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. 2026 Aug 8;17(33):9677–9685. doi: 10.1021/acs.jpclett.6c01778

Balanced Electron Donation Governs Adsorption Site Selection in CO2 Electroreduction to HCOOH on Transition Metal-Doped Magnesium Oxides

Shangqing Zhao †,‡, Yuhang Wang ‡, Songbo Ye ‡, Heng Liu ‡, Xue Jia ‡, Linda Zhang ‡,§, Bo Da ∥, Qiang Wang ⊥, Huiling Zheng †,*, Hao Li ‡,∥,*, Wen-Ying Li †,*
PMCID: PMC13501949  PMID: 42574080

Abstract

Magnesium oxide (MgO) is attractive for CO2 utilization. However, the electronic inertness has limited its application in electrochemical catalysis. Herein, spin-polarized density functional theory (DFT) calculations with the BEEF-vdW functional were performed to evaluate the activity and selectivity of transition-metal-doped MgO (TM-MgO) catalysts for electrochemical CO2 reduction reaction (eCO2RR). Co-, Pd-, and Pt-doped MgO were identified as promising candidates for selective formic acid (HCOOH) production. Mechanistic analysis shows that TM doping induces electron redistribution in MgO, activating neighboring Mg sites while enabling charge transfer between TM dopants and reaction intermediates. The competition between Mg-site activation and TM-intermediate charge transfer determines the site preference and binding strength of the key OCHO* intermediate. Bader charge, density of states, and COHP analyses support this mechanism. This work provides design principles for oxide-based electrocatalysts toward selective HCOOH production from eCO2RR.

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As an important inorganic rock-salt alkaline-earth oxide, magnesium oxide (MgO) has been widely applied in refractory materials, electrode coatings, gas separation, catalyst supports, and photocatalytic reactions, because of its remarkable structural stability as well as its distinctive chemical, electronic, and optical properties. − As a basic oxide, MgO possesses a high theoretical CO2 capture capacity (24.8 mmol/g), making it an ideal candidate for CO2 utilization technologies. − However, its application in CO2 catalytic conversion, particularly in electrocatalytic processes for the production of value-added chemicals, remains limited. This is likely because unmodified MgO is inherently inert, as it is one of the best insulators (with a band gap >7.8 eV), making it rarely used in electrocatalysis. , Although metal-doped MgO catalysts have been investigated from both experimental and theoretical perspectives, most previous studies have focused on enhancing CO2 capture or on other catalytic reactions, − such as the oxidative coupling of methane, − rather than on the catalytic conversion of CO2, particularly the electrochemical CO2 reduction reaction (eCO2RR) toward value-added products such as formic acid (HCOOH). Nevertheless, some experimental studies have shown that MgO-based materials can participate in eCO2RR when integrated with conductive carbon frameworks or modified through defect engineering, and that TM-doped or Pd-containing MgO structures can be experimentally synthesized or stabilized as doped/solid-solution phases. − In addition, doping has been widely demonstrated as an effective strategy for tuning the electronic and chemical properties of inert metal oxide materials, − such as MgO and calcium oxide (CaO). For example, the introduction of transition metals (TM) can generate additional electronic states within the band gap of metal oxides, thereby modifying their electronic structure and band alignment. These changes directly influence charge transport and optical transition behaviors, leading to variations in electrical conductivity and optical properties. Furthermore, the redistribution of electron density associated with TM incorporation can alter local bonding environments, which may subsequently affect the structural stability of the materials. − Moreover, TM can act as electron donors or acceptors, inducing localized charge redistribution that regulates the adsorption configuration and binding strength of key intermediates (e.g., OCHO* or COOH*). Such electronic modulation selectively stabilizes intermediates along the desired reaction pathway, thereby lowering the energy barrier of the rate-determining step while suppressing competing routes. − In addition, the neighboring sites around TM often exhibit modified electronic structures, which further promote selective intermediate adsorption and enhance product selectivity rather than merely increasing catalytic activity. ,

