Molybdenum Oxide Cluster Anion Reactions with C2H4 and H2O

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Molybdenum Oxide Cluster Anion Reactions with C2H4 and H2O: Cooperativity and Chemifragmentation Manisha Ray, Richard N. Schaugaard, Josey E. Topolski, Jared O. Kafader, Krishnan Raghavachari, and Caroline Chick Jarrold* Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: To probe the mechanism of sacrificial reagents in catalytic processes, product distributions from MoxOy− clusters reacting individually with C2H4 and H2O are compared with those from reactions with a C2H4 + H2O mixture, with the thermodynamics explored computationally. These molecules were chosen to model production of H2 from H2O via H2O + C2H4 → H2 + CH3CHO, mediated by MoxOy− clusters. H2O is known to sequentially oxidize MoxOy− suboxide clusters while producing H2, resulting in less reactive clusters. MoxOy− (y ∼ x) clusters undergo chemifragmentation reactions with C2H4, with MoxOyC2Hz− complexes forming as the cluster oxidation state increases. Unique species observed in reactions with the C2H4 + H2O mixture, Mo2O5C2H2− and MoO3C2H4−, suggest that the internal energy gained in new Mo−O bond formation from oxidation by H2O opens additional reaction channels. C2H3O− is observed uniquely in reactions with the C2H4 + H2O mixture, giving indirect evidence of CH3CHO formation via the cluster mediated H2O + C2H4 → H2 + CH3CHO reaction; C2H3O− can form via dissociative electron attachment to CH3CHO. Calculations support mechanisms that invoke participation of two ethylene molecules on thermodynamically favorable pathways leading to experimentally observed products.

1. INTRODUCTION Transition metal-based materials in the form of metal−organic frameworks,1 metal oxide semiconductors,2 metal nanoparticles,3 and metal−hydrogenase complexes4 have been explored and implemented as photocatalytic materials for H2 production from H2O. Among those materials, molybdenum (Mo) systems in the form of oxides and sulfides have received considerable attention as heterogeneous electrocatalysts for hydrogen evolution reactions (HER).5−10 MoO3 is a low-cost, nontoxic, and environmentally benign transition metal with high stability, and is an inexpensive potential alternative for platinum-based catalysts or cocatalysts, which are well-known and efficient photocatalytic HER systems.11,12 One approach to studying the local atomic-scale interactions that govern catalytic activation is the use of cluster models in both experimental and computational efforts.13−24 Cluster models are particularly appropriate for treating metal oxides because bonding in metal oxides is localized, and nanoparticulate or mesoporous materials often have enhanced properties, pointing to the importance of defect sites. MoO3, a lamellar material, is inherently low-dimensional, and our previous studies on MoxOy− suboxide (y < 3x) clusters25 elucidated molecular structures with a combination of bridge and terminal bonds and bond angles that were evocative of the bulk structural parameters.26 Oxygen vacancies on the metal oxides in particular, which are commonly implicated as active sites,27 are cluster-like in nature. Cluster studies are complementary to bulk surface models, which can be complicated by the dynamic nature of surfaces while undergoing catalytic processes.28 Several examples of gas phase © XXXX American Chemical Society

reactivity studies on metal oxide cations, anions and neutrals that have probed catalytic phenomena are included in refs 29−32. In the context of H2 production from H2O, our research interests lie on the investigation of molecular interactions in molybdenum (Mo) and tungsten (W) metal oxide cluster anion reactions with H2O, where we have previously observed H2 production via the following reaction associated with the oxidation of metal oxide cluster anions:33−36 MxOy− + H 2O → MxOy + 1− + H 2

(1)

However, production of trapped intermediates, MxOy+1H2−, resulting from H2O addition to certain clusters in the oxidation pathway, is more prevalent once the metal approaches its highest oxidation state, +6 for Mo and W. The reactivity data have also been interpreted computationally in terms of free energy reaction pathways, which point to the presence of high barriers to −H and −OH rearrangements in the intermediates, preventing the thermodynamically favored H2 elimination reaction.34,37,38 To maintain a continuous production of H2, we now consider implementing a sacrif icial reagent that can potentially reduce the more oxidized Mo-oxide clusters as formed during oxidation by water, maintaining the clusters in lower oxidation states necessary for H2 production. A sacrificial reagent is a reaction participant that facilitates the full-cycle Received: November 1, 2017 Revised: December 4, 2017 Published: December 4, 2017 A

DOI: 10.1021/acs.jpca.7b10798 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

of UHP helium carrier gas (30 psig stagnation pressure) issued from a pulsed molecular beam valve operated in 30 Hz repetition rate and swept into a 25 mm long, 3 mm diameter reaction channel, where the clusters form and cool. As has been observed in all previous studies on cluster anions generated from ablation of pressed refractory metal powder, no O2 addition to the He carrier gas is necessary for production of a broad range of cluster oxidation states.25,33−36,38,39,41 A second molecular beam valve is used to introduce varying quantities of C2H4, H2O or mixture of C2H4 and H2O (H2O seeded into C2H4) to the reaction channel by changing the pulse duration. The resulting gas mixture is then expanded into a vacuum chamber and skimmed by a 3 mm diameter skimmer. The anions are accelerated on the molecular beam axis to 1 keV, rereferenced to ground, and then enter a 1.2 m beammodulated time-of-flight mass spectrometer, where the anions are guided through several ion optics and finally, collide with a 25 mm microchannel plate (MCP) detector at the end of an ion drift tube. The resulting signal from the detector is then recorded with a digitizing oscilloscope. The mass resolution,

reaction but is not a desired product (H2, in this case) of the process. We have previously determined computationally the potential free energy pathway of MoxOy− reactions with H2O and C2H4, giving the overall net reaction: H 2O + C2H4 → H 2 + CH3CHO

(2)

