P. Mériaudeau

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.

Effect of support material on the catalytic combustion of diesel soot particulates

Several potential support materials were mixed with soot and subsequently the combustion temperature of the soot in 15% O2 was determined. Al2O3 and SiO2 showed no catalytic effect, TiO2 and ZrO2 lowered the soot combustion temperature with 80–90 K, whereas CeO2, La2O2CO3 and V2O5 (reference catalyst) showed a substantial activity for soot combustion. After poisoning with sulfur dioxide, the catalytic effect of CeO2 was strongly inhibited, whereas La2O2CO3 and V2O5 retained their high activity.

Dehydrocyclization of Alkanes Over Zeolite-Supported Metal Catalysts: Monofunctional or Bifunctional Route

Abstract The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the...

Catalytic combustion of diesel soot particles on copper catalysts supported on TiO2. Effect of potassium promoter on the activity

We have prepared a TiO2 supported copper catalyst and studied the effect of potassium on its activity in the oxidation of soot particles. The catalysts, with a K/Cu atomic ratio varying between 0 and 2, were calcined at 1073 K. They were characterized by BET surface area measurements, X-ray diffraction and temperature-programmed reduction under hydrogen. The catalytic activity was measured in a microbalance by means of temperature-programmed oxidation in air or argon. The catalytic activity of copper was enhanced by the presence of potassium. This effect was attributed to the formation of mixed KTi oxides which inhibit the sintering of the TiO2 support and thus increases the surface area of the catalyst. Although a redox mechanism can explain the catalytic combustion, no correlation could be established between the reducibility of the different solids and their activity in soot combustion.

Isomerisation of butene in isobutene on ferrierite catalyst: A mono- or a bimolecular process?

Abstract The isomerisation of n-butene into isobutene has been studied over a ferrierite catalyst being in a selective state (aged catalyst) or non-selective state (fresh catalyst). It is observed, by using 13 CCCC, that a bimolecular process operates on the non-selective catalyst whereas a monomolecular process exists on the selective catalyst.

Pore size and crystal size effects on the selective hydroisomerisation of C8 paraffins over Pt–Pd/SAPO-11, Pt–Pd/SAPO-41 bifunctional catalysts

Abstract The hydroconversion of n-octane over bifunctional Pt–Pd/SAPO-11 and Pt–Pd/SAPO-41 catalysts was studied. High selectivity, 90% towards isomerisation, was observed even at high conversion. SAPO-41 was found slightly more selective than SAPO-11. The grain sizes have a profound effect on conversion, small grains were more active than large grains, but had almost no significant effect on the selectivity and product distributions. It was concluded that the reaction of n-octane occurs in the SAPOs channels but the molecules did not penetrate too far in the channels. Restricted transition shape selectivity at the SAPO pore mouth is responsible for the product distributions observed. These hypotheses were further supported by the study of monoethyl heptane isomers and of dimethyl hexane isomers. Pore size effects and particle size effects indicate that it is highly probable that the reactions occurred in the channels not too deep from the surface. Restricted transition-state selectivity was the main factor governing the extent of methyl debranching in branched C8 isomers. A relatively small difference in the size of the channels resulted in a significant difference in the constraint exerted by the pore dimension, smaller the pore dimension higher the tendency for the catalyst to form monobranched isomer with the methyl at terminal position. Also high selectivity for debranching the methyl branched C8 isomers is obtained with these medium monodimensional pore SAPO materials. Small-grain molecular sieves were more effective than large-grain samples particularly if amount of acid sites on the outer surface is small.

Electron paramagnetic resonance investigation of oxygen photoadsorption and its reactivity with carbon monoxide on titanium dioxide: the O3–3 species

