Pt-Pt Bond Distance Research Articles

Investigation on High Stable Ordered Ptｍ (M=Fe, Co, Ni, Cu) Catalysts Via Operando X-Ray Absorption Spectroscopy Analysis

In the development of society, energy stands as the cornerstone for human survival and progress. Historically, traditional fossil fuels served as the primary energy source; however, the widespread utilization of fossil fuels exacerbates issues like climate change and environmental pollution and necessitates the development of new, clean, and efficient energy alternatives to supplant conventional fossil fuels. Hydrogen, with its high energy density and emission-free properties, emerges as a promising alternative energy carrier, particularly in fuel cell technology. 1 However, the widespread adoption of fuel cells is hindered by challenges, notably the high cost associated with platinum catalysts used in oxygen reduction reactions (ORR). Addressing this challenge requires innovative approaches to enhance catalytic performance while reducing cost concern. 2 This has led to the exploration of alloying Pt with non-noble metals (Fe, Co, Ni, Cu), which improves the ORR activity than benchmark Pt/C and simultaneously solves the high catalyst cost issues. 3 Previous analyses of these alloys show that the significant improvement in ORR activity is due to optimized Pt 5d orbital vacancy, which shortens the Pt-Pt bond distance and weakens the binding of intermediates. The Pt-Pt bond distance has been widely regarded as a crucial factor in analyzing the structure-activity relationship.This research aimed to explore the relationship between catalytic performance and Pt-Pt bond distance, combining electrochemical performance with the operando X-ray absorption spectroscopy analysis during ORR conditions. In this study, various ordered PtM (M=Fe, Co, Ni, Cu) alloys were synthesized by thermal treatment under NH3 atmosphere using mesoporous carbon supports, which restrict particle growth during thermal treatment. The morphological attributes analyzed by TEM images clearly show that Pt-M alloys is located inside the pores of the mesoporous carbon layer. The detailed electrochemical investigation by Cyclic voltammetry (CV) and Linear sweep voltammetry (LSV), shows three times superior activity than benchmark Pt/C. The Operando HERFD-XAS analysis, conventional EXAFS combined with electrochemical investigations shows a positive correlation between specific activity and the Pt-Pt bond, which indicated that contracted lattice structure would contribute to less formation of the Pt oxide species and ultimately results in superior ORR activity and durability. Therefore, our results demonstrate the importance of the Pt-Pt bond length in assessing ORR activity and validate that structural changes during operation can be a key enabling factor in predicting catalytic performance and durability. Acknowledgment This work is supported by the PEFC and the NEDO FC-Platform commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference: A. Risco-Bravo, C. Varela, J. Bartels and E. Zondervan, Renewable Sustainable Energy Rev., 2024, 189, 113930. J. Wang, W. Ding and Z. Wei, Acta. Phys. Sin., 2021, 37, 2009094. C. Kim, F. Dionigi, V. Beermann, X. Wang, T. Moeller and P. Strasser, Adv. Mater., 2019, 31, 1805617. Figure 1

Sulfur-doped Pt/C catalyst for propane dehydrogenation with improved atomic utilization and stability

Atomically well-dispersed platinum catalysts play a vital role in various chemical processes such as direct dehydrogenation of propane (PDH), due to their high performance originated from high atom utilization. However, these catalysts suffer from complex preparation methods, and preparing atomically dispersed Pt-based catalysts with feasible methods remains challenging. Herein, we report a convenient method to prepare efficient atomically dispersed Pt/C catalysts for PDH by sulfur doping into the carbon support by simple cyclohexane impregnation method. The prepared atomically well-dispersed catalysts possess strong interactions between Pt and carbon support, which led to improved exposure of active sites and higher propylene formation rate compared to that without sulfur doping. Sintering of the Pt species was also inhibited and stability was improved by 47%. Propylene selectivity also showed a positive correlation with the amount of sulfur doping. The propane conversion first increased and then decreased, which was due to the loss of sulfur during the reaction that caused the agglomeration of Pt and the evolution of structure. It was found that the maximum catalytic activity appeared with Pt clusters of ∼ 0.71 nm with an average Pt-Pt coordination number of 3.1 and Pt-Pt bond distance of 3.09 Å.

CeO2/Pt/Al2O3 catalysts for the WGS reaction: Improving understanding of the Pt-O-Ce-Ox interface as an active site

In this work, we demonstrate that knowledge of the structural parameters of Pt nanoparticles (PtNp) supported on ceria/alumina is essential for understanding the surface structure, the electronic properties of PtNp, and the effect of ceria on the catalytic activity in the water-gas shift reaction. The similar Pt-Pt and Pt-O bond distances for different PtNp sizes suggested similar electronic density of the Pt sites, indicating that changes of the electronic properties of the Pt sites, due to the size effect, were compensated by the presence of Pt-O species. The turnover frequency based on Pt0 sites varied between 2 and 10 times higher, compared to unpromoted catalysts, and was dependent on the PtNp size and morphology, which determined the density of oxygen on the surface and the density of Pt-O-Ce sites. The effect of ceria in increasing the activity could be explained by the creation of active Pt-O-Ce sites at the surface.

