Pd Rh Research Articles

Correlation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators

Recently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of -molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal -band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The -band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

Structural optimization of Pt–Pd–Rh trimetallic nanoparticles using improved genetic algorithm

Metallic nanoparticles have received considerable attention due to their superior catalytic properties. In this study, we proposed an improved genetic algorithm to optimize atomic structures of Pt–Pd–Rh trimetallic nanoparticles with various sizes and compositions. In the improved genetic algorithm, a layered coordinate ranking method and an effective fitness function were introduced to enhance its searching capability. The optimized results show that Pt and Pd atoms prefer to segregate on the surface, while Rh atoms tend to aggregate inside. Meanwhile, Pd atoms exhibit stronger surface segregation than Pt ones. Furthermore, the distribution of atomic coordination number was investigated.

Bimetallic Pd–Rh nanoparticles onto reduced graphene oxide nanosheets as electrocatalysts for methanol oxidation

Abstract An advanced electrode for methanol oxidation in alkali medium, comprising Pd–Rh nanoparticles and reduced graphene oxide (r-GO) sheets, has been successfully synthesized by pulse microwave polyol (MP) technique. The MP technique is capable of depositing Pd–Rh nanoparticles on the surface of r-GO sheets under microwave irradiation. The atomic ratio of Pd:Rh is chosen as a crucial factor to examine the catalytic activity toward H adsorption/desorption and methanol oxidation. The Pd–Rh catalyst (with Pd:Rh ratio = 75:25) displays the highest alloying degree (~ 41.5%), favoring the formation of Pd–Rh pairs in the binary crystals. On the Pd–Rh catalyst, the Pd site acts as major contributor toward hydrogen adsorption and methanol oxidation, whereas the neighboring Rh site serves as a promoting center to strip oxygenate species (e.g., Pd − (CO) ads sites). The enhanced catalytic activity on the Pd–Rh catalyst is attributed to the fact that the binary catalyst possesses a high alloying degree, leading to high catalytic coverage, high-level of CO tolerance, and superior cyclic durability. The Tafel slope of as-made Pd–Rh catalyst could reach as low as 63 mV dec − 1 among the electrodes, based on the analysis of Tafel plots. The Pd–Rh catalyst also exhibits both lower equivalent diffusion resistance and equivalent series resistance, as compared with pure Pd one.

Steam-biogas reforming over a metal-foam-coated (Pd–Rh)/(CeZrO2–Al2O3) catalyst compared with pellet type alumina-supported Ru and Ni catalysts

Metal-foam-coated 0.09wt% [Pd(7)–Rh(1)]/(CeZrO2–Al2O3) catalyst was compared with commercially available alumina-supported 8.0wt% Ru and 13.0wt% Ni catalysts for steam reforming of a model biogas. The experiments were conducted in a tubular reactor using a feedstock with the steam-to-methane ratio of 1.50 at 1atm pressure and 20,000h−1 GHSV. CH4 reforming reactions by steam alone and by CO2 alone were also studied over these catalysts in order to explain the catalyst activity. The Pd–Rh catalyst is superior in a number of ways, including better activity for CH4 and CO2 conversion in spite of much less ingredient loading per unit volume. The Pd–Rh catalyst also exhibited higher syngas production index [(H2 LHV+CO LHV)/(Feed CH4 LHV)], lower syngas H2/CO ratio and coke formation. The results were supported by BET measurement, CO chemisorption and electron microscopy. The experimental results demonstrate that the metal-foam-coated Pd–Rh/(CeZrO2–Al2O3) catalyst performs better for syngas production through steam-biogas reforming compared to the Ru and Ni catalysts.

Deposition of binary Pd–Rh catalysts on nanostructured carbon supports for non-enzymatic glucose oxidation

An efficient microwave-assisted (MA) method was adopted to synthesize binary Pd–Rh catalysts onto carbon powders (CPs), carbon nanotubes (CNTs), and reduced graphene oxide (GO) sheets for glucose oxidation reaction (GOR) in alkali electrolyte. The MA method is capable of preparing binary Pd–Rh alloys with well-defined atomic ratio of Pd:Rh (75:25) under a pulse microwave irradiation. It is observed that the catalyst electrode, consisted of well-dispersed Pd–Rh alloy and CNT framework, offers higher catalytic activity and superior cyclic stability, as compared to the other supports (i.e., CPs and GO sheets). This improved catalytic activity is ascribed to the fact that the Pd–Rh/CNT catalyst creates a unique three-dimensional framework, providing one-dimensional charge transfer and more active sites for the GOR. Analyzed by the Tafel plots, the Pd–Rh/CNT catalyst delivers not only the high exchange current but also the low Tafel slope, confirming the superior catalytic activity and excellent reaction kinetics toward the GOR. Accordingly, this study proves that the selection of carbon support plays a crucial role in affecting the catalytic activity and cycleability of GOR.

