No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Progress on renewable synthetic fuels
Developing biomass economy is conducive to economic growth and rural development; it reduces fossil energy consumption and improves ecological environment. Biomass economy has great development potentials, but it still faces various challenges. In this article, the biomass resources in China are comprehensively analyzed, the industry and its current situation and trend are evaluated, and the strategic objectives, key projects, and policy suggestions for industrial development are proposed. China is rich in biomass resources, and biomass briquette fuel, large-scale biogas engineering, biomass power generation and cogeneration technologies are mature. These technologies have exhibited a good momentum of large-scale development and will be the main utilization approaches for a period of time in the future. The development idea proposed in this article for the biomass economy in China regards each county-level region as a unit and considers regional resource endowment, economic development level, and industrial development requirements. The development goals of the biomass economy in China are clarified as becoming mature, system innovation, and large-scale replacement by 2025, 2035, and 2050, respectively. Furthermore, policy suggestions are proposed in terms of technology research and development, incentive policy, market cultivation, and capital investment.
Biomass and other carbonaceous materials can be gasified to produce syngas with high concentrations of CO and H2. Feedstock materials include wood, dedicated energy crops, grain wastes, manufacturing or municipal wastes, natural gas, petroleum and chemical wastes, lignin, coal and tires. Syngas fermentation converts CO and H2 to alcohols and organic acids and uses concepts applicable in fermentation of gas phase substrates. The growth of chemoautotrophic microbes produces a wide range of chemicals from the enzyme platform of native organisms. In this review paper, the Wood–Ljungdahl biochemical pathway used by chemoautotrophs is described including balanced reactions, reaction sites physically located within the cell and cell mechanisms for energy conservation that govern production. Important concepts discussed include gas solubility, mass transfer, thermodynamics of enzyme-catalyzed reactions, electrochemistry and cellular electron carriers and fermentation kinetics. Potential applications of these concepts include acid and alcohol production, hydrogen generation and conversion of methane to liquids or hydrogen.
The aqueous electrocatalytic reduction of CO2 into alcohol and hydrocarbon fuels presents a sustainable route towards energy-rich chemical feedstocks. Cu is the only material able to catalyse the substantial formation of multicarbon products (C2/C3), but competing proton reduction to hydrogen is an ever-present drain on selectivity. Here, a superhydrophobic surface was generated by 1-octadecanethiol treatment of hierarchically structured Cu dendrites, inspired by the structure of gas-trapping cuticles on subaquatic spiders. The hydrophobic electrode attained a 56% Faradaic efficiency for ethylene and 17% for ethanol production at neutral pH, compared to 9% and 4% on a hydrophilic, wettable equivalent. These observations are assigned to trapped gases at the hydrophobic Cu surface, which increase the concentration of CO2 at the electrode–solution interface and consequently increase CO2 reduction selectivity. Hydrophobicity is thus proposed as a governing factor in CO2 reduction selectivity and can help explain trends seen on previously reported electrocatalysts.
The electrochemical CO2 reduction reaction (CO2RR) to produce CO and H2 (syngas) is a promising method for clean energy, but challenges remain, such as controlling the CO/H2 ratios required for the syngas yield. Herein, hydrophobic exfoliated MoS2 (H‐E‐MoS2) nanosheets are fabricated from bulk MoS2 by a cost‐effective ball‐milling method, followed by decoration with fluorosilane (FAS). H‐E‐MoS2 is a cost‐effective electrocatalyst capable of directly reducing CO2 and H2O for tuneable syngas production with a wide range of CO/H2 ratios (from 1:2 to 4:1). In addition, H‐E‐MoS2 shows a high current density, 61 mA cm−2 at −1.1 V, and the highest CO FE of 81.2% at −0.9 V, which are higher than those of unmodified MoS2. According to density functional theory calculations, FAS decoration on the surface of MoS2 electrode can change the electronic properties of the edge Mo atom, which facilitates the rate‐limiting CO‐desorption step, thus promoting CO2RR. Moreover, the hydrophobic surface of H‐E‐MoS2 depressed the H2 evolution reaction and created abundant three‐phase contact points that provided sufficient CO2. The hydrophobization of the electrode may provide an effective strategy for easily tuning the CO/H2 ratio of syngas in a large range for the direct electroreduction CO2 to syngas with an optimized CO/H2 ratio. A hydrophobic exfoliated MoS2 (H‐E‐MoS2) electrode is successfully developed as a robust electrocatalyst for the electroreduction of CO2 into syngas with controlled CO/H2 ratios between 1:2 and 4:1. According to density functional theory calculations, fluorosilane decoration on the surface of the exfoliated MoS2 (E‐MoS2) electrode facilitates the desorption of the CO product.
A very basic pathway from CO 2 to ethylene
Ethylene is an important commodity chemical for plastics. It is considered a tractable target for synthesizing renewably from carbon dioxide (CO 2 ). The challenge is that the performance of the copper electrocatalysts used for this conversion under the required basic reaction conditions suffers from the competing reaction of CO 2 with the base to form bicarbonate. Dinh et al. designed an electrode that tolerates the base by optimizing CO 2 diffusion to the catalytic sites (see the Perspective by Ager and Lapkin). This catalyst design delivers 70% efficiency for 150 hours.
Science , this issue p. 783 ; see also p. 707
Electrocatalytic carbon dioxide reduction to formate is desirable but challenging. Current attention is mostly focused on tin-based materials, which, unfortunately, often suffer from limited Faradaic efficiency. The potential of bismuth in carbon dioxide reduction has been suggested but remained understudied. Here, we report that ultrathin bismuth nanosheets are prepared from the in situ topotactic transformation of bismuth oxyiodide nanosheets. They process single crystallinity and enlarged surface areas. Such an advantageous nanostructure affords the material with excellent electrocatalytic performance for carbon dioxide reduction to formate. High selectivity (~100%) and large current density are measured over a broad potential, as well as excellent durability for >10 h. Its selectivity for formate is also understood by density functional theory calculations. In addition, bismuth nanosheets were coupled with an iridium-based oxygen evolution electrocatalyst to achieve efficient full-cell electrolysis. When powered by two AA-size alkaline batteries, the full cell exhibits impressive Faradaic efficiency and electricity-to-formate conversion efficiency.
