CO2/CH4 Reforming over Ni–La2O3/5A: An Investigation on Carbon Deposition and Reaction Steps

Abstract

Carbon deposition and reaction pathways in CO2/CH4 reforming over Ni–La2O3/5A have been studied by means of XRD, in situ TG, pulse experiments, chemical trapping, TEM, and EPR. The XRD results revealed that due to the formation of perovskite-like La2NiO4 phase in Ni–La2O3/5A, the small-size (ca. 9 nm) Ni0 crystallites formed in H2 reduction remained unsintered during 48 h of on-stream reaction at 800°C. The accumulation of carbon on the active sites was the main reason for Ni–La2O3/5A deactivation. The detection of 13CO2 and CO2 in O2 pulsing onto a sample pretreated with 13CH4/CO2 confirmed that the deposited carbon was from both CH4 and CO2. The 13CO2/CO2 molar ratio decreased with the rise in temperature, indicating that the contribution of CO2 toward deposited carbon was larger than that of CH4 at higher temperatures. In CO and CO2/CH4 atmospheres, we observed similar TG patterns and obtained identical TEM images of deposited carbon; we propose that carbon deposition is mainly via CO disproportionation. The observation of CD3COOH in CD3I chemical trapping experiments suggested that HCOO was an intermediate of CO2/CH4 reforming. The amount of CO2 converted was roughly proportional to the amount of H present on the catalyst surface. These results indicate that CO2 activation could be H-assisted. Pulsing CH4 onto a H2-reduced Ni–La2O3/5A catalyst and a similar catalyst treated with CO2, we found that CH4 conversion was higher in the latter case. Hence, the idea of oxygen-assisted CH4 dissoc iation is plausible. As for methane conversion, kH/kD of 1.2 and 1.1 at 600 and 700°C, respectively, were observed, implying that C–H cleavages are slow kinetic steps in CH4/CO2 reforming. Based on these experimental results, we have derived reaction pathways for CO2/CH4 reforming, the decomposition of CHxO (x=1 or 2) is considered to be the rate-determining step for syngas formation.