Hydrogen-Promoted Oxygen Activation by Free Gold Cluster Cations

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip-Enhanced Raman Spectroscopy

Gaining mechanistic understanding of oxygen activation on metal surfaces is a topical area of research in surface science. However, direct investigation of on-surface oxidation processes at the nanoscale and the empirical validation of oxygen activation pathways remain challenging for the conventional analytical tools. In this study, we applied tip-enhanced Raman spectroscopy (TERS) to gain mechanistic insights into oxygen activation on bulk Au(111) surface. Specifically, oxidation of 4-aminothiophenol (4-ATP) to 4-nitrothiophenol (4-NTP) on Au(111) surface was investigated using hyperspectral TERS imaging. Nanoscale TERS images revealed a markedly higher oxidation efficiency in disordered 4-ATP adlayers compared to the ordered adlayers signifying that the oxidation of 4-ATP molecules proceeds via interaction with the on-surface oxidative species. These results were further validated via direct oxidation of the 4-ATP adlayers with H2O2 solution. Finally, TERS measurements of oxidized 4-ATP adlayers in the presence of H2O18 provided the first empirical evidence for the generation of oxidative species on bulk Au(111) surface via water-mediated activation of molecular oxygen. This study expands our mechanistic understanding of oxidation chemistry on bulk Au surface by elucidating the oxygen activation pathway.

Pd 4d Orbital Overlapping Modulation on Au@Pd Nanowires for Efficient H2O2 Production

Isolating Pd atoms has been shown to be crucial for the design of a Pd-based electrocatalyst toward 2e- oxygen reduction reaction (ORR). However, there are limited studies focusing on the systematic compositional design that leads to an optimal balance between activity and selectivity. Herein, we design a series of Au@Pd core@shell structures to investigate the influence of the Pd 4d orbital overlapping degree on 2e- ORR performance. Density functional theory (DFT) calculations indicate that enhanced H2O2 selectivity and activity are achieved at Pdn clusters with n ≤ 3, and Pd clusters larger than Pd3 should be active for 4e- ORR. However, experimental results show that Au@Pd nanowires (NWs) with Pd4 as the primary structure exhibit the optimal H2O2 performance in an acidic electrolyte with a high mass activity (7.05 A mg-1 at 0.4 V) and H2O2 selectivity (nearly 95%). Thus, we report that Pd4, instead of Pd3, is the upper threshold of Pd cluster size for an ideal 2e- ORR. It results from the oxygen coverage on the catalyst surface during the ORR process, and such an oxygen coverage phenomenon causes electron redistribution and weakened *OOH binding strength on active sites, leading to enhanced activity of Pd4 with only 0.06 V overpotential in acidic media.

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.

Room-Temperature Conversion of Methane to Methanediol by [FeO2]. J Phys Chem Lett 2023;14:1633-1640. [PMID: 36752636 DOI: 10.1021/acs.jpclett.2c03786] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Inspired by the activities of P-450 enzyme and Rieske oxygenases in nature, in which the high-valent Fe-oxo complexes play a key role for oxidation of alkanes, the oxidation process of methane by the high-valent iron oxide cation [FeO₂]⁺ has been explored by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously reported [FeO]⁺/CH₄ and [Fe(O)OH]⁺/CH₄ systems, which afford [FeOH]⁺ as the main product, the generation of Fe⁺ dominates the reaction of [FeO₂]⁺ with CH₄. Theoretical calculations suggest a novel "oxygen rebound" pathway for the liberation of methanediol. In particular, the inevitable valence increase of Fe prior to C-H activation is similar to the cytochrome P-450 mediated processes. To our best knowledge, this study provides the first example of methane activation by the high-valent Fe(V)-oxo species in the gas phase, which may thus bridge the gas-phase model and the condensed-phase biosystems.

