Small pore zeolite
SSZ-13 supported Pd as highly stable low-temperature methane combustion
catalysts
Yanran Cui, Bo Peng, Libor Kovarik, Yong Wang, Feng Gao*
Institute
for Integrated Catalysis, Pacific Northwest
National Laboratory
Richland, WA 99352, USA
*Corresponding author: feng.gao@pnnl.gov
Abstract
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].
In this work, hydrothermally
stable small pore SSZ-13 zeolites are used as the PdO support. Si/Al ratios of
the supports are systematically varied from 6 to 36 to manipulate support
hydrophobicity, which is 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. By increasing Si/Al ratio to increase
support hydrophobicity, Pd progressively occupies zeolite external surfaces as
PdO particles. Pd/SSZ-13 catalysts with Si/Al = 6 and 12 still suffer from
water-induced deactivation, the same as the Pd/Al2O3
reference catalyst studied here; at Si/Al ratios of 24 and 36, however, the
Pd/SSZ-13 catalysts become much more stable than Pd/Al2O3
in the presence of water vapor. To obtain insights into the stability
improvement of these latter Pd/SSZ-13 catalysts, detailed low-temperature methane
combustion kinetics are conducted. For Pd/Al2O3, the powder-law
dependence on H2O partial pressure is found at -1, fully consistent
with the inhibiting role of water vapor. For the Pd/SSZ-13 catalysts (Si/Al =
24 and 36), the ~0 powder-law dependence on H2O pressure explains
their improved stability. It is concluded that hydrophobicity of the support
plays a key role in promoting the stability and activity of methane combustion
catalysts.
References:
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