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Nano Research 1 NanoDOI 10.1007/s12274Res -015-0873-0

Hydrogenation of molecular to hydroperoxyl:

An alternative pathway for O2 activation on nanogold catalysts

Chun-Ran Chang1,2 (), Zheng-Qing, Huang1, and Jun Li2 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0873-0 http://www.thenanoresearch.com on August 4, 2015

© Tsinghua University Press 2015

Just Accepted

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TABLE OF CONTENTS (TOC)

Hydrogenation of Molecular Oxygen to Hydroperoxyl:

An Alternative Pathway for O2 Activation on Nanogold Catalysts

Chun-Ran Chang1,2*, Zheng-Qing, Huang1, and Jun Li2*

1 School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China 2 Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

We report that molecular oxygen (dioxygen) can be feasibly activated through a hydroperoxyl (OOH) species by abstracting a atom from H-containing coadsorbates on Au nanoparticles. The formed OOH oxidant either directly undergoes oxidation reactions through the end-on oxygen atom or dissociates into atomic oxygen and hydroxyl for further oxidation.

Nano Research

DOI (automatically inserted by the publisher) Research Article

Hydrogenation of Molecular Oxygen to Hydroperoxyl:

An Alternative Pathway for O2 Activation on Nanogold Catalysts

Chun-Ran Chang1,2 (), Zheng-Qing, Huang1, and Jun Li2 ()

Received: day month year ABSTRACT

Revised: day month year Activation of molecular O2 is the most critical step in the gold-catalyzed Accepted: day month year oxidation reactions and the underlying mechanisms remain under debate. In (automatically inserted by this work, we reveal an alternative pathway of O2 activation with the assistance the publisher) of H-containing substrates using density functional theory (DFT). It is demonstrated that the coadsorbed H-containing substrates (R–H) can not only © Tsinghua University Press enhance the adsorption of O2 but also transfer a hydrogen atom to the adjacent and Springer-Verlag Berlin O2, leading to the O2 activation through a hydroperoxyl (OOH) radical species. Heidelberg 2015 The activation barriers of the H-transfer from 16 selected R-H compounds (H2O, CH3OH, NH2CHCOOH, CH3CH=CH2, (CH3)2SiH2, etc) to the coadsorbed O2 are KEYWORDS lower than 0.50 eV in most cases, indicative of the feasibility of the activation of O2 via OOH under mild conditions. The formed OOH oxidant, with an O2 activation, gold cluster, increased O–O bond length of ~1.45 Å , either directly undertakes oxidation adsorption, dissociation, reactions through the end-on oxygen atom or dissociates into atomic oxygen hydroperoxyl and hydroxyl (OH) by crossing a fairly low barrier of 0.24 eV. Using CO oxidation as a probe, we find that OOH has superior activity than activated O2 and atomic oxygen. This study uncovers a new pathway for the activation of O2 and may provide insights for understanding the oxidation catalysis of nanosized gold.

three decades, gold nanoparticles and subnanometer 1 Introduction particles have been successfully applied to a wide range of reactions, including low-temperature CO While bulk gold is chemically inert, the unique oxidation [3], epoxidation [4, 5], selective catalytic properties of gold nanoparticles (NPs) have hydrogenation and oxidation [6-8], -gas shift [9, attracted extensive attention since the pioneering 10]. Among them, the utilization of gold in oxidation works by Haruta [1] and Hutchings [2]. In the past reactions has aroused particular interest because gold

Address correspondence to Chun-Ran Chang, [email protected]; Jun Li, [email protected]

