Crimson Publishers Publish With Us Reprints e-Books Video articles

Full Text

Progress in Petrochemical Science

Photo-catalytic Conversion of CO2 to Hydrocarbons: Introduction, Challenges and Possible Approaches

Xiangchao Meng and Zisheng Zhang*

Department of Chemical and Biological Engineering, University of Ottawa, Canada

*Corresponding author: Zisheng Zhang, Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, K1N6N5, Canada

Submission: April 19, 2018;Published: May 08, 2018

DOI: 10.31031/PPS.2018.01.000520

ISSN 2637-8035
Volume1 Issue4

Abstract

Photo-catalytic reduction of CO2 to produce valuable hydrocarbons is a promising process to not only alleviate the issue of massive CO2 emissions, but also provide a strategy to convert solar energy to chemical energy. It has been extensively studied in last 10 years. In this brief review, the basic concepts as well as an introduction of this advanced process are included. Meanwhile, challenges and possible approaches are also pointed out. Future works were also suggested at last.

Keywords: Photocatalysis; CO2; CO2 reduction; Solar energy

Introduction of Photo-catalysis and its mechanism in Co2 reduction

Photo-catalysis has extensively studied since the first report in 1972 [1]. This technique theoretically aims to convert light energy into chemical energy. As for the mechanism, it can be interpreted as electrons on the conduction band (CB) of a semiconductor may be activated and jumped to its valence band (VB) with positive holes left behind. The separated charge carriers (electrons and holes) may be recombined, or transfer to the bulk surface to react with adsorbed water/oxygen to produce oxidative free radicals (such as •OH, •O2- etc.), or other adsorbed species (such as organics, CO2, NO, bacteria, etc.) [2-8].

From the aspect of types of reactions, the applications of photocatalysis can be simply divided into two categories, namely, advanced oxidation processes (AOPs) and photo-reduction processes. As for the AOPs, the effective species are photo-generated holes and generated oxidative free radicals, these processes are widely applied in decomposition of pollutants in waste water and polluted air, and bacteria inactivation or disinfection. And for the reduction processes, it mainly includes water splitting to evolve H2, heavy metal ions recovery, CO2 reduction and N2 fixation. As a promising approach to convert CO2 to valuable hydrocarbons, photo-catalytic reduction of CO2 has been immensely studied in last 10 years. As shown in Figure 1, it summarized the number of documents about photo-catalytic reduction/ conversion of CO2 by year, the number of related documents in the year of 2017 is about 7 times higher than that in the year of 2010. It suggests this research area is a promising and rapidly being developed topic in recent years and will possibly last long.

figure 1:Documents by year with search the keywords ‘CO2 reduction photocatalysis’ or ‘CO2 conversion photocatalysis’ on April 18, 2017 using Scopus


In Inoue et al. [9] for the first time, reported on photo-catalytic reduction of CO2 to produce hydrocarbons. The photo-catalytic reactions occurred in CO2 reduction can be considered to the follow schemes. The separated electrons and holes (reaction 1) can separately react with dissolved CO2 and adsorbed water (reaction 2). It is more complicated for the reductions, formic acid, formaldehyde and methyl alcohol may be produced (Reaction 3-6). The feasibility of CO2 reduction for a semiconductor is significantly determined by the conduction band potential. As shown in Figure 2 [10], only electrons on the conduction band with higher reduction ability are capable of convert CO2 to hydrocarbons. The formation of products is different, which depends on the number of electrons and protons taking part in the chemical reactions. For example, to form CH4 from CO2, eight electrons and eight protons are required (Reaction 3-6). The selectivity of products is a significant problem for photo-catalytic reduction of CO2, which is affected by many factors, such as reaction conditions, red-ox potentials and the type of substrates. From the point of thermo dynamic, CO2 is too stable to be reacted. The structure of CO2 (O=C=O) consists of a linear connection between carbon and two oxygen atoms. The Gibbs free energy for the CH4 and CH3OH is -51 and -166kJ/mol, which is higher than that for water splitting. This determines CO2 reduction is an incredible endothermic process, and more complicated compared to the water splitting process [11,12].

figure 2:Band structures of various semiconductors relative to the redox potentials at pH 7 of compounds involved in CO2 reduction (Adapted from [10], © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).


