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The oxidation of CO by O 2 was studied for a Pt/γ -Al 2 O 3 catalyst and for a commercially available Pt/Rh/CeO 2 /γ -Al 2 O 3 three-way catalyst. Kinetic experiments were carried out in an isothermal fixed-bed microreactor under intrinsic conditions, i.e., in the absence of mass and heat transfer limitations, in the temperature range from 436 to 503 K, with CO and O 2 inlet partial pressures between 0.12 and 8.3 kPa and H 2 O and CO 2 inlet partial pressures between 0 and 10 kPa. For the Pt/γ -Al 2 O 3 catalyst, the CO 2 production rate was found to be essentially proportional to the oxygen and inversely proportional to the carbon monoxide partial pressures, although at large CO and small O 2 partial pressures deviations occur. A kinetic model, based on elementary reaction steps, was constructed. It was concluded that for the experimental conditions considered, the noble metal surface is almost completely covered with CO, the CO adsorption being in quasi-equilibrium, and that irreversible molecular adsorption of oxygen is the rate-determining step, followed by potentially instantaneous dissociation. The presence of steam was found to enhance the reaction rate. For the experiments carried out over Pt/Rh/CeO 2 /γ -Al 2 O 3 in the presence 10 kPa H 2 O and 10 kPa CO 2 , it was found that the CO 2 production rate becomes zero order in CO at high CO partial pressures. The partial reaction order in O 2 is approximately 0.5. The experimental observations were explained by the existence of a second bifunctional reaction path next to the reaction path catalyzed by the noble metal only. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal and oxygen from ceria at the noble metal/ceria interface. The experiments could be described adequately over the investigated range of conditions by a kinetic model incorporating the monoand bifunctional reaction paths. For the quantification and understanding of the changes in the partial reaction orders in CO and O 2 as a function of the experimental conditions, a kinetic model based on elementary reaction steps is necessary.

. M. M. (1997). The reaction mechanism of the partial oxidation of methane to synthesis gas: a transient kinetic study over rhodium and a comparison with platinum. Journal of Catalysis, 167(1), 43-56. DOI: 10.100643-56. DOI: 10. /jcat.199743-56. DOI: 10. .1533 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The partial oxidation of methane to synthesis gas over rhodium sponge has been investigated by admitting pulses of pure methane and pure oxygen as well as mixtures of methane and oxygen to rhodium sponge at temperatures from 873 to 1023 K. Moreover, pulses of oxygen followed by methane and vice versa as well as pulses of mixtures of methane and labelled oxygen were applied to study the role of chemisorbed oxygen and incorporated oxygen in the reaction mechanism. The decomposition of methane on reduced rhodium results in the formation of carbon and hydrogen adatoms. During the interaction of pure dioxygen with rhodium the catalyst is almost completely oxidized to Rh 2 O 3 . In addition to rhodium oxide, oxygen is also present in the form of chemisorbed oxygen species. During the simultaneous interaction of methane and dioxygen at a stoichiometric feed ratio and a temperature of 973 K only 0.4 wt% Rh 2 O 3 is present. The chemisorbed oxygen species are completely desorbed after 2 s. A Mars-Van Krevelen mechanism is postulated: methane reduces the rhodium oxide, which is reoxidized by dioxygen. Synthesis gas is produced as primary product. Hydrogen is formed via the associative desorption of two hydrogen adatoms from reduced rhodium and the reaction between carbon adatoms and oxygen present as rhodium oxide results in the formation of carbon monoxide. The consecutive oxidation of CO and H 2 proceeds via both chemisorbed oxygen and oxygen present as rhodium oxide. Continuous flow experiments were performed to compare rhodium and platinum. When compared to platinum, rhodium shows a higher conversion to methane at a comparable temperature and also a higher selectivity to both CO and H 2 , the difference for CO being most pronounced. The observed differences in methane conversion and selectivities for the two catalysts are ascribed to the higher activation energy for methane decomposition on platinum compared to rhodium. An additional explanation for the difference in H 2 selectivit...

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