Rhodium Oxide Cluster Ions Studied by Thermal Desorption Spectrometry

Direct reduction of NO into N2 catalyzed by fullerene-supported rhodium clusters

Catalytic conversion of NO has long been a focus of atmospheric pollution control and diesel vehicle exhaust treatment. Rhodium is one of the most effective metals for catalyzing NO reduction, and understanding the nature of the active sites and underlying mechanisms can help improve the design of Rh-based catalysts towards NO reduction. In this work, we investigated the detailed catalytic mechanisms for the direct reduction of NO to N2 by fullerene-supported rhodium clusters, C60Rh4+, with density functional theory calculations. We found that the presence of C60 facilitates the smooth reduction of NO into N2 and O2, as well as their subsequent desorption, recovering the catalyst C60Rh4+. Such a process fails to be completed by free Rh4+, emphasizing the critical importance of C60 support. We attribute the novel performance of C60Rh4+ to the electron sponge effect of C60, providing useful guidance for designing efficient catalysts for the direct reduction of NO.

Hydrogen Storage Capacity of Single-Nb-Atom-Doped Al Clusters in the Gas Phase Revealed by Thermal Desorption Spectrometry

Hydrogen is a promising energy resource as a substitute for fossil fuels, and metal alloy hydrides are considered to be good candidates as hydrogen storage materials. In the hydrogen storage processes, hydrogen desorption is as important as hydrogen adsorption. In order to understand the hydrogen desorption features of those clusters, here, single-Nb-atom-doped Al clusters were prepared in the gas phase and their reaction with hydrogen was investigated using thermal desorption spectrometry (TDS). On average, six to eight H atoms were adsorbed in Al_nNb⁺ (n = 4-18) clusters, and most H atoms were released upon heating of the clusters to 800 K. Two types of desorption features of Al_nNb⁺ clusters were found, which related to the flexibility of the clusters. This study demonstrated the potential of Nb-doped Al alloy as an efficient hydrogen storage material with high storage capacity, thermal stability at room temperature, and hydrogen desorption ability upon moderate heating.

Pyrolysis of Mass-Selected (V2O5)NO− (N = 1−6) Clusters in a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap that can stably run up to 873 K was newly designed and installed into a homemade reflectron time-of-flight mass spectrometer coupled with a laser ablation cluster source and a quadrupole mass filter. The instrument was used to study the pyrolysis behavior of mass-selected (V2O5) NO− ( N = 1−6) cluster anions and the dissociation channels were clarified with atomistic precision. Similar to the dissociation behavior of the heated metal oxide cluster cations reported in literature, the desorption of either atomic oxygen atom or molecular O2 prevailed for the (V2O5) NO− clusters with N = 2−5 at 873 K. However, novel dissociation channels involving fragmentation of (V2O5) NO− to small-sized V xO y− anions concurrent with the release of neutral vanadium oxide species were identified for the clusters with N = 3−6. Significant variations of branching ratios for different dissociation channels were observed as a function of cluster size. The kinetic studies indicate that the dissociation rates of (V2O5) NO− monotonically increased with the increase of cluster size. The internal energies carried by the (V2O5) NO− clusters at 873 K as well as the energetics data for dissociation channels have been theoretically calculated to rationalize the experimental observations. The decomposition behavior of vanadium oxide clusters from this study can provide new insights into the pyrolysis mechanism of metal oxide nanoparticles that are widely used in high temperature catalysis.

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.

Ladder Oxygenation of Group VIII Metal Clusters and the Formation of Metalloxocubes M13O8. J Phys Chem Lett 2022;13:733-739. [PMID: 35025527 DOI: 10.1021/acs.jpclett.1c04098]

The diversity of valence and bonding of transition metals makes their oxidation processes perplexing at reduced sizes. Here we report a comprehensive study on the oxidation reactions of rhodium clusters Rh_n^± (n = 3-30) and find that Rh_3,4O₄⁺, Rh_5-7O₆⁺, and Rh_8-13O₈⁺ always dominate the mass distributions showing size-dependent ladder oxygenation which is closely associated with the O-binding modes. While the Rh_8-13O₈⁺ clusters display a μ₃-O binding mode (hollow site adsorption), Rh_3-4O₄⁺ and Rh_5-7O₆⁺ favor the μ₂-O binding mode (edge-site adsorption) or a mixture of the two modes. The μ₃-O binding mode is inclined to yield a cubic Rh₁₃O₈, while the μ₂-O binding mode gives rise to oxygen-bridge protection for the metal clusters. Such ladder oxidation was also observed for Pt_n⁺, Fe_n⁺, Co_n⁺, and Ni_n⁺ clusters. We propose a three-dimensional diagram for the oxidation states and O-binding modes of metals, and highlight the metalloxocubes M₁₃O₈⁺ for cluster-genetic materials.

Substitution of O with a Single Au Atom as an Electron Acceptor in Al Oxide Clusters

Al atoms generally adopt the +3 oxidation state and form stoichiometric oxides such as Al₂O₃ in the bulk phase. Among small cationic gas-phase clusters, near-stoichiometric clusters such as Al₃O₄⁺, Al₃O₅⁺, Al₄O₆⁺, Al₄O₇⁺, Al₅O₇⁺, and Al₅O₈⁺ have been readily generated in experimental studies. However, when a single Au atom was included in the clusters, oxygen-deficient clusters such as AuAl₄O₅⁺ were formed in high abundance; in these clusters, the Au atom accepted electron density from the Al atoms. The geometrical structures and atomic charges in the clusters suggest that a single Au atom can substitute for O atoms in Al oxide clusters. This propensity originates from the high electron and low oxygen affinities, which, together, constitute an unusual property of Au.

Thermal stability of iron-sulfur clusters

The thermal decomposition of free cationic iron-sulfur clusters Fe_xS_y⁺ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric Fe_xS_y⁺ (y = x) or near stoichiometric Fe_xS_y⁺ (y = x ± 1) clusters is observed. In particular, Fe₄S₄⁺ represents the most abundant product up to 600 K, Fe₃S₃⁺ and Fe₃S₂⁺ are preferably formed between 600 K and 800 K, and Fe₂S₂⁺ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe₂S₂⁺. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O₂ molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable Fe_xO_y⁺ clusters at high temperatures.

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.

Catalytic Decomposition of NO by Cationic Platinum Oxide Cluster Pt3O4. J Phys Chem Lett 2017;8:2143-2147. [PMID: 28445054 DOI: 10.1021/acs.jpclett.7b00591]

The catalytic decomposition of NO by cationic platinum oxide cluster Pt₃O₄⁺ was investigated by mass spectrometry and thermal desorption spectrometry. Upon reaction with two NO molecules, molecular oxygen desorbed from the cluster at room temperature to form Pt₃O₄N₂⁺. Then, at temperatures above 400 K, desorption of N₂ from Pt₃O₄N₂⁺ was observed. These processes were confirmed by isotope-labeling experiments, and the energetics of O₂ and N₂ release were determined by density functional calculations. The combination of these elementary steps resulted in the catalytic decomposition of NO by Pt₃O₄⁺.

Structure, fragmentation patterns, and magnetic properties of small nickel oxide clusters

We report a comprehensive theoretical study of the structural and electronic properties of neutral and charged nickel oxide clusters, Ni_nO_m^0/± (n = 3–8 and m = 1–10), in the context of recent experiments of photodissociation and Ion Mobility Mass Spectrometry.