AVS2013 Session SS2-ThA: Surface Dynamics and Non-adiabatic Processes

It is well known that formate (HCOO*) specie is the important intermediate for methanol synthesis by hydrogenation of CO₂ on copper catalysts. Because the formate synthesis by reaction of CO₂ with adsorbed hydrogen atom has been suggested to be an Eley – Rideal type mechanism ^[1], we try to clarify the dynamics of formate decomposition on Cu（110）, Cu（100） and Cu（111）surfaces in this study. That is, both formate synthesis and formate decomposition are expected to be thermal non-equilibrium processes. We have thus measured angle-resolved temperature programmed desorption (AR-TPD) of CO₂ formed by decomposition of formate. Formate was prepared by decomposition of ¹³C-formic acid (H¹³COOH) on oxygen pre-adsorbed or oxygen-free Cu(110), Cu(100) and Cu(111) surfaces. AR-TPD experiments were then carried out for ¹³CO₂. The signal intensity of ¹³C¹⁶O₂ in distribution was plotted for each polar angle in the range -80° to 80°( 5° as interval angle) from TPD spectrum. It is found that identical sharp distributions of ¹³CO₂ were observed Cu(110), Cu(100) and Cu(111) surfaces, showing distribution normal to the surfaces with the function of cos⁸θ. These results indicate that the decomposition of formate is a thermal non-equilibrium process, in which CO₂ molecules desorb with hyper-thermal energy due to a repulsive force from the surfaces. It is also suggested that the local structures just before the CO₂ desorption near the transition states (TS) are the same on Cu（110）, Cu（100） and Cu（111）. That is, ¹³C of ¹³COO is bound with a hydrogen adatom, while the O-Cu bond has been already broken just before the CO₂ desorption. The TS structures may be free to rotate and therefore desorbed ¹³CO₂ distribution is insensitive to the surface structure. The structure insensitivity is consistent with that measured for the kinetics of formate synthesis. Density functional theory (DFT) calculations are also consistent with the proposed model. [1] H. Nakano, J. Nakamura et al., J. Phys. Chem. B, 105 (2001) 1355-1365.

The effects of dynamics, surface temperature, and tunneling on the dissociative chemisorption of hydrogen on Cu(111) are explored using a dynamically-biased precursor mediated microcanonical trapping (d-PMMT) model. Transition state vibrational frequencies were taken from recent GGA-DFT electronic structure calculations and the model’s few remaining parameters were fixed by optimizing simulations to a limited number of quantum-state-resolved associative desorption experiments. The d-PMMT model reproduces a diverse variety of dissociative chemisorption and associative desorption experimental results, and, importantly, largely captures the surface temperature dependence of quantum-state-resolved dissociative sticking coefficients. Molecular translational energy parallel to the surface was treated as a spectator degree of freedom. The efficacy of molecular rotational energy to promote dissociation, relative to normal translational energy, varied monotonically from -45% to 33% as the rotational energy increased. The molecular vibrational and surface phonon efficacies were 60%. Efficacies did not vary with isotope change from H₂ to D₂. The thermal dissociative sticking coefficient for H₂/Cu(111) is predicted to vary as S(T) = S₀ exp(-E_a/RT) where S₀ = 0.075 and E_a = 49.2 kJ/mol over the 300 K ≤ T ≤ 1000 K temperature range. Dynamical effects are significant and suppress S(T) by ~2 orders of magnitude as compared to statistical expectations. For thermal dissociative chemisorption of H₂/Cu(111) at 1000 K, a temperature of catalytic interest, normal translational energy is calculated to provide 74% of the energy necessary to react, surface phonons 17%, molecular rotation 5%, and vibration 4%. Tunneling is calculated to account for 13% of S(T) at 1000 K, but more than 50% at temperatures below 400 K.

The femtosecond dynamics of CO adsorbed on a Ru(0001) surface after excitation by an optical laser have been studied using the Linac Coherent Light Source (LCLS) free electron x-ray laser. Changes in the occupied and unoccupied electronic structure were monitored using ultrafast soft x-ray absorption and emission spectroscopy. We found that the CO molecules, upon excitation by the optical laser, are excited to a weakly adsorbed precursor state occurring on a time scale of >2 ps, and at longer timescales, desorbed from this precursor state. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process. Within the first couple picoseconds, the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, each of these states can be selectively probed and the optical laser fluence dependent transient populations can be determined. At high optical laser fluences, a large fraction of the molecules (30%) were trapped in the transient precursor state that precedes desorption. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate. About half the molecules in the precursor state desorbed from the surface while the other half return to the chemisorbed state. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells - chemisorbed and precursor state separated by an entropy barrier - explain the anomalously high prefactors often observed in desorption of molecules from metals.

As an example for a hybrid system of inorganic and organic semiconductors we investigate the electronic structure and dynamics at the ZnO(10-10) surface and the influence of hydrogen and pyridine adsorption. Hydrogen termination leads to the formation a metallic surface band, whereas pyridine adsorption results in a quite substantial work function reduction (up to 2.9 eV), which may be useful for controlling the energy level alignment at inorganic/organic interfaces. Furthermore we directly monitor the ultrafast relaxation of hot photocarriers in the ZnO conduction band and the subsequenst formation of an excitonic state at the ZNO surface on ps timescales.

