Low-temperature combustion of hydrogen on supported Pd catalysts

Low-temperature ( 2 /O 2 mixtures diluted in N 2 has been studied experimentally using a microreactor with transient exhaust monitoring using mass spectroscopy. Experimental results using a γ-Al 2 O 3 washcoat-supported PdO x catalyst reveal the importance of transient measurements for elucidating features of the catalytic combustion mechanism and, in particular, the effects of H 2 O adsorption/desorption and of the Pd oxidation state. For the cases studied, experiments indicate that H 2 conversion depends on equivalence ratio ( Φ ) only at low temperatures ( T in T in >125°C, mass transfer limitations become more significant, and thus, as T in rises, conversion becomes relatively independent of Φ . Addition of H 2 O vapor to the inlet flow causes a reduction in conversion for lower T in , but for T in >125°C, it only delays catalyst light-off and the rapid transition to high steady-state conversion. A final set of experiments indicated very high (and apparently steady) H 2 conversion at T in as low as 50°C by starting with a prereduced catalyst. Efforts to understand the experimental results with a transient one-dimensional reactor model using detailed surface chemistry indicates the importance of the relative adsorption rates of H 2 and O 2 as well as the H 2 O adsorption/desorption. The model captures the trends for conversion with respect to temperature but fails to predict well the influence of inlet H 2 O concentrations. Implications on further mechanism development of the discrepancies between the model predictions and experiments results are discussed. Nonetheless, these low-temperature H 2 combustion studies provide a starting point to further Pd surface chemistry for combustion of other fuels for a wide range of applications.

[1]  G. Veser,et al.  On the oxidation–reduction kinetics of palladium , 1999 .

[2]  G. Groppi,et al.  High-temperature combustion of CH4 over PdO/Al2O3: kinetic measurements in a structured annular reactor , 2001 .

[3]  L. Pfefferle,et al.  Support and water effects on palladium based methane combustion catalysts , 2001 .

[4]  D. Vlachos,et al.  The autothermal behavior of platinum catalyzed hydrogen oxidation: experiments and modeling , 1999 .

[5]  Abhaya K. Datye,et al.  Catalyst microstructure and methane oxidation reactivity during the Pd↔PdO transformation on alumina supports , 2000 .

[6]  D. Sekiba,et al.  Adsorption and desorption kinetics of oxygen on the Pd(110) surface , 1999 .

[7]  Fabio H. Ribeiro,et al.  Kinetics of the Complete Oxidation of Methane over Supported Palladium Catalysts , 1994 .

[8]  J. G. McCarty,et al.  Dispersion of palladium on alumina surfaces , 1994 .

[9]  E. Nowicka,et al.  Hydrogen segregation on a palladium hydride surface , 1998 .

[10]  I. Lundström,et al.  Competition between hydrogen and oxygen dissociation on palladium surfaces at atmospheric pressures , 1983 .

[11]  Frank Behrendt,et al.  NUMERICAL MODELING OF CATALYTIC IGNITION , 1996 .

[12]  Francisco J. Urbano,et al.  Investigation of the active state of supported palladium catalysts in the combustion of methane , 1995 .

[13]  Arun S. Mujumdar,et al.  Introduction to Surface Chemistry and Catalysis , 1994 .

[14]  P. Deuflhard,et al.  One-step and extrapolation methods for differential-algebraic systems , 1987 .

[15]  P. Forzatti,et al.  Analysis of a catalytic annular reactor for very short contact times , 1999 .

[16]  Bernd Emonts,et al.  Catalytic radiant burner for stationary and mobile applications , 1999 .

[17]  Jens Hartmann,et al.  The interaction of oxygen with alumina-supported palladium particles , 2001 .

[18]  J. Geus,et al.  The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane , 1999 .