Nickel oxide, adsorption catalytic oxidation

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

The energetic nonuniformity of catalytically active nickel oxide was investigated by Refer (161). The kinetics of activated adsorption was studied under constant pressure at several temperatures and the data were described by the expression... 

In the catalysis community, it is generally accepted that there are two types of support materials for heterogeneous oxidation catalysts [84]. One variety is the reducible supports such as iron, titanium, and nickel oxide. These materials have the capacity to adsorb and store large quantities of molecules. The adsorbed molecules diffuse across the surface of the support to the catalyst particle where they are activated to a superoxide or atomically bound state. The catalytic reaction then takes place between the reactant molecules and the activated on the catalyst particle. Irreducible supports, in contrast, have a very low ability to adsorb O. Therefore, can only become available for reaction through direct adsorption onto the catalyst particle. For this reason, catalysts deposited on irreducible supports generally exhibit turnover frequencies that are much lower than those deposited on reducible supports [84]. More recent efforts in our laboratory are focused on characterizing catalyst support materials that are commonly used in industry. These studies are aimed at deciphering how specific catalyst and support material combinations result in superior catalytic activity and selectivity. 

It has been shown in Section III, A that a fraction of oxygen ions irreversibly adsorbed on nickel oxide at elevated temperatures (250°) reacts at room temperature with carbon monoxide to form adsorbed carbon dioxide. This interaction evidently also occurs on the surface of oxygenated or regenerated samples during the catalytic reaction (76). It has been observed, for instance, that adsorption of carbon monoxide, at room temperature, on the regenerated sample, although it decreases its electrical conductivity from 10- to IO-12 ohm- cm-, does not... 

All results presented in the various sections of this article concern the adsorption and the catalytic reaction of simple gases (oxygen, carbon monoxide, nitrous oxide) on the surface of divided nickel oxides. It may seem that we have limited our investigations to a very minute... 

Winter 10) relates isotopic exchange of molecular oxygen with magnesium oxide, zinc oxide, chromium oxide, nickel oxide, and iron oxide. He also compares the rates of isotopic exchange of these oxides with oxygen and the rates of adsorption and catalytic activity relating to the oxidation of CO and the decomposition of NgO. 

The jS-D-galactosidase from Aspergillus niger immobilized by adsorption onto either alumina or titania-treated stainless steel retained essentially all of its activity when stored under water at 23 °C for long periods little activity was lost when cheese whey was treated with the immobilized enzyme at 55 °C over several weeks. The catalytic activity of the immobilized enzyme (maximum activity at pH 3 and 60 °C) was unaffected by frequent sanitizations. Nickel-nickel oxide sinters were also used to immobilize this j8-D-galactosidase. Gluco-amylase was also successfully immobilized on these matrices. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Some metal oxides (notably alumina, magnesia and silica) can be readily prepared in a stable state of high specific surface area. Because of their technical importance as adsorbents, they have been featured in many fundamental and applied investigations of adsorption. Other oxides (e.g. those of chromium, iron, nickel, titanium and zinc) tend to give surfaces of lower area, but exhibit specific adsorbent and catalytic activity. These oxides have also attracted considerable interest. 

Aluminium dissolves with H2 evolution, and this hydrogen remains chemisorbed on nickel, presumably in a dissociated form. Raney nickel catalysts are often doped with other metals in order to improve the catalytic activity the selectivity decreases in the order. Mo > Cr > Fe > Cu > Co. These metals are fused with the Ni-Al alloy and remain on the final catalyst, probably as oxides. It is believed that the role of the doping metals is to strengthen the selective adsorption of nitrogenous substrates. 

Two examples in which adsorptive or catalytic properties have been affected by radiation-induced desorption or decomposition are the destruction of catalytic centers on copper and nickel (55, 60) already described (Section III,A,3,c) and the effect of radiolysis of surface water on adsorption of carbon monoxide by zinc oxide (153). 

Recently there have appeared papers by other authors in which volcano-shaped curves have been obtained. Fahrenfort, van Reijen, and Sachtler (467) have carried out complex kinetic, IR spectroscopic, calorimetric, and mass spectrometric investigations on the decomposition of formic acid on various metals. The authors come to the conclusion that the reaction proceeds via the intermediate formation of an adsorption complex of the surface nickel formate type. By comparing the heat of formation of the formate of the corresponding metal with the temperature Tr at which a fixed depth of conversion r (log r = —0.8) is reached, the authors have obtained a broken line similar to the Balandin volcano-shaped curves (Fig. 63). The catalyst half-covered with the adsorption complex is the most active one. The reaction investigated by the authors differs from those investigated by us. It is characteristic, however, that in the case of oxides the selectivity is the same with respect to the decomposition of alcohols and of formic acid [Fig. 1 in Mars (468)). In their report at the Paris Congress on Catalysis Sachtler and Fahrenfort (469) give additional data on volcano-shaped curves for a number of reactions and point out that this relationship between the catalytic activity and the stability of the intermediate complex has been qualitatively predicted by Balandin. ... 

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