Mechanism of poisoning of the V 2O 5/TiO 2 catalyst for the reduction of NO by NH 3

Abstract

Alkali metals are among the strongest poisons to the V 2O 5/TiO 2 catalyst for selective catalytic reduction of NO by NH 3. The strength of the poison is directly related to its basicity. SO 2, in contrast, promotes the activity. The chemisorbed NH 3 on the catalyst is predominantly NH 4 +, bonded to the Brønsted acid site of VOH. A direct correlation exists between the amount of chemisorbed ammonia and the activity of the poison-doped catalyst. Furthermore, dehydroxylation of the catalyst by heat treatment eliminates its activity, which is restored rapidly by exposure to water vapor. Extended Hückel molecular orbital (EHMO) calculation was performed on a model V 2O 5/TiO 2 surface. The extraction energy for proton from the VOH group and the net charge of H in the VOH group are used as indices for Brønsted acidity. The EHMO results show decreases in the Brønsted acidity by the addition of alkali metals, and the order of the decrease follows the order of the basicity of the alkali metal. SO 2, in contrast, increases the Brønsted acidity. These results indicate that the Brønsted acid sites are the active sites for the reaction. Alkali metals poison the catalyst by decreasing its Brønsted acidity. SO 2 promotes the activity by increasing the Brønsted acidity.