Challenges and Possible Approaches

Photocatalytic activity

One of the ideal energy sources for photo-catalysis is solar light. The solar-driven photo-catalytic reduction of CO2 to hydrocarbons is also called artificial photosynthesis. One of the primary hindrances for this process is its low activity under visible light, as for TiO2 is a typical and commercialized photo-catalyst, only responsive to ultraviolet (UV). Approaches to overcome this shortage were adopted, and can be divided into two categories, namely, modification of TiO2 [13-15] and preparation of visible light-responsive photo-catalysts [16-19]. Since CO2 is a stable molecule, the photo-catalytic activity of a bare semiconductor is poor, and various modifications are generally required to improve the photo-catalytic activity in the CO2 reduction. As for the effective approaches, they are comprehensively reviewed in [20]

Selectivity

As electrons on the conduction band are capable of react with various adsorbed species and the various hydrocarbons may be formed. It is significant to improve the selectivity of specific products. Meanwhile, hydrogen may also be generated, which will decrease of selectivity of hydrocarbon products. Dong et al. [21] have reported on improve the selectivity of CH4 in photo-catalytic reduction of CO2 via adjusting the size of platinum nanoparticles (Figure 3). Kumar et al. [22] have reported on photo-catalytic reduction of CO2 to selectively produce methanol without addition of sacrificial agents on graphene oxide supported heteroleptic ruthenium complex.

figure 3:Correlations between the selectivity for CH4 and surface site proportion as functions of the size of Pt NPs (Adapted from [21], Copyright © 2018, Springer Nature).


Experimental Conditions

Various experimental conditions influenced the photo-catalytic reduction of CO2. For example, Mizuno et al. [23] reported on the influence of CO2 pressure on the photo-catalytic reduction CO2. They found that with increase the pressure of CO2, it will accelerate the CO2 reduction process. Another critical issue for photo-catalytic reduction is the carbon source, carbonaceous residues on the photo-catalysts surface which may be from the synthesis process and the laboratory atmosphere, significantly influence the CO2 reduction and possibly contribute to the overall product yield.

Conclusion and Outlook

Photo-catalytic reduction of CO2 to produce valuable hydrocarbons is a promising process. However, as for the low conversion efficiency, low selectivity of a specific product, and unsatisfactory light harvesting, more and more efforts should be done before it’s widely applied in practice. Also, mechanisms for this complex processes should be clearly clarified in future works. The key to boost the feasibility of this process is to develop advanced materials with high visible light-responsive activity and high selectivity, such as recently developed plasmonic photo-catalyst. Try to combine with other advanced techniques such as electro catalysis would be another approach to enhance the development of this process. To use it in industrial scale, photo reactor design would be another issue to be conducted.

Acknowledgement

This work was financially supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada. Xiangchao Meng was the recipient of a scholarship from the China Scholarship Council (CSC) for the duration of this work.

