A Study of Synthesis & [and] Performance of Flame-Made Semiconducting BiVO4 Nanoparticles for the Photocatalytic Degradation of Aqueous Organic Pollutants

Semiconductor photocatalysis is an Advanced Oxidation Technology that uses light as a source of energy to accelerate the degradation of harmful organic pollutants. Current research addresses the development of photocatalysts capable of harvesting visible light (Chapter 1). Bismuth vanadate (BiVO4) is a semiconductor that can absorb visible light up to 500 nm. It has been successfully tested for the oxidation of water and some dyes. So far, this material has been synthesized with low specific surface areas (SSA). Flame Spray Synthesis is a cost-effective method for the synthesis of nanoparticles and can be easily scaled up. The Flame Spray Synthesis of BiVO4 nanoparticles is described in detail. Careful adjustment of the process parameters allowed to control particle size and crystallinity of the as-prepared BiVO4 powders. Temperature of the collection site was found to hold a critical role on the control of these two properties (Chapter 3). Photocatalytic activity of the as-synthesized powders was then evaluated for the degradation of the Methylene Blue dye under visible-light irradiation. Photocatalyst properties controlled by synthesis parameters were confronted with their photocatalytic activity. It was found that both crystalline and amorphous powders could carry out the N-demethylation of Methylene Blue. Higher SSA lead to higher activity. A crystalline sample with moderate SSA was also found to degrade phenol to some extent at acidic pH but was subject to deactivation. Addition of different electron acceptors improved its photocatalytic activity. In particular, hydrogen peroxide (H2O2) could be used as a powerful yet rapidly consumed electron scavenger that enabled the degradation of phenol and dichloroacetate at neutral pH (Chapter 4). Advanced electrochemical and photochemical methods gave evidence of the poor reducing power of conduction-band electrons of BiVO4. Besides, the presence of deep traps associated with the change of coordination geometry of vanadium surface atoms upon photoexcitation has been correlated with the apparent photocatalytic activity of flame-made BiVO4 at pH 2. Finally, a mechanism involving the regneration of MB after its reduction when used as an electron acceptor has been proposed (Chapter 5). The flame spray synthesis of BiVO4 might be developed to produce high-SSA amorphous photocatalysts for the selective N-demethylation of Methylene Blue. Regarding the development of novel photocatalysts, the understanding of the role of the atomic surface structure should become a priority to increase their activity.

On the photocatalytic degradation of phenol and dichloroacetate by BiVO4: The need of a sacrificial electron acceptor

Nikola Castillo, Laura Ding, Andre Heel, César Pulgarin

The photodegradation of phenol and dichloroacetic acid (DCAA) by \ce{BiVO4} was studied in the absence as well as presence of selected electron scavengers. The experiments were performed under the visible (vis) irradiation of aqueous solutions over a wide pH range (1--13). Phenol was photocatalytically degraded by \ce{BiVO4} into \emph{p}-benzoquinone below pH~3 and into an open-ring structure at pH~13. Methylene blue (MB) accelerated the reaction below the isoelectric point of \ce{BiVO4} and did not undergo significant degradation. In presence of \ce{H2O2}, phenol was rapidly degraded up to pH~9. The degradation rates are two orders of magnitude higher than in absence of electron scavenger. The degradation of dichloroacetic acid was only possible in presence of \ce{H2O2}. High initial concentrations of \ce{H2O2} inhibit the reaction and its consumption is very fast. Sequential additions of this sacrificial electron acceptor (SEA) enables the total degradation of a 1~mM DCAA solution.

Elsevier2010

Accelerated methylene blue (MB) degradation by Fenton reagent exposed to UV or VUV/UV light in an innovative micro photo-reactor

Juan Kiwi, Mengkai Li, César Pulgarin

This study presents the accelerated discoloration/degradation of methylene blue (MB) in solution by Fenton reagent under exposure to ultraviolet (UV) or Vacuum-UV/UV (VUV/UV) light in an innovative micro photo-reactor. The MB degradation was kinetically faster when using VUV/UV light at 254/185 nm compared to UV-irradiation at 254 nm. Oxidative radicals produced by the photo-Fenton process were identified with appropriate scavengers. The addition of benzoquinone (BQ) at millimolar (mM) concentrations to MB solutions precluded completely the MB photo-induced bleaching, while tert-butanol hindered to a lesser extent the MB degradation suggesting that HO2 center dot was the predominant intermediate leading to MB degradation. The VUV/UV micro photo-reactor comprised mercury resonance lines at 185 and 254 nm. The photon percentages absorbed by water were estimated to be 16.8% at 254nm and 78.3% at 185 nm by water, while those absorbed by H2O2 were 0.9% and 2.2%, correspondingly. The solution parameters affecting the features of MB degradation, such as MB, Fe, and H2O2 concentrations, were explored and reaction mechanism was proposed. The lifetimes of (OH)-O-center dot and HO2 center dot were estimated to be 2 ns and 0.38 s under an optimized solution with 0.016 mM MB, 0.147 mM H2O2 and 0.05 mM Fe3+. Moreover, an estimation of the mean free paths of these radicals in solution provided the evidence that it was the radical lifetimes and mean-free paths, not their oxidation potentials that controlled the MB degradation kinetics. This study shows the potential of this VUV/UV assistant photo-Fenton process for the degradation of diluted organic compounds in aqueous solution. (C) 2016 Elsevier B.V. All rights reserved.

Elsevier2016

Flame-assisted synthesis of nanoscale, amorphous and crystalline, spherical BiVO4 with visible-light photocatalytic activity

Nikola Castillo, Andre Heel, César Pulgarin

The synthesis of bismuth vanadate (BiVO4) nanoparticles has drawn considerable attention to their application as a visible-light-driven photocatalyst. Several techniques are addressing the enlargement of surface area, but some of them can cause impurities and lower the performance of the material. In this work, flame spray synthesis technique was used as a simple, easy upscalable technique to produce BiVO 4 powders. The effect of process parameters on particle properties such as size, morphology and crystallinity were investigated by several techniques: BET, XRD, DSC, TEM, DRS and ? potential. Spherical BiVO 4 nanoparticles with either amorphous or monoclinic phase-pure crystal structure, along with specific surface areas (SSA) between 10 and 75 m2 g-1 were obtained by a setup including an in situ crystallization step at T ? 270? on the powder collection site, sometimes followed by a mild or severe annealing post-treatment. Traditional synthesis routes usually require such annealing post-treatment which implies considerable loss of SSA. The photocatalytic activity of the as-prepared powders was investigated by the degradation of the cationicdye methylene blue (MB). N-demethylation of the dye was clearly identified as one of the degradation pathways, while ring cleavage was only observed with crystalline samples. Crystallinity and SSA were crucial parameters for the photocatalytic activity of different samples of BiVO 4 depending on the pH of the solutions. The control of these two parameters during the synthesis of BiVO4 qualifies this method for a potential large scale production.

Elsevier2010