Akiko Aramata

Electrooxidation of methanol on platinum bonded to the solid polymer electrolyte, Nafion

The electrooxidation of methanol was enhanced on PtSn-SPE, PtRu-SPE and PtIr-SPE in sulfuric acid solution, when compared with the activity of Pt-SPE, which has already been shown to have a higher activity than a Pt electrode. SPE is an abbreviation for Nafion, a solid polymer electrolyte. It is suggested that this dual enhancement of the oxidation rate for PtSn-SPE and PtRu-SPE catalysts is due to the modification of the oxidation state of Pt by Sn and Ru and to the presence of H2O and CH3OH, both modified by the SPE matrix. This modification appears to weaken their hydrogen bonds in solution. Both Pt and Ir have catalytic properties for methanol oxidation, but a PtIr-SPE catalyst showed a more enhanced catalytic activity than either of them. This will be discussed in terms of Ir, oxidized at relatively low positive potentials, assisting the redox process of Pt0/Pt2+ or Pt2+/Pt4+ in the SPE matrix, where CH3OH and H2O are present in modified forms. For comparison, IrPd-SPE was also used as an electrode and showed a higher activity than Ir alone, although Pd did not have any activity toward methanol oxidation in sulfuric acid solution. Irrespective of the kind of Pt-SPEs, the Tafel slope was approximately 120 mV; the CH3OH concentration dependence was of the order of 0.2–0.6. The pH dependence was nearly 0.5 against NHE. The activation energy of the Pt-SPEs for the reaction ranged between 20 and 33 kJ mol−1.

Adsorption of hydrogen on platinum single crystal electrodes in acid and alkali solutions

Abstract The electrochemical behaviour of hydrogen and oxygen adsorption on platinum low- and high-index planes was examined in acid and alkali solutions. The features varied systematically with the surface structure and the pH of solution and are discussed in terms of steps and terraces of the respective surfaces. With respect to adsorbed hydrogen, Pt(111) provides mainly a long-range ordered adsorption site, Pt(100) both long-range and short-range ordered sites and Pt(110) mainly a short-range ordered site, respectively. The hydrogen wave observed on Pt(111) from 0.05 to 0.35 V has a very flat shape, like a platform, and is common in H 2 SO 4 , HClO 4 and NaOH solutions. A monoatomic (111) step was confirmed to provide (110) sub-step sites along the step. With respect to adsorbed oxygen species, two peaks, P O,1 and P O,2 , are observed at Pt(111) only in alkali solution. They are attributed to the adsorption of OH from water and O from OH, respectively. Interestingly, these species do not affect the surface structure, leaving the original surface when they are removed.

The electroreduction of carbon dioxide by macrocyclic cobalt complexes chemically modified on a glassy carbon electrode

Abstract Various macrocyclic cobalt complexes were chemically bonded to a glassy carbon (GC) electrode, on which CO 2 , electroreduction was carried out to give CO in aqueous phosphate buffer solution of pH 6.3. The macrocyclic cobalt complexes were naphthalocyanato cobalt(II), phthalocyanato cobalt(II), dibromo(11-hydroxyimino-4,10-dipropyl-5,9-diazatrideca-4,9-diea-3-one oximato) cobalt(III) (denoted as CoDO), two kinds of hydrophobic vitamin B 12 s (heptamethyl cobyrinate perchlorate and heptapropyl cobyrinate perchlorate 5,10,15,20-tetraphenylporphyrinato cobalt(II), and 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrinato cobalt(II). Their redox potentials Co(I)/Co(II) are in the above order from positive to negative potentials in a 0.05 M TBAP DMSO solution, being between -0.23 and -1.0 V (SCE). These complexes were chemically bonded to GC through -CONH-pyridine which locates perpendicularly to a planar semi-planar complex structure, where the N of the pyridine forms a coordinate bond with the Co atom of the above complexes as a fift ligand. The catalytic activity for hydrogen evolution in aqueous solution was observed to be high on Co naphthalocyanine and C phthalocyanine modified GC electrodes; the latter gave H 2 evolution at the most positive potentials among the Co complexes employee. When the lower potential limit in cyclic voltammetry became less than the hydrogen evolution potential, in the reverse positive-going sweep, and anodic lump current was oberved at -0.35 ∼ -0.78 V, which is assigned to a cobalt hydride oxidation process; the hydrid is suggested to form when hydrogen evolution takes place, and the hump disappeared after the introduction of CO 2 into the solution, was observed that the Co complex chemically bonded on GC can give CO from CO 2 only at relatively low overvoltages, except for CoDO which was not able to reduce CO 2 ; phthalocyanato cobalt(II) gave CO at E = − 1.0V (0.26 V as overvoltage) at 20% current efficiency. The highest CO current efficiency was observed in the case of tetraphenyl-porphyrinato cobalt(II) chemically modified GC.