Tailoring lattice strain in ultra-fine high-entropy alloys for active and stable methanol oxidation

High-entropy alloys (HEAs) have been widely studied due to their unconventional compositions and unique physicochemical properties for various applications. Herein, for the first time, we propose a surface strain strategy to tune the electrocatalytic activity of HEAs for methanol oxidation reaction (MOR). High-resolution aberration-corrected scanning transmission electron microscopy (STEM) and elemental mapping demonstrate both uniform atomic dispersion and the formation of a face-centered cubic (FCC) crystalline structure in PtFeCoNiCu HEAs. The HEAs obtained by heat treatment at 700°C (HEA-700) exhibit 0.94% compressive strain compared with that obtained at 400°C (HEA-400). As expected, the specific activity and mass activity of HEA-700 is higher than that of HEA-400 and most of the state-of-the-art catalysts. The enhanced MOR activity can be attributed to a shorter Pt-Pt bond distance in HEA-700 resulting from compressive strain. The nonprecious metal atoms in the core could generate compressive strain and down shift d-band centers via electron transfer to surface Pt layer. This work presents a new perspective for the design of high-performance HEAs electrocatalysts.

Development of Pt-Co catalysts supported on carbon nanotubes using the polyol method—tuning the conditions for optimum properties

Pt alloys with transition metals supported on carbon substrates are used as improved cathode electrocatalysts for fuel cells. Enhanced catalytic activity is attributed to the structure (Pt-Pt bond distance) and/or electronic effect (Pt d-electron vacancy). This work focuses on the development of Pt3Co/f-MWCNT catalysts (functionalized multiwalled carbon nanotubes [f-MWCNT]) using ethylene glycol as the dispersing and reducing agent. The aim is to in parallel achieve fine dispersion, quantitative deposition and alloy formation. As described herein, the pH value of the reaction suspension has a critical effect on the composition and morphology of the synthesized nanoparticles. High pH values favor the formation of Pt3Co alloy, nevertheless negatively influencing the dispersion. A discussion is made on the reduction/deposition mechanism and how to control the conditions to result in optimum properties.

Rational design of porous structures via molecular layer deposition as an effective stabilizer for enhancing Pt ORR performance

Pt-based catalysts are widely applied in fuel cells as key components of both anode and cathode. However, commercial Pt/C catalysts with Pt nanoparticles dispersed on carbon support exhibit poor durability due to the Ostwald ripening effect, detachment and agglomeration of Pt nanoparticles during fuel cell operation. Herein, we demonstrate for the first time the application of molecular layer deposition (MLD) to stabilize Pt catalysts. By atomic layer deposition (ALD) of Pt particles on MLD-derived interlayer, the Pt catalysts show significantly enhanced oxygen reduction reaction (ORR) activity and durability compared to that without the MLD interlayer. The MLD-derived surfaces with enriched pores are helpful for anchoring the Pt nanoparticles and to avoid agglomeration or detachment from the surface sites. In addition, X-ray adsorption spectroscopy (XAS) results show that the deposition of Pt on the MLD-derived interlayer caused the electron transfer from Pt to substrates, which results in a decrease in the number of electrons in the d orbital of Pt. In addition, the extended X-ray absorption fine structure (EXAFS) result indicates that PtPt bond distance is shortened by the deposition on the MLD-derived NCNT, which leads to the enhanced activity of Pt catalysts. This work provided a novel route for stabilizing Pt catalysts through MLD technique.

Temperature Effect of Oxygen Reduction Reaction Activity on Pt-Pd/C Core-Shell Catalyst Investigated By X-Ray Absorption Spectroscopy

