Understanding the complementarities of surface-enhanced infrared and Raman spectroscopies in CO adsorption and electrochemical reduction

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

are frequently employed to identify reaction intermediates in electrocatalytic systems, which are then correlated with reaction activities and used to deduce reaction mechanisms 3,4 . Infrared spectroscopy (IR) involves the interaction of infrared radiation (usually mid-IR within the wavenumber range of 400 -4000 cm -1 ) with the substrate. IR radiation is able to excite to quantum transitions of electrons in the substrate among vibrational levels if the energy differences between these levels match the energy of photons in the IR beam, thus generating fingerprint spectra of these vibrational modes 5 . In order for a vibrational mode to be IR active, the mode must be associated with changes in the dipole moment. In contrast to IR, Raman spectroscopy is based on the inelastic scattering of photons from sample surface. Typically, a source of monochromatic light (usually a laser in the visible, near infrared, or ultraviolet range) is shined on the sample. Photons excite the substrate into an excited energy state, and photons are emitted when the substrate returns to the ground state. Scattered photons with energy distinct from the incident light contain information regarding the vibration levels of the substrate, and are used to generate Raman spectra.
For a vibrational mode to be Raman active, the mode must be associated with a change in the polarizability.
Complementarity of IR and Raman Spectroscopies. IR and Raman spectroscopies are generally recognized as complementary vibrational techniques for probing molecular structure. For example, S3 the symmetric stretching vibration of a homo-diatomic molecule does not have a permanent dipole moment due to its symmetry, which makes this mode IR inactive. In contrast, the polarizability of a homo-diatomic molecule is expected to change along the normal coordinate in the stretching mode due to the displacement of nuclei, making the vibration mode Raman active. For antisymmetric stretching or bending vibrations, the dipole moment changes sign, and thus the corresponding modes are IR active. Although the changes of the polarizability in such modes are also non-zero, they are symmetrical upon inversion of the sign of the reaction coordinate, leading to an approximately harmonic changes of the polarizability for small displacements. Therefore, both the asymmetric stretching and bending vibrations are typically Raman inactive. In principle, as for large molecules, the dissection of bond dipoles and bond polarizabilities can be conducted in a similar conceptual approach. For modes that are both IR and Raman active of a molecular species, the IR and Raman peaks are expected to appear at the same wavenumber 6 .

Challenges and Significance of Combining Surface Enhanced IR and Raman Spectroscopies.
Although IR and Raman spectroscopies are generally considered complementary, they are seldomly used together when investigating electrochemical reactions. The utilization of IR spectroscopy in aqueous solutions is challenging due to the intense absorption of water, leading to to severe loss of spectral signal with any significant light path through water. In order to minimize the path length of the IR light through aqueous electrolytes, two different configurations of infrared reflection absorption spectroscopy (IRAS) and attenuated total reflection (ATR) were developed, which are applicable to a wide range of surfaces, including well-defined single-crystal facets 5 . In contrast to surface enhanced IR spectroscopy, surface enhanced Raman spectrscopy is limited to a relatively few substrates with surface plasmonic effects, such as Au, Ag, and Cu, and requires S4 sufficient surface roughness due to the low intensity of scattering light 7,8 . Therefore, the electrode surface structures, experimental setups and conditions are usually different between IR and Raman experiments.
In electrochemical catalysis, reactive molecules are expected to adsorb on catalyst surface prior to the subsequent bond activations, which could significantly alter their vibrational modes compared to those in their free molecular states and lead to shifts of vibrational frequencies 9