Adsorption And Reaction Of Iron Protoporphyrin IX On Electrode Surfaces With In Situ Vibrational Spectroscopies

Surface electrochemistry is a rapidly growing inter-discipline of surface scienceand electrochemistry. A recent hot topic in surface electrochemistry is the interfacialbioelectrochemistry which involves the construction, characterization and utilizationof bioactive electrode surfaces modified with functional biomolecules and theirrelated model compounds. These functional electrode/electrolyte interfaces providenot only platforms for the study of the electron transfer but also for the potentialapplications in electrocatalysis and bio-sensing.Metalloporphyrins exist in many organisms as prosthetic groups of proteins.Specifically, iron protoporphyrinâ…¨(simplified as FePP) is the common active centerof myoglobin, hemoglobin, cytochrome C and peroxide enzymes. FePP has manyimportant functions such as transportation and biocatalysis in vivo. Recent years,FePP modified electrodes were found to exhibit excellent electrocatalytic effects onthe reduction of dioxygen, hydrogen peroxide, nitrite, carbon dioxide and so on. Thestudy of potential dependent structure and reaction of FePP adlayer at electrodes notonly helps to understand its biocatalytic functions at molecular level but also lay thebasis for this functional electrode to be used in catalysis and sensing aspects.ATR-surface enhanced IR absorption spectroscopy (ATR-SEIRAS) and surfaceenhanced Raman spectroscopy (SERS) have high surface sensitivity which willfacilitate in situ monitoring of surface adsorption and reaction. Most of the skeletalvibrations of FePP are Raman-active. Strong SER effects are produced mainly oncoinage metals which limited the study of versatile functional surfaces with FePP.The complicated surface selection rule of SERS makes it difficult to deduce theadsorption configuration of adsorbates. Further more, it is quite difficult if notpossible to obtain directly the binding information of small ligands toward FePPadlayer by SERS which can only be judged by the change in relative intensities ofporphyrin ring bands. SEIRAS is very sensitive to the highly polar small molecules,including CO and NO, facilitating the study of surface coordination of FePP with COor NO. In addition, SEIRAS is also suitable for detecting the peripheral propionicand propionate groups of FePP adlayer, thus may help to understand the adsorptionconfiguration of this molecule. So far, lead acid batteries remain the most frequently used batteries of all with itshigh performance-price ratio, secure and reliable operation. However, the seriousanodic corrosion and the high impedance layer formed on surface of the grid alloy arethe main factors to affect the discharge performance and deep charge/discharge cyclelife of the batteries. The formation of the high impedance layer is related to thecomposition and the properties of the alloy, as well as the growth mechanism of theanodic oxide film formed on the alloy. It is part of our thesis work to study the growthmechanism of anodic film and select the suitable alloying elements from rare-earthmetals to tackle the anodic corrosion problem.The main results and conclusions of the dissertation are summarized as follows:(â… ) In situ ATR-SEIRAS study on FePP adlayer self-assembled on a Au electrodeIn situ ATR surface enhanced IR absorption spectroscopy (ATR-SEIRAS) wasfirstly applied to probe potential dependent adsorption configuration of FePP adlayeron Au (FePP/Au) electrodes in 0.1 M HClO₄ (pH 1) and PBS (pH 7).A SEIRA-active Au nanoparticle film (ca. 60-nm-thick) was deposited on thetotal-reflecting plane of hemicylindrical Si prism with electroless plating method. TheFePP adlayer immobilized on Au nanoparticle film by soaking the latter in a basicsolution containing dilute FePP as the working electrode.FePP is of approximately D_4h symmetry structure, IR-active in-plane stretchingvibrations of porphyrin ring skeleton are very weak, only the peripheral propionicacid groups of FePP can be observed in pH 1 solution. At lower potentials, statistically,two peripheral propionic acid groups predominate, FePP mainly lie nearly flat on theAu