Strategies toward the sustainable electrochemical oxidation of methane to methanol

Published in: Current Opinion in Green and Sustainable Chemistry DOI: 10.1016/j.cogsc.2021.100489 Publication date: 2021 Document version Publisher's PDF, also known as Version of record Document license: CC BY Citation for published version (APA): Arminio Ravelo, J. A., & Escudero Escribano, M. (2021). Strategies toward the sustainable electrochemical oxidation of methane to methanol. Current Opinion in Green and Sustainable Chemistry, 30, [100489]. https://doi.org/10.1016/j.cogsc.2021.100489


Introduction
The direct conversion of greenhouse gases into green fuels and valuable chemicals is of paramount interest to achieve a decarbonized future [1,2]. Methane emissions are the second largest cause of global warming (IEA; URL: https:// www.iea.org/articles/global-methane-emissions-from-oiland-gas) after CO 2 emissions. Although methane emissions represent only 10% of the global greenhouse gas emissions (US EPA; URL: https://www.epa.gov/ ghgemissions/overview-greenhouse-gases), methane has a potential climate impact 28 times higher than CO 2 in 100-year term (European Commission; URL: https://ec. europa.eu/energy/topics/oil-gas-and-coal/methane-emissi ons_en). Thus, policies are being created to reduce the global annual emissions of methane up to 75% by 2030 (IEA; URL: https://www.iea.org/reports/methane-emissions-from-oil-and-gas). Nowadays, the energy sector is one of the principal sources of methane in the atmosphere. Typically, methane surplus is destined to gas flaring, contributing to 1% of CO 2 annual emissions (The World Bank, https://www.worldbank.org/en/programs/ gasflaringreduction). It is crucial to find more sustainable solutions for the use and conversion of methane to fulfill the energy demand with a negligible carbon footprint.
Direct conversion of methane to methanol is one of the most appealing alternatives to gas flaring. Currently, methanol is commercially produced via a two-step process involving the oxidation of methane to synthesis gas (syngas), a mixture of CO and H 2 [3], followed by its catalytic reduction to methanol. This process is high energy consuming and expensive [3]. At present, methane to methanol plants are economically justified only at large-scale productions of methanol when generating at least 2500 metric tons per day [4]. The possibility to convert directly methane to methanol would open the opportunity to reduce the energy consumption and costs of production [4]. However, this process remains a great challenge because of the inertness of methane's CeH bonds and the difficulty to selectively oxidize it to methanol [4,5].
In the field of catalysis, methane to methanol conversion strategies can be divided into five areas: biocatalysis, homogeneous catalysis, thermal heterogeneous catalysis, photocatalysis, and electrocatalysis, summarized in Figures 1 and 2. In biocatalysis, methane monooxygenase (MMO) enzyme stands out for its ability to selectively convert methane to methanol [6,7]. In homogeneous catalysis, a great part of the work is based on the use of platinum (Pt) complexes [8,9]. These catalysts are capable to activate the CeH bond in methane and convert it selectively to methanol or methanol derivatives at mild conditions (~100 Ce200 C) [9]. Inspired by the active centers of the MMO enzyme, heterogeneous catalysis has traditionally used catalyst materials based on Cu and Fe [10,11]. These catalysts can activate methane at mild to high temperature and pressure conditions. In photocatalysis, the driving force is the ultraviolet light. Typically, semiconductor materials are combined with metal or metal oxides that allow efficient electron transfers and enhance the formation of radicals [12,13]. Thus, methane is activated by radical chain reactions [14].
Electrocatalytic approaches for methane to methanol oxidation are inspired by biological, heterogeneous, and homogeneous catalytic systems [4]. Here, the driving force is an electrical potential difference. Electrochemical strategies are particularly appealing, as they can operate at low to mild temperatures. Yet, the reaction is extremely challenging. Methane is highly stable and less reactive than methanol. The Gibbs free energy (DG) and the electrode potential (E rev ) versus the reversible hydrogen electrode (RHE) in reactions 1 and 2 show that the oxidation of methane to methanol is less favorable than the full oxidation to CO 2 . However, it is possible to oxidize selectively methane by tuning the reaction mechanism   Methane to methanol direct conversion strategies. (a) Direct methane activation. (b) Indirect methane activation: by using catalytic species capable to regenerate on the electrode surface (pink arrows) or by the electrogeneration of radical species (red arrows). and enhancing the formation of intermediates that lead to the formation of methanol rather than CO 2 [15,16]**. Therefore, electrocatalytic approaches focus on the selection of electrode materials that activate methane and generate adequate intermediates to enhance the production of methanol.
The electrochemical oxidation of methane takes place by direct or indirect electrochemical methane activation ( Figure 2). On the one hand, direct methane activation is focused on its direct adsorption on the electrode active sites ( Figure 2a); on the other hand, the indirect methane activation involves the electrochemical generation of highly reactive species capable to activate methane in the interface neighborhoods or/and the bulk of the solution (Figure 2b) [5]. Herein, we present a brief overview of the developments of the electrocatalytic conversion of methane to methanol at mild to low temperatures and discuss the strategies toward both the direct and indirect electrochemical methane activation.

