This paper analyzes the mechanism of deactivation of a new Cu-Al2O3-CuAl2O4 water-gas shift (WGS) catalyst recently developed for smallscale hydrogen generation applications, such as fuel cell-based residential power generators and hydrogen fueling stations. The catalyst has good potential for use in small-scale fuel processors working under essentially steady-state conditions, such as on-site hydrogen generation systems for hydrogen filling stations. However, transient regimes with frequent starts and stops of the fuel processing system, common for the residential fuel cell applications, can result in aging with a decline in activity of this (and most likely of any) copper-based WGS catalyst. The study described in this paper reveals that the aging resulting in deactivation of Cu-Al2O3-CuAl2O4 under start-stop conditions is due to passivation of the catalytically active surface by the dense shell of hydrogen-bonded hydroxides strongly adsorbed over copper oxide crystallites. This deactivation phenomenon, likely to be typical for all copper-based WGS catalysts operating with frequent starts and stops, is a serious issue that will likely prevent use of such catalysts in residential fuel cell applications. However, the deactivated Cu-Al2O3-CuAl2O4 catalyst can be regenerated by calcining in air, with full recovery of the activity. This regenerationj which is impossible for conventional copper-zinc-alumina catalysts, is a significant practical advantage and also supports the conclusion on the role of surface hydroxides in the deactivation mechanism. (c) 2007 Elsevier Inc. All rights reserved.