Study On Preparation And Catalytic Performance Of UiO-66 And MnO_x/UiO-66 Composites For DeNO_x

As one of the major atmospheric pollutants,nitrogen oxides?NO_x?are hazardous to environment and human health.Energy demand and its consumption are growing annually,resulting in larger volumes of NO_x released into the atmosphere.Therefore,it is urgent to remove NO_x from flue gas.The most widely used and efficeint technology for NO_x removal is selective catalytic reduction?SCR?,and the development of efficient low-temperature SCR catalyst with good SO₂ resistance has attracted extensive attentions.Metal-organic frameworks?MOFs?have exhibited outstanding properties including large surface areas,high porosity,highly dispersed metal sites and regular pore structures,which make them as the potential catalysts for the SCR reaction.However,the stability of MOFs has been the main drawback which limits their extensive applications.Therefore,the UiO-66 which exhibits excellent stabilities and possess a significant amount of missing linker defects is investigated for DeNO_x.Firstly,the UiO-66 is prepared with a coordination modulation process in which acetic acid as a modulator.The effects of acetic acid on the structure and properties of UiO-66 are studied.The characterization results indicate that the size and the morphology of the products can be controlled by varying the amount of acetic acid.The amounts of defects as well as the specific surface area and pore volumes of UiO-66 samples prepared by the coordination modulation process are increased,which are beneficial for SCR reaction.The activity test results indicate that UiO-66 exhibits high activity only at higher temperatures while the catalytic performance was poor at low temperatures.The UiO-66 samples prepared by the coordination modulation process show a better catalytic performance with the highest NO conversion of 95%.Secondly,MnO_x is supported on UiO-66 to prepare the MnO_x/UiO-66 catalysts by the impregnation method and the in-situ deposition method.The structures and properties of the prepared MnO_x/UiO-66 catalyst samples are investigated and their catalytic performance for NH₃-SCR are also studied.Moreover,the loading of MnO_x are optimized for above two preparation methods.The characterization results indicate that the UiO-66 still retains its inherent lattice phase and octahedral structure during manganese oxide loading process.X-ray photoelectron spectroscopy?XPS?analysis indicated that there exists a strong interaction between MnO_x and UiO-66,which has a positive effect on the catalytic activity for NH₃-SCR at low temperatures.Compared with the MnO_x/UiO-66 samples prepared by the impregnation method,the catalytic materials prepared with the in-situ deposition method show larger specific surface areas and pore volumes as well as better thermal stability.Compared with UiO-66,the catalytic activity of the MnO_x/UiO-66 prepared by the impregnation method is greatly improved at low temperatures.The MnO_x/UiO-66 material prepared with 25%manganese nitrate shows the highest SCR catalytic activity which reaches99%NO conversion in the temperature range between 250?and 290?.The MnO_x/UiO-66 catalysts prepared by the in-situ deposition method show both high catalytic activity and a wide active temperature window.The catalyst with 8.5 wt%MnO_x loading shows nearly 100%NO conversion in the temperature between 125?and 250?.On the whole,the catalysts prepared with the deposition method show better low-temperature SCR catalytic activity and wider active temperature windows.Both of the MnO_x/UiO-66 catalyst samples synthesized by the above two preparation methods exhibit good resistance to SO₂ and H₂O.The influence of H₂O on the catalytic activity was more obvious.However,the NO_x conversion still maintained above 83%.SO₂ just has a slightly influence on the performance of the MnO_x/UiO-66 catalysts,especially for the catalysts prepared by the in-situ deposition method.