Catalyst utilization factor

This indicates that cos GhjO increases with the increase in oxygen content and decreases with the decrease in hydrogen content. This relationship was confirmed by the results of Kinoshita et al. [94] on the electrochemical treatment of carbon blacks. Hydrophilic carbons provide high catalyst utilization factors, whereas hydrophobic carbons (or graphitized carbons) allow easy water removal, avoid flooding of the CLs, and ensure better resistance to corrosion [8,96,97]. 

Another reason, which affects the utilization factor, is the structure of the carbon particles themselves. Rao et al. [17] have demonstrated that the catalyst utilization factor may vary significantly depending on the porosity of carbon materials. They have prepared a series of Pt-Ru (1 1) catalysts supported on carbon materials from the Sibunit family with grossly different BET surface areas, ranging from 20 to 400 m /g, which were utilized as the anode catalysts in liquid-fed DMFC. To be able to distinguish between the influence on cell performance of the metal dispersion and the carbon support porosity, the metal dispersion was kept constant and close to 0.3. It was demonstrated that the catalyst utiUzation factor reached 100% for low-surface-area supports but dropped down to 10% for the high-surface-area Sibunit carbon. As a result, in methanol electrooxidation, both the mass activity (Ag Ru) and specific activity increased with a... 

Figure 2.7. Catalyst utilization at macroscopic scale, i.e. the catalyst utilization factor /(Xptc, Xei) (>Sr)/Xptc in the exchange current density (top panel, cf. Eqs. (2.65) and (2.66)), and oxygen diffusion coefficient (bottom panel, cf. Eq. (2.63)) as functions of the liquid water saturation, plotted for the three different pore size distributions in Figure 2.2 [35]. The plots reveal the effect of porous structure on the basic competition between activity (top) and mass transport (bottom) in the CCL. The structure with a large fraction of primary pores is beneficial for catalyst utilization and detrimental for gas diffusion, and vice versa.

The size of the cataly.st particle influences the observed rate of reaction the smaller the particle, the less time required for the reactants to move to the active catalyst sites and for the products to diffuse out of the particle. Furthermore, with relatively fast reactions in large particles the reactants may never reach the interior of the particle, thus decreasing the catalyst utilization. Catalyst utilization is expressed as the internal effectiveness factor //,. This factor is defined as follows ... 

In order for diffusional limitations to be negligible, the effectiveness factor must be close to 1, i.e. nearly complete catalyst utilization, which requires that the Thiele modulus is suffieiently small (< ca. 0.5), see Figure 3.32. Therefore, the surface-over-volume ratio must be as large as possible (particle size as small as possible) from a diffusion (and heat-transfer) point of view. There are many different catalyst shapes that have different SA/V ratios for a given size. 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

The primary optimization target of CLs is the effectiveness factor of Pt utilization, Tcl- It includes a factor, that accounts for statistical limitations of catalyst utilization that arise on a hierarchy of scales, as specified in the following equafion. defermines the exchange current density ... 

When the effective reaction rate is controlled by pore diffusion, then the asymptotic solution of the catalyst effectiveness factor as a function of the generalized Thiele modulus can be utilized (cq 108). This (approximate) relationship has been derived in Section 6.2.3.1. It is valid for arbitrary order of reaction and arbitrary pellet shape. 

Internal and external mass transfer resistances are important factors affecting the catalyst performance. These are determined mainly by the properties of the fluids in the reaction system, the gas-liquid contact area, which is very high for monolith reactors, and the diffusion lengths, which are short in monoliths. The monolith reactor is expected to provide apparent reaction rates near those of intrinsic kinetics due to its simplicity and the absence of diffusional limitations. The high mass transfer rates obtained in the monolith reactors result in higher catalyst utilization and possibly improved selectivity. 

The bed effectiveness is an important factor for an optimal design of a PPR for a given application. A lower bed effectiveness means that more catalyst and a larger reactor is needed for the same process duty. Another important factor is pressure drop. As discussed above, a PPR with thin catalyst layers will have a high degree of catalyst utilization, so the amounts of catalyst and reactor space required are low. However, with thinner catalyst layers the construction cost of the PPR will be higher. 

