Operando Spectroscopy of Catalysts Exploiting Multi-technique and Modulated Excitation Approaches.

Operando spectroscopy combines the in situ determination of material structure by spectroscopy/diffraction techniques with the measurement of material performance, which is conversion/selectivity in the field of heterogeneous catalysis. A central question in operando spectroscopy is whether the signatures visible by the characterization methods are responsible for catalyst performance. Individual analytical methods can provide useful information, but their combination (multi-technique approach) is essential to obtain a complete perspective on molecular reaction mechanisms. This approach must be coupled to experimental protocols and mathematical algorithms enabling the ability to disentangle the contribution of the active structure from the unresponsive one. Here, we report an account with examples from our own research activities in catalysis science.

Surface species in direct liquid phase synthesis of dimethyl carbonate from methanol and CO2: an MCR-ALS augmented ATR-IR study.

The reaction mechanism of dimethyl carbonate (DMC) production over ZrO2 from CO2 and CH3OH is well-known, but the level of understanding has not improved in the last decade. Most commonly, the reaction mechanism has been explored in the gas phase, whilst DMC production occurs in the liquid phase. To overcome this contradiction, we exploited in situ ATR-IR spectroscopy to study DMC formation over ZrO2 in the liquid phase. A multiple curve resolution-alternate least square (MCR-ALS) approach was applied to spectra collected during the CO2/CH3OH interaction with the catalyst surface, leading to the identification of five pure components with their respective concentration profiles. CO2 and CH3OH activation to carbonates and methoxide species was found to strongly depend on the reaction temperature. Low temperature prevents methanol dissociation leaving a catalyst covered with stable carbonates, whilst higher temperature decreases the stability of the carbonates and enhances the formation of methoxides. A reaction path involving the methoxide/carbonate interaction at the surface was observed at low temperature (≤50 °C). We propose that a different reaction path, independent of carbonate formation and involving the direct CO2/methoxide interplay, occurs at 70 °C.

Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments.

In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VOx) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VOx species appeared very clear at 30-100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ/operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.

Interconversion between Lewis and Brønsted-Lowry acid sites on vanadia-based catalysts.

Lewis acid sites (LAS) and Brønsted-Lowry acid sites (BAS) play key roles in many catalytic processes, particularly in the selective catalytic reduction (SCR) of nitrogen oxides with ammonia. Here we show that temperature, gas feed, and catalyst composition affect the interplay between LAS and BAS on vanadia-based materials under SCR-relevant conditions. While different LAS typically manifest as a single collective peak in the steady-state spectra, their individual signals could be isolated through the increased sensitivity of transient experimentation. Furthermore, water could increase BAS not just by converting pre-existing LAS, but also by generating spontaneously new acid sites. These results pave the way for understanding the relationship between LAS and BAS, and how their ratio determines the reactivity of vanadia-based catalysts not just in SCR but in other chemical transformations as well.

Fluorescence-detected quick-scanning X-ray absorption spectroscopy.

Time-resolved X-ray absorption spectroscopy (XAS) offers the possibility to monitor the state of materials during chemical reactions. While this technique has been established for transmission measurements for a number of years, XAS measurements in fluorescence mode are challenging because of limitations in signal collection as well as detectors. Nevertheless, measurements in fluorescence mode are often the only option to study complex materials containing heavy matrices or in samples where the element of interest is in low concentration. Here, it has been demonstrated that high-quality quick-scanning full extended X-ray absorption fine-structure data can be readily obtained with sub-second time resolution in fluorescence mode, even for highly diluted samples. It has also been demonstrated that in challenging samples, where transmission measurements are not feasible, quick fluorescence can yield significant insight in reaction kinetics. By studying the fast high-temperature oxidation of a reduced LaFe0.8Ni0.8O3 perovskite type, an example where the perovskite matrix elements prevent measurements in fluorescence, it is shown that it is now possible to follow the state of Ni in situ at a 3 s time resolution.

CuO/La0.5Sr0.5CoO3: precursor of efficient NO reduction catalyst studied by operando high energy X-ray diffraction under three-way catalytic conditions.

Substitution of critical raw materials such as platinum group metals in automotive catalysts is challenging. In this work we prepared a nanocomposite in which CuO nanoparticles are highly dispersed on a La0.5Sr0.5CoO3 perovskite-type oxide. The behaviour and reactivity under three way catalyst conditions was monitored by operando time-resolved high-energy X-ray diffraction under oscillating rich/lean feed. The reducing environment converted CuO into Cu(0) in a two step process: Cu(ii) to Cu(i) and to Cu(0), while the perovskite evolved to an oxygen deficient brownmillerite phase. These structural transformations are shown to be crucial for catalytic activity. The in situ generated Cu(0)/Cu(i)/brownmillerite nanocomposite is active for NO reduction above 300 °C, reaching 90% NO conversion at 450 °C. The effect of feed composition on the diffraction patterns was studied by Rietveld refinement in order to rationalize the experimental observations under TWC conditions.

