Modulation Of The Surface Interface Of Cobalt-based Catalysts And Their Catalytic Toluene Oxidation Performance By Metal-organic Template Method

As a major air pollutant,volatile organic compounds(VOCs)are attracting more and more attention worldwide for their harmful effects on human health and the environment.Among the many methods to treat VOCs,catalytic oxidation has become one of the most promising methods due to its advantages of lower operating temperature and fewer by-products,the key to which is the development of efficient catalysts.In the past,researchers have mainly focused on the effects of topography and size on the performance of catalysts,but have seldom explored the regulatory mechanisms at the surface interface.In this paper,toluene was used as the target pollutant,cobalt-based catalysts were studied,and the metal-organic template method was used as the core method to investigate the changes in the surface interface of catalysts and their influence on catalytic activity by controlling the template precursor synthesis process and template pyrolysis.The main conclusions are as follows:(1)Hexagonal tube-shaped Co₃O₄ was prepared by metal-organic template derivatization, and the formation process of single-crystal precursors of special hexagonal tube-shaped two-dimensional metal-organic layers(2D MOLs)was followed by time-dependent experiments,and found that it contains two complementary formation mechanisms:the transformation of crystals to crystals in solution and the dissolution of inner crystals+migration(toward the outer surface)+recrystallization of inner crystallites to reduce the surface energy,the Gibbs free energy calculation also shows that the crystal-to-crystal transition is spontaneous,and finally a macroscopic hexagonal tube morphology is formed under the combined action of Me CN.Compared to other Co₃O₄with different morphologies prepared by other interfering agents,Co-Me CN-O shows better catalytic oxidation activity of toluene(T₉₀=227?,E_a=69.5 k J mol^-1)and good stability.A series of characterizations proved that Co-Me CN-O has more structural defects,more surface adsorbed oxygen species,lower reduction temperature and greater surface Co³⁺concentration than other synthetic Co₃O₄ catalysts,which together lead to its excellent activity.(2)The Pt-Co₃O₄ catalyst namely Pt-Co(OH)₂-O was successfully prepared by the overall metal-organic template conversion.The template conversion resulted in the conversion of the catalyst morphology from rhombic dodecahedron to flower clusters composed of ultra-thin nanosheets,completing the conversion from three-dimensional morphology to two-dimensional topography.This process makes Pt nanoparticles(NPs)more exposed and further enhances the strong metal support interaction(SMSI)between Pt NPs and Co₃O₄ support.Pt-Co(OH)₂-O shows excellent toluene catalytic performance(T₉₀=167?,E_a=40.85 k J mol^-1,TOF_Pt=2.68×10-3 s^-1)and good stability.The XPS test proved the enhancement of SMSI caused by template conversion,and a series of characterizations proved that it led to the highest Pt⁰ content,more oxygen species,higher structural defect rate and lower reduction temperature performance than other synthetic Pt-Co₃O₄ catalysts in the Pt-Co(OH)₂-O catalyst,which will facilitate the catalytic reaction.The in situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)experiments show that the introduction of Pt NPs to form SMSI weakens the Co-O bond and facilitates oxygen migration.The change of binding energy and the degree of oxygen migration in the quasi in situ XPS experiment further confirmed that the Pt-Co(OH)₂-O catalyst has stronger SMSI.These characteristics together lead to excellent catalytic performance,which provides a reference for adjusting the SMSI of supported catalyst.(3)Two types of Pt-Co₃O₄ catalysts,namely PCMO and P-CMO,were produced by pre-adsorption of Pt NPs onto metal-organic templates(Co-MOF-74,namely CM)and Co₃O₄supports,respectively,assisted by metal-organic templates,followed by roasting under different conditions.Among them,the PCMO catalyst with a template pyrolysis process has better catalytic oxidation activity of toluene than the P-CMO catalyst without a template pyrolysis process,and the PCMO400 catalyst shows the most excellent activity(T₉₀=180?,E_a=41.29 k J mol^-1)and shows good stability.By comparing the two types of catalysts,it is found that the dissipation of carbon during the pyrolysis of the metal-organic template of the PCMO catalyst promotes the stability of Pt NPs and is beneficial to the formation of SMSI.However,due to the impregnation method,the contact area between Pt NPs and the support is limited,after the calcination temperature is further increased,it is unable to further form a stronger SMSI,the catalyst as a whole is oxidized,the active material Pt⁰ decreases,and the low-temperature reduction performance decreases,which ultimately leads to a decrease in the toluene catalytic oxidation ability of the catalyst.Further studies are needed to prove the correlation between SMSI and catalytic activity.(4)A series of Pt-Co₃O₄ catalysts,namely PCPO,were obtained by controlled pyrolysis of metal-organic templates,and in situ grown Pt NPs loaded with metal-organic precursors were roasted at different temperatures in an empty atmosphere.Among them,PCPO400 shows the most excellent toluene catalytic oxidation performance(T₉₀=187?,E_a=45.42 k J mol^-1)and good stability.The catalyst formation process was traced by TG-MS,during pyrolysis,the metal-organic template first loses all water molecules,then the frame collapse organic part is completely decomposed and Co²⁺is oxidatively recombined to form the final product,lowering the heating rate will be beneficial for the frame collapse and the formation of Pt-Co interface.By selecting a more stable metal-organic template,the retention temperature of carbon elements in the roasting process is increased,providing a reducing environment during the heating process,protecting Pt NPs and facilitating the formation of SMSI.Upon further elevation of the roasting temperature,the oxidative SMSI effect is enhanced,which counteracts the reduction effect of the carbon elements.A series of characterizations such as XPS and ESR demonstrate that with elevation of the roasting temperature,oxidative SMSI between catalyst Pt NPs and Co₃O₄ support is enhanced,electron transfer capacity is intensified,and oxygen vacancies are increased,but excessive SMSI formation decreased the proportion of Co³⁺and Pt⁰ species,the degree of exposure of their active sites and their adsorption capacity,which ultimately led to a decrease in toluene catalytic activity.It can be seen that the SMSI formed at the surface interface of the loaded catalyst does not have an absolute facilitating effect in non-homogeneous catalysis,but excessive SMSI will affect the catalytic reaction.