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Solid-liquid interfaces : SLIMCAT lifts the veil on molecular-scale mechanisms

January 2018

The molecular scale understanding of chemical phenomena taking place at the solid-liquid interface (SLI) is a common challenge present in many scientific domains, especially in heterogeneous catalysis. For instance, the industrial production of energy from fossil and biological resources implies chemical reactions occurring either at a catalyst-liquid interface or yielding water as an important by-product (see for example aqueous phase reforming or Fischer-Tropsch catalysis). Moreover, the synthesis and functionalization steps of industrial supported catalysts are most often done by aqueous phase impregnation techniques on  Y-alumina supports.

The latter is the goal of the ANR1 funded SLIMCAT project (Solid Liquid Interface at Molecular-scale for Catalysis), which involves IFP Energies nouvelles and 4 French academic partners, each of them specialized in cutting-edge theoretical and characterization techniques at the SLI.

Research within the SLIMCAT project focuses on the description of the impregnation processes of the Y-alumina support with various precursors and molecules, key step of the catalyst preparation. This is a challenging topic since, not only SLI phenomena are difficult to unravel and quantify, but also Y-alumina is a complex material for which results from most standard characterization techniques are either unavailable or difficult to interpret.
In this context, theoretical calculations at the atomic scale are a valuable asset and help to fill in the gaps of experiment based results.

Based on our previous experience on the modelization of Y-alumina2, and in collaboration with Dr. Dominique Costa from the Paris Chemical Research Institute, we performed Ab Initio Molecular Dynamics (AIMD) simulations* of the Y-alumina/water interface that gave valuable insights about the reactivity of surface sites of this material at the SLI.
This complex technique gives us an atomic-scale representation of the average structure and properties of a material at realistic conditions. This is particularly useful for complex systems with many degrees of freedom (a large amount of atoms) and for which more standard computational techniques are difficult to apply. In the present case, it shows us the average structure of Y-alumina reactive sites in presence of water and their interaction with the solvent.

Results reported in our recent publication3 show that water interacts in a different manner with the different faces of Y-alumina crystallites (see the below figure). Our structural analysis pointed out that surface hydroxyl groups (OH) on the (110) facet interact easily with water molecules. This is not the case for the (100) orientation, where surface OH groups prefer to interact with one another through a network of intra-surface hydrogen bonds. Hence, according to our computed vibrational spectra, the solvation of OH groups on the (110) facet induces a more important red-shift (see red arrows in the below figure) in the stretching frequencies of this functional groups (with respect to their vibrational frequency in vacuum). This explains why the reactivity of the two interfaces with incoming organic molecular adsorbates: ongoing research seems to confirm this trend. Therefore, these results offer new insights for the interpretation of experimental data about chemical phenomena at the SLI of this complex material and perspectives for the preparation of tailor-made supported catalysts.

                                          Click on the picture to enlarge it.

   

This example is of one among many other research topics, dealing with the SLI, we want to cover in the incoming International Conference SLIMAIA, held in March 2018.
   
                                   >> See SLIMAIA’s website for details.

  
* AIMD simulations consist of a sequence of resolutions of the Schrödringer’s equation (in our case within the framework of Density Functional Theory) for a set of structures generated by a simulation of the thermal evolution of the atomic model representing the SLI.

Publications

  1. French National Research Agency (Agence Nationale de la Recherche Scientifique). Grant n° ANR-14-CE08-0019.
       
  2. X. Krokidis, P. Raybaud, A.E. Gobichon, B. Rebours, P. Euzen, H. Toulhoat, Theoretical Study of the Dehydration Process of Boehmite to γ-Alumina, J. Phys. Chem. B 105 (2001) 5121-5130 and M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat, Hydroxyl Groups on γ-Alumina Surfaces: A DFT Study, J. Catal. 211 (2002) 1-5.
        
  3. B. F. Ngouana-Wakou, P. Cornette, M. Corral Valero, D. Costa and P. Raybaud, An Atomistic Description of the γ-Alumina/Water Interface Revealed by Ab Initio Molecular Dynamics, J. Phys. Chem. C 121 (2017) 10351−10363.

 

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