The discrepancy of NH3 oxidation mechanism between SAPO-34 and Cu/SAPO-34

The difference of NH3 oxidation mechanism over SAPO-34 and Cu-SAPO-34 was studied. XRD (X-ray diffraction), SEM (scanning electron microscopy) and H2-TPR (H2-temperature programmed desorption) were conducted to estimate the Cu species distribution. The quantity of individual Cu2+ ions escalated with the elevation of silicon content in the Cu/SAPO-34 catalysts, leading to an enhancement in the activity of the NH3-SCR (ammonia-selective catalytic reduction) process. This augmentation in activity can be attributed to the increased presence of isolated Cu2+ species, which are pivotal in facilitating the catalytic reaction. In addition, the kinetic test of NH3 oxidation indicated that the CuO species were the active sites for NH3 oxidation. Specifically, the strong structural Brønsted acid sites were the NH3 oxidation active sites over the SAPO-34 support, and the NH3 reacted with the O2 on the Brønsted acid sites to produce the NO mainly. While the NH3 oxidation mechanism over Cu/SAPO-34 consisted of two steps: firstly, NH3 reacted with O2 on CuO sites or residual Brønsted acid sites to form NO as the product; subsequently, the generated NO was reduced by NH3 into N2 on isolated Cu2+ sites. Simultaneously, the isolated Cu2+ sites might demonstrate a significant function in the NH3 oxidation process to form N2. The identification of active sites and corresponding mechanism could deepen the understanding of excellent performance of NH3-SCR over the Cu/SAPO-34 catalyst at high temperature.

Enhancing Photocatalytic Properties of TiO2 Photocatalyst and Heterojunctions: A Comprehensive Review of the Impact of Biphasic Systems in Aerogels and Xerogels Synthesis, Methods, and Mechanisms for Environmental Applications

This review provides a detailed exploration of titanium dioxide (TiO2) photocatalysts, emphasizing structural phases, heterophase junctions, and their impact on efficiency. Key points include diverse synthesis methods, with a focus on the sol-gel route and variants like low-temperature hydrothermal synthesis (LTHT). The review delves into the influence of acid-base donors on gelation, dissects crucial drying techniques for TiO2 aerogel or xerogel catalysts, and meticulously examines mechanisms underlying photocatalytic activity. It highlights the role of physicochemical properties in charge diffusion, carrier recombination, and the impact of scavengers in photo-oxidation/reduction. Additionally, TiO2 doping techniques and heterostructures and their potential for enhancing efficiency are briefly discussed, all within the context of environmental applications.

A facile two-step hydrothermal preparation of 2D/2D heterostructure of Bi2WO6/WS2 for the efficient photodegradation of methylene blue under sunlight

A facile two-step hydrothermal method was successfully used to prepare a photocatalyst Bi2WO6/WS2 heterojunction for methyl blue (MB) photodegradation. Fabricated photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). Band gap measurements were carried out by diffuse reflectance spectroscopy (DRS). Results indicated that the prepared heterostructure photocatalyst has increased visible light absorption. Photocatalytic performance was evaluated under sunlight irradiation for methylene blue (MB) degradation as a model dye. Variations in pH (4-10), amount of catalyst (0.025-0.1 g/L), and initial MB concentrations (5-20 ppm) were carried out, whereas all prepared catalysts were used to conduct the tests with a visible spectrophotometer. Degradation activity improved with the pH increase; the optimum pH was approximately 8. Catalyst concentration is directly related to degradation efficiency and reached 93.56% with 0.075 g of the catalyst. Among tested catalysts, 0.01 Bi2WO6/WS2 has exhibited the highest activity and a degradation efficiency of 99.0% in 40 min (min) for MB. MB photodegradation follows pseudo-first-order kinetics, and obtained values of kapp were 0.0482 min-1, 0.0337 min-1, 0.0205 min-1, and 0.0087 min-1 for initial concentrations of 5 ppm, 10 ppm, 15 ppm, and 20 ppm, respectively. The catalyst was reused for six cycles with a negligible decrease in the degradation activity. Heterostructure 0.01 Bi2WO6/WS2 has exhibited a photocurrent density of 16 μA cm-2, significantly higher than 2.0 and 4.5 μA cm-2 for the pristine WS2 and Bi2WO6, respectively. The findings from these investigations may serve as a crucial stepping stone towards the remediation of polluted water facilitated by implementing such highly efficient photocatalysts.

