Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts: Oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium

We compare Rh and Pt as catalysts for the partial oxidation of methane to syngas at millisecond contact times. The basis for the comparison are species and temperature profiles, with a spatial resolution of about 300 pm measured along the centerline of an adiabatically operated metal-coated alpha-Al2O3 foam using a capillary sampling technique with mass spectrometric species measurement. Gas temperature profiles are measured with a thermocouple. Investigated stoichiometries range from C/O = 0.6 to 2.6 at constant flow rate of 4.7 slpm and atmospheric pressure. Rh and Pt are compared with respect to (i) profile development at syngas stoichiometry, (ii) profile development at varying stoichiometries from C/O = 0.6-2.6, (iii) product selectivities and yields in the oxidation zone, (iv) contribution of partial oxidation and steam reforming to the final syngas yield, (v) mass transport limitations, and (vi) approach to thermodynamic equilibrium. Independent of C/O and metal, all profiles show an oxidation zone and a steam-reforming zone. H-2 and CO are formed in the presence of gas-phase oxygen by partial oxidation and in the absence of gas-phase oxygen by steam reforming. CO2 reforming is not observed. At the same C/O, H-2 and CO selectivities and yields are higher in the oxidation zone on Rh than on Pt. As the C/O ratio increases, the catalyst temperature decreases and selectivities to H-2 and CO in the oxidation zone decrease. The decrease is larger on Pt than on Rh. Because Rh is also the better steam-reforming catalyst, H-2 and CO yields are generally higher on Rh than on Pt. The rate of O-2 conversion at the catalyst entrance is largely mass transport- controlled on Rh but not on Pt. In the oxidation zone on Pt, the methane CPO is kinetically controlled with a constant reaction rate. An average O-2 mass transport coefficient is calculated and compared with literature values on foam catalysts. Finally, exit species flow rates and temperatures are compared with thermodynamic calculations at constant pressure and enthalpy. Rh brings the methane oxidation close to equilibrium if C/O <= 1.0, whereas Pt reaches equilibrium only at very high catalyst temperatures if C/O <= 0.7. At higher C/O, deviations from equilibrium are observed mainly because steam-reforming slows, but also because water-gas shift equilibrium is not established. (c) 2007 Elsevier Inc. All rights reserved.