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 otivated by the potential value of H 2 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 H ad (1) or the discharge reaction of H 3 O + ions (formed by the dissociation of water) 8 + → + + − HO e H H O 3 ad 2 (2) followed by (ii) either a recombination reaction (Tafel step) + → H H H ad ad 2 (3) and/or an electrochemical desorption reaction (Heyrovsky reaction) + + → + + → + + − − − H H e H HO H e H OH ad 2 2 ad 2 (4) where H ad refers to an adsorbed H atom. The HER of individual Al atoms with water has been studied by laser- induced 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 HAl n OH(H 2 O) 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 H 2 release according to the reaction Al n - + 2H 2 O → Al n (OH) 2 - +H 2 . 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 simulta- neously to enable a comparison of their reactivity and Received: November 9, 2012 Accepted: December 6, 2012 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 3818 dx.doi.org/10.1021/jz301830v | J. Phys. Chem. Lett. 2012, 3, 3818-3821