(513ai) Promoter and Support Effects on Transition Metal Carbides for CO2 Hydrogenation

Carbon dioxide hydrogenation is a reaction of interest to produce chemicals and fuels as a substitute for fossil fuel-sourced production. To make this reaction pathway competitive with existing production methods, low-cost transition metal carbides (TMCs) have been employed to yield high selectivity toward desired products (CO, CH₄ or hydrocarbons). One example is the reverse water-gas shift (RWGS) reaction, where supported Mo₂C and WC exhibit high selectivity toward CO.^{1, 2} However, little is known about the carbide-support physicochemical interactions that promote high CO selectivity.

To investigate these interactions, Mo₂C is synthesized on a variety of metal oxide supports using incipient wetness impregnation. Catalyst supports are selected to vary both chemical (Lewis and Brønsted acidity in H-ZSM-5 (Si/Al: 50 and 200) and structural (amorphous SiO₂ vs. silicalite-1) properties to determine their effect on RWGS performance. Ion exchange effects are also investigated within supports using M-ZSM-5 (M: Na, K or Rb). NH₃, CO₂ and CO pulse chemisorption and temperature programmed desorption experiments are used to determine acidic properties and adsorption behavior of the reactant and product. These measurements are combined with X-ray characterization techniques (XRD, XAFS and XPS) to characterize the bulk and surface properties of the catalysts to correlate adsorption behavior to support properties and catalytic performance. Packed bed reactor experiments illustrate that varying the chemical properties of the support has little influence on reactor performance, but the support phase dramatically affects the catalytic performance, with SiO₂ resulting in a CO STY of 0.7 L CO produced min^-1 g^-1, whereas silicalite-1 results in significantly decreased performance (0.01 L CO produced min^-1 g^-1).

References

1. Porosoff, M. D.; Yang, X.; Boscoboinik, J. A.; Chen, J. G., Angew Chemie Int. Ed. 2014, 126 (26), 6823-6827.
2. Morse, J. R.; Juneau, M.; Baldwin, J. W.; Porosoff, M. D.; Willauer, H. D., J CO2 Util 2020, 35, 38-46.