Construction Of Cu-rare Earth Oxide(Cu-REO) Catalysts For Methanol Steam Reforming To Produce Hydrogen

Over recent years,hydrogen has attracted considerable attention as an energy carrier,particularly for the energy supply to proton exchange membrane fuel cells(PEMFCs).However,PEMFCs require high-purity hydrogen as fuel,but the storage and transportation of hydrogen have still some limitations.To solve these problems,the technology of on-board alcohol steam reforming to produce hydrogen has attracted people’s attention,which has the potential to be industrialized.Methanol,as a liquid hydrogen storage fuel,possesses a high H/C ratio and low conversion temperature.The development of high efficiency catalysts is the core issue to realize the technology of methanol steam reforming(MSR)for hydrogen production.Cu-based catalysts have been widely used in MSR reaction,due to the high activity and low CO selectivity.Whereas,some fundamental scientific aspects are still unclear for the using of Cu-based catalysts in MSR reaction,and people are in short of systematic and deep understanding on the surface-active sites and structure-reactivity relationship.Therefore,in this thesis,several series of Cu-rare earth oxide(REO)catalysts have been designed and fabricated,and explored the Cu-REO interaction,the dispersion and valence state distribution of Cu,the redox property,surface basic sites,active oxygen species,and their influence on the catalytic performance in MSR reaction.The main contents are summarized here:Part 1:Five rare earth metal oxide(REO)supports,namely La2O3,Pr6O11,Sm2O3,Y2O3 and CeO2,were prepared by precipitation method.Subsequently,5 wt.%Cu was loaded onto the supports using a wet impregnation method to obtain 5%Cu/REO catalysts.The catalytic performance tests revealed that 5%Cu/Sm2O3 and 5%Cu/CeO2 exhibited higher methanol conversion and hydrogen yields than other samples.XRD,XPS,EPR and N2O titration results have indicated that the high dispersion of Cu species and abundant Cu active sites were the main factors improving the catalytic performance.Moreover,the numbers of the active oxygen species(O2-)and basic sites on the catalyst surfaces are also crucial factors to enhance the reaction performance.Based on the In-situ DRIFTS results,the adsorption of CH3OH likely could occur on Cu+ sites,and form several intermediates such as methoxy,aldehyde groups and formates,which will eventually be converted into the final CO2 product.Compared with the results achieved on 5%Cu/CeO2 and 5%Cu/Y2O3,it was found that the monodentate formate(m-COOH*)exhibits higher activity than the bidentate formate(bi-COOH*),which can explain that 5%Cu/CeO2 has much better MSR performance.In contrast,H2O could be adsorbed and dissociated on the metallic Cu0 sites,leading to the generation of H2.Therefore,on these catalysts,the co-existence of Cu+ and Cu0 sites is indispensable for the reaction.Moreover,compared with other catalysts,5%Cu/Sm2O3 and 5%Cu/CeO2,the two catalysts with better reaction performance,have also higher Cu+/(Cu0+Cu+)ratios.This proves that a higher Cu+ content can facilitate the catalytic performance as well.Part 2:Ce-Cu-O solid solutions having different Ce/Cu ratios were designed and prepared,and their structure-performance relationship was investigated with XRD extrapolation method established by our lab.It is quantified that the lattice capacity of Ce-Cu-O solid solution is 0.109 g CuO/g CeO2,equaling to a Ce/Cu mole ratio of 81/19.Therefore,Cu2+cations can maximally substitute 19%Ce4+in the cubic fluorite CeO2 matrix to form a stable solid solution in pure phase.Below the lattice capacity,the quantity of surface defects/oxygen vacancies improves by increasing the lattice Cu2+amount until it reaches the lattice capacity.Further increasing CuO content leads to the generation of surface CuO species.The formation of pure phase solid solution is beneficial to generate surface active oxygen species(O2-)and alkaline sites,which is vital for the reaction.The sample with a lattice capacity amount of CuO(Ce0.8Cu0.2Ox)possesses the largest amount of these surface sites.In addition,the reduced Ce0.95Cu0.05Ox sample is mainly composed of Cu+,while Ce0.3Cu0.7Ox sample is mainly composed of Cu0.Whereas,the Ce0.8Cu0.2Ox sample has appropriate amounts of both Cu+ and Cu0 after reducing,being favorable for the redox cycle during the reaction.Therefore,two important conclusions can be drawn from this part of work:(1)Ce-CuO solutions exhibit a significant lattice capacity threshold effect in MSR reaction;(2)The presence of both Cu+ and Cu0 sites are necessary to proceed the reaction smoothly,and a catalysts with better performance usually have a higher content of Cu+ centers,which is in agreement to the conclusions drawn from the Part 1.Part 3:To achieve better catalysts,the influence of the phase structure of REO on the Cu active component was investigated.Three groups of Cu/REO catalysts were prepared with cubic and monoclinic Gd2O3,Eu2O3 and Sm2O3 supports for MSR reaction to produce H2.Based on CH3OH conversion and H2 yield,the reaction performance of the catalysts ranks as Cu/Sm2O3-M>Cu/Sm2O3-C>Cu/Gd2O3-M>Cu/Gd2O3-C>Cu/Eu2O3-M>Cu/Eu2O3-C.For the same kind of REO,Cu supported on the monoclinic support shows better performance than on the cubic one.Despite the phase structure difference,Sm2O3 is the best support among all the three kinds of REOs.Compared with Cu/REO catalysts prepared with cubic supports,the corresponding catalysts prepared with monoclinic supports generally possess more surface oxygen vacancies,which can generate more surface-active oxygen(O2-)and moderate basic sites.Moreover,the contents of Cu+ on the catalysts follow the same sequence.The reaction performance is positively related to the amount of these three types of surface sites.But metallic Cu0 species is necessary to maintain the Cu+?Cu0 redox cycle.Furthermore,on a catalyst with good performance,those vital surface reaction intermediates can be stabilized during the reaction.Cu/Sm2O3-M possesses the largest quantities of these surface sites,and has the appropriate amount of Cu+ and Cu0 after reduction,thereby displaying the optimal performance in all the catalysts.In conclusion,evident support crystalline structure effect is observed for Cu/REO catalysts,and a monoclinic phase REO is a better support than the respective cubic phase one.