Oxygen-Controlled Hydrogen Evolution Reaction: Molecular Oxygen

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An Oxygen-Controlled Hydrogen Evolution Reaction: Molecular Oxygen Promotes Hydrogen Production from Formaldehyde Solution Using Ag/MgO Nanocatalyst Renhong Li, Xiaohui Zhu, Hisayoshi Kobayashi, Shohei Yoshida, Wenxing Chen, Leilei Du, Kaicheng Qian, Xiaoqing Yan, Biling Wu, Shihui Zou, Linfang Lu, Wuzhong Yi, Yuheng zhou, and Jie Fan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03370 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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An Oxygen-Controlled Hydrogen Evolution Reaction: Molecular Oxygen Promotes Hydrogen Production from Formaldehyde Solution Using Ag/MgO Nanocatalyst Renhong Li,*,† Xiaohui Zhu,† Xiaoqing Yan,† Hisayoshi Kobayashi*,‡ Shohei Yoshida,‡ Wenxing Chen,*,† Leilei Du,† Kaicheng Qian,† Biling Wu,† Shihui Zou,§ Linfang Lu,§ Wuzhong Yi,§ Yuheng Zhou,§ Jie Fan*,§ †

Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China ‡ Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 6068585, Japan § Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China ABSTRACT: Molecular hydrogen is one of the essential reactants in the chemical industry, and its generation from renewable sources such as biomass materials and water is of great benefit to the future society. Generally, molecular oxygen should be preeliminated in the hydrogen evolution reactions (HER) in order to avoid reverse hydrogen oxidation reaction (HOR). Here, we report a highly efficient HER from formaldehyde/water mixture using MgO supported Ag nanoparticles (AgNPs/MgO) as the catalyst and molecular oxygen as a promoter. The HER rate depends almost linearly on the oxygen partial pressure, and the optimal turnover frequency (TOF) of the silver catalyst exceeds 6,600 h–1. Based on the experimental and theoretical results, a surface stabilized MgO/Ag–•OOH complex is suggested to be the main catalytically active species for the HER. KEYWORDS: hydrogen evolution reaction, Ag, MgO, formaldehyde, oxygen promotion

1. INTRODUCTION Molecular hydrogen (H2) is of critical importance in the chemical industry and might play vital role in future for benign and secure energy technologies.1 Today, more than 80% of the hydrogen energy consumed worldwide is mainly achieved by steam reforming and coal gasification, which depends on limited fossil resources.2 On a mid- to long-term basis, there is an increasing demand for alternative technologies to generate H2 in a more mild and sustainable manner.3 In early 1990s, a new hydrogen evolution reaction (HER) emerged when chemists investigated the origin of the frequent-occurring detonation during storage of nuclear wastes.4 It finally led to the conclusion that the combination of water and formaldehyde (HCHO) can be reduced into H2 gas in highly basic media (HCHO + H2O → HCOOH + H2).4 Notably, this specific dehydrogenation reaction meets the required standards of “hydrogeneconomy” because water acts as one of the hydrogen donor,4 yielding a theoretical hydrogen weight efficiency of 8.4%, which is higher than formic acid (4.4wt %).5 A range of heterogeneous catalytic systems were recently developed for HER from alkaline aldehyde solution.6 However, its practical application is still restricted by several intrinsic limitations, such as the low catalytic efficiency and the use of high concentration soluble bases (e.g., NaOH). More importantly, the fundamen-

tal chemical principles responsible for the supported metal nanoparticles mediated HER from HCHO aqueous solution remain unclear. In nature, hydrogenase, such as green algae, can photolyze water into H2 with high efficiency, but it completely loses function in the presence of molecular oxygen due to the reverse hydrogen oxidation reaction (HOR, 2H2 + O2 → 2H2O) occurs on catalytically active centers.7 The same drawback is also present in artificial organic or inorganic catalysts, hence anaerobic condition is usually necessary to achieve higher HER efficiency.8 Herein, we report highly efficient H2 generation from HCHO/H2O mixture at room temperature using MgO supported AgNPs (AgNPs/MgO) as an all-solid-state catalyst without any soluble base additives. The HER rate can be dramatically enhanced by increasing the O2 partial pressure (pO2), and an excellent turnover frequencies (TOF, 6,641 h−1) for Ag catalyst is achieved with pO2 at 5 atm. In stark contrast to most of HER processes, O2 is not consumed during the catalytic reaction. Based on the experimental observations and DFT calculations, we show for the first time that a surface stabilized reactive oxygen species, MgO/Ag–•OOH complex, having radical character and originate from a two-electron

