PtCu Alloy Research Articles

Platinum-Group Mineral Characterization in Concentrates from High-Grade PGE Al-rich Chromitites of Korydallos Area in the Pindos Ophiolite Complex (NW Greece)

The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly nonstoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu 4 ), Pd-bearing tetraauricupride [(Au,Pd)Cu], nielsenite (PdCu 3 ), sperrylite (PtAs 2 ), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd) Cu 3 ], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd 11 Sb 2 As 2 ), zvyagintsevite (Pd 3 Pb), native Au, keithconnite (Pd 20 Te 7 ), naldrettite (Pd 2 Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 µm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphurbearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian

THE PLATINIFEROUS Au-Pd BELT OF MINAS GERAIS, BRAZIL, AND GENESIS OF ITS BOTRYOIDAL Pt-Pd AGGREGATES

We delineate an approximately 240-km-long, north-trending Au-Pd-Pt belt in Minas Gerais, Brazil, which hosts hydrothermal Pd-Pt-bearing gold mineralization in its southern part (Quadrilatero Ferrifero and Itabira district) and alluvial Pd-Pt-Au mineralization in its northern part (southern Serra do Espinhaco). The hydrothermal Pd-Pt-bearing gold mineralization, locally known as jacutinga, occurs as late-orogenic quartz-hematite-(talc-kaolinite) veins, which truncate the Brasiliano regional tectonic foliation of the itabiritic host rocks of the Paleoproterozoic Itabira Iron Formation. The alluvial Pd-Pt-Au mineralization occurs as botryoidal Pd-Pt nuggets and delicate palladiferous gold aggregates in clastic and organic-rich valley infill on quartzite and hematitic rocks of the Paleo- to Mesoproterozoic Espinhaco Supergroup. The Au-Pd-Pt belt follows the regional trend of major thrust faults related to the ~0.6 Ga Brasiliano orogeny, and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum. Secondary botryoidal and arborescent Pt-Pd aggegates and primary (detrital) Au-Pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the Serra do Espinhaco. We suggest that their source was jacutinga-style vein mineralization, as deduced from abundant detrital hematite and rutile, and the discovery of detrital hongshiite, a Pt-Cu alloy that forms bonanza pockets within jacutinga at Itabira. Reconnaissance laser ablation-inductively coupled plasma-mass spectrometry analyses for trace metals in the botryoidal Pt-Pd grains show that they have elevated Tl and Se contents of 0.9 to 354 and 54 to 2,700 ppm, respectively. These elements correlate positively, with a Tl/Se ratio of about 0.08, a value that contrasts with Tl/Se >1 for hongshiite from the hydrothermal jacutinga system at Itabira. The low and roughly constant Tl/Se ratios of the alluvial samples are close to the average value of 0.12 for fluvial waters. We suggest that the botryoidal Pt-Pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying Pt and Pd from the dissolution of detrital Pt-Pd minerals from jacutinga. The connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for Pd-Pt-bearing gold mineralization reaching from Ouro Preto to Diamantina. The very high oxidation state required for efficient low-temperature aqueous mobilization of Pt, Pd, and Au was apparently controlled by the quartzitic and hematitic rocks in the Serra do Espinhaco, but not in the southern belt where greenstone rocks buffered fluids to moderate pH and lower oxidation states.

Analysis of thermoreflectance signals and characterization of thermal conductivity of metal thin films

The temperature response of the thermal conductivity (lambda) of metal thin films has been investigated by the thermoreflectance (TR) method. The phase lag of the TR signals depends on the thermal diffusivity when the heating area is small, while on the thermal effusivity when the heating area is large. This enables us to evaluate lambda by analyzing the three-dimensional thermal propagation in the film on the substrate. We show that by analyzing the TR signals, lambda of Cu-Pt alloy thin films formed on glass substrates can be estimated. The estimated lambda drastically decreases with an increase in the Pt concentration. Furthermore, we discuss these results by considering the crystallographic properties of the abovementioned thin films investigated by transmission electron microscopy and x-ray diffraction.

Synthesis of CuPt Nanorod Catalysts with Tunable Lengths

Solution chemistry methods have been used to synthesize bimetallic CuPt alloy nanoparticle catalysts with controllable sizes and shapes. By variation of the relative ratios of the oleylamine and oleic acid stabilizers, solvent, and reduction rate, the nanoparticles could be tuned from approximately 2 nm spherical particles to nanorods with diameters of approximately 2.5 nm and aspect ratios tunable from 5:1 to 25:1. These mixed-metal nanoparticles show excellent catalytic properties for CO oxidation, with light-off temperatures that are nearly 200 K below those of conventional supported Pt catalysts.

