Mechanism Of Photocatalytic Oxidation Research Articles

Effect of photocatalysts on electrochemical properties and chemical mechanical polishing rate of GaN

The improvement of polishing rate during chemical mechanical polishing of GaN materials has been a research hotspot. Efficient and inexpensive catalysts are important for large-scale industrial production. In this paper, the effects of ZnO, TiO2 and ZrO2 catalysts on the polishing rate in photocatalytic oxidation polishing were studied, and the mechanism of UV photocatalytic oxidation of GaN was analyzed. In combination with ultraviolet light irradiation, three different kinds of catalysts were used in both electrochemical and chemical mechanical polishing experiments with varying pH values in both H2O2 and K2S2O8 systems. The experimental results show that ZrO2 can effectively improve the chemical mechanical polishing rate of GaN wafers. A slurry containing 0.15 M K2S2O8 and 0.05 wt% ZrO2 had a polishing rate of 487.9 nm/h at pH 10.

Photocatalytic degradation mechanism of benzene over ZnWO4: Revealing the synergistic effects of Na-doping and oxygen vacancies

Metal ion doping and vacancy-engineering are effective strategies to promote photocatalytic performance. ZnWO4 is an important inorganic photocatalytic material. Herein, ZnWO4 with Na-doping and oxygen vacancies (OVs) was prepared by a facile one-step hydrothermal strategy via adding Na2WO4. Due to the synergistic effects of Na-doping and OVs, the photocatalytic benzene removal rate of the ZnWO4 samples is much higher than that of pure ZnWO4. According to the characterization results, the Na-doping and OVs not only promote the migration of electrons but also activate H2O and O2 molecules to generate a large number of reactive radicals. The results of in situ DRIFT spectroscopy and DFT calculations clearly demonstrate that the Na-doping and OVs enhance the adsorption and activation of reactants and intermediates. The photocatalytic benzene oxidation mechanism is proposed. Benzene is completely oxidized following the order of phenol, catechol (quinol), benzoquinone, and small molecule organics such as acetic acid, eventually producing H2O and CO2. The work could provide new mechanistic understanding for photocatalytic VOCs decomposition.

La-doping induced localized excess electrons on (BiO)2CO3 for efficient photocatalytic NO removal and toxic intermediates suppression

Photocatalysis technology has been extensively adopted to abate typical air pollutants. Nevertheless, it is a challenge to develop photocatalysts aiming to simultaneously improve photocatalytic selectivity and efficiency. In this study, to improve the photocatalytic selectivity and the performance of (BiO)2CO3 in the oxidation of NO to target products (NO2− /NO3−), we developed a novel method to construct La-doped (BiO)2CO3 (La-BOC) for forming localized excess electrons (Ex) on (BiO)2CO3 surface. The results indicate that the Ex could effectively accelerate the activation of reactants and promote charge separation and transfer. Under visible light, the gas molecules could capture the Ex and get activated to produce reactive oxygen species (ROS) with high oxidation ability, which enables complete oxidation of NO to target products instead of producing other toxic by-products. Due to the functionality of the Ex, the photocatalytic selectivity and efficiency of La-BOC have been synchronously improved. Combining experimental and theoretical methods, this work unravels the pathway of charge carriers transportation/transformation and elucidates the photocatalytic NO oxidation mechanism. The present work could provide a novel method to improve photocatalytic selectivity and activity for safe air pollutant abatement.

S-scheme Sb2WO6/g-C3N4 photocatalysts with enhanced visible-light-induced photocatalytic NO oxidation performance

Normal photocatalysts cannot effectively remove low-concentration NO because of the high recombination rate of the photogenerated carriers. To overcome this problem, S-scheme composites have been developed to fabricate photocatalysts. Herein, a novel S-scheme Sb2WO6/g-C3N4 nanocomposite was fabricated by an ultrasound-assisted method, which exhibited excellent performance for photocatalytic ppb-level NO removal. Compared with the pure constituents of the nanocomposite, the as-prepared 15%-Sb2WO6/g-C3N4 photocatalyst could remove more than 68% continuous-flowing NO (initial concentration: 400 ppb) under visible-light irradiation in 30 min. The findings of the trapping experiments confirmed that •O2− and h+ were the important active species in the NO oxidation reaction. Meanwhile, the transient photocurrent response and PL spectroscopy analyses proved that the unique S-scheme structure of the samples could enhance the charge separation efficiency. In situ DRIFTS revealed that the photocatalytic reaction pathway of NO removal over the Sb2WO6/g-C3N4 nanocomposite occurred via an oxygen-induced route. The present work proposes a new concept for fabricating efficient photocatalysts for photocatalytic ppb-level NO oxidation and provides deeper insights into the mechanism of photocatalytic NO oxidation.

