Silver Nanoparticles with Surface-bonded Oxygen for Highly Selective

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Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective CO2 Reduction Kun Jiang,† Priti Kharel,†,‡ Yande Peng,†,§ Mahesh K. Gangishetty,† Hao-Yu Greg Lin,∥ Eli Stavitski,⊥ Klaus Attenkofer,⊥ and Haotian Wang*,† †

Rowland Institute, Harvard University, 100 Edwin H. Land Boulevard, Cambridge, Massachusetts 02142, United States Hamilton College, 198 College Hill Road, Clinton, New York 13323, United States § Department of Chemistry, University of Science and Technology of China, 96 JinZhai Road, Hefei, Anhui 230026, China ∥ Center for Nanoscale Systems, Harvard University, 11 Oxford Street, Cambridge, Massachusetts 02138, United States ⊥ Brookhaven National Laboratory, Upton, New York 11973, United States ‡

S Supporting Information *

ABSTRACT: The surface electronic structures of catalysts need to be carefully engineered in CO2 reduction reaction (CO2RR), where the hydrogen evolution side reaction usually takes over under a significant overpotential, and thus dramatically lowers the reaction selectivity. Surface oxides can play a critical role in tuning the surface oxidation state of metal catalysts for a proper binding with CO2RR reaction intermediates, which may significantly improve the catalytic activity and selectivity. Here, we demonstrate the importance of surface-bonded oxygen on silver nanoparticles in altering the reaction pathways and improving the CO2RR performances. A comparative investigation on air-annealed Ag (Air-Ag) catalyst with or without the post-treatment of H2 thermal annealing (H2-Ag) was performed. In Air-Ag, the subsurface chemically bonded O species (O− Agδ+) was identified by angle resolved X-ray photoelectron spectroscopy and X-ray absorption spectroscopy techniques, and contributed to the improved CO selectivity rather than H2 in CO2RR electrolysis. As a result, though the maximal CO Faradaic efficiency of H2-Ag is at ∼30%, the Air-Ag catalyst presented a high CO selectivity of more than 90% under a current density of ∼21 mA/cm2. KEYWORDS: Carbon dioxide reduction, Silver catalyst, Angle resolved X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Surface bonded oxygen, Reaction selectivity

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INTRODUCTION Using renewable energy and peak valley electricity to efficiently drive room temperature carbon dioxide conversion into valueadded chemicals and fuels is not only environmentally benign to close the anthropogenic carbon circle but also becoming an economical option for energy storage and utilization.1−6 The key to advance the electrochemical CO2 reduction reaction (CO2RR) relies on developing active catalytic materials to deliver both high activity and selectivity toward desired products. It is known that bulk Au and Ag metals are capable of converting CO2 to CO at relative high Faradaic efficiencies (FEs) in aqueous electrolyte.7,8 Along this line, lots of efforts have been made to engineer Au- and Ag-based electrocatalysts toward a higher CO FE at lower overpotentials, such as the tipinduced electric field enhancement,9,10 nanoparticle size11−13 and micro/meso-structure optimizations,14−17 and so on. Among these different methods, oxide-derived (OD) catalysts show a promising prospect due to their significant improved CO2RR performance.18,19 Nevertheless, it is also noted that the origin for these improvements on OD catalysts is still under © 2017 American Chemical Society

intense investigation, i.e., generated grain boundaries during the in situ reduction process,20−22 remained (sub)surface oxygen species,23−27 and/or reconstructed surface step sites exposure.28−30 One recent report on CuO-derived Cu nanowire catalyst31 comparatively investigates how the precursor reducing method, i.e., forming gas (containing 5% H2) annealing vs electrochemical reduction, alters the obtained crystalline structure of Cu wire. Possible Cu2O residues from electrochemical reduction are suggested to account for the overall improved CO2RR selectivity.31,32 On the other hand, it is generally accepted that many metal oxides will be electrochemically reduced into their metallic phases under CO2RR conditions (usually under significant negative potentials) according to the Pourbaix diagrams.22,33 Take another OD-Ag report for example,19 which is electrochemically reduced from an alkaline media anodized Ag2O precursor, the obtained metallic Ag surface is postulated to be the active sites Received: July 15, 2017 Revised: August 18, 2017 Published: September 12, 2017 8529

