Ru-Xin
Meng‡
a,
Lan-Cheng
Zhao‡
b,
Li-Pan
Luo
c,
Yi-Qi
Tian
a,
Yong-Liang
Shao
d,
Qing
Tang
*c,
Likai
Wang
*b,
Jun
Yan
a and
Chao
Liu
*a
aCollege of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, P. R. China. E-mail: chaoliu@csu.edu.cn
bSchool of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255049, Shandong, P. R. China. E-mail: lkwangchem@sdut.edu.cn
cCollege of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, P. R. China. E-mail: qingtang@cqu.edu.cn
dSchool of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China
First published on 22nd January 2025
Precise control over the distribution of active metal sites on catalyst surfaces is essential for maximizing catalytic efficiency. Addressing the limitations of traditional cluster catalysts with core-embedded catalytic sites, this work presents a strategy to position catalytic sites on the surfaces of oxide clusters. We utilize a calixarene-stabilized titanium–oxo cluster (Ti12L6) as a scaffold to anchor Ag1+in situ, forming the unique nanocluster Ti12Ag4.5 with six surface-exposed Ag1+ sites. The in situ transformation from Ti12L6 into Ti12Ag4.5 clusters was traced through mass spectrometry, revealing a solvent-mediated dynamic process of disintegration and reassembly of the Ti12L6 macrocycle. The unique Ti12Ag4.5 cluster, featuring a surface-exposed catalytic site configuration, efficiently catalyzes the electroreduction of CO2 to CO over a broad potential window, achieving CO faradaic efficiencies exceeding 82.0% between −0.4 V and −1.8 V. Its catalytic performance surpasses that of bimetallic Ti2Ag2, which features a more conventional design with Ag1+ sites embedded within the cluster. Theoretical calculations indicate that the synergy between the titanium–oxo support and the single Ag1+ sites lowers the activation energy, facilitating the formation of the *COOH intermediate. This work reveals that engineered interactions between active surface metal and the oxide support could amplify catalytic activity, potentially defining a new paradigm in catalyst design.
To effectively tackle these challenges, it is crucial to develop molecular analogues for oxide-supported single-atom materials. Recent advances in TiO2 analogues,16–19 particularly through the development of crystalline titanium oxide clusters (TOCs), have facilitated comprehensive investigations of TiO2 structures and reactivities at the molecular level.20–24 These TOCs, when modified with catalytically active single metals, hold promise as effective molecular mimics for oxide-supported SACs. However, heterometal-doped TOCs are typically created via a one-pot solvothermal method, with heterometal sites embedded deep within the cluster core, often in fully coordinated states.25–29 Such configurations limit the interactions with reactants, exposing a significant shortfall in their capacity to act as true molecular proxies. This insight highlights the urgent need for a new class of crystalline, cluster-stabilized single-atom materials, designed to more precisely emulate the structural and reactive properties of oxide-supported SACs at a molecular level.30,31
Here, we present an approach designed to strategically place catalytic Ag1+ sites on the surfaces of oxide clusters (Scheme 1). Employing thiacalix[4]arene (TC4A) as a protective ligand,32–38 we meticulously engineered a titanium–oxo macrocycle, Ti12L6, through a one-step solvothermal method, which is enriched with surface O and S sites. We utilized Ti12L6 as a scaffold for the in situ loading of Ag1+ ions, revealing a solvent-mediated assembly process through mass spectrometry. In N,N-dimethylformamide (DMF) solution, the scaffold undergoes decomposition under the cleavage of Ag1+ ions; however, in acetonitrile, the scaffold remains stable and coordinates six accessible single Ag1+ sites on the cluster surface, ultimately transforming into a Ti12Ag4.5 cluster. Ti12Ag4.5 has proven to be an exceptional catalyst for the electrochemical reduction of CO2, exhibiting superior reactivity and CO selectivity compared to its bimetallic counterpart, Ti2Ag2, which possesses a more conventional structure with Ag1+ sites embedded within the cluster matrix. Notably, Ti12Ag4.5 exhibits high selectivity for CO across a wide voltage range, with CO faradaic efficiency (FECO) consistently exceeding 82.0% from −0.4 V to −1.8 V vs. RHE, reaching a peak FECO of 92.7%. At an overpotential of ∼−1.4 V, the system remained stable for continuous electrolysis over 11 h with a CO partial current density exceeding 100.0 mA cm−2, while the FECO consistently remains above 85.0%. We elucidated the reaction path using in situ ATR-SEIRAS technology and comprehensively calculated the Gibbs free energy changes for each elementary step of CO2 conversion to CO, highlighting the critical contribution of exposed Ag centers to the observed catalytic prowess.
