Palladium-rhodium Alloys Research Articles

Dihydride formation in the palladium–rhodium alloys under high hydrogen pressure

Palladium, rhodium and their alloys were studied in a diamond anvil cell in a high pressure hydrogen atmosphere up to 20 GPa by X-ray diffraction at room temperature. Formation of a monohydride with H/(Rh + Pd) = 1 ratio was observed for each alloy at hydrogen pressures less than 3 GPa. The corresponding volume expansion of the metal lattice was 2.2–2.8 Å3 per metal atom. A formation of the dihydride with H/(Rh + Pd) = 2 was observed at a hydrogen pressure around 10 GPa for the alloys with rhodium content Rh/(Rh + Pd) ≥ 50 at.%. The dihydride formation is accompanied by a volume expansion of metal lattice by 3.2–3.9 Å3 per metal atom. For Pd-rich alloys with Rh/(Rh + Pd) ≤ 25 at.% and pure Pd the dihydride formation was not observed up to maximum reached hydrogen pressure. All observed phases had a fcc structure of metal lattice.

Hydrogen sorption characteristics of nanostructured Pd–10Rh processed by cryomilling

Palladium and its alloys are model systems for studying the solid-state storage of hydrogen. Mechanical milling is commonly used to process complex powder systems for solid-state hydrogen storage; however, milling can also be used to evolve nanostructured powder to modify hydrogen sorption characteristics. In the present study, cryomilling (mechanical attrition milling in a cryogenic liquid) is used to produce nanostructured palladium–rhodium alloy powder. Characterization of the cryomilled Pd–10Rh using electron microscopy, X-ray diffraction and surface area analysis reveal that (i) particle morphology evolves from spherical to flattened disk-like particles; while (ii) crystallite size decreases from several microns to less than 100nm; and (iii) dislocation density increases with increased cryomilling time. Hydrogen absorption and desorption isotherms as well as the time scales for absorption were measured for cryomilled Pd–10Rh, and correlated with observed microstructural changes induced by the cryomilling process. In short, as the microstructure of the Pd–10Rh alloy is refined by cryomilling: (i) the maximum hydrogen concentration in the α-phase increases, (ii) the pressure plateau becomes flatter and (iii) the equilibrium hydrogen capacity increases at pressure of 101.3kPa. Additionally, the rate of hydrogen absorption was reduced by an order of magnitude compared to non-cryomilled (atomized) powder.

Influence of temperature on hydrogen electrosorption into palladium-noble metal alloys. Part 3: Palladium–rhodium alloys

Hydrogen electrosorption into Pd–Rh alloys (containing more than 60at.% Pd in the bulk) was studied in acidic solution (0.5M H2SO4) using cyclic voltammetry and chronoamperometry. Alloys were obtained by electrodeposition from baths containing PdCl2, RhCl3 and HCl. The influence of temperature (in range between 283K and 328K) and alloy bulk composition on parameters of hydrogen sorption in Pd–Rh alloys was examined. It has been found that for the temperature range studied the maximum hydrogen absorption capacity is the highest in 283K and decreases with temperature increase. It has been established that potentials of hydrogen phase transitions (α→β and β→α) are shifted negatively with increasing temperature. The potential of absorbed hydrogen oxidation peak is shifted negatively with increasing temperature. With increasing Rh bulk content the phase transition potentials and hydrogen oxidation peak are shifted into lower potentials. An alloy containing 92.6at.% Pd is characterized by the highest hydrogen absorption capacity with H/M ratio exceeding 0.80.