Sunlight-Driven Generation of Hypochlorous Acid on Plasmonic Au/AgCl Catalysts in Aerated Chloride Solution

HClO is typically manufactured from Cl₂ gas generated by the electrochemical oxidation of Cl^- using considerable electrical energy with a large concomitant emission of CO₂. Therefore, renewable energy-driven HClO generation is desirable. In this study, we developed a strategy for stable HClO generation by sunlight irradiation of a plasmonic Au/AgCl photocatalyst in an aerated Cl^- solution at ambient temperature. Plasmon-activated Au particles by visible light generate hot electrons, which are consumed by O₂ reduction, and hot holes, which oxidize the lattice Cl^- of AgCl adjacent to the Au particles. The formed Cl₂ is disproportionated to afford HClO, and the removed lattice Cl^- are compensated by the Cl^- in the solution, thus promoting a catalytic HClO generation cycle. A solar-to-HClO conversion efficiency of ∼0.03% was achieved by simulated sunlight irradiation, where the resultant solution contained >38 ppm (>0.73 mM) of HClO and exhibited bactericidal and bleaching activities. The strategy based on the Cl^- oxidation/compensation cycles will pave the way for sunlight-driven clean, sustainable HClO generation.

In Situ Construction of Dye-Sensitized BiOCl/Rutile-TiO2 Nanorod Heterojunctions with Highly Enhanced Photocatalytic Activity for Treating Persistent Organic Pollutants

The construction of efficient and stable heterojunction photocatalysts with a controllable close contact interface and visible-light response is a challenging research topic in the field of photocatalysis. Herein, a series of BiOCl/rutile-TiO₂ (R-TiO₂) nanorod heterojunctions were constructed using R-TiO₂ nanorods as supporting frameworks followed by selective adsorption of Cl^- on R-TiO₂(110) facets and in situ growth of BiOCl on the surface of TiO₂ nanorods. The strong affinity of rhodamine B (RhB) as a photosensitizer for BiOCl allowed the prepared BiOCl/R-TiO₂ heterojunctions to work efficiently under visible-light irradiation. The dye-sensitized BiOCl/R-TiO₂ nanorod heterojunctions displayed promising photocatalytic performance for simultaneously treating RhB and the persistent organic pollutant 2-sec-butyl-4,6-dinitrophenol (DNBP). The highly enhanced photodegradation activity of the BiOCl/R-TiO₂ system was mainly attributed to the efficient RhB-photosensitization effect, the enhanced heterojunction effect, and the suitable conduction band match between BiOCl and R-TiO₂, which facilitated electron transfer from the excited RhB to the catalyst surface and charge separation across the BiOCl/R-TiO₂ interface, thus promoting the formation of ^•O₂^- and h⁺ as dominant active species in the reaction system for degradation of pollutants. The results demonstrate that the construction of a dye-sensitized BiOCl/R-TiO₂ heterojunction system is an effective strategy for improving the photocatalytic potential.

Selective Photocatalytic Reduction of CO2 to CH4 Modulated by Chloride Modification on Bi2WO6 Nanosheets

Solar-driven photocatalytic CO₂ reduction into CH₄ with H₂O is considered to be a promising way to alleviate the energy crisis and greenhouse effect. However, current CO₂ photoreduction technologies tend to overlook the role of photooxidation half reaction as well as the effect of the protons produced by water oxidation on CH₄ generation, resulting in low CO₂ conversion efficiency and poor CH₄ selectivity. In the present study, a series of chloride-modified Bi₂WO₆ nanosheets were constructed in view of chloride-assisted photocatalytic water oxidation. The results show that the CH₄ yield of the synthesized sample can be enhanced up to about 10 times compared to that with no Cl^- modification. Besides, the selectivity of CH₄ can be regulated by the loading amount of chloride, varying from 51.29% for Bi₂WO₆ to 94.98% for the maximum. The increase of product yield is attributed to chloride modification, which not only changed the morphology of the catalyst, but also modified the pathway of water oxidation. Further studies on intermediate products and the density functional theory calculation confirm that the Cl^- ions on Bi₂WO₆ nanosheets not only promote H₂O oxidation, but also lower the energy barrier for intermediate *CHO generation, thus facilitating CH₄ production. The results gained herein may provide some illuminating insights into the design of a highly selective photocatalyst for efficient CO₂ reduction.

