The decentralized production of synthesis gas, a mixture of CO and H₂, is of great technological importance for gas to liquid transformations in remote natural gas reservoirs. Synthesis gas production by steam reforming is not feasible for such application, because steam reformers are large and expensive plants that can not be scaled down for small scale operation. The catalytic partial oxidation (CPO) of methane, the main component of natural gas, (Eqn. 1) has received considerable attention for decentralized synthesis gas production because it provides close to 100% methane conversion and >90% synthesis gas yields in millisecond contact times [1]. Compared to contact times of seconds in a steam reformer, CPO reactors can be three orders of magnitude smaller processing the same amount of synthesis gas. In addition to reduced investment costs, methane CPO supplies a H₂/CO ratio of 2/1, which is favorable for methanol or Fischer-Tropsch synthesis. Group VIII metal catalysts (Rh, Pt, Ru, Ni) on highly porous ceramic foam monoliths are ideal catalyst composites for small high throughput reactors in decentralized applications.

CH₄ + ½ O₂ → CO + 2 H₂ ΔrH° = -36 kJ/mol^-1 (1)

In this contribution we compare Rh and Pt coated, autothermally operated alpha-Al₂O₃ foam catalysts for the partial oxidation of methane to synthesis gas [2]. Basis for the comparison are species and temperature profiles measured inside the foams using a recently developed capillary sampling technique with mass spectrometric species measurement [3]. A spatial resolution of about 300 µm was achieved. Gas temperature profiles were measured with a thermocouple. Thirteen different stoichiometries ranging from C/O= 0.6 to 2.6 at a constant flow rate of 4.7 slpm and atmospheric pressure were investigated. Spatial profiles, measured in 10 mm long, 80 ppi Rh and Pt foams, each containing 5-wt % noble metal, are compared in respect of:

i) profile development at synthesis gas stoichiometry ii) profile development at varying stoichiometries from C/O = 0.6 to 2.6 iii) product selectivities and yields in the oxidation zone iv) contribution of partial oxidation and steam reforming to the final synthesis gas yield v) mass transport limitations vi) approach to thermodynamic equilibrium.

All profiles show an oxidation zone and a steam reforming zone, independent of the C/O ratio. H₂, CO and H₂O are the main products in the oxidation zone. Only small amounts of CO₂ are formed. At the point of complete O₂ conversion, the H₂O profile passes through a maximum and becomes co-reactant to methane in a steam reforming reaction. CO₂ reforming is not observed. For the same reactant stoichiometry, H₂ and CO selectivities and yields are higher in the oxidation zone on Rh than on Pt. As the C/O ratio is increased, the catalyst temperature decreases and selectivities to H₂ and CO in the oxidation zone decrease. The decrease in selectivity to partial oxidation products is stronger on Pt than on Rh. Because Rh is also the better steam reforming catalyst, H₂ and CO yields are generally higher on Rh than on Pt. Analysis of the O₂ profiles reveals that the reaction is O₂ mass transport limited in the oxidation zone on Rh but kinetically controlled on Pt. An average O₂ mass transport coefficient, calculated from the O₂ profiles, is in good agreement with literature values on foam catalysts. As a last point in the comparison we compare exit species flow rates and temperatures with thermodynamic calculations. The latter are performed 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. Deviations from equilibrium at higher C/O result, because steam reforming slows down but also because water gas shift equilibrium is no longer established.

[1] Hickmann D. A.; Schmidt L. D. Science 1993, 259, 343. [2] Horn R.; Williams K. A.; Degenstein N. J.; Bitsch-Larsen A.; Dalle-Nogare D.; Tupy S. A.; Schmidt L. D. J. Catal. in press [3] Horn R.; Degenstein N. J.; Williams K. A.; Schmidt L. D. Catal. Lett. 2006, 110, 169.