Titanium Oxycarbide as Platinum-Free Electrocatalyst for Ethanol Oxidation

The compound material titanium oxycarbide (TiOC) is found to be an effective electrocatalyst for the electrochemical oxidation of ethanol to CO2. The complete course of this reaction is one of the main challenges in direct ethanol fuel cells (DEFCs). While TiOC has previously been investigated as catalyst support material only, in this study we show that TiOC alone is able to oxidize ethanol to acetaldehyde without the need of expensive noble metal catalysts like Pt. It is suggested that this behavior is attributed to the presence of both undercoordinated sites, which allow ethanol to adsorb, and oxygenated sites, which facilitate the activation of water. This is a milestone in DEFC research and development and opens up innovative possibilities for the design of catalyst materials for intermediate temperature fuel cells.

A laboratory-based multifunctional near ambient pressure X-ray photoelectron spectroscopy system for electrochemical, catalytic, and cryogenic studies

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15 mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately -190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

Zirconium Carbide Mediates Coke‐Resistant Methane Dry Reforming on Nickel‐Zirconium Catalysts

Graphitic deposits anti-segregate into Ni⁰ nanoparticles to provide restored CH₄ adsorption sites and near-surface/dissolved C atoms, which migrate to the Ni⁰ /ZrO₂ interface and induce local Zr_x C_y formation. The resulting oxygen-deficient carbidic phase boundary sites assist in the kinetically enhanced CO₂ activation toward CO(g). This interface carbide mechanism allows for enhanced spillover of carbon to the ZrO₂ support, and represents an alternative catalyst regeneration pathway with respect to the reverse oxygen spillover on Ni-CeZr_x O_y catalysts. It is therefore rather likely on supports with limited oxygen storage/exchange kinetics but significant carbothermal reducibility.

Porous Silicon Oxycarbonitride Ceramics with Palladium and Pd2Si Nanoparticles for Dry Reforming of Methane

Pd-containing precursor has been synthesized from palladium acetate and poly(vinly)silazane (Durazane 1800) in an ice bath under an argon atmosphere. The results of ATR-FTIR and NMR characterizations reveal the chemical reaction between palladium acetate and vinyl groups in poly(vinyl)silazane and the hydrolyzation reaction between –Si–H and –Si–CH=CH₂ groups in poly(vinyl)silazane. The palladium nanoparticles are in situ formed in the synthesized precursors as confirmed by XRD, XPS, and TEM. Pd- and Pd₂Si-containing SiOCN ceramic nanocomposites are obtained by pyrolysis of the synthesized precursors at 700 °C, 900 °C–1100 °C in an argon atmosphere. The pyrolyzed nanocomposites display good catalytic activity towards the dry reforming of methane. The sample pyrolyzed at 700 °C possesses the best catalytic performance, which can be attributed to the in situ formed palladium nanoparticles and high BET surface area of about 233 m² g⁻¹.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper–Mixed Oxide Interfaces

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu_xMn_1–xO₃ (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu_yPd_x/perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N₂ selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N₂O at low temperatures, enhancing the selectivity toward N₂. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal–mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

Properties of Bulk In-Pt Intermetallic Compounds in Methanol Steam Reforming

Heterogeneous catalysts are often complex materials containing different compounds. While this can lead to highly beneficial interfaces, it is difficult to identify the role of single components. In methanol steam reforming (MSR), the interplay between intermetallic compounds, supporting oxides and redox reactions leads to highly active and CO₂‐selective materials. Herein, the intrinsic catalytic properties of unsupported In₃Pt₂, In₂Pt, and In₇Pt₃ as model systems for Pt/In₂O₃‐based catalytic materials in MSR are addressed. In₂Pt was identified as the essential compound responsible for the reported excellent CO₂‐selectivity of 99.5 % at 300 °C in supported systems, showing a CO₂‐selectivity above 99 % even at 400 °C. Additionally, the partial oxidation of In₇Pt₃ revealed that too much In₂O₃ is detrimental for the catalytic properties. The study highlights the crucial role of intermetallic In−Pt compounds in Pt/In₂O₃ materials with excellent CO₂‐selectivity.

Atomic-Scale Insights into Nickel Exsolution on LaNiO3 Catalysts via In Situ Electron Microscopy

Using a combination of in situ bulk and surface characterization techniques, we provide atomic-scale insight into the complex surface and bulk dynamics of a LaNiO₃ perovskite material during heating in vacuo. Driven by the outstanding activity LaNiO₃ in the methane dry reforming reaction (DRM), attributable to the decomposition of LaNiO₃ during DRM operation into a Ni//La₂O₃ composite, we reveal the Ni exsolution dynamics both on a local and global scale by in situ electron microscopy, in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy. To reduce the complexity and disentangle thermal from self-activation and reaction-induced effects, we embarked on a heating experiment in vacuo under comparable experimental conditions in all methods. Associated with the Ni exsolution, the remaining perovskite grains suffer a drastic shrinkage of the grain volume and compression of the structure. Ni particles mainly evolve at grain boundaries and stacking faults. Sophisticated structure analysis of the elemental composition by electron-energy loss mapping allows us to disentangle the distribution of the different structures resulting from LaNiO₃ decomposition on a local scale. Important for explaining the DRM activity, our results indicate that most of the Ni moieties are oxidized and that the formation of NiO occurs preferentially at grain edges, resulting from the reaction of the exsolved Ni particles with oxygen released from the perovskite lattice during decomposition via a spillover process from the perovskite to the Ni particles. Correlating electron microscopy and X-ray diffraction data allows us to establish a sequential two-step process in the decomposition of LaNiO₃ via a Ruddlesden-Popper La₂NiO₄ intermediate structure. Exemplified for the archetypical LaNiO₃ perovskite material, our results underscore the importance of focusing on both surface and bulk characterization for a thorough understanding of the catalyst dynamics and set the stage for a generalized concept in the understanding of state-of-the art catalyst materials on an atomic level.

