Photoreaction Study of Methanol Adsorption Complexes on VO2

Article pubs.acs.org/JPCC

Photoreaction Study of Methanol Adsorption Complexes on VO2(V2O5)n+ (n = 1−3) Clusters at 355 nm Bo Xu,†,‡ Jing-Heng Meng,†,‡ 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, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: VO2(V2O5)nCH3OH+ (n = 1−3) complexes are generated from the interactions of pregenerated VO2(V2O5)n+ clusters with CH3OH in a fast flow reactor. The photoreactions of these three adsorption complexes at 355 nm are characterized by a high-resolution tandem time-of-flight mass spectrometer. The major photoproducts of V3O7CH3OH+ are CH2O and V3O7H2+, whereas those of V5O12CH3OH+ and V7O17CH3OH+ are CH3OH and the corresponding VxOy+ (x, y = 5,12 and 7,17). Collision-induced dissociation and density functional theory/ RRKM calculations suggest that CH3OH is dissociatively adsorbed on V3O7+, whereas it is nondissociatively adsorbed on V5O12+ and V7O17+. This study suggests that the thermal dissociation of CH3OH to CH3O is the prerequisite for its photo-oxidation into CH2O, which may shed light on the photochemistry of methanol over transition metal oxide surfaces.

T

mechanisms of vanadium oxide clusters with CH3OH have been revealed in many studies;40−44 and (3) the natural abundance of 51V is 99.75%, and this is a benefit for mass spectrometry. Combined with density functional theory (DFT) and Rice−Ramsberger−Kassel−Marcus (RRKM) computations, we hope that this study can shed light on the photoreaction mechanism of CH3OH over TMO surfaces.

ransition metal oxides (TMOs) are widely used as both catalysts1−3 and photocatalysts.4,5 Numerous investigations have been carried out to understand the catalytic processes over TMO surfaces. Due to the complexities of the catalytic surfaces, the reaction mechanisms of these catalytic processes remain poorly understood in details. As one appealing approach, gas-phase studies of metal oxide clusters and their reaction behavior with small molecules can enhance our understanding of molecular level mechanisms for condensed-phase catalytic reactions. Several reviews on the studies of TMO clusters are available in the literature.6−15 Among these studies of the reactions of TMO clusters with small molecules, most of the attention was concentrated upon thermal reactivity, whereas photoreactivity was almost unexplored. To the best of our knowledge, only the photo-oxidation of propene on a V4O11− cluster with a femtosecond laser was studied.16,17 To understand the details of the photoreaction processes, investigations on photoreactions of small molecule adsorption complexes on TMO clusters are very important. Methanol is one of the simplest organic molecules and plays an important role in photocatalytic reactions on TMO surfaces.18,19 Many condensed-phase studies have focused on the photocatalytic processes of CH3OH on TMO surfaces.20−25 However, there are many debates about the photochemistry of CH3OH.21,22 To understand the molecular level details of CH3OH photocatalysis on a TMO surface, we conducted the photoreaction of methanol adsorption complexes on fully oxidized VO2(V2O5)n+ (n = 1−3) clusters at 355 nm. The reasons to study vanadium oxide cluster systems are (1) many experimental and theoretical investigations of vanadium oxide clusters have been reported, and the structures of vanadium oxide clusters are known;26−39 (2) the thermal reaction © 2014 American Chemical Society

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METHODS Experimental Methods. The laser ablation and supersonic expansion cluster source coupled with a fast flow reactor is the same as that in our previous works,39,45 whereas the tandem time-of-flight mass spectrometer (TOF-MS) was upgraded recently. A schematic diagram of the tandem TOF-MS is shown in Figure 1. Briefly, the VxOy+ clusters are pulse-generated by laser ablation of a rotating and translating vanadium disk in the presence of about 0.5% O2 seeded in a He carrier gas (99.999%) with a backing pressure of 5 atm. The clusters generated in a gas channel are expanded and reacted with helium-diluted CH3OH (CD3OH) in a fast flow reactor. By using the method in ref 46, the instantaneous total gas pressure in the fast flow reactor is estimated to be around 400 Pa at T = 300 K. The collision rate that a cluster (radius = 0.5 nm) experiences with the bath gas (radius = 0.05 nm, T = 300 K, P = 400 Pa) in the reactor is about 9.2 × 107 s−1 for an approaching velocity of 1 km/s. Because 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 Received: January 26, 2014 Revised: June 19, 2014 Published: July 21, 2014 18488

dx.doi.org/10.1021/jp5009187 | J. Phys. Chem. C 2014, 118, 18488−18495

The Journal of Physical Chemistry C

Article

Figure 1. Schematic diagram of the tandem TOF-MS coupled with a crossed gas beam and an ultraviolet laser system. Parts: P1, electrodes for accelerating ions; P2, deflectors; P3 and P7, einzel lenses; P4, P5, and P8, reflectors; P6, mass gate; P9, dual microchannel plate detector. The red line denotes the ion trajectory in the tandem TOF-MS.

