Low-Temperature Selective Oxidative Dehydrogenation of Cyclohexene by Titania-Supported Nanostructured Pd, Pt, and Pt-Pd Catalytic Films.

Films of titania-supported monometallic Pd, Pt, and bimetallic Pt-Pd catalysts made of metallic nanoparticles were prepared by magnetron sputtering and studied in the oxidative dehydrogenation (ODH) of cyclohexene. Pd/TiOx and Pt-Pd/TiOx were found active at as low temperature as 150 °C and showed high catalytic activity with high conversion (up to 81%) and benzene selectivity exceeding 97% above 200 °C. In turn, the Pt/TiOx catalyst performed poorly with the onset of benzene production at 200 °C only and conversions not exceeding 5%. The activity of bimetallic Pt-Pd catalysts far exceeded all of the other investigated catalysts at temperatures below 250 °C. However, the production of benzene significantly dropped with a further temperature increase due to the enhanced combustion of CO2 at the expense of benzene formation. As in situ NAP-XPS measurement of the Pt-Pd/TiOx catalyst in the reaction conditions of the ODH of cyclohexene revealed Pd surface enrichment during the first temperature ramp, we assume that Pd surface enrichment is responsible for enhanced activity at low temperatures in the bimetallic catalyst. At the same time, the Pt constituent contributes to stronger cyclohexene adsorption and oxygen activation at elevated temperatures, leading to changes in conversion and selectivity with a drop in benzene formation and increased combustion to CO2. Both the monometallic Pd and the Pt-Pd-based catalysts produced a small amount of the second valuable product, cyclohexadiene, and below 250 °C produced only a negligible amount of CO2 (<0.2%). To summarize, Pd- and Pt-Pd-based catalysts were found to be promising candidates for highly selective low-temperature dehydrogenation of cyclic hydrocarbons that showcased reproducibility and stability after the temperature activation. Importantly, these catalysts were fabricated by utilizing proven methods suitable for large-scale production on extended surfaces.

Binding energies of O2 and CO to small gold, silver, and binary silver-gold cluster anions from temperature dependent reaction kinetics measurements.

A detailed analysis of experimentally obtained temperature-dependent gas-phase kinetic data for the oxygen and carbon monoxide adsorption on small anionic gold (Au(n)(-), n = 1-3), silver (Ag(n)(-), n = 1-5), and binary silver-gold (Ag(n)Au(m)(-), n + m = 2, 3) clusters is presented. The Lindemann energy transfer model in conjunction with statistical unimolecular reaction rate theory is employed to determine the bond dissociation energies E(0) of the observed metal cluster complexes with O(2) and CO. The accuracy limits of the obtained binding energies are evaluated by applying different transition-state models. The assumptions involved in the data evaluation procedure as well as possible sources of error are discussed. The thus-derived binding energies of O(2) to pure silver and binary silver-gold cluster anions are generally in excellent agreement with previously reported theoretical values. In marked contrast, the binding energies of O(2) and CO to Au(2)(-) and Au(3)(-) determined via temperature-dependent reaction kinetics are consistently lower than the theoretically predicted values.

Ultrafast nuclear dynamics induced by photodetachment of Ag2- and Ag2O2-: oxygen desorption from a molecular silver surface.

Femtosecond nuclear dynamics of mass-selected neutral Ag2 and Ag2O2 clusters are investigated with the 'negative ion-to neutral-to positive ion'(NeNePo) technique. For the bare silver dimer, wave packet dynamics occurring in the neutral electronic ground state and in the first excited triplet state are observed after photodetachment from the anion with 3.05 eV photon energy. While the dynamics in the ground state lead to an oscillatory structure in the NeNePo-pump-probe spectra with a vibrational constant of 185 cm-1, the dynamics in the triplet state are assigned to a bound-free transition leading to dissociation. Photodetachment from the Ag2O2- complex results in the desorption of O2. The experimental data clearly show the influence of the desorbing oxygen ligand on the nuclear dynamics of the silver dimer inducing a red shift in the vibrational frequency and an intensity enhancement of the oscillatory signal.

Femtosecond time-resolved geometry relaxation and ultrafast intramolecular energy redistribution in Ag2Au.

The ultrafast dynamics of the bimetallic cluster Ag2Au is investigated by pump-probe negative ion-to-neutral-to-positive ion (NeNePo) spectroscopy. Preparation of the neutral cluster in a highly nonequilibrium state by electron detachment from the mass-selected anion, and subsequent probing of the neutral nuclear dynamics through two-photon ionization to the cationic state, leads to strongly probe-energy-dependent transient cation-abundance signals. The origin of this pronounced time and wavelength dependence of the ionization probability on the femtosecond scale is revealed by ab initio theoretical simulations of the transient spectra. Based on the analysis of underlying dynamics, two fundamental processes involving geometry relaxation from linear to triangular structure followed by ultrafast intramolecular vibrational energy redistribution (IVR) have been identified and for the first time experimentally observed in the frame of NeNePo spectroscopy under conditions close to zero electron kinetic energy.

Issues in implementing TANF in New York: the perspective of frontline workers.

The study discussed in this article examined the perspectives of front-line welfare workers on issues related to the implementation of TANF: domestic violence, work requirements, time limits for cash benefits, and functions of workers. Based on focus groups held in upstate New York, findings suggest a lack of criteria for the granting of exemptions from TANF requirements, worker resistance to serving those caught in the cycle of violence, and congruence between the legislation's "work first" strategy and worker preference. However, participants identified limitations to employment-focused welfare programs, including restrictions on education and job preparation. Further research is needed on the implementation of TANF, including use of the Family Violence Option, and on the use of administrative discretion by front-line workers.

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses.

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.

Strongly cluster size dependent reaction behavior of CO with O2 on free silver cluster anions.

Reactions of free silver anions Agn- (n = 1 - 13) with O2, CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Agn- (n = 1 - 11) readily react with molecular oxygen to yield AgnOm- (m = 2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O2 and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Agn(CO)O2- are detected for cluster sizes with n = 4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag7-, Ag9-, and Ag11-, the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle.

Catalytic CO oxidation by free Au2-: experiment and theory.

Temperature-dependent rf-ion trap mass spectrometry and first-principles simulations reveal the detailed reaction mechanism of the catalytic gas-phase oxidation of CO by free Au(2)(-) ions in the presence of O(2). A metastable intermediate with a mass of Au(2)CO(3)(-) was observed at low temperatures. Two alternative structures corresponding to digold carbonate or peroxyformate are predicted for this intermediate. Both structures are characterized by low activation barriers for the formation of CO(2). These combined experimental and theoretical investigations provide a comprehensive understanding of the kinetics, energetics, and atomic arrangements along the reaction path, thus allowing a formulation of the catalytic cycle for the oxidation reaction.

Cooperative effects in the activation of molecular oxygen by anionic silver clusters.

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.