Reaction mechanisms of dissociative chemisorption of HI, I2, and CH3I on a magic cluster Al 13− (original) (raw)
Related papers
Journal of the American Chemical Society, 2006
Recently, the icosahedral Al13cluster has been shown to possess some unusual characteristics due to its special stability (Bergeron, D. E.; et al. Science 2004, 304, 84-87; 2005, 307, 231-235). Here we present reactions of isolated Al13clusters with hydrogen chloride, following their oxidation through the application of Fourier transform ion cyclotron resonance mass spectrometry. Due to the ultra-low-pressure conditions, the reaction time can be expanded to make one intermediate after another come into view. The following intermediates are generated sequentially, releasing AlCl and H 2: Al13HCl-, Al12H-, Al12H2Cl-, Al11H2-, Al12Cl-, and Al11-. The resulting reaction scheme proves to be a molecular model for the dissolution of a metal in an acid, revealing the initial steps of a heretofore unknown fundamental heterogeneous reaction.
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.
The Journal of Physical Chemistry C, 2011
Reactivity of aluminum clusters has been found to exhibit size-sensitive variations. N 2 reduction is a hard process, and its dissociation on the Al surface is one of the few chemical methods available under nonhazardous conditions. In this context, we attempt to understand the adsorption behavior of N 2 molecules as a function of varying size and shape of Al clusters using a Density Functional Theory (DFT) based method. During the complex formation, various N 2 adsorption modes are examined. The results clearly demonstrate that, while the interaction energy does not vary with respect to the cluster size, shape of the cluster is highly contributive toward the chemisorption (a prerequisite for the reactivity) of the N 2 molecule. The underlying electronic and structural factors influencing the adsorption of N 2 molecules on the Al clusters are analyzed with the help of the Electron Localization Function (ELF) and Frontier Molecular Orbitals. As an illustration, the activation barrier calculations on various Al 13 conformations are calculated, and results confirm the experimental propositions that high-energy structures (depending upon their geometrical and electronic orientation) are more favorable for N 2 reduction.
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.
Interaction of Dioxygen with Al Clusters and Al(111): A Comparative Theoretical Study
Journal of Physical Chemistry C, 2008
We have studied the interaction of oxygen molecules with Al clusters and Al(111) using both wavefunction based quantum chemistry methods and density functional theory (DFT). These calculations were motivated by the fact that molecular beam experiments indicate that the adsorption of O2 on Al(111) should be activated whereas periodic DFT calculations yield purely attractive adsorption paths for almost all impact configurations of O2 on Al(111). On small Al4 clusters, accurate wavefunction based quantum chemistry methods find a non-vanishing barrier in the O2 adsorption. The DFT calculations for slabs and larger Al clusters confirm the important role of spin effects for the O2 dissociation barrier on Al. The results indicate that exchange-correlation effects play a crucial role for the determination of the adsorption barrier in the system O2/Al but their determination is hampered by serious technical problems that are discussed in detail.
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.
Chemisorption of oxygen atoms on aluminum (110): A molecular orbital cluster study
Surface Science, 1980
Self-consistent-field-Xo-scattered-wave calculations have been performed for clusters containing up to 22 aluminum atoms representing the (110) surface of aluminum in interaction with an oxygen atom adsorbed over the trough of the surface. Two types of adsorption site were considered: (i) directly atop the central aluminum atom of the trough and (ii) the so-called long bridge site. For each type of site two surface-oxygen distances were considered. Comparisons with available photoemission data (peaks at about-7 eV and roughly-10 eV with respect to the Fermi level) indicate that the more likely site is of the latter type with the oxygen atom coplanar, or nearly so, with the ridge aluminum atoms, i.e. at the center of an octahedral type interstice which is missing two neighboring atoms. Additional oxygen-related structure, attributed to essentially non-bonding oxygen orbitals is calculated near-3 eV similar to the case of low-coverage oxygen chemisorption on Al(100). There is some evidence for this feature in recent photoemission spectra although it is much weaker than the peaks at higher binding energy. A possible explanation of this fact is proposed in terms of the character of the associated wave functions and recent intensity calculations by Li and Tong on related systems. Decomposition of the various peaks into contributions from oxygen px, pY and pz orbitals is given, which should prove useful in the interpretation of future studies of the light polarization dependence of photoemission from Al(110) + 0 at low coverage.
The Journal of Physical Chemistry A, 2013
Aluminum clusters are now technologically important due to their high catalytic activity. Our present study on the small-sized aluminum clusters applies density functional theory (DFT)-based reactivity descriptors to identify potential sites for adsorption and eventual chemical reaction. Depending on symmetry, susceptibility of various type of reactive sites within a cluster toward an impending electrophilic and/or nucleophilic attack is predicted using the reactivity descriptors. In addition, the study devises general rules as to how the size, shape, and charge of the cluster influences the number of available sites for an electrophilic and/or nucleophilic attack. The predictions by reactivity descriptors are validated by performing an explicit adsorption of water molecule on Al clusters with four atoms. The adsorption studies demonstrate that the most stable water−cluster complex is obtained when the molecule is adsorbed through an oxygen atom on the site with the highest relative electrophilicity.
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.