Surface Density Of Active Sites Research Articles

Kinetic study of the catalytic oxidation of alkanes over nickel, palladium, and platinum foils

The kinetics of the total oxidation of alkanes (methane, ethane, propane, n-butane and isobutane) over Ni, Pd, and Pt foils was studied under fuel-lean conditions by using a recirculating batch reactor with mass spectrometry detection. Approximate first- and zeroth-order kinetics with respect to the hydrocarbon and oxygen concentrations, respectively, was observed in all cases. Significantly slower rates were seen for methane conversion, but other subtle changes were also identified among the other hydrocarbons as well. The reactivity trends could be clearly correlated with C-H bond strengths, since higher rates were seen for the longer and more branched hydrocarbon chains. Also, platinum was found to be the most active catalyst for the oxidation of all the compounds 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-exponential factor, not the activation energy, suggesting that they have to do with the surface density of active sites on the surface. The nature of the active catalyst during reaction was determined by simple inspection to be the metallic phase in the cases of Pt and Ni but an oxide layer in the case of Pd.

Exchange reaction of methane on some oxide catalysts

The hydrogen exchange reaction between CD 4 and CH 4 was investigated over alumina, silica, titanium oxide, magnesia, and alumina modified with acid and base. When evacuated at 600°C, alumina was found to be most active followed by magnesia while silica and titanium oxide were totally inactive at any temperatures up to 600°C. Alumina modified with 9% of K 2CO 3 retained about 20% of the original activity, but one impregnated with 5% of H 2SO 4 completely lost the activity. It was found that the surface density of active sites of alumina measured with carbon dioxide with the aid of TPD was one order of magnitude smaller than that estimated previously. The exchange reaction between CD4 and the surface hydroxyl groups was also studied and the involvement of the surface hydroxyl groups in the CD 4-CH 4 exchange reaction was discussed.