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1.
We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters. 相似文献
2.
D. A. Pichugina S. A. Nikolaev D. F. Mukhamedzyanova N. E. Kuz’menko 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(6):959-964
The interaction of ethylene and acetylene molecules with planar (2D) and nonplanar (3D) gold clusters Au n (n = 10, 12, 20) was studied by the density functional theory (DFT) method. The coordination of hydrocarbons at the vertices, edges, and fragments of the Au3 cluster was shown to form π, di-σ, and μ type complexes, respectively. The standard Gibbs energy and the C-C bond length of the hydrocarbon change during its adsorption in the series μ > di-σ > π complexes. The highest selectivity in adsorption of acetylene relative to that of ethylene was achieved on Au12 (3D) and Au20 (2D) clusters. 相似文献
3.
Visart de Bocarmé T Chau TD Tielens F Andrés J Gaspard P Wang RL Kreuzer HJ Kruse N 《The Journal of chemical physics》2006,125(5):054703
We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra. 相似文献
4.
De Haeck J Veldeman N Claes P Janssens E Andersson M Lievens P 《The journal of physical chemistry. A》2011,115(11):2103-2109
Well controlled gas phase experiments of the size and dopant dependent reactivity of gold clusters can shed light on the surprising discovery that nanometer sized gold particles are catalytically active. Most studies that investigate the reactivity of gold clusters in the gas phase focused on charged, small sized clusters. Here, reactivity measurements in a low-pressure reaction cell were performed to investigate carbon monoxide adsorption on neutral bare and silver doped gold clusters (Au(n)Ag(m); n = 10-45; m = 0, 1, 2) at 140 K. The size dependence of the reaction probabilities reflects the role of the electronic shells for the carbon monoxide adsorption, with closed electronic shell systems being the most reactive. In addition, the cluster's reaction probability is reduced upon substitution of gold atoms for silver. Inclusion of a single silver atom causes significant changes in the reactivity only for a few cluster sizes, whereas there is a more general reduction in the reactivity with two silver atoms in the cluster. The experimental observations are qualitatively explained on the basis of a Blyholder model, which includes dopant induced features such as electron transfer from silver to gold, reduced s-d hybrization, and changes in the cluster geometry. 相似文献
5.
We report on experiments probing the reactivity of neutral Au(n) clusters, n = 9-68, with carbon monoxide. The gold clusters are produced in a pulsed laser vaporization cluster source, operated at room temperature (RT) or at liquid-nitrogen temperature (LNT), pass through a low-pressure reaction cell containing CO gas, and are subsequently laser ionized. The reaction probabilities are determined by recording mass abundance spectra with time-of-flight mass spectrometry. The main observations are a strong temperature dependence and a remarkable size dependence. Upon cooling of the cluster source to LNT, the reactivity increases substantially. At LNT, the reaction probabilities for Au(n) with the first CO molecule are about a factor 10 higher than at RT. Moreover, adsorption of two, three, and even four CO molecules is observed, in contrast to RT clusters which at most adsorb one CO molecule. This temperature dependence is related to the lifetime of the cluster-molecule complexes, being much longer for cold clusters. The observed striking size dependence is similar at both temperatures and is discussed in terms of the electronic structure effects. 相似文献
6.
Le HT Lang SM De Haeck J Lievens P Janssens E 《Physical chemistry chemical physics : PCCP》2012,14(26):9350-9358
The effect of a single vanadium dopant atom on the reactivity of small gold clusters is studied in the gas phase. In particular we investigated carbon monoxide adsorption on vanadium doped gold clusters using a low-pressure collision cell. Employing this technique the reactivity of both neutral and cationic clusters was studied under the same experimental conditions. Analysis of the kinetic data as a function of the pressure in the reaction cell shows that the reaction mechanism is composed of a fast adsorption and a delayed dissociation reaction. It is demonstrated that the reactivity of positively charged Au(n)V(m)(+) (n = 8-30, m = 0-3) is greatly enhanced as compared to the corresponding neutral species and that dissociation rates decrease with decreasing temperatures. While the overall magnitude of the reactivity does not change upon doping with vanadium clusters, the size dependence is significantly affected. The neutral singly vanadium doped gold clusters show a sudden drop after size Au(13)V, followed by a smooth increase, in contrast to the extended odd-even staggering for bare gold clusters. This difference can be explained by changes in the electronic structure of the clusters, related to the partly filled 3d shell of the vanadium dopant atom. 相似文献
7.