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Scheme 1 Schematic representation of the cluster assembly featuring a Ti–oxo core with surface-exposed Ag1+ catalytic sites. |
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Fig. 2 (A) The overall structure of Ti2Ag2; (B) coordination mode of oxidized TC4A in Ti2Ag2. (C) Comparison of the positions of the Ag and Ti sites in Ti2Ag2. |
Interestingly, using Ti12L6 crystals in reactions with Ag(I) salts yielded distinctly different outcomes. Introducing Ag2SO4 into DMF solution along with Ti12L6 crystals and allowing the mixture to react at 80 °C for three days resulted in the formation of the known clusters Ti2Ag2-DMF and Ti2Ag4.34 Subsequently, by switching the solvent to CH3CN, rhombic Ti12Ag4.5 crystals were obtained, emphasizing the critical role of solvent selection in steering the chemical pathway. Ti2Ag2-DMF and Ti2Ag4 have typical bimetallic configurations, with two or four Ag1+ ions embedded between two {Ti(TC4A)} units. In contrast, in Ti12Ag4.5, multiple Ag1+ ions are effectively anchored onto the surface of the Ti12 core without altering its intrinsic structure. SCXRD analysis revealed the structure of Ti12Ag4.5 to be [H1.5Ti12Ag4.5O18(HTC4A)6(CH3CN)4], with the cluster containing three crystallographically distinct sites for Ag. Structural analysis demonstrated that the Ti–oxo core of Ti12Ag4.5 closely mirrors the macrocyclic structure of Ti12L6, albeit with minor deviations (Fig. 3A). The Ag1+ sites are symmetrically divided and positioned on both the upper and lower facets of the Ti–oxo macrocycle (Fig. 4B). Ag1 and Ag2, coordinated through phenoxide, sulfur, and μ-O22−, are placed between two TC4A ligands, exhibiting coordination numbers of 3 and 4, respectively. The distances between Ag1/Ag2 and the Ti–oxo core range from 2.392 to 2.607 Å. Notably, the Ag3 site, with an O3N2 coordination environment, is defined by two phenoxide groups, one μ-O2−, and two CH3CN, with Ag–O bond lengths ranging from 2.649 to 2.774 Å and Ag–N distances of 2.056 and 2.215 Å. The spatial arrangement of the Ag sites, especially the Ag3 sites proximal to the cluster surface as depicted in Fig. 3C, highlights their potential as catalytically active sites. This is particularly significant given the labile nature of the CH3CN ligand, which may facilitate dynamic catalytic processes. Sites Ag1 and Ag2 exhibit full occupancy, while Ag3 displays a partial occupancy of 0.25. As a result, the average number of Ag sites in the cluster is 4.5, which is confirmed by the mass spectrometry analysis of Ti12Ag4.5. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis reveals six distinct peaks with a mass difference of 107.86, corresponding to the ion {H4Ti12Ag6O16(TC4A)6(CH2Cl2)}+ (Fig. 3D) (x = 1–6). This pattern suggests that the cluster contains six Ag sites, which can sequentially detach under ionization conditions. Additionally, the atomic ratio of Ag to Ti in the cluster, as determined by ICP analysis, is 2.63, which is in excellent agreement with the theoretical value of 2.66 (Table S2†).
