Al Valence Controls the Coordination and Stability of Cationic

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Al Valence Controls the Coordination and Stability of Cationic Aluminum-Oxygen Clusters in Reactions of Al with Oxygen n+

Albert Armstrong, Hanyu Zhang, Arthur C Reber, Yuhan Jia, Haiming Wu, Zhixun Luo, and Shiv N. Khanna J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05646 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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The Journal of Physical Chemistry

Al Valence Controls the Coordination and Stability of Cationic Aluminum-Oxygen Clusters in Reactions of Aln+ with Oxygen Albert Armstrong,†ξ Hanyu Zhang,‡ξ Arthur C. Reber,† Yuhan Jia,‡ Haiming Wu,‡ Zhixun Luo,‡* and Shiv N. Khanna†* †

Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA

‡

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry,

Chinese Academy of Sciences; University of Chinese Academy of Sciences, Beijing 100190, P. R. China. §These

authors contributed equally to this work.

ABSTRACT: The reactivity of cationic aluminum clusters with oxygen is studied via a customized time-of-flight mass spectrometer (TOFMS). Unlike the etching effect for anionic aluminum clusters exposed to oxygen, here the cationic Aln+ clusters react and produce a range of small AlnOm+ clusters. Relatively large mass abundances are found for Al3O4+, Al4O5+, and Al5O7+ at lower O2 reactivity; while at higher O2 concentration oxygen addition leads to Al2O7+, Al3O6,8,9,10+ and Al4O7,9+ showing relatively high abundance, and Al5O7+ remains as a stable species dominating the Al5Om+ distribution. In order to understand these results, we have investigated the structures and stabilities of the AlnOm+ clusters. Firstprinciples theoretical investigations reveal the structures, HOMO-LUMO gaps, fragmentation energies, ionization energies, and Hirshfield charge of the AlnOm+ clusters (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). Energetically, Al3O4+, Al4O5+, and Al5O7+ are calculated to be most stable with high fragmentation energies, however ACS Paragon Plus Environment

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they still allow for chemisorption of additional O2 with large binding energies leading to clusters with higher O/Al ratios. The stability of the species is consistent with Al possessing three valence electrons while O typically accepts two, leading to the expectation that Al3O4+, Al5O7+, and Al7O10+ are reasonably stable. In addition to this, Al3O+, Al5O3+ and Al7O5+ are found to exhibit large HOMOLUMO gaps associated with the different oxidation states of Al. The oxygen-rich species such as Al2O7+, Al3O10+ and Al4O9+ all display superoxide structures providing further insights into the oxidation of aluminum clusters.

1. Introduction The chemistry of aluminum oxides is significant in catalysis, materials, and synthesis. Aluminum oxide (Al2O3), or alumina, is a long-established catalytic support,1-4 and is also used in protective coatings.5-7 These materials come in a range of different structural motifs, with α-Al2O3, the most stable from, where O atoms are arranged in hexagonal close-packed planes, and the Al atoms occupy octahedral interstitial sites. In γ-Al2O3, and δ-Al2O3 the Al may sit in tetrahedral sites,8 and thin films of Al2O3 exhibit a variety of 4-fold, 5-fold, and 6-fold coordination.9-10 These atomic structures indicate that aluminum oxide materials may adopt different structural motifs, and may even form wide-band gap materials with a stoichiometry of Al10O13 when supported on NiAl.9 In particular, the identification of particularly stable cluster motifs may be valuable in understanding defects in aluminum oxide materials, and in understanding the combustion of aluminum. The reaction of aluminum clusters with oxygen is highly exothermic with aluminum having the second largest energy density by volume of any element.