The reaction, selected to demonstrate the proof of principle, is marginally endothermic by ∼5 kcal/mol with acetaldehyde as product of ethylene oxidation, and is mediated by the Mo2O4− and Mo2O5− clusters.39 The formation of a deeply trapped Mo2O5C2H4− (+H2) species found computationally might be expected to interrupt the production of CH3CHO necessary to achieve the full catalytic cycle. From an experimental standpoint, however, due to the high internal energy of the cluster complexes gained upon new bond formation (in this case Mo−O and Mo−C bonds) in the gas phase, it is conceivable that the vibrationally hot Mo2O5−−C2H4 intermediate might undergo CH3CHO loss and thus participate in a full cycle of H2 production. Here, we report the experimental results of MoxOy− cluster reactions with C2H4 and H2O motivated by the calculations on Mo2O4−/Mo2O5− cluster couple with H2O and C2H4. Reactions are performed separately with C2H4 and H2O followed by a mixture of two reactants (C2H4 + H2O) to compare the evolution of reaction products in individual reactions vs when the reactants are introduced together. Rate constants determined for MoxOy− + C2H4 reactions are determined to be approximately 2 orders of magnitude smaller than rate constants for MoxOy− + H2O reactions (compensated for in this study by running coreactions with C2H4 number densities maintained at 102 times the number density of H2O in the reactor) but are commensurate with rate constants measured for reactions between cluster anions by other groups.14 The primary reaction products, acetylene (−C2H2) and ethylidene (−C2H4) metal oxo complexes observed in MoxOy− + C2H4 reactions are substantially reduced when H2O is present with C2H4. Additionally, cluster fragmentation takes place in reactions with ethylene, resulting in monomolybdenum complexes, which by extension accompanies the elimination of a neutral species. Several fragmentation reactions are evident in reactions with the C2H4 + H2O gas mixture that are different from those observed in reactions with only C2H4. We also observe C2H3O− in the low-mass region of the mass spectrum, an anion that would form via dissociative attachment of an electron to neutral CH3CHO, providing indirect evidence that C2H4 oxidation in the presence of H2O is occurring, though we do not have direct evidence of the branching ratio for CH3CHO production in MxOy− + H2O + C2H4 reactions. Calculations are performed using density functional theory (DFT) to evaluate the free energies of reactions that may lead to some of the chemi-fragmentation products observed experimentally.

( Δmm ) of the TOF mass spectrometer in the range of the ion masses for this study is 300. Initial cluster distributions were recorded prior to introduction of reactants (C2H4, H2O, C2H4 + H2O), and mass spectra were subsequently recorded at controlled incremental increases of reactant pressure to the reaction channel. The ethylene was diluted by varying amounts in pure helium to reach reaction rates that were comparable to cluster−water reaction rates when H2O is at the equilibrium vapor pressure (ca. 3000 Pa). The mass spectra shown result from diluting the ethylene to a mole fraction of 0.85 in UHP He. The number of cluster−C2H4 collisions was approximated using the Langevin collisional rate coefficient, kL,43−45 ⎛ α ⎞1/2 ⎛ ⎞1/2 3 −1 −9 α kL = 2πq⎜ ⎟ = 2.34 × 10 ⎜ ⎟ cm s ⎝ μ⎠ ⎝ μ⎠

(3)

For a singly charged ion, with μ in amu, polarizability in Å3 (ref 46), kL = 9.5 × 10−10 cm3 s−1. With the same expression, kL is 6.8 × 10−10 cm3 s−1 for H2O, but because H2O has a permanent dipole moment, we use the variation-treatment collision rate constant summarized by Chesnavich et al.,44 which takes into account the permanent dipole of the collision partner of the ion, μD. For water,

μD

( ) = 1.54 Debye Å α

−9

−3/2

, and at 300 K, kvar

−1

∼ 3.5·kL = is 2.4 × 10 cm s . The number of cluster− reactant collisions is then c = kL or var·Nreactant ·t

3

(4)

where Nreactant is the number density of the H2O or C2H4, and t is the residence time in the reactor, which we approximate to be initially ca. 25 μs prior to introduction of the reactants, with an increase with increasing reactant pressure approximated by the change in timing of the ion rereferencing circuit necessary to maintain constant mass ranges. Mass spectra were analyzed with OriginPro 9.2, a data analysis and graphics program package. Overlapping peaks were fit with Gaussian functions to determine the contribution of each mass to the total peak intensity. II.B. Computational Details. Reaction free energy calculations were performed using unrestricted Kohn−Sham

II. METHODS II.A. Experimental Details. Clusters are generated, undergo reactions with gas-phase molecules, then are mass analyzed using a home-built experimental apparatus that has been described previously. 40−42 Briefly, MoxOy− clusters are generated using an 8 mJ/pulse energy of second harmonic output of a Nd:YAG laser operated at a 30 Hz repetition rate ablated on a pressed rotating 98Mo metal target (Trace Sciences). The resulting plasma is then entrained in a pulse B


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with C2H4 (red trace), H2O (blue trace), and a C2H4 + H2O mixture (green trace). The C2H4 was diluted in He to 0.85 for these particular measurements. The conditions under which the three product distributions shown in Figure 1 were measured were chosen such that the number of cluster−H2O collisions in the blue and green traces are comparable, and the number of cluster C2H4 collisions in the red and green traces are comparable. In the mass spectrum of the initial cluster distribution, the ions indicated with asterisks (*) are stoichiometric (y = 3x) clusters for each x-value, and the suboxide clusters are at 16 amu mass steps to lower masses. As will be shown below, MoxOy− (x = 2, 3) suboxide clusters undergo cluster fragmentation and addition reactions in the presence of C2H4. The fragmentation patterns change in reactions with the C2H4 + H2O mixture. However, the stoichiometric clusters (y = 3x) do not appear to undergo any significant intensity changes. Therefore, the evolutions of the reaction products in each x series are compared, keeping the intensity of the stoichiometric clusters (MoO3−, Mo2O6− and Mo3O9−) constant at each series of the mass spectrum. The next several sections describe the products formed under the various conditions, compare the relative reaction rates, show evidence that some of the products formed arise from reactions with more than one C2H4 molecule, and present reaction thermodynamics supporting the involvement of two C2H4 molecules. III.A.1. MoOy− + C2H4/H2O/C2H4 + H2O. Figure 2 shows the mass spectra in the x = 1 mass region on an expanded scale,

