Gas-Phase Photoelectrocatalysis for Breaking Down Nitric Oxide

Microwave assisted synthesis of PQ-GDY@NH2-UIO-66(Zr) for improved photocatalytic removal of NOx under visible light

Pyrazinoquinoxaline-based graphdiyne (PQ-GDY) contains a fixed number of sp-sp2 hybridized carbon atoms and pyrazine-like sp2 hybridized N atoms. In this paper, NH2-UIO-66(Zr) on PQ-GDY substrate was successfully constructed with the help of microwave-assisted heating. PQ-GDY surface acts as a microwave antenna under microwave irradiation to rapidly absorb microwave energy and form hot spots (hot spot effect), which facilitates the formation of well-dispersed NH2-UIO-66(Zr) with good crystallinity. Transient absorption spectra show that high hole transport property of PQ-GDY can accelerate the migration of photogenerated holes from NH2-UIO-66(Zr) to PQ-GDY and greatly reduce the recombination rate of photogenerated electrons and holes due to the strong interaction between PQ-GDY and NH2-UIO-66(Zr). Under visible light (λ ≥ 420 nm), PQ-GDY@NH2-UIO-66(Zr) shows high photocatalytic stability and high NOx removal rate up to 74%, which is 44% higher than that of primitive NH2-UIO-66(Zr). At the same time, it inhibits the formation of toxic by-products (NO2) and limits its concentration to a low level.

One-pot synthesis of CdS/CeO2 heterojunction nanocomposite with tunable bandgap for the enhanced advanced oxidation process

Herein, a binary nanocomposite CdS/CeO₂ has been fabricated via a one-pot co-precipitation method for the degradation of Rose Bengal (RB) dye. The structure, surface morphology, composition, and surface area of the prepared composite were characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, Brunaur-Emmett-Teller analysis UV-Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The prepared CdS/CeO₂(1:1) nanocomposite has a particle size of 8.9 ± 0.3 nm and a surface area of 51.30 m²/g. All the tests indicated the agglomeration of CdS nanoparticles over the surface of CeO₂. The prepared composite showed excellent photocatalytic activity in the presence of hydrogen peroxide under solar irradiation towards the degradation of Rose Bengal. Near to about complete degradation of 190 ppm of RB dye could be achieved within 60 min under optimum conditions. The enhanced photocatalytic activity was attributed to the delayed charge recombination rate and a lower bandgap of the photocatalyst. The degradation process was found to follow pseudo-first-order kinetics with a rate constant of 0.05824 min^-1. The prepared sample showed excellent stability and reusability and maintained about 87% of the photocatalytic efficiency till the fifth cycle. A plausible mechanism for the degradation of the dye is also presented based on the scavenger experiments.

Activation of chloride by oxygen vacancies-enriched TiO2 photoanode for efficient photoelectrochemical treatment of persistent organic pollutants and simultaneous H2 generation

Photoelectrochemical (PEC) activation of chloride ions (Cl^-) to degrade persistent organic pollutants (POPs) is a promising strategy for the treatment of industrial saline organic wastewater. However, the wide application of this technology is greatly restricted due to the general photoanode activation of Cl^- with poor capability, the propensity to produce toxic by-products chlorates, and the narrow pH range. Herein, oxygen vacancies-enriched titanium dioxide (O_v-TiO₂) photoanode is explored to strongly activate Cl^- to drive the deep mineralization of POPs wastewater in a wide pH range (2-12) with simultaneous production of H₂. More importantly, nearly no toxic by-product of chlorates was produced during such PEC-Cl system. The degradation efficiency of 4-CP and H₂ generation rate by O_v-TiO₂ were 99.9% within 60 min and 198.2 μmol h^-1 cm^-2, respectively, which are far superior to that on the TiO₂ (33.1% within 60 min, 27.5 μmol h^-1 cm^-2) working electrode. DFT calculation and capture experiments revealed that O_v-TiO₂ with abundant oxygen vacancies is conducive to the activation of Cl^- to produce more reactive chlorine species, evidenced by its high production of free chlorine (48.7 mg L^-1 vs 7.5 mg L^-1 of TiO₂). The as-designed PEC-Cl system in this work is expected to realize the purification of industrial saline organic wastewater coupling with green energy H₂ evolution.

