Chapter Six - Mechanistic studies toward the rational design of oxide catalysts for carbon dioxide hydrogenation
Introduction
Carbon dioxide (CO2) capture and sequestration (CCS) has been proposed many decades ago as a major approach in reducing the CO2 concentration in the atmosphere and ever since been intensively studied. Despite the continuing concern with its cost-effectiveness, it may become a reality if the widespread utilization of renewable energy can significantly drive down the energy cost. Furthermore, to make a transition from a fossil fuel-based energy and chemical industry to a renewable energy-based one, the captured CO2 must be further utilized to produce the carbon-based fuels and chemicals, which are the essential ingredients of our modern society. This is the essence of CO2 capture, utilization, and sequestration (CCUS), where CO2 utilization is just as important as its sequestration.
Either from general chemistry or in the actual industrial production, CO2 is closely related to carbon monoxide (CO), and CO2 hydrogenation has an intimate relation with the conversion of syngas, a mixture of CO and molecular hydrogen (H2). An interesting bit is the discovery by Spencer and co-workers that the well-known methanol synthesis (MS) reaction from syngas conversion over the classical Cu/ZnO/Al2O3 catalyst proceeds from CO2 rather than CO (1), and this catalyst and similar oxide-supported Cu catalysts remain the most studied heterogeneous catalysts for CO2 hydrogenation to methanol (2).
The role of the different components in the above-mentioned oxide-supported Cu catalysts remains a major concern, and in the case of the Cu/ZnO-based catalysts, the nature and the role of the interaction between the Cu and ZnO components continue to receive significant interests in recent works 3, 4, 5, 6, 7, 8, 9. Using high-pressure operando techniques including steady-state isotope transient kinetic analysis and Fourier-transform infrared spectroscopy (SSITKA-FTIR) and time-resolved spectroscopies including X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (XRD), along with electron microscopy and theoretical modeling, Zabilskiy and van Bokhoven et al. 5, 8 show that the high activity of the Cu/ZnO catalyst in the CO2 hydrogenation to methanol reaction can be attributed to the close contact between the metallic copper (Cu0) and zinc oxide (ZnO) phases, which are responsible for the dissociative adsorption of H2 and the formation/hydrogenation of the zinc-formate intermediate, respectively. In particular, the copper-zinc (CuZn) alloy phase was not observed under the typical reaction conditions, and was thus ruled out as one of the active phases, in line with an early study on these catalysts (10). Besides oxide-supported Cu catalysts, oxide-supported Co, Pd, and Pt catalysts are also objects of recent studies for the CO2 hydrogenation to methanol reaction 11, 12, 13.
One of the key difficulties in the industrial applications of the oxide-supported Cu catalysts for the CO2 hydrogenation to methanol reaction is their lower stability compared to that for the syngas conversion to methanol reaction, usually attributed to catalyst sintering due to the formation of a significant amount of water (2), which is apparent when comparing the two reactions, i.e., CO2 + 3H2 → CH3OH + H2O and CO + 2H2 → CH3OH. In syngas conversion, despite the discovery that methanol mainly comes from CO2 rather than CO, from the overall equation water formed during the CO2 hydrogenation to methanol reaction is likely further consumed by the water-gas shift (WGS) reaction (CO + H2O → CO2 + H2), thus preventing the accumulation of a significant amount of water in the reaction stream and avoiding catalyst sintering. One way to improve the stability of the Cu/ZnO-based catalysts is to introduce other oxide supports such as zirconia (ZrO2) and ceria (CeO2), which were found to enhance the catalytic performance for the CO2 hydrogenation to methanol reaction 14, 15, 16, 17, 18, 19. Nevertheless, one recent interesting finding (6) suggests a suitable amount of water in the feed can actually promote methanol formation in the CO2 hydrogenation reaction over the classical Cu/ZnO/Al2O3 catalyst, where the role of water is proposed to be the hydrolysis of surface methoxy (CH3O*) intermediate, leading to methanol formation. However, this conclusion appears to contradict the general notion that the formation of a significant amount of water leads to catalyst sintering and deactivation, and has yet to be further verified and may apply only to certain situations (2).
