High throughput methods have been used to measure the kinetics of H2-D2 exchange across AgPd alloys spanning alloy composition space; i.e. AgxPd1-x , x = 0 to 1. In particular, we have focuss on the reaction orders measured over a wide range of H2:D2 ratios. These yield results that are inconsistent with a simple Langmuir Hinshelwood (LH) mechanism. Mechanisms for catalytic H2-D2 exchange involving subsurface hydrogen, H’(D’), have been analyzed to predict the ranges of reaction conditions over which the reaction orders, nH2 and nD2, can be used to distinguish among the various possible mechanism. Four different mechanisms and combinations thereof have been considered: a LH model, a breakthrough model invoking direct reaction between surface H(D) and subsurface H’(D’), and two models invoking activation of H(D) by H’(D’). For conditions with PH2 » PD2, the reaction orders are found to be nH2 ≈ 0 and nD2 ≈ 1. The value of nH2 ≈ 0 is inconsistent with a LH mechanism under conditions with a hydrogen coverage of θ ≈ 1. A mechanism in which two subsurface H’(D’) atoms promote recombinative desorption of H(D) atoms on the surface is consistent with the observed reaction orders under conditions in which the coverages on the surface and in the subsurface are θ ≈ 1 and θ' ≈ 0, respectively.

Kinetic fingerprints of catalysis by subsurface hydrogen on Pd-Ag alloys

2019 Spring National Meeting

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