Atom-dispersed copper and nano-palladium in the boron-carbon-nitrogen matric cooperate to realize the efficient purification of nitrate wastewater and the electrochemical synthesis of ammonia

Electrochemical nitrate reduction reaction (NIRR) driven by sustainable energy is not only expected to realize the green production of ammonia under ambient conditions, but also a promising way to purify nitrate wastewater. The ammonia yield rate and Faradaic efficiency of NIRR catalyzed by Pd10Cu/BCN constructed with structural constraints and pre-embedded reducing agent strategies were as high as 102,153 μg h^-1 mg_cat.^-1 and 91.47%, respectively. Pd10Cu/BCN can remove nearly 100% of 50 mg L^-1 NO₃^- without NO₂^- residue within 10 h, and the realization of this effect does not require the participation of any chloride. Control experiments and DFT calculations explain the efficient operation mechanism of NIRR on Pd10Cu/BCN, where the Pd and CuN4 sites play the role of synergistic catalysis. Compared with the reported literature, Pd10Cu/BCN with good biocompatibility has become an outstanding representative of NIRR catalyst, which provides an alternative way for the green production of ammonia and the purification of nitrate wastewater.

Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NOx

Learning from the important role of porphyrin-based chromophores in natural photosynthesis, a bionic photocatalytic system based on tetrakis (4-carboxyphenyl) porphyrin-coupled TiO₂ was designed for photo-induced treating low-concentration NO_x indoor gas (550 parts per billion), achieving a high NO removal rate of 91% and a long stability under visible-light (λ ≥ 420 nm) irradiation. Besides the great contribution of the conventional ^•O₂^- reactive species, a synergic effect between a singlet oxygen (¹O₂) and mobile hydroxyl radicals (^•OH_f) was first illustrated for removing NO_x indoor gas (¹O₂ + 2NO → 2NO₂, NO₂ + ^•OH_f → HNO₃), inhibiting the production of the byproducts of NO₂. This work is helpful for understanding the surface mechanism of photocatalytic NO_x oxidation and provides a new perspective for the development of highly efficient air purification systems.

Removal and Recovery of Gaseous Elemental Mercury Using a Cl-Doped Protonated Polypyrrole@MWCNTs Composite Membrane

Due to the restrictions on mercury mining, recovering the mercury from mercury-containing waste is attracting increasing attention. This study successfully achieved the removal and recovery of gaseous elemental mercury (Hg⁰) by using membrane technology. A novel composite membrane of Cl-doped protonated polypyrrole-coated multiwall carbon nanotubes (Cl-PPy@MWCNTs) was fabricated in which MWCNTs acted as the framework to support the active component Cl-PPy. The morphology, structure, and composition of the prepared membranes were determined by field emission scanning electron microcopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, etc. The composite membrane exhibited an excellent performance in Hg⁰ removal (97.3%) at a high space velocity of 200,000 h^-1. The dynamical adsorption capacity of Hg⁰ was 3.87 mg/g when the Hg⁰ breakthrough reached 10%. The adsorbed Hg⁰ could be recovered/enriched via a leaching process using acidic NaCl solution; meanwhile, the membrane was regenerated. The recovered mercury was identified in the form of Hg²⁺, with a recovery efficiency of over 99%. Density functional theory calculations and mechanism analysis clarified that the electrons of Hg⁰ transported to the delocalized electron orbits of protonated PPy and then combined with Cl^- to form Hg₂Cl₂/HgCl₂. Finally, we first demonstrated that the analogous protonated conductive polymers (e.g., polyaniline) also possessed good Hg⁰ removal ability, implying that such species may offer more outstanding answers and attract attention in future.

