G. Maire

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.

A comparative study of LaCoO3, Co3O4 and LaCoO3—Co3O4: I. Preparation, characterisation and catalytic properties for the oxidation of CO

Abstract LaCoO 3 , Co 3 O 4 and a mix of them, were synthesized and studied in the case of automotive exhaust reactions. Two preparation methods were used: simultaneous precipitation and a sol-gel type. The samples were characterised by X-ray diffraction, TPR, TPO and microanalysis. The CO oxidation reaction was performed under dry or wet atmospheres for a binary CO + O 2 and a complete CO + NO + C 3 H 8 + O 2 gas mixture. In all cases, the cobalt oxide pointed out the best performances. A quenching effect of the surface of the sol-gel catalysts induced activity at room temperature under the CO + O 2 dry mixture. Under the complete wet gas mixture, the perovskite LaCoO 3 showed a lower activation energy for the CO oxidation reaction than for Co 3 O 4 .

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.

Isomerization and hydrogenolysis of C6 alkanes on PtAl2O3 catalysts and Pt single-crystal faces

Abstract The contact reactions of C 6 hydrocarbons (isomerization, dehydrocyclization, hydrocracking) were studied under the same experimental conditions, on platinum-alumina catalysts of low or high dispersion, on Pt(557), Pt(119), Pt(111) surfaces, and on a polycrystalline foil of platinum. The experiments on these well-characterized surfaces were performed up to atmospheric pressure in an isolation cell housed within the main UHV chamber equipped with LEED, AES, and ELS facilities. The reaction mixture was analyzed by GLC and part of it was used for mass spectrometric location of the 13 C in each molecule. The results have shown that isomerization by bond-shift or cyclic mechanisms and hydrogenolysis take place on single-crystal faces of platinum, simulating very well the data from PtAl 2 O 3 catalysts of large size aggregates of Pt. Isomerization by bond shift is much more important on stepped surfaces. The contributions of the relative percentages of bond-shift and cyclic mechanisms are discussed as a function of different models of adsorbed species evolving from one site to multisites of chemisorption, differences in local density of states of the edges, and differences in the electronic charges of the carbon atoms in the hydrocarbons. These models are correlated to the classical ones of Anderson, Rooney, and Gault. The peculiar properties of the highly dispersed 0.2% PtAl 2 O 3 catalyst were never simulated by single crystals of platinum.

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.

Characterization of the standard platinum/silica catalyst EUROPT-1: VI. Catalytic properties

Abstract Catalytic properties of EUROPT-1, a standard 6.3% Pt/SiO 2 catalyst, have been studied in a number of European laboratories affiliated to the Council of Europes Research Group on Catalysis. The hydrogenation of benzene proceeds with a turnover frequency (TOF) of 66 molecules h −1 Pt −1 s at 323 K and an activation energy of 45 kJ mol −1 p H 2 =0.78 atm (1 atm=101 kPa), p B =0.09 atm, but with increasing temperature the rate passes through a maximum, the temperature of which is dependent on hydrogen pressure. Results are also reported for the exchange and addition of deuterium to benzene, the hydrogenation of toluene, the dehydrogenation of cyclohexane and the epimerization of 1,2-dimethylcyclohexane. Values of TOF and Arrhenius parameters are also presented for reactions of linear alkanes (C 2 –C 6 ) with hydrogen. Rates of hydrogenolysis at 573 K p H 2 ca. 0.9 atm, p HC ca. 0.06 atm increase from 0.2 to 9.5 molecules h −1 Pt −1 s with increasing carbon number, while activation energies decrease from 210 to ca. 115 kJ mol −1 . The reactions of methylcyclopentane, 2-methylpentane, isomeric heptanes, 3-methylhexane and 4-methylheptane have also been investigated, and product distributions are given: these are as expected for highly dispersed platinum ( d =1.8 nm). The behaviour of EUROPT-1 in these reactions is compared with that of other supported platinum catalysts as described in the literature. Sufficient information is now available on EUROPT-1 for it to be regarded as a reliable reference catalyst.

