Dehydrogenation of Ammonia on Ru(0001) by Electronic Excitations

Article pubs.acs.org/JPCC

Dehydrogenation of Ammonia on Ru(0001) by Electronic Excitations Sabine Maier,†,‡ Ingeborg Stass,†,§ Xiaofeng Feng,†,∥ Aaron Sisto,⊥ Alexey Zayak,⊥,# Jeffrey B. Neaton,⊥ and Miquel Salmeron*,†,∥ †

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States Department of Physics, University of Erlangen-Nürnberg, Erlangen, Germany § Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany ∥ Department of Materials Science and Engineering, University of California, Berkeley, United States ⊥ The Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, Department of Physics, University of California, and Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California, United States # Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403, United States ‡

S Supporting Information *

ABSTRACT: We study the mechanism leading to the breaking of the N−H bonds in ammonia on Ru(0001) by means of scanning tunneling microscopy (STM). Our results support a model where injection of electrons or holes into antibonding (LUMO) and bonding (HOMO) orbitals of the molecule is far more effective than thermal excitations for molecular dissociation. We also found that a critical electric field between tip and surface is necessary to shape the tunneling barrier to obtain efficient rates of electrons from the tip or surface. First principle DFT calculations allow us to explain the observations and show that the applied electric field cannot by itself account for the observed dissociation. Since electron injection is the process governing photo- and electrochemical reactions, our STM study provides a detailed view of the reaction mechanism at the single molecular level in these nonthermal reactions.

■

has to take into account the effect of the strong electric field between the tip and the sample. The effect of the electric field on reactions at the single-molecule level has received little attention in STM experiments. In this paper we study the mechanism leading to the breaking of N−H bonds of single NH3 on Ru(0001). Several transition metal surfaces have been studied as a catalyst facilitating NH3 dissociation,17−20 and ruthenium has been identified as an optimal one.21,22 Here we show that in this case it is possible to elucidate the dissociation mechanism of the molecule because diffusion and desorption are not outcompeting dissociation when the energy of the tunneling electron is high enough to populate (depopulate) antibonding (bonding) orbitals of the molecule.

INTRODUCTION Chemical reactions are normally activated thermally to overcome the barriers for bond breaking. Heating populates all vibrational modes following a Bose−Einstein distribution and all electronic states following a Fermi−Dirac distribution. Usually, however, only one particular vibration mode or a particular electronic excitation is crucial to overcome the reaction barrier. Scanning tunneling microscopy offers the possibility to overcome the unselective nature of thermal chemistry and makes possible to study in detail these mechanisms at the single molecular level by virtue of its spectroscopic capabilities. A case of particular interest is the process where electrons or holes of appropriate energy are injected into antibonding (LUMO) or bonding (HOMO) orbitals of a molecule. This nonthermal process governs the photo- and electrochemical reactions. Traditionally laser pulses have been used to prepare reactants in specific vibrational states to study chemical reactions.1−3 A molecule vibrationally excited by a tunneling electron can dissociate,4−8 diffuse,9 rotate,10,11 or desorb12,13 from the surface. There are only a few reports of dissociation of molecules by direct injection of electrons or holes to antibonding or bonding orbitals in STM experiments with single molecules.14−16 This is because energetic tunneling electrons often induce diffusion and desorption of the target molecule away from the tip apex before vibration levels can be excited highly enough to cause dissociation. In addition, one © XXXX American Chemical Society

■

EXPERIMENTAL AND THEORETICAL METHODS The dehydrogenation mechanism of ammonia was studied using a home-built low temperature scanning tunneling microscope (STM) operated in ultrahigh vacuum.23 The ruthenium crystal was initially cleaned by cycles of argon ion sputtering (1 keV, pressure: 3 × 10−5 Torr) and annealing at 1600 K. Repeated annealing and cooling cycles between 770 Received: March 30, 2015 Revised: April 21, 2015

