Deposition of Gold Nanoparticles on Niobium Pentoxide with Different Crystal Structures for Room-Temperature Carbon Monoxide Oxidation

CO Oxidation at Low Temperatures over the Au Cluster Supported on Crystalline Silicotitanate

Crystalline silicotitanate (CST) with a sitinakite structure has attracted considerable attention as an ion exchanger because of its anionic surface owing to SiO4 tetrahedral and TiO6 octahedral linked structures. In particular, the anionic surface of CST is advantageous in immobilizing single Au atoms derived from a cationic precursor such as Au(en)2Cl3 (en = ethylenediamine). In this study, a Au/CST catalyst was prepared and evaluated for CO oxidation. The transmission electron microscopy and X-ray photoelectron spectroscopy results suggest that Au single atoms and clusters (Auδ+, 0 < δ < 1) were supported on Na+-CST particles. The 1.0 wt % Au/CST catalyst yielded high catalytic activity for CO oxidation, where 50% CO oxidation was achieved at a low temperature of -13 °C. In the low-temperature region (from -84 to 20 °C), CO oxidation over Au/CST may progress through the well-known Langmuir-Hinshelwood mechanism. Conversely, the experimental results of CO oxidation without O2 confirmed that the reaction proceeded according to the Au-assisted Mars-van Krevelen mechanism using the lattice oxygens of the CST particles at temperatures above 20 °C. Therefore, this work contributes to the design of highly dispersed single-atom or cluster catalysts using the anionic surface of the microporous CST.

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

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.

Ultrafine-clustered metallic Pt species are considered to be more active than oxidized Pt single ions for CO oxidation reaction.