Solid-State Construction of CuOx/Cu1.5Mn1.5O4 Nanocomposite with Abundant Surface CuOx Species and Oxygen Vacancies to Promote CO Oxidation Activity

Carbon monoxide (CO) oxidation performance heavily depends on the surface-active species and the oxygen vacancies of nanocomposites. Herein, the CuOx/Cu1.5Mn1.5O4 were fabricated via solid-state strategy. It is manifested that the construction of CuOx/Cu1.5Mn1.5O4 nanocomposite can produce abundant surface CuOx species and a number of oxygen vacancies, resulting in substantially enhanced CO oxidation activity. The CO is completely converted to carbon dioxide (CO2) at 75 °C when CuOx/Cu1.5Mn1.5O4 nanocomposites were involved, which is higher than individual CuOx, MnOx, and Cu1.5Mn1.5O4. Density function theory (DFT) calculations suggest that CO and O2 are adsorbed on CuOx/Cu1.5Mn1.5O4 surface with relatively optimal adsorption energy, which is more beneficial for CO oxidation activity. This work presents an effective way to prepare heterogeneous metal oxides with promising application in catalysis.


Introduction
Transition metal oxide catalysts for eliminating carbon monoxide (CO) at lower temperatures have attracted enormous attention in the past decades for their inexpensive cost and wide applications in catalytic applications and environmental protection [1][2][3]. Many techniques, such as morphology control [4][5][6][7], engineering defects [8][9][10], and construction of composite oxides [11][12][13] have been developed to improve the CO oxidation performance of transition metal oxide catalysts. Particularly, heterogeneous metal oxides that have exhibited excellent performances in CO oxidation fields [14][15][16] are the most widely studied because of their interactions of components [17]. Previous works have gradually demonstrated that heterogeneous metal oxides with synergistic interactions between two components are crucial for promoting catalytic performances [12,18,19]. The catalytic activity of nanocomposites depends significantly on surface active species and oxygen vacancies [20]. Therefore, the manipulation of the surface-active species and the oxygen vacancies of nanocomposites by simple strategy to optimize their catalytic performance is of great importance to meet the application in practice.
In the past few decades, many transition metal oxide catalysts have been developed, mainly including CeO 2 [13,21,22], MnO 2 [23][24][25], Co 3 O 4 [26][27][28], CuO [29][30][31], Fe 2 O 3 [32][33][34] [16,35], Cu-Mn [36], Ce-Cu [37,38], and Ce-Mn [39,40] composite oxides. Different active metals and carriers will result in different interactions between metals and carriers and different exposed active sites, thus making them have different reactivity for CO catalytic oxidation. Yu [16] synthesized the catalyst of Co 3 O 4 -CeO 2 nanocomposite, which showed good catalytic activity due to its special hollow multishell structure and the interaction between the two components. Chen [41] constructed CuO x -CeO 2 nanorods and explained the relationship between reduction treatment and catalytic activity, indicating that reduction treatment accelerates the generation of active sites. In our previous works, we fabricated a CuO x -CeO 2 catalyst via the solid-state method [22], and investigated the influence of heating rate on catalytic performance. It was demonstrated that the heating rate can regulate the surface dispersion of CuO x on CeO 2 surface, resulting in enhanced catalytic performance. Copper-manganese mixed oxide catalyst is a typical transition metal-based catalyst in the CO oxidation reaction, which is known for its high activity at high temperature and low cost [36,42]. However, current commercial copper-manganese catalysts exhibit relatively low catalytic activity at low temperatures for CO oxidation. Furthermore, specific deactivation frequently occurs during the catalytic process [43,44]. The catalytic performance of heterogeneous catalysts is closely associated with their synergistic interactions and oxygen vacancies. In addition, many controllable synthetic strategies, such as direct calcination [30] and hydrothermal/solvothermal synthesis [45,46], have been developed for the fabrication of heterogeneous catalysts with the active two-phase interface, controllable size, shape, and composition in view of the purpose of improving catalytic performance. These synthetic routes are usually low-producing, time-consuming, and high-energy-consuming [47,48]. Solid-state synthesis integrated the advantages of low cost, eco-friendly and large-scale and have aroused wide concern in recent years [49][50][51][52][53].
Herein, a solid-state synthesis was developed to fabricate the CuO x /Cu 1.5 Mn 1.5 O 4 , which was implemented by the straightforward grinding of copper salt, manganese salt, and potassium hydroxide at ambient conditions. The metal oxide catalysts fabricated by solid-state synthesis are considered a simple and economical approach because they are without complicated procedures and organic solvents. The as-prepared CuO x /Cu 1.5 Mn 1.5 O 4 exhibit significant advantages compared to other methods. The catalytic performance was obviously promoted, which can be attributed to the surface CuO x species and the number of oxygen vacancies. More importantly, this work presents us with an effective way to prepare heterogeneous metal oxides with outstanding catalytic performance.

