2.4. Photoelectrochemical reduction of CO2 to methanol

anode, that would be used in electrocatalytic CO2 reduction process [65]. Several experiments have been conducted to test the ability of some semiconductors and metal oxides for CO2 conversion to methanol. This include silicon carbide [66], TiO2 [67–70], WO3 [71], NiO [70], ZnO [70], and InTaO4 [72] either by themselves or they can be combined with different heterogeneous catalysts to achieve the same goal. The main challenge in methanol production on semiconductors by using solar energy is that the formation reaction is reversible. Thus, in order to mitigate the methanol oxidation, it is very essential to find new strategies to achieve a

Gondal et al. [66] proved that the granular silicon carbide is a promising photocatalyst for CO2 reduction to methanol. The granular silicon carbide (α6H-SiC) has been tested as a photocatalyst to reduce CO2 and convert it into methanol using a 355-nm laser. The reaction cell was filled with α6H-SiC granules, pressurized with CO2 gas at 50 psi and distilled water. Therefore, they mentioned that a pair of competitive reactions which are photo-oxidation and photo-reduction are existed in the photochemical process, as shown in Figure 11. When the reaction starts, the photooxidation rates (Ko) will be slower than the photoreduction rates (Kr) because of the low concentration of produced methanol. The obtained results showed that the maximum molar concentration of methanol and photonic efficiencies of CO2 conversion into

CdS/TiO2 and Bi2S3/TiO2 nanotube photocatalysts were tested by Li et al. [67], and their photocatalytic activities that reduce CO2 to methanol under visible light irradiation have been studied. The obtained results proved that the synthetical TNTs are almost a good material to be act as photoreduction to convert CO2 into methanol. The largest methanol production on

Figure 11. Schematic illustration of the photoreduction and photooxidation reactions in the photochemical process [67].

methanol achieved was around 1.25 mmol/l and 1.95%, respectively.

practical industrial process [66, 70].

52 Carbon Dioxide Chemistry, Capture and Oil Recovery

The photoelectrocatalytic CO2 reduction process is a combination of the photocatalytic and electrocatalytic methods together. Many research works were focused to find the best semiconductor material that can be used as a photoelectrode to convert CO2 into methanol using any solar energy in PEC cell; however, no tested semiconductor met the desired stability and efficiency [73]. In fact, the photoelectrochemically reduction of CO2 need around 1.5 eV of thermodynamic energy input. Therefore, the PEC cell needs greater energy input to make up the losses that causes by band bending (which is needed for charge separation at the surface of semiconductor), overvoltage potentials, and resistance losses [61, 74–81]. The first important step for the reduction of CO2 to methanol by the photoelectrochemical (PEC) method is the hydrogen ions and electrons generation by the solar irradiance of semiconductor which is used as photocathode. The semiconductor (e.g., GaP, SiC) is illuminated by light as the source of energy that is higher than the semiconductor's band gap. In that case, the electrons in semiconductor will be excited and transferred to conduction band from the valance band, and it will reach the cathode counter electrode through an external electrical wire. Furthermore, in order to produce the electrochemical reduction and oxidation reactions, the produced electronhole pairs at or near the interface will be separated by the semiconductor and will be injected into the electrolyte [82–84]. A major problem in using the photoelectrochemical cells is the ability of n-type semiconductor materials to generate holes on the surface that can oxidize the

Figure 12. The two-compartment photoelectrochemical cell for CO2 reduction [87].

methanol can be used instead of the fossil fuel, thus reducing the dependence on fossil fuel and contribute in the market growth of CO2 utilization. Herein, a complete literature of different methods for CO2 conversion into methanol is reported in this section. This include homogeneous/heterogeneous catalytic, electrochemical, photochemical, and photoelectrochemical reduction. However, the high performance in CO2 conversion process can be achieved by using an effective catalyst. In general, the development of required catalyst can be used as a solution if the catalyst is already used, but it is required high cost to be scaled up or it does not exist and await discovery thus the challenges in catalytic processes are huge indeed. The poor product selectivity and the low/high reaction temperatures are considered to be the main barriers in the heterogeneous CO2 reduction process. However, the above discussion shows that among various methods proposed for CO2 conversion to methanol or to any valuable chemical, the electrochemical cells are the preferable over other methods. Nevertheless, many barriers still exist in the CO2 electrochemical reduction in which the electrocatalyst is needed to be used at higher selectivity as well as lower over potentials. Various heterogeneous electrocatalysts are selective, fast and energy-efficient, but they are considered to be unstable catalysts. Therefore, in the future, the electricity needed for electrochemical CO2 reduction process on a large scale can come from different renewable energy sources such as hydro, wind, wave, geothermal, tides, and so on. In this sense, many research works should be focused on new electrocatalytic materials that can be used to allow working at higher current densities without loss of Faradaic efficiency. On the other hand, photochemical processes offer an attractive approach to reduce CO2 to methanol using solar energy. However, this method is not widely used due to its critical conditions to absorb the required amount of solar energy. Otherwise, the prospects to develop the successful technologies for the efficient CO2 conversion using solar energy are certainly long term (>5 years out). Nonetheless, photoelectrochemical reduction processes are discovered to be attractive approaches for the reduction of CO2 to methanol. At present, the applications of solar photoelectrochemical devices are very limited due to its high cost and several reasons, as discussed above. However, it is very important for research efforts to continue in these areas because this technology will be

Carbon Dioxide Conversion to Methanol: Opportunities and Fundamental Challenges

http://dx.doi.org/10.5772/intechopen.74779

55

extremely needed for efficient reduction of CO2 in the coming years.

