Primary Reaction Steps of Al13- Clusters in an HCl Atmosphere: Snapshots of the Dissolution of a Base Metal (original) (raw)
Related papers
Reaction mechanisms of dissociative chemisorption of HI, I2, and CH3I on a magic cluster Al 13−
Journal of Computational Chemistry, 2008
We have investigated the transition-state structures and reaction mechanisms for the dissociative chemisorption reactions of HI, I 2 , and CH 3 I on the magic cluster Al À 13. The HI, I 2 , and CH 3 I molecules approach Al À 13 with an end-on orientation rather than a side-on orientation because of the more effective orbital overlap in the end-on orientation. The reactions of Al À 13 with HI and I 2 would produce Al 13 HIand Al 13 I 2 2 , respectively, because of large exothermic energy changes and relatively small activation energies. The reaction of Al À 13 with CH 3 I is unlikely to take place because of the low mobility of CH 3 on Al À 13 and the high activation barrier for the S N 2-type reaction. The dissociative chemisorption reactions are preferred thermodynamically to the abstractive chemisorption reactions.
Hydrogen dissociation on small aluminum clusters
Chem Phys, 2010
Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Aln clusters considered, with n =2-6. The aluminum dimer, which shows a Π3u electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox et al., J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al6 cluster. The electronic structure of the Aln-H2 systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Aln clusters (with n even) exhibit the lowest barriers to H2 dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σu molecular orbital of H2.
The Journal of Physical Chemistry A, 2001
High-level ab initio quantum chemical calculations have been used to investigate possible reactions in the Al-H-Cl system. Transition states or barrierless reaction paths have been identified for essentially all feasible reactions in this system involving a single aluminum atom. Structures, energies, and vibrational frequencies for reactants, products, and transition states in this system are presented. These results provide a basis for the estimation of reaction rate parameters for this system using transition state theory and related unimolecular reaction rate theories and thereby construct a reaction mechanism useful for detailed chemical kinetic modeling of aluminum combustion in HCl and chemical vapor deposition using AlCl 3 in H 2 . In the few cases where previous experimental or theoretical results have been published, the present work is consistent with previous work.
The Journal of Chemical Physics, 2010
We have studied the interaction of CH 4 with Al 2 and Al 3 neutral and charged clusters in the two lowest lying spin states using density functional theory. These calculations, via extended search, are used to determine the stable positions of H and CH 3 near the cluster, and the transition state to break the H -CH 3 bond. In all cases, stable methyl-aluminum-hydrides are possible. The H desorption is studied by means of vibration analysis and application of transition state theory. A common observed trend is that, in breaking the H -CH 3 bond, the interacting H atom is attached to the "surface" of the clusters attracting some negative charge of Ϸ0.2e. The charge transfer is illustrated using the corresponding orbitals near the transition state in conjunction with the computed Mulliken population analysis. Thermal vibrations, generally, do not enhance the reaction. In all exothermic cases, the binding energy toward CH 3 + HAl n charge increases with increasing charge of the original Al n ͑q=−1,0,1͒ cluster. Although Al lacks occupied d-orbitals, the small Al clusters reduce the ͑free methane͒ CH 3 -H dissociation barrier except for Al 3 ͑q=−1,0͒ . The relevant reactions in desorption require ϳ400-700°C.
1994
The role of solvent effects in association re,?ctions is studied in atom-cluster collisions. Classical trajectory studies of the systems'H+Cl(Ar), (n=1,12) are used to investigate the influence of size, structure, and internal energy of the "microsolvation" on the H+Cl association reaction. The following effects of solvating the chlorine in an Ar, cluster are found. (1) In the H+Cl& system there is a large "third body" effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found'at low energies is a substantial fraction of the gas-kinetic cross section. The mole&e is Produced in highiy excited vibrational-rotational states. (2) Some production of long-lived HCl***Ar complkxes, with lifetimes of 1 ps and larger, is found for the H-t ClAr collisions. Weti coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HClfAr. (3) At low collision energy (E-10 kJ/mol) for H+Cl(Ar),,,>the H+Cl association shows a sharp threshold. effect with cluster temperature. For temperatures T345 K the cluster IS liquidlike, and the reaction probability is high. For TS40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E=lOO kJ/mol) H atoms can penetrate also the solidlike Cl(Ar),, cluster: At this energy, the solid-liquid phase change is found not to increase the reaction probability.
