Hydrothermally Stable Pd and Pt/CeO2(core)@ZrO2(shell) Catalysts for Low Temperature TWC Applications

Chih-Han  Liu, University at Buffalo

Hydrothermally Stable Pd and Pt/CeO2(core)@ZrO2(shell) Catalysts for Low Temperature TWC Applications

 

Chih-Han Liu1, Todd J. Toops2 and Eleni A. Kyriakidou1,*

1Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA

2National Transportation Research Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

*elenikyr@buffalo.edu

 

Figure 1.  (a) EDS elemental map and (b) Ce, Zr elemental distributions across CeO2@ZrO2 supports.

As a result of the continued improvements in vehicle engine efficiency, catalysts need to perform effectively at low exhaust temperatures in order to meet the strict emission standards introduced by the Environmental Protection Agency.  Therefore, US DRIVE established a goal of achieving 90% conversion of hazardous emissions at 150 ºC; so-called “150 ºC challenge”.  Fluctuations in the air to fuel ratio in stoichiometric gasoline engine aftertreatment systems result in a decreased efficiency of the catalytic converter [1].  CeO2 is thus a promising catalyst support due to its high oxygen storage capacity (OSC) that buffers O2 during rich/lean cycling [2].  Specifically, Zhan et al. reported that under stoichiometric conditions, the T90 of CO, total hydrocarbons (THCs) and NOx was 229, 320 and 235 oC, respectively, over 1 wt.% Pd/CeO2 [3].  However, CeO2 suffers from poor thermal stability as indicated by a decrease in its surface area from 50 to 2 m2/g with an increase in calcination temperature from 550 to 800 oC [4].  A potential solution for improving the thermal stability of CeO2 is to incorporate ZrO2 into CeO2.

Herein, CeO2(core)@ZrO2(shell) structured supports were synthesized by using a solvothermal synthesis method for the synthesis of CeO2-spheres, followed by a sol-gel process that ZrO2 shell was formed by the hydrolysis and condensation of zirconium butoxide (Fig. 1a-b).  1 wt.% Pd/CeO2@ZrO2 and 1.8 wt.% Pt/CeO2@ZrO2 catalysts were synthesized by wet impregnation.  Pd and Pt supported on CeO2-spheres and commercial CeO2 were synthesized for comparison purposes.  The catalytic performance of the studied catalysts was evaluated using the low temperature oxidation catalyst test protocol defined by US DRIVE under stoichiometric gasoline direct injection (S-GDI) conditions [5].  The T90’s of CO, THC and NOx over degreened 1.8 wt. % Pt/CeO2@ZrO2 were 59, 100 and 152 oC lower than the T90’s of CO, THC and NOx over degreened 1 wt. % Pd/CeO2@ZrO2.  Specifically, the T90’s of CO, THC and NOx over 1.8 wt. % Pt/CeO2@ZrO2 were 203, 253 and 287 oC, respectively.  Hydrothermally aged catalysts showed a slight deactivation with T90’s of CO, THC and NOx at 317, 374 and 374 oC (1 wt. % Pd/CeO2@ZrO2) and 261, 328 and 342 oC (1.8 wt. % Pt/CeO2@ZrO2), respectively.  Overall, this work illustrates the potential of developing CeO2-sphere and CeO2@ZrO2 supported catalysts for stoichiometric gasoline oxidation reaction.
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References:

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[5] Aftertreatment Protocols for Catalyst Characterization and Performance Evaluation: Low-Temperature Oxidation Catalyst Test Protocol: https://cleers.org/wp-content/uploads/2015_LTAT-Oxidation-Catalyst-Characterization-Protocol.pdf.