This work is supported by the Natural Science Foundation of China (No. 21506194, 21676255).

Caution: Please consult all relevant Material Safety Data Sheets (MSDS) before operation. Several chemicals are flammable and highly corrosive. Concentrating light can cause harmful light intensity and temperature increases. Please use all appropriate safety devices such as personal protective equipment (safety glasses, gloves, lab coats, pants, etc.).
1. Catalyst Preparation
<ol>
	<li>Preparation of TiO2 by anodization 
	Note: Anodization uses metal foils and a Pt foil as two counter electrodes. The two electrodes are put into the electrolyte. Using electricity, the metal foils at the anode site are oxidized.
	<ol>
		<li>Dissolve 0.3 g of NH4F and 2 mL of H2O into 100 mL of glycol in a 200-mL beaker with a stirrer to form the electrolyte. Put the beaker with the electrolyte into a 45 &#176;C water bath.</li>
		<li>Trim the Ti foil (50 x 250 mm size) with scissors to 25 x 25 mm.</li>
		<li>Polish the Ti foil surface with a 7,000-mesh sandpaper to remove the surface impurities.</li>
		<li>Submerge the Ti foil in a volumetric flask containing 15 mL of ethanol, then a flask with 15 mL of acetone, then treat it for 15 min with an ultrasonic cleaner. Take out the Ti foil, rinse it 3 - 5x with deionized water, and place it in a volumetric flask containing 20 mL of ethanol.</li>
		<li>Dissolve 10 mL of H2O, 5 mL of HNO3, 3 mL of H2O2, 1 mL of 18% wt (NH2)2CO, and 1 mL of 18% wt NH4F into a 100-mL beaker to form a polishing solution.</li>
		<li>Take out the Ti foil from the ethanol flask, rinse it 3x with deionized water, and put it into the polishing solution for 2 - 3 min. Remove the Ti foil and wash it with deionized water for 3x.</li>
		<li>Use an anode alligator clip to hold the pretreated Ti foil and another clip to hold a Pt foil (25 x 25 mm). Place the two foils face to face in the electrolyte at a distance of 2 cm from each other. Turn on the direct-current (DC) stabilized current power source, tune the voltage to 50 V, and electrolyze for 30 min.</li>
		<li>After the anodization has finished, close the power and take out the TiO2 foil</li>
		<li>Submerge the Ti foil in a volumetric flask containing 15 mL of ethanol, then a flask with 15 mL of acetone, then treat it for 15 min with an ultrasonic cleaner. Take out the Ti foil, rinse it 3 - 5x with deionized water, and place it in a 50-mL crucible.</li>
		<li>Put the crucible in an oven at 60 &#176;C for 12 h to let the foil dry.</li>
		<li>Calcine the TiO2 foil in a muffle furnace under 400 &#176;C for 2 h with a heating rate of 2 &#176;C/min.</li>
	</ol>
	</li>
</ol>
2. Catalytic Tests and Product Analysis
<ol>
	<li>Catalytic tests under concentrating light
	<ol>
		<li>Clean the stainless cylinder-shaped reactor (inner diameter = 5.5 cm, volume = 100 mL) with deionized water then dry it in an oven at 60 &#176;C for 10 min, to ensure no interference from other carbon sources.</li>
		<li>Take out the reactor from oven, add 2 mL H2O, a stirrer, and a catalyst holder (a small shelf that holds the catalyst in the reactor), and put a quartz glass with pores (diameter = 2 cm) on the bottom of the holder and the TiO2 catalyst (diameter = 1 cm) on the center of the quartz glass. Put a thermocouple through an opening on the reactor wall on the catalyst surface. Add a Fresnel lens on the top of the holder and seal the reactor with a quartz glass window.</li>
		<li>Put the reactor on the electromagnetic apparatus. Check the air tightness with nitrogen (N2).</li>
		<li>Feed the CO2 (99.99%) into the reactor through a mass flow controller (MFC) and flush the reactor at least 3x to change the gas in the reactor to CO2.</li>
		<li>Place the Xe lamp 2 cm directly above the reactor, open the Xe lamp power and adjust its current to 15 A, and turn on the magnetic stirrer switch to start the reaction.</li>
		<li>Record the temperature change on the catalyst surface and in the gas.</li>
	</ol>
	</li>
	<li>Product analysis
	<ol>
		<li>Analyze the product every 1 h using a gas chromatography (GC), which is equipped with a flame-ionized detector (FID) and a capillary column (see Table of Materials) for separation of C1-C6 hydrocarbons.</li>
		<li>Calculate the number of products by the external standard line method. Before quantifying the product, build a standard curve of methane (CH4).</li>
	</ol>
	</li>
	<li>Catalytic tests under concentrating light with pretreatment 
	Note: This procedure is similar to 2.1, with differences noted.
	<ol>
		<li>Wash reactor as in step 2.1.1.</li>
		<li>Assemble reactor as in step 2.1.2, except without adding H2O.</li>
		<li>Check the air tightness as in step 2.1.3.</li>
		<li>Feed the pretreatment gas (such as air, N2 and H2O) into the reactor through an MFC and exchange the gas three times in succession to make the reactor pure pretreatment gas.</li>
		<li>Adjust the lamp as in step 2.1.5.</li>
		<li>Keep the catalyst under light (10 concentrating ratio) illumination for 1 h in air atmosphere, then turn off the Xe lamp and magnetic stirrer to finish pretreatment.</li>
		<li>Feed the CO2 (99.99%) into the reactor as in step 2.1.4.</li>
		<li>Inject 2 mL H2O into the reactor from the opening of the wall. Open the Xe lamp and magnetic stirrer power to start the reaction as step 2.1.5.</li>
		<li>Record the temperature change as in step 2.1.6.</li>
	</ol>
	</li>
</ol>

