Tin underpotential deposition on platinum and its catalytic influence on the kinetics of molecular oxygen electroreduction

The formation and dissolution of tin ad-layers on polycrystalline platinum were analysed by cyclic voltammetry in aqueous 10^–4 M tin(II)/1 M sulfuric acid in the 0.05–0.70 V versus RHE range. At this concentration level it was possible to observe that platinum sites involving (110) planes are mainly related to tin underpotential deposition. In contrast to previous results, no irreversible adsorption was found in the course of the electrodeposition. Thermodynamic adsorption parameters were calculated from the potential dependence of tin surface coverage. Catalytic properties of this new surface were studied on the basis of oxygen electroreduction as a model. Kinetic runs were performed with rotating ring-disk electrodes on bare and tin-modified platinum surfaces. Molecular oxygen reduction on tin-modified platinum takes place through the production of both water and hydrogen peroxide. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electronic Publication     

Impact of the Pt catalyst on the oxygen electroreduction reaction kinetics on various carbon supports

Micro- and mesoporous carbide-derived carbons synthesized from molybdenum and tungsten carbides were used as porous supports for a platinum catalyst. Synthesized materials were compared with commercial Vulcan XC72R conducting furnace black. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and low-temperature N₂ adsorption methods were applied to characterize the structure of catalysts prepared. The kinetics of oxygen electroreduction in 0.5 M H₂SO₄ solution was studied using cyclic voltammetry and rotating disk electrode methods. The synthesized carbide-derived carbons exhibited high specific surface area and narrow pore size distribution. The platinum catalyst was deposited onto the surface of a carbon support in the form of nanoparticles or agglomerates of nanoparticles. Comparison of carbide-derived carbons and Vulcan XC72R as a support showed that the catalysts prepared using carbide-derived carbons are more active towards oxygen electroreduction. It was shown that the structure of the carbon support has a great influence on the activity of the catalyst towards oxygen electroreduction.     

Mechanism of oxygen electroreduction on gold surfaces in basic media

The mechanism of the electroreduction of oxygen on Au surfaces in basic media is examined using surface-enhanced Raman scattering (SERS) measurements and density functional theory (DFT) calculations. The spectroscopy reveals superoxide species as a reduction intermediate throughout the oxygen electroreduction, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O(2) on Au(100) in base arises from a disproportionation mechanism which is enhanced on Au(100) relative to the other two low Miller index faces of Au.     

氧-硼共修饰多壁碳纳米管载铂催化剂的制备及氧还原活性

采用一种简便的方法,合成了氧-硼共修饰的多壁碳纳米管材料,以此为载体制备的铂基催化剂具有更小的铂粒径、更高的电化学表面积(40 m²·g_Pt^-1)和更高的氧还原活性(0.3 A·mg_Pt^-1)。氧、硼在提高碳纳米管的载体分散性、控制铂颗粒的均匀性和粒径、促进氧还原反应的氧吸附/解离方面发挥着重要的作用。     

氧-硼共修饰多壁碳纳米管载铂催化剂的制备及氧还原活性

采用一种简便的方法,合成了氧-硼共修饰的多壁碳纳米管材料,以此为载体制备的铂基催化剂具有更小的铂粒径、更高的电化学表面积(40 m²·g_Pt^-1)和更高的氧还原活性(0.3 A·mg_Pt^-1)。氧、硼在提高碳纳米管的载体分散性、控制铂颗粒的均匀性和粒径、促进氧还原反应的氧吸附/解离方面发挥着重要的作用。     

Single-wall carbon nanotubes supported platinum nanoparticles with improved electrocatalytic activity for oxygen reduction reaction

Significant enhancement in the electrocatalytic activity of Pt particles toward oxygen reduction reaction (ORR) has been achieved by depositing them on a single wall carbon nanotubes (SWCNT) support. Compared to a commercial Pt/carbon black catalyst, Pt/SWCNT films cast on a rotating disk electrode exhibit a lower onset potential and a higher electron-transfer rate constant for oxygen reduction. Improved stability of the SWCNT support is also confirmed from the minimal change in the oxygen reduction current during repeated cycling over a period of 36 h. These studies open up ways to utilize SWCNT/Pt electrocatalyst as a cathode in the proton-exchange-membrane-based hydrogen and methanol fuel cells.     

Electro-deposition of platinum nanoparticles on 4-mercaptobenzene-functionalized multi-walled carbon nanotubes

A new method to electro-deposit platinum nanoparticles on the surface of multi-walled carbon nanotubes (MWNTs) functionalized with 4-mercaptobenzene has been described. X-ray photoelectron spectroscopy results reveal that 4-mercaptobenzene was attached to the surface of MWNTs. Transmission electron microscope and X-ray diffraction analysis confirm that platinum nanoparticles were highly dispersed on the surface of MWNTs, and the average size of the platinum particle is 4.2 nm. The electrocatalytic properties of the Pt/MWNT composite electrode for methanol oxidation were investigated by cyclic voltammetry, and the results show that the fabricated composites exhibit high catalytic activity and good long-term stability. The study provides a feasible approach to fabricate Pt/MWNT composite electrode for direct methanol fuel cell.     

