Carbon-induced restructuring

Figure 13. The carbon induced restructuring of the nickel (100) surface. (Reproduced with permission from Lawrence Berkeley Laboratory.)...

An extreme case of chemisorption-induced restructuring of metal surfaces is coirosive chemisorption as observed by SFG. In this circumstance, metal atoms break away from step or kink surface sites and form bonds with several adsorbate molecules. Carbon monoxide can form several carbonyl ligand bonds with platinum atoms leading to the creation of metal-carbonyl species. Thus, metal-metal bonds are broken in favor of forming metal-carbonyl clusters that are more stable at high CO pressures. The SFG vibrational spectra detect the reversible formation of new adsorb carbon monoxide species above 1(X) Torr on Pt(l 11), that appear to be platinum-carbonyl clusters Pt (CO) , with (m/n) > 1 and a CO commensurate overlayer. 

Chemisorption-induced restructuring can be very well seen using a small metal tip and field ion microscopy. In Figure 6.1 lb the field ion microscope picture of a rhodium tip is shown when clean and after exposure to carbon monoxide at 420 K at low pressures ( 10 Pa) [8]. The metal tip has been completely reshaped as a result of CO chemisorption. The tip becomes faceted and rougher, the step density is reduced, and extended low-Miller-index terraees are formed. 

Adsorbate Induced Restructuring. Perhaps, the most striking observation of recent years is the adsorbate induced restructuring of surfaces. This can be demonstrated by the restructuring of the nickel (100) face(13) in the presence of half a monolayer of carbon... 

The adsorption of carbon on Ni(lOO) is described in references [173, 214]. Discuss the nature of surface restructuring induced by carbon adsorption, and find values for the strength of the Ni-Ni and Ni-C bonds [215, 216]. 

Ethylidyne restructures the Rh(l 11) crystal face [30], sulfur restructures the Fe(l 10) face [7], and carbon restructures the Ni(lOO) face [6, 46]. The surface metal atoms move into new equilibrium p>ositions upon chemisorption in different ways, and there is evidence of restructuring even in the second substrate layer under the surface. Review the available data and point out the important electronic and structural parameters that influence the nature and magnitude of chemisorption-induced surface restructuring. 

Thus, an intriguing feature of the coenzyme Bi2-dependent enzymes is the dramatic (> lO -fold) labihzation of the bound organometallic cofactor towards homolysis of the Co-C bond [119,123,173]. The mechanism of the enzyme (and substrate-induced) labihzation of this Co-C bond is stiU a key problem, and much discussed, in Bn-chemistry. Evidence for covalent restructuring of the bound cofactor (except for the formation of the base-off/His-on form in the carbon skeleton mutases) is not available [75,119,123, 173,194]. In addition protein and solvent molecules can only weakly stabihze a radical center [240]. Steric distortions of the protein-bound cofactor were discussed as means for the enhanced rate of Co - C bond homolysis [51,119, 163,217]. Halpem s theory of an upwards conformational distortion of the... 

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