In situ X-ray Powder Diffraction Study of the Reactions of CrO2 and TlPd3O4 with Hydrogen

Hydrogen is an important reducing agent for transition metal oxides, however, details on reaction pathways are often unknown. Therefore, the reduction of CrO₂ and TlPd₃O₄ was investigated by in situ X-ray powder diffraction and subsequent Rietveld analysis. CrO₂ is reduced by hydrogen gas (0.3 MPa) starting at 180 °C according to 2 CrO₂ + H₂ → 2 CrOOH. The reaction is complete at 250 °C and there are no signs for intermediates or non-stoichiometry. In nitrogen atmosphere the reaction TlPd₃O₄ → TlPd₃ + 2 O₂ occurs without intermediates in one step starting at about 670 °C. Thermal volume expansion is determined to be V(TlPd₃O₄) = 880.7(1) 10⁶ pm³ + 1.64(7) 10⁴ T pm³/K + 10.7(9) T² pm³/K² and V(TlPd₃) = 258.6(1) 10⁶ pm³ + 8.5(7) 10³ T pm³/K + 2.6(7) T² pm³/K² for 25 °C ≤ T ≤ 730 °C. The formation of β-TlPd₃H from TlPd₃O₄ in 0.3 MPa hydrogen gas at 75 °C occurs very fast. Unit cell parameters indicate the occurrence of a metastable α-TlPd₃H_≈0.2 with a hydrogen-filled ZrAl₃ type. Cubic anti-perovskite type β-TlPd₃H reacts in air to TlPd₃ with a possible hydrogen deficient intermediate β-TlPd₃H_1–x and hints for remaining hydrogen in the tetragonal ZrAl₃ type intermetallic compound. In situ methods thus give a deeper insight in the TlPd₃-O₂-H₂ system with the identification of possible candidates for interesting intermediate phases and more detailed information on thermal stabilities.