Ultrasmall Silver Clusters Stabilized on MgO for Robust Oxygen

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Energy, Environmental, and Catalysis Applications

Ultrasmall Silver Clusters Stabilized on MgO for Robust OxygenPromoted Hydrogen Production from Formaldehyde Reforming Shuang Chen, Shipan Liang, Biling Wu, Zhuohuang Lan, Ziwei Guo, Hisayoshi Kobayashi, Xiaoqing Yan, and Renhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11023 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Ultrasmall Silver Clusters Stabilized on MgO for Robust OxygenPromoted Hydrogen Production from Formaldehyde Reforming Shuang Chen,† Shipan Liang,† Biling Wu,† Zhuohuang Lan,† Ziwei Guo,† Hisayoshi Kobayashi,*,‡ Xiaoqing Yan,§ Renhong Li*,† †Department

of Materials Engineering, College of Material and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018,

China. ‡Emeritus

Professor of Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan §Department

of Chemistry, College of Science, Zhejiang Sci-Tech University, Hangzhou 310018, China

ABSTRACT: Efficient molecular hydrogen generation from renewable biomass derived resources and water is of great importance to the sustainable development of future society. Herein, ultrasmall Ag nanoclusters supported on defect-rich MgO matrix (AgUCs/MgO) are synthesized by a facile impregnation/calcination method and are applied to robust oxygen-promoted formaldehyde reforming into H2 at room temperature. DFT calculations and experimental observations show that the catalyst spatially builds up a channel for directional electron transfer from electron-rich Ag sites to anti-bonding π orbital of chemisorbed bridged O2 molecules, leading to the implementation of low temperature O2 adsorption and activation. The catalytically active species, •OOH, is thus selectively generated via a preferential two-electron reduction of O2 with low energy barrier on Ag sites involving an unusual longrange proton coupled electron transfer process. The •OOH–AgUCs/MgO active center is efficient for the subsequent C–H activation and H2 generation, leading to a 3-fold improvement of turnover frequency as compared with its analogous AgNPs/MgO catalyst. Our atomic-level design and synthetic strategy provide a platform that facilitates construction of electron-proton transfer channel for catalysis, altered adsorption configurations of activated reactants and enhancement of catalytic hydrogen generation activity, extending a promising direction for the development of next-generation energy catalysts. KEYWORDS: cluster, formaldehyde, reforming, hydrogen production, proton coupled electron transfer

1. INTRODUCTION As a clean and renewable energy resource, molecular hydrogen (H2) has been considered as an attractive alternative energy supply for a range of applications,1-4 while the problem of safe storage and transportation limits its large-scale implementation.5 A possible solution to this bottleneck is in situ release of the required H2 from a stable liquid, which is ideally composed of aqueous-phase biomass-derived resources.6-18 In this regard, the use of formaldehyde solution has drawn intense interest, because it is inexpensive and can reform itself with water to release hydrogen with a high gravimetric density of 8.4 per cent by weight.6-13 Noble metal nanomaterials, such as Ag,67 Pd,8-9 Au,10-11 etc., are perhaps the most common heterogeneous catalysts for formaldehyde decomposition to produce high level hydrogen gas. However, these catalysts still face practical problems regarding harsh reaction conditions since the catalyst needs to be active and selective towards stable H2 production during continuous reaction process. Therefore, developing an efficient heterogeneous catalyst to optimize the kinetics of the formaldehyde dehydrogenation process and thus

achieve robust H2 evolution at mild conditions is of great importance. Noble metal nanoclusters (NCs) consisted of limited (several~100) metal atoms are recently emerged as a new frontier in such catalysis,19-22 with the added economic benefit that the precious metal is diluted at the atomic limit. With small sizes on metal oxide supports, this few-atom metal construction offers the following advantages as compared with their nanoparticles (NPs) and bulk counterparts: (i) a significant reduction in catalytic metal usage by maximizing the active metal utilization efficiency, (ii) the high activity and selectivity by their low-coordination, unsaturated sites as well as metalsupport interactions and (iii) well-defined active sites for mechanistic studies. The unique electronic structure and unsaturated coordination environments of the active sites endow NCs very different and often enhanced surface process, including facile dissociation of reactant molecules and weak binding of intermediate species and/or products.23 In particular, the coordinatively unsaturated metal atoms can offer d-orbitals with electrons to bind with orbitals of molecules such as oxygen, thereby achieving selective chemisorption. The formed bonds