Studies on PtNi alloys: AES and photoemission determination of the surface composition and interaction with oxygen and hydrogen

Abstract Three PtNi alloys with 20, 40 and 70 at% Ni were studied by Auger and photoelectron spectroscopies. The clean surfaces were found to be enriched in platinum in an amount increasing with increasing platinum concentration in the bulk. On the contrary oxidation treatments resulted in a nickel surface segregation which was only slightly offset after reduction. Chemical shifts characteristic of oxidation were observed on XPS peaks of both platinum and nickel under experimental conditions where pure platinum could not be oxidized.

Monodisperse colloidal metal particles from nonaqueous solutions: Catalytic behavior in hydrogenation of but-1-ene of platinum, palladium, and rhodium particles supported on pumice

Abstract Metal catalysts have been prepared by depositing monodisperse particles of platinum (2–3 nm), rhodium (2–3 nm), or palladium (5 nm) prepared in reversed micellar solutions on pumice. The particles are well dispersed on the support whereas particles deposited from aqueous or alcoholic solution give large aggregates. The catalytic properties of these different catalysts in the deuteration, isomerization, and hydrogen-deuterium exchange of but-1-ene have been compared. The activities calculated per metal surface atom are similar. However, platinum prepared from microemulsions show unusually high selectivity in the isomerization reaction, and for such particles dehydrogenated species are active in the exchange reaction. The specificity of rhodium and palladium catalysts is independent of the mode of preparation. The reaction mechanisms are discussed.

Auger electron spectroscopy study of chemisorption-induced segregation in a PdAu alloy

Abstract The chemisorption of oxygen has been studied by Auger electron spectroscopy at 500 °C and pressures varying from 10 −7 to 1 Torr on a 22 atom % PdAu alloy. A quantitative method has been used for calculating the Pd and Au surface concentrations. For the clean surface it agrees well with the bulk alloy composition even after ion bombardments and annealing. Significant Pd surface enrichment due to oxygen chemisorption has been observed. This result may provide an explanation for the influence of preoxidation treatments on the catalytic behavior of such alloys. Hydrogen chemisorption on the surface after oxygen treatment causes the oxygen and thus palladium surface concentrations to increase further.

Preparation of colloidal platinum/palladium alloy particles from non-ionic microemulsions: Characterization and catalytic behaviour

Abstract Bimetallic particles of platinum and palladium have been prepared by reduction, with hydrazine at arabient temperature, of H 2 PtCl 6 and PdCl 2 dissolved in microemulsions consisting of water, hexadecane and pentacthylene glycol dodecyl ether. The particles were deposited onto Al 2 O 3 . Examination by scanning transmission electron microscopy and extended X-ray adsorption fine structure measurements of particles with atomic ratio Pt:Pd = 35:65 showed that the particles are true Pt/Pd alloys. Indirect evidence was also obtained that particles containing only 4% Pd are alloys. The diameter of the particles ranges from 10 to 100 nm; there is evidence that they are formed by aggregation of particles in the diameter range 2–5 nm. The catalytic properties of the Pt/Pd/Al 2 O 3 catalysts prepared from microemulsions were similar to those of catalysts prepared by conventional impregnation. Catalysts with low Pd content had the same selectivity in the isomerization of 2-methylpentane and hydrogenolysis of methylcyclopentane as a highly dispersed Pt catalyst (or a pure Pd catalyst). The catalytic cracking of 2-methylpentane indicates that the particle surfaces behave as Pt surfaces up to a Pd concentration of 60% in the particles. Isomerization of hexanes is dominated by the cyclic mechanism even at Pd concentrations as low as 4%; for these very low Pd concentrations hydrogenolysis of methylcyclopentane is the same as observed for very small Pt particles. It is concluded that Pd is enriched in the particle surfaces in such a way that they consist of Pd and some highly dispersed Pt. even at bulk Pd concentrations as low is 4%.