Collision-Induced Dissociation and Density Functional Theory Studies

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Collision-Induced Dissociation and Density Functional Theory Studies of CO Adsorption over Zirconium Oxide Cluster Ions: Oxidative and Nonoxidative Adsorption Xiao-Nan Wu,†,‡ Jia-Bi Ma,†,‡ Bo Xu,†,‡ Yan-Xia Zhao,†,‡ Xun-Lei Ding,† and Sheng-Gui He*,† †

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, P. R. China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China ABSTRACT: Zirconium oxide cluster cations and anions are produced by laser ablation and reacted with CO in a fast flow reactor. The CO adsorption products ZrxOyCOþ are observed for most of the generated cationic clusters (ZrxOyþ = Zr2O5,6þ, Zr3O7,8þ, Zr4O9,10þ...) while only specific anionic systems (ZrxOy = Zr3O7, Zr4O9...) absorb CO to produce ZrxOyCO. To study how the CO molecule is adsorbed on the clusters, the ZrxOyCO( products are mass-selected by a time-of-flight mass spectrometer (TOF-MS) and collided with a crossed helium beam. The fragment ions from collision-induced dissociation (CID) are detected by a secondary TOF-MS. Loss of CO and CO2 is observed upon the collision of the helium beam with ZrxOyCOþ and ZrxO2xþ1CO, respectively. Density functional theory calculations indicate that oxidative and nonoxidative adsorption of CO takes place over Zr3O7 and Zr3O7þ, respectively. This is consistent with the CID experiments.

1. INTRODUCTION Transition metal oxides (TMOs) are widely used as both catalysts and catalytic supports.1 To understand elementary reactions over the TMO surfaces, which can be useful for rational design of relevant catalysts, one important way is to study the bonding and reactivity of TMO clusters under isolated, controlled, and reproducible conditions (see ref 2 for recent reviews). Meanwhile, study of TM-containing species can also establish very useful concepts such as two-state reactivity (TSR)3 and the importance of relativistic effects4 in chemistry. As an important type of reaction, CO oxidation over TMO clusters has been extensively studied by both experimental and theoretical methods. Many oxide clusters of transition metals including iron,5,6 tungsten,7 zirconium,8,9 gold,10 cerium,11 and others12,13 have been reported to be able to deliver oxygen atoms to CO molecules to form CO2 at thermal (or close to thermal) collision conditions based on mass spectrometric techniques. In addition to the direct oxidation of CO to form gas phase CO2, the reactions of TMO clusters MxOyq (M is a metal and q is the charge number) with CO often generate the CO adsorbed complex MxOyCOq.8,11,1315 However, little attention has been paid to study the CO attached TMO clusters.16 The CO adsorption is an important step for CO catalytic oxidation over TMO-based materials.17 As a result, study of the MxOyCOq complex may be important to understand the nature of the CO adsorption over bulk oxides at a molecular level. This work focuses on the CO adsorption on zirconium oxide clusters cations (ZrxOyþ) and anions (ZrxOy). Condensed phase zirconia (ZrO2) is employed as both catalyst and catalystsupport material for many reactions including CO oxidation.18 The structural and reactivity properties of gas phase and matrix isolated r 2011 American Chemical Society

zirconium oxide species are extensively reported in the literature.8,9,1921 For interaction of CO with zirconium oxide clusters, Johnson and co-workers have recently identified CO oxidation by ZrxO2xþ (x = 25)8 and ZrxO2xþ1 (x = 14)9 clusters to form gas phase CO2 by using guided-ion-beam mass spectrometry and density functional theory (DFT) calculations. Minor products with negligible intensities resulting from the association of CO onto the ZrxO2xþ (x = 25) cations were also reported,8 and there was no report of CO adsorption onto the ZrxO2xþ1 (x = 14) anions.9 In this study, CO association products ZrxOyCO( are produced from the interaction of pregenerated ZrxOy( with CO in a fast flow reactor and characterized by collision-induced dissociation (CID). It is noticeable that characterization of product clusters is important, and usually further information about the reaction mechanisms could be achieved.16,2225 Infrared multiphoton dissociation23 and anion photoelectron spectroscopy16,24 have been recently reported to study the product clusters such as V2O5C2H4þ, V4O10H2C3H8þ, MoO2CO1,2, and Al11,12NH3, while the CID is a traditional mass spectrometry-based method. There are several variations of CID that usually correspond to different types of mass analyzers.22,26,27 Time-of-flight (TOF) mass spectrometry is possibly the simplest but very sensitive method for mass analyzing clusters, and we use a TOF/TOF tandem mass spectrometer (MS) to carry out the CID experiments that are described below. Received: January 29, 2011 Revised: March 17, 2011 Published: May 10, 2011 5238

