Photocatalysts For Conversion Research Articles

Enriching surface-ordered defects on WO3 for photocatalytic CO2-to-CH4 conversion by water

Defect engineering has been widely applied in semiconductors to improve photocatalytic properties by altering the surface structures. This study is about the transformation of inactive WO3 nanosheets to a highly effective CO2-to-CH4 conversion photocatalyst by introducing surface-ordered defects in abundance. The nonstoichiometric WO3-x samples were examined by using aberration-corrected electron microscopy. Results unveil abundant surface-ordered terminations derived from the periodic {013} stacking faults with a defect density of 20.2%. The {002} surface-ordered line defects are the active sites for fixation CO2, transforming the inactive WO3 nanosheets into a highly active catalyst (CH4: O2 = 8.2: 16.7 μmol h-1). We believe that the formation of the W-O-C-W-O species is a critical step in the catalytic pathways. This work provides an atomic-level comprehension of the structural defects of catalysts for activating small molecules.

Metal Doping in SrTiO3 Photocatalyst to Improve the Photocatalytic Activity for CO2 Conversion

Among various strategies toward finding new energy sources to replace conventional fossil fuels, photocatalytic conversion of CO2 into useful chemicals using water as an electron donor and a proton source has been increasingly attracting attentions because it can convert light energy to chemical energy. The photocatalytic conversion of CO2 involves photoabsorption (light harvesting), charge separation, and H2O oxidation, as well as the consumption of the photogenerated electrons for reduction. The evolution of H2 rather than CO is preferred when H2O is used as the electron donor, since the redox potential of CO2/CO (−0.51 V vs NHE, at pH 7) is more negative than that of H+/H2 (−0.41 V vs NHE, at pH 7) in an aqueous solution. Therefore, “high selectivity toward CO2 reduction” is one of the most important requirements for the photocatalytic system to achieve the CO2 conversion in water. Various mixed oxides including tantalum-based oxides, titanium-based oxides, gallium-based oxides, and niobium-based oxides were found to be excellent photocatalysts for the conversion of CO2 into CO in the presence of Ag cocatalyst. However, these materials show significant activity for CO evolution only when irradiated with deep region UV-light (λ < 300 nm). Therefore, a series of photocatalysts which can drive under the photoirradiation with UV-light at λ > 300 nm is strongly desired in the research area of the photocatalytic conversion of CO2. As continuous efforts to develop effective photocatalysts for the photocatalytic conversion of CO2 to CO in water, in this study, we investigated metal doping in the SrTiO3 photocatalyst, and evaluated the photocatalytic activity of the doped SrTiO3 photocatalysts for the conversion of CO2 to CO under UV-LED-light irradiation at 365 nm. SrTiO3 photocatalyst was prepared by a previously reported solid-state method (Journal of Power Sources, 2008, 183, 701−707). SrCO3 and TiO2 were ground for 15 min in a mortar and pestle, and the mixture was transferred to an alumina crucible to calcine at 1373 K for 10 h in air. The resulting powder was washed three times with hot ultrapure water and dried overnight at room temperature. M-doped SrTiO3 (M−SrTiO3, M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) was synthesized via a flux method (ACS Applied Energy Materials, 2020, 3, 1468−1475.). Excess amount of SrCl2 flux was added to a mixture of the prepared SrTiO3 and a metal oxide as a doping source. The mixture was ground for 15 min in a mortar and pestle, transferred to an yttria crucible, and calcined at 1418 K for 15 h. The obtained powder was washed three times with hot ultrapure water, and dried overnight at 353 K. Photocatalytic activity of AgCo/M−SrTiO3 photocatalyst was evaluated by using an external-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 0.2 L of an aqueous NaHCO3 solution, and the dissolved air in the suspension was degassed by a flow of high-purity CO2 gas. CO2 gas was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a monochromatic UV-LED lamp at 365 nm (IRS-1000, CELL System Co., Ltd., Japan). The gaseous products in the outlet gas, e.g., CO, H2, and O2, were analyzed by gas chromatography (FID-GC with methanizer: CO, TCD-GC: H2 and O2). The attached figure shows a formation rate of CO (red), H2 (blue), and O2 (green) in the photocatalytic conversion of CO2 with H2O using the M−SrTiO3 photocatalysts (M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) in the presence of AgCo cocatalyst. Zn−, Li−, Mn−, W−, Ca−, and Y−SrTiO3 showed extremely low photocatalytic activities for the conversion of CO2, whereas Al− and Mg−SrTiO3 produced certain amount of CO and O2. Mg−SrTiO3 exhibited the highest activity for CO2 conversion, which has been observed as a CO formation rate of ca. 20 μmol h−1. However, H2 evolution was thoroughly suppressed (0.055 μmol h−1) in the presence of AgCo/Mg−SrTiO3, where the selectivity toward CO evolution was higher than 99%. These results indicated that Mg-doping into SrTiO3 improved the photocatalytic activity for the conversion of CO2. Moreover, stoichiometric O2 evolution (8.9 μmol h−1) was found in the presence of AgCo/Mg−SrTiO3, suggesting that H2O acts as both an electron donor and a proton source for the photocatalytic conversion of CO2 in water. The apparent quantum efficiency (AQE) of AgCo/Mg−SrTiO3 photocatalyst in the photocatalytic conversion of CO2 under monochromatic UV-light irradiation (365 nm) was determined to be 0.05%. Figure 1