Aqueous-phase electrochemical reduction of carbon dioxide requires an active, earth-abundant electrocatalyst, as well as highly efficient mass transport. Here we report the design of a porous hollow fibre copper electrode with a compact three-dimensional geometry, which provides a large area, three-phase boundary for gas-liquid reactions. The performance of the copper electrode is significantly enhanced; at overpotentials between 200 and 400 mV, faradaic efficiencies for carbon dioxide reduction up to 85% are obtained. Moreover, the carbon monoxide formation rate is at least one order of magnitude larger when compared with state-of-the-art nanocrystalline copper electrodes. Copper hollow fibre electrodes can be prepared via a facile method that is compatible with existing large-scale production processes. The results of this study may inspire the development of new types of microtubular electrodes for electrochemical processes in which at least one gas-phase reactant is involved, such as in fuel cell technology.
In present study, repeated-batch fermentation in biofilm reactor was evaluated for ethanol production by using carob extract. Response surface method (RSM) was used to determine optimum initial sugar concentration, pH, and agitation in the constructed biofilm reactor. The optimum conditions for ethanol production in biofilm reactor were determined as initial sugar content of 7.71°Bx, pH of 5.18 and agitation of 120 rpm. The ethanol production (P), the yield (YP/S) and production rate (QP) were found as 24.51 g/L 48.59% and 2.14 g/L/h at the optimized conditions, respectively. The fermentation time for maximum ethanol production in carob extract was reduced to 12 h by using a biofilm reactor compared the published data in the literature, which was 30 and 24 h by using suspended and immobilized yeasts in a stirred tank reactor, respectively. Furthermore, the effect of various nitrogen sources and enrichment were also evaluated and YP/S, QS and QP were significantly decreased compared to the results obtained from optimum conditions in biofilm reactor. Overall, results showed that a biofilm reactor for ethanol production from carob extract can be successfully implemented in point of reducing fermentation time, eliminate re-inoculation of the typical batch fermentation as well as increasing production yield.
Lignocellulosic waste (LCW) is an abundant, low-cost, and inedible substrate for the induction of lignocellulolytic enzymes for cellulosic bioethanol production using an efficient, environmentally friendly, and economical biological approach. In this study, 30 different lignocellulose-degrading bacterial and 18 fungal isolates were quantitatively screened individually for the saccharification of four different ball-milled straw substrates: wheat, rice, sugarcane, and pea straw. Rice and sugarcane straws which had similar Fourier transform-infrared spectroscopy profiles were more degradable, and resulted in more hydrolytic enzyme production than wheat and pea straws. Crude enzyme produced on native straws performed better than those on artificial substrates (such as cellulose and xylan). Four fungal and five bacterial isolates were selected (based on their high strawase activities) for constructing dual and triple microbial combinations to investigate microbial synergistic effects on saccharification. Combinations such as FUNG16-FUNG17 (Neosartorya fischeri-Myceliophthora thermophila) and RMIT10-RMIT11 (Aeromonas hydrophila-Pseudomonas poae) enhanced saccharification (3- and 6.6-folds, respectively) compared with their monocultures indicating the beneficial effects of synergism between those isolates. Dual isolate combinations were more efficient at straw saccharification than triple combinations in both bacterial and fungal assays. Overall, co-culturing can result in significant increases in saccharification which may offer significant commercial potential for the use of microbial consortia.
To develop a more sustainable bio-based economy, an increasing amount of carbon for industrial applications and biofuel will be obtained from bioenergy crops. This may result in intensified land use and potential conflicts with other ecosystem services provided by soil, such as control of greenhouse gas emissions, carbon sequestra-tion, and nutrient dynamics. A growing number of studies examine how bioenergy crops influence carbon and nitrogen cycling. Few studies, however, have combined such assessments with analysing both the immediate effects on the provisioning of soil ecosystem services as well as the legacy effects for subsequent crops in the rotation. Here, we present results from field and laboratory experiments on effects of a standard first-generation bioenergy crop (maize) and three different second-generation bioenergy crops (willow short rotation coppice (SRC), Miscanthus 9 giganteus, switchgrass) on key soil quality parameters: soil structure, organic matter, biodiversity and growth and disease susceptibility of a major follow-up crop, wheat (Triticum aestivum). We analysed a 6-year field experiment and show that willow SRC, Miscanthus, and maize maintained a high yield over this period. Soil quality parameters and legacy effects of Miscanthus and switchgrass were similar or performed worse than maize. In contrast, willow SRC enhanced soil organic carbon concentration (0–5 cm), soil fertility, and soil biodiversity in the upper soil layer when compared to maize. In a greenhouse experiment, wheat grown in willow soil had higher biomass production than when grown in maize or Miscanthus soil and exhibited no growth reduction in response to introduction of a soil-borne (Rhizoctonia solani) or a leaf pathogen (Mycosphaerel-la graminicola). We conclude that the choice of bioenergy crops can greatly influence provisioning of soil ecosystem services and legacy effects in soil. Our results imply that bioenergy crops with specific traits might even enhance ecosystem properties through positive legacy effects.
The photocatalytic reduction of carbon dioxide with water to fuels and chemicals is a longstanding challenge. This article focuses on the effects of cocatalysts and reaction modes on photocatalytic behaviors of TiO2 with an emphasis on the selectivity of photogenerated electrons for CO2 reduction in the presence of H2O, which has been overlooked in most of the published papers. Our results clarified that the reaction using H2O vapor exhibited significantly higher selectivity for CO2 reduction than that by immersing the photocatalyst into liquid H2O. We examined the effect of noble metal cocatalysts and found that the rate of CH4 formation increased in the sequence of Ag < Rh < Au < Pd < Pt, corresponding well to the increase in the efficiency of electron–hole separation. This indicates that Pt is the most effective cocatalyst to extract photogenerated electrons for CO2 reduction. The selectivity of CH4 in CO2 reduction was also enhanced by Pt. The size and loading amount of Pt affected the activity; a smaller mean size of Pt particles and an appropriate loading amount favored the formation of reduction products. The reduction of H2O to H2 was accelerated more than the reduction of CO2 in the presence of Pt, leading to a lower selectivity for CO2 reduction and limited increases in CH4 formation rate. We demonstrated that the addition of MgO into the Pt–TiO2 catalyst could further enhance the formation of CH4. The formation of H2 was suppressed simultaneously, suggesting the increase in the selectivity for CO2 reduction in the presence of MgO. Furthermore, the MgO- and Pt-modified TiO2 catalyst exhibited a higher CH4 selectivity in CO2 reduction.