Elucidating the role of earth alkaline doping in perovskite-based methane dry reforming catalysts

To elucidate the role of earth alkaline doping in perovskite-based dry reforming of methane (DRM) catalysts, we embarked on a comparative and exemplary study of a Ni-based Sm perovskite with and without Sr doping. While the Sr-doped material appears as a structure-pure Sm_1.5Sr_0.5NiO₄ Ruddlesden Popper structure, the undoped material is a NiO/monoclinic Sm₂O₃ composite. Hydrogen pre-reduction or direct activation in the DRM mixture in all cases yields either active Ni/Sm₂O₃ or Ni/Sm₂O₃/SrCO₃ materials, with albeit different short-term stability and deactivation behavior. The much smaller Ni particle size after hydrogen reduction of Sm_1.5Sr_0.5NiO₄, and of generally all undoped materials stabilizes the short and long-term DRM activity. Carbon dioxide reactivity manifests itself in the direct formation of SrCO₃ in the case of Sm_1.5Sr_0.5NiO₄, which is dominant at high temperatures. For Sm_1.5Sr_0.5NiO₄, the CO : H₂ ratio exceeds 1 at these temperatures, which is attributed to faster direct carbon dioxide conversion to SrCO₃ without catalytic DRM reactivity. As no Sm₂O₂CO₃ surface or bulk phase as a result of carbon dioxide activation was observed for any material – in contrast to La₂O₂CO₃ – we suggest that oxy-carbonate formation plays only a minor role for DRM reactivity. Rather, we identify surface graphitic carbon as the potentially reactive intermediate. Graphitic carbon has already been shown as a crucial reaction intermediate in metal-oxide DRM catalysts and appears both for Sm_1.5Sr_0.5NiO₄ and NiO/monoclinic Sm₂O₃ after reaction as crystalline structure. It is significantly more pronounced for the latter due to the higher amount of oxygen-deficient monoclinic Sm₂O₃ facilitating carbon dioxide activation. Despite the often reported beneficial role of earth alkaline dopants in DRM catalysis, we show that the situation is more complex. In our studies, the detrimental role of earth alkaline doping manifests itself in the exclusive formation of the sole stable carbonated species and a general destabilization of the Ni/monoclinic Sm₂O₃ interface by favoring Ni particle sintering.

To elucidate the role of earth alkaline doping in perovskite-based dry reforming of methane (DRM) catalysts, we embarked on a comparative and exemplary study of a Ni-based Sm perovskite with and without Sr doping.

How the in situ monitoring of bulk crystalline phases during catalyst activation results in a better understanding of heterogeneous catalysis

The present Highlight article shows the importance of the in situ monitoring of bulk crystalline compounds for a more thorough understanding of heterogeneous catalysts at the intersection of catalysis, materials science, crystallography and inorganic chemistry. Although catalytic action is widely regarded as a purely surface-bound phenomenon, there is increasing evidence that bulk processes can detrimentally or beneficially influence the catalytic properties of various material classes. Such bulk processes include polymorphic transformations, formation of oxygen-deficient structures, transient phases and the formation of a metal-oxide composite. The monitoring of these processes and the subsequent establishment of structure-property relationships are most effective if carried out in situ under real operation conditions. By focusing on synchrotron-based in situ X-ray diffraction as the perfect tool to follow the evolution of crystalline species, we exemplify the strength of the concept with five examples from various areas of catalytic research. As catalyst activation studies are increasingly becoming a hot topic in heterogeneous catalysis, the (self-)activation of oxide- and intermetallic compound-based materials during methanol steam and methane dry reforming is highlighted. The perovskite LaNiO3 is selected as an example to show the complex structural dynamics before and during methane dry reforming, which is only revealed upon monitoring all intermediate crystalline species in the transformation from LaNiO3 into Ni/La2O3/La2O2CO3. ZrO2-based materials form the second group, indicating the in situ decomposition of the intermetallic compound Cu51Zr14 into an epitaxially stabilized Cu/tetragonal ZrO2 composite during methanol steam reforming, the stability of a ZrO0.31C0.69 oxycarbide and the gas-phase dependence of the tetragonal-to-monoclinic ZrO2 polymorphic transformation. The latter is the key parameter to the catalytic understanding of ZrO2 and is only appreciated in full detail once it is possible to follow the individual steps of the transformation between the crystalline polymorphic structures. A selected example is devoted to how the monitoring of crystalline reactive carbon during methane dry reforming operation aids in the mechanistic understanding of a Ni/MnO catalyst. The most important aspect is the strict use of in situ monitoring of the structural changes occurring during (self-)activation to establish meaningful structure-property relationships allowing conclusions beyond isolated surface chemical aspects.

