NH3 formation pathways from NO reduction by CO in the presence of water over Rh/Al2O3
Steady-state kinetic measurements with varying NO (0.05-0.2 kPa), CO (0.2-0.5 kPa), and water (1.5-9 kPa) pressures at 598 K rationalize that N2/N2O selectivity is determined by NO pressure while the NH3/(N2O + N2) selectivity is a single-valued function of NO/CO ratio. The existence of NCO* species on Al2O3 is confirmed by IR spectra. Monotonically increasing NH3/Rh ratios from H2O recirculation over the NCO* accrued in transient reaction studies on Rh/Al2O3 upon exposure to NO/CO mixtures suggests NH3 formation occurs by hydrolysis of isocyanate species bound on Al2O3. These findings demonstrate the critical role of H2O and NO/CO ratio in determining NH3 selectivity for operation of three-way automotive exhaust catalysts at rich air/fuel ratios.