Study On The Hydrogenation Of Carbon Oxides By In-situ X-Ray Absorption Fine Structure (XAFS) Method

With the rapid changes and development in modern industry,more and more fossil fuels are consumed;the environmental issue caused by carbon oxides?COx?has become a major concern.How to realize the effective utilization of carbon oxides has attracted worldwide research interest.Converting COx into useful feedstock chemicals and fuels by hydrogenation reactions is an important strategy for removing carbon oxides from atmosphere,and for reducing dependence on petrochemicals.Currently,much work in this area has focused on the hydrogenation of carbon oxides.Hydrogenation of carbon oxides is a complex reaction system,whose products are widely distributed.Although many researches have been carried out on the reaction system,it is still a great challenge to determine the nature of the active sites,the role of promoters,and to improve the catalyst activity and selectivity of target product.X-ray absorption fine structure based on the synchrotron radiation facility is well positioned to study the chemical state and local geometry structure of the absorber atoms;In addition,it is well suitable for the in-situ study to dynamic monitor the structural evolutions of materials.Currently,in-situ XAFS method is widely used in the study of the structural evolution of catalyst.In this paper,we firstly designed and developed two kinds of in-situ XAFS apparatus.Combination X-ray diffraction?XRD?,transmission electron microscopy?TEM?and Temperature-programmed reduction?TPR?methods,the dynamic structural evolution of Cu-and Co-based catalysts was studied by in-situ XAFS technique.Maim results of this paper are shown as following:?1?Two kinds of in-situ XAFS devices were successfully designed and developed for the catalysis reactions.Based on the differences in the window materials,they were divided into: higher pressure and atomospheric pressure in-situ cells.The devices were easy to operate,and both of them could be used both in transmission mode and fluorescence mode.By the in-situ XAFS invesitigations of the commercial Cu O/Zn O/Al₂O₃ catalyst for methanol synthesis,higher quality XAFS data were obtained,which suggested the higher performance of the home-made devices.To account for the deviation from harmonic behavior,the cumulant expansion technique was used to correct the error in the bond contraction.In addition,In order to elucidate the contributions of the structure and thermal components to the Debye-Waller factors,the Einstein method was also used to analyze the data that provides a straightforward proof regarding the effect of the temperature on the in-situ XAFS experiment.?2?The CuO/ZnO/Al₂O₃ catalyst prepared by an oxalate gel co-precipitation method,whose activity increased as a function of the reaction temperature and pressure,were studied at calcined,reduced and reacted conditions.TEM images and additional EDX images showed that the uniform distributions of copper were achieved at the calcined and reduced conditions.TPR profile revealed two obvious reduction peaks located at 165 o C and 195 o C for copper species.QXAFS results demonstrated that the calcined form consists mainly of a mixed Cu O and Zn O,and it was progressively transformed into Cu metal particles and dispersed Zn O species as the reduction treatment.It was demonstrated that activation of the catalyst precursor occurred via a Cu+ intermediate,and the active catalyst predominantly consisted of metallic Cu and Zn O even under higher pressures.In addition,the structure of the active catalyst did not change with temperature or pressure which indicates that the role of the Zn was mainly to improve Cu dispersion.It also indicated the potential of QXAFS method in following the operate structure evolutions for the catalysts in methanol synthesis.?3?Structural evolution in a Co/Mn O model catalyst prepared by incipient wetness impregnation method was investigated by in-situ quick X-ray absorption spectroscopy under pressurized condition.During catalytic test of Fischer-Tropsch synthesis?FTS?over this Co/Mn O catalyst,an obvious induction period at higher pressure?10 bar?was observed that CO conversion increased almost twice compared with the initial value?1 bar?,while the selectivity for different products was almost constant.QXAFS measurements combined with in-situ XRD and HADDF-STEM demonstrated that the cobalt species with well distributions were gradually reduced to the mixture of Co⁰ and Mn1-x Cox O-type solid solution through in-situ treatment,and the metallic Co fraction in the catalyst was stable at75% in both ambient and pressurized conditions.However,the Mn O phase showed intimate interaction with cobalt species in the whole process,and the partial reduction of surface Mn O in the Co/Mn O catalyst took place only at pressurized condition.It is interesting to find a reconstruction phenomenon for the small cobalt particles after pressurized condition,which could be induced by the surface reduction of manganese oxide under higher reaction pressure.The presence of oxygen vacancy on manganese prompted the local migration of the cobalt atoms that contributed to the higher performance for FTS under pressurized conditions.?4?The structural evolutions of Co/Cu the binary-catalyst for FTS were investigated by in-situ XAFS method.Ex-situ XAFS results showed that Co3O4 and Cu O were formed at calcined state.After the reduction treatment,Co species was a mixture of Co⁰ and Co2+,while Cu species was totally Cu O.Co species could be completely reduced to metallic state at in-situ reduction treatment.EXAFS analysis further demonstrated the formation of Co-Cu alloy.It suggested that the addition of copper species in Co-based catalyst not only improve the distribution of cobalt but also reduce the reduction temperature of Co Ox.Hence,Co-Cu alloy created in Co/Cu binary-catalyst for FTS was identified as the main active site for its higher SMSI effect.