NO adsorption/desorption property of TiO2–ZrO2 having tolerance to SO2 poisoning

The effect of SO2 on the adsorption and desorption properties of NO was studied over TiO2–ZrO2 and γ-Al2O3. The microporous binary oxide, TiO2–ZrO2, exhibited a large amount of sorptive NOx uptake (ca. 1 mmol g−1) at 25 °C in the presence of O2 and SO2. The irreversible chemisorption as nitrate (NO3) is a main cause for the large NO uptake. Desorption of NOx from TiO2–ZrO2 occurred at lower temperature of ca.450 °C, compared to ≥500 °C required for that from γ-Al2O3. A similar trend was observed for the SOx desorption, because SO2 adsorbed on γ-Al2O3 is a stable sulfate-type (SO4), whereas less stable sulfite (SO3) is dominant species on the surface of TiO2–ZrO2. The SOx desorption could further be facilitated in the presence of reducing agents like CO. The thermal swing adsorption/desorption cycle experiment demonstrated that TiO2–ZrO2 can be used as an NOx adsorbent having improved tolerance to SO2. The SO2 tolerance can be explained by the weak basicity of TiO2–ZrO2 and the low activity to oxidize SO2 to form sulfate species on the surface.