Theoretical Study of Plasmon-Enhanced Surface Catalytic Coupling

Yuchao ZhangShuai HeWenxiao GuoYue HuJiawei HuangJustin R. MulcahyWei ... Zhiyang Zhang , Virginia Merk , Anja Hermanns , Wolfgang E. S. Unger , and ...
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Letter pubs.acs.org/JPCL

Theoretical Study of Plasmon-Enhanced Surface Catalytic Coupling Reactions of Aromatic Amines and Nitro Compounds Liu-Bin Zhao,† Meng Zhang,† Yi-Fan Huang,† Christopher T. Williams,‡ De-Yin Wu,*,† Bin Ren,† and Zhong-Qun Tian† †

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China ‡ Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Taking advantage of the unique capacity of surface plasmon resonance, plasmon-enhanced heterogeneous catalysis has recently come into focus as a promising technique for high performance light-energy conversion. This work performs a theoretical study on the reaction mechanism for conversions of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to aromatic azo species, p,p′-dimercaptoazobenzene (DMAB). In the absence of O2 or H2, the plasmon-driven photocatalysis mechanism (hot electron−hole reactions) is the major reaction channel. In the presence of O2 or H2, the plasmon-assisted surface catalysis mechanism (activated oxygen/ hydrogen reactions) is the major reaction channel. The present results show that the coupling reactions of PATP and PNTP strongly depend on the solution pH, the irradiation wavelength, the irradiation power, and the nature of metal substrates as well as the surrounding atmosphere. The present study has drawn a fundamental physical picture for understanding plasmon-enhanced heterogeneous catalysis. SECTION: Spectroscopy, Photochemistry, and Excited States

P

performance light-energy conversion. Despite the importance of plasmon-enhanced photocatalysis/catalysis, the fundamental physical picture is still unclear.2 In this study, typical aromatic compounds p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) are chosen as model molecules. It has previously been demonstrated from surface-enhanced Raman spectroscopy (SERS) studies that both PATP and PNTP adsorbed on plasmonic nanostructures can be selectively converted to an azo species p,p′dimercaptoazobenzene (DMAB).12−18 The reactions are strongly influenced by the solution pH,19−22 the irradiation wavelength,23,24 the irradiation power,25,26 the substrate materials,15,27,28 and the surrounding atmosphere.17,29 However, the reaction mechanism is still debated, and the system is especially lacking in detailed thermodynamic and kinetic data. Theoretical studies on model reactions help to elucidate the reaction mechanism, and thus facilities optimizing experimental conditions and improving reaction yield and selectivity. In this Letter, density functional theory (DFT) calculations have been performed to investigate the surface catalytic coupling reactions of PATP and PNTP. Two theoretical models (plasmon-driven photocatalysis and plasmon-assisted surface catalysis) are proposed to explore the reaction

lasmonic nanostructures are characterized by their excellent absorption of visible light through an excitation of surface plasmon resonance (SPR). This resonance can be described as coherent oscillation of the surface conduction electrons excited by the incident electromagnetic radiation when the excitation frequency matches the characteristic frequency of metal nanostructures. The excitation of SPR brings several significant benefits to photocatalysis and catalysis where the plasmonic nanostructures are employed to enhance the surface chemical reactions.1−3 First, the collective oscillation of valence electrons that resonate with incident light gives rise to a significant enhancement of local electromagnetic field, thus strongly enhancing the absorption and photochemistry at the molecule−metal interface.4,5 Second, charge-transfer photochemistry can occur through metal plasmon excitation. The generation of electron−hole pairs opens unique pathways of photoexcitation and photoreaction.6,7 Third, noble metal nanoparticles, such as gold and silver, exhibit high catalytic performance in many catalyzed oxidation and reduction reactions.8,9 Finally, in the presence of plasmon excitation, photothermal conversion allows for very rapid, localized heating, and provides the activation energy that is required for surface chemical reactions.10,11 Plasmon-enhanced photocatalysis/catalytic reactions have become a very interesting and active direction of research due to their high throughput and low energy requirement. Taking the advantage of SPR, plasmonic photocatalysis has recently come into focus as a promising technique for high © 2014 American Chemical Society

