Experimental and Theoretical Study of the Reactions between Cerium

ARTICLE pubs.acs.org/JPCC

Experimental and Theoretical Study of the Reactions between Cerium Oxide Cluster Anions and Carbon Monoxide: Size-Dependent Reactivity of CenO2n+1
(n = 1
21) Xiao-Nan Wu,†,‡ Xun-Lei Ding,† Shu-Ming Bai,†,‡ Bo Xu,†,‡ Sheng-Gui He,*,† and Qiang Shi*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China

bS Supporting Information ABSTRACT: Reactions of cerium oxide cluster anions with carbon monoxide are investigated by time-of-flight mass spectrometry and density functional theory computations aided with molecular dynamics simulations. Interesting size-dependent reactivity of the CenO2n+1
cluster series with n = 1
21 is observed: (1) the small n = 1
3 clusters have no or very low reactivity toward CO, (2) the large n = 4
21 clusters can oxidize CO to produce CO2, and (3) the n = 4 (Ce4O9
), 6 (Ce6O13
), 7 (Ce7O15
), and 12 (Ce12O25
) clusters have relatively higher reactivity than their neighboring systems Ce3O7
, Ce5O11
, Ce8O17
, etc. Theoretical study indicates that the CenO2n+1
clusters contain oxygen-centered radicals (O
•) and the nature of the spin density distributions within the clusters controls the experimentally observed size-dependent reactivity. The experiment and theory in this study suggest that the metal oxide clusters as large as Ce21O43
can contain the reactive O
• centers, at which the size may be large enough to mimic related active sites in condensed phase catalysts. Oxidation of CO by O2 at low temperature is of widespread importance and reactive oxygen species including O
• are usually involved. The nature of the O
• radicals is demonstrated to be able to further address the goodness of nanocrystalline CeO2 in the low-termperautre CO oxidation.

1. INTRODUCTION Oxidation of carbon monoxide by molecular oxygen at low temperature (room temperature and below) is important in terms of scientific, economic, and environmental considerations.1 As an important type of catalyst,2 cerium dioxide and CeO2containing materials have been investigated to catalyze CO oxidation at temperatures as low as possible. The CeO2 nanorods3 and nanotubes4 have been recently reported to be active for CO oxidation around 150 °C. In addition, the nanocrystalline CeO2 supported Au was demonstrated to be able to catalyze CO oxidation at 5 °C.5 Superoxides (O2
•), peroxides (O22
), and mononuclear oxygen-centered radicals (O
•) are common reactive oxygen species (ROS)6 over metal oxides and the ROS are proposed to be involved in the low-temperature oxidation of the stable CO molecules over CeO2 catalytic materials.7 However, the electronic, thermodynamic, and kinetic details involved with the ROS over CeO28 at the molecular level are far from clear and thus remain to be addressed in order to further understand the catalytic behavior of CeO2 in low-temperature CO oxidation. To address the nature of ROS and related active sites over metal oxide surfaces at the molecular level, one important way is to study the bonding and reactivity of metal oxide clusters under isolated, controlled, and reproducible conditions,9,10 which is very useful in the case that the surface ROS have too low concentrations r 2011 American Chemical Society

or too short lifetimes for traditional investigations. In a previous work,11 we studied the reactivity of positively charged cerium oxide clusters and identified that the active sites of CenO2n+ (n = 2
6) clusters are (OCeO)• heteroatom radicals rather than the (pure) oxygen-centered radicals O
• as usually identified in oxide clusters of d-block transition metals and main group elements.9g,10e,12
14 This implies that the electronic structure and reactivity of the ROS over bulk CeO2 can be complex. To have a further understanding of the ROS over CeO2 surfaces, it is desirable to study the negatively charged cerium oxide clusters, which is the main topic of this study. In addition, the mechanistic studies for the Au/CeO2 catalysts indicated that there is charge transfer in the interface between gold and CeO2, which causes positively charged Au and negatively charged CeO2 in oxidative environment.15 Since charge states may influence the bonding and reactivity of atomic clusters significantly,12,16 it is necessary to study the anionic cerium oxide clusters in order to better understand the nature of negatively charged CeO2 systems. It is noticeable that the reactions of the anionic metal oxide clusters12c,17 have been much less studied compared with those Received: March 4, 2011 Revised: June 9, 2011 Published: June 09, 2011 13329

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The Journal of Physical Chemistry C of the cationic ones partly due to relatively lower oxidative reactivity of anions versus cations. Strong size-dependent reactivity has been often identified for metal clusters Mnq (M is a metal atom and q is the charge number) and the size dependence can be interpreted in terms of electronic and geometric issues, such as shell structures,18 spin conservations,19 and complementary active sites.20 For metal oxide clusters MnOmq, the situation is more complex as there are two independent size variables n and m. Recently, we have shown that for metal oxide clusters MnOmq, it is useful to define a value Δ  2m
nx + q (in which x is the number of valence electrons of M) to clarify the oxygen richness or poorness for the cluster systems.9g,21 Available experimental12
14,17 and theoretical21 results indicate that oxide clusters of early transition metals (groups 3
7 and 3d
5d metals except Cr and Mn) with Δ = 1 are the O
• radical species. In addition, the Δ = 2 and 3 cluster series can be the O22
and O2
• species, respectively.9g It thus suggests that one should consider the metal oxide clusters with the same Δ values when investigating the size-dependent reactivity. When the definition of Δ is applied to the cerium oxide cluster system CenOmq, the Δ = 1 clusters are CenO2n+ and CenO2n+1
of which the former series has been studied.11 We will show that the CenO2n+1
series in a size region from n = 1 up to n = 21 has very significant and interesting size-dependent reactivity toward CO under near room-temperature conditions. Such size-dependent reactivity can be well rationalized based on the DFT calculated electronic and geometric structures of the O
• radical species over the CenO2n+1
clusters. The nature of the O
• species discovered in this study may be in parallel with the goodness of nanocrystalline CeO2 for low-temperature CO oxidation.5 Note that monocerium oxides were extensively studied22 and a few investigations on larger systems CenOm (n e 6),23 CenOm(OH)k
(n e 16 and k 6¼ 0),24 and CenOm+ (n e 16)11 have been recently reported.