Density Functional Study of Size-Dependent Hydrogen Adsorption on Ag n Cr (n = 1-12) Clusters

Increasing interest has been paid for hydrogen adsorption on atomically controlled nanoalloys due to their potential applications in catalytic processes and energy storage. In this work, we investigate the interaction of H₂ with small-sized Ag _n Cr (n = 1-12) using density functional theory calculations. It is found that the cluster structures are preserved during the adsorption of H₂ either molecularly or dissociatively. Ag₃Cr-H₂, Ag₆Cr-H₂, and Ag₉Cr-H₂ clusters are identified to be relatively more stable from computed binding energies and second-order energy difference. The dissociation of adsorbed H₂ on Ag₂Cr, Ag₃Cr, Ag₆Cr, and Ag₇Cr clusters is favored both thermodynamically and kinetically. The dissociative adsorption is unlikely to occur because of a considerable energy barrier before reaching the final state for Ag₄Cr or due to energetic preferences for n = 1, 5, and 8-12 species. Comprehensive analysis shows that the geometric structure of clusters, the relative electronegativity, and the coordination number of the Cr impurity play a decisive role in determining the preferred adsorption configuration.

Synthesis of active, robust and cationic Au25 cluster catalysts on double metal hydroxide by long-term oxidative aging of Au25(SR)18

Synthesis of an atomically precise Au₂₅ cluster catalyst was attempted by long-term, low-temperature aging of Au₂₅(BaET)₁₈ (BaET-H = 2-(Boc-amino)ethanethiol) on various double metal hydroxide (DMH) supports. X-ray absorption fine structure analysis revealed that bare Au₂₅ clusters with high loading (1 wt%) were successfully generated on the DMH containing Co and Ce (Co₃Ce) by oxidative aging in air at 150 °C for >12 h. X-ray absorption near-edge structure and X-ray photoelectron spectroscopies showed that the Au₂₅ clusters on Co₃Ce were positively charged. The Au₂₅/Co₃Ce catalyst thus synthesized exhibited superior catalytic performance in the aerobic oxidation of benzyl alcohol under ambient conditions (TOF = 1097 h^-1 with >97% selectivity to benzoic acid) and high durability owing to a strong anchoring effect. Based on kinetic experiments, we propose that abstraction of hydride from α-carbon of benzyl alkoxide by Au₂₅ is the rate-determining step of benzyl alcohol oxidation by Au₂₅/Co₃Ce.

Infrared action spectroscopy of nitrous oxide on cationic gold and cobalt clusters

Understanding the catalytic decomposition of nitrous oxide on finely divided transition metals is an important environmental issue. In this study, we present the results of a combined infrared action spectroscopy and quantum chemical investigation of molecular N2O binding to isolated Aun+ (n ≤ 7) and Con+ (n ≤ 5) clusters. Infrared multiple-photon dissociation spectra have been recorded in the regions of both the N[double bond, length as m-dash]O (1000-1400 cm-1) and N[double bond, length as m-dash]N (2100-2450 cm-1) stretching modes of nitrous oxide. In the case of Aun+ clusters only the ground electronic state plays a role, while the involvement of energetically low-lying excited states in binding to the Con+ clusters cannot be ruled out. There is a clear preference for N-binding to clusters of both metals but some O-bound isomers are observed in the case of smaller Con(N2O)+ clusters.

Reaction between Ag17+ and acetylene outside the mass spectrometer: dehydrogenation in the gas phase

We present the first example of acetylide protected silver clusters by a reaction between Ag₁₇⁺ and acetylene, conducted around atmospheric pressure. The products were obtained after dehydrogenation of acetylene in the gas phase. The observed reaction mechanism may be helpful to design new catalysts useful in organometallic chemistry.

Co-adsorption of O2 and C2H4 on a Free Gold Dimer Probed via Infrared Photodissociation Spectroscopy

Infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with density functional theory (DFT) calculations has been employed to study the activation of molecular oxygen and ethylene co-adsorbed on a free gold dimer cation Au₂⁺. Both studied complexes, Au₂O₂(C₂H₄)⁺ and Au₂O₂(C₂H₄)₂⁺, show distinct features of both intact O₂ and ethylene co-adsorbed on the cluster. However, the ethylene C=C double bond is activated, increasing in length by up to 0.07 Å compared with the free molecule, and the red shift of the O-O vibration frequency increases with the number of adsorbed ethylene molecules, indicating a small but increasing activation of the O-O bond. The small O₂ activation and the rather weak interaction between O₂ and C₂H₄ are also reflected in the calculated electronic structure of the co-adsorption complexes which shows only a small occupation of the empty anti-bonding O₂ 2π^*_2p orbital as well as the localization of most of the Kohn-Sham orbitals on O₂ and C₂H₄, respectively, with only limited mixing between O₂ and C₂H₄ orbitals. The results are compared with theoretical studies on neutral Au_xO₂(C₂H₄) (x = 3, 5, 7, 9) complexes.

