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In Situ Scanning Tunneling Microscopy Imaging of Well-Defined Rh(111) Electrodes in KNO2-Containing 0.5 M Hydrofluoric Acid Solutions Ze-Haw Zang, Zi-Lin Wu, and Shueh-Lin Yau* Department of Chemistry, National Central University, Chungli, Taiwan Received April 12, 1999. In Final Form: July 22, 1999 This work examined the spatial structure and bonding configuration of nitric oxide (NO) at a welldefined Rh(111) electrode surface by in situ scanning tunneling microscope (STM) imaging under potential control. Immersing Rh(111) electrodes into acidic KNO2 solutions (pH 2, 0.5 M HF) resulted in a long-range ordered (3 × 3) structure, possibly owing to irreversibly adsorbed NO molecules. Although this structure predominated between 1.0 and 0.3 V, cathodic polarization to 0.2 V or more negative caused local roughening. In the absence of HNO2, cathodic polarization of a Rh(111) electrode to 0.05 V completely reduced the surface-bound NO molecules. Coulometric and in situ STM measurements revealed a saturated coverage of 0.48 and 0.44, respectively, for NO molecules in the ordered (3 × 3) structure. This work also proposed a tentative model of the (3 × 3) structure containing 4 NO molecules. One fourth of the NO molecules adsorbed at near-top sites, whereas the remaining resided at 2-fold bridging sites. Real-time in situ STM imaging provided a direct view of the reduction processes at potential negative of 0.2 V. Reactions preferentially occurred at atomically flat terraces, rather than at surface defects such as step edges, kinks, and vacancies. Moreover, the initial reaction fronts were spatially concentrated, rather than randomly distributed.
Introduction A serious pollutant emitting from automobiles and power plants, nitric oxide has been thoroughly examined at group VIII metal surfaces in ultrahigh vacuum (UHV).1-8 Among the materials studied, rhodium exhibits a high activity toward decomposing NO molecules.9 Studies using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS) demonstrate that the surface structure of substrates, amount of dosage, and temperature influence the chemical bonding and reactivity of NO molecules.4-7 While dissociating to yield nitrogen and oxygen atoms at stepped rhodium surfaces, nitric oxide molecularly adsorbs at Rh(111) at room temperature.4-7 Nitric oxide forms two well-ordered structures, c(4 × 2) and (2 × 2), at coverages of 0.5 and 0.75, respectively.7 HREELS results indicate that although NO molecules predominantly adsorb at bridge sites having a coverage of less than 0.5, they start to occupy near-top sites at a higher coverage.5-7 According to a detailed dynamic LEED analysis, the (2 × 2) structure contains two molecules at near-top sites and one at a 2-fold bridge site.7 All the NO molecules orient vertically with their N-end down. Coadsorbed species such as oxygen atoms or carbon monoxide molecules markedly influence the binding preference of NO molecules.5,6 Oxygen atoms, which predominantly adsorb at 3-fold * To whom correspondence and reprint requests should be addressed. Tel: 886-3-422-7151-5909. Fax: 886-3-422-7664; email:
[email protected]. (1) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (2) Serri, J. A.; Cardillo, M. J.; Becker, G. E. J. Chem. Phys. 1982, 77, 2175. (3) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1984, 136, 285. (4) DeLouise, L. A.; Winograd, N. Surf. Sci. 1985, 159, 199. (5) Root, T. W.; Fisher, G. B.; Schmidt, L. D. J. Chem. Phys. 1986, 85, 4679. (6) Root, T. W.; Fisher, G. B.; Schmidt, L. D. J. Chem. Phys. 1986, 85, 4687. (7) Kao, C. T.; Blackman, G. S.; Van Hove, M. A.; Somorjai, G. A.; Chan, C. M. Surf. Sci. 1989, 224, 77. (8) Agrawal, V. K.; Trenary, M. Surf. Sci. 1991, 259, 116. (9) Harrison, B.; Wyatt, M.; Gough, K. G. Catalysis 1982, 5, 127.
