Explaining the Activity Dependence on Particle Size for Phenol Hydrogenation on Pt and Rh By Identifying the Active Facet

Bio-oils derived from lignin biomass contain oxygen-rich organic compounds that can be upgraded through thermocatalytic (TCH) or electrocatalytic hydrogenation (ECH) to energy-dense, hydrogen-rich transportation fuels. However, both TCH and ECH of bio-oils are performed on expensive precious metals such as Pt and Rh nanoparticles (NPs), yet they are still plagued with low catalytic turnover frequency (<1 s^–1). Because these nanoparticles have several exposed surface facets, it is challenging to discern the active facet that contributes most to the catalytic turnover and consequently unclear how to design new catalysts with improved activity.

To this end we identify the active facet for phenol ECH and TCH on Pt and Rh that has optimal surface properties (high reaction rate constant and optimal surface coverages of the reacting bio-oil species). We measured the phenol TCH turnover frequency on Rh in the kinetic regime at varying phenol concentrations and fit the rate data to a Langmuir-Hinshelwood model, to extract the phenol adsorption energy on the active facet of Rh. We showed that the active facet for phenol TCH and ECH on both Rh and Pt is the facet that binds phenol very weakly due to water displacement, which we attribute to the (111) facets through density functional theory and microkinetic modelling. Finally, we show that as we increase the fraction of the active (111) facets by increasing the Pt and Rh NPs sizes, the measured ECH turnover frequencies are much higher compared to the smaller particles, further confirming that the (111) facets dominate the catalytic turnover. Ultimately our studies show that Pt and Rh nanoparticles are almost inactive at small particle sizes (e.g., 2 nm) because of the high abundance of inactive step sites.