surface; while at higher potentials, one of them is deprotonated with its twooxygen atoms attached on Au surface, yielding a kind of reorientation (tilting) of theporphyrin ring. The steric hindrance probably prevents the simultaneous adsorption oftwo carboxylate groups on Au surface even at higher potentials.Slight different spectral features can be found at pH 7. Firstly, the band forperipheral propionate groups is much stronger at higher potentials at pH 7; Secondly,the porphyrin ring may re-orientate to a relatively large extent by tilting up ring planethrough anchoring a carboxylate group on the metal surface at ca. 0.5 V at pH 1 and-0.4 V at pH7, respectively; Thirdly, the band 1563 cm^-1, assigned toν_asCOO-,appeared at the potential higher than -0.4 V at pH 7. Chemical dissociation based on pKa of the two peripheral propionic acid groups at pH 7 may also contribute to thedifferent spectral features observed.(â…¡) in situ ATR-SEIRAS on CO adducts of FePP adlayer self assembled on a AuelectrodeCoordination chemistry of CO with metalloporphyrins is a biologically andchemically important topic as promoted by the interest in comparing the coordinationproperties of metalloporphyrins with heme proteins. Historically, studies on CObinding with metalloporphyrins were mainly limited to bulk phase reactions.The surface coordination chemistry of carbon monoxide with the reduced form(Fe^â…¡PP) of iron(â…¢) protoporphyrinâ…¨(Fe^â…¢pp) monolayer self-assembled on a Auelectrode in 0.1 M HClO₄ was studied for the first time by using in situ ATR- surfaceenhanced infrared absorption spectroscopy (ATR-SEIRAS).Both mono- and bis- carbonyl adducts [simplified as Fe^â…¡(CO)PP andFe^â…¡(CO)₂PP, respectively] were detected on the FePP/Au(prepared with 50μMFePP in 0.1M borax), depending on the history of potential control. Initially theFe^â…¡(CO)PP predominates, and the intermediate transition potential for the conversionof Fe^â…¡(CO)PP to Fe^â…¢PP and CO was spectrally determined to be ca. 0.09 V (vs. SCE).The ratio of Fe^â…¡(CO)₂PP and Fe^â…¡(CO)PP increases after a potential excursion to asufficiently positive value. Fe^â…¡(CO)₂PP is much more stable against itselectro-oxidation to Fe^â…¢PP than its counterpart Fe^â…¡(CO)PP with increasing potential.The observed change of coordination properties may be ascribed to an irreversiblestructural reorganization of the FePP adlayer caused by the potential excursion.The great difference of adsorption of FePP on Au can be found with FePP/Auprepared with saturated FePP in acetone. Several relative weak IR-active in-planestretching vibrations of porphyrin ring skeleton can be observed instead of theperipheral propionic acid groups. Fe^â…¡(CO)PP can also be found but rather weak, nopeaks ascribed to Fe^â…¡(CO)₂PP can be observed with this method, suggesting thestrong coordination of acetone to FePP.(â…¢) in situ ATR-SEIRAS on NO adduets of FePP adlayer self assembled on aAu electrodeNitric oxide is unique among diatomic molecules in that it can bind to manymetals of different oxidation states and different electron configurations. The coordination chemistry of NO with iron porphyrins has been intensively investigated,inspired by both fundamental and technological interests ranging from relevantbiological functions including vasodilatation and neuronal communication toenvironmental concerns about (bio)catalytic reduction and oxidation of nitrites. TheNO coordination with FePP/Au is complicated: first, NO can coordinate to both ferricand ferrous protoporphyrins; second, multiple isoelectronic forms are present for aNO adducts of either ferric or ferrous protoporphyrin; third,ν_N-O of NO coordinatedto Fe^â…¡pp is close toδ_HOH of water andν_C-O of peripheral carboxylic groups, and thusshould be dealt with care.Based on potential controlled ATR-SEIRAS on independent FePP/Au electrodesand multi-step ATR-SEIRAS measurement on one same FePP/Au electrode, for thefirst time, up to three IR bands corresponding to three types of nitrosyl adducts ofFePP have been identified with their intensities (concentrations) varied with thepotential applied. The 1915 cm^-1 band, which shows up at relatively positivepotentials and stabilizes