Kinetic Fingerprints of Catalysis by Subsurface Hydrogen

Abstract

Mechanisms for catalytic H 2 −D 2 exchange involving subsurface hydrogen, H′ (D′), have been analyzed to predict the ranges of reaction conditions over which the reaction orders, n H 2 and n D 2 , 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 H 2 −D 2 exchange have been measured over 90 Ag x Pd 1−x alloy film samples with a continuous range of compositions: x = 0 → 1. For conditions with P H 2 ≫ P D 2 , the reaction orders are found to be n H 2 ≅ 0 and n D 2 ≅ 1. The value of n H 2 ≅ 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.

H2–D2 Exchange Activity and Electronic Structure of AgxPd1–x Alloy Catalysts Spanning Composition Space

“…Fundamentally, the subsurface hydrogen mechanisms require the presence of either one or two adjacent H′ or D′ species in the immediate subsurface to facilitate the adsorption and desorption occurring on the top surface. Analysis of the rate laws derived from these mechanisms under conditions where P H2 ≫ P D2 and θ ≅ 1 revealed that the 2H′ mechanism is consistent with n H2 = 0 observed in the earlier work using this Ag x Pd 1– x CSAF …”

“…We have also measured the average valence band energy, ε̅ v , of each catalyst so that its H 2 –D 2 exchange activity and associated kinetic parameters can be linked to the electronic structure of the alloy. Detailed explanations of CSAF preparation, CSAF characterization, measurement of the H 2 –D 2 exchange kinetics, and kinetic parameter estimation can be found elsewhere. , Below, we briefly summarize the experimental sections pertinent to this work with the addition of a section for the characterization of the electronic structure of the valence band which has not been previously discussed.…”

“…Kinetics across the CSAF. The H 2 −D 2 exchange activity of the Ag x Pd 1−x CSAF was measured at 90 different alloy compositions 49 using a high-throughput 100-channel microreactor array which has been described in detail elsewhere. 11 In that study, only 90 channels of the reactor were in use because the inlet flow to 1 row of 10 reactors was blocked.…”

“…Previously, we explored reaction mechanisms for H 2 –D 2 exchange that include the presence of subsurface hydrogen, herein denoted by H′, to resolve deviations from the LH framework . We proposed the Single Subsurface Hydrogen (1H′) mechanism and the Dual Subsurface Hydrogen (2H′) mechanism, which both incorporate the diffusion of surface H and D atoms into and out of the subsurface with an equilibrium constant, K ss .…”

Decoding reactive structures in dilute alloy catalysts

“…2, Supplementary Methods). Moreover, migration into the subsurface 37,38 was not considered in our dilute systems, where Pd largely remains dispersed as isolated atoms in the interior of Au. A recent study of Pd/Ag(111) showed that subsurface H can become metastable only in the presence of locally extended Pd under high H 2 pressures 39 .…”