Correlation of electronic structure with catalytic activity: H₂-D₂ exchange across Cu_xPd_{1- x} composition space

The relationship between alloy catalyst activity and valence band electronic structure has been investigated experimentally across a broad, continuous span of Cu_xPd_1-x composition space. Cu_xPd_1-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 Cu_xPd_1-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 Cu_xPd_1-x CSAF and used to estimate the average energy of the filled states of the valence band as a function of alloy composition, ε_v(x). The energy of the v-band center shifted monotonically from ε_v = -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 ε_v(x) across alloy composition space yields ΔEads^‡(ε_v) which decreases as the v-band energy shifts toward the Fermi level.