Fiorenzo Bregani

Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction

Abstract The adsorption of nitric oxide, nitrogen dioxide and ammonia and their coadsorption on vanadia-titania have been studied by FT-IR spectroscopy. Upon nitric oxide adsorption, a surface nitrosyl species is formed rapidly and nitrates are formed slowly by oxidation. Nitrogen dioxide adsorption forms nitrate species by oxidation and nitric oxide. Lewis-bonded molecular species and ammonium ions are formed upon ammonia adsorption. Coordinated ammonia is thermally more stable than ammonium ions and can lose an hydrogen atom to give an amide species. Adsorption on a water-covered sample shows that ammonia displaces water from Lewis sites. Experiments of nitric oxide adsorption on ammonia-covered vanadia-titania show that ammonia poisons the nitric oxide adsorption sites and that NO 3 - species are formed by nitric oxide oxidation on vanadyl sites. By heating the ammonia-covered sample in the presence of gaseous nitric oxide coordinated ammonia reacts via the amide species, while ammonium ions do not. The following reaction mechanism is proposed: VO 2 + + NH 3 = [HO-V-NH 2 ] 2+ NO + [HO-V-NH 2 ] 2 + = [HO-V-NH 2 -NO] 2+ [HO-V-NH 2 -NO] 2+ = N 2 + H 2 O + [VOH] 2+ [VOH] 2+ +1/4 O 2 = VO 2+ +1/2 H 2 O This reaction sequence is believed to operate during the selective catalytic reduction of NO x over vanadia-titania based catalysts.

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

Abstract The physico-chemical characteristics and the reactivity of sub-monolayer V2O5-WO3/TiO2 deNOx catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO2 surface. The presence of tungsten oxide stabilizes the surface VIV and modifies the redox properties of V2O5/TiO2 samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V2O5-WO3/TiO2 catalysts exhibits higher activity than the binary V2O5/TiO2 and WO3/TiO2 reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.

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.

On the effect of dopants and additives on the state of surface vanadyl centers of vanadia-titania catalysts

The effect of dopants on the V=O stretching frequencies of vanadyls on low-V-loading vanadia-titania catalysts is shown. Alkali and alkali-earth metal cations decrease ν(V=O) while oxo-anions like sulphates, phosphates and arsenates increase it. Acid-type cations, like Al3+, MoO4+ and WO4+ have a small effect. A relation with the effect of these dopants on the catalytic activity in the SCR of NOx is proposed.

A study of the abatement of VOC over V2O5-WO3-TiO2 and alternative SCR catalysts

Abstract The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO2 commercial SCR catalysts differing in the V content and over Mn–TiO2 alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO2. The sample higher in V is more active. Mn–TiO2 is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO2 as the only detectable product. V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO2 can be proposed for DeNOx and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.

Theoretical and experimental study of the interaction between NOx reduction and SO2 oxidation over DeNOx-SCR catalysts

The interaction between NOx reduction and the undesired SO2 oxidation over DeNOx-SCR catalysts is investigated experimentally during catalyst conditioning and by transient and steady-state activity tests, as well as by means of model simulations in order to evaluate possible methods to minimize formation of SO3.

Influence of the process parameters on the extrusion of ceramic catalysts

Summary A systematic and quantitative study of the extrusion of Ti0 2 based ceramic pastes in the form of honeycomb is presented. The rheological properties of the paste are correlated to its extrudability and to the morphological and mechanical characteristics of the final compound. A mathematical model of the single screw extruder is developed for scale up purposes and validated against data.