hollow sites of Rh(111), make a 2-fold coordination of NO unfavorable and enforce occupation of near-top sites. Notably, the amount of coadsorbed oxygen atoms dominates the extent of the site shift.6 At the liquid-solid interfaces, electrochemical means and in situ Fourier transform (FT)-infrared spectroscopy were used to probe the chemical bonding and reduction reaction of nitric oxide at well-defined Rh, Pt, and Ir electrodes.10,11 Nitric oxide, similar to carbon monoxide, possesses strong vibrational transitions between 1400 and 1900 cm-1, making it another molecular probe for interfacial structures. According to a related investigation, immersion of Rh electrodes into acidic KNO2 solutions leads to adsorption of a monolayer of NO molecules.11 In the presence of HNO2, although three bipolar bands centered at 1800, 1671, and 1572 cm-1 are found, that investigation could not ensure the identity of the second feature. Potential only slightly changes the IR spectra, indicating its minor effect on the binding of NO molecules. Coulometric measurements reveal a coverage of 0.48 ( 0.03 (per Rh atom), which nearly equals that of the c(4 × 2) structure. Thus, NO molecules were thought to form a c(4 × 2) structure.11 In contrast, in the absence of HNO2 in solutions, a band appearing near 1570 cm-1 prevails between 0.2 and 1.0 V, although a weak band near 1800 cm-1 gradually emerges at a positive potential of 0.7 V. Although another in situ IR study also confirmed these results, the effect of potential on the coordination of NO molecules appears to be more drastic.12 Because 0.7 V marks the onset of oxidation of Rh(111) electrodes,13-16 the 1800 cm-1 band, possibly stemming from near-top NO (10) Gomez, R.; Weaver, M. J. J. Phys. Chem. 1998, 102, 3754. (11) Gomez, R.; Rodes, A.; Perez, J. M.; Feliu, J. M. J. Electroanal. Chem. 1995, 393, 123. (12) Tang, C. Ph.D. Thesis, National Tsinghwa University, August 1998. (13) Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1987, 227, 259. (14) Wan, L. J.; Yau, S. L.; Itaya, K. J. Electroanal. Chem. 1995, 381, 105. (15) Clavilier, J.; Wasberg, M.; Petit, M.; Klein, L. H. J. Electroanal. Chem. 1994, 374, 123.
10.1021/la990417m CCC: $18.00 © 1999 American Chemical Society Published on Web 10/29/1999
STM Imaging of Rh(111) Electrodes
molecules, can be associated with partial oxidation of Rh(111) electrode. Its intensity grows with anodic potential at the expense of the 1570 cm-1 band. This finding resembles the effect of coadsorbed oxygen atoms on the binding of NO molecules in UHV.6 Although it was used successfully to probe the realspace structure of well-defined electrified interfaces,17,18 in situ STM was used to examine the structure and chemical bonding of nitric oxide at clean Rh(111) electrodes. A long-range ordered adlattice of nitric oxide was imaged with molecular resolution in KNO2-containing 0.5 M hydrofluoric acid solutions (pH 2). This structure was determined to be (3 × 3), θ ) 0.44. Because this structure differs from the pure NO structures found in UHV,7 we believe that this structure could be a mixed form of NO molecules and OH species, as derived from decomposition of HNO2 at Rh(111) surfaces. This mixed (3 × 3) structure was stable between 1.0 and 0.3 V. In situ STM imaging also facilitated real-time monitoring of the reduction processes at Rh(111) surfaces under cathodic polarization to 0.2 V or more negative.
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Figure 1. Cyclic voltammogram of an ordered Rh(111) electrode at 50 mV/s in 0.5 M HF containing 1 mM KNO2. The first sweep goes negative, starting from the open-circuit potential (0.8 V).
