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Catalytic CO Oxidation by Gas-Phase Metal Oxide Clusters Xiao-Na Li, Li-Na Wang, Li-Hui Mou, and Sheng-Gui He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05185 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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The Journal of Physical Chemistry

Catalytic CO Oxidation by Gas-Phase Metal Oxide Clusters Xiao-Na Li,†,§ Li-Na Wang,†,‡,§ Li-Hui Mou,†,‡,§ Sheng-Gui He*,†,‡,§ †State

Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ‡University §Beijing

of Chinese Academy of Sciences, Beijing 100049, China.

National Laboratory for Molecular Sciences and CAS Research/Education Center of

Excellence in Molecular Sciences, Beijing 100190, China. Corresponding author: [email protected]

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ABSTRACT Oxidation of CO into CO2 is a prototypical reaction in heterogeneous catalysis and is one of the extensively studied reactions in gas phase to explore the underlying mechanisms of related catalysts. In this article, we present and discuss our recent advances in the fundamental understandings of catalytic CO oxidation by O2 mediated with heteronuclear metal oxide clusters (HMOCs) using state-of-the-art mass spectrometry and quantum chemistry calculations. The HMOCs can be considered as ideal models to provide mechanistic insights into the elementary steps of mixed or oxide supported catalysts at a strictly molecular level. A concept of Electronegativity-Ladder effect was proposed to account for the enhanced reactivity of noble metal (NM) doped HMOCs, and then this effect was successfully extended in the design of NM-free HMOCs in catalytic CO oxidation by O2. The future directions and the challenges are also discussed.

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1. INTRODUCTION The oxidation of CO into CO2 is not only practically important in industrial applications but also fundamentally interesting in the field of scientific research.1 The oxidative removal of toxic CO became a necessity in pollution control with the development of petrochemical industry during World War I and then the investigations to oxidize CO at room or lower temperatures were carried out.2 The oxidation of CO into CO2 on copper oxide in 1878 represented the earliest research in this field,3 to the best of our knowledge, and then noble metal (NM) catalysts were developed 2, 4 and are still being actively used because of their extraordinary activity. A lot of research efforts have been devoted to elucidating the mechanisms of CO oxidation over NM catalysts and the important Langmuir-Hinshelwood kinetics5 was identified unambiguously. The discovery of the oscillation kinetics in CO oxidation6 boosted the evolution of a new subject of nonlinear dynamics in surface science.7 CO oxidation is also a widely used probe reaction8-9 to characterize the nature of metal catalysts. Since the breakthrough made by Haruta,10-12 oxide-supported NM nanoclusters13-16 and even single NM atoms17-22 have attracted enormous interests in catalytic CO oxidation, in particular with molecular O2 as oxidant (2CO + O2 → 2CO2). However, the details regarding the nature of the active sites on catalysts and the reaction mechanisms of this seemingly simple oxidation process have not been fully understood. One of the central topics is the nature of the catalytically active oxygen species [O2 → O2−• (superoxide) → O22− (peroxide) → O−• (atomic oxygen radical) + O2− (lattice oxygen)] involved in CO oxidation under working conditions. Direct participation of molecularly adsorbed oxygen,23 O2−• and O22−,24-26 and atomic oxygen species (O2− and O−• )27-28 has been individually evidenced, whereas a clear picture on the reactions of molecular oxygen species (O2−• and O22−) in CO oxidation has not been fully characterized. It is challenging to capture 3 / 36



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a molecular-level mechanism for the reactions on condensed-phase catalysts because of the complexity of the bulk system. From a microscopic perspective, a chemical reaction generally takes place on an active site that is composed of a handful of atoms. As a result, gas-phase atomic clusters with precisely defined number of atoms can be considered as the ideal models of active sites on real-life catalysts.29-34 Cluster reactions can be studied under isolated conditions to provide the mechanistic understanding of related heterogeneous systems. Conversion of CO into CO2 is one of the best studied catalytic reactions in the gas phase. In 1981, Kappes and Staley reported the first example of such a catalytic reaction that the atomic Fe+ cation can catalyze the oxygen atom transfer in reaction CO + N2O → CO2 + N2.35 Later, many metal cations and homonuclear metal oxide clusters were studied in catalytic CO oxidation, and the important results have been summarized in recent review articles.36-40 Without the participation of NM atoms, the O−• radical41-42 on homonuclear metal clusters can readily react with CO, while it is energetically demanding for the reactions of the O2−•, O22−, and O2− species with CO under thermal collision conditions. Thus, N2O was frequently used as the oxidant in catalytic CO oxidation (N2−O bond: 1.73 eV).40, 43-45 At least two NM atoms were required when the catalytic reaction 2CO + O2 → 2CO2 (O−O bond: 5.16 eV) was mediated with homonuclear metal clusters, including Au2−,46-47 Au6−,48 Au3(CO)2−5,49 Agn− (n = 7, 9, 11),50 PtnOm− (n = 3−6; m = 0−2),51 and Pd6O3−5+.52 Heteronuclear metal oxide clusters (HMOCs)38 were also studied to explore the mechanisms of CO oxidation, particularly on mixed or supported metal catalysts. In this paper, we present and discuss our recent advances in catalytic CO oxidation by O2 mediated with single NM atom-doped53-56 and even NM-free HMOCs57-59 by using state-of-the-art mass spectrometry and quantum chemistry calculations. The cooperative interaction between NM atoms and the base-metal cluster support was 4 / 36


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elucidated and a concept of Electronegativity-Ladder (E-Ladder) effect54 was proposed to account for the enhanced reactivity of NM catalysis. The E-Ladder effect was then used for rational design of NM-free HMOCs.57-59 The CO-assisted catalysis in NM-free systems was also discovered and elucidated. Our studies highlighted the finding that the O2−• or O22− species55,

58

have to be

dissociated into atomic oxygen species (O2− or O−•), which can then oxidize CO directly. 2. METHODS 2.1 Experimental Section The HMOC ions were generated by laser ablation of the mixed-metal disks compressed with different metal powders in the presence of O2 seeded in the He carrier gas with the backing pressures of about 5-8 atm. The molar ratio of different metals (1:1 ~ 10:1) and the percentage of O2 (0.02% ~ 2%) used in the He carrier gas were dependent on the nature of different HMOCs. The cluster ions of interest were mass-selected using a quadrupole mass filter and then entered into a linear ion trap (LIT) reactor,60 where they were confined and thermalized by collisions with a pulse of He gas and then interacted with a pulse of CO, O2 or a mixture of CO and O2 under thermal collision conditions. The instantaneous gas pressure of He in the reactor was around 2~4 Pa, and the partial pressures of the reactant molecules were also dependent on the reaction systems (< 10 Pa). The temperature of cooling gas (He), reactant gases, and the LIT reactor was around 298 K. The cluster ions ejected from the LIT were detected by a reflectron time-of-flight mass spectrometer (TOF-MS). The structures for some cluster ions were characterized by photoelectron spectroscopy61 (for Cu2VO4−)57 or collision induced dissociation (for CuAl4O7−9−).58 The neutral Au-Ti-O clusters were also generated by pulsed laser ablation of the mixed-metal disks in the presence of O2 seeded in the He carrier gas with the backing pressure of about 4 atm. 5 / 36



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The clusters generated in a gas channel were expanded and reacted with pure reactant gases of CO, O2, or a mixture of CO/N2 or CO/O2 in a fast flow reactor, where the total pressure was about 60-100 Pa. After the reactions, the charged clusters were removed from the molecular beams by two deflection plates. The neutral reactants and products were skimmed into the vacuum system of the reflectron TOF mass spectrometer and ionized by four VUV laser beams (118 nm, 10.5 eV/photon) generated with an intense λ = 355 nm laser beam in a gas cell containing the Xe/Ar mixture. After single photon ionization, the cluster ions passed through the reflector and then were detected by a dual microchannel plate detector. Note that most of the metal oxide clusters have ionization energies below 10.5 eV,62-63 thus most of them can be ionized and detected. 2.2 Computational Details Density functional theory (DFT) calculations using the Gaussian 09 program64 were carried out to investigate the mechanistic details for catalytic CO oxidation by O2 mediated with the HMOCs. To find an appropriate functional for a particular system, the bond dissociation energies of M−O, O−O, and O−CO were generally computed by various functionals and compared with available experimental data. The TZVP basis set65 was generally used for base metals, C, and O atoms, and the Stuttgart/Dresden relativistic effective core potentials (denoted as SDD in Gaussian software)66 was used for NM atoms (Au, Pt, and Rh). A Fortran code based on a genetic algorithm67 was used to generate initial guess structures of each HMOC. The relaxed potential energy surface scan was used extensively to obtain good guess structures for intermediates (IMs) and transition states (TSs) along the pathways. The TSs were optimized by using the Berny algorithm.68 Intrinsic reaction coordinate calculations69 were performed so that each TS connects two appropriate local minima. Vibrational frequency calculations were carried out to check that IMs and TSs have zero and only 6 / 36