density functional theory (UKSDFT) with the quantum chemical program package ORCA 3.0.3 (ref 47) using the hybrid exchange−correlation functional B3LYP.48−50 Geometry optimizations of the various chemical species, vibrational mode calculations, and energy evaluations were carried out with the def2-SVP basis set.51,52 For computational expediency the core electrons of Mo were modeled using the Stuttgart/Dresden effective core potential,53 and minimum energy structures were verified as having no imaginary frequencies in the vibrational analysis. Entropy and zero-point energy (ZPE) were obtained for polyatomic species from vibrational frequency calculations performed numerically. For single-atom species, translational entropy is estimated from the Sackur−Tetrode equation by assuming free range of motion. The thermally corrected free energy of each species is reported by combining the SCF energy, ZPE, and the entropic contribution at 298.15 K. The ΔG values are in the standard state with all species in the gas phase.

III. RESULTS AND ANALYSIS Analysis of the mass spectra of initial cluster distributions and subsequently formed products from reactions with (1) C2H4, (2) C2H4 + H2O, and (3) H2O is complicated by the fact that all cluster reactions are running in parallel, and in the case of (2) C2H4 + H2O, competing reactions are occurring at comparable rates. In addition, as observed previously in reactions with hydrocarbons,54 cluster fragmentation occurs in reactions with C2H4. We therefore first consider the products formed and then attempt to determine unambiguously the reactions that lead to the product formation based on kinetic analysis. III.A. Reaction Product Distribution. Figure 1 shows a broad overview of the initial MoxOy− cluster distribution (black trace) and the resulting product distributions from reactions

Figure 2. Initial 98MoOy− cluster distribution (black trace) along with products resulting from reactions with C2H4 (red), H2O (blue), and a C2H4 + H2O mixture (green).

with the initial cluster distribution (black trace), and product distributions measured after approximately 3 × 103 MoOy−− H2O (blue and green traces) and 1 × 105 MoOy−−C2H4 collisions (green and red traces). The initial ion distribution includes MoO2−, MoO3− (most intense peak), and several hydroxides (MoO3H− and MoO4H−). The MoOy− + H2O product distribution is consistent with previous measure-

Figure 1. Overview of initial 98MoxOy− cluster distribution (black trace) along with products resulting from reactions with C2H4 (red), H2O (blue) and a C2H4 + H2O mixture (green). Stoichiometric (y = 3x) clusters are indicated with asterisks (*). C


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The Journal of Physical Chemistry A ments,33 Given the low initial abundance of MoO2− and absence of MoO−, the distinct appearance of MoOC2H2−, MoO2C2H2−, and MoO2C4H6− suggests that these species arise from chemi-fragmentation of larger clusters, an effect observed in previous studies on MoxOy− cluster reactions with methane and ethane.54,55 As further support, intensities of MoOy− ions remain constant throughout the course of increasing reactant concentration, as shown in the Supporting Information (S2). This observation clearly rules out their formation via precursor MoOy− ions. III.A.2. Mo2Oy− + C2H4/H2O/C2H4 + H2O Product Distributions. Figure 3 shows the initial Mo2Oy− cluster distribution

Figure 3. Initial 98Mo2Oy− cluster distribution (black trace) along with products resulting from reactions with C2H4 (red), H2O (blue) and a C2H4 + H2O mixture (green).

Figure 4. Sequences of mass spectra in the 98Mo2Oy− cluster mass range (a) with increasing C2H4 + H2O and (b) with increasing C2H4 injected into the fast flow reactor. The numbers next to each trace are the cluster−C2H4 collisions approximated in the way described in text.

(black trace) and product distributions from reactions with C2H4 (red trace), the C2H4 + H2O mixture (green trace) and H2O (blue trace) under conditions yielding approximately 3 × 103 Mo2Oy−−H2O and 1 × 105 Mo2Oy−−C2H4 collisions. As observed previously,33,36 reactions with H2O (blue trace) result in sequential oxidation of the suboxide clusters, the sequence terminating with Mo2O5− + H2O → Mo2O6H2− reaction. The product distribution observed in this snapshot of reactions with C2H4 (red trace) at first glance suggests addition product formation. With the lowest oxides, product stoichiometry further suggests H2 evolution, whereas simple addition dominates for the more oxidized clusters:

complicated processes are also occurring. Figure 4 shows the series of mass spectra recorded with incremental increases in (a) the C2H4 + H2O gas mixture and (b) C2H4 introduced to the fast flow reaction channel. These are additionally presented in comparison with mass spectra measured at incremental increases in H2O (in the absence of C2H4) in the Supporting Information (S3). The numeric values are the approximated number of Mo2Oy−−C2H4 collisions. In Figure 4a, the number of Mo2Oy−−H2O collisions is approximately 0.03 times the Mo2Oy−−C2H4 collisions (though the number density of H2O is approximately 1% that of C2H4, the collision rate coefficient is ∼2.5 times higher, vide supra). Both sequences of mass spectra exhibit the appearance of ions with masses consistent with Mo2OyC2H2n− (n = 0, 1, 2) between peaks in the Mo2Oy− (y = 2−6) cluster series, growing in intensity at intermediate cluster−C2H4 collision numbers, and decreasing in intensity at higher cluster−C2H4 collisions. However, the increases in the Mo2OyC2H2n− peak intensities do not balance with the decrease in the putative precursor Mo2Oy− peak intensities. Figure 5 shows plots of the Mo2Oy− cluster intensity as a function of C2H4 (and/or H2O) exposure, arranged for consistency with the mass spectra shown in Figures 1 and 2. The panels in the left column show the intensities of the Mo2Oy− clusters as a function of reactant exposure, and the

Mo2Oy− + C2H4 → Mo2Oy C2H 2n− + (2 − n)H 2 (y = 2−4; n = 0, 1)

Mo2O−y + C2H4 → Mo2Oy C2H4 −

(5)

