Rh Layers Research Articles

Steady-state multiplicity phenomena during the electrochemical promotion of NO reduction by C 2H 4 in presence of O 2 on thin Rh and Pt catalyst-electrodes in a monolithic electropromoted reactor

The electrochemical promotion of catalysis (EPOC) was used to promote the selective reduction of NO by hydrocarbons in presence of oxygen using thin (∼40 nm) porous Rh and Pt catalyst layers sputtered on the opposite surfaces of thin (0.25 mm) solid electrolyte (YSZ) plates serving as electrocatalytic elements of a monolithic electrochemically promoted reactor (MEPR). Using 22 Rh/YSZ/Pt type cells it was found that the reduction of NO in presence of 1.1 kPa O 2 and 0.36 kPa C 2H 4 can be efficiently electropromoted with 340% rate enhancement, reaching 95% NO conversion with 100% selectivity to N 2 in the temperature range from 280 to 340 °C. The apparent Faradaic efficiency is larger than unity for both the NO reduction and the C 2H 4 oxidation reaction. At elevated temperatures (≥300 °C) and high reactant conversions it was found that after current interruption, the catalytic rates do not return to their initial values but remain in a new highly active steady state. It appears that this highly active state is not a genuine intrinsic permanent NEMCA state but is manifestation of steady-state multiplicity in the monolithic reactor resulting from near complete gaseous O 2 consumption. Thus the low and high activity steady states corresponding to zero applied potential appear to correspond to high and low average P O 2 in the reactor. The latter is the result of the near complete reactant conversion under the preceding electropromoted operation. These highly active permanent NEMCA states may be quite useful for practical applications.

Growth and sacrificial oxidation of transition metal nanolayers

Growth and oxidation of Au, Pt, Pd, Rh, Cu, Ru, Ni and Co layers of 0.3–4.3 nm thickness on Mo have been investigated with ARPES and AFM. Co and Ni layers oxidize while the Mo remains metallic. For nobler metals, the on top O and oxidation state of subsurface Mo increase, suggesting sacrificial e − donation by Mo. Au and Cu, in spite of their significantly lower surface free energy, grow in islands on Mo and actually promote Mo oxidation. Applications of the sacrificial oxidation in nanometer thin layers exist in a range of nanoscopic devices, such as nano-electronics and protection of e.g. multilayer X-ray optics for astronomy, medicine and lithography.

Electrochemical behavior of thin polycrystalline rhodium layers studied by cyclic voltammetry and quartz crystal microbalance

Electrochemical behavior of thin polycrystalline Rh layers has been studied in 0.5 M H 2SO 4 solution by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The properties of surface oxide formed on freshly electrodeposited Rh are different than on electrochemically aged Rh. The analysis of frequency changes in both hydrogen and oxygen regions is presented. It is suggested that hydrogen desorption occurs simultaneously with the adsorption of HSO 4 − ions, whose maximum surface coverage reaches ca. 8%. The EQCM results indicate that RhO is the main species formed during Rh surface oxidation. Metal dissolution proceeds during electrode cycling to sufficiently high potentials. The amount of dissolved metal increases with an increase in potential and a decrease in scan rate.

First-principles study of the electronic structure of heavy fermionYbRh2Si2

The electronic properties of YbRh2Si2 are studied by means of band structure calculations based on thedensity functional theory within the LDA (local density approximation),LDA+U, and fully relativistic scheme. The 4f derived bands are studied within a relativisticframework which yields flat and spin–orbit split bands, and a correlated band method(LDA+U) that includes correlation corrections. In both cases the uppermost4f band is located roughly 150 meV below the Fermi energyEF, and hybridizes weakly with the dispersive Rh 4d bands and a Rh4dxy band that does not disperse perpendicular to the Rh layers. When weapplied the fully relativistic scheme, the 4f derived bands split intoj = 7/2 and 5/2 excitations due to spin–orbit coupling effects. Thef7/2 multiplet is locatedmuch closer to EF, hybridizing anisotropically with a Rh 4d derived conductionband. This mixing induces a non-integer occupation of the f levelsnf and hencereveals that YbRh2Si2 is a mixed-valence heavy fermion. The 4f electrons can be delocalized through the hybridization withconduction electrons. And the hybridization between f and conduction d electrons plays a importantrole in YbRh2Si2. The Fermi surface splits into three different non-touching sheets. There are two tinycylinder Fermi surfaces around Z points which are from the Rh 4d bands and the largerFermi surfaces are from the 4f derived bands.

