The role of Ni on the performance of automotive catalysts: evaluating the ethanol oxidation reaction

The role of the nickel oxide on the ethanol oxidation reaction over different model catalysts and commercial catalyst was studied. At 473 K, the catalytic activity followed the order: Pd/Al2O3≈Pd/CeO2/Al2O3≈Pd/Al2O3+NiO/Al2O3 physical mixture>Pd/NiO/Al2O3≈Pd/NiO/CeO2/Al2O3≈commercial catalyst. NiO/Al2O3 and CeO2/Al2O3 catalysts were not active at this temperature. Nickel addition strongly decreased the catalytic activity of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts. Temperature-programmed reduction (TPR) and oxygen storage capacity (OSC) measurements showed that palladium promoted the nickel oxide reduction. This strong interaction between both metals was destroyed under the reaction conditions leading to a nickel oxide surface segregation. The decrease of ethanol conversion in the presence of nickel was due to the coverage of palladium particles by nickel oxide, as demonstrated by IR analysis. The catalytic behavior of commercial catalyst was well represented by the Pd/NiO/CeO2/Al2O3 model catalyst. The main products of ethanol oxidation were acetaldehyde, acetic acid, carbon oxides and ethyl acetate. Ethanol oxidation proceeds through parallel and series reactions. The nickel addition shifted all the reaction system towards high temperature due to the decrease of activity.