Characterization of Oxidation and Reduction of a Palladium–Rhodium Alloy by Atom-Probe Tomography

Platinum group metals (PGMs) are used in numerous catalyst applications, including conversion of engine exhaust gases and hydrocarbon reforming. Reducing the loading of PGMs without diminishing the overall catalyst activity is a major challenge. Fundamental studies of PGMs under reactive conditions can assist the design/synthesis of “nanoengineered” catalysts, tunable and optimized for cost, stability, and performance. In the present study, the oxidation and reduction behavior of a Pd–6.4 at. % Rh alloy is investigated following treatment at 873 K for various exposure times using atom-probe tomography. For short oxidation times (10 min), an oxide layer with PdO stoichiometry grows on the surface. As the oxidation time increases, two phases with stoichiometries of (Rh1Pd1)O2 and (Pd2O) evolve. When the alloy is subsequently reduced in hydrogen, a nanoscale dispersion of Rh-rich metallic regions remains. This provides a route for the synthesis of multifunctional catalysts with different nanosurface regions in close proximity to one another.

Electrosorption of hydrogen into palladium–rhodium alloys: Part 2. Pd-rich electrodes of various thickness

Hydrogen electrosorption into Pd-rich (≥80% Pd) Pd–Rh alloy layers of various thickness has been studied in acidic solutions (0.5 M H 2SO 4) using cyclic voltammetry and chronoamperometry. Pd–Rh electrodes were prepared as limited volume electrodes (LVEs) by electrochemical deposition. The influence of electrode potential and alloy thickness on the amount of electrosorbed hydrogen was investigated. For the thickness range examined (0.11–1.55 μm) no distinct effect on alloy absorption properties was found. The influence of scan rate on hydrogen oxidation charge was also studied. The results obtained suggest that the layer of subsurface hydrogen in Pd–Rh alloys is much less developed than in pure Pd and direct hydrogen absorption/desorption path is more probable.

Electrosorption of hydrogen into palladium–rhodium alloys

Hydrogen electrosorption into Pd–Rh alloys has been studied in acidic solutions (0.5 M H 2SO 4) using cyclic voltammetry and chronoamperometry. Pd–Rh electrodes were prepared as thin alloy electrodeposits on Au wires. The influence of electrode potential on the amount of electrosorbed hydrogen was investigated. One can distinguish potential regions of α- and β-phase existence as well as the region of α–β phase transition. For alloys containing less than 20 at.% Rh in the bulk the hydrogen-to-metal ratio, H/(Pd + Rh), is greater than for pure Pd. The maximum value of the H/(Pd + Pt) ratio is 0.80 for an alloy containing in the bulk 94 at.% Pd and 6 at.% Rh. Due to a smaller lattice parameter of the Pd–Rh alloy with respect to pure Pd the α–β phase transition occurs at lower potentials than in Pd. The hysteresis is observed in chronoamperometric absorption and desorption experiments; the effect of hysteresis decreases with the increase in Rh content. Additionally, preliminary results are presented concerning the electrochemical quartz crystal microbalance measurements of hydrogen absorption/desorption into/from Pd–Rh alloys in comparison with Pd.

Deuterium solubility and electrical resistance of palladium-rhodium alloys

The deuterium solubility and the corresponding change in electrical resistance of Pd-5.0 and -10.0 at.% Rh alloys have been investigated at different temperatures by a gas phase method. In these studies, a decrease in low pressure deuterium solubilities and increase in plateau pressures with increasing Rh content have been observed. The decrease in lattice size with increasing Rh content seemed responsible for it. The miscibility gap decreased with increasing Rh content. Higher deuterium solubility was attained for lower Rh content alloy. The studied systems showed remarkable p-c hysteresis. The loss in energy due to hysteresis for the Pd-10.0 at.% Rh–D system showed lower values than that of the Pd-5.0 at.% Rh–D system. The α max resistance values are gradually decreased with increasing Rh content and increased with increasing temperature. The initiation of β deuteride creates pronounced lattice deformation and R/ R 0 values increased sharply. For a higher Rh content alloy, the increase in R/ R 0 values during β hydride initiation is higher, probably for its smaller lattice size. At higher temperatures, a lowering of resistance occurred when the system entered from a coherent state to an incoherent state. In the main plateau region, the change in R/ R 0 showed small values. From D/M content of about 0.55, a pronounced lowering of resistance, was observed, probably due to approaching of the β phase structure to a NaCl type lattice. Remarkable resistance hysteresis was observed at the studied temperatures. For the Pd-5.0 at.% Rh–D system a larger resistance hysteresis appeared than that of the Pd-10.0 at.% Rh–D system. Larger lattice expansion and the creation of higher lattice deformation in the β phase region due to a higher amount of deuterium absorption seemed responsible for it. The R/ R 0-c behavior showed a good similarity with that of their hydrogen system found by the same method, but dissimilar to that reported by the electrochemical method.

