Poisons with oxide surfaces

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

Poisons. - Unlike secondary surface metal oxide additives that indirectly interact with the surface vanadia sites via lateral interactions, poisons are surface metal oxide additives that directly interact with the surface vanadia sites and decrease the TOF. For example, the addition of surface potassium oxide to supported vanadia catalysts results in both a structural change and a reactivity change of the surface metal oxide species.50 This interaction, at submonolayer coverages, reflects the attractive interaction between these two surface metal oxide species. The presence of the surface potassium oxide poison alters the V-O bond lengths and the ratio of polymeric and isolated surface vanadia species (favoring isolated surface vanadia species). The interaction of the surface potassium oxide poison with the surface vanadia species is schematically shown in Figure 5. 

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. 

These catalysts are extremely sensitive to catalyst poisons, which reduce chemisorption of hydrogen and nitrogen on the active surfaces of the catalyst and thereby reduce its activity. Gaseous oxygen-, sulfur-, phosphorus-and chlorine compounds, such as water, carbon monoxide, carbon dioxide, the latter being reduced to water under ammonia synthesis conditions, are particularly troublesome in this regard. Catalyst poisoned with oxide compounds can be reactivated by reduction with pure synthesis gas. 

Rodriguez JA, Hrbek J (1999) Interaction of sulfur with well-defined metal and oxide surfaces Unravehng the mysteries behind catalyst poisoning and desulfurization. Acc Chem Res 32 719-728... 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

The rate of slow chemisorption is sharply decreased on a surface of zinc oxide poisoned with water (11). This observation is in agreement with the necessity of activating zinc oxide before it becomes an adsorbent for hydrogen. 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

When the purpose of the working electrode is to act as an inert electron sink, as in the detection of catecholamines, carbon is the preferred electrode material. On occasions when the electrode plays a direct role in the reaction, the precious metals are chosen. For example, silver can be oxidized to silver cyanide in the presence of cyanide ions. A major consideration when choosing an electrode material is its ability to maintain an active surface. Electrodes will develop a layer of surface oxide at positive applied potentials. The oxide layer will inhibit the oxidation of the analyte, and the response will decrease with repeated injections. The active surface can be renewed by polishing the electrode. Since glassy carbon electrodes are more resistant to poisoning by oxide formation, they are the electrode of choice for direct current amperometry. 

The vapor-phase, high-space-velocity oxidation to the thermodynamic reaction products should be contrasted with kinetically controlled oxidation of chemical feedstocks when the active metal is purposely poisoned or the surface area reduced. 

Carbon dioxide, C02, is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C02 adsorption onto oxide surfaces leads to a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of C02 and possible surface species are summarized in Table VI. The wave-number range below 1000 cm"1 was usually not accessible in studies on adsorbed C02 because of the strong absorption of the oxides at lower wave numbers. 

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). 

The oxidation of CO on the surface of hopcalite is a reaction of zero order. Both a stoichiometric process resulting in the reduction of the oxide surface and a catalytic process with oxygen of the gas phase have been observed. The heat of activation for the catalytic process varies for different samples of hopcalite within the limits of 5 to 7 kcal. (64). Poisoning of the hopcalite surface by adsorption of water vapor is independent of the temperature. The catalytic reaction takes place on a definite and constant part of the surface and no additional active sections of surface are brought in to play if the temperature is increased. From a determination of the heats of wetting of water on hopcalite, it is apparent that the surface can be subdivided into two types with different heats of wetting (64). 

DOT CLASSIFICATION 8 Label Corrosive SAFETY PROFILE Poison by intravenous route. Moderately toxic by ingestion and skin contact. An experimental teratogen. Experimental reproductive effects. Mutation data reported. A corrosive irritant to skin, eyes, and mucous membranes. Causes skin sensitization. Combustible when exposed to heat or flame. Ignites on contact with cellulose nitrate of high surface area. Can react with oxidizing materials. To fight fire, use CO2, dry chemical, alcohol foam. When heated to decomposition it emits toxic fumes of NOx. 

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