Volcano-type activity curve

Pig. 3.27 Classical volcano type activity curve. Promoters AI2O3 K2O temperature 450°C space velocity 10,000 pressure 100 atm... 

It will be seen from above discussion that the activity of ammonia synthesis catalyst correlates not only with the chemical compositions, but also the crystal types and crystal structure of iron oxides. The relationships between the activity and the Fe +/Fe + ratio can be interpreted perfectly by the molecular ratio / of the iron oxides, which have the different crystal structures in their precursors. At the same time, it also gives the theoretical explanation for those results of the classical catalysts (Fig. 3.27). For example, for the classical volcano-type activity curve, when Fe +/Fe + = 0.5, then / = f (Eqs. 3.16 and 3.17), so the catalyst has the good activity both sides at Fe +/Fe + = 0.5, due to / < 1, so the activity of the catalyst decreases. 

The mechanism often invoked on zeolitic-type catalysts with a partially oxidised hydrocarbon as a possible intermediate of the reaction could account for these results. Indeed, there are similarities such as the conversion of NO which follows a volcano-type curve and there is an increase of activity parallel to an increase of the oxygen coneentration in the reacting mixture. 

The magnitude of adsorption heat corresponds to the strength of adsorption bond, and so there should also be some relationship between adsorption and catalytic activity. The catal3dic activity is reversely proportional to adsorption strength when the surface coverage of reaction molecule reaches certain level, as indicated by Sabatier s theory of intermediate complex and experience. On the other hand, if adsorption is too weak, it is difficult to activate adsorbed molecules. The best activity can be obtained only in the case with suitable adsorption strength. This relationship is usually called as a volcano type curve, as shown in Fig. 2.4. 

Since the last century, it has been commonly believed that the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most similar to those of magnetite. The relationship between the activity and the ratio (Fe +/Fe +) is a volcano type curve, which seems to be an unquestioned... 

Hydrogen evolution is the only reaction for which a complete theory of electrocatalysis has been developed [33]. The reason is that the reaction proceeds through a limited number of steps with possibly only one type of intermediate. The theory predicts that the electrocatalytic activity depends on the heat of adsorption of the intermediate on the electrode surface in a way giving rise to the well known volcano curve. The prediction has been verified experimentally [54] (Fig. 2) and the volcano curve remains the main predictive basis on which the catalytic activity is discussed [41, 55],... 

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. ... 

The relationship between thermodynamics and kinetics in chemical reactions is usually expressed by the Bronsted equation (eq. 3.52 in chapter 3.4) k = gKa, where k is the rate constant, K is the equilibrium constant of the elementary stage, and g and a (Polanyi parameter) are constant values for a serious of reactions. These constants are determined by parameters characterizing the elementary mechanism (composition and structure of the activated complexes, etc.) thus allowing for the existence of an optimum catalyst, on which the rate of catalytic reaction per unit of surface has a maximum value. Equations of the type (3.52) were used for the explanation of "volcano-curves", when catalytic activity as a function of thermodynamic characteristics follows a curve with a maximum. An example for a volcano curve in methanation of CO is given in Figure 7.6. 

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