Catalytic activity surface impurities

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

These impurity phases deteriorate the catalytic activity due to reduction of surface areas. 

The adsorption and accumulation of various impurities from the electrolyte or surrounding atmosphere on the catalyst surface. The rate of accumulation of impurities on the catalyst surface depends on its activity for adsorption, which often is parallel to its catalytic activity. 

It should be noted immediately that not all the frequencies absorbed by a semiconductor are photocatalytically active, but only those that are also photoelectrically active, i.e., that cause an internal photoelectric effect in the semiconductor. Note further that the sign and magnitude of the photo-catalytic effect depend on the past history of the specimen exposed to illumination i.e., they depend on the external influences to which the specimen in question was subjected in the course of the whole of its life, and also on the conditions of the experiment (temperature, intensity of illumination, etc.). For example, by introducing into the semiconductor an impurity of any concentration or by adsorbing foreign gases on its surface it is possible to render its catalytic activity more or less sensitive to illumination. 

The introduction of an impurity into a crystal causes a displacement of the Fermi level both inside the crystal and, generally speaking, at its surface [in this case the Fermi level is displaced in the same direction both at the surface and in the bulk of the crystal, see reference (1) ]. This results, according to (63) and (5), in a change of g0. A donor impurity displaces the Fermi level upward, while an acceptor impurity shifts it in the opposite direction. The same impurity exerts diametrically opposite influences on the catalytic activity in acceptor and donor reactions. 

In the older literature one of the strongest supports for active points or active centers has been the finding that poisons such as CO destroy the catalytic activity completely, even if they are in such small amounts as to cover only a fraction of the surface. This would indicate that the catalyst surfaces referred to in the older literature were either very impure or very heterogeneous, or that since the surfaces were often measured by hydrogen adsorption, surfaces very much too high were obtained because the absorption of hydrogen into the interior of the structure as discussed earlier in this article was not realized. 

In spite of much effort, the nature of the active sites on acid—base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced to a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described. 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

The promoter is a substance that by itself does not have catalytic properties however, when it is added to the catalyst, it improves its properties. The promoter can be related to the stabilization of the catalyst structure or to the modification of the chemical properties of the catalyst surface [2], The term impurity is reserved for trace quantities of other components over which the investigator or the manufacturer has little control. The reaction rate under specified conditions of temperature, composition, and pressure is used to measure the catalytic activity of the solid, as described later. 

Decomposition due to contamination or contact with active surfaces. The rate of decomposition can be increased by the presence of soluble impurities and/or contact with active surfaces. High and low pH will also destabilize hydrogen peroxide. pH affects the activity of the catalytic impurities and the stabilizers which are present.47 Self-heating can rapidly accelerate the decomposition rate of destabilized hydrogen peroxide. Large amounts of oxygen and steam can be formed quickly (Table 1.4). 

Impurities can be divided into two groups those that are electroactive in the range of potential of interest, and those that may interfere with measurement by adsorbing on the surface and poisoning it (a more general and perhaps less ominous phrase, would be "changing its catalytic activity"). 

Tlic results also suggest that substitutional formation is most favorable on the (110) surface. This supports the view that the (110) surface will be more catalytically active than the (111) surface, as impurities segregate preferentially to this surface. Sayle et al. note that the segregation energies (i.e. the differences between bulk and surface energies) are larger for the Af cations than for cations due to elec-... 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

---