2.3. Results and Discussion
2.3.2. Activation of CO 2
It is believed that CO2 adsorption on the catalytic surface and subsequent activation are necessary for efficient CO2RR, though the inertness of CO2 molecule makes this step challenging. Surface-active sites may have different configurations for adsorbed CO2 molecules. Binding of CO2 is governed by the orientation of its highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) with respect to the surface. The possible adsorption conformations that can be formed on the activation of CO2 on any surface are depicted in Figure 2.5. In this study, we investigated the adsorption and activation of CO2, by computing the Eads of the proposed catalytic surfaces, as well as analyzing the optimized geometries of CO2 adsorbed systems. According to our DFT optimized geometries, CO2 may interact with the catalytic surface either through physisorption or chemisorption.
Although, for its catalytic conversion to useful products (hydrocarbons and alcohols), chemisorption of CO2 is significant. It is noted that CO2 moved away (parallel) from the surface at a significant distance (> 3.4 Å) in the TM-SACs over which it was physisorbed. SACs embedded in Sc/Fe/Co/Ni/Cu/Zn/Y/Pd/Ag/Cd/Pt/Au showed nearly linear CO2 relationships upon adsorption of CO2
(Figure 2.5). The bond lengths of C=O and the bond angles of O=C=O (φOCO) are noted to be ~1.18 and ~180°respectively, which are consistent with the geometric parameters of a gas-phase molecule of CO2. Thus, activation of CO2 over the aforementioned SACs is challenging. A relatively strong surface- adsorbate interaction was found for the catalysts with chemisorbed configurations with increasing CO2
adsorption energies for Ta>Nb>Re>Mo>Ti> Hf>Zr>V>W>Mn>Ru>Rh>Os>Cr>Ir. Interestingly, the mechanism of CO2 chemisorption was not identical across all these catalysts. It is noted that CO2
molecules can also be stabilized by the Ga atoms on the surface instead of TM active sites. The structural optimization through DFT revealed that the CO2 molecule interacts with TM atoms through its C atom in Ru/Rh/Re and Os-SACs, while the surface Ga atom stabilizes the adsorbed CO2 molecule through Ga-O coordination which further facilitates the CO2 activation. Furthermore, the Ti/V/Cr/Nb/Mo/Ta/W/Ir embedded SACs showed that the CO2 is adsorbed in a bidentate fashion with C and O atoms bonded with a single TM active site. Interestingly, the geometrical structure of CO2
molecule changed significantly across all the chemisorbed configurations with φOCO < 180° and the C=O bond length > 1.20 Å. The calculated geometrical parameters for both physisorbed and chemisorbed structures are provided in Table 2.3, and their optimized configurations are presented in Figure 2.5. Table 2.3 provides the calculated geometrical parameters of both chemisorbed and physisorbed CO2, as calculated from their optimized geometries.
20
Figure 2.5: Schematic illustration of possible CO2 adsorption configurations and optimized CO2
adsorbed geometries of all the considered systems. The golden filled line represents the physisorbed systems. The chemisorbed systems exhibit different types of CO2 adsorption configurations either by sharing single metal active site in the mono- and bi-dentate fashion or the surface Ga atom stabilized configuration through Ga-O coordination (e.g Ru, Rh, Re and Os-SACs). (Color code: transition metal, golden; Ga, light green; N, blue; C, gray; O, red).
21
Table 2.3: Geometrical parameters of CO2 molecule after adsorption over various single atom catalysts (SACs). Adsorption energy of CO2 (𝐸ads[∗CO2]), angle between O=C=O (𝜑𝑂𝐶𝑂), distance from transition metal to carbon atom of adsorbed CO2 (dTM-C), distance between oxygen and carbon atoms (dO-C), and the corresponding configurations of CO2•–species.
Catalysts
𝜟𝑬𝐚𝐝𝐬[∗𝐂𝐎𝟐]
(eV)
𝝋𝑶𝑪𝑶 (deg.)
