In situ IR spectroscopic and XPS study of surface complexes and their transformations during ammonia oxidation to nitrous oxide over an Mn-Bi-O/α-Al2O3 catalyst

Surface complexes resulting from the interaction between ammonia and a manganese-bismuth oxide catalyst were studied by IR spectroscopy and XPS. At the first stage, ammonia reacts with the catalyst to form the surface complexes [NH] and [NH2] via abstraction of hydrogen atoms even at room temperature. Bringing the catalyst into contact with flowing air at room temperature or with helium under heating results in further hydrogen abstraction and simultaneous formation of [N] from [NH2] and [NH]. The nitrogen atoms are localized on both reduced (Mn2+) and oxidized (Mnδ+, 2 < δ < 3) sites. Atomic nitrogen is highly mobile and reacts readily with the weakly bound oxygen of the oxidized (Mnδ+-N) active site. The nitrogen atoms localized on oxidized sites play the key role in N2O formation. Nitrous oxide is readily formed through the interaction between two Mnδ+-N species. N2 molecules result from the recombination of nitrogen atoms localized on reduced (Mn2+-N) sites.