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Gas/Surface reactions probed by reflection absorption infrared spectroscopy

Harmina Vejayan

In my thesis, I present an investigation of the dissociation reactions of gas phase molecules on single crystal metal surfaces studied by a molecular beam in combination with Reflection Absorption Infrared Spectroscopy (RAIRS). Two gas/surface systems were investigated namely water (D2O) dissociation on a flat Cu(111) surface and methane dissociation on a stepped Pt(211) surface.The dissociation of water molecules on an oxygen covered Cu(111) surface is presented first. In this study, I explain how I confirmed the reaction mechanism which leads to facile water dissociation on a Cu(111) surface pre-covered with oxygen atoms using isotopic labeling of the oxygen atoms coupled with RAIRS detection. Direct observation of the two different hydroxyl isotopologues on the surface showed that the water molecules dissociate on the surface by transferring an H-atom onto the oxygen adsorbate. At a surface temperature of 180 K, the reverse disproportionation reaction was observed to occur simultaneously with the dissociative adsorption of water molecules which resulted in the removal on the pre-covered oxygen isotope on the surface with continuous surface exposure to water molecules.The study of water dissociation is continued but for a clean Cu(111) surface i.e. no pre-adsorbed oxygen atoms. The sticking probabilities of the dissociated water molecules were investigated by controlling the translational energy and the vibrational energy introduced into the incident water molecules. This experiment showed that the translational energy is better in promoting the dissociative chemisorption of D2O on Cu(111) surface than vibrational energy. The interaction between methylidyne-CH(ads) co-adsorbed with H(ads) atoms on a Pt(211) surface was investigated. Three distinct RAIRS absorption peaks were observed for the CH(ads) on the surface where the intensity of the peaks could be transferred from one to the other by changing the H atoms coverage on the surface. These peaks were then assigned to the methylidyne with 0, 1 or 2 neighbouring hydrogen atoms. The three hydrogen environments arose as a result of the limited one-dimensional diffusion of hydrogen atoms on the steps of the Pt(211) surface.I conclude by summarizing the results from the studies presented in this thesis and showing future experiments that one could perform to understand the water molecules reaction on Cu surfaces better.

EPFL2023

Methylidyne Adsorption on Pt(211) Probed by Reflection Absorption Infrared Spectroscopy (RAIRS)

Rainer Beck, Ana Gutiérrez González, Harmina Vejayan

Methylidyne, CH(ads), adsorbed on a Pt(211) surface and its interaction with chemisorbed hydrogen atoms was studied by reflection absorption infrared spectroscopy (RAIRS). Methylidyne was formed on Pt(211) by methane dissociation from a molecular beam followed by thermal decomposition of the methane dissociation products. CH(ads) was detected by RAIRS via its symmetric C−H stretch vibration resulting in three discrete absorption peaks in the region −1 of 2950−2970 cm . While the frequencies of the three C−H stretch peaks remain fixed, their relative intensities depend on the H(ads) co- coverage. This differs markedly from what was observed previously for the RAIR spectra of CH(ads) adsorbed on Pt(111) by the group of Trenary,1 who observed a single C−H stretch peak, which showed a continuous blue shift with increasing H(ads) coverage. Based on our experimental results and density functional theory (DFT) calculations, we propose that the three discrete absorption peaks on Pt(211) are due to the adsorption of methylidyne on the steps of Pt(211) forming one-dimensional rows of adsorbates. Depending on the H(ads) coverage, the CH(ads) species on the step sites can have either zero, one, or two neighboring H(ads) atoms, leading to three different vibrational C−H stretch frequencies and a reversible shift in relative peak intensity depending on the H(ads) coverage.

2022

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3)

Rainer Beck, Harmina Vejayan

Reflection–absorption infrared spectroscopy (RAIRS) is widely used to identify molecular adsorbates on metals during surface chemical reactions, but the interpretation of RAIRS data can be difficult with experiment alone. Here, we reveal from first-principles calculations the origin of the contrasting RAIRS spectra of methyl adsorbed on Pt(111) and Ni(111). We find that the dynamic dipole associated with the symmetric C–H stretch vibration of CH3 along surface normal is significant on Pt(111) but negligibly small on Ni(111), explaining the strong IR activity in the former and the absence of any RAIRS peaks in the latter. This difference is correlated to different charge transfer patterns between metals and the adsorbate, which are determined by the different preferred adsorption sites of methyl on the two surfaces. This work highlights the need of electronic structure calculations in interpreting RAIRS spectra of adsorbates on metal surfaces.

2021

Gas/Surface reactions probed by reflection absorption infrared spectroscopy

Harmina Vejayan

EPFL2023