Efficient Photocatalytic Oxidation Research Articles

Photothermal effect and continuous hot electrons injection synergistically induced enhanced molecular oxygen activation for efficient selective oxidation of benzyl alcohol over plasmonic W18O49/ZnIn2S4 photocatalyst

The molecular oxygen activation (MOA) process tends to be the rate-limiting step for photocatalytic selective organic transformation such as aromatic alcohol oxidation. Herein, using the plasmonic 0D/2D W18O49/ZnIn2S4 (WOZ) heterojunctions as a model system, we firstly shed light on the ultra-efficient MOA origin of the synergism of photothermal effect and continuous hot electrons injection through both experimental and DFT calculations. This synergistic action not only enhances the generation rate of •O2¯ and reduces the activation energy barrier, but also significantly increases the adsorption of the O2 and activates it more easily, thus achieving efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde. Meanwhile, the charge separation and transfer processes involved in O2 activation can be further optimized with the Z-scheme transfer route. This work would provide an exciting opportunity to construct more active photothermal catalysts with ultra-high efficiency of MOA for a wide variety of organic transformations with solar energy.

A Titania-Supported Polyoxometalate and Au Cocatalyst for Efficient Photocatalytic Environmental Remediation

Photocatalysis has been considered an effective method for environmental purification and pollutant removal, with many experiments having being performed. The sustainable development of environmentally friendly materials that can photocatalytically oxidize and degrade contaminants is widely studied. Here, we report the results of the photocatalytic oxidation of contaminants (over 99% conversion of the contaminants was achieved) on a tri-component photocatalyst by the simultaneous decoration of Au nanoparticles and a new type of Sn-substituted Keggin structure polyoxometalate (POM) on a TiO2 semiconductor (denoted as AuPT). The light absorption and the electron–hole pair separation capacity of TiO2 was significantly ameliorated on AuPT. The synergistic effect of the Au resonance energy transfer (RET) course and the POM redox transformation can be advantageous to the efficient transmission of photogenerated electrons and holes in a way that achieves efficient photocatalytic oxidation of contaminants.

Fe2TiO5/Fe2O3 (Shell/Shell) and (Shell/Core) Heterostructured for Efficient Oxygen Evolution.

A delicate synthesis strategy of Fe2TiO5/Fe2O3 (shell/shell (s/s)) and (shell/core (s/c)) heterostructures was proposed to realize efficient photocatalytic water oxidation. The hierarchical structure increases the light absorption depth of material, the strongly coupled components promoted the separation of photoinduced excitons, and the high surface area with adequate porosity allowed the diffusion of electrolyte to adsorb at rich active sites. Furthermore, Fe2TiO5/Fe2O3 (s/s) was coated by graphitic carbon nitride (g-CN) subunits and exhibited good photocatalytic water reduction, resulting from the contact interface accelerating the effective separation of electrons and holes.

Efficient photocatalytic toluene selective oxidation over Cs3Bi1.8Sb0.2Br9 Nanosheets: Enhanced charge carriers generation and C–H bond dissociation

Inert C–H bonds activation at room temperature has been regarded as the control step in photocatalytic generation of valuable products from hydrocarbons. Herein, lead-free perovskite Cs3Bi1.8Sb0.2Br9 nanosheets with a proper amount of Sb in Cs3Bi2Br9 not only improves optical absorption due to anomalous band gap behavior, but also promotes the generation of holes (h+). The convergence of h+ on the surface of Cs3Bi1.8Sb0.2Br9 is crucial for the dissociation of toluene C(sp3)–H bond, which was proved by the density functional theory calculation and isotopic labelling results. The excellent activity in photocatalytic selective oxidation of toluene with a conversion rate of 5.83 mmol g−1h−1 under visible light is the highest ever achieved. This work sheds light on the modification of halide perovskites through the regulation of B-site cations for photocatalytic applications.