dx.doi.org/10.1021/jp200984r | J. Phys. Chem. A 2011, 115, 5238–5246

The Journal of Physical Chemistry A

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Figure 1. A schematic diagram of the TOF/TOF-MS (Chamber II) coupled with a cluster source (Chamber I) and a crossed helium beam (Chamber III). Parts (Ps): 1, 6, and 28: pulsed values (general value, series 9); P 2: stainless steel assembly to hold carrier gas and laser ablated plasmas; P 3: sample disk for laser ablation; P 4: cluster formation channel (adjustable length); P 5: fast flow reactor; Ps 7, 8, 29, and 30: skimmers (3 mm diameter); Ps 911 and 2224: electrodes for accelerating ions; Ps 12 and 25: deflection plates; Ps 13, 14, and 26: einzel lens; Ps 1618: mass gate; Ps 1921 electrodes to control cluster kinetic energy; Ps 15 and 27: dual microchannel plate detectors. Some critical distances (d): d(910) = d(1011) = d(2223) = d(2324) = 13.5 mm, d(1617) = d(1718) = d(1819) = d(1920)/2 = d(2021) = 6.5 mm, d(2122/left edge) = 10.0 mm, d(1116) = 2.05 m, and d(2427) = 0.65 m. Typical potentials (U) for cations (similar for anions unless specified): U(9) = 1450 V (positive pulse, rise time∼100 ns); U(10) = 1350 V (positive pulse, rise time∼100 ns); U(11, 16, 18) = 0 V, U(17) = 1800 V (negative pulse, fall and rise time∼100 ns, pulse width ∼ 800 ns); U(19, 20, 21) = 800 V, U(22) = 1000 V, U(23) = 760 V, and U(24) = 2500 V (þ1500 V for anions).

2. METHODS 2.1. Experimental Section. The schematic diagram of the TOF/TOF-MS is shown in Figure 1. The primary TOF-MS (Chamber II and Parts 815) equipped with the cluster source (Chamber I and Parts 17) was used in our previous works for the reactions of TMO clusters with small molecules.11,15,20,28,29 The secondary TOF-MS (Parts 1627) and a separate vacuum chamber (Chamber III and Parts 2830) for generation of a helium supersonic beam are attached to the primary TOF-MS in this work. The procedures for cluster generation, reaction, mass analysis, mass selection, and CID may be briefly described as follows. The zirconium oxide cluster cations and anions are generated by laser ablation of a translating and rotating zirconium disk (Part 3 in Figure 1) in the presence of 0.11% O2 seeded in a He carrier gas with backing pressure of 300 kPa. For generation of oxygen-rich zirconium oxide anions (ZrxOy, y > 2x) that are almost inert toward N2O, the N2O instead of O2 can also be used as the oxygen source. A 532 nm (second harmonic of Nd3þ: yttrium aluminum garnet (YAG)) laser with energy of 58 mJ/pulse and repetition rate of 10 Hz is used. The prepared gas mixture O2/He or N2O/He is passed through a 10 m long copper tube coil at low temperature (T = 77 K for O2 and 195 K for N2O) before entering into a valve (Part 1) that generates the gas pulses. This method nearly eliminates contributions of undesirable hydroxo species ZrxOy(HO)z( (z > 0) in our cluster distribution possibly by trapping down residual water in the gas handling system. Similar treatment (T = 195240 K) is also applied in the use of the reactant gas (see below). The clusters formed in a gas channel (2 mm diameter 25 mm length, Part 4 in Figure 1) are expanded and reacted with He-

diluted CO (5%) or other small molecules in a fast flow reactor (6 mm diameter 60 mm length, Part 5). The reactant gases are pulsed into the reactor 20 mm downstream from the exit of the narrow cluster formation channel by a second valve (Part 6). By using the method in ref 6, the instantaneous total gas pressure in the 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 (kcollision) of 5.0 107 s1 for an approaching velocity of 1 km/s. Since the length (60 mm) of the reactor is much longer than 1 mm, the intracluster vibrations are likely equilibrated (cooled or heated, depending on the vibrational temperature after exiting Part 4 with a supersonic expansion) to close to the bath gas temperature before reacting with the diluted CO molecules. The bath gas temperature is around 300400 K considering that the carrier gas can be heated during the process of laser ablation.30 After reacting in the fast flow reactor, the reactant and product cluster ions exiting from the reactor are skimmed (Parts 7 and 8) into the ion source region of the primary TOF-MS. The clusters are accelerated (Parts 911), deflected (Part 12), focused (Parts 13 and 14), and can be detected directly with a first dual microchannel plate detector (Part 15) for primary mass and abundance analysis. A digital oscilloscope (LeCroy WaveSurfer 62Xs) is used to record the signals from the first detector (or the second one Part 27) by averaging 5001000 traces of independent mass spectra (each corresponds to one laser shot). For the CID measurements, the clusters of interest are massselected with a mass gate (Parts 1618) and decelerated into a field 5239