These effects suggest that TM-doped MgO (TM-MgO) can provide a tunable platform for regulating eCO2RR selectivity. Recent advances in descriptor-based and AI-assisted catalyst discovery have emphasized the value of theoretical descriptors and computational screening for rational electrocatalyst design. − Previous theoretical studies have commonly evaluated eCO2RR activity and selectivity using Gibbs free energies of key intermediates and limiting potentials derived from reaction free-energy profiles. , For two-electron eCO2RR products, COOH* is generally associated with the CO formation pathway, whereas OCHO* is widely regarded as a key intermediate for HCOOH formation. , Therefore, the competition between OCHO* and COOH* adsorption provides an important criterion for understanding the selectivity between HCOOH and CO. Based on this mechanistic understanding, OCHO* was selected as a primary descriptor for screening TM-MgO systems toward HCOOH formation.

Motivated by the tunability of TM-MgO and the descriptor-based understanding of eCO2RR selectivity, we constructed a series of single TM-MgO catalysts by replacing one surface Mg atom with 3d, 4d, and 5d TMs. Spin-polarized density functional theory (DFT) calculations with the BEEF-vdW were then performed to evaluate their theoretical activity and selectivity for eCO2RR to HCOOH. Although BEEF-vdW is computationally more expensive than commonly used functionals such as PBE and RPBE, its reported accuracy makes it suitable for describing the energies of CO2-related catalytic processes. Combined with a two-dimensional eCO2RR volcano model, Co-, Pd-, and Pt-doped MgO were identified as promising catalysts with improved activity and selectivity toward HCOOH compared with undoped MgO. Further theoretical analysis revealed that the electron transfer between the doped TM and intermediates, particularly the electron-donating effect of the TM, played a crucial role in tuning the binding energy (E b) of the OCHO*, thereby optimizing the eCO2RR pathway toward HCOOH production. In summary, this study provides theoretical insights into the design of efficient catalysts for eCO2RR to HCOOH, offering promising prospects for producing high-value-added chemicals while contributing to the mitigation of climate issues.

eCO2RR Selectivity and Activity of TM-MgO Catalysts

Compared to the (110) and (111) surfaces of MgO, the (100) surface is the most stable and predominantly exposed. − Therefore, we focused on investigating the effects of TM doping on the MgO (100) surface (hereafter referred to simply as MgO) in this work. Subsequently, we calculated the formation energies of 21 TM-MgO models to evaluate the thermodynamic accessibility of TM substitution in MgO (the details are shown in Figure S1 and Table S1). Formation energies were evaluated under both low and high oxygen chemical potential conditions using bulk TM metals and stable TM oxides as TM reference states, respectively, with both surface and subsurface substitution configurations considered. Negative formation energies were obtained for all considered TM-MgO models, indicating that single substitutional TM dopants in MgO are thermodynamically accessible under the adopted chemical potential references. The surface-doped structures are generally more stable than the corresponding subsurface-doped structures; therefore, surface-substituted TM-MgO models were used for the following adsorption and reaction analyses.

In eCO2RR to HCOOH, there is a competition between the main product HCOOH and the byproducts carbon monoxide (CO) and hydrogen (H2) from the hydrogen evolution reaction (HER). Therefore, the catalyst selectivity was evaluated based on the free energy of key adsorbed intermediates (OCHO*, COOH*, and H*). Figure shows the adsorption competition analyses for 21 TM-MgO catalysts with respect to the selectivity of the products (HCOOH, CO, and H2). All Gibbs free energies were obtained at the most stable adsorption sites (the possible adsorption sites for the adsorbates and their corresponding E b values are shown in Figure S2 and Table S2). Figure a–c shows that, for most TM-MgO catalysts (except for Os-MgO), the Gibbs free energy of OCHO* is lower than that of COOH*. This suggests that the reaction proceeds via the OCHO* intermediate, thereby favoring HCOOH production over CO formation. In the selectivity comparison between CO and H2 (Figure d–f), only a few catalysts, such as Mn-MgO and Au-MgO, favor H2 production. Furthermore, the Gibbs free energy for the formation of OCHO* is significantly more negative than those of H* across all investigated catalysts (Figure g–i). This thermodynamic preference indicates that the HCOOH pathway is energetically much more favorable than the competing HER. The suppression of HER can be attributed to the stronger binding affinity of CO2-derived intermediates on the active sites, which effectively outcompetes proton reduction. Consequently, the active sites are predominantly occupied by OCHO* species, leading to high selectivity for HCOOH while minimizing H2 evolution.

