Characterization and reactivity of oxygen-centred radicals over transition metal oxide clusters

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 1925–1938 1925

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 1925–1938

Characterization and reactivity of oxygen-centred radicals over transition

metal oxide clusters

Yan-Xia Zhao,ab

Xiao-Nan Wu,ab

Jia-Bi Ma,ab

Sheng-Gui He*aand

Xun-Lei Ding*a

Received 14th July 2010, Accepted 24th November 2010

DOI: 10.1039/c0cp01171a

We introduce chemical structures and reactivity of oxygen-centred radicals (O��) over transition

metal oxide (TMO) clusters based on mass spectrometric and density functional theory studies.

Two main issues will be discussed: (1) the compositions of TMO clusters that have the bonding

characteristics of (or contain) the O�� radicals; and (2) the dependences (cluster structures, sizes,

charge states, metal types, etc.) of the chemical reactivity and selectivity for the O�� radicals over

TMO clusters. One of the goals of cluster chemistry is to understand the elementary reactions

involved with complex heterogeneous catalysis. The study of the O�� containing TMO clusters

permits rather detailed descriptions for how mono-nuclear oxygen-centred radicals may exist and

react with small molecules over TMO based catalysts.

1. Introduction

Transition metal oxides (TMOs) are widely used as both

catalysts and catalytic support materials.1–7 The catalytic role

of the oxide surface is in terms of forming or providing oxygen

in an activated state.8 In a general scheme of O2 dissociation: O2

(molecular oxygen)-O2�� (superoxide)-O2

2� (peroxide)-

2O�� (mono-nuclear oxygen-centred radical) - 2O2� (lattice

oxygen), the O2��, O2

2�, and O�� are considered as reactive

oxygen species (ROS).9–11 Note that O2�� is usually denoted as

O2� while O�� is often simplified as O� or O� in the literature.

In condensed-phase studies, spectroscopy-based methods

including infrared, Raman, electron spin resonance, etc. have

been widely used to characterize the role of the ROS in surface

reactions and the achievements have been frequently

reviewed.8,9,11–14 It has been generally considered that

oxidations or oxidative transformations of very stable

molecules (such as CO12,15–17 and CH4)18–23 over TMO

surfaces at low temperature often must involve ROS.24–26

However, in some cases, ROS may not be generated with

sufficient concentrations or their lifetimes may be too short9,27

for condensed phase studies. As a result, it is very useful to

adopt alternative ways to investigate the chemistry of the ROS

involved with the condensed-phase systems.

One such alternative way is to study the TMO clusters, in

order to understand the elementary steps involved in the ROS

a Beijing National Laboratory for Molecular Science (BNLMS), StateKey Laboratory for Structural Chemistry of Unstable and StableSpecies, Institute of Chemistry, Chinese Academy of Sciences, Beijing100190, People’s Republic of China. E-mail: [email protected],[email protected]; Fax: +86-10-62559373; Tel: +86-10-62536990

bGraduate School of Chinese Academy of Sciences, Beijing 100039,People’s Republic of China

Yan-Xia Zhao

Yan-Xia Zhao received her BSdegree in chemistry fromShanxi Normal University in2005 and her MS degreein chemistry from BeijingNormal University in 2008.She is now a PhD candidateat the Institute of Chemistry,Chinese Academy of Science.Her research interests mainlyfocus on the chemistry oftransition metal oxide speciesin the gas phase.

Xiao-Nan Wu

Xiao-Nan Wu received aBS degree in chemistryfrom Beijing University ofChemical Technology in 2007.He is now a PhD student at theInstitute of Chemistry, ChineseAcademy of Sciences. Hisresearch activities are on thestudy of chemical reactions oftransition metal oxide clusters,especially lanthanide metaloxide clusters by using time offlight mass spectrometry.

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1926 Phys. Chem. Chem. Phys., 2011, 13, 1925–1938 This journal is c the Owner Societies 2011

over the TMO catalysts. The TMO materials, especially those

with metals being in the highest oxidation states (such as TiO2

and V2O5) are usually insulators or semiconductors in which

the motion of the majority of the electrons is localized in

nature. As a result, small TMO clusters may well be effective

models of real surface species or active sites over the bulk TMO

materials.28–30 This article is to review the research progress

from the cluster chemistry to understand one of the ROS, the

mono-nuclear oxygen-centred radical species O��, over the

TMO surfaces. It is noticeable that in addition to the O2

(or other molecules such as N2O)9,12 dissociation, the O��

species may also be generated through photon irradiation in

photocatalytic processes because the lattice oxygen O2� may

be converted to O�� directly (O2� + hn - O�� + e�)

or through recombination with pre-generated holes (O2� +

h+ - O��).9,14,31,32 Meanwhile, the dehydroxylation of the

surface hydroxyl groups by evacuation as a pretreatment of

materials at high temperature may also generate O��

centres.33–35

Fig. 1a shows the electron configuration of the simplest O��

species, the free O� anion system, of which the unpaired

electron or the spin densities (SDs) are located in one of the

O 2p orbitals. Density functional theory (DFT) calculations36

predicted (Fig. 1b–d) that the oxide clusters of metals, such as

the first d-block transition metal Sc, can have essentially the

same characteristics for the SD distributions as the free

O� does: the unpaired electron or SD values of about 1 mB in

each of Sc2O4�, Sc3O5, and Sc4O6

+ clusters are also

distributed in the 2p orbital of one of the oxygen atoms. As

a result, one can consider that each of these clusters contains

one unit of O��.

The O�� radicals can be terminally bonded (denoted as Ot��,

Fig. 1b and c) or bridgingly bonded (denoted as Ob��, Fig. 1d)

over the TMO clusters. The O�� containing clusters can be

positively (Fig. 1d) or negatively (Fig. 1b) charged, or overall

neutral (Fig. 1c), which may correspond to the surface O��

species in different local charge environments.37–39 One can

also imagine that the sizes of the O�� containing clusters can

vary from several to dozens of atoms. The strength of the O��

binding onto the clusters may depend on cluster sizes,

structures, charge states, and the types of the transition

metals. All of these issues can ultimately influence the

chemical reactivity and selectivity of the O�� radicals over

the TMO clusters. The O�� containing TMO clusters are thus

ideal models to investigate a detailed and possibly rich

chemistry of the surface O�� species, under isolated,

controlled, and reproducible conditions.

