Development and characterization of mixed oxide catalysts for the selective catalytic reduction of nitric oxide from stationary sources using ammonia

Seven first row transition metals were deposited on various commercial TiO₂, SiO₂, and Al₂O₃ supports to create mono- and bimetallic catalysts that were compared in the selective catalytic reduction of nitric oxide using ammonia at low temperatures ranging from 373–523 K. The catalyst with the highest activity both in the absence and presence of water in the feed was 20 wt.% Mn/Hombikat TiO ₂ synthesized from a nitrate precursor and calcined below 673 K. Under those conditions, it was capable of achieving 100% NO conversion at 393 K. Numerous surface characterization techniques were used to identify the surface properties that result in highly active and selective low temperature SCR catalysts. The deposition of manganese as MnO₂, the ease of reducibility of the metal oxide, and the symmetric deformation of ammonia coordinated to Lewis acid sites at 1167 cm−1, were all found to be important for good catalytic performance. No synergistic effects were observed from combinations of the three most active transition metals. However, MnO _x-NiO/TiO₂ had an extended lifetime relative to MnO _x/TiO₂ in feeds containing SO₂. The extensive data collected from in-situ FTIR experiments in the presence of NO and NH₃ were used to propose a reaction mechanism for MnO _x/TiO₂ that begins with the coordination of NH₃ over Mn+4 species and proceeds through the formation of bridged nitrates.; A combination of potentiometric titrations and UV/Vis spectroscopy were used to quantify the reduction of V+5 to V+4 after the addition of oxalic acid as the solution is aged. After approximately four hours, the aging vanadium oxalate solution reaches steady state, and the final distribution of the vanadium present is 89% V+4 and 11% V+5. TiO₂ supported monolayer catalysts synthesized from the aged (V+4) vanadium oxalate solution consistently outperformed catalysts made from freshly prepared (V+5) vanadium oxalate solutions. Surface characterization revealed that surface acid sites increase in strength and vanadia reduces more easily in catalysts synthesized from aged vanadium oxalate solutions, which enhances reaction mechanism depends upon acid sites and redox operation.