Finding the optimal catalyst composition for a given application is the major challenge in multicomponent catalyst design due to the need to perform many catalyst preparation, characterization and reactivity measurements across composition space. To accelerate this search Composition Spread Alloy Films (CSAFs) are prepared which are thin multicomponent films that have composition gradients parallel to their surfaces, AxByC1-x-y with x = 0 → 1 and y = 0 → 1-x. In order to study alloy catalysis across a CSAF, a 100 channel microreactor array has been developed to run steady state catalytic reactions at different positions or alloy compositions on the CSAF. With this reactor we can sample product distributions from 100 different alloy catalysts in the space in about 15 minutes. CuxPd1-x and CuxAuyPd1-x-y CSAFs spanning all of binary and ternary composition space have been prepared using a rotating shadow mask CSAF deposition tool developed in house. The relationship between alloy catalyst activity and electronic structure is investigated experimentally across CuxPd1-x, and CuxAuyPd1-x-y CSAFs. H2-D2 exchange kinetics was measured at 100 discrete compositions on the CSAFs over a temperature range of 333 – 597 K. The characterization of the CSAFs were done using energy dispersive X-ray spectroscopy and X-ray photoemission spectroscopy. H2 conversion was chosen to be the indicator of activity and it was found to decrease monotonically with decreasing Pd content. A microkinetic model that has been validated using a number of single component Cu-Pd catalysts in a fixed bed reactor was used to estimate the energy barriers to dissociative adsorption and associative desorption of H2 as functions of alloy composition, x and y. Across CuxAuyPd1-x-y CSAF, increasing Pd content from 0 to 1 was found to decrease adsorption barrier from 0.44 to 0.12 eV and increase desorption barrier from 0.4 to 0.74 eV which suggests H2-D2 exchange reaction is limited by H2 desorption. Spatially resolved UV photoemission spectra obtained from the CSAFs were used to estimate the energy of the valence-band center as a function of alloy composition. The v-band center shifted monotonically from -2.0 to -3.5 eV across the CuxPd1-x CSAF. The barrier to dissociative adsorption of H2 was found to decrease as the v-band energy increases. This data provides the first experimental correlation of elementary reaction barriers with valence band energy across a continuous span of alloy composition space.

Alloy catalysis across composition space

251st National Meeting (2016)

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