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 NO 2 with polycrystalline surfaces of metallic zinc and zinc oxide. NO 2 exhibits a complex chemistry on metallic zinc. After adsorbing nitrogen dioxide, N, O, NO, NO 2, and NO 3 are present on the surface of the metal. At room temperature the NO 2 molecule mainly dissociates into O adatoms and gaseous NO, whereas at low temperatures (<250 K) chemisorbed NO 2 and NO 3 dominate on the surface. NO 2 is a very good oxidizing agent for preparing ZnO from metallic zinc. Zn reacts more vigorously with NO 2 than metals, such as Rh, Pd, or Pt which are typical catalysts for the removal of NO x molecules (DeNO x process). At 300 K, the main product of the reaction of NO 2 with polycrystalline ZnO is adsorbed NO 3 with little NO 2 or NO present on the surface of the oxide. No evidence was found for the full decomposition of the NO 2 molecule (i.e., no NO 2 → N + 2O). The results of density functional (DF-GGA) calculations for the adsorption of NO 2 on a six-layer slab of ZnO, or INDO/S calculations for NO 2 on a Zn 37 O 37 cluster, show stronger chemisorption bonds on (0001) Zn-terminated terraces than on (0001̄) O-terminated terraces. The Zn ↔ NO 2 interactions on ZnO are strong and the Zn sites probably get oxidized and nitrated as a result of them. It appears that NO 2 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.