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subsequently construct a channel for driving the electron transfer from d-orbitals of metal atoms to the lowest unoccupied molecular orbitals of chemisorbed molecules, facilitating molecular bending and/or bond dissociation.24-25 Although size-selected NCs (or even individual single atoms) soft-landed on oxide substrates have been extensively used for fundamental studies,26-29 such a high-cost and low-yield fabrication method is not suitable for practical applications. Herein, we report a facile impregnation/calcination route to stably immobilize the ultrasmall Ag nanoclusters on defect-rich MgO (denoted as AgUCs/MgO) for catalytic oxidative formaldehyde reforming into H2 at low temperatures. Importantly, the exceptionally high density of electron-rich surface Ag sites not only render efficient sites for synchronous adsorption and activation of O2 and HCHO molecules, but also facilitate the transfer of electron from Ag center to chemisorbed bridged O2 to form superoxide species (O2•−). The rate determining step of the whole reaction, i.e., C–H bond cleavage of HCHO is subsequently achieved by abstraction of hydrogen via O2•− to the formation of peroxide species, which is crucial to the success of the H2 generation. Therefore, a sequential proton coupled electron transfer (PCET) is realized in the catalytic reaction that involves intermolecular proton transfer process with significant contributions from excited electronproton vibration states between two guest molecules. These catalytic features substantially lower the energy barriers of reactants dissociation as well as products desorption on the catalyst surface, resulting in its catalytic performance far superior to AgNPs/MgO. 2. EXPERIMENTAL SECTION Chemicals. All reagents were analytical grade quality. MgO (99.9%, Sigma-Aldrich), AgNO3 (99.8%, Sigma-Aldrich), ethanol [analytical reagent (AR)], formaldehyde (AR), acetaldehyde (AR), benzaldehyde (AR), propanal (97%, Sigma-Aldrich), isobutyraldehyde (99.5%, Sigma-Aldrich), cinnamaldehyde (≥ 95%, Sigma-Aldrich), cyclohexanecarboxaldehyde (97%, Sigma-Aldrich), were used as received without further purification. Preparation of AgUCs/MgO. The AgUCs/MgO were synthesized by incipient wetness impregnation method. During the synthesis process, 0.25 M (13 mg) of AgNO3 and 20 M (1 g) of MgO were dissolved in 30 mL of ethanol, respectively. When stirring at 300 r·min-1, AgNO3 solution were dosed at a rate of 10 g·min-1 into MgO alcohol solution. The resulting solution kept at that rotating speed for 3 h, thus forming a homogeneous dark yellow solution and the precursors were separated from the solution by centrifugation (8000 r·min-1, 5 min) with ethanol for 3 times, followed by drying in an oven at 60 oC overnight. The obtained yellow solid was calcined in a pipe furnace at 300 oC for 3 h in a flow of air with a heating ramp of 4 °C·min−1 and then cooling to room temperature.

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Preparation of AgNPs/MgO. Typically, AgNPs/MgO was prepared with the same synthesis condition of AgUCs/MgO except that the calcination condition of air was replaced by H2/Ar. The silver concentration of the prepared AgNPs/MgO catalyst is 0.8 wt%. Characterizations. Powder X-ray diffraction patterns of samples were determined using a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The morphologies and sizes of samples were characterized by JEOL JEM-1230 operated at 100 kV. The high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) and HRTEM combined with EDS measurements were performed on a FEI TITAN Cs-corrected ChemiSTEM equipped with an energy dispersive X-ray (EDX) spectroscope, operating at 200 kV. Before microscopy examination, the samples were dry dispersed onto a copper grid coated with a thin holey carbon film. UV-vis absorption spectra were measured on a PerkinElmer Lambda 35 UV-vis spectrophotometer. The XPS experiment was performed on a VG Scientific ESCALAB Mark II spectrometer. All binding energies were calibrated using the adventitious carbon to the C1s peak at 284.8 eV. X-ray absorption fine structure (XAFS) spectra (Ag K-edge) were collected at BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF) operated at 3.5 GeV with injection currents of 140-210 mA. A Si(111) double-crystal monochromator was employed for energy selection. Ag foil and AgNPs/MgO were used as reference samples and measured in the transmission mode, and AgUCs/MgO was measured in fluorescence excitation mode. Prior to the experiments, the powder sample was reduced at 200 oC for 30 minutes in a Ushaped tube with valves on each end. After it was cooled to room temperature, the reactor was evacuated and transferred into an argon-filled glove box. All of the samples were then sealed with Kapton membrane and subjected to EXAFS measurement. The XAS spectrum was processed and analyzed using Athena and Artemis. The coordination number, distance and Debye-Waller factor were the fitted variables and fitting range in k space was 3-12 Å-1 and in r space was 1-4 Å. All spectra were collected in ambient conditions. Raman spectra were excited by a nanosecond Nd:YAG (Innova 305, Coherent, with adjustable wavelength) and recorded on a T-64000 spectrometer (Jobin Yvon Instruments, SA) equipped with a liquid nitrogen cooled CCD camera and a triple stage spectrograph. The excitation laser beam was loosely focused to the quartz tube containing formaldehyde solution and/or the solid catalyst as a function of reaction time. Background curves and spikes were subtracted from the presented spectra. The calibration of the spectrometer was done with the known vibrational frequencies of the dichloromethane solvent Raman bands before each set of measurements. The oxygen temperature-programmed desorption (O2-TPD) was performed on a Quantachrome automated chemisorption


