Rhodium Films Research Articles

Electrochemical modification of the catalytic activity of TiO2/YSZ supported rhodium films

The electrochemical activation of ethylene oxidation was studied over rhodium catalysts of different thickness (40, 100 and 160 nm) sputtered on top of a thin layer of TiO2 deposited on YSZ. The strong relationship between catalytic activity and oxidation state of rhodium was confirmed. Under open-circuit operation the catalyst potential appears as a suitable indicator of the surface oxidation state of rhodium allowing a prediction of the catalytic behavior from solid electrolyte potentiometric measurements. Under closed-circuit conditions the catalyst potential was used as a tool to tune the catalytic activity of rhodium which showed increasing promotional efficiency with decreasing catalyst film thickness.

Partial oxidation of n-hexadecane at short contact times: Catalyst and washcoat loading and catalyst morphology

Rhodium supported on alumina foam is an exceptionally active catalytic partial oxidation catalyst, giving high selectivities to syngas or olefins by changing reaction stoichiometry or catalyst properties such as rhodium loading and washcoat loading. Rhodium catalyst loading and γ-alumina washcoat loading were varied systematically on α-alumina foams to investigate resulting microstructure and products of the partial oxidation of n-hexadecane. At conditions favorable for producing syngas, varying rhodium loading between 0.05 and 10.0 wt.% (a factor of 200) had little affect on H 2 and CO selectivities or fuel conversions. However, non-washcoated catalysts gave low selectivities to syngas. Increasing washcoat loading results in high selectivities of olefins, while varying rhodium loading has little affect on production of olefins. Selectivitiy data from multi-metallic catalysts, Rh–Ce and Rh–Pt, was also investigated. The morphology of catalysts was examined using scanning electron microscopy. The micrographs indicate that increasing rhodium loading above a few percent led to the formation of a rhodium film on the catalyst support. The presence of washcoat seems to keep rhodium spread on the surface at high reaction temperatures enhancing heterogeneous products. Very high washcoat loadings (10 wt.%) decrease pore diameters, increasing the likelihood of reactant contact with the rhodium surface, leading to increased H 2 and decreased olefin selectivities.

Characterization of metal and metal alloy films as contact materials in MEMS switches

This study presents a basic step toward the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between two important parameters, resistivity and hardness, since they provide the guidelines and assessment of contact resistance, wear, deformation and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals, platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au), were investigated. The interrelationship between resistivity and hardness was established for three levels of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh) and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Ultrathin Rh films on Ru(0001): Oxidation in confinement

Ultrathin rhodium films with a thickness ranging from 1 to a few monolayers were deposited on a single-crystal Ru(0001) surface in order to investigate the oxidation behavior of ultrathin epitaxial films on a dissimilar substrate. It is found that rhodium grows on Ru(0001) initially layer by layer, adapting the in-plane lattice parameters of Ru(0001). When exposing Rh films to oxygen environment (approximately 4.8 x 10(6) L O2 exposure) at 660 K, 2-4 ML Rh films form a surface oxide composed of (9 x 9) O-Rh-O trilayers. Quite in contrast, oxidation of the 1 ML RhRu(0001) film leads to a poorly ordered oxide with a rutile structure reminiscent of RuO2(110) on Ru(0001). The oxidized 1 ML RhRu(0001) film contains much more oxygen than the oxidized thicker Rh films. Lower temperatures (535 K) and high doses of oxygen lead to a (1 x 1)-O overlayer on the 1 ML RhRu(0001) surface, whose atomic geometry resembles closely that of the (1 x 1)-O phase on clean Ru(0001).

Application of membranes and filtering films for gas sensors improvements

It is possible to improve the performances of gas sensors using filtering films deposited above the sensing material. This paper is a review of several recent studies performed by the authors in this sense in order to point out the major difficulties on this subject. The difference between thin and thick film technologies is well underlined. Firstly, an example of a thick porous layer is presented with the objective to develop a porous protective layer for soot protection in car exhausts. By another way, the use of metallic films has been successfully used to improve the selectivity of SnO 2 sensors. Even if thin films of rhodium allow to solve the problem of the dual response CO/NO 2, the best results are obtained generally with thick films. This is partly due to several problems resulting of the direct contact of the metallic films with the sensing material. The use of SiO 2 thick insulating layer allows to improve the selectivity to CH 4 with a fully agreement with catalytic activities of a platinum filter in regards the CO interference. A last example demonstrates also the interest of thin SiO 2 films for the selective detection of H 2.

