Density functional theory analysis of reactivity of PtxPdy alloy clusters

We investigate the reactivity of various Pt_xPd_y combinations (with x + y = 10 and various x:y ratios) towards the adsorption of specific intermediates of the oxygen reduction, using the B3PW91 hybrid density functional theory. The reactivity is shown to be not only sensitive to the composition of the cluster, but also to the atomic distribution. The calculations indicate that two different ensembles: one ordered and one randomly mixed, with overall composition Pt₃Pd₇ are thermodynamically more favorable than pure Pt₁₀ for the oxygen reduction reaction. The reasons for this behavior are clearly explained in terms of the atomic and electronic distribution, which makes the Pd atoms to act as electron donors both to Pt atoms and to the adsorbates, thus the reactivity of the Pd atoms in such environment becomes intermediate between Pt and Pd. Moreover, it is found that in a mixed Pt₃Pd₇ state the electronic distribution makes the average atom more similar to Pt than to Pd, whereas in an ordered Pt₃Pd₇ cluster, the average atom is more similar to Pd than to Pt.     

Nuclear spin-lattice relaxation of60Co in ferromagnetic alloys Pt99Co1 and Pd99Co1

Nuclear spin-lattice relaxation (SLR) of⁶⁰Co in the systems Pt₉₉Co₁, Pd₉₉Co₁ has been studied by very low temperature nuclear orientation technique. The thermal cycling method has been used. The difference of two orders of magnitude of Korringa constant has been found between Pt₉₉Co₁ and Pd₉₉Co₁.     

A Novel Fluorescent Probe for the Highly Selective and Sensitive Detection of Palladium in Aqueous Medium

Based on the Pd⁰-catalyzed Tsuji-Trost allylic oxidative insertion reaction, we developed a fluorescent probe PdL1 for sensing Pd⁰. As expected, probe PdL1 exhibited high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Pd⁰ in CH₃CH₂OH/PBS (10 mM, pH = 7.4, 6:4, v/v) solution. The detection limit was calculated to be as low as 15 nM, which can meet the selective requirements for practical application.     

Structure Studies of Platinothioneins by Using Extended X-Ray Absorption Fine Structure Spectroscopy

The metal binding sites in the two Pt containing metallothioneins (Pt₇MT and Pt₁₄MT) were examined by means of Pt(II) L₃-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Comparisons between the phase and amplitude functions derived from the isolated shells to those of Pt…Pt, Pt-S and Pt-N model components showed that each platinum in Pt₇MT was coordinated by four sulfur atoms at a distance of 2.31±0.01 Å. Analysis of the outer shell data of platinum atom in Pt⁷MT indicated backscattering platinum atom at approximate 4.29 Å. Strikingly different structural parameters had been obtained for the Pt¹⁴MT species, fitting of the first shell revealed that each platinum was coordinated by two sulfurs at the distance of 2.30±0.02 Å and two nitrogens at 2.02±0.02 Å. The results of the work provided the detailed information concerning the local environments of the coordinated Pt(II) in these two platinothioneins.     

Mössbauer effect studies of spin disturbances in dilute ferromagnetic alloys

¹¹⁹Sn Mössbauer effect studies of dilute Pt Fe, Pd Fe and Ni Fe alloys are reported. For Pt Fe an oscillatory spatial dependence of the spin-polarization about the Fe atoms is observed. The range increases with increasing temperature. For Pd Fe and Ni Fe the host properties are masked by the magnetic disturbances induced by the tin probe atoms.     

Nuclear quadrupole interactions of 111In/Cd solute atoms in a series of rare-earth palladium alloys

Nuclear quadrupole interactions were measured at ¹¹¹In/Cd probe atoms in rare-earth palladium phases RPd₃ having the L1₂ structure using the technique of perturbed angular correlation of gamma rays (PAC). Measurements were made for pairs of samples having compositions of the Pd-poorer and Pd-rich boundaries of the RPd₃ phase fields, typically 75 and 78 at.% Pd. Two signals were detected in most phases, corresponding to probe atoms on cubic R- and non-cubic Pd-sublattices. Site preferences of parent In-probe atoms were characterized by site-fractions of probes on the R- and Pd-sublattices. For all Pd-rich samples, probes exclusively occupied the R-sublattice, consistent with a heuristic rule that solute atoms tend to occupy the sublattice of an element in which there is a deficiency. For Pd-poorer alloys with R = Tb, Er, Yb, Lu, Y and Sc, probes exclusively occupied the Pd-sublattice. For Pd-poorer alloys with R = Ce, Pr, Nd, Sm and Eu, probes occupied both sublattices, with site fractions varying as a function of temperature. In contrast, probes only occupied the R-sublattice in Pd-poorer LaPd₃. Jump frequencies on the Pd-sublattice of daughter Cd-probes were determined from nuclear relaxation caused by fluctuating electric field gradients. Activation enthalpies for diffusional jumps were determined from temperature dependences and found to increase linearly as the lattice parameter decreases along the series Pr, Nd, Eu and Sm. Jump frequencies are believed to have been even higher in CePd₃ than in PrPd₃, but were too low to be detectable in Tb, Er, Yb, Lu, Y and Sc palladides. A correlation between site preferences and jump frequencies is noted and discussed. This paper provides a complete account of measurements of jump frequencies of Cd-probe atoms and comparisons with similar measurements made on other series of L1₂ phases.     

