Leny Yuliati

Photocatalytic conversion of methane

Various efforts have been carried out to convert methane to more useful chemicals and hydrogen. However, due to its high stability, high energy is usually consumed for its conversion, which still remains as a problem to be solved. Recently, photocatalysis has been proposed to be one of the answers to break the thermodynamic barrier. This tutorial review provides a brief history about developments in the methane conversion and specially highlights the developments in the photocatalytic conversion of methane, such as methane coupling and methane conversion with other molecules.

A urea precursor to synthesize carbon nitride with mesoporosity for enhanced activity in the photocatalytic removal of phenol.

A urea precursor was used for the first time to prepare mesoporous carbon nitride (MCN) by a thermal polymerization process with silica nanospheres as a hard template. Although the prepared MCN samples have similar structures and optical properties, it was revealed that the specific surface area, pore-size distribution, and morphology of the MCN samples depend on the initial mass ratio of urea to silica. Compared to the bulk carbon nitride (BCN) that only gave 20% phenol removal (6 h of irradiation), the activities can be enhanced up to 74% on MCN samples for photocatalytic removal of phenol under visible-light irradiation. The highest conversion was obtained on MCN with an initial mass ratio of urea to silica of 5, which has high surface area of 191 m(2) g(-1) and a nanoporous structure with uniform pore-size distribution of 7 nm. In addition to the high activity, the MCN sample also showed high photocatalytic stability.

Highly dispersed magnesium oxide species on silica as photoactive sites for photoinduced direct methane coupling and photoluminescence

Photoinduced direct methane coupling proceeded around room temperature over highly dispersed magnesium oxide species on silica, which exhibited fine structure in photoluminescence emission spectra. It was found that increasing the emission intensity tends to give an increase in the photoactivity for this reaction. The emission sites in the silica-supported magnesia have vibrational energy around 950 cm(-1) and lifetime of excited state around 38 ms, which were similar properties to the previously reported other silica-based photoactive systems for this reaction, such as silica-alumina and silica-supported zirconia. These photoluminescence spectra could be similarly quenched by methane molecules. Thus, it is commonly suggested that in the systems of highly dispersed metal oxide species (MOx) on silica, the surface Si-O-M bonds are deeply related to the dominant photoactive sites for both the fine structural photoluminescence spectra and photoinduced direct methane coupling.

Mesoporous carbon nitride for adsorption and fluorescence sensor of N-nitrosopyrrolidine

A metal-free mesoporous carbon nitride (MCN) was investigated for the first time as an adsorbent for N-nitrosopyrrolidine (NPYR), which is one of the nitrosamine pollutants. Under the same condition, the adsorption capability of the MCN was found to be higher than that of the MCM-41. Since the adsorption isotherm was consistent with Langmuir and Freundlich model equations, it was suggested that the adsorption of NPYR molecules on the MCN occurred in the form of mono-molecular layer on the heterogeneous surface sites. It was proposed that MCN with suitable adsorption sites was beneficial for the adsorption of NPYR. The evidence on the interaction between the NPYR molecules and the MCN was supported by fluorescence spectroscopy. Two excitation wavelengths owing to the terminal N-C and N=C groups were used to monitor the interactions between the emission sites of the MCN and the NPYR molecules. It was confirmed that the intensity of the emission sites was quenched almost linearly with the concentration of NPYR. This result obviously suggested that the MCN would be applicable as a fluorescence sensor for detection of the NPYR molecules. From the Stern-Volmer plot, the quenching rate constant of terminal N-C groups was determined to be ca. two times higher than that of the N=C groups on MCN, suggesting that the terminal N-C groups on MCN would be the favoured sites interacted with the NPYR. Since initial concentration can be easily recovered, the interactions of NPYR on MCN were weak and might only involve electrostatic interactions.

High photocatalytic activity of Fe2O3/TiO2 nanocomposites prepared by photodeposition for degradation of 2,4-dichlorophenoxyacetic acid

Two series of Fe2O3/TiO2 samples were prepared via impregnation and photodeposition methods. The effect of preparation method on the properties and performance of Fe2O3/TiO2 for photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) under UV light irradiation was examined. The Fe2O3/TiO2 nanocomposites prepared by impregnation showed lower activity than the unmodified TiO2, mainly due to lower specific surface area caused by heat treatment. On the other hand, the Fe2O3/TiO2 nanocomposites prepared by photodeposition showed higher photocatalytic activity than the unmodified TiO2. Three times higher photocatalytic activity was obtained on the best photocatalyst, Fe2O3(0.5)/TiO2. The improved activity of TiO2 after photodeposition of Fe2O3 was contributed to the formation of a heterojunction between the Fe2O3 and TiO2 nanoparticles that improved charge transfer and suppressed electron–hole recombination. A further investigation on the role of the active species on Fe2O3/TiO2 confirmed that the crucial active species were both holes and superoxide radicals. The Fe2O3(0.5)/TiO2 sample also showed good stability and reusability, suggesting its potential for water purification applications.

