Spectroelectrochemical Flow Cell Research Articles

Operando Investigations of Reversible and Irreversible Transformations of Metal Organic Framework Based Catalysts during the Oxygen Evolution Reaction

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

Understanding the Transition Process of Cu Oxide to Metallic Under the CO2 Reduction Conditions Probed By Operando Quick-XAS

During the last decades, significant efforts have been made to directly convert CO2 as a potential feedstock into hydrocarbons as fuels and/or basic chemicals for industrial applications. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative for a large-scale production of hydrocarbons. However, there are still some challenges including poor product selectivity and highly complex multiple-step reaction mechanisms.[1] In order to convert CO2 on a Cu electrode, high overpotentials up to 1.0 V are required, which make the energy efficiency still rather poor and lead to competition with the H2 evolution.[2] Additionally, the morphology of the Cu materials i.e. structure and ‘chemical state’ (metallic vs oxidized, high vs low coordinated) strongly influence the performance of the CO2RR. Recently, we have shown the critical potential of oxide-metal transition processes for Cu oxide foam annealed at 300°C probed by operando XAS, XRD and Raman spectroscopy.[3,4] All three operando techniques showed an entire reduction of Cu oxide to metallic before the production of hydrocarbons starts. [3,4]In this work, we have investigated the kinetics of both electrochemical oxide-metal reduction and CO2RR on nanoporous Cu foams as catalyst precursor annealed at four different temperatures (100°C, 200°C, 300°C, 450°C) in air using operando Quick X-ray Adsorption Spectroscopy (Quick-XAS). The Quick-XAS measurements were carried out in a custom-made spectro-electrochemical flow cell using 0.5 M KHCO3 as the electrolyte, while the XAS-Spectra were measured in transmission mode. The Quick-XANES data was analyzed by linear combination fit (LCF) and principle component analysis (PCA) to monitor the potential dependent changes of the chemical state and coordination number of the Cu species. Based on the Cu K-edge XANES and EXAFS data, we show that the annealing temperature strongly influences the chemical state of the Cu species. More precisely, the population of the Cu(II) species within the as prepared foams increases with increasing annealing temperature. Starting from the different ratios of Cu(0):Cu(I): Cu(II), the oxide-metal transition processes are shifted in the cathodic direction by applying potential steps of 100 mV. With an increase in annealing temperature, this oxide-metal transition is more rapid and occurs at lower cathodic overpotentials, but still before the production of hydrocarbons begins. In contrast, the potential jump experiments of several hundreds of mV lead to different kinetics of the oxide-metal reduction of Cu species. These transition processes and the resulting structure of porous Cu foams have a huge impact on the product distribution for CO2RR. Altogether, our results provide deeper insights into the oxide-metal transition processes to form the catalytically active Cu species for hydrocarbon formation during CO2RR.Reference:[1] S. Nitopi et al., Chemical Reviews, 2019, 119, 7610. [2] C. W. Li et al., Journal of the American Chemical Society, 2012, 134, 7231. [3] A. Dutta et al., Chimia, 2021, 75, 733. [4] A. Dutta et al., Journal of Catalysis, 2020, 389, 592.

Selective Anodic Oxidation of Solketal as Acetal‐Protected Glycerol over Nickel Boride in Alkaline Media to Glyceric Acid**

AbstractAlkaline anodic oxidation of glycerol suffers from facile C−C bond cleavage, especially when using non‐precious metal electrocatalysts, which limits the yield of more valuable C3 oxygenates. Usually, a high C3 selectivity is a tradeoff with conversion for most catalysts. Thus, we used solketal as the reactant, which is acetal‐protected glycerol with acetone. CV experiments showed that solketal is oxidized over nickel boride (NixB) at potentials where NiOOH is formed. Electrolysis over NixB in a thin‐film spectroelectrochemical flow cell at 1.58 V vs. RHE to avoid pronounced oxygen evolution showed a stable current density of ca. 6 mA cm−2. Simultaneously recorded ATR‐FTIR spectra revealed solketal conversion to solketalate and formate. Indeed, 59 % conversion and 77 % selectivity to glyceric acid were determined by HPLC after acidic cleavage of the acetal, resulting in a yield of 45 %. Therefore, solketal is a promising reactant for the selective electrosynthesis of glyceric acid.

