Pure Rh Research Articles

Local and chemical environment dependence of the magnetic properties of CoRh core-shell nanoparticles

The ground-state magnetic properties of Co${}_{x}$Rh${}_{1\ensuremath{-}x}$ nanoparticles having sizes in the range of 0.8--2 nm ($N=43$ and 273 atoms) and Co concentrations $x\ensuremath{\approx}0$, $0.25$, $0.5$, $0.75$, and 1 are investigated in the framework of density-functional theory by using a fixed-moment method. Electron correlation effects are explored by comparing the results of the local spin-density and generalized-gradient approximations to the exchange and correlation functional. The role of chemical order on the magnetic behavior is investigated by considering a variety of core-shell atomic arrangements with nearly spherical CoRh interfaces. A local relaxation of the cluster geometry is performed by taking face-centered cubic structures as starting configurations. All considered Co${}_{x}$Rh${}_{1\ensuremath{-}x}$ clusters are found to be magnetic with an average spin moment per CoRh unit that is larger than in macroscopic alloys having similar concentrations. This is a consequence of both, the enhancement of the Co moments and the occurrence of important induced Rh moments, which couple parallel to the Co moments. The distribution of the local magnetic moments within the clusters is found to depend strongly on the local and chemical environment of the atoms. In particular, the Rh moments show a nontrivial dependence as a function of the distance to the CoRh interface. The results for the local magnetic moments are correlated to the electronic densities of states, which reflect the concentration and chemical-order dependence of the cluster electronic structure. Finally, the effects of coating and of the 3$d$-4$d$ interface are analyzed by comparing the magnetic behaviors of core-shell particles with those of the corresponding pure Co and Rh cores.

Nanometer size 3d–4d and 3d–5d substitutional clusters: Promising candidates for magnetic storageapplications

Spin-polarized density-functional calculations including spin–orbit coupling (SOC) have been performed for FemRhn and FemPtn clusters having N=m+n,N≤19 atoms. The spin magnetic moments, orbital magnetic moments, and the magnetic anisotropy energies have been determined. A significant enhancement of magnetic anisotropy energies is found by the substitutional nanoalloying of Fe with Rh and Pt atoms. We obtained a remarkable non-monotonous dependence of the MAE as a function of Fe content, i.e., upon going from pure Fe to pure Rh and Pt. The substitutional nanoalloying boost the magnetic anisotropy energies by creating significant cluster symmetry lowerings.

Microwave synthesis of Au–Rh core–shell nanoparticles and implications of the shell thickness in hydrogenation catalysis

A microwave-assisted heating method allows for the convenient and reproducible synthesis of defined Au-Rh core-shell metallic nanoparticles with tuneable shell thicknesses. Nanoparticles with shells as thin as two Rh monolayers can be prepared, which are effective in vapour-phase hydrogenation catalysis at room temperature without the need for pre-treatment. Particles with Rh shells consisting of two or four Rh overlayers show similar catalytic properties and are both significantly more highly active than pure Rh nanoparticles, per mol of Rh employed.

Fabrication of Rh based solid-solution bimetallic alloy nanoparticles with fully-tunable composition through femtosecond laser irradiation in aqueous solution

Many late transition binary alloy nanoparticles (NPs) have been fabricated through a wide variety of techniques. Various steps are involved in the fabrication of such NPs. Here, we used a simple and green route to fabricate solid-solution Rh–Pd and Rh–Pt bimetallic alloy NPs through femtosecond laser irradiation in a solution without any chemicals like reducing agents. X-ray diffraction (XRD) peaks of NPs obtained in the solutions with different ratios of Rh–Pd and Rh–Pt ions monotonically varied from the position of pure Rh to those of Pd and to Pt which respectively indicated that these NPs were alloy. Composition of fabricated NPs was fully tuned over the entire range of Rh1−x –Pd x , and Rh1−x –Pt x with varying the mixing ratio of metal ions in the solution. Studies of Rh–Pd and Rh–Pt solid-solution system suggest that the alloy formation occurs through the nucleation of Rh and then followed by the diffusion of Rh, Pd and Rh, Pt to form a homogeneous alloy. The variety of average size of the alloy NPs for different compositions could be attributed to different reduction rate and surface energies of metal ions. Our result implies that femtosecond laser irradiation in aqueous solution is one of the potential methodologies to form multimetallic solid-solution alloy NPs with fully tunable composition.

Ab Initio Thermodynamics Examination of Sulfur Species Present on Rh, Ni, and Binary Rh–Ni Surfaces under Steam Reforming Reaction Conditions

The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO(x) (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO(x) adsorption under steam reforming conditions. The transition temperature from a clean surface (<(1)/(9) ML) to S adsorption was identified on Rh(111), Ni(111), Rh(1)Ni(2)(111), and Rh(2)Ni(1)(111) metals at various P(H(2))/P(H(2)S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with (1)/(3) ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh(1)Ni(2) is covered with (1)/(9) ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh(4) cluster and CeO(2)(111) surface. SO(x) surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.

