P. Forzatti

Present status and perspectives in de-NOx SCR catalysis

Abstract This paper surveys the present status and the perspective of de-NO x SCR catalysis for stationary sources, that is based on the reduction of NO x by NH 3 to form water and nitrogen. After a brief description of the SCR chemistry the characteristics of commercial SCR catalysts, their physico-chemical properties and reactivity, and their performances in both NO x reduction and SO 2 oxidation are presented. The mechanism of the SCR reactions, the mathematical modeling of the reactor and the arrangements of the reactor and process are then described. Finally the emerging technologies for NO x removal and the future perspectives of the SCR catalysis are outlined.

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.

Higher Alcohol Synthesis

Abstract It has been known for many years that it is possible to produce mixtures of methanol and higher alcohols from synthesis gas by alkali promotion of the methanol synthesis catalysts and by appropriate modification of the reaction conditions [l]. From 1927 to 1945 plants were in operation in the United States and Germany, that were dismantled with the coming of different feedstocks and the necessity of obtaining pure alcohols for chemical use. In the last decade the chemical and petroleum industry has shown a renewed and growing interest in the use of mixtures of methanol and higher aliphatic alcohols. The original goal in the late 70s was mainly to reduce oil dependence by producing synthetic components for gasoline blends. More recently the trend to lead phase-down due to environmental protection has focused the attention on the product performances of such alcohol mixtures as high octane blending stock for gasoline. It has been shown that the addition of higher alcohols to methanol increases the...

Catalytic combustion for the production of energy

Abstract In this paper, attention is focused on adiabatic lean-premixed catalytic combustion, which has attracted interest in the last decades as an environmentally friendly and cost-effective alternative to flame combustion for power generation by gas turbines. The different configurations of the combustion systems for gas turbines are presented. and the results of pilot tests, full-scale bench tests and field trials with retrofitted machines are illustrated. The relevant physico-chemical and catalytic properties of highly active supported PdO catalysts, including reversible PdO–Pd transformation and low-temperature activity, are then addressed. The structural and catalytic properties of highly stable metal substituted hexaaluminates are also discussed. The use of fuel alternative to natural gas is briefly covered. The relevant features of mathematical models for both, the catalyst section and the homogeneous section, that can be used in the design and analysis of the catalytic combustor, are described and the major conclusions of the modelling activity are outlined. Finally, the research opportunities in the area are discussed.

Preparation and characterization of hexaaluminate-based materials for catalytic combustion

A simple preparation method for barium hexaaluminate-based catalysts to be used in catalytic combustion is described. Morphological and physico-chemical characterization data are given for Ba-Al-O (withBa/Al = 1/12) and for Ba-Mn-Al-O (with Ba/Mn/Al = 1/0.5/11.5; 1/1/11; 1/2/10) samples calcined at different temperatures. The preparation method here proposed provides stable materials with a surface area of 15 m2/g after calcination at 1400°C. The structural and morphological properties, as well as the results of activity tests in methane combustion, compare well with those reported in the literature for samples with the same composition and prepared according to a more complex preparation route.

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.

Experimental and theoretical investigation of the dynamics of the SCR - DeNOx reaction

Abstract The kinetics of NH 3 -adsorption -desorption over model V 2 O 5 -( WO 3 ) TiO 2 catalysts for the Selective Catalytic Reduction of NO X has been studied by transient response techniques, and described according to rate expressions which account for heterogeneity of the catalyst surface. A dynamic mathematical model of the industrial SCR monolith reactor has been derived, using the results of the transient study of NH 3 adsorption as well as other independently estimated parameters. It has been successfully validated against laboratory data concerning unsteady operation of commercial SCR honeycomb catalysts, and applied to the simulation of typical transients of industrial SCR reactors. It is shown that the variations of the NO X emissions are always much faster than the dynamics of NH 3 .

Status and perspectives of catalytic combustion for gas turbines

This paper provides a review of the status and of the perspectives of catalytic combustion for gas turbines. First the development activities of catalytic combustion systems carried out in the last few years are reported. Then the relevant characteristics of PdO supported catalysts and of transition metal-substituted hexaaluminates (HAs), that have been most extensively considered for this application, are addressed. Next the use of mathematical modelling as a tool for the design and analysis of catalytic combustors is discussed. Finally a novel fuel-rich approach to catalytic combustion is illustrated and the perspectives for this technology are briefly outlined.

A new sequential experimental design procedure for discriminating among rival models

Abstract A new procedure to be used in the sequential design of experiments for discriminating among rival models is presented. The procedure consists of a new design criterion and a model adequacy test based on statistical principles. The method is illustrated with examples and compared to previous methods.