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reduction of O2 with the assistance of water and HCHO molecules, acts as the catalytically active center in the HER process. The interplay of catalytic oxygen activation with the transition-metal-catalyzed dehydrogenation not only provides a low temperature path for the liberation of H2 gas from earthabundant hydrogen storage chemicals, but also offers new opportunities to achieve an efficient bio-inspired HER catalyst with improved oxygen tolerances and decreased intrinsic oxygen sensitivities. 2. EXPERIMENTAL SECTION Materials synthesis In a typical synthesis of MgO NPs, 6.2 mL of magnesium methoxide and 100 mL of methanol were placed in a sonic bath for 10 min. The mixture was then hydrolyzed with 200 mL of deionized water at 80 oC for 6 h. The resulting hydrolysate was washed by ethanol and deionized water for several times, and collected after centrifuging. The powder was dried at 80 oC overnight and then calcined at 450 oC in air for 5 h. In a typical synthesis of AgNPs, 110 mg of AgCF3COO was mixed with 250 µL of dodecanethiol in 50 mL of benzene to form a clear solution under a flow of H2/Ar (5 v/v%) gas, to which 435 mg of borane-tert-butylamine complex (BTBC) was added in one portion. The mixture was heated with stirring at 55 oC for 2 h before the reaction system was cooled to room temperature. AgNPs were precipitated out from the reaction mixture as black solid powders by addition of 20 mL of ethanol. The precipitate was separated by centrifuge, washed with ethanol, and dried in H2/Ar flow. For the synthesis of PtNPs, 100 mg of Pt(acac)2 was dissolved in 30 mL oleylamine at 110 oC for 10 mins, then cooled to 100 oC. The mixture was heated to 140 oC and kept for 4 h after the addition of 2 mL oleylamine containing 200 mg BTBC as the reducing agent. After cooling to room temperature, PtNPs were precipitated from the mixture by addition of 60 mL anhydrous ethanol, and then collected by centrifuge and ethanol washing, and drying in vacuum (60 oC) over night. AgNPs were loaded onto MgO NPs (or other supports) by a colloid deposition method. In a typical preparation, 5 mg AgNPs were dissolved in 25 mL chloroform. To this solution, 495 mg MgO NPs was added. After 30 min stirring, the solid product was centrifuged and dried in N2. The supported AgNPs were calcined in 5 v/v% H2/Ar mixed gas at 450 oC for 5 h to obtain AgNPs/MgO nanocatalysts. The actual loading concentration is 0.8wt % as confirmed by ICP-MS analysis. The same procedure was applied for the preparation of PtNPs/MgO nanocatalysts except for calcination in open air at 450 oC for 5 h. Characterizations TEM images were recorded on a JEOL JEM-1230 operated at 100 kV. HR-TEM and HAADF-STEM combined with EDS measurements were proceeded on FEI TITAN Cs-corrected ChemiSTEM equipped with an energy dispersive X-ray (EDX) spectroscope, operating at 200 kV. Wide-angle XRD patterns were recorded on a Bruker D8 diffractometer using CuKα radiation. Inductively coupled plasma mass spectrometry (ICP-MS) was carried out in Agilent 7700 using the dissolving solution containing calculated AgNPs/MgO in nitric acid to determine the exact Ag loading amount. The 13C solid state

