L. Hilaire

Reduction of CeO2 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and X-ray photoelectron spectroscopy measurements

The reduction of CeO2 by hydrogen has been studied from 300–1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV–VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO2 samples were used with B.E.T. surface areas of 115 and 5 m2 g–1, respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the CeIII photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO1.83. Temperatures higher than 923 K were required to reduce the ceria further. The surface CeIII content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17 000 cm–1 band in the UV–VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO1.83 led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum.

The multi-surface structure and catalytic properties of partially reduced WO3, WO2 and WC + O2 or W + O2 as characterized by XPS

The XPS at different experimental conditions of WO3, WO2, W(0), WC,W(0) + O2 and WC + O2 are reported. Extensive hydrogenolysis reactions of 2-methylpentane yielding methane a major product were observed on relatively pure W(0) and WC surfaces. Isomerization reactions for the same reactant were observed on WO2 or partially oxydized W(0) and WC surfaces, while bulk WO3 does not show any catalytic reactivity at the beginning of the reaction. The catalytic activity of WO2 is attributed to the presence of a certain density of states of the 5d, 6s electrons present at the Fermi-level on the W4+ cation and observed in the XPS of the valence band energy region of WO2. Such reduction by H2 and the hydrocarbon reactant of bulk WO3 or that present on W(0) or WC surfaces results in the presence of WO2 and to a lesser extent WO and W(0), which explains the isomerization reactions carried on these modified surfaces.

Photoemission on 3d core levels of Cerium: An experimental and theoretical investigation of the reduction of cerium dioxide

Abstract The oxidation state of ceria (CeO2) used as a support of catalysts is investigated by X-Ray Photoelectron Spectroscopy (XPS) on the 3d core level of Cerium. A sequence of 3d-XPS spectra is obtained for a ceria converted progressively (and partially) to Ce2O3 under increasing temperatures of reduction (up to 900°C). The spectra are analysed theoretically by using a filled-band impurity Anderson model and by chosing appropriate parameter values.

Hydrogenation of But-1-yne and But-1-ene on Palladium Catalysts: Particle Size Effect

Abstract The hydrogenation of but-1-yne and but-1-ene was studied on highly (D = 97%) and lowly (D = 26%) dispersed palladium on alumina catalysts. The activity calculated in terms of accessible palladium atoms was found to depend on the particle size. For the hydrogenation of but-1-ene, the turnover number was higher on highly dispersed catalysts than on the lowly dispersed one. The reverse was true for the hydrogenation of but-1-yne. During the hydrogenation of but-1-yne the quantity of but-1-ene produced without further hydrogenation was higher on small particles. A study by X-ray photoemission spectroscopy showed an increase of the binding energy by 1 eV for the 3d core level of palladium in 0.2% Pd/Al2O3 (D > 80%) , either before or after a catalytic run (no coalescence occurred). The catalytic results were interpreted as being due to the variation of the adsorption strength of the hydrocarbons. The unsaturated hydrocarbons could be more strongly adsorbed on very small palladium particles which may be considered as “electron deficient”. A calculation showed that the sticking coefficient of but-1-yne was at least 2.5 times as high on small particles as on larger ones.

Molybdenum based catalysts. I. MoO2 as the active species in the reforming of hydrocarbons

The XPS spectra of bulk MoO2 and MoO3 are reported. Commercial MoO2 reveals the presence of multilayers of MoO3 covering bulk MoO2. In situ reduction of bulk MoO3 by H2 at temperatures equal to 623 K and less than 673 K reduces the superficial layer(s) to mainly MoO2. Such phase is characterised by a certain density of states in terms of the free 4d, 5s electrons localised mainly on the Mo atoms and observed in the valence band energy region at the Fermi-level in the XP spectrum of this system. Catalytic reactions on the MoO2 phase of 2-methylpentane at atmospheric pressure using a mixture of the hydrocarbon (5 Torr) with hydrogen (755 Torr) yield 3-methylpentane andn-hexane as the main products in roughly equal amounts. These results are interpreted in terms of dual sites on the MoO2 surface.

