Collisional Activation of N2O Decomposition and CO Oxidation Reactions on Isolated Rhodium Clusters

A Combined Infrared and Computational Study of Gas-Phase Mixed-Ligand Rhodium Complexes: Rh(CO)n(N2O)m+ (n = 1-5, m = 1-4)

In this study, mixed carbonyl and nitrous oxide complexes with Rh+ were studied by mass-selective infrared photodissociation spectroscopy in a molecular beam. The infrared spectra, recorded in the region of the CO and N2O N═N stretches, were assigned and interpreted with the aid of simulated spectra of low-energy structural isomers. Clear evidence of an inner coordination shell of four ligands is observed. The observed vibrational structure can be understood on the basis of local mode vibrations in the two ligands. However, there is also evidence of multiple low-lying isomers and cooperative binding effects between the two ligands. In particular, σ donation from directly coordinated nitrous oxide ligands drives more classical carbonyl bonding than has been observed in pure carbonyl complexes. The observed fragmentation branching ratios following resonant infrared absorption are explained by simple statistical and energetic arguments, providing a contrast with those of equivalent Au+ complexes.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

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.

Spectroscopy and Infrared Photofragmentation Dynamics of Mixed Ligand Ion-Molecule Complexes: Au(CO)x(N2O)y. J Phys Chem A 2021;125:7266-7277. [PMID: 34433267 DOI: 10.1021/acs.jpca.1c05800]

We report a combined experimental and computational study of the structure and fragmentation dynamics of mixed ligand gas-phase ion-molecule complexes. Specifically, we have studied the infrared spectroscopy and vibrationally induced photofragmentation dynamics of mass-selected Au(CO)_x(N₂O)_y⁺ complexes. The structures can be understood on the basis of local CO and N₂O chromophores in different solvation shells with CO found preferentially in the core. Rich fragmentation dynamics are observed as a function of complex composition and the vibrational mode excited. The dynamics are characterized in terms of branching ratios for different ligand loss channels in light of calculated internal energy distributions. Intramolecular vibrational redistribution appears to be rapid, and dissociation is observed into all energetically accessible channels with little or no evidence for preferential breaking of the weakest intermolecular interactions.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

Free electron laser infrared action spectroscopy of nitrous oxide binding to platinum clusters, Ptn(N2O)+

Infrared multiple-photon dissociation spectroscopy has been applied to study Pt_n(N₂O)⁺ (n = 1–8) clusters which represent entrance-channel complexes on the reactive potential energy surface for nitrous oxide decomposition on platinum.

Electrochemical supermolecular templating of mesoporous Rh films

Making mesoporous rhodium (Rh) with traditional soft-templating methods is challenging because Rh has a high surface energy compared to other metals. Here, we report a synthetic concept to generate mesoporous Rh films (MRFs) by electrochemical co-deposition of Rh precursors and block copolymer micelles. We investigate the effect of deposition potentials and pH on the resulting mesoporous structures. Controlled electrodeposition enables us to conformally coat the entire surface of the electrode with a homogeneous mesoporous Rh film with any arbitrary thickness up to ∼840 nm. The average pore size of the MRF is ∼14 nm, with an average wall thickness of ∼9.5 nm. Since the MRFs are directly deposited on conducting substrates, they can be used as porous electrodes for various important electrocatalytic reactions. We examine the performance of these MRFs for the electrochemical methanol oxidation reaction (MOR) and find that they have a mass-normalized peak current density ∼4 times higher than a commercial Rh black (RhB) catalyst.

Structural isomers and low-lying electronic states of gas-phase M+(N2O)n (M = Co, Rh, Ir) ion–molecule complexes

The structures of gas-phase group nine cation–nitrous oxide metal–ligand complexes, M⁺(N₂O)_n (M = Co, Rh, Ir; n = 2–7) have been determined by a combination of infrared photodissociation spectroscopy and density functional theory.

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⁺.

Catalytic oxidation of CO with N2O on isolated copper cluster anions

A catalytic redox reaction involving N2O and CO on size-selected copper cluster anions, Cun(-), was investigated in the gas phase using a guided ion-beam tandem mass spectrometer. When Cun(-) is exposed to a mixture of N2O and CO, CunO(-) is produced via the decomposition of N2O. Increase of the CO partial pressure results in the reproduction of Cun(-) and decrease of CunO(-) through the oxidation of CO. The present results demonstrate that a full catalytic cycle for the reaction, N2O + CO → N2 + CO2, takes place on copper cluster anions. Furthermore, in the investigations of the elementary reactions of Cun(-) + N2O and CunO(-) + CO, we found that the catalytic oxidation of CO with N2O on Cun(-) proceeds most efficiently at n = 7 in the size range of n = 5-16.