Measurements of hydrogen solubility and electrical resistance of some palladium–rhodium alloys by a gas phase technique

The hydrogen solubility and the corresponding change in electrical resistance of Pd–5 and 10 at.% Rh alloys have been investigated by a gas phase technique. It was observed that low pressure hydrogen solubilities decreased with increasing Rh content, whereas, the increment of main plateau pressures showed opposite behavior. The α max and β min contents were increased with increasing Rh content but the widths of the miscibility gaps showed almost similar values. Higher hydrogen solubility was attained for higher Rh content alloy. It seemed that pressure hysteresis was increased with increasing Rh content but practically it does not, because the loss in useful available energy for the 10 at.% Rh–H system showed lower values than that for the 5 at.% Rh–H system. The ( R⧸ R 0) α max values are gradually increased with increasing temperature but decreased with increasing Rh content. During the initiation of β hydride, pronounced increments of R⧸ R 0 values are observed probably due to lattice deformation. However, extent of resistance increment due to β hydride initiation increased with increasing Rh content but decreased with increasing temperature. At higher temperature, a lowering of resistance occurred when the system entered from coherent state to incoherent state. In the main plateau region, change of R⧸ R 0 value is small but from H⧸ M contents about 0.55, a pronounced lowering of resistance was observed due to approaching of β phase structure to NaCl type. A remarkable resistance hysteresis appeared between 323–373 K, whereas almost equal R⧸ R 0 values were attained for the Pd–5 at.% Rh–H system at 398 K. It occurred probably due to the annihilation of plastic deformations during hydride decomposition which were created during β hydride formation. For the 5 at.% Rh–H system a larger resistance hysteresis appeared than that of the 10 at.% Rh–H system. A larger lattice expansion and concurrent creation of a higher lattice deformation in the β phase region of 5 at.% Rh–H system seemed responsible. At 348 K, an analogous R⧸ R 0 – c behavior was observed between the Pd–10 at.% Rh–H(D) systems.

Hydrogen-Induced Phase Separation of Palladium-Rhodium Alloys Using an Environmental Cell TEM

The palladium-rhodium system has been extensively studied for its hydrogen absorption characteristics. However, the phase diagram of the palladium-rhodium system has not been conclusively determined below 800 K. Shield and Williams have experimentally determined the incoherent miscibility gap in Pd-Rh alloys using electrical resistivity studies, however the coherent miscibility gap and spinodal have not been determined. Recently work by Noh and Flanagan has suggested that hydrogen enhances metal atom mobility and may increase the kinetics of phase separation in Pd-Rh alloys. Field and Thoma found that hydrogen causes a Pd-10%Rh alloy to decompose during an in situhydrogen charging experiment in an environmental cell TEM. According to the calculations by Gonis et al. of the miscibility gap for the palladium-rhodium system, the Pd-10%Rh alloy may be within the chemical spinodal at room temperature.In this work, two palladium-rhodium compositions were investigated. The first was a Pd-10 at.% Rh alloy produced by melt-spinning, and the second was a Pd-30at.%Rh alloy which had been arc-melted and cold rolled followed by an annealing treatment to homogenize the material. TEM specimens were prepared by punching 3 mm disks from the material.

Structure of palladium-rhodium alloys below their melting points.

A new method for calculating the boundaries between two solid alloy phases has been developed. It is first principles in that the only input is the atomic numbers of the constituents, and differs from previous efforts in that no interaction potentials are introduced. It is used to calculate the miscibility gap in the palladium-rhodium phase diagram.