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NMR spectrum was recorded with a Bruker Ascend 400 (400 MHz) and Bruker Avance III HD 400M spectrometer by using adamantane (δ = 38.5 ppm) as an internal standard. Quantitycross Polarization method was used to quatify the carbon species on the surface of catalyst. X-band EPR signals were recorded on a Bruker EPR A-300 spectrometer. The settings for the EPR spectrometer were as follows: center field, 3511.39 G; sweep width, 100 G; microwave frequency, 9.86 G; modulation frequency, 100 kHz; power, 101 mW; conversion time, 10 msec. The solid EPR analysis was carried out by packing AgNPs/MgO powders into a glass capillary tube, which was placed in the EPR chamber for testing. Liquid nitrogen was used to keep the chamber temperature at ~77 K. The location and the intensity of g factors were determined by Bruker’s WINEPR program based on hν = gβH, where h is Planck’s constant, H is the applied magnetic field, and β is the Bohr magneton. The spin trapping experiments were performed as follows: 5,5–Dimethyl-pyrroline-N-oxide (DMPO) spin-trapping reagent and other chemicals were purchased from Sigma-Aldrich Chemical Co. and used without further purification. The formaldehyde solution containing calculated catalyst and ice-cooled DMPO solution (0.08 M) was quickly transferred into a glass capillary tube and tested by EPR spectroscopy at room temperature. Catalytic HER experiments Catalytic H2 production from HCHO/H2O solution was carried out with 20 mg of catalysts suspended in 5 mL of aqueous HCHO solution (1.0 M) in a 55 mL Pyrex test tube under stirring (400 ± 10 rpm). A water bath was used to maintain the reaction temperature at 25 ± 0.5 oC (or 50 ± 0.5 oC). The oxygen content in the reaction tube was adjusted by bubbling with pure O2 gas for calculated time, and the tubes were finally sealed with silica gel stoppers. High pressure HER reaction was carried out in a quartz reactor (200 mL) equipped with a steel cap. O2 pressure was adjusted by an external gas cylinder containing 99.99% O2. Gas volumes of 400 µL were extracted from the test tubes using a microliter syringe at regular intervals, and GC-TCD was employed for evaluating the gas evolution amount, including H2, O2, CO2 and CO. The TOF reported here is an apparent TOF value based on the surface number of Ag atoms in catalyst, which is calculated by the following equation: TOF = (nH2/nAg×t)×(d/1.3), where nH2 and nAg represent the molar amounts of evolved H2 within t h and Ag catalyst, respectively, and d is the average diameter of AgNPs (~20.4 nm calculated using Scherrer’s equation based on the peak width of Ag(111) reflection); 1.3/d is the empirical surface atom dispersion degree for nanoscale particles.The recycling experiment is carried as follows: after 5 h reaction (1 cycle), the test tube was opened and then purged with air for 5 min to remove residual H2, and again sealed for the next cycle. DFT calculations DFT calculations with the periodic boundary conditions were carried out using a plane wave based program, Castep.[9] The Perdew-Burke-Ernzerhof (PBE) functional[10] was used together with the ultrasoft-core potentials.[11] Grimme’s dispersion (van der Waals) correction was employed.[12] Since MgO(001) surface is stable and inert, Ag cluster was desorbed from MgO slab without the dispersion correction. The basis set cutoff energies were set to 310 eV. The electron configurations of the atoms were H: 1s1, C: 2s22p2, O: 2s22p4, Mg:

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2p63s2, and Ag 4d104s1. Two types of catalyst model are employed, i.e., “oxide” and “hydroxide”. The oxide type unit cell consists of three layer MgO(001) slab with a concave type step structure, (MgO)34, and Ag10 hexagonal shaped cluster overlaid. The hydroxide type unit cell was constructed by adding three dissociated water molecules, and has composition of (MgO)34Ag10(HOH)3. For both catalyst models, geometry optimization was carried out with respect to the top MgO layer, Ag10 cluster, and adsorbed molecules. The lattice constants were fixed to those derived from bulk MgO crystal. In the following energetics, the reference energy is taken as the sum of energies of O2, H2O, HCHO, and (MgO)34Ag10 or (MgO)34Ag10(HOH)3. Thus in the latter, adsorption energy of three dissociated water molecules is included in the reference energy. The activation energy for the transition state (TS) is defined as the difference between the TS and the preceding local minimum. 3. RESULTS AND DISCUSSION

Figure 1. The characterizations of 0.8wt % AgNPs/MgO: (a) HAADF-STEM image (inset shows as-synthesized monodispersed AgNPs) and (b-d) EDS spectra showing highly dispersed AgNPs on MgO surface (scale bar, 200 nm), (e) HRTEM image showing a single AgNP on MgO surface with corresponding SAED image (inset), and (f) XRD pattern. AgNPs/MgO catalyst was prepared by a colloid deposition method, followed by calcination under H2/Ar gas (see Supporting Information, SI, for details). The high dispersion of AgNPs (monodispersed AgNPs are shown in the inset of Figure 1a) on MgO surface was verified by HAADF-STEM analysis coupled with detailed element mapping studies (Figure 1a-d). High-resolution TEM (HR-TEM) reveals the crystalline nature of a typical supporting AgNP (Figure 1e). The lattice fringes with an interplanar lattice spacing of 0.239 and 0.212 nm correspond to the Ag(111) and MgO(200) atomic planes, respectively. The result of selective area electron diffraction (SAED) of AgNPs/MgO (inset, Figure 1e) further indicates the final product is with good crystallization. XRD pattern for AgNPs/MgO is shown in Figure 1f. All the diffraction peaks of MgO can be readily indexed to the standard diffraction of cubic MgO (PDF card No. 65-0476). Average crystalline size calculated from the broadening of the (200) XRD peak of MgO is 24.8 nm based on the Scherrer equation. The XRD pattern also shows the diffraction peaks of (111), (200) and (220) for face-centered cubic silver (PDF card No. 04-0783), indicating the successful deposition of AgNPs on MgO surface.

