Manganese-promoted rhodium/NaY zeolite catalysts. An IR spectroscopic study

Abstract

Manganese-promoted rhodium/NaY catalysts have been prepared by ion exchange and subsequent H₂ reduction and by chemical vapour deposition (CVD) of [Rh(CO)₂(acac)] and Mn₂(CO)₁₀ followed by thermal decomposition. Carbonyl FTIR spectra reveal significant differences between monometallic Rh/NaY and Mn/NaY and their bimetallic counterparts and they demonstrate profound effects of the synthetic methodology.

The H₂-reduced Rh/NaY samples prepared by ion exchange contain an appreciable amount of protons. As a consequence Rh⁺(CO)₂ complexes are formed in the presence of CO. In contrast, samples prepared by CVD contain only Rh₄(CO)₁₂ and Rh₆(CO)₁₆ which can easily interconvert.

The samples prepared by ion exchange and containing both Rh and Mn, exhibit bands at 1800 and 1830 cm^–1 characteristic of bridging CO ligands which are always accompanied by low-frequency bands at 1684 and 1700 cm^–1. These bands are attributed to an Rh₂—CO—Mn²⁺ complex. It is suggested that these η²-CO species are formed by interaction between bridging CO ligands and Rh_n–carbonyl clusters (n= 4 and 6) with Mn²⁺ ions via the oxygen end.

This complex is totally absent in the bimetallic samples prepared by CVD of neutral organometallic precursors. These materials do not contain Mn²⁺ ions since the Mn₂(CO)₁₀ precursor is decomposed on previously deposited Rh° particles, thus forming bimetallic particles, the surfaces of which are presumably enriched in Mn. The number of Rh₃ ensembles is therefore low and bridging CO ligands are not formed in the presence of CO.

The possible relevance of these results for the catalytic conversion of CO–H₂ mixtures on manganese-promoted Rh/NaY catalysts is discussed.