Catalytic Activity and Characterizations of Ni/MTixOy-Al2O3 (M=K, Ca) Catalyst for Steam Methane Reforming

Abstract

Steam reforming of hydrocarbons has mainly considered as recent topic for fuel processing due to high productivity of hydrogen gas. Although the development of various catalysts for the process has studied, the catalyst deactivation is still a major drawback of technical advancement. Therefore, the objective of this study was to investigate the catalytic stability of modified nickel catalysts containing titanate and the effects of additives such as potassium and calcium. The activities and stabilities of Ni/MTixOy-Al2O3 (M=K and Ca) catalysts were compared with those of a commercial catalyst for methane steam reforming. Nickel catalysts supported on the MTixOy-Al2O3 (M= K and/or Ca) were prepared by the wet impregnation method for steam methane reforming to produce hydrogen. X-ray diffraction, N2 physisorption, scanning electron microscopy with energy dispersive spectroscopy, the H2 temperature-programmed reduction technique, and X-ray photoelectron spectroscopy were employed for the characterization of catalyst samples. The catalytic performance of the Ni/K2TixOy-Al2O3 catalysts was comparable to that of FCR-4 under the mild reaction condition. In particular, a stability test at 800℃ and the reactanct flow with the steam-to-carbon (S/C) feed ratio of 1.0 indicated that the Ni/K2TixOy-Al2O3 catalysts were more active, thermally stable and resistant to deactivation than the non-promoted Ni/Al2O3. It is considered that the appropriate interaction strength between nickel and the modified support and proper K2TixOy phases with a surface monolayer coverage achieved at ca. 15 wt.% loading in the support play important roles in promoting the steam methane reforming activity as well as suppressing the sintering of the catalyst. The Ni/K2-xCax/2TiyOz(20)-Al2O3(80) catalysts have complex phases of K-Ca-Ti-O system and their nickel species were strongly interacted with the K2-xCax/2TiyOz(20)-Al2O3(80) support, indicating their low reducibilities. The Ni/K2-xCax/2TiyOz(20)-Al2O3(80) catalysts showed acceptable activities for steam methane reforming under the mild reaction condition. For the catalytic stability test, the Ni/CaTiyOz(20)-Al2O3(80) represented the superior stability than FCR-4, Ni/Al2O3, and any other catalysts in this work. For the Ni/K2-xCax/2TiyOz-Al2O3 catalyst (having higher than x of 0.6), the presence of perovskite oxide (CaTiO3) on the catalyst may derive acceptable catalytic activity with high resistance to coking due to its oxygen mobility. The Ni/CaTixOy-Al2O3 catalysts have the medium interaction strength between nickel and support, weaker than that of NiO/Al2O3, and their nickel species were well-dispersed in the support. They showed relatively lower activities for steam methane reforming under the mild reaction condition. However, the Ni/CaTixOy-Al2O3 catalysts presented good performance at a higher reaction temperature than 800℃, closed to thermodynamic limit. A sulfur-tolerance test result for the Ni/CaTixOy-Al2O3 catalysts under a severe reaction condition: at 750℃, in a reactant flow with S/C feed ratio of 2.5 containing sulfur compounds of 7 ppmv, and at a GHSV of 15,000 h-1 demonstrated that they maintained their good stability for the reaction time of 300 min. On the other side, significant loss in activity was observed over the Ni/Al2O3 catalyst. In particular, over the Ni/CaTixOy(20)-Al2O3(80) catalyst, particle size of Ni metal was maintained to be same during the sulfur-tolerance test, indicating its superior resistance to sulfur. Consequently, both potassium titanate (K2TixOy) and calcium titanate (CaTixOy) would be a promising additive material of alumina supported nickel catalyst for steam methane reforming reaction, effectively inhibiting deactivation from sintering of catalyst as well as sulfur poisoning.