(545d) High-Throughput Characterization of Composition, Structure and Reactivity of PdxCu1-X Alloys
Alloy catalysts often display better activity, selectivity, and stability than their pure components. Most experimental studies of alloy catalysts try to correlate changes in catalytic activity with electronic and/or structural properties using a limited number of single-composition alloy samples. In this work, we use high-throughput techniques, developed in our laboratory, to discern relationships among catalytic activity, microstructure and electronic structure across continuous composition space.
Alloy libraries were prepared as Pd–based binary Composition Spread Alloy Films (CSAFs) which include all possible compositions of the alloy in the form of a thin film deposited on a substrate. PdxCu1-x CSAFs were prepared by evaporative co-deposition of the components onto Mo substrates, followed by annealing at 800 K. To evaluate alloy activity for H2 dissociation as a function of alloy composition, H2-D2 exchange (H2 + D2 → 2HD) kinetics were measured at 100 discrete locations (alloy compositions) across the CSAF surface using a unique multichannel microreactor array. Microstructure and electronic structure were characterized as functions of composition across the CSAF surface by spatially resolved X-ray photoemission spectroscopy (XPS), UV photoemission spectroscopy (UPS), and electron backscatter diffraction (EBSD).
Binary PdxCu1-x CSAFs, spanning the composition range x= 0.05→0.95, possess the features of the bulk PdCu phase diagram at the annealing temperature (800 K). H2-D2 exchange experiments, performed at atmospheric pressure and over a temperature range from 300 to 600 K, reveal maximum exchange activity at x ~ 0.70. Activation barriers to H2 dissociative adsorption and recombinative desorption, estimated from microkinetic analysis of the reaction data, correlate with valence band electronic structure, characterized by UPS, across composition space. For example, the barrier to dissociative adsorption decreases as average d-band energy increases at Pd-rich (high x) compositions. This work illustrates use of a high-throughput methodology to construct composition-structure-activity relationships across broad composition and structure space for optimization and fundamental understanding of multicomponent catalytic materials.