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Fig. 4 Time-dependent ESI-MS for the reaction of the Ti12L6 crystals and Ag2SO4 in CH3CN (A) or DMF (B) solution at 80 °C at 0 h, 1 h and 6 h. |
From the above results, it is evident that the solvent choice significantly influences the reaction pathway of Ti12L6 with Ag(I) salts; CH3CN primarily leads to Ti12Ag4.5, whereas solvents like DMF result in different structures of Ti2Ag2-DMF and Ti2Ag4. The significant impact of seemingly minor variations in solvent choice on the synthesis outcomes is fascinating. To elucidate the formation mechanism of Ti12Ag4.5, its evolutionary process was monitored using ESI-MS.39 Time-resolved ESI-MS analysis of the reaction mixture, containing Ti12L6 crystals and Ag2SO4 in CH3CN, captured data at various intervals (Fig. 4A). Initially, ESI-MS detected two principal signal sets corresponding to the +2 and +3 charge states of {HxTi12O18−y(TC4A)6}, indicating that Ti12L6 maintained its integrity in the early stages of the reaction. After heating at 80 °C for one hour, new signals emerged, with a strong peak corresponding to {HTi2(TC4A)2}+, resulting from the fragmentation of Ti12L6. Additionally, subtle signals in the m/z range of 2000–2500, likely representing {Ti12AgxO20−z(TC4A)6}2+, were observed. By the six-hour mark, the spectrum displayed distinct signal sets aligned with the +2 and +3 charge states of the {HxTi12AgyO20−z(TC4A)6} species, indicating a complete transformation of Ti12L6 into Ti12Ag4.5.
Conversely, in a DMF environment, ESI-MS revealed a completely different reaction scenario (Fig. 4B). Initially, Ti12L6 also maintained its structure in DMF (Fig. S34†). One hour post Ag(I) salt introduction, ESI-MS revealed the breakdown of the macrocycle, detecting {HxTi6Oz(TC4A)3}+ and {HxTi2Oz(TC4A)2}+ species. Notably, the emergence of {HxTi2AgyOz(TC4A)2}+ (x = 2–4) clarified the formation of Ti2Ag4 and Ti2Ag2-DMF clusters. By the six-hour mark, these signals intensified significantly, highlighting the progressive formation of these clusters in the DMF solution.
These findings indicate that the synthesis of Ti12Ag4.5 is not merely a simple process of Ag1+ adsorption onto the Ti12L6 carrier. Instead, it involves a complex sequence of fragmentation and subsequent reassembly (Fig. 5). Under solvothermal conditions, the Ti12L6 framework undergoes a fragmentation process, resulting in the formation of {Ti2O(TC4A)} units. A dynamic equilibrium forms between the larger {Ti12O12(TC4A)6} structure and these smaller {Ti2O(TC4A)} units. In the presence of CH3CN solvent, during the reassembly phase, these {Ti2O(TC4A)} fragments coordinate with Ag1+ ions, facilitating the formation of Ti12Ag4.5. Conversely, using DMF as the solvent leads to a transformation of the {Ti2O(TC4A)} units into more complex {Ti2O(TC4A)2} structures. These structures then complex with Ag1+ ions to form Ti2Ag2-DMF and Ti2Ag4, highlighting a distinct synthetic pathway that is significantly influenced by the choice of solvent.
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Fig. 5 Schematic diagram illustrating the solvent-mediated self-assembly process transitioning from Ti12L6 to Ti12Ag4.5. |
Powder X-ray diffraction (PXRD) analysis shows that the Ti12Ag4.5 crystal maintains its crystalline structure even after exposure to a highly alkaline 1 M KOH solution for 24 hours (Fig. S35†). Given the resilience of Ti12Ag4.5, its eCO2RR activity was assessed in a three-compartment flow cell with 1 M KOH as the electrolyte. Linear sweep voltammetry (LSV) results indicated a significantly higher current density and more positive onset potential for both the Ti12Ag4.5 and Ti2Ag2 clusters in the CO2 flow electrolyzer compared to the N2-purged system, confirming their CO2 reduction capability, as illustrated in Fig. 6A. Gas chromatography detected only CO and H2 as products, with no other liquid products identified by 1H NMR spectroscopy (Fig. S39†). Control experiments under N2-saturated conditions yielded no carbon-reduction products. Isotopic tracing experiments with 13CO2 through GC-MS confirmed the production of 13CO (m/z = 29) (Fig. S40†). Additionally, the primary electrocatalytic product of the Ti12L6 catalyst was identified as H2, indicating that the Ag1+ components of these clusters predominantly drive the catalytic activity (Fig. S37†).
