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1.
1H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of TiOx/SiO2 catalysts suggest the interaction of surface TiOx. species with Si-OH groups of the silica. Simultaneously, Ti-OH groups from surface titania species appear. The distribution of TiOx species over SiO2 is non-uniform, since a considerable part of surface OH groups remains unreacted with supported titania. Supported vanadia species interact both with Si-OH and Ti-OH groups. 51V NMR spectra suggest the interaction of vanadia with supported titania species and show the non-uniform distribution of titania over the SiO2 surface. Deposition of titania as well as vanadia produces strong electron-accepting (Lewis) sites which interact with the terminal N atom of adsorbed N2O molecules, resulting in a downfield shift of the resonance in 15N NMR spectra. The acid strength of electron-accepting sites is similar in both cases. Only about 10% of the total amount of supported titania and vanadia create Lewis sites.  相似文献   

2.
Ag-impurity effects on the first- and second-order quadrupole interaction (QI) at 23Na site in an isomorphic mixed system, Na1−xAgxNO2 (x=0, 0.0084, 0.026, 0.079, 0.094, 0.16), have been investigated by employing 23Na (I=3/2) magic angle spinning nuclear magnetic resonance (MAS NMR) technique. The central transition (CT) and satellite transition (ST) are simultaneously observed with this system. From the spectral analysis, the quadrupole parameter and its distribution width are obtained as a function of Ag concentration. From the intensity loss of CT MAS centerband and of the envelope function of ST MAS sidebands due to impurities, the range of their influence on the second- and first-order QI is estimated. The estimated ranges contain the second and first neighbouring Na sites from the resonating 23Na nucleus for the first- and second-order QI, respectively.  相似文献   

3.
A report is presented on the observation of Hahn echoes from the following quadrupolar nuclei of half integer spin (I) in polycrystalline solids in the large static magnetic field gradient (37.5 T/m) which exists in the fringe field of a superconducting solenoid: 7Li, 23Na, 11B, 65Cu (I = 3/2); 27Al (I = 5/2); 51V, 59Co (I = 7/2); and 115In (I = 9/2). 23Na echo-trains from NaCl (with non-selective excitation) and from Na2SO4 (with selective excitation) are compared quantitatively for two different RF pulse sequences: 90x-(τ-90y-τ-echo-)n and 90x-(τ-90x-τ-echo-)n. The signals obtained from RF pulses corresponding to non-selective 90 ° pulses were shown to be quantitative, whereas in the selective case smaller signals were obtained since only the central transition contributed. The loss of signal from this cause can be distinguished from small signals resulting from low density of nuclei by use of the second sequence. A 7Li image obtained from LiF in a cylindrical glass-vial is shown.  相似文献   

4.
A. Mrz  S. Mrz 《Surface science》1994,320(3):307-314
Polar profiles of directional elastic peak electron spectroscopy (DEPES) and directional Auger electron spectroscopy (DAES) were measured for the Cu(011) face in the vicinity of particular close-packed directions in the sample and for sample temperatures of 400<T<1200 K. For all directions investigated, the ln C versus T curve, where C is the contrast: C = 2(ImaxImax)/(Imax + Imax) for a particular maximum in the polar profile, is composed of three linear parts. The slope of this particular part is different for the different maxima investigated but the breaks between the linear parts occur always at the temperatures T1 ≈ 700 K and T2 ≈ 1000 K. The former break is ascribed to the appearance of anharmonic thermal vibrations while the latter one indicates the surface roughening transition.

DEPES was applied for the first time in the investigation of the surface roughening transition and it was found to be a very convenient and effective method. The surface roughening temperature TR ≈ 1000 K was found for the Cu(011) face, in an excellent agreement with theoretical predictions.  相似文献   


