With the intensifying global need for alternative energy and fuels, there is strong interest in the development of efficient catalysts that can drive the chemical conversion of renewable resources into useful products. One promising pathway is to catalytically convert synthesis gas (syngas, CO + H₂) to oxygenates. We study the conversion of syngas to synthetic liquid fuels and high-value chemicals using supported metal, heterogeneous catalysts. Several different catalyst systems will be described. The promotion of rhodium-, copper- and cobalt-based catalysts are explored using metal oxides deposited by atomic layer deposition (ALD). In the case of Rh promoted by MoO₃, we show that the promoted catalysts exhibit an increase in both activity and oxygenate selectivity relative to unpromoted Rh nanoparticles, and that this is the result of formation of a novel catalyst structure. We also report the promotion of catalytic metals by Fe₂O₃. Fe₂O₃ ALD on SiO₂-supported Cu catalysts occurs selectively on the SiO₂ and both promotes higher alcohol synthesis and stabilizes the catalyst against deactivation through sintering. On the other hand, Fe₂O₃ ALD on SiO₂-supported Rh catalysts occurs selectively on the Rh, yet also has a beneficial effect on selectivity and activity as well as stability. We also explore cobalt-based catalysts, particularly Co₂C catalysts, for oxygenate production. We will describe how introducing a ZnO promoting overlayer on Co/SiO₂ by ALD significantly enhances oxygenate selectivity by enabling Co₂C formation during syngas reaction. By exploring several different common support materials, we show that the composition of the support surface strongly controls whether ZnO-promoted Co can transform into the catalytically-selective Co₂C. Synchrotron-based studies provide insight into the mechanisms by which the support and promoters change reactivity, while density functional theory (DFT) calculations reveal the reaction pathways on the ALD-modified catalyst surfaces. The outlook for atomic scale surface modification using ALD to synthesize and study catalytic materials will be discussed.

Arun Asundi

Appearances