Holger Kohlmann

In the industrial production of molybdenum the reduction of molybdenum trioxide with hydrogen is a two‐step process. In order to get a deeper understanding of the chemistry involved, in situ studies were performed on the thermal behavior of molybdenum trioxide and the first reduction step to molybdenum dioxide. MoO 3 shows a very strong anisotropy for thermal expansion (linear expansion coefficients α in 10 −4 K −1 : 1.32(4) for a , 5.5(1) for b , −0.246(5) for c ). Upon heating in air, preferred orientation in XRPD patterns decreases and the color changes from grey to yellow, probably caused by decreasing oxygen vacancy concentration. The reduction of MoO 3 by hydrogen was investigated via in situ X‐ray powder diffraction for varying heating rate, hydrogen flow and potassium content. The formation of the intermediate γ ‐Mo 4 O 11 shows a higher rate for increasing potassium content; it does not play a role whether potassium is present in the starting material MoO 3 or in MoO 2 formed in the reaction. The high‐temperature modification of K 2 MoO 4 was found to crystallize in the α‐K 2 SO 4 type ( P 6 3 / mmc , a =6.3463(2) Å, c = 8.1331(2) Å). Heating up MoO 3 in vacuum (1 Pa) with the same T , t protocol as above yields MoO 2 even without the presence of hydrogen.