H₂ Adsorption on 3d Transition Metal Clusters:  A Combined Infrared Spectroscopy and Density Functional Study

Abstract

The adsorption of H 2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H 2 is found to be exclusively dissociative on Sc n O + , V n + , Fe n + , and Co n + , and both atomic and molecularly chemisorbed hydrogen is present in Ni n H m + complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2-or 3-fold coordinated to cationic metal clusters.

Molecular adsorption of H2 on small cationic nickel clusters

“…14 ) stretch vibration, n(Ni-(H 2 )). 9 Comparison of the IR-MPD spectrum with simulated spectra supports this assignment. The molecular adsorption of H 2 on Ni 4 + is remarkable as sites for dissociatively bound H atoms are available and as the dissociative chemisorption of H 2 on nickel surfaces does not proceed via a molecular precursor state.…”

“…For the complexes with 5 and 6 Ni atoms, intense absorption bands are observed in the 1000-1500 cm À1 range, implying that the H is predominantly dissociatively bound in bridging or face sites. 9 For the Ni 4 D 2 + complex, a band located at 2460 cm À1 is observed, proving that there is at least one isomer present in which the D 2 molecule is molecularly bound. The frequency of the D-D stretch band is shifted considerably to lower frequency compared to the frequency of free D 2 (2941 cm À1 ).…”

“…It has been shown that this approach gives good agreement with experiment. 9,10 Initial complex geometries were generated based on bare metal structures proposed in the literature. For each geometric isomer, calculations were performed for several different spin states.…”

The adsorption of CO on transition metal clusters: A case study of cluster surface chemistry

“…In most cases co-adsorption of hydrogen leads to an increase of the C-O bond strength and a blue-shift of ν(CO), as illustrated in Figure 5 for cationic cobalt clusters. Hydrogen is dissociating on the surface of the cobalt clusters [82] leading to a localization of 3d electron density into Co-H bonds. Co-adsorbed CO molecules are sensitive to that effect.…”

Perspective: Methods for large-scale density functional calculations on metallic systems

“…The slab model has been applied successfully in screening metal and metal alloys for a variety of reactions, providing guidance on how to improve current catalysts or identifying novel materials or compositions. 9 However, these types of models, while providing useful insight, do not capture the complexity of the nanoparticle, 10 for example, the effect the particle size has on properties or edge effects between different crystal planes. The influence of the support can modify the electronic structure and geometry of the nanoparticle as well as cause "spillover effects" which may, in turn, affect catalytic activity.…”