13C-nuclear-magnetic-resonance of carbon monoxide on platinum: Study of a highly dispersed catalyst
type: CreativeWork
description: The 13C Nuclear Magnetic Resonance lines, spin-lattice relaxation times Tl and spin-spin relaxation times T2 of CO chemisorbed on the surface layer of small particles of Pt metal have been observed by the method of spin echoes as a function of magnetic field, temperature, CO coverage and Pt dispersion. The samples are industrial catalysts: Pt supported on eta-alumina, with Pt dispersions ranging from 4% to 76%. After cleaning, the samples are exposed to CO isotopically enriched to 90% 13C. The 13C line shapes and spin-lattice relaxation times establish the presence of a 13C Knight shift AH. The spin-lattice relaxation times are independent of the applied field Ho and follow the Korringa relation, T1T(AH/H)2 = constant. From 77 K to 300 K, the relaxation of the omagnetization is multiexponential: the distribution in T1's reflects a distribution in Knight shifts arising from a distribution of CO chemisorption sites at the Pt surface. Above 300 K, the relaxation is exponential, with TIT = 46±3 (sK), from which we deduce an average isotropic Knight shift (AH/Ho) = 301±10 ppm, in reasonable agreement with the line shape data. The presence of a 13C Knight shift indicates that unpaired CO electrons leak off onto the platinum, requiring a mixing between CO molecular orbitals and Pt conduction electron wave functions at the Fermi surface. From the observed value of (AH/Ho)' we have made a crude estimate of the size of the mixing coefficients. The 13C lines in an applied field Ho of 83 kG are centered 28±1.5 G downfield from the resonance of the reference compound !MS. The broad peak in the lines has a FWHM of 30±1 G. The position and width of the 13C lines is roughly independent of CO coverage and Pt dispersion from 77 K to 445 K. This shows that the 13C Knight shift is independent of particle size for particles a few angstroms to a few hundred angstroms in diameter, and that the surface Local Density of States depends only on the local environment. We conclude that the CO b…
abstract: The 13C Nuclear Magnetic Resonance lines, spin-lattice relaxation times Tl and spin-spin relaxation times T2 of CO chemisorbed on the surface layer of small particles of Pt metal have been observed by the method of spin echoes as a function of magnetic field, temperature, CO coverage and Pt dispersion. The samples are industrial catalysts: Pt supported on eta-alumina, with Pt dispersions ranging from 4% to 76%. After cleaning, the samples are exposed to CO isotopically enriched to 90% 13C. The 13C line shapes and spin-lattice relaxation times establish the presence of a 13C Knight shift AH. The spin-lattice relaxation times are independent of the applied field Ho and follow the Korringa relation, T1T(AH/H)2 = constant. From 77 K to 300 K, the relaxation of the omagnetization is multiexponential: the distribution in T1's reflects a distribution in Knight shifts arising from a distribution of CO chemisorption sites at the Pt surface. Above 300 K, the relaxation is exponential, with TIT = 46±3 (sK), from which we deduce an average isotropic Knight shift (AH/Ho) = 301±10 ppm, in reasonable agreement with the line shape data. The presence of a 13C Knight shift indicates that unpaired CO electrons leak off onto the platinum, requiring a mixing between CO molecular orbitals and Pt conduction electron wave functions at the Fermi surface. From the observed value of (AH/Ho)' we have made a crude estimate of the size of the mixing coefficients. The 13C lines in an applied field Ho of 83 kG are centered 28±1.5 G downfield from the resonance of the reference compound !MS. The broad peak in the lines has a FWHM of 30±1 G. The position and width of the 13C lines is roughly independent of CO coverage and Pt dispersion from 77 K to 445 K. This shows that the 13C Knight shift is independent of particle size for particles a few angstroms to a few hundred angstroms in diameter, and that the surface Local Density of States depends only on the local environment. We conclude that the CO b…
author: Rudaz, Serge Laurent
language: en
keywords: Carbon-13 (13C) nuclear magnetic resonance, carbon monoxide on platinum, highly dispersed catalyst, spin-lattice relaxation
date published: 2011-06-07T17:47:30Z
date modified: 2026-07-22T22:25:24Z
provider: University of Illinois - Urbana-Champaign
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Illinois at Urbana-Champaign

PropertyValue
property ID: Handle
value: http://hdl.handle.net/2142/25342