A structure investigation of Pt-Co bimetallic catalysts fabricated by mechanical alloying (original) (raw)
Related papers
High surface area mechanically alloyed Pt-based catalyst
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
Extensive efforts are being made to reduce the amount of the platinum catalyst currently used in polymer electrolyte membrane fuel cell (PEMFC) while maintaining high power level, in an attempt to reduce the cost of the fuel cell. To achieve this, a mechanical alloying technique was employed to refine the catalyst microstructure (i.e., increasing the effective catalyst surface area). In the present work, an investigation is carried out to study the relationship between the mechanical alloying process parameters and catalyst microstructural refinement, surface area and electrocatalytic behavior. Pt-Co alloys were fabricated from high purity Pt and Co powders using high-energy ball milling. The alloy catalysts were characterized using TEM, SEM, EDX, XRD and BET techniques. A high surface area Pt-based catalyst with improved catalytic activities has been achieved.
Alloying and microstructural changes in platinum–titanium milled and annealed powders
Journal of Alloys and Compounds, 2012
Equiatomic platinum-titanium powder mixtures were processed by high energy ball milling under argon atmosphere and sintered under vacuum. Evolution of the crystal structures and microstructures of the products formed were investigated by XRD and SEM techniques, respectively. The HCP crystals of Ti were first deformed and then a disordered metastable FCC Pt(Ti) solid solution was formed during milling due to semi-coherency of FCC lattices. A nanostructured Pt(Ti) product was formed after long milling time, which contained 44-47 at.% Ti and 53-56 at.% Pt. An ordered PtTi intermetallic was formed by annealing the metastable Pt(Ti) at temperature above 1300 • C. The crystal structure and microstructure of the TiPt phase depended on the milling time, annealing temperature and the cooling rate. The B19 PtTi plate martensite was formed after annealing at 1500 • C and quenching at a cooling rate of 23 • C/min to 200 • C/min for short time milled products. The width of martensite features was smaller at high cooling rate. In PtTi products milled for longer time, no martensitic transformation was observed on cooling the annealed samples. Small amounts of Pt 5 Ti 3 were formed in the powders milled for 16 h or more, followed by annealing at 1500 • C and furnace cooling at ∼2 • C/min.
Study of the atomic structure and morphology of the Pt 3 Co nanocatalyst
Journal of Physics: Conference Series, 2009
It has been shown that Pt3Co nanoparticles used as a catalyst for cathode of Proton Exchange Membrane Fuel Cells (PEMFC) enhance oxygen reduction reaction (ORR) activity even by a factor of two compared to pure Pt nanoparticles. The local structure and chemical disorder of a commercially available Pt3Co nanocatalyst supported on high surface area carbon were investigated. High-quality XAFS spectra were collected at the ELETTRA synchrotron XAFS 11.1 beamline. XAFS spectra analysis have been performed accounting for the reduction of the coordination number and degeneracy of three-body configurations, resulting from transmission electron microscopy (TEM) and x-ray diffraction (XRD) extracted mean particles diameter, size distribution and expected surface atom contributions. The presence of a Co-Co first neighbour EXAFS signal is shown to be related to the degree of the alloy's chemical disorder. This is a good starting point for analyzing the atomic structure of Pt3Co nanocrystalline system and their changes as a function of alloy preparation or working conditions when they operate as a catalyst in PEMFC.
Identification of a Pt3Co Surface Intermetallic Alloy in Pt–Co Propane Dehydrogenation Catalysts
ACS Catalysis, 2019
Bimetallic Pt-Co nano-particles (NP's) were prepared and characterized by scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, in situ synchrotron X-ray diffraction and catalytic conversion for propane dehydrogenation with and without added H 2. In addition, the surface extended X-ray absorption fine structure (EXAFS) obtained by fitting the difference spectrum between the fully reduced and room temperature oxidized catalysts suggest that the surface structure remains Pt 3 Co, although the core changes from Pt, to Pt 3 Co and to PtCo. At low Co loading, the bimetallic nano-particles form a Pt 3 Co intermetallic surface alloy with Pt-rich core. With increasing Co loading, a full alloy forms where both the surface and NP compositions are Pt 3 Co. A further increase in Co loading leads to a Co-rich NP core, likely PtCo, with a surface of Pt 3 Co. Although Pt-Co intermetallic alloys form two different phases and several morphologies, the surface structure is similar in all catalysts. Although both monometallic Pt and Co are active for alkane dehydrogenation, all bimetallic Pt-Co catalysts are significantly more olefin selective than either single metal. The turnover rates of the bimetallic catalysts indicate that Pt is the active atom with little contribution from Co atoms. The high olefin selectivity is suggested to be due to Co acting as a less active structural promoter to break-up large Pt ensembles in bimetallic NP's.