Diffusion Mobilities in fcc Cu-Au and fcc Cu-Pt Alloys

Abundant diffusion data in binary fcc Cu-Au and fcc Cu-Pt alloys have been assessed to obtain the atomic diffusion mobilities of fcc Cu-Au and fcc Cu-Pt alloys by the CALPHAD method. The obtained mobilities are expressed as functions of compositions and temperatures. Good agreements are obtained from comprehensive comparisons between the calculated and experimentally measured self-diffusion coefficients, impurity diffusion coefficients, tracer diffusion coefficients, interdiffusion coefficients, and concentration curves. Accordingly, the developed mobility parameters, in conjunction with the CALPHAD-type thermodynamic descriptions, can be used to simulate diffusion behaviors at high temperatures. It is believed that the results of this work contribute to the design of a general Cu mobility database.

The Influence of Platinum Surface Morphology on the Electrooxidation of Methanol in Alkaline Solutions

In this work we compare methanol oxidation characteristics in pH 0 and pH 14 electrolytes, and examine the effect of methanol concentration and platinum surface condition (e.g. H-UPD, OH adsorption, Pt oxidation/reduction, planar, nanoporous) on the oxidation current. We observe that in rotating disc electrode (RDE) experiments the oxidation currents on smooth platinum decrease with increasing rotation rate. We show that this decrease is associated with increasing the rate at which oxidation intermediates are swept into the bulk electrolyte before being fully oxidized. To increase the dwell time of intermediates a platinum electrode with 3 nm length scale surface porosity was synthesized by dealloying a Pt-Cu alloy. We then perform the same RDE experiments and show that in this case the methanol oxidation current increases with increasing convection. We attribute this behavior to the trapping of reaction intermediates that otherwise were swept away on the polished sample.

An attempt to solve Andersen’s puzzle in surface segregation during alloy sputtering

The paper addresses CuPt alloy sputtering by Ar ions and discusses the well-known experiment performed by Andersen et al. 25 years ago, but not yet properly explained. The atomistic (binary-encounter) simulation has been applied to extract the concentrations of surface Cu and Pt atoms from the experimental data. The results of simulations favor segregation of Cu at all bombarding energies studied experimentally (1.25–320 keV). It has been shown that some mysterious results of the experiment can be explained by a reconstruction of the surface undergoing sputtering. For forecasting purposes, the sputtering of CuPt alloy with 0.25–1 keV Ar ions is also considered.

Aging Studies of Voltammetrically Dealloyed Pt-Cu Nanoparticle ORR Electrocatalysts

Voltammetrically dealloyed Pt-Cu alloy nanoparticles show significantly improved Pt-mass and specific activities for the oxygen reduction reaction in acidic media in both RDE and MEA configurations. The active phase of the catalyst is prepared from very Cu-rich precursor materials which are formulated into catalyst inks and electrode layers. The high Cu content of the precursor materials raises fundamental and practical questions as to the molecular stability / oxidation state of the Cu rich catalyst surface and the shelf-life of precursor powders, inks and electrode layers. Here we present aging studies of a carbon-supported Pt25Cu75 nanoparticle catalyst precursor. We monitored how the surface area (ECSA) and the oxygen electroreduction (ORR) reactivity of dealloyed precursors (active form of catalyst) change as a function of the exposure time to air of the Pt25Cu75 precursor at room temperature. Similarly, we aimed to investigate how ECSA and ORR of the active catalyst changed with the time the liquid catalyst ink was exposed to air. We find little detrimental effect of air aging of the precursor on ORR activity over the time scales investigated. The effect of air aging of the catalyst inks is more complex, however. Initially, aged inks show increased ORR reactivity, that is, aged inks result in more active catalyst layers. After about 4-5 days of aging of the inks in air, however, we observed a drop in ORR activity of the resulting catalyst layers. This complex aging behavior of the inks seems independent of prior air aging of the dry powder precursor.

Voltammetric surface dealloying of Pt bimetallic nanoparticles: an experimental and DFT computational analysis