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one imaginary frequency, respectively. 3. RESULTS AND DISCUSSION 3.1 Catalytic CO oxidation by single NM atom-doped HMOCs: the E-Ladder effect Single atom catalysis (SAC) by using atomically dispersed metal sites can maximize the effective use of precious metals and achieve satisfying performance in catalytic reactions,9, 17-18, 20, 70-71

and the SAC can bridge the homogeneous and heterogeneous catalysis.20,

71

Since the

successful generation of the first single-atom catalyst (Pt1/FeOx) that was very effective in catalytic CO oxidation,17 a number of single atom catalytic systems have been reported in condensed-phase studies. Recently, our gas-phase studies have identified a few catalytic cycles for CO oxidation by single Au53 or Pt54 atoms doped HMOCs. Such studies provided the strictly molecular-level understanding of SAC process. It is noteworthy that only a few Au1 or Pt1 doped gas-phase systems are catalytic for CO oxidation, and many Au1-Ce-O72 and Au1-Ti-O73 HMOC ions can only adsorb CO. In the ion trap reactor, the laser-ablation generated and mass-selected AuAl3O5+ cluster can oxidize two CO molecules consecutively to give rise to AuAl3O3+ that can then react with O2 to regenerate AuAl3O5+, which forms a catalytic cycle shown in Figure 1a. The electronic structure analysis identified that the catalysis was driven by electron cycling primarily through the alterative formation of Au+−O (AuAl3O5+) bond and Au−−Al bond (AuAl3O3+ and AuAl3O4+). The pronounced relativistic effect74-75 of Au and its relatively large electronegativity (2.54) with respect to Al (1.61)76 can result in the coexistence of a reductive Au−Al bond and an oxidative O−• radical in AuAl3O4+. Thus, a pair of electrons can be stored in the strong Au−Al bond (3.37 eV)77 and the stored electrons can be released with the expose of AuAl3O3+ to O2. In contrast, the smaller 7 / 36



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electronegativity (1.93)76 of Ag and the weak Ag−Al bond (1.87 eV)77 allow the formation of an Ag−O bond in AgAl3O4+, which is not able to oxidize CO to form a catalytic cycle.53 The oxidation state of gold is sensitive to the coordination environments, and the cationic,78-79 anionic,80-82 and neutral83 gold has been contradictorily proposed to be reactive in catalytic CO oxidation. In this work,53 we identified that gold can change its oxidation state flexibly and dynamically during the catalysis to facilitate both steps of oxidation and reduction (Figure 1a). A similar behavior can be observed for the catalysis mediated with the PtAl3O5−7− cluster anions (Figure 1b).54 After oxidation of the first CO molecule by the PtAl3O7− cluster, the product PtAl3O6− cluster involves an O−• radical and a Pt atom that is not fully oxidized. Thus, PtAl3O6− can oxidize the second CO molecule to form a catalytic cycle shown in Figure 1b. However, the un-doped Al4O7− cluster54 can oxidize only one CO molecule and the Al4O5−7− clusters cannot drive catalytic CO oxidation. The capability to transfer at least two oxygen atoms consecutively to CO from a single HMOC is the prerequisite to catalyze CO oxidation by O2 (2CO + O2 → 2CO2). Based on these studies, we proposed a concept of E-Ladder effect to account for the enhanced reactivity of NM atoms doped HMOCs to drive a consecutive oxidation process (Figure 1c). For a particular M1-M2-O cluster where M2 represents a NM atom, the E-Ladder effect originates from the well-fitting electronegativity of M2 (2.2~2.5, the electronegativity for Ag is 1.93) in between that of M1 (base metal atoms, such as 3d- or main group metal atoms, 1.3~1.9) and O (3.44), thus M2 can accumulate the transferred electrons (O2− + CO → CO2 + 2e− or O−• + CO → CO2 + e−) gradually and promote the oxidation of two or more CO molecules by M1-M2-O. The RhAl2O6+ cluster that can oxidize five CO molecules further demonstrated the fact that Rh can function as a rather effective Ladder to accept electrons during the consecutive oxidation process (Rh3+ in 8 / 36


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RhAl2O6+ → Rh− in RhAl2O+).84 In contrast, for the YAlO3+ 85 or VAlO4+ clusters,44, 86 the Y and V atoms are not the effective Ladders because of their much weaker electron-withdrawing ability (electronegativity: Y/1.22 and V/1.63). Thus, each of the two clusters can oxidize only one CO molecule. Theoretical studies also predicted that each of the clusters ZrScO4 and ZrNbO5 can oxidize only one CO molecule.87 A series of studies have identified that the E-Ladder effect was also operative for the consecutive oxidation of two or more H2 molecules by AuNbO4+,88 Au2VO4+,89 AuVO4+,90 and AuTi3O8−.91 Electronegativity is a fundamental concept in chemistry to predict the reactivity and charge transfer event among atoms and then guide the development of new materials. The researches related to electronegativity have attracted great interests.92-93 In a recent report,93 electronegativity was newly defined by Rahm and co-workers as the average binding energy of valence electrons in the ground-state, providing a better understanding of electronic structure-function relationship. In our study, electronegativity was highlighted to measure the relative electron-withdrawing capability of particular atoms in HMOCs in order to drive a consecutive oxidation process. Note that the E-Ladder effect was originally proposed by studying the charged clusters.54 The cluster charge states have significant effect on their reactivity and mechanisms.94-96 Thus, the reactivity study of neutral HMOCs is vital to further understand this effect. However, it is experimentally challenging to study neutral HMOCs due to the difficulties in cluster ionization and detection without fragmentation.62 The VCoO497 and CeAlO498 clusters were the only available examples of HMOCs, each of which can oxidize one CO molecule. Very recently, benefiting from the homemade time-of-flight mass spectrometer coupled with an improved vacuum ultraviolet laser system,99 the

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neutral AuTi2O3−6 clusters were successfully generated and detected, and the catalytic oxidation 2CO + O2 → 2CO2 mediated with AuTi2O3−6 was identified.55 The typical mass spectra for the interactions of AuxTiyOz with CO, O2, and a gas mixture (CO/N2 or CO/O2) in the fast flow reactor are shown in Figure 2a-e. On the interactions with CO (Figure 2b), the appearance of signals AuTi2O3 and AuTi2O4CO and the simultaneously decreased intensities of AuTi2O5 and AuTi2O4 indicated that AuTi2O5 may oxidize CO to generate AuTi2O4 and the formed AuTi2O4 can react with CO to give rise to AuTi2O3 and AuTi2O4CO. The interactions of AuxTiyOz with a gas mixture (CO/O2, Figure 2d) indicated that AuTi2O3 can pick up one O2 molecule to generate AuTi2O5 with respect to the reference spectrum (CO/N2, Figure 2c). The reactivity of AuTi2O4 toward O2 to generate AuTi2O6 can be clearly observed in Figure 2e. The absence of AuTi2O6 in Figure 2d indicated that AuTi2O6 can react with CO favorably to produce AuTi2O5. Thus, these elementary reactions can comprise the catalytic cycles of CO oxidation by O2 mediated with AuTi2O3−6 (Figure 2f). The DFT calculations show that the lowest-lying isomer of AuTi2O6 contains an O2−• radical (O−O bond: 134 pm) (Figure 3a). For CO oxidation by AuTi2O6, it is interesting to find that the AuCO unit in I1 prefers to move to the terminal atom (labeled as O2, I1→TS1→I2),55 and then CO can be oxidized by the opposite terminal O1 atom (I2→TS2→I3). This behavior can be traced back to the fact that the five-fold coordinated Ti site prefers to remove one oxygen atom by CO oxidation. Note that the O2−• radical in product AuTi2O5 is nearly intact as that in AuTi2O6. The activation and dissociation of chemically adsorbed oxygen (O2−• and O22−) are generally regarded as the crucial steps in catalytic CO oxidation by gold-containing species.27-28, 100-102

Previous studies reported that the Au2-dimer101-102 in HMOCs can activate and dissociate the

O2−• or O22− species during consecutive CO oxidation. Herein, the O2−• unit in AuTi2O5 can be 10 / 36


