Pd/ZrO2-Wox katalizör üzerinde methane ile seçici katalitik indirgenmede oluşan ara ürünlerin FT-IR spektroskopik karakterizasyonu

This work involves in situ FT-IR spectroscopic study of the routes offormation, composition and thermal stability of strongly bound NOx complexes onthe surface of Pd/tungstated zirconia, and transformation of the surface NOxcomplexes in the presence of methane in order to elucidate the mechanism ofselective catalytic reduction of NO with methane. Sol-gel polymer-templatesynthesis was chosen to obtain high surface area in the preparation of thetungstated zirconia used as support (WO3 nominal content of 18.6 wt %). ThePd(II) ions (0.1 wt%) have been deposited by impregnation. PXRDcharacterization shows that the support and the catalyst are tetragonal and containmesoporous phase.The adsorption of NO at room temperature on the tungstated zirconiashows presence of coordinatively unsaturated Zr(IV) ions. The spectrum of NOadsorbed on palladium modified on tungstated zirconia reveals the existence oftwo types of Pd(II) sites. No exposed Zr(IV) ions are observed. The surface NOxspecies (N2O3, nitro and nitrito ions) on both samples are produced at roomtemperature by oxidation of NO with the W6+=O species. In the case of thePd/tungstated zirconia, palladium(II) can oxidize NO to NO2 at 623 K.vThe adsorption of NO/O2 mixture at room temperature on the samplesstudied leads to formation of various kinds of surface nitrates characterized bydifferent modes of coordination. The thermal stability of the nitrate speciesformed on both samples is comparable: They disappear after dynamic evacuationat 673 K. However, lower concentration of the surface nitrates on thePd/tungstated zirconia compared to that on the tungstated zirconia indicates that inthe former case the nitrates are coordinated to the support.The experimental results show that methane interacts differently withthe NOx-precovered tungstated zirconia and Pd/tungstated zirconia although bothmaterials are able to activate methane at the same temperature in absence ofadsorbed NOx species. In the case of the tungstated zirconia the surface nitratessuppress the oxidation of methane, whereas the NOx-precovered Pd/tungstatedzirconia catalyzes the formation of nitromethane. The latter compound isconsidered as a key intermediate in the selective catalytic reduction of NO withmethane in excess oxygen. A mechanism, which involves direct activation ofmethane by the catalyst, leading to the products of the selective reduction (N2,CO2 and H2O) is proposed.Keywords: Adsorption of NO and NO/O2; In situ FT-IR spectroscopy; tungstatedzirconia; Pd supported on tungstated zirconia; NO selective catalytic reduction bymethane; Mechanism; Nitromethane.