Received: February 16, 2014 Accepted: March 21, 2014 Published: March 21, 2014 1259

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Figure 1. (a) Schematic diagram of the plasmon-driven photocatalysis mechanism. Light absorption by plasmonic metal substrate creates electron− hole pairs. The optically excited hot electron can attach to an unoccupied molecular state. Similarly the excited hot hole can be captured by an occupied molecular state. (b) Simulated absorption spectrum of Ag38 cluster. The maximum excitation lines around 320 nm are associated to the surface plasmon resonance. (c) Energy levels of Ag38 cluster and frontier orbitals of PATP and PNTP on silver. The blue lines are energy ranges of excited hot electrons, which are close to the LUMO level of PNTP. The red lines are energy range of excited hot holes, which are close to the HOMO level of PATP.

The above two mechanisms are influenced by the surrounding atmosphere. Light irradiation on metal nanostructures adsorbed with molecules triggers both surface plasmon resonance and charge transfer (CT) processes.30 Two possible photoexcitation channels are involved at molecule−metal interfaces. One arises from direct absorption of light energy to excite the electron from metal valence band to the unoccupied orbital of adsorbate (metal-to-molecule CT) or from the occupied orbital of adsorbate to the metal unoccupied band (molecule-to-metal CT) via the usual dipole transition. The direct photoinduced CT rate is proportional to the transition dipole moment and

mechanisms under different experimental conditions. In the absence of O2 or H2, the plasmon-driven photocatalysis mechanism (hot electron−hole reactions) is the major reaction channel. In the presence of O2 or H2, the plasmon-assisted surface catalysis mechanism (activated oxygen/hydrogen reactions) is the major reaction channel. In the former mechanism, the photoreactions are directly driven by the excited electron−hole pairs from surface plasmon relaxation. In the latter one, surface coupling reactions are assisted from chemical redox agents such as adsorbed oxygen/hydrogen species, which are first activated by surface plasmon relaxation. 1260

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Table 1. Calculated Changes in Enthalpy (ΔH0, kcal·mol−1), Entropy (ΔS0, cal·mol−1·K−1), and Gibbs Energies (ΔG0, kcal· mol−1) for Conversions of PATP and PNTP to DMAB under Different Conditions ΔH0

ΔS0

ΔG0

reactions

Au

Ag

Au

Ag

Au

Ag

2M5−PATP → M5−DMAB−M5 + 2H2 2M5−PATP+ O2 → M5−DMAB−M5 + 2H2O 2M5−PNTP → M5−DMAB−M5 + 2O2 2M5−PNTP + 4H2 → M5−DMAB−M5 + 4H2O

56.81 −50.51 58.44 −156.20

52.29 −54.33 60.70 −153.94

15.16 −5.92 35.31 −6.86

3.91 −17.18 25.48 −13.56

52.29 −48.75 47.91 −154.17

51.83 −49.21 52.18 −149.90

Figure 2. Energy diagram illustrating the energy distribution of excited electron−hole pair on silver (a) and gold (b) by light irradiation at 633 nm, together with the initial oxidation potential of PATP, the initial reduction potential of PNTP, and the redox potential of water splitting as a function of solution pH.

