Modifying the Electrocatalytic Selectivity of Oxidation Reactions with Ionic Liquids

Ionic liquids (ILs), salts with melting points below 100 °C, represent a fascinating class of liquid materials typically characterized by an extremely low vapor pressure. Besides their application as new solvents or as electrolytes for electrochemical purposes, there are two important concepts of using ILs in catalysis: Liquid–liquid biphasic catalysis and IL thin film catalysis. Liquid–liquid biphasic catalysis enables either a very efficient manner to apply catalytic ILs, e.g. in Friedel–Crafts reactions, or to apply ionic transition metal catalyst solutions. In both cases, phase separation after reaction allows an easy separation of reaction products and catalyst re-use. One problem of liquid–liquid biphasic catalysis is mass transfer limitation. If the chemical reaction is much faster than the liquid–liquid mass transfer the latter limits the overall reaction rate. This problem is overcome in IL thin film catalysis where diffusion pathways and thus the characteristic time of diffusion are short. Here, Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL) are the two most important concepts. In both, a high surface area solid substrate is covered with a thin IL film, which contains either a homogeneously dissolved transition metal complex for SILP, or which modifies catalytically active surface sites at the support for SCILL. In each concept, interface phenomena play a very important role: These may concern the interface of an IL phase with an organic phase in the case of liquid–liquid biphasic catalysis. For IL thin film catalysis, the interfaces of the IL with the gas phase and with catalytic nanoparticles and/or support materials are of critical importance. It has recently been demonstrated that these interfaces and also the bulk of ILs can be investigated in great detail using surface science studies, which greatly contributed to the fundamental understanding of the catalytic properties of ILs and supported IL materials. Exemplary results concerning the IL/vacuum or IL/gas interface, the solubility and surface enrichment of dissolved metal complexes, the IL/support interface and the in situ monitoring of chemical reactions in ILs are presented. Important concepts in catalysis with ionic liquids are reviewed, including Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL), along with the detailed analysis of the relevant interfaces using surface science methods.

This work examines important physicochemical and thermophysical properties of ultrathin ionic liquid (IL) layers under confinement into the pore structure of siliceous supports and brings significant advances toward understanding the effects of these properties on the gas separation and catalytic performance of the developed supported ionic liquid phase (SILP) and solid catalysts with ionic liquid layers (SCILL). SILPs were developed by making use of functionalized and nonfunctionalized ILs, such as 1-(silylpropyl)-3-methyl-imidazolium hexafluorophosphate and 1-butyl-3-methyl-imidazolium hexafluorophosphate ILs, whereas the SCILL was prepared by effectively dispersing gold nanoparticles (AuNPs) onto the IL layers inside the open pores of the SILP. The information derived from the gas absorption/diffusivity and heterogeneous catalysis experiments was exemplified in relation to the liquid crystalline ordering and orientation of the IL molecules, investigated by X-ray diffraction (XRD) and modulated differential scanning calorimetry (MDSC). The extent of pore blocking was elucidated with small angle neutron scattering (SANS) and was proven to be a decisive factor for the gas separation efficiency of the SILPs. CO2/CO separation values above 50 were obtained in cases where liquid crystalline ordering of the IL layers and extended pore blocking had occurred. The presence of the IL layer in the developed SCILL assisted the formation of ultrasmall (2–3 nm) and well-stabilized AuNPs. The low-temperature CO oxidation efficiency was 22%. The catalytic experiments showed an additional functionality of the IL, acting as an “in-situ trap” that abstracts the product (CO2) from the reaction site and improves yield.

