Rh Foil Research Articles

Catalytic Etching and Oxidation of Platinum Group Metals: Morphology, Chemical Composition and Structure of Pt, Pd and Rh Foils in the O2 Atmosphere and during NH3 Oxidation with Air at 1133 K

Platinum alloy gauzes are employed for the high-temperature oxidation of NH3 to NO used in the industrial production of HNO3 for application in agricultural fertilizers. To enhance the efficiency of NH3 oxidation, various Pt–Pd–Rh alloys are employed for the production of such catalytic gauzes. To understand the role of these metals in NH3 oxidation, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction were applied to investigate the morphology, composition and structure of Pt, Pd and Rh foils after annealing in O2 and oxidation of NH3 with air at 1133 K. After annealing in O2 and NH3 oxidation, the metallic microgranular structure was detected on Pt(poly), whereas oxide layers of Rh2O3 and PdO were observed on Rh(poly) and Pd(poly). At the onset of NH3 oxidation (t = 1 h), fibrous metal-oxide agglomerates of nanofibers formed on these oxide layers. The long-term (5–10 h) oxidation of NH3 led to the formation of a continuous layer of pyramidal crystals on Rh(poly) and palladium “cauliflowers” on Pd(poly). The highly exothermic reaction of NH3 with oxygen on metals and PdO or Rh2O3 initiates strong catalytic etching forming grains and facets on Pt, fibrous metal-oxide agglomerates, pyramidal crystals and metallic “cauliflowers” on Rh and Pd.

Pattern Formation in Catalytic H2 Oxidation on Rh: Zooming in by Correlative Microscopy.

Spatio-temporal nonuniformities in H2 oxidation on individual Rh(h k l) domains of a polycrystalline Rh foil were studied in the 10–6 mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations.

Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching

Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching

Tailoring the Pt Surface Oxophilicity Via Single-Atom Rh Doping for Boosting Hydrogen Oxidation/Evolution Reaction in Alkaline Electrolyte

Recently, the rapid development of the anion exchange membrane (AEM) shows a promising future in the commercialization of low cost and long-term stable AEM based fuel cells and water electrolyzers. AEM features an alkaline electrolyte, where non-precious metal ORR catalysts possess high ORR activity and can replace Pt catalysts. However, Pt is still the best selection for HOR catalysts in the alkaline medium. The HOR activity of Pt is highly pH-dependent and decreases over two orders of magnitude when pH increases from 1 to 14. Thus, the sluggish anodic HOR reaction becomes the most critical issue in the development of AEMFCs. The major reason for the slow HER/HOR process in the alkaline electrolyte is ascribed to the positive shift of potential of zero free charge (PZFC) while pH value goes up from pH = 1 to 14. As PZFC shifts to a more positive value, it becomes more difficult for OHads to get adsorbed on the Pt surface to facilitate the water dissociation or the removal of Hads. Tailoring Pt surface with a second transition metal hydroxides has been proved to be an effective method to enhance the HOR activity on Pt, but at the sacrifice of tremendous active surface Pt sites (~50%), leading to the waste of electrochemical surface area (ECSA) and the decrease of the maximum mass activity (MA). Herein, taking advantage of the single-atom tailoring strategy, we have achieved single Rh atom tailoring of Pt nanowires for the optimization of surface oxophilicity for OH adsorption to boost the HER/HOR activity in the alkaline condition.The HOR activity of PtRh nanowires is highly sensitive to the oxophilicity of Pt surface and the structure of surface Rh species. Pt surface with higher oxophilicity usually shows a higher HER/HOR rate in alkaline conditions because the sluggish Volmer step can be accelerated by the early adsorption of OH in the Hupd potential region. However, both the HOR and HER activity of the PtRh nanowires reach the maximum value only when the Rh is in the single-atom form and the Pt surface is under the optimum oxophilicity.It was revealed by the XANES spectra that the single Rh atoms under 0 V vs. RHE are positively charged. The in-situ and ex-situ XANES spectra of Rh L edge show that Rh atoms are redox-active and electroactive. The positive shift of the edge position from Rh foil to Rh single atoms under 0 V vs. RHE indicates that the single Rh atoms are slightly oxidized and coordinated with eight Pt and one O, even under 0 V vs. RHE. When the applied potential increases, Rh-O bond intensity increases, and Rh-Pt bond intensity decreases, as expected from the oxidation from slightly charged single atom Rh to single Rh(OH)x species. We also noted that the Rh-O peak shift positively when potentials shifts from 0.54 V to 0 V vs. RHE, indicating an elongated Rh-O bond at a lower overpotential, which indicates that the adsorbed oxygen species under 0 V are not the OH from Rh(OH)x and should be weakly adsorbed OH or H2O↓. The fitting of in-situ EXAFS of Rh L edge on the SARh-PtNWs also confirms that the Rh atoms are bonded with eight Pt atoms and one O, and no Rh-Rh interaction is found throughout the whole HOR range.The existence of single atom Rh-OHads or Rh-H2O↓ under 0 V vs. RHE significantly facilitates the sluggish Volmer step at the minimal sacrifice of the active Pt sites where H2 dissociation and H adsorption takes place. When the surface Rh concentration surpasses a critical point, singly dispersed Rh atoms start to aggregate to a continuously distributed Rh layer which significantly increases the surface oxophilicity for earlier OH adsorption, but at the sacrificial of highly active Pt site for H2 dissociation and high Hads surface coverage. HOR/HER requires an equilibrium of H adsorption and OH/H2O adsorption on the Pt surface to reach the maximum oxidation/production rate of H2. However, both the Tafel step and the Volmer step on the Rh rich Pt surface are far away from the optimum condition, Also, the excess Rh and adsorbed water may form a rigid water layer that covers the Pt surface and prevent it from accessing the H2, results in the decreased overall HOR activity.In short, we have synthesized single atom Rh tailored Pt nanowires and used in-situ EXAFS and XANES to prove the existence of the surface Rh-OHads or Rh-H2O↓ which significantly facilitate the HER and HOR process. Figure 1