Subnanometric Bismuth Clusters Confined in Pyrochlore‐Bi2Sn2O7 Enable Remarkable CO2 Photoreduction

AbstractThe development of highly efficient photocatalysts for conversion of carbon dioxide (CO2) with water (H2O) into chemical fuels is of great importance for energy sustainability and carbon resource utilization. Herein, we demonstrated a facile hydrothermal method for in situ construction of subnanometric Bi metallic clusters in pyrochlore‐Bi2Sn2O7 frameworks, leading to the remarkable improvements of photocatalytic performances for CO2 reduction into CO in the absence of sacrificial reagent. More specifically, an outstanding CO evolution activity of 114.1 μmol g−1 h−1 has been achieved, more than 20‐fold improvement compared with the pristine Bi2Sn2O7 (5.7 μmol g−1 h−1). Detailed experiments together with in situ characterizations reveal that the spatially confined Bi clusters could significantly promote charge‐separation/electron‐enrichment and adsorption/activation of CO2 molecules, which provides highly efficient reaction channels to facilitate the generation of *COOH intermediate as well as the subsequent desorption of *CO towards CO formation. These demonstrations provide an important knowledge for precise design and fabrication of highly efficient photocatalysts for CO2 conversion into solar fuels.

Subnanometric Bismuth Clusters Confined in Pyrochlore-Bi2 Sn2 O7 Enable Remarkable CO2 Photoreduction.

The development of highly efficient photocatalysts for conversion of carbon dioxide (CO2 ) with water (H2 O) into chemical fuels is of great importance for energy sustainability and carbon resource utilization. Herein, we demonstrated a facile hydrothermal method for in situ construction of subnanometric Bi metallic clusters in pyrochlore-Bi2 Sn2 O7 frameworks, leading to the remarkable improvements of photocatalytic performances for CO2 reduction into CO in the absence of sacrificial reagent. More specifically, an outstanding CO evolution activity of 114.1 μmol g-1 h-1 has been achieved, more than 20-fold improvement compared with the pristine Bi2 Sn2 O7 (5.7 μmol g-1 h-1 ). Detailed experiments together with in situ characterizations reveal that the spatially confined Bi clusters could significantly promote charge-separation/electron-enrichment and adsorption/activation of CO2 molecules, which provides highly efficient reaction channels to facilitate the generation of *COOH intermediate as well as the subsequent desorption of *CO towards CO formation. These demonstrations provide an important knowledge for precise design and fabrication of highly efficient photocatalysts for CO2 conversion into solar fuels.