Segregation of Solutes in Two-Phase Mixtures

Fractions of indium solutes in each phase of a mixture of two binary phases were measured using perturbed angular correlation of gamma rays. Measurements of phase fractions were made on Pd₃Ga₇–PdGa, PdGa–Pd₅Ga₃, and FeAl₂–FeAl mixtures as a function of composition. The phase fractions were analyzed using a thermodynamic model that takes into account differences between energies of solute atoms in the two phases. From the model, segregation coefficients were obtained for the systems studied. Also, earlier measurements on Ni₂Al₃–NiAl were reanalyzed. Large differences are found among the segregation coefficients. This revised version was published online in September 2006 with corrections to the Cover Date.     

Investigation of structure and magnetic characteristics of Ni-implanted AlGaN films

The structural, valence of elements and magnetic characteristics of Ni-implanted Al_0.5Ga_0.5N films, deposited on Al₂O₃ substrates by metalorganic chemical vapor deposition (MOCVD), were reported. Ni ions were implanted into Al_0.5Ga_0.5N films by Metal Vapor Arc (MEVVA) sources under the energy of 100 keV for 3 h. The films were annealed at 900 K in the furnace for the transference of Ni ions from interstitial sites to substitutional sites in AlGaN and activating the Ni³⁺ ions. Characterizations were carried out in situ using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and Vibrating sample magnetometer (VSM), indicating that the films have wurtzite structure without forming a secondary phase after annealing. Ni ions were successfully implanted into substitutional sites of Al_0.5Ga_0.5N films and the chemical bonding states of Ni³⁺ is Ni–N. The apparent hysteresis loops prove the films exhibited ferromagnetism at 300 K. The room temperature (RT) M_s and H_c obtained were approximately 0.22 emu/g and 32.97 Oe, respectively. From the first-principles calculation, A total magnetic moment of 2.86 μB per supercell is calculated: the local magnetic moment of NiN₄ tetrahedron, 2.38 μB, makes the primary contribution. The doped Ni atom hybridizes with its four nearby N atoms in NiN₄ tetrahedron, then N atoms are spin polarized and couple with Ni atom with strong magnetization, which result in ferromagnetism. Therefore, the p-d exchange mechanism is responsible for ferromagnetism in Ni-doped AlGaN. It is expected that the room temperature ferromagnetic Ni-doped Al_0.5Ga_0.5N films can make it possible to the applications for the spin electric devices.     

Effect of alloying on the stabilities and catalytic properties of Pt–Au bimetallic subnanoclusters: a theoretical investigation

Density functional theory (DFT) has been applied to study the geometrical and electronic structures and the catalytic properties for NO oxidation of pure Pt and PtAu clusters. The calculated results suggest that Pt₁₀ clusters shows the most stable structure among the pure Pt _n (n = 2–13) clusters with the local maximum Δ₂ E value. The doping of Au atoms reduces the stability of the clusters, and Pt₆Au₄ cluster has the most stable structure among Pt_10?n Au _n (n = 1–7) clusters, due to the closest band centers between Pt and Au atoms (0.83 eV) and the obvious s–p resonance peaks near the Fermi level. Pt₆Au₄ cluster displays the strongest activation of O₂ molecules among Pt_10?n Au _n (n = 0–7) clusters, owing to the clear overlap between O 2p and Pt 6 s and Au 6 s near the Fermi level, and the more positive d band center than the others. The interaction between NO and metals changes slightly in NO/Pt_10-nAu_n (n = 2–7) systems, which is weaker than that in NO/Pt₉Au system, as a result of the decreasing resonance peaks of sp hybridization near the Fermi level. Compared to pure Pt₁₀ cluster, the lower energy barriers and larger reaction energies on Pt₆Au₄ cluster suggest a higher catalytic activity of PtAu cluster for the O₂ dissociation and NO oxidation reactions. Our study provides atomic-scale insights into the nature of the interfacial effect that determines NO oxidation on PtAu cluster catalysts.     

