At the Karlsruhe Institute of Technology (KIT), scientists have studied cerium dioxide nanoparticles using probe molecules and a sophisticated ultra-high vacuum infrared measurement system and gained new insights into their surface structure and chemical activity. Their work is reported in three papers in the journal Angewandte Chemie.
Cerium oxide, oxygen compounds and the rare earth metal cerium are the most important oxides for technical applications. Cerium dioxide is mainly used in heterogeneous catalysis, such as exhaust gas catalytic converters in passenger cars, photocatalysis in solar cells, water decomposition or decomposition of pollutants. Cerium dioxide used in catalytic converters is in powder form. It consists of nanoscale particles with a highly complex structure. A special arrangement of metal and oxygen atoms on the surface determines the physical and chemical properties of cerium dioxide. However, until now, it has not been possible to accurately analyze the rearrangement and reconstruction processes that occur on the surface of nanoparticles.
Scientists at KIT, Institute of Functional Interfaces (IFG) under the guidance of Prof. ChristofWll have established new methods for studying the chemical properties of oxide surfaces in recent years. They use small molecules such as carbon monoxide (CO), molecular oxygen (O2) or nitrous oxide (N2O) as probe molecules. These molecules attach to the surface of the oxide nanoparticles. The researchers then determined the frequency at which the probe molecules vibrate. “This approach has made significant progress in understanding the surface properties of cerium dioxide nanoparticles,” ChristofWll said.
Together with scientists from KIT’s Institute of Catalytic Research and Technology (IKFT), Humboldt University in Berlin, University of Udine in Italy and The Polytechnic University of Barcelona/Catalonia in Spain, IFG researchers studied the structure and chemical activity of cerium dioxide nanoparticles on various aspects of the surface
The main reason for this progress is that the researchers succeeded in verifying the vibration frequencies of powder measurements by making measurements using precisely defined model substances. They use a unique ultra-high vacuum infrared system. In addition, they used quantum mechanical calculations to allocate previously unknown vibration bands of oxide particles. In this way, they gained new insights into the surface chemistry of cerium dioxide nanoparticles.
The scientists demonstrated that the rod-like cerium dioxide nanoparticles have many surface defects, such as zigzag nanofibers, oxygen vacancies, corners and edges. These irregularities may lead to the high catalytic activity of such nanoparticles. In addition, the researchers found that the photoreactivity of cerium dioxide could be significantly enhanced by the generation of oxygen vacancies, or unoccupied oxygen sites. Another study provided basic information on the locations of oxygen vacancies on various cerium oxides and their correlation with oxygen activation. “Based on the results of our study, we can now systematically further develop and optimize nanoscale cerium oxide catalytic converters and photocatalysts,” said Professor Wall.