Characterization of Gas-Solid Reactions using In Situ Powder X-ray Diffraction

In situ neutron diffraction to investigate the solid-state synthesis of Ni-rich cathode materials

Studying chemical reactions in real time can provide unparalleled insight into the evolution of intermediate species and can provide guidance to optimize the reaction conditions. For solid-state synthesis reactions, powder diffraction has been demonstrated as an effective tool for resolving the structural evolution taking place upon heating. The synthesis of layered Ni-rich transition-metal oxides at a large scale (grams to kilograms) is highly relevant as these materials are commonly employed as cathodes for Li-ion batteries. In this work, in situ neutron diffraction was used to monitor the reaction mechanism during the high-temperature synthesis of Ni-rich cathode materials with a varying ratio of Ni:Mn from industrially relevant hydroxide precursors. Rietveld refinement was further used to model the observed phase evolution during synthesis and compare the behaviour of the materials as a function of temperature. The results presented herein confirm the suitability of in situ neutron diffraction to investigate the synthesis of batches of several grams of electrode materials with well-controlled stoichiometry. Furthermore, monitoring the structural evolution of the mixtures with varying Ni:Mn content in real time reveals a delayed onset of li-thia-tion as the Mn content is increased, necessitating the use of higher annealing temperatures to achieve layering.

Synchrotron X-ray powder diffraction under high pressures up to 33 MPa for mechanoresponsive materials

Equipment for synchrotron X-ray diffraction at high pressures up to 33 MPa with an accuracy of ±0.1 MPa using a liquid as a pressure-transmitting medium has been developed. This equipment enables atomic-scale observation of the structural change of mechanoresponsive materials under applied pressures. The validity of the equipment is demonstrated by observation of the pressure dependence of the lattice parameters of copper. The observed bulk modulus of copper was found to be 139 (13) GPa which is a good agreement with the literature value. The developed equipment was subsequently applied to a repeatable mechanoluminescence material, Li_0.12Na_0.88NbO₃:Pr³⁺. The bulk modulus and compressibility along the a and c axes were determined as 79 (9) GPa, 0.0048 (6) GPa^-1 and 0.0030 (9) GPa^-1, respectively, for the R3c phase. The advance of high-pressure X-ray diffraction will play an important role in understanding mechanoresponsive materials towards their atomic-scale design.

A double-walled sapphire single-crystal gas-pressure cell (type III) for in situ neutron diffraction

A new sapphire single-crystal gas-pressure cell for elastic neutron scattering to study solid–gas reactions is presented and evaluated; it allows conditions of 298 K and 9.5 MPa hydrogen pressure and 1110 K at ambient pressure.

In situ neutron diffraction is an important characterization technique for the investigation of many functional materials, e.g. for hydrogen uptake and release in hydrogen storage materials. A new sapphire single-crystal gas-pressure cell for elastic neutron scattering has been developed and evaluated; it allows conditions of 298 K and 9.5 MPa hydrogen pressure and 1110 K at ambient pressure. The pressure vessel consists of a sapphire single-crystal tube of 35 mm radius and a sapphire single-crystal crucible as sample holder. Heating is realized by two 100 W diode lasers. It is optimized for the D20 diffractometer, ILL, Grenoble, France, and requires the use of a radial oscillating collimator. Its advantages over earlier sapphire single-crystal gas-pressure cells are higher maximum temperatures and lower background at low and high diffraction angles. The deuterium uptake in palladium was followed in situ for validation, proving the potential of the type-III gas-pressure cell for in situ neutron diffraction on solid–gas reactions.

Simultaneous neutron powder diffraction and Raman spectroscopy – an approach of combining two complementary techniques

Diffraction techniques are well-established methods for crystal structure determination as well as phase identification and quantification. Raman spectroscopy can be a valuable complementary characterization technique, because in contrast to the former it yields also information on amorphous materials and it is a probe for short-range structural effects. The herein presented setup allows for simultaneous neutron diffraction and Raman spectroscopy, shown with a sample of lead sulfate under ambient conditions as a proof of principle. In order to fulfil requirements of both methods, a sapphire single-crystal is used as a sample holder. Practical considerations for successful simultaneous in situ neutron diffraction and Raman spectroscopic measurements are given.

