Ultrahigh Electrocatalytic Conversion of Methane at Room Temperature

Abstract

Due to the greenhouse effect, enormous efforts are done for carbon dioxide reduction. By contrast, more attention should be paid for the methane oxidation and conversion, which can help the effective utilization of methane without emission. However, methane conversion and utilization under ambient conditions remains a challenge. Here, this study designs a Co₃O₄/ZrO₂ nanocomposite for the electrochemical oxidation of methane gas using a carbonate electrolyte at room temperature. Co₃O₄ activated the highly efficient oxidation of methane under mild electric energy with the help of carbonate as an oxidant, which is delivered by ZrO₂. Based on the experimental results, acetaldehyde is the key intermediate product. Subsequent nucleophilic addition and free radical addition reactions accounted for the generation of 2‐propanol and 1‐propanol, respectively. Surprisingly, this work achieves a production efficiency of over 60% in the conversion of methane to produce these long‐term stable products. The as‐proposed regional electrochemical methane oxidation provides a new pathway for the synthesis of higher alcohols with high production efficiencies under ambient conditions.

1. Introduction

Methane (CH₄) is the primary component of natural gas, which constitutes 21.4% of the total primary energy sources in the world,1 and is widely used as an important fuel in both industrial chemical process and human daily life. Compared with other fossil fuels, such as oil and coal, the combustion of natural gas provides lower carbon dioxide (CO₂) emissions,2 making natural gas a suitable alternative transitional energy source until carbon‐free energy sources are sufficiently mature to be deployed.3 However, the emission of CH₄ gas has long been ignored and regarded as a trivial matter,4 even though its effect as a greenhouse gas is over 30 times more potent than that of CO₂.5 In particular, global warming and the exploitation of shale gas aggravate this emission. Recently, more attention has been given to the negative impact of CH₄ emissions due to increasing environmental pollution and climate change.6, 7, 8 Hence, efforts have focused on the conversion of atmospheric CH₄ into an equimolar amount of CO₂ through thermocatalysis or photocatalysis. However, conventional CH₄ conversion processes still suffer from various drawbacks, including the use of precious metal catalysts, high reaction temperatures that are far from ambient conditions and extremely low conversion efficiencies.9, 10, 11 In this regard, the oxidation and conversion of CH₄ to liquid alcohols, such as methanol, ethanol, and propanol, is much more economical and energy‐efficient. Among liquid alcohols, higher alcohols with high energy densities have wide applications in fabricating commodity chemicals.12

Methanol, as a product of CH₄ conversion, has been widely investigated. Currently, the syngas reaction is the main route for the industrial production of methanol, whereby syngas is produced through CH₄ steam reforming. The two reactions for CH₄ conversion to methanol are given in Equations (1) and (2), 13, 14

$${\mathrm{CH}}_4(\mathrm{g})\hspace{0.17em}+\hspace{0.17em}{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\hspace{0.17em}\stackrel{\mathrm{Ni}}{\to }\hspace{0.17em}\mathrm{CO}(\mathrm{g})\hspace{0.17em}+\hspace{0.17em}3{\mathrm{H}}_2(\mathrm{g})\hspace{0.17em}\mathrm{\Delta }{H}_{\mathrm{298}}\hspace{0.17em}=\hspace{0.17em}\mathrm{49}.\mathrm{3}\hspace{0.17em}\mathrm{kcal}\hspace{0.17em}{\mathrm{mol}}^{-1}$$ (1)

$$\mathrm{CO}(\mathrm{g})\hspace{0.17em}+\hspace{0.17em}2{\mathrm{H}}_2(\mathrm{g})\hspace{0.17em}\stackrel{{\mathrm{Cu/ZnO/Al}}_2^{}\hspace{0.17em}{\mathrm{O}}_3^{}}{\to }\hspace{0.17em}{\mathrm{CH}}_3\mathrm{OH}(\mathrm{g})\hspace{0.17em}\mathrm{\Delta }{H}_{\mathrm{298}}\hspace{0.17em}=\hspace{0.17em}-\mathrm{21}.\mathrm{7}\hspace{0.17em}\mathrm{kcal}\hspace{0.17em}{\mathrm{mol}}^{-1}$$ (2)

