The Ruthenium Catalyst Of Low-temperature Catalytic Combustion Of Volatile Organic Compounds Containing Chlorine

Volatile chlorinated organics have extensive applications in the industrial production, but their residues are considered as pollutants leading to long-term contamination of the environment, destroying the ozone layer in the atmosphere. Among various available technologies treating the CVOCs as pollutant, catalytic combustion as one of the effective methods has the characteristics such as treating low concentration of CVOCs at low temperature with low energy consumption and good efficiency in simple process installation. Nevertheless, there are still problems to be solved, such as the short life of the employed catalysts and the formation of polychlorinated compound during this technological process. Research on the catalysts of high-performance is still necessary in the catalytic combustion of CVOCs, a promising technology in realization of environmental protection.In this paper, Ru was adopted as catalysts in study of the catalytic combustion of CVOCs. By using catalytic combustion of dichloromethane (DCM) as a model of chlorinated alkanes, the Ru/Al2O3, Ru/Ce-Al2O3and Ru/TiO2catalysts were studied in terms in catalytic performance and mechanism in DCM catalytic combustion. Some achieved results make contribution to the scientific fundamentals for development of the catalysts of high-activity and long life span available for low-temperature catalytic combustion of CVOCs. The results achieved in this paper are listed as follows:1. Low-temperature Combustion of DCM over Ru/Al2O3The Ru/Al2O3catalysts were prepared by incipient wetness (IM) and co-precipitation (CP). Characterization by XRD, H2-TPR, XPS and BET indicates that Ru/Al2O3-CP catalyst possesses larger BET surface, more acid sites and smaller RuO2particles. The catalytic combustion of DCM was carried out over the synthesized the Ru/Al2O3catalysts in the feed stream of750ppm DCM,10%O2with the balance of N2. The experimental results indicate that all the catalysts exhibited good activity with complete conversion at350℃. Of these two catalysts, Ru/Al2O3-CP presents more highly active and90%conversion occurs only at308℃. Main products from oxidation were CO2, H2O, HC1and Cl2, but trace CH3Cl and CHCl3were formed at higher temperature. The activity of Ru/Al2O3catalyst cannot be maintained below285℃due to Cl deposition. With raising temperature up to285℃, the removal of Cl species from Al2O3can be promoted by Ru species in the form of HC1and Cl2.2. The effect of Ce on Catalytic decomposition of DCM over Ru/Al2O3Ce-Al2O3mixed oxides catalysts with various Ce content were prepared by co-precipitate, then RuO2catalysts supported on Ce-Al2O3were prepared by incipient wetness with RuCl3aqueous solution. The characterization by XRD, N2adsorption, TEM/HRTEM, H2-TPR. Raman. NH3-TPD and XPS shows that there exists the interaction between Ce species and Al2O3, resulting in the formation of a considerable amount of Ce+-O-A1species. The incorporation of Ru makes Ce3+-O-Al species disappear. As a result, Ru species enter CeOx lattice and the dispersion of Ru species increases. The oxygen mobility of Ru/Ce-Al2O3increases greatly and the acidity increases to some extent. During the catalytic decomposition of DCM, Ru/Ce-Al2O3catalysts present an outstanding catalytic activity, of which, Ru/7%Ce-Al2O3catalyst is most active, and its Tso and T90are247and284℃. The investigation of reaction kinetics shows that DCM decomposition over Ru/Ce-Al2O3catalyst is largely dependent on gas oxygen concentration. High stable activity of Ru/Ce-Al2O3catalysts can be ascribed to the synergistic effect of high acidity, high oxygen mobility of CeOx and the stability of RuOx exposed to Cl species.3. Catalytic decomposition of CH2Cl2over supported Ru/TiO2catalystThe TiO2catalysts were prepared by hydrothermal, sol-gel and hydrolysis methods. RuO2catalysts supported on TiO2was prepared by incipient wetness with RuCl3aqueous solution. RuO2species dispersed on TiO2-hy composed of88%anatase and12%brookite presents high activity with50%and90%conversion at235℃and267℃, respectively, lower than Ru/Al2O3and other all catalysts reported in literatures. The dependence of reaction rate on DCM concentration on Ru/TiO2-hy is close to first order, and hence, high efficiency of DCM decomposition can be obtained within the range of DCM concentration of750-6000ppm. The interface between RuO2and TiO2is where the most activity sites lie, and the removal of Cl species produced during DCM decomposition in the form of HC1and Cl2is readily, probably due to the formation of oxychloride RuO2-xClx. Ru/TiO2catalyst, because of high activity of DCM combustion and high stability, become useful in the practice of removal of alkyl chlorides.A mechanism consisting of six elementary steps is schematized:(1) DCM absorbs on active hydroxyl groups, leading to the formation of chloromethoxy species and HC1;(2) chloromethoxy species is transformed further into a hemiacetal specie and HC1;(3) hemiacetal specie is decomposed to CO;(4) oxygen is dissociated into active oxygen species by which CO is oxidized into CO2on Ru species;(5) HC1transfers to Ru species where chlorination occurs and oxychloride RuO2-xClx is formed;(6) Cl species of RuO2-xClx reacts with active oxygen through Deacon process to form Cl2and H2O at high temperature and Cl species escapes from RuO2-xClx in the form of HCl.