Thirty years ago Castleman and co-workers11

observed that selected small aluminum cluster anions such as Al13− are highly stable surviving the oxygen etching reactions,11-13 named as double magic cluster (i.e., both electronic and geometric closed shells);14 on the other hand, a few other Al clusters such as Al17− and Al12− find enhanced reactivity with water pertaining to a mechanism based on complementary active sites.15 The primary product of these aluminum etching experiments with anions is the loss of two Al2O fragments after the adsorption of an ACS Paragon Plus Environment

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O2 molecule, with detachment of an electron as the other major process.16-17 As the abundance of the neutral clusters cannot be measured with mass spectroscopy, the products in the oxygen etching of small aluminum cluster anions is less clear due to electron detachment. However, if aluminum cation clusters are considered, the products can be investigated more directly. Besides, there are numerous studies illustrating the stability and reactivity of size dependent Al clusters.17-41 However, despite the importance of aluminum oxide, the question remains whether there exist AlnOm+ compositions with unique stablity.42-43 An alternative strategy for identifying electronically stable AlnOm+ clusters is to find multiple valence clusters, where some of the Al atoms are in the +1 oxidation state, while others are in the +3 oxidation state.44 Aluminum oxide is usually found in an Al to O ratio of 2:3, which is rooted in the preferred oxidation state of Al being +3, and O being -2.44-50 For cations, Al7O10+, Al5O7+, and Al3O4+ are expected to have Al3+ and O2− and be highly stable. Because there is a large gap of 3.6 eV between the 3s2 and 3p1 shells in an Al atom, it is possible that in small clusters the valence of Al may be +1, leading to electronically stable clusters with multiple valences.44, 51 Al+ compounds are well-known including Al-cyclopentadiene and Al-halides which may serve as precursors for forming metalloid clusters,52-54 demonstrating that aluminum in the +1 oxidation state can indeed be stabilized.55 In neutral AlnOm clusters, Al2O, Al4O4, Al6O4, and Al6O5 have been found to be electronically stable due to multiple valence Al+1 and Al+3. Among others, the complexes of two-coordinate, three-coordinate, fourcoordinate and six-coordinate Al cations supported by various ligands have been found to present open axial coordination sites that attract Lewis-basic substrates allowing for broadened catalysis. This raises the question whether multiple valence AlnOm+ clusters may be observed with significant abundance in gas phase studies In the present study, we explore the structures and stability of cationic aluminum oxide clusters containing up to 10 atoms of oxygen, combining first-principles calculations and gas-phase experiments via a flow tube reactor coupled with mass spectrometry. Experiments were conducted for a wide range ACS Paragon Plus Environment


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of distributions of Aln+ clusters which react with oxygen giving rise to oxygen etching effect along with a few oxygen-rich cationic products. As results, the experimental observations accord well with the theoretical prediction. These findings on stable AlnOm+ species clarify the reactivity of Aln+ cations with oxygen and reveals concepts for understanding the formation of alumina-based catalysts for a variety of application. 2. Experimental and Theoretical Methods 2.1. Experimental method The experiments are conducted on a customized reflection time-of-flight mass spectrometer (ReTOFMS),56 combined with well-defined laser vaporization (LaVa) cluster source and compact reaction tube, of which the schematic diagram is illustrated in Figure S1 (ESI). A 10Hz 532 nm Nd: YAG laser with an average energy of 15−20mJ/pulse was used for laser ablation of an aluminum disk (Φ16mm, 99.999% purity). 5% O2/He with a pressure of 0.1MPa was injected into the system by another pulsed General Valve (Parker, Serial 9) to react with the laser ablated products of aluminum in the reaction tube. The He buffer gas was controlled by a pulsed General Valve (Parker, Serial 9). The signals from the MCP detector are recorded with a digital oscilloscope (Teledyne LeCroy HDO6000). 2.2. Computational method The calculations used the Amsterdam Density functional (ADF) set of codes.57 Here, the electronic orbitals of the cluster are constructed from a linear combination of atomic orbitals that are formed from Slater type orbitals centered at the atomic sites. Care is taken to ensure that the basis set of Slater type orbitals is sufficiently complete to accurately represent the cluster electronic wave-functions. The basis set employed in these calculations was ZORA/TZ2P. We have used the gradient corrected density functional proposed by Perdew-Burke-Ernzerhof (PBE)58 to include exchange and correlation effects, which has been found to be adequate to represent electronic structure of Aln− clusters. Since the present studies involve multiple Al and O atoms, we investigated a large number of possible structures ACS Paragon Plus Environment

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generated by hand and included additional structures motivated by previous work on aluminum oxide clusters. In each case, multiple spin configurations were examined to ensure that the ground state atomic structure and multiplicity have been identified. After the minimum energy structures were identified, we used

various

markers

to

examine

the

stability

of

the

clusters.

