Study On The Hydrogen Production From DME Steam Reforming Over Cu-based Catalysts Combined With Solid Acids

Hydrogen is a kind of clean and renewable energy, which is more suitable for future energy development. Rational use of hydrogen is beneficial to both relieving energy shortage and environment protection. Dimethyl ether steam reforming (DME SR) is an effective way to produce hydrogen. Compared with other technologies, steam reforming has its own merits, such as mild reaction condition, low cost, few by-products and easy separation. Therefore, it attracts more and more attention in recent years. For the production of hydrogen via DME steam reforming, the development of highly efficient catalysts is crucial. Currently, the greatest challenge is to improve the catalytic performance of the catalysts, including low-temperature activity and selectivity. In this work, a kind of complex catalysts consisting of metal (Cu-Ce) and solid acid (γ-Al2O3) is employed for DME SR reaction, and systematically studied from several aspects. For the metal catalysts, the effect of preparation methods, contents of active components, reaction conditions and active phase modifications is investigated carefully; while for the solid-acid catalysts, the acidity is also optimized based on their catalytic performance. The structures and the properties of the catalysts are well correlated.Firstly, for the metal catalysts, preparation methods, contents of active components and reaction conditions are investigated. The results show that the metal catalyst prepared by urea combustion exhibits much higher DME conversion, H2 yield and CO₂ selectivity. XPS results indicate that the above-mentioned catalyst contains more active copper species and adsorbed oxygen species on the surface than those prepared by other methods. The catalyst with 30wt.% CuO content shows the highest DME conversion at the reaction temperature not higher than 400oC. It is found from the XRD results that the particle size of CuO in this catalyst is the smallest. H2-TPR results suggest that the proper interaction between copper and ceria in the catalyst makes more copper species reducible in the catalyst. The catalysts which are pre-reduced in H2 for 0.5h show the best performance for DME SR reaction at a low space velocity (SV=6000h-1). In this case, the DME conversion can reach 100% at the reaction temperature of 450oC. Meanwhile, the stability of the catalysts is also significantly improved.Metal catalysts are modified to achieve better catalytic activity and selectivity. In the first step, Ce was partially substituted by Co or Mn. By comparing the catalytic activity of the catalysts, it is confirmed that the addition of Co or Mn to the catalysts remarkably increases the DME conversion and H2 yield at 400oC or below. H2-TPR and XRD results indicate that the presence of Co or Mn could enhance both the dispersion and the reducibility of copper species. It is revealed by the results of XPS and CO-pulse chemisorption that there is more surface lattice oxygen and Cu+ species on the catalyst doped with Mn, which facilitates the formation of CO₂. In the second step, bimetallic catalysts Cu-M (M=Zn, Co, Mn, Zr, K, Cs) are employed. It is found that the bimetallic catalysts show lower DME conversion but much higher CO₂ selectivity, especially the catalyst doped with Zn. From the XPS results, it is proved that the higher selectivity to CO₂ is related to the higher content of surface lattice oxygen on this catalyst.To improve the catalytic activity of the catalysts for DME hydrolysis, metal oxides are added to modify the acidity of alumina. The catalysts Al-M (M=Co,Cr,Ni,La) are prepared by sol-gel method. It is found that the reaction of DME hydrolysis is suppressed at low temperature over the modified catalysts. The results NH3-TPD and pyridine-IR indicate that the main acid sites on the catalysts are weak Lewis acid sites. The amount for acid sites is enhanced when Cr is doped in the catalyst, as a result, the catalyst exhibits higher DME hydrolysis activity.