Effect of Cu doping on CO₂ hydrogenation to methanol over NHC-Cu-H functionalized ZIF-90: A DFT mechanistic study

Industrial activities have led to increasing CO₂ emissions, making the development of efficient conversion pathways essential. In this study, density functional theory (DFT) calculations were employed to explore the effect of Cu doping on the adsorption, activation, and hydrogenation of CO₂ over NHC-Cu-H functionalized ZIF-90 (denoted as NHC-Cu-H/ZIF-90). Comparative analysis of NHC-Cu-H/ZIF-90 and NHC-Cu-H/Cu-doped ZIF-90 was carried out to clarify the effect of Cu doping on the catalyst’s electronic structure and reaction pathway. DFT-based electronic structure analysis revealed that Cu doping in the ZIF-90 framework modifies the electronic environment of the NHC-Cu-H active site, increasing Cu-H bond polarization and strengthening Cu-H orbital overlap. Mechanistic analysis revealed that for both catalytic systems, the carboxyl pathway is kinetically unfavorable, and that CO₂ hydrogenation preferentially proceeds via the formate pathway. Along this pathway, the concerted cleavage of the C-O and. O-H bonds in the CH₂OHOH intermediate, leading to the formation of HCHO and H₂O, were identified as the rate-determining step for both catalysts, with activation energy barriers of 1.63 and 1.70 eV for NHC-Cu-H/Cu-doped ZIF-90 and NHC-Cu-H/ZIF-90, respectively. Energetic span analysis further showed that Cu doping reduces the energetic span from 0.77 to 0.63 eV, leading to an estimated increase in the turnover frequency (TOF) from 0.60 to 143 s⁻¹ , corresponding to an enhancement of more than two orders of magnitude in catalytic activity. Furthermore, the results showed that Cu doping facilitates the heterolytic dissociation of H₂ and improves hydrogen activation. Kinetic analysis revealed that Cu doping decreases the activation free energy barrier (ΔG ‡ ) for hydride transfer to CO₂ from 0.09 to 0.05 eV, thereby increasing the corresponding rate constant. In contrast, Cu doping slightly increases ΔG ‡ for hydride transfer to HCOOH and HCHO, leading to lower rate constants for these subsequent hydrogenation steps. Nevertheless, Cu doping improves the overall catalytic behavior by facilitating H₂ activation, promoting the initial hydrogenation of CO₂, and slightly lowering the activation energy barrier of the rate-determining step.