The Flower-like Co₃O₄ Hierarchical Microspheres for Methane Catalytic Oxidation

Abstract

The development of non-noble Co₃O₄ catalysts exposing highly active crystal planes to low-temperature methane oxidation is still a challenge. Hence, a facile solvothermal method was adapted to construe flower-like Co₃O₄ hierarchical microspheres (Co₃O₄-FL), which are composed of nanosheets with dominantly exposed {112} crystal planes. The flower-like hierarchical structure not only promotes the desorption of high levels of active surface oxygen and enhances reducibility, but also facilitates an increase in lattice oxygen as the active species. As a result, Co₃O₄-FL catalysts offer improved methane oxidation, with a half methane conversion temperature (T₅₀) of 380 °C (21,000 mL g⁻¹ h⁻¹), which is much lower than that of commercial Co₃O₄ catalysts (Co₃O₄-C). This study will provide guidance for non-noble metal catalyst design and preparation for methane oxidation and other oxidative reactions.

1. Introduction

The emission of volatile organic compounds (VOCs) is a global environmental issue related to atmospheric pollution and is harmful to human health and the environment due to toxic and/or carcinogenic smog and greenhouse gasses. Thus, it is necessary to develop various methods to eliminate VOCs, including incineration, catalytic removal, adsorption, absorption, condensation, and biofiltration, among which catalytic oxidation is regarded as the most efficient process, and it is especially effective in addressing low concentrations of VOCs [1,2].

Methane, with ultralow concentrations in air, is a highly chemically stable compound, with the highest C–H bond energy of ~435 kJ mol⁻¹ in its hydrocarbons, and it has 28~36 times the greenhouse effect of carbon dioxide [3]. Therefore, designed catalysts should be capable of the catalytic oxidation of methane, with a high activity, low catalytic temperature and excellent selectivity for carbon dioxide. Before now, the catalysts for methane have been divisible into noble metal and transition metal oxides. The former achieves high catalytic performance at low temperatures; however, they are expensive, and prone to deactivation. The latter have high potential activity, low costs and thermal stability [4,5].

Among the transition metal oxide catalysts, Co₃O₄ has attracted wide attention for the catalytic oxidation of VOCs (methane, toluene, n-hexanal), given its different potential morphologies, its spinel structure with strong Co³⁺/Co²⁺ redox properties, and its unique exposed crystal planes [6,7,8,9,10,11].

Studies have shown that Co₃O₄ nanosheets exhibit high catalytic activity, despite their lower special surface area compared with Co₃O₄ nanobelts and nanocubes [12]. The main reason for the different capacity for catalytic oxidation shown by methane is that the {112} exposed planes of nanosheets are more reactive than the exposed planes in the other morphologies, due to the low energy required for breaking the C–H bond [13]. Meanwhile, Co₃O₄ nanotubes also present better catalytic activity than Co₃O₄ nanorods and nanoparticles during methane oxidation, which is not only related to the presence of {112} exposed planes, but also its open structure [14]. Nevertheless, using lattice oxygen as the active species and the presence of defects should induce the improvement of catalytic activity [15].

The objective of this work is to study the preparation of flower-like Co₃O₄ (FL) hierarchical microspheres stacked with mass nanosheets via a simple solvothermal method and to examine its catalytic activity for methane oxidation. The as-synthesized Co₃O₄-FL was characterized by N₂ physisorption, X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR) and O₂ temperature-programmed desorption (O₂-TPD) in order to investigate the physical-chemical properties. The enhanced catalytic performance can be attributed to the highly exposed {112} planes of the nanosheets, together with the active lattice oxygen and derivative oxygen vacancies on their surfaces.