<ol><li>De-Richter, R. K., Ming, T., Caillol, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fighting+global+warming+by+photocatalytic+reduction+of+CO2%2C+using+giant+photocatalytic+reactors%5BTitle%5D)+AND+%22Renewable+%26+Sustainable+Energy+Reviews%22%5BJournal%5D)">Fighting global warming by photocatalytic reduction of CO2, using giant photocatalytic reactors.</a> Renewable & Sustainable Energy Reviews. 19 (1), 82-106 (2013).</li>
<li>Fang, Y., Wang, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photocatalytic+CO2+conversion+by+polymeric+carbon+nitrides%5BTitle%5D)+AND+%22Chemical+Communications%22%5BJournal%5D)">Photocatalytic CO2 conversion by polymeric carbon nitrides.</a> Chemical Communications. 54 (45), 5674-5687 (2018).</li>
<li>Kondratenko, E. V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Status+and+perspectives+of+CO2+conversion+into+fuels+and+chemicals+by+catalytic%2C+photocatalytic+and+electrocatalytic+processes%5BTitle%5D)+AND+%22Energy+%26+Environmental+Science%22%5BJournal%5D)">Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes.</a> Energy & Environmental Science. 6 (11), 3112-3135 (2013).</li>
<li>Izumi, Y. Recent Advances (2012-2015) in the Photocatalytic Conversion of Carbon Dioxide to Fuels Using Solar Energy: Feasibilty for a New Energy. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aIzumi%2C+Y+%22Advances+in+CO2+Capture%2C+Sequestration%2C+and+Conversion%22">Advances in CO2 Capture, Sequestration, and Conversion</a>. Jin, F., He, L. -N., Hu, Y. H. , American Chemical Society. 1-46 (2015).</li>
<li>White, J. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Light-Driven+Heterogeneous+Reduction+of+Carbon+Dioxide%3A+Photocatalysts+and+Photoelectrodes%5BTitle%5D)+AND+%22Chemical+Reviews%22%5BJournal%5D)">Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes.</a> Chemical Reviews. 115 (23), 12888-12935 (2015).</li>
<li>Habisreutinger, S. N., Schmidtmende, L., Stolarczyk, J. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photocatalytic+Reduction+of+CO2+on+TiO2+and+Other+Semiconductors%5BTitle%5D)+AND+%22Angewandte+Chemie+International+Edition%22%5BJournal%5D)">Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors.</a> Angewandte Chemie International Edition. 52 (29), 7372-7408 (2013).</li>
<li>Weinstein, L. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Concentrating+Solar+Power%5BTitle%5D)+AND+%22Chemical+Reviews%22%5BJournal%5D)">Concentrating Solar Power.</a> Chemical Reviews. 115 (23), 12797-12838 (2015).</li>
<li>Herrmann, J. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((TiO2+-based+solar+photocatalytic+detoxification+of+water+containing+organic+pollutants.+Case+studies+of+2%2C+4-dichlorophenoxyaceticacid+%282%2C+4+-+D%29+and+of+benzofuran%5BTitle%5D)+AND+%22Applied+Catalysis+B+Environmental%22%5BJournal%5D)">TiO2 -based solar photocatalytic detoxification of water containing organic pollutants. Case studies of 2, 4-dichlorophenoxyaceticacid (2, 4 - D) and of benzofuran.</a> Applied Catalysis B Environmental. 17 (1-2), 15-23 (1998).</li>
<li>Zeng, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Combined+effects+of+initial+water+content+and+heating+parameters+on+solar+pyrolysis+of+beech+wood%5BTitle%5D)+AND+%22Energy%22%5BJournal%5D)">Combined effects of initial water content and heating parameters on solar pyrolysis of beech wood.</a> Energy. 125, 552-561 (2017).</li>
<li>Zeng, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Characterization+of+solar+fuels+obtained+from+beech+wood+solar+pyrolysis%5BTitle%5D)+AND+%22Fuel%22%5BJournal%5D)">Characterization of solar fuels obtained from beech wood solar pyrolysis.</a> Fuel. 188, 285-293 (2017).</li>
<li>Nguyen, T. V., Wu, J. C. S., Chiou, C. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photoreduction+of+CO+over+Ruthenium+dye-sensitized+TiO-based+catalysts+under+concentrated+natural+sunlight%5BTitle%5D)+AND+%22Catalysis+Communications%22%5BJournal%5D)">Photoreduction of CO over Ruthenium dye-sensitized TiO-based catalysts under concentrated natural sunlight.</a> Catalysis Communications. 9 (10), 2073-2076 (2008).</li>
<li>Guan, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photoreduction+of+carbon+dioxide+with+water+over+K2Ti6O13%2C+photocatalyst+combined+with+Cu%2FZnO+catalyst+under+concentrated+sunlight%5BTitle%5D)+AND+%22Applied+Catalysis+A%3A+General%22%5BJournal%5D)">Photoreduction of carbon dioxide with water over K2Ti6O13, photocatalyst combined with Cu/ZnO catalyst under concentrated sunlight.</a> Applied Catalysis A: General. 249 (1), 11-18 (2003).</li>
<li>Han, S., Chen, Y. F., Abanades, S., Zhang, Z. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Improving+photoreduction+of+CO2+with+water+to+CH4+in+a+novel+concentrated+solar+reactor%5BTitle%5D)+AND+%22Journal+of+Energy+Chemistry%22%5BJournal%5D)">Improving photoreduction of CO2 with water to CH4 in a novel concentrated solar reactor.</a> Journal of Energy Chemistry. 26 (4), 743-749 (2017).</li>
<li>Roy, S. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Toward+solar+fuels%3A+photocatalytic+conversion+of+carbon+dioxide+to+hydrocarbons%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons.</a> ACS Nano. 4 (3), 1259-1278 (2010).</li>
<li>Li, D., Chen, Y. F., Abanades, S., Zhang, Z. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Enhanced+activity+of+TiO2+by+concentrating+light+for+photoreduction+of+CO2+with+H2O+to+CH4%5BTitle%5D)+AND+%22Catalysis+Communications%22%5BJournal%5D)">Enhanced activity of TiO2 by concentrating light for photoreduction of CO2 with H2O to CH4.</a> Catalysis Communications. 113, 6-9 (2018).</li>
</ol>

