Oxidation of CO by Nickel Oxide Clusters Revealed by Post Heating

Zooming in on the initial steps of catalytic NO reduction using metal clusters

The study of reactions relevant to heterogeneous catalysis on the surface of well-defined metal clusters with full control over the number of consituent atoms and elemental composition can lead to a detailed insight into the interactions between metal and reactants. We here review experimental and theoretical studies involving the adsorption of NO molecules on mostly rhodium-based clusters under near-thermal conditions in a molecular beam. We show how IR spectrosopic characterization can give information on the binding nature of NO to the clusters for at least the first three NO molecules. The complementary technique of thermal desorption spectrometry reveals at what temperatures multiple NO molecules on the cluster surface desorb or combine to form rhodium oxides followed by N₂ elimination. Variation of the cluster elemental composition can be a powerful method to identify how the propensity of the critical first step of NO dissociation can be increased. The testing of such concepts with atomic detail can be of great help in guiding the choices in rational catalyst design.

The study of reactions relevant to heterogeneous catalysis on metal clusters with full control over the number of constituent atoms and elemental composition can lead to a detailed insight into the interactions governing catalytic functionality.

Conversion of carbon dioxide to a novel molecule NCNBO- mediated by NbBN2- anions at room temperature

The activation of carbon dioxide (CO₂) mediated by NbBN₂^- cluster anions under the conditions of thermal collision has been investigated by time-of-flight mass spectrometry combined with density functional theory calculations. Two CO double bonds in the CO₂ molecule are completely broken and two C-N bonds are further generated to form the novel molecule NCNBO^-. To the best of our knowledge, this new molecule is synthesized and reported for the first time. In addition, one oxygen atom transfer channel produces another product, NbBN₂O^-. Both of the Nb and B atoms in NbBN₂^- donate electrons to reduce CO₂, and the carbon atom originating from CO₂ serves as an electron reservoir. The reaction of NbB^- with N₂ was also investigated theoretically, and the formation of NbBN₂^- from this reaction is thermodynamically and kinetically quite favorable, indicating that NCNBO^- might be produced from the coupling of N₂ and CO₂ mediated by NbB^- anions.

Hydrogen- and oxygen-atom transfers in the thermal activation of benzene mediated by Cu2O2+ cations

The mass-selected copper oxide cluster cations Cu₂O₂⁺ are successfully prepared by laser ablation and reacted with benzene in a linear ion trap reactor. The cluster reaction is characterized by reflectron mass spectrometry in conjunction with density functional theory calculations. Four types of reaction channels are observed: (1) Cu₂OH⁺ + C₆H₅O˙, (2) Cu₂C₆H₆⁺ + O₂, (3) Cu(C₆H₆)₂⁺ + Cu and (4) Cu₂O₂C₆H₆⁺, in which the first one is the major product. Observation of the products Cu₂OH⁺ indicates that oxygen atom transfer (OAT) accompanying hydrogen atom transfer (HAT) occurs, and the oxygen-centered radical (O^-˙) in Cu₂O₂⁺ is crucial to the cleavages of C-H and Cu-O bonds. It is interesting that HAT and OAT occur in a single reaction channel to form C₆H₅O˙, which has not been reported in the reactions of gas-phase ions with benzene. The calculated results are in agreement with the experimental observations, and no two-state reactivity scenario is present in the potential energy surface of channel 1. This work can provide useful insights into the nature of active sites over copper oxides and reaction mechanisms in the corresponding heterogeneous reactions.

