Synthesis of Cobalt-Based Nanoparticles as Catalysts for Methanol Synthesis from CO₂ Hydrogenation

1. Introduction

Nowadays, one of the main worldwide concerns is the slow but unstoppable rise in global average temperature, a direct cause of climate change that seems almost unavoidable. The high quantities of greenhouse gases emitted to the atmosphere, of which carbon dioxide emissions are the most important, predict an increase in the Earth’s temperature by 2040 of approximately 1.5 ºC compared to the data recorded at the end of the 19^th century [1]. Such predictions have alerted the scientific community to develop protocols to lower carbon dioxide emissions, which can be classified according to whether they are carbon capture and storage (CCS) or carbon capture and utilization (CCU) methods. On one hand, CCS methods consist of capturing and storing the gas, and it is so efficient that it could account for almost 20% of the carbon dioxide reduction. Nevertheless, it can result quite costly because industrial-scale installations have to be built. Unfortunately, fossil fuels are still needed as an energy source for this treatment, so carbon dioxide reduction will never be fully completed [2].

On the other hand, CCU results are remarkably interesting, since it does not only deals with the storage of carbon dioxide, but it takes advantage of this gas as a valuable carbon resource for chemical conversion into other products. This is a more viable strategy as it could not only keep the atmospheric concentration at acceptable levels, but also provides high added-value chemical/fuel products, such as methane and methanol [3,4]. Among all the technologies, carbon dioxide hydrogenation has been considered a promising alternative for obtaining some products from carbon dioxide. An attractive possibility is the production of methanol, a chemical compound that has a wide range of applications. For instance, it can be used as a solvent to obtain several chemicals (formaldehyde, acetic acid, etc.), as well as in the conversion process to olefins, which can be used to produce hydrocarbon fuels and their derivatives, which are currently obtained from petroleum [5,6].

The hydrogenation of carbon dioxide to methanol consists of two main competing reactions. The first is the synthesis of methanol from carbon dioxide and hydrogen:

$$C{O}_2+3H_2 \to C{H}_{3}OH+H_{2}O \mathbf{Equation} \mathbf{1} {∆H^0}_{298 K}=-49.5 kJ/mol$$

The second reaction is the reverse water-gas shift reaction (RWGS), which produces carbon monoxide:

$$C{O}_2+H_2\to {CO+H}_{2}O \mathbf{Equation} \mathbf{2} {∆H^0}_{298 K}=41.2 kJ/mol$$

Furthermore, catalytic hydrogenation of carbon dioxide into methanol can also occur indirectly from carbon monoxide formed through the RWGS reaction [7]:

$$CO+2H_2\to C{H}_{3}OH \mathbf{Equation} \mathbf{3} {∆H^0}_{298 K}=-90.6 kJ/mol$$

The synthesis of methanol is thermodynamically favoured at low temperatures and high pressures [8,9].

In recent years, bimetallic catalysts have been extensively investigated due to their chemical, electronic and structural properties, as well as the synergistic effects between metals leading to the catalytic system. Unlike monometallic systems, the bimetallic catalytic system has advantages in terms of selectivity, activity and stability [10]. The catalysts mainly chosen for carbon dioxide hydrogenation to methanol studies are Cu-Zn based systems, although other bimetallic catalysts such as Pd-Zn, Pd-Ga, Cu-Ni and Ni-Ga have been also investigated [7]. In addition to these mentioned catalysts, methanol has also been obtained using cobalt-based catalysts. Stangeland et al. [11] demonstrated that Co₃O₄/MnOx was efficient in the production of methanol at mild pressures, but a variety of by-products are also obtained. These authors achieved a 10-fold increase in methanol yield with Co₃O₄/MnOx catalysts compared to Cu/Zn based catalysts under similar conditions [11]. Wang et al. [12] investigated silica-supported cobalt catalysts with the aim of accelerating the selectivity of the hydrogenation of carbon dioxide to methanol, and they showed that the inclusion of silica in the cobalt catalysts improved both carbon dioxide conversion and selectivity towards methanol [12].

