13C-nuclear-magnetic-resonance of carbon monoxide on platinum: Study of a highly dispersed catalyst
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 bond to Pt is localized. Additionally, T2 is independent of temperature, CO coverage. and Pt dispersion from 77 K to 384 K. with T2 = 700 to 800 microseconds. Finally, we obtain by NMR a description of CO diffusion on the surface of supported Pt particles. This description is entirely consistent with the diffusion activation energies deduced from the Field Emission data of Lewis and Gomer on polycrystalline Pt.
Carbon-13 (13C) nuclear magnetic resonance, carbon monoxide on platinum, highly dispersed catalyst, spin-lattice relaxation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign