Theoretical characterization of the surface composition of ruthenium nanoparticles in equilibrium with syngas

Abstract

A deeper understanding of the relationship between experimental reaction conditions and the surface composition of nanoparticles is crucial in order to elucidate mechanisms involved in nanocatalysis. In the framework of the Fischer-Tropsch synthesis, a resolution of this complex puzzle requires a detailed understanding of the interaction of CO and H with the surface of the catalyst. In this context, the single- and co-adsorption of CO and H to the surface of a 1 nm ruthenium nanoparticle has been investigated with density functional theory. Using several indexes (d-band center, crystal overlap Hamilton population, density of states), a systematic analysis of the bond properties and of the electronic states has also been done, in order to bring an understanding of structure/property relationships at the nanoscale. The H : CO surface composition of this ruthenium nanoparticle exposed to syngas has been evaluated according to a thermodynamic model fed with DFT energies. Such ab initio thermodynamic calculations give access to the optimal H : CO coverage values under a wide range of experimental conditions, through the construction of free energy phase diagrams. Surprisingly, under the Fischer-Tropsch synthesis experimental conditions, and in agreement with new experiments, only CO species are adsorbed at the surface of the nanoparticle. These findings shed new light on the possible reaction pathways underlying the Fischer-Tropsch synthesis, and specifically the initiation of the reaction. It is finally shown that the joint knowledge of the surface composition and energy descriptors can help to identify possible reaction intermediates.

Shape, electronic structure and steric effects of organometallic nanocatalysts: relevant tools to improve the synergy between theory and experiment

“…We have recently demonstrated the usefulness of this approach in the case of RuNPs. 12 We are now going to confirm the accuracy of the pDOS and pCOHP analysis provided by Lobster on Ru 4 (µ 3 −H) 4 (η 6 −C 6 H 6 ) 4 , which belongs to the cluster realm, thoroughly investigated for decades [84][85][86][87] . This 60-electron Ru 4 H 4 L m tetrahedral cluster, stable according to the Mingos-Wade rules, 88,89 exhibits six skeletal MOs as well as four Ru-H MOs.…”

“…Beyond these specific cases, a careful comparison between the MOs and associated energy levels on one hand and the pCOHP(ε) and pDOS(ε) profiles on the other hand does not reveal a wrong projection of the wavefunction on the local basis set. Owing to the relevant and accurate description of the bonding provided by the Lobster package on this test case, it can be safely applied to more realistic models of NPs, as we will be exemplified in section 4.2 (see Cusinato et al 12 ).…”

“…4 Several features are now firmly established by various experimental and theoretical approaches and can be used as a guide to define a theoretical model for dressed metal NPs: (i) the RuNPs we are concerned with have a very small metal core (typically between 1 nm 5 and 4 nm 6,7 ), which characterization by X-ray diffraction shows a crystalline character with the expected hcp structure of bulk ruthenium; 8 (ii) they can accommodate at least 1.3 hydride per surface ruthenium atom, a value probably slightly underestimated owing to the method employed; 9 (iii) a combination of 1 H and 2 H NMR has shown that hydrides are very mobile at the surface of RuNPs and that hydride-deuteride exchange between adsorbed hydrides and an isotopic D 2 gas source occurs under mild conditions; 10 (iv) a joint DFT/NMR study has proven the coexistence of ontop, edge-bridging, and face-capping deuterides at NPs surfaces, in contrast to what is observed and calculated on bare Ru(0001) surfaces; 11 (v) in the context of the Fischer-Tropsch reaction, ab initio thermodynamics and gas-phase chromatography measurements have shown that surface hydrides are replaced by CO ligands under a syngas flow. 12 This remarkable and broad knowledge of the metal core, of the surface composition and of the role of protecting ligands both on the size, on the shape, and on the catalytic properties has been collected during several years of experimental and theoretical studies performed on ruthenium NPs. On the theoretical side, there is a need to speed up the rationalization and optimization processes by integrating several tools and models into a rational framework.…”

Zwitterionic amidinates as effective ligands for platinum nanoparticle hydrogenation catalysts

“…We initially considered a Ru 55 (CO) 66 cluster, with a coverage corresponding to 1.5 CO per surface Ru atom, as established in previous ab initio thermodynamics calculations. 12 The adsorption energies of μ 2 -κ 1 N, κ 1 N′ bicoordinated ICy· (Ph) NCN, κ 1 N monocoordinated Z / Z conformer and the CO ins compound are slightly endothermic by 3–5 kcal mol –1 whatever the ligand, probably owing to the crowding of carbon monoxides on the surface. This is why we considered the adsorption properties on a Ru 55 (CO) 59 cluster.…”

“…11 In the context of the nanoparticle surface chemistry, the theoretical calculations provide detailed information not only on the coordination mode of the species bound to the nanoparticle surface, 12 but also on the chemical reactions that may take place on these surfaces. 13 …”