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analyzer (chemBET pulsar TPR/TPD) to investigate the interaction between O2 and catalyst surface. Approximately 100 mg sample was first treated at 350 oC for 60 min under Ar atmosphere to remove residual moisture. After cooled to room temperature, 10 vol% O2/Ar was flowed at 50 mL/min through the sample for 60 min. Finally, the sample was heated to 550 oC at a ramp rate of 10 oC/min. X-band electron paramagnetic resonanc (EPR) signals were recorded at an ambient temperature in 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 ms. AgUCs/MgO, AgNPs/MgO and MgO powders were packed within cylindrical quartz EPR tubes respectively, and then place int the EPR chamber for the solid EPR testing. The location and the intensity of the 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 carried out by recording the EPR signals of free radicals generated during the formaldehyde-reforming process trapped by 5,5-dimethyl1-pyrroline N-oxide (DMPO) at room temperature. Electrochemical measurements were carried out with a threeelectrode system on a CHI750e electrochemical workstation. Catalytic H2 evolution reaction experiments. In the experiments of H2 production activity, 20 mg of catalysts was dispersed in 5 mL of aqueous HCHO solution in a 55 mL Pyrex test tube under continuous magnetic stirring. After bubbled with pure oxygen gas for a calculated time, the hydrogen generation proceeded at certain temperatures (20, 25, 30, 40 and 50 oC) with a water bath. For quantifying the gas evolution amount, 400 μL of gases were taken out from the vessel using a microliter syringe and tested by GC-TCD at regular intervals. The TOF reported is calculated by the following equation: TOF = (nH2/nAg × t), where nH2 and nAg represent the molar amounts of evolved H2 within t h and Ag-based catalysts, respectively. For the recycling experiment, after 3 h of reaction (1 cycle), AgUCs/MgO catalyst was separated from the reaction solution by centrifugation and dried under vacuum at 60 oC. The dried catalyst was weighed and again added formaldehyde solution for the next cycle. DFT calculations. DFT calculations with the periodic boundary conditions were carried out using a plane wave based program, Castep.30-31 The Perdew−Burke−Ernzerhof (PBE) functional32 was used together with the ultrasoft-core potentials and Grimme’s dispersion (van der Waals) correction was employed.33-34 The basis set cutoff energy was set to 310 and 340 eV for geometry optimization and post-energy calculation, respectively. The electron configurations of the atoms were H: 1s1, C: 2s22p2, O: 2s22p4, Mg: 2p63s2, and Ag: 4d105s1. The lattice parameters were a = 12.6336 Å, b = 8.4224 Å, and α = β = γ = 90°. The surface normal was taken in the direction c, and

