Fundamental Deactivation Processes

Promoted magnetite in its reduced state, used as an ammonia synthesis catalyst, undergoes deactivation for a number of reasons. Like other high-surface area structures, it may sinter. Species present in the gas stream may chemisorp upon the catalytic surface and thereby poison it, and furthermore, access to the pore system may be blocked by deposits. These processes of sintering, poisoning, and fouling are discussed below. 

In its active state, an ammonia synthesis catalyst is a high-density, medium-surface area composite, which is produced by the reduction of promoted magnetite (see Chapter 2). With a resulting surface area of about 10 m g and a bulk density around 2.8 g cm , an approximate surface area of 2-3 x 10 m per m is available. On a microscopic scale this corresponds to an average crystallite size of about 20-30 nm. 

Such small particles are generally not thermodynamically favored due to their excess surface energy. The sintering process results in an irreversible increase in average crystallite size. Mass transport is involved, and this may occur through diffusion (surface or bulk) or the formation of volatile intermediaries. Sintering is discussed in more detail later in the chapter and by Hughes.  

The catalytic activity of an ammonia catalyst may be reduced in the presence of certain chemical compounds, referred to as poisons. These may be gaseous, occurring as minor components of the synthesis gas, or as solids introduced to the catalyst during the manufacturing process, as impurities in the natural magnetite from which the catalyst is made. The latter will not be dealt with here, since they are already covered in Chapter 2. 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen 

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Abstract Peroxidases use H2O2 as electron acceptor in order to catalyze a variety of oxidative reactions through a catalytic cycle with two intermediates. Additionally to these intermediates, a third species (Compound III) is produced when ferric peroxidases are exposed to an excess of H2O2. Compound III is a peroxy-Fe111 porphyrin free radical, the best described of the intermediates leading to the irreversible deactivation of the enzymes. This chapter aims to describe the structure, stability, formation, and decay of Compound III as fundamental knowledge required to understand, and potentially to control, the peroxidases H202-dependent deactivation process. 

Catalyst deactivation is of major concern in catalyst development and design of packed bed reactors. Decay of catalytic activity with time can be caused by several mechanisms such as fouling, sintering and poisoning. Although much fundamental experimental work has been done on deactivation,very little attention has been focused on modelling and systematic analysis of nonadiabatic fixed bed reactors where a deactivation process occurs. 

Conventional heterogeneous catalysis and empiricism could provide a starting point in the selection of electrocatalysts for new unexplored processes for chemical production, energy generation or conservation, and environmental control. However, a fundamental understanding of adsorption characteristics, electrode kinetics, mechanisms, adsorbate-support interactions, and deactivation processes are needed for improved electrocatalyst... 

Here, ki2 and k2i correspond to diffusion in solution, k23 and k32 represent the rate constants of electron transfer in the respective encounter complex, and k3Q is the sum of those of rapid deactivation processes. The fundamental theory for elucidating these electron transfer processes was proposed by Marcus (3). It considers that the steps described by k23/ 32 30 outer-sphere electron... 

As stated, one of the fundamental problems encountered in the direct oxidation of hydrocarbon fuels in SOFCs is carbon deposition on the anode, which quickly deactivates the anode and degrades cell performance. The possible buildup of carbon can lead to failure of the fuel-cell operation. Applying excess steam or oxidant reagents to regenerate anode materials would incur significant cost to SOFC operation. The development of carbon tolerant anode materials was summarized very well in several previous reviews and are not repeated here [7-9], In this section, the focus will be on theoretical studies directed toward understanding the carbon deposition processes in the gas-surface interfacial reactions, which is critical to the... 

In recent years, a large body of work emphasized the use of zeolites for production of fine chemicals (refs.1-4). The interests stand in replacement of liquid acids to lower corrosion of equipment and pollution, and to reach specific selectivities. However, the hopes raised up in a rapid development of processes seems restrained nowadays. Many patents claimed zeolites as catalysts but very few have received industrial applications. Actually, basic research on the stability, the origin of deactivation, the regenerability of the catalysts have to be developed. Moreover, fundamental aspects of the mechanism of this new kind of reactions are lacking, in particular, the possibility of radical mechanisms, which are rather scarce with hydrocarbons, but can likely occur when heteroatoms are involved in the reactant. Those were our objectives in the study of the isomerisation of substituted halobenzenes on zeolites (refs.5-7). Indeed this reaction was claimed to occur readily on zeolites (refs.8-9), but it is supposed that no industrial development has followed. 

As outlined above, the electrochemical properties of this redox species are strongly pH-dependent and this behavior can be used to illustrate the supramolecular nature of the interaction between the polymer backbone and the pendent redox center. The cyclic voltammetry data shown in Figure 4.17 are obtained at pH = 0, where the polymer has an open structure and the free pyridine units are protonated (pKa(PVP) = 3.3). The cyclic voltammograms obtained for the same experiment carried out at pH 5.7 are shown in Figure 4.18. At this pH, the polymer backbone is not protonated and upon aquation of the metal center the layer becomes redox-inactive, since protons are involved in this redox process. This interaction between the redox center and the polymer backbone is typical for these types of materials. Such an interaction is of fundamental importance for the electrochemical behavior of these layers and highlights the supramolecular principles which control the chemistry of thin films of these redox-active polymers. Finally, it is important to note that the photophysical properties of polymer films are very similar to those observed in solution. Since the layer thickness is much more than that of a monolayer, deactivation by the solid substrate is not observed. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

In contrast to so-called microkinetic analyses, an important aspect of chemical reaction engineering involves the use of semiempirical rate expressions (e.g., power law rate expressions) to conduct detailed analyses of reactor performance, incorporating such effects as heat and mass transport, catalyst deactivation, and reactor stability. Accordingly, microkinetic analyses should not be considered to be more fundamental than analyses based on semiempirical rate expressions. Instead, microkinetic analyses are simply conducted for different purposes than analyses based on semiempirical rate expressions. In this review, we focus on reaction kinetics analyses based on molecular-level descriptions of catalytic processes. 

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

It is rewarding that both fundamental and applied aspects are dealt with. The deactivation of catalysts in important industrial processes like fluid bed catalytic cracking, hydrotreatment, hydrodesul furization, catalytic reforming, hydrodenitrogenation, steam reforming,... 

The reasoning process may be modeled by a computer algorithm (see Section 2.6.2.1). In order to propose catalyst components for a given reaction on a more fundamental basis, reaction steps have to be identified which lead to the desired products or which should be avoided because they are not selective or because they result in deactivation of the catalyst. These reaction steps ought to be elementary reactions steps however, in the case that such elementary reaction steps are not known, simplified reaction schemes may be useful, too. 

SC-CO2 is also becoming increasingly important as reaction media [7] for a great variety of fundamental chemical reactions ranging from catalysis to polymerization, [8,9] to synthesis and growth of inorganic materials [1,2], to nanoparticle production and preparation processes [1,2,10,11], and to biotechnological applications such as activation and deactivation of enzymes [12], biomass conversion, and biocatalysis [1,2,13],... 

When we say that a catalyst deactivates, we mean that under conditions of constant temperature and reaction mixture composition, the conversion of the reactants into products decreases with time. The first question that is asked after a substance has been identified as a catalyst is, "How long will it last ". Accordingly, the phenomenon of deactivation has been the object of an enormous number of research and development projects by companies involved with catalytic processes throughout the world and a lesser, but still significant number of studies, in universities and research institutes. Catalysts are widely used in processes and their finite lifetime is a concern from an economic viewpoint. Therefore, a fundamental under-... 

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