Liana D. Socaciu

Coadsorption of CO and O2 on small free gold cluster anions at cryogenic temperatures: Model complexes for catalytic CO oxidation

The cluster ions Aun(CO)m(O2)u− (n = 2,3; m = 1, u = 1,2) are successfully synthesized via coadsorption of oxygen and carbon monoxide onto mass-selected gold clusters inside a temperature controlled ion trap at 100 K. The investigated gold clusters Au1–3− show a significant size dependence in the reactivity toward oxygen and carbon monoxide. Whereas no reaction products are detected for the monomer ion, simultaneous adsorption of CO and O2 onto the dimer leads to the formation of Au2(CO)(O2)− which is regarded as a model complex to study the mechanism of catalytic CO oxidation on gold clusters. In the case of Au3− the conditioning of the cluster by CO preadsorption is found essential to enable O2 coadsorption.

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.

Reaction mechanism for the oxidation of free silver dimers

Abstract The kinetics of the interaction of Ag 2 + with O 2 is studied in the gas phase under multi-collision conditions at various temperatures. A new experimental scheme is employed, which consists of an octopole ion trap of variable temperature inserted into a tandem quadrupole mass spectrometer. From the time evolution of the reactant and product molecule concentrations at different temperatures the corresponding reaction mechanism is extracted. Surprisingly, the product Ag 2 O + is detected, which is formed after molecular adsorption and dissociation of O 2 . We can clearly identify Ag 2 O 2 + as an intermediate in this reaction. In addition, absolute rate coefficients and activation energies for the molecular adsorption of O 2 onto Ag 2 + are presented.

Time-resolved explosion dynamics of H 2 O 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.

Catalysis with small free noble metal clusters

A review; in general, reaction mechanisms in heterogeneous catalysis are difficult to reveal, because of the complexity of the reaction system. Our approach employs mass-selected metal clusters as well-defined model reaction centers. Chem. reactions on these clusters are investigated in a temp. controlled radio frequency (rf)-ion trap setup. Product ion concns. as a function of storage time enable the detn. of reaction kinetics and reaction mechanisms of free clusters. As an example, the CO combustion reaction on small gold clusters is studied. It is known that supported gold clusters with few atoms up to nm size exhibit relevant catalytic properties. Our ion trap measurements reveal catalytic activity already for neg. charged gold dimers. When the reaction kinetics are investigated as a function of temp., an intermediate product with CO and O2 coadsorbed can be isolated. In contrast, small at. silver clusters have not been found relevant for catalytic oxidn. processes so far. Our investigations show for the first time evidence for a strongly size dependent catalytic activity of Agn-. [on SciFinder(R)]