Phosphorus on platinum surfaces – from molecular adsorption to catalyst poisoning

Platinum is a catalytically active metal widely used in technology. An important application is automotive exhaust gas catalysis, aiming at reducing combustion engine emissions. Platinum is used to catalyse oxidation reactions of carbon monoxide and hydrocarbons, for instance. On the other hand, phosphorus is a catalyst poison. It is adsorbed on catalyst surfaces, causing decrease in the catalytic activity. In practice, phosphorus is commonly found in lubricant oils and additives, which makes phosphorus poisoning a true problem in exhaust gas catalysis. In addition to real catalysts, studying phosphorus adsorption on platinum single crystal surfaces may give insight into the mechanism of poisoning. However, so far the number of such studies has remained relatively limited.In this work, molecular phosphorus adsorption on platinum (111) and (110) surfaces has been investigated. In addition, effects of phosphorus as well as sulfur on real platinum and platinum-palladium catalysts for diesel and natural gas oxidation have been studied. Furthermore, one section is devoted to maleic anhydride adsorption on platinum (111). Maleic anhydride is an organic molecule, used as an intermediate in many large-scale chemical manufacturing processes, related for instance to lubricant oils and biofuels. The studies featured in this work have been carried out with standard techniques of surface science, such as X-ray photoelectron spectroscopy, low-energy electron diffraction, scanning tunnelling microscopy and temperature-programmed desorption. Furthermore, results acquired with additional experimental and computational methods by collaborators are also reported. Phosphorus adsorption on both single crystal surfaces was found to be more efficient on elevated temperatures. Annealing the surface after deposition resulted in formation of ordered structures. Several different superstructures were found, depending on the phosphorus coverage which eventually saturated on both surfaces. Phosphorus was observed to have a tendency towards forming large clusters. On the real catalyst surfaces, phosphorus was found to form phosphate compounds with particles in the catalyst support. It also induced morphological changes, such as decreasing the specific surface areas. The magnitude of deactivation varied from moderate to severe depending on the catalyst and the reaction. Sulfur was found to mitigate the poisoning effect caused by phosphorus. Maleic anhydride formed unordered multilayer structures on platinum (111). All molecules desorbed or dissociated below room temperature, above which desorption of molecule fragments was observed in a wide temperature range. The results gained in this work give insight into how catalyst poisoning induced by phosphorus takes place. Moreover, they also give answers to some previously uncharted problems in fundamental surface science.