dx.doi.org/10.1021/jp200984r |J. Phys. Chem. A 2011, 115, 5238–5246

The Journal of Physical Chemistry A free region (Parts 19 and 20), where the cluster is collided with a crossed He beam that is at a right angle and formed by a third pulsed value (Part 28) and two skimmers (Parts 29 and 30). The daughter (fragment) and parent ions can be kinetic-energy-readjusted (accelerated or decelerated) with an electrode (Part 21) and enter into the ion source region (Parts 2224) of the secondary TOF-MS for mass analysis. The deflection and focusing assemblies (Parts 25 and 26) are also designed in the secondary flight tube that is perpendicular to the primary flight tube and the He beam. The secondary TOF-MS runs at a floating-ground mode, i.e., the electrode of Part 24 and the secondary flight tube are applied with a constant DC voltage (2500 V and þ1500 V for cations and anions, respectively in this work). The design of the apparatus is similar to the ones used for photoinduced dissociation (PID,31,32 using laser beam instead of molecular beam) and other CID27 studies employing the TOF/TOF-MS27,31 or reflectron TOF-MS.32 Compared with the CID apparatus in literature,27 we move the collision region out of the ion source assembly of the secondary TOF-MS and use a separate chamber to generate the molecular beam possibly with more intensities. Some geometrical and electrical parameters used for our apparatus are listed below Figure 1. The synchronization of the pulsed events (laser firing, three value openings, cluster accelerating and mass-selecting) are managed with commercially available and homemade electronics. In the reported PID31 or CID27 experiments employing TOF/TOF-MS, a pulsed voltage was usually applied to the first electrode (such as Part 22 in Figure 1) in the ion source assembly of the secondary TOF-MS. We find that application of a constant DC voltage to Part 22 (as well as to Part 23) can also generate reasonably good signals for daughter and parent ions and this simplifies the operation of the apparatus. Otherwise, for different mass-selected clusters or different deceleration/acceleration voltages over Parts 1921, the timing conditions for the pulsed voltage over Part 22 have to be reoptimized and this actually does not improve the quality of the mass spectra in our experiments. The application of constant DC voltages to Parts 22 and 23 leads to loss of zero-time (when the ions get accelerated) for the secondary TOF-MS. In this case, a CID mass spectrum is recorded with zero-time determined by the opening of the mass gate (pulsed voltage over Part 17) and the final flight times are calibrated by shifting a constant value so that the parent ion has an ideal flight time simulated with the geometrical and electrical parameters for the secondary TOF-MS. 2.2. Computational. The DFT calculations are performed for the structures, energies, and mechanisms for the interactions of CO with a few selected zirconium oxide cluster cations and anions. The B3LYP functional33 and Gaussian 03 program34 are used. The TZVP basis sets35 are selected for C and O atoms, and D95 V basis set combined with Stuttgart/Dresden relativistic effective core potential (28-electron core, denoted as SDD in Gaussian 03) is selected for Zr.36 The justification of the computational method for zirconium oxide system can be found in previous studies.20 Geometry optimization with full relaxation of all atoms is performed. Transition state optimizations are performed by using the Berny algorithm37 from initial structures obtained through potential energy surface (PES) scans with appropriate internal coordinates. Vibrational frequencies are calculated to check that reaction intermediates and transition state species have zero and only one imaginary frequency, respectively. Intrinsic reaction coordinate calculations38 are also performed to confirm that each transition state connects two appropriate local minima. The zero-point vibration corrected energies (ΔH0K) are reported in this work.

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Figure 2. TOF mass spectra for reactions of Zr3Oyþ with 0.6 Pa CO (a), Zr3Oy with 0.4 Pa CO (c), and Zr3Oy with 0.7 Pa CO2 (e) in the fast flow reactor. The reference spectra with pure He in the reactor are given in panels (b), (d), and (f). The inset (g) plots the simulated Zr3 isotopomers. The ZrxOy( and ZrxOyX( are marked with x,y and x,yX (X = CO or CO2), respectively.

3. RESULTS 3.1. Experimental Section. 3.1.1. Results with the Primary TOF-MS (without CID). Figure 2 plots the mass spectra obtained

with the primary TOF-MS for reactions of zirconium oxide cluster cations (panels a and b) with CO and anions (panels cf) with CO as well as CO2 in a selected mass region that covers Zr3O610(. Zirconium has five stable isotopes: 90Zr/51.5%, 91Zr/11.2%, 92 Zr/17.1%, 94Zr/17.4%, and 96Zr/2.8%. The Zr3 isotopomers (simulated results in Figure 2g) can be partially resolved with the mass spectrometer. The experiment indicates that CO can be readily adsorbed onto the oxygen-rich zirconium oxide cluster cations (ZrxOyþ, y > 2x) such as ZrxO2xþ1þ and ZrxO2xþ2þ (x = 2, 3, 4...) generated by the cluster source; for example, Zr3O7,8COþ are produced (with depletion of Zr3O7,8þ) after the interaction of the clusters with 0.6 Pa CO in the reactor (Figure 2a). By contrast, very few oxygen-rich zirconium oxide cluster anions can associate with CO under similar conditions. Figure 2c indicates that only Zr3O7 reacts with CO to form Zr3O7CO, while Zr3O8 is nearly inert with CO (Zr3O8CO is not observable). It is noticeable that for Zr2Oy clusters including Zr2O5 that has one less ZrO2 unit than Zr3O7, no association products Zr2OyCO are observed. For large clusters ZrxOy (x > 3), only products ZrxO2xþ1CO are apparently observed. Figure 1e shows that zirconium oxide cluster anions such as Zr3O68 can absorb CO2 molecules quite efficiently. The first-order rate constant (k1) for the cluster depletion in the fast flow reactor can be estimated by using the following equation: k1 ¼ lnðI0 =IÞ=ðF ΔtÞ

ð1Þ

in which I and I0 are signal magnitudes of the clusters in the presence and absence of reactant gas (CO or CO2 in this study), respectively; F is the molecular density of reactant gas 5240



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Figure 3. TOF mass spectra for collision of the crossed He beam with mass selected Zr2O5þ (a), Zr2O5COþ (b), Zr2O7þ (c), Zr2O5(CO)2þ (d), Zr3O7þ (e), Zr3O7COþ (f), and Zr3O9þ (g). The ZrxOyþ and ZrxOyXþ peaks are marked with x,y and x,yX [X = CO or (CO)2], respectively. The neutral loss (such as loss of O2 from Zr2O5þ to produce Zr2O3þ in panel a) is indicated for each of the daughter ion peaks.

