Gianguido Ramis

Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review

Abstract The open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed. Catalytic systems based on supported V2O5 (including the industrial TiO2-supported V2O5–WO3 and/or V2O5–MoO3 catalysts) and catalysts containing Fe2O3, CuO, MnOx and CrOx are considered. The results of spectroscopic studies of the adsorbed surface species, adsorption–desorption measurements, flow reactor and kinetic experiments are analyzed. The proposed reaction mechanisms are described and critically discussed. Points of convergence and of disagreement are underlined.

An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

Abstract The adsorption and the oxidation of ammonia over sub-monolayer TiO2-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NOx by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SCO process). For comparison, the adsorption of ammonia and hydrazine over the TiO2-anatase support has also been studied. CrOx TiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe2O3 TiO2, CrOx TiO2, CoOx TiO2 and CuO TiO2, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions,N−2. NiOx TiO2 and MnOx TiO2 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SCO processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SCO activity.

An FT-IR study of the adsorption of urea and ammonia over V2O5–MoO3–TiO2 SCR catalysts

Abstract A model V2O5–MoO3–TiO2 DeNOx catalyst has been prepared and briefly characterized. The adsorption of urea on it (and of ammonia for comparison) has also been investigated. It has been observed that urea can adsorb over the catalyst in an anionic form that can later decompose to isocyanate anions and ammonia and ammonium ion species. This mechanism suggests that the potential reactivity of the anionic form of urea and of isocyanate anions with VOCs, if they are present in the waste gas, possibly rise to polluting compounds must be taken into account for applications of the urea/DeNOx process.

FT Raman and FTIR studies of titanias and metatitanate powders

FT Raman and FTIR/FTFIR skeletal spectra of different TiO2 powders, of both synthetic and commercial origin, and of Sr, Ba, Co and Ni metatitanates are reported and discussed in relation to the predictions of factor group analyses. The ability of vibrational techniques to show the presence of brookite impurities in both anatase and rutile, and of rutile in anatase, as well as to give morphological and surface information is emphasized. The spectra of SrTiO3, BaTiO3, NiTiO3 and CoTiO3 are also discussed in relation to their different structures (pervoskite and ilmenite-type).

Characterization and composition of commercial V2O5zWO3zTiO2 SCR catalysts

Abstract The surface and bulk structure and composition of a commercial VzWzTi oxide catalyst used for denitrification of waste gases from power plants has been investigated by XRD, XRF, ICP, TG-DTA, FT-IR and SEM-EDS analyses and by comparison with ‘model’ catalysts. The catalyst powder consists of a full ‘monolayer’ of surface complexes, mainly constituted by wolframyl species with the addition of small amounts of vanadyl and sulphate species over high-area TiO2-anatase. The characteristics of the surface vanadyl, wolframyl and sulphate species fully consist with those previously reported for model VzTi, WzTi, VzWzTi and sulphated titania catalysts. The catalyst powder is mixed with glass-like particles, playing the role of mechanical promoters. The possibility of contamination of the catalyst particles by Mg or other alkali or alkali earth cations possibly arising from the glass particles during monolith preparation and during reactor start-up and shut-off procedures as well as during catalyst operation is suggested.

On the role of acidity in catalytic oxidation

Abstract The role of the catalyst surface acid-base properties on the heterogeneously-catalyzed oxidation reaction mechanisms is discussed. Acid-base properties depend on the covalent/ionic character of the metal-oxygen bonds and are involved in some steps of the oxidation reactions, such as the activation of the CH hydrocarbon bonds, the step associated with the evolution of alkoxide species and the desorption/overoxidation of the partial oxidation products. Thus they participate with the cation redox properties in determining the selective/unselective catalyst behavior.

An FT-IR study of the adsorption and oxidation of N-containing compounds over Fe2O3-TiO2 SCR catalysts

Abstract The interaction of NO, ammonia (NH3), hydrazine (N2H4), hydroxylamine (NH2OH), urea (H2NCONH2), acetamide (CH3CONH2) and acetonitrile (CH3CN) on Fe2O3-TiO2 model catalysts was studied. The data support the idea that hydrazine can be intermediate in the oxidation of ammonia to nitrogen, while hydroxylamine-type species, formed at higher temperature, can be intermediate in the oxidation of ammonia to NO. Bronsted acid sites do not appear to be involved in this chemistry.

Potassium doping of vanadia/titania de-NOxing catalysts: Surface characterisation and reactivity study☆

An investigation of the effect of K-doping on the surface and catalytic properties of sub-monolayer vanadia/titania de-NOxing catalysts is carried out. Samples having different vanadium and potassium loadings have been investigated by means of Fourier-transform infrared spectroscopy (FT-IR), temperature-programmed desorption (TPD), temperature-programmed surface reaction (TPSR) and temperature-programmed reaction (TPR) techniques. Isolated vanadyls and polymeric metavanadate species are present on the surface of undoped catalysts; both species increase on increasing the V2O5 loading in the range 0.28–5.3% w/w. TPSR and TPR data provide evidence for a greater reactivity of polymeric metavanadate species as compared to isolated vanadyls. Besides, a different type of isolated vanadyl appears to form at high loadings. Ammonia is adsorbed at vanadium sites in the form of molecularly coordinated species and of ammonium ions. Coordinated species show a higher thermal stability than ammonium ions. Ammonia is also coordinated at titanium sites to give a more weakly held species. Upon K-doping the stretching frequencies of surface vanadyls are lowered due to the production of strong oxide basic anions on the catalyst surface. This reduces the Lewis acidity of vanadium ions. IR and TPD experiments indicate that both molecularly chemisorbed ammonia and ammonium ions are present in much lower amounts and are less strongly held on K-doped samples. TPD and TPSR data further indicate that the alkali dopant poisons preferentially Lewis acid sites associated with vanadium rather than with Ti4+ ions. TPSR and TPR data show that alkali doping reduces markedly the nitric oxide conversion (associated with the number of active sites), but not the temperature threshold of the SCR reaction (associated with the intrinsic reactivity of the active sites). The lower number of active sites on alkali-doped catalyst can be related to the poisoning of both Bronsted and Lewis vanadium acid sites due to alkali addition, which results in a lower ammonia surface coverage.

FT-IR study of the surface properties of polycrystalline vanadia

Abstract The surface properties of a fumed vanadium oxide sample have been studied by FT-IR spectroscopy. It has been found that evacuation at temperatures up to 673 K causes a partial reduction of the sample, with a decrease in transmission of the IR light corresponding to the formation of ESR-detectable paramagnetic centers. The surface of activated samples does not show well-defined v(OH) bands. However, the adsorption of pyridine, pivalonitrile, acetonitrile and CO evidences the presence of both Lewis and Bronsted acid centers. The adsorption of CO2. does not result in the formation of detectable carbonate species. It is concluded that vanadia displays predominantly the character of an acidic anhydride, with a high covalency of the V-O-V bonds. A comparison is made with the properties of oxide-supported vanadia catalysts.

Transition metal mixed oxides as combustion catalysts : preparation, characterization and activity mechanisms

Abstract A number of transition metal mixed oxides with spinel-type, corundum-type and perovskite-type structures have been prepared and characterized. Attempts have been made to improve their morphological properties and their stability. Some of them have been tested in the catalytic combustion of methane, CO and H 2 (perovskites), of propane and of phenantrene. FT-IR experiments allowed to obtain a quite complete picture of the mechanism of catalytic combustion of C3 organic compounds on spinel-type oxides MgCr 2 O 4 and Co 3 O 4 . Nucleophilic oxygen species (lattice oxygen) is thought to be involved in both partial and total oxidation.