Steering the methanol steam reforming reactivity of intermetallic Cu-In compounds by redox activation: stability vs. formation of an intermetallic compound-oxide interface

To compare the inherent methanol steam reforming properties of intermetallic compounds and a corresponding intermetallic compound–oxide interface, we selected the Cu–In system as a model to correlate the stability limits, self-activation and redox activation properties with the catalytic performance. Three distinct intermetallic Cu–In compounds – Cu₇In₃, Cu₂In and Cu₁₁In₉ – were studied both in an untreated and redox-activated state resulting from alternating oxidation–reduction cycles. The stability of all studied intermetallic compounds during methanol steam reforming (MSR) operation is essentially independent of the initial stoichiometry and all accordingly resist substantial structural changes. The inherent activity under batch MSR conditions is highest for Cu₂In, corroborating the results of a Cu₂In/In₂O₃ sample accessed through reactive metal–support interaction. Under flow MSR operation, Cu₇In₃ displays considerable deactivation, while Cu₂In and Cu₁₁In₉ feature stable performance at simultaneously high CO₂ selectivity. The missing significant self-activation is most evident in the operando thermogravimetric experiments, where no oxidation is detected for any of the intermetallic compounds. In situ X-ray diffraction allowed us to monitor the partial decomposition and redox activation of the Cu–In intermetallic compounds into Cu0.9In0.1/In₂O₃ (from Cu₇In₃), Cu₇In₃/In₂O₃ (from Cu₂In) and Cu₇In₃/Cu0.9In0.1/In₂O₃ (from Cu₁₁In₉) interfaces with superior MSR performance compared to the untreated samples. Although the catalytic profiles appear surprisingly similar, the latter interface with the highest indium content exhibits the least deactivation, which we explain by formation of stabilizing In₂O₃ patches under MSR conditions.

To compare the properties of intermetallic compounds and intermetallic compound–oxide interfaces, Cu–In was used as a model to correlate stability limits, self-activation and redox activation with the inherent methanol steam reforming performance.

The sol-gel autocombustion as a route towards highly CO2-selective, active and long-term stable Cu/ZrO2 methanol steam reforming catalysts

The adaption of the sol-gel autocombustion method to the Cu/ZrO₂ system opens new pathways for the specific optimisation of the activity, long-term stability and CO₂ selectivity of methanol steam reforming (MSR) catalysts. Calcination of the same post-combustion precursor at 400 °C, 600 °C or 800 °C allows accessing Cu/ZrO₂ interfaces of metallic Cu with either amorphous, tetragonal or monoclinic ZrO₂, influencing the CO₂ selectivity and the MSR activity distinctly different. While the CO₂ selectivity is less affected, the impact of the post-combustion calcination temperature on the Cu and ZrO₂ catalyst morphology is more pronounced. A porous and largely amorphous ZrO₂ structure in the sample, characteristic for sol-gel autocombustion processes, is obtained at 400 °C. This directly translates into superior activity and long-term stability in MSR compared to Cu/tetragonal ZrO₂ and Cu/monoclinic ZrO₂ obtained by calcination at 600 °C and 800 °C. The morphology of the latter Cu/ZrO₂ catalysts consists of much larger, agglomerated and non-porous crystalline particles. Based on aberration-corrected electron microscopy, we attribute the beneficial catalytic properties of the Cu/amorphous ZrO₂ material partially to the enhanced sintering resistance of copper particles provided by the porous support morphology.

Steering the Catalytic Properties of Intermetallic Compounds and Alloys in Reforming Reactions by Controlled in Situ Decomposition and Self-Activation

Based on the increasing importance of intermetallic compounds and alloys in heterogeneous catalysis, we explore the possibilities of using selected intermetallic compounds and alloy structures and phases as catalyst precursors to prepare highly active and CO2-selective methanol steam reforming (MSR) as well as dry reforming of methane (DRM) catalyst entities by controlled in situ decomposition and self-activation. The exemplary discussed examples (Cu51Zr14, CuZn, Pd2Zr, GaPd2, Cu2In, ZnPd, and InPd) show both the advantages and pitfalls of this approach and how the concept can be generalized to encompass a wider set of intermetallic compounds and alloy structures. Despite the common feature of all systems being the more or less pronounced decomposition of the intermetallic compound surface and bulk structure and the in situ formation of much more complex catalyst entities, differences arise due to the oxidation propensity and general thermodynamic stability of the chosen intermetallic compound/alloy and their constituents. The metastability and intrinsic reactivity of the evolving oxide polymorph introduced upon decomposition and the surface and bulk reactivity of carbon, governed by the nature of the metal/intermetallic compound-oxide interfacial sites, are of equal importance. Structural and chemical rearrangements, dictating the catalytic performance of the resulting entity, are present in the form of a complete destruction of the intermetallic compound bulk structure (Cu51Zr14) and the formation of an metal/oxide (Cu51Zr14, InPd) or intermetallic compound/oxide (ZnPd, Cu2In, CuZn) interface or the intertranformation of intermetallic compounds with varying composition (Pd2Zr) before the formation of Pd/ZrO2. In this Perspective, the prerequisites to obtain a leading theme for pronounced CO2 selectivity and high activity will be reviewed. Special focus will be put on raising awareness of the intrinsic properties of the discussed catalyst systems that need to be controlled to obtain catalytically prospective materials. The use of model systems to bridge the material's gap in catalysis will also be highlighted for selected examples.

True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity

Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as -1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.

Operando Fourier-transform infrared-mass spectrometry reactor cell setup for heterogeneous catalysis with glovebox transfer process to surface-chemical characterization

We describe a new type of operando Fourier transform infrared (FTIR)-mass spectrometry setup for surface-chemical and reactivity characterization of heterogeneous catalysts. On the basis of a sophisticated all-quartz FTIR reactor cell, capable of operating between room temperature and 1000 °C in reactive gas atmospheres, the setup offers a unique opportunity to simultaneously collect and accordingly correlate FTIR surface-chemical adsorption data of the active catalyst state and FTIR gas phase data with complementary reactivity data obtained via mass spectrometry in situ. The full set of catalytic operation modes (recirculating static and flow reactor conditions) is accessible and can be complemented with a variety of temperature-programmed reaction modes or thermal desorption. Due to the unique transfer process involving a home-built portable glovebox to avoid air exposure, a variety of complementary quasi in situ characterization methods for the pre- and post-reaction catalyst states become accessible. We exemplify the capabilities for additional x-ray photoelectron spectroscopy characterization of surface-chemical states, highlighting the unique strength of combining adsorption, electronic structure, and reactivity data to gain detailed insight into the reactive state of a Cu/ZrO₂ heterogeneous catalyst during methanol steam reforming operation.