(the method to calculate F has been described in ref 6); Δt is the 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 estimated rate constants of k1(Zr3O7þ þ CO), k1(Zr3O7 þ CO), and k1(Zr3O6 þ CO2) are 1.0 1010, 1.1 1010, and 4.8 1011 cm3 molecule1s1, respectively. These k1 values may be systematically under- or overestimated due to systematic deviation in determining the F and Δt values in eq 1, while the deviation is 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 CeO2þ þ C2H4 f CeOþ þ C2H4O11,39 and V4O10þ þ CH4 f V4O10Hþ þ CH3.29,40 3.1.2. Results with the Secondary TOF-MS (with CID). Figure 3 plots the typical mass spectra obtained with the secondary TOFMS for the CID of mass-selected zirconium oxide cluster cations and their association products with CO. The results for the anionic species are shown in Figure 4. For collision of a cluster with the collision gas, the center-of-mass kinetic energy (Ec) can be calculated by using eq 2 below. Ec ¼ jU1 U2 j m=M

ð2Þ

in which U1 is the average potential applied to Parts 9 and 10 in Figure 1, U2 is the potential applied to Parts 19 and 20, M is the mass of the cluster, and m is the reduced mass of the cluster with collision gas (He). The spectra in Figures 3 and 4 are obtained with the |U1 U2| value around 600 V that corresponds to Ec values of 9.0 and 5.7 eV for collisions of He with Zr2O5þ and Zr3O9þ, respectively. The CID experiments with conditions similar to those used for Figures 3 and 4 have also been carried out on a lot of other zirconium oxide cluster cations and anions including ZrxO2xþ and ZrxO2xþ1 (x = 24) and no fragment ions are observed

Figure 4. TOF mass spectra for collision of the crossed He beam with mass selected Zr2O4CO2 (a), Zr2O7 (b), Zr3O6CO2 (c), Zr3O7CO (d), and Zr3O7CO2 (e). The ZrxOy and ZrxOyX peaks are marked with x,y and x,yX (X = CO or CO2), respectively. See caption of Figure 3 for an explanation of the notations. Note that Zr3O7CO (d) and ZrxOyCO2 (a,c,e) are generated from reactions of the clusters anions with CO (Figure 2c) and CO2 (Figure 2e), respectively.

for the collision of the oxygen-poor or oxygen-slightly rich clusters (ZrxOyþ and ZrxOyþ1, y < 2x) with the crossed He beam. However, if the He beam is changed to an Ar beam, which leads to increase of the Ec by a factor of about 10 and possibly more efficient conversion of the Ec into the cluster vibrational energy, fragmentation of the clusters into smaller ones with less metal atoms can be observed. Since we are interested in studying how the small molecules (CO, CO2...) are absorbed over atomic clusters, the hard collision (with Ar beam) that leads to sever cluster fragmentation is not helpful, and the relatively soft collision with He beam may be used. Figures 3c and 3g indicate that loss of O2 is quite efficient upon the collisions of the oxygen-very-rich clusters Zr2O7þ and Zr3O9þ with He. In contrast, for oxygen-less-rich clusters Zr2O5þ (Figure 3a) and Zr3O7þ (Figure 3e), loss of O2 can be identified, but this is much more difficult, especially for the Zr3O7þ system. The experiments thus suggest that there are relatively strongly and weakly bonded O2 units in clusters ZrxO2xþ1þ and ZrxO2xþ3þ (x = 2, 3), respectively. This is generally in agreement with the result that there are O2 units in most of the oxygen-rich metal oxide clusters.2h,20c,15 The experiments (Figure 3b,f) indicate that loss of CO rather than CO2 is observed upon the collision of Zr2O5COþ and Zr3O7COþ with He beam. Loss of two units of CO is also observed for the CID of secondary association product Zr2O5(CO)2þ (Figure 3d). The above results indicate that the CO molecules are all weakly bonded over the cluster cations. It should be pointed out that the daughter ion Zr2O5þ in Figure 3b (marked with “CO”) or in Figure 3c (marked with “O2”) should have the same flight time as the parent Zr2O5þ does (Figure 1a). However, Figure 3 indicates that the daughter ions (Zr2O5þ in Figure 3b,c, as well as Zr3O7þ in Figure 3f,g) have slightly longer flight times (corresponding to mass-up-shifts 5241



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Table 1. DFT-Calculated Dissociation Energies (Enthalpies of Reactions) ΔH0K (eV)

commentb

Zr2O4þ f 2Zr2O3þ þ 3O

3.93

N

Zr2O5þ f 2Zr2O4þ þ 3O 2 Zr2O5þ f 2Zr2O3þ þ 3O2

3.49 2.31

N Y

reactiona

# T1

2

T2 T3

2

T4

2

T5

2

T6

2

T7

2

T8

2

T9

2

0.78

Y

T10 T11

2

Zr2O7 f 2Zr2O5 þ 3O2 2 Zr3O7 f 2Zr3O6 þ 3O

0.90c 4.84

Y N

T12

2

Zr3O7CO (S1)f 2Zr3O7 þ 1CO

1.78

N

T13

2

Zr3O7CO (S1)f 2Zr3O6 þ 1CO2

1.18

Y

T14

2

Zr3O7CO (S2)f 2Zr3O7 þ 1CO

0.35

N

Zr2O5COþ f 2Zr2O5þ þ 1CO

0.90

Y

Zr2O7þ f 2Zr2O5þ þ 3O2

0.41

Y


4.26

N


3.59

N

Zr3O7þ f 2Zr3O5þ þ 3O2

2.74

Y (weak)

Zr3O7COþ f 2Zr3O7þ þ 1CO

a

Superscripts indicate the spin multiplicities. The lowest energy structure is used for each species unless specified for Zr3O7CO of which the S1 and S2 structures (Figure 5) are used. b Y and N denote the experimentally observed and nonobserved fragmentation channels, respectively. c Structures and energies of Zr2O5 and Zr2O7 are taken from ref 20c.

