HCHO Adsorption-Desorption And Catalytic Oxidation Performance Over Ag Based Catalysts

Formaldehyde (HCHO) is a kind of serious indoor air pollutant, which is harmful for the environment and human health. Therefore, the HCHO elimination becomes the hot issue. The catalytic oxidation technology is known as the effective method to eliminate HCHO in indoor air due to no secondary pollution, easy operation and low energy consumption. Low-cost Ag catalyst is identified as a potential catalyst due to its high catalytic performance in many oxidation reactions. However, the catalytic activity of Ag catalysts for HCHO catalytic oxidation at low temperatures is still low. Therefore, it is challenge to prepare Ag catalyst with high activity at low temperatures for HCHO catalytic oxidation. In order to solve this problem, the effects of support and pretreatment conditions, HCHO adsorption and oxidation path over Ag catalysts, as well as the HCHO catalytic oxidation performance in AgCo bimetallic catalysts are investigated in this work. The obtained results are listed below:(1) The HCHO adsorption and desorption performance in different supports are studied by HCHO-TPD and HCHO adsorption breakthrough curves. It is indicated that the surface areas and pore structures affect the HCHO adsorption-desorption performance in different support materials. The higher surface areas are, the stronger HCHO adsorption capacity is. The smaller pore size is, the narrower pore size distribution is, the more difficult HCHO desorption is. Compared with the other supports, MCM-41, with high surface areas and well defined mesopore channel, has the highest HCHO adsorption capacity. It is noteworthy to point out that the Ag catalysts supported on MCM-41increase the HCHO adsorption at low temperature. Therefore, MCM-41is a possible support material for HCHO catalytic oxidation over Ag catalysts.(2) The HCHO adsorption and oxidation mechanism with time and temperature is systematically investigated by in situ FT-IR and HCHO-TPSR methods on pure MCM-41and Ag/MCM-41catalysts. It is found that the Ag-O structure would increase the HCHO adsorption and activation performance then improve the formation of formate and adsorbed CO species. The presence of surface hydroxyl groups offers a new path for the formation of formate species over Ag/MCM-41catalysts. The formation and conversion of formate and adsorbed CO species is the rate control step for HCHO catalytic oxidation reaction over Ag/MCM-41catalysts. Moreover, the generation of CO2over Ag/MCM-41is considered as the paralleled pathways:1) by the reaction of adsorbed CO species, which produced formate species dissociation;2) by the reaction of formate species directly. The surface hydroxyl groups and Ag-O structure are the important factors to improve the HCHO catalytic oxidation activity over Ag/MCM-41catalysts. (3) The effect of oxygen pretreatment on HCHO adsorption and catalytic oxidation of Ag/MCM-41catalysts has been investigated from500℃to800℃. The formation of Ag2O clusters is strongly related to the O2pretreatment temperature. With the increase of O2pretreatment temperature from500℃to700℃, the Ag2O clusters occur the diffusion from outside to inside of the channels, and the amount of Ag2O clusters inside the channels becomes maximum at600℃. It is suggested that Ag2O clusters inside of MCM-41pores improve the HCHO adsorption capacity and accelerates the formation of HCHO adsorption intermediates, particularly DOM ad-species. Importantly, it is found that the more small Ag nanoparticles (<5nm) coexist with large particles when the catalyst is pretreated at700℃. And the formed silver particles with smaller size at700℃plays an essential effect for the formation of formate intermediate and surface reaction activity enhancement.(4) Highly active bimetallic AgCo/APTES@MCM-41catalyst for HCHO catalytic oxidation is prepared by two step chemical reduction method. It is indicated that the HCHO catalytic activity in AgCo bimetallic catalysts is higher than any other monometallic Ag and Co catalysts. Complete oxidation of formaldehyde is achieved at90℃over the bimetallic catalysts with Ag/Co=3/1(mass ratio). It is lower than monometallic Ag catalysts(150℃). The formation of the strong metal-metal interaction (SMMI) between Ag and Co in the bimetallic catalysts due to the electron transfer between each other results in the good low-temperature reducibility of Co oxides and the formation of more surface active oxygen species (Osurf). All these factors give rise to the high formaldehyde catalytic oxidation activity in AgCo bimetallic catalysts.