K-PtCo/Al2O3 catalyst coatings were studied concerning preferential oxidation of carbon monoxide (PROX) in hydrogen-rich gas streams. It has been found that the addition of potassium can enhance the catalytic activities at temperatures below 413 K, thus can widen the window of operation in terms of reaction temperatures. The most active catalyst coating of 1.5K-Pt2Co (Pt 1 wt.%, Co 2 wt.%, and K/Co molar ratio of 1.5) can reduce carbon monoxide concentrations from initial 1% to less than 10 ppm at a high GHSV of 120,000 ml g(-1) h(-1) in a temperature range 393-433 K. The structure of catalyst coatings has been thoroughly characterized using multi-techniques, such as high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), in situ laser Raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The addition of K has evidenced to promote the formation of Pt3Co and increase the particle size as identified by TEM and XPS. Pt3Co plays an important role in PROX. For PtCo/Al2O3 catalyst coatings, a redox equilibrium for Co during PROX was found. In contrast, with the addition of K, this redox equilibrium disappeared, probably because of the increased amount of Pt3Co phase in the catalyst surface region. (C) 2013 Elsevier B.V. All rights reserved.