Hydrogen and anion adsorption at platinum single crystal electrodes in phosphate solutions over a wide range of pH

Abstract The adsorption of hydrogen and anions at platinum single crystal, mainly Pt(111), electrodes in phosphate buffer solution was studied by electrochemical and in-situ FTIR measurements over a wide range of pH and potential. Phosphoric acid and its anions behaved essentially similarly to sulphuric acid, being different from perchloric acid. The weakly adsorbed hydrogen was not affected by the kind of anions and pH. Strongly adsorbed hydrogen on Pt(111) split into two parts at pH = ca. 5. These two parts united again at pH > 12, giving the same peak (P 0,1 ) as observed in 0.1 M NaOH. A potential shift in the positive direction past the double layer region caused a change of the adsorbed PO 4 3− to the protonated adsorbate, HPO 4 2− at high pH and of the adsorbed HPO 4 2− to H 2 PO 4 − at neutral pH, respectively. Comparison of the electrochemical results with the in-situ FTIR results suggested the importance of adsorbed OH in the change of the adsorbed phosphate species. The effect of the surface structure was not significant for phosphate adsorption.

Study of methanol electrooxidation on RhSn oxide, PtSn oxide, and IrSn oxide in comparison with that on the Pt metals

Abstract Catalytic activities of RhSn oxide, IrSn oxide, and PtSn oxide were studied with respect to their tin oxide effect in comparison with those of the Pt metals and each other. Surface chemical states were observed using XPS for the RhSn and PtSn oxides. Electrochemical and XPS results were compared to each other. Tin oxide showed a pronounced enhancement of the catalytic activity toward methanol electrooxidation on PtSn oxide with respect to Pt itself in acidic solutions, but not in alkaline solutions. In the case of RhSn oxide, tin oxide had a negative catalytic activity effect, with respect to Rh, and had no effect in the case of IrSn oxide, with respect to Ir. This difference was considered to be correlated to the presence or the absence of redox coupling of the respective Pt group metal species. Formaldehyde electrooxidation was also studied on PtSn oxide in acidic solution. In alkaline solution, PtSn oxide was also used for not only formaldehyde, but also formate electrooxidation and its catalytic activity was discussed in comparison with Pt itself.

Methanol electrooxidation on platinum directly bonded to a solid polymer electrolyte membrane

Abstract Methanol electrooxidation in perchloric acid solution was performed on platinum directly bonded to a solid polymer electrolyte (Pt-SPE) membrane. The initial catalytic activity of platinum on either a cation or anion-exchange membrane of the SPE was comparable with that of platinized platinum(pt-Pt). After an initial sharp deactivation in a short time at the polarization, the Pt-SPE was found to retain high activity for a long duration, whereas the pt-Pt showed a considerable decrease in the activity during the polarization. A surface mediator action of Pt 0 and Pt 2+ was stressed as essential for the durability of a high catalytic activity of Pt-SPE, and it was suggested that the matrix stabilized the platinum at a higher oxidation state, Pt 2+

Aminopyridyl cation radical method for bridging between metal complex and glassy carbon: cobalt(II) tetraphenylporphyrin bonded on glassy carbon for enhancement of CO2 electroreduction

