Semiconducting oxide photocatalysts for reduction of CO<Subscript>2</Subscript> to methanol

The explosive growth in anthropogenic energy consumption, coupled with the consequent environmental pollution, have been acknowledged as two impending challenges confronting humanity. Photocatalytic CO₂ reduction to produce value-added hydrocarbon fuels, by using abundant solar energy and redundant atmospheric CO₂, is an innovative way to satisfy global energy requirements whilst simultaneously reducing atmospheric CO₂ levels. Although this notion is several decades old, it has unfortunately been lingering in a state of infancy due to inherently poor CO₂-to-fuel conversion efficiencies, and the generation of low-value products (e.g., CO, HCHO). These pitfalls hamper this process from any potential commercial breakthrough and are primarily fuelled by the lack of progress in developing high-performance photocatalytic materials. Fortunately, the advent of nanotechnology has recently introduced many promising novel materials for this purpose. Here, we review photocatalysts with proven potential for converting CO₂ into methanol, a high-value, energy-dense hydrocarbon fuel that is easily transported using existing pipeline infrastructure. Methanol possesses multifarious applications in the automobile, industrial and petrochemical sector. In addition, the development of direct methanol fuel cells (DMFCs) has introduced the tantalizing prospect of using methanol as a medium for storing solar energy that is easily converted to electricity via DMFCs. As such, methanol is an ideal fuel, with numerous advantages over its counterparts. This article reviews several photocatalysts that have been reported for this environmentally sustainable process of converting CO₂ into methanol by photocatalysis. Specifically, the performance enhancement effected by adding dopant atoms, forming heterostructured composites and nanostructures, is investigated in terms of four key areas: (1) enhanced visible light sensitivity, (2) improved adsorption of reactants on the catalytic surface, (3) lowered electron–hole recombination and (4) increased CO₂ reduction kinetics. The trends deduced therein are invaluable for researchers developing novel photocatalytic materials, which will utilize sunlight to convert CO₂ into methanol with enhanced efficiency, thus ushering in the era of a green methanol-based economy.     

Conversion of plant materials into hydroxymethylfurfural using ionic liquids

The use of fossil fuels now induces two major issues. First, fossil fuel burning is increasing atmospheric carbon dioxide (CO₂) concentrations and, in turn, global warming. Second, fossil fuel resources are limited and will thus decrease in the long run. As a potential solution, there is a need for ecological manufacturing processes that convert raw plant materials into chemical products. For instance, raw plants can be directly converted into hydroxymethylfurfural, which is a versatile intermediate for the synthesis of valuable biofuels such as dimethylfuran and 5-ethoxymethyl-2-furfural. This technology has two benefits for chemical sustainability. First, the pretreatment step is eliminated, thus contributing to reduction of CO₂ emissions. Second, plants are sustainable resources versus fossil fuels, which are limited. Here, we review current sustainable technologies for the production of biobased products and hydroxymethylfurfural from plants, using in particular ionic liquids. Plant sources include poplar, switchgrass, miscanthus, weed plants, and agave species.     

Selective reduction of NO by photo-SCR with ammonia in an annular fixed-film photoreactor

Gaseous NO was photocatalytically reduced at room temperature by photo-assisted selective catalytic reduction (photo-SCR) with ammonia over TiO₂ in this study. NO reduction efficiency and N₂ selectivity were determined from gases composition at the outlet stream of photoreactor. Effect of operating conditions, e.g. light intensity and inlet concentrations of ammonia and oxygen, on the NO reduction efficiency and N₂ selectivity were discussed to determine the feasible operating condition for photocatalytic reduction of NO. Experimental results showed that selective catalytic reduction of NO with ammonia over TiO₂ in the presence of oxygen was a spontaneous reaction in dark. The photoirradiation on the TiO₂ surface caused remarkable photocatalytic reduction of NO to form N₂, NO₂, and N₂O under 254 nm UV illuminations, while almost 90% of N₂ selectivity was achieved in this study. The ammonia and oxygen molecules played the roles of reductant and oxidant for NO reduction and active sites regeneration, respectively. The reduction of NO was found to be increased with the increase of inlet ammonia and oxygen concentrations until specific concentrations because of the limited active sites on the surface of TiO₂. The kinetic model proposed in this study can be used to reasonably describe the reaction mechanism of photo-SCR.     

