Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

IR study of polycrystalline ceria properties in oxidised and reduced states

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

Understanding Ostwald ripening and disintegration of supported metal particles under operating conditions has been of central importance in the study of sintering and dispersion of heterogeneous catalysts for long-term industrial implementation. To achieve a quantitative description of these complicated processes, an atomistic and generic theory taking into account the reaction environment, particle size and morphology, and metal–support interaction is developed. It includes (1) energetics of supported metal particles, (2) formation of monomers (both the metal adatoms and metal–reactant complexes) on supports, and (3) corresponding sintering rate equations and total activation energies, in the presence of reactants at arbitrary temperature and pressure. The thermodynamic criteria for the reactant assisted Ostwald ripening and induced disintegration are formulated, and the influence of reactants on sintering kinetics and redispersion are mapped out. Most energetics and kinetics barriers in the theory can b...

Atomistic theory of Ostwald ripening and disintegration of supported metal particles under reaction conditions.

Support effects on hydrotreating catalysts

Low-temperature carbon monoxide adsorption and state of molybdena supported on alumina, titania, ceria, and zirconia. An infrared spectroscopic investigation

The state and dispersion of a highly dispersed Rh/Al/sub 2/O/sub 3/ catalyst in various reactive atmospheres were investigated by low-temperature infrared spectroscopy of chemisorbed CO. Rh metal particles disintegrate in the presence of CO, and Rh/sup +/ species form as a result of oxidation by surface -OH groups at temperatures less than or equal to 300 K. The predominant oxidation process at higher temperatures is presumably related to the dissociative chemisorption of CO. The rates of these processes are strongly dependent on the initial reduction conditions, being less for the more severely reduced samples containing the larger Rh metal particles. A cyclic process of CO adsorption and H/sub 2/ reduction leads to particle growth and increasing resistance of the larger particles to disintegration and oxidation in CO. Simultaneous exposure of the samples to CO and H/sub 2/ does not prevent the oxidative conversion of some of the Rh/sup 0/ into Rh/sup +/ at temperatures greater than or equal to 473 K, but complete conversion of the Rh/sup 0/ is hindered because the CO ligands are removed by reaction with the H/sub 2/. Water vapor in the presence of CO assists the formation of Rh/sup +/ species.

H. Knoezinger

Papers

Low-temperature carbon monoxide adsorption and state of molybdena supported on alumina, titania, ceria, and zirconia. An infrared spectroscopic investigation

Highly dispersed rhodium on alumina catalysts: influence of the atmosphere on the state and dispersion of rhodium

Low-Temperature CO Adsorption and State of Molybdena Supported on Alumina, Titania, Ceria and Zirconia.

Highly Dispersed Rhodium on Alumina Catalysts: Influence of the Atmosphere on the State and Dispersion of Rhodium.