Introduction Environment and global warming issue is one of the main global issues around the world. The polymer electrolyte fuel cells (PEFCs) supposed to be one of the main devices used in the global renewable energy system and pure hydrogen energy society in the future. PEFC cathode catalyst requires an excess amount of platinum due to the degradation of oxygen reduction reaction (ORR) activity. High performance, high durability and low cost cathode catalyst is expected for the wide use of PEFC devices. Therefore, it is important to reduce the use of platinum in the catalyst and improve its catalytic activity as well as durability. Core-shell structure catalyst is one of the solutions to achieve a higher catalytic activity and less use of platinum. Compare with the common used platinum catalyst, Pd-core/Pt-shell catalyst is one of the promised catalyst to use less platinum, reduce the cost and possess high performance. However, the factor of controlling ORR activity in core-shell catalyst still has not well understood such as potential/temperature-dependent surface structures and oxidation states of platinum. Experimental In this research, we synthesized the 1 mono-layer Pt shell on carbon-supported palladium(Pd) core by copper under potential deposition (Cu-UPD) method, investigated its oxygen reduction reaction (ORR) activity at various temperatures and the oxygen coverage of platinum by rotating disk electrode (RDE) electrochemical measurements and operando X-ray absorption fine structure (XAFS, Fig. 1) respectively. Through the results of electrochemical measurements and XAFS, the relationship among ORR activity and oxygen coverage of the surface Pt shell at different temperature, the change of electronic structure and local structure of Pt atoms in Pt-Pd/C core-shell catalyst at different temperature in the same condition with electrochemical measurement (N2-saturated) and real working condition (O2-saturated) have been discussed. Results and discussion The result of electrochemical measurements shows Pt-Pd/C core-shell catalyst possess higher ORR activity and lower oxygen coverage than common Pt/C catalyst. In the high temperature (50~70°C), the ORR activity of Pt-Pd/C catalyst decrease with the increase of oxygen coverage and temperature, shows the decrease of active sites on the surface of Pt shell and the change of mechanism or structure of Pt-Pd/C catalyst. The result of XAFS shows the core-shell catalyst possess compressive strain and less Pt-Pt bond distances than common Pt/C catalyst in N2-saturated electrolyte, the expand of Pd core and Pt-Pt bond extend occurs at high temperature, explained the high ORR activity and less oxygen coverage of Pt-Pd/C core-shell catalyst in room temperature and the activity loss in the high temperature. In O2-saturated electrolyte, XAFS result shows Pt-O bond formation is dramatically increase and in the high temperature almost all the Pt shell oxidized to PtO2, Pt-Pd bond remarkable extends, shows the loss of ORR activity of the Pt-Pd/C catalyst in high temperature (60°C) in O2-saturated condition. Conclusion This research investigated ORR activity on Pd core-Pt shell with the change of temperature. The core-shell catalyst had a compressive strain and less Pt-Pt bond distances than common Pt/C catalyst in N2-saturated electrolyte, however, the expand of Pd core and Pt-Pt bond extend occurs at high temperature. Acknowledgments This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

A Combined SAXS and XAS Setup for Operando Electrocatalyst Degradation Studies

The noble metal nanoparticles (NPs, including oxides and/or alloys) customarily used to catalyze the electrochemical reactions at play in low temperature fuel cells and electrolyzers are prone to potential-induced dissolution1 which leads to changes in their composition and morphology. Minimizing the impact of these processes on the devices’ operation requires a better understanding of their mechanism under application-relevant operando conditions. In this regard, small angle X-ray scattering (SAXS) provides information about a sample’s NP size distribution and agglomeration, while X-ray absorption spectroscopy (XAS) is sensitive to the electronic (oxidation state) and geometric (identity and number of coordinating atoms, bond distances) structure of the element of interest. The combination of these two techniques in an operando setup would result in novel insight on the degradation mechanisms affecting noble metal nanoparticles. With this motivation, we have developed a novel setup in which the gas ionization chambers and flight tube plus 2D-detector required for XAS and SAXS acquisition respectively, are mounted in parallel on a movable table that allows to transition among techniques within a couple of minutes. This configuration allows to use both techniques in their optimal configuration, without losing signal quality while compromising on one technique while combining it with the other (as done in the past).2 To validate this approach it was used to study the operando degradation of two carbon-supported Pt-NP (Pt/C) catalysts customarily used in polymer electrolyte fuel cells, which constitute a relatively well understood system owing to the large volume of work that has been devoted to its study.3 Specifically, commercial Pt on Vulcan and Pt on graphitized Black Pearls catalysts (Pt/V and Pt/BP-g, respectively – both with 30 wt. % Pt) were processed into electrodes and electrochemically tested in an operando flow cell developed in-house.4 The degradation protocol consisted of 250 potential cycles between 0.5 and 1.5 V vs. the reversible hydrogen electrode (RHE) at 50 mV∙s-1, using 0.1 M HClO4 at 60 °C as the electrolyte. Complementing results derived from both techniques with respect to changes in particle diameter as well as oxidation state of the catalyst particles are then used to quantifiy NP-growth rates, correlation of particle size with Pt-Pt bond distance as well as NP oxide coverages. In summary, this contribution will introduce a novel, combined SAXS and XAS setup devoted to (electro)catalyst research, and exemplify its potential application based on the results derived for two Pt/C catalysts, which are fully consistent with previous literature on such materials. Cherevko, A. R. Zeradjanin, A. A. Topalov, N. Kulyk, I. Katsounaros, and K. J. J. Mayrhofer, ChemCatChem 6, 2219 (2014).Nikitenko, A. M. Beale, A. M. J. van der Eerden, S. D. M. Jacques, O. Leynaud, and M. G. O‘Brien, J. Synchrotron Rad. 15, 632 (2008).Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, F. Holby, and D. Morgan, Top. Catal. 46, 285 (2007).Binninger, E. Fabbri, A. Patru, M. Garganourakis, J. Han, D. Abbott, O. Sereda, R. Kötz, A. Menzel, M. Nachtegaal, and T. J. Schmidt, J. Electrochem. Soc. 163, H906 (2016).A. Gilbert, N. N. Kariuki, R. Subbaraman, A. J. Kropf, M. C. Smith, E. F. Holby, D. Morgan, and D. J. Myers, J. Am. Chem. Soc. 134, 14823 (2012).Lei, J. Jelic, L. C. Nitsche, R. Meyer, and J. Miller, Top. Catal. 54, 334 (2011).

