Jean-Louis Portefaix

Correlations between X-ray photoelectron spectroscopy data and catalytic properties in selective oxidation on SbSnO catalysts

Abstract A series of SbSnO catalysts has been studied by X-ray photoelectron spectroscopy (XPS) as a function of Sb content (from 1.7 to 39.7 atom%) and activation temperature. At low activation temperature (500 °C) surface and bulk compositions are comparable, while at higher activation temperatures strong surface enrichment in Sb occurs. Detailed analysis of the XPS line shapes shows that two phases may exist at the surface of the catalyst. The first one, which is observed at low Sb content and low activation temperature (500 °C), is assigned to Sb(V) in solid solution in the SnO 2 lattice. The second one, which is observed for Sb contents larger than 5% or for high-temperature activations, is assigned to an Sb 2 O 4 phase. Catalytic properties for selective oxidation of propylene were studied in the 350 to 400 °C range. The selectivity for acrolein increases when surface Sb content is increased either by enhancement of the Sb concentration or by higher activation temperature. It is then postulated that the catalytic phase, which gives selective oxidation, consists of an Sb 2 O 4 phase lying at the surface of a solid solution of Sb (V) in SnO 2 .

Structure sensitivity of mild oxidation reactions on oxide catalysts. A review

Abstract In recent years, numerous bibliographic reviews have been devoted to mild oxidation catalysts. Some correlations between their physicochemical and catalytic properties have been put forward, but no clear picture has emerged. More recently, the new concept of the structure sensitivity of mild oxidation reactions appeared. The amount of work dealing with this topic is now considerable. The aim of this review is to gather examples of structure sensitive catalytic reactions on oxides. The consequences of this new concept on the problems associated with the preparation of oxide catalysts are discussed.

Catalytic properties of niobium sulphides in the conversion of nitrogen containing molecules

Abstract The catalytic properties of niobium sulphides, i.e. NbS 3 and Nb 1.12 S 2 have been examined in the conversion of nitrogen-containing molecules (n-pentylamine and pyridine), and in the hydrogenation of biphenyl. Compared to MoS 2 , NbS 3 and Nb 1.12 S 2 show higher activities for all these reactions and a peculiar ability to perform CN and CC bond cleavage. The activities for these last reactions have been related to the acid-base properties of niobium compounds. Moreover, the trisulphide is more active than Nb 1.12 S 2 . These differences in activities are discussed in relation to the structural features of both sulphides and the existence in NbS 3 of S 2 2− groups and metal-metal pairing.

Electrical behaviour of powdered tin–antimony mixed oxide catalysts

A series of tin + antimony mixed oxide powders, calcined at 773 K, has been studied by electrical conductivity. A continuity in the electrical behaviour is found between semiconducting SnO2 and insulating Sb2O4. Pure stannic oxide is an n-type semiconductor and its free electrons come from the first ionization of anionic vacancies whose concentration is ≈ 1018 cm–3 at 608 K under 2.13 × 104 Pa O2. The enthalpy of formation of these vacancies and the ionization energy of their 2nd electrons have been estimated. As the Sb content increases, antimony dissolves into the SnO2 structure in the 5+ state, which increases the conductivity, σ, up to a maximum corresponding to 6.1 Sb atom %. Around this value formation of the Sb2O4 phase begins. The increased conductivity of mixed oxides with high Sb content (>20 atom %), compared with that of Sb2O4, which is an insulator, is attributed to a doping effect by Sn4+ cations in Sb3+ lattice positions of Sb2O4. Comparison of how both σ and catalytic properties vary with Sb content shows that electron transfer between catalyst and adsorbed species is not the rate limiting step in propene oxidation and that the solid solution of Sb5+ in SnO2 cannot constitute the active phase for acrolein formation.

Enhancement of the catalytic properties of NiMo and CoMo alumina-supported sulfide catalysts by addition of ruthenium sulfide dispersed in Y zeolites

Abstract Y zeolite- or alumina-supported ruthenium sulfide catalysts were investigated for the hydrogenation of pyridine to piperidine. Their activity was compared with that of commercial NiMo or CoMo alumina-supported catalysts and they were found to be more active than the latter. RuKY exhibited higher intrinsic activities than RuHY and Ru Al 2 O 3 catalysts. Mechanical mixtures consisting of RuKY and commercial alumina-supported NiMo- or CoMo-based catalysts exhibited synergy for the hydrogenation of pyridine. This phenomenon was not observed for the other supports. This observation is related to the particular ability of highly dispersed ruthenium sulfide for hydrogen activation.

