Activation of CH4_4, NH3_3, and N2_2 by Tantalum Ions, Clusters and Their Oxides: What Can Be Learnt from Studies of Ions in the Gas Phase

The emission control of harmful compounds and greenhouse gases and the development of alternative, sustainable fuel sources is a major focus in current research. A solution for this problem lies in the development of efficient catalytic materials. Here, gas phase model systems represent prominent examples for obtaining fundamental insights on reaction properties of prospective catalytic systems. In this work, we review results from studies of tantalum clusters and their oxides in the gas phase and discuss insights with a potential relevance for applied systems. We focus on reactions that are essential for sustainable chemistry in the future. In detail, we address the activation of methane, which may enable the transformation of a greenhouse gas to a chemical feedstock, and we discuss the activation of NH$_3$, which may function as an alternative energy carrier whose unwanted emission needs to be curbed in future applications. Finally, we consider the activation of N$_2$ as a third reaction, since reducing the high energy demand of ammonia synthesis still bears significant challenges. While tantalum may be an interesting catalytic material, the discussed studies may also serve as benchmark for investigations of other materials

Communication: In search of four-atom chiral metal clusters

A combined study utilizing anion photoelectron spectroscopy and density functional theory was conducted to search for four-atom, chiral, metal, and mostly metal clusters. The clusters considered were AuCoMnBi−/0, AlAuMnO−/0, AgMnOAl−/0, and AuAlPtAg−/0, where the superscripts, −/0, refer to anionic and neutral cluster species, respectively. Based on the agreement of experimentally and theoretically determined values of both electron affinities and vertical detachment energies, the calculated cluster geometries were validated and examined for chirality. Among both anionic and neutral clusters, five structures were identified as beingchiral

Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study

Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their crystalline state are among the most promising materials for organic opto-electronics. Following previous works on oligoacenes we present a systematic comparative study of the electronic, optical, and transport properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using density functional theory (DFT) and time-dependent DFT we computed: (i) electron affinities and first ionization energies; (ii) quasiparticle correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv) electronic absorption spectra of neutral and $\pm1$ charged systems. The excitonic effects are estimated by comparing the optical gap and the quasiparticle corrected HOMO-LUMO energy gap. For each molecular property computed, general trends as a function of molecular size and charge state are discussed. Overall, we find that circumacenes have the best transport properties, displaying a steeper decrease of the molecular reorganization energy at increasing sizes, while oligorylenes are much more efficient in absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical Physics (14/04/2011

Tuning SMSI Kinetics on Pt-loaded TiO2_2(110) by Choosing the Pressure: A Combined UHV / Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined Pt/TiO$_2$(110) catalysts at pressures of up to 0.1 Torr. By tuning the O$_2$ pressure, we can either selectively oxidize the TiO$_2$ support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10$^{-5}$ Torr O$_2$, but orders of magnitudes less effective at higher O$_2$ pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred

From C-H Bond Insertion to Hydrogen Atom Transfer: Tuning the Reaction Mechanisms of Methane Activation by the Oxidation of Ta₂⁺

The activation of methane under mild conditions is a challenging but rewarding goal; the underlying key parameters, however, remain elusive. In this study on isolated tantalum Ta2+ compounds exposed to methane in a ring-electrode ion trap, strong changes in the reactivity are observed depending on the compound\u27s degree of oxidation. While the general reaction behavior is presented for species ranging from Ta2+ to Ta2O6+ based on experimental kinetic studies, we focus in more detail on the dehydrogenation reactions occurring on Ta2O2+ and the hydrogen atom transfer (HAT) on Ta2O5+, for which density functional theory calculations were performed. In the first part, we elucidate the role of Ta–C–Ta bridging motifs in product structures as driving forces for the dehydrogenation of methane on Ta2O2+; in the second part, we investigate the origins of the HAT – a hitherto unknown reaction scheme for binary tantalum oxides. For the latter, we show that the reactivity originates from the spin density on oxygen atoms, which is a typical characteristic of the reaction on other metal oxides. This reflects a change in the reactivity from oxidized metallic systems to metal oxides and demonstrates that chemical modifications of tantalum compounds can achieve different methane activation schemes

IR-photodissociation and photodetachment spectroscopy of Cl− · (NH3)x (IR:

