Catalytic Performance Of Pd-Cu/hydroxyapatite Catalyst For Low-temperature CO(Preferential) Oxidation

Since the implementation of Air Pollution Prevention and Control Action Plan and Three-Year Action Plan for Winning the Blue Sky War from 2013,the environmental air quality of China has been improved significantly.However,with the rapid development of the national economy,the living condition of people has been greatly improved,and the number of motor vehicles has increased significantly.The traditional automobiles brings convenience to mankind,produces a large amount of toxic and harmful gas containing CO,and causes serious environmental pollution problem.While mankind is actively eliminating the environmental pollutant CO,electric vehicles powered by proton exchange membrane fuel cells(PEMFC)are emerging.The fuel(hydrogen)used in the proton exchange membrane fuel cells is mainly obtained through the chemical processes such as methane steam reforming and water gas shift reactions,so the obtained hydrogen usually contains a certain amount of CO.However,proton exchange membrane fuel cells have extremely stringent requirement for CO content(<10 ppm),the presence of trace amounts of CO would poison or even deactivate Pt electrode.It can be seen that CO elimination in the atmospheric environment and hydrogen source has become very important and necessary.Regardless of whether there is hydrogen in the feed gas,catalytic oxidation is one of the most direct and effective methods for CO elimination.Among low-temperature CO(preferential)oxidation catalysts,supported Wacker catalysts(Pd-Cu/support)have advantages of resistance to water vapor and halide poisoning,and the choice of support is critical to the catalytic performance of supported Wacker catalysts for low-temperature CO(preferential)oxidation.Hydroxyapatite(HAP)is a non-toxic and highly biocompatible calcium phosphate,simultaneously,the zeolite-like pores contained in the structure make it exhibit strong ion exchange capacity.Therefore,HAP has been widely used in the repair and reconstruction of biomedical hard tissues,and the treatment of water and soil pollution caused by heavy metal ions(Cd²⁺,Hg²⁺,Pb²⁺,etc.).In addition,due to the tunable acid/base property and special thermal stability,HAP has been widely used as a support in numerous catalytic reactions,such as water gas shift,methane reforming,NO_xreduction,alcohol oxidation,aldehyde oxidation and hydrocarbon oxidation,etc.It can be seen that HAP is a promising support with excellent performance.However,there is no report on supported Wacker catalysts using HAP as a support.Based on this,HAP synthesized by different methods was used as the support,and a variety of supported Wacker catalysts(Pd-Cu/HAP)were prepared by the impregnation method.The effects of the modification of alkaline metals(K,Na,Mg),the preparation methods of support(hydrothermal method and sol-gel method)and the modification of transition metal salt promoters(FeCl₃,CoCl₂,SnCl₄and Fe(NO₃)₃)on the catalytic performance of Pd-Cu/HAP catalysts for low-temperature CO(preferential)oxidation were investigated.The Pd-Cu/HAP catalysts were characterized by N₂-physisorption,XRD,FT-IR,Raman,TEM,HAADF-STEM,EDS-Mapping,H₂-TPR,CO₂-TPD,XPS and In-situ CO-DRIFTS,and the catalytic performance and characterization results of catalysts are correlated.The main conclusions obtained are as follows:1.Effect of Na promoter on the structure of Pd-Cu/hydroxyapatite catalyst and catalytic performance for low-temperature CO oxidationBy modifying the HAP support with alkaline metals(K,Na,Mg),it is found that compared with K and Mg,the Na modification can significantly promote the catalytic performance of Pd-Cu/HAP catalyst for low-temperature CO oxidation.Among them,the catalyst with a sodium content of 1.74 wt%behaves above 50%CO conversion.Further increasing the sodium content to 2.19 wt%,the CO conversion of the catalyst shows above 70%CO conversion,while the CO conversion of the unmodified catalyst is only 40%.The characterization results show that the active Cu phase in unmodified Pd-Cu/HAP catalyst is in the form of Cu₂Cl(OH)₃,while the Cu phase in the sodium-modified Pd-Cu/HAP catalysts is the more active Cu(OH)Cl species.More importantly,the higher active surface Pd⁺and more surface Cu⁺species exist on the sodium-modified Pd-Cu/HAP catalysts.Moreover,the sodium-modified Pd-Cu/HAP catalysts possess the less weak basic sites,which is conducive to the desorption of CO₂.Therefore,it can be concluded that the active Cu phase,the surface composition and surface basicity of Pd-Cu/HAP catalyst can be changed by the addition of Na promoter,which is an effective method to promote the catalytic performance for low-temperature CO oxidation.2.Effect of support preparation method on the structure of Pd-Cu/hydroxyapatite catalyst and catalytic performance for low-temperature CO oxidationHydroxyapatite supports(HAP-SG,HAP-HT)were prepared by sol-gel and hydrothermal methods,respectively,Pd-Cu/HAP catalysts(PC-SG,PC-HT)were synthesized by an impregnation method,and the catalytic performance toward low-temperature CO oxidation were investigated.The results indicate that PC-HT exhibits better catalytic performance for low-temperature CO oxidation than PC-SG.Under the reaction conditions of 0.5 vol%CO,3.3 vol%H₂O,25?and GHSV6000 h^-1,the CO conversion of PC-HT is about 70%,while the CO conversion of PC-SG maintains at about 50%.The characterization results show that PC-HT possesses the larger specific surface area and pore volume,contains more Cu₂Cl(OH)₃species with stronger interaction with Pd species and HAP support.More importantly,In-situ CO-DRIFTS results confirm that Pd⁺species with stronger CO activation ability and more Cu⁺species with stronger redox property present in PC-HT,which is crucial for the establishment of a good Wacker cycle of PC-HT.In addition,the fewer and weaker basic sites of PC-HT facilitate the desorption of CO₂,thus promoting the progress of low-temperature CO oxidation reaction.3.Effect of Fe modification on the structure of Pd-Cu/hydroxyapatite catalyst and catalytic performance for low-temperature CO preferential oxidationCompared with CoCl₂,SnCl₄and Fe(NO₃)₃,the addition of FeCl₃is the best choice for improving the low-temperature CO preferential oxidation(CO-PROX)catalytic performance of Pd-Cu/HAP catalyst.When the Fe/Cu atomic ratio in the Pd-Cu-Fe/HAP catalyst is 1:1,the Pd-Cu-Fe/HAP catalyst exhibits the better low-temperature CO-PROX catalytic performance,which presents the CO conversion of 40%and CO₂selectivity of 100%at 30°C in the presence of water.However,the CO conversion of the unmodified Pd-Cu/HAP catalyst rapidly decreases from 56%to 21%within 2 h.The characterization results show that the formation of Fe₂O₃species increases the specific surface area and produces a strong interaction in Pd-Cu-Fe/HAP catalyst.More importantly,the presence of Fe₂O₃in Pd-Cu-Fe/HAP catalyst not only makes Pd⁰species easily oxidized,but also leads to the electron cloud density of Cu⁺species decreased.This could avoid the interruption of reoxidation caused by H₂adsorption on Pd⁰species,maintain CO adsorption and activation,and make for the oxidation of Cu⁺species.Compared with fresh and used Pd-Cu/HAP and Pd-Cu-Fe/HAP catalysts,it can be found that the formation of Fe₂O₃species improves the structural stability of active Cu species.