# Sol-gel process immobilization of TEMPO catalysts on multi-walled carbon nanotubes for electrocatalytic alcohol oxidation

Hichem Ichou, Garen Suna, Stéphane Diring, Clément Maheu, Fabrice Odobel

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Hichem Ichou, Garen Suna, Stéphane Diring, Clément Maheu, Fabrice Odobel. Sol-gel process immobilization of TEMPO catalysts on multi-walled carbon nanotubes for electrocatalytic alcohol oxidation. Chemical Communications, 2025, 61 (28), pp.5325-5328. 10.1039/d5cc00330j. hal-05018524

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# Sol-Gel Process Immobilization of TEMPO Catalysts on Multi-walled Carbon Nanotubes for Electrocatalytic Alcohol Oxidation

Received 00th January 20xx,
Accepted 00th January 20xx

Hichem Ichou,ª# Garen Suna,ªᵇ# Stéphane Diring,ª Clément Maheu,ᶜ Fabrice Odobelª*

DOI: 10.1039/x0xx00000x

Immobilizing catalysts on electrodes remains a crucial step in developing electrosynthetic and photoelectrocatalytic systems. An innovative method for immobilizing 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), functionnalized by an alkoxysilane group, around multi-walled carbon nanotubes *via* a sol-gel reaction is described. The system demonstrates impressive electrocatalytic performance of benzyl alcohols in carbonate buffer at pH 9.

The immobilization of molecular electrocatalysts on electrode surfaces is a key strategy in electrosynthesis and photoelectrocatalysis, as it merges the high efficiency and selectivity of molecular catalysts with their recoverability and often leads to improved overall performance.¹ The grafting of molecular catalysts on electrodes present several advantages such as: i) allow for the use of minimal amount of catalyst, reducing material costs, ii) facilitate the recovery and, thus, the recyclability and, iii) bypass the solubility limitations of the catalyst, since insoluble catalysts in the electrolyte can still be effectively used upon immobilization. However, developing such hybrid systems is still a challenging endeavor. The process must feature robust binding to the electrode to ensure stability, good electronic communication to allow quantitative charge extraction by the catalyst, and good accessibility to reagents to avoid mass transport limitations. In the present study, we report an innovative strategy to immobilize catalysts on multi-walled carbon nanotubes (simplified notation: CNTs) based on a sol-gel process. As a proof of concept, 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) functionalized by a triethoxysilane group (TEMPO-silane) was selected as model catalyst to explore the feasibility of this new approach (Scheme 1). Nitroxide derivatives, such as TEMPO, have garnered significant attention as invaluable catalysts for selective oxidation reactions in both laboratory and industrial contexts.² The low cost and ready availability of TEMPO

derivatives make them highly attractive catalysts for the clean and selective oxidation of alcohols into aldehydes, ketones, carboxylic acids, and nitriles, which are valuable functional groups in organic synthesis.²ᵃ, ²ᵇ

Besides the oxidation of organic molecules, TEMPO has been used in catalytic depolymerization processes of biopolymers such as lignin and cellulose.³ TEMPO was immobilized either by covalent bonds or by non-covalent interactions on various supports⁴ such as carbon-based materials (carbon nanotube,⁵ graphene oxide,⁶), polymers,⁷ metal oxides (indium tin oxide (ITO),⁸ titanium dioxide⁹) and silica.¹⁰

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Four main strategies to immobilize TEMPO on electrodes. m-ITO (NPs): mesoporous ITO nanoparticles.</figcaption></figure>

For example, metal oxides generally offer conductivity, transparency to visible light and facilitates the covalent bonding of TEMPO functionalized by alkoxysilane⁸ or phosphonic acid⁹ through covalent bonds. In this context, Reisner and co-workers developed an original electrolyzer consisting in a cathode for CO₂ reduction with concomitant alcohol oxidation at the anode.⁸ The latter is based on a mesoporous ITO film coated with a TEMPO catalyst upon condensation of a silatrane-anchor (Scheme 1a). In a different

