Investigation of the selective catalytic reduction of NO with NH3 over the WO3/Ce0.68Zr0.32O2 catalyst: the role of H2O in SO2 inhibition

Abstract

The effect of H₂O and SO₂ on the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) over WO₃/Ce_0.68Zr_0.32O₂ at 250 °C was systematically investigated using various characterization techniques. NH₃-SCR activity revealed that H₂O in the reaction gas had little impact on the NO_x conversion at 250 °C, whereas, the presence of SO₂ slowly decreased the NO_x conversion from 99.6% to 85.0% in 40 h. Particularly, the NO_x conversion linearly declined to 75% with the co-existence of SO₂ and H₂O in 16 h. Thus, the presence of H₂O obviously accelerated the deactivation of WO₃/Ce_0.68Zr_0.32O₂ compared with only the presence of SO₂ under NH₃-SCR conditions. Nevertheless, the above deactivation resulting from SO₂ and H₂O could be mostly eliminated by thermal treatment. The characterization via XRD, NO/CO₂-TPD, XPS, H₂-TPR, OSC and ex/in situ DRIFT demonstrated that (NH₄)₂SO₄ and Ce₂(SO₄)₃ were deposited on the surface of the sulfated catalysts, which decreased the number of active sites and weakened the redox properties of WO₃/Ce_0.68Zr_0.32O₂; thus, resulting in the deactivation of the WO₃/Ce_0.68Zr_0.32O₂ catalyst. Importantly, the presence of H₂O accelerated the generation of sulfates on the surface of WO₃/Ce_0.68Zr_0.32O₂, which led to the serious deactivation of the catalyst in the presence of both H₂O and SO₂. As a result, the Langmuir–Hinshelwood route of NH₃-SCR over the catalyst was disrupted due to the shielding of sulfate species.