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1.
《Vibrational Spectroscopy》2007,43(1):140-151
The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1–16 wt.% V has been systematically investigated by infrared, Raman, UV–vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V–OH groups, characterized by a band at 3660 cm−1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H2O. Further hydration resulted in the appearance of a V–O–V vibration band located around 700 cm−1 pointing to the formation of di- or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a V⋯V contribution was detected for the hydrated species. The IR, Raman and UV–vis data suggested a pyramidal anchoring of the dehydrated VOx species, whereas, the EXAFS data pointed to the presence of single V–O–Si bonded VOx species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. Combining the results of this study with data from our previous studies [D.E. Keller, F.M.F. de Groot, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 10223; D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313] as well as literature led to a reaction scheme in which a monomeric VOx species anchored by three Si–O–V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VOx species anchored by one Si–O–V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V–O–H and Si–O–H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure (i.e., Si–O–VO(OH)2) could be present under catalytic conditions where H2O is a reaction product (e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si–O)3–VO) and the umbrella (Si–O–VO(OH)2) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions.  相似文献   

2.
The CuII hydration shell structure has been studied by means of classical molecular dynamics (MD) simulations including three-body corrections and hybrid quantum-mechanical/molecular-mechanical (QM/MM) molecular dynamics (MD) simulations at the Hartree–Fock level. The copper(II ) ion is found to be six-fold coordinated and [Cu(H2O)6]2+ exhibits a distorted octahedral structure. The QM/MM MD approach reproduces correctly the experimentally observed Jahn–Teller effect but exhibits faster inversions (<200 fs) and a more complex behaviour than expected from experimental data. The dynamic Jahn–Teller effect causes the high lability of [Cu(H2O)6]2+ with a ligand-exchange rate constant some orders of magnitude higher than its neighbouring ions NiII and ZnII. Nevertheless, no first-shell water exchange occurred during a 30-ps simulation. The structure of the hydrated ion is discussed in terms of radial distribution functions, coordination numbers, and various angular distributions and the dynamical properties as librational and vibrational motions and reorientational times were evaluated, which lead to detailed information about the first hydration shell. Second-shell water-exchange processes could be observed within the simulation time scale and yielded a mean ligand residence time of ≈20 ps.  相似文献   

3.
With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH2)2CO?H2O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm3 (44% NaCl) and 1.800÷1.849 g/cm3 (67% Na2SO4). In a saturated urea solution, NaNO3, NaCl, and Na2SO4 complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO3, NaCl, and Na2SO4. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.  相似文献   

4.
Structures and electronic excitation energies of the benzophenone–water (Bp–H2O) and benzophenone–methanol (Bp–CH3OH) complexes have been investigated by means of density functional theory calculations. The CAM-B3LYP/6-311++G(d,p) and higher level calculations were carried out for the system. The calculations indicate that free Bp has a nonplanar structure with twist angle of 54.2° for two phenyl rings (referred to as ?). In the case of the Bp–H2O system, the twist angle of the phenyl rings and structure of the Bp skeleton were hardly changed by hydration (? = 55.1° for Bp–H2O). However, the excitation energies of Bp were drastically changed by this solvation. The time-dependent density functional calculations show that the n–π* transition (S1 state) is blue-shifted by the solvation, whereas two π–π* transitions (S2 and S3) were red-shifted. The origin of the specific spectral shifts is discussed on the basis of the theoretical results.  相似文献   

5.
X-ray diffraction data from a H2SO4 solution were examined. Peaks in the range 3–5 A in the correlation function reveal the presence of sulphate-H2O interactions. Each sulphate ion interacts with eight water molecules. A model of hydration similar to that present in the crystal structure of H2SO4 - 4H2O is shown to be consistent with these results.  相似文献   

6.
《Fluid Phase Equilibria》2005,233(1):34-46
A series of bicanonical ensemble Monte Carlo (BC MC) simulations has been performed to calculate Na+ hydration Gibbs energy in aqueous solution. The hydration Gibbs energy of Na+ ion in aqueous solution is the difference between formation free energies of Na+ (H2O)n and (H2O)n clusters at n  α. The convergence of the hydration free energy to bulk water value is fast, and the results at n = 60 turned out to be in good agreement with experimental ones and those calculated using free energy perturbation method [1]. The ion–water interaction has been described by Aqvist's pair potential [1] and SPC model [2] has been used for water–water interactions. The behaviour of the absolute Gibbs energy, the entropy, the internal energy of the clusters and the development of hydration shells’ structure with the increase of the number of water molecules are discussed.  相似文献   

7.
The solubility of YPO4 · 2H2O, LaPO4 · 1.5H2O, CePO4 · 1.5H2O, and NdPO4 · H2O 10–60 wt % in sulfuric-phosphoric acid solutions containing 10–60 wt % H2SO4 and 13.8–41.4 H3PO4 was studied.  相似文献   