<footnote>ª Nantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, France.</footnote>

<footnote>ᵇ Dr. G. Suna, Present address: Istanbul University, Department of Chemistry, 34134 Istanbul, Turkey</footnote>

<footnote>ᶜ Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France</footnote>

<footnote>#These authors contributed equally to this work</footnote>

<footnote>Supplementary Information available: [Experimental details; materials, instrumentations, methods and synthesis of the compounds; additional characterizations by electrochemistry, FT-IR, XPS and SEM]. See
DOI: 10.1039/x0xx00000x</footnote>
approach, Stahl and co-workers levered on π-π stacking interactions between pyrene and carbon nanotubes (CNTs) to develop an efficient and stable electrocatalytic system with TEMPO catalyst (Scheme 1b).⁵ª In another interesting example, Pagliaro and co-workers reported electrocatalytically active and stable electrode made of hybrid mesoporous silica functionalized with a TEMPO moiety exhibiting selective oxidation of alcohols (Scheme 1c).¹⁰ᵇ The present study proposes an innovative immobilization approach of TEMPO-silane on carbon nanotubes (CNTs) for an easy fabrication in mild conditions of a catalytic electrode that achieves significant activity (Scheme 1d). This method represents a notable advancement in the field of heterogeneous catalysis as it is quick, it eliminates the need for applying a negative electric bias to induce electrode polymerization and avoids high-temperature drying steps like in Pagliaro and colleagues' approach. Such a procedure is particularly valuable for applications involving photoelectrodes, which are sometimes incompatible with these conditions.

To investigate the potential polymerization of TEMPO-silane by hydrolysis condensation reactions around CNTs, we first prepared an ink consisting of CNTs dispersed in ethanol with 1% Nafion (details in the Supporting Information). This ink was drop-cast onto an FTO substrate, which was subsequently immersed in a TEMPO-silane solution for 15 minutes, rinsed with toluene, dried under a nitrogen flow and the edge of the CNT film is delineated by a deposit of epoxy glue (Scheme S1). A multiple-scan cyclic voltammetry (CV) experiment in carbonate buffer at pH 8.2 reveals the oxidation wave of TEMPO at 0.42 V vs. SCE, indicative of the reversible formation of N-oxoammonium species (Figure 1a). Notably, over approximately 10 scans, the current remained stable ($J = 0.5$ mA/cm²), suggesting effective immobilization of TEMPO-silane around CNTs. The same behavior was observed even at higher pH 10 (Figure 1a). Subsequently, the experiment was conducted in acetonitrile (ACN) where TEMPO-silane is more soluble (purple trace), and once again the signal of TEMPO-silane oxidation remains stable, *albeit*, anodically shifted by about 200 mV owing to the lower electrostatic stabilization of the N-oxoammonium in this less polar solvent. The observed linear relationship ($R²=0.99$) of anodic and cathodic peak currents with scan rate is consistent with a redox species adsorbed onto the electrode surface (Figure 1b).

<figure><img src="image_3.png" /></figure>

(ACN/TBAPF₆ 0.1M). (a) Peak current variation with the scan rate (b). Multi-scan CV control experiment of the FTO/CNT/TEMPO-NHAc in carbonate buffer (pH 10) (c). CVs of the FTO/CNT/TEMPO-silane at different amount of Nafion (d). NaHCO₃ 0.1M, pH 10, scan rate: 50mV/s, CE: Pt mesh, SCE as reference electrode. Surface: 0.28 cm².

To investigate the role of the alkoxysilane function in promoting the grafting of TEMPO-silane on CNT, a FTO/CNT electrode was immersed in a solution of 4-acetamido-TEMPO (TEMPO-NHAc), washed and dried under N₂ flow. The multi-scan CV experiments of this electrode features the redox wave of TEMPO-NHAc, but its intensity steadily decreases from 0.3 mA/cm² to 0.08 mA/cm² after 8 scans, proving the leaching of TEMPO-NHAc from the electrode (Figure 1c). A similar experiment in ACN electrolyte showed that redox feature of TEMPO-NHAc was also lost only after only 2 scans due to the absence of grafting and its higher solubility (Figure S1).

