Surface Plasmon Enabling Nitrogen Fixation in Pure Water through a

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Surface Plasmon Enabling Nitrogen Fixation in Pure Water through a Dissociative Mechanism under Mild Conditions Canyu Hu, Xing Chen, Jianbo Jin, Yong Han, Shuangming Chen, Huanxin Ju, Jun Cai, Yunrui Qiu, Chao Gao, Chengming Wang, Zeming Qi, Ran Long, Li Song, Zhi Liu, and Yujie Xiong J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Journal of the American Chemical Society

Surface Plasmon Enabling Nitrogen Fixation in Pure Water through a Dissociative Mechanism under Mild Conditions Canyu Hu,†, ‡ Xing Chen,¶, ‡ Jianbo Jin,†, ‡ Yong Han,§ Shuangming Chen,† Huanxin Ju,† Jun Cai,§, ¦ Yunrui Qiu,† Chao Gao,† Chengming Wang,† Zeming Qi,† Ran Long,*, † Li Song,† Zhi Liu, §, ¦ and Yujie Xiong*, † †Hefei

National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, and National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. ¶Department §School

of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States.

of Physical Science and Technology, Shanghai Tech University, Shanghai 201203, China.

¦State

Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China. KEYWORDS: surface plasmon; N2 fixation; nanostructure; hot electron; ammonia.

ABSTRACT: Nitrogen fixation in a simulated natural environment (i.e., near ambient pressure, room temperature, pure water and incident light) would provide a desirable approach to future nitrogen conversion. As NN triple bond has a thermodynamically high cleavage energy, nitrogen reduction under such mild conditions typically undergoes associative alternating or distal pathways rather than follows a dissociative mechanism. Here we report that surface plasmon can supply sufficient energy to activate N2 through a dissociative mechanism in the presence of water and incident light, as evidenced by in-situ synchrotron radiation-based infrared spectroscopy and near ambient pressure X-ray photoelectron spectroscopy. Theoretical simulation indicates that the electric field enhanced by surface plasmon, together with plasmonic hot electrons and interfacial hybridization, may play a critical role in NN dissociation. Specifically, AuRu core-antenna nanostructures with broaden light adsorption cross section and active sites achieve an ammonia production rate of 101.4 μmol·g-1·h-1 without any sacrificial agent at room temperature and 2-atm pressure. This work highlights the significance of surface plasmon to activation of inert molecules, serving as a promising platform for developing novel catalytic systems.

INTRODUCTION Ammonia (NH3) synthesis is a highly important process to agriculture and chemical production, as well as supplies a liquefiable and carbon-free medium for hydrogen storage.14 Although nitrogen is constituting ~78% of the atmosphere, the utilization efficiency of nitrogen is highly limited by its strong nonpolar N≡N bond toward dissociation or activation.5-7 Conventional heterogeneous catalysis provides sufficient energy for activating the N≡N bond by maintaining high temperature on catalyst surface; however, such a system cannot selectively deposit phonon energy into the N≡N bond. Given the extremely large bond energy of 941 kJ mol−1, the industrial Haber-Bosch process that converts nitrogen (N2) and hydrogen (H2) into NH3 requires high temperature (over 300 C) and pressure (over 100 atm) to achieve N2 dissociation, resulting in massive energy consumption.3 It remains a grand challenge to develop sustainable approaches toward N2 fixation into ammonia under mild conditions. Among the recently developed reaction systems, photocatalysis and

electrocatalysis that can supply electrons for N2 activation provide an opportunity for the conversion of N2 into ammonia under simulated natural condition (near ambient pressure, room temperature and pure water).1-3, 813 However, the electrons transferred to N molecules in 2 photocatalysis and electrocatalysis do not have sufficiently high energy to break N≡N bonds. As such, photocatalytic and electrocatalytic N2 fixation mainly undergo associative alternating or distal pathways (i.e., N≡N bond cleavage after NH formation). Although the associative pathways provide an important opportunity for reducing energy input, the dissociation of N2 molecules by breaking the ultrastable N≡N bond under mild conditions is still a highly important research topic from the perspective of fundamental understanding. If achievable, the related findings would provide insights for modifying the conventional techniques (e.g., reducing the energy input for the Haber-Bosch process). To achieve N2 dissociation without high temperature and pressure, it is imperative to develop a new mechanism for catalyst design. In principle, the N≡N bond cleavage can be

