Efficient Pt And Co-based Catalysts For Formaldehyde Oxidation At Room Temperature

Formaldehydehy（HCHO）is a typical indoor air pollutant,which has strong stimulating,allergic,toxic,teratogenic and cancer-causing effects.If people stay in an indoor environment with excessive formaldehyde for a long time,it may cause people to suffer from various diseases.Common methods for removing formaldehyde mainly include catalytic oxidation,activated carbon adsorption,high temperature plasma,and photocatalysis.Among them,catalytic oxidation technology uses catalyst to activate oxygen to completely convert formaldehyde into non-toxic and harmless CO₂ and H₂O.At present,developing efficient catalysts and clarifying the reaction mechanism are the hotspots and difficulties in the research of formaldehyde catalytic oxidation.In this paper,Pt based and Co based catalysts were prepared by optimizing the conditions,respectively.The results showed that both types of catalysts showed excellent activity and stability in the catalytic oxidation of formaldehyde.In addition,we also explored their reaction mechanism by characterization techniques such as in situ DRIFTS,and elucidated that the microscopic environment around the catalyst’s active center was crucial to their performance.This study provides a certain idea for the design and development of high-performance formaldehyde oxidation catalysts.The main research contents and results of this paper are as follows:（1）Oxides are widely used as supports for noble metal catalysts due to the presence of a large number of oxygen vacancies on the surface and a large specific surface area.In the catalytic oxidation reaction of formaldehyde,oxides have been deeply studied.The hydroxyl groups on its surface can promote the adsorption of HCHO and the decomposition of formate intermediates produced in the reaction,while the synergistic effect of non-oxide supports is rarely studied.In this paper,a series of catalysts with different carbon nitride supports and different Pt loadings were prepared by controlling the conditions.The activity test results showed that Pt/C₃N₄-650 had the most excellent catalytic performance.When the loading amount of Pt was as low as 0.48 wt%,80 ppm of formaldehyde could be completely converted at room temperature,and the space velocity was as high as 50000 m L·g^-1·h^-1.Through studying the mechanism,it was found that the pyridine nitrogen in carbon nitride could create Lewis base sites,which could effectively adsorb and activate oxygen molecules,and then cooperated with Pt nanoparticles to achieve efficient catalytic oxidation of formaldehyde at room temperature.The content of pyridine nitrogen increased with the increase of carbon nitride preparation temperature.（2）Since the low reserves and high prices of precious metals limit their practical applications,it is of practical significance to develop high-performance non-precious metal catalysts.In the past research on formaldehyde oxidation catalysts,most of the non-noble metal catalysts used transition metal oxides（such as Mn O₂,Co₃O₄,etc.）,and the non-transition metal oxides received little attention.It could be seen from the experiments that the Co@NC catalyst with a certain catalytic activity for the oxidation of formaldehyde.Meantime,in order to further optimize the structure of Co@NC and improve the activity of the catalyst,the number of nitrogen atoms coordinated with the cobalt atom in the active center was regulated by changing the preparation temperature of the catalyst.When the preparation temperature was raised to 800°C,900°C and1000°C respectively,Co N₄/C,Co N₃/C,and Co N₂/C were prepared.The activity test results showed that Co N₂/C had the best catalytic activity and stability.The conversion rate of 150 ppm formaldehyde could reach 88%at a space velocity of 60000 m L·g^-1·h^-1and it still maintain more than 70%within 55 hours.Through a series of characterization methods and theoretical calculations,it could be seen that the evaporation of nitrogen atoms in Co-N_x/C will form nitrogen vacancies that were conducive to oxygen adsorption and activation,thereby improving the activity of the catalyst for the catalytic oxidation of formaldehyde.In addition,Co-N_x/C was a single-atom catalyst,which not only with a very low active center loading,but also increased the utilization rate of metal atoms,thereby improving the catalytic oxidation activity of formaldehyde.This was also the first time that Co single-atom catalyst had been used in the catalytic oxidation of formaldehyde.