2. Experimental

2.1. Co₃O₄-FL Preparation

All reagents were purchased from Aladdin Co., Ltd. (Shanghai, China) and used without further purification. In a typical procedure, 2 mmol CoCl₂ and 0.01 g polyvinyl alcohol (PVA) was dissolved with stirring for 30 min in a 30 mL mixture of deionized water and ethylene glycol, with a volume ratio of 2:1, to form a homogeneous solution, and then the solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C for 24 h. After cooling to room temperature, the precipitate was then collected by centrifugation and washed with deionized water and ethanol several times, then dried in an oven at 60 °C overnight to obtain the Co(OH)₂ precursor. Co₃O₄-FL was obtained by calcining the as-prepared precursor at 450 °C for 2 h in air. The commercial Co₃O₄ (Co₃O₄-C) was made from agglomerate nanoparticles with grain sizes of 30 nm on a micro scale, which were provided for comparison by Aladdin Co., Ltd. (Shanghai, China). In a typical procedure, the precipitate of the Co(OH)₂ precursor collected after the solvothermal process is approximately 0.15 g. The mass of the final Co₃O₄-FL after calcining is about 0.13 g. Both the Co₃O₄-FL and Co₃O₄-C were utilized directly as catalysts for methane oxidation without further treatment.

2.2. Characterizations

The X-ray diffraction (XRD) pattern of the catalyst was determined on a Bruker D8 diffractometer (Billerica, MA, USA) with Cu Kα radiation (λ = 0.154184 nm). Scanning electron microscopy (SEM) was performed on an FEI Inspect F50 microscope (Hillsboro, OR, USA), while transmission electron microscopy (TEM) was performed on an FEI Tecnai F30 microscope (Hillsboro, OR, USA). The specific surface areas were measured at liquid nitrogen temperature using a ASAP2020 Micromeritics instrument (Norcross, GA, USA). Specific surface areas of the samples were calculated using the BET equation. The hydrogen temperature-programmed reduction (H₂-TPR) was performed on a Quantachrome Chembet Pulsar (Boynton Beach, FL, USA) using 10 vol.% H₂/Ar (50 mL∙min⁻¹) as the reducing gas. The reduction temperature increased from 50 to 600 °C at a rate of 10 °C∙min⁻¹. The oxygen temperature-programmed desorption (O₂-TPD) experiment was performed on a Quantachrome Chembet Pulsar (Boynton Beach, FL, USA). Firstly, 100 mg of catalyst was pretreated in a 3 vol.% O₂/He flow at 450 °C for 30 min. After cooling to 50 °C under the same oxidative atmosphere, the catalyst was purged by a stream of purified He (30 mL∙min⁻¹). Then, the reactor was heated up to 600 °C at an increasing rate of 10 °C∙min⁻¹. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Fisher ESCALAB 250Xi spectrometer (Waltham, MA, USA) using an Al Kα (1486.6 eV) radiation source. The binding energy of the C 1s electron (284.6 eV) was used to calibrate the spectra.

2.3. Catalytic Tests

The methane oxidation reaction was performed on a fixed bed reactor operated with 0.1 g of catalysts at atmospheric pressure. The reactant gas consisted of 2 vol.% of CH₄, 20 vol.% of O₂ and 78 vol.% of Ar passed through the catalysts with a gas flow rate of 35 mL min⁻¹, which corresponds to a weight hourly space velocity (WHSV) of 21,000 mL∙g⁻¹∙h⁻¹. The reaction temperature was raised from 50 to 550 °C at a rate of 2 °C∙min⁻¹. The reactant and products were analyzed online with a gas chromatograph (Shimadzu, GC-2014, Kyoto, Japan) equipped with a flame ionization detector (FID) and a chromatographic column (GDX 502, 2 m × 3 mm). The conversion of methane was determined every 30 °C with the following equation:

CH₄ conversion (%) = (1 − [CH₄]_out/[CH₄]_in) × 100

(1)