Biodiesel is a renewable transportation fuel consisting of fatty acid methyl esters (FAME), generally produced by transesterification of vegetable oils and animal fats. In this review, the fatty acid (FA) profiles of 12 common biodiesel feedstocks were summarized. Considerable compositional variability exists across the range of feedstocks. For example, coconut, palm and tallow contain high amounts of saturated FA; while corn, rapeseed, safflower, soy, and sunflower are dominated by unsaturated FA. Much less information is available regarding the FA profiles of algal lipids that could serve as biodiesel feedstocks. However, some algal species contain considerably higher levels of poly-unsaturated FA than is typically found in vegetable oils.Differences in chemical and physical properties among biodiesel fuels can be explained largely by the fuels’ FA profiles. Two features that are especially influential are the size distribution and the degree of unsaturation within the FA structures. For the 12 biodiesel types reviewed here, it was shown that several fuel properties – including viscosity, specific gravity, cetane number, iodine value, and low temperature performance metrics – are highly correlated with the average unsaturation of the FAME profiles. Due to opposing effects of certain FAME structural features, it is not possible to define a single composition that is optimum with respect to all important fuel properties. However, to ensure satisfactory in-use performance with respect to low temperature operability and oxidative stability, biodiesel should contain relatively low concentrations of both long-chain saturated FAME and poly-unsaturated FAME.
Pure TiO2 anatase particles with a crystallite diameters ranging from 4.5 to 29nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO2 photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.
Air-blown gasifier suffers from lower heating value of producer gas (PG) compared to steam gasification, but at the expense of the addition of steam boiler for the latter. The performance of an air-blown double walled downdraft biomass gasifier was characterized experimentally. Air fogging unit was added to the annulus of the gasifier allowing for superheated steam to be generated as additional oxidizer. Steam-to-biomass (S/B) ratio has been introduced to investigate the optimum amount of injected water needed to produce the optimum quality of PG. Comparisons were made on the composition and heating value of PG with and without water injection which were analysed and calculated with the aid of gas chromatograph. Highest heating value of PG was 4.72 MJ/Nm³ at S/B ratio of 0.2 which corresponded to about 10% increment. Various S/B ratios were investigated in this study in the range of 0.1–0.3. However, as S/B ratio exceeded 0.25, it resulted adversely to the quality of PG. The effect of water injection on tar contamination in PG was also investigated. Tar reduction was proportional to the amount of the injected water resulting in about 8% reduction at maximum S/B ratio corresponding to 0.65 g/m³ tar yield.
Electrochemical reduction of CO2 to liquid products is an attractive approach for achieving a carbon‐neutral energy cycle. However, the CO2 reduction reaction (CO2RR) on most electrocatalysts usually suffer from poor catalytic activity, low Faradaic efficiency (FE) and energy efficiency (EE) as well as inadequate stability. Herein, Bi2O3 nanosheets have been successfully grown on a conductive multiple channel carbon matrix (MCCM) for CO2RR. The obtained electrocatayst shows a desirable partial current density of ~17.7 mA cm–2 at a moderate overpotential, and it is highly selective towards HCOOH formation with FE approaching 90% in a wide potential window and its maximum value of 93.8% at −1.256 V. Additionally, it also exhibits a maximum EE of 55.3% at an overpotential of 0.846 V and long‐term stability of 12 h with negligible degradation. The superior performance is mainly attributed to the synergistic contribution of the interwoven MCCM and the hierarchical Bi2O3 nanosheets, where the MCCM provides an accelerated electron transfer, increased CO2 adsorption and a high ratio of pyrrolic‐N and pyridinic‐N, while ultrathin Bi2O3 nanosheets offer abundant active sites, lowered contact resistance and work function as well as shortened diffusion pathways for electrolyte.
Flowing CO 2 boosts a molecular catalyst
Molecular electrocatalysts for CO 2 reduction have often appeared to lack sufficient activity or stability for practical application. Ren et al. now show that design of the surrounding electrochemical cell can substantially boost both features. They directly exposed a known molecular catalyst, cobalt phthalocyanine, to gaseous CO 2 in a flow cell architecture, rather than an aqueous electrolyte. The configuration accommodated current densities exceeding 150 milliamperes per square centimeter, with longevity limited by local proton concentration rather than catalyst stability.
Science , this issue p. 367
ConspectusFeS proteins are metalloproteins prevalent in the metabolic pathways of most organisms, playing key roles in a wide range of essential cellular processes. A member of this protein family, the Fe protein of nitrogenase, is a homodimer that contains a redox-active [Fe 4 S 4 ] cluster at the subunit interface and an ATP-binding site within each subunit. During catalysis, the Fe protein serves as the obligate electron donor for its catalytic partner, transferring electrons concomitant with ATP hydrolysis to the cofactor site of the catalytic component to enable substrate reduction. The effectiveness of Fe protein in electron transfer is reflected by the unique reactivity of nitrogenase toward small-molecule substrates. Most notably, nitrogenase is capable of catalyzing the ambient reduction of N 2 and CO into NH 4⁺ and hydrocarbons, respectively, in reactions that parallel the important industrial Haber-Bosch and Fischer-Tropsch processes. Other than participating in nitrogenase catalysis, the Fe protein also functions as an essential factor in nitrogenase assembly, which again highlights its capacity as an effective, ATP-dependent electron donor.Recently, the Fe protein of a soil bacterium, Azotobacter vinelandii, was shown to act as a reductase on its own and catalyze the ambient conversion of CO 2 to CO at its [Fe 4 S 4 ] cluster either under in vitro conditions when a strong reductant is supplied or under in vivo conditions through the action of an unknown electron donor(s) in the cell. Subsequently, the Fe protein of a mesophilic methanogenic organism, Methanosarcina acetivorans, was shown to catalyze the in vitro reduction of CO 2 and CO into hydrocarbons under ambient conditions, illustrating an impact of protein scaffold on the redox properties of the [Fe 4 S 4 ] cluster and the reactivity of the cluster toward C1 substrates. This reactivity was further traced to the [Fe 4 S 4 ] cluster itself, as a synthetic [Fe 4 S 4 ] compound was shown to catalyze the reduction of CO 2 and CO to hydrocarbons in solutions in the presence of a strong reductant. Together, these observations pointed to an inherent ability of the [Fe 4 S 4 ] clusters and, possibly, the FeS clusters in general to catalyze C1-substrate reduction. Theoretical calculations have led to the proposal of a plausible reaction pathway that involves the formation of hydrocarbons via aldehyde-like intermediates, providing an important framework for further mechanistic investigations of FeS-based activation and reduction of C1 substrates.In this Account, we summarize the recent work leading to the discovery of C1-substrate reduction by protein-bound and free [Fe 4 S 4 ] clusters as well as the current mechanistic understanding of this FeS-based reactivity. In addition, we briefly discuss the evolutionary implications of this discovery and potential applications that could be developed to enable FeS-based strategies for the ambient recycling of unwanted C1 waste into useful chemical commodities.