Wagemans RW van Lenthe JH de Jongh PE van Dillen AJ de Jong KP 《Journal of the American Chemical Society》2005,127(47):16675-16680
Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than approximately 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrogen desorption temperature is an important step toward the application of Mg as a hydrogen storage material. 相似文献
8.
Vindigni F Manzoli M Chiorino A Tabakova T Boccuzzi F 《The journal of physical chemistry. B》2006,110(46):23329-23336
Fourier transform infrared spectra of CO adsorption from 120 K up to room temperature on two gold catalysts supported on different mixed ceria-titania oxides are discussed in comparison with those obtained on Au/TiO(2) and Au/Fe(2)O(3) reference catalysts provided by the World Gold Council. The spectra of adsorbed CO, run on the different samples before preliminary treatment, are shown and compared with those of the untreated catalysts and of the samples reduced either in CO or in hydrogen. Big differences have been found between the ceria-titania supported samples and the reference ones: unusual absorption bands, irreversible to outgassing, have been detected after CO interaction on the untreated and oxidized ceria containing samples. These absorptions are assigned to CO on Au(n)(+) small clusters stabilized at the ceria defects. By reduction in hydrogen, negatively charged Au(n)(-) species are produced on the same sample. Oxidized small particles are present on the reference catalysts, but only on the untreated samples; after treatment, only metallic step sites are evident. 相似文献
9.
Bokwon Yoon Dr. Pekka Koskinen Dr. Bernd Huber Oleg Kostko Bernd von Issendorff Prof. Hannu Häkkinen Prof. Michael Moseler Dr. Uzi Landman Prof. 《Chemphyschem》2007,8(1):157-161
Ground-state structures and other experimentally relevant isomers of Au(15) (-) to Au(24) (-) clusters are determined through joint first-principles density functional theory and photoelectron spectroscopy measurements. Subsequent calculations of molecular O(2) adsorption to the optimal cluster structures reveal a size-dependent reactivity pattern that agrees well with earlier experiments. A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons. 相似文献
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Prestianni A Martorana A Labat F Ciofini I Adamo C 《The journal of physical chemistry. B》2006,110(25):12240-12248
With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O2 and CO, on model neutral and positively charged clusters (Au(n)(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor. 相似文献
12.
Structural Chemistry - In this review, we give a brief summary of methodological and computational aspects of determination of structure and different size-dependent properties of small atomic... 相似文献
13.
The electronic structure of zeolite A is developed in a step by step procedure from the simple OhH8Si8O12 molecule, to the ∞ 1 [(-O)2H4[Si8O12)] chain, to the ∞ 2 [(-O4)(Si8O12)] layer, and finally to the silica zeolite A framework ∞ 3 [Si24O48]. It is remarkable how well the calculated band structures of both, ∞ 2 [(-O)4(Si8O12)] and ∞ 3 [Si24O48] correspond to the experimentally determined band structure of α-quartz with a Fermi level of -10.55 eV. The HOMO region consists in each case of nonbonding 2p-oxygen bands which in a localized language can be denoted as oxygen lone pairs ( | O<). We observe in each case the typical behaviour of an insulator with saturated valencies whose electronic structure can be described as being localized and is already present in the starting Oh-H8Si8O12 molecule. The double-8-rings D8R of the ∞ 2 [(-O)4(Si8O12)] layer have a pore diameter of 4.1 Å, the same as the pore opening of zeolite A. It is large enough to accept up to four Ag, forming ∞ 2 [(-O)4(Si8O12)Agn], n = 1, 2, 3, 4, layers, suitable for modelling the electronic interactions between the zeolite cavity embedded silver clusters and between the clusters and the zeolite framework. With one Ag per D8R the band structure is simply a superposition of the 4d, 5s and 5p levels of a layer of nearly noninteracting Ag and the silicon dioxide layer. The Ag-d band lies below the oxygen lone pairs, the Ag-s band lies about 3 eV above the oxygen lone pairs, and the Ag-5p bands are in the antibonding silicon dioxide region. The first electronic transition is of oxygen lone pair to Ag-5s LMCT type. Increasing silver content results in progressive splitting of the 5σ Ag bands and shifts the first (Agm+ n)? ← (| O<) charge transfer transition to lower energies. The filled Ag 4d-bands lie always significantly below the (| O<) HOCOs (highest occupied crystal orbitals) but their band width increases with increasing silver content. In all cases the zeolite environment separates the Ag clusters through antibonding Ag-(← O<) interactions so that the coupling remains weak and it makes sense to describe the Ag clusters in the D8R as quantum dots weakly interacting with each other. 相似文献
