Potassium doping of vanadia/titania de-NO xing catalysts: Surface characterisation and reactivity study

Abstract

An investigation of the effect of K-doping on the surface and catalytic 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 spectroscopy (FT-IR), temperature-programmed desorption (TPD), temperature-programmed 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 V 2O 5 loading in the range 0.28–5.3% w/w. TPSR and TPR data provide evidence for a greater reactivity of polymeric metavanadate species as compared to isolated vanadyls. Besides, a different type of isolated vanadyl appears to form at high loadings. Ammonia is adsorbed at vanadium sites in the form of molecularly coordinated species and of ammonium ions. Coordinated species show a higher thermal stability than ammonium ions. Ammonia is also coordinated at titanium sites to give a more weakly held species. Upon K-doping the stretching frequencies of surface vanadyls are lowered due to the production of strong oxide basic anions on the catalyst surface. This reduces the Lewis acidity of vanadium ions. IR and TPD experiments indicate that both molecularly chemisorbed ammonia and ammonium ions are present in much lower amounts and are less strongly held on K-doped samples. TPD and TPSR data further indicate that the alkali dopant poisons preferentially Lewis acid sites associated with vanadium rather than with Ti 4+ ions. TPSR and TPR data show that alkali doping reduces markedly the nitric oxide conversion (associated with the number of active sites), but not the temperature threshold of the SCR reaction (associated with the intrinsic reactivity of the active sites). The lower number of active sites on alkali-doped catalyst can be related to the poisoning of both Brønsted and Lewis vanadium acid sites due to alkali addition, which results in a lower ammonia surface coverage.