Combined experimental (mostly mass spectrometry based)

and theoretical (such as DFT based) methods can be used to

study the bonding and reactivity of the TMO clusters in

different charge states. The mass spectrometric study of

neutral TMO cluster reactivity is difficult and progress has

been made recently by Prof. Elliot R. Bernstein’s group40–46 by

using single-photon (soft) ionisation which prevents severe

cluster fragmentation that would occur in the process of

electron-impact or multi-photon (hard) ionisation.47 It

should be noted that spectroscopic method is often very

useful for studying the structures and reactivity of neutral

TMO clusters.48 For example, important mechanistic details

involved in MO + CH4 2 M + CH3OH reactions can been

obtained with matrix isolation infrared absorption

spectroscopy.49 The transition metal species are challengingJia-Bi Ma

Jia-Bi Ma received her BSdegree in chemistry from JilinUniversity in 2008. She is nowa PhD student at the Instituteof Chemistry, Chinese Academyof Sciences. Her researchactivities focus on experimentaland theoretical investigationsof the reactions of thetransition metal oxide clusterswith small molecules.

Sheng-Gui He

Sheng-Gui He received his BSdegree in physics and PhDdegree in chemistry from theUniversity of Science andTechnology of China in 1997and 2002, respectively. Afterpostdoctoral stays at theUniversity of Kentucky(September 2002–February2005) with Prof. Dennis J.Clouthier and at ColoradoState University (February2005–January 2007) withProf. Elliot R. Bernstein,he joined the Institute ofChemistry, Chinese Academy

of Science in January 2007. His research interests are onexperimental and theoretical studies of reactive intermediatesincluding free radicals and atomic clusters.

Xun-Lei Ding

Xun-Lei Ding received his BSdegree in physics in 1999 fromthe University of Science andTechnology of China (USTC)and PhD degree in chemistryin 2004 under the supervisionof Professor Jin-Long Yang.After a one-year postdoctoralstay at USTC with ProfessorJian-Guo Hou and a two-yearpostdoctoral stay at theTheory@Elettra group ofCNR-INFM DEMOCRITOS(Italy) with Professor MariaPeressi, he joined the Instituteof Chemistry, Chinese Academy

of Sciences in 2007. His research interests include first principlestudies on structural and reactivity properties of clusters andsurfaces.

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systems for state-of-the-art quantum chemistry calculations.50

The experimental data, such as bond and ionisation energies,

electron affinities, vibrational spectra, and reaction channels

and rate constants may be used to gauge the validity of chosen

theoretical methods (functionals of DFT, basis sets, etc.).51–54

We do not attempt to go into further details of the

experimental and theoretical methods for studying the TMO

clusters and readers may refer to the original references. The

study of bonding and reactivity of atomic clusters including

many of those without oxygen-centred radicals has broad

scientific significance55,56 and several aspects of recent

achievements have been reviewed29,30,48,49,57–68 and selected

individual contributions may be found in refs. 69–85. This

review article focuses on the research progress in bonding and

reactivity of the O�� radicals over TMO clusters based on our

own works and other closely related investigations,

particularly those of the groups headed by Prof. Helmut

Schwarz, Prof. A. Welford Castleman, Jr., and their

collaborators.

2. homonuclear transition metal oxide clusters

2.1 d-block transition metal oxide clusters

2.1.1 Compositions of O�� containing TMO clusters.

To consider possible compositions of the TMO clusters

(MxOyq, M is a metal atom and q is the charge number,

q = 0, �1) with the oxygen-centred radical O��, one may

quickly find the stoichiometric metal oxide cluster cations such

as Sc2kO3k+, TikO2k

+, V2kO5k+, CrkO3k

+, and Mn2kO7k+

because these species can be generated by exciting one electron

from each of the stoichiometric neutral clusters into vacuum.

The unpaired electron left in each of the cations can be

localized on oxygen atom(s) to form an oxygen-centred

radical under the conditions that all of the metal atoms are

in the maximum oxidation state and electron excitation does

not result in significant structure relaxation. One may also

think that the above predicted O�� containing clusters are

oxygen-rich by (averagely) a half oxygen atom if one counts

the total valence numbers of metal and oxygen atoms and takes

into account the net charge. To follow a rule that O��

containing clusters are oxygen-rich by a half oxygen atom,

one finds thatMxOyq clusters with O�� may satisfy the general

equation:36

D R 2y � nx + q = 1, (1)

in which n is the number of valence electrons of elementM that

can be oxidized by oxygen to the +n oxidation state. The

D=1 anions are Sc2kO3k+1�, TikO2k+1

�, V2kO5k+1�, etc. and

the D = 1 neutrals are Sc2kO3kScO2, V2kO5kVO3, etc. (note

that there is no D = 1 neutral oxide cluster for groups 4 and 6

metals). The D value for a general MxOyq cluster can be

used to judge whether the cluster is oxygen-rich (D > 0) or

oxygen-poor (D o 0) and the extent of the richness and

Fig. 1 DFT calculated profiles of spin density distributions for free

O� anion (a), and Sc2O4� (b), Sc3O5 (c), and Sc4O6

+ (d) clusters. The

electron configuration of O� is also shown. The structures of the

clusters are adapted from ref. 36 and some of the Sc–O bond lengths

are given in pm.

Fig. 2 DFT calculated profiles of SOMOs forM3Oyq (D=1) clusters. TheMulliken spin density values greater than 0.5 mB over oxygen atoms are

given in parentheses. Adapted from ref. 36.

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poorness. The D value may be called extent of cluster over-

oxidation.

A DFT study with a hybrid functional B3LYP86,87

predicted that for early transition metals (M = groups 3–7

and 3d–5d metals), except for M = Cr and Mn, all of the

investigated oxide clusters MxOyq with D = 1 and x = 1–3

(x = 1–6 for M = Sc) have the character of oxygen-centred

radicals.36 Fig. 2 shows the profiles of singly occupied

molecular orbitals (SOMOs) for the D = 1 clusters with

three metal atoms. Like in the free O� anion, the unpaired

electrons in these clusters are mainly located over the O 2p

orbitals. Note that in the D = 1 oxide clusters (such as V3O8

and W3O9+) of groups 5–7 metals, the unpaired electron can

be distributed over two oxygen atoms in each cluster. This

is reasonable if one considers that adsorption of a free O�

onto a cluster (such as onto V3O7+ to form V3O8) can result

in re-distribution of the SDs. Each of these clusters in Fig. 2

can be considered to contain one (or equivalently one) unit of

O�� radical.

