Catalytic removal of NO

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

Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides and represent an useful model system to better understand the fundamental aspects of the chemistry and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on the reactivity of copper-based catalysts (supported and unsupported copper oxide, Cu-zeolites, cuprates and other copper compounds) in various reactions of conversion of nitrogen oxides, four main sub-topics are discussed in detail: (i) nature of copper species, (ii) chemisorption and surface transformations of NO, (iii) relationship between copper species and activity in the conversion of nitrogen oxides and (iv) mechanism of reduction of nitrogen oxides to N 2 . Five reactions of transformation of nitrogen oxides are discussed in detail: (i) decomposition of NO, (ii) reduction of NO with ammonia in the presence or not of oxygen, (iii) reduction of NO with hydrocarbons in the presence of oxygen, (iv) reduction of NO with CO and (v) decomposition of N 2 O. The mechanism of reduction of nitrite and N 2 O by copper enzymes is also discussed, with a view to provide some useful insights on the chemistry of transformation. In this review particular attention is directed towards controversial points in the literature, underestimated questions, and hypothesis and theories which do not allow interpretation of all sets of experimental data. Discussion is also focused on the presence of multiple and competitive pathways of transformation, the relative roles of which depend on reaction conditions.

Nature of active species in copper-based catalysts and their chemistry of transformation of nitrogen oxides

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al2O3, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H2, O2) and on their DeNOx activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu2+ and Cu0, while Cu+ is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation. The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu+ ions; the isolated Cu2+ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N2 are characterized by the presence of isolated Cun+ ions beside the additional influence of the zeolitic framework; after the formation of Cu+ ions the redox cycles are reversible but, after the formation of Cu0, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions.

Surface and bulk properties of Cu–ZSM-5 and Cu/Al2O3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

The interaction of NO with copper ions in ZSM5 : an EPR and IR investigation

In CuY zeolites degassed at temperatures above 650 K cupric ions are reduced to some extent to cuprous ions.Evidence for this process of auto-reduction was taken from the decrease in weight of the sample, and the detection of a desorbing gas in a volumetric system. The latter was identified as oxygen. U.v. reflectance spectroscopy showed directly the decrease of the Cu2+ species upon outgassing and their increase after oxygen adsorption. Excitation spectra showed the presence of Cu+ ions after high temperature degassing. Infrared spectroscopic techniques showed that Lewis sites are also formed upon degassing. A mechanism is proposed that accounts for the observations.

Redox behaviour of transition metal ions in zeolites. Part 3.—Auto-reduction of cupric ions in Y zeolites

Spectroscopic characterization and thermal stability of copper(II) ethylenediamine complexes on solid surfaces. 1. Synthetic faujasites types X and Y

Ionic conduction in dehydrated partially Ca2+-exchanged synthetic faujasites is due to Na+ ions on sites II. The mechanism is a vacancy diffusion. Activation energies lie in the range 38–59 kJ mol–1 and are mainly determined by the positive charge density in the hexagonal prisms and in the cubo-octahedra. Deep bed heating prior to the conduction experiments leads to an increase of the activation energy for migration of Na+ ions in X zeolites but not in Y zeolites. Two relaxations are observed. The low frequency relaxation is a Maxwell–Wagner effect. The high frequency relaxation is ascribed to the movement of Ca2+ ions on sites I and I′ between neighbouring sites. The activation energies for this relaxation range between 84 and 106 kJ mol–1. The electrical permittivities, uncorrected for pellet heterogeneity, are in the range 2.32–2.84.

Willy de Wilde

Papers

Redox behaviour of transition metal ions in zeolites. Part 3.—Auto-reduction of cupric ions in Y zeolites

Spectroscopic characterization and thermal stability of copper(II) ethylenediamine complexes on solid surfaces. 1. Synthetic faujasites types X and Y

Cation Motion in Dehydrated, Partially Ca2+-exchanged Zeolites

Conductivity and Dielectric Dispersions of Hydrated and Partially Hydrated Synthetic Faujasites