Abstract:This contribution is dedicated to the memory of Prof. Roger J. Griffin.Methylene Blue has a long history as a photochemical reagent and is known to undergo photofading on exposure to visible light in aqueous solution. Under aerobic conditions, photobleaching occurs by way of a two-step process involving intermediary formation of singlet oxygen. The first step is ascribed to regio-selective addition of singlet oxygen within the precursor complex. This geminate reaction ultimately leads to formation of the leuco… Show more
“…With no added TiO 2 in Figure a, we still observe a significant pseudo-first-order rate constant for MB dye decomposition upon UV irradiation. Under these conditions, the mechanism of MB dye photodegradation in aqueous solution has been established to occur through O 2 •– . When the same experiment is now conducted with a fixed amount (1 mg/100 mL of MB solution) of Ce 2 (CO 3 ) 3 ·8H 2 O and varying amounts of TiO 2 , a completely different behavior is observed, as shown in Figure b, and we discuss these data below in parts.…”
Section: Resultsmentioning
confidence: 91%
“…•− . 42 When the same experiment is now conducted with a fixed amount (1 mg/100 mL of MB solution) of Ce 2 (CO 3 ) 3 • 8H 2 O and varying amounts of TiO 2 , a completely different behavior is observed, as shown in Figure 4b, and we discuss these data below in parts.…”
While photoactive metal oxides such as TiO 2 find widespread use in paints and coatings as well as cosmetics and suncare products, they also generate reactive oxygen species (ROS), which degrade materials and are associated with humanhealth pathologies. Here, we demonstrate a robust and potent catalytic antioxidant consisting of earth-abundant cerium carbonate nanoparticles and micron-size Ce 2 (CO 3 ) 3 . 8H 2 O, which are characterized by powder X-ray diffraction and scanning nanobeam electron diffraction. When dispersed with photoactive metal oxides, these cerium carbonate catalysts decrease the photodecomposition rate of organic dyes and commercial pigment colorants in aqueous media by up to 820-fold, as well as in acrylic coatings. X-ray photoelectron spectroscopy and kinetic experiments support the same tandem catalysis mechanism of photoprotection when using both micron-size Ce 2 (CO 3 ) 3 . 8H 2 O and cerium carbonate nanoparticles. This mechanism involves ROS disproportionation (catalyzed by cerium carbonate) and H 2 O 2 decomposition (partially catalyzed by TiO 2 ) pathways, both of which cerium carbonate also catalyzes on its own, crudely mimicking the function of the cascade system of superoxide dismutase and catalase enzymes. When cerium carbonate nanoparticles were dispersed at 2 wt % in polymethylmethacrylate, the transparency of the polymer film was preserved and the photo-oxidative degradation of the polymer was prevented following UV irradiation at 254 nm, which otherwise resulted in the loss of optical properties and hydroxylation as characterized by ATR−FTIR spectroscopy, in the control polymer lacking cerium carbonate. Similar observations were made regarding color preservation in paint films comprising dye and insoluble commercial colorant pigments. The material chemistry associated with this photoprotection catalysis is subtle and emphasizes the importance of both Ce(III) and carbonate together, as both CePO 4 and Na 2 CO 3 are inactive. This emphasis is also apparent in comparisons of photoprotection catalysis with previously reported cerium oxide nanoparticles, which are significantly less active compared with Ce 2 (CO 3 ) 3 . 8H 2 O under the same conditions.
