Design And Application Of Layered MnCoLDHs-based Catalysts For The Room Temperature Catalytic Oxidation Of Formaldehyde

Indoor pollutants have become one of the five most important environmental factors affecting public health,among which formaldehyde is an indoor pollutant with significant adverse effects on the environment and humans,and how to eliminate formaldehyde at room temperature in a green and efficient manner has become a hot research topic.Among the reported formaldehyde elimination technologies,catalytic oxidation based on loaded precious metal catalysts is currently one of the most effective methods to treat indoor formaldehyde.Catalytic oxidation of formaldehyde at room temperature is considered to be the most promising method for formaldehyde removal due to its unique advantages,which can completely oxidise formaldehyde to CO₂ and H₂O without secondary pollutants being generated.The efficiency of catalytic oxidation of formaldehyde depends on the performance of the catalyst,of which precious metal loaded catalysts have strong low temperature oxidation activity and stability,but their expensive price limits their application in real life,while non-precious metal catalysts,although inexpensive,have poor thermal stability,unstable structure and easy loss of active components,resulting in low oxidation activity of formaldehyde.Therefore,in the process of catalyst preparation,transition metal-based catalysts are used as carriers,and active oxygen species（adsorbed oxygen,hydroxyl group,lattice oxygen,etc.）are introduced into the catalysts by suitable methods to optimise the formaldehyde oxidation pathway and enhance the catalytic activity.In this thesis,based on the properties of Mn Co LDH such as layered structure and abundant surface hydroxyl groups,the active sites were introduced and the content of active oxygen species was enhanced by a small amount of noble metal loading or doping,non-precious metal doping and reduction treatment,etc.The noble metal Pd-loaded,noble metal Ag-doped and non-precious metal M（M=Fe,Cr,Ce）-doped Mn Co LDH nanocatalysts were successfully constructed and explored The conformational relationship between the catalyst surface structure and the oxidation performance of formaldehyde,as well as the catalytic oxidation mechanism of formaldehyde on the catalyst were investigated.Details of the study are as follows:（1）Hydroxyl-rich Mn Co LDH was prepared by co-precipitation with Na BH₄ solution pre-reduction carriers,and 0.1 wt.%,0.3 wt.%and 0.5 wt.%Pd loading was used to modulate the noble metal dispersion and oxygen vacancies by deposition-precipitation,and to promote the ambient catalytic oxidation of formaldehyde by low content Pd-based catalysts in comparison with catalysts directly loaded with Pd by Mn Co LDH,to obtain the optimum catalyst was 0.3Pd/Mn Co LDH-Na BH₄（0.3Pd LN）.The results of the formaldehyde activity test showed that the catalyst completely oxidised formaldehyde at a concentration of 50 ppm at a vacancy rate of 24 000 m L/（g·h）at 30°C and was stable for 24 h.The characterisation results showed that the catalyst had a higher specific surface area,lower Pd valence and better Pd dispersion,and the electron transfer efficiency between LDH and Pd was improved after reduction,forming more oxygen vacancies to activate oxygen,while retaining the abundant hydroxyl groups on Mn Co LDH to enhance the catalytic oxidation activity of the catalyst.The structural characterization of the catalyst was combined with activity testing to recognize the key role of adsorbed hydroxyl groups and adsorbed oxygen on the catalyst in the oxidation of formaldehyde,and to propose a possible mechanism of the catalytic oxidation reaction,where the presence of Pd promoted the formation of surface adsorbed oxygen,and the pre-reduction of Mn Co LDH by Na BH₄ retained the abundant surface hydroxyl groups,and the formaldehyde was decomposed by the synergistic effect of surface adsorbed oxygen and surface hydroxyl groups.（2）The catalyst in the first system suffers from the high price of the precious metal Pd and the poor stability of the catalyst.Based on this,in the second system the active oxygen,which plays a major role in the catalyst,was changed by improving the preparation method and choosing the less expensive noble metal Ag.A series of Ag Mn Co LDH（Ag MCL）catalysts with different Ag contents were prepared by a one-step co-precipitation method,and Ag Mn Co LDH-S（Ag MCL-S）catalysts were obtained by doping Ag into Mn Co LDH by a conventional impregnation method,and the two catalysts were investigated for formaldehyde oxidation after H₂ reduction treatment.The results of the comparative activity tests showed that the catalytic oxidation activity of formaldehyde increased with increasing Ag doping and H₂ reduction treatment in the Ag MCL and Ag MCL-S series catalysts,with 8Ag MCL-H being the optimum catalyst for the complete oxidation of formaldehyde at 30°C at a concentration of 30 ppm and a vacancy rate of 22 200 m L/（g·h）and stable for more than 100 h,which was much better than 8Ag MCL-S-H has a much better formaldehyde oxidation performance.Moreover,the Ag in 8Ag MCL-H was more uniformly dispersed,with more surface hydroxyl groups,adsorbed oxygen and lattice oxygen,easier electrochemical reaction and better electron transfer capacity,which facilitated the redox reaction.The structural characterization of the catalyst was combined with the activity characterization to cognize the synergistic effect of adsorbed oxygen,lattice oxygen and hydroxyl groups on the catalyst in the oxidation of formaldehyde to achieve the efficient degradation of formaldehyde by the catalyst.The mechanism of the catalytic oxidation of formaldehyde was also proposed by XPS and in-situ DRIFTS results.（3）After exploring the first two systems it was found that the key to the catalytic oxidation of formaldehyde is the introduction and retention of the oxygen active species.The use of noble metals was still an issue for the second system,so the third system was chosen to be doped with non-precious metals to further investigate the oxygen species at play.MMn Co LDH（M=Fe,Cr,Ce）catalysts were prepared using a one-step co-precipitation method by doping Mn Co LDH with different non-precious metals,and active oxygen species with abundant hydroxyl and lattice oxygen were constructed by H₂ reduction treatment.The catalysts were tested for their performance in the catalytic oxidation of formaldehyde and0.2Ce Mn Co LDH-H（0.2Ce MCL-H）had the optimum capacity to oxidise formaldehyde.Formaldehyde at a concentration of 30 ppm and a vacancy rate of 22 200 m L/（g·h）was completely oxidised at 30°C and stabilised for more than 120 h.Comparing x Ce MCL with different Ce content doping（x=0.1,0.2,0.3）,the H₂ reduction treatment revealed that excessive Ce doping rather limited the formaldehyde oxidation performance.The characterisation results show that 0.2Ce MCL-H has higher specific surface area,pore volume and average pore size,higher lattice oxygen,and electron transfer occurs between Co²⁺/Co³⁺,Mn³⁺/Mn⁴⁺and Ce³⁺/Ce⁴⁺,favouring formaldehyde oxidation.