Structural stability and mutual transformations of molybdenum carbide, nitride and phosphide

doi:10.1016/j.materresbull.2011.07.023

Materials Research Bulletin

Volume 46, Issue 11, November 2011, Pages 1938-1941

https://doi.org/10.1016/j.materresbull.2011.07.023 Get rights and content

Abstract

The structural stability and transformations of Mo carbide, nitride and phosphide were investigated under various atmosphere conditions by X-ray diffraction (XRD). The results indicated that the order of structural stability of these Mo-based compounds was as follows: Mo₂N < Mo₂C < MoP. Both Mo₂C and Mo₂N can be transformed to MoP, whereas the reverse transformations did not occur. Noticeably, compared with those Mo sources containing oxygen, the use of Mo₂C/Mo₂N as Mo-source can produce finely dispersed MoP nanoparticles by the temperature-programmed reaction (TPR) method. The result was probably due to the fact that lower-levels H₂O generated during synthesis process can avoid strong hydrothermal sintering. The influence of formation energy had been considered and was found to relate to the structural stability and transformations of these Mo-based compounds.

Graphical abstract

Both Mo₂C and Mo₂N can be transformed to MoP, whereas the reverse changes are inviable, which is used to develop a promising and practical pathway for preparing MoP nanoparticles.

Highlights

► Mo carbide, nitride and phosphide are prepared. ► The structural stability increases in the order of Mo₂N < Mo₂C < MoP. ► Both Mo₂C and Mo₂N can be transformed to MoP, whereas the reverse changes are inviable. ► This study develops a promising and practical pathway for preparing MoP nanoparticles.

Introduction

Transition metal interstitial compounds, including carbides, nitrides and phosphides, are produced by introducing C, N and P atoms into the metal lattices. These materials have unique physical and chemical properties which give them a wide variety of application, such as mechanical, electronic and magnetic materials, superconductors and catalysts [1], [2], [3], [4], [5], [6]. Noticeably, the application of these materials as catalysts is always of interest because of their Pt-like catalytic activities in many reactions [1], such as hydrodesulfurization (HDS) [7], NH₃ synthesis [8], methane reforming [9], cellulose conversion [10], and NO removal [11]. However, it was found that the structural stability of these materials was low in the mentioned above reactions. It was reported that Mo₂C and Mo₂N catalysts were significantly more active than MoS₂ for the HDS of thiophene, while the surface and even the bulk of Mo₂C/Mo₂N catalyst could be sulfided to MoS₂ [7]. As for carbide-catalyzed methane reforming reactions, Darujati and Thomson found that Mo₂C possessed catalytic activity comparable to noble metals, but deactivation occurred due to oxidation of Mo₂C into inactive MoO₂ [9]. Analogous conclusions have been drawn by our research group in Mo₂C-catalyzed NO decomposition, and our recent studies have focused on the oxidative stability of carbide, nitride and phosphide catalysts under NO decomposition/reduction conditions [11], [12], [13], [14], [15]. In addition, Hargreaves et al. [8] reported the Mo₂N and Co₃Mo₃N systems for NH₃ synthesis, in which they showed that the lattice nitrogen can be removed to generate low-nitrogen containing phases in reactive gas atmospheres. Therefore, investigation of structural stability of carbide, nitride and phosphide under different atmospheres is very important for their application.

In this study, we focus on the investigation of structural stability and mutual transformations of Mo carbide, nitride and phosphide. The selection of Mo-based compounds is due to their considerable application in catalysis. To our knowledge, this is the first report on the mutual transformations of these Mo-based compounds.

Section snippets

Experimental

Mo₂C and Mo₂N samples were prepared from MoO₃ by temperature-programmed reaction (TPR) in CH₄ + H₂ and NH₃, respectively [11], [14]. Typically, about 2.0 g of MoO₃ precursor was placed in a micro-reactor and a flow of 30% CH₄ + 70% H₂ mixture (150 ml/min) or pure NH₃ (150 ml/min) was introduced into the system. The temperature was increased from room temperature (RT) to 300 °C over a period of 30 min followed by a rise in temperature from 300 to 450 °C at a rate of 0.67 °C/min, and a further increase from

Structural stability

XRD patterns of Mo carbide, nitride and phosphide obtained from MoO₃ are shown in Fig. 1(A, C and E). The peak positions were consistent with the crystalline phases of β-Mo₂C (2θ = 34.5, 38.1, 39.5, 52.3 and 61.8°, JCPDS72-1683), γ-Mo₂N (2θ = 37.4, 43.5, 63.1, 75.7 and 79.7°, JCPDS25-1366) and MoP (2θ = 27.9, 32.2, 43.1, 74.3 and 85.7°, JCPDS24-0771). There were no peaks that can be assigned to Mo oxide(s) or other Mo carbide, nitride and phosphide phases, indicating that these Mo-based compounds

Conclusions

We have investigated the structural stability and mutual transformations of Mo carbide, nitride and phosphide under various atmospheres using XRD. The results indicate that the structural stability of these Mo-based compounds increases in the following order: Mo₂N < Mo₂C < MoP. Both Mo₂C and Mo₂N can be transformed to MoP, whereas the reverse transformations did not occur. The structural stability and transformations were found to correlate with the formation energy of these Mo-based compounds.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (No. 21006032), China Postdoctoral Science Foundation (No. 20100470916) and the Fundamental Research Funds for the Central Universities of China (2009ZZ0032).

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Structural stability and mutual transformations of molybdenum carbide, nitride and phosphide

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experimental

Structural stability

Conclusions

Acknowledgements

J. Alloys Compd.

Mater. Res. Bull.

Mater. Res. Bull.

Mater. Res. Bull.

J. Catal.

J. Solid State Chem.

Appl. Catal. A

J. Alloys Compd.

J. Solid State Chem.

J. Alloys Compd.

Fuel

Catal. Commun.

J. Catal.