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Besides, surface adsorbed oxygen species was identified by scanning TEM coupled energy dispersive X-ray (STEM-EDX) mapping as shown in Figure S4. The lattice spacing by averaging the 10 fringes indicated in Figure 1c was measured to be ∼2.32 Å (Figures 1D and S5), consistent with the spacing of Ag(111). Negligible changes in morphology or size distribution are observed after the H2 annealing treatment as shown in Figures 1B and S6, neither any introduced surface impurities indicated by XPS survey spectrum (Figure S7), ruling out the potential effects of structure changes on catalytic activities. Although the morphologies, structures and phases of both Air-Ag and H2-Ag remain the same, a dramatic difference in CO2RR selectivity was observed. Electrochemical CO2RR performance was evaluated in a customized H-cell with CO2saturated 0.1 M KHCO3 as the electrolyte. Figure 2A shows the overall steady-state currents at each applied potential with iRcompensation during electrolysis. To better exclude the interference from exposed Ag surface area variation in H2-Ag and Air-Ag samples, electrochemical surface area determination was also carried out by integrating Pb2+ underpotential deposition (UPD) charges on Ag surfaces. As shown in Figure S8, the electrochemical surface areas for H2-Ag and Air-Ag are calculated to be 0.36 and 0.39 cm2, by assuming a charge of 400 μC cm−2 for Pb(II)UPD on Ag,35,36 suggesting a minor surface area effect. Only H2 and CO were detected as the major reduction product, in agreement with previous reported bulk Ag catalysts.7,37,38 For H2-Ag, CO2-to-CO conversion starts at ∼ −0.44 V vs RHE, with CO FE slowly climbing to 31.4% at −1.02 V (Figure 2B), whereas H2 evolution dominates the reduction reaction throughout the whole potential range. In contrast, the onset potential for CO evolution move forward to ∼ −0.35 V on Air-Ag (ca. overpotential of 230 mV, Figure 2C), with its FE readily increasing to 91.7% under a current density of ∼21.2 mA/cm2 at −0.97 V, overwhelming the side reaction of H2 evolution. As shown in Figure 2D, nearly identical Tafel slopes were observed on both samples, suggesting the similar reaction kinetics and rate-limiting steps of initial electron transfer toward surface adsorbed *COO− intermediate on both Air-Ag and H2Ag.16,39,40 Nevertheless, as compared to H2-Ag, the largely improved CO FE and the forward onset potential on Air-Ag clearly point out the favorable stabilization and conversion of *COO− active intermediate over air-annealed Ag surface sites. Figure 2E shows the stability test of Air-Ag under continuous electrolysis at −0.86 V, with the overall current density gradually increased from 13.2 to 17.9 mA/cm2 while maintaining a relatively high CO FE of ∼80%. The postcatalysis SEM image of Air-Ag in Figure S2B, together with its size distribution histogram in Figure S6C show no noticeable morphology changes. Because the morphological structures and the metallic phases are quite similar for the two Ag NP samples, it is very important to explore the wedge of such different CO2RR activity and selectivity. The electronic structure of the catalyst surface might play a critical role in the differences. Herein, surface-sensitive AR-XPS was employed to investigate Ag/Ag+ and Ag-O species distribution in the near surface layers (several angstroms to few nanometers) by plotting their angle/depth profile,41,42 and further reinforced by synchrotron-based extended X-ray absorption fine structure (EXAFS) analysis. The 90° is the typical emission angle for normal XPS measurements. With a smaller of incident angle, the X-ray detection depth would

