CO2 Adsorption on PtCu Sub-Nanoclusters Deposited on Pyridinic N-Doped Graphene: A DFT Investigation.

To reduce the CO2 concentration in the atmosphere, its conversion to different value-added chemicals plays a very important role. Nevertheless, the stable nature of this molecule limits its conversion. Therefore, the design of highly efficient and selective catalysts for the conversion of CO2 to value-added chemicals is required. Hence, in this work, the CO2 adsorption on Pt4-xCux (x = 0-4) sub-nanoclusters deposited on pyridinic N-doped graphene (PNG) was studied using the density functional theory. First, the stability of Pt4-xCux (x = 0-4) sub-nanoclusters supported on PNG was analyzed. Subsequently, the CO2 adsorption on Pt4-xCux (x = 0-4) sub-nanoclusters deposited on PNG was computed. According to the binding energies of the Pt4-xCux (x = 0-4) sub-nanoclusters on PNG, it was observed that PNG is a good material to stabilize the Pt4-xCux (x = 0-4) sub-nanoclusters. In addition, charge transfer occurred from Pt4-xCux (x = 0-4) sub-nanoclusters to the PNG. When the CO2 molecule was adsorbed on the Pt4-xCux (x = 0-4) sub-nanoclusters supported on the PNG, the CO2 underwent a bond length elongation and variations in what bending angle is concerned. In addition, the charge transfer from Pt4-xCux (x = 0-4) sub-nanoclusters supported on PNG to the CO2 molecule was observed, which suggests the activation of the CO2 molecule. These results proved that Pt4-xCux (x = 0-4) sub-nanoclusters supported on PNG are adequate candidates for CO2 adsorption and activation.

1. Introduction

Due to human activity associated with the usage of fossil fuels and industrialization, the concentration of CO2 in the atmosphere has increased considerably. It is causing environmental problems such as the greenhouse effect, global warming, and climate change among others [1,2,3]. Therefore, in order to reduce the CO2 concentration in the atmosphere, various investigations and technologies are being developed such as the CO2 sequestration process [4,5], and CO2 conversion into different value-added chemicals is another strategy widely used [6,7,8]. Nevertheless, the stable nature of the CO2 molecule limits its conversion [9,10]. Therefore, the design of highly efficient and selective catalysts for the conversion of CO2 into value-added chemicals is required.

Currently, many catalysts have been designed for CO2 conversion into value-added chemical products, among them the transition metal nanoparticles-based catalysts can be highlighted [11,12]. However, more recently, it has been documented that alloy catalysts have more outstanding catalytic activities than monometallic nanoparticles for CO2 conversion [13,14]. Among the different bimetallic catalysts studied, PtCu nanoparticles have become very important because they present good catalytic properties for the CO2 conversion [15,16,17]. For example, Cu–Pt alloy nanocubes with a relatively broad range of composition ratios were synthesized and evaluated for CO2 electroreduction reaction [15]. It was found that the Cu–Pt alloys exhibit compositional-dependent activities towards CO2 electroreduction. In another study, Cu–Pt nanocrystals with different amounts of Cu and Pt were prepared and evaluated for CO2 electroreduction reaction [16]. Guo and coworkers highlighted the importance of the compositional effect of Cu–Pt nanocrystals on their catalytical activities in what CO2 electroreduction is concerned. In addition, it was demonstrated that the Cu–Pt (3:1) nanocrystals exhibited the highest activity and faradaic efficiency in the CO2 electroreduction reaction among all the as-prepared Cu–Pt samples. Recently, a density functional theory (DFT) study of CO2 adsorption on Cu4-xPtx (x = 0–4) clusters was performed [18]. It was computed that the gas phase linear CO2 molecule was deformed upon adsorption, with its bend angle varying from about 134° to 145°, which could favor the CO2 dissociation. It can be inferred from these studies that Cu–Pt alloys are good candidates for the conversion of CO2 to value-added products.

