POTASSIUM DOPING OF VANADIA TITANIA DE-NO(X)ING CATALYSTS - SURFACE CHARACTERIZATION AND REACTIVITY STUDY/

An investigation of the effect of K-doping on the surface and catalyti c properties of sub-monolayer vanadia/titania de-NO(x)ing catalysts is carried out. Samples having different vanadium and potassium loadings have been investigated by means of Fourier-transform infrared spectro scopy (FT-IR), temperature-programmed desorption (TPD), temperature-pr ogrammed surface reaction (TPSR) and temperature-programmed reaction ( TPR) techniques. Isolated vanadyls and polymeric metavanadate species are present on the surface of undoped catalysts; both species increase on increasing the V2O5 loading in the range 0.28-5.3% w/w. TPSR and T PR data provide evidence for a greater reactivity of polymeric metavan adate species as compared to isolated vanadyls. Besides, a different t ype of isolated vanadyl appears to form at high loadings. Ammonia is a dsorbed at vanadium sites in the form of molecularly coordinated speci es and of ammonium ions. Coordinated species show a higher thermal sta bility than ammonium ions. Ammonia is also coordinated at titanium sit es to give a more weakly held species. Upon K-doping the stretching fr equencies of surface vanadyls are lowered due to the production of str ong oxide basic anions on the catalyst surface. This reduces the Lewis acidity of vanadium ions. IR and TPD experiments indicate that both m olecularly chemisorbed ammonia and ammonium ions are present in much l ower amounts and are less strongly held on K-doped samples. TPD and TP SR data further indicate that the alkali dopant poisons preferentially Lewis acid sites associated with vanadium rather than with Ti4+ ions. TPSR and TPR data show that alkali doping reduces markedly the nitric oxide conversion (associated with the number of active sites), but no t the temperature threshold of the SCR reaction (associated with the i ntrinsic reactivity of the active sites). The lower number of active s ites on alkali-doped catalyst can be related to the poisoning of both Bronsted and Lewis vanadium acid sites due to alkali addition, which r esults in a lower ammonia surface coverage.