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.

Solid–gas reactions in synthetic chemistry: what can we learn from reaction pathways?

The knowledge of reaction pathways in the preparation of solids is usually rather scarce, which hinders synthesis planning and process control. This is particularly true for metastable compounds, which are a challenge for chemical synthesis, especially in the solid state. In situ studies can help in exploring the energy landscape around their local minimum by investigating formation and decomposition. Screening the multi-parameter space in synthetic chemistry is much more efficient using in as compared to ex situ methods. Studying solid–gas reactions in situ is demanding due to the oftentimes harsh conditions as for temperature and gas pressure. Examples are given for a variety of solids and applications, e.g., metal hydrides (hydrogen storage, hydrogenation – decomposition – desorption – recombination), intermetallics (heterogeneous catalysis), metal nitrides, nitride oxides and oxides (magnetic materials, photocatalysts). Many new metastable compounds with intriguing properties were discovered by such in situ studies in flowing or static gas atmosphere (H2, Ar, NH3, air) at elevated pressures and temperatures using a variety of in situ methods such as X-ray and neutron powder diffraction, thermal analysis, environmental scanning electron microscopy, Raman, NMR, UV-VIS and X-ray absorption fine structure spectroscopy. The potential of unravelling reaction pathways of solid–gas reactions for improving syntheses and controlling chemical processes is demonstrated.

The bibliography includes 48 references.

Based on a talk given at the 5th EUCHEMS Inorganic Chemistry Conference (EICC-5, Moscow, Russia, 2019).

Potassium octahydridotriborate: diverse polymorphism in a potential hydrogen storage material and potassium ion conductor

Hydrogen storage properties and polymorphism in KB₃H₈. The order–disorder polymorphic transition results in disordered B₃H₈⁻ anions, facilitating cation mobility.

Synthesis, structure, and polymorphic transitions of praseodymium(iii) and neodymium(iii) borohydride, Pr(BH4)3 and Nd(BH4)3

In this work, praseodymium(iii) borohydride, Pr(BH4)3, and an isotopically enriched analogue, Pr(11BD4)3, are prepared by a new route via a solvate complex, Pr(11BD4)3S(CH3)2. Nd(BH4)3 was synthesized using the same method and the structures, polymorphic transformations, and thermal stabilities of these compounds are investigated in detail. α-Pr(BH4)3 and α-Nd(BH4)3 are isostructural with cubic unit cells (Pa3[combining macron]) stable at room temperature (RT) and a unit cell volume per formula unit (V/Z) of 180.1 and 175.8 Å3, respectively. Heating α-Pr(BH4)3 to T ∼ 190 °C, p(Ar) = 1 bar, introduces a transition to a rhombohedral polymorph, r-Pr(BH4)3 (R3[combining macron]c) with a smaller unit cell volume and a denser structure, V/Z = 156.06 Å3. A similar transition was not observed for Nd(BH4)3. However, heat treatment of α-Pr(BH4)3, at T ∼ 190 °C, p(H2) = 40 bar and α-Nd(BH4)3, at T ∼ 270 °C, p(H2) = 98 bar facilitates reversible formation of another three cubic polymorph, denoted as β, β' and β''-RE(BH4)3 (Fm3[combining macron]c). Moreover, the transition β- to β'- to β''- is considered a rare example of stepwise negative thermal expansion. For Pr(BH4)3, ∼2/3 of the sample takes this route of transformation whereas in argon only ∼5 wt%, and the remaining transforms directly from α- to r-Pr(BH4)3. The β-polymorphs are porous with V/Z = 172.4 and 172.7 Å3 for β''-RE(BH4)3, RE = Pr or Nd, respectively, and are stabilized by the elevated hydrogen pressures. The polymorphic transitions occur due to rotation of RE(BH4)6 octahedra without breaking or forming chemical bonds. Structural DFT optimization reveals the decreasing stability of α-Pr(BH4)3 > β-Pr(BH4)3 > r-Pr(BH4)3.

