RH and H2 Production in Reactions between ROH and Small

Article pubs.acs.org/JPCA

RH and H2 Production in Reactions between ROH and Small Molybdenum Oxide Cluster Anions Sarah E. Waller and Caroline C. Jarrold* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: To test recent computational studies on the mechanism of metal oxide cluster anion reactions with water [Ramabhadran, R. O.; et al. J. Phys. Chem. Lett. 2010, 1, 3066; Ramabhadran, R. O.; et al. J. Am. Chem. Soc. 2013, 135, 17039], the reactivity of molybdenum oxo−cluster anions, MoxOy− (x = 1 − 4; y ≤ 3x) toward both methanol (MeOH) and ethanol (EtOH) has been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. The size-dependent product distributions are compared to previous MoxOy− + H2O/D2O reactivity studies, with particular emphasis on the Mo2Oy− and Mo3Oy− series. In general, sequential oxidation, MoxOy− + ROH → MoxOy+1− + RH, and addition reactions, MoxOy− + ROH → MoxOy+1RH−, largely corresponded with previously studied MoxOy− + H2O/D2O reactions [Rothgeb, D. W.; Mann, J. E.; Jarrold, C. C. J. Chem. Phys. 2010, 133, 054305], though with much lower rate constants than those determined for MoxOy− + H2O/D2O reactions. This finding is consistent with the computational studies that suggested that −H mobility on the cluster−water complex was an important feature in the overall reactivity. There were several notable differences between cluster−ROH and cluster−water reactions associated with lower R−OH bond dissociation energies relative to the HO−H dissociation energy.

1. INTRODUCTION

Previous gas-phase reactivity studies of TMO clusters have been performed on cationic,15−23 anionic,24−26 and neutral27,28 species. Castleman and co-workers reported the effects of size and stoichiometry of vanadium and niobium oxide cluster cations with MeOH.21 Production of H2CO and C2H6O, both of which are common products in condensed-phase reactions, were inferred from their experiments. A study conducted by Goncharov et al. on molybdenum oxide cluster cation reactions with MeOH indicated that the most reduced clusters exhibited preferential C−H bond insertion, which resulted in alcohol dehydrogenation and aldehyde elimination, while clusters with near-stoichiometric composition favored C−O bond insertion, resulting in the formation of metal−alkyl groups and alcohol dehydrogenation.29 In both studies, the number of coordination sites and the oxidation state of the transition metal proved to be important factors that affect reactivity. Bernstein and co-workers investigated the reactions of neutral iron oxide clusters with MeOH and found evidence of MeOH dehydrogenation, among other reactions.30 Ferreira et al. used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to study first row TMO (Mn, Fe, Co, Ni, and Cu) anion reactivity with MeOH and also found MeOH dehydrogenation to be the primary process.26 Willet et al.

Transition-metal oxides (TMOs) and transition-metal-based materials are of interest in a wide range of areas including catalysis,1−3 solar cells,4,5 and organic electronic devices.6,7 The catalytic activity of TMOs has been attributed to their ability to support multiple low-energy electronic states and the ease with which electrons can be donated/accepted in the process of making/breaking bonds.8 Methanol (MeOH) and ethanol (EtOH) are commonly used to test the catalytic activity of bulk TMO materials,9−11 and direct alcohol fuel cells (DAFCs) are of interest as they are more compact and portable than hydrogen fuel cells and because alcohols have a higher volumetric energy density than hydrogen.12 There is ample motivation to explore the properties of TMO− alcohol interactions in more detail. However, bulk surfaces are difficult to characterize due to ambiguity over the nature and abundance of catalytically active surface sites and the dynamic nature of TMO surfaces during catalysis.13 Surface defect sites, such as oxygen vacancies, have been implicated in catalytic activity.14 Because surface chemical bonds are believed to have cluster-like properties,8 gas-phase clusters have been used as model systems for bulk catalyst surface sites. Cluster studies have resulted in detailed information about the interactions between transition-metal centers in various oxidation environments and a reactant. This information may ultimately inform efforts to design catalysts with improved selectivity, lower operating temperatures, and poison resistance. © 2014 American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: February 26, 2014 Revised: March 24, 2014 Published: March 24, 2014 8493

dx.doi.org/10.1021/jp502021k | J. Phys. Chem. A 2014, 118, 8493−8504

The Journal of Physical Chemistry A

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(Macron, anhydrous, ACS reagent grade) or EtOH (PharmcoAaper, absolute, ACS/USP grade) seeded in UHP He. Reactant gas mixtures were made at room temperature by exposing an evacuated mix tank to an alcohol reservoir for 10 min after static pressure was reached on a vacuum gauge (Grainger, −100 to 0 kPa, part no. 4FLT5) connected to the system. The final pressure of the alcohol in the mix tank was assumed to be the vapor pressure of the alcohol at room temperature, 13.02 and 5.95 kPa for MeOH and EtOH, respectively. The mix tank was then isolated, and 400 psig of UHP He was added to the tank. This process yielded the maximum concentration of 0.47% MeOH/He (1.888 psi of MeOH seeded in 400 psi of He) used in this study. Lower concentrations ranging from 0.24 to 0.05% were obtained through serial dilutions of the 0.47% mix. Equivalent EtOH/He mixtures range from 0.22% (0.863 psi EtOH/400 psi He) to 0.03%. The number density of MeOH or EtOH was varied both by changing the concentration of the alcohol in the reactant gas mixture and by varying the duration of the gas pulse issued from valve II (ca. from 3x) and hydroxides (MoO3H−, MoO4H−) are observed. After exposure to MeOH, the hydroxides increase in intensity, and MoO3CH3−, MoO4CH3−, MoO4CH4−, and MoO4C2H6− become evident in the mass spectra. An analogous change in the mass distribution is observed after EtOH reactions. The monohydroxides and alkoxides are indicative of −R, −H, −OR, or −OH abstraction by the bare oxides (MoOy−), which would require breaking a ∼4 eV bond43,44 to produce the neutral radical. However, the resulting hydroxide or alkoxide anion would be particularly stable. Wang and co-workers did an extensive study on MoO4H− and MoO4CH3− in particular and measured >4.5 eV electron affinities for these particularly stable anions in which the Mo center is in a +6 oxidation state.45

