Recent Progress in Using Photocatalytic Carbon Dioxide Reduction

Technology to Compound Methane

Zhenning Zhang

School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610097, China

Keywords:

Carbon Dioxide Reduction, Photocatalytic Materials, Photocatalysts.

Abstract: With the development of modernization process, Massive combustion of fossil fuels not only causes heavy

load of carbon dioxide emission in the air, but also causes the worry about the exhausted of future energy

resource. By using reduction of carbon dioxide can solve both these problems. Of the various methods used

for reducing carbon dioxide, photocatalytic carbon dioxide reduction is more environmentally friendly. In

this article, the author begins by outlining the functions and difficulty points of carbon dioxide to methane

reduction, then introduce the characteristics introduce how the photocatalytic reduction of CO2 to

synthesize CH4 works.Moreover, certain characteristics and more recent developments in the field of

photocatalysts of reducing CO2 into CH4, such as TiO2 based photocatalysts, vanadium based

photocatalysts and Tungsten based photocatalytic materials and their doping, co-doping, surface

modification or nano-structure technology will be presented.The author of this paper additionally concludes

with some potential directions for photocatalyst development.

1 INTRODUCTION

With ongoing technological advancements and the

accelerating pace of industrialization, the

widespread utilization of fossil fuels has become a

reality. However, this rapid depletion results in a

significant surge of carbon dioxide in the

atmosphere. The absence of effective measures to

control carbon dioxide emissions will lead to a

tripling of these emissions by 2050 compared to

1990. The steady rise in global carbon dioxide

emissions will severely impact Earth's temperature

over the next few decades, potentially causing the

thawing of Arctic permafrost and releasing an

unknown quantity of carbon dioxide. Although

global carbon emission efficiency has improved, the

absolute levels of emissions have not significantly

decreased. Current environmental energy sources

and technologies have not yet reached a level

sufficient to mitigate the intensity of carbon dioxide

emissions. Additionally, concerns are mounting

regarding the potential depletion of fossil fuels in the

future, by their extensive use.

Therefore, to address the greenhouse effect and

air pollution stemming from the extensive emissions

of greenhouse gases like carbon dioxide, and to seek

sustainable and innovative eco-friendly energy

sources, researchers have begun exploring the

efficient utilization of green energies such as solar,

wind, and hydrogen. Carbon dioxide reduction

technology offers a novel strategy to tackle the

emission issue of greenhouse gases like carbon

dioxide, which involves tackling the source of

emissions proactively. Additionally, CO2 has been

converted into other valuable products by this

technology, including methane and other fuels,

thereby reclaiming resources. Given that methane

yields a significant amount of heat through

combustion reactions, the synthesis of methane via

carbon dioxide reduction technology represents a

more efficacious method for fuel production.

Carbon dioxide is a non-combustible gas with

the chemical formula CO2. The optimal solvent for

carbon dioxide is organic liquid. Gaseous carbon

dioxide has stable chemical properties under

conventional conditions, therefore it cannot directly

undergo decomposition and redox reactions.

Therefore, in most cases, the reaction of carbon

dioxide needs to be carried out under high

temperature or catalyst conditions. For example,

under high temperature conditions, carbon dioxide

reacts with carbon to produce carbon monoxide;

Under the presence of copper zinc catalysts, carbon

268

Zhang, Z.

Recent Progress in Using Photocatalytic Carbon Dioxide Reduction Technology to Compound Methane.

DOI: 10.5220/0013913800004914

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Renewable Energy and Ecosystem (ICREE 2024), pages 268-272

ISBN: 978-989-758-776-4

dioxide reacts with hydrogen to produce products

such as methanol and water.

The carbon dioxide reduction reaction is an

uphill reaction, which means that additional energy

and catalyst are required for reduction. This reaction

can be classified based on the source of energy.

Common carbon dioxide reduction technologies

include thermal catalysis, electrocatalysis, and

photocatalysis. Thermal catalytic technology has

high efficiency and large output, but high

temperature and energy are required, and has poor

environmental protection. Electrocatalysis and

photocatalytic technology can adapt at room

temperature and normal pressure, and have good

environmental friendliness. Electrocatalytic

technology requires electrical energy support, which

may result in energy loss and increased costs.

Photocatalytic technology directly utilizes sunlight

for catalysis, but is affected by the diurnal

periodicity of sunlight. In addition, there are also

some hybrid reduction technologies, such as

photothermal catalysis, microbial

photoelectrochemical catalysis, and photosynthesis.