and exhibits an 80% CO2-to-CO selectivity at a moderate overpotential of 0.49 V. These controversies in the literature indicate that more detailed structure−activity relationship study is important for further development of CO2RR electrocatalysts. In this work, we conduct a comparative study of CO2RR performance on air-annealed Ag (noted as Air-Ag thereafter) catalysts with or without the post-treatment of H2 thermal annealing (H2-Ag). In Air-Ag, the subsurface bonded O species is identified by angle resolved X-ray photoelectron spectroscopy (AR-XPS) and X-ray absorption spectroscopy (XAS) techniques, and contributes to the improved selectivity of CO evolution rather than H2 generation. As a result, though the maximal CO FE of H2-Ag is at ∼30%, the Air-Ag catalyst presented a high CO selectivity of more than 90% at a current density of ∼21 mA/cm2.

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RESULTS AND DISCUSSION Ag nanoparticles (NPs) supported on carbon fiber papers (CFPs) were prepared via a dip-coating and thermal annealing method.34 Typically, O2-plasma treated CFP pieces (1 cm × 2 cm) were dipped into a silver nitrate solution containing 10 wt % AgNO3 and 4 wt % PVP followed by vacuum drying, and a 1 h thermal annealing at 300 °C in air (Air-Ag). The control sample of H2-Ag was underwent another 1 h of annealing at 300 °C in forming gas (5% H2 balanced with Ar). Both Air-Ag and H2-Ag were identified by X-ray diffraction (XRD) as metallic phases (PDF No. 04-0783, Figure S1). Figure 1a shows

Figure 1. Characterization of Ag catalysts. (A) SEM images of Air-Ag/ CFP and (B) H2-Ag/CFP. (C) TEM image of an Air-Ag NP and (D) the average lattice spacing of (111) plotted by integrating the image pixels along 10 atomic layers as indicated by the white arrows. An enlarged view of Air-Ag NP lattice is provided in Figure S5.

the scanning electron microscopy (SEM) image of Air-Ag NPs with hundreds of nanometers in diameter (see also the lowmagnification SEM image in Figure S2A). It is interesting to note that those NPs were embedded in hollow carbon caves on the carbon fiber substrate, possibly due to the violent Ag2O decomposition reaction during the annealing process. Each NP is single crystalline as suggested by transmission electron microscopy (TEM) in Figure 1C, where some boundaries are observed when two individual NPs merge together (Figure S3). 8530

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Figure 2. (A) Averaged steady-state current densities during CO2RR at different potentials, together with H2 and CO FEs over (B) H2-Ag and (C) Air-Ag samples. (D) Comparison of Tafel slopes for CO evolution. (E) Stability test of Air-Ag under −0.86 V vs RHE. The geometric area of working electrode is 1 cm × 1 cm, in CO2-saturated 0.1 M KHCO3 electrolyte.

lattice spacing results, their magnitudes of the Fouriertransformed EXAFS spectra largely resemble the feature of metallic Ag foil reference except for this Ag−O correlation. AirAg shows the largest Ag−O peak intensity as compared to H2 annealed counterpart and Ag reference, but is yet weaker as compared to its Ag−Ag pair, which is due to the decomposition of AgO to Ag under annealed temperatures.19,43 Corresponding O 1s XPS spectra are also plotted in Figure S9, with a representative deconvolution of XPS peaks taken at the emission angel of 60° shown in Figure 3C, where a reasonable increase in O signals was noticed in Air-Ag vs H2Ag. Related literature by Rocha et al.44 has demonstrated five distinct atomic oxygen species with different electronic structures on Ag catalyst surfaces, with the low binding energy (BE) components at 528.1−529.2 eV assigned to chemically bonded O(−Agδ+) species, whereas the atomic adsorbed surface oxygen species located at ∼530.8 eV40,41 and other oxygen species above 532.0 eV assigned to the residual O(−H) of H2O.25 Because bonded oxygen processes a strong electron

