Surface Interface Design Of Cobalt-Based Catalysts And Study On Their Catalytic Oxidation Behaviors

As China is still in the period of rapid economic development,the growth trend of anthropogenic emissions of air pollutants（such as VOCs and CO,etc.）is significant and will continue for a long time.In order to improve ambient air quality,it is very critical to control pollutant emission standards.At present,adsorptive concentration and catalytic oxidation technologies are combined to treat low concentration air pollutants.However,there are still some problems in catalysts,such as unsatisfactory low temperature catalytic performance,poor sintering resistance and the poisoning of catalysts by water vapor in the reaction atmosphere.Therefore,it is urgent to break through the constraints of the key catalyst materials for pollution control and form innovative air pollutants emission reduction cutting-edge technology.According to the principle of nano-catalyst interface design,a series of cobalt-based catalysts with different surface/interface structures are accurately constructed around the above industrial problems.Herein,toluene,CH₄ and CO are selected as reaction model pollutants for basic research.Combing experiments with theoretical calculations reveal the structural-activity relationship between surface structure and catalytic behavior.The main research contents and conclusions are as follows:1.Surface Cu decorated Co₃O₄ catalyst（Cu-Co₃O₄-Q）is obtained by a simple quenching process.The hot Co₃O₄nanosheets are poured into an aqueous solution of copper nitrate to obtain Cu-Co₃O₄-Q catalyst.The Cu-Co₃O₄-Q catalyst exhibits a significant photothermocatalytic activity for toluene oxidation under the irradiation of full solar spectrum with light of 834 mw/cm^-2.The reaction rate of Cu-Co₃O₄-Q is 1.76 times higher than that of Co₃O₄.The Cu decoration is confined to the limited surface layers of Co₃O₄,different from a traditional hydrothermal synthesis method（Cu-Co₃O₄-T）.Cu²⁺substitution for Co²⁺is preferred over Co³⁺on the surface of Cu-Co₃O₄-Q and thus produces more active sites,exhibiting better low-temperature catalytic oxidation activity.Moreover,Cu-Co₃O₄-Q catalyst shows comparably catalytic activity as partial supported noble metal catalysts.2.Co³⁺-rich exposed crystal facets of Co₃O₄ hexagonal nanosheets evoluted from ZIF-67are prepared by a facile and controllable reflux method（Co₃O₄ HN-T,T is the reflux reaction temperature）.The structure-activity relationship between active crystal planes of Co₃O₄ catalyst and water resistance is preliminically determined.The protons from the hydrolysis of metal ions etch the edge of Co（OH）₂ hexagonal nanosheets with releasing partial Co ions,then reacting with hydroxyl ions released from the protonation of dimethylimidazole（2MI）in pure water,thus giving rise to Co（OH）₂ with abundance of Co OOH.Transformation effect induced by Co OOH plays a key role in the transformation of the exposed（111）crystal facets of Co₃O₄ into（112）planes.The derivative Co₃O₄ HN-100 catalyst exhibits enhanced catalytic activity for CO and hydrocarbons（methane and toluene）oxidation,which is attributed to more Co³⁺active sites on the（112）planes.Furthermore,the Co₃O₄ HN-100 catalyst enclosed with（112）planes remains better catalytic activity than Co₃O₄ HN-60 in rich-moisture feed gas.Temperature-programmed reduction techniques together with DFT calculations are performed to further investigate the structure-activity relationship between the catalyst and catalytic activity.The first C-H bond activation to form oxygen vacancies（1.84 e V）in surface lattice oxygen on Co₃O₄（112）is easier than that（3.12 e V）on Co₃O₄（111）,while activation barrier（2.00 e V）on Co₃O₄（112）from CO₂ to bicarbonates is higher than that（0.60 e V）on Co₃O₄（111）,as a result of boosting catalytic oxidation and water resistance.3.Cu O_x/Co₃O₄ catalyst with Cu（I）-O-Co interfaces is further prepared via a facile co-precipitation,achieving the significant enhancement of low-temperature catalytic activity and water resistance.Experiment results together with DFT calculations not only verify that Cu⁺species are stabilized over Cu（I）-O-Co interfaces because of the strong interaction between Cu₂O₁and Co₃O₄,but also demonstrate that the Cu（I）-O-Co interface facilitates oxygen species activation for promoting catalytic oxidation and reduces the accumulation of hydroxyl and bicarbonate species on the surface of Cu O_x/Co₃O₄.DFT calculations also reveal that the CO oxidation rate-limiting step via the dissociation pathway under dry conditions is the Co-OO-Cu species dissociation while the CO coupling with the adsorbed O₂ becomes the rate determining step for CO oxidation through the direct pathway over Cu（I）-O-Co interfaces under humid conditions.Further,the Cu O_x/Co₃O₄ catalyst also exhibits superior and stable catalytic activity for toluene oxidation,comparable with partial supported noble metal catalysts(T₉₀=190°C).4.A Cobalt-based high-entropy oxide（HEO）support-supported Pt catalyst with continuous interface structure is obtained via a sol-gel assisted mechanical milling strategy to achieve both superior CO low-temperature catalytic activity,water resistance and high hydrothermal stability,promoting the application of nano-catalyst in harsh catalytic environment.Specifically,the interaction between HEO and Al₂O₃ is used to inhibit further growth of HEO microparticles while another interaction between Pt and HEO is designed to stabilize Pt species on HEO surface under harsh conditions.Comprehensive characterizations including STEM,XAFS and in-situ CO DRIFTS demonstrate the existence of isolated Pt species on HEO.Temperature-programmed techniques further verify the existence of Pt-O-M bonds（M=Co,Fe,Ni,Cu,Zn）and surface lattice oxygen species can more easily extracted from the surface of HEO.The obtained Pt-HEO/Al₂O₃ heterogeneous catalyst not only shows high hydrothermal stability at elevated temperature（750°C,10%H₂O,10 h）but also has ultrahigh durability（540 h）.Cofeeding water obviously enhances CO low-temperature catalytic activity and 100%conversion can be obtained at 150°C-achieving the“150°C challenge”.Remarkably,the self-evolving of ionic Pt species to stable dual oxidic-metallic Pt phase boosts low-temperature catalytic activity of Pt-HEO/Al₂O₃during long-term activity tests.