Study On Performance And Mechanism Of Selective Photocatalytic Oxidation Of NO By Single Pd(Pt) Atom Catalyst

With the large-scale expansion of industrial production in modern society,air pollution problems have become progressively more serious.In particular,a large amount of NO_x emissions（of which NO accounts for 90%-95%）,resulting in a series of environmental problems and health problems.Therefore,it is urgent to reduce NO_xemissions economically and effectively.Technically,visible photocatalytic oxidation of NO provides a cost-effective strategy for NO_x removal.However,existing catalysts have some problems such as low removal efficiency,poor selectivity,and high cost.Therefore,the development of efficient and highly selective photocatalysts for the catalytic oxidation of NO remains a great challenge.Single atom catalysts（SACs）provide an effective strategy for the efficient utilization of noble metals due to their most exposed active sites,highest atom utilization and unique catalytic activity,opening a new direction for obtaining highly active and stable catalysts.Based on this,in this thesis,ultrathin Bi₂WO₆（BWO）nanosheets with oxygen（O）vacancies,metal-organic framework（MOF）-derived ZnO/C and g-C₃N₄ catalyst carriers are used to load noble metal Pd（Pt）single atoms through three stabilization strategies,namely,O vacancy formation,carbon（C）-bridging,and N coordination,respectively.This synthesized SACs were used for selective photocatalytic oxidation of NO and revealed the mechanism of single atom enhanced photocatalytic oxidation activity of NO and the reason for its high selectivity.The main research of this thesis is as follows:Pd-V_O-UBWO catalysts are obtained by loading single Pd atoms onto ultrathin BWO（UBWO）nanosheets with O vacancies via impregnation followed by calcination for selective photocatalytic oxidation of NO to NO₃^-.Atomic force microscopy（AFM）and electron spin resonance（ESR）determines the thickness of the UBWO nanosheets and the presence of O vacancies,respectively.Aberration-correction high-angle-annular-dark field scanning transmission electron microscopy（AC-HADDF-STEM）and X-ray photoelectron spectroscopy（XPS）are used to prove the existence of Pd single atoms.single Pd atom anchoring to the O vacancy of Vo-UBWO determined by ESR tests and density functional theory（DFT）calculations.The Pd-V_O-UBWO catalysts achieve high photocatalytic catalytic activity for NO oxidation and high selectivity for NO₃^-formation.Within 1 h of visible light irradiation,the conversion rate of NO by the Pd-V_O-UBWO catalyst（89.5%）is much higher than that of Vo-UBWO（45.7%）at an NO inlet concentration of 48 ppm.The selectivity of NO₃^-formation is 78.5%over Pd-V_O-UBWO,which is much higher than that of NO₂（7.4%）and HNO₂（5.3%）.In addition,the ESR show that single Pd atoms enhance the production of O₂^·-and OH^·.DFT calculations show that the single Pd atoms promoted the adsorption and activation of NO on the catalyst surface,thus improving the NO conversion and selectivity.Pt-ZnO/C catalysts are prepared by C bridging atomically dispersed Pt onto ZnO/C derived from MOF for selective photocatalytic oxidation of NO to NO₃^-.The MOF-derived ZnO/C has smaller particle size and larger specific surface area than regular ZnO.The presence of C enhancing the absorption of visible light and facilitating the migration of photogenerated e^--h⁺pairs.Pt exists as Pt single atoms and nanoclusters in Pt-ZnO/C catalysts.The Pt-ZnO/C catalyst has a high NO conversion rate and a selectivity for NO₃^-formation.The conversion rate of NO achieved by Pt-ZnO/C catalyst is 81.9%,which is much higher than ZnO/C（31.4%）and ZnO/C loaded with Pt nanoparticles（Pt NPs-ZnO/C,58.6%）.Moreover,the selectivity of NO₃^-formation over the Pt-ZnO/C catalyst is as high as 85.1%,which is higher than that of than that of NO₂（1.4%）and NO₂^-（13.5%）.In addition,the catalytic activity evaluation experiments and ESR results indicates that the introduction of Pt single atoms and nanoclusters promote the production of O₂^·-and OH^·,which enhance the NO conversion.Meanwhile,DFT results show that single Pt atom promotes the adsorption and activation of O₂ and H₂O to form O₂^·-and OH^·.More importantly,OH^·facilitates the selective oxidation of NO to NO₃^-.Moreover,the projected density of states（PDOS）of single Pt atoms loaded on the ZnO/C catalysts show that C atom bridge favors the stability of single Pt atoms due to the interaction between C and Pt atoms.The selective oxidation of NO to NO₃^-,which covers the active site of the catalyst,results in catalyst deactivation.To improve the stability of the catalysts,the0.1CNPd SAC is obtained by anchoring single Pd atoms onto g-C₃N₄（CN）by impregnation combined with calcination for the selective photocatalytic oxidation of NO to NO₂.The generated NO₂ can be subsequently used to produce high value-added nitric acid by uptake into water.AC-HADDF-STEM and X-ray absorption fine structure spectroscopy（XAFS）are employed to determine the synthesis of single Pd atoms.Pd atoms tend to form coordination bonds with N atoms compared to C atoms in 0.1CNPd SAC.The 0.1CNPd SAC exhibited high NO conversion rate and selectivity for NO₂ formation as well as long stability under visible light and simulated sunlight irradiation.At a NO inlet concentration of 21 ppm,the conversion rate of NO is 88.9%and the selectivity of NO₂ formation is 79.3%over 0.1CNPd SACs within 5 h of visible light irradiation.At a NO inlet concentration of 12 ppm,the NO removal efficiency and NO₂ instantaneous selectivity over 0.1 CNPd SAC are83%and 92.8%,respectively,after 117.6 h of simulated solar irradiation.This light irradiation time is about 23.5 times the longest duration tested for other catalysts in the literature.DFT calculations show that the strong interaction between single Pd atoms and the surrounding N atoms promotes the generation of photogenerated electrons,thus improving the photocatalytic activity of the 0.1CNPd SAC.Moreover,the DFT calculations show that NO₃^-generated on the surface of the 0.1CNPd SAC react with NO to produce NO₂,thus improving the selectivity of NO₂ formation and the stability of the SAC.The thesis includes 73 Figures,19 Tables,and 300 References.