From Single Atoms to Clusters: Measuring Catalytic Activity for Energy Conversion

Johannes Novak Hansen

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Developing active, selective and stable catalysts to drive key reactions is essential for a future sustainable energy landscape. Catalysts in the size range of single atoms to clusters show interesting possibilities in this regard. In this thesis, the challenges and opportunities of such small entities, as well as more general issues in measuring catalytic activity, are discussed. The thesis starts out by briefly outlining the climate crisis and motivating the need for catalysts to drive eletrochemical reactions for energy conversion and storage. Subsequently, a brief introduction is given to some fundamentals on catalysis, electrochemistry and the experimental methods used for the work herein. The following chapters present three studies within the field of electrocatalysis:

Oxygen Reduction Reaction (ORR): The two-electron ORR provides a sustainable way to synthesize H2O2. A study of a single atom palladium catalyst is presented, demonstrating high selectivity (80% faradaic efficiency) and activity toward H2O2. Using various characterization techniques and density functional theory (DFT), the geometric effect of the single atom site and the surrounding coordination environment of nitrogen species was rationalized to be the cause of this.

Hydrogen Evolution Reaction (HER): Geometric current densities are often reported for novel HER catalysts, without consideration for the applied loading of the catalyst. To address this issue as well as other common pitfalls, a benchmark study of HER was conducted using 3.8 nm mass-selected Pt nanoparticles and commercial Pt/C. It was demonstrated that platinum has an intrinsic activity at least three orders of magnitude larger than earth-abundant catalyst, however, the true activity of platinum is still underestimated.

Developing a Model System for Small Entities: To gain fundamental insight into small entities, a model system of Pt1-3 on nitrogen-doped highly oriented pyrolytic graphite (HOPG) was investigated. It was shown that the system displays HER activity correlating with the presence of Pt species and that atomic nitrogen-defects can be created in the HOPG lattice by NH3-sputtering and subsequent annealing. Furthermore, the possibilities for further developing the model system and elucidating the active site structure are discussed.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDepartment of Physics, Technical University of Denmark
Number of pages170
Publication statusPublished - 2021

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