One of the key aspects of cobalt-based catalysts is their ability to catalyse various carbon dioxide conversion reactions, including methanation [13,14], synthesis of higher alcohols [15,16] or methanol synthesis [11,12,17,18]. In these investigations, it was shown that the selectivity of these catalysts can be influenced by the use of different supports. In the present study, different supports including zeolite, manganese oxide and cerium oxide have been investigated to increase the catalytic activity of cobalt-based catalysts. It is hypothesiezed that different metal-support interactions between cobalf nanoparticles and the support will enhance the catalytic sites, leading to higher methnaol yield and selectivity. Zeolite is a microporous aluminosilicate with a three-dimensional structure based on SiO₄ and AlO₄ and it is known for its catalytic and adsorption properties [19]. Cerium oxide is a non-toxic oxide, which is of great importance in catalysis, due to its ability to store and supply oxygen [20,21]. It provides a large number of oxygen vacancies on the surface which can fucntion as sites for adsorption and activation of carbon dioxide. Manganese oxide is a mesoporous support like zeolite and cerium oxide its efffect as support for nanocatalysts used for carbon dioxide hydrogenation to methanol has not been reported [11].

On the other hand, capping agents are of utmost importance as stabilisers since they counteract the attraction between nanoparticles, thus inhibiting their overgrowth and aggregation. These agents are amphipathic molecules that are characterized by having a polar head group and a non-polar tail, and due to this amphipathic property, they improve the compatibility with other phases. Different types of protection agents have been implemented in the synthesis of nanomaterials (surfactants, polymers, polysaccharides, etc.), aiming to obtain nanoparticles of smaller sizes, which will lead to the highest surface area of the nanoparticles [19,22]. In this study, the agglomeration of the synthesized nanoparticles was controlled by steric stabilisation using polyvinylpyrrolidone (PVP).

The present work is focused on studying the catalytic activity of supported and unsupported cobalt-based nanoparticels. The aim is to evaluate this type of nanomaterials in order to obtain high methanol production yields and high selectivity under mild pressure and temperature conditions.

2. Materials and Methods

2.1. Materials

Sodium borohydride (NaBH₄) (99.0%), cobalt (II) chloride hexahydrate (CoCl₂·6H₂O) (99.0%), polyvinylpyrrolidone (PVP), sodium carbonate (Na₂CO₃) (99.5%), zeolite, cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O (99.0%), sodium hydroxide (NaOH) (98.0%), manganese (II) sulphate monohydrate (MnSO₄·H₂O) (99.0%), and potassium permanganate (KMnO₄) (99.5%) were all purchased from Sigma-Aldrich (Barcelona, Spain). A mixed-gas bottle of carbon dioxide and hydrogen with a molar ratio of 1:3, respectively, was provided by Carburos Metálicos S.A. (Barcelona, Spain).

2.2. Synthesis of nanocomposites

2.2.1. Co nanoparticles synthesis

Co nanoparticles were synthesized using the chemical reduction method. A total of 1.10 g of cobalt (II) chloride hexahydrate was dissolved in a beaker with 100 mL of deionised water with magnetic stirring and nitrogen bubbling to avoid the oxidation of cobalt. After 15 min of stirring, the reducing agent, sodium borohydride, was added. A total of 0.96 g of sodium borohydride was dissolved in 100 mL of deionised water and then added dropwise with a Pasteur pipette. Once the reducing agent was added, the solution was left to stir for 20 min to ensure that all of the cobalt had been reduced [23]. Once the reaction was complete, cobalt nanoparticles were separated from the water using a magnet. The product was washed three times with deionised water to remove any impurities and dried in an oven at 105 ºC overnight. The synthesis of Co nanoparticles with PVP was performed with the same method, but 1.10 g of PVP was added to the cobalt (II) chloride hexahydrate solution.

2.2.2. Co₃O₄ nanoparticles synthesis

Co₃O₄ nanoparticles were synthesized using the co-precipitation method. Briefly, in 100 mL of deionised water, 0.61 g of cobalt (II) chloride hexahydrate was dissolved using magnetic stirring for 20 min and in another 100 mL of deionised water was dissolved the precipitation agent, 0.52 g of sodium carbonate at a concentration of 1 M. The synthesis was performed in a 500 mL Scharlau Minireactor HME-R/500 with mechanical stirring and heating. The 100 mL cobalt (II) chloride hexahydrate solution in deionised water was added to the reactor. Then, the NaOH solution was added at a flow rate of 5 mL/min using a peristaltic pump (Watson Marlow SCI 400), and the reaction was carried out at 60 ºC for 5 h with constant stirring at 120 rpm. The product was centrifuged (Beckman Avanti J-20 Centrifuge) three times for 15 min at 5000 rpm to remove any impurities and dried in an oven at 105º C overnight [23]. The synthesis of Co₃O₄ with PVP was performed with the same method, but 0.62 g of PVP was added to the cobalt (II) chloride hexahydrate solution.

