Patent Application
20140308597
The invention relates to a process for producing hydrogen by steam reforming of methanol, a catalyst for use in the process, and a process for producing the catalyst.
Global efforts are currently under way to minimize the emissions of NOx, SOx, hydrocarbons, CO, and CO2. The use of hydrogen as an environmentally friendly energy carrier has been massively encouraged over the last years. Hydrogen is considered as the best fuel because of no emission of pollutants and also offers high efficiency when used in proton exchange membrane (PEM) fuel cells. Particularly, for portable applications such as cell phones, mp3-players, laptop computers and similar niche products, the use of PEM fuel cells is deemed to be more energy efficient than battery technology (Zhao, T. S. Microfuel cells: Principles and Applications, Elsevier, USA, 2009). Low temperature PEM fuel cells and micro fabrication technologies are potentially the preferred choices for these consumer products. There are a number of ways of obtaining hydrogen from both renewable and non-renewable sources on a large industrial scale, but the storage and transfer of hydrogen in solid systems for mobile use are problematic because of poor volumetric and weight energy densities (Van den Berg, A. W. C. & Arean, C. O. Chem. Commun, 668-681, 2008). In addition, ultra-pure hydrogen gas is required by the PEM fuel cells. In particular, the gas stream has to be free from CO gas (<10 ppm) otherwise the catalytic performance of the fuel cells is significantly degraded (Springer, T. E., Rockward, T., Zawodzinski, T. A. & Gottesfeld, S. J. Electrochem. Soc. 148, A11-A23, 2001). The use of on-board reforming of organic compounds with downstream multistage CO post-treatments such as the water gas shift (WGS) reaction, selective oxidation of CO to CO2 (SELOX), hydrogenation of CO to methane or membrane technology, etc. is not applicable. This is because these cumbersome multistage processes commonly taken place at elevated temperatures, which precludes their adaptation in the small portable devices where space and heat management are at a premium.
Hydrogen stored in a chemical form as liquid organic compounds and released in-situ on demand at room temperature without CO contamination appears to be a more promising direction for mobile fuel cells. The primary liquid fuel can be stored in a disposable or recycled cartridge, which is changeable and logistically easily available. The production of hydrogen from organic compounds such as formic acid, which is nontoxic and a liquid at room temperature, with a density of 1.22 g. mL−3, has recently been demonstrated. The use of methanol, however, which is a key platform chemical for present fuel and chemical infrastructures with high energy content 5420 kcal kg−1, is economically more attractive (Olah, G. A.; Coeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy, Wiley-VCH, 2nd Edit. Los Angeles, Calif., USA, 1998; WO 98/29333). Although methanol is currently produced industrially from non-renewable fuels over Cu/ZnO/Al2O3 catalysts (Toyir, J.; Ramirez de la Piscina, P.; Fierro, J. L. S.; Homs, N. Appl. Catal. B: Env. 29, 207-215, 2001; Saito, M.; Fujitani, T.; Takeuchi, M.; Watanabe, T. Appl. Catal. A: Gen. 138, 311-318, 1996; Sloczyn'ski, J.; Grabowski, R.; Olszewski, P.; Kozlowska, A. Stoch, J.; Lachowska, M.; Skrzypek, J. Appl. Catal. A: Gen. 310, 127-137 2006) in-situ hydrogen from methanol facilitates efficient on site energy conversion and cleaner emission until green methanol can be produced.
Low temperature steam reforming of methanol is seen as a promising route to hydrogen production. In particular, the direct, catalytic production of hydrogen by Non-Syngas Direct Steam Reforming (NSGDSR) of methanol is an attractive proposition. This route offers many advantages over other methods in terms of energy efficiency, CO mitigation and safety considerations. It is also in clear contrast with the conventional complex route involving steam reformation to syngas, followed by water gas shift and CO cleanup stages for the hydrogen production.
In PEM fuel cells, the on-board steam reforming process (Eq. 1) can provide a source of hydrogen in-situ, which is then combined downstream with oxygen to produce water (Eq. 2), with an accompanying release of energy:
CH3OH+H2O⇄3H2+CO2 ΔH=+49.7 kJ mol−1 (Eq. 1)
H2+½O2⇄H2O ΔH=−286 kJ mol−1 (Eq. 2)
CO is considered to be formed via methanol decomposition (Eq. 3) and the reverse WGS reaction (Eq. 4):
CH3OH⇄CO+2H2 ΔH=+90.2 kJ mol−1 (Eq. 3)
CO2+H2⇄CO+H2O ΔH=+41.2 kJ mol−1 (Eq. 4)
CO production must be minimised as much as possible, since levels greater than 10 ppm in the gas stream may poison the Pt-based catalyst used in the downstream reaction (Eq. 2) and severely impair its performance. A key challenge therefore is to provide a suitable steam-reforming catalyst that can operate at low temperatures (150-200° C.) and minimise CO production without the need for complicated downstream multi-stage CO post-treatment. The catalysts currently used for methanol steam reforming fall into two main categories: Cu-based and group 8-10 metals. Cu-based catalysts generally achieve higher activity but are unstable, being susceptible to deactivation over time due to thermal sintering, whereas group 8-10 metals provide greater stability at the expense of activity. The mechanism for CO production with Cu-based catalysts remains a controversial topic. It is not yet clear whether CO is produced primarily via methanol decomposition or the reverse WGS reaction. FT-IR studies with CuZnZrAlOx have shown that CO is formed as a secondary product via the reverse WGS reaction only, and not via methanol decomposition, but in general the matter is still far from settled. A key challenge therefore is to develop efficient catalysts for the production of hydrogen from steam-methanol reformation, in which the product gas stream contains a very low concentration (towards less than 10 ppm) of CO gas. Lachowska, M., Reac Kinet Mech Cat (2010) 101:85-91 describes a study of steam reformation of methanol over a mixed Cu/Zn/Zr/Ga oxide catalyst, which contains 65.3 wt. % CuO, 26.3 wt % ZnO, 4.5 wt. % ZrO2 and 3.9 wt. % Ga2O3 (i.e. 67.2 at. % Cu, 26.4 at. % Zn, 3.0 at. % Zr and 3.4 at. % Ga, relative to the total number of metal atoms in the oxide). However, the study only provides calculated carbon monoxide concentrations presented at the parts per hundred (percent) level, indicating that the level of CO production has not been controlled at the parts per million (ppm) level. It remains an important challenge therefore to provide a suitable steam-reforming catalyst that can operate at low temperatures (150-200° C.) and minimise CO production at the ppm level, without the need for complicated downstream multi-stage CO post-treatments.
The inventors have provided a heterogeneous catalyst which is active for the production of hydrogen by Non-Syngas Direct Steam Reforming (NSGDSR) of methanol at low temperatures and atmospheric pressure. Little or no CO is produced at the ppm level, making the NSGDSR process suitable for use in PEM fuel cell applications.
Accordingly, the invention provides a process for producing H2 by steam reforming of methanol, which process comprises contacting a gas phase comprising (a) CH3OH and (b) H2O with a solid catalyst, which solid catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
In another aspect, the invention provides a catalyst for use in a process for producing H2 by steam reforming of methanol, which catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
In another aspect, the invention provides a process for producing a catalyst, which catalyst is suitable for use in a process for producing H2 by steam reforming of methanol, and which catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %, which process comprises:
(1) a co-precipitation step, comprising contacting: (a) a solution of copper nitrate, zinc nitrate and gallium nitrate, wherein the atomic percentage of copper relative to the total number of metal atoms in said solution is from 20 at. % to 55 at. %, with (b) a metal carbonate, to produce a co-precipitate comprising said copper, zinc and gallium;
(2) a separation step, comprising separating the co-precipitate from solution; and
(3) a calcination step, comprising calcining the co-precipitate by heating the co-precipitate in air.
The process of the invention for producing a catalyst optionally further comprises:
(4) a reduction step, comprising heating the calcined product in the presence of H2.
In another aspect, the invention provides a catalyst which is obtainable by the process of the invention for producing a catalyst.
In another aspect, the invention provides the use of a catalyst of the invention as defined above in a process for producing H2 by steam reforming of methanol.
In another aspect, the invention provides a fuel cell system comprising a fuel cell and a methanol reformer, which methanol reformer comprises a catalyst of the invention. Typically, the fuel cell is a proton exchange membrane (PEM) fuel cell.
In another aspect, the invention provides a portable electronic device comprising a fuel cell system of the invention.
In another aspect, the invention provides the use of a catalyst in a process for producing methanol by the hydrogenation of carbon dioxide, which catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
In another aspect, the invention provides a process for producing methanol by the hydrogenation of carbon dioxide, which process comprises contacting a gas phase comprising (a) CO2 and (b) H2, with a solid catalyst, which solid catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
).
The invention provides a process for producing H2 (hydrogen) by steam reforming of methanol, as defined above. In the process of the invention, the H2 is advantageously produced directly, in a single step, by Non-Syngas Direct Steam Reforming (NSGDSR) of methanol in accordance with the following reaction:
CH3OH+H2O→CO2+3H2
This in clear contrast with the conventional complex route involving steam reformation to syngas, followed by water gas shift and CO cleanup stages for the hydrogen production.