A

DOI: 10.1021/acs.jpcc.5b03054 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C and 1770 K in a partial oxygen atmosphere (4 × 10−8 Torr) were performed in order to deplete the first subsurface layers from carbon impurities. The remaining oxygen on the surface was removed by annealing the sample to 1720 K in ultrahigh vacuum (UHV). After preparation, the sample was transferred to the STM, in a connected UHV chamber, for imaging and dosing ammonia. All STM images presented in this paper are acquired in the constant current mode at a sample temperature of around 7 K using electrochemically etched Pt−Rh (80− 20%) tips. Ammonia molecules (99.99% anhydrous ammonia, Sigma-Aldrich) were dosed through a tubular doser in situ in the microscope at sample temperatures of 7 K. The clean Ru(0001) surface was exposed to NH3 gas to produce a coverage typically below 0.1 ML. After imaging to locate the NH3 molecules, the tip was positioned above one of them at the imaging distance (typically 0.5−1.0 nm, tunneling resistance 3 GΩ), the feedback was disabled, and a step voltage was applied to the sample. After a suitable time, the sample was imaged again to identify the nature of the reaction products. The identification of the different NHx (x = 0−3) species and their adsorption sites are discussed in detail in ref 24. We started with voltage pulse strengths below the vibrational modes of NH3 and gradually increased their strength until successive dissociation events were observed. The onset voltage of successive dissociation is referred to as dissociation threshold. The threshold has been determined in this way for each distance several times using different microscopic tips. Voltage pulses with a duration of up to 60 s have been applied. Density functional theory (DFT) calculations have been performed using the projector augmented wave (PAW) method25 and the Perdew−Burke−Ernzerhof (PBE)26 implementation of the generalized gradient approximation to represent exchange-correlation effects, as implemented in the VASP package.27−29 We modeled the Ru(0001) surface as a four-layer slab with periodic boundary conditions. Ammonia molecules are placed at on-top sites on the top ruthenium layer, with 10 Å of vacuum in the direction normal to the surface. To prevent ammonia molecules from interacting with each other, we chose a 4 × 4 supercell on the surface. The planewave energy cutoff is set to 500 eV for relaxations and self-consistent total energy calculations with high accuracy. For the Brillouin zone sampling, a Monkhorst−Pack integration scheme is used with a 2 × 2 × 1 k-points grid and Gaussian smearing with a width of 0.2 eV. The electronic convergence criterion for all calculations is set to 1 × 10−9 eV. To model a static electric field in the surface normal direction, a sawtooth external potential is used, varying along the surface normal direction.30 Spurious dipole interactions arising from the periodic boundary conditions cause both forces at the surface and the potential in the vacuum to be incorrect. To eliminate these effects, a self-consistent correction to the external potential is applied by inserting a fictitious dipole plane in the vacuum region between periodic slabs. This acts to eliminate the interaction between periodic images and retain the periodicity of the input external potential profile.

threshold voltage close to ±3 V at 3 GΩ tunneling resistance, marked by an abrupt change of the tunneling current and a notable change in the image contrast, which changed from a protrusion for NH3 to a low contrast feature for NH2 and a depression for NH and N (Figure 1). The NH2 molecule

Figure 1. Dissociation of an ammonia molecule into NH2 using a voltage pulse of −3.5 V. STM image showing 4 NH3 molecules before (a) and after (b) dissociation, where the top molecule dissociated to form NH2 a few lattice distances away; (c) corresponding current vs time diagram, where the steep drop of the current reflects the dissociation event. The contrast of the product changes from a protrusion for NH3 to a weaker one for NH2. The STM contrast of NH2 has the shape of a dumbbell with two lobes aligned perpendicular to the compact lattice direction of the Ru substrate, as shown magnified in the inset of (b). The STM imaging parameters: −36 mV, 12 pA.

