Ga2O3-modified Cu/SiO2 catalysts with low CO selectivity for catalytic steam reforming

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 °C. 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.