MANGANESE-PROMOTED RHODIUM NAY-ZEOLITE CATALYSTS - AN IR SPECTROSCOPIC STUDY

Manganese-promoted rhodium/NaY catalysts have been prepared by ion exc hange and subsequent H-2 reduction and by chemical vapour deposition ( CVD) of [Rh(CO)2(acac)] and Mn2(CO)10 followed by thermal decompositio n. Carbonyl FTIR spectra reveal significant differences between monome tallic Rh/NaY and Mn/NaY and their bimetallic counterparts and they de monstrate profound effects of the synthetic methodology. The H-2-reduc ed Rh/NaY samples prepared by ion exchange contain an appreciable amou nt of protons. As a consequence Rh+(CO)2 Complexes are formed in the p resence of CO. In contrast, samples prepared by CVD contain only Rh4(C O)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 a ccompanied by low-frequency bands at 1684 and 1700 cm-1. These bands a re attributed to an Rh2-Co-Mn2+ complex. It is suggested that these et a2-CO species are formed by interaction between bridging CO ligands an d 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 c ontain Mn2+ ions since the Mn2(CO)10 precursor is decomposed on previo usly deposited Rh(degrees) particles, thus forming bimetallic particle s, the surfaces of which are presumably enriched in Mn. The number of Rh, 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-H-2 mixtures on manganese-promoted Rh/NaY catalysts is discussed.