Zeolite-modified Electrodes Research Articles

Role of ZSM-5/AC hybrid support on the catalytic activity of Pd-Ag electrocatalysts towards ethanol oxidation: An experimental and kinetic study

Ethanol is a promising future fuel for Direct Alcohol Fuel Cells (DAFCs) and one of the significant half-cell reactions of this type of FC is ethanol oxidation reaction (EOR). However, the high cost, low selectivity of the catalyst towards complete oxidation of ethanol to CO2, and poor electrode kinetics are the key challenges in commercializing this technology. The way to mitigate these issues is to design affordable, stable, and highly efficient electrocatalysts. In this prospect, we developed a zeolite-carbon (ZSM-5/AC) supported Pd-Ag binary alloy catalysts via a simple co-impregnation followed by co-reduction method. The influence of ZSM-5/AC composite support and its composition on the activity of bimetallic Pd-Ag catalyst has been studied in detail. The PdAg/ZSM-5/AC electrocatalyst with 1:3 wt ratio of ZSM-5:AC exhibited better catalytic performance (10.9 mA cm−2) and tolerance towards intermediate species than PdAg/AC and Pd/AC electrocatalysts. The larger jf/jb (forward to backward current density ratio) of 2.8 for PdAg/ZSM-5/AC indicates lower carbonaceous species accumulation on the zeolite-modified electrode. Additionally, the material possessed a good long-term cyclic stability up to 500 cycles. The high activity observed for PdAg/ZSM-5/AC could be attributed to synergism between bimetallic catalysts and high surface area zeolite support. Overall, the study reveals that the Pd-Ag bimetallic and ZSM-5/AC are potentially efficient electrocatalyst and support, respectively, for the ethanol oxidation reaction in DEFC application.

Application of LTA zeolite-modified electrode for sensitive detection of retinoic acid in tap water.

An electrochemical method based on a Linde Type-A zeolite-modified glass carbon electrode (LTA/GCE) was introduced for the determination of retinoic acid (RA). LTA zeolite could be synthesized through a hydrothermal method and served as a commercial electrochemical sensor with high stability and sensitivity in electrochemical progress. The as-synthesized product was characterized by scanning electron microscopy (SEM), differential thermal analysis (DTA), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Under optimal conditions, a detection limit of 0.8 μM was obtained for RA with a linear range of 0.8-20.1 μM. This electrochemical method for determining RA was simpler and cheaper than previously reported methods. Furthermore, the modified electrode could be applied to the detection of RA in tap water, achieving a linear range of 1.4-15.0 μM with a detection limit of 1.4 μM and good recovery. The modified electrode designed by this method provided good selectivity, stability, and reproducibility for RA determination and reliable application for the analysis of RA in environmental water.

Fe(III)-exchanged zeolites as efficient electrocatalysts for Fenton-like oxidation of dyes in aqueous phase

Electrochemical oxidation, a powerful tool for the conversion of several of organic dye compounds using metal-exchanged zeolite modified electrodes, has shown great potential due to its capability of high efficiency degradation, a global priority for a sustainable environment. In this study, the role of the Fe(III)-exchanged-zeolite-modified electrodes using two different zeolite structures (such as FAU (Y) and MFI (ZSM-5)) on the Fenton-like oxidation of Congo Red dye (C32H22N6Na2O6S2) was investigated at room temperature. To clarify the electrocatalytic trend observed by constant potential electrolysis, the surface acidity of the catalysts prepared by ion-exchange method was determined by microcalorimetric measurements of ammonia adsorption. The different acid properties deriving from the presence of different cations (NH4+, H+ and Na+) in the case of MFI structure were found to enhance the ion-exchange capacity as well as the oxidation reaction. MFI catalysts - Fe(Na)ZSM-5, Fe(H)ZSM-5 and Fe(NH4)ZSM-5 - exhibited excellent activity and stability at the end of the electrolysis (within 60 min), with a total dye degradation and an higher mineralization on Fe(H)ZSM-5 (64% of TOC), compared with Fe(Na)Ynano, which takes twice as long (120 min) for total degradation, with 19% of TOC removal. This work provides an effective route for the development of stable Fe(III)-zeolite-modified electrodes for electro-Fenton oxidation, with a better stabilization of Fe3+ ions within the framework, without the aggregation of iron and the addition of H2O2, at room temperature.