the magnitude of the local optical field |E|2. Although the calculated oscillator strengths for the photodriven CT transitions are relatively small, the magnitude of CT transitions can be greatly amplified by the enhanced optical field.13,14 This phenomenon is explained as surface plasmon-enhanced spectroscopy and photochemistry.31,32 The photoinduced surface catalytic coupling reactions of aromatic nitro and amino compounds based on the direct CT model have been investigated in our previous study.13,14 Except for the direct photoinduced CT mechanism, a stepwise CT channel through photoexcited electron−hole pair is also involved at the molecule−metal interface. This mechanism was recently suggested to explain the plasmondriven photocatalysis on silver and gold nanostructures.3,33−35As shown in Figure 1a, light absorption by the metal substrates creates a distribution of hot electrons and hot holes. Especially, the electron−hole pairs can be efficiently created in gold and silver nanostructures upon surface plasmon decay. Following optical excitation, the hot electrons transiently occupy the empty states above the Fermi level while leaving the hot holes below the Fermi level. If a nearby electron acceptor or donor with a suitable energy level is present, the transfer of electron between surface plasmons and adsorbate could happen. The hot electrons can diffuse to the surface and subsequently attach to the unoccupied molecular state. Similarly, a hot hole in the metal substrate can attach to an occupied molecular state.6,7 Figure 1b presents the simulated absorption spectrum of Ag38 cluster36 by TD-DFT calculation. The Ag38 cluster shows very strong absorption lines around 320 nm, which is coincident with a previous study on Ag20 cluster.37 It has been demonstrated that the discrete electronic transition in

small metal clusters can be employed as an analogue of plasmon frequency38 and thus serve as a model system to understand the excitation of surface plasmons. The molecular orbitals involved in the strongest absorption of Ag38 cluster are shown in Figure 1c. The energy levels of excited hot electrons and hot holes are presented with blue lines and red lines, respectively. The collective electronic transition leads to a wide distribution of hot electrons above the Fermi level and the remaining hot holes below the Fermi level. PNTP and PATP adsorbed on gold and silver surfaces can provide suitable orbitals for the electron transfer and the hole transfer. According to our calculation, the HOMO of PATP is 1.2 eV lower than the Fermi level of silver, which means a proper matching with the energy level of hot holes. By contrast, the LUMO level of PNTP is 1.6 eV higher than the Fermi level of silver, which also indicates a high coherence with the energy level of hot electrons. Thus the photooxidation of PATP and photoreduction of PNTP could be driven via the plasmondriven CT reactions. The plasmon-driven photooxidation of PATP and photoreduction of PNTP can be viewed as follows. The formation of energetic charge carriers (electron−hole pair) on the surface of photocatalyst is the initial step. PATP is then oxidized by the hot hole, while water as a sacrificial agent is reduced by the hot electron to release hydrogen. The overall reaction of photocatalysis of PATP is therefore actually a dehydrogenation process. However, when O2 is present, it has the priority to be reduced by the hot electron. Accordingly, the overall reaction turns to an oxygenation process. In the case of PNTP, it is reduced by the hot electron, and water as a sacrificial agent is oxidized by the hot hole to produce oxygen. The overall reaction of photocatalysis of PNTP is a deoxygenation process. 1261

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Figure 3. Gibbs free energies for oxidation of PATP to DMAB by excited “hot” hole as a function of irradiation wavelength and solution pH.

Nevertheless, in the presence of H2, H2 is first oxidized by the hot hole. In this case, the overall reaction becomes a hydrogenation process. Table 1 lists the changes in enthalpy, entropy, and free energies for conversions from PATP and PNTP to DMAB under different conditions. The oxygenation of PATP and hydrogenation of PNTP are thermodynamically exothermic and spontaneous reactions, and could occur without light irradiation.39 The dehydrogenation of PATP and deoxygenation of PNTP are thermodynamically endothermic and nonspontaneous reactions. However, these two reactions can be driven via energetic electron−hole pairs created by specific light irradiation. Note that data in Table 1 and other calculations in this Letter for the adsorbed PATP and PNTP on silver and gold surfaces are modeled as surface metalmolecule complexes M5−PATP(PNTP) (see Figure S1 in the Supporting Information).12 Next, the half-reaction of PATP oxidized by hot holes and PNTP reduced by hot electrons was examined. The photoelectrochemical reactions of PATP and PNTP involve multiple electron transfer and proton transfer steps, in which the electron energy is determined by the irradiation wavelength and the chemical potential of the proton depends on the solution pH. Electrochemical voltammetry experiments verified that the initial oxidation/reduction process is the most difficult step during the overall electrochemical reactions of PATP and PNTP.40,41 Figure 2 shows the oxidation potential of PATP/ PATP(NH) and the reduction potential of PNTP/PNTP(H) as a function of solution pH (see Supporting Information for molecular abbreviations).

