Structure and properties of bimetallic titanium and vanadium oxide clusters.

By employing a genetic algorithm together with density functional theory (B3LYP), we investigate the most stable minimum structures of several bimetallic titanium and vanadium oxide clusters that contain four metal atoms. The following compositions are studied: VnTin-4O10(-) (n = 1-4), (TiO2)VOn(-) (n = 1-4), and (TiO2)VOn(+) (n = 1-3). Apart from (TiO2)3VO(-), vanadium oxo groups are always part of the most stable minimum structures when vanadium is present. Anti-ferromagnetic coupling lowers the energy substantially if spin centers are located at neighbored metal atoms rather than at distant oxygen radical sites. Vanadium-rich or oxygen-poor compositions prefer symmetric adamantane-like cage structures, some of which have already been proposed in a previous study. In contrast, vanadium-poor and oxygen-rich compositions show versatile structural motifs that cannot be intuitively derived from the symmetric cage motif. Particularly, for Ti4O10(-) there are several non-symmetric and distorted cages that have an up to 68 kJ mol(-1) lower energy than the symmetric adamantane-like cage structure. Nevertheless, for the adamantane-like cage the simulated infra-red spectrum (within the harmonic approximation) agrees best with the experimental vibrational spectrum. The oxidative power of the (TiO2)3VO3(-) and (TiO2)3VO2(+) clusters as measured by the energy of removing 1/2 O2 (297 and 227 kJ mol(-1), respectively) is less than that of the pure vanadium oxide clusters (V2O5)VO3(-) and (V2O5)VO2(+) (283 and 165 kJ mol(-1), respectively).

Structural variability in transition metal oxide clusters: gas phase vibrational spectroscopy of V3O(6-8)+.

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.

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.

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.

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 [(triple bond)SiO)2ZrH2] is much faster than the monohydride [(triple bond)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.

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.

V2O5/SiO2 surface inspired, silsesquioxane-derived oxovanadium complexes and their properties.

Inspired by surface species proposed to occur on heterogeneous catalysts novel oxovanadium(v) silsesquioxanes were synthesised. Reaction of a T8-silsequioxane containing two geminal OH groups with O=V(O(i)Pr)3 led to a dinuclear compound where the geminal disiloxide functions of two silsesquioxanes are bridging two O=V(O(i)Pr) moieties (2). Formation of 2 shows that--in contrast to proposals made for silica surfaces--in molecular chemistry a bidentate coordination of geminal siloxides to one vanadium centre is not favourable. With the background that species being doubly anchored to a support have been suggested to play active roles on V2O5/SiO2 catalysts an anionic complex has been prepared where a divalent dioxovanadium unit replaces one Si corner of a (RSiO1.5)8, cube (a Si-OH function remains pending) (3). 3 has been intensely investigated by vibrational spectroscopy, and to support assignments not only of the v(V=O) bands but also of the v(V-O-Si) bands, whose positions are of interest in the area of heterogeneous catalysis, isotopic enrichment studies and DFT calculations have been performed. The corresponding investigations were aided by the synthesis and analysis of a silylated derivative of 3, 4. Moreover, with regard to their potential as structural and spectroscopic models all complexes were characterised by single crystal X-ray diffraction. Finally, 2 and 3 were tested as potential catalysts for the photooxidation of cyclohexane and benzene with O2. While 2 shows a slightly higher activity than vanadylacetylacetonate, 3 leads to significantly increased turnover numbers for the conversion of benzene to phenol.

Probing the electronic structure of early transition-metal oxide clusters: polyhedral cages of (V2O5)n(-) (n = 2-4) and (M2O5(2)(-) (M = Nb, Ta).

Vanadium oxide clusters, (V2O5)n, have been predicted to possess interesting polyhedral cage structures, which may serve as ideal molecular models for oxide surfaces and catalysts. Here we examine the electronic properties of these oxide clusters via anion photoelectron spectroscopy for (V2O5)n(-) (n = 2-4), as well as for the 4d/5d species, Nb4O10(-) and Ta4O10(-). Well-resolved photoelectron spectra have been obtained at 193 and 157 nm and used to compare with density functional calculations. Very high electron affinities and large HOMO-LUMO gaps are observed for all the (V2O5)n clusters. The HOMO-LUMO gaps of (V2O5)n, all exceeding that of the band gap of the bulk oxide, are found to increase with cluster size from n = 2-4. For the M4O10 clusters, we find that the Nb/Ta species yield similar spectra, both possessing lower electron affinities and larger HOMO-LUMO gaps relative to V4O10. The structures of the anionic and neutral clusters are optimized; the calculated electron binding energies and excitation spectra for the global minimum cage structures are in good agreement with the experiment. Evidence is also observed for the predicted trend of electron delocalization versus localization in the (V2O5)n(-) clusters. Further insights are provided pertaining to the potential chemical reactivities of the oxide clusters and properties of the bulk oxides.

