Effects of transition metal addition on the solid-state transformation of molybdenum trioxide to molybdenum carbides

Abstract

Direct conversion Of MoO3 to molybdenum carbides by temperature-programmed reaction (TPR) with a reacting gas mixture of CH4/4H(2) has been studied in the presence of a transition metal selected from Pt, Pd, Ni, Co, Cu, and pre-synthesized Mo2C loaded on MoO3. Loading of the metals reduced the temperatures of MoO3 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 alpha-MoC1-x, while Co, Cu, or pre-synthesized Mo2C formed hexagonal beta-Mo2C, which was the same phase as that produced from the direct transformation Of MoO3 itself without any metal loading. The nature of transition metals loaded on MoO3 also led to different TPR patterns for the transformation to molybdenum carbide, suggesting that different reaction routes were involved. Thus, MoO3 loaded with Co, Cu, or Mo2C showed TPR patterns similar to that of MoO3 itself, except for slightly lower temperatures for the initiation of the reduction. With these metals, the transformation Of MoO3 to carbides followed a nontopotactic route involving the MoO2 intermediate phase. By contrast, loading of Pt, Pd, or Ni to MoO3 drove MoO3 reduction into a topotactic transformation through a MoOxCy intermediate phase. However, unlike the well-established topotactic MoO3 --> Mo2N --> alpha-MoC1-x transformation, the pseudomorphism was only partly maintained in this metal-promoted, direct MoO3 --> alpha-MoC1-x transformation.