Catalyst utilization Several factors limit the efficiency of catalyst utilization ... 

Advanced design Control of the catalyst layer thickness and composition provide the means to optimize catalyst utilization and performance issues. Taking together all factors listed above, only 10-20% of the catalyst is effectively... 

A novel design route expanding on such a two-phase concept has already been followed by the company 3M for some time. The benefits in terms of catalyst utilization and performance enhancement are evident from that work [147]. The activity per total mass of Pt of the 3M layers is larger by about a factor 6 than the mass activity of conventional three-phase CCLs. This affirms the awful inactivity of Pt in conventional CCLs. 

The highest electronic resistances were observed at low Nafion loadings, indicating that the ionomer played a significant role as a binder [211], Meanwhile, kinetic losses pass throngh a minimnm correlated with the electrochemically active snrface area of the catalyst estimated from cyclic voltammograms [209] The higher the electrochemically active surface area, the lower the kinetic losses. This volcano type of cnrve reflects the optimnm in the metal utilization factor u. Below, we try to nnderstand how carbon properties may influence these characteristics. 

It shonld be noted that high utilization factors measnred with cyclic voltammetry by no means warrant the assnmption that nnder dynamic conditions of fnel cell operation the CLs deliver the same cnrrent as they wonld without mass transport and ohmic constraints. To acconnt for the latter, Gloagnen et al. [185] employed the effectiveness factor the ratio of the actnal reaction rate to the rate expected in the absence of mass and ionic transport limitations. The effectiveness factor is a fnnction of the total catalyst area, the exchange cnrrent density, the overpotential, the diffusion coefficient D, the concentration of electroactive species Co, the thickness of the CL, and the proton conductivity of the electrolyte, and drops sharply below 100% with increased exchange current density and decreased the product DCq. 

It is worth noting that the remarkable effect described for the carbon support porosity on the metal utilization factor and hence on the specific electrocat-alytic activity in methanol electrooxidation was only observed when the catalysts were incorporated in ME As and measured in a single cell. The measurements performed for thin catalytic layers in a conventional electrochemical cell with liquid electrolyte provided similar specific catalytic activities for Pt-Ru/C samples with similar metal dispersions but different BET surface areas of carbon supports [223]. The conclusions drawn from measurements performed in liquid electrolytes are thus not always directly transferable to PEM fuel cells, where catalytic particles are in contact with a solid electrolyte. Discrepancies between the measurements performed with liquid and solid electrolytes may arise from (1) different utilization factors (higher utilization factors are usually expected in the former case), (2) different solubilities and diffusion coefficients, and (3) different electrode structures. Thus, to access the influence of carbon support porosity... 

The lifetime of catalysts is a key factor in the economics of many industrial processes especially when coke forms and deposits over the catalyst. It is therefore necessary to model the deactivation and regeneration of the catalysts utilized for the purpose of design, operation, control and optimization of these processes. 

It is also possible to base a utilization factor on the bulk gas phase composition, much in the same way as was done already with the c-concept for reaction and transport around and inside a catalyst particle. Let this global utilization factor. 

Heterogeneous catalytic reactors are the most important single class of reactors utilized by the chemical industry. Whether their importance is measured by the wholesale value of the goods produced, the processing capacity, or the overall investment in the reactors and associated peripheral equipment, there is no doubt as to the prime economic role that reactors of this type play in modem industry. The focus of this chapter is the design of heterogeneous catalytic reactors. Particular emphasis is placed on the concept of catalyst effectiveness factors and the implications of heat and mass transfer processes for fixed bed reactor design. 

The real-to-apparent surface area ratio of a distributed electrode, frequently also termed heterogeneity or roughness factor, corresponds to the ratio y0/y0 catalyst utilization is... 

At macroscopic scale, catalyst utilization is severely limited by the statistical constraints imposed by the random structure of the three-phase composite. Only catalyst particles that are simultaneously accessible to electrons, protons, and oxygen could be electrochemically active. These requirements are included in the factors/(Yptc, X )/Yptc and g(Sr) in Eq. (2.27). Figure 2.7 reveals that due to these factors alone the upper limit of catalyst utilization lies in the range of 20%. 

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