RESUMEN
The selective dehydrogenation of hydrocarbons and their functionalized derivatives is a promising pathway in the realization of endothermic fuel systems for powering important technologies such as hypersonic aircraft. The recent surge in interest in single atom catalysts (SACs) over the past decade offers the opportunity to achieve the ultimate levels of selectivity through the subnanoscale design tailoring of novel catalysts. Experimental techniques capable of investigating the fundamental nature of the active sites of novel SACs in well-controlled model studies offer the chance to reveal promising insights. We report here an approach to accomplish this through the soft landing of mass-selected, ultrasmall metal oxide cluster ions, in which a single noble metal atom bound to a metal oxide moiety serves as a model SAC active site. This method allows the preparation of model catalysts in which monodispersed neutral SAC model active sites are decorated across an inert electrically conductive support at submonolayer surface coverage, in this case, Pt1Zr2O7 clusters supported on highly oriented pyrolytic graphite (HOPG). The results contained herein show the characterization of the Pt1Zr2O7/HOPG model catalyst by X-ray photoelectron spectroscopy (XPS), along with an investigation of its reactivity toward the functionalized hydrocarbon molecule, 1-propanamine. Through temperature-programmed desorption/reaction (TPD/R) experiments it was shown that Pt1Zr2O7/HOPG decomposes 1-propanamine exclusively into propionitrile and H2, which desorb at 425 and 550 K, respectively. Conversely, clusters without the single platinum atom, that is, Zr2O7/HOPG, exhibited no reactivity toward 1-propanamine. Hence, the single platinum atom in Pt1Zr2O7/HOPG was found to play a critical role in the observed reactivity.
RESUMEN
We present a newly developed magnetic-bottle time-of-flight electron spectrometer suitable for continuous or quasicontinuous photoionization sources such as synchrotrons. A strong magnetic field collects almost all photoelectrons from a well-defined ionization volume and quantitatively suppresses background electrons which originate outside of this interaction region. Although it is a pulsed instrument, a relatively high duty cycle is achieved by storing the photoelectrons generated between two cycles in an electromagnetic trap. This makes the new instrument suitable for experiments with very low sample densities. Another advantage is the high energy resolution, 50 meV in the first version of the spectrometer described here, which simply depends on the length of the time-of-flight instrument.
RESUMEN
A systematic analysis of the average linewidth of features in the photoelectron spectra of size-selected elemental clusters consisting of up to 10 atoms is presented. With increasing atomic weight, the average linewidth decreases. Several possible reasons for this trend are discussed. Obvious effects such as experimental resolution, vibrational temperature, and lifetime broadening can be excluded. The only remaining explanation is a mass-dependence of the Franck-Condon envelope. Each photoelectron peak corresponds to an electronic transition, which exhibits a Frank-Condon envelope. Its full width of half maximum depends on the spatial expansion of the nuclear wave functions in the initial state. With increasing atomic mass, the nuclear wave functions narrow down.
RESUMEN
Bare metal clusters have properties that make them interesting for applications in photochemistry and photovoltaics. Long-lived excited states are a prerequisite for such applications, because in them the energy of the photon can be stored. Clusters have a low density of states and long-lived excited states should therefore occur frequently. However, in fact, such states are a rarity, as indicated by time-resolved photoelectron data of mass-selected cluster anions. And there is another puzzling observation: only clusters with narrow peaks in their photoelectron spectra exhibit long-lived excited states. Both findings can be explained if internal conversion, i.e. the conversion of electronic excitation energy into vibrational excitations, is the major relaxation mechanism in clusters. It becomes more likely, if a change of the electronic configuration results in a large geometry change, which is probably the case for most clusters. Only clusters with a weak coupling between geometric and electronic structure may have long-lived excited states and narrow peaks.
RESUMEN
Boron aluminum hydride clusters are studied through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations. Boron aluminum hydride cluster anions, BxAlyHz(-), were generated in a pulsed arc cluster ionization source and identified by time-of-flight mass spectrometry. After mass selection, their photoelectron spectra were measured by a magnetic bottle-type electron energy analyzer. The resultant photoelectron spectra as well as calculations on a selected series of stoichiometries reveal significant geometrical changes upon substitution of aluminum atoms by boron atoms.
RESUMEN
Research on homogeneous and heterogeneous catalysis is indeed convergent and finds subnanometric particles to be at the heart of catalytically active species. Here, monodisperse gold clusters are deposited from the gas phase onto porous titania generating well-defined model systems and the resulting composite materials exhibit a sharp size-dependency on the number of gold atoms per cluster and exceptionally high-turnovers toward the bromination of 1,4-dimethoxybenzene are observed. This indicates that the deliberate generation of active centres is of utmost importance for the creation of the most "gold-efficient" catalysts.
