Chemisorption, transition-metal sulfide

Apparent SMSI effects have also been reported with non-Transition Metal oxide supports.Those obtained with magnesia were almost certainly due to evolution of traces of hydrogen sulfide, or of iron ions, from the bulk at the high temperatures used, but real effects have been seen with silica when very high pre-treatment temperatures were employed the rate of the ethane reaction decreased more quickly than that of hydrogen chemisorption, pointing clearly to the need for multiatomic sites in hydrogenolysis. 

The toxicity of an element such as sulfur is dependent on the presence, in the valency shell of the toxic element, of free electron pairs which are evidently necessary for the formation of the link with the catalyst. The toxicity—i.e., the power of forming a relatively strong chemisorptive bond—disappears if the structure of the molecule is of a shielded type in which this element is already associated with a completely shared electron octet. Thus, it appears (Maxted, 8) that the chemical bond by means of which the poison is linked to the metallic surface resembles the ordinary dative bond in which the poison is the donor. In the case of methyl sulhde adsorbed on palladium, indications have been obtained (Dilke, Eley, and Maxted, 9) by means of magnetic susceptibility measurements that electrons from the methyl sulfide enter the d-band of the adsorbing metal to give a coordinate link, the process being probably accompanied (Maxted, 10) by a filling up of the fractional deficiencies or holes in the d-band of the metal due to d- -band overlap which seem to be responmble for the catalytic activity of the transition metals (11). 

The differences in chemisorption between transition metal and transition-metal oxide surfaces, as discussed above, are quite general and are very similar for the other oxides and for sulfides and other ionic solids. The electronic factors that control this behavior can be captured by relating the cations in the oxide to the metal atoms in well-defined coordination complexes. 

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