2. METHODS 2.1. Experimental. The experimental setup for a pulsed laser ablation/supersonic nozzle coupled with a fast flow reactor is similar to the one described in our previous studies.11,14,16 Only a brief outline of the experiments is given below. The CenOm
clusters are generated by laser ablation of a rotating and translating cerium metal disk in the presence of about 1% O2 seeded in a helium carrier gas with a backing pressure of 5 atm. A 532 nm (second harmonic of Nd3+:yttrium aluminum garnet, YAG) laser with energy of 5
8 mJ/pulse and repetition rate of 10 Hz is used. The prepared gas mixture O2/He is passed through a 10 m long copper tube coil at low temperature (T = 77 K) before entering into a valve (General Valve, series 9) that generates the gas pulses. This method nearly eliminates contributions of undesirable hydroxo species CexOy(HO)zq (z > 0) in our cluster distribution possibly by trapping down residual water in the gas handling system. Similar treatment (T ≈ 200
260 K) is also applied in the use of the reactant gases (see below). The clusters formed in a gas channel (2 mm diameter  25 mm length) are expanded and reacted with He diluted CO or other small molecules in a fast flow reactor (6 mm diameter  60 mm length). The reactant gases are pulsed into the reactor 20 mm downstream from the exit of the narrow cluster formation channel by a second valve (General Valve, series 9). By use of the method in ref 25, the instantaneous total gas pressure in the

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fast flow reactor is estimated to be around 260 Pa at T = 350 K. The number of collisions that a cluster (radius = 0.5 nm) experiences with the bath gas (radius = 0.05 nm, T = 350 K, P = 260 Pa) in the fast flow reactor is about 50 per 1 mm of forward motion. This corresponds to a collision rate of 5  107 s
1 for an approaching velocity of 1 km/s. Since the reactor length (60 mm) is much longer than 1 mm, the intracluster vibrations are likely equilibrated to close to the bath gas temperature before reacting with the diluted CO molecules. The bath gas temperature is around 300
400 K considering that the carrier gas can be heated during the process of laser ablation.26 After reaction in the fast flow reactor, the reactant and product ions exiting from the reactor are skimmed (3 mm diameter) into a vacuum system of a time-of-flight mass spectrometer (TOFMS) for mass and abundance measurements. Ion signals are generated by a dual microchannel plate detector and recorded with a digital oscilloscope (LeCroy WaveSurfer 62Xs) by averaging about 2000 traces of independent mass spectra (each corresponds to one laser shot). The uncertainties of the relative peak intensities are within 10%. The CenOm
clusters with mass range of 150
4000 amu can be generated and detected by the TOF-MS with sufficient resolution to resolve oxygen. 2.2. Computational. The DFT and molecular dynamics (MD) calculations have been employed to study the structures of CenO2n+1
clusters. The DFT calculations are performed with the Gaussian 03 program27 and the hybrid B3LYP exchangecorrelation functional.28 The triple-ζ basis set with Stuttgart/ Dresden relativistic effective core potentials29 is adopted for Ce (as well as for Au in test calculations). The all electron TZVP basis sets30 are used for O and all other related light atoms. The choice of the (conventional) DFT functional and the basis sets for the cerium oxide cluster system has been validated in the previous studies.11,23a The DFT calculations are carried out only for the small clusters CenO2n+1
(n e 6) and some related CenOmq (n e 6) systems. In the DFT geometry optimizations, a high number of guess structures of the CenO2n+1
(n e 6) clusters are constructed based on chemical intuition and a systematic consideration of the topological conformations14b and are from the MD simulations. The MD simulations are performed with a simulated annealing method31 using a modified version of the DL_POLY program.32 The CenO2n+1
cluster consists of nCe4+, 2nO2
, and one O
ion, and the shell model33 is used for the interatom potentials. The parameters for Ce4+
Ce4+, Ce4+
O2
, and O2

O2
interactions are based on those in ref 34, which have been used to study cerium oxide nanoparticles.23b,35 Details of the potential model and the MD simulation can be found in the Supporting Information. The MD calculations determine several structures for each of the CenO2n+1
(n = 2
6) clusters and these MD geometries are further optimized by the DFT calculations (see above). It is noticeable that for each structure isomer, the geometrical parameters (bond lengths and angles) by the MD are generally close to those by the DFT. In addition, the global minimum structures from the MD simulations also correspond to the lowest or the low-lying energy structures from the DFT calculations. These indicate that the adopted potential model and parameters are quite useful and can be used to predict geometrical structures for larger CenO2n+1
(n > 6) clusters for which the DFT calculations are too slow to perform with our computational facilities. The structures of Ce12O25
are thus determined with the MD calculations. 13330

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reactor,39 the peak intensities of CenO2n+1
(n = 4
21) clusters generally decrease while those of CenO2n
(n = 4
21) increase. It also tells that the presence of CO in the reactor does not cause apparent signal depletion for the oxygen-rich clusters CenO2n+2
and CenO2n+3
generated in the cluster source (Figure 1a), indicating that these clusters are not reactive with CO under the experimental conditions. For each of the n values from 4 to 21, the magnitude of signal increase for CenO2n
roughly equals that of the signal decrease for CenO2n+1
and the magnitude also varies strongly with n. Different CO pressures (0.3
2.0 Pa) in the reactor are also tested, and the above experimental results are well reproducible. The experiments suggest the following oxygen atom transfer reactions with size-dependent rate constants Cen O2n+1
+ CO f Cen O2n
+ CO2 , n ¼ 4
21

