Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1 + ν3 symmetric plus antisymmetric C–H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1 + ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C–H bond, making them very reactive. This behavior results from the weakening of the reacting C–H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C–H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.

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.

EPFL2019

Vibrational Activation of Methane Chemisorption: The Role of Symmetry

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

Quantum state-resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Pt(111). IR–IR double resonance excitation in a molecular beam is used to prepare CH4 in all three different vibrational symmetry components A1, E, and F2 of the 2ν3 antisymmetric stretch overtone vibration. Methyl dissociation products chemisorbed on the cold Pt(111) surface are detected via reflection absorption infrared spectroscopy (RAIRS). We observe similar reactivity for CH4 prepared in the A1 and F2 sublevels but up to a factor of 2 lower reactivity for excitation of the E sublevel. It is suggested that differences in the localization of the C–H stretch amplitudes for the three states at the transition state leads to the observed difference in reactivity rather than state-specific vibrational energy transfer to electronic excitation of the metal

Amer Chemical Soc2014

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

Rainer Beck, Li Chen, Hirokazu Ueta

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (Ts) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (Et) and vibrational state (n3 anti-symmetric C–H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(y) and the CH3(ads) saturation coverage ysat. We observe that both S0 and ysat increase with increasing Et as well as upon n3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Royal Society of Chemistry2013

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

Ana Gutiérrez González

EPFL2019