On the surface of copper, carbon dioxide molecules can stick and break up into carbon monoxide and oxygen. This is the first step in converting this greenhouse gas into other useful chemicals. Researchers have used the HIPPIE experiment station of MAX IV to study which properties of the copper surface makes the reaction most efficient.
One way of meeting the challenge of reducing the carbon dioxide emission to the atmosphere is to convert the greenhouse gas into other useful chemicals. However, carbon dioxide is a stubborn molecule that does not readily react to form other compounds. The first step is, therefore, to break it into carbon monoxide and oxygen. The copper surface can act as a catalyst for the reaction which means that once stuck to the surface the molecule more easily breaks into smaller parts. However, this only happens at specific places on the surface, so-called active sites. The researchers, therefore, try to understand what the active sites look like and how to prepare the surface to have as many active sites as possible.
A piece of metal can be cut in many directions, and the resulting surface will have a different number of active sites. For example, cutting along specific directions makes for a surface with many atomic-scale steps. In the experiments, the researchers used a curved surface which means that they can study many equivalents of the surfaces that would arise if cutting straight through the metal, at the same time. It also makes the experiment conditions closer to the real conditions, such as in a nanoparticle where many different surfaces would exist simultaneously. The method used for the experiments are X-ray Photoemission Spectroscopy (XPS).
“We got promising results that point to higher CO2 dissociation rate at copper surfaces with a higher density of active sites. Now we need to disentangle the variety of surface chemical species, particularly oxides, that are detected with XPS, and their dependence on the surface plane and therefore active site density,” says Enrique Ortega, one of the members in the team with researchers from Lund University and Universidad del País Vasco in San Sebastian, Spain.
Surface science experiments in the soft X-ray regime usually take place under ultrahigh vacuum, partly because the photoelectrons generated in the experiment do not travel very far in higher pressures. However, in real life, this catalytic surface would interact with gasses under pressures closer to atmospheric pressure. The dynamics of the surface under these pressures are significantly different. With the help of clever engineering, XPS at higher pressures is possible, and at the HIPPIE beamline setup, the gas pressure can be up to 30 millibars. When the pressures are higher the method is called Ambient Pressure XPS (AP-XPS).
“We have used the Ambient Pressure X-ray Photoemission (AP-XPS) system at the HIPPIE beamline. This setup features a sophisticated gas-dosing system coupled to an in-vacuum reactor, which is state-of-the-art in AP-XPS, a recently developed technique that is revolutionizing research in surface chemistry and heterogeneous gas-surface catalysis,” says Ortega.