Structural Evolution of GaOx-Shell and Intermetallic Phases in Ga-Pt Supported Catalytically Active Liquid Metal Solutions

We present a comprehensive scale-bridging characterization approach for supported catalytically active liquid metal solutions (SCALMS) which combines lab-based X-ray microscopy, nano X-ray computed tomography (nano-CT), and correlative analytical transmission electron microscopy. SCALMS catalysts consist of low-melting alloy particles and have demonstrated high catalytic activity, selectivity, and long-term stability in propane dehydrogenation (PDH). We established an identical-location nano-CT workflow which allows us to reveal site-specific changes of Ga-Pt SCALMS before and after PDH. These observations are complemented by analytical transmission electron microscopy investigations providing information on the structure, chemical composition, and phase distribution of individual SCALMS particles. Key findings of this combined microscopic approach include (i) structural evolution of the SCALMS particles' GaOx shell, (ii) Pt segregation toward the oxide shell leading to the formation of Ga-Pt intermetallic phases, and (iii) cracking of the oxide shell accompanied by the release of liquid Ga-Pt toward the porous support.

Kinetics of dehydrogenation of n-heptane over GaPt supported catalytically active liquid metal solutions (SCALMS)

The concept of Supported Catalytically Active Liquid Metal Solutions (SCALMS) was explored for the catalytic dehydrogenation of n-heptane. For this purpose, a GaPt on alumina (Ga84Pt/Al2O3) was compared with a Pt on alumina catalyst at different reaction temperatures and feed compositions. While the observed activation energies with both catalysts for the overall n-heptane depletion rate were similar with both catalysts, the SCALMS systems provides a lower activation energy for the desired dehydrogenation path and significantly higher activation energies for the undesired aromatization and cracking reaction. Thus, the SCALMS catalyst under investigation shows technically interesting features, in particular at high temperature operation. The partial pressure variation revealed an effective reaction order of around 0.7 for n-heptane for both catalysts, while the effective order for hydrogen was 0.35 for Pt/Al2O3 and almost zero for SCALMS.

Toward High-Energy-Density Fuels for Direct Liquid Organic Hydrogen Carrier Fuel Cells: Electrooxidation of 1-Cyclohexylethanol

Electrochemically active liquid organic hydrogen carriers (EC-LOHCs) can be used directly in fuel cells; so far, however, they have rather low hydrogen storage capacities. In this work, we study the electrooxidation of a potential EC-LOHC with increased energy density, 1-cyclohexylethanol, which consists of two storage functionalities (a secondary alcohol and a cyclohexyl group). We investigated the product spectrum on low-index Pt single-crystal surfaces in an acidic environment by combining cyclic voltammetry, chronoamperometry, and in situ infrared spectroscopy, supported by density functional theory. We show that the electrooxidation of 1-cyclohexylethanol is a highly structure-sensitive reaction with activities Pt(111) ≫ Pt(100) > Pt(110). Most importantly, we demonstrate that 1-cyclohexylethanol can be directly converted to acetophenone, which desorbs from the electrode surface. However, decomposition products are formed, which lead to poisoning. If the latter side reactions could be suppressed, the electrooxidation of 1-cyclohexylethanol would enable the development of EC-LOHCs with greatly increased hydrogen storage capacities.

Supraparticles on beads for supported catalytically active liquid metal solutions - the SCALMS suprabead concept

A novel GaPt-based supported catalytically active liquid metal solution (SCALMS) material is developed by exploiting the suprabead concept: Supraparticles, i.e. micrometer-sized particles composed of nanoparticles assembled by spray-drying, are bonded to millimeter-sized beads. The suprabeads combine macroscale size with catalytic properties of nanoscale GaPt particles entrapped in their silica framework.