PtxGd alloy formation on Pt(111): Preparation and structural characterization
Surface Science, 2016
Pt x Gd single crystals have been prepared in ultra high vacuum (UHV). This alloy shows promising catalytic properties for the oxygen reduction reaction. The samples were prepared by using vacuum deposition of a thick layer of Gd on a sputter cleaned Pt(111) single crystal, resulting in a ∼63 nm thick alloy layer. Subsequently the surfaces were characterized using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), ion scattering spectroscopy (ISS) and temperature programmed desorption (TPD) of CO. A Pt terminated alloy was observed upon annealing the sample to 600 • C. The LEED and synchrotron XRD experiments have shown that a slightly compressed (2×2) alloy appear. The alloy film followed the orientation of the Pt(111) substrate half the time, otherwise it was rotated by 30 •. The TPD spectra show a well-defined peak shifted down 200 • C in temperature. The crystal structure of the alloy was investigated using ex-situ X-ray diffraction experiments, which revealed an in-plane compression and a complicated stacking sequence. The crystallites in the crystal are very small, and a high degree of twinning by merohedry was observed.
2007
This study investigates the relative electrochemical activity of ordered and disordered Pt-Co alloy phases coexisting in multi-phase catalyst materials. Of particular interest is the effect of the relative distribution between ordered and disordered Pt-Co alloy phases on the observed electrocatalytic activity for the oxygen reduction reaction (ORR). Three Pt-Co catalysts with identical overall composition, Pt 50 Co 50 , but with distinct distributions between two disordered face centered cubic (fcc) Pt-Co alloy phases and one ordered face centered tetragonal (fct) alloy phase are considered. Comparing the structure of the catalysts with their electrocatalytic activity for ORR suggests that the Co-rich (60-80 at% Co) disordered phase is linked to the observed 3× activity enhancement compared to a pure Pt catalyst. If the ordered fct phase outweighs the Co-rich disordered phase the activity drops drastically. It is concluded that Co-rich disordered phases are the preferred Pt-Co alloy phases with respect to catalyst activity.
Local Ordering Changes in Pt–Co Nanocatalyst Induced by Fuel Cell Working Conditions
The Journal of Physical Chemistry C, 2012
A dx.doi.org/10.1021/jp2099569 | J. Phys. Chem. C XXXX, XXX, XXX−XXX dmadmin | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2:3679 | 2.0 alpha 39) 2012/05/08 15:22:00 | PROD-JCA1 | rq_334126 | 5/23/2012 14:45:21 | 12 56 A major source of catalyst degradation is the loss of 57 electrochemically active surface area (ECSA). In several studies 58 the loss of active platinum by dissolution of Pt was 59 observed. 6−10 Dissolved Pt species can migrate and leave the 60 catalytically active zone (e.g., Pt particles can be detected in the 61 ionomer phase, outside the conductive carbon support 7,8 ) or 62 redeposited on other (larger) particles due to their higher 63 equilibrium potential for dissolution (Ostwald ripening). 6,10 64 There is also evidence for ECSA loss due to other mechanisms, 65 such as agglomeration (due to carbon corrosion or the 66 migration of nanoparticles on the carbon support) and 67 coalescence, which was demonstrated. e.g.. in refs 7−14. 68 Attempts to distinguish the relative contribution of each 69 mechanism have also been made, 6,8 but the matter is still under 70 debate. Many works have shown that alloying Pt with transition 71 metals can also improve the stability of the catalyst, particularly 72 those containing cobalt (see, e.g,. refs 3, 12, and 13 and 73 references therein). 74 Several hypotheses have been put forward in the literature to 75 justify the reasons for Pt-based alloys activity and durability 76 increase with respect to pure Pt catalyst. Many experimental 77 and theoretical works show that this enhancement could be 78 attributed mainly to changes in the surface structure and 79 chemical composition of the near-surface region (segregation to 80 the surface and dissolution of atoms and atomic species) and 81 also (as a consequence) in the local geometric structure (i.e., 82 Pt−Pt bond distance, number of Pt nearest neighbors), 83 electronic structure (electron density of states in the Pt 5d 84 band, strength of interaction between the Pt and the 3d-85 transition metal atoms), and nature and coverage of surface 86 oxide layers. 3,4,15−20 However, many of these works considered 87 extended alloy surfaces, and obviously, the mechanisms 88 established for bulk surfaces need not be applicable to 89 nanoparticles. Moreover, often the catalyst was degraded 90 (dealloyed, sintered, annealed, acid treated) in conditions 91 which only mimic the real fuel cell environment. 92 a R [Å] is the mean bond length, σ 2 [10 −3 Å 2 ] is the Debye−Waller-like parameter, and N c is the coordination number.