Figure 2. (a) AgNPs/MgO (0.8wt %) catalyzed H2 evolution from HCHO/H2O solution (FA, 1 M) or paraformaldehyde solution (PF, ~1 M); the effect of pO2 on the HER rate: (b) below 1 atm as a function of reaction time and (c) from 1 to 5 atm within 2 h reaction, and (d) the double logarithmic plots of the initial H2 evolution rate against pO2 (inset shows the O2 content evolution as a function of reaction time). AgNPs/MgO nanocatalyst shows an excellent HER activity in HCHO/H2O solution at room temperature (~25 oC) and in air (pO2 = 0.22 atm). Figure 2a reveals the steady H2 evolution from HCHO/H2O solution (1 M) over 0.8wt % AgNPs/MgO. We find that AgNPs/MgO also effectively catalyzes paraformaldehyde (PF) solution into H2 under identical reaction conditions. In commercial HCHO solution (i.e., formalin), methanol (MeOH) is pre-added to prohibit the oligomerization of HCHO molecules. Thus it can be inferred that MeOH has no adverse effect on the HER in HCHO solution. On addition of the catalyst into the mixture, constant H2 gas is immediately generated without any induction period, and no detrimental carbon monoxide is detected during the entire reaction period. The catalyst exhibits high HER activity with TOF = 548 h–1 within 6 h reaction (see the Experimental Section for calculation methodology). MgO alone or replacing the support by silica does not show any HER activity, indicating a synergic effect between Ag and MgO. The influencing factors (e.g., substrate concentration, reaction temperature and metal loading) are given in Figure S1 to S3 (SI). In detail, a positive feature of the AgNPs/MgO catalyst is its soluble base-free condition and resulting high tolerance toward HCHO concentration (Figure S1). Base additives (NaOH or KOH) are commonly used in HER from HCHO solution (Table S1). The use of base additives leads to the undesired disproportionation of the aldehyde (Cannizzaro reaction), which follows second order kinetic in aldehyde. Consequently, high HCHO concentration (> 1 M) is disadvantageous for the HER in these catalytic systems. To achieve high energy density, concentrated HCHO solution is necessary. Due to its soluble base-free reaction condition, the HER rate over AgNPs/MgO catalyst displays an optimal HER activity at high HCHO concentration (2.5 M, corresponding to a TOF of 582 h–1). Even at a much higher HCHO concentration (e.g., 10 M), the TOF of the silver catalyst is still higher than most reported values (Table S1).

