Self-Activation and Improved Stability Investigation of a Novel Plate-Type Al2O3/Al Monolith Supported Cu-Based Catalyst in Steam Reforming of Dimethyl Ether

Hydrogen is considered as a clean source of 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. Generally, steam reforming of dimethyl ether (DME SR) (Eq. (1)) consists of two moderate endothermic reactions: DME hydrolysis (Eq. (2)) to methanol over a solid acid catalyst, followed by steam reforming of methanol (MSR, Eq. (3)) over a steam reforming 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)

For DME hydrolysis system, a novel plate-type anodic γ-Al₂O₃ as monolithic support was prepared through the anodization technology, and its catalytic performance for hydrolysis of dimethyl ether (DME) was compared with that of the commercial γ-Al₂O₃. The results show that the anodic Al₂O₃/Al monolith exhibited higher catalytic activity than the commercial γ-Al₂O₃ due to its stronger acidity, better hydrophilicity and lower activation energy. Moreover, a 120h stability evolution was carried out and the results show that the plate-type anodic alumina has an excellent ability for coke suppression, which demonstrated that the novel anodic γ-Al₂O₃/Al monolith was an excellent acidic catalyst and support for DME SR.

  For DME SR system, a series of Cu/γ-Al₂O₃/Al bi-functional catalysts were prepared by impregnation method. The catalytic performance of Cu/γ-Al₂O₃/Al catalysts both with and without H₂ pre-reduction was compared. It was found that these two composite catalysts exhibited the similar catalytic activity. Meanwhile, according to XRD and XPS results, copper species over un-reduced catalyst was reduced to metallic Cu after DME SR reaction, indicating the in situ reduction of CuO in the reaction condition of DME SR. However, Cu-based catalysts have poor thermal stability and can easily lose catalytic activity due to the sintering of metallic Cu above 300°C. To improve the thermal stability of Cu, several components (e.g. Mn, Cr, Ni) were added to the Cu-based catalyst and their catalytic performance for DME SR was studied. It was found that Ni-doped catalyst had excellent catalytic activity. Moreover, the durability test of Cu/Ni/γ-Al₂O₃/Al was carried out under critical conditions (400°C) and the results show that it has an excellent stability for 100h with a 100% DME conversion, which demonstrated that the novel plate anodic 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.