“…With no added TiO 2 in Figure a, we still observe a significant pseudo-first-order rate constant for MB dye decomposition upon UV irradiation. Under these conditions, the mechanism of MB dye photodegradation in aqueous solution has been established to occur through O 2 •– . When the same experiment is now conducted with a fixed amount (1 mg/100 mL of MB solution) of Ce 2 (CO 3 ) 3 ·8H 2 O and varying amounts of TiO 2 , a completely different behavior is observed, as shown in Figure b, and we discuss these data below in parts.…”
Section: Resultsmentioning
confidence: 91%
“…•− . 42 When the same experiment is now conducted with a fixed amount (1 mg/100 mL of MB solution) of Ce 2 (CO 3 ) 3 • 8H 2 O and varying amounts of TiO 2 , a completely different behavior is observed, as shown in Figure 4b, and we discuss these data below in parts.…”
While photoactive metal oxides such as TiO 2 find widespread use in paints and coatings as well as cosmetics and suncare products, they also generate reactive oxygen species (ROS), which degrade materials and are associated with humanhealth pathologies. Here, we demonstrate a robust and potent catalytic antioxidant consisting of earth-abundant cerium carbonate nanoparticles and micron-size Ce 2 (CO 3 ) 3 . 8H 2 O, which are characterized by powder X-ray diffraction and scanning nanobeam electron diffraction. When dispersed with photoactive metal oxides, these cerium carbonate catalysts decrease the photodecomposition rate of organic dyes and commercial pigment colorants in aqueous media by up to 820-fold, as well as in acrylic coatings. X-ray photoelectron spectroscopy and kinetic experiments support the same tandem catalysis mechanism of photoprotection when using both micron-size Ce 2 (CO 3 ) 3 . 8H 2 O and cerium carbonate nanoparticles. This mechanism involves ROS disproportionation (catalyzed by cerium carbonate) and H 2 O 2 decomposition (partially catalyzed by TiO 2 ) pathways, both of which cerium carbonate also catalyzes on its own, crudely mimicking the function of the cascade system of superoxide dismutase and catalase enzymes. When cerium carbonate nanoparticles were dispersed at 2 wt % in polymethylmethacrylate, the transparency of the polymer film was preserved and the photo-oxidative degradation of the polymer was prevented following UV irradiation at 254 nm, which otherwise resulted in the loss of optical properties and hydroxylation as characterized by ATR−FTIR spectroscopy, in the control polymer lacking cerium carbonate. Similar observations were made regarding color preservation in paint films comprising dye and insoluble commercial colorant pigments. The material chemistry associated with this photoprotection catalysis is subtle and emphasizes the importance of both Ce(III) and carbonate together, as both CePO 4 and Na 2 CO 3 are inactive. This emphasis is also apparent in comparisons of photoprotection catalysis with previously reported cerium oxide nanoparticles, which are significantly less active compared with Ce 2 (CO 3 ) 3 . 8H 2 O under the same conditions.
“…The MB dye is degraded in water by ROS species and, then, carbon dioxide is released as a reaction product [32] , [34] , [37] . The uncoated CF sample indicates photobleaching of methylene blue under irradiation, involving 60% photodegradation efficiency of the MB dye due to the auto-generation of singlet oxygen [69] , [70] . Fig.…”
“…MB produces singlet oxygen ( 1 O 2 ) with a good quantum yield (Φ Δ 0.60 in CH 3 CN, Rossi et al, 2008) and has been applied in 1 O 2 photooxidation reactions on several occasions (Nilsson et al, 1972;Matheson et al, 1975;Jahnke and Frenkel, 1978;Cocquet et al, 2000;Oelgemöller et al, 2006;Lancefield et al, 2012;Schachtner et al, 2016). The main drawback of MB is its tendency to agglomerate and its intermolecular reactivity leads to the formation of leuco-forms which account for a low stability in solution (Nassar et al, 2019). Building on our previously developed non-covalent immobilization approach of cationic dyes on SiO 2 , we succeeded in dispersing MB on micro-and nano-SiO 2 particles by simply mixing the two components together in water (Figure 1A).…”
Section: Resultsmentioning
confidence: 99%
“…It is also available with very high purity which is useful for studying its photochemical properties. However, it is known to be relatively unstable and therefore it represents an interesting model for understanding which reaction parameters affect PC stability the most (Nassar et al, 2019).…”
Methylene blue was efficiently immobilized on silica micro- and nanoparticles by electrostatic interactions and the performances of the heterogenized photocatalysts were compared against the homogeneous conditions using the photooxidation of citronellol as a model reaction under red light, in a batch and a continuous flow photochemical reactor. In batch, the heterogeneous photocatalyst outperforms the homogeneous one, presumably due to kinetic and stability effects. The two catalytic systems are also compared in a flow reactor displaying improved mass transfer properties. We demonstrate that this results in a dramatic enhancement in photocatalyst stability, reactivity and productivity. This study highlights the importance of photocatalyst stability under homogeneous versus heterogenized conditions and in batch versus flow photochemistry.
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