Correlation of Electronic Structure with Catalytic Activity: H2-D2 Exchange across CuxPd1-x Composition Space

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 H-2-D-2 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 H-2-D-2 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 H-2 adsorption, Delta E-ads(double dagger), and recombinative H-2 desorption, Delta E-des(double dagger), as functions of alloy composition, x. Their values fall in the ranges Delta E-ads(double dagger)(x) = 0.15 to 0.45 eV and Delta E-des(double dagger) (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, epsilon(v)(x). The energy of the v-band center shifted monotonically from epsilon(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 Delta E-ads(double dagger)(x) with epsilon(v)(x) across alloy composition space yields Delta E-ads(double dagger)(epsilon(v)) which decreases as the v-band energy shifts toward the Fermi level.