J. Phys. Chem. C 2009, 113, 19277–19280
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Vibration-Assisted Rotation and Deprotonation of a Single Formic Acid Molecule Adsorbed on Ni(110) Studied by Scanning Tunneling Microscopy Satoshi Katano,†,‡ Yousoo Kim,† Yuma Kagata,† and Maki Kawai*,†,§ Surface Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198 Japan, Research Institute of Electrical Communication, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 Japan, and Department of AdVanced Materials Science, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8651, Japan ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: August 31, 2009
We have studied the reaction of a single molecule of formic acid adsorbed on the Ni(110) surface using a scanning tunneling microscope (STM) at cryogenic temperatures. At 50 K, formic acid molecules, having an O-H bond, were converted to formate via thermal dissociation of the O-H bond, whereas deuterated formic acid molecules, having O-D, are adsorbed intact. By injecting tunneling electrons from the STM tip into individual formic acid molecules, we have selectively controlled two types of reactions: rotational motion and O-H bond dissociation to form formate. Injection of the tunneling electrons at various voltages indicates that significant enhancements in the reaction probability are due to the excitation of specific vibrational modes, that is, C-O stretching and O-D stretching modes of formic acid. 1. Introduction An initial process in heterogeneous catalytic reactions, that is, dissociative adsorption of molecules on transition-metal surfaces, has been extensively studied because of its importance in fundamental surface chemistry and in industrial applications.1,2 Such a dynamic process involves energy dissipation and transfer between molecules and metal surfaces. In particular, the collision of incident gas-phase molecules to surfaces is known to accompany the distribution of higher translational energy to lower molecular vibrational and rotational energies.3 It is noteworthy that highly excited vibrational and rotational modes overcome the activation barrier to dissociative adsorption. A microscopic understanding of the mechanism behind vibrationassisted dissociation at surfaces would provide new insights into direct or precursor-mediated reactions. Recently, scanning tunneling microscopy (STM) has been utilized as means not only to image a molecule with a high spatial resolution but also to excite a specific vibrational mode of an individual molecule by tuning a bias voltage between the STM tip and the substrate.4-10 Excitation of vibrational modes can be examined using inelastic electron tunneling microscopy (IETS) for the detection of small conductance changes.4,8 IETS provides the most direct and chemically sensitive analysis of single molecules on the surface, allowing us to elucidate the intrinsic properties of molecules and the nature of heterogeneous catalysis at the nanoscale. Moreover, action spectroscopy that measures responses of a molecule as a function of applied bias voltages has also been used to investigate the coupling of molecular vibrations with motions and reactions of molecules.8,11,12 Multiple excitations of vibrational modes induce molecular motions and chemical reactions that relate closely to the fundamental surface reaction process mentioned above. * Corresponding author. E-mail:
[email protected]. Tel: +81-48-467-9405. Fax: +81-48-462-4663. † RIKEN. ‡ Tohoku University. § University of Tokyo.
