Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective

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Letter

Silver Nanoparticles with Surface-bonded Oxygen for Highly Selective CO Reduction 2

Kun Jiang, Priti Kharel, Yande Peng, Mahesh Gangisetty, HaoYu Greg Lin, Eli Stavitski, Klaus Attenkofer, and Haotian Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02380 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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ACS Sustainable Chemistry & Engineering

Silver Nanoparticles with Surface-bonded Oxygen for Highly Selective CO2 Reduction Kun Jiang,1 Priti Kharel,1, 2 Yande Peng,1, 3 Mahesh K. Gangishetty,1 Hao-Yu Greg Lin,4 Eli Stavitski,5 Klaus Attenkofer,5 and Haotian Wang*,1

1

Rowland Institute, Harvard University, 100 Edwin H. Land Boulevard, Cambridge, MA 02142, United States 2

Hamilton College, 198 College Hill Road, Clinton, NY 13323, United States

3

Department of Chemistry, University of Science and Technology of China, 96 JinZhai Road, Hefei, Anhui 230026, China 4

Center for Nanoscale Systems, Harvard University, 11 Oxford Street, Cambridge, MA 02138, United States 5

Brookhaven National Laboratory, Upton, NY 11973, United States

*

Corresponding author. E-mail: [email protected]

ACS Paragon Plus Environment

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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 lows 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 catalyst 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 posttreatment 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, while 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 value-added 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 towards 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 has been made to engineer Au- and Ag-based electrocatalysts toward a higher CO FE at lower overpotentials, such as the tip-induced electric field enhancement,9, 10 nanoparticle size11-13 & micro-/meso-structure optimizations,14-17 and so on. Among these different methods, oxidederived (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 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 is 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 believed to be the active sites and exhibits an 80% CO2-to-CO selectivity at a moderate overpotential of 0.49 V. These controversies in literatures indicate that more detailed structure-activity relationship study is thereby important for further development of CO2RR electrocatalysts. Here in this work, we conduct a comparative study of CO2RR performance on air-annealed Ag (Air-Ag) catalysts with or without the post-treatment of H2 thermal annealing (H2-Ag). In


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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 contribute to the improved selectivity of CO evolution rather than H2 generation. As a result, while 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.

Results and Discussion Ag nanoparticles (NPs) supported on carbon fiber papers (CFPs) were prepared via a dipcoating 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 (noted as Air-Ag thereafter). The control sample of H2-Ag was undergoing another 1-h 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 the scanning electron microscopy (SEM) image of Air-Ag NPs with hundreds of nanometers in diameter (see also the low-magnification 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). 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

(Figures 1D and S5), consistent with the spacing of Ag(111). Negligible

changes in morphology or size distribution is 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.


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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.

Although the morphologies, structures, and phases of both Air-Ag and H2-Ag remain the same, a dramatically difference in CO2RR selectivity was observed. Electrochemical CO2RR performance was evaluated in a customized H-cell with CO2-saturated 0.1 M KHCO3 as the electrolyte. Figure 2A shows the overall steady-state currents at each applied potentials 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


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starts at ~ -0.44 V vs. RHE, with CO FE slowly climbing to 31.4 % at -1.02 V (Figure 2B), while 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.

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.


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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 H2-Ag.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 active *COO− intermediate over air annealed Ag surface sites. Figure 2E shows 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 post-catalysis SEM image of Air-Ag in Figure S2B together with its size distribution histogram in Figure S6C show no noticeable morphology changes.

Figure 3. (A) Angle-resolved core-level XPS spectra for Ag 3d region of H2-Ag (left) and AirAg (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.


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Since the morphological structures and the metallic phases are quite similar for the two Ag NP samples, it is very important and interesting to explore the wedge of such a 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 Xray 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 become the shallower, presenting more electronic and chemical information in surface region. The core-level AR-XPS spectra of Ag 3d were 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, ascribing 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 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. Ag foil spectrum is predominant in 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 above XRD and lattice spacing results, their magnitudes of the Fourier-transformed EXAFS spectra largely resemble the feature of metallic Ag foil reference except for this Ag-O correlation. Air-Ag 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 readily decomposition of AgO to Ag under annealed temperatures.19, 43 Corresponding O 1s XPS spectra were 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 were noticed in Air-Ag vs. H2-Ag. Related literatures by Rocha et al.44 have demonstrated five distinct atomic oxygen species with different electronic structures


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on Ag catalyst surfaces, with the low binding energy (BE) components at 528.1–529.2 eV assigned to chemically bonded O(-Agδ+) species, while 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 Since bonded oxygen processes a strong electron 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 on 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 sub-surface 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 course of 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 oxygen embedded in the topmost Ag layer is mainly responsible for the methanol dehydrogenation reaction activity. In a 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 a strong competition with hydrogen evolution reaction. Given that a variety of transition metal oxide catalyst candidates for CO2 reduction, our present work shall advance the understanding of structure-activity relationship for the design of more efficient CO2RR electrocatalysts.

Supporting Information Experimental details, additional material characterizations including XRD patterns, SEM, TEM images, EDX mapping, Ag particle size distribution histograms and XPS spectra, together with


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electrochemical surface are determination. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements 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. ECS-0335765. 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. DE-SC0012704.