Results and Discussion
The preparation process of CuO x /Cu 1.5 Mn 1.5 O 4 is schematically illustrated in Scheme 1. The CuO x /Cu 1.5 Mn 1.5 O 4 can be efficiently synthesized by the solvent-free strategy. The corresponding X-ray powder diffraction (XRD) patterns of CuO x /Cu 1.5 Mn 1.5 O 4 are exhibited in Figure 1 Figure 1b, compared with individual CuO species, the major diffraction peaks of the products with different Cu/Mn mole ratios were slightly shifted to a higher degree, which can be ascribed to the change of the lattice parameters. In addition, XRD patterns show the characteristic peaks of CuO and MnO x , as shown in Figure S1. The ratios of Cu 2+ /Cu + in Cu 2 O/CuO can be controlled by adjusting the types of copper salts. The energy-dispersive X-ray spectrum (EDS) mapping analyses was implemented to identify the elementary composition of the CuO x /Cu 1.5 Mn 1.5 O 4 . The Cu/Mn molar ratio in the as-prepared samples is close to 1.0 ( Figure S2), which has no significant difference compared to the theoretical value during synthesis.   Figures S4-S6, which also exhibit ir-regular nanoparticles.
The CO catalytic performance of the as-obtained CuO x /Cu 1.5 Mn 1.5 O 4 were firstly evaluated. As shown in Figure S7a,b, the best CO catalytic activity is CuO x /Cu 1.5 Mn 1.5 O 4 with Cu/Mn molar ratio of 1:1 calcined at 400 • C. The CuO x /Cu 1.5 Mn 1.5 O 4 can completely convert CO to CO 2 at 75 • C, especially at low temperatures. T 50 (the temperature of 50% of CO conversion) is only 41 • C. The CO 2 yield in the CO oxidation has been shown in Figure S8a, which presents nearly 100% yield. Moreover, they have been compared with previous works (Table S1), which also presents a better catalytic property. Other samples exhibit a relatively lower catalytic activity performance with higher T 100 (the temperature of 100% of CO conversion) and T 50 (the temperature of 50% of CO conversion) in Figure 3. The 100% CO conversion was accomplished for individual CuO, Cu 2 O-CuO, and MnO x samples at 140 • C, 130 • C, and 200 • C, respectively. The individual CuO x and MnO x particles show poorer performance than CuO x /Cu 1.5 Mn 1.5 O 4 , implying that the synergistic effect between CuO x and MnO x may promote its catalytic activity that is not presented in the individual components. As shown in Figure S8b, the sample of physical mixing of CuO x + MnO x was also prepared, which exhibits the poor catalytic performance for CO oxidation. The stability of the CuO x /Cu 1.5 Mn 1.5 O 4 was also tested at 60 • C. The negligible decline of activity can be observed during 30 h testing from Figure S9, which implies the excellent stability of CuO x /Cu 1.5 Mn 1.5 O 4 for CO oxidation reaction.  The X-ray photoelectron spectra (XPS) were used to investigate the chemical states of samples. The XPS spectrum in Figure 4 and Figure S10 indicate the coexistence of the Cu, Mn, and O elements. Two peaks at about 931.1 and 950.9 eV, respectively, shown in Figure 4a refer to the Cu + or Cu • due to the fact that their binding energies are basically the same [31,54]. Cu • is unstable at room temperature and easily oxidized to copper oxide. The CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposites were acquired after being calcined at high temperature in air. Therefore, the peak is assigned to Cu + because of the successful synthesis of CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposites. The two main peaks have small shoulder peaks that appeared at 933.3 and 953.4 eV, relating to the Cu 2+ [55][56][57][58]. The XPS analysis results imply that Cu + and Cu 2+ coexist on the surfaces of the CuO x /Cu 1.5 Mn 1.5 O 4 . As shown in Figure 4b, the asymmetrical Mn 2p spectra of individual MnO x catalysts could be fitted into four components based on their binding energies. The binding energies of 640.2 eV, 641.2 eV, 642.5 eV, and 646.0 eV correspond to Mn 2+ , Mn 3+ , Mn 4+ species, and the satellite peak, respectively [59]. The O 1s XPS spectrum of samples can be divided into three single peaks (Figure 4c), corresponding to surface lattice oxygen (O α ), surface adsorbed oxygen (O β ), and adsorbed molecular water species (O γ ), respectively [14,[60][61][62]. The CuO x /Cu 1.5 Mn 1.5 O 4 demonstrates the highest surface adsorbed oxygen, which is beneficial to the adsorption of O 2 molecules, and thus help to improve catalytic performance. As shown in Figure 5a, the reduction property of prepared samples was investigated by hydrogen temperature-programmed reduction (H 2 -TPR). A peak at 200 • C-400 • C was presented for an individual CuO sample, attributing to the gradual reduction of copper oxide [30]. In addition, two H 2 reduction peaks at 245 • C and 364 • C occurred for an MnO x sample, which correspond to the gradual reduction of  Figure 5b, the temperature below 200 • C is due to the desorption of surface oxygen species (O β ) [64]. The second peak, appearing at 250-550 • C, corresponds to the overflow of surface lattice oxygen (O α ). The high-temperature zone at above 550 • C is related to the bulk lattice oxygen species [63]. The CuO x /Cu 1.5 Mn 1.5 O 4 shows the highest amount of adsorbed oxygen species compared with other samples, confirming the higher oxygen capacity that is conducive to the promotion of catalytic performance.
