Electrochemical NMR Research Articles

Probing Spatially-Resolved Pt Distribution in PtRu Nanoparticles with 195Pt EC-NMR

We report the first quantitative analysis of spatially resolved local Pt distribution in graphite and carbon nanofibers supported PtRu alloy nanoparticles using slow-beat measurements by in situ 195Pt electrochemical NMR. While the two samples had the same nominal 1:1 atomic ratio and similar average particle size, it was found that the former had a rather homogeneous Pt distribution but not the latter, which had Pt segregation at the core and depletion at the surface of the nanoparticles.

Seven-Coordinate Iron Complex as a Ditopic Receptor for Lithium Salts: Study of Host−Guest Interactions and Substitution Behavior

Interactions between the seven-coordinate tweezerlike [Fe(dapsox)(H2O)2]ClO4 complex (H2dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with different lithium salts (LiOTf, LiClO4, LiBF4, and LiPF6) in CH3CN have been investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has been demonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensing capability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the order LiOTf < LiClO4 < LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants for the Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host-guest interaction with the reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of the hydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at -15 degrees C in acetonitrile in the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k(1) = 411 +/- 14 M(-1) s(-1); DeltaH(1) not equal = 9 +/- 2 kJ mol(-1); DeltaS1 not equal = -159 +/- 6 J K(-1) mol(-1); k(2) = 52 +/- 1 M(-1) s(-1); DeltaH(2) not equal = 4 +/- 1 kJ mol(-1); DeltaS(2) not equal = -195 +/- 3 J K(-1) mol(-1). The very negative DeltaS not equal values are consistent with an associative (A) mechanism. Under the same conditions but with 0.2 M LiOTf, k1Li and k2Li are 1605 +/- 51 and 106 +/- 2 M(-1) s(-1), respectively. The increased rate constants for the {[Fe(dapsox)(CH3CN)2] x LiOTf}+ adduct are in agreement with an associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction between the lithium salt and the anion-substituted forms, [Fe(dapsox)(CH3CN)(NCS)] and [Fe(dapsox)(NCS)2]-, of the complex. These experiments demonstrate that in nonaqueous media lithium salts cannot be simply used as supporting electrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that the negatively charged alpha-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereas the two terminal amide groups bind the anion via hydrogen bonding.

Electrochemical and 19F NMR Studies of Ionic Associations in Low Molecular Weight Analogs of PEO-Based Polymeric Electrolytes

not Available.

Electrochemical and solid-state NMR studies on LiCoO 2 coated with Al 2O 3 derived from carboxylate-alumoxane

The surface of LiCoO 2 cathodes was coated with various wt.% of Al 2O 3 derived from methoxyethoxy acetate-alumoxane (MEA-alumoxane) by a mechano-thermal coating procedure, followed by calcination at 723 K in air for 10 h. The structure and morphology of the surface modified LiCoO 2 samples have been characterized with XRD, SEM, EDS, TEM, BET, XPS/ESCA and solid-state 27Al magic angle spinning (MAS) NMR techniques. The Al 2O 3 coating forms a thin layer on the surface of the core material with an average thickness of 20 nm. The corresponding 27Al MAS NMR spectrum basically exhibited the same characteristics as the spectrum for pristine Al 2O 3 derived from MEA-alumoxane, indicating that the local environment of aluminum atoms was not significantly changed at coating levels below 1 wt.%. This provides direct evidence that Al 2O 3 was on the surface of the core materials. The LiCoO 2 coated with 1 wt.% Al 2O 3 sustained continuous cycle stability 13 times longer than pristine LiCoO 2. A comparison of the electrochemical impedance behavior of the pristine and coated materials revealed that the failure of pristine cathode performance is associated with an increase in the particle–particle resistance upon continuous cycling. Coating improved the cathode performance by suppressing the characteristic structural phase transitions (hexagonal to monoclinic to hexagonal) that occur in pristine LiCoO 2 during the charge–discharge processes.

High mobility I − / I 3− redox couple in a molecular plastic crystal: A potential new generation of electrolyte for solid-state photoelectrochemical cells

A series of new electrolyte materials based on a molecular plastic crystal doped by different iodide salts together with iodine have been prepared and characterized by thermal analysis, ionic conductivity, electrochemical and solid-state NMR diffusion measurements. In these materials, the plastic crystal phase of succinonitrile acts as a good matrix for the quaternary ammonium based iodides and iodine and appears to act in some cases as a solid-state “solvent” for the binary dopants. The materials were prepared by mixing the components in the molten state with subsequent cooling into the plastic crystalline state. This resulted in waxy-solid electrolytes in the temperature range from − 40 to 60 °C. The combination of structural variation of the cations, and fast redox couple diffusion (comparable with liquid-based electrolytes), as well as a high ionic conductivity of up to 3 × 10 − 3 S cm − 1 at ambient temperature, make these materials very attractive for potential use in solid-state photoelectrochemical cells.