set to c = 24.2 Å including the vacuum region. Two types of catalyst models were employed. MgO support was modeled by a three-layer slab with atomic composition of (MgO)36 representing the MgO(001) surface. In Type-1 model, the two Ag atoms and the nearest two Mg atoms interchanged their atomic positions to make the alloy. In Type-2 model, four Ag atoms were just deposited on the surface Mg atoms. In all the configurations for both catalyst models, the geometry of lower two layers (drawn by thin lines in Figure S9) was fixed to the crystalline structure of bulk MgO, and geometry optimization was carried out with respect to the top layer of MgO slab, Ag4 cluster, and substrate molecules. In the following energetics, the realtive energy is referred to the sum of the energies of O2, H2O, HCHO, and (MgO)36Ag4. 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 As illustrated in Figure 1a, the synthesis of defect-rich MgO supported isolated Ag nanoclusters (AgUCs/MgO) begins with the formation of AgOH hydroxide intermediate on the surface of MgO via an incipient wetness impregnation (IWI) method and a subsequent low temperature calcination strategy (see the Experimental section). Meanwhile, for comparison, Ag NPs of ~12 nm stabilized by MgO (denoted AgNPs/MgO) with the same loading is prepared (Figure S1). The XRD patterns of AgUCs/MgO and AgNPs/MgO are shown in Figure 1b. All characteristic peaks of MgO can be indexed to the standard diffraction of cubic MgO (PDF card No. 65-0476). However, no reflections attributable to any Ag phase are detectable within AgUCs/MgO catalyst, indicating the presence of highly dispersed Ag species. As compared with bare MgO (Figure 1c), a broad and strong absorption in the visible region (300–800 nm) appears in AgNPs/MgO due to the localized surface plasmon resonance of Ag NPs,35 while AgUCs/MgO does not show any significant change, confirming that all silver species are isolated and do not form large NPs over the entire MgO surface. Transmission electron microscopy (TEM; Figure S2a) and high-resolution TEM (HR-TEM; Figure S2b) are then employed to study the detailed structure of Ag on MgO. The TEM and HR-TEM images show that the AgUCs/MgO catalyst possesses stepped edges, kinks, and single/multiple vacancies across the whole catalyst surface, which is further confirmed by XPS (Figure S3) and ESR (Figure 4a) measurements. No Agderived NPs are observed, and the existence of a high density of ultrasmall Ag clusters is evidenced by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Due to the Z-contrast, slightly brighter island-like aggregates (marked in red circles) can be distinguished from MgO matrix (Figure 1d-1f and S2c-2e). The fully dispersed Ag clusters show a narrow particle size distribution and high homogeneity, with an average size of 2.31 nm. Atomic-resolution energy-dispersive spectroscopy (EDS)



element mapping shown in Figures 1g, 1h and S2f further corroborates the highly random distribution of ultrasmall Ag clusters on MgO surface.

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that the metallic Ag is maintained and there is no sign of oxidation silver phase on the AgUCs/MgO catalyst. X-ray absorption near edge structure (XANES) and XPS are employed to determine the electronic properties of Ag and Mg. It can be seen that AgNPs/MgO exhibits similar Ag-K absorption relative to Ag foil (first-order derivative peak appears at 25524.18 eV, Figure 2b), suggesting that AgNPs are in the metallic state. In contrast, the slightly downshifted white line intensity of AgUCs/MgO clearly shows that the electron density of Ag is higher than that of Ag foil and AgNPs/MgO. The XPS analysis further confirms the accumulation of electrons in Ag clusters (Figure 2c). A red shift (~0.50 eV) of the Ag3d peak is indicative of electron transfer to more electronegative silver atoms present within ultrasmall AgUCs/MgO.41-43 This hypothesis is also consistent with the concomitant increase in Mg1s binding energy in AgUCs/MgO (Figure S3).

Figure 1. (a) Schematic illustration of the synthesis process for AgUCs/MgO, (b) XRD and (c) UV-vis patterns of AgUCs/MgO, AgNPs/MgO and MgO, (d-f) aberrationcorrected HAADF-STEM and (g-h) EDS mapping images of AgUCs/MgO. The local structures of Ag (r space) are determined by Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectroscopy (Figure 2a). As listed in Table S1, two prominent peaks at ~2.49 Å and ~3.31 Å are from the Ag–Mg and Ag–Ag contributions, respectively. In comparison with AgNPs/MgO, a monotonic decrease in the coordination number of the 1st shell for Ag–Ag interactions (from 8.97 to 1.48), and an increase in the Ag–Mg coordination number (from 0 to 2.85) are observed for AgUCs/MgO, suggesting that Ag clusters interact more strongly with MgO than that of Ag NPs. The Ag– Mg coordination is most likely originated from the few-atom Ag clusters occupied an oxygen vacancy, leading to the spontaneous bonding between Ag and metallic element in the supports.23, 36-37 The decrease of significant Ag–Ag scattering in AgUCs/MgO indicates that Ag NPs are almost eliminated from the catalyst and that isolated ultrasmall Ag clusters become the dominant structure, in consistent with the result of HAADFSTEM analysis. Meanwhile, the contribution from Ag–O can be excluded because it is usually located around ~1.50 Å.38 The Ag3d XPS spectrum (Figure S4) for AgUCs/MgO shows two obvious peaks at the binding energies of 368.3 eV and 374.1 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively. The 5.8 eV splitting energy of the 3d doublet also suggests the metallic nature of Ag clusters.39-40 Temperature programmed reduction (TPR; Figure 2d) and fourier-transform infrared (FTIR; Figure 7) measurements further provide the evidence

