Continuously-variable survival exponent for random walks with movable partial reflectors

We study a one-dimensional lattice random walk with an absorbing boundary at the origin and a movable partial reflector. On encountering the reflector, at site x, the walker is reflected (with probability r) to x-1 and the reflector is simultaneously pushed to x+1. Iteration of the transition matrix, and asymptotic analysis of the probability generating function show that the critical exponent delta governing the survival probability varies continuously between 1/2 and 1 as r varies between 0 and 1. Our study suggests a mechanism for nonuniversal kinetic critical behavior, observed in models with an infinite number of absorbing configurations.Comment: 5 pages, 3 figure

Synthesis and Characterization of Pt Catalysts on SnO2 Based Supports for Oxygen Reduction Reaction

The oxygen reduction reaction was studied at Pt nanocatalysts on two different tin oxide based supports, Sb-SnO2 and Ru-SnO2, in acid solution. Tin oxide based supports were synthesized by hydrazine reduction method. Physical characterization of the supports was performed by BET, X-ray diffraction and TEM techniques. SnO2 belonging peaks were detected in Sb-SnO2 powder, while Ru-SnO2 XRD diffraction patterns contained peaks of RuO2 and SnO2. The average crystallite sizes, determined by Scherrer equation, were 3 nm and 4 nm for Sb-SnO2 and Ru-SnO2, respectively. Pt catalysts on Sb-SnO2 and Ru-SnO2 supports were synthesized by borohydride reduction method. TEM analysis revealed homogeneous particle size distribution, with average particle size of 2.9 and 5.4 nm, for Sb-SnO2 and Ru-SnO2, respectively. Electrocatalytic activity and stability of these catalysts for oxygen reduction were studied by cyclic voltammetry and linear sweep voltammetry at rotating disk electrode (RDE). Pt catalysts on Sb and Ru doped SnO2 support exhibited catalytic activities comparable to Pt on commercial carbon based support. Stability tests were also performed. Determined small loss of electrochemical active surface area of the Pt catalyst on Sb doped tin oxide support, after repetitive cycling, indicated high stability and durability of this cathode for-prospective fuel cells application. (C) 2013 The Electrochemical Society. All rights reserved

Novel Pt catalyst on ruthenium doped TiO2 support for oxygen reduction reaction

Ruthenium doped titanium oxide support was synthesized. The support was characterized by BET (Brunauer, Emmett, Teller) and X-ray diffraction techniques (XRD). Determined specific surface area was 41 m(2) g(-1). XRD revealed presence mainly TiO2 anatase phase and some peaks belonging to rutile phase. No Ru compounds have been detected. Platinum based catalyst on this support was prepared by borohydride reduction method. The catalyst was characterized by scanning transmission electron microscopy (STEM, HAADF) and electron energy loss spectroscopy (EELS). Homogenous Pt particle distribution over the support, with average Pt nanoparticle diameter of 3 nm was found. This novel catalyst was tested for oxygen reduction in acid solution. It exhibited remarkable higher catalytic activity in comparison with Pt/C, as well as with Pt nanocatalysts at titanium oxide based supports, reported in literature. (C) 2013 Elsevier B.V. All rights reserved

Synthesis and characterization of MoOx-Pt/C and TiOx-Pt/C nano-catalysts for oxygen reduction

The oxygen reduction reaction (ORR) was studied at carbon supported MoOx-Pt/C and TiOx-Pt nanocatalysts in 0.5 mol dm(-3) HClO4 solution, at 25 degrees C. The MoOx-Pt/C and TiOx-Pt/C catalysts were prepared by the polyole method combined by MoOx or TiOx post-deposition. Home made catalysts were characterized by TEM and EDX techniques. It was found that catalyst nanoparticles were homogenously distributed over the carbon support with a mean particle size about 2.5 nm. Quite similar distribution and particle size was previously obtained for Pt/C catalyst. Results confirmed that MoOx and TiOx post-deposition did not lead to a significant growth of the Pt nanoparticles. The ORR kinetics was investigated by cyclic voltammetry and linear sweep voltammetry at the rotating disc electrode. These results showed the existence of two E - logj regions, usually observed with polycrystalline Pt in acid solution. It was proposed that the main path in the ORR mechanism on MoOx-Pt/C and TiOx-Pt/C catalysts was the direct four-electron process with the transfer of the first electron as the rate-determining step. The increase in catalytic activity for ORR on MoOx-Pt/C and TiOx-Pt/C catalysts, in comparison with Pt/C catalyst, was explained by synergetic effects due to the formation of the interface between the platinum and oxide materials and by spillover due to the surface diffusion of oxygen reaction intermediates. (C) 2008 Elsevier Ltd. All rights reserved.Euro-Interfinish 2007 Conference, Oct 18-19, 2007, Athens, Greec

Nb-TiO2 supported platinum nanocatalyst for oxygen reduction reaction in alkaline solutions

