Steam Reforming of Dimethyl Ether over a Novel Plate-Type γ-Al2O3/Al Monolith Supported Cu-Based Bi-Functional Catalyst

Nowadays, the increasing global demand of energy accelerates the depletion of fossil fuels, thus boosting the extensive exploration of new energy alternatives. Hydrogen is considered as a clean energy and steam reforming of dimethyl ether (DME) is regarded as a promising method to produce hydrogen for fuel cell applications. Unlike many other fuels (e.g. methanol, methane and bio-ethanol), DME is non-toxic and provides high H/C ratio and high energy density. Moreover, the well-developed infrastructure of LPG can readily be used for the distribution of DME due to their similar physical properties. Generally, steam reforming of dimethyl ether (DME SR) (Eq. (1)) can be expressed as a combination of DME hydrolysis (Eq. (2)) to methanol over a solid acid catalyst and steam reforming of methanol (MSR, Eq. (3)) over a metallic catalyst. Therefore, a bi-functional catalyst consisting of acid sites and MSR sites is required for overall DME SR process.

DME steam reforming: CH₃OCH₃ + 3H₂O ↔ 6H₂ + 2CO₂ △H_r = +135kJ/mol (1)

DME hydrolysis: CH₃OCH₃ + H₂O ↔ 2CH₃OH △H_r = +37kJ/mol (2)

MeOH steam reforming: CH₃OH + H₂O ↔ 3H₂ + CO₂ △H_r = +49kJ/mol (3)

In our previous works,^1-3 a plate-type γ-Al₂O₃/Al monolith was prepared through anodization technology. The structured monolith was proven to have an excellent activity in DME hydrolysis. Moreover, the anodic alumina supported Cu composite catalyst exhibited a satisfied performance in DME SR system.

In this study, the self-reduction behavior of fresh Cu/γ-Al₂O₃/Al catalyst during DME SR was further studied and the self-activation mechanism was proposed based on XRD and XPS analysis. However, the Cu/γ-Al₂O₃/Al catalyst showed a poor thermal stability at 350 °C due to the sintering of copper. As an approach, nickel was added to the Cu-based catalyst, and then the effect of nickel loading and chemical state on the activity of Cu/Ni/γ-Al₂O₃/Al catalyst was investigated. It was found that the proper amount of nickel could improve the dispersion of copper species, and thus enhancing the catalytic activity. For catalysts with lower nickel loadings, the thermal stability of Cu-based catalyst could not be improved. While, for catalysts with higher nickel loadings, part of nickel species would aggregate and block the active sites of copper. Moreover, a small amount of metallic Ni appeared, which resulted in the occurrence of side-reaction of CO methanation. Finally, the durability evaluation of optimized Cu/Ni/γ-Al₂O₃/Al was carried out under critical conditions (400 °C) and the results showed that it has an excellent stability during a 120 h test, which demonstrated that the novel plate-type γ-Al₂O₃/Al monolith supported Cu and Ni composite catalyst was an excellent catalyst for DME SR. Furthermore, it would be very promising for the application of micro-channel reformer for the domestic fuel cell system as for its outstanding shape flexibility and catalytic performance.

References

[1] F.Y. Fan, Q. Zhang, J.J. Xu et al., Catal. Today, 216 (2013) 194-9.

[2] Q. Zhang, F.Y. Fan, G.M. Xu et al., Int. J. Hydrogen Energy, 38 (2013) 10305-14.

[3] Q. Zhang, J.J. Xu, F.Y. Fan et al., Fuel Process. Technol., 119 (2014) 52-9.