Catalytic Oxidation Of Formaldehyde At Low Temperature Over Supported Nano Gold Catalysts

Formaldehyde is one of the most serious indoor pollutants, which is harmful to the human health and air condition. Thus, elimination of formaldehyde in the indoor air at the ambient temperature has attracted much attention. Among the present methods of formaldehyde elimination, catalytic oxidation technique is desirable because it can decompose formaldehyde to harmless CO2 and H2O at low temperature and atmospheric pressure. In the field of catalytic oxidation of formaldehyde, supported gold catalysts are promising for their excellent catalytic oxidation properties. However, the reaction temperature for complete oxidation of formaldehyde over gold catalysts is still high, while the nature of the catalysts remains unclear. In this work, a series of supported nano gold catalysts were prepared by a deposition-precipitation method and tested for formaldehyde oxidation. The effect of support and the reaction mechanism were discussed. The detailed results are as follows:1. The effect of Au loading on the catalytic properties of Au/CeO2 catalysts. It was found that the particle size of Au increased with increasing Au loading. The reactivity reached its maximum at the Au loading of 2%, with complete oxidation of HCHO to CO2 and H2O at 150℃. It was due to the fact the catalyst contained more active sites and less aggregated Au particles.2. The effect of support on the catalytic properties of gold catalysts. A series of gold catalysts were prepared by a deposition-precipitation method and an ammonia-complex method on different metal oxides (Al2O3, TiO2, CeO2 and SiO2) and tested for low temperature HCHO oxidation. It was found that among these catalysts, the Au/CeO2 catalyst exhibited the highest reactivity, with 80% of conversion obtained even at 40℃. It was found that the reactivity of HCHO oxidation was influenced by the chemical state of Au species and the properties of the supports. The high activity obtained on the Au/CeO2 catalyst was probably due to the presence of cationic Au species (Au3+) in the catalyst and the formation of AuxCei1-xO2-δsolid solution, which generates oxygen vacancies and activates the oxygen species and consequently enhance the reactivity.3. The effect of specific surface area of CeO2 on the catalytic properties of gold catalysts. It was found that the reactivity of the catalyst was enhanced by increasing surface area. For example, on a Au/CeO2-270 catalyst with the highest surface area of 270 m2/g, HCHO was completely decompsed into CO2 and H2O at 50℃. The characterization results (Raman, H2-TPR and XPS) indicated that the high surface area Au/CeO2 catalyst contained higher concerntration of oxygen vacancies and cationic Au species compared to that with low surface area, which may faciliate the adsorption of HCHO and the activation of active oxygen species, thus to promote the reactivity.4. The possible reaction mechanism for the catalytic oxidation of formaldehdye over Au/CeO2 catalyst. It was proposed that the HCHO molecule adsorbed on the catalyst, then reacted with the active oxygen species to form carbonate species; finally, the carbonates could be decomposed into gaseous CO2 and H2O in the mild temperature.