Chemistry of SO2 and NO2 on ZnO(0001)-Zn and ZnO powders: changes in reactivity with surface structure and composition

Synchrotron-based high-resolution photoemission and X-ray absorption near-e dge spectroscopy (XANES) have been used to study the interaction of SO2 and NO2 with ZnO(0 0 0 1)-Zn and polycrystalline surfaces of zinc oxide (films and powders). Important differences are observed when comparing the chemic al behavior of the adsorbates on these oxide surfaces. These differences ar e in part a consequence of changes in structural properties (flat versus ro ugh surfaces), but in some cases they clearly originate in variations in su rface composition (zinc tt adsorbate versus oxygen tt adsorbate interaction s). For example, the Zn-terminated (0 0 0 1) crystal face of ZnO is much le ss reactive towards SO2 than polycrystalline ZnO. On ZnO(0 0 0 1)-Zn and po lycrystalline ZnO, the Zn <-> SO2 bonding interactions are weak. Adsorption of SO2 on Zn sites was seen only at temperatures below 200 K. In contrast, the SO2 molecules react readily with O sites of Ar+ sputtered ZnO(0 0 0 1) -Zn or polycrystalline ZnO forming very stable SO3 species. Due to its radi cal nature, adsorbed NO2 is more chemically active than SO2. After dosing n itrogen dioxide to ZnO(0 0 0 1)-Zn at 100 K, chemisorbed NO2 and NO3 coexis ts on the surface. A partial NO2. ads --> NO3, ads transformation is observ ed from 150 to 300 K. The data for the NO2/ZnO(0 0 0 1)-Zn system clearly p rove that large quantities of NO3 can be formed on metal sites of an oxide surface as a consequence of partial decomposition or disproportionation of NO2. The routes for the formation of SO3 and NO3 on ZnO can be different, b ut these species have in common a high stability and decompose at temperatu res well above 500 K. Thus, ZnO powders can be useful as sorbents in DeSO(x ) and DeNO(x) operations. (C) 2001 Elsevier Science B.V. All rights reserve d.