Nanomaterials in electro-organic synthesis

Volta, Faraday, and Kolbe were the pioneers of successful electro-organic synthesis. Green chemistry has recently revived this area of research. The use of clean energy sources (electricity and light) for synthetic purposes reduces the consumption of toxic oxidants and reductants, thus proceeding reactions under mild conditions. Hence, due to the eco-friendly aspect and efficient organic transformations, electro-organic synthesis attained significant importance worldwide. It should be realized that eliminating even just one step from the multistep synthetic methods in the industry can decrease energy consumption, making the process economically more feasible.

The emerging field of nanomaterials as catalysts and its expansion for organic synthesis will have a significant impact in the future on the chemical industry. The nanomaterials have prominent electrical, magnetic, thermal and optical properties with fascinating outlooks in various fields such as catalysis, microelectronics, optoelectronics, and sensors. However, the preparation and application of nanomaterials at the industrial level have not been achieved yet due to some obvious limitations such as the use of green methodologies for synthesis, development of appropriate homogeneous structure on the nanometer scale, and the effect of morphology and valence states of nanomaterials on catalytic activity. Moreover, the reusability of catalysts used, exploration of in-expensive nanomaterials as an alternative to costly noble metal-based catalysts for economic productivity are constraints too. Besides, the expansion of nanomaterials in terms of investigation and construction, their use in the electrosynthesis of organic substances is not well explored. So far, only a few studies have investigated the role of nanomaterials as substituting toxic redox reagents, and there is still a long way to realize the industrial application of nanomaterials for complex electro-organic synthesis. However, nanomaterials have a huge potential for the electrosynthesis of organic materials, and it could be a significant way of developing interesting organic substances.

Although electro-organic synthesis has been investigated by various research groups worldwide, here we have tried to summarize the significant developments in the last decade in organic electrosynthesis by employing just nanomaterials. The literature coverage is enough to present the scope of nanocatalysis in the latest versatile trends in organic electrosynthesis.

Electrochemical organic synthesis is accepted as an eco-friendly method for producing diverse molecules driven by electricity. The electrochemical cells are used for this process, comprising of anode and cathode for a two-electrode cell, whereas for a three-electrode cell, a reference electrode is also used. The oxidation reaction occurs at the anode compartment, and oxidation species are produced (protons and electrons). The protons are usually transferred towards the cathode, where proton consumption occurs, while the electrons move in an external circuit to generate the reduced species. The potential applied and current measured happens between the working electrode and reference electrode for a two-electrode system. In a typical three-electrode cell, the working, counter, and reference electrodes are placed in the same electrolyte solution, and the current flow takes place between the working and counter electrode. The working potential is determined with respect to the reference electrode. The two-electrode systems are more feasible and efficient than the three-electrode system. Normally, a three-electrode system doesn't give cell efficiency; instead, they show the efficiency of the working electrode, while, in the two-electrode system, the potential shown is the cell voltage. The electrochemical reactions are always paired, i.e., an oxidation half-reaction and a reduction half-reaction. Principally, in an electrochemical transformation, the rate of the electron transfer is mainly dependent on the electrochemical process, the electrode surface, the substrate used and the electrolyte. All the key parameters and different aspects in electro-organic synthesis were discussed in detail by Pollok and Waldvogel and a very comprehensive overview is presented in Fig. 1.¹ 

The electro-organic synthesis has promoted the preparation of a variety of electrode materials for the redox reactions of reactants, including both metallic and non-metallic. Depending upon the potential values, the electrodes can oxidize or reduce the chemical reagents for a respective chemical reaction. Usually, the non-metallic electrodes are graphite or carbon-based materials, whereas metallic electrodes are made up of metals like Cu, Co, Fe, Ni, Pt, Pd, Ir, Ag, Au etc. At an appropriate potential value, these electrodes generate reactive species by either oxidizing or reducing reactants to undergo subsequent reactions. The electrodes also reduce the stable organic complexes and generate reactive intermediates (difficult to produce otherwise) that further participates in the organic transformation.

During direct electrosynthesis, heterogeneous electron transfer occurs between electrode and substrate, always giving higher overpotential. Consequently, more by-products are formed. Besides, indirect electrosynthesis is considered another promising strategy, which involves a redox mediator during electrochemical transformations. Mostly, redox mediators are used for the efficient transfer of electrons or holes between the electrodes and the organic compound. The use of suitable mediators (with redox potential lesser than the substrate) facilitates electrolysis to take place in moderate conditions, making it industrially more feasible. The electrochemical synthesis of organic compounds is usually identified and quantified by GC, HPLC, NMR, etc., and conversion yields and faradaic efficiency are calculated to determine the efficiency of the process.

In an electrochemical process, water-based solvent systems are usually used, and oxygen is generated at the anode. Nevertheless, the oxygen evolution reaction (OER) requires a large overpotential. Various studies have been carried out to develop a more thermodynamically and economically feasible oxidation process other than OER at the anode. The electrochemical oxidation of organic compounds is one alternative to OER, but with few constraints, like the lower onset potential than water oxidation and solubility in water at room temperature. Besides, the value-added products should be produced economically compared to traditional synthetic methods. In this regard, the electro-oxidation of alkenes, amines, alcohols, ketones, hydrazine, etc., could be considered.

Moreover, if toxic substances are not used, this electro-oxidation could turn out totally as a green process. Furthermore, with the optimization of reaction conditions (applied potential, temperature, pH, electrolyte, solvent) and catalyst involved, high selectivity of the desired product could be attained.