The process comprises contacting a gas phase comprising (a) CH3OH and (b) H2O, with a solid catalyst. The gas phase may or may not comprise other gases, in addition to the methanol and steam. For instance, the gas phase may comprise an inert gas, e.g. nitrogen or argon, which could for example be present as a carrier gas. The inert gas, when present, is typically nitrogen. Additionally or alternatively, the gas phase may further comprise oxygen. Blending oxygen or air into the gas phase may encourage combustion and may also balance the total thermodynamic requirements of the NSGDSR system. Thus, the gas phase may further comprise oxygen or air.
The gases in the gas phase may be pre-mixed, i.e. mixed together before the mixture is brought into contact with the catalyst. Alternatively, the gases can be fed into a reactor separately, so that the reactant gases are mixed together in the presence of the solid catalyst.
Typically, the step of contacting said gas phase with said solid catalyst comprises passing said gas phase through a reactor comprising said catalyst.
In the process of the invention, H2 is usually produced in the gaseous state. Thus, the process of the invention for producing H2 is typically a process for producing hydrogen gas.
The solid catalyst used in the process of the invention for producing H2 comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium. The mixed metal oxide may comprise a plurality of oxide phases. Thus, for instance, in one preferred embodiment, the mixed metal oxide includes a non-stoichiometric cubic spinel phase comprising excess interstitial Cu+ ions that are highly dispersed and highly reducible, leading to an enhanced surface area of Cu in the catalyst. The mixed metal oxide may comprise other metals, in addition to copper, zinc and gallium, for instance zirconium. However, in some embodiments, the catalyst does not contain any zirconium. In other embodiments, the mixed metal oxide contains little or no zirconium. Thus, the mixed metal oxide may in some embodiments contain less than or equal to 2.0 at. % Zr, relative to the total number of metal atoms in the mixed metal oxide. Alternatively, copper, zinc and gallium may be the only metals in the mixed metal oxide. As will be understood by the skilled person, much of the copper, zinc and gallium in the mixed metal oxide will be present as cations in the oxide structure. However, the mixed metal oxide may additionally comprise particles of these metals in the oxidation state zero, such as for instance particles of copper metal, Cu0. Such Cu0 particles may be present on the surface of the mixed metal oxide. The Cu0 particles will typically have a mean particle size of less than or equal to 10 nm, or for instance less than or equal to 5 nm.
The atomic percentage of copper in the mixed metal oxide (and “copper” here refers to all copper, including copper cations as well as any Cu0 present) relative to the total number of metal atoms in the mixed metal oxide (and “metal atoms” here means all metal atoms, including metal ions and any metal present in oxidation state zero) is from 20 at. % to 55 at. %. The inventors have demonstrated good MeOH conversion levels for such catalysts, and little or no CO production at the ppm level. The inventors have also observed a close correlation between MeOH conversion, CO suppression, and the specific surface area of Cu in the oxide. It has been found that deviations from the range of 20 at. % to 55 at. % copper in either direction leads to a reduced specific surface area of Cu, and reduced MeOH conversion. The inventors expected at first that increasing the Cu content would lead to an increase specific surface area of Cu. However, it is a finding of the present invention that beyond ˜50% Cu loading, the surface area starts to decrease again, and so does the MeOH conversion in the steam reformation of methanol. Also, the inventors have observed that in general, CO production decreases as MeOH conversion increases, which is in turn dependent on the surface area of Cu in the catalyst. Thus, the catalysts of the invention, which are used in the process of the present invention for producing H2, contain an advantageous level of Cu of from 20 at. % to 55 at. %, which provides for a high surface area, good MeOH conversion and suppression of CO production.
Typically, the atomic percentage of copper in the mixed metal oxide, relative to the total number of metal atoms in the oxide the atomic percentage of copper is from 30 at. % to 55 at. %. More typically, it is from 35 at. % to 55 at. %. The inventors have found that such catalysts have particularly high specific surface areas of Cu and particularly good MeOH conversion levels and suppression of CO. The atomic percentage of copper may for instance be from 40 at. % to 52 at %.
Typically, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is equal to or greater than 5 at. %. The inventors have also found that including gallium in the catalyst leads to suppression of CO production and that increasing the level of gallium leads to particularly low CO levels. In particular, the inventors have found that CO production is not only related to SAcat, but also strongly related to Ga content, with CO levels decreasing as the Ga content is increased. This is an important discovery, as it means that that Ga in the catalyst is playing an active role in the catalytic process by somehow suppressing CO formation. The inventors have observed from AC impedance that Ga in the support leads to decreasing conductivity and increasing thermal activation energy. This is because as the Ga content increases, the poorly crystalline tetragonal NSS phase begins to predominate, which has reduced Cu mobility due to its lack of ordered crystallinity. The oxygen mobility will also be reduced for the same reason. O-vacancies are believed to play a key role in CO formation; therefore the lack of available O-vacancies in the support due to the poorly crystalline nature of the tetragonal NSS phase, caused by the abundance of Ga, would explain the downward trend in CO formation as Ga content is increased. There is, therefore a balance to be struck between maximising the αCu surface area in order to promote MeOH conversion, and maximising the Ga content so as to suppress CO formation. The presence of gallium is also thought to stabilise a highly dispersed, reducible form of Cu.
Thus, preferably, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is equal to or greater than 10 at. %. The atomic percentage of gallium relative to the total number of metal atoms in the oxide may for instance be equal to or greater than 15 at. %. In another embodiment, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is equal to or greater than 18 at. %. In some embodiments, it is equal to or greater than 20 at. %.
Typically, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is from 10 at. % to 40 at. %. The atomic percentage of gallium relative to the total number of metal atoms in the oxide may for instance be from 10 at. % to 35 at. %, or for instance from 15 at % to 35 at %.
The zinc present in the mixed metal oxide is also thought to stabilise a highly dispersed, reducible form of Cu. Thus, the atomic percentage of zinc relative to the total number of metal atoms in the oxide is typically from 10 at. % to 50 at. %. More typically, the atomic percentage of zinc relative to the total number of metal atoms in the oxide is from 15 at. % to 45 at. %.
Particularly high activities and suppression of CO production have been found in catalyst wherein the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the mixed metal oxide are from 30 to 55 at. % copper, at least 15 at. % zinc, and at least 10 at. % gallium. Preferably, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide are from 40 to 55 at. % copper, at least 20 at. % zinc, and at least 15 at. % gallium. In some embodiments, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide may be from 40 to 52 at. % copper, at least 25 at. % zinc, and at least 15 at. % gallium.
Preferably, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide are from 30 to 55 at. % copper, from 15 to 45 at. % zinc, and from 10 to 40 at. % gallium. The atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide may for instance be from 40 to 55 at. % copper, from 20 to 45 at. % zinc, and from 15 to 35 at. % gallium.
In some embodiments of the process of the invention for producing H2, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the mixed metal oxide are from 40 to 52 at. % copper, from 20 to 40 at. % zinc, and from 15 to 30 at. % gallium, preferably wherein said atomic percentages are from 47 to 52 at. % copper, from 27 to 33 at. % zinc, and from 17 to 23 at. % gallium, more preferably wherein said atomic percentages are about 49 at. % copper, about 31 at. % zinc, and about 20 at. % gallium.
In some embodiments, the metal atoms in the mixed metal oxide used in the process of the invention for producing H2 consist essentially of said copper, zinc and gallium.
In some embodiments, the metal atoms in the mixed metal oxide used in the process of the invention for producing H2 consist of said copper, zinc and gallium, i.e. said copper, zinc and gallium may in some embodiments be the only metals in the mixed metal oxide. In one embodiment, the metal atoms in the mixed metal oxide consist of from 20 to 55 at. % copper, from 5 to 40 at. % zinc, and from 5 to 40 at. % gallium. In another embodiment, the metal atoms in the mixed metal oxide consist of x at. % copper, y at. % gallium, and z at. % zinc, wherein x is from 30 to 55, y is from 10 to 40, and z is 100−(x+y). The metal atoms in the mixed metal oxide may for instance consist of x at. % copper, y at. % gallium, and z at. % zinc, wherein x is from 40 to 55, y is from 15 to 35, and z is 100−(x+y). In one preferred embodiment, x is from 40 to 52, y is from 15 to 35, and z is 100−(x+y). In another preferred embodiment, x is from 40 to 55, y is from 15 to 25, and z is 100−(x+y).
Typically, in the process of the invention for producing H2 (hydrogen) by steam reforming of methanol, the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 40 m2/g-catalyst. The mixed metal oxide may for instance have a specific copper metal surface area, as measured by N2O chemisorption, of at least 50 m2/g-catalyst, or for instance at least 60 m2/g-catalyst. More preferably, the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 65 m2/g catalyst. In a particularly preferred embodiment the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 70 m2/g catalyst.