Photocatalytic degradation of the herbicide cinosulfuron in aqueous TiO<Subscript>2</Subscript> suspension

The photocatalytic degradation of a sulfonylurea herbicide, cinosulfuron, has been studied in TiO₂ aqueous suspensions. A first order kinetic law was found. The influence of the initial concentration of cinosulfuron and of the initial radiant flux on the kinetics were evaluated. The identification of the intermediate products was based on high performance liquid chromatography coupled with mass spectrometry analyses (HPLC-MS). The mineralization of cinosulfuron was traced using ion chromatography and total organic carbon (TOC) measurements. These results indicate that the photocatalytic degradation of cinosulfuron leads to CO₂, NO₃ ⁻ and SO₄ ²⁻ as final products, and in addition cyanuric acid (C₃H₃O₃N₃), confirming previous results on triazinic ring-containing compounds. Electronic Publication     

TiO2 photocatalyzed degradation of diazinon in an aqueous medium

The photocatalytic degradation of diazinon was studied over TiO₂ catalysts. The kinetics obtained demonstrated that powder titania (t_1/2 = 9.7 min) was more efficient compared to pure titania thin film catalysts (t_1/2 = 29.4 min). Mineralization of organic carbon to CO₂ after 360 min of irradiation was found to be 75% while heteroatoms (P, S, N) were mineralized into phosphate, sulfate and nitrate ions, respectively. A microtox test was performed to evaluate the toxicity of solutions treated by catalysts. Illumination of diazinon in the presence of TiO₂ gave rise to several intermediates that have been identified by means of solid phase extraction and gas chromatography-mass spectrometry, while a simple degradation pathway is proposed.     

Embodied energy and emergy evaluation of a typical biodiesel production chain in China

Biodiesel from non-grain feedstock has been considered as one of the proper substitutes for fossil fuels associated with a series of activities emerging in China in order to meet the resource shortage and develop the energy crops. This paper presents an ecological accounting framework based on embodied energy, emergy, and CO₂ emission for the whole production chain of biodiesel made from Jatropha curcas L. (JCL) oil. The energy and materials invested in and CO₂ emission from the whole process, including cropping, transportation, extraction, and production, are accounted and calculated. Also, E_mCO₂, the ratio of real CO₂ released to the emergy-based sustainability indicator per joule biodiesel, is proposed in this paper to present a new goal function for low-carbon system optimization. Finally, the results are compared with those of the bioethanol (wheat) production in Henan Province, China, and bioethanol (corn) production in Italy in view of the indices of embodied energy, emergy and CO₂ emissions and E_mCO₂.     

Photodegradation of an azo dye by silver-doped nano-particulate titanium dioxide

The synthesis of silver doped nano-particulate titanium dioxide (Ag/TiO₂) using a microemulsion method and an investigation of its photocatalytic activity for the degradation of Acid Red 27 in distilled water under UV-irradiation is reported. The prepared Ag/TiO₂ is characterized using transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. The size of the Ag nanoparticles is around 5–15?nm, with almost uniform distribution on the TiO₂ particles. The efficiency of the photocatalytic process is evaluated to establish the optimum conditions, found to be at 2?wt% of Ag loading on TiO₂, catalyst dosage of 400?mg?L^?1, and calcination temperature of 300°C. Complete decolorization of the dye solution on Ag/TiO₂ was observed in 20?min of UV irradiation in the optimum conditions.     

Photocatalytic degradation of phenol with mesoporous TiO2?xBx

We report a facile approach for preparing mesoporous boron-doped TiO₂ materials by combining the sol?Cgel process with the dehydration of glucose. Specifically a high surface carbon material was formed by dehydration of glucose, then used as template. This material and the TiO₂ dry gel were calcinated to produce porous TiO₂. The as-synthesized boron-doped TiO₂ was in pure anatase crystallite phase with high surface area. X-ray photoelectron spectroscopy (XPS) results showed that boron was incorporated into the anatase TiO₂ lattice to form TiO_2?xB_x. The absorption spectra of TiO_2?xB_x extended into the visible region to 460?nm. The TiO_2?xB_x exhibited much higher photocatalytic activity on phenol degradation than pure TiO₂. It showed that the phenol degradation by-products of TiO_2?xB_x were different from that of pure TiO₂. Mechanism of the photocatalytic degradation of phenol at TiO_2?xB_x was also proposed.     