Experimental Section Rh electrodes were prepared according to a procedure described elsewhere.14,15 The electrodes were annealed with a hydrogen torch and quenched into hydrogen-saturated Millipore water. Rh single crystalline beads were used for STM imaging experiments, whereas cut-and-polished Rh(111) electrodes were used for voltammetric measurements. Rh electrodes after annealing and quenching were quickly transported into the electrochemical or STM cell under protection of a thin water film. To ensure a long-range ordered NO adlayer, the potential of a Rh electrode was made negatively to 0.05 V to reduce the oxide species produced by the quenching process, followed by potential holding at 0.6 V in a pH 2, 0.5 M HF solution. According to a previous report,11 immersion of Rh and Pt electrodes into KNO2-containing acidic solutions could produce a monolayer of nitrogen monoxide. In this study, this procedure was adopted to prepare NO overlayers at Rh(111) electrodes for electrochemical and in situ STM experiments. Ultrapure hydrofluoric acid from Merck Inc. (Germany) and KNO2 from Fisher (New Jersey) were used as received. Millipore triple-distilled water (resistivity > 18.2 MΩ) was used to prepare all the needed solutions. The length of time that the electrodes were submerged in the KNO2-dosing solutions could influence the amount of NO adsorbed at Rh(111) electrodes. The Rh electrodes were exposed to the solutions for at least 10 min to ensure the saturated coverage. Before the dosing of KNO2, Rh(111) electrodes were diagnosed by acquiring cyclic voltammograms in pure HF solutions. In addition, cyclic voltammetry (CV) data, the same as reported previously,13-15 were obtained herein. Hydrofluoric acid rather than perchlorate acid solutions were used in this experiment because rhodium-catalyzed reduction of perchlorate to chloride anions can be avoided.15,19 The STM was a Nanoscope-E (Santa Barbara, CA) and the tip was made of tungsten (diameter, 0.3 mm) prepared by electrochemical etching in 2 M KOH. After thorough rinsing with water and acetone, a tip was further painted with nail polish for insulation. The leakage current of the tip at the open circuit potential was less than 0.05 nA. More than 80% of the as-prepared tips yielded a good resolution. Reversible hydrogen electrodes (RHE) were used in the electrochemical and STM measurements and all the potentials in the next section refer to a RHE scale.
Results Cyclic Voltammetry. Figure 1 shows the cyclic voltammogram of a Rh(111) electrode in a pH 2, 0.5 M HF (16) Hourni, M.; Wieckowski, A. J. Electroanal. Chem. 1988, 244, 147. (17) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (18) Itaya, K. Prog. Surf. Sci. 1998, 53, 121. (19) Rhee, C. K.; Wasberg, M.; Zelenay, P.; Wieckowski, A. Catal. Lett. 1991, 10, 149.
Figure 2. Two consecutive cyclic voltammograms of a NOcovered Rh(111) electrode at 50 mV/s in 0.5 M HF. The dotted and the solid traces represent the first and second scans.
solution containing 1 mM KNO2. Although this current vs potential profile is essentially featureless between 0.8 and 0.2 V, the cathodic current increases precipitously at 0.20 V because of the irreversible reduction of surfacebound NO molecules and nitrite species in the solution. At the positive potential of 0.8 V, a small but persistent anodic current of approximately 20 µA/cm2 is noted, possibly arising from oxidation of Rh(111) electrode and/ or adsorption of counteranions of fluoride and nitrite ions. This CV result correlates with the literature.11 As for Rh(111) electrodes covered with a monolayer of NO, the first potential sweeping toward the negative from open-circuit potential (0.85 V) to 0.05 V consistently gives rise to a poorly resolved doublet-reduction feature at 0.12 V, as revealed by the dotted trace in the voltammograms of Figure 2. In this case, the scan rate was 25 mV/s. A higher scan rate of 50 mV/s shifted the peak potential to 0.08 V, whereas sweeping at 10 mV/s led to a peak at 0.16 V. This doublet feature merged into a single peak at scan rates exceeding 50 mV/s. Notably, the cyclic voltammograms after the first potential excursion to 0.05 V resemble that of a clean Rh(111) electrode, including the highly reversible current spike at 0.63 V and the well-defined cathodic peak at 0.13 V. These features appearing in the solid trace are attributed to the adsorption/desorption of OH-like species and adsorption of a monolayer of hydrogen atoms, respectively.14,15 According to these findings, a single potential sweeping to 0.05 V completely reduces NO molecules at the surface. The coverage of surfacebound NO molecules can be quantified by integrating the charges between 0.3 and 0.05 V under the first reduction CV profile. In this study, a value of 612 µC/cm2 was obtained after subtracting the amount of charges associated with hydrogen discharge occurring between 0.2 and 0.05 V. At the anodic end, the CV profile was essentially featureless for potential cycling between 0.4 and 1.0 V, implying a stable NO adlayer within this range of potentials. Oxidation of adsorbed NO molecules to higher oxidized forms does not proceed to a detectable extent at potentials as positive as 1.2 V.
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Figure 3. Two consecutive cyclic voltammograms of a disordered Rh(111) electrode at 50 mV/s in 0.5 M HF. The dotted and the solid traces are the first and second scans.