Selective ring opening of methylcyclohexane and decalin over Rh–Pd supported catalysts: Effect of the preparation method

Bimetallic Rh–Pd catalysts prepared by various impregnation methods were evaluated in the selective ring-opening of decalin and methylcyclohexane (MCH) used as model molecules of hydrogenated aromatics of the Light Cycle Oil fraction. Rh and Pt were deposited on Al2O3 and SiO2–Al2O3 (S40) by coimpregnation (CI) or successive impregnations (SI) with different orders of metal addition. Catalysts were characterized by H2 chemisorption, temperature-programmed reduction, temperature-programmed desorption of pyridine, and test reactions of cyclopentane hydrogenolysis and isomerization of 3,3-dimethyl-1-butene. Their catalytic behaviors for the ring-opening reaction were deeply influenced by the acidity of the support, and in a lesser extent by the metal deposition method. On both supports, the CI catalysts, displaying the highest dispersion values, exhibited the best ring opening performances. In the case of the SI catalysts, the addition order of both metals modified in a moderate way the properties of the catalyst. The yield to ring opening products obtained in MCH ring opening with CI catalysts supported on Al2O3 was 39–69% higher than those prepared by SI (conversion=55–60%) while for the catalysts supported on S40, the yield to ring opening products was 8–48% higher for the CI catalysts compared to SI catalysts (conversion=72–77%). The conversion of decalin for the bimetallic catalysts supported on Al2O3 was lower than 20% being dehydrogenated compounds the main reaction products regardless the preparation method. The catalyst supported on S40 showed decalin conversion values between 46 –53% and yield to ring opening near to 30%. Moreover, the CI catalysts have yield to ring opening 10 to 20% higher than the catalysts prepared by SI. The catalyst prepared by coimpregnation supported on S40 was the most appropriate for opening the MCH and decalin due to an optimal balance between the metal and acid functions.

Thermodynamic aspects of hydrogen electrosorption into Pd–Rh alloys

Abstract Pd–Rh alloys were electrodeposited potentiostatically from baths containing a mixture of aqueous solutions of PdCl 2 , RhCl 3 and HCl in various proportions. The thickness of the deposited layers was ca. 1 μm. The processes of hydrogen electrosorption were studied in 0.5 M H 2 SO 4 solution in the temperature range of 283–328 K, using chronoamperometry and cycling voltammetry. Thermodynamic parameters of hydride formation and decomposition (Gibbs energy — ΔG α → β , ΔG β → α , enthalpy — ΔH α → β , ΔH β → α and entropy — ΔS α → β , ΔS β → α ) were calculated from the potentials of the α → β and β → α phase transitions. The absolute values of Gibbs energy and enthalpy distinctly decrease with increasing bulk content of Rh, while the entropy values are only slightly affected by the alloy bulk composition. It means that with increasing bulk content of Rh the hydride phase formation in Pd–Rh alloys becomes less exothermic and the hydride itself becomes thermodynamically less stable. Absorbed hydrogen probably occupies both Pd and Rh neighboring interstices.

Partitioning of rhodium and ruthenium between Pd–Rh–Ru and (Ru,Rh)O2 solid solutions in high-level radioactive waste glass

The partitioning of rhodium and ruthenium between Pd–Rh–Ru alloy with a face-centered cubic (FCC) structure and (Ru,Rh)O2 solid solution has been investigated between 1273 and 1573 K at atmospheric oxygen fugacity. The rhodium and ruthenium contents in FCC increase, while the RhO2 content in (Ru,Rh)O2 decreases with increasing temperature due to progressive reduction of the system. Based on the experimental results and previously reported thermodynamic data, the thermodynamic mixing properties of FCC phase and (Ru,Rh)O2 have been calibrated in an internally consistent manner. Phase equilibrium of platinum grope metals in an HLW glass was calculated by using the obtained thermodynamic parameters.