M5−S−C6H4−NO2 + H+ + e− → M5−S−C6H4−NO2 H (2)

The equilibrium potentials of reactions 1 and 2 with respect to the standard hydrogen electrode (SHE) can be calculated from the Gibbs free energies of the corresponding hydrogenation reactions.42 The calculated standard electrode potentials of reaction 1 on silver and gold are 1.11 and 1.30 V vs SHE, respectively. The cyclic voltammogram of PATP on gold in a pH = 7.2 buffer solution shows an anodic peak at 0.50 V vs Ag/AgCl,40 which is 1.12 V vs SHE at pH = 0. The calculated standard electrode potentials for reaction 2 on silver and gold are −0.36 V and −0.45 V, respectively. The cyclic voltammogram of PNTP on gold in a pH = 2 buffer solution shows a cathodic peak at −0.45 V vs SCE,41 which is −0.09 V vs SHE at pH = 0. According to the TDDFT calculation of the Ag38 cluster, the excited electron (hole) has a wide energy distribution above (below) the metal Fermi level. Here a primary hypothesis was made to relate the light irradiation wavelength with the energy levels of excited energetic carriers. The hot electron is assumed to have an average energy of Ee = EF + αℏω, and the hot hole is assumed to have an average energy of Eh = EF − (1 − α)ℏω, where 0 ≤ α ≤ 1 is the fraction of the light irradiation that drives the photoreaction.43 For a simple treatment, the value of α is set approximately to 0.5. As shown in Figure 2, the hot electron with energy higher than the reduction potential of PNTP can drive the photoreduction process, while the hot hole with energy lower than the oxidation potential of PATP can drive the photooxidation process. The redox potentials for PATP oxidation40 and PNTP reduction41 shift negatively with the increase of pH, so the proton transfer accompanies the electron transfer process. Figure 2 also shows that the oxidation of PATP is favorable in basic solution, while the reduction of PNTP is

M5−S−C6H4−NH 2 → M5−S−C6H4−NH + H+ + e− (1) 1262

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dehydrogenation (deoxygenation) steps and convert to the final coupling product DMAB. Figure 4 presents the potential energy surface for the oxidation coupling reaction of PATP to DMAB by M2O. The