Selective CO2 photoreduction to C2 hydrocarbon via synergy between metastable ordered oxygen vacancies and hydrogen spillover over TiO2 nanobelts

The development of photocatalysts with desirable compositions and structures to improve the selectivity of CO2 conversion to hydrocarbon remains a challenging goal. In this study, we present a strategy of defected TiO2 nanobelts with metastable ordered oxygen vacancies by a simple Pd-catalyzed oxygen reduction method under anoxic condition. Ordered oxygen vacancies not only provide the driving force and channel for the directional movement of photoinduced electrons by modulating built-in electric field, but also promote CO2 adsorption and activation. Pd nanoparticles (Pd NPs) boosted dissociation of H2O to increase H* coverage and promoted the protonation of the intermediates via hydrogen spillover. The synergistic effect of ordered oxygen vacancies and hydrogen spillover contributed to the enhanced photocatalytic reduction of CO2 toward C2 products. The maximum selectivity of C2 products can reach 51.45% with an evolution rate of 25.987 μmol·g−1·h−1. This strategy provides a convenient method for designing a selective CO2 conversion photocatalyst.

Solar-light-driven photocatalytic conversion of monosaccharide by oxygen-doped graphitic carbon nitride/artificial steel converter slag composite

The development of high performance, low-cost and resource-efficient photocatalysts with improved solar light harvesting capabilities can significantly enhance the practicability and viability of photocatalytic conversion of biomass-based substances to value-added chemicals. This work reports the construction of heterojunction between oxygen-doped graphitic carbon nitride and artificial steel converter slag (OCN/artCS) as efficient photocatalysts for conversion of biomass-derived monosaccharide to lactic acid under the simulated solar light. The OCN/artCS composites were facilely fabricated by the thermal polymerization of the mixtures of dicyanamide (DA) and oxalic acid (OA), which are precursors of OCN. The artCS acts as photocatalyst support for OCN to grow on its surface and can provide Ca2Fe2O5 semiconductor to possibly form the OCN/Ca2Fe2O5-containing artCS heterojunction. Compared with pristine OCN, the OCN/artCS composites exhibited higher photocatalytic performance for selective production of lactic acid, possibly due to the effective separation of photogenerated charge carriers after creation of OCN/artCS heterojunction. At the optimal content of artCS in the composites, the OCN/5%artCS delivered 99% glucose conversion and 90% lactic acid yield within 60 min at 50 °C under the simulated sunlight irradiation. This work can be a platform for the efficient utilization of industrial solid wastes for production of high-performance photocatalysts, enabling their promising applications in photobiorefinery and other photocatalytic processes.

Highly Selective Nitrogen-Doped Graphene Quantum Dots/Eriochrome Cyanine Composite Photocatalyst for NADH Regeneration and Coupling of Benzylamine in Aerobic Condition under Solar Light

Photocatalysis is an ecofriendly and sustainable pathway for utilizing solar energy to convert organic molecules. In this context, using solar light responsive graphene-based materials for C–N bond activation and coenzyme regeneration (nicotinamide adenine dinucleotide hydrogen; NADH) is one of the utmost important and challenging tasks in this century. Herein, we report the synthesis of nitrogen-doped graphene quantum dots (NGQDs)-eriochrome cyanine (EC) solar light active highly efficient “NGQDs@EC” composite photocatalyst for the conversion of 4-chloro benzylamine into 4-chloro benzylamine, accompanied by the regeneration of NADH from NAD+, respectively. The NGQDs@EC composite photocatalyst system is utilized in a highly efficient and stereospecific solar light responsive manner, leading to the conversion of imine (98.5%) and NADH regeneration (55%) in comparison to NGQDs. The present research work highlights the improvements in the use of NGQDs@EC composite photocatalyst for stereospecific NADH regeneration and conversion of imine under solar light.

