Efficiency of Cu and Pd substitution in Fe-based perovskites to promote N2 formation during NH3 selective catalytic oxidation (NH3-SCO)

Abstract

Iron-based perovskites, of LaFe1-xB′xO3-δ (B′=Cu, Pd) formula, are proposed as effective materials for the ammonia selective catalytic oxidation to nitrogen (NH3-SCO). Effects on N2 yield, of copper or palladium substitutions in B position, and of perovskite dispersion over Al2O3 support, are reported. Copper and palladium substitution in perovskite lattice significantly promotes the NH3 conversion rate, owing to the outstanding redox capacity displayed by the substituted compositions at low temperature (T<300°C). While N2 yield decreases upon Cu-doping, it retains as high as 80–90% over Pd-containing catalysts. Copper substitution enhances low-temperature oxygen mobility, which is favorable to NH bond fracture of adsorbed −ONH3 species that results in high NO formation. Palladium substitution results in an opposite effect, and high selectivity towards N2 is obtained. Additionally, N2 yield is significantly improved at high temperature, when perovskite active phase is dispersed over Al2O3 support. Combining in-situ DRIFTS and density functional theory (DFT) calculations, NH3-SCO to N2 reaction pathway over Fe-based perovskites is proposed to follow an Eley-Rideal (E-R) mechanism, during which gaseous NH3 reacts with adsorbed −ONH2 species to form surface diazo species (NN). For LaFeO3, the rate-determining step is the −ON2H2 to −ON2H reaction (overcoming an energy barrier of 3.48eV), while for LaFe0.95Pd0.05O3, the rate-determining step is ON bond cleavage (energy barrier of 1.55eV) that explains then higher N2 yield measured for the Pd-containing perovskite catalyst.