Figure 2. 98Mo2Oy− cluster distributions collected before (solid black traces) and after exposure to (a) MeOH and (b) EtOH (red dotted traces). Product distributions were collected with approximately 9800 MeOH collisions and 3300 EtOH collisions, respectively.

distribution measured after ROH exposure [red dotted traces; (a) MeOH, (b) EtOH]. The product distributions shown were measured with approximately 9800 MeOH collisions and 3300 EtOH collisions, respectively. Full series of mass spectra in the range of the x = 2 manifold measured with incremental increases in NROH in the reaction channel for both MeOH and EtOH are included in the Supporting Information (SI 3 and 4). The initial Mo2Oy− cluster distributions in panels (a) and (b) of Figure 2 exhibit oxides ranging from y = 2 to 7. The peak at 358 amu is Mo3O4−. Due to the mass coincidence of MeOH addition and oxidation products, EtOH reactions will be considered and used as a starting point for analyzing the MeOH reaction products. With EtOH exposure (Figure 2b), intensities of the lower oxides decrease in a manner similar to what was 8495



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observed in Mo2Oy− + H2O reactions, suggesting that sequential oxidation with C2H6 production via 98

Mo2Oy− + R OH → 98Mo2Oy + 1− + RH

The formation of Mo2O5C2H6− raises the possibility that a portion of the ions with a mass of 292 amu after MeOH exposure arises from Mo2O5CH4− product formation. The appearance of Mo2O6C4H10− and Mo2O7C4H10−, in conjunction with the observation of the two addition products, suggests that both Mo2O4− and Mo2O5− react sequentially with two EtOH molecules to produce a diethoxy complex and H2, as observed in the x = 1 series (eq 1). MeOH reactivity is found to be largely analogous to EtOH reactivity. Comparing the general intensities of the initial cluster distributions with the product distributions of both alcohols in the mass spectra reveals an interesting mise en scène. The mass spectrum measured after exposure to EtOH was collected with roughly one-third of the metal−alcohol collisions, yet the intensity of the lower oxides seemed to be more depleted, and the alcohol product intensities were similar to those in the mass spectrum measured after MeOH exposure. This suggests that EtOH reacts faster with Mo2Oy− clusters than MeOH. This observation is supported by the relative integrated intensities of (a) Mo2Oy− clusters and (b) primary addition reaction products plotted against the approximate number of MoxOy−−EtOH collisions, shown in Figure 3. Similar plots for

(2)

may be occurring for the more reduced portion of the Mo2Oy− series. Ions that are not observed in the initial cluster distribution fall into three categories that can be described as abstraction, ROH addition, and double addition products. Abstraction products are low-abundance and include Mo2O5H− (277 amu) Mo2O6H− (293 amu), Mo2O7H− (309 amu), and Mo2O7C2H5− (337 amu). Addition products include the low-intensity Mo2O5C2H6− (306 amu) complex and the more predominant Mo2O6C2H6− (322 amu) complex. Double addition products, Mo2O6C4H10− (350 amu) and Mo2O7C4H10− (366 amu), suggest that loss of H2 is involved. Masses appearing 2−6 amu lower than the bare Mo2Oy− peaks are possible addition/dehydrogenation products, Mo2Oy−1CHz− (z = 0 − 2) or Mo2Oy−2C2Hz− (z = 2 − 4), or carbonyls (carbonyl formation from similarly performed reactions between Pd− and MeOH have been observed in the past).46 The Mo2Oy−1CHz− and Mo2Oy−2C2Hz− peaks are more evident in MeOH studies (Figure 2a) than those in EtOH studies, presumably due to the higher mobility and vapor pressure of MeOH in the experiment, potentially allowing small quantities of MeOH to migrate to the higher-energy ablation/cluster cooling region of the source. Finally, much lower- intensity Mo2OyH2− (y = 5 and 6) products were observed, which could be evidence of ROH dehydrogenation, as previously interpreted by Bernstein and co-workers and Ferreira et al.,26,30 or of condensation, which appears feasible because of the observation of two ROH addition products. However, they are very minor products and will not be further discussed. The appearance of Mo2O5H− suggests that either Mo2O4− abstracts a −OH or that Mo2O5− abstracts a −H from EtOH. The bond dissociation energy for C2H5−OH is 0.41 eV lower than the C2H5O−H bond dissociation energy (CH3−OH bond dissociation is 0.58 eV lower than the CH3O− H bond dissociation energy);47 therefore, −OH abstraction is energetically favored over −H abstraction. Similarly, the appearance of Mo2O6H− and Mo2O7H− could be due to either −OH or −H abstraction processes. In both cases, a stable, closedshell anion is formed, and in particular, Mo2O7H− and Mo2O7R− products permit both Mo centers to be in +6 oxidation states. However, the intensity of the stoichiometric Mo2O6− is fairly constant (vide infra), and the abstraction products are not very intense. The HO−H bond dissociation energy is 1.17 eV higher than the CH3−OH bond dissociation energy, which may explain why abstraction products were not observed in the cluster−water reactivity studies.36,37 The most predominant product peak, Mo2O6C2H6− (Figure 2b) is consistent with the addition reaction 98

