Catalyst active surface area

Trend changes in catalyst activity, surface area, rare earth, and metals content. Consider adding/increasing metals inhibitor. 

Pt mass and catalyst active surface area were reduced by about 3 and 2 times, respectively, when the RH was dropped from 100% to 50% at a cell temperature of 60°C. The degree of Pt oxidation was also found to increase significantly when RH went from 20% to 72%, but further increase was not apparent above 72% (Xu et al, 2007). 

Exchange current density per unit catalyst active surface area (intrinsic activity) (A cm )... 

FIGURE 11.3 Impact of (grating conditions on catalyst active surface area loss, (a) Pt surface area as a function of stack luntime (b) impact of RH and higti-temperatuie operation on Pt surface area loss of Pt/C as a function of potential cycles [51]. (For color version of this figure, tiie reader is referred to tiie online veraon of this book.)... 

This equation can be used to modify the effective diffusion coefficient an catalyst active surface area under flooded conditions. Another important parameter is irreducible liquid saturation, also called the immobile saturation (Ji ), which represents the amount of isolated trapped water in the pores of the PM. That is, even when a high flow rate of gas is introduced into the porous media, some fraction of liquid will remain (unless evaporated) primarily due to discontinuity or isolation with the rest of the pores. The irreducible fraction does not represent the fraetion of liquid in the porous media which cannot be removed from the fuel cell media. In fact, removal from drag forces is not possible, but removal from evaporation is. 

Catalyst Particle Size. Catalyst activity increases as catalyst particles decrease in size and the ratio of the catalyst s surface area to its volume increases. Small catalyst particles also have a lower resistance to mass transfer within the catalyst pore stmcture. Catalysts are available in a wide range of sizes. Axial flow converters predorninanfly use those in the 6—10 mm range whereas the radial and horizontal designs take advantage of the increased activity of the 1.5—3.0 mm size. 

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

For an identical fresh catalyst, the surface area of an E-cat is an indirect measurement of its activity. The SA is the sum of zeolite and... 

Decrease in E-cat activity, surface area, fresh catalyst activity, and rare earth content... 

As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area AG but also the length, t, of the three-phase-boundaries between the solid electrolyte, the catalyst film and the gas phase (Fig. 4.7). 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

In this section, we demonstrate the real ORR activities (apparent rate constant per real active surface area, fe pp) and P(H202) at bulk Pt and nanosized Pt catalysts dispersed on carbon black (Pt/CB) with dp,= 1.6 + 0.4, 2.6 + 0.7, and 4.8 1.0 nm in the practical temperature range 30-110 °C [Yano et al., 2006b]. The use of a channel flow double-electrode (CFDE) cell allowed us to evaluate fe pp and P(H202) precisely. 

The hydrogenation processes were performed at a relatively low temperature and pressure in the presence of promoted Raney Ni 2400 and Raney Co 2724 catalysts (13) in this study but any common nitrile hydrogenation catalysts (e.g. Fe, Ru, Rh, bulk or supported catalysts) could be used. The advantage of using a low temperature and pressure process is that it lowers the investment cost of an industrial process. Raney Ni 2400 is promoted with Cr and Raney Co 2724 is promoted with Ni and Cr. The particle sizes for both catalysts were in the range 25 - 55 pm. The BET surface area of Raney Ni 2400 and Raney Co 2724 are 140 m2/g and 76 m2/g, respectively, and the active surface area of the Ni and Co catalysts are 52 and 18 m2/g, respectively, based on CO chemisorption (Grace Davison Raney Technical Manual, 4th Edition, 1996). 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

Comparative methods may be effectively used for measurements of partial surface areas, Ac, of components in porous composites, for example for active surface area in supported catalysts. The traditional methods of Ac measurements are based on chemisorption of H2, 02, CO, NOr. and some other gases that chemisorb on an active component, and have negligible adsorption on a support [5,54], The calculation of Ac is fulfilled by an equation similar to Equation 9.18 assuming some values of w and atomic stoichiometry of chemisorption [54]. But, unfortunately chemisorption is extremely sensitive to insignificant variations of chemical composition and structure of surface, which alters the results of the measurements. 

The stability during potential cycling and ORR activity of Pt (20 wt°/o) supported on MWCNTs and carbon black was also investigated [136]. Two different potential cycling conditions were used, namely lifetime (0.5 to 1.0 V vs. RHE) and start-up (0.5 to 1.5 V vs. RHE). Pt supported on MWCNTs catalyst exhibited a significantly lower drop in normalized electrochemically active surface area (ECA) values compared to Pt supported on Vulcan (Fig. 14.10), showing that MWCNTs possess superior stability to commercial carbon black under normal and severe potential cycling conditions [137]. 

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

In addition, the reduction of NOj is a very fast reaction and is controlled by external and internal diffusion [27, 30]. In contrast, the oxidation of SO2 is very slow and is controlled by the chemical kinetics [31]. Accordingly, the SCR activity is increased by increasing the catalyst external surface area (i.e. the cell density) to favor gas-solid mass transfer while the activity in the oxidation of SO2 is reduced by decreasing the volume of the catalyst (i.e. the wall thickness) this does not affect negatively the activity in NO removal because significant ammonia concentrations are confined near the external geometric surface of the catalyst. 

Accordingly, the catalytic activity in a given catalytic reaction depends on only four factors. Two of them are specific for the system as a whole the activation energy and the reaction order. The latter may be reduced to the heat of adsorption, as b0 is a nearly universal constant. The other two factors are, at least in first approximation, properties of the catalyst its surface area F and its energy distribution function. Future work will have to answer the question of which parameters control, qualitatively and quantitatively, these four factors. 

Since the solid surface is responsible for catalytic activity, a large readily accessible surface in easily handled materials is desirable. By a variety of methods, active surface areas the size of football fields can be obtained per cubic centimeter of catalyst. 

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