Rhodium Foil Research Articles

Coexisting multi-states in catalytic hydrogen oxidation on rhodium

Catalytic hydrogen oxidation on a polycrystalline rhodium foil used as a surface structure library is studied by scanning photoelectron microscopy (SPEM) in the 10−6 mbar pressure range, yielding spatially resolved X-ray photoemission spectroscopy (XPS) measurements. Here we report an observation of a previously unknown coexistence of four different states on adjacent differently oriented domains of the same Rh sample at the exactly same conditions. A catalytically active steady state, a catalytically inactive steady state and multifrequential oscillating states are simultaneously observed. Our results thus demonstrate the general possibility of multi-states in a catalytic reaction. This highly unusual behaviour is explained on the basis of peculiarities of the formation and depletion of subsurface oxygen on differently structured Rh surfaces. The experimental findings are supported by mean-field micro-kinetic modelling. The present observations raise the interdisciplinary question of how self-organising dynamic processes in a heterogeneous system are influenced by the permeability of the borders confining the adjacent regions.

XPS Studies of the Initial Oxidation of Polycrystalline Rh Surface

Increased interest in the oxidation process of polycrystalline rhodium, observed in recent years, is the result of its application in exhaust catalytic converters. However, most studies have involved sample surfaces with low Miller indices. In our research, we investigated polycrystalline rhodium foil containing crystallographically different, highly stepped, µm-sized crystallites. These crystallites were exposed to identical oxidizing conditions. To determine crystallographic orientation, the electron backscattering diffraction (EBSD) method was used. To investigate the initial stages of oxidation on the individual crystallites of Rh, X-ray photoelectron spectroscopy (XPS) studies were performed. The results obtained for the individual crystallites were compared and analyzed using chemical state quantification of XPS data and multivariate statistical analysis (MVA).

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.

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.

Validation of calculated self-shielding factors for Rh foils

Rhodium foils of about 5 mm diameter were obtained from IRMM. One foil had thickness of 0.006 mm and three were 0.112 mm thick. They were irradiated in the pneumatic transfer system and in the carousel facility of the TRIGA reactor at the Jožef Stefan Institute. The foils were irradiated bare and enclosed in small cadmium boxes (about 2 g weight) of 1 mm thickness to minimise the perturbation of the local neutron flux. They were co-irradiated with 5 mm diameter and 0.2 mm thick Al–Au (0.1%) alloy monitor foils. The resonance self-shielding corrections for the 0.006 and 0.112 mm thick samples were calculated by the Monte Carlo simulation and amount to about 10% and 60%, respectively. The consistency of measurements confirmed the validity of self-shielding factors. Trial estimates of Q 0 and k 0 factors for the 555.8 keV gamma line of 104Rh were made and amount to 6.65±0.18 and (6.61±0.12)×10 –2, respectively.

Production of Palladium-103 (103Pd) from a thin rhodium foil target – Improved cooling concept

Palladium-103 (103Pd) is one of the commonly used radioisotopes for prostate cancer treatment. The current irradiation technique used to produce this radioisotope suffers from several serious inherent drawbacks, of which one is the low beam current due to cooling limitation and the other is the electroplating process used to prepare the target. A liquid-metal jet impingement target cooling system developed at Soreq NRC demonstrated recently a cooling capacity of 8.4kW/cm2 while the pressure of the cooling liquid on the target back was less than 1bar. The latter value implies that the target can be made very thin and that the copper back-plate might be removed. Hence, we propose a new target design based on the use of a thin rhodium foil directly cooled by a liquid-metal such as gallium.

Methane Partial Oxidation Catalyzed by Platinum and Rhodium in a High-Temperature Stagnation Flow Reactor

A series of measurements investigating the partial oxidation chemistry of methane and oxygen over platinum and rhodium foils was performed in a stagnation flow reactor at a pressure of 30 Torr. Pro...

Cyclotron production of no-carrier-added palladium-103 by bombardment of rhodium-103 target

Electroplated rhodium foil was employed as the target for cyclotron production of palladium-103. An electrodissolution apparatus was found better than other dissolution methods in terms of personnel shielding and palladium-103 yield. The ion-exchange column chromatography method was simple and effective for the purification of palladium-103 and the final stripping agent of NH4Cl+NH3(1:1) was more efficient than other agents.

