*3.5.1 Bimetallic C-NCs*

*Colloids - Types, Preparation and Applications*

*with permission from the Royal Society of Chemistry.*

catalytic activity and stability.

**Figure 5.**

cube, octahedral, and rhombic dodecahedral, respectively. These low index surfaces are said to be less energetic, that is, catalytically less active. However, the plains lying on three sidelines of triangle and one which is residing inside the triangle are known as high index surfaces, that is, catalytically more active and stable. The high index planes (310), (311), (331), and (321) are correlated with tetrahedron, trapezohedron, trisoctahedron, and hexoctahedron, respectively, as illustrated in **Figure 5(b).** The high energy of these polyhedron is attributed to predominance of atomic steps, islands and kinks on their surface. For example, a concave rhombic dodecahedron Au C-NC enclosed with multiple high index planes showed excellent activity and selectivity towards CO formation [50]. In addition, this particular structure of Au C-NC was found durable for longer period of time. Summing up, low coordinating high index planes with CN less than 7 exhibits high catalytic activity, selectivity, stability whereas low index planes with higher CN greater than 6 exhibits lower

*(a) Unit stereographic triangle of fcc single-crystal and models of surface atomic arrangement. (b) Unit stereographic triangle of polyhedral nanocrystals bounded by different crystal planes. Reproduced form [49]* 

Lattice strain has evolved as another factor that have a significant influence on electrocatalytic properties of C-NCs towards ECO2RR. It was studied that strain generates a distortion in lattice, which, in turn, alters d-band center of metallic C-NCs. The altered d-band center either facilitates or lowers the adsorption of intermediates, as a result of that changing catalytic properties. Typically, upshifting of the d-band center facilitates adsorption of reaction intermediates on the surface of C-NCs. This is because, as the d band center approaches the Fermi level, which is the highest occupied state, the antibonding orbitals move over it, causing them to empty. This, as a result, strengthens the binding strength of the reaction intermediate on the catalytic surface and, therefore, enhances the reaction kinetics [51] In general, lattice strain in C-NCs can be induced by shaping C-NC in various morphologies, which leads to the inward displacement of atoms at high-energy locations, such as corners and edges, while the outward displacement of atoms on planes to gain overall crystal stability. These compression and expansion in the atomic arrangement in the NCS induce aeolotropic strain gradients that may improve the catalytic efficiency/selectivity of C-NCs [45]. Octahedral and Icosahedron C-NC of Pd with similar sizes were examined by huang et al. to investigate the effect of strain on ECO2RR. They observed that icosahedral/C C-NC shows higher FE (91% with −0.81 V vs. RHE) for CO production than octahedral/C C-NC in ECO2RR. The molecular simulations and DFT calculations showed that surface strain in icosahedral C-NC enhanced catalytic selectivity due to shifting in d-band center, which, in turn, facilitates absorption of a key intermediate (COOH\*

**200**

Until now, several bimetallic NCs have been investigated for ECO2RR. For example, Kortlever et al. have found an optimal composition of a novel Pd70Pt30/C electrocatalyst highly active and selective towards HCOOH production. Moreover, this has a remarkably lower onset potential close to 0 V vs. RHE, making it best catalyst till the date [53]. However, usage of noble high-cost metals discourages its application on economical scale. Incorporation of non-noble metals such as Cu, Ni, Fe, etc., in bimetallic system could serve a better alternative to address high cost and stability of electrocatalyst. Kim et al. synthesized different composition of NCs including Au, AuCu, AuCu3, Au3Cu, and Cu, which were assembled in a monolayer on glassy carbon while keeping precise control over morphology (Size, Shape etc.) [44]. When considering electronic effect solely, pristine Au NC should have shown higher activity due to its optimal binding with COOH and CO intermediates. Interestingly, they have observed higher activity for Au3Cu bimetallic C-NC than expected one. Therefore, electronic effect mere does not explain volcanic acti vity correlation for C-NCs. The geometric effect that works synergistically along with electronic effect ensure further stabilization of intermediates, thus, explaining optimal activity of Au3Cu bimetallic C-NC towards electrochemical CO2 reduction, among others. Previous studies have shown excellent properties of In based metal catalysts for selective CO2 conversion into HCOOH. However, these metal catalysts suffer from limited current density and poor stability.

