An investigation of the deactivation of Rh/alumina catalysts under strong oxidising conditions

A range of physical and chemical techniques have been used to investigate the interaction between rhodium and alumina at high temperature under oxidising conditions. Rh/Al2O3 catalysts have been studied by EXAFS, TPR, and H-2 chemisorption after different calcination treatments, and catalytic reactions (CH4, CO, and C3H6 oxidation, and NO reduction with C3H6 in the presence of excess oxygen) have been used as highly sensitive and specific chemical probes. It has been found for 1% Rh/Al2O3, that after low temperature calcination (300 degrees C) the rhodium is present as small easily reduced oxide clusters spread out on the alumina surface. The resulting catalyst has high activity for oxidation reactions due to the presence of reactive terminal (Rh-O) oxygens. However, the high oxidation activity leads to poor selectivity in the reduction of NO with C3H6 in excess oxygen. Calcination at higher temperatures (500 to 700 degrees C) causes the rhodium to interact strongly with the alumina support and become increasingly difficult to reduce. These catalysts have low activity for oxidation reactions but are highly selective for the reduction of NO to N-2. EXAFS data show that the rhodium is almost all present as isolated ions with high co-ordination to both oxygen and aluminium. These results are taken to show that, whilst the rhodium is still exposed at the surface, it is now in the alumina surface layer, but strongly bound and thus difficult to reduce. During this high temperature calcination, it is thought that the rhodium spreads over the support and diffuses into defect sites in the alumina surface.