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facilitated by surface plasmon through two different mechanisms. A mechanism is the indirect energy transfer scheme working through the injection of highly energetic electrons into molecular orbitals.14 It is anticipated that the hot electrons generated through the decay of surface plasmon resonance in metallic nanostructures,6, 15-17 which have been demonstrated versatile for various reactions,18-19 may play such an important role. While this indirect energy transfer mediated by hot electrons may impact on multiple reaction steps, N≡N bond cleavage can be more selectively achieved through another direct energy transfer mechanism that directly turns N2 into vibrationally or electronically excited states.6 In this second mechanism, plasmonic metal can still make an important contribution by forming a complex with adsorbate, within which charge carriers are directly excited from the hybridized metaladsorbate interface.14 No matter whether N2 would be dissociated through the indirect or direct energy transfer, N2 should be chemisorbed in the region very close to plasmonic metal (rather than mediated by semiconductor9), enabling the preservation of hot electron energy and the formation of metaladsorbate complex. Herein, we report that AuRu core-antenna nanostructures can effectively catalyze the dissociation of N2 toward NH3 production in pure water under mild conditions (2-atm pressure, room temperature and incident light). The design of AuRu core-antenna nanostructures is the key to achieving the N2 dissociation. In our work, Au nanocrystals are selected as plasmonic nanostructures to harvest light in a wide range,20 but are incapable of well adsorbing N2 molecules. For this reason, we integrate plasmonic Au with catalytic Ru for surface plasmon-mediated N2 dissociation as Ru atoms have been recognized as active sites with promising catalytic activity in N2 fixation.10 The hybrid structure with Au core and Ru antenna provides a suitable platform for fine-tuning the energy flow from plasmonic Au to neighbouring catalytic Ru sites and then to adsorbed N2, a key process to plasmonic catalysis,16, 21-22 through composition control. It turns out that AuRu core-antenna nanostructures with 31% Ru (molar ratio) achieve the highest NH3 production rates of 101.4 μmol·g-1·h-1 for N2 fixation without any sacrificial agent.

EXPERIMENTAL SECTION Synthesis of AuRu0.31. In a typical synthetic process for AuRu0.31 sample, 40 mL diethylene glycol (DEG) solution containing 177.6 mg polyvinyl pyrrolidone (PVP) was kept in a 100 mL three-necked flask under magnetic stirring at 215 °C. Subsequently, 4 mL DEG solution containing 10.39 mg HAuCl4·4H2O and 3.92 mg RuCl3·xH2O was injected into the flask using a syringe pump at a rate of 1.8 mL·min-1. The reaction solution was initially light red and then gradually turned brown. After reaction for 15 min, the product was collected through centrifugation once with acetone and three times with deionized water at 8,000 rpm for 9 min before re-dispersion in 2 mL DI water.

