Selective poisoning as a tool for the characterization of surface active sites of metallic catalysts

✍ Scribed by F. Figueras; B. Coq

• Publisher: Elsevier Science
• Year: 1992
• Tongue: English
• Weight: 996 KB
• Volume: 12
• Category: Article
• ISSN: 0920-5861
• DOI: 10.1016/0920-5861(92)85042-k

✦ Synopsis

A consistent description of catalysis by metals should include the effects of particle size, support, alloy formation and poisoning. A simple and useful tool for the study of a reaction mechanism is to determine the nature and the efficiency of the poisons of this class of reactions. The concept of toxicity of poisons has been introduced long ago by Maxted [l, 21. Toxicity is a measure of the efficiency of a poison for a particular reaction and can be defined by the relationship: v/v(j= 1 -UC in which v,, is the rate of unpoisoned reaction, v the rate of the reaction in presence of a concentration C of poison, and a the toxicity. Maurel and Barbier [3-61 introduced the notion of selective poisoning, based on the different shapes of these curves. A linear decrease of activity with C shows a uniform surface for the poison: all the sites are equivalent and the poison is then nonselective. A good example of this behaviour can be found with the adsorption of mercaptans, or H,S, on platinum which poison to the same extent (Table 1) the three different reactions of hydrogenation, H-D exchange and C-C hydrogenolysis.

In some cases, a non linear poisoning is obtained: a fraction of the surface remains free of poison, and can then catalyxe some reactions, whereas other reaction paths are suppressed.

For instance, elemental sulphur, or Pb 3+, have a low toxicity for benzene-D, exchange, but selectively poison hydrogenolysis of cyclopentane, while Pb(CH,), has the reverse behaviour.

Poisoning can then be used to control the selectivity in industrial catalysis. The effect of poisons, mainly sulphur and metallic ions, has been reviewed earlier by Barbier [7, 81. The question arises whether the poison simply covers some of the surface sites, with little modification of the neighbouring atoms, or induces a modification of the electronic properties of these atoms. Former interpretations based on the electronic theory of catalysis