Relation between propylene oxidation performance of copper-based bimetallic catalysts and the redox behaviour of their surfaces

Abstract

Propylene oxidation and surface oxidation–reduction rate processes on supported bimetallic catalysts which consist of Cu as substrate with small amounts of adherent Ag, Au or Rh have been studied and the relation between them discussed

The adhering components enhanced the rates of both the reduction of a preoxidized surface and of propylene oxidation in the order Rh Au > Ag, but exerted no effect on the oxidation rate of a prereduced surface.

The reduction processes were kinetically divided into three stages as follows: (1) the accelerating reduction rate period (ARP; fraction reduced x [graphic omitted] 0–0.3), (2) the maximal reduction rate period (MRP; x [graphic omitted] 0.3–0.8), (3) the decelerating reduction rate period (DRP; x [graphic omitted] 0.65–0.95).

For all the catalysts, Arrhenius parameters of each reduction rate coefficient in the period MRP and DRP, and the rate of propylene oxidation showed compensation effects: log A=E/(2.3 RT_s)+ log C_s and indicated the same isokinetic temperature T_s, (615 ± 9 K). E for the initial reduction rate (E_k) was correlated with that of the rate of propylene oxidation (E_o) for each catalyst by the expression E_k=E_o+(8.5 ± 0.9) kcal mol^–1.

The accelerating effects of the adhering components were observed during the incubation period in the early stages of ARP. This indicates that small amounts of a metal which is more difficult to oxidize than Cu, having a dispersed structure on the surface of Cu substrate exhibit a spillover of the reductant gas, and perform the role of initiation nuclei for reaction with the surface oxygen.