Electrocatalytic reduction of PhCH₂Cl on Ag-ZSM-5 zeolite modified electrode

Abstract

An Ag-ZSM-5/SS zeolite modified electrode has been prepared by a facile one-step method, during which Ag nanoparticles were formed in situ with a ZSM-5 film grown on a stainless steel substrate. The morphology and composition were characterized by XRD, SEM, TEM, EDX, H₂-TPR, XPS, etc. Ag-ZSM-5/SS possessed much higher catalytic activity towards the electroreduction and electrocarboxylation of PhCH₂Cl compared with an Ag bulk electrode.

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Most organic halides have good chemical stability and strong toxic carcinogenic teratogenic effects, and can also pollute the environment. Among the investigations of electrochemical reduction and carboxylation of organic halides, both homogeneous and heterogeneous catalytic methods have been tried,^1–4 due to the negative reduction potential of organic halides. Although good current efficiencies could be obtained by using Ni, Pd, Co and other complexes,^5–8 the separation processes are very complicated. Nowadays, Ag is recognized as a powerful catalytic material for the electroreduction and electrocarboxylation of organic halides. Moreover, a higher electrocatalytic activity was obtained on nano Ag electrodes due to their higher number of active sites on the surface.^9–12 However, nanoparticles are likely to aggregate without suitable substrates, which will decrease the catalytic activity.

Zeolites are stable crystalline aluminosilicates with ordered micropores or/and nanosized cages, which makes them good supports for nanoscale metals.^13,14 Zeolite modified electrodes (ZMEs), for their unique size, shape and charge selectivity and high ion exchange capacity, have been used in electroanalysis and electrocatalysis.¹⁵ Many metal doped ZMEs, such as Ni-ZMEs,¹⁶ Cu-ZMEs,¹⁷ Pd-ZMEs¹⁸ and Fe-ZMEs,¹⁹ have been investigated for their catalytic activities. However, most of the preparation methods of ZMEs involve dip coating and secondary growth, which is complex and tedious.

In this paper, as part of our continuous study concentrating on ZMEs,²⁰ Ag-ZSM-5/SS has been prepared by a one-step synthesis method, and was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂-temperature programmed reduction (H₂-TPR), inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption isotherms. Ag nanoparticles could be formed during the growth of a ZSM-5 film on a stainless steel substrate. The electrocatalytic activities towards the electroreduction and electrocarboxylation of organic halides were tested by cyclic voltammograms (CV) and bulk electrolysis (BE).

The synthesis of Ag-ZSM-5/SS was carried out directly on stainless steel in an aqueous solution containing AgNO₃ with TPAOH (tetrapropylammonium hydroxide), NaOH, NaAlO₂ and TEOS (tetraethylorthosilicate) at 175 °C for 16 h. The presence of Ag, Si, Al, Na and O could be confirmed by energy dispersive X-ray spectroscopy (EDX) (Fig. S1†) and ICP. The XRD patterns of the synthesized ZSM-5/SS and Ag-ZSM-5/SS are illustrated in Fig. 1A. This indicates that the ZSM-5 film is successfully fabricated on the stainless steel substrate, and without the effect of Ag⁺ in the synthesis solution. In addition, four diffraction peaks at 2θ = 38.1, 44.3, 64.4 and 77.4°, which correspond to the (111), (200), (220) and (311) crystal faces of crystalline Ag, respectively, are observed for Ag-ZSM-5/SS. The results reveal that Ag is formed simultaneously during the in situ growth of the ZSM-5 film. Typical ZSM-5 crystals can be seen in the SEM images (Fig. 1B and C), which also indicates complete coverage over the entire substrate surface. The film thickness of Ag-ZSM-5/SS is characterized in Fig. 1D, and is about 10 μm and homogeneous. Moreover, many clustered particles, which might be Ag, are coated on ZSM-5 in the case of Ag-ZSM-5/SS (Fig. 1C). Black dots that should come from Ag nanoparticles with an average diameter of 9 nm can be observed by TEM (Fig. 1E). The particle size histogram is presented in the inset. The surface area of 32.9 m² g⁻¹ and pore volume of 0.0201 cm³ g⁻¹ for the synthesized Ag-ZSM-5, which are smaller than those of ZSM-5, were measured by N₂ adsorption–desorption isotherms (Fig. S2†). These smaller values may be caused by the dilution effect and/or pore blocking with Ag.

Fig. 1 XRD patterns (A) and SEM images (B–D) of ZSM-5/SS (B) and Ag-ZSM-5/SS (C and D); TEM image (E), H₂-TPR profile (F) and XPS spectra (G) of Ag-ZSM-5/SS.

To ascertain the state of Ag in Ag-ZSM-5/SS, H₂-TPR and XPS have been carried out. As shown in Fig. 1F, the peak at ca. 330 °C is ascribed to the reduction of Ag⁺ to Ag⁰, indicating the existence of Ag⁺. In combination with the XRD results, which show four diffraction peaks corresponding to Ag⁰, this indicates that Ag⁺ and Ag particles (Ag⁰) coexist in Ag-ZSM-5/SS. Four peaks, attributed to the Ag 3d_5/2 and Ag 3d_3/2 binding energies respectively, could be found in the XPS spectrum for the as-synthesized Ag-ZSM-5/SS (Fig. 1G). The two peaks at 367.2 eV and 373.2 eV are characteristic of Ag⁺, while the two peaks at 368.5 eV and 374.7 eV are characteristic of Ag⁰. However, in the case of Ag-ZSM-5/SS with etching, only the peaks of Ag⁺ can be seen, and the peaks of Ag⁰ disappeared. This may be because the Ag on the surface of zeolite fell off when the Ag-ZSM-5/SS was etched. All of these points indicate that the Ag⁺ exists in the zeolite skeleton and the Ag⁰ exists on the zeolite surface.