Phase equilibrium experiments on the simulated high-level waste glass containing platinum group elements

Phase equilibrium experiments for the simulated high-level waste (HLW) glass containing palladium, rhodium and ruthenium discharged from a full scale mock up melter have been carried out between 1073 and 1473 K under air and CO2 atmosphere. The chemical compositions of Pd–Rh–Ru–Te (Pd metal phase) and (Ru, Rh)O2 (Ru oxide phase) solid solutions and glass matrix were measured by electron probe micro analysis. Palladium content in the Pd metal phase decreased and ruthenium content in the Ru oxide phase increased with increasing temperature and decreasing oxygen fugacity due to progressive reduction of rhodium. The chemical compositions of these crystalline phases are independent of that of borosilicate glass matrix. Partition coefficient of rhodium between Pd metal phase and Ru oxide phase (KRh) can be expressed bywhere T and represent absolute temperature and oxygen fugacity. Based on the comparison of crystal compositions in the HLW glass with experimental results, it can be expected that the HLW glass examined in this study had been equilibrated in the temperature range from 1273 to 1373 K and oxygen fugacity close to that of air just before discharging from the melter.

All-proportional solid-solution Rh–Pd–Pt alloy nanoparticles by femtosecond laser irradiation of aqueous solution with surfactant

Formation of Rh–Pd–Pt solid-solution alloy nanoparticles (NPs) by femtosecond laser irradiation of aqueous solution in the presence of polyvinylpyrrolidone (PVP) or citrate as a stabilizer was studied. It was found that the addition of surfactant (PVP or citrate) significantly contributed to reduce the mean size of the particles to 3 nm for PVP and 10 nm for citrate, which was much smaller than that of the particles fabricated without any surfactants (20 nm), and improved the dispersion state as well as the colloidal stability. The solid-solution formation of the Rh–Pd–Pt alloy NPs was confirmed by the XRD results that the diffraction pattern was a single peak, which was found between the positions corresponding to each pure Rh, Pd, and Pt NPs. Moreover, all the elements were homogeneously distributed in every particle by STEM-EDS elemental mapping, strongly indicating the formation of homogeneous solid-solution alloy. Although the Rh–Pd–Pt alloy NPs fabricated with PVP was found to be Pt rich by EDS observation, the composition of NPs fabricated with citrate almost exactly preserved the feeding ratio of ions in the mixed solution. To our best knowledge, these results demonstrated for the first time, the formation of all-proportional solid-solution Rh–Pd–Pt alloy NPs with well size control.

Nanostructure of metallic particles in light water reactor used nuclear fuel

An extraordinary nano-structure has been observed in the metallic (Mo–Tc–Ru–Rh–Pd) particles that are known to form during irradiated in light water nuclear reactor fuels. This structure points possible high catalytic reactivity through the occurrence of a very high surface area as well as defect sites. We have analyzed separated metallic particles from dissolved high burn-up spent nuclear fuel using scanning and transmission electron microscopy. The larger particles vary in diameter between ∼10 and ∼300nm and possess a hexagonally close packed epsilon-ruthenium structure. These particles are not always single crystals but often consist of much smaller crystallites on the order of 1–3nm in diameter with evidence suggesting the occurrence of some amorphous regions. It is possible that neutron irradiation and fission product recoils generated the unusual small crystallite size. The composition of the metallic particles was variable with low levels of uranium present in some of the particles. We hypothesize that the uranium may have induced the formation of the amorphous (or frustrated) metal structure. This unique nano-structure may play an important role in the environmental behavior of nuclear fuels.

Shaping Single-Crystalline Trimetallic Pt–Pd–Rh Nanocrystals toward High-Efficiency C–C Splitting of Ethanol in Conversion to CO2

Atomic-scale construction and high-throughput screening of robust multicomponent nanocatalysts with tunable well-defined surface structures and associated active sites for the ethanol electro-oxidation reaction (EOR) in high activity and selectivity, referring to C–C bond cleavage and full oxidation of ethanol as a clean and sustainable energy source, has remained a great challenge. Herein, we demonstrate a powerful conceptual approach to design, synthesize, and optimize single-crystalline Pt–Pd–Rh nanocrystals of altered shapes and compositions for enhanced EOR performance, based on combined density functional theory (DFT) calculations and experiment study. We prepared (111)-terminated Pt–Pd–Rh nanotruncated-octahedrons (NTOs) and (100)-terminated Pt–Pd–Rh nanocubes (NCs) with varied-compositions by regulating the reduction tendency of metal precursors in a facile hydrothermal method. Aided by DFT calculations, Pt3PdRh NTOs, PtPdRh NTO, and 8.8 nm PtPdRh NCs-200 were screened to be the best performing ca...

Controlled synthesis of bimetallic Pd-Rh nanoframes and nanoboxes with high catalytic performances.