The authors have nothing to disclose.

Concentrating light reduces the light incident area and requires the use of a disc-shaped catalyst or a so-called fixed-bed reactor to hold the catalyst. Since the light source is usually a round-shaped lamp, the shape of the catalyst should also be round. To obtain a round disc, it is possible to press the powder into a disk by tableting or to change the metal foil into an oxide by anodization. The anodization method uses electricity to oxidize the metal to an oxide semiconductor. As the metal precursor is already a sheet or foil, it can be trimmed more easily after oxidation without breaking it.
Another factor that needs to be considered is the intensity measurements. We have not given the light intensity after concentration because the use of a commercial detector of light intensity has some limitations. Such a detector often has a large surface area (ID = 1 cm) and a wall to protect it, which will also block much of the light when it is used to measure the concentrating light. Also, when the concentrating ratio is large, the small size of the Xe lamp (which often has an ID of 5 cm) will concentrate the light to a very small area, which may be smaller than the detector area. Therefore, to further investigate the concentrating light technique, large-sized lamps will have to be used and the intensity detector will have to be improved.
After the implementation of the protocol presented here, the CH4 yield rate was clearly enhanced using a suitable concentration ratio, which means that the concentrated light can, to some extent, reduce the amount of catalyst. Of course, a higher light intensity is not always beneficial for a catalytic performance; there is an optimal concentration ratio. Many factors may contribute to the appearance of the optimal concentration ratio. It is known that for the photocatalytic reactions, the reaction order of the light intensity often decreases while the light intensity increases, until it reaches zero. The high intensity also causes the rapid generation and recombination of e--h+ pairs.
To summarize, we have demonstrated a concentrating light method to improve CO2 photoreduction behavior. Considering the meaning of reducing the catalyst amount and increasing the reaction rate, the method might be useful for the photocatalytic decomposition of H2O, the reduction of CO2, and degradation of volatile organic compounds (VOCs) under real sunlight. At present, there are few studies on photocatalysis under real sunlight, and the yield is very low. Concentration can vastly reduce the volume of the reactor and save costs; in addition, it can increase the light intensity and temperature and, thus, greatly improve the photocatalytic efficiency, but it may be necessary to add an automatic solar tracking system in consideration of the movement of sunlight.

