Rhodium Clusters Research Articles

Possible large magnetic moments in 4d transition metal clusters

The electronic structure and magnetism of 13 atom clusters of ruthenium, rhodium and palladium having face centered cubic(fcc) geometry has been studied using a Gaussian orbital basis and the local spin density approximation. Calculations were done for the lattice spacings relevant to the bulk crystal lattice. Using the fixed moment states as input potentials, as many as 3 self-consistent states were obtained for these clusters. The 3 converged states of Rh13 cluster is found to have magnetic moments of 0.69 μB , 1.00 μB and 1.46 μB . Out of these states, 0.69 μB moment state is found to be the ground state. But the total energy difference between the 0.69 μB and 1.00 μB state is very small. The 1.46 μB moment state coincides with the state reported previously by other authors which was obtained using the discrete variational method. The experimentally observed moment was around 0.47 μB . Our calculated moment is closer to the experimentally observed moment than the previously reported moment, but is still a bit larger. Ru13 cluster is also found to have large moments, and 3 self-consistent states are also obtained for this cluster. The 3 magnetic moments of the Ru13 cluster are 0.46 μB , 0.62 μB and 1.08 μB . Out of these states, 0.62 μB moment state is found to be the ground state. For the Pd13 cluster, in addition to the nonmagnetic state previously reported, a state with magnetic moment of 0.46 μB is also found to exist indicating possible magnetism in cluster phase.

Synthesis and characterization of high-nuclearity osmium–rhodium mixed-metal carbonyl clusters; molecular and crystal structures of [Os12Rh9(CO)44(μ3-Cl)]·2CH2Cl2 and [Os4Rh3(μ3-H)(CO)14(μ3-CO)(η4-C7H8)2

The anionic triosmium cluster [N(PPh3)2][Os3(µ-H)(CO)11] reacts with [Rh(nbd)Cl)2 (nbd = norbornadiene) in the presence of AgPF6 to give two new high-nuclearity osmium–rhodium clusters [Os12Rh9(CO)44(µ3-Cl)] 1 and [Os4Rh3(µ3-H)(CO)14(µ3-CO)(η4-C7H8)2] 2 in moderate yields.

An AFM investigation of the deposition of nanometer-sized rhodium and copper clusters by spin coating

We have used atomic force microscopy to investigate the deposition of nanometer-sized clusters by spin coating and it is shown that it is possible to produce a homogeneous distribution of nanometer-sized Cu and Rh particles using this technique. However, the formation of particles with a uniform size and distribution is not only dependent on the solute concentration and spin frequency, as has been discussed previously, but also on a number of other factors, including: atmospheric humidity, solvent properties and the chemistry of the solute. In the case of the simple salts, Cu(NO 3) 2 and RhCl 3 dissolved in ethanol, particles precipitate out of the solution during spin coating and deposit onto the substrate. However, AFM and XPS analysis reveals that the use of a Cu(acetate) 2 precursor results in the formation of a layer of Cu(acetate) 2 on the substrate. This behaviour is attributed to the existence of a larger metastable super-saturated region in the Cu(ac) 2 solution resulting from the presence of the acetate ligands. The layer of Cu(ac) 2 is observed to form particles on calcination, the particle size and distribution being sensitive to the calcination rate. Possible factors responsible for the ramp rate sensitivity are discussed. AFM imaging of the surface following oxidation and reduction of the larger Rh particles indicate that these particles break up as a result of the treatment, where this behaviour is consistent with previous studies. Following this treatment, or after direct reduction in H 2, the Rh particles are observed to exhibit a particle-substrate interaction, indicated by the inability of the AFM to sweep the particles across the surface. A similar behaviour is also determined to occur following oxidation of the Cu particles.