Susceptibility and relaxation measurements on rhodium metal at positive and negative spin temperatures in the nanokelvin range

We have measured the susceptibility of polycrystalline rhodium foils, down to nuclear spin temperaturesT = 280 pK and “up” toT = −750 pK. AtT > 0, the static susceptibility follows the antiferromagnetic Curie-Weiss law with θ = −1.8±0.3 nK. AtT < 0, the expected ferromagnetic behavior in the vicinity ofT = −0 changes into antiferromgnetic tendency around −6 nK. If only nearest and next nearest neighbor interactions are assumed, our data yieldJn/h = −17±3 Hz andJnnn/h = 10±3 Hz for the exchange interaction coefficients. We have also investigated the field and polarization dependence of the spin-lattice relaxation time τ1 at positive and negative nanokelvin temperatures; surprisingly τ1 is longer whenT< 0. Infields below 1 mT, the Korringa constant shows a strong decrease, which becomes more pronounced as the conduction electron temperature decreases. Observed behavior is consistent with conduction electrons scattering from magnetic impurities, provided that the impurity-induced contribution to spin-lattice relaxation is proportional to the inverse of the nuclear spin temperature in small fields.

Surface Oxidation of Rhodium at Ambient Pressures as Probed by Surface-Enhanced Raman and X-Ray Photoelectron Spectroscopies

Presented here is a comprehensive study of ambient-pressure thermal oxidation of rhodium surfaces utilizing surface-enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS) as complementary techniques. Real-time SERS provides details regarding surface bonding and the time-dependent evolution of the surface oxides under in situ conditions, while XPS is used to obtain quantitative information about relative amounts of the oxides and extent of oxidation. The surfaces examined are electrochemically deposited rhodium thin films on gold substrates, and polycrystalline rhodium foil. Surface oxidation involved heating in a flowing stream of oxygen or oxygen/argon mixtures at atmospheric pressure, and temperatures up to 300°C. Raman spectra from the oxidized thin film exhibit several features in the 200-1000 cm−1 region that may be attributed to rhodium-oxygen vibrations of surface oxygen and oxides. Bands at 530 and 800 cm−1 are assigned to symmetric and asymmetric Rh-O stretches, respectively, in Rh2O3. A third band at 290 cm−1, attributed to a Rh-O bending mode, arises from a behaviorally distinct oxide species. The corresponding XPS spectra exhibit several features that can be correlated with the SERS bands. The presence of Rh2O3 is inferred from the appearance of a RH(3d) peak at 308.6 eV and a matching O(1s) peak at 530 eV. An additional O(1s) feature appears at 531.7 eV, which apparently correlates with the 290-cm−1 SERS band. While a clearcut assignment of the latter has proven to be difficult, possible species are chemisorbed oxygen along with RhOOH or RhO2. A differing reactivity of the spectrally distinct oxygen species towards CO was observed. A model for the spatial oxide composition is proposed based on a quantitative analysis of the XPS data.

Nuclear antiferromagnetism in rhodium metal at positive and negative nanokelvin temperatures

We have measured the nuclear susceptibility of polycrystalline rhodium foils down to 2809 pK and up to -750 pK. In spite of these record-low and -high spin temperatures, the ordered states were not reached. At T>0, the measured susceptibility follows the antiferromagnetic Curie-Weiss law with θ=-(1.8±0.3)nK. At T<0, however, ferromagnetic behavior at high entropy crosses over to antiferromagnetic tendency around ¦T¦<6nK. We obtain J nn/h = −17±3Hz and J nnn/h = 10±3Hz if only nearest and next nearest neighbor interactions are assumed.

Nuclear antiferromagnetism in rhodium metal at positive and negative nanokelvin temperatures.