In-situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). NAP-XPS measurements were performed using a system located at State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences (SIMIT, CAS). This system was manufactured by SPECS Surface Nano Analysis GmbH, Germany. The facility is composed of two chambers, an analysis chamber and a quick sample load-lock chamber. The analysis chamber is equipped with a PHOIBOS NAP hemispherical electron energy analyzer, a microfocus monochromatized Al Kα X-ray source with beam size of 300 μm, a SPECS IQE-11A ion gun and an infrared laser heater. The catalyst was dropped onto a very clear silicon wafer, and dried at room temperature. Subsequently, the sample was treated with Ar plasmon for 10 min, and stored in the atmosphere of argon prior to further characterization. After the sample was delivered to the analysis chamber, high-purity N2 was sequentially introduced into the analysis chamber with the partial pressure up to 0.5 mbar. The catalyst on the silicon wafer was irradiated using a 300 W Xe lamp (PLS-SXE300, Perfect Light). N 1s spectra were acquired in situ at each stage. In addition, Au 4f spectra and Ru 3d spectra were also be recorded before and after the irradiation. Infrared spectroscopy characterization. Infrared spectroscopy measurements were performed using a Bruker IFS 66v Fourier-transform spectrometer equipped with a Harrick diffuse reflectance accessory at the Infrared Spectroscopy and Microspectroscopy Endstation (BL01B) in NSRL in Hefei, China. Each spectrum was recorded by averaging 128 scans at a resolution of 2 cm−1. To perform in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) characterization, the samples were fully dried (or mixed with dried KBr), and were held in a custom-fabricated IR reaction chamber which was specifically designed to examine highly scattering powder samples in the diffuse reflection mode. The chamber was sealed with two ZnSe windows. For the spectra in the absence of H2O, the fully dried sample was firstly measured to obtain a background spectrum. For the spectra in the presence of H2O, water-saturated N2 or Ar gas was introduced into the chamber with the samples. The samples were then measured to obtain background spectra. During the in-situ characterization, pure N2 (99.999%) was continually introduced into the chamber. The spectra were collected under dark condition or after a certain irradiation time using a 300 W Xe lamp (PLSSXE300, Perfect Light). To perform low-temperature Fourier-transform infrared (LT-FTIR) spectroscopy, the samples were held in a Praying Mantis™ low temperature reaction chamber. The chamber with samples was evacuated under vacuum at 575 K for 1 h. After the chamber was cooled to room temperature naturally, pure Ar (99.999%) was introduced into the chamber as protective gas up to atmospheric pressure. Subsequently, the chamber was cooled down to 130 K, and a spectrum was measured as a background before N2 absorption. Then pure N2 (99.999%) was introduced into the chamber. After


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Journal of the American Chemical Society N2 adsorption equilibrium, the spectra were collected immediately. The spectra were obtained by subtracting the background. Catalytic N2 fixation measurements. The light-driven catalytic N2 reduction was performed in a homemade quartz tube in N2 atmosphere (2 atm) at room temperature. Typically, 0.2 mg AuRux sample (based on ICP-MS measurements) was dispersed in DI water in the tube to reach a total suspension volume of 3 mL, which was then vacuumed and saturated with pure N2 for three times and sealed with a glass stopper with 2 atm pure N2. The suspension was under vigorously stirred, and irradiated using a 300 W Xe lamp (PLS-SXE300, Perfect Light) at the intensity of 400 mW·cm-2 (full-spectrum light). The product was then analyzed by cation chromatography (IC 1040, Techcomp). The control experiments were performed under the same conditions except the exclusion of light with an aluminum foil or the exclusion of N2 with pure argon.

ordered lattice fringes with a spacing of 2.32 Å and 2.01 Å in Figure 1d can be assigned to the (111) and (200) planes, respectively. Assuming that the lattice constant follows the Vegard’s law,24 the Au/Ru atomic ratio of the core is calculated to be 0.753:0.247, indicating that the core is a Au-concentrated AuRu alloy nanocrystal. Meanwhile, Figure 1f and 1g show the magnified image and corresponding FFT relevant pattern taken from the antenna region (i.e., pattern 2 marked in Figure 1c). In Figure 1g, we can also obtain a FFT pattern of fcc single crystal except that the view direction is along zone axis [111]. Correspondingly, the spacing of lattice fringes for the antenna is about 1.37 Å, which can be assigned on the (022) planes of AuRu alloy (Figure 1f).24 Following the Vegard’s law, the Au/Ru atomic ratio of antenna is calculated to be 0.124:0.876. Based on the results, we can concluded that the antenna possesses a higher Ru content, consistent with the EDS mapping results in Figure 1b.