In this paper, we present the results of our study of STMtip-induced vibrational excitation of formic acid adsorbed on Ni(110) at cryogenic temperatures. The adsorption and interaction of formic acid (HCOOH) with transition-metal surfaces has attracted considerable interest because formate (HCOO-), which forms via the dissociation of the hydroxyl bond, appears as an intermediate in the water shift reaction, methanol synthesis, and other processes.13-22 Experiments of particular relevance to this STM study have been published previously.21,22 As HCOOH is exposed to the Ni(110) surface at 50 K, deprotonation of the carboxyl group readily occurrs, leading to the formation of HCOO- at both short-bridged and long-bridged sites.21 STM measurements performed at several substrate temperatures indicate that formate adsorbed at a long-bridged site is a metastable species that can diffuse even at low substrate temperatures (>15 K).22 In this study, we observed intact adsorption of formic acid at 50 K when exposing to HCOOD and DCOOD, indicating kinetic isotope effects on the hydroxyl bond. Furthermore, we demonstrated selective reactions of formic acid, including rotation and deprotonation, using the STM tip. Action spectra indicate that reaction probability is significantly enhanced by the excitation of a specific vibrational mode. Rotational motion and formate conversion occur via excitation of C-O and O-D stretching modes, respectively. Here, we discuss the adsorption and reaction behaviors of formic acid on Ni(110) in vibrationally excited states. 2. Experimental Section All experiments were performed in an ultra-high-vacuum (UHV) chamber with a base pressure of 3 × 10-11 Torr. The Ni(110) surface was cleaned by repeated Ar ion sputtering and annealing at 1000 K, until a clean surface was confirmed in the STM image. Several isomers of formic acid (HCOOH, DCOOH, HCOOD, and DCOOD) were purified by the freeze-pump-thaw method before being used. We exposed formic acid to the clean Ni(110) surface at 50 K through a dosing tube located near the substrate. STM experiments were carried out using a lowtemperature STM system (LT-STM, Omicron GmbH) at 4.7
10.1021/jp906627g CCC: $40.75 2009 American Chemical Society Published on Web 10/09/2009
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Figure 1. STM images obtained after exposing the Ni(110) surface to formic acid, (a) DCOOH, and (b) DCOOD at 50 K. S and L indicate formate (DCOO-) adsorbed at the short-bridged and the long-bridged sites, respectively. The molecule indicated by F in (b) is formic acid (DCOOD) adsorbed at the trough of the Ni(110) surface. The atomically resolved STM image of the Ni(110) substrate is also depicted in an inset of (b). All STM images were acquired at 4.7 K with a sample bias of 20 mV and a tunneling current of 1.0 nA.
K. All STM images were obtained in constant-current mode with an electrochemically etched tungsten tip. 3. Results and Discussion Figure 1a shows the STM image obtained after exposing Ni(110) to DCOOH at 50 K. An individual molecule appears as a bright protrusion, accompanied on either side with depressions. It was found that the two depressions always align parallel to the [11j0] and [001] directions. Typical examples of these species are indicated in Figure 1 by S and L, respectively. We note that the STM image shows the same characteristics as the image derived from exposure to HCOOH. This indicates that the observed species can be assigned to the formate (DCOO-), which is formed via the dissociation of the O-H bond when the formic acid molecule is adsorbed. Drawing on previous reports, we attribute S and L to the formate adsorbed at the short-bridged and long-bridged sites, respectively.21,22 However, we found that a molecule assumes a different shape in an STM image when formic acid molecules, having a deuteride hydroxyl group (HCOOD or DCOOD), are exposed to Ni(110). The STM image obtained after exposing DCOOD to Ni(110) at 50 K is shown in Figure 1b. In addition to S and L formate, the species indicated by F appears. The STM image of F shows a 0.2 Å higher protrusion and a 0.1 Å lower depression compared with S formate. Weak depressions observed in the STM image of F are probably explained by the small perturbation on the electronic states of the metal when the formic acid molecule was adsorbed. The fraction of F on Ni(110) was measured by counting the number of molecules extended to the several images and was found to be 87%. The number of F species molecules decreased and were transformed into S formate upon annealing at temperatures above 120 K. Previous vibrational studies revealed that the Ni(110) surface is covered only with formate above 120 K, indicating that the F species is intact formic acid.14,16 The fact that DCOOD and HCOOD exist as stable adsorbates at 50 K can be explained by the kinetic isotope effect. Heavier atoms, such as deuterium, yield a lower zero-point energy, which results in a higher activation barrier for O-D bond dissociation.18,23,24 Figure 2a shows an STM image of S and F coadsorbed on Ni(110) obtained after exposure to DCOOD at 50 K. The center of F is located in the trough of Ni(110). We found that two equivalent adsorption orientations of F, reflecting the symmetry of the Ni substrate, appear simultaneously; these are indicated by Fa and Fb in Figure 2a. The longitudinal axis of F, defined
Figure 2. (a) An STM image of formate (S) and formic acid (Fa and Fb) coadsorbed on the Ni(110) surface. Fa and Fb shown in (a) are basically in the same adsorption configuration except that their axes are orientated symmetrically to the [001] direction. The expanded STM image focused on Fa and Fb is shown in (b). (c) An STM image of (a) after the STM tip was fixed above the marked point (X) shown in (a), followed by the injection of tunneling electrons (200 mV, 0.9 nA) from the STM tip for 5 s. (d) The adsorption model of formate (S, L) and formic acid (Fa and Fb) on the Ni(110) surface. The current profile during the injection of tunneling electrons is shown in (e). All STM images were acquired with a sample bias of 20 mV and a tunneling current of 1.0 nA.
