Gerd Ganteför

Origin of unusual catalytic activities of Au-based catalysts

Abstract Experimental evidences for the non-dissociative chemisorption of O 2 are presented on even-numbered free Au anion clusters (Au n − , n =number of atoms) up to Au 20 − at room temperature. Our result indicates that the formation of the activated di-oxygen species is the key of the unusual catalytic activities of Au-based catalysts. No correlation between geometrical structures of Au n − and the activities towards O 2 adsorption was found, showing that site-specific chemistry disappears for Au-nanocatalysis. We demonstrate that interplay between cluster physics and surface chemistry is a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2

The reactivity pattern of small (∼10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH–, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

Hyperhalogens : Discovery of a new class of highly electronegative species

Electron affinity (EA), defined as the amount of energy necessary to remove an electron from an anion, plays a dominant role in reactivity. This is evidenced by halogen atoms, whose electron affinities are larger than those of any other element in the periodic table. However, there exists a class of molecules whose electron affinities are even larger. Bartlett and Lohmann were among the first to study such molecules nearly half a century ago. They showed that PtF6 can even oxidize O2 molecules [1] and Xe atoms. Several years later, Gutsev and Boldyrev coined the term superhalogen to describe these highly electronegative species. According to these authors, a superhalogen consists of a central metal atom surrounded by halogen atoms. When the number of these halogen atoms exceeds the maximal valence of the metal atom, the molecule possesses electron affinities that are much larger than that of the halogen atoms. In a series of subsequent theoretical studies, Boldyrev and coworkers showed that a large number of superhalogens, in which the central metal atom is typically an sp element, are possible. The first photoelectron spectrum of MX2 anions (M = Li, Na; X = Cl, Br, I) was reported by Wang and coworkers. Subsequent photoelectron spectroscopic and theoretical studies have further confirmed the existence of superhalogens in the gas phase. In a very recent theoretical study, it was proposed that the hydrogen atom can act as the central atom to form a superhalogen, and it was shown that the vertical detachment energies of [HnFn+1] anions can be extremely high. Numerous other superhalogen anions, such as permanganate (MnO4 ), perchlorate (ClO4 ), hexafluorides (AuF6 and PtF6 ), BO2 , and MgxCly [20] have also been reported. In a joint experimental and theoretical study, we have presented unusually stable Aun(BO2) clusters (n = 1–5) that exhibit superhalogen characteristics. Because of their high EAs, superhalogens almost always exist as negative ions, usually as the anionic portions of salts. Because salts composed of superhalogens have highly oxidative properties, there is considerable interest in the synthesis of species with high EAs. Herein we show that a new class of highly electronegative species can be synthesized if the peripheral halogen atoms are replaced by superhalogen moieties. We name this new class of electronegative species “hyperhalogens”, because their electron affinities can even be larger than those of their superhalogen building blocks and thus can serve as ingredients in the synthesis of new superoxidizing agents. Using density functional theory (DFT) and photoelectron spectroscopy (PES) experiments with a cluster beam, we demonstrate this possibility by concentrating on a gold atom and also a gold cluster decorated with BO2 superhalogens. The BO2 molecule, like MnO4, has a large electron affinity of 4.32 eV, 21] whilst its anionic counterpart, BO2 , being isoelectronic with CO2, is a very stable anion. It has been shown recently that the EA of an XFn cluster (X = Cu, Ag, Au; n = 1–6) increases as the central coinage metal atom is decorated successively with fluorine atoms. This happens as the extra electron is delocalized over several halogen atoms. We questioned whether the electron affinity would increase even further if the metal atom is decorated with superhalogen molecules instead. In this case, the extra electron will be delocalized over superhalogen moieties. What would happen if some but not all of the halogens atoms were replaced with superhalogen molecules; would the electron affinity lie in between the two? For example, would the electron affinity of Au(BO2)2 be much larger than that of AuO2? Similarly, would the electron affinity of AuO(BO2) be in between that of AuO2 and Au(BO2)2? From our DFT-based calculations (Table 1), we found that the electron affinity of Au(BO2)2 is 5.54 eV, which is 1.6 times larger than that of AuO2. [24] On the other hand, the electron affinity of AuO(BO2) is 4.21 eV, which lies between that of

Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters

Photoelectron spectra of Ag−n clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag−2–Ag−9 can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag−21.

