Boris Imelik

X-ray diffraction study of cupric ion migrations in two Y-type zeolites containing adsorbed reagents

Abstract X-Ray diffraction study of two copper-exchanged Y zeolites (Cu 16 Na 24 Y and Cu 12 Na 5 H 27 Y) in the dehydrated state showed that the cupric ions strongly prefer S I sites in the sodalite cages where they are firmly bonded to three framework oxygen atoms. The effect of sorbed molecules on the cation distribution was studied by adding ammonia, pyridine, naphthalene and butene to the dehydrated zeolite. Adsorption of a small amount of ammonia which can enter the sodalite cages produced slight but significant changes in the copper position. Addition of pyridine, naphthalene or butene induced an important migration of copper ions from S I and S I sites to the supercages where they can interact with sorbed molecules. The nature of the cation-organic molecule associations was investigated: infrared measurements and X-ray results provided evidence of a proton transfer from the organometallic complexes to the framework oxygen atoms.

Adsorption of hydrocarbons and various gases on Ni-SiO2 catalysts studied by high field magnetic methods

Abstract The adsorption of various gases (H 2 , H 2 S, alcanes, alcenes and alcynes containing up to five carbons, benzene, cyclohexane, cyclopentane, and some mixtures) on Ni-SiO 2 catalysts has been studied by measuring variations of saturation magnetization. The adsorption has been performed at −78 °C and the system heated in steps. When coverage is small intermediate adsorbed compounds thermally stable have been detected. The observed bond number is even (2, 4, 6, 8) and it corresponds to strongly dehydrogenated states for alcanes, alcenes, cyclanes and benzene, and to an associative model for acetylene and propyne. Complete cracking occurs at relatively low temperatures (50 °C to 200 °C) with formation of Ni 3 C and NiH: the greater the saturation of the hydrocarbon, the smaller its carbon number, and the lower is its cracking temperature. Sulfur of H 2 S is rapidly and irreversibly dissolved into the bulk of nickel. These results which are not always in good agreement with those obtained by low field techniques, strongly supports the following view: nickel atoms, on which a molecule is adsorbed cease to participate to the collective magnetism.

Electron spin resonance studies on titanium dioxide and magnesium oxide—Electron donor properties

The electron donor properties of TiO2 and MgO have been investigated by tetracyanoethylene and trinitrobenzene adsorption. The electron donor centers are associated with OH− groups present on the surfaces of the solids activated at low temperature (<300 °). During the dehydration of TiO2 and MgO at higher temperature, weakly coordinated O2- ions are formed on their surfaces, and these are responsible for the reducing properties of these solids.

Magnetic and infrared study of CO chemisorption on silica supported nickel-copper alloys

CO adsorption at room temperature on Ni-Cu alloys supported on SiO2 is studied by two complementary techniques, infra-red spectroscopy and magnetic methods (saturation magnetization). The bond number between CO and the metallic surface calculated from magnetic data decreases from 1.8 to 1 as the Cu content increases. Two bands attributed to CO bonded to Ni are observed (the A band in the 2000–2050 cm−1 region, and the B band in the 1950–1900 cm−1 region). A small band assigned to CO bonded to Cu is also detected. As Cu content increases, the intensity of the B band decreases, and a noticeable and continuous frequency shift of the three bands is observed. Experimental results are fully accounted for assuming that: (i) two adsorbed species of CO on Ni, a monodentate and a bridged species (with small amounts of other multicentered species) are formed, as suggested by Eischens and Pliskin; (ii) dilution of Ni by Cu decreases the relative abundance of the bridged (and multicentered) species for some geometric reasons previously invoked by Soma-Noto and Sachtler; (iii) surface complexes are formed between CO and Ni; however Ni remains in its metallic state. The surface complex is sensitive to the electronic environment of the metallic atom, with a frequency shift of the three infra-red bands upon alloying as a consequence.

X-ray photoelectron spectroscopy study of Pd and Pt ions in type Y-zeolite. Electron transfer between metal aggregates and the support as evidenced by X-ray photoelectron spectroscopy and electron spin resonance

Spectroscopic study by ESCA and e.s.r. techniques was performed for Pt and Pd ions in type Y-zeolites. Pt and Pd ions were found to be ionically bonded to lattice oxygen ions. Atomically dispersed PtO and PdO were shown to give significant X.P.S. positive chemical shifts: +1.3 and +1.4 eV for Pt 4f and Pd 3d lines, respectively. These shifts were assigned to smaller electron relaxation energy.Metal aggregates of small diameter (ϕ⩽ 20 A) and narrow size distribution were prepared in zeolite or on silica. The X.P.S. chemical shift was quite significant (∼+0.7 eV) for Pt on zeolite support with respect to those (< +0.3 eV) for Pd on zeolite or Pt or Pd on silica support and did not depend on the particle size. Electron donor and electron acceptor properties of the materials were studied by following the e.s.r. formation of charge transfer complexes (radical ion) as a function of Pt or Pd loading. Pt aggregates were shown to form charge transfer complexes with Lewis acid sites and to increase electron donor properties of the zeolite no matter what the particle size. This increase was shown to arise from electron donation from the metal to the zeolite lattice. However, Pd aggregates were shown by e.s.r. not to give rise to significant electron transfer.

