The CO content of hydrogen feed to proton exchange membrane fuel cells (PEMFC) must be kept under 1-100 ppm for their proper operation. This can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen (PROX). The present study reports on the mechanism of PROX reaction on Pt/CeO 2 catalyst, by using catalytic tests, in-situ DRIFTS, high-pressure XPS, HRTEM and TDS techniques. Bulk metallic, pronounced adsorbate-induced surface Pt and a small amount of oxidized Pt sites were detected by in-situ, high pressure XPS under PROX conditions. The pre-oxidized ceria surface was strongly reduced in pure H 2 but significantly re-oxidized under PROX conditions (i.e. O 2 +CO in excess hydrogen) at T=358 K. The remaining small amount of Ce 3+ decreased with increasing temperature. HRTEM found well-crystallized CeO 2 particles (8-10 nm) in the case of activated (pre-oxidized) sample that transformed in a large extent to an oxygen deficient ceria super-cell structure after PROX reaction. Metallic Pt particles (2-3 nm) and small (0.5-0.6 nm) Pt clusters were found by HRTEM. These findings were in accordance with the variations in relative intensity of the corresponding Pt-CO bands (DRIFTS). Different types of carbonate and formate species were detected (XPS and DRIFTS). Their possible role in the reaction mechanism is discussed. Resolved OH bands could not be found by DRIFT in the PROX reaction mixture indicating significant amount of adsorbed water in a hydrogen-bonded structure. Its presence seems to suppress hydrogen oxidation while CO oxidation still takes place, as the metallic particles are covered by CO (DRIFTS). The direct contribution of surface water in a low-temperature water-gas-shift (LTWGS) type reaction in the PROX mixture is proposed.

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Pt black catalysts have been characterized by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The spectra measured after standard purification (O2 and H2 at 600 K) compared well with those of a purified reference Pt foil. All samples exhibited pronounced Fermi-edge intensities in UPS although only 60–70% Pt was detected on their surfaces by XPS, the remainder being C and O. Line analysis of the C 1s XPS, region showed the presence of partly oxidized graphite and hydrocarbon polymer, likely in three-dimensional islands. OH/H2O species attached to the metallic Pt sites were detected by UPS bands, in agreement with O 1s XPS line analysis. Similar spectral features are recorded at 600 K. Carbon could not be removed entirely by O2 up to 850 K; hydrogen did not remove surface oxygen even up to 750 K. UPS features of C on Pt used in hydrocarbon reactions were similar to those reported for amorphous hydrogenated carbon overlayers. Consequences of the present findings for the catalytic properties of Pt in n-hexane reactions and the quantification of H2–O2 titration are discussed briefly