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The effect of the reaction temperature on HER is investigated in the range of 0–70 oC (Figure S2). It is noted that the HER is very sensitive to the temperature. The initiate HER rate increases greatly with the increasing reaction temperature. As the temperature increases from 0 to 50 oC, the steady-state HER rate increases rapidly from 100.6 to 120 mmol h–1 g–1 catalyst. However, further increasing the temperature to 70 oC leads to the rapid decrease of HER rate, possibly due to the instability of •OOH radicals at high temperatures. We will show later that •OOH radical is the main active species to the success of the HER. As the metal loading increases from 0 to 2.0wt %, the HER rate increases steadily (Figure S3a). However, when the HER rate was normalized into mass specific activity by a given MNP, the 0.8wt % catalyst is found to outperform other catalysts (Figure S3b). Thus we use this specific sample for further studies. The most distinctive feature of AgNPs/MgO catalyst is its oxygen-promotion effect (OPE). In the absence of O2, no H2 is produced from HCHO solution (Figure 2b). The HER rate is significantly enhanced by increasing pO2 from 0 to 1 atm, corresponding to a TOF increase from 0 to 1,969 h–1. Such OPE still works under pressurized conditions. The high-pressure reaction is carried out in a home-made quartz reactor that can stand at most 5 atm. It is found that the HER rate keeps ascending steadily as pO2 increases continuously from 1 to 5 atm (Figure 2c). A linear dependency with a slope of 0.85 has been obtained from double logarithmic plots of the initial rate against pO2 (Figure 2d), suggesting a quasi-first order kinetics of the HER rate. The TOF value is as high as 6,641 h–1 under pressurized condition (5 atm), corresponding to an excellent H2 production rate of 1,378 LH2 h−1 gAg−1. To our knowledge, this reaction rate is the highest value ever reported for HER from HCHO/H2O solution using a heterogeneous catalyst (Table S1). Meanwhile, the O2 content remains almost unaffected within 6 h reaction irrespective of its initial concentration (inset of Figure 1d, where three experimental conditions, 0.03, 0.295 and 0.505 mmol O2 present in the reaction system, are selected), indicating that O2 has not been consumed during the entire reaction. The OPE and the constant O2 concentration make O2 a sort of “catalyst” in the reaction. We primarily expect that reactive oxygen species (ROS) such as •OOH radicals derived from molecular O2 reduction over catalyst surface play decisive roles in reaction kinetics. To verify our proposal, we added H2O2, an analogous counterpart to •OOH radical, into HCHO/H2O mixture and found that H2O2 has similar promoting effect on the HER (Figure 3a), although H2O2 is normally a robust oxidant.13 The HER rate is linearly proportional to the concentration of H2O2 (inset, Figure 3a), similar to the HER studies carried by AgNPs/MgO catalyst. This coincidence confirms our speculation that adsorbed ROS may be the real catalyst in HER. If this is true, the TOF of O2 catalyst can be calculated to be 20–41 h–1 with respective to dissolved molecular O2 in HCHO solution (Table S2), which is about 40–300 fold lower than the TOF of the AgNPs. It implies that only a small amount of dissolved O2 molecules interacts with surface Ag atoms and actively participates in the reaction process.

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Figure 3. (a) HER in aqueous HCHO (1 M) with different concentrations of H2O2 (inset shows the simulating curve of HER rate against H2O2 concentration); (b) solid EPR spectra of different samples carried out at 77 K or at 298 K in open air; operando EPR spectra of (c) DMPO adducts recorded in the ternary systems containing HCHO, H2O and AgNPs/MgO catalyst in open air, and (d) DMPO adducts recorded for AgNPs/MgO or PtNPs/MgO reaction systems from 0 to 10 min. Next, we try to understand how ROS is generated in the reaction system and further elucidate the origin of their catalytic functions. The solid electron paramagnetic resonance (EPR) spectra collected at 77 K provides spectroscopic evidence of the ROS on AgNPs/MgO catalyst. As illustrated in Figure 3b (blue line), an anisotropic species is observed in the AgNPs/MgO sample on exposure to air, which is very likely to be the surface localized superoxide radical anion O2•– exhibiting g‖ at 1.999 and g⊥ at 2.255. This is in accord with the ability of MgO to locate oxygen derived species such as superoxide.14 On the other hand, no apparent EPR signals are detected on bare MgO or AgNPs/SiO2 samples, suggesting the synergetic effect between Ag and MgO for the electron transfer to adsorbed O2 molecules. In addition, a relatively weak but apparent signal at g ⊥ = 2.255 is also observed for the AgNPs/MgO sample at 298 K, indicating that O2 can be easily activated on the catalyst surface even at room temperature. More mechanistic information is obtained by identifying surface intermediates using operando liquid phase EPR spintrapping experiments. This examination is carried out with a suspension containing AgNPs/MgO catalyst, HCHO/H2O mixture, and DMPO in air. As shown in Figure 3c, a pronounced nine-fold signal labeled by the stars characteristic of the DMPO–•H radical (hyperfine splitting constants: αN = 16.57 G; αH(1) = αH(2) = 22.58 G), appears as soon as the EPR recorded. In addition, a group of signals labeled by squares are also emerged, which can be assigned to the DMPO–O2•− adduct (αN = 19.4 G; αH = 16.7 G). The intensity of the radical signal decreases as a function of operation time, and completely disappears after 300 s. It is noted when the suspension was purged with N2 in advance, no prominent DMPO–•H signal was found under otherwise identical reaction conditions, which confirms the decisive role of O2 in the dehydrogenation kinetics. Importantly, a spin adduct with coupling constants of aN = 14.8 G,