RESULTS AND DISCUSSION Our AuRu core-antenna nanostructures are synthesized via a simple method that involves PVP as a stabilizer, DEG as a solvent and reducing agent, and HAuCl4·4H2O and RuCl3·xH2O as metal precursors. The mixture of Au and Ru precursors in a DEG solution is injected into the preheated PVP solution using a syringe pump. During the synthetic process, a core-antenna structure is spontaneously formed since Au can be more easily reduced (redox potentials: [AuCl4]-/Au 1.002 V versus SHE; Ru3+/Ru 0.386 V versus SHE).23 Figure 1a show the transmission electron microscopy (TEM) images of a typical sample with the Ru/Au molar ratio of 0.31 obtained at the speed of 1.8 mLmin-1. The obtained nanostructure contains a nanoparticle as the core and some branched-out whiskers as the antennas, constituting the so-called core-antenna nanostructure. In such a protocol, the addition of Ru plays an important role in the formation of core-antenna nanostructures as indicated by the comparison of samples with various Ru/Au ratios (Figure S1 and S2). While X-ray diffraction (Figure S3) reveals the formation of AuRu alloy, we name the core-antenna nanostructures as AuRux where x represents the molar content of Ru atoms as determined by inductively coupled plasma mass spectrometry (Table S1). This protocol allows us to subtly control the content of Ru by adjusting the amount of added Ru precursor. To examine the spatial distribution of Au and Ru, we have collected energy-dispersive spectroscopy (EDS) mapping images (Figure 1b). The Ru element is more concentrated at the antennas, suggesting the deposition of AuRu alloy antennas over the surface of Au-based core. We further characterize the sample with high-resolution TEM (HRTEM, Figure 1c). Figure 1d and 1e show the magnified image and corresponding fast Fourier transform (FFT) pattern obtained from the core region (i.e., pattern 1 marked in Figure 1c). The FFT pattern in Figure 1e fits a pure face-centered cubic (fcc) crystal structure with a ＿ symmetry of Fm3m, viewed along the [011] direction. The

Figure 1. (a) TEM image of AuRu0.31 core-antenna nanostructures. (b) EDS elemental mapping profiles of AuRu0.31 nanostructure showing the distribution of Au (orange) and Ru (purple) elements. (c) HRTEM image of AuRu0.31 core-antenna nanostructures. (d) Magnified HRTEM image and (e) corresponding FFT pattern taken from pattern 1. f) Magnified HRTEM image and g) corresponding FFT pattern taken from pattern 2.

To resolve the local structural information for Au and Ru atoms, the samples are further characterized by synchrotron radiation-based X-ray absorption fine structure (XAFS) spectroscopy. Figure S4a and S4b show the Au L3-edge and Ru K-edge X-ray absorption near-edge structure (XANES) spectra for the AuRu0.31 core-antenna nanostructures in reference to standard Au and Ru foils, respectively. Processed through a Fourier-transform (FT), the Au L3-edge and Ru K-edge extended XAFS (EXAFS) spectra are obtained. The main peak in the Au L3-edge spectra of AuRu0.31 sample (Figure S4c) slightly shifts to a



shorter bond length (2.52 Å), in comparison with the characteristics of Au foil (2.57 Å). Meanwhile, the main peak in Ru K-edge spectra of AuRu0.31 sample (Figure S4d) fits the characteristics of Ru foil (2.35 Å) with obviously lower intensity. To obtain the local bonding information in our core-antenna nanostructures, we extract metalmetal bond lengths and metal coordination numbers from EXAFS curve fitting. As listed in Table 1, the Au L3-edge spectrum reveals that the total coordination number (CN) of Au (CNAu-Au + CNAu-Ru) in our AuRu0.31 sample is 11.8. This indicates that Au atoms are mostly fully coordinated, as Au atoms are gathered in the core part. Meanwhile, the total CN of Ru (CNRu-Au + CNRu-Ru) in our AuRu0.31 sample is 9.5, as Ru atoms are more concentrated in the outermost antenna layer as coordinatively unsaturated sites.

water, the reaction rate is maintained higher in the first 3 hours. The coefficient of determination (r2) for the first 3 hours is 0.99, indicating that the amount of produced ammonia follows a clear linear relationship with reaction time. When the dissolved N2 is consumed, the mass transfer of N2 from gas phase to liquid phase will become the rate-limiting process, leading to a reduced reaction rate at 6-24 h. Nevertheless, the amount of produced ammonia is still linearly increased by prolonging reaction time (r2=0.97 between 6-24 h), corroborating the excellent durability of our plasmonic catalysts. It is worth mentioning that the obtained AuRu nanostructures show negligible activity decay after a long period of N2 fixation reactions, indicating an excellent stability of the catalyst (Figure S6). Note that no N2H4 has been detected during the long-period photocatalytic reaction, underlining the high selectivity for ammonia generation.

