Sanja I. Stevanović

Structural Effect in Electrocatalysis: Formic Acid Oxidation on Pt Electrodeposited on Glassy Carbon Support

The structural effect of Pt nanoparticles on formic acid oxidation was studied using Pt electrochemically deposited on glassy carbon as a model system. The morphology of Pt deposited on glassy carbon is defined by agglomerates whose number, size, and distribution depend on Pt loading and support pretreatment as revealed by atomic force microscopy characterization. A scanning tunneling microscopy analysis of the electrodes showed that an increase of Pt loading leads to an increase of Pt particles size and their coalescences. Electrochemical treatment of the support prior to Pt deposition results in a decrease of the particle size on acidic treated support and in their negligible change on alkaline treated support. The coalescences of the particles detected cause the formation of different defects. The most active are the electrodes with the smallest Pt loading, and with the support treated in acid having the lowest defected surface and the highest contribution of high coordinated (111) facets exposed to the reaction. The activity of the electrode decreases as the number of defects grows with increasing of the loading or with alkaline pretreatment of support, i.e., coalescence of the particles. The results obtained suggest that the ratio between the facets and the defect sites rather than particle size determines the rate of the formic acid oxidation.

Electrochemical and Crystallographic Aspects of Lead Granular Growth

Lead granules synthesized by the potentiostatic regime of electrolysis were characterized by the scanning electron microscopy technique. Effect of the different parameters of electrolysis, such as solution composition, overpotential of electrodeposition, and quantity of the electricity, on lead granular growth has been systematically investigated. Aside from the electrochemical aspects of lead granular growth, crystallographic aspects of the obtained granules were also analyzed. In the dependence of the electrodeposition conditions, granules of various shapes were obtained. The granules, such as octahedrons and hexagons, as well as many various types of twinned particles: single-twinned, multiply-twinned, lamellar-twinned, and many other complicated shapes denoted as polyparticles, were synthesized through regulation of the parameters of electrolysis. Increasing both the concentration of Pb2+ ions and overpotential of the electrodeposition favored the formation of more complicated forms. Formation of granules of specified crystallographic characteristics was also correlated with the basic principle of metal electrocrystallization.

Characterization of nanoporous carbon fibrous materials obtained by chemical activation of plane tree seed under ultrasonic irradiation

An ultrasonic irradiation was applied for the impregnation by chemical agents in the chemical activation process of new type of active carbon precursor. Plane tree seed, due to the unique fibrous structure and low cost is a promising eco-friendly raw material for the preparation of activated carbon materials. Ultrasonic irradiation was used for the impregnation step allowing the chemical activation by different agents: potassium or sodium hydroxide, hydrogen peroxide and pyrogallol. The porous structures were examined by nitrogen adsorption/desorption isotherms at 77 K and electrochemically by cyclic voltammetry. The textures of these materials were observed by scanning electron microscopy. The application of ultrasonic irradiation in the impregnation step increased surface area of the final material more than two times in comparison to the material which impregnation in the activation process was by conventional stirring. Ultrasonic irradiation enhances the chemical activation process and the activated carbon fibrous materials with nanoporous structure were obtained by impregnation of seeds with alkaline hydroxides. Total surface areas of these samples were 976 m(2) g(-1) and 1130 m(2) g(-1). These fibers have total specific capacitance as high as 125 F g(-1) and 53 F g(-1) which major fraction in both cases originate from internal micropores structure.

Influence of a low content of PEO segment on the thermal, surface and morphological properties of triblock and diblock PCL copolymers

Two series, one of triblock (PCL/PEO/PCL) and the other of diblock (PCL/PEO) copolymers were prepared by ring-opening polymerization of ε-caprolactone catalized with tin(II) octoate and by using dihydroxy or monohydroxy poly(ethylene oxide) as the macroinitiator. The PEO block length was fixed (Mn 1,000 g/mol) and the PCL block lengths (Mn 10,000-40,000 g/mol) were tailored by changing weight ratio of ε-CL/PEO. The copolymers’ structure was confirmed by 1H and quantitative 13C NMR spectroscopy while their molecular weights were determined by GPC analysis. The thermal properties and the degree of crystallinity of the copolymers were investigated and compared by using DSC and WAXS. Both types of copolymers were semicrystalline with the orthorhombic PCL crystal lattice. The surface morphology of the copolymer films was investigated by using optical microscopy and AFM analysis, which confirmed the spherulitic lamellar structure with spherulites of different diameters. Data indicated that a low content of PEO segment had an influence on thermal degradation behavior, crystallinity and morphology of copolymers. Roughness of copolymer films was affected by the content of PEO and correlated with the spherulites’ diameter. The small changes in water and moisture absorption properties of copolymers compared to homopolymer PCL were observed.

