Controlling the Bonding of CO on Cobalt Clusters by Coadsorption of H2

Understanding the effect of adsorption sites of CO at cobalt surface on its reactivity with H2/H by DFT calculations

Cobalt (Co) is widely used in Fischer-Tropsch synthesis (FTS), converting synthesis gas, carbon monoxide + hydrogen (CO + H2), to long-chain hydrocarbons. The adsorption of CO on the Co surface is the key step in FTS. In this work, the effect of CO adsorption sites on the reactions between CO and H2 was investigated by using density functional theory (DFT). The energetics and structures of the reactions between the adsorbed CO (CO*) and H2/adsorbed H2 (H2*)/adsorbed H atom (H*) were calculated. The results show that the reaction between CO* and H2 is initiated by the molecular adsorption of H2 on the Co surface. The reactions between CO* and H2*/H* are influenced by CO adsorption sites. For the reaction system of CO* + H2*, it has the lowest reaction barrier when CO is adsorbed at the hcp site, while for CO* + H*, it has the lowest reaction barrier when CO is adsorbed on the top site. Kinetic analysis indicates that to improve the reactivity of CO + H2 in FTS, the adsorption of CO should be controlled to favour the top and bridge sites. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Screening of Subnanoscale Metal Hydride Formation for Late Transition Metals Using Dimer Cations─Group IX Element

The energetically stable structures of M2Hm+ (M = Co, Rh, Ir; m = 2, 4, 6, ...) were investigated using density functional theory calculations, and possible reaction pathways for the sequential adsorption of H2 molecules on M2+ were proposed. Based on the most stable structures, adsorption energies of H2 were calculated for each adsorption step, and the maximum numbers of adsorbed H atoms on Co2+, Rh2+, and Ir2+ were estimated to be 14, 16, and 16, respectively. Compared to group XI elements (M = Cu, Ag, and Au), which are conceivably inert to H2, more H atoms were bound to Co2+, Rh2+, and Ir2+. The adsorption of H2 on M2+ (M = Co, Rh, Ir, or Cu) in the gas phase was investigated experimentally at 300 K using mass spectrometry. Although Rh2+ and Ir2+ stored numerous H2 molecules as predicted by calculations, Co2+ was found to adsorb no H atoms. It was probably due to the insufficient adsorption energy of Co2+ and the kinetic effect in the H2 adsorption process. Thus, computational calculations can overestimate the number of adsorbed H atoms.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

The spin magnetic order of Con+ (n ≤ 5) clusters

The magnetism of transition metal clusters has been for decades a complicated puzzle, with experimental results disagreeing with calculations performed within the density functional theory formalism. In this work, we provide a key to this puzzle by investigating the lowest-energy spin states of cobalt cluster, Co_n⁺ (n ≤ 5), using CASSCF/NEVPT2 calculations with very large active spaces. The geometries as well as the spin configurations adopted by the clusters in their ground-state are known from experiments, making Co_n⁺ clusters an ideal model system for theoretical investigation. Here, using the experimentally known geometries determined by far-infrared spectroscopy as inputs, we calculated the lowest-energy spin configurations of the clusters, revealing that the CASSCF/NEVPT2 formalism correctly predicts the preferred electronic configuration of the clusters known experimentally. This is in contrast to the widely used density functional theory, with results that depend on the selected exchange-correlation functional. The reasons for the failure of density functional theory, in opposition to CASSCF/NEVPT2, are discussed, providing a solid framework for investigating other transition metal and transition metal oxide clusters.

Preadsorption Effect of Carbon Monoxide on Reactivity of Cobalt Cluster Cations toward Hydrogen

We report gas-phase reactions of free Co_n(CO)_m⁺ (n = 3-11, m = 0-2) with H₂, expecting a catalytic reaction of coadsorbed CO and H₂ on Co_n⁺. Preadsorption of CO molecules is found to promote H₂ adsorption, in particular, on Co_n(CO)⁺ (n = 5, 8-10). Density functional theory (DFT) calculations reveal that the reactivity is governed by the molecular-orbital energy of Co_n⁺, which is tuned by preadsorbed CO molecules. Collision-induced-dissociation experiments performed on Co_nCOH₂⁺ (n = 8-10) imply that at least some of the CO and H₂ molecules are bound together on Co_n⁺.

Interaction of carbon monoxide with doped metal clusters

Highlight of experimental and computational studies about the interaction of CO with transition and coinage metal clusters, particularly discussing the influence of dopant atoms.

Spectroscopic properties and activated mechanism of NO on isolated cationic tantalum clusters: A first-principles study

The adsorption and dissociation of NO on the cationic Ta₁₅⁺ cluster were investigated using the density-functional theory (DFT) calculations, and the Ta-centered bicapped hexagonal antiprism (BHA) structure of cationic Ta₁₅⁺ cluster can be identified as the global minimum, which reproduces well the infrared multiple photo dissociation (IR-MPD) spectrum. Our results show that the cationic BHATa₁₅⁺ cluster provides the hollow region for NO to interact effectively, and possess larger adsorption strength on the region than other sites. The density of states, charge density differences and frontier molecular orbitals were analyzed to understand the electronic properties of the stable NO-adsorbed isomers. The characteristic IR peaks of the firstly two low-lying isomers are properly assigned, in which the strongest IR peak originates from the N - O stretching vibration. For the dissociation of NO on the BHATa₁₅⁺ cluster, it is found that the reaction path II easily occurs rather than path I due to small reaction barrier, and the cluster may possess the great catalytic behavior to dissociate NO molecule. The present results will inevitably stimulate future theoretical and experimental studies for the design of novel Ta-based catalytic materials for the NO dissociation.

IR spectroscopy on isolated Co(n)(alcohol)m cluster anions (n=1-4, m=1-3): structures and spin states

Isolated cobalt-alcohol cluster anions containing n=1-4 cobalt and m=1-3 alcohol molecules (alcohol=methanol, ethanol, propanol) are produced in a supersonic beam by using a laser ablation source. By applying IR photodissociation spectroscopy vibrational spectra in the OH stretching region are obtained. Several structures in different spin states are discussed for the (n,m) clusters. In comparison with density functional theory calculations applied to both the Co/alcohol clusters and the naked Co cluster anions, an unambiguous structural assignment is achieved. It turns out that structures are preferred with a maximum number of hydrogen bonds between the OH groups and the Co···Co units. These hydrogen bonds are typical for anionic species leading to an activation of the OH groups which is indicated by large red-shifts of the OH stretching frequencies compared to the naked alcohols. For each (n,m) cluster, the frequency shifts systematically with respect to the different alcohols, but the type of structure is identical for all alcohol ligands. The application of IR spectroscopy turns out to be an ideal tool not only as a probe for structures but also for spin states which significantly influence the predicted OH stretching frequencies.

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study

The adsorption of H2 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 H2 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.