Onset of High Reactivity in Palladium Catalyzed Low-temperature Methane Combustion

Feng  Gao, PNNL

Onset of High Reactivity in Palladium Catalyzed Low-temperature Methane Combustion

 

Yanran Cui,1 Bo Peng,1 Yilin Wang,1 Arun Devaraj,1 Libor Kovarik,1 Donghai Mei,1,2 Johnny Zhu Chen,3 Jeffrey T. Miller,3 János Szanyi,1 Yong Wang,1,4 Feng Gao1,*

 

  1. Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  2. State Key Laboratory of Membrane Separation and Membrane Processes, School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China
  3. School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA
  4. The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, USA

*Corresponding author: feng.gao@pnnl.gov

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., PdO sintering or the transformation of the active PdO to an inactive Pd(OH)2 phase, or hydroxyl group accumulation on the alumina supports [3, 4]. Recently, it has been discovered that small-pore zeolites can be used as supports to synthesize catalysts that show improved stability as compared to Pd/Al2O3 [5, 6].

For small-pore zeolite SSZ-13 supported Pd catalysts, Pd dispersion is greatly influenced by Si/Al ratio of the SSZ-13 support. At low Si/Al ratios (≤ 12), low-loaded Pd (≤ 1wt%) largely presents as isolated Pd(II) cations in zeolite exchange sites; whereas at high Si/Al ratios (> 12), Pd largely presents as PdO particles on the zeolite external surface. The PdO particles catalyze low-temperature methane combustion much more efficiently than isolated Pd(II) ions. On low Si/Al ratio supports, when Pd loading increases to ~2-5 wt%, the majority of Pd still diffuses into the zeolite bulk. However, in addition to isolated Pd ions, extremely small PdOx clusters also form in this case. The detection of such PdOx clusters via atomic probe tomography (APT) and X-ray absorption spectroscopy (XAS) coincides well with the abrupt reactivity increase of the Pd/SSZ-13 catalysts. Therefore, it can be concluded that lattice oxygen associated with Pd is critically important for the observed high reactivity. Finally, theoretical calculations via density functional theory (DFT) provide important molecular-level insights into the reactivity difference between isolated and multinuclear Pd sites.

 

Key words: methane combustion, Pd catalyst, zeolite, support, particle size

 

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