It is shown that both the materials and the pressure gaps can be bridged for ruthenium in heterogeneous oxidation catalysis using the oxidation of carbon monoxide as a model reaction. Polycrystalline catalysts, such as supported Ru catalysts and micrometer-sized Ru powder, were compared to single-crystalline ultrathin RuO 2 films serving as model catalysts. The microscopic reaction steps on RuO 2 were identified by a combined experimental and theoretical approach applying density functional theory. Steady-state CO oxidation and transient kinetic experiments such as temperature-programmed desorption were performed with polycrystalline catalysts and single-crystal surfaces and analyzed on the basis of a microkinetic model. Infrared spectroscopy turned out to be a valuable tool allowing us to identify adsorption sites and adsorbed species under reaction conditions both for practical catalysts and for the model catalyst over a wide temperature and pressure range. The close interplay of the experimental and theoretical surface science approach with the kinetic and spectroscopic research on catalysts applied in plug-flow reactors provides a synergistic strategy for improving the performance of Ru-based catalysts. The most active and stable state was identified with an ultrathin RuO 2 shell coating a metallic Ru core. The microscopic processes causing the structural deactivation of Ru-based catalysts while oxidizing CO have been identified.

Competition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene iodide (CH2I2) in solution has been measured in real time. After excitation of the C−H− stretch overtone and C−H− stretch containing combination bands of CH2I2 between 1.7 and 2.4 μm an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initially excited degrees of freedom to vibrational states with larger Franck-Condon (FC) factors for the electronic transition (long wavelength wing) and energy loss due to energy transfer to the solvent. A model based upon the dependence of the electronic absorption on the internal energy 〈E〉 of CH2I2 has been used to determine the times for intramolecular vibrational energy redistribution and intermolecular energy transfer to the solvent. In the simplest version of our model the internal energy of the molecule probed by the population of the FC-active modes rises and decays exponentially on a picosecond (ps) time scale, which reflects the initial intramolecular vibrational energy redistribution and the subsequent energy transfer to the solvent. This simple approach was able to accurately describe the measured transient absorption for all solvents and excitation wavelengths. Overall time constants for IVR have been found to be on the order of 9−10 ps, almost independent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overall time constants for VET have been determined in the range between 60 and 120 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited.

The oxidation of CO over Ru/MgO and Ru/SiO2 catalysts was used as a simple model reaction to derive turnover frequencies at atmospheric pressure, which were observed to agree with kinetic data obtained under high-vacuum conditions with supported ruthenium catalysts and the RuO2(110) single-crystal surface. Thus, it was possible to bridge both the pressure and the materials gap. However, a partial deactivation was observed initially, which was identified as an activated process, both under net reducing and net oxidizing conditions. Temperature-programmed reduction (TPR) experiments were performed subsequently in the same reactor, to monitor the degree of oxidation, as a function of the reaction temperature and the CO/O2 reactant feed ratio. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, the structural changes of the ruthenium catalysts during the oxidation of CO were confirmed, under relevant reaction conditions. Under net reducing conditions, only domains of RuO2 seem to exist on the metallic ruthenium particles, whereas, under net oxidizing conditions, the ruthenium particles were fully oxidized to bulk RuO2 particles, which may expose less-active facets, such as the RuO2(100)−c(2 × 2) surface.

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Transient femtosecond IR-pump-UV-probe spectroscopy is employed to investigate the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of benzene, toluene (CH 3 -C 6 H 5 ), and R,R,R,-trifluorotoluene (CF 3 -C 6 H 5 ) selectively excited in overtones or combination bands of C-H stretch vibrations in solution. Global IVR and VET rate coefficients are derived from the measured transient absorption profiles using a simple kinetic model. The study reveals the effect of a methyl rotor and the effect of methyl rotor fluorination on the mechanisms and time scales of IVR and VET in aromatic model systems. For the present case, it turned out that the methyl rotor in toluene is not simply an enhancer for IVR; however, its fluorination accelerates IVR significantly. These results suggest that the methyl rotor effect on an aromatic ring in solution is more subtle than expected from previous gas-phase studies. In particular, the corresponding relaxation rates in this case are not simply governed by the number of lowest order resonances, such as found for aliphatic molecules. Instead, in aromatic molecules also, the very large number of higher order anharmonic resonances may play a pronounced role. Because the IVR rates are not at all correlated with the total density of states, we conclude that intramolecular vibrational energy relaxation of a zeroth order C-H stretch overtone or combination vibration in these molecules is not in its statistical limit and that hierarchical IVR, such as known for isolated molecules, still survives to some extent in solution.Our results further suggest that VET rates are not always simply correlated with the lowest frequency modes of the molecules.

The light-induced (266 nm) ultrafast decarboxylation of two peroxides R 1 -C(O)O-OR 2 , with R 1 ) phenyl and R 2 ) benzoyl or tert-butyl, in solution has been studied on the picosecond time scale by absorption spectroscopy with a time resolution typically of 100 to 200 fs. The reaction was investigated in various solvents of different polarity and viscosity to elucidate the influence of the solvent environment on the decarboxylation rate. Transient intermediate benzoyloxy radicals, R 1 -CO 2 , were monitored at wavelengths between 300 and 1000 nm. While the primary dissociation of the peroxide is too fast to be resolved, the dissociation of intermediate benzoyloxy radicals is clearly detected on the picosecond time scale. The mechanism of light-induced two-step dissociation is discussed, as is the dependence of reaction dynamics on the type of substituent R 2 as well as the branching ratio between prompt and delayed CO 2 formation. A model for the decarboxylation process is presented that is based on molecular structure parameters and energies. The latter quantities, which are obtained from density functional theory calculations, serve as input data for calculations of the specific decomposition rate coefficients of benzoyloxy intermediates via statistical unimolecular rate theory. The predicted benzoyloxy radical decay data are compared with corresponding experimental concentration versus time traces.

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