Low-temperature selective catalytic reduction of NO with NH3 over V/ZrO2 prepared by flame-assisted spray pyrolysis: Structural and catalytic properties

Abstract

A series of V/ZrO2 (V/Zr atomic ratio=0.05, 0.11, 0.17, 0.25, 0.33, 0.42), V-WOx(0.66)/ZrO2, and V-MoOx(0.66)/ZrO2 were synthesized by adopting a one-step flame spray pyrolysis technique and investigated for the low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Our XRD results suggest that the vanadia species are in amorphous state as dominant surface monomeric VOx species on zirconia support in V/ZrO2 samples with V/Zr≤0.17. Further increase in vanadia content led to the formation of ZrV2O7 solid solution as a result of zirconia migration into the V2O5 crystallites. H2-TPR data results are highly consistent with the XRD results that the low temperature shifts in the reduction peaks of 15% V/ZrO2 catalyst attributed to the presence of more easily reducible dominant surface monomeric VOx species. The evolution of a new reduction peak at high temperature indicates the formation of dominantly isolated polymeric V oxides species in V/ZrO2 samples with V/Zr≥0.25. Our H2-TPR profiles for tungsten-promoted V/ZrO2 catalyst revealed the shift of (T1) peak position to lower temperatures, suggesting that the reduction potential of vanadium oxide species is increased compared to V/ZrO2 catalyst. This occurrence signifying that the addition of tungsten to V/ZrO2 promoted the catalyst for the formation of monomeric surface vanadia species, whereas the addition of Mo promoted the formation of polymeric VOx species and thus inhibit the SCR activity. An intense sharp band characteristic of coordinatively held NO2 species is evident at 1630cm−1, no gaseous N2O species were detected at 2224cm−1, 1286cm−1 in the in-situ FT-IR spectra of NO adsorbed over the V/ZrO2. Our in-situ FT-IR studies of NO+NH3 co-adsorption demonstrate that the strong signals at 1435 and 1714cm−1 (bending vibration of NH4+) have seen to decrease with respect to temperature. This observation reveals that the NH4+ species bound to Brönsted acid sites are responsible for the enhancement in the SCR reaction over the vanadia-zirconia surfaces. The low temperature NO catalytic reduction activity would indicate that an optimal dispersion of monomeric VOx species on zirconia is attained with the 15% of V in the V/ZrO2 catalyst. The introduction of Zr (seven fold coordination) into V2O5 may lead to the reduction of elemental number in crystal grain and deviation of adjacent oxygen atoms. The change in lattice parameter, crystal anisotropy and negative thermal expansion can block the release of labile oxygen, this seems to be the reason for the decrease in catalytic activity in V/ZrO2 catalysts with V/Zr≥0.25. The addition of tungsten has a strong influence on the NO conversion, since tungsten loading with the WOx/VOx atomic ratio=0.66 exhibits a maximum conversion of ∼98% in the temperature range 180–240°C, whereas the molybdenum loading showed an inhibition effect on the SCR activity of V/ZrO2 catalyst.