NMR of Small Platinum Particles
- PDF / 272,726 Bytes
- 6 Pages / 416.88 x 635.4 pts Page_size
- 98 Downloads / 200 Views
253
NMR OF SMALL PLATINUM PARTICLES
HAROLD T. STOKES, HOWARD E. RHODES,* PO-KANG WANG AND CHARLES P. SLICUTER Department of Physics and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 J. H. SINFELT Exxon Research and Engineering Company,
Linden, New Jersey 07036
ABSTRACT 19 5
We present Pt NMR lineshapes as well as relaxation data in three different samples of platinum metal particles We (46%, 26%, and 15% dispersion) supported on alumina. show that the electronic properties of these particles are very much different from those of bulk Pt metal. A prominent peak in the lineshape has been identified as a "surface resonance" which arises from Pt nuclei on the surface of the Pt particles. We find that these surface Pt atoms are "nonmetallic" when coated with adsorbed molecules.
INTRODUCTION Heterogeneous catalysis is a field of great interest because of its imporHowever, the fundamental understanding of tant technological applications. In an effort to learn more catalytic phenomena is not well developed [1]. about the microscopic details of heterogeneous catalysis, we have been applying nuclear magnetic resonance (NMR) to the study of a typical catalyst, platinum 195 Pt in small platinum In this paper, we present NMR data [2-4] on metal. electronic properties the that show We on alumina. metal particles supported of these particles are very much different from those of bulk platinum metal. Furthermore, the Pt atoms on the surface are "nonmetallic" when coated with adsorbed molecules. SAMPLES We have used three different samples (prepared by one of the authors, These samples consist of small Pt particles JHS) which we label 1, 2, and 3. The (All samples are 10% Pt-90% alumina by weight.) supported on alumina. dispersion (fraction of Pt atoms which are on the surface of the particles) of samples 1, 2, and 3 was measured by hydrogen chemisorption and was found After the samples were prepared, they to be 46%, 26%, and 15%, respectively. were opened to air. The subsequent catalytic reaction of atmospheric oxygen with adsorbed hydrogen probably caused the Pt particles in these samples to be coated with water. KNIGHT SHIFT When a material is placed in an applied magnetic field H0, interactions between the nuclei and nearby electrons give rise to a displacement of the NMR Polarization of the electron orbital moments gives rise to a frequency vQ. In metals, polarization of the conducdisplacemeLnt called the chemical sbift. Presently at Eastman Kodak Co.,
Rochester,
New York 14650
254 tion-electron spins gives rise to an additional displacement called the Knight shift. For a given nucleus, Knight shifts are usually an order of magnitude larger than chemical shifts [5]. 1 95 In non-metallic Pt compounds, the Pt nuclear resonance is usually observed between Ho/v0 = 1.093 and 1.100 kG/MHz, a spread of about 0.6% [6]. This spread of resonances is due to chemical shifts alone, since the Knight shift is zero in an insulator. By convention, the zero chemical shift (and 19 5 zero Knight shift) of
Data Loading...