Enhanced Dissolution of Rh from RhZn3 Formed Through Zn Vapor Pretreatment

A Rh-Zn compound was synthesized by exposing Rh to a fixed pressure of Zn vapor. After keeping Rh and Zn in a sealed container for 10 days at 1173 K and 1093 K (900 °C and 820 °C), respectively, an unreported compound was obtained. Its chemical composition was determined to be RhZn3, and X-ray diffraction analysis revealed it to have a face-centered tetragonal structure (a = b = 3.78 A and c = 3.88 A). The dissolution rate of Rh from the RhZn3 in hydrochloric acid was measured using a channel flow, double-electrode method, and it was found to be more than 100 times greater than that from pure Rh. The result verified the possibility of a Rh recovery process in which Rh is exposed to Zn vapor prior to acid leaching to enhance its dissolution.

P/S ligands derived from carbohydrates in Rh-catalyzed hydrosilylation of ketones

Reported is the synthesis of a number of diastereomerically pure cationic Rh(I)-complexes I starting from phosphinite thioglycosides. These complexes were used in the asymmetric hydrosilylation of prochiral ketones. The reactivity and enantioselectivity of the reaction was shown to be dependent on the pyranose ring, the substituent at the sulfur atom, the hydroxylic protective groups and most significantly on the alkene co-ligand.

XPS and TPD investigation of CO adsorption on mixed Rh–V layers supported by gamma-alumina

Alumina-supported mixed bimetallic Rh–V thin films, with the overall thickness of 0.8ML, were prepared under the ultrahigh vacuum (UHV) conditions and characterized with respect to their electronic and CO adsorption properties. X-ray photoelectron spectroscopy (XPS) was utilized to characterize electronic changes accompanying bimetallic Rh–V interaction and interaction between metal and polycrystalline γ-Al2O3 substrate. The chemisorption properties were probed by temperature-programmed desorption spectroscopy (TPD) of CO molecules. The electronic and chemisorption properties of the mixed layers were compared with pure Rh and V layers grown on the same γ-Al2O3 substrate and with a model bimetallic Rh–V system prepared by V deposition on a polycrystalline Rh foil. By varying the preparation conditions, we observed a strong dependence of the studied properties on the position of the V atoms. The presence of V atoms on the surface led to a fast deactivation, while vanadium presented under the surface resulted in a weakening of CO-metal surface bond, a change in the proportion of the adsorption side species, and an increase of CO dissociation.

Structural and magnetic properties of FemYn (m + n = 7, Y = Ru, Rh, Pd, and Pt) nanoalloys

The structural and magnetic properties of the small binary clusters Fe m Y n (with m + n = 7, Y = Ru, Rh, Pd, Pt) were studied through extensive ab initio calculations, by means of the fully unconstrained version of the density-functional method, as implemented in the SIESTA code, within the generalized gradient approximation. The lowest energy state geometries, the chemical ordering, and the electronic and the magnetic structures were calculated. We found that the lowest energy geometrical structures for the pure Ru, Rh, Pd, Pt, and Fe heptamers, are a cube without an apex, a triangular prism capped on a square face, a decahedron, a side capped double square, and a decahedron, respectively. Starting from these geometries of the pure element heptamers, we followed the changes in the geometric structure as a function of the chemical composition. We analyzed all the different chemical arrangements, which depend on the particular geometry, and magnetic moment orientations, in the whole range of compositions. In general, there are important modifications to the magnetic moment of the Y atoms as soon as one of them is substituted by an Fe atom in the cluster. In contrast, under the same circumstances, the Fe magnetic moment takes values larger than 3 μ B and keeps almost this value, insensitive to the structure, composition and chemical order of the system.

Enantiomerically Pure Cyclopentadienyl- and Indenyl-Functionalized N-Heterocyclic Carbene Complexes of Iridium and Rhodium

Novel enantiomerically pure cyclopentadienyl- and indenyl-functionalized N-heterocyclic carbene ligands have been prepared by reaction of a chiral imidazole tosylate derivative with the corresponding cyclopentadienyl and indenyl lithium salts. Coordination of the Cp-functionalized NHC ligand to iridium and rhodium allowed the preparation of enantiomerically pure chelating cyclopentadienyl-functionalized Ir(III) and Rh(III) metal complexes. In contrast, the indenyl-functionalized NHC coordinates to iridium in a monodentate fashion, giving an Ir(I)-NHC complex containing a dangling indene group.

