Tracking Minority Species in Redox Reactions: A Quantitative Combined XAS and Modulation Excitation Study.

Understanding nature of intermediates/active species in reactions is a major challenge in chemistry. This is because spectator species typically dominate the experimentally derived data and consequently active phase contributions are masked. Transient methods offer a means to bypass this difficulty. In particular, modulation excitation with phase-sensitive detection (ME-PSD) provides a mechanism to distinguish between spectator and reacting species. Herein, modulation excitation (ME) time-resolved (energy dispersive) X-ray absorption spectroscopy, assisted by phase sensitive detection (PSD) analysis, has been applied to the study of a liquid phase process; in this case the classic ferrocyanide/ferricyanide redox couple. Periodic switches of the electrical potential (anodic/cathodic) enabled the use of the ME approach. Structural changes at fractions as low as 2% of the total number of electroactive species were detected within the X-ray beam probe volume containing ~30 pmol of Fe(II)/Fe(III).

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework.

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 µmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 µmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

Heterogenised catalysts for the H-transfer reduction reaction of aldehydes: influence of solvent and solvation effects on reaction performances.

Heterogenisation of homogeneous catalysts onto solid supports represents a potential strategy to make the homogeneous catalytic function recyclable and reuseable. Yet, it is usually the case that immobilised catalysts have much lower catalytic activity than their homogeneous counterpart. In addition, the presence of a solid interface introduces a higher degree of complexity by modulating solid/fluid interactions, which can often influence adsorption properties of solvents and reactive species and, ultimately, catalytic activity. In this work, the influence of support and solvent in the H-transfer reduction of propionaldehyde over Al(OiPr)3-SiO2, Al(OiPr)3-TiO2 and Al(OiPr)3-Al2O3 heterogenised catalysts has been studied. Reaction studies are coupled with both NMR relaxation measurements as well as molecular dynamics (MD) simulations in order to unravel surface and solvation effects during the reaction. The results show that, whilst the choice of the support does not influence significantly catalytic activity, reactions carried out in solvents with high affinity for the catalyst surface, or able to hinder access to active sites due to solvation effects, have a lower activity. MD calculations provide key insights into bulk solvation effects involved in such reactions, which are thought to play an important role in determining the catalytic behaviour. The activity of the heterogenised catalysts was found to be comparable with that of the homogeneous Al(OiPr)3 catalysts for all supports used, showing that for the type of reaction studied immobilisation of the homogeneous catalyst onto solid supports is a viable, robust and effective strategy.

Microstructural and thermodynamic characterization of wormlike micelles formed by polydisperse ionic surfactant solutions.

For industrial applications of self-assembled wormlike micelles, measurement and characterization of a micellar material's microstructure and rheology are paramount for the development and deployment of new high-performing and cost-effective formulations. Within this workflow, there are significant bottlenecks associated with experimental delays and a lack of transferability of results from one chemistry to another. In this work, we outline a process to predict microscopic and thermodynamic characteristics of wormlike micelles directly from rheological data by combining a more robust and efficient fitting algorithm with a recently published constitutive model called the Toy Shuffling model [J. D. Peterson and M. E. Cates, J. Rheol. 64, 1465-1496 (2020) and J. D. Peterson and M. E. Cates, J. Rheol. 65, 633-662 (2021)]. To support this work, linear rheology measurements were taken for 143 samples comprising a common base formulation of commercial sodium lauryl ether sulfate, cocamidopropyl betaine, and salt (NaCl). The steady state zero shear viscosity evident in linear rheology was measured in duplicate via direct steady and oscillatory shear experiments. Fitting the collected data to the model, we found trends in the microstructural and thermodynamic characteristics that agree with molecular dynamics simulations. These trends validate our new perspective on the parameters that inform the study of the relationship between chemical formulation and rheology. This work, when implemented at scale, can potentially be used to inform and test strategies for predicting self-assembled micellar structures based on chemical formulation.

Shielding Protection by Mesoporous Catalysts for Improving Plasma-Catalytic Ambient Ammonia Synthesis.

Plasma catalysis is a promising technology for decentralized small-scale ammonia (NH3) synthesis under mild conditions using renewable energy, and it shows great potential as an alternative to the conventional Haber-Bosch process. To date, this emerging process still suffers from a low NH3 yield due to a lack of knowledge in the design of highly efficient catalysts and the in situ plasma-induced reverse reaction (i.e., NH3 decomposition). Here, we demonstrate that a bespoke design of supported Ni catalysts using mesoporous MCM-41 could enable efficient plasma-catalytic NH3 production at 35 °C and 1 bar with >5% NH3 yield at 60 kJ/L. Specifically, the Ni active sites were deliberately deposited on the external surface of MCM-41 to enhance plasma-catalyst interactions and thus NH3 production. The desorbed NH3 could then diffuse into the ordered mesopores of MCM-41 to be shielded from decomposition due to the absence of plasma discharge in the mesopores of MCM-41, that is, "shielding protection", thus driving the reaction forward effectively. This promising strategy sheds light on the importance of a rational design of catalysts specifically for improving plasma-catalytic processes.

