Pt-Pt Bond Length Research Articles

High-Entropy Alloys as Oxygen Reduction Reaction Electrocatalysts for Proton Exchange Membrane Fuel Cells Application

High entropy alloys (HEA) are complex multi-element materials. Owing to their multi-elemental nature, they can finetune surface adsorption energy and active sites which make them appealing to various catalytic reactions. HEI is a subclass of HEA that consist of an ordered alloy structure rather than a random alloy structure. The order-ness may induce more stable structures and uniform active sites, which in turn enhance the durability and activity under catalytic conditions. Here, we have synthesized a HEI(Pt40Ni20Co20Mn10Ir10) supported on porous carbon via a seed-mediated impregnation method. The intermetallic characteristics with periodic trends can be seen in the HAADF-STEM image (Fig. 1a). Based on the atomic arrangement, the HEI resembled a L12 structure. The corresponding EDX mapping the catalyst also showed the presence of all alloying species in the nanoparticles. The catalyst performance of the catalysts was evaluated in a 3-electrode liquid cell set-up with catalyst coated glassy carbon rotating disk electrode, Ag/AgCl and Pt mesh as working, reference and counter electrode, respectively. The catalyst’s mass activity (MA) was calculated based on the result obtained from ORR polarization test conducted under O2 saturated 0.1M HClO4. The initial mass activities for PtCo and HEA in ORR were 0.81 and 1.23 A mg-1 Pt, respectively (Fig. 1b), while that of Pt/C was only 0.26 A mg-1 Pt. A subsequent accelerated degradation test (ADT) was conducted under Ar-saturated 0.1M HClO4 with 0.6-1.0 V vs RHE square-wave cycling (3s hold at each potential) for 30k cycles. MA was measured after 10k and 30k cycles. HEA retained a MA of 0.48 A mg-1 Pt after the ADT, above the DOE target (0.44 A mg-1 Pt). We also compared our HEI samples synthesized with different annealing temperatures and duration (Fig. 1c), and it showed that 800◦C with 20 mins temperature hold showed the highest MA and electrochemical surface area (ECSA).Extended X-ray absorption spectroscopy (EXAFS) for the catalyst has also been conducted to further elucidate the structural-performance relationship of HEI (Fig.1d). From Pt L3 edge EXAFS fitting, the fitted R space showed that the Pt-Pt bond length was around 2.704 ± 0.004 Å, which was same as that of the PtCo (Fig. 1e). Therefore, it is believed that both HEI and PtCo has the same strain effect but HEI possessed stronger ligand effect than PtCo due to its unique Pt-M coordination, hence resulting in superior ORR performance over PtCo and Pt. Figure 1

Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time.

Photoexcited molecular trajectories on potential energy surfaces (PESs) prior to thermalization are intimately connected to the photochemical reaction outcome. The excited-state trajectories of a diplatinum complex featuring photo-activated metal-metal σ-bond formation and associated Pt-Pt stretching motions were detected in real time using femtosecond wide-angle X-ray solution scattering. The observed motions correspond well with coherent vibrational wavepacket motions detected by femtosecond optical transient absorption. Two key coordinates for intersystem crossing have been identified, the Pt-Pt bond length and the orientation of the ligands coordinated with the platinum centers, along which the excited-state trajectories can be projected onto the calculated PESs of the excited states. This investigation has gleaned novel insight into electronic transitions occurring on the time scales of vibrational motions measured in real time, revealing ultrafast nonadiabatic or non-equilibrium processes along excited-state trajectories involving multiple excited-state PESs.

Operando X-Ray Absorption Spectroscopic Study of Pt-Based Nanowire Catalysts Under Oxygen Reduction Reaction

