Estimation of the fractions of different nuclear iron species in uniformly metal-exchanged Fe-ZSM-5 samples based on a Poisson distribution

A simple calculation method is presented which allows for estimation of the fractions of different iron species in Fe-ZSM-5 based on statistical considerations. Under the assumption of uniformly distributed iron, the probability with which Fe-O-Fe bridged and higher nuclear Fe-species are formed in a Fe-ZSM-5 sample may be calculated based on a simple Poisson distribution. The results of these calculations were compared with DR-UV/vis and DRIFT spectroscopy measurements on a variety of Fe-ZSM-5 samples with different Fe/Al ratios. Moreover, the calculated concentration profiles were checked with literature data from other studies. It is shown that the fractions of different iron species in Fe-ZSM-5 can be estimated by this calculation method with satisfactory accuracy, provided that the metal-exchange process resulted in a uniform distribution of iron in the zeolite. The method may be useful for verifying of analytical results, understanding of observed activity patterns and may facilitate the identification of activity-structure relationships. (C) 2009 Elsevier B.V. All rights reserved