Energy catalysis serves as an essential technological bridge connecting the global transition toward renewable resources with long-term sustainable development, providing a critical foundation for establishing a zero-carbon human society. Addressing modern environmental degradation and fossil fuel depletion requires converting renewable inputs such as solar, wind, and biomass energy into storable chemical fuels like green hydrogen and methanol. However, realizing these transformations demands sophisticated catalysts capable of lowering thermodynamic reaction barriers and accelerating sluggish kinetic pathways. Historically, catalyst optimization has targeted strategies such as increasing local reactant concentrations, tuning electronic structures, stabilizing reactive intermediate species, and suppressing parasitic side reactions. In response to these analytical hurdles, researchers increasingly rely on in situ and operando molecular vibrational spectroscopy techniques to track dynamic surface evolutions, detect chemical bond vibrations, and establish rigorous structure-performance correlations under real operating environments.
Vibrational characterization methods possess distinct diagnostic capabilities across catalytic research, complementing atomic-resolution transmission electron microscopy, structural X-ray diffraction, and electronic-state X-ray absorption and photoelectron spectroscopies. In situ infrared spectroscopy excels at identifying functional group transformations through bond-specific vibrational modes, enabling direct kinetic analyses across broad spectral windows. Concurrently, in situ Raman spectroscopy probes rotational and low-frequency modes with minimal interference from aqueous solvents, offering distinct utility for electrocatalysis. Sum frequency generation spectroscopy provides exceptional surface selectivity and sub-monolayer sensitivity by harnessing second-order nonlinear optical processes that isolate interfacial phenomena from bulk media. The physical foundation of infrared absorption rests on interactions between infrared radiation and oscillating molecular dipole moments. Quantum mechanically, absorption transitions predominantly occur from the ground vibrational state to the first excited vibrational state, representing the fundamental absorption band, whereas transitions to higher excited states produce considerably weaker overtone bands. Spectroscopic conventions plot infrared absorbance against wavenumber in reciprocal centimeters, representing the reciprocal of radiation wavelength across mid-infrared regions to monitor surface chemical changes during ongoing catalytic reactions.
Implementing in situ infrared techniques in catalytic environments requires careful selection of detection geometries, categorized broadly into transmission and reflection modes. Transmission mode captures bulk-phase absorption, but weak intermediate signals and intense infrared absorption from liquid electrolytes, specifically water bending and stretching bands at approximately 1640 and 3400 reciprocal centimeters, severely obscure interfacial signatures. Specular reflection, utilized in infrared reflection-absorption spectroscopy, employs a thin electrolyte layer between one and ten micrometers thick, yet faces compromised time resolution, gas-liquid segregation from gas evolution, elevated interfacial impedance, and solvent evaporation artifacts. Conversely, attenuated total reflection geometry mitigates electrolyte solvent interference by directing light through an internal reflection element, generating an evanescent wave whose characteristic penetration depth probes surface species adjacent to a thin metallic working electrode. Pioneered by Yeager in the 1970s and subsequently integrated by Pons and Osawa into surface-enhanced infrared absorption spectroscopy, this approach facilitates molecular-level interfacial detection. Complementarily, diffuse reflectance infrared Fourier transform spectroscopy accommodates gas-solid interfaces without special pelletization or morphology destruction, serving as a flexible analytical tool across wide temperature, pressure, and atmospheric ranges in photocatalytic and thermocatalytic investigations.
In electrochemical water splitting, the cathodic hydrogen evolution reaction offers an eco-friendly route to generate clean hydrogen fuel boasting an energy density of 141.8 megajoules per kilogram, yet reaction kinetics depend intimately on the spatial coverage and configuration of adsorbed hydrogen intermediates. Employing in situ Fourier transform infrared spectroscopy and surface-enhanced infrared absorption spectroscopy, Zhu and colleagues evaluated ruthenium-modified platinum surfaces across varying ruthenium coverage levels. Pristine platinum electrodes exhibited an on-top adsorbed hydrogen vibrational band between 2000 and 2040 reciprocal centimeters at potentials below 0.2 volts, while pristine ruthenium films displayed an on-top hydrogen band at a lower wavenumber of 1821 reciprocal centimeters between 0.025 and minus 0.05 volts. On ruthenium-modified platinum films, both absorption bands appeared concurrently; the platinum-associated band position remained stationary, showing that ruthenium did not alter the electronic state of uncovered platinum, whereas the ruthenium-associated band shifted upward by approximately 40 reciprocal centimeters, demonstrating enhanced hydrogen-binding strength and cooperative participation of both metal sites during hydrogen evolution across various electrolyte pH values.