Polymer electrolyte fuel cells (PEFCs) are attracting attention as an environmentally friendly electrochemical device that uses hydrogen as a fuel. The application of PEFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum reserves. It has been noted that the interaction between the cathode catalyst and the oxygen intermediate species results in an overpotential in the ORR process. Therefore, it is very important to evaluate the interaction between the oxygen species and the catalyst based on the electronic structure. Pt nanowires are known to be highly active as ORR catalysts because they tend to expose more active planes for ORR, such as (111) planes1. Pt oxides formed on Pt nanowires with such 1D anisotropy are different from conventional nanoparticle catalysts in terms of structure and electronic structure2. However, this has not been fully clarified yet under oxygen reduction conditions. In this study, we analyzed the oxidation state of Pt nanowire catalysts in the ORR by operando analysis and analyzed the electronic and local structures.Synthesis of Pt nanowire catalyst : Oleylamine (5 mL) was used as a solvent and Pt(acac)2, Ni(acac)2, W(CO)6 and CTAC (hexadecyltrimethylammonium chloride) were added and sonicated for 1 hour. The product was dissolved and allowed to react in an oil bath at 160°C for 2 hours. The product particles were collected by centrifugation, and the washing and centrifugation were repeated three times with a mixture of cyclohexane: ethanol = 1:9. The nanowires were then loaded on Vulcan XC-72. ORR activity evaluation and XAS measurement: The rotating disk electrode (RDE) method was used for electrochemical measurements. A tripolar cell was used for the measurements. catalyst ink containing Pt nanowires was prepared and applied to a glassy carbon electrode for the RDE to prepare the electrode. A catalyst-coated electrode was used as the working electrode, a platinum mesh as the counter electrode, and a reversible hydrogen electrode (RHE) as the reference electrode. The electrolyte was 0.1 M HClO4. The cell temperature was controlled at 25 oC. XAS measurements were performed using a home-made measuring cell and the same electrochemical measurement conditions as for the RDE.The morphology and structure of the prepared Pt nanowires were observed by electron microscopy. As shown in Figure 1, the nanowires were uniformly distributed on the carbon and no significant aggregation was observed. The average diameter of the nanowires was 2.0 nm.To analyze the local structure of the Pt nanowire catalyst, the Pt-Pt bond length was estimated from EXAFS. Figure 2 shows the fitting results of the Pt-Pt bond lengths of Pt nanowire and Pt nanoparticle (TEC10V30E, TKK) catalysts in the range 0.5-1.1 V vs. RHE. With increasing potential, the Pt-Pt bond lengths of both catalysts gradually increased, but the Pt nanowire catalyst maintained a short Pt-Pt bond, and this nanowire characteristic structure may be the reason for its higher ORR activity than the nanoparticle. Acknowledgements: This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Operando X-Ray Absorption Spectroscopic Study on Electrochemical Oxygen Reduction Mechanism of Novel Platinum-Based Nanostructured Catalysts

Polymer electrolyte fuel cells (PEFCs) have been developed for electric device applications such as fuel cell vehicles (FCV). PEFC cathode catalyst requires an excess Pt due to the degradation of oxygen reduction reaction (ORR) activity. Many research efforts aim to design Pt-based nanomaterials with controllable composition and structure to increase their activity while reducing the amount of Pt, like core-shell structure. The core-shell structure is obtained by Adzic’s method (Cu-UPD)1, where the underpotential deposition (UPD) of Cu mediator on Pt and the displacement of Cu mediator by Pt are conducted. This method requires the highly precise potential control for the UPD of Cu by means of potentiostats. In contrast, our previous research demonstrated the spontaneous two-step method (Galvanic cell) to synthesize spherical Pd@Pt core-shell catalyst without a special control of electrochemical equipment and reducing/stabilizing agents2. In this study, this Galvanic cell method was applied for Pd@Pt core-shell nanowire catalysts. Compared with the conventional Cu-UPD method, this method can synthesize the catalysts on a larger scale and form more uniform monolayer Pt shells, resulting in a higher ORR activity.Ultrathin Pd nanowires were prepared using surfactants as templates3. Briefly, palladium nitrate, ODA, and DTAB were dissolved in toluene under strong magnetic stirring. The mixture was placed in an inert atmosphere and sonicated for 20 min. NaBH4 was added to the above solution as a reducing agent. One hour later, the organic phase was recovered using distilled water and chloroform as extractants. After centrifugation, Pd nanowires can be obtained. After mixing the catalyst with carbon, the Pd/C powder is dispersed in the N2-saturated 0.01M CuSO4 and 0.5 M H2SO4 solution, with continual stirring. Then, a Cu wire is immersed in the above mixture2. Near 5 hours later, Cu wire is extracted, and successively N2-saturated 10 mM K2PtCl4 solution is added to the suspension to obtain Pd-nanowire@Pt/C. Electrochemical tests were performed in 0.1 M HClO4, and the catalyst ORR activity was evaluated at 0.9 V. XAS measurements of Pt L-edge were also performed under the same conditions.In contrast to the previously reported method (Cu-UPD)4, our proposed method does not require electrochemical equipment, or reducers/stabilizers, and uniform Pt shells in Pd-nanowire@Pt/C can be obtained. As shown in Figure 1, Core-shell catalysts have a lower white line intensity than Pt/C, meaning that they are more difficult to oxidize. The ligand effect causes surface Pt compression, and after calculating, the first-shell Pt-Pt bond length in the Pd-NW@Pt Gal. catalyst is shorter than that of the nanoparticle, which means a stable structure and less oxygen species generation. So, the catalysts synthesized using the Galvanic Cells method can provide higher ORR activity (specific activities: 0.8 mA/cm2 and mass activities: 1.3 A/mgPt) compared to the Cu-UPD method. Acknowledgement This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References; [1] Zhang, J.; Mo, Y.; Vukmirovic M. B.; Klie R; Sasaki K.; Adzic R R.; J. Phys. Chem. B 2004 108 (30), 10955-10964.[2] Wang, X.; Orikasa, Y.; Inaba, M.; Uchimoto, Y.; et al. ACS Catal., 2020, 10, 430– 434.[3] Teng, X.; Han, W.; Ku, W.; Hucker, M.; et al. Angew. Chem., Int. Ed. 2008, 47, 2055– 2058.[4] Koenigsmann, C., Adzic, R. R.; et al. J. Am. Chem. Soc., 2011 , 133, 9783– 9795. Figure 1

