Redox Properties of Vanadium Ions in SBA-15-Supported Vanadium Oxide: An FTIR Spectroscopic Study

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• 作者：Tzvetomir V. Venkov ; Christian Hess ; Friederike C. Jentoft
• 刊名：Langmuir
• 出版年：2007
• 出版时间：February 13, 2007
• 年：2007
• 卷：23
• 期：4
• 页码：1768 - 1777
• 全文大小：227K
• 年卷期：v.23,no.4(February 13, 2007)
• ISSN：1520-5827

文摘

The state of vanadium ions in V_xO_y/SBA-15 (2.7 wt % V) was studied with FTIR spectroscopy using CO and NOas probe molecules. Neither CO (at 85 K) nor NO (at RT) adsorb on the oxidized sample because of the coordinativesaturation of V⁵⁺ ions and the covalent character of the V⁵⁺=O bond. After treatment of the sample in 50 kPa H₂at 673 K, the V⁵⁺ ions are reduced to two different types of V³⁺ sites, as manifested by carbonyl bands at 2189 and2177 cm^-1. In the presence of O₂ at 85 K, thus formed V³⁺ ions are partly oxidized to V⁴⁺ sites showing carbonylicbands at 2202 and 2190 cm^-1. When the reduced sample is exposed to O₂ at room temperature, the V³⁺ ions are fullyoxidized to V⁵⁺. The adsorption of NO on the reduced V_xO_y/SBA-15 shows that the V³⁺ and V⁴⁺ ions possess twoeffective coordinative vacancies and as a result can adsorb two NO molecules forming the respective V³⁺(NO)₂ andV⁴⁺(NO)₂ dinitrosyls. The introduction of O₂ to the V_xO_y/SBA-15-NO system leads to reoxidation of the V³⁺ andV⁴⁺ ions to V⁵⁺ and formation of bridged (1639 cm^-1) and bidentate (1573 cm^-1) surface nitrates. After coadsorptionof CO and NO on the reduced sample the formation of surface mixed carbonyl-nitrosyls (2108 and 1723 cm^-1) wasobserved for the first time.