Methane Activation on Bimetallic Catalysts: Properties and Functions of Surface Ni-Ag Alloy

The paper presents a fundamental study of the properties and functions of well-defined Ni-Ag surface alloys in methane decomposition and steam reforming, aiming at providing a better understanding of the principle for manipulating the catalytic activity of steam reforming and suppressing carbon formation. A better insight of structure-property relationship was obtained by a kinetic study of the reactions on well-defined surface Ni-Ag alloys, which were synthesized by surface redox reaction to selectively introduce Ag atoms into the surface of Ni particles supported on hydrotalcite derived support. The effects of Ni surface alloy with Ag are three-folds in general. Replacement of Ni by Ag reduces the number of active site exponentially with increasing Ag site coverage. Ag site is not only inactive, but also significantly reduces the activity of adjacent Ni sites for methane activation in both methane decomposition and steam reforming. The third effect is to block the active sites for the nucleation and growth of the filamentous carbon. The rate of methane activation at Ni step sites was found to be 16-19 times of that on Ni terrace sites. The carbon formation rate decreased linearly with Ag site coverage and the effect of Ag is divided into two regions. At low Ag site coverages (0 to 0.055), Ag atoms preferentially deposit on Ni step sites, which has a significant effect on the methane activation compared to Ag atoms on the Ni terrace sites. The results reveal that the effects of Ag site on the carbon formation in the two regions are mostly caused by the different effects of Ag on the activity of Ni step sites and terrace sites, respectively, rather than the different ensemble sizes for carbon formation proposed in the literature.