Origin of dielectric relaxations in KHSeO4 above room temperature

The dispersion curves of the dielectric response for KHSeO₄ were obtained in the radio frequency range at several isotherms below the fast proton conducting phase (T?<?415 K). The results reveal a distinct dielectric relaxation at low frequency, which is about 682 Hz at 320 K, and then, it shifts to higher frequencies (～10 kHz) as the temperature increases. The f _max vs. reciprocal T shows an activated relaxation process with an activation energy of 0.5 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be attributed to polarization induced by the proton jump and selenate tetrahedral reorientations. The displacement of mobile H⁺ proton accompanied by SeO ₄ ^??2 tetrahedra reorientations creates structural distortion in both sublattices which induce localized dipoles like HSeO ₄ ^? .     

PAC Probes as Diffusion Tracers in Solids

Perturbed angular correlation (PAC) probe atoms have been used as tracers to study diffusion in solids. The method works for diffusion on a sublattice for which the point symmetry is noncubic and the electric field gradient (EFG) at the probe nucleus reorients in each jump. Such motion leads to relaxation of the nuclear quadrupole interaction. Precise values of the tracer jump frequency have been obtained from fits of measured PAC perturbation functions. Results obtained to date are reviewed for Cd tracer atoms in rare-earth indides such as LaIn3 that have the L1₂ crystal structure, for which each jump on the In-sublattice reorients the EFG by 90°. New results are presented for LaSn3 and prospects for future studies are outlined.     

Kernresonanz-und leitfähigkeitsmessungen zu diffusion und fehlordnung in LiBr

Electrical conductivity and nuclear magnetic relaxation rates were measured with pure and doped LiBr between 400 K and the melting point (824 K). Prevalent intrinsinc disorder was observed down to 470 K. The degree of thermal disorder is 5.10⁻⁷ at 470 K and 5.10⁻³ at the melting point. From the relaxation rates of ⁷Li, which are caused by Li-diffusion and nuclear dipole interaction, mean jump frequencies of the cations are derived. Conductivities calculated from these frequencies for a jump process via neighbouing cation vacancies are in perfect agreement with directly measured conductivities. From relaxation rates of ⁸¹Br with MgBr₂-doped crystals jump frequencies of vacancies were obtained which are again in good agreement with those derived from the conductivity data. The energies resulting from the measurements are (0.43 ± 0.03) eV for migration of cation vacancies and (1.46 ± 0.05) eV for thermal disorder. From motional narrowing of the ⁸¹Br absorption line the jump frequency of the anions is obtained, which is much smaller than for the cations. Since this motional narrowing is not influenced by any doping, it is concluded that anion transport mainly occurs via pairs of cation and anion vacancies.     

Electronic structure and half-metallicity of the new Heusler alloys PtZrTiAl,PdZrTiAl and Pt0.5Pd0.5ZrTiAl

The electronic structure and magnetic properties of the PtZrTiAl, PdZrTiAl and Pt_0.5Pd_0.5ZrTiAl Heusler alloys were investigated using the full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA). For the PtZrTiAl, and PdZrTiAl alloys, the results showed that these Heusler alloys were stable in the Type I structure. The (Pt, Pd)ZrTiAl Heusler alloys are found to exhibit half-metallic ferromagnetism for both the Type I and Type II structure. The total magnetic moments of the PtZrTiAl and PdZrTiAl alloys were obtained to be 3 μ_B per formula unit, which are in agreement with the Slater-Pauling rule m_tot = (Nv ? 18). The half-metalliciy characteristic exists in the relatively wide ranges of 6.06–6.78 Å, and 6.13–6.73 Å for the PtZrTiAl and PdZrTiAl alloys, respectively. To complete the fundamental characteristics of these alloys, Pt_0.5Pd_0.5ZrTiAl is predicted to be a half-metallic ferromagnet with an energy gap of 0.90 eV in the minority spin and a complete spin polarization at the Fermi level. These new Heusler alloys may become ideal candidate material for future spintronic applications.     

Dissociation of PtSi,NiSi and PdGe in presence of Pt,Ni and Pd films,respectively

⁴He⁺ ions backscattering spectrometry and x-ray diffractometry were used to study interactions between PtSi and Pt, NiSi and Ni, PdGe and Pd. Due to the dissociation of the compound the formation of a phase richer in metal was observed to grow at the original compound/metal interface in the temperature range considered, 280–325°C for Pt₂Si, 325°C for Ni₂Si and 180–260°C for Pd₂Ge. The growth kinetics of these new phases (Pt₂Si and Pd₂Ge) follow a parabolic relation between thickness and annealing time. At a given temperature the growth rate of Pt₂Si and Pd₂Ge in compound-metal structure is a factor higher than in the usual semiconductor-metal structure. Partially supported by Consiglio nazionale delle Ricerche (Italy) and by Commission of the European Communities.     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters — a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt _n and Pd _n clusters (n=2–9, 13). Upon full saturation by H atoms, the calculated H2 dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂ dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃ from the icosahedral to fcc-like structures at certain H coverage was also investigated.      

