Selective catalytic reduction of NO by NH₃ with WO₃-TiO₂ catalysts: Influence of catalyst synthesis method

A series of supported WO₃/TiO₂ catalysts was prepared by a new synthesis procedure involving co-precipitation of an aqueous TiO(OH)₂ and (NH₄)₁₀W₁₂O₄₁*5H₂O slurry under controlled pH conditions. The morphological properties, surface WO_x molecular structures, surface acidity and surface chemistry of the co-precipitated WO₃/TiO₂ catalysts were determined with BET, in situ Raman, in situ IR, steady-state NO/NH₃/O₂ SCR and NO/NH₃-temperature-programmed surface reaction (TPSR) spectroscopy, respectively. Time-resolved isotopic ¹⁸O-¹⁶O exchange with IR spectroscopy demonstrated that tungsten oxide was present as surface WO_x sites on the TiO₂ support with mono-oxo O = WO₄ coordination. In contrast to previous studies employing impregnation synthesis that found only surface one mono-oxo O = WO₄ site (1010-1016 cm^-1) on TiO₂, the co-precipitation procedure resulted in the formation of two distinct surface WO_x sites: mono-oxo O = WO₄ (~1012-1014 cm^-1) and a second mono-oxo O = WO₄ (~983-985 cm^-1). The new surface mono-oxo O = WO₄ (~983-985 cm^-1) site is thought to be associated with surface defects on the co-precipitated titania support. The co-precipitated catalysts exhibited slightly enhanced SCR reactivity that is thought to be related to the presence of the new surface O = WO₄ sites. Additional factors, however, may also be contributing. This is the first study that attempts to relate the molecular level structural properties of co-precipitated WO₃-TiO₂ catalysts with their surface reactivity for SCR.