Effects of Transition Metal Addition on the Solid-State Transformation of Molybdenum Trioxide to Molybdenum Carbides

Direct conversion of MoO 3 to molybdenum carbides by temperature-programmed reaction (TPR) with a reacting gas mixture of CH 4 /4H 2 has been studied in the presence of a transition metal selected from Pt, Pd, Ni, Co, Cu, and pre-synthesized Mo 2 C loaded on MoO 3 . Loading of the metals reduced the temperatures of MoO 3 reduction and increased the specific surface area of produced carbides. However, the obtained phase of molybdenum carbides differed depending on the employed transition metal; Pt, Pd, or Ni produced cubic α-MoC 1 - x, while Co, Cu, or pre-synthesized Mo 2 C formed hexagonal β-Mo 2 C, which was the same phase as that produced from the direct transformation of MoO 3 itself without any metal loading. The nature of transition metals loaded on MoO 3 also led to different TPR patterns for the transformation to molybdenum carbide, suggesting that different reaction routes were involved. Thus, MoO 3 loaded with Co, Cu, or Mo 2 C showed TPR patterns similar to that of MoO 3 itself, except for slightly lower temperatures for the initiation of the reduction. With these metals, the transformation of MoO 3 to carbides followed a nontopotactic route involving the MoO 2 intermediate phase. By contrast, loading of Pt, Pd, or Ni to MoO 3 drove MoO 3 reduction into a topotactic transformation through a MoO x C y intermediate phase. However, unlike the well-established topotactic MoO 3 → Mo 2 N → α−MoC 1 - x transformation, the pseudomorphism was only partly maintained in this metal-promoted, direct MoO 3 → α-MoC 1 - x transformation.