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Surface Structure Dependence of Selective Oxidation of Ethanol on Faceted CeO2 Nanocrystals

Scientific Achievements: 

Shaped nanoparticles of cerium oxide were synthesized and compared as catalysts for oxidative dehydrogenation of ethanol to understand the role of surface structure upon catalytic behavior
.

Significance and Impact:

Synthetic control of the shapes of oxide catalysts has been shown to be a route to vary selectivity in ethanol partial oxidation reactions.

 Research Details:

•  Shaped nanoparticles of CeO2 (rods, cubes and octahedra) present different crystallographic faces that are hypothesized to alter their catalytic reactivity and selectivity

•  Nanoshaped CeO2 was synthesized and compared in steady state and temperature programmed conditions to explore differences in surface chemistry and catalytic behavior

•  Surface intermediates along with desorption or reaction products varied between the different shapes

•  Differences were found in types and stability of ethoxide intermediates, temperature of reaction onset, dehydrogenation: dehydration selectivity and ratio of methane: (CO + CO2) in C1 decomposition products

•  Results are described by structure dependent competition between a-CH and ß-CH scission, and relative ease of oxygen removal

Dynamics of Quinone of OLC
Selectivity in the oxidative dehydrogenation of ethanol to acetaldehyde (AcH), ethylene or
CO2 depends upon the
surface structure of CeO2.

Additional Description:

We have outlined the pathways for ethanol selective oxidation as a function of temperature and under conditions of sub-stoichiometric amounts of O2, as catalyzed by CeO2 nanoshapes with different morphologies. Different nanoshapes present different crystallographic faces and therefore comparing these shaped particles provides a method of probing the effects of site geometry and surface structure upon catalytic behavior. We find many differences in the surface interactions and reactions leading to different catalytic product selectivity. These include differences in types and stability of ethoxide intermediates, temperature of reaction onset, dehydrogenation: dehydration selectivity as determined by competition between a-CH and ß-CH scission, branching between two pathways for ethylene formation, and ratio of methane: (CO + CO2) in C1 decomposition product. For all shapes, the dominant surface species during both TPD and under reaction conditions (TPSR) conditions are ethoxide below 300 C which is eventually reacted away and displaced by acetate and carbonates at higher temperature. Furthermore, significant differences between TPD in He and TPSR in reaction mixture are observed including inhibition of ethylene formation under selective oxidation reaction conditions compared to TPD, yielding pronounced differences in acetaldehyde: ethylene: CO2 selectivity. Factors that may lead to the observed structural differences are the oxygen storage and availability especially as it relates to H2O/H2 selectivity, base strength, and mobility of oxygen anions on different surface terminations, variation in acid strength of the Ce cation sites caused by variation in their oxygen coordination, geometry of the adsorption site relative to the molecules and intermediates, and the relative numbers of vacancy or active sites created from oxygen removal during the reaction.

Credit:
This work was published by Meijun Li, Zili Wu, and S. H. Overbury “Surface structure dependence of selective oxidation of ethanol on faceted CeO2 nanocrystals” Journal of Catalysis, 2013, 306, 164–176.  Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy
.

Surface Chemistry and Catalysis R&D Projects

Provided by Oak Ridge National Laboratory's Chemical Sciences Division
Rev:   March 2014