Dependent Relationship between Quantitative Lattice Contraction and Enhanced Oxygen Reduction Activity over Pt-Cu Alloy Catalysts.

Lattice contraction has been regarded as an important factor influencing oxygen reduction reaction (ORR) activity of Pt-based alloys. However, the relationship between quantitative lattice contraction and ORR activity has rarely been reported. Herein, using Pt-Cu alloy nanoparticles (NPs) with similar particle sizes but different compositions as examples, we investigated the relationship between quantitative lattice contraction and ORR activity by defining the shrinking percentage of Pt-Pt bond distance as lattice contraction percentage. The results show that the ORR activities of Pt-Cu alloy NPs exhibit a well-defined volcano-type dependent relationship toward the lattice contraction percentage. The dependent correlation can be explained by the Sabatier principle. This study not only proposes a valid descriptor to bridge the activity and atomic composition but also provides a reference for understanding the composition-structure-activity relationship of Pt-based alloys.

Metal and Metal Oxide Interactions and Their Catalytic Consequences for Oxygen Reduction Reaction.

Many industrial catalysts are composed of metal particles supported on metal oxides (MMO). It is known that the catalytic activity of MMO materials is governed by metal and metal oxide interactions (MMOI), but how to optimize MMO systems via manipulation of MMOI remains unclear, due primarily to the ambiguous nature of MMOI. Herein, we develop a Pt/NbOx/C system with tunable structural and electronic properties via a modified arc plasma deposition method. We unravel the nature of MMOI by characterizing this system under reactive conditions utilizing combined electrochemical, microscopy, and in situ spectroscopy. We show that Pt interacts with the Nb in unsaturated NbOx owing to the oxygen deficiency in the MMO interface, whereas Pt interacts with the O in nearly saturated NbOx, and further interacts with Nb when the oxygen atoms penetrate into the Pt cluster at elevated potentials. While the Pt-Nb interactions do not benefit the inherent activity of Pt toward oxygen reduction reaction (ORR), the Pt-O interactions improve the ORR activity by shortening the Pt-Pt bond distance. Pt donates electrons to NbOx in both Pt-Nb and Pt-O cases. The resultant electron eficiency stabilizes low-coordinated Pt sites, hereby stabilizing small Pt particles. This determines the two characteristic features of MMO systems: dispersion of small metal particles and high catalytic durability. These findings contribute to our understandings of MMO catalytic systems.

Pt Hollow Nanospheres As an Electrocatalyst for the Oxygen Reduction Reaction

H2-fed proton exchange membrane fuel cells (PEMFCs) represent the most advanced fuel cell technology and have a great deal of potential applications in low/zero-emission electric vehicles, distributed home power generators, and power sources for small and portable electronics. However, the commercialization of PEMFC technology has been greatly hindered by some challenges, mainly sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode and the high cost of the noble metal Pt (1, 2). Alloying platinum with non-noble metals such as Co is an effective approach to improve the catalytic performance and reduce the usage of Pt. In addition, through transferring Pt nanoparticles (NPs) into a hollow nanostructure, electrocatalytic performance can be improved greatly due to its relatively lower density and higher surface area-to-volume ratio than its solid counterpart (i.e., NPs). In this work, Pt hollow nanospheres (HNSs) were fabricated through a replacement reaction of Co atoms by PtCl6 2- ions to reduce Pt usage and improve the catalytic activity towards the ORR. Carbon black Vulcan XC-72R (VC) was introduced into a solution prior to the addition of Co(II) and the formation of Co NPs and the replacement of Co by PtCl6 2-ions for a uniform dispersion of Co NPs and the Pt HNSs on the carbon support. Some Co atoms have alloyed Pt in the synthesis and exist in the Pt HNSs (3). The hollow mesoporous core in the Pt HNSs can be utilized as an electrolyte solution buffering reservoir to minimize the diffusion distance to the interior surface of the porous shell of Pt crystallites while the porous nanochannels (i.e., the micropores between the Pt crystallites) in the shell open to the mesoporous hollow core form fast mass transport networks providing more accessible sites for oxygen transfer. In addition, triple phase boundaries (i.e., gas-electrolyte-Pt NPs) can be developed more easily in the Pt HNS enabling individual Pt crystallites in the shell to be accessible to electrolyte ions and oxygen, and thus catalytically active. Furthermore, alloying Pt with Co might result in a significant lattice shrinking because of the change in Pt-Pt bond distance which also contributes to the improved electrocatalytic activity. In contrast, for the state-of-the-art Pt NPs/VC catalyst, the Pt NPs can agglomerate more easily to form larger particles, resulting in reduced active sites. Besides, the interior voids between Pt NPs may not be accessible to electrolyte ions and do not contribute to the ORR activity due to the lack of triple phase boundaries. As a result, the as-developed PtCo (20 wt%) HNS/VC catalyst outperforms significantly the state-of-the-art Pt(20 wt%)NP/VC catalyst.