appears as a small dumbbell-shaped protrusion in STM images (see inset of Figure 1b). The identification of the different NHx (x = 0−3) species and their adsorption sites is discussed in detail in ref 24. The high electric field between tip and surface during the voltage pulses raises the question of whether Stark shift effects are important enough to substantially affect the electronic structure of the molecule and influence the reaction. To study the electric field dependence we performed experiments at various tip−surface distances by retracting the tip from its initial value corresponding to a tunnel resistance of 3 GΩ (imaging conditions; Figure 2a). As the distance was increased, we found that the threshold bias for dissociation increased also with a linear dependence (Figure 2b), while the emission current decreased by several orders of magnitude (Figure 2c), requiring pulse durations ranging from 1 ms to 60 s for the dissociation reaction to occur. The linear dependence of the threshold voltage on the tip− sample separation is significant, as it indicates that there is a critical value of the field for dissociation to occur. From the slopes of the curves in Figure 2b we obtain fields of 0.11 ± 0.01

■

RESULTS AND DISCUSSION Following excitation during the voltage pulse, several competing reactions including diffusion, desorption, and dissociation of NH3 were observed to occur depending on the strength and polarity in the STM experiments. Dissociation into various NHx (x = 0−2) products was observed above a B


Article

The Journal of Physical Chemistry C

Figure 2. Measured dissociation voltage threshold and corresponding current during the dissociation of NH3 to NHx (x = 0−2) fragments on Ru(0001). (a) Sketch of the setup defining the bias polarity. (b) Dissociation voltage threshold dependence on the tip retraction distance for both voltage polarities. (c) Measured current for the data points in (b). The STM parameters defining the initial tip surface distance (origin of x-axis) were 36 mV and 12 pA.

Figure 3. (a) Initial, (b) transition, and (c) final state of the ammonia dissociation process calculated using climbing image nudge-elastic band theory in absence of an electric field.

and −0.30 ± 0.01 V/Å for positive and negative bias, respectively. Although this calculation is valid only for parallel plate electrodes, the good reproducibility of the dissociation threshold for several different tips indicates that the tip radius is sufficiently larger than the tip−sample distance and that the approximation is still valid. The electrons emitted at the dissociation threshold have an energy substantially larger than that required to excite stretch and umbrella vibrational modes of the NH3 molecules, which are below 450 meV.31 Although these electrons could enhance dissociation through multiple quanta excitations,32,33 the linear dependence of threshold voltage with distance and the asymmetry between positive and negative polarity excludes dissociation by excitation of molecular vibrations. Having thus excluded direct vibrational excitation as the dissociation mechanism we performed a theoretical study of the effect of the applied electric field on the electronic structure of NH3, its bonding energy, vibrational modes, dissociation, desorption, and diffusion barriers. In the calculations we used density functional theory within the generalized gradient approximation of Perdew, Burke, and Ernzerhof (DFT-PBE). In agreement with previous DFT calculations,24,34,35 we found the on-top site to be the equilibrium zero-field adsorption site for NH3 and the bridge site for NH2 molecules. Figure 3 shows the minimum energy adsorption sites of NH3 and NH2, as well as the transition state between the two minimum energy configurations, calculated using the climbing image nudgedelastic band method, in the absence of an electric field. The other reaction products, NH and N, occupy fcc hollow sites. Our binding energies of NH2 and NH are 0.1 and 0.2 eV lower than that of NH3, respectively. We also found that due to the high screening from the metal and the relatively small protrusion of NHx relative to the field-

induced image charge plane, the structure of the bound species was unchanged. The forces on NH3 due to the electric field were found to be relatively small, that is, less than 0.5 eV/Å for field strengths up to ±1 V/Å (Figure S1), producing a change of less than 3% in the calculated N−Ru bond length (2.22 Å) and H−N−H bond angle (108.84°), and negligible changes in the calculated N−H bond length (1.02 Å), see Table 1. Table 1. Changes in the Equilibrium Geometry of NH3 on Ru(0001) Due to an Electric Field E = 0 (V/Å) E = 1 (V/Å) E = −1 (V/Å)

α(H−N−H) (deg)

d(N−H) (Å)

d(Ru−N) (Å)