Spin polarization, structure and chemical composition of the Rh/Fe(1 0 0) interface

We report on spin- and energy-resolved electron emission induced by grazingly scattered 100 keV protons from ultrathin Rh layers on Fe(1 0 0). A finite spin polarization of emitted electrons is observed up to a nominal coverage of 3 Rh layers. The growth of Rh at room temperature is studied by grazing scattering of 50 keV He + ions. We find that growth of Rh on Fe(1 0 0) deviates from perfect pseudomorphic layer-by-layer growth. The chemical composition is investigated by Auger electron spectroscopy using 5 keV electrons and grazingly scattered 200 keV protons.

Ni 단층이 삽입된 Rh 박막의 전자구조와 자성

A single slab in which one Ni(001) atom layer embedded between two of four Rh layers is considered to examine the oscillation of magnetic moment in each layer. The all electron total-energy full-potential linearized augmented plane wave(FLAPW) method was used to calculate the spin densities, magnetic moments, density of states(DOS), and the number of electrons within each muffin-tin(MT) sphere. The magnetic moment of the center layer Ni(C) in the system of 4Rh/Ni/4Rh is calculated to be 0.34, which is 40% have magnetic moment at the interface layers by strong band hybridization with Ni(C) when Ni(001) monolayers is inserted, and the magnetic moment shows a damped oscillation as we go from center Ni(C) layer to the surface Rh(S). From the calculated density of states, it is found that the Fermi level shifts inside the energy band of the Ni(C) in affection of Rh(001).

Mössbauer study of magnetism of Rh layers in Ag(0 0 1)/Rh/Sn/Rh superlattice structures

Abstract The magnetism of Rh(0 0 1) layers epitaxially grown on Ag(0 0 1) in multilayered structures [Ag(20 A)/Rh( t )/Sn(2 A)/Rh( t )] ×30 /Ag(50 A), t =2, 4 A was studied using the 119 Sn monatomic layers as a Mossbauer probe. The crystallographic structure of the multilayers was examined by X-ray diffraction in the high and low angle regions. The 119 Sn Mossbauer spectra show that the Rh(0 0 1) layers in the grown multilayers are nonmagnetic at room temperature.

Ｃｏ／Ｒｈ（１１１）多層膜の層間磁気結合と磁気異方性

It is known that a Co/Rh multiplayers system show strong interlayer exchange coupling and perpendicular magnetic anisotropy. In this study, we fabricated two series of Co/Rh(111) multilayers: one with a wedge-shaped Rh layer and a Co layer of constant thickness, and the other with Rh layers constant thickness and wedge-shaped Co layers. The former were used for investigation of the interlayer coupling and the latter for investigation of magnetic perpendicular anisotropy. For the Rh wedged multilayers, spin flip was observed in the M-H curve, indicating the coexistence of anti-ferro magnetic coupling and perpendicular anisotropy. The Co wedged multilayers exhibited the perpendicular magnetic anisotropy for Co layer thicknesses less than 12 A. From the Co layer thickness dependence of the magnetic perpendicular anisotropy, we found that the perpendicular anisotropy of Co/Rh (111) can be dominated by the strain produced by lattice mismatch.

AES and TDS study of CO interaction with Rh layers on Al substrate

We have evaporated Rh atoms onto polycrystalline aluminium substrate and studied the surface composition by Auger electron spectroscopy (AES). We have also explored the adsorption of CO molecules by thermal desorption spectroscopy (TDS). Both AES and TDS results exhibit dramatic changes during the sample heating and CO adsorption/desorption experiments. A Rh–Al alloy is probably formed on the surface of aluminium covered by the Rh deposit if the sample is annealed at 600K. Activation energy of CO desorption is found lower than on Rh particles on Al2O3. The aluminium Auger peak shift and the oxygen Auger peak increase indicate the temperature stimulated sample oxidation. This oxidation causes an increase of the activation energy of CO desorption with respect to that observed previously on Rh particles on an Al2O3 substrate.

Morphology and magnetic properties of thin films of Rh on highly oriented pyrolitic graphite

The structure and magnetic properties of ultrathin Rh layers deposited on highly oriented pyrolitic graphite (HOPG) have been investigated by means of core-level photoemission and scanning tunneling microscopy. The Rh growth on HOPG follows the Volmer-Weber mode at 300 K for any coverage, while Rh may form a commensurate $p(1\ifmmode\times\else\texttimes\fi{}1)$ ordered structure when deposited at 150 K for a coverage up to one monolayer. For submonolayer or monolayer Rh films on HOPG the linear magnetic dichroism in the angle distribution of photoelectrons shows no evidence of in-plane long-range magnetic ordering.