Performance modelling of zeolite-based potentiometric sensors

Developing simple and cost-effective electrochemical sensors for widespread on-site application is of considerable practical importance in agriculture, environmental monitoring and food science. Among multiple sensing platforms, potentiometry is particularly effective in terms of simplicity, low cost and mass-production. This work is focused on a systematic analysis of the structure – performance relationship using chemometric techniques, which can be applied to sensor arrays with varying response patterns. The potentiometric sensitivity of zeolite-modified electrodes, containing thirteen synthetic and three natural zeolites, in aqueous solutions of Na+, K+, NH4+, Ca2+ and Mg2+ has been correlated with a range of zeolite characteristics using PCA and PLS modelling, thus demonstrating how structural and physical properties impact the performance of zeolite-modified potentiometric sensors. In addition to steric factors, e.g. zeolite pore size, the important characteristics governing the sensor performance are the Si/Al ratio and the presence of specific extraframework cations. For instance, K+ and Na+ show a strong effect on the potentiometric sensitivity towards Ca2+. The level of precision achieved by the PLS models indicates that semi-quantitative predictions are feasible. To improve the computational models, larger sets of data with a wider range of zeolite-modified sensors are necessary. The constituent materials of such sensors should have a set of well-defined properties, which can be controlled and tuned for a particular application. It is anticipated that synthetic rather than natural zeolites would satisfy such requirements.

Fabrication of a highly b-oriented MFI-type zeolite film-modified electrode with molecular sieving properties by Langmuir–Blodgett method

A novel highly compact and b-oriented MFI-type zeolite monolayer was obtained by assembling sec-butanol-modified rounded-coffin-shaped silicalite-1 microcrystals by Langmuir–Blodgett (LB) method. The LB monolayer was subsequently tested as a seed layer for the secondary growth of uniformly b-oriented and defect-free MFI zeolite films on stainless steel substrates and platinum electrodes. Three different methods were evaluated for identifying a suitable synthesis route to suppress the undesired a-oriented twin growth during the synthesis of MFI films. The crystal intergrowth and orientation of the MFI films were characterized by scanning electron microscopy and X-ray diffraction (XRD). The crystallographic preferred orientation indices and preferred orientation coverage values calculated using the XRD data were used to quantitatively determine the preferred orientation and surface coverage of the prepared zeolite films. The experimental results confirm that the low tetrapropylammonium hydroxide (TPAOH) method is the most effective route to suppress the undesired a-oriented twin growth, when the highly b-oriented LB monolayer was used as the seed layer for the secondary growth of the b-oriented MFI film. The highly b-oriented MFI-type zeolite film-modified electrodes were further tested as the working electrodes in electrochemical experiments. Cyclic voltammetry experimental results confirmed that the b-oriented MFI films fabricated at low TPAOH concentration were defect free and possessed a distinct molecular sieving property after calcination. The LB method, a highly reproducible and controllable method for the organization of zeolite crystals, can be further applied as an effective platform for the promotion and development of zeolite-modified electrodes as selective sensors.

An Efficient Amperometric Sensor for Hydrogen Peroxide by Using a Carbon Paste Electrode Modified with Cobalt Impregnated Zeolite

Cobalt impregnated zeolite-modified electrode was prepared by mixing cobalt–zeolite (Co–Z) and graphite powder with different percentages. Using the cyclic voltammetric technique, the electrochemical oxidation of hydrogen peroxide at such electrodes was investigated. Experiments on zeolite show that it is not electrochemically active towards hydrogen peroxide oxidation in NaOH solution. The presence of cobalt ions in the zeolite matrix, by soaking the electrode in an aqueous Co(NO 3)2 solution, markedly enhances the electrocatalytic activity which was found to depend on the cobalt content. On the other hand, the presence of zeolite and/or Co metals in the catalyst is essential; however, the electro-catalytic activity depends on different percentages of Co–Z. Under the selected conditions, the anodic peak current was linearly dependent on the concentration of the hydrogen peroxide in the range 0.03–9 and 0.006-0.1 mM with CV and amperometric methods, respectively. The detection limits (S/N=3) were also estimated to be 17 and 2.5 µM.