The utilization of fossil fuels is accompanied by large amounts of CO2 emission, contributing greatly to global warming. CO2 capture, storage, and conversion are essential to reduce the CO2 content in the atmosphere1. The photoreduction of CO2 to hydrocarbons can reduce CO2, convert CO2 to fuels, and save solar energy. However, CO2 is an extremely stable molecule. Its C=O bond possesses a higher dissociation energy (about 750 kJ/mol)2. This means that CO2 is very hard to be activated and transformed, and only short wavelength lights with high energy can be functional during the process. Therefore, CO2 photoreduction studies suffer from low conversion efficiencies and reaction rates at present. Most reported CH4 yield rates are only at several &#181;mol&#183;gcata-1&#183;h-1 levels on a TiO2 catalyst3,4. The design and fabrication of photocatalytic systems with high conversion efficiency and reaction rate for CO2 reduction remain a challenge.
One popular area of research into CO2 photoreduction catalysts is to broaden the available light band to the visible spectrum and enhance the utilization efficiency of these wavelengths5,6. Instead, in this manuscript, we try to increase the reaction rate by enhancing the light intensity. As the driving force of a photocatalytic reaction is solar light, the basic idea is to use concentration technology to raise the incident solar light intensity and, therefore, increase the reaction rate. This is similar to a thermocatalytic process, where the reaction rate can be increased by increasing the temperature. Of course, the temperature effect cannot be not increased infinitely, and likewise with the light intensity; a major goal of this research is to find a suitable light intensity or concentration ratio.
This is not the first experiment that uses concentrating technology. In fact, it has been widely used in concentrating solar power and waste water treatment7,8. Biomaterials such as beech wood sawdust can be pyrolyzed in a solar reactor9,10. Some previous reports have mentioned the method for CO2 photoreduction11,12,13. One sample exhibited a 50% increment in the product yield when the light intensity was doubled14. Our group has found that concentrating light can raise CH4 yield rate with an up to 12-fold increase in intensity. In addition, pretreatment of catalyst before reaction by concentrating light can further increase the CH4 yield rate15. Here, we demonstrate the experimental system and method in detail.

