In heterogeneous catalysis, the interaction between metal nanoparticles and metal oxides (MOx) can strongly influence the catalytic activity and selectivity. In this study, we investigate the effects of MOx supports and promoters on rhodium (Rh) nanoparticles for direct ethanol synthesis from syngas. Syngas conversion to higher alcohols serves as a key pathway for turning coal, shale gas and biomass into clean liquid fuels and other valuable fine chemicals. Supported Rh nanoparticles with proper MOx promoters or supports have been shown to give considerable selectivity to ethanol. The composition and concentration of the MOx largely determines the activity and product distribution of the Rh catalysts. However, the interaction between Rh and these MOx are still subject to debate. In this work, we specifically look into the promoting effects of manganese oxide (MnOx) and titanium oxide (TiO2) for C2-oxygenate products. Atomic layer deposition (ALD) is applied to deposit MOx layers on silica supported Rh nanoparticles in order to change the surface and interface composition of the Rh catalysts. Based on a self-saturating layer-by-layer growth model, ALD has the unique capability to achieve uniform coating on high surface area substrates. This allows controllable surface and interface modification compared to traditional synthesis methods. The catalysts are tested in the syngas conversion reaction. With thin MnOx or TiO2 coated on the Rh surface, the overall reaction rate increased significantly compared to bare Rh nanoparticles. Moreover, MnOx enhances selectivity to C2-oxygenates, while TiO2 does not. We also compared catalysts modified by ALD to those prepared by co-impregnation. Our results show that ALD-modified catalysts can achieve even higher oxygenate yield. A higher surface concentration of MOx-Rh interfacial sites and protection from sintering, enabled by the ALD method, are likely to be responsible for the increase in oxygenate production. The catalyst structure and surface properties are characterized, and their relationship to the activity and selectivity enhancements will be discussed.

Investigating the function of metal oxide promoters on supported Rh catalysts for syngas conversion to oxygenates through surface and interface modification

249th National Meeting (2015)

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