Theoretical studies on dihedral angle-bending isomers of M2Pt20/− clusters

The structures and electronic properties of the gaseous M2Pt20/− clusters (M represents the alkaline earth metal) were investigated using the density functional theory (B3LYP and PBE0) and wave function theory (SCS-MP2, CCSD and CCSD (T)). The results indicate that the D2h isomers with the planar structures are more stable than the C2v isomers with smaller dihedral angles and shorter Pt-Pt bond lengths. The mutual competition of M(s,p)-Pt(5d) interaction and Pt-Pt covalent bonding contributes to the different stabilizations of the two kinds of isomers. The M(s,p)-Pt(5d) interaction favors the planar isomers with D2h symmetry, while the Pt-Pt covalent bonding leads to the C2v isomers with bending structures. Two different crossing points are determined in the potential energy curves of Be2Pt2 with the singlet and triplet states. But there is just one crossing point in potential energy curves of Ra2Pt2 and Ca2Pt2− because of flatter potential energy curves of Ra2Pt2 with the triplet state or Ca2Pt2− with quartet state. The results reveal a unique example of dihedral angle-bending isomers with the smallest number of atoms and may help the understanding of the bonding properties of other potential angle-bending isomers.

Orbital Control and Coherent Charge Transport in Transition Metal Platinum(II)–Platinum(II) Lantern Complexes in Molecular Junctions

Platinum (II) dimers in organometallic complexes exhibit oscillatory vibrational charge transfer with optical excitation and whose nature depends strongly on the Pt-Pt distance. Yet the electronic transport of such devices has not been studied. We take a well-reported Pt(II) dimer complex that is ideal for transport in a molecular junction and subject it to a battery of electronic structure property calculations. With DFT, we find the orbital nature of the HOMO level to not be solely induced by the 5dz2 orbital as long thought, but rather a complex mixture with significant contributions from 6s orbital states from the Au electrodes that have not been considered before. We probe how the chemical tuning of the ligand affects the Pt-Pt bond length and thus conduction, by systematically modifying both the bridging ligands (BL) and cyclometallating ligands (CMLs) with donor (methyl) and acceptor (fluorine) derivatives. With nonequilibrium green’s functions (NEGF-DFT), we find patterns between the conductivity,...

Pt8 cluster on alumina under a pressure of hydrogen: Support-dependent reconstruction from first-principles global optimization.