Very stable, highly electroactive polymers of zinc(II)-5,15-bisthienylphenyl porphyrin exhibiting charge-trapping effects

Two new thiophene-substituted porphyrins and their zinc complexes were synthesized and electropolymerized via thiophene units on different electrode surfaces. Electrochemical characterizations show that the resultant polythiophenemetalloporphyrin hybrid materials are highly electroactive and exhibit remarkable higher electrochemical stability. Interestingly, they exhibit a charge-trapping effect. A mechanism for the observed charge-trapping phenomenon was proposed and supported by using model compounds. The deposited polymer films on different electrode surfaces exhibit a homogeneous morphology and possess good processability (soluble in polar solvents such as DMSO or DMF). Besides electrochemical method, UV/vis, NMR, FTIR, TEM and SEM were employed to characterize the new hybrid material.

Study on the reaction of 2,3-dihydroxybenzoic acid with molybdenum in aqueous solution. I. Synthesis and characterization of the oligomeric complexes formed

Dimeric or oligomeric oxo-complexes of Mo(VI) with 2,3-dihydroxybenzoic acid were prepared in aqueous solutions in the presence or not of K 2S 2O 5 (acting as a reducing agent) in various conditions. The complexes were found to contain the cis - ( Mo 2 O 5 ) 2 + core and the ligands in the catecholate, semiquinonate or mixed valence oxidation form, depending on the reaction conditions and especially on the presence or not of the reductant. The isolated complexes in the presence or absence of reductant and the oxidation products in solution in the presence of air were studied via elemental, thermogravimetric and electrochemical analysis, Infrared, Raman, NMR and ESR spectroscopies and Electrospray Mass Spectra. The general molecular formula for the complexes is {[(PPh 4) 2(Mo 2O 5L 2X 2] · xH 2O)} n , where the coordinated ligand’s L oxidation form varies and X involves coordinated water or hydroxyl group depending on the ligand oxidation state.

Organic Phase Cyclopentadienylnickelthiolate Sensor System for Electrochemical Determination of Sulfur Dioxide

AbstractA series of cyclopentadienylnickelthiolate complexes, [Ni(PBu3)(η5‐C5H5)(SC6H4X‐4)] (X=F, Cl, Br, NH2), were shown to express stable reversible electrochemical properties even after formation of SO2 adducts in organic phase consisting of argon purged CH2Cl2/0.1 M [n‐Bu4N][BF4]. The formal potentials (E°′) values of the compounds ranged from 265 to 431 mV/Ag‐AgCl depending on the para substituent of the benzene thiolate ligand. Electrochemical, UV‐vis and 1H NMR spectroscopic analyses show that the formation of SO2 adducts causes the perturbation of the electronic density of the nickel metal center, indicated by shifts in the E°′ values of the NiII/III redox couple that is dependent on SO2 concentration. The detection limits of the resulting organic phase electrochemical gas sensor system was as low as 0.56 ppm SO2 for the fluoro complex, while the linear range was as high as 700–2000 ppm SO2 for the amino complex.

Binding energy of ruthenium submonolayers deposited on a Pt(111) electrode

We investigated the 3d5/2 core-level binding energy of Ru in Ru nanoislands spontaneously deposited on a Pt(111) electrode [Pt(111)/Ru], and the binding energies of 3d5/2 iodine and 1s CO adsorbed on Pt(111)/Ru by the use of X-ray photoelectron spectroscopy. Both iodine and CO were used as surface probes of the electronic properties of Pt(111)/Ru. Little difference was found in the binding energy of Ru in Pt(111)/Ru and in Ru(0001). However, the addition of Ru to Pt(111) induces major changes in the core-level binding energies of chemisorbed iodine and CO as referenced to those adsorbed on Ru(0001). We conclude that the iodine 3d5/2 and CO 1s C core levels experience higher electronic charge on Pt(111)/Ru than on Ru(0001), suggesting a charge transfer from Pt to Ru, or to a Ru-I “surface molecule” within the deposit. The charge transfer from Pt to Ru is in agreement with the result of previous in situ electrochemical NMR investigations [P.K. Babu, H.S. Kim, A. Wieckowski, E. Oldfield (2003) J. Phys. Chem. B 107:7595] and confirms the general trend of reduction in the density of states of Pt due to alloying with Ru [J. McBreen, S. Mukerjee (1995) J. Electrochem. Soc. 142:3399]. Theoretical calculations are in progress to further interpret the origin of the binding-energy shifts observed in this study.

