Rhodium Clusters Research Articles

Size and Topological Effects of Rhodium Surfaces, Clusters and Nanoparticles on the Dissociation of CO

The present density functional theory study provides insight into the reactivity of the surface metal atoms of extended/periodic Rh surfaces, clusters, and nanoparticles toward CO adsorption and dissociation. Our results demonstrate that the defect site in a B5 configuration is the most active one for CO dissociation on all three considered systems. However, the reactivity of the B5 site for CO dissociation depends critically on the size of the system. The barrier for CO dissociation barrier on the B5 site increases for smaller particles. The lowest barrier is found for the B5 site of a stepped Rh (211) surface. CO dissociation on this site occurred with a barrier below the desorption energy of CO.

Octahedral (cyclopentadienyl)rhodium clusters [Rh6Cp6(μ6-C)]2+ and [Rh6Cp6(μ3-CO)2]2+: Synthesis, structures and electrochemistry

The 86-electron dicationic octahedral rhodium clusters containing Cp (Cp=C5H5) ligands and either an interstitial carbon atom, [Rh6Cp6(μ6-C)]2+ ([1]2+), or two carbonyl groups, [Rh6Cp6(μ3-CO)2]2+ ([2]2+), were synthesized in low yields by reactions of Rh3Cp3(μ-CO)3 with RhCp(C2H4)2 or [RuCp∗(MeCN)3]+ (Cp∗=C5Me5), respectively. The structures of [1]2+ and [2]2+ were determined by X-ray diffraction. Their electrochemical behavior proved that they possess a rather extended electron transfer activity. In accordance with DFT calculations, the nearly octahedral structure of [1]2+ and [2]2+ is retained both upon oxidation (2+/3+) and the first reduction (2+/+); however, the second reduction (+/0) results in the breaking of one (for [1]0) or two (for [2]0) Rh–Rh bonds. In the case of the related Dahl’s nickel cluster Ni6Cp6 the nearly octahedral structure is retained upon all redox steps (3+/2+/+/0/−/2−).

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Zeolite-Supported Rhodium Complexes and Clusters: Switching Catalytic Selectivity by Controlling Structures of Essentially Molecular Species

Precise synthesis and characterization of site-isolated rhodium complexes and extremely small rhodium clusters supported on zeolite HY allow control of the catalyst selectivity in the conversion of ethene to n-butene or ethane, respectively, as a result of tuning the structure of the active sites at a molecular level.

Impurity Atoms on Small Transition Metal Clusters. Insights from Density Functional Model Studies

We review our computational studies at the DFT level on small isolated metal clusters of late transition metals that contain atomic (H, C, O) or diatomic (CO, N2) ligands. These investigations were initiated by the clarification of the structure of iridium and rhodium clusters, as characterized by EXAFS, and then were extended to clusters of other transition metals (Ni, Ru, Pd, Os, Pt). The results suggest that a single H atom hardly changes the structure of a small metal cluster, while the presence of O and C impurity atoms causes large variations in the metal–metal distances. The adsorption of single atoms results in a partial oxidation of the metal moiety, yet addition of an atomic impurity only moderately modifies the electronic properties of small clusters, whereas stronger modifications of the properties are caused when the charge of the metal cluster is varied. The dissociative adsorption of larger amounts of hydrogen, up to 6 H2 molecules, on metal tetramers causes an elongation of the (average) inter-metallic distances, bringing them close to the experimental values. This body of computational results can be helpful for elucidating the structures of experimentally observed species and for rationalizing their electronic and catalytic properties.

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

Carbon nanotubes decorated with gold, platinum and rhodium clusters by injection ofcolloidal solutions into the post-discharge of an RF atmospheric plasma

In this paper, we present a new, simple, robust and efficient technique to decoratemulti-wall carbon nanotubes (MWCNT) with metal nanoparticles. As case studies, Au, Ptand Rh nanoparticles are grafted onto MWCNTs by spraying a colloidal solution intothe post-discharge of an atmospheric argon or argon/oxygen RF plasma. Themethod that we introduce here is different from those usually described in theliterature, since the treatment is operated at atmospheric pressure, allowing therealization in only one step of the surface activation and the deposition processes.We demonstrate experimentally that the addition of oxygen gas in the plasmaincreases significantly the amount of grafted metal nanoparticles. Moreover, TEMpictures clearly show that the grafted nanoparticles are well controlled in size.

