Insights into the reaction mechanisms of methanol decomposition, methanol oxidation and steam reforming of methanol on Cu(111): A density functional theory study
Introduction
Recently, alternative energy sources have received great attention due to the decline of fossil fuel resources and their increasing cost [1]. Methanol is the smallest alcohol molecule, and is being studied as a viable alternative to commercial fossil fuel resources. To be used as a clean fuel, methanol is often first transferred into hydrogen [2]. There are three primary ways to realize such a transfer; methanol decomposition (CH3OH = CO + 2H2) [3], [4], [5], methanol oxidation (CH3OH + 1/2O2 = CO2 + 2H2) [6], [7], [8] and methanol steam reforming (MSR, CH3OH + H2O = CO2 + 3H2) [9], [10], [11], [12]. Due to the importance of hydrogen, much attention has been paid to methanol dissociation on transition metal surfaces, such as Pt [13], [14], [15], [16], Pd [17], [18], [19], [20], Cu [18], [21], [22], [23], [24], Ir [25], [26] and various alloys [2], [20], [27], [28].
There are three types of catalysts for methanol decomposition; Pt-, Pd- and Cu-based catalysts [4], [5], [7], [8], [9], [10], [11], [12], [27]. However, the application of Pt- and Pd-based catalysts is restricted because Pt and Pd are precious metals. Therefore, much interest and concern have been focused on the study of methanol scission on Cu-based catalysts [7], [8], [9], [12]. Previous work involving methanol decomposition on the Cu(110) surface finds that a number of preadsorbed CH3OH and small amounts of CH2O desorb from the surface after heating, indicating that clean Cu(110) is not very active in methanol dissociation. However, when the surface is covered by O atoms, methanol dissociation is significantly facilitated, and various products have been observed, such as H2O, H2 and CH2O [29], [30], [31], [32], [33], [34]. The reaction mechanism of methanol dissociation is described as first involving O–H bond scission in methanol followed by sequential dehydrogenation to CH2O, then to CO or CO2.
Previous theoretical calculations have been performed to investigate methanol decomposition on Cu(11), (110) and (100) surfaces. Greeley and Mavrikakis found that the most favourable route for methanol direct scission on a clean Cu(111) surface is described as follows: CH3OH → CH3O → CH2O → CHO → CO, and methoxy dehydrogenation is the rate-limiting step [23]. Mei et al. also found the same reaction mechanism on a clean Cu(110) surface [21]. Sakong and Gross proposed that methanol oxidation may proceed through CH3OH → CH3O → CH2O → CH2OO → CHOO → CO2, in which CHOO dehydrogenation is the rate-limiting step [22], [35]. For MSR, Bo et al. studied CH2O dissociation on hydroxyl-covered and oxygen-covered Cu(100) surfaces, and found that formaldehyde reaction with surface O atoms is thermodynamically more favourable compared with the reaction path of formaldehyde with OH [36]. Gu and Li studied methanol dissociation on a hydroxyl-covered Cu(111) surface, and found that CH2O tends to react with hydroxyl to form hydroxymethoxy followed by its dissociation to CO2 [18]. However, they consider that preadsorbed OH only reacts with CH2O, and do not study OH reaction with other intermediates. To the best of our knowledge, a detailed understanding of the mechanism of MSR is still lacking.
It has been generally agreed that the active component in Cu-based catalysts for various reactions, including methanol decomposition, methanol oxidation and MSR, is metallic copper [18], [37]. Meanwhile, X-ray diffraction (XRD) characterization has proven that Cu(111) is the main component on the surface of copper [38], [39], and has been widely used in previous theoretical investigations regarding molecule adsorptions on transition metal surfaces [20], [21], [24], [40], [41]. In the paper, we have systematically identified possible reaction paths for the thermodynamics and dynamics involved in the steam reforming of methanol on a Cu(111) surface at the molecular level. In order to better understand the differences of underlying mechanism for the three reactions and give a guide to the rational design of Cu-based catalysts, methanol decomposition and methanol oxidation are also studied on Cu(111) surface using density functional theory (DFT).
Section snippets
Calculation models and methods
All calculations were performed using the Dmol3 package in Material Studio [42], [43], which has already been extensively used to study CO2 hydrogenation on Cu surfaces [44], [45], H2S desulfurization on ZnO surfaces [46], [47], and water gas shift reaction on Au surfaces [48], [49]. The all-electron relativistic DFT [42], [43] was used for core electrons by employing the generalized gradient approximation and the Perdew and Wang function (PW91) [50]. The electronic structures were obtained by
Adsorption of reactants
Fig. 2 shows the adsorption configuration of the intermediates involved in methanol decomposition, methanol oxidation and MSR on a Cu(111) surface at their favourable sites, and the corresponding adsorption energies are listed in Table 1. As shown in Fig. 2, methanol and H2O prefer to adsorb at the top site with an O bond on the clean Cu(111) surface, and O2 prefers to adsorb at the fcc site with an O bond on the clean Cu(111) surface. For methanol adsorption, the bond length of Cu–O is
Conclusions
In this paper, the mechanisms of methanol decomposition, methanol oxidation and MSR on a Cu(111) surface have been systematically investigated by using DFT calculations at the molecular level. We identify the preferred reaction paths of methanol decomposition, methanol oxidation and MSR. For methanol decomposition, the main reaction path is as follows: CH3OH → CH3O → CH2O → CHO → CO; for methanol oxidation, the main reaction path is found to favourably proceed as follows: CH3OH → CH3O → CH2
Acknowledgements
The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (20676087 and 21306125), China Postdoctoral Science Foundation Funded Project (2012M510784), Natural Science Foundation of Shanxi (Grant No. 2012011046-1 and 012021005-1), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (2013107), Special/Youth Foundation of Taiyuan University of Technology (No. 2012L042).
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