Correction: Unravelling potential reaction intermediates during catalytic pyrolysis of polypropylene with microscopy and spectroscopy

[This corrects the article DOI: 10.1039/D3CY01473H.].

Alternative nano-lithographic tools for shell-isolated nanoparticle enhanced Raman spectroscopy substrates

Chemically synthesized metal nanoparticles (MNPs) have been widely used as surface-enhanced Raman spectroscopy (SERS) substrates for monitoring catalytic reactions. In some applications, however, the SERS MNPs, besides being plasmonically active, can also be catalytically active and result in Raman signals from undesired side products. The MNPs are typically insulated with a thin (∼3 nm), in principle pin-hole-free shell to prevent this. This approach, which is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), offers many advantages, such as better thermal and chemical stability of the plasmonic nanoparticle. However, having both a high enhancement factor and ensuring that the shell is pin-hole-free is challenging because there is a trade-off between the two when considering the shell thickness. So far in the literature, shell insulation has been successfully applied only to chemically synthesized MNPs. In this work, we alternatively study different combinations of chemical synthesis (bottom-up) and lithographic (top-down) routes to obtain shell-isolated plasmonic nanostructures that offer chemical sensing capabilities. The three approaches we study in this work include (1) chemically synthesized MNPs + chemical shell, (2) lithographic substrate + chemical shell, and (3) lithographic substrate + atomic layer deposition (ALD) shell. We find that ALD allows us to fabricate controllable and reproducible pin-hole-free shells. We showcase the ability to fabricate lithographic SHINER substrates which report an enhancement factor of 7.5 × 103 ± 17% for our gold nanodot substrates coated with a 2.8 nm aluminium oxide shell. Lastly, by introducing a gold etchant solution to our fabricated SHINER substrate, we verified that the shells fabricated with ALD are truly pin-hole-free.

Oxidative Conversion of Polyethylene Towards Di-Carboxylic Acids: A Multi-Analytical Approach

To reduce the pressure on the environment created by the increasing amount of plastic waste, the need to develop suitable plastic recycling methods has become more evident. However, the chemical recycling toolbox for polyethylene (PE), the most abundant type of plastic waste, remains underdeveloped. In this work, analytical methods were developed to explore the possibility to oxidatively convert PE into di-carboxylic acids as reaction products. A multi-analytical approach including gas chromatography-mass spectrometry, gas chromatography-flame ionization detection, several (2D) nuclear magnetic resonance methods as well as in-situ transmission infrared spectroscopy was used. This led to a thorough qualitative and quantitative analysis on the product mixture, which extends and clarifies the existing literature. Without a catalyst (thermally) already up to 7 mol % di-carboxylic acids can be formed. Furthermore, it was found that the majority of the oxidized functionalities are carboxylic acids, (methyl) ketones, γ-lactones, γ-ketones and esters. An intra-molecular hydrogen shift seemed key in the cleavage step and the formation of late-stage side products. In addition, crosslinking reactions due to esterification reactions seem to limit the di-carboxylic acid yield. Therefore, these two handles can be taken into account to study and design similar (catalytic) systems for the oxidative conversion of plastic waste.

Unravelling potential reaction intermediates during catalytic pyrolysis of polypropylene with microscopy and spectroscopy

While plastics-to-plastics recycling via melting and re-extrusion is often the preferred option due to a relatively low CO2 footprint, this technique requires a highly sorted waste stream and plastic properties can often not be maintained. Obtaining aromatics, such as benzene, toluene, and xylene (BTX), via catalytic pyrolysis of polyolefins, such as polypropylene and polyethylene, offers another attractive recycling technology. In this process, a discarded crude oil refinery catalyst (ECAT) was previously shown to lower the unwanted formation of deactivating coke species compared to a fresh crude oil refinery catalyst (FCC-cat), while yielding 20 wt% aromatics from polypropylene. In this work, we study the underlying reaction mechanism for this chemical recycling process over the fresh and used refinery catalyst as well as a model system, not containing any zeolite material, using a combination of microscopy and spectroscopy. More specifically, by using in situ fluorescence microscopy, in situ infrared spectroscopy, in situ ultraviolet-visible spectroscopy as well as ex situ solid-state nuclear magnetic resonance, we observe highly fluorescent methylated aromatic intermediates that differ for the three catalyst materials under study both in their fluorescence, IR, UV-vis, and NMR spectroscopy features. This detailed micro-spectroscopic comparison informs which potential reaction intermediates lead to increased coke formation. Our results suggests that a next generation of catalyst materials for this process would profit from a higher accessibility and a milder acidity compared to an FCC-cat and shows the great potential of using ECAT to reduce coking and obtain a BTX stream, which could be become the chemical building blocks for the manufacturing of e.g., plastics and coating materials.

Green Additives in Chitosan-based Bioplastic Films: Long-term Stability Assessment and Aging Effects

Although biomass-based alternatives for the manufacturing of bioplastic films are an important aspect of a more sustainable future, their physicochemical properties need to be able to compete with the existing market to establish them as a viable alternative. One important factor that is often neglected is the long-term stability of renewables-based functional materials, as they should neither degrade after a day or week, nor last forever. One material showing high potential in this regard, also due to its intrinsic biodegradability and antibacterial properties, is chitosan, which can form stable, self-standing films. We previously showed that green additives introduce a broad tunability of the chitosan-based material properties. In this work, we investigate the long-term stability and related degradation processes of chitosan-based bioplastics by assessing their physicochemical properties over 400 days. It was found that the film properties change similarly for samples stored in the fridge (4 °C, dark) as at ambient conditions (20 °C, light/dark cycles of the day). Additives with high vapor pressure, such as glycerol, evaporate and degrade, causing both brittleness and discoloration. In contrast, films with the addition of crosslinking additives, such as citric acid, show high stability also over a long time, bearing great preconditions for practical applications. This knowledge serves as a stepping-stone to utilizing chitosan as an alternative material for renewable-resourced bioplastic products.

