Full Length ArticleElectron doped C2N monolayer as efficient noble metal-free catalysts for CO oxidation
Graphical abstract
Introduction
Over the past few decades, CO oxidation has become one of the most investigated research topics, with enormous efforts being devoted in designing efficient catalyst, in order to overcome the growing environmental problems caused by the emission of toxic CO from automobiles, industrial process etc [1], [2], [3]. Some noble metal based catalysts (Pt [1], [4], [5], [6], [7], Pd [7], [8], [9], [10], Rh [7], [11], [12], Au [2], [13], [14] etc.) have been proposed and studied extensively by many researchers for the oxidation of CO. However, these noble metal based catalysts require high reaction temperature for significant operations because of their high energy barriers [4], [15], [16]. Besides, the high cost and low abundance of these noble metal catalyst, imposes limitations for their usage in large scale CO oxidation. Thus for practical realization of CO oxidation at room temperature, scientific community is on the lookout for noble metal-free catalysts that are inexpensive and environment friendly [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].
Two dimensional (2D) graphene has drawn considerable attention in the field of heterogeneous catalysis because of its high surface-to-volume ratio [26], [27], [28], [29]. Pristine graphene is considered to be chemically inert and prevents the adsorption of many gas molecules on it [30], [31], [32]. Therefore, numerous efforts have been made to functionalize graphene to expand its application area. Doping foreign element in graphene is one of the most widely used routes to enhance its chemical reactivity. Heteroatom doping modifies the electronic structure and increases the charge transfer from the surface to the reactant, thereby enhancing the activity of the host, for the oxidation of CO to CO2 [27], [28], [29]. Analogous to the case of graphene, electron doped 2D hexagonal boron nitride sheet (h-BN) (inorganic analogue of graphene) has been reported to be efficient catalyst for CO oxidation, notwithstanding from the chemical inertness of pristine h-BN [22], [33]. In order to design a new catalyst for CO oxidation, it is desirable to have precise understanding of adsorption energies of the reactants, reaction energies and reaction intermediates [34]. Now coming to the mechanisms of CO oxidation, it is either Langmuir-Hinshelwood (LH) mechanism or the Eley-Ridel (ER) mechanism, depending on the adsorptions of O2 and CO. The LH mechanism involves the adsorption of both O2 and CO on the surface of the catalyst followed by a surface reaction, where as in ER mechanism, CO molecule in gas phase reacts directly with a pre-adsorbed O2 molecule on the catalyst leading to the formation of CO2 [35]. In order to gain an insight into the details of different reaction steps and reaction mechanisms, state-of-the-art density functional theory (DFT) based approach has been used with great success and good predictive power, for designing new catalysts [36].
Recently, Baek et al. reported a noble layered 2D structure having uniformly distributed holes and nitrogen atoms and a C2N stoichiometry [37]. Density functional based phonon calculation has confirmed its high structural stability [38]. Moreover, the high surface-to-volume ratio of this 2D C2N monolayer and the evenly distributed holes in C2N, make it a potential candidate to design noble metal-free catalyst [39]. Recent theoretical studies have shown the promising potential of transition metal embedded C2N monolayer as a catalyst for CO oxidation and oxygen reduction reaction (ORR) [39], [40].
In this work, we have explored the catalytic activity of electron doped C2N monolayer. We have shown that pristine C2N exhibits chemical inertness towards O2 and CO adsorption. So we tailored the electronic structure of C2N by incorporating substitutional oxygen doping (O → N) and noticed significant enhancement of its chemical reactivity. Chemical activation of O2 molecule was observed when adsorbed on the oxygen doped C2N, and CO oxidation was found to occur via ER mechanism with negligibly small energy barrier.
Section snippets
Computational details
For investigating the catalytic activity, the model structure was constructed using 2 × 2 × 1 supercell of C2N monolayer. The spurious interaction between periodic images was avoided by incorporating a vacuum of length 18 Å in the direction perpendicular to C2N surface. Electron doping in C2N was realized by replacing two N atoms from 2 × 2 × 1 C2N supercell, with two O atoms.
The geometry relaxation, total energy calculations, and the electronic structure calculations were carried out using spin
Pristine C2N: geometry and electronic structure
First, we briefly discuss the structural and electronic properties pristine C2N monolayer. Despite some previous theoretical and experimental reports on pristine C2N, we repeated the calculation in order to ensure validity of our computational approach. The primitive unit cell of C2N monolayer (shown in Fig. 1(a)) contains six nitrogen atoms and twelve carbon atoms and 2 × 2 supercell forms a hole which is surrounded by six nitrogen atoms as shown in Fig. 1(a). The optimized lattice parameter of C
Conclusion
Electron doped C2N surface has been found to be efficient catalyst for CO oxidation. Pristine C2N surface shows chemical inertness towards O2 and CO molecules. Two oxygen atoms (two electrons) doping in C2N surface (2OC2N) is found to enhance the chemical activity of the surface towards CO oxidation. We have considered two configurations of 2OC2N, viz. 2OC2N_1 and 2OC2N_2, with two different O doping positions. Both 2OC2N_1 and 2OC2N_2 surfaces adsorb an incoming O2 molecule, resulting in
Acknowledgements
SC is financially supported by a CSIR fellowship 09/080(0787)/2011-EMR-I. TD is financially supported by DST INSPIRE fellowship. RT thanks SERB for the financial support (Grant no: SB/FTP/PS028/2013). GPD acknowledges the support from IBIQUS project from the Dept. of Atomic Energy, Govt. of India (DAE).
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