108.84 106.58 111.13

1.02 1.02 1.02

2.22 2.23 2.22

Furthermore, only a weak vibrational mode softening in particular for the umbrella mode was observed for applied fields up to ±1 V/Å, a mechanism that could induce dissociation or desorption for modes such as the N−H stretch, umbrella, or N−Ru stretch (Figure S2). Therefore, the electric field does not induce significant structural changes capable of inducing dissociation. This agrees with a previous study of Widdra et al.36 who observed a Stark shift in the symmetric deformation mode of ammonia produced by the static NH3 dipoles within a monolayer without observing dissociation due to an electric field. The calculations of the binding energy of NH3 in on-top sites (equal to the desorption energy barrier at the low temperature of our experiments), reveal a linear dependence on the electric field, as shown in Figure 4, with the binding energy changing by about ±0.15 eV for fields of ±1 V/Å. A negative polarity field decreases the binding energy and destabilizes the molecule, C


Article


The trend in the calculated binding energies of NH3 in the presence of the field compelled further study of potential fieldinduced changes in the molecular electronic structure. The projected density of states (PDOS) from the HOMO and LUMO orbitals of the adsorbed NH3 molecule is shown in Figure 5. The PDOS of both orbitals have energy tails

Figure 4. Dissociation, diffusion, and desorption barriers of NH3 on Ru(0001) as a function of the electric field from DFT-PBE.

while positive polarity has the opposite effect. This polarity dependence reflects a change in the NH3−Ru interaction unique to the ionic binding character that involves the N lone pair and Ru 4d-electrons.37 In addition to the binding energy we calculated the energy barriers for the competing reactions of dissociation (NH3 → NH2 + H) and diffusion, as a function of the electric field, using the climbing-image nudged elastic band method.38 We found that, at the transition state for dissociation, a H atom is located equidistant between two adjacent Ru atoms, while NH2 is pulled slightly off from the initial on-top position (Figure 3). This geometry is similar to the transition state calculated for NH3 dissociation on a Pt surface.39 Although the transition state geometry is relatively unaffected by the application of an electric field, there is a linear change in the reaction barrier with the applied electric field. The trends in the computed barriers with electric fields of these reactions are shown in Figure 4. The barrier for diffusion of NH3 between on-top sites is computed to be 0.59 eV at zero field, a value that does not change significantly with fields of either polarity because the field-induced change in binding energy at the initial state is identical to the change at the transition state. The barriers for both dissociation and desorption (equal to the binding energy) decrease with the negative polarity field. At −1.03 V/Å the barrier for desorption decreases below that for diffusion, making this reaction kinetically favorable. These results agree with the experimental findings that below the dissociation threshold preferentially diffusion for positive voltage pulses and desorption for negative voltage pulses, respectively, was observed in the event of an occasional change in the NH3 adsorption site. The computed zero-field barrier for dissociation is 1.37 eV, in agreement with previous DFT studies.40 Although the dissociation barrier is found to decrease with negative polarity, the trend is reversed with positive polarity. Furthermore, the dissociation barrier in the presence of the field remains much higher than the barriers for desorption and diffusion due to the relatively small change in the barrier with respect to the applied field. This is consistent with the trend of increased stability of NH3 on the surface with positive polarity, as the initial state in the reaction decreases in energy relative to the transition state. As a result, we conclude that dissociation due to field-induced changes in the reaction barrier is unlikely within the range of fields applied in these experiments.

