Catalytic Oxidation Of Formaldehyde At Ambient Temperature Over Alkali Promoted Pt Catalysts

Formaldehyde is a kind of carcinogenic substance, and it strongly threatens the safety of human life when its concentration in air exceeds a definite amount. Therefore, it is of great importance to reduce the formaldehyde in indoor air. Catalytic oxidation is considered to be the most effective method for this purpose, and design and research of the catalysts with high activity and catalytic stability at low temperature is a common target of the researchers. It’s also a burning question to clarify the reaction route and the role of each catalyst component in the research field. As water vapor is coexisted in the indoor circumstance with formaldehyde, investigating the effect of water on the activity of catalyst for formaldehyde oxidation is also important in the view of application.In this thesis, the catalytic activity of xPt/TiO2and xPt-Na/TiO2catalysts for formaldehyde oxidation in the system of600ppm HCHO+18.9%O2/N2was investigated at25℃. It was found that the activity of catalyst can be significantly improved by adding alkali-metals to xPt/TiO2in the catalyst preparation, particularly in the case of modifying the xPt/TiO2with sodium.The precursor of sodium being added to the catalyst and the reducing method were optimized. The results indicate that NaNO3is better than Na2CO3and Na2SO4when it is used as the sodium precursor modifying the catalyst, and the Na2O·Pt/TiO2catalysts prepared by9%H2/N2reduction are more active than those reduced whichever by HCHO, HCOOH, or NaBH4in aqueous solution. Due to the promotional effect of3wt.%Na2O, the HCHO conversion over the3%Na2O·0.5%Pt/TiO2catalyst, which is obtained from the optimal preparation conditions, drastically increased from22%to100%when the gas mixture of600ppm HCHO+18.9%O2/N2was fed to the catalyst bed at25℃with gas hourly space velocity (GHSV) of60000mL/g-h.It was found that the water vapor being present in the gas mixture not only promotes but also stabilizes the activity of the3%Na2O·0.5%Pt/TiO2catalyst. The formaldehyde in the gas mixture of600ppm HCHO+2.8%H2O+18.9%O2/N2at GHSV of90000mL/g-h was100%removed at ambient temperature over the catalyst. No formaldehyde conversion declining with time on stream was found in the tested period of7h. In situ FTIR reveals that the formaldehyde adsorption over the0.1%Pt/TiO2and3%Na2O·0.1%Pt/TiO2leads to formation of dioxymethylene (DOM) species and consumption of hydroxyl groups of the catalysts. It also reveals that some of the hydroxyl groups are released when the DOM species are further converted to formate intermediate species. It was found that the addition of Na2O component to0.1%Pt/TiO2significantly accelerates the formation rate of formate intermediate species during the formaldehyde adsorption. Based on the characterization, it was proposed that the hydroxyl group increasing on the catalyst and the C-H bond weakening of the DOM species being caused by the Na2O component addition to0.1%Pt/TiO2are responsible for the above improving result. Moreover, it was found that water vapor enhancing the aimed reaction only works over the Na2O doped Pt/TiO2catalyst, which can be explained as well by the hydroxyl groups being increased on the catalyst.In addition, the function of water vapor in the formaldehyde oxidation process was also studied in detail by the FTIR. It was proposed and demonstrated that water vapor improves not only the formation of formate species but also the rate-determining step converting the species towards to the final product CO2. Just because of the improving effect on the step of formate species oxidation, water vapor being present in the feed gas can effectively prevent the3%Na2O·0.5%Pt/TiO2catalyst from deactivation caused by the accumulation of formate species on the catalyst surface.