The Research Of CO Oxidation On The Surface Of Co₃O₄-based Catalysts: Reaction Process And Catalytic Performance

With the development of industrial society, air pollution has become very prominent and one of the main pollutants is CO. It is a very economic and environmental method to eliminate CO through the technology of catalysis at low temperature and the key to this method is the highly efficient catalyst for CO oxidation. Co3O4is considered as the most potential candidate as the alternative to the noble metal catalysts due to its high activity for CO oxidation. In this thesis, the methods of doping and supporting Au were used to modify the surface structure and surface chemical property of Co3O4and then tune CO adsorption and O2activation in order to further promote the catalytic performance and unravel the nature of the catalytic activity of Co3O4for CO oxidation. The results of our study are as follows:First of all, the Co3O4and Bi2O3-Co3O4samples were prepared by precipitation and co-precipitation method. The deposition of Bi2O3greatly enhanced the activity and stability of Co3O4for CO oxidation and the catalytic performance changed in the form of volcano curve with increasing doping amount of Bi2O3.20wt.%Bi2O3-Co3O4could completely convert CO to CO2at as low as-89℃and sustain the complete conversion for10hours at-75℃. XRD and Raman results showed moderate Bi2O3could enter the lattice of Co3O4, and promote the formation of the lattice distortion and structural defect. H2-TPR results showed that reduction of Co3O4was promoted and the diffusion of oxygen was accelerated due to the strong interaction between Bi2O3and Co3O4. XPS and EPR results showed the surface oxidation states of Cobalt species and the amount of Co2+in20wt.%Bi2O3-Co3O4were increased, which suggested the presence of structural defects. The kinetic data showed not only the amount of surface active sites were increased on the surface of Co3O4but also the catalytic ability of single active site was enhanced greatly.Secondly, aiming at achieving higher catalytic activity on CO oxidation at lower temperature, not only were sufficient CO adsorption and effective O2activation necessary, but also low energy barrier of CO reacting with active oxygen and rapid CO2desorption should be assured. Based on this rational, Based on this rational, we proposed the following three foundamental principles to meet the demands above:(I) lower M-O (metal-oxygen) bonding energy;(Ⅱ) larger cation radius;(Ⅲ) relatively lower electronegativity if possible. In2O3was chosen to simultaneously tune the activity of CO oxidation over doped Co3O4. As a result, the sample of25wt.%In2O3-Co3O4could completely convert CO to CO2at-105℃, which was the first time to achieve100%CO conversion below-100℃meanwhile the stability was greatly promoted. The results of characterization indicated that the doping of In2O3made the lattice of Co3O4expanded, which altered Raman symmetry of the tetrahedral sites (CoO4) and octahedral sites (CoO6) as well as the UV-visible absorption. Therefore the electron transfer from O2-to Co3+and/or Co2+became easier and the bond strength of Co-O was weakened. The doping of In2O3also made d-band center of Co downshift, which correspondingly weakened the adsorption of CO2and obviously inhibited the accumulation of surface carbonate species. The DFT calculations also confirmed the adsorption energy of CO, formation energy of oxygen vacancy and the reaction barrier of CO oxidation over In2O3-Co3O4surface were lowered.Last but not least, tuning surface oxygen vacancies over Co3O4played a very crucial role in stabilizing Au catalysts. According to this rational, the highly dispersed Au particles and single-atom Au catalysts over Co3O4were prepared. Single-atom Au could completely convert CO to CO2at as low as-100℃and sustain100%conversion at-75℃for20h. Also, Au catalysts could still exhibit good catalytic performance after pretreatment at different temperatures (25-300℃) in oxidative atmosphere for4times, which suggested the sintering effects were inhibited. Our XPS data confirmed that Auδ+species played a very important role in the catalytic reaction for CO oxidation. O2-TPD and CO-TPR experiments exhibited that the amount of oxygen desorbed from Au/Co3O4was higher than that from pure Co3O4, which suggested that the activation of oxygen was greatly enhanced and more active oxygen species could be supplied for CO oxidation. Meanwhile, the isotopic (18O2) results suggested that there might be two reaction channels for CO oxidation over single-atom Au/Co3O4:1) CO adsorbed on Co3+sites preferentially reacted O species adsorbed on single-atom Au, which was the main reaction mechanism;2) If the Au could not supply adequate active oxygen species to the catalytic cycle, partial lattice oxygen would participate the catalytic reaction, reacting with CO adsorbed on Co3+sites, which was the secondary reaction mechanism.