Diffusion of chemisorbed oxygen into Pd sub-surfaces and its influence in oxidation catalysis

Abstract

Oxygen migration to subsurface layers of palladium and its influence in catalytic oxidation has been addressed in this mini review. Oxygen dissolution in palladium is a well-known phenomenon; many researchers observed the presence of surface, subsurface and bulk oxides by different methods on palladium single crystals as well as in powder catalysts. Opinion is divided on deciding the nature of palladium that is responsible for catalytic oxidation reactions. Nonetheless it is tilting towards oxidized palladium, which is likely to be responsible for the oxidation catalytic activity, and increasing numbers of evidences are available. Oxygen covered in the subsurface layers of palladium show extended activity regime to higher temperature and it has been demonstrated by molecular beam methods. CO oxidation studied on Pd surfaces with CO+O₂ mixture displays oxidation activity up to 900 K, highlighting a significant increase in CO adsorption capacity on the above surfaces. In this mini review we highlight the oxygen diffusion into Pd-sub surfaces and its characterization by several methods. Further how the subsurface oxygen could influence the electronic structure and hence catalytic activity has been briefly discussed through CO oxidation reactions.