2 THE REACTION

MECHANISMS

One way to think of the technology used in

photocatalytic carbon dioxide reduction is as a

man-made version of photosynthesis. In essence,

light energy is transformed into chemical energy via

the procedure of photocatalytic reduction of carbon

dioxide, which permits carbon dioxide to undertake

processes of decomposition or synthesis with other

substances. The earliest research on the use of light

for carbon dioxide reduction originated from

Halmann. Initially, p-GaP photocatalysts were used

to reduce carbon dioxide to produce methanol. Later,

photocatalysts based on titanium dioxide, zinc oxide,

tungsten, vanadium, silicon carbide, etc. gradually

emerged, and the mechanism of carbon dioxide

photoreduction was gradually determined. The

reduction products were also gradually enriched,

such as methane, formic acid, formaldehyde, etc.

The mechanism of photocatalytic carbon dioxide

reduction originates from the electron hole pairs

generated by semiconductor materials under

illumination conditions. By successfully

transforming light energy into chemical power, this

process facilitates the synthesis reaction with other

molecules or the breakdown of carbon dioxide.

When the illumination energy exceeds the

semiconductor bandgap, electrons are excited by

light energy and move from valence band to

conduction band, leaving holes. Photo-generated

charge carriers relocated to the exterior of the

catalyst, but recombination also occurs.

Photo-generated electrons have strong reducibility

and can react with CO2 and H2O to form

hydrocarbons. Types of catalyst affects the specific

reduction path (Tang et al, 2021).

However, if the transition rate is lower than the

pace at which photo-generated carriers recombine,

the reaction is difficult to proceed. In addition, the

reactions between photo generated holes, photo

generated electrons, catalysts, and the influence of

surrounding medium can easily lead to photo

corrosion of photocatalysts. To prevent occurrence

of photo corrosion, as well as decrease the rate at

which photo-generated carriers recombine, the

number of photo-generated holes and

photo-generated electrons needs to be matched as

much as possible. In terms of thermodynamics, top

potential of valence band and conduction band

bottom potential of semiconductor materials need to

be between the oxidation reaction potential of water

and reduction reaction potential of CO2. In terms of

dynamics, it is required that semiconductor materials

have good ability to generate electron hole pairs, as

well as sufficient sites on surface of photocatalysts

to react with CO2 (Tang et al, 2021).

The large Gibbs free energy of carbon dioxide

makes it difficult to produce methane and methanol

under photoreduction circumstances; however, the

reaction can be made easier by using light energy.

The oxidation of H2O, which produces oxygen,

electrons, and hydrogen ions, is the first step of the

reduction reaction of CO2 to make CH4. CO2 then

combines with hydrogen ions and electrons to

produce methane and water. This process can be

thought of as a series of reactions involving carbon

dioxide and water, which produce methane and

oxygen.

The reaction equation for the oxidation reaction

of water is:2H



O→O



+4H



+4e



, in which the

oxidation-reduction potential is 0.82V vs. NHE. The

reaction equation for the reduction reaction of

carbon dioxide is: CO



+4H



+4e



→CH





O , in this reaction, the oxidation-reduction

potential is -0.24V vs. NHE. Overall, the equation

for the reaction is that the required Gibbs free energy

for this reaction is -51.75 Kcal/mol.

Recent Progress in Using Photocatalytic Carbon Dioxide Reduction Technology to Compound Methane

269

Figure 1. The reaction mechanism of reduction procedures

A reaction pathway for the reduction reaction of

CO2 to produce CH3OH or CH4 is depicted in the

above image. As we can see from the image, H2O

undergoes oxidation at the hole, producing hydroxyl

groups and hydrogen ions. Following the reduction

of carbon dioxide, the hydrogen ions react to form

hydroxyl groups and CO. They eventually react with

hydrogen or hydroxyl atoms to generate methane or

methanol after going through a number of similar

processes.

3 CATALYTIC PATHWAY

Photocatalysts are generally semiconductors that can

activate chemical properties through light radiation

and promote the occurrence of redox reactions. If

photosynthesis is used as an analogy, chlorophyll

can be seen as a photocatalyst for photosynthesis.

The definition of photocatalyst includes

photosensitization, which is the process in which the

photochemical quantity changes due to the

absorption of radiation by photosensitizer molecules.

The catalyst field for photocatalytic carbon

dioxide reduction is generally divided into titanium

dioxide based series photocatalytic materials,

V-based series photocatalytic materials, W-based

series photocatalytic materials, carbon nitride based

series photocatalytic materials, and other materials.

