Jens Döbler

Structure and reactivity of V2O5: bulk solid, nanosized clusters, species supported on silica and alumina, cluster cations and anions

Vanadyl bond dissociation energies are calculated by density functional theory (DFT). While the hybrid (B3LYP) functional results are close to the available reference data, gradient corrected functionals (BP86, PBE) yield large errors (about 50 to 100 kJ mol−1), but reproduce trends correctly. PBE calculations on a V20O62H24 cluster model for the (001) surface of V2O5 crystals virtually reproduce periodic slab calculations. The low bond dissociation energy (formation of oxygen surface defect) of 113 kJ mol−1 (B3LYP) is due to substantial structure relaxations leading to formation of V–O–V bonds between the V2O5 layers of the crystal. This relaxation cannot occur in polyhedral (V2O5)n clusters and also not for V2O5 species supported on silica or alumina (represented by cage-type models) for which bond dissociation energies of 250–300 kJ mol−1 are calculated. The OV(OCH3)3 molecule and its dimer are also considered. Radical cations V2O5+ and V4O10+ have very low bond dissociation energies (22 and 14 kJ mol−1, respectively), while the corresponding radical anions have higher dissociation energies (about 330 kJ mol−1) than the neutral clusters. The bond dissociation energies of the closed shell V3O7+ cation (165 kJ mol−1) and the closed shell V3O8− anion (283 kJ mol−1) are closest to the values of the neutral clusters. This makes them suitable for gas phase studies which aim at comparisons with V2O5 species on supporting oxides.

Gas phase vibrational spectroscopy of mass-selected vanadium oxide anions

The vibrational spectra of vanadium oxide anions ranging from V(2)O(6)(-) to V(8)O(20)(-) are studied in the region from 555 to 1670 cm(-1) by infrared multiple photon photodissociation (IRMPD) spectroscopy. The cluster structures are assigned and structural trends identified by comparison of the experimental IRMPD spectra with simulated linear IR absorption spectra derived from density functional calculations, aided by energy calculations at higher levels of theory. Overall, the IR absorption of the V(m)O(n)(-) clusters can be grouped in three spectral regions. The transitions of (i) superoxo, (ii) vanadyl and (iii) V-O-V and V-O single bond modes are found at approximately 1100 cm(-1), 1020 to 870 cm(-1), and 950 to 580 cm(-1), respectively. A structural transition from open structures, including at least one vanadium atom forming two vanadyl bonds, to caged structures, with only one vanadyl bond per vanadium atom, is observed in-between tri- and tetravanadium oxide anions. Both the closed shell (V(2)O(5))(2,3)VO(3)(-) and open shell (V(2)O(5))(2-4)(-) anions prefer cage-like structures. The (V(2)O(5))(3,4)(-) anions have symmetry-broken minimum energy structures (C(s)) connected by low-energy transition structures of C(2v) symmetry. These double well potentials for V-O-V modes lead to IR transitions substantially red-shifted from their harmonic values. For the oxygen rich clusters, the IRMPD spectra prove the presence of a superoxo group in V(2)O(7)(-), but the absence of the expected peroxo group in V(4)O(11)(-). For V(4)O(11)(-), use of a genetic algorithm was necessary for finding a non-intuitive energy minimum structure with sufficient agreement between experiment and theory.

Structural diversity and flexibility of mgo gas-phase clusters

Magnesium oxide (MgO) is a prototype material of (simple) metal oxides. The NaCl-type structure of bulk MgO is the only phase observed in experiments up to a pressure of 227 GPa. 2] This indicates that MgO has an inherent structural stability, which can be expected to persist when passing from the bulk solid to molecular clusters. Indeed, mass spectra of (MgO)n + and (MgO)nMg + cluster ions along with calculations using rigid ion and polarizable ion shell model potentials indicate compact cubic structures similar to fragments of the MgO crystal lattice, with the most abundant clusters based on a (MgO)3 subunit. [4] The spectra and cluster compositions observed in IR resonance-enhanced multiphoton ionization experiments on large neutral (MgO)n clusters (n 15) have also given indications for cubic structures. Up to now, computational studies have almost exclusively investigated neutral MgO clusters, without direct comparison to experiment, 6–8, 10–17] despite the fact that most experiments were performed on cationic clusters. The main conclusion from these studies has been that the most stable structures for a given value of n are cubelike, except for (MgO)3n clusters, for which rings and stacks of rings are preferred. The geometric structures of the cationic MgO clusters have been assumed to be the same as for neutral ones (vertical ionization approximation), except for small hypermagnesium ions. So far, no systematic theoretical studies of the stoichiometric cationic clusters have been reported. Herein we demonstrate that, in contrast to the bulk material, neutral and cationic gas-phase clusters of MgO display unusual structural diversity and flexibility. Not only are the structures of the clusters in most cases noncubic, but the neutral and charged ones also differ. The atomic structures of cationic stoichiometric (MgO)n + (n = 2–7) clusters were determined by combining quantum chemical calculations with infrared multiple photon dissociation (IRMPD) experiments. In particular, global structure optimizations using density functional theory (DFT) have been performed on all the cluster sizes. Although several of the geometric structures reported here (but not all of them) have been found before with neutral 7, 10–17] and anionic clusters by different computational techniques, our calculations reveal unequivocally the global minima of all these configurations. Cationic clusters and their weakly bound complexes with Ar and O2 have been investigated experimentally in a molecular beam. Changes in this cluster distribution induced by the interaction with tunable infrared radiation were used to obtain the cluster-size-specific IR-MPD spectra. Figure 1 shows the global minimum structures of neutral (MgO)n and cationic (MgO)n + clusters with n = 2–7; for other low-energy isomers see Figures 1S and 2S in the Supporting Information. Figures 2 and 3 show a comparison between the experimental IR-MPD spectra and the calculated linear IR

