Effects of surface structure, promoters and supports in rhodium catalysts for higher oxygenates synthesis from syngas [electronic resource]
Keywords: Materials Science and Engineering.

Synthesis gas (CO + H2) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels and high-value chemicals. However, to enable industrial scale application, catalysts for higher oxygenate production still need to be improved. Rhodium (Rh) is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates, and supported Rh catalysts have shown relatively high selectivity and stability towards higher oxygenates compared to other syngas conversion catalysts. Controlled synthesis and modification, as well as atomic level understanding, are highly desirable for the development and optimization of syngas conversion catalysts. We first investigated the intrinsic selectivity and structure sensitivity of silica-supported Rh catalysts by combining experiments and theoretical calculations. A variety of Rh/SiO2 catalysts were synthesized and a strong inverse correlation between catalytic activity and C2+ oxygenate selectivity was observed experimentally. DFT calculation shows the Rh (211) surface to be ~6 orders of magnitude more active than the (111) surface, but highly selectivity towards methane, while the Rh (111) surface is intrinsically selective toward acetaldehyde. Therefore, Rh is constrained by such activity-selectivity tradeoff, and appropriate supports or promoters would be required to improve C2+ oxygenate production. Sodium oxide impurities were found to play a key role in modulating the active site distribution by preferentially blocking step or other defect sites. Excessive sodium oxide species on surface terrace sites can facilitate CO dissociation and therefore, depending on its concentration, sodium oxide was shown to result in considerably different activity and selectivity patterns. We also applied atomic layer deposition (ALD) to study the effects of promoters and supports, which largely determined the performance of Rh catalysts. ALD has the capability to achieve uniform coatings on high surface area substrates and hence enables controllable design and synthesis of heterogeneous catalysts. Two types of MnO-promoted Rh catalyst were synthesized using ALD: MnO as a support modification layer and MnO as an overlayer. Compared to the conventional co-impregnation synthesis, an ultrathin MnO support layer does not affect the Rh nanoparticle size distribution, reducibility or chemisorption capabilities. Both experimental characterizations and DFT calculations indicate the interface sites between the Rh and MnO support are crucial for promoting C2+ oxygenate production. In addition, we deposited ultrathin layers of titania and alumina by ALD to achieve chemical modification of the catalyst support without changing its physical properties. An inverse catalyst structure was also accomplished on high surface area catalysts. The surface chemical properties of the titania and alumina supports were found to change the nanoparticle size, catalytic activity and selectivity. These results advanced the understanding of the active sites and the role of metal oxide promoters and supports on Rh catalysts, and may help improve the performance of syngas conversion catalysts.