Voltammetric dealloying of bimetallic platinum-copper (Pt-Cu) alloys has been shown to be an effective strategy to modify the surface electrocatalytic reactivity of Pt bimetallic nanoparticles (S. Koh and P. Strasser, J. Am. Chem. Soc., 2007, 129, 12624). Using cyclic voltammetry and structural XRD studies, we systematically characterize the Pt-Cu precursor compounds as well as the early stages of the selective Cu surface dissolution (dealloying) process for Pt(25)Cu(75), Pt(50)Cu(50), and Pt(75)Cu(25) alloy nanoparticles annealed at both low and high temperature. We also assess the impact of the synthesis conditions on the electrocatalytic reactivity for the oxygen reduction reaction (ORR). To gain atomistic insight into the observed voltammetric profiles, we compare our experimental results with periodic DFT calculations of trends in the thermodynamics of surface Cu dissolution potentials from highly stepped and kinked Pt(854) single crystal surfaces. The modeling suggests a dependence of the electrochemical Cu dissolution potentials on the detailed atomic environment (coordination number, nature of coordinating atoms) of the bimetallic Pt-Cu surfaces. The DFT-predicted shifts in electrochemical Cu dissolution potentials are shown to qualitatively account for the observed voltammetric profiles during Cu dealloying. Our study suggests that metal-specific energetics have to be taken into account to explain the detailed dealloying behavior of bimetallic surfaces.

Surface segregation in CuPt alloys by means of an improved modified embedded atom method

We present a procedure to investigate surface structures in CuPt alloys by combining the modified embedded atom method (MEAM) and the cluster expansion method (CEM). While the MEAM provides structural information for the description of extended anisotropic defects, the CEM improves the ability to correctly reproduce the relevant ground state structures in agreement with ab initio data. The procedure is validated with the reproduction of surface energies of pure Cu and Pt, the prediction of ${T}_{C}$ for order-disorder transitions, the surface and segregation energies in ordered CuPt alloys, and Monte Carlo (MC) simulations of temperature-dependent surface segregation profiles. A complete MEAM-CEM/MC study of the surface segregation in ${\mathrm{Cu}}_{3}\mathrm{Pt}$, CuPt, and $\mathrm{Cu}{\mathrm{Pt}}_{3}$ alloys is presented, engaging only 11 composition- and volume-independent alloy-specific parameters. Results are critically compared with experimental data from literature and with an independent set of ab initio periodic density functional theory calculations.

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of 13C18O+12C16O mixtures on the Pt(2.9%)/γ-Al2O3, (Pt(2.6%)+Cu(2.7%))/γ-Al2O3, and (Pt(2.6%)+Cu(5.1%))/γ-Al2O3 catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated 13C18O molecules adsorbed on Pt are ∼1940 and ∼1970 cm−1, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed 13C18O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt.

Corrosion and ORR activity of Pt alloy electrocatalysts during voltammetric pretreatment

This paper addresses the impact of voltammetric pretreatment of Pt alloy electrocatalyst for the oxygen reduction reaction on the emergence of stable voltammetric profiles and sustained activity characteristics in de-aerated and oxygen-saturated acidic electrolytes. The evolution of metal-corrosion related features during sweep voltammetry and cyclic voltammetry (CV) is monitored for Pt50Co50 and Cu-rich Pt-Cu alloy electrocatalysts a t various extents of CV pretreatment. While the Co-containing catalyst requires essentially only one slow anodic CV to attain a stable profile, Cu containing alloys are found to require more extensive pretreatment until reproducible surface areas and activities are obtained. The study underscores the need for customized pretreatment protocols for optimal Pt alloy activity testing.

High surface area nanoporous platinum: facile fabrication and electrocatalyticactivity

In this paper, nanoporous platinum films with an enhanced surface area wereobtained through a facile route by selective anodic dissolution of Cu from PtCualloy. A homogeneous PtCu alloy was electrochemically deposited at a glassycarbon electrode at room temperature, then Cu was electrochemically etchedfrom the PtCu alloy resulting in the fabrication of a nanoporous platinum filmwith a much higher surface area. The surface area of the dealloyed nanoporousplatinum film was highly enhanced by up to 500 times compared with that of apolycrystalline platinum electrode. This nanoporous platinum film exhibited highstability and remarkable catalytic activity for oxygen electro-reduction and methanolelectro-oxidation, with promising applications in fuel cells and biosensors, etc.

Facile Fabrication of Nanoporous Platinum by Selective Anodic Dissolution of Cu from Electrodeposited PtCu Alloy

not Available.