Typically, the mixed metal oxide has a specific copper metal surface area per gram copper, as measured by N2O chemisorption, of at least 100 m2/g-Cu, more typically at least 120 m2/g-Cu or for instance at least 140 m2/g-Cu. Preferably, the surface area per gram copper, as measured by N2O chemisorption, is at least 150 m2/g-Cu. In some embodiments, it is at least 160 m2/g-Cu, or for instance at least 180 m2/g-Cu. In particularly preferred embodiments the mixed metal oxide has a specific copper metal surface area per gram copper, as measured by N2O chemisorption, of at least 200 m2/g-Cu. The specific copper metal surface area per gram copper may for instance be at least 210 m2/g-Cu.
Typically, in the process of the invention, the mixed metal oxide comprises particles of said copper, wherein the copper particles have a mean particle size of less than or equal to 40 nm. The particles usually however have a mean particle size of less than or equal to 30 nm, or more typically less than or equal to 20 nm. The term “particle size” as used herein means the diameter of the particle if the particle is spherical or, if the particle is non-spherical, the volume-based particle size. The volume-based particle size is the diameter of the sphere that has the same volume as the non-spherical particle in question.
The mixed metal oxide may for instance comprise particles of said copper, wherein the copper particles have a mean particle size of less than or equal to 10 nm. Preferably, the copper particles have a mean particle size of less than or equal to 5 nm.
Typically, in the process of the invention, the mixed metal oxide has a copper metal dispersion of at least 15%, preferably at least 20%.
Typically, in the process of the invention, the mixed metal oxide comprises Cu2+, interstitial Cu+ and Cu0.
Usually, the mixed metal oxide comprises a non-stoichiometric spinel phase comprising copper, zinc and gallium.
Typically, the mixed metal oxide comprises a non-stoichiometric cubic spinel phase comprising copper, zinc and gallium.
Usually, the spinel phase comprises interstitial Cu+. The interstitial Cu+ ions are easily reducible to Cu metal (Cu0) leading to high Cu0 dispersion and surface area.
Thus, the mixed metal oxide typically comprises a spinel phase comprising copper, zinc and gallium, which spinel phase comprises interstitial Cu+ and Cu0.
The spinel phase typically also comprises octahedral Cu2+.
Thus, the mixed metal oxide typically comprises a spinel phase comprising copper, zinc and gallium, which comprises octahedral Cu2+, interstitial Cu+ and Cu0.
The spinel phase typically comprises particles of copper metal (Cu0). These copper particles typically have a mean particle size of less than or equal to 40 nm, or for instance less than or equal to 30 nm, or preferably less than or equal to 20 nm. In one preferred embodiment the spinel phase comprises particles of copper metal (Cu0) which have a mean particle size of less than or equal to 10 nm. More preferably the copper particles have a mean particle size of less than or equal to 5 nm.
The catalyst used in the process of the invention for producing H2 may or may not further comprise a solid support material, in addition to said mixed metal oxide. Any suitable support material may be used. In other embodiments, the catalyst does not further comprise a solid support material. Thus, the mixed metal oxide may be unsupported.
The catalyst used in the process of the invention for producing H2 may be a catalyst which is obtainable by the process of the invention as defined herein for producing a catalyst suitable for use in a process for producing H2 by steam reforming of methanol.
Usually, in the process of the invention for producing H2 by steam reforming of methanol, the step of contacting the gas phase with the solid catalyst is performed at atmospheric pressure. However, pressures other than atmospheric pressure may also be used.
The inventors have shown that, in the process of the invention for producing H2 by steam reforming of methanol, the catalyst can advantageously be used to generate hydrogen, with little or no production of CO, at temperatures of less than or equal to 200° C. However, higher temperatures may in principle be used. Typically, therefore, the step of contacting the gas phase with the solid catalyst is performed at a temperature which does not exceed 200° C.
Usually, the step of contacting the gas phase with the solid catalyst is performed at a temperature of from 80° C. to 200° C., more typically from 100° C. to 200° C.
In some embodiments, the step of contacting the gas phase with the solid catalyst is performed at a temperature of from 120° C. to 200° C., more typically from 130° C. to 200° C. For instance, the step of contacting the gas phase with the solid catalyst may be performed at a temperature of from 140° C. to 200° C.
Example 1 herein also shows however that CO formation can be suppressed totally or reduced by decreasing reaction temperature, in order to discourage the slow RWGS reaction;
The process of the invention for producing H2 preferably occurs substantially without any formation of carbon monoxide. Thus, the process of the invention typically occurs substantially without any formation of carbon monoxide either via methanol decomposition, as follows:
CH3OH→CO+2H2
or via the reverse water-gas shift reaction as follows:
CO2+H2→CO+H2O
Typically, the gaseous product mixture comprises no more than 100 ppm by volume of carbon monoxide, more typically no more than 50 ppm by volume. In preferred embodiments, the gaseous product mixture comprises no more than 10 ppm by volume of carbon monoxide.
Thus, usually, in the process of the invention for producing H2 by steam reforming of methanol, the process occurs substantially without any formation of carbon monoxide. Typically, the level of CO produced does not exceed 100 ppm. More typically, the level of CO produced does not exceed 50 ppm. In particularly preferred embodiments, the level of CO produced does not exceed 10 ppm.
Typically, in the process of the invention for producing H2, the percent conversion of methanol is at least 20%. More typically, it is at least 30%.
Usually, in the process of the invention for producing H2, the molar ratio of H2O to CH3OH in said gas phase is equal to or greater than 1. In some embodiments however the molar ratio of H2O to CH3OH in said gas phase is equal to or greater than 10:1, preferably equal to or greater than 20:1. Such molar ratios were found to promote the methanol conversion;
In the process of the invention for producing H2, the gas phase which comprises H2O and CH3OH may be generated by feeding a liquid phase comprising said H2O and CH3OH through a heated zone, which causes evaporation of the liquid phase to produce said gas phase. Typically, in such embodiments, the liquid phase is fed into said heated zone at a feed rate which is equal to or greater than 0.01 mL/minute. However, it is clear from the experiments in Example 1 herein that CO formation can be suppressed or severely reduced by decreasing contact time in order to discourage the slow RWGS reaction that produces CO. Thus, preferably, the liquid phase is fed into said heated zone at a feed rate which is equal to or greater than 0.04 mL/minute, more preferably at a feed rate which is equal to or greater than 0.06 mL/minute, or for instance at a feed rate which is equal to or greater than 0.08 mL/minute. In some embodiments, the liquid phase is fed into said heated zone at a feed rate which is equal to or greater than 0.1 mL/minute, for instance equal to or greater than 0.12 mL/minute.
The process of the invention for producing H2 by steam reforming of methanol may further comprise recovering said H2. Typically, the process of the invention produces a mixture of gases comprising H2 and CO2. The step of recovering said H2 typically therefore comprises collecting the product gas mixture and separating the H2 from said mixture. The separation may be effected by any suitable method known in the art, for instance by using a filter material which selectively retains contaminants and lets the hydrogen pass through. The separated H2 gas may also for instance be compressed and/or stored for later use.
The process of the invention for producing H2 by steam reforming of methanol may further comprise using the H2 produced as a fuel. For instance, the H2 produced may be used to power a fuel cell, such as a PEM fuel cell.
The catalysts used in the process of the invention for producing H2 are themselves novel. Accordingly, the invention further provides a catalyst for use in a process for producing H2 by steam reforming of methanol, which catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
The catalyst of the invention may be as further defined hereinbefore, in the discussion of the process of the invention for producing H2 by steam reforming of methanol. Thus, the catalyst of the invention comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium. The mixed metal oxide may comprise a plurality of oxide phases. Thus, for instance, in one preferred embodiment, the mixed metal oxide in the catalyst of the invention includes a non-stoichiometric cubic spinel phase comprising excess interstitial Cu+ ions that are highly dispersed and highly reducible, leading to an enhanced surface area of Cu in the catalyst. The mixed metal oxide may comprise other metals, in addition to copper, zinc and gallium, for instance zirconium. In other embodiments, the mixed metal oxide contains little or no zirconium. Thus, the mixed metal oxide may in some embodiments contain less than or equal to 2.0 at. % Zr, relative to the total number of metal atoms in the mixed metal oxide. In one embodiment, the catalyst does not contain any zirconium. Alternatively, copper, zinc and gallium may be the only metals in the mixed metal oxide. As will be understood by the skilled person, much of the copper, zinc and gallium in the mixed metal oxide will be present as cations in the oxide structure. However, the mixed metal oxide may additionally comprise particles of these metals in the oxidation state zero, such as for instance particles of copper metal, Cu0. Such Cu0 particles may be present on the surface of the mixed metal oxide. The Cu0 particles will typically have a mean particle size of less than or equal to 10 nm, or for instance less than or equal to 5 nm.
The atomic percentage of copper in the mixed metal oxide of the catalyst of the invention, relative to the total number of metal atoms in the mixed metal oxide, is from 20 at. % to 55 at. %. More typically, the atomic percentage of copper in the mixed metal oxide, relative to the total number of metal atoms in the oxide the atomic percentage of copper is from 30 at. % to 55 at. %. Even more typically, it is from 35 at. % to 55 at. %. The atomic percentage of copper may for instance be from 40 at. % to 52 at %.
