The remarkable effect of oxygen on the N2 selectivity of water catalytic denitrification by hydrogen.

The selective catalytic reduction of nitrates (NO 3 - ) in pure water toward N 2 formation by the use of gaseous H 2 and in the presence of O 2 (air) at 1 atm total pressure and 25 °C has been investigated over Pd-Cu supported on various mixed metal oxides, x wt % MO x /γ-Al 2 O 3 (MO x = CeO 2 , SrO, Mn 2 O 3 , Cr 2 O 3 , Y 2 O 3 , and TiO 2 ). It is demonstrated forthefirsttimethata remarkable improvement in N 2 reaction selectivity (by 80 percentage units) can be achieved when oxygen is present in the reducing feed gas stream. In particular, significantly lower reaction selectivities toward NH 4 + and NO 2 - can be obtained, whereas the rate of NO 3 - conversion is not significantly affected. Moreover, itwas shown that the same effect is obtained over the Pd-Cu-supported catalysts irrespective to the chemical composition of support and the initial concentration of nitrates in water used. The Pd-Cu clusters supported on 4.8 wt%TiO 2 /γ-Al 2 O 3 resulted in a solid with the best catalytic behavior compared with the rest of supports examined, both in the presence and in the absence of oxygen in the reducing feed gas stream. DRIFTS studies performed following catalytic reduction by H 2 of NO 3 - in water revealed that the presence of TiO 2 in the Pd-Cu/TiO 2 -Al 2 O 3 system enhanced the reactivity of adsorbed bidentate nitrate species toward H 2 . Nitrosyl species adsorbed on the alumina and titania support surfaces are considered as active intermediate species of the selective catalytic reduction of NO 3 - by H 2 in water. Pd-Cu/TiO 2 -Al 2 O 3 appears to be the most selective catalyst ever reported in the literature for the reduction of nitrates present in pure water into N 2 by a reducing gas mixture of H2/air.