Energy Conversion Processes with Perovskite-type Materials.

Mixed oxides derived from the perovskite structure by combination of A- and B-site elements and by partial substitution of oxygen provide an immense playground of physico-chemical properties. Here, we give an account of our own research conducted at the Paul Scherrer Institute on perovskite-type oxides and oxynitrides used in electrochemical, photo(electro)chemical and catalytic processes aimed at facing energy relevant issues.

Structural Reversibility of LaCo1-x Cux O3 Followed by In Situ X-ray Diffraction and Absorption Spectroscopy.

Combinations of perovskite-type oxides with transition and precious metals exhibit a remarkable self-regenerable property that could be exploited for numerous practical applications. The objective of the present work was to study the reversibility of structural changes of perovskite-type oxides under cyclic reducing/oxidizing atmosphere by taking advantage of the reducibility of LaCoO3 . LaCoO3±Î´ and LaCo0.8 Cu0.2 O3±Î´ were prepared by ultrasonic spray combustion and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed that copper adopted the coordination environment of cobalt at the B-site of the rhombohedral LaCoO3 under the selected synthesis conditions. The structural evolution under reducing atmosphere was studied by in situ XRD and XANES supporting the assignment of the observed structural changes to the reduction of the perovskite-type oxide from ABB'O3 (B'=Cu) to B'0 /ABO3 and to B'0 B0 /A2 O3 . Successive redox cycles allowed the observation of a nearly complete reversibility of the perovskite phase, i. e. copper was able to revert into LaCoO3 upon oxidation. The reversible reduction/segregation of copper and incorporation at the B-site of the perovskite-type oxides could be used in chemical processes where the material can be functionalized by segregation of Cu and protected against irreversible structural changes upon re-oxidation.

Mitigation of Secondary Organic Aerosol Formation from Log Wood Burning Emissions by Catalytic Removal of Aromatic Hydrocarbons.

Log wood burning is a significant source of volatile organic compounds including aromatic hydrocarbons (ArHC). ArHC are harmful, are reactive in the ambient atmosphere, and are important secondary organic aerosol (SOA) precursors. Consequently, SOA represents a major fraction of the sub-micron organic aerosol pollution from log wood burning. ArHC reduction is thus critical in the mitigation of adverse health and environmental effects of log wood burning. In this study, two Pt-based catalytic converters were prepared and tested for the mitigation of real-world log wood burning emissions, including ArHC and SOA formation, as well as toxic carbon monoxide and methane, a greenhouse gas. Substantial removal of mono- and polycyclic ArHC and phenolic compounds was achieved with both catalysts operated at realistic chimney temperatures (50% conversion was achieved at 200 and 300 °C for non-methane hydrocarbons in our experiments for Pt/Al2O3 and Pt/CeO2-Al2O3, respectively). The catalytically cleaned emissions exhibited a substantially reduced SOA formation already at temperatures as low as 185-310 °C. This reduces the sub-micron PM burden of log wood burning significantly. Thus, catalytic converters can effectively reduce primary and secondary log wood burning pollutants and, thereby, their adverse health impacts and environmental effects.

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment.

The combination of spectroscopic and diffraction methods to study chemical transformations is fundamental for the understanding of reaction mechanisms. The identification of short-lived species, likely active species, is often hindered by the contribution of spectator species not directly involved in the reaction. The present study considers two different approaches to obtain increased sensitivity to transient species for experiments obeying the modulated excitation paradigm and exploiting phase sensitive detection (PSD). First, the variation of the frequency of the external stimulation (ω) during the experiment is considered. We demonstrate using the Fourier analysis that the increase of ω, i.e., the decrease of the modulation period T, enhances the sensitivity to short-lived species. The second alternative is the use of a single stimulation frequency (ω) during the measurement and the variation of the demodulation frequency (nω) during data analysis. The absolute intensity of the phase-resolved signals is reduced by increasing n. However, species with slow kinetics are more attenuated than species with fast kinetics. Thus, transient species possessing fast kinetics are enhanced relative to other components and can be, in principle, discerned with improved sensitivity in the phase-resolved data obtained with n > 1. Experimental results in the field of heterogeneous catalysis are provided that support our findings.

Genesis of a Co-Salicylaldimine Complex on Silica Followed in Situ by FTIR and XAS.

