Photocatalytic selective hydrogenation of phenylacetylene over sulfur-doped TiO₂ supported Ni nanoparticles catalyst

The selective hydrogenation of alkynes to alkenes, as one of the basic hydrogenation processes, is of great importance in the fine chemical, pharmaceutical, coal chemical and environmental protection industries. For example, polystyrene is one of the world's five most used general-purpose plastics, and is widely used in the manufacture of rigid plastics for household appliances, films and foam products. However, styrene products often contain a certain amount of phenylacetylene, which can lead to catalyst poisoning in the polymerisation of styrene. Due to the similar properties of styrene and phenylacetylene, it is very difficult to separate trace amounts of phenylacetylene from styrene. The best method to solve this problem is to convert phenylacetylene to styrene by selective hydrogenation. In recent years, metal photocatalysis has demonstrated great potential in the field of organic synthesis due to its mild reaction conditions. When metal nanoparticles are exposed to light irradiation with a wavelength greater than their size, collective oscillations of electrons on the surface of the metal particles are induced. Localised surface plasmon resonance (LSPR) occurs when the collective oscillations of the surface electrons match the electric field of the incident light. It is a new type of photocatalytic technology different from the traditional semiconductor photocatalytic charge separation mechanism. Light-excited high-energy electrons are distributed on the metal surface, which makes the highest intensity of photoelectric field on the metal surface and favours the reaction to proceed on the surface. The metal nanoparticles absorb the light energy and make it enriched on the metal surface, which is finally converted into the excited high-energy electron energy. The energetic electrons directly activate the reactant molecules adsorbed on the surface of the metal nanoparticles to initiate the reaction. Therefore, it is of great interest to achieve selective hydrogenation of phenylacetylene under mild reaction conditions by metal photocatalysis. Catalyst support is an integral part of catalyst and plays a very important role in loaded metal catalysts. To date, the effect of sulphur species in sulphur-doped TiO₂ on the catalytic performance is not well understood. The present work provides an effective method to achieve efficient and highly selective hydrogenation of styrene to phenylacetylene under mild conditions. A series of sulfur-doped TiO₂ loaded Ni nanoparticle catalysts were prepared by roasting titanium sulfate oxyhydrate precursors. The effects of roasting temperatures on the content of sulfur species on the TiO₂ surface and the performance of photocatalytic selective hydrogenation of phenylacetylene were investigated. The sulfur species on the TiO₂ surface underwent electron transfer to the Ni nanoparticles, and the electron-rich Ni facilitated the desorption of styrene from the surface of the catalysts, which improved the selectivity of styrene. Visible light excitation of Ni nanoparticles generates “hot electrons”, which can effectively promote the dissociation and activation of H₂ and facilitate the hydrogenation reaction. The present study reveals the relationship between the sulfur species on the surface of the support and the catalytic hydrogenation performance of phenylacetylene, which demonstrates that the presence of sulfur species on the support is not necessarily harmful, and the rational use of them can help to design catalysts with high reactivity to meet the needs of potential applications.