Computational modeling predicts the stability of both Pd⁺ and Pd²⁺ ion-exchanged into H-CHA

Abstract

Quantum chemical simulations show that the Al configuration determines whether Pd+ or Pd2+ is thermodynamically most favorable in Pd/H-CHA.

Assessing the stability of Pd-exchanged sites in zeolites with the aid of a high throughput quantum chemistry workflow

“…Al pairs in the 6MR arrangements, especially in the NNNN position, provide the most favorable host for the Pd cation, in which case the Pd resides at the center of the 6MR (Figure 5a and b and Figure S10a and b). This finding is consistent with other recent studies [27,28]. This geometry provides the most number of oxygen atoms in close proximity to the cation, but not too close.…”

“…We start by reporting our results for the location of Pd cations exchanged into CHA [27,28]. eV less stable due to the proton occupying a different location (Figure S9).…”

“…The nature of the oxidation state of the Pd cations and their location in the zeolite continue to be debated in the scientific literature [27,28,40]. Here, we attempt to shed some light on the subject based on the results of our high throughput screening.…”

“…Pd-CHA and Pd-BEA were chosen because they are good candidates for passive NO adsorbers (PNAs) that can be used to trap the emissions of NO from automobile exhaust during cold startup and before the three-way catalytic converter becomes effective [24][25][26]. The speciation of Pd in these materials is recognized as a challenging, and still controversial problem [27,28]. We limit our discussion to the following Pd species: Pd + associated with isolated charge-exchange sites, Pd + H + pairs associated with two charge-exchange sites involving next nearest neighbor (NNN) or next-next nearest neighbor (NNNN) pairs of Al atoms, and Pd +2 associated with two charge-exchange sites also involving NNN or NNNN Al pairs.…”

Single‐Site vs. Cluster Catalysis in High Temperature Oxidations

“…>400 °C). As we and others have suggested, [21, 44] the Al sites in CHA are the most likely grafting points to stabilize single Pt or Pd atoms. This observation is rather common in instances where the extra‐framework species are cationic, for example in Cu dimers and trimers, [27, 45] and single atoms of noble metals under specific synthesis and reaction conditions [5] .…”