2.2.3. MnO₂ nanoparticles synthesis

MnO₂ nanoparticles were synthesized using the co-precipitation method. In 100 mL of deionised water, 3.12 g of manganese (II) sulphate monohydrate were dissolved and in another 100 mL of deionised water 2.18 g of potassium permanganate were dissolved. The potassium permanganate dissolution was added to the above dissolution dropwise, and the mixture was vigorously stirred in a reactor Scharlau Minireactor HME-R/500 (Barcelona, Spain) at 80 ºC for 5 hours. Then, a sodium hydroxide solution was added dropwise to adjust the pH to 11. Once the reaction was completed, the nanoparticles were centrifuged and washed three times with deionised water. Finally, the nanoparticles obtained were dried at 105 ºC for 12 hours [24].

2.2.4. Co₃O₄/Zeolite nanocomposite synthesis

To immobilize the Co₃O₄ nanoparticles onto zeolite the same Co₃O₄ synthesis described above was followed. In 200 mL of ultrapure water, 1 g of zeolite was dispersed in an ultrasound bath for 15 minutes. Once zeolite was dispersed, it was transferred to the reactor, and the same Co₃O₄ synthesis procedure was performed.

2.2.5. CeO₂ nanoparticle synthesis

CeO₂ nanoparticles were synthesized using the co-precipitation method from an aqueous solution of cerium (III) nitrate hexahydrate. In 100 mL of deionised water, 5.12 g of cerium (III) nitrate hexahydrate were dissolved, and in another 100 mL of deionised water 1.88 g of the precipitant agent, sodium hydroxide, were dissolved. The synthesis was carried out in a reactor (Scharlau Minireactor HME-R/500) with mechanical agitation. The precursor solution and 100 mL of deionised water was added to the reactor, and then, by a peristaltic pump, the sodium hydroxide solution was added with a 7mL/min flow. Once the entire precipitating agent was added, the dissolution was left agitating for 15 minutes. Eventually, the precipitates obtained were leaked, washed with deionised water three times, and then dried at 105 ºC overnight [25].

2.2.6. Co₃O₄ /CeO₂ nanocomposite synthesis

To immobilize the Co₃O₄ nanoparticles onto CeO₂, the same Co₃O₄ synthesis procedure was followed. In 200 mL of deionised water, the nanoparticles of CeO₂ were scattered in an ultrasound bath for 15 minutes. Once dispersed, the same Co₃O₄ synthesis procedure was carried out, but first the solution of the CeO₂ nanoparticles was transferred to the reactor.

2.2.7. Co₃O₄ /MnO₂ nanocomposite synthesis

Co₃O₄ nanocomposites were synthesized using the co-precipitation method. The same Co₃O₄ synthesis procedure followed, but first the previously synthesized MnO₂ nanoparticles were dispersed in 200 mL of deionised water in an ultrasound bath for 15 minutes. Subsequently, the same Co₃O₄ synthesis was followed after the dissolution of MnO₂ was added to the reactor.

2.3. Characterization of catalysts

X-ray diffraction (XRD) was used to perform a structural analysis of the nanoparticles and their crystallographic structure. The X-ray diffraction patterns were recorded on a diffractometer (PANalytical X’Pert) using Cu-Kα radiation. The measurements were carried out at room temperature in a range of 10.0º - 80.0º on 2θ with a step size of 0.026º. All XRD results were analysed with the simulation of the nanoparticles obtained using the X’Pert High Score (PANalytical) software. A scanning electron microscopy (SEM) (FEI Quanta 650F ESEM) equipped with an energy-dispersive spectroscopy (EDS) source was used to determine the morphology, size distribution and composition of the nanoparticles. Samples were prepared on copper and graphite grids (TED PELLA, Inc., Redding, CA, USA).

