Increasing reactivity of plasmonic hot holes by a trapping strategy

Plasmonic photocatalysis has emerged as a promising solution for global energy crisis and environment pollution by facilitating wide ranging chemical transformations using photons in a broad region of solar spectrum. Despite numerous successful examples on improvement of electron-driven photochemistry, effective utilization of plasmonic hot holes is a long-standing challenge due to their ultrafast relaxation and short lifetime. Herein, we report that the reactivity of plasmonic hot holes can be greatly enhanced by a novel hot hole trapping strategy. We demonstrate a new concept of a metal-adsorbate interfacial structure that can be in situ constructed on gold (Au) surface in the presence of molecular hydrogen (H2) under plasmonic excitation, where the key is to employ an electron-filled antibonding state hybridized by H1s and Au5d as a localized “trap” to improve utilization efficiency of plasmonic hot holes. This interfacial structure is evidenced by light-induced H2 spillover and d-band model analysis. The prolonged lifetime and preserved oxidation power of plasmonic hot holes was evidenced by superior photocatalytic activity for methylene blue (MB) degradation in the presence of H2 which was accelerated by over 5 times. In addition, FTIR coupled with CO molecular probe reveals that the physical location of hole trapping are low coordinated positions sites on Au nanoparticles. These findings could provide an innovative pathway to increase utilization efficiency of hot holes for visible-light-driven photocatalysis applications

Modification of bituminous coal by air oxidation to increase ammonia capture

The development of low-cost animal bedding materials for ammonia (NH3) mitigation in livestock facilities is highly desirable from both environmental and waste management perspectives. In this study, we developed an air oxidation method as a simple but effective approach to increase the NH3 adsorption capacity of relatively ubiquitous bituminous coal (BC). The BC samples were oxidized in air at a range of temperatures, from 100 to 550 °C, and the highest NH3 adsorption was observed from the BC modified at 300 °C, with an 11-fold increase (49.7 mg g−1) compared with the initial BC. Samples were characterized using microscopy, BET, FT-IR, XPS, and Boehm titration. The results indicated that NH3 adsorption was dominated by the acidic surface functional groups generated by air oxidation. Ammonium ion (NH4+) adsorption was described by the Langmuir adsorption isotherm and the modified BC retained 5 times more NH4+ than the initial BC. These findings provide insights for the modification of coal materials for the capture of NH3 by simple air oxidation and show promise for future utilization of modified BC for mitigation of NH3 emissions from intensive livestock systems

Lignite ammonia adsorption and surface chemistry after dewatering

Ammonia (NH3) emissions from intensive cattle feedlots can be significantly reduced by the application of lignite to the bedding materials. Unfortunately, the high-water content of lignite leads to high transportation costs that preclude the use of this otherwise low-cost and efficient NH3 absorbing material. Efficient dewatering of lignite is required so that its transportation from mining sites to feedlots is economically feasible. It is important that dewatering of lignite does not decrease the NH3 capture capacity or increase potential safety hazards. This work evaluates the effects of an aerobic drying on dewatering, NH3 adsorption capacity, risk of spontaneous combustion, and the underlying surface chemistry of lignite. We found that aerobic dewatering at 200 °C decreased the lignite water content from 61.6% to 4.2% with reduced risk of spontaneous combustion, while ammonium (NH4+) and NH3 adsorption by lignite increased significantly by 65.8% and 28.8%, respectively. Chemical characterization indicated that these enhancements originated from partial oxidation of the dried lignite surfaces. The linear correlation between NH4+ adsorption and the concentration of oxygen-containing functional groups provides insight into the surface chemistry of NH3 adsorption, which is critically important for designing efficient lignite-based feedlot bedding materials