Figure 1. The mass spectra of cationic aluminum clusters produced via a LaVa-source (a), and the spectra after exposure to different quantities of oxygen (b/c). The series of Al2-6Ox+ are labeled in black, red, green, azure, blue.

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Figure 2. Optimized ground-state atomic structures of AlnOm+ (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). Aluminum is gray, Oxygen in Red.

This includes the HOMO-LUMO gap that is generally used as an indicator of chemical stability as a high gap indicates a cluster that is resilient to donating or accepting charge. We also examined the fragmentation channels involving the energy required to break a cluster of n Al and m O atoms into fragments containing x Al and y O, and the other fragment containing n-x Al and m-y O atoms. The fragmentation energies ΔFrag for an AlnOm+ cluster into AlxOy+ was calculated via the equation 1, ∆Frag(𝐴𝑙𝑛𝑂𝑚 + ) = E(𝐴𝑙𝑥𝑂𝑦 + ) + E(𝐴𝑙𝑛 ― 𝑥𝑂𝑚 ― 𝑦) ― E(𝐴𝑙𝑛𝑂𝑚 + )

(1)

where E(AlnOm)+ is the total energy of a cluster of n Al and m O atoms. All possible fragmentation channels are considered.


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3. Results and Discussion Figure 1a presents a typical mass spectrum of the cationic Aln+ (6≤n≤50) clusters (for full spectrum see Figure S1, ESI), where well-resolved clean clusters are successfully obtained. As a comparison, Figure 1b and c display the mass distribution of these clusters at the presence of different amount of reactant oxygen. As is shown, the mass abundances of all Aln+ clusters are reduced at the increasing amount of oxygen, due to collision-induced dissociation and chemical reactions forming different products.59-60 At the same time, several AlnOm+ are produced as seen in the low-mass range, which shows dependence on the quantity of oxygen being introduced. The observation of AlnOm+ species on the small mass range, instead of large mass range, manifest the existence of etching-dominant channel for Aln+ in reacting with O2. Previously published studies have ascertained that the dominant reaction pathway for anionic aluminum clusters with O2 follows “Aln− + O2  Aln-4− + 2Al2O”,33, 61 while that of anionic cobalt clusters corresponds to “Con− + O2  CoO2− + Con-1”,61-65 In boron clusters,66-71 the bond strengths are larger than that of aluminum, so the reactivity with O2 most commonly results in the fragmentation into 2 BO molecules in the cations, “Bn+ + O2  Bn-2+ + 2BO”, or BO2 in the anions, “Bn− + O2  Bn-1− + BO2”.72-75 Considering that small aluminum clusters are very reactive with oxygen, the reactivity of cationic Al clusters with O2 may generate small oxides AlnOm+ (even oxygen-rich species such as Al2O7+), written as, 𝐴𝑙𝑛 + + y𝑂2 [𝐴𝑙𝑛 ― 𝑚]0, + + [𝐴𝑙𝑚𝑂2𝑦] +,0

𝐴𝑙𝑛 + + y𝑂2 [𝐴𝑙𝑚𝑂𝑥] +,0 + [𝐴𝑙𝑛 ― 𝑚𝑂2𝑦 ― 𝑥]

(2) 0, +

(3)


