Iron clusters chemisorption reactivity

In the context of this model we thus expect that the addition of ammonia to an iron cluster will lower its ionization threshold. The magnitude of the IP decrease will be dependent on the number of ammonia molecules chemisorbed and on the size of the cluster. We except that if the ionization thresholds and reactivities toward hydrogen were measured for Fe (NH3) or Fe H2 (n variable, m constant), an IP-ln (rate constant) anticorrelation would be found. Expieriments to date have shown that, upon ammoniation, the minimum in reactivity of iron clusters toward hydrogen shifts to smaller cluster sizes and that the rate constant for hydrogen chemisorption for these ammoniated clusters is about a factor of 10 lower than that for the bare clusters. However, the number of chemisorbed ammonias is different for each cluster. Experiments involving metal clusters with a fixed number of chemisorbed ammonias is a needed probe of the detailed interaction between NH3 and a cluster. 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

Finally, the overall trend is toward antipathetic structure sensitivity iron atoms (n = 1), which have the highest IP, are not active (17). Of course, for supported small particles the size distribution would not ordinarily permit the observation of the detailed oscillations of Fig. 23. Other results for H2 chemisorption on bare cobalt and niobium clusters have been reported by Geusic et al. (16), and the reaction with 02 and H2S has been studied by Whetten et al. (355a). For the latter, the oscillations as the reactivity increases with n are much smaller than those of Fig. 23. 

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