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Article

Ag Nanoparticle-Modified Metal Azole Framework for Enhancing the Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide

by
Xu Han
,
Haotian Wang
,
Yijie Zhang
,
Yuting Tan
and
Xiaomeng Lv
*
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212003, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(1), 32; https://doi.org/10.3390/catal15010032
Submission received: 29 October 2024 / Revised: 18 December 2024 / Accepted: 27 December 2024 / Published: 2 January 2025
(This article belongs to the Special Issue Advanced Electrocatalysts for Energy-Related Applications)
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Abstract

The electrocatalytic reduction of carbon dioxide (CO2RR) into high-value-added products is considered to be a promising way to mitigate carbon emissions. However, it remains a challenge to design an efficient catalyst with an excellent performance. In this work, we synthesized a metal azole framework (MAF) by changing 2-methylimidazole ligands into 5-mercapto-1-methyltetrazole (MMT) for use as organic linkers and mercaptan groups as anchoring sites for the Ag nanoparticles. The Ag NPs@ MAF-MMT material displayed a wide potential window from −0.6 to −1.2 V vs. RHE, with a maximum CO Faradaic efficiency (FECO) over 90.5%, and a current density of 18 mA cm−2 at −1.1 V vs. RHE for 11 h in an H-cell. This work provides a new option to immobilize Ag nanoparticles in MAFs material for the exploration of carbon dioxide reduction catalysts.

1. Introduction

The excessive use of fossil fuels leads to abundant carbon dioxide emissions, which has caused serious ecological and environmental problems. An electrochemical method powered by the electricity generated from renewable energy converts carbon dioxide into carbon monoxide, formic acid, methanol, ethylene and other high-value-added products, providing a promising technology for alleviating the greenhouse effect and coping with energy shortages [1,2]. Among different products, CO is considered to be one of the most practicable reactions in the chemical industry due to its technical and economic feasibility [3,4]. However, the CO2 molecule has a symmetrical carbon–oxygen double bond structure and is extremely thermodynamically stable, so the development of a highly efficient catalyst is a pivotal factor in achieving this conversion.
Metal–organic frameworks (MOFs) are porous materials consisting of metal ions or clusters as nodes and organic compounds as linkers, such as Zeolitic Imidazolate Framework-8 (ZIF-8), which is composed of 2-methylimidazole and Zn2⁺. Due to their special structure and composition, MOFs and their composites have the advantages of high crystallinity, high porosity and specific surface area, a designable framework, and structural diversity; they are already widely applied in gas storage, gas separation, catalysis, and drug delivery [5,6,7,8]. Despite its abundant channels and framework, the structure can facilitate the transport of reaction substrates/products; the poor conductivity and stability of MOFs hinder their development in CO2 electrocatalysis in some way. Up until now, several strategies have been proposed to heighten the conductivity of MOF composite materials: (1) constructing MOFs with electroactive molecules as building block [9]; (2) establishing two-dimensional MOFs with excellent charge transport properties [10,11]; (3) introducing conductive guests into MOFs to optimize charge transport capacity [12,13,14]; (4) employing functionalized connectors to achieve better charge delocalization [15]; (5) using conductive materials as substrates to support MOFs [16]. As to the strategy of incorporation of metal, the inherent permanent porosity of MOFs provides inherent conditions for anchoring small metal nanoparticles, and the crystalline porous structure of MOFs is capable of preventing the migration and accumulation of metal nanoparticles [17,18]. The synthesis of metal nanoparticles in MOFs materials has been widely reported and can mainly be categorized in two ways: (a) The metal nanoparticles coated with surfactants are synthesized first, which is followed by MOF epitaxial growth or assembly [19]. (b) Metal precursors are introduced into MOFs and then reduced in situ, forming composite encapsuled metal nanoparticles [20]. The composite synthesized the latter way, without any surfactants, in order to obtain clean and accessible surfaces of metal nanoparticles generally shows better catalytic properties [21].
In their pioneering work on CO2RR, Hori et al. found that silver (Ag) is a promising candidate for constructing a high-efficiency MOF-based catalyst because of its moderate activation energy in the formation of *COOH intermediates and weak adsorption energy for CO [20]. However, the tendency to lower the surface energy, leading to aggregation under reaction conditions and deterioration in activity, severely impeded the commercialization of Ag-based catalysts, and further modification was needed for high performance of Ag-based materials [22]. Confining these Ag nanoparticles into a porous MOF material may be a good way to prevent MOF aggregation but enhance the conductivity of MOFs.
Herein, we synthesized a metal azole framework (MAF) material by changing 2-methylimidazole ligands in ZIF-8 into 5-mercapto-1-methyltetrazole (MMT) as organic linkers and mercaptan groups as anchoring sites for the Ag nanoparticles. We used commercial carbon black as the carrier to explore its performance in electrocatalytic carbon dioxide reduction to carbon monoxide. Compared with the pristine MOF materials without Ag, the catalytic activity and selectivity were improved significantly. The carbon monoxide Faraday efficiency (FECO) was 90.5% at 0.7 V vs. RHE potential, and the stability was maintained 11 h in the constant current test without obvious decline. This study exhibits a new composite material to explore enhanced catalytic CO2RR performance.