Direct hydroxylation of benzene to phenol mediated by nanosized vanadium oxide cluster ions at room temperature

Gas-phase vanadium oxide cluster cations and anions are prepared by laser ablation. The small cluster ions (<1000 amu) are mass-selected using a quadrupole mass filter and reacted with benzene in a linear ion trap reactor; large clusters (>1000 amu) with no mass selection are reacted with C₆H₆ in a fast flow reactor. Rich product variety is encountered in these reactions, and the reaction channels for small cationic and anionic systems are different. For large clusters, the reactivity patterns of (V₂O₅) _n⁺ (n = 6-25) and (V₂O₅) _n O^- (n = 6-24) cluster series are very similar to each other, indicating that the charge state has little influence on the oxidation of benzene. In sharp contrast to the dramatic changes of reactivity of small clusters, a weakly size dependent reaction behavior of large (V₂O₅)_6-25⁺ and (V₂O₅)_6-24O^- clusters is observed. Therefore, the charge state and the size are not the major factors influencing the reactivity of nanosized vanadium oxide cluster ions toward C₆H₆, which is not common in cluster science. In the reactions with benzene, the small and large reactive vanadium oxide cations show similar reactivity of hydroxyl radicals (OH^•) toward C₆H₆ at higher and lower temperatures, respectively; different numbers of vibrational degrees of freedom and the released energy during the formation of adduct complexes can explain this intriguing correlation. The reactions investigated herein might be used as the models of how to realize the partial oxidation of benzene to phenol in a single step, and the observed mechanisms are helpful to understand the corresponding heterogeneous reactions, such as those over vanadium oxide aerosols and vanadium oxide catalysts.

Design and Application of a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap reactor with hexapole design was homemade to study ion-molecule reactions at variable temperatures. The highest temperature for the trapped ions is up to 773 K, which is much higher than those in available reports. The reaction between V₂O₆^- cluster anions and CO at different temperatures was investigated to evaluate the performance of this reactor. The apparent activation energy was determined to be 0.10 ± 0.02 eV, which is consistent with the barrier of 0.12 eV calculated by density functional theory. This indicates that the current experimental apparatus is prospective to study ion-molecule reactions at variable temperatures, and more kinetic details can be obtained to have a better understanding of chemical reactions that have overall barriers. Graphical Abstract.

Controllable green synthesis of supported hollow NiO crystals with shape evolution from octahedral to novel truncated octahedral

A synthesis method for supported hollow NiO crystals from an octahedral morphology to their different truncated shapes was obtained by utilizing a wet-impregnation method via controlling the calcination temperature.

A comparative study of small 3d-metal oxide (FeO)n, (CoO)n, and (NiO)n clusters

Geometrical and electronic structures of the 3d-metal oxide clusters (FeO)_n, (CoO)_n, and (NiO)_n are computed using density functional theory with the generalized gradient approximation in the range of 1 ≤ n ≤ 10.

Release of Oxygen from Palladium Oxide Cluster Ions by Heat

Palladium oxide cluster ions, PdnOm(+), were prepared in the gas phase using laser ablation of a palladium rod in the presence of oxygen. The cluster ions were heated to 1000 K downstream from the cluster source (post heating), and the abundance of PdnOm(+) (n = 2-7) was examined using mass spectrometry. Temperature-programmed desorption experiments revealed that an oxygen molecule is released from oxygen-rich PdnOm(+), forming oxygen-deficient PdnOm-2(+). It was found that Pd6O4(+) was thermally stable up to 1000 K. The activation energy for oxygen molecule desorption has been obtained and compared with previous results by Lang et al.

Atomically precise cluster catalysis towards quantum controlled catalysts

Catalysis of atomically precise clusters supported on a substrate is reviewed in relation to the type of reactions. The catalytic activity of supported clusters has generally been discussed in terms of electronic structure. Several lines of evidence have indicated that the electronic structure of clusters and the geometry of clusters on a support, including the accompanying cluster-support interaction, are strongly correlated with catalytic activity. The electronic states of small clusters would be easily affected by cluster-support interactions. Several studies have suggested that it is possible to tune the electronic structure through atomic control of the cluster size. It is promising to tune not only the number of cluster atoms, but also the hybridization between the electronic states of the adsorbed reactant molecules and clusters in order to realize a quantum-controlled catalyst.