First-principles DFT approaches for understanding the reactivity of transition metal nanoclusters Are we close to an in silico design of efficient nanocatalysts?

L. Cusinato, I.C. Gerber, I. del Rosa, R. Poteau
Université Paul Sabatier, FR

Keywords: nanomaterials, catalysis

Summary:

Nanocatalysis has recently emerged as a major new field for the rational design of improved catalysts. Provided that understanding at the atomic level reactive processes that occur on their surface allows the fine-tuning of nanocatalysts, first principle calcula-tions can guide the conception of nanocatalysts, both in terms of activity and selectivity.1 However, on the contrary to state-of-the art theoretical studies in homogeneous and heterogeneous catalysis, finding all chemical events that may occur on the surface of nanoparticles (NPs) is a task currently out of reach, as will be shown in the case of RuNPs-catalyzed ethylene hydrogenation. A relevant alternative is to develop a conceptual DFT approach that could be a useful guide to design efficient nanocatalysts with sites having a specific activity. We recently developed theoretical descriptors for adsorption strength, which are derived from the d-band center model of Hammer and Nørskov.2 Such quantities are in line with the Sabatier principle, the Brönsted-Evans-Polanyi relationship and the resulting volcano-shape activity that is one of the most fundamental discovery in heterogeneous catalysis. According to the qualitative concept of Paul Sabatier, catalytic properties will be hindered if the reactants adsorb too strongly, whereas no reaction will occur if the interaction is too weak. The Sabatier principle applied to the NP case is illustrated by the monoelectronic descriptors introduced in this work, which will be depicted as color maps which give a straightforward point of view of possible reactivity spots.3 The main outcome of this approach is the definition of effective d atomic orbitals for each surface atom, which energy depends on the field generated by the other metal atoms and by the surface ligands. On the basis of our DFT calculations, we conjecture that a catalytic activity will be maximized by optimizing the energy of the effective d AOs of the active sites. This can be achieved by modulating the ligandfield exerted on the surface metal atoms. Some examples will be shown for cobalt and ruthenium nano clusters. We will in particular compare the dissociation CO on various sites, a reaction of utmost importance in the context of the Fischer-Tropsch reaction.4 In summary, such approach provides a feasible way to search for efficient nanocatalysts.