References

  1. Fujishima A, Honda K (1972) Electro-chemical photolysis of water at a semiconductor electrode. Nature 238(5358): 37-38.
  2. Meng X, Zhang Z, Li X (2015) Synergetic photoelectrocatalytic reactors for environmental remediation: A review. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 24: 83-101.
  3. Hu X, Meng X, Zhang Z (2016) Synthesis and characterization of graphene oxide-modified Bi2WO6 and its use as photocatalyst. International Journal of Photo energy 2016: 1-8.
  4. Meng X, Zhang Z (2016) Bismuth-based photocatalytic semiconductors: introduction, challenges and possible approaches. Journal of Molecular Catalysis A: Chemical 423: 533-549.
  5. Li Z, Meng X, Zhang Z (2017) Recent development on MoS2-based photocatalysis: a review. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 35: 39-55.
  6. Meng X, Li Z, Zeng H, Chen J, Zhang Z (2017) MoS2 quantum dotsinterspersed Bi2WO6 hetero structures for visible light-induced detoxification and disinfection. Applied Catalysis B: Environmental 210: 160-172.
  7. Li Z, Meng X, Zhang Z (2018) Few-layer MoS2 nanosheets-deposited on Bi2MoO6 microspheres: A Z-scheme visible-light photocatalyst with enhanced activity. Catalysis Today.
  8. Meng X, Zisheng Z (2018) Two dimensional graphitic materials for photoelectrocatalysis: a short review. Catalysis Today.
  9. Inoue T, Fujishima A, Konishi S, Honda K (1979) Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature 277: 637-638.
  10. Tu W, Zhou Y, Zou Z (2014) Photocatalytic conversion of CO2 into renewable hydrocarbon fuels: state‐of‐the‐art accomplishment, challenges, and prospects. Adv Mater 26(27): 4607-4626.
  11. Nahar S, Zain M, Kadhum A, Hasan H, Hasan M (2017) Advances in photocatalytic CO2 reduction with water: a review. Materials 10(6): 629.
  12. Hasan M, Zain M, Hamid R, Nahar S, Kaish A (2004) Recent Advances in Photo-catalytic Materials for Artificial Photosynthesis.
  13. Tseng IH, Wu JCS, Chou HY (2004) Effects of sol-gel procedures on the photocatalysis of Cu/TiO2 in CO2 photoreduction. Journal of Catalysis 221(2): 432-440.
  14. Li X, Liu H, Luo D, Li J, Huang Y, et al. (2012) Adsorption of CO2 on heterostructure CdS(Bi2S3)/TiO2 nanotube photocatalysts and their photocatalytic activities in the reduction of CO2 to methanol under visible light irradiation. Chemical Engineering Journal 180: 151-158.
  15. Zhang Q, Li Y, Ackerman EA, Gajdardziska Josifovska M, Li H (2011) Visible light responsive iodine-doped TiO2 for photocatalytic reduction of CO2 to fuels. Applied Catalysis A: General 400(1-2): 195-202.
  16. Fu Y, Sun D, Chen Y, Huang R, Ding Z, et al. (2012) An amine‐functionalized titanium metal organic framework photocatalyst with visible‐lightinduced activity for CO2 reduction. Angew Chem Int Ed Engl 51(14): 3364-3367.
  17. Sato S, Morikawa T, Saeki S, Kajino T, Motohiro T (2010) Visiblelight‐ induced selective CO2 reduction utilizing a ruthenium complex electrocatalyst linked to a p‐type nitrogen‐doped Ta2O5 semiconductor. Angew Chem Int Ed Engl 49(30): 5101-5105.
  18. Sato S, Morikawa T, Kajino T, Ishitani O (2013) A highly efficient mononuclear iridium complex photocatalyst for CO2 reduction under visible light. Angew Chem Int Ed Engl 52(3): 988-992.
  19. Wang S, Hou Y, Wang X (2015) Development of a Stable MnCo2O4 Cocatalyst for Photocatalytic CO2 Reduction with Visible Light. ACS Appl Mater Interfaces 7(7): 4327-4335.
  20. Li K, Peng B, Peng T (2016) Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels. ACS Catalysis 6(11): 7485- 7527.
  21. Dong C, Lian C, Hu S, Deng Z, Gong J, et al. (2018) Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles. Nature Communications 9(1): 1252.
  22. Kumar P, Bansiwal A, Labhsetwar N, Jain SL (2015) Visible light assisted photocatalytic reduction of CO2 using a graphene oxide supported heteroleptic ruthenium complex. Green Chemistry 17(3): 1605-1609.
  23. Mizuno T, Adachi K, Ohta K, Saji A (1996) Effect of CO2 pressure on photocatalytic reduction of CO2 using TiO2 in aqueous solutions. Journal of Photochemistry and Photobiology A: Chemistry 98(1-2): 87-90.

© 2018 Zisheng Zhang. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially.

About Crimson

We at Crimson Publishing are a group of people with a combined passion for science and research, who wants to bring to the world a unified platform where all scientific know-how is available read more...

Leave a comment

Contact Info

  • Crimson Publishers, LLC
  • 555 Madison Avenue, 5th floor
  •     New York, NY 10022, USA
  • +1 (929) 600-8049
  • +1 (929) 447-1137
  • info@crimsonpublishers.com
  • www.crimsonpublishers.com