2 Nano Res. is capable of catalyzing a variety of oxidation even-odd alternation correlates well to the trend in reactions under extremely mild conditions by use of the electron affinities of gold clusters [33], suggesting molecular oxygen as a feedstock and often with a the electron transfer from anionic cluster to O2 might high selectivity towards desired products [11]. be the primary reason for the activation of O2. This Despite numerous experimental and theoretical picture was evidenced by anion photoelectron studies dedicated to the nature of the catalytic (PES) and infrared multiple photon oxidation on gold, the underlying mechanism dissociation (IR-MPD) studies [34, 35]. Specifically, remains unclear. Even for the simplest CO oxidation Zeng and coworkers revealed that molecular O2 can reaction, the active site and the exact mechanism are be activated to superoxo- or peroxo-state by small still much debated. A main controversial issue is how even-sized Aun– (n = 2 – 18) clusters and the two molecular O2 is activated on nanogold catalysts. states can be converted from one to the other on Au8–, Unlike other transition-metal catalysts, molecular which further confirms the electron transfer O2 neither adsorbs nor dissociates on bulk gold occurring between anion clusters and adsorbed O2 surface. Therefore, the activation of O2 is often the [36]. Because the high activity of nanogold catalysts rate-determining step in nanogold catalysis. For might involve cationic gold [37-39], thus the oxide-supported gold catalysts, it is generally interaction between O2 and cationic gold clusters was accepted that O2 is activated at the interface between also studied in literature. Yoon et al theoretically Au nanoparticles and supports [12-14] or at the reported that positively charged Au clusters can bind oxygen defects of reducible oxides [15, 16]. The O2 strongly, with a binding energy of 0.46 eV for hydroxyls produced on oxide supports were also Au6O2+, albeit no activation of the O–O bond [40]. shown to have substantial effects on the activation of Similar conclusion was also drawn by Ding et al O2 [16, 17]. Another role of oxide supports involves in using hybrid functional DFT calculations [32]. modifying the catalytic behavior of Au clusters by Nevertheless, a recent joint experimental and adding [18-22] or removing electrons [9, 23-27]. theoretical work by Woodham et al claim that However, it is difficult to identify which charge or cationic gold clusters are capable of activating O2 to valance state of Au is mainly responsible for the moieties when multiple oxygen ligands catalytic performance. Very recently, Wang et al are complexed with Au clusters [41]. presents an interesting picture regarding the charge Compared to the charged gold clusters, little is transfer in the whole CO oxidation process using ab known about the interaction between neutral gold initio molecular dynamics (AIMD) simulations[28]. clusters and molecular O2 in part due to the lack of They have demonstrated that the charge state of the direct experimental detection for uncharged species. supported Au cluster is dynamically changing Therefore, theoretical studies are needed to provide during the catalytic cycle, where the data for assessing such interaction. Hybrid DFT charging/discharging of Au cluster not only controls calculations show that an oscillation behavior also the amount of O2 adsorbed at the cluster/oxide exists in the interaction between small neutral Aun (n interface but also strongly influences the energetic of = 1–6) clusters and O2 [32]. The binding energies of O2 all the redox steps. with odd-sized Au3 (0.25 eV) and Au5 (0.64 eV) are To gain an explicit understanding on the much higher that that with even-sized Au2, 4, 6 clusters interaction between Au NPs and O2, one simple (< 0.1 eV) [32]. This phenomenon was confirmed by approach is to address this issue on bare gold high-level ab initio coupled-cluster (CC) calculations clusters. It has been shown that small-sized anionic [42] and IR-MPD spectra [43]. For larger neutral Au gold clusters with even-numbered atoms are reactive clusters dissociative adsorption of O2 is predicted to with O2, whereas anionic clusters with old-numbered be more favorable than molecular adsorption, but the atoms are inert toward O2 because of lack of dissociation barriers are expected to be ~1 eV or low-lying unpaired electrons [29-31]. The binding higher [40, 44]. Barrio et al. employed Au14, Au25, and energies of O2 with even-sized Aun– (n = 2, 4, 6) Au29 as model systems to show the importance of anions are calculated to be higher than 0.7 eV using unsaturated sites for O2 adsorption [45]. Roldán et al hybrid functional DFT calculations [32]. The studied the activation of O2 on a series of neutral

