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1.
M. L. Kimble N. A. Moore A. W. Castleman Jr C. Bürgel R. Mitrić V. Bonačić-Koutecký 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):205-208
In this paper,
we present results from our joint experimental and theoretical study of the reactivity
of anionic gold oxide clusters Au2,3O1-4
- towards CO. We provide clear
evidence that, although O–O bond
weakening/dissociation is important to enable CO oxidation,
the presence of atomic oxygen can be favorable but is not always sufficient. Furthermore, we show that with the
addition of a single gold atom the reactivity channels can be changed. As a
consequence, in contrast to CO oxidation in the case of anionic gold dimer oxides,
association of CO or replacement of O2 by CO become
the dominant reaction channels for Au3On
-. This demonstrates the nonscalable
properties of gold clusters in the size regime in which each atom counts. 相似文献
2.
R. Mitri C. Bürgel J. Burda V. Bonai-Koutecký P. Fantucci 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):41-44
Bimetallic silver-gold clusters are well suited to study
changes in metallic versus
ionic properties involving charge transfer as a function of
the size and the composition. We present structures, ionization
potentials (IP) and vertical detachment energies (VDE) for
neutral and charged bimetallic
AunAgm (
2(n +
m)5) clusters obtained from
density functional level of theory. In the stable structures of
these clusters Au atoms assume positions which favor charge
transfer from Ag atoms. In clusters with equal numbers of hetero
atoms (n =
m = 1- 4) heteronuclear
bonding is preferred to homonuclear bonding, giving rise to
large values of ionization potentials. For larger clusters
(n=m=5, 10) stable structures do not favor
neither hetero bonding nor segregation into the single
components, although they exhibit more metallic than ionic
features. This remains valid also for
Au8Ag12 cluster
characterized by strong charge transfer to gold subunit. The
influence of doping of pure gold clusters with silver atoms on
VDE and IP values is discussed in context of their reactivity
towards O2 and CO molecules. As a
starting point we consider reactivity towards CO and
O2 molecules on the example of
AgAu- dimer. The results show that
the catalytic cycle can be fullfilled. 相似文献
3.
K.?Koyasu M.?Niemietz W.?Westh?user G.?Gantef?r 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,53(1):59-62
The decay path of an Ag8(O2)- cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results
in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure.
With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O2 and Ag8- on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher
binding energy while the O2 and Ag8- separate. Finally, the spectrum of bare Ag8- appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow
for direct dissociation. 相似文献
4.
M. Hartmann R. Mitrić B. Stanca V. Bonačić-Koutecký 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):151-155
Ultrafast ground state nuclear dynamics of Au4 and Ag4 is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy
based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment
of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations “on the fly” on the
neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed
probe pulse. In Au4, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations
in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ∼1 ps for the structural relaxation towards the stable rhombic D2h neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy
of the rhombic structure. In contrast, the relaxation dynamics in Ag4 is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground
state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural
behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy.
Received 22 December 2000 相似文献
5.
C.-K. Siu S. J. Reitmeier I. Balteanu V. E. Bondybey M. K. Beyer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):189-192
Pt4
- catalyses the conversion of CO and
N2O to CO2 and N2 in the gas phase, as observed by Fourier
transform ion cyclotron (FT-ICR) mass spectrometry. The partial pressures of
CO and N2O determine the extent of poisoning and the turnover numbers
that can be achieved. The catalytic conversion terminates as soon as two CO
are adsorbed on the cluster. With N2O, the reactivity of
Pt4O2
- and Pt4O3
- is reduced to 41% and
34% compared to Pt4O-, respectively, and with
Pt4O4
- this value is reduced to 1%. In contrast,
Pt4
+ shows no apparent catalytic activity. Density functional
theory calculations of Pt4
+/- with CO and N2O adsorbates
reveal significantly different stabilities of the reaction intermediates for
the different charge states. 相似文献
6.
Kiichirou?Koyasu Wilko?Westh?user Marco?Niemietz Jan?Heinen Gerd?Gantef?r 《Applied Physics A: Materials Science & Processing》2009,96(3):679-684
Using time-resolved photoelectron spectroscopy, the decay channels of AuO2− and Au2O2− following photoexcitation with 3.1-eV photons have been studied. For AuO2−, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption
can be excluded. For Au2O2−, the spectra indicate O2 desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au
n
O2− with analogous data on Ag
n
O2− clusters, a discernible pattern emerges: for dissociatively bound O2(AuO2−, Ag3O2−), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au2O2−, Ag2O2−, Ag4O2−, Ag8O2−), the 3.1-eV photoexcitation triggers fast O2 desorption with a high quantum yield. 相似文献
7.
