The Reaction Mechanism of the Partial Oxidation of Methane to Synthesis Gas: A Transient Kinetic Study over Rhodium and a Comparison with Platinum

Abstract

. 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...

Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals

“…In fact, the catalytic effect of Rh for dissociation of methane or hydrogen is stronger than Pd and Pt. In literature, it can be found that catalytic activity of Pd and Pt on methane or hydrogen is more or less the same [128][129][130][131]. But Figure 14 shows a higher response with Pd-Ag than Pt and thus contradicts the reported results.…”

“…Therefore, the devices using Pt as catalytic metal exhibit lower response and relatively longer response time than Pd-Ag for all the cases. The OH formation energy of Rh is higher than both Pd-Ag and Pt [128][129][130][131] and so Rh catalytic metal contact gives higher response than Pd-Ag and Pt. This corroborates the experimental results obtained by Bhattacharyya et al [120].…”

“…Further, Rh contact showed the higher response (83.6%) than Pd-Ag and Pt ( Figure 13). It was further reported that the response and recovery time are shorter for Pd-Ag compared to Rh and Pt contacts, most probably due to the fact that solubility and diffusivity of hydrogen obtained from methane dissociation is higher for Pd-Ag compared to both Pt and Rh [128][129][130][131].…”

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