Electrocatalytic Hydrogenation of Model Bio-Oil Compounds on Pt and Rh

Bio-oil is recognized as a sustainable resource to offset fossil fuel demand and reduce greenhouse gas emissions. To convert the biomass into valuable fuels or chemicals, catalytic hydrogenation and deoxygenation of bio-oil compounds such as phenol and benzaldehyde are necessary. The H₂ for hydrogenation is typically obtained from methane steam reforming, which is energy intensive and produces CO₂. If instead hydrogen equivalents are provided by the reduction of protons via aqueous phase electrocatalytic hydrogenation (ECH), biomass hydrogenation could be driven using renewable electricity, opening an attractive route to produce high-energy density transportation fuels in a sustainable manner. The main challenge hindering the widespread use of ECH for bio-oil conversion is the high cost relative to fossil fuels. By understanding and increasing ECH activity and selectivity, the economics can be drastically improved.

In this project, the thermodynamics and kinetics of aqueous-phase ECH of phenol to cyclohexanol on platinum and rhodium metals are investigated via density functional theory (DFT) calculations and first-principles microkinetic simulations. The experimentally measured intrinsic rate for ECH of phenol on Pt/C and Rh/C nanoparticles decreases as the average particle size decreases. Therefore, we hypothesize that the active sites for phenol hydrogenation are (111) and (100) terraces, which are more prevalent than step sites on the surfaces of larger particles. To test this hypothesis, we perform DFT calculations of phenol hydrogenation on the (111) terraces, (100) terraces, and (553) step of Pt and Rh. All DFT calculations used the Perdew-Burke-Ernzerhof functional with the semi-empirical D3 dispersion correction (PBE-D3). The effect of applied potential on the thermodynamics and kinetics was incorporated using the computational hydrogen electrode and the Butler-Volmer formalism. We predict that the platinum terraces are more active than the step sites, in agreement with our experimental observations. For Rh, a large dependence on the phenol hydrogenation kinetics is observed depending on hydrogen coverage. Ultimately, these findings provide atomistic insight into the activity differences between steps and terraces of Pt and Rh toward phenol ECH, as well as the impact of hydrogen coverage on ECH thermodynamics and kinetics.