Syngas fermentation for fuels and chemicals is limited by the low rate of gas-to-liquid mass transfer. In this work, a unique bulk-gas-to-atomized-liquid (BGAL) contactor was developed to enhance mass transfer. In the BGAL system, liquid is atomized into discrete droplets, which significantly increases the interface between the liquid and bulk gas. Using oxygen as a model gas, the BGAL contactor achieved an oxygen transfer rate (OTR) of 569 mg·L ⁻¹ ·min ⁻¹ and a mass transfer coefficient (K L a) of 2.28 sec ⁻¹ , which are values as much as 100-fold greater than achieved in other kinds of reactors. The BGAL contactor was then combined with a packed bed to implement syngas fermentation, with packing material supporting a biofilm upon which gas saturated liquid is dispersed. This combination avoids dispersing these gas-saturated droplets into the bulk liquid, which would significantly dilute the dissolved gas concentration. Although this combination reduced overall K L a to 0.45–1.0 sec ⁻¹ , it is still nearly 20 times higher than achieved in a stirred tank reactor. The BGAL contactor/packed bed bioreactor was also more energy efficient in transferring gas to the liquid phase, requiring 8.63–26.32 J mg ⁻¹ O 2 dissolved, which is as much as four-fold reduction in energy requirement compared to a stirred tank reactor. Fermentation of syngas to ethanol was evaluated in the BGAL contactor/packed bed bioreactor using Clostridium carboxidivorans P7. Ethanol productivity reached 746 mg·L ⁻¹ ·h ⁻¹ with an ethanol/acetic acid molar ratio of 7.6. The ethanol productivity was two-fold high than the highest level previously reported. The exceptional capability of BGAL contactor to enhance mass transfer in these experiments suggests its utility in syngas fermentation as well as other gas-liquid contacting processes.
Photoreduction of CO 2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO 2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO 2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO 2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO 2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO 2 and complements the existing reviews on various semiconductor photocatalysts.
Solar-driven electrochemical CO2 reduction to fuels has considerable potential to satisfy future renewable energy needs and mitigate CO2 emissions, but current approaches face significant challenges, such as poor selectivity and large overpotential for the catalytic reactions. Here we present a hybrid microbial-photoelectrochemical system that selectively converts CO2 into methane in a single step without any by-products. The hybrid system employs a biocathode that was capable of reducing CO2 into CH4 with an unprecedentedly low overpotential (< 50 mV) and a TiO2 nanowire array photoanode for light harvesting and water oxidation. Using sunlight as the sole energy input, we demonstrate highly selective CO2 reduction to CH4 with an overall Faradaic efficiency of up to 96%. With a continuous supply of CO2, the hybrid system generates a stable current over 90 h, demonstrating its long-term stability.
Catalyst deactivation by coke deposition was the main barrier in the catalytic pyrolysis of biomass for hydrocarbons, thus deactivated catalysts should be regenerated timely. In this study, several measures were applied in the regeneration process to obtain higher yield of hydrocarbons in the catalytic conversion of biomass-derivates. The effect of oxygen concentration in the regeneration atmosphere on catalytic performance was firstly investigated, and then steam was introduced on the basis of the optimal oxygen concentration to further decrease the internal temperature of catalyst particle. Finally the deactivated catalysts were partially regenerated by retaining some coke. The results showed that 15% was the optimal oxygen concentration, at which the catalysts still showed steady production of olefins and aromatics after 30 cycles of catalysis-regeneration. When 15% oxygen and 5% steam was combined in the regeneration atmosphere, the temperature of catalyst bed was reduced efficiently. The stable yield of hydrocarbons by combined regeneration method was boosted by 31.3% compared with pure 15% oxygen. At retaining coke amount of 2.18%, yield of hydrocarbons was promoted by 27.4% compared with completely regenerated. With the combined oxygen/steam atmosphere and controlled regeneration, the temperature of catalyst particles was well regulated and stable catalysis-regeneration process to produce hydrocarbons via thermochemical utilization of biomass can be realized.
Globally, there is increasing awareness that renewable energy and energy efficiency are vital for both creating new economic opportunities and controlling the environmental pollution. AD technology is the biochemical process of biogas production which can change the complex organic materials into a clean and renewable source of energy. AcoD process is a reliable alternative option to resolve the disadvantages of single substrate digestion system related to substrate characteristics and system optimization. This paper reviewed the research progress and challenges of AcoD technology, and the contribution of different techniques in biogas production engineering. As the applicability and demand of the AcoD technology increases, the complexity of the system becomes increased, and the characterization of organic materials becomes volatile which requires advanced methods for investigation. Numerous publications have been noted that ADM1 model and its modified version becomes the most powerful tool to optimize the AcoD process of biogas production, and indicating that the disintegration and hydrolysis steps are the limiting factors of co-digestion process. Biochemical methane potential (BMP) test is promising method to determine the biodegradability and decomposition rate of organic materials. The addition of different environmentally friendly nanoparticles can improve the stability and performance of the AcoD system. The process optimization and improvement of biogas production still seek further investigations. Furthermore, using advanced simulation approaches and characterization methods of organic wastes can accelerate the transformation to industrializations, and realize the significant improvement of biogas production as a renewable source and economically feasible energy in developing countries, like China. Finally, the review reveals, designing and developing a framework, including various aspects to improve the biogas production is essential.
