Effect of Ce on 5 wt% Ni/ZSM-5 catalysts in the CO_2 reforming of CH_4 reaction

摘要

The aim of this study is to investigate the promotional effect of Ce on Ni/ZSM-5 catalysts in the CO_2 reforming of CH_4 reaction. The evaluation of the catalytic performances of the composite catalysts was conducted in a fixed-bed reactor at atmospheric pressure. The influencing factors, including temperature, Ni and Ce loadings, molar feed ratio of CO_2/CH_4, and time-on-stream (TOS), were investigated. The characteristics of the catalysts were checked with Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The reduction and the basic properties of the composite catalysts were elucidated by temperature-programmed reduction by H_2 (H_2-TPR) and temperature-programmed desorption of CO_2 (CO_2-TPD), respectively. The reactivity of deposited carbon was studied by sequential temperature-programmed surface reaction of CH_4 (CH_4-TPSR) and temperature-programmed oxidation using CO_2 and O_2 (CO_2-TPO and O_2-TPO). Results indicate that higher CH_4 conversion, H_2 selectivity, and desired H_2/CO ratio for 5 wt% Ni & 5 wt% Ce/ZSM-5 could be achieved with CO_2/CH_4 feed ratio close to unity over the temperature range of 500-900 ℃. Moreover, the addition of Ce could not only promote CH_4 decomposition for H_2 production but also the gasification of deposited carbon with CO_2. The dispersion of Ni particles could be improved with Ce presence as well. A partial reduction of CeO_2 to CeAlO_3 was observed from XPS spectra over 5 wt% Ni & 5 wt% Ce/ZSM-5 after H_2 reduction and 24 h CO_2-CH_4 reforming reaction. Benefiting from the introduction of 5 wt% Ce, the calculated apparent activation energies of CH_4 and CO_2 over the temperature range of 700-900 ℃ could be reduced by 30% and 40%, respectively.