Mo2O5− + C2H5OH → 98Mo2O6 C2H6−

(3)

which is analogous to the water addition reaction that terminated the sequential oxidation of the Mo2Oy− suboxide clusters, which was also at y = 5. Comparing the intensity of the Mo2O6C2H6− peak to the mass spectrum collected after MeOH exposure (Figure 2a), we infer that most of the intensity associated with the peak at 308 amu, which coincides with the mass of Mo2O7−, is Mo2O6CH4−. Additionally, we observe Mo2O5C2H6− (306 amu), which is evidently due to addition of EtOH to Mo2O4−. An analogous peak was not observed in water reactions.

Figure 3. Plot of the relative integrated intensities of (a) 98Mo2Oy− and (b) primary reactivity products as a function of the approximate number of cluster−EtOH collisions. Note the difference in the y-axis scales.

MoxOy−−MeOH distributions, which are complicated by mass coincidences, can be found in the Supporting Information (SI 5). Suboxide (y < 6) cluster intensities essentially decay toward zero intensity. This observation, coupled with the fairly constant 8496



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Table 1. Pseudo-First-Order Rate Constants for Mo2Oy− + EtOH Reactions Determined from Equations 14 and 15 and Second-Order Rate Constants Determined by Multiplying Pseudo-1° Rate Constants by the Collisional Volume Per Time, 2 × 10−16 m3·s−1a pseudo-1° rate constant (collision−1)

reaction

a

+ CH3CH 2OH →

Mo2O−3

+ CH3CH 2OH →

Mo2O−4

2° rate constant (m3·s−1)

−3

4.4(8) × 10−19

−3

1.53(2) × 10

3.06(9) × 10−19

7.5(2.3) × 10−3

1.5(5) × 10−20

−3

3.8(3) × 10

7.6(6) × 10−19

2.2(4) × 10

k1

Mo2O−2

k2

Mo2O−3

k3a

Mo2O−4 + CH3CH 2OH → Mo2O−5 + C2H6

k3b

Mo2O−4

k3c

Mo2O−4 + CH3CH 2OH → Mo2O5C2H−6

2.1(1) × 10−3

4.2(2) × 10−19

k4

− Mo2O5C2H−6 + CH3CH 2OH → Mo2O6C4 H10 + H2

4.5(7) × 10−4

9.0(1.4) × 10−20

k5a

Mo2O−5

−4

5.7(1.2) × 10−20

k5b

Mo2O−5 + CH3CH 2OH → Mo2O6H− + C2H5

k5c

Mo2O−5

k6

− Mo2O6C2H−6 + CH3CH 2OH → Mo2O7 C4 H10 + H2

k7a

Mo2O−6

k7b

Mo2O−6 + CH3CH 2OH → Mo2O7 H− + C2H5

k7c

Mo2O−6

+ C2 H6

+ C2 H6 −

+ CH3CH 2OH → Mo2O5H + C2H5

+ CH3CH 2OH →

+ CH3CH 2OH → + CH3CH 2OH → + CH3CH 2OH →

Mo2O−6

2.87(6) × 10

+ C2 H6

2.5(1) × 10−5

Mo2O−7

Mo2O7 C2H−5

6.2(1.0) × 10−19

3.1(6) × 10−4

6.2(1) × 10−20

−4

7.2(3.0) × 10−20

2.0(3) × 10−4

4.0(6) × 10−20

7.1(4.4) × 10−5

1.4(9) × 10−20

3.6(1.5) × 10

+ C2 H6 +H

5.0(2) × 1021

−3

3.1(5) × 10

Mo2O6C2H−6

Values in parentheses reflect the uncertainty in the last digit.

Mo2O6− intensity, is very similar to what was observed in previous Mo2Oy− + H2O reactions37 in which sequential oxidation with H2 production (Mo2Oy− + H2O → Mo2Oy+1 + H2) occurred for y < 5, while Mo2O5− added water to form Mo 2 O 6 H 2 − . Mo 2 O 5 C 2 H 6 − , Mo 2 O 6 C 2 H 6 − , Mo 2 O 7 C 2 H 5 − , Mo2O6C4H10−, and Mo2O7C4H10− peak intensities increase with the number of collisions, and the increases observed for Mo2O6C2H6− in particular closely follow the decrease in the Mo2O5− signal intensity, suggesting a direct reactant−product relationship. The increase in what appear to be double EtOH addition products is, as expected, slower than the single addition product observed. The inferred sequence of reactions for Mo2Oy− clusters with EtOH is summarized explicitly in Table 1. Mo3Oy− + ROH. The initial distribution of the Mo3Oy− cluster series (solid black traces) and the distribution after exposure to ROH [red dotted trace; (a) MeOH, (b) EtOH] are shown in Figure 4. The product distributions were collected with approximately 10 700 MeOH and 4000 EtOH cluster−alcohol collisions, respectively. A series of mass spectra collected with incremental increases in MeOH− and EtOH−cluster collisions are included in the Supporting Information (SI 6 and 7). The initial cluster distributions (black traces) exhibit Mo3Oy− clusters ranging from y = 3 to 10, with the stoichiometric oxide, Mo3O9−, having the greatest intensity. Because of the mass coincidence of MeOH addition and oxidation products, EtOH reaction patterns will again be considered first and used to inform the analysis of the MeOH reaction patterns. Upon exposure to EtOH (Figure 4b), evidence of sequential oxidation, abstraction, EtOH addition, and double addition reactions similar to those inferred in the Mo2Oy− + ROH series is observed. Abstraction products formed include Mo3O5H− (385 amu) and Mo3O7H− (407 amu). Addition products Mo3O6C2H6−, Mo3O8C2H6−, and Mo3O9C2H6− are the most intense products, while the Mo3O10C2H5− and Mo3O9C4H10− double alcohol addition products are lower in intensity. The bare suboxide, Mo3O6−, is the most intense cluster after EtOH exposure. Carbon- and hydrogen-containing metal−oxo species are again observed at masses 2−6 amu lighter than Mo3Oy− peaks. As with the x = 2 series, Mo3OyH2− (y = 6, 8, and 9) products, which may be