Among them, the catalysts that can efficiently

synthesize methane through carbon dioxide

reduction are titanium dioxide based series

photocatalytic materials, V-based series

photocatalytic materials, and W-based series

photocatalytic materials (Tang et al, 2021). This

article will mainly introduce some research progress

on the three catalytic materials mentioned above.

3.1 Titanium Dioxide Based

Photocatalysts

The basic principle of titanium dioxide based

catalysts is similar to that of photocatalysis

mentioned earlier. Titanium dioxide is a

semiconductor material that, when exposed to a

certain intensity of light, causes the separation of

hole and electron carriers. Compared with general

conductive materials, titanium dioxide carriers are

more difficult to composite. The photo induced

holes on the surface of titanium dioxide have strong

oxidizing properties, which can oxidize donor

molecules, while electrons on the conduction band

of titanium dioxide can be used in reduction of

receptor molecules.

Titanium dioxide, as a catalyst, has the

characteristics of good safety, low cost, and high

stability. There are three crystal forms of titanium

dioxide in nature: brookite, rutile, and anatase. Of

these three crystal forms, anatase or a mixed anatase

and rutile crystal structure performs relatively well.

In particular, the mixed anatase and rutile crystal

form performs exceptionally well in converting

carbon dioxide to produce methane and methanol.

However, in order to separate holes from electron

carriers, a wavelength of light less than 388nm is

required, and this range of light is near-ultraviolet

light. Therefore, titanium dioxide may face harsh

usage conditions during the catalytic process (Tang

et al, 2021).

To address the above issues, researchers mainly

focus on changing the structure and state of titanium

dioxide. By using the above methods, the efficiency

of titanium dioxide as a catalyst can be improved,

and the applicability of titanium dioxide can be

expanded. Now, researchers have attempted various

techniques, such as doping, co doping, surface

modification, etc., to broaden the photon absorption

range of TiO2 and reduce recombination effect of

electrons and holes. For example, by doping

elements such as iron, cobalt, nickel, manganese,

vanadium, and nitrogen into titanium dioxide, the

maximum frequency of photons absorbed by TiO2

can be increased to the range of visible light

(Abdullah et al, 2017). Akple et al. (2015) conducted

N doping and surface fluorination modification on

TiO2. On the one hand, nitrogen doping reduced the

requirement for light energy absorption, and on the

other hand, the fluorine ions generated by surface

fluorination modification became an intermediary

for surface charge separation. Additionally, surface

heterojunctions enhanced electron hole separation.

They found that the titanium dioxide catalyst after

surface modification and N doping had better

activity under visible light conditions. In addition,

the efficiency of methane generation can be

effectively improved by transforming the structure

of titanium dioxide into sponge or titanium dioxide

hollow ball structure, while the nanotube structured

titanium dioxide formed by solvothermal method,

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sol gel method, direct oxidation method, deposition

and other methods, supplemented by Pt as a

cocatalyst, can effectively select methane products

(Tang et al, 2021).

3.2 Vanadium Based Photocatalysts

Vanadium, as a transition element, has a lower 3d

energy band and is therefore considered a promising

synthetic element for visible light catalytic materials.

Therefore, vanadium based photocatalytic materials

have better application prospects in visible light

photocatalytic carbon dioxide reduction. Although

vanadium oxide is not an ideal option for reducing

carbon dioxide due to its conduction band

characteristics, vanadates such as ZnV2O4, InVO4,

ZnV2O6, exhibit good activity in carbon dioxide

reduction (Tang et al, 2021).

Tahir et al. (2019) synthesized graded 3D

microspheres, which not only exhibited good

stability and photoactivity under visible light

conditions, but also successfully achieved selective

reduction of carbon monoxide, methane, and

methanol.

Du et al. (2022) prepared vanadates such as

copper vanadate, nickel vanadate, chromium

vanadate, and zinc vanadate, and tested and

compared their catalytic activities. Finally, it was

found that nickel vanadate had the best catalytic

activity, while copper vanadate was unable to

catalyze carbon dioxide reduction technology due to

its conduction band properties. This can to some

extent verify the Tanabe hypothesis, which suggests

that the properties of vanadates as catalysts may

mainly come from the properties of their cations.

Tantalum nitride doped with vanadium was

developed by Nguyen et al. (2017) in order to the

reduction reaction of CO2. Methane, CO, and other

products were effectively produced under visible

light conditions by reducing carbon dioxide. It was

determined by comparison that tantalum nitride

doped with vanadium exhibits greater photocatalytic

activity.

