Thiago L.R. Hewer

Ag3PO4 sunlight-induced photocatalyst for degradation of phenol

Micrometric particles of Ag3PO4 were prepared by a simple precipitation method, and the as-prepared material was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FEG-SEM), nitrogen sorption isotherms, Raman spectroscopy and UV-visible diffuse reflectance spectrophotometry. Ag3PO4 particles showed excellent catalytic activity for degradation of phenol using simulated sunlight irradiation (Xenon lamp). After one hour of treatment, approximately 75% of phenol was completely mineralized. Additional experiments were conducted in order to evaluate the influence of dissolved oxygen in the photocatalytic process. During photodegradation assisted by Ag3PO4, it was considered that oxygen production and phenol oxidation were competitive processes.

Cério: propriedades catalíticas, aplicações tecnológicas e ambientais

Cerium based-compounds have great importance in a wide range of technological applications, such as: fuel cell devices development; metallurgic processes, petroleum refining; glass and ceramic production. Recently, its catalytic properties have been also explored for environmental applications, especially those to prevent or to control atmospheric and water pollution. Subjects covered in this work include a brief description of the fundaments of cerium catalytic properties and some relevant technological applications. Special attention is given to its photocatalytic activity and its ability to degrade pollutants. Recent results and future prospect about these applications are also evaluated.

One-pot green synthesis of cerium oxide–carbon microspheres and their catalytic ozonation activity

Monodisperse carbon microspheres surrounded by CeO2 nanoparticles were prepared by a one-pot green hydrothermal method. The hybrid microspheres were synthesized using a mixed D(+)-glucose and Ce(NO3)3 aqueous solution with subsequent hydrothermal treatment at 160 °C for 12 h. The as-prepared material was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. The results showed that microsphere growth is catalyzed by Ce3+ ions, for the same hydrothermal treatment period the diameters of hybrid microspheres were 5 μm, 25 times higher than for pure carbon spheres. These hybrid materials exhibited excellent catalytic activity in salicylic acid (SA) degradation by ozone. Ozonation studies showed when the CS20Ce microsphere was used as a catalyst, the mineralization rate of AS reached 80% after 60 minutes of treatment. Cerium oxide–carbon microspheres catalytic activity was attributed to the synergic effect of the carbon and CeO2 interface structures that were capable of enhancing radical species generation in the ozonation process.

Synthesis and characterization of silver phosphate/calcium phosphate mixed particles capable of silver nanoparticle formation by photoreduction

Silver phosphate is a semi-conductor sensitive to UV-Vis radiation (<530nm). Exposure to radiation removes electrons from the oxygen valence shell, which are scavenged by silver cations (Ag+), forming metallic silver (Ag0) nanoparticles. The possibility of silver nanoparticle formation in situ by a photoreduction process was the basis for the application of mixed calcium phosphate/silver phosphate particles as remineralizing and antibacterial fillers in resin-based dental materials. Mixed phosphate particles were synthesized, characterized and added to a dimethacrylate resin in 20% or 30% mass fractions to investigate their efficacy as ion-releasing fillers for dental remineralization and antibacterial activity. The formation of metallic silver nanoparticles after exposure to visible radiation from a dental curing unit (peak emission: 470nm) was demonstrated by particle X-ray diffraction and scanning electron microscopy analysis of the composite fractured surface. Calcium and phosphate release from materials containing the mixed particles were similar to those containing pure CaP particles, whereas Streptococcus mutans colonies were reduced by three orders of magnitude in relation to the control, which can be attributed to silver release. As expected, the optical properties of the materials containing mixed phosphate particles were compromised by the presence of silver. Nevertheless, materials containing mixed phosphate particles presented higher fracture strength and elastic modulus than those with pure CaP particles.

Ion-releasing dental restorative composites containing functionalized brushite nanoparticles for improved mechanical strength

OBJECTIVES This study describes the synthesis of brushite nanoparticles (CaHPO4·2H2O) functionalized with triethylene glycol dimethacrylate (TEGDMA) and their application in dental restorative composites with remineralizing capabilities. METHODS Nanoparticles were synthesized, with TEGDMA being added to one of the precursor solutions at three different molar ratios (0:1, 0.5:1 and 1:1, in relation to the ammonium phosphate precursor). Then, they were added (10 vol%) to a photocurable dimethacrylate matrix containing 50 vol% of reinforcing glass particles. The resulting composites were tested for degree of conversion, biaxial flexural strength and elastic modulus (after 24h and 28days in water), and ion release (over a 28-day period). Commercial composites (one microhybrid and one microfilled) were tested as controls. RESULTS The final TEGDMA content in the functionalizing layer was modulated by the molar ratio added to the precursor solution. Functionalization reduced nanoparticle size, but did not reduce agglomeration. Improved mechanical properties were found for the composite containing nanoparticles with higher TEGDMA level in comparison to the composite containing non-functionalized nanoparticles or those with a low TEGDMA level. All brushite composites presented statistically significant reductions in strength after 28 days in water, but only the material with high-TEGDMA nanoparticles retained strength similar to the microhybrid commercial control. Overall, ion release was not affected by functionalization and presented steady levels for 28 days. SIGNIFICANCE Though agglomeration was not reduced by functionalization, the improvement in the matrix-nanoparticle interface allowed for a stronger material, without compromising its remineralizing potential.

A comparison between the four Geldart groups on the performance of a gas-phase annular fluidized bed photoreactor for volatile organic compound oxidation

Heterogeneous photocatalytic oxidation (PCO) is a widely studied alternative for the elimination of volatile organic compounds (VOC) in air. In this context, research on novel photoreactor arrangements to enhance PCO rates is desired. Annular fluidized bed photoreactors (AFBPR) have yielded prominent results when compared to conventional thin film reactors. However, very few works aimed at optimizing AFBPR operation. In this study, TiO2 photocalytic agglomerates were synthesized and segregated in specific size distributions to behave as Geldart groups A, B, C, and D fluidization. The TiO2 agglomerates were characterized by XRD, FTIR spectra, and N2 adsorption. Photocatalyst performances were compared in a 10-mm gapped AFBPR for degrading the model pollutant methyl-ethyl-ketone (MEK), using a 254-nm radiation source. Geldart group C showed to be inadequate for AFBPR operation due to the short operation range between fluidization and elutriation. In all the cases, photocatalytic reaction rates were superior to sole UV photolysis. Group A and group B demonstrated the highest reaction rates. Considerations based on mass transfer suggested that the reasons were enhanced UV distribution within the bed at lower flow rates and superior catalyst surface area at higher flow rates. Results also revealed that groups A, B, and D perform equally per catalyst area within an AFBPR if the fluidization numbers (FN) are high enough.