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gold clusters Aun (n = 5 – 79) and demonstrated that computational model because it has all the necessary Au38 is the critical size for O2 adsorption and sites and surfaces of a FCC crystal. In addition, size dissociation [46, 47]. 38 is a “magic” number for cubo-octahedral structure Although extensive works have been done to study and is often taken as the representative of large-sized the interaction between O2 and Au clusters, an clusters [59-64]. Au38 (~1 nm) possesses a high overall understanding on the activation of O2 is still symmetry of Oh and exposes both (111) and (100) elusive. Actually, the presence of coadsorbates, facets and is identified as a critical particle for the including the reactant itself may have a activation of molecular oxygen [46, 47]. During substantial effect on the activation of O2. For example, geometry optimizations, the whole cluster together the dissociation of O2 can be dramatically promoted with the adsorbate(s) was allowed to relax. The by the coadsorbed C2H4, CO, H2O, and atomic convergences of energy, gradient, and maximum oxygen [48-51]. Our previous studies have also displacement were set to 10–5 hartree, 810–4 shown that a coadsorbed water or molecule hartree/Å , and 510–3 Å , respectively. The adsorption not only favors the adsorption of O2 but also transfers energy Ead of an adsorbate was determined from Ead = a hydrogen atom to adjacent O2 [52, 53], implying Eads/cluster – (Eads + Ecluster), where Eads/cluster is the total that the hydrogenation of O2 to OOH might be a new energy of the Au38 covered with the adsorbate, Eads pathway for the activation of O2. To further elucidate the total energy of the adsorbate in the gas phase, this OOH pathway for O2 activation, here we carry and Ecluster the total energy of the bare Au38 cluster. out a systematic study on the activation of O2 by The coadsorption energy Ecoad of two adsorbates was various H-containing substrates, including water, determined from Ecoad = Ecoads/cluster – (Eads1 + Eads2 + alcohols, organic acids, amines, and . It is Ecluster), where Ecoads/cluster is the total energy of the Au38 shown that O2 is capable of abstracting a hydrogen covered with the two adsorbates, Eads1 and Eads2 the atom from most of the selected H-containing total energy of the first adsorbate and the second substrates with unexpectedly low barriers. adsorbate in the gas phase, respectively. With these Importantly, in the probe CO oxidation reaction definitions, a negative value of Ead or Ecoad means a OOH exhibits superior reactivity than other activated release of energy or a stable adsorption on the cluster oxygen species. This study uncovers an alternative following the thermodynamic convention. pathway for O2 activation on gold clusters and nanoparticles.

2 Computational details

All the calculations were performed using DMol3 code of the Material Studio package [54, 55]. The electron exchange and correlation were treated within generalized-gradient approximation (GGA) in the form of PBE functional [56]. The localized double-numerical quality basis set with polarization functions (DNP) was used. The core electrons of Figure 1 Geometry of Au38 cluster. metal atoms were described using effective core potentials (ECP) developed by Berger et al [57], in The catalytic mechanisms were explored with the which the mass-velocity and Darwin relativistic calculations of transition states (TS) and corrections were introduced. A thermal smearing of intermediates, where the TSs were determined by 0.002 hartree and a real-space cutoff of 4.5 Å were using complete LST/QST (linear synchronous transit applied in our calculations. and quadratic synchronous transit) approach[65] and a mode-eigenvector following (MEF) method. [66] All A neutral Au38 cluster (Figure 1) with the transition states were confirmed to possess only cubo-octahedral shape, albeit not the global one imaginary frequency and the corresponding minimum structure [58], was selected as the

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vibration mode was verified to indeed connect the adsorption energy of O2 is calculated to be –0.32 eV, reactant and product. The activation barrier Ea is suggesting a weak interaction with gold cluster. defined as the energy difference between the TS and However, the O–O bond length, 1.31 Å , is already the initial state (IS). The reaction energy ΔE is the elongated by 0.11 Å with respect to that of gas-phase energy difference between the final state (FS) and the O2, indicative of a partial activation of adsorbed O2. IS. Therefore, a negative value of ΔE means a As the activation of O2 can be sensitively probed thermodynamically favorable process. using spectroscopy, herein we calculated the vibrational frequencies of adsorbed O2 on Au38. Experimentally, free O2 has a stretching frequency of 3 Results and discussion 1556 cm-1, while the electron transfer into the π* orbital of O2 lowers the frequency to 1074 cm-1 for a 3.1 Adsorption and dissociation of O2 on Au38 superoxo (O2–), or 866 cm-1 for a peroxo (O22–) species

Before exploring new pathways for O2 activation, we [34]. Therefore, the calculated O–O stretching -1 1 1 first discuss the adsorption of O2 on Au38 cluster. As frequency (1090 cm ) in η μ mode together with the previously reported [46, 47], molecular O2 negative charge (–0.25 e) accumulated on adsorbed preferentially adsorbs at the low-coordinated edge O2 suggest that the molecular O2 is activated to a sites shared by (111) and (100) facets on Au38. superoxo-like species. Moreover, the O–O bond Therefore, we considered three possible modes for length (1.31 Å ) in η1μ1 mode is close to that in the adsorption of O2, i.e., η1μ1, η2μ2, η2μ4, which metal-superoxo complexes in the range of 1.25–1.35 involve one, two, and four Au–O bond(s), Å [67]. respectively, as shown in Table 1. For η1μ1, the

