detected, with peaks at δppm=0.93 (3H), 1.36 (2H), 1.64 (2H), and 3.17 (2H). Analysis of 13C NMR after enhancement with 13CO2 showed four peaks at δppm=12.86, 19.15, 23.11, and 58.07. Given the integration and chemical shifts of this product, a metal-bound butanol is likely the primary product of the reaction.
Further characterization via 1H-1H COSY (see
ICP-OES of the electrolyte before and after electrolysis showed several metal ion impurities at ppb levels; however, control experiments showed no effect on the observed catalysis. Critically, this system is copper-free. Notably, nickel was found to be roughly an order of magnitude more concentrated versus background in samples utilizing a nickel-enhanced electrode. This data further suggests that the metal-bound species contains either Ga(III) or Ni(II).
To further investigate the identity of the metal-bound species, positive-ion electrospray mass spectrometry (ESIMS+) was employed. The resulting solution from the CPC experiment was serially diluted two times with a 90/9.9/0.1 mixture of water/acetonitrile/formic acid. After dilution, 0.5 μL of this solution was injected into an AGILENT® 6230 LC/TOF system. Results were compared to a purine (121.050873) and hexakis (1H,1H,3H-tetrafluropropoxy)-phosphazine (922.009798) standard. This assignment was confirmed using a Scientific Instrument Services mass spectrometry simulator. ICP-OES measurements were performed using an Agilent® 5800 ICP-OES system. Standards (1000 μg/mL) for each element were purchased from Agilent. The post-electrolysis sample (5.0 mL) was dissolved in 0.26 mL of HNO3 and then digested for 3 min at 200° C. using a DISCOVER® SP-D digester. Electrodes were soaked overnight in 5.0 mL of aqua regia, and then 5.0 mL of fresh aqua regia was added prior to digestion. Samples were then subjected to ICP-OES.
The base peak at m/z=239 is assigned to a trigonal fragment containing both butanol and butyraldehyde, [Ni(CH3CH2CH2CH2OH)—(CH3CH2CH2CHO)Cl], formed by dissociation of a chloride anion from the molecular species Ni(CH3CH2CH2CH2OH)—(CH3CH2CH2CHO)Cl2. This identification is supported by density functional theory (DFT) calculations for both the ground-state Ni(II) dichloride precursor complex and the monochloride fragment in vacuo. A single imaginary frequency supports the identification of the three-coordinate monochloride Ni(II) complex being a transition state; such species have been previously observed by ESIMS. In summary, the major C4 product obtained from the Ni-enhanced (Cr2O3)3(Ga2O3) system at −1.48 V has been identified as nickel-bound 1-butanol, as determined by the combination of NMR, ICP-OES, and MS data.
Ni-enhanced electrodes (from Example 1) were subjected to linear sweep voltammetry (LSV) under Ar and CO2 atmospheres. The current onset potential under CO2 was found to be ˜−0.60 V vs Ag/AgCl. To ensure that ˜1.0 C would pass over the selected 20 h reaction time, potentials were chosen that generated at least 0.70 mA/cm2 current. As a result, potentials from ˜0.90 to ˜1.60 V vs Ag/AgCl at pH 4.1 were examined (see
The starting pH for the reaction was varied from 3.8 (the pH of CO2-saturated electrolyte) to 5.0 (see
Five control experiments were conducted using nickel-free (Cr2O3)3(Ga2O3), Cr2O3, Ga2O3, Ni-enhanced Cr2O3, and Ni-enhanced Ga2O3. The results of (Cr2O3)3(Ga2O3) are summarized in
To probe the overall durability of the interfaces of interest and further access the reactivity of the system, time dependence measurements were completed over a period of 7 days. Using controlled potential coulometry (CPC), the catholyte exposed to a Ni-enhanced (Cr2O3)3(Ga2O3) electrode (pH=4.1, V=−1.48 V vs Ag/AgCl) was sampled every 4 h for the first 24 h and then every subsequent day (see
The route 1 electrode was olive-green in color, with a foam-like appearance. SEM imaging showed large porous islands. This morphology was distinctly different from that obtained in previous studies of (Cr2O3)3(Ga2O3), which presented a “dried lake bed” morphology. SEM images after CPC showed no structural change. To explore the amount of nickel present in the interfacial assembly, a glassy carbon electrode was electroplated and etched with HCl following the route 1 synthesis protocol. The surface so generated was examined using X-ray photoelectron spectroscopy (XPS). Two peaks were observed in the Ni 2p region having a 17.3 eV separation (Ni 2p1/2 and Ni 2p3/2 at binding energies of 875.5 and 858.2 eV, consistent with the presence of Ni(0). The low intensity of the observed signal is as expected given the trace amount of nickel present on the electrode surface. In a separate set of experiments, a set of route 1 electrodes (five electrodes) were treated with concentrated aqua regia overnight to dissolve the catalyst layer. The solutions so obtained were digested and subjected to ICPOES analysis, with nickel quantities reported in Table 1, below.
