Preparation Of Birnessite-based Catalytic Materials And Their Thermal-catalytic/Photo-catalytic Performances

Volatile organic compounds（VOCs）pose a serious threat to human health.Developing efficient catalytic materials is an important way to solve the problem of VOCs pollution,which has significant practical significance and application prospects.VOCs are mainly hydrocarbons,and the key to their degradation lies in the fracture and recombination of C-H,C-O or C-C bond.This work investigates the relationship between the structure and efficiency of birnessite-type Mn O₂,designs and prepares a novel porous birnessite nanospheres catalyst with high photocatalytic/thermal catalytic performance.The photocatalytic/thermal catalytic activity of the birnessite is further improved by ion doping,heterojunction construction.Formaldehyde is the most common VOCs,and its molecular structure contains C-H and C-O bond.Therefore,this paper takes formaldehyde as a representative,and evaluates the photocatalytic/thermal catalytic oxidation performance of the catalyst based on its ability to catalyze the oxidation of formaldehyde,and further discusses its degradation mechanism.The main conclusions and innovative points of the paper are as follows:（1）Based on density functional theory,the molecular structure of Mn O₂ catalyst was designed,and the influence of the type of water,manganese vacancy and exposed crystal surface on its photocatalytic/thermal catalytic oxidation activity was revealed.birnessite contains three different types of water:adsorbed water,interlayer water,and structural hydroxyl groups.The surface hydroxyl groups can improve the ability to adsorb formaldehyde and promote the conversion of formaldehyde to formate.Adsorbed water and interlayer water can supplement the consumed hydroxyl groups,enabling birnessite to have the ability to continuously catalyze oxidation;The manganese vacancies in birnessite can serve as capture centers under visible light irradiation,accelerating electron transport and generating a large number of active radicals,promoting the activation of molecular oxygen and increasing the number of surface superoxide radicals;There are more oxygen vacancies on the（002）crystal plane,which can react with water molecules to promote the formation of hydroxyl radical,and the catalytic activity of birnessite with more（002）crystal planes is stronger.（2）The seed crystal mediated growth method was developed to directly synthesize porous birnessite nanospheres from the acid pyrolusite-pyrite leaching solution.By selectively recovering Mn²⁺ions from the leaching solution through oxidation-reduction reaction,the crystal seeds of birnessite are obtained,and then further crystallized to form birnessite-type Mn O₂ through mediation method.With the extension of reaction time,the crystal seeds of birnessite are first transformed into block-like Mn O₂ with relatively smooth surfaces,and then porous nanospherical particles are formed;as the reaction temperature increases and the reaction time prolongs,the average oxidation state of Mn decreases.Low valent manganese species are more likely to form oxygen vacancies,increase the content of active groups,and enhance photocatalytic/thermal catalytic activity.（3）The process and mechanism of catalytic degradation of formaldehyde by Fe³⁺,Co²⁺,and Ni²⁺ion doped birnessite were elucidated.Based on L-H and E-R mechanism,five possible reaction paths of HCHO catalytic oxidation were identified,and the energy change and reaction heat of each step,as well as the speed control step of each path,were further studied through the transition state search method;Revealed the mechanism of high catalytic activity of Fe³⁺doped birnessite;Fe³⁺doped birnessite with high redox reversibility（Fe（III）（?）Fe（II）（?）Fe（I））can capture more photo excited electrons,accelerate the interface charge transfer process,and generate a large number of activated oxygen species,thereby improving the photocatalytic/thermal catalytic performance of HCHO oxidation.（4）The birnessite was uniformly loaded onto the hydrated calcium silicate substrate using in-situ loading method.Furthermore,it was also preparedα-Fe₂O₃/Ti O₂/birnessite@hydrated calcium silicate composite material.The composite material can completely remove formaldehyde gas under visible light irradiation and has excellent catalytic stability.This composite material uses hydrated calcium silicate as the base material,and can be directly pressed into calcium silicate boards without any adhesive or surfactant,making it have broad application prospects in the field of indoor decoration.