Mechanism of 1,1-d2 propene oxidation over oxide catalysts

Abstract CD 2 CHCH 3 was oxidized over bismuth molybdate, tin-antimony mixed oxides, and supported molybdenum and vanadium oxide catalysts. The deuterium retention is high (>90%) in the recovered propene. Percentage retentions of deuterium in the acrolein agree with literature data when bismuth molybdate is used as catalyst. On SbSnO and supported Mo and V oxides, no isotope effect is noticed for the abstraction of the second hydrogen from the olefin. The slow step of the reaction may therefore be different for the oxidation of propene on BiMoO and SbSnO. The ethanal produced by oxidation of CD 2 CHCH 3 contains only minor amounts of deuterium, whatever the catalyst used. It is suggested that partial oxidation of propene to acrolein and CC bond rupture are parallel reactions which involve different intermediates. Possible mechanisms adapted from organic chemistry are presented to explain these findings.

Catalytic oxidation of propene over SbSnO mixed oxides

Abstract The surface composition of Sb Sn O mixed catalysts has been investigated by Auger electron spectrometry. The results show a surface enrichment in antimony upon calcination, which is consistent with previous XPS analysis. AES can therefore be used to study the surface of real catalysts, provided a low intensity for the primary beam is used. The selectivity for the oxidation of propene to acrolein changes with the temperature at which the reaction is performed, and the variations depend on the temperature of the calcination step in the preparation of the catalyst. Two classes of catalysts appear: those calcined at 773 K exhibit, as with SnO 2 samples, a decrease in selectivity for acrolein when the reaction temperature increases; those calcined at 1023 K and above show, as with Sb 2 O 4 , an increase in selectivity when the reaction temperature increases. The previous hypothesis that the active phase is an oriented film of Sb 2 O 4 on the Sb SnO 2 solid solution agrees with this observation. Substantial differences are observed when comparing the selectivity pattern for the allylic oxidation of propene obtained in this work to that of allylic oxidations of butenes and o-xylene described in previous articles. This experimental fact is discussed in terms of different structure sensitivities for these reactions.

Influence of bulk and surface structure of tungsten trioxide in the oxidation of propene

Abstract Four samples of pure tungsten trioxide microcrystals were investigated for partial catalytic oxidation of propene by air at 375 °C in a flow reactor at low conversion. Two of these catalysts were hexagonal WO 3 with well-developed (100) and (001) crystal faces, respectively. The other two were monoclinic WO 3 and differed only by their particle habits and specific surface areas and had well-developed (100) crystal faces. It was found that all the samples are selective for acrolein formation. The activity and selectivity are significantly different for the two hexagonal catalysts which differ by their exposed faces; as expected they are the same for the monoclinic samples. Selectivity appears to be equal for samples of different bulk structures but similar surface arrangements, i.e., WO 3 hex. (100) and WO 3 mono. (100); this correlation is not observed for activity.

Hydroformylation of ethylene over cobalt, nickel, molybdenum, CoMo and NiMo alumina supported sulfide catalysts

Abstract Vapour phase hydroformylation of ethylene was studied with alumina supported Co, Ni, Mo, CoMo and NiMo sulfide catalysts in a flow reactor with fixed bed at 513–563 K, 10·105 Pa of overall pressure in the presence of H2S in the feed. With this reaction mixture three different kinds of reactions were observed, hydrogenation, hydroformylation and H2S addition. Nickel and cobalt catalysts presented the best hydroformylation activity. The addition of molybdenum enhanced the formation of ethane and sulfur compounds but decreased the hydroformylation reaction. This last result is discussed in relation with the formation of a NiMoS or CoMoS phase.

Mechanism of carbon sp2-heteroatom bond cleavage in hydroprocessing of substituted benzenes over unsupported transition metal sulfides

Abstract Hydroprocessing of substituted benzenes like aniline, phenol, diphenylsulfide and chlorobenzene was performed in a batch reactor over unsupported transition metal sulfides, namely Co, Ni, Nb, Mo, Ru, Rh, Pd and W sulfides at 280°C and 70 bar of hydrogen pressure. Under these experimental conditions, diphenylsulfide and chlorobenzene mainly react through initial hydrogenolysis of the carbon-substituent bond whereas aniline and phenol react through initial hydrogenation of the aromatic ring. Such a behavior was already reported for conventional sulfided cobalt- and nickel-molybdenum alumina supported catalysts. Nevertheless, these new results confirm the preponderant influence of mesomeric effects on the reactivity of organic models toward sulfided catalysts. In addition to the results obtained over the supported bimetallic sulfides, it was found from quantum chemical calculations that the hydrogenolysis rate constants correlate with the π-electron density on the carbon bearing the substituent and with the overall calculated π-electron transfer between the substituents and the benzene ring. It is thus assumed that hydrogenolysis of carbon sp 2 -substituent bonds results from the attack, by a soft nucleophilic species like a hydride ion, on the carbon bearing the substituent.