Catalytic Formaldehyde Removal By "Storage-oxidation" Cycling Process And Catalytic Oxidation At Room Temperature

Formaldehyde is one of the most common and most toxic indoor gaseous pollutants. Long-term exposure to indoor air containing HCHO may cause adverse effects on human health. Significant efforts have been made to remove low concentration of HCHO.Physical absorption and heterogeneous catalytic oxidation are regarded as the most promising HCHO removal technology. However, HCHO oxidation requires the use of elevated temperatures for most of the catalysts reported to date, though several supported noble metal catalysts have been demonstrated to be effective for HCHO oxidation at room temperature. The way of removal of indoor HCHO by adsorption is limited for a short life time and the absorbents’regeneration.In this work, a novel "storage-oxidation" cycling process was applied to remove HCHO based on a bi-functional catalyst. The bi-functions of the catalysts should include storage and partial oxidation of HCHO into the formates during the storage period at room temperature; and complete oxidation of the stored HCHO into CO2and H2O at elevated temperatures. Through the "storage-oxidation" cycling process, the catalyst could be in situ regenerated and the problem of competitive adsorption of H2O with HCHO was resolved. In addition, the different types of oxides supported Au catalysts prepared by deposition-precipitation and co-precipitation were investigated as catalysts for complete oxidation of HCHO at room temperature. The results were summarized as follows:(1) Catalytic removal of indoor HCHO was proposed to proceed in a "storage-oxidation" cycling process. At room temperature, HCHO is partially oxidized into the formate species over non-noble metals and stored on the catalyst. When the catalyst reaches saturation, it is regenerated in situ by heating, and the stored formate species are completely oxidized into CO2and H2O. Due to the highly dispersed silver clusters formed and its good redox properties, the Ag-MnOx-CeO2catalyst showed better HCHO oxidation properties in both the storage phase (HCHO partial oxidation to the formate at room temperature) and oxidation-regeneration phase (total oxidation of the formates into CO2and H2O at elevated temperatures). The presence of H2O (RH=50%,25℃) was found to enhance the HCHO storage capacity for Ag-MnOx-CeO2catalyst, while competitive adsorption of HCHO with H2O was observed over Ag/HZSM-5catalyst. The results of DRIFTS indicate that the partial oxidation of HCHO into the formate is accelerated by the presence of H2O over the Ag-MnOx-CeO2catalyst. The breakthrough time of the Ag-MnOx-CeO2catalyst was-20h (1275min) under the condition of17ppm HCHO/21%O2/H2O(RH=50%,25℃))/N2. Moreover, the "storage-oxidation" capacity of the Ag-MnOx-CeO2catalyst remained virtually unchanged for the five test cycles.(2) Two kinds of Au/CeO2, prepared by deposition-precipitation (DP) using urea (U) or NaOH (N) as precipitants were investigated as catalysts for HCHO oxidation. H2-TPR and XPS techniques were used to characterize the Au/CeO2samples. Due to the generation of increased amounts of active surface oxygen species resulting from the strong Au-Ce interaction, the Au/CeO2(DPU) catalyst showed higher activity than the Au/CeO2(DPN) catalyst, achieving100%conversion of HCHO into CO2and H2O at room temperature, even in the presence of water and at high GHSV (143,000h-1); moreover, the conversion was stable for at least60h. The reaction mechanism and the rate limiting steps for HCHO oxidation over the Au/CeO2catalysts were identified by means of in situ DRIFTS studies. The influence of oxygen and water on the formation and consumption of the formate reaction intermediates was also investigated.(3) FeOx-supported Au catalysts prepared by co-precipitation (CP) using Na2CO3as precipitant were investigated for catalytic HCHO oxidation. The applied calcination temperature was found to greatly influence both the chemical properties and microstructure of the catalysts. Characterization using XRD, H2-TPR and XPS suggested that lower calcination temperature improves the reducibility of the catalysts, and favors the presence of surface hydroxyl groups. Consequently, an Au/FeOx catalyst calcined at200℃afforded100%conversion of HCHO into CO2and H2O at room temperature and under humid air. In situ DRIFTS studies suggested that the moisture was essential for deep oxidation of the formate intermediates into CO2and H2O, this being the rate limiting step for catalytic HCHO oxidation.(4) Au supported on γ-Al2O3prepared by deposition-precipitation (DP) using urea is found to be a highly active catalyst for the total oxidation of HCHO at room temperature under humid air, without the need for a reducible oxide as support. In-situ DRIFTS studies suggested that the surface hydroxyl groups played key role in the partial oxidation of HCHO into the formate intermediates, which can be further oxidized into CO2and H2O with participation of nano-Au. This study challenges the traditional idea of supporting noble metals on reducible oxides for HCHO oxidation at room temperature.