However, it is well known that metallic nanoparticles tend to agglomerate due to their high surface energies, which in turn involves the coarsening of larger particles from those of smaller size [19,20]. Therefore, to overcome the agglomeration problems, it is necessary to disperse or support these nanoparticles on materials with high surface area. To this end, graphene is considered a good support material due to its high specific surface area, excellent electrical conductivity, and resistance to corrosion [21,22], however, this material has a limited chemical reactivity [23]. Consequently, various approaches have been implemented to improve its activity, e.g., functionalization and doping among others [24,25,26]. Specifically, among the different dopants used to modify graphene reactivity, pyridinic-type N doping can be highlighted because it enhances both the stability and reactivity of metallic nanoparticles [27,28]. Nowadays, there is a sizeable number of theoretical studies that analyze the stability and reactivity of metal nanoparticles supported on pyridinic N-doped graphene (PNG) [29,30,31]. These studies show the potential of PNG to improve not only the stability, but also the reactivity of metal nanoparticles for different applications [29,30,31].

According to the literature, the reactivity and stability of Pt4-xCux (x = 0–4) clusters supported on PNG substrate was investigated using the DFT [32]. It was demonstrated that Pt–Cu nanoparticles supported on PNG are good candidates to adsorb the glyphosate molecule and PNG stabilized the Pt–Cu nanoparticles as well [32]. However, to the best of our knowledge, there are no theoretical studies on CO2 adsorption on Pt–Cu clusters supported on PNG using the DFT calculations. Therefore, in this work, the CO2 adsorption on Pt–Cu sub-nanoclusters deposited on PNG was studied using the DFT calculations. In order to achieve this goal, the most stable interaction between the Pt4-xCux (x = 0–4) sub-nanoclusters and the PNG was investigated. Furthermore, DFT calculations were used to bring light into the CO2 adsorption on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG.

2. Computational Methodology

All calculations were carried out within the DFT implemented in the ORCA 5.0.0 package [33]. All the electronic structure calculations were addressed through the revised Perdew–Burke–Ernzerhof exchange correlation functional (revPBE) [34]. For the C, H, N, and O atoms, the Ahlrichs basis sets def2-SVP were used for the calculations and def2-TZVP for the Cu atoms [35], whereas the Pt ones were treated using the basis set LANL2DZ for effective core potentials [36]. The convergence tolerances for geometry optimization were energy change = 5 × 10−6 Eh, max. gradient = 3 × 10−4 Eh/Bohr, rms gradient = 1 × 10−4 Eh/Bohr, max. displacement = 4 × 10−3 Bohr, and rms displacement = 2 × 10−3 Bohr.

To investigate the stability of Pt4-xCux (x = 0–4) sub-nanoclusters on PNG, the most stable structures for the Pt4-xCux (x = 0–4) sub-nanoclusters were obtained from a previous study [18]. However, it is worth highlighting that these structures were reoptimized employing the methodology used in this study, which are depicted in Figure 1.

Figure 1

Structures and spin multiplicity (M) of Pt4-xCux (x = 0–4) sub-nanoclusters. (a) Pt4, (b) Pt3Cu, (c) Pt2Cu2, (d) PtCu3, and (e) Cu4.

The pyridinic-type N doping can be located anywhere on the graphene (e.g., edge or center). Here, we used graphene as the support material, therefore, the doping was localized in the center of the graphene. In this case, different numbers of nitrogen atoms (e.g., 1, 2, or 3) can be used. In this work, we used pyridinic-type doping with three N atoms, as it has been a widely used structure [29,30,31,37]. In this sense, circumcoronene (C54H18) was used as model of graphene. To obtain the PNG structure, a C atom was removed from the center of the graphene to create a vacancy, then the hanging C atoms were replaced by N ones, as shown in Figure 2.

Figure 2

Pyridinic N3-doped graphene structure.

The binding energies (Eb) between the Pt4-xCux (x = 0–4) sub-nanoclusters and the PNG were calculated as follows: where , , and are the energies of the Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, Pt4-xCux (x = 0–4) sub-nanoclusters, and the PNG structure, respectively.

The adsorption energies (Eads) of CO2 on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG were obtained as: where is the energy of CO2 adsorbed on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, while and are the energies as a single point calculation of the free-standing Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG and the CO2 molecule from the optimized structure of the Pt4-xCux (x = 0–4)/PNG+CO2 composite, respectively.

To analyze the molecular interactions of the sub-nanoclusters supported on PNG and the CO2 adsorption over Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, the Quantum Theory of Atoms in Molecules (QTAIM) developed by Bader was employed for the charge transfer analyses; to this end, the Multiwfn program was used [38].

