A Model gamma-Alumina-Supported Rhenium-Platinum Catalyst Prepared from [Re2Pt(CO)12]: 1. Synthesis and Spectroscopic Characterization.

Abstract

Catalysts supported on -Al2O3 were prepared from [Re2Pt(CO)12], and from Pt (NH3)4(NO3)2 and NH4ReO4. The former samples were characterized by infrared and X-ray photoelectron spectroscopies (XPS) and by temperature-programmed reduction (TPR); the latter were characterized by TPR. [Re2Pt(CO)12] was initially chemisorbed on the -Al2O3 surface. Upon treatment in H2 at about 150°C, the cluster fragmented and formed rhenium subcarbonyls, and at about 400°C the sample was decarbonylated. Adsorption of CO and of NO as probe molecules gave evidence of metallic Pt, but there was no evidence of adsorption on Re. The XPS data indicating the Re binding energies give evidence of the presence of low-valent cationic Re in the sample after the treatment at 400°C in H2. In contrast, when a mixture of samples of Re on -Al2O3 and Pt on -Al2O3 prepared from [H3Re3(CO)12] and [(CH3)2Pt(COD)], respectively, was treated under equivalent conditions, the Re was present in a high-valent cationic form (Re7+), and Pt was metallic. It is concluded that Pt facilitated the reduction of Re and that Pt was likely near the rhenium in the sample prepared from [Re2Pt(CO)12]. The TPR data are consistent with the foregoing results. The TPR data characterizing the samples prepared from the metal salts show that the degree of hydroxylation the -Al2O3 support significantly influenced the reduction of the Re and the Pt, but these data are not sufficient to determine the interactions between the two metals.