3.2.1Concerted Reaction Mechanism
The concerted reaction mechanism is proposed to take place in a single step of the CO2 hydrogenation to form the formic acid. The reaction starts with the CO2 adsorbed on the Cu ion active site with the energy of −13.5 kcal/mol that is previously mentioned. The hydrogen molecule (H2) is then coadsorbed next to the adsorbed CO2 to form the coadsorption complex (Coads), as shown in Figure 3a. In this complex, the H1–H2 distance slightly increases from the isolated H2 molecule by 0.01 Å upon absorption. The intermolecular distances of C•••H1 and H2•••O2 are 2.93 and 2.33 Å, respectively. The coadsorption energy with respect to isolated reactants is calculated to be −13.9 kcal/mol. In the transition state (TS_C) of this pathway, the CO2 is simultaneously hydrogenated at the C and Oc atoms to form the product of formic acid. The H1–H2 bond distance is lengthened from 0.75 to 1.00 Å, while the distance of the C–H1 and Oc-H2 is shortened to be 1.33 and 1.36 Å, respectively. The angle of Oc-C-Oc′ is reduced from 178.3 to 143.2°. Normal mode analysis for this transition state is at 2327.6i cm–1, corresponding to the breaking of the H1–H2 bond and simultaneously the H1 and H2 are moving toward the C and Oc atoms, respectively. The activation energy is calculated to be 67.2 kcal/mol. Then, the formic acid product (Prod) is formed over the Cu active site of Cu-MOF-5 with the interaction energy of −9.8 kcal/mol. Finally, the adsorbed formic acid is desorbed endothermically and requires an energy of 18.0 kcal/mol.
3.2.1Concerted Reaction Mechanism
The concerted reaction mechanism is proposed to take place in a single step of the CO2 hydrogenation to form the formic acid. The reaction starts with the CO2 adsorbed on the Cu ion active site with the energy of −13.5 kcal/mol that is previously mentioned. The hydrogen molecule (H2) is then coadsorbed next to the adsorbed CO2 to form the coadsorption complex (Coads), as shown in Figure 3a. In this complex, the H1–H2 distance slightly increases from the isolated H2 molecule by 0.01 Å upon absorption. The intermolecular distances of C•••H1 and H2•••O2 are 2.93 and 2.33 Å, respectively. The coadsorption energy with respect to isolated reactants is calculated to be −13.9 kcal/mol. In the transition state (TS_C) of this pathway, the CO2 is simultaneously hydrogenated at the C and Oc atoms to form the product of formic acid. The H1–H2 bond distance is lengthened from 0.75 to 1.00 Å, while the distance of the C–H1 and Oc-H2 is shortened to be 1.33 and 1.36 Å, respectively. The angle of Oc-C-Oc′ is reduced from 178.3 to 143.2°. Normal mode analysis for this transition state is at 2327.6i cm–1, corresponding to the breaking of the H1–H2 bond and simultaneously the H1 and H2 are moving toward the C and Oc atoms, respectively. The activation energy is calculated to be 67.2 kcal/mol. Then, the formic acid product (Prod) is formed over the Cu active site of Cu-MOF-5 with the interaction energy of −9.8 kcal/mol. Finally, the adsorbed formic acid is desorbed endothermically and requires an energy of 18.0 kcal/mol.
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