3. Results

3.1. Stability of Pt4-xCux (x = 0–4) Sub-Nanoclusters on PNG

The most stable interaction between Pt4-xCux (n = 0–4) sub-nanoclusters and PNG was determined using several configurations. Figure 3 and Figure 4 illustrate the most stable interactions between the Pt4-xCux (n = 0–4) sub-clusters and PNG. It was found that the most stable interaction between the Pt4 sub-cluster and the PNG was with a Pt atom trapped in the vacancy of the PNG, which is consistent with a previous study reported in literature [39]. It is also investigated that the most stable interaction between the Pt3Cu sub-nanocluster and PNG is with a Pt atom trapped in the vacancy of the PNG. For the case of Pt2Cu2 sub-nanocluster deposited on PNG, two isoenergetic structures were found as the most stable structures, see Figure 3c,d. In the first structure, the interaction occurred with two Cu atoms joined with PNG, where one of the atoms is anchored in the vacancy, while in another structure located at only 0.05 eV above the most stable structure, the interaction is with one atom of Cu and one of Pt, in this case the Cu atom is anchored into the vacancy.

Figure 3

Top and side views of the most stable configurations of the adsorption of Pt4-xCux (x = 0–2) sub-nanoclusters on PNG. (a) Pt4/PNG, (b) Pt3Cu/PNG, and (c,d) Pt2Cu2/PNG. Note that (d) is a quasi-degenerated state of the Pt2Cu2/PNG system with a difference in energy of 0.05 eV.

Figure 4

Top and side views of the most stable adsorption sites of Pt4-xCux (x = 3 and 4) sub-nanoclusters on PNG. (a,b) PtCu3/PNG and (c) Cu4/PNG. Note that (b) is a quasi-degenerated state of the PtCu3/PNG system with a difference in energy of 0.01 eV.

For PtCu3 sub-nanoclusters supported on PNG, two isoenergetic configurations were also computed as the most stable structures, see Figure 4a,b. In the case of the most stable interaction, it is observed that the interaction between the PtCu3 sub-nanoclusters and PNG occurred with three Cu atoms (Figure 4a), while in another structure, the interaction between the sub-nanocluster and the PNG occurred via two Cu atoms, see Figure 4b. Finally, for Cu4 sub-nanocluster deposited on PNG, two Cu atoms interacted with the PNG. In addition, the Eb between the Pt4-xCux (x = 0–4) sub-nanoclusters and the PNG were calculated, see Table 1. It is observed that Eb are substantially higher than those reported in previous findings for Pt-based sub-nanoclusters supported on pristine graphene [40,41]. Therefore, it can be inferred that PNG is a good support material for Pt-based nanoclusters. In addition, the calculated Eb between the Pt4 and the PNG is −3.61 eV, which is similar to that reported in the literature with a value of −4.40 eV [39].

Table 1

Binding energies (Eb) and charge transfer between the Pt4-xCux (x = 0–4) sub-nanoclusters and the PNG.

System	Eb (eV)	QTAIM Charge (e)
Pt4/PNG	−3.61	0.23
Pt3Cu/PNG	−3.01	0.26
Pt2Cu2/PNG	−2.65	0.52
PtCu3/PNG	−3.26	0.69
Cu4/PNG	−2.44	0.57

The interaction between the Pt4-xCux (x = 0–4) sub-nanoclusters and PNG was further investigated by the QTAIM charge transfer, see Table 1. The results suggest that Pt4-xCux (x = 0–4) sub-nanoclusters transfer charge to the PNG structure since these ended with a total positive charge, which can be attributed to the large electronegativity of the N atoms. Furthermore, it is observed that as the content of Cu in the sub-nanoclusters increases, the charge transfer from sub-nanoclusters to the PNG tends to increase as well, which can be attributed to the low electronegativity of the Cu atoms.