The DFT study suggested that the D = 1 clusters of CrxOyq

[(CrO3)1–3+ and (CrO3)2–3O

�] and MnxOyq (MnO4, Mn2O7

+,

Mn2O8�, and Mn3O11) have unbroken O–O bonds and these

clusters do not have O�� radicals.36 This means that chromium

(Cr) and manganese (Mn) of the 3d metals do not belong to the

type of transition metals of which all of the oxide clusters

MxOyqwith D=1 contain O�� radicals. This is in parallel with

the smallest bond energies of CrO (4.78 eV) and MnO

(3.83 eV)88 among MO (M = groups 3–7 and 3d–5d), as

well as quite large bond energy of O2 (5.12 eV).89

An analysis of the geometrical parameters (such as in

Fig. 1b–c, see details in ref. 36) indicates that the M–Ot�� or

M–Ob�� bond in each cluster is significantly longer than the

normal MQOt double or M–Ob single bond, respectively. For

example, the Sc–Ot�� bond length in Sc2O4

� (Fig. 1b) is

206 pm in contrast to the much shorter length (176 pm) of

the other Sc–O terminal bond in the same cluster. The

elongation of the M–Ot�� bonds in the D = 1 clusters is also

consistent with the result of Wiberg bond order analysis for

selected clusters: the bond order values are 1.14, 1.06, 0.80,

0.75 for Ta–Ot�� in TaO3, Mo–Ot

�� in MoO4�, Sc–Ot

�� in

ScO2, and Zr–Ot�� in ZrO3

�, respectively.36 This suggests that

the Ot�� (as well as Ob

��) atoms with spin density values close

to one unit mB are singly bonded with the transition metal

centres, in agreement with the picture that one of the 2p

orbitals of Ot�� or Ob

�� atom is singly occupied and such

electronic structure (or electron configuration) is the same as

that for the free O� anion (Fig. 1a).

The Ot�� (or Ob

��) atom with elongated M–Ot��

(or M–Ob��) bond and a locally open shell electronic

structure is expected to be the active site of a D = 1 cluster

in the reaction with molecules such as CO, CH4, C2H4, and so

on. Oxygen atom transfer is an important type of chemical

reaction, so it is useful for predicting the M–O bond

dissociation energies [energy cost for O atom loss: MxOyq

(D = 1) - MxOy–1q (D = �1)+O, M = groups 3–7 metals

and xr 3] in order to study the reactivity of the D=1 clusters

with O�� centres. The computedM–O dissociation energies by

B3LYP cover a broad energy range from 1.90 eV (Tc3O10) to

5.06 eV (Ta2O6�, see ref. 36 for details), suggesting a possibly

rich chemistry for the early transition metal oxide clusters with

oxygen-centred radicals.

2.1.2 Reactivity of O�� radicals over cationic TMO

clusters. A systematic verification of the theoretical results36

came from an experimental study of the reactions of early

transition metal oxide cluster cations (MxOy+,M= Sc, Y, La;

Ti, Zr, Hf; V, Nb, Ta; Cr, Mo, W; Mn, Re) with methane

(CH4) under near room-temperature conditions.90 The CH4

molecule is chemically very stable21,65,67,91 and not too many

oxide clusters (see a list in Table 1) are able to activate CH4

at low- or room-temperature.92–105 By using a time of flight

mass spectrometer that is coupled with a laser ablation

cluster source and a fast flow reactor,106 several series of

stoichiometric metal oxide cluster cations (D = 1 clusters)

(TiO2)1–5+, (ZrO2)1–4

+, (HfO2)1–2+, (V2O5)1–5

+,

(Nb2O5)1–3+, (Ta2O5)1–2

+, (MoO3)1–2+, (WO3)1–3

+, and

Re2O7+ have been identified to be able to abstract a

hydrogen atom from CH4 to produce CH3 under near room-

temperature conditions.90 The mass spectra that suggest

reactions of (V2O5)2–5+ + CH4 - (V2O5)2–5H

+ + CH3

are given in Fig. 3. Note that a lot of other cluster cations

with D = �1, 0, 2, etc. were also generated in the clusters

source and reacted/collided with the CH4 in the fast flow

reactor. However, there was no evidence of the CH4

activation by any of these D a 1 clusters.

The experiments also showed that the D = 1 clusters of

(M2O3)1–2+ (M = Sc, Y, La), (CrO3)1–2

+, and Mn2O7+ are

inert or react very slowly with CH4 under near room-

temperature conditions. The inertness of (CrO3)1–2+ and

Mn2O7+ [in contrast to high reactivity of (MoO3)1–2

+,

(WO3)1–3+, and Re2O7

+] toward the CH4 activation is in

agreement with the B3LYP predictions36 that there are no

O�� radicals in these chromium or manganese species. A few

DFT studies indicated that the homolytic C–H bond activation

of CH4 by the O�� radicals over the cluster cations [such as

V4O10+,98 (Al2O3)3–5

+,99 VAlO4+,103 (V2O5)(SiO2)1–4

+,104

and so on] is (overall) barrierless. Except for MnO+, the

available theoretical calculations36,98–105 suggested that all

of the clusters listed in Table 1 contain terminally bonded

Table 1 Oxide cations that are able to activate methane under nearroom-temperature conditions

Year Homonuclear oxide clusters References

1989 OsO4+ 92

1992 FeO+ 931995 MnO+ 941997 MoO3

+ 951999 TiO2

+, ZrO2+ 96

2006 MgO+ 97V4O10

+ 982008 (Al2O3)3–5

+ 992009 SO2

+ 100P4O10

+ 1012010 (TiO2)1–5

+, (V2O5)1–5+, 90

(ZrO2)1–4+, (Nb2O5)1–3

+, (MoO3)1–2+

(HfO2)1–2+, (Ta2O5)1–2

+, (WO3)1–3+, Re2O7

+

(CeO2)2–4+ 102

Heteronuclear oxide clusters2010 VAlO4

+ 103V2O5(SiO2)1–4

+, (V2O5)2SiO2+ 104

V3PO10+ 105

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O�� radicals (Ot��) while the O�� in (Sc2O3)1–3

+, Y2O3+, and

La2O3+ are bridgingly bonded (Ob

��).36 This rationalizes the

inertness of (M2O3)1–2+ (M = Sc, Y, La) toward the low-

temperature CH4 activation although the DFT calculations

predicted that there are O�� radicals in these clusters. Our

recent experiments have indicated that some of the D=1 oxide

clusters of group 3 metals such as (Sc2O3)2,3+ are able to

abstract the hydrogen atoms from larger alkanes C2H6 and

C4H10 at low-temperature.107 This indicates that Ob�� radicals

are still highly reactive toward the C–H activation although

they are not as reactive as the Ot�� radicals.