by the alignment of depression-protrusion-depression, is parallel to the diagonal of Ni atoms in the Ni(110) unit cell. On the basis of STM image analysis, we propose the adsorption model for F as shown in Figure 2d. In previous reports, the two depressions appearing in the STM image of formate can be ascribed to the contribution from the electronic states localized at the oxygen atom.21,22 Note that those depressions are associated with the hybridization of electronic states between 3d orbitals of Ni atom and delocalized π and π* orbitals of formate. Although each oxygen atom in a formic acid molecule is in a different chemical environment (i.e., -CdO and -O-D), the two depressions in the STM image of F give essentially the same contrast. This is probably due to the delocalization of the π orbital around the carboxyl functional group, which is caused by the formation of the metal-substrate bonds through the oxygen atoms. The expanded STM image focused on Fa and Fb is shown in Figure 2b. Careful STM image analysis revealed
Single Formic Acid Molecule Adsorbed on Ni(110)
Figure 3. (a, d) STM images of formate (DCOO) and formic acid (DCOOD) coadsorbed on the Ni(110) surface. S and L indicate formate (DCOO-) adsorbed at the short-bridged and long-bridged sites, respectively. (b, e) STM images obtained after the injection of tunneling electrons (500 mV, 3 nA, 1 s) from the STM tip to formic acid, indicated by F in (a, b), respectively. Formic acid was transformed to (b) S formate or (e) L formate. The current profile during the injection of tunneling electrons (500 mV) is shown in (c). All STM images were acquired with a sample bias of 20 mV and tunneling current of 1.0 nA.
that Fa and Fb also have a weak depression, which is spread to the direction perpendicular to the longitudinal axis. It should be noted that the depression appears a little intensely at one side, as indicated by the arrow in Figure 2b. This suggests that the OCO place of F is slightly inclined with respect to the surface normal direction. It is interesting to note that the two equivalent orientations of F can be switched by the injection of tunneling electrons from the STM tip. We fixed the STM tip above the formic acid molecule indicated by Fa in Figure 2a, in which X denotes the exact tip position, slightly off-center of the molecule. The profile during the injection of tunneling electrons is shown in Figure 2e. The tunneling current was suddenly decreased (to 0.8 nA) during injection of the tunneling electrons, with a sample bias of 200 mV and tunneling current of 0.9 nA. The STM image obtained after the injection of tunneling electrons is shown in Figure 2c. Judging from the direction of the longitudinal axis, the formic acid molecule Fa changed its orientation to Fb. We induced another type of reaction when we applied a high bias voltage from the STM tip. Figure 3a shows the STM image of S, L, and F coadsorbed on Ni(110) obtained after exposure to DCOOD. As in the experiment detailed above (Figure 2), the STM tip was precisely fixed above F before application of a voltage pulse of 500 mV with the current set at 3 nA. The STM image obtained after electron injection in this experiment indicates that F species molecules were converted into S (Figure 3b). Here, we clearly confirm that formic acid changed into formate adsorbed at the short-bridged site, accompanying the cleavage of the O-D bond. The current profile during the injection of tunneling electrons is shown in Figure 3c. The large decrease in the tunneling current is due to the significant chemical and site changes induced by the STM tip. We also
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Figure 4. (a) Reaction yields per injected electron for the rotation (from Fa to Fb, indicated by the open circles) and OD bond dissociation of formic acid (HCOOD) on Ni(110) (formation of S and L indicated by the filled circles). The tunneling current was set at 3 nA. (b) Isotope effect was also examined using DCOOD (open squares, rotation; filled squares, dissociation).
