Cr-containing magnetites Fe_3−xCr_xO₄: The role of Cr³⁺ and Fe²⁺ on the stability and reactivity towards H₂O₂ reactions

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• 作者：F. Magalhã ; es ; M.C. Pereira ; S.E.C. Botrel ; J.D. Fabris ; W.A. Macedo ; R. Mendonç ; a ; R.M. Lago ; L.C.A. Oliveira
• 关键词：Magnetite ; Chromium ; Fenton reaction
• 刊名：Applied Catalysis A ; General
• 出版年：2007
• 期刊代码：4_0926860x
• 类别：ch
• 出版时间：1 November 2007
• 卷：332
• 期：1
• 页码：115-123
• 文件大小：1467 K

摘要

A series of Cr-containing magnetites, Fe_3−xCr_xO₄ (x = 0.00, 0.07, 0.26, 0.42 and 0.51) has been prepared using conventional co-precipitation method. Mössbauer and powder X-ray diffraction measurements suggested the formation of the spinel crystalline phase with initial substitution of d=retrieve&_udi=B6TF5-4PCPFHM-1&_mathId=mml1&_user=10&_cdi=5217&_rdoc=16&_acct=C000050221&_version=1&_userid=10&md5=88b06d5a1adeff54b32b8655c82c8400">direct.com/cache/MiamiImageURL/B6TF5-4PCPFHM-1-2/0?wchp=dGLzVzz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=19 width=32> by Cr³⁺ and for higher Cr contents, chromium also replaces d=retrieve&_udi=B6TF5-4PCPFHM-1&_mathId=mml2&_user=10&_cdi=5217&_rdoc=16&_acct=C000050221&_version=1&_userid=10&md5=2e16737f347ea020805b3a7ce6a58452">direct.com/cache/MiamiImageURL/B6TF5-4PCPFHM-1-3/0?wchp=dGLzVzz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=19 width=103> in the crystalline structure. N₂ adsorption/desorption revealed that the presence of Cr has a remarkable effect on the texturial properties of the material decreasing the pore diameter from meso- to micropore with a significant increase on the BET surface area. Thermal analyses (TG and DTA) coupled with XRD and Mössbauer showed that thermal treatment up to 270 °C with O₂ leads to the oxidation of d=retrieve&_udi=B6TF5-4PCPFHM-1&_mathId=mml3&_user=10&_cdi=5217&_rdoc=16&_acct=C000050221&_version=1&_userid=10&md5=185e706a19bc4c0f89cb7a68c04b1fcc">direct.com/cache/MiamiImageURL/B6TF5-4PCPFHM-1-4/0?wchp=dGLzVzz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=19 width=32> producing Cr-substituted maghemite but at 600 °C the cubic maghemite is converted to the hexagonal hematite and Cr is expelled from the iron oxide structure. The reactivity of Fe_3−xCr_xO₄ was investigated using two H₂O₂ reactions, i.e. the decomposition to O₂ and the oxidation of the dye methylene blue used as model contaminant. The obtained results showed that the presence of Cr directly promoted the H₂O₂ decomposition. On the other hand, the presence of small concentration of Cr, i.e. Cr_0.07 and Cr_0.21, caused a significant increase on the activity for the oxidation of the dyes with complete discoloration and high degree of mineralization. The higher activity of Fe_2.93Cr_0.07O₄ was discussed in terms of a coupling of the redox pairs Fe³⁺/Fe²⁺ and Cr²⁺/Cr³⁺ producing a more efficient regeneration of the Fenton active specie Fe²⁺.