Several strategies have been proposed to replace soluble metallorganic complexes in organic solvents by similar molecular entities immobilized on non-reactive solids. The characterization of these complexes at atomic and molecular level during synthesis is demanding but essential to guide rational design. In the present work, the formation of cobalt salicylaldimine complex on γ-aminopropyl modified silica (SiO2 ) was monitored in ethanol on-line by Fourier transform infrared spectroscopy (FTIR) and in situ X-ray absorption spectroscopy (XAS) simultaneously using two independent cells. The organic ligand was monitored by FTIR to follow the stepwise synthesis of the Co-salicylaldimine complex. The oxidation state of Co, obtained by XANES, was found to be +2, while different coordination environments were observed in the presence or absence of the pendant organic ligand produced in situ on SiO2 . EXAFS analysis inferred that the oxidation state and the local structure of the Co2+ ion on the modified SiO2 surface was similar to that of a salen-complex with four Co-O/N bonds.

Kinetic Studies of the Pt Carbonate-Mediated, Room-Temperature Oxidation of Carbon Monoxide by Oxygen over Pt/Al2O3 Using Combined, Time-Resolved XAFS, DRIFTS, and Mass Spectrometry.

The kinetics involved in a recently revealed ambient-temperature mechanism for the catalytic oxidation of carbon monoxide by oxygen over a 5 wt % Pt/Al2O3 catalyst are evaluated within a periodic, plug flow, redox operation paradigm using combined mass spectrometry (MS), diffuse reflectance infrared spectroscopy (DRIFTS), and time-resolved Pt L3-edge XAFS. The species that are the most active at room temperature are shown to be a high-wavenumber (ca. 1690 cm-1) carbonate that we associate directly with a room-temperature redox process occurring in a fraction of the Pt atoms present in the catalyst. Our results, however, do not exclude the participation of carbonate species native to the Al2O3 support, though these species tend to store CO at ambient temperature and become significant participants in CO oxidation catalysis only at slightly higher temperatures (323-333 K). Pt carbonate formation (1690 cm-1) under CO and the reaction to yield CO2 is shown to be extremely rapid and subject to an average apparent activation energy (Eapp), across the techniques applied, of 8.7 kJ mol-1, within the temperature range investigated (276-343 K). Reoxidation of Pt (XANES) and subsequent CO2 production mediated by Pt carbonates under O2 (MS/IR) displays a first-order dependence upon O2 partial pressure and a negative dependence upon the coverage of CO adsorbed on the Pt nanoparticles also present in this catalyst. This oxidative regeneration/CO2 production step is subject to an apparent activation energy (Eapp) of 56.5 (±5) kJ mol-1, is kinetically limited by the desorption of molecular CO from Pt nanoparticles, and also is shown to be dependent upon the partial pressure of O2 present in the oxidizing half of the cycle that we associate with the direct interaction of O2 with molecular CO adsorbed on the nanoparticles that promotes their desorption. Finally, a minority reactive state producing CO2 in the oxidizing cycle that displays no dependence upon the CO coverage of the nanoparticles can be induced through simple thermal treatment of the catalyst. These results are discussed in terms of the number and types of species present within the reactive system and in terms of the wider possibilities for the development of effective low-temperature CO oxidation using Pt/Al2O3 catalysts.

An operando emission spectroscopy study of Pt/Al2O3 and Pt/CeO2/Al2O3.

In situ time-resolved spectroscopic examination of catalysts based on well dispersed nanoparticles on metal oxides under transient conditions significantly facilitates the elucidation of reaction mechanisms. In this contribution, we demonstrate the level of structural information that can be obtained using high-energy resolution off-resonant spectroscopy (HEROS) to study 1.3 wt% Pt/Al2O3 and 1.3 wt% Pt/20 wt% CeO2/Al2O3 catalysts subjected to redox pulsing. First, HEROS is compared with XANES in a temperature programmed reduction experiment to demonstrate the increased sensitivity and time resolution of HEROS. Second, modulation excitation spectroscopy is exploited by redox pulsing to enhance the sensitivity of HEROS to structural changes by the application of phase sensitive detection (PSD) to the time-resolved HEROS data set. The HEROS measurements were complemented by resonant X-ray emission (RXES) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements performed under identical conditions and in a single reactor cell in order to probe different aspects of the catalyst materials under the selected experimental conditions.

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium-Based Catalysts.

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy Elucidate the CO2 Promotion on a Commercial Methanol Synthesis Catalyst.

Optimal amounts of CO2 are added to syngas to boost the methanol synthesis rate on Cu-ZnO-Al2 O3 in the industrial process. The reason for CO2 promotion is not sufficiently understood at the particle level due to the catalyst complexity and the high demands of characterization under true reaction conditions. Herein, we applied operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy on a commercial catalyst to gain insights into its morphology and surface chemistry. These studies unveiled that Cu and ZnO agglomerate and ZnO particles flatten under CO/H2 and/or CO2 /H2 . Under the optimal CO/CO2 /H2 mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H2 O byproduct, enhancing the water-gas shift activity and thus the methanol production. Our results provide a rationale to the CO2 promotion emphasizing the importance of advanced analytical methods to establish structure-performance relations in heterogeneous catalysis.