<table><tbody><tr><td>Ti foil, 99.99%</td><td>Hebei Metal Technology Co., Ltd.</td><td></td><td></td></tr><tr><td>Pt foil, 99.99%</td><td>Tianjin Aida Henghao Technology Co., Ltd.</td><td></td><td></td></tr><tr><td>Ammonium fluoride, 98%</td><td>Aladdin</td><td>A111758</td><td>Humidity sensitive</td></tr><tr><td>Glycol, &gt;99.9%</td><td>Aladdin</td><td>E103323</td><td></td></tr><tr><td>Anhydrous ethanol,&gt;99.9%</td><td>Aladdin</td><td>E111977</td><td>Flammable</td></tr><tr><td>Acetone, &gt;99.5%</td><td>Hangzhou Shuanglin Chemical Co., Ltd.</td><td>200-662-2</td><td>Irritating smell</td></tr><tr><td>Nitric acid, 65.0%-68.0%</td><td>Hangzhou Shuanglin Chemical Co., Ltd.</td><td>231-714-2</td><td>Humidity sensitive</td></tr><tr><td>Hydrogen peroxide, 30 wt. % in H2O</td><td>Aladdin</td><td>H112515</td><td>Strong oxidative</td></tr><tr><td>Urea, 99%</td><td>Aladdin</td><td>U111897</td><td></td></tr><tr><td>De-ionized water, 99.00%</td><td>Laboratory made</td><td></td><td></td></tr><tr><td>Xe lamp, CELHXF300/CELHXUV300</td><td>Beijing Zhongjiao Jinyuan Co., Ltd.</td><td></td><td></td></tr><tr><td>Stainless cylinder reactor, CEL-GPPC</td><td>Beijing Zhongjiao Jinyuan Co., Ltd.</td><td></td><td></td></tr><tr><td>Fresnel lens, MYlens</td><td>Meiying Technology Co., Ltd.</td><td></td><td></td></tr><tr><td>7000 mesh sandpaper</td><td>Zibo Taichuan Abrasives Co., Ltd.</td><td></td><td></td></tr><tr><td>Ultrasonic cleaner, SK2210HP</td><td>Shanghai Kedao Ultrasonic Instrument Co., Ltd.</td><td></td><td></td></tr><tr><td>Thermostatical water bath, DF-101S</td><td>Boncie Instrument Technology Co., Ltd.</td><td></td><td></td></tr><tr><td>Alligator clip</td><td>Guangzhou Rongyu Co., Ltd.</td><td></td><td></td></tr><tr><td>DC constant voltage source, DY-150V 2A</td><td>Shanghai Anding Electric Co., Ltd.</td><td></td><td></td></tr><tr><td>Muffle furnace, KSL-1200X</td><td>Hefei Kejing Materials Technolgy Co., Ltd.</td><td></td><td></td></tr><tr><td>Quartz glass</td><td>Lianyungang Weida Quartz Products Co., Ltd.</td><td></td><td></td></tr><tr><td>Thermocouples, WRNK-191K</td><td>Feiyang Electric Accessories Co., Ltd.</td><td></td><td></td></tr><tr><td>Electronmagnetic stirrer, 85-2</td><td>Shanghai Zhiwei Electric Appliance Co., Ltd.</td><td></td><td></td></tr><tr><td>Vacuum pump,SHB-IIIA</td><td>Henan Province Taikang science and education equipment factory</td><td></td><td></td></tr><tr><td>Gas Chromatograph, GC2014</td><td>SHIMAPZU</td><td></td><td></td></tr><tr><td>HT-PLOT Q capillary column</td><td>Hychrom</td><td></td><td></td></tr><tr><td>Optical power meter,CEL-NP2000</td><td>Beijing Zhongjiao Jinyuan Co., Ltd.</td><td></td><td></td></tr><tr><td>Electronic scale, JJ124BC</td><td>Shanghai Jingtian Electronic Instrument Co., Ltd.</td><td></td><td></td></tr></tbody></table>

co2 photoreduction to ch4 performance under concentrating solar light

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea is to use concentration technology to raise the incident solar light intensity. Concentrating a large-area light onto a small area cannot only increase light intensity, but also reduce the catalyst amount, as well as the reactor volume, and increase the surface temperature. The concentration of light can be realized by different devices. In this manuscript, it is realized by a Fresnel lens. The light penetrates the lens and is concentrated on a disc-shaped catalyst. The results show that both the reaction rate and the total yield are efficiently increased. The method can be applied to most CO2 photoreduction catalysts, as well as to similar reactions with a low reaction rate at natural light.

The original photocatalytic reactor system mainly contains two components, a Xe lamp and a stainless cylinder reactor. For the concentrating light reactor system, we added a Fresnel lens and a catalyst holder, as shown in Figure 1. The Fresnel lens is used to concentrate the light in a smaller area. As the light has been concentrated, the catalyst must be placed in a lit area; therefore, the catalyst is made into disc shape, and a holder is used to hold the catalyst in this area.
When the anodization method was used, a layer of TiO2 nanotube arrays would form on the foil. Figure 2 displays some characterization results. However, more importantly, TiO2 arrays or other semiconductors could stick on the foil for easy cutting into discs of various sizes without breaking.
We have tested the catalytic performance of as-prepared TiO2 and other semiconductors under concentrating light. Figure 3 displays typical results of CH4 yield versus irradiation time under different concentration ratios (the ratio of the area of light source to the area of the catalyst). The reaction rates of methane on different catalysts were significantly improved under the concentrating conditions. In the case of TiO2, the maximum methane production rate reached 34.56 &#181;mol&#183;gcata-1&#183;h-1. In the case of Fe2O3, the maximum methane production rate reached 19.15 &#181;mol&#183;gcata-1&#183;h-1, which is about 18 times the rate under nature light15. If the catalyst is pretreated with suitable gas (air), the methane production rate can be further increased. The effect is considered to be from the change in surface properties, but more research is needed to prove this.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58661/58661fig1.jpg" /> 
Figure 1: Concentrating light reactor system for the photocatalytic reduction of CO2. (A) Photograph of the set-up. (B) Schematic of the set-up. 1 = Xe lamp, 2 = tquartz glass window, 3 = Fresnel lens, 4 = holder, 5 = photocatalyst, 6 = stainless steel reactor, 7 = H2O, and 8 = magnetic stirrer. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58661/58661fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58661/58661fig2.jpg" /> 
Figure 2: X-ray diffraction (XRD, left) and scanning electron microscope (SEM, right) of TiO2 by anodization. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58661/58661fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58661/58661fig3.jpg" /> 
Figure 3: Representative results for the CH4 yield at different concentration ratios (CR). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58661/58661fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light at JoVE.com