algorithm method.48 Intrinsic reaction coordinate (IRC) calculations49,50 are also performed to verify that the TS actually connects two appropriate local minima in the reaction pathway. Time-dependent DFT51 is used to optimize the first singlet excited state of one IM in the reaction pathway of V3O7+ with CH3OH. The initial guess structure of the excited-state IM is from the optimized structure of IM at ground state. Vibrational frequency calculations are performed to check that reaction IMs and TSs have zero and only one imaginary frequency, respectively. Zero-point vibration corrected energies (ΔH0K) are reported in this work. All of the above computations are performed by employing hybrid B3LYP exchange-correlation functional52−54 and TZVP basis sets55 with the use of the Gaussian 09 program.56 The RRKM calculations are performed to estimate the rates for IM transformation and dissociation by using the equation

temperature after exiting the cluster formation channel with a supersonic expansion) to close to the bath gas temperature before reacting with the CH3OH molecules. Our recent experiments indicated that the cluster vibrational temperature in the reactor can be close to 300 K.45 After reacting in the fast flow reactor, the reactant and product ions are skimmed into the vacuum system of the tandem TOF-MS. The clusters of interest pass through two identical reflectors with a Z-shaped configuration39,47 and then are mass selected with a mass gate. The mass-selected ions (the adsorption complexes in this study) can interact with a crossed gas beam for structural characterization by collision-induced dissociation (CID). They can also be irradiated by a 355 nm (third harmonic output of a Nd:YAG laser, Continuum, Surelite II) beam to induce the photoreaction. The laser beam with the spot diameter of 7.0 ± 0.5 mm is not focused. In the photoreaction experiments, the energies of the laser pulse are varied in the range of 2.8−22.3 mJ (the shot-to-shot energy stability is about ±5%) with a pulse duration of 4−6 ns and a line width of 1 cm−1. After interacting with the gas beam or the laser beam, the daughter (fragment) and the parent ions pass through the third reflector and then are detected by a dual microchannel plate detector. The signals from the detector are recorded with a digital oscilloscope (LeCroy WaveSurfer 62Xs) by averaging 1000 traces of independent mass spectra. The uncertainties of the reported relative ion signals are about 10%. The overall mass resolution of the tandem TOF-MS is about 2000 (m/Δm). The mass resolution (Rs) of the secondary TOF-MS by which the daughter ion is analyzed can be defined by the equation Rs =

k(E) =

(2)

in which ρ(E) denotes the density of states of IM at energy E [= E1 (vibrational energy of clusters and CH3OH) + E2 (binding energy of IM) + E3 (center-of-mass collision energy between clusters and CH3OH)], N#(E − E#) represents the total number of states of the TS with a barrier E#, and h is the Planck constant. The direct count method proposed by Beyer and Swinehart57 is used for determining the number (N#) and density (ρ) of states under the approximation of harmonic vibrations. E1 (T = 298 K), E2, E#, and vibrational frequencies are all from the DFT calculations. For the IM dissociation, in which no distinct TS exists on the potential energy surface, the variational transition state theory58 is adopted.

mp(t p − td) (mp − md )Δtd

N # (E − E # ) hρ(E)

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(1)

RESULTS AND DISCUSSION Figure 2a displays the mass spectra of selected regions that cover VxOy+ (x = 3, 5, and 7) clusters. When helium-diluted CH3OH is pulsed into the reactor, the absolute intensities of V3O7+, V5O12+, and V7O17+ decrease while the signals at the position of Δmass = +32 amu increase simultaneously (Figure 2b). This suggests that each of these clusters can adsorb one CH3OH molecule:

in which mp and md are the masses of the parent and daughter ions, respectively; tp and td are the flight times of the parent and daughter ions, respectively; and Δtd is the full width at halfmaximum of the daughter ion peak. Using eq 1, Rs is calculated to be about 750 for the parent and daughter ions with masses of 479 and 447 amu, respectively. Computational Methods. The DFT computations are carried out to study thermal reaction pathways of V3O7+ and V5O12+ with CH3OH. The geometry optimizations of reaction intermediates (IMs) and transition states (TSs) are involved in the calculations of the reaction pathways. The initial guess structures of the TS species are obtained generally through relaxed potential energy surface (PES) scans using appropriate internal coordinates and then optimized using the Berny