H. Nagao K. Awazu S. Hayakawa K. Iwamoto M. Toyoda T. Ichihara 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,45(2):279-287
Various dissociation channels of silver bromide cluster ion
Ag2Br+ and silver cluster ion Ag3
+ were observed in
high-energy collisionally-activated dissociation (CAD) using a Cs target.
The fragment patterns of the high-energy CAD were compared with those of the
metastable dissociation and low-energy CAD. The difference in the fragment
patterns between the high-energy CAD and the other dissociation methods was
explained in terms of the internal energy distributions. The dissociation
mechanisms of neutral silver bromide cluster Ag2Br and silver cluster
Ag3 were also investigated by charge inversion mass spectrometry using
the Cs target. While the fragment ions AgBr- and Ag2
- were
dominantly observed in the charge inversion spectrum of Ag2Br+,
the undissociated ion Ag3
- was observed as a predominant peak in
the case of Ag3
+. The dissociation behavior of Ag2Br*
can be explained on the basis of the calculated thermochemical data.
Contrary to this, the predominant existence of the undissociated
Ag3
- cannot be explained by the reported thermochemical data. The
existence of undissociated Ag3
- suggests that the dissociation
barrier is higher than the internal energy of Ag3
*
(theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization
potentials of Ag3 and Cs. 相似文献
8.
Dissociation of molecular hydrogen (H2) is extensively studied to understand the mechanism of hydrogenation reactions. In this study, H2 dissociation by Au1-doped closed-shell titanium oxide cluster anions AuTi3O7- and AuTi3O8- has been identified by mass spectrometry and quantum chemistry calculations. The clusters were generated by laser ablation and massselected to react with H2 in an ion trap reactor. In the reaction of AuTi3O8- with H2, the ion pair Au+-O22- rather than Au+-O2- is the active site to promote H2 dissociation. This finding is in contrast with the previous result that the lattice oxygen is usually the reactive oxygen species in H2 dissociation. The higher reactivity of the peroxide species is further supported by frontier molecular orbital analysis. This study provides new insights into gold catalysis involving H2 activation and dissociation. 相似文献
9.
The stability and mechanical properties of the compounds in the Ag/SnO2 composite by reaction synthesis are investigated with the advantage of density functional theory. Cohesive energy and formation enthalpy show that the calculated compounds are stable and the stability is aligned as Ag2O<SnO∼Ag2SnO3<Ag6O2∼SnO2. Similar results are also obtained by the Milliken population analysis. The crystal structures of Ag8Sn4O12 (supercell of Ag2SnO3) could be produced by the reaction between 2×Ag6O2 and 4×SnO2; the formation energy is about −164.95 kJ/mol. The thermodynamical data of these compounds are calculated using Debye's quasi-harmonic approximation, such as heat capacity and Gibbs free energy. Bulk modulus, Young's modulus and shearing modulus values are obtained by the Voigt-Reuss-Hill method. 相似文献
10.
The interactions between AgnO-(n=1-8) and H2 (or D2) were explored by combination of the mass spectroscopy experiments and density function theory (DFT) calculations. The experiments found that all oxygen atoms in AgnO-(n=1-8) are inert in the interactions with H2 or D2 at the low temperature of 150 K, which is in contrast to their high reactivity with CO under the same condition. These observations are parallel with the preferential oxidation (PROX) of CO in excess hydrogen catalyzed by dispersed silver species in the condensed phase. Possible reaction paths between AgnO-(n=1-8) and H2 were explored using DFT calculations. The results indicated that adsorption of H2 on any site of AgnO-(n=1-8) is extremely weak, and oxidation of H2 by any kind of oxygen in AgnO-(n=1-8) has an apparent barrier strongly dependent on the adsorption style of the "O". These experiments and theoretical results about cluster reactions provided molecule-level insights into the activity of atomic oxygen on real silver catalysts. 相似文献
11.
