Strong metal-support interactions in nickel/titania catalysts: The importance of interfacial phenomena

Abstract

Nickel catalysts supported on silica and titania have been prepared by wet impregnation. The uncalcined catalysts, after an initial mild reduction at 623 K, have been treated by heating in hydrogen or argon at successively higher temperatures up to 923 K. The activity and selectivity of the catalysts have been determined under flow conditions at 1 bar pressure for the hydrogenolysis of n-hexane at 548 K, for the hydrogenolysis of ethane over the temperature range 478–673 K, and for the CO/hydrogen reaction at 548 K. Hydrogen adsorption isotherms have been measured and the specific metal surface areas determined using the Langmuir equation for dissociative adsorption to determine the monolayer coverage. Mean crystallite diameters have been determined by X-ray line broadening. Ni/titania catalysts exhibit normal hydrogen chemisorption properties after reduction at 723 K, although adsorption is suppressed after thermal treatment at 923 K. X-Ray line broadening measurements indicate a mean particle size of 8.0 nm after reduction at 723 K, and 13.3 nm after heating to 923 K. Activity measurements, after reduction at 723 K, show excellent agreement between the specific activities of the Ni/silica and Ni/titania catalysts for the hydrogenolysis of n-hexane and ethane. All these results are consistent with literature data for these reactions. However, for the CO/hydrogen reaction the specific activity of the Ni/titania catalyst is a factor of 50 higher than that of the Ni/silica catalyst. After heating at 923 K, there was a large decrease in the activity of the Ni/titania catalysts for all the reactions investigated. It is concluded that the enhanced activity of the Ni/titania catalysts for the CO/hydrogen reaction cannot be due to strong metal-support interactions (SMSI) as defined in the literature. A model of the Ni/titania system is presented which emphasizes the importance of considering the interface between the metal and the partially reduced support. It is proposed that new active sites created at the interface are responsible for the high specific activity of these catalysts for the CO/hydrogen reaction.