A series of cobalt-promoted Pt catalysts supported on multi-walled carbon nanotubes was synthesized, and their performance was evaluated for the water-gas shift (WGS) reaction. Compared to the monometallic Pt catalyst, the WGS turnover rate (TOR) at 300 degrees C was promoted by a factor of 10 at a Pt:Co molar ratio of 1:3. X-ray absorption spectroscopy and XRD showed the presence of a Pt3Co alloy along with a partially oxidized cobalt and a free cobalt metal phase after reduction pretreatment. In order to determine the dominant active site over the Co-promoted catalysts, selective leaching of partially oxidized Co (designated as CoOxHy) and Co metal was performed with a 5% wt. acetic acid solution, while preserving the Pt-rich phases. The WGS TOR at 300 degrees C for the Co-promoted catalysts after leaching was observed to be even lower than that of the monometallic Pt catalyst. Thus, the alloy formation was determined to be inconsequential towards promotion in the WGS TOR, while the dominant active site was determined to be a PtCo alloy in intimate contact with the CoOxHy phase. Combined Density Functional Theory (DFT) calculations and ab-initio thermodynamic phase diagrams point to a monolayer of CoOH being the most stable Co phase on Pt(111) under WGS conditions. Calculations of OH binding energies on Pt(111), Pt3Co(111), and at the interface between CoOH overlayers and Pt(111) show that trends in the WGS activity of these catalysts are linked to the strength of OH binding, with the strongest OH binding found at the interface between CoOH and Pt, supporting the conclusion that a similar interface is the source of enhanced WGS activity in the PtCo bimetallic system. (C) 2020 Elsevier Inc. All rights reserved.