ð1Þ

Note that the CenO2n+1
(n = 1
3) clusters are also generated in the cluster source while there is no indication of the above oxygen atom transfer reaction for these small systems. The firstorder rate constant (k1) of a reaction in the fast flow reactor can be estimated by using the equation k1 ¼ lnðI0 =IÞ=ðFΔtÞ Figure 1. (a) Time of flight mass spectrum for distribution of CenOm
clusters in absence of CO in the reactor, (b) difference spectrum (spectrum in presence of 1.0 Pa CO in the reactor minus spectrum in panel a), and (c) relative rate constants k1rel [=k1(CenO2n+1
+ CO)/ k1(Ce4O9
+ CO)] determined with 0.3 Pa CO (hollow circle) and 1.0 Pa CO (solid circle) in the reactor. The inset in panel a indicates the multiple peaks for Ce4Om
cluster series with m = 8
11.

The DFT method at the same level of theory for the cluster structure optimizations has also be used to study the reaction mechanisms for a few reactions involved with the cerium oxide clusters. This includes geometry optimizations of reaction intermediates and transition states (TSs) through which the intermediates transfer to each other. The TS optimizations are performed by employing the Berny algorithm.36 The initial guess structures of the TS species are obtained through relaxed potential energy surface scans using appropriate internal coordinates. Vibrational frequency calculations are performed to check that reaction intermediates and TS species have zero and one imaginary frequency, respectively. Intrinsic reaction coordinate calculations37 are also performed so that a TS connects two appropriate local minima. Test calculations indicate that basis set superposition error (BSSE)38 is negligible, so the BSSE is not taken into consideration in this study. The DFT calculated energies reported in this study are the zero-point vibration corrected (ΔH0K) or the relative enthalpies/free energies (ΔH298K/ΔG298K) of formation under standard conditions (298.15 K, 1 atm).

3. RESULTS 3.1. Experimental Results. Figure 1a presents a typical TOF mass spectrum for the distribution of CenOm
(n = 3
21) clusters generated under the condition of 1% O2 seeded in 5 atm He. For each of the Cen cluster series, there are multiple peaks that can be assigned with m = 2n, 2n + 1, 2n + 2, and 2n + 3. For example, Ce4O8
, Ce4O9
, Ce4O10
, and Ce4O11
are generated for n = 4 cluster series. The difference spectrum in Figure 1b indicates that upon the reaction with 1.0 Pa CO in the fast flow

ð2Þ

in which I and I0 are signal magnitudes of the clusters in the presence and absence of reactant gas (CO or other molecules), respectively, F is the molecular density of the reactant gas (see ref 25 for the method to calculate F), and Δt is the effective reaction time that is estimated as l/v (l is the reactor length ≈60 mm and v is the cluster beam velocity ≈1 km/s). The determined rate constant of k1(Ce4O9
+ CO) is 7.8  10
11 or 8.6  10
11 cm3 molecule
1 s
1 by using the experimental data obtained with 1.0 or 0.3 Pa CO in the reactor, respectively. Because it is hard to determine accurately the F and Δt values in the pulse experiment, the absolute k1 value can be systematically under- or overestimated. The systematic deviation may be within a factor of 5 by comparing the rate constants from our fast flow reaction experiments with those from other independent experiments for known reactions such as CeO2+ + C2H4 f CeO+ + C2H4O11,22c and V4O10+ + CH4 f V4O10H+ + CH3.13a,14a,b The relative rate constants [k1(CenO2n+1
+ CO)/ k1(Ce4O9
+ CO)] that are independent of the F and Δt values are plotted in Figure 1c for two different CO pressures. The k1 values determined with low CO pressure (0.3 Pa) in the reactor are generally larger than those with high pressure (1.0 Pa), and the rate differences are well above the experimental uncertainties for the reactions of CO with Ce6O13
, Ce7O15
, and Ce12O25
. This indicates that these clusters may have isomeric structures significantly populated in the experiments, which will be further discussed based on the theoretical results below. Figure 1c indicates that no matter how low or high the CO pressure (0.3
1.0 Pa) used, the Ce4O9
, Ce6O13
and Ce7O15
, and Ce12O25
are significantly more reactive than their neighboring clusters (Ce3O7
, Ce5O11
, Ce8O17
, Ce11O23
, and Ce13O27
). To further confirm the above size-dependent reactivity of the CenO2n+1
clusters, the reactions of CenOm
with a few small hydrocarbon molecules are also studied. All of the generated CenOm
clusters (Figure 1a) do not oxidize or associate with unsaturated hydrocarbons C2H2 and C2H4 under similar conditions used for the reactions with CO. Figure S1 (Supporting Information) shows that the deuterium atom abstraction from n-C4D10 by CenO2n+1
is apparently observed for n = 4 rather 13331

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Figure 2. Density functional theory calculated structures and profiles of spin density distributions for CenO2n+1
(n = 1
6) clusters. For each CenO2n+1
, the notation n-i denotes the ith isomer in terms of relative energy that is given below the structure and in electronvolts. For the lowest energy isomers, the spin density values over oxygen atoms in μB and some Ce
O bond lengths in picometers are given. More details including symmetries and electronic states can be found in the Supporting Information.