New Strategies for Surface-Enhanced Raman Scattering at Transition-Metal Interfaces: Thickness-Dependent Characteristics of Electrodeposited Pt-Group Films on Gold and Carbon

The surface-enhanced Raman scattering (SERS) properties of rhodium films electrodeposited onto a SERS-active gold substrate are examined as a function of overlayer thickness, using carbon monoxide and thiocyanate as chemisorbate probes in aqueous solution. Although the adsorbate Raman signals obtained for ultrathin (≤10 monolayers, ML) Rh layers emanate from the gold template, a significant or even dominant SERS component for thicker films is identified as arising instead from the transition-metal overlayer itself. The latter contribution was diagnosed and characterized from similar experiments undertaken using SERS-inactive gold and carbon substrates, and from the observation of comparable Raman enhancements obtained with green (514.5 nm) as well as red (632.8 nm) excitation. While the surface enhancement factors for these layers, of the order of 103, are markedly less than those obtained for ultrathin films on SERS-active gold, the signals even for very weak Raman scatterers (such as adsorbed hydrogen) ...

Growth and thermal stability of Fe, Ni, Rh and Pd layers on the (111)W crystal face

LEED, AES and Δ φ measurements were used to investigate the growth of Fe, Ni, Rh and Pd layers on the tungsten (111) surface. The thermal stabilities of the layers and of the substrate were also examined. Both were found to be thermally unstable at coverages above one monolayer. In addition, Rh and Pd were found to cause faceting of the substrate surface. In thick Fe layers, a non-wetting-wetting transition was observed.

Laser Raman spectroscopy – a powerful tool for in situ studies of catalytic materials

Advantages and limitations of laser Raman spectroscopy (LRS) as an in situ vibrational spectroscopy for the study of catalytic materials and surfaces under working conditions are discussed. Measurements can be carried out at temperatures as high as 1200 K in controlled atmospheres. Modern instrumentation permits time resolutions in the sub‐second regime for materials with high Raman cross sections. Transient studies are thus possible. Several examples are presented of in situ LRS studies including the phase analysis of bismuth molybdate and VPO oxidation catalysts, synergy effects and oxygen exchange in Sb2O3/MoO3 oxide mixtures, intermediates in oxidative coupling of methane, NO decomposition on Ba/MgO catalysts, and transient SERS studies of partial oxidation of methanol on Ag single crystal surfaces and of the reduction of oxide overlayers on electrodeposited Rh layers.

Interfacial polarisation effect on the interlayer couplings in Co/Rh sandwiches

The structural, magnetic and transport properties of Co/Rh sandwiches grown by ultra high vacuum evaporation and sputtering have been studied. High-energy electron diffraction observations during the growth reveal that both Co and Rh layers have been stabilised in the (111) fcc structure for the evaporated sandwiches. X-ray measurements performed on sputtered samples show a predominant fcc polycrystalline structure of the stacks with a preferential (111) texture. Magnetisation and magnetoresistance measurements show a very strong antiferromagnetic exchange coupling for thin Rh layers, reaching for 4.8A Rh, the strongest ever observed in exchange coupled systems. This value is in good agreement with the value of obtained by ab initio calculations for Co/Rh (hcp) superlattices. This is explained by the magnetic nature of the Co/Rh interfaces. Indeed, the variation of the measured saturation magnetisation as a function of the Co layer thickness shows no evidence of Co moment reduction for the Co atoms located at the interfaces, even for the very thin layers. The value of the preserved magnetic moments of the cobalt atoms at the interfaces is confirmed by ab initio calculations for Co/Rh superlattices taking the intermixing into account.

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.

The structure of (√3 × √3)R30°-sulphur on Rh(111) surface studied by X-ray standing wavefield absorption and surface EXAFS

The surface structure of (√3 × √3)R30°-S-Rh(111) has been investigated by normal incidence X-ray standing wavefield (NIXSW) absorption and surface-extended X-ray absorption fine structure (SEXAFS). NIXSW measurements show that the most likely site of adsorption is at the threefold “fcc” hollow, with some lateral disorder. The location of the S atoms at the “fcc” hollow site is consistent with S adsorption on the neighbouring fcc (111) transition metal surfaces. The coherent position relative to the Rh(111) lattice plane was measured to be 1.73±0.05 A ̊ . SEXAFS analysis revealed an S-Rh nearest neighbour bond distance of 2.25±0.04 A ̊ . The combined results of the NIXSW and SEXAFS measurements suggest that there is a net outward surface relaxation of the topmost Rh(111) layers of 0.1±0.05 A ̊ .

