Experimental And Mechanistic Studies Of Formaldehyde Catalytic Removal On The Doped Lanthanum-Manganese Perovskite-type Catalyst

The emission of formaldehyde（HCHO）greatly threatens human health and the ecological environment.Catalytic oxidation shows high efficiency for HCHO removal and is regarded as a promising technology for HCHO emission control.The key challenge of catalytic oxidation is to develop catalysts with high efficiency,low cost,good thermal stability,and water resistance.In this work,a combination of experiments,theoretical calculations,and microkinetic analysis was employed to develop the doped perovskite-type catalyst.The doping effects of HCHO catalytic oxidation over lanthanum-manganese perovskite（La Mn O₃）were revealed at a molecular level.The Mn site of La Mn O₃ perovskite was considered as the active site.Mn-site doping can modulate the catalytic activity of La Mn O₃perovskite.The transition metal atoms in the fourth period（Sc、Ti、V、Cr、Fe、Co、Ni、Cu、Zn）were selected as the dopants.The oxygen vacancy formation energy of the Mn-site doped La Mn O₃ catalyst and the activation energies of HCHO dehydrogenation was calculated through the density functional theory（DFT）calculation method.A negative linear correlation between the surface oxygen vacancy formation energy of catalysts and the descriptorΦof dopants was found.The descriptorΦwas constructed of atomic electronegativity（EN_M）and atomic radius（R_M）（Φ=EN_M/R_M）.The Bronsted-Evans-Polanyi（BEP）relationship was found between the activation energy and the reaction heat of HCHO dehydrogenation steps.The oxygen vacancy formation energy can be predicted by the descriptorΦof the dopants,and the activation energy of HCHO dehydrogenation can be predicted by the reaction heat.Further,the dopants that can both promote the formation of oxygen vacancy and reduce the activation energy of HCHO dehydrogenation were selected,including Ni,Cu,and Zn.The Ni-,Cu-,and Zn-doped La Mn O₃ perovskites were synthesized based on the above results,and the HCHO catalytic oxidation performance of catalysts was tested.The HCHO catalytic performance of La Mn O₃ was promoted by the dopants.The Cu-doped La Mn O₃catalyst showed the highest catalytic performance.The complete oxidation of HCHO was achieved by Cu-doped La Mn O₃at 180°C,which was 40°C lower than that by La Mn O₃.The interaction between metal and oxygen atoms was weakened and the redox ability was improved by the dopants.The characterization and experimental results were consistent with the DFT calculation results,demonstrating the feasibility of the screening strategy.The La atoms in the La Mn O₃ perovskite played a role in stabilizing the crystal structure.The La-site doping was proved to enhance the catalytic activity of La Mn O₃ catalyst for HCHO oxidation.However,the mechanistic understanding about the effect of La-site doping on catalyst activity was still unclear.The DFT calculations and microkinetic analysis were used to screen the eleven kinds of La-site dopants（Y、Zr、Nb、Mo、Ru、Rh、Pd、Ag、Sr、In、Sn）.The oxygen mobility was facilitated by Sr,Ag,and Sn dopants.The Sr dopant could promote the oxygen migration and HCHO dissociation.The second dehydrogenation step of HCHO has the largest degree of rate control value at low temperature and the oxygen adsorption step controls the whole reaction at high temperature.Although the La-site doping could promote the catalytic activity of La Mn O₃for HCHO oxidation,the stability was depressed after La-site doping.In order to improve catalyst activity and stability,the Cu and Sr atoms were respectively used as Mn-and La-site dopants in La Mn O₃ catalysts.The Cu and Sr co-doped La Mn O₃catalysts were prepared for HCHO catalytic oxidation.The catalyst with the Cu substitution content of 0.2 presented the best HCHO oxidation performance and achieved the complete oxidation of HCHO at 160°C.Meanwhile,this catalyst presented good stability and the HCHO oxidation efficiency remained above 95%within 24 h.The in-situ FTIR and DFT calculation was performed to provide the mechanistic insight into the effect of Cu and Sr dopants on HCHO catalytic oxidation.The synergistic effect between Cu and Sr would accelerate the charge transfer between HCHO and catalyst surface,thus promoting HCHO adsorption and dissociation.The acid treatment was employed to improve the low-temperature activity of La Mn O₃further.The catalyst morphologies and HCHO catalytic oxidation activity were significantly influenced by the nitric acid treatment（HNO₃-La Mn O₃）.The HNO₃-La Mn O₃ showed excellent low-temperature activity.Compared with the undoped La Mn O₃catalyst,the temperature that achieved 50%HCHO converision on HNO₃-La Mn O₃ catalyst was reduced by 67°C,and the complete conversion temperature was reduced by 40°C.The surface La and Sr cations were selectively dissolved by the dilute nitric acid,and the specific surface area was improved by the diluted nitric-acid treatment.These are beneficial for HCHO adsorption and oxidation.The stability test showed that the oxidation efficiency of HNO₃-La Mn O₃ remained above 95%within 48 h.After the injection of water vapor,the catalytic efficiency also remained above 95%within 24 h.Thus,HNO₃-La Mn O₃ showed excellent stability and water resistance.The research layed a foundation for the practical application of the doped La Mn O₃ perovskite in HCHO removal from industrial emissions.