Efficient photocatalytic degradation of PFOA in N-doped In2O3/simulated sunlight irradiation system and its mechanism

Abstract

In this work, the kinetics and mechanisms of photocatalytic degradation of perfluorooctanoic acid (PFOA) by nitrogen-doped indium trioxide (N-doped In2O3 or NH4NO3-In2O3) under Xe lamp irradiation were systematically investigated. Nitrogen originating from various inorganic and organic nitrogen compounds was doped into In2O3 to examine the catalytic effect. The NH4NO3-In2O3 catalyst was characterized by SEM, TEM, FT-IR, UV-DRS, BET surface area, XPS, and XRD, and showed the best catalytic performance. After 18 h of irradiation, PFOA was almost completely degraded with addition of 5 mg catalyst, while it was hardly degraded by simulated sunlight alone. The low photocatalytic degradation efficiency of PFOA was observed in different natural waters, which was due to the inhibitory effect of high pH and common anions like SO42−. The calculation results of Materials Studio showed that a shoulder-to-shoulder π system between nitrogen atom and oxygen atom (NO π*) was formed after nitrogen doping into interstitial sites of In2O3, therefore resulting in a reduction in band gap energy. Occupied NO π* states were responsible for the red shift, while unoccupied NO π* levels restrained the recombination of electrons (e−) and holes (h+). Nitrogen doping reduced the excitation energy for electronic transition, and thus accelerated the reaction. According to mass spectrometry analysis, decarboxylation and elimination were still the main photodegradation pathways of PFOA. To the best of our knowledge, this report is the first to explore the reaction mechanisms of PFOA in an N-doped In2O3/Xe lamp irradiation system.