Science | June 7th, 2017
Electronic Effects on the Oxidation of Ethanol

Results are important for the production of fuel cells to direct ethanol

In internal combustion engines, various toxic substances – such as $ \rm CO$, $\rm NO_2 $ – are produced due to the incomplete breakdown of fuels. By decreasing the activation energy required for complete combustion to occur, catalysts aid in transforming these products into less toxic gases – such as carbon dioxide ($\rm CO_2 $), methane ($\rm CH_4 $).

Nevertheless, $\rm CO_2 $ and $\rm CH_4 $ produced mainly by human activity are currently considered to be responsible for the rise in the average temperature of the planet, which leads to the intensification of extreme climatic events. Hence, the search for more efficient energy sources that can significantly reduce or even eliminate the emission of these gases has intensified.

One of the alternatives to combustion engines are the so-called fuel cells that transform chemical energy into electricity similarly to batteries, but with reactants stored externally to the cell and continuously supplied and consumed.

Hydrogen gas ($\rm H_2 $) is considered the best fuel for these cells since its oxidation has as final product only water vapor. However, the distribution of hydrogen for use in vehicles is itself a technological challenge.

To avoid some of these difficulties, other substances can be used in fuel cells both indirectly – as a source of hydrogen (by processes collectively called reform) – and directly. In particular, the use of biofuels, such as ethanol, in fuel cells enables taking advantage of the entire fueling infrastructure already established for combustion engines. This alternative, however, requires the development of substantially more efficient catalysts than those currently available for the ethanol oxidation reaction.

The ethanol oxidation reaction involves parallel reaction steps in which the products of the incomplete reaction are predominant, rather than $\rm CO_2$ and water. For the reaction to start, the ethanol must adsorb, i.e., adhere to the surface of the catalyst, and for the oxidation to be complete, the intermediate molecules must also remain attached to the catalyst to be oxidized. In addition, the adsorption of a given molecule depends on its interaction with the electrons of the outermost orbitals of the catalyst. Changes in the electronic occupation of the orbital affect the adsorption energy and may favor intermediate reactions that lead to the complete reaction and its products.

Thus, researchers from the São Paulo State University (UNESP) used the facilities of the Brazilian Synchrotron Light Laboratory (LNLS) to investigate the ethanol oxidation reaction catalyzed by palladium (Pd) nanoparticles supported on carbon-oxide hybrids containing antimony tin oxide (ATO) for use in the so-called direct ethanol fuel cells.

The catalysts were prepared containing identical palladium nanoparticles in equal amounts but with different amounts of ATO. The group studied the catalytic activity in the oxidation of ethanol and verified the increase of the activity with the increase of ATO in the support. On the other hand, spectroscopic analyzes showed a higher production of $ \rm CO_2 $, which evidenced the occurrence of changes in the adsorption energies that favored the complete reaction.

Due to the way the catalysts were prepared, the changes in activity must be associated with the differences in the composition of the supports. Thus, the electronic properties of the different catalyst preparations were studied in the LNLS’ SXS Beamline. It was observed the increase in the electron occupation of the 4d band of the palladium with the increase of the amount of ATO in the support, revealing the electron transfer from support to nanoparticles. According to the researchers, the results show for the first time the strong correlation between the electronic properties of palladium nanoparticles and their activity in the oxidation of ethanol.

Source: Gabriel M. Alvarenga, Irã B. Coutinho Gallo, Hebe M. Villullas, Enhancement of ethanol oxidation on Pd nanoparticles supported on carbon-antimony tin oxide hybrids unveils the relevance of electronic effects, Journal of Catalysis, Volume 348, April 2017, Pages 1-8. DOI: 10.1016/j.jcat.2017.02.002.

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