PtMo bimetallic catalysts supported on Y-zeolite: I. Characterization of the catalysts

Bimetallic PtMoY catalysts have been prepared by adsorption and decomposition of Mo(CO)6 in a Y-type zeolite containing 10-A Pt particles trapped in the zeolite cages. Part of the Mo atoms are deposited onto the Pt particles which produces a weak increase of the particle size measured by CTEM and STEM and a progressive decrease of the amount of H2 or CO chemisorbed on platinum. At high Mo contents these chemisorptive properties are completely masked. It was checked by XPS, X-ray absorption edge spectroscopy, and ir spectroscopy with CO and NO as probe molecules that the Pt particles are initially electron deficient but the electronic structure becomes similar to that of bulk platinum once the Mo atoms were added. This is probably because the Pt atoms are then shielded from the electron acceptor sites of the support. The Mo atoms bonded to Pt are probably in a low valence state, but there is a fraction of Mo not bonded to Pt, especially in the zeolite outer layers of Mo-rich samples, which is oxidized to MoIV-MoVI by oxido-reduction reactions with the zeolite protons.

Pt10Ni90(111) single crystal alloy: Determination of the surface composition by AES, XPS and ISS

Abstract The surface composition of the Pt 10 Ni 90 (111) single crystal alloy has been determined from AES, XPS and ISS experiments. The clean surface is largely enriched with platinum: 30–40% in the top layer instead of 10% in the bulk. This enrichment concerns mainly the ultimate surface layer and appears to be only slightly dependent on the sample temperature.

A study of the texture and structure of Raney nickel

Abstract The texture of Raney nickel was simultaneously approached by nitrogen adsorption, magnetic measurements, X-ray diffraction line-broadening, X-ray small angle scattering and electron microscopy. The grain of Raney nickel, with a size larger than 1000 A, takes the form of a collection of crystallites ranging in size from 25 to 150 A, held together in the shape of a sponge. The average pore radius is 34 A, but individual elements possessing an average radius of 7 to 8 A also appear. Electron and X-ray diffraction patterns correspond to face-centered cubic nickel.

Pt-Mo bimetallic catalysts supported on Y-zeolite: II. Activity and selectivity in n-butane conversion

Pt-Mo bimetallic catalysts prepared by adsorption and decomposition of Mo(CO)6 on 1-nm Pt aggregates supported on Y-zeolite exhibit enhanced hydrogenolysis activity with respect to PtY zeolite in n-butane conversion. The activity curve vs PtMo composition is volcano-shaped with a maximum near equiatomic composition. The rate at maximum is 7 and 34 times larger than on PtNaHY and PtSiO2, respectively. The reaction orders with respect to n-C4 and H2 are positive in a large range of pressures. The kinetic parameters are interpreted in terms of the participation of both Pt and Mo atoms in the reaction mechanism. Mo atoms act primarily as strong adsorption sites for the hydrocarbon whereas hydrogen atoms dissociated on Pt are required to hydrogenate the hydrocarbon fragments.

Propene-V2O2 interactions studied by infrared emission spectroscopy

Abstract Infrared emission spectroscopy was used to investigate the interactions between propene and vanadium hemipentoxide. The spectrum obtained for V 2 O 5 alone was very similar to the spectrum given by KBr disc transmission method. Reaction of propene on V 2 O 5 was performed between 110 and 250°C; IR spectra were recorded in situ. Reduction of the oxide occurred and its bands were strongly altered. The bands attributed to the terminal oxygen (1018 cm −1 ) and to the doubly bridged oxygen (820 cm −1 ) were mainly affected by the reaction with propene. For a reaction temperature of 250°C, the recorded spectrum was close to that given by hydrogen reduction at 230°C. It can be concluded that V 2 O 5 was reduced by propene with the formation of a superficial suboxide. By oxygen treatment, the reduced form was restored to the initial V 2 O 5 sample. Infrared emission spectroscopy appears as a very suitable method for studying the interactions of the reactants with the catalysts.