Z-scheme BiFeO3-CNTs-PPy as a highly effective and stable photocatalyst for selective oxidation of benzyl alcohol under visible-light irradiation

Z-scheme BiFeO3-CNTs-PPy photocatalysts were prepared via hydrothermal reaction and oxidative polymerization, and characterized by physico-chemical techniques to detect their structural and optical properties. The formed Z-scheme could efficiently enhance the separation of photo-generated electron-hole pairs, promote holes to directly participate in the oxidation reaction and avoid the formation of non-selective hydroxyl radicals. The photocatalytic performance of Z-scheme BiFeO3-CNTs-PPy photocatalysts was inspected by selective oxidation of benzyl alcohol under visible-light irradiation. In optimal condition, the highest conversion rate of benzyl alcohol was up to 83.2%, much higher than those of PPy (2.3%), BiFeO3 (15.2%), BiFeO3-CNTs (24.6%) and BiFeO3-PPy (29.4%), while the selectivity was above 99%. Simultaneously, BiFeO3-CNTs-PPy also presented good stability and could be recycled up to 5 times without remarkable loss of activity and change in structure. The relationship between solvents, reaction temperature, light wavelength, substituent type, and photocatalytic efficiency was investigated. The results showed that the best solvent and reaction temperature were ethyl acetate and 303 K, respectively. In the conversion of benzyl alcohol, 360−560 nm light played the most important role and the present of electron-withdrawing groups was beneficial to the selective oxidation. Moreover, according to the conclusions of the free radical capture experiments, the coumarin conversion experiments and electron spin resonance (ESR) spectra, the main active species in the photocatalytic process were determined. Based on these, the possible mechanism for photocatalytic selective oxidation of benzyl alcohol over BiFeO3-CNTs-PPy was proposed.

Effect of temperature and reaction time during solvothermal synthesis of BiOCl on microspheres formation: implications in the photocatalytic oxidation of gallic acid under simulated solar radiation

Synthesis and characterization of bismuth oxychloride (BiOCl) has received much attention due to its excellent photocatalytic properties, and low toxicity that allow its potential application for environmental decontamination processes. In this work, experimental conditions (temperature and reaction time) were established to synthesize BiOCl microspheres by a solvothermal method with high photocatalytic efficiency on the degradation of 3,4,5-trihydroxybenzoic acid (gallic acid). BiOCl materials were synthesized according to a design of experiment (DoE) where temperature and reaction time were selected as varying parameters. Obtained BiOCl materials with the highest and lowest degradation toward gallic acid, were characterized using several techniques. Results showed that the applied temperature is the most important parameter during solvothermal synthesis, which influences not only morphology and structure of BiOCl, but also its thermal stability and optical properties. Response surface methodology (RSM) analysis indicated that the highest photocatalytic efficiency of synthesized BiOCl material, is obtained when the temperature and reaction time are fixed at 155 °C and 18 h, respectively. Finally, a reaction mechanism of photocatalytic oxidation of gallic acid was proposed based on an experimental tests by adding different radical’ scavengers.

Preparation of a AgCl/PbMoO4 Composite and Investigation of Its Photocatalytic Oxidative Desulfurization Performance.

