Edge-Induced Active Sites Enhance the Reactivity of Large Aluminum

Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience...
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Edge-Induced Active Sites Enhance the Reactivity of Large Aluminum Cluster Anions with Alcohols Arthur C. Reber,† Patrick J. Roach,‡ W. Hunter Woodward,‡ Shiv N. Khanna,†,* and A. W. Castleman, Jr.‡ †

Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States Department of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States

‡

ABSTRACT: Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience. We have measured the size-selective reactivity of aluminum cluster anions with alcohols. All clusters with more than 20 atoms are found to be reactive, while Al11−, Al13−, and Al20− show enhanced resistance to oxidation at smaller sizes. The reactivity of aluminum cluster anions with water, methanol, and tertbutyl alcohol all exhibit patterns that require complementary active sites (Lewis acid, Lewis base) on adjacent atoms. Theoretical investigations reveal that at small sizes, the location of reactive pairs occurs on specific active sites, but at larger sizes the reactive pairs begin to accumulate on the edges between facets, marking the transition from the nonscalable size-dependent regime to the scalable regime where the nanoparticles are universally reactive.

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INTRODUCTION One of the major challenges in nanoscience is the understanding of emergent phenomena that arise as the size of a particle is reduced from the bulk regime all the way to clusters which contain only a handful of atoms.1−6 The cluster regime is marked by its “non-scalable” behavior in which adding or removing a single atom may drastically change the cluster’s properties.7−11 The origin of such behaviors in metal clusters is primarily due to quantum confinement of a metallic “Nearly Free Electron” (NFE) gas that results in bunching of electronic states into electronic shells.2,12,13 Shell effects manifest in the stability of clusters as seen through magic numbers as well as in their chemical behavior. For example, while bulk aluminum readily oxidizes, selected aluminum cluster anions with closed electronic shells have greatly reduced reactivity with oxygen.8,9,14,15 The quantum confinement also affects the reactivity with other oxidants, such as water, by producing active sites in clusters characterized by an uneven distribution of charge.16−21 While the reactivity of small aluminum clusters is strongly dependent on size, studies on larger clusters/nanoparticles find consistently high reactivity at all sizes.22−27 The transition between size selected and universal reactivity in aluminum nanoparticles occurs in the cluster size regime allowing the study of this phenomenon. The present investigations were partly motivated by our recent findings on the reactivity of water with aluminum cluster anions.16,17 In spherical clusters, the quantum confinement results in the grouping of electron levels into shells that result in closed electronic shells at electron counts of 2, 8, 18, 20, 34, 40, 70, ... electrons. The reactivity of aluminum cluster anions Aln− with oxygen is known to exhibit sharply reduced reactivity at n = 13, 23, and 37, consistent with shell closures at 40, 70, © 2012 American Chemical Society

and 112 electrons. The reactivity with water, however, showed that clusters with open electronic shells such as Al20− (61 valence electrons) do not display any significant reactivity while clusters with closed electronic shells, such as Al23− and Al37−, which are inert in oxygen reactive experiments, exhibit significant reactivity. Theoretical investigations revealed that the new reactivity pattern was associated with geometrical factors as it involved a pair of neighboring active sites where a Lewis acid active site preferentially bound the O atom while an adjacent Lewis base active site donated the charge to the H atom, breaking the OH bond. Clusters with geometrical structures where the Lewis acid and Lewis base sites were farther separated showed reduced reactivity. Similar reduced reactivity occurs in spherical clusters as the charge is distributed evenly over the surface of the cluster, greatly reducing reactivity, as no site on the cluster may behave preferentially as a Lewis acid or Lewis base. The presence of complementary active sites is also intimately linked with the cluster’s atomic structure. Clusters where the atomic arrangements have incomplete geometrical shells are marked by vacancies or adatoms on the surface that break the symmetry resulting in sites with an uneven distribution of charge and promoting complementary pairs. These results motivate additional questions, such as if the phenomenon is specific to water or can such complementary pairs break OH bonds in other systems, e.g., alcohols? Second, how does the location of such pairs evolve with size since experiments indicate that all the clusters at large sizes lead to the observed reactivity without regard to size? Here, we not Received: May 15, 2012 Revised: July 6, 2012 Published: July 10, 2012 8085

