NMR study of platinum catalysts

Abstract

Abstract The author reports on studies by his group of the 195 Pt NMR of several samples of Pt supported on alumina at frequencies ranging from 9 to 74 MHz, and at 77 and 4.2 K, using spin-echoes at fixed frequency. The samples were prepared by Dr. John Sinfelt of the Exxon Research Laboratory, who has characterized their dispersions chemically as 46%, 26%, and 15% for samples 1, 2, and 3 respectively. The NMR spectra are exceedingly broad, extending from the value characteristic of bulk metallic Pt to a value characteristic of diamagnetic Pt compounds (a range of 4.5 kG at 74 MHz). The detailed shapes vary drastically with average particle size, and exhibit structure which depends on the chemical treatment of the Pt surface. The spin-spin and spin-lattice relaxation times T 2 and T 1 respectively, vary across the absorption line but, for a given surface treatment, and at a given position on the absorption line, T 1 and T 2 are the same in all three samples. In sample 3, there is a sharp peak at the position of the 195 Pt resonance in bulk metal, but it is missing in the samples of smaller particle size. In samples exposed to the air or to hydrogen, there is a strong peak near the normal position of the 195 Pt resonance in diamagnetic compounds which we believe arises from the surface layer of Pt atoms since its area relative to the total absorption line agrees with Sinfelt's chemical determination of the sample dispersion and since the peak disappears when the surface is cleaned chemically. When the surface is covered, the electrons of the surface platinum atoms are tied up in chemical bonds (i.e., are non-metallic) since the surface peak occurs at the position characteristic of 195 Pt in diamagnetic compounds, possesses a long T 1 , and has T 1 whose temperature and magnetic field dependence differ drastically from T 1 in metals.