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition

The structural and morphological characterization of individual catalyst particles for olefin polymerization, as well as for the reverse process of polyolefin decomposition, can provide an improved understanding for how these catalyst materials operate under relevant reaction conditions. In this review, we discuss an emerging analytical toolbox of 2D and 3D chemical imaging techniques that is suitable for investigating the chemistry and reactivity of related catalyst systems. While synchrotron-based X-ray microscopy still provides unparalleled spatial resolutions in 2D and 3D, a number of laboratory-based techniques, most notably focused ion beam-scanning electron microscopy, confocal fluorescence microscopy, infrared photoinduced force microscopy and laboratory-based X-ray nano-computed tomography, have helped to significantly expand the arsenal of analytical tools available to scientists in heterogeneous catalysis and polymer science. In terms of future research, the review outlines the role and impact of in situ and operando (spectro-)microscopy experiments, involving sophisticated reactors as well as online reactant and product analysis, to obtain real-time information on the formation, decomposition, and mobility of polymer phases within single catalyst particles. Furthermore, the potential of fluorescence microscopy, X-ray microscopy and optical microscopy is highlighted for the high-throughput characterization of olefin polymerization and polyolefin decomposition catalysts. By combining these chemical imaging techniques with, for example, chemical staining methodologies, selective probe molecules as well as particle sorting approaches, representative structure-activity relationships can be derived at the level of single catalyst particles.

Fluorescent-Probe Characterization for Pore-Space Mapping with Single-Particle Tracking

Porous solids often contain complex pore networks with pores of various sizes. Tracking individual fluorescent probes as they diffuse through porous materials can be used to characterize pore networks at tens of nanometers resolution. However, understanding the motion behavior of fluorescent probes in confinement is crucial to reliably derive pore network properties. Here, we introduce well-defined lithography-made model pores developed to study probe behavior in confinement. We investigated the influence of probe-host interactions on diffusion and trapping of confined single-emitter quantum-dot probes. Using the pH-responsiveness of the probes, we were able to largely suppress trapping at the pore walls. This enabled us to define experimental conditions for mapping of the accessible pore space of a one-dimensional pore array as well as a real-life polymerization-catalyst-support particle.

Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals

We have used confocal laser scanning microscopy on the small, fluorescent resorufin dye molecule to visualize molecular accessibility and diffusion in the hierarchical, anisotropic pore structure of large (~10 μm-sized) zeolite-β crystals. The resorufin dye is widely used in life and materials science, but only in its deprotonated form because the protonated molecule is barely fluorescent in aqueous solution. In this work, we show that protonated resorufin is in fact strongly fluorescent when confined within zeolite micropores, thus enabling fluorescence microimaging experiments. We find that J-aggregation guest-guest interactions lead to a decrease in the measured fluorescence intensity that can be prevented by using non-fluorescent spacer molecules. We characterized the pore space by introducing resorufin from the outside solution and following its diffusion into zeolite-β crystals. The eventual homogeneous distribution of resorufin molecules throughout the zeolite indicates a fully accessible pore network. This enables the quantification of the diffusion coefficient in the straight pores of zeolite-β without the need for complex analysis, and we found a value of 3×10-15  m2  s-1 . Furthermore, we saw that diffusion through the straight pores of zeolite-β is impeded when crossing the boundaries between zeolite subunits.

Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals

Invited for the cover of this issue is the group of Professor Bert Weckhuysen at Utrecht University. The image depicts the change in fluorescence color of a resorufin dye molecule when it is protonated and confined inside the micropores of zeolite-β. Read the full text of the article at 10.1002/chem.202302553.

Operando time-gated Raman spectroscopy of solid catalysts

Operando Raman spectroscopy is a powerful analytical tool to provide new insights in the working and deactivation principles of solid catalysts. Intense fluorescence can obscure Raman spectra to the extent that they become uninterpretable. Time-gated Raman spectroscopy, based on pulsed excitation and time-gated detection, suppresses background fluorescence based on its slower time dynamics compared to Raman scattering. In this work, we demonstrate and quantify the benefit of time gating for operando Raman spectroscopy, using the propane dehydrogenation reaction over Pt-Sn-based catalyst materials as a case study. Experimental time-gated Raman spectroscopy data are fitted to a time-trace model that is used to optimize time gating for the maximum signal-to-background-noise ratio. Time-gated Raman spectra of a spent propane dehydrogenation catalyst material show lower background fluorescence compared to the time-integrated Raman spectra counterparts. Simultaneous operando time-gated and time-integrated Raman spectroscopy experiments demonstrate the benefit of time gating to obtain more distinct Raman features, especially in the early coking stages where spectra are dominated by background fluorescence.

Probing nearby molecular vibrations with lanthanide-doped nanocrystals

The photoluminescence (PL) of lanthanide-doped nanocrystals can be quenched by energy transfer to vibrations of molecules located within a few nanometers from the dopants. Such short-range electronic-to-vibrational energy transfer (EVET) is often undesired as it reduces the photoluminescence efficiency. On the other hand, EVET may be exploited to extract information about molecular vibrations in the local environment of the nanocrystals. Here, we investigate the influence of solvent and gas environments on the PL properties of NaYF4:Er3+,Yb3+ upconversion nanocrystals. We relate changes in the PL spectrum and excited-state lifetimes in different solvents and their deuterated analogues to quenching of specific lanthanide levels by EVET to molecular vibrations. Similar but weaker changes are induced when we expose a film of nanocrystals to a gas environment with different amounts of H2O or D2O vapor. Quenching of green- and red-emitting levels of Er3+ can be explained in terms of EVET-mediated quenching that involves molecular vibrations with energies resonant with the gap between the energy levels of the lanthanide. Quenching of the near-infrared-emitting level is more complex and may involve EVET to combination-vibrations or defect-mediated quenching. EVET-mediated quenching holds promise as a mechanism to probe the local chemical environment-both for nanocrystals dispersed in a liquid and for nanocrystals exposed to gaseous molecules that adsorb onto the nanocrystal surface.

Mapping Temperature Heterogeneities during Catalytic CO2 Methanation with Operando Luminescence Thermometry

Controlling and understanding reaction temperature variations in catalytic processes are crucial for assessing the performance of a catalyst material. Local temperature measurements are challenging, however. Luminescence thermometry is a promising remote-sensing tool, but it is cross-sensitive to the optical properties of a sample and other external parameters. In this work, we measure spatial variations in the local temperature on the micrometer length scale during carbon dioxide (CO2) methanation over a TiO2-supported Ni catalyst and link them to variations in catalytic performance. We extract local temperatures from the temperature-dependent emission of Y2O3:Nd3+ particles, which are mixed with the CO2 methanation catalyst. Scanning, where a near-infrared laser locally excites the emitting Nd3+ ions, produces a temperature map with a micrometer pixel size. We first designed the Y2O3:Nd3+ particles for optimal temperature precision and characterized cross-sensitivity of the measured signal to parameters other than temperature, such as light absorption by the blackened sample due to coke deposition at elevated temperatures. Introducing reaction gases causes a local temperature increase of the catalyst of on average 6-25 K, increasing with the reactor set temperature in the range of 550-640 K. Pixel-to-pixel variations in the temperature increase show a standard deviation of up to 1.5 K, which are attributed to local variations in the catalytic reaction rate. Mapping and understanding such temperature variations are crucial for the optimization of overall catalyst performance on the nano- and macroscopic scale.

