Effective and selective electrocatalytic nitrate reduction to ammonia on urchin-like and defect-enriched titanium oxide microparticles

This work reported a facile approach to surface oxygen vacancy (OV)-enriched urchin-like TiO2 microparticles (U-TiO2), which were highly effective and durable in catalyzing selective nitrate reduction to ammonia (NO3RR). Specifically, the U-TiO2 delivered a mass activity of 1.15 min−1 mg−1catalyst, a low yield of toxic NO2−-N intermediate (≤0.4 mg/L) and an exceptional high NH3-N selectivity of 98.1% in treating 22.5 mg/L of NO3−-N under a potential of -0.60 V vs. RHE, outperforming most of the reported oxide-based catalysts. When comparing the performance of U-TiO2 with that of the solid amorphous TiO2 counterpart (A-TiO2) that had close particle size but more OV on surfaces, we identified that the OV was the reactive sites, but rather than its content, the NO3RR kinetics were primarily limited by the electron and mass transfer at U-TiO2/water interfaces. Accordingly, the superior performance of U-TiO2 to A-TiO2 could be ascribed to the hierarchical urchin-like structure in U-TiO2. The in-situ DEMS test revealed that the NO3RR on U-TiO2 followed a pathway of *NO3− → *NO2−→ *NO → *N → *NH → *NH2 → *NH3. We also demonstrated that the U-TiO2 could keep its robust performance under a wide NO3−-N concentration range and in the presence of some co-existing ions (such as Ca2+, Cl−, Mg2+). However, the presence of humic acid and CO32− in water slowed down the NO3RR on U-TiO2. This work provides a more fundamental insight into the OV-driven NO3RR process on TiO2, which should benefit for the development of efficient TiO2-based catalysts.