Specific Poisons with Oxide Surfaces

Interaction of Specific Poisons with Oxide Surfaces 

The adsorption of potential poisons listed in Table I according to the HSAB concept is discussed in this section. From the information accumulated regarding the modes of interaction of any one of these adsorbates with oxide surfaces, it will be concluded whether a particular molecule may be suitable as an effective specific poison or as a probe molecule for the characterization of certain surface properties. 

Interaction of Water with Oxide SuRFACES-SuRFACE MODELS 

It is well known that the presence of water on an oxide catalyst surface effectively reduces the catalytic activity versus a wide variety of reactions. On the other hand, promotional effects have also been observed, so that criterion h (Section II.C.l) is invalidated. Water can be held by incompletely coordinated 

The 6 modification has also frequently been used in adsorption studies. The 5 phase may be regarded as a superstructure of three Al203-spinel cells containing an integer number of aluminum ions (111). 

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IV. Interaction of Specific Poisons with Oxide Surfaces. 203... 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

Electronic promoters, for example, the alkali oxides, enhance the specific activity of iron-alumina catalysts. However, they reduce the inner surface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are reduced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanendy. 

In order to substantiate this measure of chromia area, the rates of carbon monoxide oxidation over the various catalysts were measured. It was found that the alumina portion of the surface could be rendered inactive by selective poisoning with water and, under these conditions, the reaction was catalyzed exclusively by the ehromia surface. Since the activation energy was independent of the chromium content, it was reasonable to expect a linear variation of specific activity (i.e., activity per unit total surface area) with the fraction 0 (Table I) of the total surface contributed by the chromia phase. In Fig. 3 the specific rate is... 

In the specific case of biomass gasification, several alkaline salts and heavy metals and metal oxides particles may act as additional poisons by enhancing the sintering of the Ni crystallites or by being adsorbed on the Ni sites [44]. While acid supports such as alumina react with alkali to form crystalline phases, basic supports (like MgO) do not react directly with them however, alkali causes coverage of the surface and plugging of the pores. 

The hydrogenolysis of EtaSiH over silica-supported Pd and Pt catalysts resulted in significant poisoning, specifically, the loss of activity in the hydrogenation of cyclohexene.375 Oxidation, however, fully restored the activity of catalysts with small metal particles (>50% dispersion) as a result of surface reconstruction. 

The verification of a suspected mechanism and the determination of the rate determining step are important for the selection of the most suitable base catalyst and its modification by dopes. For most practical purposes, a high specific rate per unit area is desirable although other factors must also be considered especially the possibility to prepare the catalyst with a high surface area per unit weight, a relatively low sensitivity against poisons, a reasonable prize, and eventually a high selectivity for the production of a wanted product if several overall reactions occur in parallel, see, e.g., the oxidation of ethylene, Section V.E. 

For the supported metal systems the influence of metal sintering (see Figure 4) with the concomitant loss of active metal area is the most common morphological change (as compared with the specific influence of poisons, as discussed elsewhere in this chapter). The ability of a metal either to sinter or to retain its acdve surface is controlled by its thermal history, the degree of interaction with the support, and the exposure to specific environments (reducing or oxidizing, for example). 

A wide variety of analytical probes are used to study, characterize and monitor catalysts and catalyst surfaces. Our intent here is to discuss some of the more common and routine techniques. Much more detail and many more techniques can be found in specialized books 33, 32, 27, 12]. A catalyst functions through the highly specific interactions the active sites have with the reactants. The catalyst might be a metal dispersed on an inert carrier, a polycrystalline or amorphous mixture of metal oxides, or a zeolite (a crystalline and highly porous oxide). The experimentalist is typically interested in the catalyst composition, structure of the catalyst, distribution of active sites, presence of poisons/impurities after the catalyst has been used, and number of active sites parameters that influence the catalytic activity. 

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