Alcohol oxidation reactions are widely used for the preparation of aldehydes and ketones, and carboxylic acid, hence, they are of immense importance. These reactions result in the production of carbonyl and carboxylic acid groups and usually happen at lower potentials. Hence, the development and investigation of effective catalysts are significantly required in this aspect. A recent study by Yin et al. has realized the efficient synthesis of the first example of commercially significant 1,1-diethoxyethane (DEE). They have prepared an efficient, stable bifunctional platinum iridium-based nanocatalyst with 1 nm dimension (PtIr nanowire) and 85% of faradaic efficiency (FE) for DEE, and 94% for the hydrogen evolution reaction (HER). The synthesis of DEE occurs at the anode and hydrogen at the cathode, with a low voltage of 0.61 V at 10 mA cm⁻². The pristine Pt nanowires and commercial Pt/C are reported to have higher potentials of 0.85 V and 0.86 V, respectively, higher than the PtIr nanowires.²  Moreover, the mechanistic study by in situ FTIR revealed that PtIr nanowires facilitate the activation of O–H and C–H bonds in ethanol, first forming the acetaldehyde intermediate, and then DEE.

Recently, Rajeev et al. reviewed electrochemical alcohol oxidation and discussed various chemically modified electrodes. The electro-oxidation of alcohols has gained huge attention due to the industrial importance of the products attained (e.g., aldehydes) due to their applications in cosmetic, agrochemical industries, and pharmacological applications. Various nobel metal-based, non-noble metal-based and carbon-based electrocatalysts have been investigated so far, either as the catalyst used during the reaction or used on the surface of an electrode for the oxidation of alcohols.³  Pt–RuO₂/Ti electrodes were reported for the methanol oxidation. The authors explained that Pt nanoparticles were dispersed in RuO₂ and on the surface, no metallic Ru and Pt–Ru alloy was observed. The detailed study revealed the enhanced catalytic activity for methanol oxidation was mainly regulated by the oxygen species donor hydrous ruthenium oxide, thereby promoting the oxidation of CO to CO₂.⁴ 

Another PEDOT and bimetallic Pt–Ru composite was prepared by electrochemical co-deposition on carbon paper substrate using chloroplatinic acid and ruthenium chloride. The formation of thin layer PEDOT on carbon paper substrate instigated uniform, distinct, and dispersed nanoclusters of Pt–Ru. In contrast, agglomerated nanoclusters were detected in the absence of PEDOT. Further investigations revealed greater peak currents of methanol oxidation for PtRu–PEDOT electrode than Pt–Ru electrode.⁵  As the Pt-based materials find a significant place in electrocatalysis, Pt and Mn composite was prepared and claimed to deliver higher peak current density and lower overpotential for methanol oxidation. The authors prepared Pt nanoparticles composited with MnO₂ nanowire arrayed 3D brush shaped electrode (PME) by electrodeposition on Si substrate and exhibited far better performance than bare Pt nanowire arrayed electrode with the same amount of Pt nanoparticles. Interestingly PME presented better tolerance towards catalyst poisoning and achieved higher resistance. Thus, it was considered economical as it settled the issues regarding the cost of metal used and poisoning of the catalyst.⁶  Another interesting work was presented by Wang et al. using carbon spheres as support for Pt particles for ethanol electro-oxidation in an acidic medium. To enlarge and effectively use the surface area, the catalyst (Pt) was dispersed on conductive support like carbon. They developed a novel approach of composite-molten salt (CMS) for the preparation of carbon nanospheres. This performance was attributed to the high carbonization, conductivity, and development of porous structure by nanospheres.⁷  Soleimani-Lashkenari described the development of Pd incorporated PANI/TiO₂ composite for methanol oxidation. They deposited Pd on PANI on a modified glassy carbon electrode and studied the oxidation ability in an alkaline media.⁸  The Pd/PANI/TiO₂ composite showed better oxidation activity and higher stability compared to Pd or only PANI/TiO₂ composites.

Pd nanoparticles are found to have a higher curvature; thus, it is usually complicated to grow them on one-dimensional CNTs or carbon spheres. So, the layered structures are used for the anchoring of such nanomaterials. Song et al. carried out this idea and developed porous carbon nanoflakes (a layered structure) and dispersed the Pd nanoparticles in the inner laminar region of carbon nanoflakes (Pd@PDCX). It effectively reduced the resistance and enhanced the catalytic activity during the methanol oxidation reactions.⁹  Mohammadi et al. demonstrated the preparation and catalytic performance of another Ni/Co-based bimetallic catalyst and zeolite-4A modified carbon paste for methanol electro-oxidation. Nickel cobaltite nanoparticles (NiCo₂O₄ NPs) have been employed for catalytic applications due to their tunable properties such as morphological, textural and redox properties that help in better adsorption on substrate materials. The integration of NiCo₂O₄ nanoparticles and zeolite-4A created binary electroactive sites, hence long term stability with high surface area and good electron conductivity was delivered for methanol oxidation.¹⁰  Besides a recent study demonstrated the used of Cobalt based bifunctional nanosheets for the paired hydrogen production with methanol oxidation to the valueable formate ions. Moreover, low overpotential of 155 mV for methanol conversion and 148 mV for HER at a current density of 10 mA cm⁻² with combined 100% FE was attained.¹¹ 

The selective electro-oxidation of glycerol holds a significant value worldwide as it is an inexpensive by-product of the soap and biodiesel industry. Various researchers have investigated different catalysts for the products of glycerol. Many catalysts have been reported so far, but amongst the nanocatalysts, Zhang et al. developed a Au-supported carbon nanotube-based catalyst that displayed good selectivity of glycolate of about 85% at mild reaction conditions.¹²  Besides, GLY electro-oxidation has also been explored for bimetallic and trimetallic catalysts other than monometallic catalysts. In this aspect, Dai et al. employed a AuPt based catalyst that possessed a selectivity of 70% for lactic acid.¹³  Likewise, PtRuSn/C, a ternary catalyst, has been developed by Kim et al. and presented high current densities with lower onset potentials.¹⁴  The synergistic effects of Ru, Sn and Pt resulted in the superior activity that facilitated the complete oxidation of CO or CO-like groups.

