Catalytic hydrogenation of nitrate in water. Improvement of the activity and selectivity to N2 by using Rh(III)-hexamolybdate supported on ZrO2-Al2O3

Catalysts prepared on ZrO2, Al2O3 and ZrO2-Al2O3 (ZrAl-10) supports with Anderson heteropolyanion (RhMo6) as active phase were investigated for the elimination of NO3- from water. Raman characterization of pure and supported RhMo6 phase showed the presence of polymolybdic species of different degrees of complexity when RhMo6 was supported. The temperature-programmed reduction study revealed the synergic effect between Rh and Mo species, through which the reducibility of Mo was promoted by Rh, and different phase/support interactions were verified. Among the supports, ZrAl-10 presented the highest acidity due to the presence of ZrO2 in the tetragonal modification and high specific surface area (due to Al2O3), favouring rhodium-molybdenum active phase/support interaction and high dispersion. All catalysts prepared were active in removing NO3-, the one prepared with the RhMo6 phase on the ZrAl-10 support being the most active. These results point to the formation of an active surface with a high dispersion of Rh and Mo. The highest selectivity to N2 (99.3) exhibited by the RhMo6/ZrAl-10 catalyst is proposed to be related to the high Rh dispersion (0.755) and to the presence of Lewis acid sites (oxygen vacancies) of the tetragonal ZrO2 modification that favour NO3- adsorption through electrostatic interactions.