Catalyst layer Introduction

In order to control heat removal and therefore the catalyst temperature, multiple-tube reactors (Lurgi process) or quench reactors with several catalyst layers and introduction of cold gas (ICI process) are mainly used. Catalyst performance in modern larger reactors is 1.3-1.5 kg of methanol per liter per hour, and large-scale plants have capacities of up to 10 fra, which reflects the position of methanol as a key product of Ci chemistry. 

Hui et al. [29] conducted toluene contamination tests at different toluene concentrations. They also studied the effects of different operational conditions on toluene contamination, including the effects of fuel cell relative humidity (RH), of Pt loading in the cathode catalyst layer, of back pressure, and of air stoichiometry [30]. Figure 3.8 shows a set of representative results of contamination tests at 1.0 A cm with various levels of toluene concentration in the air. It can be seen that the cell voltage starts to decline immediately after the introduction of toluene, and then reaches a plateau (steady state). These plateau voltages indicate the saturated nature of the toluene contamination. For example, the cell voltage drops from 0.645 V to 0.522 V at 1.0 A cm-2 within 30 min of the cathode... 

We begin with the discussion of cell thermodynamics and electrochemistry basics (Chapter 1). This chapter may serve as an introduction to the field and we hope it would be useful for the general reader interested in the problem. Chapter 2 is devoted to basic principles of structure and operation of the polymer electrolyte membrane. Chapter 3 discusses micro- and mesoscale phenomena in catalyst layers. Chapter 4 presents recent results in performance modeling of catalyst layers, and in Chapter 5 the reader will find several applications of the modeling approaches developed in the preceding chapters. 

An alternative method to conventional thin-film techniques is the colloidal method. Typically, the catalyst layers are applied as a solution. It is well known that Nafion forms a solution in solvents with dielectric constants greater than 10. When a solvent which has a dielectric constant of 5.01 is employed as the solvent, a colloid forms in lieu of a solution. Shin et al. (2002) suggested that in the conventional solution method the catalyst particles could be excessively covered with ionomer, which leads to under-utilization of platinum. In addition, it was proposed that in the colloidal method the ionomer colloid absorbs the catalyst particles and larger Pt/C agglomerates are formed. The colloidal method is known to cast a continuous network of ionomer that enhances proton transport. The thickness of a catalyst layer that Shin et al. (2002) formed by the colloidal ink was twice that of the 0.020 nun thick layer formed with solution ink, In addition, the size of Pt/C agglomerates increased from 550 to 736 nm with the introduction of the colloidal method. The colloidal method dramatically outperformed the solution method at high current densities in single cell experiments. 

A further development in GDL manufacture was the introduction of a micro porous hydrophobic layer applied to one side of the carbon backing which faced the catalyst layer (Lister and McLean, 2004 Song et al., 2001). This layer typically consisted of PTFE and carbon black. Carbon black imparts good electrical properties and improves the gas transport to the catalyst layer. The type of carbon powder used also impacts the cell performance. Antolini et al. (2002) investigated two different carbon powders (oil furnace carbon black and acetylene black) and reported that acetylene black gave better cell performance than oil furnace carbon black. Carbon is a critical material in fuel cells since it has the necessary properties of electrical conductivity, corrosion resistance, surface properties and low cost factors which could make an inexpensive fuel cell a reality. Carbon is a nonmetallic substance with a wide range of crystalline and amorphous... 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

A complex nanostructured catalyst for ammonia synthesis consists of ruthenium nanoclusters dispersed on a boron nitride support (Ru/BN) with barium added as a promoter (33). It was observed that the introduction of barium promoters results in an increase of the catalytic activity by 2—3 orders of magnitude. The multi-phase catalyst was first investigated by means of conventional HRTEM, but this technique did not succeed in identifying a barium-rich phase (34). It was even difficult to determine how the catalyst could be active, because the ruthenium clusters were encapsulated by layers of the boron nitride support. By HRTEM imaging of the catalyst during exposure to ammonia synthesis conditions, it was found that the... 

A composite material (denoted as Y/MCM-41) composed of a core of zeolite Y particle and a thin layer of MCM-41 have been prepared by the crystallization of the reaction mixture of MCM-41 and zeolite Y particles. The Y/MCM-41 particle size increases with the increase of the Si02/Al203 ratio of MCM-41. Introduction of hydroxymethyl fiber into the zeolite Y particle favors the significant increase of its strength, but zeolite p easily formed. The adsorption property of Y/MCM-41 is different from those of zeolite Y and MCM-41. H(Y/MCM-41) as a catalyst is highly selective to C4-C5 hydrocarbons and slowly deactivated in the cracking of n-heptane compared to the mechanical mixture particles of HY and HMCM-41 (designated as H(Y+MCM-41)). 

All the available experimental and theoretical work performed on NEMCA leads to the conclusion that electrochemical promotion is caused by electrocatalytic introduction of promoting species like O2 or Na+ from the solid electrolyte to the catalyst/gas interface where a double layer is formed, of which density and internal electric field vary with the applied potential. The latter affects the work function at the surface and therewith the bond strength of adsorbing reactants and intermediates. This causes the dramatic and reversible modification in catalytic rate (Vayenas and Koutsodontis, 2008 see Figure 28). 

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