Guido Busca

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.

Technologies for the removal of phenol from fluid streams: A short review of recent developments

The available technologies for the abatement of phenol from water and gaseous streams are briefly reviewed, and the recent advancements summarized. Separation technologies such as distillation, liquid-liquid extraction with different solvents, adsorption over activated carbons and polymeric and inorganic adsorbents, membrane pervaporation and membrane-solvent extraction, have been discussed. Destruction technologies such as non-catalytic, supercritical and catalytic wet air oxidation, ozonation, non-catalytic, catalytic and enzymatic peroxide wet oxidation, electrochemical and photocatalytic oxidation, supercritical wet gasification, destruction with electron discharges as well as biochemical treatments have been considered. As for the abatement of phenol from gases, condensation, absorption in liquids, adsorption on solids, membrane separation, thermal, catalytic, photocatalytic and biological oxidation have also been considered. The experimental conditions and the performances of the different techniques have been compared.

Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces

Abstract Published data on the infrared spectra and coordination of carbonate, bicarbonate and formate ions, and of CO2 in metal complexes are reported. The usual assignments of absorption bands due to products of CO and CO2 reactive adsorption on metal oxide surfaces are also critically reexamined.

Spectroscopic characterization of the acid properties of metal oxide catalysts

Abstract The IR spectroscopic methods for the characterization of the surface acidity of metal oxide catalysts are briefly reviewed. The use of different basic probe molecules whose IR spectra are sensitive to protonation and/or to the strength of Lewis acid–base interaction is described. The results obtained for the characterization of the Lewis acid strength of more than 30 binary and ternary mixed oxides are interpreted on the basis of the different polarizing powers of the involved cations. The characterization of the Bronsted acidity of several oxides and zeolites made by using the “basic strength method”, the “hydrogen bonding method” and the “olefin polymerization method” is also described.

FT-IR characterization of the surface acidity of different titanium dioxide anatase preparations

The surface acidity of four different anatase preparations has been studied by FT-IR spectroscopy of adsorbed benzene, ammonia, pyridine, carbon monoxide and dioxide. Our results indicate that surface chemistry of anatase, in particular acidity, greatly depends on the preparation of the sample, both through the resulting morphology and the presence of surface impurities. Different morphologies may result in different types of Lewis acid sites. If pure, anatase does not show Bronsted acidity. The presence of silica impurities is responsible for an enhancement of Bronsted acidity on several samples, and may be evaluated by the intensity of an IR band situated near 3730 cm−1. Very strong Bronsted acidity is generated by the presence of sulfate impurities.

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.

Exploring, Tuning, and Exploiting the Basicity of Hydrotalcites for Applications in Heterogeneous Catalysis.

Basic catalysis! The basic properties of hydrotalcites (see picture) make them attractive for numerous catalytic applications. Probing the basicity of the catalysts is crucial to understand the base-catalysed processes and to optimise the catalyst preparation. Various parameters can be employed to tune the basic properties of hydrotalcite-based catalysts towards the basicity demanded by each target chemical reaction.Hydrotalcites offer unique basic properties that make them very attractive for catalytic applications. It is of primary interest to make use of accurate tools for probing the basicity of hydrotalcite-based catalysts for the purpose of 1) fundamental understanding of base-catalysed processes with hydrotalcites and 2) optimisation of the catalytic performance achieved in reactions of industrial interest. Techniques based on probe molecules, titration techniques and test reactions along with physicochemical characterisation are overviewed in the first part of this review. The aim is to provide the tools for understanding how series of parameters involved in the preparation of hydrotalcite-based catalytic materials can be employed to control and adapt the basic properties of the catalyst towards the basicity demanded by each target chemical reaction. An overview of recent and significant achievements in that perspective is presented in the second part of the paper.

The surface acidity of solid oxides and its characterization by IR spectroscopic methods. An attempt at systematization

The bases of IR spectroscopic methods for the characterization of the surface acidity, of the Lewis type and of the Bronsted type, of solid, simple and mixed oxides are briefly reviewed. The results are described and a systematization is proposed. The surface acidity is essentially associated with the ionicity/covalency of the element–oxygen bond, mainly affected by the size and charge of the cation.

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

Abstract The adsorption and transformation of ammonia over V 2 O 5 , V 2 O 5 /TiO 2 , V 2 O 5 -WO 3 /TiO 2 and CuO/TiO 2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH 2 amide species or to its dimeric form N 2 H 4 , hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N 2 − and azide anions N 3 − are further observed over CuO/TiO 2 . The comparison of the infrared spectra of the species arising from both NH 3 and N 2 H 4 adsorbed over CuO/TiO 2 strongly suggest that N 2 H 4 is an intermediate in NH 3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH 2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V 2 O 5 -based systems, but not over CuO/TiO 2 , in spite of the high SCR and SCO activity of this catalyst. Consequently Bronsted acidity is not necessary for the SCR activity.

Magnetic properties of partially-inverted zinc ferrite aerogel powders

Fine powders of ZnFe2O4 with an average particle size of 10 nm and inversion parameter of 0.21 were synthesized by the aerogel procedure. Portions of the powders were calcined in air at 500 and 800 °C and other portions were ball-milled for 10 h. The materials were characterized by x-ray diffractometry, vibrating sample, and SQUID magnetometry, Mossbauer spectrometry, and low temperature calorimetry. Upon calcination the powders underwent significant changes in grain size, inversion parameter, and hence magnetic properties. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. Upon ball-milling the grain size varied widely and the inversion parameter attained a value of 0.55. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe2O4 powders having comparable grain size and inversion parameters.