Overlayer Bimetallic Catalysts or Particle Size Effects? A Comparison of Re@Pd Catalysts with Different Particle Sizes

摘要

Alumina supported Re@Pd bimetallic over-layer catalysts of different loading, dispersion, and particle size were synthesized using the directed deposition technique. Past computational and empirical studies have shown that through depositing a monoatomic pseudomor-phic overlayer of one metal atop a different base metal it is possible to alter the adsorption and catalytic properties of the system when compared to both monometallic counterparts. It has been shown that a 2 wt% Re@Pd overlayer catalyst was capable of having lower hydrogen adsorption strength and lower activity for ethylene hydrogenation than its pure Pd counterpart. Particle size effects have long been known to play a role in the behavior of various catalysts as smaller particles tend to have more unsaturated, highly reactive, metal sites (e.g., kinks, corners, edges, etc.). In order to demonstrate that the observed changes in activity were not merely particle size effects, 7 wt% Re@Pd catalysts have been synthesized through similar methods and consist of particles much larger than those in the 2 wt% sample (viz., 6.3 and 2.7 nm). Similar reductions in hydrogen adsorption quantity and strength were seen for both Re@Pd samples and ethylene hydrogenation activity proved consistent. Both results indicate that the formation of a Pd overlayer was reason for this observed change in adsorption and activity and the effect cannot be merely attributed to particle size effects.