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 μm measured along the centerline of an adiabatically operated metal-coated α-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. H2 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, H2 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 H2 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, H2 and CO yields are generally higher on Rh than on Pt. The rate of O2 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 O2 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⩽1.0C/O⩽1.0, whereas Pt reaches equilibrium only at very high catalyst temperatures if C/O⩽0.7C/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.

[1]  Olaf Deutschmann,et al.  Modeling the partial oxidation of methane in a short‐contact‐time reactor , 1998 .

[2]  O. Deutschmann,et al.  Modeling the partial oxidation of methane in a fixed bed with detailed chemistry , 2004 .

[3]  D G Vlachos,et al.  Hierarchical multiscale mechanism development for methane partial oxidation and reforming and for thermal decomposition of oxygenates on Rh. , 2005, The journal of physical chemistry. B.

[4]  Vito Specchia,et al.  Modeling a transport phenomena limited reactivity in short contact time catalytic partial oxidation reactors , 2003 .

[5]  G. Marin,et al.  Partial oxidation of methane to synthesis gas over Rh/α-Al2O3 at high temperatures , 1997 .

[6]  G. Saracco,et al.  Short-Contact-Time Catalytic Partial Oxidation of Methane:Analysis of Transport Phenomena Effects , 2002 .

[7]  H. Wan,et al.  Mechanistic study of partial oxidation of methane to synthesis gas over supported rhodium and ruthenium catalysts using in situ time-resolved FTIR spectroscopy , 2000 .

[8]  Kenneth A. Williams,et al.  Spatial and temporal profiles in millisecond partial oxidation processes , 2006 .

[9]  L. Schmidt,et al.  Production of Syngas by Direct Catalytic Oxidation of Methane , 1993, Science.

[10]  Kenneth A. Williams,et al.  Syngas by catalytic partial oxidation of methane on rhodium: Mechanistic conclusions from spatially resolved measurements and numerical simulations , 2006 .

[11]  Lanny D. Schmidt,et al.  Modeling homogeneous and heterogeneous chemistry in the production of syngas from methane , 2000 .

[12]  G. Froment,et al.  Reaction Mechanism and Role of the Support in the Partial Oxidation of Methane on Rh/Al2O3 , 1996 .

[13]  Anders Holmen,et al.  Partial oxidation of methane to synthesis gas over rhodium catalysts , 1998 .

[14]  Gbmm Guy Marin,et al.  The Reaction Mechanism of the Partial Oxidation of Methane to Synthesis Gas: A Transient Kinetic Study over Rhodium and a Comparison with Platinum , 1997 .

[15]  M. Baerns,et al.  External mass and heat transfer limitations of the partial oxidation of methane over a Pt/MgO catalyst-consequences for adiabatic reactor operation , 1997 .

[16]  Jörg Frauhammer,et al.  Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor , 2000 .

[17]  Martin Schmal,et al.  Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts , 2005 .

[18]  Subir Roychoudhury,et al.  Catalytic partial “oxidation of methane to syngas” at elevated pressures , 2005 .

[19]  Alfons Baiker,et al.  2D-mapping of the catalyst structure inside a catalytic microreactor at work: partial oxidation of methane over Rh/Al2O3. , 2006, The journal of physical chemistry. B.

[20]  Kenneth A. Williams,et al.  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 , 2007 .

[21]  L. Schmidt,et al.  Partial oxidation of n-hexadecane at short contact times: Catalyst and washcoat loading and catalyst morphology , 2006 .

[22]  Olaf Deutschmann,et al.  Natural Gas Conversion in Monolithic Catalysts: Interaction of Chemical Reactions and Transport Phenomena , 2001 .

[23]  Malcolm L. H. Green,et al.  Brief Overview of the Partial Oxidation of Methane to Synthesis Gas , 2003 .

[24]  S. Reyes,et al.  Evolution of Processes for Synthesis Gas Production: Recent Developments in an Old Technology , 2003 .

[25]  G. Groppi,et al.  Mass-Transfer Characterization of Metallic Foams as Supports for Structured Catalysts , 2005 .

[26]  G. Groppi,et al.  Comparison among structured and packed-bed reactors for the catalytic partial oxidation of CH4 at short contact times , 2005 .