Methanol electro-oxidation catalysts. Platinum promoted by tin

Abstract

Platinum catalysts modified by tin have been prepared by two unconventional techniques, (a) by pseudo-plating onto the platinum catalyst from H₂SO₄+ SnSO₄ solutions and (b) by immersion of the platinum catalyst in anhydrous SnCl₄. Technique (a) worked very effectively for bulk Adams platinum and platinum dispersed on a conducting support. However, it was found that two types of tin were incorporated when this method was used, one being loosely bound and a poison, the other being strongly bound and promoting the activity of the platinum. By appropriate anodization the poisoning tin could be removed without undue effect on the promoting tin. Method (b) worked effectively for unsupported platinum upon which only the promoting type of tin was deposited. However, immersion in anhydrous SnCl₄ produced no improvement in carbon supported platinum; apparently the carbon preferentially adsorbed the complexing tin species from solution.

With the technique using anhydrous SnCl₄, oxygen covered platinum was the preferred condition of the platinum substrate to take up the promoting tin species, giving an oxygen containing platinum + tin complex. This is in agreement with previous spectroscopic studies of platinum + tin electro-deposited catalysts which showed predominantly the presence of oxidized tin. However, other published work has presented convincing evidence that the tin promoter in such catalysts is present as Sn^o. It is probable that under reaction conditions the oxygenated platinum + tin complex is reduced to give a surface alloy.