Atomic-dispersed copper simultaneously achieve high-efficiency removal and high-value-added conversion to ammonia of nitrate in sewage

Environmentally friendly electrochemical reduction pathways from NO₃^- to NH₃ or N₂ have provided feasible strategy into the green production of ammonia or the treatment of nitrate wastewater. Here, we anchored single-atom Cu with boron carbon nitride on carbon nanotube (BCN@Cu/CNT), and achieved the efficient operation of electrochemical nitrate reduction reaction (NIRR). BCN@Cu/CNT can efficiently catalyze the selective conversion of high-concentration nitrate into high-value-added ammonia, where the ammonia yield rate and Faradaic efficiency are as high as 172,226.5 μg h^-1 mg_cat.^-1 and 95.32% (at -0.6 V), respectively. BCN@Cu/CNT also shows the ability to efficiently remove low-concentration nitrates in sewage. Specifically, here only takes 5 h to nearly 100% (99.32%) eliminate NO₃^- (50 mg L^-1) in sewage without any residual NO₂^-. The excellent catalytic activity and physicochemical stability of BCN@Cu/CNT for NIRR suggest the promising industrial application prospects, including the green production of ammonia and the purification of nitrate wastewater.

Highly efficient photoelectrochemical removal of atrazine and the mechanism investigation: Bias potential effect and reactive species

In this work, efficient photoelectrochemical (PEC) removal of atrazine, one of the most widely used chemical herbicides in the world, was obtained by adjusting the bias potential applied on the photo-anode, and the optimal atrazine removal efficiency reached 96.8% at the potential of 0.2 V vs. SCE in 2 h with the reaction rate constant of 1.72 h^-1. The results indicated at the optimal potential, the separation efficiency of photo-generated holes and electrons was the highest with the lowest electron transfer resistance. Mechanism investigation revealed that superoxide radicals, hydroxyl radicals and holes all contributed to atrazine degradation, and the bias potential on the photo-anode could influence atrazine removal efficiency by changing the generation amount and distribution of the reactive oxygen species (ROS). It was presumed the nucleophilicity of superoxide radical played an important role in atrazine dechlorination, leading to the enhanced removal efficiency. However, the bias potential did not show obvious influence on the degradation intermediates of atrazine in the PEC system compared with that in photocatalytic oxidation, since it was actually an electro-assisted photocatalytic process in the potential range investigated. The work will provide fundamental basis for establishing efficient PEC system for pollutant remediation experimentally and theoretically.

Mechanism study on TiO2 inducing O2- and OH radicals in O3/H2O2 system for high-efficiency NO oxidation

To achieve high NO oxidation efficiency, excessive O₃ must be used, which would lead to the high cost and escape of ozone. Herein, we adopted low cost and environmental-friendly TiO₂ as the catalyst of low concentration O₃ and H₂O₂ system for high-efficiency NO_x oxidation. The Ti sites on TiO₂ were the deprotonation sites of H₂O₂ and H₂O into Ti-OOH and Ti-OH species, respectively. We found that the surface of rutile phase TiO₂ had a low concentration Ti-OOH component but a large amount of Ti-OH after contacting with H₂O₂ solution, thus lots of ·OH and a few O₂^- radicals formed with introducing O₃ molecules. H₂O₂ solution induced the formation of a large amount of Ti-OOH and Ti-OH species on the anatase phase TiO₂ surface, thus lots of O₂^- generated in the O₃/H₂O₂ system. O₂^- and OH radicals could efficiently oxidize NO, in which O₂^- radicals could oxidize NO to NO₃^- in one step with high selectively. Therefore, anatase TiO₂ had better performance in NO_x oxidation than rutile phase TiO₂. The effect of temperature and SO₂ concentration on NO oxidation was also investigated, the results showed that TiO₂-A/O₃/H₂O₂ system promoted NO oxidation at a low temperature and a low concentration of SO₂.

Efficient Self-Driving Photoelectrocatalytic Reactor for Synergistic Water Purification and H2 Evolution

The photoelectrocatalytic (PEC) technique has attracted much attention to getting clear energy and environmental purification. Simultaneous reactions of solar energy generation could be used to apply for practical applications to maximize the functionality of reactor systems. Herein, we crafted a self-driving photoelectrocatalytic reactor system, comprising platinum (Pt) modified p-Si nanowires (Pt/Si-NWs) as a photocathode and TiO₂ nanotube arrays (TiO₂-NTAs) as a photoanode for synergistic H₂ evolution and water purification, respectively. Hydrogen evolution in the cathode chamber and environmental remediation in the anode chamber were achieved with the aid of appropriate bandgap illumination and self-built bias voltage. The mismatch of Fermi levels between TiO₂-NTAs and Si-NWs reduced the recombination rates of photoinduced electrons and holes through the formation of Z scheme and inner electric filed. The synergistic PEC reactions exhibited much higher activities than those achieved using other systems so far. This basic principal could be applied for fabricating other PEC reactors in photoelectro conversion devices and be established as design guidelines for reactors to maximize the PEC performance.