From the equations, the initial reaction in syngas reforming requires more energy than that released from the second reaction, illustrating that additional energy input is necessary for the conversion of CH₄ to methanol. Some scientists have employed a class of bacteria known as methanotrophs with methane mono‐oxygenases to convert CH₄ into methanol under ambient conditions using oxygen.15 The methods mentioned above require complex processes, extra energy consumption, or enzyme cultivation and critical control of the conditions.13, 16 In fact, the direct oxidation of CH₄ by oxygen gas (O₂) is always accompanied by substantial overoxidation, which is both kinetically and thermodynamically favorable.13 The conversion of CH₄ to methanol with O₂ is triggered by energy input and proceeds spontaneously with a substantial release of energy, which makes it exergonic for the further oxidation of methanol to formaldehyde, formic acid, carbon monoxide (CO), and CO₂. Controlling CH₄ oxidation to obtain methanol is extremely difficult. Thus, compared with the route of inhibiting CH₄ overoxidation to obtain methanol, the control of further oxidation processes under mild conditions may enable the production of more useful higher alcohols, such as propanol. Electrochemical oxidation in aqueous electrolyte has been demonstrated to be a suitable method for the conversion of CH₄ or other volatile organic compounds (VOCs) at low temperature using simple reaction instruments.17, 18, 19

The C—H bonds in CH₄ have a high dissociation energy of 104 kcal mol⁻¹, which makes CH₄ extremely inert. When O₂ is used as an oxidizing agent for CH₄ oxidation, the reaction of triplet O₂ with singlet CH₄ to form singlet methanol is a spin‐forbidden process.13 Thus, protons in CH₄ are not expected to be readily abstracted by O₂ under mild conditions, such as low temperature. Therefore, other oxidizing agents are necessary to replace O₂. In conventional alkaline electrochemical systems, which are always performed at room temperature, hydroxide (HO⁻) generally functions as the oxidizing agent. However, hydroxide has previously been demonstrated to have negligible activity for abstracting protons from CH₄ under mild conditions.20 By contrast, carbonate (CO₃ ²⁻) oxidizes species by donating a charged oxygen atom accompanied by CO₂ release, resulting in a large enthalpy of reaction and favorable oxidation kinetics.21, 22, 23 Therefore, CO₃ ²⁻ may be an attractive alternative to OH⁻ for alkaline electrochemical reactions. In addition, to realize the donation of an oxidizing agent from CO₃ ²⁻, zirconia (ZrO₂) should be employed to facilitate CO₃ ²⁻ adsorption because of its surface Lewis acid sites and electron‐accepting capabilities.24, 25 Based on the above results, Mustain's group selected nickel oxide (NiO) as the catalyst and prepared NiO/ZrO₂ for the electrochemical oxidation of CH₄ with CO₃ ²⁻ as the oxidizing agent.26 However, from their results, NiO did not exhibit satisfactory selectivity for CH₄ oxidation, and the reaction mechanism was also unclear. Thus, more efficient catalysts with high activity and selectivity should be employed.

Among metal oxide catalysts, cobalt oxide (Co₃O₄) has been demonstrated to be one of the most efficient catalysts for the oxidation of VOCs.27 In addition, Co₃O₄ has a strong surface adsorption capacity for formaldehyde.28 According to theoretical observations of the reduction of CO₂ on transition metal surfaces,29 formaldehyde tends to be oxidized to CO. However, when adsorbed on the surface of Co₃O₄, formaldehyde, converted from methanol, may be more active for further additional reactions to higher alcohols, illustrating the regional selectivity of Co₃O₄ for CH₄ oxidation. Thus, we designed a Co₃O₄/ZrO₂ composite for the electrochemical oxidation of CH₄, using CO₃ ²⁻ as the oxidizing agent source, that resulted in high selectivity for the production of 1‐propoanl and 2‐propanol with over 60% production efficiency. The reaction process is shown in Scheme 1 .

Scheme 1.

The reaction process of electrochemical oxidation of methane gas.