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framework. The electronic orbitals in clusters are expressed as a linear combination of atomic orbitals that were, in turn, formed via a linear combination of Gaussian functions located at the atomic sites. The exchange correlation contributions are included within the GGA-PBE gradient corrected density functional formalism.39 The calculations were carried out at an all electron level, using the Naval Research Laboratory Molecular Orbital Library (NRLMOL) developed by Pederson and co-workers.40,41 The basis sets are built from a variable number of primitive Gaussians and are based on a total-energy minimization for free atoms, further optimized for all electron density functional calculations.42 The basis set for Al had 6s, 5p, and 3d Gaussians; that for C and O had 6s, 5p, and 3d Gaussians; and the basis set for H had 4s, 3p, and 1d Gaussians. The basis sets were supplemented with a diffuse d-Gaussian. Some supplemental calculations were done by using the deMon2K density functional code.43 The presented results are ground states geometries in which the geometry was fully optimized starting from a number of initial geometries. Numerous sites for the transition state and binding site for the aluminum clusters were tested. Transition states were calculated by using a linear transit approach in which the O−H bond distance was varied. Smooth reaction surfaces with only small rearrangements of the aluminum core were found. Aluminum cluster geometries have been identified from previous studies.44,45 Binding energies were calculated by using the equation

only demonstrate the general nature of active site chemistry but also that as the cluster size is increased, the complementary sites begin to accumulate at the edges marking the beginning of the scalable regime.28−32 In this work, we have studied the reactivity of water, methanol, and tert-butyl alcohol (H 2 O, CH 3 OH, and (CH3)3COH) with Aln− anions. The observed similarities in the reaction rates allow us to generalize the complementary active site concept to additional species with hydroxyl groups. These reactions also produce surface-bound alkoxy groups that can protect the aluminum cluster from further attack, in much the same way as thiols are used as capping ligands on gold clusters.33−36 Theoretical investigations examine energetics including the reaction barriers with methanol for Aln− clusters, where n = 6−20, 23, 25, and 27, with special focus on the reactivity of the larger clusters. The largest cluster that shows resistance to alcohol etching is Al20−, which is found to have a high barrier to O−H cleavage due to the even distribution of Lewis acid sites around the equator of the cage, and the physical separation from preferred Lewis base sites which are located on the polar cap of the double cage structure. Al23− has a closed electronic shell and is one of the smallest clusters with bulk-like packing. Its high reactivity is a consequence of the localization of charge density of the frontier orbitals on the exposed edges of the cluster. An analysis of the transition states on various features of the cluster reveals that the reactivity on the edges is greatly enhanced with respect to that on the facets. As all larger clusters follow from facets with edges, the universal reactivity of the larger aluminum clusters is explained.

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E b = E(products) − E(reactants)

(1)

Reactants are the pure Aln− cluster and CH3OH, unless otherwise specified. Zero-point energy corrections and basis set superposition errors were found to be negligible for comparing reactivity between different clusters, and are not included.

EXPERIMENTAL METHODS

Our experimental setup has been discussed in detail previously,37,38 and is only briefly discussed here. Clusters were produced from an aluminum rod (99.999%, Puratronic) in a laser vaporization source with the presence of a continuous (8000 standard cubic centimeters per minute) flow of helium (High Purity, Praxiar, Inc.). The clusters were carried out of the source through an expansion nozzle and into a laminar flow tube that was maintained at a pressure of 0.7 Torr by a highvolume pump. The clusters were thermalized via collisions with the helium carrier gas before being exposed to a selected reactant introduced through a reactant gas inlet. The three reactants used in these experiments were deuterium oxide (99.9%, Cambridge Isotope Laboratories, Inc.), methanol-d (99.5%, Alfa Aesar), and tert-butyl alcohol (99.5%, Baker). Although all mass spectra presented involving water and methanol are of deuterated species, similar results were obtained with H2O and CH3OH, respectively. In order to control the flow of these reactant gases into the reaction vessel, they were heated or cooled to a point where their vapor pressure was approximately 100 Torr. Flow was controlled by using a medium-flow needle valve (SS-4MG-MH, Swagelok Co.). Reactants and products were sampled through a 2 mm extraction orifice via a custom conical octopole and guided through a series of differentially pumped RF octopoles and electrostatic lenses before being analyzed via quadrupole mass spectrometry (Extrel, CMS).

