Anna Białas

Evolution of the surface species of the V2O5–WO3 catalysts

Abstract Vanadia-related species formed as a result of vanadium segregation at the surface of V–W oxide bronze crystallites were investigated. The structures of these species and their transformations induced by oxygen removal and oxygen adsorption were monitored using photoelectron spectroscopy and the FT Raman technique. Assignments of the MeO vibrational bands, based on the results of DFT calculations for model clusters, have been proposed. Two kinds of surface species are dominant depending on the tungsten content: V 4+ –O–W 6+ at low tungsten content and V 5+ –O–W 5+ at higher tungsten concentration.

Effect of Fe3+ ions present in the structure of poly(acrylic acid)/montmorillonite composites on their thermal decomposition

Poly(acrylic acid)/montmorillonite (MMT) composites with various polymer contents were synthesized by in situ polymerization technique. The structure of obtained materials was characterized by powder X-ray diffraction and infrared spectroscopy (FTIR). It was found that only a limited amount of hydrogel could be introduced between the clay layers. The remaining part of polymer was deposited on the external surface of clay particles. The introduction of the polymer modifier significantly increased the adsorption capacity of MMT in the elimination of Fe3+ ions from aqueous solution. The thermal behavior of the samples before and after the Fe3+ adsorption was examined by thermogravimetry and differential thermal analysis. Moreover, the composition of gaseous products evolved during decomposition was determined by FTIR. The materials after Fe3+ adsorption exhibited different thermal stability in oxidizing atmosphere than the fresh samples. Fe3+ cations, forming FeOx species during thermal treatment, appeared to be effective catalysts of polymer oxidation.

Structural studies of V2O5–WO3 and WO3–V2O5 solid solutions

Powder X-ray diffraction, Fourier transform Raman spectroscopy, solid-state NMR and X-ray photoelectron spectroscopy were used to determine the composition and structure of a mixed oxide containing 10 mol% V2O5 and 90 mol% WO3. The parent sample consists of hybrid crystallites of a tungsta-bronze isostructural with tetragonal WO3 but containing atomic vanadium in the bulk of the crystals and V(III) in the surface region, and of small crystals of strongly reduced vanadia-like surface species (V2O5–x with x=0.42). Further thermal treatment causes progressive surface segregation of vanadium from the V2O5–WO3 solid solution, accompanied by the formation of a WO3–V2O5 solid solution and partial tetragonal–monoclinic structural transformation of the bronze.

Competition between NO reduction and NO decomposition over reduced V-W-O catalysts

The SCR of NO and NO decomposition were investigated over a V-W-O/Ti(Sn)O 2 catalyst on a Cr-Al steel monolith. The conversions of NO and NH 3 over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH 3 was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V 10 O 31 H 12 (V-V) and V 9 WO 31 H 12 (V-W) modelling (001) surfaces of vanadia and WO 3 -V 2 O 5 solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W-N bond.

Evolution of Ti–Sn-rutile-supported V2O5–WO3 catalyst during its use in nitric oxide reduction by ammonia

This paper concerns the relation between surface structure of crystalline vanadia-like active species on vanadia–tungsta catalyst and their activity in the selective reduction of NO by ammonia to nitrogen. The investigations were performed for Ti–Sn-rutile-supported isopropoxy-derived catalyst. The SCR activity and surface species structure were determined for the freshly prepared catalyst, for the catalyst previously used in NO reduction by ammonia (320 ppm NO, 335 ppm NH3 and 2.35 vol% O2) at 573 K as well as for the catalyst previously annealed at 573 K in helium stream containing 2.35 vol% O2. The crystalline islands, exposing main V2O5 surface, with some tungsten atoms substituted for V-ones, were found, with XPS and FT Raman spectroscopy, to be present at the surface of the freshly prepared catalyst. A profound evolution of the active species during the catalyst use at 573 K was observed. Dissociative water adsorption on V5+OW6+ sites is discussed as mainly responsible for the catalyst activity at 473 K and that on both V5+OW6+ and V4+OW6+ sites as determining the activity at 523 K.

Thermal stability and pollutant adsorption efficiency of nanocomposites consisted of clay and polymeric quaternary ammonium salts

Thermal stability and pollutant adsorption efficiency of nanocomposites consisted of clay and polymeric quaternary ammonium salts Polymer/layered silicate nanocomposites based on polymeric quaternary ammonium salts (ionenes) intercalated into the interlayer galleries of montmorillonite were synthesized. Zeta potential measurements were conducted to determine the amount of ionenes required to neutralize the negative charge of clay. The composition and structure of the obtained nanocomposites were examined by elemental analysis, ATR-FTIR and XRD. High dye sorption capacity was observed for the composite containing ionene 6,2.