Typically, the atomic percentage of gallium relative to the total number of metal atoms in the mixed metal oxide of the catalyst of the invention is equal to or greater than 5 at. %. Preferably, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is equal to or greater than 10 at. %. The atomic percentage of gallium relative to the total number of metal atoms in the oxide may for instance be equal to or greater than 15 at. %. In another embodiment, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is equal to or greater than 18 at. %. In some embodiments, it is equal to or greater than 20 at. %.
Typically, the atomic percentage of gallium relative to the total number of metal atoms in the oxide is from 10 at. % to 40 at. %. The atomic percentage of gallium relative to the total number of metal atoms in the oxide may for instance be from 10 at. % to 35 at. %, or for instance from 15 at % to 35 at %.
The atomic percentage of zinc relative to the total number of metal atoms in the oxide is typically from 10 at. % to 50 at. %. More typically, the atomic percentage of zinc relative to the total number of metal atoms in the oxide is from 15 at. % to 45 at. %.
In some embodiments of the catalyst of the invention, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the mixed metal oxide are from 30 to 55 at. % copper, at least 15 at. % zinc, and at least 10 at. % gallium. Preferably, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide are from 40 to 55 at. % copper, at least 20 at. % zinc, and at least 15 at. % gallium. In some embodiments, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide may be from 40 to 52 at. % copper, at least 25 at. % zinc, and at least 15 at. % gallium.
More preferably, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide are from 30 to 55 at. % copper, from 15 to 45 at. % zinc, and from 10 to 40 at. % gallium. The atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the oxide may for instance be from 40 to 55 at. % copper, from 20 to 45 at. % zinc, and from 15 to 35 at. % gallium.
In some embodiments of the catalyst of the invention, the atomic percentages of copper, zinc and gallium relative to the total number of metal atoms in the mixed metal oxide are from 40 to 52 at. % copper, from 20 to 40 at. % zinc, and from 15 to 30 at. % gallium, preferably wherein said atomic percentages are from 47 to 52 at. % copper, from 27 to 33 at. % zinc, and from 17 to 23 at. % gallium, more preferably wherein said atomic percentages are about 49 at. % copper, about 31 at. % zinc, and about 20 at. % gallium.
In some embodiments of the catalyst of the invention, the metal atoms in the mixed metal oxide consist essentially of said copper, zinc and gallium. In some embodiments, the metal atoms in the mixed metal oxide consist of said copper, zinc and gallium, i.e. said copper, zinc and gallium may in some embodiments be the only metals in the mixed metal oxide. Thus, the metal atoms in the mixed metal oxide may consist of from 20 to 55 at. % copper, from 5 to 40 at. % zinc, and from 5 to 40 at. % gallium. In another embodiment, the metal atoms in the mixed metal oxide consist of x at. % copper, y at. % gallium, and z at. % zinc, wherein x is from 30 to 55, y is from 10 to 40, and z is 100−(x+y). The metal atoms in the mixed metal oxide may for instance consist of x at. % copper, y at, % and z at. % zinc, wherein x is from 40 to 55, y is from 15 to 35, and z is 100−(x+y). In one preferred embodiment, x is from 40 to 52, y is from 15 to 35, and z is 100−(x+y). In another preferred embodiment, x is from 40 to 55, y is from 15 to 25, and z is 100−(x+y).
Typically, in the catalyst of the invention, the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 40 m2/g-catalyst. The mixed metal oxide may for instance have a specific copper metal surface area, as measured by N2O chemisorption, of at least 50 m2/g-catalyst, or for instance at least 60 m2 g-catalyst. More preferably, the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 65 m2/g catalyst. In a particularly preferred embodiment the mixed metal oxide has a specific copper metal surface area, as measured by N2O chemisorption, of at least 70 m2/g catalyst.
Typically, the mixed metal oxide has a specific copper metal surface area per gram copper, as measured by N2O chemisorption, of at least 100 m2/g-Cu, more typically at least 120 m2/g-Cu or for instance at least 140 m2/g-Cu. Preferably, the surface area per gram copper, as measured by N2O chemisorption, is at least 150 m2/g-Cu. In some embodiments, it is at least 160 m2/g-Cu, or for instance at least 180 m2/g-Cu. In particularly preferred embodiments the mixed metal oxide has a specific copper metal surface area per gram copper, as measured by N2O chemisorption, of at least 200 m2/g-Cu. The specific copper metal surface area per gram copper may for instance be at least 210 m2/g-Cu.
Typically, in the catalyst of the invention, the mixed metal oxide comprises particles of said copper, wherein the copper particles have a mean particle size of less than or equal to 40 nm. The particles usually however have a mean particle size of less than or equal to 30 nm, or more typically less than or equal to 20 nm. The term “particle size” as used herein means the diameter of the particle if the particle is spherical or, if the particle is non-spherical, the volume-based particle size. The volume-based particle size is the diameter of the sphere that has the same volume as the non-spherical particle in question.
The mixed metal oxide may for instance comprise particles of said copper, wherein the copper particles have a mean particle size of less than or equal to 10 nm. Preferably, the copper particles have a mean particle size of less than or equal to 5 nm.
Typically, in the catalyst of the invention, the mixed metal oxide has a copper metal dispersion of at least 15%, preferably at least 20%.
Typically, in the catalyst of the invention, the mixed metal oxide comprises Cu2+, Cu+ and Cu0. The Cu+ is typically interstitial Cu+.
Usually, the mixed metal oxide comprises a non-stoichiometric spinel phase comprising copper, zinc and gallium.
Typically, the mixed metal oxide comprises a non-stoichiometric cubic spinet phase comprising copper, zinc and gallium.
Usually, the spinel phase comprises interstitial Cu+. The interstitial Cu+ ions are easily reducible to Cu metal (Cu0) leading to high Cu0 dispersion and surface area.
Thus, the mixed metal oxide typically comprises a spinel phase comprising copper, zinc and gallium, which spinel phase comprises interstitial Cu+ and Cu0.
The spinel phase typically also comprises octahedral Cu2+.
Thus, the mixed metal oxide typically comprises a spinel phase comprising copper, zinc and gallium, which comprises octahedral Cu2+, interstitial Cu+ and Cu0.
The spinel phase typically comprises particles of copper metal (Cu0). These copper particles typically have a mean particle size of less than or equal to 40 nm, or for instance less than or equal to 30 nm, or preferably less than or equal to 20 nm. In one preferred embodiment the spinel phase comprises particles of copper metal (Cu0) which have a mean particle size of less than or equal to 10 nm. More preferably the copper particles have a mean particle size of less than or equal to 5 nm.
The catalyst of the invention may or may not further comprise a solid support material, in addition to said mixed metal oxide. Any suitable support material may be used. In other embodiments, the catalyst of the invention does not further comprise a solid support material. Thus, the mixed metal oxide may be unsupported.
Typically, the catalyst of the invention is obtainable by the process of the invention defined herein for producing a catalyst, which catalyst suitable for use in a process for producing H2 by steam reforming of methanol.
Thus, further provided is a process for producing a catalyst, which catalyst is suitable for use in a process for producing H2 by steam reforming of methanol, and which catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %, which process comprises: (1) a co-precipitation step, comprising contacting: (a) a solution of copper nitrate, zinc nitrate and gallium nitrate, wherein the atomic percentage of copper relative to the total number of metal atoms in said solution is from 20 at. % to 55 at. %, with (b) a metal carbonate, to produce a co-precipitate comprising said copper, zinc and gallium; (2) a separation step, comprising separating the co-precipitate from solution; and (3) a calcination step, comprising calcining the co-precipitate by heating the co-precipitate in air. The process usually further comprises: (4) a reduction step, comprising heating the calcined product in the presence of H2.
As the skilled person will appreciate, the proportions of copper, zinc and gallium in the catalyst can be accurately controlled by varying the proportions of copper nitrate, zinc nitrate and gallium nitrate employed in the solution used in step (1). The atomic percentages of copper, zinc and gallium in the final mixed metal oxide product should be the same as the atomic percentages of copper, zinc and gallium in the solution of the respective nitrates. Thus, any of the catalysts of the invention defined above, having any of the abovementioned atomic percentages of copper, zinc and gallium, can be produced by dissolving the correct amounts of copper nitrate, zinc nitrate and gallium nitrate in the solution used in step (1).
Accordingly, in the process of the invention for producing a catalyst, the atomic percentages of copper, zinc and/or gallium in said solution of copper nitrate, zinc nitrate and gallium nitrate, relative to the total number of metal atoms in said solution, may be the same as any of the atomic percentages of copper, zinc and/or gallium defined herein for the catalyst of the invention.