A series of graphene nanosheets-based catalysts have been reported by Zhou et al. They developed monometallic Pt-based, bimetallic PtNi, PtRu, PtRh, and trimetallic PtRuNi, PtRhNi and graphene-based composite catalysts and employed for glycerol oxidation in basic solution.¹⁵  Among all the prepared catalysts, platinum and rhodium-based bimetallic PtRh/GNS and trimetallic PtRhNi/GNS catalysts displayed higher current densities and lower onset potentials and were claimed to have superior activity. The authors explained that products produced can be readily tuned via reasonably optimizing catalyst composition and selecting applied potentials. The different products of glycerol oxidation produced at potentials of 0.4, 0.1, and 0.2 V, were examined by high-performance liquid chromatography (HPLC). Five oxidation products (tartronic acid, glyceraldehyde, glycolic acid, glyceric acid and oxalate acid) were achieved from glycerol electro-oxidation.

Recently a hetero-nanocrystal was developed by combining Co₃Se₄ NCs of about ∼2 nm and Cu₂Se NCs of about ∼15 nm. Various heterojunctions with lattice defects were developed as a charge transport channel. The authors explained the presence of hydroxyl groups and some species like Cu*–OOH, Co*–OOH, and SeO_x on the hetero nanocatalyst interface participating in synergistic activity.¹⁶  Mathew et al. attempted a highly selective synthesis of pyridyl carbinal, attained by oxidation of pyridyl carbinol in a facile single step by using carbon nanospheres decorated by Pd nanoparticles. Interestingly, the carbon spheres were achieved utilizing Areca nut, and enhanced surface sites were revealed due to mesoporous structure. This porous structure was acclaimed to stimulate the adsorption of carbinol molecules hence facilitating the selective oxidation towards carbonyl group.¹⁷ 

Polythiophene is a versatile, highly stable, processable, and conducting polymer widely used in electrocatalytic applications. Various attempts have been made to develop metal and polythiophene-based catalysts for different applications. In this regard, the nanoclusters of Pt nanoparticles dispersed on polythiophene adsorbed on stainless steel electrodes were successfully employed for benzylalcohol oxidation. It was confirmed that the modified Pt-polythiophene-based stainless steel electrode displayed far better performance as compared with simple stainless steel electrode when TEMPO was used as a mediator.¹⁸ 

Similarly, polythiophene and PANI-modified electrodes have also been investigated for electrocatalytic applications. PANI and CNTs based composite materials are widely studied. In an attempt to oxidize glycerol, an electrocatalyst based on PANI and MWCNTS was employed as a substrate with Pt nanoparticles electrodeposited. In an attempt to develop a PANI-based electrocatalytic system, Pd deposition was made for the electrocatalytic TEMPO-based oxidation of pyridyl methanol. Moreover, the suggested composite achieved single-step oxidation, yielding biologically important pyridyl carboxaldehyde.¹⁹  Likewise, Pd and PEDOT composite has also been investigated electrocatalytic oxidation of aliphatic alcohols and glycerol in alkaline medium. It was claimed that nanodendrites of Pd on a thin layer of PEDOT possessed various uniform surface defects that caused better adsorption and better catalytic activity of the catalyst, and a comparison was made between Pd/C and Pd-PEDOT/C, with excellent performance with Pd-PEDOT/C. The electro-oxidation of propanol, propanediol and glycerol were successfully carried out.³ 

Morin oxidation was also investigated by electrodeposited Ir-PEDOT nanograins on carbon paper. The authors demonstrated that the oxidation of morin is a highly selective, single irreversible step and adsorption dependent. Moreover, the prepared catalyst was employed for the detection of morin in grape wine, mulberry and guava leaves.²⁰ 

The use of bimetallic palladium and molybdenum nanocubes for the ethylene glycol electro-oxidation demonstrated a 9.3 times higher current density as compared to commercially available Pd/C. The authors also explained the synergistic effects of the amorphous structure and the presence of molybdenum oxides improved the activity of PdMo/C.²¹  Besides this Pd based nanocatalysts using carbon nanotubes and carbon nano dots are recently explored for ethylene glycol oxidation.²² 

Another interesting subject in the electro-oxidation of alcohols is the oxidation in the form of nanoemulsions (a stable isotropic liquid dispersion of water, oil and surfactant). Microemulsions have also been studied but nanoemulsions provide a higher active surface area, so a reaction takes place fastly.^23,24  TEMPO and bromide mediated electro-oxidation of alcohols using a nanoemulsion was studied successfully. Sugars and ethylene glycol that formed nanoemulsions were oxidized whereas, the alcohols that didn't nanoemulsify were not oxidized. The authors prepared water-soluble tempo compounds by derivatization with ammonium or sulfonic acid groups. This strategy could be promising as the main solvent is the water, and non-aqueous solvents could be avoided, making the process cost-effective. Moreover, the mediator could be reused as it remained intact in the aqueous solution and was attained by extraction. The electro-oxidation of an industrially significant polyhydroxy compound, sorbitol, has been investigated by developing monometallic Pt, Au and bimetallic (PtAu) with different compositions as nanocatalysts.²⁵  The prepared nanocatalysts were of homogenous size with an average size of 5.6 to 6.5 nm. They realized that bimetallic catalysts with composition Pt₄₀Au₆₀/C displayed the best catalytic activity for electro-oxidation with 40 mA mg⁻¹ at 0.1 M sorbitol, 2 M KOH having an onset potential of −0.50 V vs. NHE. It was claimed that the observed onset potential, when compared to reported values of other polyhydroxy compounds such as ethylene glycol and glycerol, was found to be more negative.

