Mass-Selected Model Systems in Catalysis: from Nanoparticles to Single Atoms

A quick glimpse into the literature on heterogeneous catalysis research reveals many different approaches to developing novel catalysts. This Ph.D. thesis describes an approach based on applying mass-selected clusters and nanoparticles in model systems, to gain increased fundamental understanding of relevant catalytic processes. The thesis opens with a brief introduction to the exponentially increasing global energy demand leading to the necessary transition to sustainable energy sources, including chemical energy storage. This is followed by a brief introduction to the fundamentals of catalysis and the experimental techniques applied in this work. The subsequent chapters present studies within different topics in catalysis and will therefore be summarized individually:
Platinum Dissolution During the Oxygen Reduction Reaction: One of the major degradation phenomena in a polymer electrolyte membrane fuel cell is dissolution of the platinum based oxygen reduction reaction (ORR) catalyst. Together with other research groups we studied the dissolution of Pt nanoparticles under ORR conditions using scanning flow cell measurements with online inductively coupled plasma mass spectrometry. Mass-selected Pt nanoparticles from 2-10 nm, combined with simulations and an industrially relevant 2 nm Pt catalyst, showed that the dissolution rate is proportional to the edge-to-edge interparticle distances. Furthermore, the mass-selected Pt nanoparticles were used to show that the standard dissolution testing protocols create a misleading volcano shaped particle size effect.
Benchmarking the Hydrogen Evolution Activity of Platinum: Platinum is an excellent catalyst for the hydrogen evolution reaction (HER), but its natural abundance is far from ideal for the necessary terawatt scale applications. Instead many novel HER catalysts based on earth-abundant materials are being reported with misleading claims of platinum-like activity based on the overpotential at 10 mA/cm² geometric current density, η_10mA/cm² . A mass-selected 3.8 nm Pt nanopar-
ticle ”benchmark” catalyst is used to show, that this metric is heavily affected by the amount of catalyst material loaded onto the working electrode. Meanwhile, catalyst loading is rarely reported in literature, leading to erroneous comparison.
The ”benchmark” catalyst displays η_10mA/cm² from 16 mV to 150 mV for catalyst loadings from 5000 ng/cm² to 13ng/cm², measured in a rotating disc electrode setup. The HER activity is reported as η_10mA/cm² , mass activity and intrinsic activity, to allow for proper scientific comparison.
Improving the CO Oxidation Performance of Gold: The lack of stability of Au nanoparticles during CO oxidation is adressed by alloying Au with Ti. This study uses a microreactor to investigate the CO oxidation activity and stability of mass-selected AuTi nanoparticles. Preliminary results show that 2.5 nm AuTi nanoparticles on a TiO_x support are superior in mass-activity compared to Au nanoparticles of similar size. Additionally, AuTi nanoparticles on an SiO_x support display activities similar to Au nanoparticles on TiO_x support.
Developing a Model System for Single Atoms and Clusters: This chapter motivates and presents the on-going development of a model system for single atom and cluster catalysts. The model system is based on anchoring the active species with nitrogen defects in a highly oriented pyrolitic graphite (HOPG) support. Pre-liminary results show, that nitrogen defects of different densities can be created by NH₃ sputtering. XPS and STM characterization indicates that the defect types are similar to those observed in different single atom catalysts reported in literature. Additionally, the ability to measure the electrochemical activity of these model systems is shown.