In-situ characterization of water-gas shift catalysts using time-resolved X-ray diffraction

Time-resolved X-ray diffraction (XRD) has emerged as a powerful technique for studying the behavior of heterogeneous catalysts (metal oxides, sulfides, carbides, phosphides, zeolites, etc.) in-situ during reaction conditions. The technique can identify the active phase of a heterogeneous catalyst and how its structure changes after interacting with the reactants and products (80 K < T < 1200 K; P < 50 atm). In this article, we review a series of recent works that use in-situ time-resolved XRD for studying the water-gas shift reaction (WGS, CO + H₂O → H₂ + CO₂) over several mixed-metal oxides: CuMoO₄, NiMoO₄, Ce_1-xCu_xO_2-δ and CuFe₂O₄. Under reaction conditions the oxides undergo partial reduction. Neutral Cu⁰ (i.e. no Cu¹⁺ or Cu²⁺ cations) and Ni⁰ are the active species in the catalysts, but interactions with the oxide support are necessary in order to obtain high catalytic activity. These studies illustrate the important role played by O vacancies in the mechanism for the WGS. In the case of Ce_1-xCu_xO_2-δ, Rietveld refinement shows expansions/contractions in the oxide lattice which track steps within the WGS process: CO(gas) + O(oxi) → CO₂(gas) + O(vac); H₂O(gas) + O(vac) → O(oxi) + H₂(gas).