Understanding electrochemical stability of Cu+ on zeolite modified electrode with Cu-ZSM5

In the present work the system Cu-ZSM5 was prepared by aqueous ion-exchange method from zeolite H-ZSM5. The solids were characterized by X-ray diffraction, nitrogen physisorption, temperature-programmed reduction with hydrogen (TPR-H2) and electron paramagnetic resonance (EPR). These porous materials were mixed with poly (methacrylic acid methyl ester) and methyl acrylate (MA), and immobilized on a glassy carbon electrode in order to obtain the so-called zeolite-modified electrode (ZME). The as-prepared electrodes were characterized by infrared spectroscopy and electrochemical techniques as cyclic voltammetry and chronocoulometry. The presence of copper in the zeolite was confirmed by TPR-H2. The XRD results indicate not important structural changes in the zeolite ZSM5 due to copper incorporation. The EPR spectroscopy showed that copper in Cu-ZSM5 is as isolated form of Cu2+ ions. The results of nitrogen physisorption suggest that the Cu2+ cations are occupying the exchange sites in zeolite ZSM5. On the other hand, the IR spectroscopy revealed the presence of CO and CC groups in the mixture Cu-zeolite/polymer. The electrochemical profiles showed that reduction of Cu2+ to Cu0 occurs by two steps in presence of chloride due to stabilization of Cu+. The influence of anion in the electrolyte suggests that the redox processes Cu2+ to Cu+ and Cu+ to Cu0 occurs on the zeolite-modified electrode even at nitrate- and sulfate-based solutions at the glassy carbon/zeolite interface. This stabilization is mainly associated to the interactions between Cu+ and CC group and the zeolite framework acting as “electron reservoir”.

Zeolite-modified electrodes with analytical applications

Zeolite-modified electrodes (ZMEs) have been widely investigated because of their chemical, physical, and structural characteristics (shape, size, and charge selectivities; physical and chemical stabilities; high ion-exchange capacity; hydrophilic character; etc.), which make them of high interest in the design of electroanalytical systems. The paper presents recent literature data about fundamental and practical aspects related to the obtaining and applications of ZMEs. Some new ZMEs based on carbon paste incorporating soluble pheno-thiazinic dyes adsorbed on X-type zeolites are assessed comparatively, and the influence of some experimental parameters on the electrochemical response of these electrodes was investigated. The kinetic parameters for the heterogeneous electron-transfer process corresponding to the surface-immobilized mediators were determined, and all observed differences were used as evidence of the influence of the mediator structure and of the zeolite nature on the electrochemical activity of the new electrodes and on their electrocatalytic properties toward β-nicotinamide adenine dinucleotide (NADH) or ascorbic acid (AA) electro-oxidation and H2O2 electroreduction.

APPLICATIONS OF ZMEs IN ELECTROANALYSIS OF ORGANICS

In this paper, the role of Ag/Cu-modified zeolite in order to improve the electroanalytical performances of zeolite-modified electrodes (ZMEs) to detect organic pollutants is discussed. Two types of ZMEs, i.e., Ag/Cu-modified zeolite-expanded graphiteepoxy composite electrodes are discussed in comparison with expanded graphite-epoxy composite electrode. Function of the electroactivity of target species, two major characteristics of the zeolite was exploited, e.g., sorption capacity and catalytic properties of Cu/Ag-modified zeolite. Thus, for the electrochemical determination of 4-chlorophenol, the sensitivity was improved by applying a preconcentration step before electrochemical quantification. Ag-modified zeolite-expanded graphitecomposite electrode exhibited both the electrocatalytic and preconcentration effect toward urea oxidation that led to its determination in aqueous solution, and Cu-modified zeolite-expanded graphite-composite electrode allowed the electrochemical determination of glucose. These modified electrodes impart both catalytic properties to the electrode surface for the electrooxidation of organics and/or selective sorption capacity to preconcentrate the target analyte prior its sensitive electrochemical detection.