Design and synthesis of metal-free ethene-based covalent organic framework photocatalysts for efficient, selective, and long-term stable CO2 conversion into methane

The efficient and selective photocatalytic CO2 conversion into higher-valued hydrocarbon products (e.g., methane and ethane) over covalent organic frameworks (COFs) remains a challenge, with all previously reported attempts producing carbon monoxide as the dominant product. Herein, we report a new ethene-based COF, through polycondensation of electron-rich (E)-1,2‑diphenylethene and 1,3,6,8‑tetraphenylpyrene units. The synthesized ethene-based COF functioned as an efficient metal-free photocatalyst for the conversion of CO2 into methane under visible light irradiation, with a selectivity of 100 %, a production rate of 14.7 µmol g–1h–1, and an apparent quantum yield of c.a. 0.99 % at 489.5 nm, which are the most promising values reported for CO2 conversion by a metal-free COF photocatalyst, without any support from a co-catalyst. The carbon origin of CH4 product is confirmed by isotope tracer 13CO2 experiment. Moreover, the photocatalytic system consistently produces methane for > 14 h with recyclability.

Metal-Free Photocatalysts for Conversion of H2 O into Hydrogen Peroxide.

Hydrogen peroxide (H2 O2 ) is an important green oxidizing agent for environmental protection and chemical production. In comparison to the traditional anthraquinone method, photosynthesis is a green and energy-saving process for H2 O2 production. To improve the stability and practical application value of the H2 O2 synthesized by photocatalysis, the H2 O2 photosynthesis should be conducted in pure water without involving any sacrificial reagents. In this regard, organic semiconducting catalysts pose as a suitable candidate for photocatalytic H2 O2 synthesis owing to their metal-free nature to prevent H2 O2 decomposition by the metal ions. In this Perspective, the H2 O2 photosynthesis history is firstly introduced, followed by a review of the organic semiconductor photocatalysts reported to date. Finally, the main problems to thwart the advances of current pure H2 O-to-H2 O2 photosynthesis are discussed, followed by proposed solutions to address these issues in order to pave new ways for the development of highly efficient metal-free organic photocatalysts for sustainable pure H2 O-to-H2 O2 conversion.

Novel Cerium-Based Sulfide Nano-Photocatalyst for Highly Efficient CO2 Reduction.

To address the environmental crisis caused by excessive emissions of CO2 , the development of effective photocatalysts for the conversion of CO2 into chemicals has emerged as one of the most promising strategies. Herein, beyond those well-studied materials, a rare-earth sulfide-based nanocrystal NaCeS2 is fabricated and investigated for efficient and selective conversion of CO2 into CO, where the role of Ce ions is crucial. Firstly, the hybridization of Ce 4f and Ce 5d orbitals contributes to the photoresponsive band structure of NaCeS2 . Secondly, due to the charge rearrangement supplied by the incompletely filled 4f orbitals of Ce ions, NaCeS2 exhibits excellent charge separation efficiency and CO2 adsorption affinity, reducing the energy barrier for the conversion from CO2 to CO. Moreover, a NaCeS2 -MoS2 heterostructure is also designed to further boost the electron transfer from the Mo site to the Ce site, which results in an improvement of the catalytic reduction yield from 7.24 to 23.42µmol g-1 within 9 h (both better than TiO2 controls). This work offers a platform for the development of rare-earth-based photocatalysts for CO2 conversion.

Construction of the Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite and its photocatalytic reduction of CO2

The Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite can be used as a photocatalyst for conversion of CO2 in order to deal with energy and environmental issues.

