Synthesis,performance And Catalytic Mechanism Of Supported Pt Catalysts For Formaldehyde Oxidation

Formaldehyde（HCHO）is one of the most common and toxic indoor air pollutants,which has been identified as carcinogen by the World Health Organization.However,as an important chemical feedstock,HCHO is widely used in indoor construction/decoration materials and textile industry,which are the main emission sources of HCHO.Therefore,seeking safe and efficient means for eliminating HCHO to improve indoor air quality has become an important and urgent environmental problem.Among the known HCHO elimination technologies,the thermal catalytic oxidation method is widely recognized as the most promising option owing to its high HCHO removal efficiency,free-of toxic byproducts and saving energy.Crucial to enabling this vision is the development of HCHO oxidation catalyst with high activity,stability and moisture tolerance,as well as low-cost.Precious Pt is recognized as the most active catalyst for HCHO oxidation.For decades,a series of oxide-supported Pt catalysts that can catalyze the complete oxidation of formaldehyde at room temperature have been successfully developed by optimizing the Pt active component and the properties of support materials.But as a whole,the catalytic performance and price of the existing supported Pt catalysts is still far below the requirements of commercial application.In terms of mechanism research,although the catalytic reaction pathway of HCHO oxidation was proposed,the understanding of the nature of the active site was lacking,which led to the lack of guidance for optimization design of catalyst composition/structure.This seriously restricts the exploration of highly efficient Pt-based catalysts for HCHO oxidation.Based on the current research status,this thesis aims to develop a highly efficient and inexpensive HCHO oxidation catalyst,choosing the transition metal oxide supported Pt catalyst as the research objects,focusing on the scientific/technical issues such as the nature of active site,the regulation of the defect structure of oxide and the construction of high-density synergistic active sites.The main progress achieved is as follows:（1）A hollow octadecahedralα-Fe₂O₃ supported Pt catalyst was synthesized using a simple hydrothermal method followed by an impregnation-reduction process.It was found that the octadecahedrons selectively exposed{113}and{104}high-index facets,which could help to improve the dispersed state of Pt,as well as to increase the surface O vacancy concentration.Benefitting from the successful regulation of the high-index facets,the Pt/Fe₂O₃ catalyst enabled a 100%conversion of 90 ppm HCHO in a feed stream with a gas hourly space velocity（GHSV）120 L g^-1_cat h^-1 at a temperature as low as 20 ^oC,and its mass-specific reaction rate reached up to 37.09μmol g _Pt^-1 s^-1 at room temperature,which was five-fold higher than that of the commercial Fe₂O₃ supported Pt catalyst.This work indicates the catalytic performance of HCHO oxidation catalysts can be effectively improved by tailoring the surface structure of the support materials,which provides a feasible solution for the development of highly efficient HCHO oxidation catalysts.（2）The Pt/FeO_x was also selected as the research object.A supported Pt catalyst on Fe-W-O amorphous nanosheets was synthesized using a one-step solvothermal method by employing a combination of amorphization and second element doping modification strategies.It was found that the Pt nanoparticles with the size of 1～2 nm were well dispersed on the Fe-W-O surface.The control experimental results revealed that amorphization and W-doping could jointly promote the formation of oxygen vacancy in FeO_x substrate,thus increasing the density of Pt/O vacancy synergistic active sites for HCHO oxidation.The optimal catalyst(Pt/a-Fe W_0.08O_x)exhibited an extraordinarily high specific reaction rate of 68.3μmol g _Pt^-1 s^-1（25°C）and excellent stability during 24 h continuous test([HCHO]=120 ppm,GHSV=900 L g^-1_cath^-1),outperforming most existing HCHO oxidation catalysts.（3）In an effort to identify the catalytic active site and better understand the catalytic mechanism,the representative Pt/Ti O₂ catalysts were selected as the research object.A series of Pt/Ti O_2-x catalysts with different oxygen vacancy concentration and spatial distribution were prepared via the hydrogen spillover effect of Pt site over Pt/Ti O₂ catalyst during annealing treatment under H₂ atmosphere.A combination of XPS,EPR and an atomic resolution STEM-EELS analyses have been conducted to probe the concentration and spatial distribution of oxygen vacancy.The microscopic and spectroscopic results revealed that the spatial extent of oxygen vacancy zone preferentially existing in the vicinity of Pt nanoparticles and gradually expanded from the interface to the region far from Pt.The results of catalytic performance test and control experiment showed that Pt/Ti O_2-x catalyst exhibited a positive linear relationship between catalytic activity and oxygen vacancy concentration in the low reduction temperature range.However,there are no relationship between the catalytic activity and the concentration of oxygen vacancy,while the reduction temperature was increased to higher than 200°C.In situ spectroscopy in combination with online GC analysis further provided complementary insights into the functional roles of Pt and oxygen vacancy in HCHO oxidation:oxygen vacancy serves as active site for chemically absorbing HCHO molecules and the adjacent Pt site dissociatively activates O₂.These results revealed that HCHO oxidation occurs at Pt/Ti O_2-xinterface via a cooperative mechanism,whereas the oxygen vacancies located at far away from Pt may not contribute to the catalytic reaction,due to the limited transfer of active oxygen species on oxide surface.A reasonable explanation for this phenomenon is that the limited transfer of active O species on Ti O₂ surface makes them inaccessible to the sites far away from Pt.Hence,the key role of metal/support interfacial structure in the HCHO oxidation has been successfully elucidated by using the advanced analytical techniques and design experiments,which provides a general guideline for the rational design and controllable synthesis of highly active catalyst for HCHO oxidation.（4）Under the guidance of the catalytic mechanism obtained in the previous chapter,a H₂Ti_xO_x nanotube supported Pt catalyst with high density Pt/O-vacancy synergistic active sites was designed and synthesized by via a three-step procedure（hydrothermal-impregnation-H₂reduction treatment）.It was found that the specific surface area of the H₂Ti_xO_x nanotube reached up to 173.6 m² g^-1.Moreover,a highly dispersed Pt nanoparticle（1.4±0.3 nm）supported on the H₂Ti_xO_x surface could be readily achieved by using Na₂[Pt（OH）₆]as Pt precursor.This in combination with the created oxygen vacancy in the vicinity of Pt nanoparticles via hydrogen spillover effect,thus constructing a high-density Pt/O-vacancy synergistic activity site.As a consequence,the Pt/H₂Ti_xO_x catalyst exhibited an excellent performance for HCHO oxidation,its specific activity reached up to 113.0μmol g _Pt^-1 s^-1.In addition,the Pt/H₂Ti_xO_x catalyst displayed an excellent stability and good moisture tolerance towards HCHO oxidation at room temperature.The comprehensive catalytic performance of the Pt/H₂Ti_xO_x catalyst outperformed most existing HCHO oxidation catalysts.This work may lay technical basis for the design and synthesis of practical noble metal catalysts for HCHO oxidation.