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In order to determine the stability of the cationic and neutral clusters, we have investigated the ground state atomic structures of AlnOm+ clusters containing up to 7 Al atoms and up to 10 oxygen atoms, as shown in Figure 2. One of the general features is the breaking of the Al-Al bonds as successive O atoms are added. As the Al-O bond is much stronger than the Al-Al bond, with the binding energy of an Al2 molecule being 1.48 eV compared to 7.83 eV for the AlO molecule. Starting from Al2, the first two O atoms insert into the Al-Al bond, while additional O is added to this Al2O2+ core, first as an atom, and then as an O2 molecule and then as an ozone molecule for Al2O7+. A similar progression is seen in the addition of O to Al3 cluster, with an Al3O4+ core being built up, to which O2 molecules are gradually added. For larger series, the core corresponds to Al4O6+, Al5O7+, and Al6O9+. Several clusters are found to have planar structures, the Al4 series up to 4 oxygen atoms, Al5O3+, and Al5O4+, and Al7O5+. Most strikingly, the Al7O5+ cluster is unusually large for a planar structure, and in the neutral clusters, the largest planar structure is Al6O5. We now consider the origin of stability and we begin by examining the HOMO-LUMO gap, in that a cluster with a large gap is an indicator of low chemical reactivity, especially with O2.13, 63 Figure 3A shows the HOMO-LUMO gap in the AlnOm+ clusters. The clusters with unusually large HOMO-LUMO gaps in this study are Al3O+, Al3O4+, Al3O6+, Al5O3+, and Al5O7+, Al7O4+, Al7O5+ and Al7O10+. Because aluminum atoms have an odd number of valence electrons, only cationic clusters with an odd number of aluminum atoms may have an even number of electrons. For this reason, only clusters with an odd number of aluminum atoms are likely to have large HOMO-LUMO gaps. In smaller clusters marked by lower coordination, the Al sites may be monovalent while for larger sizes Al typically acquires a trivalent character at higher coordination. In the present work,


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the stability of Al3O+ (other experimental evidence in Figure S2, ESI) can be looked upon as due to monovalent Al sites that each have an oxidation state of +1 while the O site has an oxidation state of -2. As the cluster is a cation, the cluster is stabilized by this valence. In the case of Al5O3+, we see an example of multiple valence Al, with one Al atom bound to 3 O atoms, and an oxidation state of +3, and 4 Al atoms singly bound to O, with and oxidation state of +1. The 3 O atoms then take 6 electrons and the last electron is gone because the cluster is cationic. In the case of Al7O4+, and Al7O5+, a similar argument may be made. Al7O4+ has one highly coordinated Al site and 6 low coordinated Al sites. Assuming that the highly coordinated Al site is trivalent while the low coordinated Al sites have an oxidation state of +1, the Al sites donate 8 electrons to 4 O atoms again satisfying the chemical valence. For the case of Al7O5+, we find 5 monovalent Al and 2 trivalent Al, so the Al needs to give away 10 electrons after ionization. As there are five O atoms, this also satisfies the valence. The remaining stable clusters Al3O4+, Al5O7+, and Al7O10+ (for other experimental evidence see Figure S2 and S3, ESI) can also be looked upon as each Al being in a +3-valence state while each O in a -2 oxidation state. All this points to the fact in moderate oxygen coverages, the stability is rooted in the multivalent character of the Al sites, while at higher O coverages the stability is rooted in the +3 oxidation state of aluminum. In actual experiments, there is a coexistence of cations, anions, and neutrals generated from the LaVa source, therefore, not only does the relative stabilities of the ionic clusters relative to each other determine the observed mass abundances of Aln±,0, but also the subsequent ion-molecule reaction products may result in dissociation and neutralization processes in the rich-pressure collision cell. To this end, we examined the adiabatic ionization energies (I.E.) of the AlnOm clusters, which are obtained as the energy difference between the ground state of the neutral and


