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The authors report use of 17 0 and l3 C NMR to study the bonding of CO on Pd particles. By l7 0-13 C double resonance, they measure the CO bond length to be 1.20 ±0.03 A. The 13 C resonance frequency is exceptionally high, 310 ppm above values typical for metal carbonyls. Evidence that the shift arises from electron-spin polarization is given from studies of the magnitude and the dependence on temperature and frequency of the 13 C spin-lattice relaxation time. A diffusion energy of 6 ± 2 kcal/mol, half that of CO on Pt, is deduced from motional narrowing of the 13 C NMR line. PACS numbers: 82.65Jv, 76.70.Fz We report NMR studies of CO chemisorbed on Pd metal for samples consisting of small metal particles supported on alumina. We have determined the C-O bond length in an experiment which directly measures the magnetic dipolar coupling between 17 0 and 13 C nuclei in isotopically enriched 13 C 17 0 molecules. Since the dipolar coupling depends strongly on the internuclear distance, this experiment is a sensitive probe of the bond length. The C-O bond length is expected to be strongly influenced by bonding to the metal surface, so that measurement of the bond length provides an important test of theories of the bonding of CO to the metal.To our knowledge, the only other measurement of the bond length of CO chemisorbed on metals is that of Behm and co-workers. x They measured the bond length of CO chemisorbed on a Pd(100) single-crystal surface using low-energy electron diffraction (LEED), and found 1.15 ± 0.10 A. Our result represents a threefold increase in the accuracy of the measurement, and allows comparisons with the bond length of CO in other cases.We use an 17 0-13 C spin-echo double resonance (SEDOR) experiment in which we observe the effect on the 17 0 NMR of flipping the spins of the 13 C nuclei. This is the first reported use of 17 0 NMR to study chemisorption on metal surfaces. We discuss how we have solved the problems posed by the particularly weak 17 0 signal and the complications arising from the quadrupole coupling of the 17 0 nuclei.The 13 C NMR of the chemisorbed CO has some unusual features which give further information about the bonding of the CO to the Pd surface. The center of the 13 C resonance line is 540-ppm higher in frequency than the 13 C resonance of tetramethylsilane (TMS), which is commonly used as a reference. This is the largest reported shift for 13 C in molecules that do not contain paramagnetic centers, and fails far outside the range of known 13 C chemical shifts. We show that the shift arises from electron spin polarization (a Knight shift) which results from the mixing of the Pd conduction band with the CO molecular orbitals. The resonance narrows at higher temperatures because of the diffusion of the CO on the Pd surface, but the details of the narrowing differ strongly from conventional cases. We show that this difference results from the distribution in the size of our Pd particles. From the narrowing, we deduce an activation energy for diffusion of 6 ± 2 kcal/mol, onl...

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We report spectra isolating the LIII x-ray absorption threshold resonance of the elements iridium, platinum, and gold in the pure metallic state and in a variety of compounds. When normalized spectra obtained on the metals are subtracted from those obtained on the compounds, the resulting difference spectra are related to differences in the electronic structure of the absorber atom in the two types of environment. The change in area of a threshold resonance line obtained from such a difference spectrum can be related to the ionicity of the bonds of the absorber atom in its compounds. Measurements on supported platinum and iridium catalysts provide information on electronic changes in the metal due to the small size of the metal clusters or to interaction with the support material. Information on electronic changes due to interaction of the catalysts with gas molecules may also be obtained.

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