confirmed the formation of L formate produced from formic acid. The STM tip was fixed above the formic acid, indicated by F in Figure 3d. After the injection of tunneling current into F with a sample bias of 500 mV and tunneling current of 3 nA (the same conditions as shown in Figure 3b), F was found to be converted into L (Figure 3e). We note that the probabilities of the STM tip-induced reaction of formic acid, that is, the rotation of F and the formation of formate (S or L) via OD bond dissociation, clearly depend on the applied voltage. The reaction yields of observed events as a function of the applied voltage are shown in Figure 4. The reaction yields measured by using HCOOD are shown in Figure 4a. Clear enhancements in the reaction yield were verified for the rotational motion at 140-160 meV and for the formation of formate at 260-280 meV. We confirmed that DCOOD provides similar voltage dependencies (Figure 4b), indicating that the C-H bond in formic acid is not responsible for the tunneling current-induced reactions. It is noteworthy that the observed threshold energies are close to the specific vibrational energies of formic acid adsorbed on metal surfaces. We have considered the possibility that C-O and O-D stretching modes are responsible for the STM-tip-induced rotation and formation of formate, respectively. Although the vibrational energy of the O-D stretch is reported to be 329 meV in the gas phase,25 this value is shifted to lower energy, that is, 236-277 meV, when the formic acid molecule exists on the metal surface or in crystal phase.26,27 Such a large vibrational energy shift stems from the interaction with metal and neighboring molecules, indicating
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Katano et al. cleavage of the O-H bond, whereas formic acid molecules, having an O-D bond, show intact adsorption. We demonstrated the selective reaction of formic acid using the STM tip. A formic acid molecule was switched between two equivalent adsorption configurations, accompanying the rotational motion at low bias voltage. However, O-D bond dissociation, followed by the formation of formate, can be induced at high voltage. Action spectra of these processes indicate that the reaction probability is significantly enhanced by the excitation of specific vibrational modes. Rotational motion and formate conversion take place via excitation of C-O and O-D stretching modes, respectively.
Figure 5. Schematic model of the rotation of formic acid and the formation of formate by tunneling electrons from the STM tip.
that the energy of the O-D stretching mode is sensitively affected by the local environments. A schematic model of STM-tip-induced reactions of formic acid adsorbed on Ni(110) is shown in Figure 5. It demonstrates that rotational motion and O-D bond cleavage can be induced selectively by tuning to an appropriate voltage. This selectivity can be explained in terms of the difference in the reaction probability, as shown in Figure 4. Rotational motion is preferentially excited at a lower voltage due to the higher tunneling-electron-induced reaction probability compared with the O-D bond dissociation. Although we found that excitation of the C-O stretching mode leads to rotational motion of formic acid, the reaction coordinate of this mode does not coincide with the rotational motion. We expect that dissipation of the energy from the high-frequency (C-O stretching) to a lowfrequency mode, such as the hindered rotational, will induce rotational motion. This kind of anharmonic coupling between vibrational modes has already been reported.5,7,11 On the other hand, formate formation observed in the highenergy region can be induced by the excitation of the O-D bond. This vibrational mode directly corresponds to the reaction coordinate. Subsequent to breaking the O-D bond, monodentate formate was transiently formed. This