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aβH = 14.7 G, and aγH = 1.6 G is observed from the mid-term to the end of 10 min reaction, which is consistent with an oxygen-centered species, most likely •OOH radicals, over the catalyst surface. Notably, the intensity of the •OOH radicals is enhanced thereafter the disappearance of •H radicals, and keeps almost constant with prolonged reaction time. Thus, the resulting •OOH radicals are supposed to be the main ROS for the dehydrogenation of HCHO/H2O mixture. We notice that no hydroxyl radicals (•OH) are generated during the entire examination period, suggesting that the intermediacy of •OOH does not give rise to •OH, via a Fentontype reaction over the silver surface. In contrast to the AgNPs/MgO system, an intensive DMPO-•OH adduct is generated after 10 min reaction in the PtNPs/MgO catalytic system (Figure 3d), indicating that •OOH is apt to decompose into •OH over the Pt surface because it is much more active than that of Ag at low temperatures. Apparently, the resulting •OH species could not only oxidize HCHO into CO2 (HCHO + 4•OH → 3H2O + 2CO2), but also quench •H into H2O (•H + •OH → H2O), as the oxidation potential of •OH is as large as 2.8 V while •OOH is only 0.87 V in basic media. This is the possible reason that no H2 is generated in the PtNPs/MgO catalytic system even though •H radicals are formed at the beginning (purple line, 0 min). Thus, it can be concluded that the resulting surface stabilized MgO/Ag–•OOH complex actively participate in the HCHO/H2O dehydrogenation reaction rather than total oxidation.

process suggests a four-electron pathway for the oxygen reduction at the Pt/MgO electrode, as also evidenced by the corresponding negligible current for HO2– oxidation recorded at the Pt ring electrode.15 The transferred electron number (n) per O2 molecule involved in the oxygen reduction is calculated from Eq. (1) to be 2.3 and 3.8 for the Ag/MgO and Pt/MgO electrode (at the potential of –0.50 V), respectively, suggesting a dominant two-electron reduction process for the Ag electrocatalyst and a four-electron reduction for the Pt catalyst. n = 4ID/(ID + IR/N) Eq. (1) where N = 0.3 is the collection efficiency, ID is the faradic disk current, and IR is the faradic ring current. We further employ aqueous NaNO3 (0.1 M) as a neutral electrolyte for studying the electrocatalytic properties. As shown in Figure 4b, Ag/MgO still exhibits a two-step process for the oxygen reduction with the onset potential of about – 0.32 V and –0.59 V, respectively, while Pt/MgO presents a distinct one-step process at –0.32 V. The transferred electron number per O2 molecule over Ag/MgO and Pt/MgO is calculated to be 2.7 and 3.74, respectively, similar to the KOH system.

Figure 4. RRDE voltammograms and the corresponding amperometric responses (inset) for oxygen reduction in oxygensaturated (a) 0.1 M KOH and (b) 0.1 M NaNO3 at the Ag/MgO (black lines) and Pt/MgO (red lines) electrodes. The selective generation of peroxy and hydroxy speices on AgNPs/MgO and PtNPs/MgO catalysts surface is further demonstrated by rotating ring-disk electrode (RRDE) voltammograms. Figure 4a shows the steady-state voltammograms for the Ag/MgO and Pt/MgO electrocatalysts supported by a glassy carbon electrode in O2 saturated 0.1 M KOH electrolyte. The Ag/MgO electrode exhibits a two-step process for the oxygen reduction with the onset potential of about –0.32 V and –0.59 V, respectively. The first sharp step over –0.32 V can be attributed to the two-electron reduction of O2 to HO2–, as supported by a substantial concomitant increase in the oxidation current over about –0.36 to –0.58 V at the ring electrode (inset of Figure 4a). The current increase corresponds to the amperometric responses for the oxidation of hydrogen peroxide ions (HO2–) measured with a Pt ring electrode at the potential of 0.50 V.15 Unlike the Ag/MgO electrode, the Pt/MgO electrode shows a one-step process over –0.63 V for the oxygen reduction with the steady-state diffusion current that is more than 1.3 times higher than that obtained at the Ag/MgO electrode. The observed one-step