Table 1 Calculated characteristics of O2 adsorption on Au38 cluster

a b Adsorption Optimized Ead(O2) d(Au–O) d(O–O) ν(O–O) q(O2) mode geometry /eV /Å /Å /cm-1 /e

η1μ1 –0.32 2.23 1.31 1090 –0.25

2.16 η2μ2 –0.56 1.34 971 –0.36 2.17

2.34 2.34 η2μ4 –0.61 1.42 733 –0.50 2.35 2.35 a b Stretching frequency of adsorbed O2. Mulliken charge of adsorbed O2.

Compared to η1μ1 mode, the adsorption of O2 in and the decreased O–O stretching frequency (971 η2μ2 mode is slightly improved in light of the cm-1). These features are still close to those of increased adsorption energy (–0.56 eV), the longer superoxo, thus the activated O2 in η2μ2 mode can also O–O bond (1.34 Å ), the more negative q(O2) (–0.36 e), be classified as a superoxo-like species. While for the

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η2μ4 mode, the adsorption of O2 is further enhanced oxygen atoms all bind strongly at the three-fold with an adsorption energy of –0.61 eV. Each oxygen hollow sites of gold, which explains why the atom of O2 is bridged over two Au atoms. In dissociative adsorption of O2 is energetically particular, the O–O bond is notably elongated by 0.21 favorable on some larger gold clusters [40]. Å with respect to that of free O2, close to the typical value (1.40–1.50 Å ) of metal-peroxo complexes [67]. Therefore, from η1μ1 to η2μ4 the adsorbed O2 experienced a transition from superoxo- to peroxo-adsorption, similar to the episode on anion Au8– cluster [36]. The η2μ2 adsorption can be regarded as an intermediate for this transition. From the abovementioned results, it is clear that molecular O2 can be partially activated through chemisorption on gold clusters when it becomes feasible to transfer electron(s) to triplet O2. In appearance, the degree of the O2 activation is strongly dependent on the adsorption modes, but actually the amount of electrons transferred from gold cluster to O2 is the underlying determining factor. As shown in Table 1, the more electrons accumulated on adsorbed O2, the longer O–O bond length and the lower O–O stretching frequency will Figure 2 Energy profiles for the dissociation of O2 on Au38. 2 be, indicative of the better activation of O . Moreover, The zero energy level refers to the total energy of bare Au38 and as the negative charge on O2 increases, the gas-phase O2. adsorption of O2 on gold cluster becomes more stable. 3.2 Hydrogenation of O2 to OOH on Au38 These results well support the viewpoint that the electronic structure is the link between the physical While the dissociation of O2 is feasible on Au38 cluster, structure of a material and the functionality [68]. it becomes more difficult on larger or smaller gold clusters due to the high barriers [40, 47]. Therefore, in Although O2 can be partially activated via this section we discuss a new pathway for the adsorption, the dissociation of O2 into atomic oxygen activation of O2 by hydrogen-abstraction from is usually considered as the complete pathway for O2 H-containing substrates (R–H) that avoids direct O2 activation. Figure 2 depicts the energy profile for the dissociation. The R–H substrates are selected from dissociation of O2 on Au38 upon the η2μ2 and η2μ4 the most commonly used solvents or reactants that modes. We find that the direct dissociation of O2 may involve in oxidation reactions, including water, from η2μ2 mode needs to overcome a high barrier of alcohols, amines, amino acids, and . 2.06 eV, although the thermodynamics is favored by Table 2 lists the activation barriers (Ea) and reaction 0.66 eV. However, the dissociation of O2 from η2μ4 energies (ΔE) for O2 reacting with 16 selected R–H mode is more feasible with an activation barrier of substrates. Figure 3 displays the optimized structures 0.64 eV and an energy release of –0.62 eV, comparable of the initial states (IS), transition states (TS) and final with previous study [46, 47]. The large deviation states (FS) of the 16 reactions. between the two barriers can be ascribed to the varied structure of the transition states. In TS1, two For a catalytic reaction involving two or more nearly separated O atoms adsorb at the top sites of reactants, trapping the reactants within a suitable Au, which is extremely unstable due to the electron region is always a necessary step. After searching for unsaturation. While in TS2, each O atom locates at a several possible adsorption sites, we find that O2 and bridge site of two Au atoms, where the O–O bond R–H prefer to coadsorbed on the low-coordinated cleavage can be effectively compensated by Au–O (100) facet of Au38, as shown in the IS structures of interactions. For the two pathways, the dissociated Figure 3. Importantly, the two neighboring Au atoms