Algae may be fermented to produce hydrogen. However micro-algae (such as Arthrospira platensis) are rich in proteins and have a low carbon/nitrogen (C/N) ratio, which is not ideal for hydrogen fermentation. Co-fermentation with macro-algae (such as Laminaria digitata), which are rich in carbohydrates with a high (C/N) ratio, improves the performance of hydrogen production. Algal biomass, pre-treated with 2.5% dilute H2SO4 at 135 °C for 15 min, effected a total yield of carbohydrate monomers (CMs) of 0.268 g/g volatile solids (VS). The CMs were dominating by glucose and mannitol and most (ca. 95%) were consumed by anaerobic fermentative micro-organisms during subsequent fermentation. An optimal specific hydrogen yield (SHY) of 85.0 mL/g VS was obtained at an algal C/N ratio of 26.2 and an algal concentration of 20 g VS/L. The overall energy conversion efficiency increased from 31.3% to 54.5% with decreasing algal concentration from 40 to 5 VS g/L.
Photocatalytic reduction of CO2 is a promising technology to capture CO2 and convert it into solar fuels simultaneously. However, current photoreactors usually face the problems of low specific surface area, non-uniform light distribution and poor photon transfer. To address these issues, a novel optofluidic membrane microreactor with high surface-area-to-volume ratio, enhanced photon and mass transport and uniform light distribution was proposed in this work by combining optofluidics with the membrane reactor technology for the photocatalytic reduction of CO2 with liquid water. A TiO2/carbon paper composite membrane was prepared as the photocatalytic membrane via coating TiO2 onto the carbon paper followed by hydrophobic treatment by poly-tetrafluoroethylene (PTFE) for the separation of the gas/liquid phases. The performance of the proposed optofluidic membrane microreactor was evaluated by measuring the methanol yield. The effects of the liquid water flow rate, light intensity and catalyst loading on the methanol yield were also studied. It was shown that a maximum methanol yield of 111.0 μmole/g-cat·h was achieved at a flow rate of 25 μL/min and under the light intensity of 8 mW/cm2, which is among the top in comparison to the reported data. Results obtained fully demonstrate the feasibility and superiority of the proposed optofluidic membrane microreactor for the photocatalytic reduction of CO2.
In this study, the influence of various physical process parameters on the liquefaction of lignocellulosic biomass (pine wood) in supercritical ethanol was investigated. The parameters include reaction temperature (280-400 degrees C), initial nitrogen pressure (0.4-7.5 MPa), reaction time (0-240 min), and biomass-to-solvent ratio (0.06-0.25 g/g). The reaction temperature and residence time were found to have a more significant effect on biomass conversion and product yield than pressure and biomass-to-solvent ratio had; conversion in the range 34.0-98.1% and biocrude yield in the range 15.8-59.9 wt% were observed depending on the process parameters. Despite the absence of catalysts and external hydrogen source, solid biomass to liquid and gaseous products conversion of 98.1%, and a high biocrude yield of approximately 65.8 wt% were achieved at 400 degrees C, 120 min, and a biomass-to-solvent ratio of 0.06 g/g. Moreover, the biocrude contained considerably lower amounts of oxygen and higher amounts of carbon and hydrogen, resulting in a substantially higher heating value (>30 MJ/kg) as compared to raw feed-stock (20.4 MJ/kg). A comparison with sub- or supercritical water-based liquefaction revealed that supercritical ethanol produced biocrude with a lower molecular weight and much better yield. Finally, a new biomass liquefaction reaction mechanism associated with supercritical ethanol is proposed. Crown Copyright
The possibility of economically deriving fuel from cultivating algae biomass is an attractive addition to the range of measures to relieve the current reliance on fossil fuels. Algae biofuels avoid some of the previous drawbacks associated with crop-based biofuels as the algae do not compete with food crops. The favourable growing conditions found in many developing countries has led to a great deal of speculation about their potentials for reducing oil imports, stimulating rural economies, and even tackling hunger and poverty. By reviewing the status of this technology we suggest that the large uncertainties make it currently unsuitable as a priority for many developing countries. Using bibliometric and patent data analysis, we indicate that many developing countries lack the human capital to develop their own algae industry or adequately prepare policies to support imported technology. Also, we discuss the potential of modern biotechnology, especially genetic modification (GM) to produce new algal strains that are easier to harvest and yield more oil. Controversy surrounding the use of GM and weak biosafety regulatory system represents a significant challenge to adoption of GM technology in developing countries. A range of policy measures are also suggested to ensure that future progress in algae biofuels can contribute to sustainable development.
This research was designed for the first time to investigate the activities of photocatalytic composite, Ag3PO4/g-C3N4, in converting CO2 to fuels under simulated sunlight irradiation. The composite was synthesized using a simple in-situ deposition method and characterized by various techniques including Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), and an electrochemical method. Thorough investigation indicated that the composite consisted of Ag3PO4, Ag, and g-C3N4. The introduction of Ag3PO4 on g-C3N4 promoted its light absorption performance. However, more significant was the formation of hetero-junction structure between Ag3PO4 and g-C3N4, which efficiently promoted the separation of electron-hole pairs by a Z-scheme mechanism, and ultimately enhanced the photocatalytic CO2 reduction performance of the Ag3PO4/g-C3N4. The optimal Ag3PO4/g-C3N4 photocatalyst showed a CO2 conversion rate of 57.5 μmol•h-1•gcat-1, which was 6.1 and 10.4 times higher than those of g-C3N4 and P25, respectively, under simulated sunlight irradiation. The work found a new application of the photocatalyst, Ag3PO4/g-C3N4, in simultaneous environmental protection and energy production.
