INDEX catalytic processes

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

Water is non-combustible and nonflammable, odorless and colorless, and is universally available important prerequisites for the solvent of choice in catalytic processes. The dielectric constant or the refi"active index can be important in particular reactions and in analyzing them. The favorable thermal properties of water make it highly suitable for its simultaneous double function as a mobile support and heat transfer fluid, a feature that is utilized in the RCH/RP process (see below). 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

In electrocatalysis, the presence of surface defects is important to activate electron transfer in complex reactions. This is the consequence of the adsorbed species involved during the electro-catalytic process. Thus, Pt(l 10), the low Miller index plane, deserves special attention in theoretical studies. 

The polymerization process is a low temperature catalytic reaction. The type of polymer produced is strongly affected by the reaction temperature. Low temperatures give low molecular weight polymers, the kind useful in caulking compounds and as a viscosity index improver for motor oils. 

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