Alumina supported Pt nanoclusters under a hydrogen environment play a crucial role in many heterogeneous catalysis applications. We conducted grand canonical genetic algorithm simulations for supported Pt8 clusters in a hydrogen gas environment to study the intracluster, cluster-support, and cluster-adsorbate interactions. Two alumina surfaces, α-Al2O3(0001) and γ-Al2O3(100), and two conditions, T = 600 °C, pH2 = 0.1 bar and T = 25 °C, pH2 = 1.0 bar, were considered corresponding to low and high hydrogen chemical potential μH, respectively. The low free energy ensemble of Pt8 is decorated by a medium (2-12 H), respectively, high (20-30 H), number of hydrogen atoms under equilibrium at low μH, respectively, high μH, and undergoes different morphological transformations on the two surfaces. On α-Al2O3(0001), Pt8 is mostly 3D but very fluxional in structure at low μH and converts to open one-layer 2D structures with minimal fluxionality at high μH, whereas on γ-Al2O3(100), the exact opposite occurs: Pt8 clusters present one-layer 2D shapes at low μH and switch to compact 3D shapes under high μH, during which the Pt8 cluster preserves moderate fluxionality. Further analysis reveals a similar Pt-Pt bond length increase when switching from low μH to high μH on both surfaces although morphological transformations are different. Electronic structure analysis shows the existence of bonding interactions between Pt and Lewis acidic Al3+ sites along with the Pt-O interaction, which implies the necessity to include Al neighbors to discuss the electronic structure of small Pt clusters.

ORR Activity Enhancement of PtML/Pd/C Core-Shell Catalysts By Potential Cycling

It is now widely recognized that the amount of expensive Pt catalyst in PEFCs should be decreased for cost reduction. Pd core/Pt shell structured catalyst supported on carbon (Pt/Pd/C) is a strong candidate for improving the utilization of Pt. It has been also reported that the activity oxygen reduction reaction (ORR) activity of the Pt shell is enhanced by the strain effect of the Pd core [1,2]. Recently, we developed a new synthetic method for carbon supported Pd core/PtMLshell structured catalyst (Pt/Pd/C) using a modified Cu-UPD process [3] and found that ORR activity of the Pt/Pd/C catalyst was drastically enhanced after a potential cycling accelerated durability test (ADT) [4]. In this study, we investigated microstructural changes of the Pt/Pd/C catalyst after potential cycling and discussed the mechanism for the enhanced ORR activity. In addition, we optimized the protocol for potential cycling to further enhance the ORR activity. Pt/Pd/C catalyst was obtained from Pd/C core (4.5 nm, 30 wt.%, Ishifuku Metal Industry) using the modified Cu-UPD method [3]. The Pd/C core and Pt/Pd/C catalyst were characterized by TG, XRD, XRF, TEM, TEM-EDX and CV. The ORR activity of the catalyst was measured by the RDE technique at 1,600 rpm in O2 saturated 0.1 M HClO4 at 298 K. The ADT was performed using rectangular-wave potential cycling (FCCJ, 0.6 V (3 s)/1.0 V (3 s) vs. RHE for 10,000 cycles) in Ar saturated 0.1 M HClO4at 353 K. The ORR activity of the Pt/Pd/C catalyst was enhanced after ADT, showing by 2-fold higher ORR mass activity (512 A/g-Pt) than that of a commercially available Pt/C catalyst (2.8 nm, 46 wt.%, TEC10E50E, TKK). Morphological changes of the Pt/Pd/C catalyst after ADT is shown in Figure 1. The average size of the catalyst decreased from 5.6 to 4.9 nm and the catalyst particles changed into spherical particles. TEM-EDX analysis revealed that the Pd core dissolved out by 73 % and the Pt shell was thickened (to approximately 2.3 ML) after ADT. The microstructural change of the Pt/Pd/C catalyst after ADT is schematically illustrated in Figure 2. The Pd core dissolved out through defects on the Pt shell during ADT. As a result, the Pt atoms at the shell were rearranged to give a defect-free surface to the Pt shell, by which the number of lower coordinated Pt atoms decreased. EXAFS analysis revealed that the Pt-Pt bond length decreased after ADT. It was concluded that the enhanced ORR activity after ADT was caused by a moderately induced compressive strain at the Pt shell, in addition to a decreased number of the lower coordinated Pt atoms. Though the ADT protocol gave an enhanced mass activity (~ x2) to the Pt/Pd/C catalyst, there remains one problem that the electrochemical surface area (ECSA) decreased from 140 initially to 40-50 m2/g-Pt after ADT. Hence we optimized the potential cycling protocol to minimize the decrease in ECSA during ADT. The optimized protocol (0.4 V (300 s)/1.0 V (300 s) for 60 cycles) gave a high ECSA (> 80 m2/g) and a very high ORR mass activity (1002 A/g-Pt, x4 of Pt/C) after potential cycling. Acknowledgment This study was supported by NEDO, Japan.

Synthesis-atomic structure-properties relationships in metallic nanoparticles by total scattering experiments and 3D computer simulations: case of Pt-Ru nanoalloy catalysts.