(y = 4, 5)

(6)

In the snapshot of the mass distribution observed in reactions with both H2O and C2H4 (green trace), the lower oxides are evidently depleted by oxidation, reducing the appearance of C2H4 addition products. A unique product observed only in the cluster reactions with the C 2 H 4 + H 2 O mixture is Mo2O5C2H2−. Though the reactions described in eq 5 and 6 may be occurring, a survey of a sequence of mass spectra collected with incremental increases in reactant pressure suggests that more D


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Figure 5. Plots of the relative integrated intensities of 98Mo2Oy− reactants and primary C2Hn addition products from (a) 98Mo2Oy− + C2H4 reactions and their comparison with (b) 98Mo2Oy− + C2H4 + H2O reactions as a function of reduced cluster−reactant collisions (α, see text).

right panel shows the product intensity. For direct comparison, the abscissae are labeled from α = 0 to 1, reflecting the minimum to maximum exposure to H2O (0 to ca. 5.5 × 103 cluster−H2O collisions in Figure 5a,b) and C2H4 (0 to ca. 1.8 × 105 cluster−C2H4 collisions in Figure 5b,c). Note that there are numerous approximations associated with the calculations of the number of collisions, which we expect to be within an order of magnitude of the actual number of collisions, and additional uncertainty associated with the typical shot to shot fluctuations in the two pulsed molecular beam valves as well as the ablation source, and the plots are meant to convey the general trends extracted from the mass spectra.

Figure 5a shows the straightforward evolution of cluster intensities, consistent with sequential oxidation reactions. That is, Mo2O2−, the most reduced cluster decays exponentially, and the intermediate clusters initially plateau or increase in intensity due to their production via oxidation of the less oxidized clusters, followed by a near exponential decrease in intensity associated with oxidation by water after depletion of the more reduced species. In the right panel of Figure 5a, the total suboxide cluster intensity (solid black trace) is plotted with the terminal Mo2O6H2− product intensity (solid blue trace), showing the overall mass balance. That is, the sum of suboxide species at α = 0 is equal to the Mo2O6H2− intensity at α = 1. E


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such a trapped intermediate supports the potential free energy reaction pathway of Mo2O5− + C2H4 reaction as calculated by Ray et al.39 III.A.3. Mo3Oy− + C2H4/H2O/C2H4 + H2O. Figure 6 shows the initial Mo3Oy− cluster distribution (black dotted trace) on an

The dashed traces show the same information for reactions with the C2H4 + H2O gas mixture and will be discussed below. A similar treatment of the Mo2Oy− + C2H4 cluster and product intensities (Figure 5c) shows that the Mo2Oy− cluster intensities, for the most part, decay with C2H4 collisions, though there is a reproducible increase in suboxide cluster intensity within the first incremental increase in C2H4 pressure in the reactor. However, the evolution of the Mo2OyC2H2n− reaction products, shown in the right panel, appears with higher intensity than the decrease in what at first glance would be the Mo2Oy− precursor between α = 0.1 and α = 0.5. It therefore appears that the less oxidized Mo2OyC2H2n− complexes are forming via chemi-fragmentation of larger cluster anions that are not detected efficiently in the experimental apparatus, or by reactions with neutral species followed by free electron detachment in the source. These products, by virtue of their decay with increasing C2H4 pressure in the reactor, are subsequently consumed in reactions with C2H4. Another unexpected group of products formed in Mo2Oy− + C2H4 reactions but not observed in reactions with H2O are Mo2O4H2 and Mo2O5H2−, which can be seen clearly in Figure 4b at 2 amu higher than Mo2O4− and Mo2O5−. These new species may be due to abstraction from C2H4 with C2H2 elimination or, as above, chemi-fragmentation products from larger clusters. Figure 5b shows that the Mo2Oy− suboxide ion intensities in reactions with the C2H4 + H2O gas mixture follow a similar pattern to the Mo2Oy− + H2O reactions (Figure 5a), though all are depleted at lower values of “α” compared to Figure 5a, due to consumption in parallel reactions with both H2O and C2H4. Plots were also constructed for reactions with C2H4 (χC2H4 = 0.47 in He) + H2O gas mixtures and exhibit ion decay patterns that are intermediate between Figure 5a,b, as shown in the Supporting Information (S4). The product evolution, shown in the right panel of Figure 5b, again features an increase in Mo2OyC2H2n− species that does not correlate with the decrease in Mo2Oy− reactant intensities. The sum of the Mo2Oy− cluster intensities at α = 0 is greater than the Mo2O6H2− terminal product in reactions with H2O at α = 1 (dashed traces, Figure 5a) by an amount equal to the sum of addition products at α = 1, which again is consistent with the Mo2OyC2H2n− (y = 2−4) species observed at α = 0.1−0.4 being reactive toward C2H4. As noted above, Mo2O5C2H2− is a unique product observed only in reactions with the C2H4 + H2O gas mixture. Its complete absence in Mo2Oy− + C2H4 reaction rules out C2H4 addition to Mo2O5− to produce Mo2O5C2H2− + H2 under all conditions, and because Mo2O4C2H2− produced in C2H4 reactions is much lower in intensity than Mo2O5C2H2− produced in reactions with the C2H4 + H2O mixture, it is unlikely that the latter forms from a reaction between water and Mo2O4C2H2−. Possible explanations include (i) Mo2O5C2H2− formation only via reactions with the C2H4 + H2O mixture may be due to reactions with newly oxidized, and therefore high-internal-energy larger clusters (MoxOy−; x > 2) resulting in fragmentation to form Mo2O5C2H2− + Mox−2Oy−5H2 or Mox−2Oy−5 + H2. Such a scenario relies on high internal energy tipping the reaction over a barrier to H2 release. (ii) In a similar vein, the high internal energy of nascent Mo2O5− clusters produced from Mo2O4− + H2O, may open the Mo2O5− (Eint ≈ EMo−O bond) + C2H4 → Mo2O5C2H2− + H2 reaction pathway. The formation of

