NaBr poisoning of Au/TiO ₂ catalysts: Effects on kinetics, poisoning mechanism, and estimation of the number of catalytic active sites

Sodium bromide was used to intentionally poison a commercial Au/TiO ₂ catalyst with the goals of understanding the nature of halide poisoning and evaluating the number and nature of the catalytic active sites. A series of eight poisoned catalysts were prepared by impregnating the parent catalyst with methanolic solutions of NaBr. Each catalyst was tested with CO oxidation catalysis under differential reactor conditions; O ₂ reaction orders and Arrhenius activation energies were determined for each material. All of the kinetic data, including a Michaelis-Menten analysis, indicated that the primary effect of adding NaBr was to reduce the number of catalytically active sites. Density functional theory calculations, employed to evaluate likely binding sites for NaBr, showed that NaBr binds more strongly to Au corner and edge atoms than it does to the titania support or to exposed Au face atoms. Infrared spectroscopy of adsorbed CO, along with a Temkin analysis of the data, was also used to evaluate changes to the catalyst upon NaBr deposition. These studies suggested that NaBr addition induces some subtle changes in the coverage dependent properties of CO adsorption, but that these did not substantially impact the CO coverage of the CO binding sites. The experimental and computational results are discussed in terms of possible poisoning mechanisms (site-blocking vs off-site binding and modification); the nature and number of active sites are also discussed in the context of the results.