Interaction of SO2 with ZnO Nanoshapes: Impact of Surface Polarity

As petroleum-related products dominate the global fuel market, acid gas resistant materials are in demand. ZnO is one of the widely used materials as SO2 sorbent and catalyst/catalyst support. To understand the surface structure sensitivity in interaction with SO2, morphology controlled ZnO wire and plate with nonpolar/polar facets have been investigated as model materials in this work. Morphology and crystal structure of nano- to micro-sized ZnO wire and plate were confirmed by SEM, XRD, and Raman characterizations. SO2 interactions with the ZnO plate (dominated by polar facet {0001}) and ZnO wire (dominated by nonpolar facet {10 (1) over bar 10}) were investigated by in situ IR coupled with temperature-programmed desorption (TPD) as well as XPS. SO2 interaction with ZnO resulted in more sulfate species on the wire than on the plate while some sulfates and sulfites still remained on both surfaces after heating to 350 degrees C. TPD showed higher SO2 desorption capacity on the ZnO plate surface at a lower temperature than on the nonpolar wire surface, indicating a stronger interaction between SO2 and the nonpolar surface of ZnO. Acid-base properties probed by adsorption microcalorimetry and reducibility of ZnO probed by H-2-TPR were analyzed to understand the difference in SO2 behavior on different ZnO surfaces. The similar reducibilities of the two ZnO surfaces from H-2-TPR indicate that the SO2-ZnO interaction is more related to the surface structure and acid-base property of ZnO, supported by the higher density of both acid and base sites probed by NH3 and CO, on the ZnO wire than on the plate. The results from this work suggest that polar surface of ZnO can be the preferred facet when engineering ZnO-based materials with enhanced SO2-resistance.