Nuclear magnetic resonance on oriented189,191 Pt in iron

Nuclear magnetic resonance measurements have been performed for¹⁸⁹Pt and¹⁹¹Pt oriented at 7 and 15 mk in iron host. The magnetic hyperfine splitting frequencies, ν=¦μB_HF/Ih¦, of the¹⁸⁹Pt and¹⁹¹Pt ground states were determined to be 277.61(5) and 319.88(3) MHz. With the hyperfine field of B_HF=-1280(26) kG the nuclear magnetic moments were deduced to be: ¦μ(¹⁸⁹Pt;3/2^?)¦=0.427(9) μ^N; ¦μ(¹⁹¹Pt,3/2^?) ¦=0.492(10) μ_N. The effective spinlattice relaxation time for¹⁹¹PtFe at 7 mK in a polarizing magnetic field of 2 kG has been found to be 30(2) s using a single-exponential fit.     

195Pt,15N NMR spectroscopy of aqueous solutions of mixed valent oxonitrocomplexes,Pt(II,IV): K5[(NO2)3PtIV(μ-O)3 Pt 3 II (NO2)6] and K8[(NO2)6Pt 3 II (μ-O)3PtIV(μ-O)3Pt 3 II (NO2)6]

Two aqueous solutions of oxonitrocomplexes comprising di- and tetravalent platinum atoms, K₅[(NO₂)₃Pt^IV(μ-O)₃Pt ₃ ^II (NO₂)₆]· 3H₂O and K₈[(NO₂)₆Pt ₃ ^II (μ-O)₃Pt^IV (μ-O)₃Pt ₃ ^II (NO₂)₆] · 7H₂O, have been studied by¹⁹⁵Pt and¹⁵N NMR technique. Both compounds, enriched with the¹⁵N (95%) isotope, were synthesized by thermal denitrozylation of K₂[Pt(NO₂)₄]. Constants of spin-spin scalar coupling,¹⁹⁵Pt-¹⁵N and¹⁹⁵Pt^II-¹⁹⁵Pt^IV, were obtained for tetra-and heptameric complexes in aqueous solutions through multiplet simulation. The¹⁹⁵Pt and¹⁵N NMR spectra of the same solutions obtained two months after their preparation show signals from the hydrolytic forms of the tetrameric complex as well as the signals from mono- di- and trimeric oxonitrocomplex forms of divalent platinum. All signals of species formed as a result of hydrolysis were identified.     

Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn₇Ir₃ crystal structure (D8_f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D8_1???3, cI52) or the Sn₇Ir₃ structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of ¹¹¹In/Cd probe atoms, and demonstrated a common, characteristic “signature” of the Sn₇Ir₃ structure in all three alloys. The Sn₇Ir₃ structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn₇Ir₃ structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Hyperfine interaction and phase transitions in oxide films of zirconium alloys

Trimetallic Pt/Al₂O₃SnIn–Cl naphtha-reforming catalysts were prepared via co-precipitation route. Platinum and chlorine were introduced by the incipient wetness technique on the alumina support already doped with about 0.3 %wt of Sn to obtain about 0.3 %wt of Pt and 1.5 %wt of Cl. For the same Pt, Sn and Cl composition, indium loading ranged from 0.06 to 0.6 wt.%. The obtained catalysts were investigated by ¹¹⁹Sn Mössbauer spectroscopy between 95 and 300 K. Two Sn(IV), Sn(II) and Sn(0) environments have been identified and well characterized by their hyperfine parameters. The Lamb-Mössbauer factors have been determined for each environment and found to be 0.53, 0.27 and 0.31 for Sn(IV), Sn(II) and Sn(0) respectively. The addition of indium has been found to favour the formation of Pt_xSn alloys. ¹¹⁹Sn Mössbauer spectroscopy results show that addition of even small amount of In (0.06 wt.%) leads to the formation of a Pt₃Sn alloy. At higher indium loadings, higher amounts of Pt_xSn alloys of almost equal Pt and Sn atomic concentrations were detected. The increasing formation of Pt_xSn alloys with higher indium loading is in good correlation with a decrease of catalyst’s overall conversion and selectivity to C1 and C3–C4 paraffins and increase of isomerization selectivity.