Template-free Synthesis of Stable Cobalt Manganese Spinel Hollow Nanostructured Catalysts for Highly Water-Resistant CO Oxidation.

Development of spinel oxides as low-cost and high-efficiency catalysts is highly desirable; however, rational synthesis of efficient and stable spinel systems with precisely controlled structure and components remains challenging. We demonstrate the design of complex nanostructured cobalt-based bimetallic spinel catalysts for low-temperature CO oxidation by a simple template-free method. The self-assembled multi-shelled mesoporous spinel nanostructures provide high surface area (203.5 m2/g) and favorable unique surface chemistry for producing abundant active sites and lead to the creation of robust microsphere configured by 16-nm spinel nanosheets, which achieve satisfactory water-resisting property and catalytic activity. Theoretical models show that O vacancies at exposed {110} facets in cubic spinel phase guarantee the strong adsorption of reactive oxygen species on the surface of catalysts and play a key role in the prevention of deactivation under moisture-rich conditions. The design concept with architecture and composition control can be extended to other mixed transition metal oxide compositions.

Introduction

Transition metal and mixed transition metal oxides are among the immensely important compounds in the world and have offered significant application values in extensive fields like catalysis, energy storage, electronics, sensors, and magnetism (Poizot et al., 2000, de la Cruz et al., 2008, Wu et al., 2018). As one of the most intriguing groups of reducible transition metal oxides, which contain trivalent and bivalent cations in the lattice structure, spinel oxides exhibit more plentiful and complex surface compositions and structures than other materials (Bragg, 1915, Zhao et al., 2017, Xie et al., 2009). The morphology-dependent exposed crystal planes determine not only the surface geometric structure but also the surface composition and strongly affect the catalytic properties (Cargnello et al., 2013, Hu et al., 2008). Moreover, the concentration variation of the surface defects closely related to the crystal structure can significantly affect the catalytic activity in various catalytic processes (Huang, 2016). Compared with their monometallic analogs, bimetallic spinel oxide systems are superior in geometry and electronic characteristics, so that bimetallic systems can be designed as ordered mesoporous materials and possess distinctive chemical activities (Yu et al., 2017). Traditionally, the spinel compounds are generally synthesized by a solid-state method, which requires harsh conditions including elevated temperature, complicated procedures, and prolonged time to overcome reaction energy barriers. The produced spinels thus reveal large particle size, low surface area, and irregular shape, which might severely deteriorate their physicochemical properties (Wiley and Kaner, 1992, Lu et al., 2014). For decades many efforts have been made and some alternative moderate synthetic routes like sol-gel, solvothermal, and co-precipitation approaches have been developed (Lavela et al., 2007, Indra et al., 2014, Li et al., 2009). However, although spinel nanoparticles with metastable phases and tunable size can be obtained to some extent, the mild and rational synthesis of efficient and stable bimetallic spinel systems, especially with precise control over the lattice structures, chemical compositions, and morphologies of individual nanocrystals, still remains a big challenge.

The catalytic oxidation of combustion pollutants like carbon monoxide (CO) has drawn great attention, not only because of the relevance in vast industrial applications but also due to the importance as a key model reaction for grasping the mechanisms of catalysis (Xie et al., 2009, Cargnello et al., 2012, Christopher et al., 2011, Nie et al., 2017, Zhang et al., 2018, Saavedra et al., 2018). Noble metals function as high-performance catalysts in oxidation reactions, and the reaction temperature to complete oxidation can be sharply reduced. However, the high cost and the scarcity of noble metals usually makes the produced catalysts less desirable (Song et al., 2014). Instead, spinel-type metal oxides free of noble metals are being urgently developed as high-performance and cost-efficient catalysts, technologically and pragmatically (Chen et al., 2011, Li et al., 2015a). Co3O4 proves to be one of the most active nonprecious spinel-type catalysts for low-temperature CO oxidation (Xie et al., 2009, Song et al., 2014). The cobalt-based occupancies such as (CoM)3O4 (M = Fe, Cu, Cr) are interesting bimetallic spinel oxide systems, which display specific influence of charge and coordinating state of cobalt on catalysis (Gu et al., 2015). Recently, attempts have also been made to put nanostructured Co3O4 on proper substrates like graphene to form bifunctional oxygen-involving electrocatalysts (Li et al., 2016a), but the structure collapsing and fast performance decay problems are usually caused by relatively simple architecture and weak interfacial interactions of the components, which result in irreversible deactivation during catalytic process. Moreover, although spinel oxides can deliver competitive catalytic performances compared with noble metal catalysts, the water poisoning problem generally faced by metal oxides and supported metal catalysts (including noble metals) developed for low-temperature CO oxidation greatly limited their practical application (Xie et al., 2009, Song et al., 2014). Even if the surface hydrophobicity modification realized by utilizing polymers emerges as a promising strategy to prevent deactivation of catalysts in the presence of water, the catalytic activity of these catalysts might be easily reduced due to the blockage of active sites and surface area caused by hydrophobic coatings (Chen et al., 2010, Kuo et al., 2014). Therefore, it is of paramount importance in the development of high-efficiency and stable non-precious catalytic systems for low-temperature CO oxidation reaction.

