USE OF POSITRON-EMITTING N-13 FOR STUDIES OF THE SELECTIVE REDUCTION OF NO BY NH3 OVER VANADIA-TITANIA CATALYST AT VERY-LOW REACTANT CONCENTRATIONS

Positron-emitting (NO)-N-13 molecules produced with a cyclotron were used to investigate the selective catalytic reduction (SCR) of NO by NH3 over vanadia/titania at very low reactant concentrations. The (NO)-N-13 concentration was on the order of 5 x 10(-9) ppm, which is more than 11 orders of magnitude lower than the usually used concentrations. Catalyst samples were pretreated with H-2 or NH3, without adding these reagents during the conversion experiments. Under these conditions the vanadia/titania catalyst pretreated with NH3 kept its full activity for at least 5 h. At the low NO concentrations present, 50% conversion was already achieved at 80-degrees-C, with complete conversion at 150-degrees-C. The reaction rate at 92-degrees-C was 1.9 x 10(-18) mol of NO (g of catalyst)-1 s-1. This value is in good agreement with the value expected from measurements with a similarly prepared catalyst assuming a first-order reaction in NO. The activation energy was calculated to be 37 kJ mol-1, which compares fairly well with the one measured on a similarly prepared catalyst under usual reactant concentrations (43 kJ mol-1). In contrast, vanadia/titania pretreated with H2 did not show any NO conversion. These results indicate that NH3 is necessary for NO conversion and that one N atom of the formed N2 originates from NH3. Using a thermochromatographic apparatus, adsorption enthalpies (at zero coverage) for NO and NO2 on various oxides were determined. Values of -21 +/- 2 and -33 +/- 3 kJ mol-1 were found for NO and NO2, respectively, independent of the material.