Michel Dufaux

Metal-support interaction. Catalytic properties of Ti02-supported platinum, iridium, and rhodium

Abstract The reduction of Pt TiO 2 , Ir TiO 2 , Rh TiO 2 in the range 473–773 K of temperature induces a strong metal-support interaction as evidenced by the change in catalytic properties of these metals used for ethylene, benzene, and styrene hydrogenations, cyclohexane dehydrogenation, and butane hydrogenolysis. Treatment under O 2 partially restores the usual properties of these metals. The origin of this strong metal-support interaction is discussed.

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.

EPR evidence for the isomorphous substitution of titanium in silicalite structure

TS-1 zeolite has been synthesized following the literature procedure and characterized via X-ray diffraction, IR spectrometry and 29Si NMR and shown to match previous findings. In addition titanium-exchanged HZSM-5 was prepared. Both solids were examined by EPR spectroscopy following CO reduction. Whereas the titanium-exchanged ZSM-5 exhibited the usual EPR signal consistent with trivalent titanium in tetragonally distorted octahedral symmetry with g⊥ = 1.97 and g∥ = 1.94, the Ti-substituted silicalite exhibited an axially symmetric signal with a split g∥ component (g∥)1 = 1.98, (g∥)2 = 1.97 and g⊥ = 1.913 best interpreted as arising from a trivalent titanium in a tetrahedral coordination, indicating that the precursor tetravalent ion was indeed in a T site. Reactivity towards O2 was also monitored and differences in the gzz values for O2− adsorbed on titanium confirm the different crystal fields prevailing at each titanium siting. It is also concluded that reduction of framework titanium with CO does not result in its removal from the lattice.

Effect of pore size and aluminium content on the production of linear alkylbenzenes over HY, H-ZSM-5 and H-ZSM-12 zeolites: Alkylation of benzene with 1-dodecene

Abstract H-ZSM-5 (Si/Al = 15 and 30), H-ZSM-12 (Si/Al = 80) and HY zeolites (Si/Al ratios in the range 2.7-26.4) were evaluated in the benzene alkylation reaction with 1-dodecene for linear alkyl-benzene-LAB production. A series of dealuminated Y-zeolites was prepared under strictly controlled conditions by treatment with silicon tetrachloride. The materials thus obtained were washed and exchanged with ammonium chloride. The catalysts were characterized by X-ray diffraction, infrared spectroscopy of the framework vibrations,29Si and27Al-nuclear magnetic resonance. Temperature-programmed desorption of ammonia and elemental analysis. H-ZSM-5 and H-ZSM-12 catalysts showed very low activity. H-ZSM-12 zeolite exhibits both product and transition state type shapeselectivity, favouring the formation of the least bulky 2-phenyl isomer. With HY-zeolites, the alkylation activity increased linearly with the number of lattice Al per unit cell over a range of Si/Al ratios from 2.7 to 26.4, so a practically constant turnover frequency is obtained. The HY-zeolites also gave 97–98% selectivity towards LAB and produced branched dodecene, traces of branched alkylates and dialkylbenzenes as by-products. Neither dialkyltetralins nor dialkylindanes were found among the products. HY-zeolites did not exhibit shape-selectivity effects. The apparent activation energy was 15 kcal/mol. Small changes in the low Na content of the zeolites were responsible for considerable increases in activity. Dealuminated HY zeolites showed the highest selectivity towards 2-phenyldo-decane. In this case we suggest that the alkylation reaction competed effectively with the migration of the double bond across the chain, so that some of the initial alkylcarbenium ions formed alkylated benzene without undergoing isomerization. The isomerization of the phenylalkane was not observed after alkylation.

Oxygen species adsorbed on CeO2/SiO2 supported catalysts

The nature of the oxygen species adsorbed on slightly reduced CeO2/SiO2 supported catalysts has been investigated using oxygen enriched with 17O2. The observed hyperfine splitting was found to be associated with the central component of the g tensor; it indicated that the corresponding orbital contained the unpaired electron and therefore the central component was defined as gxx. With this assumption, the e.s.r. spectra are consistent with the adsorption of oxygen as O–2 at 77 K with gyy= 2.0109, gxx= 2.0158 and gzz= 2.028 with a hyperfine splitting (Axx) of 75 G about gxx. The O–2 is adsorbed with the internuclear axis along the surface of the catalyst at a site corresponding to a cerium ion. There is some indication that interaction with the metal ion orbitals is perturbing the energy levels of the adsorbed O–2 ion.

An electron spin resonance study of cation radicals on the surface of MoO3Al2O3 and MoO3SiO2 catalysts

Abstract ESR spectra of polynuclear aromatic hydrocarbons adsorbed on MoO 3 Al 2 O 3 and MoO 3 SiO 2 have been recorded. Formation of the corresponding cation radicals indicates the presence of strong electron-deficient sites on the surface of these catalysts. The effect of activation temperature on radical forming ability of these solids has been studied. In the electron transfer reaction, Mo 6+ is reduced to Mo 5+ . Molybdenum is therefore directly involved in the electron transfer process. The radical concentration was dependent upon the molybdenum content and its oxidation state. The effect of oxygen on radical-ion formation has also been studied.

Catalytic oxidation of nicotine over zeolite-supported platinum

Platinum loaded NaY, HY and dealuminated HY were shown to be active in the total oxidation of nicotine at 573 K into CO2, H2O and N2 at the exclusion of CO and NOx. The activity (turnover frequency (TOF)) of a surface metal atom was independent of the method of metal loading (ion-exchange or impregnation) but increased with the metal particle size. Compared to non-porous silica, zeolites exerted a beneficial effect on metal particles of similar sizes, presumably, because a shielding effect of the acidic zeolite matrix with respect to the nicotine base.