Aminopyridyl compound immobilization on glassy carbon (GC) was carried out by amine cation radical formation through electro-oxidation at GC in ethanol solution, and subsequently by chemical bonding of the radical to GC. The aminopyridyl group immobilized on GC takes part as a bridge between cobalt tetraphenylporphyrin (Co(II)TPP) and GC, where the nitrogen of the pyridyl group is coordinated as a fifth ligand of Co(II)TPP, being perpendicular to the porphyrin plane. The Co(II)TPP modified GC electrode was thus prepared by a bridging amine of 4-aminopyridine, 4-aminoethylpyridine, or imidazole in the electrochemical and chemical processes, which is denoted ‘CoTPP bonded GC’. Whereas Co(II)TPP physically attached to GC, denoted CoTPP|GC, was not able to reduce CO2 atE = −1.1V(SCE) in pH 6.3 phosphate buffer solution, ‘CoTPP bonded GC’ gave CO from CO2 electroreduction at that potential with 60% current efficiency. By polarization of ‘CoTPP bonded GC’ at hydrogen evolution potentials, Co(II)TPP stacked on ‘CoTPP bonded ’ is suggested to be converted to HCo(II)TPP, but Co(II)TPP on CoTPP|GC remains the same. Hydrogenated ‘CoTPP bonded GC’ is suggested to play an important role as one of the intermediates of CO2 electroreduction.

Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode: Part II. Emirs and polarization-modulated irras stuidy of the adsorbed co layer in acidic and alkaline solutions

Infrared spectra of CO adsorbed on a gold electrode in 1 M HClo4, 0.2 M NaOH and 0.2 M (CH3)4NOH have confirmed that the adsorption is greatly enhanced in the alkaline solutions compared to the acidic solution. The enhanced adsorption is apparently assisted by OH− ions while there is little effect from the alkali metal ions. The adsorption reaches a maximum around −0.1 V (RHE). The CO stretching frequency at a constant potential shifts to lower frequencies by ca. 50–60 cm −1 with the decrease of CO coverage. The observed frequency shift with coverage is opposite in nature to that found in the gas phase for the Au/CO system. Oxidation of the CO layer established initially at –0.1 V, proceeds randomly in the adsorbed layer from ca. 0.7 V, giving rise to a steep decrease of the intensity of the CO stretching band with a concomitant large shift of the CO stretching frequency to lower wavenumbers. The potential dependence of the CO stretching frequency is linear, 64 cm−1/V, both in the acidic and alkaline solutions, between −1.1 V and +0.7 V(SHE). The infrared frequencies of the CO stretching vibration are lower that the frequencies observed by SERS (surface-enhanced Raman spectroscopy) by more than 50 cm−1

Effects of adsorbed CO on hydrogen ionization and CO oxidation reactions at Pt single-crystal electrodes in acidic solution

CO was adsorbed on the three low index planes of Pt single-crystal electrodes, and the hydrogen ionization and the dissolved CO oxidation reactions on the CO-covered electrodes were investigated in acid solution. The oxidation wave of CO adsorbed at 50 mV vs. a reversible hydrogen electrode had two peaks; a small prepeak at around 0.5 V and a main oxidation peak at around 0.7 V. Fourier transform IR measurement and the number of electrons per site for adsorbed CO indicated that these oxidation peaks could not be assigned individually to the linear and the bridged CO species. These peaks reflect the difference in the kinetic stability among the adsorbed CO. The stripping of the prepeak CO (CO1(a)) initiated both the hydrogen ionization and the dissolved CO oxidation reactions to a value close to the diffusion-limited values. These reactions required only a very small number of free sites which are provided in the present case by the removal of the kinetically unstable CO1(a). CO1(a) was absent in the adsorbed CO at 0.4 V. The voltammograms for the dissolved CO oxidation on the CO-covered electrodes revealed the presence of other two adsorbed states, COII(a) and COIII(a), where COII(a) was most stable and gave the main peak of the oxidation wave of the adsorbed CO. COIII(a) was very sensitive to the experimental conditions and was responsible for the hump that appeared in the dissolved CO oxidation.

Electrocatalytic activity of CoII TPP-pyridine complex modified carbon electrode for CO2 reduction

Abstract A cobalt(II)tetraphenylporphyrin (CoIITPP)-pyridine complex modified glassy carbon electrode (CoTPP/py/GC) was prepared and characterized in the form of CoIITPP-py-NHCO-GC. The catalytic activity for the electrochemical reduction of CO2 was studied in phosphate buffer; the electrode showed a high catalytic activity for CO2 reduction to CO at potentials more negative than −1.0 V vs. SCE (∮.4 V vs. RHE), with a current efficiency of 92% for CO production at −1.1 V vs. SCE. CoTPP/py/GC also showed a high durability of the catalytic activity and the overall turnover number (mol of CO produced/mol of CoIITPP on GC) exceeded 107. The role of the pyridine in CoTPP/py/GC is discussed in connection with the activity and stability as an electrode.