Gas-Phase Photoelectrocatalytic Oxidation of NO via TiO2 Nanorod Array/FTO Photoanodes

Most photoelectrocatalytic (PEC) reactions are performed in the liquid phase for convenient electron transfer in an electrolyte solution. Herein, a novel PEC reactor involving a tandem combination of TiO₂ nanorod array/fluorine-doped tin oxide (TiO₂-NR/FTO) working electrodes and an electrochemical auxiliary cell was constructed to drive the highly efficient PEC oxidation of indoor gas (NO_x). With the aid of a low bias voltage (0.3 V), the as-formed PEC reactor exhibited an 80% removal rate for oxidizing NO (500 ppb) under light irradiation, which is much higher than that of the traditional photocatalytic (PC) process. Upon being irradiated by light, the photogenerated electrons are quickly separated from the holes and transferred to the counter electrode (Pt) owing to the applied bias voltage, leaving photogenerated holes in the TiO₂-NR/FTO electrode for oxidizing NO molecules. Moreover, both dry and humid NO could be effectively removed by the tandem TiO₂-NR/FTO-based gas-phase PEC reactor, indicating that the NO molecules could also be directly oxidized by photogenerated holes in addition to hydroxyl radicals. The presence of trace amounts of water could promote the PEC oxidation of NO owing to the formation of hydroxyl radicals induced by reactions between the water and holes, which could further oxidize NO. This PEC reactor offers an energy-saving, environmentally friendly, and efficient route to treat air polluted with low concentrations of gases (NO_x and SO_x).

Combination of Pd-Cu Catalysis and Electrolytic H2 Evolution for Selective Nitrate Reduction Using Protonated Polypyrrole as a Cathode

Pd-Cu catalysis is combined with in situ electrolytic H₂ evolution for NO₃^- reduction with protonated polypyrrole (PPy) as a cathode. The surface of PPy is not only beneficial for H₂ evolution, but exclusive for NO₃^- adsorption, and thus inhibits NO₃^- reduction. Meanwhile, the in situ H₂ generation exhibits a much higher utilization efficiency because of the smaller bubble size and higher dispersion. The Pd-Cu catalysts with the ratios of 6:1 and 4:1 exhibit the highest NO₃^--N removal (100%) and N₂ selectivity (93-95%) after 90 min. In comparison with the results obtained with other cathode materials (Ti, Cu, Co₃O₄, and Fe₂O₃) and obtained by other researchers, the new process shows a faster NO₃^--N reduction rate and much higher N₂ selectivity. However, the O₂ generated on the anode can oxidize Cu to Cu₂O that may work as the catalyst for NO₃^--N reduction to NH₄⁺-N by H₂, resulting in more than 60% NH₄⁺-N generated without a proton exchange membrane. Both the PPy film and Pd-Cu catalyst exhibit good stability and there is no Cu²⁺ or Pd²⁺ in solution after reaction. Real industrial wastewater is further treated in this system, the NO₃^--N is reduced from 670 mg L^-1 to less than 100 mg L^-1 in 90 min, and only little amount of NH₄⁺-N is generated.

Strong Hollow Spherical La2NiO4 Photocatalytic Microreactor for Round-the-Clock Environmental Remediation

This work reports a moderate round-the-clock route to treating organic pollutants by utilizing a La₂NiO₄ hollow-sphere microreactor. A glycerol-assisted solvothermal route followed by an annealing process was applied for fabricating the catalyst. Both the physicochemical properties and the catalytic performance of the as-obtained microreactor for treating pollutants were discussed. The microreactor exhibited a strong ability to degrade phenol and anionic dyes in the absence of light irradiation, owing to its high surface area and positively charged surface. With the aid of visible-light irradiation, the degradation rate of the organic pollutants could be further accelerated due to the light multireflection in a hollow structure, which enhances the utilization of light. The present work indicates that the hollow-sphere La₂NiO₄ microreactor is effectively energy saving for environmental remediation.