Sum Frequency Laser Spectroscopy during Chemical Reactions on Surfaces

Abstract

The surface specificity of vibrational sum frequency generation (VSFG) spectroscopy allows one to characterize adsorbed and reacting molecules on catalyst surfaces while the catalyst functions at high pressure and high temperature. VSFG spectroscopy can be carried out in different modes, including scanning, broadband, time-resolved, and polarization-dependent, and has been applied to various active surfaces. Single-crystal and nanoparticle model catalysts have mostly been used, which are typically prepared under ultrahigh vacuum, but applications to powder materials have been reported recently. In this article, the fundamentals and technical aspects of VSFG are summarized, and its benefits are illustrated by case studies of elementary processes of heterogeneous catalysis.involved electrons, atoms, ions, etc.) or, if compatible, are then dominated by the gas-phase contribution, which obscures the surface species information.For many years, VSFG remained one of the very few techniques that allowed for in situ (or operando) 13,14 studies of welldefined surfaces. Recent advances in the high-pressure (HP) variants of, for instance, polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), x-ray photoelectron spectroscopy (HP-XPS), scanning tunneling microscopy (HP-STM), or transmission electron microscopy (HP-TEM) have filled the surface scientist's toolbox for now. For a description of these methods, we refer to the works cited in References 11 and 15, and we focus on VSFG below. In this article, we restrict ourselves to well-defined surfaces such as single crystals and evaporated metal oxide systems and exclude conventional high-surface-area catalysts. The high surface area of the active component in technical catalysts facilitates in situ vibrational studies, for example, by infrared or Raman spectroscopy. 16 Vibrational Sum Frequency Generation Spectroscopy: PrinciplesVSFG makes use of the second-order nonlinear optical process of sum frequency generation, that is, two light waves at different frequencies interact in a medium characterized by a nonlinear susceptibility tensor χ (2) and generate a wave at the sum of their frequencies. 1 Because this nonlinear process typically produces only a small signal, high incident light intensities (i.e., pulsed lasers) are required. To acquire an SFG vibrational spectrum of adsorbed/reacting molecules on a metal catalyst, two short (e.g., picosecond) laser pulses are spatially and temporally overlapped on the sample (Figure 1a). One input beam is in the visible range at fixed frequency (ω vis ), and the second is tunable in the mid-IR region (ω IR ) to probe the vibrational modes of the surface species. In a simplified picture, when the IR beam is tuned through a vibrational resonance of the adsorbate, it induces a vibrational transition from the ground state to an excited state, and simultaneously, the visible beam induces a transition to a higher-energy virtual state through an anti-Stokes Raman process (see transition from higher to lower vibrational ...

Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy

“…Over the past decades, considerable progress has been made in the adaptation of vacuum-based surface science techniques to investigations of surfaces under realistic relative humidity and even liquid/solid interfaces. Surface sensitive spectroscopies that can be used under these conditions include optical methods, such as infrared spectroscopy (IR) 2,3 and vibrational sum-frequency generation (VSFG) 4,5 ; X-ray emission spectroscopy (XES) 6 ; near edge X-ray absorption fine structure (NEXAFS) with fluorescence detection 7 as well as surface X-ray diffraction (SXRD) 8 . Among the imaging techniques that can be used to study surfaces in the presence of gases and liquids are scanning force microscopy (SFM) in both contact 9 and noncontact 10 modes, as well as scanning tunneling microscopy (STM) 11 , and in recent years also transmission electron microscopy (TEM) 12 and scanning electron microscopy (SEM) 13 .…”

In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

“…On the other hand, vibrational SFG technique utilizes a second-order nonlinear optical process in which two light waves at different frequencies interact in a medium characterized by a nonlinear susceptibility tensor v (2) resulting in a wave corresponding to the sum of the frequencies of the interacting waves [28,29,35]. In order to obtain a SFG vibrational spectrum of adsorbates on a planar model catalyst, two picosecond laser pulses are spatially and temporally overlapped on the sample (Fig.…”

“…This means SFG signal can be detected for the adsorbates at the gas/solid or liquid/solid interfaces where inversion symmetry is broken. However SFG is not allowed for the media having inversion symmetry such as bulk solids, liquids and gases [35].…”

“…2 Basic operational principles of SFG. a IR-vis SFG process [29], b description of an SFG spectrometer based on a Nd:YAG picosecond laser system [29], c various modes of operation for SFG: scanning, broadband, pump-probe and polarization-dependent operational modes [35] similar experiments were extended to even higher CO partial pressures (i.e. P CO = 450 Torr) by Ozensoy et al [19] ( Fig.…”

The ReactorAFM: Non-contact atomic force microscope operating under high-pressure and high-temperature catalytic conditions

“…Examples are vibrational sum frequency generation laser spectroscopy 5 and near-ambient pressure X-ray photoelectron spectroscopy, 6 giving information on the vibrational states of adsorbed molecules and the chemical state of atoms, respectively. A third example is surface X-ray diffraction (SXRD) 7 that provides information on surface structure.…”