Likewise, a novel MWCNTS based nanocatalyst has been developed for alcohol oxidation and investigated by CV and bulk electrolysis methods. The authors have used the pi–pi stacking and developed pyrene-tethered TEMPO/MWCNTs and used this functionalized electrode for the oxidation of benzylalcohol.²⁶  Pyrene was first adsorbed onto MWCNTs via π–π stacking and then drop-casted onto a glassy carbon electrode. Afterward, the GC-MWCNT electrode was dipped into a solution of pyrene-Tempo, and followed by thorough rinsing. The successful functionalization was determined by the CV, which revealed a reversible oxidation feature. A linear relationship between the peak current and scan rate ascertained the effective functionalization of the electrode by pyrene tempo derivative. Afterward, this electrode was used to oxidize a variety of benzylalcohols, and in particular, the oxidation of a hydroxymethylpyrimidine starting material to rosuvastatin (CrestorS) and a reductase inhibitor blockbuster HMG-CoA was also evaluated.

5-Hydroxymethylfurfural (HMF) is a representative starting material obtained from biomass hydrolysis and after electro-oxidation, 2,5-diformylfuran (DFF), furandicarboxylic acid (FDCA) could be formed, a valuable green monomer. Various nanocatalysts have been investigated so far, for instance, Pt, Pd Au, Pd–Au nanoparticles, PdAu₂ have been found with the good activity of 83% yield and 100% conversion at 0.9 V (vs. RHE) in basic solution.^27,28  Besides, Non-noble-metal-based Ni, Fe Co-based catalysts and co-catalysts have also been explored for the oxidation of HMF at more positive potential almost higher than 1.4V vs. RHE.^29–32 

Recently, selective oxidation of HMF by a bi-phase electrochemical flow system with high reaction and energy utilization efficiency was reported by Zai and co-workers. In order to increase catalytic efficiency towards I/I₂-mediated HMF oxidation and HER, CoS₂/CoS and platinum nanoparticles modified GF were used as electrodes and consumed only 28.4% energy compared to neat GF electrodes. Furthermore, significantly reduced energy consumption from 8.8 to 2.5 kJ for the gram-scale synthesis of DFF was attained. The authors claimed that a bi-phasic flow system could avoid over-oxidation to achieve high selectivity (Fig. 2a,b).³³  The same group successfully optimized the electrochemical setup for the generation of Fe³⁺ reduction with selective HMF oxidation through the mediator I⁻/I³⁻ and Fe²⁺/Fe³⁺ using Fe and Co-based nanomaterials (Fig. 2e,f).³⁴ 

Ni₂P nanoparticle arrays deposited on nickel foam as a bifunctional catalyst have been explored for paired HMF oxidation and HER.³⁵  It was stated that the current was increased during HMF oxidation and onset potential shifted to a lower value as compared to the oxygen evolution reaction (OER), indicating the feasibility of HMF oxidation over OER. In comparison to water electrolysis, it was also observed that less than 200 mV input voltage was required to reach the same current density of 100 mA cm⁻². Moreover, the in situ generation of oxidized Ni species was found to be catalytic sites for HMF oxidation by XRD and XPS analysis. A recent report by Fu and Luo presented nickel and cobalt-based LDH nanostructures realized a 100% faradaic efficiency for HER and FDCA production.³⁶  The electronic structure of Ni and Co were tuned and a high current density of 87 mA cm⁻² can be obtained at 1.3 V vs. RHE by facilitating charge/mass transfer with Cu_xS core NiCo-LDH shell nanostructures.

Formaldehyde is a carcinogenic compound with associated environmental risks. However, from the perspective of the methanol-based fuel cell, formaldehyde electro-oxidation finds significant attraction. TiO₂ and nickel-based nanotube arrays were described for formaldehyde oxidation. The authors have explored the combined effect of Ni(OH)₂/Ni/TNAs towards formaldehyde oxidation with an effectively low applied potential of (0.35 V) and high electrocatalytic rate constant of 5.36 × 10⁵ cm³ mol⁻¹ s⁻¹.³⁷  Gongxuan Lu et al. achieved a successful synthesis of four different shapes of silver nano products with significant electro-oxidation activities.³⁸  The authors tested the catalytic properties by CV and silver nanorods were found to possess superior activity for formaldehyde electro-oxidation in alkaline media amongst other Ag nanoproducts. In another synthesis attempt, Ag NPs dispersed on a p-xylylene diamine (Px) functionalized GO surface and then used for formaldehyde oxidation in alkaline media without forming poisonous CO intermediate.²²  The authors also compared the anodic current density with Pt and Pd achieved a higher anodic current with lower onset potential, ascribed to the good dispersion of Ag nanoparticles over the large surface area of GOPx. Recently, NiWO₄-NPs were also prepared via simple synthetic method by Daemi, et al.³  The modified electrode developed by mixing CPE and NiWO₄-NPs was employed for the oxidation of formaldehyde. Besides, the NiWO₄-NPs/CPE electrode was presented as low cost, simple synthetic protocol, stable and reproducible.

