Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS and XAFS Spectroscopy.

Abstract

During the preparation of alumina supported platinum catalysts, the precursor [Pt(NH3)4](OH)2 decomposes to a neutral Pt(NH3)zO species during the drying process at 120 'C. Treatment in flowing hydrogen at 180 '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 H2 at 200 'C. The final reduction at 350 'C causes the total disappearance of the platinum precursor with a further increase in platinum particle size. The direct reduction at 350 'C yields the biggest metal particles (35 A) while calcination before reduction produces a much higher dispersion (metal particle diameter 10 A). The beneficial effect of calcination, already observed by many authxs when using [Pt(NH3)4](OH)2 as a precursor for the preparation of highly dispersed WyA1203, can now be explained because this treatment avoids the formation of the mobile neutral Pt(NH3)zO complex. The metal particles produced by treatment in flowing hydrogen at 180 'C present a metal-oxygen contribution at 2.7 A 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 A. This is a consequence of the presence of the first shell metal-oxygen at 2.7 8, and implies a [ 11 11 epitaxy in the metal-support interface.