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activated with the help of the mobile AuCO unit (I8→TS7→I9, Figure 3b). The electrons (−0.47 e) required to reduce O2−• to O22− come dominantly from the gold atom in the AuCO unit (natural charge: Au/−0.01 e in I8 → Au/+0.38 e in I9), indicating that the mobility of AuCO can facilitate charge transfer. The transfer of AuCO is also crucial for CO oxidation by AuTi2O5 (I10→TS9→I11) to produce AuTi2O4, which can react with O2 to regenerate AuTi2O6 (Figure 3c). For O2−• and O22− containing HMOCs, including Au2VO4−,101 Au2TiO4−,102 RhAl2O6+,84 and AuTi2O6,55 available studies demonstrated that O2−• and O22− are not able to oxidize CO directly and they have to be dissociated into the atomic oxygen species (O−• or O2−) that can oxidize CO directly. The gold−CO interaction that may commonly exist in heterogeneous catalysis can influence the reactivity of gold catalyst, and the AuCO unit can be dynamically generated under reaction conditions.103-107 However, the mobility of the whole AuCO unit during the catalysis of CO oxidation has not been discovered in previous gas-phase studies. Herein, by studying the reactions of neutral HMOCs in catalytic CO oxidation, we identified that CO adsorption can promote the mobility of the AuCO unit. For example, the dissociation energy of the AuCO (2.53 eV) unit from I1 is much smaller than that to evaporate an Au atom (3.22 eV) from AuTi2O6. The electronic structure analysis found that the gold atom in the mobile AuCO unit can function as a more flexible Ladder to control electrons during the catalysis and the oxidation state of gold can switch several times (Au+→Au−→Au+→Au0→Au+, Figure 4). This electron cycling process is accompanied with the alternative formation of Au−O bond and Au−Ti bond (Figure 3). This is the similar case for CO oxidation catalyzed by AuTi2O5.55 Note that some fundamental chemistry such as interchange of Au−M bond with Au−O bond to drive catalytic oxidation may be the same for both the neutral55-56 and the charged clusters.53 11 / 36



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3.2 Catalytic CO oxidation by NM-free HMOCs Condensed-phase studies have demonstrated that base metal catalysts can also exhibit outstanding performances in a wide range of reactions, such as catalytic CO oxidation.103-104, 107 A fundamental understanding on base metal catalysis is of scientific and practical importance to guide the design of cost-effective heterogeneous catalysts. Note that the electronegativity for late 3d transition metal atoms (Fe, Co, Ni, and Cu) also locates in between that of main group or early transition metal atoms and the oxygen atom, as shown in Figure 5. Thus, the E-Ladder effect may be used to rationally design NM-free HMOCs in catalytic CO oxidation. Previous studies identified that most of the generated NM-free HMOCs were not capable of catalyzing CO oxidation by O2. Recently, we reported the first example that the copper-vanadium bimetallic oxide clusters Cu2VO3−5− can catalyze CO oxidation by O2.57 The Cu−V system was chosen because of the tremendous success of Cu-based catalysts in a large range of practical applications.104, 108-112 For catalytic CO oxidation by AuAl3O3−5+ (Figure 1a), the charge cycling on the gold atom (Au+ ↔ Au−) can drive the catalytic cycle. There is a similar mechanism in the Cu2VO3−5− catalytic system. The DFT calculations demonstrate that there is a Cu2-dimer in the Cu2VO3−5− clusters with Cu−Cu bond of 255 pm in Cu2VO5−, 238 pm in Cu2VO4−, and 234 pm in Cu2VO3−. Natural charge analysis demonstrates that the Cu2-dimer is positively charged while the VO3−5 moiety is negatively charged in Cu2VO3−5−, and this well charge separation can facilitate charge transfer interaction during the reactions. Thus, the Cu2-dimer can also accept a significant amount of negative charge during consecutive CO oxidation (Cu2VO5− → Cu2VO4− → Cu2VO3−) and the stored electron can be released during O2 activation (Cu2VO3− → Cu2VO5−) benefiting from the dynamic change of

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Cu−Cu bond in Cu2VO3−5− under catalytic conditions, suggesting that the Cu2-dimer functions effectively as a single Au atom to promote the catalytic reaction. Ni (1.91) and Cu (1.90) have very similar electronegativities,76 while the Ni2VO3−5− clusters59 exhibited a dramatically different pattern in catalytic CO oxidation by O2, as shown in Figure 6. This indicates that a subtle change in electronic structure of 3d metals will bring about a remarkable difference of their reactivity in the catalysis. Ni2VO5− can oxidize CO to generate Ni2VO4− (reaction 1, Figure 6b) with respect to the reference spectrum shown in Figure 6a, whereas the absence of Ni2VO3− indicated that Ni2VO4− can only adsorb CO to give rise to Ni2VO4CO− (reaction 2) and then Ni2VO4CO− can oxidize CO to produce Ni2VO3CO− and CO2 (reaction 3). The generation of Ni2VO3CO− from the reaction of Ni2VO4CO− with CO can be further evidenced by the interactions of mass-selected Ni2VO4− (Figure 6c,d) and Ni2VO4CO− (Figure 6e,f) with CO. The reaction of Ni2VO3CO− with O2 can regenerate Ni2VO5− (Figure 6h, reaction 4). Thus, the elementary reactions 1-4 can comprise a CO self-promoted catalysis mediated with the Ni2VO4,5− clusters. Note that Ni2VO3− suffered from low abundance in the experiment and cannot be generated starting from Ni2VO5−, Ni2VO4−, and Ni2VO4CO−, thus Ni2VO3− was absent in this catalysis. The DFT calculations show that there is also a Ni2-dimer in the four catalytically related species Ni2VO5−, Ni2VO4−, Ni2VO4CO−, and Ni2VO3CO− (Figure 7a). Ni2VO5− can oxidize CO favorably59 to produce the experimentally observed Ni2VO4− (Figure 6b). In contrast, Ni2VO4− can only oxidize CO into a bent CO2− unit attached on the cluster (I16→TS14→I17, Figure 7b), whereas it is challenging to evaporate CO2 directly (I17→TS15’→I18’). With the help of a secondly adsorbed CO (I17→I18), a gas-phase CO2 molecule can be released breezily. Theoretical calculations support the experimental result that Ni2VO4CO− rather than Ni2VO4− can oxidize CO into CO2. 13 / 36



Ni2VO5− + CO → Ni2VO4− + CO2

(1)

Ni2VO4− + CO → Ni2VO4CO−

(2)

Ni2VO4CO− + CO → Ni2VO3CO− + CO2

(3)

Ni2VO3CO− + O2 → Ni2VO5− + CO

(4)

The CO-promoted catalysis may be ubiquitous in heterogeneous processes.111,

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113-115

In

gas-phase studies, the capability of the attached CO to improve the reactivity of atomic clusters has also been reported previously.116-117 Herein, the ability of I17 (Figure 7b) to evaporate the CO2 molecule lies in the ability of product Ni2VO3− to accommodate the left electrons118 that are stored originally in the CO2− unit (natural charge: −0.94 e). Figure 7c shows that the Ni2-unit in Ni2VO3− is reluctant to function as an effective Ladder to accept the transferred electrons (−0.82 e), the process of which has to surmount the high barriers. With CO addition, the Ni2-dimer in Ni2VO3CO− can accept the transferred electrons (−0.78 e) readily and step by step. The attached CO is not a spectator but an indispensable assistant to share the burden with the Ni2-unit to withdraw electrons. This can be reflected by the result that the attached CO is positively charged (+0.20 e) in I18 and becomes negatively charged (−0.14 e) in Ni2VO3CO− (Figure 7c). The attached CO can modify the electronic structure of Ni2VO4CO− in I18 and makes the whole CO−Ni2 moiety resemble the behavior of a NM atom in terms of electron storage during oxygen atom transfer. The formal oxidation states for Ni are +2 and +3, thus Ni prefers to be in the oxidized states. While Cu is more adaptive to the change of the coordination surroundings because of the multivalent nature of Cu with Cu0, Cu1+, Cu2+, and even Cu3+ oxidation state.119-120 In Cu-containing mixed-metal catalysts, Cu was commonly viewed as the active component with easily tuned chemical states.121-122 Note that the Ni−Ni bond in the Ni2-dimer in four catalytically related species suffers from a remarkable 14 / 36


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stretching (232 pm in Ni2VO3CO− versus 272 pm in Ni2VO4CO−), which is the result of a significant change of electronic structures. The Ni2-dimer can also store the negative charge during consecutive CO oxidation (+1.8 e in Ni2VO5− → +0.6 e in Ni2VO3CO−) and the stored electron can then be released with the expose of Ni2VO3CO− toward O2 to drive the catalysis. This indicates that the Ni2-dimer behaves dynamically like that of the Cu2-dimer57 in Cu2VO3−5− in terms of electron storage and release by changing the Ni−Ni bond, whereas this dynamic behavior has to be assisted by CO adsorption. For catalytic CO oxidation by single Cu atoms doped CuAl4O7−9− clusters, the CuAl4O9− cluster with an O2−• unit is also kinetically less favorable to oxidize CO directly into CO2. The attachment of another CO on CuAl4O9CO− is helpful to dissociate O2−• into O22− and then atomic oxygen species that can then oxidize CO directly.58 The soul of fundamental studies is to uncover the underlying mechanisms that will then be used to tailor catalysts with improved performance. Different mechanisms for catalytic reaction 2CO + O2 → 2CO2 mediated with atomic clusters have been identified,59 as summarized in Figure 8, and our recent studies discovered the CO-assisted catalysis in NM-free systems (Mechanism III for the CuAl4O7−9− systems58 and Mechanism IV for the Ni2VO4,5− systems59). The dynamic response of catalysts towards reactant molecules under real-life environments has a strong influence on their catalytic reactivity.9, 123-124

