Highly Efficient RuO_x/NbO_x-ZrO_x Catalysts for Ammonia Removal via Tuning Acidic Active Species

Abstract

NH3 elimination is highly desired during hazy pollution periods. However, the enhanced low-temperature activity and in-depth understanding of the reaction mechanism remain a tough challenge for NH3 selective catalytic oxidation (NH3–SCO) to N2. Here, a zirconium–niobium composite oxide-supported 0.2 wt % RuOx component (R/Z-N) was synthesized and systematically investigated. Compared with its counterparts (RuOx/ZrO2 and RuOx/Nb2O5), R/Z-N presented superior NH3 removal performance, obtaining complete conversion of NH3 at 300 °C and 90% N2 selectivity at a wide temperature window of 175–300 °C, followed by a lower apparent activation energy (79.2 kJ/mol). Thorough structural and surface property analysis revealed that the unique amorphous Nb2Zr6O17 structure was formed over R/Z-N through strong zirconia–niobium oxide interaction, resulting in its highest fraction of medium strong acid, surface adsorbed oxygen, and RuO2 catalytic active sites among these samples. Detailed situ DRIFTS studies further revealed that NH4+ species adsorption on Bro̷nsted acid sites exhibited higher activity compared to NH3 on Lewis. Thus, the plentiful Bro̷nsted acid sites over R/Z-N could accelerate the activation and dehydrogenation of NH3, further promoting the fast NH3–SCO process via the i-SCR mechanism, thereby improving activity. This work could guide the development of a high-efficiency catalyst for NH3 purification via tuning acidic active species.