KINETIC-STUDY OF THE CATALYTIC-OXIDATION OF ALKANES OVER NICKEL, PALLADIUM, AND PLATINUM FOILS

The kinetics of the total oxidation of alkanes (methane, ethane, propa ne, n-butane and isobutane) over Ni, Pd, and Pt foils was studied unde r fuel-lean conditions by using a recirculating batch reactor with mas s spectrometry detection. Approximate first- and zeroth-order kinetics with respect to the hydrocarbon and oxygen concentrations, respective ly, was observed in ail cases. Significantly slower rates were seen fo r methane conversion, but other subtle changes were also identified am ong the other hydrocarbons as well. The reactivity trends could be cle arly correlated with C-H bond strengths, since higher rates were seen for the longer and more branched hydrocarbon chains. Also, platinum wa s found to be the most active catalyst for the oxidation of all the co m pounds studied here except methane, which is oxidized faster on the nickel substrate. The variations in activity among the three catalysts were shown to be associated mostly with changes in the pre-exponentia l factor, not the activation energy, suggesting that they have to do w ith the surface density of active sites on the surface. The nature of the active catalyst during reaction was determined by simple inspectio n to be the metallic phase in the cases of Pt and Ni but an oxide laye r in the case of Pd.