Near-infrared emission from binuclear platinum (II) complexes containing pyrenylpyridine and pyridylthiolate units: Synthesis, photo-physical and electroluminescent properties

Two novel binuclear platinum (II) complexes of (Rpypy)2Pt2(pyt)2 were synthesized and characterized, in which Rpypy represents 2-(1-pyrenyl) pyridine (pypy) and 2-(7-tert-butyl-1-pyrenyl)pyridine (tBupypy), pyt refers to pyridyl-2-thiolate, respectively. Their binuclear framework was evidenced by the single crystal of (pypy)2Pt2(pyt)2 with a short PtPt bond distance of 2.8814(3) Å. The intense near-infrared (NIR) emission peaks at 692 and 753 nm were exhibited for both dinuclear platinum complexes, which is similar to the emission of the mono-nuclear platinum complex of (pypy)Pt(acac) under photo-excitation. However, a red-shifted NIR electroluminescence (EL) peaked at 692 nm with a shoulder at 753 nm was observed in their doped polymer light-emitting diodes using PVK/OXD-7 as the host blend, which presented intrinsic character of MMLCT. Better EL performances with the maximum external quantum efficiency of 0.74% and a radiate emittance of 123.4 μw.cm−2 were obtained in the (tBupypy)2Pt2(pyt)2 doped device. To our best knowledge, this is a first reported example on the NIR EL of binuclear platinum complex using pyridyl-2-thiolate as auxiliary ligand.

Operando X-Ray Absorption Spectroscopy Measurement Studies on Oxygen Reduction Reaction of Ionic-Liquid-Encapsulated Pd/Pt Core Shell Catalysts

For the widespread use of Polymer Electrolyte Fuel Cell (PEFC), it is needed to reduce the amount of platinum used for electrode catalyst materials from the viewpoint of improving oxygen reduction reaction (ORR) activity of platinum. Recently, it is reported by Erlebacher1), 2) that the ORR activity of nanoporous NiPt alloy catalysts encapsulated by ionic liquid (IL) is higher than that of this material not encapsulated by. Based on the report3) that promoting the oxygen solubility at the interface between catalysts and electrolyte improves the ORR activity, this result implies that the high oxygen solubility of IL mainly contributes to high ORR activity. However, the reason of improvement of the ORR activity has not been clarified yet. In this study, we prepared Pd/Pt core shell catalyst, which is one of the most notable nanomaterials showing substantially high ORR activity, and encapsulated the surface of this catalyst with IL. Then we studied on the ORR of IL-encapsulated Pd/Pt core shell nanoparticles (NPs) by operando X-ray absorption spectroscopy (XAS) method. Pd/C (particle diameter of Pd: 3.6 nm) was applied in this study. Cu monolayer has been deposited on Pd NPs using under potential deposition (UPD) of Cu in 10 mM CuSO4 solution. A single UPD Cu monolayer replacement with Pt in K2PtCl4 solution formed Pt monolayer film on Pd NPs (Pt/Pd/C). The surface of Pt/Pd/C was coated with [MTBD][NTf2] 2), 3) (IL). Convection voltammetry was performed at 25 ˚C in O2-saturated 0.1 M HClO4 aq and H2SO4 aq using rotating disc electrode (RDE). RDE was rotated at 300, 500, 700, 900, 1200, 1600, and 2500 rpm. Electrochemical activity value at 0.9 V vs. Reversible Hydrogen Electrode (RHE) was calculated from Koutecky-Levich plot. Operando XAS measurements for Pt L Ⅲ -edge and L Ⅱ -edge of Pt/Pd/C catalysts were carried out by using the beamline BL14B2 in SPring-8 (Japan). These measurements were performed by fluorescence method in O2-saturated 0.1 M HClO4 aq and H2SO4 aq. Potentials were maintained at 0.5, 0.85, and 1.15 V vs. RHE. Specific activity value of Pt/Pd/C in 0.1 M H2SO4 aq was much lower than that of Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distances were calculated by analyzing EXAFS spectra of Pt L Ⅲ -edge. At each potentials, there are no significant differences between Pt-Pt bond distances of Pt/Pd/C in 0.1 M HClO4 aq and H2SO4 aq. It implied that the irreversible specific adsorption of SO4 2- on the surface of Pd/Pt NPs decreased the number of active site of catalysts. The specific activity value of IL-encapsulated Pt/Pd/C was 1.31 times higher than that of non-encapsulated Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distance estimated by EXAFS spectra of Pt L Ⅲ -edge of IL-encapsulated Pt/Pd/C was definitely longer than that of non-encapsulated Pt/Pd/C. These results suggested that coating with IL on the surface of Pt/Pd/C have lattice strain. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity and the strain effect induced d-band shift regulates the adsorption properties of rate-limiting intermediates in catalytic processes.4)