Direct electrochemical methane activation
Direct electrochemical methane to methanol strategies focus on tuning the electrode surface structure and composition [15,16**,17]. The materials selected need to (1) activate methane and (2) stabilize the intermediates involved in the formation of methanol to avoid the formation of CO 2 [4,18]. These strategies offer the opportunity to integrate methane to methanol conversion in electrochemical devices such as fuel cells [16]**. This would not only allow the production of methanol but also the generation of electricity ( Figure 3a). Here, we discuss the advances on the three main materials used as electrodes for direct methane to methanol conversion: metallic surfaces, metal oxides/ promoted oxygenated surfaces, and ZrO 2 composites.

Metallic surfaces
The use of metallic surfaces for direct methane activation has emerged as a collateral result of the studies on the electro-oxidation of hydrocarbons for fuel cell applications [19,20]. These studies were the base for understanding the potential dependence of methane adsorption and activation. Studies on Pt electrodes have shown that methane adsorption is a slow process and potential dependent [15,19]. Methane adsorbs at potentials higher than 0.2 V versus RHE and oxidizes to CO 2 around 0.6e0.8 V versus RHE ( Figure 3b) [15,19]. Both adsorption and activation potentials are sensitive to the temperature and electrolyte used [15,19,21]. Kinetic studies show that methane oxidation has an initial electron transfer step to form adsorbed CH 3 * (Reaction 3, the rate-determining step) [15,20], followed by the formation of CO* and/or COH* (Reaction 4) before its oxidation to CO 2 (Reactions 5e7) [15,21,22].
Recent theoretical calculations suggest that methane is thermochemically activated on Pt surfaces (Reaction 3) [15]. This means that the rate of methane activation is independent of the potential applied but dependent on the temperature. Methane activation is also sensitive to the electrode surface structure and composition. It has been reported that Pt terraces can activate the CeH bonds, but only on Pt (100) single-crystalline surfaces, CH x * intermediates can combine with oxygen species with a low energy barrier [23]**. This shows that direct electrochemical methane activation presents a strong structure sensitivity, similar to other electrocatalytic reactions such as CO 2 electroreduction [24]. Moreover, the electric field on the interface can modify importantly the efficiency of methane activation, being Polymer electrolyte membrane fuel cell for direct methane to methanol conversion using ZrO 2 composites and CO 3 2 exchange membrane. Right: Mechanism suggested for methane to methanol conversion using CO 3 2− as oxidizing agent. Adapted with permission from Ref. [17]. Further permissions related to the material excerpted should be directed to the ACS. favored on low oxygen content where metallic Pt predominates [25].
Although it is possible to activate methane electrochemically on Pt surfaces, the adsorption rates of methane tend to be extremely slow compared with the oxidation to CO 2 [15,19e22,23**]. As a consequence, none of the studies at low temperature has shown selectivity to methanol thus far. The mechanism on Pt surfaces suggests that once CH 3 * is adsorbed, it rather losses all its hydrogens rather than forming methoxy-alike species [15,23]**. Although there is almost nonexisting literature on other metallic surfaces, in situ Fourier-Transform Infrared Spectroscopy (FTIR) studies have revealed that the activation of methane on other metals is possible [22]. Still, based on theoretical calculations, Pt shows the highest rates of methane activation [15]. Boyd et al. noted that, in transition metals, the decrease of the methane activation barrier strengthens the CO* binding. They suggest that the best catalysts for methane oxidation are those that diverge from the scaling relationship between methane activation and CO* formation [15]. Notwithstanding, the challenge is higher for methane to methanol conversion. Designing and tuning electrocatalytic surfaces that favor the adsorption and activation of methane and avoid the formation of the CO* intermediate, linked to the full methane oxidation [15,23**,25], remains a challenge.
Metal oxides and oxygen-promoted surfaces: activation through water discharge The use of metal oxides or oxygen-promoted surfaces to convert methane to methanol is a promising approach. The use of metal oxides was inspired by the electrooxidation of light alkenes to oxygenated species at potentials close to the oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER). Here, the discharge of water forms oxygenated active species responsible for the oxidation process [26,27].