To prepare high-performance catalysts with resistance to deactivity and deformation for target reactions under harsh environment, the understanding of morphology complexity and surface structure would be crucial to uncover the atomic-scale characteristics. The hollow micro- or nanostructures with controllable size, shape, composition, and interior architecture have important implications in vast fields (Hu et al., 2011, Li and Shi, 2014, Dai et al., 2015, Li et al., 2016b). Encouraged by the observation on building novel 3D nanostructures such as foams and urchins to overcome the structure collapsing problems (Lu and Zhao, 2015, You et al., 2016), we go forward to think that it a feasible strategy to design spinel oxide systems on the basis of forming robust multi-shelled hollow microsphere structure to enhance the catalytic activity and stability during long-term oxidation process. Meanwhile, largely due to the water strongly binding to metal-oxygen sites, monometallic catalysts often suffer severely from water poisoning at low temperature (Goodman et al., 2017). We also deem it necessary to explore suitable bimetallic systems to sharply improve the catalytic activity and minimize deactivation in the presence of steam. In this regard, the highly exposed active sites on the multi-shelled hollow nanostructures greatly improve the physicochemical characteristics and the formed robust hollow structure can prevent the possibility of collapsing during catalytic reactions. In addition, the synergistic and coordinative effects of metal oxides' self-support interface from bimetallic spinels may be maximized, which is critical in catalytic performance.

Herein, we report the preparation of hierarchical multi-shelled hollow microsphere mesoporous nanocatalysts from cobalt-based bimetallic spinels, which are long-term stable in the presence of steam. By tuning the structure and composition in a facile manner, we carried out CO oxidation to explore the catalytic activities and stabilities of different series of cobalt-based spinel hollow microsphere nanocatalysts. The self-assembled multi-shelled hollow nanostructured catalysts were prepared in two steps (Figure 1): (1) synthesis of glycerine Co-based bimetallic nanostructures with different secondary units via a solvothermal approach that involves facile solution-based ion redistribution and coordination processes and (2) slow heating process induced by the crystallization of spinel phase and formation of cobalt-based spinel multi-shelled hollow microspheres at mild condition. The ultimate goal of this work was a simple and facile template-free method to design complex nanostructured bimetallic spinel catalysts for low-temperature CO oxidation. The robust bimetallic spinel microspheres consisted of bistratal mesoporous shells with large surface area, wide tunable size range, and abundant oxygen vacancies. The reagent molecules can easily enter the interior of hollow microspheres through the mesopores within cobalt-based bimetallic spinel shells, and the product can exit back out through these mesopores. Significantly, the outstanding water-resisting property enabled the study of synergistic effects and particle agglomeration behavior during the spinel-catalyzed CO oxidation in the presence of steam. The high chemical stability, as well as uniform mesoporous hierarchical multi-shelled structure with rich morphologies, suggests that the cobalt-based bimetallic spinel multi-shell hollow microspheres are an excellent spinel system for catalytic oxidation reactions at low temperature.

Figure 1

Scheme for Template-free Synthesis of Different Kinds of CoxMnyO4 Mesoporous Nanostructured Materials

(A) The synthesis of CoxMnyO4 spinel multi-shell hollow microspheres. Blue sphere: Co, green sphere: Mn, chain: glycerol.

(B) XRD patterns of as-prepared CoxMnyO4 spinel with tetragonal and cubic phases. See also Figure S2.

(C) Transformation procedure in tetragonal and cubic phase CoxMnyO4 spinel.

See also Figure S4.