112, 115

Recent advances in in situ characterization technologies112,

make it possible to explore the response of catalyst surface toward gas-phase reactants under

reaction conditions. Our recent studies55,

58-59

are thus important to understand the essence of

catalyst-reactant interactions at a strictly molecular level. 4. CONCLUSTION AND FUTURE DIRECTIONS In this article, we have emphasized the importance of Electronegativity-Ladder (E-Ladder) 15 / 36



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effect to account for the enhanced reactivity of heteronuclear metal oxide clusters (HMOCs) in catalyzing CO oxidation by O2. A clear picture on the reactivity of molecular oxygen species [superoxide (O2−•) and peroxide (O22−)] in CO oxidation was obtained. The O2−• and O22− species have to be dissociated into atomic oxygen species, which can then oxidize CO directly. The E-Ladder effect may be promising to predict HMOCs used in other redox processes and provide the fundamental understandings. For example, the underlying mechanisms may be used in rational design of effective catalysts for the water-gas shift reaction (CO + H2O → CO2 + H2)125-127 that is industrially important in the production of H2. An appropriate system should facilitate both steps of oxidation (CO → CO2) and reduction (H2O → H2), while an effective cluster catalyst has not been reported for this important reaction. The doping of clusters with appropriate compositions and structures is important to identify catalytic water-gas shift reaction in the gas phase. Note that electronegativity may not be the dominant factor to evaluate the reactivity of HMOCs with nearly the same electronegativities of different metal atoms. Our newly report demonstrated that the MnVO5− and ZnVO5− clusters128 (electronegativity: Mn/1.55, V/1.63, and Zn/1.65) can also catalyze CO oxidation by O2, and the catalytic reactivity is highly dependent on their electronic structures rather than electronegativity. Thus, there is still a long way to go to further understand this seemingly simple catalytic reaction. The development of new methods can be crucial to accelerate the identification of new type of cluster catalysis. We found that only a few experimentally generated clusters were catalytically reactive and most of the clusters were inert or only reactive in elementary reactions. In recent years, the machine learning methods were129 successfully used to solve chemistry problems, for example, guiding chemical synthesis,130 enhancing theoretical chemistry,131 and assisting the discovery of 16 / 36


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successful chemical reaction from failed experiments.132 The future application of machine learning in cluster science will be greatly helpful to accelerate the discovery of cluster catalysts. The previously identified large amounts of clusters, including those of the reactive and the inert species, can be used to build a cluster space to train the machine learning models that can predict new cluster catalytic cycles. The state-of-the-art mass spectrometry can determine the composition and reactivity of cluster species in the gas phase, while mass spectrometry cannot provide detailed information for cluster geometrical and electronic structures. Photoelectron spectroscopy is versatile and convenient to provide direct information on electron affinities and energy levels of cluster vibronic states, while it is usually suitable for studying the negatively charged clusters and the related neutral species. Infrared photon dissociation spectroscopy with tunable infrared lasers133 is another commonly used method to probe structural information for neutral and charged species in the gas phase. The reported cluster catalysts usually had low abundance (fluence) in the cluster source and it was difficult to characterize these clusters, particularly the catalytic intermediates with spectroscopic methods that usually need more intense cluster species than mass spectrometric characterization. The development of new cluster sources to enhance the cluster fluence is very important and the experimental characterization of the catalytic intermediate can be possible. Such characterization can be used to further understand the catalytic mechanisms. AUTHOR INFORMATION Corresponding author *Sheng-Gui He. E-mail: [email protected] ORCID Xiao-Na Li: 0000-0002-0316-5762 17 / 36



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Li-Na Wang: 0000-0002-9194-3527 Sheng-Gui He: 0000-0002-9919-6909 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21773254, 21833011, and 21573246), Beijing Natural Science Foundation (No. 2172059), and the Youth Innovation Promotion Association CAS (No. 2016030).

REFERENCES (1) Freund, H. J.; Meijer, G.; Scheffler, M.; Schlögl, R.; Wolf, M. CO Oxidation as A Prototypical Reaction for Heterogeneous Processes. Angew. Chem. Int. Ed. 2011, 50, 10064-10094. (2) Ray, A. B.; Anderegg, F. O. The Oxidation of Carbon Monoxide by Passage with Oxygen or Air through the Silent Discharge and over Ozone Decomposing Catalysts. J. Am. Chem. Soc. 1921, 43, 967-978. (3) Wright, C. R. A.; Luff, A. P. I-First Report to the Chemical Society on “Researches on Some Points in Chemical Dynamics.” J. Chem. Soc. Trans. 1878, 33, 1-27. (4) McKinney, P. V. Reduction of Palladium Oxide by Carbon Monoxide. J. Am. Chem. Soc. 1932, 54, 4498-4504. (5) Engel, T.; Ertl, G. A Molecular Beam Investigation of the Catalytic Oxidation of CO on Pd (111). J. Chem. Phys. 1978, 9, 1267-1281. (6) Imbihl, R.; Ertl, G. Oscillatory Kinetics in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 697-733. (7) Jakubith, S.; Rotermund, H. H.; Engel, W.; von Oertzen, A.; Ertl, G. Spatiotemporal Concentration Patterns in A Surface Reaction: Propagating and Standing Waves, Rotating Spirals, and Turbulence. Phys. Rev. Lett. 1990, 65, 3013-3016. (8) Hashmi, A. S. K.; Hutchings, G. J. Gold Catalysis. Angew. Chem. Int. Ed. 2006, 45, 7896-7936. (9) DeRita, L.; Resasco, J.; Dai, S.; Boubnov, A.; Thang, H. V.; Hoffman, A. S.; Ro, I.; Graham, G. W.; Bare, S. R.; Pacchioni, G.; et al. Structural Evolution of Atomically Dispersed Pt Catalysts Dictates Reactivity. Nat. Mater. 2019, 18, 746-751. (10) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at A Temperature Far Below 0 °C. Chem. Lett. 1987, 16, 405-408. (11) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153-166. (12) Daté, M.; Haruta, M. Moisture Effect on CO Oxidation over Au/TiO2 Catalyst. J. Catal. 2001, 201, 221-224. 18 / 36


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W. D. Catalytic Oxidation of Carbon Monoxide on Monodispersed Platinum Clusters: Each Atom Counts. J. Am. Chem. Soc. 1999, 121, 3214-3217. (14) Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation. Science 2008, 321, 1331-1335. (15) Green, I. X.; Tang, W.; Neurock, M.; Yates Jr., J. T. Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on A Au/TiO2 Catalyst. Science 2011, 333, 736-739. (16) Du, Y.; Sheng, H.; Astruc, D.; Zhu, M. Atomically Precise Noble Metal Nanoclusters as Efficient Catalysts: A Bridge between Structure and Properties. Chem. Rev. 2019, DOI: 10.1021/acs.chemrev.8b00726. (17) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation using Pt1/FeOx. Nat. Chem. 2011, 3, 634-641. (18) Yang, X. F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740-1748. (19) Jones, J.; Xiong, H.; DeLaRiva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G.; Oh, S.; Wiebenga, M. H.; Pereira Hernández, X. I.; et al. Thermally Stable Single-Atom Platinum-on-Ceria Catalysts via Atom Trapping. Science 2016, 353, 150-154. (20) Liu, J. Catalysis by Supported Single Metal Atoms. ACS Catal. 2017, 7, 34-59. (21) Nie, L.; Mei, D.; Xiong, H.; Peng, B.; Ren, Z.; Hernandez, X. I. P.; DeLaRiva, A.; Wang, M.; Engelhard, M. H.; Kovarik, L.; et al. Activation of Surface Lattice Oxygen in Single-Atom Pt/CeO2 for Low-Temperature CO Oxidation. Science 2017, 358, 1419-1423. (22) Cao, L.; Liu, W.; Luo, Q.; Yin, R.; Wang, B.; Weissenrieder, J.; Soldemo, M.; Yan, H.; Lin, Y.; Sun, Z.; et al. Atomically Dispersed Iron Hydroxide Anchored on Pt for Preferential Oxidation of CO in H2. Nature 2019, 565, 631-634. (23) Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. Reaction of CO with Molecularly Chemisorbed Oxygen on TiO2-Supported Gold Nanoclusters. J. Am. Chem. Soc. 2004, 126, 13574-13575. (24) Liu, H.; Kozlov, A. I.; Kozlova, A. P.; Shido, T.; Asakura, K.; Iwasawa, Y. Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on A TiO2-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and As-precipitated Titanium Hydroxide. J. Catal. 1999, 185, 252-264. (25) Okumura, M.; Coronado, J. M.; Soria, J.; Haruta, M.; Conesa, J. C. EPR Study of CO and O2 Interaction with Supported Au Catalysts. J. Catal. 2001, 203, 168-174. 19 / 36



