Gas-surface reaction dynamics and surface science
The local ordering of atoms at the surface of a metallic particle determines its catalytic activity and selectivity. As energy systems of the future will be based on efficient catalytic conversion of small molecules in closed cycles, we study how structural effects of catalysts can be used to our advantage.
- Ludo Juurlink
Real catalyst particles are generally on the order of several nanometers in diameter. As they occur with a distribution of diameters and varying shapes, relating their activity and selectivity to local ordering of surface atoms is difficult. Hence, we use macroscopic single crystal surfaces to relate reactivity and selectivity to the exact ordering of the metal atoms. Our surfaces consist of a single type or at most three types of arrangements of surface atoms.
In our studies of reactivity of small gaseous molecules, we combine surface science techniques with supersonic molecular beam methods. Beams of molecules, e.g. H2 (D2), O2, and CO2, impact with controllable and well-defined kinetic energy onto metal surfaces. We measure the reactivity and its dependence on kinetic energy and other variables. This helps us determine by what mechanism the impacting molecule reflects, adsorbs, or breaks apart, and how we may steer the interaction to the desired outcome. Beyond heterogeneous catalysis, these studies for hydrogen also relate to chemical reactions occurring at the walls inside energy-producing fusion reactors.
We then vary the surface structure using flat single crystals that contain different atomically flat terraces separated by mono-atomic high steps. Comparing results for various surfaces allows us to relate reactivity to the local structure of surface atoms and their positions in atomic steps and terraces. In the newest project, we also gain control over the internal energy of CO2 using state-resolved laser excitation prior to impact onto a catalytically active surface. This new project helps use determine which form of energy is most efficient in reducing CO2 back into useful chemicals.
We develop a new method in studies of structure-activity and structure-selectivity relations by using curved single crystal metal samples. We search for methods to produce samples that are curved due to monoatomic steps. A single sample thus allows us to probe reactivity on a wide range of step-to-terrace ratios. We analyze which curved metal crystal can be used by diffraction techniques and scanning tunnelling microscopy. We subsequently study local reactivity on small parts of these curved surfaces using two molecular beams impacting simultaneously on the same local structure. The gas phase products from the catalytic reaction inform us of reactivity and selectivity dependencies on surface structure. We currently focus our attention on Ni, Ag, Pt and Co samples as they are important to various catalytic processes involved in reactions of small molecules.
In our last project we study how surface structure influences the ordering of water and the reactivity at the metal-water interface. Using both flat and curved metal samples in combination with various surface sensitive techniques, we determine how surface structure governs the ordering of the first layers of water molecules. We also add hydrogen or oxygen atoms to the interface and investigate reactions between water and these adatoms. These studies help us understand electrochemical oxidation and reduction of water, which is considered one of the most important reactions in renewable energy systems.