Sulfate formation on SO_x trapping materials studied by Cu and S K-edge XAFS

The elementary steps during oxidative chemisorption of SO₂ by a novel composite material consisting of highly disordered benzene tri-carboxylate metal organic framework materials with Cu as central cation and BaCl ₂ as a second component (Ba/Cu-BTC) and by a conventional BaCO ₃/Al₂O₃/Pt based material were investigated. EXAFS analysis on the Cu K-edge in Ba/Cu-BTC indicates the opening of the majority of the Cu-Cu pairs present in the parent Cu-BTC. Compared to Cu-BTC, the BaCl₂ loaded material has hardly any micropores and has higher disorder, but it has better accessibility of the Cu²⁺ cations. This results from the partial destruction of the MOF structure by reaction between BaCl₂ and the Cu cations. The SO₂ uptake in oxidative atmosphere was higher for the Ba/Cu-BTC sample than for the BaCO ₃/Al₂O₃/Pt based material. XRD showed that on Ba/Cu-BTC the formation of BaSO₄ and CuSO₄ occurs in parallel to the destruction of the crystalline structure. With BaCO ₃/Al₂O₃/Pt the disappearance of carbonates was accompanied with the formation of Ba- and Al-sulfates. XANES at the S K-edge was used to determine the oxidation states of sulfur and to differentiate between the sulfate species formed. At low temperatures (473 K) BaSO₄ was formed preferentially (53 mol% BaSO₄, 47 mol% CuSO₄), while at higher temperatures (and higher sulfate loading) CuSO₄ was the most abundant species (42 mol% BaSO₄, 58 mol% CuSO₄). In contrast, on the BaCO₃/Al₂O₃/Pt based material the relative concentration of the sulfate species (i.e., BaSO ₄ and Al₂(SO₄)₃) as function of the temperature remained constant.