Figure 5. Calculated DFT-PBE HOMO and LUMO resonances projected density of states of NH3 adsorbed on Ru(0001) as a function of applied electric field. The tails of the PDOS extend to near 3 eV from the Fermi level and experience only small changes (within 0.5 eV) with electric field.

extending to 3−4 eV from the Fermi level, and shifted by 0.5 eV or less by the applied fields. Even accounting for the fact that DFT is well-known to place such resonance energies too close to the Fermi level,41−43 our calculations, nonetheless, indicate no significant changes in the energy position of these levels with applied field. The theoretical results discussed above, together with the experimental findings of a threshold voltage close to ±3 V for dissociation, strongly point to a model where the tunneling electrons resonate with the LUMO orbitals at + bias, and with the HOMO orbitals at − bias, leading to the molecular dissociation. This supports also the observation of nonlocal dissociation14 for pulse voltages well above the threshold, where in a radius of several tens of nanometers around the STM tip multiple molecules are dissociated (Figure S3). The capital role of the electric field can be understood by considering the tunneling probability dependence on the shape of the tunneling barrier, which changes from a roughly rectangular one at low electric fields, to a triangular one at high electric fields. The critical field values are those producing a barrier width such that electrons can tunnel from the tip or from the surface, respectively, at a rate sufficiently high to dissociate NH3. An asymmetry in the sign of the field is to be expected because in the + polarity the barrier is determined by the work function of the tip, while for polarity, it is determined by the work function of the sample. The changing value of the current emission shown in Figure 2c (∼4 orders of magnitude) is compensated by a corresponding increase in the increased time of the required pulse duration (also ∼4 orders). Other variables are the fact that the electron energies also change with bias, spanning the range of the region of LUMO and HOMO energy spectra.

■

CONCLUSIONS In conclusion, our study of the dissociation of single ammonia molecules adsorbed on flat Ru(0001) terraces using electron D


Article


(7) Hla, S. W.; Bartels, L.; Meyer, G.; Rieder, K. H. Inducing All Steps of a Chemical Reaction with the Scanning Tunneling Microscope Tip: Towards Single Molecule Engineering. Phys. Rev. Lett. 2000, 85, 2777−2780. (8) Sloan, P. A.; P, R. E. Two-Electron Dissociation of Single Molecules by Atomic Manipulation at Room Temperature. Nature 2005, 434, 367−371. (9) Komeda, T.; Kim, Y.; Kawai, M.; Persson, B. N. J.; Ueba, H. Lateral Hopping of Molecules Induced by Excitation of Internal Vibration Mode. Science 2002, 295, 2055−2058. (10) Stipe, B. C.; Rezaei, M. A.; Ho, W. Inducing and Viewing the Rotational