Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Abstract

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O2 at 700 °C, oxidative fragmentation occurs, and Pt2+ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt2+ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H2 to Ptδ+ monocarbonyls. This conversion, which weakens the platinum–zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta-Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst

“…We used DFT calculations to obtain initial estimates of the Ni−O bond distances (see the Supporting Information for details); the QuantEXAFS methodology was not used because the precise coordination environment is not known. 44,45 Our conventional EXAFS analysis indicates that Ni- S3. These results, along with HAADF-STEM and CO adsorption, show that Ni is reoccupying silanol nests with a distorted tetrahedral geometry.…”

“…The EXAFS fit of oxidized Ni-[DeAl]-beta is also consistent with distorted tetrahedral Ni sites. We used DFT calculations to obtain initial estimates of the Ni–O bond distances (see the Supporting Information for details); the QuantEXAFS methodology was not used because the precise coordination environment is not known. , …”

Bridging the Gap between the X-ray Absorption Spectroscopy and the Computational Catalysis Communities in Heterogeneous Catalysis: A Perspective on the Current and Future Research Directions

“…116,117 While uniform atomically dispersed catalysts are more amenable to analysis by QuantEX-AFS, this approach has been extended to more complex systems such as Pt/ZSM-5. 118 The latter study is particularly interesting due to the complexity of the zeolite support; the authors use a zeolite-specific simulation codebase 119 to enumerate all possible bonding sites of Pt 2+ and [Pt−OH] 1+ species in the MFI topology. Although questions about the uniformity of the proposed Pt 2+ site remain open, a key advantage of QuantEXAFS is the ability to compare the modeled EXAFS spectra of different types of sites (e.g., all possible Pt 2+ vs all possible [PtOH] 1+ ) rather than only considering a few plausible configurations.…”

“…QuantEXAFS is a recently reported theory-guided Python-based EXAFS analysis approach that uses the open-source Larch software for EXAFS preprocessing, analysis, and fitting . This method begins by creating an exhaustive database of DFT-optimized structures that can be used to model the experimental EXAFS. , While uniform atomically dispersed catalysts are more amenable to analysis by QuantEXAFS, this approach has been extended to more complex systems such as Pt/ZSM-5 . The latter study is particularly interesting due to the complexity of the zeolite support; the authors use a zeolite-specific simulation codebase to enumerate all possible bonding sites of Pt 2+ and [Pt–OH] 1+ species in the MFI topology.…”

Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation

“…It is becoming more recognized that catalyst structure can be dynamic, changing with time on stream and reactor and reaction conditions. − These catalyst structural changes can alter the catalyst activity and the selectivity toward desired products. , Of course, they can also complicate kinetic analysis due to site type and density changes. Therefore, studying catalyst dynamics under reaction conditions is critical in understanding reactions, mechanisms, and designing catalysts.…”