Selective oxidation of CO in hydrogen, obtained from hydrocarbons, is an important reaction for producing hydrogen appropriate for use in fuel cells. This paper reports on the variations in activity and selectivity of alumina-supported Art catalysts for the selective oxidation of CO due to the presence/absence of chloride (Cl-) species and to different An particle sizes [dispersion, D (%)]. This study made use of elemental analysis, chemisorption, kinetic studies, and steady-state isotopic transient kinetic analysis (SSITKA) to explore these variations. A comparison of An catalysts with different Au loadings to 5 wt% Pt/gamma-Al2O3 is also made. Magnesium citrate was added during preparation of one of the Au catalysts [referred to as Au(Mg)], because it was found to displace Cl- species, resulting in smaller Au particle sizes. Rapid partial deactivation was observed for all of the catalysts during the first 30 min of time-on-stream irrespective of the presence or absence of Cl- species. A reactivation after initial rapid deactivation was observed during reaction only for the Au(Mg) catalyst. This catalyst, the Au catalyst with the smallest particle size (D > 90%) and the lowest Cl- content, was more active than the catalysts with larger particle sizes (lower %D) at steady state. But all Au catalysts had similar initial activity. A decrease in the concentration of reactive intermediates (NI-CO2) along with a very large decrease in intrinsic site activity (k), both obtained from SSITKA, appear to have been the cause of the initial deactivation of the Au catalysts. For the Pt catalyst, the decreased concentration of reactive intermediates was the main cause of initial deactivation. Although the precise reasons for the increase in activity with time-on-stream after initial deactivation of the An catalyst prepared with magnesium citrate are not clear, it can be hypothesized that increasing amounts of water formed during the reaction may have helped in decomposing deactivating An carbonate species or in restructuring/reconstructing the active sites. The smaller An particle size and/or absence of Cl- species may have contributed to this. Although there was a slight increase in the number of active CO2 intermediates, most of the increase in rate for the Au(Mg) catalyst was due to an increase in average site activity. There is no evidence that Mg functions as a chemical promoter. (c) 2005 Elsevier Inc. All rights reserved.