Olefin epoxidation is a significant transformation and, over the years, has gained a great interest among the scientific community for the key role of epoxide in the chemical industry. Metal oxide materials as heterogeneous electrocatalysts for energy-related chemical transformations have been extensively studied (e.g., oxygen evolution). Recently, Manthiram et al. used a heterogeneous catalytic approach employing water as an O-source for electro-epoxidation of alkenes using H₂O.³⁹  In this system, Mn₃O₄ nanoparticles were deposited on the carbon anode, and electrogenerated MnIVO was claimed to be the active epoxidation agent. The authors carried out electrokinetic studies and the mechanism was proposed. It was suggested that epoxidation was an approximate first-order reaction, and the rate of reaction is dependent on the concentration of water and substrate. The authors demonstrated the cyclooctene conversion of ∼50% in about 4 hours. Moreover, various linear, aliphatic and cyclic alkenes were also employed for the electroepoxidation by this new method and synthetically useful yields were achieved.

Besides, various electro-epoxidation attempts have been made for ethylene and propylene. For instance, Leow et al.⁴⁰  employed a heterogeneous: homogeneous interface while using chloride as a redox mediator at the anode to oxidize ethylene. The authors achieved a current density of 1 A cm⁻² with ∼70% FE and selectivity of ∼97%. When run at 300 mA cm⁻² for 100 hours, the system maintained a 71(±1)%Faradaic efficiency throughout.

Recently, a CoS₂/CoS heterojunction-based electrocatalyst as a working electrode has been reported for a benign electrosynthesis of epoxide at ambient conditions. NaBr was employed as a mediator in a green solvent system (Water: MeCN). 97% selectivity of styrene oxide was achieved at a constant current of 30 mA. The applied voltage was reduced to 4–5 V at 30 mA cm⁻², which effectively reduced the energy consumption of the reaction. Besides, the authors made detailed investigations using various combinations of electrodes for epoxidation (GF–FeS₂, GF–CoS, GF–CoS₂, GF–NiS₂, GF–Cu₇S₄, GF–CdS, GF–MnS) and among all combinations, CoS₂/CoS paired with carbon rod displayed the best selectivity.⁴¹  The synergistic cooperation between bromide oxidation at the anode, HER and oxygen reduction reaction (ORR) at the cathode was demonstrated. Various substrates have also been studied, and substituent effect was determined. Therefore, the simplicity of the method allows easy scale-up.

For the synthesis of various functionalities like amine oxides, oximes, nitriles, imines, amides, and azo compounds, the electrochemical oxidation of amines has gained huge attention worldwide. The amines oxidation reaction has lower oxidation potential than OER, hence, it can be paired with HER.⁴²  The electrochemical preparation of various aromatic nitriles with electron donating/withdrawing groups by oxidation of aromatic amines was demonstrated by Zhang et al.⁴³  The authors used NiSe nanorods by direct selenization of nickel foam. The authors also claim that this strategy is easily scalable as the amine oxidation reaction was carried out without using any oxidants, organic solvent and with higher yields of 93%.

Zhou and He et al.⁴⁴  recently used the WO₃ based photoanode and carried out the oxidation of industrially important 3-methyl pyridine in a divided electrolyzer with Pt wire as the counter and SCE as reference electrodes, proton exchange membrane as PEC reactor with 300 W xenon lamp as an illuminator. The authors described that 3-methylpyridine was oxidized to a 3-pyridine carboxylic acid with co-generation of hydrogen. They demonstrated the production of 3-pyridine carboxylic acid was more favorable both kinetically and thermodynamically as compared with PEC water splitting, as indicated by lower onset oxidation potential 170 mV. Besides, the conversion efficiency of 3-pyridine carboxylic acid was enhanced from 45% to 69% by adding the Cr2O₇⁻², a common oxidant.

Li and co-workers have also studied the synthesis of xanthone by electrocatalysis using AuPt based alloy nanoparticles. These nanoparticles were prepared at a low potential of −0.30 V vs. Pt at 30 °C in the ethylene glycol and choline chloride (ethaline) by one-step electrochemical reduction. As-prepared nanomaterials were directly deposited on the GCE used in the material preparation to develop AuPt NFs/GCE electrode, further used for electro-oxidation of xanthene. The electro-oxidation was carried out at ambient conditions using a constant potential of 0.80 V vs. Ag/AgCl, rendering this methodology environment friendly with an isolated yield of 87% (Table 1).⁴⁵ 

Ammonia (NH₃) is highly significant for fertilizer production and to date, the Haber–Bosch process is the main synthetic route for the large-scale production of ammonia. However, it is believed that if less demanding conditions could be used for ammonia synthesis, then there would be the potential for smaller devices for ammonia production.⁴⁶  Zhang et al. reported a 3D noble metal-free nitride-based VN nanosheet array catalytic system for NRR under mild conditions. This catalyst was demonstrated to achieve a surpassing efficiency than aqueous-based catalyst systems in terms of faradaic efficiency and NH₃ formation. Moreover, this catalyst system showed electrochemical stability of 8 h and worked via the Mars–van Krevelen mechanism. The catalyst also showed high selectivity (i.e. no formation of N₂H₄).⁴⁷  The Litch group,⁴⁸  Sun group,⁴⁹  Perathoner et al.,⁴⁹  Zhang et al.,⁴⁷  and Li et al.⁵⁰  made significant efforts in establishing the mechanism for Nitrogen reduction reaction by exploiting iron based nanoparticles (Fe₂O₃) in various forms, (nanorods, nanoparticles, composites with rGO and CNTs) in the presence of an applied potential. The electron transfer from the cathode to the Fe₂O₃ nanoparticles plays a key role, and the particle size also affects the ammonia production at a rate that increases with decreasing particle size. The Wang group, has also obtained some significant results for ammonia production. They have used nitrogen-doped carbon material in an acidic medium and achieved a Faradaic efficiency of 5.4% with a rate of ammonia generation of 0.08 g m⁻² h⁻¹. Furthermore, functionalization of the NCM with gold nanoparticles (Au NPs) was carried out, which notably increased the FE up to 22% and the rate to 0.36 gm⁻² h⁻¹. The authors ascertained this protocol to be scalable for industrial applications.⁵¹ 