Steering the Methane Dry Reforming Reactivity of Ni/La2O3 Catalysts by Controlled In Situ Decomposition of Doped La2NiO4 Precursor Structures

The influence of A- and/or B-site doping of Ruddlesden-Popper perovskite materials on the crystal structure, stability, and dry reforming of methane (DRM) reactivity of specific A2BO4 phases (A = La, Ba; B = Cu, Ni) has been evaluated by a combination of catalytic experiments, in situ X-ray diffraction, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and aberration-corrected electron microscopy. At room temperature, B-site doping of La2NiO4 with Cu stabilizes the orthorhombic structure (Fmmm) of the perovskite, while A-site doping with Ba yields a tetragonal space group (I4/mmm). We observed the orthorhombic-to-tetragonal transformation above 170 °C for La2Ni0.9Cu0.1O4 and La2Ni0.8Cu0.2O4, slightly higher than for undoped La2NiO4. Loss of oxygen in interstitial sites of the tetragonal structure causes further structure transformations for all samples before decomposition in the temperature range of 400 °C-600 °C. Controlled in situ decomposition of the parent or A/B-site doped perovskite structures in a DRM mixture (CH4:CO2 = 1:1) in all cases yields an active phase consisting of exsolved nanocrystalline metallic Ni particles in contact with hexagonal La2O3 and a mixture of (oxy)carbonate phases (hexagonal and monoclinic La2O2CO3, BaCO3). Differences in the catalytic activity evolve because of (i) the in situ formation of Ni-Cu alloy phases (in a composition of >7:1 = Ni:Cu) for La2Ni0.9Cu0.1O4, La2Ni0.8Cu0.2O4, and La1.8Ba0.2Ni0.9Cu0.1O4, (ii) the resulting Ni particle size and amount of exsolved Ni, and (iii) the inherently different reactivity of the present (oxy)carbonate species. Based on the onset temperature of catalytic DRM activity, the latter decreases in the order of La2Ni0.9Cu0.1O4 ∼ La2Ni0.8Cu0.2O4 ≥ La1.8Ba0.2Ni0.9Cu0.1O4 > La2NiO4 > La1.8Ba0.2NiO4. Simple A-site doped La1.8Ba0.2NiO4 is essentially DRM inactive. The Ni particle size can be efficiently influenced by introducing Ba into the A site of the respective Ruddlesden-Popper structures, allowing us to control the Ni particle size between 10 nm and 30 nm both for simple B-site and A-site doped structures. Hence, it is possible to steer both the extent of the metal-oxide-(oxy)carbonate interface and its chemical composition and reactivity. Counteracting the limitation of the larger Ni particle size, the activity can, however, be improved by additional Cu-doping on the B-site, enhancing the carbon reactivity. Exemplified for the La2NiO4 based systems, we show how the delicate antagonistic balance of doping with Cu (rendering the La2NiO4 structure less stable and suppressing coking by efficiently removing surface carbon) and Ba (rendering the La2NiO4 structure more stable and forming unreactive surface or interfacial carbonates) can be used to tailor prospective DRM-active catalysts.

Increasing Complexity Approach to the Fundamental Surface and Interface Chemistry on SOFC Anode Materials

In this Account, we demonstrate an increasing complexity approach to gain insight into the principal aspects of the surface and interface chemistry and catalysis of solid oxide fuel cell (SOFC) anode and electrolyte materials based on selected oxide, intermetallic, and metal–oxide systems at different levels of material complexity, as well as into the fundamental microkinetic reaction steps and intermediates at catalytically active surface and interface sites. To dismantle the complexity, we highlight our deconstructing step-by-step approach, which allows one to deduce synergistic properties of complex composite materials from the individual surface catalytic properties of the single constituents, representing the lowest complexity level: pure oxides and pure metallic materials. Upon mixing and doping the latter, directly leading to formation of intermetallic compounds/alloys in the case of metals and oxygen ion conductors/mixed ionic and electronic conductors for oxides, a second complexity level is reached. Finally, the introduction of an (inter)metall(ic)–(mixed) oxide interface leads to the third complexity level. A shell-like model featuring three levels of complexity with the unveiled surface and interface chemistry at its core evolves. As the shift to increased complexity decreases the number of different materials, the interconnections between the studied materials become more convoluted, but the resulting picture of surface chemistry becomes clearer. The materials featured in our investigations are all either already used technologically important or prospective components of SOFCs (such as yttria-stabilized zirconia, perovskites, or Ni–Cu alloys) or their basic constituents (e.g., ZrO₂), or they are formed by reactions of other compounds (for instance, pyrochlores are thought to be formed at the YSZ/perovskite phase boundary). We elaborate three representative case studies based on ZrO₂, Y₂O₃, and Y-doped ZrO₂ in detail from all three complexity levels. By interconnection of results, we are able to derive common principles of the influence of surface and interface chemistry on the catalytic operation of SOFC anode materials. In situ measurements of the reactivity of water and carbon surface species on ZrO₂- and Y₂O₃-based materials represent levels 1 and 2. The highest degree of complexity at level 3 is exemplified by combined surface science and catalytic studies of metal–oxide systems, oxidatively derived from intermetallic Cu–Zr and Pd–Zr compounds and featuring a large number of phases and interfaces. We show that only by appreciating insight into the basic building blocks of the catalyst materials at lower levels, a full understanding of the catalytic operation of the most complex materials at the highest level is possible.