2. Results and Discussion

2.1. Fabrication and Characterization of the Co₃O₄/ZrO₂ Nanocomposite

Coprecipitation and hydrothermal methods were employed to fabricate the Co₃O₄/ZrO₂ nanocomposite (for details, see the Experimental Section). Scanning electron microscopy (SEM) was used to observe the morphology of different samples fabricated using different ZrO₂/Co₃O₄ ratios, denoted 1–2 ZrO₂/Co₃O₄, 1–4 ZrO₂/Co₃O₄, and 1–6 ZrO₂/Co₃O₄, as shown in Figure 1 . In all samples, oval‐shaped ZrO₂ nanoparticles with uniform sizes were formed and were adsorbed on the surface of the Co₃O₄ plates. Upon increasing the amount of Co₃O₄, the particle size of Co₃O₄ gradually increased until it reached bulk (Figure 1; and Figure S1, Supporting Information), which may affect the catalytic properties of the ZrO₂/Co₃O₄ nanocomposite. Pure Co₃O₄ powder was also prepared for comparison (Figure S2, Supporting Information) and showed large particles that were more than 10 µm in size. The size of the Co₃O₄ plates can be controlled through coprecipitation with ZrO₂. The elemental ratios of Zr/Co for all samples were obtained through EDS (energy‐dispersive X‐ray spectroscopy) measurement (Table S1 and Figure S3, Supporting Information). The Zr/Co ratio decreased upon increasing the amount of Co precursor. In the 1–6 ZrO₂/Co₃O₄ sample, big Co₃O₄ plates were surrounded by small ZrO₂ particles, which may have caused the sudden decrease in the Zr/Co ratio. The EDS measurement was focused on the surface of the samples, which would be affected much by the surface state of the materials. Thus, the ratios showed different value when compared to the stoichiometric ratio of elements calculated from the reactants described in the Experimental Section. In order to obtain the stoichiometric ratio of Zr/Co, ICP‐OES (inductively coupled plasma optical emission spectrometry) measurement was conducted, shown in Table S1 (Supporting Information). The 1–4 ZrO₂/Co₃O₄ sample exhibited relatively small particle sizes and a suitable amount of Co₃O₄ and showed the best electrochemical catalytic performance for CH₄ oxidation, which will be discussed next.

Figure 1.

Morphologies of the ZrO₂/Co₃O₄ nanocomposites with different ratios. SEM images of the a,b) 1–2 ZrO₂/Co₃O₄, c,d) 1–4 ZrO₂/Co₃O₄, and e,f) 1–6 ZrO₂/Co₃O₄ samples. The scale bars in (a)–(f) are 1 µm. g) HR‐TEM image and h) TEM image with elemental mappings of the 1–4 ZrO₂/Co₃O₄ sample. The inserts are fast‐Fourier transformation patterns.

For further examination of ZrO₂ and Co₃O₄, 1–4 ZrO₂/Co₃O₄ was examined by transmission electron microscopy (TEM) (Figure 1g,h). In the TEM image (Figure 1h), the Co₃O₄ plate can be easily distinguished from the surrounding ZrO₂ nanoparticles. From the high‐resolution TEM (HR‐TEM) image (Figure 1g), the lattice constant of ZrO₂ was found to be 0.252 nm, corresponding to the (002) facet, and that of Co₃O₄ was 0.238 nm, corresponding to the (022) facet. In addition, elemental mapping was performed for Co, Zr, and O atoms (Figure 1h). From the elemental distribution, the Co₃O₄ plates and ZrO₂ nanoparticles were confirmed to have different structures. From the SEM and TEM images, well physical connection between ZrO₂ and Co₃O₄ could be demonstrated, which may lead to synergistic effects in the electrochemical oxidation of CH₄. In addition, from the X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) spectra (Figure 2 ), peaks of each sample showed almost same position without shifting, illustrating no obvious chemical bondings can be observed between ZrO₂ and Co₃O₄.

Figure 2.

Characteristics of the ZrO₂/Co₃O₄ nanocomposite. XRD spectra of a) the 1–2 ZrO₂/Co₃O₄, 1–4 ZrO₂/Co₃O₄, and 1–6 ZrO₂/Co₃O₄ samples and b) pure Co₃O₄. c) Co 2p XPS signals of pure Co₃O₄ and ZrO₂/Co₃O₄ samples with different ratios of 1–2, 1–4, and 1–6. d) Zr 3d XPS signals of ZrO₂/Co₃O₄ samples with different ratios of 1–2, 1–4, and 1–6. e) Deconvoluted O 1s XPS signals of pure Co₃O₄ and ZrO₂/Co₃O₄ samples with different ratios of 1–2, 1–4, and 1–6.