ORR Activity of Large-Scale Synthesized Pd Core-Pt Shell Catalysts

Introduction It is of great importance to decrease Pt usage in the polymer electrolyte fuel cells (PEFCs) for their cost reduction. Pd core-Pt shell catalyst (Pt/Pd/C) is a promising candidate for the decrease due to its high Pt utilization efficiency and enhancement of oxygen reduction reaction (ORR) activity [1, 2]. Recently, we have reported that ORR specific activity of the Pt/Pd/C catalyst is drastically enhanced with an accelerated durability test (ADT) [3]. It is considered that the enhancement arises from rearrangement of the Pt shell associated with the Pd core dissolution, in which number of low-coordinated surface Pt atoms decreased and a compressive strain was induced in the Pt shell [3].In our previous study, the Pt/Pd/C catalysts were successfully synthesized with a modified Cu-UPD/Pt replacement process in a large-scale (50-100 g/batch) [4]. However, the degree of the ORR activity enhancement after the ADT had poor reproducibility although their initial physical and electrochemical properties were almost equivalent. In this study, structural change of the Pt/Pd/C catalysts with the ADT was analyzed using STEM-EDX and XAFS techniques to understand the poor reproducibility in ORR performance after the ADT. Experiment 80 g of carbon supported Pd core (Pd/C; particle size: 6.0 nm, Pd loading: 30 wt.%) was ultrasonically dispersed in 50 mM H2SO4 containing 20 mM CuSO4 and stirred at 5oC with coexistence of a metallic Cu sheet under Ar atmosphere. After pre-determined stirring time, the Cu sheet was removed and K2PtCl4 was added to replace under-potentially deposited Cu monolayer on the Pd core surface with Pt monolayer, forming Pt/Pd/C core-shell catalyst. The Pt/Pd/C catalyst was characterized with ICP, XRD and CO adsorption. ORR activity of the Pt/Pd/C catalyst was evaluated with RDE technique in O2 saturated 0.1 M HClO4 at 25oC. Accelerated durability test (ADT) was performed using a rectangular wave potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE at 60oC in Ar saturated 0.1 M HClO4for 10,000 cycles. Structure of the Pt/Pd/C catalyst was analyzed by STEM, STEM-EDX and XAFS (BL14B2 beam line at SPring-8). Results and Discussion Physical and electrochemical properties of two Pt/Pd/C catalysts, (a) and (b), which were synthesized in different batches (100 g/batch), are summarized in Table 1. Although initial properties are almost equivalent, ORR specific activity of (a) Pt/Pd/C catalyst is much enhanced by the ADT (434→966 µA/cm2, 2.2-fold) compared with that of (b) Pt/Pd/C catalyst (437→731 µA/cm2, 1.7-fold).HAADF-STEM images and STEM-EDX line analyses of the catalysts after the ADT are demonstrated in Fig. 1, revealing that there are two different structures in the catalysts. One retained Pd core-Pt shell structure with thickened Pt shell (Fig. 1 (A)) and the other transformed to a Pt enriched nanoparticle with large Pd dissolution (Fig. 1 (B)). The STEM-EDX analysis showed that the core-shell structure was much retained in the (a) Pt/Pd/C catalyst. Furthermore, XAFS analysis indicated that Pt-Pt bond distance of the Pt shell was more shortened in the (a) Pt/Pd/C catalyst after the ADT. Therefore, it can be concluded that the higher ORR specific activity of the (a) Pt/Pd/C catalyst after the ADT arises from retention of the core-shell structure and the shortened Pt-Pt bond distance (i.e., higher compressive strain in the Pt shell).Since it is considered that the inferior ORR specific activity of the (b) Pt/Pd/C catalyst after the ADT is mainly due to size distribution of the Pd core nanoparticles, synthesis of the Pd nanoparticles with narrower size distribution should be developed. At the meeting, cell performance using the Pt/Pd/C catalysts will be presented. Acknowledgement This work was supported by NEDO, Japan.