Kown et al. synthesized In2O3-ZnO C-NCs that showed excellent selectivity towards formation of HCOOH [54]. The XRD pattern showed that pre reduction of these C-NCs during electrolysis leads to the formation of In-Zn bimetallic C-NCs. Among all Zn1-xInx NCs, In0.05Zn0.95 offered remarkable selectivity for HCOOH production at FE of 95% (−1.2 V vs. RHE) as well as with higher current density. The higher catalytic activity was seen in both Zn1-xInxO and Zn1-xInx with decreasing value of x. The XPS data has revealed predominance of O vacancies in bimetallic systems at lower x, which, in turn, decreases thickness of oxide layers in Zn1-xInx. Consequently, conductivity of Zn1-xInx C-NCs increases at lower x, therefore, facilitates rapid electron transfer processes in ECO2RR. Thus, highest catalytic activity of Zn0.95In0.05 C-NC is attributed to its remarkable conductivity. DFT calculations

)

revealed that tight binding of OCHO\* on pristine In, which impedes HCOOH production, has weakened by introduction of Zn in bimetallic C-NC. Therefore, mutual synergies of In with Zn in bimetallic system enhanced its catalytic selectivity in CDRR as compared to In [54]. Guo et al. have shown compositional effect in Cu3Pt C-NCs is responsible for improved activity and selectivity for CH4 [55]. It was observed that increasing Cu contents in bimetallic NC leads to desorption of more CO\* intermediates that subsequently gets protonated into CH4. Moreover, Pt which shows higher affinity for H<sup>+</sup> has significantly accelerated protonation of CO\* . However, higher Cu content beyond ratio of Cu and Pt (3:1) raised CO\* poisoning, and thus, declining in CH4 production [55].

### *3.5.2 Dopped C-NCs*

In doped C-NCs, extrinsic or intrinsic introduction of impurities can cause a change in the electronic structure in a way that can enhance catalytic efficiency/ selectivity and durability. For example, the incorporation of impurities can provide additional electrons (n-type) or additional vacancies (p-type), thereby, increasing the conductivity of the catalyst that would otherwise be poor in conductivity. Therefore, the increased conductivity may accelerate the electron transfer process in the ECO2RR. Although many advances have been made in the field of doped C-NCs, some studies conducted in ECO2RR. Recently, a group led by Kim et al. reported an unprecedented selectivity of vanadium (23V) doped In2O3 C-NCs towards CH3OH formation in addition to HCOOH and CO, previously not known with pristine In and In2O3 [56]. This can be understood using the commonly suggested scheme for producing CH3OH shown in the **Figure 3.** The CO\* intermediate needs to be stabilized on the surface to proceed towards CH3OH formation otherwise it may release as CO gas and terminate the reaction. The introduction of V3+ into In2O3 strengthens the binding of CO\* intermediate with NCs possibly due to the π-back donation from dopant to intermediate CO\* , and thus, reaction proceeds towards CH3OH (FE of 15.8% at −0.83 V vs. RHS) [56].

#### **3.6 C-NCs/metal organic framework hybrid**

In the past years, molecular organic frameworks (MOFs) have received significant attention in the field of catalysis, including ECO2RR, where they have been used primarily to provide solid support for molecular catalysts [57, 58]. The combination of MOF with C-NCs creates a class of hybrid materials that have demonstrated great potential to become future class of electrocatalysts with improved efficiency/selectivity and durability. Concerning this, a deep understanding of the material design and working principle of these novel hybrid systems is indispensable prior to implementing them practically. Recently, Guntern et al. investigated catalytic selectivity and durability of Ag C-NCs/Al PMOF hybrid system for electrochemical CO2 reduction [59]. Based on UV visible and XPS data, it was concluded that electron transfer from MOF to Ag C-NC in this hybrid system increases electron density at Ag C-NC, thereby, facilitates electron transfer to CO2 .− intermediate. As in result, selectivity of Ag C-NCs/Al PMOF towards CO increases than pristine Ag C-NCs while decreases for H2ER. Additionally, a small contribution of transport events due to diffusion of reactants and products within pores of MOF adds in the selectivity of NC/MOF hybrid towards CO. Besides, NC/MOF hybrid system showed better stability compared to bare Ag C-NCs at lower potential [59].

**203**

C-NCs [64].

