Effect of Ag promoter on redox properties and catalytic performance of Ag-Mo-P-O catalysts for oxidative dehydrogenation of propane

The influence of addition of sliver ions on the redox properties and catalytic performance of molybdenum-phosphate catalyst in oxidative dehydrogenation (ODH) of propane has been investigated. MoO₃ and AgMoO₂PO₄ have been detected in Ag-doped Mo-P-O catalysts. The redox properties have been characterized by H₂-temperature-programmed reduction (TPR), electronic paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of MoO₃, Ag₂O, AgMoO₂PO₄ and Ag-doped Mo-P-O catalysts has also been examined. The incorporation of Ag improved the number of Mo⁵⁺ species and the reducibility of the catalysts. The Ag-doped catalysts favor activity and selectivity in ODH of propane higher than the undoped catalyst. The best catalyst was obtained with Ag/Mo ratio of 0.3 and MoO₃/AgMoO₂PO₄ ratio of 1.1. The influence of promoter Ag⁺ on redox properties and catalytic performance is due to forming redox couple “Ag⁰+Mo⁶⁺Ag⁺+Mo⁵⁺” and the synergetic effect originating from “coherent interface” between MoO₃ and AgMoO₂PO₄.     

Photoproduction of Θ and other pentaquark states

We present constraints on the relative photoproduction cross sections of positive parity pentaquark states, Σ₅, Λ₅, and N₅, based on a minimum phenomenology gained in and their baryon–meson couplings as in the work of Close and Dudek. The possibility of anomalous signals in γp→K⁰_S(Θ⁺;Σ_d⁺) is discussed. We emphasize the importance of comparing with “conventional” states such as γN→KΣ(1660).     

Measurement of the cascade time of ∑ in liquid helium

In a study of ∑⁻π⁺ production by stopped K⁻ in liquid helium, (19 ± 6)% of stopped ∑⁻ were observed to decay. This corresponds to a “cascade time” of 28 ± 8 psec.     

Temperature dependence of the electric dipole moment of paraelectric Ag and OH centers in rubidium halide crystals

Sensitive dielectric measurements were performed in diluted substitutional impurity dipole systems (10¹⁷ – 10¹⁸ cm^-3) in rubidium halides. Both, molecular OH^- and monatomic Ag⁺ off-center dipole systems were studied at low frequencies (up to 100 KHz) and at temperatures between 1.5 and 300 K. A best fit to the experiment yields for all the above systems a constant dipole moment over a wide temperature range. Even for the shallow Ag⁺ off-center systems no indication for an off-center to on-center transition was found.     

Measurement of the average polarization of B produced by polarized muons in the μ + C → νμ + B reaction

The average polarization of ¹²B produced by the capture of polarized muons in ¹²C has been measured using recoil implantation techniques. From the result we deduce the average polarization of the ¹²B ground state ¹²B(0) corresponding to the ¹²C → ¹²B(0) Gamow-Teller reaction: J_μ(0) = 0.43 ± 0.10. The sizeable deviation of this polarization from the value of 2/3, characteristic of a “bare” 0⁺ → 1⁺ process, is a fair evidence for induced axial-vector interaction(s) in muon capture. The ratio of the induced pseudoscalar and the axial-vector coupling constants is deduced to be: g^μ_P/g^μ_A = 12 ± 5.     

Scintillation properties of PbWO4 crystals doped with the rare-earth ions

In PbWO₄(PWO) crystals grown by Czochralski method the influence of atmosphere of the growth (O₂, air) and doping with the rare-earth ions of different types (A³⁺=Lu³⁺, Gd³⁺,Tb³⁺,Eu³⁺ as well as doubly doped A³⁺–Li⁺) on light yield and luminescence decay were analyzed. PWO scintillator with the ultra-fast (τ=0.5 ns) main component of luminescence decay (87% of total light yield) was obtained using the O₂-growth atmosphere and doping by Eu₂O₃ at a concentration of 5000 ppm. It is concluded that the decrease of decay constant of the main scintillation component is the result of the resonant energy transfer between the centers of “blue” PWO luminescence (λ_max=420 nm) and the 4f–4f-transitions of Eu³⁺ ions in this spectral region.     

Importance of the photo-beta process for the synthesis of “p” elements in stellar conditions

In this work, we compare positon capture and photo-beta disintegration probabilities in several stellar conditions.

We show that the second process can be neglected with regard to the first one in strongly endothermic nuclear transitions, whereas photo-beta disintegration can be competitive with positon capture and even can have a greater likelihood than the latter process in weakly endothermic and exothermic transitions.

