Elsevier

Surface Science

Volume 704, February 2021, 121756
Surface Science

Theoretical study of methanol activation catalyzed by B11N11 and B14N14 nano-cages

https://doi.org/10.1016/j.susc.2020.121756Get rights and content

Highlights

Abstract

Many computational studies have been performed to examine the mechanism of methanol oxidation at various surfaces. In this work, the methanol decomposition mechanism on B11N11 and B14N14 nano-cages in gas and solvent phases by the DFT-D method has been performed. A complete evaluation of methanol decomposition was performed from five possible pathways, and kinetic and thermodynamic properties were investigated. Our calculations show that the initial scission of the O‒H bond in methanol decomposition, is the lowest energy barrier and the highest rate constant, and reaction proceeds via CH3OH → CH3O → CH2O → CHO→ CO. In general, Methanol oxidation is investigated on the B11N11 surface is more desirable. Remarkable differences in kinetic and thermodynamic result on methanol decomposition in B11N11 and B14N14 nano-cages indicate that the dehydrogenation of methanol might be structure-sensitive.

Introduction

In recent years, direct methanol fuel cells are very favorite due to environmental effects and relatively high energy efficiency. Natural exodus gases are the most important reasons for scientist's attention to direct methanol fuel cells. Methanol decomposition reaction, as a hydrogenation process in fuel cells, the industry-wide need for platinum-based electrocatalysts, prevents commercial-scale use of fuel cells due to expensive and limited resources. Therefore, non-platinum catalysts with high oxidation activity are interesting [1], [2], [3], [4], [5]. High hydrogenation, sufficient resources, and considerable safety have become methanol as a significant energy source in direct methanol fuel cell [6], [7], [8]. Carbon materials doped with Nitrogen, Boron, Phosphor, Sulfide, and Iodine have been represented to be impressive catalysts [9], [10], [11], [12], [13], [14] and define great CO tolerance [14]. Studies show that doping causes electron structure deformation and increase catalytic rate of reaction [15]. Replacing carbon atoms with Nitrogen and Boron atoms, are created boron nitride compounds. Boron nitride compounds, because of larger bandgap than carbon compounds, have been studied less; large bandgap does not cause such compounds to be rejected or disesteem. By using atomically thin BN materials, the bandgap can be significantly reduced [16,17]. Previous studies have used boron nitride materials based on conductive electrodes as electrocatalysts [18]. Therefore, boron nitride nano-cages can act as catalysts and can be used to separate charge between nitrogen and boron atoms. A theory review by Fowler et al. [19] shows that boron nitride (BN)x nano-cages (4>30) can have a large number of stable structures. The structures of B11N11 and B14N14 are geometrically and electronically, a suitable and durable candidate. Density Functional Theory (DFT) studies show that boron nitride nano-cages can serve as an electrocatalyst [20,21]. The present study is performed to determine the appropriate electrocatalyst in B11N11 and B14N14 nano-cages. The B11N11 structure has only one isomer with symmetry of CS. The B14N14 structure has three isomers, the most stable of which is the symmetry of CS [19].

The direct methanol fuel cell is made up of two parts anode, and cathode. In the anode, liquid methanol is oxidized with water and produces hydrogen ions and carbon dioxide. The water in the anode prevents the system from drying out and affects the mass transfer mechanism [22].

In this work, we also investigated the solvent effect (water) on the methanol decomposition. Theory studies of the methanol decomposition in the gas and liquid phases show that the methanol dehydrogenation is highly dependent on the reaction environment [23,24]. The solvent effect may change the reaction mechanism without affecting the reaction rate, or it may change the reaction rate without affecting the reaction mechanism, or it may change the reaction rate and the reaction mechanism simultaneously [25]. Water molecules play an essential role in the methanol oxidation by creating hydrogen bonds and short- range and long-range interactions as well as modifying the number of active sites on the surface [26].

Section snippets

Computational details

In the present study, DFT calculations have been performed using Materials Studio software and Dmol3 module for structural optimization, transition state, rate constant, and computational details of methanol, water, and boron nitride nano-cages. Grimme was the first to introduce the DFT-D approach [27]. In the Grimm method, the dispersion value is added to the calculated energy from the DFT method (EDFT-D):EDFTD=EDFT+EDisp,EDFT is the energy generated by DFT and EDisp calculated energy of the

Result and discussion

Since little studies have been performed to show the adsorption properties of methanol decomposition mediators on the B11N11 and B14N14 surfaces, in this section, the geometry structures and energy characteristics of all intermediates involved in the dehydrogenation process were discussed in detail.

Conclusions

DFT-D calculations have been performed to fully investigate the methanol decomposition on B11N11 and B14N14 nano-cages. The reaction of methanol decomposition was investigated from three ways Osingle bondH, Csingle bondH, and Csingle bondO bond scission and structural features and kinetic and thermodynamic properties were studied. After comparing the results, it was concluded that Osingle bondH bond scission has the lowest activation energy, and reaction proceeds via CH3OH → CH3O → CH2O → CHO→ CO. In general, B11N11 nano-cages is a

Author statements

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Surface Science journal.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The financial support from University of Mazandaran is gratefully acknowledged.

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