Active Sites and Elementary Steps in Heterogeneous Catalysis: What Can We Learn From Computational Chemistry?
Heterogeneous catalysis is today at the core of sustainable chemistry, since it allows a fine control of chemical bond forming and breaking processes. This opens energy efficient or selective chemical processes. Computational chemistry is today a key method, among other physical chemistry characterisation tools to reach such an understanding at the molecular level of elementary processes occurring during the catalytic act.
The nature of the surface active site, and the molecular reaction mechanisms are the two complementary pillars of fundamental catalysis. In this lecture we will explore the duality between these two aspects by first principle calculations.
First one needs to understand the nature of the surface of a metal or oxide catalytic particle, under a pressure of gas at a given temperature. In the recent years, the combination of DFT calculations and thermodynamic approaches allowed the understanding or prediction of the nature of the most stable surface of catalysts in specific reaction conditions, giving insights very complementary to in situ characterization methods. Examples of formation of a surface carbide on various transition metals will be presented in the framework of selective hydrogenation1,2 or Fischer-Tropsch synthesis3.
We will the move from extended surfaces to small Pt particles (of size 1 to 13 atoms) on a -alumina support. We show that the presence and the chemical nature of the support surface strongly impact the stability and the shape of the particle.4 More specifically, the presence of chlorine on the (110) surface of -alumina stabilizes small platinum clusters. This stabilization originates from the simultaneous migrations of chlorine atoms and protons from the support towards the Pt clusters. In particular, this trend leads to a local energy minimum, as a function of cluster size, for the Pt3 cluster. The shape of the particle in not only affected by the support, but also by the reactants. When submitted to a pressure of hydrogen, a Pt13 particle on alumina yields a much stronger adsorption of hydrogen compared to extended surface hence leading to a high H coverage (up to 3 H/Pt) and the formation of a surface hydride.5 Hydrogen uptake is associated with a change of the shape of the cluster and a weakening of the cluster/support interaction.
The final part will concern molecular reactivity. The mechanism of the hydrogenation of butadiene will be presented and we will show how modelling can explain why Pt in not selective for these reactions, while the PtSn alloy is.6,7 New C-H bond formation pathways, where the C=C bond is not coordinated to the surface, play a central role for the specific selectivity of the alloy.
1. D. Teschner, Z. Révay, J. Borsodi, M. Hävecker, A. Knop-Gericke, R. Schlögl, D. Milroy, S. David Jackson, D. Torres, P. Sautet, Angew. Chem. Int. Ed. 47 (2008) 9274
2. P. Sautet, F. Cinquini, ChemCatChem 2, 636-639 (2010)
3. E. de Smit, M. M. van Schooneveld, F. Cinquini, H. Bluhm, P. Sautet, F. M. F. de Groot and B. M. Weckhuysen , Angewandte Chemie International Edition, 50, 1584 (2011)
4. C. Mager-Maury, G. Bonnard, C. Chizallet, P. Sautet and P. Raybaud, ACS Catalysis 2, 1346 (2012)
5. C. Mager-Maury, G. Bonnard, C. Chizallet, P. Sautet and P. Raybaud, ChemCatChem 3, 200 (2011)
6. F. Delbecq, D. Loffreda, P. Sautet, J. Phys. Chem. Lett., 1 (2010) 323-326 F. Vigné, J. Haubrich, D. Loffreda, P. Sautet, F. Delbecq, J. Catal. 275, 129 (2010)