Reaction of NO2 with Zn and ZnO:: Photoemission, XANES, and density functional studies on the formation of NO3

Synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) have been used to study the interaction of NO2 with polycrystalline surfaces of metallic zinc and zinc oxide. NO2 exhibits a complex chemistry on metallic zinc. After adsorbing nitrogen dioxide, N, O, NO, NO2, and NO3 are present on the surface of the metal. At room temperature the NO2 molecule mainly dissociates into O adatoms and gaseous NO, whereas at low temperatures (< 250 K) chemisorbed NO2 and NO3 dominate on the surface. NO2 is a very good oxidizing agent for preparing ZnO from metallic zinc. Zn reacts more vigorously with NO2 than metals, such as Rh, Pd, or Pt which are typical catalysts for the removal of NOx molecules (DeNO(x) process). At 300 K, the main product of the reaction of NO2 with polycrystalline ZnO is adsorbed NO3 with little NO2 or NO present on the surface of the oxide. No evidence was found for the full decomposition of the NO2 molecule (i.e., no NO2 -> N + 2O). The results of density functional (DF-GGA) calculations for the adsorption of NO2 on a six-layer slab of ZnO, or INDO/S calculations for NO2 on a Zn37O37 cluster, show stronger chemisorption bonds on (0001) Zn-terminated terraces than on (000 (1) over bar) O-terminated terraces. The Zn -> NO2 interactions on ZnO are strong and the Zn sites probably get oxidized and nitrated as a result of them. It appears that NO2 is very efficient for fully oxidizing metal centers that are missing O neighbors in oxide surfaces. On zinc oxide, the nitrate species are stable up to temperatures near 700 K. ZnO can be useful as a sorbent in DeNO(x) operations.