Figure 4. 98Mo3Oy− cluster distributions collected before (solid black traces) and after exposure to (a) MeOH and (b) EtOH (red dotted traces). Product distributions were collected with approximately 10 700 MeOH collisions and 4025 EtOH collisions, respectively.

indicative of ROH dehydrogenation or 2ROH condensation, were observed in much lower quantities. The lowest oxides, Mo3O3−, Mo3O4−, and Mo3O5−, are depleted after ROH exposure. By comparing the appearance of EtOH addition products with the changes in peak intensities in 8497



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the MeOH reactions, we can infer that both Mo3O3− and Mo3O4− primarily undergo oxidation with RH production, though Mo3O4− may also abstract a −OH to form Mo3O5H−, while Mo3O5− adds ROH to produce Mo3O6RH−. The mass of the Mo3O5− + MeOH addition product coincides with the mass of Mo3O7−, which appears to increase in intensity with MeOH exposure in Figure 4a, while the Mo3O5− + EtOH addition product, Mo3O6C2H6−, is clearly evident in Figure 4b. Additional EtOH addition reaction products can be attributed to Mo3O7− + EtOH → Mo3O8C2H6− (452 amu) and Mo3O8−+ EtOH → Mo3O9C2H6− (468 amu). We therefore assert that Mo3O7− is very likely reacting with MeOH to form Mo3O8CH4− (mass coincident with Mo3O9−), while the apparent Mo3O7− peak intensity is maintained by the coinciding Mo3O5− + CH3OH addition reaction. The apparent increase in Mo3O10− after MeOH exposure arises from the Mo3O8− + MeOH → Mo3O9CH4− addition product. Both ROH reaction product distributions exhibit a peak that can be attributed to sequential reaction with two ROH molecules. In Figure 4a, the appearance of Mo3O9C2H6− (468 amu) is consistent with the reaction 98

Mo3O7− + 2CH3OH → 98Mo3O9C2H6− + H 2

(4)

−

An analogous Mo3O7 + 2EtOH addition product is also observed, and low-intensity peaks at masses consistent with Mo3O8− + 2MeOH (484 amu, Figure 4a) and Mo3O8− + 2EtOH addition products are also observed. The intensity of the suboxide cluster, Mo3O6−, decreases slightly with EtOH exposure, though it remains fairly constant with MeOH exposure. The formation of Mo3O7H− (407 amu) is a more pronounced product after EtOH exposure; it is less evident after MeOH exposure, which may in part be due to the intensity of the nearby Mo3O6CH4− ion peak. Regardless, this result suggests that Mo3O6− abstracts −OH from EtOH. Mo3O6− was found to be unreactive toward water; therefore, −OH abstraction represents a significant difference. The intensity of Mo3O9− also remains fairly constant after ROH exposure. The ethoxy complex, Mo3O10C2H5−, possibly formed from abstraction by Mo3O9−, is observed in small quantities, but the analogous Mo3O10CH3− methoxy product is not observed. As noted before, the product distributions shown in Figure 4a and b were collected with approximately 10 700 MeOH and 4000 EtOH collisions, respectively. However, the MeOH products are less intense than the EtOH analogues. This again suggests that Mo3Oy− + EtOH reactions are faster than MeOH reactions. Figure 5 shows the relative integrated intensities of (a) Mo3Oy− clusters and (b) major reactivity products as a function of approximate oxide−EtOH collisions. Similar plots of Mo3Oy− clusters and major MeOH reactivity products can be found in the Supporting Information (SI 8). The suboxide clusters, except Mo3O6−, exhibit a general loss in intensity. The Mo3O6− and Mo3O9− peak intensities change a small amount but over the long-term are fairly constant. The increase in the Mo3O6C2H6− peak intensity closely follows the decline of Mo3O5−. Furthermore, Mo3O8C2H6− and Mo3O9C2H6− intensities also closely follow the loss of Mo3O7− and Mo3O8−, respectively. Once again, intensities that trend one another are indicative of a reactant− product relationship. Predicted reactions for Mo3Oy− clusters with EtOH are expressed in Table 2. Mo4Oy− + ROH. Figure 6 shows the initial Mo4Oy− cluster distribution (solid black traces) and distributions observed after exposure to ROH [dotted red traces, (a) MeOH and (b) EtOH].

Figure 5. Plot of the relative integrated intensities of (a) 98Mo3Oy− and (b) primary reactivity products as a function of the approximate number of cluster−EtOH collisions. Please note the difference in the y-axis scales.