In the range of temperature we consider here (T ≈ 10⁹ °K), it appears that the lifetime ratio τ(ph)/τ(e_c⁺) against photo-beta disintegration and positon capture is the smallest for densities in the neighbourhood of 10⁶ g/cm³.

Thus, we arrive at the conclusion that the photo-beta process can play a role in the synthesis of two “p” elements at least, ₆₂¹⁴⁴Sm and ₈₀¹⁹⁶Hg, for which the ratios τ(ph)/τ(e_c⁺) are close to 10 and 20, respectively, in the most favourable stellar conditions.     

Relationship between electronic and crystal structure in Cu–Ni–Co–Mn–O spinels: Part B: Binding energy anomaly in Cu photoemission spectrum

In very rare circumstances, X-ray photoemission spectra of copper in spinel oxides exhibit a “negative binding energy shift”. The origin of such an anomalous XPS chemical shift was investigated. A metastable Ni_0.48Co_0.24Cu_0.6+xMn_1.68−xO₄ (0 < x < 0.6) spinel was fabricated at 600 °C using a low-temperature solution technique. The binding energy of the 2p_3/2 level of copper (930.8 eV) is found 1.9 eV lower than that of Cu⁰ (932.7 eV). XPS and EXAFS studies revealed that the post-thermal annealing between 600 and 800 °C undergoes an irreversible cubic-to-tetragonal phase transformation through oxidation–reduction reaction Cu¹⁺ + Mn⁴⁺  Cu²⁺ + Mn³⁺, and only tetrahedral Cu¹⁺ species in the cubic spinel shows this anomalous chemical shift. The negative shift of the core levels was correlated to an equal shift of the Cu 3d valence band levels. XPS valence bands from the samples annealed at different temperatures were compared to DOS calculations. The DOS computations were performed with FEFF-8.1 code using experimental crystal parameters established by the EXAFS analysis. It was found that the tetrahedral Cu¹⁺ in the 600 °C annealed sample exhibits localization of the 3d orbitals showing behavior characteristic to zinc. The completely filled and isolated 3d electron shell appears as a false valence band edge in the XPS spectrum. The position of the Cu 3d, and other core levels, is established by oxygen pinning the Cu valence band levels and by the fixed value of the p–d gap characteristic to the tetrahedral copper environment in this spinel.     

Outer curvature and conformal geometry of an imbedding

The differential geometry of an imbedded (e.g. string or membrane world sheet) surface in a higher-dimensional background is shown to be conveniently describable (except in the null limit case) in terms of what are designated as its first, second, and third fundamental tensors, which will have the respective symmetry properties η_μν = η_(μν) as a trivial algebraic identity, K_μν^ρ = K_(μν)^ρ as the “generalised Weingarten identity”, which is the (Frobenius type) integrability condition for the imbedding, and Ξ_λμν^ρ = Ξ_(λμν)^ρ as a “generalised Codazzi equation”, which depends on the background geometry being flat or of constant curvature, needing replacement by a more complicated expression for a generic value of the background curvature B_κλ^μ_ν. The “generalised Gauss equation” expressing the dependence on this background curvature of the internal curvature tensor R_κλ^μ_ν of the imbedded surface is converted into terms of the first and second fundamental tensors, and it is thereby demonstrated that the vanishing of the (conformally invariant) “conformation tensor”, i.e. the trace free part C_μν^ρ of the second fundamental tensor K_μv^ρ, is a sufficient condition for conformal flatness of the imbedded surface (and thus in particular for the vanishing of its (Weyl type) conformal curvature tensor C_κλ^μ_ν) provided the background is itself conformally flat. In a trio of which the first two members are the generalised Gauss and Codazzi equations, the “third” member is shown to give an expression in terms of C_μν^ρ for the (trace free, conformally invariant) “outer curvature” tensor Ω_κλ^μ_ν whose vanishing is the condition for feasibility of the natural generalisation of the Walker frame transportation ansatz. The vanishing of C_μν^ρ is shown to be sufficient in a conformally flat background for the vanishing also of Ω_κλ^μ_ν.     

Ionic conduction and dielectric relaxation in Ag/Na ion-exchanged aluminosilicate glasses: mixed mobile ion effect and KWW relaxation

Ag⁺/Na⁺ ion-exchanged aluminosilicate glasses with uniform concentration profiles were prepared, and their electrical conductivities were investigated as functions of the ion-exchange ratio and the initial glass compositions. In the case of the ion-exchanged glasses of x20Ag₂O–(1−x)20Na₂O–10Al₂O₃–70SiO₂ in mol%, the conductivity, σ, and its activation energy, E_σ, showed a minimum and a maximum at the same ion-exchange ratio x=0.3, respectively, and the mixed mobile ion effect (MMIE) was observed. The fully ion-exchanged sample attained σ=3.5×10⁻⁵ S/cm at 200 °C, which was 1.5 orders of magnitude larger than that of initial glass. In the case of x25Ag₂O–(1−x)25Na₂O–25Al₂O₃–50SiO₂, the mixed mobile ion effect was also observed at x=0.5. The maximum conductivity of 2×10⁻⁴ S/cm at 200 °C was obtained in the fully ion-exchanged glass sample.

