TiO2-supported metal oxide catalysts for low-temperature selective catalytic reduction of NO with NH3. I. Evaluation and characterization of first row transition metals

IR spectroscopy in catalysis

Titania-supported tungsten and vanadia oxides with different W and V loadings and calcined at different temperatures have been prepared by the sol–gel method and characterized. Large amount of tungsten (9%; w/w) provides thermal stability to WO3/TiO2 systems upon addition of vanadia. It is found that WO3 and V2O5 crystallites are formed when their concentrations are higher than those corresponding to three monolayers. DTA analyses have shown that the presence of V2O5 crystallites is not essential for the transformation of titania anatase into rutile. High reactivity in limited temperature range (225–350 °C) has been observed for the catalyst with 8% (w/w) V2O5 with a high formation of N2O amount during the SCR reaction. The catalyst with 3% (w/w) V2O5 exhibits a slight lower reactivity but with a high selectivity to N2 preserved in all working temperature.

Effect of vanadia and tungsten loadings on the physical and chemical characteristics of V2O5-WO3/TiO2 catalysts

Low temperature SCR of NO with NH3 over carbon nanotubes supported vanadium oxides

This paper reports, for the first time, a systematic investigation (screening) of the catalytic activity of 40 different formulations of transition metal oxides-based supported catalysts in the course of the total oxidation of benzene as a model molecule for dioxin. The catalysts consisted in 10 different transition metal oxides (CrOx, MnOx, VOx, SnOx, WOx, NbOx, TaOx, MoOx, ZrOx, and BiOx) supported on 4 different supports (2 kinds of TiO2, Al2O3 and SiO2). A theoretical coverage of 0.75 monolayer of active phase was chosen to minimize the formation of crystallites. XPS and XRD characterizations demonstrated the better spreading as monolayer of the active phases at the surface of titania than of Al2O3 and SiO2. The latter induces a poor dispersion of almost all the active phases as crystallites. The variation of spreading of the active phases on the different supports is governed by the difference in surface free energy and is fully explained by the "solid-solid wetting" concept. For almost all active phases, the conversion of benzene progressively improves when the support is changed from SiO2 to Al2O3 and, finally, titanias. The performances of the active phases exhibiting this behavior, are clearly dictated by their presence as well-dispersed monolayers at the surface of the support. The screening revealed CrOx, VOx and MnOx as the most active phases. The first two exhibit their best activity when spread as monolayer at the surface of TiO2 based supports (classical behavior). At the opposite, MnOx works best when present in the form of Mn3O4 crystallites on the SiO2 support. (c) 2006 Elsevier B.V. All rights reserved.

Systematic investigation of supported transition metal oxide based formulations for the catalytic oxidative elimination of (chloro)-aromatics: Part I: Identification of the optimal main active phases and supports

The performance of several TiO 2 -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, V 5+ in the precursor solution yields lower-performance catalysts compared to the case of V 4+ under identical conditions. Aging the vanadium precursor solution, which is associated with the reduction of V 5+ to V 4+ (VO 2 +  → VO 2+ ), 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 TiO 2 -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 WO 3 and Al 2 O 3 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, NH 3 -temperature programmed desorption, XRD, and UV/VIS were used to characterize the catalysts tested in this work.

N Economidis

Papers

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