The crystalline structures of the ZrO₂/Co₃O₄ composites with different ratios were analyzed by powder XRD using Cu Kα radiation and then compared with that of pure Co₃O₄ (Figure 2a,b). The diffraction peaks of ZrO₂ corresponded to the monoclinic phase (JCPDS No. 37‐1484), and the peaks of Co₃O₄ were indexed to the cubic structure (JCPDS No. 42‐1467). In the XRD patterns, the typical (001), (100), (011), (−111), and (022) planes of ZrO₂ were observed at ≈17.5°, 24.2°, 24.6°, 28.3°, and 50.3°. All related peaks gradually decreased in intensity as the amount of Co₃O₄ increased. The typical (111), (311), and (440) planes of Co₃O₄ were observed at ≈19.0°, 36.9°, and 65.2°, which also showed the same change in peak intensity with an increase in ZrO₂.

The abovementioned data explained the microscopic and crystalline structures of the ZrO₂/Co₃O₄ nanocomposite. Therefore, we next set out to determine the surface state, which is responsible for the unique regional selectivity of CH₄ oxidation, by XPS, as shown in Figure 2c–e. XPS spectra of the Co 2p (Figure 2c), Zr 3d (Figure 2d), and O 1s (Figure 2e) core levels, along with a survey scan (Figure S4a, Supporting Information), were obtained for the samples with different ZrO₂/Co₃O₄ ratios. The binding energies of the Co and Zr signals did not greatly shift upon changing the component ratio. However, the intensity of the Co signals clearly changed as the amount of Co decreased. The Zr signals also changed, but not as much as the Co signals, which may be due to ZrO₂ being on top of the Co₃O₄ surface. Additionally, the O 1s spectroscopic signals were affected. The O 1s peaks at ≈530 and 532 eV are ascribed to lattice oxygen and nonuniform surface sites, respectively.30 The peak at ≈532 eV is related to defect sites on the surface, such as chemisorbed or dissociated oxygen or hydroxyl species.31 The surface of Co₃O₄ is known to be readily covered with a monolayer of negatively charged chemisorbed oxygen.28 In this case, the peak at ≈532 eV in the O 1s spectrum of pure Co₃O₄, shown in Figure 2e, can be clearly deconvoluted to demonstrate the surface adsorption ability of Co₃O₄. In addition, after coprecipitation with ZrO₂, the O 1s signals of all ZrO₂/Co₃O₄ composites showed enhanced peaks at ≈532 eV, which can be observed more clearly in the overlapping plots in Figure S4b (Supporting Information). This result shows the surface electron accepting capabilities of ZrO₂, as reported previously,24, 25 indicating its strong adsorption of CO₃ ²⁻ electrolyte during the electrochemical oxidation of methane.

2.2. Electrochemical Performance for CH₄ Oxidation

To measure the electrochemical performance of CH₄ oxidation, a glassy carbon disc was employed to load the catalyst, forming the working electrode. Details of the preparation and measurement process are provided in the Experimental Section. As mentioned above, NiO is not an efficient catalyst for CH₄ oxidation. Thus, we prepared a ZrO₂/NiO composite and compared its catalytic ability with that of ZrO₂/Co₃O₄, as shown in Figure S5 (Supporting Information). In the measurement of the current–voltage (J–V) curves and Nyquist plots (for impedance analysis), argon (Ar) saturation was used to analyze the water oxidation and CH₄ saturation was used to analyze the CH₄ oxidation competing with water oxidation. Additional anodic activity under CH₄ saturation indicated that both NiO and Co₃O₄ have CH₄ oxidizing activity. However, CH₄ oxidation performed better on the surface of Co₃O₄ than NiO. The same results were observed via electrochemical impedance spectroscopy (EIS) (Figure S5b,d, Supporting Information), which illustrated the superior activity of Co₃O₄ for CH₄ electrochemical oxidation.