Green Additives in Chitosan-Based Bioplastic Films: Physical, Mechanical, and Chemical Properties

To switch to alternatives for fossil-fuel-based polymer materials, renewable raw materials from green resources should be utilized. Chitosan is such a material that is a strong, but workable derivative from chitin, obtained from crustaceans. However, various applications ask for specific plastic properties, such as certain flexibility, hardness and transparency. With different additives, also obtainable from green resources, chitosan-based composites in the form of self-supporting films, ranging from very hard and brittle to soft and flexible were successfully produced. The additives turned out to belong to one of three categories, namely linear, non-linear, or crosslinking additives. The non-linear additives could only be taken up to a certain relative amount, whereas the uptake of linear additives was not limited within the range of our experiments. Additives with multiple functional groups tend to crosslink chitosan even at room temperature in an acidic medium. Finally, it was shown that dissolving the chitosan in acetic acid and subsequently drying the matrix as a film results in reacetylation compared to the starting chitosan source, resulting in a harder material. With these findings, it is possible to tune the properties of chitosan-based polymer materials, making a big step towards application of this renewable polymer within consumer goods.

The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks

Capturing and converting CO2 through artificial photosynthesis using photoactive, porous materials is a promising approach for addressing increasing CO2 concentrations. Porphyrinic Zr-based metal-organic frameworks (MOFs) are of particular interest as they incorporate a photosensitizer in the porous structure. Herein, the initial step of the artificial photosynthesis is studied: CO2 sorption and activation in the presence of water. A combined vibrational and visible spectroscopic approach was used to monitor the adsorption of CO2 into PCN-222 and PCN-223 MOFs, and the photophysical changes of the porphyrinic linker as a function of water concentration. A shift in CO2 sorption site and bending of the porphyrin macrocycle in response to humidity was observed, and CO2/H2O competition experiments revealed that the exchange of CO2 with H2O is pore-size dependent. Therefore, humidity and pore-size can be used to tune CO2 sorption, CO2 capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO2 photoreduction.

Transport limitations in polyolefin cracking at the single catalyst particle level

Catalytic cracking is a promising approach to chemically recycle polyolefins by converting them into smaller hydrocarbons like naphtha, and important precursors of various platform chemicals, such as aromatics. Cracking catalysts, commonly used in the modern refinery and petrochemical industry, are tailored to process gaseous or liquid feedstock. Polyolefins, however, are very large macromolecules that form highly viscous melts at the temperatures required to break their backbone C-C bonds. Therefore, mass transport is expected to limit the performance of traditional cracking catalysts when applied to the conversion of polymers. In this work, we study these effects during the cracking of polypropylene (PP) over catalysts utilized in the fluid catalytic cracking (FCC) process. Thermogravimetric experiments using PP of varying molecular weight (Mw) and catalysts of varying accessibility showed that low Mw model polymers can be cracked below 275 °C, while PP of higher Mw required a 150 °C higher temperature. We propose that this difference is linked to different degrees of mass transport limitations and investigated this at length scales ranging from milli- to nanometers, utilizing in situ optical microscopy and electron microscopy to inspect cut open catalyst-polymer composites. We identified the main cause of transport limitations as the significantly higher melt viscosity of high Mw polymers, which prohibits efficient catalyst-polymer contact. Additionally, the high Mw polymer does not enter the inner pore system of the catalyst particles, severely limiting utilization of the active sites located there. Our results demonstrate that utilizing low Mw polymers can lead to a significant overestimation of catalyst activity, and suggest that polyolefins might need to undergo a viscosity reducing pre-treatment in order to be cracked efficiently.

Correction to "Correlating the Morphological Evolution of Individual Catalyst Particles to the Kinetic Behavior of Metallocene-Based Ethylene Polymerization Catalysts"

[This corrects the article DOI: 10.1021/jacsau.1c00324.].

Elucidating the Structure and Composition of Individual Bimetallic Nanoparticles in Supported Catalysts by Atom Probe Tomography

Understanding and controlling the structure and composition of nanoparticles in supported metal catalysts are crucial to improve chemical processes. For this, atom probe tomography (APT) is a unique tool, as it allows for spatially resolved three-dimensional chemical imaging of materials with sub-nanometer resolution. However, thus far APT has not been applied for mesoporous oxide-supported metal catalyst materials, due to the size and number of pores resulting in sample fracture during experiments. To overcome these issues, we developed a high-pressure resin impregnation strategy and showcased the applicability to high-porous supported Pd-Ni-based catalyst materials, which are active in CO2 hydrogenation. Within the reconstructed volume of 3 × 105 nm3, we identified over 400 Pd-Ni clusters, with compositions ranging from 0 to 16 atom % Pd and a size distribution of 2.6 ± 1.6 nm. These results illustrate that APT is capable of quantitatively assessing the size, composition, and metal distribution for a large number of nanoparticles at the sub-nm scale in industrial catalysts. Furthermore, we showcase that metal segregation occurred predominately between nanoparticles, shedding light on the mechanism of metal segregation. We envision that the presented methodology expands the capabilities of APT to investigate porous functional nanomaterials, including but not limited to solid catalysts.

