DECOMPOSITION OF THE PRECURSOR [PF(NH3)(3)](OH)(2), GENESIS AND STRUCTURE OF THE METAL-SUPPORT INTERFACE OF ALUMINA-SUPPORTED PLATINUM PARTICLES - A STRUCTURAL STUDY USING TPR, MS, AND XAFS SPECTROSCOPY

During the preparation of alumina supported platinum catalysts, the pr ecursor [Pt(NH3)(4)](OH)(2) decomposes to a neutral Pt(NH3)(2)O specie s during the drying process at 120 degrees C. Treatment in flowing hyd rogen at 180 degrees C leads to partial reduction of the platinum ammi ne complex and formation of platinum metal particles. A large increase in metal particle size is observed after a treatment under flowing H- 2 at 200 degrees C. The final reduction at 350 degrees C causes the to tal disappearance of the platinum precursor with a further increase in platinum particle size. The direct reduction at 350 degrees C yields the biggest metal particles (35 Angstrom) while calcination before red uction produces a much higher dispersion (metal particle diameter 10 A ngstrom). The beneficial effect of calcination, already observed by ma ny authors when using [Pt(NH3)(4)](OH)(2) as a precursor for the prepa ration of highly dispersed Pt/gamma-Al2O3, can now be explained becaus e this treatment avoids the formation of the mobile neutral Pt(NH3)(2) O complex. The metal particles produced by treatment in flowing hydrog en at 180 degrees C present a metal-oxygen contribution at 2.7 Angstro m formed at the metal-support interface. This long distance is assumed to be caused by the presence of hydrogen in the metal-support interfa ce based upon our results in combination with other TPD and EXAFS stud ies. A second metal-oxygen contribution with similar coordination numb er is detected at 3.86 Angstrom. This is a consequence of the presence of the first shell metal-oxygen at 2.7 Angstrom and implies a [111] e pitaxy in the metal-support interface.