than for n = 2, 3, and 5. This is consistent with the relatively high reactivity of Ce4O9
toward CO shown in Figure 1. 3.2. Computational Results. The structures and profiles of spin density (SD) distributions for the lowest and some low-lying energy isomers of CenO2n+1
(n = 1
6) are shown in Figure 2. The results for higher-lying energy isomers are deposited in the Supporting Information. The CeO3
(isomer 1-1) has a planar (C2v) geometry and the SDs of the cluster are almost equally distributed over the three oxygen atoms. The lowest energy structure of Ce2O5
(2-1) has a high symmetry (D3h), and the SDs are distributed over all of the five O atoms. The SDs are still delocalized over three oxygen atoms in Ce3O7
(3-1) while in Ce4O9
(4-1) the SDs are highly localized in the 2p orbital of a single terminal oxygen atom (Ot). Such an Ot atom with SDs close to one unit μB is a mononuclear oxygen centered radical (denoted as Ot
•) because it has similar electronic structure as the free O
ion does. The Ot atoms with a fraction (0.12
0.40 μB) of the SDs in CeO3
(1-1), Ce2O5
(2-1), and Ce3O7
(3-1) may be denoted as Ot
•f. The Ce
Ot
• bond in 4-1 is significantly longer than the Ce
Ot
•f bonds in 1-1 (194 and 195 pm), 2-1 (187 pm), and 3-1 (192 pm), so it is expected that the Ot
• radical (or 4-1) is more oxidative than the Ot
•f radicals (or 1-1, 2-1, and 3-1) in the reaction with small molecules. The DFT calculated reaction pathways for Ce3O7
(3-1) + CO f Ce3O6
+ CO2 and Ce4O9
(4-1) + CO f Ce4O8
+ CO2 are plotted in Figure 3. Both of the reactions are highly exothermic. The approach of CO with the Ot
• radical in 4-1 or the Ot
•f in 3-1 to form a second C
O bond (IM1f TS1f IM2 or IM10 f TS10 f IM20 ) is expected to be the rate-limiting step

since the subsequent process to form CO2 is driven by the highly favorable thermodynamics. The energy (ΔH0K) of TS1 is negative (
0.066 eV) with respect to the separated reactants Ce4O9
(4-1) and CO. In addition, the TS1 is only above IM1 by 0.003 eV in terms of the ΔH0K. These indicate that the approach of CO with Ce4O9
(4-1) to form IM2 is highly favorable. In contrast, the TS10 is above the separated reactants [Ce3O7
(3-1) + CO] and IM10 by 0.03 and 0.10 eV, respectively, suggesting an unfavorable (or much less favorable with respect to Ce4 system) process for the approach of CO with Ce3O7
(3-1). The DFT calculations indicate that the approach of CO with the Ot
•f radicals in Ce2O5
(2-1) is even more difficult because the transition state that corresponds to TS1 or TS10 in Figure 3 is 0.22 eV (ΔH0K) higher in energy than the separated reactants Ce2O5
(2-1) and CO. The reaction mechanism study thus confirms that the CO oxidation by Ce4O9
(4-1) that has highly localized SDs (Figure 2) is much more favorable than the oxidation by Ce2O5
(2-1) or Ce3O7
(3-1) that has delocalized SDs, which supports the experimental result that Ce4O9
can deliver an oxygen atom to CO quite easily (Figure 1) while there is no indication of such oxygen atom transfer process in the reaction of CenO2n+1
(n = 1
3) clusters with CO. The reactions of Ce3O7
(3-1) + n-C4H10 f Ce3O7H
+ 2-C4H9 and Ce4O9
(4-1) + n-C4H10 f Ce4O9H
+ 2-C4H9 are also studied by the DFT calculations (Figure S11, Supporting Information) that support the experimental results that Ce4O9
is reactive while Ce3O7
is unreactive in the reaction with n-C4D10 (Figure S1, Supporting Information). The size-dependent reactivity observed for the reactions of small clusters CenO2n+1
(n = 1
4) 13332

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Figure 3. Density functional theory calculated reaction pathways for [Ce4O9
(4-1; Cs, 2A00 ) + CO (1Σ+)] (R1) f [Ce4O8
(Cs, 2A00 ) + CO2 (1Σg+)] (P1) and [Ce3O7
(3-1; Cs, 2A0 ) + CO (1Σ+)] (R10 ) f[Ce3O6
(Cs, 2A0 ) + CO2 (1Σg+)] (P10 ). The relative energies of the reaction intermediates (IM) and transition states (TS) in terms of ΔH0K (eV) and ΔG298K (eV) are given as ΔH0K/ΔG298K. The structures (some bond lengths in picometers and bond angles in deg) are given for the Ce4 reaction system.

with CO and n-C4D10 has now been well rationalized. This also suggests that the adopted DFT method for the electronic structure calculations (Figure 2) is reliable. The DFT calculations predict an interesting electronic structure for the lowest energy isomer of Ce5O11
(5-1): most of the SDs (0.89 μB) are distributed over one 2p orbital of a bridgingly bonded oxygen atom (Ob). Such an Ob atom can also be considered as a mononuclear oxygen-centered radical (denoted as Ob
•). Unlike the Ot
• radical of 4-1 that is 1-fold coordinated with Ce, the Ob
• radical of 5-1 is 2-fold coordinated and it is expected that the approaching of CO with Ob
• to form an OCO moiety is less favorable than the process of IM1f TS1f IM2 in Figure 3. The DFT calculations indicate that the energy (ΔH0K) of the transition state for CO approaching the Ob
• radical in 5-1 (Figure S12) is positive (0.07 eV) with respect to the separated reactants (Ce5O11
/5-1 + CO), in contrast to the negative energy (
0.07 eV) of TS1 in Figure 3 for the Ce4 reaction system. This is consistent with the sharp decrease of the rate constants from n = 4 to n = 5 in Figure 1c. The cluster isomer 5-2 has an Ot
• radical and is expected to be as reactive as 4-1 in the reaction with CO. However, the 5-2 may be only slightly populated (≈9%)39 in the cluster source, which rationalizes the observed reactivity of Ce5O11
(Figure 1). The DFT calculations suggest that there is an Ot
• radical in the lowest energy isomer of Ce6O13
(6-1) and the isomer 6-2 with an Ob
• radical is slightly higher in energy, which is an opposite situation in comparison with the Ce5O11
cluster system. This explains the sharp increase of the rate constants from n = 5 to n = 6 in Figure 1c since the Ot
• radical is highly reactive with CO as shown for Ce4O9
+ CO in Figure 3. The cluster isomer (such as 6-2) with Ob
• radical that is much less reactive than Ot
• may also be populated in the cluster source. The experimental rate constant k1(Ce6O13
+ CO) determined with eq 2 is thus between k1(Ce6O13
/6-1+CO) (denoted as k1t) and