Table 1. Fitting results of Au L3-edge and Ru K-edge EXAFS data. The lengths of Au-Au, Au-Ru, Ru-Au and Ru-Ru bonds and coordination numbers of Au and Ru atoms are extracted from the curve-fitting for Au L3-edge and Ru K-edge EXAFS data (Figure S4), respectively. R, the lengths of Au-Au, Au-Ru, Ru-Au and Ru-Ru bonds; CN, the coordination numbers of Au and Ru atoms; σ2, the Debye-Waller factor. Error bounds (accuracies) are estimated as CN, 5 %; R, 1 %; 2, 1 %.

Bond

CN

Bond length (Å)

Au-Au

10.7

2.84

8.9

Au-Ru

1.1

2.78

16.8

Ru-Au

2.8

2.78

16.8

Ru-Ru

6.7

2.68

7.9

Au foil

Au-Au

12

2.86

7.6

Ru foil

Ru-Ru

12

2.68

4.5

sample

AuRu0.31

σ2 Å2)

(10-3

We are now in a position to assess the efficacy of surface plasmon-driven N2 reduction based on our core-antenna nanostructures. The catalytic experiments are performed in water under full-spectrum irradiation (400 mW∙cm-2) at relatively low N2 pressure (2 atm). As displayed in Figure 2a, AuRu nanostructures significantly outperform bare Au nanoparticles in catalytic ammonia production as determined by ion chromatography. As Ru is incorporated into Au nanoparticles up to 31%, ammonia production rate is gradually promoted owing to the increase of Ru catalytic sites. In pure water without any sacrificial agent, AuRu0.31 achieves an ammonia production rate of 101.4 μmol·g-1·h-1, about 7.3-fold higher than that of bare Au nanocrystals (13.8 μmol·g-1·h-1). However, when the content of Ru atoms reaches 39%, the light harvesting of AuRu nanostructures is significantly reduced (Figure S5), leading to a lower activity of N2 fixation. We further carry out timedependent measurements with continued light irradiation up to 24 hours over AuRu0.31. As shown in Figure 2b, the reaction rates can fall into 2 regimes (i.e., the first 3 h, and 6-24 h). Each regime exhibits linear relationship with a different slope. Since the reaction starts with N2-saturated

Figure 2. (a) Catalytic ammonia production rates by bare Au, AuRu0.14, AuRu0.23, AuRu0.31 and AuRu0.39 in the first 2 hours (full-spectrum, 400 mW∙cm-2, pure water, 2 atm N2). (b) Time-dependent photocatalytic ammonia production by AuRu0.31 in 24 hours (full-spectrum, 400 mW∙cm-2, pure water, 2 atm N2). (c) Photocatalytic ammonia production rates by AuRu0.31 in the first 2 hours under different light intensity. (d) Calculated AQEs (black dots) for N2 fixation over AuRu0.31 in pure water under 20 mW∙cm-2 monochromatic light irradiation, with its UV-vis extinction spectrum (red line) as a reference.

In the meantime, we look into the oxidation product for the surface plasmon-driven N2 fixation reaction. As we have not observed the production of sufficient oxygen gas, we employ electron spin resonance (ESR) spectroscopy to detect the intermediates formed during reaction of AuRu0.31 in water, using 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as spin-trapping agents. While no signal for singlet oxygen can be detected by 4-oxo-TMP, a nearly 1:2:2:1 quartet signal is observed for nitroxide-OH spin adduct (DMPO/∙OH, aN=aH=1.49 mT) (Figure S7). This indicates that hydroxyl radical (∙OH) rather than other