Shedding Light on the Electronic Structure and Local Configuration of Fe/N/C Active Sites – an in Situ X-Ray Absorption Spectroscopy Study

In recent years, the search for platinum group metal-free oxygen reduction reaction (ORR) catalysts as an inexpensive and sustainable replacement for state-of-the-art Pt-based materials has inspired a lot of work in the field of single-atom Fe-based catalysts (often referred to as Fe/N/C). While promising initial ORR-activities were achieved with different synthesis approaches,[1,2] tackling the lack of durability[3] will require a strategic optimization of the employed preparation routes. A comprehensive fundamental understanding of the catalysts active sites’ electronic structure and local configuration under operating conditions lies at the heart of this process and remains a subject of vivid debate. Several studies employing both ex and in situ Mössbauer and X-ray absorption spectroscopies (XAS)[4–7] have brought insights regarding these questions, but a full unambiguous conclusion has not been reached and the community is seeking for novel approaches to unravel such structure-activity and structure-stability relations.With this motivation, we have used in situ XAS to study two catalysts prepared with distinctively different synthesis approaches and widely free of detectable inorganic side phases,[5,8] and that therefore constitute ideal model systems to draw conclusions applicable to this catalyst family. We focused our attention on the spectral 1s → 3d pre-edge feature of the X-ray absorption near edge structure (XANES) spectrum that is sensitive to any changes in the local site symmetry, orbital occupancy, and spin state of the absorbing atom,[9,10] but that to the best of our knowledge has so far not been looked into in the field. In order to achieve adequate energy resolution to discern the relevant features, the spectra were acquired with a Si(311) monochromator at the SuperXAS beamline of the Swiss Light Source employing our group’s spectroelectrochemical flow cell.[11] As illustrated in Fig. 1a, pseudo-Voigt component fits allow tracking the spectral changes as a function of the applied polarization conditions, as qualitatively indicated with green arrows.When using bulk-sensitive techniques like XAS, one must bear in mind that while the catalytic process and expected site changes of interest take place only on the surface sites in contact with the electrolyte, the observed signal also contains the spectral contributions of those sites that remain unaffected by the potential. This is particularly relevant when studying such single-atom Fe-based catalysts, since it is expected that a relatively large (and hard to quantify) fraction of their sites are buried in the catalysts’ bulk. Herein we overcome this intrinsic challenge by performing modulation excitation (ME) experiments using the applied potential as a stimulus (illustrated in Fig. 1b). The technique allows to greatly enhance the signal of the species that changes with said stimulus,[12] and consequently to effectively surmount the lack of surface sensitivity described above. Furthermore, these ME experiments acquired with fluorescence-detected quick-scanning XAS[13] complemented with a multivariate curve resolution (MCR) analysis of the data provide quantitative information about the kinetics of the structural changes undergone by the sites in these Fe/N/C catalysts upon modifying the potential (Fig. 1c).In summary, this work will provide a novel perspective by exploiting the sensitivity of pre-edge XANES features and ME-experiments for the first time for the analysis of Fe/N/C-catalysts allowing insights that will broaden the greatly-needed understanding of the potential-dependent structure of the active sites.[1] E. Proietti et al., Nat. Commun. 2011, 2:416.[2] J. Shui et al., Proc. Natl. Acad. Sci. 2015, 112, 10629–10634.[3] D. Banham et al., J. Power Sources 2015, 285, 334–348.[4] U. I. Kramm et al., Phys. Chem. Chem. Phys. 2012, 14, 11673–88.[5] A. Zitolo et al., Nat. Mater. 20 15, 14, 937–42.[6] Q. Jia et al., Nano Energy 2016, 29, 65–82.[7] S. Wagner et al., Angew. Chemie Int. Ed. 2019, 58, 10486–10492.[8] S. Wagner et al., Hyperfine Interact. 2018, 239:10.[9] T. E. Westre et al., J. Am. Chem. Soc. 1997, 119, 6297–6314.[10] M. Wilke et al., Am. Mineral. 2001, 86, 714–730.[11] T. Binninger et al., J. Electrochem. Soc. 2016, 163, H913–H920.[12] A. Urakawa et al., Chem. Eng. Sci. 2008, 63, 4902–4909.[13] A. H. Clark et al., J. Synchrotron Radiat. 2020, 27, 1–8. Figure 1

A Combined Spectroscopic and Electrochemical Approach to Investigate the Selectivity Determining Factors for CO2 Electroreduction on Pd-Nanoparticles

