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Effects of Biofuel Blends on Performance of Exhaust Gas Catalyst: Ethanol and Acetaldehyde Reactions
Abstract:
The use of biofuels in internal combustion engines changes the composition of the engine exhaust gas. When burning a biofuel blend, significant amounts of oxygenated hydrocarbons such as alcohols, ethers and aldehydes are present in the exhaust gas. It is known, that these compounds influence catalytic processes in exhaust gas converters. In this work we propose a global kinetic model for ethanol and acetaldehyde oxidation on commonly used Pt, PtPd and Pd-based catalytic oxidation converters of automobile exhaust gases. The mechanism is based on two steps: (i) partial oxidation of ethanol to acetaldehyde, and (ii) complete oxidation of acetaldehyde to CO₂ and H₂O. Kinetic parameters of ethanol and acetaldehyde reactions are evaluated on the basis of laboratory light-off experiments with several catalytic monolith samples (noble metal loading 9-140 g/cft; Pt, Pd, and PtPd; at space velocity 30 000-240 000 h-₁).
Net formation of acetaldehyde from ethanol is observed around the light-off temperature. The Pd-based catalysts exhibit slightly higher selectivity to acetaldehyde. At higher temperatures both ethanol and acetaldehyde are almost fully oxidized, however, internal diffusion effects in 50 μm washcoat layer significantly affect the selectivity of ethanol oxidation and acetaldehyde conversions. The calibrated model is then used for simulation of the NEDC (New European Driving Cycle) test driving cycle for different converter configurations (monolith size, noble metal loading). With very low noble metal activity (low loading or aged catalyst) the acetaldehyde conversion is negative, i.e., more acetaldehyde is produced in the converter by partial oxidation of ethanol. For medium to higher activity of noble metals the net effect of the catalyst is positive - acetaldehyde is oxidized to CO₂ and H₂O.