Hydrogen-induced metal atom mobility in palladium-rhodium alloys

Hydrogen-induced metal atom mobility in palladium-rhodium alloys

Magnetostriction of dilute hydrogenated palladium-rhodium alloys

The longitudinal and transverse magnetostriction of polycrystalline Pd, Pd99Rh1, Pd97Rh3 and Pd95Rh5 is measured at 4.2K before and after charging with about 1 at.% H. The alloys are believed to be in the alpha phase and the systematic decrease in the volume magnetostriction is explained in a semi-rigid band model as being due to filling the 4d band at a rate of 0.3+or-0.2 states per H atom. The shape magnetostriction is unchanged by hydrogenation.

The effect of microsegregation of the hydrogen solubility of palladium-rhodium alloys

The effect of microsegregation of the hydrogen solubility of palladium-rhodium alloys

Paramagnetostriction of palladium and palladium-rhodium alloys

The longitudinal and transverse magnetostriction of single-crystal Pd and three PdRh alloys was measured. The volume magnetostriction was found to increase as Rh was added to Pd up to a maximum value about a factor of four larger than that of Pd at about 5 at.% Rh in Pd. The shape magnetostriction remained constant and roughly equal to the large value observed in Pd. This strong anisotropy of the magnetostriction is therefore not due to the stress-dependent anisotropy of the exchange-enhanced spin susceptibility, and is more likely to result from the stress-independent anisotropy of the spin-orbit interaction.

Change in composition of surface layer of palladium-rhodium alloys during catalytic reaction

1. A substantial enrichment of the presurface layer in rhodium of palladium alloys, containing 5 and 10% Rh, after cyclohexane was dehydrogenated on them, was detected by the x-ray photoelectronic spectroscopy (XPES) method. 2. Enrichment of the surface of the foil in rhodium is accompanied by an increase in the dehydrogenation rate of cyclohexane to benzene.

Influence of the composition and conditions of heat treatment of palladium-rhodium alloys on their catalytic activity with respect to the dehydrogenation of cyclohexane

1. The catalytic activity with respect to the dehydrogenation of cyclohexane on metallurgical alloys of Pd, containing from 7 to 20% Rh, decreases with the removal of H2 from the alloy and is entirely restored after treatment of the alloys with H2. The catalytic activity of the alloys increases when the temperature of their treatment with air is increased from 350 to 750‡. 2. An increase in the catalytic activity of Pd-Rh alloys was detected after experiments conducted at 450–470‡ in a stream of argon, evidently associated with the removal of firmly adsorbed hydrogen.

The oxidation of ethylene over evaporated palladium-rhodium alloy films: II. Variation of activity with film composition and structure

Rates of CO 2 formation when ethylene was completely oxidized in a static system over evaporated PdRh alloy films showed a complex variation with alloy composition. This activity pattern generally reflected the variations in activation energy, E, with composition; values of E and log (frequency factor) fell on a linear plot, indicating a compensation effect, with the exception of pure Rh which is discussed. The activity pattern can be interpreted in terms of the film structure and properties occurring in the three composition ranges distinguished as follows: 0–30% Rh; increased activity compared with pure Pd, expected from bulk electronic properties and previous results with PdAg alloy films, was not observed. Instead the activity passes through a minimum at 10–15% Rh. It is believed that at this composition the alloy has a reasonable capacity to dissolve hydrogen (from C 2H 4) which would reduce the activity by filling d-band holes, combined with a decreased ability to effect H 2O 2 combination. An alternative explanation, based on the onset of phase-separation at the surface at lower Rh contents than expected, was tested by CO oxidation. 30–80% Rh; the activity decreased continuously with increasing Rh content in alloys composed of two “phases,” the Rh-rich Phase II is believed to form the kernel of the crystallites, surrounded by a variable-composition Phase I. The activity results were re-plotted assuming that the surface composition can be equated with the composition of Phase I given by X-ray data in Part I. Apparently anomalous results fitted into place and a maximum in activity below 30% Rh, which would occur in the absence of hydrogen solubility, was indicated. 80–100% Rh; some enrichment of the surface by Pd may occur; the decrease in activity between 98% and pure Rh is discussed, as well as its very low absolute activity.