The photoreduction of CO2 on inorganic semiconductors has been researched for several decades, but the conversion efficiency is still low due to the recombination of photo-generated electron–hole pairs, low utilization efficiency of solar energy and weak adsorption of CO2. Here we for the first time demonstrate that metal–organic frameworks such as ZIF-8 can effectively adsorb CO2 dissolved in water, and promote photocatalytic activity of a semiconductor catalyst in CO2 reduction into liquid fuels in an aqueous medium. In particular, Zn2GeO4/ZIF-8 hybrid nanorods were successfully synthesized by growing ZIF-8 nanoparticles on Zn2GeO4 nanorods. The Zn2GeO4/ZIF-8 nanocomposite inherits both high CO2 adsorption capacity of ZIF-8 nanoparticles and high crystallinity of Zn2GeO4 nanorods. The Zn2GeO4/ZIF-8 hybrid nanorods containing 25 wt% ZIF-8 exhibit 3.8 times higher dissolved CO2 adsorption capacity than the bare Zn2GeO4 nanorods, resulting in a 62% enhancement in photocatalytic conversion of CO2 into liquid CH3OH fuel. The strategy reported here is promising for developing more active photocatalysts for improving CO2 conversion efficiency by taking advantage of excellent adsorption property of metal–organic frameworks in aqueous media.
The photoelectrocatalytic (PEC) reduction of CO2 into high-value chemicals is beneficial in alleviating global warming and advancing a low-carbon economy. In this work, Pt-modified reduced graphene oxide (Pt-RGO) and Pt-modified TiO2 nanotubes (Pt-TNT) were combined as cathode and photoanode catalysts, respectively, to form a PEC reactor for converting CO2 into valuable chemicals. XRD, XPS, TEM, AFM and SEM were employed to characterize the microstructures of the Pt-RGO and Pt-TNT catalysts. Reduction products, such as C2H5OH and CH3COOH, were obtained from CO2 under band gap illumination and biased voltage. A combined liquid product generation rate (CH3OH, C2H5OH, HCOOH and CH3COOH) of approximately 600 nmol/(h cm2) was observed. Carbon atom conversion rate reached 1,130 nmol/(h cm2), which were much higher than those achieved using Pt-modified carbon nanotubes and platinum carbon as cathode catalysts.
Efficient solar conversion of carbon dioxide and water vapor to methane and other hydrocarbons is achieved using nitrogen-doped titania nanotube arrays, with a wall thickness low enough to facilitate effective carrier transfer to the adsorbing species, surface-loaded with nanodimensional islands of cocatalysts platinum and/or copper. All experiments are conducted in outdoor sunlight at University Park, PA. Intermediate reaction products, hydrogen and carbon monoxide, are also detected with their relative concentrations underlying hydrocarbon production rates and dependent upon the nature of the cocatalysts on the nanotube array surface. Using outdoor global AM 1.5 sunlight, 100 mW/cm(2), a hydrocarbon production rate of 111 ppm cm(-2) h(-1), or approximately 160 microL/(g h), is obtained when the nanotube array samples are loaded with both Cu and Pt nanoparticles. This rate of CO(2) to hydrocarbon production obtained under outdoor sunlight is at least 20 times higher than previous published reports, which were conducted under laboratory conditions using UV illumination.
Global warming caused by anthropogenic CO2 emission has been one of the most important issues in the fields of science, environment and even international economics and politics. To control and reduce CO2 emissions, intensive carbon dioxide capture and storage (CCS) technologies have been comprehensively developed for sequestration of CO2 especially from combustion flue gas.Microalgae-based CO2 biological fixation is regarded as a potential way to not only reduce CO2 emission but also achieve energy utilization of microalgal biomass. However, in this approach culture process of microalgae plays an important role as it is directly related to the mechanism of microalgal-CO2 fixation and characteristics of microalgal biomass production. The aim of this work is to present a state-of-the-art review on the process effect, especially on the effects of photobiochemical process, microalgal species, physicochemical process and hydrodynamic process on the performance of microalgal-CO2 fixation and biomass production. Also, the perspectives are proposed in order to provide a positive reference on developing its fundamental research and key technology.
The liquefaction of “green tide” macroalgae Enteromorpha prolifera in sub-/supercritical alcohols in a batch reactor had been investigated. Effects of the temperature and algae/solvent ratio on the liquefaction yields in methanol and ethanol were studied. The results showed that, under the conditions of the reaction time of 15 min and algae/solvent ratio set at 1:10, the macroalgae in methanol at 280 °C produced a bio-oil yield at 31.1 wt % of dry weight and the ethanol at 300 °C yielded bio-oil at 35.3 wt %. Different from bio-oils obtained by hydrothermal liquefaction of microalgae as well as macroalgae in our previous work, the bio-oils obtained by liquefaction of macroalgae in alcohols are mainly composed of ester compounds. A variety of fatty acid (C3–C22) esters (methyl or ethyl) in the bio-oils obtained in methanol and ethanol, respectively, were qualified by gas chromatography–mass spectrometry, and their relative contents are above 60% of the total area for each bio-oil. In addition, some N-containing compounds, sugars, fatty alcohols/ketones, and very few hydrocarbons were also qualified. Overall, bio-oils obtained in two alcohols are much similar to biodiesel on the composition. The elemental analysis of bio-oils indicated that bio-oils still have high oxygen contents. Moreover, the bio-oils are found to contain a considerable fraction of light components using thermogravimetric analysis (TGA), and the contents of low-boiling-point (bp < 350 °C) compounds are up to 70% of the weight for both bio-oils; therefore, it might help for the further separation and refining of bio-oils to produce fuels and chemicals.
