New Nickel Catalysts by Step Blocking

In Chapter 2 (Section VI. C) of this volume, Hansen et al. also discuss detailed TEM results that show how the steps of a supported nickel cluster play an important role as centers for carbon growth in a real supported catalyst (73). 

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. 

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