Results and Discussion

Among different varieties of mixed valence spinel oxides, manganese-based compounds have been proposed as a sort of interesting materials with versatile applications in view of their high abundance, low price, low toxicity, and environment friendliness. The physicochemical properties of CoxMnyO4 spinels are highly sensitive to crystal phase and chemical composition, which greatly depend on the synthetic conditions (Chen et al., 2011, Li et al., 2015a). Tetragonal and cubic-phase CoxMnyO4 spinels were first synthesized by tuning Co/Mn molar ratio in a wide range of 0.4–2.5, and the as-prepared spinel samples with different compositions were labeled as CoMn2.5O-T, CoMn2O-T, CoMnO-C, Co2MnO-C, and Co2.5MnO-C. After increasing the concentration of cobalt in microspheres, the crystal phase transformed from tetragonal to cubic phase (Figure 1C), which was confirmed by the color change of solution after solvothermal treatment (Figure S1). The corresponding X-ray diffraction (XRD) patterns (Figure 1B) display the Mn-rich samples with tetragonal phase (space group I41/amd (141)) and Co-rich hierarchical microspheres with cubic phase (space group Fd-3m (227)) (Menezes et al., 2015). In the fabrication of tetragonal CoxMnyO4 spinels, no intermediate was found during the synthesis process (Figure S2A). The preferred orientation can be deduced from the intensity ratio of different facets. As depicted in Figure 1B, the intensity ratio of (112)/(211) was 0.43. The value of intensity ratio of (112)/(211) is larger than that of the standard (0.35) (JCPDS 77-0471), implying that the preferential orientation of CoMn2.5O-T is [112] direction. As for CoMn2O-T sample, the values of intensity ratios of (101)/(211) and (103)/(211) are 0.28 and 0.69, which are both greater than the standard, indicating that the b-axis orientation along the {010} plane is preferred. On the other hand, Mn ions in CoMn-glycerine complex were gradually oxidized to cubic Mn7O13 in Co-rich environment; the formation of cubic cobalt manganese spinels was attributed to the cubic crystal structure and high Mn valence of intermediate Mn7O13 (Figure S2B). The intensity ratios of (220)/(311) and (222)/(311) are 0.46 and 0.15 for cubic Co2MnO-C spinels, respectively, which are also greater than the standard (JCPDS 23-1237). This change indicates the crystal of Co2MnO-C orientated in the [10] or [10] direction. Energy-dispersive X-ray spectroscopy and elemental analysis from X-ray photoelectron spectroscopy (XPS) prove the average atomic ratio of Co and Mn in as-prepared spinel compounds. Unlike cubic cobalt manganese spinels made by traditional solid-state methods, whose crystals are not stable at low temperature and easily transform into the tetragonal phase in quenching process (Vila and Rojas, 1996), the as-prepared cubic CoxMnyO4 spinels in this strategy are very stable, even at 0°C.

Controlled Morphology and Structure by Phase and Composition Adjustment

The morphological properties of as-prepared CoxMnyO4 spinel hierarchical hollow microspheres were analyzed to confirm the structure controllability of catalysts, which strongly affects the physicochemical properties (Zhang and Lou, 2014). As observed in Figures 2 and S3, the shape, size, and interior structure of these microspheres were largely influenced by tuning the phase and composition. When the Co/Mn ratio was set as 0.4, dented CoMn2.5O-T microspheres with close double shells of 80 nm interdistance and 500–700 nm diameter were produced (Figures 2A and 2B). The nanoparticles as secondary units stacked loosely on the surface to form a thin shell, which may be attributed to the low stacking fault energy under Mn-rich environment. After enhancing the Co/Mn ratio to 0.5, smooth CoMn2O-T microspheres with a diameter of 400 nm and similar interdistances between the adjacent shells could be observed (Figures 2E and 2F). To further illustrate the configurations of hollow microspheres, transmission electron microscopic (TEM) and high-resolution TEM (HRTEM) analysis were employed to explore more details concerning the crystal structure of samples. The dominant exposed plane of CoMn2.5O-T sample is {112}, which only exhibits a lattice distance of 0.30 nm (Figure 2D). Similarly, CoMn2O-T spheres are observed to be disintegrated into a mass of small nanocrystals with clear lattice fringes of 0.24 nm, and 0.27 nm corresponds to (202) and (103) crystal facets of CoxMnyO4with tetragonal phase spinel structure (JCPDS 77-0471) (Chen et al., 2011, Li et al., 2015a), demonstrating that the exposure facet is mostly {010} (Figure 2H). When the Co/Mn ratio reached 1.0, glycerine-CoMn outer shell shrank more slowly and converted into cobalt manganese spinel shell. Compared with CoxMnyO4-T samples, the thickness of shell for CoxMnyO4-C microsphere increased to two times (Figure 2K). Interestingly, for flower-like Co2MnO-C hollow microspheres (Figures 2M and 2N), the generated thin radially standing nanoflakes (16 nm) acted as 2D secondary building blocks and constructed highly rough outer shell due to the minimization of their surface energies by an oriented attachment. After elevating the content of cobalt, it was found that the nanoparticles were orderly arranged to form smooth surface again. The contraction effect was minimized, and the thickness of inner shell was enlarged as well as the outer shell, resulting in a minimum interdistance of ∼20 nm between shells (Figures 2S and S4). Moreover, the exposed crystal facets of CoxMnyO4 spinel microspheres changed, whereas the crystal phase transformed from tetragonal to cubic phase. According to the results of measured lattice distance in HRTEM images, the exposure facets of CoMnO-C, Co2MnO-C, and Co2.5MnO-C samples are mostly {112}, {110}, and {112}, respectively. These different reactive crystal facets in the selective synthesis of spinel nanomaterials play an important role in the formation of physicochemical characteristics of catalysts, indicating the highly active nature of the cobalt manganese spinel shell (Xie et al., 2009, Chen et al., 2011, Li et al., 2016c).