Nitrobenzene (NB) is one of the 129 toxic pollutants by United States Environmental Protection Agency (USEPA) with its maximum tolerable concentration in waste-water as 1 mg L⁻¹ and is of global environmental concern. Besides NB, other nitroaromatics also have environmental risks inciting the remediation of nitroaromatic compounds (NACs). Thus, many research groups have been investigating the electrocatalytic reduction of NACs. Hence, the detection of NB and conversion of NACs to anilines is beneficial and is of concern amongst the scientific community. Nevertheless, different nitro oxidation products such as azo, amine, azoxy derivatives are significant organic molecules with various industrial applications like dyestuffs, pharmaceuticals, etc., due to their distinctive properties from the N–N–O and NN moieties in azoxy- and azo-molecules. Electrochemical methods are the preferred method for the conversion of NB to such important substances (anilines, azoxy, azo compounds) over traditional synthetic protocols due to their robustness, cost-effectiveness, simplicity and sensitivity. The electrochemical reduction of nitroarene in an aqueous solution is just similarly operated in either three-electrode or two-electrode cells. The reduction of nitroarene happens at the cathode, while water oxidation becomes an anodic reaction, where the protons from water combined with electrons play a key role in the reduction of the nitro group. Normally, the working potential range is in the range of hydrogen evolution from water splitting (0 V vs. RHE). Thus, the reduction of nitroarene and water are two competitive reactions. Highly efficient electrocatalytic reduction is mainly dependent on the cathodic structural properties, and so far, various metal-based materials such as Fe, Ni, Pd, Pt, Ag, Au etc., and various carbon-based materials (graphene, CNTs, N doped carbon materials) have been developed as cathode materials. Cu-based catalysts have also been investigated and were found to be efficient cathode materials for this purpose. Chen et al. developed stable Cu-based catalysts with different morphologies deposited on Ti plate by tuning the applied voltage via electrodeposition technique. Amongst all prepared materials, the dendritic nano-structured Cu catalysts prepared at high operating potentials revealed an exceptional efficiency and selectivity for the reduction of nitrobenzene to aniline. The authors observed about 97% aniline selectivity with a high rate constant of 0.0251 min⁻¹.⁵² 

Another Cu-based catalyst was prepared using precursor Cu₇₀Zr₃₀ alloy via chemical itching by Jiang et al.⁵³  The effective electrocatalytic activity of obtained amorphous alloy was mainly due to the presence of Cu-rich large surface area. The reduction products (i.e., hydroxylamine, amine, nitro, azo, azoxybenzene, p-aminophenol) of nitroarenes are reaction condition-dependent hence, the selectivity control is still challenging. The alkaline reaction conditions are known to produce amino groups along with by-products (azoxy-, azo- and hydrazobenzene), whereas the acidic medium produces p-aminophenol with aniline and azoxybenzene group mainly on solid cathodes (Cu, Pb and Hg).⁵⁴ 

In another report, N-doped carbons with deposited transition metals (Fe, Co or Cu) were constructed as electrocatalysts for the co-production of aniline. Besides the low cost and abundance of these metals, several oxidation states could also be attained and promote the reduction reaction. Researchers have also investigated the use of Cu, Pt-based nanoparticles and MWCNTs supported copper and Pt nanoparticles as electrocatalysts for the nitrobenzene reduction reaction.^55–57  However, the multi-walled carbon nanotubes were replaced by N-doped carbons as a support due to lower selectivity. Moreover, they also attempted to improve further and employed iron and cobalt, as they are more electropositive than copper.⁵⁸  Nephrolepis leaf-like silver microstructures were also successfully prepared via electrodeposition method and Ag microstructures deposited on GCE displayed a significant improvement for the electroreduction of nitrobenzene.⁵⁹  Titania-based cheap and stable catalysts are also suitable for the electro-reduction of nitroarene. Yu et al. prepared oxygen-deficient TiO₂ based single crystals as cathode, where the active atomic H* gets adsorbed and plays a key role in electro-reduction of nitrobenzene.⁶⁰ 

For instance, Sang et al., described MWNTs modified glassy carbon electrodes (MWNTs/GCEs) as functional carbon materials to examine the electro-reduction of nitrobenzene in an acidic medium.⁶¹  Various MWNT samples were studied, including pristine MWNT (MWNT-P having OH groups), acid-treated MWNT (MWNT-T mainly had COOH groups besides OH) and hydroxyl-containing MWNT (MWNT-OH mainly having OH). It was found that the highest electrocatalytic activity was presented by MWNT-OH amongst all, owing to the oxygen-containing functional group and weak capacitive feature. The authors investigated the reduction mechanism of NB on MWNTs/GCEs by using the cyclic voltammetry (CV) technique to detect and degrade it practically, where phenylhydroxylamine, azobenzene or p-aminophenol could be the products by regulating the potential windows.

In another attempt to investigate carbonaceous materials for electro-reduction of nitroaromatics, the effect of simultaneous oxygen reduction has also been assessed. Notably, an efficient activity for electro-reduction of nitrobenzene was revealed by the activated graphite sheets, with an acceptable linear range around 0.05–1100 mM, 0.05–147 mM and 0.05–145 mM, in the N₂, O₂ and aerated atmospheres, respectively. Besides 4-nitro aniline and 4-nitro phenol, this protocol was also applied to biologically important molecules like the anti-cancer drug flutamide and a pesticide methyl parathion.