Spectro-electrochemical setup for in situ and operando mechanistic studies on metal oxide electrode surfaces

This work shows a combined setup of Diffuse Reflectance FT-IR Spectroscopy (DRIFTS) and electrochemical characterization by AC and DC methods for in situ and operando investigations of surface species during CO₂ electrolysis on metal oxide electrodes and their correlation with electrochemical activity. A high-temperature reaction chamber enables conducting DRIFTS and electrochemical experiments simultaneously at temperatures up to 1000 °C in both reductive and oxidative reaction atmospheres and under anodic and cathodic polarization conditions. A dedicated gas- and electrical feedthrough solution is presented, which is the key element required for recording electrochemical AC and DC characteristics using an electrochemical cell, which is simultaneously studied by DRIFTS experiments under realistic operation conditions. Selected results, obtained on a gadolinium doped ceria model solid oxide electrolysis cell upon different polarization states, demonstrate the basic functionality and capabilities of the setup and show how the simultaneous DRIFT-spectroscopic and electrochemical investigation of the surface and bulk chemistry on electrode materials leads to increased insight in the population of potential intermediates during CO₂ electrolysis. With infrared spectroscopy and impedance spectroscopy as common and complementary spectroscopic methods in material science, the setup is considered to exhibit a huge potential in a wide field of fundamental and applied mechanistic research.

Synthesis and crystal structure of ABW-type SrFe1.40V0.60O4

Single crystals of SrFe1.40V0.60O4, strontium tetra-oxidodi[ferrate(III)/vanad-ate(III)], have been obtained as a side product in the course of sinter experiments aimed at the synthesis of double perovskites in the system SrO-Fe2O3-V2O5. The crystal structure can be characterized by layers of six-membered rings of TO4-tetra-hedra (T: FeIII, VIII) perpendicular to [100]. Stacking of the layers along [100] results in a three-dimensional framework enclosing tunnel-like cavities in which SrII cations are incorporated for charge compensation. The sequence of directedness of up (U) and down (D) pointing vertices of neighboring tetra-hedra in a single six-membered ring is UUUDDD. The topology of the tetra-hedral framework belongs to the zeolite-type ABW.

Elucidating the physical properties of the molybdenum oxide Mo4O11 and its tantalum substituted variant Mo2Ta2O11

Although γ/η-Mo4O11 and Mo2Ta2O11 are used in a variety of industrial applications and can easily be synthesized in a chemical vapour transport (CVT) process or reactions in silica ampoules, respectively, only few data are available concerning their physical properties. In this paper, we further explore the properties of the three compounds with respect to their thermal and magnetic behavior, surface composition, and Raman spectroscopic properties.

Zirconium Oxycarbide: A Highly Stable Catalyst Material for Electrochemical Energy Conversion

Metal carbides and oxycarbides have recently gained considerable interest due to their (electro)catalytic properties that differ from those of transition metals and that have potential to outperform them as well. The stability of zirconium oxycarbide nanopowders (ZrO_0.31C_0.69), synthesized via a hybrid solid‐liquid route, is investigated in different gas atmospheres from room temperature to 800 °C by using in‐situ X‐ray diffraction and in‐situ electrical impedance spectroscopy. To feature the properties of a structurally stable Zr oxycarbide with high oxygen content, a stoichiometry of ZrO_0.31C_0.69 has been selected. ZrO_0.31C_0.69 is stable in reducing gases with only minor amounts of tetragonal ZrO₂ being formed at high temperatures, whereas it decomposes in CO₂ and O₂ gas atmosphere. From online differential electrochemical mass spectrometry measurements, the hydrogen evolution reaction (HER) onset potential is determined at −0.4 V_RHE. CO₂ formation is detected at potentials as positive as 1.9 V_RHE as ZrO_0.31C_0.69 decomposition product, and oxygen is anodically formed at 2.5 V_RHE, which shows the high electrochemical stability of this material in acidic electrolyte. This peopwery makes the material suited for electrocatalytic reactions at anodic potentials, such as CO and alcohol oxidation reactions, in general.

CO2 Reduction by Hydrogen Pre-Reduced Acceptor-Doped Ceria

The reactivity of H₂ pre-reduced acceptor-doped ceria materials Gd_0.10 Ce_0.90 O_2-δ (GDC10) and Sm_0.15 Ce_0.85 O_2-δ (SDC15) was tested with respect to the reduction of CO₂ to CO in the context of the reverse water-gas shift reaction. It was demonstrated that not only oxygen vacancies, but also dissolved hydrogen is a reactive species for the reduction of CO₂ . Dissolved hydrogen must be considered upon discussion of the mechanism of the reverse water-gas shift reaction on ceria-derived materials apart from oxygen vacancies and formates. The reduction of CO₂ is preceded by the formation of carbonate species of different thermal stability and reactivity. The stability of these carbonates was directly demonstrated by in situ infrared spectroscopy and revealed the largely reversible nature of CO₂ ad- and desorption. In comparison to pre-reduced samples, decreased carbonate coverage is obtained after oxidative treatments of GDC10 and SDC15. No significant effect of the sample treatment (O₂ oxidation or H₂ reduction) on the surface carbonate stability was noticed. Mono-dentate carbonates and carboxylates appear to be more easily formed on pre-reduced (i. e. defective) samples. Ce⁴⁺ reduction to Ce³⁺ (by H₂ ) and re-oxidation to Ce⁴⁺ (by CO₂ ) on GDC10/SDC15 were directly monitored by infrared spectroscopic analysis of a distinct, IR-active electronic transition of Ce³⁺ . These results show the complex interplay of oxygen vacancy/dissolved hydrogen reactivity and surface chemical aspects in acceptor-doped ceria materials.

