Christian Kerpal

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Activated methane on small cationic platinum clusters.

The catalytic activation of C H bonds in small hydrocarbons, particularly methane, is a reaction that is of significant technological interest, as it allows valuable, functionalized products to be made from plentiful, cheap feedstocks. However, even on well-characterized platinum surfaces, the determination of the details of methane activation, in particular the earliest steps, remains difficult. Challenges include the weak physisorption of molecular methane on platinum surfaces, its ready dissociation, and the difficulty associated with determining hydrogen atom positions in many surface experiments, as hydrogen atoms are weak scatterers of X-rays or electrons and have no electronic core levels. Despite these challenges, Yoshinobu et al. have used infrared reflection absorption spectroscopy to show that CH4 adsorbed on Pt(111) has at most C3v symmetry. [4] strçm et al. have determined the adsorption geometry of methane on Pt(977) using X-ray absorption spectroscopy, reporting methane to bind by a single hydrogen atom, though they were unable to determine whether it was bound atop or in hollow sites. Partially dehydrogenated reaction intermediates/products, including methyl, methylene, and methylidyne, have been extensively studied (see for example Ref. [6]). The reactions of methane with platinum atoms and clusters have been studied in some detail. In the case of small ionic clusters reacting with CH4 under single collision conditions, Ptn[C,2H] + complexes were found to be the favored products. There have been a number of computational studies of the interactions of platinum clusters and surfaces 14, 15] with methane, primarily using density functional theory (DFT). Such calculations are challenging, owing to the large system size, number of electrons and possible paths, and the fact that several electronic states and crossings between them may need to be treated. Experimental spectroscopic characterization of these species, particularly the reaction intermediates, can therefore provide important information about their structures and benchmark data for theory. Recently, we have demonstrated the possibility of forming reactive intermediate species under thermalized conditions in a flow reactor. This approach was unsuccessful for the Ptn +

Structures of Platinum Oxide Clusters in the Gas Phase

The structures of small gas-phase Pt(n)O(2m)(+) (n = 1-6, m = 1, 2) cluster cations have been investigated in a combined infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) study. On the basis of the infrared spectra obtained, it is concluded that in most clusters oxygen is bound dissociatively, preferring 2-fold bridge binding motifs, sometimes combined with singly coordinated terminal binding. Comparison of the oxide cluster structures with those of bare cationic platinum clusters reported previously reveals major structural changes induced in the platinum core upon oxygen binding. For some cluster sizes the presence of the Ar messenger atom(s) is found to induce a significant change in the observed cluster structure.

Unusual Bonding in Platinum Carbido Clusters.

Vibrational spectroscopy and density functional theory calculations are used to determine the structures of small gas-phase platinum carbido clusters PtnC(+), n = 3-5. The carbon atom is found to prefer three-coordinate binding sites near the center of the cluster, in contrast to most previously investigated adatoms on transition metal clusters. The Pt3C unit is particularly stable, and binding of the carbon atom also leads to significant rearrangement of the metal framework when compared to the bare clusters.

Communication: The structures of small cationic gas-phase platinum clusters

The structures of small platinum clusters Pt(3-5)(+) are determined using far-infrared multiple photon dissociation spectroscopy of their argon complexes combined with density functional theory calculations. The clusters are found to have compact structures, and Pt(4)(+) and Pt(5)(+) already favor three-dimensional geometries, in contrast to a number of earlier predictions. Challenges in applying density functional theory to 3rd row transition metal clusters are addressed. Preliminary calculations suggest that the effects of spin-orbit coupling do not change the favoured lowest-energy isomers.

Small platinum cluster hydrides in the gas phase.

The reactions of small cationic platinum clusters (Pt2(+)-Pt7(+)) with molecular hydrogen were investigated, and the structures of the hydride complexes were analyzed using IR spectroscopy. We determined the relative reaction rates for the addition of the first H2 molecule to the platinum clusters, and we report the hydrogen saturation coverages observed at high H2 concentration. High H atom per Pt atom ratios were observed, similar to earlier measurements on other group-10 transition metals. The structures of the fully saturated complexes of Pt2(+)-Pt5(+) were investigated using a combination of infrared multiple-photon dissociation (IR-MPD) spectroscopy in the frequency range of 550-2050 cm(-1) and density functional theory-based calculations. We found molecularly bound hydrogen alongside bridge and often atop binding of hydrogen atoms for all of the low-energy structures, in contrast to earlier theoretical predictions.