Kinetic Fingerprints of Catalysis by Subsurface Hydrogen

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 mechanisms. Four different mechanisms and combinations thereof have been considered: a Langmuir−Hinshelwood (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′). In parallel, the kinetics of H2–D2 exchange have been measured over 90 AgxPd1–x alloy film samples with a continuous range of compositions: x = 0 → 1. 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 an 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