Water, one of the most popular species in our planet, can play a catalytic role in many reactions,including reactions in heterogeneous catalysis. In a recent experimental work, Bergeld, Kasemo, andChakarov demonstrated that water is able to promote CO oxidation under low temperatures (200K). In this study, we choose CO oxidation on Pt(111) in the presence of water as a model system toaddress the catalytic role of water for surface reactions in general using density functional theory.Many elementary steps possibly involved in the CO oxidation on Pt(111) at low temperatures havebeen investigated. We find the following. First, in the presence of water, the CO oxidation barrier isreduced to 0.33 eV (without water the barrier is 0.80 eV). This barrier reduction is mainly due to theH-bonding between the H in the H2O and the O at the transition state (TS), which stabilizes the TS.Second, CO can readily react with OH with a barrier of 0.44 eV, while COOH dissociation toproduce CO2 is not easy (the barrier is 1.02 eV). Third, in the H2O1OH mixed phase, CO can beeasily converted into CO2 . It occurs through two steps: CO reacts with OH, forming COOH; andCOOH transfers the H to a nearby H2O and, at the same time, an H in the H2O transfers to a OH,leading to CO2 formation. The reaction barrier of this process is 0.60 eV under CO coverage of 1/6ML and 0.33 eV under CO coverage of 1/3 ML. The mechanism of CO oxidation at lowtemperatures is discussed. On the basis of our calculations, we propose that the water promotioneffect can in general be divided into two classes: (i) By H-bonding between the H of H2O and anelectron negative species such as the O in the reaction of CO1O1H2O!CO21H2O, H2O canstabilize the TS of the reaction and hence reduce the barrier. (ii) H2O first dissociates into H and OHand then OH or H participates directly in the reaction to induce new reaction mechanism with morefavorable routes, in which OH or H can act as an intermediate. ? 2003 American Institute ofPhysics.