facilitated in acidic solution. Because the Fermi level of silver (−4.3 eV) is higher than that of gold (−5.0 eV),44 the photocatalysis of PATP occurs more easily on gold while the photocatalysis of PNTP occurs more easily on silver. It is also noticed that plasmon-driven water splitting can be also driven by visible light irradiation on silver (basic solution) and gold (acidic solution) nanostructures. Figure 3 shows the Gibbs free energies as a function of irradiation wavelength and solution pH for plasmon-driven oxidation of PATP to DMAB by excited holes. The potential energy surfaces for plasmon-driven reduction of PNTP to DMAB by excited electrons can be found in the Supporting Information. The oxidation of PATP to PATP(NH) is the rate determining step among the oxidation of PATP to DMAB. The coupling of PATP(NH) gives DMHAB, which can be further oxidized to DMAB. Figure 3a,b are the wavelength-dependent reaction energy surfaces on silver and gold electrodes in pH = 7 solution. The calculated oxidation potentials are 0.70 and 0.89 V vs SHE at pH = 7 on silver and gold electrodes, respectively. On silver, the photooxidation of PATP can be driven only by 514 and 633 nm irradiation, whereas on gold, the photooxidation reaction can occur even for 1064 nm irradiation. Figure 3c,d are the pH-dependent reaction energy surfaces on silver and gold electrodes under 633 nm irradiation. The oxidation potential of PATP decreases with increasing pH. PATP is easier to be oxidized in basic solutions. On silver, the photooxidation of PATP is spontaneous when solution pH values are larger than 4. On gold, the photooxidation reaction occurs spontaneously even in pH = 1 solution. The present calculations coincide with previous pH-dependent19,20 and wavelength-dependent23 SERS experiments of PATP. As shown in Figure 3, the oxidation of PATP by hot holes is thermodynamically more facile on gold because its Fermi level is lower than that of silver. Nevertheless, as demonstrated from SERS experiments, PATP is more easily converted to DMAB on silver substrates than on gold substrates.45 One possible reason is that hot holes are assumed to possess an average energy of Eh = EF − (1−α)ℏω. For a simple treatment, the value of α is set approximately to 0.5 in calculating the energy diagram of photoelectrochemical reaction of PATP. However, the value of parameter α strongly depends on the electronic structures of metal substrates. If α of gold is smaller than that of silver, the oxidation ability of hot holes in the former will decrease. The conversions of PATP and PNTP to DMAB in the presence of chemical redox agents O2 and H2 involve another reaction mechanism. In this case, the adsorbed oxygen/ hydrogen species are first activated by surface plasmon relaxation and then react with adsorbed PATP and PNTP molecules. This mechanism is named as plasmon-assisted surface catalysis. It has been reported that plasmon-induced dissociations of O2 and H2 via hot electrons transferring into the molecular antibonding orbitals eventually produce adsorbed O and H atoms.46,47 Our experimental studies have evidenced that SPR-assisted activation of oxygen gas plays a critical role in the selective oxidation of PATP to DMAB.29 Here, the reaction mechanisms for conversions of PATP and PNTP by surface activated oxygen/hydrogen species are explored using DFT calculations. M2O and M3H clusters (see Figure S1 in the Supporting Information) were employed in our calculations to model the surface active atomic oxygen and hydrogen species. Two nearby PATP (PNTP) molecules undergo four sequential

Figure 4. Potential energy surfaces of the oxidation coupling reaction of PATP to DMAB catalyzed by M2O (M2O = Au2O or Ag2O).

abstraction of amino hydrogen atom from PATP by M2O gives rise to PATP(NH) radical. In the transition state (TS1) of PATP and M2O reaction, the M2O cluster and the N−H bond of PATP construct a four-membered ring structure N−H−O− M−N, in which the oxygen atom of M2O approaches the hydrogen atom of PATP and the nitrogen atom of PATP binds to the metal atom of M2O through the nitrogen lone pair (see Figure S3 in the Supporting Information). The formation of new O−H and N−M bonds along with the breakage of N−H and O−M bonds generates PATP(NH)−M and MOH. MOH can react with another nearby PATP molecule through transition state (TS2) to produce PATP(NH)−M and water. The calculated ΔG value for the reaction of PATP with Au2O is comparatively negative to that for the reaction of PATP with Ag2O, as the product PATP(NH) has stronger interactions with gold. However, the activation energy Ea for the dehydrogenation reaction by Ag2O is ∼10 kcal/mol smaller than by Au2O, indicating a stronger oxidative ability of Ag2O than Au2O. Two PATP(NH) radicals occur N−N coupling reaction to produce hydrazo species DMHAB with a ΔG of −20 kcal/mol. DMHAB undergoes two consecutive deoxygenation processes to yield DMAB(H) and DMAB. During the reaction of DMHAB with M2O, a N−H−O linear transition state (TS3) is formed. The hydrogen atom is transferred from DMHAB to M2O. It was noted that the TS3 energy is lower than the reactant due to the interaction of the metal atom with the benzene ring. A similar case occurs at the TS4 state. As a result, two hydrogen atoms in DMHAB are removed by M2O to give the final product DMAB. Except for the coupling route, PATP(NH) can undergo further dehydrogenation reaction to produce nitrene-like species PNETP48 (see Figure S4 in the Supporting Information). However, the reaction barriers of PATP(NH) with M2O and MOH are higher than that of its coupling product DMHAB. Because the formation of DMHAB is a thermodynamically spontaneous reaction and the reaction barriers for the subsequent dehydrogenation reactions are reduced, the reaction mechanism as shown in Figure 4 should be the more favorable reaction route for the surface catalytic oxidation of PATP. In order to verify the surface catalytic oxidation abilities of M2O (MOH), the direct oxidation of PATP by free O2 was also 1263

