Physical Vapor Deposition of [EMIM][Tf2N]: A New Approach to the Modification of Surface Properties with Ultrathin Ionic Liquid Films

Ultrathin films of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][Tf₂N], are prepared on a glass substrate by means of an in situ thermal‐evaporation/condensation process under ultrahigh‐vacuum conditions. By using X‐ray photoelectron spectroscopy (XPS), it is demonstrated that the first layer of the IL film grows two dimensionally, followed by the three‐dimensional growth of successive layers. The first molecular layer consists of a bilayer, with the [EMIM]⁺ cations in contact to the surface and the [Tf₂N]^? anions at the vacuum side. The ultrathin IL films are found to be stable under ambient conditions.     

Formation and Surface Behavior of Pt and Pd Complexes with Ligand Systems Derived from Nitrile-functionalized Ionic Liquids Studied by XPS

We studied the formation and surface behavior of Pt(II) and Pd(II) complexes with ligand systems derived from two nitrile-functionalized ionic liquids (ILs) in solution using angle-resolved X-ray photoelectron spectroscopy (ARXPS). These ligand systems enabled a high solubility of the metal complexes in IL solution. The complexes were prepared by simple ligand substitution under vacuum conditions in defined excess of the coordinating ILs, [C₃CNC₁Im][Tf₂N] and [C₁CNC₁Pip][Tf₂N], to immediately yield solutions of the final products. The ILs differ in the cationic head group and the chain length of the functionalized substituent. Our XPS measurements on the neat ILs gave insights in the electronic properties of the coordinating substituents revealing differences in donation capability and stability of the complexes. Investigations on the composition of the outermost surface layers using ARXPS revealed no surface affinity of the nitrile-functionalized chains in the neat ILs. Solutions of the formed complexes in the nitrile ILs showed homogeneous distribution of the solute at the surface with the heterocyclic moieties preferentially orientated towards the vacuum, while the metal centers are rather located further away from the IL/vacuum interface.     

Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non‐functionalized ionic liquids (ILs). From angle‐resolved X‐ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium‐based ILs methylated at the C2 position, a phenyl‐functionalized IL, an alkoxysilane‐functionalized IL, halo‐functionalized ILs, thioether‐functionalized ILs, and amine‐functionalized ILs. The results are compared with the results for corresponding non‐functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.     

Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro‐N‐ethylcarbazole on Pd(111)

Dodecahydro‐N‐ethylcarbazole (H₁₂‐NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H₁₂‐NEC on Pd(111) by using a surface‐science approach under ultrahigh vacuum conditions. By combining infrared reflection–absorption spectroscopy, density functional theory calculations and X‐ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H₁₂‐NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C? H bonds is observed within the five‐membered ring. Rapid dehydrogenation occurs to octahydro‐N‐ethylcarbazole (H₈‐NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H₈‐NEC proceeds within the six‐membered rings. Starting from clean Pd(111), C? N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature‐programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H₈‐NEC as the main product desorbing from Pd(111). The onset temperature for H₈‐NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H₈‐NEC is observed even above the temperature threshold for H₈‐NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.     

Temperature-Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non-Fluorinated Ionic Liquids

Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we investigate the topmost nanometers of various binary ionic liquid (IL) mixtures at different temperatures in the liquid state. The mixtures consist of ILs with the same [PF₆]⁻ anion but two different cations, namely 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF₆], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C₄C₁Im][PF₆], with 10, 25, 50 and 75 mol % content of [PFBMIm][PF₆]. We observe a preferential enrichment of the fluorinated chain in the topmost layer, relative to the bulk composition, which is most pronounced for the lowest content of [PFBMIm][PF₆]. Upon cooling the mixtures stepwise from 95 °C until surface charging effects in XPS indicate solidification, we observe a pronounced increase in surface enrichment of the fluorinated chain with decreasing temperature in the liquid state. In contrast to the mixtures with lower [PFBMIm][PF₆] contents, cooling the 75 mol % mixture additionally shows an abrupt decrease of the fluorinated chain signal before complete solidification occurs, which is assigned to partial precipitation effects.     

过氧化氢直接氧化苯制备苯酚的多相催化研究进展*

作为一种绿色、洁净的氧化剂,使用过氧化氢直接氧化苯来制备苯酚的过程极具吸引力。综述了分子筛、碳基材料、金属氧化物、金属有机框架等多相催化剂在氧化苯制备苯酚反应中的应用,主要讨论了催化剂的合成路线、反应机理和催化性能,希望能为新的多相催化体系的合理设计提供有益的视角。     

Crystallography Aided by Atomic Core-Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Br?nsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.     

