Ultra-fine platinum species supported on niobium pentoxide for CO oxidation

Abstract

Platinum oxide supported on a Lewis acid niobium oxide (Nb₂O₅) support has been used for various heterogeneous and homogeneous catalysts. In this work, we used urea as a precipitating agent to obtain crystallized Nb₂O₅ with high surface area via a hydrothermal route. Nb₂O₅-supported Pt catalysts were subsequently synthesized via an incipient wetness impregnation approach. Multiple characterizations including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption confirmed the identical structural and textural properties of the Nb₂O₅ support before and after the impregnation process. Furthermore, the X-ray absorption fine structure technique (XAFS) results with related data analysis indicate that the platinum species in the fresh and H₂-pretreated samples were in the form of single atoms or ultrafine clusters. In addition, the decrease in coordination number (CN) of the first-shell Pt–O bond, as well as the formation of Pt–Pt contribution with very low CN, after H₂-pretreatment was verified, which corresponds to the decrease of oxidation state for Pt species on the surface of supports. Thus, the ultrafine-clustered metallic Pt species are considered to be more active than the oxidized Pt single ions. The current results will be of great significance in controllable synthesis of active Pt-based catalysts for other catalytic oxidation reactions.