Application of Transient Infrared Spectroscopy To Investigate the Role of Gold in Ethanol Gas Sensing over Au/SnO₂

Diffuse reflectance infrared Fourier transform (FT-IR) spectroscopy (DRIFTS) was used in combination with resistance measurements to study the mechanism of Au/SnO2 during ethanol gas sensing and to elucidate the influence of gold on the sensor response. Time-resolved DRIFT spectra during ethanol gas sensing reveal significant differences between Au/SnO2 and bare SnO2 regarding the amount of C–H-containing adsorbates, which are less abundant on Au/SnO2 because of their consumption by the adsorbed oxygen species. Modulation excitation DRIFT spectroscopy (ME-DRIFTS) was applied to Au/SnO2 in comparison to bare SnO2, enabling a distinction of the temporal behavior of different C–H-containing surface adsorbates such as acetate and formate. ME-DRIFTS reveals the presence of a new surface species at 2030–2060 cm–1, not detected for unloaded SnO2 and associated with CO adsorbed on negatively charged gold particles. X-ray photoelectron spectroscopy (XPS) and ultraviolet/visible (UV/vis) spectra confirm the presence of metallic gold, which makes an influence on the electronic properties of the SnO2 sensor material unlikely. Based on our spectroscopic findings, we postulate a detailed ethanol gas-sensing mechanism and attribute the increase in the sensor response to an oxygen spillover from gold to the surface of tin oxide

Structure of Isolated Vanadia and Titania: A Deep UV Raman, UV–Vis, and IR Spectroscopic Study

Dispersed vanadia and titania are of great interest due to their (photo)­catalytic properties. The structure of isolated (highly dispersed) vanadia and titania has been studied using a combination of deep UV Raman, UV–vis, and IR spectroscopy. Highly dispersed vanadia and titania were prepared by incipient wetness impregnation of the corresponding isopropoxide precursors onto silica SBA-15. Using resonance Raman spectroscopy, a wide range of vanadium (0.00001–0.7 V/nm²) and titanium (0.001–0.7 Ti/nm²) loadings can be analyzed. At very low loadings (<0.05 M/nm²) the structure of both highly dispersed vanadia and titania is characterized by tetrahedral coordination of the central atom and anchoring to the support by one or more M–O–S linkages. At higher loadings, titania partly forms oligomeric surface structures according to UV–vis results. In the case of highly dispersed vanadia samples (<0.1 V/nm²), Raman spectroscopy reveals distinct differences in the vanadia surface structures under ambient and dehydrated conditions. Corresponding UV–vis spectra indicate the formation of isolated and oligomeric vanadia surface structures. A smaller dependence of the titania surface structure on the environmental conditions (ambient/dehydrated) was observed. For the 0.7 M/nm² samples, the presence of hydroxylated vanadia and titania structures in the dehydrated state was verified by IR spectroscopy. The assignment of the vibrational bands was facilitated by the results of a normal-mode analysis using models based on polyhedral oligomeric silsesquioxane (POSS). Our study clearly demonstrates the potential of resonance Raman spectroscopy using 217.5 nm deep UV excitation to study the surface structure of transition metal oxide species even in isolated conditions

DataSheet1_Operando spectroelectrochemistry of bulk-exfoliated 2D SnS2 for anodes within alkali metal ion batteries reveals unusual tin (III) states.docx

In this study we report an affordable synthesis and preparation of an electrochemically exfoliated few-layer 2-dimensional (2D) SnS2 anode material of high cycling durability and demonstrate its performance on the example of alkali metal batteries. The metalation mechanism consists of highly unusual and previously only speculated Sn (III)-state grasped by operando Raman spectroelectrochemistry aided by symmetry analysis. The prepared 2D material flakes were characterized by high resolution transmission electron microscopy, X-ray photoelectron and Raman spectroscopies. The operando Raman spectroelectrochemistry was chosen as a dedicated tool for the investigation of alkali-metal-ion intercalation (Li, Na, K), whereby the distortion of the A1g Raman active mode (out-of-plane S-Sn-S vibration) during battery charging exhibited a substantial dependence on the electrochemically applied potential. As a result of the structural dynamics a considerable Raman red-shift of 17.6 cm−1 was observed during metalation. Linewidth changes were used to evaluate the expansion caused by metalation, which in case of sodium and potassium were found to be minimal compared to lithium. Based on the spectroscopic and electrochemical results, a mechanism for the de-/intercalation of lithium, sodium and potassium is proposed which includes alloying in few-layer 2D SnS2 materials and the generation of point-defects.</p

