Hannes Frey

Last Name

Frey

First Name

Hannes

ORCID

0000-0001-7011-3615

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02293 - Catalysis Hub / Catalysis Hub

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Publications 1 - 8 of 8

Sintering behavior of carbon-supported Pt nanoparticles and the effect of surface overcoating
Liu, Qiang; Rzepka, Przemyslaw; Frey, Hannes; et al. (2022)
Materials Today Nano
The significance of supported metal catalysts in heterogeneous catalysis calls for in-depth understanding of metal particle sintering behavior in order to rationalize the design of stable nanocatalysts. Here, we show that the sintering behavior of supported metal nanoparticles (NPs), exemplified by studying the carbon-supported Pt NPs, becomes markedly accelerated when the interparticle distance between neighboring metal NPs is shortened. Pt NPs with close proximity are prone to form aggregates and further coalescence to fewer, larger crystals under high temperature conditions. Enlarging interparticle distance on the support and engineering NP surface with overcoatings are beneficial for the preparation of sinter-resistant catalysts. The former can be achieved through reducing the metal loadings or utilizing suitable synthetic technologies that allow for the homogeneous distribution of supported metal particles. The surface overcoating engineering relied on the carbon nanoshell formed on the Pt particle surface by in situ pyrolysis of an organic polymer. The evolution of carbon nanoshell formation was visualized by in situ transmission electron microscopy measurements. We also show the potentials of surface overcoating for enhancing catalytic performance.
Strong-metal-support-effects in heterogeneous catalysis studied by in situ electron microscopy
Frey, Hannes (2023)
Heterogeneous catalysts are instrumental in the production of chemicals, fuels, and energy transformation, with the dynamic interactions between the catalyst and its environment at their core. Despite its centrality, numerous questions linger regarding the effects of variables such as hydrogen or oxygen pressure, gas composition, and temperature on catalyst configurations. This investigation delves into these complexities, centering on the behavior of platinum supported on titania, a reducible oxide. The role of platinum catalysts in hydrogenation and oxidation reactions is well-documented, with their properties being significantly influenced by their oxide support. The platinum-titania interaction, particularly at elevated temperatures, leads to the emergence of the Strong Metal-Support Interaction (SMSI) state. However, the mechanisms, especially the debated encapsulation process of platinum under oxygen and the state of encapsulation under reactive conditions warrant further scrutiny. It was found that the state of the overlayer varies with the environment. High temperatures and varying atmospheric compositions, specifically the presence of oxygen or hydrogen, can trigger particle encapsulation by the support material. Yet, when exposed to a mixture of both gases, a pronounced particle mobility is observed. In-situ Transmission Electron Microscopy (TEM) provides invaluable insights into these dynamics, emphasizing the importance of crystallographic orientation between particle and support in governing the dynamic behavior of the particle. Chapter 1 investigates Pt nanoparticles on Platinum-Silica Nitrate (Pt-SiNx) and Platinum-Titania (Pt-TiO2) substrates, spotlighting the profound influence of the substrate and the presence or absence of oxygen in the environment on particle growth and stability. Chapter 2 explores the behavior of nanoparticles on a titanium dioxide (TiO2) substrate in hydrogen–oxygen redox conditions. This chapter challenges traditional understandings of SMSI states and sheds light on the transformative effects of a redox-active environment. Chapter 3 focuses on catalyst degradation, emphasizing the role of the redox atmosphere and support type in particle migration and sintering. Chapter 4 examines the effects of various (pre)treatment protocols on Pt-TiO2 catalysts, stressing the importance of encapsulation in preventing platinum evaporation under high-temperature oxidative conditions. In conclusion, this research offers comprehensive insights into the behaviors, dynamics, and adaptions of catalytic Pt nanoparticles to their environment, enriching the understanding in the realm of catalysis and material science.
Controlling the Strong Metal-Support Interaction Overlayer Structure in Pt/TiO2 Catalysts Prevents Particle Evaporation
Beck, Arik; Frey, Hannes; Huang, Xing; et al. (2023)
Angewandte Chemie. International Edition
Platinum nanoparticles (NPs) supported by titania exhibit a strong metal-support interaction (SMSI)[1] that can induce overlayer formation and encapsulation of the NP's with a thin layer of support material. This encapsulation modifies the catalyst's properties, such as increasing its chemoselectivity[2] and stabilizing it against sintering.[3] Encapsulation is typically induced during high-temperature reductive activation and can be reversed through oxidative treatments.[1] However, recent findings indicate that the overlayer can be stable in oxygen.[4, 5] Using in situ transmission electron microscopy, we investigated how the overlayer changes with varying conditions. We found that exposure to oxygen below 400 °C caused disorder and removal of the overlayer upon subsequent hydrogen treatment. In contrast, elevating the temperature to 900 °C while maintaining the oxygen atmosphere preserved the overlayer, preventing platinum evaporation when exposed to oxygen. Our findings demonstrate how different treatments can influence the stability of nanoparticles with or without titania overlayers. expanding the concept of SMSI and enabling noble metal catalysts to operate in harsh environments without evaporation associated losses during burn-off cycling.
Dynamic Structural Changes due to Metal-Support Interaction under Reactive Conditions
