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Ruthenium Cluster Structure Change Induced by Hydrogen Adsorption: Ru Dennis Bumueller, Anna-Sophia Hehn, Eugen Waldt, Reinhart Ahlrichs, Manfred M. Kappes, and Detlef Schooss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09521 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016
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Ruthenium Cluster Structure Change Induced by Hydrogen Adsorption: Ru19Dennis Bumüller1, Anna-Sophia Hehn2, Eugen Waldt1, Reinhart Ahlrichs2, Manfred M. Kappes1,2, and Detlef Schooss1,2,* 1
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
2
Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany
ABSTRACT: The effect of hydrogenation on the structure of Ru19- has been studied using a combination of trapped ion electron diffraction and density functional computations. While the bare Ru19- cluster has a closed shell octahedral geometry, hydrogenation of the cluster changes the structure type of the ruthenium core towards an icosahedral motif. The experiments show a gradual structural transition depending on the number of adsorbed hydrogen atoms. Density functional theory computations reveal the driving force behind this process to be the larger hydrogen adsorption energies for the bi-icosahedral structure and predict a corresponding structural rearrangement at around 20 adsorbed hydrogen atoms which is consistent with the experimental findings. Additionally, the computations provide insight into the hydrogen binding
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situation. They show that hydrogen is preferentially atomically bound only to surface Ru-atoms. H2 binding is predicted only at high hydrogen loadings.
INTRODUCTION More than 100 years after its large-scale industrial realization, the iron-catalyst-based HaberBosch process (HBP) continues to produce most of the world’s ammonia while consuming 1-2% of its annual energy supply. Ammonia can be generated more efficiently, e.g. with the Kellog Advanced Ammonia Process (KAAP) at lower pressures and temperatures than HBP, thus potentially saving energy. KAAP makes use of a supported ruthenium nanoparticle catalyst.1-2 Model studies have established that the optimal nanoparticle size range is on the order of a few nm
3-4
. This has been rationalized in terms of the number density of active sites for dissociative
chemisorption of N2 present on the surface of the Ru nanoparticles. These “B5 sites” correspond to a specific stepped surface configuration which has in turn been shown to facilitate N2 activation on ruthenium single crystals5. Calculations indicate that the number of B5 sites per total ruthenium mass should be strongly dependent on particle size.6 Such calculations generally assume that the corresponding ruthenium nanocrystals are Wulff-polyhedral fragments of the bulk hcp lattice3, 7. However, it is unclear whether this structural model remains valid for particle sizes extending down into the cluster range (< 2 nm). In particular, the occurrence of B5 sites is directly linked to close-packing of the constituent atoms (fcc or hcp) 5. Cluster structures may in fact differ significantly from the bulk lattice motif as we have shown for a series of 55 atom transition metal clusters in gas phase 8. Even if B5-like sites do exist on small Ru clusters 9, it is not obvious that they would retain the chemical/electronic character of the B5 sites present on
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the surface of larger nanoparticles. Finally, as clusters consist mainly of surface atoms, the interaction with the support or with the (gaseous) chemical environment may become strong enough to induce further structural changes as particle size decreases. In the real system, catalyst polydispersity often masks such effects. One approach to unravel them is to studying rigorously size- and composition selected clusters in gas phase. Such reductionist studies can help to clarify the underlying reaction mechanisms and thus offer a systematic route towards modelling complex catalytic processes. The present study is motivated along these lines. It explores hydrogen adsorption onto ruthenium clusters – an important secondary reaction in ruthenium catalyzed ammonia synthesis. Hydrogen adsorption onto bulk ruthenium surfaces has been intensively studied experimentally.10-14 Ruthenium is known to form only surface hydrides, a bulk hydride phase is stable only at very high H2 pressures (> 14 GPa)15. It is commonly accepted, that hydrogen is preferentially adsorbed atomically to ruthenium surfaces at three-fold coordinated fcc and hcp sites independent of coverage. Hydrogen adsorption onto Ru nanoparticles has also been studied - in particular for particle sizes larger than a few nanometers which are thought to be of either hcp or fcc structure type16-17. These adsorb up to 2 H per surface atom18. Hydrogen adsorption on ruthenium surfaces has been described theoretically using infinite slab surface models
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. The preferred adsorption sites on Ru (0001)
10, 19, 21
were found to be fcc
sites (µ3) with adsorption energies of ~0.6 eV, followed by hcp (0.47eV), bridging (0.44 eV) and atop (0.14eV) sites. Similar results were obtained for larger Ru nanoparticles which are known to retain the hexagonal close-packed structure of bulk ruthenium 22-23. In contrast to larger Ru nanoparticles, experimental investigations of hydrogen adsorption on Ru clusters are scarce. In fact, to our knowledge, there have been no detailed studies of hydrogen
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adsorption on small Ru clusters. Experimental insight into hydrogen binding was only obtained indirectly - from vibrational spectroscopy of hydrido-ruthenium complexes RuHLn 24 which have been studied as model systems for Ru clusters and nanoparticles. RunH2 clusters with n