Electrochemical sensing of cadmium and lead ions at zeolite-modified electrodes: Optimization and field measurements

The scope for using a zeolite NH4-Y modified carbon paste electrode (ZYMCPE) in the voltammetric detection of Cd2+ and Pb2+ ions in ground water resources and industrial effluents is examined. The experimental conditions with respect to zeolite to graphite ratio in the electrode preparation, preconcentration time and electrolyte are optimized to maximize the sensitivity of the measurement. The detection limits are 1.0μgL−1 for Cd2+ ion and 3.6μgL−1 for Pb2+ ion. The excellent reproducibility of results obtained in the present study offers the possibility to develop commercial voltammetric sensors based on ZYMCPE for the two ions. The analytical results for two real samples (ground water and industrial effluent) agree very well with those obtained by atomic absorption spectroscopy.

Zeolite-Modified Carbon-Paste Electrode as a Selective Voltammetric Sensor for Detection of Tryptophan in Pharmaceutical Preparations

Several synthetic zeolites such as mazzite, mordenite, zeolite L, zeolite beta, and MCM-41 were tested as electrode modifiers in voltammetric determination of tryptophan. It was found that addition of zeolite beta to the carbon paste would generate the peak current of Trp because of its catalytic effect. The anodic peak currents were proportional to Trp concentrations in the range of 5.0 × 10−7 to 5.0 × 10−3 M. The detection limit was 1.0 × 10−7 M. The influence of several species, especially other amino acids, were tested. The proposed method was applied successfully to the determination of tryptophan in pharmaceutical formulations.

USING ZEOLITE-MODIFIED ELECTRODE FOR THE ELECTROCHEMICAL DETERMINATION OF 4-AMINOPHENOL FROM WATER

Two types of zeolite-modified electrodes, e.g., Ag-doped zeolite-expanded graphite-epoxy (AgZEG) and Cu-doped zeoliteexpanded graphite-epoxy (CuZEG) composite electrodes were tested for the application of the electrochemical determination of 4aminophenol (4-AP). The electrochemical techniques used for the electrochemical determination of 4-AP were cyclic voltammetry (CV) and chronoamperometry (CA). Some aspects related to the mechanistic behaviour are discussed. The electrochemical determination of 4-aminophenol on AgZEG and CuZEG composite electrodes was achieved in the concentration range 0.05 to 0.4 mM, with a sensitivity of about 0.2 mA*cm*mM. The electroanalytical performance of the determination of 4-AP on zeolite-modified electrodes was not influenced by the type of electrode.

A new method for electrocatalytic oxidation of ascorbic acid at the Cu(II) zeolite-modified electrode

A new method is developed for the catalytic oxidation of ascorbic acid at graphite zeolite-modified electrode, doped with copper(II) (Cu 2+A/ZCME). Copper(II) exchanged in zeolite type A acts as catalyst to oxidize ascorbic acid. The modified electrode lowered the overpotential of the reaction by ∼400 mV. First, the electrochemical behavior of copper(II), incorporated in the zeolite type A modified electrode, was studied. The results illustrate that diffusion can control the copper(II)/copper(0) redox process at the Cu 2+A/ZCME. Then, the behavior of electrocatalytic oxidation reaction for ascorbic acid was researched. The electrode was employed to study electrocatalytic oxidation of ascorbic acid, using cyclic voltammetry and chronoamperometry as diagnostic techniques. The diffusion coefficient of ascorbic acid was equal to 1.028 × 10 −5 cm 2 s −1. A linear calibration graph was obtained over the ascorbic acid with a concentration range of 0.003–6.00 mmol L −1. The detection limit (DL) of ascorbic acid was estimated as 2.76 × 10 −7 mol L −1. The relative standard deviations of 10 replicate measurements (performed on a single electrode at several ascorbic acid concentrations between 3.0 and 200 μmol L −1) were measured between 1.0 and 2.4%.

