The Studies Of HCHO Adsorption-Desorption And Surface Reaction Performance Over The Supported Silver Catalysts

Formaldehyde (HCHO) is an important volatile organic compounds (VOCs), which is known to show harmful effect on the human health and ecological environment. Therefore, the effective removal of formaldehyde is very important for the environmental protection field. In this paper, the HCHO adsorption-desorption and surface reaction properties were studied on the supported silver catalysts.To study the effect of the supports on the formaldehyde catalytic oxidation performances, the silver catalysts were supported on different oxide and zeolites supports (TiO2, MCM-41, SBA-15, NaY, SiO2) using incipient-wetness impregnation method. It could be concluded that there existed the well-dispersed Ag2O (below XRD detection limit), metallic Ag particles (Ag0) with subsurface oxygen species, some isolated silver ions (Ag+) and few Agnδ+clusters on the Ag based catalysts. The states, the dispersion and the amount of these silver species are obviously different on different supports. There were much more metallic Ag particles (Ag0) (8-10 nm) with subsurface oxygen species and well-dispersed Ag2O on Ag/MCM-41 and Ag/SiO2 samples. Well dispersed Ag2O, metallic Ag particles (Ag0) with subsurface oxygen species, some isolated silver ions (Ag+) and few Agnδ+clusters existed on Ag/NaY. The Ag/SBA-15 sample possessed well dispersed Ag2O, metallic Ag particles (Ag0) (about 5 nm) with subsurface oxygen species. Few well dispersed Ag2O, metallic Ag particles (Ag0) (about 3 nm), some Agnδ+clusters existed on Ag/TiO2. Ag/MCM-41 showed the excellent low temperature adsorption-desorption and surface reaction performances.The HCHO adsorption-desorption and surface reaction performances were studied over Ag/MCM-41 with different silver loading. Based on the experimental results, silver particles with subsurface oxygen species (about 8-10 nm) showed the best HCHO adsorption-desorption and surface reaction performances. The new desorption peak for HCHO moved to lower temperature from 90℃to 77℃when the silver loading was changed from 2 wt.%to 8wt.%. When the silver loading further increased, the desorption temperature shifted to higher temperature (from 77℃to 100℃), meanwhile its desorption amount sharply decreased.8 wt.%Ag/MCM-41 catalysts showed highest surface reaction activity. Thus an appropriate silver loading and particle dispersion will be essential to obtain high catalytic activity for HCHO oxidation at low temperatures.The HCHO adsorption behavior over supported silver catalysts was studied by TPD and FT-IR techniques to investigate the HCHO catalytic oxidation mechnism. It was found that the adsorbed species and their amount showed an important role in HCHO catalytic oxidation. There existed four kinds of intermediates, conclding molecular HCHO, dioxymethylene (DOM), formate ions and polyoxymethylene (POM) during the HCHO adsorption process on the silver catalysts. The silver species accelerated the conversion of dioxymethylene to formate ions which promote the formaldehyde catalytic oxidation elimination.