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 (ZrO0.31C0.69), 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 ZrO0.31C0.69 has been selected. ZrO0.31C0.69 is stable in reducing gases with only minor amounts of tetragonal ZrO2 being formed at high temperatures, while 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 VRHE. CO2 formation is detected at potentials as positive as 1.9 VRHE as ZrO0.31C0.69 decomposition product, and oxygen is anodically formed at 2.5 VRHE, which shows the high electrochemical stability of this material in acidic electrolyte. This makes the material suited for electrocatalytic reactions at anodic potentials, such as CO and alcohol oxidation reactions, in general.
A time-resolved series of high-resolution transmission electron microscopy (HRTEM) images is used to monitor phase and morphology transformation of rod-like and spherical particles with initial orthorhombic InOOH phase in-situ under continuous illumination with high-energy electrons in a transmission electron microscope. For both particles types, the electron-beam irradiation induces a fast InOOH to rh-In2O3 decomposition accompanied with the formation of voids within the particle/rod center. After illumination time intervals of about 1-2 min (i.e. electron dose 6.3-12.6∙10⁷ e∙nm⁻²) for particles and 8 min (4.3∙10⁸ e∙nm⁻²) for rods, respectively, several small empty cavities become visible in the particle/rod center. The cavities coalesce and form a large hollow space/canal after further illumination. Time-resolved in-situ HRTEM unambiguously shows that the formation of internal voids in both nanoparticle types is a consequence of the structural InOOH-to-rh-In2O3 phase transition that starts at the surface of the corresponding particle. The as-formed oxide phase encapsulates the untransformed hydroxylated phase. Its decomposition does not follow the Kirkendall mechanism; the matter transferred outwards is removed in form of water, leading to void formation inside without an increase of the particle size.
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 gCu⁻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 gCu⁻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.
We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-d). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(IV) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(III). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.
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 Ce0.583+Ce0.424+O1.71 and Sm0.153+Ce0.393+Ce0.464+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 Ce0.663+Ce0.344+O1.67, rh-Ce0.603+Ce0.404+O1.70 and tri-Ce0.483+Ce0.524+O1.76 (for pure CeO2) or bixbyite Sm0.153+Ce0.473+Ce0.384+O1.69 and tri-Sm0.153+Ce0.313+Ce0.544+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.
Kohlenstoff‐gesättigte Pd0‐Nanopartikel mit ausgedehnter Phasengrenze zu ZrO2 bilden sich unter CH4‐Trockenreformierbedingungen aus einem intermetallischen Pd0Zr0‐Präkatalysator. Der resultierende, hochaktive Katalysatorzustand arbeitet bifunktionell: CO2 wird an oxidischen Phasengrenzplätzen effizient aktiviert, und PdxC gewährleistet die rasche Versorgung der Phasengrenze mit reaktiven C‐Atomen. ZrO2‐geförderte Methan‐Trockenreformierung auf PdxC: Kohlenstoff‐gesättigte Pd0‐Nanopartikel mit ausgedehnter Phasengrenze zu ZrO2 entstehen unter CH4‐Trockenreformierbedingungen aus einem intermetallischen Pd0Zr0‐Präkatalysator. Der so gebildete hochaktive Katalysatorzustand arbeitet bifunktionell: CO2 wird an oxidischen Phasengrenzplätzen effizient aktiviert, und PdxC gewährleistet die rasche Versorgung der Phasengrenze mit reaktiven C‐Atomen.
Structural and chemical consequences of reactive metal-support interaction (RMSI) effects occurring under hydrogen reduction in a metastable metal-oxide system have been exemplified for small PdO particles loaded onto rhombohedral In2O3 (PdO/rh-In2O3) using synchrotron-based in situ X-ray diffraction experiments at temperatures up to 500 °C. rh-In2O3 is a meta-stable In2O3 polymorph that is prone to gas-phase dependent phase transformation to cubic In2O3 (c-In2O3). Cross-influence of metal-support interaction and phase transformation can therefore be expected in similar temperature regimes. To separate both effects, comparable experiments have also been conducted on pure rh-In2O3. Phase transformation of pure rh-In2O3 to cubic In2O3 in hydrogen occurs between 415 °C and 450 °C. On Pd/rh-In2O3, a sequence of PdO reduction to Pd metal, followed by PdH0.706 hydride formation and subsequently, InPd and In3Pd2 intermetallic compound formation have been observed between 30 °C and 500 °C. After the intermetallic compound formation is finished at around 400 °C, the phase transformation to c-In2O3 sets in at exactly the same temperature as on pure rh-In2O3 and extends over the same temperature range. This proves that the phase transformation of rh-In2O3 to c-In2O3 is not influenced by the reduction and the intermetallic compound formation. In contrast to Pd on c-In2O3, In-Pd compound formation from rh-In2O3 occurs at much lower temperatures (230 °C vs. 300 °C), despite finally approaching the same compound stoichiometries (InPd and In3Pd2). This points to a high structural stability of reduced rh-In2O3/stability of oxygen vacancies (compared to c-In2O3) as well as to facilitated diffusion of reduced In(-O) species at low temperatures.
The intermetallic compound (IC) formation between Pd and Ce via reductive metal-support interaction (RMSI) has been studied using two different Pd/CeO2 materials: A thin film of Pd particles embedded in fluorite-type CeO2, grown on vacuum-cleaved NaCl(001) facets and CeO2 powder impregnated with small Pd particles. The reduction of CeO2 and formation of different Pd-Ce intermetallic compounds has been monitored by diffraction in an electron microscope and synchrotron-based in situ X-ray diffraction, and is complemented by density functional theoretical calculations (DFT) of the formation enthalpies to judge the relative stabilities of the formed intermetallic compounds. On both studied materials, the formation of similar ICs, namely CePd3 and CePd5, has been observed for the first time at temperatures of 1100 K following RMSI and, thus, indicates that any catalytic pre-treatment in hydrogen, provided the reduction temperature is high enough, must also be considered for the Pd-CeO2 system. The studies are complemented by a brief review of the Pd-Ce phase diagram to discuss the potential pathways of Pd-Ce intermetallic compound formation.
