PdO self-assembly on zeolite SSZ-13 with rows of O3Al(IV)OH selectively incorporated in PdO(101) facets for moisture-resistant methane oxidation

Abstract

We describe an efficient way to prepare moisture-tolerant methane (hydrocarbon) combustion catalysts based on PdO nanoparticles supported on siliceous SSZ-13 zeolite. Only zeolites with high Si/Al ratios >15 are hydrophobic enough to exclude the Pd from the micropores while forming well-faceted PdO nanoparticles. Simultaneously, during self-assembly mobile Al hydroxo species get incorporated into the as-formed PdO nanoparticles. For the first time, we reveal selective incorporation of rows of O3Al(IV)-OHbridging aluminum hydroxo-species into the (101) facets of PdO nanoparticles that form during thermal self-assembly in Pd/SSZ-13 using state-of-the-art atomically-resolved HAADF-STEM imaging, solid-state NMR, DFT calculations and reactivity measurements. The Al+3-OH moieties form atom-thin rows in place of tri-coordinate Pd ions Pd+2 in Pd1O3 on (101) facets: these tri-coordinate Pd1+2O3 are responsible for C-H bond dissociation of methane and hydrocarbons during catalytic methane oxidation. However, on unmodified or non-zeolite supported PdO nanoparticles in the presence of water vapor from engine exhaust, water competes with methane by forming a deactivated Pdtetra(OH)(H2O)Pdtetra site with two water molecules on contiguous 3-coordinate Pd, which is not active for C-H bond activation. When Al-OH moieties are present in place of some tri-coordinate Pd1O3 sites, water dissociation becomes kinetically unfavorable due to disruption of Pdtetra(OH)(H2O)Pdtetra species formation. Consequently, our catalytic measurements reveal a significantly more stable performance of such catalysts in methane combustion in the presence of water vapor. Our findings provide an unprecedented atomic-level insight into structure-property relationships for supported PdO materials in catalytic methane oxidation and offer a new strategy to prepare moisture-tolerant Pd-containing methane combustion catalysts for green-house gas mitigation by selectively doping atomically thin rows of non-precious metal into specific facets of PdO.

Understanding of Active Sites and Interconversion of Pd and PdO during CH4 Oxidation

“…We should point out that, in the oxidized sample, the catalytic activity decreases from 40% to around 28% after 8 h of reaction. The issue with the deactivation of the Pd catalyst in the investigation reaction is complex and generally agreed to be related to the presence of water vapor [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ], which forms during methane oxidation. Water competes with methane for the active sites on the catalyst surface, since it can dissociate and block the catalytically active undercoordinated Pd sites, forming a spectator Pd-OH species; the effective removal of strongly adsorbed OH species from the PdO surface occurs only at temperatures in excess of 500 °C [ 31 , 32 ].…”