CO₂ Photoreduction to CH₄ Performance Under Concentrating Solar Light

Summary

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

Abstract

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.

Introduction

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 µmol·g_cata^-1·h^-1 levels on a TiO₂ catalyst^3,4. 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 wavelengths^5,6. 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 treatment^7,8. Biomaterials such as beech wood sawdust can be pyrolyzed in a solar reactor^9,10. Some previous reports have mentioned the method for CO₂ photoreduction^11,12,13. 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.

Protocol

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

1. 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.
   1. 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 °C water bath.
   2. Trim the Ti foil (50 x 250 mm size) with scissors to 25 x 25 mm.
   3. Polish the Ti foil surface with a 7,000-mesh sandpaper to remove the surface impurities.
   4. 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.
   5. 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.
   6. 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.
   7. 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.
   8. After the anodization has finished, close the power and take out the TiO₂ foil
   9. 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.
   10. Put the crucible in an oven at 60 °C for 12 h to let the foil dry.
   11. Calcine the TiO₂ foil in a muffle furnace under 400 °C for 2 h with a heating rate of 2 °C/min.

2. Catalytic Tests and Product Analysis

1. Catalytic tests under concentrating light
   1. 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 °C for 10 min, to ensure no interference from other carbon sources.
   2. 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.
   3. Put the reactor on the electromagnetic apparatus. Check the air tightness with nitrogen (N₂).
   4. 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.
   5. 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.
   6. Record the temperature change on the catalyst surface and in the gas.
2. Product analysis
   1. 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.
   2. Calculate the number of products by the external standard line method. Before quantifying the product, build a standard curve of methane (CH₄).
3. Catalytic tests under concentrating light with pretreatment
   Note: This procedure is similar to 2.1, with differences noted.
   1. Wash reactor as in step 2.1.1.
   2. Assemble reactor as in step 2.1.2, except without adding H₂O.
   3. Check the air tightness as in step 2.1.3.
   4. 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.
   5. Adjust the lamp as in step 2.1.5.
   6. 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.
   7. Feed the CO₂ (99.99%) into the reactor as in step 2.1.4.
   8. 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.
   9. Record the temperature change as in step 2.1.6.

Representative Results

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 µmol·g_cata^-1·h^-1. In the case of Fe₂O₃, the maximum methane production rate reached 19.15 µmol·g_cata^-1·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.

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. Please click here to view a larger version of this figure.

Figure 2: X-ray diffraction (XRD, left) and scanning electron microscope (SEM, right) of TiO₂ by anodization. Please click here to view a larger version of this figure.

Figure 3: Representative results for the CH₄ yield at different concentration ratios (CR). Please click here to view a larger version of this figure.

Discussion

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.

Materials

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