Nuclear Magnetic Resonance Study of Promoted Catalysts.

We have used NMR to study the properties of promoted catalysts prepared by adding alkali metals (Na, K, Rb, or Cs) to silica supported Pt clusters. We have also examined the effect of the alkali metals on CO and C_ {2}H_{4} adsorbed on the Pt. The properties of the promoter were studied in samples containing ^{23 }Na. By performing a ^{23 }Na-^{13}C doubled resonance with adsorbed CO, we determined that at least 22% of the NA in the samples is on the Pt. The ^{23}Na line shows no indication of a Knight shift and the Na spin-lattice relaxation varies as 1/T^2, suggesting that the Na orbitals mix with the Pt conduction band to a much lesser degree than do those of adsorbed CO. Consideration of several results leads us to conclude that the Na forms three dimensional structures on the Pt, most likely Na _{2}O molecules or crystallites. We observed the effect of the promoters Na, K, Rb, and Cs on the NMR properties of adsorbed CO. In all cases the effects are small. The CO line is narrowed and shifted to higher frequency in the presence of the promoter while the CO spin lattice relaxation is slowed. The effect of the promoter is greater in the samples with the heavier alkali metals K, Rb, and Cs. In the case of CO and Rb, for which the effect of the alkali metal is greatest, a modest change of about 10-15% in the parameters used to describe the NMR results is adequate to account for the effect of the Rb. We investigated C_{2 }H_{rm 4} adsorbed on Pt both with and without K. By performing ^{13}C-^ {1}H double resonance we discovered that the C_{2}H_ {4} exists as a CCH_{3 }^ecies on the surface in both samples. The results of an annealing experiment, conducted to study C-C bond scission, indicate about a 2% reduction in the activation energy for C-C bond scission for C_ {2}H_{4} coadsorbed with K.