Catalysis by Imaging: From Meso- to Nano-scale

In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10–5–10–6 mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.

An XPS Study of the Interaction of Rhodium Foil with NO2

The interaction of rhodium foil with nitrogen dioxide over a temperature range of 30–450°C at three NO2 pressures of 10–6, 10–5, and 10–4 mbar was studied using X-ray photoelectron spectroscopy (XPS). At all of the three pressures, a three-dimensional surface oxide film with the Rh2O3 stoichiometry was formed on the foil surface starting from a temperature of 150°C. As the interaction temperature was increased at an NO2 pressure of 10–6 mbar, the film thickness d initially increased to reach a maximum of ~2.0 nm at 350°C and then decreased due to the decomposition of Rh2O3 to the metal. At pressures of 10–5 and 10–4 mbar, a monotonic increase in d up to 3.0 or 4.7 nm, respectively, was observed in a temperature range of 150–450°C. In the case of the interaction with the participation of molecular oxygen at a pressure of 10–4 mbar and temperatures of 30–450°C, the Rh foil surface was not oxidized. The results of the study on the interaction of NO2 with Rh were compared with the data obtained earlier for Pd and Pt under comparable conditions.

CO Oxidation on Stepped Rh Surfaces: \u03bcm-Scale Versus Nanoscale

The catalytic CO oxidation reaction on stepped Rh surfaces in the 10−6 mbar pressure range was studied in situ on individual μm-sized high-Miller-index domains of a polycrystalline Rh foil and on nm-sized facets of a Rh tip, employing photoemission electron microscopy (PEEM) and field-ion/field-emission microscopy (FIM/FEM), respectively. Such approach permits a direct comparison of the reaction kinetics for crystallographically different regions under identical reaction conditions. The catalytic activity of the different Rh surfaces, particularly their tolerance towards poisoning by CO, was found to be strongly dependent on the density of steps and defects, as well as on the size (µm vs. nm) of the respective catalytically active surface.Graphic

Towards chemically neutral carbon cleaning processes: plasma cleaning of Ni, Rh and Al reflective optical coatings and thin Al filters for free-electron lasers and synchrotron beamline applications

The choice of a reflective optical coating or filter material has to be adapted to the intended field of application. This is mainly determined by the required photon energy range or by the required reflection angle. Among various materials, nickel and rhodium are common materials used as reflective coatings for (soft) X-ray mirrors. Similarly, aluminium is one of the most commonly used materials for extreme ultraviolet and soft X-ray transmission filters. However, both of these types of optics are subject to carbon contamination, which can be increasingly problematic for the operation of the high-performance free-electron laser and synchrotron beamlines. As an attempt to remove this type of contamination, an inductively coupled plasma source has been used in conjunction with N2/O2/H2 and N2/H2 feedstock gas plasmas. Results from the chemical surface analysis of the above materials before and after plasma treatment using X-ray photoelectron spectroscopy are reported. It is concluded that a favorable combination of an N2/H2 plasma feedstock gas mixture leads to the best chemical surface preservation of Ni, Rh and Al while removing the carbon contamination. However, this feedstock gas mixture does not remove C contamination as rapidly as, for example, an N2/O2/H2 plasma which induces the surface formation of NiO and NiOOH in Ni and RhOOH in Rh foils. As an applied case, the successful carbon removal from ultrathin Al filters previously used at the FERMI FEL1 using an N2/H2 plasma is demonstrated.

