Chemisorption of hydrogen(2) and carbon monoxide on positive transition metal cluster ions in an FT-ICR apparatus
Weiss, Falk Dietrich
Smalley, Richard E.
Doctor of Philosophy
A new FT-ICR apparatus (Fourier Transform - Ion Cyclotron Resonance) has been developed, where clusters produced in a laser vaporization supersonic cluster source were efficiently injected and trapped in an elongated cylindrical Penning type ion trap located in the center of a magnetic field of 6 Tesla. The successful injection of multiple cluster pulses into the ion trap makes the experiment largely independent of the intensity of the supersonic cluster source. Long trapping times of up to 30 minutes and a high mass resolution are additional features of this apparatus. In a first application, the apparatus was used to systematically investigate the chemisorption behavior of positively-charged group V and group IX transition metal cluster ions towards H$\sb2$. The absolute rate constant of the chemisorption process for the addition of the first hydrogen molecule as well as the saturation values for the complete hydrogenation of the clusters were measured. In the cases of Vanadium, Niobium and Cobalt, the reactivity of the positive cluster ions were then compared to the reactivity of the neutral cluster species. The patterns in reactivity for the positive as well as for the neutral clusters showed striking similarities. This led to the conclusion that the reactivity of these clusters is by and large independent of their charge state. Within the niobium and tantalum cluster series isomeric forms of a particular cluster species were found exhibiting a large difference in reactivity towards hydrogen. In the third series of cluster experiments performed on the FT-ICR apparatus, the chemisorption of CO on positively-charged group IX transition metal cluster ions was investigated. In the case of rhodium, the measured saturation values for these gas phase cluster carbonyls scaled parallel to the saturation values found for the rhodium carbonyl clusters in the liquid phase indicating similar structures for both gas phase as well as liquid phase clusters. In the presence of oxygen which is dissociatively chemisorbed on the clusters' surface, formation and desorption of CO$\sb2$ were found during the CO chemisorption process on all three group IX metals.
Physical chemistry; Molecular physics