2.4. Catalytic activity test

The catalytic experiments were carried out in a stainless-steel-packed bed reactor with an internal diameter of 5.25 mm, a height of 8.90 cm, and, thus, a volume of 1.92 cm³. All samples were calcinated at 500 °C for 4 h and and those containing elemental copper were reduced by a hydrogen flow of 40 mL/min at 350 °C for 2h. A certain amount of catalyst was added and a small amount of glass wool was used at each end of the reactor to avoid possible leakage of the catalysts. In order to investigate the impact of pressure, the work was carried out under two moderate pressure values: 10 and 15 bar. The flow rate of the stoichiometric H₂/CO₂ mixture was 10 mL/min, and the effect of temperature on the catalytic activity of the catalyts was tested by changing the temperature ranging from 180 to 280 °C. The gas samples were collected using sampling bags (SKC FlexFoil PLUS Sample Bag), and the obtained methanol was analysed in a gas chromatograph (Shimadzu GC-2010) with a flame ionisation detector (FID) using helium as carrier gas. The software used was Chromeleon, the inlet temperature was 260 °C, and the flow was 50 mL/min; the detector temperature was 280 °C. An Agilent 7890B GC System chromatograph served to detect carbon monoxide and dioxide by using a thermal conductivity detector (TCD) with helium as the carrier gas. The software used was OpenLab, the inlet temperature was 120 °C, the inlet flow was 20 mL/min and the detector temperature was 150 °C. To study the catalytic activity, methanol space–time yield (STY) indicating the mass of methanol obtained per mass of catalyst per hour as well as methanol selectivity being the percentage of carbon dioxide that was converted to methanol rather than other products (in this case carbon monoxide) were caclulated accrding to the following equations.

$${CH}_{3}OH STY \left(\frac{g}{k{g}_{cat}\times h}\right)=\left(\frac{Mass of methanol \left(g\right) formed}{W_{cat}\left(kg\right)\times Hour}\right)$$

(1)

$${CH}_{3}OH Selectivity \left(\%\right)=\left(\frac{ moles of methanol formed }{n{\left[{CO}_2\right]}_{in }-n{\left[{CO}_2\right]}_{out }}\right)\times 100$$

(2)

3. Results and Discussion

3.1. Structural and morphological characterization of nanomaterials

The XRD patterns obtained for cobalt and cobalt oxide with and without capping aganet (PVP) and their corresponding simulations using PANALYTICAL X’Pert High Score software are presented in Figure 1. As it is shown, the peaks at diffraction angles of 2θ of 19.05º, 31.27º, 36.90º, 38.56º, 44.82º, 55.70º, 59.40º, 65.30º, 74.14º and 77.14º correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (620) and (533) planes of Co₃O₄ [11,23,26]. Furthermore, the peaks at 2θ of 44.23º (111), 51.52º (200) and 75.85º (220) corresponed to Co nanoparticles [23]. Regarding the Co₃O₄ and Co with and without PVP, it was observed that PVP did not interfere with the crystallinity of cobalt and cobalt oxide nanoparticles. As can be seen, there is no presence of cobalt oxide in the cobalt XRD patterns, indicating that the synthesis of cobalt nanoparticels was satisfactory and all the nanoparticles were completely reduced.

The X-ray diffraction patterns of all the supports are shown in Figure 2. Regarding the first support, CeO₂, it is observed that the peaks obtained in the sample synthesized and the simulation are identical. The same phenomenon occurs with zeolite. However, not all the peaks observed in the MnO₂ simulation correspond to the peaks observed in the synthesized sample, which indicates that this sample is not completely crystalline.

The X-ray diffraction patterns of the bimetallic nanoparticles and their supports are shown in Figure 3. Regarding the highest peaks of Co₃O₄/zeolite, it is observed that the first peaks are associated to zeolite since the X’Pert High Score software determined that they correspond to the two typical elements of zeolite (aluminum and silicon). Small peaks corresponding to Co₃O₄ were also observed at the following angles: 19.05º, 31.27º, 36.90º, 38.56º, 44.82º, 55.70º , 59.40º and 65.30º. Regarding the XRD patterns of Co₃O₄ /CeO₂, it is observed that the peaks situated at the angles 28.50º, 33.10º, 47.50º, 56.30º, 69.40º, 76.70º and 79.10º belong to CeO₂. They are associated with planes (111), (200), (220), (311), (400), (331) and (420), respectively (Figure 3) [27]. Moreover, other characteristic peaks of Co₃O₄ are seen at angles 19.05º, 31.27º, 36.90º, 38.56º, 44.82º and 65.30º (Figure 3) [28]. Finally, in the Co₃O₄/MnO₂ patterns, the peaks of both materials can also be visualised (Figure 3). The angles 24.50º, 41.60º, 50.30º, 54.70º, 63.70º, 72.30º and 79.19º correspond to Co/MnO₂, and are related to the (110), (120), (220), (231), (130), (343) and (330) planes (Figure 3) [29]. Nevertheless, the other profiles observed correspond to the cobalt oxides present in the sample, since they coincide with the Co₃O₄ angles described above. This means that there is a coexistence of elemental cobalt and its oxidized species in the sample.

