Graphitic carbon nitride (g-C3N4) has been regarded as a promising material in various applications, most notably in photocatalysis [1,2], electrocatalysis [3] and recently as catalyst support in thermo-catalytic hydrogenation reactions [4,5]. It possesses a unique polymeric structure, with carbon and nitrogen bonded strongly together in the form of s-triazine or tri-s-triazine/heptazine units [6,7]. Consequently, g-C3N4 has advantages such as good thermal and chemical stability, and tunable electronic structure [5,8]. As a catalyst support, g-C3N4 possesses nitrogen-rich basic sites and modifiable surface area and pore volume, which can provide high dispersion to metal nanoparticles [9,10]. Additionally, g-C3N4 (Fig. 1) can be simply synthesized by thermal polymerization of organic precursors containing carbon and nitrogen, such as urea [11] and melamine [12].
Methanation (also known as Sabatier reaction) has been extensively studied in recent years due to the great interest in the production of synthetic natural gas (SNG). Carbon monoxide (CO) methanation is the hydrogenation reaction of CO to CH4 (Eq. 1), which is the main component in SNG. It is well known that nickel (Ni) is considered the most practical catalyst material for CO methanation, owing to its good catalytic performance as well as low cost. However, traditional nickel catalysts often suffer from deactivation at elevated temperature due to the highly exothermic CO methanation reaction [13]. Therefore, the effects of support materials, metal dopants, and synthesis methods have been studied to improve the activity and stability of Ni-based catalysts.
\(CO+3{H}_{2}\to {CH}_{4}+{H}_{2}O {\varDelta H}_{r}^{0}=-206 kJ/mol\)
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(1)
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There have been several studies on the utilization of g-C3N4 as the support material. Chen et al. [5] and Park et al. [4] had successfully prepared the mesoporous g-C3N4 supported iron catalysts (Fe/g-C3N4) for the Fischer Tropsch synthesis (FTS). Chen and coworkers [5] had discussed the interaction between nitrogen-containing base of g-C3N4 support and iron catalyst can enhance the FTS performance by demonstrating high olefin/paraffin ratio. Furthermore, the presence of abundant lone-pair electron sites in g-C3N4 support could facilitate the reduction of iron oxide to a more reduced form [4].
Carbon monoxide is a very reactive molecule and has been utilized as a feedstock in many chemical reactions. CO is also regarded as a strong reducing agent aside from classical reducing agents such as hydrogen and metal hydrides [14]. Abu Tahari et al. [15] have investigated the comparison of reduction behavior between reductants CO and H2 in the reduction process of iron oxide. They found that CO is a faster reducer at a lower temperature of reduction, whereas H2 is a good reductant in completing the reduction. As for the methanation catalysts, it is interesting to study the behavior of CO molecules either in reduction or adsorption-desorption on the surface of catalysts, since CO is one of the reactants.
In a recent study, a nickel supported on graphitic carbon nitride (Ni/g-C3N4) catalyst was prepared by a simple impregnation procedure. The desorption and reduction by CO were studied by temperature-programmed desorption (CO-TPD) and reduction (CO-TPR), and the characterization was conducted by X-ray diffraction (XRD), N2 physisorption, and X-ray photoelectron spectroscopy (XPS) analyses. Finally, the catalyst performance in CO methanation was evaluated using a temperature-programmed experiment under atmospheric conditions.