Thiol protected platinum black and palladium black catalysts in oxidation catalysis

Platinum and palladium metals are common catalysts for formic acid wet oxidation, exhibiting high activity and good stability. However, in the presence of some reactive organic substances, even in low concentrations, they suffer severe deactivation. This paper demonstrates the feasibility of protecting metal catalysts from aggressive media by creating a tertiary structure over the active sites. The approach is based on the known principle of metal surface modification by self-assembled organic monolayer films. We have found that self-assembled monolayers (SAMs) fabricated from heterocyclic thiols containing nitrogen, e.g., 6-mercaptopurine (6MP), can protect platinum and palladium black catalysts for formic acid oxidation in the presence of iminobis[methylenephosphonic acid] (IDMPA). IDMPA was chosen as a model inhibitor because it displays strong chelating properties. Catalysts protected with 6MP demonstrate higher activity and stability and have significantly lower metal leaching (up to 15 times in case of palladium black) than their unprotected counterparts. The surface structure of the catalysts and their surface composition were characterized using X-ray photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). (C) 2000 Elsevier Science B.V.