Engineering bimetallic catalysts for the selective hydrogenation of carbon monoxide and carbon dioxide to alcohol products
Keywords: Chemical Engineering.

As human population and industrialization grows, so too do our global demands for fuels and chemicals. Increasing consumption of fossil fuels has led to rising CO2 levels in the atmosphere, a major challenge due to the role of CO2 as a significant greenhouse gas contributing to climate change. To address anthropogenic CO2 production, new processes to capture, sequester, and utilize carbon dioxide are needed. CO2 utilization in particular is an attractive approach as it also creates a value-added product. This utilization could take the form of direct CO2 hydrogenation or a two-step process whereby the CO2 is first converted to CO via the water-gas shift reaction. To enable these processes, the discovery and development of efficient catalysts that can selectively reduce CO and CO2 to high value, oxygenated products is necessary. Towards this goal, the investigation of new catalyst formulations is crucial. Furthermore, the characterization of these materials is necessary to understand the drivers of catalyst performance and predict future targets for study. In Chapter 3, we begin with an investigation into the design of Co-Cu catalysts for CO hydrogenation to higher alcohols. To improve control over particle properties, a liquid phase nanoparticle synthesis based on the polyol method was selected to synthesize Co2.5Cu particles, which were then supported onto a variety of metal oxide supports. The results show alloyed phases were obtained using the polyol method, resulting in selectivity towards higher alcohols, as high as 11.3% when supported on alumina. However, segregation of cobalt and the formation of cobalt carbide were observed in the catalysts after catalytic testing, which limit performance compared to the desired alloy phase. In Chapter 4, our focus shifts towards understanding the surface properties of a newly discovered Ni5Ga3 catalyst for CO2 hydrogenation to methanol. Results revealed that upon air exposure Ga migrates from the subsurface region to the surface of the nanoparticles forming a Ga-oxide shell surrounding a metallic core. Reduction of this shell results in a surface enrichment of Ga. By varying reduction temperatures, it was found that partial reductions gave low CO uptakes but high methanol activity, indicating a promotional effect of the oxide phase. In Chapter 5, the investigation into the role of metal oxides in methanol synthesis continues with a study of In-Pd catalysts. We present the promotional effect of Pd on In2O3 catalysts and investigate structure-performance relationships therein. Catalysts were synthesized with varying In:Pd ratios, and it was found that In2Pd/SiO2 showed the highest activity (5.1 umol MeOH/gInPd/sec) and selectivity toward methanol (61%). Based on microkinetic modeling, operando X-ray absorption spectroscopy and ex situ characterization, the active phase is proposed to be a bimetallic In-Pd particle with a Pd-rich core and a surface In2O3 phase. A non-precious metal containing In-Ni system was also developed and displayed similar composition-activity trends to the In-Pd system. Both palladium and nickel were found to form a bimetallic catalyst with enhanced methanol activity and selectivity relative to indium oxide. Overall, this dissertation presents catalyst syntheses, advanced characterizations, and catalytic hydrogenation experiments which led to fundamental insights into the activity and selectivity of heterogeneous, bimetallic catalysts for alcohols synthesis. This work provides key understanding towards the development of catalyst theory and materials for the hydrogenation of CO and CO2.