Point defects in CaF2 and CeO2 investigated by the periodic electrostatic embedded cluster method

A periodic electrostatic embedding scheme is presented that uses the periodic fast multipole method. The convergence of properties with increasing cluster size is examined for cluster models of calcium fluoride. Properties investigated are the electron density, the density of states, the electronic excitation of color centers, and energies of defect formation. The embedded cluster method is applied to CeO(2) and oxygen vacancies in bulk CeO(2) as well as on its (111) surface. Employing the PBE0 functional, vacancy formation energies of 3.0 and 3.3 eV have been obtained for the bulk and the (111) surface, respectively. Formation of subsurface defects requires 3.33 eV (singlet open shell). The localization of the electrons left behind on defect formation in Ce 4f states is discussed. Occupied Ce 4f states are well localized on nearest Ce atoms for surface and subsurface vacancies. Localization apart from the vacancy was obtained for bulk. The total CPU time spent on the embedding part did not exceed 30 s on a single CPU even if 8000 basis functions of the cluster are involved.

Structural variability in transition metal oxide clusters

We present gas phase vibrational spectra of the trinuclear vanadium oxide cations V(3)O(6)(+)·He(1-4), V(3)O(7)(+)·Ar(0,1), and V(3)O(8)(+)·Ar(0,2) between 350 and 1200 cm(-1). Cluster structures are assigned based on a comparison of the experimental and simulated IR spectra. The latter are derived from B3LYP/TZVP calculations on energetically low-lying isomers identified in a rigorous search of the respective configurational space, using higher level calculations when necessary. V(3)O(7)(+) has a cage-like structure of C(3v) symmetry. Removal or addition of an O-atom results in a substantial increase in the number of energetically low-lying structural isomers. V(3)O(8)(+) also exhibits the cage motif, but with an O(2) unit replacing one of the vanadyl oxygen atoms. A chain isomer is found to be most stable for V(3)O(6)(+). The binding of the rare gas atoms to V(3)O(6-8)(+) clusters is found to be strong, up to 55 kJ/mol for Ar, and markedly isomer-dependent, resulting in two interesting effects. First, for V(3)O(7)(+)·Ar and V(3)O(8)(+)·Ar an energetic reordering of the isomers compared to the bare ion is observed, making the ring motif the most stable one. Second, different isomers bind different number of rare gas atoms. We demonstrate how both effects can be exploited to isolate and assign the contributions from multiple isomers to the vibrational spectrum. The present results exemplify the structural variability of vanadium oxide clusters, in particular, the sensitivity of their structure on small perturbations in their environment.

Formaldehyde Formation on Vanadium Oxide Surfaces V2O3(0001) and V2O5(001): How does the Stable Methoxy Intermediate Form?†

Hydroxy-mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O-H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.

Reactions of H2, CH4, C2H6, and C3H8 with [(MgO)n]+ Clusters Studied by Density Functional Theory

The reactions of CH4 with [(MgO)n]+ cluster cations (n=1–5, or 7) and those of C2H6 and C3H8 with [(MgO)2]+ are studied by density functional theory (B3LYP functional/TZVP basis set). Comparison with CCSD(T) calculations shows that these results are affected by errors between −6.5 and +5.1 kJ mol−1. The barriers of −26.1 and −27.0 kJ mol−1 predicted for CH4/(MgO)+ and C3H8/[(MgO)2]+, respectively, are in agreement with the observed hydrogen abstraction reaction. The reactions of H2 with [(MgO)n]+ cluster cations are studied for comparison, and the energy of hydrogenation is considered as reactivity descriptor for the metal oxide clusters in this type of reaction.

Partial oxidation of methanol on well-ordered V2O5(001)/Au(111) thin films

The partial oxidation of methanol to formaldehyde on well-ordered thin V(2)O(5)(001) films supported on Au(111) was studied. Temperature-programmed desorption shows that bulk-terminated surfaces are not reactive, whereas reduced surfaces produce formaldehyde. Formaldehyde desorption occurs between 400 K and 550 K, without evidence for reaction products other than formaldehyde and water. Scanning tunnelling microscopy shows that methanol forms methoxy groups on vanadyl oxygen vacancies. If methanol is adsorbed at low temperature, the available adsorption sites are only partly covered with methoxy groups after warming up to room temperature, whereas prolonged methanol dosing at room temperature leads to full coverage. In order to explain these findings we present a model that essentially comprises recombination of methoxy and hydrogen to methanol in competition with the reaction of two surface hydroxyl groups to form water.

Identification of Conical Structures in Small Aluminum Oxide Clusters: Infrared Spectroscopy of (Al2O3)1−4(AlO)+

The vibrational spectroscopy of the electronically closed-shell (Al 2O 3) n (AlO) (+) cations with n = 1-4 is studied in the 530-1200 cm (-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C 3 v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.

Methane activation by silica-supported Zr(IV) hydrides: the dihydride [(SiO)2ZrH2] is much faster than the monohydride [(SiO)3ZrH]

The silica-supported Zr(iv) dihydride [(triple bond)SiO)2ZrH2] reacts quickly and completely with methane to yield [(triple bond)SiO)2ZrMe2] through the intermediate [(triple bond)SiO)2ZrHMe], while its monohydride analogue [(triple bond)SiO)3ZrH] yields the monomethylated product [(triple bond)SiO)3ZrMe] slowly and incompletely.