All in all, these experiments support that the alkoxysilane groups play a crucial role in mediating the stabilization of TEMPO-silane on CNT. Since alkoxysilane is unlikely to react with the sp² carbons of CNTs, the most plausible scenario is a hydrolysis-condensation reaction in the vicinity of CNTs induced by the acidity of Nafion. Nafion is a polymer containing sulfonic acid groups and a fluorinated backbone, displaying acidity in its protonated form.¹¹ Alkoxysilanes are known to polymerize under acidic or basic conditions *via* hydrolysis into silanol, followed by condensation to produce a polysiloxane layer.¹² Therefore, Nafion's Brönsted acidity likely catalyzes the polymerization of alkoxysilane groups around CNT surfaces, forming a polysiloxane shell responsible of the robust immobilization of TEMPO-silane.¹³ To test this hypothesis, CNT inks was mixed with 0%, 1%, and 5% Nafion and the above deposition procedure on FTO substrates was repeated (Figure 1d). CV experiments in carbonate buffer (pH 10) of inks with 1% or 5% Nafion exhibited the redox wave of TEMPO oxidation, with the highest current recorded with 5% Nafion, while that with 0% Nafion ink showed no redox signal, indicating the indispensable role of Nafion in TEMPO-silane immobilization.

The hydrolysis-condensation reaction of the alkoxysilanes has been well-studied using ²⁹Si NMR,¹²,¹⁴ and FTIR spectroscopy.¹⁵ However, due to the paramagnetic nature of the TEMPO-silane molecule and its low quantity in the hybrid material, NMR spectroscopy was unsuitable for characterizing the hybrid material. Therefore, a qualitative analysis using IR-ATR spectroscopy was conducted (Figures 2a and S2). TEMPO-silane was dissolved in methanol, Nafion was added and the mixture was dried. The crude mixture and pure TEMPO-silane were then characterized using IR-ATR spectroscopy to monitor spectral changes (Figure S2). A zoomed-in view of the 400-2000 cm⁻¹ region of TEMPO-silane (black) and the TEMPO Silane+Nafion mixture (red) showed that bands at 1560 cm⁻¹ and 1650 cm⁻¹, corresponding to the stretching vibrations of TEMPO's N-H and C=O bonds, are still present after polymerization (Figure 2a). Additionally, the intensity of peaks at 773 cm⁻¹ and 880 cm⁻¹, associated with the vibration of the
ethoxysilane and pure EtOH, decreased as hydrolysis progressed, while new bands appeared between 1130 cm⁻¹ and 1215 cm⁻¹, corresponding to Si-O-Si/Si-C bond vibrations.¹⁵ The primary broad band signal at 1060 cm⁻¹, attributed to the C-C-O skeletal motion of ethanol and the stretching vibration of Si-O-Et in ethoxysilane, diminished upon silica formation, giving way to a narrower peak at 1130 cm⁻¹ corresponding to Si-O-Si stretching modes.¹⁵a Despite being qualitative, disappearance of Si-OEt bonds and appearance of new bands attributed to Si-O-Si provides evidence of TEMPO-silane's hydrolysis and its condensation.

To investigate further the polymerization of TEMPO-silane with Nafion, X-ray photoelectron spectroscopy (XPS) was used on TEMPO-silane samples with and without Nafion (Figure S3). Similarly, to the IR-ATR measurements, a new emission line is observed in the O1s range at 535.7 eV. In the Si 2p energy range two doublets are distinguished, one at 102.0 and 102.6 eV and one at 102.7 eV and 103.3 eV. In the absence of Nafion these emission lines correspond to two silane environments.¹⁶ After Nafion addition, the ratio of the high energy doublet increases from 32 % to 45 %, this is explained by the polymerization process inducing Si-O-Si environment. The observation of these two phenomena in the O1s and Si2p energy range provides further evidence of the condensation of TEMPO silane following the addition of Nafion and support the results of IR-ATR.

