Carbon-Based Nanocomposites as Heterogeneous Catalysts
4.4 Carbon-Based Nanocomposites for Reduction Reactions
4.4.2 CO 2 Reduction
In another report, the conversion of nitroarenes to aromatic amines under environmentally benign conditions was achieved by using mag- netic iron oxide nanoparticles decorated on beet root derived AC (Fe3O4@ BRAC) [47]. The highly stable and recyclable catalyst has been utilized in the presence of base (Figure 4.8), wherein isopropyl alcohol (IPA) acts both as solvent and the hydrogen donor in the reaction. Also, it is easy to separate the catalyst from the reaction mixture because of the magnetic nature of the catalyst.
Furthermore, the carbon-based materials have also been explored for environmental remediation, such as AC has the effective chemisorption capacity of NOx which is one of the main component of the polluted air [48]. The use of carbon-based material for the removal of NOx from the environment is one of the important strategies for pollutant abate- ment [49–52]. Urea supported on AC fibers (ACFs) was synthesized by Shirahama and coworkers [53] for the reduction of NO2 to N2 under ambi- ent conditions. The reduction reaction continued until urea used is not consumed completely. Small amounts of NO and NO3 were also formed as byproducts. The reduction rate of NO2 group was affected by the humidity in air which accelerated the formation of the HNO3 on the ACF surface.
Due to the large surface area of ACF, it behaves as an appropriate support for urea and activated urea for oxidative interactions with NO2.
of using these metal catalyst, including low efficiency, selectivity, and recyclability. The decoration of transition metal nanoparticles over carbon support-based materials can construct a cascade photoreaction system which can improve the efficiency and recyclability of the catalyst. For example, Lin and the coworkers [56] used cobalt doped graphitic car- bon nitride (g-C3N4) both as a capturing and activating substrate for CO2 gas molecules. Here, g-C3N4 photocatalyst was doped with cobalt species which acts as oxidative and reductive promoters to accelerate the separa- tion and transfer rates of the charge carriers (Figure 4.9). The CO2 to CO conversion has been done under mild conditions in environment friendly solvent. The optimization of the catalysts offers a cost-effective material for artificial photosynthesis. The g-C3N4 used here is for light harvesting and the cobalt playing the role of the reductive cofactor for CO2 to CO conversion.
In addition to g-C3N4, graphene-based nanoheterostructures have also been utilized for CO2 reduction [57]. It was observed that the oxygenated functional group on GO introduced band gap in the catalyst due to its iso- lated sp2 domains. The inherent band gap of GO makes the photocatalytic conversion of CO2 to methanol possible. Thus, GO is playing two roles, one as the solar energy harvester and other as the reductant of CO2. The amount of the CO2 conversion to methanol is determined by using gas chromatography-mass spectrometry (GC-MS). Further, a possible photo catalytic mechanism has been formulated for better understanding of the CO2 photoreduction process. The GO was modified with the surplus
CO
CoOx
Co(bpy)32+
D+ D
g-CN
CO2 O
OC
N N
N N
H N
N N N
N N
N N N
N N N
N N
N
N N
N N
NH
H
– +
Figure 4.9 Cooperative effect of cobalt redox catalysis and g-CN photocatalysis for activation and reduction of CO2 to CO under visible light irradiation. Reprinted with permission from ref. [56] Copyright 2014 American Chemical Society Publishers.
oxygenated components and thus able to generate the electrons hole pairs which serves as the reducing and oxidizing agents. Due to the lower reduc- tion potential value of GO than the reduction potential of CO2/CH3OH, GO acts as donor for this conversion. On the other hand, the oxidation potential of the holes in GO is higher than the H2O/O2; therefore, it can acts as acceptor for this conversion. Thus, the electrons and holes gener- ated upon irradiation of GO results in the formation of CH3OH on reaction with adsorbed CO2 and H2O molecules. Thus, the hypothetical interpreta- tion of CO2 reduction is well supported by the photocatalytic performance of the GO catalyst.
Another class of carbon-based support materials, CNT, immobilized with pyrene-modified Ir complex has been used for electrochemical reduc- tion of CO2 to formate [58]. An impressive conversion rate and very high TONs of formate formation can be attributed to the large surface area of CNTs. Due to noncovalent interactions between the CNT electrode and Ir complex, it is easy to remove the Ir catalyst by soaking the electrode in THF to get clean CNT electrode. The prepared catalyst has shown high efficiency, excellent mechanical stability, and recyclability for the produc- tion of formate from CO2. A high current density of 15 mA cm−2 has been achieved by the developed photocatalyst with high formate selectivity. But, still more efforts are needed to utilize this electrolysis device for practical use in large scale.
In another report, CNTs doped with nonmetal can acts as an efficient, selective, and stable catalyst for CO2 reduction [59]. The successful syn- thesis of nitrogen doped CNT has been confirmed by morphological and structural characterizations as shown in Figure 4.10. The high electric conductivity, more catalytic site (due to nitrogen defects), and low free energy for CO2 activation make this conversion easy and convenient.
From the deconvoluted X-ray photo electron spectroscopy, it was observed that three different configurations of nitrogen were introduced, i.e., gra- phitic, pyrrolic, and pyridinic. Further, theoretical studies concluded that the pyridinic configuration is playing the major role in CO2 conversion to CO. Overall, the prepared catalyst was showing excellent selectivity and stability for CO2 conversion to CO.
At present, the reduction of the anthropogenic CO2 emission is one of the challenging task in order to address the consequences of climate change. The most feasible option to reduce carbon emission is to capture and restore CO2. The modified surface of the CNTs can be utilized effi- ciently for capturing CO2 emissions [60]. Nitrogen functionalities intro- duced into the CNTs improves the selectivity and capacity of the catalyst for CO2 capturing. It was observed that the nitrogen functionalities improve
the CO2 capture capacities of the catalyst without bringing any change in its textural properties.