Motion of a Single Molecule. Science 1998, 279, 1907− 1909. (11) Stipe, B. C.; Rezaei, M. A.; Ho, W. Coupling of Vibrational Excitation to the Rotational Motion of a Single Adsorbed Molecule. Phys. Rev. Lett. 1998, 81, 1263−1266. (12) Bartels, L.; Wolf, M.; Klamroth, T.; Saalfrank, P.; Kühnle, A.; Meyer, G.; Rieder, K. H. Atomic-Scale Chemistry: Desorption of Ammonia from Cu(111) Induced by Tunneling Electrons. Chem. Phys. Lett. 1999, 313, 544−552. (13) Pascual, J. I.; Lorente, N.; Song, Z.; Conrad, H.; Rust, H. P. Selectivity in Vibrationally Mediated Single-Molecule Chemistry. Nature 2003, 423, 525−528. (14) Maksymovych, P.; Dougherty, D. B.; Zhu, X. Y.; Yates, J. T. Nonlocal Dissociative Chemistry of Adsorbed Molecules Induced by Localized Electron Injection into Metal Surfaces. Phys. Rev. Lett. 2007, 99, 016101. (15) Hahn, J. R.; Ho, W. Orbital Specific Chemistry: Controlling the Pathway in Single-Molecule Dissociation. J. Chem. Phys. 2005, 122, 244704. (16) MacLeod, J. M.; Lipton-Duffin, J.; Fu, C.; Rosei, F. Inducing Nonlocal Reactions with a Local Probe. ACS Nano 2009, 3, 3347− 3351. (17) Loffler, D. G.; Schmidt, L. D. Kinetics of NH3 Decomposition on Iron at High-Temperatures. J. Catal. 1976, 44, 244−258. (18) Ertl, G.; Huber, M. Mechanism and Kinetics of Ammonia Decomposition on Iron. J. Catal. 1980, 61, 537−539. (19) Hashimoto, K.; Toukai, N. Decomposition of Ammonia over a Catalyst Consisting of Ruthenium Metal and Cerium Oxides Supported on Y-Form Zeolite. J. Mol. Catal. A: Chem. 2000, 161, 171−178. (20) Bradford, M. C. J.; Fanning, P. E.; Vannice, M. A. Kinetics of NH3 Decomposition over Well Dispersed Ru. J. Catal. 1997, 172, 479−484. (21) Choudhary, T. V.; Sivadinarayana, C.; Goodman, D. W. Catalytic Ammonia Decomposition: COx-Free Hydrogen Production for Fuel Cell Applications. Catal. Lett. 2001, 72, 197−201. (22) Logadottir, A.; Rod, T. H.; Norskov, J. K.; Hammer, B.; Dahl, S.; Jacobsen, C. J. H. The Bronsted-Evans-Polanyi Relation and the Volcano Plot for Ammonia Synthesis over Transition Metal Catalysts. J. Catal. 2001, 197, 229−231. (23) Shimizu, T. K.; Mugarza, A.; Cerda, J. I.; Heyde, M.; Qi, Y. B.; Schwarz, U. D.; Ogletree, D. F.; Salmeron, M. Surface Species Formed by the Adsorption and Dissociation of Water Molecules on a Ru(0001) Surface Containing a Small Coverage of Carbon Atoms Studied by Scanning Tunneling Microscopy. J. Phys. Chem. C 2008, 112, 7445−7454. (24) Maier, S.; Stass, I.; Cerda, J.; Salmeron, M. Bonding of Ammonia and Its Dehydrogenated Fragments on Ru(0001). J. Phys. Chem. C 2012, 116, 25395−25400. (25) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (27) Kresse, G.; Hafner, J. Ab-Initio Molecular-Dynamics Simulation of the Liquid-Metal Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251−14269. (28) Kresse, G.; Hafner, J. Ab-Initio Molecular-Dynamics for OpenShell Transition-Metals. Phys. Rev. B 1993, 48, 13115−13118.

