Tandem Catalytic Antioxidant Nanoparticles Comprising Cerium Carbonate and Photoactive Metal Oxides

Abstract

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.

Uniform titania-supported Ce(iii) carbonate cluster catalysts for degradation of reactive oxygen species

“…2 and 3. 14,[33][34][35][36] Given the unit-cell dimensions for cerium carbonate, 37 we surmise that the deposited clusters are about two unit-cell layers thick. Under this scenario, one of these is interacting with the support, whereas the other is an overlayer on top.…”

“…29 Based on the similarity of our synthesis conditions to those that we have previously used to synthesize Ce(III) carbonate as well as the known stabilization of the Ce(III) valence state by carbonate ligands, these Ce-rich domains are hypothesized to consist of Ce(III) carbonate, which is highly insoluble in water. 14,32 We characterized the mixed carbonate material using ATR-FTIR spectroscopy. We observe a broad band centered around 1400 cm −1 , with full width at half max of more than 75 cm −1 , which reects the heterogeneity in the mixed carbonate material (see Fig.…”

“…This frequency range coincides with previously reported bands in carbonate materials. 14,33 In addition, we characterized the cerium-rich domains of the material using electron energy loss spectroscopy (EELS). EELS data in Fig.…”

“…13 Recently, we demonstrated highly active tandem photoprotection catalysts based on a combination of Ce(III) carbonate and TiO 2 . 14 In our approach, Ce(III) carbonate takes on the role of a SOD mimic, whereas TiO 2 functions as a catalase mimic. The SOD activity of Ce(III) carbonate is related to the function of small cerium-oxide nanoparticles in the literature, which are surface-enriched in Ce(III) rather than Ce(IV).…”

“…Previously, when using Ce(III) carbonate and TiO 2 as tandem catalytic antioxidants, both of these insoluble solids were dispersed in aqueous suspension as separate particles. 14 In that conguration, the ROS generation and catalase-mimetic functions of TiO 2 are spatially segregated from the SOD mimetic function of the Ce(III) carbonate. We posited that if instead the Ce(III) carbonate and TiO 2 could be synthesized with close spatial proximity, the resulting tandem catalysts could potentially function with higher efficiency.…”