Catalysts, general surfaces

The available surface area of the catalyst gready affects the rate of a hydrogenation reaction. The surface area is dependent on both the amount of catalyst used and the surface characteristics of the catalyst. Generally, a large surface area is desired to minimize the amount of catalyst needed. This can be accomphshed by using either a catalyst with a small particle size or one with a porous surface. Catalysts with a small particle size, however, can be difficult to recover from the material being reduced. Therefore, larger particle size catalyst with a porous surface is often preferred. A common example of such a catalyst is Raney nickel. 

The larger the value of kj, the stronger is the adsorption of j on the catalyst-electrode surface. More generally the Langmuir isotherm (6.23) can be written as ... 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Natural products and common industrial chemicals in massive form are seldom useful as catalysts because they have low specific surface areas, may contain various amounts of impurities that have deleterious effects on catalyst performance, do not usually have the exact chemical composition desired, or are too expensive to use in bulk form. The preparation of an industrial catalyst generally involves a series of operations designed to overcome such problems. Many catalysts can be produced by several routes. The actual choice of technique for the manufacture of a given catalyst is based on ease of preparation, homogeneity of the final catalyst, stability of the catalyst, reproducibility... 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

It was obvious that the catalysts had to be optimized for North Sea atmospheric residues. In order to find a more nsefnl catalyst than the reference catalyst, two new catalysts were tested. The first one, Catalyst B, was selected based on resnlts from the previous tests the matrix surface area was reduced to an optimal size. The zeolite surface area was however kept constant. The second new catalyst. Catalyst C, was selected according to the old general recommendation for residne catalysts, and both the zeolite and matrix snrface areas were increased compared to the reference catalyst. The surface areas for the three catalysts are shown in Table 3.10. 

The surface properties of importance for adsorbents, catalysts, adherent surfaces, and corrodable surfaces are those properties which control interactions with adsorbable species. These interactions always involve dispersion force interactions and may or may not involve specific interactions. The ability of a surface to interact with another material can be determined at present best by observing its interactions with test materials, and these observations are never done in high vacuum and generally involve wet chemical techniques. 

In general, a high order in CP (e.g. +0.87, as was found in the case of the Pt/SiC>2 [big] catalyst), indicates that the surface contains on average a very low concentration of CP by doubling the amount of CP molecules in the gasphase, the surface concentration is doubled. This can only occur if during the adsorption process the CP molecule does not encounter another CP molecule which is already adsorbed. The surface is virtually covered with atomic D (indicated by the order in D2 of-0.82 for Pt/SiC>2 [big]), and the surface concentration of D is unaffected by the CP partial pressure. The low surface concentration of CP also explains the observation that the exchange pattern of CP is unaffected by Pep for this Pt/SiC>2 [big] catalyst the surface contains only a small amount of CP and adsorbed CP molecules do not influence each other due to mutual (lateral) interactions. 

Among the more important catalysts are metals, which may be promoted by other metals, or by oxides and oxides, which are usually rendered more effective by mixing with other oxides. It is usual to distinguish between supported catalysts, generally metals in a finely divided condition on the surface of silicate minerals, and promoted catalysts, where an oxide, or occasionally some other compound, is mixed with the metal the mixture being sometimes also supported on an inert refractory support. The distinction is not, however, absolutely sharp. 

The reaction of silicon and chloromethane proceeds phenomenologically in a way that the copper catalyst forms precipitates on the most activated areas of the silicon surface, which are the areas around intermetallic phases, grain boundaries or defects. In these areas one observes the fastet consumption of the silicon particle, also the general surface reacts, but much slower [5]. 

Nature of the Catalyst Support. - Of equal importance to the ionic character of the impregnating solution is the ionic exchange type and capacity of the catalyst-support surface. This is, of course, directly related to the chemical structure of the support surface. To say that information in the catalyst literature on the chemical structure of support surfaces is sparse is almost an understatement. Evaluations of this important catalyst-preparation parameter are almost without exception entirely overlooked in the published literature on catalyst preparations. A very limited amount of information can be found in a review of 1970. This review is, however, primarily concerned with the physical rather than the chemical structure of catalyst supports. The chemical reactivity of an oxide support surface would appear to depend upon the extent of its hydroxylation. This in turn depends upon the chemical type of the support, the way it was made, and particularly upon its previous thermal history. A few generalizations can be made, as follows. 

As carried out industrially, the processes pose problems in almost all their aspects. The catalysts generally operate between 800 and 1100 °C and at very high space velocities (>100 000 h ) with contact times of the order of 10" — 10 s the question arises therefore whether the reactions are wholly surface catalysed, or whether surface initiated gas-phase reactions are important. Since there is a considerable reorganization of atoms in reactants during their conversion to products, the nature of the reaction intermediates has been the subject of considerable speculation over many years. Reaction theories for ammonia oxidation were named, prior to 1960, after the principle intermediate proposed, viz. the imide (NH), nitroxyl (HNO), and hydroxylamine (NH2OH) theories. Similarly, alternative theories for the Andrussow cyanide process have proposed methylene-imine (CH2=NH) and nitrosomethane (CH3.NO) as reaction intermediates. Modern techniques might now reasonably be expected to discriminate amongst these hypotheses. 

Until Atkins and co-workers published that Cl atoms lowered the O2 desorption peak maximum temperature from 513 K (240 °C) to 481 K (208 °C) [3], there had been nothing published linking the effect of Cl atoms on any measurable physical property of the catalyst. Generally, the comments were nuanced along the lines that, because Cl atoms (which were considered to be on the surface of... 

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