Letter pubs.acs.org/JPCL
Reactivity of Aluminum Clusters with Water and Alcohols: Competition and Catalysis? Zhixun Luo,† Jordan C. Smith,† W. Hunter Woodward,† and A. W. Castleman, Jr.*,†,‡ †
Department of Chemistry and ‡Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: An in-depth investigation is presented on the hydrogen evolution reaction of aluminum clusters with water and methanol/isopropanol. Aluminum clusters were found to undertake an etching effect in the presence of methanol, but also resulted in an addition reaction with isopropanol. Such reactivity without producing hydrogen is different than water, although they all contain an OH group. Further, we studied the competition of water versus alcohols reacting with Al clusters by simultaneously introducing them into a fast-flow tube reactor. Water dominates the competitive reaction with Al clusters, and the O−H bond in water is readily activated to form aluminum hydroxide cluster products. Also found is that water functions as a catalyst in the activation of the O−H bond in alcohol molecules.
SECTION: Molecular Structure, Quantum Chemistry, and General Theory
M
HAlnOH(H2O)x species in which the additional water molecules play a catalytic role.10,11 Further insight into the general HER mechanism can be gleaned from studies on correlative cluster reactivity in the gas phase. Recently, Roach et al.12 examined the reaction of Al clusters with water and showed an origin of H2 release according to the reaction Aln− + 2H2O → Aln(OH)2− + H2. This mechanism is based on complementary active sites that refer to two locations on the cluster surface where one location behaves like a Lewis acid (accepting the electrons from the oxygen) while an adjacent location behaves as a Lewis base (donating electrons to the hydrogen).13 This established theory predicts interesting size-selective reactivity between Al clusters and water. Further investigations demonstrated that the location of reactive pairs occurs on specific active sites for small-sized Al clusters, but the reactive pairs begin to accumulate on the edges between facets for larger-sized Al clusters.14 In addition to the size selectivity of the clusters themselves, the reactivity of metal clusters with organic molecules may differ even if the reactants have similar functional groups.15 In the present study, we investigated the reactivity of water and alcohols with Al cluster anions in the gas phase carried by helium through a fast-flow tube apparatus. In particular, we studied the reactivity of both reactants introduced simultaneously to enable a comparison of their reactivity and
otivated by the potential value of H2 as a powerful “green” fuel, the hydrogen evolution reaction (HER) has been meticulously studied.1 The HER is also significant in electrochemical energy conversion, electrosynthesis, metal deposition, and corrosion.2,3 However, an in-depth and complete understanding of the HER mechanism is still elusive despite nearly a century of study.1,4,5 The general process of the HER on a metal electrode involves the following steps:6,7 (i) a discharge reaction (Volmer step)
H+ + e− → Had
(1)
or the discharge reaction of H3O dissociation of water)8
+
ions (formed by the
H3O+ + e− → Had + H 2O
(2)
followed by (ii) either a recombination reaction (Tafel step) Had + Had → H 2
(3)
and/or an electrochemical desorption reaction (Heyrovsky reaction) H+ + Had + e− → H 2 H 2O + Had + e− → H 2 + OH−
(4)
where Had refers to an adsorbed H atom. The HER of individual Al atoms with water has been studied by laserinduced fluorescence (LIF), suggesting that the major product appears to be “AlOH + H” with fragmentation of the HAlOH molecule.9−11 Moreover, the reactivity of Al clusters with multiple water molecules was studied, and it was found that the first step in such a reaction is the generation of a © XXXX American Chemical Society
Received: November 9, 2012 Accepted: December 6, 2012
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that of methanol (118.8 versus 104.6 kcal/mol, respectively).20 In order to further probe hydroxyl group reactivity with Al clusters, we simultaneously introduced a 1:1 mixture of H2O and CH3OH into our fast-flow tube apparatus. If vapor−liquid equilibrium conditions are acquired, a mixture of 77% water and 23% methanol maintained at a temperature of 84.5 °C will achieve a 1:1 mol ratio of water and methanol in the gas phase.21,22 Two mass spectra showing aluminum clusters in the absence and presence of the water−methanol vapor are displayed in Figure 2. It is notable that most of the Al2n−
interactions with Al clusters. Our focus on aluminum, water, methanol, and isopropanol is motivated by their potential as inexpensive energetic precursors. As the simplest alcohol, methanol has been considered as one of the most important candidates for storage and production of hydrogen.16 An indepth study on the reactivity of Al clusters with methanol (and water) is important in understanding the HER mechanism. Figure 1 presents mass spectra of Al clusters in the absence and presence of methanol. It is clearly seen that methanol
Figure 1. Reaction of methanol with aluminum clusters. The green spectrum represents the original Aln− spectrum before the reaction. The red spectrum represents the products after introduction of methanol. The insets display Al13−, the HOMO of Al15CH3OH−, and the LUMO+1 of Al17−.