The electrochemical conversion of carbon dioxide into added-value products keeps on receiving increased interest as a means to valorize CO2 while diminishing its anthropogenic emissions. A broad spectrum of CO2-electroreduction products (e.g., CH4, C2H4) has been reported, but only a few are expected to be economically competitive with regards to industrially established chemical production pathways [1]. Interestingly, Pd-based nanoparticle catalysts are particularly well suited for the production of two of such products, since they feature the unique ability to selectively produce formate and carbon monoxide (CO) at low vs. high overpotentials, respectively [2]. This switching of the selectivity with the applied potential has been linked to a difference in the nature of the Pd catalyst and corresponding reaction mechanism. Specifically, at potentials below − 0.4 VRHE at which CO is the main reduction product [2,3], the reaction is believed to proceed on a fully CO-covered Pd-surface [4]. At higher potentials between − 0.4 and 0 VRHE, though, formate is predominantly produced [2,3] through a hypothesized CO2-hydrogenation mechanism on palladium hydride (PdHx) [5].Chiefly, the design of better-performing catalysts requires a better understanding of the relation between the precise stoichiometry of this hydride phase (i.e., the value of x in PdHx) and the applied potential, reaction duration and corresponding coverage of surface adsorbates (e.g., CO).To determine the relation between the above-described parameters, in this study we used a commercial carbon-supported palladium catalyst (40 % Pd/C) that displayed the above-described switchable selectivity between formate and CO in online gas chromatography measurements in a newly designed parallel plate cell (Figure 1). Specifically, very high selectivities of up to 90 % for formate were found at very low overpotentials (− 0.1 to − 0.3 VRHE), while at high overpotentials (− 0.7 to − 0.9 VRHE) CO is the main product with faradaic efficiencies of over 60 %, and only low amounts of formate are found. The interplay between surface adsorbates and palladium hydride formation was then investigated by quantifying the catalyst’s time and potential-dependent hydride content using chronoamperometry complemented with operando X-ray absorption spectroscopy (XAS) in our group's spectroelectrochemical flow cell [6]. Here, we investigated the influence of CO2 presence in the electrolyte acquiring XAS over 10 minutes of potential hold in both N2- and CO2-saturated potassium bicarbonate.As an example of our results, Figure 2 shows the X-ray absorption near edge structure (XANES) spectra after ten minutes of holding potential at − 0.1 VRHE. While we observed full hydride formation in N2-saturated KHCO3, the palladium in the CO2 saturated electrolyte remained in full metallic state, suggesting that in the presence of carbon dioxide surface adsorbates prevent the formation of PdHx. Additionally, we were able to monitor changes of the palladium hydride stoichiometry in the seconds- to minutes-timescales, thereby enabling us to investigate the hydride formation speeds with an unprecedented time resolution. Finally, the strongly adsorbed surface species formed during CO2 electroreduction were identified and their surface coverage quantified using a novel combined rotating disk electrode (RDE) cyclic voltammetry and in situ external reflection Fourier-transform infrared spectroscopy (FTIR) setup. This allowed us to determine that in the CO2 saturated bicarbonate electrolyte the Pd-surface is covered by a full monolayer of surface-adsorbed CO that prevents hydrogen adsorption and subsequently palladium hydride formation, as unveiled by our XAS results.In conclusion, we have combined electrochemical with spectroscopic methods providing us with the means to improve our understanding of the palladium hydride stoichiometry and adsorbed surface species under CO2 electroreduction conditions. Thus, this contribution will provide novel insight into the relation among the time-dependent bulk and surface composition of Pd-nanoparticles and their link to their CO2-reduction selectivity.Literature:[1] J. Durst et al., Chimia, 2015, 69, 769. [2] M. Rahaman et al., ChemSusChem, 2017, 10, 1733. [3] D. Gao et al., NanoResearch, 2017, 10, 2181. [4] R. Kortlever et al., Catalysis Today, 2015, 244, 58. [5] X. Min et al., J. Am. Chem. Soc., 2015, 137, 4701. [6] T. Binninger et al., J. Elec. Soc., 2016, 163, H906. Figure 1

In Situ Surface-Enhanced Raman Spectroscopic Studies of Electrochemically Formed Germanene

Germanene is a 2D allotrope of germanium atoms arranged in a honeycomb structure with Dirac fermion characteristics analogous to graphene and silicene. Formation of germanene is a recent advance, based on evaporation of Ge in an ultrahigh vacuum environment. An alternative method, developed by authors, is its electrochemical growth from aqueous solutions. Initial studies were performed on Au(111) substrates in aqueous solutions of H2GeO3 using in situ scanning tunneling microscopy, voltammetry, surface X-ray diffraction, and coulometry. These results indicated that electrochemically formed germanene domains were formed and covered the surface, although the coherence lengths were short because of the presence of domain walls composed of five- and seven-membered defect rings. The present study has made use of an operando spectroelectrochemical flow cell with a Raman microscope to follow the growth of germanene. Deposits were formed on a surface-enhanced Raman spectroscopy-active Au substrate. Initial spectr...