<figure><img src="image_4.png" /><figcaption>Figure 2. (a) FT-IR spectra of the TEMPO-silane (black) and TEMPO silane+Nafion (red). (b). Zoom on the top-view SEM image of the CNT (1% Nafion)/TEMPO-silane substrate 15.0 kV. x20000.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3. EDX mapping of a CNT/TEMPO-silane sample.</figcaption></figure>

SEM was utilized to examine the size and morphology of this hybrid system. Figure 2b and Figure S4 presents top-view of the CNT (1% Nafion)/TEMPO-silane substrate, revealing the presence of a novel material with sizes ranging from 1 to 3 µm

on the surface of the CNTs. This is most likely polysiloxane, as EDX element mapping show that silicon, a major constituent of TEMPO-silane, predominantly matches the shape of the new material integrated into the CNTs (Figure 3). Taken altogether, these findings support that the observed material corresponds to the siloxane structure formed upon hydrolysis-condensation of TEMPO-silane.

A pH-dependent investigation of the electrocatalytic properties of the resulting hybrid electrode was conducted at 3 different pH values (8.2, 9 and 10). Current density increases with pH, and the onset potential of *para* methoxy-benzyl alcohol oxidation cathodically shifts from 0.4 V vs. SCE at pH 8.2 to 0.32 V vs. SCE at pH 10 (Figure S6). Controlled potential electrolysis (CPE) at the applied potential (E~app~) of 1.48 V vs. RHE displays a good stability at pH 8.2 and 9, with no significant loss in current density over one hour, while pH 10 showed a 30% reduction after one hour (Figure 4a). The lower stability at high pH could be attributed to the hydrolysis of the urea function⁸ or to the degradation of the silica backbone in high alkaline medium.¹⁰b The current densities increase with pH, indicating faster oxidation rates in basic conditions, in agreement with the well-documented mechanism of alcohol oxidation by N-oxoammonium with proton release.²a Product quantification by gas chromatography (GC) after CPE of the electrolyte composition indicated a high faradaic efficiency (FE ~99%) at all pHs, with 4-methoxybenzylaldehyde as the sole product demonstrating high selectivity and efficiency of this electrocatalytic system (Table 1 and Figure S9).

Naturally and in agreement with previous reports,⁵a, ⁸ the rate increases with the pH 10 owing to the release of protons upon oxidation of alcohol (Scheme S2). After one hour of CPE, outstanding TON and TOF values of 25,000 and 6.1 s⁻¹, respectively, were achieved at pH 9, going beyond the state of art.⁴b, ⁵a, ⁸ Figure 4b illustrates that the current density logically increases with MeOBenzOH concentration, while the onset potential remains unchanged. The linear fitting of the peak current with respect to alcohol concentration confirms that the reaction rate is first-order (Figure S7).

<figure><img src="image_6.png" /><figcaption>Figure 4. (a) CPE of the FTO/CNT/TEMPO-silane at +1.48 V vs. RHE at different pHs in 0.1 M carbonate buffer; (b) CVs of the FTO/CNT/TEMPO-silane at different MeOBenzOH concentration. 50 mV/s of scan rate. 16h of catalyst soaking. 50mV/s scan rate, CE: Pt mesh, SCE as reference electrode. Surface: 0.28 cm².</figcaption></figure>

Then, the catalyst soaking time of the CNT film into the TEMPO-silane solution was investigated from 15 minutes to 48 hours and it was observed that optimum was reached after 16 hours (Figure S8). All durations yielded similar stability, with
constant FE and aldehyde as the sole oxidation product (Table S1).

