Small pore zeolite SSZ-13 supported Pd as highly stable low-temperature methane combustion catalysts

Yanran  Cui, Pacific Northwest National Laboratory

The low-temperature catalytic combustion of methane has been extensively studied for reducing methane emissions from lean-burn natural gas engine exhausts. Pd supported on Al₂O₃ has been known to be the most active, and therefore, the most commonly used catalyst for this application [1]. However below ~450 ºC, water vapor in engine exhausts severely deactivates this catalyst [2]. The deactivation has been attributed to transformation of the active PdO phase, e.g., sintering or the formation of an inactive Pd(OH)₂ phase, or hydroxyl group accumulation on the alumina supports [3, 4]. While catalyst deactivation caused by the PdO phase transformation is more difficult to prevent, that caused by –OH accumulation on the support material can be readily circumvented by using hydrophobic supports.

 

In this work, hydrothermally stable small pore SSZ-13 was used as support for PdO. Si/Al ratios of the supports were systematically varied from 6 to 36 to manipulate their hydrophobicity. Si/Al ratio was found to play a decisive role in Pd dispersion. For a hydrophilic support at Si/Al = 6, Pd largely presents as atomically dispersed cations in zeolite exchange positions. With increasing Si/Al ratio (i.e., support hydrophobicity), PdO particles on zeolite external surfaces gradually dominate. At Si/Al ratios of 24 and 36, the Pd/SSZ-13 catalysts demonstrated much higher stability than a Pd/Al₂O₃ reference at the same Pd loading. To obtain insights into the stability improvement of the Pd/SSZ-13 catalysts, detailed low-temperature methane combustion kinetics were conducted. For Pd/Al₂O₃, the -1 order rate dependence on H₂O partial pressure is fully consistent with its inhibiting role. For the Pd/SSZ-13 catalysts (Si/Al = 24 and 36), the ~0 order rate dependence on H₂O partial pressure provides compelling evidence in supporting the argument that surface –OH accumulation on the support is the primary cause for low-temperature catalyst deactivation. It is concluded that hydrophobicity of the support plays a key role in promoting the stability and activity of methane combustion catalysts.

References:

[1] Gélin, P., Primet, M., Appl. Catal. B: Environ. 39: 1-37, 2002

[2] Mihai, O., Smedler, G., Nylén, U., Olofsson, M., Olsson, L., Catal. Sci. Technol. 7: 3084-3096, 2017

[3] Roth, D., Gélin, P., Primet, M., Tena, E., Appl. Catal. A: General 203: 37-45, 2000

[4] Schwartz, W. R., Ciuparu, D., Pfefferle, L. D., J. Phys. Chem. C 116: 8587-8593, 2012

 

The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. Computing time was granted by a user proposal at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC). The experimental studies described in this paper were performed in the EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.