Insight into electrocatalytic stability of low loading Pt-Bi/GC and Pt/GC clusters in formic acid oxidation

Formic acid oxidation was examined on platinum-bismuth deposits on glassy carbon substrate prepared by two-step process, i.e., electrochemical deposition of Bi followed by electrochemical deposition of Pt as described in our previous article (J Electrochem Soc 161:H547–H554, 2014). Upon treatment of as-prepared clusters by slow anodic sweep, bimetallic structure consisting of Bi core occluded by Pt and Bi-oxide was obtained and exhibited significant activity and exceptional stability in HCOOH oxidation. In order to explain such electrocatalytic stability, in this work, the electrochemical properties of Pt@Bi/GC catalyst were investigated applying same protocols in supporting electrolyte with or without HCOOH and compared with Pt/GC. The protocols comprised potentiodynamic, quasi-steady-state, and chronoamperometric measurements combined with the surface characterization by COads stripping voltammetry. Application of potential cycling at Pt@Bi/GC electrode in supporting electrolyte containing HCOOH leads to minor change in surface morphology, mildly leaching of Bi from the electrode surface, and negligible decrease in activity. On the other hand, significant Bi dissolution and considerable decrease in activity are the effects of the same treatment without HCOOH. Contrary to Pt@Bi/GC, Pt/GC electrodes subjected to the same protocols exhibit completely opposite properties being more stabile during potential cycling without HCOOH than in the presence of this acid. Exceptional stability in formic acid oxidation of Pt@Bi/GC catalyst is thus most probably the result of the combination of predominant dehydrogenation path of the reaction, suppressed Bi leaching, and compensation of dissolved Bi from the core as its source due to which surface morphology endured minor changes.

Enhanced activity in ethanol oxidation of Pt3Sn electrocatalysts synthesized by microwave irradiation

High surface area carbon supported Pt and Pt3Sn catalysts were synthesized by microwave irradiation and investigated in the ethanol electro-oxidation reaction. The catalysts were obtained using a modified polyol method in an ethylene glycol solution and were characterized in terms of structure, morphology and composition by employing XRD, STM and EDX techniques. The diffraction peaks of Pt3Sn/C catalyst in XRD patterns are shifted to lower 2θ values with respect to the corresponding peaks at Pt/C catalyst as a consequence of alloy formation between Pt and Sn. Particle size analysis from STM and XRD shows that Pt and Pt3Sn clusters are of a small diameter (∼2 nm) with a narrow size distribution. Pt3Sn/C catalyst is highly active in ethanol oxidation with the onset potential shifted for ∼150 mV to more negative values and with ∼2 times higher currents in comparison to Pt/C.

Platinum electrocatalyst supported on glassy carbon: a dynamic response analysis of Pt activity promoted by substrate anodization

In previous investigations the physicochemical state of electrochemically activated glassy carbon (GC) has been found to affect the electrochemical activity of GC-supported Pt particles. It has been assumed that carbon functional groups (CFGs) generated by GC anodization are able to renew the Pt surface through bifunctional catalysis. In order to provide evidence for the intimate electrocatalytic relationship between Pt and anodized GC and reveal the cause of CFG-induced enhancement of Pt activity, the dynamic response of Pt black supported on differently anodized GC is analysed in this paper by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in acidic solution. It was found that the capacitive properties of Pt black are not affected by modest GC anodization, but the pore resistance of the Pt layer is considerably affected. Clear evidence for the promoting influence of activated GC (i.e., CFGs) on the Pt desorption capability toward reverse hydrogen spillover at a Pt/CFGs-decorated GC interface in the Pt double layer region is elucidated by both EIS and CV measurements. The extent of GC anodization influences, in a quite similar way, both reverse hydrogen spillover desorption parameters (gained by EIS and CV) and the methanol oxidation rate as it has influence on the parameters describing the particular state of activated GC itself. Namely, the pore resistance of the Pt layer and GC resistance due to the presence of CFGs the highest when GC was moderately anodized, whereas the charge transfer resistance for hydrogen spillover desorption is the lowest. The CFGs of the anodized GC are able to “permeate” the above-applied Pt layer, thus increasing the Pt/CFGs-decorated GC interface responsible for the enhancement of Pt electrochemical activity.