The mass spectra collected after ROH exposure were measured with approximately 10 600 MeOH and 4000 EtOH collisions, respectively. The mass spectra have a poor signal-to-noise ratio, and the resolution in this range is insufficient to unambiguously distinguish Mo4Oy−1 + EtOH → Mo4OyC2H6− products from bare Mo4Oy+2− bare oxides; therefore, the analysis of this cluster series will be very brief. The initial distribution shows Mo4Oy− clusters ranging from y = 5 to 12, with the suboxide Mo4O9− cluster being the most abundant. Mo5O8− (618 amu) and Mo5O9− (634 amu) are also observed in this mass range. After exposure to MeOH (Figure 6a), the intensity of Mo4O5− and Mo4O6− decreases to zero, while the intensity of Mo4O8− (or Mo4O7CH3−) becomes comparable to that of Mo4O9−. The other ion intensities do not change significantly. One new product peak appears, which, given the lower mass resolution, could correspond to Mo4O12CH3− (599 amu) or Mo4O12CH4− or Mo4O13− (600 amu). The product distribution after exposure to EtOH (Figure 6b) differs in that the Mo4O7− signal decreases and the intensity at the approximate mass of Mo4O10− (or Mo4O8C2H6−) increases. Because of the width of this peak and by comparing the overall intensity distribution to the MeOH product distribution, we can infer that the Mo4O7− + ROH reaction yields Mo4O8RH−. Overall, the trends in ion intensity with ROH exposure are similar to what was observed in Mo4Oy− + D2O reactions, where Mo4O8− and Mo4O8D2− appeared to be kinetic bottlenecks in water 8498



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Table 2. Pseudo-First-Order Rate Constants for Mo3Oy− + EtOH Reactions Determined from Equations 14 and 15 and Second-Order Rate Constants Determined by Multiplying Pseudo-1° Rate Constants by the Collisional Volume Per Time, 2 × 10−16 m3·s−1a pseudo-1° rate constant (collision−1)

reaction

a

+ CH3CH 2OH →

Mo3O−4

+ CH3CH 2OH →

Mo3O−5


−3

2.3(2) × 10−19

−3

2.5(1) × 10−19

2.8(5) × 10−4

5.6(6) × 10−20

−4

1.44(6) × 10−19

1.2(3) × 10−3

2.4(6) × 10−19

7.7(1.2) × 10−5

1.5(2) × 10−20

−4

2.2(7) × 10

4.4(1.4) × 10−20

6.0(2) × 10−4

1.20(4) × 10−19

−4

6.6(7) × 10

1.3(1) × 10−19

1.3(3) × 10−4

2.6(6) × 10−20

−4

3.5(1) × 10

7.0(2) × 10−20

1.14(8) × 10

k1

Mo3O−3

k2a

Mo3O−4

k2b

Mo3O−4 + CH3CH 2OH → Mo3O5H− + C2H5

k3a

Mo3O−5

k3b

Mo3O−5 + CH3CH 2OH → Mo3O6 C2H−6

k4a


k4b

Mo3O−6

k5a


k5b

Mo3O−7

k6


k7a

Mo3O−8

k7b

Mo3O−8 + CH3CH 2OH → Mo3O9C2H−6

5.9(6) × 10−4

1.2(1) × 10−20

k8


1.4(3) × 10−4

2.8(6) × 10−19

k9

Mo3O−9

−4

7.0(6) × 10−20

+ CH3CH 2OH →

Mo3O−6

+ C2 H6

1.26(7) × 10

+ C2 H6

7.2(3) × 10

+ C2 H6

−

+ CH3CH 2OH → Mo3O7 H + C2H5

+ CH3CH 2OH → + CH3CH 2OH →

+ CH3CH 2OH →

Mo3O8C2H−6 Mo3O−9

+ C2 H 6

Mo3O10 C2H−5

3.5(3) × 10

+H

Values in parentheses reflect the uncertainty in the last digit(s).

which is consistent with ROH addition to Mo4O9−. Additionally, in Figure 4b, a low-intensity product peak with mass corresponding to Mo4O10C4Hz− (z ≈ 12) is observed, suggesting that two alcohols may be able to add sequentially to Mo4O8−. However, we are unable to unambiguously assign this mass, and we cannot definitively assert whether the product reflects two complete EtOH additions or two additions with H2 production. B. Reaction/Rate Constant Analysis for MoxOy− (x = 2 and 3). Simple collision theory was invoked in previous MoxOy− + H2O/D2O reactivity studies to determine relative rate constants for cluster−water reactions.36,37 To compare those rate constants to the rates of MoxOy− + ROH reactions, the same analysis will be applied here. Individual oxide peaks in the x = 2 and 3 manifolds of MoxOy− vary as a function of alcohol (ROH) number density present in the reactor. On the basis of the products observed, five different reactions are suggested. Oxidation of the less oxidized clusters to form a more oxidized cluster via k

Mox Oy− + R OH → Mox Oy + 1− + R H

(5)

−

is the primary reaction for Mo2Oy (y = 2−4) clusters and for the Mo3O3− and Mo3O4− clusters. ROH addition via k

Mox Oy− + R OH → Mox Oy + 1R H−

(6)

is the primary Mo2O5− + ROH reaction, Mo3Oy− (y = 5, 7, and 8) + ROH reactions, and a minor reaction for the Mo2O4− and Mo2O6− + EtOH reactions. MoxOy+1R− product peaks indicate an alkoxy addition pathway of the form k

Mox Oy− + R OH → Mox Oy + 1R− + •H

Figure 6. 98Mo4Oy− cluster distributions collected before (solid black traces) and after exposure to (a) MeOH and (b) EtOH (red dotted traces). Product distributions were collected with approximately 10 580 MeOH collisions and (b) 3968 EtOH collisions, respectively.