Synthesis and structural characterisation of redox-responsive N-ferrocenoyl amino acid esters; the X-ray crystal structure of N-ferrocenoyl alanine methyl ester; electrochemical anion recognition and 1H NMR complexation studies

The N-ferrocenoyl amino acid ester derivatives FcCOR {Fc=(η 5-C 5H 5)Fe(η 5-C 5H 4)} where R=Gly(OMe) 1, Gly(OEt) 2, Gly(OBn) 3, l-Ala(OMe) 4, l-Ala(OEt) 5, l-Leu(OMe) 6, l-Leu(OEt) 7, l-Leu(OBn) 8, l-Phe(OMe) 9 and l-Phe(OEt) 10, were prepared by coupling ferrocene carboxylic acid with the appropriate amino acid ester starting materials using the 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) protocol and these have been characterised by spectroscopic techniques. The electrochemical anion sensing behaviour of compounds 1– 10 with several anions using a platinum microdisk working electrode is described, together with 1H NMR anion complexation studies. The X-ray single crystal structure of N-ferrocenoyl- l-alanine methyl ester 4 has been determined and contains two molecules which differ slightly in conformation in the asymmetric unit of space group P2 1 (No. 4); principal dimensions are amide N(H)CO 1.224(6) and 1.231(6) Å, ester CO 1.220(10) and 1.190(7) Å, with N–H⋯OC(amide) as the primary intermolecular hydrogen bond, N⋯O 2.992(6) and 2.971(6) Å and with graph set C(4).

Acid-base and counter ion dependent solid-state assembly of a ferrocene β-aminoalcohol and the corresponding ammonium salt

2-(Ferrocenylmethyl)amino-2-methylpropan-1-ol was synthesized and converted to the respective ammonium bromide ([ 1H]Br ≡ 2) and dihydrogenphosphate ([ 1H]H 2PO 4 ≡ 3). The solid-state structures of 1, 2 and the solvated salt 3 · 1/6Et 2O ( 3a) have been determined by X-ray diffraction. The solid-state assemblies of 1 and 2 are dominated by infinite ladder-like arrays interconnected by hydrogen bonds whereas the solid-state structure of 3a is built up from linear hydrogen-bonded dihydrogenphosphate chains, which are interlinked via hydrogen bonds to the cations [ 1H] + into a complicated three-dimensional network. Compound 1 and its interactions with Bu 4NBr and Bu 4NH 2PO 4 in solution were further studied by electrochemical methods and NMR titrations.

Electronic Alterations Caused by Ruthenium in Pt−Ru Alloy Nanoparticles as Revealed by Electrochemical NMR

We have carried out a series of 195Pt and 13C NMR spectroscopic and electrochemical experiments on commercial Pt−Ru alloy nanoparticles and compared the results with those on Pt-black samples having similar particle sizes. The Pt NMR spectrum of the alloy nanoparticles consists of a single Gaussian peak, completely different from the broad “multi-Gaussian” NMR spectra, which are generally observed for carbon-supported Pt catalysts. Spin−echo decay measurements show that the intrinsic spin−spin relaxation time (T2) is much larger in the alloy compared to Pt-black. A “slow-beat” is observed in the spin−echo decay curve of the alloy, implying that the NMR frequencies of spin−spin coupled Pt nuclei in the alloy nanoparticles are quite similar, unlike the situation found with Pt-black. These 195Pt NMR results strongly suggest that there is a surface enrichment of Pt atoms in the Pt−Ru alloy nanoparticles. The CO-stripping cyclic voltammogram (CV) of the Pt−Ru alloy nanoparticles is broader than that observed w...

Electrochemical and solid state NMR characterization of composite PEO-based polymer electrolytes