Coating heterogeneous catalysts with ionic liquids (ILs), a strategy known as 'solid catalysts with ionic liquid layers', can fine-tune catalytic selectivity. Introducing functional groups into ILs enhances their interaction with reactants, but precise control over their positioning is crucial. The structural formation in the IL wetting layer of the carbonyl-functionalized IL [5-oxo-C6C1Im][NTf2] on Au(111) is investigated using infrared reflection absorption spectroscopy and scanning tunneling microscopy under ultrahigh vacuum conditions, supported by density functional theory and molecular dynamics simulations. At low temperatures (<130 K), the IL forms disordered islands, which coalesce into ordered films near ambient temperature. At low coverage, the IL adopts flat, space-demanding adsorption geometries. Upon forming a closed film, adsorption shifts to more compact configurations, with the carbonyl group tilting toward the vacuum while the ring remains surface-bound. Deposition at 300 K forms crystalline structures in the sub-monolayer regime, where the cation side chain can either stand upright or lie flat depending on the coverage. The IL remains thermally stable and desorbs completely at 500 K without decomposition. These findings highlight how IL coverage and deposition conditions tune functional group orientation at the catalyst interface, optimizing SCILL performance.

Introduction Hydrogen fuel cells could play a key role in the decarbonization of the energy sector. However, their commercialization is hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and by the requirement of platinum, which is expensive and unsustainable. Alternative catalysts based on transition metals have proven promising, but their activity and durability is still far from that of platinum.Recently, thin layers of ionic liquids have been successfully used to improve both the durability and activity of noble-metal free ORR catalysts[1]. As shown in Figure 1, the presence of the ionic liquid layer can influence the reaction kinetics through multiple effects: (i) O2 transport, (ii) water transport (iii) proton transport and (iv) binding to the reaction intermediate.These competing effects are generally convoluted. To that end, we have recently developed models to quantify the effect of the ionic liquid on both oxygen transport and reaction kinetics. Effect of Ionic liquids on reaction kinetics The ionic liquids tested for oxygen reduction are hydrophobic and their degree of water uptake is largely controlled by the cation. By decreasing the concentration of water at the active sites, they can cause the dehydration of the *OH intermediate, ultimately weakening the *OH bond[2]. For those catalysts such as Pt, which sit on the strong binding side of the volcano, this weakening of OH causes an increase in activity[3].To the contrary, we have now shown that ionic liquid layers decrease the activity of catalyst on the weak binding side of the volcano, such as iron phtalocyanine (see Figure 2). By changing ionic liquid, we can control hydrophobicity and by monitoring the position of the voltammetric peak for *OH adsorption we can probe the strength of *OH binding. In this way, we are able to confirm the earlier hypothesis that hydrophobicity controls *OH binding[2]. We hence provide a new lever for tailoring the reaction kinetics of oxygen reduction for any transition metal catalyst using ionic liquids. Oxygen Transport in ionic liquid layers The ideal ionic liquid should transport oxygen quickly, while featuring high oxygen solubility. However, it has been shown that O2 solubility and diffusivity cannot be independently tuned, as, for example, fluorination of the anion improves oxygen concentration, at the expense of diffusivity[4]. The optimum balance between these two parameters has been so far unclear. To that end, we have developed a way to deconvolute the effect of oxygen solubility and diffusivity.Using gravimetric, volumetric and electrochemical techniques, we reliably characterized oxygen transport in ionic liquids, and we were able to experimentally distinguish between oxygen activity and concentration. By using Fick’s diffusion and the Faraday law, we obtained a prediction for the diffusion-limited current in a rotating disc-electrode, as a function of the nature of the ionic liquid, its thickness and the rotational speed. As shown in in Figure 3, the diffusion-limited current density correlates to the permeability of oxygen in the ionic liquid layer; with our experimental data confirming the model.When looking at the diffusion-limited region, this model leads to the trivial conclusion that oxygen transport can be improved by maximizing oxygen permeability in the ionic liquid and increasing the thickness to that of the diffusion layer. However, real fuel cell devices are operated at a potential where both oxygen transport and kinetics are rate-limiting.In this case, the reaction rate is proportional to oxygen activity in the ionic liquid. Using the model developed, we were able to conclude that oxygen activity should be maximised over transport and that a single monolayer is the ideal ionic liquid coverage.Finally, we used a sorption analyser to study the ionic liquid distribution on the surface of the catalyst. We observed that rather than forming the ideal monolayer, the ionic liquid fills up the smallest pores first and tends to block bigger pores. Our current work is focusing on improving catalyst wettability by the ionic liquid to approach the ideal distribution. Conclusion In conclusion, the cation and anion of ionic liquids can be independently tailored to control hydrophobicity on one side and O2 activity on the other. This can ultimately allow to optimize the binding energy of key intermediates and maximize the reactant concentration at the active site. Our approach provides a rational method to improve the activity of noble-metal-free oxygen reduction catalysts. [1] Favero et al., Adv. Energy Sustainability Res. 2021, 2, 2000062 [2] Casalongue et al., Nat. Commun., 2013, 4, 2817 [3] Huang et al., J. Electrochem. Soc., 2017, 164, F1448 [4] Vanhouette et al., RCS Adv., 2018, 8, 4525 Figure 1