shows an etching effect toward all of the Al cluster anions except Al13−. This etching-based reactivity between aluminum clusters and methanol differs from previously reported results of Aln− reacting with water.12 Instead, methanol closely resembles the reaction of Aln− toward oxygen, where selective species (e.g., Al13−) exhibit resistance to the etching effect and the cluster’s increasing intensity is due to its being a uniquely stable product after the dissociation of larger Al clusters.17−19 In contrast, Al23− has demonstrated to be a stable cluster in the presence of oxygen but turns out to be reactive when exposed to methanol, forming Al23(CH3OH)4−. Al23− has a structure with hexagonal packing of Al atoms, where the three-fold longitudinal edges (function as Lewis acid sites) exhibit lowenergy transition states for the attaching of −OH group molecules.14 Other Aln− species were also found to undergo the attachment of one or multiple CH3OH molecules, as seen for Al15−, Al16−, Al17−, Al19−, and Al21−. Al15−18− have previously been found to be highly reactive species with strong tendencies for the chemisorption of water molecules.12,13 It is interesting to note that Al15− attaches only one methanol molecule to form Al15CH3OH−, but Al17− gives rise to Al17(CH3OH)3−. The cluster, Al17−, has been identified as one of the most reactive species in Al cluster anions, and there are more active sites on Al17− than Al15−, resulting in less steric hindrance for the Al17− cluster to attach multiple methanol molecules.13 The methanol products from Al17− and Al15− coincide well with the previously established theory that complementary active sites support sizeselective reactivity of aluminum cluster anions with water.12 The products observed in Figure 1 indicate that methanol reacts with Aln− at room temperature but does not produce H2, which is in sharp contrast with the abundant HER products for Aln− reacting with water.12 Comparing the −OH group in water and methanol, however, it has been demonstrated that H2O has a slightly larger O−H bond dissociation energy compared to
Figure 2. Reaction of H2O and CH3OH (1:1 molar ratio) with Al cluster anions: (a) the original Aln− spectrum before the reaction, and (b) the spectrum showing the products after simultaneous exposure to the reactants.
clusters (even number of Al atoms) exhibit higher reactivity compared to their Al2n+1− counterparts, ascribed to the spin accommodation effect as the Al2n− clusters have half-filled HOMOs.13,23 The increased intensity of Al13− with the reactant mixture is similar to that for the methanol-only conditions in Figure 1; Al23− and Al37− also exhibit resistance to the etching.17,18 In addition to the etching reactivity seen in Figure 2, we also observe addition chemistry between Aln− clusters and the mixture of reactants. In particular, the Al27(H2O)−, Al27(OH)2−, Al29(H2O)−, and Al29(OH)2− species display relatively strong intensity, indicating preferential reactivity of water with Al27− and Al29− rather than methanol. This agrees with a previously reported investigation that found that, at larger sizes, Al24−, Al27−, and Al29− are the most reactive species.13 The theoretical results by W. Zhang et al. have also shown that medium-sized Al clusters in the size range of Al27−−Al30− favor a doubletetrahedron motif, with a tendency for the cluster surface to form a fcc(111) structure, but Al28− and Al30−, whose symmetries are both D3h, are more stable than Al27− and Al29−.24 The other reaction products comprised of Aln(OH)x− (Figure 2) also differ from the result of the water-only work.12 It is interesting to note that the observation of Aln(CH3O)x− species in the present study indicates an occurrence of HER from the methanol. In order to further explore this finding, we also examined the reaction between aluminum cluster anions with a mixture of isopropanol and water as a reactant mixture (approximately 1:1 molar ratio in 3819
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the gas phase). The result is displayed in Figure 3. As a comparison, Figure 3b displays the reaction between Aln− and
Figure 4. A reaction coordinate diagram showing the production of H2 from Al27− + 2 H2O. (a) The LUMO of Al27−, (b) the HOMO for the Al27(H2O)− complex, (c) a transition state, (d) the dissociated chemisorbed product and LUMO+1, (e) the HOMO surface after the second water molecule is bound to the active site, (f) another transition state, (g) a complex with two dissociative chemisorbed water molecules, (h) the transition state for H2 release, and (i) the cluster after H2 is released. The energy values were corrected with zero-point vibrations. The insets at the bottom display the structures of Al27−, Al28−, Al29−, the HOMO of Al29H2O−, and Al30−. Figure 3. (a) Size distribution of Aln− clusters, (b) the products after reaction with C3H7OH, and (c) the reaction results of Aln− clusters with water and isopropanol (1:1 molar ratio in the gas phase).