Bimetallic nanoframes and nanoboxes of Pd-Rh are synthesized by selective removal of Pd cores from different Pd-Rh nanocubes prepared by a hydrothermal reaction of PdCl2, RhCl3 and HCHO. HCHO in the procedure alters the reaction kinetics and the growth behavior of Pd and Rh, resulting in different nanocubes that determine the following hollow nanostructures, nanoframes or nanoboxes. The catalytic properties of the hollow nanostructures are investigated using the oxidation of o-phenylenediamine (OPDA) to 2,3-diaminophenazine (DAP) as a model reaction. The resulting bimetallic nanoframes and nanoboxes show enhanced conversion efficiencies compared to their solid counterparts. This method offers a convenient way for mass production of bimetallic hollow nanomaterials.

Kinetically-controlled growth of cubic and octahedral Rh-Pd alloy oxygen reduction electrocatalysts with high activity and durability.

Rh is a promising candidate as an indispensible component in bimetallic catalysts due to its unique capability to resist against the aggressive corrosion from the reaction medium. However, Rh has a very strong oxygen binding ability and is generally not suitable for the oxygen reduction reaction (ORR). Here, we have demonstrated shape-controlled synthesis of Rh-Pd alloy nanocrystals with high activity and durability for ORR by retarding the reaction kinetics at an ultra-slow injection rate of metal salts using a syringe pump. Under precise control of sluggish reaction kinetics, Pd followed a preferential overgrowth along the <100> direction, whereas the growth behavior of Rh was dominant along the <111> direction. These different kinetically-controlled growth behaviors associated with Rh and Pd were essential for achieving the shape transition between the cube and the octahedron of their alloys. The Rh8Pd92 alloy octahedra exhibited the highest mass activity with a value of 0.18 mA μg(-1) in terms of the equivalent Pt cost, and were two-fold higher than that of commercial Pt/C. Significantly, all Rh-Pd alloy nanocrystals were highly stable with only less than 25% loss in mass activity after 30,000 CV cycles in O2 saturated acid solution compared to ∼ 56% loss of the commercial Pt/C (E-TEK). Indeed, the mass activity of Rh8Pd92 was 3.3 times higher than that of commercial Pt/C after the accelerated stability test (ADT). This improvement in activity and durability may arise possibly from synergistic effects between the facet and the surface composition.

Influence of the support on the selective ring opening of methylcyclohexane and decalin catalyzed by Rh–Pd catalysts

Rh–Pd catalysts supported on Al2O3, SiO2 and SiO2–Al2O3 (SIRAL 40) were studied for methylcyclohexane (MCH) and decalin ring opening reactions. It was found that the Rh/Pd atomic ratios were similar to the theoretically expected one and the metal dispersion values varied between 30 and 50%. On bimetallic catalysts supported on Al2O3 and SiO2, Pd and Rh were present mainly in the form of monometallic particles, whereas a heterogeneous distribution constituted of large Pd particles and small bimetallic ones were observed on SIRAL 40. Total and Bronsted acidities followed the order: SIRAL 40>>Al2O3>SiO2. Al2O3 supported catalysts were the most suitable for MCH ring opening, while the opening of decalin was favored by using SiO2–Al2O3 (SIRAL 40). The catalysts supported on SiO2 produced mainly dehydrogenated compounds. The higher total and Bronsted acidities of SIRAL 40 series had a great impact on the opening of bicyclic naphthenes, whereas the metal function of the catalyst and the hydrogenolytic activity of Rh had a major role during MCH reaction. Using bimetallic catalysts in the MCH reaction decreased the formation of cracking and dehydrogenated products, and the selective formation of RO products could be optimized by tuning the working temperature.

Determination of the equilibrium miscibility gap in the Pd–Rh alloy system using metal nanopowders obtained by decomposition of coordination compounds

The Pd–Rh phase diagram has been reinvestigated in the subsolidus region using X-ray diffraction, scanning and transmission electron microscopy. The true equilibrium at the miscibility boundary was achieved with the two-way approach. Nanosized powders of metastable solid solutions and two-phase palladium–rhodium mixtures were used to shorten the time required to equilibrate the system. The initial samples were prepared by decomposition of coordination compounds [Pd(NH3)2Cl2], [Rh(NH3)5Cl]Cl2, [Pd(NH3)4]3[Rh(NO2)6]2 and [Pd(NH3)4][Rh(NH3)(NO2)5].The obtained phase diagram exhibits miscibility gap wider than generally accepted with the critical point of solubility at 58at.% Rh and 820°C.