Photoelectron spectroscopy of silver and palladium cluster anions : electron delocalization versus localization

Photoelectrons (PE) from jet-cooled mass-identified silver and palladium cluster anions (number of atoms, n⩽ 21) were detached by UV laser light and energy-analysed in a time-of-flight (TOF) electron spectrometer of the magnetic-bottle type. For palladium PE threshold energies smoothly increase with n; for Ag, they show clear evidence of shell effects as well as an ‘even–odd oscillation’. The PE energy spectra are strongly structured, the structures being attributed to transitions involving the neutral ground states as well as contributions of low-lying excited neutral states. For silver the results can, in part, be qualitatively understood in terms of a delocalized electron Fermi-gas within the ellipsoidal deformed cluster. This picture fails for the more localized d electrons of palladium. For a thorough interpretation more elaborate calculations are necessary. The first results are available for alkali-metal clusters and will be compared to the silver and copper data.

New experimental setup for photoelectron spectroscopy on cluster anions

We describe a new experimental setup for photoelectron spectroscopy on mass selected clusters. The recently developed pulsed arc cluster ion source (PACIS) for metal and semiconductor clusters is used as an anion source. The design of the PACIS is optimized for maximum intensity of cluster ion production and a minimum internal temperature of the particles. A simple modification allows vaporization of liquid and low melting point metals. The produced anions are mass selected via an inline time‐of‐flight setup with the option of using a reflectron for increased mass resolution. Photoelectron spectra of the mass selected cluster anions are collected in a ‘‘magnetic bottle’’ type electron spectrometer. First results on copper clusters are presented.

Interactions of Au cluster anions with oxygen

Experimental and theoretical evidence is presented for the nondissociative chemisorption of O2 on free Au cluster anions (Aun-, n=number of atoms) with n=2, 4, 6 at room temperature, indicating that the stabilization of the activated di-oxygen species is the key for the unusual catalytic activities of Au-based catalysts. In contrast to Aun- with n=2, 4, 6, O2 adsorbs atomically on Au monomer anions. For the Au monomer neutral, calculations based on density functional theory reveal that oxygen should be molecularly bound. On Au dimer and tetramer neutrals, oxygen is molecularly bound with the O-O bond being less activated with respect to their anionic counterparts, suggesting that the excess electron in the anionic state plays a crucial role for the O-O activation. We demonstrate that interplay between experiments on gas phase clusters and theoretical approach can be a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

VIBRATIONAL SPECTROSCOPY OF CLUSTERS USING A MAGNETIC BOTTLE ELECTRON SPECTROMETER

The design of a high resolution ‘‘magnetic‐bottle’’‐type time‐of‐flight electron spectrometer suitable for the study of mass‐separated metal and semiconductor cluster anions is described. A high collection efficiency is achieved by using magnetic fields to guide the photoelectrons, so that vibrationally resolved photoelectron spectra can be recorded at a low laser pulse energy (<10 μJ focused to 1 mm2) avoiding multiphoton processes. Spectra of clusters with a very low relative abundance, for example the products of chemical reactions involving clusters, can be recorded and an energy resolution of 6 meV (48 cm−1) achieved.

Photoelectron spectroscopy of Cu−n clusters: Comparison with jellium model predictions

We present a comparison of the electronic level structure of Cu−n clusters with the jellium model using photoelectron spectroscopy of metal cluster anions. The spectra are recorded at an energy resolution of 30 meV using photon energies of up to 6.4 eV. We obtain a well resolved picture of the electronic structure of the 4s derived electronic states in the energy region between the localized 3d derived states and the highest occupied molecular orbital. The observed features can be assigned to the 1s, 1p, and 1d shells predicted by the jellium model if ellipsoidal distortions and effects like shake‐up processes, multiplet splittings and the s–d hybridization are taken into consideration.

Appearance of bulk properties in small tungsten oxide clusters

Contrary to the conventional understanding that atomic clusters usually differ in properties and structure from the bulk constituents of which they are comprised, we show that even a dimer of tungsten oxide (WO(3))(2) possesses bulklike features and the geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, WO(3). This observation, based on a synergistic approach involving mass distributions under quasisteady state conditions, photoelectron spectroscopy, and first principles molecular orbital theory, not only illustrates the existence of a class of strongly covalent or ionic materials whose embryonic forms are tiny clusters but also lends the possibility that a fundamental understanding of complex processes such as catalytic reactions on surfaces may be achieved on an atomic scale with clusters as model systems.