An Au22(L8)6 nanocluster with in situ uncoordinated Au as a highly active catalyst for O2 activation and CO oxidation

Ligand-capped gold nanoclusters with atomic precision have attracted great interest as a new type of nanocatalyst to elucidate mechanisms and establish structure-reactivity correlations. In most cases, however, the catalytic activity of fully protected gold nanoclusters is severely hindered due to the blocking effects of the ligands. Alternatively, partially protected gold nanoclusters with the creation of in situ coordination unsaturated (cus) Au are highly promising for nanocatalysis. In this study, via density functional theory (DFT) calculations, we investigated the reactivity of a diphosphine-protected Au22(L8)6 nanocluster using oxygen activation and CO oxidation as the testing probes. The results showed that the cus Au at the interface shows strong adsorption and activation of oxygen, promoting the dissociation of two O2 molecules into fully oxidized Au22(L8)6O4 nanoclusters with a moderate activation barrier (0.6-0.7 eV). The adsorption of additional O2 prefers molecular adsorption and locates at the terminal Au11 unit around the Au-P framework. Furthermore, our results indicate that the Au22(L8)6 cluster shows very high activity and a low energy barrier (0.51 eV) for CO oxidation. The facile O2 activation and CO oxidation over the Au22(L8)6 nanocatalyst will aid the rational design of partially protected metal nanoclusters with coordination unsaturated metal centers in oxidation or other catalytic reactions for important practical applications.

On the nature of active sites for formic acid decomposition on gold catalysts

Atomic scale size-sensitivity of the catalytic properties of sub-nanometer gold clusters for HCOOH decomposition.

Hydrogen-Induced Adsorption of Carbon Monoxide on the Gold Dimer Cation: A Joint Experimental and DFT Investigation

It is demonstrated, using tandem mass spectrometry and radio frequency ion trap, that the adsorption of a H atom on the gold dimer cation, Au₂H⁺, prevents its dissociation and allows for adsorption of CO. Reaction kinetics are measured by employing a radio frequency ion trap, where Au₂⁺ and CO interact for a given reaction time. The effect of a hydrogen atom is evaluated by comparing reaction rate constants measured for Au₂⁺ and Au₂H⁺. The theoretical results for the adsorption of CO molecules and their reaction characteristics with Au₂⁺ and Au₂H⁺ are found to agree with the experimental findings. The joint investigations provide insights into hydrogen atom adsorption effects and consequent reaction mechanisms.

Consecutive reactions of small, free tantalum clusters with dioxygen controlled by relaxation dynamics

The reaction of small cationic tantalum clusters (Ta_n⁺, n = 4–8) with molecular oxygen is studied under multi-collision conditions in the gas phase, and the reaction kinetics are analyzed in order to elucidate underlying mechanisms.

From oxidative degradation to direct oxidation: size regimes in the consecutive reaction of cationic tantalum clusters with dioxygen

Cationic tantalum clusters (Ta_9–12⁺) are reacted with molecular oxygen under multi-collision conditions in the gas phase in order to analyze the reaction kinetics.

Dioxygen insertion into the gold(i)-hydride bond: spin orbit coupling effects in the spotlight for oxidative addition

O2 insertion into a Au(i)-H bond occurs through an oxidative addition/recombination mechanism, showing peculiar differences with respect to Pd(ii)-H, for which O2 insertion takes place through a hydrogen abstraction mechanism in the triplet potential energy surface with a pure spin transition state. We demonstrate that the spin-forbidden Au(i)-hydride O2 insertion reaction can only be described accurately by inclusion of spin orbit coupling (SOC) effects. We further find that a new mechanism involving two O2 molecules is also feasible, and this result, together with the unexpectedly high experimental entropic activation parameter, suggests the possibility that a third species could be involved in the rate determining step of the reaction. Finally, we show that the O2 oxidative addition into a Au(i)-alkyl (CH3) bond also occurs but the following recombination process using O2 is unfeasible and the metastable intermediate Au(iii) species will revert to reactants, thus accounting for the experimental inertness of Au-alkyl complexes toward oxygen, as frequently observed in catalytic applications. We believe that this study can pave the way for further theoretical and experimental investigations in the field of Au(i)/Au(iii) oxidation reactions, including ligand, additive and solvent effects.