k1(Ce6O13
/6-2+CO) (denoted as k1b and k1b , k1t). At the low pressure limit (k1tFΔt ≈ 0), the experimental k1(Ce6O13
+ CO) value can be very close to k1t while at high CO pressures (k1tFΔt . 1), k1(Ce6O13
+ CO) can be significantly smaller than k1t or even close to k1b. This can be responsible for the decrease of the observed rate constant k1(Ce6O13
+ CO) [as well as k1(Ce7O15
+ CO)] by a factor of about 2 when CO pressure in the reactor increases from 0.3 to 1.0 Pa (Figure 1c). The relatively high reactivity of Ce7O15
and Ce12O25
clusters shown in Figure 1 suggests that the lowest energy isomer for each of them contains one Ot
• radical (as in 4-1 and 6-1). The MD simulation indicates that the global minimum of Ce12O25
(Figure S5, Supporting Information) is composed of a well-structured (Ce4+)12(O2
)24 moiety and an O
ion that is terminally bonded (Ot
• radical) in the cluster.

4. DISCUSSION 4.1. Spin Density Distribution Controls the Size-Dependent Reactivity. The experiment (Figure 1) identifies interest-

ing size-dependent reactivity for reactions of CenO2n+1
with CO in the size region of n = 3
6: (1) reactivity increase or appearance at n = 3 f 4, (2) reactivity decrease at n = 4 f 5, and (3) reactivity reincrease at n = 5 f 6. Table 1 lists the DFT calculated Ce
O bond energies [D0(Ce
O)] of CenO2n+1
as well as the electron detachment energies for possible photoelectron spectroscopic studies. The oxidation of CO by CenO2n+1
(eq 1) may be controlled by the bond energies while the D0(Ce
O) values of CenO2n+1
decrease monotonically for n = 3
6, which does not correlate with the experimentally observed oscillation of the rate constants (Figure 1c). In addition, the O
CO bond energy is 5.45 eV40 (5.44 eV by B3LYP) and the oxidation of CO by CenO2n+1
with n = 1
6 are all highly exothermic. These indicate that the size-dependent reactivity 13333

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Table 1. Density Functional Theory Calculated Bond Dissociation (ΔH0K) and Vertical Electron Detachment Energies (in eV) of the CenO2n+1
Clusters CenO2n+1
f

2

CenO2n+1
f

2

CenO2n


3




1

2

n

2

3

+ O

CenO2n+1 + e

Table 2. The Nearest Ce
Ce Distance (dmin), Mulliken Atomic Charge (Q), Electron Population (Ne), and Spin Density Value (σ) for Selected Cluster Isomers in Figure 2

CenO2n+1
f

1-1

CenO2n+1 + e


dmin(Ce
Ce)