A Uniform Bimetallic Rhodium/Iron Nanoparticle Catalyst for the Hydrogenation of Olefins and Nitroarenes

Mix and more than match: Relative to the catalytic activity of pure Rh nanoparticles in a dendrimer cage, Rh/Fe bimetallic nanoparticles in dendrimers have improved catalytic activity towards the hydrogenation of olefins, and unlike Wilkinson catalyst could catalyze nitroarene hydrogenation (see scheme, G4=4th generation dendrimer).

Coordination-dependent bond energies derived from DFT surface-energy data for use in computations of surface segregation phenomena in nanoclusters

Theoretical computations of alloy surface phenomena, such as elemental segregation, within atomic pair-interaction models, necessitate the use of reliable bond energies as input. This work introduces the idea to extract the coordination dependence of bond energies from density-functional theory (DFT) computed surface energy anisotropy. Polynomial functions are fitted to DFT data reported recently for surface energies of pure Pt, Rh and Pd. Compared to other approaches, the proposed method is highly transparent, and is expected to yield better insight into the origin of alloy segregation phenomena at surfaces of bulk and nanoclusters. Reference to this paper should be made as follows: Polak, M. and Rubinovich, L. (2011) 'Coordination-dependent bond energies derived from DFT surface-energy data for use in computations of surface segregation phenomena in nanoclusters', Int. J. Nanotechnol., Vol. 8, Nos. 10/11/12, pp.898-906.

Rh1−xPdxnanoparticle composition dependence in CO oxidation by oxygen: catalytic activity enhancement in bimetallic systems

Bimetallic 15 nm Rh(1-x)Pd(x) nanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O(2) in two pressure regimes: atmospheric pressure and 100-200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100-200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O(2) reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd-Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.

Adsorption and desorption of CO on Ni decorated stepped Rh(553)

Vicinal surfaces are important in surface science, as they show interesting electronic structures and reactivities due to the steps. In this paper the adsorption and desorption of carbon monoxide on the stepped Rh(553) surface decorated with Ni is reported. With 0.1 to 0.3 monolayer Ni on Rh(553) one and two atoms broad Ni wires along the Rh steps are formed. The adsorption and desorption of carbon monoxide on these surfaces is investigated using thermal desorption spectroscopy (TDS) and reflection absorption infra red spectroscopy (RAIRS). TDS shows a marked change from just one broad TDS peak on pure Rh(553) to 4 distinct peaks with increasing Ni decoration. RAIRS shows that already at 0.1 monolayer Ni the CO adsorption states on bridge sites on the Rh step atoms are completely quenched. In addition it is shown that with Ni films up to 3 monolayer the on top adsorption sites for CO on Ni are preferred over the bridge and hollow adsorption sites in contrast to what is known from the Ni(111) surface.

Density functional theory study of sulfur tolerance of CO adsorption and dissociation on Rh–Ni binary metals

The effect of Ni addition to improve the sulfur tolerance of a Rh catalyst for CO dissociation was studied using density functional theory (DFT) methods. Adsorption and dissociation were considered over the (1 1 1) surfaces of binary Rh 1Ni 2 and Rh 2Ni 1 metals with comparison to pure Rh and Ni surfaces. Sulfur adsorption on the Rh 1Ni 2(1 1 1) surface is 0.21 eV more endothermic than on the Rh(1 1 1) surface, suggesting that a Rh 1Ni 2 bimetallic catalyst has a higher sulfur tolerance than pure Rh catalysts due to a lower surface coverage of the sulfur poison. To compare catalytic activity in the presence of adsorbed sulfur, the CO dissociation rates over the binary and pure metals were calculated with 1/9 sulfur coverage. CO dissociation is fastest on the pure Rh surface under sulfur-free conditions, whereas among sulfur poisoned surfaces, the Rh 1Ni 2 surface shows the fastest CO dissociation rate. The CO dissociation barrier on Rh 1Ni 2 is destabilized less by a S coadsorbate than for the other metals. The addition of Ni atoms to a Rh catalyst improves the sulfur tolerance of the catalyst for CO dissociation by minimizing the repulsion between the adsorbed S atom and the CO dissociation transition state, as evidenced through a projected density of states analysis. The Rh 1Ni 2(2 2 1) stepped surface also shows a lower activation barrier and higher CO dissociation rate in the presence of sulfur than the Rh(2 2 1) stepped surface.

