Crystallite Disorder in Catalysts

Measurements of Crystallite Disorder in Catalysts. - Many authors have speculated that the unusual activity of a particular catalyst preparation might be related to the presence of microstrain within individual catalyst particles. Experimental observations to support this speculation are few however, since in any highly dispersed material it is difficult to separate the effects of microstrain from other effects such as crystallite size and active site concentration. One careful study measured the microstrain in nickel and copper catalysts49 but failed to connect the results explicitly with activity data. 

Potentially of equal importance is the relationship between strain and catalyst stability. A calculation of the contribution to the total free energy of a catalyst crystal caused by the presence of strain-inducing microscopic precipitates50 showed that the extra free energy increases with the size of the crystal and inhibits it from sintering. This theory is an interesting one since it provides a mechanism which the catalyst scientist can exploit in his search for stable, high surface-area materials. The theory predicts the equilibrium crystallite size of the iron crystals of an ammonia synthesis catalyst with acceptable accuracy. 

It has also been suggested that the strain associated with a high concentration of such microscopic precipitates could be described by paracrystallinity theory,51 which would imply that the diffraction pattern of a catalyst containing such precipitates would have a characteristic form which would 

Hosemann, W. Vegel, D. Weick, and F. J. Balta-Calleja, Acta Cryst. Sect. A, 1981, 37, 85, and refs therein. 

Fischer, R. Hosemann, W. Vogel, J. Koutecky, J. Pohl, and M. Raleck, Proceedings VII International Congress on Catalysis, Tokyo, 1980 , Elsevier, Amsterdam, 1981, p. 475. 

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