Characterization and catalytic properties of Rh–Sn/Al2O3 catalyst prepared by organometallic grafting

Sn was added to a 1% Rh/Al2O3 catalyst (D = 75%) by controlled surface reaction of Sn(n-C4H9)4 in amounts corresponding nominally to 1/4, 1/2, 1 and 2 monolayers. The Sn/Rh ratios obtained were 0.19, 0.34, 0.60 and 0.75, respectively. The Sn–Rh catalysts were characterized by H2 chemisorption, TEM, CO-FTIR, 119Sn-Mössbauer spectroscopy and in the test reaction of methylcyclopentane (MCP). Three different Sn phases were found after reduction: SnO2, Sn-poor (most probably Rh2Sn alloy) and Sn-rich phase (most probably RhSn4 alloy). Tin transformed entirely to SnO2 upon oxidation. The effect of oxidation (373 K) and reduction (at 473 K) appeared to be reversible. Neither RhSn4 nor Rh2Sn was active catalytically in MCP ring-opening, but their position on the surface affected the performance of Rh not interacting with Sn. On the catalyst with Sn/Rh = 0.19, the strongly bonded adsorption of intermediates was hindered by the selective deposition of Sn on low-Miller-index micro-facets (as indicated also by CO-FTIR), causing increased activity in selective ring-opening reaction of MCP and decreasing the production of fragments. With Sn/Rh = 0.34, sites active in ring-opening reactions (low-coordination facets) were also partly blocked by tin addition. On catalysts with even higher Sn loading, tin also penetrated into the particles forming possibly alloy-like phases. On Sn/Rh = 0.6 some surface Rh still remained Sn-free, and was active in non-selective ring-opening of MCP. The catalyst with Sn/Rh = 0.75 was inactive in MCP ring-opening reaction. The latter two samples represent “bulk-bimetallic” catalysts, as opposed to “surface-bimetallic” with lower amounts of tin. The present results indicate the co-existence of electronic and geometric effects, inasmuch as chemically interacting Rh–Sn patches located on certain surface sites influence the catalytic behavior.

Three different phases were found on bimetallic Rh–Sn/Al2O3 catalysts: SnO2, Rh2Sn and RhSn4 alloys. Neither RhSn4 nor Rh2Sn was active in MCP ring-opening (RO). Their position affected the performance of Sn-free Rh: on the surface of low-Miller-index micro-facets if Rh/Sn = 0.2 – selective RO, random surface deposition when Sn/Rh = 0.34, and in bulk position when Sn/Rh = 0.6 – unselective RO. Sn/Rh = 0.75 was inactive. Figure optionsDownload as PowerPoint slide