(265c) Kinetic Fingerprints of Subsurface Hydrogen Active in Hydrogenation

Reaction kinetic data obtained for the elementary H₂ - D₂ exchange reaction, over Cu_xAu_yPd_1-x-y and Ag_xPd_1-x alloy catalysts spanning composition space. Measurements have been made using alloy films with lateral composition gradients - Composition Spread Alloy Films (CSAFs). These CSAFs are created such that all possible binary or ternary alloy composition are present at points distributed across the alloy surface. Catalytic activity has been measured using a unique multichannel microreactor array designed to isolate 100 composition regions across the CSAF and to establish steady-state reaction conditions at all compositions. Reaction kinetics can then be measured over a range of temperatures and reactant partial pressures at all 100 alloy compositions concurrently. The presentation will discuss the design of the multichannel microreactor and its operation in concert with CSAF catalyst libraries. Our measurements of reaction kinetics for H₂ - D₂ exchange across Ag_xPd_1-x alloy catalysts with composition in the range x = 0 to x=0.8 suggest that the reaction mechanism is not as elementary as commonly supposed. Langmuir Hinshelwood kinetics do not apply. This corroborates a number of other observations made in other laboratories. Measurements of the reaction orders show that for P_H2 >> P_D2 the reaction orders are n_D2 = 1 and n_H2 = 0. The value of n_H2 is inconsistent with a standard Langmuir Hinshelwood mechanism that would predict n_H2 = -1. 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%. We have analyzed the kinetics of four different mechanisms, and various combinations thereof, including three that explicitly involve subsurface hydrogen. Of these, the only mechanistic model that predicts the observed reaction order of n_H2 = 0 with P_H2 >> P_D2 is a mechanism in which adjacent H and D atoms on the surface are destabilized by two atoms in the subsurface such that recombination of these surface atoms is much more rapid than that of surface hydrogen atoms with only one or no hydrogen atoms in the immediate subsurface. The origin of the reaction order n_H2 = 0 is clearly understood. These observations and models suggest the conditions for measurements that would identity the role of subsurface hydrogen in reactions such as ethylene or acetylene hydrogenation.