Figure 5. DFT-optimized structures of Zr2O5þ, Zr2O5COþ, Zr2O7þ, Zr3O7þ, Zr3O7COþ, Zr3O7, and Zr3O7CO. The ZrxOy( and ZrxOyCO( are denoted as x,y( and x,yCO(, respectively. S1, S2, etc. denote different structure isomers, and the relative energy in eV is given for each isomer. Some bond lengths in pm are given. All of the species are in doublet spin multiplicity.

of about 23 amu if flight times are converted to masses). This can be explained by slightly larger deceleration of the daughters (such as Zr2O5þ) with respect to the parents (such as Zr2O5COþ) by the electric field between Part 21 and the region formed by Parts 22 and 23 (Figure 1) in the experiment. In this case, the daughters fly slightly behind the parents before the acceleration to the secondary detector (Part 27). It is possible to get correct flight times for the daughters if we further increase the voltages on Parts 1921 in Figure 1 so that the deceleration mentioned above is negligible. However, this causes signal decrease for the daughter ions and poor mass resolution for the secondary TOF-MS in our experiment and is thus not used. Figure 4d shows that loss of CO2 rather than CO is observed in the CID of Zr3O7CO that is formed from the interaction of Zr3O7 with CO in the fast flow reactor (Figure 2c). To confirm the assignment of the CO2 loss, the CID of Zr3O6CO2 that is from association of Zr3O6 with CO2 (Figure 2e) is also studied. Figure 4c,d indicates that identical peak positions for the daughter ions are obtained in the CID of Zr3O6CO2 and Zr3O7CO. A few other CID spectra for Zr2O4CO2, Zr2O7, and Zr3O7CO2 are also shown in Figure 4, and the fragmentation channels are as expected. 3.1. Computational Results. Figure 5 plots the DFT optimized structures and relative energies for Zr2O5þ, Zr2O5COþ, Zr2O7þ, Zr3O7þ, Zr3O7COþ, Zr3O7, and Zr3O7CO. Table 1

lists DFT calculated dissociation energies (D0) for a few clusters that are studied with the CID. The singlet O and O2 are higher in energy than the triplet O and O2 by 2.76 and 1.68 eV (B3LYP calculated values), respectively. As a result, the dissociation channels involving O and O2 in Table 1 are only listed for triplet O (3O) and O2 (3O2). It is noticeable that a dissociation such as 2 Zr2O4þ f 2Zr2O3þ þ 3O (channel T1 in Table 1) does not have to involve two PESs with different spin multiplicities (TSR3 with doublet and quartet PESs in this case) because coupling of spin angular momenta 1 p (3O) and 1/2 p (2Zr2O3þ) can result in 3/2 p (4Zr2O4þ) as well as 1/2 p (2Zr2O4þ). The TSR is very important for interpreting the reactivity of TMO clusters such as VO2þ3bd and NbO3.3e However, the above consideration indicates that the listed dissociation channels involving 3O and 3 O2 in Table 1 can take place without participation of the quartet PESs. The DFT-calculated structural and energetic results are generally in agreement with the CID experiments. For example, there is one O2 unit in the ground or energetically low-lying isomeric structure of Zr2O5þ (2,5þ /S1 or 2,5þ /S2 in Figure 5) and the O2 loss (channel T3 in Table 1) is more favorable than the O loss (channel T2). As a result, the O2 loss rather than O loss is observed in the CID of Zr2O5þ. Moreover, one O2 unit is very loosely bonded (D0 = 0.41 eV, channel T5) in Zr2O7þ while the O2 unit is relatively strongly bonded in Zr2O5þ (D0 = 2.31 eV). This is consistent with the more efficient fragmentation of Zr2O7þ (Figure 3c) versus Zr2O5þ (Figure 3a). Table 1 indicates that, except for channels T12 and T14, the dissociation channels with D0 values greater than 3.0 eV are all nonobservable in the CID experiments. It is noticeable that channel T8 has a D0 value (2.74 eV) that is close to 3.0 eV, and this channel is only weakly observed (Figure 3e). Loss of CO2 (channel T13 in Table 1) from Zr3O7CO/S1 (see Figure 5) is energetically more favorable than the loss of CO (channel T12). In addition, the Zr3O7CO/S2 cannot be efficiently generated in the fast flow reactor (see more discussion below). As a result, 5242



Figure 6. DFT-calculated energetics for absorption and oxidation of CO over Zr3O7þ (a) and Zr3O7 (b). The energies in eV with respect to the separate reactants Zr3O7( þ CO are given. The structures (bond length in pm) of the reaction intermediates and transition states for Zr3O7þ (2A) þ CO (1Σþ) f Zr3O6þ (2A) þ CO2 (1Σgþ) are given in panel (c), while those of Zr3O7þ/S1, Zr3O7/S1, and Zr3O7/S2 are in Figure 5.

CO2 loss rather than CO loss (channels T12 and T14) from Zr3O7CO is observed in the CID experiment (Figure 4d). The DFT results indicate that CO is coordinated with the Zr4þ sites through ZrC interaction (ZrO interaction has been tested to be much weaker) in the cationic clusters Zr2O5COþ and Zr3O7COþ (Figure 5). The binding energies are around 0.780.90 eV for the lowest energy isomers (Table 1). The CID of these species formed in the fast flow reactor leads to loss of CO as suggested by the spectra in Figure 3. The CO molecule may also coordinate with the Zr4þ sites in the anionic cluster Zr3O7CO with structures S2 and S3 (Figure 5). However, the binding energies for such anionic species are very small (e0.35 eV), which leads to very short lifetime for these species (see Discussion below).

