Periodic Reporting for period 4 - ILID (Ionic Liquid Interface Dynamcis)
Periodo di rendicontazione: 2021-04-01 al 2022-01-31
The aim of the project was to take the ground-breaking step from our present knowledge of static properties to the understanding and control of dynamical processes at ionic liquid (IL) interfaces. ILs are chosen as model systems for liquids in general for two reasons: First, their structural diversity allows for tailoring their properties over a wide range, and second, they can be studied using the extremely powerful methods of surface science in ultra-high vacuum due to their low vapor pressure. Such studies cannot be performed for conventional liquids, since they evaporate.
ILs are not only relevant from a fundamental point of view, but also for a variety of real world applications. In catalysis, two concepts have been put forward recently: Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL). In both, a high surface area solid substrate is covered with a thin IL film, which contains a dissolved metal complex for SILP, or which modifies active sites at the support for SCILL. For these and other applications, a fundamental understanding of the dynamical processes at the gas/IL and/or IL/support interfaces was strongly needed, but did not exist. Equally important, but even more challenging, is the investigation of the dynamics of chemical reactions in ILs, also under electrochemical conditions.
The applicant proposed a multi-method approach with new and unique setups to follow these dynamical processes in real time. Four key topics were addressed:
(A) How do gases pass through the gas/liquid interface?
(B) How does the liquid/solid interface form?
(C) Real-time studies of reactions in ILs, and
(D) Real-time studies of electrochemical processes in ILs.
ILs are not only relevant from a fundamental point of view, but also for a variety of real world applications. In catalysis, two concepts have been put forward recently: Supported Ionic Liquid Phase (SILP) and Solid Catalyst with Ionic Liquid Layer (SCILL). In both, a high surface area solid substrate is covered with a thin IL film, which contains a dissolved metal complex for SILP, or which modifies active sites at the support for SCILL. For these and other applications, a fundamental understanding of the dynamical processes at the gas/IL and/or IL/support interfaces was strongly needed, but did not exist. Equally important, but even more challenging, is the investigation of the dynamics of chemical reactions in ILs, also under electrochemical conditions.
The applicant proposed a multi-method approach with new and unique setups to follow these dynamical processes in real time. Four key topics were addressed:
(A) How do gases pass through the gas/liquid interface?
(B) How does the liquid/solid interface form?
(C) Real-time studies of reactions in ILs, and
(D) Real-time studies of electrochemical processes in ILs.
Overall, the project ran very smoothly. Some delays due to the Corona pandemic were compensated by a cost-neutral extension by four months.
Topic A “How do gases pass through the gas/liquid interface?” required to build a new ultra-high vacuum-based molecular beam setup which allows for studying the interaction of gases with IL surfaces. The setup is in operation since Oct. 1, 2017. Three subtopics were addressed. The first concerned CO2 uptake of amine-based ILs, and turned out to be very challenging, since the sticking coefficients are very small, and CO2 absorption could only be verified by XPS. The second topic concerned the adsorption/sticking of hydrocarbons at IL surfaces. We investigated the interaction of n-butane and imidazolium-based ILs with different alkyl chain lengths (C1 to C8) at various temperatures. Adsorption at low temperatures occurs only for ILs with chains lengths of three and larger; we assigned this behavior to the dispersive interactions of butane with the alkyl chains of the IL. We also studied to role of the anions and compared the behavior of n‑butane, iso‑butane and 1‑butene. The third subtopic addressed the termination of the IL/vacuum interface. For mixtures of ILs with alkyl chains and fluorinated alkyl chains, we found a preferential enrichment of the fluorinated chains. This surface enrichment is driven by the lower surface tension of the fluorinated moieties; it decreases at higher temperatures due to entropic reasons. A related study addressed the surface behavior of low-temperature molten salt mixtures during the transition from liquid to solid.
Topic B “How does the liquid/solid interface form?” addressed the fundamental understanding of adsorption and wetting of ILs on solid surfaces. Novel results were obtained concerning the anion exchange at the liquid/solid interface of ultrathin IL films on Ag(111). We addressed the stability of IL layers against replacement, which is of highest relevance for catalytic applications [P3]. By studying different IL combinations, we observed exchange phenomena, and identified the adsorption energy of ions at the IL/solid interface and the surface free energy at the IL/vacuum interface as driving forces. This approach can be used to remove a specific IL from the IL/solid interface and desorb it to the gas phase without decomposition by an appropriate heating treatment. This opens important new insights into the stability of molecular systems on surfaces in general. We expanded our XPS studies towards variable temperature scanning tunneling and scanning force microscopy, and obtained novel results concerning surface order and growth between 100 and 300 K, along with time-dependent changes in the wetting behavior, which occur on the time scale of up to days.
Topic C “Real-time studies of reactions in ILs” also ran very successful. In a first subproject, we studied the reactions of a polyhalide ILs with Cu, Ag, Au and Mo surfaces. All three group 11 metals are strongly corroded by the IL at moderate temperatures to give a very high content of dissolved CuI, AgI, and AuI 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 the dynamics of metal oxidation by dibromoiodide anions to form molecular iodine and anionic [Br-M-Br]-complexes. In contrast, molybdenum is inert against the trihalide IL, due to surface passivation. In a second subproject, we showed with the group of Dr. G. Abellan and Prof. A. Hirsch that ILs can act as extremely effective media for the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state due to protection from water and air by the ionic liquid. These few layer pnictogen materials act as 2D catalysts for the alkylation of a variety of acid–sensitive organic molecules. In a third subproject, we studied chemical reactions within ultrathin IL layers on metal surfaces, and in particular addressed protic ILs and mixtures of protic and aprotic ILs. Here, we could demonstrate the on-surface metathesis of an IL on Ag(111).
