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 precursor [Pt(NH3)(4)](OH)(2) decomposes to a neutral Pt(NH3)(2)O species during the drying process at 120 degrees C. Treatment in flowing hydrogen at 180 degrees C leads to partial reduction of the platinum ammine 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 total 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 reduction produces a much higher dispersion (metal particle diameter 10 Angstrom). The beneficial effect of calcination, already observed by many authors when using [Pt(NH3)(4)](OH)(2) as a precursor for the preparation of highly dispersed Pt/gamma-Al2O3, can now be explained because this treatment avoids the formation of the mobile neutral Pt(NH3)(2)O complex. The metal particles produced by treatment in flowing hydrogen at 180 degrees C present a metal-oxygen contribution at 2.7 Angstrom formed at the metal-support interface. This long distance is assumed to be caused by the presence of hydrogen in the metal-support interface based upon our results in combination with other TPD and EXAFS studies. A second metal-oxygen contribution with similar coordination number 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] epitaxy in the metal-support interface.