Features of the Mechanism of Catalytic Oxidation of Carbon Monoxide on Pd/Al2O3 Catalysts

Abstract

The mechanism of catalytic low-temperature oxidation of CO on Pd/Al2O3 was studied. A combined reaction mechanism through the formation and decomposition of carboxyl-type structures was proposed, in which adsorbed oxygen reoxidizes reduced palladium atoms and ensures the decomposition of carboxyl structures with the formation of CO2 and the regeneration of OH groups. It was shown that with a small addition of Pd the activity of the catalyst increases significantly. The positive effect is due to the activation of CO or a change in the state of oxygen on the surface of the catalyst. Palladium was estimated by the value of hydrogen adsorption at 70°C. The reaction rate of catalytic oxidation of carbon monoxide, as the main characteristic of the activity and efficiency of the studied catalysts, was measured on a circulation-static unit using the method of continuous analysis of the gas mixture. The higher SCA of supported palladium in comparison with massive palladium in the reaction of low-temperature oxidation of CO may be associated with the written-off mechanism. This assumption is consistent with the TDA data. Close values ??of the adsorption energy of CO on the compared objects indicate a practically identical state of adsorbed CO on supported and massive palladium. However, the concentration of this form of CO, active in the oxidation reaction, on Pd/Al2O3 is approximately two orders of magnitude higher. Pd black adsorbs CO mainly in a weakly bound, low-activity form, which is reversible already at 100°C. For Pd black, which has no hydroxyl coating, a mechanism is realized with the participation of CO molecules and adsorbed oxygen atoms in intermediate complexes. The decomposition of these complexes is consistent with the high activation energy. The effect of superadditive adsorption capacity for carbon monoxide and specific catalytic activity in the reaction of low-temperature oxidation of CO was found. It was also discovered that the low-temperature oxidation reaction of CO on catalysts of the Pd/Al2O3 system can proceed via a non-trivial mechanism, which includes the participation of hydroxyl groups of the Al2O3 surface, coordinated by palladium, in the reaction.