In addition to the hydrogen atom abstraction reactions,

oxygen atom transfer processes were also experimentally

identified in the reactions of the D = 1 cluster cations with

carbon monoxide and (or) unsaturated hydrocarbon molecules

at thermal collision conditions. Some typical such reactions are

V2kO5k++C2H2,4-V2kO5k–1

++C2H2,4O (k=1–3),108–110

ZrkO2k+ + C2H2,4 - ZrkO2k–1

+ + C2H2,4O (k = 1–5) and

ZrkO2k+ + CO - ZrkO2k–1

+ + CO2 (k = 2–5),111

and WkO3k+ + C3H6 - WkO3k–1

+ + C3H6O (k = 1–3)

and WkO3k+ + CO - WkO3k–1

+ + CO2 (k = 1–3).112 The

available DFT calculations indicated that all of the above

reactions are both kinetically and thermodynamically favorable

and the approaching of the small molecules (CO, C2H4, etc.) to

the Ot�� centres in these clusters is all barrierless.109–111

Meanwhile, it has been demonstrated that some of the reduced

clusters such as ZrkO2k–1+ (with D= �1) can react with N2O at

thermal collision conditions to regenerate the D = 1 clusters.111

This means that the D = 1 and D = �1 cluster couples can be

good model catalysts for the oxygen atom transfer reactions.

2.1.3 Reactivity of O�� radicals over neutral TMO clusters.

The story of the neutral vanadium oxide clusters (V2kO5kVO3)

that have the Ot�� radical centres is quite different from that of

the cationic counterparts (V2kO5k+). The experimental and

theoretical studies indicated that CQC double bond cleavage

takes place under near room-temperature conditions in the

reaction of V2kO5kVO3 (k = 0–2) with alkene molecules such

as C2H4: V2kO5kVO3 + C2H4 - V2kO5kVO2CH2 +

HCHO,41,43,113 in contrast to the relatively simple oxygen

atom transfer reactions for the cationic system: V2kO5k+ +

C2H4 - V2kO5k–1+ + C2H4O (k = 1–3).108,109 The DFT

calculations suggested that the formation of acetaldehyde

(oxygen atom transfer) is thermodynamically more favorable

than that of the formaldehyde (CQC double bond cleavage

and oxygen atom transfer) in the reaction of C2H4 with VO341

and V3O8:113

VO3 + C2H4 - VO2CH2 + HCHO DH298 K = �0.25 eV

(2a)

VO3 +C2H4 - VO2 + CH3CHO DH298 K =�0.83 eV(2b)

V3O8+C2H4-V3O7CH2+HCHO DH298 K=�0.04 eV(3a)

V3O8 + C2H4 - V3O7 + CH3CHO DH298 K = �1.32 eV

(3b)

However, the study of the detailed reaction mechanisms

(see Fig. 4 for reaction (3)) identified that reactions (2b) and

(3b) are kinetically less favorable than (2a) and (3a),

respectively, in agreement with the experimental identification

of reactions channels (2a) and (3a). Note that the formation of

a 3 (OVO) + 2 (CQC) cyclo-addition intermediate (such as

3 in Fig. 4) is a critical step to cause the CQC double bond

cleavage of the alkenes.

The DG298 K and DH0 K values given in Fig. 4 show that due

to the entropy loss (DS o 0, loss of degrees of freedom) in the

formation of reaction intermediates from the separated

reactants (V3O8 and C2H4), the entropic contribution

(�DS � T > 0) shifts the relative free energy (DG298 K)

above the relative enthalpy (DH0 K, very close to DH298 K)

by about 0.4–0.5 eV for the reaction intermediates and

transition states, whereas the relative energies of the

separated products (P1 and P2) do not change significantly.

The transition state 2/6 is much more stable than the product

Fig. 3 Time of flight mass spectra for reactions of V4O10,11+, V6O15,16

+, V8O19,20+, and V10O24,25

+ with (a) He (for reference), (b) CH4, and

(c) CD4. To get the spectra in the V4O10,11+ region, the partial pressures of CH4 and CD4 in the fast flow reactor are about 0.25 and 0.3 Pa,

respectively. About 1 Pa CH4 and 3 Pa CD4 are used to get the spectra in the V6, V8, and V10 regions. Numbers x, y denote VxOy+ and x, yX denote

VxOyX+ in which X = H, D, H2O, or CD4. Adapted from ref. 90.

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P1 (V3O7CH2+HCHO) in terms of DH0 K (�0.43 eV versus

�0.03 eV) while 2/6 is less stable than P1 in terms of DG298 K

(+0.03 eV versus�0.05 eV). The experimental identification of

V3O7CH2 (reaction channel 3a)41 suggests that the entropic

contribution to the free energies possibly plays an important

role. Otherwise, a much faster conversion 2 - 2/6 - 6 - P2

(reaction channel 3b) would make the reaction channel 3a

incompatible and not observable.

Partial oxidation of propylene (C3H6) catalyzed by the

simplest O�� containing vanadium oxide cluster VO3 was

also studied by DFT calculations.114 The overall barrierless

catalytic cycles at room-temperature are summarized in Fig. 5.

The calculations predicted that, in the model catalytic cycles,

the most favorable products are acetaldehyde (CH3CHO) and

CO2, which are also the major products in propylene selective

oxidation over V2O5/SiO2 catalyst.115 The match of the simple

gas phase model catalysis with the complex condensed phase

heterogeneous reactions may not be a coincidence. The match

can be rationalized by similar chemistry of the O�� radicals

over condensed phase system and the gas phase clusters. The

electron paramagnetic resonance (EPR) spectroscopy

characterized that the interaction of N2O with VO2 centres in

zeolite BEA (Si/Al = 30) generates vanadium(V) bound O��

radicals with gx = 2.0202, gy = 2.0173, and gz = 2.0284

(anisotropic g-factors); and |Ax| = 1.65, |Ay| = 1.58, and

|Az| = 1.49 mT (hyperfine structure constants).116 In the case

Fig. 4 DFT calculated potential energy profiles (a) and structures of reaction intermediates and transition states (b) for V3O8 + C2H4. An integer

n is used to denote reaction intermediate and two integer combination n1/n2 is used to denote transition state that connects reaction intermediates n1and n2. The relative Gibbs free energies (DG298 K in eV) and zero-point vibration corrected energies (DH0 K in eV) with respect to V3O8 + C2H4

are given in parentheses as (DG298 K/DH0 K). The bond lengths are given in 0.1 nm. Adapted from ref. 113.

