Partial reduction of a vanadia/silica-titania catalyst with ammonia + nitric oxide at 473 K studied by electrical conductance and ESR measurements

Abstract

Partial reduction of vanadia supported on silica-titania by NH3 + NO has been studied by electrical conductance and ESR measurements. The relationship between electrical conductance and degree of reduction was determined by oxidative and reductive titrations of V(IV) and V(V) species leached from catalyst samples which had been reduced to different levels.In situ monitoring of the steady-state V(1V) ion concentration by electrical conductance during reduction with NH3 + NO in the concentration range M O O ppm for each reactant in a 2 vol 7% 02/Ar carrier gas was performed. In a large excess of NH,, the V(IV) ion concentration increased sharply with even small additions of NO. In a large excess of NO, the NH, + NO mixture exhibited first an oxidizing character, and the V(IV) ion concentration increased when PNHl > The reaction order with respect to the NO concentration, determined from both the NO conversion and the initial rate of catalyst reduction in excess NH3, was found to be less than unity. Determination of the stoichiometry of the reaction with respect to O2 indicated that the gas-phase O2 concentration required to balance the reducing character of NH3 + NO mixtures on the surface was significantly higher than predicted by the balanced equation describing the reaction. ESR measurements on the catalyst detected V(IV) ions as vanadyl groups having two different coordination spheres. Reduction of the catalyst with NH, + NO caused an increase in the V(IV) signal and a decrease of the hyperfine structure. latroduction Selective catalytic reduction of nitrogen oxides by ammonia (SCR) is an established technology for reducing emissions from sb,tatiow power stations, A thoroUa description of the catalysts and details of the reaction have recently been published.' Vanadia-based catalysts, supported on Ti02, exhibit high activity for the SCR reaction in the Presence of 1-6 VOl % oxygen. Commercial catalysts are promoted with tungstate to reduce the activity for oxidation of SO,? and work has recently been reported on optimizing the catalyst pore structure to increase the resistance against SO2 and As203 p~isoning.~ Commercial reactors operate in the temperature range 620-670 K with water vapor concentrations of 6-12 vol %.

Selective Catalytic Reduction of NO by NH3 over Aerogels of Titania, Silica, and Vanadia

“…Both Shikada et al (21) and Bjorklund et al (22) have prepared titaniasilica, containing 50 mol % of each component, by co-precipitation and used it as a support for vanadia in SCR. But they do not report structural data for their supports, nor compare their activities directly with that of titania.…”