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on (100) facet distanced by ~3 Å are capable of coadsorption energy of O2 and C6H5NH2, –1.25 eV, is trapping O2 and R-H within a reactive region. Our 0.31 eV (in absolute value) higher than that of calculation results reveal that the coadsorption of O2 separated adsorptions. Such cooperative effect in and R–H exhibits a cooperative effect in most cases. coadsorption is ascribed to the hydrogen bonding For example, the coadsorption energy of O2 and H2O interaction between R–H and O2, which is more is calculated to be –0.98 eV, which is 0.40 eV (in notable for substrates involving O–H and N–H bonds absolute value) higher than the sum of the separated than the ones involving C–H and Si–H bonds, as adsorption energy of H2O and O2, indicating that the revealed by the calculation results in Table 2. coadsorption is cooperative but not competitive. The

Table 2 Calculated coadsorption energies (Ecoad) of H-containing substrates (R-H) and O2, the activation barriers (Ea), and reaction * * * * energies (ΔE) of O2 + R-H → OOH + R on Au38

a R-H Reactions Ecoad / eV Ea / eV ΔE / eV

H-abstraction from O-H bond * * * * 1 water O2 + H2O → OOH + OH –0.98 0.26 0.19 * * * * 2 methanol O2 + CH3OH → OOH + CH3O –1.03 0.29 0.23 * * * * 3 phenol O2 + C6H5OH → OOH + C6H5O –0.95 0.21 –0.01 * * * * 4 O2 + HCOOH → OOH + HCOO –0.79 0.25 –0.13 * * * * 5 O2 + NH2CHCOOH → OOH + NH2CHCOO –0.87 0.22 –0.11

H-abstraction from N-H bond * * * * 6 glycine O2 + NH2CHCOOH → OOH + NHCHCOOH –1.19 0.22 0.21 * * * * 7 phenylamine O2 + C6H5NH2 → OOH + C6H5NH –1.25 0.32 0.25

H-abstraction from C-H bond * * * * 8 O2 + CH4 → OOH + CH3 –0.43 1.03 –0.24 * * * * 9 O2 + C2H4 → OOH + C2H3 –0.85 0.68 –0.04 * * * * 10 O2 + C2H2 → OOH + C2H –0.87 0.19 –0.54 * * * * 11 O2 + CH3CHO → OOH + CH2CHO –0.52 0.20 0.07 * * * * 12 O2 + CH3COCH3 → OOH + CH2COCH3 –0.54 0.29 0.20 * * * * 13 propylene O2 + CH3CH=CH2 → OOH + CH2CH=CH2 –0.80 0.28 0.11 * * * * 14 ethylbenzene O2 + C6H5CH2CH3 → OOH + C6H5CHCH3 –0.52 0.42 0.02

H-abstraction from Si-H bond * * * * 15 O2 + SiH4 → OOH + SiH3 –0.39 0.31 –1.21 * * * * 16 dimethylsilane O2 + (CH3)2SiH2 → OOH + (CH3)2SiH –0.51 0.06 –1.40 a An asterisk (*) represents the adsorbed state.

In the coadsorbed structures, O2 is only slightly investigated thermodynamically and kinetically. activated in light of the O–O bond length being Depending on the origin of the H atoms, these around 1.30 Å , thus further activation is needed. reactions are classified into four groups, i.e., Subsequently, the reactions between R–H and O2 are H-abstraction from O–H bond, N–H bond, C–H bond,