Surface oxygenous groups modified graphitic carbon nitride with significant positive shift of valence band for efficient photocatalytic oxidation

Graphitic carbon nitride (g-C3N4) is a remarkable metal-free photocatalyst which is widely used for photocatalytic reductive reaction due to its suitable potential of conduction band, but also suffers from intrinsically weak photocatalytic oxidation ability. Here, by simply refluxing g-C3N4 in H2O2, oxygenous groups are introduced into g-C3N4. The resultant oxygenous groups modified g-C3N4 (g-C3N4(H2O2)) exhibits significantly improved hydrophilic ability and photocatalytic oxidative ability. It is well evidenced that valence band position is positive shifted significantly by introduction of surface oxygenous groups. However, the photocatalytic activity for oxygen evolution is not improved obviously, suggesting that the driving force for water oxidation is not increased simultaneously. Luckily, the photocatalytic oxidative activity of the modified g-C3N4 (include oxygen evolution, pollutant degradation and antibacterial property) could be further improved after forming TiO2-containing nanocomposites during the H2O2 reflux treatment with the addition of tertrabutyl titanate (g-C3N4/TiO2(H2O2) due to much increased driving force and accelerated charge migration. Herein, a facile and simple method is proposed to engineer the band structure of g-C3N4 for improving its photocatalytic oxidative ability, which expand the application of carbon nitride.

Light‐Induced Dynamic Stability of Oxygen Vacancies in BiSbO4 for Efficient Photocatalytic Formaldehyde Degradation

Defect engineering has been regarded as a versatile strategy to maneuver the photocatalytic activity. However, there are a few studies concerning how to maintain the stability of defects, which is important to ensure sustainable photocatalytic performance. Here, a novel strategy to modulate the structural properties of BiSbO4 using light‐induced dynamic oxygen vacancies is reported by us for efficient and stable photocatalytic oxidation of formaldehyde. Interestingly, the continuous consumption and replenishment of vacancies (namely dynamic vacancies) ensure the dynamic stability of oxygen vacancies, thus guaranteeing the excellent photocatalytic stability. The oxygen vacancies could also accelerate the electron migration, inhibit the photogenerated electron/hole recombination, widen the light absorption spectra, and thus improve the photocatalytic formaldehyde removal performance. Combined with the results of in situ DRIFTS, the reaction mechanism for each step of formaldehyde oxidation is revealed. As supported by DFT calculation of Gibbs free energy, the introduction of oxygen vacancies into BiSbO4 can promote spontaneous process of formaldehyde oxidation. Our work highlights a promising approach for stabilizing the defects and proposes the photocatalytic reaction mechanism in combination with the thermodynamic functions.

Synergistic effect of iodine doped TiO2 nanoparticle/g-C3N4 nanosheets with upgraded visible-light-sensitive performance toward highly efficient and selective photocatalytic oxidation of aromatic alcohols under blue LED irradiation

Selective oxidation of alcohols via photocatalytic processes by O2 molecules as a green oxidant and visible light has a great potential to dissolve the environmental crisis and worldwide energy. Herein, I-TiO2 Nanoparticle/g-C3N4 Nanosheets (I-TiO2/CNNSs) heterostructured composite with different contents of graphitic carbon nitride nanosheets has been successfully prepared via a facile and simple procedure. The structure of the I-TiO2/CNNSs nanocomposite was analyzed by FT-IR, XRD, EDX, BET, TGA, FESEM, DRS, and PL techniques. The resultant nanocomposite (I-TiO2/(20 wt%)CNNSs) exhibits superior photocatalytic efficiency than its pristine g-C3N4 and iodine doped TiO2 for the oxidation of benzyl alcohols under the illumination of visible light (λ ≥ 400 nm). Introduction of the I-TiO2 on the surface of CNNSs can improve the absorption ability in the visible region and separation efficiency of charge carriers during the photocatalytic oxidation. Hence, these obtained results show that the I-TiO2/(20 wt%)CNNSs nanocomposite possess high photocatalytic performance and excellent reusability in oxidation reactions and other sustainable energy-related applications.

