A new approach of selective Ge deposition for RhGe/Al2O3 catalysts:: characterization and testing in 2,2,3-trimethylbutane hydrogenolysis

A series of Rh monometallic and RhGe/Al2O3 catalysts was prepared in a dynamic reactor by anchoring Ge(n-C4H9)(4) on Rh surface, containing preadsorbed hydrogen and by subsequent hydrogenolysis of the surface organometallic complexes. Different amounts of organometallic Ge compound were introduced, corresponding nominally to 1/20, 1/2, 2 Ge monolayers. The samples were characterized by FTIR of CO, transmission electron microscopy (TEM), hydrogen chemisorption and hydrogenolysis of 2,2,3-trimethylbutane (223TMB) as a test reaction. Hydrogen chemisorption and TEM results are consistent with (i) the effect of preparation procedure on the morphology of the monometallic sample, (ii) the surface deposition of Ge on Rh particles. Concerning the effect of preparation, the grafting procedure led for blank experiments (Ge-free catalysts) to a slight sintering which can be avoided by Ge anchoring. On the other hand, we used the relative normalized intensities of linear and bridged CO species of FTIR measurements to identify the location of Ge deposits. The amount of the Ge compound introduced had a pronounced effect on the final landing position of Ge. Excess Ge(n-C4H9)(4) resulted in a statistical deposition on the surface of Rh or a slight diffusion in Rh lattice, while low-Miller-index microfacets were preferentially occupied when the amount of tetra-n-butylgermanium was markedly lower than that for monolayer coverage. The presence of quaternary carbon atom in the reactant molecule (223TMB) induced changes in the chemisorption geometry deduced from the shift of maximum activity towards lower p(H-2). In the wide negative hydrogen order region breaking of C-C bonds can be the slowest step. Low-Miller-index microfacets of Rh particles may be active in multiple C-C bond cleavage during one residence time of the reactant molecule. (C) 2002 Elsevier Science B.V. All rights reserved.