Electrocatalytic Reduction of Nitrate on Activated Rhodium Electrode Surfaces

Electrodeposited rhodium films on titanium substrates have been electrochemically activated to produce a high area surface with a specific activity for nitrate electroreduction directly to N2. The activation process involves oxidation/reduction cycles in an alkaline, KCl electrolyte containing nitrate ions. Surfaces of up to 230 times the geometric area are achieved, together with a surface morphological modification. While the active surface, once formed, is intrinsically unstable during long-term nitrate reduction, its activity can be maintained in situ by an electrochemical cycling procedure. The high area rhodium has the form of a nano-structured ‘sponge’, with a surface area of ca. 19 m2 g−1. The morphological modification is evidenced by a change in the hydrogen UPD structure, changes in the surface redox behaviour associated with OH adsorption, and a reduction in the activation energy for nitrate reduction from ca. 47 to 20 kJ mol−1. The reduction in activation energy, however, is accompanied by a decrease in the pre-exponential factor, and this apparent compensation effect results in similar rate constants on the activated and unactivated surfaces. The enhancement in the catalyst's activity for nitrate reduction results from an increase in the relative activity of nitrate reduction over water reduction. The activated catalyst sustains steady state nitrate reduction at an increased over-potential before the reaction to N2 decays, and hydrogen evolution and reduction to ammonia take place. The presence of nitrate ions is essential for the formation of the active surface, and specifically adsorbed nitrate ions and reductive intermediates are present at the surface when it is reformed. A mechanism for the elementary surface reaction steps involved in nitrate reduction, and the apparent ‘habit’ growth of the active surface phase in the nitrate containing solution is discussed.

Preparation and characterization of H 1–e rhodium films

Mesoposous films of rhodium were prepared by electrochemical deposition from rhodium (III) chloride dissolved in the H 1 lyotropic liquid crystalline phase of octaethyleneglycol monohexadecyl ether (C 16EO 8) or Brij ® 56. Characterization of the electrochemically deposited films using SEM, TEM, small angle X-ray diffraction and cyclic voltammetry show that they are smooth, strongly adherent, with a high surface area (32 m 2 g −1) and containing a regular hexagonal array of cylindrical pores with a pore centre to pore centre separation of around 6 nm. Preliminary studies show that the films can be used to carry out the electrochemical reduction of nitrate in basic solution.

Electrochemical Deposition of Nanostructured (H1-e) Layers of Two Metals in Which Pores within the Two Layers Interconnect

Electrochemical deposition of metals from the H1 hexagonal phase of nonionic lyotropic liquid-crystalline phases has been shown to produce films which contain regular arrays of uniform pores whose dimensions are determined by those of the micelles in the lyotropic phase used as the template. In this paper we report results for a study of the deposition of one H1-e metal film on top of another. Using H1-e films of palladium and rhodium as a model system we show, using electrochemical measurements, that the surface of the pores of the inner metal film are not blocked by deposition of an overlayer of the second mesoporous metal. By studying the evolution of the voltammetry of the mesoporous metallic bilayers as the surfactant is leached out of the pores over time, we conclude that the pores in the outer H1-e metal film connect to those in the inner layer so that we have a hexagonal array of continuous pores running though both metal layers in the structure.

CO oxidation over the Pt-Rh system. 3. Reaction on a heterophase surface

The reaction of the CO oxidation over a Pt-Rh heterophase surface, which represents 10-20 nm platinum particles located on a rhodium film, which in turn is deposited on an inert support, has been investigated at low pressures (P < 2'10-5 mbar). The results are compared with the data for the clean surfaces of Pt and Rh. In the high-temperature range, the rate of CO2 formation on the heterophase surface is found to be higher than the sum of the rates on individual metals corrected by the surface area of the different metals. The nature of the synergistic effect in the CO oxidation is discussed.

Abnormal Infrared Effects of Nanostructured Rhodium Thin Films for CO Adsorption at Solid/Gas Interfaces

Nanometer-scale thin films of rhodium supported on glassy carbon (nm-Rh/GC) were prepared by electrochemical deposition using cyclic voltammetry. STM studies demonstrated that the Rh film is made of layered crystallites of an average size 230 nm (l) × 90 nm (w) × 10−30 nm (h). In situ FTIR spectroscopic investigations revealed, for the first time, that the nanostructured film exhibits abnormal infrared effects (AIREs) for CO adsorption at solid/gas interfaces. The AIREs are characterized by the inverting of the direction of IR bands of adsorbed CO (COad), an enhancement of IR absorption intensity, and an increase in the full width at half-maximum (fwhm) of the bands. The inversion of the direction of the absorption bands and the increase in the fwhm are observed in spectra of CO on nm-Rh/GC of different Rh film thicknesses; the enhancement of IR absorption depends strongly on the thickness of the Rh film. The maximum enhancement factor has been determined to be 10.38 for an Rh film thickness of 79 nm. The...