3d 및 4d 전이금속과 Pd가 c(2×2) 합금을 이룬 단층의 자성에 대한 제일원리 연구

Pd과 3d(V, Ti) 및 4d(Mo, Ru, Rh) 전이금속이 c(<TEX>$2{\times}2$</TEX>) 합금을 이루고 있는 단층의 자성을 제일원리적 FLAPW 에너지띠 방법을 이용하여 연구하였다. 이들 합금과 비교를 위하여 Pd과 V, Ti 및 Mo, Ru, Rh 원자로만 이루어진 단층의 전자구조도 계산하였다. 3d 원소인 V와 Ti 만으로 이루어진 단층의 경우, V은 반강자성, Ti의 경우는 강자성상태가 안정적이었으나, 이들이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이루었을 경우 모두 자기모멘트가 반대 방향을 가지는 준강자성 상태가 안정적이었다. 반면에 4d 원자인 Mo, Ru, Rh이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이룰 경우에는 자기모멘트들이 같은 방향을 가졌다. 자기모멘트 값을 보면, Ru이나 Rh의 경우 순수단층이나 Pd과 합금을 이룬 경우 그리 큰 차이를 보이지 않았으나, Mo의 경우 <TEX>$0.02\;{\mu}_B$</TEX>에서 <TEX>$2.98\;{\mu}_B$</TEX>로 급격히 증가하였다. 합금을 이루는 두 원소사이의 전하이동은 전기음성도에 따르게 됨을 알았다. We investigated the electronic structure and magnetism of the (3d, 4d)-Pd alloyed c(<TEX>$2{\times}2$</TEX>) monolayer systems, by use of the FLAPW band method. For comparison, pure 3d- and 4d-transition metal monolayers are also considered. We found that the antiferromagnetic configuration of pure V monolayers is sustained in the V-Pd alloy system, while the Ti-Pd alloy system is changed to antiferromagnetic configuration from the ferromagnetic state in pure Ti monolayer. The 4d TM (Mo, Ru, Rh)-Pd monolayers are found to be stable in ferromagnetic configurations. The magnetic moments of Ru and Rh atoms in Ru-Pd and Rh-Pd systems are almost same with those of pure Ru and Rh monolayers, while the magnetic moment of Mo atom is increased to <TEX>$2.98\;{\mu}_B$</TEX> in Mo-Pd alloyed system from the value of Mo monolayer, <TEX>$0.02\;{\mu}_B$</TEX>.

Temperature dependence of electrochemically promoted NO reduction by C 3H 6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C 3H 6–NO–O 2 reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated. Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C 3H 6 oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.

Analysis of the influence of rhodium addition to platinum on its activity towards methanol electrooxidation by EIS

Influence of rhodium addition to platinum on the activity of the alloy in methanol electrooxidation has been studied using Pt–Rh/Au limited volume electrodes with various surface compositions including the pure Pt and Rh metals. Electrochemical impedance spectroscopy (EIS) was used in the study. In the case of the Pt–Rh alloy, the impedance picture of methanol oxidation is qualitatively the same as for the pure Pt electrode. However, impedance spectra strongly depend on alloy composition. Equivalent circuits suitable for methanol oxidation on Pt were also used in the case of Pt–Rh and similar fitting results were obtained. A reaction mechanism suggested in the literature for Pt, which involves two strongly adsorbed intermediates competing for the same adsorption sites, is likely also for the Pt–Rh alloys. However, fittings with a corresponding impedance model were unsuccessful for both Pt and Pt–Rh because of mathematical caveats, such that quantitative comparisons were not possible. Nevertheless, EIS results suggest that Rh inhibits the kinetics of formation of reactive oxygen species at the surface of the alloy.

Asymmetric Enamide Hydrogenation Using Phosphinite Thioglycosides: Synthesis of d- and l-Aminoesters Using d-Sugars as Catalyst Precursors

Diastereomerically pure cationic Rh(I) complexes derived from phosphinite thioglycosides I were used as catalysts in highly enantioselective hydrogenations of enamides. The conformational similarity of alpha-D-arabinopyranose with beta-L-galactopyranose allows the synthesis of both enantiomers of alpha-amino acid derivatives such as D- and L-DOPA in excellent ee (97% and 98%), using derivatives of the former sugar as catalyst precursors.

Anodic oxidation of Pd alloys with Pt and Rh

Anodic oxidation of Pd alloys with Pt and Rh has been studied in acidic solution (0.5 M H 2SO 4) using cyclic voltammetry coupled with the electrochemical quartz crystal microbalance (EQCM). The influence of electrode potential and alloy composition on the processes of surface oxide formation/reduction and electrochemical dissolution of metals was investigated. For Pd–Rh alloys containing up to ca. 85–90% Pd on the surface the oxidation of electrode material starts at potentials 200–300 mV lower than on pure Pd or Pd–Pt alloys and 50–100 mV higher than on pure Rh. The formation of a monolayer of surface oxide is completed at ca. 1.36 V for Rh-rich Pd–Rh alloys, and ca. 1.46 V for Pd–Pt and Pd-rich Pd–Rh alloys. The EQCM results indicate that surface oxide formed on Pd alloys has MO stoichiometry. Both Pd and Rh dissolve during alloy polarization to the potential region of surface oxidation. The amount of dissolved metal is inversely proportional to scan rate and increases with the increase in electrode potential and Pd surface content in alloys. Electrochemical dissolution of metals from the alloys proceeds differently than from pure metal electrodes.