Catalytic Activity of Pt and Tungsto-Phosphoric Acid Supported on MCM-41 for the Reduction of NO

Abstract

The catalytic reduction of NO _x is one of the most rapidly growing applications in the field of environmental catalysis, an area which accounts in total for more than one-third of the world catalyst market today. For NO _X emissions from stationary sources, V ₂ O ₅ /TiO ₂ catalysts and NH ₃ as reducing agent are regularly used, however, extensive research is necessary to develop catalysts in which more environmentally more acceptable molecules such as hydrocarbons can be used as reducing agents. Initially, research focused on transition metal-loaded zeolites, but the drastic loss in activity when H ₂O and/or SO ₂ are present during the reaction has led to the development of alternative catalysts such as Pt group metals supported on metal oxides (SiO ₂ and A1 ₂ O ₃ ). In order to combine the advantages of zeolites with oxide-based systems, the catalytic properties of noble and transition metals supported on mesoporous molecular sieves with MCM-41 type structure were investigated. To generate strong Brønsted acid sites, the catalysts were co-impregnated with tungstophosphoric acid. These catalysts showed promising properties with respect to activity and stability in presence of water vapor. The mesoporous molecular sieve was found to act as a high surface area support for the metal and acid clusters, which were, due to the special characteristics of MCM-41 type materials, highly stabilized against thermal sintering and pore blocking. The structure and the activity/selectivity of these catalysts with respect to the influence of the water vapor are discussed in order to explain the unique properties of these materials in the reduction of NO _x with hydrocarbons in the presence of water vapor.