Electrodeposition and electrocatalytic properties of platinum nanoparticles on multi-walled carbon nanotubes: effect of the deposition conditions

Platinum (Pt) nanoparticles were electrochemically deposited on multi-walled carbon nanotubes (MWCNTs) through a three-step process, including an electrochemical treatment of MWCNT, electro-oxidation of PtCl₄ ²⁻ to Pt(IV) complex, and an electro-conversion of Pt(0) on MWCNT. The effect of formation conditions for Pt(IV) complexes on the Pt nanoparticals transformed was investigated. The structure and elemental composition of the resulting Pt/MWCNT electrode were characterized by transmission electron micrograph (TEM) and energy dispersive X-ray spectroscopy (EDX). The electrocatalytic properties of the resulting Pt/MWCNT electrode for methanol oxidation have been investigated. The high electrocatalytic activity and good stability of Pt/MWCNT electrode may be attributed to the high dispersion of platinum nanoparticles and the particular properties of the MWCNT supports.     

Electrochemical DNA biosensors based on platinum nanoparticles combined carbon nanotubes

Platinum nanoparticles were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced electrochemical DNA biosensor. Multi-walled carbon nanotubes and platinum nanoparticles were dispersed in Nafion, which were used to fabricate the modification of the glassy carbon electrode (GCE) surface. Oligonucleotides with amino groups at the 5′ end were covalently linked onto carboxylic groups of MWCNTs on the electrode. The hybridization events were monitored by differential pulse voltammetry (DPV) measurement of the intercalated daunomycin. Due to the ability of carbon nanotubes to promote electron-transfer reactions, the high catalytic activities of platinum nanoparticles for chemical reactions, the sensitivity of presented electrochemical DNA biosensors was remarkably improved. The detection limit of the method for target DNA was 1.0 × 10⁻¹¹ mol l⁻¹.     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.     

Development of porous cathode powders for SOFC and influence of cathode structure on the oxygen electroreduction kinetics

Meso/macroporous La_1−xSr_xCoO_3−δ powder with the specific surface area higher than 140 m² g⁻¹ has been synthesized from the corresponding nitrates, using solution thermal decomposition method. These nanopowders have been used for preparation of SOFC cathodes, demonstrating lower oxygen electroreduction activation energy than that for less porous cathodes prepared from the powders synthesized using traditional solid state reaction method. To increase macroporosity of the cathodes the special pore forming agent has been added into the raw cathode paste. The very low total polarization resistance and activation energy values have been obtained for oxygen electroreduction, depending on the cathode porosity and potential applied.     

Electrodeposition of platinum–nickel alloy nanocomposites on polyaniline-multiwalled carbon nanotubes for carbon monoxide redox

An electrochemical method was developed to deposit platinum (Pt)–nickel (Ni) alloy nanocomposites on polyaniline-multiwalled carbon nanotubes (Pt–Ni/PAN/MWCNTs). The material was characterized by various methods including field emission scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical techniques. An appreciably improved catalysis toward oxidation of carbon monoxide (CO) was observed at the Pt–Ni/PAN/MWCNTs nanocomposites (real ratio of Pt–Ni of 17:1), which was interpreted by a mechanism based on the bifunctional catalysis. The successful preparation of Pt–Ni/PAN/MWCNTs nanocomposites opens a new path to synthesize the promising catalysts for CO.     

Kinetics and mechanism of oxygen electroreduction on PtCoCr/C catalyst containing 20–40 wt % platinum

It is shown that the mechanism of oxygen electroreduction on the PtCoCr/C systems in 0.5 M H₂SO₄ is similar to that proposed for the Pt/C catalyst. The activity of ternary catalysts is by two and more times higher than that of monoplatinum catalyst. The constant k ₁ is much larger than k ₂ (k ₁ and k ₂ are the rate constants of O₂ reduction to water and H₂O₂, respectively) for all catalysts studied. This indicates that the catalytic systems are selective with respect to O₂ reduction immediately to water in the practically important potential range from 1.0 to 0.6 V. The yield of H₂O₂ increases with a shift of potential in the cathodic direction (<0.7 V) and does not exceed 1%. The sum of rate constants of further conversion of hydrogen peroxide also increases with a shift of potential in the cathodic direction. After a corrosion attack (a treatment in the acid for 24 h), a ratio between the rate constants (k ₁/k ₂) for the PtCoCr/C catalysts increases. This is caused by a considerable increase in k ₁, which is 2.84 × 10⁻² cm/s for the catalyst containing 34 wt % Pt (against 1.5 × 10⁻² cm/s for the untreated catalyst). This can be explained by the reaction proceeding on the particle surface, which was enriched in platinum in the course of corrosion treatment. The properties of platinum clusters on the alloy surface differ from those of monoplatinum as a result of the ligand effect. The amount of oxygen chemisorbed from water on this surface is lower than on Pt/C catalyst. This is the main factor determining an increase in the activity and stability of ternary catalysts.     

Mechanism of electroreduction of allyl alcohol at platinized platinum electrode in acidic aqueous solution.

