Μελέτη της αντίδρασης αναγωγής των οξειδίων του αζώτου με προπυλένιο παρουσία περίσσειας οξυγόνου σε καταλύτες Rh

Abstract

In the present study, the catalytic reduction of NO with propylene in the presence of excess oxygen was examined, over Rh catalysts supported on doped TiO2 carriers with tungsten or calcium cations. Doped TiO2 carriers were prepared employing the solid state diffusion technique at elevated temperatures. Characterization of doped carriers consisted of specific surface (BET) area and crystalline mode (XRD) measurements as well as measurements of the specific conductivity (AC-Impedance) and surface acidity (TPD-MS). In general, doping TiO2 results in an increase of the surface acidity, in alterations of specific conductivity and in the preservation of the specific surface in a manner, which depends on dopant concentration and calcinations temperature. Supported Rh catalysts were prepared following the wet impregnation method using Rh(NO3)3 as the precursor metal compound. Characterization of the prepared catalysts consisted of dispersion and mean metal crystalline size measurements, employing hydrogen chemisorption at ambient temperature. Comparison of the doped catalysts, as far as the activity toward NO reduction and the selectivity to N2 is concerned, shows two different behaviors. Doping with W6+ cations results in higher NO reduction activity with a slightly increase of selectivity toward dinitrogen formation. Doping with Ca2+ leads to higher catalytic activity for the reduction of NO, while the selectivity was found to decrease upon increasing calcium content. The above observations are explained by the theory of dopant induced metal support interactions (DIMSI). The mechanistic pathways of the above mentioned reaction over Rh/TiO2 catalysts were studied employing infrared (IR) and mass spectroscopy (MS). The results showed, that NO is molecularly absorbed on the Rh. In the presence of reduced Rh sites NO dissociation occurs, resulting in N and O adspecies. Recombination of N species results in the production of dinitrogen in the gas phase. N2O formation is correlated with the presence of the dinitrosyl complex species on the catalytic surface. Reaction of the latter with NO from the gas phase leads to N2O production. Regeneration of oxidized Rh sites is achieved via oxidation of propylene to acrolein. Further oxidation of the latter leads to the formation of carboxylates.