Novel photodeposited catalysts for carbon monoxide oxidation and preferential oxidation of carbon monoxide in the presence of hydrogen (PROX)

It has been shown that gold nanoparticles supported on metal oxides with sizes below 5 nm exhibit unique catalytic properties for CO oxidation, water-gas shift, and epoxidation of propylene. In particular, Au/TiO2 exhibits CO oxidation activity at temperature as low as 90 K. Recent research reported that silver nanoparticles supported on TiO2 are similar to Au nanoparticles supported on TiO2. Conventional methods for preparing Ag/TiO 2 and Au/TiO2 supported catalysts include deposition-precipitation, wet impregnation and co-precipitation. Among these methods deposition-precipitation has shown the best results for these reactions due to the narrow size distribution of Ag and Au. The major disadvantage of the deposition-precipitation method is that Au and Ag particle sizes are very sensitive to precursor concentration. Preparation of Au particles less than 5 nm requires a low precursor concentration and low Au loading. In this work, a different approach, photodeposition, was used to prepare Ag and Au nanoparticles supported on TiO2. The main shortcoming of photodeposition is that its efficiency is very sensitive to both bulk and surface structural features. However, such problems can be overcome by reducing the size of TiO2 particles to the nanoscale.;Photodeposition of Ag nanoparticles on commercial TiO2 particles and nanoparticles was performed on microscope grid-supported samples in order to provide direct visualization of the spatial distribution of photoactive sites on sub-micron-scale and nanoscale TiO2 particle surfaces and to create materials for potential catalytic applications. HRTEM (high-resolution transmission electron microscopy) and HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) were used to characterize these materials. The size and spatial distributions of the Ag nanoparticles on the commercial TiO2 were not uniform; the concentration of Ag was higher on grain boundaries and at the edges of these submicrometer particles. In the case of TiO2 nanoparticles, the size distribution of the Ag nanoparticles deposited was relatively uniform and independent of irradiation time and photon energy. The amount of Ag deposited on TiO2 nanoparticles was at least 6 times higher than that on commercial samples for comparable irradiation conditions. Compared to the case of Ag photodeposition, the difference in the amount of Au photodeposited on TiO2 particles and nanoparticles was even greater, especially at low precursor concentration. Photodeposition on TiO2 nanoparticles is suggested as a potential method for the preparation of Ag/TiO2 and Au/TiO2 catalysts. The high photoreactivity of TiO2 nanoparticles makes them an ideal material for preparation of supported metal catalysts when highly uniform nanoscale particle size is required.;Extending the previous work, the Ag and Au photodeposition on TiO 2 nanoparticles were scaled-up from micro scale to laboratory scale to generate sample powders in sufficient quantity to be utilized in catalytic reactor. Most of the methods published for the preparation of metal-decorated TiO2 by photodeposition involve suspended particles in a aqueous media. Our approach for the scale-up process was different, and was developed based on the sample preparation for metal photodeposition on microscope grids. A thin film of TiO2 nanoparticle powders coated on a glass plate was obtained by using a spin-coating technique before metal photodeposition. Several commercial TiO2 nanoparticles were used in this study, in order to choose appropriate samples for the scale-up experiments. Preliminary studies of the photoreactivity of commercial TiO2 nanoparticles for Ag photodeposition on a microscope grid were carried out prior to the scale-up process. The effects of Ag photoreduction conditions, in particular the radiation source and the metal precursor solution concentration, on Ag photodeposition were investigated to determine the optimum conditions for the scale-up process. The size and spatial distribution of Ag nanoparticles, as well as the Ag loading on commercial TiO2 nanoparticles for the scale up process under various conditions were determined using TEM and HAADF-STEM techniques.;After the optimal conditions for the scale-up process were established, Ag/TiO2 and Au/TiO2 nanoparticles obtained from the scaled-up process were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS) and HAADF. In addition, Ag/TiO2 and Au/TiO2 nanoparticles were also prepared using traditional deposition-precipitation methods. The catalytic performance of Ag/TiO2 and Au/TiO2 nanoparticles prepared using two different methods for CO oxidation and preferential oxidation of CO in the presence of H2 (PROX) were investigated using a high-throughput reactor. Results obtained from this study showed that the size of Ag particles, Ag loading, as well as preparation method contributed to the difference in activity of Ag/TiO2 for CO oxidation and PROX. In the case of Au/TiO 2 the difference in activity for CO oxidation and PROX is mainly due to the preparation method and Au loading.