Spatiotemporal Mapping of Local Heterogeneities during Electrochemical Carbon Dioxide Reduction

The activity and selectivity of a copper electrocatalyst during the electrochemical CO₂ reduction reaction (eCO₂RR) are largely dominated by the interplay between local reaction environment, the catalyst surface, and the adsorbed intermediates. In situ characterization studies have revealed many aspects of this intimate relationship between surface reactivity and adsorbed species, but these investigations are often limited by the spatial and temporal resolution of the analytical technique of choice. Here, Raman spectroscopy with both space and time resolution was used to reveal the distribution of adsorbed species and potential reaction intermediates on a copper electrode during eCO₂RR. Principal component analysis (PCA) of the in situ Raman spectra revealed that a working electrocatalyst exhibits spatial heterogeneities in adsorbed species, and that the electrode surface can be divided into CO-dominant (mainly located at dendrite structures) and C-C dominant regions (mainly located at the roughened electrode surface). Our spectral evaluation further showed that in the CO-dominant regions, linear CO was observed (as characterized by a band at ∼2090 cm^-1), accompanied by the more classical Cu-CO bending and stretching vibrations located at ∼280 and ∼360 cm^-1, respectively. In contrast, in the C-C directing region, these three Raman bands are suppressed, while at the same time a band at ∼495 cm^-1 and a broad Cu-CO band at ∼2050 cm^-1 dominate the Raman spectra. Furthermore, PCA revealed that anodization creates more C-C dominant regions, and labeling experiments confirmed that the 495 cm^-1 band originates from the presence of a Cu-C intermediate. These results indicate that a copper electrode at work is very dynamic, thereby clearly displaying spatiotemporal heterogeneities, and that in situ micro-spectroscopic techniques are crucial for understanding the eCO₂RR mechanism of working electrocatalyst materials.

How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper

Copper is a unique catalyst for the electrochemical CO₂ reduction reaction (CO2RR) as it can produce multi-carbon products, such as ethylene and propanol. As practical electrolyzers will likely operate at elevated temperatures, the effect of reaction temperature on the product distribution and activity of CO2RR on copper is important to elucidate. In this study, we have performed electrolysis experiments at different reaction temperatures and potentials. We show that there are two distinct temperature regimes. From 18 up to ∼48 °C, C2+ products are produced with higher Faradaic efficiency, while methane and formic acid selectivity decreases and hydrogen selectivity stays approximately constant. From 48 to 70 °C, it was found that HER dominates and the activity of CO2RR decreases. Moreover, the CO2RR products produced in this higher temperature range are mainly the C1 products, namely, CO and HCOOH. We argue that CO surface coverage, local pH, and kinetics play an important role in the lower-temperature regime, while the second regime appears most likely to be related to structural changes in the copper surface.

Restructuring of titanium oxide overlayers over nickel nanoparticles during catalysis

Reducible supports can affect the performance of metal catalysts by the formation of suboxide overlayers upon reduction, a process referred to as the strong metal-support interaction (SMSI). A combination of operando electron microscopy and vibrational spectroscopy revealed that thin TiO_x overlayers formed on nickel/titanium dioxide catalysts during 400°C reduction were completely removed under carbon dioxide hydrogenation conditions. Conversely, after 600°C reduction, exposure to carbon dioxide hydrogenation reaction conditions led to only partial reexposure of nickel, forming interfacial sites in contact with TiO_x and favoring carbon-carbon coupling by providing a carbon species reservoir. Our findings challenge the conventional understanding of SMSIs and call for more-detailed operando investigations of nanocatalysts at the single-particle level to revisit static models of structure-activity relationships.

Addition of Pore-Forming Agents and Their Effect on the Pore Architecture and Catalytic Behavior of Shaped Zeolite-Based Catalyst Bodies

Porous materials, such as solid catalysts, are used in various chemical reactions in industry to produce chemicals, materials, and fuels. Understanding the interplay between pore architecture and catalytic behavior is of great importance for synthesizing a better industrial-grade catalyst material. In this study, we have investigated the modification of the pore architecture of zeolite-based alumina-bound shaped catalyst bodies via the addition of different starches as pore-forming agents. A combination of microscopy techniques allowed us to visualize the morphological changes induced and make a link between pore architecture, molecular transport, and catalytic performance. As for the catalytic performance in the methanol-to-hydrocarbons (MTH) reaction, pore-forming agents resulted in up to ∼12% higher conversion, an increase of 74% and 77% in yield (14% and 13% compared to 8.6% and 7.7% of the reference sample in absolute yields) toward ethylene and propylene, respectively, and an improved lifetime of the catalyst materials.

Silicalite-1 Layer Secures the Bifunctional Nature of a CO2 Hydrogenation Catalyst

Close proximity usually shortens the travel distance of reaction intermediates, thus able to promote the catalytic performance of CO₂ hydrogenation by a bifunctional catalyst, such as the widely reported In₂O₃/H-ZSM-5. However, nanoscale proximity (e.g., powder mixing, PM) more likely causes the fast deactivation of the catalyst, probably due to the migration of metals (e.g., In) that not only neutralizes the acid sites of zeolites but also leads to the reconstruction of the In₂O₃ surface, thus resulting in catalyst deactivation. Additionally, zeolite coking is another potential deactivation factor when dealing with this methanol-mediated CO₂ hydrogenation process. Herein, we reported a facile approach to overcome these three challenges by coating a layer of silicalite-1 (S-1) shell outside a zeolite H-ZSM-5 crystal for the In₂O₃/H-ZSM-5-catalyzed CO₂ hydrogenation. More specifically, the S-1 layer (1) restrains the migration of indium that preserved the acidity of H-ZSM-5 and at the same time (2) prevents the over-reduction of the In₂O₃ phase and (3) improves the catalyst lifetime by suppressing the aromatic cycle in a methanol-to-hydrocarbon conversion step. As such, the activity for the synthesis of C₂ ⁺ hydrocarbons under nanoscale proximity (PM) was successfully obtained. Moreover, an enhanced performance was observed for the S-1-coated catalyst under microscale proximity (e.g., granule mixing, GM) in comparison to the S-1-coating-free counterpart. This work highlights an effective shielding strategy to secure the bifunctional nature of a CO₂ hydrogenation catalyst.

Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO₂ reduction, but its impact on the performance of CO₂ electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO₂ electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO₂ electrolysis performance, resulting in variations of C₂₊ product Faradaic efficiency (FE) by >3×, with C₂₊ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO₂ reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO₂/H₂O concentration ratio and promoting high CO surface coverage to facilitate C₂₊ production in long-term CO₂ electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO₂ reduction systems.

