Ultraviolet-A light/oligomeric melem vs. visible light/graphitic carbon nitride towards H2O2 photo-production and pollutants degradation: sometimes less is more

Currently, there is a convention on performing photocatalytic reactions with graphitic carbon nitride (g-C3N4) with visible light only, in spite of the synthesis efforts required to obtain acceptable results on its applications. Therefore, in this work, we have compared the photocatalytic performance, under UVA and visible light irradiation, of carbon nitrides prepared from melamine calcination at different temperatures, 350 – 650 °C, by means of H2O2 photo-production (with methanol as hole scavenger) and of photocatalytic phenol oxidation, respectively. The photocatalysts were characterised by several techniques, and Principal Component Analysis (PCA) was employed to highlight correlations among the huge amount of generated data and obtain rapid conclusions on the overall trend. Results demonstrated that the compound obtained from melamine calcination at 425 °C (oligomeric melem) under UVA irradiation led to an outstanding 2 mM h−1 H2O2 production rate in presence of 3 M of methanol, as well as the fastest phenol 100 μM oxidation rates (kobs = 4 × 10−2 min−1); higher polymerisation temperatures (i.e., lower band gap energies, C:N ≈ 3:4) resulted in lower photocatalytic behaviours under UVA irradiation. Furthermore, the PCA indicated that the presence of melem oligomers in any material was detrimental to observe high photocatalytic activities, being negligible in those with scarce polymerisation degree (melam oligomer) or high one (melon). Taking advantage of UV lamps and low molecular weight carbon nitride materials, we believe that a change in the literature focus should be done due to the here reported advantages of UVA/oligomeric melem system over the visible light/g-C3N4 one.