Electrodes catalytical activity

Gomes JF, Gasparotto LHS, Tremiliosi-Filho G (2013) Glycerol electro-oxidation over glassy-carbon-supported Au nanoparticles direct influence of the carbon support on the electrode catalytic activity. Phys Chem Chem Phys 15 10339-10349... 

Table 7.1 lists redox potentials for the catalysis of organohalides and phenols using MPc and MP catalysts. The reduction of tran5-l,2-dibromocyclohexanes was catalyzed by Co(II) porphrin monomers at potentials > 1 V positive of the uncatalyzed case. High catalytic currents were obtained when using siloxane films on carbon electrodes containing specially designed diethylene glycol spaced Co porphyrin. Fe TAPP monomers adsorbed onto carbon electrodes and following reduction of Fe to Fe, showed catalytic activity towards the reduction of alky halides such as benzyl bromide. Table 7.1. A decrease in overvoltage of up to 1.0 V was obtained compared to unmodified carbon electrode. Catalytic activity was also observed for the Fe species . ...

Regarding the electrocatalyst, the similar concepts about the catalytic activity can be defined in the similar ways as Eqns (3.1)—(3.4). Actually, electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. However, in this book, we are only focused on the heterogeneous electrocatalysts. The role of electrocatalyst is to assist in transferring electrons between the electrode catalytic active sites and reactants, and/or facilitates an intermediate chemical transformation. One important difference between catalytic chemical reaction and electrocatalytic reaction is that the electrode potential of the electrocatalyst can also assist in the reaction. By changing the potential of the electrocatalyst, which is attached onto the electrode surface, the electrocatalytic activity can be enhanced or depressed significantly. 

Molecular-level SOFC models aim to understand (i) the kinetics of the reaction at the interface between electrode and electrolyte, (ii) the conduction process in the electrolyte, and (iii) the conduction process in the electrodes. Catalytic activity at TPB, activation energy for oxygen ion transport, and surface exchange current are application examples for such models. 

The hydrogen electrode consists of an electrode of platinum foil (approximately 1 X 1 X 0-002 cm) welded to a platinum wire which is fused into a glass tube. In order to increase its catalytic activity it is platinised by making it cathodic in a solution of chloroplatinic acid (2% chloroplatinic acid in 2 N HCl) frequently lead acetate is added to the solution (0-02%) and this appears to facilitate the deposition of an even and very finely divided layer... 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

At that time it was first reported that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in a very pronounced, reversible and, to some extent, predictable manner by applying electrical currents or potentials (typically up to 2 V) between the catalyst and a second electronic conductor (counter electrode) also deposited... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

Thus the driving force for O2 backspillover from YSZ to the gas exposed, i.e. catalytically active, electrode surface exists and equals jJ02 (YSZ) - jl02- (M). It vanishes only when O2 backspillover has taken place and established the effective double layer shown in Fig. 3.6. 

There is an important point to be made regarding UWr vs t transients such as the ones shown in Fig. 4.15 when using Na+ conductors as the promoter donor. As will be discussed in the next section (4.4) there is in solid state electrochemistry an one-to-one correspondence between potential of the working electrode (UWr) and work function (O) of the gas exposed (catalytically active) surface of the working electrode (eAUwR=AO, eq. 4.30). Consequently the UWr vs t transients are also AO vs t transients. 

D. Tsiplakides, S. Neophytides, O. Enea, M.M. Jaksic, and C.G. Vayenas, Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) of Pt Black Electrodes Deposited on Nafion 117 Solid Polymer Electrolyte, /. Electrochem. Soc. 144(6), 2072-2088 (1997). 

It also shows that electrochemical promotion is due to electrochemically controlled migration (backspillover) of ions (acting as promoters) from the solid electrolyte to the gas-exposed catalytically active catalyst-electrode surface. 

Solid electrolyte cells can be used to alter significantly the work function catalytically active, catalyst electrode surface by polarizing the catalyst-solid electrolyte interface. 

The variation in quasireference electrode in presence of reactive gas mixtures. This is due to its high catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar ina, A.K. Demin, and A.S. Lipilin, On the origin of the Non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena. Oxygen isotope exchange on Pt electrode in cell with solid oxide electrolyte, Catal. Lett. 18, 153-164 (1993). 

At t=0 a constant anodic current I=5mA is applied between the Pt catalyst film and the counter electrode. The catalyst potential, Urhe, reaches a new steady state value Urhe=1.18 V. At the same time the rates of H2 and O consumption reach, within approximately 60s, their new steady-state values rH2-4.75T0 7 mol/s, ro=4.5T0 7 mol/s. These values are 6 and 5.5 times larger than the open-circuit catalytic rate. The increase in the rate of H2 consumption (Ar=3.95T0 7 mol H2) is 1580 % higher than the rate increase, (I/2F=2.5T0 8 mol/s), anticipated from Faraday s Law. This shows clearly that the catalytic activity of the Pt catalyst-electrode has changed substantially. The Faradaic efficiency, A, defined from ... 

As shown in Figure 12.4 this finely dispersed Pt catalyst can be electrochemically promoted with p values on the order of 3 and A values on the order of 103. The implication is that oxide ions, O2", generated or consumed via polarization at the Au/YSZ/gas three-phase-boundaries migrate (backspillover or spillover) on the gas exposed Au electrode surface and reach the finely dispersed Pt catalyst thereby promoting its catalytic activity. 

It has been recently found that direct electrical contact, via a metal wire, to the catalyst-electrode is not necessary to induce the effect of electrochemical promotion.8 11 It was found that it suffices to apply the potential, or current, between two terminal electrodes which may, or may not, be catalytically active. The concept appears to be very similar with that of the bipolar design used now routinely in aqueous electrochemistry. 

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

When a small fraction of irreversible mediator side reactions cause a rapid decrease of catalytic activity in the homogeneous case in a modified electrode this would be disastrous since there is no bulk supply of catalyst. Thus, higher turnover numbers are generally required than in the homogeneous case... 

The catalytic activity of viologen modified polypyrrole electrodes in preparative elK tror luctions has been extended from vicinal to geminal poly-... 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

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