Figure 2

As-prepared CoxMnyO4 Spinel Multi-Shell Hollow Microspheres Constructed by Different Building Blocks

(A–T) Scanning electron microscopic images of CoMn2.5O-T (A and B), CoMn2O-T (E and F), CoMnO-C (I and J), Co2MnO-C (M and N), and Co2.5MnO-C (Q and R) spinel microspheres. The mechanically broken hollow structure showing the internal cavities (inset in M) and 2D nanosheets as secondary unit constructed the flower-like hollow spheres (inset in N). See also Figure S3. TEM and HRTEM images of CoMn2.5O-T (C and D), CoMn2O-T (G and H), CoMnO-C (K and L), Co2MnO-C (O and P), and Co2.5MnO-C (S and T) spinel microspheres. The inset images are the corresponding fast Fourier transform patterns.

For nanosized materials to be catalytically active, a high-energy interfacial structure is of great significance. In previous examples of nanostructured spinel catalysts, transition metal oxides were often deposited on suitable supporting materials to obtain high-performance oxidation catalysts with structured stability (Ren et al., 2014, Liang et al., 2011). The weak interfacial interaction between different components thus might cause performance decay or deactivation by irreversible oxidation reaction during catalytic process (Guo et al., 2017). In our design of multi-shelled hollow microsphere spinel catalysts in cubic phase, the high-energy-density structures were created by combining the active Co3+ and the precipitated intermediate Mn7O13. The strong interactions between binary metal oxides facilitate the generation of highly coordinated Mn-O-Co bonds and form high-oxidation-state manganese oxide. Meanwhile, the architecture of multi-shelled hollow microsphere offers attractive stability to resist high temperature, and large surface area to promote gas-solid interactions enhancing catalytic activity.

As the reaction time usually plays an important role in producing mesoporous microspheres and influences the final yield of the products, the morphological evolution as a function of the reaction time was evaluated with TEM analysis (Figure S5). By applying Co2MnO-C as an example, it was observed that Co2MnO-C microsphere without visible pores was formed at the initial stage. After the reaction time was elevated to 3 h, a yolk-shell structure that combined a porous shell and retained the dense core of glycerate-CoMn was observed. When the reaction time was further increased to 7 h, the solid sphere in the interior slowly shrank into a small microsphere with mesoporous structure in the same way. Thus, a unique multi-shelled Co2MnO-C microsphere was finally obtained. In general, CoxMnyO4 spinel nanoparticles possess low surface areas (no more than 80 m2/g). Through a facile and rapid synthetic method, Chen and co-workers developed a series of granular CoxMnyO4 with very high surface areas (122 m2/g) (Chen et al., 2011). Comparatively, our multi-shelled Co2MnO-C sample synthesized by the template-free method possessed the highest surface area of 203.5 m2/g known up to now (Table S1 and Figures 3A and 3B), which was approximately twice those of CoxMnyO4 spinel nanoparticles in previous literatures (Chen et al., 2011, Li et al., 2015a). Moreover, the surface areas of reported single-component Fe2O3, MoS2, and V2O5 hollow microspheres constituted by 2D nanosheets were only 88.6, 31.5, and 60 m2/g (Ma et al., 2015, Wang et al., 2016, Pan et al., 2013), which were several times lower than those of our prepared Co2MnO-C sample. The results reveal that complex hollow microspheres formed by multi-component nanosheets can greatly enhance the surface area, which may be beneficial to CO oxidation. Unlike reported template approaches, here cobalt manganese spinel multi-shelled hollow microspheres were produced by a facile and template-free manner with no need for additional templates, extra multi-steps, and prolonged time. Comparatively, although Ostwald ripening procedure is considered as a useful template-free method and employed frequently to synthesize inorganic hollow spheres, it only focuses on certain special materials. Because of the anisotropy in different nanomaterials, it is very hard to assemble multi-component metal oxides into hierarchical hollow microspheres by simultaneously incorporating different components during synthesis process (Yu et al., 2017). Therefore, the strategy described here can be an effective and universal way for the controlled fabrication of various bimetallic spinel hollow microspheres.

