Photochemical generation of reactive intermediates from urban-waste bio-organic substances under UV and solar irradiation.

Singlet oxygen (1O2), hydroxyl radicals (•OH), and excited triplet states of organic matter (3OM*) play a key role in the degradation of pollutants in aquatic environments. The formation rates and quantum yields (Φ) of these reactive intermediates (RI) through photosensitized reactions of dissolved organic matter (DOM) have been reported in the literature for decades. Urban biowaste-derived substances (UW-BOS), a form of organic matter derived from vegetative and urban waste, have recently been shown to be efficient sensitizers in the photo-degradation of different contaminants. Nevertheless, no quantitative measurements of photo-oxidant generation by UW-BOS have been reported. In this study, the formation quantum yields of 1O2 and •OH, as well as quantum yield coefficients of TMP degradation (indicative of 3OM* formation), were quantified for two UW-BOS samples, under 254-nm UV radiation or simulated sunlight and compared to a DOM standard from the Suwanee River (SRNOM). Values of Φ for UW-BOS samples ranged from Φ(+1O2) = 8.0 to 8.8 × 10-3, Φ(+•OH) = 4.1 to 4.3 × 10-6, and f TMP = 1.22 to 1.23 × 102 L Einstein-1 under simulated sunlight and from Φ(+1O2) = 1.4 to 2.3 × 10-2, Φ(+•OH) = 1.3 to 3.5 × 10-3, and f TMP = 3.3 to 3.9 × 102 L Einstein-1 under UV. Although UW-BOS are not necessarily better than natural DOM regarding photosensitizing properties, they do sensitize the production of RI and could potentially be used in engineered treatment systems.

Singlet oxygen formation from wastewater organic matter.

Singlet oxygen ((1)O2) plays an important role in the inactivation of pathogens and the degradation of organic contaminants. The present study looks at the surface steady-state concentration of (1)O2 and quantum yields (ΦSO) for organic matter present in or derived from wastewater (WWOM), including those that are partially treated and after undergoing oxidation. The surface steady state concentrations of (1)O2 ranged from 1.23 to 1.43 × 10(-13) M for bulk wastewaters under simulated sunlight. The ΦSO values for these samples varied from 2.8% to 4.7% which was higher than the values observed for the natural organic matter isolates evaluated (1.6-2.1%). Size fractionation of WWOM resulted in ΦSO increases, with a value of up to 8.6% for one of the <1 kDa fractions. Furthermore, oxidation of WWOM by hypochlorous acid (HOCl) and molecular ozone also resulted in an increase in ΦSO, with the highest measured value being 9.3%. This research further explores the correlations between the photosensitizing properties of WWOM and optical characteristics (e.g., absorbance, E2:E3 ratio). Making use of easily measurable absorbance values, a model for the prediction of (1)O2 steady-state concentrations is proposed.

Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol.

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.

Shape-dependent catalytic properties of Pt nanoparticles.

Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al(2)O(3). Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis.

Solving the structure of size-selected Pt nanocatalysts synthesized by inverse micelle encapsulation.

The structure, size, and shape of gamma-Al(2)O(3)-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H(2) atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods.