Direct oxidation of methane to methanol on Co embedded N-doped graphene: Comparing the role of N₂O and O₂ as oxidants

Highlights

• The CoN₃–G shows better catalytic activity toward adsorption of gas reactants and consequently methane oxidation.

• Methane converts to methanol on CoN₃-G via a two-step pathway, employing N₂O as an oxygen- donor.

• The CH bond cleavage proceeds via the Eley-Rideal (ER) mechanism while other reactions proceed via the Langmuir-Hinshelwood (LH) mechanism.

• Methane dissociates actively on CoN₃-G results in the formation of ethane molecule in a CC coupling reaction during the oxidation process.

Abstract

In this work, the effects of N-doping into the Co-doped single vacancy (Co-SV-G) and di-vacancy graphene flake (Co-dV-G) are investigated and compared toward direct oxidation of methane to methanol (DOMM) employing two different oxidants (N₂O and O₂) using density functional theory (DFT) calculation. We found that DOMM on CoN₃-G utilizing the N₂O molecule as oxygen-donor proceeds via a two-step reaction with low activation energies. In addition, we found that although CoN₃-G might be a good catalyst for methane conversion, it can also catalyze the oxidation of methanol to CO₂ and H₂O due to the required low activation barriers. Moreover, the adsorption behaviors of CH_x (x = 0–4) species and dehydrogenation of CH_x (x = 1–4) species on CoN₃-G are investigated. We concluded that CoN₃-G can be used as an efficient catalyst for DOMM and N₂O reduction at ambient conditions which may serve as a guide for fabricating effective C/N catalysts in energy-related devices.

Introduction

Traditional conversion of methane (CH₄) to methanol (CH₃OH) is performed by a two-step process in the industry under the harsh reaction conditions such as high temperature and pressure [1,2]. Despite the fact that it is not a cost-effective procedure, there exists no industrial process capable of directly converting methane to methanol. In addition, N₂O is known as an agricultural toxic gas which may also originate from other activities like wastewater treatments, or the combustion of fossil fuels and biomass. During the last century, it is found that the global warming of CH₄ and N₂O are 28 and 265 times higher than CO₂. Therefore, investigating the conversion of these gas molecules is of great interest. Direct synthesis of CH₃OH from CH₄ at room temperature and pressure is a promising way for the industry. To date, many studies have been explored on the direct synthesis of methanol but none of them has proven cost-effective. Thus, it remains one of the considerable challenges in the sector of methane utilization due to the strong CH bond energy and its chemical inertness of methane.

It is known that although noble metals like Ir, Pt, Rh and Ru [[3], [4], [5], [6], [7]] are highly active toward DRM and resistant to carbon formation, they are economically less attractive materials for large-scale industrial use due to their scarcity, high cost and toxicity [8]. Amongst different types of catalysts, single individual metal atoms anchored to graphene-based materials [[9], [10], [11]] are discovered as novel material not only because they minimize material usage to meet the goal of cost-effective catalysis, but also because they surpass conventional catalysts in terms of having a high specific activity with a significantly reduced amount of noble metals. Recently, single metal atoms doped into monolayer surfaces have been tested as catalysts for various reactions owing to their well-defined sites, unsaturated coordination environment, and high atom efficiency. Doping single-atoms into the graphene structure may be effective to improve its catalytic properties [12,13]. Amongst an increasing number of single-atoms, most have been focusing on supporting noble metal atoms like Pt or Pd on metal oxide or metal surfaces [14,15]. Dopants such as P [16], Si [17,18], Fe [19,20], Pt [21,22], Pd [23,24], Ni [25,26] substitute with carbon atoms in the graphene sheet and can significantly enhance the properties of graphene sheet. On the other hands, tailoring the graphene sheet by introducing defects [27,28] or heteroatoms (e.g., N, B, P) [29] into the structure of graphene sheet will speed up the catalytic reactions occurred on the surface [30,31] and modulate the electronic properties of the catalyst [[32], [33], [34], [35]].

It has been reported that N-doped graphene (N-G) [36] and transition metal-coordinated nitrogen-doped graphene (TMN-G) can enhance the chemical reactivity of graphene due to their low cost, high durability, and high catalytic activity. They are widely used in molecular sensors [37,38], bio-sensing applications [39], metal-free oxygen reduction catalysis (ORR) [40,41], CO oxidation reactions [42], and in lithium batteries [[43], [44], [45]]. The introduced N atoms modify the energy distribution of electronic states and thereby the local reactivity [46]. Three common N-doped configurations are observed in N-G, i. e., pyridinic, pyrrolic, and graphitic nitrogen [47,48]. However, there are no investigations toward the differences of catalytic ability between different N defects. Recently, it was found that pyridinic-N can better anchor transition metal atoms in comparison to other types of N-G, due to their higher stability and catalytic activity toward various reactions [49,50]. For instance, Yeager et al. [51] found that metal cations coordinated by pyridinic nitrogen-doped at the defective sites of graphitic carbon are the active sites for ORR in alkaline and acid electrolytes. Experimentally, graphene or carbonic structures containing nitrogen-coordinated transition metals can be easily synthesized [33,52,53]. Recently, Fei et al. [54] worked on the synthesis of nitrogen-enriched core-shell structured cobalt-carbon nanoparticles dispersed on graphene sheets, which can be used for hydrogen evolution reaction in both acidic and basic media. Wang et al. [55] found that Co embedded N-doped carbon nanotubes have high activity toward ORR and the oxygen evolution reaction (OER) in both alkaline and neutral media due to their low cost and appropriate features to act as bifunctional catalysts for both the ORR and OER.