Dry reforming of methane over gallium-based supported catalytically active liquid metal solutions

Gallium-rich supported catalytically active liquid metal solutions (SCALMS) were recently introduced as a new way towards heterogeneous single atom catalysis. SCALMS were demonstrated to exhibit a certain resistance against coking during the dehydrogenation of alkanes using Ga-rich alloys of noble metals. Here, the conceptual catalytic application of SCALMS in dry reforming of methane (DRM) is tested with non-noble metal (Co, Cu, Fe, Ni) atoms in the gallium-rich liquid alloy. This study introduces SCALMS to high-temperature applications and an oxidative reaction environment. Most catalysts were shown to undergo severe oxidation during DRM, while Ga-Ni SCALMS retained a certain level of activity. This observation is explained by a kinetically controlled redox process, namely oxidation to gallium oxide species and re-reduction via H2 activation over Ni. Consequentially, this redox process can be shifted to the metallic side when using increasing concentrations of Ni in Ga, which strongly suppresses coke formation. Density-functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations were performed to confirm the increased availability of Ni at the liquid alloy-gas interface. However, leaching of gallium via the formation of volatile oxidic species during the hypothesised redox cycles was identified indicating a critical instability of Ga-Ni SCALMS for prolonged test durations.

Understanding the Buoy Effect of Surface-Enriched Pt Complexes in Ionic Liquids: A Combined ARXPS and Pendant Drop Study

Recently, we demonstrated that Pt catalyst complexes dissolved in the ionic liquid (IL) [C4 C1 Im][PF6 ] can be deliberately enriched at the IL surface by introducing perfluorinated substituents, which act like buoys dragging the metal complex towards the surface. Herein, we extend our previous angle-resolved X-ray photoelectron spectroscopy (ARXPS) studies at complex concentrations between 30 and 5 %mol down to 1 %mol and present complementary surface tension pendant drop (PD) measurements under ultraclean vacuum conditions. This combination allows for connecting the microscopic information on the IL/gas interface derived from ARXPS with the macroscopic property surface tension. The surface enrichment of the Pt complexes is found to be most pronounced at 1 %mol . It also displays a strong temperature dependence, which was not observed for 5 %mol and above, where the surface is already saturated with the complex. The surface enrichment deduced from ARXPS is also reflected by the pronounced decrease in surface tension with increasing concentration of the catalyst. We furthermore observe by ARXPS and PD a much stronger surface affinity of the buoy-complex as compared to the free ligands in solution. Our results are highly interesting for an optimum design of IL-based catalyst systems with large contact areas to the surrounding reactant/product phase, such as in supported IL phase (SILP) catalysis.

Structure Formation in an Ionic Liquid Wetting Layer: A Combined STM, IRAS, DFT and MD Study of [C2 C1 Im][OTf] on Au(111)

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 DFT calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2 C1 Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2 C1 Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ∼450 K. [C2 C1 Im][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 [C2 C1 Im][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.

Ga-Pt supported catalytically active liquid metal solutions (SCALMS) prepared by ultrasonication - influence of synthesis conditions on n-heptane dehydrogenation performance

Supported catalytically active liquid metal solution (SCALMS) materials represent a recently developed class of heterogeneous catalysts, where the catalytic reaction takes place at the highly dynamic interface of supported liquid alloys. Ga nuggets were dispersed into nano-droplets in propan-2-ol using ultrasonication followed by the addition of Pt in a galvanic displacement reaction - either directly into the Ga/propan-2-ol dispersion (in situ) or consecutively onto the supported Ga droplets (ex situ). The in situ galvanic displacement reaction between Ga and Pt was studied in three different reaction media, namely propan-2-ol, water, and 20 vol% water containing propan-2-ol. TEM investigations reveal that the Ga-Pt reaction in propan-2-ol resulted in the formation of Pt aggregates on top of Ga nano-droplets. In the water/propan-2-ol mixture, the desired incorporation of Pt into the Ga matrix was achieved. The ex situ prepared Ga-Pt SCALMS were tested in n-heptane dehydrogenation. Ga-Pt SCALMS synthesized in pure alcoholic solution showed equal dehydrogenation and cracking activity. Ga-Pt SCALMS prepared in pure water, in contrast, showed mainly cracking activity due to oxidation of Ga droplets. The Ga-Pt SCALMS material prepared in water/propan-2-ol resulted in high activity, n-heptene selectivity of 63%, and only low cracking tendency. This can be attributed to the supported liquid Ga-Pt alloy where Pt atoms are present in the liquid Ga matrix at the highly dynamic catalytic interface.

