Rh Monolayers Research Articles

Mechanistic study of methanol synthesis from CO2 hydrogenation on Rh-doped Cu(111) surfaces

Self-consistent periodic density functional theory (DFT) calculations were employed to explore the adsorption and reaction mechanisms of CO2 hydrogenation to methanol via the reverse water-gas-shift pathway on Cu(111) and three RhCu(111) surfaces with different doped-Rh atoms on Cu(111) surfaces, denoted as Rh3Cu6(111), Rh6Cu3(111) and Rh ML surfaces. All the possible favored adsorption sites, structures and adsorption energies of the relative species on Cu(111) and RhCu(111) were determined. H2CO*, HCO*, CO*, cis-COOH* and trans-COOH* adsorbing at Rh atoms through C atom are strengthened by the doped Rh, and the adsorption energies of species on Cu sites through O atom are not altered obviously, which can be explained by d-PDOS analysis. Through the analyses of Brønsted-Evans-Polanyi (BEP) relationships, it is found that the adsorption of trans-COOH* and co-adsorbed CO*/H* is stronger on RhCu(111) surfaces, while the energy of CO* in the TS geometry is dramatically decreased by doping with Rh atoms, leading to the alternation of rate-limiting step from CO2 hydrogenation forming trans-COOH* on Cu(111) and Rh3Cu6(111) surfaces to CO hydrogenation forming HCO* on Rh3Cu6(111) and Rh ML (monolayer) surfaces. The order of the highest activation barriers for the overall reaction is Cu(111) > Rh6Cu3(111) > Rh ML > Rh3Cu6(111). Moreover, due to the dissociative adsorption of H2, the yield of CO byproduct is suppressed. The calculated results show that the overall reaction process of CH3OH synthesis is facilitated kinetically and thermodynamically, especially on Rh3Cu6(111) surface. The present insights are helpful for the rational design and optimization of Rh-Cu bimetallic catalysts used in the process of CO2 hydrogenation to CH3OH synthesis.

Formation of Hexagonal-Close Packed (HCP) Rhodium as a Size Effect.

Previous studies on the structural and functional properties of rhodium are based on the face-centered-cubic (fcc) structure in the bulk form. Here we report the first discovery of the hexagonal-close packed (hcp) rhodium in the nanoparticle form. The hcp Rh can be directly synthesized by solvothermal reaction or by electron-beam induced decomposition of Rh monolayers. The hcp Rh nanoparticles are stable under electron beam irradiation. Compared with the fcc structure, the hcp Rh nanoparticles show a large lattice expansion (6% larger atomic volume). The first-principles calculations suggest that the lower surface energy of hcp Rh leads to the size effect in the crystal structure.

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.

Large perpendicular magnetic anisotropy of ultrathin Ru and Rh films on a NiAl(001)surface

Using the full potential linearized augmented plane wave (FLAPW) method, the magneticproperties of two-dimensional Ru and Rh monolayers (MLs) on a NiAl(001) surface havebeen investigated. It has been found that free standing one monolayer Ru andRh films have ferromagnetic ground state with magnetic moments of 2.21 and1.48 μB, respectively. The ferromagnetism is still observed even on a Ni terminatedNiAl(001) surface, while no magnetic state is found on an Al terminatedsurface. The calculated magnetic moments of Ru and Rh atoms are 1.56 and0.88 μB, respectively. In addition, an induced magnetic moment in surface Ni is observed. Ithas been found that the free standing Ru film has perpendicular magnetizationto the film surface with a magnetocrystalline anisotropy (MCA) energy of0.66 meV/atom, while an in-planeMCA energy of 0.37 meV/atom is achieved in Rh film. Very interestingly, we find that both Ru/NiAl(001) andRh/NiAl(001) films have perpendicular magnetic anisotropy and the calculated MCAenergies are 0.66 and 1.11 meV in Ru/NiAl(001) and Rh/NiAl(001), respectively. Alongwith the magnetic anisotropy, we have presented theoretically calculated x-rayabsorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) results.

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>.

Effective coordination number: A simple indicator of activation energies for NO dissociation on Rh(100) surfaces

We have used density-functional theory to compute the activation energy for the dissociation of NO on two physical and two hypothetical systems: unstrained and strained Rh(100) surfaces and monolayers of Rh atoms on strained and unstrained MgO(100) surfaces. We find that the activation energy, relative to the gas phase, is reduced when a monolayer of Rh is placed on MgO, due both to the chemical nature of the substrate and the strain imposed by the substrate. The former effect is the dominant one, though both effects are of the same order of magnitude. We find that both effects are encapsulated in a simple quantity which we term as the 'effective coordination number'(n{sub e}); the activation energy is found to vary linearly with n{sub e}. We have compared the performance of n{sub e} as a predictor of activation energy of NO dissociation on the above-mentioned Rh surfaces with the two well-established indicators, namely, the position of the d-band center and the coadsorption energy of N and O. We find that for the present systems n{sub e} performs as well as the other two indicators.