3.2. CO2 Adsorption on Pt4-xCux (x = 0–4) Sub-Nanoclusters Deposited on PNG

To analyze the adsorption and activation of the CO2 molecule on the Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, the CO2 adsorption energy, CO2 bond elongation, CO2 bending angle, and charge transfer from sub-nanoclusters supported PNG to CO2 were used as indicators of an effective CO2 dissociation process [42,43]. To obtain the most stable interaction between the CO2 and sub-nanoclusters supported PNG, several modes (e.g., top, bridge, and hollow) of CO2 adsorption on sub-nanoclusters supported on PNG were investigated. In Figure 5, the most stable CO2 adsorption on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on the PNG is reported. The results show that the CO2 molecule is deformed when it is adsorbed on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG (Figure 5), giving way to a bending angle from 135.86° up to 141.25°, see Table 2. Similar results were obtained when the CO2 molecule was adsorbed on Cu4-xPtx (x = 0–4) clusters [18]. In addition, it can be observed that the CO2 is adsorbed side-on type on Pt4-xCux (x = 1–4)/PNG composites, whereas for the Pt4/PNG composite the CO2 molecule is bonded with a Pt atom. The type of CO2 adsorption on Pt4-xCux (x = 1–4)/PNG composites is like those computed on Cu4-xPtx (x = 0–4) clusters [18]. To estimate the Eads between the CO2 molecule and the Pt4-xCux (x = 0–4) sub-nanoclusters deposited on the PNG, the Eads were calculated using Equation (2). It is observed that CO2 presented a chemisorption on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, since in all cases the Eads were higher than 1 eV. In addition, the CO2 molecule is adsorbed stronger on bimetallic Pt4-xCux (x = 1–4) sub-nanoclusters deposited on PNG than on Pt4 sub-nanocluster supported on PNG, which can be attributed to the presence of Cu atoms in bimetallic sub-nanoclusters. Moreover, an elongation of the average C-O bond length is observed when the CO2 is adsorbed on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG (Table 2). It is worth noting that the free CO2 presents an average C-O bond length of 1.20 Å. Considering the bond length elongation and the bending angle of the CO2 molecule adsorbed on Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG, it is observed that there is an activation of the CO2 molecule, which suggests that less energy is required to achieve the dissociation of this molecule. Finally, when CO2 is adsorbed on PtCu3 and Cu4 sub-nanoclusters deposited on PNG, the structures of the PtCu3 and Cu4 sub-nanoclusters presented a deformation. For instance, the structure of the Cu4 sub-nanocluster changes from planar to tetrahedral.

Figure 5

Top and side views of the most stable CO2 adsorption sites on the sub-nanoclusters supported on the PNG: (a) Pt4, (b) Pt3Cu, (c) Pt2Cu2, (d) PtCu3, and (e) Cu4.

Table 2

Properties of the CO2 adsorption on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG.

System	Eads (eV)	Charge Transfer Toward CO2 (e)	Average CO2 Bond Length (Å)	Bending Angle of CO2 (°)
CO2/Pt4/PNG	−1.06	−0.37	1.24	141.25
CO2/Pt3Cu/PNG	−2.21	−0.42	1.24	140.03
CO2/Pt2Cu2/PNG	−2.34	−0.44	1.25	139.32
CO2/PtCu3/PNG	−2.48	−0.46	1.25	135.86
CO2/Cu4/PNG	−1.81	−0.58	1.24	138.27

Finally, Table 2 shows the charge transfer between the CO2 molecule and the Pt4-xCux (x = 0–4) sub-nanoclusters supported on the PNG. The total charge of the CO2 molecule resulted in negative values for all the systems studied, which indicated that the CO2 molecule gained charge after the adsorption. Furthermore, it is observed that as the Cu content in the sub-nanoclusters increases, the charge transfer from the sub-nanoclusters supported on PNG to CO2 molecule tends to increase as well, which can be attributed to the low electronegativity of the Cu atoms. Moreover, it is found that the charge transfer plays a significant role in the activation of the CO2 molecule [42,43].

4. Conclusions

The CO2 adsorption on the Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG was studied using the density functional theory. To the best of our knowledge, this is the first study on the CO2 adsorption on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG. First, the stability of the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG was analyzed. The results revealed that PNG enhanced the stability of the Pt4-xCux (x = 0–4) sub-nanoclusters. After, the CO2 adsorption on the Pt4-xCux (x = 0–4) sub-nanoclusters deposited on PNG was computed. Numerous indicators such as Eads, average bond length elongation, angle bending, and charge transfer were used to characterize the CO2 interaction on the proposed systems. When the CO2 molecule was adsorbed on the Pt4-xCux (x = 0–4) sub-nanoclusters supported on the PNG, the CO2 underwent both bond length elongation and bending angle. In addition, the charge transfer from the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG to the CO2 molecule was observed. The results obtained with those indicators suggest that the activation of the CO2 molecule took place. Therefore, the Pt4-xCux (x = 0–4) sub-nanoclusters supported on PNG are suitable candidates for the CO2 adsorption and activation.