Effects of oxidation and reduction treatments on methylcyclopentane hydrogenolysis over Rh/TiO 2 and Rh/SiO 2 catalysts

Abstract

We have studied the effects of oxidation and reduction treatments of supported rhodium on its catalytic activity. In particular, we have studied the hydrogenolysis of methylcyclopentane over Rh/SiO 2 and Rh/TiO 2 catalysts with different metal dispersions. It has been observed that following oxidation at 673 K, a subsequent reduction at increasing temperatures causes a gradual increase in activity. When the reduction temperature reaches about 550 K a maximum in activity is observed, while a further increase in reduction temperature causes a decrease in activity. The same trend is observed on both SiO 2- and TiO 2-supported catalysts. However, the activity variations are most pronounced when the metal dispersion is high and the support is TiO 2. The changes in overall activity are accompanied by changes in selectivity. It is observed that the selectivity toward n-hexane follows the pattern opposite that of the activity, showing a minimum at reduction temperatures of about 550 K. Again, these selectivity variations are most pronounced for the titania-supported catalyst. We interpret these variations in terms of morphology changes occurring in the metal particles during the oxidation/reduction cycles. The reduction following the oxidation treatment initially renders the metal particles with a very open structure. Under these conditions the catalytic activity is relatively low. An increase in the reduction temperature causes an annealing of the metal particles, reaching an optimal configuration at about 550 K for which the catalytic activity shows a maximum. A further annealing of the metal particle caused by an increase in reduction temperature causes the activity to decrease. In the case of TiO 2, these changes are enhanced by an interaction occurring under high-temperature oxidation conditions between the oxidized Rh particles and the support. The behavior of the TiO 2-supported catalysts is also different at high reduction temperatures due to the well known SMSI effect.