become shallower, presenting more electronic and chemical information for the surface region. The core-level AR-XPS spectra of Ag 3d are plotted in Figure 3A, where both Air-Ag and H2-Ag are dominated by the Ag metal feature, i.e., with an Ag0 3d5/2 peak located at 368.4 eV. With increased emission angle, a small shoulder peak was observed at 367.3 eV in Air-Ag, ascribed to the surface Ag+ species formed during the annealing process in air. To better distinguish and identify Ag+ from bulk Ag0 signals, we carried out XAS measurements using synchrotron radiation on the Ag K-edge, and employed a bulk Ag foil as reference. The obtained EXAFS spectra are plotted in R-space (radial distribution function) and shown in Figure 3B. The Ag foil spectrum is predominant in the metallic state, and shows expected peaks at 2.08 and 2.75 Å, which are corresponding to the Ag−Ag first coordination shell.27,43 The other weak peak at ∼1.50 Å is attributed to Ag−O correlation, possibly due to the continuous exposure of Ag-foil surface to atmosphere O2. For H2-Ag and Air-Ag samples, in good harmony with the above XRD and 8531

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Figure 3. (A) Angle-resolved core-level XPS spectra for Ag 3d region of H2-Ag (left) and Air-Ag (right), (B) Ag K-edge Fourier-transformed EXAFS spectra in R-space for H2-Ag, Air-Ag and a Ag foil reference sample. (C) Representative deconvolution of O 1s region XPS spectra for H2-Ag and Air-Ag at the same emission angel of 60°, together with (D) emission angle (depth) profile of surface O(−Agδ+) species and other surface oxygen residuals in Air-Ag.

competition with hydrogen evolution reaction. Given a variety of transition metal oxide catalyst candidates for CO2 reduction, our present work shall advance the understanding of the structure−activity relationship for the design of more efficient CO2RR electrocatalysts.

affinity, it would give rise to positively charged surface Ag sites, and therefore possibly alters the CO2RR pathway with strengthened adsorption of active *CO2− intermediate. More chemically bonded O−Ag signals at 528−530 eV are observed for the Air-Ag sample, which is in agreement with our EXAFS result and further suggestive of a different surface structure as compared to H2-Ag. The atomic fraction of surface bonded O(−Ag) in the whole near surface region O residuals was further plotted vs emission angle in Figure 3D. The content of O−Ag species gradually increases with decreasing angle from 90° to 60° (detection region more approaching to surface layers), suggesting that partial oxygen atoms has dissolved into bulk substrate45 and the bonded O species is mostly enriched at subsurface layers. When the emission angel further drops to ∼30°, surface trapped trace O2 and/or H2O molecules contribute mainly to the overall O signals. In the study of the silver−oxygen system in different catalytic reactions,45−47 it has been demonstrated that a tightly held oxygen species was formed after exposure to O2 at high temperature (above 200 °C), among which these specific oxygens embedded in the topmost Ag layer are mainly responsible for the methanol dehydrogenation reaction activity. In good harmony with these previous reports, our Air-Ag and H2-Air catalysts have shown the correlation between the surface bonded O−Agδ+ species and the favorable electrochemical CO2 reduction performances. Control experiments over H2-annealed Ag catalyst with eliminated O−Agδ+ interaction leads to a sluggish CO2 conversion and a low CO selectivity under strong

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02380. Experimental details, additional material characterizations including XRD patterns, SEM, (S)TEM images, EDX mapping, Ag particle size distribution histograms and XPS spectra, together with electrochemical surface area determination (PDF)

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AUTHOR INFORMATION

Corresponding Author

*H. Wang. E-mail: [email protected]. ORCID

Kun Jiang: 0000-0003-3148-5058 Haotian Wang: 0000-0002-3552-8978 Notes

The authors declare no competing financial interest. 8532

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ACKNOWLEDGMENTS This work was supported by the Rowland Junior Fellows Program at Rowland Institute, Harvard University. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS0335765. CNS is part of Harvard University. Part of this research used 8-ID (ISS) beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DESC0012704.

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