Optical absorption of isolated silver cluster-tryptophan:
A joint experimental and theoretical study
T. Tabarin R. Antoine I. Compagnon M. Broyer P. Dugourd R. Mitrić J. Petersen V. Bonačić-Koutecký 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):275-278
We present a joint experimental and theoretical gas phase study of photoabsorption and photofragmentation of silver cluster-biomolecule
complexes. We demonstrate on the example of [ Trp.Ag3] + that binding of the metal cluster to a biomolecule leads to a significant enhancement of the photoabsorption in comparison
with [Trp.H]+ and [Trp.Ag]+. This enhancement arises due to the coupling between the excitations in the metallic subunit with charge transfer excitations
between silver cluster and tryptophan. Our experimental studies show that silver clusters up to eleven atoms can be bound
to tryptophan and we present first results on the photofragmentation of the Trp.Ag11
+ complex cation, in which properties of cluster subunit remain preserved. 相似文献
12.
Z.F. Liu W.L. Yim J.S. Tse J. Hafner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,10(1):105-114
Ab initio Molecular Dynamics (MD) method, based on density functional theory (DFT) with planewaves and pseudopotentials, was used to
study the stability and internal motion in silver cluster Agn, with n
=4-6. Calculations on the neutral, cationic and anionic silver dimer Ag2 show that the bond distance and vibrational frequency calculated by DFT are of good quality. Simulations of Ag4, Ag5, and Ag6 in canonical ensemble reveal distinct characteristics and isomerization paths for each cluster. At a temperature of 800 K,
an Ag4 has no definite structure due to internal motion, while for Ag5 and Ag6the clusters maintain the planar structure, with atomic rearrangement observed for Ag5 but not for Ag6. At a temperature of 200 K, Ag4 can exist in two planar structures whilst Ag5 is found to be stable only in the planar form. In contrast Ag6 is stable in both planar trigonal and 3D pentagonal structures. Micro-canonical MD simulation was performed for all three
clusters to obtain the vibrational density of states (DOS).
Received 5 May 1999 and Received in final form 20 August 1999 相似文献
13.
J. Moc 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,45(2):247-252
Five doublet isomers of the Al3H2 cluster lying
within a narrow range of 5 kcal/mol, along with the isomerization transition
states connecting them, have been located with the coupled-cluster CCSD(T)
and DFT methods. The two most stable doublet structures, the C2v planar
including the two Hs bound terminally and C1 non-planar showing one H
in terminal site and the other in threefold site are found to be essentially
degenerate. Although the reaction of Al3 with H2 to yield
Al3H2 is found to be significantly exothermic, by 23.5 kcal/mol,
this hydrogenation is impeded by a considerable kinetic barrier of 16 kcal/mol. Our result is consistent with the observed
lack of reactivity of
Aln towards H2(D2) for n=3 under thermal conditions [3].
The quartet Al3H2 isomers are predicted to lie 16–21 kcal/mol higher
in energy than the doublet analogues. Further dimerization of
Al3H2 to form Al6H4 has also been examined.
Electronic supplementary material Supplementary Online Material 相似文献
14.
M. Niemietz M. Engelke Y.D. Kim G. Ganteför 《Applied Physics A: Materials Science & Processing》2007,87(4):615-619
We present time-resolved photoelectron spectra of mass-selected Ag2O2 anions. The anions are photoexcited by photons with an energy of 3.1 eV, and photoelectron spectra of the excited species
Ag2O2
-
* and the subsequently appearing fragments are recorded using a probe laser pulse with a photon energy of 1.5 eV. The excited
state of Ag2O2
- has a short lifetime of 130 fs±70 fs only and decays by direct photodesorption of O2. The data demonstrate the ability of time-resolved photoelectron spectroscopy (TR-PES) to observe the breaking of chemical
bonds if the decay process of the excited state is direct (non-thermal desorption). The data are compared to recent results
of a NeNePo experiment [1] on the same system.
PACS 68.43.Tj; 78.47.+p; 33.80.Eh; 36.40.-c 相似文献
15.
V. Bonačić-Koutecký C. Bürgel L. Kronik A. E. Kuznetsov R. Mitrić 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,45(3):471-476
We present structural and optical properties of silver clusters Agn (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and FS-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible
for the absorption and emission patterns due to the specific interaction between the excitations
within the cluster and the support site which is strongly cluster size and structure dependent.
We propose Ag4 at stoichiometric site as well as Ag2, Ag4 and Ag6 at FS-center defects as good candidates for the emissive centers in the visible regime. 相似文献
16.
