Modification Of Cu/SiO₂ Catalysts With La Or Ga For DME Steam Reforming To Produce Hydrogen

Dimethyl ether?DME?is a potential clean fuel because of its excellent properties,such as high energy density,high oxygen content,low boiling point,easy storage and transportation.In addition,DME has a relatively lower toxicity compared with methanol.Thus,DME steam reforming?SR?becomes a promising way to provide on-board H₂ sources for fuel-cell vehicles.Cu-based catalysts have good catalytic performance in DME SR,but also have the disadvantages of poor activity in low temperature,poor stability and high selectivity of CO.In this work,we modified Cu/SiO₂ catalysts by La and Ga,which were applied to the DME SR.Multiple techniques,such as BET,XRD,XAFS,FTIR,TEM,H₂-TPR,XPS,TPSR and N₂O chemisorption were employed to characterize the catalysts.The effects of La and Ga modification on the catalytic performance for DME SR are systematically investigated.It is reported that the reaction temperature over the acidic catalyst of?-Al₂O₃ for DME hydrolysis ranges from 300 to 400°C.In such a temperature range,the copper species readily undergo serious aggregation,leading to the deactivation of the catalysts.Herein,we modified Cu/SiO₂ catalysts by La₂O₃,which show the high catalytic activities and long-term stabilities for DME SR.The addition of La can improve the dispersion of both metallic Cu and Cu₂O on the catalysts,and quantitatively tune the ratio of Cu⁺/?Cu⁰+Cu⁺?to enhance the catalytic activity.Additionally,we found the selectivity of CO is determined by the ratio of Cu⁺/?Cu⁰+Cu⁺?.The catalyst with the ratio of Cu⁺/?Cu⁰+Cu⁺?=0.5 exhibits the highest catalystic activity and selectivity.The La modified Cu/SiO₂ catalyst also exhibits long term stability,which is ascribed to the strong electron donor-acceptor interaction between the lanthanum and copper species.The strong electron donor-acceptor interaction can not only stabilize the copper species from aggregation,but also avoid over-reduction of the Cu₂O species to the Cu⁰.The constant ratio of Cu⁺/?Cu⁰+Cu⁺?on the La-modified Cu/SiO₂ catalyst ensures the high catalytic stability in DME SR.Generally,the byproduct of CO is common in DME SR,and a small amount of CO can poison Pt electrodes of fuel cells.Therefore,it is required that the concentration of CO in hydrogen supplied for fuel cells should be less than 20 ppm.Since Ga₂O₃ can suppress generation of CO in DME SR,we modified Cu/SiO₂catalyst with Ga₂O₃ to reduce CO generation and increase the selectivity of CO₂ in the DME SR.In addition,the interaction between the Ga and Cu species can not only improve the dispersion of copper species on the catalysts,but also benefit for formation of Cu⁺.Ga is an essential factor to quantitatively adjust the ratio of Cu⁺/?Cu⁰+Cu⁺?.When the ratio of Cu⁺/?Cu⁰+Cu⁺?equals to⁰.5,we achieve the maximum H₂ production rate.Our result reveals that the addition of Ga can also improve the WGS reaction to achieve the high selectivity towards CO₂.