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Adsorption competition analyses. (a–c) ΔG OCHO* vs ΔG COOH*, (d–f) ΔG H* vs ΔG COOH*, and (g–i) ΔG H* vs ΔG OCHO* for the TM-MgO catalysts. ΔG OCHO*, ΔG COOH*, and ΔG H* represent the changes of Gibbs free energy for CO2 to OCHO*, CO2 to COOH*, and H* formation, respectively.

To further elucidate the competition between HER and eCO2RR on TM-MgO catalysts, we evaluated their reaction activities. Figure a–c shows the Gibbs free-energy diagrams for HER on TM-MgO catalysts, indicating that Ti-, V-, and Nb-doped MgO exhibit low Gibbs free-energy differences (ΔG) of +0.13, −0.13, and −0.12 eV, respectively. These values are close to the ideal ΔG H* (0 eV), suggesting potential HER activity. The remaining TM-MgO catalysts exhibit larger absolute ΔG values, ranging from 0.20 eV (Zr-MgO) to 3.01 eV (Ni-MgO), implying lower HER activity. Although Ti-, V-, and Nb-doped MgO display ΔG values near zero, the susceptibility of these systems to HO* poisoning can lead to deactivation and consequent suppression of the HER. Accordingly, the effect of HO* poisoning was accounted for in the current density simulations. As illustrated in Figure d–f, the 2D volcano microkinetic models were generated at 0 V vs. the reversible hydrogen electrode (RHE). This method has been proven with higher accuracy in simulating HER performance on various types of catalysts (details of the simulation methods can be found in the Supporting Information). − Most catalysts are either prone to HO* poisoning (e.g., Ti-, V-, and Nb-doped MgO) or exhibit poor H* adsorption (i.e., the Volmer step, including Ni-, Ag-, and Au-doped MgO), with only a few, such as Ru-, Os-, and Ir-doped MgO catalysts, showing potential HER activity. However, during the eCO2RR process, H* adsorption competes with the bidentate adsorption of OCHO* on TM-Mg or Mg–Mg sites. In these catalysts, the thermodynamically favorable adsorption sites for H* are located on the TM atoms, yet their Gibbs free energy values are significantly weaker than those of OCHO*, as shown in Figure g–i. Consequently, no catalyst is theoretically expected to achieve ideal HER activity under eCO2RR conditions.

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TM-MgO catalysts of (a–c) Gibbs free-energy diagrams for HER, (d–f) 2D microkinetic volcano models for HER as a function of ΔG H* and ΔG HO*, simulated under an applied potential of 0 VRHE. Details of the simulation method can be found in the Supporting Information.

Following the confirmation of high HCOOH selectivity, the activity of TM-MgO catalysts toward eCO2RR was further assessed to screen for the most promising HCOOH catalysts. We derived a volcano model based on the Gibbs free energy of COOH* (or OCHO*) and CO* (or HCOOH*), as shown in Figure . According to the results in Figure a–c and the CO production activity volcano model in Figure b, although the Mo- and Rh-dopedMgO catalysts are located at the peak of the volcano model, indicating the highest activity, the Gibbs free energy of OCHO* is more negative than that of COOH*. This suggests a preference for OCHO* formation, making the catalyst unfavorable for the CO2-to-CO reaction. Figure c shows the volcano model for HCOOH production activity, in which Co-, Pd-, and Pt-doped MgO are located close to the volcano peak. Additionally, in Figure b, these three catalysts are far from the peak of the volcano model for CO production, suggesting their potential for high activity and selectivity toward HCOOH formation. Particularly, the Co-MgO catalyst exhibits both the best HCOOH production activity and the lowest CO production activity, highlighting its optimal catalytic performance for eCO2RR-to-HCOOH.