Spectroelectrochemistry of Halide Anion Adsorption and Dissolution of Single Gold Nanorods

A spectroelectrochemical flow cell is used to probe the localized surface plasmon resonance (LSPR) of the same single gold nanorods (AuNRs) in sodium fluoride, sodium chloride, and sodium bromide electrolytes using dark-field scattering microscopy. The changes in resonance energy, line width (full-width at half-maximum, fwhm), and peak intensity of a Lorentzian fit to single AuNR scattering spectra as the rods are charged are compared to determine the role of anion adsorption. We demonstrate that at positive potentials up to +0.25 V relative to a Pt quasi-reference electrode, the induced changes in the LSPR are independent of halide anion. At more positive potentials (+0.3 to +0.35 V) bromide and chloride ions damp the AuNR LSPR, observed as an increase in the line width. At the most positive potential investigated in all three electrolyte solutions (+0.35 V), the AuNR scattering intensity decreases irreversibly in bromide electrolyte, indicating dissolution. The kinetics of the bromide-mediated dissoluti...

Perfluoroalkyl-Phosphonic Acid Adsorption on Polycrystalline Platinum and Its Influence on the Oxygen Reduction Reaction

Motivated by the need for a better understanding of the oxygen reduction reaction (ORR) in high temperature proton exchange membrane fuel cells (HTPEM-FC), we have investigated the impact of anion adsorption on the ORR activity and selectivity for small concentrations of perfluoroalkylated phosphorus-based model compounds. A polycrystalline Pt surface was used to evaluate the adsorption behavior of trifluoromethyl-phosphonic acid (TFMPA) and pentafluoroethyl-phosphonic acid (PFEPA) by means of cyclic voltammetry (CV), a rotating disc electrode (RDE), and a spectro-electrochemical flow cell. In situ Fourier transform infrared spectroscopy in the attenuated total reflection (ATR-FTIRS) configuration was performed to study the impact of the adsorbed species on the ORR activity and selectivity employing simultaneous detection of hydrogen peroxide formation during the ORR. Electrochemical impedance spectroscopy (EIS) was applied to determine the charge transfer resistance RCT under ORR conditions. The presence of TFMPA, PFEPA, or H3PO4 had significant effects on the cyclic voltammograms in the Hupd and oxide formation potential region. The ORR activity, measured under well-defined mass transport conditions, revealed a doubling of the kinetic current densities for TFMPA and PFEPA compared with H3PO4, suggesting a weaker adsorption tendency for these model compounds. The ORR selectivity was found to be dependent on the nature of the adsorbed species. These observations are consistent with ATR-FTIRS and EIS results, where the reduced interaction of perfluoroalkylated compounds with the Pt surface was observed compared to H3PO4.

The Effect of Anions on the Oxygen Reduction Reaction Activity and Selectivity on a Au Electrode: A Double Disk Electrode Flow Cell ATR-FTIR Spectroscopy Study