Synthesis of one-dimensional nickel perylene diimide/iron hydroxide nanohybrid as catalyst and precursor for efficient photocatalytic and electrocatalytic water oxidation

The sluggish oxygen evolution reaction (OER) attracts increasing attention for the storage and/or conversion of clean energy. However, the whole design of precursor and product is studied a few for efficient photocatalytic and electrocatalytic water oxidation. Here, we report the large-scale synthesis of one-dimensional (1D) nanohybrids (NHs) of 50-nm nickel perylene diimide (Ni-PDI) nanowires loading iron hydroxide (Fe(OH)3) nanoparticles through one-step hydrothermal reaction. The Fe(OH)3@Ni-PDI NHs as OER photocatalyst possess the super activity, even high electrocatalytic performance after pyrolysis. Under visible light (λ > 420 nm), the photocatalytic oxygen evolution rate of 22,000 μmol g−1 h−1 can be achieved via the synergy between 1D Ni-PDI nanowires and in-situ generated iron (oxy)hydroxide as cocatalyst. Through pyrolysis of 1D Fe(OH)3@Ni-PDI NHs, the NiFe nanoparticles covered by NiFe2O4 layer can be synthesized on 30-nm nitrogen-doped carbon nanowires (NiFe/NiFe2O4@NC). After optimizing the mole ratio of nickel to iron atoms (2:1) and pyrolysis temperature (800 °C), as-obtained sample exhibits the OER activity with the overpotential of 256 mV at 10 mA cm−2 and Tafel slope of 40 mV dec−1 in 1.0 M KOH. The work provides a universal strategy to design 1D nanostructure for efficient OER catalysis.

Crystal phase-dependent generation of mobile OH radicals on TiO2: Revisiting the photocatalytic oxidation mechanism of anatase and rutile

Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial (OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2 → OHf). The surface trapped holes are not involved in OHf formation, but lead to the generation of surface-bound OH radical (OHs) on both anatase and rutile. The generation of OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the OHf generation mechanism on anatase involves the reductive pathway. Better understanding of OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.

Irregularly Shaped Mn2O3 Nanostructures with High Surface Area for Water Oxidation

Water oxidation is an energy-consuming, four-electron-transfer reaction and is essential for solar fuel production from water. Catalysts based on precious metals such as RuO2 and IrO2 show high efficiency for oxygen evolution reaction. However, these catalysts are less abundant and expensive. To date, earth-abundant water oxidation catalysts still exhibit less activity for water oxidation. Herein, we report the synthesis of high surface area Mn2O3 nanomaterials for an efficient photocatalytic water oxidation catalyst. The synthesis process involves three simple steps. In the first step, CaMnO3 is synthesized by the citrate-gel method. In the second step, CaMnO3 is transformed into freestanding layers of ε-MnO2 by selective removal of Ca2+. In the third step, these layers are converted into irregularly shaped two-dimensional Mn2O3 flakes (AD-Mn2O3) by calcination at 550 °C. These AD-Mn2O3 nanostructures show 4 times higher surface area (127 m2 g–1) when compared to the irregularly shaped Mn2O3 nanoparticles (CG-Mn2O3) synthesized by the citrate-gel method at the same temperature. The AD-Mn2O3 nanostructures show super hydrophilicity with a contact angle of zero degree. This material exhibits excellent photocatalytic water oxidation activity with a turnover frequency of 1.53 × 10–3 s–1, which is twice the activity shown by CG-Mn2O3. This study can help in developing an earth-abundant, cost-effective, efficient catalyst for overall water splitting.

Highly efficient visible-light photocatalytic ethane oxidation into ethyl hydroperoxide as a radical reservoir.

Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible light irradiation. The production of ethyl hydroperoxide (CH3CH2OOH), which is a promising radical reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile decomposition. Here, we demonstrated a design of a highly efficient visible-light-responsive photocatalyst, Au/WO3, for ethane oxidation into CH3CH2OOH, achieving an impressive yield of 1887 μmol gcat−1 in two hours under visible light irradiation at room temperature for the first time. Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force for ethane oxidation, enhancing CH3CH2OOH production by six times to 11 233 μmol gcat−1 at 100 °C and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for scalable ethane conversion into CH3CH2OOH as a potential anticancer drug.