Correlations between magnetic properties and bond formation in Rh–MgO(0 0 1)

We present the results of first principles calculations for the magnetism of Rh adlayers on MgO(0 0 1), at three different adsorption sites and three different coverages, corresponding to 1, 1/2 and 1/8 monolayers. Finite magnetization is found in all cases except that of one Rh monolayer above the oxygen site, which is also the most stable. We examine how the magnetization changes as a function of the Rh–surface distance and relate this to changes in the real-space charge density and in the density of states (DOS) as the Rh adlayer interacts with the surface. We find that increasing either the Rh–Rh interaction strength or the Rh–surface interaction strength leads to reduced magnetization, while increasing the former drives a crossover from localized (atomic) to itinerant magnetism. Neither the magnetic transition itself, nor the localized-to-itinerant magnetism crossover, is found to be directly related to the formation of Rh–surface bonds. From a practical point of view, we predict that magnetism in the Rh–MgO(0 0 1) system is most likely to be found experimentally at reduced coverages and at low temperatures.

Preparation and Electrochemical Behavior of Ordered Rh Adlayers on Pt(100) Electrodes

Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 V(RHE) in 0.1 M H(2)SO(4). In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 x 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.

Structural Changes of the W(211) Surface Induced by Ultrathin Films of Rh, Pt, and Pd

A W(211) surface covered with a thin film (between 0.5 and 1 physical monolayer) of Rh, Pt, or Pd is found to exhibit an n × 1 superstructure when annealed above a threshold temperature of ∼900 K (500 K for Pd). The superstructure is observed using low energy electron diffraction; phase diagrams are presented to indicate the coverage range and the temperature threshold where the new structure appears. Scanning tunneling microscopy results indicate that in the case of Pd/W(211) the superstructure phase is due to missing overlayer rows. In the case of Rh/W(211), there are two possible interpretations to the observed periodic structure: either the adlayer forms an ordered superstructure or the W surface itself forms a microfaceted structure with {110} faces. On the basis of our STM observations of gas adsorption on the surface, we found the explanation involving microfacets more plausible. A careful calibration of overlayer coverage is performed using a quartz crystal monitor combined with temperature programmed desorption measurements performed at high temperatures (up to 2400 K). Auger electron spectroscopy used in combination with thermal annealing series demonstrates that overlayer coverages above one physical monolayer are thermally unstable, which is explained by the formation of an ultrathin alloy film.

FIRST PRINCIPLES TOTAL ENERGY STUDY OF THE MORPHOLOGICAL INSTABILITY OF Mo(111) INDUCED BY ULTRATHIN FILMS OF Rh

By using first principles total energy calculations, the surface energies of (100), (110), (111) and (211) surfaces of Mo before and after adsorbing ultrathin films of Rh have been presented. The results show that in the case of clean Mo surfaces Mo(111) is stable against faceting to Mo(211), Mo(110) and Mo(100), while after adsorbing one physical monolayer of Rh, the surface energies of all the four surfaces decrease, and Mo(111) can facet to {211}, but not to {110} and {100} surfaces.

The Magnetism and Electronic Structures of Rh Monolayer

Recent Auger electron spectroscopy experiment suggested the possibility of magnetism in Rh overlayer on Cu(001). This experimental result was verified by our theoretical study on Rh monolayer on Cu(001). However the calculated total energy difference between para- and ferromagnetic states was found to be very small and it calls for the investigation of another state, i. e., the antiferromagnetic state. The dependence of total energies on the lattice constant was calculated for three magnetic states, i. e., para-, ferro-, and antiferromagnetic ones. It was found that there is no meaningful energy difference between para- and antiferromagnetic states for all the lattice constant. The ferromagnetic state is most stable compared to other states for the lattice constant greater than 6.68 a.u. The energy minima was found at the lattice constant of 6.57 a.u. where it is paramagnetic.

Magnetism of epitaxial Ru and Rh monolayers on graphite

Recently, Pfandzelter et al. (1995) reported the first observation of monolayer ferromagnetism of a 4d metal, namely in a Ru monolayer grown on graphite. Using the tight-binding linear-muffin-tin-orbital (TB-LMTO) method we have calculated the electronic and magnetic structure of epitaxial Ru and Rh monolayers on graphite with the experimentally determined atomic density. Monolayers of the other 4d elements were found to be non-magnetic already in the free-standing limit. The magnetic structure of the Ru and Rh monolayers is studied as a function of metal-graphite interlayer distance h. They become magnetic at h = 4.5 a.u. (Ru) and h = 4.8 a.u. (Rh) in a first-order transition. In the assumed p(2 × 2) super-structure, the moments on the “hollow” site atoms are up to four times bigger than those on the “on-top” site atoms. For h > 5.4 a.u. (Ru) and h > 5.1 a.u. (Rh) the site dependence vanishes and the moments of the free monolayers are approximately reached (1.9 μB and 1.2 μB, respectively).