Tunable selectivity of radical generation over TiO2 for photocatalysis

The realization of controllable radical generation through structural modification of photocatalyst is a challenging goal and is an important strategy for environmental remediation and noncomplete and selective photo-oxidation. Here, we control structural composition of TiO2 through crystalline modification and use photocatalytic dye degradation as a model system to investigate photocatalytic details. Importantly, modified TiO2 materials exhibit tunable mechanism pathway towards photocatalytic decomposing methylene blue (MB) monitored by radical trapping experiments. The anatase-rich TiO2 heterojunction shows preferential hole-mediated decomposition pathway in comparison with superoxide and hydroxyl radicals. In contrast, increasing ratio of rutile in TiO2 favors superoxide-facilitated MB decomposition over hole. The surface chemistry of specific surface atomic configuration is a key factor in tuning the capability of oxygen reduction and hole trapping, resulting in photocatalytic selectivity of radical generation towards photo-oxidation

Highlighting unique function of immobilized superoxide on TiO2 for selective photocatalytic degradation

Selective photocatalytic degradation reactions hold great promise for environmental control by utilizing clean and inexhaustible solar energy, but the selectivity and tunability due to uncontrollable oxidation process remains a challenge. Given the photogenerated reactive oxygen species (ROS) as major oxidants in selective degradation systems, we propose that desirable selectivity for organic pollutant degradation can be achieved by adjusting the corresponding photocatalytic radical production processes. Using the anatase-rich and rutile-rich titanium dioxide (TiO2) with methyl orange-methylene blue (MO-MB) dye aqueous mixture as a model system, we investigate tunability and mechanism details for selective degradation via photocatalytic evaluation and trapping experiments. Benefiting from the selective generation of ROS on rutile-rich TiO2 and unique properties of immobilized superoxide, the photocatalysts display outstanding tunability and selectivity. This work provides insights into the actual function of immobilized superoxide on selective photocatalytic degradation reactions by discussing a plausible rational reaction process

Adsorbent materials for ammonium and ammonia removal: A review

Ammonium (NH4+) and ammonia (NH3) are notorious hard-to-treat pollutants, leading to serious deterioration of aquatic ecosystems and significant risks to human health. While adsorption is a promising method to tackle this problem, finding suitable adsorbent materials which are abundant, low-cost and efficient remains a constant challenge. Thus, this review summarizes recent development of important adsorbent materials implemented for NH3/NH4+ removal. Advantages and disadvantages of representative adsorbent materials including bentonite, zeolite, clay, biochar, activated carbon, metal organic framework and their modified forms are compared, and the nature of their adsorption processes are discussed in context of adsorption sites, isotherm models (e.g. Langmuir and Freundlich), kinetic equations (e.g. pseudo-first order, pseudo-second order and intra-particle diffusion) and thermodynamic analysis. Future perspective on the utilization of inexpensive lignite is also conferred. Although both conventional and nanostructured materials face challenges regarding economic cost, energy consumption, secondary pollution and adsorption efficiency, these can be tackled by adopting various of advanced options. Current research on adsorption mechanisms forms a solid basis for the design and development of novel adsorbent materials. We speculate that the pursuit of strategies for effective surface modification of natural abundant resources will lead to a bright future of removal processes suited to low NH3/NH4+ concentration conditions

Synergistic effect of aeration-assisted photocatalysis: Two parallel mechanism pathways for TXP-10 surfactant removal

TXP-10 is a commercial member of alkylphenol polyethoxylate surfactant family that is widely adopted as emulsifier, detergent and dispersing agent in agriculture and industry. However, it has adverse effects on environmental and public health due to the risk of eutrophication, oestrogenic activities and anti-precipitation properties. Here, we evaluated critical parameters and addressed the mechanism of aeration-assisted photodegradation of TXP-10 by using TiO2 particulates as model photocatalysts. It was found that O2 aeration could significantly enhance photocatalytic degradation efficiencies with a high synergetic index of 3.30 at an optimum rutile composition of 90 wt.%. Trapping experiments and superoxide radical quantification suggest two parallel mechanism pathways (O2-sensitive and -insensitive) dependent on phase composition of photocatalyst and irradiation intensity. Furthermore, the anti-precipitation property of TXP-10 solution during photocatalysis was evaluated and correlated to transformation of hydrophobic groups to hydrophilic derivatives. This work would gain important enlightenment into the mechanism of synergistic effect of aeration-assisted photocatalysis and its essential role in the environmental remediation