Modeling and optimization of V2O5/TiO2 nanocatalysts for NH3-Selective catalytic reduction (SCR) of NOx by RSM and ANN techniques

In the present study, two statistical methods including the response surface method (RSM) and artificial neural network (ANN), were employed for modeling and optimization of selective catalytic reduction of NOx with NH₃ (NH₃-SCR) over V₂O₅/TiO₂ nanocatalysts. The relationship between catalyst preparation variables, such as metal loading, impregnation temperature, and calcination temperature on NO conversion were investigated. The R² value of 0.9898 was obtained for quadratic a RSM model, which proves the high agreement of the model with the experimental data. The results of Pareto analysis revealed that three factors including calcination temperature, V loading, and impregnation temperature have a considerable impact on the response. Deduced from the established RSM model, the order of influence on the NO conversion was as follows: calcination followed by V loading and impregnation temperature. The optimum condition of catalyst preparation for maximum NO conversion over V₂O₅/TiO₂ nanocatalysts was predicted to be at 0.0051 mol of V loading, an impregnation temperature of 50 °C and a calcination temperature of 491 °C. Moreover, an ANN model was created by a feed-forward back propagation network (with the topology 4, 12 and 1) to model the relation between the selected catalyst preparation variables and NH₃-SCR process temperature. The R² values for training, validation as well as test sets, were 0.99, 0.9810 and 0.9733. These high values proved the accuracy of the AAN model in modeling and estimating the NO conversion over V₂O₅/TiO₂ nanocatalysts. According to the ANN model, the relative significance of each variable on NO conversion is calcination temperature, process temperature loading, and impregnation temperature from high to low importance, respectively, corroborating the obtained results from RSM.

Reactive metal-support interaction in the Cu-In2O3 system: intermetallic compound formation and its consequences for CO2-selective methanol steam reforming

The reactive metal-support interaction in the Cu-In₂O₃ system and its implications on the CO₂ selectivity in methanol steam reforming (MSR) have been assessed using nanosized Cu particles on a powdered cubic In₂O₃ support. Reduction in hydrogen at 300 °C resulted in the formation of metallic Cu particles on In₂O₃. This system already represents a highly CO₂-selective MSR catalyst with ~93% selectivity, but only 56% methanol conversion and a maximum H₂ formation rate of 1.3 µmol g_Cu ^-1 s^-1. After reduction at 400 °C, the system enters an In₂O₃-supported intermetallic compound state with Cu₂In as the majority phase. Cu₂In exhibits markedly different self-activating properties at equally pronounced CO₂ selectivities between 92% and 94%. A methanol conversion improvement from roughly 64% to 84% accompanied by an increase in the maximum hydrogen formation rate from 1.8 to 3.8 µmol g_Cu ^-1 s^-1 has been observed from the first to the fourth consecutive runs. The presented results directly show the prospective properties of a new class of Cu-based intermetallic materials, beneficially combining the MSR properties of the catalyst's constituents Cu and In₂O₃. In essence, the results also open up the pathway to in-depth development of potentially CO₂-selective bulk intermetallic Cu-In compounds with well-defined stoichiometry in MSR.

Parameter Screening of PVDF/PVP Multi-Channel Capillary Membranes

The increasing research in the field of polymeric multi-channel membranes has shown that their mechanical stability is beneficial for a wide range of applications. The more complex interplay of formation process parameters compared to a single-channel geometry makes an investigation using Design of Experiments (DoE) appealing. In this study, seven-channel capillary membranes were fabricated in a steam–dry–wet spinning process, while varying the composition of the polymer solution and the process temperatures in a three-level fractional factorial linear screening design. The polymers polyvinylidene flouride (PVDF) was the chemically resistant main polymer and polyvinylpyrrolidone (PVP) was added as hydrophilic co-polymer. Scanning electron microscopy and atomic force microscopy were applied to study the membrane morphology. Fabrication process conditions were established to yield PVDF/PVP multi-channel membranes, which reached from high flux (permeability P = 321.4L/m2/h/bar, dextran 500 kDa retention R = 18.3%) to high retention (P = 66.8L/m2/h/bar, R = 80.0%). The concentration of the main polymer PVDF and the molecular weight of the co-polymer PVP showed linear relations with both P and R. The permeability could be increased using sodium hypochlorite post-treatment, although retention was slightly compromised. The obtained membranes may be suitable for micro- or ultra-filtration and, at the same time, demonstrate the merits and limitations of DoE for multi-channel membrane screening.