2. Results and Discussion

2.1. Structural Characterization of the Catalysts

The whole synthesis procedure is illustrated in Scheme 1. The morphologies of MAF-MMT, AgNO3@MAF-MMT and Ag NPs@MAF-MMT were characterized by scanning electron microscopy (SEM), as shown in Figure 1 and Figure S1. The MAF-MMT sample exhibited a regular columnar structure with a length of about 1 μm (Figure 1a), demonstrating the successful coordination of Zn2+ with 5-mercapto-1-methyltetrazole in order. The morphology changed after adding AgNO3 (Figure S1), indicating that the presence of Ag+ salt may influence the appearance of material. After reduction with NaBH4, the sample further evolved, displaying a superimposed sheet structure (Figure 1b).
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to reveal the microscopic morphology and internal structure of the sample. The light color indicates the flaky MAF-MMT substrate, and the dark small spots are Ag NPs (Figure 1c). The −SH group could stabilize Ag NPs via interactions between Ag and S ions [23]; therefore, 10 nm Ag NPs are regularly anchored and distributed on the MAF-MMT substrate. In the high-resolution transmission image of the Ag NPs@MAF-MMT (Figure 1d), the clear crystal fringes of the sample show good crystallinity. The crystal face spacing of the sample was measured to be 0.2143 nm, which belongs to the (111) crystal face of Ag nano-particles [20].
X-ray diffraction (XRD) was performed to analyze the crystal structures of MAF-MMT and Ag NPs@MAF-MMT. Compared with MAF-MMT, the two additional peaks at 38.18° and 44.47 were attributed to the (111) and (200) crystal faces of Ag [20] in the Ag NPs@MAF-MMT sample (Figure 2a), suggesting that Ag NPs were successfully loaded on the MAF-MMT substrate, which conformed with the result of HRTEM. At the same time, it was observed that, after the reduction, the crystallinity of the catalyst changed. Although most of the characteristics of MAF-MMT (~30° and 35°) were retained, some disordered peaks appeared around 52°. Combining with the results of HRTEM, it can be inferred that the incorporation of silver caused a change in the crystal structure of the MAF-MMT. Fourier transform infrared spectroscopy (FT-IR) was conducted to investigate the molecular structure of MAF-MMT and Ag NPs@MAF-MMT samples. The peaks at 712 cm−1 and 1180 cm−1 were ascribed to the bending vibration of the C-S bond and the stretching vibration of C-S bond (Figure 2b), respectively. The peaks at 1460 cm−1 were caused by the N=N bond in tetrazole [24]. Compared with the MAF-MMT sample, the presence of these peaks proved the successful complex recombination of the ligands. The IR characteristic peaks of the Ag NPs@MAF-MMT sample did not change significantly, indicating that although the morphology changed slightly, the functional group structure of the MAF-MMT substrate did not break due to the introduction of Ag NPs. In addition, in the XRD pattern of the Ag NPs@MAF-MMT, the peak intensity of the MAF-MMT decreased (Figure 2b), which was attributed to the surface of MAF-MMT being covered by the anchored Ag NPs with high crystallinity [24]. Particularly, the hydroxyl strength in the infrared spectrum of the composite catalyst was significantly increased due to the silver nanoparticle surface plasmon resonance effect (SPR), enhancing the hydroxyl absorption peak. This was because the silver nanoparticles absorb the incident light, resulting in an enhanced local electromagnetic field, which enhances the infrared absorption of the vibrational patterns associated with the hydroxyl group [25].
X-ray photoelectron spectroscopy (XPS) was further performed to study the chemical composition and valence bond information of the material. For all tested samples, the main peak at C 1s spectrum was corrected to 284.8 eV. The presence of Ag element in Ag NPs@MAF-MMT corroborated that Ag nanoparticles were successfully fixed into the MAF-MMT material (Figure 2c and Figure S2, Table S1). The peaks at 368.1 eV and 374.1 eV in high-resolution structure spectrum of Ag 3d belonged to the 3d5/2 and 3d2/3 of Ag0, suggesting that Ag existed in the form of Ag0 [20]. The C-S bond shifted from 284.7 eV in MAF-MMT to 286.9 eV in Ag NPs@MAF-MMT, indicating the strong binding interaction between S and Ag (Figure 2d) [24]. The emerging peak at 169.1 eV in the Ag NPs@MAF-MMT sample (Figure 2e) was ascribed to the S-Ag bond, which also proved this interaction, indicating that the sulfhydryl group in MAF-MMT successfully anchored the Ag NPs, which was in line with XRD and TEM results. The Zn 2p in MAF-MMT and Ag NPs@MAF-MMT samples showed two peaks near 1022 eV and 1045 eV, corresponding to the characteristic peaks of Zn 2p3/2 and Zn 2p1/2 (Figure 2f), respectively. The peaks of Zn 2p3/2 and Zn 2p1/2 at 1045.3eV and 1022.3 eV in Ag NPs@MAF-MMT shifted to 1045.1 eV and 1022.1 eV, which was possibly due to the binding of Ag-S [24]. This facilitated the immobilization of Ag.