Geometric and electronic properties of gold clusters doped with a single oxygen atom

A systematic theoretical study on single oxygen atom doped gold clusters showed that a single oxygen atom can be adsorbed on various sites of gold surfaces, and obtain nearly one electron from gold atoms.

Catalysis by clusters with precise numbers of atoms

Clusters that contain only a small number of atoms can exhibit unique and often unexpected properties. The clusters are of particular interest in catalysis because they can act as individual active sites, and minor changes in size and composition--such as the addition or removal of a single atom--can have a substantial influence on the activity and selectivity of a reaction. Here, we review recent progress in the synthesis and characterization of well-defined subnanometre clusters, and the understanding and exploitation of their catalytic properties. We examine work on size-selected supported clusters in ultrahigh-vacuum environments and under realistic reaction conditions, and explore the use of computational methods to provide a mechanistic understanding of their catalytic properties. We also highlight the potential of size-selected clusters to provide insights into important catalytic processes and their use in the development of novel catalytic systems.

Reactivity and Catalytic Activity of Hydrogen Atom Chemisorbed Silver Clusters

Metal clusters of silver have attracted recent interest of researchers as a result of their potential in different catalytic applications and low cost. However, due to the completely filled d orbital and very high first ionization potential of the silver atom, the silver-based catalysts interact very weakly with the reacting molecules. In the current work, density functional theory calculations were carried out to investigate the effect of hydrogen atom chemisorption on the reactivity and catalytic properties of inert silver clusters. Our results affirm that the hydrogen atom chemisorption leads to enhancement in the binding energy of the adsorbed O2 molecule on the inert silver clusters. The increase in the binding energy is also characterized by the decrease in the Ag-O and increase in the O-O bond lengths in the case of the AgnH silver clusters. Pertinent to the increase in the O-O bond length, a significant red shift in the O-O stretching frequency is also noted in the case of the AgnH silver clusters. Moreover, the hydrogen atom chemisorbed silver clusters show low reaction barriers and high heat of formation of the final products for the environmentally important CO oxidation reaction as compared to the parent catalytically inactive clusters. The obtained results were compared with those of the corresponding gold and hydrogen atom chemisorbed gold clusters obtained at the same level of theory. It is expected the current computational study will provide key insights for future advances in the design of efficient nanosilver-based catalysts through the adsorption of a small atom or a ligand.

Reactivity of Gold Hydrides: O2 Insertion into the Au-H Bond

Dioxygen reacts with the gold(I) hydride (IPr)AuH under insertion to give the hydroperoxide (IPr)AuOOH, a long-postulated reaction in gold catalysis and the first demonstration of O₂ activation by Au–H in a well-defined system. Subsequent condensation gave the peroxide (IPr)Au–OO–Au(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). The reaction kinetics are reported, as well as the reactivity of Au(I) hydrides with radical scavengers.

Density functional theory study on the activation of molecular oxygen on a stepped gold surface in an aqueous environment: a new approach for simulating reactions in solution

The activation of oxygen molecules is an important issue in the gold-catalyzed partial oxidation of alcohols in aqueous solution. The complexity of the solution arising from a large number of solvent molecules makes it difficult to study the reaction in the system. In this work, O2 activation on an Au catalyst is investigated using an effective approach to estimate the reaction barriers in the presence of solvent. Our calculations show that O2 can be activated, undergoing OOH* in the presence of water molecules. The OOH* can readily be formed on Au(211) via four possible pathways with almost equivalent free energy barriers at the aqueous-solid interface: the direct or indirect activation of O2 by surface hydrogen or the hydrolysis of O2 following a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. Among them, the Eley-Rideal mechanism may be slightly more favorable due to the restriction of the low coverage of surface H on Au(211) in the other mechanisms. The results shed light on the importance of water molecules on the activation of oxygen in gold-catalyzed systems. Solvent is found to facilitate the oxygen activation process mainly by offering extra electrons and stabilizing the transition states. A correlation between the energy barrier and the negative charge of the reaction center is found. The activation barrier is substantially reduced by the aqueous environment, in which the first solvation shell plays the most important role in the barrier reduction. Our approach may be useful for estimating the reaction barriers in aqueous systems.

The role of reducible oxide-metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.