Fig. 6B and C illustrate the trends in product distribution across varying potentials for two distinct clusters. In a CO2-saturated environment, Ti12Ag4.5 consistently exhibits a high FECO production, maintaining over 82.0% across an extensive potential range from −0.4 V to −1.8 V vs. RHE, peaking at 92.7% at a higher potential of −1.4 V (Table S3†). However, for Ti2Ag2, the predominant reaction between −0.6 V and −1.0 V is CO2 reduction, reaching its highest FECO of 85.1% at −0.8 V. Beyond this, the reaction is largely overtaken by H2 evolution in the potential range from −1.2 to −1.8 V, leading to a progressive decline in FECO as potential increases. At −1.8 V, the FECO of Ti2Ag2 plummets to merely 21.2%, whereas Ti12Ag4.5 still manages to maintain an FECO of 83.25%. Additionally, the CO partial current density (JCO) for both clusters was also analyzed (Fig. 6D). The JCO for Ti12Ag4.5 reached an impressive 130.1 mA cm−2 at −1.8 V, which is above 2.4 times greater than that of Ti2Ag2. This comparative analysis emphasizes the pronounced differences in catalytic efficiencies between two clusters, attributing Ti12Ag4.5's superior performance to its unique structural configuration and effective Ag1+ site utilization.
Electrocatalytic stability is a key indicator for evaluating the performance of electrocatalysts in the eCO2RR. We employed PXRD to analyze the reduction of Ag+ in Ti12Ag4.5 after reaction at different applied voltages (Fig. S44†). PXRD analysis reveals that the catalyst remains stable when the applied voltage is below −1.8 V. However, when the voltage exceeds −1.8 V, signals corresponding to Ag nanoparticles appear, indicating the reduction of Ag+ to metallic Ag. To further evaluate the durability of the catalyst, we conducted a rigorous 11-hour chronoamperometric test at −1.4 V. During this process, the current density remained above 100 mA cm−2, and the FE for CO remained stable above 85.0% (Fig. 6E). Additionally, ESI-MS analysis of the catalysts after electrolysis showed a signal for the {HxTi12Ag6O18−z(TC4A)6}2+ species, confirming that the structural integrity of the Ti12Ag4.5 catalyst was maintained (Fig. S45†). Further characterization by transmission electron microscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy showed that the catalyst maintained its chemical composition and structural stability during the electrolysis process (Fig. S47–S49†). Differential pulse voltammetry (DPV) measurements reveal that the electrochemical gap of Ti12Ag4.5 is 1.91 V (Fig. S50†), which is greater than the 1.32 V observed for Ti2Ag2. This larger electrochemical gap further suggests enhanced stability of Ti12Ag4.5 during the electrocatalytic process. The stability of Ag+ in the Ti12Ag4.5 clusters can be attributed to the strong electronic and structural stabilization provided by the Ti–oxo support. This unique characteristic not only preserves the active sites but also prevents the competitive reduction of Ag+, thereby enhancing the suitability of these clusters for CO2 reduction.