Usually, the co-precipitation step comprises contacting: (a) said solution of said copper nitrate, zinc nitrate and gallium nitrate, which is an aqueous solution, with (b) a second aqueous solution which comprises said metal carbonate. Said contacting may be performed by adding both solutions to water, usually deionised water. Typically, the contacting is performed whilst stirring. Typically, the contacting is carried out at a pH of from 6 to 7.
Any suitable metal carbonate may be used in the process of the invention for producing a catalyst. Typically, the metal carbonate is an alkali metal carbonate, for instance sodium carbonate.
The co-precipitation step may further comprise an aging step. The aging step typically comprises heating the co-precipitate in the presence of a solvent, typically the solvent from which it was precipitated (usually water). Typically, the co-precipitate is heated to a temperature of up to about 80° C., or for instance up to about 90° C. The co-precipitate may be heated at the temperature for up to about 24 hours.
In the separation step, any suitable means can be used to separate the co-precipitate from solution. For instance, the separation may be performed by filtration or by centrifugation. Typically, the separation step further comprises washing the co-precipitate, after separating the co-precipitate from solution. Typically, the co-precipitate is washed with distilled water. The separation step may additionally further comprise drying the co-precipitate. The co-precipitate is typically dried at a temperature of equal to or greater than 70° C., e.g. at a temperature of from 80 to 100° C. It is typically dried at the temperature for a number of hours, e.g. for 4 hours or more. It is typically dried at the temperature for 8 to 16 hours. The co-precipitate is usually dried in air.
The calcination step typically comprises heating the co-precipitate in air to a temperature of at least 250° C. More typically, the co-precipitate is heated in air to a temperature of at least 350° C., or for instance to a temperature of at least 380° C. Typically, in the calcination step, the co-precipitate is heated in air at the temperature for at least 1 hour, more typically for at least 2 hours. A typical heating programme would be ramping at 3° C./min up to 380° C. for 180 mins. The co-precipitate is typically heated to the temperature in static air.
Usually, the reduction step comprises heating the calcined product in the presence of H2 (typically in the presence of a mixture of H2 and an inert gas, such as N2, and more typically under a flowing stream of H2 and the inert gas). The calcined product may be heated in the presence of said H2 to a temperature of at least 120° C. More typically, the reduction step comprises heating the calcined product in the presence of said H2 to a temperature of at least 150° C. The calcined product is typically heated in the presence of said H2 for up to about 2 hours.
The catalyst produced by the process of the invention may be as further defined herein for the catalyst of the invention.
The process of the invention for producing a catalyst typically further comprises recovering the catalyst.
The process of the invention for producing a catalyst may further comprise using the catalyst thus produced for producing H2 by steam reforming of methanol. Thus, the process may further comprise using the catalyst thus produced in a process of the invention as defined herein for producing H2 by steam reforming of methanol.
The invention further provides a catalyst which is obtainable by a process of the invention as defined herein for producing a catalyst.
The invention further provides the use of a catalyst of the invention as defined herein in a process for producing H2 by steam reforming of methanol.
The invention further provides the use of a catalyst, which catalyst is obtainable by the process of the invention as defined herein for producing a catalyst, in a process for producing H2 by steam reforming of methanol.
The catalysts of the invention can be used to produce hydrogen, in accordance with the process of the invention for producing H2 by steam reforming of methanol, and the hydrogen thus produced may be used to power a fuel cell. Thus, the invention further provides a fuel cell system which comprises (a) a fuel cell and (b) a methanol reformer, wherein the methanol reformer comprises a catalyst of the invention as defined herein. The fuel cell is typically a proton exchange membrane (PEM) fuel cell.
Particularly, for portable applications such as cell phones, mp3-players, laptop computers and similar niche products, the use of PEM fuel cells is deemed to be more energy efficient than battery technology. Low temperature PEM fuel cells are potentially the preferred choices for these consumer products. Thus, in another aspect, the invention provides a portable electronic device comprising a fuel cell system of the invention as defined above. The portable electronic device may for instance be a laptop computer, a mobile interne device, a mobile phone, an MP3 player, a remote control device, a netbook, a video recording device, a camera, a portable military device, a satellite navigation device, or a handheld games console.
It is a further finding of the invention that the catalyst of the invention is surprisingly active for the production of methanol by the hydrogenation of carbon dioxide, in accordance with the following reaction:
CO2+3H2→CH3OH+H2O
As demonstrated in Example 3 hereinbelow, methanol was produced at a higher yield, and with a higher % conversion of CO2, and with higher % selectivity for methanol, than when a conventional industrial catalyst (Johnson Matthey HiFUELT R120 catalyst) was used under the same conditions.
Accordingly, the invention further provides the use of a catalyst of the invention as defined herein in a process for producing methanol by the hydrogenation of carbon dioxide.
The invention further provides the use of a catalyst, which catalyst is obtainable by the process of the invention as defined herein for producing a catalyst, in a process for producing methanol by the hydrogenation of carbon dioxide.
Further provided is a process for producing methanol by the hydrogenation of carbon dioxide, which process comprises contacting a gas phase comprising (a) CO2 and (b) H2, with a solid catalyst, which solid catalyst comprises a mixed metal oxide, which mixed metal oxide comprises copper, zinc and gallium, wherein the atomic percentage of copper relative to the total number of metal atoms in the oxide is from 20 at. % to 55 at. %.
The catalyst used in this process may be a catalyst of the invention as further defined herein.
Typically, in the process of the invention for producing methanol by the hydrogenation of carbon dioxide, said contacting of the gas phase with the solid catalyst is performed at elevated temperature. Said contacting step is typically performed at a temperature equal to or greater than 400K, more typically at a temperature equal to or greater than 450K. In a preferred embodiment, the process comprises contacting the gas phase with the solid catalyst at a temperature equal to or greater than 500K.
Usually, in the process of the invention for producing methanol by the hydrogenation of carbon dioxide, said contacting of the gas phase with the solid catalyst is performed at elevated pressure (i.e. greater than atmospheric pressure). Said contacting is typically performed at a pressure which is equal to or greater than 2 MPa, more typically at a pressure equal to or greater than 3 MPa. In a preferred embodiment, the process comprises contacting the gas phase with the solid catalyst at a pressure which is equal to or greater than 5 MPa.
The molar ratio of H2 to CO2 in said gas phase is usually 3:1, although other molar ratios of these reactant gases may be used as appropriate.
The process of the invention for producing methanol by the hydrogenation of carbon dioxide may further comprise recovering said methanol.
The catalyst used in the process of the invention for producing methanol may be a catalyst which is obtainable by a process of the invention as defined herein for producing a catalyst.
Also, the process of the invention as defined herein for producing a catalyst may further comprise: using the catalyst thus produced for producing methanol by the hydrogenation of carbon dioxide. In particular, the process of the invention as defined herein for producing a catalyst may further comprise: using the catalyst thus produced in a process of the invention as defined herein for producing methanol by the hydrogenation of carbon dioxide.
The present invention is further illustrated in the Examples which follow:
In this study, NSGDSR has been carried out at atmospheric pressure, temperature range of 150-200° C., steam to methanol molar ratios ranging from 1-20. Effects of reaction temperature, contact-time, steam to methanol molar ratio and catalyst composition on methanol conversion, CO selectivity, and hydrogen productivity are thus evaluated.
Typically, Cu based catalysts such as CuZnGaOx, were co-precipitated from a 100 mL aqueous solution containing 3.03 g of Cu(NO3)2.xH2O (Aldrich), 2.40 g of Zn(NO3)2.6H2O (Aldrich) and 2.15 g of Ga(NO3)3.xH2O (Aldrich) by using a Na2CO3 aqueous solution (prepared by dissolving 3.50 g of Na2CO3 in 100 mL DI water), both solutions were dispensed at 0.05 mL/sec to a high-speed stirring (1500 r/min) 300 mL DI water, with the pH controlled between 6 and 7. Then, the resulting precipitate was aged in the solution at 80-90° C. in a closed system for 24 hours. After aging, the precipitate was recovered by centrifugation, washed with 50 mL distilled water for 6-12 times under suction and dried in air (80-100° C. for 8-16 hours) and subsequently calcined in static air (at 3° C./min up to 380° C. for 180 mins) to produce the catalyst. The calcined catalyst was pre-reduced during temperature programmed reduction from room temperature to 150° C. at 20 mL/min flowing stream of 10% H2/N2 for 2 hours prior catalyst testing. Table 1 summarizes the quantities of chemical precursors for CuZnGaOx and others related materials using the same synthesis method.
Determination of catalytic activity was carried out using a home-built plug-flow reactor system. Typically, a powder mixture of 0.40 g catalyst and 0.40 g SiC sandwiched by silica wool plugs was placed in a 4 mm i.d. silica reactor housed in a temperature programmed furnace. A liquid feed of CH3OH:H2O=1:2 set at the flow rate of 0.1 mL/min generated from an HPLC pump was mixed with a N2 flow at 10 mL/min. The mixture was allowed to pass through pre-heated ⅛″ id. piping at 150° C. where all liquids generated to gas mixture before it reached to the reactor. The exited gas after a dry-ice cold trap (to remove water and methanol) was analyzed with pre-calibrated gas chromatography (GC). Thus, the H2, CO, CO2, N2 and CH4 gases were measured by GC equipped with thermal conductivity detector. A separate GC-FID (flame ionization detector) equipped with methanator with detection limit of CO below 10 ppm was also used.