RESUMEN
Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have investigated the extent to which the aluminum moieties within selected magnesium-aluminum clusters are Zintl anions. Magnesium-aluminum cluster anions were generated in a pulsed arc discharge source. After mass selection, photoelectron spectra of MgmAln (-) (m, n = 1,6; 2,5; 2,12; and 3,11) were measured by a magnetic bottle, electron energy analyzer. Calculations on these four stoichiometries provided geometric structures and full charge analyses for the cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra). Calculations revealed that, unlike the cases of recently reported sodium-aluminum clusters, the formation of aluminum Zintl anion moieties within magnesium-aluminum clusters was limited in most cases by weak charge transfer between the magnesium atoms and their aluminum cluster moieties. Only in cases of high magnesium content, e.g., in Mg3Al11 and Mg2Al12 (-), did the aluminum moieties exhibit Zintl anion-like characteristics.
RESUMEN
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 being chiral.
RESUMEN
The quantum states in small, compact metal clusters are bunched into electronic shells with electronic orbitals resembling those in atoms, enabling classification of stable clusters as superatoms. The filling of superatomic orbitals, however, does not generally follow Hund's rule, and it has been proposed that magnetic superatoms can be stabilized by doping simple metal clusters with magnetic atoms. Here, we present evidence of the existence of a magnetic superatom and the determination of its spin moment. Our approach combines first principles studies with negative ion photoelectron experiments and enables a unique identification of the ground state and spin multiplicity. The studies indicate VNa8 to be a magnetic superatom with a filled d-subshell and a magnetic moment of 5.0 µB. Its low electron affinity is consistent with filled subshell and enhanced stability. The synthesis of this species opens the pathway to investigate the spin-dependent electronics of the new magnetic motifs.
RESUMEN
Theoretical calculations based on density functional theory have found (PbS)(32) to be the smallest cubic cluster for which its inner (PbS)(4) core enjoys bulk-like coordination. Cubic (PbS)(32) is thus a "baby crystal," i.e., the smallest cluster, exhibiting sixfold coordination, that can be replicated to obtain the bulk crystal. The calculated dimensions of the (PbS)(32) cluster further provide a rubric for understanding the pattern of aggregation when (PbS)(32) clusters are deposited on a suitable surface, i.e., the formation of square and rectangular, crystalline nano-blocks with predictable dimensions. Experiments in which mass-selected (PbS)(32) clusters were soft-landed onto a highly ordered pyrolytic graphite surface and the resulting aggregates imaged by scanning tunneling microscopy provide evidence in direct support of the computational results.
RESUMEN
A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, Al(n)H(m) (-), exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in Al(n)H(m) (-) species was m=3n+1 for n=5-8, i.e., Al(n)H(3n+1) (-), and m=3n+2 for n=4, i.e., Al(4)H(14) (-), as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for Al(n)H(m) (-) species with m=3n+1 and m=3n+2 hydrogen atoms were significantly higher than those of nearby Al(n)H(m) (-) mass peaks for which m<3n+1, i.e., the ion intensities for Al(n)H(3n+1) (-) and for Al(4)H(14) (-) deviated from the roughly bell-shaped ion intensity patterns seen for most Al(n)H(m) (-) species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that Al(n)H(3n+1) (-) clusters have chain and/or double-ring polymeric structures.
RESUMEN
Reactivity of aluminum cluster anions toward ammonia was studied via mass spectrometry. Highly selective etching of Al(11)(-) and Al(12)(-) was observed at low concentrations of ammonia. However, at sufficiently high concentrations of ammonia, all other sizes of aluminum cluster anions, except for Al(13)(-), were also observed to deplete. The disappearance of Al(11)(-) and Al(12)(-) was accompanied by concurrent production of Al(11)NH(3)(-) and Al(12)NH(3)(-) species, respectively. Theoretical simulations of the photoelectron spectrum of Al(11)NH(3)(-) showed conclusively that its ammonia moiety is chemisorbed without dissociation, although in the case of Al(12)NH(3)(-), dissociation of the ammonia moiety could not be excluded. Moreover, since differences in calculated Al(n)(-) + NH(3) (n=9-12) reaction energies were not able to explain the observed selective etching of Al(11)(-) and Al(12)(-), we concluded that thermodynamics plays only a minor role in determining the observed reactivity pattern, and that kinetics is the more influential factor. In particular, the conversion from the physisorbed Al(n)(-)(NH(3)) to chemisorbed Al(n)NH(3)(-) species is proposed as the likely rate-limiting step.