Europium–Magnesium–Aluminum-Based Mixed-Metal Oxides as Highly Active Methane Oxychlorination Catalysts

Methane oxychlorination (MOC) is a promising reaction for the production of liquefied methane derivatives. Even though catalyst design is still in its early stages, the general trend is that benchmark catalyst materials have a redox-active site, with, e.g., Cu²⁺, Ce⁴⁺, and Pd²⁺ as prominent showcase examples. However, with the identification of nonreducible LaOCl moiety as an active center for MOC, it was demonstrated that a redox-active couple is not a requirement to establish a high activity. In this work, we show that Mg²⁺-Al³⁺-based mixed-metal oxide (MMO) materials are highly active and stable MOC catalysts. The synergistic interaction between Mg²⁺ and Al³⁺ could be exploited due to the fact that a homogeneous distribution of the chemical elements was achieved. This interaction was found to be crucial for the unexpectedly high MOC activity, as reference MgO and γ-Al₂O₃ materials did not show any significant activity. Operando Raman spectroscopy revealed that Mg²⁺ acted as a chlorine buffer and subsequently as a chlorinating agent for Al³⁺, which was the active metal center in the methane activation step. The addition of the redox-active Eu³⁺ to the nonreducible Mg²⁺-Al³⁺ MMO catalyst enabled further tuning of the catalytic performance and made the EuMg₃Al MMO catalyst one of the most active MOC catalyst materials reported so far. Combined operando Raman/luminescence spectroscopy revealed that the chlorination behavior of Mg²⁺ and Eu³⁺ was correlated, suggesting that Mg²⁺ also acted as a chlorinating agent for Eu³⁺. These results indicate that both redox activity and synergistic effects between Eu, Mg, and Al are required to obtain high catalytic performance. The importance of elemental synergy and redox properties is expected to be translatable to the oxychlorination of other hydrocarbons, such as light alkanes, due to large similarities in catalytic chemistry.

Droplet microreactor for high-throughput fluorescence-based measurements of single catalyst particle acidity

The particles of heterogeneous catalysts differ greatly in size, morphology, and most importantly, in activity. Studying these catalyst particles in batch typically results in ensemble averages, without any information at the level of individual catalyst particles. To date, the study of individual catalyst particles has been rewarding but is still rather slow and often cumbersome¹. Furthermore, these valuable in-depth studies at the single particle level lack statistical relevance. Here, we report the development of a droplet microreactor for high-throughput fluorescence-based measurements of the acidities of individual particles in fluid catalytic cracking (FCC) equilibrium catalysts (ECAT). This method combines systematic screening of single catalyst particles with statistical relevance. An oligomerization reaction of 4-methoxystyrene, catalyzed by the Brønsted acid sites inside the zeolite domains of the ECAT particles, was performed on-chip at 95 °C. The fluorescence signal generated by the reaction products inside the ECAT particles was detected near the outlet of the microreactor. The high-throughput acidity screening platform was capable of detecting ~1000 catalyst particles at a rate of 1 catalyst particle every 2.4 s. The number of detected catalyst particles was representative of the overall catalyst particle population with a confidence level of 95%. The measured fluorescence intensities showed a clear acidity distribution among the catalyst particles, with the majority (96.1%) showing acidity levels belonging to old, deactivated catalyst particles and a minority (3.9%) exhibiting high acidity levels. The latter are potentially of high interest, as they reveal interesting new physicochemical properties indicating why the particles were still highly acidic and reactive.

Understanding the Reactivity of Supported Late Transition Metals on a Bare Anatase (101) Surface: A Periodic Conceptual DFT Investigation

The rapidly growing interest for new heterogeneous catalytic systems providing high atomic efficiency along with high stability and reactivity triggered an impressive progress in the field of single-atom catalysis. Nevertheless, unravelling the factors governing the interaction strength between the support and the adsorbed metal atoms remains a major challenge. Based on periodic density functional theory (DFT) calculations, this paper provides insight into the adsorption of single late transition metals on a defect-free anatase surface. The obtained adsorption energies fluctuate, with the exception of Pd, between -3.11 and -3.80 eV and are indicative of a strong interaction. Depending on the considered transition metal, we could attribute the strength of this interaction with the support to i) an electron transfer towards anatase (Ru, Rh, Ni), ii) s-d orbital hybridisation effects (Pt), or iii) a synergistic effect between both factors (Fe, Co, Os, Ir). The driving forces behind the adsorption were also found to be strongly related to Klechkowsky's rule for orbital filling. In contrast, the deviating behaviour of Pd is most likely associated with the lower dissociation enthalpy of the Pd-O bond. Additionally, the reactivity of these systems was evaluated using the Fermi weighted density of states approach. The resulting softness values can be clearly related to the electron configuration of the catalytic systems as well as with the net charge on the transition metal. Finally, these indices were used to construct a model that predicts the adsorption strength of CO on these anatase-supported d-metal atoms. The values obtained from this regression model show, within a 95 % probability interval, a correlation of 84 % with the explicitly calculated CO adsorption energies.

Correction: Propane to olefins tandem catalysis: a selective route towards light olefins production

Correction for 'Propane to olefins tandem catalysis: a selective route towards light olefins production' by Matteo Monai et al., Chem. Soc. Rev., 2021, 50, 11503-11529, https://doi.org/10.1039/D1CS00357G.

Structure sensitivity in gas sorption and conversion on metal-organic frameworks

Many catalytic processes depend on the sorption and conversion of gaseous molecules on the surface of (porous) functional materials. These events often preferentially occur on specific, undercoordinated, external surface sites. Here we show the combination of in situ Photo-induced Force Microscopy (PiFM) with Density Functional Theory (DFT) calculations to study the site-specific sorption and conversion of formaldehyde on the external surfaces of well-defined faceted ZIF-8 microcrystals with nanoscale resolution. We observed preferential adsorption of formaldehyde on high index planes. Moreover, in situ PiFM allowed us to visualize unsaturated nanodomains within extended external crystal planes, showing enhanced sorption behavior on the nanoscale. Additionally, on defective ZIF-8 crystals, structure sensitive conversion of formaldehyde through a methoxy- and a formate mechanism mediated by Lewis acidity was found. Strikingly, sorption and conversion were influenced more by the external surface termination than by the concentration of defects. DFT calculations showed that this is due to the presence of specific atomic arrangements on high-index crystal surfaces. With this research, we showcase the high potential of in situ PiFM for structure sensitivity studies on porous functional materials.

New Editor-in-Chief for Catalysis Science & Technology

Executive Editor Maria Southall welcomes Bert Weckhuysen as the new Editor-in-Chief of Catalysis Science & Technology and pays tribute to the leadership and many contributions of departing Editor-in-Chief, Javier Pérez-Ramírez.

