Influence of Stabilizers on the Performance of Au/TiO2 Catalysts for CO Oxidation

Abstract

Stabilizers are commonly employed to synthesize nanocrystals with well-defined morphologies and size distributions, making them ideal tools to study structure–activity relationships in heterogeneous catalysts. While it is well documented that stabilizers can influence both the structure and size of the nanocrystals, little emphasis has been placed on how the properties of these species influence catalytic performance. Herein, different polymers (poly-sodium acrylate (PNaA), poly(vinyl alcohol) (PVA), and poly(vinyl pyrrolidone) (PVP)) and the monomer sodium acrylate (NaA) were used as stabilizers for the synthesis of Au nanoparticles supported on TiO2. The mean Au particle size in all of the catalysts was comparable regardless of the stabilizer used; however, the activity of these catalysts toward CO oxidation differed markedly. The activity decreased in the sequence Au/TiO2 (NaA) > Au/TiO2 (PNaA) > Au/TiO2 (none) > Au/TiO2 (PVA) > Au/TiO2 (PVP), despite the Au/TiO2 (none) catalyst possessing a larger Au mean particle size, suggesting active site blocking due to the steric nature of the polymer species. According to UV–vis, X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), high-resolution transmission electron microscopy (HRTEM), X-ray absorption spectroscopy (XAS), and composition analysis experiments, it was concluded that the enhanced activity of Au/TiO2 (NaA) and Au/TiO2 (PNaA) catalysts were attributed to the large proportions of low coordinate step/kink Au sites. It is to be noted that the electronic interactions between NaA and HAuCl4 facilitated the production of active Au nanoparticles. Our findings highlight that the physicochemical properties of the stabilizer can profoundly influence the reactivity of supported metal catalysts prepared by sol-immobilization. These observations highlight the influence that various stabilizers have on the morphology of supported metal nanoparticles and provides an explanation for the low activity of catalysts prepared using common forms such as PVA and PVP.