Ru Modified Mn/TiO₂ Catalysts For Low-temperature Catalytic Oxidation Of Chlorobenzene:Preparation And Catalytic Performance

Chlorinated volatile organic compounds（CVOCs）mainly come from evaporation and leakage in industrial production process and daily use process.CVOCs possess high biotoxicity and high resistance to degradation,causing great harm to the environment and human health.Catalytic oxidation is an effective method to degrade CVOCs,because of the advantages of low reaction temperature,high treatment efficiency and few by-products.The core of this technology is the catalyst.However,CVOCs are prone to condensation reaction with other organic pollutants at high temperature,producing more toxic chlorine-containing organic compounds.In addition,chlorine species readily react with the active component of the catalysts to generate the MO_xCl_yspecies,which inactivate the catalysts.Therefore,it is crucial to improve the low temperature catalytic activity and chlorosis resistance of the catalysts.Mn O_xshows good CVOCs catalytic activity due to its rich variable valence state and strong redox activity.By choosing the appropriate supports or metal doping modification is an effective way to improve the catalytic performance of Mn-based catalysts.In this paper,a series of Mn/MO_xcatalysts were prepared with Mn O_xas the active component and different metal oxides as the supports,and the catalytic performance of catalysts was evaluated with chlorobenzene（CB）.The catalytic performance of the catalysts was evaluated as follows:the concentration of CB is 500 ppm and the space velocity is 15000 m L/（g·h）.The catalytic performance of catalysts was further improved by the element modification,then the structure-activity relationship and the reaction mechanism of catalysts were studied in combination with a series of characterization methods.Firstly,three metal oxides ZrO₂,TiO₂andγ-Al₂O₃were selected as the supports to prepare Mn/MO_xcatalysts.The support with the best activity was screened by the evaluation of chlorobenzene catalytic activity,and the influence of preparation methods and preparation parameters on the catalytic activity of Mn/MO_xcatalyst were further investigated.The results showed that the Mn/Ti O₂catalyst prepared by the deposition-precipitation method exhibited the best catalytic activity.When the Mn loading amount and the calcination temperature were10 wt.%and 400℃,respectively,the synthesized Mn/Ti O₂catalyst demonstrated 90%CB conversion and 76%CO₂selectivity at 300℃.No chlorine-containing organic by-products were generated.Secondly,noble metals Pt,Pd and Ru were selected as modification components to improve the catalytic activity at low temperature and the stability.The optimal modification elements and their doping amount were selected.When the amount of Ru was 0.8 wt.%,the Ru-modified catalyst had excellent catalytic oxidation performance of chlorobenzene.When the reaction temperature was 300℃,the conversion of CB was 100%,the selectivity of CO₂was 95%and the stability was significantly improved.Finally,the relationship between the catalytic performance and physicochemical properties and the reaction mechanism of the catalysts were analyzed by a series of characterization.The modification of Ru did not change the surface morphology and pore structure of the catalysts,Ru species highly dispersed on the Mn/Ti O₂catalyst in microcrystalline state.Compared with Mn/Ti O₂catalyst,the formation of more Mn⁴⁺and Ru⁴⁺in Ru-Mn/Ti O₂catalyst indicated that the redox performance of the catalyst was improved.The formation of more oxygen vacancies and reactive oxygen species indicated that the Ru-modified Mn/Ti O₂catalyst had better oxygen migration capacity and oxidation capacity.The strong redox performance of Ru-Mn/Ti O₂catalyst was the main reason for its low temperature catalytic activity and CO₂selectivity.XPS results of the catalysts after stability test showed that the catalysts were inactivated by Cl species,and the improved stability of Ru modified Mn/Ti O₂catalysts could be attributed to Ru species promoting the removal of chlorine species.The reaction mechanism of chlorobenzene on the Ru-Mn/Ti O₂and Mn/Ti O₂catalysts was analyzed by in situ DRIFT.Ru modification did not change the reaction path of chlorobenzene on the catalyst surface.At first,CB molecules adsorbed on the active sites on the surface of the catalyst,and C-Cl bond was first broken and Cl·reacted with hydroxyl groups on the surface of the catalyst.At the same time the surface phenolate species generated by nucleophilic substitution.Then the phenolates were further oxidized into benzoquinone species and the ring cleavage reactions occurred by the attacking of nucleophilic oxygen,decomposed into carbonate bidentate and carboxylate.In the next step,they were further oxidized into lots of intermediates,such as ketone and aldehydes.Finally,the intermediates were eventually converted into CO₂,CO,H₂O.