Catalysts based on single atoms of scarce precious metals can lead to more efficient use through enhanced reactivity and selectivity. However, single atoms on catalyst supports can be mobile and aggregate into nanoparticles when heated at elevated temperatures. High temperatures are detrimental to catalyst performance unless these mobile atoms can be trapped. We used ceria powders having similar surface areas but different exposed surface facets. When mixed with a platinum/aluminum oxide catalyst and aged in air at 800°C, the platinum transferred to the ceria and was trapped. Polyhedral ceria and nanorods were more effective than ceria cubes at anchoring the platinum. Performing synthesis at high temperatures ensures that only the most stable binding sites are occupied, yielding a sinter-resistant, atomically dispersed catalyst.

We provide the first observation and characterization of super electrophilic metal cations on a solid support. For Pd/SSZ-13, the results of our combined experimental (Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd+3 and Pd+4, are, in fact, present as super electrophilic Pd+2 species (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution, we reassign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligand-free) super electrophilic Pd+2 in SSZ-13, and their complexes with CO and NO. With CO, nonclassical, highly positive [Pd­(CO)2]2+ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Nonclassical carbonyl complexes also form with Pt+2 and Ag+ in SSZ-13. The Pd+2(CO)2 complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd+2 complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd+2–carbonyl–olefin [Pd­(CO)­(C2H4)] and Pd+2–nitrosyl–olefin [Pd­(NO)­(C2H4)] complexes. Overall, we show that the zeolite framework can stabilize super electrophilic metal (Pd) cations and show the new chemistry of the Pd/SSZ-13 system with implications for adsorption and catalysis.

Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt % Pd) was synthesized. The material was characterized with Fourier transform infrared (FTIR) and cryo-scanning transmission electron microscopy. Adsorption of NO leads to the formation of Pd­(II)–NO and Pd­(I)–NO complexes as well as NO+ species that replace residual H+ (extra-framework) sites. These nitrosyl species have notable thermal stability, with resistance to decomposition under high vacuum at 200 °C. Addition of molecular oxygen to NO-containing stream improves NO x storage of the Pd/H-SSZ-13. In particular, addition of O2 to NO slightly increases the amount of Pd­(II)–NO complex with νNO at ∼1865 cm–1, whereas the low frequency νNO band at 1805 cm–1, assigned to Pd­(I)–NO, decreases in intensity. Simultaneously, polydentate nitrate species appear in small amounts, contributing to the high temperature NO x release stage during a passive NO x adsorber (PNA) cycle. The concentration of NO+ (characterized by the broad infrared band centered at 2170 cm–1), in the presence of O2, increases in intensity profoundly and contributes to the increased capacity of Pd/SSZ-13 to store NO x and release it at temperatures >140 °C. In the presence of H2O/O2, Pd/SSZ-13 does not perform satisfactorily as PNA, but the addition of CO to the stream improves the PNA storage capacity and shifts the NO x release peak temperature to >320 °C, where selective catalytic reduction catalysts are the most effective. With the aid of FTIR spectroscopy, we reveal the selective formation of a mixed carbonyl–nitrosyl complex Pd­(II)­(NO)­(CO) in the presence of CO. Because of shielding of the Pd­(II) ion from excess water and selective formation of such stable coordinatively saturated Pd­(II)­(NO)­(CO) complexes, the PNA performance is improved by CO. Therefore, we demonstrate that, besides NO species adsorbed on Pd, nitrosyl ions (NO+) in extra-framework positions of chabazite are important for PNA storage. Furthermore, the important role of CO in promoting PNA performance is elucidated, thus highlighting the utility of the combined spectroscopic approach (in addition to materials performance testing) to derive structure/PNA performance relationships and identify new avenues to improve the PNA performance.