The oxygen reduction reaction (ORR) on Au electrode exhibits a sluggish kinetics and its selectivity is sensitive to adsorbed hydroxyl intermediates and the pH [1,2]. Adsorbed anions are known to strongly affect the ORR in the kinetic region via the occupation of surface sites required for the dissociative adsorption of O2 molecules. Also the ORR selectivity for the reduction to water instead to hydrogen peroxide is affected when going from weakly adsorbing to more strongly adsorbing anions [3,4]. Application of in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS) was demonstrated recently to offer important insights on the nature of adsorbed species / intermediates in the ORR [5,6], which was recently extended to ORR studies using a spectro-electrochemical flow cell [7]. However, the application of in situ ATR-FTIRS, which allows correlation of the ORR activity and the presence of adsorbed species, so far has not provided any direct information on the impact of the adsorbed anions on the ORR selectivity. To pursue this aim, a spectroelectro-chemical dual thin layer flow cell, used for combination of in situ ATR-FTIRS, online mass spectrometry and electrochemical measurements [8], was modified for simultaneous electrochemical detection of hydrogen peroxide at the second (Pt collector) electrode [9]. A thin (~30 nm) Au film was electrolessly deposited [10] onto the face plane of a hemi-cylindrical Si prism, and then carefully annealed in a butane flame in a N2 atmosphere to improve the film stability, while maintaining a high IR sensitivity (about three-fold higher linearly adsorbed CO band intensity at 0.0 V in CO saturated solution compared to the literature [5]). A homemade mirror accessory was used within the sample chamber of a Varian 670i FTIR spectrometer, equipped with a p-polarizer and a liquid nitrogen cooled mercury cadmium telluride detector. Measurements were performed at a spectral resolution of 4 cm-1 and a temporal resolution of 1 s-1 per spectrum (by co-adding 5 interferograms). The Pt collector was biased at 1.2 V (RHE) for hydrogen peroxide oxidation (collection efficiency of ca. 20%). O2 saturated solutions of perchloric, sulfuric acids (0.5 M) or sodium hydroxide (1.0 M) were used. The ORR starts at ca. 0.5, 0.6 and 1.0 V (RHE) in perchloric, sulfuric acids, and alkaline solution, respectively. It does not reach the mass transport limitation up to 0.0 V in acidic solutions, whereas in alkaline medium the mass transport limited ORR current is reached at about 0.5-0.6 V, and slightly decreases at lower potentials due to increasing hydrogen peroxide formation. The hydrogen peroxide yields of 60 to 80% were found at potentials 0.0 to 0.5 V in both perchloric and sulfuric acid solutions, in agreement with gradually increasing integrated band intensities of the corresponding anions. In contrast, in alkaline solution the hydrogen peroxide yields are close to 0% at the onset of the ORR on Au film at high potentials and gradually increase with decreasing potential to ca. 60 % at 0.0 V. In this case, IR spectroscopy is little helpful since it cannot discriminate between adsorbed water, hydroxyl or peroxyl. A correlation between the variation of the ORR activity and selectivity in alkaline solution and the OH adsorption is derived indirectly, by i) comparing the double layer current obtained on a Au film electrode in alkaline solution, and from ii) additional measurements in CO saturated 0.1 M NaOH solution, where the mass transport limited current is reached over a wide potential range (0.5 - 1.2 V). These results strongly support the presence of adsorbed OH in this potential range, which, however, does not favor the ORR to hydrogen peroxide. Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2) [1] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Nørskov, J. Phys. Chem. Lett. 3, 2948 (2012). [2] P. Quaino, N.B. Luque, R. Nazmutdinov, E. Santos, W. Schmickler, Angew. Chem. Int. Ed. 51, 12997 (2012). [3] N.M. Markovic, P.N. Ross Jr., Surf. Sci. Rep., 45, 117 (2002). [4] J.X. Wang, N.M. Markovic, R.R. Adzic, J. Phys. Chem B 108, 4127 (2004) [5] M.H. Shao, R.R. Adzic, J. Phys. Chem B 109, 16563 (2005). [6] N. Ohta, K. Nomura, I. Yagi, J. Phys. Chem. C 116, 14390 (2012). [7] M. Nesselberger, S.J. Ashton, G.K.H. Wiberg, M. Arenz, Rev. Sci. Instr. 84, 074103 (2013). [8] M. Heinen, Y.X. Chen, Z. Jusys, R.J. Behm, Electrochim. Acta, 52, 5634 (2007). [9] Z. Jusys, J. Kaiser, R.J. Behm, Electrochim. Acta, 49, 1297 (2004). [10] K.-i. Ataka, T. Yotsuyanagi, M. Osawa, J. Phys. Chem. 100, 10664 (1996).