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Reduction Potentials in Alkaline Solutions: A Study by Seriatim ECSTM-DEMS. J. Electroanal. Chem. 2017, 793, 113-118. 29. Liu, X.; Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Nørskov, J. K., Understanding Trends in Electrochemical Carbon Dioxide Reduction Rates. Nat. Commun. 2017, 8, 15438. 30. Gunathunge, C. M.; Li, X.; Li, J. Y.; Hicks, R. P.; Ovalle, V. J.; Waegele, M. M., Spectroscopic Observation of Reversible Surface Reconstruction of Copper Electrodes under CO2 Reduction. J. Phys. Chem. C 2017, 121, 12337-12344. 31. Raciti, D.; Livi, K. J.; Wang, C., Highly Dense Cu Nanowires for Low-Overpotential CO2 Reduction. Nano Lett. 2015, 15, 6829-6835. 32. Raciti, D.; Cao, L.; Livi, K. J. T.; Rottmann, P. F.; Tang, X.; Li, C.; Hicks, Z.; Bowen, K. H.; Hemker, K. J.; Mueller, T.; Wang, C., Low-Overpotential Electroreduction of Carbon Monoxide Using Copper Nanowires. ACS Catal. 2017, 7, 4467-4472. 33. Zhang, Y.-J.; Sethuraman, V.; Michalsky, R.; Peterson, A. A., Competition between CO2 Reduction and H2 Evolution on Transition-Metal Electrocatalysts. ACS Catal. 2014, 4, 3742-3748. 34. Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y., Bifunctional NonNoble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261. 35. Herrero, E.; Buller, L. J.; Abruna, H. D., Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. 2001, 101, 1897-1930. 36. Chen, C. H.; Yang, H. L.; Chen, H. R.; Lee, C. L., Activity on Electrochemical Surface Area: Silver Nanoplates as New Catalysts for Electroless Copper Deposition. J. Electrochem. Soc. 2012, 159, D507-D511. 37. Hatsukade, T.; Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F., Insights into the Electrocatalytic Reduction of CO2 on Metallic Silver Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 13814-13819. 38. Singh, M. R.; Kwon, Y.; Lum, Y.; Ager, J. W.; Bell, A. T., Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO2 over Ag and Cu. J. Am. Chem. Soc. 2016, 138, 1300613012. 39. Li, C. W.; Kanan, M. W., CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films. J. Am. Chem. Soc. 2012, 134, 7231-7234. 40. Liu, S. B.; Tao, H. B.; Zeng, L.; Liu, Q.; Xu, Z. G.; Liu, Q. X.; Luo, J. L., Shape-Dependent Electrocatalytic Reduction of CO2 to CO on Triangular Silver Nanoplates. J. Am. Chem. Soc. 2017, 139, 2160-2163. 41. McCafferty, E.; Wightman, J., Determination of the Concentration of Surface Hydroxyl Groups on Metal Oxide Films by a Quantitative XPS Method. Surf. Interface Anal. 1998, 26, 549-564. 42. Stephens, I. E. L.; Bondarenko, A. S.; Perez-Alonso, F. J.; Calle-Vallejo, F.; Bech, L.; Johansson, T. P.; Jepsen, A. K.; Frydendal, R.; Knudsen, B. P.; Rossmeisl, J.; Chorkendorff, I., Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying. J. Am. Chem. Soc. 2011, 133, 5485-5491. 43. Gangishetty, M. K.; Scott, R. W. J.; Kelly, T. L., Thermal Degradation Mechanism of Triangular Ag@SiO2 Nanoparticles. Dalton Trans. 2016, 45, 9827-9834. 44. Rocha, T. C. R.; Oestereich, A.; Demidov, D. V.; Havecker, M.; Zafeiratos, S.; Weinberg, G.; Bukhtiyarov, V. I.; Knop-Gericke, A.; Schlogl, R., The Silver-Oxygen System in Catalysis: New Insights by near Ambient Pressure X-Ray Photoelectron Spectroscopy. Phys. Chem. Chem. Phys. 2012, 14, 45544564. 45. Bao, X.; Muhler, M.; SchedelNiedrig, T.; Schlogl, R., Interaction of Oxygen with Silver at High Temperature and Atmospheric Pressure: A Spectroscopic and Structural Analysis of a Strongly Bound Surface Species. Phys. Rev. B 1996, 54, 2249-2262. 46. Bao, X.; Barth, J. V.; Lehmpfuhl, G.; Schuster, R.; Uchida, Y.; Schlögl, R.; Ertl, G., OxygenInduced Restructuring of Ag(111). Surf. Sci. 1993, 284, 14-22. 47. Bao, X.; Muhler, M.; Pettinger, B.; Schlogl, R.; Ertl, G., On the Nature of the Active State of Silver During Catalytic-Oxidation of Methanol. Catal. Lett. 1993, 22, 215-225.


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Synopsis Surface bonded oxygen species on Ag sites (O-Agδ+) promotes CO2 reduction pathway over hydrogen evolution during electrolysis.


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Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective

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