Mechanism Of Catalytic Oxidation Of Typical VOCs On Nano-metal Supported TiO₂

Volatile organic compounds（VOCs）pose a serious threat to the environment and human health due to their toxicity.Formaldehyde and malodorous,as toxic compounds,used in a wide range of applications.including the manufacturing of sterilizing and disinfecting agents,medicines,cosmetics,and a number of consumer goods.The emission of formaldehyde and malodorous significant affects human health,which need to be removed..Catalytic oxidation is widely used to remove VOCs due to high remove efficiency,mild reaction conditions and simple operation.The study of catalytic oxidation mechanism of volatile organic compounds is of great significance for the preparation of efficient and stable catalysts.At present,there are still some problems related to the oxidation mechanism of formaldehyde and methanethiol.（1）The catalytic degradation efficiency of formaldehyde highly depends on the humidity.Therefore,it is important to study the detailed roles of water vapor in the catalytic oxidation of formaldehyde.（2）Malodorous gas is very serious environmental problem.And methanethiol is the typical malodorous gas in daily life,Therefore,efficient removal of malodorous is highly desirable,and understanding the catalytic oxidation mechanism of methanethiol is critical important for the design of efficient catalyst.The main research contents of this thesis are as follows:（1）Pt/TiO₂catalyst was synthesized by NaHB₄reduction method,and the effect of relative humidity on the catalytic oxidation of formaldehyde to CO₂was studied with space speed of 84000 m L/g·h at room temperature.When the relative humidity is 0,formaldehyde conversion rate is 100%,but mineralization rate is 0,while the relative humidity is 40%,mineralization rate is 100%.When the relative humidity increases to 50%,the CO₂generation rate decreases to 70%.It shows that water plays an important role in the mineralization of formaldehyde.（2）.The CO₂generation rate over Pt/TiO₂in the first 10 min was calculated with formaldehyde and deuterium substituted formaldehyde as raw materials.in different reaction atmosphere.K_H/K_Dkinetic isotope effect（KIE）values aof K_（C-H）/K_（C-D）is found to be 2,and K_（O-H）/K_（O-D）was calculated to be 1,indicating that C-H fracture is the rate-determining step in the whole reaction process,water can affect the depth of formaldehyde mineralization,probably due to the formation of H₂O·HCOO^-and D₂O·HCOO^-.Furthermore,In situ DRIFTS wase used to study the catalytic oxidation mechanism of formaldehyde and the detailed role of water in the process of formaldehyde degradation on Pt/Ti O₂catalyst in dry air and relative humidity of 40%was investigated.The DRIFTS results show that the degradation path of formaldehyde over Pt/Ti O₂is HCHO→DOM→HCOO^-→CO₃^2-→CO₂,in which HCOO^-is the key intermediate specie,water vapor can promote the decomposition of CO₃^2-to CO_2.The results of kinetic isotope effect assisted DRIFTS showed that dissociation of C-H in HCOO^-is a rate-determining step,water can activate the C-H bond of dioxymethylene,elongate the C-H of HCOO^-by changing the adsorption configuration,thus promoting the conversion of HCOO^-.These studies demonstrated that considerable water vapor can improve the catalytic performance and stability of formaldehyde over Pt/Ti O₂by promoting the transformation of intermedium over Pt/Ti O₂catalyst,and reducing accumulation of HCOO^-over catalyst surface.（3）Cu/TiO₂with different Cu loading amount was synthesized by NaBH₄reduction method,and the catalytic oxidation of CH₃SH over Cu/Ti O₂at room temperature was studied.Among them,0.5 wt.%Cu/Ti O₂displayed the highest catalytic activity for CH₃SH conversion,and the catalytic activity of 0.5 wt.%Cu/Ti O₂still maintained at 100%after 4cycles.Agilent SCD detector gas chromatography was used to determine the products in the tail gas.During the reaction,methanethiol is catalyzed to CH₃SSCH₃,with selectivity up to95%.The reaction mechanism and deactivation mechanism of CH₃SH over 0.5 wt.%Cu/Ti O₂were studied by DRIFTS.The oxidation pathway of methanethiol on 0.5 wt.%Cu/Ti O₂in dry air was as follows:（1）CH₃SH+Cu→CH₃SCu or CH₃SSCH₃（main）,（2）CH₃SCu+O_（ROS）→CH₃SO₃^-or SO₄^2-and HCOO^-.The deactivation of 0.5 wt.%Cu/Ti O₂can be attributed tothe accumulation of CH₃SO₃^-,SO₄^2-and HCOO^-on the catalyst surface.