To elucidate the effects of surface CuO x species and the oxygen vacancies in CuO x / Cu 1.5 Mn 1.5 O 4 in detail, the Cu 1.5 Mn 1.5 O 4 was fabricated by changing the types of copper salt in the synthesis process to investigate the structure-activity relationships. The Cu 2+ and Mn 4+ proportion was evaluated by XPS (Figure 6d). It indicates that the ratio of Cu 2+ and Mn 4+ in CuO x /Cu 1.5 Mn 1.5 O 4 is higher than Cu 1.5 Mn 1.5 O 4 sample. The higher contents of Cu 2+ and Mn 4+ are beneficial to the formation of Cu 2+ -O 2− Mn 4+ entities at the two-phase interface [36]. As reported in the studies [63,65], the presence of abundant Mn 4+ proportion can create many adsorbed oxygen species [63]. In addition, the ratios of Cu + /Cu 2+ in different samples show a change by altering the types of copper salts in the synthetic process (Table S2). The CuO x /Cu 1.5 Mn 1.5 O 4 exhibits a higher Cu 2+ ratio than Cu 1.5 Mn 1.5 O 4 , which corresponds to XRD results that the CuO x /Cu 1.5 Mn 1.5 O 4 shows the higher intensity of the CuO diffraction peaks. The XPS results indicate that the Cu 2+ and Mn 4+ proportion can be engineered by changing the component of Cu-based oxides in the synthetic process.  As shown in Figure 7a, there are obvious differences in catalytic performance after changing the copper salts. The Cu 1.5 Mn 1.5 O 4 exhibits poor catalytic activity compared with CuO x /Cu 1.5 Mn 1.5 O 4 at the same condition. Herein, the performance of catalysts can be meaningfully boosted by altering the surface CuO x species and the oxygen vacancies in CuO x /Cu 1.5 Mn 1.5 O 4 . As shown in Figure S11a (Figure 7b), while the catalytic performance showed obvious differences. The CuO x /Cu 1.5 Mn 1.5 O 4 exhibits the higher intensity of the CuO diffraction peaks than Cu 1.5 Mn 1.5 O 4 . In our previous work [31], the individual Cu 2 O/CuO nanocomposites and CuO can be fabricated by tuning the types of copper salt in the synthesis. The Cu 1.5 Mn 1.5 O 4 with different surface CuO x types were fabricated by altering the types of copper salts (+1, +2 valence state) in the synthetic process. Therefore, the different surface types in Cu 1.5 Mn 1.5 O 4 depend on the types of copper salts (+1, +2 valence state) in the synthetic process used. The higher content of CuO in CuO x /Cu 1.5 Mn 1.5 O 4 can significantly enhance redox reaction between Cu and Mn species, promoting charge transfer in nanocomposites, and thus achieving a stronger interaction. From Figure 7c, the CuO x /Cu 1.5 Mn 1.5 O 4 shows a lower reduction temperature than Cu 1.5 Mn 1.5 O 4 , implying the better reducibility. The peak areas for different samples were estimated from the H 2 -TPR results. In Table S3, the higher peak areas of first peak α is presented for CuO x /Cu 1.5 Mn 1.5 O 4 , indicating the ratio of Cu 2+ and Mn 4+ in CuO x /Cu 1.5 Mn 1.5 O 4 , which is consistent with XPS results. Therefore, we can confirm that CO oxidation activity is heavily dependent on the surface CuO x species in CuO x /Cu 1.5 Mn 1.5 O 4 . In addition, as shown in Figure    The effects of surface CuO x species and the oxygen vacancies in composite oxide can be clarified based on the above results. In Figure 9, the catalytic property of the CuO x /Cu 1.5 Mn 1.5 O 4 is improved compared to Cu 1.5 Mn 1.5 O 4 , which confirms the important role of surface CuO x species and oxygen vacancies. After construction of CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposite with abundant surface CuO x species and oxygen vacancies, the abundant Cu 2+ and Mn 4+ proportions in CuO x /Cu 1.5 Mn 1.5 O 4 are higher than in Cu 1.5 Mn 1.5 O 4 , which facilitated the formation of more (Cu 2+ -O 2− -Mn 4+ ) entities at the two interfaces. In addition, the construction of CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposites is beneficial for enhancing the synergetic interaction between MnO x species and CuO x species, which promotes the massive production of surface adsorbed oxygen species [36]. In CO oxidation, surface CuO x species and oxygen vacancies play significant roles in catalytic activity. The abundant surface CuO x species and oxygen vacancies could preferentially adsorb CO and O 2 molecules [22], and the adsorbed O 2 reacts with CO to form CO 2 , which ultimately enhances catalytic activity.

