(399a) Adsorption of Organics and Nitrate on Pt Electrodes for Electrochemical Reduction Reactions

Using renewable electricity from solar and wind to drive electrochemical reduction reactions is a promising way to sustainably and selectively transform chemicals to desired products (e.g., to make liquid fuels or destroy harmful waste). Catalysis plays a critical role in how efficiently these electrochemical reactions proceed, and having active and selective electrocatalysts is necessary for the success of these electrochemical processes. An important component to rationally design electrocatalysts is understanding how the reactants and their intermediates bind to electrocatalyst surfaces; however, gaining molecular level insight into adsorption is challenging due to the complexity of the liquid phase.

In this talk, we will discuss two reduction reactions, namely, (1) the hydrogenation of bio-oil for production of transportation fuels (using phenol and benzaldehyde as models),¹ and (2) the reduction of nitrates for wastewater remediation. Both of these reactions involve the reduction of protons as hydrogen equivalents to reduce the desired reactant, and the rate-determining step involves an adsorbed reactant species. We first measured the reduction reaction rates on different metal surfaces (e.g., Pd and Pt). After, we show how phenol, benzaldehyde, and their intermediates/products bind to Pt compared to nitrate and nitrite. We discuss how the differences in binding can begin to explain some differences in the kinetics of these reactions (i.e., the binding strength of the reactants and intermediates act as catalyst activity descriptors), and how catalysts can be designed differently for these different reduction reactions.

To examine the adsorption of reactants and their intermediates we conduct cyclic voltammogram experiments on a Pt surface where the underpotential deposition of adsorbed hydrogen is observed. Different concentrations of the reactant molecule are introduced to probe how this affects hydrogen adsorption, which indirectly measures the adsorption of the reactant molecule (e.g., for phenol). By looking at the effect of reactant concentration on coverage, isotherms are derived to determine energies of adsorption of these different species, as a first step in understanding what is occurring at the catalyst surface. We compare these experimentally determined adsorption energies to predicted adsorption energies obtained using density functional theory calculations.

We examine these bio-oil model compounds and nitrate by both near-edge (XANES) and extended X-ray absorption fine structure (EXAFS). XANES probes the electronic state of the catalyst and EXAFS allows us to detect surface coverages (e.g. from Pt-C or Pd-C scattering under operating conditions with phenol or benzaldehyde). This spectroscopy, coupled with kinetic measurements and the isotherm measurements, helps us to better understand the reduction reactions.

References

(1) Singh, N.; Song, Y.; Gutiérrez, O. Y.; Camaioni, D. M.; Campbell, C. T.; Lercher, J. A. ACS Catal. 2016, 6, 7466.