The oxidation of ethylene over evaporated palladium-rhodium alloy films: I. Film structure with special reference to phase-separation

Recent thermodynamic data for Pd-Rh alloys confirm earlier experimental observations that annealing for very prolonged periods below 850 °C causes separation into two FCC phases over a wide composition range. Palladium-rhodium alloy films, prepared by slow simultaneous evaporation of the component metals on to a glass substrate at 400 °C, show good bulk homogeneity (expected lattice constants, symmetrical X-ray diffraction profiles) at the ends of the composition range (0–30 and 80–100% Rh) but tend to phase-separation at intermediate compositions. After use as catalysts for ethylene oxidation, Pd-Rh alloy films with 30–80% Rh show clear evidence of two “phases” contributing to the diffraction profile; Phase I varied in composition and was deficient in Rh compared with the corresponding solid solution and Phase II had a composition of 88 ± 5% Rh in accord with the solubility diagram. Palladium-rich films showed strong (111) preferred orientation, weakening with increasing Rh content so that the crystallites in almost pure Rh films had approximately random orientation. The formation of Pd-Rh alloy films is considered with regard to the interpretation of the catalytic results according to composition: 0–30% Rh; diffusion during film formation and use is limited and inhibits the phase-separation indicated, giving a homogeneous alloy. Evidence was found for hydrogen solubility in alloy films with <10% Rh after carrying out ethylene oxidation with partial consumption of the reactants, indicating a subdivision of this composition range. (Films with up to ~30% Rh could be charged with hydrogen if all the oxygen was consumed, leaving unreacted ethylene.) 40–80% Rh; a mechanism is proposed involving the preferential nucleation of Rh whereby Phase II can form in conditions of limited diffusion as a Rh-rich kernel to the crystallite, surrounded by a Rh-deficient solid solution of variable composition. 80–100% Rh; the process continues but the “phases” are sufficiently close in composition to remain undetected.

Catalytic activity of noble metal alloys. Methane-deuterium exchange and propane cracking on platinum-palladium and palladium-rhodium alloys

The exchange reaction between methane and deuterium and the cracking of propane in the temperature range 100 ° to 200 ° have been used to study variations in catalytic activity in the PtPd and PdRh alloy series. Stepwise exchange was exhibited by both Pt and Pd, CH 3D being the most abundant initial product and the other deuteromethanes appearing in order. Rh showed predominantly multiple exchange at the higher temperatures with the initial appearance of CD 4. Stepwise exchange was the most important mechanism with all alloy samples although the contribution of multiple exchange increased with increasing temperature. The order of activity of the pure metals followed the sequence Pt Rh Pd at 150 ° for both exchange and cracking reactions and the binary alloys showed intermediate activities. A rough correlation between atomic radius and exchange mechanism was observed and the mechanism of the exchange reaction is discussed.

The magnetic susceptibilities of palladium and palladium-rhodium alloys from 1.85 to 293 °K

Rapid increases in the susceptibilities of palladium-rich alloys have been observed at low temperatures. From the difference in susceptibilities of two specimens of nominally pure palladium, these increases appear to be due to impurities and are consistent with the assumption that the impurities give rise to a form of super paramagnetism with carriers of moments of the order of 9 Bohr magnetons. This forms the basis for the correction of the measured susceptibilities of the alloys for impurity contribution and the extrapolation of the susceptibility values to 0 °K. By combining these results with the results of measurements of the electronic contribution to the specific heat, the variation of Stoner’s exchange parameter θ' with composition is deduced. The form of this experimentally deduced variation suggests that the previously reported peak in the density of states curve for palladium lies closer to the Fermi level for palladium than hitherto supposed.