tunneling from an STM tip has revealed the importance of several reaction mechanisms. We have shown that • NH3 molecules do not diffuse appreciably compared to dissociation during application of electric field pulses, allowing for an exploration of both vibrational and electronic excitations; • The excitation of multiple vibration quanta is not the most efficient pathway to dissociate single NH 3 molecules; • Tunneling electrons with energies of 3 eV or higher are needed to induce dissociation; • A threshold field must be applied for the effective extraction of electrons in the field-effect assisted tunneling process to dissociate the molecule; • Stark shifts affect the binding energy and barriers for several reactions, including desorption and dissociation, while at the same time showing that none of these changes are by themselves sufficient to induce dissociation; • The injection (extraction) of electrons to (from) the LUMO (HOMO) levels is the most likely explanations for the observed dissociation.

■

ASSOCIATED CONTENT

* Supporting Information S

Additional Figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03054.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. DOE under Contract No. DE-ACO2-05CH11231. Portions of this work were performed at the Molecular Foundry, a DOE Office of Science User facility. Computations performed at NERSC.

■

REFERENCES

(1) Potter, E. D.; Herek, J. L.; Pedersen, S.; Liu, Q.; Zewail, A. H. Femtosecond Laser Control of a Chemical Reaction. Nature 1992, 355, 66−68. (2) Budde, F.; Hamza, A. V.; Ferm, P. M.; Ertl, G.; Weide, D.; Andresen, P.; Freund, H. J. Photodesorption of NO from Ni(100)-O. Phys. Rev. Lett. 1988, 60, 1518−1521. (3) Buntin, S. A.; Richter, L. J.; Cavanagh, R. R.; King, D. S. Optically Driven Surface Reactions: Evidence for the Role of Hot Electrons. Phys. Rev. Lett. 1988, 61, 1321−1324. (4) Stipe, B. C.; Rezaei, M. A.; Ho, W.; Gao, S.; Persson, M.; Lundqvist, B. I. Single-Molecule Dissociation by Tunneling Electrons. Phys. Rev. Lett. 1997, 78, 4410−4413. (5) Shin, H.-J.; Jung, J.; Motobayashi, K.; Yanagisawa, S.; Morikawa, Y.; Kim, Y.; Kawai, M. State-Selective Dissociation of a Single Water Molecule on an Ultrathin MgO Film. Nat. Mater. 2010, 9, 442−447. (6) Kim, Y.; Komeda, T.; Kawai, M. Single-Molecule Reaction and Characterization by Vibrational Excitation. Phys. Rev. Lett. 2002, 89, 126104. E


Article

The Journal of Physical Chemistry C (29) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (30) Neugebauer, J.; Scheffler, M. Adsorbate-Substrate and Adsorbate-Adsorbate Interactions of Na and K Adlayers on Al(111). Phys. Rev. B 1992, 46, 16067. (31) Jacobi, K. Nitrogen on Ruthenium Single-Crystal Surfaces. Phys. Status Solidi A 2000, 177, 37−51. (32) Salam, G. P.; Persson, M.; Palmer, R. E. Possibility of Coherent Multiple Excitation in Atom Transfer with a Scanning Tunneling Microscope. Phys. Rev. B 1994, 49, 10655−10662. (33) Walkup, R. E.; Newns, D. M.; Avouris, P. Role of Multiple Inelastic Transitions in Atom Transfer with the Scanning Tunneling Microscope. Phys. Rev. B 1993, 48, 1858−1861. (34) Neyman, K. M.; Staufer, M.; Nasluzov, V. A.; Rosch, N. Structure and Vibrations of Adsorption Complexes NH3/Ru(001): Density Functional Model Cluster Studies. J. Mol. Catal. A: Chem. 1997, 119, 245−251. (35) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Norskov, J. K. Ammonia Synthesis from First-Principles Calculations. Science 2005, 307, 555− 558. (36) Widdra, W.; Moritz, T.; Kostov, K. L.; König, P.; Staufer, M.; Birkenheuer, U. Identification of a Vibrational Stark Shift within an Adsorbate Layer: NH3 on Ru(001). Surf. Sci. 1999, 430, L558−L564. (37) Van Santen, R. A.; Neurock, M. Molecular Heterogeneous Catalysis: A Conceptual and Computational Approach; Wiley-VCH Verlag GmbH & Co. KGaA: New York, 2006. (38) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (39) Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A.; NovellLeruth, G.; Ricart, J. M.; Perez-Ramirez, J. Ammonia Dissociation on Pt(100), Pt(111), and Pt(211): A Comparative Density Functional Theory Study. J. Phys. Chem. C 2007, 111, 17551−17557. (40) Logadottir, A.; Norskov, J. K. Ammonia Synthesis over a Ru(0001) Surface Studied by Density Functional Calculations. J. Catal. 2003, 220, 273−279. (41) Neaton, J. B.; Hybertsen, M. S.; Louie, S. G. Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces. Phys. Rev. Lett. 2006, 97, 216405. (42) Dell’Angela, M.; Kladnik, G.; Cossaro, A.; Verdini, A.; Kamenetska, M.; Tamblyn, I.; Quek, S. Y.; Neaton, J. B.; Cvetko, D.; Morgante, A.; et al. Relating Energy Level Alignment and AmineLinked Single Molecule Junction Conductance. Nano Lett. 2010, 10, 2470−2474. (43) Tamblyn, I.; Darancet, P.; Quek, S. Y.; Bonev, S. A.; Neaton, J. B. Electronic Energy Level Alignment at Metal-Molecule Interfaces with a GW Approach. Phys. Rev. B 2011, 84, 201402.

F


Dehydrogenation of Ammonia on Ru(0001) by Electronic Excitations

Recommend Documents