Thermodynamic Analysis of Chemical Hydrogen Storage: Energetics of Liquid Organic Hydrogen Carrier Systems Based on Methyl-Substituted Indoles

Liquid organic hydrogen carriers can store hydrogen in a safe and dense form through covalent bonds. Hydrogen uptake and release are realized by catalytic hydrogenation and dehydrogenation, respectively. Indoles have been demonstrated to be interesting candidates for this task. The enthalpy of reaction is a crucial parameter in this regard as it determines not only the heat demand for hydrogen release, but also the reaction equilibrium at given conditions. In this work, a combination of experimental measurements, quantum chemical methods and a group-additivity approach has been applied to obtain a consistent dataset on the enthalpies of formation of different methylated indole derivatives and their hydrogenated counterparts. The results show a namable influence of the number and position of methyl groups on the enthalpy of reaction. The enthalpy of reaction of the overall hydrogenation reaction varies in the range of up to 18.2 kJ·mol^-1 (corresponding to 4.6 kJ·mol(H₂)^-1). The widest range of enthalpy of reaction data for different methyl indoles has been observed for the last step (hydrogenation for the last double bond in the five-membered ring). Here a difference of up to 7.3 kJ·mol(H₂)^-1 between the highest and the lowest value was found.

Isolated Rh atoms in dehydrogenation catalysis

Isolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga₉Rh₂ phase and the hitherto not-studied Ga₃Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems.

Preparation of geometrically highly controlled Ga particle arrays on quasi-planar nanostructured surfaces as a SCALMS model system

This study establishes a preparative route towards a model system for supported catalytically active liquid metal solutions (SCALMS) on nanostructured substrates. This model is characterized by a uniquely precise geometrical control of the gallium particle size distribution. In a SCALMS system, the Ga serves as a matrix material which can be decorated with a catalytically active material subsequently. The corresponding Ga containing precursor is spin-coated on aluminum based substrates, previously nanostructured by electrochemical anodization. The highly ordered substrates are functionalized with distinct oxide coatings by atomic layer deposition (ALD) independently from the morphology. After preparation of the metal particles on the oxide interface, the characterization of our model system in terms of its geometry parameters (droplet diameter, size distribution and population density) points to SiO₂ as the best suited surface for a highly controlled geometry. This flexible model system can be functionalized with a dissolved noble metal catalyst for the application chosen.

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, [C₃ CNC₁ Im][Tf₂ N] and [C₁ CNC₁ Pip][Tf₂ N], 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.

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

The front cover artwork is provided by the groups of Prof. Hans-Peter Steinrück and Prof. Peter Wasserscheid at the Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg. The image shows substitution of volatile acetonitrile ligands by a nitrile-functionalized imidazolium cation. The formed cationic complex and the counter ions exhibit a specific preferential orientation at the ionic liquid/gas interface, which can be studied by angle-resolved XPS. Read the full text of the Research Article at 10.1002/cphc.202200391.

The Buoy Effect: Surface Enrichment of a Pt Complex in IL Solution by Ligand Design

Invited for the cover of this issue are the groups of Hans-Peter Steinrück and Peter Wasserscheid at the Friedrich-Alexander-Universität Erlangen-Nürnberg. The image depicts two Pt catalysts dissolved in an ionic liquid. For one of them, fluorinated side chains in the ligand system act as buoys leading to pronounced enrichment of the complex at the gas/IL interface, as is evidenced by strongly enhanced Pt signals in angle-resolved photoelectron spectroscopy. For the complex without fluorinated side chains, no such effect is observed. Read the full text of the article at 10.1002/chem.202203325.

The Buoy Effect: Surface Enrichment of a Pt Complex in IL Solution by Ligand Design

The targeted enrichment of a Pt complex with an ionic liquid (IL)-derived ligand system in IL solution is demonstrated by using angle-resolved X-ray photoelectron spectroscopy. When the ligand system is complemented with fluorinated side chains, the complex accumulates strongly at the IL/gas interface, while in an equivalent solution of a complex without these substituents no such effect could be observed. This buoy-like behavior induces strong population of the complex at the outermost molecular layer close to surface saturation, which was studied over a range from 5 to 30 %_mol . The surface enrichment was found to be most efficient at the lowest concentration, which is particularly favorable for catalytic applications such as supported ionic-liquid-phase (SILP) catalysis.

Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems

The observable reaction rate of heterogeneously catalyzed reactions is known to be limited either by the intrinsic kinetics of the catalytic transformation or by the rate of pore and/or film diffusion. Here, we show that in gas generation reactions from liquid reactants, the nucleation of gas bubbles in the catalyst pore structure represents an additional important rate-limiting step. This is highlighted for the example of catalytic hydrogen release from the liquid organic hydrogen carrier compound perhydro-dibenzyltoluene. A nucleation-inhibited catalytic system produces only dissolved hydrogen with fast saturation of the fluid phase around the active site, while bubble formation enhances mass transfer by more than a factor of 50 in an oscillating reaction regime. Nucleation can be efficiently triggered not only by temperature changes and catalyst surface modification but also by a mechanical stimulus. Our work sheds new light on performance-limiting factors in reactions that are of highest relevance for the future green hydrogen economy.

Electrochemically Triggered Energy Release from an Azothiophene-Based Molecular Solar Thermal System

Molecular solar thermal (MOST) systems combine solar energy conversion, storage, and release in simple one-photon one-molecule processes. Here, we address the electrochemically triggered energy release from an azothiophene-based MOST system by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and density functional theory (DFT). Specifically, the electrochemically triggered back-reaction from the energy rich (Z)-3-cyanophenylazothiophene to its energy lean (E)-isomer using highly oriented pyrolytic graphite (HOPG) as the working electrode was studied. Theory predicts that two reaction channels are accessible, an oxidative one (hole-catalyzed) and a reductive one (electron-catalyzed). Experimentally it was found that the photo-isomer decomposes during hole-catalyzed energy release. Electrochemically triggered back-conversion was possible, however, through the electron-catalyzed reaction channel. The reaction rate could be tuned by the electrode potential within two orders of magnitude. It was shown that the MOST system withstands 100 conversion cycles without detectable decomposition of the photoswitch. After 100 cycles, the photochemical conversion was still quantitative and the electrochemically triggered back-reaction reached 94 % of the original conversion level.

A New Class of Task‐Specific Imidazolium Salts and Ionic Liquids and Their Corresponding Transition‐Metal Complexes for Immobilization on Electrochemically Active Surfaces

Adding to the versatile class of ionic liquids, we report the detailed structure and property analysis of a new class of asymmetrically substituted imidazolium salts, offering interesting thermal characteristics, such as liquid crystalline behavior, polymorphism or glass transitions. A scalable general synthetic procedure for N‐polyaryl‐N’‐alkyl‐functionalized imidazolium salts with para‐substituted linker (L) moieties at the aryl chain, namely [LPh_mIm^HR]⁺ (L=Br, CN, SMe, CO₂Et, OH; m=2, 3; R=C₁₂, PEG_n; n=2, 3, 4), was developed. These imidazolium salts were studied by single‐crystal X‐ray diffraction (SC‐XRD), NMR spectroscopy and thermochemical methods (DSC, TGA). Furthermore, these imidazolium salts were used as N‐heterocyclic carbene (NHC) ligand precursors for mononuclear, first‐row transition metal complexes (Mn^II, Fe^II, Co^II, Ni^II, Zn^II, Cu^I, Ag^I, Au^I) and for the dinuclear Ti‐supported Fe‐NHC complex [(OPy)₂Ti(OPh₂ImC₁₂)₂(FeI₂)] (OPy=pyridin‐2‐ylmethanolate). The complexes were studied concerning their structural and magnetic behavior via multi‐nuclear NMR spectroscopy, SC‐XRD analyses, variable temperature and field‐dependent (VT‐VF) SQUID magnetization methods, X‐band EPR spectroscopy and, where appropriate, zero‐field ⁵⁷Fe Mössbauer spectroscopy.

Ga-Ni supported catalytically active liquid metal solutions (SCALMS) for selective ethylene oligomerization

Non-precious metal supported catalytically active liquid metal solutions exhibit attractive performance in ethylene oligomerization. It is found for the Ga-Ni system on silica that the performance depends strongly on the applied Ga/Ni ratio. Ga-rich systems forming liquid alloys exhibit a far higher Ni-based catalytic activity than solid intermetallic compounds or Ni nanoparticles.