The initial studies on oxygen-promoted surfaces focused on the production of oxygenated active species through the ORR mechanism. Pd, Pt, Rh, Au, and PdAu supported on carbon were tested as electrodes in a fuel cell configuration. Only Pd and PdAu succeeded in converting methane to methanol, but with poor selectivity [26]. Better results were attained through the OER mechanism [28]. Among the initial electrocatalysts tested, materials composed of V 2 O 5 and/or RuO 2 reported the best conversions and selectivity toward methanol [27e 29]. Interestingly, NiOeV 2 O 5 /Rh nanocomposites present high selectivity toward methanol production with current efficiencies of 91% at 100 C [30]. According to different studies on these nanocomposites, methane is activated through the oxygenated active species generated during the OER (Reactions 8e15) [16**,26,28e 30]. Alternative mechanisms suggest the formation of redox pairs on the catalyst surface [29]; however, there is not enough experimental evidence to confirm this. Recently, Arnarson et al. created a model to identify the ideal metal oxides for the electrochemical methane to methanol conversion based on their oxygen binding energy [16]**. According to their model, methane is thermochemically activated. This suggests the need for mild temperatures (>100 C) to ease the activation of methane through the oxygenated species. The electrocatalyst needs to bind oxygen weakly or to have it as radical that serves to oxidize methane. Thus, methane can only be activated and converted to methanol by the formation of O* over the electrocatalyst surface, as shown in Reactions 9 and 12. Besides, the potential applied to the electrocatalyst must be high enough to produce *O and low enough to avoid the formation of *OOH (Figure 3c) Methane to Methanol Reaction: ) to activate methane through oxygen insertion. NiO was selected because of its ability to activate methane in heterogeneous catalysis and its use for organic oxidation in alkaline media. The results showed that the composite was capable to activate and partially oxidize methane. Interestingly, the catalyst produced methanol, isopropanol, acetate, acetic acid, acetone, ethanol, and formic acid. However, all products were further oxidized in fuel cell experiments (Figure 3d) [17]. Similar experiments have been reported with ZrO 2 composites using Co 3 O 4 [32], NiCo 2 O 4 [33], and CuO 2 [34]*, showing higher catalytic activity toward partial methane oxidation compared with the composite with NiO. Still, the results showed the generation of methanol only as an intermediate to produce higher alcohols and/or carboxylic acids. In general, it seems that methanol is efficiently produced in the fuel cell configurations but rapidly oxidized [17,32,34]*.
Despite the satisfactory performance of this type of composites, there is a need for further mechanistic understanding. The reaction pathways suggested are based on possible oxidation reactions reported in the literature (Figure 3d) without theoretical nor experimental studies that support them. Recent theoretical calculations on this type of materials suggest that methane activation barrier is decreased, thanks to the high-speed electronic network formed between the oxygen atoms between ZrO 2 and the metal oxide [34]*. It seems that the use of CO 3 2À as the oxidizing agent is responsible to produce higher alcohols. Interestingly, some of the faradaic efficiencies for some of the products reported are above 100% [32,33,34]*. This suggests that more than one step of the mechanism or reaction does not occur by electron transfers at the electrodeeelectrolyte interface but by redox reactions between species in solution. In general, the results also evidence the competition between methane oxidation and the OER [34*, 35]. Yet, it is not clear if they are involved in the same mechanistic pathway or as parallel reactions.

Indirect electrochemical activation of methane
Strategies on the indirect electrochemical activation of methane focus on the generation of species capable to break or activate the CeH bond [37,38]. In contrast to direct methane activation strategies, methane is not activated on the electrode surface. Besides, the electrodes do not act as electrocatalysts but as a source of electrons for the main active species. This allows to efficiently decouple methane conversion and methanol selectivity, as they do not share the same active sites [39]. Here, we discuss the two main approaches for the indirect methane to methanol conversion: electrochemical generation of active species and electrochemical-assisted homogeneous catalysis.