Potential controlled electro-reduction of nitrobenzene by integrating CoP nanosheets as a cathode was investigated by Chong et al.³⁸  Various substrates with different functional groups were successfully prepared. Interestingly, different substrates were evaluated, and reactive functionalities like –C–Br, –CO and –CC bonds remained intact during the synthesis of amines, inferring to the appreciable chemoselectivity of CoP cathode. Particularly, the good adsorption and electron-withdrawing effect of nitro is the reason. Moreover, the otherwise challenging asymmetric azoxy-aromatic compounds were also synthesized by this strategy (Fig. 3). Another advantage of this method is the use of water as a hydrogen source, which could prove convenient for the synthesis of deuterated amines. Besides, using two-electrode cells that could be operated by a 1.5 V battery, the coupled anodic oxidation of aliphatic amines, azoxybenzene and nitriles could be prepared at gram scale level. The authors also examined nickel and iron phosphide (NiP and FeP) catalysts, but the results were not so good as compared to CoP, hence they ascertained this protocol to be facile and applicable for other electrochemical green syntheses.

Carbonyl compounds have been electro-hydrogenated enantioselectively by employing copper nanoparticles as a cathode in the presence of cinchonidine.⁶²  The process was dependent on the concentration of carbonyl compounds adsorbed on the surface of copper nanoparticles. The asymmetry was induced, and good yields with reasonable enantiomeric excess were achieved successfully. Wang and Lu, in 2016, presented a different approach by encapsulating the alkaloids into the copper nanoparticles.⁶³  Afterwards, this composite was compressed to form a coin and employed directly at the cathode for the hydrogenation of ketones. Water was used as the sole hydrogen source, and good yields of 93% with an enantiomeric excess of up to 71% of asymmetric alcohols were obtained. Moreover, the stability and recyclability of the alkaloid and Cu nanoparticles-based composite suggest this protocol as a facile method for asymmetric synthesis.⁶⁴ 

Yue et al. demonstrated the use of stable and reusable bimetallic nanocatalyst based on Pt and Cu, employed as the cathode for the asymmetric hydrogenation of aromatic ketones. The prepared catalyst was coated on the carbon paper and used as a cathode, while sacrificial magnesium was used as an anode. The authors also used Cinchonidine (CD) as it was found that CD gets adsorbed on the surface of the cathode and develops a chiral environment for the further asymmetric hydrogenation of ketone. Asymmetric trifluoromethyl benzyl alcohol was obtained with 59% enantiomeric excess (ee) and 25% yield under constant current electrolysis. Although the yields were not reasonable, however, the optimization could be done in future perspectives.^64,65 

The reduction of epoxides was explored by Huang and co-workers, as they demonstrated the use of zinc electrodes as anode and cathode in an aqueous solution. The zinc was found to play a critical role as the oxidation and reduction of the electrodes produced Zn(0) species, which developed into hierarchically organized nanostructures, however, when commercial zinc powder was used, lower yields were achieved.^66,67  Recently, a highly stable, nanostructured chiral-imprinted Pt–Ir alloy as electrode exhibiting pronounced enantioselectivity and stability was developed for the synthesis of chiral compounds. The authors claimed an improved electrosynthesis of chiral molecules while using alloys compared to pure metals. Acetophenone electro-reduction was investigated as the model, as the target product was 1-phenylethanol (1-PE) with another product might be 2,3-diphenyl-2,3-butanediol. The authors explained remarkable activity with >95%ee for asymmetric electrosynthesis.⁶⁸ 

For the past few years, other than the electrochemical synthetic protocols of various organic functionalities, photo-electrochemical techniques also gained the huge interest of scientists, and various photoanodic materials have been explored in this regard.

In a recent report, haematite has been employed as a photoanode to achieve C–H amination. Photo-electrocatalysis is considered to consume less energy than direct electrolysis due to light harvesting. In this photoanode, the holes were generated when illuminated and these holes further oxidize the electron-rich aromatic compounds to form radical cations. Subsequently, the radical cations reacting with azoles form nitrogen-containing heterocycles, which could be immensely significant from a medicinal perspective. In this reaction, the presence of hexafluoroisopropanol as a solvent was assumed to develop a H-bonding with the substrates, facilitating the ortho- product formation.⁶⁹  This synthetic protocol is significant as the earth-abundant, low cost and stable reagent was successfully employed for the late-stage functionalization of several molecules with medicinal importance. The synthesis of hydroquinone (HQ) by electro-organic conversion of PhOCH₃ has been achieved by depositing pristine-MWCNT–Nafion on the glassy carbon electrode surface. This chemically modified electrode exhibited redox-active behavior at electrode potential, Eo0 ¼ 0.45 V vs. Ag/AgCl (A2/C2). It was revealed that inherent iron present in the MWCNT participated in the reaction and developed specific interactions with the oxygen atom of PhOCH₃. Moreover, the authors carried out various techniques and controlled experiments to confirm the formation of surface-confined HQ. Recently, the electrocatalytic reduction of CO₂ with reduction of nitrogenous small molecules have been investigated for the production of organic amides such as urea and acetamide. In this aspect, Cu,⁷⁰  Cu–Pd⁷¹  and Te–Pd based nanoparticles have been already explored for the synthesis of urea and acetamides. Hu's research team has recently presented a rational perspective of constructing C–N bond by co-reduction of CO₂ and nitrogenous small molecules by electrocatalysis via developing heterogeneous dual-active site catalysts (DACs).⁷² 

In an attempt to prepare organic halides, Li et al. has recently adopted TiO₂ based photo-electrochemical catalyst. The photoanode used was oxygen vacancy rich TiO_2, and when illuminated, holes generated caused oxidation of halide ions (Cl, Br, I) to respective radicals or halogen molecules. The produced halogen radicals or molecules reacted with organic substrates attaining organic halides.⁷³  The authors examined various heteroaromatic rings, cycloalkanes, aliphatic hydrocarbons and aromatic substrates. In addition, the authors prepared a self-powered PEC system for the production of chlorocyclohexane by using seawater as a chloride source. The productivity of chlorocyclohexane attained was 412 µmol h⁻¹ paired with 9.2 mL h⁻¹ generation of H₂, this strategy could be applied for achieving organic chlorides using seawater.

