*4.3.2. Photochemical reduction of CO2*

The photocatalytic reduction of CO2 has been widely studied using different semiconductors of the following types: metal oxides, sulfides, or nitrides, layered metal materials like layered double hydroxide, metal-organic frameworks, and metal-free carbonaceous materials such as graphitic carbon nitride. The photocatalytic reduction can be summarized into five steps:


#### *4.3.3. CO2 reforming with methane*

Lastly, reforming of CO2 and CH4 into syngas (mixture of H2 /CO) can be performed by catalytic and nonthermal plasma methods or by the novel hybrid technique combining both. Catalytic methods still suffer from high thermal energy consumption, catalyst deactivation by coke deposition, and high costs. Selected examples of catalysts for reforming process are Pt, Pd, Ir, Rh, Ru, Co, and Ni. The general mechanism of the dry methane reforming (DRM) involves the adsorption of CO2 and CH4 onto the catalyst followed by dissociation of the molecules into CO, and O, C, and H atoms. The atoms recombine forming additional CO molecule and H2 gas, followed by desorption of the gases where the CO desorption constitutes the rate-determining step in the process.

Nonthermal plasma relies on electronic energy. Electrons are accelerated by an external electric field to collide with CO<sup>2</sup> and CH4 transferring their energy to induce the dissociation of the molecules when the energy exceeds 4.5 and 8.8 eV, respectively. The dissociation generates radicals and more active species, which reform the CO and H2 products. The main characteristic of this method is the low selectivity since the radicals can reform into side products such as hydrocarbons [4].

### **4.4. Biofuels from microalgae**

Both processes can proceed *via* transfer of 2, 4, 6, 8, 12, or more electrons depending on the nature of the employed catalyst and the experimental conditions, and they hence yield vari-

The kinetics of the electrochemical reduction is sluggish due to the reorganization of the lin-

transfer after the adsorption of the molecule onto the working electrode. This step is mostly

primary products. Very few electrocatalysts (e.g., Cu) ensure the further reduction of CO into hydrocarbons, but without an elucidated mechanism till now. Various electrolytic materi-

functionalized carbonaceous catalysts such as N-doped carbon nanofibers and graphene

of the following types: metal oxides, sulfides, or nitrides, layered metal materials like layered double hydroxide, metal-organic frameworks, and metal-free carbonaceous materials such as graphitic carbon nitride. The photocatalytic reduction can be summarized into five steps:

• Absorption of photons by the semiconductor photocatalyst generating the hole and elec-

into syngas (mixture of H2

lytic and nonthermal plasma methods or by the novel hybrid technique combining both. Catalytic methods still suffer from high thermal energy consumption, catalyst deactivation by coke deposition, and high costs. Selected examples of catalysts for reforming process are Pt, Pd, Ir, Rh, Ru, Co, and Ni. The general mechanism of the dry methane reforming (DRM)

identified as the rate-determining step initiating at −1.9 V, and it forms CO<sup>2</sup>

that is further protonated into HCOO• or HOOC• and reduced into HCOO<sup>−</sup>

respectively. The majority of electrochemical reductions of CO2

etc.), layered transition metal dichalcogenides (e.g., WS2

• Charge separation of the generated electrons and holes.

adsorption and transfer of electrons to the CO2

• Surface redox reaction involving the reduction of the CO2

.

and CH4

O and H2

and CH4

molecule into more active bent form, which creates overpotential to the first electron

•− anion radical

(formate) or CO,

), and heteroatom-

produce HCOOH or CO as

and the oxidation of the common

/CO) can be performed by cata-

onto the catalyst followed by dissociation of the

reduction processes including metals (Sn, Pd, Cu, Pt,

has been widely studied using different semiconductors

, and MoS2

, MoSe2

molecules.

ous products as mentioned before.

10 Carbon Dioxide Chemistry, Capture and Oil Recovery

*4.3.1. Electrochemical reduction of CO2*

als have been investigated in the CO2

*4.3.2. Photochemical reduction of CO2*

The photocatalytic reduction of CO2

quantum dots [19].

tron pairs.

reductants such as H2

Lastly, reforming of CO2

involves the adsorption of CO2

• Desorption of the products [19, 20].

 *reforming with methane*

• CO2

*4.3.3. CO2*

ear CO2

The photosynthetic microorganisms (e.g., microalgae) constitute future alternative energy sources to fossil fuels and can serve to fix CO<sup>2</sup> directly from waste streams, decreasing the high existing levels. Microalgae can transform solar energy into chemical forms *via* photosynthesis and posses faster growth rate than plants. They can be cultivated in diverse environments as open or closed ponds and photobioreactors with minimum requirement of nutrients. After cultivation, the biomass content is harvested, dried, and converted into fuels by thermochemical (e.g., pyrolysis) or biochemical (e.g., fermentation) processes. The limited cultivation areas and the costs of the harvesting stage are still burdening the large-scale routes of this prospective CO2 utilization [21].