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We present a protocol for improving the performance of CO2 photoreduction to CH4 by heightening the incident light intensity via concentrating solar energy technology.

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea ...

co2-photoreduction-to-ch4-performance-under-concentrating-solar-light

Nanyang Technological University

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JoVE Journal

Engineering

6.7K Views.  Zhejiang University of Technology. Deze methode kan helpen bij het beantwoorden van belangrijke vragen in het gebied van kunstmatige fotosynthese, zoals kooldioxide-fotoreductie op methaan. De concentratietechnologie kan niet alleen de lichtintensiteit verhogen, maar ook de katalysatorhoeveelheid en het reactorvolume verminderen, terwijl de reactietemperatuur wordt verhoogd, waardoor de reactiesnelheid voor fotoreductie wordt verhoogd. Hoewel deze methode inzicht kan geven in fotokatalytische reductie van kooldioxide, kan het ...

Video: CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

Optimize Flue Gas Settings to Promote Microalgae Growth in Photobioreactors via Computer Simulations

Flue gas from power plants can promote algal cultivation and reduce greenhouse gas emissions1. Microalgae not only capture solar energy more efficiently than plants3, but also synthesize advanced biofuels2-4. Generally, atmospheric CO2 is not a sufficient source for supporting maximal algal growth5. On the other hand, the high concentrations of CO2 in industrial exhaust gases have adverse effects on algal physiology. Consequently, both cultivation conditions (such as nutrients and light) and the control of the flue gas flow into the photo-bioreactors are important to develop an efficient &ldquo;flue gas to algae&rdquo; system. Researchers have proposed different photobioreactor configurations4,6 and cultivation strategies7,8 with flue gas. Here, we present a protocol that demonstrates how to use models to predict the microalgal growth in response to flue gas settings. We perform both experimental illustration and model simulations to determine the favorable conditions for algal growth with flue gas. We develop a Monod-based model coupled with mass transfer and light intensity equations to simulate the microalgal growth in a homogenous photo-bioreactor. The model simulation compares algal growth and flue gas consumptions under different flue-gas settings. The model illustrates: 1) how algal growth is influenced by different volumetric mass transfer coefficients of CO2; 2) how we can find optimal CO2 concentration for algal growth via the dynamic optimization approach (DOA); 3) how we can design a rectangular on-off flue gas pulse to promote algal biomass growth and to reduce the usage of flue gas. On the experimental side, we present a protocol for growing Chlorella under the flue gas (generated by natural gas combustion). The experimental results qualitatively validate the model predictions that the high frequency flue gas pulses can significantly improve algal cultivation.

Flue gas from power plants is a cheap CO2 source for algal growth. We have built prototype &#34;flue gas to algal cultivation&#34; systems and described how to scale up the algal cultivation process. We have demonstrated the use of a mass-transfer bio-reaction model to simulate and to design the optimal operation of flue gas for the growth of Chlorella sp. in algal photo-bioreactors.

Flue gas from power plants can promote algal cultivation and reduce greenhouse gas emissions1. Microalgae not only capture solar energy more efficiently ...

Environment

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is composed of two materials, a plastic and an elastomer, whose dispersion characteristics (refractive index variation with wavelength) are different. We first designed the lens geometry and then used ray-tracing simulation, based on the Monte Carlo method, to analyze its performance from the point of view of both optical efficiency and the maximum attainable concentration. Afterwards, ADG Fresnel lens prototypes were manufactured using a simple and reliable method. It consists of a prior injection of plastic parts and a consecutive lamination, together with the elastomer and a glass substrate to fabricate the parquet of ADG Fresnel lenses. The accuracy of the manufactured lens profile is examined using an optical microscope while its optical performance is evaluated using a solar simulator for concentrator photovoltaic systems. The simulator is composed of a xenon flash lamp whose emitted light is reflected by a parabolic mirror. The collimated light has a spectral distribution and an angular aperture similar to the real Sun. We were able to assess the optical performance of the ADG Fresnel lenses by taking photographs of the irradiance spot cast by the lens using a charge-coupled device (CCD) camera and measuring the photocurrent generated by several types of multi junction (MJ) solar cells, which have been previously characterized at a solar simulator for concentrator solar cells. These measurements have demonstrated the achromatic behavior of ADG Fresnel lenses and, as a consequence, the suitability of the modelling and manufacturing methods.