Thermal Diffusion of 57Co into Rh Foil for Preparing Mössbauer Source

AbstractTo establish the coating conditions for57Co, non-radioactive Co ions are dissolved in an acid solution and electroplated on to a rhodium plate. The thermal diffusion of electroplated Co into a rhodium matrix was studied to apply a57Co Mössbauer source. The procedure to form a Co deposited onto Rh foil was established using two different electroplating baths: the acid-based buffer (pH 3) containing boric acid, sodium chloride, and saccharin, and the alkaline-based buffer (pH 10) containing hydrazine hydrate and ammonium citrate. The influence of different annealing conditions was investigated. From the results, the best diffusion degree of electrodeposited Co onto the rhodium matrix was obtained in an annealing process performed at 1100°C for 3 h in vacuum over 10−5hPa.

Hydrogen Oxidation on Stepped Rh Surfaces: µm-Scale versus Nanoscale

The catalytic H2 oxidation reaction on stepped Rh surfaces in the 10−6 mbar pressure range was studied in situ on individual high-Miller-index domains of a polycrystalline Rh foil by PEEM (photoemission electron microscopy) and on a Rh nanotip by FIM/FEM (field-ion/field-emission microscopy). The activity, particularly the tolerance to poisoning by oxygen, was found to strongly depend on the density of steps and defects, as well as on the size of the catalytically active surfaces.Graphical

Wide-band-gap wrinkled nanoribbon-like structures in a continuous metallic graphene sheet

The light-like dispersion of graphene monolayer results in many novel electronic properties in it1, however, this gapless feature also limits the applications of graphene monolayer in digital electronics2. A rare working solution to generate a moderate bandgap in graphene monolayer is to cut it into one-dimensional (1D) nanometre-wide ribbons3-13. Here we show that a wide bandgap can be created in a unique 1D strained structure, i.e., graphene-nanoribbon-like (GNR-like) structure, of a continuous graphene sheet via strong interaction between graphene and the metal substrate, instead of cutting graphene monolayer. The GNR-like structures with width of only a few nanometers are observed in a continuous graphene sheet grown on Rh foil by using thermal strain engineering. Spatially-resolved scanning tunnelling spectroscopy revealed bandgap opening of a few hundreds meV in the GNR-like structure in an otherwise continuous metallic graphene sheet, directly demonstrating the realization of a metallic-semiconducting-metallic junction entirely in a graphene monolayer. We also show that it is possible to tailor the structure and electronic properties of the GNR-like structure by using scanning tunnelling microscope.

Line shape of 57Co sources exhibiting self absorption

The effect of selfabsorption in Mossbauer sources is studied in detail. Spectra were measured using an old 57Co/Rh source of 74MBq activity with an original activity of ca. 3.7GBq and a 0.15GBq57Co/α − Fe source magnetized by an in-plane magnetic field of 0.2 T. The 57Co/α − Fe source of a thickness of 25 μ was used both from the active and the inactive side giving cause to very different selfabsorption effects. The absorber was a single crystal of ferrous ammonium sulphate hexahydrate (FAS). Its absorption properties were taken over from a detailed study (Bull et al., Hyperfine Interact. 94(1–3), 1; Spiering et al. 2). FAS (space group P21/c) crystallizes as flat plates containing the (\(\overline {2}\)01) plane. The γ-direction was orthogonal to the crystal plate. The 57Co atoms of the 57Co/Rh source were assumed to be homogeneously distributed over a 6μ thick Rh foil and to follow a one dimensional diffusion profile in the 25 μ Fe-foil. The diffusion length was fitted to 10 μ. The theory follows the Blume-Kistner equations for forward scattering (Blume and Kistner, Phys. Rev. 171, 417, 3) by integrating over the source sampled up to 128 layers.

Observation of unconventional splitting of Landau levels in strained graphene

In strained graphene, lattice deformation can create pseudo-magnetic fields and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the pseudo-magnetic fields are of opposite directions in the K and K' valleys of graphene. Here, we present experimental spectroscopic measurements by scanning tunneling microscopy of strained graphene on Rh foil. We direct observed valley splitting of the Landau level induced by the coexistence of the pseudo-magnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the pseudo-magnetic fields and the external magnetic fields.

Use of Instrumental Neutron Activation Analysis to investigate the distribution of trace elements among subsamples of solid materials

The results of analytical measurements performed with solid-sampling techniques are affected by the distribution of the analytes within the matrix. The effect becomes significant in the case of determination of trace elements in small subsamples. In this framework we propose a measurement model based on Instrumental Neutron Activation Analysis to determine the relative variability of the amount of an analyte among subsamples of a material. The measurement uncertainty is evaluated and includes the counting statistics, the full-energy gamma peak efficiency and the spatial gradient of the neutron flux at the irradiation position. The data we obtained in a neutron activation experiment and showing the relative variability of As, Au, Ir, Sb and W among subsamples of a highly pure Rh foil are also presented.