Protecting Structure Analyses of Organic Molecule-protected Nanoscopic Noble Metal Clusters

The stabilizing structure of cationic surfactant-protected platinum clusters in water and tertiary amine-protected rhodium clusters in chloroform, prepared by photo- and hydrogen-reduction, respectively, was investigated. These nanoscopic noble metal clusters present a narrow size distribution and are stable. The structural information of protective organic molecules on the surface of metal clusters was studied by transmission electron microscopy and hydrodynamic radius measurements according to the Taylor dispersion method. The size of the entire cluster with the protective layer surrounding the metal surface, obtained as Stokes' radii by the Taylor dispersion method, is considered to be fairly consistent with the sum of the naked particle size, obtained by transmission electron micrographs, and the size of the adsorbed protective layer, supporting the conformational information.

Reactions of benzene with rhodium cluster cations: Competition between chemisorption and physisorption

The reactions of Rh+n cations, n=1–40, with C6H6 and C6D6 were investigated under single collision conditions in a Fourier-transform-ion-cyclotron-resonance mass spectrometer. For clusters with up to 18 rhodium atoms, dissociative chemisorption, accompanied by total or partial dehydrogenation in competition with nondissociative adsorption of an intact benzene molecule (physisorption) was observed. For clusters with n≳18 only the nondissociative adsorption was observed in the primary reaction step. Besides some enhancement of the nondissociative adsorption channel for cluster sizes n=9, 11, 12, and 13, deuteration has little effect upon the observed reactions. The atomic rhodium cation reacts after a few seconds induction time to an arene complex, which then forms a bis-arene complex in a secondary reaction step. Rhodium dimer alone exhibits some cleavage of the Rh–Rh bond, resulting also in the Rh–C6H+6 complex.

Reactions of ( η6-diphenylacetylene) chromiumtricarbonyl complexes with polynuclear carbonyls. III. Reactions of M 4(CO) 12 (M = Rh, Co) clusters with Cr(CO) 3( η6-PhC 2Ph). Crystal and molecular structure of the Rh 4(CO) 9( μ5, η6: η6-PhC 2Ph)Cr(CO) 3 cluster

Reactions of two isoelectronic tetranuclear M 4(CO) 12 clusters (M = Rh,Co) ( I, III) with the chromium-arenetricarbonyl complex Cr(Co) 3( η 6-PhC 2Ph)( II) yield a product of metal carbonyl fragments in addition to the free triple bond of diphenylacetylene. The cobalt cluster reaction results in partial destruction of the cluster tetranuclear framework and formation of the Co 2(CO) 6( μ 4, η 2: η 6-PhC 2Ph)Cr(CO) 3 complex, obtained earlier starting from the binuclear Co 2(CO) 8 compound. In the case of a rhodium cluster addition of a tetranuclear carbonyl fragment takes place, yielding the Rh 4(CO) 9( μ 5, η 2: η 6-PhC 2Ph)Cr(CO) 3 ( IV) cluster. The crystal and molecular structure of IV reveals an unusual interaction between the Cr(CO) 3 fragment and a rhodium atom of the tetranuclear Rh 4 cluster core. 13C NMR spectroscopic data show that the solid state structure is retained in solution.

RHODIUM-CLUSTER REACTIVITY: STICKING PROBABILITIES OF SOME DIATOMIC MOLECULES

The reactivity of neutral rhodium clusters towards D 2, N 2, O 2, CO , and NO has been investigated and the absolute sticking probability (S) of the first molecule on Rh 10– Rh 50 ( Rh 10– Rh 35 for D 2) was determined. For O 2, the absolute sticking probability of the second molecule was measured. The reactions were made under single-collision-like conditions where the clusters make only one or a few collisions with reactive gas molecules. All molecules but N 2 react with a high sticking probability on most cluster sizes. N 2 molecules adsorbed on clusters produced in a liquid-nitrogen-cooled cluster source, while cooling the cluster source did not significantly change S for the other molecules. The sticking probability of O 2 and NO on Rh n looked almost the same, monotonically increasing from 0.2/0.4 at n=10 to 0.8/0.9 at n=25, and the high S also remains for larger clusters. The sticking probability of CO shows small variations around S=0.6 over the entire size range. The sticking probability is somewhat lower for D 2 and substanitially lower for N 2 (also on cool clusters), and these two molecules display the most distinct variations in S as a function of cluster size, often with coinciding maxima and minima.