Oxygen absorbed on TiO2 when subjected to u.v. irradiation gives rise to three different paramagnetic species. The first species, A, has been identified as O–2(g1= 2.021, g2= 2.009 and g3= 2.001 and C1≈C2≈ 0 Oe, C3= 77 Oe for 17O hyperfine spliting), the second species, C, as O–3(g1= 2.014, g2= 2.009 and g3= 2.003, no detected hyperfine lines due to 17O) and the third, B, as O3–3(g∥= 2.008 and g⊥= 2.001, no detectable hyperfine lines due to 17O). The latter species reacts readily with CO at 77 K and gives rise to species D, identified as OI–OII–CO–III according to the reaction: O3–3+ CO → O2–latt.+ O—O—CO– where the two directly bonded oxygen atoms originate from the absorbed molecular oxygen. The e.p.r. parameters of species D are : g1= 2.0465, g2= 2.006, g3= 2.001, C1≃ 6 Oe ± 0.5C2≈C3≈ 4 Oe ± 0.5 for 13C hyperfine interaction, CI1≈CI2≈ 0 and CI3= 104 Oe, CII1C≈II2 0 and CII3≃ 42.5 Oe and CIII1≈CIII2≈CIII3≈ 0 for 17O hyperfine splitting constants of the three oxygen nuclei. It is mainly the characterization of species, D, which enables us to postulate the nature of species, B.

Paramagnetic oxygen species adsorbed on reduced SnO2

Abstract The nature of the oxygen species adsorbed on slightly reduced SnO 2 has been investigated using oxygen enriched with 17 O 2 . The oxygen adsorbs as O 2 − with g 1 = 2.024, g 2 = 2.009, g 3 = 2.0036 and a hyperfine splitting ( A xx ) about g 3 of 80.5 gauss. A second paramagnetic species is characterized by g 1 = 2.034, g 2 = 2.004 and g 3 = 1.994; this species shows no interaction with 17 O 2 and is attributed to the localization of an electron in some lattice defect by the adsorbed oxygen.

Synthesis and characterization of SAPO-41 : effect of the silicon content and the crystal size on the hydroisomerization of n-octane over Pt-Pd/SAPO-41

Silicoaluminophosphate SAPO-41 was synthesized either in the presence of surfactant or without added surfactant. Under the synthesis conditions with added surfactant small-size molecular sieve crystals were formed, while larger crystals were obtained in the absence of surfactant. Magic-angle spinning nuclear magnetic resonance spectroscopic studies showed that silica-rich domains existed for silicon-rich samples. Infrared spectra in the OH stretching vibrations and temperature-programmed desorption of ammonia revealed the presence of Bronsted acid sites. Samples prepared in the presence of surfactant exhibited higher acid strength. The hydroconversion of n-octane was perfomed over Pt–Pd-containing SAPO-41 samples. These catalysts showed high selectivity for the hydroisomerization reaction. Among the C8 isomers, monomethylheptanes were preferentially formed. The selectivity towards hydroisomerization at constant conversion level did not change with the SAPO-41 crystal size. However, a small-crystal SAPO-41 sample was considerably more active than large-crystal SAPO-41. The shape selectivity of the tubular medium-pore SAPO-41 played the major role. Pore mouth catalysis at the interior of the pore is also discussed.

Oxidation state and surface segregation of Mo ions in titania catalysts evidenced by ESR, UV and ESCA techniques

Abstract Incorporation of Mo ions into a TiO 2 matrix has been studied as a function of preparation procedure and heat treatment conditions, using ESCA, UV and ESR spectroscopies. Oxidation state, coordination geometry and incorporation of Mo ions into the TiO 2 particles have been considered. For anastase samples, UV and ESCA techniques show that surface and bulk concentrations of Mo are about identical, with the majority of Mo being in the form of Mo 6+ and a significant quantity existing as Mo 5+ in distorted octahedral symmetries. ESR spectroscopy only detects substitutional ions designated Mo s 5+ in very small amounts (∼1% of the 0.5% Mo introduced). For rutile, heated at 800°C in air, ESCA technique shows that the surface concentration is enriched in Mo in the +6 oxidation state by a factor 10 compared to the bulk, whereas UV spectroscopy shows that Mo is mainly incorporated in the bulk in the form of Mo 5+ presenting two different environments of the distorted octahedral type. ESR techniques detect substitutional Mos in very small amounts (∼1% of the 0.5% Mo introduced). For rutile samples, directly prepared by the flame method, the surface concentration of Mo is about twice that of the bulk, Mo ions being mainly in a +6 oxidation state at the surface and +5 in the bulk. Strong surface enrichment (× 10–20) occurs after heating anastase or rutile at 800°C in air. Rutile samples turn out to be composed of MoO 3 islands at the surface of TiO 2 doped with Mo(V) ions in Mo ensembles or clusters occluded in the TiO 2 .