Figure 3 shows the linear sweep voltammetry (LSV) curves of the ZrO₂/Co₃O₄ samples with different ratios in CH₄‐saturated carbonate electrolyte. The 1–4 ZrO₂/Co₃O₄ sample showed the highest electrochemical current density for CH₄ oxidation, which agrees with the sample morphology results after adjusting the component ratio, as 1–4 ZrO₂/Co₃O₄ had a relatively suitable size and amount of Co₃O₄. The current density of the 1–2 ZrO₂/Co₃O₄ sample was relatively low even after increasing the potential to a high value, which illustrated that a low amount of Co₃O₄ provided poor catalytic activity in CH₄ oxidation. By contrast, ZrO₂ showed extremely weak oxidizing activity under both H₂O and CH₄ saturation due to its high bandgap of more than 5 eV.32 The 1–6 ZrO₂/Co₃O₄ sample showed the same J–V curve trend as the 1–4 ZrO₂/Co₃O₄ sample but a lower current density value, illustrating the weaker surface adsorption ability toward the carbonate electrolyte due to the lower amount of ZrO₂. In addition, J–V curves of the optimized sample in Ar‐ and CH₄‐saturated electrolyte were recorded to demonstrate its ability for CH₄ oxidation, as shown in Figure S6a (Supporting Information). In addition, pure Co₃O₄ powder was also prepared to examine the electrochemical oxidation of CH₄ (Figure S6b, Supporting Information). Unfortunately, the Co₃O₄ sample without ZrO₂, which aids the adsorption of oxygen donors, showed no additional anodic activity in CH₄‐saturated electrolyte and even worse performance, which confirmed that the outstanding electrochemical oxidation of CH₄ resulted from the synergistic effects of ZrO₂ and Co₃O₄, i.e., carbonate adsorption and CH₄ oxidation, respectively.

Figure 3.

Electrochemical performance of CH₄ oxidation. a) J–V curves and b) magnified curves of the ZrO₂/Co₃O₄ samples with different ratios of 1–2, 1–4, and 1–6.

2.3. CH₄ Conversion Measurement and Product Analysis

In the measurement of the CH₄ conversion, the catalyst was loaded on carbon paper to form the anode, carbonate was the electrolyte and platinum foil was the counter electrode. The preparation and measurement processes are shown in the schematic diagram in Figure S7 (Supporting Information), and further details are provided in the Experimental Section. CH₄ conversion using the Co₃O₄ catalyst was performed to obtain more stable and useful higher alcohols. Therefore, a sealed reactor instrument is more suitable for the conversion and for tracking the CH₄ oxidation progress in real time. To determine the optimal potential for long reaction times, the difference in the current density of the optimized sample in Ar‐ and CH₄‐saturated electrolyte (see Figure S6a, Supporting Information) was calculated, and the curves are provided in Figure S8 (Supporting Information). Based on the curves, 2.0 V was selected as the suitable potential for CH₄ electrochemical oxidation to obtain a relatively high current density and low competition with water oxidation. The products were collected after reaction for 3, 6, and 12 h with vigorous stirring. I–t curves for 12 h measurement was shown in Figure S9 (Supporting Information). To identify the products, proton nuclear magnetic resonance (¹H‐NMR) spectroscopy was performed. Figure 4 a shows the ¹H‐NMR spectrum of the products obtained after 12 h of reaction. The main products were 1‐propanol and 2‐propanol.33, 34 However, by‐products, such as methanol, ethanol, acetaldehyde, and acetone, were also observed. The typical ¹H‐NMR peak of methanol is located at ≈3.3–3.5 ppm,34 which may overlap with that of 1‐propanol. The ¹H‐NMR peaks of ethanol appear at the same positions as the 2‐propanol peaks.34 However, the subpeak numbers of ethanol and 2‐propanol are different; thus, the product can be identified as 2‐propanol. However, small ethanol peaks with weak peak intensity may be obscured by the 2‐propanol peaks. The small peak at ≈2.2 ppm can be ascribed to acetaldehyde and acetone.33 Compared with the main products (1‐propanol and 2‐propanol), all the by‐products were observed in negligible quantities. In order to eliminate other influence factors, control experiment was conducted at the same condition for 12 h long‐term reaction except the presence of CH₄. The ¹H‐NMR result of the products from control experiment was shown in Figure S10a (Supporting Information). In addition, the pure carbonate electrolyte before reaction was also detected with ¹H‐NMR spectrum for comparison (Figure S10b, Supporting Information). To confirm the amount of CH₄ consumed and products generated, gas chromatography (GC) and GC‐mass spectrometry (MS) were performed. The amount of CH₄ remaining in the reactor after 3, 6, and 12 h of reaction is shown in Figure 4b. In addition, the reference line of the amount of CH₄, measured by GC, is shown in Figure S11 (Supporting Information). CH₄ gas was mostly consumed, and the amount decreased gradually with the reaction time. After 12 h of reaction, almost 40% of the CH₄ gas was converted. Meanwhile, the amount of various products measured by the GC‐MS system is shown in Table 1 .

Figure 4.

Product analysis and production efficiency. a) ¹H‐NMR spectrum of the products after 12 h. b) The amount of CH₄ remaining after electrochemical oxidation. c) Production efficiencies of the products of 1‐propanol, 2‐propanol, and acetaldehyde at different reaction times.

