A review of preparation strategies for α-MoC_1-x catalysts: Transition metal carbides attract growing attention

Transition metal carbides are attracting growing attention as robust and affordable alternative heterogeneous catalysts to platinum group metals, for a host of contemporary and established hydrogenation, dehydrogenation, and isomerisation reactions. In particular, the metastable α-MoC1-x phase has been shown to exhibit interesting catalytic properties for low temperature processes reliant on O-H and C-H bond activation. While demonstrating exciting catalytic properties, a significant challenge exists in the application of metastable carbides, namely the challenging procedure for their preparation. In this review we will briefly discuss the properties and catalytic applications of α-MoC1-x, followed by a more detailed discussion on available synthesis methods and important parameters that influence carbide properties. Techniques are contrasted with properties of phase, surface area, morphology and Mo:C being considered. Further, we briefly relate these observations to experimental and theoretical studies of α-MoC1-x in catalytic applications. Synthetic strategies discussed are, the original temperature programmed ammonolysis followed by carburisation, alternative oxycarbide or hydrogen bronze precursor phases, heat treatment of moybdate-amide compounds and other low temperature synthetic routes. The importance of carbon removal and catalyst passivation in relation to surface and bulk properties are also discussed. Novel techniques that by-pass the apparent bottle neck of ammonolysis are reported, however a clear understanding of intermediate phases is required to be able to fully apply these techniques. Pragmatically, the scaled application of these techniques requires the pre-pyrolysis wet chemistry to be simple and scalable. Further, there is a clear opportunity to correlate observed morphologies/phases and catalytic properties with findings from computational theoretical studies. Detailed characterisation throughout the synthetic process is essential and will undoubtedly provide fundamental insights that can be used for the controllable and scalable synthesis of metastable α-MoC1-x.