Ru surface density effect on ammonia synthesis activity and hydrogen poisoning of ceria-supported Ru catalysts

Evaluating the effect of metal surface density on catalytic performance is critical for designing high-activity metal-based catalysts. In this study, a series of ceria (CeO2)-supported Ru catalysts (Ru/CeO2) were prepared to analyze the effect of Ru surface density on the catalytic performance of Ru/CeO2 for ammonia synthesis. For the Ru/CeO2 catalysts with Ru surface densities lower than 0.68 Ru nm−2, the Ru layers were in close contact with CeO2, and electrons were transferred directly from the CeO2 defect sites to the Ru species. In such cases, the adsorption of hydrogen species on the Ru sites in the vicinity of O atoms was high, leading to a high ammonia synthesis activity and strong hydrogen poisoning. In contrast, the preferential aggregation of Ru species into large particles on top of the Ru overlayer resulted in the coexistence of Ru clusters and particles, for catalysts with a Ru surface density higher than 1.4 Ru nm−2, for which Ru particles were isolated from the direct electronic influence of CeO2. Consequently, the Ru−CeO2 interactions were weak, and hydrogen poisoning can be significantly alleviated. Overall, electron transfer and hydrogen adsorption synergistically affected the synthesis of ammonia over Ru/CeO2 catalysts, and catalyst samples with a Ru surface density lower than 0.31 Ru nm−2 or exactly 2.1 Ru nm−2 exhibited high catalytic activity for ammonia synthesis.