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(a) Representative DFT-optimized adsorption configurations of OCHO*. Orange, red, blue, brown, white, teal, and magenta represent Mg, O from MgO, O from OCHO*, C, H, TM from Mg-site, and TM from TM-site atoms, respectively. (b, c) 2D volcano models on TM-MgO catalysts for CO and HCOOH formation. (d) Gibbs free-energy diagrams of the eCO2RR. A detailed description of the modeling and computational method can be found in the Supporting Information.

Interestingly, the incorporation of specific transition metals, such as Ti, Zr, and Hf, diminishes the HCOOH production activity compared with pristine MgO. To elucidate the origin of this performance degradation, we compared these less active catalysts with the high-performance Co-, Pd-, and Pt-doped MgO systems in the following discussion. For TM-MgO catalysts, the key intermediate OCHO* sometimes preferentially adsorbs at the Mg-site (e.g., Co-, Pd-, and Pt-doped MgO), whereas, in other cases, it tends to adsorb at the TM-site (e.g., Ti, Zr, and Hf-doped MgO), as indicated by the teal (Mg-site) and magenta (TM-site) coding in Figure d. It can be clearly seen that OCHO* adsorbed on TM-site has a highly negative Gibbs free energy, leading to a significant ΔG (>2.0 eV) in the second protonation step (i.e., OCHO* + H* + e– → HCOOH*), which is unfavorable for HCOOH formation. In contrast, OCHO* adsorbed on Mg-site shows moderate binding strength, with maximum ΔG (G MAX) of 0.36, 0.52, and 0.67 eV for Co-, Pd-, and Pt-doped MgO, respectively, all lower than that of pure MgO (0.86 eV), implying an optimal HCOOH activity of these catalysts.

Electronic Structure Analyses of TM-MgO Catalysts

To elucidate the origin of the enhanced activity, we performed a comprehensive electronic structure analysis, including the Bader charges, density of states (DOS), and crystal orbital Hamiltonian population (COHP) analysis. Here, Bader charge analysis was used to quantify dopant-induced charge redistribution and electrostatic modulation of the adsorption sites, while DOS and band-center analyses were used to reveal the electronic-state variation before and after OCHO* adsorption. In addition, COHP analysis was employed to evaluate the Mg–O and TM–O bonding strengths between OCHO* and the surface sites. These complementary descriptors provide quantitative support for distinguishing moderate Mg-site stabilization from excessive TM–O bonding-induced overstabilization of OCHO*. Figure a depicts the relationship between the Bader charges of O atoms in bidentate-adsorbed OCHO* and the corresponding G OCHO*. We found that, compared with pure MgO, the Bader charges of TM–MgO with Mg-site adsorption (highlighted by the teal oval) show no significant differences. In contrast, for OCHO* adsorption on TM-site (highlighted by the magenta oval), a more positive Bader charge of the O atoms corresponds to more stable OCHO* adsorption, showing significant differences, with charge values ranging from 2.32 to 2.52 e–.

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G OCHO* as a function of (a) Bader charge of O atoms, (b) charge transfer, (c) Δp-band center, (d) Δd-band center, and (e, f) Pauling electronegativity. (g–i) Calculated projected density of states of pure MgO (g), Co-MgO (h), and Ti-MgO (i) before/after adsorbing OCHO*.

To further elucidate this phenomenon, we analyzed the charge transfer of the introduced TM atom before and after adsorption, as shown in Figure b and Table S3. The selected TM-MgO catalysts (Ti-, Zr-, Hf-, Co-, Pd-, and Pt-doped MgO) span both extremes in HCOOH production activity (from poor to excellent) and encompass both early and late TM. Interestingly, we noted a clear proportional relationship between the TM charge transfer and the GOCHO*, indicating that greater charge transfer leads to more stable OCHO* adsorption. This suggests that the electrons donated by the TM dopant may contribute to the bonding of OCHO*, thereby strengthening its adsorption. Additionally, compared with pure MgO, the Co-, Pd-, and Pt-doped MgO exhibit moderate electron-donating effects, which may fine-tune the electronic environment of adjacent Mg-site and enhance OCHO* adsorption. However, their electron-donating capacity is notably lower than that of Ti-, Zr-, and Hf-doped MgO, indicating limited charge transfer to the adsorbate OCHO* and therefore a smaller influence on the Gibbs free energy value. This explains why the Bader charges of TM–MgO with Mg-site adsorption show no significant differences.