GaPt Supported Catalytically Active Liquid Metal Solution Catalysis for Propane Dehydrogenation-Support Influence and Coking Studies

Supported catalytically active liquid metal solutions (SCALMS) of Pt in Ga (2 at.-% Pt) were studied in the temperature range of 500 to 600 °C for propane dehydrogenation. A facile synthesis procedure using ultrasonication was implemented and compared to a previously reported organo-chemical route for gallium deposition. The procedure was applied to synthesize GaPt-SCALMS catalyst on silica (SiO₂), alumina (Al₂O₃), and silicon carbide (SiC) to investigate the effect of the support material on the catalytic performance. The SiC-based SCALMS catalyst showed the highest activity, while SiO₂-based SCALMS showed the highest stability and lowest cracking tendency at higher temperatures. The selectivity toward propene for the SiO₂-based catalyst remained above 93% at 600 °C. The catalysts were analyzed for coke content after use by temperature-programmed oxidation (TPO) and Raman spectroscopy. While the SiC- and SiO₂-supported SCALMS systems showed hardly any coke formation, the Al₂O₃-supported systems suffered from pronounced coking. SEM-EDX analyses of the catalysts before and after reaction indicated that no perceivable morphological changes occur during reaction. The SCALMS catalysts under investigation are compared with supported Pt and supported GaPt solid-phase catalyst, and possible deactivation pathways are discussed.

Adsorption Motifs and Molecular Orientation at the Ionic Liquid/Noble Metal Interface: [C2C1Im][NTf2] on Pt(111)

In solid catalysts with ionic liquid layers (SCILLs), ionic liquid (IL) thin films are used to modify the activity and selectivity of catalytic materials. In this work, we investigated the adsorption behavior of the IL 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁Im][NTf₂] on Pt(111) by combining experimental and theoretical studies. Under ultrahigh vacuum (UHV) conditions, the IL was deposited onto a Pt(111) single crystal surface by physical vapor deposition (PVD) at different surface temperatures (200, 300, and 400 K). The adsorption process was monitored by in situ infrared reflection absorption spectroscopy (IRAS). Complementary to the IRAS studies, we performed density functional theory (DFT) calculations and analyzed the adsorption motifs and orientation of the IL ions. In total, we calculated four different systems: (a) [C₂C₁Im]⁺ and [NTf₂]^- ions in the gas phase; [NTf₂]^- anions in (b) small (4 × 4) and (c) large (6 × 6) Pt(111) supercells; and (d) a complete ion pair of [C₂C₁Im][NTf₂] in a (6 × 6) Pt(111) supercell. Based on DFT, we simulated IR spectra and compared them to the experimental data. Our results suggest that the binding motif and orientation of the IL is strongly dependent on the actual IL coverage. In the monolayer (ML), [NTf₂]^- interacts strongly with the metal surface and adopts a specific orientation in which it interacts with the Pt surface via the SO₂ groups. Also the [C₂C₁Im]⁺ cations adopt a preferential orientation up to coverages of 1 ML. Upon transition to the multilayer region, the specific orientation of the ions is gradually lost.

Interaction between Ionic Liquids and a Pt(111) Surface Probed by Coadsorbed CO as a Test Molecule

We used temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) to study the desorption behavior of CO on Pt(111) coadsorbed with four kinds of ionic liquids (ILs), namely 1-butyl-1-methyl-pyrrolidinium-bis(trifluoromethylsulfonyl)imide ([C₄C₁Pyr][NTf₂]), 1-ethyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide ([C₂C₁Im][NTf₂]), 1-butyl-1-methyl-pyrrolidinium-trifluoro-methanesulfonate ([C₄C₁Pyr][OTf]), and 1-butyl-1-methyl-pyrrolidinium-hexafluorophosphate ([C₄C₁Pyr][PF₆]). We found that CO desorbs earlier from a Pt(111) surface with coadsorbed ILs than without. In addition, the CO desorption temperature varies between different types of coadsorbed ILs, which follows the order: [C₄C₁Pyr][PF₆] (365 K) > [C₄C₁Pyr][NTf₂] (362 K) > [C₂C₁Im][NTf₂] (352 K) > [C₄C₁Pyr][OTf] (348 K). We ascribe the difference in CO desorption temperature to the different interaction strength between ILs and the Pt(111) surface. A stronger IL-Pt(111) interaction leads to a lower CO desorption temperature. We suggest that TP-IRAS experiments of CO coadsorbed with ILs can be a useful method to aid the characterization of the interaction strength between ILs and metal surfaces such as Pt(111).