The characterization and morphology study of the materials was performed using a scanning electron microscope (SEM). Figure 4C shows the SEM images of the Co₃O₄ nanoparticles, which presented an irregular shape with a high degree of agglomeration, with average sizes of 36-64 nm, as previously reported [23]. Nevertheless, when the PVP was added, the nanoparticles were smaller (19 - 25 nm) with a more spherical geometry (Figure 4D). It is reported in literature that by adding a capping agent, the nanoparticles are smaller and this effect leads to an increase in the surface area of the catalyst, which means that the active sites are more exposed. Therefore, the catalytic activity of the catalyst increases [19,22]. The morphology of cobalt nanoparticles with and without PVP is shown in Figure 4A and Figure 4B, respectively. The same effect is observed when PVP was added to the synthesized material: the morphology obtained is well defined and the particles are smaller.

SEM images of nanoparticles embedded in a support are presented in Figure 5. Co₃O₄ nanoparticles immobilised on zeolite show a granular morphology with an average diameter of 24-30 nm (Figure 5A). It is shown that the use of support helps to obtain nanoparticles with better distribution and a higher surface area. In Figure 5C, Co₃O₄ nanoparticles supported on CeO₂ can be observed, demonstrating that the nanoparticles have a very small size and better distribution compared to Co₃O₄ nanoparticles without support (Figure 4C) [28]. Co₃O₄ nanoparticles embedded in MnO₂ show an aggregated morphology, as previously reported in literature (Figure 5B) [24].

The composition of the chemical elements present in the catalysts was determined by energy dispersive spectrometry (EDX). This analysis was performed on the nanoparticles immobilised on the three tested different supports. As observed with XRD, the EDX spectrum of Co₃O₄/Zeolite confirms the presence of expected chemical elements (Co, Al, Si) (Table 3) [30]. Nevertheless, a very small presence of sodium from the reducing agent used during the synthesis was also detected, indicating that the material could have been washed more times. Co and Ce were detected in the Co₃O₄/CeO₂ sample, as expected (Table 2). In the Co₃O₄/MnO₂ sample (Table 1), the presence of Co and Mn elements was detected, as well as a very small amount of sodium and potassium.

3.2. Catalytic activity of the catalysts

To study the catalytic activity of the samples, methanol STY and selectivity were obtained. Figure 6 presents methanol STY for cobalt and its oxide as a function of operating temperature at pressures of 10 and 15 bar, respectively. The effect of reaction temperature was also investigated and as can be seen for most of the catalysts methnaol STY increases gradually as the temperature rises. However, for Co₃O₄ catalyst, a decrease in STY was observed at 280 ºC compared to 260 ºC, which is not in accord to the behaviour of other catalysts. This can attributed to the formation of more carbon monoxide at high temperatures due to the endothermic nature of reverse water gas shoft reaction as the competeing reaction for methnaol syntheis as presented before. The influence of pressure was also investigated, and the results showed that a better catalytic activity is obtained at 15 bar, as the use of high pressures is advantageous due to the exothermic nature of the reaction [8,11].

Furthemore. the catalytic activity of cobalt and cobalt oxide was compared in terms of STY and selectivity to understand which cobalt species presents the active sites more favorable for methnaol synthesis. As can be observed, cobalt nanoparticles give methnaol STY of 3.2 g.kg_catalyst^-1h^-1, while only 0.25 g.kg_catalyst^-1h^-1 was obtained for cobalt oxide nanoparticles. Hence, metalic cobalt posseses active sites catalyzing the methnaol syntheis from carbon dioxide hydrogenation reaction more efficiently. Using elemental Co, a higher STY was obtained if compared to Co₃O₄ catalyst, since the selectivity of the reaction is much more favoured for methanol formation than for carbon monoxide production when this material is used instead of Co₃O₄, because the latter material is highly selective towards methane and carbon monoxide formation [31,32]. In many studies, this effect has been attributed to a lower carbon dioxide adsorption on the cobalt surface compared to Co₃O₄, which favours the formation of other products such as methane when using the oxide [33]. However, the catalyst selected for the study was cobalt oxide due to its high stability compared to elemental cobalt [35].