Figure 3

Specific Surface Area of Different Multi-shell Hollow Microspheres and CO Oxidation Catalysis under Moisture-Rich Conditions

(A and B) Adsorption/desorption isotherms (A) and Barrett-Joyner-Halenda (BJH) pore size distribution (B) for different kinds of spinel hollow microspheres. See also Table S1 and Figure S5.

(C) The catalytic activities of different CoxMnyO4 samples for CO oxidation under moisture-rich conditions (~2% H2O). See also Figure S8.

(D) Long-term durability test of as-prepared CoxMnyO4 spinel catalysts under moisture-rich conditions (~2% H2O). See also Figures S11 and S12.

On the basis of our synthetic route, other cobalt-based spinel multi-shelled hollow microspheres with high surface areas and mesoporous structure, such as CoxFeyO4, CoxZnyO4, and CoxCuyO4, were also successfully synthesized (Figure S6). Moreover, 2D nanoflakes as secondary units were directly assembled into hierarchical bimetallic hollow structures, which greatly enhanced their surface areas and numbers of active sites and prevented self-aggregation behavior of low-dimensional subunits. In general, higher surface area can contribute to higher catalytic activity due to more exposed active sites and weakened metal-oxygen bond strength (Yan et al., 2013). For this reason, a catalytic experiment toward CO oxidation was carried out to clarify the relationship between the performance and structure. As expected, the catalytic activity in carbon monoxide oxidation followed the same tendency as the surface area (Figure S7). The amount of active sites can be enhanced with the increase of generated small pores, therefore the formed mesoporous structure greatly improved the mass transfer and led to the much better catalytic performance.

CO Oxidation Catalysis

The catalytic activity tests of all prepared spinel microspheres were carried out for CO catalytic oxidation. As depicted in the Figure S8, hierarchical Co2MnO-C microsphere exhibits the highest activity among all samples in the temperature range of 30°C–90°C and the lowest apparent activation energy (30.1 kJ/mol) toward CO oxidation, which is fixed by the corresponding Arrhenius plots. Co2MnO-C sample also gives specific rates of 54.95% conversion at 90°C, which is 15 times higher than that of CoMn2O-T and at least two orders of magnitude more active than that of CoMn2.5O-T catalyst. The Co2MnO-C multi-shelled hollow microsphere possesses a high value of turnover frequencies (TOFs) (9.52×10−4 s−1), which is 10 times more active than Co3O4 hollow microsphere (Figure S9), indicating the synergistic effect in CoxMnyO4 spinel catalysts. The high activity at low temperature is consistent with the US Department of Energy road map, which gives the aim for light off below 150°C (USDRIVE, 2013). The incorporation of Mn enhances the catalytic activity, which reaches the highest point at Co/Mn = 2:1, creating bimetallic spinel oxide catalyst with largely improved surface atom fractions, which consequently leads to high catalytic activity. Moreover, the performance of other bimetallic spinel nanostructures, in which Fe, Zn, and Cu were incorporated as major constituents, was also evaluated in the experiment. The results showed that these spinel catalysts were at least four times less active than Co2MnO-C sample (Figure S10). The durability tests were used to explore the stability of the high-efficiency state of synthesized spinel catalysts; the as-prepared Co2MnO-C samples can sustain the total conversion at 120°C for a long period of 71 h with similar morphology to that of the pristine samples under dry feed gas condition (Figures S11 and S12), showing superior stability compared with synthesized Co2MnO-C solid microspheres, which failed in keeping the conversion after 19 h (Figure S13). Furthermore, although the prepared catalysts were deactivated after operation owing to the formation of formate or carbonate-like species (Figure S14) (Li et al., 2015b, Poyraz et al., 2013), the high activity with 100% CO conversion can be easily renewed and kept for at least up to five cycles by subsequent regenerations after oxidation treatment (Figure S15).

