Catalytic stepwise nitrate hydrogenation in batch-recycle fixed-bed reactors

Pd (1.0 wt.%)-Cu (0.3 wt.%) bimetallic and Pd (1.0 wt.%) monometallic catalysts were synthesized by means of incipient-wetness impregnation technique and deposited on alumina spheres (dp = 1.7 mm). The prepared catalysts were tested at T = 298 K and p(H2) = 1.0 bar in the integrated process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions, in which the denitration step was carried out consecutively in separate, single-flow fixed-bed reactor units operating in a batch-recycle mode. In the first reactor packed with a Pd-Cu bimetallic catalyst, nitrate ions were transformed to nitrites at pH 12.5 with a selectivity as high as 93%; the rest was found in the form of ammonium ions. Liquid-phase nitrite hydrogenation to nitrogen in the second reactor unit packed with a Pd monometallic catalyst was conducted at low pH values of 3.7 and 4.5, respectively. Although these values are well below the pHpzc of examined catalyst (6.1), which assured that the nitrite reduction was carried out over a positively charged catalyst surface, up to 15% (23% in the presence of 5.0 g/l NaCl in the solution) of initial nitrite content was converted to undesired ammonium ions. Since a negligible amount of these species (below 0.5 mg/l) was produced at identical operating conditions over a powdered Pd/γ-Al2O3 catalyst, it is believed that the enhanced production of ammonium ions observed in the second fixed-bed reactor is due to the build-up of pH gradients in liquid-filled pores of spherical catalyst particles. Both Pd-Cu bimetallic and Pd monometallic catalysts were chemically resistant in the investigated range of pH values.