4. DISCUSSION 4.1. Absorption and Oxidation of CO by Zirconium Oxide Cluster Cations. The DFT results indicate that Zr2O5þ or Zr3O7þ

(Figure 5) contains one η2-O2 moiety in which each O atom has Mulliken spin density values close to 0.5 μB. Such η2-O2 moiety can be considered as a superoxide radical (O2•). It was demonstrated by condensed phase studies41 that the O2• radicals are usually involved in the oxidation of CO by O2 over metal oxide based catalysts. Since CO can be trapped (absorbed) by zirconium oxide cluster cations such as Zr3O7þ (Figure 2a), we further consider the possibility of CO oxidation by the O2• radical over the Zr3O7þ

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cluster with DFT calculations (Figure 6). The results indicate that the oxidation to form Zr3O6þ and CO2 is very exothermic (ΔH0K = 1.85 eV) while the whole process is subject to high reaction barriers of about 1.0 eV (see TS1 and TS2 in Figure 6a). It should be pointed out that in Figure 6a the reaction intermediates (IM1, IM2, and 3,7COþ/S1) and transition states (TS1 and TS2) are all doublets, and the corresponding quartets are all higher (>2.3 eV) in energy. As a result, the TSR3 is not further considered for the reaction of Zr3O7þ with CO and the crossing of the two curves (Curve 1: 3,7þ þ CO f 3,7COþ f 3,7þ þ CO; Curve 2: 3,7þ þ CO f IM1 f TS1 f IM2 f TS2 f IM3 f 3,6þ þ CO2) in Figure 6a does not mean a TSR. Figure 6c indicates that the high barriers that are involved in Zr3O7þþCOfZr3O6þþCO2 can be considered to be involved with the activation of the OO bond in the η2-O2 moiety since the OO bond length increases by more than 10 pm in each of the following critical reaction steps: IM1fTS1, TS1fIM2, and IM2fTS2. This also suggests that activation of OO bond may well be an important factor that should be considered in the oxidation of CO by O2.6,17 The unfavorable kinetics for oxidation of CO by O2• radical over the Zr3O7þ cluster further supports that CO is simply trapped by the Zr4þ sites, as in Zr3O7COþ complex with S1 structure (Figures 5 and 6a). This also further agrees with the CID result that loss of CO rather than CO2 from Zr3O7COþ is observed (Figure 3f). The nature of the interaction of CO with metal centers has been well studied in the literature.42 In our case, the DFT-calculated CO stretching vibrational frequencies of Zr2O5COþ/S1 and Zr3O7COþ/S1 are 2323.1 and 2308.5 cm1, respectively. These values are blue-shifted by more than 90 cm1 with respect to the vibrational frequency of the free CO molecule (2218.3 cm1 by DFT), indicating an electrostatic bonding between CO and the Zr4þ sites in these cationic clusters.42b The initially formed metastable reaction intermediate, such as Zr3O7COþ/S1 carries vibrational energies (Evib) of Zr3O7þ and CO, binding energy (Eb = 0.779 eV) between Zr3O7þ and CO, and the center of mass kinetic energy (Ek = μv2/2, μ-reduced mass of CO with Zr3O7þ, v ≈ 1 km/s, and Ek ≈ 0.135 eV). Using the DFT-calculated vibrational frequencies, the Evib values are estimated to be 0.343 eV for vibrational temperature (Tvib) of 298 K. This means that the total excess energy (Et = Eb þ Evib þ Ek = 1.257 eV) is well above the binding energy (0.779 eV) and the dissociation of the metastable complex Zr3O7COþ/S1 back into Zr3O7þ and CO should be considered. The RiceRamsbergerKasselMarcus (RRKM) theory with a modification to variational transition state theory (VTST)43 may be used to estimate the dissociation rate (kdis) of Zr3O7COþ/S1. By using a procedure described in ref 6, kdis(Zr3O7COþ/S1 f Zr3O7þ þ CO) at Tvib = 298 K is determined to be 5.6 107 s1, which is close to the rate for cluster collision with the He carrier gas in the fast flow reactor (kcollision ≈ 5.0 107 s1, see subsection 2.1). As a result, the collisions by the carrier gas have a chance to stabilize (cool down) the metastable complex Zr3O7COþ/S1. Note that the experimentally determined rate of Zr3O7þ depletion that corresponds to Zr3O7COþ formation is about 1.0 1010 cm3 molecule1 s1 (subsection 3.1.1). The efficiency for formation of Zr3O7COþ upon each collision with CO can be estimated to be 8.5% by using effective diameters of Zr3O7þ and CO as 1.0 and 0.22 nm, respectively. Since one collision with He may not be enough to stabilize the metastable complex, the relatively low efficiency (8.5%) is reasonable. 5243