The electroreduction of allyl alcohol to form propene at the platinized platinum electrode in acidic aqueous solution has been studied using CV plots, IR, ESR, and MS spectra, and a semiempricial MO method (MOPAC7/AM1, PM3). From the determinations of charge-transfer coefficients, reaction orders and apparent activation energy for the given reaction, the detection of the intermediates such as C(3)H(5)(+), C(3)H(5)(*), and C(3)H(5)(-) species, and PM3 calculations of charge distribution and frontier orbital energies of the reaction species C(3)H(5)OH and C(3)H(5)(+), the authors suggest that in acidic aqueous solution the production of propene via reductive splitting of the C-OH bond situated in the allyl position of allyl alcohol obeys a carbonium ion-carbanion mechanism.     

The beneficial effect of the addition of tungsten carbides to Pt catalysts on the oxygen electroreduction

Tungsten carbide nanocrystal modified Pt catalysts have been prepared by an intermittent microwave heating (IMH) method and show an improved activity for oxygen electroreduction in alkaline media.     

Mechanism of adsorption,electroreduction and hydrogenation of compounds with ethylenic bonds on platinum and rhodium: Part II. Comparison of the electroreduction and catalytic hydrogenation processes

The catalytic liquid-phase hydrogenation of maleic acid on platinum and rhodium has been investigated. It is shown that the rate-determining step of this process as well as of the electroreduction process of maleic acid is the interaction of the chemisorbed particle of maleic acid with the adsorbed hydrogen which is formed at the preceding rapid stage of either the dissociative adsorption of molecular hydrogen, or the electrochemical stage of hydrogen ion discharge. The rate of the process with the same degree of surface coverage with hydrogen and chemisorbed particles of maleic acid does not depend on whether the process is carried out catalytically or electrochemically, on whether maleic acid and hydrogen were preliminarily adsorbed on the surface of the electrode-catalyst or not. With due regard for the mutual influence of chemisorbed particles participating in the rate-determining stage, the main kinetic equations for the electroreduction and catalytic hydrogenation processes have been derived. The difference in the rates of electroreduction of maleic acid on platinum and rhodium, with the same degree of electrode surface coverage with reactants, is shown to be the result of differences in the adsorption heats (or bonding strength with the surface) of hydrogen and maleic acid on these two metals. Experimental procedures are described in Part I [1].     

Effect of carbon nanotubes content on crystallization kinetics and morphology of polypropylene

Thermoplastic nanocomposites were prepared in a laboratory mixer using polypropylene (PP) and different amounts of single-walled carbon nanotubes (SWNT) in the range 0.25–2 wt%. The effect of SWNT content on the thermal and mechanical properties and also morphology of the PP/SWNT nanocomposites were studied. The results obtained from nonisothermal crystallization of PP and the nanocomposites, which were carried out using the differential scanning calorimetry technique, showed that not only the overall rate of crystallization of PP increased when SWNT was added to the polymer but also the rate of nucleation was higher and the crystallite size distribution was more uniform for the nanocomposites than for PP. From the optical microscopy studies, it was found that the PP spherulites decreased in size when SWNT was introduced into the polymer and also the mature spherical shaped crystals of PP changed in part to the immature kidney- or bean-shaped crystal forms in the nanocomposites. In addition, the crystallization kinetics was also studied by using isothermal spherulitic growth rate, and the values of nucleation constant, K_g, and end surface free energy, σ_e, were calculated for PP and the nanocomposites according to Lauritzen–Hoffman theory. The reductions of these two parameters were in agreement with the fact that the rate of crystallization of PP in nanocomposites was higher than that of the pristine polymer.     

Effect of multi-walled carbon nanotubes on non-isothermal crystallization kinetics of polyamide 6

The non-isothermal crystallization behaviors of multi-walled carbon nanotubes (MWNTs)/polyamide 6 (PA6) composites were investigated by differential scanning calorimetry (DSC). Three methods, namely, Avrami, Ozawa and Mo, were carried out to analyze the non-isothermal crystallization data. The results showed that the MWNTs in PA6 acted as effective nucleation agents. However the crystallization rate of composites obtained was lower than that of the neat PA6. It is indicated that the presence of MWNTs influenced the mechanism of nucleation and the growth of PA6 crystallites.     

Regulation of singlet oxygen generation using single-walled carbon nanotubes

We have designed a novel photodynamic therapy (PDT) agent using protein binding aptamer, photosensitizer, and single-walled carbon nanotube (SWNT). The PDT is based on covalently linking a photosensitizer with an aptamer then wrapping onto the surface of SWNTs, such that the photosensitizer can only be activated by light upon target binding. We have chosen the human alpha-thrombin aptamer and covalently linked it with Chlorin e6 (Ce6), which is a second generation photosensitizer. Our results showed that SWNTs are great quenchers to singlet oxygen generation (SOG). In the presence of its target, the binding of target thrombin will disturb the DNA interaction with the SWNTs and cause the DNA aptamer to fall off the SWNT surface, resulting in the restoration of SOG. This study validated the potential of our design as a novel PDT agent with regulation by target molecules, enhanced specificity, and efficacy of therapeutic function, which directs the development of photodynamic therapy to be safer and more selective.