Redox behaviour of ceria-titania mixed oxides

The reducibility of the supports is one of the properties influencing the catalytic behaviour of platinum catalysts in the selective hydrogenation of α,β-unsaturated aldehydes. Platinum on reducible supports has shown higher selectivities in carbonyl hydrogenation compared to Pt/Al2O3 and Pt/SiO2 catalysts. We report here a study on the redox properties of mixtures of titania and ceria, precursors of Pt/TiO2–CeO2 catalysts. Two samples, with atomic compositions Ti/Ce 8/2 and 5/5, were prepared using a sol–gel process. They were treated under hydrogen and air in a wide range of temperatures. Surface compositions and the percentage of reduced ceria were determined by X-ray photoelectron spectroscopy. Temperature programmed reduction (TPR) profiles, and X-ray diffraction patterns were interpreted in terms of the formation and stability of several mixed compounds.

Effect of surface treatments, photon and electron impacts on the ceria 3d core level

Abstract We have investigated the effect of surface treatments and damage induced on the Ce3d core level of ceria samples by the X-ray power density, the exposure time to the X-ray beam and the sample charge effect. We show that a CeO2 surface requires passivation through a mild oxidant exposure such as CO2 to yield a pure Ce(IV)3d spectrum. Thermal “in vacuo” treatments lead to partial chemical reduction of the surface. Damage induced by the X-ray beam and sample charging produces similar effects. We have studied in more detail the effect of sample charging. We mainly assign these Ce3d core level changes to a photon-stimulated oxygen desorption (PSD) via an Auger interatomic decay. However, owing to the partially reversible character of this effect and to main modifications of the Ce3d final states obtained through hybridization of the Ce4f-O2p states U′ and U″ (V′ and V″, respectively), we propose alternative pathways based on intra-atomic rearrangements or on oxygen migration towards interstitial sites of the fluorite lattice.

Iron molybdate catalysts for methanol to formaldehyde oxidation: effects of Mo excess on catalytic behaviour

Two iron molybdate catalysts have been prepared by coprecipitation, one stoichiometric (Mo/Fe atomic ratioD1.5) and the other with a typical industrial composition (Mo/Fe atomic ratioD3). Physicochemical characterisation shows that Mo excess brings about an increase in the surface area and deconvolution of NH3 TPD curves evidences some changes in acidity. On the other hand, surprisingly, Mo excess does not cause significant changes in the activity for methanol oxidation per unit surface area. This provides evidence that Fe2(MoO4)3 is in fact the active phase of the catalyst. The catalyst with Mo excess leads to higher selectivity for HCHO to the detriment of dimethyl ether selectivity. This result is attributable to larger oxygen availability at the catalyst surface and changes in acidity. The catalytic behaviour of the prepared catalysts was compared with that of an industrial catalyst. It was found that the prepared catalyst with an Mo/Fe composition similar to the industrial one is more active due to larger surface area.

Interaction of oxygen and hydrogen with palladium

Abstract The interaction of oxygen and hydrogen with Pd(111) and polycrystalline palladium was studied in a wide domain of temperatures and pressures with low energy electron diffraction, Auger electron spectroscopy (AES), and photoelectron spectroscopy (XPS and UPS). It was shown that the exact knowledge of the influence of possible contaminants (S, C, O) is necessary to prove unambiguously the origin of the structures induced by the adsorbates. Results on the electronic structure of palladium are presented with special attention on surface effects. The different steps of the formation of palladium oxide were characterized, from weak interactions to reconstruction leading to a bulk compound. The study of the Pd-H system confirmed the existence of supplementary states in the bottom of the metal band.

Influence of the support on the catalytic properties of nickel/ceria in carbon monoxide and benzene hydrogenation

Abstract In this work the catalytic properties of nickel supported on various supports (Al 2 O 3 , SiO 2 , CeO 2 ) in syngas conversion are compared. The influence of the temperature of reduction pretreatment was studied. The characterization of the catalysts was performed by temperature programmed reduction, isothermal reduction, CO and H 2 chemisorption, X-ray diffraction, X-ray absorption spectroscopy, magnetization and X-ray photoelectron spectroscopy. The modification of the catalytic properties of Ni/CeO 2 catalysts with reduction pretreatment is correlated to the transformation of the CeO 2 support and to strong interactions between these species and metal particles.