Page 20 of 36

(26) Carrettin, S.; Hao, Y.; Aguilar-Guerrero, V.; Gates, B. C.; Trasobares, S.; Calvino, J. J.; Corma, A. Increasing the Number of Oxygen Vacancies on TiO2 by Doping with Iron Increases the Activity of Supported Gold for CO Oxidation. Chem. Eur. J. 2007, 13, 7771-7779. (27) Widmann, D.; Behm, R. J. Active Oxygen on A Au/TiO2 Catalyst: Formation, Stability, and CO Oxidation Activity. Angew. Chem. Int. Ed. 2011, 50, 10241-10245. (28) Widmann, D.; Behm, R. J. Activation of Molecular Oxygen and the Nature of the Active Oxygen Species for CO Oxidation on Oxide Supported Au Catalysts. Acc. Chem. Res. 2014, 47, 740-749. (29) Yin, S.; Bernstein, E. R. Gas Phase Chemistry of Neutral Metal Clusters: Distribution, Reactivity and Catalysis. Int. J. Mass Spectrom. 2012, 321-322, 49-65. (30) Lang, S. M.; Bernhardt, T. M. Gas Phase Metal Cluster Model Systems for Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2012, 14, 9255-9269. (31) Harding, D. J.; Fielicke, A. Platinum Group Metal Clusters: From Gas-Phase Structures and Reactivities towards Model Catalysts. Chem. Eur. J. 2014, 20, 3258-3267. (32) Zavras, A.; Khairllah, G. N.; Krstić, M.; Girod, M.; Daly, S.; Antoine, R.; Maitre, P.; Mulder, R. J.; Alexander, S. A.; Bonačić-Koutecký, V.; et al. Ligand-Induced Substrate Steering and Reshaping of [Ag2(H)]+ Scaffold for Selective CO2 Extrusion from Formic Acid. Nat. Commun. 2016, 7, 11746. (33) Chi, C.; Qu, H.; Meng, L.; Kong, F.; Luo, M.; Zhou, M. CO Oxidation by Group 3 Metal Monoxide Cations Supported on [Fe(CO)4]2–. Angew. Chem. Int. Ed. 2017, 56, 14096-14101. (34) Schwarz, H. Ménage-à-trois: Single-Atom Catalysis, Mass Spectrometry, and Computational Chemistry. Catal. Sci. Technol. 2017, 7, 4302-4314. (35) Kappes, M. M.; Staley, R. H. Gas-phase Oxidation Catalysis by Transition-Metal Cations. J. Am. Chem. Soc. 1981, 103, 1286-1287. (36) Schwarz, H.; Böhme, D. K. Gas-Phase Catalysis by Atomic and Cluster Metal Ions: the Ultimate Single-Site Catalysts. Angew. Chem. Int. Ed. 2005, 44, 2336-2354. (37) Liu, Q. Y.; He, S. G. Oxidation of Carbon Monoxide by Atomic Clusters. Chem. J. Chin. Univ. 2014, 35, 665-688. (38) Schwarz, H. Doping Effects in Cluster-Mediated Bond Activation. Angew. Chem. Int. Ed. 2015, 54, 10090-10100. (39) O’Hair, R. A. J. Mass Spectrometry Based Studies of Gas Phase Metal Catalyzed Reactions. Int. J. Mass Spectrom. 2015, 377, 121-129. (40) Li, X. N.; Zou, X. P.; He, S. G. Metal-Mediated Catalysis in the Gas Phase: A Review. Chin. J. Catal. 2017, 38, 1515-1527. (41) Zhao, Y. X.; Wu, X. N.; Ma, J. B.; He, S. G.; Ding, X. L. Characterization and Reactivity of Oxygen-Centred Radicals over Transition Metal Oxide Clusters. Phys. Chem. Chem. Phys. 2011, 13, 1925-1938. 20 / 36


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Ma, J. B.; Xu, B.; Meng, J. H.; Wu, X. N.; Ding, X. L.; Li, X. N.; He, S. G. Reactivity of Atomic Oxygen Radical Anions Bound to Titania and Zirconia Nanoparticles in the Gas Phase: Low-Temperature Oxidation of Carbon Monoxide. J. Am. Chem. Soc. 2013, 135, 2991-2998. (43) Sun, X.; Zhou, S.; Yue, L.; Schlangen, M.; Schwarz, H. Metal-Free, Room-Temperature Oxygen-Atom Transfer in the N2O/CO Redox Couple as Catalyzed by [Si2Ox] •+ (x = 2–5). Angew. Chem. Int. Ed. 2017, 56, 9990-9993. (44) Debnath, S.; Knorke, H.; Schöllkopf, W.; Zhou, S.; Asmis, K. R.; Schwarz, H. Experimental Identification of the Active Site in the Heteronuclear Redox Couples [AlVOx]+•/CO/N2O (x = 3, 4) by Gas-Phase IR Spectroscopy. Angew. Chem. Int. Ed. 2018, 57, 7448-7452. (45) Sun, X.; Zhou, S.; Yue, L.; Guo, C.; Schlangen, M.; Schwarz, H. On the Remarkable Role of the Nitrogen Ligand in the Gas-Phase Redox Reaction of the N2O/CO Couple Catalyzed by [NbN]+. Angew. Chem. Int. Ed. 2019, 58, 3635-3639. (46) Häkkinen, H.; Landman, U. Gas-Phase Catalytic Oxidation of CO by Au2–. J. Am. Chem. Soc. 2001, 123, 9704-9705. (47) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Wöste, L.; Heiz, U.; Häkkinen, H.; Landman, U. Catalytic CO Oxidation by Free Au2–: Experiment and Theory. J. Am. Chem. Soc. 2003, 125, 10437-10445. (48) Wallace, W. T.; Whetten, R. L. Coadsorption of CO and O2 on Selected Gold Clusters: Evidence for Efficient Room-Temperature CO2 Generation. J. Am. Chem. Soc. 2002, 124, 7499-7505. (49) Xie, Y.; Dong, F.; Bernstein, E. R. Experimental and Theory Studies of the Oxidation Reaction of Neutral Gold Carbonyl Clusters in the Gas Phase. Catal. Today 2011, 177, 64-71. (50) Socaciu, L. D.; Hagen, J.; Le Roux, J.; Popolan, D.; Bernhardt, T. M.; Wöste, L.; Vajda, S. Strongly Cluster Size Dependent Reaction Behavior of CO with O2 on Free Silver Cluster Anions. J. Chem. Phys. 2004, 120, 2078-2081. (51) Shi, Y.; Ervin, K. M. Catalytic Oxidation of Carbon Monoxide by Platinum Cluster Anions. J. Chem. Phys. 1998, 108, 1757-1760. (52) Lang, S. M.; Fleischer, I.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Pd6O4+: An Oxidation Resistant yet Highly Catalytically Active Nano-Oxide Cluster. J. Am. Chem. Soc. 2012, 134, 20654-20659. (53) Li, Z. Y.; Yuan, Z.; Li, X. N.; Zhao, Y. X.; He, S. G. CO Oxidation Catalyzed by Single Gold Atoms Supported on Aluminum Oxide Clusters. J. Am. Chem. Soc. 2014, 136, 14307-14313. (54) Li, X. N.; Yuan, Z.; Meng, J. H.; Li, Z. Y.; He, S. G. Catalytic CO Oxidation on Single Pt-Atom Doped Aluminum Oxide Clusters: Electronegativity-Ladder Effect. J. Phys. Chem. C 2015, 119, 15414-15420. (55) Chen, J. J.; Li, X. N.; Chen, Q.; Liu, Q. Y.; Jiang, L. X.; He, S. G. Neutral Au1-Doped Cluster Catalysts AuTi2O3–6 for CO Oxidation by O2. J. Am. Chem. Soc. 2019, 141, 2027-2034. (56) Ou, S. H.; Chen. J. J.; Li, X. N.; Wang, L. N.; Ma, T. M.; He, S. G. CO Oxidation by Neutral Gold-Vanadium Oxide Clusters. Chin. J. Chem. Phys. 2019, 32, 207-212. (57) Wang, L. N.; Li, X. N.; Jiang, L. X.; Yang, B.; Liu, Q. Y.; Xu, H. G.; Zheng, W. J.; He, S. G. Catalytic CO Oxidation by O2 Mediated by Noble-Metal-Free Cluster Anions Cu2VO3–5−. Angew. Chem. Int. Ed. 2018, 57, 3349-3353. 21 / 36