Spectroelectrochemical flow cell with temperature control for investigation of electrocatalytic systems with surface-enhanced Raman spectroscopy

We describe a method for investigating the reaction mechanism of fuel cell systems by designing a spectroelectrochemical cell with functions of temperature and flow control to mimic the reaction condition of fuel cell systems and utilizing Au core Pt shell (Au@Pt) nanoparticles to enhance the Raman signal of the surface species on the surface of electrocatalysts. The cell consists of three parts: a thin-layer spectroelectrochemical reaction chamber with an optical window for Raman measurement, the heating chamber right beneath the reaction chamber, and a long spiral flow channel to preheat the solution to the desired temperature and effectively exchange the solution. The temperature of the solution can be easily controlled from room temperature to 80 degrees C, and the flow rate can be as high as 945 microl s(-1). The temperature and flow control is demonstrated by monitoring the changes in the cyclic voltammograms and the Raman signals. By synthesizing Au@Pt nanoparticles and assembling them on a Pt substrate, we can significantly enhance the Raman signal of surface species on the Pt shell surface, which allows us to detect strong signal of CO as the dissociative product of formic acid as well as the intermediate species of the oxidation process. The further development and perspectives of using SERS to study the electrocatalytic systems are discussed.

Active-site structure, binding and redox activity of the heme-thiolate enzyme CYP2D6 immobilized on coated Ag electrodes: a surface-enhanced resonance Raman scattering study.

Surface-enhance resonance Raman scattering spectra of the heme–thiolate enzyme cytochrome P450 2D6 (CYP2D6) adsorbed on Ag electrodes coated with 11-mercaptoundecanoic acid (MUA) were obtained in various experimental conditions. An analysis of these spectra, and a comparison between them and the RR spectra of CYP2D6 in solution, indicated that the enzyme’s active site retained its nature of six-coordinated low-spin heme upon immobilization. Moreover, the spectral changes detected in the presence of dextromethorphan (a CYP2D6 substrate) and imidazole (an exogenous heme axial ligand) indicated that the immobilized enzyme also preserved its ability to reversibly bind a substrate and form a heme–imidazole complex. The reversibility of these processes could be easily verified by flowing alternately solutions of the various compounds and the buffer through a home-built spectroelectrochemical flow cell which contained a sample of immobilized protein, without the need to disassemble the cell between consecutive spectral data acquisitions. Despite immobilized CYP2D6 being effectively reduced by a sodium dithionite solution, electrochemical reduction via the Ag electrode was not able to completely reduce the enzyme, and led to its extensive inactivation. This behavior indicated that although the enzyme’s ability to exchange electrons is not altered by immobilization per se, MUA-coated electrodes are not suited to perform direct electrochemistry of CYP2D6.Electronic supplementary materialThe online version of this article (doi:10.1007/s00775-007-0303-1) contains supplementary material, which is available to authorized users.

Electropolymerization of aniline on polyaniline-modified electrodes under hydrodynamic conditions

A new procedure for the electrosynthesis of polyaniline (PANI), based on the electropolymerization of aniline on a PANI-modified electrode, is proposed. The procedure is controlled by the chemical reaction between the aniline and the highest oxidized form of PANI. A spectroelectrochemical flow cell has been used to study the influence of the flow rate on this process, which has enabled direct observation of the autocatalytic mechanism. Unexpectedly, the hydrodynamic conditions do not seem to have an effect on the resulting polymer film. Similar results were obtained by carrying out the electropolymerization in nitric and perchloric acid.

Electropolymerization and characterization of polyaniline films using a spectroelectrochemical flow cell

A new spectroelectrochemical flow cell is presented and its capability to provide a better understanding of reaction mechanisms is illustrated with the study of the electrosynthesis and characterization of a conducting polymer, polyaniline (PANI). A spectroelectrochemical study of electropolymerization of aniline under flow conditions has been performed for the first time. The significant influence that the flow rate of feeding monomer solution has on the electropolymerization process and, consequently, on the electrochromic properties of the resulting polymer has been demonstrated.

In situ rapid-scan time-resolved microscope FTIR spectroelectrochemistry: study of the dynamic processes of methanol oxidation on a nanostructured Pt electrode

In this paper we report the development of an electrochemical in situ rapid-scan time-resolved microscope infrared reflection spectroscopy (RS-TR-MFTIRS), which consists of a FTIR spectrometer with rapid-scan facility, an infrared microscope in combination with a nanostructured Pt microelectrode, and a versatile external reflection (or thin-layer) spectroelectrochemical flow cell. It has been demonstrated that the RS-TR-MFTIRS can successfully overcome the disadvantages in mass transport, transient response and uniformity of current flow that arise from a conventional external reflection infrared cell. The dynamic process of methanol oxidation on a nanostructured platinum microelectrode in alkaline solution was employed as a test reaction to characterize the performance of the newly developed technique. The results illustrate that cyclic voltammograms (CVs) and infrared spectra within a potential interval of 2.6 mV can be simultaneously recorded at a potential scan rate up to 200 mV s −1. The dynamic processes of the poisoning intermediate (adsorbed CO species) and reactive intermediate (formate ions) involved in methanol oxidation in alkaline solutions were studied intensively. It has been revealed that the bridge-bonded CO (CO B) species and formate ions are generated preferentially at the onset potential (−0.54 V vs SCE) of methanol oxidation, while the linearly bonded CO (CO L) species originate at more positive potentials that are close to the anodic current peak potential (−0.17 V vs SCE) of methanol oxidation.

