C.W. Kwong

2D phosphorene as a water splitting photocatalyst: fundamentals to applications

Hydrogen from direct splitting of water molecules using photons is reckoned to be a sustainable and renewable energy solution for the post fossil-fuel era. Efficient photocatalysts, including metal-free photocatalysts, are key determinants of cost-effective hydrogen generation at a large-scale. The search for new materials that are metal-free is therefore ongoing. Recently, 2D phosphorene, a phosphorus analogue of graphene, has been added as a new semiconductor to the family of monolayer-flatland materials. In this review, we focus on analysing the fundamental electronic, optical and chemical properties of 2D phosphorene and assess its suitability as a metal-free water splitting photocatalyst. We also critically analyse its stability against claims from environmental antagonists and attempt to predict its future as a photocatalyst. This review provides timely information for researchers, scientists and professionals devoted to materials research for photocatalysis.

Chemical looping combustion of biomass-derived syngas using ceria-supported oxygen carriers.

Cu, Ni and Fe oxides supported on ceria were investigated for their performance as oxygen carriers during the chemical looping combustion of biomass-derived syngas. A complex gas mixture containing CO, H2, CO2, CH4 and other hydrocarbons was used to simulate the complex fuel gas environment derived from biomass gasification. Results show that the transfer of the stored oxygen into oxidants for the supported Cu and Ni oxides at 800°C for the combustion of syngas was effective (>85%). The unsupported Cu oxide showed high oxygen carrying capacity but particle sintering was observed at 800°C. A reaction temperature of 950°C was required for the supported Fe oxides to transfer the stored oxygen into oxidants effectively. Also, for the complex fuel gas environment, the supported Ni oxide was somewhat effective in reforming CH4 and other light hydrocarbons into CO, which may have benefits for the reduction of tar produced during biomass pyrolysis.

Fly-ash products from biomass co-combustion for VOC control

Experiments were conducted in a continuous flow reactor at room temperature to evaluate the elimination of low-concentration toluene in the gas phase to verify if fly-ash products from biomass combustion in an ozonation system could be used in the removal of volatile organic compounds. The fly-ash products from pure biomass combustion (Ash(100)) demonstrated the highest ozonation activities upon the removal of low-concentration toluene (1.5 ppmv), followed by the fly-ash products from co-combustion (Ash(30)) and the coal combustion (Ash(0)). Kinetic experiments showed that the activation energy of the toluene elimination process was substantially reduced with the use of ozone and the reaction intermediates, such as formic acids, aldehydes, etc. Results also showed that the intermediates were reduced with increasing humidity level. The combined use of fly-ash products and zeolite 13X enhanced the removal of toluene to above 90% and suppressed the release of residual ozone and intermediates by holding them in the adsorbed phase.

Catalytic Combustion of Methane with Ozone Using Pd-Exchanged Zeolite X: Experimental Investigation and Kinetics Model

The authors experimentally investigated the catalytic combustion of methane on a 3-wt%-Pd-ion-exchanged zeolite 13X catalyst under various reaction temperatures, ozone/methane concentration ratios, and gas hourly space velocities. A model based on Langmuir-Hinshelwood kinetics was developed to analyze the performance of the system. Without the catalyst, the activation energy for methane oxidation with ozone is 148 kJ/mol. Adding catalysts but using oxygen only as oxidant, the activation energy reduces to 114 kJ/mol. Adding ozone to the system greatly reduces the activation energy to 63 kJ/mol. The enhanced methane conversion by adding ozone with catalysts could be attributed to the decomposition of ozone to atomic oxygen species. With the catalyst, at temperatures below 400°C, methane conversion is mainly achieved by reaction with ozone. Adding ozone at this low temperature range can enhance the energy efficiency of methane conversion. Reaction with oxygen becomes increasingly important at higher temperature and becomes the dominant mechanism at above 500°C.

Methane emission abatement by Pd-ion-exchanged zeolite 13X with ozone

The performance of 3 wt% Pd-ion-exchanged zeolite 13X for methane emission abatement under various parameters including reaction temperature (295–435 °C), inlet ozone concentration (0–4 vol%), space velocity (12 500–62 400 h−1) and inlet methane concentration (200 and 1400 ppm) were investigated. Without zeolite, gas phase reaction among methane, air and ozone gave 1–11% of methane conversion at 295–435 °C. The presence of inlet ozone (1.1–4 vol%) to the zeolite 13X system enhanced methane removal efficiency by 24–56%, giving an overall performance of 93% in this low temperature range. There existed a range of reaction temperatures where methane conversion was most enhanced by ozone in the zeolite system. Without ozone, methane oxidation showed an activation energy of 136 kJ mol−1. This was reduced to 116 kJ mol−1 by using 4 vol% of ozone. The enhanced methane removal efficiency was attributed to the decomposition of ozone to form atomic oxygen reacting with adsorbed methane. Higher percentage methane conversion was observed at lower space velocities under various inlet ozone concentrations. As the inlet ozone and methane concentration increased, the methane removal efficiency was limited by the number of available active sites in the zeolite, which governs the amount of adsorbed methane and atomic oxygen available for subsequent reactions. The system can be used for energy efficient green house gas and industrial effluent treatment.

