Elsevier

Electrochimica Acta

Volume 354, 10 September 2020, 136624
Electrochimica Acta

TEMPO mediated electrocatalytic oxidation of pyridyl carbinol using palladium nanoparticles dispersed on biomass derived porous nanoparticles

https://doi.org/10.1016/j.electacta.2020.136624Get rights and content

Abstract

Remarkable electrocatalytic property of Pd nanostructures dispersed on CNSareca coated CFP electrode towards TEMPO mediated electrooxidation of pyridyl carbinol was reported for the first time. Carbon nanospheres (CNSs) derived from Areca catechu decorated with Pd nanoparticles were coated on carbon fiber paper (CFP) and was employed for electrooxidation of pyridyl carbinol in aqueous acidic medium. An environmentally benign and economic strategy was utilized for the preparation of CNSs obtained from Areca catechu. The physical characterizations, electronic state and chemical composition of the modified electrode were studied using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) techniques were used for analyzing the morphology of modified electrode. The electrochemical characterizations of the modified electrodes were performed by Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS). Pd decorated CNSareca dispersed on CFP electrode has exhibited strong electrocatalytic activity towards TEMPO mediated oxidation of pyridyl carbinol.

Introduction

Nitrogen-containing compounds possess excellent biological properties, because of which they are used as structural components for pharmaceuticals and agrochemicals. The demand for a number of nitrogen-containing chemicals, ranging from simple structured compounds as pyridine derivatives to complex compounds used as pharmaceutical ingredients is growing rapidly [1]. Among the nitrogen containing compounds, pyridine derivatives are produced in large quantities for various applications such as herbicides, insecticides, vitamins like nicotinic acid and nicotinic acid amide, pharmaceuticals and adhesives. Pharmaceutically important compounds produced by oxidation of molecules like pyridyl carbinol, pyridyl methane, pyridyl nitriles, thiophene methanol have the potential to meet the requirements of pharmaceutical industries. However, production of these significant molecules face challenges with respect to the use of environmentally benign oxidants offering high selectivity and good yield in a single step [2]. Recently, carbon-based materials such as diamond, nanotubes, nanofibers, graphene and nanospheres have triggered substantial interest in developing various energy conversion devices owing to their low density, specific and high surface areas, and high flexibility along with their increased stability and mechanical strength [3]. Among these listed materials, carbon nanospheres (CNSs) are attractive candidates for developing functional materials to be applied in different areas of modern science and technology due to their simple production methods, high thermal stability, large packing density and excellent electronic and ionic conductivities [4]. ∖These materials were used as reinforcement substances for anode materials in Na/Li-ion batteries, supercapacitors, energy storage materials and many non-energy applications including novel supports of catalytic systems, adsorbents, cell delivery systems, multiphoton bioimaging and photoluminescence applications [5]. Moreover, CNSs have already been explored as materials for electrode assemblies in polymer electrolyte membrane fuel cells (PEMFCs) because of their high surface area and their role in serving as the core for metal nanoparticles [6]. Also, CNSs have been used as electrode materials for fabricating energy storage devices such as supercapacitors [[7], [8], [9], [10], [11]].

On the other hand, metal nanoparticles dispersed on electrically conducting substrates have received considerable attention for applications as biosensors, electrochemical capacitors, anti-corrosive coatings and electrocatalysts [2]. Palladium nanostructures and nanocomposites are studied intensively because of their significance as catalytic materials [[12], [13], [14], [15], [16], [17]], sensors [18,19] and hydrogen storage materials [[20], [21], [22], [23]]. They have emerged as an effective alternate to Pt for electroxidation of organic molecules. Therefore, it is necessary to exploit the advantages of high surface-to-volume ratio and enhanced catalytic activity coming from porous nanostructures. Henceforth, a method for the preparation of Pd nanostructures that is either solution-based or coated on suitable substrates is of considerable interest among various research groups [[24], [25], [26], [27], [28], [29], [30]]. Carbon-based supports are generally used for dispersing the nanoparticles in order to achieve high catalytic activity. In this regard, CNSs can be used as an effective substitute to the carbon supports like graphene, carbon nanotubes, owing to their uniform dispersive ability on the electrode substrate and their porous nanostructures can attract the organic molecules towards the electrode surface.