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of propylene adsorption onto the zeolite with the V–O��

radicals, single oxygen transfer leading to the rupture of the

CQC double bond was observed. The mechanism of the

propylene CQC bond rupture rather than the formation of

C3H6O (acetone, propanal, etc.) by the V–O�� radicals in the

condensed phase system116 can be well interpreted by the gas

phase studies of the reactions of C3H6 with VO3(V2O5)0–2(=VO3, V3O8, and V5O13) clusters that are exactly the

V–O�� radical species.41,43,113

2.1.4 Reactivity of O�� radicals over anionic TMO clusters.

The reactivity of the O�� radicals over negatively charged

TMO clusters is generally much lower than that of the O��

centres over the positively charged clusters in the reaction with

hydrocarbon molecules. The O�� containing anions V2O6�

and V4O11�,108,117,118 and ZrkO2k+1

� (k = 1–4)119 display

association or minor oxygen atom transfer channels with

unsaturated hydrocarbon molecules such as C2H2 and C2H4,

in contrast to the efficient oxygen atom transfer processes in

the reactions of (V2O5)1–3+ as well as (ZrO2)1–5

+ with these

molecules.108–111 In addition, the activation of the most

stable alkane molecule CH4 by (V2O5)1–5+, (Nb2O5)1–3

+,

and (Ta2O5)1–2+ is very fast (the first order rate constants

k1 B 10�10 cm3 molecule�1 s�1)90,98 while it was considered

that the group 5 metal oxide cluster anions are inert toward

saturated hydrocarbon molecules such as C4H10.108,120

The DFT results121–125 in Fig. 6a show that the reactivity

(both thermodynamically and kinetically) of the O��

containing clusters toward CH4 activation depends

significantly on the charge states. The cationic (V2O5+) and

anionic (V2O6�) clusters have the highest and lowest reactivity,

respectively, while the reactivity of the neutral one (V3O8) is in

the middle. The C–H activation barrier (0.26 eV) in V2O6�+

CH4 is still very small as compared with the large barriers

(1.7–3.8 eV)126,127 of CH4 activation by the MQO double

bonds in TMO cluster species V3O6Cl3, Cr3O9, Mo3O9, and

W3O9 without the O�� centres. The barriers for the reactions of

larger alkanes (C2H6, n-C4H10, etc.) with O�� containing

clusters (such as V2O6�, see Fig. 6b) can be small enough to

provide a chance to observe the C–H activation by TMO

anions at low-temperature. Note that the examples for the

activation of alkanes by anions are scarce.68

With the above considerations in mind and by increasing the

pressures of alkanes in the fast flow reactor, we were able to

observe evidences for the C–H activation of C2H6 and n-C4H10

by V2O6� and V4O11

� 118 as well as by Zr2O5� and Zr3O7

� 125

(see Fig. 7 for one example). The estimated rate constants (k1)

for V2O6� + C4H10 and Zr2O5

� + C4H10 are 1 � 10�12 and

2 � 10�11 cm3 molecule�1 s�1, respectively. Since the methane

activation is a holy grail in alkane chemistry, it is interesting to

consider the rate of CH4 activation by these O�� containing

cluster anions. The k1(V2O6�+CH4) value may be scaled from

k1(V2O6�+C4H10) by a factor of exp(�DEa/kBT), in which

DEa is the difference of the C–H activation barrier (0.245 eV,

Fig. 6b), kB is the Boltzmann constant, and T is the

temperature. For T = 298 K, k1(V2O6� + CH4) is estimated

to be 7.2 � 10�17 cm3 molecule�1 s�1. Similarly, with the DFT

calculated DEa = 0.206 eV for the Zr2O5� reaction systems,

the k1(Zr2O5�+CH4) at room temperature can be estimated to

be 6.6 � 10�15 cm3 molecule�1 s�1, which is close to

k1(OH+CH4) = 7.89 � 10�15 cm3 molecule�1 s�1.128,129

This means that some of the O�� containing cluster anions

may be as reactive as the free OH radical toward the C–H

activation. The higher reactivity of Zr2O5� versus V2O6

� in the

C–H s bond activation is in parallel with the localized SDs

(over one O atom) in the former and delocalized (over two

O atoms) SDs in the latter.36 The influence of the SD

localization and delocalization on the cluster reactivity will

be discussed in more detail in the text below. It should be

pointed out that the reactivity of the O�� containing TMO

anions (ZrkO2k+1�, k = 1–4) and cations (ZrkO2k

+, k = 2–5)

toward the CO oxidation can be very similar and the

rate constants were measured to be on the order of

10�12 B 10�13 cm3 molecule�1 s�1 in refs. 111 and 119.

2.2 Cerium oxide cluster cations

Despite the fact that there are comprehensive data available on

the chemistry of d-block transition metal oxide clusters, the

investigations on oxide clusters of f-block metals (lanthanides

and actinides) are very limited.130–137 Cerium [Xe]4f15d16s2 is

the first 4f-element and its oxides (such as ceria, CeO2) are

widely used as catalysts and as promoters of heterogeneous

catalytic reactions,5,138 so the understanding of structure-

reactivity relationship of cerium–oxygen systems is very

important.139,140 Based on the chemistry of d-block TMO

clusters with O�� radicals, one would expect that the

stoichiometric cerium oxide cluster cations CekO2k+ contain

oxygen-centred radicals that are very reactive toward CH4,

C2H4, CO, etc. Meanwhile, the bulk CeO2 and ZrO2 can have

the same cubic fluorite (CaF2) structure141 while zirconium

([Kr]4d25s2) has one more d and one less f electrons than

cerium does. Is the behavior of f electron in cerium very similar

to that of the additional d electron in zirconium in terms of the

oxide cluster reactivity?

The experimental study of the reactions between cerium

oxide cluster cations and a series of small molecules under

near room-temperature conditions indicated that the CekO2k+

cluster can transfer one oxygen atom to C2H4 (see the spectra

Fig. 5 Schematic diagram showing barrierless reaction channels that

form catalytic cycles for propylene (C3H6) partial oxidation over the

VO3 cluster under gas phase, room-temperature conditions. Adapted

from ref. 114.