The effect of ionic liquid on alumina supported copper catalysts for the competitive hydrogenation of octanal in the presence of octene

A new concept of a solid catalyst with ionic liquid layer (SCILL) as a novel method to improve the selectivity of heterogeneous catalysts is presented. The sequential hydrogenation of cyclooctadiene (COD) to cyclooctene (COE) and cyclooctane on a commercial Ni catalyst coated with the ionic liquid [BMIM][n‐C8H17OSO3] was tested as first model system. Compared to the original catalyst, the coating of the internal surface with the ionic liquid (IL) strongly enhances the maximum intrinsic COE yield from 40 to 70 %. This effect is already achieved for a pore filling degree of only 10 % and cannot be explained by pore diffusion, as shown by experiments with different particle sizes and theoretical considerations. The IL layer is very robust and no leaching into the organic phase was detectable.

In a solid catalyst with ionic liquid layer (SCILL), ionic liquid (IL) coatings are used to improve the selectivity of noble metal catalysts. To understand the origins of this selectivity control, we performed model studies by surface science methods in ultrahigh vacuum (UHV). We investigated the growth and thermal stability of ultrathin IL films by infrared reflection absorption spectroscopy (IRAS). We combined these experiments with scanning tunneling microscopy (STM) to obtain information on the orientation of the ions, the interactions with the surface, the intermolecular interactions, and the structure formation. Additionally, we performed density-functional theory (DFT) calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2C1Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ~450 K. [C2C1Im][OTf] preferentially adsorbs at the step edges and elbows of the herringbone reconstruction of Au(111). The anion adsorbs via the SO3 group with the molecular axis perpendicular to the surface. At low coverage, the [C2C1Im][OTf] crystallizes in a glass-like 2D phase with short-range order. At higher coverage, we observe a phase transition to a 6-membered ring structure with long-range order.

Materials making use of thin ionic liquid (IL) films as support-modifying functional layer open up a variety of new possibilities in heterogeneous catalysis, which range from the tailoring of gas-surface interactions to the immobilization of molecularly defined reactive sites. The present report reviews recent progress towards an understanding of "supported ionic liquid phase (SILP)" and "solid catalysts with ionic liquid layer (SCILL)" materials at the microscopic level, using a surface science and model catalysis type of approach. Thin film IL systems can be prepared not only ex-situ, but also in-situ under ultrahigh vacuum (UHV) conditions using atomically well-defined surfaces as substrates, for example by physical vapor deposition (PVD). Due to their low vapor pressure, these systems can be studied in UHV using the full spectrum of surface science techniques. We discuss general strategies and considerations of this approach and exemplify the information available from complementary methods, specifically photoelectron spectroscopy and surface vibrational spectroscopy.