Effect of Atomic Ordering on the Catalytic Activity of Carbon Supported PtM (M: Fe, Co, Ni, and Cu) Alloys for Oxygen Reduction in PEMFCs.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of Atomic Ordering on the Catalytic Activity of Carbon Supported PtM (M = Fe , Co, Ni, and Cu) Alloys for Oxygen Reduction in PEMFCs

Carbon-supported PtM (, Co, Ni and Cu) catalysts have been synthesized by precipitating the ions as hydroxides in presence of a commercial carbon-supported Pt catalyst followed by heat treatment in a 90% Ar-10% mixture at for . X-ray diffraction indicates the PtFe and PtCo alloys thus obtained to have an ordered structure while the PtNi and PtCu alloys have a disordered FCC structure. Structural analysis of PtFe and PtCo after annealing at various temperatures for indicates the ordering to be maximized for an annealing temperature of around . Evaluation of the PtM alloy catalysts for oxygen reduction in proton exchange membrane fuel cells (PEMFCs) indicates that the alloys with the ordered structures have higher catalytic activity with lower polarization losses than Pt and the disordered PtM alloys. With the ordered phases, the activity increases with the extent of ordering. The enhanced catalytic activity is explained based on an optimal structural and electronic features consisting of optimum number of Pt and M nearest neighbors, d-electron density in Pt, atomic configuration on the surface, and Pt-Pt distance.

Effect of cold working on ordering of an equiatomic CuPt alloy

Effect of cold working on the ordering of an equiatomic CuPt alloy during continuous heating was studied by means of electrical resistivity measurements, hardness tests, differential scanning calorimetry, and X-ray diffraction. The ordering after cold working occurred in three stages with remarkable acceleration, namely, the migration and the annihilation of excess vacancies (I a), those of secondary defects (I b) and the migration of equilibrium vacancies (II) as well as that of quenched sample. The lattice defects introduced by cold working advanced the stages I a and I b, and therefore decreased the stage II. There existed two temperature regions with softening due to ordering enhanced recrystallization and normal recrystallization.

Ordering and segregation inXPt (X=V, Cu, and Au) random alloys

We examine the phase stability and the ordering tendencies of some Pt-based fcc random alloys using the generalized perturbation method (GPM) implemented in the linear muffin-tin orbitals (LMTO) basis. The reference medium for the GPM is chosen as the completely disordered state of the alloy and its electronic structure is described in the coherent potential approximation (CPA). Ordering tendencies and phase stability are examined via effective pair interactions and their lattice Fourier transforms. Relativistic effects on the ground state cohesive properties and the ordering tendencies are determined by carrying out nonrelativistic and fully relativistic (in some cases also scalar-relativistic) LMTO-CPA calculations. In all cases considered, namely $X\mathrm{Pt}$ with $X=\mathrm{V},$ Cu, and Au, the correct ordering tendency is obtained. The ordering tendency is found to be somewhat overestimated in the GPM. Relativistic effects are found to be most prominent in AuPt, where the nonrelativistic LMTO-CPA-GPM description shows a tendency towards ${L1}_{1}$ ordering and the correct result, i.e., phase segregation, is obtained only in the fully relativistic description. The sensitivity of the ordering tendency to factors such as lattice relaxation and volume per atom is examined briefly. Finally, the effect on the phase stability of adding a third component, such as V or Au to CuPt alloy, is studied by extending the formalism to the case of a ternary alloy.

LONG-PERIOD STRUCTURES IN NOBLE-METAL ALLOYS: NEW ASPECTS

Proceeding from precise calculation of the electronic structure and the generalized electronic susceptibility, a systematic study of the relationship between the nesting properties of the Fermi surface (FS) and the character of long-period ordered superstructures (LPSs) in noble-metal alloys, Cu‐Au, Cu‐Pd, and Ag‐Mg, is performed. An answer is provided as to why in Cu-Au, Cu-Pd, and Cu-Pt alloys so-called incommensurate LPSs, characterized by an irrational period and smeared antiphase boundaries, are formed. The reasons for occurrence of such LPSs within a short temperature range only are clarified. The dependence of the long period M on the long-range order parameter η in Cu‐Au alloys is accounted for. The causes for formation of two-dimensional LPSs in Au3Cu and Cu3Pd alloys are found. It is shown that their stability may be explained for by energy gaps opening on coinciding patches of FS in two mutually perpendicular directions. Arguments are drawn in favor of the fact that among quasicrystalline substances, phases with incommensurate LPSs occupy an intermediate position between incommensurate systems and quasicrystals. An attempt is made to identify the reasons for qualitatively different behavior of LPSs with large and small superperiods. It is shown that the dependence of LPS’s behavior on the value of 2M is controlled by the quality of nesting on FS and the nesting vector sensitivity (2k F) to variations in the long-range order parameter η.

Disorder broadening of core level photoemission spectra in CuxPt1−x alloys

Abstract High resolution photoelectron spectroscopy has shown the presence of disorder broadening of the Cu 2p 3/2 photoelectron line in CuPt alloys. The broadening is greatest for the Cu 50 Pt 50 composition as predicted by the charge correlated model. The broadening is less in Cu 75 Pt 25 than in Cu 25 Pt 75 probably due to the tendency to order in Cu 75 Pt 25 .