Oxidation of CO Catalysed by Nanometric Mono and Bi-Noble Metals on Various Oxide Supports

Abstract

Air pollution results mainly from the injurious smoke emitted by cars, buses, trucks, factories etc. This smoke contains carbon monoxide, unburned hydrocarbons and oxides of nitrogen and sulphur. In case of cars, catalytic converter is used to convert the noxious gases generated by the engine into harmless emissions, which are released from the exhaust. The catalyst used in the converter comprises of precious metals such as palladium, platinum and rhodium. But, due to the high cost of these metals, the cost of converter increases significantly. Hence the design and development of an active, stable, and cost-effective catalyst is a must in order to meet the commercial requirements. The objective of this work was to develop active, stable and cost-effective nanocatalysts for CO oxidation reaction. Oxides used as support for dispersion of noble metals were selected from the different families of materials. In the first work, different morphologies of CeO2, namely nanorods with exposed (110 + 100) planes, nanocubes with (100) planes, and nano octahedra with (111) planes were synthesized using a hydrothermal method. Platinum was nucleated on these morphologies using a microwave-assisted method and these nanohybrids were tested for CO oxidation with the help of in house gas chromatography (GC) setup. CeO2/Pt with nanorods as support was found to be the most active catalyst. With the aid of XPS and IR spectroscopy techniques it was concluded that the presence of oxygen-deficient sites and the formation of the least stable carbonates contribute to the superior performance of these nanorods. CO oxidation was also tested using Au/ZnO nanorod catalyst with different loadings of gold. Solvothermal route was used to synthesize ZnO nanorods followed by microwave assisted method to nucleate gold. It was found that the catalyst with lower loading of Au was stable in terms of performance. Fractional conversion of CO with respect to weight of the catalyst was observed. The effect of varying the concentration of CO keeping the concentration of O2 fixed on the CO conversion was studied; similarly, the effect of varying the concentration of O2 keeping the concentration of CO fixed was studied. A DRIFTS study helped in explaining the observations made. The effect of iso-valent substitution on the performance of SrTiO3 support for CO oxidation was tested. Substituted perovskite, Sr1-xBaxTiO3 (x= 0, 0.1, 0.25, 0.5, 0.75, 0.9 and 1), supports were synthesized and the support with x= 0.5 was found to convert CO to CO2 at a much lower temperature than the other compositions. XPS analysis for this support confirmed the presence of the Ti3+ oxidation state, which contributed to better performance of this support. Mono-metallic and bi-metallic catalysts were synthesized using this composition as support. In case of the bi-metallic nanocatalyst, the issues of activity and stability were overcome, and the reason for this was attributed to the synergistic effect between the metals. The effect of time and temperature on the growth of SrTiO3 nanoshapes was studied. In case of the ex situ approach, surface faceting along with the growth of internal voids as a function of increase in heating time during reaction was observed with the help of TEM imaging, and XPS analysis provides information about the availability of two different Ti4+ oxidation states resulting from TiO2 and SrTiO3. In situ heating in the TEM helped in observing real time changes in the shape and position of internal cavities. It was observed that internal cavities tend to merge together at lower heating temperatures attaining a stable shape, in line with the external surface at higher temperatures. It was observed that the external surface was stable till 800 ºC, and then surface modification was observed at the corners of the nanoshape. β- MnO2 is generally known to be the least active polymorph of MnO2 due to its phase structure. In this work, the reducibility efficiency of MnO2 was altered with the help of Pt nucleation. TEM imaging confirmed the nucleation of ultrafine Pt nanoparticles. This modified support was tested for CO oxidation and it was observed that the temperature for the full conversion of CO drops significantly. This was attributed to the strong interaction between Pt and MnO2.