Next, the extent to which surface structure affects the reduction of NO molecules at Rh electrodes was examined. Potential excursion to 1.3 V vastly modified the CV characteristics of an ordered Rh(111) electrode, including loss of the fine structure at 0.63 V, a more pronounced oxide reduction wave at 0.5 V, and broadening of the hydrogen adsorption profile (Figure 3). These changes can be attributed to the interruption of a long-range ordered Rh(111) surface, possibly producing a high population of step defects. A monolayer of NO molecules was adsorbed from an acidic KNO2 solution, after which a potential sweeping toward the negative was implemented in 0.5 M HF solution. Starting from the OCP of 0.7 V, cathodic current increased instantaneously, followed by three reduction waves at 0.42, 0.23, and 0.12 V, before the reversal of potential at 0.05 V. The following CV profiles are the same as those of a roughened but clean Rh(111) electrode, implying a complete and irreversible reduction of NO molecules. Evidently, this NO-stripping CV profile markedly differs from that of an ordered Rh(111) electrode (Figure 2). Compared with an ordered Rh(111) electrode, NO molecules at roughened Rh(111) electrodes began to decrease at less negative potentials. In addition, the reduction features at 0.42 and 0.23 V can arise from reduction of NO at uncharacterized defect sites. More thoroughly examining other well-defined Rh electrodes can elucidate the relationship between structure and reactivity. In situ STM Imaging. Before STM imaging of NO adsorbate, we identified the orientation of a Rh(111) electrode at 0.15 V in 0.5 M HF. A topography scan in Figure 4a reveals the typical surface morphology of a Rh(111) facet. The terrace and step features are obvious, and only a single step defect occurs to separate two extended terraces within an imaged area of 250 nm2. The step height of 0.23 nm indicates that this step defect is monatomic. This image was obtained with a bias voltage of 200 mV and a set point current of 3 nA. High-resolution STM scans, which reveal atomic features at terrace sites, justify the (111) orientation of the Rh electrode. An ordered array of protrusions along with a depression at the bottom of the image is clearly observed in Figure 4b, as obtained at 0.15 V with a bias voltage and feedback current of 20 mV and 20 nA. The hexagonal (60 ( 2°) array with a nearest neighbor spacing of 0.27 nm indicates that this ordered structure arises from atomic features of a Rh(111) electrode. This finding is essential to assign the structure of adsorbate over this hexagonal substrate lattice. Because Figure 4b displays a constantcurrent STM image, each protrusion gives rise to a 0.02-nm corrugation height, whereas the STM tip dropped 0.04 nm at the vacancy defect. Incidentally, the direction of the step line in Figure 4a is nearly parallel to that of a close-packed atomic row of Rh atoms. An electrochemically clean environment is essential to achieve STM atomic
Figure 4. STM topography scan (a) and atomic resolution (b) of Rh(111) in 0.5 M HF. The constant-current STM image in b reveals atomic features of the Rh(111) substrate and a vacancy defect at the bottom of the image. These images were obtained at 0.15 V with the tip biased 50 mV more positive than the Rh electrode. The set point current was 3 nA.
resolution of a Rh(111) substrate. In addition, the presence of a saturated amount of hydrogen at the surface at 0.15 V seemed to help achieve high-quality STM resolution. This STM atomic resolution can be used to calibrate the x- and y-sensitivity of piezo scanners. Shortly after introducing KNO2 solution, STM imaging of a Rh(111) electrode at 0.6 V unveiled a well-ordered structure on terraces spanning as much as 100 nm2, as indicated by the constant-current STM image in Figure 5a. This finding reveals that exposure to acidic KNO2 solution facilitates the growth of an organized adlayer on an initially ordered Rh(111) surface. A two-dimensional network was clearly imaged all over the 50 × 50 nm area. Figure 5b illustrates in detail the ordered structure at the central terrace. Despite the long-range ordering of the superlattice, many vacancy defects appear in the image. Such defects could stem from either vacancy defects at the Rh(111) substrate or missing adsorbate-NO molecules. These defects were immobile within an imaging period of 5 min. Meanwhile, a domain wall, highlighted by a vertical-running dotted curve in the image, segregates two ordered domains. Two line segments drawn in the images reveal the spatial misalignment between these two domains by a Rh atomic distance of 0.27 nm. Further high-magnification STM imaging could resolve the internal molecular features of the ordered array (Figure 6a). This 5 × 5 nm constant-current STM image is original, while Figure 6b presents a zoom-in of Figure 6a in which a 2D Fourier transform filtering method was used to remove noise with 0.2-nm spacing or closer. The rhombus in this figure represents the unit mesh of the long-range ordered NO adlayer structure. The internal angles of rhombus are determined to be 60 and 120 ( 2°, indicating the hexagonal arrangement of the NO overlayer. The unit vectors of the rhombus are parallel to the close-packed atomic rows of Rh(111) and the length of 0.81 nm triples the diameter of a rhodium atom (0.268 nm). These results unambiguously identify a (3 × 3) superlattice, which
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Figure 6. Original (a) and filtered (b) constant-height STM molecular resolution of NO chemisorbed at Rh(111). The rhombus in the image of b depicts the unit cell of the adlayer structure. The image was acquired at 0.5 V with a bias voltage of -100 mV and a feedback current of 10 nA. Figure 5. In situ constant-height STM images of Rh(111) in 0.5 M HF containing 1 mM KNO2. These images were obtained at 0.6 V with a bias voltage of -200 mV and a feedback current of 15 nA.