Metal oxides with complex structures or rich morphologies have recently been explored for catalytic oxidation applications. Examples include Co3O4 nanorods, Co3O4/graphene nanocomposites, integrated spinel nanoarrays, interfacial Pt-NiO1-x nanostructures, Pt@mSiO2 core-shell nanospheres, and nickel-iron foams (Xie et al., 2009, Lu and Zhao, 2015, Ren et al., 2014, Liang et al., 2011, Joo et al., 2009, Kim et al., 2018, Wang et al., 2012). Our present design of bimetallic spinel oxide multi-shelled hollow microspheres with wide tailorability and functionality enables the reactive molecules to directly enter their interior space by offering more open channels and less diffusion obstruction so as to take full advantage of highly exposed active sites and enhanced synergetic effect of components in such mixed metal oxides. In addition, the blockage of active sites and the conversion of surface lattice oxygen to hydroxy groups caused by the adsorption of water molecules are cut down to a large extent in the presence of Mn (Tao et al., 2019). The interactions between adsorbed CO and hydroxyl group on the surface were greatly weakened, which prevented the usually inevitable deactivation in CO oxidation reaction by trace concentrations of moisture, exhibiting great potential for practical use in oxidation reactions.

Water-Resisting Property of Cobalt Manganese Spinel Hollow Nanostructured Catalysts

In practice, metal oxide-based exhaust treatment catalysts are normally not water tolerant. Water-induced deactivation of the oxide catalysts still remains a big problem, especially for cobalt-based nanocatalysts. H2O dissociative adsorption occurred in the oxygen vacancies and Co3+, resulting in the formation of carbonates (Xie et al., 2009, Song et al., 2014). To evaluate water-resisting property of the prepared spinel nanocatalysts, the deactivation behavior was investigated in the presence of water conditions (∼2% water vapor) (Figure 3C). Even though the existence of H2O in feed gas lowered the conversion of CO, Co2MnO-C multi-shelled hollow microsphere catalysts can still reach a total conversion at 170°C in moisture-rich conditions. By enhancing the content of Mn, the adsorbed H2O decreases, which is evidenced by Fourier transform infrared spectroscopy (FTIR) analysis and water adsorption experiments (Figures S16 and S17). Therefore, we suppose that the behaviors of H2O direct dissociation and adsorption on the oxygen vacancies and metal cations (Co or Mn) can be reduced, thus ensuring the availability of active sites for the adsorption of CO; this phenomenon is similar to the reported literature (Tao et al., 2019). As mentioned above, Co2MnO-C multi-shelled hollow microsphere catalysts reached a total conversion at 120°C in normal conditions, therefore the difference between reaction temperature of complete oxidation in normal and moisture environment (ΔT) was as small as 50°C. After the incorporation of Mn in the formation of multi-shelled hollow microspheres, more active sites and surface defects were generated, thus increasing the reactivity of surface-adsorbed oxygen species and promoting their catalytic activity even under moisture-rich conditions. Compared with literature findings, Xie and co-workers synthesized very-high-activity Co3O4 nanorods with predominantly exposed {110} planes; however, the ΔT of Co3O4 nanorods catalysts was as large as 277°C (Xie et al., 2009). Meanwhile, the ΔT of most reported spinel nanoparticles was also very high and in the range of 100°C–178°C (Table S2) (Song et al., 2014, Kuo et al., 2014, Shen et al., 2017, Wang et al., 2014, Yu et al., 2009, Biemelt et al., 2016). Comparatively, the ΔT values of our CoxMnyO4 sample can reach as low as 20°C, exhibiting the excellent water-resistent ability of CoxMnyO4 multi-shelled hollow microsphere catalysts. Apart from these, despite the fact that the conversion of CO was reduced in the presence of water conditions, the prepared Co2MnO-C multi-shelled hollow microsphere catalysts can still keep very high activity (TOF = 1.75×10−4 s−1) among reported metal oxide and noble metal catalysts under moisture-rich conditions. Moreover, the as-prepared CoxMnyO4 spinel catalysts showed long-term stability in the presence of steam and maintained the activity with little fluctuation for as long as 30 h (Figure 3D). Compared with the performance of reported spinel-type catalysts, which often decayed in the first 10 h (Shen et al., 2017, Wang et al., 2014), the results demonstrated that as-prepared cobalt manganese spinel multi-shelled hollow microsphere catalysts were much more stable and very promising for practical application. The robust shield of bistratal shells can effectively retard the water poisoning, thus preserving high catalytic efficiency during CO oxidation, even under moisture-rich conditions.