Conductivity is a key issue for a material used as electrode. The resistance of ZSM-5/SS is very high (>20 000 Ω sq⁻¹), due to the non-conductivity of normal zeolite. Thanks to the doped Ag particles running through the whole zeolite film during the in situ preparation step, the resistance of Ag-ZSM-5/SS is 4 mΩ sq⁻¹, which makes it possible to be used in electrochemistry. The electrocatalytic ability of the material has been tested towards the electroreduction and electrocarboxylation of organic halides. Here, PhCH₂Cl has been chosen as a model substrate. For comparison, an Ag bulk electrode, which is recognized as powerful electrocatalytic material, is checked too.

As shown in Fig. 2, an irreversible reduction peak could be observed for all electrodes. The peak potential for PhCH₂Cl reduction on the Ag electrode is −1.438 V (curve f). When Ag-ZSM-5/SS is used as the working electrode (curve c), the PhCH₂Cl reduction peak is shifted to an even more positive potential (−0.988 V), which suggests that the Ag-ZSM-5/SS has better electrocatalytic activity for the reduction of PhCH₂Cl than Ag. Here, Ag nanoparticles may be recognized as the catalytic sites, while the ZSM-5 provides more places for dispersing and stabilizing the Ag nanoparticles, which may promote the Ag nanoparticles’ catalytic activity. Moreover, the peak current varies linearly with υ^1/2, which confirmed the diffusion control for the electroreduction processes.

Fig. 2 Cyclic voltammograms of 2.6 mM PhCH₂Cl–0.1 M TEAP–MeCN on Ag-ZSM-5/SS (0.0125 M) (a), Ag-ZSM-5/SS (0.025 M) (b), Ag-ZSM-5/SS (0.05 M) (c), Ag-ZSM-5/SS (0.0725 M) (d), Ag-ZSM-5/SS (0.1 M) (e) and Ag (f) electrodes.

To further elucidate the relationship between the Ag nanoparticles and the catalytic activity, a series of Ag-ZSM-5/SS electrodes were prepared under different AgNO₃ concentrations. Similar diffraction peaks could be found by XRD (Fig. S3†), except for the intensity of the peaks. At a relatively low AgNO₃ concentration of 0.0125 M, few Ag nanoparticles are found on ZSM-5 (Fig. 3a). By increasing the AgNO₃ concentration gradually to 0.05 M, the silver nanoparticles become bigger and also increase in number (Fig. 3b and c). Aggregation of the Ag nanoparticles, which decreases the electrochemically active surface area (ECSA) rapidly, could be evidenced when the AgNO₃ concentration further increased to 0.1 M (Fig. 3d and e). Fig. 2 shows CVs of a series of Ag-ZSM-5/SS ZME materials in a solution of 2.6 mM PhCH₂Cl–0.1 M TEAP–MeCN. The highest peak current and most positive peak potential could be found on Ag-ZSM-5/SS with an AgNO₃ concentration of 0.05 M. Both a higher and lower AgNO₃ concentration will decrease the peak current and make the peak potential move to a more negative position. The result of the ECSA sequence is same as that of the PhCH₂Cl reduction peak potential, indicating that the high catalytic activity is coming from the large ECSA.

Fig. 3 TEM images of Ag-ZSM-5/SS samples prepared under different AgNO₃ concentrations. (a) Ag-ZSM-5 (0.0125 M), (b) Ag-ZSM-5 (0.025 M), (c) Ag-ZSM-5 (0.05 M), (d) Ag-ZSM-5 (0.075 M), (e) Ag-ZSM-5 (0.1 M).

Then, the Ag-ZSM-5/SS electrode was used for the electrocarboxylation of PhCH₂Cl, and compared with an Ag bulk electrode. As shown in Fig. 4, the highest electrocarboxylation yield of 61% could be obtained at −1.1 V on Ag-ZSM-5/SS, while a lower yield of 40% is obtained at a more negative potential of −1.5 V on the Ag bulk electrode. These data further prove that the electrochemical catalytic performance of the Ag-ZSM-5/SS is better than an Ag plate. After electrolysis, Ag-ZSM-5/SS was characterized by XRD and SEM (Fig. S4†), and the results obtained were almost the same as before, indicating that Ag-ZSM-5/SS has good stability.

Fig. 4 Electrocarboxylation of PhCH₂Cl on Ag-ZSM-5/SS and Ag electrodes.

Conclusions

In summary, an Ag-ZSM-5 zeolite membrane modified electrode was successfully prepared by a facile one-step method on a stainless steel substrate. Moreover, Ag-ZSM-5/SS exhibits remarkable electrocatalytic activity towards the electroreduction and electrocarboxylation of PhCH₂Cl.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21203066, 21373090, 21473060) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Materials, instruments, general procedure and characterization of Ag-ZSM-5/SS. See DOI: 10.1039/c6ra09141e

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