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems…*

Capping agents such as, organic ligands, surfactants or polymers are engendering factors of surface anchoring molecules that are crucial in synthesis of C-NCs based catalysts with well-defined shape, uniform size distribution and different composition. After synthesis, these capping agents can significantly alter C-NCs catalytic efficiency in both positive and negative way by staying absorbed on their surface. For example, Wang et al. reported that catalytic activity of Ag NCs modified with capping agents increased by 53-fold compared to pristine Ag NCs towards ECO2RR [60]. In Several studies it has been shown that presence of surface molecule can affect active site of NC in numerous ways: 1) by perturbing electronic structure of active sites 2) by introducing steric hindrance that impedes diffusion of absorbates reaching at active center 3) by blocking selective facets of C-NCs, which, in turn, could enhance selectivity and activity. Moreover, chiral ligands can be used to produce stereoselective products. Therefore, surface functionalized NCs has opened a new window in the field of electrocatalytic reactions where unprecedented control over efficiency/selectivity can be achieved by ligand design [61, 62]. However, researchers still have a limited understanding of how these anchoring ligands affect the local electronic environment of NC and how the backbone of anchoring ligands regulates reactivity between NC and surrounding reactants, or

Pankhurst et al. have tuned ECO2RR selectivity of Ag C-NCs using different imidazolium ligands [63]. Here they were able to introduce different organic component by varying tail and anchoring groups on imidazole motif. When performed ECO2RR, the ligand bearing NO2 anchoring group with octyl tail-found highly selective towards CO with FE of 92%. It was concluded that interaction between cationic imidazolium group with CO2 increases population of this reactant over the catalytic surface. Furthermore, an optimal chain length of ligand tail-group increased hydrophobicity of surface, and thus, increases selectivity and efficiency for ECO2RR by inhibiting EH2ER. However, electronic changes induced by ligand anchoring-group did not improve significantly properties of electrocatalyst. The next example of ligand surface functionalization for ECO2RR discusses N-heterocyclic-carbine functionalized Au C-NCs (Au C-NC-Cb) from group led by Cho et al. The significant downfield shifting in 13C NMR peaks of NHC reveals strong electron donation from ligand to metal, making Au C-NCs surface electron-rich. As in result, it facilitates the electron transfer process to CO2, and thus, enhances efficiency of Au C-NC-Cb relative to bare Au

The concept of using electric current to control various chemical reactions achieved much attention, since the time when humankind invented first power resources. Splitting of water to produce hydrogen and oxygen gas started much earlier, but now it has started at large scale in industrial process and seems to play a crucial role in combating the future energy crisis. The increasing demand for energy day by day and the shortage of fossil fuels have encouraged scientists to

Hydrogen is considered a clean source of energy because by-product of H2 combustion is H2O and the starting material to obtain H2 is water, therefore, an efficient and clean source to supplant the depleting fossil fuels [65–67].

**4. C-NCs-based heterogeneous catalyst for EH2ER**

develop a renewable and clean source of energy.

*DOI: http://dx.doi.org/10.5772/intechopen.95338*

**3.7 Ligand functionalized C-NCs**

reaction intermediates.

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems… DOI: http://dx.doi.org/10.5772/intechopen.95338*

### **3.7 Ligand functionalized C-NCs**

*Colloids - Types, Preparation and Applications*

which shows higher affinity for H<sup>+</sup>

and thus, declining in CH4 production [55].

more CO\*

The CO\*

RHS) [56].

CO\*

*3.5.2 Dopped C-NCs*

revealed that tight binding of OCHO\* on pristine In, which impedes HCOOH production, has weakened by introduction of Zn in bimetallic C-NC. Therefore, mutual synergies of In with Zn in bimetallic system enhanced its catalytic selectivity in CDRR as compared to In [54]. Guo et al. have shown compositional effect in Cu3Pt C-NCs is responsible for improved activity and selectivity for CH4 [55]. It was observed that increasing Cu contents in bimetallic NC leads to desorption of

However, higher Cu content beyond ratio of Cu and Pt (3:1) raised CO\*

intermediates that subsequently gets protonated into CH4. Moreover, Pt

In doped C-NCs, extrinsic or intrinsic introduction of impurities can cause a change in the electronic structure in a way that can enhance catalytic efficiency/ selectivity and durability. For example, the incorporation of impurities can provide additional electrons (n-type) or additional vacancies (p-type), thereby, increasing the conductivity of the catalyst that would otherwise be poor in conductivity. Therefore, the increased conductivity may accelerate the electron transfer process in the ECO2RR. Although many advances have been made in the field of doped C-NCs, some studies conducted in ECO2RR. Recently, a group led by Kim et al. reported an unprecedented selectivity of vanadium (23V) doped In2O3 C-NCs towards CH3OH formation in addition to HCOOH and CO, previously not known with pristine In and In2O3 [56]. This can be understood using the commonly suggested scheme for producing CH3OH shown in the **Figure 3.**

 intermediate needs to be stabilized on the surface to proceed towards CH3OH formation otherwise it may release as CO gas and terminate the reaction.

