Quantum state-resolved gas/surface reaction dynamics probed by reflection absorption infrared spectroscopy

Rainer Beck, Régis Bisson, Li Chen, Hirokazu Ueta
American Institute of Physics, 2013
Article

Résumé

We report the design and characterization of a new molecular-beam/surface-science apparatus for quantum state-resolved studies of gas/surface reaction dynamics combining optical state-specific reactant preparation in a molecular beam by rapid adiabatic passage (RAP) with detection of surface-bound reaction products by reflection absorption infrared spectroscopy (RAIRS). RAIRS is a non-invasive infrared spectroscopic detection technique that enables online monitoring of the buildup of reaction products on the target surface during reactant deposition by a molecular beam. The product uptake rate obtained by calibrated RAIRS detection yields the coverage dependent state-resolved reaction probability S(θ). Furthermore, the infrared absorption spectra of the adsorbed products obtained by the RAIRS technique provide structural information, which help to identify nascent reaction products, investigate reaction pathways, and determine branching ratios for different pathways of a chemisorption reaction. Measurements of the dissociative chemisorption of methane on Pt(111) with this new apparatus are presented to illustrate the utility of RAIRS detection for highly detailed studies of chemical reactions at the gas/surface interface.

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https://infoscience.epfl.ch/record/186427?ln=fr

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Molecular-beam/surface-science apparatus for state-resolved chemisorption studies using pulsed-laser preparation

Rainer Beck, Plinio Maroni, Thomas Rizzo, Mathieu Schmid

We describe a new apparatus that combines pulsed laser excitation in a molecular beam with surface-science methods for preparation of clean single-crystal surfaces and detection of adsorbates to enable state-selected studies of gas-surface reaction dynamics. Reactant molecules are prepared in specific vibrationally excited states via overtone pumping using tunable, narrow-band laser radiation. The collision-free environment of the molecular beam prevents relaxation of the prepared molecules before impact on the target surface and enables complete control over the collision energy and incidence angle. Chemisorption products are detected after a given deposition time by Auger electron spectroscopy. To achieve sufficient beam flux of state-selected reactant molecules for product detection by standard surface-science techniques, we use a high-intensity, short-pulse molecular-beam source matched to the low duty cycle of the pulsed lasers used in our experiments. We present the design and characterization of this new apparatus together with a scheme for generating infrared laser pulses of high spectral brightness for saturating weak vibrational overtone transitions within a significant volume of the molecular beam. The effectiveness of our approach is demonstrated by state-resolved sticking coefficient measurements for overtone-excited (2nu(3)) CH4 on Ni(100) for a range of impact energies. (C) 2003 American Institute of Physics.

2003

Chargement

In this thesis, I describe the design and first applications of a new molecular-beam/surface-science ultra-high vacuum apparatus. The new machine combines rovibrational state-selective infrared pumping of gas-phase reactants with reflection absorption infrared spectroscopy (RAIRS) detection of surface-bound reaction products for quantum-state resolved studies of chemical reactions occurring at the gas/surface interface. RAIRS enables the online detection of adsorbates formed by chemisorption reactions of reactants prepared in a molecular beam that are incident on a single crystal metal surface with well-defined translational energy and angle of incidence. For state-resolved reactivity studies, the incident reactants can be prepared in selected rovibrational states using infrared pumping by a tunable, continuous-wave optical parametric oscillator. Our RAIRS setup achieves sub-monolayer detection sensitivity (2×10-4 ML CO and 1×10-2 ML CH3) within a 35 seconds acquisition time which enables us to record uptake curves of chemisorption products in real time during a molecular beam exposure. I obtained assigned RAIRS spectra of the nascent dissociation products of CH4, CH3D, CH2D2, CHD3 and CD4 on Pt(111) at 150 K; the assignments for the partially deuterated methane species are reported for the first time. I used the capability of distinguishing different surface species by RAIRS together with the quantum state-specific preparation of gas-phase reagents to probe for vibrational bond-selectivity in the dissociative chemisorption of CH3D, CH2D2 and CHD3 on Pt(111). For the three partially deuterated methane isotopologues, I observed that incident kinetic and/or thermal vibrational energy produce a nearly statistical distribution of C-H and C-D bond cleavage products. In contrast, activation by a single quantum of C-H stretch normal mode excitation leads to bond-selective dissociation via C-H bond cleavage. I also explored the capability of RAIRS to quantify the state-resolved reactivity in a gas/surface reaction. Using RAIRS, I measured the laser-off sticking coefficient S0 and the state-resolved S0 for the vibrationally excited 3(v=1, J=2) state of CH4 on Pt(111) at 150 K for different normal incident kinetic energies in the range of 21.6 - 67.3 kJ/mol. The increase of S0(laser-off) at Ts=150 K with kinetic energy is observed I to be steeper than what previously measured at higher surface temperatures Ts=578-800 K, demonstrating the effect of surface phonon excitation on the dissociative chemisorption of methane on Pt(111). I also report an intriguing dependence of the saturation coverage of the methyl products on the reactivity of the incident CH4. Furthermore, isotope effects for methane dissociation on Pt(111) were studied by comparing the S0(laser-off) of CH4, CH3D, CH2D2, CHD3 and CD4 at the same total incident energy (~70 kJ/mol), showing a decrease in S0 with increasing deuteration. Furthermore, the RAIRS results enabled a preliminary comparison of the relative vibrational state-specific reactivity of CH4, CH3D, CH2D2 and CHD3, each containing a single quantum of C-H stretch excitation but delocalized over increasing number of C-H bonds. Finally, RAIRS was also used to detect the transient physisorption of CH4 on Pt(111) and to exam the role of vibrational energy in the physisorption of CH4 on Pt(111) at 77 K. The results showed that one quantum of C-H stretch (v3) excitation has no measurable effect on the physisorption of methane on Pt(111), in sharp contrast to the vibrationally bond-selective chemisorption. The results presented in this thesis demonstrate that RAIRS is a powerful detection method for state-resolved gas/surface reaction dynamics studies which will be applicable to the study of many other molecule/surface systems.

EPFL2012

Chargement

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

Chargement

EPFL2019

EPFL2012