contains four protrusions per unit cell. This implies a coverage of 4/9 or 0.44. Closely examining the unit cell reveals that the intensity of the protrusions are different. The four corners of the unit cell, labeled 1, appear to be equally bright whereas the other three spots 2, 3, and 4 exhibit similar intensity. 1 gives rise to a corrugation height 0.02 nm higher than 2, 3, and 4. The appearance of this STM image somewhat resembles that of iodine chemisorbed at Pt(111), in particular the asymmetric (3 × 3) structure. Both of the (3 × 3) structures contain four spots arranged in a similar manner. In the asymmetric Pt(111)-(3 × 3)-iodine adlayer,20 an iodine atom resides at a 3-fold hollow site at the center of the unit cell, although it is not clearly imaged. Consequently, this study has spent much effort to determine whether, like iodine at Pt(111), a NO molecule adsorbed in the middle of the (3 × 3) unit cell. However, after more than 200 h imaging this (3 × 3) structure, a high-quality STM image could not be obtained to resolve this issue. The difficulty might arise from the influence of solvent molecules-water, which could form hydrogen bonds with adsorbed NO molecules. In addition, NO molecules are not as rigid as iodine atoms and their wiggling motion thereby degraded the quality of STM imaging. This structure was stable against potential cycling between 1.0 and 0.2 V, although surface roughness apparently increases at a positive potential of 0.9 V. Although STM could not identify the chemical nature of these unknown species, they could be counteranions of F- and/or NO2- or caused by partial oxidation of the Rh(111) electrode. (20) Schardt, B. C.; Yau, S. L.; Rinaldi, F. Science 1989, 243, 1050.
Time-Dependent in Situ STM Imaging of NO Reduction. Real-time and real-space imaging capability enables STM imaging of the dynamics of interfacial events, particularly the initial stage of electrochemical processes. Many STM investigations have examined how surface structure affects the reactivity of interfacial reactions in UHV21,22 and electrochemical environments.23-26 Surface defects of steps and vacancies frequently behave differently from those of terrace sites. In terms of our attempt to reduce NO molecules at Rh(111) electrode surface, in situ STM allowed us to synchronously monitor a local surface area as potential was increased from 0.3 to 0.2 V or more negative potentials in 1 mM KNO2 and 0.5 M HF. Because the instantaneous cathodic current at 0.2 V is only 30 µA/cm2, according to the CV profile in Figure 1, only submonolayer of NO molecules underwent reduction, likely to occur at the interface. The locations of the reaction sites for these reduction processes is of particular interest in this article. First, the 60 × 60 nm in situ constant-current STM images in Figure 7a reveal not only well-defined terraceand-step features, but also well-structured molecular arrays. Internal structure of the ordered arrays, i.e., essentially the same as that found at 0.6 V (Figures 6a and 6b), is not discussed further. No change was noted for a 10-min imaging period at this potential. However, once the potential was decreased to 0.2 V, noticeable changes were observed immediately, as revealed by the in situ STM image presented in Figure 7b, as obtained 30 s after the potential step. The nearly identical morphological (21) Stranick, S. J.; Kamna, M. M.; Weiss, P. S. Surf. Sci. 1995, 338, 41. (22) Altman, E. I.; Colton, R. J. Surf. Sci. 1993, 295, 13. (23) Yau, S. L.; Moffat, T.; Bard, A. J. J. Phys. Chem. 1994, 98, 5493. (24) Kaji, K.; Yau, S. L.; Itaya, K. J. Appl. Phys. 1995, 78, 125. (25) Gao, X. P.; Hamelin, A.; Weaver, M. J. Phys. Rev. Lett. 1991, 67, 618. (26) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376.
