/ 2 O 2 f CO 2 ) on a FeO x /Pt/TiO 2 catalyst (∼140 wt % FeO x on 1 wt % Pt/TiO 2 ) was studied at 60°C by using in situ DRIFT spectroscopy. The PROX reaction of CO occurs via a HCOO intermediate and its oxidation with OH instead of oxygen, which is different from ordinary oxidation of CO with O 2 (CO + 1 / 2 O 2 f CO 2 ). The mechanism is as follows: (i) CO(a) + OH -f HCOO + e, (ii) O + e + H + f OH, and (iii) HCOO + OH f CO 2 + H 2 O, and step (iii) is the rate-determining step. Providing HCOO intermediates by a reaction of CO(a) with the OH -anion, step (i), was deduced by combining an electroconductive PROX catalyst (Pt/CNT(carbon nanotube)) with a hydrogen fuel cell. The oxidation of CO enhanced by H 2 and H 2 O and the hydrogen isotope effect by H 2 /D 2 and H 2 O/ D 2 O on the FeO x /Pt/TiO 2 catalyst are well explained by this mechanism.

Oxidation of CO on the FeO x /Pt/TiO 2 catalyst is markedly enhanced by H 2 and/or H 2 O at 60°C, but no such enhancement is observed on the Pt/TiO 2 catalyst, but shift reaction (CO + H 2 O fi H 2 + CO 2 ) does not occur on the FeO x /Pt/TiO 2 catalyst at 60°C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO x loaded Pt/TiO 2 catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O 2 , and the reaction of the bridge-bonded CO with O 2 as well as of the linearly bonded CO is markedly enhanced by adding H 2 to a flow of CO + O 2 . From these results, we deduced that the promoting effect of H 2 and/or H 2 O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO x /Pt/TiO 2 catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H 2 O. KEY WORDS: preferential oxidation (PROX) reaction of CO in H 2 ; CO oxidation enhanced by H 2 O; CO oxidation enhanced by H 2 ; FeO x loaded Pt/TiO 2 catalyst; In-situ DRIFT IR; hydroxyl carbonyl intermediate; bi-carbonate intermediate.

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