Table 1.

Concentrations of various products after the electrochemical oxidation of CH₄ for 3, 6, and 12 h

Time [h]	Methanol [μg mL−1]	Formaldehyde [μg m L−1]	Ethanol [μg mL−1]	Acetaldehyde [μg mL−1]	1‐Propanol [μg mL−1]	2‐Propanol [μg mL−1]	Acetone [μg mL−1]
3	29.95	0.88	0	261.50	0.50	19.53	0
6	33.15	1.11	0.49	170.34	56.51	101.74	3.82
12	33.71	1.10	35.21	153.42	1336.12	1315.56	14.16

As described in Table 1, seven products were detected: methanol, formaldehyde, ethanol, acetaldehyde, 1‐propanol, 2‐propanol, and acetone. The amount of methanol and formaldehyde (products containing one carbon atom) did not greatly change with reaction time, illustrating a balance between generation and consumption. Thus, methanol and formaldehyde are the primary products of CH₄ oxidization. As detailed in a previous study, formaldehyde should be the methanol oxidation product.23 After comparing the amount of ethanol and acetaldehyde (products containing two carbon atoms), acetaldehyde can be confirmed as the main product from the addition reaction of CH₄ and formaldehyde. Moreover, the amount of acetaldehyde decreased with the reaction time, which illustrates that acetaldehyde plays a pivotal role in the production of 1‐propanol and 2‐propanol, indicating that 1‐propanol and 2‐propanol were converted from acetaldehyde. Then, after 12 h of reaction, 1‐propanol, and 2‐propanol were the main stable products of CH₄ oxidation, which agrees with the ¹H‐NMR results. The conversion efficiencies for acetaldehyde, 1‐propanol and 2‐propanol were calculated and are shown in Figure 4c. After 12 h of reaction, the main products, 1‐propanol and 2‐propanol, showed total production efficiency of over 60%.

2.4. Reaction Mechanism Analysis

As observed in the results in Table 1, acetaldehyde was the key product. The reactions involved in the formation of acetaldehyde are shown below, according to previous investigations35

$${\mathrm{CH}}_4\hspace{0.17em}\stackrel{\mathrm{oxidant}}{\to }\hspace{0.17em}{\mathrm{CH}}_3\mathrm{OH}\hspace{0.17em}\stackrel{\mathrm{oxidant}}{\to }\hspace{0.17em}\mathrm{HCHO}$$ (3)

$${\mathrm{CH}}_4\hspace{0.17em}+\hspace{0.17em}{\mathrm{CH}}_3\mathrm{OH}\hspace{0.17em}\stackrel{\mathrm{oxidant}}{\to }\hspace{0.17em}{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\hspace{0.17em}\stackrel{\mathrm{oxidant}}{\to }\hspace{0.17em}{\mathrm{CH}}_3\mathrm{CHO}$$ (4)

$${\mathrm{CH}}_{\mathrm{4}}\hspace{0.17em}+\hspace{0.17em}\mathrm{HCHO}\hspace{0.17em}\stackrel{\mathrm{oxidant}}{\to }\hspace{0.17em}{\mathrm{CH}}_3\mathrm{CHO}$$ (5)

$${\mathrm{CH}}_3\mathrm{OH}\hspace{0.17em}+\hspace{0.17em}\mathrm{HCHO}\hspace{0.17em}\stackrel{{}^{\mathrm{dehydration}}}{\to }\hspace{0.17em}{\mathrm{CH}}_3\mathrm{CHO}$$ (6)

In the primary reaction involved in CH₄ oxidation, CH₄ was oxidized by the oxidant (carbonate in this work) to form CH₃OH, which was subsequently oxidized to HCHO. The production mechanism has been investigated in detail, and there are several ways to achieve the reaction.36 After that, several reactions can occur to generate acetaldehyde by employing the reactants CH₄, methanol, and formaldehyde. Thus, the generation and accumulation of acetaldehyde is rapid and large, which is in accordance with the results in Table 1. For comprehensive understanding of the reaction processes, theoretical potentials of several oxidation reactions related to the methane conversion were listed in Table S2 (Supporting Information).