2.2. Electrochemical Performances

The electrocatalytic performance of Ag NPs@MAF-MMT catalysts was assessed in a standard three-electrode H-type electrolytic cell with a CO2-saturated 0.5 M KHCO3 (pH = 7.2) electrolyte, in which two chambers were separated by a Nafion 117 proton exchange membrane. The performance of the electrocatalysis for CO2 reduction was preliminarily evaluated by a line sweep voltammetry (LSV) curve. As shown in Figure 3a, the two catalysts showed significantly different responses to CO2 reduction. MAF-MMT presented a quite low current response, limited by poor electrical conductivity, while Ag NPs@MAF-MMT displayed an obviously enhanced current density. The onset potentials of MAF-MMT and Ag NPs@MAF-MMT were −0.72 V vs. RHE and −0.59 V vs. RHE, respectively, indicating the lower energy barrier in electrocatalytic process for Ag NPs@MAF-MMT. At −1.2 V vs. RHE potential, the current density of Ag NPs@MAF-MMT catalyst was 3.2 times that of MAF-MMT catalyst, showing higher activity than original MAF-MMT.
The MAF-MMT and Ag NPs@MAF-MMT catalysts were tested at constant voltages from the potential range of −0.6 V vs. RHE to −1.2 V vs. RHE, and the gas products were detected by online gas chromatography (GC). The liquid products were analyzed by nuclear magnetic resonance (1H-NMR, Figure S3). The results showed that only CO and H2 were detected in the reduction products. The FECO of MAF-MMT catalyst at all the applied potentials was lower than 40%, while the Ag NPs@MAF-MMT catalyst exhibited a good FECO (Figure 3b), in which the FECO reached a maximum of 90.5% at −0.7 V vs. RHE. At each test potential, the FEH2 of Ag NPs@MAF-MMT decreased markedly compared to MAF-MMT, showing that the hydrogen evolution reaction was significantly inhibited (Figure 3c). Stability is also an important factor for assessing catalyst performance. As shown in Figure 3d, Ag NPs@MAF-MMT retained prominent catalytic stability over 11 h with FECO of about 90% without obvious decline during the test. After the stability test, the morphology, infrared spectroscopy and XRD pattern of the catalyst did not change significantly (Figures S4 and S5), indicating the excellent stability of the catalyst. The performance exceeded that of most reported MOF-based catalysts (Figure 3e, Table S2) [20,26,27,28,29,30,31,32].
The Tafel slope of Ag NPs@MAF-MMT was fitted to be 161 mV dec−1 (Figure 4a), much lower than that of MAF-MMT (426.7 mV dec−1, Figure S6). The comparatively smaller Tafel slope of the Ag NPs@MAF-MMT material indicated that the catalyst delivered less kinetic resistance during the reaction, which was conducive to the reaction. The electrochemical impendence spectroscopy (EIS) spectra displayed that Ag NPs@MAF-MMT presented a smaller semicircle than that of MAF-MMT (Figure 4b), indicating a smaller charge transfer resistance (Rct) during the catalytic process, in line with the results of the Tafel slope. The Ag NPs@MAF-MMT nanocomposite exhibited an Rct value of 12.7 Ω, while MAF-MMT exhibited an Rct value of 15.9 Ω. As described in Figure 4c, the double-layer capacitance (Cdl) of Ag NPs@MAF-MMT (0.67 mF cm−2) was much larger than that of MAF-MMT (0.23 mF cm−2). The electrochemical layer capacitance (Cdl) was proportional to the electrochemical active surface areas (ECSA), and it could be concluded that the anchored silver nanoparticles ensure Ag NPs@MAF-MMT more active sites, accounting for the improved performance for CO2RR (Figure S7). In addition, we also made a comparison of different silver loading, and the results are included in the Supplementary Information (Figure S8).