An approach to determining the 3D atomic structure of metallic nanoparticles (NPs) in fine detail and using the unique knowledge obtained for rationalizing their synthesis and properties targeted for optimization is described and exemplified on Pt-Ru alloy NPs of importance to the development of devices for clean energy conversion such as fuel cells. In particular, PtxRu100-x alloy NPs, where x = 31, 49 and 75, are synthesized by wet chemistry and activated catalytically by a post-synthesis treatment involving heating under controlled N2-H2 atmosphere. So-activated NPs are evaluated as catalysts for gas-phase CO oxidation and ethanol electro-oxidation reactions taking place in fuel cells. Both as-synthesized and activated NPs are characterized structurally by total scattering experiments involving high-energy synchrotron X-ray diffraction coupled to atomic pair distribution functions (PDFs) analysis. 3D structure models both for as-synthesized and activated NPs are built by molecular dynamics simulations based on the archetypal for current theoretical modelling Sutton-Chen method. Models are refined against the experimental PDF data by reverse Monte Carlo simulations and analysed in terms of prime structural characteristics such as metal-to-metal bond lengths, bond angles and first coordination numbers for Pt and Ru atoms. Analysis indicates that, though of a similar type, the atomic structure of as-synthesized and respective activated NPs differ in several details of importance to NP catalytic properties. Structural characteristics of activated NPs and data for their catalytic activity are compared side by side and strong evidence found that electronic effects, indicated by significant changes in Pt-Pt and Ru-Ru metal bond lengths at NP surface, and practically unrecognized so far atomic ensemble effects, indicated by distinct stacking of atomic layers near NP surface and prevalence of particular configurations of Pt and Ru atoms in these layers, contribute to the observed enhancement of the catalytic activity of PtxRu100-x alloy NPs at x ∼ 50. Implications of so-established relationships between the atomic structure and catalytic activity of Pt-Ru alloy NPs on efforts aimed at improving further the latter by tuning-up the former are discussed and the usefulness of detailed NP structure studies to advancing science and technology of metallic NPs - exemplified.

Pt–Au Alloying at the Nanoscale

The formation of nanosized alloys between a pair of elements, which are largely immiscible in bulk, is examined in the archetypical case of Pt and Au. Element specific resonant high-energy X-ray diffraction experiments coupled to atomic pair distribution functions analysis and computer simulations prove the formation of Pt-Au alloys in particles less than 10 nm in size. In the alloys, Au-Au and Pt-Pt bond lengths differing in 0.1 Å are present leading to extra structural distortions as compared to pure Pt and Au particles. The alloys are found to be stable over a wide range of Pt-Au compositions and temperatures contrary to what current theory predicts. The alloy-type structure of Pt-Au nanoparticles comes along with a high catalytic activity for electrooxidation of methanol making an excellent example of the synergistic effect of alloying at the nanoscale on functional properties.

Structural and Electronic Properties of Pt<sub>3</sub> and Pt<sub>3</sub>M Clusters (M=Au, Ag, Sn, Fe)

With density functional theory, the structural and electronic properties of Pt3 and Pt3M clusters (M=Au, Ag, Sn, Fe) have been studied. It is found that all the Pt3M clusters show planar and tetrahedral structures and their Pt-Pt bond lengths are larger than the bond lengths of Pt3. The effect of M doping on electronic properties of Pt3 clusters have been investigated. It is found that by adding one M atom, energy gaps of the corresponding clusters become smaller. The calculated results also indicate that M atom makes the highest energy level of Pt atoms shift highly, except for tetrahedral Pt3Au clusters. These may be good for analyzing adsorption problems

Oxygen adsorption on small PtNi nanoalloys

The adsorption of an oxygen molecule on nanoclusters of Pt and PtNi, in the size range between 13 and 55 atoms, has been studied using first-principle simulations. The structures have been obtained as a function of size and chemical composition of the clusters by means of the parallel excitable-walkers basin hopping method. O(2) preferentially adsorbs along the edge between two (111) facets due to a massive distortion of the Pt-Pt bond length. This bond elongation favours the adsorption in such a way that the binding energy of oxygen on a pure 55-atom cluster is still twice the value on the clean Pt(111). On the other hand, on 55-Pt(shell)Ni(core) nanoparticles, the O(2) binding energy is slightly lower than on Pt(111), because nickel core inhibits the stretching of the Pt-bond because of their size mismatch. However, as soon as its concentration is increased, Ni appears at the surface and its oxyphilic nature contributes to bind the oxygen molecule stronger.

Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by X-ray Transient Absorption Spectroscopy

The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(μ-(t)Bu(2)pz)](2) (ppy = 2-phenylpyridine; (t)Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal-metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt-Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 Å in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.

Unprecedented Near-Infrared (NIR) Emission in Diplatinum(III) (d7−d7) Complexes at Room Temperature

The synthesis and single-crystal X-ray structures of the first family of efficient NIR emitters with tunable emission energy based on dihalodiplatinum(III) (5d(7)-5d(7)) complexes of general formulae [Pt(2)(mu-C(6)H(3)-5-R-2-AsPh(2))(4)X(2)] (R = Me or CHMe(2); X = Cl, Br or I), together with that of their diplatinum(II) (5d(8)-5d(8)) precursors ([Pt(2)(mu-C(6)H(3)-5-R-2-AsPh(2))(4)]) and cyano counterparts (X = CN), are reported. The diplatinum(II) complexes with isopropyl groups are isolated initially as a mixture of two species, one being a half-lantern structure containing two bridging and two chelate C(6)H(3)-5-CHMe(2)-2-AsPh(2) ligands (1b) that exists in two crystalline modifications [d(Pt...Pt) = 3.4298(2) A and 4.3843(2) A]; the other is a full-lantern or paddle-wheel structure having four bridging C(6)H(3)-5-CHMe(2)-2-AsPh(2) ligands (2b) [d(Pt...Pt) = 2.94795(12) A]. Complete conversion of the isomers into 2b occurs in hot toluene. The Pt-Pt bond distances in the diplatinum(III) complexes are less than that in 2b and increase in the order X = Cl (3b) [2.6896(2) A] < Br (4b) [2.7526(3) A] < I (5b) [2.7927(7) A] approximately CN (6b) [2.7823(2), 2.7924(2) A for two independent molecules]. Comparison with the corresponding data for our previously reported series of complexes 3a-6a (R = Me) indicates that the Pt-Pt bond lengths obtained from single-crystal X-ray analysis are influenced both by the axial ligand and by intermolecular lattice effects. Like [Pt(2)(mu-pop)(4)](4-) [pop = pyrophosphite, (P(2)O(5)H(2))(2-)], the diplatinum(II) complexes [Pt(2)(mu-C(6)H(3)-5-R-2-AsPh(2))(4)] [R = Me (2a), CHMe(2) (2b)] display intense green phosphorescence, both as solids and in solution, and at room temperature and 77 K, with the emission maxima in the range 501-532 nm. In contrast to the reported dihalodiplatinum(III) complexes [Pt(2)(mu-pop)(4)X(2)](4-) that exhibit red luminescence only at 77 K in a glass or as a solid, complexes 3a-6a and 3b-6b are phosphorescent in the visible to near-infrared region at both room and low temperatures. The electronic spectra and photoemissive behavior are discussed on the basis of time-dependent density functional theory (TDDFT) calculations at the B3YLP level. The photoemissive states for the halide analogues 3a,b-5a,b involve a moderate to extensive mixing of XMMCT character and MC [d sigma-d sigma*] character, whereas the cyano complexes 6a and 6b are thought to involve relatively less mixing of the XMMCT character into the MC [d sigma-d sigma*] state.

The Emergence of Nonbulk Properties in Supported Metal Clusters: Negative Thermal Expansion and Atomic Disorder in Pt Nanoclusters Supported on γ-Al2O3

