Support Acidity Improves Pt Activity In Propane Combustion In The Presence Of Steam By Reducing Water Coverage On The Active Sites

Support effects in catalysis are important in determining the catalytic activity of supported phases through geometric and electronic effects. Unveiling and quantifying support effects lead to the opportunity to finely tune and optimize the activity of supported catalysts. The study of support effects needs to be conducted in conditions where other important parameters, such as size and composition of the active phase, are not influencing the activity. This requirement is particularly important when the activity is affected by reactants or products that have implications in the reaction mechanism, as is the case of water in hydrocarbon combustion. Here, we present a systematic study on the effects of support acidity and water on propane combustion catalyzed by platinum-based catalysts that reveal support effects beyond the conventional geometric and electronic effects. A set of catalysts prepared from colloidal Pt nanoparticles supported on alumina, silica-alumina, and tungsten oxide-modified silica-alumina with increasing controlled Bronsted acid site density was prepared. We observed a monotonic increase in reaction rate as the same Pt particles were supported on progressively more acidic supports, with an improvement of up to 40 times in reaction rate for the sample with the highest Bronsted acid site density [Pt/WO3(10%)/30% SiO2/Al2O3] compared to the bare Pt/Al2O3 sample. Based on kinetic measurements on samples with different Pt particle sizes, we propose that (i) the metal-support perimeter participates in the reaction, and (ii) the interaction between metal and Bronsted acid sites is responsible for the increase in activity. The origin of such rate increase was found to be related to the enhanced resistance to water poisoning introduced by the acid sites: samples with higher acidity displayed nearly zero water rate order and more stable rates in the presence of steam. Using a Langmuir-Hinshelwood model, the most acidic sample was shown to display the lowest water coverage on the Pt surface. Summarizing all the observations, we propose that the Bronsted acid sites help reduce water coverage on the Pt surface through a spillover-like mechanism, which results in available sites for propane adsorption and activation and thus higher reaction rates in propane combustion. This work highlights how supports play a role not only in modifying electronic and geometric properties of supported phases, but also in the reaction mechanism through active roles in modifying surface coverages.