Model Studies on the Ozone-Mediated Synthesis of Cobalt Oxide Nanoparticles from Dicobalt Octacarbonyl in Ionic Liquids

Low-temperature synthesis in ionic liquids (ILs) offers an efficient route for the preparation of metal oxide nanomaterials with tailor-made properties in a water-free environment. In this work, we investigated the role of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4 C1 Pyr][NTf2 ] in the synthesis of cobalt oxide nanoparticles from the molecular precursor Co2 (CO)8 with ozone. We performed a model study in ultra-clean, ultrahigh vacuum (UHV) conditions by infrared reflection absorption spectroscopy (IRAS) using Au(111) as a substrate. Exposure of the pure precursor to ozone at low temperatures results in the oxidation of the first layers, leading to the formation of a disordered Cox Oy passivation layer. Similar protection to ozone is also achieved by deposition of an IL layer onto a precursor film prior to ozone exposure. With increasing temperature, the IL gets permeable for ozone and a cobalt oxide film forms at the IL/precursor interface. We show that the interaction with the IL mediates the oxidation and leads to a more densely packed Cox Oy film compared to a direct oxidation of the precursor.

Kinetic modelling of reactions for the synthesis of 2-methyl-5-ethyl pyridine

The kinetics of the reactions for the synthesis of 2-methyl-5-ethyl pyridine (MEP) were investigated systematically by means of operando Raman spectroscopy.

Capturing spatially resolved kinetic data and coking of Ga–Pt supported catalytically active liquid metal solutions during propane dehydrogenation in situ

Spatially resolved kinetic data of gallium–platinum SCALMS was captured while elucidating the effect of carrier material on coke formation.

Enhancing the feasibility of Pd/C-catalyzed formic acid decomposition for hydrogen generation – catalyst pretreatment, deactivation, and regeneration

Formic acid decomposition (FAD) generates H₂ at low temperatures. CO poisoning inhibits FAD but is lifted under oxidative treatment.

Coke Formation during Propane Dehydrogenation over Ga-Rh Supported Catalytically Active Liquid Metal Solutions

Supported Catalytically Active Liquid Metal Solutions (SCALMS) were recently described as a new class of heterogeneous catalysts, where the catalytic transformation takes place at the highly dynamic interface of a liquid alloy. Their application in alkane dehydrogenation has been claimed to be superior to classical heterogeneous catalysts, because the single atom nature of Rh dissolved in liquid Ga hinders the formation of significant amounts of coke, e. g. by oligomerisation of carbon fragments and excessive dehydrogenation. In the present study, we investigate the coking behaviour of Ga-Rh SCALMS during dehydrogenation of propane in detail by means of high-resolution thermogravimetry. We report that the application of Ga-Rh SCALMS indeed limits the formation of coke when compared to the Ga-free Rh catalyst, in particular when relating coke formation to the catalytic performance. Furthermore, the formed coke has been shown to be highly reactive during temperature programmed oxidation in 21 % O2/He with onset temperatures of approx. 150 °C enabling a regeneration of the Ga-Rh SCALMS system under mild conditions. The activation energy of the oxidation lies in the lower range of values reported for spent cracking catalysts. Monitoring the formation of coke and performance of SCALMS in situ via thermogravimetry coupled with mass spectrometry revealed the continuous formation of coke, which becomes the only process affecting the net weight change after a certain time on stream.

Influence of gas impurities on the hydrogenation of CO2 to methanol using indium-based catalysts

In this study, the performance of an In₂O₃/ZrO₂ catalyst for hydrogenation of CO₂ to methanol is reported in the presence of typical impurities of industrial CO₂ feed gas streams.

Cu carbonyls enhance the performance of Ru-based SILP water–gas shift catalysts: a combined in situ DRIFTS and DFT study

In situ DRIFT spectroscopy and DFT identify Cu carbonyl shuttles that enhance the performance of Ru-based SILP water–gas shift catalysts.