According to Lu et al. (2014), titanium dioxide

nanotube arrays doped with vanadium nitrogen

perform better at catalyzing the shift between CO2

to CH4 due to the efficient separation of electrons

and holes that occurs after doping.

Le Chi et al. (2019) prepared and compared the

performance of TaON and vanadium doped TaON in

photocatalytic production of methane, hydrogen,

oxygen and other products from carbon dioxide.

After research, it was found that due to vanadium

doping, vanadium doped TaON has a smaller

bandgap energy, which increases catalyst activity.

Researchers also prepared 1.5V-TaON materials,

which have good efficiency in reducing carbon

dioxide to produce methane. The above experiments

and research cases demonstrate that vanadium

doping is beneficial for improve activity of catalysts,

and vanadates as catalysts for photocatalytic

generation of methane from CO2 also have certain

development prospects.

3.3 Tungsten Based Photocatalytic

Materials

The first photocatalytic compounds based on

tungsten were created in 1979. For the first time,

Inoue (Inoue et al, 1979)used elements like tungsten

oxide and titanium dioxide in 1979 to successfully

reduce carbon dioxide aqueous solutions to fuels like

methane and carbon monoxide.

Tungsten based photocatalytic materials mainly

include tungstate salts (such as MnWO4) and

tungsten oxide materials.

When tungsten oxide is used as a photocatalytic

material, the commonly used forms of tungsten

oxide include: WO3, W18O49, WO3∙0.33H2O, and

other forms (Yang et al, 2021). For block WO3

materials, due to their higher conduction band

potential compared to the oxidation-reduction

reaction of CO2 to produce CH4 (greater than

-0.24V vs NHE), block WO3 materials are unable to

reduce carbon dioxide to produce methane.

However, by preparing WO3 ultra-thin nanosheets,

the conduction band potential of WO3 can be

reduced, allowing it to be used for catalyzing the

reduction of CO2 to compound CH4 (Tang et al,

2021). In contrast to using W18O49 alone as a

catalyst to catalyze the generation of methane from

carbon dioxide, using W18O49 as a co catalyst has a

better effect. For example, catalyzing with Cu2O,

carbon nitride, Cu and other catalysts can increase

the ability of carbon dioxide reduction to produce

methane (Bhavani et al, 2023).

After the study by Jiang et al. (2020), it was

found that W18O49 can help increase the selectivity

of methane during carbon dioxide reduction

catalysis on {1, 1, 1} of Cu2O. WO3∙0.33H2O

material has been proven to have promising

prospects in photocatalytic materials due to its

excellent ability to conduct electrons and protons.

Doping WO3 ∙ 0.33H2O can further enhance its

catalytic performance (Wang et al, 2019).

For tungstate catalysts, materials such as bismuth

tungstate, copper tungstate, and zinc tungstate are

used to catalyze reduction reaction. This type of

Recent Progress in Using Photocatalytic Carbon Dioxide Reduction Technology to Compound Methane

271

material has the characteristics of low cost and high

stability. However, for some tungstate catalysts, the

potential of their valence and conduction bands is

not a good option for catalyzing the reduction of

CO2 to CH4, such as MnWO4. Its conduction band

position prevents direct catalysis, and before it can

be utilized for carbon dioxide reduction, it

frequently needs to undergo a number of additional

treatments. Even if some tungstate salts are capable

of catalytic processes, doping treatment can greatly

enhance their performance.

4 CONCLUSIONS

To summarize, photocatalytic carbon dioxide

reduction technology uses light-induced electron

hole pairs produced by semiconductors to finish the

carbon dioxide reduction process. Methane, carbon

monoxide, methanol, and other fuels with high value

can all be produced in large quantities using

photocatalytic carbon dioxide reduction technology.

The mechanism of photocatalytic carbon dioxide

reduction to compound methane and various

catalysts that can be used for this process are

introduced in this article along with the

photocatalytic reduction of CO2 to CH4 as well as

its catalyst

conditions (i.e. meeting the top

potential of valence band conduction band

bottom potential between oxidation reaction

potential of H2O and the reduction reaction potential

of CO2). Regarding photocatalytic carbon dioxide

reduction catalysts, future research directions will

mainly focus on the following points:

(1) By doping, co doping and other means, make

the catalyst have a wider range of applications and

better activity and efficiency;

(2) By changing the structure of the catalyst,

such as constructing nanostructures, improve the

performance of the catalyst;

(3)Research and develop new catalysts, such as

organic compounds and organic complexes.

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