in a rather narrow potential range can be reasonably assignedto the Fe^â…¡(NO)(OH₂)PP species or its isoelectronic format Fe^â…¡(NO)⁺(OH₂)PP. Theother two bands with much lower frequencies, which can stabilize over a much widerpotential range and exhibit nearly opposite potential dependent intensities, arebasically characteristic of nitrosyl adducts of ferrous FePP. One band at ca. 1670 cm^-1with insignificant Stark effect, can be attributed to Fe^â…¡(NO)PP. The other above 1705cm^-1 with significant Stark effect, could be ascribed to Fe^â…¡(NO)₂PP. The multi nitrosyladductions may be caused by the largely inhomogeneous structure of the FePP adlayeron Au electrodes.The exchange of NO and CO on FePP/Au in 0.1M HClO₄ are brieflyinvestigated. Stronger binding of NO with FePP adlayer prevents the CO coordinationwith the latter, and the formation of Fe^â…¡(CO)PP was only found at potentials negativeof -0.2V where Fe^â…¡(NO)PP is partially reduced. The exchange of CO and NO onFePP/Au in 0.1M HClO₄ didn't arouse the change of adsorption orientation of FePPon Au surface.(â…£) in situ SERS and in situ ATR-SEIRAS study of iron protoporphyrinmonolayer adsorption on polycrystalline Pt electrodeSelf-assembly of molecules including thiols and porphyrins on Au and Agelectrodes has been intensively studied. To broaden the types of functional electrodes,FePP self-assembled on Pt electrode is among the targets of our thesis work. SERS is suitable for detecting the skeletal vibrations of FePP adlayer whereas SEIRAS is goodfor detecting polar ligands and peripheral groups of FePP adlayer. For the first time,both in situ SERS and in situ ATR-SEIRAS were applied to probe adsorption of FePPadlayer on Pt electrodes in 0.1 M HClO₄.The potential dependent SERS spectra of FePP adlayer with the enhanced factornearly 10³ were acquired on a roughened Pt electrode. Analyses of spectral datarecorded at pH1 over a wide potential region made it possible to obtain potentialdependent adsorption isotherms, from which the stranded redox potential was foundto be ca. -0.2 V. Up-tilted orientation of FePP on Pt electrode was assumed based onin situ SERS and in situ ATR-SEIRAS measurements. Unlike FePP on Au electrode,FePP on Pt electrode hardly reorients within the potential range investigated.SERS was also extended to the measurements of FePP adsorption on roughenedPt electrode performed in pH 3 and pH 7 solutions (PBS). The intermediate transitionpotential of Fe³⁺/Fe²⁺ moves to a lower potential at higher pH.(â…¤) Study of polarization behavior of the anodic Pb(â…¡) film on Pb eletrode andthe effect of alloying element Yb on the growth of anodic Pb(â…¡) and PbO₂films on Pb alloy electrode in 4.5 M H₂SO₄The anodic polarization behavior of anodic Pb(â…¡) film preformed on Pbelectrode was studied by using fast speed linear sweep voltammetry (LSV),electrochemical impedance spectroscopy and (EIS) scanning electron microscope(SEM). The mechanism of the formation of the anodic PbO film is suggested asfollows: (1) Pb+OH^-→Pb(OH)_ad+e^-; (2) Pb(OH)_ad+OH^-→PbOOH^-+H⁺+e^-(rds); (3)PbOOH^-→PbO+OH^-.The anodic Pb(â…¡) films formed on Pb and Pb-Yb electrode at 0.9V (vs Hg/Hg₂SO₄) in sulfuric acid solution were studied. The results showed that the additionof Yb can decrease the value of Z' for the anodic Pb(â…¡) film and inhibit the growth ofthe film. According to above mechanism proposed for the formation of the anodicPbO film, the inhibition effect of this alloy element can be explained as follows: thesaturated concentration of Yb³⁺ is far smaller than that of PbOOH^-, so Yb³⁺ willprecipitate earlier than PbOOH^- at the innermost of film. Once the concentration ofPbOOH^- is close to the saturated concentration of Yb³⁺, PbOOH^- will co-precipitatewith Yb³⁺ at the innermost of film, inhibiting the film growth.The study of anodic PbO₂ film (α-PbO₂ andβ-PbO₂) formed on Pb-Yb electrode at the deep charge potential of 1.4 V in sulfuric acid solution was carried out. Theresults showed that the addition of Yb can promote the growth ofα-PbO₂ but inhibitthe growth ofβ-PbO₂. The growth of PbO under-layer can be inhibited at 1.4V asjudged from the impedance decease after alloying Pb with Yb.