2-1 298

3-1 349

4-1 360

5-2 357

6-1 374

4-2 347

/pm

4.32

3.38

3.15

2

4.38

3.92

4.09

Q(Ce)/|e|a

0.70

0.87

0.96

1.13

1.18

1.13

3

3.65

4.42

4.78

Ne(Ce 4f)a

1.09

1.06

0.99

0.87

0.85

0.81

1.01

σ(Ce)/μBa

0.09

0.04

0.02
0.04
0.04
0.03

0.13

σ(Ot
• or Ot
•f) 0.32

0.12

0.40

0.44

6

3.22 3.11 3.00

4.51 4.50 4.81

5.35 5.00 5.37

is not caused by the thermodynamics while the kinetics should play an important role. The theoretical results such as those in Figures 2 and 3 suggest that the kinetics is mainly controlled by the electronic structure, specifically, the SD distribution within the clusters. 4.1.1. Spin Density Delocalization versus Localization. The DFT study indicates that the SDs are delocalized in small CenO2n+1
clusters (1-1, 2-1, and 3-1) while the SDs are highly localized in large CenO2n+1
systems (4-1 and 6-1), which correlates with relatively large rate constants of Ce4O9
and Ce6O13
with CO (Figure 1c). It is important to note that the anionic Ce4O9
(Δ = 1 cluster)21 is also much more reactive than the cationic counterpart Ce4O8+ (Δ = 1) in the reaction with CO: k1(Ce4O9
+ CO) and k1(Ce4O8+ + CO) values are about 8  10
11 and 1  10
12 cm3 molecule
1 s
1, respectively.41 This is in sharp contrast with the general consideration that cationic metal oxide clusters are usually much more oxidative than the corresponding anionic systems as discovered from the investigations of many d-block transition metal oxide clusters.9g,12
14,17 For example, the rate constant of Zr2O4+ (Δ =1) + CO is larger than that of Zr2O5
(Δ = 1) + CO by more than one order of magnitude.12b,c The reactivity anomaly for the cerium system [k1(Ce4O9
+ CO) . k1(Ce4O8+ + CO)] can be well explained if we recall that Ce4O8+ is a heteroatom radical in which the SDs are delocalized over one Ce and two O atoms.11 It may be now a good time to establish a concept that the issue of SD (or unpaired electron) delocalization versus localization really counts (exists and plays important role) when one interprets the chemistry of atomic clusters as well as relevant surfaces.2c This concept has been demonstrated in our previous work11 by comparing the reactivity of different types of metal oxide clusters: ZrnO2n+ and CenO2n+ with n = 2
5 although one may argue that the reactivity difference for different MnO2n+ (M = Zr and Ce) systems can also be related with other issues such as M
O bond energies and lengths, details of the reaction mechanisms for MnO2n+ + CO, etc. This study firmly indicates that the concept of SD delocalization versus localization has to be considered for the same cluster system CenO2n+1
to explain the reactivity appearance for CenO2n+1
+ CO at n = 4 (Figure 1c). Table 2 shows that the issue of SD delocalization versus localization is well correlated with the cluster size. The nearest Ce
Ce distances (dmin) of the small clusters (2-1 and 3-1) are much shorter than those of the bulk CeO242 in which the Ce atoms can be considered to be Ce4+. The short Ce
Ce distances could favor (and vise versa) less charge distributions on Ce (the third row of Table 2), which reduces the Columb repulsion between Ce atoms. In addition, Ce3+ ([Xe]4f1) is also a quite stable oxidation state due to relatively large fourth ionization energy of

383b

a

1

4 5

bulk

0.99

0.99

0.99

1.01

/μB a

Only the distances involving the cerium atom that is bonded with the Ot
• or Ot
•f atom are considered. The Q, N, and σ(Ce) values are also listed for such cerium atom. b Reference 42.

the Ce atom.43 As a result, in small clusters (1-1, 2-1, and 3-1), the cerium atoms can have relatively high electron population (Ne g 0.99) on the 4f orbital. Such Ce atoms tend to share the SDs with the O atoms as can be seen from the fact that the SD values (σ) on Ce and Ot in 1-1, 2-1, and 3-1 have the same sign (either alpha or beta). It is thus reasonable to understand the SD delocalization in small clusters 1-1, 2-1, and 3-1. In contrast, for large systems (4-1 and 6-1), the increased size and Ce
Ce distances (1) favor more charge distributions on Ce, (2) reduce the 4f occupancy, and (3) change the sign of SD values on Ce with respect to those on O atom. These correlate with the highly localized SDs over O atoms in 4-1 and 6-1. Note that high symmetry of the small clusters (1-1/C2v and 2-1/D3h) may also cause the SD delocalization. However, in the cluster isomer 4-2 that has no symmetry and is topologically very similar to 4-1, the SD delocalization is again correlated with the small dmin and large values of Q, Ne, and σ(Ce). 4.1.2. Spin Density Localization over Ot versus Ob. The above consideration of the cluster size and its effect on the Ce 4f occupancy lead to the tendency of SD localization in large CenO2n+1
clusters, which is generally consistent with the experimental observation of the oxygen atom transfer reactions for n g 4 (reaction 1). However, there is significant oscillation of the rate constants in Figure 1c. The DFT study on the Ce5O11
cluster isomers (Figure 2) suggests that in addition to the SD delocalization versus localization, the issue of SD localization over Ot versus Ob also plays an important role. Figures 3 and S12 (Supporting Information) indicate that the Ot
• radical is more reactive toward CO than the Ob
•, which is reasonable if one considers the stereohindrance effect. In all of the reported O
• containing clusters series,9g,10e,12
14 such as in (V2O5)n+ (n = 1
5)13a,14a and ZrnO2n+1
(n = 1
4),12c the O
• radicals are terminally (Ot
•) rather than bridgingly (Ob
•) bonded and the transition between Ot
• and Ob
• has never been demonstrated previously. The rate constant oscillation observed for CenO2n+1
+ CO (n = 4
21, Figure 1c) may be a good example for this. Figure 2 shows that for Ce4O9
, the cluster isomer with Ot
• (4
1) is more stable than the one with Ob
• (4
4) by a significant value of 0.39 eV. In contrast, for further large clusters Ce5O11
and Ce6O13
, the isomers with Ot
• and Ob
• (5-1 and 5-2; 6-1 and 6-2) are very close in energy (0.07
0.08 eV). Note that the DFT accuracy may not be sufficient enough to tell whether 5-1 (Ob
•) and 6-1 (Ot
•) are more stable than 5-2 (Ot
•) and 6-2 (Ob
•), respectively. However, the reactivity study (Figure 1) supports that the DFT predicted relative energetics 13334