<table><caption>Table 1. Electrochemical data after 1-hour CPE of FTO/CNT/TEMPO-silane. E<sub>app</sub>= 1.48 V vs. RHE.</caption><thead><tr><th>pH</th><th>Charge (C)</th><th>FE (%)</th><th>TON</th><th>TOF (s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>8.2</td><td>1.8±0.2</td><td>98±2</td><td>15 500 ± 2100</td><td>4.3±1.1</td></tr><tr><td>9</td><td>2.9±0.2</td><td>98±2</td><td>25 000 ± 3700</td><td>6.1±1.2</td></tr><tr><td>10</td><td>3.97±0.5</td><td>99±2</td><td>34 000 ± 4000</td><td>9.4±1.1</td></tr></tbody></table>

Catalyst loading: 0.6 nmol; NaHCO<sub>3</sub> 0.1M, 50 mM MeOBenzOH. 16h of catalyst soaking. Ref. SCE. CE Pt mesh. Surface 0.28 cm<sup>2</sup>.

To compare our system with literature, benzyl alcohol (BenzOH) was used as another substrate. In a 1-hour CPE experiment at 0.65 V vs. SCE at pH 9 BenzOH showed 1.5 mA/cm<sup>2</sup> current density. This trend aligns with alcohol oxidation potential, where electron-donor groups enhance the oxidation rate. Particularly through the nucleophilic attack of the oxygen of the alcohol on the N-oxoammonium cation.<sup>10a</sup> To conclude, this study discloses a novel approach to immobilize TEMPO catalyst within CNTs for alcohol oxidation in aqueous medium according to an original sol-gel process. The resulting hybrid electrode was characterized by CV, IR-ATR, XPS, and SEM, demonstrating impressive electrocatalytic performance metrics with 91% faradaic efficiency, a TON of 12,000, and a TOF of 3.3 s<sup>-1</sup> at pH 9 with benzyl alcohol. This system is approximately 5 times faster than the CNT/TEMPO pyrene system<sup>5a</sup> and the ITO/TEMPO-silane system<sup>8</sup> under comparable conditions (Table S2). The functionalization of molecules with alkoxysilane groups can be readily achieved using inexpensive, commercially available alkoxysilane reagents through amidation, click chemistry, or reactions with a thiocyanate like in this example. This innovative immobilization strategy holds great potential for application to other catalysts functionalized with alkoxysilane groups, paving the way for the development of electrocatalytic hybrid systems tailored for proton reduction, CO<sub>2</sub> reduction, or water oxidation.

## Author Contributions

Hichem Ichou synthesized the compounds and made preliminary electrochemical studies. Garen Suna carried out the detailed electrochemical studies. Stéphane Diring supervised HI and GS. Clément Maheu realized and analyzed the results of the XPS study. Fabrice Odobel designed the study, proposed the experiments, analyzed the results and wrote the first draft of the manuscript. All authors provided critical feedback and helped in shaping the research, analyzing the results and preparing the final manuscript.

## Acknowledgments

This work received financial support from the French National Agency for Research for the project PECALO (ANR-20-CE05-0019-01) and for the EUR LUMOMAT project (Investments for the Future

program ANR-18-EURE-0012). C. Maheu thanks the Marie Skłodowska-Curie action for the project OMATSOLFUEL (n° 101105640). The authors greatly acknowledge AMaCC platform's team (CEISAM UMR CNRS 6230, University of Nantes) for their chromatography analyses contributions to this work. XPS measurements were performed using the IMN's characterization platform, PLASSMAT, Nantes, France.

## Conflicts of interest

There are no conflicts to declare.

## Data availability

The data supporting this article have been included as part of the Supplementary Information.

## Notes and references

1. (a) R. M. Bullock, A. K. Das and A. M. Appel, *Chem. Eur. J.*, 2017, **23**, 7626-7641; (b) D. Grammatico, A. J. Bagnall, L. Riccardi, M. Fontecave, B.-L. Su and L. Billon, *Angew. Chem. Int. Ed.*, 2022, **61**, e202206399; (c) L. Rotundo, A. Barbero, C. Nervi and R. Gobetto, *Catalysts*, 2022, **12**, 1448.