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Journal of the American Chemical Society type of reactive oxygen species is mainly produced from water oxidation by photogenerated holes. To further prove the universality, we synthesize other hybrid structures by modifying Au nanorods with Ru atoms (Figure S8a) or modifying Au nanospheres with Ru atoms (Figure S8b), which also show the activities of 18.2 and 83.6 μmol·g-1·h-1 in the reduction of N2 to ammonia under the same reaction conditions, respectively (Table S2). Even when AuRu0.31 nanostructures are prepared in the absence of PVP (Figure S8c), the catalyst still shows a fairly good activity (66.1 μmol·g-1·h-1) in N2 fixation. To examine how photon energy is coupled into N2 activation, we perform catalytic N2 reduction by altering photon flux (i.e., light intensity) as the rate of chargecarrier formation in plasmonic metals usually scales linearly with the photon flux.25-26 Figure 2c shows that the N2 fixation catalyzed by AuRu0.31 nanostructures has nearly linear power law dependence on light intensity. The linear rate–intensity relationship is a typical indicator for the induction of a reaction by a single charge-carrier.25-26 This reveals that the reduction N2 to ammonia is driven by plasmonic hot electrons. To evaluate the light utilization efficiency, the wavelength-dependent apparent quantum efficiencies (AQEs) of AuRu0.31 are determined by measuring ammonia production rates under various monochromatic light irradiation in pure water (Figure 2d). Typically, the photocatalytic N2 fixation by semiconductor is limited by its intrinsic absorption region.27-28 In a sharp contrast, the AQEs by our plasmonic catalysts well match the full extinction spectra range of AuRu nanostructures, indicating a relatively high utilization efficiency of incident light. Specifically, the AQEs at 350 nm and 550 nm are determined to be 0.21‰ and 0.17‰, respectively, comparable to the recent reports for associative pathways in N2 fixation.27-28 Another key aspect to nitrogen fixation application is to verify the origin of generated ammonia. To this end, we perform the catalytic N2 reduction using isotopic 15N2 as a nitrogen source. As revealed by 1H nuclear magnetic resonance (1H-NMR) spectroscopy (Figure 3a), the detected 15NH4+ in acidic solution identifies the role of 15N2 as the nitrogen source.27 Moreover, when we employ a mixture of 15N2 + 14N2 or 14N2 as the nitrogen source to perform the photocatalytic reactions, a combination 14NH ++15NH + or 14NH4+ alone can be detected, 4 4 respectively, confirming the nitrogen source. Furthermore, control experiments show that N2 reduction cannot be triggered in the system in the absence of catalysts, light irradiation or N2 environment (Table S2). These reaffirm that the N2 fixation process takes place through our AuRux plasmonic catalysts by coupling photon energy indeed. Upon identifying the fact that surface plasmon drives N2 fixation, we further investigate the activation process of N2 molecules in such a system. We first employ lowtemperature Fourier-transform infrared (LT-FTIR) spectroscopy at 130 K to examine the vibrational mode of chemisorbed N2 (Figure 3b), which can help understand the adsorbed state of N2 on AuRu0.31 nanostructures. The

negative band at about 2300 cm-1 can be assigned to the deduction of environmental CO2 from the background. This negative band remains consistent during experiments, indicating a good stability of reaction system. In principle, there are two configurations for an adsorbed N2 molecule: end-on and side-on. Normally, the N≡N vibration frequency of free N2 molecules is located at 2359 cm-1. Meanwhile, the vibration frequency of N2 molecules adsorbed at metal surface via an end-on configuration is located at 2000-2400 cm-1.29-30 The side-on configuration typically shows an even lower vibration frequency.27 In the case of our sample, the vibration peak at 2248 cm-1 after the exposure to N2 suggests an inhomogeneous line broadening of N≡N bond at an end-on configuration.30 Note that free non-polar diatomic N2 molecule is infrared forbidden so that the vibration of N ≡ N can only be measured at its adsorbed state. As the half-width is directly related to the square root of temperature31, the vibration of adsorbed N2 can be detected below 200 K.32

Figure 3. (a) 1H-NMR (400 MHz) spectra of solution after N2 fixation reaction using AuRu0.31 as a catalyst in 15N2, 15N2 + 14N2 or 14N2 atmosphere. (b) LT-FTIR spectra of N2 molecules after N2 adsorption on AuRu0.31 at 130 K. In-situ DRIFTS spectra recorded for (c) N2 and (d) N2 + H2O over AuRu0.31 nanostructures under the same irradiation condition. The background for the fully dried sample or the water-saturated system has been subtracted from the spectra in (c) or (d), respectively.