8.
《Chemical physics letters》2003,367(3-4):351-360
Influence of hydration on the Watson–Crick guanine–cytosine hydrogen bonded (h-bonded) base pair (GC) and stacked pair (G/C) was investigated in their first hydration shell. An electrostatic based approach has been used to identify the potential binding sites for water molecules around GC and G/C pairs. Several geometries of the complexes, GC…(H2O)n and G/C…(H2O)n (n=1–6) were investigated using HF/6-31G** and HF/6-31G++** methods. Further minimization calculations were performed at both B3P86/6-31G** and MP2/6-31G** levels to assess the role of electron correlation contribution in the hydration process. It can be concluded from the present findings that the stacked base-pair hydrate better than the corresponding h-bonded base pair, and DNA base pairs can accommodate up to 4–5 water molecules whereas stacked pair do accommodate 5–6 water molecules.  相似文献   

9.
Solubility was studied in the system NaCl–AlCl3–HCl–H2O at 25°C in the section 28 wt % HCl. The system is of the eutonic type and has an extensive sodium chloride crystallization region. The composition of the eutonic solution is the following, wt %: NaCl, 0.47; AlCl3 ? 6H2O, 8.88; HCl, 25.38; and H2O, 65.27. The lines of saturated solutions were approximated by polynomial equations.  相似文献   

10.
Dielectric properties of ethanol and 1-hexanol solutions containing LiCl, CaCl2·2H2O and Ca(NO3)2·4H2O, respectively, have been determined. It is found that LiCl reduces the static permittivity in ethanol, but CaCl2·2H2O and Ca(NO3)2·4H2O both give an initial increase in s. All the electrolytes studied increase the mean relaxation time of the ethanol solutions. In 1-hexanol the static permittivity is rather invariant for all studied electrolytes at low concentrations, while the same lengthening of the mean relaxation time is observed. When water is added in addition to the hydration water of the electrotyte, the static permittivity in hexanol is almost unaltered while the relaxation time is drastically shortened. The experimental result is discussed in terms of a formation of ion pairs, solvation sheaths, and kinetic depolarization, a partial release of hydration water and a structuring influence on the alcohol structure by the hydrated cation.  相似文献   

11.
Metal–organic frameworks (MOFs) with long-term stability and reversible high water uptake properties can be ideal candidates for water harvesting and indoor humidity control. Now, a mesoporous and highly stable MOF, BIT-66 is presented that has indoor humidity control capability and a photocatalytic bacteriostatic effect. BIT-66 (V3(O)3(H2O)(BTB)2), possesses prominent moisture tunability in the range of 45–60 % RH and a water uptake and working capacity of 71 and 55 wt %, respectively, showing good recyclability and excellent performance in water adsorption–desorption cycles. Importantly, this MOF demonstrates a unique photocatalytic bacteriostatic behavior under visible light, which can effectively ameliorate the bacteria and/or mold breeding problem in water adsorbing materials.  相似文献   

12.
Metal–organic frameworks (MOFs) with long‐term stability and reversible high water uptake properties can be ideal candidates for water harvesting and indoor humidity control. Now, a mesoporous and highly stable MOF, BIT‐66 is presented that has indoor humidity control capability and a photocatalytic bacteriostatic effect. BIT‐66 (V3(O)3(H2O)(BTB)2), possesses prominent moisture tunability in the range of 45–60 % RH and a water uptake and working capacity of 71 and 55 wt %, respectively, showing good recyclability and excellent performance in water adsorption–desorption cycles. Importantly, this MOF demonstrates a unique photocatalytic bacteriostatic behavior under visible light, which can effectively ameliorate the bacteria and/or mold breeding problem in water adsorbing materials.  相似文献   

13.
14.
A periodical interaction model of LDHs-Cl-yH2O has been proposed. The geometry optimization and energy of the layered double hydroxides (LDHs) were calculated using CASTEP/LDA method at the CA-PZ level. The distribution of H2O in the interlayer and the super-molecular interaction between host layer and guest anion have been investigated by analyzing the geometric parameters, charge population, energy, and density of state (DOS). The results showed that there was a strong super-molecular interaction between the host layer and the guest anion Cl. In the system of LDHs-Cl-yH2O, the interlayer distance increased gradually then tended to invariableness. And in the process of hydration of LDHs-Cl, hydrogen bonding was superior to electrostatic interaction, and layer-water type hydrogen bonding was a little stronger than anion-water type hydrogen bonding between H2O and the rest of the structure. When y was 1 or 2, Cl and the plane of water were parallel to the layer; while y was 3 or 4, distribution of Cl and water was random. Moreover, the LDHs-Cl-yH2O would change from ionic crystal to molecular crystal with the increase of number of water molecule. The hydration of LDHs-Cl would achieve a definite saturation state.  相似文献   