(7)

This reaction is observed for Mo2O6− upon exposure to both alcohols and also for Mo3O9− with EtOH (but not MeOH). Furthermore, observation of MoxOyH− species indicates either a hydroxyl or hydride abstraction process, which could possibly proceed via

addition to the least oxidized clusters, with some water addition observed for y = 9−11. The intensity of Mo4O9− decreases slightly upon exposure to EtOH, while Mo4O10C2Hz− (z = 5−6) appears,

k

Mox Oy− + R OH → Mox Oy + 1H− + •R 8499

(8)



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or k

Mox Oy− + R OH → Mox Oy H− + R O•

(9)

As pointed out above, the R−OH bond dissociation energy is lower than the RO−H bond dissociation energy for both alcohols. Finally, several addition complexes formed by reaction 6 can undergo a reaction with a second ROH, to form the dialkoxy and H2 via k

Mox Oy + 1R H− + R −OH → Mox Oy + 2 R 2− + H 2

(10)

Double alkoxy addition products are observed for Mo2O4−, Mo2O5−, Mo3O7−, and Mo3O8−. Sequential oxidation reactions (eq 5) for all but the least oxidized clusters have competing production and depletion reactions for the bare oxides. That is, MoxOy− is both produced by oxidation of MoxOy−1− and consumed by subsequent reaction with ROH, which gives a rate law dNMoxOy− dt

= −k′j ·NMoxOy−·NR OH + ki′·NMoxOy−1−·NR OH

Figure 7. Representative plot used to determine pseudo-1° rate constants for the most reduced MoxOy− cluster, here Mo2O2−. The rate constant of the most reduced cluster in the x = 2 manifold is determined by fitting eq 14

(11)

For example, Figure 7 shows a plot of ln(IMo2O2−/IMo2O2,initial−) (intensities are shown in Figure 3) as a function of number of cluster−alcohol collisions, the slope of which gives −k1 = −3.8(3) × 10−4 collision−1. The linear fit equation, R2 value, and standard deviation are included in Figure 7 in order to present the typical quality of these fits. The standard deviation included in this panel is taken as the error in the rate constant. Figure 8 shows how the rate constant for oxidation of Mo2O3−, k2, is determined graphically by plotting eq 15. Again, the linear

where NMoxOy− and NROH represent the number densities of the clusters and ROH. The reactor residence time is not directly controlled in this experiment; rather, the alcohol number density is controlled by the initial ROH concentration in the mix tank supplying the reactant to the reaction channel and by the valve II pulse duration. We therefore express the rate law in terms of (dNMoxOy−/dc) by dividing the reaction rate by the collision frequency of a cluster with ROH molecules, z = dc/dt, to give dNMoxOy− dt dNMoxOy− · = dt dc dc

(12)

The collision frequency depends on NROH. However, on the basis of the ion intensity observed at the detector, an upper limit NMoxOy− in the reactor is approximated to be on the order of 105 cm−3 (1011 m3), which is 102−103 times lower than the typical ROH number density. The collision frequency is therefore effectively constant over the cluster residence time. Dividing the rate law by the collision frequency, assuming constant NROH, results in a pseudo-1° rate law dNMoxOy− dc

= −kj·NMoxOy− + ki·NMoxOy−1−

(13)

NMoyOx− values are assumed to be proportional to the integrated peak intensities, IMoyOx−. The pseudo-1° rate constant is determined by fitting the exponential decay observed for the lowest oxide in each manifold (Mo2O2− and Mo3O3−) plotted as a function of collisions ln

IMo2O2− IMo2O2,initial−

Figure 8. Representative plot used to determine pseudo-1° rate constants for MoxOy− reactions for y-values greater than the most reduced cluster, here Mo2O3−. Using the value of k1 determined in Figure 7, k1 is determined by fitting eq 15. Please note the scaling factor for the y-axis.

= −k1·c (14)

fit, R2 value, and the standard deviation of the slope are included in the graph. The value of k2 was determined to be 6.0(2) × 10−4 collision−1. Subsequent rate constants are determined in a similar fashion. Rate constants for Mo2Oy− + EtOH and Mo3Oy− + EtOH reactions are summarized in Tables 1 and 2, respectively, along with the subsequently determined 2° rate constants (determined by multiplying pseudo-1° rate constants (in units of collision−1) by z·NROH−1, the cluster collisional volume swept per time).

where k1 is the rate constant associated with the Mo2O2− + ROH → Mo2O3− + RH reaction. Rate constants for higher oxides can be sequentially and graphically determined. For example, once k1 is determined by fitting eq 14, the rate constant for the oxidation of Mo2O3−, k2, is determined by fitting dIMo2O3− dc