VO2 (V2O5)n+ + CH3OH → VO2 (V2O5)n CH3OH+

(3)

(n = 1−3)

This reaction channel is confirmed by isotopic labeling experiment using CD3OH (Figure 2c). Note that each VO3(V2O5)n+ (n = 1−3) can also adsorb one CH3OH 18489



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photoreaction of V3O7CH3OH+ (Figure 3a) is V3O7H2+, which suggests the following reaction channel: hv

V3O7 CH3OH+ → V3O7 H 2+ + CH 2O

(4)

+

For the photoreaction of V5O12CH3OH (Figure 3c), V5O12+ and CH3OH are generated as the major products, whereas V5O12H2+ and CH2O are generated as the minor products. The reaction channel generating CH2O totally disappears, and CH3OH and V7O17+ are generated as the major products for the photoreaction of V7O17CH3OH+ (Figure 3e): hv

VxOy CH3OH+ → VxOy+ + CH3OH

(5)

(x , y = 5, 12 and 7, 17)

For the photoreaction of these three VO2(V2O5)nCH3OH+ (n = 1−3) complexes, a common channel generating CH4O2 and the corresponding VxOy−1+ is also observed. The isotopic labeling experiments with CD3OH confirm the above reaction channels. Moreover, the generation of CD2O suggests that both of the two H atoms in the generated CH2O are from the CH3 moiety in CH3OH. Panels a−c of Figure 4 show the mass spectra for the photoreaction of V 3 O 7 CH 3 OH + (see Figure S1 for V5O12CH3OH+ in the Supporting Information) at different laser fluences. It should be noted that some minor reaction channels appear at high laser fluence; for example, the channels generating CH3O and CH3OH are observed in the photoreaction of V3O7CH3OH+ (Figure 4a). The Ln-Ln plot of the power dependence of the photoreaction products of V3O7CH3OH+ and V5O12CH3OH+ is shown in Figure 4d. The linear fit yields a slope of 0.99 for the generation of CH2O from the photoreaction of V3O7CH3OH+, which suggests that CH2O is generated by the single-photon absorption process. The slope of the CH3OH channel from the photoreaction of V5O12CH3OH+ is 1.36, so the generation of CH3OH at low fluence is mainly due to the single-photon absorption process. The slopes of CH4O2 from photoreactions of V3O7CH3OH+ and V5O12CH3OH+ are 1.77 and 1.74, respectively, which indicates that the generations of CH4O2 are mainly due to twophoton absorption processes:

Figure 2. Selected TOF mass spectra for reactions of VxOy+ with He (a), CH3OH (b), and CD3OH (c). Numbers x,y denote VxOy+ and x,y,X denote VxOyX+ (X = CH3OH, CD3OH, and H2O).

molecule, but these adsorption complexes are not discussed in this study. Figure 3 shows the mass spectra for the photoreactions of VO2(V2O5)nCH3OH+ and VO2(V2O5)nCD3OH+ (n = 1−3). To display the daughter ion peaks in the mass spectra more clearly, the peak heights of the daughter ions are amplified and shown by the red lines. The major product ion for the

2hv

VxOy CH3OH+ ⎯→ ⎯ VxOy − 1+ + CH4O2

(6)

(x , y = 3, 7 and 5, 12)

In this study, we focus on the photo-oxidation of CH3OH to CH2O, so the photoreaction channels generating CH4O2 and CH3O are not discussed below. As the starting points for the photoreactions, the structures of the adsorption complexes need to be determined. The CID studies of V3O7CH3OH+ and V5O12CH3OH+ are conducted to investigate the adsorption states of CH3OH on V3O7+ and V5O12+ clusters, respectively. For collision of a cluster with the collision gas, the center-ofmass collision energy (Ecm) can be calculated by using eq 7

