METAL-PARTICLE STRUCTURE - CONTRASTING THE INFLUENCES OF CARBONS AND REFRACTORY OXIDES

An analysis of the literature suggests that there are at least three d ifferent characteristics of carbon that can be utilized to generate me tal surfaces not found on refractory oxide supports. First, on graphit ic carbon many metals interact very weakly, allowing bimetallic partic les to form structures identical to those anticipated for bulk materia ls. Of particular significance is the formation of true alloys, both i n the bulk and on the (catalytic) surface of the bimetallic particles. In contrast, on conventional refractory-oxide supports these same str uctures will not form for certain base-metal/noble-metal pairs. Instea d, a preferential and strong interaction between the more 'base' metal and the support generally leads to preferential segregation of that m etal to the refractory oxide interface and, concomitantly, dominance o f the catalytic interface by the 'more noble' metal. As a result of th ese structural differences, the catalytic chemistry, both activity and selectivity, of some bimetallic particles supported on refractory oxi des and graphitic carbons are dramatically different. Second, it is cl ear that it is possible to directly bond metals to unsaturated active sites on high surface-area carbon blacks, activated carbon, etc. This has been demonstrated to yield thermally stable particles of a unique structure. On refractory oxides, strong interaction generally leads to the creation of complex, ionic-bonded 'interface' phases. Third, carb on structure can be manipulated to generate shape-selective supports. This can be done with refractory oxides, but only carbon surfaces are neutral. Thus, only on carbon will reduced metal readily form. There i s surprisingly little research into any of these phenomena, suggesting there are many opportunities to create unique metal surfaces using ca rbon as a support. (C) 1998 Elsevier Science B.V. All rights reserved.