Visible light photocatalytic reduction of 4-Nitrophenol using CdS in the presence of Na2SO3

Kinetic study of the 4-Nitrophenol photooxidation and photoreduction reactions using CdS

“…The absorption band of the 4-NPhenolate decreased as the time progressed, while a new absorption band appeared at 300 nm, indicating the 4-Aminophenol (4-AP) formation [11]. Additionally, the presence of the absorption band at 230 nm (associated to the aromatic ring of the 4-AP) after the elimination of SO 3 2− ions in the final irradiated solution (dash line), confirms that the 4-NP was photoreduced to 4-AP during the UV light irradiation with CdS and in the presence of sulfite ions in anaerobic condition, as previously reported [17]. Similar behavior (degradation process at 4 mM and reduction process at 8 mM of Na 2 SO 3 ) was obtained when N 2 atmosphere (anaerobic condition) was used, since the sulfite concentration was also maintained during all the photocatalytic reaction.…”

“…The kinetic data such as the apparent rate constant (K app ) in both degradation and reduction reactions were obtained considering a pseudo zero order. The Langmuir-Hinshelwood kinetic model is usually applicable to describe the kinetics of 4-NP degradation or reduction on photocatalyst like CdS [12,17]. The disappearance rate (r) and the reactant concentration (C) are related to time (t):…”

“…Fig. 4 shows the UV-vis absorbance 3 2− ) and to the 4-NPhenolate ion formation, respectively, due to the alkaline conditions generated by the presence of sulfite ions [17]. During the photocatalytic reaction, the absorption band of the SO 3 2− ions decreases as the time progresses, suggesting that it is being oxidized to sulfate.…”

“…The consumption of sulfite ions may cause the modification of the pH to lower values of the irradiated solution and as a consequence it may cause the appearance of the absorption band at 315 nm (dash line in Fig. 4), associated to the 4-NP formation [11,17], thus establishing an equilibrium between the 4-NP and 4-NPhenolate species. However, the disappearance of the absorption band at 345 nm (isosbestic point, I.P.)…”

“…In contrast, the 4-NP can be eliminated by catalytic reduction by metallic nanoparticles (Au or Ag) deposited on different supports such as chitosan-coated Fe 3 O 4 , SBA-15, Halloysite nanotubes or ZnO [10][11][12][13][14], in the presence of the sodium borohydride (NaBH 4 ), forming 4-aminophenol (4-AP), which is considered as an important intermediate in the pharmaceutical industries for the manufacture of analgesic and antipyretic drugs, such as paracetamol, phenacetin, acetanilide and so on [15]. In addition, the 4-NP reduction by photocatalytic processes has been achieved with resin-supported dye under visible light [16] and alternatively, our group have reported the photocatalytic reduction of 4-NPhenolate with CdS in the presence of sodium sulfite (Na 2 SO 3 ) under visible light [17].…”