Use of positron-emitting nitrogen-13 for studies of the selective reduction of nitric oxide by ammonia over vanadia/titania catalyst at very low reactant concentrations

Abstract

Positron-emitting [sup 13]NO molecules produced with a cyclotron were used to investigate the selective catalytic reduction (SCR) of NO by NH[sub 3] over vanadia/titania at very low reactant concentrations. The [sup 13]NO concentration was on the order of 5 [times] 10[sup [minus]9] ppm, which is more than 11 orders of magnitude lower than the usually used concentrations. Catalyst samplers were pretreated with H[sub 2] or NH[sub 3], without adding these reagents during the conversion experiments. Under these conditions the vanadia/titania catalyst pretreated with NH[sub 3] 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 [times] 10[sup [minus]18] mol of NO (g of catalyst)[sup [minus]1] s[sup [minus]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[sup [minus]1], which compares fairly well with the one measured on a similarly prepared catalyst under usual reactant concentrations (43 kJ mol[sup [minus]1]). In contrast, vanadia/titania pretreated with H[sub 2] did not show any NO conversion. Thesemore » results indicate that NH[sub 3] is necessary for NO conversion and that one N atom of the formed N[sub 2] originates from NH[sub 3]. 31 refs., 6 figs., 2 tabs.« less