Structure Sensitivity of CO2 Conversion over Nickel Metal Nanoparticles Explained by Micro-Kinetics Simulations

Nickel metal nanoparticles are intensively researched for the catalytic conversion of carbon dioxide. They are commercially explored in the so-called power-to-methane application in which renewably resourced H₂ reacts with CO₂ to produce CH₄, which is better known as the Sabatier reaction. Previous work has shown that this reaction is structure-sensitive. For instance, Ni/SiO₂ catalysts reveal a maximum performance when nickel metal nanoparticles of ∼2-3 nm are used. Particularly important to a better understanding of the structure sensitivity of the Sabatier reaction over nickel-based catalysts is to understand all relevant elementary reaction steps over various nickel metal facets because this will tell as to which type of nickel facets and which elementary reaction steps are crucial for designing an efficient nickel-based methanation catalyst. In this work, we have determined by density functional theory (DFT) calculations and micro-kinetics modeling (MKM) simulations that the two terrace facets Ni(111) and Ni(100) and the stepped facet Ni(211) barely show any activity in CO₂ methanation. The stepped facet Ni(110) turned out to be the most effective in CO₂ methanation. Herein, it was found that the dominant kinetic route corresponds to a combination of the carbide and formate reaction pathways. It was found that the dissociation of H₂CO* toward CH₂* and O* is the most critical elementary reaction step on this Ni(110) facet. The calculated activity of a range of Wulff-constructed nickel metal nanoparticles, accounting for varying ratios of the different facets and undercoordinated atoms exposed, reveals the same trend of activity-versus-nanoparticle size, as was observed in previous experimental work from our research group, thereby providing an explanation for the structure-sensitive nature of the Sabatier reaction.

Bifunctional Europium for Operando Catalyst Thermometry in an Exothermic Chemical Reaction

Often the reactor or the reaction medium temperature is reported in the field of heterogeneous catalysis, even though it could vary significantly from the reactive catalyst temperature. The influence of the catalyst temperature on the catalytic performance and vice versa is therefore not always accurately known. We here apply EuOCl as both solid catalyst and thermometer, allowing for operando temperature determination. The interplay between reaction conditions and the catalyst temperature dynamics is studied. A maximum temperature difference between the catalyst and oven of +16 °C was observed due to the exothermicity of the methane oxychlorination reaction. Heat dissipation by radiation appears dominating compared to convection in this set-up, explaining the observed uniform catalyst bed temperature. Application of operando catalyst thermometry could provide a deeper mechanistic understanding of catalyst performances and allow for safer process operation in chemical industries.

Near-Unity Electrochemical CO2 to CO Conversion over Sn-Doped Copper Oxide Nanoparticles

Bimetallic electrocatalysts have emerged as a viable strategy to tune the electrocatalytic CO₂ reduction reaction (eCO₂RR) for the selective production of valuable base chemicals and fuels. However, obtaining high product selectivity and catalyst stability remain challenging, which hinders the practical application of eCO₂RR. In this work, it was found that a small doping concentration of tin (Sn) in copper oxide (CuO) has profound influence on the catalytic performance, boosting the Faradaic efficiency (FE) up to 98% for carbon monoxide (CO) at -0.75 V versus RHE, with prolonged stable performance (FE > 90%) for up to 15 h. Through a combination of ex situ and in situ characterization techniques, the in situ activation and reaction mechanism of the electrocatalyst at work was elucidated. In situ Raman spectroscopy measurements revealed that the binding energy of the crucial adsorbed *CO intermediate was lowered through Sn doping, thereby favoring gaseous CO desorption. This observation was confirmed by density functional theory, which further indicated that hydrogen adsorption and subsequent hydrogen evolution were hampered on the Sn-doped electrocatalysts, resulting in boosted CO formation. It was found that the pristine electrocatalysts consisted of CuO nanoparticles decorated with SnO₂ domains, as characterized by ex situ high-resolution scanning transmission electron microscopy and X-ray photoelectron spectroscopy measurements. These pristine nanoparticles were subsequently in situ converted into a catalytically active bimetallic Sn-doped Cu phase. Our work sheds light on the intimate relationship between the bimetallic structure and catalytic behavior, resulting in stable and selective oxide-derived Sn-doped Cu electrocatalysts.

Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography

The conversion of methanol to valuable hydrocarbon molecules is of great commercial interest, as the process serves as a sustainable alternative for the production of, for instance, the base chemicals for plastics. The reaction is catalyzed by zeolite materials. By the introduction of magnesium as a cationic metal, the properties of the zeolite, and thereby the catalytic performance, are changed. With atom probe tomography (APT), nanoscale relations within zeolite materials can be revealed: i.e., crucial information for a fundamental mechanistic understanding. We show that magnesium forms clusters within the cages of zeolite SSZ-13, while the framework elements are homogeneously distributed. These clusters of just a few nanometers were analyzed and visualized in 3-D. Magnesium atoms seem to initially be directed to the aluminum sites, after which they aggregate and fill one or two cages in the zeolite SSZ-13 structure. The presence of magnesium in zeolite SSZ-13 increases the lifetime as well as the propylene selectivity. By using operando UV-vis spectroscopy and X-ray diffraction techniques, we are able to show that these findings are related to the suppression of aromatic intermediate products, while maintaining the formation of polyaromatic compounds. Further nanoscale analysis of the spent catalysts showed indications of magnesium redistribution after catalysis. Unlike zeolite H-SSZ-13, for which only a homogeneous distribution of carbon was found, carbon can be either homogeneously or heterogeneously distributed within zeolite Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon clusters were isolated, visualized, and analyzed and were assumed to be polyaromatic compounds. Small one-cage-filling polyaromatic compounds were identified; furthermore, large-cage-crossing aromatic molecules were found by isolating large coke clusters, demonstrating the unique coking mechanism in zeolite SSZ-13. Short-length-scale evidence for the formation of polyaromatic compounds at acid sites is discovered, as clear nanoscale relations between aluminum and carbon atoms exist.

Operando Laboratory-Based Multi-Edge X-Ray Absorption Near-Edge Spectroscopy of Solid Catalysts

Laboratory-based X-ray absorption spectroscopy (XAS) and especially X-ray absorption near-edge structure (XANES) offers new opportunities in catalyst characterization and presents not only an alternative, but also a complementary approach to precious beamtime at synchrotron facilities. We successfully designed a laboratory-based setup for performing operando, quasi-simultaneous XANES analysis at multiple K-edges, more specifically, operando XANES of mono-, bi-, and trimetallic CO₂ hydrogenation catalysts containing Ni, Fe, and Cu. Detailed operando XANES studies of the multielement solid catalysts revealed metal-dependent differences in the reducibility and re-oxidation behavior and their influence on the catalytic performance in CO₂ hydrogenation. The applicability of operando laboratory-based XANES at multiple K-edges paves the way for advanced multielement catalyst characterization complementing detailed studies at synchrotron facilities.

