Catalytic Oxidation Of Toluene Over Manganese/Cerium Oxide Supported Pt Catalysts:Performance And Reaction Mechanism

Volatile organic compounds（VOCs）are the important precursors for the formation of PM_2.5 and O₃,which pose a serious threat to human health and ecological environment.At present,catalytic oxidation has become one of the most promising treatment technologies for VOCs because of its advantages of high degradation efficiency at low temperature and no secondary pollution.The key of this technology is the development of low-temperature,efficient and stable catalysts.Considering that noble metal Pt has high activation ability for VOCs pollutant molecules,and the oxygen vacancies on the surface of reducible metal oxide supports can not only anchor noble metals,but also serve as the key active sites for adsorbing and activating oxygen species,the combination of the two is an effective mean to improve the catalytic oxidation performance of VOCs.However,at present,the research on how are some important characteristics（such as,oxygen vacancy concentration of supports,valence state of noble metals and metal-support interaction）of the above types of noble metal catalysts,comprehensively affects the catalytic oxidation performance and reaction mechanism of VOCs is not deep enough,resulting in obvious restrictions on the research and development of efficient and stable catalysts and their further industrial application.Based on the above problems,representative VOCs toluene was selected as the probe molecule,and the comprehensive effects of Pt/Mn O_x or Pt/CeO₂ catalyst support/noble metal properties and metal-support interaction modulation on the oxidation performance and reaction mechanism of toluene oxidation were explored by constructed a reducible metal oxide support（manganese oxide or cerium oxide）supported noble metal platinum as the catalyst system.The main research contents and results are as follows:（1）Manganese oxides with different crystal forms（i.e.,α-MnO₂,β-MnO₂ andγ-MnO₂）were synthesized by hydrothermal method,and the effect of crystal forms on toluene oxidation activity was studied.It was found thatα-MnO₂ exhibited the highest catalytic activity with a50%toluene conversion(T₅₀)at 229°C and a 90%toluene conversion(T₉₀)at 238°C,followed byγ-MnO₂(T₉₀=252°C)andβ-MnO₂(T₉₀=278°C),demonstrated thatα-MnO₂ is more suitable to load Pt.Transient in-situ DRIFTS results indicated that the reactivity of adsorbed toluene species on the surface ofα-MnO₂ samples are more active than the other samples,which is mainly reflected in that when toluene and oxygen are fed into the in-situ reaction tank at the same time,only CO₂adsorption peak was detected onα-MnO₂,while benzoyl oxide or benzoate species could still be detected on other samples.The higher reactivity of surface adsorbed intermediates onα-MnO₂ is due to its superior redox property and high oxygen vacancy concentration.（2）α-MnO₂ was used as the support precursor,and a series of novel Pt/Mn₃O₄catalysts were prepared by one-step co-reduction of H₂Pt Cl₆·6H₂O andα-MnO₂by Na BH₄.The effect of main exposed crystal facets of support on toluene oxidation activity was studied.With the help of the acidity of H₂Pt Cl₆·6H₂O,the hydrogen production capacity of Na BH₄ was improved,in promoting the transformation ofα-MnO₂ to Mn₃O₄ with more stable spinel structure,Pt nanoparticles were firmly anchored on Mn₃O₄ at the same time.Through modulating the main exposed crystal facets ofα-MnO₂,the morphology,oxygen vacancy concentration and noble metal valence of the catalyst can be changed,so as to obtain octahedral Pt/Mn₃O₄-110 catalyst with stable structure and exhibited high efficiency at low temperature.In situ DRIFTS results show that compared with other samples,the weaker Mn-O bond,richer oxygen vacancies and higher mobility of oxygen species on Pt/Mn₃O₄-110 sample are conducive to the release of lattice oxygen from the catalyst,and then promote the oxidation process of benzaldehyde to benzoate species,and further convert to formate and bicarbonate.（3）CeO₂-DC,CeO₂-SG and CeO₂-P supports were prepared by direct calcination,sol-gel and precipitation,respectively.The effect of oxygen species on the surface of supports on the metal-support interaction of the noble metal catalysts were studied.Before Pt was not loaded,CeO₂-DC samples with the highest surface oxygen species mobility exhibited the highest toluene oxidation activity.After loading Pt,the catalyst which Pt supported on CeO₂-P with moderate surface oxygen species mobility exhibited the highest toluene oxidation activity,because the metal-support interaction between Pt and CeO₂-P can significantly weaken the Ce-O bond,improve the low-temperature redox ability of the catalyst and enrich oxygen vacancies,which make Pt/CeO₂-P sample can convert more primary toluene adsorbed species on the surface of catalyst,such as benzyl and benzyl alcohol,into benzoate or formate species at a relatively lower temperature.（4）CeO₂-P was selected as the support to prepare Pt/CeO₂ by impregnation.The metal-support interaction between Pt and CeO₂ was adjusted via switching the reduction atmospheres strength（CO,H₂,NH₃）and temperature（200,300,400℃）,and further explored the effect of changed metal-support interactions on the reaction mechanism of toluene oxidation.Some properties that dominate the activity of the catalyst can be gradually improved through variable reduction treatment,such as redox ability,the ratio of Pt⁰ and the concentration of oxygen vacancy.Pt⁰ is more favorable for toluene adsorption than Pt²⁺,but the sample with the highest proportion of Pt⁰ has not the best toluene oxidation activity,indicating that the catalyst activity is comprehensively affected by these factors.Different reduction treatments can obviously affect the degree of deep oxidation of toluene via changing the surface/lattice oxygen mobility and the activation ability towards gas-phase oxygen of Pt/CeO₂ catalysts（from benzoate to formate or monodentate carbonate）.In situ DRIFTS,in situ Raman,C₇H₈-TPSR andO₂-TPD together manifested the main activation sites of gas-phase oxygen are Pt⁰ species when the temperature is lower than 200℃.Above 200℃,the main activation sites of gas-phase oxygen expanded to oxygen vacancies and Pt⁰ species.In this study,by adjusting the crystal forms,crystal facets,surface oxygen species of supports and metal-support interaction of Pt/Mn O_x or Pt/CeO₂ catalysts,the comprehensive effects of the changes of oxygen vacancy concentration,noble metal valence and metal-support interaction on the catalytic activity and reaction mechanism of toluene oxidation were studied,so as to provide experimental and theoretical basis for the subsequent research and development of high activity and stable noble metal catalysts.