CO oxidation over a Pt-Rh system. 1. Reaction on individual metals

The reaction of CO oxidation over thin films of rhodium and platinum prepared by vacuum evaporation of the metals on an inert support has been investigated at low pressures (P < 2×10-5 mbar). Rhodium has been found to be more active than platinum in this reaction. The reasons of the higher activity of Rh are discussed.

High temperature materials for thin-film thermocouples on silicon wafers

We have developed an instrumented calibration wafer for radiometric temperature measurements in rapid thermal processing (RTP) tools for semiconductor processing. The instrumented wafers have sputter deposited thin-film thermocouples to minimize the thermal disturbance of the wafer by the sensors. The National Institute of Standards and Technology (NIST) calibration wafer also employs platinum–palladium wire thermocouples to achieve a combined standard uncertainty of 0.4°C in the temperature measurement of the thin-film thermocouple junction at 900°C. The high temperatures of the wafer has required the development of new thin-film material systems. We have reported the results of our testing and characterization of sputtered platinum, palladium, rhodium, and iridium thin films using titanium bond coats on thermally oxidized silicon wafers. Depth profiling with secondary ion mass spectrometry was used to determine the diffusion profiles from the metal film to the silicon after heat treatments as high as 1000°C. Electron microscopy and optical microscopy were used to follow the reactions and the deterioration of the thermoelectric films. In addition, performance tests up to 1000°C in the NIST RTP test bed were used to determine the stability of the material systems. Failure mechanisms and limitations of the thin-film thermocouple materials have been discussed with data on hysteresis and drift in thermometry performance. The results of our evaluations indicated that Rh/Ir thin-film thermocouples have the best properties for wafer temperatures above 900°C.

Surface-Enhanced Raman Scattering from Substrates with Conducting or Insulator Overlayers: Electromagnetic Model Predictions and Comparisons with Experiment

Model electromagnetic (EM) calculations are presented of the surface-enhanced Raman scattering (SERS) intensities expected for gold and silver particles coated with conducting as well as insulating dielectric films, with the objective of assessing their thickness-dependent properties and hence the anticipated scope of such “overlayer SERS” tactics to chemically diverse interfacial materials. Most calculations refer to Raman enhancements at the film outer edge, relevant to species adsorbed on the overlayer. Spheroidal and spherical metal particles are treated by using the electrostatic approximation, with a first-order electrodynamic correction for “radiation damping”, in vacuum and aqueous environments. The presence of simple insulating dielectric (such as organic) overlayers yields progressive decays in the calculated Raman enhancement factor, G, at the film edge with increasing thickness, d, throughout the visible optical region of experimental significance, even though the largest G values are necessarily obtained close to the plasmon resonance energy. These G–d dependencies are markedly milder than predicted for bare metal particles in water or vacuum, suggesting the potential broad-based analytical utility of nanoscale (∼ 1–10 nm thick) molecular dielectric overlayers in SERS. Furthermore, Raman enhancements increasing with film thicknesses are typically obtained at locations close to the inner film edge, relevant to moieties imbedded in dielectric overlayers. Sharper G–d decays are predicted at the film edge of transition-metal overlayers, especially near the plasmon resonance; unlike dielectric insulators, the metal overlayers progressively “quench” the optical frequency-dependent enhancement at the film edge arising from substrate plasmon resonance. Reasonable agreement is obtained in comparison with the Raman intensity-thickness dependence measured for chemisorbed carbon monoxide on rhodium films electrodeposited onto gold. The model calculations can also account qualitatively for the nonmonotonic Raman intensity-thickness dependencies observed for phonon bands from semiconducting (cadmium chalcogenide) overlayers on gold, attributed to film-induced redshifts in the substrate plasmon resonance. More general implications for the analytical utility of “overlayer SERS” tactics are also pointed out.