| www.editorialmanager.com/nare/default.asp Nano Res. 7 and Si–H bond. In the first group, water is selected as energy is calculated to be 0.32 eV and 0.25 eV, a special H-containing substance for study due to its respectively. existence in a variety of chemical systems, either as Based on these results, we discuss hydrogen moisture or solvent or reactant, or oxidation product. abstraction by O2 from C–H and Si–H bonds. Since In particular, water was shown to have a promotional are inert as the noble gases in organic effect in a number of chemical reactions [69-77]. chemistry [79], it is not surprising that the H-transfer Calculation results show that O2 can readily abstract from CH4 to O2 has a quite high barrier of 1.03 eV. In a hydrogen atom from coadsorbed H2O to form OOH, fact, CH4 only weakly physisorbs on Au38 via a with a low barrier of 0.26 eV and a reaction energy of hydrogen atom, reflecting the difficulty in activating 0.19 eV. Although this step alone is slightly C–H σ-bond. Compared to CH4, the H-abstraction endothermic, the energy cost can be compensated in from C2H4 and C2H2 are relatively easier, with other elementary steps or at temperatures higher activation barriers of 0.68 eV and 0.19 eV, respectively. than zero Kelvin. A recent joint experimental and Because α-H usually presents high activity in theoretical work by Saavedra et al. confirms the chemical reactions, we hence further inspect the generation of OOH on Au/TiO2 catalyst in the reaction between O2 and the α-H contained in presence of water [78], which is consistent with our acetaldehyde, acetone, propylene, and ethylbenzene. previous OOH mechanism.48 The hydrogen transfer Calculation results show that the α-H-abstraction by from methanol to O2 is analogous to that of H2O, the O2 is facile to take place with activation barriers activation barrier and reaction energy of which are falling between 0.20 eV and 0.42 eV. These results calculated to be 0.29 eV and 0.23 eV, respectively. In indicate that the α-H might serve as an initiator for comparison of the above results with those on bulk the activation of O2 in some systems. Although the Au(111) surface [53], the hydrogenation of O2 to other types of hydrogen atoms, such as β-H, may also OOH by H2O or CH3OH is much easier on Au38, contribute to O2 activation, the activity is much lower which is mainly attributed to the fact that the than that of α-H, thus they are not considered in the low-coordination sites on Au38 can benefit the current work. Finally, the H-abstraction from silane coadsorption of O2 and H2O (or CH3OH) as well as and dimethylsilane are also calculated, which have the H-transfer processes. While compared with our activation barriers of 0.31 and 0.06 eV, and reaction previous results on Au10 subnanometer cluster [52] , energies of –1.21 eV and –1.40 eV, respectively. the hydrogen transfer from H2O to O2 on Au38 is From the results above, one can see that the slightly more difficult, as is expected. In this group of barriers for the activation of O2 to OOH by the 16 selected species, we also investigated the selected R-H substrates are lower than 0.50 eV with H-abstraction reactions from phenol, formic acid, and the exceptions of methane and ethylene. Although glycine, which are shown to be more favorable in the DFT calculations with GGA exchange-correlation thermodynamics and kinetics than the cases of H2O functional might have underestimated the barriers, and CH3OH. The exothermicity of these reactions the comparison with the O2 dissociation pathway suggests that the activation of O2 via OOH is more does suggest that the OOH-activation pathway is favorable in an acidic environment. Of particular more favorable. After hydrogenation, the O–O bond interest is the reaction between O2 and glycine since length of OOH has been elongated to 1.45 Å , nearly O2 can abstract a hydrogen atom either from 20% longer than that of free O2, indicating that carboxyl- or amino-group. The former turns out to be molecular O2 is substantially activated. The formed a little easier than the latter according to the reaction OOH can either serve as an oxidant or dissociate into energies listed in Table 2. The H-abstraction from two oxidizing agents of O and OH for further –COOH or –NH2 can also be expected in other amino oxidation, which will be discussed in the next section. acids, which may provide novel insights into the Theoretically, the activation of O2 by an extra ligand activation of O2 in biosystems under certain has been demonstrated previously in the interaction conditions. One more example for the H-abstraction between O2 and Xe+ via a (p-π*)σ bonding [80]. from N–H bond is the reaction between O2 and Similarly, the activation of O2 by an extra H atom can phenylamine, the activation barrier and reaction be explained by the formation of a (s-π*)σ bonding,

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which leads to the π* anti-bonding MOs of O2 being H2O [70]. These results provide strong support for partially occupied and thus the O–O bond being the activation of O2 along the alternative weakened. Experimentally, the OOH species is OOH-pathway. observed by in situ UV/Vis in the presence of O2 and

Figure 3 Optimized geometries of the initial states (IS), transition states (TS), and final states (FS) of reactions 1-16 listed in Table 2.

