Catalytic deNO x properties of novel vanadium oxide based open-framework materials

The deNO _x catalytic properties of a new class of open-framework structure materials, Li₆Mn₃(H₂O)₁₂V₁₈O₄₂(XO₄)] · 24H₂O (X = V, S) (1), Fe₃(H₂O)₁₂ V₁₈O₄₂(XO₄)] · 24H₂O (X = V, S) (2), Co₃(H₂O)₁₂V₁₈O₄₂(XO₄)] · 24H₂O (X = V, S) (3), and Li₆Ni ₃ ^II (H₂O)₁₂V ₁₆ ^VI V ₂ ^V O₄₂(SO₄)] · 24H₂O (4), have been studied. The crystal structures of these novel systems consist of three-dimensional arrays of vanadium oxide clusters {V₁₈O₄₂(XO₄)} , as building block units, interlinked by {–O–M–O–} (M = Mn, 1; M = Fe, 2; M = Co, 3; M = Ni, 4) bridges. Their open-framework structures contain cavities, similar to those observed in conventional zeolites, which are occupied by exchangeable cations and/or readily removable water of hydration. The catalysts derived from these materials were tested for the selective catalytic reduction (SCR) of nitrogen oxides {NO _x } into N₂ using a hydrocarbon, propylene, as the reducing agent. The catalysts were ineffective under lean burn conditions. However, the new catalysts, especially the one derived from the cobalt derivative (3), showed intriguing deNO _x activity under rich conditions. They remove up to ~ 99% of the toxic NO _x emissions in 1.5% O₂ with 100% selectivity to N₂. The active phase of the catalysts exhibit good stability, can be readily regenerated, and are selective to the desired product-N₂. The catalytic reactions occur at moderately low temperatures (400–500 °C). The catalysts were characterized by FT-IR, temperature programmed reactions (TPR and TPO), SEM, BET surface area measurements, elemental analysis, and X-ray diffraction (XRD). Additional advanced techniques were used to further characterize the catalyst phases that showed most promising deNO _x activity and increased tolerance to oxygen.