The production of 1‐propanol and 2‐propanol from acetaldehyde is the most important reaction step in CH₄ conversion. The mechanism involves a type of addition reaction, as shown in Figure 5 . The formation of 2‐propanol is common and has been reported in previous work.23 As shown in Figure 5a, the methyl group in CH₄ acts as a nucleophilic reagent and attacks the carbonyl carbon in acetaldehyde. Then, a nucleophilic addition reaction occurs to form 2‐propanol as one of the main products in this CH₄ conversion reaction. However, considering the addition reaction mechanism, the formation of 1‐propanol from acetaldehyde and CH₄ is impossible. Therefore, we considered all the reaction conditions to determine a possible route for 1‐propanol production and found that the free radical addition reaction is suitable (Figure 5b1–b3). First, in route b1, a methyl radical is generated from CH₄ with the participation of Co₃O₄ and carbonate. A carbonate radical is generated through anodic oxidation with the help of Co₃O₄ due to the relatively low generation energy compared with that of the hydroxyl radical,37 which can be obtained from electrochemical oxidation processes.38 The carbonate radical acted as an intermediate to generate a methyl radical through reaction with CH₄. At the same time, in route b2, the as‐produced acetaldehyde equilibrates between its isomers, acetaldehyde, and vinyl alcohol. The vinyl alcohol configuration has a higher energy of 45 kJ mol⁻¹ than the acetaldehyde form but is reachable in the presence of carbonate.39, 40 In the normal electrophilic addition to alkenes, the products follow the Markovnikov rule,41 illustrating 2‐propanol as the main product when CH₄ reacts with vinyl alcohol. However, when the addition reaction is conducted through the free radical route, the products follow the anti‐Markovnikov rule, thereby producing 1‐propanol as the main product, as shown in route b3. When a methyl radical attacks carbon 1, the resulting 2‐propanol radical (free electron on carbon 2) is not the most stable state. However, when the methyl radical attacks carbon 2, the resulting 1‐propanol radical (free electron on carbon 1) is more stable than the 2‐propabol radical, illustrating 1‐propanol as the main product. 2‐Propanol can be directly converted from acetaldehyde and CH₄ through a nucleophilic addition reaction, leading to more 2‐propanol being produced than 1‐propanol at short oxidation times. However, after long reaction times, the amount of 1‐propanol exceeds that of 2‐propanol, even though 2‐propanol is more thermodynamically stable, illustrating the unique regional selectivity of 1‐propanol production through radical addition with the participation of a Co₃O₄ catalyst and carbonate electrolyte. In summary, after comprehensive analysis with the above content, the complete reaction pathways for electrochemical oxidation of methane with ZrO₂/Co₃O₄ anode and carbonate electrolyte were proposed, shown in Figure S12 (Supporting Information). The reaction process may help other researchers to have an overall understanding of the one carbon related reactions.

Figure 5.

Reaction mechanism analysis. a) Nucleophilic addition reaction of methane and acetaldehyde to form 2‐propanol. b₁–b₃) Free radical addition reaction of methane and acetaldehyde to form 1‐propoanl.

3. Conclusion

In summary, we designed a ZrO₂/Co₃O₄ nanocomposite that aids in the regional selective oxidation of CH₄ to 1‐propanol and 2‐propanol via an electrochemical method. We expected stable products, such as higher alcohols, to be formed due to the strong surface adsorption ability of Co₃O₄ and the participation of carbonate, which has a soft oxidizing ability, delivered by ZrO₂. After tracking the products for different reaction times, acetaldehyde was found to be the key intermediate. To convert acetaldehyde to 1‐propanol with the participation of CH₄, a free radical addition reaction was conducted by an electrochemical reaction, with Co₃O₄ as the catalyst and carbonate as the electrolyte. Finally, as a result of competition between reactions, both 1‐propanol and 2‐propanol were the main products. This electrochemical partial oxidation of CH₄ may aid in the synthesis of other oxygenates and long‐chain hydrocarbons.