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the cationic species. Figure 3B shows that there are large variations in the I.E. of the clusters. The high I.E. of Al2Ox and Al3Ox series with higher O coverage is due to I.E. of O2 and O3 that are 12.06 eV and 12.53 eV respectively. Neutral Al4O6 and Al6O9 correspond to the Al to O ratio of 2:3 as in bulk Al3O3 and consequently have higher I.E. Al5O3 and Al7O3, and Al7O4 have multiple valence stability in the cation, and marked by lower I.E. One can generally associate the low I.E. for low O content, especially in cases where O2 motifs are adsorbed to a cluster core. In experiments where the clusters undergo collision, the stability is primarily governed by the energetic considerations. We, therefore, calculated the fragmentation energy using Eq. (1) for all the clusters. Figure 3c plots all the fragmentation energies, and Figure 4 plots the fragmentation energies as a function of the O/Al ratio. Figure 4 also shows the lowest energy fragmentation pathway that can be used to interrogate as to what extent the ratio 2:3 for bulk Al2O3 seen in the bulk phase is present at these small sizes. The peaks in the fragmentation energy at each size correspond to the clusters requiring maximum energy to fragment the cluster into two pieces and hence is a measure of the energetically stable species. From the results, Al2O2+, Al3O3/4+, Al4O5+, Al5O7+, and Al7O10+ appear as the most stable for different Al content. Note that Al3O4+, Al4O5+, and Al5O7+ are seen in experiments at low O2 coverage while Al7O10+ is seen at high coverage (Fig. S3). In all of the cases, the energetically most stable clusters correspond to a ratio which is one less O atom than the 2:3 ratio for bulk phase Al2O3. The results bring out several interesting correlations. First, the fragmentation channels exhibit a peak at O/Al ratio of 1.5 quite similar to the ratio for the bulk phase. For aluminum-rich aluminum oxide clusters, the most likely fragmentation channel is Al2O, or the removal of Al+, in part because the ionization energy of Al is only 6.00 eV, a value lower than many AlnOm clusters. The lowest energy fragmentation is the


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removal of O near the 1.5:1 O to Al ratio, and becomes O2 loss in the oxygen rich clusters. This is understandable since both Al2O and O2 are highly stable fragments, and are favorable fragments.


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Figure 3. (A) HOMO-LUMO gaps of the AlnOm+ cationic clusters (2 ≤ n ≤ 7; 1 ≤ m ≤ 10). (B) Ionization energies of AlnOm clusters, corresponding to the electron affinities of the AlnOm+. (C) The fragmentation energies of the AlnOm+ clusters (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). We have also carried out an additional analysis of stability by investigating the minimum energy required to remove an O2 molecule. The results are shown in Figure 5. Note that there is a minimum at Al2O9+ (up to ~0.2eV) indicating that O2 growth would likely end with Al2O7+. In the case of Al3Ox+, we see that Al3O4+ is quite stable, so the consecutive addition of O2 molecules may lead to Al3O6+, Al3O8+, and Al3O10+. We note that the addition of O2 to Al3O4+ without rearranging the core leads to an O2 binding energy of 0.83 eV, 0.49 eV, and 1.04 eV, suggesting that with excess O2, Al3O10+ is a likely product. For the Al4On+ series, Al4O7+ binds O2 strongly at 2.57 eV, while Al4O9+ binds much weaker at 0.47 eV. For the Al5Om+ series, Al5O7+ is the most prominent peak in the mass spectra, which is the stable cluster without adsorbed O2.


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Figure 4. The correlation of fragmentation energies with the ratios of O/Al. The gray line indicates the 3:2 O/At ratio.

Figure 5. O2 removal energy from AlnOm+ clusters. To further analyze the effect of oxygen coverage on the charge transfer in the AlnOm+ clusters, we have plotted the Hirshfeld Charges of O and Al as a function of cluster size and number of oxygen atoms in Figure 6. Beginning with Al2, the increase in the number of O sites increases the charge transfer from Al to O, but a maximum value is reached once the O coverage leads to the formation of O-O bonds. The peak is seen at Al2O3+, Al3O4+, Al4O6+, Al5O7+, and Al6O7+. Of special interest are the clusters with multiple valence. In Al3O+, the Hirshfeld charge on the Al atoms is 0.46 e-, indicating that they have a valence on 1. In the case of Al5O3+, one singly valent Al atom have charges of 0.37 e- and 0.42 e-, while the trivalent atom has a charge of 0.63 e-. For Al7O5+, the two central Al have Hirshfeld charges of +0.66 and +0.65 e- indicating they are trivalent, while the 5 external Al have charges from +0.32 to +0.42 e- indicating that they are monovalent. An analysis of the charges is consistent with the multiple valence hypothesis.


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Figure 6. (A) Average Hirshfeld charge of Al atoms. (B) Average Hirshfeld charge of oxygen atoms.