The electric relaxation analysis was also conducted on both systems, and Kohlrausch–Williams–Watts (KWW) fractional exponent β was obtained as a function of x. The decrease of β was observed near x≈0.3 in the former system, while that of the later system was independent of the ion-exchange ratio. Based on the structural analysis results, the observed behaviors were investigated from the point of view of the occupation of Ag⁺ ions on the non-bridging oxygen-site (NBO-site) and the charge compensation-site (CC-site) of AlO₄ tetrahedral unit.     

EPR evidence of local lattice mode in K3H(SO4)2 and Rb3H(SO4)2 fast-proton conductors

The observation of an anomalous temperature dependence of Mn²⁺ EPR spectra linewidth and nonaxial crystal-field parameter in K₃H(SO₄)₂ and Rb₃H(SO₄)₂ allows one to suggest the presence of “local mode” predicted by Yamada (Ferroelectrics 170 (1995) 23). The activation energy for this kind of excitation was found and equals 11.3 (0.5) and 7.4 (0.3) meV for Mn²⁺ doped K₃H(SO₄)₂ and Rb₃H(SO₄)₂, respectively.     

ZnS:Ag的ESR及结构相变研究

ESR实验表明Ag在ZnS微晶中是以Ag⁺离子的形式存在,没有测到Ag²⁺离子的信号.以Mn²⁺为探针的ESR和X射线衍射分析研究发现,在ZnS微晶由立方点阵(β相)变为六角点阵(α相)的结构相变中,Ag⁺离子对相变的影响与掺杂浓度和煅烧温度有关,当掺Ag量为5×10^-4g/g时相变在1100~1200℃出现逆转现象.由Mn²⁺的ESR谱得出α相的朗德因子g为2.006,越精细常数a为6.76mT,β相的g为2.002,a为6.81mT.     

改性羧甲基壳聚糖复合材料的制备、表征与研究

为了提高壳聚糖的水溶性及其止血方面的性能，将壳聚糖（CS）进行羧甲基化改性，并引入具有抗菌作用的Ag⁺和TiO₂，制备出羧甲基壳聚糖复合止血材料。首先，在壳聚糖中引入羧甲基，制得羧甲基壳聚糖（CMCS），之后向其中引入Ag⁺和TiO₂，分别制备出Ag⁺-CMCS、TiO₂-CMCS和Ag⁺-TiO₂-CMCS复合材料。然后，采用FTIR、XRD、SEM等手段对复合材料的结构进行表征。最后，对复合物的凝血、止血性能进行了测试。实验结果表明：改性后的羧甲基壳聚糖的IR图谱在3 423 cm^-1和1 380 cm^-1处出现了羧甲基壳聚糖钠盐的特征吸收峰。改性后的羧甲基壳聚糖在XRD图谱中表现出了金属晶态。CMCS的粒径为14.8 nm左右，Ag⁺粒径为143.5 nm左右，纳米TiO₂的粒径为267.2 nm左右，且三者分散的很均匀。Ag⁺-TiO₂-CMCS的凝血、止血效果要优于Ag⁺-CMCS和TiO₂-CMCS，同时Ag⁺-CMCS和TiO₂-CMCS的凝血、止血效果要优于CMCS。     

Band structures and proton-neutron interactions in Ta

Six rotational bands in the odd-odd nucleus ¹⁷⁴Ta have been populated with the ¹⁶⁰Gd(¹⁹F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π1/2⁻ [541] ν1/2⁻ [521] band and semi-decoupled π1/2⁻ [541] ν7/2⁻ [633] band are observed, in addition to the high-K couplings of the π9/2⁻ [514] ν7/2⁺ [633], π9/2⁻ [514] ν5/2⁻ [512], π7/2⁺ [404] ν7/2⁺ [633], and π5/2⁺ [402] ν5/2⁻ [512] configurations. The signature splitting of the π1/2⁻ [541] ν7/2⁺ [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i_13/2 neutrons in the π1/2⁻ [541] bands of odd-Z nuclei.     