The Journal of Physical Chemistry A It should be pointed out that it is hard to determine the vibrational temperature Tvib of the laser ablation generated clusters. A spectroscopic study by using the relative intensities of the hot bands of VO2 suggested that the Tvib value for the clusters from laser ablation can be as high as 700 K.44 However, the above estimations involved with stabilization of Zr3O7COþ by collision with He propose that when the clusters interact with small molecules (CO and others), the Tvib should not be much higher than room-temperature in our experiments because kdis increases significantly as Tvib increases. For example, kdis(Zr3O7COþ/S1 f Zr3O7þ þ CO) values are calculated to be 1.5 108, 3.6 108, and 1.6 1010 s1 for Tvib = 350, 400, and 700 K, respectively. Note that the dissociation rate 3.6 108 s1 at 400 K is already much higher than the highest cooling rate (kcollision ≈ 5.0 107 s1). As a result, it could be safe to bracket the Tvib in between 300 and 400 K for the clusters generated in our previous works,11,15,20,28,29 and this study further suggests that the Tvib may be close to the lower limit (300 K). Figure 6b shows that CO may be trapped in a shallow potential well (D0 = 0.35 eV) in the anionic system Zr3O7CO/S2. However, similar VTST calculations as used for Zr3O7COþ/S1 indicate that at Tvib = 298 K, kdis(Zr3O7CO/S2fZr3O7þ CO) is about 1.6 1010 s1, which is much faster that the cooling rate in the fast flow reactor. This means that the experimentally observed Zr3O7CO (Figure 2c) can not have the structure of Zr3O7CO/S2 in Figure 5. More discussion for the observation of Zr3O7CO will be presented below. 4.2. New Insights into the Reaction of CO with Zirconium Oxide Cluster Anions. By using the guided-ion-beam mass spectrometry and DFT calculations,9 Johnson and co-workers determined that there is a mononuclear oxygen-center radical (O•)2h,7,8,21,40 in each of the ZrxO2xþ1 (x = 14) clusters (ZrO3, Zr2O5, Zr3O7, and Zr4O9), and CO molecules can approach the reactive O• centers in the ZrxO2xþ1 cluster barrierlessly to form CO2 units (such as in Zr3O7CO/S1 shown in Figure 5). The DFT-calculated total Mulliken atomic charge over the CO2 unit in Zr3O7CO/S1 is 0.70 |e|, so such a CO2 unit can be best described as CO2. Natural bond orbital analysis45 indicates that in Zr3O7CO/S1, the CO2 interacts with Zr3O6 mainly through electron donation from a lone pair (LP) orbital [O/2s (∼60%) and O/2p (∼40%)] around one related O atom in CO2 to the empty LP orbital (>90% Zr/4d) around the interacting Zr4þ site. The DFT-calculated electron affinities of Zr3O6 and CO2 are 2.74 and 0.51 eV, respectively. As a result, the dissociation channel of Zr3O7CO/S1 f Zr3O6 þ CO2 is much less favorable than Zr3O7CO/S1 f Zr3O6 þ CO2 (channel T13 in Table 1), and the dissociation involving CO2 is not further considered in interpreting the experimental data. In the reported experiments, isolated ZrxO2x and CO2 rather than association products ZrxO2xþ1CO are observed in the reactions of ZrxO2xþ1 with CO.7 Our fast flow reaction experiments for small clusters (ZrO3 and Zr2O5) do agree with the above investigations: (1) fast depletion of ZrO3 and Zr2O5 as well as the corresponding appearance of ZrO2 and Zr2O4 can be observed upon the interaction of the clusters with CO in the reactor, and (2) no apparent signals for ZrO3CO and Zr2O5CO can be observed. However, for large clusters starting from Zr3O7 (Figure 2c), the major reaction channel between ZrxO2xþ1 and CO is the association reaction under our experimental conditions. Note that in Figure 2c (left side), very weak signal appearance of Zr3O6 (from Zr3O7 þ CO f Zr3O6 þ CO2) can

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also be identified, whereas the major product that corresponds to depletion of Zr3O7 is Zr3O7CO. The DFT calculations indicate that the ZrO bond energy (D0) of ZrxO2xþ1 increases from 4.33 eV (x = 1) to 4.70 eV (x = 2) and to 4.84 eV (x = 3).46 The OCO bond energy is 5.45 eV47 (5.44 eV by B3LYP). As a result, the formation of CO2 from ZrxO2xþ1 þ CO f ZrxO2x þ CO2 becomes less and less exothermic as the cluster size increases. In addition, Figures 5 and 6b indicate that CO2 can be trapped by Zr3O6 as Zr3O7CO/S1 with a significantly binding energy of 1.18 eV (see also the observation of Zr3O6CO2 in Figure 2e). Reference 9 indicated that the binding energy of CO2 with Zr2O4 is 1.07 eV. As a result, the collisions by He in the fast flow reactor have a greater chance to stabilize the reaction intermediate Zr3O7CO than to stabilize Zr2O5CO. This is consistent with observation of the former association product rather than the latter one in our fast flow reaction experiments. Meanwhile, the CID experiment (Figure 3d) further proves that the CO molecule is really oxidatively absorbed as in Zr3O7CO/S1 rather than nonoxidatively absorbed as in Zr3O7CO/S2 (Figure 5). The highest gas pressure used in reactor for reaction of Zr3O7 with CO in the previous study9 is 20 mTorr (2.66 Pa, which is much smaller than the bath gas pressures (∼260 Pa) used in this work. The slow and fast cooling by the bath gas in different works can be one of the reasons to observe different products (isolated Zr3O6 and CO2 versus Zr3O7CO) in the reaction of Zr3O7 with CO. Note that other issues such as cluster temperature that is involved with Evib and cluster velocity that is involved with Ek (see subsection 4.1) may also be slightly different in different works.48 The reaction of Zr3O7 with CO is thus a good system to demonstrate that reaction conditions (pressure and temperature) as well as energetics (heat of formation and excess energy) are important factors involved with competition of dissociation versus stabilization for metastable reaction intermediates. The current study also suggests that in addition to the oxidation step, such as CO oxidation by oxygen-centered radicals,2h,79 the CO2 desorption step may also be very important in terms of catalytic oxidation of CO to CO2 under condensed phase and high pressure conditions.