Ionic liquids (ILs) are capable of tuning the kinetics of electroreduction processes by modifying a catalyst interface. In this work, a group of hydrophobic imidazolium-based ILs were immobilized on Ag foams by using a procedure known as "solid catalyst with ionic liquid layer" (SCILL). The derived electrocatalysts demonstrated altered selectivity and CO production rates for the electrochemical reduction of CO2 compared to the unmodified Ag foam. The activity change caused by the IL was dependent on the length of the N-alkyl substituent. The rate of CO production is optimized at moderate chain length and IL loadings. The observed trends are attributed to a local enrichment of CO2-based species in the proximity of the catalyst and a modification of the environment of its active sites. On the contrary, high loadings or long IL chains render the surface inaccessible and favor the hydrogen evolution reaction.

Solid catalysts with ionic liquid layers (SCILLs) are heterogeneous catalysts which benefit significantly in terms of selectivity from a thin coating of an ionic liquid (IL). In the present work, we study the interaction of CO with a Pd model SCILL consisting of a 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-imide ([C4C1Pyr][NTf2]) film deposited on Pd(111). We investigate the CO permeability and stability of the IL film via pressure modulation experiments by infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum (UHV) and at ambient pressure conditions by time-resolved, temperature-programmed, and polarization-modulated (PM) IRAS experiments. In addition, we performed molecular dynamics (MD) simulations to identify adsorption motifs, their abundance, and the influence of CO. We find a strongly bound IL wetting monolayer (ML) and a potentially dewetting multilayer. Molecular reorientation of the IL at the interface and multilayer dewetting allow for the accumulation of CO at the metal/IL interface. Our results confirm that co-adsorption of CO changes the molecular structure of the IL wetting layer which confirms the importance to study model SCILL systems under in situ conditions.Graphical abstract

Modifying the chemical environment of active surfaces with ionic liquids (IL) is an emerging strategy for tailoring novel electrocatalytic systems, including carbon dioxide reduction (CO2RR). Although copper (Cu) catalysts have recently gained more attention in this field, their modification with ILs is yet to be investigated. This work tested a range of common hydrophobic ILs impregnated into carbon‐supported Cu catalysts, following the “solid catalyst with ionic liquid layer” (SCILL) approach. The latter was used to showcase the applicability of real‐time product detection for CO2RR employing electrochemical mass spectrometry. The observed patterns of C1 to C3 product selectivity offered valuable insights into the intricate reaction mechanism. In addition, increasing the size of the IL cation showed an opposite and significant effect on the reaction selectivity. The obtained qualitative results were partially compared with conventional long‐term experiments.

Electric plasma assisted pyrolysis of methane represents a highly promising greener alternative to produce ethylene from biogas and renewable energies compared to conventional steam cracking of naphtha. The mediocre performance of typical Pd-Ag catalysts for the downstream purification of the substantially higher concentrated acetylene impurities (≥15 vol.-%) in those ethylene streams via selective hydrogenation is yet limiting economic interest. Following the concept of solid catalysts with ionic liquid layer (SCILL), we have modified an intrinsically non-selective palladium catalyst with imidazolium based ionic liquids varying among 10 different anions and investigated them in this reaction. The best performing [C4C1IM][MeSO4]-SCILL reaches an outstanding average ethylene selectivity over 20 h on-stream of 82 % at full acetylene conversion without any sign of deactivation, clearly outperforming conventional Pd-Ag catalysts. By varying parameters like ionic liquid (IL) loading, temperature, feed gas composition, cations, and by using XPS for surface analysis we could gain a very comprehensive understanding of the underlying mechanisms that reduce the competing over-hydrogenation and oligomerisation side-reactions.