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass ADG Fresnel Lens for Concentrating Photovoltaics

The achromatic doublet on glass (ADG) Fresnel lens makes use of two materials with differing dispersion to reduce chromatic aberration and increase attainable concentration. In this paper, a protocol for the complete characterization of the ADG Fresnel lens is presented.

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is ...

Microalgae Cultivation and Biomass Quantification in a Bench-Scale Photobioreactor with Corrosive Flue Gases

Photobioreactors are illuminated cultivation systems for experiments on phototrophic microorganisms. These systems provide a sterile environment for microalgal cultivation with temperature, pH, and gas composition and flow rate control. At bench-scale, photobioreactors are advantageous to researchers studying microalgal properties, productivity, and growth optimization. At industrial scales, photobioreactors can maintain product purity and improve production efficiency. The video describes the preparation and use of a bench-scale photobioreactor for microalgal cultivation, including the safe use of corrosive gas inputs, and details relevant biomass measurements and biomass productivity calculations. Specifically, the video illustrates microalgal culture storage and preparation for inoculation, photobioreactor assembly and sterilization, biomass concentration measurements, and a logistic model for microalgal biomass productivity with rate calculations including maximum and overall biomass productivities. Additionally, since there is growing interest in experiments to cultivate microalgae using simulated or real waste gas emissions, the video will cover the photobioreactor equipment adaptations necessary to work with corrosive gases and discuss safe sampling in such scenarios.

Bench-scale, axenic cultivation facilitates microalgal characterization and productivity optimization before subsequent process scale-up. Photobioreactors provide the necessary control for reliable and reproducible microalgal experiments and can be adapted to safely cultivate microalgae with the corrosive gases (CO2, SO2, NO2) from municipal or industrial combustion emissions.

Photobioreactors are illuminated cultivation systems for experiments on phototrophic microorganisms. These systems provide a sterile environment for ...

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity in aqueous CO2 reduction. Different from traditional metal nanocrystals, the synthesis of metal single atoms involves a matrix material that can confine those single atoms and prevent them from aggregation. We report an electrospinning and thermal annealing method to prepare Ni single atoms dispersed and coordinated in a graphene shell, as active centers for CO2 reduction to CO. During the synthesis, N dopants play a critical role in generating graphene vacancies to trap Ni atoms. Aberration-corrected scanning transmission electron microscopy and three-dimensional atom probe tomography were employed to identify the single Ni atomic sites in graphene vacancies. Detailed setup of electrochemical CO2 reduction apparatus coupled with an on-line gas chromatography is also demonstrated. Compared to metallic Ni, Ni single atom catalyst exhibit dramatically improved CO2 reduction and suppressed H2 evolution side reaction.

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

Here, we present a protocol for the synthesis and electrochemical testing of transition metal single atoms coordinated in graphene vacancies as active centers for selective carbon dioxide reduction to carbon monoxide in aqueous solutions.

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity ...

Chemistry

Coupling Carbon Capture from a Power Plant with Semi-automated Open Raceway Ponds for Microalgae Cultivation

In the United States, 35% of the total carbon dioxide (CO2) emissions come from the electrical power industry, of which 30% represent natural gas electricity generation. Microalgae can biofix CO2 10 to 15 times faster than plants and convert algal biomass to products of interest, such as biofuels. Thus, this study presents a protocol that demonstrates the potential synergies of microalgae cultivation with a natural gas power plant situated in the southwestern United States in a hot semi-arid climate. State-of-the-art technologies are used to enhance carbon capture and utilization via the green algal species Chlorella sorokiniana, which can be further processed into biofuel. We describe a protocol involving a semi-automated open raceway pond and discuss the results of its performance when it was tested at the Tucson Electric Power plant, in Tucson, Arizona. Flue gas was used as the main carbon source to control pH, and Chlorella sorokiniana was cultivated. An optimized medium was used to grow the algae. The amount of CO2 added to the system as a function of time was closely monitored. Additionally, other physicochemical factors affecting algal growth rate, biomass productivity, and carbon fixation were monitored, including optical density, dissolved oxygen (DO), electroconductivity (EC), and air and pond temperatures. The results indicate that a microalgae yield of up to 0.385 g/L ash-free dry weight is attainable, with a lipid content of 24%. Leveraging synergistic opportunities between CO2 emitters and algal farmers can provide the resources required to increase carbon capture while supporting the sustainable production of algal biofuels and bioproducts.