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Table 2. Calculated Gibbs Free Energies (ΔG, kcal/mol) and Activation Energies (Ea, kcal/mol) for Surface Catalysis of PATP and PNTP on Gold and Silver ΔG

Ea

reactions

Au

Ag

Au

Ag

M5−PATP + M2O → M5−PATP(NH)−M + MOH M5−PATP + MOH → M5−PATP(NH)−M + H2O M5−PATP(H)−M + M2O → M5−PNETP−M2 + MOH M5−PATP(H)−M + MOH → M5−PNETP−M2 + H2O M−PNTP + M3H → M−PNSTP + M3OH M−PNSTP + M3H → M−PNETP + M3OH

−5.17 −10.23 −7.51 −12.56 −5.25 6.07

−5.99 −2.16 −6.11 −2.28 −19.84 −7.82

16.28 15.29 15.46 14.89 21.74 28.53

3.66 5.58 5.26 7.03 12.19 13.34

Figure 5. Potential energy surfaces of reduction coupling reaction of PNTP to DMAB by catalyzed M3H through different reaction routes.

directly attached to the M3 cluster. Once the O−H bond moves across the curve crossing region of potential energy surface, PNTP loses one oxygen atom to give PNSTP and M3H is converted to M3OH. The subsequent deoxygenation of PNSTP is similar to PNTP. The ΔG for the reaction of PNTP with Ag3H is about ∼15 kcal/mol more negative than that of the reaction of PNTP with Au3H. This can be attributed to the strong affinity of silver cations for an oxygen anion. Meanwhile, the activation energies Ea for the deoxygenation reactions by Ag3H are ∼10 kcal/mol smaller than that by Au3H, illustrating that Ag3H has a better catalytic activity for PNTP dehydrogenation. Figure 5b presents another reaction route for surface catalysis of PNTP. In this route, the coupling of PNSTP generates the dimeric structure of PNSTP, DMABDO.51 The formation of DMABDO is a nonspontaneous reaction with a ΔG of ∼15 kcal/mol. However, DMABDO can undergo two consecutive deoxygenation processes to yield DMAOB and DMAB. The N−O bond strength in DMABDO and DMAOB (coordinate bond) are weaker than that in PNTP and PNSTP (covalent bond), so activation energies for deoxygenation of DMABDO and DMAB are smaller than that of PNTP and PNSTP. Although the formation of PNSTP dimer is a thermodynamically unfavorable reaction, the subsequent deoxygenation reactions are accelerated as a result of small reaction barrier. In order to verify the surface catalytic reduction ability of M3H, the direct reduction of PNTP by free H2 was also studied as a reference (see Figures S8 and S9 in the Supporting Information). The direct hydrogenation of PNTP by H2 needs to break the H−H bond and requires high energy input to overcome the reaction barrier. The calculated activation energies for hydrogenation of PNTP and PNSTP are about 60 and 45 kcal/mol, respectively. The reaction activity of H2, however, can be strongly enhanced via formation of interfacial atomic hydrogen. Our calculations show that the activation

