The effect of pH and zwitterionic buffers on catalytic nitrate reduction by TiO2-supported bimetallic catalyst

• TiO2/Cu/Pd catalyst showed complete nitrate reduction in 1 h.
• Catalyst prepared at low temperature showed higher nitrate removal.
• Organic buffers showed higher nitrate removal than inorganic buffers.
• Catalytic nitrate reduction decreased as suspension pH increased.
• TRIS buffer showed adverse effect on catalytic nitrate reduction.

We have investigated the effect of pH and zwitterionic buffers on catalytic nitrate reduction by TiO2-supported Cu–Pd catalyst (TiO2/Cu/Pd). TiO2/Cu/Pd prepared at 450 and 550 °C showed lower nitrate removal (76.6% and 30.8%, respectively) than that at 350 °C showing 100% removal, due to the sintering of its particles at high temperatures. Organic buffer systems (MES for pH 6 and CHES for pH 10) showed higher nitrate removal than inorganic buffer systems (phosphate for pH 6 and bicarbonate for pH 10). Nitrate reduction decreased from 100% to 72.0% as suspension pH increased from 6 to 10 of which range was kept by zwitterionic buffers (MES, MOPS, TRIS, TAPS, and CHES). Nitrate removal at pH 8 prepared with TRIS buffer dramatically dropped (57.8%) due to leaching of Cu (35.2%) from the catalyst surface, resulting in loss of catalytic sites for nitrate reduction. Nitrite selectivity dramatically increased from the pH value higher than 7, while selectivity of ammonium and nitrogen gas decreased. Nitrate removal at pH 8 prepared with TAPS buffer was much higher (91.7%) than that with TRIS buffer at the same pH confirming the adverse effect of TRIS buffer on catalytic nitrate reduction by TiO2/Cu/Pd.

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