Deactivation Studies over Ni−K/CeO2−Al2O3 Catalyst for Dry Reforming of Methane

Abstract

The activity of 13.5Ni−2K/10CeO2−Al2O3 catalyst was tested for 60 h time on stream (TOS) for the carbon dioxide reforming of methane at three different temperatures (650, 700, and 750 °C). The amount of coke deposited on the catalyst at different time intervals was estimated by thermogravimetric analysis (TGA). Results suggested that both CH4 cracking and CO disproportionation contribute to coke deposition. No appreciable deactivation was observed for the catalysts at all three temperatures for the 60-h run. The used catalysts were characterized by Brunauer−Emmett−Teller (BET) surface area, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis to understand the morphology of the coke deposited on the catalyst. XRD patterns showed that carbon formed on 13.5Ni−2K/10CeO2−Al2O3 after 60 h TOS at 700 and 750 °C dispersed well and could not be observed, while after 60 h TOS at 650 °C, mainly graphitic carbon (peak at 2θ = 26.3°) formed. XPS characterization demonstrated the existence of mainly two kinds of carbon species, graphitic (−C−C−) and oxidized carbons (−C−CO−, CO3-). TGA, XRD, and XPS studies revealed that significant amount of coke was deposited on 13.5Ni−2K/10CeO2−Al2O3 catalyst at 650 °C. However, the amount of accumulated coke with TOS did not affect the high activity of the catalyst up to 60 h. This suggested that some carbon species formed on the surface of the catalyst must be involved in the reaction to produce CO. TEM results indicated that a large part of the graphitic carbon deposited on the catalyst surface was of the filamentous form with nickel on top of these carbon filaments. This form of graphitic carbon is more active in the reforming reaction of methane, probably because of its close interaction with nickel particles. Therefore, the catalyst had high stability at 650 °C despite the coke deposited. The value of the activation energy for coke oxidation for the 13.5Ni−2K/10CeO2−Al2O3 catalyst was estimated to be in the range of 110−160 kJ/mol.