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1.
The behavior of N 2 and Cl 2 molecules in H 2O clusters was studied by the molecular dynamics method. Structural, thermodynamic, kinetic, and electrical properties of water aggregates containing N 2 and Cl 2 molecules were examined. The energy of the admixture-water interaction is negative and decreases as the cluster size increases. The electrostatic potential and the field strength undergo strong changes in the vicinity of aggregate border. The effect of hydration on the rate of some atmospheric reactions was considered. 相似文献
2.
Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al 2O 3 using in situ/operando spectroscopy and ex situ aberration-corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment. 相似文献
3.
Using DFT method, the stable adsorption configurations of Cu 4 cluster on Cr 2O 3 (0001) surface were investigated. The regular tetrahedron structure and the planar structures were considered as the initial adsorption configuration of Cu 4 cluster, respectively. The adsorption energies of the two structures were also calculated. The simulation result indicated that the adsorption energy of the regular tetrahedron structure was higher than that of the planar structure, and thus the regular tetrahedron structure was confirmed to be the stable adsorption configuration for Cu 4 cluster on Cr 2O 3 (0001) surface. Moreover, it was observed that the Cu 4 cluster showed the definite stable adsorption sites on Cr 2O 3 (0001) surface, namely 3‐fold O sites. During the adsorption process of Cu 4 cluster onto Cr 2O 3 (0001) surface, the Cu 4 cluster could bond with more Cr or O atoms on the surface, and the apparent charge transfer also occurred correspondingly. Meanwhile, the Cu 4 cluster and Cr 2O 3 (0001) surface would bond in the form of local polarization to enhance the stability of adsorption configuration. 相似文献
4.
Cluster properties of copper have been investigated using the Molecular-Dynamics md technique. The structural stability and energetics of spherical Cu n ( n = 13–135) clusters have been investigated at temperatures T = 1 K and T = 300 K. It has been found that the average interaction energy per atom in the cluster decreases and reaches an asymptotic value as cluster size increases. The melting behaviour of clusters n = 13 and n = 55 have been investigated. It has been found that the melting temperature decreases as cluster size increases, and for clusters with multishell structures melting starts from the outermost shell. In the simulation an emprical potential energy function (PEF) proposed by Erkoç has been used, which contains two-body atomic interactions. 相似文献
5.
To clear the adsorption site of CO on the surface of the nanosized SnO2 film, the structures, stability, and electronic characteristics of the CO-adsorbed (SnO2)2 clusters have been investigated by using PW91 functional. The more stable configurations of these CO(SnO2)2 clusters derive from CO are adsorbed on the lower energy excited-state (SnO2)2 clusters rather than on the ground-state (SnO2)2 clusters. The calculated adsorption energies for CO on the lower energy excite-state (SnO2)2 clusters are up to 1.363–1.454 eV which is transferred from physical adsorption to chemical adsorptions. CO adsorption increases the kinetic stability and decreases the conductivity of the lower energy excited-states (SnO2)2 clusters. 相似文献
6.
Density functional theory calculations are performed to analyze the structure and stability of Cu and Cu-K clusters with 3 to 9 atoms. The results indicate that the stability of the clusters decreases after doping with a K atom. With the increase of cluster size, the stability of the clusters shows odd-even alternation. Cu 8 and Cu 7K clusters exhibit the highest stability. Next, different adsorption sites are considered to investigate the geometry of Cu nNO and Cu n−1KNO clusters. By calculating the adsorption energy and the HOMO-LUMO energy gap, it is determined that both types of reactions are exothermic processes, indicating stable adsorption of NO. Notably, the Cu nK clusters are more active (stronger adsorption) for NO than the Cu n clusters. The most chemically active clusters among Cu nNO and Cu n−1KNO clusters are Cu 8NO and Cu 7KNO clusters. Finally, electron transfer and Mayer bond order analysis of Cu 8NO and Cu 7KNO clusters reveal that the N O bond order decreases due to electron transfer when Cu/Cu-K clusters adsorb NO. In this process, the N atom is the electron donor and the Cu atom is the electron acceptor. Fundamental insights obtained in this study can be useful in the design of Cu/Cu-K catalysts. 相似文献
7.
采用密度泛函理论(DFT)中广义梯度近似(GGA)方法, 对Pt原子与γ-Al 2O 3(001)面的相互作用及迁移性能进行了研究. 分析了各种可能吸附位及吸附构型的松弛和变形现象, 吸附能和迁移能垒的计算结果表明: Pt团簇能够稳定吸附在该表面. Pt原子在表面O位的吸附能明显较高, 这主要是由Pt向基底O原子转移了电子所致. 电荷布居分析表明, Pt原子显电正性, Pt和Al原子之间存在排斥作用, 导致与Al原子产生较弱相互作用. 计算的平均吸附能大小依赖于Pt团簇的大小和形状, 总体趋势是随着Pt原子数增多, 吸附能降低. Pt原子在γ-Al 2O 3(001)表面迁移过程所需克服的迁移能垒最高值为0.51 eV. 随着吸附的Pt原子数增多,更倾向于形成Pt团簇. 因此, Pt原子在γ-Al 2O 3(001)表面的吸附演变不可能形成光滑、均匀平铺的吸附构型, 而在一定条件下容易出现团聚. 相似文献
8.