Unraveling Mechanistic Differences in Optical and Electrical Sensors: Time-Resolved <i>Operando</i> UV–Vis Spectroscopy of p‑Type Perovskite Gas Sensors

To gain a deeper understanding of p-type chemiresistive and gasochromic sensor materials, LaFeO3 and SmFeO3 were investigated by using time-resolved operando UV–vis spectroscopy. A qualitative match in the electrical conductance and optical absorbance below the band edge was observed. The two properties are connected by changes in the electron hole concentration caused by the sensor surface reaction with ethanol, leading to a decrease in both conductance and free carrier absorption. Quantitative differences between the conductance and free carrier absorption behavior are explained based on surface effects caused by the surface band bending and surface adsorbates, which lead to significant changes in conductance but are only of minor importance for changes in the free carrier absorption. Our results demonstrate the potential of time-resolved operando UV–vis spectroscopy to develop a detailed mechanistic understanding of chemiresistive and gasochromic sensors by discriminating between surface and bulk effects and to combine their use for future gas sensor development

Oligomerization of Supported Vanadia: Structural Insight Using Surface-Science Models with Chemical Complexity

In light of the ongoing debate on the structure of supported vanadia, we report on a spectral marker enabling the direct identification of oligomeric surface structures. A series of VO_<i>x</i>/SiO₂/Si­(100) planar samples with chemical complexity was synthesized by spin-coating and investigated in detail by UV resonance Raman spectroscopy at 256.7 nm excitation as well as by X-ray photoelectron spectroscopy. The enhanced Raman sensitivity allows vibrational spectra to be recorded as a function of vanadium loading (0 ≤ <i>L</i>_V ≤ 20.2 V nm^–2) despite the small surface area of the planar model samples. At low loadings (<i>L</i>_V < 7.3 V nm^–2) the spectra are dominated by dispersed vanadia species, whereas at higher loadings the presence of crystalline V₂O₅ is also observed. We identify new spectral features at 492, 562, and 676 cm^–1, which are attributed to V–O–V-related modes of oligomeric vanadia surface species. The vanadia surface species show a linear increase with vanadium loading, saturating at a loading of <i>L</i>_V = 7.3 V nm^–2, at which V₂O₅ crystallite formation is observed to increase significantly. Our spectroscopic results are consistent with a growth model that includes oligomerization of vanadia surface species to increase the packing density, thereby reducing the number of V–O–Si linkages to the support

Unraveling the Active Vanadium Sites and Adsorbate Dynamics in VO_<i>x</i>/CeO₂ Oxidation Catalysts Using Transient IR Spectroscopy

The oxidative dehydrogenation (ODH) of propane over supported vanadia catalysts is an attractive route toward propene (propylene) with the potential of industrial application and has been extensively studied over decades. Despite numerous mechanistic studies, the active vanadyl site of the reaction has not been elucidated. In this work, we unravel the ODH reaction mechanism, including the nuclearity-dependent vanadyl and surface dynamics, over ceria-supported vanadia (VOx/CeO2) catalysts by applying (isotopic) modulation excitation IR spectroscopy supported by operando Raman and UV–vis spectroscopies. Based on our loading-dependent analysis, we were able to identify two different mechanisms leading to propylene, which are characterized by isopropyl- and acrylate-like intermediates. The modulation excitation IR approach also allows for the determination of the time evolution of the vanadia, hydroxyl, and adsorbate dynamics, underlining the intimate interplay between the surface vanadia species and the ceria support. Our results highlight the potential of transient IR spectroscopy to provide a detailed understanding of reaction mechanisms in oxidation catalysis and the dynamics of surface catalytic processes in general

Ceria and Its Defect Structure: New Insights from a Combined Spectroscopic Approach

Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different commercial ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N₂ adsorption–desorption, and optical spectroscopy (Raman, DRIFTS, UV–vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV–vis). The Raman feature at around 250 cm^–1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV–vis band gap values and the presence of Raman bulk defects was established based on the observed decrease of the band gap energy with increasing number of defects. Detailed Raman analysis revealed that the frequently mentioned linear equation for the determination of the crystal size from the half-width of the <i>F</i>_2g Raman feature is erroneous, since the <i>F</i>_2g half-width depends on ceria bulk defects. Apart from these universal observations, differences in the properties depending on synthesis conditions were observed. In particular, it is shown that the type and quantity of ceria defects are influenced not only by crystal size but also by the preparation method