Construction of defect-engineered three-dimensionally ordered macroporous WO3 for efficient photocatalytic water oxidation reaction

We report defect-engineered 3DOM WO3 photonic crystals featuring abundant oxygen vacancies, excellent charge transport properties and remarkable photocatalytic performance.

Visible-light-responsive lanthanide coordination polymers for highly efficient photocatalytic aerobic oxidation of amines and thiols

Photoinduced activation of oxygen by visible-light-responsive CPs via electron/energy transfer and its roles on aerobic oxidation of amines and thiols.

Boosting activity of molecular oxygen by pyridinium-based photocatalysts for metal-free alcohol oxidation

Pyridinium molecules are developed as electron- and energy-transfer mediators to boost the activity of air oxygen and thus achieve highly efficient photocatalytic oxidation of alcohols without any noble metal and additional co-catalysts/additives under mild conditions.

TiO2@UiO‐66 Composites with Efficient Adsorption and Photocatalytic Oxidation of VOCs: Investigation of Synergistic Effects and Reaction Mechanism

AbstractTiO2@UiO‐66 composite with intimate contact interfaces is fabricated by in situ solvothermal methods. Compared with other UiO‐based composites, TiO2@UiO‐66 exhibits superior photocatalytic activity for oxidation of toluene into CO2 during long‐time reaction under flowing conditions. This is because (1) the intimate contact interface and well‐matched band structure between TiO2 and UiO‐66 can effectively separate and transfer photogenerated electrons; (2) the synergistic effect between UiO‐66 and TiO2 promotes the adsorption of toluene and desorption of CO2 and provides sufficient active sites for photocatalytic reaction. The toluene conversion and CO2 production on 4TiO2@U are 3.27 and 4.10 times higher than that on UiO‐66, respectively. 4TiO2@U also shows the highest activity for formaldehyde and rhodamine B degradations. The in situ FTIR results exhibit that toluene can be oxidized by .O2− and h+ to benzaldehyde and benzoic acid, then further oxidized to oxalic acid, and finally mineralized into CO2 and H2O.

Efficient Photocatalytic Oxidation of Aromatic Alcohols over Thiophene‐based Covalent Triazine Frameworks with A Narrow Band Gap

AbstractPhotocatalytic selective oxidation of aromatic alcohols by covalent organic frameworks (COFs) is a sustainable strategy to replace present metal‐based heterogeneous catalytic oxidation systems. Covalent triazine‐based frameworks (CTFs), a subgroup of COFs, possess promising properties as efficient catalysts for photocatalytic oxidation. Sulfur‐containing metal‐free CTFs exhibit a good performance in photocatalysis due to their narrowed band gap, and the fast generated photoelectrons/holes separation and transfer. Here, we report the synthesis of thiophene‐based CTFs under mild conditions for photocatalytic oxidation of aromatic alcohols to the corresponding benzaldehydes using pure oxygen as the oxidant. Full conversion and a selectivity as high as 90 % to benzaldehyde were obtained paving the way for a potential application of these metal‐free photocatalysts in fine chemicals synthesis.

Synthesis of TiO2/Nanozeolite Composites for Highly Efficient Photocatalytic Oxidation of Propene in the Gas Phase.

In this work, we reported the preparation of composites based on titania (TiO2) and Zeolite Socony Mobil-5 (ZSM-5) nanozeolite, following two approaches (i.e., incorporating the presynthesized zeolite in the synthesis medium of TiO2 and incorporating presynthesized TiO2 in the synthesis medium of ZSM-5). The materials synthesized were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible spectroscopy (UV–vis), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectrometry analysis, and their photocatalytic activities were assessed in the oxidation of propene in the gas phase. It was observed that the synthesis methodology affects the final properties of the composite, which ultimately affected their photocatalytic performance in the studied application. It was found that the Nano-ZSM5/TiO2 composite was the most active among the investigated samples, which was attributed to the intimate contact between the two components of the composite, the preserved properties of the photocatalytic active phase in the final material, and the positive contribution of the nanozeolite by increasing the local concentration of propene.

