Characterization of Rh-based catalysts with EPR, TPR, IR and XPS

Rh-based catalysts to be used for the synthesis of C -oxygenates from syngas were characterized with EPR, TPR, IR and 2 XPS methods. The chemical state of the Mn component was extensively studied with EPR after in situ reduction and treatment with various probe molecules. The results indicated that on the MnrSiO catalyst, Mn can exist as isolated Mn 2 ions on the surface of SiO through the formation of coordination compounds with surface hydroxyls and H O molecules as 2 2 ligands. Thermal reduction of the MnrSiO catalyst resulted in the migration and accumulation of the Mn ions. The 2 results of TPR, IR, XPS were consistent with those of EPR, which indicated that on a Rh–MnrSiO catalyst, a Rh–Mn 2 mixed oxide was formed, which stabilized the Rh species. The formation of small clusters of the Rh–Mn mixed oxide inhibited deep reduction and accumulation of the Mn component, while at the same time increased the dispersion of the Rh component. As a promoter, Mn acts as an electron acceptor, while Li exhibits an electron-donation effect. The Li component can inhibit the formation of Rh–Mn mixed oxide and increase the concentration of Rh on the surface of SiO . The 2 existence of Li may also cause the tilted-adsorption form of CO on Rh, as well as the spillover of H from Rh to the SiO 2 2 support. q 1999 Elsevier Science B.V. All rights reserved.