Furthermore, the shift in the band center and the projected density of states (pDOS) curves (Figure c, d, and g–i) further confirm that doping alters the adsorption strength of OCHO* on MgO. Particularly, in Figure c, d, the Δp-band (Mg-site) and Δd-band center (TM-site) represent the shifts in band center positions after adsorption relative to their values before adsorption, with the adsorption strength positively correlated with these shifts. Moreover, as shown in Figure g–i, after adsorption, the valence bands of the TM-doped catalysts (arising from Mg 2p and Ti 3d orbitals, respectively) shift significantly upward toward the Fermi level (E f), indicating stronger bonding interactions. Figures S3–S5 further show that the pDOS overlap between the adsorption sites and OCHO* is larger for TM-MgO than for pure MgO, supporting enhanced OCHO* adsorption after TM doping. Although several TM-MgO surfaces retain semiconducting features in the DOS, these oxide motifs can be electronically coupled with conductive substrates in practical electrodes; therefore, the DOS results are used here mainly to evaluate dopant-induced local electronic modulation, while the catalytic performance is assessed from adsorption and reaction energies.

Interestingly, we found that for Mg-site adsorption catalysts in TM-MgO, G OCHO* shows a negative correlation with Pauling electronegativity (Figure e), whereas for TM-site adsorption, it exhibits a positive correlation (Figure f). This contrasting trend can be attributed to two factors. First, for adsorption on the Mg-site, the TM dopant modulates the electronic environment of neighboring Mg atoms, indirectly influencing OCHO* adsorption, rather than directly participating in OCHO* bonding as occurs in TM-site adsorption. Second, in TM-MgO catalysts where OCHO* preferentially adsorbs on the Mg-site, the d orbitals of late transition metals (e.g., Co, Pd, and Pt) are nearly filled, leaving few available d orbitals for electron donation or back-donation. Their higher electronegativity further restricts their ability to transfer electrons to OCHO*, resulting in weaker bonding interactions. This may explain why OCHO* tends to adsorb more stably on adjacent Mg sites rather than directly on these late-TM dopants. In contrast, for TM-site adsorption, early transition metals (e.g., Ti, Zr, and Hf) exhibit stronger OCHO* adsorption. These metals possess lower electronegativity and partially filled d orbitals, allowing them to donate electrons more readily and provide empty or half-filled d orbitals for strong orbital overlap with OCHO*. This facilitates stronger covalent interactions, thereby stabilizing OCHO* adsorption. Conversely, as the electronegativity increases and the d orbitals become more fully occupied (as in late transition metals), the adsorption strength of OCHO* decreases due to limited electron transfer and weaker orbital interactions.

The effect of TM dopants on the local electronic structure was further quantified by calculating the average Bader charge difference (defined as ΔQ = Q adj – Q ref) between Mg atoms adjacent to the TM dopant and reference Mg atoms elsewhere in the slab, as shown in Figure S6. For Co-, Pd-, and Pt-doped MgO, the ΔQ value is negative, reflecting a localized depletion of electron density from the neighboring Mg sites. The resulting increase in cationic character (showing higher Lewis acidity) enhances the electrostatic stabilization of the oxygen atoms in OCHO*, thereby anchoring the intermediate to the Mg-site. In contrast, for Ti, Zr, and Hf-doped MgO, the ΔQ values are slightly more positive than those of the aforementioned three doped catalysts, indicating an accumulation of electron density on the neighboring Mg sites. This increased electron shielding reduces the cationic character of the Mg atoms, leading to a weaker electrostatic affinity for OCHO*. Consequently, the OCHO* intermediate preferentially shifts toward the TM-site, where it can be more effectively stabilized through strong covalent interactions and orbital overlap with the TM dopants.