Electrochemical generation of highly active species
In this approach, methane is activated by homogeneous reactions with highly reactive species, such as radicals. In electrochemistry, the focus is on the electrogeneration of these species, which are typically oxygenated species that can react to methane forming methanol or to derivatives that can be easily converted to methanol.
The research on this approach traces back to the end of the 80s and the beginning of the 90s. These works involve the study of complex systems based on the formation of Cl • radicals by photochemical reactions [40], generation of HO radicals through the Fenton reaction [37] and formation of superoxide radicals, O 2 À , in highly alkaline media to activate methane [41] ( Table 1). It is important to mention that the mechanisms are based on radical chain reactions, and therefore, more than one product was generated. The experiments succeeded to produce methanol at ambient temperatures. However, the current efficiency and selectivity toward methanol were not high enough for scalable applications. It also seems relevant to limit the content of O 2 in the electrolyte to reduce the formation of side products such as formaldehyde [37,41]. Nonetheless, further understanding of how methane is activated through these oxygenated radicals is essential to improve selectivity.

Electrochemical-assisted homogeneous catalysis
This approach was inspired by previous research in homogeneous catalysis [38]. The idea is to make use of electrochemistry to overcome some of the common Table 1 Different radical-based reactions to convert methane to methanol.

Ref.
Mechanism [40] 2Cl À ðaqÞ + 2e À / Cl 2ðgÞ Cl 2ðgÞ + hn/ 2Cl limitations that present "simple" homogeneous catalytic systems. The catalysts used are typically transition metal complexes capable to activate the CeH bond and convert methane selectively to methanol or methanol derivatives at mild conditions [9]. The complex used as catalyst typically is reduced during the methane activation. To keep the complex active, it is necessary to use an external agent, the co-catalyst, capable to reoxidize it ( Figure 1b). One big limitation in homogeneous catalysis is to find an adequate co-catalyst that has a redox potential higher than the one of the catalyst [9,38]. The higher the redox potential of the co-catalyst, the higher the number of catalytic cycles the complex can go. Therefore, an appealing option is to use an electrode as the "co-catalyst" to provide the potential difference necessary to regenerate the catalyst [42]* [35,41].
Electrochemical-assisted homogeneous catalysis approaches have been recently introduced by Surendranath et al. to selectively oxidize methane to methanol [38]. The first experiments focused on the conversion of methane to two methanol derivatives: methyl bisulfate (CH 3 OSO 3 H) and methanesulfonic acid (CH 3 SO 3 H) [38]. PdSO 4 was used as a catalyst in concentrated sulfuric acid at mild temperatures (~100 C) and high methane pressure (Figure 4a) [38,43]. Remarkably, the system showed a catalytic activity 5000 times higher compared with the nonassisted homogeneous catalytic system [38]. . Some success has also been achieved in nonaqueous media [45]. The system showed the possibility of using an Rh complex as catalyst by controlling the concentration of O 2 along Si nanowires as electrodes. The results showed high selectivity to methanol at ambient conditions. However, the complexity of the catalytic system limited the conversion of methane (Figure 4b) [45].
One of the main advantages of using electrochemicalassisted homogeneous catalysis is the number of possibilities to activate methane with high selectivity to methanol. Coupling electrochemistry with homogeneous catalysis offers the possibility to avoid competition with the OER or ORR. This approach allows the production of methanol without its further oxidation. Notwithstanding, the use of homogeneous catalysis has Electrochemical-assisted homogeneous catalysis for methane to methanol conversion strategies. (a) Proposed mechanism for electrochemical methane functionalization by a putative Pd 2 III,III intermediate. Green and blue arrows indicate faradaic and nonfaradaic reaction pathways, respectively. Reproduced with permission from Ref. [38]. Copyright 2017 American Chemical Society. (b) Simplified catalytic cycle for electrochemical methane functionalization by Rh-based catalyst in nonaqueous media and Si nanowires as electrodes. Reproduced with permission from Ref. [45]. Copyright 2019 American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS. other limitations. Methane to methanol conversion depends on how fast the catalyst defuses to the electrode to be regenerated [42]*. Besides, it is important to select adequate catalysts that contain metallic centers that allow fast electron transfers to avoid high overpotentials for the regeneration of the catalyst [38].

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.