关键词: 水蒸气重整, 二甲醚, Cu/SiO₂, Ga₂O₃, 选择性

Abstract:

Dimethyl ether (DME) is considered a promising energy source and clean fuel for the next generation, with its high hydrogen content, and non-toxicity compared with methanol. In addition, it is easy to store and transport. DME steam reforming (SR) has received considerable attention for its applicability in the production of hydrogen for fuel cell applications. Generally, DME SR consists of two steps: DME hydrolysis and methanol SR. DME hydrolysis often occurs on an acidic catalyst, such as γ-Al₂O₃. Methanol SR in Cu-based catalysts requires both Cu⁰ and Cu⁺ as the active sites; moreover, the relative ratios of Cu⁰ and Cu⁺ can influence the catalytic performance. In addition, the byproduct of CO also commonly exists in DME SR, and a small amount of CO can poison Pt electrodes of fuel cells. Therefore, it is necessary to reduce the concentration of the generated CO in DME SR. Herein, using an ammonia-evaporation method, we synthesized a Cu/SiO₂ catalyst, which can simultaneously generate the dual copper species of Cu⁰ and Cu⁺ by reduction. After modification with Ga₂O₃, the xGa-Cu/SiO₂ catalysts show much improved catalytic activity and decreased CO selectivity. The Cu/SiO₂ catalyst shows a DME conversion of 90.7% and CO selectivity of 11.5% at 380 ℃. The 5Ga-Cu/SiO₂ catalyst, with a loading amount of Ga₂O₃ of 5% (w, based on the weight of Cu), shows the best performance, with a DME conversion of 99.8% and CO selectivity of 4.8% under the same conditions. The measurement of apparent activation energies shows that the addition of Ga₂O₃ cannot change the reaction path. By multiple characterization methods, we demonstrated that the improved performance can be ascribed to the following two aspects. First, our characterization results show that the loaded Ga₂O₃ is highly dispersed on the Cu/SiO₂ catalyst, which can increase the interaction between Ga and Cu species. This can not only improve the dispersion of copper species (Cu⁰ and Cu⁺) on the catalysts, but can also adjust the ratios of Cu⁺/(Cu⁰ + Cu⁺). The H₂ production rate shows a typical volcano curve owing to the ratio of Cu⁺/(Cu⁰ + Cu⁺), and reaches a maximum of 5.02 mol·g^-1·h^-1 at Cu⁺/(Cu⁰ + Cu⁺) = 0.5 for the 5Ga-Cu/SiO₂ catalyst. We conclude that the interaction between Ga and Cu species and the synergistic effect between Cu⁰ and Cu⁺ result in the promoted catalytic activity for DME SR. Second, by using a temperature-programmed surface reaction (TPSR), we showed that the addition of Ga₂O₃ can efficiently promote the water–gas shift reaction, thereby reducing the CO selectivity in DME SR. Thus, Ga₂O₃ suppresses the generation of CO, leading to the low CO selectivity and high CO₂ selectivity. In summary, the Ga₂O₃-modified Cu/SiO₂ catalyst yields reformates with low CO selectivity and high catalytic activity for DME SR. Our work provides a novel approach to designing a highly efficient Cu-based catalyst for catalytic SR systems.

Key words: Steam reforming, Dimethyl ether, Cu/SiO₂, Ga₂O₃, Selectivity