Catalysis, surface, adsorption measurements during

Adsorption Measurements during Surface Catalysis Kenzi Tamaru... 

Tamaru, Kenzi. Adsorption measurements during surface catalysis. Adv. Catal. 1964, 65. 

The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. Farnsworth Adsorption Measurements during Surface Catalysis Kenzi Tamaru... 

The coverage of A at steady state, can be found by the integration of Eqs. (2) and (4) subject to Eqs. (3), (7), and (8). Tamaru (29) referred to this aspect of the method as adsorption measurements during surface catalysis. The result depends only on the material balances. However, rather than only integrating, one can compare the experimental results during the transient period to the curves simulated by a trial sequence of steps, optimizing the values of the kinetic parameters as discussed previously and as most recently described by van der Linde et al. 27). 

In 1957, this author initiated a program of adsorption measurements during surface catalysis with simultaneous measurements of reaction rate (6). The adsorption postulated from the reaction kinetics could consequently be compared with the observed results to examine the reaction mechanism. The state and the coverage of the catalyst surface during the reaction could be followed by direct measurements, including data on the pressures of the reacting species and on the reaction rate. 

The importance of adsorption measurements during surface catalysis has been outlined so far in generality. In the following sections, the fundamental principle of the kinetic study based on this approach will be discussed together with appropriate experimental methods and results. An attempt will be made to classify the application of this approach to various cases, and emphasis will be placed on the principles of this approach rather than the detailed discussion of each reaction. 

In recent years, considerable progress has been made in the elucidation of the reaction mechanism using such tools as infrared spectroscopy 34), electric conductivity 35, 36) isotopic tracer 37-39) and adsorption measurements during surface catalysis 21, 40). 

The zero-order kinetics imply saturated adsorption on the active part of the catalyst surface, while the adsorption on the whole surface apparently depends upon the pressure of formic acid. The saturated adsorption of the copper surface calculated as one-site adsorption is shown as 0 = 1 in the results. It appears therefore that the catalytically active part is a minor part of the surface available for adsorption. In this manner, adsorption measurements during surface catalysis in the case of a zero-order reaction could lead to an estimate of the active part of the catalyst surface. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

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