To look into the reaction upon N2 adsorption, we employ in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to monitor the AuRu-catalyzed N2 reduction process. The in-situ DRIFTS spectra are recorded under N2 flow (1 mLmin-1) with incident light. In the first 20 min, no significant change can be observed for the DRIFTS spectra in the absence of H2O (Figure 3c), demonstrating that the hydrogen in ammonia truly originates from the protons in water. After H2O is



introduced into the reaction system, several absorption bands gradually arise with the irradiation. Specifically, the absorption band at 3000-3600 cm-1 corresponds to the adsorbed ammonia.1, 27, 33 Furthermore, an absorption band at 1404 cm-1 that can be assigned to the NH4+ deforming vibration is promoted with light irradiation (Figure 3d).1 Note that the background for the water-saturated system has been subtracted from the spectra in Figure 3d so that no signal for H2O is involved. In order to further confirm the source of N and the assignment of absorption bands in Figure 3d, in-situ DRIFTS spectra are recorded under water-saturated Ar flow (i.e., without N2) under the same condition (Figure S9). The absence of N-H vibrations indicates that the N-H signals observed in Figure 3d originate from the intermediates and products that are formed from N2 reduction indeed. Moreover, the DRIFTS spectrum recorded for H2O adsorbed on sample surface (Figure S10) reveals that the absorption bands of H2O are located at 1625-1800 and 3420-3850 cm-1, which are obviously different from the absorption bands of ammonia and NH4+. It is worth mentioning that the absence of N-N vibration from N2Hy species (1100-1300 cm-1, Figure 3d) in our case excludes the possibility that surface plasmondriven N2 reduction reaction follows the associative alternating pathway.34-35

We further employ in-situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to monitor the evolution of N 1s spectra while the catalytic system is illuminated. As shown in Figure 4a, the initial N 1s XPS spectrum collected in ultra-high vacuum displays a broad peak at 399.6 eV that can be identified as pyrrolic N peak in PVP, the capping agent on catalyst.36 Subsequently, high-purity N2 is introduced into analysis chamber until the partial pressure of N2 becomes stable at 0.5 mbar. As a result, the XPS peak for gaseous N2 appears at 405.1 eV (Figure 4b). Upon turning on Xe lamp, the chemisorption of atomic N (≡N) resulting from the completely dissociated N2 is observed with an XPS peak at 395.3 eV (Figure 4c). This observation suggests that surface plasmon-driven N2 fixation undergoes a dissociative mechanism.34, 36 Along with the hydrogenation process of adsorbed N atom, N-H species emerge in the XPS spectra with binding energies at 397.2 eV and 398.0 eV (Figure 4d and 4e), which can be specifically assigned to the partially hydrogenated =NH and –NH2 fragments, respectively. Finally, the XPS peak for chemisorbed ammonia appears at 400.8 eV (Figure 4e and 4f). Although N2Hy-related species cannot be reliably distinguished from other N-H species, the XPS peak for atomic N (≡N) at 395.3 eV clearly reveals the dissociation of N2 molecules. The NAP-XPS results here provide direct evidence for the dissociative pathway of surface plasmondriven N2 fixation under mild conditions.

Figure 5. The optimized structures of N2 adsorbed on Ag22Ru6 cluster, both charged by 1e and experienced with an electric field of 1.0x108 v·m-1. The electron density difference for  (spin-up) and  (spin-down) is normalized, and the absolute isovalue is set to 0.02. The red and green colors represent increase and decrease in electron density, respectively.

Figure 4. N 1s XPS spectra of AuRu0.31: (a) under ultra-high vacuum, (b) under 0.5 mbar N2 gas without light, (c-f) under 0.5 mbar N2 gas with incident light for different time.