PbMoO4 materials were synthesized by the glycerol and hydrothermal methods, and AgCl nanoparticles were loaded onto the surface of PbMoO4 by using the precipitation–deposition method. Finally, a AgCl/PbMoO4 photocatalyst was successfully prepared. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy (UV–vis-DRS) were used to characterize the phase composition, morphology, and light absorption characteristics of the catalyst. An n-octane solution of dibenzothiophene (DBT) was used to simulate fluid catalytic cracking to make gasoline. The photocatalytic oxidation performance of DBT under visible light was studied in terms of the type of light source as well as of the catalyst, substrate, and AgCl content. The mechanism of photocatalytic oxidation was also studied. The results show that AgCl loading causes a red shift of the absorption edge of PbMoO4, which improves the photocatalytic activity of the material. When the AgCl loading was 25.0%, the amount of catalyst was 1.5 g/L, and the visible light irradiation time was 2.0 h, the highest desulfurization rate of DBT reached 97.0%.

High-performance of Cu-TiO2 for photocatalytic oxidation of formaldehyde under visible light and the mechanism study

Photocatalysis is regarded as a promising method for indoor formaldehyde (HCHO) removal, and there remains a strong demand for the development of highly efficient photocatalysts for HCHO oxidation under visible light. Herein, by grafting nano CuOx clusters onto TiO2, the higher performance is achieved over Cu-TiO2 for the photocatalytic oxidation of HCHO under visible light compared with the typical photocatalysts N-TiO2 and g-C3N4. It is found that the presence of water greatly promotes the activity of HCHO oxidation on Cu-TiO2. The characterization results indicate that the CuOx clusters on TiO2 exhibit high redox reversibility between Cu (II) and Cu (I) and greatly facilitate the migration of electrons. The mechanism of photocatalytic oxidation of HCHO was explored by using in situ DRIFTS measurements and DFT calculations. It is shown that HCHO is firstly oxidized to dioxymethylene (DOM), and then DOM is transformed into formate species under visible light irradiation. DFT calculations results indicate that the OH species are responsible for surface formate decomposition, which is the rate-determining step, while O2− radical is not active in formate decomposition. Our findings improve the understanding on the photocatalytic oxidation of HCHO, and the Cu-TiO2 catalyst is a promising candidate for practical application in indoor air purification.

Green fabrication of AuNPs/CMTKP/g-C3N4 nanocomposites with enhanced photocatalytic activity for the removal of nitric oxide under visible-light irradiation

A facile and green method of in-situ photo-reduction was developed to fabrication the AuNPs/CMTKP/g-C3N4 nanocomposites catalyst. Carboxymethyl tamarind kernel polysaccharide (CMTKP) was used to regulate and stabilize the monodispersed AuNPs onto g-C3N4 NSs surface. The morphology, microstructure and composition of the Au-CMTKP-g-C3N4 nanosheets hybrids were systematically characterized. The NO removal experiments showed that AuNPs/CMTKP/g-C3N4 nanocomposites exhibited excellent visible-light response and enhanced photo-oxidization ability to inhibit the generation of NO2. Compared with g-C3N4 NSs, the removal efficiency of 4 wt% Au-CMTKP-g-C3N4 NSs was increased by 27.5%. Reactive radicals during the photocatalysis processes were identified by electron spin resonance (ESR) study. As identified by ESR, ·O2−, ·OH and photogenerated holes could be considered as the main activated species involved in NO removal. The possible mechanism for photocatalytic NO oxidation was also proposed. Meanwhile, this photocatalyst also demonstrates suitable stability, showing good potential in practical applications.

Synergistic effects of crystal structure and oxygen vacancy on Bi2O3 polymorphs: intermediates activation, photocatalytic reaction efficiency, and conversion pathway

This work unraveled the synergistic effects of crystal structure and oxygen vacancy on the photocatalytic activity of Bi2O3 polymorphs at an atomic level for the first time. The artificial oxygen vacancy is introduced into α-Bi2O3 and β-Bi2O3 via a facile method to engineer the band structures and transportation of carriers and redox reaction for highly enhanced photocatalysis. After the optimization, the photocatalytic NO removal ratio on defective β-Bi2O3 was increased from 25.2% to 52.0% under visible light irradiation. On defective α-Bi2O3, the NO removal ratio is just increased from 7.3% to 20.1%. The difference in the activity enhancement is associated with the different structure of crystal phase and oxygen vacancy. The density functional theory (DFT) calculation and experimental results confirm that the oxygen vacancy in α-Bi2O3 and β-Bi2O3 could promote the activation of reactants and intermediate as active centers. The crystal structure and oxygen vacancy could synergistically regulate the electrons transfer pathway. On defective β-Bi2O3 with tunnel structure, the reactants activation and charge transfer were more efficient than that on α-Bi2O3 with zigzag-type configuration because the defect structures on the surface of α-Bi2O3 and β-Bi2O3 were different. Moreover, the in situ FT-IR revealed the mechanisms of photocatalytic NO oxidation. The photocatalytic NO conversion pathway on α-Bi2O3 and β-Bi2O3 can be tuned by the different surface defect structures. This work could provide a novel strategy to regulate the photocatalytic activity and conversion pathway via the synergistic effects of crystal structure and oxygen vacancy.