Two ways to improve the catalytic activity working with cobalt oxide nanoparticles were studied: (a) the addition of polyvinylpyrrolidone (PVP) to control the size of the nanoparticles, and (b) the addition of a support to improve the synergistic effect between the nanoparticles and the support. A comparison of the catalysts performance with and without PVP was carried out to analyse the effect of this polymer on the materials. By adding PVP in the synthesis of Co₃O₄, higher methanol STY was obtained which is accounted for by the smaller nanoparticles of Co₃O₄ with a more spherical morphology (Figure 4C and Figure 4D), leading to higher specific surface area and therefore more availability of the catalytic sites.

To study the effect of supports on the catalytic activity of Co₃O₄ nanoparticels, thye were immobilised on different supports inculding zeolite, CeO₂ and MnO₂. As seen in Figure 7, more methanol STY was obtained when immobilising Co₃O₄ on CeO₂. This can be attributed to the metal-support interaction between Co₃O₄ and CeO₂ generating interfacial sites which can synergystically catalyze methnaol synthesis reaction. In addition, oxygen vacancies present in CeO₂ are also assumedt to facilitate the adsorption and activation of carbon dioxide [20,21,34,35,36]. Other support studied was zeolite, which resulted in an improvement in methanol production since the presence of aluminum atoms in these silicate-based materials introduces negative charges that are compensated by exchangeable cations in the pore space, and these porous characteristics in the zeolite structure are those that allow greater carbon dioxide adsorption capacity [19,37]. Co₃O₄ immobilised on MnO₂ also resulted in more methanol STY compared to Co₃O₄. This can be attributed to the synergistic effects of the two materials as the catalytic activities of the individual materials are lower and less selective for methanol (Figure 6), indicating the importance of the architecture and nature of the interface [11]. Comparing the methanol STY of the three supports studied, CeO₂ performed the best as the support which can be due to the oxygen vacancies promoting carbon dioxide adsorption and activation as well as the generation of the interfacial sites betweeen CeO₂ and Co₃O₄ being more favorable for the methnaol syntheis reaction.

Methanol selectivity for the catalyst samples is presented in Figure 9. As can bee seen, at 180 ºC and 15 bar, the methanol selectivity for all samples was 100%, indicating that no carbon monoxide was formed at this temperature. However, as the temperature increases, the selectivity decreases as the enthalpy of methanol synthesis is negative, and therefore it is an exothermic reaction, which is more favoured at lower temperatures (Figure 9) [9,38]. In case of Co₃O₄ with PVP and Co₃O₄/zeolite, a decrease in selectivity towards methanol was only observed from 220 ºC. As for the other remaining catalysts, this methanol selectivity decreased considerably because of the production of carbon monoxide at high temperatrures (Figure 9) [11,39].

4. Conclusions

In this work, a series of cobalt-based catalysts with different supports were synthesised and their catalytic activity for methanol syntheis from carbon dioxide hydrogenation was studied. XRD analysis detected the crystalline phase of the samples and confirmed the integration of Co₃O₄ and Co on the supports studied (zeolite, cerium oxide and manganese oxide). Moreover, it was demonstrated that the addition of PVP as a stabilising agent improves the catalytic capacity of Co₃O₄, as it helps to obtain more homogeneous and smaller nanoparticles. However, for the Co catalyst synthesized using PVP, this effect was not observed as a result of partial oxidation of the material due to the presence of the stabilizing agent. Comparing the effect of the supports studied, the performance of CeO₂ as support was more promisong which was accounted for by the presencec of oxygen vacancies in CeO₂ which promot carbon dioxide adsorption and activation. In addition, the higher catalytic activity of the catalyst supported by CeO₂ can be attributed to the genearation of favorbale interfacial sites betweeen CeO₂ and Co₃O₄ for the methnaol syntheis reaction.