In-situ Study Of Heterogeneous Catalytic Reaction Process By Surface Enhanced Raman Spectroscopy

Constructing a model catalytic system and using advanced spectroscopic techniques to in situ study the catalytic reaction process are of great significance for understanding the structure-activity relationship and mechanism of catalytic reactions.However,in situ study on mechanism remains a big challenge.Raman spectroscopy can provide fingerprint information of adsorbed species on the catalyst surface,but its sensitivity is too low to monitor trace adsorbed species on the catalyst surface.To solve this problem,this work established nano-model strategy for in situ studying catalysts with different scales and the surface reactions by constructing a nano-model catalytic system,combining SERS spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy(SHINERS).The gap mode or SHINERS satellite structure strategy can be used to in-situ detect the dynamic changes of intermediate products on the catalyst surface during catalytic reactions,such as interfacial hydrogen spillover,catalytic hydrogenation,and catalytic oxidation.Furthermore,by regulating the shell composition of shell isolated nanoparticles(SHINs)and the structure of the catalyst surface interface,single atom catalysts(SACs)were constructed.The structure characterization of the SACs and the in-situ observation of the reaction process were realized.The main results of this thesis are outlined as follows:1.Understanding the activation and transfer of hydrogen at the interface in catalytic hydrogenation is crucial and remains a major challenge.With the help of magnetron sputtering technology,LB film technology,and liquid-oil interface film formation technology,we have successfully prepared atomic flat Pt films,adjustable thickness TiO2 films,and single-layer Au nanoparticle films.Finally,an exquisite Au/TiO2/Pt interlayer nanostructure was successfully constructed,where surfaceenhanced Raman spectroscopy(SERS)has 10 nm spatial resolution.This model structure allows the activation of hydrogen,hydrogen spillover,and hydrogenation reaction to proceed separately on Pt,TiO2,and Au.The influence of the interfacial structure on the catalytic performance was then studied by in situ SERS using the reduction of para-nitrothiophenol(pNTP)to para-aminothiophenol(pATP)as a probe reaction.In situ SERS studies show that atomic hydrogen can effectively spill over at the Pt-TiO2-Au interface,and the final spillover distance of TiO2 is?50 nm.Combining kinetic isotope experiments and density functional theory calculations,we find that the hydrogen spillover proceeds via the water-assisted cleavage and formation of surface hydrogen-oxygen bond.More importantly,the selectivity of nitro or isocyano hydrogenation can be controlled by controlling the overflow distance of hydrogen.This work provides molecular insights into the hydrogen activation process.2.In this work,a gap-mode structure composed of SHINs/nanocatalyst/SiO2/Au film multilayer structure was prepared and used for in-situ SERS to study the structureactivity relationship of benzyl alcohol selective oxidation on Au@Pd nanocatalyst.Catalytic experiments show that the catalytic performance of Au@Pd nanocatalyst is significantly better than that of Pd nanocatalyst,and its catalytic activity has a volcanic curve relationship with the increase of Pd shell thickness.In situ SERS studies have shown that during the oxygen activation process,competition between electronic effects and tensile stress results in volcanic-like behavior.For Au@Pd with a thin shell,the electronic effect is dominant and the activation of oxygen is suppressed.As the thickness of the shell layer increases,the effect of tensile stress becomes more significant,promoting the formation of superoxide and improving the catalytic activity.Further increase in the thickness of the shell layer will cause the disappearance of electron and tensile strain effects,leading to a decrease in activity.This work not only improves the basic understanding of the structure-activity relationship for the selective oxidation of benzyl alcohol,but also provides a promising strategy for the in-situ study of nanocatalysis by Raman spectroscopy.3.Advanced characterization technology and in-situ technology are of great significance for studying the electronic and structural properties of single atom catalysts(SACs)and their catalytic processes.Using atomic layer deposition(ALD)technology,we coated Au and Ag nanoparticles with Raman-enhancing capability with a very thin TiO2,Al2O3 shell layer to prepare Au-SHINs and Ag-SHINs on a large scale.Then,atomically dispersed Pd catalytic active sites were deposited on the surface of the shell by means of wet chemical method,and the SHINERS-SACs satellite structure was constructed.Due to the different adsorption behavior of phenyl isocyanide(PIC)on the surface of single atom site and nanoparticles,the identification and in situ observation of the structure of atomic-level disperse catalysts and nanoparticle catalysts were realized.Taking the hydrogenation of pNTP to pATP as a probe reaction,we directly studied the reaction process of hydrogenation of pNTP to generate pATP on atomically dispersed Pd and Pd nanoparticles.This study provides a new strategy for the in situ study of atom-level dispersed catalysts.4.We constructed SHIENRS satellite structure by assembling the nanocatalyst on the surface of the SHINs using the electrostatic self-assembly strategy.The reaction process of CO2 hydrogenation on Pt and PtCo nanoparticles was studied by in situ SHINERS technique.In situ SERS showed that carbon dioxide on the Pt surface was adsorbed on the catalyst surface in the form of CO32-.For PtCo,a weakly adsorbed state of CO2?-was also observed apart from the CO32-adsorption form.The high stability of carbonate makes it difficult to be hydrogenated whereas the weakly adsorbed CO2?-is easily activated for hydrogenation.This can explain why the catalytic activity of PtCo is higher than that of Pt nanocatalyst.