Mechanism of poisoning of the V₂O₅/TiO₂ catalyst for the reduction of NO by NH₃

Alkali metals are among the strongest poisons to the V₂O₅/TiO₂ catalyst for selective catalytic reduction of NO by NH₃. The strength of the poison is directly related to its basicity. SO₂, in contrast, promotes the activity. The chemisorbed NH₃ on the catalyst is predominantly NH₄⁺, bonded to the Brønsted acid site of VOH. 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 V₂O₅/TiO₂ surface. The extraction energy for proton from the VOH group and the net charge of H in the VOH 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. SO₂, 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. SO₂ promotes the activity by increasing the Brønsted acidity.