5.
6.
The quenching of spin fluctuations by magnetic fields has been observed in heat capacity and electrical resistivity measurements at low temperatures for a series of highly exchange enhanced magnetic materials. These include: the weak itinerant electron ferromagnets Sc3In, Zr1−xHfxZn2 (0 x 0.2) and Ni3Al; the strong Pauli paramagnets RCo2 (R = Sc, Y and Lu), TiBe2 and Pd1−xNix (0 x 0.01); and the heavy fermion systems CeSn3, CeSix (x ≈ 1.85) an d UAl2. The reported quenching of spin fluctuations in scandium and palladium by magnetic fields is reviewed, and it appears that the initial observations and conclusions are incorrect, and that fields greater than 10 and 40 T, respectively, will be necessary to quench spin fluctuations in these metals. The behaviors of these spin fluctuators have been grouped into six classes.  相似文献   

7.
We have studied the microscopic properties of the hexagonal ZrNiAl, a model compound for a wide family of intermetallic compounds crystallizing in this type of structure, by using 27Al NMR spectroscopy. We have investigated the lineshape of static and MAS NMR spectra as a function of magnetic field strength (4.7–9.4 T) and temperature (5–300 K). Our data indicate that the 27Al NMR spectra result from a combined effect of quadrupole and anisotropic shift interactions. The 27Al nuclei are in an environment characterized by the quadrupole coupling constant e2qQ/h of 3.3 MHz, asymmetry parameter ηQ of 0.42, isotropic shift δiso of 393 ppm, shift anisotropy δanis = δzz − (δxx + δyy)/2 of 150 ppm, and asymmetry factor ηS of 0.5. They are found to be temperature independent. The spin–lattice relaxation rate measured at 7.05 T is proportional to the temperature with T1T = 135 s K. The mechanisms responsible for observed values of δiso, δanis, T1T, and the enhanced Korringa constant are discussed.  相似文献   

8.
A two-step thermochemical cycle with the ternary metal oxide system (Fe1 − xMnx)3O4/(Fe1 − xMnx)1 − yO is applied to convert solar energy to chemical energy. Experimental investigations on the water splitting reaction of (Fe1 − xMnx)1 − yO revealed temporary formation of a manganese rich rock salt phase and an iron rich spinel phase due to phase segregation processes.  相似文献   

9.
A rigorous calculation of the quantum-mechanical rotational partition function for tetrahedral XY4 molecules yields Qr = ( )(2IY + 1)4π - exp(/4), where IY is the spin of the Y nucleus, and ≡ Bhc/kT. This result is accurate to 1 per cent or better for all values of B and T such that < .  相似文献   

10.
Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBus)2EAA or Al(OBus)3/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the 27Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with 27Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the 27A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (Cqcc = e2qQ/h). Hydration of the sample results in a decrease of the average Cqcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average Cqcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.  相似文献   

11.
A review is given of the occurrence and composition of ternary rare earth compounds RT12−xMx based on the tetragonal ThMn12 structure. Most of the compounds form with T = Fe but there are also examples withT = Co, Ni or Mn. Particular attention is paid to the site preference of the nonmagnetic component M, (Al, Si, Ti, V, Cr, Mo, W, Re). A discussion is given of the magnetic properties of the RT12−xMx compounds and the corresponding nitrides obtained after changing with nitrogen gas. The important role played by the intersublattice coupling on the magnetocrystalline anisotropy of these materials is stressed. Advantages and disadvantages of the RT12−xMx compounds as permanent magnet materials are discussed.  相似文献   

12.
The ordering temperatures of alloys in the quasi-binary section Fe3(NixPt1−x) have been determined experimentally, and are found to be consistent with theoretical predictions. Lattice constants and expansion coefficients vary smothly between Fe0.75Ni0.25 and Fe0.75Pt0.25 and are basically in agreement with the predictions of the Weiss 2γ state theory except in a region very close to Fe0.75Pt0.25. The effect of heat treatments below the ordering temperature is placed in an overall context of competing phase transformations, which can also be used to rationalise conflicting views on the existence of Fe3Ni, and the role of ordering on the Invar effect in general.  相似文献   

13.
Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr2−xCexCuO4 and Sr0.9La0.1CuO2 that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a dx2y2 symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y1−xCaxBa2Cu3O7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.  相似文献   

