Mechanistic insights into the deactivation of Pd/BEA methane combustion catalysts by hydrothermal aging

Abstract

Three Pd/Beta zeolite catalysts with low, intermediate and high support Si/Al2 (SAR) ratios were synthesized for methane combustion in both fresh and hydrothermally aged forms. SAR was found to dramatically influence catalyst performance and hydrothermal stability. Via an array of structural and textural characterizations, it is discovered that (1) at low SAR, catalyst deactivation is due primarily to the transformation of catalytically active PdO to inactive Pd2+-ions via protonolysis; (2) at intermediate SAR, catalyst deactivation during aging is due partly to the protonolysis chemistry, and partly to Al-decoration of the remaining PdO particles; (3) at high SAR, both of these deactivation mechanisms are greatly mitigated, leading to much improved catalyst hydrothermal stability. Our work here demonstrates that PdO particles stabilized by silanol nests, sites that are highly active for methane combustion, can only survive harsh hydrothermal aging at very high support SAR.