from first principles PP-I-1

PP-III-74MCP Transformation on Rh-M (M=Ge, Sn) Grafted CatalystsMounir Chamam 1 , Laurence Pirault-Roy 2 , Károly Lázár 1 , Zoltán Paál 11 Institute of Isotopes, Hungary Academy of Sciences, POB. 77, Budapest, H-1525, Hungary2 LACCO-UMR, 6503, Univ. Poitiers, 44, avenue du Recteur Pineau, 86022 Poitiers, Francemounir@freemail.huIncreasing attention is paid nowadays to bimetallic ensembles prepared by “ControlledSurface Reactions” (CSR), so-called tailor-made catalysts [1, 2]. The full characterization ofsuch catalysts requires combination of physico-chemical methods with catalytic tests [3, 4].Our aim was to get closer to the understanding of Rh-Ge and Rh-Sn interactions and toexamine the effect of particle size of the monometallic parent catalyst. Three 1% Rh/Al 2 O 3catalysts were prepared with dispersion of 75, 65 and 20 %, respectively. They were modifiedby adding heptane solution of Sn(n-C 4 H 9 ) 4 or Ge(n-C 4 H 9 ) 4 in Ar atmosphere at T=323 K, inamounts nominally to 1/4, 1/2 and 1 monolayers [4]. After drying (in Ar at 393 K), thesamples were reduced in H 2 at 473 K for 4 h. The catalysts were characterized by H 2chemisorption, TEM, CO-FTIR, 119 Sn-Mössbauer spectroscopy and in transformation ofmethylcyclopentane (MCP) in a closed loop reactor [4].5P 0,25M4Ge0,5M 1M3Sn2Ge10TOF/TOF(P)D=75% D=65% D=20%Activity of 1%Rh/Al 2 O 3 catalysts with different dispersion and activity change of bimetallic samplesafter Sn or Ge grafting in methylcyclopentane reaction, p(MCP)/p(H 2 )= 1,3/32 kPa, T=483K.In the case of the parent catalyst (P), the activity related to one surface Rh atom (TOF, h -1 )increased with decreasing dispersion. Larger Rh particles produced also more fragments. Additionof second metal in small amounts increased the catalytic activity (per surface Rh atomsdetermined by H 2 chemisorption). For samples with high dispersion addition of 1/4 monolayerwhile with low dispersion 1/2 monolayer provided the highest activity. Changes were morepronounced on large particles and the effect of Sn addition was more important then that of Ge.Correlating the catalytic behavior with the observed selectivity pattern we suggest that smallamount of the second metal deposited selectively on low-Miller-index micro-facets increasing theintroduced amount leads to deposition on bigger on high-Miller-index sites (random deposition).The formation of different inter-metallic compounds is also suggested.[1] V. Ponec, G. C. Bond, in. "Catalysis by Metals and Alloys", Elsevier, Amsterdam, 1995.[2] S. Göbölös, J. L. Margitfalvi, in J. J. Spivey (Ed.) Cat. Spec. Period. Rep., 17, 1, 2004.[3] B. Coq, F. Figuéras, Coord. Chem. Rev. 178, 1753, 1998.[4] D. Teschner, L. Pirault-Roy, D. Naud, M. Guérin, Z. Paál, Appl. Catal. A, 252, 421, 2003.229