Ionic liquids are an interesting class of electrolytes both from a fundamental scientific standpoint and from an applications standpoint due to a number of relevant applications like batteries, supercapacitors and solar cells. Even though the constituents of ionic liquids are only ions, the conductivities are known to be pretty low. The suggested reasons for this low conductivity are low mobility due to high viscosity and the existence of most ions as ion pairs. Even though they have low conductivities, their electrochemical potential window and their low to non-existent vapor pressures make them very attractive as electrolytes. Because ionic liquids have no measurable vapor pressure, in-situ X-Ray Spectroscopy (XPS) can be performed directly on the ionic liquids during electrochemical experiments. This is not possible with more conventional electrolytes since any other electrolytic solution would not be conducive to running experiments inside the ultra-high vacuum (UHV) chamber. We have been doing experiments investigating both Faradaic and non-Faradaic aspects of electrochemical experiments on ionic liquids inside the XPS chamber. On the Faradaic side we have recently shown that the reduction of imidazolium based ionic liquids can yield stable carbene species (Aydogan-Gokturk et.al. New Journal of Chemistry 18(2017), 10299-10304 and Aydogan-Gokturk et.al. Electrochimica Acta 234(2017), 37-42) and that gold nanoparticles can be oxidatively synthesized starting from metal electrodes (Camci et.al. ACS Omega, 2(2017), 478-486). On the non-Faradaic front, investigations of the electrochemical double layer revealed that the applied potential has transient effects that are sensed unexpectedly far away from the electrode surface (Camci et.al. Phys. Chem. Chem. Phys. 18(2016)28434-28440). Specifically, our experiments indicate that the effects of the applied potential are felt up to millimeters away from the electrode surface during the transients. These transients can last hundreds of seconds and dominate the response. The current contribution is going to elaborate more on the unexpected electrochemical double layer effects. Even though the electrochemical double layer in electrolytes is well understood since the days of Gouy-Chapman-Stern model development, the double layer in the ionic liquid media is known to be somewhat different. There is a sizable body of recent modeling literature on the electrochemical double layer in ionic liquids. For example, work by Bazant et.al. (Phys. Rev. Lett. 106(2011) 046102), Lee et.al.(Phys. Rev. Lett. 115(2015)106101) and Gavish et.al.(J. Phys. Chem. Lett. 7(2016), 1121-1126) consider continuum type models of various sorts to model the behavior of the anions and the cations in great detail. The results indicate that there is some layering behavior that are either close to the electrode surface or throughout the region between the electrodes. The models used, though very detailed and thorough neglect ion pairing and thermal fluctuations. The effect of ion pairing is a crucial part of the explanation of the low conductivity of ionic liquids. On the other hand, thermal fluctuations are a necessary part of the conventional explanation of the diffuse double layer. In most cases, the models developed predict a layered steady state structure and also predict that the Debye length would be on the order of 10-20 times the size of a monolayer for most cases. Our experiments indicate that the effect of the applied potential is felt way farther away from the electrode in the transient times. We have developed a simpler modeling framework that model diffusion, convection and ion pairing using simple algebraic expressions on a simple grid that is set up. This framework involves setting up a grid between the two electrodes and starting with a homogeneous ionic liquid. At every time step, the algorithm iterates over the grid and treats diffusion, migration and ion pairing separately using simple constructs. Diffusion is simply handled by a random number generator that determine which way the ions will move, if at all. Then, the potential difference between neighboring points of the grid is used to evaluate the electric fields, which is used to then treat migration. Finally, for every position on the grid, a simple chemical equilibrium calculation is employed to solve for the number of ions that pair up. Using this framework to model the behavior of the ionic liquids and tuning the parameters of the simulation based on the results of transient measurements inside the XPS chamber using Squarewave applied potentials, we will report behavior of the electrochemical double layer in ionic liquids and insights about various parameters of ionic liquids such as ion-pairing constants, mobilities and diffusion coefficients. Figure 1