Page 22 of 36

(58) Zou, X. P.; Wang, L. N.; Li, X. N.; Liu, Q. Y.; Zhao, Y. X.; Ma, T. M.; He, S. G. Noble-Metal-Free Single-Atom Catalysts CuAl4O7–9− for CO Oxidation by O2. Angew. Chem. Int. Ed. 2018, 57, 10989-10993. (59) Wang L. N.; Li, X. N.; He, S. G. Catalytic CO Oxidation by Noble-Metal-Free Ni2VO4,5– Clusters: A CO Self-Promoted Mechanism. J. Phys. Chem. Lett. 2019, 10, 1133-1138. (60) Yuan, Z.; Li, Z. Y.; Zhou, Z. X.; Zhao, Y. X.; He, S. G. Thermal Reactions of (V2O5)nO– (n = 1–3) Cluster Anions with Ethylene and Propylene: Oxygen Atom Transfer versus Molecular Association. J. Phys. Chem C 2014, 118, 14967-14976. (61) Xu, H. G.; Zhang, Z. G.; Feng, Y.; Yuan, J.; Zhao, Y.; Zheng, W. J. Vanadium-Doped Small Silicon Clusters: Photoelectron Spectroscopy and Density-Functional Calculations. Chem. Phys. Lett. 2010, 487, 204-208. (62) Bernstein, E. R. Neutral Cluster Mass Spectrometry. Int. J. Mass Spectrom. 2015, 377, 248-262. (63) Chen, J. J.; Zhang, T.; Zhang, M. Q.; Liu, Q. Y.; Li, X. N.; He, S. G. Size-Dependent Reactivity of Nano-Sized Neutral Manganese Oxide Clusters toward Ethylene. Chem. Eur. J. 2017, 23, 15820-15826. (64) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.01, Gaussian, Inc.: Wallingford, CT, 2009. (65) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829-5835. (66) Dolg, M.; Stoll, H. Preuss, H. Energy-Adjusted Ab Initio Pseudopotentials for the Rare Earth Elements. J. Chem. Phys. 1989, 90, 1730-1734. (67) Ding, X. L.; Li, Z. Y.; Meng, J. H.; Zhao, Y. X.; He, S. G. Density-Functional Global Optimization of (La2O3)n Clusters. J. Chem. Phys. 2012, 137, 214311. (68) Schlegel, H. B. Optimization of Equilibrium Geometries and Transition Structures. J. Comput. Chem. 1982, 3, 214-218. (69) Gonzalez, C.; Schlegel, H. B. An Improved Algorithm for Reaction Path Following. J. Chem. Phys. 1989, 90, 2154-2161. (70) Wang, A. Q.; Li, J.; Zhang, T. Heterogeneous Single-Atom Catalysis. Nat. Rev. Chem. 2018, 2, 65-81. (71) Cui, X.; Li, W.; Ryabchuk, P.; Junge, K.; Beller , M. Bridging Homogeneous and Heterogeneous Catalysis by Heterogeneous Single-Metal-Site Catalysts. Nat. Catal. 2018, 1, 385-397. (72) Nagata, T.; Miyajima, K.; Mafuné, F. Gold Atoms Supported on Gas-Phase Cerium Oxide Cluster Ions: Stable Stoichiometry and Reactivity with CO. J. Phys. Chem. A 2016, 120, 7624-7633. (73) Himeno, H.; Miyajima, K.; Yasuike, T.; Mafuné, F. Gas Phase Synthesis of Au Clusters Deposited on Titanium Oxide Clusters and Their Reactivity with CO Molecules. J. Phys. Chem. A 2011, 115, 11479-11485. (74) Schwarz, H. Relativistic Effects in Gas-Phase Ion Chemistry: An Experimentalist's View. Angew. Chem. Int. Ed. 2003, 42, 4442-4454. (75) Pyykkö, P. Theoretical Chemistry of Gold. Angew. Chem. Int. Ed. 2004, 43, 4412-4456. (76) Allred, A. L. Electronegativity Values from Thermochemical Data. J. Inorg. Nucl. Chem. 1961, 17, 215-221. 22 / 36


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(77) Cuthill, A. M.; Fabian, D. J.; Shu-Shou-Shen, S. Bond Dissociation Energies of the Metallic Vapor Species Aluminum-Silver and Aluminum-Gold Measured by Knudsen-cell Mass Spectrometry. J. Phys. Chem. 1973, 77, 2008-2011. (78) Grünert, W.; Groβmann, D.; Noei, H.; Pohl, M. M.; Sinev, I.; Toni, A. D.; Wang, Y.; Muhler, M. Low-Temperature Oxidation of Carbon Monoxide with Gold(III) Ions Supported on Titanium Oxide. Angew. Chem. Int. Ed. 2014, 53, 3245-3249. (79) Fierro-Gonzaleza, J. C.; Guzman, J.; Gatesd, B. C. Role of Cationic Gold in Supported CO Oxidation Catalysts. Top. Catal. 2007, 44, 103-114. (80) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, Jr., J. P.; Goodman, D. W. The Role of F-Centers in Catalysis by Au Supported on MgO. J. Am. Chem. Soc. 2005, 127, 1604-1605. (81) Yoon, B.; Häkkinen, H.; Landman, U.; Wörz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO. Science 2005, 307, 403-407. (82) Zhou, X.; Shen, Q.; Yuan, K.; Yang, W.; Chen, Q.; Geng, Z.; Zhang, J.; Shao, X.; Chen, W.; Xu, G.; et al. Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation. J. Am. Chem. Soc. 2018, 140, 554-557. (83) Weiher, N.; Beesley, A. M.; Tsapatsaris, N.; Delannoy, L.; Louis, C.; van Bokhoven, J. A.; Schroeder, S. L. M. Activation of Oxygen by Metallic Gold in Au/TiO2 Catalysts. J. Am. Chem. Soc. 2007, 129, 2240-2241. (84) Li, X. N.; Zhang, H. M.; Yuan, Z.; He, S. G. A Nine-Atom Rhodium-Aluminum Oxide Cluster Oxidizes Five Carbon Monoxide Molecules. Nat. Commun. 2016, 7, 11404. (85) Ma, J. B.; Wang, Z. C.; Schlangen, M.; He, S. G.; Schwarz, H. On the Origin of the Surprisingly Sluggish Redox Reaction of the N2O/CO Couple Mediated by [Y2O2]+• and [YAlO2]+• Cluster Ions in the Gas Phase. Angew. Chem. Int. Ed. 2013, 52, 1226-1230. (86) Wang, Z. C.; Dietl, N.; Kretschmer, R.; Weiske, T.; Schlangen, M.; Schwarz, H. Catalytic Redox Reactions in the CO/N2O System Mediated by the Bimetallic Oxide-Cluster Couple AlVO3+/AlVO4+. Angew. Chem. Int. Ed. 2011, 50, 12351-12354. (87) Nöβler, M.; Mitrić, R.; Bonačić-Koutecký, V.; Johnson, G. E.; Tyo, E. C.; Castleman, Jr., A. W. Generation of Oxygen Radical Centers in Binary Neutral Metal Oxide Clusters for Catalytic Oxidation Reactions. Angew. Chem. Int. Ed. 2010, 49, 407-410. (88) Jiang, L. X.; Li, X. N.; Li, H. F.; Zhou, Z. X.; He, S. G. Generation of Hydroxyl Radicals in the Reaction of Dihydrogen with AuNbO4+ Cluster Cations. Chem. Asian J. 2016, 11, 2730-2734. (89) Zhou, X. H.; Li, Z. Y.; Ma, T. M.; He, S. G. Consecutive H2 Oxidation Mediated by Au2VO4+ Clusters. J. Phys. Chem. C. 2016, 120, 10452-10459. (90) Zou, X. P.; Li, Z. Y.; Li, X. N.; Wang, L. N.; Li, H. F.; Ma, T. M.; He, S. G. Consecutive Oxidation of Three H2 Molecules by A Gold-Vanadium Oxide Cluster Anions, Top. Catal. 2018, 61, 28-34. (91) Jiang, L. X.; Li, X. N.; Li, Z. Y.; Li, H. F.; He, S. G. H2 Dissociation by Au1-Doped Closed-Shell Titanium Oxide Cluster Anions. Chin. J. Chem. Phys. 2018, 31, 457-462.