− k1·IMo2O2− = −k 2·IMo2O3−

(15) 8500



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Table 3. Sample Pseudo-First-Order Rate Constants for MoxOy− + CH3OH (x = 2 and 3) Determined from Equations 14 and 15 and Second-Order Rate Constants Determined by Multiplying Pseudo-1° Rate Constants by the Collisional Volume Per Time, 2 × 10−16 m3·s−1a pseudo-1° rate constant (collision−1)

reaction k1

Mo2O−2

+ CH3OH →

Mo2O−3

k2

Mo2O−3

+ CH3OH →

Mo2O−4

k1

Mo3O−3 + CH3OH → Mo3O−4 + CH4

k2a

Mo3O−4

k2b

Mo3O−4 + CH3OH → Mo3O5H− + CH3

+ CH3OH →

Mo3O−5

+ CH4

+ CH4

3.8(3) × 10

7.5(5) × 10−20

6.0(2) × 10

−4

1.2(4) × 10−20

3.7(4) × 10−4

7.4(8) × 10−20

−4

1.6(8) × 10−19

1.2(2) × 10−4

2.2(4) × 10−20

7.9(4) × 10

+ CH4


−4

a Values in parentheses reflect the uncertainty in the last digit. A complete list of reactions and rate constants can be found in the Supporting Information (SI 9).

products. A table containing various bond dissociation energies of MeOH/EtOH-related molecules can be found in the Supporting Information (SI 10). The most striking difference between cluster−water and cluster−ROH reactions is the observation of dialkoxide products for which there is no correlation with double water addition products. For example, Mo2O5− adds water to form Mo2O6H2−. A second water addition accompanied by H2 production would result in Mo2O7H2−, an ion that is not observed in cluster−water reactions. However, Mo2O5− adds ROH to form Mo2O6RH−, which can add a second ROH molecule to form Mo2O7R2− + H2. Simple DFT calculations48 on the enthalpies of the oxidation reaction (MoxOy− + ROH → MoxOy+1− + RH) predict that it is exothermic for all Mo2Oy− species leading up to the Mo2O6− stoichiometric cluster; therefore, the presence of both Mo2O5RH− and Mo2O6RH− species suggests that these are trapped intermediates that live long enough to undergo the second ROH addition. A summary of the calculated enthalpies of sequential oxidation are included in the Supporting Information (SI 11). The reactivity patterns in the Mo3Oy− and Mo4Oy− oxide series manifold are generally very similar to those seen in the previous water reactivity study with respect to oxidation (reaction 4) versus addition reactions (reaction 5). The notable differences between cluster−ROH and cluster−water reactivity are activity of Mo3O6− and Mo3O9− toward ROH (these oxides were largely unreactive in the presence of water) and the formation of dialkoxy products. Once again, the higher energy required to break the HO−H bond relative to R−OH or RO−H bonds could explain these minor differences. B. Comparison of Rate Constants Determined for MoxOy− + ROH with MoxOy− + H2O Reactions. Computational studies on several MoxOy− + H2O reaction paths suggested that −H mobility on the MoxOy+1H2− addition complex plays an important role in H2 production. This is because the lowest barrier pathway where the water can add to the cluster involves formation of two dihydroxide groups, one of which must convert to a hydride group in the proximity of the hydroxide in order to lead to H2 production.39,40 The primary motivation for performing MoxOy− + ROH (R = CH3, C2H5) reactivity studies was to determine whether MoxOy− + ROH reaction rate constants were smaller than those determined for MoxOy− + H2O reactions.37 If proton mobility is indeed an important factor, the lower mobility of an −R group in a cluster complex should result in smaller MoxOy− + ROH reaction rate constants, and trapped intermediates of the form MoxOy+1RH− could be more prevalent. The current results show definitively that the cluster−water reaction rate constants are larger than the cluster−ROH rate constants. For example, the pseudo-1° rate constant for

Table 3 summarizes the same information for the simplest MeOH reactions. Rate constants for all Mo2Oy− and Mo3Oy− reactions with MeOH can be found in the Supporting Information (SI 9). Fewer MeOH rate constants were able to be determined definitively due to mass coincidences with bare MoxOy− clusters. Rate constant values are determined from data in Figures 3 and 5, for which each point is an average of five intensity measurements. The uncertainty in the fits ranged from 5 to 20%, which reflects the average spread in intensity values determined over the entire collisional range. It is important to note that the intrinsic error is associated with the approximations made in calculating the number of collisions; therefore, these numbers are interpreted as order-of-magnitude approximations for the rate constants, and uncertainty in their relative magnitude is properly reflected in the tabulated errors. As summarized in Table 1, rate constants for the Mo2Oy− + EtOH reactions were found to lie between 1.53(2) × 10−3 and 7.5(2.3) × 10−3 collision−1, with the exception of most −OH abstraction processes and the final step in the H2 evolution processes, which are smaller by an order of magnitude. EtOH addition reaction rate constants are of the same order of magnitude as direct oxidation reactions. The Mo3Oy− series of rate constant analysis was more complicated due to lower integrated peak intensities and to the higher number of reactions occurring simultaneously. Nevertheless, the rate constants are of the same order of magnitude as those for the Mo2Oy− manifold. A comparison of the EtOH rate constants to their MeOH analogues indicates that MeOH rate constants are generally lower by about an order of magnitude. This is consistent with the relative intensities of EtOH and MeOH products in all mass spectra.

