Comparison of TiO2-based oxide catalysts for the selective catalytic reduction of NO:: effect of aging the vanadium precursor solution

The performance of several TiO2-based polyoxide supports loaded with vanadia was tested for the selective catalytic reduction of NO with ammonia as reductant. It is remarkable to note that the valence of vanadium in the precursor solution during the impregnation step is crucial for the synthesis of efficient DeNO(x) catalysts. More specifically, V5+ in the precursor solution yields lower-performance catalysts compared to the case of V4+ under identical conditions. Aging the vanadium precursor solution, which is associated with the reduction of V5+ to V4+ (VO2+ --> VO2+), prior to impregnation results in catalysts with excellent catalytic behavior under identical activation and operating conditions. The present variation of loading vanadia is used for TiO2-based supports with low crystallinity. These supports, which have traditionally performed poorly, are now able to function as effective SCR catalysts. Increasing the acidity of the support by incorporating oxides such as WO3 and Al2O3 significantly improves the SCR activity and nitrogen selectivity. The concentration of tungsten used in these catalysts was higher than that typically used in industry. It was also found that the supports should be synthesized with the simultaneous precipitation of the corresponding precursors. From our work we found that the mixed oxide catalysts possess Bronsted and Lewis acid sites of comparable strength over a wide range of temperatures. Catalysts prepared from aged vanadium precursor solutions demonstrated a wider temperature window for optimum operation. FT-IR, NH3-temperature programmed desorption, XRD, and UV/VIS were used to characterize the catalysts tested in this work. (C) 1999 Elsevier Science B.V. All rights reserved.