The resulting average mass of nickel was found to be 0.012 mg, but as expected, there was variation in nickel loading from electrode to electrode. Potential factors leading to the unprecedented reactivity of the route 1 electrode are nickel concentration, morphology, and crystallography.
To probe these factors, five alternate syntheses were used—routes 2-6. In all cases, CPC experiments were performed. First, nickel was electroplated using a conventional Watts bath solution (route 2). The literature indicates that Watts bath electrodes primarily produce an exposed Ni [200] face. These electrodes were slate blue in color, and SEM imaging showed a structure of a mostly solid surface of nickel with islands of chromium and gallium. Hydrogen production was found to be a function of the active chromium-gallium catalyst coverage. Increased coverage generated greater concentrations of C2+ products and limited H2. These electrodes generated 1-butanol, albeit at lower ξ, than route 1 electrodes, with 3-hydroxybutanal seen as the primary product. To confirm that the quantity of nickel was important for selectivity of 1-butanol over 3-hydroxybutanal, a dilute Watts bath was employed; this was known as route 3. These electrodes were first electroplated with nickel via a Watts bath and followed the same treatment, post nickel plating, as the route 1 electrodes. This treatment resulted in the catalyst having an olive-green appearance. SEM imaging showed flaky islands of catalyst above a rough surface. These electrodes generated significantly higher concentrations of 1-butanol, albeit with significantly more charge passed, than route 2 electrodes. However, neither route 2 nor route 3 generated 1-butanol at the high faradaic efficiencies provided by route 1.
The possibility that the second step in the synthesis of the route 1 electrode (HCl etch followed by mechanical abrasion) generated nickel nanoparticles and might be key to the catalytic activity was considered. In synthesis route 4, commercial nickel nanoparticles (0.05 wt % suspension) were used in place of electrochemical deposition of nickel before drop-casting and heating under reducing conditions. These electrodes had a kelly-green appearance. SEM imaging showed a structure of amorphous granules in addition to the “lakebed” patterns identified previously. Production of 1-butanol reached ξ=17%, which is less than half of the faradaic yield observed using route 1. Synthesis route 5 utilized the same nanoparticles as in route 4, but they were suspended as a colloid in the metal nitrate solution before drop-casting. This was done to better intersperse the nickel sites throughout the chromium-gallium catalyst. Route 5 electrodes had a silvery appearance, and SEM showed a porous, sponge-like structure. This route produced 1-butanol, albeit at very low concentrations. Synthesis route 6 was performed by first drop-casting nickel nitrate on the glassy carbon surface and heating under reducing conditions. These electrodes then had the primary face of bulk nickel metal [111]. This resulted in a uniform, metallic nickel surface upon which the Cr/Ga nitrates were drop-casted. These electrodes appeared to be blue-gray in color and did not produce 1-butanol. SEM showed a structure of flakes on a mostly uniform nickel surface. Route 6 did not produce C4 products. Based on routes 2-6, it can be concluded that a higher amount of nickel on the surface correlates with a decrease in the generation of 1-butanol and other C4 products. Additionally, electroplated nickel outperforms the other nickel addition routes (4-6). It is hypothesized that this could be caused by the crystal face specificity of electroplated nickel. Gas phase analysis of routes 2-6 indicated H2 as the only gaseous product. All routes, except for route 6 showed formic acid, methanol, acetic acid, acetone, 1-butanol, and 3-hydroxybutanal as liquid products, as summarized in
However, in route 2-6 electrodes, 1-butanol formation was disfavored compared to route 1 electrodes. In all cases, hydrogen generation was approximately equal to or significantly higher than that of route 1 electrodes. In routes with significant nickel coverage (2, 5, and 6), hydrogen was seen as the primary product (>>50%) of the reaction. This is likely due to the exposed nickel to the solution. As exposed nickel decreased, higher overall yields for C2+ products increased (i.e., routes 1, 3, and 4). This set of experiments show that the coupling step must happen at the electrode interface.
