Bridging the Pressure and Materials Gap in Heterogeneous Catalysis: A Combined UHV, In Situ, and Operando Study Using Infrared Spectroscopy

The interactions of gas molecules with metal oxides used as catalysts or support materials in heterogeneous catalysis are highly intriguing. It is of great importance to gain detailed insight into the complex and often dynamic behavior of oxide particles under operando conditions. In this study, the understanding of CO interactions with cerium oxide surfaces is advanced by bridging the so-called materials and pressure gaps. This is accomplished by studying the influence of different types of materials, pressures, and temperatures by using different infrared spectroscopies as the primary investigation tool. Whereas low-temperature CO adsorption (<80 K) on various well-defined CeO 2 single crystal surfaces yields distinct vibrational bands that can be assigned to different adsorption sites on fully stoichiometric and also on reduced surfaces using validated ab initio calculations, strong gas-phase contributions turn the interpretation of results obtained for powders under operando conditions into a major challenge. By using a combination of UHV-IRRAS, in situ transmission infrared spectroscopy, and operando DRIFTS measurements, the reference data obtained for single-crystal surfaces under UHV conditions could be used to assign the features observed in spectra obtained for powder materials. In the next step, the different CO vibrational bands were used to monitor surface structural changes occurring at elevated pressures and temperatures. An increase in the concentration of Ce 3+ species as a result of CO-induced reduction could be directly demonstrated even at low (300 K) temperatures. Our results demonstrate important progress toward the noninvasive, nondestructive characterization of real catalysts under operando conditions.