The oxidation of alcohols to corresponding carbonyl compounds is an important transformation in organic chemistry and one of the important functional group conversions in synthetic chemistry [31]. The worldwide annual production of carbonyl compounds are mostly produced by oxidation of alcohols [32]. The oxidation of alcohols is conventionally performed using transition metal oxidants (such as K2Cr2O7, KMnO4, SeO2) [33]. However, these oxidants generate equivalent metal wastes and pose serious threat to the environment. In addition, oxidation reactions are usually carried out in halogenated organic solvent medium which are not environment friendly [[34], [35], [36]]. Therefore, there is a great demand for designing green, clean, efficient and simple synthetic routes that can employ new catalytic materials instead of toxic oxidants for oxidation of alcohols. Electrochemical organic synthesis comprises of promising environment friendly approaches when compared to conventional organic synthesis owing to the use of electrons as reagents rather than conventional toxic oxidizing and reducing reagents [37]. With the use of electron as the reagent, transformations can be carried out either directly or indirectly [38,39]. As a substitute to conventional oxidants, stable nitroxyl radicals have received great attention for application as oxidation catalysts. One of the most prominent representatives of the nitroxyl radicals was found to be 2,2,6,6- Tetramethylpiperidinyl-l-oxyl (TEMPO). The remarkable features of TEMPO is its stability in aqueous and non-aqueous media due to the presence of steric hindrance around nitroxyl moiety. An acidic medium is necessary to make the catalytic system effective; the oxoammonium salt (TEMPO+) is formed by disproportionation of TEMPO catalyzed by the acidic medium [40]. TEMPO can be reversibly oxidized by electrochemical methods [41] or by means of oxidants such as H2O2, O2 and metal halides, to generate an active oxoammonium species (TEMPO+), which can be obtained by a chemical or electrochemical one-electron oxidation of TEMPO. This species is an ideal oxidant for selective oxidation of alcohols to corresponding aldehydes or ketones without undergoing over-oxidation forming acids [[42], [43], [44]].

In the present investigation, excellent electrocatalytic activity of Pd nanoparticles dispersed on CNS film coated carbon fiber paper (CFP) electrode was demonstrated for TEMPO mediated electrooxidation of pyridyl carbinol in aqueous acidic medium. The reported methods in the literature generally involve multiple steps whereas the present method follows single step for the deposition of Pd nanoparticles on CNSareca/CFP to form a stable nanocomposite modified electrode. The process is facile, less expensive and can be scaled up easily. To the best of authors’ knowledge, this is a first report displaying the remarkable electrocatalytic property of Pd nanostructures dispersed on CNSareca coated CFP electrode towards TEMPO mediated electroxidation of pyridyl carbinol. The proposed work is schematically illustrated in Scheme 1.

Section snippets

Materials and methods

Pyridyl carbinol, Palladium chloride (PdCl2), TEMPO and polyvinylidene difluoride (PVDF) binder were procured from Sigma Aldrich-Merck company. Analytical grade potassium ferricyanide (K3 [Fe(CN)6]), potassium ferrocyanide (K4 [Fe(CN)6]‧3H2O), Sulphuric acid (H2SO4) and N-methyl-2-pyrrolidone (NMP) were obtained from SD Fine-Chem Pvt. Ltd., India. Distilled water (DW) was used for preparation of all the aqueous solutions. Areca nut biomass were collected from areca plantations at Sirsi,

Characterization of CNSareca

Thermogravimetric analysis (TGA) of the precursor, Areca catechu sample is shown in Fig. 1a. The weight loss initially is due to removal of moisture and water content. Major weight loss of the precursor is observed from 200 °C to 400 °C. Cellulose and hemicelluloses, the major components of biomass, start to decompose at ∼400 °C [46,47]. Further weight loss is only gradual and minimal after 500 °C, which could be due to removal of lignin. Hence pyrolysis of Areca catechu at 550 °C was done to

Conclusions

The present study reports a facile, highly sensitive and selective electrochemical method for the oxidation of pyridyl carbinol mediated by TEMPO. Electrochemically deposited Pd nanoparticles on CNSareca/CFP is majorly crystalline with uniform dispersion on the surface of the electrode. By using CV technique, the electrocatalytic activity of Pd-CNSareca/CFP electrode towards TEMPO mediated oxidation of pyridyl carbinol was studied. The modified electrode surface attract the molecules of pyridyl

Credit author statement

Agnus T Mathew: Electrocatalytic oxidation studies, drafting. Vinay S Bhat.: CNS synthesis and characterization and drafting. Akshaya K B: Writing the manuscript, characterization and analysis. Supriya S: XRD, TEm, SEM analysis and drafting. Maiyalagan T: XPS characterization and analysis. Anitha Varghese: Supervision, coceptualization, manuscript editing, analysis. Gurumurthy Hegde: Supervision, coceptualization, manuscript editing, analysis.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

All the authors of this manuscript have declared that there is no any conflict of interest.

Acknowledgements

The authors are grateful to CeNSE, Indian Institute of Science, Bengaluru, funded by the Ministry of Electronics and Information Technology (MeitY), Govt. of India for SEM and XRD characterizations. Gurumurthy Hegde would like to thank AISTDF-SERB for providing ASEAN grant with file number IMRC/AISTDF/CRD/2018/000019.

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