Activity enhancement for the hydrogenation of CO adsorbed on Pt/Al2O3 and Pd/Al2O3 induced by high-temperature reduction

Abstract

On the basis of the rate constant per active site determined by pulse surface reaction rate analysis (PSRA), the effect of the reduction temperature on the hydrogenation of adsorbed CO was studied for alumina-supported noble metal catalysts. It was found that the reduction temperature influenced the hydrogenation of adsorbed CO in a different way from one noble metal to another. The rate constant on Pt prepared from H2PtCl6 was more than five times larger on reduction at 773 K than at 523 K. Heating the catalyst, exhibiting the enhanced activity, in an O2 atmosphere at 723 K followed by reduction at 523 K resulted in a decrease in the rate constant to near its original value. The rate constant on Pd also varied with heat treatments in a way similar to that for Pt, although the activity enhancement was less pronounced. In contrast, no activity enhancement was observed on Rh and Ru. It was also found that the activity enhancement strongly depended on the metal precursor or preparation conditions : the Pt catalyst prepared from [Pt(NH3)4]Cl2 did not exhibit enhanced activity even after reduction at 773 K. Compared with the effect on the hydrogenation of adsorbed CO, the reduction temperature hardly influenced CO chemisorption for all noble metals examined. On the basis of the results obtained, a possible cause is discussed for the enhanced activity on Pt and Pd and for the absence of enhancement on Rh and Ru.