Electronic and magnetic properties of embedded Rh clusters in Ni matrix

The electronic structure and magnetic properties of rhodium clusters with sizes of one to 43 atoms embedded in a nickel host are studied by first-principles spin-polarized calculations within the local density functional formalism. A single Rh atom in Ni matrix is found to have a magnetic moment of 0.45 mu B. Rh13 and Rh19 clusters in Ni matrix have lower magnetic moments compared with the free ones. The most interesting finding is that Rh43 cluster, which is bulk-like and non-magnetic in vacuum, becomes ferromagnetic when embedded in the nickel host.

Calculations on the magnetic properties of rhodium clusters

The electronic structures of rhodium clusters with sizes of 6, 9, 13, 19, and 43 are studied by first-principles spin-polarized calculations within the local-density-functional formalism. The bond lengths of all clusters are optimized by minimizing the binding energies. The magnetic moments of the clusters are presented and compared with experiments. The electronic structure of the Rh43 cluster has almost the same features as bulk rhodium.

Kinetics of hydrogenolysis of methylamine on a rhodium catalyst

The kinetics of hydrogenolysis of methylamine to methane and ammonia were investigated over a catalyst consisting of small clusters of rhodium dispersed on silica. Data obtained in the temperature range 353–408 K exhibit a characteristic pattern in which the rate passes through a maximum as the hydrogen partial pressure is increased by two orders of magnitude from 0.01 to 1.0 atm. At a given temperature, the position of the maximum shifts slightly in the direction of higher hydrogen partial pressure when the methylamine partial pressure increases by one to two orders of magnitude. Of particular interest is the finding that the rate increases with decreasing methylamine partial pressure over a broad range of hydrogen partial pressures covered in the investigation. As the hydrogen pressure increases, the inverse dependence of the rate on methylamine pressure becomes less pronounced and eventually disappears at a sufficiently high hydrogen pressure. At hydrogen partial pressures somewhat higher than those at which the rate maxima are observed, there is some indication that the inverse dependence changes to a positive dependence, especially at the lowest temperatures investigated. It seems likely that the rate limiting step of the reaction changes when the hydrogen pressure varies over a wide range. At the highest hydrogen pressures studied, it is suggested that the limiting step is one in which the scission of the carbon-nitrogen bond occurs in a hydrogen deficient surface intermediate formed in the chemisorption of methylamine, with no direct participation of hydrogen as a reactant in the step. On the other hand, at the lowest hydrogen pressures investigated, it is proposed that the rate is limited by a step in which chemisorbed hydrogen does participate directly as a reactant.

Magnetic Properties of Embedded Rh Clusters in Ni Matrix

ABSTRACTThe electronic structure and magnetic properties of rhodium clusters with sizes of 1 - 43 atoms embedded in the nickel host are studied by the first-principles spin-polarized calculations within the local density functional formalism. Single Rh atom in Ni matrix is found to have magnetic moment of 0.45μB. Rh13 and Rhl 9 clusters in Ni matrix have lower magnetic moments compared with the free ones. The most interesting finding is tha.t Rh43 cluster, which is bulk-like nonmagnetic in vacuum, becomes ferromagnetic when embedded in the nickel host.

Infrared Spectroscopic Studies of CO Adsorption on Rhodium Supported by SiO 2, Al 2O 3, and TiO 2

CO adsorption on Rh (0.5 wt%) supported by SiO 2, Al 2O 3, and TiO 2 was studied by DRIFT spectroscopy at 296 K and atmospheric pressure. A support effect on the particle size distribution was evident by dominant dicarbonyl species on Rh/Al 2O 3 and Rh/TiO 2, characteristic for CO adsorption on dispersed clusters, while significant proportions of linear- and bridged-bonded CO, ascribed to CO adsorbed on crystalline rhodium, were observed on Rh/SiO 2. Adsorption and desorption of CO was nondissociative on Rh/SiO 2, while on Rh/Al 2O 3 and Rh/TiO 2 CO dissociation occurred at 296 K. CO desorption at 296 K was reductive, which led to irreversible agglomeration of rhodium clusters; in thermal CO desorption (up to 500 K), this process was strongly enhanced.

