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The steam reforming reaction of hydrocarbons and organic fuels, in general, is followed by a two-stage reaction of water gas shift, which allows increasing the hydrogen yield and a final purification step for CO removal to use hydrogen in an ammonia plant or a PEM fuel cell. This paper is focused on the CO Preferential Oxidation, CO PROX (or CO selective oxidation in excess hydrogen) reaction, considered as the simplest and cost effective process to achieve the less than 10 ppm CO. The objective of this paper is to review the performances of noble metals (Pt, Ru, Rh, Pd), gold and transition metal oxides catalysts in this reaction. Although the results reported are largely influenced by the experimental conditions (reactant flow composition, mass of catalyst, duration of experiment …) a comparison of advantages and drawbacks for each type of catalysts is proposed in terms of activity and selectivity as well as of CO 2 and H 2 O influences. A special attention will be paid to copper-doped ceria catalysts which appear to be very active and selective in a range of temperatures appropriate for fuel cell application. The performances, the stability and the low cost of these formulations compared to noble metal-based catalysts make them very attractive for an industrial application.

Oxide-supported noble metal catalysts were tested in the preferential oxidation of carbon monoxide (PROX) reaction in the temperature range between 50 and 300 • C. Both the influence of the noble metal nature (Pt, Ir, Pd), the support physical and chemical properties (redox, acidity, basicity) and the reaction conditions (oxygen stoichiometry) on the catalyst activity and selectivity was evaluated. Platinum and iridium were shown to be the most active and selective catalysts in the whole temperature range compared with palladium. Furthermore, noble metals supported over ceria-based oxides were shown to be active and selective, especially at low temperature. Additionally, it was observed that the higher the molar fraction in ceria in the oxide, the higher the activity and the selectivity in the PROX reaction. Ceria, with the highest oxygen mobility at the oxide surface, was shown to be the best support. Accordingly, on simple oxides (CeO 2 , SiO 2 -Al 2 O 3 ,Al 2 O 3 , SiO 2 ,La 2 O 3 and MgO), the induced mobility of the oxygen atoms at the surface of the support determined elsewhere, well correlated with the basicity of the support, was shown to be one key parameter for the performances of the catalysts in the PROX reaction. Finally, the formation of water (hydrogen oxidation) at high temperature and high oxygen excess was shown to be responsible for the increasing activity of the catalysts in the conversion of CO to CO 2 via the water gas shift reaction (WGSR).

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The urea method is explored for a high-yield preparation of CuO-CeO 2 catalysts precursors. Solutions containing urea, Ce(NO 3 ) 3 , and Cu(NO 3 ) 2 were aged at 363 K for 5 h, achieving a quantitative coprecipitation in the form of amorphous Cu(II)-Ce(III) basic carbonates, with Cu(II) contents up to 40%. The mixed precursors were characterized by PXRD, SEM, EDS, FTIR, and TG. Upon mild thermal treatment under air atmosphere, i.e., T ) 723 K, powders were readily converted into CuO-promoted nanometric ceria. No Tenorite (CuO) segregation is observed after annealing at 873 K. The binary oxides were characterized by means of temperature-programmed reduction (TPR) and a test reaction for the preferential oxidation of carbon monoxide (CO-PROX), achieving the best performance with samples containing around 20% in copper atoms.

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