We have investigated the role of different Pd sintering mechanisms on the regeneration process for alumina supported Pd hydrogenation catalysts. The Pd sintering mechanisms are strongly influenced by the catalyst morphology. In the case of fresh catalyst, Pd particles sit directly on the alumina surface and sintering occurs via a traditional ripening and coalescence mechanisms and was significant only at 600 degrees C or above. For the used catalyst, Pd particles are supported mostly on green oil residue and this dramatically changed the sintering mechanism during the regeneration process. Catalytic gasification of green oil residue around the Pd particles in an oxidizing environment permits movement and coalescence to take place at relatively low temperatures. During the stripping, Pd particles are passively brought into contact as hydrocarbon is gasified and the exothermic nature of the process raises the temperature to about 500 degrees C. At this temperature, chance contact between metal particles results in rapid coalescence and gives rise to particle sintering even at temperatures well below the Tamman temperature. This passive contact process is the dominant mechanism for Pd coarsening during catalyst regeneration. (c) 2004 Elsevier B.V. All rights reserved.