Catalysis Performance Of VOCs Degradation Over Zr-based UiO-66 And Its Derivatives Supported Pd Nanoparticles

Volatile organic compounds（VOCs）,as the main precursors of ozone and fine particulate matter PM2.5,are an important inducement of complex air pollution in our country,and it is urgent to control VOCs effectively.As one of the most effective end-treatment technologies for VOCs removal,catalytic oxidation technology is widely applied in VOCs elimination because of its mature technology,simple operation,high efficiency,low energy and no secondary pollution.The key to catalytic oxidation is developing an efficient catalyst.Supported noble metal catalysts（especially Pd）are usually used for the efficient degradation of VOCs at low temperature because of their high catalytic activity.However,in actual working conditions,the emission of VOCs often contains a variety of components,which will cause toxicity to the active components of the supported noble metal catalysts,resulting in the decline of activity or even deactivation of the catalysts.Metal organic frameworks（MOFs）,especially Zr-based UiO-66,are widely used as the support of supported noble metal catalyst because of its high hydrothermal stability and high specific surface area.Therefore,in this paper,a series of supported Pd catalysts was constructed by using Zr-based metal organic framework UiO-66 and its derivatives as the supports.Because of the high porosity and functionalization of UiO-66,the dispersity of Pd species was increased,and the catalytic degradation of VOCs at low temperature was enhanced.The structure-activity relationship between catalyst structure and catalytic activity was studied.The mechanism of resistance to water,SO₂ and Cl,and degradation mechanism of mixed component VOCs and their interaction mechanism were revealed.The main results are as follows:（1）The effect of reducing agent on the structure and catalytic performance of UiO-66 supported Pd catalyst.The supported Pd catalyst with 1.0 wt%Pd loading was prepared by using UiO-66 as support and H₂,Na BH₄ and ethylene glycol as reducing agents.The catalytic activity of the supported Pd catalysts was investigated by using toluene as the model pollutant.A series of characterization indicated that the reducing agent could affect the distribution of Pd species,surface Pd⁰ species content and the specific surface area in the supported Pd catalyst,and thus affect the catalytic activity for toluene.Among them,Pd-U-EG prepared with ethylene glycol as the reducing agent presented the best catalytic activity(T₉₀=198°C),excellent thermal stability,water resistance,reusability and resistance to air velocity change.The water resistance mechanism and degradation path of toluene catalyzed by Pd-U-EG were studied by toluene TPD,toluene TPSR and in situ DRIFTS spectroscopy.Its path is:toluene→benzaldehyde→benzoic acid→maleic anhydride→CO₂ and H₂O.（2）Effect of functional groups on water and SO₂ resistance for toluene degradation over supported Pd catalyst and its mechanism.The preparation cost of catalysts was further reduced by reducing the Pd loading（0.05wt%）,introducing-NH₂ and-NO₂functional groups into UiO-66 and using glycol as reducing agent.It was found that the introduction of-NH₂ resulted in the interaction between Pd and-NH₂,which weakened the adsorption of toluene.The introduction of-NO₂ provides additional adsorption sites to increase the adsorption performance of toluene.However,the dispersion of Pd species in the supported Pd catalyst and the surface Pd⁰ species content decreased,and Pd nanoparticles aggregated,which reduced the catalytic activity of toluene.Finally,the water resistance of catalysts was studied by toluene-TPD and toluene-TPSR under different conditions.Further,the anti-SO₂ poisoning ability and the mechanism of the catalyst were further investigated.Additionally,gaseous intermediates generated during toluene degradation over Pd-U surface were detected by GC-MS.It was found that residual Cl species in the catalyst would participate in the degradation reaction of toluene to produce chlorine-containing byproducts.（3）Study on migration and transformation of residual Cl species during VOCs degradation.Using UiO-66-Cl（namely UiO-66）as the support,ethylene glycol as the reducing agent to prepare Pd-U-Cl catalyst and to study the migration and transformation rule of residual Cl species during 5 kinds benzene series degradation.It was found that the residual Cl species in Pd-U-Cl originated from Zr Cl₄,the metal precursor for the preparation of UiO-66-Cl,and the residual Cl species existed in the form of bridging Cl in the catalyst and participated in the degradation reaction of VOCs to produce chlorine-containing byproducts.It was found by TD-GC-MS that methyl was preferentially substituted by residual Cl species to form chlorobenzene for toluene degradation.The H of benzene was substituted by residual Cl species to form chlorobenzene for benzene oxidation.The total amount of chlorine-containing byproducts produced by the degradation of o-xylene,m-xylene and p-xylene on Pd-U-Cl decreased in turn,but the production of dichlorobenzene,a precursor of highly toxic dioxins,increased in turn.Finally,the migration and transformation of Cl species during VOCs degradation were revealed by TD-GC-MS and DFT.（4）Study on the mechanism of Cl coordinated single Pd atom enhancing the chlorine resistance of VOCs degradation.Zr O₂-supported Pd catalyst was derived by using one-pot solvothermal method prepared Pd@UiO-66 as the sacrificial template.The Pd@Zr O₂ catalyst containing the Cl coordination single Pd atom（Pd₁-Cl）species was constructed by using the residual Cl species in Pd@UiO-66.Toluene,benzene and o-xylene were used as model pollutants and dichloromethane as the influencing factors to investigate chlorine resistance of the catalyst.The presence of Pd₁-Cl species in catalyst were proved.It was found that the calcination temperature could affect the content of Pd₁-Cl species in Pd@Zr O₂ catalyst,and then affect the chlorine resistance of the catalyst.Among them,Pd@Zr O₂-800 with the highest Pd₁-Cl species content showed the best chlorine resistance.In addition,compared with Pd@UiO-66-NO₃without Cl derived Pd@Zr O₂-N catalyst and the Pd/Zr O₂-C catalysts prepared by using commercial Zr O₂,the Pd@Zr O₂ catalyst containing Pd₁-Cl has excellent chlorine resistance.It could be found from a series of characterization and experiments that the presence of Pd₁-Cl in the catalyst is the key to the excellent chlorine resistance of the catalyst.Finally,the mechanism of chlorine resistance was investigated by in-situ DRIFTS spectroscopy.（5）Study on catalytic degradation mechanism of mixed component VOCs.The calcination and treatment conditions were further optimized,and Pd@UiO-66-M（M=H,NH₂,NO₂）synthesized by one-pot solvothermal method was used as the precursor system to prepare Pd@Zr O₂-M catalyst.The structure-activity relationship between catalysts structure and activity,and the mutual influence mechanism between toluene and acetone during their mixture degradation was investigated by using toluene and acetone as model pollutants.The results showed that different N species in Pd@UiO-66-M enter the Zr O₂ lattice,leading to the formation of different concentrations of oxygen vacancies.Among them,a large number of oxygen vacancies in Pd@Zr O₂-NH₂-350H could enhance the adsorption and activation of gaseous oxygen molecules and accelerate the oxidation of toluene.Meanwhile,it enhanced the degradation performance and water resistance of toluene and acetone mixture.However,acetone would inhibit toluene oxidation and toluene promote acetone degradation.Finally,the interaction mechanism between toluene and acetone was revealed by TD-GC-MS and in situ DRIFTS spectra.In summary,this paper prepared the supported Pd catalyst with UiO-66 and its derivatives as the support,studied the water resistance,sulfur resistance,migration and transformation of residual Cl species,Cl toxicity resistance,as well as the interaction and mechanism of mixed component VOC degradation during the catalytic degradation of VOCs.It provides experimental and theoretical basis for the development of highly efficient,economical,broad-spectrum,durable and high anti-toxicity supported noble metal catalysts.