State-Resolved Reactivity of CH4(2ν3) on Pt(111) and Ni(111): Effects of Barrier Height and Transition State Location

Quantum state-resolved sticking coefficients on Pt(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2ν3) at well defined kinetic energies in the range of 10-90 kJ/mol. The ground state reactivity of CH4 is approximately 3 orders of magnitude lower on Ni(111) than on Pt(111) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28 ± 6 kJ/mol. 2ν3 excitation of CH4 increases its reactivity by more than four orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by two orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2ν3 excited state of methane in terms of a difference in barrier height and transition state location for the dissociation reaction on the two metal surfaces.

The effect of molecular vibrations and surface structure on the chemisorption of methane on platinum

Marco Sacchi

In this thesis, I report state-resolved measurements of the chemisorption probability of CH4 on Pt(111) and Pt(110)-(1×2) for several rovibrationally excited states (2ν3, ν1+ν4, and 2ν2+ν4) in addition to the ground state. Measurements of the state resolved reactivity as function of the incident translational energy lead to state-resolved reactivity curves for each of the states under study. The relative efficacy of activating the dissociation reaction is obtained for each excited state by comparing the increase in reactivity observed upon excitation of a particular state to the effect of increasing the translational energy of CH4 in the ground state. The results provide clear evidence for mode specific reactivity with the highest efficacy for the stretch-bend combination (ν1+ν4), followed by the stretch overtone (2ν3) and the bend overtone state (2ν2+ν4). The results demonstrate that vibrational activation of CH4/Pt chemisorption process does not simply scale with the total internal energy of the incident CH4 molecule, which is a central assumption of the PC-MURT statistical model for dissociative chemisorption reactions developed by the group of Harrison [Ukraintsev et al., Chem. Phys., 1994. 101(2): p. 1564]. On the contrary, the qualitative predictions of the vibrationally adiabatic model proposed by Halonen et al. [J. Chem. Phys., 2001. 115(12): p. 5611] are in good agreement with our results. The higher efficacy of the ν1+ν4 state can also be rationalized by observing that, at the transition state, the breaking C-H bond is both stretched and bent from its equilibrium geometry, therefore I suggest that this state might have a significant projection on the reaction coordinate [Psofogiannakis et al., J. Phys. Chem. B, 2006. 110 : p. 24593 ; Anghel et al., Phys. Rev. B, 2005. 71 : p. 4]. Comparison between the state-resolved reactivity for CH4(2ν3) on Pt(111) and Ni(111) is used to obtain information about differences in barrier height and transition state location for the dissociation on the two different metals [Bisson et al., J. Phys. Chem., 2007. 111: p. 12679]. Finally, for the more corrugated Pt(110)-(1×2) surface, I determined the state-resolved sticking coefficients for different polar and azimuthal angles of incidence. Comparison between the reaction probability for incidence parallel and perpendicular to the missing rows of this surface shows shadowing effects that are consistent with predominant reactivity of the top layer Pt atoms.

EPFL2008

Bond-Selective and Mode-Specific Dissociation of CH3D and CH2D2 on Pt(111)

Rainer Beck, Phil Morten Hundt, Hirokazu Ueta, Maarten Eduard van Reijzen

Infrared laser excitation of partially deuterated methanes (CH3D and CH2D2) in a molecular beam is used to control their dissociative chemisorption on a Pt(111) single crystal and to determine the quantum state-resolved dissociation probabilities. The exclusive detection of C-H cleavage products adsorbed on the pt(111) surface by infrared absorption reflection spectroscopy indicates strong bond selectivity for both methane isotopologues upon C-H stretch excitation. Furthermore, the dissociative chemisorption of both methane isotopologues is observed to be mode-specific. Excitation-of symmetric C-H stretch modes produces a stronger reactivity increase than excitation of Me antisymmetric C-H stretch modes, whereas bend overtone excitation has a weaker effect on reactivity. The observed mode specificity and bond selectivity are rationalized by the sudden vector projection model in terms of the overlap of the reactant's normal mode vectors with the reaction coordinate at the transition state.

American Chemical Society2015

Bond-selective and surface-site-specific dissociation of methane on platinum

Ana Gutiérrez González

I present a molecular beam study of methane dissociation on differ-ent surface sites of several platinum single crystal surfaces (Pt(111), Pt(211), Pt(210), Pt(110)-(2x1)). The experiments were performed in a molecular beam/surface-science apparatus that combines rovibrational state-selective excitation of reactants with detection of the reaction products by reflection absorption infrared spectroscopy (RAIRS), Auger electron spectroscopy (AES) and King and Wells (K&W) technique. First, I explore how the barrier for methane dissociation depends on the Pt surface site (terrace, step, kink, and ridge atoms). For that, I compare the average reaction probabilities of methane on the different crystals using K&W. Moreover, I used the site-specific detection of chemisorbed methyl species by RAIRS to obtain the site-specific reactivity on steps, ridges and terraces. The highest methane reactivity and therefore the lowest barrier was observed for the surface atoms with the lowest coordination. The trans-lational energy dependence of the reactivity shows clear evidence for a direct activated mechanism on all the surface sites studied. The decreasing catalyt-ic activity for Pt atoms with increasing coordination number agrees well with theoretical predictions using first principles quantum theory calculations. Second, I present the role of rovibrational excitation of the incident methane on the chemisorption on different surface sites. For CH4, excitation of one quantum of the antisymmetric C-H stretch vibration (Îœ3) was found to be less efficient than an equivalent amount of translational energy normal to the sur-face for promoting the dissociation on all the surface sites. The vibrational efficacy was seen to be lower for dissociation on the low coordinated sites, in accordance with calculations of transition state geometries. For CH3D, the excitation of the antisymmetric C-H stretch vibration (Îœ4) led to bond selec-tivity on the steps and terraces of the Pt(211) surface. These results show how careful control of the translational energy and rovibrational state of the incident CH3D can be used for site-specific and bond-selective dissociation of methane. As predicted previously by theory but not observed before experi-mentally, the C-H bond selectivity is shown to decrease with increasing translational energy. Third, I explore the effect of surface temperature on the methane dissocia-tion. The sticking coefficient for a direct chemisorption reaction such as CH4 on Pt, might be expected to be independent of surface temperature, because the reaction happens very fast leaving very little time for the molecule to equilibrate with the surface. However, I observed an increase in methane reactivity with increasing surface temperature at low incident CH4 energies. This observation is interpreted by a âloss of coordinationâ from the atoms displaced out of the plane due to their vibrational motion when the surface is heated. The surface-site and quantum-state-specific data presented in this thesis are ideally suitable for testing theoretical models that aim to describe the me-thane-surface reaction at the microscopic level. Moreover, the characteriza-tion of different catalytically active sites for methane dissociation contrib-utes to advancing the understanding of methane reactivity towards real cata-lytic conditions, where surfaces with several different atomic terminations are used.