Waste-Derived Copper-Lead Electrocatalysts for CO2 Reduction

It remains a real challenge to control the selectivity of the electrocatalytic CO₂ reduction (eCO₂R) reaction to valuable chemicals and fuels. Most of the electrocatalysts are made of non-renewable metal resources, which hampers their large-scale implementation. Here, we report the preparation of bimetallic copper-lead (CuPb) electrocatalysts from industrial metallurgical waste. The metal ions were extracted from the metallurgical waste through simple chemical treatment with ammonium chloride, and Cu_xPb_y electrocatalysts with tunable compositions were fabricated through electrodeposition at varying cathodic potentials. X-ray spectroscopy techniques showed that the pristine electrocatalysts consist of Cu⁰, Cu¹⁺ and Pb²⁺ domains, and no evidence for alloy formation was found. We found a volcano-shape relationship between eCO₂R selectivity toward two electron products, such as CO, and the elemental ratio of Cu and Pb. A maximum Faradaic efficiency towards CO was found for Cu_9.00Pb_1.00, which was four times higher than that of pure Cu, under the same electrocatalytic conditions. In situ Raman spectroscopy revealed that the optimal amount of Pb effectively improved the reducibility of the pristine Cu¹⁺ and Pb²⁺ domains to metallic Cu and Pb, which boosted the selectivity towards CO by synergistic effects. This work provides a framework of thinking to design and tune the selectivity of bimetallic electrocatalysts for CO₂ reduction through valorization of metallurgical waste.

Probing the Dynamics of Low-Overpotential CO2-to-CO Activation on Copper Electrodes with Time-Resolved Raman Spectroscopy

Oxide-derived copper electrodes have displayed a boost in activity and selectivity toward valuable base chemicals in the electrochemical carbon dioxide reduction reaction (CO2RR), but the exact interplay between the dynamic restructuring of copper oxide electrodes and their activity and selectivity is not fully understood. In this work, we have utilized time-resolved surface-enhanced Raman spectroscopy (TR-SERS) to study the dynamic restructuring of the copper (oxide) electrode surface and the adsorption of reaction intermediates during cyclic voltammetry (CV) and pulsed electrolysis (PE). By coupling the electrochemical data to the spectral features in TR-SERS, we study the dynamic activation of and reactions on the electrode surface and find that CO₂ is already activated to carbon monoxide (CO) during PE (10% Faradaic efficiency, 1% under static applied potential) at low overpotentials (−0.35 V_RHE). PE at varying cathodic bias on different timescales revealed that stochastic CO is dominant directly after the cathodic bias onset, whereas no CO intermediates were observed after prolonged application of low overpotentials. An increase in cathodic bias (−0.55 V_RHE) resulted in the formation of static adsorbed CO intermediates, while the overall contribution of stochastic CO decreased. We attribute the low-overpotential CO₂-to-CO activation to a combination of selective Cu(111) facet exposure, partially oxidized surfaces during PE, and the formation of copper-carbonate-hydroxide complex intermediates during the anodic pulses. This work sheds light on the restructuring of oxide-derived copper electrodes and low-overpotential CO formation and highlights the power of the combination of electrochemistry and time-resolved vibrational spectroscopy to elucidate CO2RR mechanisms.

Classification-based motion analysis of single-molecule trajectories using DiffusionLab

Single-particle tracking is a powerful approach to study the motion of individual molecules and particles. It can uncover heterogeneities that are invisible to ensemble techniques, which places it uniquely among techniques to study mass transport. Analysis of the trajectories obtained with single-particle tracking in inorganic porous hosts is often challenging, because trajectories are short and/or motion is heterogeneous. We present the DiffusionLab software package for motion analysis of such challenging data sets. Trajectories are first classified into populations with similar characteristics to which the motion analysis is tailored in a second step. DiffusionLab provides tools to classify trajectories based on the motion type either with machine learning or manually. It also offers quantitative mean squared displacement analysis of the trajectories. The software can compute the diffusion constant for an individual trajectory if it is sufficiently long, or the average diffusion constant for multiple shorter trajectories. We demonstrate the DiffusionLab approach via the analysis of a simulated data set with motion types frequently observed in inorganic porous hosts, such as zeolites. The software package with graphical user interface and its documentation are freely available.

Using Biomass Gasification Mineral Residue as Catalyst to Produce Light Olefins from CO, CO2 , and H2 Mixtures

Gasification is a process to transform solids, such as agricultural and municipal waste, into gaseous feedstock for making transportation fuels. The so-called coarse solid residue (CSR) that remains after this conversion process is currently discarded as a process solid residue. In the context of transitioning from a linear to a circular society, the feasibility of using the solid process residue from waste gasification as a solid catalyst for light olefin production from CO, CO₂ , and H₂ mixtures was investigated. This CSR-derived catalyst converted biomass-derived syngas, a H₂ -poor mixture of CO, CO₂ , H₂ , and N₂ , into methane (57 %) and C₂ -C₄ olefins (43 %) at 450 °C and 20 bar. The main active ingredient of CSR was Fe, and it was discovered with operando X-ray diffraction that metallic Fe, present after pre-reduction in H₂ , transformed into an Fe carbide phase under reaction conditions. The increased formation of Fe carbides correlated with an increase in CO conversion and olefin selectivity. The presence of alkali elements, such as Na and K, in CSR-derived catalyst increased olefin production as well.

Using Biomass Gasification Mineral Residue as Catalyst to Produce Light Olefins from CO, CO2 , and H2 Mixtures

Invited for this month's cover is the group of Bert Weckhuysen at Utrecht University. The image shows how iron nanoparticles in a biomass gasification residue can convert CO, CO₂ , and H₂ mixtures into light olefins. The Research Article itself is available at 10.1002/cssc.202200436.

Hole Dynamics in Photoexcited Hematite Studied with Femtosecond Oxygen K-edge X-ray Absorption Spectroscopy

Hematite (α-Fe₂O₃) is a photoelectrode for the water splitting process because of its relatively narrow bandgap and abundance in the earth's crust. In this study, the photoexcited state of a hematite thin film was investigated with femtosecond oxygen K-edge X-ray absorption spectroscopy (XAS) at the PAL-XFEL in order to follow the dynamics of its photoexcited states. The 200 fs decay time of the hole state in the valence band was observed via its corresponding XAS feature.

