Surface area-specific activity

Catalyst Rate (nmol h g- ) Surface area ( Specific Activity (nmol h m )... 

The specific activity of pure bismuth molybdate, a-phase Bi2(Mo04)3 or y-phase Bi2Mo06, is fairly high. However, owing to its low surface area, the activity per unit weight of the catalyst is not so prominent. 

K0a as described in its simplified form in Eq. (1), assuming yocr/yoM MW0ct/ MWom to be equal to 1 and poor to be 0.820 kg/L./EC is the fraction of elemental carbon in atmospheric particles, which was estimated to be less than 5% in rural and remote sites [59], aac is the surface area of active carbon, acc is the specific surface of elemental carbon and 10-12 is a factor for unit correction. Assuming that the ratios /oct/ /om- MW0ct/MW0m and acc/a,AC are equal to 1, it is possible to predict Kp values from/oM and/EC [30-34, 52],... 

Fig. 11.12 Electrochemical activity (area-specific activity) normalized by the surface area of Pt for the most active alloys. The Pt surface area was determined electrochemically using the hydrogen adsorption integral of each catalyst.

According to the hypothesis, chemical conversions in a solid occur on the surface of a new crack (or in the layer adjacent to it), that is, the reaction rate in such a reacting system is a certain function of the specific surface area S (active surface area per unit volume). As noted above, the positive feedback in this model manifests itself in the fact that the rate of formation of cracks (i.e., the active surface growth rate in the sample volume) is proportional to the reaction velocity. Therefore, the equation describing the formation of a new surface can be written in a form analogous to that of a branched-chain process ... 

Many commercial activated carbons have been prepared with various sources of raw materials and different processing conditions. As a result, the micropore structures and specific surface areas of activated carbons, which are the most profound influences on the extent of adsorption, vary, and in general, activated carbons have a surface area of up to 3000 m2/g. The rate of adsorption increases with some function of the inverse of the radius of the activated carbon even though the adsorption capacity (i.e., equilibrium adsorption) is relatively independent of the particle diameter. However, for a highly porous adsorbent such as activated carbon, the... 

In the chemicals industry, the majority of the reactions take place at the surfaces of heterogeneous catalysts. The efficiency of a heterogeneous catalytic process is thus largely determined by the quality of the catalysts used, i.e. the exposed surface area of active phase and the stability. It is instructive to show the correlation between the exposed (specific) surface area, and the particle size of the catalytic material. Let us suppose that the active phase (density p in kg/m3) consists of uniform spherical particles. The specific area can be calculated as follows ... 

A representative structnred packing was shown in Figure 12.25. Packings of this type are designed specifically to do three things minimize form drag (pressure drop), provide considerable active surface area for mass transfer, and maintain nniform liquid distribution so that most of the surface area is active for mass transfer. 

Formation of metastable phases (H-In203, y-Fe203, SnO, H-M0O3) Developed specific surface area Elevated activity in a course of adsorption processes Increased additive solubility as compared to the equilibrium values... 

The deactivation of three way catalysts (TWC) can be studied by the determination of the change of some of their properties such as specific surface area, catalytic activity or precious metal dispersion. Two qualitative and quantitative methods have been investigated in order to characterize TWC oxygen species Temperature Programmed Desorption (O2 TPD) and Temperature Programmed Reduction in hydrogen (H2 TPR). 

The chemical and physical methods of activation allow to produce carbon materials with a highly developed specific surface area. Therefore, activated carbons are extensively used as electrode materials for supercapacitors [11,25,30-37]. Considering Eq. (1) showing that capacitance depends on the surface area of the electrodes, it should be expected that higher the specific surface area of an activated carbon, higher the capacitance. However, practical... 

The electrochemical storage capacity of various nanotextured carbons, including high surface area graphites, activated carbons, single wall and multiwalled nanotubes, poorly correlates with the BET specific surface area [125,129], From the slope of the capacity vs surface area plot (Fig. 32), an uptake of 1.5 wt% for 1000 m g" is estimated. However, Fig. 32 shows clearly that it is only a trend, several points being completely out of proportionality. 

The isoterms of adsorption of maleimide-activated immunoglobulins on the surface of Fe304/DMSA were investigated. The adsorption capacity value was A=92.12mg/g (Table 10.7), with the specific surface area specific = 130 m /g. The release of immunoglobulin, immobilized by means of sulfo-SMCC spacer, in the model medium was 8—12% for 24 h. 

Particle size is an important parameter that strOTigly influences the activity of catalyst nanoparticles. There are extensive studies on the particle size effect of Pt particles on the ORR catalysis, showing that the area-specific activity increased as the particle size increases due to a lower portion of low-coordinated Pt atoms on the edges and comers [13, 74—77]. Taking the surface area into account, this leads to a maximum mass activity at particle size of around 3-5 imi. Similar trends were also reported in PtsM alloy catalysts [13, 78, 79]. Eor dealloyed nanoparticles, situation becomes more complex as the morphologies and core-shell stractures were found to be dependent on the particle size, being an additional contribution to the overall size effect. 

In order to achieve ORR activity target of fourfold increase over the state-of-the-art Pt/C catalyst, one could either increase the Pt dispersion or increase the area-specific activity of the catalysts. It was shown that there are several pathways to the requirement. Some electrocatalysts prepared via these approaches have shown promising initial ORR activities, but none of them have yet been demonstrated to retain high activity throughout life. Loss of Pt surface area due to particle-size growth and partial loss of the less-noble metal from alloy catalysts are common phenomena. 

Durable high activity is achievable from even pure Pt electrocatalysts with particular structures, such as those with continuous layers of Pt and those with hollow Pt particles. Improved understanding of the origins of such high area-specific activity and knowledge on how broad a region of smooth surface is needed to obtain such activities may allow further improvement and practical implementation of these approaches. 

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