Influence of the Cation Na/Ca/Ag Ratio on the Ion Exchange Rate in Zeolite A-Modified Carbon Paste Electrodes

In contrast to previous electrochemical studies of silver-ion-exchanged zeolite-modified electrodes (Ag+—ZMEs), in which only two different compensating monocations in the zeolite have been used, in this work is presented a more complicated case of zeolite A containing three cation types, one electroactive soft cation and two nonelectroactive hard cocations: Ag+, Na+, and Ca2+, respectively. Na2Ca4Ag2—A, Na2Ca3Ag4—A, and Na2Ag10—A were obtained by ion exchange from the commercial sodium form of Lynde type-A (Na12—A). It was checked by 27Al and 29Si MAS NMR and X-ray diffraction (XRD) that no dealumination or loss of crystallinity occurred during the ion-exchange procedure. The exchanged zeolites were used to prepare zeolite-modified carbon paste electrodes (ZMCPEs). An alternative access to diffusion phenomena was achieved by a combination of open-circuit potential (OCP) and cyclic voltammetry (CV) measurements. Both techniques were performed in aqueous electrolytic medium 0.1 M NaNO3, both quiescent and ...

Application of a Novel Modified Electrode Based on NaY-Encapsulated Co(PAN) Complex

CoPAN complex has been entrapped in the supercages of Y zeolite and the redox properties of this zeolite-encapsulated complex were investigated by cyclic voltammetry with a new method for the preparation of carbon toray-zeolite-modified electrode. Formation of the CoPAN complex was ascertained by surface analysis (SEM, XRD), chemical analysis (CA), spectroscopy methods (FTIR and UV/vis) and cyclic voltammetry in aqueous medium with zeolite-modified electrodes. The cyclic voltammetry studies obtained with a zeolite-modified electrode shows evidence for electroactivity restricted to boundary associated CoPAN complex.

Accumulation and voltammetric determination of cobalt at zeolite-modified electrodes

Chemically modified electrodes based on zeolite-containing graphite pastes were constructed and evaluated as sensor electrodes for the voltammetric determination of trace cobalt in solution. Zeolite molecular sieves with pore sizes of 3, 4, and 5 A were all suitable for chemical deposition and subsequent voltammetric quantitation of trace Co(II). In order to cover a large range of parameters, the investigations were carried out using three zeolites (A, X, and Y), selected for their different cation-exchange capacities and ion-sieving properties. The analytical scheme, “preconcentration/voltammetric detection” for the determination of cobalt(II) species using zeolite-modified carbon paste electrodes (ZMCPE) was chosen. Compared to the corresponding unmodified electrodes, improvements in sensitivity were observed when using ZMCPEs due to analyte accumulation at the electrode’s surface by ion exchange in zeolite particles. The detection limit obtained for Co(II) was 3 ppm following 15 min of chemical deposition. A good correlation was found between the results obtained by ion exchange voltammetry and conventional ion-exchange characterization.

Redox properties of phenosafranine at zeolite-modified electrodes—Effect of surface modification and solution pH

Redox properties of cationic dye phenosafranine (3,7-diamino-5-phenylphenazenium chloride) (PS +) were studied at zeolite-modified electrodes using Zeolite-Y and NaZSM-5. The peak current and peak potential of phenosafranine-adsorbed zeolite were found to be influenced by the pH of the electrolyte solution. Observation of a second redox couple is suggested to be due to formation of new species at low concentration from the reduced phenosafranine at the zeolite-modified electrodes. Titanium dioxide nanoparticles encapsulated in the cavities of the zeolite or anchored on the external surface of the zeolite do not seem to affect the redox properties of adsorbed PS +. When the cyclic voltammograms are recorded immediately after the electrode is immersed into the solution, the redox potential of PS + is found to be sensitive to the nature of the zeolite surface. The peak potential shifts towards positive region under continuous cycles as the surface hydroxyl groups get protonated in acidic electrolyte solution thereby forcing the movement of dye molecules from the zeolite surface to the zeolite electrode solution interface. The electron transfer rate constants for the adsorbed dye at the electrode are calculated to be 2.5 ± 0.2 s −1 and 3.5 ± 0.2 s −1 for the zeolite-Y electrode and the ZSM-5 electrode, respectively by the Laviron equation.