Surface domain heterojunction on rutile TiO2 for highly efficient photocatalytic hydrogen evolution

Compared with the highly active anatase TiO2, rutile TiO2 usually presents poor photocatalytic performance due to high electron-hole recombination. Herein, we propose a surface domain heterojunction (SDH) structure between adjacent micro-domains with and without chemisorbed chlorine on rutile TiO2, which utilizes the potential difference between these domains to form a built-in field that promotes charge separation. Single-crystal rutile TiO2 nanorods assembled into radial microspheres with SDHs were fabricated, and these exhibited excellent solar-driven photocatalytic hydrogen evolution, ∼8-fold higher than that of the pristine one. Experimental results and density functional theory calculations reveal that the exceptional photocatalytic performance can be attributed to the in situ formation of chemisorbed chlorine, which forms SDHs that separate electrons and holes efficiently and results in surface reconfiguration, exposing the tri-active sites, increasing the O-site active centers and enhancing the catalytic activity of the 4-coordinated (Ti4c) and 5-coordinated Ti sites (Ti5c). This SDH strategy can extend to other halogen elements and thus provides an universal approach for the rational design of high-efficiency TiO2 photocatalysts toward sustainable solar-fuel evolution.

Toward Standardized Photocatalytic Oxygen Evolution Rates Using RuO2@TiO2 as a Benchmark

Quantitative comparison of photocatalytic performances across different photocatalysis setups is technically challenging. Here, we combine the concepts of relative and optimal photonic efficiencies to normalize activities with an internal benchmark material, RuO₂ photodeposited on a P25-TiO₂ photocatalyst, which was optimized for reproducibility of the oxygen evolution reaction (OER). Additionally, a general set of good practices was identified to ensure reliable quantification of photocatalytic OER, including photoreactor design, photocatalyst dispersion, and control of parasitic reactions caused by the sacrificial electron acceptor. Moreover, a method combining optical modeling and measurements was proposed to quantify the benchmark absorbed and scattered light (7.6% and 81.2%, respectively, of λ = 300-500 nm incident photons), rather than just incident light (≈AM 1.5G), to estimate its internal quantum efficiency (16%). We advocate the adoption of the instrumental and theoretical framework provided here to facilitate material standardization and comparison in the field of artificial photosynthesis.

Photocatalytic Dinitrogen Fixation with Water on Bismuth Oxychloride in Chloride Solutions for Solar-to-Chemical Energy Conversion

Ammonia is an indispensable chemical. Photocatalytic NH₃ production via dinitrogen fixation using water by sunlight illumination under ambient conditions is a promising strategy, although previously reported catalysts show insufficient activity. Herein, we showed that ultraviolet light irradiation of a semiconductor, bismuth oxychloride with surface oxygen vacancies (BiOCl-OVs), in water containing chloride anions (Cl^-) under N₂ flow efficiently produces NH₃. The surface OVs behave as the N₂ reduction sites by the photoformed conduction band electrons. The valence band holes are consumed by self-oxidation of interlayer Cl^- on the catalyst. The hypochloric acid (HClO) formed absorbs ultraviolet light and undergoes photodecomposition into O₂ and Cl^-. These consecutive photoreactions produce NH₃ with water as the electron donor. The Cl^- in solution compensates for the removed interlayer Cl^- and inhibits catalyst deactivation. Simulated sunlight illumination of the catalyst in seawater stably generates NH₃ with 0.05% solar-to-chemical conversion efficiency, thus exhibiting significant potential of the seawater system for artificial photosynthesis.

The development of an artificial neural network – genetic algorithm model (ANN-GA) for the adsorption and photocatalysis of methylene blue on a novel sulfur–nitrogen co-doped Fe2O3 nanostructure surface

In this research an S-N doped Fe₂O₃ nanostructure is synthesized and its adsorption ability and photocatalytic activity were evaluated. The optimum experimental conditions were obtained and an ANN-GA model was proposed for predicting experimental values.

Superior solar-to-hydrogen energy conversion efficiency by visible light-driven hydrogen production via highly reduced Ti2+/Ti3+ states in a blue titanium dioxide photocatalyst

A catalytic hydrogen production system was developed with TiO₂ that contains Ti³⁺/Ti²⁺ reduced states which act as both visible and IR light harvesting components as well as the catalytic site.