Figure 6. Initial 98Mo3Oy− cluster distribution (black trace) along with products resulting from reactions with C2H4 (red), H2O (blue), and a C2H4 + H2O mixture (green). Products apparently from chemifragmentation are indicated with asterisks (*).

expanded scale, along with the product distributions resulting from reactions with C2H4 (red), C2H4 + H2O (green), and H2O (blue) with approximately 1.8 × 105 Mo3Oy−−C2H4 collisions and 5 × 103 Mo3Oy−−H2O collisions. The initial cluster oxide distribution ranges from y = 3 to 9, with Mo3O6− being the most abundant. The Mo3Oy− + H2O reaction pattern, as described previously, is punctuated by the stability of Mo3O6−.33 Mo3O4− undergoes oxidation with H2 production, forming Mo3O5−. Mo3O5− undergoes water addition and oxidation, with a temperature dependent branching ratio.36 Mo3O6− is unreactive toward water, and Mo3O7− and Mo3O8− undergo water addition, rather than oxidation. All suboxide clusters in the x = 3 series undergo reactions with C2H4. As with the most reduced clusters in the x = 2 series, and Mo3O4− addition products do not balance with the initial and Mo3O4− abundances, the implication of which is that it may undergo chemi-fragmentation. However, the gradual intensity increases of Mo3O4C2H2− and Mo3O4C4H6− do balance with C2H 4 addition with H 2 elimination and subsequent C2H4 addition to Mo3O4−, in contrast to several of the Mo2OyC2H2n− (n = 0, 1) species observed. The observation of Mo3O5C2H4−, Mo3O6C2H4−, Mo3O7C2H4−, and Mo3O8C2H4− ions shows that a fairly wide range of oxidation states undergo addition reaction that does not involve H2 production. Mo3O5C4H8− and Mo3O6C4H8− are also observed at the highest C2H4 concentrations, as shown in the Supporting Information (S5), indicating that the Mo3O5C2H4− and Mo3O6C2H4− complexes are sufficiently reactive to undergo additional C2H4 addition. These observations are supported from their direct reactant−product relationship seen in plots of F


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Table 1. Reaction Rate Coefficients (cm3 s−1) for Consumption of Most Prominent Reactions of MoxOy− with C2H4, H2O, and C2H4 + H2O, along with Pseudo-First-Order Rate Constant in Terms of Reduced Inverse Collisions, α−1, in Reference to Figure 5 (See Text)a

integrated intensity against cluster−C2H4 collisions, included in the Supporting Information (S5). Mo3O6− notably exhibits no reactivity toward H2O; however, it is reactive toward C2H4 at a rate comparable to those of the other suboxide species (vide inf ra). As with the stoichiometric Mo2O6− cluster, Mo3O9− remains constant in the overall course of Mo3Oy− + C2H4 reactions. As in the x = 2 series, the intensities of Mo3OyC2H4− products are significantly reduced when reactions with C2H4 are competing with H2O reactions. III.B. Relative Reaction Rate Coefficients. As postulated previously,39 the cluster mediated H2O + C2H4 → H2 + C2H4O reaction could feasibly proceed via Mo2O4 − + H 2O → Mo2O5− (E int ≈ EMo − Obond) + H 2

C2H4

(7)

If this cyclic process were indeed occurring, the observed rate of consumption of Mo2O4− by H2O would decrease because it would be concurrently regenerated by reduction of nascent Mo2O5− by C2H4, and the intensity of Mo2O5− might be expected to reach a steady state with Mo2O4−. However, these theoretical results do not take into account that, experimentally, all suboxide species are consumed in reactions with C2H4 (reactions 5 and 6), complicating efforts to determine whether d[Mo2O4 −] d[Mo2O5−] and with reaction 8 is occurring. Plots of dα dα cluster intensities and with α, included in the Supporting Information (S6), reflect the sharp decrease in Mo2O4− at lower α values with increasing C2H4 when both reactions are consuming Mo2O4−. There is a slight decrease in the absolute value of the slope, which would be consistent with Mo2O4− being produced by a new reaction, such as (8). It is not significant enough, however, to take as definitive proof. Nonetheless, the rate coefficients determined for the separate C2H4 and H2O reactions show several interesting trends. The coefficients were approximated from the Mo2Oy− ion intensity versus reduced cluster−reactant collision data shown in the plots shown in Figure 5 as well as the Mo3Oy− ion intensity versus reduced collision data included in the Supporting Information (S5). The details of how the rate coefficients are determined graphically in MoxOy− + H2O reactions using data on Mo xOy − intensity versus approximate cluster−H 2O collisions have been described previously.33,35,56 In the case of cluster reactions with C2H4, a pseudo-first-order condition was again assumed, because the number density of C2H4 in the reactor is 104 to 106 times the number density of any given cluster anion. Reactions with C2H4 are simpler than reactions with water in that the higher oxides are not generated from the lower oxides. For all cluster−C2H4 reactions, therefore, we fit the intensities to IMoxOy− (IMoxOy−)initial

k, cm3 molecule−1 s−1

Mo2O2 Mo2O3− Mo2O4− Mo2O5−

+ + + +

C2H4 C2H4 C2H4 C2H4

→ → → →

products products products products

3.3 2.2 1.2 0.8

1.7 1.1 6.1 3.9

× × × ×

10−14 10−14 10−15 10−15

Mo3O4− Mo3O5− Mo3O6− Mo3O7− Mo3O8−

+ + + + +

C2H4 C2H4 C2H4 C2H4 C2H4

→ → → → →

products products products products products

3.5 1.5 0.4 0.9 0.4

1.8 7.8 2.0 4.5 2.0

× × × × ×

10−14 10−15 10−15 10−15 10−15

−

Mo2O5− (E int ≈ EMo − Obond) + C2H4 → Mo2O4− + CH3CHO (8)