Structure and Hydrogenation Properties of a HfNbTiVZr High-Entropy Alloy

A high-entropy alloy (HEA) of HfNbTiVZr was synthesized using an arc furnace followed by ball milling. The hydrogen absorption mechanism was studied by in situ X-ray diffraction at different temperatures and by in situ and ex situ neutron diffraction experiments. The body centered cubic (BCC) metal phase undergoes a phase transformation to a body centered tetragonal (BCT) hydride phase with hydrogen occupying both tetrahedral and octahedral interstitial sites in the structure. Hydrogen cycling of the alloy at 500 °C is stable. The large lattice strain in the HEA seems favorable for absorption in both octahedral and tetrahedral sites. HEAs therefore have potential as hydrogen storage materials because of favorable absorption in all interstitial sites within the structure.

Reversible hydrogenation of the Zintl phases BaGe and BaSn studied by in situ diffraction

Hydrogenation products of the Zintl phases AeTt (Ae = alkaline earth; Tt = tetrel) exhibit hydride anions on interstitial sites as well as hydrogen covalently bound to Tt which leads to a reversible hydrogenation at mild conditions. In situ thermal analysis, synchrotron and neutron powder diffraction under hydrogen (deuterium for neutrons) pressure was applied to BaTt (Tt=Ge, Sn). BaTtHy (1<y<1.67, γ-phases) were formed at 5 MPa hydrogen pressure and elevated temperatures (400–450 K). Further heating (500–550 K) leads to a hydrogen release forming the new phases β-BaGeH0.5 (Pnma, a=1319.5(2) pm, b=421.46(2) pm, c=991.54(7) pm) and α-BaSnH0.19 (Cmcm, a=522.72(6) pm, b=1293.6(2) pm, c=463.97(6) pm). Upon cooling the hydrogen rich phases are reformed. Thermal decomposition of γ-BaGeHy under vacuum leads to β-BaGeH0.5 and α-BaGeH0.13 [Cmcm, a=503.09(3) pm, b=1221.5(2) pm, c=427.38(4) pm]. At 500 K the reversible reaction α-BaGeH0.23 (vacuum)⇄β-BaGeH0.5 (0.2 MPa deuterium pressure) is fast and was observed with 10 s time resolution by in situ neutron diffraction. The phases α-BaTtHy show a pronounced phase width (at least 0.09<y<0.36). β-BaGeH0.5 and the γ-phases appear to be line phases. The hydrogen poor (α- and β-) phases show a partial occupation of Ba4 tetrahedra by hydride anions leading to a partial oxidation of polyanions and shortening of Tt–Tt bonds.

Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)

The hydrogenation properties of Laves phases LnMg₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb) were investigated by thermal analysis, X-ray, synchrotron, and neutron powder diffraction. At 14.0 MPa hydrogen gas pressure and 393 K, PrMg₂ and NdMg₂ take up hydrogen and form the colorless, ternary hydrides PrMg₂H₇ (P4₁2₁2, a = 632.386(6) pm, c = 945.722(11) pm) and NdMg₂H₇ (P4₁2₁2, a = 630.354(9) pm, c = 943.018(16) pm). The crystal structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg₂D₇, P4₁2₁2, a = 630.56(2) pm, c = 943.27(3) pm; NdMg₂D₇, P4₁2₁2, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMg₂D₇. The LaMg₂D₇ type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to lanthanide hydrides and magnesium. The Laves phase EuMg₂ forms a hydride EuMg₂H_x of black color. Its crystal structure (P2₁2₁2₁, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn₂ type) of the hydrogen-free parent intermetallic. GdMg₂ and TbMg₂ form hydrides GdMg₂H_x with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg₂H_x (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn₂ type of structure. CeMg₂H₇ and NdMg₂H₇ are paramagnetic with effective magnetic moments of 2.49(1) μ_B and 3.62(1) μ_B, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (μ_calc(Ce³⁺) = 2.54 μ_B; μ_calc(Nd³⁺) = 3.62 μ_B).