Structural, electronic, and magnetic properties of small rhodium clusters.

The structural, electronic, and magnetic properties of small Rh-N clusters (N = 2-8,10,12,13, and 19) are studied using the discrete-variational local-spin-density-functional method. The groundstate structures of Rh-2, Rh-3, and Rh-4 are obtained and found to exhibit a tendency toward higherdimensional geometries with longer bond lengths and more nearest-neighbor bonds. The equilibrium bond lengths in the chosen geometry for the Rh-5-Rh-8, Rh-10, Rh-12, and Rh-13 clusters are determined and show 4-6% bond length contractions compared with the bulk interatomic spacing. A. complex size dependence of the magnetic properties of the Rhry clusters is found to be consistent with a recent experiment. The average magnetic moment per atom of the Rh-N clusters varies from 0 mu(B) to 2 mu(B). The clusters with magnetic ground states have ferromagnetic interactions, while-the local moments of the 5s and 5p often align antiferromagnetically with that of the 4d. The calculated magnetic moments of the Rh-10, Rh-12, Rh-13, and Rh-19 clusters are compared and discussed with the experimental ones. Icosahedral growth is suggested for Rh clusters. The reactivity of the Rh-N clusters toward H-2, N-2, and CO molecules is predicted. The densities of states, exchange splittings, valence band widths, and ground-state electronic configurations are presented for all the clusters. Finally an energy difference is identified which may be used as the criterion for the existence of multiple magnetic solutions in local-spin-density-functional calculations.

Magnetism in 4d-transition metal clusters.

We present the results of a study of magnetism in rhodium, ruthenium, and palladium clusters. We report values for the magnetic moments of rhodium clusters which are free from a systematic error present in our previous measurements [Phys. Rev. Lett. 71, 923 (1993)] and show that their magnetic moments decrease to the bulk value of zero as the cluster sizes increase. Ruthenium and palladium clusters are nonmagnetic.

A quantum chemistry description of carbon monoxide and water adsorbates on single-crystal rhodium and platinum clusters

Extended Hückel molecular-orbital calculations of CO and HO adsorbed ensembles on single-crystal Pt and Rh clusters are presented. The energy ranges related to the stability of adsorbed ensembles of the type (Me)N(CO)n(OH)m, where Me stands for Rh(100), Rh(111), Pt(100) and Pt(111), were calculated for various coordination geometries and applied potential conditions. A stability inversion potential was found for each ensemble. A correlation was obtained between the stability inversion potentials resulting from the different adsorbed ensembles and the potentials of the current peaks related to the voltammetric oxidation of CO adsorbates on Pt and Rh resulting from comparable potential perturbation conditions.

Formation of NiWO4 and its effects on hydrogenation activity of Ni-W/Al2O3 catalysts

Rhodium supported on NaY and NaH(x)Y zeolites with variable proton exchange (x= 10-30% eq.) have been used used for benzene hydrogenation and for the hydrogenolysis of n-butane. The catalysts containing 2 wt.-% rhodium showed differences in metal reducibility, metal dispersion, metal location into the zeolite and catalytic activity. All the data reported suggest that the differences in catalytic performance of the partially exchanged Rh/NaH(x)Y zeolites can be explained in terms of electron deficient rhodium clusters generated upon hydrogen reduction.