Modulated excitation extended X-ray absorption fine structure spectroscopy.

The sensitivity of extended X-ray absorption fine structure spectroscopy (EXAFS) for minute structural changes can be enhanced by combination with the modulated excitation approach and making use of phase sensitive analysis. A modulated EXAFS experiment of a reversible periodic Pd to PdO partial oxidation has been simulated in order to understand the effect of the phase sensitive analysis on the shape and meaning of the resulting phase-resolved EXAFS spectra. In particular, the simulation comprises either a synchronous or a delayed sinusoidal variation of the EXAFS parameters, i.e. coordination number (N), interatomic distance (R) and Debye-Waller factor (σ(2)), of first Pd-Pd, first Pd-O, and second Pd-(O)-Pd coordination shells. The effect of these variations on the resulting phase-resolved Fourier transform EXAFS spectra is discussed. The results of the simulation are validated by an in situ EXAFS experiment at the Pd K-edge over 1.6 wt% Pd/Al2O3 undergoing reversible partial oxidation in a H2vs. O2 modulation at 573 K. It is shown that phase sensitive detection (PSD) is able to separate the minor contribution at ca. 2.8 Å corresponding to the growth of the Pd-(O)-Pd shell that is otherwise hidden under the static signal of the Pd-Pd shell of reduced Pd particles. The fitting of the phase-resolved EXAFS spectra suggests that the fast H2 to O2 switch leads the partial oxidation of the Pd surface with the formation of a PdO shell covering a metallic Pd core. Therefore, the dynamics of the full system can be described with greater detail than in conventional EXAFS. The intention of this work is to provide the tools and therefore a solid guidance to qualitatively and quantitatively understand the nature of the shape of phase-resolved FT-EXAFS spectra that may prove helpful in the analysis of a wide range of functional materials.

WO3/CeO2/TiO2 Catalysts for Selective Catalytic Reduction of NO(x) by NH3: Effect of the Synthesis Method.

WO3/CeO2/TiO2, CeO2/TiO2 and WO3/TiO2 catalysts were prepared by wet impregnation. CeO2/TiO2 and WO3/TiO2 showed activity towards the selective catalytic reduction (SCR) of NO(x) by NH3, which was significantly improved by subsequent impregnation of CeO/TiO2 with WO3. Catalytic performance, NH3 oxidation and NH3 temperature programmed desorption of wet-impregnated WO3/CeO2/TiO2 were compared to those of a flame-made counterpart. The flame-made catalyst exhibits a peculiar arrangement of W-Ce-Ti-oxides that makes it very active for NH3-SCR. Catalysts prepared by wet impregnation with the aim to mimic the structure of the flame-made catalyst were not able to fully reproduce its activity. The differences in the catalytic performance between the investigated catalysts were related to their structural properties and the different interaction of the catalyst components.

Revealing the dynamic structure of complex solid catalysts using modulated excitation X-ray diffraction.

X-ray diffraction (XRD) is typically silent towards information on low loadings of precious metals on solid catalysts because of their finely dispersed nature. When combined with a concentration modulation approach, time-resolved high-energy XRD is able to provide the detailed redox dynamics of palladium nanoparticles with a diameter of 2 nm in 2 wt % Pd/CZ (CZ = ceria-zirconia), which is a difficult sample for extended X-ray absorption fine structure (EXAFS) measurements because of the cerium component. The temporal evolution of the Pd(111) and Ce(111) reflections together with surface information from synchronous diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements reveals that Ce maintains Pd oxidized in the CO pulse, whereas reduction is detected at the beginning of the O2 pulse. Oxygen is likely transferred from Pd to Ce(3+) before the onset of Pd re-oxidation. In this context, adsorbed carbonates appear to be the rate-limiting species for re-oxidation.

Machine Learning for Quantitative Structural Information from Infrared Spectra: The Case of Palladium Hydride.

Infrared spectroscopy (IR) is a widely used technique enabling to identify specific functional groups in the molecule of interest based on their characteristic vibrational modes or the presence of a specific adsorption site based on the characteristic vibrational mode of an adsorbed probe molecule. The interpretation of an IR spectrum is generally carried out within a fingerprint paradigm by comparing the observed spectral features with the features of known references or theoretical calculations. This work demonstrates a method for extracting quantitative structural information beyond this approach by application of machine learning (ML) algorithms. Taking palladium hydride formation as an example, Pd-H pressure-composition isotherms are reconstructed using IR data collected in situ in diffuse reflectance using CO molecule as a probe. To the best of the knowledge, this is the first example of the determination of continuous structural descriptors (such as interatomic distance and stoichiometric coefficient) from the fine structure of vibrational spectra, which opens new possibilities of using IR spectra for structural analysis.