Syngas by catalytic partial oxidation of methane on rhodium: Mechanistic conclusions from spatially resolved measurements and numerical simulations

The mechanism for the catalytic partial oxidation of CH4 on Rh-coated α-Al2O3 foam monoliths was investigated by measuring species and temperature profiles along the catalyst axis and comparing them with numerical simulations. A thin quartz capillary connected to a quadrupole mass spectrometer was moved through the catalyst with a spatial resolution of ∼0.3 mm. Profiles were measured under autothermal operation for C/O ratios of 0.7, 1.0 and 1.3. The influence of the flow rate (5 vs. 10 l min−1) was studied for syngas stoichiometry (C/O = 1). Numerical simulations were performed with a 38 step surface mechanism using both a porous 2D-model with mass and heat transfer and a simple plug-flow model. The experimental profiles reveal complete O2 conversion within 2 mm of the catalyst entrance for all C/O ratios and flows. H2 and CO are formed partly in the oxidation zone and partly after O2 is fully converted by steam reforming. CO2 is formed in small amounts in the oxidation zone and remains constant thereafter, except for C/O = 0.7, where some water gas shift is observed. CO2 reforming does not occur under the experimental conditions. Based on the experimental findings, a two-zone picture of the reaction mechanism is proposed. The 2D numerical simulations and the measured profiles agree qualitatively for all experimental conditions. Quantitative agreement is best for syngas stoichiometry (C/O = 1.0) at 5 and 10 l min−1 flow rate. Some quantitative differences are observed for C/O = 0.7 and 1.3. The plug flow model is for all conditions inferior to the 2D model. The importance of spatial profiles for mechanism and reactor model validation is highlighted.