Gas-phase infrared photodissociation spectroscopy of tetravanadiumoxo and oxo-methoxo cluster anions.

The infrared spectra of the binary vanadium oxide cluster anions V(4)O(9)(-) and V(4)O(10)(-) and of the related methoxo clusters V(4)O(9)(OCH(3))(-) and V(4)O(8)(OCH(3))(2)(-) are recorded in the gas phase by photodissociation of the mass-selected ions using an infrared laser. For the oxide clusters V(4)O(9)(-) and V(4)O(10)(-), the bands of the terminal vanadyl oxygen atoms, nu(V-O(t)), and of the bridging oxygen atoms, nu(V-O(b)-V), are identified clearly. The clusters in which one or two of the oxo groups are replaced by methoxo ligands show additional absorptions which are assigned to the C-O stretch, nu(C-O). Density functional calculations are used as a complement for the experimental studies and the interpretation of the infrared spectra. The results depend in an unusual way on the functional employed (BLYP versus B3LYP), which is due to the presence of both V-O(CH(3)) single and V=O double bonds as terminal bonds and to the strong multireference character of the latter.

Degradation of ionized OV(OCH3)3 in the gas phase. From the neutral compound all the way down to the quasi-terminal fragments VO+ and VOH+.

The consecutive fragmentation of ionized trimethyl vanadate(V), OV(OCH3)3 (1), is examined by experiment and theory. After an elimination of formaldehyde from the molecular ion 1+, subsequent dissociations proceed via losses of first H2 and then two molecules of formaldehyde to finally yield the VOH+ cation; these redox reactions involve the V(II)/V(IV) manifold. At elevated energies, expulsion of CH3O* from 1+ can efficiently compete to afford OV(OCH3)2+, a formal V(V) compound, from which subsequent losses of H2 and two units of CH2O lead to bare VO+, thereby exploring the V(III)/V(V) redox manifold. Experiments using complementary mass spectrometric techniques, i.e., neutralization-reionization experiments and ion/molecule reactions, in conjunction with extensive computational studies provide deep insight into the ion structures and the relative energetics of these dissociation reactions. In particular, a quantitative energetic scheme is obtained that ranges from neutral OV(OCH3)3 all the way down to the quasi-terminal fragment ions VOH+ and VO+, respectively.

Oxidation of methanol to formaldehyde on supported vanadium oxide catalysts compared to gas phase molecules.

The oxidation of methanol to formaldehyde on silica supported vanadium oxide is studied by density functional theory. For isolated vanadium oxide species silsesquioxane-type models are adopted. The first step is dissociative adsorption of methanol yielding CH3O(O=)V(O-)2 surface complexes. This makes the O=V(OCH3)3 molecule a suited model system. The rate-limiting oxidation step involves hydrogen transfer from the methoxy group to the vanadyl oxygen atom. The transition state is biradicaloid and needs to be treated by the broken-symmetry approach. The activation energies for O=V(OCH3)3 and the silsesquioxane surface model are 147 and 154 kJ/mol. In addition, the (O=V(OCH3)3)(2) dimer (a model for polymeric vanadium oxide species) and the O=V(OCH3)3(*+) radical cation are studied. For the latter the barrier is only 80 kJ/mol, indicating a strong effect of the charge on the energy profile of the reaction and questioning the significance of gas-phase cluster studies for understanding the activity of supported oxide catalysts.

Structure and reactivity of V(2)O(5): 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 V(20)O(62)H(24) cluster model for the (001) surface of V(2)O(5) 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 V(2)O(5) layers of the crystal. This relaxation cannot occur in polyhedral (V(2)O(5))(n) clusters and also not for V(2)O(5) 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(OCH(3))(3) molecule and its dimer are also considered. Radical cations V(2)O(5)(+) and V(4)O(10)(+) 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 V(3)O(7)(+) cation (165 kJ mol(-1)) and the closed shell V(3)O(8)(-) 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 V(2)O(5) species on supporting oxides.

Ion chemistry of OV(OCH3)3 in the gas phase: molecular cations and anions and their primary fragmentations.

Trimethyl vanadate(V), OV(OCH(3))(3) (1), is examined by various mass spectrometric means. Photoionization experiments yield an ionization energy of IE(OV(OCH(3))(3)) = 9.54 +/- 0.05 eV for the neutral molecule. The primary fragmentation of the molecular cation 1(+), i.e., loss of neutral formaldehyde, can occur via two independent routes of hydrogen migrations to afford the formal V(IV) compounds HOV(OCH(3))(2)(+) and OV(OCH(3))(CH(3)OH)(+), respectively. These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies, direct V-O bond cleavage of 1(+) allows for expulsion of a methoxy radical concomitant with the generation of the cationic fragment OV(OCH(3))(2)(+), a formal V(V) compound. Trimethyl vanadate can also form a molecular anion, 1(-), whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V(V) compound OV(OCH(3))(2)O(-). Various mass spectrometric experiments and extensive theoretical studies provide detailed insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular cations and anions of 1.