Piotr Kedzierzawski

Comparative impedance spectroscopy study of rutile and anatase TiO2 film electrodes

The purpose of this work was a comparative electrochemical impedance analysis of the titanium dioxide-solution interface for two kinds of thin film samples consisting respectively of anatase and rutile. These kind of films, frequently used in photoelectrochemical experiments, have recently been the subject of detailed physical studies showing important differences in electrical properties between those two forms of TiO2. The EIS measurements were carried out over a wide frequency range (from 0.03 Hz to 10 kHz), analysing the whole spectrum.

Photoelectrochemical studies pertaining to the activity of TiO2 towards photodegradation of organic compounds

Abstract Photoanodic (water and methanol oxidation) and cathodic (oxygen reduction) processes occurring at illuminated (with near - UV light) bulk and particulate anatase and rutile TiO 2 films were analysed. (Photo) current-potential curves recorded for both anodic and cathodic processes point to large differences in activity between the two forms of TiO 2 . In particular, in the presence of various organic compounds, the photocurrent onset potential of anatase tends to shift negatively (sometimes as much as 0.3 V), thus increasing drastically the difference with respect to the potential of oxygen reduction. The situation is quite different for the rutile samples, the photocurrent onset potential of which appears practically unaffected by the addition of organic species to the solution and remains close to the onset potential of oxygen reduction. The reverse reaction with respect to the photoanodic process, involving species generated by the photo-oxidation of water and OH − ions, is shown to play often an essential role in determining the photocurrent onset potential (i.e. the operating mixed potential) of the photocatalyst. This is the prevailing situation for rutile but is much less frequent for anatase. In fact, a number of species, especially among organic compounds, appear able to compete efficiently for positive holes with OH groups adsorbed at the anatase surface, leading to a virtual suppression of the reverse reaction. In such a case, the reduction of oxygen present in the solution becomes the main cathodic process. This description is consistent with the fact that, in most cases of real photodegradation processes, the anatase form of TiO 2 exhibits much greater activity than the rutile form.

Effect of Multivalent Cations upon Reduction of Nitrate Ions at the Ag Electrode

Cathodic reduction of nitrate ions at a polycrystalline silver electrode was reexamined, particularly, in the presence of and cations in the solution. Although the onset potential for this reaction is located above the zero charge potential of Ag, a marked catalytic effect of ions, was observed, resulting in a large positive shift of the voltammetric peak associated with nitrate reduction. Electrochemical quartz crystal microbalance measurements demonstrated formation at the electrode surface, close to the onset potential for reduction, of films assigned to and/or in the solutions containing and ions, respectively. The observed effect of ions upon nitrate reduction is explained in terms of acid‐base catalysis, involving species diffusing as La(III)‐nitrate complexes (ion pairs) close to the silver electrode.

Hydrogen evolution on hot and cold consolidated Ni-Mo alloys produced by mechanical alloying

Ni80Mo20 and Ni57Mo43 alloys produced by mechanical alloying (MA) were used as the electrode materials for hydrogen evolution from NaOH within a limited temperature range of 20–60°C. To form the electrodes, the Ni–Mo alloys were consolidated in two different ways. One was ‘cold pressing’, which retained the original nanocrystalline structure of Ni80Mo20, and the amorphous structure of Ni57Mo43 and another one was ‘hot pressing’, which produced multiphase systems (Ni4Mo+MoO2) and improved the mechanical properties of the resulting electrodes too. Appreciable cathodic current densities of ∼100 mA cm−2 were measured at these electrodes. The estimated values of the apparent activation energy for cold consolidated materials were much lower, whereas those for the exchange current were much higher than the apparent activation energy values for a smooth, polycrystalline Ni plate.

Electrochemical reduction of CO2 at metallic electrodes

Publisher Summary The quantities of carbon stored in the form of atmospheric carbon dioxide, CO 2 in the hydrosphere and carbonates in the terrestrial environment, substantially exceed those of fossil fuels. In spite of this, the industrial use of carbon dioxide as a source of chemical carbon is currently limited to preparation of urea and certain carboxylic acids as well as organic carbonates and polycarbonates. However, the situation is expected to change in the future, if effective catalytic systems allowing activating carbon dioxide will become available. In this connection, the electrochemical reduction of CO 2 , requiring only an additional input of water and electrical energy, appears as an attractive possibility.

