Selective Reduction of NO into N2 Catalyzed by Rh1-Doped Cluster Anions RhCe2O3-5. J Am Chem Soc 2023;145:18658-18667. [PMID: 37572057 DOI: 10.1021/jacs.3c06565]

Catalytic conversion of toxic nitrogen oxide (NO) and carbon monoxide (CO) into nitrogen (N2) and carbon dioxide (CO2) is imperative under the weight of the increasingly stringent emission regulations, while a fundamental understanding of the nature of the active site to selectively drive N2 generation is elusive. Herein, in combination with state-of-the-art mass-spectrometric experiments and quantum-chemical calculations, we demonstrated that the rhodium-cerium oxide clusters RhCe2O3-5- can catalytically drive NO reduction by CO and give rise to N2 and CO2. This finding represents a sharp improvement in cluster science where N2O is commonly produced in the rarely established examples of catalytic NO reduction mediated with gas-phase clusters. We demonstrated the importance of the unique chemical environment in the RhCe2O3- cluster to guide the substantially improved N2 selectivity: a triatomic Lewis "acid-base-acid" Ceδ+-Rhδ--Ceδ+ site is proposed to strongly adsorb two NO molecules as well as the N2O intermediate that is attached on the Rh atom and can facilely dissociate to form N2 assisted by both Ce atoms.

Thermal Methane Conversion to Syngas Mediated by Rh1-Doped Aluminum Oxide Cluster Cations RhAl3O4<sup/>

Laser ablation generated RhAl₃O₄⁺ heteronuclear metal oxide cluster cations have been mass-selected using a quadrupole mass filter and reacted with CH₄ or CD₄ in a linear ion trap reactor under thermal collision conditions. The reactions have been characterized by state-of-the-art mass spectrometry and quantum chemistry calculations. The RhAl₃O₄⁺ cluster can activate four C-H bonds of a methane molecule and convert methane to syngas, an important intermediate product in methane conversion to value-added chemicals. The Rh atom is the active site for activation of the C-H bonds of methane. The high electron-withdrawing capability of Rh atom is the driving force to promote the conversion of methane to syngas. The polarity of Rh oxidation state is changed from positive to negative after the reaction. This study has provided the first example of methane conversion to syngas by heteronuclear metal oxide clusters under thermal collision conditions. Furthermore, the molecular level origin has been revealed for the condensed-phase experimental observation that trace amounts of Rh can promote the participation of lattice oxygen of chemically very inert support (Al₂O₃) to oxidize methane to carbon monoxide.

A nine-atom rhodium-aluminum oxide cluster oxidizes five carbon monoxide molecules

Noble metals can promote the direct participation of lattice oxygen of very stable oxide materials such as aluminum oxide, to oxidize reactant molecules, while the fundamental mechanism of noble metal catalysis is elusive. Here we report that a single atom of rhodium, a powerful noble metal catalyst, can promote the transfer of five oxygen atoms to oxidize carbon monoxide from a nine-atom rhodium-aluminum oxide cluster. This is a sharp improvement in the field of cluster science where the transfer of at most two oxygen atoms from a doped cluster is more commonly observed. Rhodium functions not only as the preferred trapping site to anchor and oxidize carbon monoxide by the oxygen atoms in direct connection with rhodium but also the primarily oxidative centre to accumulate the large amounts of electrons and the polarity of rhodium is ultimately transformed from positive to negative.

Optimal white light control of the negative to neutral to positive charge transition (NeNePo) in the electronic manifold of the silver trimer

Control over the electronic state of the Ag(3) cluster is approached via a progression of ultrafast photoinduced transitions within the full electronic manifold of the negative to the neutral and finally the cationic state of the system. High-bandwidth supercontinuum laser pulses ranging from 500 to 950 nm are employed for addressing the wide range of electronic resonance conditions associated with the ladder climbing process of a tandem photoelectron detachment and a resonance enhanced multiphoton ionization (REMPI). With the control of the phase over the full spectral envelope of the supercontinuum in a pulse shaper arrangement, pulse forms are generated with the aim of synchronizing ultrashort subpulse sequences to the characteristic dynamics of the system during charge reversal. Pulse forms ranging over several hundred femtoseconds in total duration and subpulse structures down to 15 fs duration with a variable spectral composition can be obtained for this purpose. A free optimization based on a closed-loop genetic algorithm is employed for ordering the subpulse sequences to match the structural evolution of the system. The effective control attainable in this scenario is evaluated in view of maintaining a defined sequence of electronic transitions within the complex dynamic response of the system during the photoexcitation. Further emphasis is made on analyzing the degree of control attainable in the nonlinear regime of multiphoton excitation at supercontinuum bandwidths.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.