Effect of tin on the reactivity of platinum group metals with light hydrocarbons.

摘要

The emphasis on making environmentally benign chemical processes has become more pronounced over the last few years and thus, the demand for highly selective catalysts has increased. Tin has been used as an additive in the petrochemical industry to improve the selectivity of catalysts. Furthermore, Sri is an excellent modifier to study because it is inert for the adsorption of most molecules, it forms an alloy with many metals and it can be analysed with catalyst characterisation techniques.; The deactivation of Pt/Sn catalysts during isobutane dehydrogenation was studied using reaction kinetics measurements, temperature programmed oxidation, chemisorption, and transmission electron microscopy. The combination of these techniques identified that the Pt and Pt/Sn catalysts deactivated by coke deposition on the active sites and by metal sintering. The presence of Sri significantly reduced the amount of coke that formed on the active sites, because coking is a structure-sensitive reaction that prefers large Pt ensembles. Tin forms an alloy with Pt and thereby reduces the size of the Pt ensembles. This effect of Sn is termed geometric.; To determine if Sn also had an electronic effect on Pt, ethylene adsorption was studied using heat-flow microcalorimetry and infrared spectroscopy. The results indicated that Sn changed the distribution of species formed on the surface and also weakened the interactions between Pt and these surface species (an electronic effect). These studies were extended to Pd, Pd/Sn, Ru and Ru/Sn catalysts. It was shown that the effect of Sn on Pd is similar to that on Pt, whereas on Ru, Sn has a smaller electronic effect but a larger geometric effect because of segregation of Sn on the surface.; The results of these studies provide fundamental information on Sn-modified catalysts to which other bimetallic systems can be compared. The basic information obtained on the adsorption of ethylene is relevant to other studies, such as the partial oxidation of ethane to ethylene, which is a major intermediate in the chemical industry, over Pt/Sn catalysts. In addition, this work may be useful in analysing more complicated systems, such as Pt/Sn/Ru catalysts used for direct methanol fuel cells.