3. Results and Discussion

3.1. Catalytic Characterization

The XRD patterns of the as-prepared precursors Co(OH)₂, Co₃O₄-FL, and Co₃O₄-C are presented in Figure 1. The diffraction peaks of Co₃O₄-FL observed at 19.0, 31.3, 36.9, 38.5, 44.8, 55.7, 59.3 and 65.2° can be assigned to the (111), (220), (311), (222), (400), (422), (511) and (440) lattice planes of the Co₃O₄ phase (JCPDS NO.43-1003), respectively. The Co₃O₄-FL was prepared from the as-prepared Co(OH)₂ precursor, which was formed through the hydrothermal method from CoCl₂ and PVA. All the diffraction peaks can be assigned to Co(OH)₂, in agreement with the standard diffraction file (JCPDS No. 30-0443). After calcination, no other peaks suggesting impurities were found in the XRD pattern of the Co₃O₄-FL, which indicates that the Co(OH)₂ precursor was completely transitioned into Co₃O₄ without any impurities. Co₃O₄-C was also identified without any impurities by XRD analysis. Nevertheless, there are obvious differences in the specific surface areas of Co₃O₄-FL and Co₃O₄-C catalysts, which were 22 and 12 m²·g⁻¹, respectively.

Figure 2 shows the SEM and high-resolution (HR)TEM images of the Co₃O₄-FL and Co₃O₄-C. Figure 2a shows that most Co₃O₄-FLs are monodispersed microspheres with a diameter size of 10–15 μm. Closer observations show that these hierarchical microspheres are composed of large numbers of nanosheets with smooth surfaces (Figure 2b,c). The HRTEM image of the Co₃O₄-FL is presented in Figure 2d. The dominant exposed planes are {112}, which are the only planes displayed in both the set of (220) planes with a lattice space of 0.288 nm and the set of (311) planes with a crossing lattice space of 0.245 nm [16]. Co₃O₄-C, on the other hand, has no definite shape, and just agglomerates nanoparticles (Figure 2e,f).

The H₂-TPR profiles show two reduction peaks on both the Co₃O₄-FL and the Co₃O₄-C catalysts (Figure 3). This figure shows overlapping peaks at 385 and 435 °C, observed on Co₃O₄-C, which are assigned to the reduction of Co³⁺ into Co⁰. However, there are two peaks in the Co₃O₄-FL catalyst at 342 and 406 °C, indicating the successive reduction behavior of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰, in two reduction steps. Additionally, the temperature of the reduction peaks over Co₃O₄-FL is lower than that over Co₃O₄-C, which could imply that the Co₃O₄-FL composed of nanosheets presents the better reducibility, whereby the more susceptible to reduction an oxide is, the more easily it can generate oxygen vacancies [17].

The O₂-TPD of catalysts was carried out to evaluate the mobility of oxygen species, as shown in Figure 4. Generally, there are four oxygen species on the surfaces of metal oxides, with the release order: O₂ (molecular oxygen) → O²⁻ (superoxide anion) → O⁻ (oxygen anion) → O²⁻ (lattice oxygen). The physically adsorbed O₂ species can be desorbed below 200 °C. The O²⁻ and O⁻ species are chemically adsorbed oxygen, which can be liberated in the range of 200–400 °C. The rest of the O²⁻ species are attributed to surface and/or lattice oxygen released above 400 °C [18]. It seems that the Co₃O₄-FL catalysts present desorption peaks at 206 and 345 °C, which can be attributed to the desorption of O₂⁻ and O⁻ species, respectively, whereas another peak is located at 415 °C, which can be assigned to the desorption of O²⁻ species. The desorption temperature of the Co₃O₄-C catalysts shifts from 206 to 257 °C, and from 415 to 482 °C, respectively, indicating that the desorption of O₂⁻, O⁻, and O²⁻ species was facilitated, implying an enhancement in the catalytic activity of Co₃O₄-FL through a suprafacial mechanism.