Ag–TiO2 doped photo catalytic degradation of Procion blue H-B dye in textile washwater

The photocatalytic degradation of Procion blue H-B dye in biodegraded textile washwater has been investigated for the complete removal of color and maximum reduction of chemical oxygen demand (COD). Pseudomonas putida was utilized for obtaining biodegraded textile washwater. In this process, silver-doped TiO₂ photocatalyst was prepared and experiments were carried out to study the effects of UV and mercury lamp irradiations on COD reduction and removal of color. The thus prepared silver-doped TiO₂ catalyst was characterized by thermogravimetric and differential thermal analysis, UV-visible spectrometer, X-ray diffraction, scanning electron microscope, energy dispersive X-ray microanalysis, and BET surface area techniques. Adsorption studies were also carried out to evaluate the fitness of isotherm models. The results show that the silver-doped TiO₂ has enhanced the photodegradation of Procion blue H-B dye under UV and mercury lamp irradiations. The enhanced activity of silver-doped TiO₂ is due to the enrichment of electron–hole separation by electron trapping of silver particles.     

Solar photocatalytic decomposition of two azo dyes on multi-walled carbon nanotubes (MWCNTs)/TiO<Subscript>2</Subscript> composites

Multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite photocatalysts with high photoactivity were prepared by sol-gel process and further characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), and UV-vis absorption spectra. Compared to pure TiO₂, the combination of MWCNTs with titania could cause a significant absorption shift toward the visible region. The photocatalytic performances of the MWCNTs/TiO₂ composite catalysts were evaluated for the decomposition of Reactive light yellow K-6G (K-6G) and Mordant black 7 (MB 7) azo dyes solution under solar light irradiation. The results showed that the addition of MWCNTs enhanced the adsorption and photocatalytic activity of TiO₂ for the degradation of azo dyes K-6G and MB 7. The effect of MWCNTs content, catalyst dosage, pH, and initial dye concentration were examined as operational parameters. The kinetics of photocatalytic degradation of two dyes was found to follow a pseudo-first-order rate law. The photocatalyst was used for seven cycles with photocatalytic degradation efficiency still higher than 98%. A plausible mechanism is also proposed and discussed on the basis of experimental results.     

Economic and environmental performance of electricity production in Finland: A multicriteria assessment framework

Meeting environmental, economic, and societal targets in energy policy is complex and requires a multicriteria assessment framework capable of exploring trade-offs among alternative energy options. In this study, we integrated economic analysis and biophysical accounting methods to investigate the performance of electricity production in Finland at plant and national level. Economic and environmental costs of electricity generation technologies were assessed by evaluating economic features (direct monetary production cost), direct and indirect use of fossil fuels (GER cost), environmental impact (CO₂ emissions), and global environmental support (emergy cost). Three scenarios for Finland's energy future in 2025 and 2050 were also drawn and compared with the reference year 2008. Accounting for an emission permit of 25 €/t CO₂, the production costs calculated for CHP, gas, coal, and peat power plants resulted in 42, 67, 68, and 74 €/MWh, respectively. For wind and nuclear power a production cost of 63 and 35 €/MWh were calculated. The sensitivity analysis confirmed wind power's competitiveness when the price of emission permits overcomes 20 €/t CO₂. Hydro, wind, and nuclear power were characterized by a minor dependence on fossil fuels, showing a GER cost of 0.04, 0.13, and 0.26 J/J_e, and a value of direct and indirect CO₂ emissions of 0.01, 0.04, and 0.07 t CO₂/MWh. Instead, peat, coal, gas, and CHP plants showed a GER cost of 4.18, 4.00, 2.78, and 2.33 J/J_e. At national level, a major economic and environmental load was given by CHP and nuclear power while hydro power showed a minor load in spite of its large production. The scenario analysis raised technological and environmental concerns due to the massive increase of nuclear power and wood biomass exploitation. In conclusion, we addressed the need to further develop an energy policy for Finland's energy future based on a diversified energy mix oriented to the sustainable exploitation of local, renewable, and environmentally friendly energy sources.     