Platinum based nanocatalyst at home made Nb-TiO2 support was synthesized and characterized as the catalyst for oxygen reduction reaction in 0.1 mol dm(-3) NaOH, at 25 degrees C. Nb doped TiO2 catalyst support, containing 5% of Nb, has been synthesized by modified acid-catalyzed sol-gel procedure in non-aqueous medium. BET and X-ray diffraction (XRD) techniques were applied for characterization of synthesized supporting material. XRD analysis revealed only presence of anatase TiO2 phase in synthesized support powder. Existence of any peaks belonging to Nb compounds has not been observed, indicating Nb incorporated into the lattice. Nb-TiO2 supported Pt nanocatalyst synthesized, using borohydride reduction method, was characterized by TEM and HRTEM techniques. Platinum nanoparticles distribution, over Nb doped TiO2 support, was quite homogenous. Mean particle size of about 4 nm was found with no pronounced particle agglomeration. Electrochemical techniques: cyclic voltammetry and linear sweep voltammetry at rotating disc electrode were applied in order to study kinetics and estimate catalytic activity of this new catalyst for the oxygen reduction reaction in alkaline solution. Two different Tafel slopes were found: one close to -90 mV dec(-1) in low current density region and other approximately 200 my dec(-1) in high current density region, which is in good accordance with literature results for oxygen reduction at Pt single crystals, as well as Pt nanocatalysts in alkaline solutions. Similar specific catalytic activity (expressed in term of kinetic current density per real surface area) of Nb(5%)-TiO2/Pt catalyst for oxygen reduction reaction in comparison with the carbon supported platinum (Vulcan/Pt) nanocatalyst, was found. (C) 2011 Published by Elsevier Ltd

Oxygen reduction at platinum nanoparticles supported on carbon cryogel in alkaline solution

The oxygen reduction reaction was investigated in 0.1 M NaOH solution, on a porous coated electrode formed of Pt particles supported on carbon cryogel. The Pt/C catalyst was characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and cyclic voltammetry techniques. The results demonstrated a successful reduction of Pt to metallic form and homogenous Pt particle size distribution with a mean particle size of about 2.7 nm. The ORR kinetics was investigated by linear sweep polarization at a rotating disc electrode. The results showed the existence of two E - log j regions, usually referred to polycrystalline Pt in acid and alkaline solution. At low Current densities (led), the Tafel slope was found to be close to -2.3RT/F, while at high current densities (bed) it was found to be close to -2x2.3RT/F. It is proposed that the main path in the ORR mechanism on Pt particles was the direct four-electron process, with the transfer of the first electron as the rate determining step. If the activities are expressed through the specific current densities, a small enhancement of the catalytic activity for Pt/C was observed compared to that of polycrystalline Pt. The effect of the Pt particle size on the electrocatalysis of oxygen reduction was ascribed to the predominant (111) facets of the platinum crystallites

Preparation and characterization TiOx-Pt/C catalyst for hydrogen oxidation reaction

The hydrogen oxidation reaction (HOR) was studied at the home made TiOx-Pt/C nanocatalysts in 0.5 mol dm(-3) HClO4 at 25 degrees C. Pt/C catalyst was first synthesized by modified ethylene glycol method (EG) on commercially used carbon support (Vulcan XC-72). Then TiOx-Pt/C catalyst was prepared by the polyole method followed by TiOx post-deposition. The synthesized catalyst was characterized by XRD, TEM and EDX techniques. It was found that Pt/C catalyst nanoparticles were homogenously distributed over carbon support with the mean particle size of about 2.4 nm. The quite similar, homogenous distribution and particle size were obtained for Pt/C doped by TiOx catalyst which was the confirmation that TiOx post-deposition did not lead to significant growth of the Pt nanoparticles. The electrochemically active surface area of the catalyst was determined by using the cyclic voltammetry technique. The kinetics of hydrogen oxidation was investigated by the linear sweep voltammetry technique at the rotating disc electrode (RDE). The kinetic equations used for the analysis were derived considering the reversible or irreversible nature of the kinetics of the HOR. It was found that the hydrogen oxidation reaction for an investigated catalyst proceeded as an electrochemically reversible reaction. The values determined for the kinetic parameters-Tafel slope of 28 mV dec(-1) and exchange current density about 0.4 mA cm(Pt)(-2) are in good agreement with usually reported values for a hydrogen oxidation reaction with platinum catalysts in acid solutions

Synthesis and characterization of Pt nanocatalyst on Ru0.7Ti0.3O2 support as a cathode for fuel cells application

Ruthenium oxide/titanium oxide, with a Ru:Ti atomic ratio of 7:3 was synthesized by modified sol-gel procedure and used as a support for platinum nanocatalyst for oxygen reduction reaction. The synthesized materials were characterized in terms of morphology, particle size distribution, chemical and phase composition by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high angle annular dark filed scanning transmission electron microscopy (HAADF, STEM) and electron energy loss spectroscopy (EELS). XPS spectra revealed that Ru atoms were in mainly in Ru(4+) oxidation state, the Ti atoms in Ti(4+) oxidation state, whereas the Pt-atoms were in metallic state. TEM analysis proved that platinum nanoparticles nucleated at both oxide species and homogeneous distribution was observed. The average platinum nanoparticle size was 3.05 nm. Electrochemically active surface area of platinum was 32 m(2) g(-1). Kinetics of the oxygen reduction was studied at rotating disc electrode in 0.5 mol dm(-3) HClO4 solution, at 25 degrees C. The catalytic activities expressed in terms of specific activity (per electrochemically active surface area of platinum) and mass activity (per mass of platinum) were determined and compared to Pt catalyst on carbon support. The high catalytic activity was proven by electrochemical characterization. (C) 2014 Elsevier B.V. All rights reserved