Favoring the Methane Oxychlorination Reaction over EuOCl by Synergistic Effects with Lanthanum

The direct conversion of CH4 into fuels and chemicals produces less waste, requires smaller capital investments, and has improved energy efficiency compared to multistep processes. While the methane oxychlorination (MOC) reaction has been given little attention, it offers the potential to achieve high CH4 conversion levels at high selectivities. In a continuing effort to design commercially interesting MOC catalysts, we have improved the catalyst design of EuOCl by the partial replacement of Eu3+ by La3+. A set of catalytic solid solutions of La3+ and Eu3+ (i.e., La x Eu1-x OCl, where x = 0, 0.25, 0.50, 0.75, and 1) were synthesized and tested in the MOC reaction. The La3+-Eu3+ catalysts exhibit an increased CH3Cl selectivity (i.e., 54-66 vs 41-52%), a lower CH2Cl2 selectivity (i.e., 8-24 vs 18-34%), and a comparable CO selectivity (i.e., 11-28 vs 14-28%) compared to EuOCl under the same reaction conditions and varying HCl concentrations in the feed. The La3+-Eu3+ catalysts possessed a higher CH4 conversion rate than when the individual activities of LaOCl and EuOCl are summed with a similar La3+/Eu3+ ratio (i.e., the linear combination). In the solid solution, La3+ is readily chlorinated and acts as a chlorine buffer that can transfer chlorine to the active Eu3+ phase, thereby enhancing the activity. The improved catalyst design enhances the CH3Cl yield and selectivity and reduces the catalyst cost and the separation cost of the unreacted HCl. These results showcase that, by matching intrinsic material properties, catalyst design can be altered to overcome reaction bottlenecks.

Tandem catalysis with double-shelled hollow spheres

Metal-zeolite composites with metal (oxide) and acid sites are promising catalysts for integrating multiple reactions in tandem to produce a wide variety of wanted products without separating or purifying the intermediates. However, the conventional design of such materials often leads to uncontrolled and non-ideal spatial distributions of the metal inside/on the zeolites, limiting their catalytic performance. Here we demonstrate a simple strategy for synthesizing double-shelled, contiguous metal oxide@zeolite hollow spheres (denoted as MO@ZEO DSHSs) with controllable structural parameters and chemical compositions. This involves the self-assembly of zeolite nanocrystals onto the surface of metal ion-containing carbon spheres followed by calcination and zeolite growth steps. The step-by-step formation mechanism of the material is revealed using mainly in situ Raman spectroscopy and X-ray diffraction and ex situ electron microscopy. We demonstrate that it is due to this structure that an Fe₂O₃@H-ZSM-5 DSHSs-showcase catalyst exhibits superior performance compared with various conventionally structured Fe₂O₃-H-ZSM-5 catalysts in gasoline production by the Fischer-Tropsch synthesis. This work is expected to advance the rational synthesis and research of hierarchically hollow, core-shell, multifunctional catalyst materials.

New insights into the NH3-selective catalytic reduction of NO over Cu-ZSM-5 as revealed by operando spectroscopy

To control diesel vehicle NO _x emissions, Cu-exchanged zeolites have been applied in the selective catalytic reduction (SCR) of NO using NH₃ as reductant. However, the harsh hydrothermal environment of tailpipe conditions causes irreversible catalyst deactivation. The aggregation of isolated Cu²⁺ brings about unselective ammonia oxidation along with the main NH₃-SCR reaction. An unusual 'dip' shaped NO conversion curve was observed in the steamed zeolite Cu-ZSM-5, resulting from the undesired NH₃ oxidation that produced NO. Here we gain further insights into the NH₃-SCR reaction and its deactivation by employing operando UV-vis diffuse reflectance spectroscopy (DRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on fresh and steamed zeolite Cu-ZSM-5. We found that tetragonally distorted octahedral Cu²⁺ with associated NH₃ preferentially forms during low temperature NH₃-SCR (<250 °C) in fresh Cu-ZSM-5. The high coordination number of Cu²⁺ ensures the availability for high coverage of nitrate intermediates. Whilst in the steamed Cu-ZSM-5, [Cu _x (OH)_2x-1]⁺ oligomers/clusters in pseudo-tetrahedral symmetry with coordinated NH₃ accumulated during the low-temperature NH₃-SCR reaction. These clusters presented a strong adsorption of surface NH₃ and nitrates/nitric acid at low temperatures and therefore limited the reaction between surface species in the steamed Cu-ZSM-5. Further release of NH₃ with increased reaction temperature favors NH₃ oxidation that causes the drop of NO conversion at ∼275 °C. Moreover, competitive adsorption of NH₃ and nitrates/nitric acid occurs on shared Lewis-acidic adsorption sites. Prompt removal of surface nitrates/nitric acid by NO avoids the surface blockage and tunes the selectivity by alternating nitrate-nitrite equilibrium. The formation of adsorbed NO₂ and HNO _x points to the necessity of an acid adsorbent in practical applications. The structural similarity under the NH₃-SCR reaction and unselective NH₃ oxidation confirmed the entanglement of these two reactions above 250 °C.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

Mechanistic Characterization of Zeolite-Catalyzed Aromatic Electrophilic Substitution at Realistic Operating Conditions

Zeolite-catalyzed benzene ethylation is an important industrial reaction, as it is the first step in the production of styrene for polymer manufacturing. Furthermore, it is a prototypical example of aromatic electrophilic substitution, a key reaction in the synthesis of many bulk and fine chemicals. Despite extensive research, the reaction mechanism and the nature of elusive intermediates at realistic operating conditions is not properly understood. More in detail, the existence of the elusive arenium ion (better known as Wheland complex) formed upon electrophilic attack on the aromatic ring is still a matter of debate. Temperature effects and the presence of protic guest molecules such as water are expected to impact the reaction mechanism and lifetime of the reaction intermediates. Herein, we used enhanced sampling ab initio molecular dynamics simulations to investigate the complete mechanism of benzene ethylation with ethene and ethanol in the H-ZSM-5 zeolite. We show that both the stepwise and concerted mechanisms are active at reaction conditions and that the Wheland intermediate spontaneously appears as a shallow minimum in the free energy surface after the electrophilic attack on the benzene ring. Addition of water enhances the protonation kinetics by about 1 order of magnitude at coverages of one water molecule per Brønsted acidic site. In the fully solvated regime, an overstabilization of the BAS as hydronium ion occurs and the rate enhancement disappears. The obtained results give critical atomistic insights in the role of water to selectively tune the kinetics of protonation reactions in zeolites.