CO Hydrogenation to Ethanol on Metal-Oxide Supported Catalysts

    Years of energy production from the combustion of fossil fuels, used extensively in transportation systems, has negatively impacted the environment. As a result, innovative, clean energy sources, such as biofuels, have emerged as an attractive and cost-effective alternative. Ethanol particularly is of high interest due to its high energy density, clean emissions, carbon-neutrality and compatibility with the existing infrastructure. Currently, ethanol is largely produced by biochemical paths like fermentation. However, other synthetic routes like the catalytic conversion of syn gas (CO, CO2, H2 and H2O) are promising alternatives that are currently being investigated.

Figure 1: Ethanol fuel cycle

        Several families of catalysts have been developed for syn gas conversion, mainly from carbon monoxide hydrogenation. The most studied catalysts are rhodium based. Rhodium can effectively dissociate CO and from C-C bonds, which are important steps in ethanol formation. Unfortunately, the formation of C2+ oxygenates (like ethanol) is also met with the production of large quantities of methane. In order to increase selectivity towards ethanol, metal promoters such as Fe, Mo, and Mn are used. Ethanol selectivity may also be further enhanced by the choice of metal oxide support, morphology of the metal catalysts and the reaction conditions.

        In order to investigate the roles of the Rh(Fe)/MOx catalysts, a combined spectroscopy approach is used.  The structure of the catalysts is studied via X-Ray Diffraction (XRD), Pair Distribution Function (PDF) analysis, and X-ray Absorption Spectroscopy (EXAFS/XANES). By comparing the structures of the catalysts under different conditions (as-synthesized, reduced, reaction conditions) we're able to determine what sort of a changes the catalyst undergoes, as well as gain some insight into the active phase(s) of the catalysts. Using gas chromatography under similar reaction conditions, we're able to correlate catalyst structure with any changes in CO conversion of selectivity and learn how reaction products are influences by catalyst composition.