A catalyst composed of a 3-nm-diameter ruthenium core coated with a platinum shell just one or two atomic layers thick converts CO to CO2 in the presence of hydrogen more selectively and at more than 50 °C lower than other common catalysts that mediate the same oxidation (Nat. Mat., DOI: 10.1038/nmat2156). Such catalysts could lead to new methods for preparing high-purity hydrogen as required for fuel cells, which are readily damaged by the high levels of CO typically found in commercial hydrogen supplies. Common CO-cleanup methods rely on catalysts that require relatively high temperatures and that waste some of the hydrogen fuel by converting it to water. To sidestep those problems, a research team headed by Manos Mavrikakis of the University of Wisconsin and Bryan Eichhorn of the University of Maryland used computational methods to search for novel and efficient CO-cleanup catalysts and concluded that the Ru-Pt core-shell structures would have the desirable properties. The core-shell particles (shown; Ru = gold, Pt = gray, O = red, C = black, H = green) exhibit properties that are distinct from Ru-Pt alloys and mediate CO oxidation by way of a novel mechanism based on hydroperoxy (O–O–H) intermediates.