Oxidation of formic acid on platinum surfaces decorated with cobalt(III) macrocyclic complexes

Platinum electrode decorated with three different mixed-ligand cobalt(III) complexes of the general formula [Co(Rdtc)cyclam](ClO4)2 [cyclam = 1,4,8,11-tetraazacyclotetradecane, Rdtc− = morpholine-(Morphdtc), piperidine-(Pipdtc), and 4-methylpiperidine-(4-Mepipdtc) dithiocarbamates, respectively] was used to study oxidation of formic acid in acidic solution. The complexes were adsorbed on differently prepared Pt surfaces, at open circuit potential. The preliminary results show increased catalytic activity of Pt for formic acid oxidation with complex ion adsorbed on the polycrystalline surfaces. The increase in catalytic activity depends on the structure of the complex applied and follows the order of metal-coordinated bidentate ligand as Morphdtc > Pipdtc > 4-Mepipdtc. Based on IR and NMR data, the main characteristics of the Rdtc ligands do not vary dramatically, but high symmetry of the corresponding complexes decreases in the same order. Accordingly, the complexes are distinctively more mobile, causing chemical interactions to occur on the surface with appreciable speed and enhanced selectivity. The effect of the complexes on catalytic activity presumably depends on structural changes on Pt surfaces caused by their adsorption.

The Role of SnO 2 on Electrocatalytic Activity of PtSn Catalysts

AbstractIn our previous paper, we described in detail studies of Sn influence on electrocatalytic activity of PtSn catalyst for CO and formic acid oxidation (Stevanović et al., J. Phys. Chem. C, 118 (2014) 278–289). The catalyst was composed of a Pt phase, Pt3Sn alloy and very small SnO2 particles. Different electrochemical treatment enabled studies of PtSn/C having Sn both in surface and subsurface layers and skeleton structure of this catalyst with Sn only in subsurface layers. The results obtained revealed the promotional effect of surface Sn whether alloyed or as oxide above all in preventing accumulation of CO and blocking the surface Pt atoms. As a consequence, in formic acid oxidation, the currents are not entering the plateau but increasing constantly until reaching a maximum. It was concluded that at lower potentials the effect of Sn on formic acid oxidation was predominantly electronic but with increasing the potential bi-functional mechanism prevailed due to the leading role of SnO2. This role of SnO2 is restated in the present study. Therefore, CO and formic acid oxidation were examined at PtSnO2/C catalyst. The catalyst was synthesised by the same microwave-assisted polyol procedure. According to XRD analysis, the catalyst is composed of a Pt phase and SnO2 phase. The reactions were examined on PtSnO2/C catalyst treated on the same way as PtSn/C. Comparing the results obtained, the role of SnO2 is confirmed and at the same time the significance of alloyed Sn and its electronic effect is revealed. Graphical Abstract

Influence of the synthesis procedures on pt catalyst activity for ethanol electrooxidation reaction

Carbon supported platinum catalysts for ethanol electrooxidation were synthesized by modified polyol synthesis method, assisted by microwave or reflux heating. Synthesized catalysts were characterized by XRD, STM and TGA techniques to determine their structural and morphological properties. STM and XRD investigations revealed small particle size (~3nm), while TGA showed Pt loading of 20% in both samples. Electrocatalytic activity of prepared catalysts was examined by potentiodynamic measurements and compared to commercial Pt catalyst (Pt/C-Tanaka). The highest activity for ethanol electrooxidation was observed with Pt catalyst prepared by microwave irradiation. The increase in activity can be assigned to the benefits of microwave assisted synthesis, such as small particle size and homogeneous particle distribution on the support.