Preparation of a Pt/SiO2 Catalyst: I. Interaction between Platinum Tetrammine Hydroxide and the Silica Surface

This paper describes a detailed study of exchange, drying, and thermal decomposition processes utilized to prepare Pt/SiO2 catalysts from Aerosil silica (Degussa) and platinum tetrammine hydroxide. It is shown that exchange is complete after about 1 min. However, more than 1 h is required to achieve a uniform distribution of the platinum complex over the silica surface. The exchanged complex does not lose any NH3 ligands upon drying at 25°C under vacuum. At 100°C and under vacuum the exchanged complex forms clusters of [Pt(NH3)4(OH)2]n deposited on the surface. Upon increasing the temperature under reduced pressure, the adsorbed complex looses the NH3 ligands progressively. At 200°C, about half the ammonia ligands are lost, and at 300°C the complex is fully decomposed, mainly into PtO particles of 1–2 nm. About 10% of PtO is reduced to Pt(0) by NH3 decomposition. Further reduction under hydrogen at 400°C does not alter the particle size. Chemisorption of H2 and O2, electron microscopy, and EXAFS analyses were used to characterize the various species formed. The results suggest that drying is the crucial step in the preparation of a Pt/SiO2 catalyst. This result was unexpected, because none of the earlier studies found this step to be crucial. The proposed procedure leads to a metal dispersion of 65±5%, which is close to the maximum value reported in the literature. The second part of this article presents time-resolved mass spectrometric analyses of the species released during thermal treatment in various atmospheres. They support the conclusions given here.