To gain insight into the N2 adsorption and activation/dissociation under surface plasmon, we have employed first-principles calculation31 to examine the adsorbed N2 under various simulated conditions. The simplified computational model consists of 22 Au atoms and 6 Ru atoms following the Ru/Au molar ratio of 0.31 in the typical AuRu alloy with the highest ammonia production rate. In the simulation, we first examine the adsorbed N2 on Au22Ru6 without any additional charge or electric field. It turns out that the adsorption of N2


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Journal of the American Chemical Society molecule preferentially takes place at the end-on configuration with the bond length of 1.128 Å (Figure S11), in agreement with our LT-FTIR measurement. Upon charging Au22Ru6 with one additional electron (Figure S12a), the N2 on Au22Ru6 is turned from neutral to charged state as indicated by electron density while the NN bond length is slightly stretched from 1.128 to 1.130 Å. Note that it is a simplified model to depict the electron transfer effect by plasmonic hot electrons. Nevertheless, this suggests that most likely electron transfer alone can hardly dissociate N2 molecules. We thus look into another key factor  the local electric field enhanced by surface plasmon. An electric field of 1.0×108 v·m-1 is applied to the system, pointing from N2 to Au22Ru6 to mimic the strong near field around AuRu alloy. Under the effects of electric field, the NN bond length is increased to 1.165 Å (Figure S12b), confirming the important role of local electric field in N2 activation. Since surface plasmon is a complex system involving various working mechanisms, multiple effects should be taken into consideration. Combining the constraints referring to negative charge and electric field, the bond of N2 is further elongated to 1.170 Å (Figure 5). More intuitively, the differential charge density helps us to well understand the electronic structure of N2 that may be hybridized with that of catalyst. The change of electron density mainly occurs at N2 and Au22Ru6 cluster surface, where N2 gains electrons which are potentially localized on NN anti-bonding orbital leading to the elongated NN bond. This also indicates that a highly hybridized system has been formed between N2 molecule and plasmonic catalyst. Given the information gleaned above, it is reasonable to assume that N2 molecule can be well chemisorbed at Ru sites through an end-on configuration to form a RuN2 complex near plasmonic Au in the enhanced local electric field. This hybridized state may allow surface plasmon to directly excite the charge carriers within the complex to induce N2 dissociation,14 forming chemisorbed ≡N. It has been reported that the Haber-Bosch process involves a dissociative mechanism enabled by high temperature and pressure.34 In our case, AuRu nanostructures open up an opportunity of following a similar pathway under low pressure (2 atm) and room temperature, further confirming the contribution of surface plasmon to N2 dissociation. The surface plasmon-driven N2 fixation system may involve indirect energy transfer via hot electrons, direct energy transfer through electronically excited states, and more importantly, the major contribution from local electric field. As such, the =NH and –NH2 intermediates are generated by coupling the protons through plasmonic hot electrons.

CONCLUSION In conclusion, we have demonstrated that the surface plasmon of AuRu nanostructures can effectively drive N2 reduction through a dissociative mechanism under mild conditions (room temperature, low pressure, pure water and incident light). The key to this design is the

integration of plasmonic light-harvesting Au with Ru catalytic sites. Under irradiation light, the surface plasmon, which may involve strong local electric field, direct excitation at catalystN2 interface and hot electron transfer, makes important contributions to N2 activation and hydrogenation. Our work highlights a promising and alternative approach to N2 fixation under mild conditions and provides information for further development of plasmonic catalysis.

ASSOCIATED CONTENT Detailed experimental section, characterization methods, and additional material characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]

Author Contributions ‡These authors contributed equally.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported in part by National Key R&D Program of China (2017YFA0207301), NSFC (21725102, U1832156, 21601173), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), CAS Interdisciplinary Innovation Team, Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2016FXCX003) and Chinese Universities Scientific Fund (WK2310000067). DRIFTS measurements

were performed at the Infrared Spectroscopy and Microspectroscopy Endstation (BL01B) in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China.

The simulation was conducted with Advanced Cyber Infrastructure computational resources provided by The Institute for Cyber Science at The Pennsylvania State University (http://ics.psu.edu). We are deeply grateful to Dr. Hayato Yuzawa from UVSOR synchrotron facility for his suggestions. We thank the support from USTC Center for Micro- and Nanoscale Research and Fabrication.

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Journal of the American Chemical Society SYNOPSIS TOC


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Surface Plasmon Enabling Nitrogen Fixation in Pure Water through a

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