Ecm = E k × μ/M

(7)

in which Ek is the ion kinetic energy in the laboratory frame, M is the mass of the cluster, and μ is the reduced mass of the cluster with collision gas. As shown in Figure 5a, the losses of CH3OH, CH3O, and CH2O are identified in the collision between V3O7CH3OH+ and He with Ecm of 18.0 eV (note that the collision does not lead to the severe fragment of V3O7

Figure 3. TOF mass spectra for photoreactions of mass-selected V 3 O 7 CH 3 OH + (a), V 3 O 7 CD 3 OH + (b), V 5 O 12 CH 3 OH + (c), V5O12CD3OH+ (d), V7O17CH3OH+ (e), and V7O17CD3OH+ (f) at 355 nm with laser fluence of about 20 mJ/cm2/pulse. Numbers x,y denote VxOy+ and x,y,X denote VxOyX+ (X = CH3OH and CD3OH). 18490



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Figure 4. (Left) TOF mass spectra for photoreactions of mass-selected V3O7CH3OH+ at laser fluences (mJ/cm2/pulse) of about 57.9 (a), 23.1 (b), and 9.0 (c). (Right) Ln-Ln plot of photoproduct yield (percentage of the parent ion) versus laser fluence.

increases, whereas that of CH2O decreases, in the CID of V3O7CH3OH+ with Ar (Figure 5c), which suggests that CH3O is generated from direct dissociation, whereas CH2O is generated from dissociation after isomerization because the direct dissociation of the parent ion with high internal energy is usually faster than isomerization.59 The most efficient loss of CH3O in the collision of V3O7CH3OH+ with Ar indicates that the O−H bond of CH3OH is cleaved upon formation of this cluster complex. The CID of V5O12CH3OH+ with Ar is also conducted (Figure 5e), and the loss of CH3OH is more efficient than that of CH3O, which means that CH3OH is mainly nondissociatively adsorbed on V5O12+. The CIDs of isotopic-labeling V3O7CD3OH+ and V5O12CD3OH+ (Figure 5b,d,f) confirm the above dissociation channels. The thermal reaction pathway of CH3OH with V3O7+ is computed by DFT to investigate the structure of V3O7CH3OH+. The structure of V3O7+ had been determined by infrared photodissociation study combined with DFT calculations.38 V3O7+ has C3v symmetry and closed shell electronic structure. All three V atoms are 4-fold coordinated, whereas the oxygen atoms have three different coordination numbers. One of the oxygen atoms is 3-fold coordinated (denoted O1), and the bond lengths of V−O1 are much longer than those of other V−Oi bonds. Natural bond orbital (NBO) analysis60 suggests that the natural charge distribution on V1 (Figure 6a) is 1.03 |e|. In addition, the local natural charge distribution around V1 is calculated using the equation12,61

Figure 5. TOF mass spectra for collision of mass-selected V 3 O 7 CH 3 OH + with He (a), V 3 O 7 CD 3 OH + with He (b), V 3 O 7 CH 3 OH + with Ar (c), V 3 O 7 CD 3 OH + with Ar (d), V5O12CH3OH+ with Ar (e), and V5O12CD3OH+ with Ar (f). The center-of-mass collision energies are 18.0, 17.8, 160.8, 159.4, 105.0, and 104.4 eV, respectively. The peak heights of the daughter ions are amplified and shown by the red lines.

N

moiety, because only a fraction of Ecm is actually converted into internal energy). These fragment products can be generated from direct dissociation or dissociation after isomerization, so the adsorption state of CH3OH on V3O7+ cannot be determined just from the CID of V3O7CH3OH+ with He. Because the yield of the fragment product generated from a specific dissociation type is associated with the internal excitation energy of the parent ion. The CID experiments of V3O7CH3OH+ at different collision energies should be conducted to determine the adsorption state of CH3OH on V3O7+. In our experiments, the collision gas is changed to Ar to increase the collision energy. The Ecm for the collision of V3O7CH3OH+ with Ar is about 160.8 eV, which is much larger than that of V3O7CH3OH+ with He. The yield of CH3O

Q L = Q (M) +

∑ Q (Oi)/fi i=1

(8)

in which Q(M) and Q(Oi) are the natural atomic charges on atoms M and Oi, which is directly bonded to M and f i-fold coordinated in the cluster, respectively. The calculated result suggests that the local natural charge distribution around V1 is 0.33 |e|. When nucleophilic CH3OH moves close to the positively charged V3O7+, the O atom of CH3OH bonds with V1 and the V1−O1 bond dissociates barrierlessly. (The structure arrangements caused by adsorption were also observed in the previous studies.38,39) The lengths of V1−Oi bonds in V3O7+ and in the generated IM1 are given in Table S1 (Supporting Information). The energy of 2.45 eV is released in 18491