Ultimate spatial resolution of scanning tunneling microscope (STM) enables us to observe the inner electronic, vibrational [1-6] and spin [7-9] structures of a molecule adsorbed on solid surfaces. Vibrational spectrum of a single molecule provides useful information not only for the chemical identification of the molecule [1] but also for investigating how molecular vibrations can couple with the relevant dynamical processes [2-6], where the response of vibrationally mediated molecular motion to applied bias voltage, namely an “action spectrum”, can reveal vibrational modes that are excited through STM inelastic tunneling processes, because the molecular motion is induced only via the inelastic tunneling processes [3-5]. Molecular spin can couple with electrons of metal substrates. Depending on the strength of the coupling spin state of the molecule changes. Examples are given for Iron phthalocyanine (Fe-Pc) adsorbed on Cu, Au and Ag. With strong coupling with Cu(110) the spin is completely killed whereas if decoupled as on Cu(110) 2x1-O, spin survives with change in the direction of easy axis [7]. On Au weak coupling leads to appearance of Kondo state, where a competition between the RKKY interaction affects the spin state [8]. Kondo state of FePc on Au depends on the adsorption site, where the difference was originating from the coupling with the substrate electrons [9-10].

References :

[1] Y. Kim, T. Komeda, and M. Kawai, Phys. Rev. Lett. 89 (2002) 126104. S. Katano, M. Trenary, Y. Kim and M. Kawai, Science316 (2007) 1883.

[2] T. Komeda, Y. Kim, M. Kawai, et al., Science295 (2002) 2055.

[3] Y. Sainoo, Y. Kim, T. Okawa, et al., Phys. Rev. Lett.95 (2005) 246102.

[4] M. Ohara, Y. Kim and M. Kawai, Phys. Rev. Lett.100 (2008) 136104.

[5] K. Motobayashi, Y. Kim, H. Ueba and M. Kawai, Phys. Rev. Lett.105 (2010) 076101.

[6] H.-J. Shin, J. Jung, K. Motobayashi, S. Yanagisawa, Y. Morikawa, Y. Kim and M. Kawai, Nature Materials9 (2010) 442-447.

[7] N. Tsukahara, K. Noto, M. Ohara, S. Shiraki, N. Takagi, Y. Takata, J. Miyawaki, M. Taguchi, A. Chainani, S. Shin and M. Kawai, Phys. Rev. Lett.102 (2009) 167203.

[8] N. Tsukahara, S. Shiraki, S. Itou, N. Ohta, N. Takagi, and M. Kawai, Phys. Rev. Lett.106 (2011) 187201.

[9] E. Minamitani, D. Matsunaka, N. Tsukahara, N. Takagi, M. Kawai, Y. Kim, e-J. Surf. Sci. Nanotech.10 (2012) 38.

[10] E. Minamitani, N. Tsukahara, D. Matsunaka, Y. Kim, N. Takagi, and M. Kawai, Phys. Rev. Lett.109 (2012) 086602.

Vibrationally excited molecules can play an important role in methane activation on transition metal catalysts. This talk will focus on the reactivity of vibrationally excited methane molecules physisorbed to the surface. Under thermal reaction conditions typical in a steam reforming reactor, over half of the methane molecules are vibrationally excited, but their average translational energy is low enough that their trapping probability is significant. We use seeding techniques and molecular beam scattering methods to control the translational energy of the methane molecules incident on the surface, and infrared laser excitation of molecules in the molecular beam to control the amount and type of vibrational excitation. We find that vibrationally hot molecules can undergo reaction on low-index Ir surfaces via a precursor-mediated mechanism even in the presence of efficient vibrational quenching channels on the metal surface. In another set of experiments we explore how methane molecules in the ground vibrational state are activated while physisorbed to a hot surface.

In contrast to prior suggestions, this work suggests that an increase in reactivity with gas temperature may not be an automatic signature for a direct reaction mechanism where methane dissociates in a single collision. High gas temperatures also result in significant populations of vibrationally excited molecules whose additional internal energy may enhance their precursor-mediated reactivity. Our ability to control independently the translational and vibrational energy of the methane reagent and the surface temperature allows us to deconvolute these contributions to reactivity and more clearly reveal the mechanistic basis for methane activation under conditions typical in thermal processing.

In the experiments an effusive beam of hyperthermal nitrogen atoms is aimed at a Ru(0001)-O(2x1) surface at an angle of 60 degrees from the surface normal. Measurements of the angular distribution for Argon scattering under the same conditions have revealed that this surface is atomically flat and uncorrugated. Upon N-atom impact hyperthermal (4 eV) NO is detected. The angular distribution is peaked in the forward direction with a peak at 30 degrees from the normal, indicating that part of the N-atom parallel momentum is conserved in the NO forming collision. These are strong signatures of a prompt surface reaction.

To check the efficiency of the reaction, the time-of-flight spectra of the NO produced were measured for successive intervals. It became clear that fast NO was no longer produced, even though NO is still present in the back ground. From the decay of the NO production rate as a function of time the reactive cross section for the N+O=> NO reaction could be determined. It is about 2 Ų. Such a high cross section is another evidence for an Eley-Rideal reaction.

Prompt reactions can be used to specifically remove adsorbates from surfaces. We will demonstrate that not only O-atoms but also N-atoms can be removed from surfaces.