Lanthanide-promoted sol-gel nickel catalysts supported on alumina were studied with regard to their activity and stability for propane steam reforming. X-Ray photoelectron spectroscopy, in situ X-ray diffraction, temperature-programmed oxidation, transmission electron microscopy, thermogravimetric analysis coupled with temperature-programmed reaction, and sequential temperature-programmed coking/gasification/hydrogenation were used to investigate the deactivation characteristics of the catalysts prepared. Adding small amounts (2 wt%) of lanthanide elements (i.e., La, Ce, and Yb) significantly improves steam reforming activity and stability. Although the catalysts deactivate more rapidly at higher reaction temperature, the activity loss can be suppressed by increasing the H2O/C ratio. The catalyst deactivation can be attributed to a combination of nickel sintering and coke formation. In addition, there appears to be partial oxidation of metallic sites after exposure to the reaction medium. Lanthanides cause enhanced sintering resistance, leading to smaller nickel crystallites in Ni/Al2O3 catalysts. Cerium promotion effectively inhibits coking by increasing active oxygen species on the nickel surface due to enhanced water adsorption and gasification. Incorporation of cerium also reduces the dissolution and diffusion of carbon through nickel particles, thus preventing the formation of carbon filaments. (c) 2005 Elsevier Inc. All rights reserved.