Atomically Dispersed Copper Sites in a Metal-Organic Framework for Reduction of Nitrogen Dioxide.

Metal-organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity, and designable functionality. However, the unambiguous identification of atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically dispersed copper sites in UiO-66 for the catalytic reduction of NO2 at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N2 at a 97% conversion of NO2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h-1 is achieved under nonthermal plasma activation. In situ and operando infrared, solid-state NMR, and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO2 molecules, with the formation of {Cu(I)···NO} and {Cu···NO2} adducts promoting the conversion of NO2 to N2. This study will inspire the further design and study of new efficient single-atom catalysts for NO2 abatement via detailed unravelling of their role in catalysis.

Elucidating the role of H2O in promoting the formation of methacrylic acid during the oxidation of methacrolein over heteropolyacid compounds.

The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H3PMoO12O40 catalyst was investigated using an in situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. Comparing the spectra and activity data using D2O instead of H2O during transient switching experiments has allowed us to evaluate the possible active sites where D2O is activated. It has been found that, during the cycling switches of D2O in and out of the MAL + O2 gas feed at 320 °C, the formation of MAA-OD product is increased and decreased when D2O is added and removed, respectively. This suggests that the deuterium from D2O is involved in the production of gas phase MAA-OD. In addition, the in situ DRIFTS-MS results obtained from the isotopic switches between D2O and H2O reveal changes in the characteristic infrared bands of the Keggin unit between 1200 and 600 cm-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of Mo-O-Mo unit, where water is activated for the formation of MAA. Based on the in situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H3PMoO12O40 catalyst is proposed.

Contrasting the EXAFS obtained under air and H2 environments to reveal details of the surface structure of Pt-Sn nanoparticles.

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ∼0.09 Šshorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.

Probing the dynamics and structure of confined benzene in MCM-41 based catalysts.

A combination of Molecular Dynamics (MD) simulations and Quasielastic Neutron Scattering (QENS) experiments has been used to investigate the dynamics and structure of benzene in MCM-41 based catalysts. QENS experiments of benzene as both an unconfined liquid and confined in the catalyst Pt/MCM-41 find that the mobility of benzene decreases upon confinement as shown by the decreased diffusion coefficients. Complementary MD simulations on benzene in MCM-41 show agreement with the QENS experiments when using a novel fully flexible model of MCM-41. Structural information from the MD simulations show that benzene in MCM-41 has a significantly different structure from that of the bulk liquid; with benzene molecules closer together and no prefered orientation.

Self-Limiting Growth of Two-Dimensional Palladium between Graphene Oxide Layers.

The ability of different materials to display self-limiting growth has recently attracted an enormous amount of attention because of the importance of nanoscale materials in applications for catalysis, energy conversion, (opto)electronics, and so forth. Here, we show that the electrochemical deposition of palladium (Pd) between graphene oxide (GO) sheets result in the self-limiting growth of 5-nm-thick Pd nanosheets. The self-limiting growth is found to be a consequence of the strong interaction of Pd with the confining GO sheets, which results in the bulk growth of Pd being energetically unfavorable for larger thicknesses. Furthermore, we have successfully carried out liquid exfoliation of the resulting Pd-GO laminates to isolate Pd nanosheets and have demonstrated their high efficiency in continuous flow catalysis and electrocatalysis.

Two-Dimensional Covalent Crystals by Chemical Conversion of Thin van der Waals Materials.

Most of the studied two-dimensional (2D) materials have been obtained by exfoliation of van der Waals crystals. Recently, there has been growing interest in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum. Here, we report an approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we used 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF3), which has a nonlayered, rhombohedral structure and therefore cannot  possibly be obtained by exfoliation. The conversion of InSe into InF3 is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF3 layers are doped with selenium. We study this new 2D material by optical, electron transport, and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation, into InF3 films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently bonded noncleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals.

Industrial Applications of Ionic Liquids.