A versatile surface Raman spectroelectrochemical flow cell: applications to chemisorbate kinetics

A versatile spectroelectrochemical flow cell is described, with the objective of probing adsorption–desorption and other adsorbate rate processes triggered by alterations in the solute composition, as monitored by surface-enhanced Raman spectroscopy (SERS). The device offers flexibility in electrode replacement and pretreatment, and provides rapid (ca 1 s) changes in the solution composition in contact with the surface, enabling adsorbate rate processes down to this timescale to be accessed by means of temporal SERS sequences. Some illustrative applications are briefly outlined, exploiting the now-established broad-based applicability of SERS to transition-metal films as well as coinage-metal surfaces. These include non-reactive adsorbate substitution processes, and the use of isotopic substitution to explore dissociative adsorption triggered by reactive versus non-reactive desorption, specifically regarding the catalytic role of chemisorbed CO in formic acid electrooxidation on rhodium.

In situ IR study of the electrooxidation of hypophosphite on a polycrystalline nickel electrode

Electrooxidation of the hypophosphite ions on polycrystalline nickel in alkaline solutions has been investigated using cyclic voltammetry, chronoamperometry and in situ IR spectroscopy (SNIFTIRS). A spectroelectrochemical flow cell was used in order to keep the composition of the thin layer constant. The spectra were analysed as a function of the electrode potential during the anodic, cathodic scans and at open-circuit potential as a function of time. On the assumption that solvent bands are independent of applied potential, there is some evidence at open circuit for hypophosphite ions adsorbing to the nickel surface through the hydrogen atoms; this orientation persists until oxidation takes place. Phosphite ions were the only oxidation product to be detected.

Detection of Biologically Important Compounds in Flowing Aqueous Streams by Surface-Enhanced Raman Spectroscopy at a Silver Electrode

Flow-injection analysis has been utilized to generate surface-enhanced Raman spectra of pyridine, adenosine, and adenosine 3′ monophosphate. These biologically important compounds have been recorded in a 10-μL spectroelectrochemical flow cell while under flowing conditions. Charge-coupled-device detection and low-power Ar+ laser excitation allowed an 1800-cm−1 region to be observed with 10-s integration times. A computer-controlled potential waveform which efficiently adsorbs and desorbs the analytes at the electrode surface is described. The effects of the quantity of anodic charge passed during the ex situ and in situ roughening of the silver electrode on the SERS intensity are discussed. The limit of detection for 50-μL injections of adenosine under true dynamic conditions is less than 800 pmol. The technique yields both quantitative data and structural fingerprinting information for analytes eluting through the cell.

Surface-Enhanced Raman Spectroscopy at a Silver Electrode as a Real-Time Detector in Flowing Streams

An analytical application for Surface-Enhanced Raman Spectroscopy at a silver electrode is described. Real-time SER spectra of adenine and cytosine have been recorded in a 10-µL spectroelectrochemical flow cell under flowing conditions. Charge-coupled-device detection allowed high-quality spectra spanning a ∼1200 cm−1 region to be recorded with integration times of 4 seconds. A low-power He Ne laser was used as a source. SERS at the silver electrode offers rapid time response to adsorption/desorption by appropriate potential modulation. The technique is extremely reproducible and insensitive to temperature and flow rate. The effects of incident photon energy and applied potential on the intensity of the Raman signal are discussed.

SNIFTIRS with a flow cell: the identification of the reaction intermediates in methanol oxidation at Pt anodes

A spectroelectrochemical flow cell was used with the SNIFTIRS method to obtain spectra from the adsorbed reaction intermediates involved in methanol oxidation at a platinum electrode in acid solution. The flow of solution prevented depletion of the reactant in the thin electrolyte layer and allowed active conditions for oxidation to be maintained. Adsorbed COH and CH XOH were identified as the intermediates.