Spectroscopic study of CO adsorption on palladium–ceria catalysts

Abstract

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...

Understanding the role of promoters in catalysis: operando XAFS/DRIFTS study of CeOx/Pt/Al2O3 during CO oxidation

“…Which is consistent with a loss in vibrational coupling between the CO molecules which occurs at high coverage [21,22]. The broad tail at ~2020 cm −1 may provide evidence of the close proximity of the Pt and CeO 2 , as this band has been previously assigned to CO adsorbed on Pt interacting with CeO 2 and similarly for Rh/CeO 2 samples where it was assigned to CO adsorbed on Rh atoms at the metal-support interface [20,23,24].…”

Palladium-ceria catalysts: Metal-support interactions and reactivity of palladium in selective hydrogenation of but-1-yne

“…The silica-supported ceria was prepared following a procedure presented elsewhere [20]. The preparation of the catalysts was carried out following two procedures previously described : (i) the samples with high metal fraction exposed (MFE) were obtained by anchoring of palladium acetylacetonate from a benzene solution [12][13][14] and the samples with low MFE by electroless plating using palladium tetrammine chloride solution [21,22]. The metal fraction exposed was measured by CO chemisorption [12][13].…”

“…The preparation of the catalysts was carried out following two procedures previously described : (i) the samples with high metal fraction exposed (MFE) were obtained by anchoring of palladium acetylacetonate from a benzene solution [12][13][14] and the samples with low MFE by electroless plating using palladium tetrammine chloride solution [21,22]. The metal fraction exposed was measured by CO chemisorption [12][13]. After drying, the samples were calcined in Oz flow at To~ = 673 K for 12 h, flushed by N2 at 293 K for 15 rain, reduced in H2 flow for 2 h at a fixed temperature Tr (473, 573, 673 K) and finally evacuated under N2 flow at Tr for 30 min and cooled down to room temperature.…”

“…This carrier leads to strong metal-support interactions with precious metals (SMSI) [9][10][11]. In the case of palladium, we have shown by infrared spectroscopy [12][13] that in reducing atmosphere, electron transfer occurs from ceria to highly dispersed palladium. Moreover, Pd enhances the rate of reduction of CeO2 by CO.…”

“…Concerning the interactions of hydrogen with the Pd/CeO2 system, it was found recently [14] that : (i) spillover of hydrogen occurs from dispersed palladium to ceria even at room temperature, (ii) a large amount of hydrogen is adsorbed reversibly on ceria, (iii) the presence of palladium accelerates the reduction of ceria by Hz. The aim of the present work is to discuss the effect of the support and the metal-support interactions on the reactivity of palladium in selective hydrogenation of but-l-yne, taking into account previous data obtained on Pd supported on various carriers [8,[12][13][14][15][16][17] and on ceria [l [8][9][10][11][12][13][14][15][16][17][18][19][20].…”