| In recent years,significant breakthroughs have been made in the clean and efficient utilization of carbon-based resources,including the direct conversion of syngas to high-value chemicals catalyzed by oxide-zeolite(OX-ZEO)bifunctional catalysts,mild oxidation of methane to methanol,etc.Understanding the physicochemical process of activation and transformation of these carbon-based light molecules on the catalyst surfaces at the molecular and atomic level is of great significance for screening and designing more efficient catalysts.In this thesis,we have studied the influence of differente oxide surface properties on the adsorption,activation and conversion of syngas molecules by using the density functional theory(DFT)calculations.In addition,in cooperation with experiments,the reaction mechanism of mild oxidation of methane under different reaction conditions was deeply discussed,and a new reaction mechanism was revealed.The main results and conclusions are as follows:1.ZnO(10(?)0)and Cr2O3(001)surfaces were used as models to study the activation process of hydrogen on metal oxide surfaces.Through 3d transition metal elements doped ZnO(10(?)0)and Cr2O3(001),the influence of structural effect and compositional effect on hydrogen activation were studied.It is found that hydrogen is activated by heterolytic dissociation over metal oxides.ZnO(10(?)0)surface has high intrinsic activity for hydrogen activation.The doped 3d transition metals reduce the activity of the bonded anions,thus reducing its activity of hydrogen activation.In contrast to ZnO(10(?)0)surface,Cr2O3(001)has a lower intrinsic activity of hydrogen activation,but the doped 3d transition metal elements make the surface show stronger oxidizability and enhance its ability to activate hydrogen.2.We studied the adsorption behavior of CO and CO2 on ZnO(10(?)0)surface at the atomic and molecular levels under ambient conditions.It was observed that the significant changes of pressure affected the adsorption properties of CO and CO2 on ZnO(10(?)0)surface.Atomic resolution images of ZnO(10(?)0)surface were obtained by scanning tunnel microscope(STM)under ambient conditions.Ordered(2×1)and(3×1)periodic bright spots were observed under the atmosphere of CO and CO2,respectively.More specific chemical information,including adsorption configuration,adsorption strength and the lateral interaction between adsorbed molecules,was obtained by theoretical calculation,which make up for the deficiencies of experiments.It is found that the lateral interaction between adsorbates would significantly affect the adsorption strength and adsorption structure of adsorbates.3.The mechanism of CO activation and hydrogenation on ZnO(10(?)0)surface was systematically studied.By doping 3d transition metal elements,the regulation mechanism of composition on syngas transformation was revealed.It is found that CO does not dissociate directly over ZnO(10(?)0)surface whether the surface is reduced or not.In the hydrogenation process of CO,the surface reduction degrees significantly affect the reaction path.On the pristine surface,CO is hydrogenated to methanol directly.On the reduced surface,the CO hydrogenation intermediates can break the CO bond through the oxygen vacancy to obtain methane,ketene,even acetylene,ethylene and so on.In the process of CO hydrogenation to methanol,formate is an important intermediate.The doping elements regulate the reaction activity by regulating the formation and conversion rate of formate.4.The reaction mechanism of selective oxidation of methane to methanol catalyzed by Au single atoms on black phosphorus(Au1/BP)nanosheets was deeply studied.It reveals that the water assisted the activation of O2 to generate reactive hydroxyl groups and ·OH radicals under light irradiation.Hydroxyl groups reacted with methane at Au single atoms to form water and CH3*species,followed by oxidation of CH3*via·OH radicals into methanol.Considering the recycling of water during the whole process,we can also regard water as a catalyst. |
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