Elucidating NH3 formation during NOx reduction by CO on Pt–BaO/Al2O3 in excess water

Abstract The reduction of NO was investigated with CO, H 2 , CO + H 2 O and CO + H 2 as the reducing species on a Pt–BaO/Al 2 O 3 monolith catalyst over an intermediate temperature range (200–300 °C). NH 3 is a major product of the NO + CO + H 2 O system under conditions of CO inhibition. The data are interpreted by the known NH 3 formation route in which the NO is reduced by H 2 formed by the water gas shift (WGS) reaction. In the absence of water, the strong adsorption of CO leads to sharp transitions between a high rate mass transport controlled regime and a lower rate kinetically controlled regime between 200 and 300 °C. Differential kinetics and integral experiments are reported at 270 °C. The intrinsic order with respect to CO in the latter regime is −1. When water is added with the CO feed, the regime transitions are more gradual and mitigated by the enhancement afforded by the hydrogen formed by the WGS reaction. Kinetic evidence for the effect of hydrogen is the much lower inhibition by CO during the WGS reaction (−0.23 order). When H 2 is added to the NO + CO mixture (without H 2 O) in the CO inhibited feed regime NH 3 and CO 2 are the major products even for low H 2 /NO feed ratio (∼1). Collectively, the steady-state findings are consistent with the major NH 3 formation pathway involving reaction of surface H (from WGS) and OH (from water) with adsorbed NO and N. The NH 3 formation route involving the hydrolysis of a surface isocyanate species formed from the reaction of NO and CO, is only of secondary importance.