Center for Advanced Materials (CAM), Qatar University, Doha, Qatar

The authors would like to acknowledge the support of Center for Advanced Materials, Qatar University (QU) for this work. Ms. Sajeda Alsaydeh also acknowledges QU for Graduate

Acknowledgements

Assistantship awarded to her.

Sajeda A. Al-Saydeh and Syed Javaid Zaidi\*

\*Address all correspondence to: szaidi@qu.edu.qa

Author details

Figure 13. The one-compartment photoelectrochemical cell for CO2 reduction [87].

semiconductor itself [85]. Recently, the hybrid system which consists of a semiconductor light harvester and a complex of metal co-catalyst has received a huge attention. In this system, the water is considered the main source of electron donors and protons for the reduction of CO2 at the surface of cathode. An example of hybrid system has been discussed by Zhao et al. [86]. They studied the full cell of photocathode with InP/Ru-complexes that was coupled with a TiO2/Pt based photoanode, as shown in Figure 12. In this full cell, in order to avoid the formate re-oxidation at the surface of photoanode, the proton exchange membrane was used as a separator. However, Arai et al. constructed a wireless full cell for photoelectrochemical CO2 reduction in which the system consists of the InP/Ru-complex as a hybrid photocathode and a photoanode of SrTiO3 (Figure 13). In this system, the redox reactions of CO2 and H2O will occur via sunlight irradiation without applying any bias. The obtained results showed that the conversion efficiency from solar to chemical energy in these two full cells was 0.03% and 0.14% for TiO2–InP/[RuCP] and SrTiO3–InP/[RuCP], respectively. Barton et al. [61] successfully reduced CO2 to methanol by using catalyzed p-GaP-based photoelectrochemical (PEC) cell in a process called chemical carbon mitigation. Chemical carbon mitigation term describes the photoinduced CO2 conversion to methanol without the use of additional CO2 generating power source. The obtained results showed that the methanol selectivity and CO2 conversion were found to be 100 and 95%, respectively.

### 3. Future prospective and conclusions

Carbon dioxide conversion is presenting both an opportunity and a challenge worldwide for the sustainability of environment and energy. The main strategies of CO2 reduction should focus on the utilization of CO2, the CO2 recycling combined with the renewable energy to save carbon sources, and the useful chemicals production from CO2. Therefore, the conversion of CO2 into energy product such as methanol will consume large amount of captured CO2 in which the market scale of methanol is potentially extensive. Furthermore, the generated methanol can be used instead of the fossil fuel, thus reducing the dependence on fossil fuel and contribute in the market growth of CO2 utilization. Herein, a complete literature of different methods for CO2 conversion into methanol is reported in this section. This include homogeneous/heterogeneous catalytic, electrochemical, photochemical, and photoelectrochemical reduction. However, the high performance in CO2 conversion process can be achieved by using an effective catalyst. In general, the development of required catalyst can be used as a solution if the catalyst is already used, but it is required high cost to be scaled up or it does not exist and await discovery thus the challenges in catalytic processes are huge indeed. The poor product selectivity and the low/high reaction temperatures are considered to be the main barriers in the heterogeneous CO2 reduction process. However, the above discussion shows that among various methods proposed for CO2 conversion to methanol or to any valuable chemical, the electrochemical cells are the preferable over other methods. Nevertheless, many barriers still exist in the CO2 electrochemical reduction in which the electrocatalyst is needed to be used at higher selectivity as well as lower over potentials. Various heterogeneous electrocatalysts are selective, fast and energy-efficient, but they are considered to be unstable catalysts. Therefore, in the future, the electricity needed for electrochemical CO2 reduction process on a large scale can come from different renewable energy sources such as hydro, wind, wave, geothermal, tides, and so on. In this sense, many research works should be focused on new electrocatalytic materials that can be used to allow working at higher current densities without loss of Faradaic efficiency. On the other hand, photochemical processes offer an attractive approach to reduce CO2 to methanol using solar energy. However, this method is not widely used due to its critical conditions to absorb the required amount of solar energy. Otherwise, the prospects to develop the successful technologies for the efficient CO2 conversion using solar energy are certainly long term (>5 years out). Nonetheless, photoelectrochemical reduction processes are discovered to be attractive approaches for the reduction of CO2 to methanol. At present, the applications of solar photoelectrochemical devices are very limited due to its high cost and several reasons, as discussed above. However, it is very important for research efforts to continue in these areas because this technology will be extremely needed for efficient reduction of CO2 in the coming years.
