Abstract
We have explored the impact of the incorporation of nanoporous carbons as additives to tungsten oxide on the photocatalytic degradation of two recalcitrant pollutants: rhodamine B and phenol, under simulated solar light. For this purpose, WO3/carbon mixtures were prepared using three carbon materials with different properties (in terms of porosity, structural order and surface chemistry). Despite the low carbon content used (2 wt. %), a significant increase in the photocatalytic performance of the semiconductor was observed for all the catalysts. Moreover, the influence of the carbon additive on the performance of the photocatalysts was found to be very different for the two pollutants. Carbon additives of hydrophobic nature increased the photodegradation yield of phenol compared to bare WO3, likely due to the higher affinity and stronger interactions of phenol molecules towards basic nanoporous carbons. Oppositely, the use of acidic carbon additives led to higher rhodamine B conversions due to increased acidity of the WO3/carbon mixtures and the stronger affinity of the pollutant for acidic catalyst’s surfaces. As a result, the photooxidation of rhodamine B is favored by means of a coupled (photosensitized and photocatalytic) degradation mechanism. All these results highlight the importance of favoring the interactions of the pollutant with the catalyst’s surface through a detailed design of the features of the photocatalyst.
Highlights
WO3 is a n-type semiconductor usually presenting a three-dimensional arrangement of slightly distorted corner-shared [WO6] octahedra derived from ideal cubic perovskite, which is responsible of its electrooptical, electrochromic, ferroelectric, and catalytic properties (Svensson and Granqvist, 1984; Cotton and Wilkinson, 1988; Kumar and Rao, 2015).It is considered an interesting material as a stable visible light driven photocatalyst for an efficient degradation of recalcitrant organic compounds due to its relatively large abundance, non-toxicity, physical and chemical resilience in harsh environments, and most importantly its strong absorption within the solar spectrum, and high oxidation power of the photogenerated holes (+3.1 −3.2 vs. NHE) due to a deep valence band
We have explored the role of carbon additives of varied properties on the performance and stability of WO3/carbon catalysts for the photooxidation of two recalcitrant pollutants of different characteristics, phenol and rhodamine B (RhB), in solution using simulated solar light
The incorporation of carbon additives did not modify the optical features of the semiconductor, as detected by UV-Vis diffuse reflectance spectroscopy (Figure 2), which is expected as the WO3/carbon catalysts were prepared by physical mixture of both components
Summary
WO3 is a n-type semiconductor usually presenting a three-dimensional arrangement of slightly distorted corner-shared [WO6] octahedra derived from ideal cubic perovskite (type ReO3), which is responsible of its electrooptical, electrochromic, ferroelectric, and catalytic properties (Svensson and Granqvist, 1984; Cotton and Wilkinson, 1988; Kumar and Rao, 2015).It is considered an interesting material as a stable visible light driven photocatalyst for an efficient degradation of recalcitrant organic compounds due to its relatively large abundance, non-toxicity, physical and chemical resilience in harsh environments, and most importantly its strong absorption within the solar spectrum (i.e., band gap between 2.4 and 2.8 eV), and high oxidation power of the photogenerated holes (+3.1 −3.2 vs. NHE) due to a deep valence band. Several strategies have been adopted to compensate for this limitation of WO3 as photocatalyst, including surface and interface modification, particle size and morphology control, composite and hybrid materials, transition/noble metal doping, surface sensitization (Ho et al, 2012; Wicaksana et al, 2014; Spurgeon et al, 2014), and the use of photoelectrochemical approaches based on the application of a bias potential or association with a photocathode – i.e., with a p-type semiconductor electrode – or a photovoltaic cell suited for the realization of a water splitting system under solar illumination (Gratzel, 2001; Bignozzi et al, 2013) All these approaches aim to increase the electron trapping, to inhibit the charge recombination, and to increase the selectivity of a particular product so as to improve the efficiency of photocatalytic processes. We have demonstrated the photochemical activity of certain nanoporous carbons due to their ability to promote the photochemical splitting of water (Ania et al, 2014; Velasco et al, 2014) and generate oxygen radical species capable of reacting with electron donors (i.e., pollutant in aqueous solution), boosting the photooxidation conversion (Velasco et al, 2012; Velasco et al, 2013a)
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