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 {101 0}) 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 °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 H₂-TPR were analyzed to understand the difference in SO₂ behavior on different ZnO surfaces. The similar reducibilities of the two ZnO surfaces from H₂-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 CO₂ 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.