Modification of surface activity of Cu-based amorphous alloys by chemical processes of metal degradation

Abstract Changes in morphology, local chemical composition, catalytic activity and local surface-enhanced Raman scattering (SERS) activity due to various methods of modification of surfaces of Cu–Zr, Cu–Hf and Cu–Ti amorphous alloys were investigated. Our recent investigations have shown that otherwise detrimental processes of material degradation may help to segregate clusters of Cu and to develop a large specific surface area of Cu on ZrOx or HfOx supports. This transforms the original Cu-based amorphous ribbons of low surface area into efficient and stable catalysts. The following methods of activation based on the degradation processes, and their combinations are presented: ageing/oxidation in air, anodic activation, and pre-treatment with hydrogen (hydrogenation under high hydrogen pressure, or cathodic hydrogen charging). Structural changes in the bulk of the amorphous alloys introduced by the above pre-treatments were examined by XRD. Morphological and chemical changes on the surface were followed microscopically (optical microscopy and high resolution SEM) and by using a high resolution scanning Auger microprobe (SAM) and an X-ray electron microprobe. The amount of segregated Cu on the surface was estimated by an electrochemical method. A test reaction of dehydrogenation of 2-propanol was used to compare the catalytic activity of the materials after different chemical pre-treatments. Proposed mechanisms of changes in the local adsorption properties, local enhancement of Raman spectra and the catalytic activities due to the above combined pre-treatments are discussed.

Stability of the passive state of Al–Ta and Al–Nb amorphous alloys

Abstract The effect of Cl − ions (0–1 M NaCl) on the anodic behavior of two series of sputter-deposited Al–Ta (11–46 at.%) and Al–Nb (13–46 at.%) amorphous alloys (AAs) was investigated in neutral electrolytes by using electrochemical and in situ microscopic methods. The samples were characterized by Auger electron spectrometry; a thin alumina film was found to cover the surface of the substrate, whereas transition metal was not detected therein. In solutions containing Cl − , the alloys broke down at and above the pit nucleation potential E np . The alloys exhibited a stable passivity within a much wider (Δ E >1 V) potential range than crystalline Al. In situ microscopic investigations revealed that for Al–Ta and Al–Nb AAs, the local current density within the pits was ∼10 6 higher than the current density from the remaining passive surface. The results are compared with those for Al–Mo and Al–W alloys and the mechanism of restrained passivity break down is discussed.

SYNTHESIS AND ELECTROCATALYSIS APPLICATION OF HYBRID PLATINUM/CERIUM OXIDE/MULTI-WALLED CARBON NANOTUBES

The hybrid nanomaterials of platinum/cerium oxide/multi-walled carbon nanotubes (Pt/CeO2/MWCNTs) are synthesized successfully via impregnation and polyol processes. MWCNTs serve as an excellent supporter where CeO2 nanoparticles are decorated with well-distributed Pt nanoparticles. Images show the average particle size of crystalline Pt and CeO2 on MWCNTs are 3–7 and 20–30 nm, respectively. In electrochemical reaction, the redox peak of Pt/CeO2-700°C/MWCNTs reveals lower potential and higher current density in methanol electro-oxidation than those of other Pt-based ones. The study indicates that the cerium oxide in Pt/CeO2-700°C/MWCNTs catalyst will enhance significantly the oxygen ions transportation between the interface of Pt and MWCNTs to eliminate the CO poison effect on Pt catalyst.

Erratum to "Modification of surface activity of Cu-based amorphous alloys by chemical processes of metal degradation" (Appl. Catal. A: Gen. 235 (2002) 157-169)

Erratum to “Modification of surface activity of Cu-based amorphous alloys by chemical processes of metal degradation” [Appl. Catal. A: Gen. 235 (2002) 157–169] M. Janik-Czachor a,∗, A. Szummer b, J. Bukowska c, A. Molnar d, P. Mack e, S.M. Filipek a, P. Kedzierzawski a, A. Kudelski c, M. Pisarek b, M. Dolata a, M. Varga d a Institute of Physical Chemical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland b Department of Materials Science, Technical University, Woloska 141, 02-507 Warsaw, Poland c Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland d Department of Organic Chemistry, University of Szeged, Dom ter 8, 6720 Szeged, Hungary e Thermo VG Scientific, East Grinstead, West Sussex, England, UK

Synthesis and Electrocatalytical Application of Hybrid Pd/Metal Oxides/MWCNTs

The performance of Pd electrocatalysts for formic acid electrooxidation was improved by application of metal oxide-multiwall carbon nanotubes composites as a catalyst support. Hybrid oxides/MWCNTs were synthesized by two different methods: chemical reduction method and impregnation method. Pd based catalysts were synthesized by polyol method on the MWCNTs or oxide/MWCNTs composites. The In2O3 was deposited on MWCNTs by impregnation method (In2O3/MWCNTs-IM support) and in the presence of NaBH4 (In2O3/MWCNTs-NaBH4 support). The physical properties of the Pd/In2O3/MWCNTs-IM, Pd/In2O3/MWCNTs-NaBH4, Pd/SnO2/MWCNTs, and Pd/MWCNTs catalysts were characterized and their electrocatalytical performance in formic acid oxidation was compared. During Pd deposition on In2O3/MWCNTs-NaBH4 support, InPd2 structure was formed as observed by XRD. The electrochemical tests indicate that the two Pd/ In2O3/MWCNTs electrocatalysts have higher electrocatalytic activity than those of Pd/SnO2/MWCNTs and Pd/MWCNTs. The best performance was observed for the catalyst obtained by In2O3 impregnation of MWCNTs denoted by Pd/In2O3/MWCNTs-IM.