Structural and electronic study of neutral, positive, and negative small rhodium clusters [Rhn, Rh, and Rh

AbstractWe have carried out a systematic study for the determination of the structure and the fundamental state of neutral and ionic small rhodium clusters [Rhn, Rh, Rh (n = 5–10)] using ab initio Hartree‐Fock methods with a LANL2DZ basis set. A range of spin multiplicities is studied for each cluster. We present the bond lengths and angles and geometric configuration adopted for the clusters in its minimum energy conformation showing the differences when the clusters have different number of unpaired electrons. In addition, we report the vertical ionization potential and the adiabatic potential calculated by the Koopman's theorem. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2541–2547, 2010

Infrared Induced Reactivity on the Surface of Isolated Size-Selected Clusters: Dissociation of N2O on Rhodium Clusters

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

Communications: The structure of Rh8+ in the gas phase

The geometric structure of the Rh8+ cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

Adsorption and reaction of Rh(CO)2(acac) on Al2O3/Ni3Al(111)

The Al(2)O(3)/Ni(3)Al(111) surface has been used as a template for the nucleation and growth of rhodium clusters using an organometallic precursor: Rh(CO)(2)(acac). When Rh(CO)(2)(acac) is deposited on the Al(2)O(3)/Ni(3)Al(111) surface, the molecule is observed to bind preferentially to specific sites associated with the film superstructure (known as the dot structure) and appears to be stable at temperatures up to 473 K at which point some sintering and aggregation processes begin. Annealing the sample to 673 K results in further sintering of the metal deposits as well as an apparent loss in the coverage of rhodium species possibly due to a combination of desorption and deligation. After annealing to 873 K the coverage of rhodium species decreases by about 50% with respect to the initial deposited coverage. Our results suggest that using an organometallic precursor rather than metal atoms to form deposited metal particles on oxide substrates may result in increased resistance to sintering processes.

Study of the Structural and Electronic Properties of RhN and RuN Clusters (N < 20) within the Density Functional Theory

Using the density-functional theory (DFT) with the generalized gradient approximation to exchange and correlation, we compute the geometries, electronic structure, and related properties of free-standing rhodium and ruthenium atomic clusters with sizes below 20 atoms. We explore different structural and spin isomers per size, for which we determine the interatomic distances, binding energy, magnetic moment, HOMO-LUMO gap, and electric dipole moment. For many sizes, different implementations of DFT predict different properties for the lowest-energy isomers, thus illustrating the complex nature of these 4d transition metal elements at the nanoscale. We discuss our results for rhodium clusters in the context of recent electric deflection measurements.

Defining Active Catalyst Structure and Reaction Pathways from ab Initio Molecular Dynamics and Operando XAFS: Dehydrogenation of Dimethylaminoborane by Rhodium Clusters

We present the results of a detailed operando XAFS and density functional theory (DFT)-based ab initio molecular dynamics (AIMD) investigation of a proposed mechanism of the dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh(4) cluster catalyst. Our AIMD simulations reveal that previously proposed Rh structures, based on XAFS measurements, are highly fluxional, exhibiting both metal cluster and ligand isomerizations and dissociation that can only be accounted for by examining a finite temperature ensemble. It is found that a fluxional species Rh(4)(H(2)BNMe(2))(8)(2+) is fully compatible with operando XAFS measurements, suggesting that this species may be the observed catalyst resting state. On the basis of this assignment, we propose a mechanism for catalytic DMAB dehydrogenation that exhibits an energy barrier of approximately 28 kcal/mol.

Aqueous-organic biphasic redox-chemistry of high-hydride content rhodium clusters: Towards immobilisation of redox-switchable H 2 binding materials on a surface

Water-insoluble phosphine-surface-stabilised rhodium clusters [ Rh 6 ( PR 3 ) 6 H 12 ] [ BAr 4 F ] 2 (R = cyclohexyl, Ar F = (3,5-CF 3) 2C 6H 3) can be immobilised onto suitable glassy carbon, graphite, or tin-doped indium oxide (ITO) electrode surfaces. When immersed into alcohol/aqueous electrolyte media, they display stable voltammetric responses consistent with the formation of organic microdroplet | aqueous electrolyte two-phase conditions which are similar to those observed in corresponding anhydrous (CH 2Cl 2) solution phase. This results in an electrode surface that is straightforwardly modified with cluster materials that can uptake or release dihydrogen depending on their redox-state. The results presented herein lead to a slight re-evaluation of the originally reported hydrogen store and release cycle for this system.

Density functional theory study of the interaction of H2 with rhodium clusters

The adsorption for H atoms on the Rhn cluster has been systematically investigated by density functional theory. The result indicates that the lowest energy structures of RhnH2 are generated with H atoms being adsorbed on the lowest energy structure of Rhn clusters, and the lowest energy structures of Rhn clusters are not changed by adsorbing H atoms. The total magnetic moment is affected by average bond distance. The chemisorption of H atoms on Rhn clusters belongs to dissociative adsorption. When H2 is absorbed on the Rhn clusters, the stability and chemical action of corresponding clusters are dramatically increased. The second order difference indicates 4 is magic number in RhnH2 and Rhn clusters. Among various possible adsorption sites, bridge site is energetically preferred for n≤5. The hollow site adsorption appears for n≥6.