The main objective of the present work was the study of different ZSM-5 catalytic formulations for the in situ upgrading of biomass pyrolysis vapors. An equilibrium, commercial diluted ZSM-5 catalyst was used as the base case, in comparison with a series of nickel (Ni) and cobalt (Co) modified variants at varying metal loading (1–10 wt.%). The product yields and the composition of the produced bio-oil were significantly affected by the use of all ZSM-5 catalytic materials, compared to the non-catalytic flash pyrolysis, producing less bio-oil but of better quality. Incorporation of transition metals (Ni or Co) in the commercial equilibrium/diluted ZSM-5 catalyst had an additional effect on the performance of the parent ZSM-5 catalyst, with respect to product yields and bio-oil composition, with the NiO modified catalysts being more reactive towards decreasing the organic phase and increasing the gaseous products, compared to the Co3O4 supported catalysts. However, all the metal-modified catalysts exhibited limited reactivity towards water production, while simultaneously enhancing the production of aromatics and phenols. An interesting observation was the in situ reduction of the supported metal oxides during the pyrolysis reaction that eventually led to the formation of metallic Ni and Co species on the catalysts after reaction, which was verified by detailed XRD and HRTEM analysis of the used catalysts. The Co3O4 supported ZSM-5 catalysts exhibited also a promising performance in lowering the oxygen content of the organic phase of bio-oil.
Arthrospira platensis wet biomass was subjected to microwave-assisted dilute H2SO4 pretreatment to improve saccharification by hydrolysis with glucoamylase and hydrogen production from dark-fermentation. When the hydrolyzed biomass from A. platensis was inoculated with hydrogenogens (heat-treated anaerobic sludge) to produce hydrogen during dark-fermentation, the maximum hydrogen yield of 96.6 ml H2/g DW was obtained. Because high concentration of NH4+ (31.6–56.5 mM) in the residual solution (also containing acetate and butyrate) obtained from dark-fermentation can significantly inhibit the activities of photosynthetic bacteria in sequential photo-fermentation, a modified zeolite was used to extract NH4+ by ion exchange to reduce the NH4+ content to 2.2–2.7 mM (91.8%–95.8% of NH4+ removal efficiency). The treated residual solution was reused for hydrogen production in sequential photo-fermentation. The maximum hydrogen yield from A. platensis wet biomass was significantly enhanced from 96.6 to 337.0 ml H2/g DW using a combination of dark- and photo-fermentation.
Abstract Biofilms in the environment can both cause detrimental and beneficial effects. However, their use in bioreactors provides many advantages including lesser tendencies to develop membrane fouling and lower required capital costs, their higher biomass density and operation stability, contribution to resistance of microorganisms, etc. Biofilm formation occurs naturally by the attachment of microbial cells to the support without use of any chemicals agent in biofilm reactors. Biofilm reactors have been studied and commercially used for waste water treatment and bench and pilot-scale production of value-added products in the past decades. It is important to understand the fundamentals of biofilm formation, physical and chemical properties of a biofilm matrix to run the biofilm reactor at optimum conditions. This review includes the principles of biofilm formation; properties of a biofilm matrix and their roles in the biofilm formation; factors that improve the biofilm formation, such as support materials; advantages and disadvantages of biofilm reactors; and industrial applications of biofilm reactors.
In order to obtain valuable products such as monosaccharides and hydrogen from rice straw (RS), the two-stage processing, hydrothermal treatment in the first stage and steam gasification in the second stage, was carried out. In the hydrothermal treatment, influence of hydrothermal treatment conditions and pretreatments to the raw RS sample on the product distribution was examined. Maximum yield of monosaccharides from the raw RS sample was 1.1wt.% (C basis), which was obtained by the hydrothermal treatment at 220°C for 5min. A water treatment of raw RS sample was carried out before hydrothermal treatment to increase the yield of monosaccharides, so that 9.4wt.% (C basis) of formic acid was extracted. Furthermore, in the subsequent hydrothermal treatment, the yield of monosaccharides increased up to approximately 4.5wt.% (C basis). Simultaneously, 7.9wt.% (C basis) of acids and furfural and 45.1wt.% (C basis) of other water-soluble products were also formed. In the second stage, conversion of hydrothermal-treated rice straw residue (HT-RSR) into hydrogen was performed by steam gasification using fixed-bed reactor and influence of nickel catalyst was examined. Hydrogen from HT-RSR sample without catalyst was produced above 800°C, while hydrogen from the 7.5wt.% nickel-loaded sample was evolved at lower temperature (500°C). The peak top temperature of the hydrogen evolution was shifted from 850°C for the 1.5wt.% nickel-loaded sample to 750°C for the 7.5wt.% nickel-loaded sample. Total amount of hydrogen evolved from the samples loaded with nickel more than 2wt.% was 50–60mmol/g-RSR and about three times larger than that from HT-RSR sample without catalyst. In addition, e.g. for the 2.3wt.% nickel-loaded sample, the CO, CO2, and other gaseous products were also evolved and their yields were 9.4, 21.1, and 3.1wt.% (C basis), respectively.
Titanium oxide species included within the framework of mesoporous zeolites (Ti-MCM-41 and Ti-MCM-48) prepared by a hydrothermal synthesis exhibited high and unique photocatalytic reactivity for the reduction of CO2 with H2O at 328K to produce CH4 and CH3OH in the gas phase. In situ photoluminescence, diffuse reflectance absorption, ESR and XAFS investigations indicated that the titanium oxide species are highly dispersed within the zeolite framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species played a significant role in the reduction of CO2 with H2O exhibiting a high selectivity for the formation of CH3OH.
Gasification of biomass at 600–800°C can produce an effluent gas for use in a combustor for power production after removal of the solid products of gasification by hot gas cleanup (e.g., aspen wood). This will avoid the fouling and/or corrosion often found in biomass combustion. Biomasses which form liquids below 800°C (e.g., wheat straw) require the use of additives which raise the lowest temperatures for the presence of inorganic liquids to over 800°C. This alternative path for avoiding the fouling and corrosion found during combustion of many biomasses probably can be applied to a broad range of analogous materials.
Diesel or jet fuel range branched alkanes were synthesized for the first time by the combination of hydroxyalkylation-alkylation (HAA) of 2-methylfuran with hydroxyacetone and subsequent hydrodeoxygenation. Due to the electron-withdrawing effect of the hydroxyl group, the hydroxyacetone route exhibited evident advantages (higher HAA reactivity and diesel yield) over the previous acetone route.
