LETTER pubs.acs.org/Langmuir
In Situ Observation of Water Dissociation with Lattice Incorporation at FeO Particle Edges Using Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Xingyi Deng,*,†,‡ Junseok Lee,†,‡ Congjun Wang,†,‡ Christopher Matranga,† Funda Aksoy,§,|| and Zhi Liu§ †
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National Energy Technology Laboratory (NETL), United States Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States ‡ URS, P.O. Box 618, South Park, Pennsylvania 15129, United States § Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States Department of Physics, Faculty of Arts and Sciences, Nigde University, Nigde, Turkey
bS Supporting Information ABSTRACT: The dissociation of H2O and formation of adsorbed hydroxyl groups on FeO particles grown on Au(111) were identified with in situ X-ray photoelectron spectroscopy (XPS) at water pressures ranging from 3 10-8 to 0.1 Torr. The facile dissociation of H2O takes place at FeO particle edges, and it was successfully observed in situ with atomically resolved scanning tunneling microscopy (STM). The in situ STM studies show that adsorbed hydroxyl groups were formed exclusively along the edges of the FeO particles with the O atom becoming directly incorporated into the oxide crystalline lattice. The STM results are consistent with coordinatively unsaturated ferrous (CUF) sites along the FeO particle edge causing the observed reactivity with H2O. Our results also directly illustrate how structural defects and under-coordinated sites participate in chemical reactions.
’ INTRODUCTION There is widespread interest in studying nanostructured surfaces, including supported model catalyst surfaces1 and high Miller index (HMI) stepped surfaces,2 in order to better understand the role of structural defects and under-coordinated sites in heterogeneous catalysis. Industrial catalysts usually consist of small particles possessing a high concentration of structural defects and under-coordinated sites, such as step edges, kink sites, and missing atom vacancies. In this regard, studying model nanocatalysts and HMI surfaces with in situ techniques allows one to better assess how these structural features impact the activity of industrially relevant catalysts at realistic operating conditions. Structural defects and under-coordinated sites are frequently suggested to be the catalytically active centers responsible for a wide variety of chemical reactions.3-5 One recent example of interest involves nanoparticulate gold for its exceptionally high activity toward low temperature oxidation reactions6-8 which is believed to arise from the presence of under-coordinated edges and adatoms.9 Interface confined coordinately unsaturated ferrous (CUF) centers have also been shown to have remarkable activity for O2 dissociation.10 More recently, we have observed that R-Fe2O3 nanocrystals grown on Au(111) show activity r 2011 American Chemical Society
related to the low-temperature water-gas shift (WGS) reaction, whereas continuous R-Fe2O3 films on Au(111) are inert to this reaction.11 The enhanced reactivity of the R-Fe2O3 nanocrystals is attributed to active sites at particle edges and along the Au interfacial regions which exist for the nanocrystals but are not present at significant densities in the continuous films.11 In this Letter, we show direct evidence to further demonstrate an enhanced reactivity at the edges of small catalyst particles. We selected FeO particles grown on Au(111) substrates, a well characterized two-dimensional material from our previous work,12 as a model platform to study the dissociation of water to form hydroxyl groups. This is a simple but important reaction seen as a critical step in many catalytic processes such as the low temperature WGS reaction.13 The results from in situ X-ray photoelectron spectroscopy (XPS; beamline 9.3.2, Advanced Light Source, Berkeley, CA14) studies directly observe the dissociation of H2O on FeO particles on Au(111) at partial pressures of water ranging from 3 10-8 to 0.1 Torr. In situ scanning tunneling microscopy (STM) with atomic resolution was also used to directly Received: December 15, 2010 Revised: January 19, 2011 Published: January 28, 2011 2146
dx.doi.org/10.1021/la1049716 | Langmuir 2011, 27, 2146–2149
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visualize how the OH groups from dissociated water are incorporated into the oxide lattice at the edges of the FeO particles. The possible active sites involving the CUF centers will also be discussed.