Recently, electron-deficient W₂C nanocrystals as electrode materials have been explored for the C–H activation. The electrocatalytic C–H activation was enhanced via a heterogeneous pathway.⁷⁴  The authors successfully developed Schottky heterojunction with nitrogen-doped carbon materials and the electron density was tuned in such a way that facilitated C–H functionalization. The alkoxylation of ethylbenzene with methanol was studied as the model reaction under mild reaction conditions. Afterward, benzenes with different alkyl chains, halogen-substituted, sterically hindered substrates, drug-oriented molecules and bioactive molecules could easily be methoxylated with reasonable yields and high selectivity up to 99%. The authors proposed better adsorption of benzylic C–H on the surface of nanocatalyst as the rate determining step. Moreover, this electrochemical transformation is also paired with hydrogen gas production at the cathode. The authors also proposed that the electron deficiency imparts the enhanced inherent activity of catalysts and various other catalysts systems could be developed for the zero-emission electrosynthesis systems.

In an attempt to conduct electro-oxidation of toluene, interesting nanocacti were developed by electrodeposition of platinum on carbon paper pre-coated by PANI. The developed nanocacti morphology was homogenous and efficient oxidation performance was revealed as compared to bare Pt/Carbon paper electrode. It was explained that high-energy surface sites of Pt-PANI/C attract the toluene molecules towards the electrode surface and facilitate diffusion to the polymer layer. The authors claimed this protocol to be highly selective and a good yield of Ar-CHO could be achieved in a single step in acetonitrile medium in the presence of NaNO₂/H₂SO₄.⁷⁵ 

In an attempt to develop green electrosynthesis methods, C–C bond formation has been studied, and an efficient CuPd catalyst deposited on carbon support was recently reported.⁷⁶  The prepared catalyst was then used for electrochemical allylic alkylation in an aqueous medium under mild conditions. The authors demonstrated that bimetallic catalysts with an almost equal ratio in CuPd NPs exhibited the best performance for selective cross-coupling of alkyl halides and allylic halides. The synthesized yields of the products were nearly 99%. Besides, it was also revealed that for cross-coupling between alkyl iodides and allylic chlorides, better activity and selectivity was achieved for those lacking deactivating electron-donating groups near the double bond of allylic substrates.

Although various literature reports have demonstrated the CO₂RR and successful synthesis of small molecules but to discuss the CO₂RR reduction in detail would be out of the scope of the current work. However, here we briefly discuss the synthesis of ethylene by CO₂RR, by the membrane electrode assembly (MEA) electrolyzer. (MEA) electrolyzer used has a direct cathode : membrane : anode contact that has been demonstrated for the CO₂RR for ethylene production.⁷⁷  Firstly, the Density Functional Theory was employed to evaluate the role of silica on affecting CO₂RR on Cu. Afterward, Cu–SiO_x catalyst was synthesized using one-pot coprecipitation and then integrated the CuSiO_x catalyst in a MEA electrolyzer. A Faraday efficiency of 65% at a current density of 215 mA cm⁻² for CO₂-to-ethylene conversion, almost double the ethylene productivity in MEA when compared to literature benchmarks.

Yang et al. described the CO₂ fixation in terms of developing new carboxylic acids by utilizing bromo- and chloro- substrates. The authors successfully prepared silver nanowires and treated them with dopamine and further calcined them at high temperatures (600–800 °C) to achieve the nitrogen doped carbon composite materials. Ag Nw/NCM 700, displayed the best catalytic activity for cycloaddition as well as for CO₂ fixation. The AgNW/NC700 was deposited on the glassy carbon surface and used as the cathode for electrocatalytic carboxylation and cycloaddition of CO₂. 0.1 m 1-(1-bromoethyl)-4-isopropylbenzene in acetonitrile (MeCN) solution was used as substrate and successful synthesis (95% yield and 99% selectivity) of ibuprofen, an important NSAID, was attained. Besides, the electro cycloaddition with propylene oxide was carried out under mild conditions, and propylene carbonate (widely used for polymers) was obtained.

Synthesis of various value-added chemicals and complex pharmaceutical relevant materials need the development of more efficient synthetic protocols. The use of abundant electricity to produce significant chemicals could be a game-changer in future commercial purposes. This chapter is not meant to be an extensive study of this ever-growing arena but just a brief outline towards the use of nanomaterials for electrosynthesis. This work showcases a number of interesting research ideas, representing creative ways to use versatile catalytic materials. Although various biologically and commercially significant chemical compounds have been prepared by electro-organic synthesis, we have just tried to compile some nanomaterials-based electro-organic synthesis. Organic synthesis has been directed towards sustainability and electro-organic synthesis offers unique tunability and holds great potential for synthesis in areas such as C–H functionalization, oxidation, and C–X bond formation in total synthesis. It could be noticed from electro-oxidation reactions mentioned above that besides other catalysts, mostly platinum and palladium have been employed owing to their inherent redox properties. Nevertheless, catalytic materials discussed in this chapter will hopefully encourage chemists to select suitable nanomaterials for rational application in total synthesis.

We are thankful to Prof. Xuefeng Qian, School of chemistry and chemical engineering, Shanghai Jiaotong University, for assisting in literature access.