The Research On Catalytic Combustion Performance Of CH₂Cl₂ Over Ceria-titania-based Complex Metal Oxides

Chloride Volatile Organic Compounds (CVOCs) are dangerous substance with high toxicity and have become one of the main sources of atmospheric pollutants. Catalytic combustion is one of the most effective methods for the treatment of CVOCs and the greatest challenge of this technology is the development of high-performance catalysts. Recently, the traditional catalysts usually encountered some problems, such as incomplete oxidation, generation of higher toxic byproducts and deactivation of catalyst. In this paper, in order to develop a catalyst with better low-temperature activity, selectivity and anti-toxic properties, modified ceria-titania-based catalysts were extensively explored for the catalytic combustion of dichloromethane (DCM)-model of short-chain chlorinated VOCs. We expected that the work conducted herein could make useful exploration for the design and syntheses of catalysts and the solution of catalyst poisoning for catalytic oxidation of CVOCs.Firstly, the influences of the type of active ingredients and the form of titania base on DCM catalytic combustion were investigated. Among the tested catalysts, Ceria doped on hydrothermally synthesized TiO2 nanoparticle with a Ce:Ti mole ratio of 1:19 obtained total oxidation of DCM at 335℃ and exhibited stable DCM removal activity on 100 h long-time stability tests at 330℃ without any catalyst deactivation. However, CO was still dominated in the exhaust gases during the process.Secondly, the reaction mechanism and resistance to poisoning of Cl for Ce/TiO2 catalyst was studied. It was observed that TiO2 had abundant Lewis acid sites and was responsible for the adsorption and the rupture of C-CI bonds. However, TiO2 tended to inactivation because of chloride poisoning as the adsorption and accumulation of Cl species over the surface. The poisoning of Cl for Ce/TiO2 was inhibited to some extent by CeO2 due to the rapid removal of Cl on the surface of CeO2, which was verified by NH3-IR characterization. Based on the DRIFT characterization and the catalysts activity tests, a two-step reaction pathway for the catalytic combustion of DCM on Ce/TiO2 catalyst was proposed. The process started with the adsorption and rupture of C-Cl bonds and partial dissociation of C-H bonds on the TiO2 support. The second step included the desorption of Cl atom and the fracture of C-H from byproducts, which was achieved by the presence of ceria.In order to improve the oxidation degree of CO on Ce/TiO2 catalyst, methods such as optimizing the quantity of ceria, introducing transition-metal-oxide component and modifying the preparation method were progressed. The increase of ceria content and the introduction of Cu components could greatly improve the selectivity of CO2. However, these improvements led to the loss of surface acid sites and the DCM conversion decreased accordingly. In the catalysts synthesized by different methods, CeTi-S-1:1 catalysts synthesized by solid mixing method yielded the best catalytic performance and CO2 selectivity comparing with catalysts prepared by impregnation method and hydrothermal method. However, the yield of CO2 for the best sample was only 25.0%.A novel two-stage Ce/TiO2-Cu/CeO2 catalyst system with a remarkable catalytic activity was successfully designed for the deep combustion of DCM. The degradation efficiency reached up to 99.0% at 335℃ with less undesired CO, Cl2 and byproducts (CxHyClz). The DCM conversion and the yield of CO2 was maintained during prolonged test at 330℃.In this novel two-stage Ce/TiO2-Cu/CeO2 system, the rupture of C-Cl and the total oxidation of CO were physically isolated. This separated arrangement not only avoided the decrease of acid sites on (Ce+Cu)/TiO2 catalyst, but also prevented the chlorine poisoning of TiO2 due to the strong adsorption of Cl on the CuO. Based on the comparison of the catalytic performance and catalyst characterization of Ce/TiO2 and (Ce+Cu)/TiO2, a three-step degradation mechanism was proposed for this two-stage Ce/TiO2-Cu/CeO2 catalysts system.Finally, the process parameters for the two-stage catalysts system were observed and optimized. The best catalytic performance was achieved when the space-speed was in 4500-15000 h-1, the inlet concentration of DCM 500-1000 ppm and the concentration of oxygen 10.0% or more than 10.0%. The degradation efficiency reached up to 99.0% at 335℃ with less undesired CO, Cl2 and byproducts (CxHyClz). Moreover, the Ce/TiO2-Cu/CeO2 catalysts system also exhibited good resistance to water penetration.