Effect of Sn on CO oxidation over Rh–Sn/SiO2bimetallic catalysts

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Abstract
Oxidation of CO by O2 has been carried out over Rh/SiO2 and Rh–Sn/SiO2 catalysts. The temperature at which the activity for CO oxidation appeared was lowered in the presence of Sn. The reaction kinetics were not affected by the addition of Sn, the adsorption capacity for H2 and CO decreased, whereas the capacity for O2 increased. Sn enhanced dissociative adsorption of molecular oxygen. Successive adsorption of CO and O2 indicated that the preadsorption of CO or O2 did not decrease the adsorption capacity of Rh–Sn/SiO2 for O2 and CO, respectively. On the other hand, strong inhibition of the adsorption capacity was observed on Rh/SiO2 after preadsorption. Temperature-programmed desorption of adsorbed CO and co-adsorbed CO–O2 showed that the chemisorption of CO on Rh was weakened by the addition of Sn and that a larger amount of CO2(product of CO oxidation reaction) was desorbed from Rh–Sn/SiO2 than from Rh/SiO2. The surface reaction between adsorbed CO and O was enhanced by the addition of Sn. The role of Sn in Rh–Sn/SiO2 for CO oxidation is considered as: (1) to enhance the activation of oxygen molecules, (2) to decrease the self-poisoning by CO and/or O2, (3) to enhance the surface reaction between adsorbed oxygen and weakly adsorbed CO.