Regulation of Ferroelectric Polarization to Achieve Efficient Charge Separation and Transfer in Particulate RuO2 /BiFeO3 for High Photocatalytic Water Oxidation Activity.

Exploiting spontaneous polarization of ferroelectric materials to achieve high charge separation efficiency is an intriguing but challenging research topic in solar energy conversion. This work shows that loading high work function RuO2 cocatalyst on BiFeO3 (BFO) nanoparticles enhances the intrinsic ferroelectric polarization by efficient screening of charges to RuO2 via RuO2 /BFO heterojunction. This leads to enhancement of the surface photovoltage of RuO2 /BFO single nanoparticles nearly 3 times, the driving force for charge separation and transfer in photocatalytic reactions. Consequently, efficient photocatalytic water oxidation is achieved with quantum efficiency as high as 5.36 % at 560nm, the highest activity reported so far for ferroelectric materials. This work demonstrates that, unlike low photocurrent density in film-based ferroelectric devices, high photocatalytic activity could be achieved by regulating the ferroelectric spontaneous polarization using appropriate cocatalyst to enhance driving force for efficient separation and transfer of photogenerated charges in particulate ferroelectric semiconductor materials.

Mechanisms of Interfacial Charge Transfer and Photocatalytic NO Oxidation on BiOBr/SnO2 p-n Heterojunctions.

In this work, hydrothermally prepared p-n heterojunction BiOBr/SnO2 photocatalysts were applied to eliminate NO in visible light. The as-synthesized BiOBr/SnO2 photocatalysts exhibit superior photocatalytic activity and stability through the establishment of a p-n heterojunction, resulting in a significant improvement in charge separation and transfer properties. The morphological structure and optical property of the BiOBr/SnO2 heterojunction were also investigated comprehensively. Extended light absorption into the visible range was achieved by SnO2 coating on the surface of the BiOBr microsphere through the constructed heterojunction between BiOBr and SnO2, thus achieving efficient NO removal. Moreover, the transfer channels and directions of charge at the BiOBr/SnO2 interface were determined by a combination of theoretical calculations and experimental studies. Within this p-n heterojunction, the charge in SnO2 migrates into BiOBr through the preformed electron transfer channels, thus generating an internal electric field (IEF) between SnO2 and BiOBr. Under the influence of IEF, the photogenerated electrons of BiOBr migrate from the conduction band (CB) to the CB of SnO2, thus promoting the separation of electrons (e-)-holes (h+) pairs. The intermediates and final products were monitored by the in situ DRIFTS technology in the process of removal of NO in visible light; hence, the oxidation pathways of NO were reasonably proposed. Meanwhile, the construction of the heterojunction not only achieves more efficient NO photocatalytic oxidation but also inhibits the production of more toxic NO2. This work provides mechanistic insights into the interfacial charge transfer for heterojunction photocatalysts and reaction mechanism for efficient air purification.

Facile synthesis of Bi2O3 nanocrystals for photocatalytic NO oxidation and its conversion pathway via in situ DRIFTS

In this work, the two different polymorphs of Bi2O3, including α-Bi2O3 and β-Bi2O3, were fabricated by the post-calcined the precursor at 300 and 500 °C respectively. The pathway of photocatalytic NO oxidation over the as-prepared photocatalyst (β-Bi2O3) was explored. The results showed that NO removal ratio on as-synthesis samples followed the order: β-Bi2O3 (32 %) > the precursor (13 %) > α-Bi2O3 (5 %). The ESR and in situ DRIFTS demonstrated that the superoxide (O2−), as the main radical, can react with the absorbed NO on the surface of β-Bi2O3 to generate the final products NO3− and NO2−. Therefore, such findings reveal the possible photocatalytic pathway of NO oxidation on β-Bi2O3 based on in situ DRIFTS and give insight into understanding the polymorphism of Bi2O3 in efficient photocatalytic NO oxidation under the visible light.