Daniel Gatewood

X-Ray Absorption Spectroscopy Studies of Water Activation on an Rh x S y Electrocatalyst for Oxygen Reduction Reaction Applications

The prototype chalcogenide electrocatalyst Rh{sub x}S{sub y} was probed in situ via a synchrotron-based X-ray absorption near-edge structure (XANES) technique to elucidate specific sites and modes of water activation during oxygen reduction reaction. X-ray diffraction revealed a mixture of phases (Rh{sub 2}S{sub 3}, Rh{sub 3}S{sub 4}, and Rh{sub 17}S{sub 15}). Theoretically generated XANES on a variety of geometries of O(H) adsorption on the predominant Rh{sub 3}S{sub 4} phase were compared to the experimental data. We show for the first time that the electrocatalyst first adsorbs O(H) in a one-fold configuration at lower potentials and n-fold at potentials greater than 0.80 V. This expectedly has important consequences for oxygen reduction reaction on alternative chalcogenide materials.

Support Effects on Water Activation and Oxygen Reduction over Au-SnOx Electrocatalysts Observed with X-Ray Absorption Spectroscopy

Gold nanoparticles supported on hydrous tin oxide (Au-SnOx) and mixed with Vulcan carbon are active electrocatalysts for the four-electron oxygen reduction reaction (ORR) to water in acid electrolyte below 0.55 V vs a reversible hydrogen electrode. In comparison, gold on Vulcan carbon (Au/VC) is inactive for the ORR in acid and generates mostly peroxide (i.e., two-electron reduction). These results imply that the SnOx support imparts the ORR activity of the Au. We study this metal support interaction by in situ X-ray absorption fine structure and near edge spectroscopy at the Au L3 and Sn K edges in an electrochemical cell. The results for 10 wt % (2 nm) and 20 wt % (2.7 nm) Au nanoparticles supported on SnOx suggest that a bifunctional mechanism plays the dominant role in the ORR reaction below 0.55 V. O2 adsorbs and dissociates on the SnOx surface with apparent simultaneous electron transfer from the Au and then is reduced further to water by electron transfer from the Au. Enhanced O[H] adsorption occurs at potentials above 1.2 V on the Au-SnOx catalysts. This O[H] adsorption is enhanced through a ligand mechanism involving the SnOx support but has no apparent effect on the ORR.

ED‐XAS Data Reveal In‐situ Time‐Resolved Adsorbate Coverage on Supported Molybdenum Oxide Catalysts during Propane Dehydrogenation

Energy‐Dispersive X‐ray Absorption Spectroscopy (ED‐XAS) data combined with UV/Vis, Raman, and mass spectrometry data on alumina‐ and silica‐supported molybdenum oxide catalysts under propane dehydrogenation conditions have been previously reported. A novel Δμ adsorbate isolation technique was applied here to the time‐resolved (0.1 min) Mo K‐edge ED‐XAS data by taking the difference of absorption, μ, at t>1 against the initial time, t=0. Further, full multiple scattering calculations using the FEFF 8.0 code are performed to interpret the Δμ signatures. The resulting difference spectra and interpretation provide real time propane coverage and O depletion at the MoOn surface. The propane coverage is seen to correlate with the propene and/or coke production, with the maximum coke formation occurring when the propane coverage is the largest. Combined, these data give unprecedented insight into the complicated dynamics for propane dehydrogenation.