Since their conception, ionic liquids (ILs) have been investigated for an extensive range of applications including in solvent chemistry, catalysis, and electrochemistry. This is due to their designation as designer solvents, whereby the physiochemical properties of an IL can be tuned for specific applications. This has led to significant research activity both by academia and industry from the 1990s, accelerating research in many fields and leading to the filing of numerous patents. However, while ILs have received great interest in the patent literature, only a limited number of processes are known to have been commercialised. This review aims to provide a perspective on the successful commercialisation of IL-based processes, to date, and the advantages and disadvantages associated with the use of ILs in industry.

HfN Nanoparticles: An Unexplored Catalyst for the Electrocatalytic Oxygen Evolution Reaction.

Water electrolysis is one of the most promising methods to produce H2 and O2 as high potential fuels. Comparing the two half-reactions, the oxygen evolution reaction (OER) is the more difficult to be optimized and still relies on expensive noble metal-based catalysts such as Ru or Ir. In this paper, we prepared nanoparticles of HfN and Hf2 ON2 and tested them for the OER for the first time. The HfN sample, in particular, showed the highest activity, requiring an overpotential of only 358 mV at 10 mA cm-2 in Fe-free electrolyte and, above all, exhibiting long-term stability. This result places this system amongst one of the most promising catalysts for OER tested to date, in terms of sustainability, activity and stability. The prepared nanoparticles are small (less than 15 nm in diameter), well-defined in shape and crystalline, and were characterised before and after electrochemical testing also via electron microscopy (EM), powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS).

Diffusion, Ion Pairing and Aggregation in 1-Ethyl-3-Methylimidazolium-Based Ionic Liquids Studied by 1 H and 19 F PFG NMR: Effect of Temperature, Anion and Glucose Dissolution.

In this work, using 1 H and 19 F PFG NMR, we probe the effect of temperature, ion size/type and glucose dissolution on the rate of transport in 1-ethyl-3-methylimidazolium ([EMIM]+ )-based ionic liquids by measuring self-diffusion coefficients. Using such data, we are able to establish the degree of ion pairing and quantify the extent of ionic aggregation during diffusion. For the neat 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) a strong degree of ion pairing is observed. The substitution of the [OAc]- anion with the bis{(trifluoromethyl)sulfonyl}imide ([TFSI]- ) anion reduces the pairing between the ions, which is attributed to a lower electric charge density on the [TFSI]- anion, hence a weaker electric interaction with the [EMIM]+ cation. The effect of glucose, important for applications of ionic liquids as extracting media, on the strongly paired [EMIM][OAc] sample was also investigated and it is observed that the carbohydrate decreases the degree of ion pairing, which is attributed to the ability of glucose to disrupt inter-ionic interactions by forming hydrogen bonding, particularly with the [OAc]- anion. Calculations of aggregation number from diffusion data show that the [OAc]- anion diffuses as a part of larger aggregates compared to the [EMIM]+ cation. The results and analysis presented here show the usefulness of PFG NMR in studies of ionic liquids, giving new insights into ion pairing and aggregation and the factors affecting these parameters.

Acyclic and Cyclic Alkyl and Ether-Functionalised Sulfonium Ionic Liquids Based on the [TFSI]- and [FSI]- Anions as Potential Electrolytes for Electrochemical Applications.

This work provides a study based on acyclic and cyclic sulfonium ionic liquids (ILs) with alkyl and ether-functionality on the cation paired with the bis{(trifluoromethyl)sulfonyl}imide, [TFSI]- , or the bis(fluorosulfonyl)imide, [FSI]- , as the counter anion. Herein, thermophysical characterisation of nine sulfonium-based ILs concerning the density, viscosity and conductivity and thermal properties including phase transition behaviour and decomposition temperature is reported. The electrochemical stability of the ILs was also measured by cyclic voltammetry at a glassy carbon macro-disk electrode. All of the ILs showed low melting point, low viscosity and good conductivity and could serve as potential electrolytes for energy storage devices.

Effects of heat treatment atmosphere on the structure and activity of Pt3Sn nanoparticle electrocatalysts: a characterisation case study.

Comprehensive identification of the phases and atomic configurations of bimetallic nanoparticle catalysts are critical in understanding structure-property relationships in catalysis. However, control of the structure, whilst retaining the same composition, is challenging. Here, the same carbon supported Pt3Sn catalyst is annealed under air, Ar and H2 resulting in variation of the extent of alloying of the two components. The atmosphere-induced extent of alloying is characterised using a variety of methods including TEM, XRD, XPS, XANES and EXAFS and is defined as the fraction of Sn present as Sn0 (XPS and XANES) or the ratio of the calculated composition of the bimetallic particle to the nominal composition according to the stoichiometric ratio of the preparation (TEM, XRD and EXAFS). The values obtained depend on the structural method used, but the trend air < Ar < H2 annealed samples is consistent. These results are then used to provide insights regarding the electrocatalytic activity of Pt3Sn catalysts for CO, methanol, ethanol and 1-butanol oxidation and the roles of alloyed Sn and SnO2.