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The Journal of Physical Chemistry C for 5-1/5-2 and 6-1/6-2 couples are correct. Considering the experimental evidence (Figure 1c) that isomers with quite different reactivity toward CO are populated in the cluster source44 at least for Ce6O13
, Ce7O12
, and Ce12O25
and the DFT results that isomers with Ot
• and Ob
• radicals are close in energy for Ce5O11
and Ce6O13
, we conclude that the CenO2n+1
cluster series starts to have energetically competitive isomers that contain either Ot
• or Ob
• at n = 5. Such competition may lead to existence of Ot
• radicals in the lowest energy isomers of Ce6O13
, Ce7O12
, and Ce12O25
clusters that have relatively high reactivity toward CO as observed by the experiment. 4.2. Spin Density Transfer. Figure 2 indicates that isomers 4-1, 4-2, and 4-4; 5-1 and 5-2; or 6-1, 6-2, and 6-3 are topologically very similar to each other while the SD distributions are different. This implies that the SDs may transfer within the clusters quite easily. To demonstrate this, the transition state that connects isomers 4-4 and 4-1 is optimized by the DFT (Figure S14, Supporting Information). The result indicates that the SD transfer for conversion of 4-4 to 4-1 (Ob
• f Ot
•) is subject to a small barrier of 0.17 eV (ΔH0K value). By using the Rice
Ramsberger
Kassel
Marcus (RRKM) theory and the DFT calculated vibrational frequencies and energetics, the rate of 4-4 f 4-1 conversion45 is estimated to be 2.0  109 s
1 at room temperature, which corresponds to a fast SD transfer. Note that for infinitively large clusters (or for condensed phase surface), the SD transfer may be as fast as 8.3  109 s
1 under similar conditions for the above 4-4 to 4-1 conversion.45 4.3. Considerations for Condensed Phase Systems. 4.3.1. Existence of O
• Species with Highly Localized Spin Densities (HLSDs). The O
• species can also be considered as hole centers generated by excitation of O 2p band in bulk metal oxides6a,9g while by no means one can conclude that the hole centers must have HLSDs (SDs of about one unit μB over a single oxygen atom). The previous studies provided experimental evidence for existence of the O
• species with HLSDs over oxide clusters as large as Al10O15+ 13b and V10O25+ 14a while this study further extends the size of such clusters up to Ce21O43
of which the diameter is about 1.2 nm.46 In condensed phase studies, ceria particles as small as 1 nm can be synthesized to catalyze the CO oxidation.4 This work thus concludes that the O
• species with HLSDs can exist over the practically applicable materials. Although this conclusion seems to be obvious with successful characterization of the O
• species by methods such as electron spin resonance spectroscopy in condensed phase systems,6a a gas phase study of atomic clusters under isolated, controlled, and reproducible conditions is now able to provide further evidence with the bottom-up strategy. It should be pointed out that the CenO2n+1
clusters have net charges while practical materials are usually overall neutral. However, local net charges may be formed by charge transfer in related systems such as gold nanoparticles supported by CeO2.5,7,15 Figure 4 demonstrates that Ce4O9
may be considered to be formed by charge transfer within Ce4O9Au that is overall neutral while the Ce4O9 moiety (Figure 4b) is negatively charged and contains an Ot
• radical, of which the reactivity toward CO is expected to be similar to the Ot
• center in the isolated Ce4O9
anion (Figure 4a). 4.3.2. Role of O
• Species in Low Temperature CO Oxidation. Superoxide species (O2
•) were experimentally evidenced to be involved in the low-temperature CO oxidation by O2 over Au/ CeO2 catalysts.7 It is expected that the elementary reaction of

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Figure 4. A comparison of Ce4O9
/4-1 (a) with one DFT optimized neutral cluster Ce4O9Au (b). The Mulliken atomic spin density values (μB) over oxygen, Mulliken atomic charges (|e|) over gold, and some bond lengths in picometers are given.

O2
• with CO will generate O
• and a full catalytic cycle may be written as O2
• + CO f O
• + CO2

ð3Þ

O
• + CO + Ce4+ f CO2 + Ce3 + •

ð4Þ

O2 + Ce3 + • f O2
• + Ce4+

ð5Þ
•

Raman spectroscopy indicates that the reactive O2 species are bonded with Ce (reaction 5) rather than with Au in the Au/CeO2 catalysts.7 In addition, the Au
O bond energy is only 2.3 eV47 (1.8 eV by B3LYP) while the calculated Ce
O bond energies of CenO2n+1
(n = 1
6) are all not less than 3.0 eV (Table 1). As a result, we speculate that the O
• radicals also bond with Ce so that cerium atoms can cycle between Ce4+ and Ce3+ (reactions 4 and 5). Reaction 3 that involves O
O bond activation and O
• generation may well be the rate-limiting step48 for the full catalytic cycle at low temperature because the elementary oxidation of CO by O
• with HLSDs (reaction 4) is usually facile49 from condensed phase studies.6a However, the role of the O
• species may still be important and should be taken into account in practice. Figure 3 indicates that the reaction of Ot
• with CO over Ce4O9
/4-1 to form CO2 and Ce4O8
leads to very small geometry relaxation of the Ce4O8 moiety (support). However, the reaction of Ob
• with CO can cause large geometry relaxation of the support (see Figure S13 in the Supporting Information for an example). Such relaxation could cause loss of catalytically active sites and deactivation of the catalysts. Moreover, it is demonstrated that the fast SD transfer (subsection 4.2) can lead to transition of O
• radicals from one site (O
• + e
f O2
) to another (O2

e
f O
•) over cerium oxides. This means that reactions 3 and 4 would take place at different sites, which causes coordinative-saturation and loss of the good Au
CeO2 interfaces.48 The Au/CeO2 catalysts prepared by coprecipitation of Au and conventional CeO2 were reported to be active for CO oxidation at 100 °C or higher,50 whereas the catalysts prepared by depositing Au nanoparticles on nanocrystalline CeO2 are active even at 5 °C.5 The nanocrystalline CeO2 is expected to have welldefined structure and relatively high stability, which can decrease the possibility of (1) extensive transfer of the O
• species from the Au
CeO2 interfaces to other sites such as inside CeO2 and (2) large structure relaxation of the CeO2 support. As a result, the role of O
• species discussed above can further address48 the goodness of the nanocrystalline CeO2 in low-temperature CO oxidation and the nature of the O
• radicals presented in this study may be commonly considered in design and use of relevant catalysts. 13335

