Preparation Of Ordered Mesoporous Mn-Ce Mixed Metal Oxides Catalysts And Their Catalytic Properties For Chlorobenzene Combustion

Chlorinated Volatile Organic Compounds?CVOCs?are a class of carcinogenic,teratogenic and biologically highly toxic compounds that are extremely harmful to human health.Therefore,developing effective and feasible CVOCs treatment technology is an urgent problem to be solved.So far,catalytic combustion has become one of the most promising technologies to eliminate CVOCs because of its low operating temperature and high effeciency.For catalytic combustion technology,the development of low-temperature and high-efficiency catalyst is a difficult task.Considering catalytic combustion for chlorobenzene?chlorobenzene as model compound of dioxins,dioxin is the most toxic compound of CVOCs?,is a heterogeneous catalytic oxidation reaction,of which the reaction process is diffusion-adsorption-surface reaction-desorption-diffusion.It is clear that the porous structure will help to reduce the diffusion resistance.In this paper,a series of ordered mesoporous MnO_x/CeO₂ catalysts with strong redox capacity has been prepared by using ordered mesoporous CeO₂ as support,and the catalytic activities for chlorobenzene combustion have been investigated.The optimum process conditions have also been explored,which is of great significance for the development of high active catalysts for catalytic combustion of chlorobenzene.The main work of this paper is present below.A series of ordered mesoporous CeO₂ supported MnO_x/CeO₂ catalysts with Mn/Ce molar ratios from 0.11 to 0.67 were synthesized by incipient impregnation method and CeO₂ was obtained by using KIT-6 as hard template.The effect of Mn content of MnO_x/CeO₂ catalysts on the catalytic properties for CB combustion was investigated.Similarly,MnO_x/CeO₂ nanoparticles catalyst was prepared using CeO₂ nanoparticles as the carrier,denoted as MnO_x/CeO₂-NPs.The catalytic tests show that the ordered mesoporous MnO_x/CeO₂ catalyst has higher catalytic activity and higher resistance to chlorobenzene poisoning than MnO_x/CeO₂-NPs.Ordered mesoporous MnO_x/CeO₂ catalysts with different Mn contents exhibited different activities for CB combustion.And when Mn/Ce molar ration is equal to 0.43,the MnO_x/CeO₂ catalyst is the most active.The main factors affecting the catalytic activity were the amount of adsorbed oxygen species and the dispersibility of surface manganese species.The activity of Mn/Ce?0.43?under different chlorobenzene concentration was also investigated,and the kinetic analysis was carried out.It was found that when the concentration of chlorobenzene was 500-2000 ppm,the conversion efficiency of the catalyst changed little and remained high.When the concentration of chlorobenzene was higher than 2000 ppm,the catalytic efficiency of the catalyst decreased obviously.The kinetic study shows that when the reaction temperature of the reaction is 200°C and the chlorobenzene concentration is 500-2000 ppm,the reaction can be described of well zero order reaction,and the reaction rate increases linearly with the increase of chlorobenzene concentration.When the benzene concentration is above 2000 ppm,the reaction rate is almost constant,showing first order kinetics.When the reaction temperature is 300°C and the chlorobenzene concentration is 500-2000 ppm,the reaction of chlorobenzene oxidation also shows the kinetics of the zero order.When the concentration of chlorobenzene is above 2000 ppm,the reaction rate slows down.High active Mn-Ce nanocomposites were successfully synthesized by a novel CTAB-assisted hydrothermal method with citric acid as chelating agent,and used in the catalytic combustion of chlorobenzene?CB?as a model of dioxins.The effect of hydrothermal temperature on the catalytic properties of the Mn-Ce samples has been explored.Catalytic performance results demonstrate that the Mn-Ce nanocomposites of which the hydrothermal temperature was 150°C is the most active,the T₉₀?the temperature when chlorobenzene conversion is 90%?is 215°C.T₉₀ of regular Mn-Ce composites with NaOH as precipitator is 318°C.The enhanced activity of the Mn-Ce nonocomposites is due to its high specific surface area,nanoscale size,as well as abundance of high valence MnO_x and active oxygen species.