The conversion of biomass-derived oxygenates is a promising method to sustainably form liquid fuels or useful chemicals. However, there are challenges in understanding the reaction mechanisms that inhibit the design of new, active and selective catalysts. Partially, these are related to the difficulties in probing the catalyst in the aqueous phase compared to the gas phase. Here we discuss the aqueous-phase electrocatalytic and thermal catalytic hydrogenation of bio-oil over supported metal catalysts, using phenol and benzaldehyde in water as model compounds. By a combination of kinetic measurements and <i>in-situ/operando</i> X-ray absorption spectroscopy, we show that on platinum group metals the reaction of adsorbed phenol and adsorbed hydrogen atoms is the rate determining step for both electrocatalytic and thermal catalytic hydrogenation. We employ aquoues-phase adsorption isotherm measurements and calorimetry to determine that a Langmuir-Hinshelwood mechanism accurately describes the observed kinetic behavior. We further show that tuning the adsorption energy of the adsorbed intermediate species can be used to increase the hydrogenation rate of phenol to cyclohexanone and cyclohexanol. We also compare phenol adsorption on Pt in the aqueous phase to gas-phase adsorption energy calorimetry measurements (and corresponding density functional theory calculations) to show the role of water in phenol adsorption onto Pt surfaces. For benzaldehyde hydrogenation, we also employ <i>in-situ/operando</i> spectroscopy to probe the catalyst surface and interpret the observed reaction rate data. This enables us to understand the dependence of the turnover frequency on applied potential, for example on a Pd/C electrocatalyst.

Aqueous-phase phenol and benzaldehyde hydrogenation on carbon-supported platinum group metals

2019 Fall National Meeting

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