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RESULTS AND DISCUSSION The reactions of Aln− clusters with ROH where R = H, CH3, and C(CH3)3 were monitored at both high and low reactant gas concentrations to quantify their reactivity. At low concentrations, the cluster ion intensity is conserved such that the integrated intensity of Aln− in the initial spectrum does not change significantly from the sum of the Aln− and Aln(ROH)m− intensity in the reactant spectrum. As the partial pressure of all reactant gases increased, larger values for m were observed. Upon further increase in the reactant gas partial pressure, the intensity of large Aln− clusters diminished, while the intensity of smaller species increased. This observed behavior is consistent with successive reactant gases binding to active sites on the cluster until a threshold was reached, resulting in subsequent fragmentation, most likely the loss of an AlOR molecule, or autoionization. Figure 1 shows a bar graph that represents the observed affinity of individual aluminum clusters toward the initial adsorption of reactant gases at low concentrations. The values plotted represent the phenomenological adsorption affinity, AAPh, which is defined as: AAPh(n) =

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THEORETICAL METHODS First principles electronic structure investigations were carried out by using a linear combination of atomic orbitals molecular orbital approach within a gradient corrected density functional

∑m > 0 Al n(ROH)m− Al n−0

(2)

Aln−

where is the intensity of the respective aluminum clusters prior to the reactant gas introduction. Distinct and common trends are observed in the relative intensity of AAPh for each of the three reactants. Specifically, Al12− and Al24− are measured as 8086

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Figure 1. Phenomenological adsorption affinity of ROH clusters [R = H, CH3, C(CH3)3)] with Aln−.

Figure 2. Integrated intensity of Aln− clusters after reaction with ROH [R = H, CH3, C(CH3)3)] at maximum obtainable reaction conditions. Graph intensities are arbitrary.

local maxima with all three alcohols, and Al16−, Al17−, and Al18− have markedly larger binding affinities than proximate sizes. tert-Butyl alcohol has a lower measured adsorption affinity than methanol and water, especially at smaller sizes. We believe this is related to the larger alcohol having more normal modes over which the collisional and binding energy may be distributed, so less energy is likely to be imparted into the cleavage of the O− H bond. This effect will be less significant as the size of the cluster increases, which is consistent with the observed adsorption affinities. Steric effects may also reduce the adsorption affinity of the tert-butyl alcohol. The similar sizedependent reactivity between Aln− and the different alcohols, coupled with the common hydroxyl functional group suggests that the interaction mechanism between the different reactant gases is the same. Figure 2 displays the integrated intensity of the bare aluminum clusters at a maximum reactant gas concentration, which was achieved by maximizing reactant gas flow until further increase no longer affected the spectrum. The intensities of Al11−, Al13−, and Al20− are present as local maxima in each of the etching reactions. A significant increase in the intensity of Aln− is observed for the species n ≤ 7. The increase in intensity of smaller clusters is consistent with the etching of larger species. The observed behavior reveals that Al11−, Al13−, and Al20− clusters show resistance to etching. The large intensity of Al7− suggests that it is forming as a preferential product. There are several species seen in Figure 2A that do not appear in parts B and C, for example, Aln− (n = 8−10, 14, 15). Although best

efforts were made to keep reactant gas flow consistent, this difference is most likely due to the significantly lower vapor pressure of water than of the other two reactant gases. Figure 3 displays the product distributions of all three reactant gases. We observe Al7Hm− and Al13Hm− (m = 1−4). The vigorous etching of the aluminum clusters confirms that the reactions are primarily dissociative. The appearance of hydrogen atoms on clusters that are not reactive at lower concentrations is consistent with the explanation that these clusters are being formed as etching products from larger clusters. In the reaction with methanol, a large product peak is seen at a mass corresponding to Al10(methanol)4−. In the reaction with tert-butyl alcohol, we observe Aln(tert-butyl alcohol)m−, where n,m = 15,1; 12,3; 16,2; 14,3; 21,2; and 31,2. It should be noted that the Aln(tert-butyl alcohol)m− distribution at the greatest obtainable extent of reaction contains a significantly greater number of nonpure aluminum peaks than are observed for the other reactant gases. This behavior suggests that the active sites on these species are sterically hindered from further attack, such that these species do not adsorb the required amount of reactant molecules to result in fragmentation. To understand the size-selective reactivity of Aln− clusters, where [n = 6−20, 23, 25, 27], we evaluated the binding energy with the methanol molecule intact, the transition state energy for O−H bond cleavage, and binding energy following the 8087