2d/2dtio2/G-C3n4 Ultrathin Nanosheet as an Efficient Biomimetic Photocatalyst for Conversion of Co2 and Ch4

2d/2dtio2/G-C3n4 Ultrathin Nanosheet as an Efficient Biomimetic Photocatalyst for Conversion of Co2 and Ch4

Cocatalyst Modification of AgTaO3 Photocatalyst for Conversion of Carbon Dioxide with Water

Cocatalyst Modification of AgTaO₃ Photocatalyst for Conversion of Carbon Dioxide with Water

Study of the Photoelectrochemical Properties of 1D ZnO Based Nanocomposites

Exploitation of common elements as photocatalysts for conversion of photons to electricity stimulates the development of a green energy strategy. In this paper, methods for the preparation of active coatings based on ZnO/Ag/CdS, which are used in the photocatalytic oxidation reaction, are examined. The physical and chemical properties of the resulting arrays were studied using optical spectrometers, an electron microscope, an X-ray diffractometer, and potentiostatic measurements and electrochemical impedance spectroscopy. The effectiveness of photocatalysts was calculated by the ability to liberate gas from aqueous solutions when exposed to light. The rate of degradation was indirectly measured with a conductometer.

Hybrid Phase MoS2 as a Noble Metal-Free Photocatalyst for Conversion of Nitroaromatics to Aminoaromatics

Hybrid Phase MoS₂ as a Noble Metal-Free Photocatalyst for Conversion of Nitroaromatics to Aminoaromatics

In situ Irradiated XPS Investigation on S-Scheme TiO2 @ZnIn2 S4 Photocatalyst for Efficient Photocatalytic CO2 Reduction.

Reasonable design of efficient hierarchical photocatalysts has gained significant attention. Herein, a step-scheme (S-scheme) core-shell TiO2 @ZnIn2 S4 heterojunction is designed for photocatalytic CO2 reduction. The optimized sample exhibits much higher CO2 photoreduction conversion rates (the sum yield of CO, CH3 OH, and CH4 ) than the blank control, i.e., ZnIn2 S4 and TiO2 . The improved photocatalytic performance can be attributed to the inhibited recombination of photogenerated charge carriers induced by S-scheme heterojunction. The improvement is also attributed to the large specific surface areas and abundant active sites. Meanwhile, S-scheme photogenerated charge transfer mechanism is testified by in situ irradiated X-ray photoelectron spectroscopy, work function calculation, and electron paramagnetic resonance measurements. This work provides an effective strategy for designing highly efficient heterojunction photocatalysts for conversion of solar fuels.

Zirconium-based metal-organic framework as an efficiently heterogeneous photocatalyst for oxidation of benzyl halides to aldehydes

UiO-66-NH 2 is able to catalyze the oxidation of benzyl halides to aldehydes under mild and environment friendly conditions. • Metal organic frameworks was first used for the catalytic oxidation of benzyl bromide to benzaldehyde. • UiO-66-NH2 was applied as an reusable photocatalyst for conversion of benzyl halides to aldehydes with high selectivity and conversion. • This study provides general, environmental and high selective preparation of benzaldehydes. • This study broadens the application scope of UiO-66-NH 2 catalysts. The development of efficient heterogeneous catalysts for fine chemical synthesis is critical for practical applications. Herein, for the first time, the zirconium-based metal-organic framework (UiO-66-NH 2 ) is applied as an efficiently heterogeneous photocatalyst for conversion of benzyl halides to corresponding aldehydes with high selectivity (about 80 %) and conversion (up to 99 %) in the presence of oxygen and DMF as solvent. Through a series of experiments and analysis, the reaction mechanism is proposed to involve nucleophilic attack of the N -oxide. This study provides a general, environmental and high selective method to prepare benzaldehydes and broadens the application fields of UiO-66-NH 2 .

CePMo 12 O 40 / TiO 2 catalysts for photocatalytic oxidation of methane to value‐added organic oxygenates