Figure 2. (a) EXAFS with corresponding R space fitting curve, (b) XANES spectra of AgUCs/MgO, AgNPs/MgO and Ag foil, (g) XPS spectra of Ag3d in AgUCs/MgO and AgNPs/MgO, and (d) TPR patterns of AgUCs/MgO, AgNPs/MgO and MgO. As depicted in the TPR process (Figure 2d), the AgUCs/MgO surface displays a rather uncommon behavior for the dissociation and chemisorption of H2. The optimal reduction temperature of AgUCs/MgO composite is 6.6 oC and 16.5 oC lower than that of AgNPs/MgO and bare MgO, respectively. More critically, a negative and broad peak around 220 oC is observed in the case of AgUCs/MgO, which is ascribed to the H2 spillover effect.44-46 Since this specified spillover of H2 from Ag sites onto contiguous oxide species benefits the reduction of MgO, the unique electronic structure of Ag nanoclusters is thus supposed to boost the chemisorption and activation of reactants including H2,24 leading to its high catalytic dehydrogenation performance. In addition, the dramatic increasing number of silver monodispersed entries on the


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surface also effectively increase the number of entrance and exit routes of H2 on and off the MgO surface.

Figure 3. Catalytic hydrogen evolution of AgUCs/MgO and AgNPs/MgO from a HCHO/H2O solution (1 M): (a) the effects of reaction temperature (0-50 oC) in air (i.e. pO2=0.22 atm), (b) corresponding Arrhenius plots, (c) the effects of pO2 (0.01 to 1 atm), and (d) initial hydrogen evolution against pO2 within 3 h reaction (the insert shows the double logarithmic plots of the initial H2 evolution rate against pO2) at room temperature. Catalytic oxidative HCHO reforming into H2 at low temperatures is used as a probe reaction to demonstrate the superiority of AgUCs/MgO catalyst. As summarized in Table S2, MgO is completely inactive and AgNPs/MgO exhibits a H2 production rate of 61.6 molH2 h-1 gAg-1 in air at room temperature (~25 oC), while AgUCs/MgO shows the highest catalytic H2 production efficiency of 143.0 molH2 h-1 gAg-1 within 3 h reaction. The corresponding turnover frequency (TOF) of AgUCs/MgO reaches 182.9 h-1, which is ~3 times higher than that of AgNPs/MgO (60.5 h-1). In addition, the hydrogen generation rate of AgUCs/MgO is very sensitive to the reaction temperature (Figure 3a), and the highest TOF achieves 328.2 h-1 at 50 oC. Moreover, the activation energy of AgUCs/MgO (Ea = 16.4 kJ/mol) is lower by a factor of 3.9 kJ/mol, revealing the minimal alteration of dehydrogenation reaction pathways over the two structurally and electronically different catalysts (Figure 3b). HCHO concentration and catalysts calcination temperature are also identified as the other two factors to the performance of AgUCs/MgO (Figure S5-S6).