Electrochemical promotion of Rh catalyst in gas-phase reduction of NO by propylene

The concept of nonfaradaic electrochem. modification of catalytic activity (NEMCA) was applied for the in situ control of catalytic activity of a rhodium film deposited on YSZ (yttria stabilized zirconia) solid electrolyte towards redn. of 1000 ppm NO by 1000 ppm C3H6 in presence of excess (5000 ppm) O2 at 300 Deg. A temporary heating at this feed compn. results in a long-lasting deactivation of the catalyst under open circuit conditions due to partial oxidn. of the rhodium surface. Pos. current application (5 mA) over both the active and the deactivated catalysts gives rise to an enhancement of N2 and CO2 prodn., the latter exceeding several hundred times the faradaic rate. While active rhodium exhibits a reversible behavior, electrochem. promotion on the deactivated catalyst is composed of a reversible and an irreversible part. The reversible promotion results from the steady-state accumulation of current-generated active species at the gas exposed catalyst surface whereas the irreversible effect is due to the progressive redn. of the catalyst resulting in an increased recovery rate of lost catalytic activity. The results are encouraging with respect to application of rhodium for the catalytic removal of NO from auto-exhaust gases under lean-burn conditions. [on SciFinder (R)]

Selectivity improvement of SnO 2 films by superficial metallic films

The purpose of this paper is to demonstrate the possibility of modifying the sensitivity of tin dioxide (SnO 2) films by depositing metallic catalysts on the surface of SnO 2 layer. The aim is to reduce the effect of ethanol which is considered as an interfering gas in many domestic or industrial applications. Two catalysts have been so studied (platinum and palladium), deposited on two types of SnO 2 layers which present different textures. Both catalysts reduce the sensitivities to ethanol, carbon monoxide and methane. But the decrease of the alcohol sensitivity is more significant and consequently, the relative sensitivities to CO and CH 4 are increased. This effect is more important with the SnO 2 layers which have a high porosity. An other application where tin dioxide sensors can be used is the automotive application. In this case, nitrogen oxides are considered as interfering gases for the carbon monoxide detection. Several catalysts are studied in order to decrease the influence of nitrogen oxides and especially the conductance decrease under NO 2. One of the catalysts which can be used to reduce the nitrogen oxide effect is rhodium. The results obtained with such rhodium films deposited on SnO 2 sensors allow us to consider future applications in the automotive field.

The initial stages of rhodium deposition on Au(111)

Abstract The electrochemical deposition of rhodium on Au(111) was studied by means of in-situ STM and cyclic voltammetry. A phase transition within the [RhCl6]3− adlayer was found for Au(111) in 0.1 M H2SO4+10−4 M RhCl3+10−3 M HCl at 520 mV versus SCE, where a ( 13 × 13 )R13.9°-superstructure of [RhCl6]3− is formed on the gold substrate. Rhodium starts to deposit irreversibly around 200 mV versus SCE. First a rhodium bilayer grows, the electrochemical behaviour of which is similar to that of a well-ordered Rh(111) surface. On top of the second rhodium monolayer the formation of small, three-dimensional clusters is observed. The electrocatalytic properties of rhodium films with an average thickness up to ten monolayers were studied by using CO oxidation as a test reaction. The influence of the surface structure on the CO oxidation kinetics is discussed briefly.

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) ...

Surface-Enhanced Raman Scattering as a Ubiquitous Vibrational Probe of Transition-Metal Interfaces: Benzene and Related Chemisorbates on Palladium and Rhodium in Aqueous Solution

A recently developed strategy for utilizing surface-enhanced Raman scattering (SERS) to obtain uniquely detailed vibrational information for a myriad of organic (and other) adsorbates on transition metals in electrochemical and other ambient environments is illustrated for benzene and a pair of monosubstituted benzenes, toluene and benzonitrile, on palladium and rhodium films in aqueous solution. The transition-metal layers, formed by constant-current deposition onto SERS-active gold substrates, can be sufficiently thin (3−5 monolayers, ML) so to yield near-optimal Raman scattering intensities, yet are essentially devoid of exposed “pinhole” sites, thereby eliminating spectral interferences from adsorption onto gold. Benzene was selected in view of the detailed vibrational information also available for this archetypical organic chemisorbate on transition metals in ultrahigh vacuum (UHV) by means of electron energy loss spectroscopy (EELS). Comparison of the spectral information obtained by SERS and EELS ...

An FT-i.r. study of the decomposition of polyethylene over supported rhodium

The decomposition of polyethylene over supported rhodium films was investigated using FT-i.r. as an analytical probe. The process was observed to occur at relatively low temperature (190°C) and hydrogen pressure (13–53 kPa). The products detected by i.r. were methane and gaseous lower hydrocarbons. The active catalyst is believed to be Rh metal, rather than the RhCl 3 salt. Larger yields of methane were obtained when Rh/Al 2O 3 was the catalyst than for Rh/TiO 2, in contrast to observations for hydrogenation of carbon dioxide. The work demonstrates the utility of FT-i.r. for in situ investigations of polymer decomposition processes.

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.