Electrochimica Acta, 2007
We report on synthesis-structure-activity-stability relationships of Pt 3 Co nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). We have synthesized Pt 3 Co alloy electrocatalysts using liquid impregnation techniques followed by reductive annealing at high and low temperatures. We have performed detailed structural X-ray diffraction (XRD)-based structural characterization (symmetry, lattice parameters and composition) of individual Pt-Co alloy phases before and, importantly, after electrochemical rotating disk electrode (RDE) measurements. This enables us to directly evaluate the corrosion stability of various Pt-Co alloy phases under typical fuel cell cathode conditions. Pt 3 Co prepared at low annealing temperatures (600 • C) resulted in multiple phases including (i) a disordered face-centered cubic (fcc) Pt 95 Co 5 phase and (ii) an ordered face-centered tetragonal (L1 0 ) Pt 50 Co 50 phase; high temperature annealing (950 C) resulted in a single ordered primitive cubic (L1 2 ) Pt 3 Co phase. The ordered alloy phases in both catalysts were not stable under electrochemical treatment: The ordered face-centered tetragonal (fct) phase showed corrosion and dissolution, while the ordered primitive cubic (L1 2 ) Pt 3 Co phase transformed into a disordered structure. The ordered primitive cubic structure exhibited higher resistance to sintering.
Applied Catalysis B-environmental
Carbon supported platinum and platinum alloys (PtCo, PtNi and PtCu) for PEMFC cathodes were prepared and studied for their oxygen reduction reaction activity and durability under potential cycling at 80 °C in 0.5 M HClO4. Catalysts with different metal alloy composition and particle size were synthesized by annealing at different temperatures to discriminate between the effects of alloying and particle size on the electrocatalytic activity and durability. XRD was used for the structural characterization of pristine catalysts, while the bulk compositions were analyzed by EDS before and after durability tests. XPS was employed to determine the surface composition of selected alloys after durability tests. The particle size of the fresh and aged catalysts was determined by TEM. Rapid dealloying, particularly from non-noble metal rich alloys, was already witnessed for the alloys potentially cycled at room temperature. Significant particle growth depending on the initial particle size was observed for both Pt and Pt alloys after the durability tests. For the alloys with similar initial particle size, the initial electrocatalytic activity depends on the initial alloy composition. Although a 3-fold enhancement in the ORR activity was observed for the non-noble metal rich alloys after initial dealloying, the specific activity of Pt and Pt alloys becomes quite similar at the end of the durability tests. Annealing of Pt/C and Pt alloys at 950 °C results in catalysts with the highest specific and mass activity and with the highest stability.► Fresh non-noble metal-rich alloys show enhanced oxygen reduction activity. ► The specific activity of the aged Pt/C and PtM/C annealed at a similar temperature is quite comparable. ► The alloying effect is almost completely lost due to dissolution of about 90–95 at% non-noble metals. ► Alloy surface only consists of Pt after the durability tests, and only a weak particle-size effect remains.
Effect of Me (Pt+Ru) content in Me/C catalysts on PtRu alloy formation: An XRD analysis
Journal of Materials Science Letters, 2000
Proton exchange membrane (PEM) fuel cells are highly efficient and low polluting electrical generators for mobile applications . This type of cell operates at relatively low temperature (about 80 • C) on hydrogen mostly obtained by steam reforming hydrocarbon fuels. CO elecrooxidation is a major item in the electrochemical conversion of hydrogen from these fuels. CO acts as a strong deactivating agent for the anode electrocatalyst (generally carbon supported platinum) in low temperature fuel cells . Poisoning occurs because CO binds strongly to Pt sites resulting in a high surface coverage of CO at the operating temperature of PEM fuel cells. A solution to this problem is to use Pt alloy catalysts that are more tolerant towards CO poisoning than pure Pt. Encouraging catalytic performance has been reported for PtRu alloy-based electrodes in the electro-oxidation of H 2 /CO mixtures . To achieve lower CO coverage values, two types of mechanisms have been proposed. An intrinsic mechanism postulates that Ru presence modifies H 2 and CO chemisorption properties, so as to reduce CO coverage with respect to H 2 oxidation sites . A promoted mechanism is based on the activity of PtRu alloys towards CO oxidation, related to their bifunctional properties: a nucleation at low potentials of oxygen containing species (OH ads ) on Ru atoms and the bimolecular reaction of OH ads with CO adsorbed on Pt.