Particle size, precursor, and support effects in the hydrogenolysis of alkanes over supported rhodium catalysts

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A series of Rh catalysts of widely varying dispersion has been prepared using γ-alumina as support and Rh acetylacetonate (Rh(acac) 3) as precursor. The hydrogenolyses of n-hexane ( nH), methylcyclopentane (MCP), and 2,2,3,3-tetramethylbutane (TeMB) were investigated as model reactions. Clear dependence of turnover frequency on Rh particle size is observed for nH and MCP hydrogenolysis, but only slight changes of selectivities occur with these alkanes. By contrast, large modifications of both specific activity and selectivity appear when TeMB is reacted. TeMB hydrogenolysis is thus a reliable tool for studying modifications of the surface structure of rhodium particles. This probe was used to investigate the effects of precursor and support on rhodium catalysts. The effect of chlorine is appreciable and shifts the selectivity of TeMB hydrogenolysis toward that of large particles. This is attributed to a different morphology of the rhodium particles. When the effect of dispersion of the metal is taken into account, no support effect is observed when SiO 2 or ZrO 2 is used as support. The different properties of rhodium on MgO can also be attributed to a different morphology of the particles. For Rh/TiO 2 prepared from RhCl 3 · 3H 2O, the catalytic properties are similar to those of Rh/Al 2O 3 of moderate dispersion whatever temperature is used for reduction. Rh/TiO 2 prepared from Rh(acac) 3 and reduced at 573 and 773 K simulates the catalytic properties of particles smaller than indeed observed. This effect can be interpreted by a partial coverage of the Rh surface by TiO x species (SMSI). This SMSI effect disappears upon eduction at 873 K.