Molybdenum carbide nanoparticle: Understanding the surface properties and reaction mechanism for energy production towards a sustainable future

https://doi.org/10.1016/j.rser.2018.03.106Get rights and content

Highlights

  • This review gives an insight into the surface properties and reaction mechanism of MCN.

  • A brief overview of preparation of MCNs and their properties have been given.

  • Effect of carburization condition on the nature of MNC are discussed.

  • Multiscale reaction model on MCNs gives an insight into the reaction mechanism.

Abstract

Rational design and synthesis of cheap, noble metal-free, thermal/hydrothermal stable and active catalyst for efficient hydrogenation and hydrogen production reaction is crucial towards renewable and sustainable energy generation. This gives the use of molybdenum carbide nanoparticle considerable attention as an alternative to noble metals. However, the industrial application is not yet feasible due to insufficient stability and activity coupled with the lack of detailed understanding of the reaction mechanism. This work discusses the effect of the operating parameters on the properties and morphology of molybdenum carbide nanoparticle, as well as their impact on the catalytic activity. Critical issues such as structural diversity, surface properties, and multiscale reaction modeling are also discussed for better understanding of the reaction mechanism. This is a promising strategy towards synthesis of cost-effective and efficient catalysts for renewable and sustainable energy production.

Introduction

One of the most important research gaps in the field of renewable and sustainable energy is rational design and synthesis of suitable eco-friendly and cost-effective catalysts with preserved energy and chemical functionality for prolonged applications in several industrial processes [1], [2], [3]. The unique chemical and physical properties of molybdenum carbide nanoparticle [4] have enhanced its popularity in the fields of materials and chemical science towards production of renewable and sustainable energy [5]. The outstanding properties of MCN include thermal stability, high electrical conductivity, adsorption capacity, high melting point, and hardness [6]. Moreover, the characteristics of MCNs such as resistance to nitrogen and sulfur, high catalytic current density, and durability are similar to those of noble metals, which enable their utilization in hydrogenation and hydrogen evolution reactions (HER) [7], [8]. Examples of these reactions include CO2 hydrogenation to alcohol, CO hydrogenation to alcohol [9], hydrodeoxygenation [10], electrocatalytic hydrogen evolution from water splitting [11] including oxygen evolution reaction [12], hydro-treating [13], watergas shift reaction (WGS) [14], hydrodesulfurization (HDS) [15], CH4 aromatization [16], and hydrodenitrogenation (HDN) [17]. The MCNs are also suitable for electrocatalytic reactions.

MCN has been used successfully to hydrogenate feedstock such as cellulose, indole, toluene, and cumene, which are popularly processed with group 9 and 10 noble metals (Pt, Pd, Rh) [18], [19], [20]. These being commercially available catalysts for reactions such as methane reforming, hydrocarbon isomerization, water-gas shift reactions, and CO hydrogenation. Further, MCN has been employed as an alternative to Ru, and to an extent Pt as electrocatalysts in the anode of polymer membrane fuel cells (PEMFC) [21], [22], [23] because of its platinum-like behaviors [24]. The thermal stability of MCN in the absence of oxygen is due to the delay of the sintering and attrition effects as reaction proceeds. However, the catalytic activity of MCN systems mainly depends on the nature and physiochemical properties of the catalyst.

Previously, the high temperature classical metallurgical process was used to prepare metal carbides but the products exhibit low specific surface areas and high particle size [25]. This results in the Metal carbide products exhibiting low catalytic performance in targeted catalytic processes [3]. Currently, the MCN synthesis method by Lee et al. [26], which is a temperature program reduction (TPR) carburization is most popular due to its remarkable improvement on the textural property of the product [27]. TPR carburization is a carbothermal reduction method that carburizes the Mo precursor supported on carbon in hydrogen atmosphere [28]. The Mo precursor is to be thermally treated, at increasing the controlled temperature in a reducing environment [29]. To form the carbide phase, the carbon source is mainly light hydrocarbon, while hydrogen is the reducing agent. The essence of the controlled temperature is to optimize the carburation temperature to avoid sintering of the reduced Mo particles, thereby reducing the particle size of the resulting carbide. The carburization conditions (temperature and time) control the physiochemical properties, the chemical nature, and structure of the resulting carbide phase. MCN exists in two main crystalline structures: orthorhombic and hexagonal (Mo2C) and hexagonal structure. The preparation methods of carbide-supported metal catalysts include wet impregnation [30], atomic layer deposition [31] and vacuum environment [32]. Mostly, the synthesized MCN, passivated prior to its exposure to air to prevent oxidation.