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Figure 6. DFT calculated reaction pathways for V3O7+ + CH3OH → V3O7H2+ + CH2O (a) and V5O12+ + CH3OH → V5O12H2+ + CH2O (b). The reaction intermediates and transition states are denoted IMm and TSn, respectively. A singlet to triplet crossing point is marked with an asterisk. The first singlet excited state of IM2 is denoted IM2′. The zero-point vibration corrected energies (ΔH0K in eV) are given. Some of the bond lengths are given in picometers.

Table 1. Calculated Rates of H Atom Transfer and Dissociation of IMs in the Reaction Pathways of V3O7+ and V5O12+ with CH3OH V3O7+ + CH3OH reaction step IM1 IM2 IM1 IM3

→ → → →

TS1 → IM2 TS2 → IM3 R1 P1

V5O12+ + CH3OH −1

k(s ) 6.0 1.2 3.1 6.2

× × × ×

reaction step 5

IM4 IM5 IM4 IM6

10 102 101 109

this step, which leads to the transfer of H atoms to oxygen atoms. Because the transferred H atom can be from the OH moiety or CH3 moiety, four different reaction pathways are considered in this study. The most favored reaction pathway is given in Figure 6a, whereas the others are given in Figures S2− S4 (Supporting Information). In Figure 6a, the H1 atom from

→ → → →

TS3 → IM5 TS4 → IM6 R2 P2

k(s−1) 7.0 1.3 1.2 5.3

× × × ×

102 10−3 10−3 108

the OH moiety transfers to the bridge-bonded O2 with a barrier of 1.34 eV. Then the H2 atom transfers from the CH3 moiety to the O4 atom with a barrier of 1.94 eV. (Note that there is a spin conversion from singlet TS2 to triplet IM3;62,63 the details of the spin conversion are given in Figure S5). 18492



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the generation of an oxygen-centered radical (O•−). As revealed in previous studies, the O•− is very oxidative toward the C−H bond12 and can transfer within the cluster,64,65 so it can be proposed that the H atom transfer from the CH3 moiety to O•− takes place easily and CH2O can be generated in the electronically excited state. For CH3OH on VO2(V2O5)n+ (n = 1−3) with different adsorption states, the photoproducts are different. The dissociatively adsorbed CH3OH on V3O7+ is photo-oxidized to CH2O, whereas the nondissociatively adsorbed CH3OH on V5O12+ and V7O17+ is only detached from the adsorption complexes. It can be seen from this gas-phase study that the thermal dissociation of CH3OH to CH3O is the prerequisite for its photo-oxidation into CH2O. In the condensed-phase studies concerning the photocatalytic chemistry of methanol on model TiO2 surface, CH2O is also the major photoproduct.21−25 Temperature-programmed desorption studies21,23,24 suggested that the nondissociatively adsorbed CH3OH is photoinactive and that the key step in methanol photochemistry is the thermal decomposition of CH3OH to CH3O, which was proposed to be the photochemically active form for holescavenging reactions of methanol on rutile TiO2. As a result, a two-step mechanism was proposed for the photo-oxidation of methanol to CH2O: the first step is the thermal dissociation of CH3OH to CH3O, and the second step is the C−H bond cleavage initiated photochemically. Therefore, both the gasphase work in this study and the condensed-phase studies21,23,24 suggest that a defined thermal reactivity is a crucial prerequisite for the photochemistry of CH3OH. It is noteworthy that a similar conclusion was also proposed for the photo-oxidation of C3H6 on the V4O11− cluster: the thermally motivated H abstraction and covalent linkage of the hydrocarbon to the cluster system at the dioxo structure result in the selective liberation of allyl and allyloxy radicals for photoreaction of V4O10(OH)C3H5− species.16,17

