Uncertainties in the Quantification of Photocatalytically Produced Ammonia: A Case Study of Overall Nitrogen Fixation over Defect-Rich Mo-Doped Sb2WO6 Nanobelts

Abstract

Photocatalytic nitrogen fixation is a potentially viable approach to meet the enormous demand for N-based chemicals such as ammonia and nitrates. However, light-driven nitrogen fixation is a complex reaction from both kinetic and thermodynamic perspectives. Also, there has not been much emphasis on the simultaneous production of ammonia and nitrate. In addition, there is much debate regarding the correctness of the quantification of photogenerated NH₃. In this work, we reported the overall nitrogen fixation over the surface of defect-rich Mo-doped Sb₂WO₆ nanobelts without the use of an electron or hole scavenger. One of the samples (SWMO0.1) with an optimum amount of Mo was found to possess the best catalytic activity for ammonia generation with a yield rate of 24 μmol g^–1 h^–1 as compared to the pristine Sb₂WO₆ sample (4 μmol g^–1 h^–1). Also, the solar to ammonia (STA) efficiency was found to be 0.035%. The photogenerated ammonia concentration was quantified using indophenol, Nessler’s reagent, and ion chromatography, and a detailed comparison was made to examine the reliability of these methods. The ammonia concentration measured using ion chromatography was much lower than that measured using the other two calorimetric methods (indophenol and Nessler’s reagent), indicating uncertainties in the accurate determination of ammonia concentration. Further, nitrate and nitrite species formed were also quantified. Thus, this work provided insights into the uncertainties in the quantification of photocatalytically produced ammonia using Mo-doped Sb₂WO₆ nanobelts as catalysts.