Methane steam reforming over Ni/Ce-ZrO2 catalyst:: Influences of Ce-ZrO2 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics

Ni/Ce-ZrO2 showed good methane steam reforming performance in term of stability toward the deactivation by carbon deposition. It was first observed that the catalyst with Cc/Zr ratio of 3/1 showed the best activity among Ni/Ce-ZrO2 samples with the Cc/Zr ratios of 1/0, 1/1, 1/ 3, and 3/1. Temperature-programmed oxidation (TPO) experiments indicated the excellent resistance toward carbon formation for this catalyst, compared to conventional Ni/Al2O3; the requirement of inlet H2O/CH4 to operate without the formation of carbon species is much lower. These benefits are related to the high oxygen storage capacity (OSC) of Ce-ZrO2. During the steam reforming process, in addition to the reactions on Ni surface (*), the redox reactions between the gaseous components present in the system and the lattice oxygen (O-x) on CeZrO2 surface also take place. Among these reactions, the redox reactions between the high carbon formation potential compounds (CH4, CHx-*n and CO) and the lattice oxygen (0,) can prevent the formation of carbon species from the methane decomposition and Boudard reactions, even at low inlet H2O/CH4 ratio (1.0/1.0). Regarding the intrinsic kinetic studies in the present work, the reaction order in methane over Ni/Ce-ZrO2 was observed to be approximately 1.0 in all conditions. The dependence of steam on the rate was non-monotonic, whereas addition of oxygen as an autothermal reforming promoted the rate but reduced CO and H-2 production selectivities. The addition of a small amount of hydrogen increased the conversion of methane, however, this positive effect became less pronounced and the methane conversion was eventually inhibited when high hydrogen concentration was added. Ni/Ce-ZrO2 showed significantly stronger negative impact of hydrogen than Ni/Al2O3. The redox mechanism on ceria proposed by Otsuka et al. [K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517; K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493; K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193] can explain this high inhibition. (c) 2005 Elsevier B.V. All rights reserved.