[Todos] Correccion Seminarios DQIAyQF/INQUIMAE. Lunes 20 de Noviembre, 13h

[sara elizabeth bari]

Seminarios del Dpto. de Química Inorgánica, Analítica y Química Física / INQUIMAE
  FCEyN - Universidad de Buenos Aires

  http://www.qi.fcen.uba.ar/es

   

  2do. Cuatrimestre, 2006

   

   

    

  Lunes 20 de Noviembre, 13.00h
  Aula Seminarios, DQIAyQF- INQUIMAE

  Ciudad Universitaria, Pab. II, Piso 3

   

   

  Eric M. Gaigneaux

   

  Université catholique de Louvain, Unité de catalyse et chimie des matériaux divisés,

  Croix du Sud 2/17, B-1348 Louvain-la-Neuve (Belgium).

  E-mail: gaigneaux en cata.ucl.ac.be - Tel: +32(0)10-473649 - Fax: +32(0)10-473665

   

   

  A dynamic view of oxide catalysts "at work" in oxidation processes

   

  The objective of this report is to present a large family of results aiming to: first illustrating the dynamic behavior of oxide catalysts and of their surfaces in selective oxidation reactions, second, showing the influence of this behavior on the catalytic performances and third, showing that improvements of the performances of oxide catalysts can be obtained by orienting the dynamic phenomena through well-thought-out procedures.

  Many "symptoms" as i) modifications of crystalline phases, oxidation states and crystals morphologies, ii) diffusion phenomena of metals, reaction intermediates and spillover species, iii) evolutions of catalytic performances with time-on-stream and iv) synergetic effects between phases, point without ambiguity to the obsoleteness of "static view" of catalysis, demonstrating the existence of dynamic phenomena at the surface of catalysts "at work". It is thus of an absolute priority to move towards a "dynamic view" of catalysis, that considers "working catalysts" as quasi-living objects and, in particular, takes into account the influence of the dynamic processes that animate the catalytic surfaces "under its real state" on their performances. Although being true for all kinds of catalysts and catalytic processes, the dynamic approach is particularly relevant in the case of oxide catalysts used in selective oxidation processes. Indeed, the mechanism of such systems, namely the Mars & van Krevelen cycle, itself denotes dynamic aspects. These are a continuous cyclic exchange of oxygen atoms between the gas phase and the catalytic surface, and the occurrence of re-arrangements of co-ordinations associated to the entrance and exit of oxygen into and from the superficial structure of the oxides. In addition to give a "breathing aspect" to working oxides, these phenomena intimately dictate their performances in oxidation processes.

  We present results obtained from 3 reactions: 1) the dehydration of 2-butanol to butenes in the presence of oxygen (DEHYD), 2) the selective oxidation of isobutene to methacrolein (METH) and 3) the oxidative dehydrogenation of propane to propene (ODP). The 3 reactions were carried out on Mo-based oxide catalysts: MoO3, MoO3-x, MoO2, Bi and Ni molybdates, eventually in physical contact with Sb2O4. The samples were systematically characterized before and after the catalytic reactions mainly by: XRD, XPS, confocal Raman microscopy, SEM and AFM microscopy.

  The 1st CONCEPT (using DEHYD) aims at determining the environment of Mo atoms (i.e. the oxidation state or the combination of oxidation states) stabilized under the conditions of reaction leading to the highest catalytic performances. Crucial results are: 1) MoO3 used alone gets explosively reduced to MoO2 during the reaction, which corresponds to a deactivation, 2) the surface of MoO3 used in mixture with Sb2O4 dynamically stabilizes to a MoO3-x stoichiometry and exhibits high catalytic performances, 3) Bi molybdates (a, b and g phases) used alone have superficial Mo atoms dynamically stabilized during the reaction in a slightly reduced state and exhibit high catalytic performances and 4° Bi molybdates used in mixture with Sb2O4 maintain superficial Mo atoms in a fully oxidized state and suffer dramatic decreases of performances. These observations show that: the most performant environment of Mo in oxide catalysts for the dehydration of 2-butanol is the slightly reduced sub-oxidic MoO3-x one.

  The 2nd CONCEPT (using METH) aims at understanding the origin of the highest performances in selective oxidation processes of sub-oxidic MoO3-x state. Crucial results are: 5) the performances of Mo8O23 sub-oxide used alone are initially high but decrease with time-on-stream, 6) the performances of Mo8O23 used in mixture with Sb2O4 are lower than those of Mo8O23 used alone but increase with time-on-stream, 7) in both cases, shear structures of the sub-oxide disappear under the working conditions, 8) MoO3 used in mixture with Sb2O4 has its surface stabilized in a MoO3-x stoichiometry (likely Mo18O52) and exhibits enhanced performances compared to MoO3 used alone, 9) in the presence of Sb2O4, the surface of MoO3 undergoes a dramatic restructuring of the (010) faces of its crystals to (100) facets, which corresponds to a real creation of active and selective catalytic sites. These observations show that: the high performances of MoO3-x sub-oxide state are not due to shear structures but to the dynamic "breathing" behavior of Mo atoms in the slightly reduced environment. This environment is the only one bringing enough flexibility in the catalyst to perform the Mars & van Krevelen cycles in a fast way, so explaining the best performances observed in the case of the stabilization of sub-oxide material during the reaction.

  The two CONCEPTS described above point to two procedures offering the possibility to orient the dynamic behavior of Mo-based oxides by modifying "in situ" the oxidation state of Mo atoms at work to their most performing (selective) environment. The results reported above point to a first procedure, namely the addition in the catalysts formulation of an external phase able to activate oxygen (as a-Sb2O4, BiPO4, SnO2 etc.) A second procedure, very novel, is described in the ODP on Ni molybdate. It is the injection of gaseous dopes (ppm or some %) in the reaction gas during the reaction. It is shown that 10) the injection of CO2 brings about a stabilization of Mo at a higher oxidation state with a corresponding selectivity loss while, 11) the injection of N2O brings about a stabilization of Mo in a lower oxidation state so promoting selectivity. CO2 promotes a higher oxidation state of Mo via the formation of monoatomic oxygen species (Oa) resulting from the dissociation of CO2 on the catalyst (Oa is more oxidant than O2). N2O inhibits the adsorption of O2, so limiting the formation of non-selective oxygen species (from O2) and/or the oxidation rate of the catalysts. These observations leads to a second procedure, namely adjusting the concentration of gas dopes in order to modulate the selectivity by orienting the dynamic behavior of the catalysts.

   

   

  iiiii



------------------------------------------------------------------------------


  No virus found in this incoming message.
  Checked by AVG Free Edition.
  Version: 7.1.409 / Virus Database: 268.14.6/535 - Release Date: 15-Nov-06



--------------------------------------------------------------------------------


No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.1.409 / Virus Database: 268.14.6/535 - Release Date: 15/11/2006
------------ próxima parte ------------
Se ha borrado un adjunto en formato HTML...
URL: http://mail.df.uba.ar/pipermail/todos/attachments/20061116/dd5fb2c1/attachment.html