CO2/CH4 reforming over Ni-La2O3/5A: An investigation on carbon deposition and reaction steps

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, chemic al trapping, TEM, and EPR. The XRD results revealed that due to the formati on of perovskite-like La2NiO4 phase in Ni-La2O3/5A, the small-size (ca. 9 n m) Ni-0 crystallites formed in H-2 reduction remained unsintered during 48 h of on-stream reaction at 800 degrees C. The accumulation of carbon on the active sites was the main reason for Ni-La2O3/5A deactivation. The detecti on of (CO2)-C-13 and CO2 in O-2 pulsing onto a sample pretreated with (CH4) -C-13/CO2 confirmed that the deposited carbon was from both CH4 and CO2. Th e (CO2)-C-13/CO2 molar ratio decreased with the rise in temperature, indica ting that the contribution of CO2 toward deposited carbon was larger than t hat of CH4 at higher temperatures. In CO and CO2/CH4 atmospheres, we observ ed similar TG patterns and obtained identical TEM images of deposited carbo n; we propose that carbon deposition is mainly via CO disproportionation. T he observation of CD3COOH in CD3I chemical trapping experiments suggested t hat HCOO was an intermediate of CO2/CH4 reforming. The amount of CO2 conver ted was roughly proportional to the amount of H present on the catalyst sur face. These results indicate that CO2 activation could be H-assisted. Pulsi ng CH4 onto a H-2-reduced Ni-La2O3/5A catalyst and a similar catalyst treat ed with CO2, we found that CH4 conversion was higher in the latter case. He nce, the idea of oxygen-assisted CH4 dissociation is plausible. As for meth ane conversion, k(H)/k(D) of 1.2 and 1.1 at 600 and 700 degrees C, respecti vely, were observed, implying that C-H cleavages are slow kinetic steps in CH4/CO2 reforming. Based on these experimental results, we have derived rea ction pathways for CO2/CH4 reforming, the decomposition of CHxO (x = 1 or 2 ) is considered to be the rate-determining step for syngas formation. (C) 2 000 Academic Press.