Organo-halides are used to form reactive carbon fragments in some catalytic reactions. In the Heck reaction, hydrogen at an alkene double bond can be substituted for an aryl or vinyl species by the use of palladium catalysts [1,2]. It has been observed that coupling of the aryl (or vinyl) to one or the other of the double bonded carbons can be controlled through the presence or absence of halides at the active site [3]. This often leaves the halogen atom (ion) co-ordinated to the surface where it, depending on coordination site, charge transfer and the strength of the palladium- halide interaction may act as a catalyst poison or catalyst promoter, with important implications in controlling selectivity.
Here we have used thermal desorption spectroscopy (based on Auger electron spectroscopy) and density functional theory to determine the adsorption energy of iodine on Pd(111) and Pd(110) surfaces. In addition, from calculated and experimental core level chemical shifts we determine the preferred adsorption site.
On Pd(111) iodine form a (√3x√3) structure where it occupies the three-fold hollow (fcc) site with an adsorption energy of 2.9 eV. The initial surface core level shift (0.28 eV to lower binding energy) is removed by iodine adsorption without inducing any new shifts. The iodine remains essentially neutral on the surface. On the Pd(110) surface a c(2x2) structure is formed with iodine in four-fold hollow sites. The initial surface core level shift (0.52 eV to lower binding energy) is replaced by a chemical shift 0.22 eV to higher binding energy. In this case the iodine is negatively charged.
[1] R.F. Heck, J. Am. Chem. Soc. 91 (1969) 6707.
[2] Y. Fujiwara, I. Moritani, S. Danno, R. Asano, S. Teranishi, J. Am. Chem. Soc. 91 (1969) 7166.
[3] W. Cabri, I. Candiani, Acc. Chem. Res. 28 (1995) 2.
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