Pt-Cu Core-Shell and Alloy Nanoparticles for Heterogeneous NOx Reduction: Anomalous Stability and Reactivity of a Core-Shell Nanostructure

摘要

Metallic nanoparticles of the noble metals find wide applications in heterogeneous gas-phase and solution-phase catalytic processes.[1]-[4] Small particle sizes optimize the dispersion of precious metals, such as platinum and palladium, and can also show anomalous reactivities not observed for the bulk metals. Bimetallic nanoparticles have been studied because of modified (enhanced) activities of noble metals and their abilities to catalyze multistep processes in which each metal participates in a different elementary step.[5]-[7] Toshima and Wang[8] first reported the synthesis of colloidal PtCu particles and showed their utility in the catalytic hydrogenation in solution. Recent studies[9], [10] have suggested that supported PtCu bimetallic catalysts may be effective in NOx reduction, but coprecipitation/impregnation methods typically used in these studies make it difficult to identify and evaluate the actual catalytic species. While some catalysts have been reported to have high activity for NOx reduction,[11], [12] the best activity and selectivity for N2 formation (a desirable feature) is typically found with monometallic platinum catalysts.[13]-[15] To fully understand the catalytic activity of PtCu bimetallic catalysts and compare the activities with the monometallic materials, a detailed understanding of the composition, phase behavior, activation processes, and stabilities of the catalytic components is essential. To this end, we have embarked on a systematic investigation of the synthesis and study of bimetallic PtCu nanoparticles to evaluate their heterogeneous catalytic activities for reduction of gas phase NO with H2 as the reducing agent.