Fuel Production from Seawater and Fuel Cells Using Seawater

Seawater is the most abundant resource on our planet and fuel production from seawater has the notable advantage that it would not compete with growing demands for pure water. This Review focuses on the production of fuels from seawater and their direct use in fuel cells. Electrolysis of seawater under appropriate conditions affords hydrogen and dioxygen with 100 % faradaic efficiency without oxidation of chloride. Photoelectrocatalytic production of hydrogen from seawater provides a promising way to produce hydrogen with low cost and high efficiency. Microbial solar cells (MSCs) that use biofilms produced in seawater can generate electricity from sunlight without additional fuel because the products of photosynthesis can be utilized as electrode reactants, whereas the electrode products can be utilized as photosynthetic reactants. Another important source for hydrogen is hydrogen sulfide, which is abundantly found in Black Sea deep water. Hydrogen produced by electrolysis of Black Sea deep water can also be used in hydrogen fuel cells. Production of a fuel and its direct use in a fuel cell has been made possible for the first time by a combination of photocatalytic production of hydrogen peroxide from seawater and dioxygen in the air and its direct use in one-compartment hydrogen peroxide fuel cells to obtain electric power.

Seawater usable for production and consumption of hydrogen peroxide as a solar fuel

Hydrogen peroxide (H₂O₂) in water has been proposed as a promising solar fuel instead of gaseous hydrogen because of advantages on easy storage and high energy density, being used as a fuel of a one-compartment H₂O₂ fuel cell for producing electricity on demand with emitting only dioxygen (O₂) and water. It is highly desired to utilize the most earth-abundant seawater instead of precious pure water for the practical use of H₂O₂ as a solar fuel. Here we have achieved efficient photocatalytic production of H₂O₂ from the most earth-abundant seawater instead of precious pure water and O₂ in a two-compartment photoelectrochemical cell using WO₃ as a photocatalyst for water oxidation and a cobalt complex supported on a glassy-carbon substrate for the selective two-electron reduction of O₂. The concentration of H₂O₂ produced in seawater reached 48 mM, which was high enough to operate an H₂O₂ fuel cell.

The generation and storage of energy from renewable sources with robust and accessible technology is of significant and growing concern. Here, the authors demonstrate the solar generation of hydrogen peroxide from seawater as a viable route towards this important goal.

Exceptional performance of photoelectrochemical water oxidation of single-crystal rutile TiO2 nanorods dependent on the hole trapping of modified chloride

It is highly desired to effectively trap photogenerated holes for efficient photoelectrochemical (PEC) water oxidation to evolve O₂ on oxide semiconductors. Herein, it is found for the first time mainly based on the time-resolved- and atmosphere-controlled- surface photovoltage responses that the modified chloride would effectively trap photogenerated holes so as to prolong the charge lifetime and hence promote charge separation of single-crystal rutile TiO₂ nanorods. Its strong capacity to trap holes, comparable to the widely-used methanol and Co(II) phosphate, is well responsible for the exceptional photoactivities for PEC water oxidation to evolve O₂ on rutile nanorods with a proper amount of chloride modified, about 2.5-time high as that on the resulting anatase nanoparticles, even 10-time if the surface area is considered. Moreover, it is suggested that the hole trapping role of chemically-adsorbed chloride is related to its lonely-pair electrons, and to the subsequently-produced intermediate Cl atoms with proper electronegativity for evolving O₂. Interestingly, this finding is also applicable to the chloride-modified anatase TiO₂. This work will provide a feasible strategy to design high-activity nanostructured semiconductor photoanodes for PEC water oxidation, even for overall water splitting.

Chloride capping of CdTiO3 for higher crystallinity and enhanced photocatalytic activity

The crystallinity of cadmium titanate (CdTiO₃) was greatly improved when synthesized under mild reaction conditions, in the presence of chloride.

Effect of the chloride ion as a hole scavenger on the photocatalytic conversion of CO2 in an aqueous solution over Ni–Al layered double hydroxides

The photocatalytic conversion of CO₂ to CO using a Ni–Al LDH photocatalyst in an aqueous solution of NaCl was investigated. HClO was produced as an oxidation product of Cl⁻ under photoirradiation.