Carbon Dioxide Reforming of Methane over K, Ce and Mn Promoted Ni/Al2o3 Catalysts: Catalyst Characterization and Activity

K. K. Pant, Nandini A, and Dhingra S. C. Chemical Engineering Department, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi, India

Reforming of methane with carbon dioxide into syngas over Ni/-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, temperature programmed studies (TPR, CO2-TPD, H2-TPD) 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 addition of both potassium and CeO2 or (MnO) as promoters. Keywords: CO2-CH4 reforming; characterization; Ni/Al2O3 catalysts; CeO2; MnO;

Catalytic activity and coke deposition The CH4 and CO2 conversion obtained using the different catalysts during the isothermal reaction at 700 0C are given in Table 1. The 13.5Ni/Al2O3 catalyst showed a high initial CH4 conversion of 81%, but also rapidly deactivated with time on stream. After 6 h time on stream, the CH4 conversion reduced to 74% and there was a significant amount of coke deposited (3.1 wt. %) on this catalyst. The other catalysts showed an initial conversion value slightly less than that of 13.5Ni/Al2O3; but no appreciable loss in activity with time on stream was observed for these catalysts. The amount of carbon on the used catalysts determined by TGA is also given in Table 1. Thus the most significant effect of addition of K, CeO2, MnO was the improvement in catalytic stability.

Table 1. Catalytic activity and coke content during CO2 methane reforming reaction at 700 0C (CH4:CO2:N2 = 1:1:1; W/FCH4,0 = 1.725 kg-cat.h/kgmethane) Catalyst CH4 conversion (%) CO2 conversion (%) Coke* (wt. %) 13.5Ni/Al2O3 81.0 81.0 3.1 13.5Ni-2K/Al2O3 79.9 80.8 0.6 13.5Ni/10CeO2-Al2O3 79.7 80.6 0.6 13.5Ni-2K/10CeO2-Al2O3 78.5 77.0 trace 13.5Ni-2K/5MnO-Al2O3 77.0 78.0 <0.1

Characterization of support and catalysts The BET surface areas, pore volumes and average particle sizes of the calcined catalysts is summarized in Table 2. It can be seen that the surface area and the pore volume for 13.5Ni/Al2O3 catalyst decreased compared to -Al2O3 support. Addition of the other promoters K, CeO2, MnO further slightly reduced the surface area.

Table 2. Physical properties of calcined catalysts Support/Catalyst SBET (m2/g) V (cm3/g) Dp ( )

Al2O3 227.8 0.367 64.5 13.5Ni/Al2O3 158.6 0.285 73.5 13.5Ni-2K/Al2O3 150.2 0.310 80.7 13.5Ni/10CeO2-Al2O3 130.3 0.294 92.0 13.5Ni-2K/10CeO2-Al2O3 127.2 0.289 93.0 13.5Ni-2K/5MnO-Al2O3 123.8 0.293 96.9

Comparison of the XRD patterns of 13.5Ni/Al2O3 and 13.5Ni-2K/Al2O3 catalyst with that of the 13.5Ni-2K/10CeO2-Al2O3 and 13.5Ni-2K/15MnO-Al2O3 catalyst indicated that addition of CeO2/MnO improves the reducibility of Ni/Al2O3 catalyst.

The reduction characteristics of the prepared samples were studied over the range 30-900 0C. The H2 responses as a function of temperature are given in Fig.1. The support -Al2O3 showed no reducibility. The support 10CeO2-Al2O3 showed a small peak at 255 0C assigned to the reduction of ceria. The support 5MnO-Al2O3 showed three distinct peaks with Tmax 329, 400 and 456 0C respectively, which could be ascribed to the different oxidation state of the manganese ion. For the catalysts, peaks in the range 200-480 0C, could be ascribed to the reduction of relatively free NiO species. The three peaks with Tmax 530-540 0C, 670-690 0C, 750-760 0C respectively is ascribed to complex NiOx species having strong interaction with -Al2O3. The first and third peaks appear as shoulders. Comparing the areas under the TPR curves, it can be concluded that all catalysts show similar degree of reduction and the number of the exposed metallic Ni atoms on all catalysts is approximately the same. For the 13.5Ni/Al2O3 catalyst the first peak occurs at 264 0C. This peak has shifted to a higher temperature for the other catalysts. Metallic Ni sites reduced from both NiO weakly interacting with the support and NiOx strongly interacting with the support are active sites for methane-reforming reactions. Eventhough free NiO sites are prerequisite for high activity, the increase of NiO population in high Ni loading favorably promotes Ni sintering and carbon deposition resulting in relatively low catalytic activity with time on stream. Therefore, the population of NiOx, compared with those of NiO should be higher for high catalytic activity and stability. In the present case, the fraction of nickel present as free Ni resulting from the low temperature reduction of NiO species is larger on 13.5Ni/Al2O3 catalyst compared to other catalysts. Therefore the initial activity of 13.5Ni/Al2O3 catalyst is slightly higher than the other catalysts. Since the fraction of nickel resulting from the reduction of NiOx strongly interacting with the support is higher on other catalysts compared to 13.5Ni/Al2O3; they show more stability compared to 13.5Ni/Al2O3 catalyst. Addition of promoters K, CeO2, MnO results in the increased amount of adsorbed CO2 which plays an important role in maintaining the catalytic activity for CO2 reforming of CH4.

Based on the results of the various characterization techniques the observed catalytic activity and stability of the different catalyst was related to the catalyst property.