The effect of surface steps and oxides on the catalytic activity on model Pd and Rh catalysts

Abstract: A catalyst is a substance that can speed up the rate of a chemical reaction without itself being consumed. Catalysts are crucial for chemical production industries, where about 90% of all chemicals are produced using catalysts. They are also used for exhaust gas cleaning, for instance in cars, where CO and unburned fuels are oxidized to CO2 and NOx is reduced to N2. To optimize catalysts, and develop other or better catalysts in the future, we need to know how they work on the atomic level. Industrial catalysts are complicated, consisting of active nanoparticles dispersed in a porous oxide support and working under atmospheric or higher pressures. Therefore, it is challenging to study the surface structure. The surface is crucial because it is the part of the catalyst that interacts with the surrounding gas, and hence is where the catalytic reaction occurs. To be able to study the surface in detail, researchers in the field of surface science study simplified systems, typically perfectly flat single crystals in ultra-high vacuum. This enables very detailed studies but can suffer from being too simplified. The difference in working pressures and complexity between surface science and industrial catalysis are called the pressure and material gaps.In this thesis, I have taken one step to bridge these gaps for CO oxidation over Rh and Pd. More specifically, I have studied the effect of steps, which are always present on the nanoparticles in industrial catalysts, by studying so-called vicinal single crystal surfaces with a well ordered periodic array of steps. Furthermore, I have studied the role of oxides for the catalytic activity.My results show that the presence of steps improves the catalytic activity. It is, however, not the steps as such that are most active, but the flat surface near the steps. Further, the activity of the oxides I have studied depends on their surface structure. If there are sites available for CO to adsorb, the oxide is active, while, for instance, the oxygen terminated Rh oxide shows low activity.These results may be used to optimize the structure and composition of catalysts to expose a perfect density of steps and/or active oxides.