(509ay) Explaining Size-Dependent Activity Trends and Identifying the Active Facet of Pt and Rh Nanoparticles for Hydrogenation of Phenol

Biomass is recognized as a sustainable resource to offset fossil fuel demand and reduce greenhouse gas emissions. Biomass can be broken down into a mixture of organic compounds called bio-oil through pyrolysis. To convert the bio-oil into valuable fuels or chemicals, catalytic hydrogenation and deoxygenation of bio-oil compounds such as phenol and benzaldehyde are necessary. The main challenge hindering the widespread use of upgrading of bio-oil is the high cost relative to fossil fuels. By understanding and increasing hydrogenation activity and selectivity, the economics can be drastically improved.

In this project, the thermodynamics and kinetics of thermocatalytic hydrogenation (TCH) and aqueous-phase electrocatalytic hydrogenation (ECH) of phenol to cyclohexanone on platinum and rhodium metals are investigated. TCH and ECH of phenol on platinum group metals are known to follow Langmuir-Hinshelwood mechanism, in which adsorbed hydrogen and phenol reacts on the catalyst surface. The experimentally measured rate for ECH of phenol on Pt/C and Rh/C nanoparticles decreases as the average particle size and fraction of (111) terraces decreases. Therefore, we hypothesize that the active site for phenol hydrogenation is the (111) terrace, which is more prevalent than step sites on the surfaces of larger particles. To test this hypothesis, we perform density functional theory calculations and microkinetic simulation of phenol hydrogenation on the (111) terraces and (221) steps of Pt and Rh. We find that the high activity of the (111) terraces compared to the steps is due to the optimized phenol and hydrogen coverage and faster intrinsic kinetics. Ultimately, these findings provide atomistic insight into the activity differences between steps and terraces of Pt and Rh toward phenol hydrogenation, as well as the impact of hydrogen coverage on hydrogenation thermodynamics and kinetics.