3. Experimental Section

3.1. Synthesis of Metal Azole Framework Material with 5-Mercapto-1-Methyltet-Razole Ligand (MAF-MMT)

MAF-MMT material was prepared according to the previously reported method with modification [24]; 1.4 g zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 0.52 g 5-mercaptoyl-1-methyltetrazole (MMT) were dissolved in a 40 mL methanol solution, respectively. Then, 0.25 g polyvinylpyrrolidone (PVP) was introduced to each of the two solutions. After complete dissolution, the two solutions were mixed and stirred for 24 h to obtain a homogeneous mixture at room temperature. After that, the mixture stood for another 24 h. The resulting product was collected by centrifugation, cleaned with a methanol solution 3 times, and dried in a vacuum drying oven at 60 °C for 24 h to obtain MAF-MMT material.

3.2. Synthesis of Ag NPs@MAF-MMT

Then, 34 mg AgNO3 was added to a round-bottom flask containing 20 mL of acetonitrile. Then, 60 mg MAF-MMT was introduced to the flask and the solution was stirred for 8 h. The resulting precipitate of AgNO3@MAF-MMT was washed by acetonitrile three times to remove excrescent metal salts on the surface and dried at 60 °C. The product was then transferred into 30 mL acetonitrile and stirred for 2 h to disperse at 0 °C. The freshly prepared MeOH solution of NaBH4 (5 mg in 5 mL MeOH) was quickly added to the above solution and stirred for another 5 min. The solid was collected via centrifugation and washed with acetonitrile three times. The residual material was dried at 60 °C for 12 h to obtain the final product of Ag NPs@MAF-MMT.

4. Conclusions

We synthesized the Zn-based MAF electrocatalyst by a introducing sulfhydryl group as the site of anchoring silver nanoparticles. The mercaptan functional group presents a strong interaction with Ag, allowing NP to anchor to the MAF. The Ag nanoparticles not only enhanced the conductivity, but also enriched the catalytic active site and promoted the catalytic activity. The catalyst showed quite high Faraday efficiency for CO at −0.7 V vs. RHE and had significant catalytic stability over 11 h, with no significant loss of reduction current. This work provides a new strategy for MAF-supported metal silver to enhance the application of MAFs in electrocatalytic CO2 reduction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15010032/s1, Figure S1: (a) & (b) Scanning electron microscope of AgNO3@MAF-MMT; Figure S2: Survey XPS of MAF-MMT and Ag NPs@MAF-MMT; Figure S3: The quantification of liquid-phase products of Ag NPs@MAF-MMT at −0.7 V vs. RHE for 11 h by 1H NMR; Figure S4: Scanning electron microscope of Ag NPs@MAF-MMT after stability test; Figure S5: (a) FT-IR spectra, (b) XRD pattern of Ag NPs@MAF-MMT after stability test; Figure S6: Tafel slopes of MAF-MMT; Figure S7: CV curves of (a) MAF-MMT and (b) Ag NPs@MAF-MMT at different scan rates; Figure S8: Performance of catalysts with different silver additions in the synthesis process, 1, 2, 3, 4 corresponding to 17, 34, 51, 68 mg, respectively. (a) LSV, (b) FECO, (c) jCO of catalysts with different silver additions; Table S1: Proportion of surface atoms in Ag NPs@MAF-MMT; Table S2: Performance of recently reported MOF-based electrocatalysts.