An ultra-flexible modular high vacuum setup for thin film deposition

A modular high vacuum chamber dedicated to thin film deposition is presented. We detail the vacuum and gas infrastructure required to operate two highly flexible chambers simultaneously, with a focus on evaporation techniques (thermal and electron beam) and magnetron sputtering, including baking equipment to remove residual water from the chamber. The use of O-ring-sealed flat flanges allows a tool-free assembly process, in turn enabling rapid changes of the whole setup. This leads to a high flexibility regarding the deposition techniques as the chamber can be adapted to different sources within minutes, permitting the formation of multilayer systems by consecutive depositions onto the same substrate. The central piece of the chamber is a flat flange ground glass tube or cross. The glass recipient permits optical monitoring of the deposition process. Further equipment, such as for the introduction of gases, additional pressure gauges, or evaporators, can be incorporated via specifically designed stainless steel/aluminum interconnectors and blank flanges. In the end, we demonstrate the preparation of an unsupported thin film system consisting of electron-beam-evaporated platinum nanoparticles embedded in magnetron-sputtered zirconia (ZrO₂), deposited onto NaCl single crystals, which subsequently can be removed by dissolution. These films are further analyzed by means of transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

Hybrid synthesis of zirconium oxycarbide nanopowders with defined and controlled composition

A combined synthesis strategy involving a carbothermal reduction and gelation approach with glycine as gelating agent was used to obtain Zr-based (oxy)carbide materials with defined and controlled composition. A comparatively low temperature approach (1500 °C) allows exploration of the ZrC–ZrO₂ phase diagram and reproducibly leads to zirconium (oxy)carbide phases with different C/Zr ratios, as confirmed by combined X-ray diffraction (XRD) and transmission electron microscopy (TEM) data. The latter also indicates a chemically very homogeneous distribution of oxygen and carbon throughout the sample bulk, a prerequisite for further characterization of its intrinsic physico-chemical properties. Due to the general variability of the synthesis procedure – variation of metal precursor, amount of gelating agent and carbon precursor source – it is expected that the method can be easily adapted and transferred to other metal – oxycarbide materials.

A combined synthesis strategy involving a carbothermal reduction and gelation approach with glycine as gelating agent was used to obtain Zr-based (oxy)carbide materials with defined and controlled composition.

On the structural stability of crystalline ceria phases in undoped and acceptor-doped ceria materials under in situ reduction conditions

The reduction of pure and Sm-doped ceria in hydrogen has been studied by synchrotron-based in situ X-ray diffraction to eventually prove or disprove the presence of crystalline cerium hydride (CeH x ) phases and the succession of potential structural phase (trans)formations of reduced cerium oxide phases during heating-cooling cycles up to 1273 K. Despite a recent report on the existence of bulk and surface CeH x phases during reductive treatment of pure CeO2 in H2, structural analysis by Rietveld refinement as well as additional 1H-NMR spectroscopy did not reveal the presence of any crystalline CeH x phase. Rather, a sequence of phase transformations during the re-cooling process in H2 has been observed. In both samples, the reduced/defective fluorite lattice undergoes at first a transformation into a bixbyite-type lattice with a formal stoichiometry Ce0.58 3+Ce0.42 4+O1.71 and Sm0.15 3+Ce0.39 3+Ce0.46 4+O1.73, before a transformation into rhombohedral Ce7O12 takes place in pure CeO2. This phase is clearly absent for the Sm-doped material. Finally, a triclinic Ce11O20 phase appears for both materials, which can be recovered to room temperature, and on which a phase mixture of bixbyite Ce0.66 3+Ce0.34 4+O1.67, rh-Ce0.60 3+Ce0.40 4+O1.70 and tri-Ce0.48 3+Ce0.52 4+O1.76 (for pure CeO2) or bixbyite Sm0.15 3+Ce0.47 3+Ce0.38 4+O1.69 and tri-Sm0.15 3+Ce0.31 3+Ce0.54 4+O1.77 (for Sm-doped CeO2) prevails. The absence of the rhombohedral phase indicates that Sm doping leads to the stabilization of the bixbyite phase over the rhombohedral one at this particular oxygen vacancy concentration. It is worth noting that recent work proves that hydrogen is indeed incorporated within the structures during the heat treatments, but under the chosen experimental conditions it has apparently no effect on the salient structural principles during reduction.

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3−δ) thin film and bulk model systems during iron exsolution

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La_0.6Sr_0.4FeO_3−δ).

Zirconium-Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming

C-saturated Pd⁰ nanoparticles with an extended phase boundary to ZrO₂ evolve from a Pd⁰ Zr⁰ precatalyst under CH₄ dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO₂ is efficiently activated at oxidic phase boundary sites and Pd_x C provides fast supply of C-atoms toward the latter.

Impregnated and Co-precipitated Pd-Ga2O3, Pd-In2O3 and Pd-Ga2O3-In2O3 Catalysts: Influence of the Microstructure on the CO2 Selectivity in Methanol Steam Reforming

ABSTRACT

To focus on the influence of the intermetallic compound-oxide interface of Pd-based intermetallic phases in methanol steam reforming (MSR), a co-precipitation pathway has been followed to prepare and subsequently structurally and catalytically characterize a set of nanoparticulate Ga2O3- and In2O3-supported GaPd2 and InPd catalysts, respectively. To study the possible promoting effect of In2O3, an In2O3-doped Ga2O3-supported GaPd2 catalyst has also been examined. While, upon reduction, the same intermetallic compounds are formed, the structure of especially the Ga2O3 support is strikingly different: rhombohedral and spinel-like Ga2O3 phases, as well as hexagonal GaInO3 and rhombohedral In2O3 phases are observed locally on the materials prior to methanol steam reforming by high-resolution transmission electron microscopy. Overall, the structure, phase composition and morphology of the co-precipitated catalysts are much more complex as compared to the respective impregnated counterparts. However, this induces a beneficial effect in activity and CO2 selectivity in MSR. Both Ga2O3 and In2O3 catalysts show a much higher activity, and in the case of GaPd2-Ga2O3, a much higher CO2 selectivity. The promoting effect of In2O3 is also directly detectable, as the CO2 selectivity of the co-precipitated supported Ga2O3-In2O3 catalyst is much higher and comparable to the purely In2O3-supported material, despite the more complex structure and morphology. In all studied cases, no deactivation effects have been observed even after prolonged time-on-stream for 12 h, confirming the stability of the systems.