Finally, the CH2O moiety is detached from IM3, and triplet V3O7H2+ and singlet CH2O are generated. The rates of H atom transfer and dissociation of IMs are given in Table 1. The rates of H1 atom transfer (IM1 → TS1 → IM2) and H2 atom transfer (IM2 → TS2 → IM3) are 6.0 × 105 (kt1) and 1.2 × 102 s−1 (kt2), respectively, whereas the collision rate that a cluster experiences with the bath gas in the fast flow reactor is about 9.2 × 107 s−1 (kcollision). Considering the computation errors of the rate calculations, that is, kt1 may be slightly underestimated and kcollision may be overestimated, IM1 can be converted into IM2, which is supported by the experiment that loss of CH3O dominates the mass spectrum for the collision of V3O7CH3OH+ with Ar (Figure 5c). In contrast, IM2 cannot be converted into IM3 because kt2 is too much smaller than kcollision. The CID study of V3O7CH3OH+ and the rate calculations suggest that CH3OH can be dissociatively adsorbed on V3O7+. The thermal reaction pathway of V5O12+ with CH3OH (Figure 6b) is also computed. The structure of V5O12+ had been studied using a genetic algorithm-based program.39 Similar to V3O7+, V5O12+ has a closed shell electronic structure, and there is also a 3-fold coordinated oxygen atom (denoted O5). NBO analysis suggests that the natural charge distribution on V2 in V5O12+ is 1.03 |e| and that the local natural charge distribution around V2 is 0.24 |e|. The adsorption of nucleophilic methanol on V2 site induces dissociation of the V2−O5 bond (the V2−Oj bonds in V5O12+ and IM4 are given in Table S2 in the Supporting Information). The released energy of 2.28 eV in this step is slightly smaller than the binding energy of IM1 in the reaction pathway of V3O7+ with CH3OH, which suggests that the different distributions of the natural charge around the adsorption sites in V3O7+ and V5O12+ play a minor role in the difference between the reaction pathways of V3O7+ and V5O12+ with CH3OH. The released energy leads to the transfer of two H atoms, and then singlet CH2O and triplet V5O12H2+ are generated. The energy diagram of the reaction of V5O12+ with CH3OH is similar to that of V3O7+ with CH3OH. However, due to the larger number of vibrational degrees of freedom for V5O12CH3OH+ in comparison to V3O7CH3OH+, the rate of the first H atom transfer (kt3 = 7.0 × 102 s−1, in Table 1) is much smaller than the rate of H1 atom transfer in the reaction pathway of V3O7+ with CH3OH (kt1) and the collision rate in the fast flow reactor (kcollision). Therefore, CH3OH is mainly nondissociatively adsorbed on V5O12+, which is consistent with the CID study of V5O12CH3OH+ indicating that the loss of CH3OH is the most efficient. For the thermal reaction of V7O17+ with CH3OH, the larger number of vibrational degrees of freedom will result in an even smaller rate of H atom transfer than kt3, so it can be predicted that CH3OH is nondissociatively adsorbed on V7O17+. For the photoreaction of VO2(V2O5)nCH3OH+ (n = 1−3), the reaction pathways in the electronically excited state should be considered. However, due to the great difficulty in the computation of electronically excited states, only the first singlet excited state of IM2 is calculated by TDDFT in this work (Figure 6a). The computed results suggest that the vertical excitation energy of IM2 is 3.45 eV, which is smaller than the energy of a photon with the wavelength of 355 nm (3.49 eV). The adiabatic excitation energy of the first singlet excited state is computed to be 2.66 eV, which means that there can be sufficient additional energy (0.83 eV) for the reaction in the electronically excited state. The photoexcitation involves

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CONCLUSIONS The photoreactions of VO2(V2O5)nCH3OH+ (n = 1−3) have been studied by tandem mass spectrometry combined with DFT/RRKM calculations. The dissociatively adsorbed CH3OH on V3O7+ is photo-oxidized to CH2O, whereas the nondissociatively adsorbed CH3OH on V5O12+ and V7O17+ is photoinactive. At a strictly molecular level, this study identifies the thermal dissociation of CH3OH to CH3O as a prerequisite for its photo-oxidation into CH2O, which parallels similar results of the condensed-phase system. In addition, a unique reaction channel generating CH4O2 from the two-photon absorption is also observed. The mechanisms of generating CH2O and CH4O2 in the electronically excited states will be studied in the future.

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ASSOCIATED CONTENT

* Supporting Information S

Additional TOF mass spectra, DFT-calculated cluster structures and reaction pathways, and the complete author list of ref 56. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(S.-G.H.) E-mail: [email protected]. Phone: +86-1062536990. Fax: +86-10-62559373. 18493



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21325314), Major Research Plan of China (Nos. 2013CB834603 and 2011CB932302), and the Chinese Academy of Sciences (YZ201318).

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Photoreaction Study of Methanol Adsorption Complexes on VO2

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