TPR experiments were carried out to determine Cu surface areas, to estimate the Cu dispersion and to calculate the corresponding Cu particle size of the Cu particle in the catalyst by N2O passivation method (Hinrichsen, O.; Genger, T.; Muhler, M. Chem. Eng. Technol. 23, 956-959, 2000). This method is based on the measurement of the hydrogen consumption under temperature programmed reduction of sample (pre-calcined at 380° C.) after reduction followed by surface oxidation by N2O treatment at room temperature. Here, a small quantity of catalyst sample, 26 mg, was used for the measurement to avoid saturating the instrumental detection limit. In order to get an accurate and reproducible Cu surface area, a low temperature (ramped at 10° C./min to 150° C. and kept there for 5 min before cooling to room temperature) pre-treatment of sample under He (10 mL/min) was used. This was followed by the 1st-TPR analysis using a 20 mL/min flow of 5% H2/Ar with a temperature programmed ramping rate at 10° C./min to 330° C. and dwelled there for 30 minutes before cooled to room temperature. Thus, the hydrogen consumption corresponding to the reduction of the bulk Cu2+ to Cu0 was measured. Then, the N2O treatment was performed at room temperature where the sample was flushed with a 20 mL/min of 5% N2O/Argon for 40 minutes before it was swept with He (10 mL/min) for 10 minutes to remove any un-reacted N2O. Then, a 2nd-TPR analysis was conducted using a 20 mL/min flow of 5% H2/Ar with temperature programmed ramping rate at 10° C./min to 330° C. Thus, the hydrogen consumption corresponding to the reduction of Cu+ to Cu0 was measured. Thus, after the 1st-TPR, the total amount of reducible copper oxide (CuO+H2→Cu+H2O) was determined and after the 2nd-TPR, the number of surface Cu atoms (Cu2O+H2→2Cu+H2O) was determined. The Cu metal dispersion was calculated as follows: [(Bulk Cu from 1st-TPR)÷(Surface Cu from 2nd-TPR)]×100%. The specific Cu metal surface area was calculated to be (Surface Cu atom per gram÷Surface packing density of Cu metal). The specific Cu metal surface area per gram Cu was calculated as: (Specific Cu metal surface area)÷(Cu loading). The Cu particle size (assume spherical) was calculated to be: {6÷[(Specific Cu metal surface area per gram Cu)×(Density of Cu metal)]}. It is noted that all the calculations are valid only by taking an assumption that each surface Cu atom decomposes N2O to N2 with the formation of Cu2O.
It is important to appreciate the CO content according to equilibrium thermodynamics of the steam reforming of methanol under our reaction conditions. Theoretical calculations were thus performed using commercial software to derive the equilibrium thermodynamics values (HSC Chemistry® 5.11) where only the intrinsic properties of gas species are considered. Here, 1 mole of CH3OH(g) and 2 mole of H2O(g) were considered as substrates and product species of CO(g), CO2(g) & H2(g) were taken into account (experimentally identified).
The contact time study of the methanol/water over the catalyst by alternating the liquid feed rate was carried out. As shown in
It is thus clear that the key CO formation could be totally suppressed or severely reduced either by decreasing reaction temperature or contact time where the slow RWGS reaction is discouraged. Since there appeared to give no CO formation at or below 150° C., as shown in
It is useful to appreciate the hydrogen productivity from this low temperature NSGDSR over the CuZnGaOx catalyst at 150° C. where no detectable CO is evident. Thus, the hydrogen productivities were evaluated at different methanol-water liquid feeding rates while keeping the other reaction parameters constant.
It is interesting to reveal from the catalyst testing that the Ga3+ incorporation to the CuZnOx can dramatically increase activity and selectivity for the NSGDSR reaction so that lower temperature for the reaction can be exploited. Thus, TPR analyses of the calcined samples of CuGaOx, CuZnGaOx and CuZnOx were carried out for comparison (
In situ catalytic production of hydrogen by steam reforming of methanol is an attractive option for use in decentralised production of clean electrical energy from PEM fuel cells. Present technology for methanol reformations including steam reforming, partial oxidation and their combination via syn-gas route suffer from problems that would require cumbersome CO cleanups otherwise leading to severe degradation in fuel cells performance. On the other hand, there is no sufficient room for such operation for portal fuel cell consumer products. Here we report a new non-syngas direct steam reforming (NSGDSR) route at <200° C. that can integrate the endothermic methanol steam reforming with the exothermic heat generated from the PEM fuel cells which may offer good heat transfer characteristics.
In addition, blending oxygen/air to NSGDSR to encourage combustion may also balance the total thermodynamic requirements of this system. The important finding from this work is that under this reaction condition, no CO formation (<10 ppm) is observed while a high conversion of methanol to CO2/H2 is evident. As a result, this new route can give high quality of hydrogen for the mobile fuel cells. In addition, we have identified active type of catalysts basing on CuGaZnOx which can deliver high activity and selectivity for hydrogen production from the NSGDSR route. Evidence suggests the Ga3+ incorporation in Cu/ZnO system renders the formation of a homogeneous LDHs solid structure where ultrafine copper particles can be generated from this solid precursor. It appears that it is an essential step to produce high active and selective catalyst for this reaction due to desirable metal-support interactions.
Having identified 43% Cu—ZnGaOx, as a new high performance catalyst, the focus of this project was to investigate differences between the catalysts containing differing levels of Cu, Zn, Ga and combinations thereof. It was decided that the comparisons should be made between the activities observed at 195° C. rather than 150° C., because the differences in MeOH conversion and CO production were more pronounced. To this end, a variety of characterisation techniques were employed to elucidate the structural & mechanistic properties of the catalyst, and comparisons were made by varying the support composition and changing the Cu-loading. Table 3 shows the difference in activity for a range of Cu-based catalysts:
The catalysts were prepared via a co-precipitation method. The precursor metal nitrate salts were dissolved and combined in 100 ml de-ionised water, with aqueous Na2CO3 (3.50 g in 100 ml DI water) used to produce the precipitate. Both solutions were added at 0.05 ml s−1 to a round-bottomed flask containing 300 ml DI water and a magnetic stirring bar maintained at 1,500 rpm, and heated to 80-90° C. with pH controlled between 6-7. The resulting precipitate was aged in solution at this temperature for 24 hrs. After ageing, the precipitate was centrifuged 6-12 times at 6,000 rpm for 5 min and washed with 50 ml DI water between each period to remove Na+ ions. The resulting solid was powdered, then dried in air at 80-100° C. overnight and subsequently calcined in static air at 3° C. min−1 up to 380° C. for 3 hrs to produce the catalyst.
xH2O/g
Table 4 summarises the quantities of precursor salt required to synthesise catalysts with the desired molar ratios. The realatomic content of Cu, Zn and Ga in the catalysts was calculated according to the actual amount of precursor salts added, which sometimes deviated slightly from the desired quantity. Table 5 summarises the catalysts prepared using the co-precipitation method that underwent further characterisation:
(N.B. In some cases the resulting atomic content deviated significantly from the originally desired quantity. For example, in the case of 43% Cu—ZnGaOx (Zn:Ga=1:3), the actual atomic content means that the catalyst should be renamed 52% Cu—ZnGaOx (Zn:Ga=1:2). However, for the purposes of this project, and to avoid confusion, the names shall remain as originally assigned.)
Determination of catalytic activity was carried out in a home-built plug-flow reactor system (
Prior to testing, the calcined catalyst was pre-reduced using TPR from room temperature to 195° C. under 10% H2/N2 gas stream at 20 ml min−1 for 2 hrs. Afterwards, a powder mixture of 0.40 g catalyst and 0.40 g silicon carbide (I), sandwiched between silica wool plugs, was placed in a 4 mm diameter silica reactor (G) containing a thermocouple (G), and housed in a temperature-programmable furnace (J, F). A liquid feed of CH3OH:H2O=1:2 (B) at a flow rate of 0.1 ml min−1 generated from an HPLC pump (D) was mixed with N2 flow (A) set at 10 ml min−1 by a mass-flow controller (C, E). The mixture was allowed to pass through pre-heated tubing maintained at 195° C. where all liquids were converted to gas before reaching the reactor. After passing through a dry-ice cold trap to remove water and methanol (K, L), the product gas stream was analysed on a connected PC (O) using pre-calibrated gas chromatography with a thermal conductivity detector (M). Thus, the H2, CO, CO2, N2 and CH4 gas levels could be quantified directly. A separate FID (N) with a detection limit of CO≦10 ppm was also used.
