Catalytic selectivity, electrochemical

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

Work on the electrochemical reduction of C—C double and triple bonds is rarely encountered in the patent literature since these syntheses are generally not competitive with catalytic methods. Electrochemical processes are only of interest where particular selectivities may be obtained. 

Much has been published on the selective electrochemical oxidation of a large variety of organic compounds, among which the higher alcohols (e.g. ethanol to acetic add, propanol-2 to acetone). The interesting point in the present context is that some of these conversions have also been studied purely catalytically in the liquid phase, employing catalysts such as Pt/C, with O2 or air as the oxidant. It has been remarked [152] that such systems should also be considered from an electrochemical point of view. Indeed, it stands to reason that the overall oxidation reaction is essentially the sum of the two constitutive electrochemical half-reactions [153]. In the case of alcohol oxidation we would then have... 

Conventional heterogeneous metal catalysts are commonly enhanced by the addition of so-called promoter species that are used to modify intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical promotion (EP) provides an in situ, reversible and efficacious means of catalyst promotion and it allows for a systematic study of the role of promoters in heterogeneous catalysis. EP studies relevant to the three-way catalytic chemistry i.e., control of automotive CO, NO and hydrocarbons emissions, demonstrate that major enhaneements in activity of Pt catalyst supported on a"-Al203 (a sodium ion conductor) are possible when Na is electrochemically pumped to the catalyst surface. In the case of the important reactions involving NO reduction by CO or by hydrocarbons, major enhancements in selectivity towards N2 (from 15 to 70%) have been also achieved. The promotional effect of Na is due to enhanced NO chemisorpion and pronounced NO dissociation on the Pt surface. 

The selection of a noble metallic material as the sensing element of a microfabricated electrochemical sensor is based on the catalytic and electrochemical properties of the metal. As mentioned, platinum, gold, and silver are often the metallic materials of choice. Palladium, iridium, and rhodium have also been used as electrochemical sensing elements. 

Fig. 8.6 The molecular Lego approach applied to the scaffold of P450 BM3 a to generate a P450 catalytic domain electrochemically accessible through the fusion with the electron transfer protein flavodoxin b to solubilize the human membrane-bound P450 2E1 by fusion with select-...

The cell efficiency of SC-SOFCs with interdigitated electrodes was estimated to be below 1%, with a fuel utilization of less than 0.1% [63]. In addition to insufficient catalytic selectivity of the electrode materials, the small size of SC-SOFCs with coplanar electrodes limits the electrochemical conversion of fuel. 

The catalytic oxidation/electrochemical membrane process consists of an upstream commercial sulfuric acid catalyst to convert SO2 to SO3 followed 1 a molten salt electrochemical cell using a sulfur oxide selective membrane. Removal efficiencies of 95% have been simulated. Projected economics for a 300 MW power plant burning 3.5% sulfur coal are 96/kW capital cost and 3.24 mills/kWh operating cost. Capital cost includes the catalytic converter and oleum plant and assumes cell replacement twice over a 30-year life (McHenry and Winnick, 1991). The process is in a very early stage of development, and no cortunercial or demonstration operations have been reported. 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

Despite the surprise caused by the first literature reports of such large non-Faradaic rate enhancements, often accompanied by large variations in product selectivity, in retrospect the existence of the NEMCA effect can be easily rationalized by combination of simple electrochemical and catalytic principles. 

Promotion We use the term promotion, or classical promotion, to denote the action of one or more substances, the promoter or promoters, which when added in relatively small quantities to a catalyst, improves the activity, selectivity or useful lifetime of the catalyst. In general a promoter may either augment a desired reaction or suppress an undesired one. For example, K or K2O is a promoter of Fe for the synthesis of ammonia. A promoter is not, in general, consumed during a catalytic reaction. If it does get consumed, however, as is often the case in electrochemical promotion utilizing O2 conducting solid electrolytes, then we will refer to this substance as a sacrificial promoter. 

C. Cavalca, G. Larsen, C.G. Vayenas, and G. Haller, Electrochemical Modification of CH3OH oxidation selectivity and activity on a Pt single-pellet catalytic reactor, /. Phys. Chem. 97, 6115-6119(1993). 

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

Reactions involving the catalytic reduction of nitrogen oxides are of major environmental importance for the removal of toxic emissions from both stationary and automotive sources. As shown in this section electrochemical promotion can affect dramatically the performance of Rh, Pd and Pt catalysts (commonly used as exhaust catalysts) interfaced with YSZ, an O2 ion conductor. The main feature is strong electrophilic behaviour, i.e. enhanced rate and N2 selectivity behaviour with decreasing Uwr and , due to enhanced NO dissociation. 

A typical electrochemical promotion experiment includes kinetic measurements under open and closed circuit conditions as well as study of the effect of catalyst potential or work function on catalytic rate and selectivity under steady state and transient conditions. In kinetic measurements one should change the partial pressure of each reactant while... 

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. 

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. 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

The scientific literature abounds in attempted correlations between the catalytic activities, of a series of catalytic electrode metals and some set of bulk properties, of these metals. Such correlations would help in understanding the essence of catalytic action and will enable a conscious selection of the most efficient catalysts for given electrochemical reactions. 

In electrochemical systems, many restrictions exist in the use of metal catalysts. Most metals other than the expensive noble metals are unstable at anodic potentials and cannot be nsed for anodic processes. The catalytic activity and selectivity of metal catalysts basically are determined by their chemical nature and are rarely open to adjustments. 

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