4. Conclusions We report here a comprehensive study of the reactivity of Aln+ with oxygen and emphasize on the stability of the as-produced AlnOm+ clusters. It is found that, a few small AlnOm+ clusters exhibit enhanced stability with interesting structures. Detailed theoretical calculations depict the


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HOMO-LUMO gaps, ionization energies, fragmentation energies, and Hirshfield charges. The calculation results find a few clusters, such as Al2O2+, Al3O3,4+, Al4O5+, Al5O7+ and Al6O8,9+, exhibit relatively higher stability. But the fact that the most AlnOm+ clusters still bear a large binding energy for chemisorption of additional oxygen, which leads to the successive addition of O2 to saturate many of these clusters with excess O2. This is consistent with the experimental observation of even larger oxygen-rich species such as Al2O7+, Al3O10+, Al4O7+, Al5O9+, etc. at the presence of large flow rate of oxygen into the flow tube reactor. Several multiple valence clusters, such as AlO3+, Al5O3+, and Al7O4+ are found to be electronically stable due to having both Al+1 and Al+3 oxidation states. Based on well-resolved mass spectrometric observation, this study not only clarifies the reactivity of Aln+ cations with oxygen, indicate the potential in forming alumina-based catalysts for a variety of application, and shows insights into the coordination chemistry of Al complexes. ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Experimental details of and coordinates for the AlnOm+ clusters are provided in a PDF. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] (SNK); [email protected] (ZL) Author Contributions


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These authors contributed equally.

ACKNOWLEDGMENT We gratefully acknowledge funding by the United State Air Force Office of Scientific Research, (AFOSR) Grant No. FA9550-18-1-0511. This work is also financially supported by the National Natural Science Foundation of China (Grant No. 21722308), and by Key Research Program of Frontier Sciences (CAS, Grant QYZDBSSW-SLH024). Z. Luo acknowledges the National Thousand Youth Talents Program. ABBREVIATIONS H HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; B. E., binding energy; TOFMS, time-of-flight mass spectrometer. REFERENCES 1. Pines, H.; Haag, W. O., Alumina: Catalyst and Support. I. Alumina, Its Intrinsic Acidity and Catalytic Activity. J. Am. Chem. Soc. 1960, 82, 2471-2483. 2. Harris, P. J. F., The Sintering of Platinum Particles in an Alumina-Supported Catalyst: Further Transmission Electron Microscopy Studies. J. Catalysis 1986, 97, 527-542. 3. Diao, W.; DiGiulio, C. D.; Schaal, M. T.; Ma, S.; Monnier, J. R., An Investigation on the Role of Re as a Promoter in Ag-Cs-Re/Α-Al2O3 High-Selectivity, Ethylene Epoxidation Catalysts. J. Catalysis 2015, 322, 14-23. 4. Reber, A. C.; Khanna, S. N., Effect of Embedding Platinum Clusters in Alumina on Sintering, Coking, and Activity. J. Phys. Chem. C 2017, 121, 21527-21534. 5. Atwood, D. A.; Yearwood, B. C., The Future of Aluminum Chemistry. J. Organomet. Chem. 2000, 600, 186-197. 6. Lu, J.; Liu, B.; Greeley, J. P.; Feng, Z.; Libera, J. A.; Lei, Y.; Bedzyk, M. J.; Stair, P. C.; Elam, J. W., Porous Alumina Protective Coatings on Palladium Nanoparticles by Self-Poisoned Atomic Layer Deposition. Chem. Mater. 2012, 24, 2047-2055. 7. Feng, H.; Lu, J.; Stair, P. C.; Elam, J. W., Alumina over-Coating on Pd Nanoparticle Catalysts by Atomic Layer Deposition: Enhanced Stability and Reactivity. Catalysis Letters 2011, 141, 512-517. 8. Krokidis, X.; Raybaud, P.; Gobichon, A.-E.; Rebours, B.; Euzen, P.; Toulhoat, H., Theoretical Study of the Dehydration Process of Boehmite to Γ-Alumina. J. Phys. Chem. B 2001, 105, 5121-5130. 9. Kresse, G.; Schmid, M.; Napetschnig, E.; Shishkin, M.; Köhler, L.; Varga, P., Structure of the Ultrathin Aluminum Oxide Film on NiAl(110). Science 2005, 308, 1440-1442.


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Al Valence Controls the Coordination and Stability of Cationic

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