Possible magnetism of Rh as an overlayer on Cu(0 0 1): A first-principles study

Possible two-dimensional magnetism of a Rh monolayer on the Cu(0 0 1) substrate was investigated using the full-potential linearized augmented plane-wave (FLAPW) method. The equilibrium interlayer distance between Rh and the adjacent Cu layer was determined through total-energy calculations (3.58 a.u.). We find that the Rh monolayer on Cu(0 0 1) can be magnetic, as suggested in a recent experiment. However, the total energy difference between the ferromagnetic (FM) and paramagnetic (PM) states is found to be very small and, hence, only observable at very low temperature. The calculated FM moment of Rh, 0.23 μ B, on Cu(0 0 1), is considerably smaller than that (0.96 μ B) on Ag(0 0 1) due to the much smaller lattice constant in the lateral plane and, hence, the stronger band hybridization between the Cu and Rh layers.

Theoretical approaches of magnetism of transition-metal thin films and nanostructures on semi-infinite substrate

Tremendous progress has been made in the field of magnetic materials and technology over the past few years. Superior properties and novel scientific questions arise from our ability to either synthesize artificial structures or tailor microstructures at the appropriate length scale. The study of low-dimensional magnetism of thin films, surfaces and interfaces has revealed new and sometimes unexpected properties. In this review we focus on the determination of the local magnetic moments distribution at the surface of transition metals, for very thin transition-metal films and nanostructures on noble and transition-metal substrates at T = 0 K. Two different approaches are commonly used. Ab initio and semi-empirical tight-binding (TB) calculations appear to be complementary to determine the electronic structure. The first provides very precise results for periodically ordered systems, whereas the second is able to study complex systems with a large number of non-equivalent sites. We discuss these approaches and point out recent developments which combine the versatility of the ab initio method with the rapidity of the TB calculation. Special attention is devoted to the magnetic frustration which occurs when both ferromagnetic and antiferromagnetic couplings are present. The Fe Cr interface which has been, since 10 years, one of the most studied systems is a good example. Recent experimental and theoretical efforts have addressed the question of a possible onset of ferro- or antiferromagnetism in nanostructures, free-standing clusters, thin-film structures of metals that are non-magnetic in the bulk form. V, Pd, Rh and Ru have been singled out as potential candidates. Thus particular attention will be devoted to V free-standing films, as well as V monolayer on Ag and Fe. Rh and Ru have been found magnetic experimentally in the form of cluster (Rh) and for Ru monolayer on graphite. However, Rh monolayer on Ag is found experimentally non-magnetic whereas calculations show that magnetism is killed when surface alloy is considered. Pd is another important element to consider because, being non-magnetic in the atomic form as well as in bulk form it presents high susceptibility. Therefore, it can become magnetic if its lattice parameter is slightly increased. It is believed to be magnetic in the case of a bilayer on Ag(0 0 1). Induced polarization is obtained when Pd is in contact with a magnetic metal. Because of its exotic structural and magnetic properties, Mn is an interesting candidate for thin-film growth as it is expected to accept different local configurations. Experimentally, one may attempt to stabilize normally high-temperature phases of Mn by epitaxial growth on a suitable substrate. We will show that Mn takes on the c(2 × 2) magnetic configuration for the Mn monolayer on Fe(0 0 1) and Co(0 0 1). Calculations show that Ir and Pt, in the monolayer range, are magnetic when grow on Ag. However, no experiment has displayed any kind of magnetism yet and more realistic calculation has shown that spin-orbit effect kills the magnetic moment. Therefore, only V, Cr, Mn, Co and Ni for 3d, and Ru, Rh and Pd for 4d will be considered in this review.

Magnetism of Rh and Ru atoms, clusters, and monolayers on Au and Ag surfaces

The magnetic character of Rh and Ru on the surface of Au and Ag is investigated by means of weak localization (WL) and the anomalous Hall effect (AHE). Dephasing of the conduction electrons in WL is the most sensitive method to detect magnetic surface impurities. The Au/Rh and Au/Ru systems are investigated in the range between 1/100 of a monolayer of Rh (Ru) and several monolayers of Rh (Ru). We observe a small dephasing by Rh clusters and no dephasing by single Rh surface impurities, while single Ru surface impurities cause already a finite but small dephasing. We suggest that the dephasing is caused by small fluctuating magnetic moments of the order of 0.1 (0.4)${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. For a coverage of one monolayer of Rh we observe a maximum in the dephasing. However, we do not observe a ferromagnetic Rh monolayer as predicted in a number of theoretical papers. Similarly, a Ru monolayer on the surface of Au or Ag does not show ferromagnetism. The absence of an AHE in the Rh and Ru monolayers excludes ferromagnetism as well. For Rh the dephasing increases again when the Rh coverage exceeds two monolayers, suggesting that pure bulk Rh shows spin fluctuations.

