Effect of Sulfur Adsorption on the Catalytic Activity

On metallic catalysts, sulfur is strongly adsorbed, and even if only minute amounts are found in the feedstock, accumulation can occur on a significant part of the metallic surface area. In the adsorbed state, the poison molecule will deactivate the surface on which it is adsorbed then the toxicity will depend on the number of geometrically blocked metal atoms. On the other hand, the chemisorption bond between the poison and the metal can modify the properties of the neighboring metallic atoms responsible for the adsorption of reactants. If the interaction between the poison and the metal is weak, the structure of the metal will remain unchanged, but it can induce a perturbation all around the adsorption site, which will be able to modify the catalytic properties of this surface. Yet if the interaction between the metal and the adsorbate is strong, it can go as far as to modify the metal-metal bond. The mobility of the surface atoms can be increased and a new superficial structure can appear. 

In order to obtain deeper insight into such complex effects, the definition of the deactivation extent induced by sulfur adsorption can provide a lot of information about its action. The simplest technique to characterize a poison is to define its initial toxicity as the number of accessible metal atoms deactivated through adsorption of the first molecule of poison (74). 

Determination of the Initial Toxieity of Sulfur for Different Reactions 

Ratio of the Activities of Fresh (r) and Sulfurized (r ) Catalysts for Different Reactions 

Further, Maxted (36) studied the influence of several sulfur compounds on the activity of platinum black for the hydrogenation of crotonic acid in the liquid phase. He noticed that between 15 and 50°C the toxicity remains constant for a sulfur compound, pointing out the irreversibility of sulfur adsorption. Conversely, the toxicity of various compounds increases with the molecule size. For molecules containing two sulfur atoms, losing all freedom of rotation through this double adsorption, the toxicity is less than for molecules of the same length containing only one atom of sulfur. 

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