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Palladium-alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by 0, and reduction by H, , using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species.Pd2+ and Pd+ entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [I Torr z (101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated) z acetylacetonate c chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.

CO adsorption on palladium4eria catalysts prepared from Pd acetylacetonate has been investigated by IR and UV-VIS spectroscopies and by volumetric measurements. After calcination and evacuation at 673 K, the samples contain small PdO particles which are immediately reduced upon adsorption of C O at room temperature. For samples prereduced by H,, electron-donor centres (Ce3 + ions, oxygen vacancies) are created during the pretreatment. These centres are responsible for increased CO adsorption by ceria in the presence of palladium, which leads to apparent CO : Pd values greater than unity and their interaction with palladium induces a carrier-terneta1 electron transfer. Plasmon absorption in reducible carriers hinders the detection of species adsorbed on the supported metal.The characterization of ceria-supported platinum metals suffers from particular difficulties owing to the semiconducting and basic properties of the carrier. Measurements of the metal fraction exposed (MFE) lead to apparent values greatly exceeding unity;'-3 the presence of the metal has been shown to favour the reduction of ceria3-* and hydrogen spill-over may beCeria is also able to adsorb carbon monoxide,'-'' which could otherwise be used as an alternative molecular probe. Transmission electron microscopy (TEM) examinations suffer from poor contrast between the support and the metal'-4 and when used as a reducible carrier, ceria tends to decorate the supported meta1.5,13*'4 In the case of palladium the formation of palladium hydride must be taken into account and definite conditions are imposed to avoid it.'' The aim of this work is to study the electronic state of ceriasupported palladium and to examine the nature of CO species adsorbed on the metal. These investigations should help to explain the large uncertainties associated with the determination of the metal fraction exposed on this type of catalyst. Experimental MaterialsCerium dioxide, obtained from Rhone Poulenc, has a specific surface area of 115 m' g-' after outgassing at 573 K. Palladium acetylacetonate [Pd(acac),] was prepared from palladium chloride (Engelhard France). Preparation of catalystsPreparation involved anchoring of Pd acetylacetonate on ceria uia immersion of the carrier in a benzene solution of Pd(acac), at 353 K for 2 h. This was followed by drying at 353 K for 15 h and calcination in flowing oxygen (18 1 h-') for 15 h at 673 K. The solids obtained at this stage were labelled unreduced. To obtain reduced samples, calcination was followed by purging under nitrogen at room temperature, RT, heating in flowing hydrogen (18 1 h-') at T, (573, 673 or 773 K) for 2 h and outgassing at 673 K for 15 h. The palladium content, determined by inductively-coupled plasma emission spectroscopy, was 1.58% for chemisorption measurements, but only 0.44% for infrared experiments because of the poor transmission of the samples. MethodsThe spectra were recorded at RT on a Perkin-Elmer 1730 Fourier transform spectrometer. Unless otherwise stated, the spectra presented are obtained after su...

The electrochemical electron-transfer rate constant has been determined as a function of the electrode potential for a series of simple electron-transfer processes to organic molecules in media containing acetonitrile or dimethylformamide and a quaternary ammonium salt as supporting electrolyte and using mercury as the electrode material. The reactions and the experimental conditions were selected so as to deal with outer-sphere processes and to minimize the magnitude of double-layer corrections. Convolution potential-sweep voltammetry and the impedance method were used for obtaining the kinetic data. Under these conditions, the electrochemical transfer coefficient was observed, in all cases, to vary, beyond experimental error, with the electrode potential. The magnitude of the variation is of the same order of magnitude as that predicted by the Marcus theory of outer-sphere electron transfer. A more complex reaction, the reduction of benzaldehyde in ethanol, involving dimerization steps following the initial electron transfer was also investigated. A definite variation of the transfer coefficient was again observed. This behaviour, observed for various solvents and functional groups, appears as a general phenomenon in the reduction of organic molecules in the case where charge transfer is fast and mainly governed by solvent reorganization.The present theories of electron transfer at electrodes,' such as the Hush-Marcus theory,2 -4 predict that the electrochemical transfer coefficient should vary with the electrode potential. Being based on an harmonic approximation they imply a quadratic dependence of the activation energy and therefore a linear dependence of the electrochemical transfer coefficient upon the electrode potential. They also predict the magnitude of the variation of the transfer coefficient as a function of the reorganization factor. The smaller the reorganization factor, i.e. the faster the electron transfer, the larger the variation of the transfer coefficient with potential.Over the last 15 years there have been several attempts to detect such variations experimentally and to compare their magnitude with that anticipated from the Earlier work5-11 in this area has not provided a clear answer to the question thus raised. The same systems that were first regarded as exhibiting such a potential dependence were later, after more accurate analysis, shown not to give rise to a definite variation that was clearly greater than experimental error.7 In order to obtain acceptable evidence that the transfer coefficient does or does not vary with the potential, the system under study must fulfil several requirements. The first of these is that the electrochemical reaction should follow a simple mechanism, preferably involving a single one-electron step giving rise to a chemically stable species, at least in the time range of the experiments. This does not mean that it is impossible to investigate the potential dependence of the transfer coefficient in more complex processes, involving for example follow-up ch...

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