In the last few decades, researchers have been dedicated to the synthesis of cost-effective electrocatalysts using cheaper transition metals [56] in which, cobalt-based electrocatalysts show the most promising results [57]. As of any first-row transition metal (Ni, Fe, and Cu), cobalt is a cheap, environmentally friendly, and accessible metal in comparison with noble metals [58] and it can be introduced into the graphene lattice with no difficulties [10]. Recently, Fei et al. [54] worked on the synthesis of nitrogen-enriched core-shell structured cobalt-carbon nanoparticles dispersed on graphene sheets, which can be used for hydrogen evolution reaction in both acidic and basic media. In an experimental investigation, Jurković et al. [59] reported the plasma‐activated methane partial oxidation reaction in a designed dielectric barrier discharge ionization reactor unit. They could produce valuable platform chemicals like methanol, formaldehyde, intermediate formic acid, acetic acid, and paraformaldehyde at room temperature and atmospheric pressure. Moreover, it is found that Co nanoparticles doped into carbon nanotube structures can decrease the local work function of the carbon surface because electrons transfer from cobalt to the surrounding carbon atoms, very easily [60]. Kattel et al. [61] investigated the ORR reaction pathway on M–N (M = Fe, Co, or Ni) catalytic clusters formed between pores in graphene supports. They found that O₂ molecule chemisorbs on CoN₄-graphene (CoN₄-G) and FeN₄-graphene (FeN₄-G) clusters but not on NiN₄-graphene (NiN₄-G) clusters. Therefore, the first two clusters were regarded as more active substrates toward ORR. The high stability is mainly attributed to the high conductivity of N-doped graphene and the embedded Co nanoparticles. N-doping increases the electron donor-acceptor properties of graphene and leads to the improvement of the conductance and interfacial electron transfer by doped Co nanoparticles [62]. Zhang et al. [49] calculated the activation barriers and thermodynamic properties of ORR on CoN₄-G. They demonstrated that CoN₄-G can enhance the ORR and resulted in the formation of two H₂O molecules. Kiefer et al. [63] reported that graphitic CoN₄ defects are stable at all potentials (U = 0–1.23 V) while CoN₂ defects are predicted to be unstable at high potentials. In addition, they predicted that the CoN₄ defect is the dominant in-plane graphitic defect in CoN_x/C electrocatalysts.

Graphene nano-flakes and graphene nano-ribbons are promising graphene-based materials with a size controllable energy bandgap, which might be useful for various technological applications [64,65]. They are important because of their potential for bottom-up fabrication of molecular devices, spintronic, and quantum dot technology [66]. They are cheap catalysts and being produced by a cheap method, they typically contain many defects. Because of the small size of graphene flakes they can be considered as a zero-dimensional form of graphene sheet showing different properties from graphene nanoribbons and bulk graphene. Such graphene-flakes are promising for a variety of applications such as electronic and magnetic devices with different molecular sizes and shapes, and in light absorption in photovoltaics due to their edge structure and wide spectrum. They have unique electronic, magnetic, and optical properties since their bandgap can be modified. Moreover, the saturation of the zigzag edges of graphene flakes with different atoms (like H in our investigations) or molecular groups leads to a spin-polarized ground state with a non-zero total magnetic moment, an electronic energy gap, and spin density that strongly depends on the used atomic group to passivate the dangling bonds of the C atoms [67]. In this study, we have chosen to model such a graphene flake as a polyaromatic hydrocarbon molecule (PAH).

This article describes our efforts to develop a mechanism for methane conversion to methanol on a modified graphene flake. The modification of the graphene flake has been done in three steps, i. e., introducing a single vacancy (SV) and di-vacancy (dV) defect, doping the defective flake with nitrogen heteroatoms making pyridinic N-doping graphene flake (N₃- and N₄-G), and finally introducing a single Co atom into the defective graphene flake lattice. Then, we investigated and compared the effect of nitrogen doping on the catalytic activity of the designed catalysts toward direct oxidation of CH₄ to CH₃OH using O₂ and N₂O as an oxidant. N₂O acts as an O-donor compound and gives us the ability to convert two pollutant gas simultaneously. The effects of using different oxidants have been studied in several investigations. For instance, Dasireddy et al. [68] studied the effects of oxidants (H₂O, N₂O, and O₂) on methane oxidation to methanol over the FePO₄ catalyst. They found that FePO₄ can actively convert methane to methanol using O₂ and N₂O oxidants.

It should be noted that although Ni or Cu metal surfaces would be most effective for CH bond cleavage, the activity of an individual single Ni or Cu atoms doped graphene-like surfaces will vary toward the same reaction. For instance, in our previous investigation [69], single Ni and Si atoms were chosen to compare the catalytic activity of the tuned graphene flake toward the direct conversion of methane to methanol. Interestingly, we found that single Ni atom doped graphene flake (Ni-G) cannot actively catalyze methane oxidation while Si-doped graphene flakes (Si-G) showed a better catalytic activity thanks to the lower activation energies and more favourable thermodynamic properties. Therefore, we reasonably assume that depending on the investigated graphene nano-flake (with applying different tuning methods), the activity of an arbitrary doped single metal atom and consequently the whole tailored graphene nano-flake will change. Motivated by previous investigations, in this article, a single Co atom is chosen as a single metal atom due to its considerable activity for ORR and OER. Density functional theory (DFT) calculations are utilized in order to find the most energetically favourable substrate for the direct oxidation of methane to methanol. All the reactions take place at room temperature and then the catalytic performances of these carbon material catalysts were investigated. Moreover, the detailed surface interactions and reaction pathways that occurred on the interface of the substrate were also discussed.