theoretical study on the interaction between an Al100 cluster and water molecules and also proposed that water could act as a catalyst via a relay (Grotthuss-like) molecular mechanism.30 Such a catalytic effect also resembles carbonic acid formation in the gas phase based on the reaction CO2 + H2O, where three or more H2O molecules are needed with one acting as the catalyst.31 In conclusion, we present a study on the gas-phase reactivity of water and methanol/isopropanol with Al cluster anions in a fast-flow tube apparatus that is coupled to a quadrupole mass spectrometer. In the absence of water, methanol was found to exhibit an etching effect toward the Al cluster anions with few exceptions (such as Al13−); in comparison, isopropanol tended to bind to the Al clusters. Although neither methanol-only nor isopropanol-only observed a loss of 2H in the roomtemperature fast-flow tube apparatus, interestingly, products with hydrogen released were observed in the reactions with both water + methanol and isoproanol + water systems. The use of bireactants toward Al clusters enables a comparison of their reactivity and gas-phase interaction. Furthermore, water was found to contribute to the activation of the O−H bond in alcohols when reacting with Al cluster anions. These investigations further the understanding of the HER mechanism and indicate potential application for hydrogen generation.
isopropanol (C3H7OH). It was found that the Aln− clusters react with isopropanol and mostly tend to attach one C3H7OH molecule with rare exception (Figure 3b); in contrast, the reaction changes in the presence of both water and isopropanol. First, the species Al27(H2O)− and Al29(H2O)− were again observed with reasonable intensity due to the relatively large binding energy of the water molecule on the Al27 and Al29 clusters (1.96 and 1.02 eV, respectively, from our calculations). In order to further explain this reactivity, Figure 4 shows the reaction coordinate diagram for the formation of H2 from Al27− and two H2O molecules. The consistency in our model provides additional evidence of the previously established mechanism based on the complementary active sites.12,13,25−27 The first two transition states, Figure 4c and f, detail the transfer of the hydrogen atoms onto the Lewis base sites of Al27−, while the transition state shown in Figure 4h refers to combination of the adsorbed H atoms, which also resembles the Tafel step shown in eq 3. Similar reactivity is inferred for Al29− + 2H2O in forming Al29(OH)2−, with the first water molecule taking the active site as shown in the bottom of Figure 4, whereas Al28− and Al30− are unreactive due to their unique geometry. Next, it is worth noting that the HER products observed in Figure 3c, seen as Al15(C3H7O)2−, Al25(C3H7O)2(C3H7OH)−, and Al27(C3H7O)2(C3H7OH)−, were not observed in the isopropanol-only reaction (Figure 3b). This is similar to the observation of Aln(CH3O)x− species for Aln− reacting with water and methanol (Figure 2) with no addition products present in the methanol-only reaction (Figure 1). Therefore, it is concluded that these products are due to the contribution of water, which may serve as a catalyst to activate the O−H bond in methanol. It was discovered many years ago that the introduction of water vapor aids in the dehydrogenation of methanol, resulting in more efficient conversion.28 Recently, by using a reactive force field method, Russo et al.29 performed a
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ASSOCIATED CONTENT
S Supporting Information *
Experimental and theoretical methods, detail of data and analysis, and so forth. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3820
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ACKNOWLEDGMENTS This material is based on work supported by the Air Force Office of Science Research under AFOSR Award No. FA955010-1-0071.
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