23 / 36



Page 24 of 36

(92) Onoda, J.; Ondráček, M.; Jelínek, P.; Sugimoto, Y. Electronegativity Determination of Individual Surface Atoms by Atomic Force Microscopy. Nat. Commun. 2018, 8, 15155. (93) Rahm, M.; Zeng, T.; Hoffmann, R. Electronegativity Seen as the Ground-State Average Valence Electron Binding Energy. J. Am. Chem. Soc. 2019, 141, 342-351. (94) Bürgel, C.; Reilly, N. M.; Johnson, G. E.; Mitrić, R.; Kimble, M. L.; Castleman, Jr., A. W.; Bonačić-Koutecký, V. Influence of Charge State on the Mechanism of CO Oxidation on Gold Clusters. J. Am. Chem. Soc. 2008, 130, 1694-1698. (95) Gao, G.; Wei, S.; Duan, X.; Pan, X. Influence of Charge State on Catalytic Properties of PtAu(CO)n in Reduction of SO2 by CO. Chem. Phys. Lett. 2015, 625, 128-131. (96) Gao, Y.; Zhang, L. M.; Kong, C. C.; Yang, Z. M.; Chen, Y. M. NO Adsorption and Dissociation on Palladium Clusters: the Importance of Charged State and Metal Doping. Chem. Phys. Lett. 2016, 658, 7-11. (97) Wang, Z. C.; Yin, S.; Bernstein, E. R. Gas-Phase Neutral Binary Oxide Clusters: Distribution, Structure, and Reactivity toward CO. J. Phys. Chem. Lett. 2012, 3, 2415-2419. (98) Wang, Z. C.; Yin, S.; Bernstein, E. R. Generation and Reactivity of Putative Support Systems, Ce-Al Neutral Binary Oxide Nanoclusters: CO Oxidation and C–H Bond Activation. J. Chem. Phys. 2013, 139, 194313. (99) Chen, J. J.; Yuan, Z.; Li, X. N.; He, S. G. A VUV Photoionization Time-of-Flight Mass Spectrometer for the Formation, Distribution, and Reaction of Nano-Sized Neutral Metal Oxide Clusters. Int. J. Mass Spectrom. 2017, 422, 98-104. (100)

Li, X. N.; Yuan, Z.; He, S. G. CO Oxidation Promoted by Gold Atoms Supported on Titanium Oxide

Cluster Anions. J. Am. Chem. Soc. 2014, 136, 3617-3623. (101)

Wang, L. N.; Li, Z. Y.; Liu, Q. Y.; Meng, J. H.; He, S. G.; Ma, T. M. CO Oxidation Promoted by the

Gold Dimer in Au2VO3− and Au2VO4− Clusters. Angew. Chem. Int. Ed. 2015, 54, 11720-11724. (102)

Li, X. N.; Li, Z. Y.; Li, H. F.; He, S. G. Multiple CO Oxidation Promoted by Au2 Dimers in Au2TiO4−

Cluster Anions. Chem. Eur. J. 2016, 22, 9024-9029. (103)

Zhang, Z.; Wu, H.; Yu, Z.; Song, R.; Qian, K.; Chen, X.; Tian, J.; Zhang, W.; Huang, W. Site-Resolved

Cu2O Catalysis in the Oxidation of CO. Angew. Chem. Int. Ed. 2019, 58, 4276-4280. (104)

Abdel-Mageed, A. M.; Rungtaweevoranit, B.; Parlinska-Wojtan, M.; Pei, X.; Yaghi, O. M.; Behm, R. J.

Highly Active and Stable Single-Atom Cu Catalysts Supported by A Metal–Organic Framework. J. Am. Chem. Soc. 2019, 141, 5201-5210. (105)

Hu, S.; Xiao, W.; Yang, W.; Yang, J.; Fang, Y.; Xiong, J.; Luo, Z.; Deng, H.; Guo, Y.; Zhang, L.; et al.

Molecular O2 Activation over Cu(I)-Mediated C≡N Bond for Low-Temperature CO Oxidation. ACS Appl. Mater. Interfaces 2018, 10, 17167-17174. (106)

Bu, Y.; Niemantsverdriet, J. W. H.; Fredriksson, H. O. A. Cu Model Catalyst Dynamics and CO

Oxidation Kinetics Studied by Simultaneous in Situ UV–Vis and Mass Spectroscopy. ACS Catal. 2016, 6, 2867-2876. (107)

Eren, B.; Heine, C.; Bluhm, H.; Somorjai, G. A.; Salmeron, M. Catalyst Chemical State during CO

Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy and Near Edge X-ray Adsorption Fine Structure Spectroscopy. J. Am. Chem. Soc. 2015, 137, 11186-11190. 24 / 36


Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(108)

Bhagi-Damodaran, A.; Michael, M. A.; Zhu, Q.; Reed, J.; Sandoval, B. A.; Mirts, E. N.; Chakraborty,

S.; Moënne-Loccoz, P.; Zhang, Y.; Lu, Y. Why Copper is Preferred over Iron for Oxygen Activation and Reduction in Haem-Copper Oxidases. Nat. Chem. 2017, 9, 257-263. (109)

Zhang, Z.; Wang, S. S.; Song, R.; Cao, T.; Luo, L.; Chen, X.; Gao, Y.; Lu, J.; Li, W. X.; Huang, W. The

Most Active Cu Facet for Low-Temperature Water Gas Shift Reaction. Nat. Commun. 2017, 8, 488. (110)

Qu, Y.; Li, Z.; Chen, W.; Lin, Y.; Yuan, T.; Yang, Z.; Zhao, C.; Wang, J.; Zhao, C.; Wang, X.; et al.

Direct Transformation of Bulk Copper into Copper Single Sites via Emitting and Trapping of Atoms. Nat. Catal. 2018, 1, 781-786. (111)

Eren, B.; Torres, D.; Karslıoğlu, O.; Liu, Z.; Wu, C. H.; Stacchiola, D.; Bluhm, H.; Somorjai, G. A.;

Salmeron, M. Structure of Copper–Cobalt Surface Alloys in Equilibrium with Carbon Monoxide Gas. J. Am. Chem. Soc. 2018, 140, 6575-6581. (112)

Wu, C. H.; Liu, C.; Su, D.; Xin, H. L.; Fang, H. T.; Eren, B.; Zhang, S.; Murray, C. B.; Salmeron, M. B.

Bimetallic Synergy in Cobalt–Palladium Nanocatalysts for CO Oxidation. Nat. Catal. 2019, 2, 78-85. (113)

Olmos-Asar, J. A.; Monachino, E.; Dri, C.; Peronio, A.; Africh, C.; Lacovig, P.; Comelli, G.;

Baldereschi, A.; Peressi, M.; Vesselli, E. CO on Supported Cu Nanoclusters: Coverage and Finite Size Contributions to the Formation of Carbide via the Boudouard Process. ACS Catal. 2015, 5, 2719-2726. (114)

Wang, J.; McEntee, M.; Tang, W.; Neurock, M.; Baddorf, A. P.; Maksymovych, P.; Yates, Jr. J. T.

Formation, Migration, and Reactivity of Au−CO Complexes on Gold Surfaces. J. Am. Chem. Soc. 2016, 138, 1518-1526. (115)

He, Y.; Liu, J. C.; Luo, L.; Wang, Y. G.; Zhu, J.; Du, Y.; Li, J.; Mao, S. X.; Wang, C. Size-Dependent

Dynamic Structures of Supported Gold Nanoparticles in CO Oxidation Reaction Condition. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 7700-7705. (116)

Lang, S. M.; Bernhardt, T. M.; Krstić, M.; Bonačić-Koutecký, V. The Origin of the Selectivity and

Activity of Ruthenium-Cluster Catalysts for Fuel-Cell Feed-Gas Purification: A Gas-Phase Approach. Angew. Chem. Int. Ed. 2014, 53, 5467-5471. (117)

Liu, Q. Y.; Ma, J. B.; Li, Z. Y.; Zhao, C.; Ning, C. G.; Chen, H.; He, S. G. Activation of Methane

Promoted by Adsorption of CO on Mo2C2− Cluster Anions. Angew. Chem. Int. Ed. 2016, 55, 5760-5764. (118)

Holm, R. H. Metal-Centered Oxygen Atom Transfer Reactions. Chem. Rev. 1987, 87, 1401-1449.

(119)

Elias, J. S.; Artrith, N.; Bugnet, M.; Giordano, L.; Botton, G. A.; Kolpak, A. M.; Shao-Horn, Y.

Elucidating the Nature of the Active Phase in Copper/Ceria Catalysts for CO Oxidation. ACS Catal. 2016, 6, 1675-1679. (120)

Wang, F.; Büchel, R.; Savitsky, A.; Zalibera, M.; Widmann, D.; Pratsinis, S. E.; Lubitz, W.; Schüth, F.

