The reduction of metal oxides with hydrogen is widely used for the production of fine chemicals and metals both on the laboratory and industry scale. In situ methods can help to elucidate reaction pathways and to gain control over such synthesis reactions. In this study, the reduction of WO3 and V2O5 with hydrogen was investigated by in situ X‐ray powder diffraction with regard to intermediates and the influence of heating rates and hydrogen flow rates. Mixtures of V4O9, V6O13 and VO2 in two modifications were identified as intermediates on the way to phase‐pure V2O3. None of the intermediates occurs in a single phase and therefore cannot be prepared this way. In contrast, the intermediates of the WO3 reduction, H0.23WO3 and W10O29, appear consecutively and can be isolated. For both reactions, the heating and flow rates have little influence on the formation of intermediates.

The hydrogenation properties of Laves phases LnMg (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, PrMg and NdMg take up hydrogen and form the colorless, ternary hydrides PrMgH (P422, a = 632.386(6) pm, c = 945.722(11) pm) and NdMgH (P422, 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 (PrMgD, P422, a = 630.56(2) pm, c = 943.27(3) pm; NdMgD, P422, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMgD. The LaMgD 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 EuMg forms a hydride EuMgH of black color. Its crystal structure (P222, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn type) of the hydrogen-free parent intermetallic. GdMg and TbMg form hydrides GdMgH with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMgH (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn type of structure. CeMgH and NdMgH are paramagnetic with effective magnetic moments of 2.49(1) μ and 3.62(1) μ, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (μ(Ce) = 2.54 μ; μ(Nd) = 3.62 μ).

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The reduction of V2O5 and WO3 with hydrogen, the latter of which is performed industrially in rotary kilns under a hydrogen countercurrent, has been investigated by using in situ X‐ray powder diffraction. This method yields insights into the processes illustrated by the transparent section of the furnace, thus gaining information about the reaction pathways as well as compositions and structures of intermediates. More information can be found in the Research Article by H. Kohlmann and co‐workers (DOI: 10.1002/chem.202203932).

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