The influence of precursor source and thermal parameters upon the formation of beta-phase molybdenum nitride

doi:10.1016/j.jallcom.2009.01.065

Journal of Alloys and Compounds

Volume 479, Issues 1–2, 24 June 2009, Pages 851-854

https://doi.org/10.1016/j.jallcom.2009.01.065 Get rights and content

Abstract

In the present paper the effect of the MoO₃ precursor and procedure on the synthesis of beta-phase molybdenum nitride is reported. Two different sources of MoO₃ were used, and treated under a flow of gas N₂/H₂ at 700 or 750 °C employing various heating ramp rates. All samples were characterized by XRD, SEM and their textural properties were determined by application of the BET method. The results show that, depending on the precursor, markedly different morphologies were observed, whereas both the molybdenum source and the thermal treatment, strongly influence the incorporation of nitrogen.

Introduction

Due to their physical and chemical properties, which are claimed to be similar to those of the platinum group metals, the transition metal nitrides have received increased attention in the last few decades. Their general aspects are described elsewhere [1], [2]. Metal nitrides possess interesting characteristics, for instance, as superconductors and hard materials [3], [4], as well as in terms of their catalytic properties [5], [6] Early reports detailed the synthesis of molybdenum nitride by nitridation of molybdenum powder in an NH₃ atmosphere over a temperature range of 700–1000 K [7]. More recently, Boudart and Volpe [8] reported that the reaction of MoO₃ with NH₃ can produce γ-Mo₂N with a surface area of ca. 200 m²/g and described the reaction as a topotactic transformation with preservation of the two-dimensional layers of the oxide precursor in the nitride product. Later Jaggers et al. [9] reported the synthesis of high surface area γ-Mo₂N and δ-MoN from (NH₄)₆Mo₇O₂₄·4H₂O, (NH₄)₂MoO₄ and H_xMoO₃ and observed that all precursors form the same oxynitride intermediates which were subsequently converted into nitrides. They also reported that the temperature at which the oxynitride reacts, determines the phase, forming the face centred cubic based γ-Mo₂N phase at higher temperatures than those needed for the hexagonal close packed based δ-MoN phase. Wise and Markel [10] reported the synthesis of γ-Mo₂N from MoO₃ under a mixture of N₂ and H₂ and found that high space velocities favour high surface area materials. Synthesis routes to γ-Mo₂N, β-Mo₂N_0.78 and δ-MoN have been detailed elsewhere, for example [11]. Nagai et al. [12] reported that during the synthesis of molybdenum nitride from MoO₃ and NH_3, transformation of γ-Mo₂N to β-Mo₂N_0.78 occurred when cooling under He rather than NH₃. Gong at al. [13], [14] reported the synthesis of β-Mo₂N_0.78 from 650 to 750 °C using MoO₃ as precursor, under various N₂/H₂ ratios following a three step temperature programme. They reported that the formation of β-Mo₂N_0.78 was observed at 650, 700 and 750 °C with N₂/H₂ ratios in the range of 1/1–1/4 [13]. We have previously observed that the production of β-Mo₂N_0.78 is sensitive to preparation variables and decided to study its synthesis further following various temperature treatment regimes. Furthermore, two sources of MoO₃ were used for this purpose.

Section snippets

Experimental

Microreactor testing has been performed as follows. Approximately 0.4 g of MoO₃ powder (Sigma–Aldrich, 99.5% or AnalaR, 99.5%) was placed between two plugs of quartz wool in a tubular quartz reactor. The reactor was first purged and then heated up from ambient temperature to either 700 or 750 °C, under a dry 3:1 N₂/H₂ gas mixture, (60 ml min⁻¹, BOC, H₂ 99.998%, N₂ 99.995%) applying different heating ramp rates (5, 10, 50, or 100 °C min⁻¹). Additional experiments were also performed removing the

Results and discussion

Two different commercial sources of MoO₃ were used in the present study, one from BDH Chemicals Ltd AnalaR (source A) and the other from Sigma–Aldrich (source S). Table 1 shows their quoted specifications [15], [16]. Both sources contain an assay of more than 99.5% MoO₃ and have similar maximum percentages of impurities, with an exception in the metals for which a maximum content of 0.002% and 0.005% is reported for sources A and S, respectively. The value for source A results from adding the

Conclusions

Our results demonstrate that the synthesis of beta-phase molybdenum nitrides from MoO₃ is a very sensitive process. Small differences in impurity content and/or crystallinity of the MoO₃ can strongly influence the morphology and the content of molybdenum in the final product. Molybdenum nitrides from source S show plate-like morphology with various particle sizes, whereas deformed rectangular blocks were obtained from source A. In addition, the synthesis of nitrides from source A is retarded,

Acknowledgements

J.S.J.H. and A.C. would like to acknowledge the Nuffield Undergraduate Research Bursary Scheme for providing support under project reference 34288. J.S.J.H. and D.M. would like to express their appreciation to the EPSRC for support in the area of nitride catalysis under project grant GR/S87300/01. JLR would like to acknowledge the generous support of Conacyt and Universidad Michoacana in allowing him to spend a sabbatical period at the University of Glasgow.

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The influence of precursor source and thermal parameters upon the formation of beta-phase molybdenum nitride

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Physica B

Catal. Today

J. Catal.

J. Solid State Chem.

J. Catal.

Appl. Catal. A: Gen.

J. Solid State Chem.

Appl. Surf. Sci.

Catal. Today

Transition Metal Carbides and Nitrides