CO on a Rh/Fe3O4 single-atom catalyst: high-resolution infrared spectroscopy and near-ambient-pressure scanning tunnelling microscopy

Infrared reflection absorption spectroscopy (IRAS) offers a powerful route to bridging the materials and pressure gaps between surface science and powder catalysis. Using a newly developed IRAS setup optimised for dielectric single crystals, we investigate CO adsorption on the model single-atom catalyst Rh/Fe3O4(001). IRAS resolves three species: monocarbonyls at isolated, twofold-coordinated Rh adatoms, monocarbonyls at fivefold-coordinated Rh atoms embedded in the surface, and gem-dicarbonyls at isolated, twofold-coordinated Rh adatoms. Under ultra-high vacuum (UHV) conditions, RhCO monocarbonyl species at adatom sites dominate. Rh(CO)2 gem-dicarbonyl formation is kinetically hindered and occurs predominantly through CO-induced dissociation of Rh dimers rather than sequential adsorption of two CO molecules at an isolated, twofold Rh adatom. The sequential-adsorption pathway to Rh(CO)2 becomes accessible at millibar CO pressures as evidenced by near-ambient-pressure scanning tunnelling microscopy (NAP-STM). These findings link the UHV behaviour to that expected under realistic reaction conditions. Assignments of the vibrational frequencies to individual species rely on isotopic labelling, thermal treatments, and a review of previous SPM, XPS, and TPD data, supported by density functional theory (DFT)-based calculations. While theory provides qualitative insight, such as the instability of dicarbonyls on fivefold-coordinated Rh atoms, it does not yet reproduce experimental frequencies quantitatively and is sensitive to the computational parameters, highlighting the need for robust experimental benchmarks. The spectroscopic fingerprints established here provide a reliable foundation for identifying Rh coordination environments in oxide-supported single-atom catalysts.

💡 Research Summary

This paper presents a comprehensive study of carbon monoxide adsorption on the model single‑atom catalyst Rh/Fe₃O₄(001), combining a newly developed infrared reflection‑absorption spectroscopy (IRAS) setup optimized for dielectric single crystals with near‑ambient‑pressure scanning tunneling microscopy (NAP‑STM). The authors first describe the construction of the IRAS instrument, which employs p‑polarized light at grazing incidence angles of 55°–74° to maximize signal‑to‑noise on the low‑reflectivity Fe₃O₄ surface. Spectra are recorded with 4 cm⁻¹ resolution, averaging 4000 scans (≈20 min per spectrum), enabling the detection of subtle vibrational features that have previously been inaccessible on metal‑oxide single crystals.

Using isotopically labeled CO (¹²CO, ¹³CO, and mixed ¹²CO/¹³CO) together with controlled Rh coverages (<0.5 ML) and systematic thermal treatments, three distinct CO species are identified: (i) a monocarbonyl on isolated two‑fold‑coordinated Rh adatoms (Rh²ˢᵗᵃᵈ‑CO) with a ν_CO around 2050 cm⁻¹, (ii) a monocarbonyl on five‑fold‑coordinated Rh atoms that substitute surface Fe (Rh⁵ᶠᵒˡᵈ‑CO) at ~1990 cm⁻¹, and (iii) a gem‑dicarbonyl on the same two‑fold Rh sites (Rh²ˢᵗᵃᵈ(CO)₂) giving two bands at ~2025 cm⁻¹ and ~1970 cm⁻¹. The gem‑dicarbonyl is shown to arise predominantly from CO‑induced dissociation of the minority Rh dimers present under ultra‑high vacuum (UHV) conditions, rather than from sequential adsorption of a second CO molecule on an isolated Rh adatom.

To probe pressure effects, the authors perform NAP‑STM experiments at 1–10 mbar CO and room temperature. While under UHV the Rh²ˢᵗᵃᵈ(CO)₂ species is essentially absent, at millibar pressures the STM images reveal the emergence of bright double‑lobed protrusions that correspond to Rh²ˢᵗᵃᵈ bearing two CO ligands. This provides direct, real‑time evidence that the sequential adsorption pathway to Rh(CO)₂, kinetically blocked at low pressure, becomes accessible under realistic catalytic pressures.

Density functional theory (DFT) calculations (PBE‑D3 with Hubbard U corrections: U_eff = 3.61 eV for Fe 3d and Rh 4d) are employed to model the adsorption geometries, binding energies, and harmonic vibrational frequencies. The calculations reproduce the relative stability trends (Rh²ˢᵗᵃᵈ‑CO more stable than Rh⁵ᶠᵒˡᵈ‑CO, and gem‑dicarbonyl favored on Rh dimers) but underestimate the absolute CO stretching frequencies by 20–40 cm⁻¹, necessitating an empirical scaling factor. Moreover, attempts to locate a stable gem‑dicarbonyl on Rh⁵ᶠᵒˡᵈ fail, consistent with its experimental absence. The sensitivity of the predicted frequencies to the choice of U parameters underscores the current limitations of DFT for quantitative vibrational spectroscopy of transition‑metal single‑atom systems.

In summary, the work demonstrates that (1) high‑sensitivity IRAS can resolve site‑specific CO vibrational fingerprints on oxide‑supported single‑atom catalysts, (2) increasing CO pressure activates a new adsorption pathway (Rh(CO)₂) that bridges the gap between UHV model studies and practical catalytic conditions, and (3) existing DFT methodologies, while qualitatively reliable, require further refinement to achieve quantitative agreement with experiment. The spectroscopic benchmarks established here provide a robust reference for future investigations of Rh and other metal single‑atom catalysts on oxide supports, facilitating the rational design of catalysts with tailored active‑site structures.