reaction from HCHO/H2O solution among a number of heterogeneous catalysts (Table S2). In addition, the H2 evolution over AgUCs/MgO obeys a parabolic correlation with pO2 (Figure 3d), and a fitting linear dependency with slopes of 0.93 and 0.64 corresponding to AgUCs/MgO and AgNPs/MgO are respectively obtained from double logarithmic plots of the initial reaction rate vs pO2, indicating their different O2 dependent property. Therefore, the activation and transformation of O2 into specific reactive oxygen species (ROS) is an essentially important process for such ambient reactions.47-49 O2temperature-programmed desorption (O2-TPD) is carried out to investigate the desorption behavior of active oxygen species, and the result is shown in Figure S7. A broad peak (< 200 oC) is detected in both AgUCs/MgO and AgNPs/MgO, which is ascribed to the desorption of physically/chemically adsorbed oxygen O2. In AgUCs/MgO, the desorption peak of O2−/O− species at ~450 oC is much higher than that of AgNPs/MgO.5051 This result demonstrates that individual, electronegative Ag sites is required to assist adsorption/activation of O2 and thus promote the subsequent HCHO oxidation reaction. We further resort electron paramagnetic resonance (EPR) and in-situ laser resonant Raman techniques to track radical generation and C– H bond breaking dynamics. Typical signals at g ⊥ = 2.081 and g|| = 2.104 recorded in solid-state EPR could be assigned to the superoxide species (O2•−) formed on AgNPs/MgO surface at the initial step of oxygen activation, whereas no apparent EPR signals are detected in pure MgO (Figure 4a). The intensity of O2•− becomes much intense for AgUCs/MgO, suggesting that isolated Ag site is in favor of stabilizing the ROS, which is consistent well with the TPD results. Another unusual 5-fold signal occurs in parallel (black triangles), which is potentially ascribed to the bridged adsorption configurations of O2•− on Ag sites with strong mutual electronic interactions. Accordingly, the resulting charge polarization effectively increases the interaction strength with the O2p levels upon chemisorption. It spatially builds up a channel for electron transfer from electronrich Ag sites to anti-bonding π orbital of chemisorbed O2 molecules due to the overlap of the π*− antibonding orbitals of O2 with Ag orbitals,25 leading to the implementation of low temperature O2 adsorption and activation.

In accordance with our previous work, the introduction of molecular O2 into the catalytic system facilitates the H2 production efficiency of AgUCs/MgO.7 As shown in Figure 3c, negligible H2 is produced in the absence of O2, while the corresponding TOF value reaches 540.5 h−1 once the oxygen partial pressure (pO2) reaches 1 atm at room temperature. In fact, this turnover frequency is the highest value for H2 evolution



Figure 4. (a) Solid EPR spectra of AgUCs/MgO, AgNPs/MgO and MgO carried out at 298 K in open air, (b) Operando EPR spectra of DMPO adducts recorded in the AgUCs/MgO and AgNPs/MgO dehydrogenation ternary systems, respectively, (c-d) In-situ Raman spectroscopy of AgUCs/MgO catalyzed HCHO solution (1 M) in air at room temperature in the wave number range of 350-2000 cm-1. Operando liquid-phase EPR spin-trapping experiments are carried out in open air to identify surface intermediates under reaction conditions (Figure 4b). In the suspension containing AgUCs/MgO, HCHO solution, and DMPO, a complex 9-fold signal (labeled by black dots) characteristic of the DMPO−H• radical (hyperfine splitting constants: αN = 22.18 G; αH(1) = αH(2) = 34.73 G) together with a set of superoxide signals (labeled by asterisks) attributed to the DMPO‒OOH• adduct (αN = 17.38 G; αH = 18.56 G) are recorded. It means that O2 is preferentially reduced to O2•− through one-electron transfer process. This may in turn correlate to the following C–H bond activation because of the feasible proton abstraction from HCHO by an oxyl radical to produce the relatively stable •OOH species.52-55 In principle, the proton transfer (PT) is a short-range intramolecular process. To realize an intermolecular PT, structural features such as the maximal vibrational overlap and minimal electron and/or proton transfer distance between two molecules are required.56 Consequently, the sharp increase in g value as well as the radical signals suggests an enhanced coupling of spin and orbital motion between HCHO and O2, enabling long-range transfer of the electron and proton from one molecule to the other. In-situ resonant Raman analysis further shows that the excited reactant vibration states contribute significantly to this C–H bond cleavage as well as the subsequent proton acceptance by O2•− species to form •OOH radical (Figure 4c-4d). The HCHO molecules anchored on AgUCs/MgO possess Raman bands at ∼910, ∼1039, ∼1488, and ∼1635 cm-1. The strong