Ma and coworkers designed single-atom catalysts by selectively anchoring ruthenium atoms onto three-fold face-centered cubic hollow sites and iridium atoms into oxygen vacancy positions on cobalt oxyhydroxide. In situ surface-enhanced infrared absorption spectra revealed potential-dependent bands at 1051 and 1094 reciprocal centimeters corresponding to hydroperoxyl species, where an observed blue shift on dual-metal sites indicated strengthened oxygen-oxygen bond vibration stemming from modulated intermediate adsorption. This modulation lowered the oxygen evolution overpotential to 180 millivolts at 10 milliamperes per square centimeter, compared to 220 millivolts for the single-metal analogue. Similarly, Wang and collaborators examined acidic oxygen evolution over ruthenium dioxide supported on lithium cobalt oxide versus unsupported ruthenium dioxide. Operando spectra revealed a prominent hydroperoxyl band at 1033 reciprocal centimeters on the supported catalyst, demonstrating an adsorbate evolution mechanism requiring only 150 plus or minus 2 millivolts overpotential at 10 milliamperes per square centimeter in 0.5 molar sulfuric acid, whereas unsupported ruthenium dioxide displayed dual bands at 1033 and 1122 reciprocal centimeters, reflecting mixed lattice oxygen oxidation and adsorbate mechanisms alongside higher overpotentials of 210 plus or minus 5 millivolts.
Electrochemical carbon dioxide reduction represents a transformative strategy to convert greenhouse gas emissions into high-value chemical feedstocks, relying on vibrational spectroscopy to elucidate catalytic pathways and intermediate structures. Hao and coworkers investigated single-atom zinc catalysts with varied coordination environments, demonstrating that symmetrical four-coordinate zinc sites produced intense bands at approximately 1650 reciprocal centimeters for interfacial water bending and 1380 reciprocal centimeters for carbon-oxygen stretching in carboxyl intermediates, proving that cooperative water dissociation and carbon dioxide activation drive superior reduction efficiency. Extending this analysis to iron systems, single-atom iron favored parasitic hydrogen evolution due to weak carboxyl and carbonyl signatures, while dual-atom iron maintained balanced water dissociation and carbon dioxide protonation to achieve high carbon monoxide selectivity, and iron clusters suffered from restricted water adsorption despite exhibiting bicarbonate and carbonyl features. Furthermore, Peng and colleagues utilized attenuated total reflection infrared spectroscopy on dual-atom zinc-tin catalysts, detecting an oxygen-bound formate intermediate band at 1308 reciprocal centimeters that displayed potential-dependent behavior while noting the total absence of carbonyl bands near 1950 reciprocal centimeters, thereby validating an exclusive pathway avoiding carbon monoxide intermediates.
In photocatalytic and thermocatalytic gas conversion systems, diffuse reflectance infrared spectroscopy elucidates complex bond activation sequences across heterogeneous interfaces. Cao and coworkers investigated photocatalytic carbon dioxide reduction over copper single-atom modified cadmium sulfide catalysts, detecting four distinct absorption bands between 3595 and 3729 reciprocal centimeters attributed to carbon dioxide overtones and terminal hydroxyl vibrations from water dissociation. Enhanced terminal hydroxyl signals at 3729 reciprocal centimeters over copper-sulfur site pairs confirmed that copper centers promote water splitting to deliver protons to adjacent sulfur sites, facilitating proton-coupled electron transfer and tripling carbon monoxide production rates. In thermocatalytic syngas conversion over rhodium-ceria catalysts, Wang and collaborators tracked surface species at 320 degrees Celsius, observing initial linear and bridged carbon monoxide adsorption followed by methylene bands at 2927 and 2854 reciprocal centimeters and methyl bands at 2967 and 2884 reciprocal centimeters upon hydrogen exposure. In methane functionalization over molybdenum disulfide, Mao and colleagues detected sequential transformation of methoxy, hydroxymethyl, and formate species down to sub-zero temperatures of minus 20 degrees Celsius, showing that molybdenum-oxygen edge sites actively drive low-temperature methane functionalization toward formic acid.
