Transformation of Bulk Pd to Pd Cations in Small-Pore CHA Zeolites Facilitated by NO

Atomic dispersion of metal species has attracted attention
as a
unique phenomenon that affects adsorption properties and catalytic
activities and that can be used to design so-called single atom materials.
In this work, we describe atomic dispersion of bulk Pd into small
pores of CHA zeolites. Under 4% NO flow at 600 °C, bulk Pd metal
on the outside of CHA zeolites effectively disperses, affording Pd2+ cations on Al sites with concomitant formation of N2O, as revealed by microscopic and spectroscopic characterizations
combined with mass spectroscopy. In the present method, even commercially
available submicrosized Pd black can be used as a Pd source, and importantly,
4.1 wt % of atomic Pd2+ cations, which is the highest loading
amount reported so far, can be introduced into CHA zeolites. The structural
evolution of bulk Pd metal is also investigated by in situ X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS), as well as ab initio thermodynamic analysis using density functional theory (DFT) calculations.