Frey, Hannes; Beck, Arik; Huang, Xing; et al. (2022)
Microscopy and Microanalysis
Visualizing Structural and Chemical Transformations of an Industrial Cu/ZnO/Al2O3 Pre-catalyst during Activation and CO2 Reduction
Huang, Xing; Beck, Arik; Fedorov, Alexey; et al. (2022)
ChemCatChem
The typical industrial catalyst used for methanol synthesis is a multi-component catalyst based on Cu/ZnO/Al2O3. The synergies between various phases of this catalyst play a vital role in defining the overall catalytic function and performance. To gain insights into the role and interaction between the relevant components and phases, ex situ and in situ transmission electron microscopy (TEM) was deployed to investigate the structures and phases of an industrial Cu/ZnO/Al2O3 in its precursor, activated and reaction states. High structural inhomogeneity in this material is revealed, i. e. the presence of various phases with different morphologies and compositions. It is shown how structural and compositional changes occur during hydrogen treatment and how compositional inhomogeneity in the starting material translates into differences in the local composition of the activated and working catalyst. The formation of defective metallic copper particles (stacking faults and twins) that are in an intimate contact with zinc oxide (poorly crystalline, partially reduced ZnOx and crystalline ZnO), alumina, Zn-Al oxide and carbon-zinc-oxygen containing phases (such as zinc formate) is observed. It is also uncovered that alumina plays a potentially important role in stabilizing cationic zinc species. This work provides atomic-level insight into the relevant state of an industrial methanol synthesis catalyst and the associated synergistic interplay between the involved phases in reactive atmosphere.
Dynamic interplay between metal nanoparticles and oxide support under redox conditions
Frey, Hannes; Beck, Arik; Huang, Xing; et al. (2022)
Science
The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)–induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal–oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.
General data management workflow to process tabular data in automated and high-throughput heterogeneous catalysis research
Lam, Erwin; Maury, Tanguy; Preiss, Sebastian; et al. (2025)
Digital Discovery
Data management and processing are crucial steps to implement streamlined and standardized data workflows for automated and high-throughput laboratories. Electronic laboratory notebooks (ELNs) have proven to be effective to manage data in combination with a laboratory information management system (LIMS) to connect data and inventory. However, streamlined data processing does still pose a challenge on an ELN especially with large data. Herein we present a Python library that allows streamlining and automating data management of tabular data generated within a data-driven, automated high-throughput laboratory with a focus on heterogeneous catalysis R&D. This approach speeds up data processing and avoids errors introduced by manual data processing. Through the Python library, raw data from individual instruments related to a project are downloaded from an ELN, merged in a relational database fashion, processed and re-uploaded back to the ELN. Straightforward data merging is especially important, since information stemming from multiple devices needs to be processed together. By providing a configuration file that contains all the data management information, data merging and processing of individual data sources is executed. Having established streamlined data management workflows allows standardization of data handling and contributes to the implementation and use of open research data following Findable, Accessible, Interoperable and Reusable (FAIR) principles in the field of heterogeneous catalysis.
Al-anchored platinum catalysts on reducible supports: High-temperature stability against redispersion during methane oxidation
Becker, Matthias; Frey, Hannes; Batool, Syeda Rabia; et al. (2025)
Applied Catalysis B: Environmental
The deactivation of noble metal catalysts due to the loss of active surface area, remains a significant challenge, especially under high-temperature oxidizing conditions. Such demanding conditions are often found in emission control systems for clean air application and during catalyst regeneration targeting to prolong the lifetime of a catalyst reducing the environmental costs associated to the process. The development of highly active catalysts with enhanced stability demands the synthesis of novel catalyst structures. However, novel catalyst formulations must rely on abundant additives to remain cost competitive and to find industrial application. Herein, we systematically explore the impact of doping support oxide surfaces (silica and titania) with AlOx and TiOx species on the activity and stability of platinum catalysts during complete methane oxidation. Conventional supports were compared with surface-modified silica supports with dispersed TiOx or AlOx species, prepared by facile wet chemistry-based surface modification (Pt/Ti-doped SiO2, Pt/Al-doped SiO2). While Pt/TiO2 demonstrated high initial activity, it suffered severe deactivation due to platinum redispersion. Conversely, Pt/Ti-doped SiO2 and Pt/Al-doped SiO2 showed less activity than Pt/TiO2 but enhanced stability and higher long-term activity. Consequently, AlOx-doping of the rate-promoting titania surface yielded both high activity and stability, demonstrating that AlOx surface species prevent PtOx from migrating into the titania pocket sites that stabilize mono-dispersed platinum species. Additionally, AlOx-doping prevented the partial anatase-rutile phase transformation of the TiO2, which was observed for the unmodified Pt/TiO2 catalyst. Combining the strong binding properties of alumina with the catalytic advantages of reducible oxides is a novel strategy for designing robust, high-temperature catalysts.

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Hannes Frey

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