Preparation and characterization of a new TiO2/SiO2 composite catalyst for photocatalytic degradation of indigo carmin

The TiO₂/SiO₂ composite was prepared by means of the SiO₂-particle-entrapment method. The FTIR data showed the presence of Si–O–Ti stretching vibration band at 970 cm⁻¹ in the TiO₂/SiO₂ composite, suggesting a reaction between TiO₂ and silica on the TiO₂ particle surface during the silicagel formation around the TiO₂ particles. The photocatalytic efficiency of TiO₂ immobilized in silicagel was compared with that of the conventional TiO₂ Degussa P25 catalyst. For this purpose, the degradation of indigo carmin (IC) dye was used as model molecule in the tests. The effect of operational parameters such as catalyst loading and dye concentration on the photocatalytic degradation of the model dye was investigated. The rate of degradation increased with increasing catalyst loading, and when the concentration of the dye decreases.     

Thermochemical conversion of municipal solid waste into energy and hydrogen: a review

The rising global population is inducing a fast increase in the amount of municipal waste and, in turn, issues of rising cost and environmental pollution. Therefore, alternative treatments such as waste-to-energy should be developed in the context of the circular economy. Here, we review the conversion of municipal solid waste into energy using thermochemical methods such as gasification, combustion, pyrolysis and torrefaction. Energy yield depends on operating conditions and feedstock composition. For instance, torrefaction of municipal waste at 200 °C generates a heating value of 33.01 MJ/kg, while the co-pyrolysis of cereals and peanut waste yields a heating value of 31.44 MJ/kg at 540 °C. Gasification at 800 °C shows higher carbon conversion for plastics, of 94.48%, than for waste wood and grass pellets, of 70–75%. Integrating two or more thermochemical treatments is actually gaining high momentum due to higher energy yield. We also review reforming catalysts to enhance dihydrogen production, such as nickel on support materials such as CaTiO₃, SrTiO₃, BaTiO₃, Al₂O₃, TiO₃, MgO, ZrO₂. Techno-economic analysis, sensitivity analysis and life cycle assessment are discussed.

     

Degradation of azo dye reactive violet 5 by TiO2 photocatalysis

The photocatalytic degradation of hydrolyzed reactive violet 5 (RV5) using titanium dioxide (TiO₂) was investigated in this study. The effects of various factors including the amount of photocatalyst, RV5 concentration, light intensity, and pH on photocatalytic degradation were evaluated. The photodegradation efficiency was 90% after 20 min of irradiation and reached nearly 100% after 80 min under the condition of pH 4 and temperature of 25°C. The decolorization rate typically followed first-order reaction, and increased markedly with increasing amount of photocatalyst, pH as well as light intensity. The total mineralization, based on total organic carbon (TOC) concentration was 53% after 20 min of UV light exposure and approached nearly 100% after 140 min. The final mineralization product was formylformamide. The photodegradation was faster than the mineralization, indicating that the intermediate products of decolorization were resistant to photodegradation. In this study, we found that toxicity of RV5 significantly decreased after decolorization. Our study suggests that the photocatalytic degradation treatment of RV5 with TiO₂ in wastewater is a simple and fast method.     

Preparation of effective TiO<Subscript>2</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> photocatalysts for water treatment

Photocatalytic oxidation using semiconductors is one of the advanced oxidation processes for degradation of organic pollutants in water and air. TiO₂ is an excellent photocatalyst that can mineralize a large range of organic pollutants such as pesticides and dyes. The main challenge is to improve the efficiency of the TiO₂ photocatalyst and to extend TiO₂ light absorption spectra to the visible region. A potential solution is to couple TiO₂ with a narrow band gap semiconductor possessing a higher conduction band such as bismuth oxide. Therefore, here we prepared Bi₂O₃/TiO₂ heterojunctions by the impregnation method with different Bi/Ti ratio. The prepared composites have been characterized by UV–Vis diffused reflectance spectra and X-ray diffraction. The photocatalytic activity of the heterojunction has been determined from the degradation of orange II under visible and UV light. Results show that Bi₂O₃/TiO₂ heterojunctions are more effective than pure TiO₂-anatase under UV-A irradiation, with an optimum for the Bi/Ti ratio of 5 %, for the photocatalytic degradation of Orange II. However, the photocatalytic activity under irradiation at λ higher than 420 nm is not much improved. Under UV–visible radiation, the two semiconductors are activated. We propose a mechanism explaining why our products are more effective under UV–visible irradiation. In this case the charge separation is enhanced because a part of photogenerated electrons from the conduction band of TiO₂ will go to the conduction band of bismuth oxide. In this composite, titanium dioxide is the main photocatalyst, while bismuth oxide acts as adsorbent photosensitizer under visible light.     