To illustrate the role of nickel during the synthesis, a 3:1 mixture of Cr(NO3)3 and Ga(NO3)3 on a nickel substrate was heat treated at 700° C. for 5 h. The strongest reflections in the X-ray diffraction patterns of all samples presented correspond to the space group R-3cH (167). A LeBail fit of the resulting XRD pattern was performed. Decomposition of metal-nitrate salts under reducing conditions typically generates metal oxides prior to full reduction to metallic states. In the case of Cr(NO3)3, reduction conditions were not sufficient to form Cr0, resulting in Cr2O3 as the product. The oxide is explicitly from the decomposition of the nitrate anion and not any other sources. In contrast to Cr, Ga(NO3)3 was reduced from Ga3+ to Ga0, forming a bimetallic alloy with nickel. The space group and lattice parameter of the alloy phase agree with a Ni3Ga alloy from the ICDD database. The contribution of pure nickel to this pattern is due to the exposed nickel substrate. Therefore, it is proposed that nickel on the surface of the glassy carbon electrode forms an alloy with some of gallium, which is coated in a several micrometer-thick, porous Cr2O3 and Ga2O3 layer (where the gallium oxide is a function of the nickel coverage).
Given the observation of a formate product, and the lack of observable CO, formic acid is likely the two-electron building block for the Ni-enhanced (Cr2O3)3(Ga2O3) system. To date, only a few electrocatalysts are suggested to go through a formate pathway to generate C—C-coupled products. Lee and Yeo have proposed a potential mechanism for the production of 1-butanol that starts with the electrosynthesis of formate from CO2, in which formate was transformed to formaldehyde. Two molecules of formaldehyde could then undergo aldehyde self-condensation to eventually generate acetaldehyde. Two acetaldehydes can then combine and undergo an aldol condensation, followed by hydrogenation, to generate 1-butanol.
To explore the applicability of this mechanism as a reasonable description of the chemistry occurring at the Ni-enhanced (Cr2O3)3(Ga2O3) interface, the reactivity of formate as a substitution for CO2 in the CPC experiment is first considered. To that end, a formic acid/sodium formate buffer (pKa=3.75) solution was purged with argon and adjusted to pH=4.1. This solution was electrolyzed at −1.48 V vs Ag/AgCl for 20 h. 1H NMR of the product electrolyte confirmed the presence of all species previously identified with the Ni-enhanced (Cr2O3)3(Ga2O3) in CO2 solutions.