A Planarized Triphenylborane Mesogen: Discotic Liquid Crystals with Ambipolar Charge‐Carrier Transport Properties

A discotic liquid‐crystalline (LC) material, consisting of a planarized triphenylborane mesogen, was synthesized. X‐ray diffraction analysis confirmed that this compound forms a hexagonal columnar LC phase with an interfacial distance of 3.57 Å between the discs. At ambient temperature, this boron‐centered discotic liquid crystal exhibited ambipolar carrier transport properties with electron and hole mobility values of approximately 10^?3 and 3×10^?5 cm² V^?1 s^?1, respectively.     

Reactions of a Polyhalide Ionic Liquid with Copper,Silver, and Gold

The reactions of copper, silver, and gold with the imidazolium-based polyhalide ionic liquid (IL) [C₆C₁Im][Br₂I] were investigated by using X-ray photoelectron spectroscopy (XPS), weight-loss measurements, and gas-phase mass spectrometry. All three Group 11 metals are strongly corroded by the IL at moderate temperatures to give a very high content of dissolved Cu^I, Ag^I, and Au^I species. The IL–metal solutions are stable against contact with water and air. The replacement of imidazolium with inorganic sodium cations decreased metal corrosion rates by orders of magnitude. Our results clearly indicate metal oxidation by iodide from dibromoiodide anions to form molecular iodine and anionic [Br-M^I-Br]⁻ (M=Cu, Ag, Au) complexes stabilized by imidazolium counterions. From experiments with a trihalide IL with imidazolium methylated at the 2-position, we ruled out the formation of imidazole–carbene as a cause of the observed corrosion. In contrast to Group 11 metals, molybdenum is inert against the trihalide IL, which is attributed to surface passivation.     

Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures—A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study

We investigated the surface tension and surface composition of various mixtures of the two ionic liquids (ILs) 1-methyl-3-octyl-imidazolium hexafluorophosphate [C₈C₁Im][PF₆] and 1,3-bis(polyethylene glycol)imidazolium iodide [(mPEG₂)₂Im]I in the temperature range from 230 to 370 K under ultraclean vacuum conditions. The surface tension was measured using a newly developed apparatus, and the surface composition was determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). In the pure ILs, the alkyl chains of [C₈C₁Im][PF₆] and the PEG chains of [(mPEG₂)₂Im]I are enriched at the IL/vacuum interface. In the mixtures, a strong selective surface enrichment of the alkyl chains occurs, which is most pronounced at low [C₈C₁Im][PF₆] contents. For the surface tension, strong deviations from an ideal mixing behaviour take place. By applying a simple approach based on the surface composition of the mixtures as deduced from ARXPS, we are able to predict and reproduce the experimentally measured temperature-dependent surface tension values with astonishingly high accuracy.     

Ionic‐Liquid‐Mediated Active‐Site Control of MoS2 for the Electrocatalytic Hydrogen Evolution Reaction

The layered crystal MoS₂ has been proposed as an alternative to noble metals as the electrocatalyst for the hydrogen evolution reaction (HER). However, the activity of this catalyst is limited by the number of available edge sites. It was previously shown that, by using an imidazolium ionic liquid as synthesis medium, nanometre‐size crystal layers of MoS₂ can be prepared which exhibit a very high number of active edge sites as well as a de‐layered morphology, both of which contribute to HER electrocatalytic activity. Herein, it is examined how to control these features synthetically by using a range of ionic liquids as synthesis media. Non‐coordinating ILs with a planar heterocyclic cation produced MoS₂ with the de‐layered morphology, which was subsequently shown to be highly advantageous for HER electrocatalytic activity. The results furthermore suggest that the crystallinity, and in turn the catalytic activity, of the MoS₂ layers can be improved by employing an IL with specific solvation properties. These results provide the basis for a synthetic strategy for increasing the HER electrocatalytic activity of MoS₂ by tuning its crystal properties, and thus improving its potential for use in hydrogen production technologies.     

Monitoring of liquid-phase organic reactions by photoelectron spectroscopy

There are strings attached: after linking the reacting groups to head groups of ionic liquids to drastically lower the vapour pressures of the reactants, ordinary liquid-phase organic reactions can be monitored by in situ X-ray photoelectron spectroscopy. This approach is demonstrated for the nucleophilic substitution of an alkyl amine and an alkyl chloride moiety, which are attached to the cation and anion of ionic liquids, respectively.