studied as a reference (see Figures S5 and S6 in the Supporting Information). In this scheme, the oxidants for each dehydrogenation reaction are replaced by O2 (0, charge; 3, multiplicity), •O2H (0, 2), H2O2(0, 1), and •OH (0, 2). DFT calculations indicate that •O2H and •OH radicals are more capable than O2 and H2O2 molecules in oxidation. The activation energies for the N−H bond cleavage by •O2H and •OH (about 10 kcal mol−1) are much lower than that by O2 and H2O2 (about 30 kcal mol−1). When O2 is adsorbed on noble metal surfaces, the dissociation of O2 is facilitated via a hot electron transfer from the metal nanoparticle.46 The reaction ability of O2 is then strongly enhanced by the formation of surface active oxygen species M2O or MOH. Our calculations show that the activation energies for surface catalytic oxidation of PATP reduce to 15 kcal/mol for Au2O (AuOH) and 5 kcal/mol for Ag2O (AgOH). The reaction activity increases in the order of O2 ≈ H2O2 < Au2O ≈ AuOH < •O2H ≈ •OH < Ag2O ≈ AgOH. This suggests that gold nanoparticles can perform oxidation reactions with high conversion and selectivity, such as has been shown in the aerobic oxidation of aniline to azobenzene.39 Gold and silver nanoparticles have also shown excellent catalytic abilities for other surface oxidation reactions such as ethylene epoxidation49 and methyl coupling,50 in which the surface oxygen atoms are of crucial importance. Figure 5 shows the potential energy surfaces for the reduction coupling reaction of PNTP to DMAB catalyzed by M3H through different reaction routes. In Figure 5a,the two oxygen atoms of PNTP are consecutively removed by M3H to produce nitroso species PNSTP and then nitrene analogue PNETP.48 During the reaction of PNTP with M3H, the nitro oxygen atom is inserted into the surface M-H bond and forms a N−HO−M transition state (see Figure S7 in the Supporting Information). The hydrogen atom of M3H is transferred to the nitro group of PNTP, while the oxygen atom of PNTP is 1264

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energies for surface catalytic reduction of PNTP and PNSTP reduce to 22 and 29 kcal/mol for Au3H and 12 and 13 kcal/mol for Ag3H, respectively. The reaction activity increases in the order of H2 < Au3H < Ag3H. Although azobenzene is an unstable intermediate by a direct hydrogenation of nitrobenzene,52 it can be produced by the photoinduced surface catalytic deoxygenation of nitrobenzene with high selectivity.9 The light irradiation plays an important role in surface catalysis of PATP and PNTP, which can be summarized as follows. On the one hand, O2 and H2 in the surrounding atmosphere can be activated by a strong coupling with the surface plasmon.46,47 The resulting surface active oxygen and hydrogen atoms significantly enhanced the surface catalytic performance. On the other hand, visible radiation at a specific resonant frequency can be converted to thermal energy through surface plasmon decay.10,11 The photothermal conversion allows for very rapid and localized heating on the metal surface, which can provide the necessary heat that is required for the cleavage of N−H (N−O) bonds.9 In conclusion, the plasmon-enhanced heterogeneous catalyzes of PATP and PNTP on gold and silver have been studied by DFT. Two reaction mechanisms named plasmon-driven photocatalysis (hot electron−hole reactions) and plasmonassisted surface catalysis (activated oxygen/hydrogen reactions) are proposed. The above two mechanisms are influenced by the surrounding atmosphere. In the absence of O2 or H2, the conversions of PATP and PNTP to DMAB are thermodynamically nonspontaneous reactions. The plasmon-driven photocatalysis mechanism is the major reaction channel. In the presence of O2 or H2, the oxidation of PATP and the reduction of PNTP become thermodynamically spontaneous reactions. The plasmon-assisted surface catalysis mechanism is then the major reaction channel. These findings have helped to draw a fundamental physical picture in understanding plasmoninvolved heterogeneous catalysis and may provide a highly efficient strategy for selective synthesis of aromatic azo dyes from the corresponding nitro and amino compounds by using plasmonic gold and silver catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Computational details; molecular abbreviation and plasmondriven photoreduction of PNTP; potential energy surfaces for direct oxygenation of PATP and direct hydrogenation of PNTP; reaction mechanism for surface catalytic oxidation of PATP and surface catalytic reduction of PNTP. This material is available free of charge via the Internet at http://pubs.acs.org.

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Letter

AUTHOR INFORMATION

Corresponding Author

*Fax: +86 592-2186979; Tel: +86 592-2189023; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this work by the NSF of China (Nos. 21373172, 21321062, and 91027009), and by the Ministry of Science and Technology of China (973 Program No. 2009CB930703). D.Y.W. also thanks the HPC of Xiamen University for support (2010121020). 1265

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