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3.3 CO oxidation by activated oxygen species measured at 1.45 Å , much longer than the value, 1.31 To further study how OOH involves in oxidation Å , of solely-adsorbed O2. After crossing a barrier of reactions, we select CO oxidation reaction as a probe 0.64 eV, a CO2 molecule is formed and subsequently for the mechanistic study. Figure 4 (from left to right) desorbs into gas-phase exothermically by 2.35 eV. displays the energy profiles for the oxidation of CO While for the CO oxidation with atomic O, a suitable to CO2 with activated O2, atomic O, and OOH, site for the coadsorption of the two species cannot be respectively. For CO oxidation by activated O2 located because once CO and O approach, a CO2 molecule, CO and O2 favorably coadsorb on two molecule will be generated immediately. Therefore, neighboring Au atoms of Au38 cluster with a the reaction between CO and atomic O might follow coadsorption energy of –0.94 eV. Through the an Eley-Rideal mechanism on Au38, with atomic O coadsorption structure, CO and O2 can move closer adsorbed on gold cluster and CO in the gas phase. As to each other and arrive at an O–O–C–O intermediate, shown in the middle column of Figure 4, the accompanied with an energy release of 0.80 eV. The activation barrier and reaction energy are calculated structure of this intermediate is analogous to that on to be 0.47 eV and –2.62 eV, respectively, suggesting small-sized gold clusters, as reported previously that atomic oxygen is more active than activated O2 [81-83]. The O–O bond length of O–O–C–O is in the oxidation of CO.

Figure 4 Energy profiles for the CO oxidation with activated O2, atomic oxygen, and OOH.

The CO oxidation reaction via OOH is very similar than the original O–O distance in OOH (1.45 Å ). to that by activated O2. CO and OOH also coadsorb Through this coadsorption structure, a CO2 molecule on two neighboring low-coordinated Au atoms of can be readily produced via combination of CO with Au38 with CO adjacent to the end-on oxygen atom of the end-on oxygen of OOH, leaving a hydroxyl being OOH. In the presence of coadsorbed CO, the O–O adsorbed on Au38. The activation barrier and reaction bond of OOH is elongated to 1.47 Å , even longer energy are calculated to be 0.31 eV and –3.99 eV,

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respectively. In comparison of the three energy alternative to the O2 dissociation, the hydrogenation profiles in Figure 4, we find that the CO oxidation by of O2 to OOH by hydrogen-abstraction from OOH is the most favorable to occur. The H-containing substrates (R–H) is found to be a more extraordinary behavior of OOH is attributed to the preferred pathway for facile activation of O2. The much weakened O–O bond in OOH, which causes activation barriers of H-transfer from R–H to O2 are the end-on oxygen atom of OOH being more easily less than 0.50 eV for most of the selected R-H abstracted than that of activated O2 and atomic substrates, implying the activation of O2 via OOH is oxygen. As an alternative to the OOH serving as an feasible on gold nanoparticles at room temperature. oxidative species, OOH can also dissociate into O After hydrogenation, the O–O distance of OOH is and OH by overcoming a relatively low barrier of increased by ~20% with respect to that of gas-phase 0.24 eV, as depicted in Figure 5. Although the O2, indicative of the strong activation of O–O bond. hydroxyl is also found to play an important role in The formed OOH can either directly perform oxidation catalysis by gold nanoparticles [84], we will oxidation reactions or dissociate into strong oxidants not explore this aspect in the current work because of atomic oxygen and hydroxyl. Among the CO oxidation reactions via hydroxyl were discussed in oxidation reactions via activated O2, atomic oxygen our previous work [52, 53]. and OOH, the OOH radical exhibits extraordinary activity compared to the other two oxidative species, with an activation barrier of 0.31 eV and a huge energy release of 3.99 eV. This study thus illustrates an alternative pathway for the activation of molecular O2 and may provide insights for understanding the complicated mechanisms of catalytic oxidation on gold catalysts.

Acknowledgements

This work was supported by the National Key Basic Research Special Foundations (2011CB932400), the China Postdoctoral Science Foundation (2014M562391), and the Fundamental Research Funds for the Central Universities (xjj2014064). The calculations were performed by using supercomputers at the Computer Network Information Center, Chinese Academy of Sciences, Figure 5 Energy profile for the dissociation of OOH into Tsinghua National Laboratory for Information atomic oxygen and hydroxyl. Science and Technology, and the Shanghai Supercomputing Center. 4 Conclusions References

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