Masking effect of copper oxides photodeposited on titanium dioxide: exploring UV, visible, and solar light activity

A series of copper oxide loaded TiO2 was prepared by a photodeposition method. It was clarified through XANES that the added copper species solely existed as highly dispersed Cu(II) and it remained as Cu(II) after the photocatalytic reaction. The prepared CuO/TiO2 with various contents (0.1–5 mol%) were used as photocatalysts for 2,4-dichlorophenoxyacetic acid (2,4-D) degradation under UV, visible, and solar simulator irradiation. Under UV light irradiation, the photocatalytic activity of 2,4-D decomposition increased by a maximum factor of 4.3 compared with unmodified TiO2 when TiO2 was loaded with 0.75 mol% CuO. In contrast, under visible light and solar simulator irradiation, the optimum loading of CuO was much lower (0.1 mol%) and enhancements of 22.5 and 2.4 times higher activities were observed, respectively. These results showed that the different masking effects due to the different light sources led to different optimum amounts of CuO, and the added CuO mostly contributed to the visible light activity of TiO2. Therefore, in addition to the increased charge separation and improved visible light absorption, the masking effect shall also be considered in designing highly active photocatalysts under visible and solar light irradiation.

Improved interfacial charge transfer and visible light activity of reduced graphene oxide–graphitic carbon nitride photocatalysts

Development of efficient visible light-driven photocatalysts is an important approach towards sustainability. Herein, reduced graphene oxide–graphitic carbon nitride (rGO–gCN) photocatalysts were successfully prepared by in situ photocatalytic reduction of graphene oxide (GO) in the presence of gCN as the photocatalyst. The XPS spectra revealed the presence of N-graphene, which confirmed the interaction between the rGO and the gCN. The rGO–gCN with GO loading amount of 0.1 wt% exhibited almost three times higher photocatalytic activity than gCN for degradation of phenol under visible light irradiation. Fluorescence spectroscopy, EIS, and photocurrent response studies indicated that the presence of rGO significantly promoted the electron–hole separation and improved the interfacial charge transfer on the gCN. These factors played a crucial role in enhancing the photocatalytic activity of the gCN under visible light irradiation.

High activity of Ag-doped Cd0.1Zn0.9S photocatalyst prepared by the hydrothermal method for hydrogen production under visible-light irradiation

Summary Background: The hydrothermal method was used as a new approach to prepare a series of Ag-doped Cd0.1Zn0.9S photocatalysts. The effect of Ag doping on the properties and photocatalytic activity of Cd0.1Zn0.9S was studied for the hydrogen production from water reduction under visible light irradiation. Results: Compared to the series prepared by the co-precipitation method, samples prepared by the hydrothermal method performed with a better photocatalytic activity. The sample with the optimum amount of Ag doping showed the highest hydrogen production rate of 3.91 mmol/h, which was 1.7 times higher than that of undoped Cd0.1Zn0.9S. With the Ag doping, a red shift in the optical response was observed, leading to a larger portion of the visible light absorption than that of without doping. In addition to the larger absorption in the visible-light region, the increase in photocatalytic activity of samples with Ag doping may also come from the Ag species facilitating electron–hole separation. Conclusion: This study demonstrated that Ag doping is a promising way to enhance the activity of Cd0.1Zn0.9S photocatalyst.

Photocatalytic removal of phenol under visible light irradiation on zinc phthalocyanine/mesoporous carbon nitride nanocomposites

A series of zinc phthalocyanine/mesoporous carbon nitride (ZnPc/MCN) nanocomposites was prepared successfully by an impregnation method. The addition of ZnPc (0.05–1.5 wt%) extended the absorption of MCN to longer visible light region without affected its structure. It was found that the photocatalytic activity of the nanocomposites for phenol removal depended on the loading amount of ZnPc. The photocatalytic activity of the MCN increased as the amount of ZnPc increased to 0.05 wt%, but further increase in the loading amount decreased the activity. It was suggested that the optimum amount of ZnPc acted as a good photosensitiser that effectively induced electron charge transfer and reduced the electron-hole recombination.

Preparation of isolated highly dispersed titanium oxides on silica by sol-gel method for photocatalytic non-oxidative direct methane coupling

Isolated highly dispersed titanium oxide species on silica as photocatalytic active sites for non-oxidative direct methane coupling, were successfully prepared by sol-gel method, in which hydrolysis of TEOS to form Si(OH) 4 would be completed, followed by condensation to form Si-O-Ti bonds. It was found that both hydrolysis and condensation time much affected the formation of the isolated highly dispersed species. The sample prepared by the present sol-gel method showed higher activity than those prepared by conventional impregnation method.