Additively manufactured RANEY®-type copper catalyst for methanol synthesis

We describe a novel approach to prepare cellular methanol synthesis catalysts by combining additive manufacturing (AM) of a Cu–Al alloy followed by selective leaching of Al with aqueous NaOH solutions.

Highly Effective Propane Dehydrogenation Using Ga-Rh Supported Catalytically Active Liquid Metal Solutions

Our contribution demonstrates that rhodium, an element that has barely been reported as an active metal for selective dehydrogenation of alkanes becomes a very active, selective, and robust dehydrogenation catalyst when exposed to propane in the form of single atoms at the interface of a solid-supported, highly dynamic liquid Ga-Rh mixture. We demonstrate that the transition to a fully liquid supported alloy droplet at Ga/Rh ratios above 80, results in a drastic increase in catalyst activity with high propylene selectivity. The combining results from catalytic studies, X-ray photoelectron spectroscopy, IR-spectroscopy under reaction conditions, microscopy, and density-functional theory calculations, we obtained a comprehensive microscopy picture of the working principle of the Ga-Rh supported catalytically active liquid metal solution.

Operando DRIFTS and DFT Study of Propane Dehydrogenation over Solid- and Liquid-Supported Ga x Pt y Catalysts

Supported catalytically active liquid metal solutions (SCALMS) represent a class of catalytic materials that have only recently been developed, but have already proven to be highly active, e.g., for dehydrogenation reactions. Previous studies attributed the catalytic activity to isolated noble metal atoms at the surface of a liquid and inert Ga matrix. In this study, we apply diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule to Ga/Al₂O₃, Pt/Al₂O₃, and Ga₃₇Pt/Al₂O₃ catalysts, to investigate in detail the nature of the active Pt species. Comparison of CO adsorption on Pt/Al₂O₃ and Ga₃₇Pt/Al₂O₃ shows that isolated Pt atoms are, indeed, present at the surface of the liquid SCALMS. Combining DRIFTS with online gas chromatography (GC), we investigated the Ga/Al₂O₃, Pt/Al₂O₃, and Ga₃₇Pt/Al₂O₃ systems under operando conditions during propane dehydrogenation in CO/propane and in Ar/propane. We find that the Pt/Al₂O₃ sample is rapidly poisoned by CO adsorption and coke, whereas propane dehydrogenation over Ga₃₇Pt/Al₂O₃ SCALMS leads to higher conversion with no indication of poisoning effects. We show under operando conditions that isolated Pt atoms are present at the surface of SCALMS during the dehydrogenation reaction. IR spectra and density-functional theory (DFT) suggest that both the Ga matrix and the presence of coadsorbates alter the electronic properties of the surface Pt species.

Homogeneously-catalysed hydrogen release/storage using the 2-methylindole/2-methylindoline LOHC system in molten salt-organic biphasic reaction systems

Ir-Complex catalysed hydrogen release/storage using a 2-methylindole/2-methylindoline Liquid Organic Hydrogen Carrier (LOHC) system is shown to be effective in a temperature range of 120 to 140 °C. In the form of a liquid-liquid biphasic reaction system with molten [PPh4][NTf2] as catalyst immobilisation phase, the applied cationic Ir-complex can be easily separated and recycled enabling a small amount of ionic catalyst solution to store/release a large amount of hydrogen.

Mechanism of the Water-Gas Shift Reaction Catalyzed by Efficient Ruthenium-Based Catalysts: A Computational and Experimental Study

Supported ionic liquid phase (SILP) catalysis enables a highly efficient, Ru-based, homogeneously catalyzed water-gas shift reaction (WGSR) between 100 °C and 150 °C. The active Ru-complexes have been found to exist in imidazolium chloride melts under operating conditions in a dynamic equilibrium, which is dominated by the [Ru(CO)₃ Cl₃ ]^- complex. Herein we present state-of-the-art theoretical calculations to elucidate the reaction mechanism in more detail. We show that the mechanism includes the intermediate formation and degradation of hydrogen chloride, which effectively reduces the high barrier for the formation of the requisite dihydrogen complex. The hypothesis that the rate-limiting step involves water is supported by using D₂ O in continuous catalytic WGSR experiments. The resulting mechanism constitutes a highly competitive alternative to earlier reported generic routes involving nucleophilic addition of hydroxide in the gas phase and in solution.