Hydrogen is a promising clean energy source. In domestic polymer electrolyte fuel cell systems, hydrogen is produced by reforming of natural gas; however, the reformate contains carbon monoxide (CO) as a major impurity. This CO is removed from the reformate by a combination of the water-gas shift reaction and preferential oxidation of CO (PROX). Currently, Ru-based catalysts are the most common type of PROX catalyst; however, their durability against ammonia (NH₃) as an impurity produced in situ from trace amounts of nitrogen also contained in the reformate is an important issue. Previously, we found that addition of Pt to an Ru catalyst inhibited deactivation by NH₃. Here, we conducted operando XAFS and FT-IR spectroscopic analyses with simultaneous gas analysis to investigate the cause of the deactivation of an Ru-based PROX catalyst (Ru/α-Al₂O₃) by NH₃ and the mechanism of suppression of the deactivation by adding Pt (Pt/Ru/α-Al₂O₃). We found that nitric oxide (NO) produced by oxidation of NH₃ induces oxidation of the Ru nanoparticle surface, which deactivates the catalyst via a three-step process: First, NO directly adsorbs on Ru⁰ to form NO-Ru^δ+, which then induces the formation of O-Ru ⁿ⁺ by oxidation of the surrounding Ru⁰. Then, O-Ru ^m+ is formed by oxidation of Ru⁰ starting from the O-Ru ⁿ⁺ nuclei and spreading across the surface of the nanoparticle. Pt inhibits this process by alloying with Ru and inducing the decomposition of adsorbed NO, which keeps the Ru in a metallic state.

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