Complexes of ammonia mols. and one chloride ion have been studied by photodetachment and IR-photodissocn. spectroscopy. For the smallest anionic complex, the stabilization energy with respect to the bare chloride ion and vibrational frequencies have been detd. Two bands showed a splitting due to rotational branches, which could be represented by simulation. Rotational consts. obtained by former ab initio calcns. are confirmed and rotational consts. of a vibrationally excited state are supplied. IR-photodissocn. spectra of clusters with up to four ammonia mols. per chloride ion were recorded. [on SciFinder(R)

Photodetachment photoelectron spectroscopy of the weakly bound OClO-center dot H2O complex

The weakly bound complex OClO-·H2O and its neutral counterpart are of interest for atm. chem. as well as for studies of microsolvation. Electron affinity and binding energies can be deduced from photodetachment photoelectron spectra of the bare mol. and its H2O complex. Such spectra are presented here. An unusual large binding energy of the anionic cluster of 0.75 eV was found. [on SciFinder(R)

The electron affinity of phenanthrene

Phenanthrene is studied by photodetachment-photoelectron spectroscopy. Due to the absence of a parent ion peak in the anion mass spectrum the electron affinity could not be detd. directly. However, this absence is the first indication that this mol. has a neg. electron affinity. The first three water complexes of phenanthrene were studied, supplying insights into its microsolvation property. Moreover, the electron affinity of the bare mol. could be detd. to be -0.01±0.04 eV by an extrapolation method using the water cluster data. The exptl. work is supported by ab initio calcns. for detg. the structure of the water complexes. Finally a correlation between the electron affinity and the redn. potential of polycyclic arom. hydrocarbons is investigated. [on SciFinder(R)

Anion photoelectron, photodetachment, and infrared dissociation spectra of Cl-.bul.C6H6

Anion photoelectron spectra, photodetachment spectra, and IR-dissocn. spectra of the weakly bound chloride-benzene complex were measured. An upper limit of its stabilization energy of 0.35 eV was found, in disagreement with a former exptl. value of 0.41 eV and in agreement with a theor. value of 0.34 eV [C.D. Thompson, B.L.J. Poad, C. Emmeluth, E. Bieske, Chem. Phys. Lett. 428(2006) 18]. The adiabatic detachment energy leads to a stabilization energy of 0.26 ± 0.02 eV. The absence of the linear isomeric structure in the supersonic beam and vibrational assignments of former IR spectra [C.D. Thompson, B.L.J. Poad, C. Emmeluth, E. Bieske, Chem. Phys. Lett. 428(2006) 18] could be confirmed. New spectroscopic features supply information about the intermol. stretching mode between chloride and benzene with a frequency of 130 ± 10 cm-1. [on SciFinder(R)

Laserspectroscopy on mass selected negatively charged molecules and complexes

Negativ geladene Moleküle und Komplexe wurden mit einer Kombination aus drei verschiedenen spektroskopischen Verfahren untersucht: der Photodetachmentspektroskopie, der Photodetachmentphotoelektronenspektroskopie und der IR-Dissoziationsspektroskopie. Diese erstmals realisierte Kombination lieferte umfassende Einblicke über die untersuchten Spezies. So konnten Stabilisierung¬senergien der anionischen und neutralen Molekülsysteme, Elektronenaffinitäten, Schwingungsfrequenzen, teilweise sogar Rotations¬konstanten und elektronische Zustände untersucht werden. Da alle Methoden mit derselben Apparatur durchgeführt wurden, liefern die erhaltenen Spektren in optimaler Weise sich ergänzende Informationen. Gegenstand der Untersuchungen waren Chrysen und Wassercluster von Phenanthren, der OClO-Wasser-Komplex, der Chlor-Benzol-Komplex, sowie die Chlor-Ammoniakcluster. Letztere konnten bis zu einer Größe von vier Ammoniakmolekülen pro Chloridion untersucht werden.Negatively charged molecules and complexes were studied by using a combination of three different spectroscopic methods: photodetachment spectroscopy, photodetachment-photoelectron spectroscopy and IR-dissociation spectroscopy. This combination that could be realised for the first time supplied extensive insights into the investigated species. Hence it was possible to determine stabilisation energies of the anionic and neutral molecular systems, electron affinities, vibrational frequencies, electronic states and to some extend rotational constants. Since each spectroscopic method was performed by using the same apparatus, complementary information was obtained. Chrysene, and water cluster of phenanthrene, the OClO-water complex, the chlorine-benzene complexe as well as chloride-ammonia cluster were studied. The latter were investigated up to a cluster size of four ammonia molecules per chloride ion