(422d) Using Surface Chemistry to Understand Aqueous-Phase Thermal Catalytic and Electrocatalytic Hydrogenation of Bio-Oil Model Substrates

One of the major challenges of the future is matching the increasing demand for energy using renewable resources. The energy density of hydrocarbons makes them ideal for transportation fuels, especially for aviation, where electric vehicles are not available. As the amount of energy transitions to renewable energy such as solar and wind, which are intermittent and potentially more decentralized, developing ways to store this energy also becomes important. Catalysis can play an integral role in the chemical processes required to produce renewable transportation fuels and store renewable energy. In this talk, I will discuss developments in catalysts used in a method to convert waste biomass to renewable fuels. The feedstock of this process is bio-oil, a product of pyrolysis or hydrothermal liquefaction of biomass. Bio-oil (modeled here using phenol and benzaldehyde) can be hydrogenated and then deoxygenated to yield fuel grade hydrocarbons.¹ Although hydrogen generally comes from steam methane reforming, which emits carbon dioxide, renewable electricity can be used to drive carbon-neutral (i) electrochemical hydrogen production (from water) which is then used for thermal catalytic hydrogenation (TCH), or (ii) direct electrochemical hydrogenation (ECH) of the bio-oil. Electrochemical routes are more economically competitive in decentralized applications and can make use of intermittent renewable electricity. The challenge of both electrochemical hydrogen production coupled with TCH and ECH is developing hydrogenation catalysts with high rates for commercial applications. The target is to achieve comparable rates (at approximately 80 °C) to existing commercial electrochemical processes, on the order of hundreds of mA per square centimeter (geometric current density) or turnover frequencies near 10 s^-1. In this work we discuss some advancements in the understanding of the aqueous-phase hydrogenation of phenol and benzaldehyde on different platinum group metals, and how this leads to improved rates for ECH and TCH.

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.

References

(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.