The spectra of Co 2p and O 1s are displayed in Figure 5a,b, illustrating the chemical surface compositions and valence states. The BE value of Co 2p_3/2 is 779–780 eV, and a 2p_1/2 splitting of 15 eV is characteristic of the octahedral Co³⁺ component of Co₃O₄ [19]. The satellite peaks at 785 eV evidence the existence of Co²⁺ species in the octahedral sites [20,21]. Here, the Co 2p spectra were resolved using a fitting procedure partially based on that suggested by Biesinger et al. [22]. The contributions of the Co³⁺ and Co²⁺ cations can be identified at 779.5 and 781.1 eV, respectively [23]. Additionally, two satellite peaks, S₁ and S₂, at 785.3 and 789.4 eV, appeared due to electron correlations and the final state effects in the Co²⁺ and Co³⁺ cations, respectively [24]. Meanwhile, the satellite peak S₃ indicates the spin orbit contributions of Co 2p_1/2, as do the satellites S₁ and S₂ [25,26]. The O 1s spectra in Figure 5b are also decomposed into two peaks. The peaks located at 529.7 and 531.2 eV can be ascribed to lattice oxygen (O_latt, i.e., O^2-) and oxygen adsorbed onto the surface oxygen vacancies (O_sur, i.e., O⁻, O₂²⁻, and OH⁻), respectively [27].

Based on the quantitative analysis, the Co²⁺/Co³⁺ molar ratio of the two catalysts was Co₃O₄-FL (1.46) > Co₃O₄-C (1.31). The higher molar ratio of Co²⁺/Co³⁺ than Co₃O₄-FL indicates that a higher abundance of Co²⁺ cations was presented on the surface of Co₃O₄-FL, which could manifest an increased redox activity for methane oxidation [28]. This trend is in good agreement with that of low-temperature reducibility. According to the quantitative analysis of the O 1s spectra, the O_sur/O_latt malor ratio of Co₃O₄-FL (0.50) is similar to that of Co₃O₄-C (0.51), whereas Co₃O₄-FL can provide other active sites for the surface oxygen species and boost the catalytic activity.

3.2. Methane Catalytic Oxidation

The effect of methane oxidation on Co₃O₄-FL and Co₃O₄-C catalysts is depicted in Figure 6. It seems that the activity is influenced by the different morphologies. The temperatures for the 50% conversion of methane (T₅₀) and 90% conversion of methane (T₉₀) in Co₃O₄-FL are 380 and 430 °C, respectively, which are much lower than these temperature values for Co₃O₄-C at the same conversion rate. This can be explained by the relatively high specific surface area, the greater number of active surface oxygen species, and the higher reducibility of highly exposed Co₃O₄-FL. Interestingly, Co₃O₄-FL showed nearly identical T₅₀ and T₉₀ values in the first three recycles, indicating the robust catalytic stability of Co₃O₄-FL. Meanwhile, other thermal stability tests were carried out at T₅₀, T₈₀ and T₁₀₀ under the same catalytic conditions (Figure 7). The values of T₅₀, T₈₀ and T₁₀₀ within the 20 h tests were 380, 415, and 520 °C, respectively, without variation. This result confirms that Co₃O₄-FL has excellent thermal stability, suggesting its great potential practical applicability.

Based on the previous characterizations and test results, the catalytic methane oxidation of Co₃O₄-FL can be enacted via a suprafacial mechanism, whereby the dissociatively adsorbed surface oxygen and the surface lattice oxygen act as the reaction active species. Besides this, the role of the reactive {112} plane in the Co₃O₄ nanosheets, and the oxygen vacancies derived from the mobility of lattice oxygen after the surface oxygen is consumed during methane oxidation, appear to be necessary.

4. Conclusions

In this study, flower-like Co₃O₄ hierarchical microspheres composed of nanosheets were prepared via a solvothermal method for methane oxidation. The dominantly exposed {112} crystal planes, together with the desorption of higher levels of active surface oxygen and the active species of lattice oxygen, lead to the presence of more active sites for C–H bond-breaking, which boosted the methane oxidation activity to a T₉₀ of 430 °C at 21,000 mL g⁻¹ h⁻¹. These results provide guidance for the design and preparation of non-noble metal catalysts for methane oxidation and other oxidative reactions.