Promotion Effect of Adding Cr/Al/Fe to Cu/ZnO on the Stability of Hydrogen Produciton Catalyst for Dimethyl Ether Steam Reforming

Abstract

Steam reforming (SR) of dimethyl ether (DME) is one of the promising ways to produce hydrogen for fuel cells (FCs) [1]. DME is harmless with a high H/C ratio, handling DME is easy because it is liquefied under ca. 6 atm and conventional facilities providing LPG can be used due to the similarity of DME to LPG.Coppper based materials were considered as good catalyst for DME SR [2]. However the durability of copper based catalysts is not good because of the copper sintering as reaction running. There are two ways to improve the durability of copper based catalysts, one is to enhance the dispersion of copper or to improve the thermal stability of copper by forming a spinel oxide or alloy [3]. γ-Al2O3 was used as support. The Cu/ZnO/γ-Al2O3, Cu/ZnO/Cr2O3/γ-Al2O3 and Cu/ZnO/Fe2O3/γ-Al2O3 catalysts were prepared by impregnation method. The nitrates were impregnated into support for 6 h, then dried at 323 K for 12 h, calcined at 823 K for 4h. The DME SR was carried out with a fixed bed flow reactor under atmospheric pressure using 1 g of catalyst at 773 K. After the catalyst was reduced by 12 v% H2/Ar at 503 K for 3 h, the mixed gas of DME and N2 (20 v%) were fed into the reactor with GHSV=500 ml/(g·h), the water was pumped into the reactor though a 473 K heater. The H/C ratio was 3. The catalysts were characterized by means of XRD, TPR, SEM, and BET. Steam reforming of DME was carried out at 773 K over Cu/ZnO/γ-Al2O3, Cu/ZnO/Cr2O3/γ-Al2O3 and Cu/ZnO/Fe2O3/γ-Al2O3 catalysts. The conversion of DME and the hydrogen yield reached 100% and 87% respectively for Cu/ZnO/γ-Al2O3 at the initial stage. The durability of this catalyst exhibted degradation. After running for 97 h, the hydrogen yield decreased to less than 60%. The reason for this degradation was supposed to be the sintering of copper, therefore, a metal oxide with better thermal stability was added into the catalyst to prevent the sintering of copper. The time-on-stream results indicate that the addtion of Cr2O3 or Fe2O3 can improve the durability of the catalyst for DME SR significantly. After running for 100 h, the hydrogen yield was kept at 84% and 85%, respectively. Figure 1 is the time-on-stream result of DME conversion over Cu/ZnO/ Cr2O3. Figure 1 is the time-on-stream result of DME conversion over Cu/ZnO/ Fe2O3. The mechanism of the improvement of the durability of catalysts were further investigated by XRD, SEM, BET and TPR. Figure 1