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in Fig. 8) or CO for k = 2–6 and abstract one hydrogen atom

from CH4 or C2H6 for k = 2–4 (the mass resolution is limited

for k = 5 and 6 clusters).102 However, CekO2k+ clusters have

high reactivity toward C2H4 and relatively much lower

reactivity toward CO, CH4, and C2H6. This is in sharp

contrast with the fact that ZrkO2k+ is highly reactive toward

all of the small molecules. Table 2 lists the rate constants for

reactions of Ce2O4+ as well as Zr2O4

+ with a few small

molecules in the same fast flow reactor and the uncertainties

of the relative rates are about 30–50%.102

The DFT study indicated that the SDs in CekO2k+ clusters

with k=2–5 (see Fig. 9, the large k=6 cluster was not studied

by the DFT) are mainly distributed over one Ce and two

O atoms, which forms (OCeO)� heteroatom radicals.

The terminal oxygen atom with a fraction of the SDs

(B0.1–0.6 mB) in CekO2k+ still has radical character and may

be denoted as Ot��f. Note that the SDs are primarily localized

over a single O atom (Ot�� radical) in ZrkO2k

+ (k = 2–4).111

The C–H activation of alkanes by a Ot�� or Ot

��f containing

cluster involves interactions of the cluster SOMO with the

C–H s orbital that may be written as hSOMO|H|si, in

which H is the interacting Hamiltonian.102 The SD

(or SOMO) localization (Ot��) leads to larger values

of oSOMO|H|s> than the SD delocalization (Ot��f) does

for a given distance between the C–H bond and the reacting

Ot atom. As a result, Ot�� is more reactive than Ot

��f in terms

of C–H s bond activation which rationalizes the experimental

results of k1(Zr2O4++CH4) c k1(Ce2O4

++CH4) in Table 2

as well as k1(Zr2O5� + C4H10) c k1(V2O6

�+C4H10)

discussed in subsection 2.1.4. The oxidation of CO molecule

by Ot�� or Ot

��f involves participation of carbon (2s) lone pair

orbital which has s character. It is thus expected that the

extent of SD localization also significantly affects/controls the

rate constants for CO oxidation, in agreement with a much

faster reaction of Zr2O4+ + CO - Zr2O3

+ + CO2 than

Ce2O4+ + CO - Ce2O3

+ + CO2 although the latter is more

exothermic than the former.102,111

As to the oxidation of C2H4 with a CQC p bond, the DFT

study indicated that approaching C2H4 with Ot��f in Ce2O4

+

to form a C–O bond is barrierless (Fig. 10) and the subsequent

steps to produce acetaldehyde are driven by the highly

favorable thermodynamics.102 One may conclude that for the

relatively reactive p system (alkenes), the extent of SD

localization or delocalization does not significantly affect the

rates of oxidation by Ot��f or Ot

�� containing clusters

(CekO2k+ and ZrkO2k

+). The SD values over Ot��f

(see Fig. 9) of CekO2k+ increases as the cluster size (k)

increases and the largest SD change occurs when k = 1

(CeO2+, 0.183 mB) goes to k = 2 (Ce2O4

+, 0.333 mB). Suchchange results in significantly enhanced reactivity for the C–H

s bond activation since CH4 and C2H6 activation by Ce2O4+

was observed while CeO2+ was not reactive with both CH4

and C2H6.135 In contrast, the rate constant of CeO2

+ + C2H4 -

CeO++C2H4O ison theorderof 1� 10�10 cm3molecule�1 s�1,135

which is still close to the rate of Ce2O4+ + C2H4 - Ce2O3

+ +

C2H4O.102

The nature of the electronic ground state of the bulk CeO2

has been controversial for a long time.141,142 The first scenario

features a formally fully occupied O 2p band and empty 4f

states (Ce 4f0) in the band gap,143,144 and the second one

involves a mixture of Ce 4f0 and Ce 4f1 O 2p hole

states.145,146 The unique SD distribution over CekO2k+

Fig. 6 DFT calculated energy profiles for hydrogen abstraction

reactions: (a) VxOyq + CH4 - VxOyH

q + CH3 in which

VxOyq = V2O5

+, V3O8, and V2O6�; and (b) V2O6

� + CnH2n+2 -

V2O6H�+ CnH2n+1 in which n = 1, 2, and 4 (n-butane). The relative

energies of the reactants, products, and the transition states for C–H

activation are shown.

Fig. 7 Time of flight mass spectra for reactions of Zr2Oy� with (a)

pure He, (b) 30 Pa C2H6, (c) 70 Pa C2H6, (d) 5 Pa C4H10, and (e) 70 Pa

C4H10 in a fast flow reactor. The relative signal intensities from 271 to

305 amu (for Zr2O6� and Zr2O7

�) are scaled by a factor of 1/4.

Adapted from ref. 125.

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(Fig. 9) disagrees with the first scenario, i.e. if the bulk CeO2

has the electronic ground state featured with the fully occupied

O 2p valence band (like ZrO2), one would expect that the

SOMOs of CekO2k+ have the character of pure oxygen 2p

orbitals (like ZrkO2k+), which is not true from the cluster

reactivity studies. Note that the electronic structures of a few

cerium containing compounds Ce(C8H8)2,147 Ce(C5H5)3

+,148

and Ce(C14H18)2149 are also unusual due to the special 4f

electron in cerium atom. The cluster study102 indicated that

cerium can be used to tune (by cluster sizes) the amount of SD

distributions over the Ot��f radicals that are generally reactive

with p while it can be much less reactive with s bonds of

hydrocarbon molecules. This clearly provides a clue

with which to enhance the selectivity (by decreasing the

byproducts H2O/CO2 formation that involves C–H bond

activation) without losing the reactivity of working catalysts

for partial oxidation of alkenes such as selective oxidation of

propylene that is very important in industry.150,151

3. Heteronuclear oxide clusters

The practical metal oxide catalysts are usually supported on or

mixed with other materials in heterogeneous catalytic

reactions. For example, V2O5 catalyst (active phase) may be

supported on Al2O3,152 SiO2,

153 TiO2,154 ZrO2,

155 CeO2,156

and so on. The mechanistic behavior of the support materials

in the surface reactions remains poorly understood and in

some cases, the usually considered inactive components (such

as CeO2 in VOx/CeO2 material) are finally characterized to be

of primary importance.157 The study of the O�� radicals

over heteronuclear oxide clusters (such as Vx1Six2Oyq for

V2O5/SiO2) may provide an opportunity to understand the

possible roles of both the active (such as V2O5) and inactive

(such as SiO2) phases in surface reactions.

