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

The reduction of NO was investigated with CO, H2, CO + H2O and CO + H2 as the reducing species on a Pt–BaO/Al2O3 monolith catalyst over an intermediate temperature range (200–300 °C). NH3 is a major product of the NO + CO + H2O system under conditions of CO inhibition. The data are interpreted by the known NH3 formation route in which the NO is reduced by H2 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 H2 is added to the NO + CO mixture (without H2O) in the CO inhibited feed regime NH3 and CO2 are the major products even for low H2/NO feed ratio (∼1). Collectively, the steady-state findings are consistent with the major NH3 formation pathway involving reaction of surface H (from WGS) and OH (from water) with adsorbed NO and N. The NH3 formation route involving the hydrolysis of a surface isocyanate species formed from the reaction of NO and CO, is only of secondary importance.

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► Systematic study of NO reduction by CO in presence of H2O.
► Significant inhibition by CO in absence of H2O leads to multiple rate controlling regimes including rate multiplicity under isothermal conditions.
► Enhanced NO reduction in presence of excess H2O suggests role of H2 generated by water gas shift reaction as primary route.
► Co-reduction of NO by mixture of H2 and excess CO gives NH3 as sole N- and H-containing product independent of H2/NO ratio.