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

Abstract

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 CeO2 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 Ce3+ 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.

Refining the mechanism of CO2 and H2 activation over gold-ceria catalysts by IR modulation excitation spectroscopy

“…2 reveals that, apart from the two gaseous species CO 2 (reactant) and CO (product), there are no features within the region of 2000 to 2200 cm −1 , which is typical for CO adsorbates, that are useful in clarifying reaction mechanisms by means of tracking adsorption sites and oxidation states as has been reported earlier. 11,31–34 On the other hand, further signals in the PSD spectra are located in the carbonate region below 1800 cm −1 , in the C–H stretching region at around 2800 cm −1 , and in the hydroxyl region above 3500 cm −1 . When the view is enlarged, a faint signal at 1953 cm −1 also becomes visible (see Fig.…”

Structure and Chemical Reactivity of Y‐Stabilized ZrO2 Surfaces: Importance for the Water‐Gas Shift Reaction

“…[21] It is widely recognized that in situ/operando DRIFT spectra of CO primarily exhibit intense and broad gas-phase rovibrational signals centered at approximately 2170 cm À 1 (R-branch) and 2120 cm À 1 (P-branch), posing challenges for a reliable observation and identification of CO vibrations on catalyst surfaces. [22] In this work, we effectively eliminated the CO gas-phase contribution using a novel normalization/ subtraction scheme, [23] where DRIFT spectra of both the reference (KBr) and YSZ powders were recorded at a highresolution of 2 cm À 1 (Figure S10). Briefly, the YSZ and KBr spectra were initially normalized, followed by subtracting the KBr reference data from the YSZ data.…”

Polarization-dependent effects in vibrational absorption spectra of 2D finite-size adsorbate islands on dielectric substrates

“…Comparing the simulations with experiments, on the one hand, permits us to identify the microscopic origin of the different responses between s-and p-polarized light at oblique incidence studied in the past. 15,17,21 On the other hand, by comparing the response between the infinite adsorbate layer and the finite islands, we can pinpoint the spatial extent of an adsorbate island necessary to consider it sufficiently representative of the bulk sample.…”

“…18 and 19. Recent developments in infrared spectroscopy have demonstrated the potential of IR spectroscopy, in particular diffuse reflectance IR spectroscopy (DRIFTS) for monitoring chemical processes on oxide powder surfaces, 20 especially for operando studies 21 in the context of heterogeneous catalysis. A crucial aspect is determining whether polarization-dependent shifts observed for adsorbate vibrational frequencies on single crystals are also present on smaller oxide particles, which could impact the interpretation of powder data.…”