The hypothesized mechanism for CO2 reduction to CO using Ag-based catalysts follows the pathway: CO2(g) → *COOH → *CO → CO(g).54–57 To validate this mechanism, we utilized in situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). This technique allows for real-time monitoring of the absorption of evanescent waves by substances on the catalyst surface, providing direct insight into the reaction dynamics. We set the potential range from −0.5 V to −1.3 V and monitored changes in absorption peaks with Ti12Ag4.5 and Ti2Ag2 as the electrocatalysts (Fig. 6F). The spectra display similarities, including a pronounced peak at 1225 cm−1, attributed to C–OH stretching in *COOH, which intensifies with increasing voltage. Another peak at 1711 cm−1, corresponding to CO stretching in *COOH, also increases in intensity from −0.5 V to −1.3 V, suggesting a rise in the surface coverage of *COOH species with increasing voltage.58,59 Additionally, a weak signal at 2127 cm−1, attributed to the Ag–*CO vibration mode, indicates the presence of *CO adsorbed on the catalyst surface, especially at lower potentials where the *CO band intensity shows a slight increase. Notably, the intermediate characteristic peak of Ti2Ag2 becomes distinctly observable only from an electrode potential of approximately −1.1 V, while the characteristic peaks of Ti12Ag4.5 are already evident at −0.5 V. This observation indicates that Ti12Ag4.5 demonstrates a higher reaction activity and stronger catalytic proficiency for the eCO2RR, aligning with experimental findings.
Density Functional Theory (DFT) calculations were conducted to elucidate the reactivity of two specifically designed clusters, Ti12Ag4.5 and Ti2Ag2. These models were optimized to mirror their actual crystal structures, simplified by replacing the tert-butyl groups on the TC4A with H for faster computational convergence. The results of these optimizations are depicted in the Gibbs free energy diagrams for both the eCO2RR and the HER, as presented in Fig. 7A. DFT calculations specifically focused on the energetics of each step, revealing that the formation of the *COOH intermediate is the rate-determining step in the CO2RR process. A critical finding from our study is the calculated Gibbs free energy for the formation of *COOH on the Ag3 site in Ti12Ag4.5, which was notably low at 0.55 eV. This value contrasts with the corresponding energy of 0.88 eV for the same process within Ti2Ag2. This substantial difference highlights that the exposed Ag sites on Ti12Ag4.5 are much more energetically favorable for catalyzing the conversion of CO2 to CO compared to those on bimetallic Ti2Ag2 (Fig. 7B). Additionally, the Gibbs free energies for hydrogen adsorption (*H) were calculated, showing high values of 2.46 eV for both clusters, indicating that neither Ti2Ag2 nor Ti12Ag4.5 is favorable for the formation of H2. This finding is important as it underscores the higher selectivity of both clusters for the CO2RR-to-CO pathway compared to the HER.
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Fig. 7 (A) Free energy diagrams for the eCO2RR and HER on Ti12Ag4.5 and Ti2Ag2; (B) schematic diagram of the eCO2RR process on Ti12Ag4.5. |
By comparing the Gibbs free energy diagrams of the eCO2RR and HER, the exceptional catalytic efficiency of Ti12Ag4.5 is clearly linked to its unique Ag coordination. This coordination is supported by a titanium–oxo core, with Ag sites evenly distributed across the cluster surface. Specifically, the Ag active sites in Ti12Ag4.5 exhibit a d-band center (εd) that is closer to the Fermi level at −3.04 eV. As a comparison, the bimetallic cluster Ti2Ag2 exhibits a lower εd value of −3.47 eV, likely attributed to the absence of the unique support effects found in Ti12Ag4.5. Furthermore, the projected density of states (PDOS) for Ti12Ag4.5 shows higher and narrower peaks at energies near the Fermi level, suggesting a higher and more localized density of electronic states (Fig. S53†). This enhanced electronic configuration facilitates stronger interactions with adsorbate *COOH molecules, improving the activation and subsequent transformation of CO2. These electronic characteristics are critical for reactions requiring complex electron interactions, thus positioning Ti12Ag4.5 as a more effective catalyst for CO2 reduction.
Footnotes |
† Electronic supplementary information (ESI) available: X-ray crystallographic file in CIF format, and full experimental and computational details. CCDC 2349621–2349625. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc07186g |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2025 |