dTM-C (Å)
dO-C (Å) CO2•–
Configuration 1O-C 2O-C
Sc-SAC -0.28 179.8 4.55 1.18 1.18 -
Ti-SAC -1.64 135.8 2.50 1.28 1.22 Cs
V-SAC -1.39 130.5 2.03 1.31 1.24 Cs
Cr-SAC -0.41 133.6 1.97 1.30 1.23 Cs
Mn-SAC -1.10 133.8 2.07 1.26 1.25 C2v
Fe-SAC -1.08 179.7 4.00 1.18 1.18 -
Co-SAC -0.97 179.5 3.55 1.18 1.18 -
Ni-SAC -0.24 179.7 3.50 1.18 1.18 -
Cu-SAC -0.25 179.7 3.70 1.18 1.18 -
Zn-SAC -1.22 179.6 3.82 1.18 1.18 -
Y-SAC -0.27 179.8 4.78 1.18 1.18 -
Zr-SAC -1.41 135.2 2.43 1.26 1.23 C2v
Nb-SAC -2.92 126.7 2.17 1.33 1.24 Cs
Mo-SAC -1.82 129.7 2.08 1.33 1.25 Cs
Ru-SAC -1.13 125.1 2.04 1.32 1.25 C2v
Rh-SAC -0.62 129.5 2.04 1.29 1.24 C2v
Pd-SAC -0.29 179.8 3.89 1.18 1.18 -
Ag-SAC -0.21 178.3 3.89 1.18 1.18 -
Cd-SAC -0.33 179.6 4.61 1.18 1.18 -
Hf-SAC -1.44 133.9 2.32 1.27 1.23 C2v
Ta-SAC -3.02 125.6 2.17 1.34 1.24 Cs
W-SAC -1.36 127.7 2.07 1.34 1.25 Cs
Re-SAC -2.74 125.6 2.10 1.34 1.25 C2v
Os-SAC -0.61 131.2 2.09 1.27 1.25 C2v
Ir-SAC -0.30 134.4 2.03 1.26 1.24 C2v
Pt-SAC 0.36 139.7 2.13 1.24 1.23 C2v
Au-SAC 0.02 178.9 3.79 1.18 1.18 -
22
Carbon dioxide radical anion (CO2•–) (a highly reactive intermediate) is formed as a result of charge transfer from the substrate to the CO2 molecule, resulting in geometric changes in the linear structure of CO2. For each of the proposed substrates, we performed bader charge analysis to quantify this charge transfer (Table 2.4). It affirms that among other substrates, Ta-SACs transferred the maximum charge to CO2 (1.30 |e|) and in return CO2 offered strongest adsorption (Eads = -3.02 eV) on this monolayer.
Additionally, the Bader charge analysis revealed a trend of Ta>Nb>W>MoRe>V>Os>Ru>
Cr>Mn>Rh>Hf>Ir>ZrTi directional charge transfer from the surface to chemisorbed CO2. Furthermore, bader analysis concludes that in case of physisorption there is a minimal or no charge is transferred from the substrate and thus CO2•– is not formed. In order to understand charge redistribution after CO2 adsorption, we calculated the difference of charge densities as, Δ𝜌 = 𝜌surf−ads – 𝜌surf – 𝜌ads, where the ρsurf and ρsurf-ads represent the charge densities of clean surface and CO2 adsorbed surface, respectively, while the ρads denotes the charge density of isolated CO2 molecule. The isosurfaces of different substrates after redistribution of charges are shown in Figure 2.6. Considering activation of CO2 as an obligatory step for its electrocatalytic reduction, we omitted the TM-SACs (TM = Sc, Fe, Co, Ni, Cu, Zn, Y, Pd, Ag, Cd, Pt, Au) from further studies because of their failure in the adsorption and activation of CO2 molecule.
Figure 2.6: The charge density difference plots of CO2 chemisorbed over various TM-SACs showing a significant amount of charge transfer takes place from the surface to adsorbed CO2 molecule. The isosurface value is 0.001563 e/Å3. The charge depletion and accumulation are represented by cyan and yellow colors, respectively. The values given in red color indicate net Bader charge on CO2•– species.
23
Table 2.4: Bader charge analysis of CO2 adsorbed systems. TM denotes the transition metal, and the CO2 adsorbate atoms are represented by C, 1O and 2O. The net charge transfer from surface to adsorbed CO2 is also provided.
Catalysts TM C 1O 2O Net charge on CO2
Sc-SAC 2.089 2.164 -1.092 -1.095 -0.023
Ti-SAC 1.991 1.507 -1.147 -1.149 -0.789
V-SAC 1.873 1.275 -1.129 -1.224 -1.078
Cr-SAC 1.647 1.380 -1.107 -1.204 -0.930
Mn-SAC 1.437 1.491 -1.202 -1.199 -0.911
Fe-SAC 1.341 2.156 -1.091 -1.084 -0.019
Co-SAC 1.039 2.142 -1.086 -1.090 -0.034
Ni-SAC 0.944 2.151 -1.083 -1.099 -0.031
Cu-SAC 0.884 2.149 -1.086 -1.094 -0.030
Zn-SAC 1.063 2.151 -1.084 -1.090 -0.024
Y-SAC 2.312 2.164 -1.097 -1.092 -0.025
Zr-SAC 2.630 1.524 -1.154 -1.168 -0.794
Nb-SAC 2.266 1.180 -1.178 -1.249 -1.247
Mo-SAC 1.961 1.242 -1.113 -1.236 -1.107
Ru-SAC 1.255 1.411 -1.202 -1.162 -0.953
Rh-SAC 0.075 1.510 -1.154 -1.184 -0.828
Pd-SAC 1.555 2.167 -1.073 -1.071 0.023
Ag-SAC 0.751 2.153 -1.042 -1.047 0.064
Cd-SAC 1.017 2.196 -1.108 -1.102 -0.015
Hf-SAC 2.545 1.531 -1.199 -1.149 -0.817
Ta-SAC 2.410 1.116 -1.162 -1.257 -1.304
W-SAC 2.180 1.132 -1.106 -1.245 -1.218
Re-SAC 1.713 1.282 -1.220 -1.167 -1.105
Os-SAC 2.778 1.371 -1.230 -1.211 -1.070
Ir-SAC 1.058 1.543 -1.140 -1.201 -0.798
Pt-SAC 0.742 1.721 -1.139 -1.164 -0.583
Au-SAC 1.515 2.174 -1.10 -1.103 -0.029
24