Induced magnetism of 4d transition metals: Rh and Ru/Fe(001) overlayers and sandwiches

The structural, electronic, and magnetic properties of mono and bilayer Rh and Ru transition metals deposited on a Fe(001) substrate or sandwiched by an additional Fe layer have been determined by using the highly precise local density full-potential linearized augmented plane-wave method. Significantly, the atomic force determinations demonstrate that a large interfacial relaxation plays an important role in their electronic and magnetic properties. Both mono and bilayer Ru and Rh in the systems considered are found to be able to retain large spin magnetic moments. The ground state of bilayer Rh is found to have its moments coupled antiferromagnetically. In good agreement with x-ray magnetic circular dichroism (MCD) measurements, the deposition of Rh layers on Fe(001) is found to enhance the Fe spin magnetic moments in the interface. Calculated values of the spin and orbital parts of the magnetic moments and the MCD spectra are presented, and the role of overlayer-substrate hybridization is discussed.

Giant antiferromagnetic exchange coupling in ultrahigh-vacuum grown (111) Co/Rh sandwiches

A study of the exchange coupling of Co/Rh/Co sandwiches grown by ultrahigh-vacuum evaporation on mica substrata is presented. In these sandwiches, both Co and Rh layers have a (111) fcc structure, as indicated by the RHEED pattern observed during growth. Oscillatory indirect exchange coupling and magnetoresistance as a function of the Rh thickness a observed. The most interesting result is the very strong antiferromagnetic exchange coupling reached for thin Rh space layers: 34 erg/cm 2 for 4.8 Å Rh. We attribute this result partly to the nature of the magnetic/non-magnetic interfaces shown by magnetisation measurements. Indeed, no reduction in the Co moment is found for these sandwiches although the is a large interdiffusion is expected at their interfaces. This is confirmed by ab initio calculations of the magnetisation with mixed interfaces. Such a magnetic nature of the interfaces should be favourable for electronic confinement in the space layer.

Magnetic configurations of rhodium

Two configurations are, a priori, favorable for a possible magnetic ground state of rhodium: small clusters and very thin films. In both cases, the reduction of the average coordination number could allow the onset of magnetism. For clusters, experimental works indicate that Rh N clusters with N60 have a non-zero average magnetic moment. By using a d-band Hubbard Hamiltonian, the size and the geometric dependence of the magnetic properties of Rh clusters is studied. Different structures and atomic relaxations are considered. Significant magnetic moments are found up to N50. For thin films of rhodium on silver, the Ag/Rh interdiffusion inhibits a long range magnetic order. The only favorable case is to consider a magnetic substrate. For thin Rh layers on Fe(001) only mono- and bi-layers are polarized. For larger thicknesses, only an interfacial polarization remains. Finally, in agreement with other studies, the Rh semi-infinite crystal does not display a local magnetic moment.

Tensor LEED analysis for the Rh(110)-(3 × 2)-S surface structure

A tensor LEED analysis for the Rh(110)-(3 × 2)-S surface supports an arrangement of chemisorbed S atoms at 23 monolayer coverage on a basically unreconstructed metallic structure. Alternating [11̄0] channels in the metal surface are occupied differently, although each has two S atoms per unit mesh. In one set, the S atoms occupy long-bridge and centre sites with a constant separation along the channel of 4.03 Å. In the other set of channels, all S atoms occupy equivalent positions, displaced from regular centre sites by 0.39 Å, to give successive S to S separations of 3.47 and 4.60 Å. The long-bridge site bonding is a novel feature which is facilitated by the neighbouring topmost Rh atoms relaxing laterally by about 0.27 Å perpendicular to the [11̄0] row. Bucklings of 0.20 and 0.10 Å are indicated to occur in the first and second Rh layers respectively; the latter value essentially equals that (0.11 Å) reported earlier for the corresponding half monolayer c(2 × 2) surface. For S atoms at or near centre sites in the (3 × 2) structure, the average S-Rh bond distances are 2.42 and 2.29 Å to the first and second layer Rh atoms respectively; the corresponding values at the long-bridge sites are 2.20 and 2.27 Å.