Self‐Assembly Film of Zeolite Y Nanocrystals Loading Palladium on an Au Electrode for Electrochemical Applications

AbstractNano‐scale zeolite Y crystals were synthesized, and palladium nanoparticles were prepared in the supercage of the zeolite by “ship‐in‐a‐bottle” approach. A novel method to fabricate zeolite‐modified electrode (ZME) loading Pd nanoparticles was developed, in which the zeolite Y loading Pd2+ ions was self‐assembled on (3‐mercaptopropyl) trimethoxysilane‐attached Au surface to form the stable and density packed multilayers (SAM‐ZME). The structures of zeolite Y and the SAM‐ZME were investigated by using TEM, XRD and SEM techniques. Pd2+ ions in the SAM‐ZME were converted into Pd nanoparticles (Pdn0) by two steps consisting of the electrochemical reduction as well as the succeeding admission and release of CO. The redox couple [Fe(CN)6]3−/4− was used to probe the electron‐transfer barrier properties during self‐assembling process. Moreover, the special properties of the SAM‐ZME loading Pdn0 were studied by using cyclic voltammetry and CO‐probe in situ FTIR spectroscopy. The results illustrated that Pdn0 in the SAM‐ZME exhibits higher electrocatalytic activity for oxidation of adsorbed CO than that of ZME prepared in our previous study by zeolite coating method. The present study is of importance in design and preparation of SAM‐ZME, which poccesseses excellent properties for the immobilization of electrocatalysts or biomolecules.

Anionic Organic Guests Incorporated in Zeolites: Adsorption and Reactivity of a Meisenheimer Complex in Faujasites

Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provided in support of an adsorption mechanism in which a neutral ion pair (alkali metal ion + Meisenheimer anion) is the actual species being adsorbed. In fact it appears that there is remarkable increase in the association constant for the ion-pair complex within the zeolite cavities as compared to DMF solution. Although this mechanism of adsorption as an ion-pair complex has precedents in the adsorption of some inorganic salts, what is novel is the notable increase in the stability and persistence of the Meisenheimer anion (a anionic reaction intermediate) as a result of zeolite inclusion. Adsorbed Meisenheimer complex exhibits much lower reactivity towards electron acceptors, oxygen, and water. Cyclic voltammetry of zeolite-modified electrodes reveals for the Meisenheimer complex adsorbed on LiY a reversible redox peak that is not observed in solution and has been interpreted as arising from site isolation and stabilisation of the electrochemically generated species.

Electrocatalytic characteristics of uric acid oxidation at graphite–zeolite-modified electrode doped with iron (III)

A new method is developed for the catalytic oxidation of uric acid at graphite–zeolite-modified electrode doped with iron (III) (Fe 3+Y/ZCME). Iron (III) exchanged in zeolite Y act as catalyst to oxidize uric acid. First, the electrochemical behavior of iron (III) incorporated in the zeolite Y-modified electrode was studied. The results illustrate that diffusion controls the ferric/ferrous redox process at the Fe 3+Y/ZCME. Then, the behavior of electrocatalytic oxidation reaction for uric acid has researched. The charge transfer coefficient ( α) for uric acid was calculated as 0.51. The electrode was employed to study electrocatalytic oxidation of uric acid, using cyclic voltammetry, rotating disk electrode and chronoamperometry as diagnostic techniques. The rate constant for the catalytic reaction ( k) was also determined by above methods. The diffusion coefficient of uric acid was 1.54 × 10 −6 cm 2 s −1. A linear calibration graph is obtained over the uric acid concentration range of 0.1–10.0 mM. Finally, it has been shown that using the Fe 3+Y/ZCME, UA can be estimated by amperometry with high sensitivity and good limit of detection by high level of stability of electrode.