(422d) Using Surface Chemistry to Understand Aqueous-Phase Thermal Catalytic and Electrocatalytic Hydrogenation of Bio-Oil Model Substrates
For phenol hydrogenation, ECH and TCH on rhodium or platinum have similar activation barriers due to a shared rate determining step of adsorbed H adatoms reacting with adsorbed phenol. Using a flow cell designed for X-Ray Absorption Spectroscopy (XAFS), we probe Pt nanoparticles under reaction conditions for both near-edge (XANES) and extended X-ray absorption fine structure (EXAFS). This allows us to look at the catalyst state (from XANES) and surface coverages (from Pt-C scattering that we show is due to the adsorbed reactant species) under operating conditions. This spectroscopy, coupled with kinetic measurements, affords us a better understanding of the surface chemistry and its dependence on hydrogen pressure and applied potential. Using this understanding, we demonstrate methods to prevent catalyst deactivation at temperatures at 80 °C.2 We also show how modifying the adsorption energy of the adsorbed species controls the reaction rate, and discuss how other reaction conditions (ex. pH) affect the surface coverages and rates. These results lead us to propose material properties that may result in more active electrocatalysts for phenol TCH or ECH, and how to reach reaction rates that may be necessary for a commercial ECH electrolyzer system.
For benzaldehyde, on metals such as platinum and rhodium, TCH and ECH have different benzaldehyde rate order dependence, unlike what is observed for phenol. This may be due to a difference in the reaction mechanism for the ECH and TCH of benzaldehyde on Pt and Rh, or surface coverages under operating conditions. Using XAFS (as well as traditional cyclic voltammetry), we try to understand how the surface coverage of the species may be responsible for the observed reaction rates and orders. We connect this with kinetic measurements and adsorption energies in an attempt to understand how the surface chemistry controls the reaction rates that are observed.
(1) Zhao, C.; He, J.; Lemonidou, A. A.; Li, X.; Lercher, J. A. J. Catal. 2011, 280(1), 8.
(2) Singh, N.; Song, Y.; Gutiérrez, O. Y.; Camaioni, D. M.; Campbell, C. T.; Lercher, J. A. ACS Catal. 2016, 6, 7466.