Polycyclic aromatic hydrocarbons on particulate matter emitted during the co-generation of bioenergy and biochar from rice husk

The aim of this study was to evaluate the emissions of polycyclic aromatic hydrocarbons (PAHs) bound to the particulate matter (PM) during the combustion of raw pyrolysis volatiles (bio-oil and pyrogas mixture) generated from the pyrolysis of rice husk. Five different raw pyrolysis volatiles were produced at varying pyrolysis temperatures (400-800°C) and subsequently combusted in a laboratory-scale, continuous pyrolysis-combustion facility at 850°C. 15 priority pollutant PAH levels in the resulting biochar, bio-oil, and PM were evaluated. Results showed that combustion of the raw pyrolysis volatiles produced at elevated pyrolysis temperatures resulted in greater concentrations of PM-bound PAHs (119% increase between 400 and 800°C) due to the increased PAH and oxy-aromatic content of the bio-oil fraction. Significantly increased benzo(a)pyrene (BaP) - equivalent toxicity of the biochar and PM was observed at elevated pyrolysis temperatures.

Correction: 2D phosphorene as a water splitting photocatalyst: fundamentals to applications

Correction for ‘2D phosphorene as a water splitting photocatalyst: fundamentals to applications’ by Mohammad Ziaur Rahman et al., Energy Environ. Sci., 2016, 9, 709–728.

Emission characteristics of a pyrolysis-combustion system for the co-production of biochar and bioenergy from agricultural wastes

The co-production of biochar and bioenergy using pyrolysis-combustion processes can potentially minimize the emission problems associated with conventional methods of agricultural by-product disposal. This approach also provides significant added-value potential through biochar application to soil. Despite these advantages, variations in biomass composition, including sulfur, nitrogen, ash, and volatile matter (VM) content, may significantly influence both the biochar quality and the emissions of harmful particulate matter (PM) and gaseous pollutants (SO2, H2S, NO2, NO). Using a laboratory-scale continuous pyrolysis-combustion facility, the influence of biomass composition (rice husk and grape pruning) and volatile production (pyrolysis) temperature (400-800 °C) on the biochar properties and emissions during combustion of the raw pyrolysis volatiles were evaluated. Utilization of grape pruning resulted in higher energy-based yields of PM10 than the rice husk, the majority of which consisted of the PM1.1 fraction due to the elevated pyrogas content of the volatiles. The PM emissions were found to be independent of the feedstock ash content due to its retainment in the biochar. Greater volatilization of biomass sulfur and nitrogen during pyrolysis at higher temperatures resulted in higher yields of sulfurous and nitrogenous gaseous pollutants. The energy-based yields of NO and NO2 were found to increase by 16% and 50% for rice husk and 21% and 189% for grape pruning respectively between 400 and 800 °C. The same trend was also observed for the emissions of H2S and SO2 for both feedstocks.

Influences of spinel type and polymeric surfactants on the size evolution of colloidal magnetic nanocrystals (MFe2O4, M= Fe, Mn)

Two types of polymeric surfactants, PEG300 and PVP40000, were used for the preparation of magnetic ferrite MFe2O4 (M = Mn, Fe) colloidal nanocrystals using a solvothermal reaction method. The effect of spinel type effect on the size evolution of various nanoparticles was investigated. It was found that Fe3O4 nanoparticles exhibited higher crystalinity and size evolution than MnFe2O4 nanoparticles with use of the two surfactants. It is proposed that this observation is due to fewer tendencies of surfactants on the surface of Fe3O4 building blocks nanoparticles than MnFe2O4. Less amounts of surfactant or capping agent on the surface of nanoparticles lead to the higher crystalibity and larger size. It is also suggested that the type of spinel (normal or inverted spinel) plays a key role on the affinity of the polymeric surfactant on the surface of building blocks.

Recycling biomass co-combustion fly-ash products for an integrated solar-assisted ventilation system

The potential use of biomass co-combustion derived fly-ash products and zeolite 13X for the elimination of volatile organic compounds (VOCs) using ozone was investigated for an integrated solar-assisted air purification and desiccant cooling system. Fly-ash products from rice husk-coal co-combustion at different biomass blending ratios were used as the adsorbent/catalyst materials. The material characteristics of the adsorbent/catalyst materials such as metal content and surface area were compared and correlated with the catalytic activities. It was found that the surface area and the metal constitutes have made the catalytic activities over the fly-ash products from biomass co-combustion superior to that from coal-only combustion. The elevated reaction temperatures from 25°C to 75°C also have significant effects on the removal of VOCs. The apparent activation energies of the reaction path over the fly-ash products with the addition of ozone to the air were reduced, when compared with the use of air as an oxidant. On the other hand, the potential synergy to Zeolite 13X was explored. The combined catalytic ozonation and adsorption enhanced the VOCs removal and at the same time reduced the intermediates emission. Furthermore, the hydrophilic properties of zeolite 13X could be utilized to handle the latent load of the solar-assisted ventilation system for energy conservations.Copyright