We have measured the dynamic susceptibility of polycrystalline rhodium foils down to 280 pK and up to -750 pK. These record-low and -high nuclear spin temperatures were reached by adiabatic demagnetization using initial polarizatons of 83% and -60%. At T>0, the static susceptibility, integrated from NMR spectra, displays an antiferromagnetic Curie-Weiss law, with θ=-1.8±0.3 nK. At T<0, a crossover from ferro- to antiferromagnetic tendency is found around -6 nK. We obtain J nn /h=-17±3 Hz and J nn /h=10±3 Hz if only nearest and next nearest neighbor interactions are assumed

PIXE channeling for concentration and location measurements of Zn- and Cd-dopants in InP single crystals

We present results of the determination of Cd- and Zn-dopants in InP single crystals using the PIXE and RBS spectrometry at our 2 MeV Van de Graaff accelerator. The (100) oriented crystals were doped by thermodiffusion of Cd and Zn atoms. For concentration and localization measurements we used the ion-channeling technique and energy dispersive spectrometry of proton induced X-ray emission (PIXE). Angular scans of the K-lines of In, Cd and Zn were obtained. The strong In X-rays were attenuated by a rhodium foil in front of the low energy Ge detector. The PIXE-channeling results were compared with SIMS and Hall-effect measurements.

Vanadium Oxide Deposited on an Rh Foil: CO and CO2 Hydrogenation Reactivity

The effects of submonolayer deposits of vanadia on the rates of CO and CO2 hydrogenation of an Rh foil have been examined. Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD) have been used to determine the oxide coverage, and X-ray photoelectron spectroscopy (XPS) has been used to determine the oxidation state for VOx deposits on a rhodium foil. After oxidation (3 × 10−6 O2 at 350°C), the vanadium valence state is 3+, which corresponds to an oxide stoichiometry of V2O3. CO chemisorption reduces 34% of the V3+ to V2+ at low oxide coverages. The absolute amount of V2+ in the oxide overlayer after CO titration is found to maximize at a coverage of 0.5 ML. The rates of CO and CO2 hydrogenation increase upon addition of VOx to an Rh foil. For CO hydrogenation, the rate maximizes at two times that of the rate on the clean surface at an oxide coverage of 0.4 ML. For CO2 hydrogenation, the rate maximizes at six times that of the clean surface rate at an oxide coverage of 0.6 ML. The oxide promoter also alters the activation energies, partial pressure dependences, and selectivities for the hydrogenation reactions. A comparison of the kinetic and spectroscopic data reveals a strong correlation between the degree of reducibility of the oxide overlayer and the amount of rate enhancement for CO and CO2 hydrogenation.

Neutron measurements on ZFX

The NRL ZFX Facility is being used to form dense Z pinches from single fibers composed of cryogenically frozen deuterium (D2) or deuterated polyethylene (CD2). Fiber diameters range from 40 to 125 μm. Maximum current through all these pinches has been about 1 MA with a rise time of 670 ns. Total neutron yields per shot are measured with a pair of rhodium foil activation counters mounted coaxial and perpendicular to the fiber axis. In order to include the effects of external scattering and absorption these detectors are calibrated in situ using a 252Cf neutron source at the pinch location. Total neutron yields have also been determined using bubble detector dosimeters. This is believed to be the first application of these detectors on a pulsed plasma source. Neutron energy spectroscopy and production time are determined with either a pair of scintillator/photomultiplier detectors located at two different perpendicular distances from the pinch axis or a single microchannel plate photomultiplier (μcp-pmt) detector. Both x-ray shielding and neutron flux collimation are required on these detectors. Typically the detected neutrons (up to 2×109 per shot) were emitted in two to four distinct but overlapping 150–200 ns wide bursts starting 300–400 ns after the current initiation.