Pt Hollow Nanospheres As an Electrocatalyst for the Oxygen Reduction Reaction

H2-fed proton exchange membrane fuel cells (PEMFCs) represent the most advanced fuel cell technology and have a great deal of potential applications in low/zero-emission electric vehicles, distributed home power generators, and power sources for small and portable electronics. However, the commercialization of PEMFC technology has been greatly hindered by some challenges, mainly sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode and the high cost of the noble metal Pt (1, 2). Alloying platinum with non-noble metals such as Co is an effective approach to improve the catalytic performance and reduce the usage of Pt. In addition, through transferring Pt nanoparticles (NPs) into a hollow nanostructure, electrocatalytic performance can be improved greatly due to its relatively lower density and higher surface area-to-volume ratio than its solid counterpart (i.e., NPs). In this work, Pt hollow nanospheres (HNSs) were fabricated through a replacement reaction of Co atoms by PtCl6 2- ions to reduce Pt usage and improve the catalytic activity towards the ORR. Carbon black Vulcan XC-72R (VC) was introduced into a solution prior to the addition of Co(II) and the formation of Co NPs and the replacement of Co by PtCl6 2-ions for a uniform dispersion of Co NPs and the Pt HNSs on the carbon support. Some Co atoms have alloyed Pt in the synthesis and exist in the Pt HNSs (3). The hollow mesoporous core in the Pt HNSs can be utilized as an electrolyte solution buffering reservoir to minimize the diffusion distance to the interior surface of the porous shell of Pt crystallites while the porous nanochannels (i.e., the micropores between the Pt crystallites) in the shell open to the mesoporous hollow core form fast mass transport networks providing more accessible sites for oxygen transfer. In addition, triple phase boundaries (i.e., gas-electrolyte-Pt NPs) can be developed more easily in the Pt HNS enabling individual Pt crystallites in the shell to be accessible to electrolyte ions and oxygen, and thus catalytically active. Furthermore, alloying Pt with Co might result in a significant lattice shrinking because of the change in Pt-Pt bond distance which also contributes to the improved electrocatalytic activity. In contrast, for the state-of-the-art Pt NPs/VC catalyst, the Pt NPs can agglomerate more easily to form larger particles, resulting in reduced active sites. Besides, the interior voids between Pt NPs may not be accessible to electrolyte ions and do not contribute to the ORR activity due to the lack of triple phase boundaries. As a result, the as-developed Pt(20 wt%) HNS/VC catalyst outperforms significantly the state-of-the-art Pt(20 wt%)NP/VC catalyst.

Crystallographic and structural characterization of heterometalic platinum complexes Part X. Heteropolynuclear pt complexes

Abstract This review covers heteropolynuclear platinum complexes. There are over sixty examples with heterometal atoms as partners including non- transition metals, K, Cs, Mg, Ca, Sr, Tl, Sn, Pb, Zn, Cd, and transition metals: Cu, Ag, Fe, Co, Ni, Rh and Pd. In addition, there are examples for the lanthanides, Eu and Yb. The most common are Ag (x16) and K (x14). The predominant geometries for Pt(II) is square-planar and for Pt(IV) is octahedral. The overall structures are complex. In spite of the wide variety of heterometal atoms partners of platinum, there is “real” Pt-M bonds only with silver, ranging from 2.678 to 2.943(I) Å (ave 2.855 Å). The mean Pt-Pt bond distance is 2.869 Å.

Improved Oxygen Reduction Activity and Durability of Dealloyed PtCo x Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects.

The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo3/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo3/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co. In situ X-ray absorption spectroscopy (XAS) reveals that these two catalysts possess similar Pt-Pt and Pt-Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The similar activity of the two catalysts is thus ascribed to their comparable strain, ligand, and particle size effects. Ex situ XAS performed on D-PtCo3/HSC under different voltage cycling stage shows that the continuous dissolution of Co leaves behind the NPs with a Pt-like structure after 30k cycles. The attenuated strain and/or ligand effects caused by Co dissolution are presumably counterbalanced by the particle size effects with particle growth, which likely accounts for the constant specific activity of the catalysts along with voltage cycling.

Platinum organometallic complexes: classification and analysis of crystallographic and structural data of tri- and oligomeric complexes

AbstractThis review covers almost 100 organoplatinum complexes: trimers (40 examples), tetramers (40 examples), pentamers (4 examples), hexamers (5 examples), nona- and oligomers (8 examples). Platinum is predominantly found in the oxidation states +2 and +4. A number of coordination geometries are observed, the most common being essentially square planar, especially with Pt(II), and distorted octahedral, especially with Pt(IV). The most common ligands are methyl, carbonyl, PX3 and bis(diphenylphosphine)methane. Relationships between the Pt-Pt distances, Pt-X-Pt bridge angles, Pt-L bond distances and covalent radii of coordinated atoms are discussed. The mean Pt-Pt bond distance elongates in the order of nuclearity: 269.0 pm (trimers)<270.5 pm (tetramers)<271.5 pm (dimers)<278.0 pm (oligomers). A comprehensive brief discussion on over 1600 organoplatinum complexes and over 2500 platinum coordination complexes is given. These complexes prefer to crystallize in monoclinic (53%) and triclinic (27%) crystal classes. About l0% of these 4100 plus complexes exist as isomers. It is observed that these isomers are more often stereoisomers than structural isomers and that distortion isomerism is surprisingly more common than the better known cis-trans isomerism, especially in the chemistry of Pt(II) complexes.

