Isotope effect of H 2 /D 2 and H 2 O/D 2 O for the PROX reaction of CO on the FeOx/Pt/TiO 2 catalyst
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Catalysis Letters Vol. 111, Nos. 3–4, November 2006 (Ó 2006) DOI: 10.1007/s10562-006-0152-5
Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst Masashi Shou and Ken-ichi Tanaka* Advanced Science Research Laboratory, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan
Received 31 March 2006; accepted 8 August 2006
The oxidation reaction of CO with O2 on the FeOx/Pt/TiO2 catalyst is markedly enhanced by H2 and/or H2O, but no such enhancement occurs on the Pt/TiO2 catalyst. Isotope effects were studied by H2/D2 and H2O/D2O on the FeOx/Pt/TiO2 catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H2/D2 as well as by H2O/D2O at 60 °C. This result suggests that the oxidation of CO with O2 via such intermediates as formate or bicarbonate in the presence of H2O, in which H2O or D2O acts as a molecular catalyst to promote the oxidation of CO as described below.
KEY WORDS: PROX reaction of CO; Isotope effect of H2/D2 and H2O/D2O; Molecular catalysis of H2O; A new PROX catalyst; FeOX loaded Pt/TiO2 catalyst.
1. Introduction Our previous studies discovered a new PROX catalyst (preferential oxidation of CO in H2) by loading a large amount of FeOx on supported Pt catalysts [1, 2, 3]. The oxidation of CO on this catalyst is markedly enhanced by adding H2 and/or H2O [4]. By loading a large amount of FeOx on a Pt/TiO2, the activity is extraordinarily enhanced even though the Pt content is effectively reduced by more than a half. It is tacitly assumed that the reaction on supported metal catalyst takes place only on the metal particles. Therefore, the activity is normalized by dispersion or by assuming specific active sites such as the perimeter. However, the precise role of the support remains a puzzle. A good example is the activity of supported Au catalyst. Haruta et al. [5, 6] showed the size dependent activity of Au particles in the oxidation of CO, and they explained the size effect by the contribution of specific sites located at the perimeter. In contrast, Chen and Goodman [7] recently claimed that a specific array of bi-layer Au atoms on TiO2 is responsible for the activity; and the *To whom correspondence should be addressed. E-mail: [email protected]
size as well as the perimeter are not essential for the activity. We also showed that a low activity 1 wt.% Au/ TiO2 catalyst becomes a superior active catalyst by loading a large amount of FeOx [8], which is very similar to the activation of 1 wt.% Pt/TiO2 catalyst by loading a large amount of FeOx. The catalysis has been generally explained by the adsorption and followed reaction of the adsorbed molecules. In the case of highly dispersed supported metal catalyst, transportation of molecules and/or intermediates over the support is indispensable to provide molecules to the active site. However, this important role of the support has not been taken into account. Imagine a Pt particle on a support having ‘‘Crain Bottle’’ like curvature (or R
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