Segregation and reactivity of boron impurities on rhodium foil

Segregation and reactivity of boron impurities on rhodium foil

HCN synthesis from CH 4 and NH 3 on clean Rh

A reaction chamber attached to a UHV surface analysis system is used to study kinetics of HCN synthesis on polycrystalline Rh for temperatures between 500 and 1600 K with CH 4 and NH 3 pressures between 0.05 and 10 Torr. AES is used to show that the surface is clean before reaction, and AES and TPD are used to examine surface residues after reaction. The rate of HCN production attains a maximum near a 1:1 reactant mixture, and exceeds 10 18 molecules/cm 2 sec at high temperatures. The selectivity (fraction of NH 3 reacting to form HCN) is greater than 90% at high temperatures at a total pressure of 1 Torr. The rate of HCN production is proportional to P CH 4 , and N 2 formation is strongly inhibited by CH 4. Examination of rhodium foils by SEM after reaction shows that the surface is faceted into predominantly (100) planes, and grain boundaries are pitted. Reaction is shown to occur by NH 3 or its fragments reacting with surface carbon layers because approximately one monolayer of carbon is found on the reactive surface while multilayers of carbon create a totally unreactive surface. This carbon also explains why the NH 3 decomposition reaction is suppressed compared to that on clean Rh. A model in which surface carbon is a reactant which also blocks reaction sites is shown to give a quantitative fit to rates of HCN and N 2 production. The high reaction probability of CH 4 (~10 −2) is surprising because of its low surface reactivity on Rh. It is argued that the surface reaction between NH 3 and CH 4 fragments is the major reaction under industrial HCN synthesis conditions and that no homogeneous processes are necessary to explain high yields of HCN.

CO hydrogenation over clean and oxidized rhodium foil and single crystal catalysts. Correlations of catalyst activity, selectivity, and surface composition

CO hydrogenation at 6 atm over Rh polycrystalline foil and single crystal (111) catalysts was investigated in a system where the surface structure and composition of the catalysts could be characterized both before and after the reaction. The reaction conditions (H 2:CO ratio, reaction temperature, surface pretreatment, and gas phase additives) were systematically varied to determine the conditions necessary for the formation of oxygenated hydrocarbons. Initially clean Rh catalysts showed no structure sensitivity, primarily produced methane (90 wt%) at an initial rate of 0.15 molecules site −1 sec −1 at 300 °C, and did not produce detectable amounts of oxygenated hydrocarbons. Preoxidation of the Rh (800 Torr O 2, 600 °C, 30 min) resulted in dramatically increased initial rates, a larger fraction of higher molecular weight hydrocarbons in the product distribution, formation of methanol, ethanol, and acetaldehyde, and some structure sensitivity. The different Arrhenius methanation preexponential factors and activation energies over the clean and preoxidized Rh foils indicate the methanation mechanism is different on these two surfaces. Decreasing the reaction temperature or H 2:CO ratio increased the C 2H 4 to C 2H 6 ratio and shifted the product distribution toward the higher molecular weight hydrocarbons. The addition of 1 mole% of CH 3OH, CH 3CH 2OH, or C 2H 4 to the H 2:CO reaction mixture caused a small increase in the fraction of higher molecular weight hydrocarbons.

Contaminant Dose from Incident Neutrons Associated with 22.5-Mev X-Rays from a Betatron

The flux of thermal and fast neutrons present at the patient treatment position has been determined by measurements of the activity induced in rhodium foils and in sulfur. A previously published energy spectrum of fast neutrons produced by photodisintegration reactions with 22 Mev Bremsstrahlung in heavy elements has been used in evaluating the activation information. There are 9 × 105 fast neutrons/cm.2 and 9 × 103 thermal neutrons per roentgen in the center of the collimated and compensated betatron x-ray beam at a treatment distance of 80 cm, from the target. The fast neutron spectrum has a peak intensity at 0.8 Mev. The dose produced by thermal neutrons is completely negligible. A calculation of the fast neutron dose induced in tissue has been carried out for two typical irradiation conditions: (1) four-port irradiation of the trunk; (2) bilateral irradiation of head.

Doppler Effect in the Slow Neutron Resonance in Rhodium

The variation in transmission of a rhodium foil to 1.26-ev neutrons has been measured as a function of foil temperature using a crystal spectrometer. A change in transmission of approximately two percent per 100-degree temperature change has been observed over a temperature range from 120\ifmmode^\circ\else\textdegree\fi{}K to 860\ifmmode^\circ\else\textdegree\fi{}K. After correction for instrument resolution and Doppler effect, a peak cross section of 4850\ifmmode\pm\else\textpm\fi{}200 barns results. Comparison has been made with the theory of Bethe and Placzek as extended by Lamb.