In Situ EPR Study of the Redox Properties of CuO–CeO2 Catalysts for Preferential CO Oxidation (PROX). ACS Catal. 2016, 6, 3520-3530. (121)

Rodriguez, J. A.; Grinter, D. C.; Liu, Z.; Palomino, R. M.; Senanayake, S. D. Ceria-Based Model

Catalysts: Fundamental Studies on the Importance of the Metal-Ceria Interface in CO Oxidation, the Water-Gas Shift, CO2 Hydrogenation, and Methane and Alcohol Reforming. Chem. Soc. Rev. 2017, 46, 1824-1841. 25 / 36



(122)

Page 26 of 36

Chen, A.; Yu, X.; Zhou, Y.; Miao, S.; Li, Y.; Kuld, S.; Sehested, J.; Liu, J.; Aoki, T.; Hong, S.; et al.

Structure of the Catalytically Active Copper–Ceria Interfacial Perimeter. Nat. Catal. 2019, 2, 334-341. (123)

Heine, C.; Hävecker, M.; Stotz, E.; Rosowski, F.; Knop-Gericke, A.; Trunschke, A.; Eichelbaum, M.;

Schlögl, R. Ambient-Pressure Soft X-ray Absorption Spectroscopy of A Catalyst Surface in Action: Closing the Pressure Gap in the Selective n-Butane Oxidation over Vanadyl Pyrophosphate. J. Phys. Chem. C 2014, 118, 20405-20412. (124)

Wu, C. H.; Weatherup, R. S.; Salmeron, M. B. Probing Electrode/Electrolyte Interfaces in situ by X-ray

Spectroscopies: Old Methods, New Tricks. Phys. Chem. Chem. Phys. 2015, 17, 30229-30239. (125)

Yao, S.; Zhang, X.; Zhou, W.; Gao, R.; Xu, W.; Ye, Y.; Lin, L.; Wen, X.; Liu, P.; Chen, B.; et al.

Atomic-Layered Au Clusters on α-MoC as Catalysts for the Low-Temperature Water-Gas Shift Reaction. Science 2017, 357, 389-393. (126)

Ding, K.; Gulec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G. D.; Marks, L. D.; Stair, P. C.

Identification of Active Sites in CO Oxidation and Water-Gas Shift Over Supported Pt Catalysts. Science 2015, 350, 189-192. (127)

Rodriguez, J. A.; Senanayake, S. D.; Stacchiola, D.; Liu, P.; Hrbek, J. The Activation of Gold and the

Water–Gas Shift Reaction: Insights from Studies with Model Catalysts. Acc. Chem. Res. 2014, 47, 773-782. (128)

Wang, L. N.; Chen, J. J.; Li, X. N.; Liu, Y. Z.; He, S. G. Doping Effects on the Reactivity of the MVO5−

(M = V−Zn) Clusters in CO Oxidation Reaction. J. Phys. Chem. C 2019, 123, 14180-14186. (129)

Butler, K. T.; Davies, D. W.; Cartwright, H.; Isayev, O.; Walsh, A. Machine Learning for Molecular and

Materials Science. Nature 2018, 559, 547-555. (130)

Gómez-Bombarelli, R.; Aguilera-Iparraguirre, J.; Hirzel, T. D.; Duvenaud, D.; Maclaurin, D.;

Blood-Forsythe, M. A.; Chae, H. S.; Einzinger, M.; Ha, D.-G.; Wu, T.; et al. Design of Efficient Molecular Organic Light-Emitting Diodes by A High-Throughput Virtual Screening and Experimental Approach. Nat. Mater. 2016, 15, 1120-1127. (131)

Bartók, A. P.; Payne, M. C.; Kondor, R.; Csányi, G. Gaussian Approximation Potentials: the Accuracy

of Quantum Mechanics, without the Electrons. Phys. Rev. Lett. 2010, 104, 136403. (132)

Raccuglia, P.; Elbert, K. C.; Adler, P. D. F.; Falk, C.; Wenny, M. B.; Mollo, A.; Zeller, M.; Friedler, S.

A.; Schrier, J.; Norquist, A. J. Machine-Learning-Assisted Materials Discovery using Failed Experiments. Nature 2016, 533, 73-77. (133)

Brümmer, M.; Kaposta, C.; Santambrogio, G.; Asmis, K. R. Formation and Photodepletion of Cluster

Ion-Messenger Atom Complexes in A Cold Ion Trap: Infrared Spectroscopy of VO+, VO2+, and VO3+. J. Chem. Phys. 2003, 119, 12700-12703.

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Figures

Figure 1. a) The identified cycle for catalytic CO oxidation by O2 mediated with AuAl3O3-5+. The density functional theory (DFT) calculated structures for the AuAl3O3-5+ clusters are shown. b) The identified cycle for catalytic CO oxidation by O2 mediated with PtAl3O5-7−. The DFT calculated structures for the PtAl3O5-7− clusters are shown. c) A schematic diagram for Electronegativity-Ladder effect. Reprinted with permission from refs 53 and 54. Copyrights 2014 and 2015 American Chemical Society.

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Figure 2. The time-of-flight (TOF) mass spectra for the elementary reactions of the neutral AuxTiyOz clusters (a) with CO (b), the gas mixture of CO/N2 (c) and CO/O2 (d), and O2 (e). The reaction gas pressures are shown in Pa and the reaction time is about 60 μs. The AuxTiyOz and AuxTiyOzCO species are denoted as x,y,z and x,y,zCO, respectively. The peaks marked with *, O, and ∆ are Ti4O8CO, AuTi2O5CO, and AuTi2O4(CO)2, respectively. Reprinted with permission from ref 55. Copyright 2019 American Chemical Society.

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Figure 3. The DFT calculated potential energy profiles for CO oxidation by AuTi2O6 (a) and by product AuTi2O5 (b), and O2 activation by AuTi2O4 (c). The bond lengths and the relative energies are in units of pm and eV, respectively. The structures of AuTi2O6, AuTi2O5, AuTi2O4, and reaction intermediates (I1−I15) are shown. Reprinted with permission from ref 55. Copyright 2019 American Chemical Society.

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Figure 4. The DFT calculated natural charges on the gold atoms in relate species during the catalysis shown in Figure 3. Reprinted with permission from ref 55. Copyright 2019 American Chemical Society.

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Figure 5. Electronegativity for metal atoms.

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Figure 6. The TOF mass spectra for the interactions of mass-selected Ni2VO5− (a), Ni2VO4− (c), Ni2VO4CO− (e), and Ni2VO3CO− (g) with CO or O2. The reaction time is about 1.7 ms. The NixVyOz− and NixVyOzCO− species are denoted as x,y,z and x,y,z,CO, respectively. Peaks marked by the asterisk are from the water impurity. Reprinted with permission from ref 59. Copyright 2019 American Chemical Society.

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Figure 7. a) The DFT calculated lowest-energy structures for the Ni2VO5−, Ni2VO4−, Ni2VO4CO− and Ni2VO3CO− species and b) the potential energy profiles for elementary reactions 2 and 3. Superscripts represent spin multiplicities. c) The natural charge on related moieties along the pathways shown in Figure 7b is shown. Reprinted with permission from ref 59. Copyright 2019 American Chemical Society.

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Figure 8. Identified mechanisms for catalytic CO oxidation by O2 over atomic clusters. Adapted with permission from ref 59. Copyright 2019 American Chemical Society.

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TOC Graphic

Xiao-Na Li

Xiao-Na Li received her BS degree in chemistry from Jilin Normal University in 2004, her MS degree in chemistry from Jilin University in 2007, and her PhD degree in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2010. She joined the Institute of Chemistry, Chinese Academy of Science in January 2011. Her research interests are experimental and theoretical studies on the reactions of metal oxide clusters in catalytic CO oxidation.

Li-Na Wang

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Li-Na Wang received her BS degree in chemistry from Hefei Normal University in 2013, her MS degree in chemistry from South China University of Technology in 2016, and her PhD degree in the Institute of Chemistry, Chinese Academy of Sciences in 2019. Her research interests are experimental and theoretical investigations on the reactions of noble metal free metal oxide clusters in catalytic CO oxidation.

Li-Hui Mou

Li-Hui Mou received her BS degree in chemistry from Beijing Technology and Business University in 2017. She is now a PhD student at the Institute of Chemistry, Chinese Academy of Sciences.

Sheng-Gui He

Sheng-Gui He received his BS degree in physics and PhD degree in chemistry from the University of Science and Technology of China in 1997 and 2002, respectively. After postdoctoral stays at the University of Kentucky (2002–2005) with Prof. Dennis J. Clouthier and at Colorado State University (2005– 2007) with Prof. Elliot R. Bernstein, he joined ICCAS in January 2007. His research interests are focused on experimental and theoretical studies of reactive intermediates including free radicals and atomic clusters.

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Catalytic CO Oxidation by Gas-Phase Metal Oxide Clusters - American

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