The excellent catalytic performance of cubic CoxMnyO4 spinel microsphere catalysts is considered to originate from their unique structural features and the preferred growth of CoxMnyO4 spinel crystals, which gives the orientation to study the catalytic activity toward CO oxidation with different exposed planes. As trivalent cobalt ions play an important role in the catalytic activity for CO oxidation (Xie et al., 2009, Song et al., 2014), the big difference in performance between tetragonal and cubic CoxMnyO4 spinel hollow microspheres can be explained by the amount of Co3+ that is easily oxidized. The preferential orientation of cubic Co2MnO-C sample is [10] or [10] direction; the {110} facet in one cubic spinel unit contains 8 Co atoms and 4 Mn atoms in theoretical models. The content of cobalt is higher than that of exposed tetragonal {010}, {112}, and cubic {112} planes in cobalt manganese spinels, which contain 2, 2, and 4 cobalt atoms, respectively. The results demonstrated that the cubic Co2MnO-C exposed {110} facet possessed richer Co sites, which were highly effective sites for CO oxidation as confirmed both theoretically and experimentally. The chemical state of coordinated metal species in prepared mesoporous CoxMnyO4 spinel multi-shelled hollow microspheres was evaluated by XPS analysis (Figures S18 and S19). Taking two representative spinel microspheres (CoMn2O-T and Co2MnO-C) as an example, two peaks of Co 2p3/2 and Co 2p1/2 are located around 781.4 and 796.6 eV, respectively, indicating the co-existence of two types of cobalt ions in tetragonal and cubic phase. The spectra of Co 2p (Figures S19A and S19B) reveal higher Co3+ concentration in Co2MnO-C microspheres. In general, for cobalt-based catalysts, a high amount of trivalent cobalt ions often contributes to better catalytic performance due to more exposed active sites. However, it was found that, although Co2.5MnO-C spinel multi-shelled hollow microspheres possessed the highest packs of Co3+, it was not the most active catalyst. Therefore the catalytic activity was determined not only by the number of Co3+ but also by the point defect concentration.

An important feature of physicochemical properties underlying structure formation is reflected by the variation of the defects in crystal lattice (Huang, 2016, Hu et al., 2016). To further understand the mechanisms, the oxygen vacancy concentration was further adjusted by controlling the crystal phase and plane lattice parameter of CoxMnyO4 spinel microspheres. Based on the TEM analysis in Figure 2, physical models of four kinds of crystal facets, {010} and {112} in tetragonal phase and {110} and {112} in cubic phase, are depicted in Figure 4A. The defects of microsphere samples are evaluated by FTIR. The bands at 617 cm−1 can be assigned to vibration of atoms in tetrahedral oxygen environment related to the Mn-O in spinels, whereas the absorptions at about 515 cm−1 are attributable to the Co-O stretching vibrations in the octahedral oxygen environment (Figure 4G) (Tholkappiyan et al., 2015). The stretching bands of Mn-O and Co-O shift to 658 cm−1 and 570 cm−1 with the increase of Co/Mn molar ratio to 2. The result reflects the generation of Kirkendall voids on completion of the reaction, which is evidenced by the TEM images (Figures 4B–4F). It is worth to note that the Mn-O and Co-O vibration peaks shift to the right side after further enhancing the cobalt content in catalysts, illustrating the decrement of defect concentrations in Co2.5MnO-C sample. As usual, oxygen vacancies are thought to be the best active sites for oxygen activation (Zhao et al., 2013), the fitted O 1s spectra displaying four major oxygen contributions with the corresponding peaks are depicted in Figures 4H and S20. The Co2MnO-C sample exhibits the most abundant oxygen vacancies (14.72%) on the surface (Table S3) and the reactivity of surface-adsorbed oxygen species is greatly enhanced. As XPS analysis performed under vacuum can contribute to the formation of oxygen vacancies, we employed the ambient pressure X-ray photoelectron spectroscopy (APXPS) instrument to evaluate the actual content of oxygen vacancies. The results demonstrated that the content of oxygen vacancies under vacuum condition was only a little higher that of oxygen vacancies analyzed under ambient conditions (Tables S3 and S4). It is also observed that the concentration of oxygen vacancies for these facets follows the order: C-{110}>C-{112}>T-{010}>T-{112}. Because the planes of CoMnO-C and Co2.5MnO-C are the same, the lattice oxygen mobility is lower than that of Co2MnO-C catalyst, which is in agreement with FTIR analysis. Figure 4I illustrates the cobalt and manganese cations and oxygen vacancies at atom scale; the lattice oxygen migration can easily be promoted on the surface of {110} facet of cubic phase Co2MnO-C due to the high oxygen vacancy formation energies.

