Highly dispersed Rh/SiO 2 and Rh/MnO/SiO 2 catalysts : 1. Synthesis, Characterization, and CO Hydrogenation Activity

Abstract

Investigation has been made of the influence of a high Rh dispersion in silica-based Rh and Rh/MnO catalysts (i) on the direct conversion of synthesis gas and (ii) on the activity enhancement due to the presence of MnO which has been reported for catalysts displaying a medium Rh dispersion. Highly dispersed Rh/SiO 2 and Rh/MnO/SiO 2 catalysts (2–6 wt% Rh) were prepared by deposition-precipitation techniques. Hydrogen chemisorption showed the Rh dispersion to be very high for both the monometallic and the bimetallic catalysts (H/Rh ≥1 for all cases). Electron microscopy revealed a uniform distribution of the Rh particles (∼1 nm) over the silica carrier. Since the hydrogen uptake capacities of Rh/SiO 2 and Rh/MnO/SiO 2 were fully on a par, it was concluded that probably no large amounts of MnO are present on the Rh surface in the bimetallic catalysts. Infrared spectra of adsorbed CO, however, provided evidence that the presence of MnO modifies the chemisorptive properties of metallic Rh. Comparison of the highly dispersed catalysts with catalysts of medium dispersion for the conversion of synthesis gas at elevated pressures revealed a shift toward methanol as the main product. Surprisingly the presence of MnO does not affect to a large extent the activity at high Rh dispersions, whereas at medium dispersion a five- to tenfold increase in the turnover frequency for CO hydrogenation was observed. The fact that the promoting effect of MnO is not operative with small Rh particles is attributed to the absence of MnO on the metal surface.