Topic D “Real-time studies of electrochemical processes in ILs” was the most challenging part of project. As planned, we have developed a task-specific sample holder allowing for in situ studies in an electrochemical cell, and obtained most interesting results. The first subtopic concerned potential screening at electrodes for neat ILs and for IL mixtures. A particularly important aspect was the influence of ambient conditions on the potential screening. A second subtopic dealing with an electrochemical cell with an X-ray transparent graphene window was less successful.
26 publications already appeared in international peer-reviewed journals (A:9 / B:8 / C:6 / D:3).
Topic A “How do gases pass through the gas/liquid interface?” required to build a new ultra-high vacuum-based molecular beam setup which allows for studying the interaction of gases with IL surfaces. The setup is in operation since Oct. 1, 2017. Three subtopics were addressed. The first concerned CO2 uptake of amine-based ILs, and turned out to be very challenging, since the sticking coefficients are very small, and CO2 absorption could only be verified by XPS. The second topic concerned the adsorption/sticking of hydrocarbons at IL surfaces. We investigated the interaction of n-butane and imidazolium-based ILs with different alkyl chain lengths (C1 to C8) at various temperatures. Adsorption at low temperatures occurs only for ILs with chains lengths of three and larger; we assigned this behavior to the dispersive interactions of butane with the alkyl chains of the IL. We also studied to role of the anions and compared the behavior of n‑butane, iso‑butane and 1‑butene. The third subtopic addressed the termination of the IL/vacuum interface. For mixtures of ILs with alkyl chains and fluorinated alkyl chains, we found a preferential enrichment of the fluorinated chains. This surface enrichment is driven by the lower surface tension of the fluorinated moieties; it decreases at higher temperatures due to entropic reasons. A related study addressed the surface behavior of low-temperature molten salt mixtures during the transition from liquid to solid.
Topic B “How does the liquid/solid interface form?” addressed the fundamental understanding of adsorption and wetting of ILs on solid surfaces. Novel results were obtained concerning the anion exchange at the liquid/solid interface of ultrathin IL films on Ag(111). We addressed the stability of IL layers against replacement, which is of highest relevance for catalytic applications [P3]. By studying different IL combinations, we observed exchange phenomena, and identified the adsorption energy of ions at the IL/solid interface and the surface free energy at the IL/vacuum interface as driving forces. This approach can be used to remove a specific IL from the IL/solid interface and desorb it to the gas phase without decomposition by an appropriate heating treatment. This opens important new insights into the stability of molecular systems on surfaces in general. We expanded our XPS studies towards variable temperature scanning tunneling and scanning force microscopy, and obtained novel results concerning surface order and growth between 100 and 300 K, along with time-dependent changes in the wetting behavior, which occur on the time scale of up to days.
Topic C “Real-time studies of reactions in ILs” also ran very successful. In a first subproject, we studied the reactions of a polyhalide ILs with Cu, Ag, Au and Mo surfaces. All three group 11 metals are strongly corroded by the IL at moderate temperatures to give a very high content of dissolved CuI, AgI, and AuI 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 the dynamics of metal oxidation by dibromoiodide anions to form molecular iodine and anionic [Br-M-Br]-complexes. In contrast, molybdenum is inert against the trihalide IL, due to surface passivation. In a second subproject, we showed with the group of Dr. G. Abellan and Prof. A. Hirsch that ILs can act as extremely effective media for the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state due to protection from water and air by the ionic liquid. These few layer pnictogen materials act as 2D catalysts for the alkylation of a variety of acid–sensitive organic molecules. In a third subproject, we studied chemical reactions within ultrathin IL layers on metal surfaces, and in particular addressed protic ILs and mixtures of protic and aprotic ILs. Here, we could demonstrate the on-surface metathesis of an IL on Ag(111).
Topic D “Real-time studies of electrochemical processes in ILs” was the most challenging part of project. As planned, we have developed a task-specific sample holder allowing for in situ studies in an electrochemical cell, and obtained most interesting results. The first subtopic concerned potential screening at electrodes for neat ILs and for IL mixtures. A particularly important aspect was the influence of ambient conditions on the potential screening. A second subtopic dealing with an electrochemical cell with an X-ray transparent graphene window was less successful.
26 publications already appeared in international peer-reviewed journals (A:9 / B:8 / C:6 / D:3).
For all for Topics, we have performed either novel experimental developments or/and achieved novel scientific results. In A, we built up a worldwide unique experimental setup to study the dynamics of the gas/IL interaction. In B, we followed the dynamic exchange processes of cations and anions at the IL/solid interface and deduced a molecular level understanding of the ongoing exchange, replacement and reaction processes. In C, we performed unique in situ studies of thermochromic transitions, and of the dissolution of metals in ILs. In D, we developed a new and unique sample holder for in situ XPS under electrochemical conditions and obtained novel insight in the potential screening at electrode interfaces.
In all four topics, major progress was achieved, and the obtained understanding is a breakthrough not only for ILs, but for liquid interfaces in general.
In all four topics, major progress was achieved, and the obtained understanding is a breakthrough not only for ILs, but for liquid interfaces in general.