A facile synthesis of a highly efficient β-Bi2O3/Bi2O2CO3 heterojunction with enhanced photocatalytic NO oxidation under visible light

Photocatalytic oxidation mechanism of NO on β-Bi2O3/Bi2O2CO3.

Rapid determination of dye concentration in wastewater by TiO2-KMnO4 synergistic photocatalytic oxidation system

A new method for the determination of dye concentration based on photocatalytic oxidation digestion and spectrophotometry was established by using KMnO4 synergistic TiO2 photocatalytic oxidation and methyl orange solution as simulated dye wastewater. The mechanism of photocatalytic oxidation for the determination of dye concentration was discussed, and the optimal reaction conditions for dye digestion were investigated. The optimal digestion conditions were as follows: solution pH 1.5, TiO2 amount 1.0 g/L, KMnO4 initial concentration 10.0 mmol/L, solution temperature 80°C, ultraviolet light irradiation time 10 min. Compared with potassium dichromate reflux method or potassium permanganate method, the photocatalytic digestion method of TiO2-KMnO4 synergistic system is more rapid, efficient, economical and environmentally friendly, which provides a new idea for the determination of dye concentration in wastewater.

Photocatalytic Water Splitting Cycle in a Dye-Catalyst Supramolecular Complex: Ab Initio Molecular Dynamics Simulations

A dye-sensitized photoelectrochemical cell (DS-PEC) is a promising device for direct conversion of solar energy into fuel. The basic idea, inspired by natural photosynthesis, is to couple the photoinduced charge separation process to catalytic water splitting. The photo-oxidized dye coupled to a water oxidation catalyst (WOC) should exert a thermodynamic driving force for the catalytic cycle, while water provides the electrons for regenerating the oxidized dye. These conditions impose specific energetic constraints on the molecular components of the photoanode in the DS-PEC. Here, we consider a supramolecular complex integrating a mononuclear Ru-based WOC with a fully organic naphthalene-diimide (NDI) dye that is able to perform fast photoinduced electron injection into the conduction band of the titanium-dioxide semiconductor anode. By means of constrained ab initio molecular dynamics simulations in explicit water solvent, it is shown that the oxidized NDI provides enough driving force for the whole photocatalytic water splitting cycle. The results provide strong evidence for the significant role of spin alignment and solvent rearrangement in facilitating the proton-coupled electron transfer processes. The predicted activation free energy barriers confirm that the O–O bond formation is the rate-limiting step. Our results expand the current understanding of the photocatalytic water oxidation mechanism and provide guidelines for the optimization of high-performance DS-PEC devices.

Enhanced visible light photocatalytic performance and reaction mechanism on Ag/AgCl-LaCO<sub>3</sub>OH nanorods composite