’ EXPERIMENTAL SECTION The growth of FeO on Au(111) has been described in detail previously.12 Briefly, metallic Fe was deposited on Au(111) at room temperature using an electron-beam assisted evaporator (Omicron EFM3T) from an iron rod (Goodfellow, 2.0 mm diameter, 99.99%), followed by oxidation with O2 (P = 3 10-7 Torr) at room temperature for 500 s and a final annealing step in UHV at 700 K for 10 min. Prior to FeO growth, the Au(111) surface was cleaned by cycles of Arþ sputtering (1 10-5 Torr, 1.5 keV) at room temperature, followed by annealing at 700 K for 10 min. The coverage of FeO on Au(111) was noted as a monolayer equivalent of Fe (MLE), referring to the atomic ratio of Fe and Au (only the surface Au atoms are taken into account) used to make the oxide, and was determined from the XPS data collected during Fe deposition on the clean Au(111) surface. Corrections to the XPS data were made for the atomic sensitivity factors (ASF) and the inelastic mean free path (IMFP), as described in our previous study.15 In situ XPS experiments were performed at beamline 9.3.2 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory with a specially designed photoemission spectrometer that can operate at near-ambient pressures (up to 2 Torr).14 The base pressure of the analysis chamber was ∼8 10-10 Torr before introducing H2O. H2O was introduced into the chamber through a UHV leak valve. The XP spectra O 1s and Fe 2p were recorded at photon energies of 650 and 830 eV, respectively, corresponding to a constant photoelectron kinetic energy of 120 eV to ensure the same probe depth for all spectral regions. The binding energy was calibrated using the bulk Au 4f7/2 peak (BE = 84.0 eV) recorded at the same photon energy. The XPS data analysis involved subtracting the nonlinear background (Shirley) and fitting the XP spectral peaks using mixed Gaussian-Lorentzian functions. All other experiments were performed in a commercial UHV chamber from Omicron Nanotechnology GmbH in our group at the National Energy Technology Laboratory (NETL).12 The base pressure of the chamber was 350 K) as monitored with a quadruple mass spectrometer (data not shown).
’ CONCLUSIONS In conclusion, we have directly monitored the dissociation of water to form adsorbed hydroxyl groups with in situ XPS and imaged the incorporation of these OH groups at FeO particle edges with atomically resolved in situ STM. Adsorbed OH is identified with the XPS measurements and seems stable at room temperature once formed. The presence of CUF sites at the peripheries of the FeO particles is likely responsible for the facile dissociation of H2O. Our results also provide direct evidence that the structural defects and under-coordinated sites are indeed active centers for chemical reactions.
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(14) Grass, M. E.; Karlsson, P. G.; Aksoy, F.; Lundqvist, M.; Wannberg, B.; Mun, B. S.; Hussain, Z.; Liu, Z. Rev. Sci. Instrum. 2010, 81, 053106. (15) Deng, X. Y.; Matranga, C. J. Phys. Chem. C 2009, 113, 11104. (16) Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer, Physical Electronics Division: Eden Prarie, MN, 1979. (17) Deng, X. Y.; Verdaguer, A.; Herranz, T.; Weis, C.; Bluhm, H.; Salmeron, M. Langmuir 2008, 24, 9474. (18) Xu, L. S.; Ma, Y. S.; Zhang, Y. L.; Jiang, Z. Q.; Huang, W. X. J. Am. Chem. Soc. 2009, 131, 16366. (19) Yamamoto, S.; Kendelewicz, T.; Newberg, J. T.; Ketteler, G.; Starr, D. E.; Mysak, E. R.; Andersson, K. J.; Ogasawara, H.; Bluhm, H.; Salmeron, M.; Brown, G. E.; Nilsson, A. J. Phys. Chem. C 2010, 114, 2256. (20) Bonnell, D. Scanning Probe Microscopy and Spectroscopy, Theory, Techniques, and Applications, 2nd ed.; VCH-Wiley: New York, 2001. (21) Weiss, W.; Ranke, W. Prog. Surf. Sci. 2002, 70, 1.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures S1—S5, including larger scale in situ STM images of FeO/Au(111) in the presence of 1 10-7 Torr of H2O at room temperature (100 100 nm2, Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES Contract DE FE 0004000. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. ’ REFERENCES (1) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (2) Vattuone, L.; Savio, L.; Rocca, M. Surf. Sci. Rep. 2008, 63, 101. (3) Ertl, G.; Kn€ozinger, H.; Sch€uth, F.; Weitkamp, J. Handbook of Heterogeneous Catalysis; VCH-Wiley: Weinheim, 2008. (4) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; VCH-Wiley: Weinheim, 1997. (5) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (6) Haruta, M. Catal. Today 1997, 36, 153. (7) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41. (8) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (9) Mavrikakis, M.; Stoltze, P.; Norskov, J. K. Catal. Lett. 2000 64, 101. (10) Fu, Q.; Li, W. X.; Yao, Y. X.; Liu, H. Y.; Su, H. Y.; Ma, D.; Gu, X. K.; Chen, L. M.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. H. Science 2010, 328, 1141. (11) Deng, X.; Lee, J.; Wang, C.; Matranga, C.; Aksoy, F.; Liu, Z. J. Phys. Chem. C 2010, 114, 22619. (12) Khan, N. A.; Matranga, C. Surf. Sci. 2008, 602, 932. (13) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Science 2007, 318, 1757. 2149
dx.doi.org/10.1021/la1049716 |Langmuir 2011, 27, 2146–2149