The goal of this review is to provide insight into the surface properties of MCN and its reaction mechanism for renewable and sustainable energy production towards a sustainable future. In Section 2, rational design and synthesis of MCN are discussed, highlighting the effect of operating parameters. The third section deals with a structural diversity of MCN using density functional theory (DFT) to categorize different form of MCN based on structural differences. Section 4 briefly discusses the surface properties of MCNs to determine the stability based on their structural diversity. While Section 5 gives an insight into multiscale reaction model on MCNs for a better understanding of catalytic reaction mechanism of the system, which is crucial to the commercial applications of MCN in the production of renewable and sustainable energy. Finally, we presented the catalytic activity of MCN based catalysts (both unsupported and promoted/supported) in Section 6.

Section snippets

Preparation of MCN

MCNs are popularly prepared by carbothermal reduction carburization process. This process consists of three different steps; (i) deposition of the Mo-precursor on the carbon source, (ii) carbothermal reduction of the Mo-precursor to produce MCN, and [33] the subsequent stabilization of the produced MCN by Mo-carbide surface passivation [19]. Generally, synthesis of MoO2 nanoparticles is not a difficult task; the transformation into MCN is where the major challenge is. The transformation is so

Structural diversity of molybdenum carbide

Understanding the structural diversity of MCN is an uphill task due to its complex nature and a number of metastable and stable phases [52]. However, density functional theory (DFT) based estimations with the revised Perdew–Burke–Ernzerhof (RPBE) exchange-correlation functional has been successfully employed to identify the crystal structure [53]. MCN is characterized by five different crystal structures: α-MoC1−x, α-Mo2C, β-Mo2C, γ-MoC and η-MoC [54], which arise from different preparation

Surface properties of MCNs

MCNs possess a similar surface to that of transition metal surface M(111), which is a little more reactive toward CH2/3 species when compared with metals with a similar strength of carbon adsorption. This makes MCN a suitable for hydrogenation reaction, which is less vulnerable to graphite poisoning because of an increased attraction to hydrocarbons [74]. Moreover, Mo2C (001) has a similar the O/OHx binding with that of the transition metal M(211) surfaces indicating the likelihood of oxygen

Multiscale reaction model on MCNs

For a better understanding of catalytic reaction mechanism with MCNs, there is a need to establish a multiscale reaction model, which combines quantum mechanical (QM) density functional tight-binding (DFTB) technique with a molecular mechanical (MM) force field [82], [83]. This could be accomplished by building a QM/MM model to define the MCN, the model aromatic solvent, and the surroundings. The free energy profiles of the reactions could be determined by using Umbrella sampling (US) [82]. Liu

Unsupported MCN

Recently, Posada-Pérez et al. [116] reported the use of MCN heterogeneously catalyzed hydrogenation reactions, where H2 is adsorbed and dissociated. The study was carried out using systemic DFT-PBE with or without dispersion terms, regarding the interaction and stability of H2 with orthorhombic β-Mo2C(001) and cubic δ-MoC(001) surfaces. For β-Mo2C(001), two likely Mo or C terminations are considered. Their report shows that the energy profiles for the elementary steps H2 dissociation are mainly

Conclusion

Definitely, hydrogenation and hydrogen production reactions are the modern research hotspot towards renewable and sustainable energy production and have, therefore, inspired extensive interests in rational design and synthesis of cheap, noble metal-free, thermal/hydrothermal stable and active catalysts. This will offer a remarkable relief to the renewable and sustainable energy community. One of such materials is MCN, which is a promising replacement for noble metals. Factors like carbon

Acknowledgments

The authors gratefully acknowledge the financial support from GSP-MOHE (MO008–2015) High Impact Research (HIR) grant and RP015–2012D grant, University of Malaya, Malaysia.

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