Figure 4

Structural Characterization of CoxMnyO4 Spinel Hollow Microsphere Catalysts

(A) Theoretical models of the different planes of CoxMnyO4 spinel hollow microspheres with tetragonal and cubic phases. See also Figures S21.

(B–F) TEM images of as-prepared CoxMnyO4 spinel samples showing crystal surfaces and internal voids.

(G and H) FTIR spectra and APXPS spectra of O 1s peaks for as-prepared CoxMnyO4 spinel catalysts. See also Table S4.

(I) Schematic showing the surface of Co2MnO-C spinel catalyst.

The Mars-van Krevelen (MvK) mechanism has been widely used to describe the CO oxidation reaction (Widmann and Behm, 2014, Nyathi et al., 2019). CO oxidation on prepared cobalt manganese spinels also follows the MvK mechanism, the details of which are as follows. A CO molecule interacts with a lattice oxygen in CoxMnyO4 to generate CO2, instead of directly reacting with the gas-phase O2, leaving an oxygen vacancy on the surface. Oxygen vacancies on catalyst surfaces may have a promotion effect on increasing catalytic activity. Because of the activation of the O2 at lattice vacancies, the gas-phase O2 participates in a reaction only after a rain check (Wu et al., 2014). Then the gas-phase O2 molecules are adsorbed at the oxygen vacancy sites. Subsequently, another CO molecule interacts with the adsorbed O2, generating CO2 and regenerating the surface. For catalysts with high efficiency, the catalytic active site needs to adsorb the reactant fast and desorb the product fast too. Therefore, the interaction between gas molecules (CO or CO2) and cobalt manganese spinels has great importance in determining the catalytic activity. We took molecular dynamics simulations by using the Materials Studio software package to calculate the interaction energies between gas molecules and cobalt manganese spinel exposed different facets. On account of XRD and HRTEM analysis, four kinds of models of tetragonal phase {010} and {112} surface and cubic phase {110} and {112} surface are constructed with optimized structural properties. The results demonstrated that the interaction energy (−4.66 kcal/mol) between polar CO molecules and Co2MnO-C exposed {110} plane was the strongest among established models (Table S5), revealing the strong electrostatic interaction and that the CO molecules were more easily captured on the {110} plane in the Co2MnO-C sample. We also simulated the adsorption of CO molecules by putting the models of cobalt manganese spinels into an atmosphere of gas molecules. As depicted in Figure S21, the cubic Co2MnO4 sample with exposed {110} plane exhibits faster capture of CO molecules than other samples, achieving almost complete adsorption in 5 ns. On the other hand, the rate of desorption is also very important in enhancing catalytic performance. The calculated interaction energy (−0.31 kcal/mol) between nonpolar CO2 molecules and Co2MnO-C exposed {110} was the weakest, illustrating that CO2 molecules could easily overcome the barriers to desorb on {110} plane. Moreover, Co2MnO-C sample exhibited the most abundant oxygen vacancies (14.35%) on the surface evidenced by APXPS analysis; the lattice oxygen mobility was greatly enhanced. From the above, the high adsorption of CO and fast desorption rate for CO2, as well as abundant oxygen vacancies, endowed the Co2MnO-C sample with high catalytic activity toward CO oxidation.

Conclusion

We have developed a simple and efficient methodology for the template-free production of hierarchical multi-shelled hollow nanostructured catalysts from cobalt-based bimetallic spinels. Through precise control of architecture and composition, the self-assembled robust microspheres configured by 16-nm CoxMnyO4 nanosheets demonstrated large surface area, favorable surface chemistry, and abundant oxygen vacancies (14.35%), which led to excellent catalytic performance in catalytic oxidation reactions at low temperature. In the meantime, as-prepared non-precious CoxMnyO4 spinel catalysts exhibited most attractive water-resisting property for low-temperature CO oxidation and can keep very high activity (TOF = 1.75×10−4 (s−1)) for as long as 30 h in the presence of steam, surpassing by far the best performance of all spinel nanoparticle catalysts reported in the literature. As well as being interesting in itself and cost efficient in practical application, the methodology can also be extended to design other stable mixed transition metal oxide systems with various configurations and compositions.

Limitations of the Study

So far, it has been very difficult to quantitatively analyze the specific reaction process of catalytic reaction on molecular scale by characterizations. Therefore, theoretical calculation was carried out by establishing models to further describe the reaction mechanism. Although the atomic-level models were established to make a qualitative description about the interactions between gas molecules and cobalt manganese spinel exposed different facets, more desirable theoretical models are needed for quantitatively illustrating the reaction mechanism.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.