Methane is the main component of natural gas. The effective activation of C-H bond of methane to synthesize value-added compounds under mild conditions is attractive. Compared to the most investigated noble metal catalysts (such as Au, Pd, and Rh), rare earth metal catalysts possess stronger oxidizing activity. Herein, we report the first synthesis of Ce cation-based phosphomolybdic salt (CePMo12O40) with a simple milling method. Furthermore, CePMo12O40-based TiO2 catalyst (CePMo/TiO2) exhibited excellent catalytic performance for selective conversion of methane, achieving high yield (5618.5 μmol/gcat) of organic oxygenates (CH3OOH, HCOOH, and HCHO) using O2 as an oxidant in water at 150°C under 1.5G sunlight irradiation for 2 hours. It was found that the yield of total produced organic oxygenates over CePMo/TiO2 is 119% larger with 1.5G sunlight light irradiation than that without the light irradiation. In addition, the introduction of CePMo into TiO2 increased its photocatalytic activity for organic oxygenates by 11%. The isotope-labeling evaluation revealed that both H2O and O2 participated the oxidation of methane into CH3OOH, HCOOH, and HCHO. This work provides the first example of rare earth POM-based photocatalysts for conversion of methane into liquid oxygenates.

Photocatalytic conversion of CO2 to methanol using membrane-integrated Green approach: A review on capture, conversion and purification

• Abundantly available CO 2 could be converted to renewable fuel methanol. • Photo-catalyst integrated adsorbent could be the best choice for high methanol yield from CO 2 . • Membrane based separation is able to generate very high degree of purification. • Solar based membrane distillation system could minimize the overall production cost. • Pollutant abatement with simultaneous production of high value bio-energy in single system. In the modern world, due to the ever increasing demand of electricity, industrialization and auto-mobilization, the abundance of greenhouse gases has shoot up to a critical level. A critical review has been initiated which provides a comprehensive literatures survey in the last two decades on the novel approaches available on different technologies for the anthropogenic CO 2 capturing and conversion to methanol. In addition to that, merits and demerits of existing conventional technologies for downstream separation, purification and concentration enrichment of methanol have been discussed and compared with membrane-based system to find out the best optimal conditions. Extensive literature review reveals that the development of graphene based, TiO 2 /CuSO 4 coupled photocatalyst for conversion of CO 2 to methanol (33–37 mg/g catalyst) and downstream separation and purification using microfiltration membranes (Flux 100-110 L/m 2 h) stand out to be the best possible options for catalyst recycle and product recovery. In the previous studies the conversion was found as low as in the range of 10–20 mg/g catalyst without any development of hydrogen exfoliation graphene based nanocomposite material as well as any integration of spent catalyst recycle or product purification technology based on membrane separation. Such innovation and integration of process design employing cutting-edge schemes not only reduces the concentration of CO 2 in biosphere but also produces renewable energy. These efforts towards green manufacturing while confirming the potentials of sustainable business is undeniably essential and should be stimulated.

Ternary ZnO/ZnAl2O4/ Al2O3 composite nanofiber as photocatalyst for conversion of CO2 and CH4

Electrospinning is a flexible synthesis method which not only facilitates the preparation of the nanofibers with dramatically improved surface area, but also takes advantage of the structural defects to form heterojunctions with photocatalytic activities. In this study for the first time, a continuous layer of composite ceramic nanofiber was fabricated by employing two-nozzle electrospinning method to make the nanofiber from a 12: 3: 3: 13: 87 (percent ratio) solution of aluminum acetate: boehmite nanoparticle: zinc acetate: polyacrylonitrile: dimethylformamide respectively. The step by step thermal post-processing was performed on the nanofiber to decompose the polymeric part and achieve a pure ceramic phase. Characterization of the ceramic nanofiber was conducted using Fe-SEM, FTIR, XRD, TGA-DTA, BET and UV–Vis techniques. The SEM and XRD results confirmed that the ceramic nanofiber with average diameter of 160 nm was composed of ZnO, Al2O3 and ZnAl2O4 phases. The resultant nanofiber was used as a photocatalyst for conversion of CO2 in presence of CH4 as the reductant and UV–A irradiation under mild conditions. The maximum conversion percentages of CO2 and CH4 after 240 min were 20% and 11.7% respectively.The present study proposes a method for preparing a ternary nanofiber of Al2O3, ZnO, and ZnAl2O4 with the potential to be used as a photocatalyst for conversion of CO2 in presence of CH4.