Figure 5. (a) Energy changes along the reactions occurred on (MgO)34(001)Ag10 slab (black line) and (MgO)34Ag10(HOH)3 slab (red line), and the structures of four transition states (TS’s) based on (MgO)34(001)Ag10 slab (b) and (MgO)34Ag10(HOH)3 slab (c), respectively. We perform DFT calculations to provide molecular level insights into the OPE dominated HER reaction pathway. In this case, both of (MgO)34Ag10 and (MgO)34Ag10(HOH)3 unit cells (Figure S4) are employed and then conducted to provide detailed information of energetics and structures of elementary reaction steps. The energy changes for overall reactions proceed on (MgO)34Ag10 and (MgO)34Ag10(HOH)3 model catalysts are shown in Figure 5a (black and red lines, respectively). The whole reaction consists of 12 configurations including 5 local minima and 3 transition states (TS’s) (Figure 5b and 5c), and their optimized structures are shown in the SI with configuration numbers, #1 to #12 (Figures S5 to S17). The reaction starts by adsorption of H2O on MgO site (#3 to #4). O2 is coadsorbed on Ag10 cluster with an end-on coordination, which

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is in favor of peroxide formation [#5].16 Then HCHO is further adsorbed molecularly onto this surface complex, and the total adsorption energy is calculated to be –284 (or –174) kJ/mol. Next step is H abstraction from HCHO by O2 molecule [#6 to #8]. The resulting HCO• is an unstable radical, and its behavior is a little different between the two models. For (MgO)34Ag10 catalyst, HCO• is immediately combined with OH species on Mg site to produce HCOOH, which is schematically represented as (Ag)–O2 + HCHO + HO–(Mg) → (Ag)–OOH + HCOOH + (Mg). The relative energy is below zero energy (–24 kJ/mol), and the reaction proceeds without essential energy barrier. Configuration #9 indicates an intermediate structure for •OOH migration from Ag cluster to MgO site. For (MgO)34Ag10(HOH)3 catalyst, the relative energy and activation energy for the TS are estimated to –47 and 116 kJ/mol, respectively, for the reaction (Ag)–O2 + HCHO → (Ag)–OOH + HCO•. The formed HCO• has a stable isolated configuration #8. Restarting optimization from a slightly different structure leads to HCOOH as configuration #9. Peroxide species (•OOH) is much stabilized by bonding to Mg site [#10] (–308 kJ/mol). The elementary reaction [#10 to #12] proceeds on MgO site. H2 and O2 are generated from •OOH–(Mg) and H–(O). This step is considered to be the rate determining step, whereas the relative energy is again below zero energy (–6 kJ/mol). It confirms that molecular O2 can indeed acts as a catalyst. The stoichiometry for the overall OPE dominated dehydrogenation reactions over AgNPs/MgO is thus suggested as follows: O2 + H2O + HCHO → •OOH + •H + HCOOH → O2 + H2 + HCOOH In real reaction, most of the in-situ generated HCOOH molecules are eventually converted into methyl orthoformate or other esters by reacting with the pre-existing MeOH in the reaction system. This may be one of the reasons that more H2 is generated from formalin than that of PF solution, though the same catalyst is used. Based on the GC-MS, H-NMR and solid NMR results (Figures S18 to S20), it is determined that about 95% reacted HCHO molecules are converted into HCOOH intermediates in the catalytic system. In addition, around 5.2% HCOOH can be further decomposed into H2 and CO2, and the evolved CO2 is captured by MgO to form basic magnesium carbonate. Thus, further catalyst optimizations, such as coupling with Pd or modifying the support are prerequisite to achieve a more sustainable OPE controlled HER. 4. CONCLUSIONS In summary, we show a new and highly efficient HER using AgNPs/MgO as the catalyst and HCHO solution as the substrate at ambient conditions. In contrast to normal HER, we find that molecular O2 greatly promotes the reaction rate and itself is not consumed during the reaction. We propose that a surface stabilized MgO/Ag–•OOH complex is the catalytically active center that directly participates in the liberation of H2 as well as the regeneration of O2 from the reaction system. This OPE controlled HER not only brings new opportunities to the development of high-performance heterogeneous catalysts for H2 generation from liquid-phase chemical hydrogen storage chemicals, but also shines light on the critical catalytic role of surface ROS in HER.

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ASSOCIATED CONTENT Supporting Information Reaction kinetic factors and detailed information of energetics and structures of elementary reaction steps based on the DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for financial supports from the National Science Foundation of China (21503189, 21403197, 91545113 and 51133006) and Zhejiang Provincial Natural Science Foundation of China (LY15B030009). The authors appreciate Prof. Xueqian Kong and Mr. Ming Xia for performing solid NMR measurements and helpful discussions.

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