ln

α−1

reaction

H2O α−1

k, cm3 s−1

Mo2O3− Mo2O4− Mo2O5−

5.4 9.6 7.4 7.4

2.3 4.0 3.0 3.1

× × × ×

Mo3O5− + H2 Mo3O6H2− no product Mo3O8H2− Mo3O9H2− C2H4 + H2O

7.8 8.4 0 7.3 6.2

3.3 3.5 0 3.0 2.6

× 10−12 × 10−12

Mo2O2− Mo2O3− Mo2O4− Mo2O5−

+ + + +

H2O H2O H2O H2O

→ + H2 → + H2 → + H2 → Mo2O6H2−


+ + + + +

H2O H2O H2O H2O H2O

→ → → → →


+ + + +

C2H4 → Mo2O5C2H2 (+H2) H2O → Mo2O6H2− C2H4 → Mo2O5C2H2 (+H2) H2O → Mo2O6H2−

10−12 10−12 10−12 10−12

× 10−12 × 10−12

χC2H4

k, cm3 s−1

0.85

4.8 2.0 5.3 3.1

0.49

× × × ×

10−15 10−12 10−15 10−12

a

The uncertainty in relative k values is approximately 30%; the uncertainty in the absolute values is ca. 1 order of magnitude. Note that the MoxOy− + H2O collision rate coefficient is 2.4 × 10−9 cm3 s−1, and the MoxOy− + C2H4 is 9.5 × 10−10 cm3 s−1.

Langevin collision rate coefficient, kL. The second-order rate coefficients are included in Table 1. They range from 10−14 to 10−15 cm3 s−1, compared to kL = 9.5 × 10−10 cm3 s−1, which is consistent with the high maximum number density of C2H4 molecules in the reactor, ca. 1018 to 1019 cm−3 and the 10−5 s reactor residence time. Rate constants for zirconium hyperoxide cluster anions with C2H4 were recently reported to be on the order of 10−13 cm3 s−1, for comparison.14 For a direct comparison with cluster−H2O reaction rates under the same set of experimental conditions, we followed the daisy-chain procedure described previously.33,35,56 Mo2O2− and Mo3O4−, the lowest oxides with significant abundance in the x = 2 and 3 cluster series, were assumed to decay exponentially, and rate coefficients were determined using eq 9. Beyond the lowest oxides, the following equations were fit to determine the sequential rate coefficients:

= −ki′·α (9)

where IMoxO−y is the peak integral in the mass spectrum, which we assume is proportional to ion intensity. The resulting effective pseudo first-order rate constants (k′i , units of α−1 where α is the reduced collision number) summarized in Table 1, were then converted to reaction efficiencies (1.8 × 105 = dc , dα where c is the approximate number of cluster−C2H4 collisions) then converted to second-order rate constants using the

dIMoxOi+1− dα

− ki·IMoxOi− = −ki + 1·IMoxOi+1−

(10)

For reactions that terminate with H2O addition, we also verified the rate constants by plotting the derivative of the addition G


Article

The Journal of Physical Chemistry A product intensity as a function of precursor intensity. The actual plots from which the rate coefficients were determined are included in the Supporting Information (S5, S7, S8). These rate coefficients are also summarized in Table 1. As observed previously, the rate coefficients for MoxOy− + H2O reactions do not vary significantly over the full range of oxides, with the exception of Mo3O6−, which is unreactive toward H2O. However, the reaction rate coefficients for MoxOy− + C2H4 decrease with increasing oxidation state. Therefore, in reactions with the C2H4 + H2O gas mixtures, the more oxidized clusters are relatively more reactive toward H2O than C2H4 when compared to the more reduced clusters. Finally, we compare the relative rate coefficients associated with Mo2O5− in reactions with the C2H4 + H2O mixture. The rate coefficient associated with water addition can be determined from the rate of Mo2O6H2− production as a function of Mo2O5− abundance, and we assume that there is a direct relationship between the production of Mo2O5C2H2− and Mo2O5− abundance. The rate coefficients determined with two different dilutions of C2H4 are included in Table 1. Although the Mo2O5− + C2H4 rate coefficient appears to be insensitive to the concentration of C2H4, the apparent rate coefficient for the Mo2O5− + H2O → Mo2O6H2− reaction decreases with increasing C2H4 concentration. New calculations on the reaction free energy pathway for the Mo2O5− + C2H4 + H2O reaction are underway to determine the feasibility of the Mo2O5−−Mo2O6− cluster couple-mediated full cycle reaction. III.C. Thermodynamics of Chemi-Fragmentation: Two C2H4 Are Better Than One. As noted above, the appearance of significant quantities of the monometallic complexes, MoOC2H2−, MoO2C2H2−, and MoO2C4H6− in MoxOy− + C2H4 reactions, in addition to modestly abundant MoO2C2H2− and MoO3C2H4− complexes in MoxOy− + C2H4 + H2O reactions, cannot be reconciled as direct products of MoO−, MoO2−, or MoO3−. MoO− is absent in the initial cluster distribution, and MoO2− and MoO3− intensities undergo negligible changes in intensity compared to the appearance of the monometallic complexes. Given the imbalance between the lowest oxides in the x = 2 and 3 series and C2H4 addition products noted above, we consider whether these monometallic complexes are products of MoxOy− (y ∼ x) dissociation. Similarly, in the x = 2 series, the appearance of Mo2O2C2H2n− and Mo2O3C2H2n− ions are not balanced by changes in the Mo2O2− and Mo2O3− ion intensities, and Mo2O5C2H2− observed in C2H4 + H2O reactions could potentially be due to fragmentation of x > 2 clusters. Table 2 summarizes the reaction free energies, ΔG, calculated for several potential chemi-fragmentation reactions resulting in the monometallic complexes observed in the mass spectrum (Figure 2). We first consider MoOC2H2− and MoO2C2H2− observed in Mo2Oy− + C2H4 reactions. As noted in the previous section, the most reduced clusters have the highest C2H4 reaction rate coefficients, so Mo2O2− + C2H4 and Mo2O3− + C2H4 reaction free energies were calculated using the previously determined structures of Mo2O2− and Mo2O3− (ref 25) with newly optimized Mo2OC2H2− and MoO2C2H2− structures. Several potential fragmentation reactions were explored. The striking result is that reactions involving one C2H4 molecule are thermodynamically unfavorable, whereas reactions involving two C2H4 molecules are thermodynamically favorable. Addition reactions resulting in Mo2OyC2H2n− product formation were also considered for y = 2−4. All of these