Notably, the modulation of Mg-site electronic structure does not act independently. Instead, it cooperates with the electron-donating behavior of the TM dopants revealed in Figure . Specifically, while the redistribution of charge around Mg sites tunes their Lewis acidity and electrostatic interaction with OCHO*, the TM dopants simultaneously participate in charge transfer with the adsorbate through d–p orbital interactions. The balance between these two effects determines the preferred adsorption site of OCHO*: moderate electron donation stabilizes OCHO* on Mg sites with optimal binding strength, whereas excessive electron donation favors TM-site adsorption, leading to overstabilization. Therefore, the dopant-induced electronic effects jointly govern adsorption site selection, which ultimately governs catalytic activity and selectivity.

Finally, we quantified the bonding strengths of OCHO* via the COHP analysis of the Mg–O bond (referring to Mg adjacent to TM and Mg in pure MgO) and the TM–O bond (Figure ). The integrated crystal orbital Hamiltonian population (ICOHP), obtained by integrating the bonding and antibonding regions below the Fermi level, reflects the adsorption strength to a certain extent. It is generally considered that greater orbital overlap leads to stronger interaction between an atom and the atom to which it is bonded. For TM-site adsorption, the absolute ICOHP values are significantly larger than those of Mg-site adsorption configurations, indicating much stronger OCHO* bonding. This overstabilization of OCHO* is detrimental to its further conversion, thereby limiting HCOOH production. Compared with pure MgO, the Co-, Pd-, and Pt-doped MgO exhibit slightly larger ICOHP values, suggesting moderately strengthened OCHO* bonding rather than excessive stabilization. Such a regulated adsorption strength places these catalysts near the peak of the HCOOH activity volcano, leading to reduced G MAX values and lower free energy changes for the rate-determining step, particularly for Co-MgO. This indicates that TM introduction indeed alters the adsorption behavior of the key intermediate OCHO*, thereby enabling the enhancement in catalytic activity.

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-COHP and -ICOHP values of (a) pure MgO and (b–g) TM-MgO. (h) The Mg–O and TM–O bonds considered for COHP analyses. Orange, red, blue, brown, white, and cyan spheres represent Mg, O from MgO, O from OCHO*, C, H, and TM atoms, respectively.

In summary, this work systematically evaluated TM-MgO catalysts for eCO2RR toward HCOOH production and identified Co-, Pd-, and Pt-doped MgO as the most promising candidates among the examined systems based on reaction free-energy profiles and selectivity analyses. Beyond catalyst screening, this study reveals that the catalytic behavior of TM-MgO is closely associated with the interplay between dopant-adsorbate charge transfer and dopant-induced modulation of neighboring Mg sites. Rather than being determined solely by adsorption strength, the activity and selectivity are strongly affected by the preferred adsorption site of the key OCHO* intermediate. Specifically, an appropriate balance between TM-intermediate electron donation and Mg-site activation enables favorable stabilization of reaction intermediates, whereas deviation from this balance may result in either insufficient activation or excessive stabilization. These findings suggest that regulating the spatial distribution of electronic effects is crucial for optimizing oxide-based electrocatalysts, not merely by strengthening adsorption but by controlling where and how intermediates bind. The concept of electronically regulated site selection provides a mechanistic framework for the rational design of oxide-based electrocatalysts and may be extended to other heterogeneous catalytic systems involving multisite interactions.

Supplementary Material

jz6c01778_si_001.pdf (978.9KB, pdf)

Acknowledgments

This work was supported by National Key Research and Development Program of China (2022YFE0208400), the National Natural Science Foundation of China (22202149, 22279156), the Science and Technology Cooperation and Communication Project of Shanxi Province (No. 202304041101016), the Foundation of State Key Laboratory of Coal Conversion (No. J24-25-610), JSPS KAKENHI (JP25K01737) and Suzhou MatSource Technology Co., Ltd. (Suzhou, China). Part of the calculations in this study were performed on the Numerical Materials Simulator at NIMS.

The data that support the findings of this study are available in the Supporting Information of this article.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.6c01778.

  • Detailed descriptions of computational methods, additional figures, and tables of simulation results (PDF)

The authors declare no competing financial interest.

Due to a production error, this paper was published ASAP on August 8, 2026, with several corrections missing from the Abstract. The corrected version was reposted on August 10, 2026.

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The data that support the findings of this study are available in the Supporting Information of this article.

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