The structural dynamics-cluster size and adsorbate-dependent thermal behaviors of the metal-metal (M-M) bond distances and interatomic order-of Pt nanoclusters supported on a gamma-Al(2)O(3) are described. Data from scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) studies reveal that these materials possess a dramatically nonbulklike nature. Under an inert atmosphere small, subnanometer Pt/gamma-Al(2)O(3) clusters exhibit marked relaxations of the M-M bond distances, negative thermal expansion (NTE) with an average linear thermal expansion coefficient alpha = (-2.4 +/- 0.4) x 10(-5) K(-1), large static disorder and dynamical bond (interatomic) disorder that is poorly modeled within the constraints of classical theory. The data further demonstrate a significant temperature-dependence to the electronic structure of the Pt clusters, thereby suggesting the necessity of an active model to describe the cluster/support interactions mediating the cluster's dynamical structure. The quantitative dependences of these nonbulklike behaviors on cluster size (0.9 to 2.9 nm), ambient atmosphere (He, 4% H(2) in He or 20% O(2) in He) and support identity (gamma-Al(2)O(3) or carbon black) are systematically investigated. We show that the nonbulk structural, electronic and dynamical perturbations are most dramatically evidenced for the smallest clusters. The adsorption of hydrogen on the clusters leads to an increase of the Pt-Pt bondlengths (due to a lifting of the surface relaxation) and significant attenuation of the disorder present in the system. Oxidation of these same clusters has the opposite effect, leading to an increase in Pt-Pt bond strain and subsequent enhancement in nonbulklike thermal properties. The structural and electronic properties of Pt nanoclusters supported on carbon black contrast markedly with those of the Pt/gamma-Al(2)O(3) samples in that neither NTE nor comparable levels of atomic disorder are observed. The Pt/C nanoclusters do exhibit, however, both size- and adsorbate-induced trends in bond strain that are similar to those of their Pt/gamma-Al(2)O(3) analogues. Taken together, the data highlight the significant role that electronic effects--specifically charge exchange due to both metal-support and metal-adsorbate interactions--play in mediating the structural dynamics of supported nanoscale metal clusters that are broadly used as heterogeneous catalysts.

Characteristic Features of Platinum Stereochemistry in the Structures of Organometallic Compounds

The Voronoi-Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to carry out crystal chemical analysis of 85 compounds containing 124 PtC n or PtC n Pt m coordination polyhedra in which the Pt-C and Pt-Pt bond lengths vary in the 1.82-2.43 and 2.58-2.98 A ranges. The main stereochemical features of platinum are discussed depending on the nature (C or Pt) and the number (n varying from 1 to 10; m varying from 0 to 9) of atoms in the first coordination sphere. It is shown that the VDP parameters can be used to identify the aghostic Pt···H-C interactions in the crystal structure.

The PF6−/BF4− Mixed Salt of the Cluster Cation [Pt3(μ3-AuPPh3)(μ-dppm)3]+, dppm = Ph2PCH2PPh2

Remarkably, the crystal structure of this Pt3Au cluster compound, tris[bis(diphenylphosphino)methane]2:3κ2P:P'; 2:4κ2P:P'; 3:4κ2P:P'-(triphenylphosphine)1κP-tetrahedro-1-gold-2,3,4-triplatinum(3 Au-Pt, 3 Pt-Pt) 0.75-hexafluorophosphate 0.25 tetrafluoroborate, [Au-Pt3(C25H22P2)3(C18H15P)](BF4)0.25(PF6)0.75, is based on an F-centred cubic unit cell which contains 32 [Pt3(μ3-AuPPh3)(μ-dppm)3]+ cations, together with 24 PF6− and eight BF4− anions. Each cation straddles a crystallographic threefold axis and contains a triangular Pt3 unit whose edges are bridged by the three dppm ligands. The Pt3 triangle is capped by an AuPPh3+ fragment to form a tetrahedral Pt3Au cluster with a slight trigonal elongation. The Pt-Pt and Pt-Au bond lengths are 2.618 (2) and 2.707 (2) A, respectively.

Asymmetrical A-Frame Triplatinum Clusters Bridged by Small Organic Molecules and Bis((diphenylphosphino)methyl)phenylphosphine.