Lignite, dewatered lignite and modified subbituminous coal reduce nitrogen loss from broiler litter

Broiler litter is generated in large quantities as a waste by-product of chicken meat production. N may be lost from the litter and emitted from bird housing as gaseous NH3, which can be damaging to the environment and limit the recycling of a valuable nutrient. This study investigated the effect of lignite application rate (0, 5, 10, 15, 20%) on N loss from broiler litter in a static chamber laboratory incubation. Lignite was subsequently dewatered and subbituminous coal modified by aerobic thermal oxidation and their ammoniacal N adsorption potentials were characterised. In a second static chamber incubation, the capacity of these materials (applied at 20%) to reduce N loss from litter was investigated. Finally, their potential to directly reduce NH3 emissions was examined using a chamber acid trap system. This study showed that lignite reduced N loss when applied to litter at a rate ≥ 5%, with the amount of N retained increasing with increasing lignite application rate. Litter treated with 20% lignite retained 24% more N than untreated litter. Following aerobic thermal treatment, maximum ammoniacal N adsorption capacities of the materials were as follows: lignite > dewatered lignite > modified subbituminous coal > subbituminous coal. Despite inequalities in adsorption capacity, lignite, dewatered lignite and modified subbituminous coal reduced total N loss by 17.3, 18.2 and 18.4% and NH3 emissions by 41.6, 49.1 and 29.8%, respectively. This study demonstrates the potential of coal-based materials to reduce NH3 emissions from broiler litter and increase the nutrient value of waste by reducing N loss

Modified lignite and black coal reduce ammonia volatilization from cattle manure

Modified lignite and black coal (BC) are potential amendments for animal bedding to abate ammonia (NH3) emissions due to their large adsorption capacities for ammoniacal nitrogen (N). However, the ability of modified lignite and BC in reducing NH3 volatilization from livestock manure and the underlying mechanisms remain unknown. The present study has investigated the effect of lignite, modified lignite, BC and modified BC on NH3 volatilization from cattle manure, biological immobilization of manure ammoniacal N and manure properties. Modified lignite and BC reduced the NH3 volatilization from manure by 44 and 36%, respectively, which were comparable with original lignite (43%). The biological immobilization of applied stable isotope labelled 15N in lignite, modified lignite, BC and modified BC amended manures was 15, 18, 11 and 16%, respectively, which were significantly higher than that in unamended manure (4%, P 8.2). Our results highlight that the adsorption and immobilization of manure ammoniacal N induced by amendments are the key drivers in reducing NH3 loss from manure, outweighing the pH effect. The findings of this study provide new insights into the mechanisms of coal amendments reducing NH3 loss from animal manure and their potential applications in intensive livestock systems

Temperature effects on redox potentials and implications to semiconductor photocatalysis

Semiconductor photocatalysts facilitate solar energy conversions via reduction–oxidation (redox) reactions. This requires the bandgaps of semiconductors to straddle the redox potentials of half-reactions. Such inherent thermodynamic prerequisite greatly limits utilization of visible or near-IR light. Thus, controllable redox potentials are highly desirable to obtain favorable band alignment. Here a simple thermodynamic approach was proposed to quantitatively predict temperature dependent behavior of energetics of redox half-reactions. Distinguished trends were observed at a temperature range of 298.15–1500 K for representative photocatalytic processes: H2O splitting was relatively temperature insensitive than CO2 reduction and NH3 synthesis, and the shifts in negative or positive directions as a function of temperature depend on the Gibbs energy change of the reactions. Furthermore, thermodynamic and kinetic implications on photocatalysis were discussed based on band alignment and overpotential. Understanding those thermodynamic characteristics is important for prediction and manipulation of physiochemical properties for more efficient photocatalysis