The D value (extent of cluster over-oxidation) defined for

homonuclear TMO clusters in eqn (1) may be extended as,

D R 2y � Sinixi + q = 1 (4)

for the compositions of O�� containing heteronuclear oxide

clusters M1x1M2x2M3x3. . .Oyq, in which ni is the number of

valence electrons of elementMi that can be oxidized by oxygen

to +ni oxidation state. The DFT studies have indicated that a

Fig. 8 TOF mass spectra for reactions of CexOy+ (x= 2–7) with (a) 0% (for reference), (b) 0.5%, (c) 2.0%, and (d) 4.0% C2H4 seeded in helium

carrier gas. The numbers x,y and x,y,z denote CexOy+ and CexOy(C2H4)z

+, respectively. Adapted from ref. 102.

Table 2 Estimated absolute (k1) and relative (k1rel) rate constants for

reactions of Ce2O4+ and Zr2O4

+ clusters with small molecules

X

Ce2O4+ + X Zr2O4

+ + X

k1 k1rel k1 k1

rel

C2H4 4.6 � 10�10 1.0 3.2 � 10�10 0.7C2H6 1.1 � 10�11 2.5 � 10�2 3.2 � 10�10 0.7CO 1.9 � 10�12 4.1 � 10�3 1.8 � 10�10 0.4CH4 2.2 � 10�13 5.0 � 10�4 9.1 � 10�11 0.2

a k1 is in unit of cm3 molecule�1 s�1 and k1rel is relative to

k1(Ce2O4+ + C2H4). Adapted from ref. 102.

Fig. 9 DFT calculated profiles of SOMOs for CekO2k+ (k = 1–5)

clusters. The Mulliken spin density values in mB over the relevant

terminal oxygen atoms are shown. Adapted from ref. 102.

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few selected bimetallic oxide clusters with D = 1 (such as

V2Ti2O10�,158 TiVO5 and CrVO6,

36 and ZrScO4 and

ZrNbO5)159 contain O�� radicals.

Our recent experiments have indicated that under near room-

temperature conditions, the D = 1 heteronuclear oxide clusters

cations VAlO4+,103 V2O5(SiO2)1–4

+ and (V2O5)2SiO2+,104 and

V3PO10+105 can activate methane with rate constants (k1) on the

order of 10�10B10�11 cm3 molecule�1 s�1, while the D = 1

anions V2O5(SiO2)1–4O� and (V2O5)2SiO2O

� are able to

activate C–H bonds of n-butane with k1 B 10�11 cm3

molecule�1 s�1.160 The experiments thus suggest that the

above D = 1 clusters contain Ot�� radicals. However, the

DFT studies of the cluster structures (see Fig. 11 for

examples) provided an interesting result: the Ot�� radicals are

bonded with Al, Si, or P atoms rather than the vanadium atoms

in most of the clusters including VAlO4+, V2O5(SiO2)1–4

+,

V3PO10+, V2O5(SiO2)1–4O

�, and (V2O5)2SiO2O�. This is in

contrast with the traditional concept that transition metal

oxides (such as V2O5) are active phase of catalysts (such as

V2O5/SiO2) while the main group metal or non-metal oxides

(such as SiO2) are support materials and do not participate

directly in surface reactions.

The O�� radical species were suggested to be important in the

selective oxidation of methane, ethane, and benzene.18,22,161–166

It was usually considered that the O�� species are involved

directly with transition metal atoms. For example, the study of

methane conversion to methanol and formaldehyde over

vanadium oxide and molybdenum oxide supported on

mesoporous silica suggested a mechanism that the reactions

are initiated by the formation of O�� coordinated with V

and Mo at the surface.18,22 The heteronuclear oxides

VAlO4+, V2O5(SiO2)1–4

+, V3PO10+, V2O5(SiO2)1–4O

�, and

(V2O5)2SiO2O� are with diameters ranging from 0.6 to

0.8 nm. The cluster study thus implies that for catalysts such

as V2O5/SiO2 and V2O5/Al2O3 in nano-size and further smaller

regions, or for related surface defect sites, one should take into

account that the usually considered support materials (Si–O and

Al–O) may well play an important role in surface reactions such

as in the process of C–H activation. The vanadium loaded SiO2

was used as photocatalyst for the partial oxidation of methane.

It was confirmed that the high photo-activity for formaldehyde

formation (1.8–2.6% yield) with high selectivity (80–92%) was

obtained when the loading amount of V was very low, ca.

0.01–0.1 mol%. In contrast, higher loading of V resulted in

lower activity and selectivity to formaldehyde.35 We speculate

that the O�� centres for the methane activation in the

photocatalytic conversion of methane at very low vanadium

loading can be bounded with Si rather than with V.

The study of the O�� containing heteronuclear oxide clusters

also resulted in some new mechanistic details that had not been

realized in the study of related homonuclear oxide systems.

Fig. 11 shows that in V2O5(SiO2)2O� and V2O5(SiO2)4O

�

clusters, the SDs are delocalized over two oxygen atoms

(Ot��f), which is also the case for V2O6

� cluster.36,64

However, the V2O5(SiO2)2O� and V2O5(SiO2)4O

� clusters

react much faster than V2O6� does in the reaction with

C4H10, in contrast with the idea that Ot��f radicals have

relatively slow reactivity toward the C–H activation.102 The

DFT study indicated that the SDs can be localized if one of the

two Si–Ot��f or V–Ot

��f bonds in each cluster is lengthened

and the other is shortened. However, the energy cost for

such SD localization process is significantly smaller in

V2O5(SiO2)2O� [and possibly in V2O5(SiO2)4O

�] than in

V2O6�, leading to higher reactivity of the former than the

latter toward C–H activation, in agreement with the

experiments.160 It indicates that the reactivity of Ot��f

toward C–H activation depends not only on the amount of

SDs but also on the energy cost for localizing the SDs.

The comparative study of methane activation by V3PO10+

and V4O10+ clusters105 suggested that intra-cluster SD

transfer in high symmetry V4O10+ and P4O10

+ clusters:

O1��–M–Ob–M=O2 - O1=M–Ob–M–O2

�� (M = V or P,

and O1 and O2 are different terminal oxygen atoms), is fast

(B109–1012 s�1) and the reacting CH4 molecule further

Fig. 10 DFT calculated reaction pathway for Ce2O4+ (2A) + C2H4 (

1Ag)- Ce2O3+ (2A0) + CH3CHO (1A0). The zero-point vibration corrected

energy (DH0 K in eV) and Gibbs free energy at 298 K (DG298 K in eV) are given in parentheses as (DH0 K/DG298 K). Some bond lengths in 0.1 nm are

given. Adapted from ref. 102.

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enhances the speed of the SD transfer by lowering down

the transfer barrier. In contrast, the SD transfer in the

heteronuclear cluster V3PO10+ is impossible due to

unfavorable thermodynamics. This means that the number of

effective Ot�� centres in V4O10

+ or P4O10+ is larger than that

in V3PO10+, which rationalizes the experimental result stating

that the rate constant of V4O10+ + CH4 (almost the same as

that of P4O10+ + CH4)

98,101 is larger than that of V3PO10+ +

CH4 by a factor of 2.5 � 0.2. The model cluster study also

suggested that the hole-centres (O��) that can be photo-

generated in bulk materials may transfer preferentially

toward pre-adsorbed small molecules over the surfaces.105

4. Additional remarks

It has been demonstrated that for groups 3–7 metals (except Cr

and Mn) and related heteronuclear systems, the O�� radicals

generally exist over the D = 1 oxide clusters (eqn (1) and (4)).

We have tried to identify O�� radicals over later transition

metal oxide clusters including FexOy� (such as Fe2kO3k

+167

and Fe2kO3k+1�) and CoxOy

� (x Z 2) by searching for

the cluster reactivity toward C–H activation at near room-

temperature conditions. However, none of the studied clusters

is able to activate CH4, C2H6, or C4H10. This implies that the

later transition metal oxides such as Fe2O3 and Co3O4 are

fundamentally different from the early transition metal oxides

such as Sc2O3, TiO2, etc. in terms of electron excitation and

hole (O��) formation.168–169 The later transition metal oxide

clusters (even for those with metal in the normally highest

oxidation state, such as Fe2kO3k)170 are also very difficult for

the quantum chemistry studies because of the additional

valence d electrons (3d5 for Fe3+) and their possible partial

participations in the bonding with oxygen.

For early transition metals and some main group metals, the

oxygen-more-rich (D = 2, 3,. . .) oxide clusters can be

superoxide (O2��) or peroxide (O2

2�) complexes.110,171–174

However, our experimental investigation on the early

transition metal oxide cluster cations indicated that the

D > 1 cluster systems are inert with CH4 while many of

the D = 1 systems can activate CH4 with rate constants on

the order of 10�10 cm3 molecule�1 s�1 under near-room-

temperature conditions.90 The chemical reactivity and

selectivity of the D > 1 TMO clusters remain poorly

understood although it is important to study these clusters

for interpretation of the chemistry of the surface O2�� and

O22� species.8,175 Zhou and co-authors have recently identified

that the neutral cluster TaO4 (D = 3) can spontaneously

activate dihydrogen at cryogenic temperatures85 while we

have recently characterized that the oxygen-very-rich cluster

Zr2O8� (D = 7) can activate C–H bonds of n-butane under

mild conditions.176 It is noticeable that for some of the D = 2

TMO clusters, such as WO4177 and VTi3O10

�,158 the DFT

studies suggested that there are two O�� centres (bi-radicals) in

each cluster. The reactivity study of these bi-radical species

may be important to interpret the chemistry of coupled O��

dimer species (O�� O��) over the metal surfaces. It has been

pointed out that it is difficult to characterize such adsorbed

dimer species due to complications such as heterogeneity of

isolated O�� sites and transition of a pair of O�� to peroxide

ion O22� in the condensed phase study,9 while a gas phase

cluster study may provide mechanistic details for such species.

5. Conclusions

The mono-nuclear oxygen-centred radicals (O��) generally

exist over oxide clusters of groups 3–7 metals (except Cr

and Mn) and related heteronuclear systems of which the

compositions satisfy eqn (1) or (4). In terms of the C–H

activation of alkanes such as CH4, the charge state of the

TMO clusters and the extent of spin density localization largely

influence the reactivity of O�� radicals: (1) the anions are

generally much less reactive than the corresponding cations

while the neutrals may be in the middle; (2) the O�� containing

cluster anions may still be as reactive as the OH free radical;

and (3) the localized spin densities over a single O atom in the

cluster lead to relatively high reactivity. The cerium element

may be used to tune the amount of spin densities over the O��

radicals. The oxygen atom transfer usually occurs in the

reactions of the O�� containing clusters with CO, alkenes

(sometimes with CQC bond cleavage), etc. The intra-cluster

spin density transfer in some of the O�� containing clusters can

be fast and the speed of the transfer can be enhanced by the

interacting small molecules. The O�� radicals are bonded with

the Al, Si, or P rather than the V atoms in most of the studied

V–Al, V–Si, and V–P heteronuclear oxide clusters. The

chemistry of O�� containing TMO clusters can be closely

related with that of the surface O�� species in terms of C–H

activation, oxygen atom transfer, alkene CQC double bond

cleavage, hole transfer, support effect, etc. in catalysis or

Fig. 11 DFT calculated structures and profiles of SOMOs for

V2O5(SiO2)1–4+ (a) and V2O5(SiO2)1–4O

� (b) clusters. The Mulliken

spin density values in mB over the relevant oxygen atoms are given in

parentheses. Some bond lengths are given in 0.1 nm. Adapted from

refs. 104 and 160.

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photocatalysis. In general, the investigation of O�� containing

TMO clusters provides a good chance to learn the chemistry of

the reactive O�� species in a bulk system as well as that of the

metal valence d and f electrons.

Acknowledgements

This work was supported by the Chinese Academy of Sciences

(Hundred Talents Fund), the National Natural Science

Foundation of China (Nos. 20703048, 20803083, and

20933008), CMS Foundation of the ICCAS (No. CMS-

LX200902), and the 973 Program (No. 2011CB932302).

Notes and references

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2 J. L. G. Fierro, Metal Oxides, Taylor & Francis Group, BocaRaton, FL, 2006.

3 R. A. van Santen and M. Neurock, Molecular HeterogeneousCatalysis, Wiley-VCH, Weinheim, Germany, 2006.

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Documents

Characterization and reactivity of oxygen-centred radicals over transition metal oxide clusters