The selection of a nitrogen precursor for the construction of N-doped titanium dioxide with enhanced photocatalytic activity for the removal of gaseous toluene

Abstract

The preparation of nitrogen-doped TiO2 (i.e., N–TiO2) catalysts is a highly effective option to improve the photocatalytic activity of TiO2. Nonetheless, relatively little is known about the effects of dopant precursors selected for their preparation with regard to the photocatalytic efficacy. In this study, three types of dopants are selected and used as N sources (urea (U), melamine (M), and aqueous ammonia (A)) for N–TiO2 samples with the name codes of NTU, NTM, and NTA, respectively. The photocatalytic efficacy of these N–TiO2 samples is examined against toluene in a packed bed flow reactor. Under optimal conditions (e.g., relative humidity (RH) = 20% and gas hourly space velocity (GHSV) = 1698 h−1), the superiority of NTA is evident over others with a quantum efficiency (QE) of 7.03 × 10−4 molecules photon−1, a space time yield (STY) of 1.38 × 10−4 molecules photon−1 mg−1, and a specific clean air delivery rate (SCADR) of 1148.8 L g−1 h−1. The analysis based on in-situ diffuse reflectance infrared Fourier transform spectroscopy and gas chromatography-mass spectrometry confirms the formation of several intermediates such as benzyl alcohol, benzaldehyde, benzoic acid, and alkane species through ring opening reactions. In addition, the prepared NTA photocatalyst exhibits the highest toluene photocatalytic degradation efficiency among all TiO2-based catalysts surveyed to date. Overall, this study offers as a valuable guideline for the development of advanced TiO2 catalytic systems (such as N–TiO2) for the treatment of aromatic hydrocarbons in indoor air.