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The properties, interactions, and reactions of cyclic water clusters (H(2)O)(n=1-5) on model systems for a graphite surface have been studied using pure B3LYP, dispersion-augmented density functional tight binding (DFTB-D), and integrated ONIOM(B3LYP:DFTB-D) methods. Coronene C(24)H(12) as well as polycircumcoronenes C(96)H(24) and C(216)H(36) in monolayer, bilayer, and trilayer arrangements were used as model systems to simulate ABA bulk graphite. Structures, binding energies, and vibrational frequencies of water clusters on mono- and bilayer graphite models have been calculated, and structural changes and frequency shifts due to the water cluster-graphite interactions are discussed. ONIOM(B3LYP:DFTB-D) with coronene and water in the high level and C(96)H(24) in the low level mimics the effect of extended graphite pi-conjugation on the water-graphite interaction very reasonably and suggests that water clusters only weakly interact with graphite surfaces, as suggested by the fact that water is an excellent graphite lubricant. We use the ONIOM(B3LYP:DFTB-D) method to predict rate constants for model pathways of water dissociative adsorption on graphite. Quantum chemical molecular dynamics (QM/MD) simulations of water clusters and water addition products on the C(96)H(24) graphite model are presented using the DFTB-D method. A three-stage strategy is devised for a priori investigations of high temperature corrosion processes of graphite surfaces due to interaction with water molecules and fragments. 相似文献
17.
Gao F 《Inorganic chemistry》2010,49(22):10409-10414
Size dependence effects in semiconductor clusters have been a subject of extensive studies for the last two decades. However, it is still difficult to employ the existing theoretical models to give reliable results of energies for clusters in the whole nanometer region. Here we offer a new theoretical method for the quantum size effects based on the idea that the energy gap shift of the cluster arises from the sum of the surface effect shift and quantum effect shift parts. We express the effects through algebraic relations rather than through variational solutions of the wave equation, without the use of any special adjustable parameter. Results reveal for the first time that the shape of the energy gap shift curve is dominated by the surface energy shift. Our method can also predict quantitatively the size dependence of dielectric constant. The new theoretical findings in the ultrasmall (<1 nm) anatase TiO(2) and the silicon clusters cannot be explained using previous theories. 相似文献
18.
Dhivya Manogaran 《Journal of computational chemistry》2019,40(16):1556-1569
We present a combined quantum chemical and molecular dynamics study of cyclic and noncyclic water n-mers ([(H2O]n, n = 2–6) at four different temperatures and showcase that the dynamics of small water clusters can reproduce the known properties of bulk water reasonably well. We investigate the making and breaking of the water clusters by computing the hydrogen bond strengths, average lifetimes, and relative stabilities, which are important to understand the complex solution dynamics. We compare the behavior of water clusters in the gas phase and in the solution phase as well as the variation in the properties as a function of cluster size and highlight the notably more interesting cluster dynamics of the water trimer when compared to the other water clusters. © 2019 Wiley Periodicals, Inc. 相似文献
19.
Summary Certain chelate compounds, containing nickel and zinc, can serve as chemical models of alcohol dehydrogenase in the dehydrogenation of ethyl alcohol.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimisheskaya, No. 7, pp. 1273–1274, July, 1965 相似文献
20.
Liu L Qiao B Ma Y Zhang J Deng Y 《Dalton transactions (Cambridge, England : 2003)》2008,(19):2542-2548
An attempt to prepare ferric hydroxide supported Au subnano clusters via modified co-precipitation without any calcination was made. High resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the structure and chemical states of these catalysts. No Au species could be observed in the HRTEM image nor from the XRD pattern, suggesting that the sizes of the Au species in and on the ferric hydroxide support were less than or around 1 nm. Chemoselective hydrogenation of aromatic nitro compounds and alpha,beta-unsaturated aldehydes was selected as a probe reaction to examine the catalytic properties of this catalyst. Under the same reaction conditions, such as 100 degrees C and 1 MPa H2 in the hydrogenation of aromatic nitro compounds, a 96-99% conversion (except for 4-nitrobenzonitrile) with 99% selectivity was obtained over the ferric hydroxide supported Au catalyst, and the TOF values were 2-6 times higher than that of the corresponding ferric oxide supported catalyst with 3-5 nm size Au particles. For further evaluation of this Au catalyst in the hydrogenation of citral and cinnamaldehyde, selectivity towards unsaturated alcohols was 2-20 times higher than that of the corresponding ferric oxide Au catalyst. 相似文献