Molecular Aspects in Syn-Gas Production: The CO 2-Reforming Reaction Case

CO 2-reforming reactions have been studied in a plug-flow reactor with catalysts containing less than a monolayer of Rh, Ru, or Ir deposited from tetrametallic dodecacarbonyl clusters onto low surface area polycrystalline α-Al 2O 3, MgO, CeO 2, La 2O 3, and TiO 2. The catalytic reactions are studied in conditions at which the carbon formation quickly deactivates Ni-based catalysts but does not deactivate noble metal based materials. Diffuse-reflectance-infrared-Fourier-transform (DRIFT) spectroscopy and mass spectrometry experiments inside a high-temperature high-pressure (HTHP) chamber have cast light on molecular aspects of the CO 2-reforming reaction which have been embodied in a reaction mechanism. The formation of highly reactive oxidic species from the dissociation of CO 2 is showed, and their role in inhibiting the production of large carbon aggregates is discussed.

POLARIZATION OF DIFFERENT INTERFACES OF Rh AND Ag STUDIED BY RECURSION TECHNIQUES

We investigate the reasons for the discrepancy between experimental and theoretical findings concerning the onset of magnetic moment when Rh , which is nonmagnetic in the bulk form, is coated on Ag (001). To perform the calculation we use a recently developed recursion method in the reciprocal space for the derivation of the density of states of multilayered systems. The local magnetic moments are deduced versus the exchange integral J. We found that a single-monolayer of Rh epitaxially grown on Ag (001) presents a high magnetic moment. This magnetic moment decreases when the Rh monolayer is covered by a Ag overlayer. However, the magnetic moment is not entirely killed as obtained also previously by recent full-potential linearized-augmented-plane-wave calculations. Therefore, there seems to be, discrepancy with recent surface-magneto-optic-Kerr effect experiments which have not detected any long-range order in the case of a monolayer of Rh on Ag (001). We study intermixing effects between Rh and Ag using different model structures containing ordered Ag .50 Rh .50 layers. The disappearance of magnetism in experimentally studied samples can be explained by such intermixing effects.

Ferromagnetism of imperfect ultrathin Ru and Rh films on a Ag(001) substrate.

We present a systematic study of the influence of various types of structural imperfections on the magnetic properties of ultrathin transition-metal films on noble-metal substrates. As a case study, we show by means of ab initio electronic structure calculations that recently predicted ferromagnetism of perfect Ru and Rh monolayers on a Ag(001) substrate is highly sensitive to noninteger coverage by additional Ru or Rh atoms and overlayer-substrate interdiffusion. Both types of structural imperfections strongly reduce the local magnetic moments and lead, in some cases, to a complete extinction of the ferromagnetic state.

Metal-ceramic interfaces: Overlayer-induced reconstruction and magnetism of 4d transition-metal monolayers.

Structural, electronic, and magnetic properties of metal-ceramic interfaces, M/MgO(001) (M=Pd, Rh, and Ru), have been investigated using the full potential linearized augmented-plane-wave method. Ru and Rh monolayers are found to be able to retain large spin magnetic moments on MgO(001) (1.95${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and 1.21${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for Ru and Rh, respectively)---indicating, in principle, the potential application of MgO(001) as a benign substrate for 4d monolayer magnetism. Significantly, according to our atomic-force determinations, the metal overlayers induce a sizable buckling reconstruction in the interfacial MgO layer, which enhances the M-MgO binding energy by 0.1 eV. The weak M-O interaction is mainly via tail effects; however, it affects the density of states at the Fermi level for Pd/MgO(001) significantly and completely eliminates the small magnetic moment of the free Pd monolayer (0.34${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$).

Magnetism of Rh islands and ordered Rh 0.5Ag 0.5 alloy on Ag(001)

Theoretical calculations show a high magnetic moment for Rh overlayer on Ag(001) which is reduced when it is coated by an Ag layer. Surface magneto-optic Kerr effect (SMOKE) and spin and angle-resolved photoemission did not detect any spin polarization for epitaxial Rh monolayers on Ag(001) and Au(001). The influence of the real structure of the overlayer on the magnetic properties of rhodium on Ag(001) is discussed.