Catalytic Activity of Highly Dispersed Palladium. II. X-Ray Photoelectron Spectroscopic and Thermal Desorption Studies of the Effects of ZrO2 Added to α-Alumina-supported Palladium

Abstract

The effects of ZrO2 upon the catalytic properties of α-alumina-supported palladium for the hydrogenation of gaseous cyclohexene were studied by means of detailed kinetic analysis, thermal desorption (TD), and X-ray photoelectron spectroscopy (XPS). The catalysts, with ZrO2 added in amounts ranging from 0.1 to 45 in Zr/Pd ratio, were characterized by CO chemisorption and X-ray diffraction. The addition of ZrO2 at the Zr/Pd ratio of 0.5 enhanced the turn-over frequency of the hydrogenation by a factor of 7 and changed the hydrogen order of the reaction from 1.0 to 0.75 and the cyclohexene order from 0 to 0.6. The reaction with D2 exhibited deuteriumdistribution patterns similar to those for the reaction on the ZrO2-supported Pd catalysts. The hydrogenation pathway on the ZrO2-added catalyst was described by an associative mechanism in which the slow step was a surface reaction between a cyclohexyl radical and a hydrogen atom. In the thermal desorption spectra, the addition of ZrO2 caused a broadening of the adsorbed hydrogen peak as well as a shift of the dissolved hydrogen peak toward a lower temperature. The desorption of cyclohexene from Pd/α-Al2O3 catalysts underwent a disproportionation to produce benzene around 318 K and cyclohexane at 368 K, whereas the latter peak shifted by 70 K to the lowertemperature side upon the addition of ZrO2. The XPS study showed that the presence of ZrO2 provided Pd metal atoms in a negatively-charged state. The support effect was discussed on the basis of the interaction between Pd metal atoms and ZrO2 oxide.

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