Density functional theory calculations of vibrational spectra of rhodium oxide clusters

Predicted infrared vibrational spectra of low-energy isomers of rhodium cluster oxides, Rh6Om+(m=1–4), are presented, based on structures calculated using density functional theory. The oxygen stretching frequencies are found to be characteristic of the oxygen binding mode. These simulated spectra provide a guide to future experimental measurements of the infrared multi-photon dissociation spectra, enabling assignment of the binding sites of oxygen atoms on the cluster. In favourable cases these calculations suggest that a determination of the underlying metal framework structure is possible.

Preadsorbed Water-Promoted Mechanism of the Water−Gas Shift Reaction

In this work, a detailed first principle study of the mechanism of the heterogeneous catalytic water−gas shift (WGS) reaction on a rhodium cluster is presented. A large number of possible reaction mechanisms relevant to the WGS reaction are explored, and as many as 34 possible pathways are located due to the unsaturated nature of the metal cluster and the multitude of intermediate species binding configurations. Bronsted−Evans−Polanyi relationships and the Sabatier principle are used to locate the kinetic and thermodynamic paths occurring on the rhodium cluster. A detailed potential energy diagram of the kinetically favored mechanism is presented that shows that the RDS of the reaction are the water dissociation, formate association and formate decomposition elementary reactions, with free energy barriers (ΔG‡) of 24.2, 25.9, and 27.0 kcal/mol, respectively. The poisoning effect of coadsorbed CO and the beneficial effect of preadsorbed water on the kinetic rate of this reaction is demonstrated and a water...

Time-Resolved Structural Characterization of Formation and Break-up of Rhodium Clusters Supported in Highly Dealuminated Y Zeolite

Mononuclear rhodium complexes incorporating two ethylene ligands and anchored to dealuminated zeolite Y by two Rh-O bonds were characterized by transient extended X-ray absorption fine structure (EXAFS) spectroscopy and infrared (IR) spectroscopy as they were converted in the presence of H{sub 2}. EXAFS spectra indicate reduction of the rhodium in the complex at 298 K to form rhodium clusters less than 3 {angstrom} in average diameter. Contacting of the resultant clusters with C{sub 2}H{sub 4} led to their oxidative fragmentation, and the process was reversible. When the H{sub 2} treatment was carried out at a higher temperature (373 K), larger clusters formed. The reduction and oxidation of the rhodium were confirmed by X-ray absorption near edge spectra. During the ethylene treatments, ethyl groups formed on the rhodium, as indicated by IR spectra; treatment in H{sub 2} led to hydrogenation of these groups to form ethane, and the ethyl groups are inferred to be intermediates in the catalytic hydrogenation of ethylene. Ethylene in the gas phase helps to stabilize rhodium in the form of mononuclear complexes on the zeolite during catalysis, hindering the cluster formation.

Oxides of small Rhodium clusters: Theoretical investigation of experimental reactivities

Density functional theory is used to investigate the structures of cationic rhodium cluster oxides, Rh(6)O(m) (+) (m=1,4). On the monoxide and dioxide, the oxygen atoms occupy bridge sites, while on trioxide and tetroxide clusters, high-coordination sites are favored. A range of spin multiplicities are investigated for each cluster, with high spin multiplicities found to be less favored for the oxides compared with the naked metal clusters. The dissociation of nitric oxide on low-energy isomers of Rh(6)O(4) (+) is investigated and found to be unfavorable compared to molecular adsorption due to a combination of thermodynamic and kinetic factors. These calculations are consistent with, and help to account for, the experimentally observed reactivity of rhodium and rhodium oxide clusters with nitric oxide [M. S. Ford et al., Phys. Chem. Chem. Phys. 7, 975 (2005)].

THEORETICAL STUDY OF NITROGEN MONOXIDE ADSORPTION ON RHODIUM CLUSTERS AT DIFFERENT SITES

The adsorption of nitrogen monoxide NO with charged and neutral [Formula: see text] clusters at atop, bridge, and threefold hollow sites had been investigated by density functional theory calculations. The results showed that rhodium clusters had strong orbital interactions with NO and formed the complex [ Rh n NO ]-/0/+. The stretching vibrational frequencies of the N–O bonds changed with the different adsorption sites and clusters sizes. The interactions between rhodium clusters and NO molecular could be described through the donation and back-donation of their frontier orbitals. The more back donation from Rh to NO , the weaker the N–O bonds, exhibiting that the lengthening of the N–O bond length and the lowering of its vibrational frequency. In general, the donation and back-donation interactions followed the tendencies: anionic > neutral > cationic, big size > small size, threefold hollow site > bridge site > atop site.