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.
Stoichiometric (MnGa2O4 and MnAl2O4) and Mn-rich (Mn1.3Ga1.7O4 and Mn1.4Al1.6O4) spinels with a small inversion degree (0.14-0.21) are obtained via a co-precipitation route followed by calcination of the as-synthesized coprecipitates at 700-1000 °C in different gas atmospheres (air, N2 or argon). In situ synchrotron XRD at elevated temperatures reveals the conditions for synthesizing phase-pure materials. The stoichiometry of the samples is confirmed by inductively coupled plasma optical emission spectrometry as well as by structure refinement of neutron diffraction data of phase-pure specimens. The XANES characterization reveals an average oxidation state of manganese +2.2 and 2.3 in the Mn1.3Ga1.7O4 and Mn1.4Al1.6O4 spinels, respectively. The mixed Mn2+–Mn3+ valence states are responsible for the ferrimagnetic properties of Mn1.3Ga1.7O4 and Mn1.4Al1.6O4 samples below 48 and 55 K, respectively, as well as for a smaller optical band gap if compared to stoichiometric spinels.
The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La0.6Sr0.4FeO3−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 (Rc to Pmm) 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.
The iron exsolution behavior of lanthanum strontium ferrite under reducing conditions is investigated using multiple ex situ and in situ methods. During heating experiments, an irreversible phase transition in oxygen-deficient environments from the rhombohedral to the cubic polymorph is witnessed in the form of a non-linear shift of the reflexes prior to the exsolution of metallic iron. Since this is not the case in oxidative environments, it leads to the conclusion that oxygen vacancies in the lattice stabilize the cubic phase. Also, the exsolution is shown to exhibit an Avramilike behavior, suggesting a diffusion-limited process, where the transport of iron to the Fe-depleted surface-near region is the slowest step.
To account for the explanation of an eventual sensoring and catalytic behavior of rhombohedral In2O3 (rh-In2O3) and the dependence of the metastability of the latter on gas atmosphere, in situ electrochemical impedance spectroscopic (EIS), Fourier-transform infrared spectroscopic (FT-IR), in situ X-ray diffraction and in situ thermogravimetric characterization in inert (helium) and reactive gases (hydrogen, carbon monoxide and carbon dioxide) have been conducted to link the gas-dependent electrical conductivity features and the surface chemical properties to its metastability towards cubic In2O3. Especially for highly reducible oxides such as In2O3, for which not only the formation of oxygen vacancies, but deep reduction to metallic state (i.e. metallic indium) has to be taken into account, this approach is imperative. Temperature-dependent impedance features are strongly dependent on the respective gas composition and are assigned to distinct changes in either surface adsorbates or free charge carrier absorbance, allowing to differentiate and distinguish between bulk reduction-related features from those directly arising from surface chemical alterations. For the measurements in an inert gas atmosphere, this analysis specifically also included monitoring the fate of differently bonded, and hence, differently reactive, hydroxyl groups. Reduction of rh-In2O3 proceeds to a large extent indirectly via rh-In2O3 → c-In2O3 → In metal. As deduced from the CO and CO2 adsorption experiments, rhombohedral In2O3 exhibits predominantly Lewis acidic surface sites. The basic character is less pronounced, directly explaining the previously observed high (inverse) water-gas shift activity and the low CO2 selectivity in methanol steam reforming.
We describe the development and implementation of a compact, low power, infrared heated tube furnace for in situ powder X-ray diffraction experiments. Our silicon carbide (SiC) based furnace design exhibits outstanding thermal performance in terms of accuracy control and temperature ramping rates while simultaneously being easy to use, robust to abuse and, due to its small size and low power, producing minimal impact on surrounding equipment. Temperatures in air in excess of 1100 °C can be controlled at an accuracy of better than 1%, with temperature ramping rates up to 100 °C/s. The complete “add-in” device, minus power supply, fits in a cylindrical volume approximately 15 cm long and 6 cm in diameter and resides as close as 1 cm from other sensitive components of our experimental synchrotron endstation without adverse effects.
The surface chemical properties of undoped tetragonal ZrO2 and the gas-phase dependence of the tetragonal-to-monoclinic transformation are studied using a tetragonal ZrO2 polymorph synthesized via a sol-gel method from an alkoxide precursor. The obtained phase-pure tetragonal ZrO2 is defective and strongly hydroxylated with pronounced Lewis acidic and Brønsted basic surface sites. Combined in situ FT-infrared and electrochemical impedance measurements reveal effective blocking of coordinatively unsaturated sites by both CO and CO2, as well as low conductivity. The transformation into monoclinic ZrO2 is suppressed up to temperatures of ∼723 K independent of the gas phase composition, in contrast to at higher temperatures. In inert atmospheres, the persisting structural defectivity leads to a high stability of tetragonal ZrO2, even after a heating-cooling cycle up to 1273 K. Treatments in CO2 and H2 increase the amount of monoclinic ZrO2 upon cooling (>85 wt%) and the associated formation of either Zr-surface-(oxy-)carbide or dissolved hydrogen. The transformation is strongly affected by the sintering/pressing history of the sample, due to significant agglomeration of small crystals on the surface of sintered pellets. Two factors dominate the properties of tetragonal ZrO2: defect chemistry and hydroxylation degree. In particular, moist conditions promote the phase transformation, although at significantly higher temperatures as previously reported for doped tetragonal ZrO2.