Impact of SCILL catalysts for the S-S coupling of thiols to disulfides.

This study reports the behaviour of SCILL based catalysts in the oxidative S-S coupling of aliphatic and aromatic thiols, namely 1-butanethiol and thiophenol, to dibutyl disulfide and diphenyl disulfide. A range of ionic liquids (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) and metal supported catalysts (5% Pt/SiO2; 5% Ru/SiO2; 5% Ru/C; 5% Pt/OMS-2) were used to prepare the SCILL catalysts and all were found to be active for the reaction following the trend 5% Pt-OMS-2 > 5% Pt/SiO2 > 5% Ru/C > 5% Ru/SiO2. The presence of SCILL catalysts afforded high selectivity to the disulfide, and the activity of the SCILL catalyst was dependent on the ionic liquid used. A significant increase in the stability of all the supported metal catalysts was found in the presence of the ionic liquid, and there was no change in the selectivity towards disulfides. This demonstrated that the ionic liquids protect the active sites of the catalyst against sulfation, thus providing more stable and active catalysts.

Non-thermal-plasma-activated de-NOx catalysis.

The combination of non-thermal plasma (NTP) with catalyst systems as an alternative technology to remove NOx emissions in the exhaust of lean-burn stationary and mobile sources is reviewed. Several factors, such as low exhaust gas temperatures (below 300°C), low selectivity to N2 and the presence of impurities, make current thermally activated technologies inefficient. Various hybrid plasma-catalyst systems have been examined and shown to have a synergistic effect on de-NOx efficiency when compared with NTP or catalyst-alone systems. The NTP is believed to form oxygenated species, such as aldehydes and nitrogen-containing organic species, and to convert NO to NO2, which improves the reduction efficiency of N2 during hydrocarbon-selective catalytic reduction reactions. The NTP has been used as a pretreatment to convert NO to its higher oxidation states such as NO2 to improve NOx reduction efficiency in the subsequent processes, e.g. NH3-selective catalytic reduction. It has been applied to the lean phase of the NOx storage to improve the adsorption capacity of the catalyst by conversion of NO to NO2 Alternatively, a catalyst with high adsorption capacity is chosen and the NTP is applied to the rich phase to improve the reduction activity of the catalyst at low temperature.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Confinement Effects on the Benzene Orientational Structure.

Liquids under confinement exhibit different properties compared with their corresponding bulk phases, for example, miscibility, phase transitions, and diffusion. The underlying cause is the local ordering of molecules, which is usually only studied using pure simulation methods. Herein, we derive experimentally the structure of benzene confined in MCM-41 using total neutron scattering measurements. The study reveals a layering of molecules across a pore, and four concentric cylindrical shells can be distinguished for a pore with the radius of 18 Å. The nanoscale confinement of the liquid has a major effect on the spatial and orientational correlations observed between the molecules, when compared with the structure of the bulk liquid. These differences are most marked for molecules in parallel configurations, and this suggests differences in chemical reactivity between the confined and bulk liquids.

Thermophysical and Electrochemical Properties of Ethereal Functionalised Cyclic Alkylammonium-based Ionic Liquids as Potential Electrolytes for Electrochemical Applications.

A series of hydrophobic room temperature ionic liquids (ILs) based on ethereal functionalised pyrrolidinium, piperidinium and azepanium cations bearing the bis[(trifluoromethyl)sulfonyl]imide, [TFSI]- , anion were synthesized and characterized. Their physicochemical properties such as density, viscosity and electrolytic conductivity, and thermal properties including phase transition behaviour and decomposition temperature have been measured. All of the ILs showed low melting point, low viscosity and good conductivity and the latter properties have been discussed in terms of the IL fragility, an important electrolyte feature of the transport properties of glass-forming ILs. Furthermore, the studied [TFSI]- -based ILs generally exhibit good electrochemical stabilities and, by coupling electrochemical experiments and DFT calculations, the effect of ether functionalisation at the IL cation on the electrochemical stability of the IL is discussed. Preliminary investigations into the Li-redox chemistry at a Cu working electrode are also reported as a function of ether-functionality within the pyrrolidinium-based IL family. Overall, the results show that these ionic liquids are suitable for electrochemical devices such as battery systems, fuel cells or supercapacitors.