Formation mechanisms of N2O and NH3 on Pd/ZrO2 and Pd/Al2O3 for NO reduction

Abstract

Reducible and nonreducible supports usually display distinct properties on the size, valence, and coordination of Platinum group elements in supported metal oxide. However, there are few reports on the regulation of main and side reaction rates and inhibition of byproducts from the perspective of reaction kinetics. Here, we compare the N2O and NH3 formations of Pd supported on Al2O3 and ZrO2 in NO reductions. Pd/Al2O3 produces less N2O and NH3 in quantity than Pd/ZrO2, which could be due to the moderate metal-support interactions between Pd with higher average valence and Al2O3 with lower ability of electrons transfer. ZrO2 acted as an electron acceptor in NO reduction facilitating the formations of unstable nitrates and surface hydroxyls. In-situ DRIFTS and NAP-XPS results suggest that the N2O generation is directly related to nitrates with different stabilities: bridging, bidentate, and partial monodentate nitrates are active on Pd/ZrO2, whereas, only nitro and partial bridging nitrates are active on Pd/Al2O3. The NH3 is mainly originated from the reaction of nitrates to surface hydroxyls with different coordination numbers. This work clarifies the byproducts formation mechanism in NO reduction and opens up the research topics using in-situ spectroscopy methodology.