Sum frequency generation spectroscopy provides unique surface diagnostic power as a second-order nonlinear optical probe operating under electric dipole approximations. Because second-order nonlinear susceptibility vanishes in centrosymmetric bulk media, sum frequency signals arise exclusively at interfaces where inversion symmetry is inherently broken. When a broadband femtosecond infrared pulse and a narrowband visible pulse overlap spatially and temporally on a sample, an output signal is emitted at the sum frequency under strict phase-matching constraints. Vibrational transitions require simultaneous infrared and Raman activity, effectively up-converting molecular vibrations into the visible detection regime. The resulting spectral intensity reflects the squared modulus of the second-order nonlinear susceptibility, comprising non-resonant and resonant components governed by vibrational amplitude, phase, and damping factors. Spectroelectrochemical sum frequency generation cells employ external thin-layer reflection geometries, which encounter electrolyte absorption distortions and mass transport limitations, or internal reflection designs that enhance stability and sensitivity when paired with transparent or ultra-thin electrodes. These configurations enable researchers to isolate true interfacial intermediates during complex catalytic reactions without interference from spectator molecules in surrounding liquid or gas phases.
Applying sum frequency generation spectroscopy to unsaturated aldehyde conversions, Baker and coworkers resolved distinct reaction pathways during furfuraldehyde conversion over platinum-supported metal oxide catalysts. Platinum on silica exhibited a dominant vinylic carbon-hydrogen stretch at 3030 reciprocal centimeters and a minor propylene methyl mode at 2860 reciprocal centimeters, driving unselective decarbonylation to carbon monoxide and furan. Conversely, bare titanium dioxide supports displayed aromatic and methylene carbon-hydrogen signatures at 3130 and 2920 reciprocal centimeters, identifying a furfuryl-oxy intermediate formed via carbonyl oxygen coordination. On platinum-titania catalysts, overlapping spectral features and negative resonant interference bands revealed interfacial cooperativity, wherein hydrogen spillover from platinum to titania activated the furfuryl-oxy species to selectively produce furfuryl alcohol. Studies on copper and gold electrodes tracked carbonyl and ethoxy intermediates, identifying standing and lying configurations of ethoxy species on copper that confirm key dual-carbon pathways leading toward ethanol production.
Advanced cascade catalytic designs further underscore the analytical precision of operando sum frequency generation in tracking multi-site surface reactions. Li and colleagues examined carbon dioxide electroreduction to methanol over cobalt and nickel phthalocyanine molecules co-loaded onto multiwalled carbon nanotubes. Single-site cobalt complexes revealed two potential-dependent carbonyl bands: an inactive species in the diffuse layer at 1940 reciprocal centimeters with a Stark tuning rate of 52 reciprocal centimeters per volt, and a catalytically active intermediate in the Stern layer at 2020 reciprocal centimeters exhibiting a Stark tuning rate of 175 reciprocal centimeters per volt. In the dual-site cascade catalyst, proximal co-loading enabled short-range carbon monoxide spillover across two-nanometer distances from nickel to cobalt sites, doubling the active-to-inactive carbonyl population ratio to approximately one compared to 0.6 in physical mixtures, which directly correlated with enhanced methanol production rates. Looking ahead, the broader adoption of in situ vibrational techniques requires overcoming spatiotemporal limitations, advancing from microsecond to femtosecond time-resolved sum frequency generation and nanoscale tip-enhanced or atomic force microscopy-infrared imaging. Integrating multi-modal spectroscopic platforms that simultaneously probe surface vibrations and electronic states, combined with machine learning models that link spectral descriptors to adsorption energies, promises to unravel complex catalytic interfaces and accelerate the rational design of high-performance energy materials.
Subject of Research: In situ infrared and sum frequency generation spectroscopy for characterization of energy catalysis mechanisms
Article Title: In situ infrared spectroscopy and sum frequency generation spectroscopy in energy catalysis: fundamentals, challenges, and perspectives
Article References: Guo, W., Ma, P., Cao, H., & Bao, J. (2025). In situ infrared spectroscopy and sum frequency generation spectroscopy in energy catalysis: fundamentals, challenges, and perspectives. Discover Catalysis, 2(1), Article 21. https://doi.org/10.1007/s44344-025-00026-3
Image Credits: AI Generated
DOI: 10.1007/s44344-025-00026-3
Keywords: in situ spectroscopy, infrared spectroscopy, sum frequency generation, energy catalysis, reaction intermediates, electrocatalysis, photocatalysis, thermocatalysis, surface chemistry, operando characterization, situ, infrared