intermediate species contacts with the metal substrate by a single Ni-O bond and is known to be unstable; thus, it can be observed only in the compressed layer.28,29 Time-resolved laser spectroscopy confirms the transient formation of the monodentate.19,20 It is reasonable to conclude that S and L formate can be produced via the rotation of the monodentate, with the Ni-O bond as the rotational axis. However, the lifetime of the monodentate is reported to be about 100 ps.20 Thus, we cannot detect such a short-lived species at a transition state with STM. Note that the L formate is produced with a slightly higher probability (about 60%) than S formate. The observed imbalance in the production of formate indicates that the potential barrier for the formation of L formate is of a lower energy compared with that for S formate. 4. Conclusion We have studied the STM-tip-induced single-molecule reaction of formic acid adsorbed on Ni(110) at cryogenic temperatures. Kinetic isotope effects were clearly observed for the dissociation of hydroxyl bonds at 50 K. Formic acid molecules, having an O-H bond, were converted to formate with the
Acknowledgment. The present work was supported, in part, by a Grant-in-Aid for Scientific Research for Young Scientists B (No. 16750022) from the Ministry of Education, Culture, Sports, Science and by a Technology and International Joint Research Grant “Molecular wire” project (03BR1) from the New Energy Development Organization (NEDO) of Japan. References and Notes (1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons Ltd.: New York, 1994. (2) Ertl, G.; Kno¨zinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 1997; Vol. 5. (3) Nolan, P. D.; Wheeler, M. C.; Davis, J. E.; Mullins, C. B. Acc. Chem. Res. 1998, 31, 798. (4) Stipe, B. C.; Rezaei, M. A.; Ho, W. Science 1998, 280, 1732. (5) Stipe, B. C.; Rezaei, M. A.; Ho, W. Phys. ReV. Lett. 1998, 81, 1263. (6) Ho, W. J. Chem. Phys. 2002, 117, 11033. (7) Komeda, T.; Kim, Y.; Kawai, M.; Persson, B. N. J.; Ueba, H. Science 2002, 295, 2055. (8) Kim, Y.; Komeda, T.; Kawai, M. Phys. ReV. Lett. 2002, 89, 4. (9) Katano, S.; Kim, Y.; Hori, M.; Trenary, M.; Kawai, M. Science 2007, 316, 1883. (10) Pascual, J. I.; Lorente, N.; Song, Z.; Conrad, H.; Rust, H. P. Nature 2003, 423, 525. (11) Stipe, B. C.; Rezaei, M. A.; Ho, W. Science 1998, 279, 1907. (12) Motobayashi, K.; Matsumoto, C.; Kim, Y.; Kawai, M. Surf. Sci. 2008, 602, 3136. (13) Sexton, B. A. Surf. Sci. 1979, 88, 319. (14) Jones, T. S.; Ashton, M. R.; Richardson, N. V. J. Chem. Phys. 1989, 90, 7564. (15) Columbia, M. R.; Thiel, P. A. J. Electroanal. Chem. 1994, 369, 1. (16) Haq, S.; Love, J. G.; Sanders, H. E.; King, D. A. Surf. Sci. 1995, 325, 230. (17) Poulston, S.; Bennett, R. A.; Jones, A. H.; Bowker, M. Phys. ReV. B 1997, 55, 12888. (18) Yamakata, A.; Kubota, J.; Kondo, J. N.; Hirose, C.; Domen, K.; Wakabayashi, F. J. Phys. Chem. B 1997, 101, 5177. (19) Bandara, A.; Kubota, J.; Onda, K.; Wada, A.; Kano, S. S.; Domen, K.; Hirose, C. J. Phys. Chem. B 1998, 102, 5951. (20) Hirose, C.; Bandara, A.; Katano, S.; Kubota, J.; Wada, A.; Domen, K. Appl. Phys. B: Lasers Opt. 1999, 68, 559. (21) Katano, S.; Kim, Y.; Kagata, Y.; Kawai, M. Chem. Phys. Lett. 2006, 427, 379. (22) Katano, S.; Kim, Y.; Kagata, Y.; Kawai, M. Chem. Lett. 2008, 37, 914. (23) Madix, R. J.; Telford, S. G. Surf. Sci. 1992, 277, 246. (24) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Collins Publishers: London, 1988. (25) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; NSRDS-NBS: 1972. (26) Zelsmann, H. R.; Marechal, Y.; Chosson, A.; Faure, P. J. Mol. Struct. 1975, 29, 357. (27) Sexton, B. A.; Madix, R. J. Surf. Sci. 1981, 105, 177. (28) Uetsuka, H.; Sasahara, A.; Yamakata, A.; Onishi, H. J. Phys. Chem. B 2002, 106, 11549. (29) Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. B 1997, 101, 361.
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