THE non-biological reduction of carbon dioxide to organic compounds is of interest, as an alternative to natural photosynthesis, for the production of organic raw materials or fuel. In one approach, the required energy was supplied by irradiation with UV light, in the presence of ferrous salts, and resulted in the production of formic acid and of formaldehyde1. In another approach, the energy was supplied from an external power source by electrochemical reduction of aqueous carbon dioxide. The reduction of carbon dioxide and production of formic acid during the electrolysis of sodium bicarbonate in aqueous solutions has also been reported2, and a study of the reduction of carbon dioxide on a mercury cathode reviews earlier work3. Polarographic measurements on mercury electrodes showed that carbon dioxide, rather than the bicarbonate ion, is the electroactive species, with a half-wave reduction potential of −2.1 V (relative to SCE), and that formic acid is the only product4. We report here the photoassisted electrolytic reduction of aqueous carbon dioxide, achieved using p-type gallium phosphide as a photocathode, with part or all of the energy being supplied by light. The products were formic acid, formaldehyde and methanol.
Biomass gasification in a fluidized bed with steam−O2 mixtures has been studied in detail at pilot plant scale. The gasifier used was 15 cm i.d. and 3.2 m high, and it was fed with pine wood chips at flow rates of 5−20 kg/h. Main operating variables studied were gasifier bed temperature (780−890 °C), steam to oxygen in the feeding ratio (2−3 mol/mol), and gasifying agent (H2O + O2) to biomass fed ratio (0.6−1.6 kg/kg daf). Product distribution here shown includes gas, tar and char yields, gas composition (H2, CO, CO2, CH4, steam, ...) and heating value, tar composition and content in the flue gas, gas heating value, apparent thermal efficiency, etc. Under good operating conditions the following gas is obtained: tar content of 5 g/Nm3, 30 vol % H2, heating value of 16.0 MJ/Nm3 (dry basis), gas yield of 1.2 Nm3 (dry basis)/kg biomass fed.
This paper presents bio-oil preparation by direct liquefaction of Dunaliella tertiolecta (D. tertiolecta) with sub/supercritical ethanol-water as the medium in a batch autoclave with high temperature and high pressure. The results indicated that ethanol and water showed synergistic effects on direct liquefaction of D. tertiolecta. The maximum bio-oil yield was 64.68%, with an optimal D. tertiolecta conversion of 98.24% in sub/supercritical ethanol-water. The detailed chemical compositional analysis of the bio-oil was performed using an EA, FT-IR, and GC-MS. The empirical formulas of the bio-oil obtained using the ethanol-water co-solvent (40%, v/v) and sole water as the reaction medium were CH(1.52)O(0.14)N(0.06) and CH(1.43)O(0.23)N(0.09), with calorific values of 34.96 and 29.80MJkg(-1), respectively. XPS and SEM results showed that ethanol-water is a very effective reaction medium in the liquefaction. A plausible reaction mechanism of the main chemical component in D. tertiolecta is proposed based on our results and the literatures.
The cracking removal of tar component in high-temperature fuel gas cleanup is one of the most crucial problems in developing cleanest advanced power technology. Five catalysts were evaluated to tar com-ponent removal from high-temperature fuel gas in a fixed-bed reactor. 1-Methylnaphthalene was chosen as a model of tar component. The Y-zeolite and NiMo catalysts were found to be the most effective catalysts. Two catalysts almost removed 100% tar component at 550 °C. The process variables, temperature and space velocity, have very significant effects on tar component removal with catalysts. The long-term du-rability shows that two catalysts maintain more than 95% removal conversion at 550 °C in 168 h. The combustion study of coke deposited on catalysts by thermal gravimetric analysis technology shows that very small amount buildup of coke appears on two catalysts surface. Using a first-order kinetic model, the apparent energies of activation and pre-exponential factors for tar component removal reaction and coke combustion on catalysts were obtained for the most active catalysts.
Biomass resources include wood and wood wastes, agricultural crops and their waste byproducts, municipal solid waste, animal wastes, waste from food processing and aquatic plants and algae. Biomass is used to meet a variety of energy needs, including generating electricity, heating homes, fueling vehicles and providing process heat for industrial facilities. The conversion technologies for utilizing biomass can be separated into four basic categories: direct combustion processes, thermochemical processes, biochemical processes and agrochemical processes. Thermochemical conversion processes can be subdivided into gasification, pyrolysis, supercritical fluid extraction and direct liquefaction. Pyrolysis is the thermochemical process that converts biomass into liquid, charcoal and non-condensable gases, acetic acid, acetone and methanol by heating the biomass to about 750 K in the absence of air. If the purpose is to maximize the yield of liquid products resulting from biomass pyrolysis, a low temperature, high heating rate, short gas residence time process would be required. For high char production, a low temperature, low heating rate process would be chosen. If the purpose is to maximize the yield of fuel gas resulting from pyrolysis, a high temperature, low heating rate, long gas residence time process would be preferred.
A study of the steam gasification of Cynara cardunculus L. was carried out in order to characterise the gas phase with a view to its energy use, analysing the influence of water partial pressure, particle size, and temperature. The main gases generated were H2, CH4, CO, and CO2, with a higher heating value between 10 and 11 MJ/N m3. The gas in greatest proportion was H2; that in the smallest proportion was methane, which was pyrolytic in origin. Within the range of variables studied, the particle size had no significant effect on the process. Temperature and water partial pressure exerted positive effects on the main parameters of the process, increasing the reaction rate, the gas yield and production, the conversion and the energy generated per kilogram of initial residue (Cynara). The experimental results show that the water–gas shift reaction is the main determinant of the composition of the gases. For this reaction, an increase in temperature leads to a greater formation of CO, and an increase in water partial pressure to a greater formation of CO2. The energy yield of the gasification process presented values between 0.5 and 0.85. Temperature and water partial pressure had a positive effect on this parameter.
Burn, baby, burn! A new bifunctional catalyst (Ga/ZSM-5) displays increased selectivity for the production of aromatic compounds during the catalytic fast pyrolysis of biomass. With this Ga-promoted ZSM-5 catalyst, olefins such as ethylene and propylene, which are produced as intermediates, are more efficiently converted into aromatics, especially benzene. Ga/ZSM-5 also promotes decarbonylation and olefin-aromatization reactions.
Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.