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The water–gas shift (WGS) reaction plays a key role in hydrogen economy. Owing to the exothermic nature of the reaction, low-temperature WGS catalysts are highly desired. Zn-modified Pd-based catalysts are promising candidates for low-temperature WGS. Herein, the effect of Zn addition on the WGS catalysis is systematically studied by using the Pd(111) and PdZn(111) surface as models. Owing to the addition of Zn, the electron-accepting ability of the catalyst is weakened, while the electron-donating ability is increased. As a result, the adsorptions of electron-donor adsorbates, including H2O, CO, H, cis-COOH, trans-COOH, and H2, are weakened, while the adsorptions of electron-acceptor adsorbates, including O and OH, are strengthened. The same most favorable reaction path is found on Pd(111) and PdZn(111), which is the associative mechanism with the carboxyl dehydrogenation assisted by adsorbed OH. Although the most favorable path is the same, the weakening of CO adsorption makes the rate-determining step change from the association of CO and OH forming cis-COOH on Pd(111) to the dissociation of H2O on PdZn(111). The rate-determining step on PdZn(111) has an energy barrier lower than the rate-determining step on Pd(111). The promotion mechanism of the PdZn alloy for WGS is therefore attributed to the fact that the addition of Zn weakens the adsorption of CO and thereby alters the rate-determining step.