Author Contributions

X.H.: Conceptualization, methodology, funding acquisition, writing—original draft; H.W.: Conceptualization, formal analysis; Y.Z.: investigation, validation; Y.T.: sources; X.L.: funding acquisition, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

Zhenjiang Key Research and Development Program (GY2021004), Industrial Center of Jiangsu University Innovation Experiment Fund (ZXJG2022022) and Jiangsu University Research Project (22A071).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00032 sch001
Scheme 1. The synthesis procedure of Ag NPs@MAF-MMT.
Scheme 1. The synthesis procedure of Ag NPs@MAF-MMT.
Catalysts 15 00032 sch001
Catalysts 15 00032 g001
Figure 1. Scanning electron microscopy scan of (a) MAF-MMT and (b) AgNPs@MAF-MMT; (c) transmission electron microscopy scan and (d) high-resolution electron microscopy image of Ag NPs@MAF-MMT.
Figure 1. Scanning electron microscopy scan of (a) MAF-MMT and (b) AgNPs@MAF-MMT; (c) transmission electron microscopy scan and (d) high-resolution electron microscopy image of Ag NPs@MAF-MMT.
Catalysts 15 00032 g001
Catalysts 15 00032 g002
Figure 2. (a) XRD patterns of MAF-MMT and Ag NPs@MAF-MMT. (b) FT-IR spectra of MAF-MMT and Ag NPs@MAF-MMT. The high resolution XPS spectra of (c) Ag 3d, (d) C 1s, (e) S 2p, and (f) Zn 2p in MAF-MMT and Ag NPs@MAF-MMT.
Figure 2. (a) XRD patterns of MAF-MMT and Ag NPs@MAF-MMT. (b) FT-IR spectra of MAF-MMT and Ag NPs@MAF-MMT. The high resolution XPS spectra of (c) Ag 3d, (d) C 1s, (e) S 2p, and (f) Zn 2p in MAF-MMT and Ag NPs@MAF-MMT.
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Figure 3. (a) LSV, (b) FECO, and (c) FEH2 of the catalysts at different reduction potentials. (d) A catalytic reaction was conducted at −0.7 V vs. RHE for 11 h and CO2RR products measured at 1 h intervals. (e) FECO and stability of Ag NPs@MAF-MMT compared with those of representative MOF-based catalysts [20,26,27,28,29,30,31,32].
Figure 3. (a) LSV, (b) FECO, and (c) FEH2 of the catalysts at different reduction potentials. (d) A catalytic reaction was conducted at −0.7 V vs. RHE for 11 h and CO2RR products measured at 1 h intervals. (e) FECO and stability of Ag NPs@MAF-MMT compared with those of representative MOF-based catalysts [20,26,27,28,29,30,31,32].
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Figure 4. (a) Tafel slopes. (b) Nyquist plots and the equivalent circuit diagram. (c) Capacitance values of Ag NPs@MAF-MMT and MAF-MMT.
Figure 4. (a) Tafel slopes. (b) Nyquist plots and the equivalent circuit diagram. (c) Capacitance values of Ag NPs@MAF-MMT and MAF-MMT.
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Han, X.; Wang, H.; Zhang, Y.; Tan, Y.; Lv, X. Ag Nanoparticle-Modified Metal Azole Framework for Enhancing the Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide. Catalysts 2025, 15, 32. https://doi.org/10.3390/catal15010032

AMA Style

Han X, Wang H, Zhang Y, Tan Y, Lv X. Ag Nanoparticle-Modified Metal Azole Framework for Enhancing the Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide. Catalysts. 2025; 15(1):32. https://doi.org/10.3390/catal15010032

Chicago/Turabian Style

Han, Xu, Haotian Wang, Yijie Zhang, Yuting Tan, and Xiaomeng Lv. 2025. "Ag Nanoparticle-Modified Metal Azole Framework for Enhancing the Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide" Catalysts 15, no. 1: 32. https://doi.org/10.3390/catal15010032

APA Style

Han, X., Wang, H., Zhang, Y., Tan, Y., & Lv, X. (2025). Ag Nanoparticle-Modified Metal Azole Framework for Enhancing the Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide. Catalysts, 15(1), 32. https://doi.org/10.3390/catal15010032

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