Nickel step edges

Fig. 6. Image sequence of a growing carbon nanofiber (movies are available as supplementary information to Reference (52) and can be found in Reference (57)). Images (A)-(H) illustrate one cycle in the elongation/contraction process. Drawings are included to guide the eye in locating the positions of mono-atomic nickel step edges at the carbon-nickel interface. The images are acquired in situ with CH4 H2 = 1 1 at a total pressure of 2.1 mbar with the sample heated to 809 K. All images were obtained at a rate of 2 frames/s. Scale bar = 5 nm. Reprinted with permission from Reference (52).

Fig. 7. Growth mechanism of graphitic carbon nanofibers. The illustration highlights the observation of spontaneous nickel step edge formation at the carbon-nickel interface. The observations in Reference (52) are consistent with a growth mechanism involving surface transport of carbon and nickel atoms along the graphene-nickel interface.

A mechanism which proceeds through surface reconstruction of the substrate has been identified for Ni deposition on Au(lll) [120, 121]. The process begins with place exchange of nickel into a particular position in the reconstructed Au(lll) surface, followed by deposition of Ni islands on top of the imbedded atom. At higher overpotentials, nucleation occurs instead at step edges, so that control of the potential allows control of the nucleation process and the distribution of Ni in the early stages of growth. In this instance, the nucleation process has been captured by STM on the atomic scale. 

Fig. 6. (a) STM image (200Ax200A) of a nickel (111) surface after exposure to ethene (1.3 X 10 "bar 100 s) at room temperature, (b) STM image (400Ax400A) of a nickel(l 1 1) surface with the step edges blocked by silver atoms. 

The selectivity of the nickel(l 1 1) surface may thus be controlled by modification of the number of free step sites, and this notion was tested experimentally by blocking the steps with small amounts of silver (84). In other STM investigations it was found that when silver was deposited on nickelfl 1 1) at room temperature, the silver preferentially nucleated and grew as islands at the step edges. When this system was post-annealed to 800 K, the silver atoms were observed to become highly mobile and decorate all the step edges of nickelfl 1 1), as shown in Fig. 6b. 

To test the altered selectivity, this silver/nickel-modified surface was exposed to ethene at room temperature. When all the step edges were decorated with silver, STM showed no evidence of an ethene-induced brim structure, either at the step edges or at the terrace sites. This result clearly indicates that the step sites are indeed the active sites for the decomposition of ethene, and the experiments showed that addition of silver effectively blocks these sites and changes the overall selectivity of the stepped nickel(l 1 1) surface. 

Earlier we discussed the increased 2ir orbital population due to interaction with the d-atomic orbitals from the step atoms. This, taken together with the lowered surface dipole, enhances the probability for CO dissociation on steps. On nickel, steps have been shown to be more active than the planar surface, in contrast with the high-workfunction metals Pt and Ir. On the latter metals CO prefers adsorp tion on the edge atoms of the steps rather than on the valley atoms, where the CO 2x orbitals would get a favorable interaction with the step-edge d orbitals (see Fig.(3.17)). Whereas this interaction is favorable, the interaction with the It CO orbitals that are doubly occupied is repulsive with d-valence electron bands of high electron occupation. For a low-workfunction metal such as Ni this repulsion may be overcome by the 2t interaction. On high-wofkfunction metals with less backdona-tion the overall effect may be that CO molecules experience a repulsive interaction... 

The temperature programmed reduction (TPR) of NiST proceeds in two steps with maxima at around 700 and 815 K (Fig. 2). The observed TPR peaks are indicative for the presence of Ni with different reactivity for hydrogen toward the zero-valent state. Since the non-crystalline silica-alumina phase gave a much poorly resolved TPR profile (Fig. 2), the peaks should be related to the crystal structure of NiST. The nickel cations placed inside the octahedral sheets being coordinated with sbc framework oxygen atoms should be more stable than Ni exposed at the edges of the clay platelets. Consequently, the first reduction step can most likely be attributed to the reduction of the N cations located at those positions. Since the reduction of NiO takes place at approximately 570 K, these data show... 

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