Preparation Of Loaded Noble Metal Or Metal Oxide Catalysts And Their Catalytic Performance For The Oxidation Of Volatile Organic Compounds

Volatile organic compounds（VOCs）are the main precursors to the formation of PM_2.5,photochemical smog and ozone,causing serious harm to human health and the environment.During the"14th Five-Year"period period,China has formally incorporated the control of VOCs into the new urban air quality assessment index.The treatment of VOCs has become a key research object in the environmental field at home and abroad,among which catalytic oxidation is widely used because of its simple operation,low cost and no secondary pollution,etc.The key of this technology is the selection of catalysts.Currently,precious metal catalysts have good activity,but there are problems such as high cost,complicated preparation process and low environmental friendliness.Therefore,simple preparation methods and inexpensive carriers are the main research directions at present.Manganese oxides have high redox performance and are easy to synthesize,and the catalytic performance can be significantly improved by loading a small amount of noble metals.In this study,manganese oxide loaded noble metal catalysts were firstly investigated,and then seafoam loaded metal oxide catalysts were investigated.The catalytic activity,stability and anti-poisoning properties of the catalysts for typical VOCs（ethyl acetate,toluene and formaldehyde）were investigated to explore the catalytic reaction mechanism,and the resulting catalysts have higher catalytic performance than domestic and foreign catalysts,which provide references for the design and preparation of new high-performance catalysts.1)The x Ag-yr GO/δ-MnO₂ catalysts were prepared by a PVA-protected reduction method,and the physicochemical properties of the catalysts were determined by various characterization techniques to evaluate their catalytic oxidation performance on ethyl acetate.The results showed that the doping of r GO could significantly enhance the strong metal-carrier interaction（SMSI）between Ag andδ-MnO₂,increase the specific surface area of the samples and improve the catalytic performance of the samples.The activity results showed that 1Ag-1.0r GO/δ-MnO₂showed excellent catalytic performance(T_90%=160°C,r_cat=4.97×10^-7 mol_{ethyl acetate}/(g_cat s),TOF_Ag=4.99×10^-2 s^-1and E_a=39.8 k J/mol),which is better than the literature Ag-based catalysts,which is related to their high Mn³⁺/Mn⁴⁺or O_ads/O_lattmolar ratios and good low-temperature reduction performance.The samples showed no change in the number of active sites after 55 h stability test.There was no significant difference in catalytic activity when the catalyst was treated by increasing or decreasing the temperature.TG results indicated that the loaded r GO samples were stable under the reaction conditions of this study.However,the activity decreased in the presence of water vapor,which may be due to the competitive adsorption of water and reactants on the catalyst surface.In situ DRIFTS characterization showed that the catalytic oxidation of ethyl acetate over 1Ag-1.0r GO/δ-MnO₂ with acetate salts as the main intermediate products and CO₂ and H₂O as the final products.2)Since r GO doping significantly enhanced the catalytic activity,to further improve the activity ofδ-MnO₂ loaded noble metal catalysts,Pd-based xPd-yr GO/δ-MnO₂ catalysts were prepared by a PVA-protected reduction method and evaluated for the catalytic oxidation activity of toluene,which is difficult to oxidize in VOCs.It was found that the doping of appropriate amount of r GO could significantly enhance the SMSI between Pd andδ-MnO₂ and improve the catalytic activity of the samples.0.48Pd-1.0r GO/δ-MnO₂ exhibited the best catalytic performance(T_90%=189°C,r_cat=6.35×10^-4 mol _Toluene/(g_cat s),TOF_Pd=9.81×10^-2s^-1and E_a=67.7 k J/mol),which was significantly better than 0.45Pd/δ-MnO₂ andδ-MnO₂ and Pd-based catalysts in the literature.r GO doping increased the Mn³⁺/Mn⁴⁺or O_ads/O_latt