A comprehensive matrix of composite poly(ethyleneoxide) (PEO)-based solid-state electrolytes was developed in order to systematically study a number of variables and their impact upon the electrochemical properties of the resulting materials. The different parameters studied in the fabrication of these materials include: (i) the lithium electrolyte salt type, (ii) the ether oxygen to lithium ratio, (iii) the molecular weight of PEO, (iv) the type of ceramic additive used, either aluminum oxide (Al 2O 3), silicon oxide (SiO 2), or titanium oxide (TiO 2), (v) the particle size of the additives used, and (vi) the concentration of additive (wt.%). The standard lithium salt used for the preparation of these electrolytes was lithium trifluoromethanesulfonate (lithium triflate or LiSO 3CF 3), which served as the baseline electrolyte salt. Other lithium salts investigated include: lithium perchlorate (LiClO 4) and lithium bis-trifluoromethanesulfonimide (LiN(SO 2CF 3) 2). Conductivity measurements were performed for each electrolyte membrane over a wide temperature range (23–100 °C). In addition, cyclic voltammetry measurements were performed on selected PEO membranes as a function of temperature to determine the impact of various parameters upon the electrochemical stability. It was observed that the parameter that displayed the most significant effect upon the PEO-base polymer conductivity was the lithium salt type employed. The lithium triflate salt-containing PEO polymers demonstrated the best mechanical properties before and after heat treatment. Ceramic fillers also appear to enhance the mechanical properties of PEO polymer electrolytes at temperatures above the melting point of PEO (60–70 °C). In addition to investigating the electrochemical characteristics of the composite membrane, solid state 7Li NMR characterization was performed to study ionic mobility by measuring spectral line widths and lithium self-diffusion coefficients. It was determined that ceramic nanoparticle additives can enhance the Li + diffusivity without a corresponding increase in polymer segmental mobility.

Reactivities and Structural and Electrochemical Studies of Coordinatively Unsaturated Arene(dithiolato)Ru(II) Complexes [(η6-p-cymene)Ru(1,2-S2C2B10H10-S,S‘)] and [(η6-C6Me6)Ru(1,2-S2C2B10H10-S,S‘)

The addition reactions of 16-electron complexes [(η6-arene)Ru(CabS,S‘)] [3: arene = p-cymene (3a), C6Me6 (3b); CabS,S‘ = 1,2-S2C2B10H10-S,S‘] with the nucleophiles PEt3, CNBut, and CO lead to the formation of the corresponding 18-electron complexes 4−6 [(η6-arene)Ru(CabS,S‘)(L)] (L = PEt3, CNBut, CO), which have been characterized by X-ray crystallography. Investigation of the redox process in a series of these base-adduct metalladithiolene complexes by cyclic voltammetry reveals a large dependence of the redox potentials on the nature of the ancillary η6-arene and incoming L ligand. In an analogous manner, the reactivity of 3a toward arylamines (7) has been found to produce [(η6-p-cymene)Ru(CabS,S‘)(L)] (8) [L = η1-NH2(CH2CH2)Ar, Ar = C6H5 (8a); C6H4OMe (8b); C6H3(OMe)2 (8c); C8H6N (8d)], having molecular structures closely related to those of 4 and 5, as deduced by an electrochemical analysis and NMR studies. In addition, complex 3 reacts with a variety of substrates such as alkynes and a diazoalkane, generating a new class of dithiolato ruthenium(II) complexes incorporating alkene and alkylidene units. Thus, the syntheses of the half-sandwich ruthenium(II) complexes [(η6-arene)Ru{η3-(S,S,C‘)-SC2B10H10SC‘R2}] [C‘R2 = (COOMe)CC(COOMe) (9: arene = p-cymene (9a), C6Me6 (9b)), HCCPh (10: arene = C6Me6 (10b)), CH2SiMe3 (11: arene = p-cymene (11a), C6Me6 (11b))] are reported, and the structure of 11b has been established by an X-ray diffraction study. The formation of 9 and 10 has also been electrochemically investigated.

UHV, Electrochemical NMR, and Electrochemical Studies of Platinum/Ruthenium Fuel Cell Catalysts

It is well-known that platinum/ruthenium fuel cell catalysts show enhanced CO tolerance compared to pure platinum electrodes, but the reasons are still being debated. We have combined cyclic voltammetry (CV), temperature programmed desorption (TPD), electrochemical nuclear magnetic resonance, and radio active labeling to probe the origin of the ruthenium enhancement in Pt electrodes modified through Ru deposition. The results prove that the addition of ruthenium not only modifies the electronic structure of all the platinum atoms but also leads to the creation of a new form of adsorbed CO. This new form of CO may be ascribed to CO chemisorbed onto the “Ru” region of the electrode surface. TPD and CV results show that the binding of hydrogen is substantially modified due to the presence of Ru. Surprisingly though, TPD indicates that the binding energy of CO on platinum is only weakly affected. Therefore, the changes in the bond energy of CO due to the ligand effect only play a small role in enhancing CO to...

Diffusion on a nanoparticle surface as revealed by electrochemical NMR.