Despite the crucial role of the oxide components in the above oxide-supported metal catalysts for the CO2 hydrogenation to methanol reaction, they are usually not used alone for this reaction. This is not surprising considering the fact that oxides as the active phase are usually associated with redox and acid-base reactions 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. Nevertheless, Ge and co-workers in their early computational studies on the indium oxide (In2O3) (110) surface proposed oxygen vacancy (VO) as the active site for the CO2 hydrogenation to methanol reaction based on available experimental evidence for its catalyzing the methanol reforming reaction to exclusively form CO2 and their computational studies on the pathways of CO2 adsorption and hydrogenation to form methanol 37, 38. It should also be pointed out that oxygen vacancy as an active site on the ZnO(000-1) surface for the methanol synthesis reaction from syngas has been proposed earlier in the works of Meyer, Frenzel, and co-workers 39, 40, which were followed by their extended works on this topic 41, 42. The importance of these theory-based proposals is that they were soon validated by experiments, and In2O3-based catalysts and other notable oxide-based catalysts were found to be efficient catalysts for the CO2 hydrogenation to methanol reaction. For In2O3-based catalysts, Pérez-Ramírez and co-workers were the first to investigate pure and ZrO2-supported In2O3 catalysts and show the latter to be more efficient for the CO2 hydrogenation to methanol reaction with CO2 conversion of 5.5% and methanol selectivity of nearly 100% at 300 °C (43). For ZnO-based catalysts, Li and co-workers developed an efficient ZnO-ZrO2 solid solution catalyst for the CO2 hydrogenation to methanol reaction with CO2 conversion of 10% and methanol selectivity of 86–91% at 315–320 °C (44).
One of the major applications of the above-mentioned oxide-based catalysts in CO2 conversion is to be used in tandem catalysis when combined with acidic zeolites for the direct hydrogenation of CO2 into products of higher values than methanol including gasoline (linear alkanes of C5–C11), simple olefins (ethylene, propylene, and butylene), and aromatics 45, 46, 47, 48, 49, 50, among others. As shown in Fig. 1A, in this process, CO2 is first hydrogenated into methanol as a reaction intermediate over the oxide component of the bifunctional catalyst, which is further transformed without separation into gasoline, olefins, or aromatics at the acidic sites of the zeolites via the methanol-to-hydrocarbon (MTH) reaction. As shown in Fig. 1B, by combining In2O3 with H-form Zeolite Socony Mobil-5 (HZSM-5), a high CO-free selectivity of 78.6% for the gasoline-range hydrocarbons (C5–C11) were obtained at 340 °C with a very low methane selectivity of 1% (45). Similarly, as shown in Fig. 1C, by combining ZrO2-supported In2O3 with SAPO-34 zeolite, a high CO-free selectivity of 80% for the simple olefins (C2=–C4=) were obtained at high CO2 conversion of 35% at 380–400 °C (46). Furthermore, by combining the ZnO-ZrO2 solid solution catalyst with HZSM-5, a high selectivity of up to 75% for aromatics was obtained at CO2 conversion of 17.5% at 315 °C with a much lower CO selectivity of 23.8% (49).
In the above-mentioned tandem reactions for the direct hydrogenation of CO2 into higher value products, a compromise has to made between the one-pass CO2 conversion, CO byproduct selectivity, and target product selectivity. As the reaction temperature increases, CO2 conversion usually increases, but CO byproduct selectivity often increases significantly. Furthermore, in order to convert the methanol intermediate to hydrocarbons at the acidic sites of the zeolites, the reaction temperature for the tandem reaction (315–380 °C) is considerably higher than the typical value (200–300 °C) for methanol synthesis from syngas over the classical Cu/ZnO/Al2O3 catalyst. Thus, it is highly desirable to design more efficient catalysts to suppress the RWGS reaction especially at relatively high reaction temperature.
To this end, further mechanistic studies are necessary to reveal the detailed catalytic mechanism of the CO2 hydrogenation reaction including both the formation of methanol via the methanol synthesis reaction, that of the byproduct CO via the RWGS reaction, and preferably also that of the byproduct CH4 via the methanation reaction. It is also important to be able to reliably predict the catalytic activity and product selectivity by performing first principles-based mean-field microkinetic simulations or kinetic Monte Carlo simulations. Once the above goals are reached, mechanism-based rational design of more efficient catalysts may be realized, which can be further validated by experimental synthesis, characterization and performance evaluation.