We investigated the dependence of the electrostatic potential on the size and the shape of various cluster models of the MgO(100) surface. Both Mg 2+ and O 2− adsorption sites have been considered. The clusters were embedded in a large array of point charges to provide a representation of the Madelung potential. We found that the electrostatic potential in the adsorption region shows a marked dependence on the size of the cluster, in particular, for non-stoichiometric clusters where the number of cations and anions differs considerably. These oscillations are due to (a) the different contribution to the electrostatic potential given by a point charge or by an extended ion, and (b) by the polarization of the ions at the cluster border. The effect of the oscillations in the electrostatic potential on the chemisorption properties was investigated for the case of CO 2 interacting with surface and defect O 2− sites of the MgO surface. © 1996 John Wiley & Sons, Inc. 相似文献
9.
The interactions between the neutral and charged (?2, ?1, +1, and +2) Ti n ( n = 1–7) clusters and one O 2 molecule were investigated by density functional theory. The calculated results show that the oxygen molecule is dissociative on the neutral Ti n clusters. Geometrically, the two O atoms are distributed at the two sides across the neutral Ti n cluster for n = 1–4 and the oxygen atom favors the three‐fold hollow site for n = 5, 6, and 7. The binding energy per atom ( Eb) and energy gap ( Egap) show higher stability and lower chemical activity of the neutral Ti nO 2 ( n = 1–7) systems compared with the corresponding Ti n clusters. The adsorption energies ( Ead) exhibit a continuously ascending tendency except for n = 4. The results of the addition of different charges (?2, ?1, +1, and +2) on the most stable neutral Ti nO 2 ( n = 1–7) systems indicate that their geometries are usually perturbed. The stabilities of the neutral Ti nO 2 systems are enhanced by adding one negative charge. The strongest interaction of the charged Ti n clusters (?2, ?1, +1, and +2) with O 2 molecule is found at charge +2. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
10.
We present the results of a density functional calculation on adsorption of O 2, CO, and their coadsorption at various sites of neutral, cationic, and anionic Pd 4 clusters. For all the clusters, the dissociative adsorption of oxygen sitting on Pd bridge sites is found to be preferable. Both O 2 and CO binding energies are found to be higher for the anionic Pd 4 cluster followed by cationic and neutral cluster. However, binding energies of O 2 or CO in the coadsorption complexes follow the trend: anionic > neutral > cationic. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
11.
Adsorption of pyridine on Ni n‐clusters (with n = 2,3,4) is studied by quantum chemical calculations at B3LYP/LANL2DZ and B3LYP/6‐311G ** levels. First, Ni n‐clusters are investigated for accessible structure and electronic states. The lowest electronic state with four unpaired electrons is predicted for Ni 4‐cluster based on geometry and electronic structure, showing that the cluster stability nicely depends on number of unpaired electrons. Correction for basis set superposition error of metal‐metal bond is appreciable and has increasing effect on cluster binding energy. Next, adsorption of pyridine in planar and vertical adsorption modes is investigated on rhombus Ni 4‐cluster. The vertical mode is found (at B3LYP/6‐311G ** level) as the most favorable adsorption mode. Adsorption energy (Δ Eads) depends on cluster size; adsorption on Ni 4‐cluster is most favorable with Δ Eads = ?207.33 kJ/mol. The natural bond orbital analysis reveals the charge transfer in adsorbate/metal‐cluster. Results of investigations for the Ni 2‐ and Ni 3‐cluster are also presented. © 2012 Wiley Periodicals, Inc. 相似文献
12.
Mercaptosuccinic acid (MSA) protected silver clusters (Ag@MSA) were synthesized and their stability was monitored both in an aqueous and in a toluene phase. In the former, the pH of the cluster solution was tuned between 3 and 10 using ammonium acetate buffer, and the cluster stability was tested. The clusters were unstable at all measured pH values, whereas in the toluene phase, by using sterically hindered counter ion (tertraoctylammonium, TOA), the stability of the clusters was improved significantly. The phase transferred clusters in the toluene phase showed very high stability over months at room temperature. Transmission electron microscopy images show an average cluster size of ~ 2.05 nm. Size exclusion chromatography and Electrospray Ionisation Mass Spectrometry (ESI–MS) measurements were employed to analyse the phase transferred clusters. Importantly, Ag 4MSA 4 ? type stable fragments were observed in ESI–MS measurements. The [Ag 8(MSA) 8(TOA) 5] 2? was the largest cluster identified by ESI–MS. 相似文献
13.