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Figure 7. Sequence of in situ STM images revealing reduction of NO at Rh(111) in 1 mM KNO2 + 0.5 M HF: (a) at 0.3 V; (b) at 0.2 V, 30 s after the potential step; (c) at 0.2 V, 40 s later than b; (d) at 0.17 V, 20 s after the potential step; (e) at 0.17 V, 40 s later than d; (f) at 0.6 V, 5 min waiting. The acquisition time for each image is 12 s.
features of these two images indicate that the STM was primarily imaging the same area of the Rh electrode with slightly upward drifting. The surface morphology remains mostly unchanged, except that a portion of the terrace in the middle of the image suddenly becomes rough. We believe that this potential-induced roughening process signals the adsorption of incoming chemical species, such as protons (in a hydrated form) needed to reduce the NO molecules. As 40 s elapsed, numerous pits gradually replaced the roughness of this particular area, as indicated by the STM image in Figure 7c. Continuous imaging revealed a diminishing of pits, which is attributed to a gradual deposition of NO molecules back to the Rh surface. However, stepping the potential negatively from 0.2 to 0.17 V triggered reductive reactions again, resulting in even more pits at about the same area (Figure 7d). Maintaining the potential at 0.17 V for another 40 s rendered a smoother surface, as revealed by Figure 7e. Located only a few angstroms away from the Rh surface, the tip could block diffusion of protons from bulk into the interfacial region, rendering a slower reduction rate of NO(ads). Clear STM images could be obtained at a potential as negatively as 100 mV where the reduction reaction was still at a slow gear. However, stepping potential to 0.05 V speeded up the processes to the extent that STM imaging could not follow it and no stable STM image could be obtained. Stepping potential back to 0.6 V and holding for 4 min renders patches of ordered structures at most terraces, as revealed by Figure 7f. Holding the potential
at 0.6 V for 5 more min eventually led to a nearly saturated NO monolayer. High-resolution STM imaging revealed that the ordered structure was (3 × 3), i.e., the same as that before the potential step. The electrode surface now seems to contain somewhat higher vacancy defects than initially, along with many unidentified blotches. Discussion The cyclic voltammetric results in this study reveal several intriguing points concerning the reduction of nitric oxide at Rh electrodes. First, according to Figure 2, Rh(111) electrodes effectively catalyze reduction of NO molecules; a single potential sweeping to 0.05 V in HF solutions essentially removed all the surface NO molecules. The fact that disordered Rh(111) electrodes result in a different CV profile for NO reduction indicates that the surface structure of Rh electrodes heavily influences the kinetics of NO reduction. Although potential-induced roughening of ordered Rh(111) surfaces remains uncharacterized, reducing NO molecules clearly hinges on surface structures. If anodic polarization produced a higher step density on Rh(111), as known for Pt(111) electrodes,27,28 (110)-like atomic arrangements are then responsible for the reduction of CV features appearing at less negative potentials (Figure 3). This view may correlate with the (27) Aberdam, D.; Durand, R.; Faure, R.; El-omar, F. Surf. Sci. 1986, 171, 303. (28) Wagner, F.; Ross, P. Surf. Sci. 1985, 160, 305.
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Figure 8. Real-space model of the Rh(111)-(3 × 3)-4NO + OH adlayer. Large and small circles represent NO and OH, respectively.