14.
31P and 207Pb MAS and static 207Pb NMR spectra of Pb3(PO4)2 and (Pb1−xBax)3(PO4)2 (x=0.08, 0.12) are analysed. The resonances stemming from different cation sites are correlated with the corresponding local symmetry and their oxygen neighbours. The coordination sphere of Pb(1) consists of 12 oxygen atoms and shows characteristics of a near-axial arrangement with a comparatively low anisotropy. The tenfold coordinated Pb(2) atoms are located in a more anisotropically-coordinated site. In Pb-diluted lead phosphate crystals the Pb(2) positions appear to be preferentially substituted by barium. There are indications that the cation distributions in the diluted samples are inhomogeneous. Furthermore, 31P MAS NMR experiments indicate a single phosphorus position.  相似文献   

15.
Variable-temperature solid-state MAS NMR studies on some yttrium-dihydride phases YH2+x are reported and yield evidence that 89Y CP MAS NMR techniques are an experimentally feasible route to investigate order–disorder phenomena in such metal-hydride phases.  相似文献   

16.
Zhengyang Wan 《中国物理 B》2021,30(11):117304-117304
The successfully experimental fabrication of two-dimensional Te monolayer films [Phys. Rev. Lett. 119 106101 (2017)] has promoted the researches on the group-VI monolayer materials. In this work, the electronic structures and topological properties of a group-VI binary compound of TeSe2 monolayers are studied based on the density functional theory and Wannier function method. Three types of structures, namely, α-TeSe2, β-TeSe2, and γ-TeSe2, are proposed for the TeSe2 monolayer among which the α-TeSe2 is found being the most stable. All the three structures are semiconductors with indirect band gaps. Very interestingly, the γ-TeSe2 monolayer becomes a quantum spin Hall (QSH) insulator with a global nontrivial energy gap of 0.14 eV when a 3.5% compressive strain is applied. The opening of the global band gap is understood by the competition between the decrease of the local band dispersion and the weakening of the interactions between the Se px, py orbitals and Te px, py orbitals during the process. Our work realizes topological states in the group-VI monolayers and promotes the potential applications of the materials in spintronics and quantum computations.  相似文献   

17.
Experiences obtained from recent improvements in the performance of solid-state (14)N MAS NMR spectroscopy have been used in a natural abundance (33)S MAS NMR investigation of the satellite transitions for this interesting spin I=3/2 isotope. This study reports the first observation of manifolds of spinning sidebands for these transitions in (33)S MAS NMR as observed for the two alums XAl(SO(4))(2) x 12H(2)O with X=NH(4) and K. For the NH(4)-alum a variable temperature (33)S MAS NMR study, employing the satellite transitions, shows that the (33)S quadrupole coupling constant (C(Q)) exhibits a linear temperature dependence (in the range -35 degrees C to 70 degrees C) with a temperature gradient of 3.1 kHz/ degrees C and undergoes a sign change with zero-crossing for C(Q) at 4 degrees C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of C(Q) with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/ degrees C. Finally, for optimization purposes, a study on the effect of the applied pulse widths at constant rf field strength on the intensity and variation in second-order quadrupolar lineshape for the central (1/2<-->-1/2) transition of the K-alum has been performed.  相似文献   

18.
A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi2V1 − xCoxO5.5 − 3x/2 (x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi2O2]n2n+ and [V1 − xCoxO3.5 − 3x/2]n2n with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral.  相似文献   

19.
利用29Si MAS NMR及27Al MAS NMR技术研究了丝光沸石催化剂制备过程中的结构变化,并利用Al的四极作用大小来区分重叠在一起的不同的Al物种.  相似文献   

20.
While liquid-state 29Si NMR of phosphorus-bearing organosilicon compounds with more than one phosphorus per molecule can take advantage of the presence of J-coupling nJ(31P29Si) for purposes of structural assignment from J-coupling patterns, conventional 29Si CP/MAS spectra of such molecular solids do not reveal structural details in a straightforward manner. For such compounds it is necessary to obtain 29Si CP/MAS spectra under conditions of simultaneous 1H- and 31P-high power decoupling in order to derive reliable 29Si chemical shift information. 29Si CP/MAS NMR spectra, obtained with and without 31P high power decoupling during the acquisition time, of several organosilicon compounds containing SixPy (x = 1 −10, y = 1 −10) moieties are reported.  相似文献   

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