molar ratio and improved the catalytic activity.The activity and physicochemical properties were stable by stability experiments,cycling experiments,warming and cooling treatment of the samples and analysis of the catalysts before and after use,and TG experiments showed that the catalysts were thermally stable in the reaction temperature range.However,the activity of the catalyst decreased in the presence of CO₂,SO₂ or NH₃,and the in situ DRIFTS characterization showed that this was due to the formation of carbonate,sulfate and ammonia species on the catalyst surface covering the active site and oxygen vacancies,thus reducing the ability to bind to toluene.The in situ DRIFTS results show that the mechanism of catalytic oxidation of toluene is as follows:the methyl group is first cleaved to methylene,then further oxidized to intermediate products（e.g.,benzaldehyde and benzoate）,and finally converted to CO₂ and H₂O.3)Since Pd-based catalysts have good activity,but species such as water and sulfur dioxide have a strong influence on the catalytic activity,an alloy catalyst（xPd–y Pt/α-MnO₂）was prepared by a reduction method using PVA as a protective agent and explored for its activity and resistance to water and sulfur in the catalytic oxidation of toluene.The results showed that the 0.93Pd-0.89Pt/α-MnO2 catalyst exhibited excellent catalytic activity(T_90%=156°C,r_cat=6.34×10^-4 mol _Toluene/(g_cat s),TOF_Pd-Pt=4.34×10^-2 s^-1and E_a=31.9 k J/mol).Significantly better than the monometallic loaded catalysts（xPd/α-MnO₂ and y Pt/α-MnO₂）and the loaded alloy catalysts reported in the literature.The characterization results showed that the Pd-Pt alloy improved the catalytic activity by increasing the ability of the catalyst to adsorb and activate oxygen and increasing the Mn³⁺/Mn⁴⁺and O_ads/O_latt molar ratios.Various stability tests showed that the xPd-y Pt/α-MnO₂ catalyst has good stability in the reaction temperature range.The results of water and sulfur toxicity resistance show that the 0.47Pd–0.46Pt/α-MnO₂ samples have better sulfur resistance than0.93Pd/α-MnO₂ andα-MnO₂.XRD,XPS,TG and in-situ DRIFTS characterization results indicate that the xPd–y Pt/α-MnO₂ catalysts have excellent resistance to neutrophilic toxicity due to the preferential adsorption of H₂O or SO₂ onto theα-MnO₂ carrier,which reduces the adsorption to the active site.studies on the mechanism of SO₂ poisoning indicate that the partial deactivation of0.47Pd–0.46Pt/α-MnO₂ was due to the generation of metallic manganese sulfate species,which reduces the specific surface area of the catalyst,Mn³⁺/Mn⁴⁺and O_ads/O_latt molar ratios,thus reducing the binding ability of the catalyst to O₂ and toluene.4)In order to reduce the cost of catalytic oxidation catalysts for VOCs,sodic seafoam-loaded transition metal oxide TM/Na Sep（TM=Cu,Fe,Ni,Mn,Co）and rare earth oxide RE/Na Sep（RE=La,Eu,Dy,Tm）catalysts were prepared by impregnation method and evaluated for their catalytic oxidation activities on HCHO,respectively.Among the TM/Na Sep and RE/Na Sep series catalysts,the Cu/Na Sep and Eu/Na Sep samples showed excellent catalytic performance,with the catalytic activity of Cu/Na Sep(T_100%=100°C,r_cat=2.89×10^-6 mol _HCHO/(g_cat s))and Eu/Na Sep Na Sep(T_100%=150°C,r_cat=3.54×10^-6 mol _HCHO/(g_cat s))were significantly better than the carrier Na Sep and the loaded metal oxide catalysts reported in the literature.In particular,the catalytic activity remained better at high air velocity（GHSV=480 000 m L/（g h））and high HCHO concentration（6000 ppm）,which was closely related to the good acidity of the catalyst surface,better redox property,more adsorbed oxygen species and good adsorption ability of Na Sep on HCHO.The stability experimental results showed that the catalyst had good thermal stability in the reaction temperature range.The in-situ DRIFTS results showed that the HCHO adsorbed on the catalyst surface was firstly oxidized to dialdehyde,then to formate species and carbonate species,and finally converted to CO₂ and H₂O.