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Mechanism of H₂ and CO formation in the catalytic partial oxidation of CH₄ on Rh probed by steady-state spatial profiles and spatially resolved transients
Publikationstyp
Journal Article
Date Issued
2007-03-01
Sprache
English
Author(s)
Journal
Volume
62
Issue
5
Start Page
1298
End Page
1307
Citation
Chemical Engineering Science 62 (5): 1298-1307 (2007)
Publisher DOI
Scopus ID
ISSN
00092509
Spatially resolved species and temperature profiles have been measured for the catalytic partial oxidation of CH4 on autothermally operated Rh-coated α - Al₂ O₃ foams in both steady state and transient mode. A probe consisting of a thin quartz capillary and a thermocouple was moved with sub-mm resolution through the catalyst/heat shield stack to sample the gases into a mass spectrometer and measure temperature. The probe was also used to follow the relaxation of the system upon perturbation by stepping the stoichiometry of the reactants periodically. The spatial profiles in steady state show that H₂ and CO are formed in presence of gas phase oxygen (oxidation zone) and after total oxygen conversion by steam reforming (reforming zone). The stepwise change of the reactant stoichiometry in the transient experiments allowed decoupling chemistry (ms timescale) and temperature (s timescale). By exposing a catalyst, initially operating very hot at C / O = 0.6, suddenly to a CH₄ rich feed (C / O = 1.4), it was possible to follow how the integral H₂ and CO production rates decrease with decreasing temperature of the catalyst. The reverse switch from C / O = 1.4 to 0.6 showed how the integral H2 and CO production rates increased with increasing temperature of the catalyst. Differential reaction rates were obtained by performing these transients spatially resolved at two adjacent points in the catalyst. For C / O = 0.6 → 1.4, H2 and CO formation show a strict linear Arrhenius behavior over the entire temperature range from ∼ 1100 to ∼ 600 C. For C / O = 1.4 → 0.6, the Arrhenius plots show two straight segments, a steep increase in the formation rate from ∼ 630 to ∼ 710 C and a weak increase from ∼ 725 to ∼ 1000 C. In both cases, H2 formation is more activated than CO formation indicating two different rate determining steps.
Subjects
Catalytic partial oxidation
Mechanism
Methane
Rhodium
Spatially resolved transients
Spatial profiles
Syngas
DDC Class
540: Chemistry