Sulfate formation on SOx 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 BaCl2 as a second component (Ba/Cu-BTC) and by a conventional BaCO3/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 BaCO3/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 BaCO3/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., BaSO4 and Al2(SO4)3) as function of the temperature remained constant.
Department of Chemistry, Technische Universität München, Lichtenbergstr. 4, D-85747 Garching, Germany.
Hendrik
H
Dathe
Andreas
A
Jentys
Johannes A
JA
Lercher
D017609:Barium Compounds / Q000737:chemistry; D002627:Chemistry, Physical / Q000379:methods; D002712:Chlorides / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D003460:Crystallization; D008422:Materials Testing; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry; D013431:Sulfates / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry; D013696:Temperature; D014961:X-Ray Diffraction
D017609:Barium Compounds; D002712:Chlorides; D013431:Sulfates; C024986:barium chloride; D013455:Sulfur; D003300:Copper; D010100:Oxygen