Unraveling Multiscale Kinetics over Subnanometer Cluster Catalysts: H₂ Desorption from Pt₃(-H)₂/γ-Al₂O₃(110)

Abstract

Despite the attractiveness of highly dispersed supported metal catalysts due to the efficient usage of the active metal component, the structural complexity of subnanometer metal cluster active sites and the interconnectedness of reaction networks over many active site configurations elude detailed understanding. Here, we perform density functional theory (DFT) calculations and state-based kinetic simulations of the desorption of H2 from Pt3(-H)2 clusters supported on dehydroxylated γ-Al2O3(110), serving as a prototype of such coupled reaction networks. Different from ideal low Miller index metal surfaces and highly symmetric gas-phase clusters, we find many unique H binding sites on the supported Pt3 clusters, resulting in an ensemble of metastable Pt3(-H)2 cluster configurations interwoven within a network of H diffusion, active site restructuring, and H2 desorption elementary steps. Simulations and spectral analysis show that the catalyst and chemistry expose three principal time scales, corresponding to the diffusion of H, restructuring of Pt3(-H)2, and desorption of H2. Free energy span-based interpretations of the reaction pathways and sensitivity analysis of the eigenvalues uncover favorable Pt3(-H)2 restructuring and H2 desorption processes as being kinetically relevant at intermediate and long times. Interestingly, H2 desorption implicates catalyst restructuring as a prerequisite for forming more favorable desorption channels. We introduce simplified ensemble-based models and effective rate constants for the modeling of such multiscale reaction processes.

Thermodynamic Equilibrium versus Kinetic Trapping: Thermalization of Cluster Catalyst Ensembles Can Extend Beyond Reaction Time Scales

“…This means that when considering reactivity involving adsorbate migration, cluster isomerization can be an important aspect of the reaction coordinate, as was recently shown for Pt 3 H 2 . 23 However, for Pt 4 H x , we find much less separation between H migration and Pt restructuring. kMC sampling is performed on the shown networks in order to explore the time evolution of isomer populations across a range of temperatures.…”

“…These networks provide useful general chemical information: for instance, somewhat surprisingly, the barriers for H migration on the Pt 4 core are not significantly smaller than the barriers involving restructuring of the Pt 4 core. This means that when considering reactivity involving adsorbate migration, cluster isomerization can be an important aspect of the reaction coordinate, as was recently shown for Pt 3 H 2 . However, for Pt 4 H x , we find much less separation between H migration and Pt restructuring.…”

“…Furthermore, different isomers can have distinct reactivities where metastable isomers have the potential to be more reactive than the most abundant stable structure. ,− The importance of structures to reactivity has also been seen for single-nanoparticle catalysts, indicating the importance of taking into account the full structural diversity when attempting to identify structure–property relationships. , Naturally, determining the exact structure on the atomistic level is difficult . Additionally, catalyst dynamics are an important part of the reaction coordinate; dynamic restructuring has been observed experimentally, − and multiple DFT studies have supported the fact that restructuring of active sites is relevant for many catalytic reactions. − Furthermore, Imaoka et al have explicitly shown the isomerization of a Pt 4 cluster in real time using HAADF-STEM, visualizing the exact kinds of structural change addressed in this paper.…”

“…This suggests that simply using the equilibrium populations of isomers is likely insufficient to predict which isomers in the ensemble contribute the most to the observed reactivity. Furthermore, recent work by Yan and Vlachos investigated the populations of γ-Al 2 O 3 -supported Pt 3 H 0,2 clusters as they underwent isomerization in conjunction with H 2 adsorption and desorption. They identified that multiple time scales are important for the restructuring of the Pt 3 cluster upon the adsorption and desorption of H 2 , finding that the restructuring of Pt 3 H 2 was essential for the favorable desorption of H 2 under conditions relevant for the dehydrogenation of propane.…”

Mechanism research reveals the role of Fen (n = 2–5) supported C2N as single-cluster catalysts (SCCs) for the non-oxidative propane dehydrogenation in the optimization of catalytic performance

“…In addition, the low energy barrier of the H 2 formation on Fe 2 -C 2 N indicates that H 2 can be desorbed from the catalyst in time, which is conducive to the occurrence of propane dehydrogenation reaction. 64,65 Overall, Fe 2 -C 2 N exhibits an excellent catalytic performance and high propylene selectivity in the PDH reaction process, and is expected to become a potential catalyst for non-oxidative PDH reactions.…”