Recent experimental results on the structural, chemical, and optical properties of II–VI compound clusters containing between 2 and 24 molecules are presented. Stability patterns in the mass spectra of MgO clusters correlate with stable structures predicted for ionic clusters. The time dependence of H 2O adsorption onto MgO clusters is measured and found to vary strongly as a function of the number of adsorbed molecules. A sharp peak in the photodissociation probability is observed for metal-excess SrO and CaO cluster ions near 2.0 eV. 相似文献
15.
Computer-implemented cluster models have been devised for ~2-nm anatase nanoparticles with exposed (001) and (100) faces. The Lewis acid sites occurring in these faces have been characterized by calculating the enthalpy of CO adsorption. In the Ti 114O 228 and Ti 187O 376H 4 clusters, the corner oxygen atoms compensating the electric charge are bound to titanium atoms by double bonds with a length of approximately 1.7 Å, which is in agreement with experimental data. The average enthalpy of CO adsorption on the (001) and (100) faces at a zero coverage is ?87.62 and ?135.31 kJ/mol, respectively. The deviation from the average value is 20.2 and 8.8%, respectively. The average enthalpy of CO adsorption for the Ti 114O 228 cluster is ?129.40 kJ/mol, and that for the Ti 187O 376H 4 cluster is ?119.79 kJ/mol. 相似文献
17.
Absorption of oxygen molecules by water clusters with sizes of 10 ≤ i ≤ 50 is studied by the molecular dynamics method using the modified TIP4P model. It is revealed that the total dipole moment
of the clusters nonmonotonically increases with their sizes. Absorption of O 2 molecules tends to raise the static permittivity of the ultradispersed medium formed by the clusters. The real and imaginary
parts of the permittivity of water clusters with absorbed O 2 molecules are aperiodic functions of frequency. The permittivity components turn out to be nonmonotonic functions of cluster
sizes. The IR absorption and reflectance spectra are calculated for clusters of pure water and aggregates with absorbed O 2 molecules. After the addition of oxygen molecules, the absorption coefficient of the clusters decreases, while the reflection
coefficient increases. It is concluded that the capture of oxygen molecules by atmospheric moisture may reduce the greenhouse
effect.
Original Russian Text ? A.E. Galashev, V.N. Chukanov, O.A. Galasheva, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68,
No. 2, pp. 155–160. 相似文献
18.
The stability and structure of water clusters absorbing nitrogen molecules or argon atoms was analyzed by molecular dynamics simulation at 233 K. The (?μ/? i) V, T derivative of the chemical potential, a value characterizing the stability of a cluster with respect to its size, depends linearly on the number of molecules i. According to this criterion, the clusters under study become stable near i = 40. The average length of H-bonds increases monotonically in the growing cluster of pure water and exhibits oscillatory behavior if the growing cluster contains N 2 molecules or Ar atoms. The number of H-bonds per molecule oscillates between one and six as the cluster size changes. These oscillations are damped in pure water and sustained for clusters containing impurities, especially argon. 相似文献
19.
Two new substituted polyoxotantalate clusters, [Ti 2Ta 8O 28] 8? and [Ti 12Ta 6O 44] 10?, considerably expand the pH range where tantalates persist in aqueous solution. The structures of [Ti 2Ta 8O 28] 8? and [Ti 12Ta 6O 44] 10? are reported as tetramethylammonium salts after synthesis at hydrothermal conditions in aqueous solution. These Ti‐substituted polyoxotantalate clusters have analogues among recently discovered niobates, but are slightly larger and more persistent in solution. Most importantly, they exhibit a much wider range of pH stability than the familiar hexatantalate cluster, which is the only other tantalate known to be stable at highly basic pH conditions. These molecules are kinetically stable to near‐neutral pH, making them excellent synthons for further development into materials and catalysts, and an significant advance in adapting tantalates for use in aqueous solutions. 相似文献
20.
The stability, infrared spectra and electronic structures of (ZrO 2) n ( n=3–6) clusters have been investigated by using density‐functional theory (DFT) at B3LYP/6‐31G* level. The lowest‐energy structures have been recognized by considering a number of structural isomers for each cluster size. It is found that the lowest‐energy (ZrO 2) 5 cluster is the most stable among the (ZrO 2) n ( n=3–6) clusters. The vibration spectra of Zr? O stretching motion from terminal oxygen atom locate between 900 and 1000 cm ?1, and the vibrational band of Zr? O? Zr? O four member ring is obtained at 600–700 cm ?1, which are in good agreement with the experimental results. Mulliken populations and NBO charges of (ZrO 2) n clusters indicate that the charge transfers occur between 4d orbital of Zr atoms and 2p orbital of O atoms. HOMO‐LUMO gaps illustrate that chemical stabilities of the lowest‐energy (ZrO 2) n ( n=3–6) clusters display an even‐odd alternating pattern with increasing cluster size. 相似文献
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