dx.doi.org/10.1021/jp202077s |J. Phys. Chem. C 2011, 115, 13329–13337

The Journal of Physical Chemistry C

5. CONCLUSIONS Significant and interesting size-dependent reactivity of CenO2n+1
clusters toward CO is observed by experiment and well interpreted by theory (note that size-dependent reactivity is often observed for pure metal clusters while this is a first good example to demonstrate the size-dependent reactivity for metal oxide clusters). The delocalized spin densities lead to no or very low reactivity for small CenO2n+1
clusters with n = 1
3. The highly localized spin densities start to occur at n = 4 while there is competition between spin density localization over terminal and bridging oxygen atoms. The n = 4, 6, 7, and 12 clusters favor spin density localization over terminal oxygen atoms in the lowest energy isomers, which corresponds to relatively high reactivity of these clusters toward CO. This study extends the size of the metal oxide clusters that contain the O
• radical species with highly localized spin densities up to Ce21O43
of which the geometrical size is similar to those of some practically applicable ceria materials. The spin density transfer that causes movement of the O
• radicals from one site to another can be fast (109 s
1) in cerium oxides at room temperature. The O
• radicals can be intermediates formed from the elementary reactions of superoxide species O2
• with CO. The reaction of bridge bonded O
• radicals with CO and the fast O
• movement can cause deactivation or loss of catalytically active sites, which can further address the goodness of the nanocrystalline CeO2 that has relatively high structure stability in the low-temperature CO oxidation. The O
• radicals are an important type of reactive intermediates over metal oxides. The nature of the O
• radicals revealed in this work may be considered in the design and use of relevant catalysts. ’ ASSOCIATED CONTENT

bS

Supporting Information. Details of the molecular dynamics simulations and additional experimental and theoretical results. This material is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.-G. He); [email protected] (Q. Shi).

’ ACKNOWLEDGMENT This work was supported by the Chinese Academy of Sciences (Knowledge Innovation Program No. KJCX2-EW-H01, Hundred Talents Fund), the National Natural Science Foundation of China (Nos. 20803083 and 20933008), and the Major Research Plan of China (No. 2011CB932302). ’ REFERENCES (1) (a) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (b) Min, B. K.; Friend, C. M. Chem. Rev. 2007, 107, 2709. (c) Xie, X. W.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. J. Nature 2009, 458, 746. (d) Jia, C. J.; Liu, Y.; Bongard, H.; Schuith, _ F. J. Am. Chem. Soc. 2010, 132, 1520. (e) Camellone, M. F.; Fabris, S. J. Am. Chem. Soc. 2009, 131, 10473. (2) (a) Trovarelli, A. Catal. Rev. Sci. Eng. 1996, 38, 439. (b) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (c) Esch, F.; Fabris, S.; Zhou, L.; Montini, T.; Africh, C.; Fornasiero, P.; Comelli, G.; Rosei, R. Science 2005, 309, 752.

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1 s
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(43) Walter, M. D.; Booth, C. H.; Lukens, W. W.; Andersen, R. A. Organometallics 2009, 28, 698. (44) (a) Note that coexistence of structural isomers for atomic clusters was often reported in literature: (b) Fiedler, A.; Kretzschmar, I.; Schr€oder, D.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 9941. (c) Wu, H. B.; Wang, L. S. J. Phys. Chem. A 1998, 102, 9129. (d) Berg, C.; Schindler, T.; Kantlehner, M.; Niedner-Schatteburg, G.; Bondybey, V. E. Chem. Phys. 2000, 262, 143. (45) The method to use the RRKM theory to calculate the rate of intracluster SD transfer can be found in ref 25. The rate of SD transfer within an infinitively large system may be estimated with kBT/h exp(
E0/kBT), in which kB and h are Boltzmann and Planck constants, respectively; T = 298.15 K; and E0 = 0.17 eV (as calculated for Ce4O9
). (46) To estimate the cluster size (diameter), the Ce21O43
system is considered to be a round ball with the CeO2 density of 7.215 g/cm3 (cubic fluorite structure). (47) Lide, D. R. Handbook of chemistry and physics, 74th ed.; CRC Press: Boca Raton, FL, 1994. (48) (a) A synergistic effect associated with interfaces between Au and nanocrystalline CeO2 in promoting formation of reactive oxygen species (end-on bonded O2
• and O22
) was proposed7a to interpret the excellent performance of the Au/CeO2 catalyst that is active at 5 °C for CO oxidation.5 Figure 1 shows that the anionic clusters CenO2n+2
(with side-on bonded O2
• by test calculations) are inert toward CO, in agreement with the DFT result (Figure S15, Supporting Information) that the O
O bond activation in the CO oxidation by O2
• over Ce4O10
is subject to a high reaction barrier (0.99 eV). Meanwhile, ref 48b indicated that the cationic clusters AunOm+ are able to associate with rather than oxidize CO under thermal collision conditions. One may propose that the Au
CeO2 interfaces (rather than the single CeO2 or Au phase) are important for reaction 3 that involves O
O activation and O
• generation at low-temperature. Further investigations on Au
Ce bimetallic oxide clusters may be able to address this. (b) Kimble, M. L.; Castleman, A. W. Int. J. Mass Spectrom. 2004, 233, 99. (49) Note that there is no firm evidence for oxidation of CO by Ob
• over Ce5O10
(Figure 1) because of the short reaction time (60 μs) in the fast flow reactor while the reaction time can be much longer for practical surface reactions. The overall reaction barrier for CO oxidation by Ob
• is very small (0.07 eV in Figure S12, Supporting Information) compared with the high barrier (0.99 eV) for CO oxidation by O2
• (Figure S15, Supporting Information). As a result, it is unlikely that reaction 4 is the rate-limiting step.48 (50) (a) Liu, W.; Flytzani-Stephanopoulos, M. J. Catal. 1995, 153, 304. (b) Bera, P.; Hegde, M. S. Catal. Lett. 2002, 79, 75.

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