Table 2. Summary of Free Energies Calculated for Possible Mo2Oy− + C2H4 Reactions Resulting in ChemiFragmentation and Acetylene (−C2H2) or Ethylidene (−C2H4) Addition Complexes ΔGrxn°, kcal/mol

reactions Mo2O2− Mo2O2− Mo2O2− Mo2O2− Mo2O2− Mo2O2−

+ + + + + +


+ + + +


+ + + + +

Mo2O2− Reactions C2H4 → MoOC2H2− + MoOH2 C2H4 → MoO2C2H2− + MoH2 2C2H4 → MoO2C2H2− + MoC2H6 2C2H4 → MoO2C4H6− + MoH2 C2H4 → Mo2O2C2H2− + H2 2C2H4 → Mo2O2C2H2− + C2H6 Mo2O3− Reactions C2H4 → MoO2C2H2− + MoOH2 2C2H4 → MoO2C2H2− + MoOC2H6 C2H4 → Mo2O3C2H2− + H2 2C2H4 → Mo2O3C2H2− + C2H6 Mo2O4− Reactions C2H4 → Mo2O4C2H2− + H2 2C2H4 → Mo2O4C2H2− + C2H6 C2H4 → Mo2O4C2H4− C2H4 + H2O → Mo2O5C2H2− + 2H2 2C2H4 + H2O → Mo2O5C2H2− + C2H6 + H2

+21.3 +12.6 −12.2 −8.3 −5.1 −30.8 +10.3 −7.4 −12.8 −38.5 −9.2 −34.9 −29.0 −57.5 −83.2

reactions were modestly favorable but became significantly more favorable when two C2H4 molecules were implemented in the calculations. Calculations on full free energy pathways including barriers are currently underway to evaluate the kinetic feasibility of these reactions and will be the subject of a subsequent report. However, there is experimental evidence that multiple C2H4 molecules are participating in the production of these chemi-fragmentation products. Mass spectra measured with sequential increases in C2H4/He pressure in the reaction channel measured with different dilutions of C2H4 (Supporting Information, S10 and S11) show a nonlinear relationship between the depletion of MoxOy− clusters and the appearance of the putative chemi-fragmentation products. In addition, we observe a decrease in the intensities of the lowest oxides that cannot be balanced with the appearance of product ions, suggesting that chemi-detachment may also be occurring. This process is possible if the internal energy gained in the reaction exceeds the product electron binding energy and is analogous to well-known chemiionization reactions.57 Calculations on this process are also underway. Finally, we consider a possible reaction product that appears only in the presence of both C2H4 and H2O, Mo2O5C2H2−. Though this ion may be produced by chemi-fragmentation of larger clusters (vide supra), it may also form by oxidation of Mo2O4− by H2O, producing internally hot Mo2O5−, which then undergoes C2H4 addition with H2 elimination. This reaction is favorable assuming a single C2H4 participant, but again, invoking two C2H4 molecules in the reaction results in a strikingly favorable reaction, ΔGrxno = −83.2 kcal mol−1. III.D. Is C2H4 Reducing MoxOy− Clusters as They Are Oxidized by H2O? As noted previously, the postulated overall cluster-mediated reaction, C2H4 + H2O → CH3CHO + H2, involves all neutral species, and direct evidence of this process occurring by the cyclic reaction path in the reactions expressed in eqs 7 and 8 would be expected to be in the form of an apparent reduction in the rate of the oxidation of MoxOy− by H


Article


near-thermoneutral C2H4 + H2O → CH3CHO + H2 reaction, among several others, mediated by the Mo2O4−/Mo2O5− cluster couple.39 Computationally, the step involving C2H4 reduction of Mo2O5− was calculated to have an entrance barrier of 27.1 kcal mol−1, which would predict a very low reaction efficiency unless the nascent Mo2O5− had very high internal temperature. What was observed experimentally in the current study on MoxOy− cluster reactions with C2H4 (in the absence of H2O) at room temperature is that all of the suboxide clusters undergo various reactions with C2H4 at rates that are more consistent with barriers below 10 kcal mol−1. This value is a upper limit based on T = 300 K and k = kL·exp(−E/RT). Formation of Mo2O5C2H4− was predicted from previous calculations on the Mo2O5− + C2H4 reaction,39 and this ion is observed experimentally in MoxOy− + C2H4 reactions. However, Mo2O5C2H4− is low in intensity compared to Mo2O5C2H2− in MoxOy− + C2H4 + H2O product mass spectra. This result indicates that the actual reactions that are occurring are more complex than previously hypothesized. However, internal energy gained in the cluster oxidation reactions with H 2 O is evident from the production of this unique Mo2O5C2H2− species. There is abundant evidence in this study that the clusters undergo chemi-fragmentation in reactions with C2H4, and the nonlinear chemi-fragment product intensity dependence on the concentration of C2H4 in the buffer gas is consistent with the reactions being in part due to the participation of two C2H4 molecules. On the basis of computational results, from a thermodynamic standpoint, cluster−C2H4 reactions are significantly more favorable when two C2H4 molecules are involved. Because cluster−C2H4 collisions are 30−40 times more frequent than cluster−H 2O collisions, dual C 2 H 4 reactions are certainly feasible. Calculations also indicate that oxidative addition of a second C2H4 to the Mo centers favors subsequent Mo−Mo bond cleavage, yielding a stable neutral fragment. As an example of one of many reaction pathways that will be considered in the report on our computational studies of the reaction mechanisms, the following reaction is favorable by 3 kcal mol−1:

H2O, which is complicated by unanticipated reactivity of all suboxide clusters toward C2H4. However, the reaction channel will inevitably have a certain abundance of free electrons from the ablation-generated plasma or chemi-detachment, and evidence of the neutral CH3CHO may be reflected in the appearance of the radical anion formed via dissociative attachment of an electron.58−60 We therefore measured mass spectra for the low-mass region ranges from 20 to 100 amu during cluster reactions with both C2H4 and the C2H4 + H2O mixture. Figure 7 shows the mass spectra in the 20−100 amu regions in reactions with C2H4, the most abundant species observed are

Figure 7. Low-mass region of the mass spectra measured for MoxOy− generated in He (black trace), while undergoing reactions with C2H4 (red), and the C2H4+H2O mixture (green).