Temperature-programmed reduction is a useful technique for studying the reducibility of solid materials. In heterogeneous catalysis, the solid catalyst powder usually exists as a precursor metal oxide under ambient conditions, which is inactive to the desired catalytic process, and so must first be ‘pre-reduced’ to produce the active material. For the low temperature SRM reaction, it is desirable to achieve the lowest possible temperature for pre-reduction of the catalyst, so that as much of the inactive metal oxide as possible is converted into the active phase, and then maintained during catalysis without deactivation due to re-oxidation or thermal sintering. Therefore by studying the reducing properties of the different Cu-based catalysts, we may begin to understand the differences between them in terms of activity.
TPR measurements were carried out on a ThermoQuest TPDRO 1100 instrument. 0.026 g of the solid powder sample was sandwiched between two tufts of glass wool inside the TPR tube, accompanied by a thermocouple, and inserted into the instrument. Helium pre-treatment (10° C. min−1 at 10 ml min−1 from 20-150° C., then held for 5 min before allowing to cool) was carried out first to remove any ambient gas molecules adsorbed on the catalyst surface. Then reduction of the copper oxide within the material (Eq. A) was achieved by running 5% H2 in Argon through the TPR tube at 5 ml min−1 at a temperature ramp of 2° C. min−1 from 40-800° C., then held at 800° C. for 30 min before allowing to cool to room temperature.
Cu(II)O+H2→Cu0+H2O (Eq. A)
The change in conductivity of the gas stream due to the consumption of hydrogen was measured as a function of both time and temperature, and the results plotted in the form of a TPR profile.
Chemisorption techniques are used in heterogeneous catalysis to investigate the properties of the active metal. Important properties such as Cu dispersion, surface area and particle size may be revealed via chemisorption methods. To determine these properties, a chemisorption technique using N2O was carried out in a manner similar to literature methods.11 0.026 g of sample was firstly pre-treated with He, as before, then pre-reduced under hydrogen using the normal TPR method, up to 330° C. Once the sample had cooled down to room temperature, 5% N2O/Ar at 20 ml min−1 was allowed to flow through the TPR tube for 40 min in order to re-oxidise the exposed Cu only, via dissociative chemisorption (Eq. B):
N2O(g)+2Cu(s)→Cu2O(s)+N2(g) (Eq. B)
After N2O treatment, He pre-treatment (10 ml min−1 for 10 min at room temp.) was again carried out to remove any adsorbed N2O, followed by 2nd TPR treatment up to 330° C., with the gas stream conductivity measured as a function of time and temperature. In order to determine the Cu surface area, it was necessary to calibrate a Cu(II)O standard, with known Cu content, against which the samples could be compared. TPR was performed on 0.005 g, 0.010 g and 0.015 g of Cu(II)O from Aldrich, and the number of moles of hydrogen consumed was calculated via knowledge of the Cu content. From this, a simple 1st order calibration was found by plotting the number of moles H2 consumed versus the TPR integrated peak area. By comparing the TPR integrated peak area of the catalyst sample with the calibration file, it was possible to determine the Cu dispersion, specific surface area and particle size.
Powder X-ray diffraction data was obtained for catalyst materials before and after calcination in order to investigate the crystal structure of the bulk material. In general, the precursor material will be a quasi-amorphous structure composed of various carbonates and hydroxycarbonates, which release CO2 and H2O upon calcination to produce the more crystalline active catalyst. It is likely that the final catalyst will be composed of more than one crystal phase, and it is possible either that one of these phases will be the most active, or that a combination of different phases is in fact required for the most effective catalysis.
To obtain the diffraction peaks, a small quantity of sample was placed onto an aluminium plate, and the X-ray diffraction data was collected on a Philips PW-1729 diffractometer using a monochromated Cu Kα beam. The aluminium plate gives characteristic diffraction peaks which can be used as a reference marker against which the peaks of the material under investigation can be compared.
The impedance of a material describes its resistance to alternating current. It is represented by the complex quantity, Z, and by plotting the real component (resistance, Z1) versus the imaginary component (reactance, Z2) of a material's impedance across a range of frequencies we obtain a semi-circular spectrum known as a Nyquist plot, By obtaining such spectra over a range of temperatures, it is possible to gain information about the charge carrying properties of the material. In the case of the present study, the predominant mechanisms for electrical conductivity within the calcined catalyst materials are either via mobile Cu ions, in which Cu is able to move between either substitution-able lattice sites or interstices, or via oxygen mobility, where O2− anions are able to ‘hop’ into adjacent vacant lattice sites. These processes have associated thermal activation energies, which describe the ease with which a mobile ion can move between sites. If we model the impedance of the material by a simple Arrhenius equation, then an Arrhenius plot of ln|Z2∥ vs. 1/T at fixed frequency should yield a straight line of gradient Ea/R, from which the activation energy associated with the ionic mobility of the material may be extracted.
To obtain the impedance measurements over a range of temperatures, the calcined catalyst was pressed at 5 tonne pressure into a pellet of approx. 1 mm thickness and 30 mm diameter, then held between two platinum electrodes inside a quartz tube containing a thermocouple, and wrapped in an electrical thermal jacket attached to a programmable heating furnace. The electrodes were connected to an Ivium CompactStat electrochemical interface, which in turn was connected to a PC with the corresponding IviumSoft software. For each material studied, the sample was heated up to 220° C. and maintained at that temperature for 1 hr, then allowed to cool by 20° increments and held at each temperature for 20 min before taking a measurement. The frequency range used was from 100,000 Hz to 2 Hz, the current range was 100 μA and the frequency scan amplitude was 0.5V.
In the presence of an external magnetic field (B0), the electrons within a material will align their magnetic moments either parallel (ms=−½) or anti-parallel (ms=+½) to the applied field. These alignments have different energies, and the phenomenon is known as the Zeeman effect.
The separation between the energy states can be written in terms of the g-factor (ge) and the Bohr magneton (μB):
Unpaired electrons within the material can move between the energy levels by absorbing a photon that satisfies the resonance condition (Eq. E):
hν=geμBB0 (Eq. E)
The statistical distribution of unpaired electrons within a paramagnetic sample is described by the Boltzmann distribution (Eq. F):
In practice there will be a slightly larger population in the lower energy state than the upper one. This means that in an external magnetic field there will be a net absorption of energy as transitions from the lower to upper state are more probable. This net absorption is measurable and forms the basis of EPR spectroscopy. The g-factor depends not only on the external magnetic field, but also local fields within the material, therefore by measuring the g-factor it is possible to investigate species in different electronic environments. The electronic environment of paramagnetic transition metal ions such as Cu2+ in the Cu/Zn/Ga oxide system may be investigated using EPR. By looking at the electronic environments of the Cu ions, we may be able to distinguish between different lattice sites, and possibly identify the most active site for the steam reforming reaction. Samples were analysed in a Bruker EM/Y X-band CW spectrometer using 100 mg solid sample in a quartz tube.
Table 6 summarises the activity and conversion properties of the synthesised catalysts, tested at 195° C., 0.1 ml min−1 feed rate with a 1:2 methanol/water molar ratio. The accompanying activity scatter plot is shown in
The CO concentration was measured directly via calibrated GC and FID. Methanol conversion was calculated according to the following equation (Eq. G):
The results show that, once again, we observe a wide range of catalytic behaviour. As before, 43CuZnGaOx was the most active catalyst, achieving 33.4% MeOH conversion with only 108 ppm CO concentration. 60CuZnGaOx achieved similar conversion (31.9%), but CO production was significantly higher at 176 ppm. 43CuZn1Ga3Ox and 43CuGaOx achieved lower CO production, but MeOH conversion was significantly reduced. 43CuZnOx performed poorly, achieving only 3.6% conversion with 423 ppm CO production.
The TPR profiles of a range of Zn/Ga oxides with and without Cu were obtained over the temperature range 40-800° C. Both the Zn/Ga ratio and the Cu-loading were varied in order to investigate the change in reducibility. The following profiles show the change in gas stream conductivity as a function of temperature.
Here we observe much more significant reduction peaks, appearing at lower temperature, corresponding to the reduction of copper species within the materials. For 43CuZnO, the reduction begins at −200° C. and reaches its maximum at 300° C. For both 43CuGaOx and 43CuZnGaOx, reduction begins earlier at ˜150° C., reaching a single maximum at 220° C. in the case of 43CuZnGaOx, and 250° C. for 43CuGaOx. For both 43CuGaOx and 43CuZnGaOx we observe an initial gradual reduction, followed by a ‘kink’ at ˜200° C. where the reduction peak suddenly becomes steeper. In all cases the total peak area was the same after extensive reduction, indicating that all the Cu in the materials had been reduced. The TPR profile for 0.005 g Cu(II)O is shown for reference; it can be seen that Cu reduction in the Zn/Ga systems takes place at significantly lower temperature than for CuO.
From
The N2O chemisorption data was obtained for a range of Cu-containing Zn/Ga oxide materials, with the TPR profiles before and after N2O treatment displayed as a function of time rather than temperature, since reference to the calibrated Cu(II)O standard for surface area calculations was more reliable using this method.