Over the past few years, in collaboration with the experimental team, we have taken significant efforts to systematically investigate the mechanism and microkinetics of the oxide-catalyzed CO2 hydrogenation to methanol reaction, especially for the relatively simple single phase In2O3 catalyst, although we also examined the electronic effects of the metal dopant on the catalytic activity and product selectivity of some of these oxide catalysts. One of our major achievements is the computer-assisted rational design of In2O3 catalyst in the less common hexagonal phase (h-In2O3) preferentially exposing the {104} facet, which enables considerably higher methanol selectivity even at the very high reaction temperature of 360 °C, thus significantly increases the productivity of the CO2 hydrogenation to methanol reaction over the In2O3 catalyst (51).
In the following sections, we first provide a detailed account on the catalytic mechanism of the CO2 hydrogenation reaction over the In2O3 catalyst including the underlying ideas for the computer-aided rational design of the above-mentioned highly efficient h-In2O3 catalyst. This is followed by mechanistic studies on the ZrO2-supported/doped In2O3 catalyst and several solid solution catalysts including the In- and Ga-ZrO2 and the Zn- and Cd-ZrO2 solid solutions, which have been found to have superior performance for the CO2 hydrogenation to methanol reaction. We aim at a better understandings of the catalytic mechanism and the structure-property relationship, which form the bases of computer-aided rational design of more efficient catalysts. We end our discussion by examining the challenges and possible future directions in the mechanistic studies of these oxide-based catalysts toward the rational design of heterogeneous catalysts for CO2 valorization.
Section snippets
Indium oxide catalysts for CO2 hydrogenation
As mentioned above, In2O3 as a catalyst used for the CO2 hydrogenation to methanol reaction was discovered only a few years ago (43), and what is more remarkable of this discovery is the fact that it was first proposed by first principles-based computational studies (37). Thus, it exemplifies the power of insights gained and predictions made by theory and computation, even if the detailed mechanism of the In2O3-catalyzed CO2 hydrogenation to methanol reaction remains under debate to this date
Rational design of a better In2O3 catalyst
As mentioned in earlier sections, the thermodynamically less stable h-In2O3 phase can either be stabilized by high pressure, or it may be stabilized under ambient pressure when prepared using special chemical synthesis methods, either by controlling the size of the nanoparticle, using special organic templates, or from suitable precursors. It has been suggested that the metastable h-In2O3 phase might be stabilized relatively to the c-In2O3 phase due to the presence of structural defects,
Mixed oxide catalysts for CO2 hydrogenation
The discovery of In2O3 as an effective catalyst for the CO2 hydrogenation to methanol reaction brings a whole new class of catalysts for this reaction, which have previously been dominated by oxide-supported metal catalysts, although they remained the most studied catalysts for this reaction and continued to receive both experimental and theoretical interests (94). Moreover, pure oxides such as In2O3 are usually not the most efficient catalysts for this reaction, and often supports or dopants
Challenges and perspectives
In this chapter, we provide a comprehensive and detailed discussion on the various mechanistic aspects of the In2O3 catalyst for the CO2 hydrogenation to methanol reaction aiming at the computer-assisted rational design of more efficient catalysts for this reaction. In addition, we also offer an in-depth theoretical analysis of the catalytic mechanisms of a series of solid solution catalysts, which are traditionally considered as mixed oxide catalysts or ZrO2-supported oxide (ZnO, CdO, In2O3, Ga
Acknowledgments
The authors are especially thankful for the funding from the “Frontier Science” program of Shell Global Solutions International B.V. (No. CW373032). They are also very grateful for the helpful discussions with Dr. Joost Smits, Dr. Alexander van der Made, and Dr. Alexander Petrus van Bavel from Shell Corporation as well as Prof. Peng Gao from Shanghai Advanced Research Institute and Prof. Yong Yang from ShanghaiTech University. The authors would also like to thank the funding support from the
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