The Preparation of CuO x /Cu 1.5 Mn 1.5 O 4 Nanocomposite with Various Surface CuO x Species and the Oxygen Vacancies
As shown in Scheme 1, in a typical procedure, 1.70 g of CuCl 2 (10 mmol) and 1.25 g of MnCl 2 (10 mmol) were mixed well in an agate mortar by grinding. Then 4.49 g of KOH (60 mmol) was added into the agate mortar. After continuous grinding for about 1 h, the resulting solid products were sufficiently washed with deionized water and anhydrous ethanol to clear the residual Cl or K species, and then dried at ambient temperature overnight. The final CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposites were acquired after calcining the mixtures in the air at 400 • C for 2 h (5 • C/min). In addition, the CuO x /Cu 1.5 Mn 1.5 O 4 with different Cu/Mn mole ratios (Cu/Mn = 1:2 and 2:1) were calcined at 300 • C or 500 • C.
The Cu 1.5 Mn 1.5 O 4 nanocomposite (containing some CuO) was also obtained, and only the CuCl 2 was replaced by CuCl during the solvent-free synthesis route.

The Preparation of MnO x
As a comparison, the individual MnO x particles were also fabricated by straightforward grinding MnCl 2 with KOH under a similar process.

The Preparation of Cu 2 O/CuO and CuO
The Cu 2 O/CuO nanocomposite was fabricated according to our previous work [31]. The 0.99 g of CuCl (10 mmol) and 1.68 g of KOH (30 mmol) were ground in the agate mortar for 1 h. The other parameters are consistent with the CuO x /Cu 1.5 Mn 1.5 O 4 nanocomposite above.
In addition, the CuO was also prepared by mixing CuCl 2 and KOH in the agate mortar. The sample of physical mixing of CuO x + MnO x was also prepared by straightforward grinding CuO and MnO x , and then calcining the mixtures in the air at 400 • C for 2 h (5 • C/min).

Conclusions
In summary, the Cu 1.5 Mn 1.5 O 4 with different surface CuO x types were fabricated by altering the types of copper salts (+1, +2 valence state) in the synthetic process. The higher content of CuO in CuO x /Cu 1.5 Mn 1.5 O 4 can significantly enhance redox reaction between Cu and Mn species, promoting charge transfer in nanocomposites, thus achieving a stronger interaction. In addition, the higher ratio of Cu 2+ and Mn 4+ is beneficial to the formation of Cu 2+ -O 2− -Mn 4+ entities at the two-phase interface, which produced abundant surface CuO x species and oxygen vacancies. DFT calculations suggest that CO and O 2 molecules are adsorbed on the CuO x /Cu 1.5 Mn 1.5 O 4 surface with relatively optimal adsorption energy, resulting in the highest CO oxidation activity. The as-synthesized CuO x /Cu 1.5 Mn 1.5 O 4 delivers excellent CO catalytic performance compared with individual CuO x and MnO x particles. The CO is completely converted to CO 2 at 75 • C when CuO x /Cu 1.5 Mn 1.5 O 4 is involved. This work opens new avenues for the efficient and sustainable production of heterogeneous metal oxides with an outstanding catalytic performance.