The introduction of V3+ into In2O3 strengthens the binding of CO\*

**3.6 C-NCs/metal organic framework hybrid**

with NCs possibly due to the π-back donation from dopant to intermediate

, and thus, reaction proceeds towards CH3OH (FE of 15.8% at −0.83 V vs.

In the past years, molecular organic frameworks (MOFs) have received significant attention in the field of catalysis, including ECO2RR, where they have been used primarily to provide solid support for molecular catalysts [57, 58]. The combination of MOF with C-NCs creates a class of hybrid materials that have demonstrated great potential to become future class of electrocatalysts with improved efficiency/selectivity and durability. Concerning this, a deep understanding of the material design and working principle of these novel hybrid systems is indispensable prior to implementing them practically. Recently, Guntern et al. investigated catalytic selectivity and durability of Ag C-NCs/Al PMOF hybrid system for electrochemical CO2 reduction [59]. Based on UV visible and XPS data, it was concluded that electron transfer from MOF to Ag C-NC in this hybrid system increases electron density at Ag C-NC, thereby, facilitates electron transfer to

.− intermediate. As in result, selectivity of Ag C-NCs/Al PMOF towards CO increases than pristine Ag C-NCs while decreases for H2ER. Additionally, a small contribution of transport events due to diffusion of reactants and products within pores of MOF adds in the selectivity of NC/MOF hybrid towards CO. Besides, NC/MOF hybrid system showed better stability compared to bare Ag C-NCs at

has significantly accelerated protonation of CO\*

.

poisoning,

intermediate

**202**

lower potential [59].

CO2

Capping agents such as, organic ligands, surfactants or polymers are engendering factors of surface anchoring molecules that are crucial in synthesis of C-NCs based catalysts with well-defined shape, uniform size distribution and different composition. After synthesis, these capping agents can significantly alter C-NCs catalytic efficiency in both positive and negative way by staying absorbed on their surface. For example, Wang et al. reported that catalytic activity of Ag NCs modified with capping agents increased by 53-fold compared to pristine Ag NCs towards ECO2RR [60]. In Several studies it has been shown that presence of surface molecule can affect active site of NC in numerous ways: 1) by perturbing electronic structure of active sites 2) by introducing steric hindrance that impedes diffusion of absorbates reaching at active center 3) by blocking selective facets of C-NCs, which, in turn, could enhance selectivity and activity. Moreover, chiral ligands can be used to produce stereoselective products. Therefore, surface functionalized NCs has opened a new window in the field of electrocatalytic reactions where unprecedented control over efficiency/selectivity can be achieved by ligand design [61, 62]. However, researchers still have a limited understanding of how these anchoring ligands affect the local electronic environment of NC and how the backbone of anchoring ligands regulates reactivity between NC and surrounding reactants, or reaction intermediates.

Pankhurst et al. have tuned ECO2RR selectivity of Ag C-NCs using different imidazolium ligands [63]. Here they were able to introduce different organic component by varying tail and anchoring groups on imidazole motif. When performed ECO2RR, the ligand bearing NO2 anchoring group with octyl tail-found highly selective towards CO with FE of 92%. It was concluded that interaction between cationic imidazolium group with CO2 increases population of this reactant over the catalytic surface. Furthermore, an optimal chain length of ligand tail-group increased hydrophobicity of surface, and thus, increases selectivity and efficiency for ECO2RR by inhibiting EH2ER. However, electronic changes induced by ligand anchoring-group did not improve significantly properties of electrocatalyst. The next example of ligand surface functionalization for ECO2RR discusses N-heterocyclic-carbine functionalized Au C-NCs (Au C-NC-Cb) from group led by Cho et al. The significant downfield shifting in 13C NMR peaks of NHC reveals strong electron donation from ligand to metal, making Au C-NCs surface electron-rich. As in result, it facilitates the electron transfer process to CO2, and thus, enhances efficiency of Au C-NC-Cb relative to bare Au C-NCs [64].
