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Improved catalyst performance: a major leap forward for supported metal catalyst preparation

May 2015

Catalyst researchers at IFP Energies nouvelles (IFPEN) have, for the first time, uncovered a molecular recognition phenomenon between cobalt-based catalyst precursors and the alumina support surface. The discovery paves the way for catalyst improvements. This research led to a paper being published in the Angewandte Chemie journal, International Edition[1] (appearing online on 23 April), classed as a "Highly Important Paper" by the reviewers.

The interaction between transition metal complexes and oxide surfaces is a phenomenon used in a variety of fields:

  • geochemistry,
  • radioactive waste treatment,
  • or heterogeneous catalysis.

In the latter, this mainly concerns the preparation of supported metal catalysts, such as cobalt-based catalysts supported on alumina, used in numerous refining processes (hydrotreatment, Fischer-Tropsch synthesis) and chemical processes (conversion of molecules with a single carbon atom, in particular).

For these catalysts, ab initio quantum calculations, performed within IFP Energies nouvelles' Catalysis and Separation Division, have identified a molecular recognition phenomenon between cobalt-based catalyst precursors and the alumina support surface.

This result led to a paper being published in the prestigious Angewandte Chemie journal, International Edition[1]. Furthermore, it was classed as a "Highly Important Paper" by the reviewers.

On theoretical gamma-alumina surface models[2], calculations based on Density Functional Theory (DFT) tend to demonstrate that the metal precursor coordination modes often invoked (coordination at surface hydroxyls) do not generate the octahedral cobalt species observed experimentally. The most stable grafts are obtained by the additional incorporation, in the metal coordination sphere, of oxygen atoms from the alumina network, resulting in a molecular recognition phenomenon, as illustrated in the figure. In their recent publication[1], the authors propose a mechanism of epitactic cobalt hydroxide layer growth with the support, explaining the multiple experimental results, and predicting the geometry of the grafting sites together with the way these layers will be oriented with respect to the alumina support.

A passivation effect of the silica is also demonstrated by calculation of the interaction of the same cobalt precursors with amorphous silica-alumina surface models developed at IFPEN.[3,4] In fact, the amorphous nature of the surface limits the occurrences of the molecular recognition phenomenon.

These studies represent an advance in terms of the rationalization of the interface phenomena involved in the drying step, during the preparation of heterogeneous catalysts, and pave the way for new opportunities in terms of future investigations during the liquid medium impregnation steps.[5]

ligne de séparation orange

[1] Kim Larmier, Céline Chizallet, Pascal Raybaud, Tuning the Metal-Support Interaction by Structural Recognition of Cobalt-Based Catalysts Precursors, Angew. Chemie, Int. Ed, 2015, sous presse
>> DOI: 10.1002/anie.201502069

Results obtained Kim Larmier during his internship year (ENS Ulm student).

[2] Mathieu Digne, Philippe Sautet, Pascal Raybaud, Patrick Euzen, Hervé Toulhoat, Hydroxyl groups on gamma-alumina surfaces : a DFT study, J. Catal., 2012, 211, 1-5.
>> DOI:10.1006/jcat.2002.3741

[3] Céline Chizallet, Pascal Raybaud, Pseudo-bridging silanols as versatile Brønsted acid sites of amorphous aluminosilicates surfaces, Angew. Chemie, Int. Ed, 2009, 48, 2891-2893.
>> DOI: 10.1002/anie.200804580

[4] Céline Chizallet, Pascal Raybaud, Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage, Catal. Sci. Technol., 2014, 4, 2797-2913
>> DOI: 10.1039/C3CY00965C

[5] Le projet ANR-14-CE08-0019 SLIMCAT, subventionné par l’Agence Nationale de la Recherche depuis 2014 et coordonné par IFPEN, traite de ces aspects.


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