A protocol is described to utilize the carbon dioxide in natural gas power plant flue gas to cultivate microalgae in open raceway ponds. Flue gas injection is controlled with a pH sensor, and microalgae growth is monitored with real time measurements of optical density.

In the United States, 35% of the total carbon dioxide (CO2) emissions come from the electrical power industry, of which 30% represent natural gas ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Microalgal Cultivation for Biomethane Obtention from Swine Waste Biogas

Biogas Purification through the use of a Microalgae-Bacterial System in Semi-Industrial High Rate Algal Ponds

In recent years, a number of technologies have emerged to purify biogas into biomethane. This purification entails a reduction in the concentration of polluting gases such as carbon dioxide and hydrogen sulfide to increase the content of methane. In this study, we used a microalgal cultivation technology to treat and purify biogas produced from organic waste from the swine industry to obtain ready-to-use biomethane. For cultivation and purification, two 22.2 m3 open-pond photobioreactors coupled with an absorption-desorption column system were set up in San Juan de los Lagos, Mexico. Several recirculation liquid/biogas ratios (L/G) were tested to obtain the highest removal efficiencies; other parameters, such as pH, dissolved oxygen (DO), temperature, and biomass growth, were measured. The most efficient L/Gs were 1.6 and 2.5, resulting in a treated biogas effluent with a composition of 6.8%vol and 6.6%vol in CO2, respectively, and removal efficiencies for H2S up to 98.9%, as well as maintaining O2 contamination values of less than 2%vol. We found that pH greatly determines CO2 removal, more so than L/G, during cultivation because of its participation in the photosynthetic process of microalgae and its ability to vary pH when solubilized due to its acidic nature. DO, and temperature oscillated as expected from the light-dark natural cycles of photosynthesis and the time of day, respectively. Biomass growth varied with CO2 and nutrient feeding as well as reactor harvesting; however, the trend remained primed for growth.

Author Spotlight: Scaling Microalgal Biotechnology for Enhanced Biomethane Production 

Air pollution impacts the quality of life of all organisms. Here, we describe the use of microalgae biotechnology for the treatment of biogas (simultaneous removal of carbon dioxide and hydrogen sulfide) and the production of biomethane through semi-industrial open high-rate algal ponds and subsequent analysis of treatment efficiency, pH, dissolved oxygen, and microalgae growth.

In recent years, a number of technologies have emerged to purify biogas into biomethane. This purification entails a reduction in the concentration of ...

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number of photoactive assemblies have emerged where a photosensitizer and cobaloxime-based H2 production catalysts work in tandem to convert light energy into the H-H chemical bonds. However, the long-term instability of these assemblies and the need for hazardous proton sources have limited their usage. Here, in this work, we have integrated a stilbene-based organic dye into the periphery of a cobaloxime core via a distinct axial pyridine linkage. This strategy allowed us to develop a photosensitizer-catalyst hybrid structure with the same molecular framework. In this article, we have explained the detailed procedure of the synthesis of this hybrid molecule in addition to its comprehensive chemical characterization. The structural and optical studies have exhibited an intense electronic interaction between the cobaloxime core and the organic photosensitizer. The cobaloxime was active for H2 production even in the presence of water as the proton source. Here, we have developed a simple airtight system connected with an online H2 detector for the investigation of the photocatalytic activity by this hybrid complex. This photosensitizer-catalyst dyad present in the experimental setup continuously produced H2 once it was exposed in the natural sunlight. This photocatalytic H2 production by the hybrid complex was observed in aqueous/organic mixture media in the presence of a sacrificial electron donor under complete aerobic conditions. Thus, this photocatalysis measurement system along with the photosensitizer-catalyst dyad provide valuable insight for the development of next generation photocatalytic H2 production devices.

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H2 production. We have also developed a simple experimental setup for evaluating the light-driven H2 production by photocatalytic assemblies.

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number ...

CO₂ Photoreduction to CH₄ Performance Under Concentrating Solar Light

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