KNiCa catalyst

CO2 behavior on supported KNiCa catalyst in the carbon dioxide reforming of methane... 

Zeolite-supported Ni and KNiCa catalysts were prepared by molten-salt method [2], which were designated as Ni/ZSI and KNiCa/ZSI, respectively, hereinafter. The support was a high siliceous ZSM-5 zeolite (UOP S-115) mixed with an alumina. 

In this study it was found that dissociation of CO2 and CH4 is an elementary step in the CO2 reforming of methane and that an active site for the dissociation of CO2 and CH4 (eqns. (1) and (2)) is metallic Ni on the KNiCa catalyst. Ni surface of KNiCa/ZSI catalyst was mostly occupied by adsorbed C and O species as intermediates during the reaction. Surface reaction of these species produced carbon monoxide and simultaneously rejuvenated nickel species (eqn. (5)), which was considered to be rate-determining step under the following reaction scheme. 

Experiments for pulse reaction on reduced KNiCa/ZSI catalyst were conducted with a gas sampling valve in a pulse microreactor, which was incorporated between the sample inlet and the column of gas chromatograph. FT-IR spectroscopic studies were performed in a quartz vacuum cell using self-supported wafers which were treated under vacuum or underwent the... 

In the previous study, KNiCa/ZSI catalyst exhibited high activity and high resistance to coke in the CO2 reforming of methane at 700 C [2]. Its high activity showed near equilibrium conversions of CO2 and CH4 as well as near equilibrium yields on CO and H2, which were unchanged during over 140 h. On the other hand, Ni/ZSl catalyst was also highly active, but this showed severe coke deposition within several hours. 

Upon the in-situ FT-IR analysis of KNiCa/ZSl catalyst, no C-H bands were detected in the region of 2700 - 3100 cm corresponding to CHx adsorbed species after the reaction and CH4 introduction at 700°C into in-situ cell. The adsorbed species formed upon CH4 adsorption was postulated mainly as a type of Ni-C. 

Figure 1 shows FT-IR spectra of reduced Ni/ZSl and KNiCa/ZSI catalysts before and after CO2 adsorption, and under the C02-reforming conditions. The behavior of adsorbed CO2 on KNiCa/ZSl was different from that on Ni/ZSI. FT-IR spectrum of KNiCa/ZSI even after reduction at 700 °C showed two evident bands at 1480 and 1410 cm which did not appear that on Ni/ZSI catalyst. These bands could be assigned to mainly asymmetric (Vas(OCO)) and symmetric stretching vibration (vs(OCO)) modes of monodentate carbonate species coordinated to Ca oxide or K oxide of the catalyst, respectively [3]. The intensities of these... 

The metallic Ni and CaO sites adjacent to reduced nickel metal sites on KNiCa/ZSl catalyst were considered as major sites of CO2 chemisorption. The chemisorbed amounts of CO2 on KNiCa/ZSl catalyst were increased to a value 40% higher than that of Ni/ZSl catalyst. As shown in the CO2 desorption profiles in Figure 2, the integrated amounts of CO2... 

Figure 3. Pulse test on the dissociation of CO2 and the oxidation of CH4 0ver reduced KNiCa/ZSI catalyst at 600 C.

From the results of both pulse reaction and adsorption experiments, it could be confirmed that Ni has a strong affinity with methane, while alkali promoters with carbon dioxide. The retardation of coke deposition on KNiCa/ZSI catalyst must be ascribed to the abundantly adsorbed CO2 species. This explanation is similar to the suggestion of Horiuchi et al. [5], showing that the surface of the Ni cat2ilyst with basic metal oxides was labile to CO2 adsorption, while the surface without them was labile to CH4 adsorption. Since coke deposition was mainly caused by methane decomposition, the catalyst surface covered with adsorbed CO2 or reactive oxygen species from the dissociation of CO2 would suppress coke deposition. The addition of alkaline promoters also seemed to greatly suppress the activity of supported Ni catalyst for the direct decomposition of methane. 

Therefore, it can be concluded that carbonates species formed on mainly Ca promoter of KNiCa/ZSI catalyst contributed to high catalyst stability due to enriching surface oxygen from CO2 by providing great ability to the CO2 adsorption. It was also confirmed that the oxidation step of surface carbon with gaseous CO2 or surface carbonates over KNiCa/ZSI catalyst was essential to effectively eliminate surface carbon species. 

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