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peaks at 910 and 1039 cm-1 are ascribed to the ν(O–C–O) symmetric and anti-symmetric stretching modes within hydrated formaldehyde aqueous solution, respectively.12 Another main peak at 1488 cm-1 and the small one at 1635 cm-1 are referred to the δ(H–C–H) coupled with the δ(O–H) bending mode in HCHO and water.57-58 The C–H and O–H bonds remain almost unchanged within 4 min reaction. The intensity of C–O bond keeps weakening from 0 to 4 min. These results indicate that a C–H and O–H vibration process occurs at the beginning of the activation. Following the vibration process, both the C– H and O–H bonds intensities keep decreasing while C–O bond remains almost unchanged (4-10 min). It implies that the selective cleavage of C–H and O–H bonds can lead to the formation of more stabilized radical leaving group (most likely corresponding to the proton coupling step: HCHO + O2•− → HCO• + •OOH and H2O dissociation process: H2O → •OH + •H, respectively). Therefore, it is concluded that PT is induced by an electron-transfer process, i.e., a nontypical sequential PCET, as illustrated in Scheme 1. Subsequently, the resulting •OOH intermediates continuously uptakes and combines with •H radicals to realize efficient H2 production as well as O2 regeneration. To further confirm the regioselectivity of the reaction, other substrate molecules are selected and the catalytic results are summarized in Figure S8.

Scheme 1 Schematic illustration of facile oxidative C–H cleavage over AgUCs/MgO involving PCET.

Figure 5. RRDE voltammograms for oxygen reduction in oxygen-saturated 0.1 M KOH over AgUCs/MgO, AgNPs/MgO and MgO electrodes. The selective generation of peroxy species on AgUCs/MgO catalysts surfaces is further demonstrated by rotating ring-disk electrode (RRDE) voltammograms (Figure 5). AgUCs/MgO electrodes exhibit a two-step process for ORR with onset potential at about –0.32 V and –0.59 V, respectively. The first sharp step is attributed to the two-electron reduction of O2 to •OOH (rate-limiting step), and the following step to the


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reduction of •OOH to •O.59 When adding HCHO as a proton donor under the same conditions, AgUCs/MgO exhibits quite different ORR performance with a 0.13 V onset potential and delivers a limit current density of 0.88 mA cm−2. It should be mentioned that AgUCs/MgO also exhibits obviously positive current value in the voltage range of −0.4~0 V, which is mainly caused by occurrence of the formaldehyde oxidation during the electrocatalytic reaction process. In contrast, the AgNPs/MgO and MgO exhibit inferior ORR activity. In detail, the limit current density is 0.62 mA cm−2 for AgNPs/MgO and 0.71 mA cm−2 for MgO, respectively, and the onset potentials of both catalysts are ~0.32V. These results suggest that the adsorption and transformation of O2 to •OOH are both easier on fully exposed electron-rich Ag sites, thus a more favorable ORR process with lower overpotential, increased kinetics, and preferential two-electron pathway can be readily achieved by AgUCs/MgO.

Figure 6. (a) O2 adsorption energies for two (MgO)36Ag4 unit cells (Type-1 model: red line; Type-2 model: blue line), (b) energies of the two models for C–H bond cleavage, and (c) stepby-step HCHO reforming process and the structures of transition states (TS) based on the two models (red and blue frameworks indicate the Type-1 and Type-2 models, respectively). DFT calculations are performed to gain fundamental insights into the reaction mechanism. In this case, we construct two catalyst models to provide detailed information on the energetics and structures of the elementary reaction steps. MgO support is modeled by a three-layer slab with atomic composition of (MgO)36 representing the MgO(001) surface, where two Ag atoms and the nearest two Mg atoms are alloyed by interchanging their atomic positions (Type-1 model). In another model named Type-2 model, a small Ag4 cluster is simply deposited on the MgO slab (Figure S9). The whole reaction consists of 12 configurations including 8 local minima and 4 transition states (TS’s) (Figure 6c), and the calculated relative activation energies of TS’s for the two models are listed in Table S3. The computational details are shown in Supporting Information (Figure S10-S16). The reaction is initiated by adsorption of O2 on Ag sites in a bridged configuration. The