Reactions of the linear triplatinum complex [Pt(3)(&mgr;-dpmp)(2)(XylNC)(2)](2+) (3) with small organic molecules led to formation of asymmetrical A-frame triplatinum complexes with an additional bridge across one of the metal-metal bonds, where dpmp is bis((diphenylphosphino)methyl)phenylphosphine. Reaction of complex 3 with electron deficient alkynes (R(1)C&tbd1;CR(2): R(1) = R(2) = CO(2)Me; R(1) = H, R(2) = CO(2)Me; R(1) = R(2) = CO(2)Et) afforded a new series of triplatinum clusters formulated as [Pt(3)(&mgr;-dpmp)(2)(&mgr;-R(1)CCR(2))(XylNC)(2)](PF(6))(2) (5a, R(1) = R(2) = CO(2)Me; 5b, R(1) = H, R(2) = CO(2)Me; 5c, R(1) = R(2) = CO(2)Et) in good yields. The complex cation of 5b was characterized by X-ray crystallography to have an asymmetrical A-frame structure comprising three Pt atoms bridged by two dpmp ligands, in which an acetylene molecule was inserted into one of the Pt-Pt bonds (triclinic, P&onemacr;, a = 19.507(3) Å, b = 20.327(4) Å, c = 14.499(4) Å, alpha = 107.69(2) degrees, beta = 102.08(2) degrees, gamma = 71.30(1) degrees, V = 5148 Å(3), Z = 2, R = 0.070, and R(w) = 0.084). The Pt-Pt bond length is 2.718(1) Å and the Pt.Pt nonbonded distance is 3.582(1) Å. Treatment of 3 with an excess of HBF(4).Et(2)O gave the asymmetrical cluster [Pt(3)(&mgr;-dpmp)(2)(&mgr;-H)(XylNC)(2)](BF(4))(3).CH(2)Cl(2) (6.CH(2)Cl(2)), in 61% yield, and a similar reaction with p-NO(2)C(6)H(4)NC led to the formation of [Pt(3)(&mgr;-dpmp)(2)(&mgr;-R(3)NC)(XylNC)(2)](PF(6))(2).CH(2)Cl(2) (7.CH(2)Cl(2)) in 94% yield (R(3) = p-NO(2)C(6)H(4)). Complexes 6 and 7 are assumed to have a single atom-bridged, asymmetrical A-frame structures. Reaction of the complex syn-[Pt(2)(&mgr;-dpmp)(2)(XylNC)(2)](2+) (1) with [MCl(2)(cod)] (M = Pt, Pd) gave the dimer-monomer combined trinuclear cluster [Pt(2)MCl(2)(&mgr;-dpmp)(2)(XylNC)(2)](PF(6))(2) (8a, M = Pt, 89%; 8b, M = Pd, 55%). The structure of 8a was determined by X-ray crystallography to be comprised of a metal-metal-bonded diplatinum core and a monomeric platinum center bridged by two dpmp ligands with a face-to-face arrangement (triclinic, P&onemacr;, a = 18.082(7) Å, b = 19.765(6) Å, c = 15.662(4) Å, alpha = 98.51(2) degrees, beta = 94.24(3) degrees, gamma = 109.82(2) degrees, V = 5161 Å(3), Z = 2, R = 0.069, and R(w) = 0.080). The Pt-Pt bond length is 2.681(2) Å and the Pt.Pt nonbonded distance is 3.219(2) Å. The heteronuclear complex 8b was transformed to an A-frame trinuclear cluster, [Pt(2)PdCl(&mgr;-Cl)(&mgr;-dpmp)(2)(XylNC)](PF(6))(2) (9), which was characterized by X-ray crystallography (monoclinic, C2/c, a = 33.750(9) Å, b = 28.289(9) Å, c = 23.845(8) Å, beta = 118.19(4) degrees, V = 20066 Å(3), Z = 8, R = 0.082, and R(w) = 0.077). The diplatinum unit (Pt-Pt = 2.606(2) Å) is connected to the mononuclear Pd center by a chloride bridge (Pt.Pd = 3.103(3) Å, Pt-Cl-Pd = 79.6(3) degrees ).

X-ray absorption spectroscopic study of platinum supported on sulfate ion-treated zirconium oxide

Platinum particles supported on a sulfate ion-treated zirconium oxide (SO 4 2− -ZrO 2) were characterized by means of X-ray absorption near edge structure (XANES )/extended X-ray absorption fine structure (EXAFS) and temperature-programmed reduction (TPR) in order to elucidate the nature of the sulfar-aided metal-support interactions. X-ray edge shift and area calculation, which give the number of unoccupied d states of the platinum particles, are suggestive of electron transfer from the platinum particle to the support and/or existence of the platinum unreduced cations. Fourier transforms (FTs) of k 3-weighted EXAFS reveal that the Pt particles on the (SO 4 2− -ZrO 2 support after hydrogen treatment at 623 K are a mixture of Pt metal ( Pt-Pt bond length = 2.77A˚) and Pt oxides, whereas the Pt particles on ZrO 2 exist as metal (Pt-Pt bond length = 2.73A˚). TEM reveals that the presence of sulfate ion causes an increase of the Pt particle size (30A˚→50A˚). The low reducibility of the Pt particles on the (SO 2- 4-ZrO 2 support is evidenced by TPR results. The nature of the platinum-support interaction in the Pt/(SO 4 2− -ZrO 2 catalyst is understood in terms of a Redox (reduction-oxidation) metal-support interaction.