Probing the H2-Induced Restructuring of Pt Nanoclusters by H2-TPD

Strong response of Pt clusters to the environment and conditions, formation of metastable states, and simple methods to trace the reversible changes

Subnanometric metal particles, the so-called "clusters", are known to be responsive to their surroundings, but the detection of occurring changes, understanding the causes, and predicting the consequences are still extremely difficult for such small particles. Our study was aimed at estimating the potential of adsorption-based methods for these purposes. Using carbon monoxide as a probing molecule, which readily adsorbs on both bare and H-covered Pt surface, we have probed the adsorption properties of highly dispersed Pt/γ-Al₂O₃ samples after treatments under different atmospheres and temperatures (H₂ or inert gas, 25-500 °C). The combined results of CO-chemisorption measurements, CO TPD, CO TPO, H₂-by-CO displacement, and H₂ TPD suggest that the system shuttles between two states: one with oxygen vacancies in the support and the other one with redox-active oxygen near the Pt clusters. These extreme states can be reversibly created and deleted, giving rise to innumerable intermediate structures that differ in the amount, binding strength, and/or reactivity of adsorbed species. Two adsorbates could act cooperatively, resulting in hydrogen spillover onto the support and making the adsorbate-metal-support interactions even more complex. Implications for better understanding the dynamic behavior of oxide-supported clusters and nanoparticles are discussed.

Interfacial Proton Transfer for Hydrogen Evolution at the Sub-Nanometric Platinum/Electrolyte Interface

Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm² and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)-O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.

Suppressing Dehydroisomerization Boosts n-Butane Dehydrogenation with High Butadiene Selectivity

Butadiene (BD) is a critical raw material in chemical industry, which is conventionally produced from naphtha cracking. The fast-growing demand of BD and the limited oil reserve motivate chemists to develop alternative methods for BD production. Shale gas, which mainly consists of light alkanes, has been considered as cheap raw materials to replace oil for BD production via n-butane direct dehydrogenation (n-BDH). However, the quest for highly-efficient catalysts for n-BDH is driven by the current drawback of low BD selectivity. Here, we demonstrate a strategy for boosting the selectivity of BD by suppressing dehydroisomerization, an inevitable step in the conventional n-BDH process which largely reduces the selectivity of BD. Detailed investigations show that the addition of alkali-earth metals (e. g., Mg and Ca) into Pt-Ga₂ O₃ /S10 catalysts increases Pt dispersity, suppresses coke deposition and dehydroisomerization, and thus leads to the significant increase of BD selectivity. The optimized catalyst displays an initial BD selectivity of 34.7 % at a n-butane conversion of 82.1 % at 625 °C, which outperforms the reported catalysts in literatures. This work not only provides efficient catalysts for BD production via n-BDH, but also promotes the researches on catalyst design in heterogeneous catalysis.

On the origin of high-temperature phenomena in Pt/Al2O3

Treatments of Pt/γ-Al₂O₃ with H₂ under harsh conditions have long been known to strongly influence the properties of this important catalytic system, but the true causes of the high-temperature effects still remain unclear. We have performed a more detailed study of this issue, having used H₂-TPD as a sensitive probe of metal-support interactions. The experimental results are in accordance with previous studies and demonstrate strong changes in adsorption and catalytic properties of Pt/γ-Al₂O₃ after high-temperature H₂ treatments, as well as the possibility to reverse the changes, completely or in part, through O₂ and H₂O treatments. Thorough examination has shown that such behaviour is an intrinsic property of Pt/γ-Al₂O₃ and cannot be attributed to impurities or experimental artifacts. Moreover, there is no abrupt transition to a high-temperature state, but the system undergoes smooth and gradual changes upon increasing the H₂-treatment temperature (T_TR), with the changes being already apparent at a T_TR of ∼ 300 °C. The results suggest that hydrogen can generate oxygen vacancies on the surface of the support in close vicinity to the Pt particles, and the system appears under equilibrium to be kinetically driven by temperature and thermodynamically driven by the P_H2/P_H2O ratio or local concentration of surface hydroxyls near Pt particles. The generated vacancies change the properties of contacting particles, and the changes are most pronounced for sub-nanometric Pt clusters and single atoms. Implications of the phenomena for the synthesis, study, and use of Pt/γ-Al₂O₃ and its related nanosystems are discussed.