The role of K2O in the selective reduction of NO with NH3 over a V2O5(WO3)/TiO2 commercial selective catalytic reduction catalyst

To elucidate the nature of the acid sites of the V2O5(WO3)/TiO2 catalyst up on K2O addition and its relation to the selective reduction of NO with NH3, measurements were made by means of infrared and Raman spectroscopy, NH3 ch emisorption, and NO reduction measurements as a function of the K2O loading . The catalytic activity was found to decrease rapidly with the K2O loading , irrespective of the similar textural properties of all samples. Addition of K2O modified the vanadium species on the catalyst surface. For large add itions of K2O, the potassium partially reacted with V2O5 to form KVO3. The amount of NH3 chemisorbed on the catalyst was observed to decrease with bot h the loading of K2O and the temperature. The adsorption of NH3 on both Bro nsted and Lewis acid sites was confirmed. The strength and the number of Br onsted acid sites decrease largely with the loading of K2O in parallel with the decrease of the SCR activity, suggesting that the SCR reaction involve s NH3 adsorption on the Bronsted acid sires. At low surface coverage of NH3 , the isosteric heat of NH3 chemisorption was determined to be 370 kJ/mol f or 0 wt.% K2O addition. With increasing K2O amount, the heat of adsorption decreased and was 150 kJ/mol for the catalyst with higher amounts of K2O ad dition. The results obtained imply that potassium disturbs the formation of the active ammonia intermediates, NH4+, resulting in deactivation of the c atalyst. (C) 1999 Elsevier Science B.V. All rights reserved.