(560bu) Promoter and Support Effects on Transition Metal Carbides for CO2 Hydrogenation

Carbon dioxide hydrogenation is a reaction of interest as an alternative route to produce chemicals and fuels. To make this reaction pathway competitive with existing production methods, heterogeneous catalysts have been employed to manipulate the reaction mechanism and promote high product selectivity.

Transition metal carbides (TMCs) have been shown to outperform traditional precious bimetallic catalysts and are highly active and selective for the reverse water-gas shift (RWGS) reaction, the first step of CO₂ hydrogenation to light olefins.^[1-2] Additionally, by modifying TMCs with alkali metal promoters, the CO yield increases significantly and approaches the thermodynamic limit at 300 °C. These experimental results are supported by density functional theory (DFT) calculations, which show enhanced CO₂ adsorption and reduced CO₂ dissociation barriers on the alkali metal-promoted catalyst, compared to the pristine carbidic surfaces.^[3]

Although supported TMCs (i.e. WC and Mo₂C) have been shown to be effective RWGS catalysts, little is known about the effects of electronic promotion and the support-metal carbide physicochemical interactions on promoting high CO product selectivity. To investigate these structure-property relationships, Mo₂C and WC are first synthesized on a variety of metal oxide supports to vary both chemical (Lewis and Brønsted acidity) and structural properties (amorphous SiO₂ vs. silicalite-1). Acidic and basic properties of the catalysts are probed via NH­₃, pyridine and CO₂ pulse chemisorption and temperature programmed desorption, while bulk and surface-sensitive X-ray characterization techniques are used to describe the metal carbide crystalline structure, phase and oxidation state with varying support composition. By combining the above techniques with reactor studies, in situ diffuse reflectance FTIR and synchrotron-based XAFS measurements, structure-property relationships are developed relating physicochemical interactions between the carbidic active phase and metal oxide support to determine adsorption behavior and catalytic performance. Further research in this area will lead to a framework to direct rational design of future catalysts for CO₂ hydrogenation to chemicals and fuels.

1. M. D. Porosoff, J. W. Baldwin, X. Peng, G. Mpourmpakis, and H. D. Willauer, ChemSusChem, 10 (11), pp. 2408–2415, (2017).

2. S. Posada-Perez, F. Vines, P. J. Ramirez, A. B. Vidal, J. A. Rodriguez, F. Illas, Physical Chemistry Chemical Physics, 16, pp. 14912-14921, (2014).

3. M. D. Porosoff, J. W. Baldwin, X. Peng, G. Mpourmpakis, H. D. Willauer, ChemSusChem, 10, pp. 2408-2415, (2017).