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of the C−O bond in methanol is 0.75 eV higher than the cleavage of the O−H bond, confirming that the O−H cleavage is the primary mechanism. Al12− is found to have the lowest barrier, consistent with this cluster being most reactive in Figure 1, and the series of Aln−, where n = 16−18, all have low energy barriers, again consistent with experiment. Al11−, Al13−, and Al20− have relatively high energy barriers to O−H cleavage and all exhibit resistance to etching at high vapor pressures. As similar results for the reactivity of water with these clusters have been previously investigated for n = 7−18, we will not repeat those results here, except to note that the results with methanol and tert-butyl alcohol are consistent with the complementary active site hypothesis. The size-selective reactivity is understood to occur in clusters which have a pair of complementary active sites in neighboring locations; however, as the size of the cluster increases the geometric structure changes from an icosahedral core with decorating atoms to hexagonal fragments with facets that join at well-defined edges. The larger size of the cluster also results in a tighter spacing of the electronic shells, which results in more states near the frontier orbitals, and orbitals which are distributed over larger regions. Further, the extra electron of the anion is spread out over a larger number of atoms resulting in the cluster becoming a better Lewis acid. To understand the effect of increasing the cluster size on the reactivity, we first focus on Al20−, which is the largest cluster that we have found that is resistant to etching. Figure 5 shows

Figure 3. Full etching spectra with nonpure aluminum peaks Aln(ROH)m− labeled as (n,m): (A) R = H; (B) R = CH3; (C) R = (CH3)3C.

cleavage of the O−H bond, and plotted the results in Figure 4. We note that on the surface of Al12−, the barrier for the cleavage

Figure 5. The reaction pathway for Al20− with methanol. (a) The Al20− cluster with LUMO and LUMO+1 plotted in red-black and bluewhite. (b) Methanol with O−H intact bound to the Al20− cluster, with HOMO charge density plotted. (c) The transition state for O−H cleavage. (d) Final state where O−H is broken.

the structure of Al20−, which is a defect-free double-cage structure that may be thought of as a double icosahedron with an atom embedded, or as a “1515161” structure. We have plotted the LUMO and LUMO+1 states which are delocalized along the equator of the prolate cluster. As these states are indicative of Lewis acid sites, we expect the oxygen to prefer to bind here during cleavage. Figure 5b shows the geometry and HOMO after a methanol molecule has bound nondissociatively to the site that is adjacent to the lowest barrier. The HOMO is located on the polar atoms, which fails to provide an adjacent Lewis base site, reducing reactivity. The transition state shown in Figure 5c is the lowest found, and its total energy is 0.14 eV higher than the total energy of Al20− and an isolated methanol molecule. We note that the O−H cleavage is exothermic, but is

Figure 4. The binding energy (B.E.) of an intact methanol molecule to an Aln− cluster, the transition state energy (T.S.) for breaking the O− H bond, and the binding energy (Rlx.) after the O−H bond has been broken. 8088

