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

• WO3-TiO2 catalysts were prepared by co-precipitation at varying pH values.
• Catalysts made by co-precipitation have two distinct surface WOx species.
• SCR activity of co-precipitated WO3-TiO2 catalysts is higher at high preparation pH.
• Co-precipitated catalysts have higher ammonia capacity than incipient wetness analogs.

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 WOx 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 WOx 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 WOx 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.

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