CARBON-13 - Nuclear - Magnetic - Resonance of Carbon Monoxide on Platinum: Study of a Highly Dispersed Catalyst

The ('13)C Nuclear Magnetic Resonance lines, spin -lattice relaxation times T(,1) and spin-spin relaxation times T(,2) 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% ('13)C. The ('13)C line shapes and spin-lattice relaxation times establish the presence of a ('13)C Knight shift (DELTA)H. The spin -lattice relaxation times are independent of the applied field H(,0) and follow the Korringa relation, T(,1)T((DELTA)H/H(,0))('2) = constant. From 77 K to 300 K, the relaxation of the magnetization is multiexponential: the distribution in T(,1)'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 T(,1)T = 46 (+OR-) 3 (sK), from which we deduce an average isotropic Knight shift ((DELTA)H/H(,0)) = 301 (+OR-) 10 ppm, in reasonable agreement with the line shape data. The presence of a ('13)C 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 ((DELTA)H/H(,0)), we have made a crude estimate of the size of the mixing coefficients. The ('13)C lines in an applied field H(,0) of 83 kG are centered 28 (+OR -) 1.5 G downfield from the resonance of the reference compound TMS. The broad peak in the lines has a FWHM of 30 (+OR -) 1 G. The position and width of the ('13)C lines is roughly independent of CO coverage and Pt dispersion from 77 K to 445 K. This shows that the ('13)C 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, T(,2) is independent of temperature, CO coverage, and Pt dispersion from 77 K to 384 K, with T(,2) = 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.