Direct Evidence for Active Support Participation in Oxide Catalysis: Multiple <i>Operando</i> Spectroscopy of VO_<i>x</i>/Ceria

Ceria-supported vanadia is catalytically active in oxidative dehydrogenation (ODH) reactions. Here, we provide direct spectroscopic evidence for the participation of the ceria support in the redox catalysis. To unravel the structural dynamics of vanadia/ceria (VOx/CeO2) catalysts during ethanol ODH, we have applied a combination of operando multiwavelength Raman and operando UV–vis spectroscopy. Our approach consists of the targeted use of different Raman excitation wavelengths, enabling the selective enhancement of ceria (at 385 nm) and vanadia (at 515 nm) vibrational features. As part of the support dynamics, ceria lattice oxygen is shown to directly participate in the ODH reaction, while V–O–Ce interface bonds are broken during substrate adsorption, resulting in ethoxide formation. The presence of V–O–Ce bonds is considered to be crucial for the observed synergy effect in catalytic performance, allowing ceria to act as an oxygen buffer stabilizing the vanadium center. By providing an experimental basis for a detailed understanding of working VOx/CeO2 catalysts, our results highlight the importance of active support participation in oxide catalysis

Approaching C1 Reaction Mechanisms Using Combined <i>Operando</i> and Transient Analysis: A Case Study on Cu/CeO₂ Catalysts during the LT-Water–Gas Shift Reaction

The elucidation of reaction mechanisms is an essential part of catalysis research, providing approaches to improve catalysts or, ultimately, to design catalysts based on a profound understanding of their mode of operation. In the context of C1 processes, redox and/or associative mechanisms have been proposed in the literature, but their critical assessment has been a major challenge. Here, we highlight the importance of applying a combination of techniques suited to address both the redox properties and intermediate/adsorbate dynamics in a targeted manner. We illustrate our approach by exploring the mechanism of LT-WGS over low-loaded Cu/CeO2 catalysts using different ceria morphologies (sheets, polyhedra, cubes, and rods) to study the influence of the surface termination. While the results from operando Raman and UV–vis spectroscopy are consistent with a redox mechanism, there is no direct correlation of activity with reducibility. Probing the subsurface/bulk oxygen dynamics using operando Raman F2g analysis coupled with H218O highlights the importance of transport properties and the availability of oxygen at the surface. Transient IR spectra reveal the presence of different surface carbonates, none of which are directly involved in the reaction but rather act as spectator species, blocking active sites, as supported by the facet-dependent analysis. From transient IR spectroscopy there is no indication of the involvement of copper, suggesting that the catalytic effect of copper is mainly based on electronic effects. The results from the operando and transient analysis unequivocally support a redox mechanism for LT-WGS over Cu/CeO2 catalysts and demonstrate the potential of our combined spectroscopic approach to distinguish between redox and associative mechanisms in oxide-supported metal catalysts

Rational Design of Mesoporous CuO–CeO₂ Catalysts for NH₃‑SCR Applications Guided by Multiple <i>In Situ</i> Spectroscopies

Efficient nontoxic catalysts for low-temperature NH3 selective catalytic reduction (NH3-SCR) applications are of great interest. Owing to their promising redox and low-temperature activity, we prepared CuO–CeO2 catalysts on a mesoporous SBA-15 support using targeted solid-state impregnation (SSI), guided by multiple in situ spectroscopy. The use of template P123 allowed dedicated modification of the surface properties of the SBA-15 matrix, resulting in a changed reactivity behavior of the metal precursors during the calcination process. To unravel the details of the transformation of the precursors to the final catalyst material, we applied in situ diffuse reflectance infrared Fourier transform (DRIFT), UV–visible (UV–vis), and Raman spectroscopies as well as online Fourier transform infrared (FTIR) monitoring of the gas-phase composition, in addition to ex situ surface, porosity, and structural analysis. The in situ analysis reveals two types of nitrate decomposition mechanisms: a nitrate-bridging route leading to the formation of a CuO–CeO2 solid solution with increased low-temperature NH3-SCR activity, and a hydrolysis route, which facilitates the formation of binary oxides CuO + CeO2 showing activity over a broader temperature window peaking at higher temperatures. Our findings demonstrate that a detailed understanding of catalytic performance requires a profound knowledge of the calcination step and that the use of in situ analysis facilitates the rational design of catalytic properties