2. (a) J. E. Nutting, M. Rafiee and S. S. Stahl, *Chem. Rev.*, 2018, **118**, 4834-4885; (b) D. Leifert and A. Studer, *Chem. Rev.*, 2023, **123**, 10302-10380; (c) S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan and D. H. B. Ripin, *Chem. Rev.*, 2006, **106**, 2943-2989.

3. (a) M. Rafiee, M. Alherech, S. D. Karlen and S. S. Stahl, *J. Am. Chem. Soc.*, 2019, **141**, 15266-15276; (b) S. Gharekhani, Y. Zhang and P. Fatehi, *Prog. Energy Combust. Sci.*, 2019, **72**, 59-89; (c) A. Isogai, T. Hänninen, S. Fujisawa and T. Saito, *Progress in Polymer Sci.*, 2018, **86**, 122-148.

4. (a) H. A. Beejapur, Q. Zhang, K. Hu, L. Zhu, J. Wang and Z. Ye, *ACS Catal.*, 2019, **9**, 2777-2830; (b) E. Megiel, *Adv. Colloid Interface Sci.*, 2017, **250**, 158-184.

5. (a) A. Das and S. S. Stahl, *Angew. Chem. Int. Ed.*, 2017, **56**, 8892-8897; (b) Y. Wang, X. Song, S. Shao, H. Zhong and F. Lin, *RSC Adv.*, 2012, **2**, 7693-7698.

6. A. J. Shakir, D. C. Culita, J. Calderon-Moreno, A. Musuc, O. Carp, G. Ionita and P. Ionita, *Carbon*, 2016, **105**, 607-614.

7. (a) D. P. Hickey, R. D. Milton, D. Chen, M. S. Sigman and S. D. Minteer, *ACS Catal.*, 2015, **5**, 5519-5524; (b) N. Mohebbati, A. Prudlik, A. Scherkus, A. Gudkova and R. Francke, *ChemElectroChem*, 2021, **8**, 3837-3843; (c) S. D. Place and P. Kavanagh, *Electroanal.*, 2024, **36**, e202300195.

8. M. A. Bajada, S. Roy, J. Warnan, K. Abdiaziz, A. Wagner, M. M. Roessler and E. Reisner, *Angew. Chem. Int. Ed.*, 2020, **59**, 15633-15641.

9. M. Omri, M. Becuwe, M. Courty, G. Pourceau and A. Wadouachi, *ACS Appl. Nano Mater.*, 2019, **2**, 5200-5205.

10. (a) B. Karimi, M. Rafiee, S. Alizadeh and H. Vali, *Green Chem.*, 2015, **17**, 991-1000; (b) B. Karimi, M. Ghahremani, R. Ciriminna and M. Pagliaro, *Org. Process Res. Dev.*, 2018, **22**, 1298-1305.

11. K. A. Mauritz and R. B. Moore, *Chem. Rev.*, 2004, **104**, 4535-4586.

12. L. L. Hench and J. K. West, *Chem. Rev.*, 1990, **90**, 33-72.

13. G. Gelbard, *Ind. Eng. Chem. Res.*, 2005, **44**, 8468-8498.

14. X. Shi, D. Graiver and R. Narayan, *Silicon*, 2012, **4**, 109-119.
15. (a) H. Jiang, Z. Zheng, Z. Li and X. Wang, *Ind. Eng. Chem. Res.*, 2006, **45**, 8617-8622; (b) M. P. Huber, S. Kelch and H. Berke, *Int. J. Adhes. Adhes.*, 2016, **64**, 153-162.

16. P. M. Dietrich, S. Glamsch, C. Ehlert, A. Lippitz, N. Kulak and W. E. S. Unger, *Appl. Surf. Sci.*, 2016, **363**, 406-411.