Surface diffusion of chemisorbed CO (from MeOH electrochemisorption) on pure and Ru-modified nanoscale Pt electrocatalyst surfaces has been investigated by solid-state electrochemical NMR (EC-NMR) in the presence of supporting electrolyte. Temperature-dependent nuclear spin-spin and spin-lattice relaxation measurements enable the diffusion activation energy, E, to be deduced. It is shown that the activation energy E correlates with the steady state current for MeOH electro-oxidation. A simple two-dimensional collision theory model is proposed to explain this intriguing observation, which may provide new mechanistic insights into the promotion of CO-tolerance in Pt/Ru fuel cell catalysts.

A polymerizable pyrrole–cobaltocenium receptor for the electrochemical recognition of anions in solution and immobilised onto electrode surfaces

Electrochemical and 1H NMR investigations showed that the first example of cobaltocenium moiety covalently attached to a pyrrole group ( 1) recognises and senses anionic guest species not only in solution but also immobilised onto electrode surfaces by electropolymerisation.

NMR of Electrocatalysts

It is now possible to investigate in detail metal/liquid interfaces by solid-state NMR under external potentiostatic control, in the presence of an electrolyte, by using the electrochemical NMR (EC NMR) technique. This permits the determination of the electronic properties of electrodes and of adsorbates as well as the study of the surface diffusion of adsorbates. The method also provides useful information on the dispersion of platinum fuel cell catalysts, on electrochemically generated sintering effects, and on the electronic effects caused by the presence of different surface adatoms. In general, EC NMR provides information on the following: (1) metal surface Ef LDOS, the chemical significance of which is that these describe the local frontier orbitals of the metal surface; (2) adsorbate structure/bonding details, such as 5σ and 2π* Ef LDOS, metal-surface bonding to atoms in the adsorbate, and even bond lengths; (3) rates and activation energies for surface diffusion; and (4) electrode potential effects on the electronic properties of the electrochemical interface, as seen by both metal and ligand NMR. We review here, therefore, the current status of the EC NMR technique, together with its applications to investigating the metal/electrolyte interface.

Nanostructured electrode surfaces studied by electrochemical NMR

Electrochemical nuclear magnetic resonance (EC-NMR) is a powerful local probe, which combines solid state NMR with electrochemistry. It is a unique technique that permits a unified, electronic-level study of the metal and adsorbate side of the electrochemical interface. Experiments can be performed either under direct potentiostatic control and in situ potential adjustment, or with samples prepared in a separate electrochemical cell and transferred to an NMR cell, where the potential is both known and constant. A phenomenological two-band model was applied to the NMR parameters to yield quantitative information about the Fermi level local density of states ( E f-LDOS) that are relevant to the type of chemisorption bond involved in the systems under investigations. A layer-model analysis was found to be effective in interpreting the 195Pt-NMR spectra of carbon-supported Pt nanoparticles. The surface peak of the 195Pt-NMR spectrum was found to be very sensitive to the chemical nature of the adsorbate present. The 195Pt Knight shifts show a direct correlation with the electronegativity of the adsorbate, and the 13C Knight shift of the CO adsorbate shows a correlation with the clean metal surface E f-LDOS. The electrode potential dependence of 13C-NMR spectra of CO adsorbed on Pt and Pd black show evidence of the alterations to the electrochemical interface by the application of the electric field. EC-NMR of Pt electrode surfaces modified by spontaneous deposition of ruthenium has provided new insights into the enhancement in CO-tolerance of these catalysts for methanol oxidation.

An NMR investigation of CO tolerance in a Pt/Ru fuel cell catalyst.

We report the first combined application of solid-state electrochemical NMR (EC NMR), cyclic voltammetry (CV), and potentiostatic current generation to investigate the topic of the ruthenium promotion of MeOH electro-oxidation over nanoscale platinum catalysts. The CV and EC NMR results give evidence for two types of CO: CO on essentially pure Pt and CO on Pt/Ru islands. There is no NMR evidence for rapid exchange between the two CO populations. CO molecules on the primarily Pt domains behave much like CO on pure Pt, with there being little effect of Ru on the Knight shift or on Korringa relaxation. In sharp contrast, COs on Pt/Ru have highly shifted (13)C NMR resonances, much weaker Korringa relaxation, and, at higher temperatures, they undergo thermally activated surface diffusion. For CO on Pt, the correlation observed between the 2pi* Fermi level local density of states and the steady-state current suggests a role for Ru in weakening the Pt-CO bond, thereby increasing the CO oxidation rate (current). The combined EC NMR/electrochemistry approach thus provides new insights into the promotion of CO tolerance in Pt/Ru fuel cell catalysts, in addition to providing a novel route to investigating promotion in heterogeneous catalysis in general.