Molecular dynamics simulations of ion impact on a supported rhodium cluster

Small metal clusters on a support are commonly used in heterogeneous catalysis. The outermost layer of these clusters largely determines its catalytic properties. Surface analytic techniques such as secondary ion mass spectrometry (SIMS) and low-energy ion scattering (LEIS or ISS) are used to study the atomic composition and structure. With these techniques the surface is bombarded with low-energy (0.1–10 keV) noble gas ions. The ions deposit part of their kinetic energy and may damage the cluster, thereby influencing the measurements when a subsequent ion impinges on the same cluster. In this paper molecular dynamics simulations are used to study the damage formation in a rhodium cluster containing 147 atoms after the impact of He, Ne and Ar ions of 0.5–5 keV. The simulations show that the metal cluster is very fragile. The rhodium sputter yield is considerably higher than for bulk rhodium and the kinetic energy distribution of the sputtered atoms is strongly affected by the shape of the cluster. It is also shown that for surface analysis low ion doses (7 × 10 12 ion scm 2 ) are required.

Polymer-Immobilised Clusters of the Platinum Group Metals

In this review major developments associated with the synthesis, properties, structure and applications of polymer-immobilised clusters of platinum, palladium, ridium, rhodium, osmium and ruthenium, are presented. Special attention is paid to polymer analogous reactions with metal clusters and new directions involving the polymerisation and copolymerisation of cluster-containing monomers. Some specific features of fixing heterometallic clusters on polymers are examined and the more interesting application of PCNM in catalysis, and future developments in this direction, are discussed.

Methylcyclohexane dehydrogenation on Rh/γ‐Al2O3 catalysts

AbstractThe influence of the Rh loading on the surface properties and catalytic behaviour of Rh/γ‐Al2O3 catalysts has been studied. The series of catalysts presents differences in metal dispersion, reducibility, surface composition and catalytic activity. All the data reported suggest that the differences in catalytic behaviour in the methylcyclohexane dehydrogenation reaction can be explained in terms of electron‐deficient rhodium clusters, essentially when the metal particle size becomes smaller than 15 Å.

Metal cluster topology 14. Fusion of octahedra in metal carbonyl clusters

Two metal carbonyl cluster octahedra can be fused by sharing a vertex to give an M 11 cluster, by sharing an edge to give an M 10 cluster, or by sharing a face to give an M 9 cluster. In a globally delocalized metal carbonyl cluster consisting of fused octahedra, vertices unique to a single octahedron use three internal orbitals, vertices of a face shared by two octahedra use four internal orbitals, vertices of an edge shared by two octahedra use five internal orbitals, and vertices of a face shared by two octahedra use six internal orbitals. Using this principle the iridium carbonyl clusters Ir 9(CO) 20 3− and Ir 12(CO) 26 2−, constructed by face-sharing fusion of two and three octahedra, respectively, are seen to be electron- and orbital-precise and thus may be regarded as three-dimensional analogues of naphthalene and anthracene, respectively. These iridium carbonyl clusters are the second and third members of a homologous series Ir 3+3 n (CO) (21+11 n)/2 ; the end member of this series is the polymer [Ir 6(CO) 11] n consisting of an infinite chain of octahedra sharing opposite faces. Other electron- and orbital-precise clusters constructed by the face-sharing fusion of two octahedra are the mixed metal derivative Ir 3Ni 6(CO) 17 3− and the Ni 9 core in nickel clusters Ni 12(CO) 21H 4− n n− ; in the latter clusters the three symmetry-related edges of the central NJ 9 core are bridged by Ni(CO) 2 groups. The rhodium carbonyl clusters Rh 9(CO) 19 3− and H 2Rh 12(CO) 25 both have two skeletal electrons less than their iridium carbonyl counterparts Ir 9(CO) 20 3− and Ir 12(CO) 26 2−, respectively, and can both be regarded as missing a pair of core bonding electrons; this arises by interaction between the symmetric S° core molecular orbitals in adjacent face-fused octahedra in the rhodium clusters which raises the energy of one these orbitals to antibonding levels. The gold-nickel ‘spiro’ mixed cluster Au 6Ni 12(CO) 24 2− consists of four Ni 3Au 3 octahedra which are fused by sharing each of the six gold vertices between two octahedra leading to overall T d symmetry; this cluster as well as Ru 10C 2(CO) 24 2− consisting of two edge-sharing octahedra are electron- and orbital-precise.