A method for detection of spectral spin diffusion from minor peaks, and its application to F MAS NMR of antimony(III)-doped fluorapatite

A new technique for detecting spectral spin diffusion in solids under MAS NMR conditions that is particularly well suited for accurately measuring cross-relaxation from minor spectral components is presented. The pulse sequence, SINK (Saturation Inter-Nuclear Kinetics), selectively saturates the magnetization of a minor spectral component with a series of rotor-synchronized DANTE pulse trains and monitors spin diffusion to other peaks with a non-selective 90° pulse. We have used SINK with ¹⁹F MAS NMR on samples of calcium fluorapatite doped with Sb³⁺ to measure spin diffusion between a weak peak at 68.6 ppm due to fluoride ions associated with Sb³⁺ and other peaks in the spectrum. The SINK experiment clearly demonstrates that spin diffusion from the former peak to the main resonance of fluorapatite at 64.0 ppm is faster than spin diffusion to a second antimony-related peak at 65.6 ppm. These results strengthen our previous conclusion that antimony(III) occupies a phosphate site in the apatite lattice, with an SbO₃³⁻ group replacing a PO₄³⁻ group. The SINK experiment also enables the detection of a “hidden” peak at approximately 62.9 ppm that is otherwise obscured by the intense main peak at 64.0 ppm.     

Long-time correlation tails in crystals

Nonequilibrium thermodynamics and the Onsager hypothesis are utilized to demonstrate that long-time correlation “tails”, decaying as t^−3/2, should be expected in the velocity autocorrelation function of crystals. In the simplest situation, this behavior is due to the non-propagating (Rayleigh) mode. In the general case, it is argued that “diffusive” modes decaying in wavevector space as exp (−tK²), where is a constant, should lead to the same behavior.     

Double quantization of CP type orbits by generalized Verma modules

It is known that symmetric orbits in g^* for any simple Lie algebra g are equipped with a Poisson pencil generated by the Kirillov-Kostant-Souriau bracket and the reduced Sklyanin bracket associated to the “canonical” R-matrix. We realize quantization of the Poisson pencil CPⁿ type orbits (i.e. orbits in sl(n + 1)^* whose real compact form is CPⁿ) by means of q-deformed Verma modules.     

Non-radiative transitions as Förster''s energy transfer to solvent vibrations

New experimental evidence is given to the approach suggested by the authors to non-radiative transitions in rare earth and transition metal ions as radiationless energy transfer from ion to ligand and solvent molecule vibrations. Using non-radiative transitions in Yb³⁺ as an example the participation of combinations of vibrations in the electronic energy dissipation was investigated. Dy³⁺ in solutions was used to show the role of anharmonicity in non-radiative transitions. It was proved that the semi-empirical calculations of non-radiative transition rates by Förster's formula gave also good results for Mn²⁺ ions luminescence which is an example of “strong” vibronic interaction. The suggested approach permits to explain the anomalous low probability of the ⁵D₁−⁵D₀ non-radiative transition in the Eu³⁺ ion.     

Escape of charged particles at stopping annihilation of antiprotons in the heavy nuclei of nuclear photoemulsion

“Rare” annihilation channels for antiprotons stopping on heavy (Ag, Br) nuclei of photoemulsion, have been sought; 4872 stops of antiprotons on photoemulsion nuclei are analysed. Events of formation and decay of the hyperfragment _Λ⁴H, escape of ⁸He and ⁸Li nuclei, one-prong stars with the mean range 79.5±5.1 μm of secondary slow “b” particles are found among the annihilation stars at capture on nuclei (Ag, Br). The lower limits for the production probability of _Λ⁴H and ⁸He, ⁸Li nuclei per antiproton stopping in the nuclei (Ag, Br) are

W_Λ⁴H2×10⁻⁴ and W_⁸He,⁸Li=(1.3±0.6)×10⁻³.

The branching ratio for the production of one-prong stars with the secondary “b” particles is at least (1.3±0.6)×10⁻³. Possible mechanisms for a production of these events in annihilation processes are considered.     

咔唑-硫脲席夫碱荧光探针对Hg~(2+)和Ag~+的可视化识别

以3-溴代-N-丁基咔唑和4-苯基-3-氨基硫脲为原料,合成了一种新型含咔唑基的硫脲席夫碱L,使用紫外-可见光谱、荧光光谱、核磁氢谱以及质谱等对其成分结构、离子识别性能、识别模式进行了研究。结果表明:L对Hg2+和Ag+具有快速响应的可视化选择性识别。当Hg2+或Ag+加入后,L溶液的颜色由无色立刻变为黄色;同时,L溶液中加入Hg2+和Ag+后,荧光光谱有不同变化,且其他离子的存在并不干扰L对Hg2+和Ag+的选择性识别。研究还发现,L对Hg2+和Ag+的荧光识别模式不同:加入Ag+后,L与Ag+生成了配合物,发生荧光猝灭;加入Hg2+后,L先与Hg2+络合发生荧光猝灭,然后脱去Hg S。