5. CONCLUSIONS The interactions between CO and the Zr4þ centers in zirconium oxide cluster cations are strong enough (provide sufficient time for cooling by bath gas) to stabilize the association products ZrxOyCOþ after the reaction of the generated ZrxOyþ with CO in the fast flow reactor. The CO oxidation by superoxide radicals (O2•) in Zr3O7þ and possibly other ZrxOyþ clusters can not happen at low temperature due to high reaction barriers and loss of CO rather than CO2 takes place upon CID of ZrxOyCOþ by the crossed He beam. The interactions between CO and the Zr4þ centers in ZrxOy are weak so that ZrxOyCO are generally not (or not efficiently) produced in the fast flow reaction experiments. However, CO can be easily oxidized to CO2 by the mononuclear oxygen-centered radical (O•) over specific clusters ZrxO2xþ1 (Zr3O7, Zr4O9...) and the large binding energy of CO2 with ZrxO2x and relatively small heat of reaction (ZrxO2xþ1 þ CO f ZrxO2x þ CO2) can result in stabilization of reaction intermediates ZrxO2xþ1CO or ZrxO2xCO2 that lose CO2 rather than CO in the CID experiments. The desorption of CO2 may be an important step in the catalytic CO oxidation to CO2 under condensed phase and high 5244


The Journal of Physical Chemistry A pressure conditions. The CID with the crossed helium beam has been demonstrated to be a relatively soft collision method that may be further used to study oxidative and nonoxidative adsorption of other small molecules over atomic clusters.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: þ86-10-62536990; fax: þ86-10-62559373.

’ 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), Major Research Plan of China (No. 2011CB932302), and the Project Sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry. ’ REFERENCES (1) (a) Ertl, G.; Knozinger, H.; Weikamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 1997. (b) Fierro, J. L. G. Metal Oxides; Taylor & Francis Group: Boca Raton, FL, 2006. (c) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (d) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25. (2) (a) Armentrout, P. B. Annu. Rev. Phys. Chem. 2001, 52, 423. (b) Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. B 2002, 106, 6136. (c) O’Hair, R. A. J.; Khairallah, G. N. J. Cluster Sci. 2004, 15, 331. (d) B€ohme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336. (e) Gong, Y.; Zhou, M. F.; Andrews, L. Chem. Rev. 2009, 109, 6765. (f) Roithova, J.; Schr€oder, D. Chem. Rev. 2010, 110, 1170. (g) Zhai, H.-J.; Wang, L.-S. Chem. Phys. Lett. 2010, 500, 185. (h) Zhao, Y.-X.; Wu, X.-N.; Ma, J.-B.; He, S.-G.; Ding, X.-L. Phys. Chem. Chem. Phys. 2011, 13, 1925. (3) (a) Schr€oder, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139. (b) Gracia, L.; Andres, J.; Safont, V. S.; Beltran, A. Organometallics 2004, 23, 730. (c) Gracia, L.; Sambrano, J. R.; Andres, J.; Beltran, A. Organometallics 2006, 25, 1643. (d) Gracia, L.; Polo, V.; Sambrano, J. R.; Andres, J. J. Phys. Chem. A 2008, 112, 1808. (e) Sambrano, J. R.; Gracia, L.; Andres, J.; Beltran, A. J. Phys. Chem. A 2004, 108, 10850. (4) Schwarz, H. Angew. Chem., Int. Ed. 2003, 42, 4442. (5) (a) Reilly, N. M.; Reveles, J. U.; Johnson, G. E.; Khanna, S. N.; Castleman, A. W., Jr. Chem. Phys. Lett. 2007, 435, 295. (b) Reilly, N. M.; Reveles, J. U.; Johnson, G. E.; Khanna, S. N.; Castleman, A. W., Jr. J. Phys. Chem. A 2007, 111, 4158. (c) Reilly, N. M.; Reveles, J. U.; Johnson, G. E.; del Campo, J. M.; Khanna, S. N.; Castleman, A. W., Jr. J. Phys. Chem. C 2007, 111, 19086. (6) Xue, W.; Wang, Z.-C.; He, S.-G.; Xie, Y.; Bernstein, E. R. J. Am. Chem. Soc. 2008, 130, 15879. (7) Johnson, G. E.; Tyo, E. C.; Castleman, A. W., Jr. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18108. (8) Johnson, G. E.; Mitric, R.; Tyo, E. C.; Bonacic-Koutecky, V.; Castleman, A. W., Jr. J. Am. Chem. Soc. 2008, 130, 13912. (9) Johnson, G. E.; Mitric, R.; N€ossler, M.; Tyo, E. C.; Bonacic-Koutecky, V.; Castleman, A. W., Jr. J. Am. Chem. Soc. 2009, 131, 5460. (10) (a) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124, 7499. (b) Hagen, J.; Socaciu, L. D.; Elijazyfer, M.; Heiz, U.; Bernhardt, T. M.; W€oste, L. Phys. Chem. Chem. Phys. 2002, 4, 1707. (c) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; W€oste, L.; Heiz, U.; H€akkinen, H.; Landman, U. J. Am. Chem. Soc. 2003, 125, 10437. (d) Kimble, M. L.; Castleman, A. W., Jr.; Mitric, R.; B€urgel, C.; Bonacic-Koutecky, V. J. Am. Chem. Soc. 2004, 126, 2526. (e) Kimble, M. L.; Moore, N. A.; Johnson, G. E.; Castleman, A. W., Jr.; B€urgel, C.; Mitric, R.; Bonacic-Koutecky, V. J. Chem. Phys. 2006, 125, 204311.

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Collision-Induced Dissociation and Density Functional Theory Studies

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