Crystallographic and structural characterization of heterometallic platinum compounds Part VII. Heterohepta- and heterooctanuclear clusters

Abstract This review classifies and analyzes over fifty heterohepta- and heterooctanuclear platinum clusters. There are eight types of metal combinations in heteroheptanuclear: Pt6M, Pt5M2, Pt4M3, Pt3M4, Pt2M5, PtM6, Pt3Hg2Ru2 and Pt2Os3Fe2. The seven metal atoms are in a wide variety of arrangements, with the most common being one in which the central M atom (mostly M(I)) is sandwiched by two M3 triangles. Another arrangement often found is an octahedron of M6 atoms asymmetrically capped by an M atom. The shortest Pt-M bond distances (non-transition and transition) are 2.326(1) Å (M = Ga) and 2.537(6) Å (M = Fe). The shortest Pt-Pt bond distance is 2.576(2) Å. In heterooctanuclear platinum clusters there are eight types of metal combinations: Pt6M2, Pt4M4, Pt3Ru5, Pt2M6, PtM7, Pt2W4Ni2, PtAu6Hg and PtAu5Hg2. From a structural point of view, the clusters are complex with bicapped octahedrons of eight metal atoms prevailing. The shortest Pt-M bond distances (non-transition and transition) are 2.651(3) Å (M = Hg) and 2.624(1) Å (M = Os). The shortest Pt-Pt bond distance is 2.622(1) Å. These values are somewhat longer than those in the heteroheptanuclear clusters. Several relationships between the structural parameters were found, and are discussed and compared with the smaller heterometallic platinum clusters

Structural Characterization and Enhanced Oxygen Reduction Activity of Chemically-Ordered Intermetallic PtFeNi Catalysts

The polymer electrolyte fuel cells (PEFCs) are excellent candidates as next-generation electric generators for vehicles, residential uses, laptop, etc. However, there are many issues to be solved to enlarge commercially uses. One of issues is related to Pt catalysts for oxygen reduction reaction (ORR) on a cathode electrode. Pt catalysts show low ORR activity and low durability, leading to lower performance in PEFCs. Our earlier studies found the chemically ordered face centered tetragonal (fct) intermetallic Pt-alloy catalysts (L10 type super-lattice structure) can satisfy both of higher ORR activity and long-term durability, whereas many of the previously-reported Pt-alloys cannot. It is considered that the high performance of fct-Pt alloy catalysts would result from the chemically-ordered structures, but it is unclear how they work to enhance ORR activity and durability. Thus, in this presentation, we will discuss the relationship between the ORR activities and surface structures in fct-Pt alloys by the detailed characterization of the catalyst’s surface structures.The fct-PtFeNi catalysts with different atomic compositions of Fe and Ni (Pt50Fe35Ni15, Pt50Fe25Ni25, Pt50Fe15Ni35) were prepared using a simple solid-state impregnation method; the mixture of metal (Pt, Fe, Ni)-salts and carbon blacks were annealed under H2/N2 at a temperature of 800°C. The prepared fct-PtFeNi catalysts exhibit about 3 times higher ORR mass activity, compared with the commercial Pt catalyst. Especially, the mass activity of Pt50Fe35Ni15 is about 0.62 A/mgPt; it is higher than that of fct-Pt50Fe25Ni25 and fct-Pt50Fe15Ni35 (about 0.5 A/mgPt). To discuss more about the enhancement of the ORR activities in the fct-PtFeNi alloys, the surface structures of the catalysts were examined by in-situ XAFS measurements. The EXAFS analysis of Pt-L3 edge reveals that the distance of Pt-Pt bonds in the fct-PtFeNi catalysts is about 2.72 nm, which is shorter than that of commercial Pt catalyst. Interestingly, the coordination number of Pt oxides (Pt-O or Pt-OH) in the fct-Pt50Fe35Ni15 is more increased at high potential of 0.8~1.2V, compared with other fct-PtFeNi catalysts with the lower content of Fe. The tendency of the formation of Pt oxides is in accord with that of the ORR activities in the fct-PtFeNi with the different compositions of Fe and Ni. Therefore, it is inferred that fct-PtFeNi with higher content of Fe would form more Pt oxides, leading to a faster reaction rate of oxygen reduction. This study brought out new insights; the surface structures of chemically-ordered fct-Pt alloys, such as the Pt-Pt bond distance and the formation of Pt oxides, strongly affect the ORR activities, as those of the previously-reported disordered Pt-alloys do as well. It is believed that our findings can aid in developing fct-Pt alloys with more enhanced catalytic activity and durability for PEFCs.