Condition-Dependent Pd Speciation and NO Adsorption in Pd/Zeolites

Metal ions exchanged on zeolites
represent a unique bridge between
heterogeneous solid materials and homogeneous inorganic chemistry.
The complexing of exchanged metal ions with H2O or NO,
is of particular relevance for a number of reactions, including the
ubiquitous presence of both gases in pollution remediation technologies.
Here, we interrogate the molecular structure of Pd cations in SSZ-13
zeolites and their interaction with H2O and NO using experimental
and computational analyses. Density functional theory (DFT) and spectroscopic
characterization establish that Pd cations preferentially populate
two Al (2Al) sites in the six-membered ring as PdII. In
situ spectroscopic and kinetic analyses follow the Pd coordination
environment and reactivity as a function of environmental conditions,
and molecular structures are rationalized through ab initio molecular
dynamics and first-principles thermodynamic modeling. Experiment and
computational modeling together reveal that, at temperatures <573
K, Pd ions are solvated and mobilized by H2O molecules,
promoting catalytic CO oxidation, and form molecular complexes akin
to their Pd homogeneous analogues. Exposure to NO promotes transformation
from 2Al → 1Al charge-compensated H2O-solvated Pd-nitrosyl
complexes, which desorb NO at higher temperatures and inhibit CO adsorption
and oxidation. A comparison with Pd-BEA and Pd-ZSM-5 zeolites demonstrates
a heterogeneous distribution of Pd-NO complexes under dry conditions
that coalesce into homogeneous H2O-solvated Pd-nitrosyl
complexes upon exposure to H2O.