The activity of 13.5Ni-2K/10CeO 2 -Al 2 O 3 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 CH 4 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/10CeO 2 -Al 2 O 3 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-, CO 3 -). TGA, XRD, and XPS studies revealed that significant amount of coke was deposited on 13.5Ni-2K/10CeO 2 -Al 2 O 3 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/10CeO 2 -Al 2 O 3 catalyst was estimated to be in the range of 110-160 kJ/mol.
Reforming of methane with carbon dioxide into syngas over Ni/γ-Al2O3 catalysts modified by potassium, MnO, and CeO2 was studied. The catalysts were prepared by impregnation technique and were characterized by BET surface area, pore volume, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, temperature-programmed studies, and pulse chemisorption. The performance of these catalysts was evaluated by conducting the reforming reaction in a fixed-bed reactor. Results of the investigation suggested that stable Ni/Al2O3 catalysts for the carbon dioxide reforming of methane can be prepared by the addition of both potassium and CeO2 (or MnO) as promoters. The results of the various characterization techniques were used to relate the observed catalytic activity and stability to the catalyst property. The stability and lower amounts of coking on promoted catalysts were attributed to partial coverage of the surface of nickel by patches of promoters, strong metal−support interaction (TPR, H2 pulse chemisorption, H2-TPD), and their increased CO2 adsorption (CO2-TPD). For the stable 13.5Ni−2K/10CeO2−Al2O3 catalyst, the effect of reaction temperature and contact time on conversion and product yield was studied. It was found that the conversion and product yield increased with increasing reaction temperature and W/F CH 4 ,0 and reached equilibrium at W/F CH 4 ,0 = 1.7 kg-cat·h/kgmethane. The mechanism of the CH4/CO2 reaction has been proposed, based on which a kinetic model was developed to estimate the kinetic parameters. The estimated kinetic parameters predicted the product yields satisfactorily. CH4 activation to form CH x and CH x O decomposition are suggested to be the rate-determining steps of the CH4/CO2 reaction over the 13.5Ni−2K/10CeO2−Al2O3 catalyst. The activation energy for methane adsorption and dissociation (E k 1L ), CH x O decomposition (E k 7L ), and reverse water gas shift reaction (E k r ) were estimated to be 113.8 ± 5.5, 119.3 ± 4.7, and 155.3 ± 7.0 kJ/mol, respectively.
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