The effect of reduction temperature on a supported platinum catalyst



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A series of cycloalkenes characterized by increasing steric hindrance were hydrogenated on four alumina-supported platinum catalysts reduced at different temperatures (250°C, 350°C, 450°C and 550°C) in order to ascertain the effect of reduction temperature on the properties of the catalyst. Kinetic data on the hydrogenation of cyclopentene, 3-methylcyclopentene, 1-methylcyclopentene and 1,2-dimethylcyclopentene were obtained and reaction mechanisms were scrutinized. The kinetic results for each catalyst were compared and analyzed, based on the information obtained from the catalyst characterizations, which included surface area measurements, platinum content determinations and microscopic structure examinations on the catalyst supports. Reduction temperatures were found to affect the catalyst activity and hydrogenation rates were obviously retarded by steric hindrance. All four catalysts exhibited [gamma] type hydrogen adsorption, and the catalyst with the most extensive heat treatment showed an additional [delta] type adsorption. The kinetic results revealed that only the [gamma] type adsorbed hydrogen participated in the hydrogenation reactions. No structural change of the alumina support was found upon reduction at different temperatures. The differences in hydrogenation rates were believed due to the differences in the structure of the surface platinum on the catalyst. The activation energies for 1,2-dimethylcyclopentene hydrogenation using different catalysts revealed that the rate differences were affected by the variations of activation energies. Competitive hydrogenations gave relative adsorption coefficients for 1-methylcyclopentene and 3-methylcyclopentene which did not differ much over the series of the four catalysts. The hydrogenation rates were therefore believed not to be controlled by adsorption. It is proposed that as the reduction temperature increases, the surface rearranges to expose more active sites that either give the adsorbed hindered cycloalkenes a correct orientation to react with hydrogen atoms or promote the desorption of the hindered cycloalkane products. The relative adsorption coefficient decreased in the order of cyclopentene, 3-methylcyclopentene and 1-methylcyclopentene, which was the same decreasing order as in the hydrogenation rates. The product analysis for 1,2-dimethylcyclopentene hydrogenation further confirmed the effect of the catalyst reduction temperature because changes in the product distributions were found using different catalysts. Finally the atomic migration model was suggested for causing the surface rearrangement during the catalyst reducing process.