Air pollution has become a focused issue worldwide with the rapid development of industry. As one of the major air pollutants, nitric oxide (NO) could cause to serious atmospheric problems, such as acid rain, haze and photochemical smog. Photocatalysis as a green technology has received increasing attention, because it has represented greatly potential application over the late decades. As a novel photocatalyst, LaCO3OH possesses relatively negative conduction band and positive valence band edges, which can be enable to sufficient potential for photocatalytic air decomposition. But the band gap of LaCO3OH (4.1 eV) allows only a small portion of solar spectrum in the ultraviolet (UV) light region to be absorbed. To extend the light absorption spectra into the visible light region, the combination of narrow band gap Ag/AgCl with the wide band gap LaCO3OH to build heterojunction could utilize abundant solar light. Noble metal nanoparticles (NPs) showed strong visible light absorption due to surface Plasmon resonance effect (SPR). In particular, silver NPs show SPR in the visible region, which has been utilized to develop efficient plasmonic photocatalysts. In this study, a three-component Ag/AgCl-LaCO3OH nanorod photocatalyst has been fabricated successfully through a chemical precipitation treatment and applied to photocatalytic NO removal. The as-prepared samples were characterized by XRD, SEM, TEM, XPS, UV-vis DRS, PL and ESR. The photocatalytic NO oxidation process was monitored by in situ DRIFTS. The results indicated that the visible-light harvesting and the photogenerated carrier separation can be enhanced simultaneously, which could result from introduction of Ag/AgCl into LaCO3OH (LCO) nanorod photocatalyst. Under visible-light irradiation, the photogenerated electron-hole pairs separation is promoted by the plasma resonance (SPR) effect of the metallic Ag0. The hot electrons would be transferred to the defect level endowed by the oxygen vacancies of the LCO. Subsequently, the electron can be transferred to the surface of LCO, captured by O2 to form ·O2− radicals, which plays a dominant role in the photocatalytic NO oxidation. In addition, the holes would be transferred to the surface of AgCl, and interact with Cl− to transform Cl0, which can directly participate in the oxidation of NO. Furthermore, Cl0 can be reduced to Cl− and react with Ag+ to forming AgCl, avoiding the photocorrosion of AgCl effectively. Meanwhile, the effect of the mol ratio of Ag/AgCl on the photocatalytic performance was also evaluated. The optimized Ag/AgCl-LCO (mol ratio at 75%) nanorods demonstrated a high NO removal ratio of 54.0%, which far exceeding that of the individual LCO (3.1%) or Ag/AgCl (18.0%) photocatalyst. Moreover, the in situ FT-IR spectra was used to dynamically monitor the intermediate toxic by-products, in combination with the active species, and further reveal the mechanism of photocatalytic NO oxidation from the molecular level. The reaction mechanism can be described as Ag + visible light → e−+h+, e−+O2→ ·O2−, h++AgCl→Cl0+Ag+, ·O2−+NO→NO3−, Cl0+ NO→NO3−+Cl−, NO+2·OH→NO2+H2O, NO2+·OH→NO3−+H+ and Cl−+Ag+→AgCl. The present work could provide navel perspectives for advancing the photocatalysis efficiency, offer a new insight of the photocatalytic reaction mechanism and promote large-scale environmental decontamination applications of plasma-based semiconductor composites.

Reactant activation and photocatalysis mechanisms on Bi-metal@Bi2GeO5 with oxygen vacancies: A combined experimental and theoretical investigation

The photocatalysis process follows two fundamental sequences: reactant adsorption and surface photocatalysis. In this work, Bi metal-deposited Bi2GeO5 (Bi@BiGeO) is prepared as a model photocatalyst to understand the adsorption and activation mechanisms of the reactants and the photocatalytic oxidation of NO under visible light irradiation. Density functional theory and analytical approaches are employed to reveal the electronic structure and photo-induced carrier transfer processes. The introduction of Bi metal and the generation of oxygen vacancies (OVs) in BiGeO were achieved simultaneously via a facile method. The Bi metal served as a visible light antenna and as an electron sink and promoted the carrier separation and transfer. The OVs promote reactant (H2O and O2) activation, thereby reinforcing the generation of reactive oxygen species (ROS). The NO molecules are actively adsorbed at the coordinative unsaturated sites on Bi@BiGeO and get activated via electron exchange. The photocatalytic NO oxidation mechanism on Bi@BiGeO is revealed based on the reaction intermediates and final products monitored using in-situ FTIR. This work highlights the importance of reactant activation as a new strategy for the design of highly efficient photocatalysts to overcome the bottlenecks in environmental applications.

