Correlation of electronic structure with catalytic activity: H2-D2 exchange across CuxPd1- x composition space

Abstract

The relationship between alloy catalyst activity and valence band electronic structure has been investigated experimentally across a broad, continuous span of CuxPd1-x composition space. CuxPd1-x composition spread alloy films (CSAFs) were used as catalyst libraries with a 100 channel microreactor to measure the H2-D2 exchange kinetics over a temperature range of 333-593 K at 100 discrete CuxPd1-x compositions spanning the range x = 0.30-0.97. The H2-D2 exchange activity exhibits a monotonic decrease over the composition range x = 0.30-0.97. A steady state, microkinetic model was used to estimate the energy barriers to dissociative H2 adsorption, ΔEads‡, and recombinative H2 desorption, ΔEdes‡, as functions of alloy composition, x. Their values fall in the ranges ΔEads‡(x) = 0.15 to 0.45 eV and ΔEdes‡(x) = 0.55-0.65 eV. Spatially resolved UV photoemission spectra were obtained from the CuxPd1-x CSAF and used to estimate the average energy of the filled states of the valence band as a function of alloy composition, εv(x). The energy of the v-band center shifted monotonically from εv = -3.3 to -3.9 eV across the composition range x = 0.30-0.97. This monotonic shift and its magnitude were corroborated by DFT calculations. The correlation of ΔEads‡(x) with εv(x) across alloy composition space yields ΔEads‡(εv) which decreases as the v-band energy shifts toward the Fermi level.