Catalysts Electrocatalysts, specific

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

Electrocatalyst see also specific catalysts adsorbate-support interactions, 30 273-279 adsorption, 30 240-264 isotherms, 30 241-243 bimetallic activity, 30 275... 

Pt/Ru electrocatalysts are currently used in DMFC stacks of a few watts to a few kilowatts. The atomic ratio between Pt and Ru, the particle si2 e and the metal loading of carbon-supported anodes play a key role in their electrocatalytic behavior. Commercial electrocatalysts (e.g. from E-Tek) consist of 1 1 Pt/Ru catalysts dispersed on an electron-conducting substrate, for example carbon powder such as Vulcan XC72 (specific surface area of 200-250 m g ). However, fundamental studies carried out in our laboratory [13] showed that a 4 1 Pt/Ru ratio gives higher current and power densities (Figure 1.6). 

In conclusion, although results on the use of Ti02 nanotube array catalysts or electrocatalysts are still limited, the future is promising. In several cases, unusual behavior was demonstrated and associated with the specific characteristics of the nanostructure. 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

Another important aspect of electrocatalysis is the study of dispersed high specific area and supported, both metal and non-metal, electrocatalysts. A high degree of dispersion brings about enhancement of the catalytic activity because of the specific area and energetics of active sites [140] and decrease of susceptibility of poisoning because of the improved ratio of catalyst area to impurities in solution. 

The most important operational features are the relationships between the crystallite sizes of the platinum electrocatalysts to the specific (A.real m"2 Pt) and the mass (Ag 1 Pt) activities. These features are most directly applicable to the efficiency and utilization of the catalyst in operating fuel-cells. 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

One-step facile hydrothermal methods can synthesize novel nanoporous Ptdr and Pt on RuO2(110) catalysts for ORR. Pt-free catalysts such as Pd-Co, Pd-Ni, Pd-Cr, Pd-Ta, Co-Ni, and CoPds metal alloys"" " as well as the chalcogenide Ru-S and Ru-Se catalysts" " are believed to be promising candidates. Modem studies on ORR electrocatalysts deal with nano-porous gold particles (10-20 nm) with large specific surface area that reduce... 

Platinum and platinum alloy electrocatalysts are by far the most studied in the PEFC and in model, bulk, metal catalyst/ionomer interfacial systems devised to study the electrocatalytic process in a PEFC environment. The reason is that, to date, no other electrocatalyst has been demonstrated to approach the specific activity exhibited by platinum (or platinum alloys) in PEFCs, particularly in the more demanding... 

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

In the first group belong the techniques which are also used in heterogeneous catalysis for determining the surface area of catalysts. Two such techniques are widely used The Brunauer-Emett-Teller (BET) method, based on the physical adsorption of N2 or Ar at very low temperatures [8, 44] and the H2 or CO chemisorption method [8, 44], The first method leads to the total catalyst surface area, whereas the second leads to the specific (active metal) surface area. In the case of supported electrocatalysts (e.g., Pt/C electrocatalysts used as anodes in PEM fuel cells) the two techniques are complementary, as the former can lead to the total electrocatalyst surface... 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

For electrogenerative processing it is often necessary to use a different, more selective catalyst than that used in fuel cells. Although the extensive screening of various electrocatalysts for fuel cells (3-13 can provide useful guidelines for choosing selective electrocatalysts, the specificity of such catalysts is not well characterized, with few exceptions (28, 29, 31, 54. ... 

For surface structure studies, perhaps the most popular technique has been LEED (373). Elastically diffracted electrons from a monoenergetic beam directed to a single-crystal surface reveal structural properties of the surface that may differ from those of the bulk. Some applications of LEED to electrocatalyst characterization were cited in Section IV (106,148,386). Other, less specific, but valuable surface examination techniques, such as scanning electron microscopy (SEM) and X-ray microprobe analysis, have not been used in electrocatalytic studies. They could provide information on surface changes caused by reaction, some of which may lead to catalyst deactivation (256,257). Since these techniques use an electron beam, they can be coupled with previously discussed methods (e.g. AES or XPS) to obtain a qualitative mapping of the structure and composition of a catalytic surface. 

Insufficient information exists currently for complex selective reactions, limited to phenomenological results with few electrocatalysts and reactants. The design of polymetallic clusters and of catalysts with controlled crystallite size, the exploration of redox catalysts, the tailoring of the physical catalyst structure, and the selection of reactors and operating conditions to enhance or suppress multiple reaction paths await further study. The exploitation of unconventional reduction or oxidation potential regimes for specificity control, which has been only occasionally attempted or appreciated, appears to be especially attractive. 

In addition to having a good CO tolerance, Pt-Ru electrocatalysts must also have a high activity for H2 oxidation. Comparison of the mass-specific activity of a PtRu2o electrocatalyst with a commercial Pt-Ru 1 1 alloy electrocatalyst for the oxidation of pure H2 showed that its activity is tluee times that of the commercial alloy. This indicates that even for a low Pt coverage on Ru, its activity for H2 oxidation is preserved, a prerequisite for an active CO tolerant catalyst. Comparing the CO tolerance of the PtRu2o electrocatalyst with that of two commercial Pt-Ru alloy electrocatalysts for the oxidation of 1095 ppm CO in H2 confirmed the exceptional stability of the former (Fig. 20) the measurements... 

The data summarized in this paper have established that the oxide pyrochlores under discussion substantially reduce the activation energy overvoltages associated with oxygen electrocatalysis. Specifically, it is found that these catalysts, in aqueous alkaline media near ambient temperature, are superior to any other oxygen evolution catalyst and are equal in performance to the best known oxygen reduction catalysts. As bidirectional oxygen electrocatalysts, they appear to be unmatched. 

The optimization of an electrochemical reactor calls for a full description of the process to accomplish the specific objective. The problem of the optimization of the electrocatalyst is of real importance in most of the recently developed technical electrodes that were prepared without detailed studies. It must be borne in mind that the strong experimental conditions in which the large electrical currents and large ionic forces of the electrolytes prevail change the morphology and the composition of the catalyst. 

Electrocatalysts with a 1/8 of monolayer Pt loading on Ru nanoparticles have been synthesized by spontaneous deposition of Pt on a Ru surface, each of which have at least three times larger mass-specific activity for H2 oxidation than two commercial catalysts and a larger CO tolerance, as determined by thin film rotating disk electrode measurements. 

Figure 4 displays a mass-specific activity of PtRu2o catalyst in comparison to Commercial A s PtRu 1 1 alloy and Commercial B s Pt3Ru2 electrocatalysts for the oxidation of H2 in 0.5M H2SO4 A considerably higher mass-specific activity of the PtRu2o electrocatalyst has been observed. 

Immobilization is, as a rule, accompanied by a declining activity of the enzymes. However, at present there are numerous examples of successful immobilization with the preservation of enzyme specificity and activity within 10 to 90% of its activity in the native state. This permits us to think that the problem of obtaining enzymic preparations which are close in physicochemical properties to heterogeneous catalysts and electrocatalysts, particularly as regards the preservation of enzymatic activity and specificity, can be successfully solved in each specific case. 

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