Al 13 H − : Hydrogen atom site selectivity and the shell model
The Journal of Chemical Physics, 2009
Using a combination of anion photoelectron spectroscopy and density functional theory calculations, we explored the influence of the shell model on H atom site selectivity in Al13H−. Photoelectron spectra revealed that Al13H− has two anionic isomers and for both of them provided vertical detachment energies (VDEs). Theoretical calculations found that the structures of these anionic isomers differ by the position of the hydrogen atom. In one, the hydrogen atom is radially bonded, while in the other, hydrogen caps a triangular face. VDEs for both anionic isomers as well as other energetic relationships were also calculated. Comparison of the measured versus calculated VDE values permitted the structure of each isomer to be confirmed and correlated with its observed photoelectron spectrum. Shell model, electron-counting considerations correctly predicted the relative stabilities of the anionic isomers and identified the stable structure of neutral Al13H.
Journal of the American Society for Mass Spectrometry, 2002
Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, [Rb-35 Cl-M i ] ϩ , where M i ϭ Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 Ϯ 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 Ϯ 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single-and multiplecollision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T3 V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T3 V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using MassKi
The Journal of Chemical Physics, 2011
The dynamics of the oxidation of micro-hydrated Al atoms has been studied taking into account the effect of tunneling. Neutral aggregates of the type Al • (H 2 O) n , n = {1-8} and Al • (H 2 O) n • m(H 2 O) have been considered, where Al • (H 2 O) n has been treated by density functional theory (DFT) theory and the other m = {52, 56} waters have been represented by an effective fragment potential (EFP). The results indicate that oxidation may take place quite fast by a relay-type mechanism occurring within a ring of water molecules which involves the Al atom, in which a H atom is transferred. The inclusion of water molecules to form the ring from n = 1 to n = 3 tends to reduce the barrier height but results in lower tunneling transmission factors. The "optimal" ring is the one containing three waters; the four-water one produces lower rates. Coordination of additional waters to Al forming a second ring does not appear to have a further catalytic effect. The inclusion of many additional waters as EFPs, to simulate larger aggregates, increases the rates significantly. The extrapolation to bulk conditions and the possible impact of ionic mechanisms have also been discussed.
We report the results of a theoretical study of neutral and anionic Al 3 O n (n) 6-8) and an experimental investigation of Al 3 O 6 H 2-clusters, focusing on their structural and electronic properties. Our results, based on density functional calculations, reveal that sequential oxidation of Al 3 O 5 induces significant structural changes in the cluster configurations in which an O 2 molecule tends to replace an O atom. The neutral Al 3 O n (n) 6-8) clusters are found to be in doublet electronic states, with a planar to three-dimensional close-packed structure being most stable. The triplet state is found to be the optimum electronic state for the ground state of anionic Al 3 O n (n) 6-8). The clusters showed an energetic preference for a twisted-pair rhombic structure, although for n) 6 and 8, a planar hexagonal structure was only 0.16 eV higher in energy. It is also shown that the strength of the oxygen-oxygen bond dominates the preferred fragmentation path for both neutral and anionic clusters. The hydration behavior of an n) 6 cluster Al 3 O 6 H 2-was examined experimentally using an ion trap-secondary ion mass spectrometer under vacuum conditions, and the gas-phase clusters were shown to add three H 2 O molecules. Since H 2 O addition is consistent with the presence of under-coordinated metals in oxide clusters, the experimental result for n) 6 was consistent with the planar hexagonal structure, which contained three under-coordinated Al sites.
Journal of the American Chemical Society, 2008
Alanes are believed to be the mass transport intermediate in many hydrogen storage reactions and thus important for understanding rehydrogenation kinetics for alanates and AlH 3. Combining density functional theory (DFT) and surface infrared (IR) spectroscopy, we provide atomistic details about the formation of alanes on the Al(111) surface, a model environment for the rehydrogenation reactions. At low coverage, DFT predicts a 2-fold bridge site adsorption for atomic hydrogen at 1150 cm -1 , which is too weak to be detected by IR but was previously observed in electron energy loss spectroscopy. At higher coverage, steps are the most favorable adsorption sites for atomic H adsorption, and it is likely that the AlH 3 molecules form (initially strongly bound to steps) at saturation. With increasing exposures AlH3 is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state, accounting for step etching observed in previous STM studies. The mobility of these weakly bound AlH 3 molecules is the key factor leading to the growth of larger alanes through AlH 3 oligomerization. The subsequent decomposition and desorption of alanes is also investigated and compared to previous temperature programmed desorption studies.