2.3. Results and Discussion
2.3.3. CO 2 RR Mechanistic Analysis
2.3.3.2. CO 2 RR via Six and Eight-Electron Pathways
Following the moderate adsorption of *CO on a catalytic surface, the successive reduction mechanism then proceeds via different pathways leading towards the formation of various products, including CH3OH and CH4. Herein, analyzing the G values of several reaction intermediates we constructed four reduction pathways (P1 through P4) for six-electron product (CH3OH) and six reduction pathways (R1 through R6) for eight-electron product (CH4). Figures 2.11-2.14 demonstrate the free energy profiles for various reaction pathways. To determine the final reduction product, we compared the free energies of various intermediates(Table 2.8) adsorbed at the surface of Os and Ir-SACs.
Free energy profiles revealed that the protonation of *CO in the third step leads to either *CHO or
*COH, determined by the coupling of C–H or O–H, respectively. It is demonstrated that formation of
*COH is more endergonic than *CHO over Os- and Ir-SACs by free energy change of 1.26 eV and 1.46 eV, respectively, thus making the reduction unfavorable via this route. Pathway P1 and the pathways R1 and R2 involve the *COH intermediate, showing that these routes are energetically unfavorable to produce CH3OH and CH4, respectively. Pathways P2 and P3 possessed same UL however, their reaction species differ from each other in the fifth protonation step, where in the former *OCH2 protonates to *CH2OH and in the latter it protonates to *OCH3. Probing the G values of these intermediates revealed that Os- SAC catalyst completes the formation of methanol through pathway P3 with a G*OCH3 of -0.64 eV that is more exothermic than the G*CH2OH (-0.41 eV) whereas, Ir-SAC prefers to proceed through *CH2OH to form CH3OH (G*OCH3 = 0.14 vs G*CH2OH = -0.05). In pathway P4, *CHO hydrogenates to *CHOH rather than *OCH2 with lower G increase of 0.85 eV and 0.48 eV over the catalytic surfaces of Os- SAC and Ir-SAC, respectively. Therefore, amongst these pathways, formation of methanol on Os-SAC and Ir-SAC is energetically more favorable through pathway P4 (*CO2 → *COOH → *CO →
*CHO → *CHOH→ *CH2OH → *CH3OH) with the UL of -0.85 V and -0.48 V, respectively.
33
Figure 2.11: Free energy profiles of CO2 reduction to CH3OH on Os-SAC at zero and applied potential via different reduction pathways (P1 through P4). Solid black lines indicate the potential-limiting step (PLS) of the corresponding pathway. (Color code: Os, light pink; Ga, light green; N, blue; C, gray; O, red; H, cyan).
34
Figure 2.12: Free energy profiles of CO2 reduction to CH3OH over Ir-SAC at zero and applied potential via different reduction pathways (P1 through P4). Solid black lines indicate the potential-limiting step (PLS) of the corresponding pathway. (Color code: Ir, golden-brown; Ga, light green; N, blue; C, gray;
O, red; H, cyan).
35
To confirm the final product of our SACs, we then inspected the pathways that generated CH4 after passing through eight hydrogenation steps. Figures 2.13 and 2.14 illustrates the comparison of free energy profiles of these reduction pathways based on their G values. The first few steps are same as that in the protonation of CO2 to CH3OH i.e., *CO protonates to *COH with a high energy barrier that ultimately becomes the PLS for the first two pathways (R1 and R2). Meanwhile, in the remaining pathways (R3-R6) a proton couples with the C atom of *CO and results in the formation of *CHO that later passes through five more hydrogenation steps with varying intermediates to generate CH4. It is worth mentioning that the reaction process till the fifth protonation step in pathways R3 and R4 is similar to the corresponding pathways P2 and P3 of CH3OH formation. However, a comparison showed that in the sixth protonation step *CH2OH preferably reduced to *CH2 with the release of a water molecule rather than reducing to *CH3OH. Similarly, the pathways P3 and R4 also showed that the sixth protonation occurs more likely at the C-end of the *OCH3 intermediate that favorably reduces into CH4 and leaves *O intermediate on the surface. To sustain the catalytic cycle, adsorbed *O species further protonate to *OH and then to H2O. As mentioned earlier, thermodynamically *CHO is easier to reduce to *CHOH than to *OCH2, that generates the possibility of two more reduction pathways (R5 and R6) towards CH4 product. Proportionately, *CHO → *CHOH → *CH + H2O → *CH2 → *CH3 →
*CH4 is the competitive reduction pathway against *CHO → *CHOH → *CH2OH → *CH2 + H2O →
*CH3 → *CH4. Comparative analysis of G values corroborated that production of CH4 over Os-SAC and Ir-SAC is highly desirable through pathway R6 with UL(s) of -0.85 V and -0.48 V, respectively.
36
Figure 2.13: Free energy profiles of CO2 reduction to CH4 on Os-SAC at zero and applied potential via different reduction pathways (R1 through R6). Solid black lines indicate the potential-limiting step (PLS) of the corresponding pathway. (Color code: Os, light pink; Ga, light green; N, blue; C, gray; O, red; H, cyan).
37
Figure 2.14: Free energy profiles of CO2 reduction to CH4 on Ir-SAC at zero and applied potential via different reduction pathways (R1 through R6). Solid black lines indicate the potential-limiting step (PLS) of the corresponding pathway. (Color code: Ir, golden-brown; Ga, light green; N, blue; C, gray;
O, red; H, cyan).
38
Figure 2.15: Optimized configurations of various adsorbates over Os-SAC. (Color code: Os, light pink;
Ga, light green; N, blue; C, gray; O, red; H, cyan).
Figure 2.16: Optimized configurations of various adsorbates over Ir-SAC. (Color code: Ir, golden- brown; Ga, light green; N, blue; C, gray; O, red; H, cyan).
39
Table 2.8: Adsorption Gibbs free energies of elementary reaction steps of CO2RR over Os and Ir-SAC.
Elementary reaction step
G (eV)
Os-SAC Ir-SAC
*CO2•– + H+ → *COOH 0.63 0.45
*CO2•– + H+ → *OCHO 1.27 0.62
*COOH + H+ + e- → *CO + H2O -0.34 -0.29
*CO + H+ + e- → *COH 1.26 1.46
*CO + H+ + e- → *CHO 0.31 0.19
*COH + H+ + e- → *C + H2O 0.19 0.07
*COH + H+ + e- → *CHOH -0.11 -0.80
*C + H+ + e- → *CH -0.32 -0.44
*CHOH + H+ + e- → *CH + H2O -0.02 0.42
*CHO + H+ + e- → *OCH2 1.15 0.54
*CHO + H+ + e- → *CHOH 0.85 0.48
*OCH2 + H+ + e- → *CH2OH -0.41 -0.05
*OCH2 + H+ + e- → *OCH3 -0.64 0.14
*OCH3 + H+ + e- → *CH3OH 0.19 -0.69
*OCH3 + H+ + e- → *O + CH4 -1.82 -1.80
*CH2OH + H+ + e- → *CH2 + H2O -0.52 -0.63
*CH2OH + H+ + e- → *CH3OH -0.14 -0.61
*CH + H+ + e- → *CH2 -0.62 -1.05
*CH2 + H+ + e- → *CH3 -0.41 -0.21
*CH3 + H+ + e- → *CH4 -0.55 -0.60
*CH3OH→ CH3OH + * -0.90 -0.24
*CH4→ CH4 + * -0.81 -0.65
40