finding that dissociative adsorption of NO molecules is much more pronounced at stepped Rh surfaces than at Rh(111).4 Further studies using other well-defined Rh electrodes can elucidate the relationship between reactivity of adsorbate and atomic structure of the substrate. By assuming that reduction of adsorbed NO molecules completely follows the reaction NO(ad) + 6H+ + 5 e- f NH4+ + H2O, one can estimate the coverage of NO molecules from the charges under the reduction wave at 0.1 V. Granted that 256 µC/cm2 charges are required for a one electron per Rh atom redox process, the 610 µC/cm2 net charges associated with NO reduction in Figure 2 results in a coverage of 0.48, same as the previously reported value.11 On the other hand, coverage of adsorbate can also be extrapolated from high-quality STM results, if all atomic or molecular features are resolved. In this case, the STM appearance of the Rh(111)-(3 × 3)-NO structure resembles that of the asymmetric Pt(111)-(3 × 3) iodine overlayer.20 Because the space inside the (3 × 3) unit cell can accommodate at least one NO molecule, we can infer that, like iodine at Pt(111), a NO molecule also adsorbs at a 3-fold hollow site within the unit cell. STM might have difficulty in imaging this particular NO molecule because of its spatial arrangement, as described for iodine at Pt(111). This hypothesis would result in a coverage of 0.55, about 16% higher than that calculated from electrochemical measurements. The value may be acceptable in view of the saturated coverage of 0.75 observed in UHV.7 However, the 16% error in coulometric measurements seems to be too high. Furthermore, because pure NO molecules only form two structures, c(4 × 2) and (2 × 2), in UHV,7 the (3 × 3) structure could be a mixed adlattice of NO and other species, such as OH, produced from decomposition of HNO2 at the Rh(111) surface. The adsorption of NO is apparently favored so that its coverage exceeds that of OH. This model is in line with previous IR results,11,12 which show no 3-fold coordinated NO molecules. Figure 8 presents a real-space ball model to account for the STM results in Figure 6b. Empirically, the relative corrugation height of protrusions in STM atomic images frequently reflects the coordination environments of adsorbate. Adsorbates at on-top or near-top sites are the brightest, followed by 2-fold bridge and 3-fold hollow ones. This model has been used to successfully account for STM results of many adsorbed atoms, ions, and molecules, including iodine,20,29 bromine,30 carbon monoxide,31,32 cyanide,33 thiocyanate,34 sulfate,35 and benzene36 chemisorbed at Pt(111) and Rh(111). STM measurements of
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carbon monoxide chemisorbed at Pt(111) even elucidate in situ IR results.32 Herein, this empirical rule is adopted to explain our STM results of NO molecules at Rh(111). The STM molecular resolution of the (3 × 3)-NO structure presented in Figure 6b reveals that the four corners, 1, exhibit a higher intensity than the remaining, labeled as 2, 3, and 4, of a similar intensity. Thus, NO molecules only reside at two types of binding sites. A previous IR study positively identified two IR bands near 1800 and 1580 cm-1 in the presence of HNO2, likely to arise from NO molecules chemisorbed at on-top (or near-top) and 2-fold bridge sites.11 The spatial structure depicted in Figure 8, although not unique, has the optimal fit to the previous IR results11 and STM images in this study. The relative intensity of the two IR bands also tentatively suggests the relative populations of these two binding configurations. According to the model in Figure 8, the on-top and 2-fold ratio would be 1 to 3, which qualitatively correlates with the IR results.11 With the van der Waals diameter of all the NO molecules as 0.31 nm, 15% larger than the atomic diameter of a Rh atom (0.27 nm), a local steric strain among the NO molecules is expected. This steric effect tends to force on-top NO molecules to asymmetric near-top sites, possibly resulting in a slight tilt of NO adsorbate. This crowded arrangement of NO molecules is unexpected, given the space available within each unit cell. As mentioned earlier, the structural information of NO adlayers obtained in UHV-coadsorbed oxygen atoms can induce shifting of adsorption sites of NO from 2-fold bridge to on-top linear sites.5,6 These results support our hypothesis that the (3 × 3)-NO adlayer contains some foreign species, possibly OH molecules at 3-fold sites within the unit mesh. The exact registry of this OH species is difficult to determine because this feature is poorly imaged. Despite the lack of direct evidence to support coadsorption of NO and OH at Rh(111), an early UHV study found that NO2 molecules decompose at Pt(111), giving rise to NO+O adlattice.37 Because of the difficulty in detecting -OH species chemisorbed at metal electrodes in situ, the ex situ approach which uses HREELS is more appropriate for characterizing the composition of the present Rh(111) electrodes if this structure remains intact upon emersion. This study again demonstrates that in situ STM is an effective means of probing interfacial events, as the cathodic processes occurring at negative potentials of 0.2 V are visualized directly through high-quality STM imaging. First, STM (Figure 7) revealed realistic surface structures at single-crystal electrodes, including not only extended terraces but also surface defects of steps, kinks, and vacancies. More importantly, in situ STM indicates that a potential step from 0.3 to 0.2 V initiates reduction reactions preferentially at the middle of terraces, not surface defects of step edges and kinks. The population of vacancy defects at the reaction sites appears to be static; no extraordinarily high density of pits was found at the (29) Yau, S. L.; Vitus, C. M.; Schardt, B. C. J. Am. Chem. Soc. 1990, 112, 3677. (30) Tanaka, K.; Yau, S. L.; Itaya, K. J. Electroanal. Chem. 1995, 396, 125. (31) Yau, S. L.; Gao, X. P.; Chiang, S. C.; Schardt, B. C.; Weaver, M. J.J. Am. Chem. Soc. 1991, 113, 6049. (32) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (33) Kim, Y. K.; Yau, S. L.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 393. (34) Yau, S. L.; Kim, Y. G.; Itaya, K. Anal. Sci. Technol. 1995, 8, 723. (35) Wan, L. J.; Yau, S. L.; Itaya, K. J. Phys. Chem.1995, 99, 9507. (36) Yau, S. L.; Kim, Y. G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (37) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; John Wiley & Sons: New York, 1996; p 464.