Simultaneous removal of tetracycline and Cr(VI) by a novel three-dimensional AgI/BiVO4 p-n junction photocatalyst and insight into the photocatalytic mechanism

Simultaneous combination of photocatalytic oxidation of tetracycline (TC) and reduction of Cr(VI) over the AgI/BiVO4 p-n junction photocatalysts were studied systematically. The result showed that the pollutant degradation efficiency of the photocatalyst was greatly enhanced when the system contained both TC and Cr(VI) compared to that containing Cr(VI) or TC alone. It is proposed that the co-existence of oxidizable and reducible species, such as TC and Cr(VI), ensured efficient use of the photoinduced electrons and holes, which leaded high efficiencies for both TC oxidation and Cr(VI) reduction. Among these photocatalysts, the 0.3-AgI/BiVO4 exhibits excellent photocatalytic activity, which is ascribed to the increased lifetime of the charge carriers confirmed by the result of time-resolved fluorescence emission decay spectra. In addition, seven degradation products of TC were found by GC–MS. Moreover, the band structure of the photocatalyst was obtained by the DFT calculation, VB-XPS spectra and the values of the Fermi level obtained by CASTEP. Finally, simultaneous photocatalytic oxidation and reduction enhancing mechanism over the AgI/BiVO4 photocatalyst was discussed in detail. The strategy of forming novel p-n junction photocatalyst for both the Cr(VI) reduction and TC oxidation will benefit the development of the other new photocatalysts for the purpose of controlling and improving the environment.

Photocatalytic Decomposition of Gaseous HCHO over Ag Modified TiO₂ Nanosheets at Ambient Temperature.

Ag nanoparticles loaded onto TiO2 nanosheets with exposed {001} facets were synthesized by solvothermal hydrolysis and photoreduction deposition methods. The results suggested that Ag NPs were uniformly dispersed on the surface of anatase TiO2 NSs with a metallic state. The Raman scattering and visible light absorption performances of Ag/TiO2 NSs were enhanced by Ag NPs due to their surface plasmon resonance effect. Photocatalytic oxidation experiments for HCHO were carried out under visible light, and the enhanced photocatalytic activity of Ag/TiO2 NSs can be attributed to the synergistic effects of the following factors: (1) the {001} facets, which possessed higher surface energy, showed higher photocatalytic activity; (2) the Ag NPs, the increased oxygen vacancies, and O2 adsorption on {001} facets can trap photoelectrons, thus inhibiting the recombination of photoelectrons and holes; (3) the Ag NPs can extend the light response range of TiO2 into visible light. The in situ FTIR results showed that higher mineralization efficiency of HCHO was achieved on Ag/TiO2 NSs than on Ag/TiO2 NPs. Additionally, the mechanism for HCHO photocatalytic oxidation was also discussed.

Kinetics and mechanisms of catalytic water oxidation.

The kinetics and mechanisms of thermal and photochemical oxidation of water with homogeneous and heterogeneous catalysts, including conversion from homogeneous to heterogeneous catalysts in the course of water oxidation, are discussed in this review article. Molecular and homogeneous catalysts have the advantage to clarify the catalytic mechanisms by detecting active intermediates in catalytic water oxidation. On the other hand, heterogeneous nanoparticle catalysts have advantages for practical applications due to high catalytic activity, robustness and easier separation of catalysts by filtration as compared with molecular homogeneous precursors. Ligand oxidation of homogeneous catalysts sometimes results in the dissociation of ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation. Since it is quite difficult to identify active intermediates on the heterogeneous catalyst surface, the mechanism of water oxidation has hardly been clarified under heterogeneous catalytic conditions. This review focuses on the kinetics and mechanisms of catalytic water oxidation with homogeneous catalysts, which may be converted to heterogeneous nanoparticle catalysts depending on various reaction conditions.

Mechanism of photocatalytic toluene oxidation with ZnWO4: a combined experimental and theoretical investigation

ZnWO4 was found to adsorb O2 and H2O, and activate them to produce active radicals. The reactions to form benzyl alcohol and benzoic acid could be carried out spontaneously. The ring opening reaction is more likely to be carried out on benzoic acid.

Interfacial Oxygen Vacancy of Bi2O2CO3/PPy and its Visible-light Photocatalytic NO Oxidation Mechanism

Interfacial Oxygen Vacancy of Bi2O2CO3/PPy and its Visible-light Photocatalytic NO Oxidation Mechanism