Abstract
Solid surfaces are extremely important in heterogeneous catalysis. They represent the boundary between lid catalyst and surrounding gaseous or liquid environment, and heavily influence chemisorption, surface diffusion, reaction and desorption processes involved in the catalytic cycle. All of these key steps critically depend on the surface composition, structure and morphology. In many cases, catalyst surfaces are dynamic, whereby the structure and composition of the surface can change with time under reaction conditions. Surface structural dynamics reflect the chemical interactions between reactants and catalysts. When catalysts are exposed to mixtures of oxidizing and reducing agents, counteracting effects of the reactants can result in fluctuations in the catalyst's chemical state. In the present work, simple model reactions involving hydrogen and oxygen were used to study surface morphological dynamics. These phenomena were studied on copper, platinum, and nickel catalysts.
The morphological dynamics were studied using a modified environmental scanning electron microscope (ESEM) by which snapshots of dynamic processes on the surface can be recorded and visualized in near real time, while composition of gas phase is simultaneously monitored using a quadrupole mass spectrometer. Near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) was utilized to identify the chemical composition of the surface phases formed in the relevant conditions.
In the case of copper, three different surface phases were identified, including metallic copper, oxygen terminated copper and cuprous oxides islands. These phases co-exist on the surface and reversibly transition among the three phases under hydrogen rich conditions (4% oxygen) at 700 °C. While copper has a high oxygen activity and low hydrogen activity, the case of platinum demonstrates such dynamics also exist for a metal that has a low oxygen activity and high hydrogen activity. However, the morphological dynamics observed for platinum are distinctly different. The surface becomes very rough in oxygen rich conditions (85% oxygen) and exhibits highest morphological dynamics at 50% oxygen. No oxide is observed, and the surface dynamics are likely a result of transitions between different surface terminations. Depending on the surface phase present, the interaction with adsorbates changes and acts as a feedback mechanism to change the morphological dynamics. The morphological dynamics are found to correlate with the catalytic activity, where the maximum in H2 consumption is correlated to the maximum rate of morphological change. This observation highlights the relevance of morphological dynamics to catalytic processes.
In the case of nickel, the hydrogen and oxygen activities are intermediate to those of copper and platinum. In hydrogen oxidation conditions, the surface exhibits global morphological oscillations at 11% oxygen concentration synchronizing with those of the gas composition, where the surface changes between metallic and oxide states at different temperatures.
As morphological changes reflect phase changes, direct visualization of the surface morphological variations provides insights into chemical dynamics of the surfaces. For instance, these findings provide insight into how dynamic phase co-existence is influenced by the reactants present, how the reactivity of the phases to the respective reactants influences the presence and dynamics of phase transitions, and how the resulting surfaces can influence catalytic activity. Further instrumental developments will enable more complex reactions to be studied in this way to achieve a comprehensive understanding of dynamics in catalysis.