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reaction sites. Therefore, we believe that NO reduction preferentially initiates at atomically flat terraces, rather than surface defects. Catalytic activity for terrace and defects is a contentious issue and the relationship between structure and reactivity has been not been elucidated.38 A key parameter considered important in previous studies has been the unequal bonding strength of adsorbate at terrace and defect sites. Possibly, NO molecules bind relatively stronger at defects than those at terraces do so that they are more resistive toward reduction. On the other hand, because protons are required to reduce the number of NO molecules, the roughening observed at 0.2 V may be related to the adsorption of incoming protons. The time-dependent STM results then simply reflect the preference of adsorption of hydrated H+. These views are correlated with our previous findings of naphthalene at Rh(111), where adsorption of protons and desorption of naphthalene preferentially occur at terrace sites under cathodic polarization. Naphthalene molecules at upper step edges or near vacancy defects appear to stick to Rh surfaces much more tightly than those at atomically flat terraces. Naphthalene molecules at defects are the last ones to desorb. Furthermore, the reduction reactions are initially concentrated at some specific sites on terraces, rather than dispersing randomly. Consequently, this phenomenon may reflect the attractive interaction of protons, present in hydrated clusters linked with hydrogen bonds, at Rh(111) electrodes. By considering the requirement of having six protons to reduce one NO molecule, this property or structure should facilitate the reduction of NO molecules. In contrast, discharge of protons at Pt(111) follows a different route, leading to random desorption of naphthalene molecules. We are currently investigating the reduction processes of NO at Pt(111) to elucidate how the chemical nature of electrode materials affects their electrocatalytic activity. The extent of po(38) Yau, S. L.; Itaya, K. Colloids Surf. A 1998, 134, 21.
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tential polarization dominates the spatial distribution of reaction fronts. If potential is made negative enough, a reaction can be made so quickly that reactions proceed at terraces and defects indiscriminately. Nitric oxide is reduced nearly completely to NH4+ ions by a potential step to 0.05 V so that well-ordered (3 × 3) structure returned as the potential was stepped back to 0.6 V. Conclusion In situ STM revealed a well-ordered (3 × 3) nitric oxide adlayer at Rh(111) electrodes in acidic KNO2 solutions. High-quality STM molecular resolution identified four NO molecules per unit cell, leading to a saturated coverage of 0.44. The corrugation heights of these features indicate that three of the four molecules reside at 2-fold bridging sites whereas the remaining one chemisorbed at a neartop site. In the presence of HNO2 the (3 × 3) structure remained stable between 1.0 and 0.3 V, the onset of reduction reactions. In contrast to the c(4 × 2) and (2 × 2) structure formed by pure NO molecules in UHV, the (3 × 3)-NO adlattice could result from coadsorption with OH from decomposition of HNO2. Time-dependent in situ STM imaging of the reduction processes of NO highlights the effect of surface structure on the interfacial events. The reduction processes predominantly occurred at terrace sites, rather than defect sites such as step edges, kinks, and vacancies. Reduction of NO molecules was irreversible, leading to soluble products such as NH4+ ions. Although vacancy defects still persisted, the ordered structure of (3 × 3)-NO was retrieved after the potential of Rh was increased from 0.05 to 0.6 V. Acknowledgment. The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 88-2113-M-008-006. LA990417M
