Effect of zirconia support crystal structure on the alloying of rhodium and iridium for the improvement of three-way catalysts†

Three-way catalysts reduce emissions of air pollutants from automobiles. Rhodium has been the main component of these catalysts. However, because the price of rhodium has recently increased, there is now a desire to save rhodium without sacrificing catalytic performance. Alloying rhodium with other metals is a way to reduce the need for rhodium and improve catalytic performance. However, because rhodium is generally immiscible with other metals, preparation of such catalysts is laborious. In this study, we first demonstrated the effect of the support-crystal-structure of zirconia on the alloying of rhodium and iridium. Monoclinic zirconia with a low specific surface area enabled the formation of rhodium–iridium solid-solution oxides after calcination. These oxides were transformed into metallic random alloys by hydrogen pretreatment. The light-off temperature of the alloy catalyst with approximately one-third the rhodium atoms was similar to that of the pure rhodium catalyst. In situ Fourier transform infrared analysis detected concerted adsorption of carbon monoxide and nitric oxide on alloyed rhodium and iridium, respectively. These adsorbed species reacted smoothly, in contrast to the non-alloyed catalyst. The successful realization of the conventional impregnation-based alloying approach opens new horizons for the design of novel bimetallic catalysts with properties tailored for advanced catalytic applications.