The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

Pd/CeO2–TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents

New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce1-xCuxO2 sample) and an impregnation method (impregnated CuOx/CeO2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-Ovacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the Ovacancy sites or the Cu-Ovacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.

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In Situ Studies of the Active Sites for the Water Gas Shift Reaction over Cu−CeO2 Catalysts: Complex Interaction between Metallic Copper and Oxygen Vacancies of Ceria

Some recent results on metal/support interaction effects in NM/CeO2 (NM: noble metal) catalysts

Metal atoms dispersed on the oxide supports constitute a large category of single-atom catalysts. In this review, oxide supported single-atom catalysts are discussed about their synthetic procedures, characterizations, and reaction mechanism in thermocatalysis, such as water-gas shift reaction, selective oxidation/hydrogenation, and coupling reactions. Some typical oxide materials, including ferric oxide, cerium oxide, titanium dioxide, aluminum oxide, and so on, are intentionally mentioned for the unique roles as supports in anchoring metal atoms and taking part in the catalytic reactions. The interactions between metal atoms and oxide supports are summarized to give a picture on how to stabilize the atomic metal centers, and rationally tune the geometric structures and electronic states of single atoms. Furthermore, several directions in fabricating single-atom catalysts with improved performance are proposed on the basis of state-of-the-art understanding in metal-oxide interactions.

Single-Atom Catalysts Based on the Metal-Oxide Interaction.

Palladium–alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by O2 and reduction by H2, using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species. Pd2+ and Pd– entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [1 Torr ≈(101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated)≈ acetylacetonate < chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.

Spectroscopic study of the interaction of carbon monoxide with cationic and metallic palladium in palladium–alumina catalysts

CO adsorption on palladium–ceria catalysts prepared from Pd acetylacetonate has been investigated by IR and UV–VIS spectroscopies and by volumetric measurements. After calcination and evacuation at 673 K, the samples contain small PdO particles which are immediately reduced upon adsorption of CO at room temperature. For samples prereduced by H2, electron-donor centres (Ce3+ ions, oxygen vacancies) are created during the pretreatment. These centres are responsible for increased CO adsorption by ceria in the presence of palladium, which leads to apparent CO: Pd values greater than unity and their interaction with palladium induces a carrier–to–metal electron transfer. Plasmon absorption in reducible carriers hinders the detection of species adsorbed on the supported metal.

Didier Tessier

Papers

Spectroscopic study of the interaction of carbon monoxide with cationic and metallic palladium in palladium–alumina catalysts

Spectroscopic study of CO adsorption on palladium–ceria catalysts