Superlattice Dirac points and space-dependent Fermi velocity in a corrugated graphene monolayer

Recent studies show that periodic potentials can generate superlattice Dirac points at energies in graphene (is the Fermi velocity of graphene and G is the reciprocal superlattice vector). Here, we perform scanning tunneling microscopy and spectroscopy studies of a corrugated graphene monolayer on Rh foil. We show that the quasi-periodic ripples of nanometer wavelength in the corrugated graphene give rise to weak one-dimensional (1D) electronic potentials and thereby lead to the emergence of the superlattice Dirac points. The position of the superlattice Dirac point is space-dependent and shows a wide distribution of values. We demonstrated that the space-dependent superlattice Dirac points is closely related to the space-dependent Fermi velocity, which may arise from the effect of the local strain and the strong electron-electron interaction in the corrugated graphene.

Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process

We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities.

Preparation of a boron nitride single layer on a polycrystalline Rh surface

The segregation of boron and its reactivity toward nitric oxide have been investigated by means of high-resolution Auger spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS). The segregation of boron from a Rh foil started from 700K. Its presence altered the surface behaviors of Rh; the uptake of NO increased by about 30–37%. Whereas the dissociation of NO was about 3–10% on a clean, boron-free surface, the extent of dissociation (at saturation) at highest boron level was almost 98%. This feature strongly suggest a direct interaction between NO and boron on the surface. The presence of boron greatly stabilized the adsorbed nitrogen and oxygen formed in NO dissociation. Boron oxide (BO, B2O2) sublimated from the surface below 1000K. Clean, single BN layer formed on the surface close to a monolayer regime, presumable in nanomash structure.

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.

X-ray attenuation around K-edge of Zr, Nb, Mo and Pd: A comparative study using proton-induced X-ray emission and 241Am gamma rays

Mass attenuation coefficients (µ/ρ) for Zr, Nb, Mo and Pd elements around their K-edges are measured at 14 energies in the range 15.744–28.564 keV using secondary excitation from thin Zr, Nb, Mo, Rh, Pd, Cd and Sn foils. The measurements were carried out at the Kα and Kβ energy values of the target elements by two techniques: (1) Proton-induced X-ray emission (PIXE) and (2) 241Am (300 mCi) source. In PIXE, 2 MeV proton-excited X-rays were detected by a Si(Li) detector. In the second case, X-rays excited by 59.54 keV photons from the targets were counted by an HPGe detector under a narrow beam good geometry set-up with sufficient shielding. The results are consistent with theoretical values derived from the XCOM package and indicate that the PIXE data have better statistical accuracy.

Investigation of the Formation Process of Photodeposited Rh Nanoparticles on TiO2 by In Situ Time-Resolved Energy-Dispersive XAFS Analysis

The photodeposition process of Rh metal nanoparticles on a TiO(2) photocatalyst from RhCl(3) aqueous solution in the presence of methanol as a sacrificial oxidant, which consists of the direct reduction of Rh(3+) ions to Rh metal and the formation of Rh nanoparticles, was uncovered by in situ time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) analysis in a liquid-solid suspension state. The fractions of Rh metal particles and Rh(3+) precursor were estimated by the least-squares fitting of each X-ray absorption near-edge structure (XANES) spectrum by a linear combination of authentic spectra corresponding to Rh(0) and Rh(3+). The fraction of Rh metal linearly increased with photoirradiation time and saturated after 90 min of photoirradiation. The coordination number (Rh-Rh pair) was evaluated by the curve fitting of the Rh-Rh scattering at 2.45 A in the Fourier transforms (FT) of extended XAFS (EXAFS) spectra. The coordination number linearly increased with photoirradiation time and attained a constant value of 10 after 90 min of photoirradiation. This value is lower than that for the Rh foil (12). These suggest the formation of fine Rh metal nanoparticles on TiO(2). In addition, the diminution rate of Rh(3+) as determined by ICP analysis was in good agreement with the increased rates for the fraction of Rh metal particles estimated by XANES spectra and the coordination number (Rh-Rh pair) evaluated by FT-EXAFS spectra. This result strongly supports the fact that electrons generated by charge separation reduce the Rh(3+) precursor to an Rh metal particle at a moment in time and at a constant rate. The Rh particles do not grow in incremental steps, but Rh particles with a uniform size appear one after another on the surface.