Oxidation and Surface Segregation Behavior of a Pt–Pd–Rh Alloy Catalyst

Platinum gauze catalysts are used extensively in the production of nitric acid from ammonia, where they are subject to harsh operating conditions combining elevated temperatures and oxidizing environments. These cause significant loss of metal species as volatile oxides. How different metallic species behave in these environments at a fundamental, atomic-scale level is not well understood. In this work, we study the early stages of oxidation of a Pt–Rh–Pd gauze at temperatures of 873–1273 K. Using a combination of advanced experimental methods, we explore how the oxidation behavior can strongly influence the surface and near-surface gauze microstructure. We show that Rh and Pd can segregate on different areas of the same surface and discuss how such atomic migration can be linked to mechanisms of metal loss from such alloys.

Effect of Mo content on thermal and mechanical properties of Mo–Ru–Rh–Pd alloys

Metallic inclusions are precipitated in irradiated oxide fuels. The composition of the phases varies with the burnup and the conditions such as temperature gradients and oxygen potential of the fuel. In the present work, Mox0.7+x(Ru0.5Rh0.1Pd0.1)0.70.7+x (x=0, 0.05, 0.1, 0.15, 0.2, and 0.25) alloys were prepared by arc melting, followed by annealing in a high vacuum. The thermal and mechanical properties of the alloys such as elastic moduli, Debye temperature, micro-Vickers hardness, electrical resistivity, and thermal conductivity have been evaluated to elucidate the effect of Mo content on these physical properties of the alloys. The alloys with lower Mo contents show higher thermal conductivity. The thermal conductivity of the alloy with x=0 is almost twice of that of the alloy with x=0.25. The thermal conductivities of the alloys are dominated by electronic contribution, which has been evaluated using the Wiedemann–Franz–Lorenz relation from the electrical resistivity data. It is confirmed that the variation of the Mo contents of the alloys considerably affects the mechanical and thermal properties of the alloys.

Exergetic study of catalytic steam reforming of bio-ethanol over Pd–Rh/CeO2 with hydrogen purification in a membrane reactor

An exergy analysis of the ethanol steam reforming process using a Pd–Rh catalyst supported on ceria in a Pd–Ag membrane reactor has been performed based on experimental data. Both chemical and physical exergy for chemical species together with thermodynamic loss calculations have been considered to evaluate the exergy efficiency of the process. The system has been studied under different operational conditions in terms of temperature (873–923 K), pressure (4–12 bar), and feed flow rate using a water–ethanol mixture of 50–50% volume (S/C = 1.6). A significant exergy efficiency dependency on the fuel flow rate and pressure has been encountered when considering the heat loss from the reactor. The best results regarding the exergetic and thermal efficiency have been obtained at high flow rates and 8–12 bar, which has been attributed to the higher hydrogen permeation through the membrane. The chemical reactions involved in the reforming process and the heat losses are the main sources of exergy destruction.

Effects of Pd–Rh Composition and CeZrO2-Modification of Al2O3 on Performance of Metal-Foam-Coated Pd–Rh/Al2O3 Catalyst for Steam Reforming of Model Biogas

Catalysts prepared by coating 1.31 wt% (Pd–Rh)/alumina composites on metal foam substrate were tested for steam reforming of model biogas (SRB) at 1 atm pressure to study the effects of Pd–Rh composition and CeZrO2-modification of Al2O3. Among the catalysts supported on unmodified alumina, [Pd(7)–Rh(1)] catalyst performed slightly better with respect to CH4 conversion as well as H2 and CO yields, whereas [Pd(3)–Rh(1)] catalyst performed slightly better for suppression of coke formation. Modification of Al2O3 with 49 wt% CeZrO2 improved catalytic activity and metal dispersion of [Pd(7)–Rh(1)] catalyst, despite 25 % loss of BET surface area. For the biogas-reforming reaction at 998 K and GHSV 1,400 h−1 with steam/methane (S/C) ratio of 1.50 in the feed, [Pd(7)–Rh(1)]/(CeZrO2·Al2O3) exhibited 92 % CH4 conversion in a microchannel-structured heat exchanger platform reactor, 20 % more than the conversion obtained over [Pd(7)–Rh(1)]/Al2O3. H2 and CO yields were also enhanced by CeZrO2-modification of Al2O3, however, these enhancements were accompanied by H2/CO selectivity decrease and CO/(CO + CO2) selectivity increase. Variation of Pd–Rh composition and CeZrO2-modification of Al2O3 did not affect catalytic stability in the feed with S/C = 1.50 for 200 h at 1,023 K and GHSV 20,000 h−1 in a tubular packed-bed reactor, while CeZrO2-modification of Al2O3 improved resistance to deterioration of surface area, pore structure and metal dispersion.