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0.48 eV higher in energy than that on the edge. Figure 6e summarizes the results for all of the barriers calculated on the cluster in a heat map in which the color coding indicates the relative barrier where the oxygen atom is bound to that particular atom. Red indicates the lowest barrier and highest reactivity, while blue indicates a high barrier with little expected reactivity. Figure 6f summarizes the barriers for the Lewis base sites, in which red indicates the lowest barrier heights, and blue indicates large barriers for the reactivity where the hydrogen atom is bound to the particular atom. The heat map demonstrates that the reactions are most likely to occur on the edges of the cluster and not on the facets. The barriers for reactivity on the edges of the cluster are all much lower than the surfaces, because the charge density associated with the frontier orbitals accumulates on the edges of the clusters. This occurs because as the number of atoms and electrons in the cluster increases, the number of nodes in the frontier electronic states increases. This pushes the charge density of these higher energy states away from the center of the cluster, resulting in greater density on the periphery of the cluster, and this concentrates the charge density along the edges. In addition to this, the atomic p orbitals of the aluminum on the edges overlap more effectively with neighboring states when pointed directly out of the cluster surface, while the overlap of p orbitals on the facet better overlaps along the surface than perpendicular to the surface, resulting in a decrease in charge density expanding from the facet into the vacuum. To further test the hypothesis that active sites converge on edges, we next examine the reactivity of Al25− and Al27− with methanol. The ground state geometry of Al27− is a tetrahedral fragment based on Al30, but is missing the two capping atoms and a corner atom.46 Al25− has well-defined edges along different sections of the cluster. Parts a and b of Figure 7 show the LUMO and LUMO+1 of Al25− and Al27− which are located primarily along the edges of the cluster. Parts c and d of Figure 7 show the HOMO and HOMO-1 of these clusters which are also localized along the edges. Parts e and f of Figure 7 show the lowest energy transition states for the cleavage of the O−H bond of methanol. In Al25− the O atom binds at the site marked by the darker blue LUMO node on the top site of the cluster. For that site, the HOMO+1 charge density is quite strong on the adjacent atom, which indicates the lowest transition state site. In Al27−, the O binds at the site marked by a large dark blue lobe of LUMO density. A site adjacent to it has significant HOMO charge density, and that indicates the site with the lowest transition state energy for cleaving the O−H bond on Al27−. This lends further support to our contention that the active sites on aluminum clusters congregate on the edges of facets as the size of the clusters increase. The barrier heights of these transition states are 0.06 and 0.21 eV less than the binding energy of the intact methanol indicating that both of these barriers may be easily overcome and that these clusters will react rapidly with methanol. We note that extensive structural searches of larger aluminum clusters have revealed results in which the cluster geometries have well-defined edges in all clusters larger than around Al21, indicating that edgeinduced active sites are expected to be universally found on larger aluminum clusters.45,47,48

inhibited by a high barrier. The cluster has a small HOMO− LUMO gap of only 0.56 eV indicating that it is the geometric separation of the Lewis acid and Lewis base sites and not the HOMO−LUMO gap that results in it being much less reactive. The reactivity of Al23− with methanol is of special interest because it has a closed electronic shell and has resistance of etching with molecular oxygen. The structure of Al23− is a hexagonal packing of the aluminum atoms, with 3-fold longitudinal edges as seen in Figure 6c. To reveal the active

Figure 6. (a) The LUMO and LUMO+1 charge density of Al23−. (b) The HOMO, HOMO-1, and HOMO-2 charge density on Al23−. (c) The lowest energy transition state for the splitting of methanol on Al23−. (d) The transition state for dissociation on a surface site. (e) The lowest transition state energy where O is bound to the selected atom. (f) The lowest transition state energy where H is dissociating to the selected atom.

sites, we have plotted the nearly degenerate LUMO and LUMO+1 in Figure 6a, and the closely spaced HOMO and HOMO+1 are plotted in Figure 6b. The Lewis acid sites shown in Figure 6c are positioned primarily on the three longitudinal edges of the cluster. The Lewis base sites are also located on the 3-fold longitudinal edges, with some additional density on the more obtuse equatorial surface. To understand the origin of reactivity of Al23−, we have calculated the transition state energy for splitting methanol at all sites, the lowest of which is shown in Figure 6c. This is located on the edge, consistent with the sites predicted by the Frontier and near-Frontier orbitals. On the other hand, the transition state on the equatorial surface is

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CONCLUSION The reactivities of aluminum cluster anions with water, methanol, and tert-butyl alcohol have been studied and the general ability of complementary active sites to perform OH 8089

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Figure 7. The LUMO and LUMO+1 of (a) Al25− and (b) Al27−. The HOMO and HOMO-1 of (c) Al25− and (d) Al27−. The lowest energy transition state for the splitting of methanol on (e) Al25− and f)Al27−.

bond breaking in alcohols, ROH [R = H, CH3, C(CH3)3)] has been demonstrated. These results are consistent with the hypothesis that the O−H bond is cleaved by complementary Lewis acid and Lewis base active sites. At small sizes, such active pairs occur at precise locations on the geometry of the cluster. However, all clusters larger than Al20− react rapidly with ROH species. This reactivity in larger clusters is caused by the accumulation of complementary pairs on edges, which marks the beginning of the scalable regime. We believe that the current findings open a new era of reactive site chemistry and further work is needed to reveal their full potential and applications.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] (S.N.K.); [email protected] (A.W.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Air Force Office of Scientific Research (AFOSR) Award No. FA9550-101-0071 (A.W.C.) and MURI Grant no. FA9550-08-1-0400 (S.N.K.).

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dx.doi.org/10.1021/jp3047196 | J. Phys. Chem. A 2012, 116, 8085−8091