Investigation Of Highly Dispersed Ir Catalysts For CO Preferential Oxidation Reaction

Hydrogen energy as a clean energy plays an increasing role in the global energy consumption due to the increasing demand for energy and urgent concern toward environmental issues.For utilization of hydrogen energy,proton exchange membrane fuel cell（PEMFCs）is considered as one of the most efficient methods to convert hydrogen energy to mobile power.However,Pt anode of PEMFCs is sensitive to the unavoidable CO（～1 vol%）remained in industrial hydrogen.An effective and economic method to convert this problem is preferential oxidation of CO（CO-PROX）,which could selectively eliminate CO in hydrogen-rich atmosphere.For catalysts investigated for CO-PROX,Ir-based catalysts exhibit excellent activity at low temperatures while poor activity at high temperatures.Nevertheless,Ir metal has a higher surface energy and melting point than the same 5d metal of Pt,favoring its dispersion and interaction with supports.Therefore,in this work,a series of highly dispersed Ir-based catalysts were synthesized based on the concept of metal-support interaction and then investigated in CO-PROX.The main results are summarized as follows:1.A series of Ir/hexaaluminate catalysts(Ir/Ba Fe_xAl_12-xO₁₉,x=0,1,2,3)were prepared via a deposition-precipitation method using Fe-substituted Ba-hexaaluminates as supports.The results show that Ir nanoparticles could be highly dispersed on hexaaluminate supports.When investigated in CO-PROX,the 3 wt%Ir/BaFeAl₁₁O₁₉（3Ir/BFA）catalyst with x=1 shows the best performance among catalysts investigated,which exhibits complete conversion of CO with a wide working temperature window from 20 to 200℃,excellent long-term durability and good resistance to CO₂ and H₂O.Extending the Fe-substituted Ba-hexaaluminate as support for other noble metal catalyst,Pt species still could be highly dispersed on support without aggregation even the loading of Pt reaches up to 5wt%.Furthermore,5 wt%Pt/BaFeAl₁₁O₁₉ also displays an excellent catalytic performance in CO-PROX,suggesting the universality of hexaaluminate as supports for noble metal catalysts.2.The reaction mechanism of CO-PROX performed on 3 wt%Ir/BaFeAl₁₁O₁₉（3Ir/BFA）catalyst was investigated by XPS,M?ssbauer spectroscopy,in-situ DRIFTS and Density Functional Theory（DFT）calculations.The results suggest that the framework Fe cations in mirror planes of hexaaluminate interacted strongly with Ir species,resulting in that the Fe ions in mirror planes could be reduced to Fe²⁺to provide active sites for O₂adsorption,and the chemical state of Ir species could be modulated to provide a moderate CO bond strength to increase its competitive oxidation ability with H₂.Such interaction leads to a noncompetitive L-H mechanism in CO-PROX reaction for 3Ir/BFA.The DFT calculation results further show that the 3Ir/BFA catalyst has a higher adsorption energy of CO than H₂（2.32 e V vs 1.12 e V）,indicating the better oxidation ability of CO than that for H₂,which is consistent with the experimental results.Notably,it also found that too strong CO adsorption leads to a poor activity at low temperatures.Therefore,proper interaction between hexaaluminate and Ir is critical for the excellent performance of 3Ir/BFA in CO-PROX reaction.3.To further increasing the performance of Ir-basd catalysts,the sol-gel method derived Cu-doped Ce O₂ were employed to support Ir metal and investigated in CO-PROX.It was found that the highly dispersed Cu species could promote the reduction of bulk oxygen species and oxygen mobility,which could enhance the adsorption and activation of O₂.Furthermore,the synergistic interaction between Ir and Cu increases CO bond strength,which is beneficial for the adsorption and activation of CO.As a result,the Ir-based catalyst with a Cu to Ce molar ratio of 1:10 shows a distinctive working window for the complete conversion of CO within 80-180℃.