Here, we examine Pd cations exchanged in zeolites and their complexing with H2O and NO at a molecular level, employing experimental and computational techniques. Density functional theory (DFT) analyses aided by spectroscopic characterization show that the exchanged Pd ions preferentially charge-compensate two Al (2Al) sites in the six-membered ring of SSZ-13 as Pd(II), akin to their similarly sized Cu and Co analogues. We further probe the reaction environment-dependent Pd coordination chemistry and reactivity using in-situ spectroscopy and kinetic studies, along with ab initio molecular dynamics, wave function theory (WFT) calculations and first-principles based thermodynamic analyses to arrive at probable molecular structures. Computational modeling and experimental results demonstrate that Pd cations are solvated by H2O below 573 K, forming four-fold coordinated square planar complexes as observed in homogeneous Pd compounds, detached from the zeolite framework. The presence of NO facilitates transformation of the 2Al to 1Al sites by the resulting H2O-solvated Pd-nitrosyl complex, in the form of (Pd(II)(NO-)(H2O)3)+. These complexes exhibit higher desorption temperatures than their dehydrated counterparts, while also inhibiting competitive CO adsorption. In-situ infrared spectra of zeolites of different topologies demonstrate formation of similar H2O-solvated Pd-nitrosyl complexes, reinforcing the role of H2O in solvation and homogenization of Pd ions in the presence of NO across zeolites of varying frameworks. Lastly, we use computational thermodynamic modeling to determine the conditions that promote Pd cation formation from Pd nanoparticles, the major product of deactivation modes for these materials.

Condition-dependent Pd speciation and no adsorption in Pd/zeolites

ACS Spring Meeting 2021

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