CONICET | Buscador de Institutos y Recursos Humanos

The dehydrogenation of isobutane was studied under severely deactivating conditions, i.e., high temperatures and in the absence of addedH2, over silica-supported Pt?Sn catalysts. Several preparation methods were investigated. It was found that the impregnation method employed has a strong influence on the degree of Pt?Sn interaction and the fraction of Pt that remains unalloyed after the calcination and reduction process. The co-impregnation methods investigated were significantly superior to the sequential method. It was found that it is important to minimize the amount of unalloyed Pt left on the catalyst because this fraction rapidly forms coke and deactivates. At the same time, the fraction of unalloyed Pt, rather than the one alloyed with Sn, is responsible for most of the CO and hydrogen adsorbed at room temperature in typical chemisorption measurements. As a consequence, the TOF values based on this type of measurements are in error because they are not related to the density of sites that are responsible for long term activity. It was also found that the high-temperature reduction/oxidation treatments usually employed to regenerate spent catalysts can have a detrimental effect on the activity and selectivity of the Pt?Sn/SiO22, over silica-supported Pt?Sn catalysts. Several preparation methods were investigated. It was found that the impregnation method employed has a strong influence on the degree of Pt?Sn interaction and the fraction of Pt that remains unalloyed after the calcination and reduction process. The co-impregnation methods investigated were significantly superior to the sequential method. It was found that it is important to minimize the amount of unalloyed Pt left on the catalyst because this fraction rapidly forms coke and deactivates. At the same time, the fraction of unalloyed Pt, rather than the one alloyed with Sn, is responsible for most of the CO and hydrogen adsorbed at room temperature in typical chemisorption measurements. As a consequence, the TOF values based on this type of measurements are in error because they are not related to the density of sites that are responsible for long term activity. It was also found that the high-temperature reduction/oxidation treatments usually employed to regenerate spent catalysts can have a detrimental effect on the activity and selectivity of the Pt?Sn/SiO22 catalysts. It is postulated that such thermal treatments lead to the disruption of the Pt?Sn alloys causing an increase in the fraction of unalloyed surface Pt. As a result, the rates of coke formation and deactivation drastically increase. The monometallic (Pt only) catalysts are also affected by the high-temperature reduction/oxidation processes. The oxidation treatment results in an increased rate of coke formation and deactivation, while the regeneration process results in a much smaller effect. This difference may be due to carbon residues left on the surface. These residues may disrupt Pt ensembles and cause a decrease in the rate of undesired reactions, such as hydrogenolysis and coking, that require a large ensemble of Pt atoms.

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