Photocatalytic degradation of the diuron pesticide

The occurrence of chlorinated pesticides in wellwaters is a major problem of public health in Ivory Coast and other African countries. Here, we studied the photocatalytic degradation of the pesticide diuron in aqueous solution in presence of two commercial TiO₂ catalysts, P25 and PC500. The capacity of diuron adsorption at the TiO₂ surface is lower for both photocatalysts. The efficiency of photocatalytic degradation of diuron, it is higher using P25 Degussa than PC500 Millenium TiO₂ catalyst.     

Urea formation from carbon dioxide and ammonia at atmospheric pressure

Urea synthesis, currently the largest use of carbon dioxide in organic synthesis, is conventionally operated at high pressure and high temperature. Here, we report for the first time that urea forms at atmosphere and ambient temperatures by negative corona discharge in gas phase. The conversion of CO₂ and yields of a solid mixture of urea and ammonium carbamate, which was identified by the ¹³C NMR spectrum, rise with reducing temperatures and increasing molar ratios of NH₃/CO₂ and discharge frequencies. The conversion of carbon dioxide was found to be 82.16?% at 20?°C and 1?atm with a molar flow ratio of n(NH₃)/n(CO₂) of 2.5. High pressure and high temperature as energy inputs are not necessary.     

Long-run dynamics of renewable energy consumption on carbon emissions and economic growth in the European union

This paper examines long-run and short-run dynamics of renewable energy consumption on carbon dioxide (CO₂) emissions and economic growth in the European Union. This study employs cointegration tests, Granger causality tests and vector error correction estimates to examine the direction of Granger causality, the long-run dynamics of economic growth and energy variables on carbon emissions. This study analyses time series data from the World Development Indicators over the period from1961 to 2012. The results of this study support a link between renewable energy consumption, economic growth, industrialization, exports and CO₂ emissions in the long-run and short-run. The results support that the sign of the long-run dynamics from the endogenous variables to the CO₂ emissions variable is negative and significant, which implies that the energy and environmental policies of the European Union aimed at curbing CO₂ emissions must have been effective in the long-term. Furthermore, renewable energy consumption and exports have significant negative impact on CO₂ emissions in the short-run. However, industrialization and economic growth have positive impact on CO₂ emissions in the short-run. The results suggest that both economic growth and industrialization must have been achieved at the cost of harming the environment. The finding suggests that the increasing consumption of renewable energy tends to play an important role in curbing carbon emissions in the region.     

Decomposition analysis of energy-related carbon dioxide emissions in the iron and steel industry in China

This work aims to identify the main factors influencing the energy-related carbon dioxide (CO₂) emissions from the iron and steel industry in China during the period of 1995–2007. The logarithmic mean divisia index (LMDI) technique was applied with period-wise analysis and time-series analysis. Changes in energyrelated CO₂ emissions were decomposed into four factors: emission factor effect, energy structure effect, energy consumption effect, and the steel production effect. The results show that steel production is the major factor responsible for the rise in CO₂ emissions during the sampling period; on the other hand the energy consumption is the largest contributor to the decrease in CO₂ emissions. To a lesser extent, the emission factor and energy structure effects have both negative and positive contributions to CO₂ emissions, respectively. Policy implications are provided regarding the reduction of CO₂ emissions from the iron and steel industry in China, such as controlling the overgrowth of steel production, improving energy-saving technologies, and introducing low-carbon energy sources into the iron and steel industry.     

Photocatalytic mineralization of a water pollutant,Sunset Yellow dye by an advanced oxidation process using a modified catalyst

ZnS-loaded TiO₂ (ZnS–TiO₂) was synthesized by a sol–gel method. The catalyst was characterized by using different techniques (XRD, HR-SEM, EDS, DRS, PL, XPS, and BET methods). The photocatalytic activity of ZnS–TiO₂ was investigated for the degradation of Sunset Yellow FCF (SY) dye in an aqueous solution using ultraviolet light. ZnS–TiO₂ is found to be more efficient than prepared TiO₂, TiO₂–P25, TiO₂ (Merck), and ZnS at pH 7 for the mineralization of SY. The effects of operational parameters such as the amount of photocatalyst, dye concentration, and initial pH on photo mineralization of SY have been analyzed. The mineralization of SY has been confirmed by chemical oxygen demand measurements. The catalyst is found to be reusable.