O. Lopez-Acevedo H. H?kkinen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,63(2):311-314
Loss of small fragments (like AuL, Au2L3,
Au4L4) have been found systematically in several MALDI and FAB
experiments on thiolate-protected gold clusters of different sizes. When using the cluster
Au25L18
-1 as
parent cluster, the fragmented cluster
Au21L14
-1 has
been reported to be obtained in high proportion (L = SCH2CH2Ph).
Here we analyse a few possible fragmentation patterns of the well-known parent cluster
Au25L18
-1 (L =
SCH3). Using DFT calculations we study the different atomic configurations
obtained after a AuL fragment is lost from
Au25L18
-1. We
found energetically favourable configurations that can be written as
Au13 [Au2L3]6-z
[AuL2]
z
-1,
where the modification can be described as a replacement of the long protecting unit by a
short one (Au2L3 → AuL2). A full replacement
(z = 6) gives rise to a protected
Au19L12
-1
cluster. This mechanism does not modify the super-atomic electronic structure of the gold
core, i.e., all these fragments remain an 8 electron super-atom clusters exactly like the
parent Au25L18
-1. We
suggest that the Au19L12
-1 cluster
could be realized by using a bulky thiolate, such as the tert-butyl thiolate
SC(CH3)3
. 相似文献
17.
I.M. Curelaru E. Suonien E. Minni K.-G. Strid T. Rönnhult 《Journal of luminescence》1983,28(2):123-133
We have measured the electron energy loss spectra of Ca2V2O7 in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V2O74- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V2O74- ion than in its component VO43- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V2O74- ion, or direct transitions to available states of the Ca2+ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range. 相似文献
18.
D. Cheng D. Cao 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(1):17-23
Canonical ensemble Monte Carlo simulations are applied to investigate the
melting of the icosahedral 55-atom Ag-Cu-Au clusters. The clusters are
modeled by the second-moment approximation of the tight-binding (TB-SMA)
many-body potentials. Results show that the introduction of the only Cu atom
of the third alloying metal in the bimetallic Ag43Au12 cluster,
forming the Ag42Cu1Au12 cluster, can greatly increase the
melting point of the cluster by about 100 K. It is also found that the
substitution of the only Cu atom of the third alloying metal in the
Ag1Au54 clusters, forming the Ag1Cu1Au53 cluster,
can result in an increase of 40 K in the melting point. It can be concluded
that the melting points of the bimetallic clusters can be tuned by the third
metal impurities doping. In addition, the surface segregation of Ag atoms in
the Ag-Cu-Au trimetallic clusters occurs even after melting. 相似文献
19.
The structural and electronic properties of silver-doped gold clusters Au n Ag v (2?≤?n?≤?10; v?=?0,?±1) have been systematically investigated using density functional theory. The results show that the ground state optimal structures of the cationic and neutral clusters are found to be planar up to n?=?3 and 9, respectively. However, for the anionic clusters, no three-dimensional lowest-energy structures are obtained according to DFT calculations. The calculated binding energy and dissociation energy as a function of cluster size exhibit odd–even alternations. The natural population analysis indicates that in Au n Ag v clusters charges transfer from the Ag atom to the Au frames. The trends for the vertical detachment energies, adiabatic electron affinities, adiabatic ionization potentials, and chemical hardness of Au n Ag v clusters, as the cluster size increases, are studied in detail and compared with the available experimental data. 相似文献
20.
采用化学自燃烧法制备了不同Ag+掺杂浓度的Y2O3:Eu纳米晶体粉末样品([Y3+]∶[Eu3+]∶[Ag+]=99∶1∶X,X=0—3.5×10-2),以及通过退火处理得到了相应的体材料.根据X射线衍射谱确定所得纳米和体材料样品均为纯立方相.实验表明在纳米尺寸样品中随着Ag离子浓度的增加,荧光发射强度随之增加,当X=2×10-2时达到最大值,其发光强度比X=0时提高了近50%.当Ag离子浓度继续增加,样品发光强度保持不变.在相应的体材料样品中则没有观察到此现象.通过对各样品的发射光谱,激发光谱,X射线衍射图谱,透射电镜(TEM)照片和荧光衰减曲线的研究,分析了引起纳米样品荧光强度变化的原因是由于Ag离子与表面悬键氧结合,从而使这一无辐射通道阻断,使发光中心Eu3+的量子效率提高;Ag+的引入所带来的另一个效应是使激发更为有效.这两方面原因使发光效率得到了提高. 相似文献