GRAPHICAL ABSTRACT

The presence of a variety of distinct supported intermetallic InPd and GaPd2 particle phases is not detrimental to activity/selectivity in methanol steam reforming as long as the appropriate intermetallic phases are present and they exhibit optimized intermetallic-support phase boundary dimensions.

Transmission in situ and operando high temperature X-ray powder diffraction in variable gaseous environments

This work describes a device for time-resolved synchrotron-based in situ and operando X-ray powder diffraction measurements at elevated temperatures under controllable gaseous environments. The respective gaseous sample environment is realized via a gas-tight capillary-in-capillary design, where the gas flow is achieved through an open-end 0.5 mm capillary located inside a 0.7 mm capillary filled with a sample powder. Thermal mass flow controllers provide appropriate gas flows and computer-controlled on-the-fly gas mixing capabilities. The capillary system is centered inside an infrared heated, proportional integral differential-controlled capillary furnace allowing access to temperatures up to 1000 °C.

Role of Precursor Carbides for Graphene Growth on Ni(111)

Surface X-ray Diffraction was used to study the transformation of a carbon-supersaturated carbidic precursor toward a complete single layer of graphene in the temperature region below 703 K without carbon supply from the gas phase. The excess carbon beyond the 0.45  monolayers of C atoms within a single Ni2C layer is accompanied by sharpened reflections of the |4772| superstructure, along with ring-like diffraction features resulting from non-coincidence rotated Ni2C-type domains. A dynamic Ni2C reordering process, accompanied by slow carbon loss to subsurface regions, is proposed to increase the Ni2C 2D carbide long-range order via ripening toward coherent domains, and to increase the local supersaturation of near-surface dissolved carbon required for spontaneous graphene nucleation and growth. Upon transformation, the intensities of the surface carbide reflections and of specific powder-like diffraction rings vanish. The associated change of the specular X-ray reflectivity allows to quantify a single, fully surface-covering layer of graphene (2 ML C) without diffraction contributions of rotated domains. The simultaneous presence of top-fcc and bridge-top configurations of graphene explains the crystal truncation rod data of the graphene-covered surface. Structure determination of the |4772| precursor surface-carbide using density functional theory is in perfect agreement with the experimentally derived X-ray structure factors.

H2 reduction of Gd- and Sm-doped ceria compared to pure CeO2 at high temperatures: effect on structure, oxygen nonstoichiometry, hydrogen solubility and hydroxyl chemistry

Acceptor doping of CeO₂ substantially influences defect chemistry, bulk structure, hydrogen solubility and hydroxyl chemistry in hydrogen atmospheres.

Structural investigations of La0.6Sr0.4FeO3−δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La_0.6Sr_0.4FeO_3−δ, LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (R3̄c to Pm3̄m) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

A dependence of structural transformation and iron exsolution on chemical environment and reducing conditions is proven for the perovskite La_0.6Sr_0.4FeO_3−δ.

Preferentially Oriented TiO2 Nanotubes as Anode Material for Li-Ion Batteries: Insight into Li-Ion Storage and Lithiation Kinetics

Self-organized TiO₂ nanotubes (NTs) with a preferential orientation along the [001] direction are anodically grown by controlling the water content in the fluoride-containing electrolyte. The intrinsic kinetic and thermodynamic properties of the Li intercalation process in the preferentially oriented (PO) TiO₂ NTs and in a randomly oriented (RO) TiO₂ NT reference are determined by combining complementary electrochemical methods, including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic cycling. PO TiO₂ NTs demonstrate an enhanced performance as anode material in Li-ion batteries due to faster interfacial Li insertion/extraction kinetics. It is shown that the thermodynamic properties, which describe the ability of the host material to intercalate Li ions, have a negligible influence on the superior performance of PO NTs. This work presents a straightforward approach for gaining important insight into the influence of the crystallographic orientation on lithiation/delithiation characteristics of nanostructured TiO₂ based anode materials for Li-ion batteries. The introduced methodology has high potential for the evaluation of battery materials in terms of their lithiation/delithiation thermodynamics and kinetics in general.

Li3Co1.06(1)TeO6: synthesis, single-crystal structure and physical properties of a new tellurate compound with CoII/CoIII mixed valence and orthogonally oriented Li-ion channels

A tellurate compound with Co^II/Co^III mixed valence states and lithium ions within orthogonally oriented channels was realized in Li₃Co_1.06(1)TeO₆. The single-crystal structure determination revealed two independent and interpenetrating Li/O and (Co,Te)/O substructures with octahedral oxygen coordination of the metal atoms. In contrast to other mixed oxides, a honeycomb-like ordering of CoO₆ and TeO₆ octahedra was not observed. Li₃Co_1.06(1)TeO₆ crystallizes orthorhombically with the following unit cell parameters and refinement results: Fddd, a = 588.6(2), b = 856.7(2), c = 1781.5(4) pm, R₁ = 0.0174, wR₂ = 0.0462, 608 F² values, and 33 variables. Additional electron density in tetrahedral voids in combination with neighboring face-linked and under-occupied octahedral lithium sites offers an excellent possible diffusion pathway for lithium ions. According to the symmetry of the crystal structure the diffusion pathways in Li₃Co_1.06(1)TeO₆ were found in two orthogonal orientations. The Co^II/Co^III mixed valence was investigated via X-ray photoelectron spectroscopy (XPS), revealing a composition comparable to that derived from single-crystal X-ray diffractometry. Magnetic susceptibility measurements underlined the coexistence of Co^II and Co^III, the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of Li₃Co_1.06(1)TeO₆ was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10^-6 S cm^-1 at 573 K.