The Cu dispersion is defined as the fraction of Cu atoms exposed to the surface. It was calculated as follows:
The specific Cu metal surface area of the catalyst was calculated as:
The specific Cu metal surface area of Cu in the catalyst was calculated as:
The Cu particle size was calculated as being the average diameter of the Cu particles on the surface, assuming spherical geometry:
(NB. The above calculations are valid only by making the assumption that N2O is decomposed to N2, with the simultaneous oxidation of surface Cu to Cu2O.)
There are several trends in Table 7 that are worth highlighting. Firstly, we can see that both DCu and SACu decrease as the Cu-loading is increased. This is reflected in the increasing Cu particle size as Cu-loading increases. However, SAcat is at its maximum at 43% Cu-loading. Secondly, we observe that as the Zn/Ga ratio is varied, DCu gradually increases as Ga content is increased until a maximum for 43CuZnGaOx, which contains 50% Cu, 30% Zn and 20% Ga. This is accompanied by increasing SACu and SAcat and decreasing particle size. However, as Ga content is increased beyond 20% the dispersion, SACu and SAcat decrease again and the particle size increases.
The AC impedance spectroscopy results were obtained, in air, for a range of catalyst and metal oxide materials pre-calcined in air at 330° C. The data obtained for each material are summarised in the table below.
From Table 8 it can be seen that the conductivity of the Cu-containing materials is greatly enhanced by several orders of magnitude relative to their non-Cu analogues, with a concomitant lowering of the thermal activation energy. This can be attributed to the presence of mobile Cu ions, which act as the dominant charge carrier. For the non-Cu analogues the dominant charge carrying mechanism is via O-vacancies, but the activation energy for the migration of vacancy lattice defects is much higher than for mobile Cu ions. The addition of Ga to these systems causes a decrease in the conductivity, the reasons for which will be explained in the discussion section.
The EPR spectra obtained with 43CuZnOx, 43CuZnGaOx and 43CuGaOx are shown in
The catalytic properties of a material are linked to its structure, since the structure plays an important role in dictating the surface area, the nature and availability of active sites and the strength of metal-support interactions. Table 9 summarises all of the main results from Example 2.
From the table above it can be seen that the surface area of Cu in the catalyst (SAcat) is an important factor in determining MeOH conversion. Therefore it is necessary to understand the factors that determine SAcat, in order to be able to rationally design more effective SRM catalysts in the future.
The spinel structure consists of a cubic close-packed oxide with general formula AB2O4 containing one Oh site and two Td sites per oxide (
CuGa2O4⇄Cu1−2x(Cui+)2xGa2O4−x+½xO2 (Eq. L)
Excess Cu2+ from CuO may then enter the spinel phase to fill the vacant lattice site, forming a non-stoichiometric phase. Cu2+ (d9) in an Oh environment exhibits the Jahn-Teller effect, causing elongation and hence weakening of the Cu—O bonds along the axial plane. This axial distortion is what drives the formation of the tetragonal spinel phase observed in the case of CuGa2O4. However, partially substituting Cu2+ for Zn2+ (d10) helps to remove the instability caused by the Jahn-Teller effect, therefore stabilising the material in the cubic NSS phase observed for 43CuZnGaOx. It was shown from the TPR that for Cu/Zn/Ga oxide materials, Cu can exist in three separate environments with different reducibilities. The low temperature reduction site, αCu, appears to correspond with Cu+ existing on or near the surface of the material. This environment is identified as interstitial Cu+ ions within the NSS phase, which will be easily reducible to Cu metal. In the case of 43CuGaOx, which consists of poorly crystalline tetragonal CuGa2O4, the spinel structure cannot be effectively maintained upon the reduction of interstitial Cu+, leading to significant Cu aggregation and poor activity. However, in the case of 43CuZnGaOx, the presence of Zn allows the structure to maintain its cubic NSS structure upon reduction, leading to high Cu dispersion and surface area. It is this αCu site that is critical to the activity of the catalyst, since the steam reforming is taking place at 195° C., therefore βCu and γCu will not be reduced under the reaction conditions. It was observed that 43CuZnGaOx displayed a broad EPR signal not seen in either 43CuGaOx or 43CuZnOx, and it was also seen from TPR that neither 43CuGaOx nor 43CuZnOx displayed a reduction peak corresponding with the βCu environment. We can therefore conclude that βCu corresponds with the Cu2+ ions existing within the cubic NSS phase. The γCu environment, which displayed the strongest TPR peak with 43CuGaOx, corresponds with Cu2+ ions in the tetragonal NSS phase. This tetragonal spinel phase was shown from XRD and EPR to also be present in 43CuZnGaOx to some extent. However, no γCu TPR peak was observed for 43CuZnGaOx. It may be the case that this peak was obscured by the larger βCu peak, since it was observed that both 43CuZn3Ga2Ox and 43CuZn1Ga3Ox contained a γCu peak in the TPR. The αCu environment, which was observed in the TPR for all the Ga-containing catalysts, does not give an EPR signal because Cu+ (d10) is not paramagnetic.
Oxygen vacancies are believed to play an important role in the steam reforming cycle.
CO formation is supposedly produced exclusively as a secondary product, via the reverse WGS reaction.
Therefore, in order to maximise CO2 formation and minimise CO formation, the Cu dispersion and surface area should be as high as possible, to maximise the possibility that an adsorbed species will be at the metal-support interface. We have observed herein that MeOH conversion is strongly related to SAcat, and that maximum conversion occurred for 43CuZnGaOx, which contains highly dispersed, reducible αCu, which upon reduction will exist as very small Cu metal particles on the surface, whilst the CuZnGaOx cubic NSS phase is retained beneath. The reduction of the interstitial Cu+ will lead to the production of O-vacancies at the metal support interface, thus providing the active sites required for SRM (Eq. M):
Another way to prevent CO formation would be to minimise the number of O-vacancies that are not in the vicinity of Cu. We observed earlier that the addition of Ga suppresses CO formation, which was explained by the reduction in O-mobility caused by the increasing formation of a poorly crystalline tetragonal spinel phase. Therefore, the Zn/Ga support is playing a dual role; not only does it encourage the formation of the cubic NSS phase, leading to a highly dispersed, highly reducible form of Cu, it also provides enough O-vacancies at the metal-support interface that the steam reforming process can proceed effectively, but few available O-vacancies in the bulk support, so that CO production via the reverse WGS reaction is effectively suppressed.
In this Example it has been shown that the enhanced performance of 43CuZnGaOx is due to the formation of a stable cubic NSS phase containing highly reducible, well-dispersed interstitial Cu+ ions, and that the presence of Ga effectively reduces the availability of O-vacancies in the bulk support leading to suppression of CO formation. αCu was identified as interstitial Cu ions in the NSS phases, βCu as Oh Cu2+ in the cubic NSS phase and γCu as Oh Cu2+ in the poorly crystalline tetragonal NSS phase. 43CuZnOx exists as a Cu/ZnO solid solution containing highly mobile Td Cu2+ ions. The presence of Zn in 43CuZnGaOx stabilised the cubic NSS phase, leading to enhanced Cu dispersion and surface area upon reduction of αCu. 43CuGaOx, although containing reducible αCu, existed as a poorly crystalline tetragonal NSS phase, and was therefore unable to maintain high Cu dispersion upon H2 reduction, leading to poor MeOH conversion. In 43CuZnOx, the combination of low metal reducibility and high O-mobility led to the low MeOH conversion and high CO production observed. In 43CuGaOx the low O-mobility helped suppress CO formation, but the aggregation of Cu diminished the MeOH conversion. In 43CuZnGaOx the reduction of αCu led to the formation of O-vacancies at the metal-support interface, but the availability of O-vacancies in the bulk support was reduced due to the high Cu dispersion, leading to the suppression of CO formation.
It has been shown that 43CuZnGaOx can achieve H2 productivity of 393.6 ml g−1 cat. hr−1 with <10 ppm CO formation at 150° C. By demonstrating here that CO production can be suppressed to <10 ppm whilst maintaining significant H2 production, the invention has provided commercially viable low temperature SRM catalysts for use in PEM fuel cells. Future work will involve the optimisation of the catalyst SRM.
In conclusion, a series of Cu/Zn/Ga mixed metal oxide solid catalysts were prepared via a co-precipitation method and tested for their activity in the low temperature steam reforming of methanol (195° C., 0.1 mil min′ reactant flow rate, 1:2 methanol/water ratio). 43CuZnGaOx containing ˜50% Cu, 30% Zn and 20% Ga was found to be the most active catalyst, displaying high activity combined with very little CO production. This can be explained in terms of the catalyst structure, a heterogeneous mixture of various phases including a non-stoichiometric cubic spinel phase, stabilised by the presence of Zn2+, containing excess interstitial Cu+ ions that were highly dispersed and highly reducible, leading to an enhanced surface area of Cu in the catalyst. The suppression of CO formation was explained in terms of the mechanism, whereby the high Cu dispersion led to an abundance of O-vacancy active sites at the metal-support interface for the steam reforming process, but few O-vacancies available in the bulk support for the reverse WGS reaction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1111781.9 | Jul 2011 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2012/051601 | 7/6/2012 | WO | 00 | 4/16/2014 |
