Low Temperature Catalytic Oxidation Of Formaldehyde Over Supported Gold Catalysts

Formaldehyde has become one of the most important and serious indoor air pollutants, which is emitted from the widely used constructive and decorative materials. Long-term exposure to indoor air containing of HCHO may be detrimental to human health, resulting in diseases such as cancer and respiratory tract tumors. Thus, great efforts have been made to reduce the indoor emission of HCHO. There are many methods for formaldehyde elimination, but most of them have disadvantages. Among the methods for formaldehyde abatement, catalytic oxidation of HCHO using heterogeneous catalysis appears to be a promising technology. Catalysts applied for HCHO oxidation include supported transition and noble metals, among which supported Au catalysts have attracted much attention. Facts affecting the reactivities of Au catalysts include preparation methods, type of supports and morphology of Au particles, and so on. However, the effects of interactions between Au and the supports, oxygen vacancy on the supports, morphology and oxidation states of Au species on the catalytic performance of the catalysts have not been studied in detail. Therefore, in this work, a series of Au catalysts supported on CeO2-SiO2were prepared by a deposition precipitation method and tested for formaldehyde oxidation. The catalytic behaviors were correlated to the Au particle size deduced by CeO2entities and the redox properties of the catalysts. In addition, Au catalysts supproted on rare earth elements doped CexM1-xO2-d mixed oxides (M=Zr, La, Pr) were tested for formaldehyde oxidation and the effects of support modification on the catalytic performance were investigated. The main contents of the thesis are as follows:1. The effect of CeO2dispersion on oxidation of formaldehyde over Au/CeO2-SiO2 catalysts.A series of CeO2-SiO2supports with different particle sizes of CeO2were prepared using an impregnation method and supported gold catalysts were prepared by a deposition-precipitation method. The catalysts were characterized by X-ray diffraction, inductively coupled plasma, N2adsorption, temperature-programmed reduction, transmission electron microscopy and Raman spectroscopy techniques. It was found that with the CeO2grain and Au particle sizes decreased with increasing dispersion of CeO2, and Au species with high oxidation states (Au+/Au3+) were formed. It was also found that a0.8Au/0.06CeO2-SiO2catalyst exhibited the best catalytic activity, which could be associated with its redox properties derived from the oxygen vacancies, Au+/Au3+species in the catalyst, as well as the contact interface between the small Au entities and CeO2support.2. The effect of rare earth elements doping on oxidation of formaldehyde over Au/CexM1-xO2-d catalysts.A series of CexM1-xO2-d mixed oxides doped with Pr, Zr, La were prepared using a sol-gel method, and Au/CexM1-xO2-d catalysts were prepared by deposition precipitation method and tested for formaldehyde oxidation. It was found that the Zr-doped2.4Au/Ce0.9Zr0.1O2-d catalyst had higher activity for the reaction compared to the undoped Au/CeO2catalyst. In addition, a series of Au/CexZr1-xO2-d catalysts with different Zr contents were prepared and tested for oxidation of formaldehyde. It was found that the2.4Au/Ce0.9Zr0.1O2exhibited the highest activity, and the reactivities of the catalysts decreased with the increasing Zr contents in the catalyst. This was probably due to the changes of surface area and grain sizes of the support with Zr content in the catalyst, which consequently affects the redox property and the nature of the Au species. The TPR results showed that the redox property of the2.4Au/Ce0.9Zr0.1O2was higher than the high amount doped catalyst, and the TEM results showed that the particle size of Au in the low Zr content catalyst was significantly smaller than those in other catalysts.