Polymer electrolyte fuel cells (PEFCs) are expected as one of the clean, highly effective energy conversion systems, and have been actively studied. Pt nanoparticles on carbon supports (Pt/C) are commonly used as a cathode catalyst for PEFCs, because of the high activity on oxygen reduction reaction (ORR). However, its durability in long term driving and high mass activity for ORR have not sufficiently fulfilled at the present. More improvements of Pt/C catalyst for ORR activity and the durability are required for the spread of PEFCs To improve the activity and durability of Pt/C, ionic liquids (ILs) modified catalysts, called Pt/C-Solid catalysis with an IL layer (SCILL) catalysts, have recently attracted much attention [1]. ILs are liquid salts around room temperature, which have high ionic conductivity and electrochemical stability. We focused on hydrophobic ionic liquids with quaternary phosphonium cations [2]. Quaternary phosphonium cations ionic liquids have higher ionic conductivity and hydrophobicity than quaternary ammonium ionic liquids with the same structure and the same counter ions. Because of interference in the oxygen reduction reaction on platinum by water, increasing the hydrophobicity of the surface is expected to improve the activity on Pt/C catalysis. Hence, by using the quaternary phosphonium cations ILs for Pt/C-SCILL catalysts, it promises to improve the ORR activities for the catalyst. On the other hand, proton conduction and oxygen permeation rate are also important factors for ORR activity enhancement. In this study, we report the ORR activity for Pt/C-SCILL catalysts with ILs composed of highly hydrophobic cations and hydrophobic anions. Quaternary phosphonium based ILs (PXXXY+TFSA-[alkyl chain X= 4 Y= 1, 12, 16]) were used for Pt/C-SCILL modified electrodes. Commercially available 20 wt% or 30 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The ultrasonicated Pt/C catalyst suspension was dropped on the mirror polished glassy carbon disk electrodes. Pt/C-SCILL modified electrodes were prepared by recasting the ILs-2-propanol solution on the bare Pt/C modified electrodes or adding ILs to the catalyst suspension. Moreover, the mixture of the ILs and bis(trifluoromethanesulfonyl)imide (HTFSA) were also examined. The thickness of the ILs layer was calculated relative to the BET surface area of the carbon support. A platinum wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. CV and LSV were carried out in 0.1 M HClO4 solution under Ar (CV) or O2 (LSV) atmosphere, respectively. The Pt/C-SCILL catalyst was prepared by recasting (thin layers, ~0.4 nm) or mixing (thick layers, 1nm~) methods. In the case of the thin layer (~0.4 nm) of ILs, the ORR activity increased as the size of the cations increased. However, for the thicker (1 nm ~) ILs layers, the ORR activity was lower than for the bare Pt/C catalysts. It seems that the aprotic ILs interfered with the proton supply, making the four-electron reduction reaction of oxygen less likely to occur. Therefore, we attempted to supply protons to the thick ILs layer by mixing an ionic liquid with a proton source. HTFSA was dissolved in P444(12)TFSA, which showed relatively good ORR activity in thin film studies. The Pt/C-SCILL catalyst was prepared by adding the mixture with a catalyst dispersion. This proton mixing Pt/C-SCILL catalyst showed a significant improvement in ORR activity compared to bare Pt/C catalyst.

Supported Ionic Liquid Phase (SILP) catalysts and Solid Catalyst with Ionic Liquid Layer (SCILL) both consist of an ionic liquid dispersed as a thin film on the inner surface of a highly porous solid material. In SILP catalysis the ionic liquid film contains a dissolved homogeneous transition metal complex and in this way a macroscopic solid adopts the chemical reactivity of a homogeneous, ligand-controlled catalyst. In SCILL materials the ionic liquid film modifies the catalytic sites of a classical heterogeneous catalyst in a ligand-like manner. In addition, the specific solubility of the IL film adjusts the local feedstock and product concentrations at the active center. Both, SILP and SCILL materials can be handled like classical heterogeneous catalysts and are suitable for classical fixed-bed technologies, a fact that makes transition to technical applications in larger scale very attractive. Our contribution focuses on some common key technical aspects of both SILP and SCILL technologies. The influence of support structure and support surface chemistry on the functionality of the prepared catalysts is highlighted. Optimization of the ionic liquid loading and large scale preparation technologies for these materials are other important aspects of the chapter.