4. DISCUSSION A. Comparison of Products Formed in MoxOy− + ROH and MoxOy− + H2O Reactions. While most of the products observed in the current cluster−ROH study can be correlated with products observed in cluster−water reactions, the cluster− ROH notably produces larger quantities of abstraction products. Monohydroxide species (Mo2Oy+1H−) are either trace or absent in reactions with water, while a number of monohydroxides (and several monoalkoxides) were observed in reactions with ROH. As mentioned above, the bond dissociation energy of HO−H, 497.10 kJ mol−1 (5.178 eV), is considerably higher than the bond dissociation energies of H3C−OH and H3CH2C−OH, which are 384.93 (4.010 eV) and 391.2 kJ mol−1 (4.075 eV), respectively (the RO−H bond dissociations being ∼50 kJ mol−1 higher in both cases).47 This should result in a lower barrier to abstraction in reactions with ROH. Associated with these lower bond dissociation energies is the appearance of −R or −OR abstraction 8501



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Mo2O2− + H2O → Mo2O3− + H2 was determined to be 0.10(4) collision−1 (i.e., 1 in every 10 collisions results in product formation). This is several orders of magnitude larger than the values determined for Mo2O2− + MeOH → Mo2O3− + CH4, 3.8(3) × 10−4 collision−1 (1 in every 2500 collisions results in product formation) and Mo2O2− + EtOH → Mo2O3− + C2H6, 2.2(4) × 10−3 collision−1 (1 in every 500 collisions results in product formation). However, it is not clear why cluster−EtOH reactions occur systematically at a higher rate than cluster−MeOH reactions. On the basis of the calculations done on cluster−H2O reactions, charge−dipole and dipole−dipole interactions will govern the initial orientation between the cluster and ROH molecule. MeOH and EtOH have the same dipole moment, 1.69 D, which is comparable to the dipole moment of water, 1.85 D.47 DFT calculations were used to compute the relative reaction enthalpy as a function of cluster oxygen content (see Supporting Information SI 11) for the sequential oxidation processes and were found to be very similar. The collisional cross section of EtOH, 38.4 Å2, is larger than that of MeOH, 23.5 Å2, but by much less than an order of magnitude. Calculations are currently being done in an effort to determine whether enhanced van der Waals interactions or the larger vibrational density of states in cluster− EtOH complexes could contribute to the disparate rate constants determined for MeOH and EtOH reactions. Because the mobility of −H atoms was found to be an important factor in both MoxOy− + H2O → MoxOy+1− + H2 reactions, in addition to lower rate constants, we also anticipated the possibility of observing trapped intermediates or addition products (e.g., MoxOy+1RH− species) not observed in the cluster−water reactions. In the current study, we did indeed observe an ion attributed to the Mo2O4− + ROH addition product, Mo2O5RH−, which was also found to undergo a second ROH addition with H2 production. However, the prevalence of abstraction products, double ROH products, and the unexpected disparate rate constants determined for cluster− EtOH and cluster−MeOH reactions reflects the higher complexity of the cluster−ROH system relative to the cluster−H2O reaction system. Calculations are currently being done on cluster−ROH interactions in an effort to understand what additional forces govern reactions with alcohols.

The cluster−ROH reaction products could largely be correlated to previously studied MoxOy− + H2O (and D2O) reactions.37 In particular, sequential oxidation of the lower oxides via MoxOy− + ROH → MoxOy+1− + RH was observed, and addition products formed via MoxOy− + ROH → MoxOy+1RH− were observed for the higher oxide clusters. However, there are a few key differences. First, several monohydroxide products suggesting cluster−ROH abstraction reactions were observed, in contrast to cluster−water reactions, which exhibited trace or no monohydroxide products. This difference is attributed to lower R−OH bond dissociation energies relative to HO−H, resulting in lower barriers to abstraction reactions. Second, dialkoxy products suggesting sequential addition of two ROH molecules accompanied by H2 production were observed. Third, we observed the Mo2O4− + EtOH addition product, Mo2O5C2H6− (the mass of the analogous Mo2O4− + MeOH addition product coincides with the mass of the abundant Mo2O6− cluster), which is a precursor to Mo2O6C4H10− + H2 formation; no analogous Mo2O5H2− addition product was observed in cluster−water reactions. Finally, the stoichiometric clusters, Mo2O6− and Mo3O9−, along with the relatively inert suboxide cluster, Mo3O6−, did exhibit small levels of ROH addition, in contrast to their inertness toward water.

■

ASSOCIATED CONTENT

S Supporting Information *

MoOy− distributions before and after exposure to MeOH and EtOH, expanded views of the Mo2Oy− and Mo3Oy− manifolds with incremental increases in exposure to MeOH and EtOH, collisional plots of MoxOy− + MeOH (x = 2 and 3), tabulated MoxOy− + MeOH rate constants, bond dissociation energies for MeOH/EtOH-related molecules, and relative enthalpy change as a function of oxygen content for Mo2Oy− + ROH → Mo2Oy+1− + RH reactions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 812-855-8300. Notes

The authors declare no competing financial interest.

■

5. CONCLUSIONS Kinetics of reactions between small molybdenum oxide cluster anions (MoxOy−; x = 1−4; y ≤ 3x) and alcohols (MeOH and EtOH) were investigated using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. These studies were motivated by recent computational studies on MoxOy− + H2O reaction pathways,39,40 which suggested that −H mobility on cluster−adsorbate complexes was an important factor in subsequent H2 production. An analogous set of products were formed in cluster−MeOH and cluster−EtOH reactions, though the rate constants determined for cluster− EtOH reactions were determined to be an order of magnitude higher than rate constants determined for analogous cluster− MeOH reactions. However, the rate constants for cluster−EtOH reactions were found to be 2 orders of magnitude lower than analogous cluster−water reactions. The lower rate constants for cluster−EtOH and cluster−MeOH reactions relative to H2O are attributed to the lower mobility of the −R group compared to −H. The disparity between cluster−EtOH and cluster−MeOH reaction rate constants is currently the subject of a detailed computational study.

ACKNOWLEDGMENTS The authors gratefully acknowledge support for this research from the U.S. Department of Energy (Grant No. DE-FG0207ER15889).

■

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