Catalytic Oxidation Of CVOCs Over CeO₂-CrO_x Mixed Oxide Catalysts

As a significant fraction of toxic air containments, chlorinated volatile organic compounds (CVOCs) not only have extremely serious effect on the environment, but also damage human body a lot. With the development of industry, chlorinated VOCs are widely applied in chemical process due to their favorable dissolubility and reactionlessness. However, considerable attention should be paid to the terrible environmental pollution which chlorinated hydrocarbons lead to. Therefore, legislation has been introduced in many countries setting very low emissions limits in process exhaust gases in recent years. Among various methods of chlorocarbons disposal, catalytic oxidation has been perceived as one of the most promising ways to destruct CVOCs, due to its low energy consumption and no production of more toxic by-products. So many researchers have focused on the relevant catalytic materials these years.In this paper, CeO2-CrOx mixed oxides were prepared by coprecipitation method to study systematically the influences of different Ce/Cr molar ratios on the catalytic performance for DCE decomposition. Based on the above results, further investigation of the effect of different preparation methods and conditions on the catalytic performance for DCE destruction was evaluated. Moreover, the mechanisms of catalytic decomposition of DCE, DCM, TCE and CB over CeO2-CrOx catalysts were also studied, by in-situ FTIR technique. The texture-structure, surface acidity distribution and redox properties of these catalysts were characterized by XRD, BET, XPS, UV-Raman, SEM, H2-TPR. NH3-TPD and DCE-TPSR techniques. Some specific conclusions from this work are drawn as follows:1. Catalytic performances for DCE destruction over CeO2-CrOx catalysts with different Ce/Cr molar ratios were evaluated. The results indicate that CeOi-CrOx mixed oxides with different Ce:Cr molar ratios showed higher performance for the removal of DCE compared with pure CeO2. The CeO2-CrOx mixed oxide with Ce:Cr molar ratio of2:1exhibited the highest catalytic activity of the samples, generating only small amounts of chlorinated by-products during the decomposition of DCE. Therefore, the composition of Ce-Cr with an appropriate molar ratio increased the mobility of reactive oxygen species, reducing ability, surface acid content and the ratio of strong/weak acidity, promoting the adsorption, activation and deep oxidation of DCE as well as the durability of the catalysts.2. Coprecipitation (supercritical-drying), microemulsion, sol-gel and homogeneous precipitation were chosen to synthesize CeO2-CrOx catalysts (Ce/Cr-2/1), on which the catalytic performance of DCE decomposition was investigated. It is found that CeO2-CrOx mixed oxides prepared by coprecipitation (supercritical-drying) method have more Cr6+species and oxygen vacacies as well as the higher ratio of strong/weak acidity and reducing ability, to promote the further deep oxidation of DCE,which exhibits best performance for DCE decomposition. For further study, the effect of the pH and drying conditions during coprecipitation method on the catalytic performance of CeO2-CrOx catalysts (Ce/Cr-2/1) for DCE decomposition was investigated. The results show that CeO2-CrOx catalysts (Ce/Cr-2/1) prepared by coprecipitation method under the condition that the pH value is10.0and use supercritical-drying procedure exhibits the best performance. And the supercritical-drying procedure leads to form more more Cr6+species and oxygen vacancies as well as the higher ratio of strong/weak acidity and reducing ability on the catalysts, which is benefit to the further oxidation of DCE.3. The catalytic performance of different kinds of CVOCs (DCM, DCE, TCE. CB) on CeO2, Cr2O3and CeO2-CrOx catalysts (Ce/Cr-2/1) was carried out. And the mechanisms of catalytic decomposition of DCE, DCM. TCE and CB over CeO2-CrOx catalysts were also studied preliminarily. The results indicate that the catalytic activity for the oxidation of different CVOCs over CeO2-CrOx catalyst (Ce/Cr-2/1) is displayed as follows:TCE> DCE> CB> DCM. Generally, CVOCs adsorbs on the acid sites of the catalysts and then dehydrochlorination reaction occurs to form carbonyl species at first. After that, the carbonyl species will be oxidated to carboxylic acid compound in the presence of surface oxygen species, and at last converts to COx and H2O.