(413c) Alloy Catalysis Spanning Composition Space

The activity and selectivities of alloy catalysts are dictated by their composition, stature and morphology. The development of a predictive understanding of alloy catalysis must be able to span composition space. One of the factors limiting the development of an understanding at this level is the scarcity of catalytic reaction data that has been collected across alloy composition space with high composition resolution. We have combined the use of Composition Spread Alloy Films (CSAFs) with a high throughput multichannel microreactor array to collect measurements of catalytic reaction kinetics, r(T;P) , at 100 different compositions of binary and ternary alloy catalysts. CSAFs are high throughput libraries in the form of alloy films with composition gradients across their surfaces; A_xB_yC_1-x-y with x = 0 → 1 and y = 0 → 1-x. These can span entire binary or ternary alloy composition spaces. The multichannel microreactor array allows spatial resolution of catalytic activity across 100 isolated alloy compositions.

The presentation will discuss the design of the multichannel microreactor and its operation in concert with CSAF catalyst libraries. Reaction kinetic data obtained for the elementary H₂ - D₂ exchange reaction, over Cu_xAu_yPd_1-x-y and Ag_xPd_1-x CSAFs suggest that this reaction is not as elementary as commonly supposed. Measurements of the reaction orders show that for P_H2 >> P_D2 the reaction orders are n_D2 = 1 and n_H2 = 0. These kinetics have also been reported in other recent studies of H₂ - D₂ exchange. The value of n_H2 is inconsistent with standard Langmuir Hinshelwood mechanism and suggests that the reaction involves subsurface hydrogen species. The interesting feature of this is that these kinetics hold for all alloy compositions for which H₂ - D₂ exchange kinetics can be measured; Pd > 20 at%. Recent measurements of ethylene hydrogenation kinetics suggest that subsurface hydrogen may also play a role in olefin hydrogenation.