Confined Preparation And Catalytic Performance For CuO-CeO₂ Mixed Oxides

The preferential oxidation of CO in H2-rich stream is an effective method to prevent the poisoning of anodes in proton exchange membrane fuel cell.With high activity and selectivity for CO preferential oxidation at low temperature,CuO-CeO₂ catalysts consisting of rare earth and transition metal oxides have been recognized as promising catalysts.However,it still has a series of problems such as unsatisfactory-activity at low temperature,narrow temperature range of CO complete conversion and poor selectivity at high temperature.Based on modification of unique tubular structure of carbon nanotubes on activity,selectivity and stability of catalysts,the confined preparation of CuO-CeO₂ composite oxides was realized and applied to CO preferential oxidation in H2-rich stream in this work by means of optimization of active composition,surface functionalization of carbon nanotubes and modulation of preparation conditions.The catalysts were characterized systematically by X-ray powder diffraction（XRD）,laser Raman spectroscopy（LRS）,high resolution transmission microscope（HRTRM）,X-ray photoelectron spectroscopy（XPS）and temperature programmed reduction（H2-TPR）.Relationship between the confined preparation of CuO-CeO₂ catalysts,microstructure and catalytic performance has been investigated emphatically.The main conclusions were obtained as follows:（1）The effects of Cu-Ce molar ratios on structure and catalytic performance of CNTs confined CuO-CeO₂ catalysts were investigated.The results show that of the average grain size of CeO₂ and dispersion of CuO species decrease with the increase of Cu-Ce molar ratios,When the Cu-Ce molar ratio is 4:6,Cu2+ enters the lattice of CeO₂ and forms Cu-O-Ce solid solution and oxygen vacancy on the surface of catalyst.Most CuO and CeO₂ particles are confined to the tube cavity of CNTs and interact with the inner surface,which enhances the reduction ability of the catalyst at low temperature.Meanwhile,there are more Ce³⁺ species and lattice oxygen on the catalyst surface.The catalyst exhibits excellent catalytic performance for CO preferential oxidation.The corresponding temperature of 50%CO conversion is 90℃,and the temperature window of total CO conversion is 40 ℃（140 ℃～180 ℃）.Moreover,the high selectivity is maintained to 140 ℃.（2）By studying the influence of active component loading on the structure and performance of CuO-CeO₂/CNTs catalysts,it is found that when the CuO-CeO₂ content is relatively low,CuO has higher dispersion on the carrier surface or exists in the form of Cu-Ce-O solid solution.With increasing loading,the grain size of CuO and CeO₂ increases gradually,and the synergistic interaction between CuO and Ceo2 promotes the reduction of CuO.In the experimental samples,the catalyst has higher activity at low temperature when the loading of CuO-CeO₂ is 20 wt.%,over which Tso is as low as 97 ℃,and the temperature window of total CO conversion is 40℃（140 ℃～180 ℃）.Moreover,the high selectivity is maintained to 140 ℃.（3）A large number of defects exist on the surface of CNTs treated with concentrated nitric acid,which is conductive to the infiltration of catalyst precursor.The increase of the contents of-OH and-COOH is beneficial to the dispersion and deposition of active species in CNTs.Compared with commercial carbon nanotubes supported catalysts with the same composition and content,the pretreated CNTs supported CuO-CeO₂ catalyst shows higher CuO dispersion,smaller grain size and more CO oxidation active sites.Total CO conversion is realized in the temperature range from 140℃ to 180 ℃.The high selectivity is maintained to 140 ℃and the catalyst has good stability.（4）CNTs confined CuO-CeO₂ catalysts for CO preferential oxidation were prepared through organic solvents with different surface tension.The results show that the active CuO species had smaller grain size and higher filling efficiency of CNTs in the preparation of catalyst with ethanol as the solvent.Most CuO and CeO₂ deposit in the tube cavity of CNTs.The catalyst has excellent activity and selectivity at low temperature.The temperature window of total CO conversion is 40 ℃（140 ℃～180 ℃）.Meanwhile,with analyzing the relative content of lattice oxygen and Ce³⁺ in the catalyst prepared with acetone as the solvent before and after the reaction,it can be found that the catalyst has certain stability.（5）Structure and performance of CuO-CeO₂/CNTs catalysts at different heat treatment temperatures were investigated.It is found that the average grain size of CeO₂ and CuO increases with the heat treatment temperature.CuO has good dispersion on the carrier surface at lower temperature.The catalyst has good performance when the heat treatment temperature is 550℃.Total CO conversion is realized in the temperature range from 140 ℃ to 180 ℃,and the selectivity is still high at 140 ℃.When the heat treatment temperature rises to 650 ℃,the sintering of CuO becomes serious and the particle size increases obviously,which is not conductive to the dispersion of CuO and the catalytic performance decreases.