relative energy for adsorption was –195.0 kJ /mol, which is ~135.6 kJ /mol nagtive to that of Type-2 model (Figure 6a), but both are thermodynamically favorable. Such a parallel diatomic adsorption mode effectively weakens the O–O bonding, and thereby facilitates the transfer of electron from the Ag3d orbital to the empty π* orbital of O2,60 resulting in the feasible formation of superoxide species (O2•−). Co-adsorption of HCHO on the Ag site modulates the configuration of O2 from end-on to side-on with a single Mg atom. In Type-1 model, the C–H bond in HCHO molecule stretches and is subsequently cleaved into H• and HCO• species on the Ag site, and •OOH radical is formed by coupling H• and O2. The activation energy is estimated to be 277.2 kJ /mol, which is the rate-determining step of the whole reaction. However, the reaction proceeds in the stable energy region with the relative energy of –2.4 kJ /mol (TS–1, Figure 6b). With further co-adsorption of H2O, stabilization is increased to – 617.8 kJ /mol. MgO provides highly active sites for water dissociation (TS–2, –465.3 kJ /mol), producing abundant surface hydroxyls and thus accelerating the HCHO/H2O reforming reaction at the interface. Since HCO• is unstable, it is immediately combined with •OH to produce HCOOH (TS–3, – 304.2 kJ /mol). Finally, H2 is produced and O2 is regenerated by the reaction between •H and •OOH (TS–4, –230.5 kJ /mol). In Type-2 model, the potential energy surface is much higher than that for Type-1 model. The relative energies for the three TS’s among the four locate in the unstable region, i. e, above zero. In this sense, the reaction path along Type-1 is more stable. The stoichiometry for the overall dehydrogenation reactions over AgUCs/MgO is thus suggested to be as follows: HCHO + O2 + H2O → •OOH + •H + HCOOH → O2 + H2 +HCOOH

Figure 7. FTIR spectra of CO adsorption for (a) AgUCs/MgO and (b) AgNPs/MgO. The re-usability and stability of the catalysts is also verified, showing that 94% of the catalytic activity is preserved even after 5 cycles (Figure S17). The recycled catalyst is fully characterized by XRD (Figure S18), HR-TEM (Figure S19), HAADF-STEM (Figure S19) and XPS (Figure S20). As a common poisoning species to most of noble metal catalysts, CO is a possible reaction intermediate during HCHO reforming.18 Hence the stability of AgUCs/MgO is further verified by determining the CO adsorption behavior using FTIR



spectroscopy (Figure 7). For AgNPs/MgO, a strong vibration band at 2014 cm-1 is ascribed to a linearly bonded CO species on Ag0, and two other weak bands at 2120 cm-1 and 2169 cm-1 are caused by bridged adsorption of CO on two Ag+ sites, respectively.18, 61 However, Ag clusters on MgO surface shows a weak band at 2014 cm-1 whereas no other peaks are found within 1800‒2300 cm‒1 frequency range, implying that electron-rich Ag sites have lower affinity to CO than that of AgNPs. An easier desorption of CO ultimately reduces the catalyst poisoning effect and benefites the high reactivity at low temperatures. 4. CONCLUSIONS In summary, a facile synthetic strategy is successfully applied to realize the highly dispersed Ag ultrasmall clusters on defectrich MgO matrix, which is robust and stable for HCHO reforming into H2 in the presence of O2 at room temperature. Its TOF value reaches 540.5 h−1 at pO2 = 1 atm, far superior to its counterpart catalyst AgNPs/MgO. Based on experimental observations and theoretical calculations, it is found that the unique electronegative Ag clusters can spatially build up a channel for electron transfer from electron-rich Ag sites to antibonding π orbital of chemisorbed bridged O2 molecules, leading to the preferential two-electron reduction of O2 into peroxide, which acts as a key active species to the success of dehydrogenation reaction. Therefore, a nontypical sequential proton coupled electron transfer involving a long-range intermolecular proton transfer process is achieved during the catalysis. Moreover, the rate-determining step of the whole reaction, i.e., activation and cleavage of C–H bond in HCHO is readily implemented on Ag sites because of its lower energy barrier than that on AgNPs. The present catalysts based on Ag clusters paves a new way to employ well-defined, controllable ultrasmall active centers in catalytic design for complicated chemical reactions.

ASSOCIATED CONTENT Supporting Information Characterizations, 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] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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We are grateful for the financial support from the National Natural Science Foundation of China (21872123, 21503189 and 21403197), the Zhejiang Provincial Natural Science Foundation of China (LY18B030007 and LY15B030009), the Fundamental Research Funds of Zhejiang Sci-Tech University (2019Q005) and the Start-up Foundation of Zhejiang Sci-Tech University (16062096-Y). We also thank Dr. Yu Tang and Dr. Yihu Dai for their devotion on EAXFS, FT-IR and O2-TPD analysis.

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Ultrasmall Silver Clusters Stabilized on MgO for Robust Oxygen

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