In the interior of a nuclear reactor, hydrogen gas is continuously generated by the radiolytic decomposition of water molecules. Therefore, the amount of hydrogen gas must be reduced to prevent hydrogen explosions. Hydrogen recombination catalysts, which facilitate the catalytic reduction of hydrogen gas, are considered a promising hydrogen-reduction tool because they can function even during power outages. In the event of a severe accident, carbon monoxide may be generated through high-temperature reactions. Because carbon monoxide has a poisoning effect on catalytic activities, new catalysts that can resist performance deterioration are highly desired.
 In this study, we developed a new catalyst (Pt-Fe/CeZrYO_x) composed of ceria-zirconia-yttria complex oxides and Pt-Fe alloy nanoparticles. This catalyst shows good catalytic performance even in the presence of carbon monoxide. To elucidate the reaction mechanism of the developed catalyst and a conventional catalyst (Pt/Al₂O₃), we analyzed the local structure around Pt atoms using synchrotron radiation X-rays from SPring-8 (Hyogo, Japan). The X-ray absorption spectra indicate that carbon monoxide molecules adsorb on Pt nanoparticle surface in the conventional catalyst, inhibiting the hydrogen recombination reaction (Fig. 1a). By contrast, in the developed catalyst, hydrogen atoms are adsorbed on the metal nanoparticle surface instead of carbon monoxide, and these hydrogen atoms serve as active sites for the hydrogen recombination reaction (Fig. 1b).
  We hope to utilize the results of this study to develop high-performance catalysts that enable safe nuclear reactor operations.