In situ X‐ray Powder Diffraction Study of the Reactions of CrO 2 and TlPd 3 O 4 with Hydrogen

Hydrogen is an important reducing agent for transition metal oxides, however, details on reaction pathways are often unknown. Therefore, the reduction of CrO 2 and TlPd 3 O 4 was investigated by in situ X‐ray powder diffraction and subsequent Rietveld analysis. CrO 2 is reduced by hydrogen gas (0.3 MPa) starting at 180 °C according to 2 CrO 2 + H 2 → 2 CrOOH. The reaction is complete at 250 °C and there are no signs for intermediates or non‐stoichiometry. In nitrogen atmosphere the reaction TlPd 3 O 4 → TlPd 3 + 2 O 2 occurs without intermediates in one step starting at about 670 °C. Thermal volume expansion is determined to be V (TlPd 3 O 4 ) = 880.7(1) 10 6 pm 3 + 1.64(7) 10 4 T pm 3 /K + 10.7(9) T 2 pm 3 /K 2 and V (TlPd 3 ) = 258.6(1) 10 6 pm 3 + 8.5(7) 10 3 T pm 3 /K + 2.6(7) T 2 pm 3 /K 2 for 25 °C ≤ T ≤ 730 °C. The formation of β‐TlPd 3 H from TlPd 3 O 4 in 0.3 MPa hydrogen gas at 75 °C occurs very fast. Unit cell parameters indicate the occurrence of a metastable α‐TlPd 3 H ≈0.2 with a hydrogen‐filled ZrAl 3 type. Cubic anti ‐perovskite type β‐TlPd 3 H reacts in air to TlPd 3 with a possible hydrogen deficient intermediate β‐TlPd 3 H 1– x and hints for remaining hydrogen in the tetragonal ZrAl 3 type intermetallic compound. In situ methods thus give a deeper insight in the TlPd 3 ‐O 2 ‐H 2 system with the identification of possible candidates for interesting intermediate phases and more detailed information on thermal stabilities.