4. Experimental Section

Fabrication of Electrocatalyst Materials: All reagents were used as received without further treatment. The ZrO₂/Co₃O₄ nanocomposite was synthesized using precipitation and a hydrothermal method. For the 1–2 ZrO₂/Co₃O₄ sample, 0.1611 g ZrOCl₂·8H₂O (99.0%, Junsei, Japan), 0.291 g Co(NO₃)₂·6H₂O (98%, Aldrich, US), and 9.6 g NaOH (96%, Samchun, Korea) were dissolved in 40 mL deionized (DI) water with vigorous stirring for 30 min. Then, the solution was transferred to a 60 mL autoclave container and heated at 180 °C for 24 h. After that, the powder was collected by centrifuge and washed with DI water 3 times. Finally, the 1–2 ZrO₂/Co₃O₄ sample was obtained after thermal annealing at 500 °C for 3 h. For the ZrO₂/Co₃O₄ nanocomposites with different ratios, the amount of Co(NO₃)₂·6H₂O was adjusted to 0.582 g for 1–4 ZrO₂/Co₃O₄ and 0.873 g for 1–6 ZrO₂/Co₃O₄, with no changes in the other conditions. For comparison, pure Co₃O₄ and ZrO₂/NiO samples were prepared by the same method as 1–4 ZrO₂/Co₃O₄, without the addition of ZrOCl₂·8H₂O and with the addition of 0.582 g Ni(NO₃)₂·6H₂O (97%, Aldrich, US) instead of Co(NO₃)₂·6H₂O, respectively.

Electrochemical Test: LSV and EIS tests for the comparison of ZrO₂/Co₃O₄ and ZrO₂/NiO samples were conducted in a three‐electrode system using a potentiostat (CH Instrument, CHI 660) with a glassy carbon electrode as the working electrode, Ag/AgCl as the reference electrode, a Pt foil as the counter electrode and 0.5 m Na₂CO₃ solution as the electrolyte. Meanwhile, LSV tests for ZrO₂/Co₃O₄ samples with different component ratio were conducted in a two‐electrode system without the utilization of Ag/AgCl reference electrode. All working electrodes were prepared by dispersing the powder samples in DI water in a concentration of 3 mg mL⁻¹ with vigorous stirring for 30 min and then dropping 20 µL of the dispersed solution on the surface of the glassy carbon electrode (area = 0.07 cm²), followed by drying at room temperature. Next, 10 µL of a 5% Nafion 117 solution (Aldrich) was deposited on the surface of the glassy carbon electrode to cover the sample films, followed by drying at room temperature. Ultrahigh‐purity argon gas (Ar, 99.999%) and methane gas (CH₄, 99.999%) were used. Before each electrochemical test, the electrolyte was bubbled with Ar or CH₄ for 1 h to prepare the Ar‐ or CH₄‐saturated electrolyte.

CH₄ Conversion Test: The long‐term electrochemical oxidation of CH₄ was conducted in a two‐electrode system with a closed reaction instrument, employing carbon paper (Alfa) as the working electrode, Pt foil as the counter electrode and 30 mL 0.5 m Na₂CO₃ solution (pH around 12.0 before reaction and about 11.9 after 12 h reaction) as the electrolyte. The working electrode was prepared by dispersing the powder sample in DI water in a concentration of 3 mg mL⁻¹ with vigorous stirring for 30 min and then dropping 5.7 mL of the dispersed solution on the surface of the carbon paper (area = 20 cm²), followed by drying at room temperature. Next, 3 mL of a 5% Nafion 117 solution was deposited to cover the sample film on the carbon paper, followed by drying at room temperature. Before the electrochemical reaction, the electrolyte was bubbled with CH₄ for 1.5 h to remove the oxygen and fill the space in the reaction instrument. In this case, after the consumption of saturated CH₄ in aqueous solution, the gas phase CH₄ could dissolve in the electrolyte continually, guaranteeing the adequate reactant. Electrochemical oxidation was conducted at 2.0 V versus Pt for 3, 6, or 12 h.

Characterization and Products Analysis: Morphology analyses of the samples were carried out using field‐emission scanning electron microscopy (FESEM, JSM‐7000F, Japan) and a JEOL JEM‐2100F (Japan) electron microscope. EDS spectra and ICP‐OES measurements were employed for the elements ratio detection. XRD measurements were conducted using a Siemens diffractometer D500/5000 in a Bragg–Brentano geometry. XPS data were obtained from a K‐alpha instrument (Thermo Scientific Inc., UK). ¹H‐NMR was conducted using an Avance III HD 400 FT‐NMR instrument (Bruker Biospin), where the sample was prepared by mixing 0.4 mL of the product solution with 0.2 mL D₂O. The amount of methane was determined using a 7890B GC instrument (Agilent Technologies). The product was examined using a 7890B‐5977A GC‐MS instrument (Agilent Technologies).

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary

Ma M., Jin B. J., Li P., Jung M. S., Kim J. I., Cho Y., Kim S., Moon J. H., Park J. H., Adv. Sci. 2017, 4, 1700379 https://doi.org/10.1002/advs.408