C2−, C2H−, C2H2−, C4H3−, and Mo−. Bare Mo− is low in abundance and clearly arises as a minor anionic product of reactions resulting in fragmentation; it is not observed in the absence of C2H4. The intensities of the C2−, C2H−, and C2H2− ions do not vary with the pressure of the reactants and may be due to the minor deposition of carbon in the source. In contrast, the C2H3O− (43 amu) ion, which can form from dissociative attachment of an electron to its neutral precursor C2H4O is observed in reactions with the C2H4 + H2O mixture. The presence of C2H3O− exclusively in reactions with the C2H4 + H2O mixture supports the possibility that cluster-mediated C2H4 oxidation is occurring. We note here that observation of C2H3O− is not definitive evidence of the cluster mediated process (eqs 7 and 8). For example, C2H− is also present in the low-mass region of the spectrum, and it may react with H2O to yield C2H3O−. However, no C2H4O− is observed, despite the greater abundance of C2H2−.

Mo2O2− + 2C2H4 → MoO2 C2H 2− + H 2MoC2H4

(11)

These results from the current study, in particular, the fragmentation complexes that are observed, will inform future efforts to characterize the reaction mechanism computationally. We note again that the overall H2 production reaction (eq 2) involves neutral species, and the evolution of CH3CHO is not observed directly in the mass spectrometric analysis. However, the observation of C2H3O− only during MoxOy− + C2H4 + H2O reactions supports the possibility that the cluster-mediated process is occurring. On the basis of rate constant analysis, there is no compelling evidence that Mo2O4− is participating in the process; however, the apparent decrease in the rate coefficient associated with the Mo2O5− + H2O → Mo2O6H2− reaction may indicate that the presence of C2H4 during this reaction results in some regeneration of Mo2O5−. Finally, it is worth noting that the neighboring second-row transition metal, anionic or cationic NbxOy− or TaxOy− clusters are unreactive toward ethylene and ethane,61 whereas MoxOy− is reactive toward both.54 This disparity might be explained by the lower Mo−Mo (4.2 eV) bond energy compared to the NbNb bond energy (5.3 eV)46 and direct metal−metal bonding in the most reduced MoxOy− clusters.25,62 Oxygen transfer to C2H4 resulting in H2CO (from the CC bond

IV. DISCUSSION This study is the first experimental test of the previously reported computational study on the mechanism of sacrificial reagent participation in the catalytic production of H2 from H2O decomposition. The computational study focused on the I



■

cleavage) or CH3CHO (from C2H4 oxidation) was predicted to occur in mostly cationic or neutral oxygen-rich clusters, forming a peroxo group through a radical oxygen transfer,21,63 a mechanism not available in reactions with the suboxide clusters in this study. In a separate study (manuscript under review), we report the anion PE spectra of several of the monometallic−C2H2 complexes, which give insight into the cluster−ethylene interactions that lead to chemifragmentation. The results are currently informing ongoing computational efforts to understand the complex web of reactions that are occurring with these suboxide cluster models of oxygen vacancies on metal oxide catalysts.

AUTHOR INFORMATION

Corresponding Author

* C. C. Jarrold. E-mail: [email protected]. Phone: (812) 856-1190. Fax: (812)855-8300. ORCID

Krishnan Raghavachari: 0000-0003-3275-1426 Caroline Chick Jarrold: 0000-0001-9725-4581 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported in its entirety by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No. DE-FG02-07ER15889, and was performed at Indiana University, Bloomington.

V. CONCLUSIONS A comparison of product distributions and reaction rate coefficients of MoxOy− reactions with C2H4, H2O, and a C2H4 + H2O mixture was completed, and the thermodynamics of several reactions leading to apparent chemi-fragmentation products was explored computationally. The underlying motivation was to evaluate the mechanism of the nearthermoneutral Mo2O4−−Mo2O5− cluster-mediated C2H4 + H2O → CH3CHO + H2 reaction for H2 production in a full catalytic cycle using C2H4 as a sacrificial reagent, a process that had been explored previously in a computational study.39 Although the results of the previous theoretical study predicted a deeply trapped Mo2O5C2H4− complex that would require retaining and channeling high internal energy gained along the reaction pathway to ultimately release CH3CHO, experimentally, Mo2O5C2H2− emerged as a unique complex that appeared only in reactions with the C2H4 + H2O gas mixture. Further, experimental evidence that two C2H 4 molecules are participating in a number of the products observed in both MoxOy− + C2H4 and MoxOy− + C2H4 + H2O reactions was supported by calculations on reaction thermochemistry, warranting and informing further computational investigations into the reaction mechanisms. All MoxOy− suboxide clusters were observed to be reactive toward C2H4, including Mo3O6−, which was previously found to be unreactive toward H2O, with reaction rate coefficients ca. 10−2 times the rate coefficients for reactions with water, and decreasing in value with increasing oxidation state. Finally, from an experimental standpoint, the unique observation of the C2H3O− anion, which could form from dissociative electron detachment to C2H4O in the reaction channel, is indirect evidence that the cluster mediated MoxOy− + C2H4 + H2O → MoxOy− + CH3CHO + H2 could be occurring.

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Article

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b10798. Representative series of MoxOy− + C2H4/H2O/C2H4 + H2O (x = 1, 2 series) mass spectra with incremental increases in C2H4, H2O, and C2H4 + H2O partial pressures; integrated intensities of Mo3Oy− clusters and reaction products plotted against reactant concentration, plots for relative rate coefficient determination (PDF) J


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