This finding provided further evidence that formate can serve as the primary reactive intermediate as reported by Lee et al. To further determine the role of (Cr2O3)3(Ga2O3), a nickel-free electrode was electrolyzed at −1.48 V for 16 h in a formic acid/formate buffer under CO2-free conditions. NMR analysis of the electrolyte showed methanol, ethanol, acetic acid, acetaldehyde, and acetone as products. Ethanol was likely formed through hydrogenation of acetaldehyde. It should be noted that in a typical experiment, 1-5 C are passed by the electrode, whereas in this experiment, 48 C of charge passed. This is indicative of a larger HER contribution, and therefore, generation of ethanol through hydrogenation of acetaldehyde is to be expected. This suggests that the “role” of (Cr2O3)3(Ga2O3) is to facilitate the reduction of formate to acetaldehyde. Notably, Ni-free (Cr2O3)3Ga2O3 is unable to couple acetaldehyde into C4 products. In a similar experiment using aqueous acetaldehyde (10 mM, pH=7) in a cell held under an argon atmosphere, acetic acid, acetone, and 1-butanol were observed as product. This observation supports the key role played by acetaldehyde in the formation of 1-butanol but does not identify which component(s) of the catalyst are responsible for the transformation of acetaldehyde to 1-butanol. A second study was done using a nickel-free (Cr2O3)3(Ga2O3) electrode under the same conditions. While acetic acid was observed to form in this study, 1-butanol was not detected. Acetone was also not detected, but the acetone NMR signal was most likely obscured by the acetaldehyde. This study demonstrated that nickel was an essential component for 1-butanol synthesis. To explore the role of nickel in the aldol condensation step, two different working electrodes were examined: a coiled nickel wire and a planar glassy carbon electrode electroplated with nickel via a Watts bath. All experiments were performed with acetaldehyde in water under an argon atmosphere at pH=7. The nickel coil produced a minimal amount of 1-butanol, whereas the Watts bath electrode generated significantly more 1-butanol. This is notable given that the nickel coil had much larger surface area than the nickel-modified carbon electrode. These results suggest that certain crystal faces or nickel grain boundaries are critical variables for butanol production. As a control, zero bias experiments were carried out, resulting in no products formed, indicating that the catalysis requires the electrode to be at a reducing potential.
The experiments described above provided the following four insights. First, formic acid is the necessary two-electron building block for generation of all other species. Second, formic acid is transformed into acetaldehyde, which can be coupled in the presence of nickel to generate 3-hydroxybutanal. This species is then further reduced to 1-butanol. Third, acetaldehyde reacts in water to form acetic acid, and finally, acetaldehyde couples with either methanol or formaldehyde to form acetone. This is complemented by the Ni-free (Cr2O3)3(Ga2O3) study that observed acetone and acetic acid as products. Thus, (Cr2O3)3(Ga2O3) is the component that generates acetaldehyde, but Ni is necessary to further generate 3-hydroxybutanal and then 1-butanol. Therefore, the Ni-enhanced transition metal oxide electrocatalytic mechanism likely involves a cascade scheme where the transition metal oxide forms acetaldehyde, which can then migrate to the nickel surface for aldol condensation and hydrogenation. Finally, it is noted that if D2O is utilized as the proton source, the KIE of 0.58 is found, which is consistent with the formation of a surface metal-hydride.
Based on these observations, the preliminary mechanism provided in
The Ni-enhanced (Cr2O3)3(Ga2O3) electrocatalyst was proposed to operate via a cascade catalysis mechanism for the generation of 1-butanol. The (Cr2O3)3(Ga2O3) interfacial layer was identified as the source of acetaldehyde from CO2. The nickel phase then promotes the coupling of two acetaldehydes, followed by hydrogenation, to generate 1-butanol. Moreover, nickel topology and concentration are the driving factors for CO2RR as seen in routes 1 and 3, while larger nickel concentrations and varying nickel topology result in a significant shift toward HER.
By introducing the nickel additive, previously reported intermetallic oxides have been tuned to achieve reactions that are seldom seen without copper. This opens the door for synthesis and design of new catalysts that bring together materials that are not active for CO2 reduction to generate C2+ products.
Various modifications may be made to the systems, methods, apparatus, mechanisms, techniques and portions thereof described herein with respect to the various figures, such modifications being contemplated as being within the scope of the invention. For example, while a specific order of steps or arrangement of functional elements is presented in the various embodiments described herein, various other orders/arrangements of steps or functional elements may be utilized within the context of the various embodiments. Further, while modifications to embodiments may be discussed individually, various embodiments may use multiple modifications contemporaneously or in sequence, compound modifications and the like.
Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. Thus, while the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims.
The present application claims priority to U.S. Provisional Patent App. No. 63/462,646, filed Apr. 28, 2023, the contents of which are incorporated by reference herein in its entirety.
This invention was made with government support under Grant No. CHE-1308652 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63462646 | Apr 2023 | US |