100 years in situ diffraction

The X-ray diffraction experiment of iron at temperatures up to 1000°C, which Albert Hull conducted 100 years ago, in 1917, may be regarded as the first in situ diffraction experiment. Ever since, diffraction methods matured and became widely used and powerful tools for materials characterization and structure determination. Considerable progress was made in radiation source brilliance and diffraction instrumentation, enabling time-dependent in situ studies of a wide range of compounds and processes today. In this contribution, we will give a brief historical sketch of the first in situ diffraction experiment and present some modern-day examples, highlighting the impact of this investigation technique to solid-state sciences.

In Situ Hydrogenation of the Zintl Phase SrGe

Hydrides (deuterides) of the CrB-type Zintl phases AeTt (Ae = alkaline earth; Tt = tetrel) show interesting bonding properties with novel polyanions. In SrGeD_4/3-x (γ phase), three zigzag chains of Ge atoms are condensed and terminated by covalently bound D atoms. A combination of in situ techniques (thermal analysis and synchrotron and neutron powder diffraction) revealed the existence of two further hydride (deuteride) phases with lower H (D) content (called α and β phases). Both are structurally related to the parent Zintl phase SrGe and to the ZrNiH structure type containing variable amounts of H (D) in Sr₄ tetrahedra. For α-SrGeD_y, the highest D content y = 0.29 was found at 575(2) K under 5.0(1) MPa of D₂ pressure, and β-SrGeD_y shows a homogeneity range of 0.47 < y < 0.63. Upon decomposition of SrGeD_4/3-x (γ-SrGeD_y), tetrahedral Sr₄ voids stay filled, while the Ge-bound D4 site loses D. When reaching the lower D content limit, SrGeD_4/3-x (γ phase) with 0.10 < x < 0.17, decomposes to the β phase. All three hydrides (deuterides) of SrGe show variable H (D) content.

Perovskite alkali metal samarium borohydrides: crystal structures and thermal decomposition

Synthesis and characterisation of samarium containing perovskite-type bimetallic borohydrides for hydrogen storage.

Synthesis, structures and thermal decomposition of ammine MxB12H12complexes (M = Li, Na, Ca)

This work presents the structural and thermal properties of ammine metal dodecahydro-closo-dodecaboranes and their reversible ammonia (or hydrogen) storage.

In situ studies of materials for high temperature CO2 capture and storage

Carbon capture and storage (CCS) offers a possible solution to curb the CO₂ emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO₂ capture, such as the CaO–CaCO₃ system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation–regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions.

Synthesis and thermal stability of perovskite alkali metal strontium borohydrides

Synthesis of new thermally stable perovskite-type metal strontium borohydrides, MSr(BH₄)₃ (M = K, Rb, Cs).

In situX-ray diffraction environments for high-pressure reactions

New sample environments and techniques specifically designed forin situpowder X-ray diffraction studies up to 1000 bar (1 bar = 105 Pa) gas pressure are reported and discussed. The cells can be utilized for multiple purposes in a range of research fields. Specifically, investigations of gas–solid reactions and sample handling under inert conditions are undertaken here. Sample containers allowing the introduction of gas from one or both ends are considered, enabling the possibility of flow-through studies. Various containment materials are evaluated,e.g.capillaries of single-crystal sapphire (Al2O3), quartz glass (SiO2), stainless steel (S316) and glassy carbon (Sigradur K), and burst pressures are calculated and tested for the different tube materials. In these studies, high hydrogen pressure is generated with a metal hydride hydrogen compressor mounted in a closed system, which allows reuse of the hydrogen gas. The advantages and design considerations of thein situcells are discussed and their usage is illustrated by a case study.