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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...

Continuous flow experiments for the oxidative coupling of methane in the absence of catalyst and at low methane conversion were carried out in empty tubular quartz reactors at atmospheric pressure, temperatures from 873 to 1123 K, and inlet molar ratios of CH,/O2 from 4 to 10 and of He/CH4 from 0 to 1.25. The methane conversion varied from 2 to 15% and the oxygen conversion from 10 to 100%. A reaction network was constructed on the basis of elementary free-radical reactions. Arrhenius parameters were estimated for the most important reactions by regression of experimental data. The effects of the process conditions on the conversions of methane and oxygen as well as on the selectivities toward products were simulated adequateliy by considering 33 elementary reactions.Ethane is mainly formed from the recombination of methyl radicals arising from degenerate branched chains involving OH and H 0 2 as main chain carriers. Etlhene originates from ethane mainly via a pyrolytic chain, while the oxidative dehydrogenation contributes to a much lower extent. Carbon monoxide originates from the oxidation of methyl radicals, whereas the contribution of the consecutive oxidation is not significant a t the conversion levels investigated. IntroductionThe oxidative coupling of methane aimed at the production of higher hydrocarbons has attracted much attention during the past years. This reaction is usually carried out in the presence of catalysts and requires temperatures up to 1150 K. At these temperatures, noncatalytic reactions in the gas phase, however, play an essential role in the formation of higher hydrocarbons. It is generally accepted that the most widely studied alkali metal/&aline earth metal oxide and the rare earth metal oxide catalysts act as a methane activator and in particular as a producer of methyl radicals, with the subsequent reactions taking place in the gas phase (Ito et al., 1985). Whether or not the reducible multivalent metal oxides catalyze the reactions in the same way is still in debate.The importance of the gas-phase reactions has been well recognized and is reflected in several recent papers on the oxidative coupling of methane in the absence of catalyst (Labinger and Ott, 1987;Lane and Wolf, 1988;Geerts et al., 1990;Zanthoff and Baerns, 1990). Furthermore, in a recent review, Lunsford (1990) has pointed out the importance of branched-chain reactions in the gas phase with respect to the generation of methyl radicals. Consequently, Lunsford proposed that catalysts might be an important initiator of the chain reaction, but not a major source of methyl radicals. Kinetic models based on the free-radical mechanism have been set up in order to understand the role played by the gas-phase reactions and the interaction between the gas-phase reactions and the reactions on the catalyst surface. The Arrhenius parameters were selected from data bases in the literature that originate mainly from combustion kinetics (Tsang and Hampson, 1986; Warnatz, 1984). While the dependence of the coupling selectivity on co...

. M. M. (1995). An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum. Catalysis Letters,, 291-304. DOI: 10.1007/BF00814232 General rightsCopyright 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 has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoiehiometric feed ratio the dominant pathways are the direct formation of CO and H2 followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.