SUPPORTED AU-PT CATALYSTS CHARACTERIZATION AND HYDROGEN TRANSFER ACTIVITY BETWEEN BENZENE AND CYCLOHEXANE

Two series of AuPt catalysts supported on SiO2 were prepared by the incipient wetness method. In one series the total amount of metal (Au + Pt) was kept constant at about 0.7 wt% (Series A), while in a second series the amount of Pt was kept constant at about 0.7 wt% (Series B). In the latter the total amount of metal ranged between 1.03 to 6.13 wt%. The two series were characterized by H2 and O2 chemisorption, wide-angle X-ray scattering, and electron spectroscopy for chemical analysis. There was indication of the presence of mixed AuPt particles as well as of particles containing Au only. In depth compositional profile of Series B showed homogeneous composition extending to at least about 12 Å below the surface, while in Series A compositional inhomogeneities were found. The rate of the carbon-14 equilibration between benzene and cychohexane and the rate of equilibration between H2 and D2 were measured on both catalyst series in a flow system at a total pressure of 1 atm. For the carbon-14 equilibration reaction the temperature was 205 °C, contact time in the range of 10 to 30 sec. The influence of the ratio cyclohexane to benzene on the reaction rate was recorded. For the deuterium equilibration the temperature was -78 °C and a ratio of pD2 pH2 = 1 was employed. The rate of the carbon-14 equilibration was found to be independent of surface composition of the AuPt particles for Series A, while for Series B the rate increased upon addition of Au to Pt. Differences in rate up to about one order of magnitude were observed between catalysts from Series A and B possessing a similar overall Au Pt ratio. No difference in the rate of deuterium equilibration was found between Series A and Series B catalysts. The kinetic behavior of Series B is discussed, and suggestions of the nature of the AuPt interaction, responsible for the enhancement of the reactivity of Pt, are advanced. The concept of reaction sensitivity to surface structure is viewed as a dynamic factor which includes the position of the reaction steady state and the corresponding degree of reaction conversion. Consequently, a reaction may be sensitive or insensitive to the surface structure depending upon its conversion. © 1979.