Reduction of Nitric Oxide by Acetylene on Ir Surfaces with Different Morphologies: Comparison with Reduction of NO by CO

Reduction of nitric oxide (NO) by acetylene (C2H2) has been investigated by temperature-programmed desorption (TPD) on planar Ir(210) and faceted Ir(210) with tunable sizes of three-sided nanopyramids exposing (311), (311), and (110) faces. Upon adsorption, C2H2 dissociates to form acetylide (CCH) and H species on the Ir surfaces at low C2H2 precoverage. For adsorption of NO on C2H2-covered Ir, both planar and faceted Ir(210) exhibit high reactivity for reduction of NO with high selectivity to N-2 at low C2H2 precoverage, although the reaction is completely inhibited at high C2H2 precoverage. Coadsorbed C2H2 significantly influences dissociation of NO. The N-, H-, and C-containing TPD products are dominated by N-2, H-2, CO, and CO2 together with small amounts of H2O. For adsorption of NO on C-covered Ir(210) at fractional C precoverage, formation of CO2 is promoted while production of CO is reduced. Reduction of NO by C2H2 is structure sensitive on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in the reduction of NO by C2H2 on faceted Ir(210) for average facet sizes of 5 nm and 14 nm. The results are compared with reduction of NO by CO on the same Ir surfaces. As for NO+C2H2, the Ir surfaces are very active for reduction of NO by CO with high selectivity to N-2 and the reaction is structure sensitive, but dear evidence is found for size effects in the reduction of NO by CO on the nanometer scale. Furthermore, coadsorbed CO does not affect dissociation of NO at low CO precoverage whereas coadsorbed CO considerably influences dissociation of NO at high CO precoverage. The adsorption sites of CCH+H on Jr are characterized by density functional theory.