15.
Micro hydration structures of the sodium ion, [Na(H2O) n ]+, n = 1–12, were probed by density functional theory (DFT) at B3LYP/aug-cc-pVDZ level in both gaseous and aqueous phase. The predicted equilibrium sodium–oxygen distance of 0.240 nm at the present level of theory. The four-, five- and six-coordinated cluster can transform from each other at the ambient condition. The analysis of the successive water binding energy and natural charge population (NBO) on Na+ clearly shows that the influence of Na+ on the surrounding water molecules goes beyond the first hydration shell with the hydration number of 6. The Car-Parrinello molecular dynamic simulation shows that only the first hydration sphere can be found, and the hydration number of Na+ is 5.2 and the hydration distance (rNa–O) is 0.235 nm. All our simulations mentioned in the present paper show an excellent agreement with the diffraction result from X-ray scattering study.  相似文献   

16.
We present a new metal–organic framework (MOF) built from lanthanum and pyrazine‐2,5‐dicarboxylate (pyzdc) ions. This MOF, [La(pyzdc)1.5(H2O)2] ? 2 H2O, is microporous, with 1D channels that easily accommodate water molecules. Its framework is highly robust to dehydration/hydration cycles. Unusually for a MOF, it also features a high hydrothermal stability. This makes it an ideal candidate for air drying as well as for separating water/alcohol mixtures. The ability of the activated MOF to adsorb water selectively was evaluated by means of thermogravimetric analysis, powder and single‐crystal X‐ray diffraction and adsorption studies, indicating a maximum uptake of 1.2 mmol g?1 MOF. These results are in agreement with the microporous structure, which permits only water molecules to enter the channels (alcohols, including methanol, are simply too large). Transient breakthrough simulations using water/methanol mixtures confirm that such mixtures can be separated cleanly using this new MOF.  相似文献   

17.
The new anosovite‐type polymorph of the title compound is synthesized by reaction of either V2F6·4H2O or a mixture of 60 wt.% VF2·4H2O and 40 wt.% VF3·3H2O with a flowing water‐saturated gaseous mixture of 15—20 vol% H2 in argon (588 K, 14—18 h).  相似文献   

18.
Calculations are presented for the structure and the isomerization reaction of various conformers of the bare serine, neutral serine–(H2O)n and serine zwitterion–(H2O)n (n = 1, 2) clusters. The effects of binding water molecules on the relative stability and the isomerization processes are examined. Hydrogen bonding between serine and the water molecule(s) may significantly affect the relative stability of conformers of the neutral serine–(H2O)n (n = 1, 2) clusters. The sidechain (OH group) in serine is found to have a profound effect on the structure and isomerization of serine–(H2O)n (n = 1, 2) clusters. Conformers with the hydrogen bonding between water and the hydroxyl group of serine are predicted. A detailed analysis is presented of the isomerization (proton transfer) pathways between the neutral serine–(H2O)2 and serine zwitterion–(H2O)2 clusters by carrying out the intrinsic reaction coordinate analysis. At least two water molecules need to bind to produce the stable serine zwitterion–water cluster in the gas phase. The isomerization for the serine–(H2O)2 cluster proceeds by the concerted double and triple proton transfer mechanism occurring via the binding water molecules, or via the hydroxyl group. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005  相似文献   

19.
A global optimization called fast hybrid global optimization algorithm was proposed based on genetic algorithm, fast simulated algorithm and conjugated gradient algorithm. We employ it to search the global minimum energy structures of Ba2+(H2O)n clusters for n = 1–30 within the TIP4P model. The results show that Ba2+(H2O)n clusters have the n+0 structure while n = 1–8. When n is in the range 9 ≤ n ≤ 18, the number of water molecules in the first shell around the barium ion is 8 and the other water molecules arrange in the outer shell. In the global minimum structure of Ba2+(H2O)19, the number of the first shell water molecules adds up to 9, and the value is kept until n = 30. According to the computational results, a conclusion that hydration numbers for Ba2+ is 9 can be drawn, which is in agreement with the result by a Monte Carlo simulation.  相似文献   

20.
The solubility in the quaternary water-salt systems LaCl3-NdCl3-HCl-H2O (1) and LaCl3-PrCl3-HCl-H2O (2) at 25°C was studied in the section of 40 wt % hydrochloric acid, a system with a eutonic discontinuity. The composition at the point of discontinuity for the eutonic solution is the following. In system 1: 4.67 wt % LaCl3 · 7H2O, 0.37 wt % PrCl3 · 7H2O, 37.98 wt % HCl, and 56.98 wt % H2O; in system 2: 4.37 wt % LaCl3 · 7H2O, 0.93 wt % NdCl3 · 6H2O, 37.88 wt % HCl, and 56.82 wt % H2O.  相似文献   

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