A New Look at Catalyst Surfaces at Work: Introducing Mixed Isotope Operando Infrared Spectroscopy (MIOIRS)

Abstract

This Perspective focuses on the characterization of supported metal catalysts by operando and CO infrared (IR) spectroscopy. CO IR spectroscopy is a powerful technique for probing catalyst surfaces and is used to identify single-atom catalysts, estimate metal surface availability to the gas phase, and measure catalyst Lewis acidity. However, the interpretation of CO IR spectra on metal surfaces is not trivial and is influenced by dipole–dipole interactions among CO molecules at medium to high coverage. Such a phenomenon results in spectral distortions, such as intensity transfer among IR bands, the appearance of spurious bands, and shifts in band position. Dipole–dipole interactions were widely investigated and understood from the 1950s to 1990s, but the implications for operando spectroscopy have been seemingly overlooked in the literature, with a few exceptions. Inspired by seminal studies in the field, I propose here the use of mixed isotopic streams, such as diluted ¹³CO in ¹²CO, to reduce dipole coupling effects and retrieve more information from operando IR spectra in reactions involving CO, such as CO oxidation or hydrogenation reactions. Similarly, mixed ¹³CO₂/¹²CO₂ streams may be applied in CO₂ hydrogenation, where adsorbed CO is commonly observed. The proposed name of the technique is Mixed Isotope Operando IR Spectroscopy, MIOIRS. In this Perspective, I will first summarize the nature of dipole–dipole interactions in adsorbed CO layers and their effects on CO IR spectra. Then, I will briefly describe how diluted isotopic mixtures of CO can partially break the coupling among adsorbates and reduce spectral distortion. In both sections, I will give a few showcases of the implications of vibrational coupling in the characterization of heterogeneous catalysts. Finally, I will discuss the possible implications of MIOIRS for the detection and quantification of defect and surface sites on metal nanoparticles, the characterization of bimetallic nanoparticles surfaces, and the kinetics of CO intermediates adsorbed on different active sites. Notably, MIOIRS may be expanded to other reactions in which adsorbates have strong permanent dipoles, such as in self-catalytic reduction for NO_x abatement.