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Article

Effect of Mn/Cu Molar Ratios on CO Oxidation Activity of Mn-Cu Bimetallic Catalysts

by
Cong Liang
1,
Yingchun Sun
2,3,
Peiyuan Li
2,3,
Ye Jiang
2,3,
Xin Sun
2,3 and
Zhengda Yang
2,3,*
1
College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China
2
College of New Energy, China University of Petroleum (East China), Qingdao 266580, China
3
Qingdao Engineering Research Center of Efficient and clean Utilization of Fossil Energy, Qingdao 266580, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(4), 353; https://doi.org/10.3390/catal15040353
Submission received: 10 March 2025 / Revised: 30 March 2025 / Accepted: 3 April 2025 / Published: 4 April 2025
(This article belongs to the Special Issue Advanced Catalysts in Environmental Purification)
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Abstract

:
The steel manufacturing industry is a major source of global air pollution, with sintering processes contributing over 70% of emissions, primarily carbon monoxide (CO), a significant uncontrolled pollutant. This study explores Mn-Cu bimetallic catalysts as a cost-effective and environmentally friendly alternative to noble metal-based systems, addressing the urgent need for efficient CO oxidation catalysts. Mn-Cu catalysts with different Mn/Cu molar ratios were synthesized via hydrothermal methods and systematically characterized using XRD, XPS, BET, H2-TPR, etc., to assess their physicochemical properties and catalytic performance. The Mn4Cu1 catalyst demonstrated the highest CO oxidation activity, achieving complete conversion at 175 °C. This performance is attributed to its optimal Mn/Cu molar ratio, high specific surface area, abundant oxygen vacancies, and superior redox properties. The catalyst’s enhanced performance is further supported by its low reduction temperature and high Mn3+ and Cu+ content, which promote efficient electron transfer and oxygen activation. These findings highlight the crucial role of Mn/Cu molar ratios in optimizing catalytic performance and offer valuable insights for designing high-efficiency, low-cost catalysts to reduce CO emissions in industrial applications.

Graphical Abstract

1. Introduction

In 2023, China produced approximately 1.03 billion tons of crude steel, representing about 54% of global output [1,2,3]. Steel manufacturing comprises multiple stages, with sintering being the largest contributor to air pollutant emissions, exceeding 70% [4,5,6]. This stage mainly emits CO, SO2, NOx, and particulate matter. Although SO2, NOx, and particulate matter emissions meet ultra-low standards, CO emissions remain uncontrolled, posing significant environmental risks [7]. Sintering flue gas contains approximately 0.4~1% CO (6000~10,000 mg·m−3), making CO a critical pollutant requiring urgent control in sintering gas treatment [8].
Developing highly efficient CO purification catalysts with long-term stability, cost-effectiveness, and environmental compatibility is crucial for reducing CO emissions from flue gases. The oxidation mechanism of CO can be categorized into three main types: the Langmuir–Hinshelwood (L-H) mechanism, the Eley–Rideal mechanism, and the Mars–van Krevelen mechanism [9,10]. Among these, the L-H mechanism is most commonly observed in CO oxidation over noble metal catalysts, such as Pt and Pd [11,12]. In this mechanism, CO molecules and O2 molecules are first adsorbed on the catalyst surface. The CO molecules remain in an adsorbed state (denoted as COads), while O2 molecules dissociate into adsorbed oxygen atoms (Oads). These adsorbed species compete for active sites on the catalyst surface. Subsequently, the chemically adsorbed COads reacts with Oads to form CO2, which then desorbs from the surface. Notably, lattice oxygen does not participate in this reaction pathway. Currently, CO oxidation catalysts mainly include noble metals (e.g., Au [9], Ag [10], Pt [11], Pd [12] and Ru [13]) and non-noble metals (e.g., Cu [14,15], Mn [16,17] and Co [18]). However, the practical use of noble metal catalysts is constrained by their low abundance in the Earth’s crust and high preparation costs [8]. In contrast, transition metal oxide-based composite catalysts have attracted considerable interest due to their superior redox properties [19,20,21]. Previous studies indicate that synergistic interactions between multiple metals significantly enhance catalytic performance [22,23]. Compared to single-metal systems, binary and multi-metal oxides create more active catalytic sites via interface effects and electronic structure modulation. Among composite metal oxides (e.g., Mn-Ce [24], Cu-Ce [25] and Mn-Cu [26]), Mn-Cu catalysts demonstrate significant potential due to their unique lattice structures and surface properties. Notably, Mn-Cu catalysts enhance CO oxidation at low temperatures and can even catalyze the reaction effectively at room temperature [27]. CuMnOx (hopcalite) is among the earliest known low-temperature CO oxidation catalysts. Shi et al. [28] demonstrated that in the CuMnOx system, copper doping regulates the distribution of alkaline sites on α-MnO2, reduces H2O competitive adsorption on surface oxygen atoms, and significantly enhances CO oxidation efficiency.
A significant structure–activity relationship exists between catalyst synthesis parameters (e.g., calcination temperature, precursor ratio) and both microstructure and surface active site distribution [8,29,30]. For instance, Ye et al. [31] examined how three preparation methods (citrate sol–gel, hydrothermal, and hydrothermal–citrate complexation) affect Mn-Ce catalyst performance. The results showed that the catalyst synthesized via hydrothermal–citrate complexation achieved the highest CO oxidation activity and remained stable for over 1400 min. Similarly, Yang et al. [32] synthesized Mn-Cu bimetallic catalysts via three methods (redox precipitation, hydrothermal, and oxalate complexation), and performed a comparative analysis. The results revealed that the Mn-Cu catalyst synthesized via redox precipitation exhibited the highest low-temperature activity, achieving complete CO conversion at 175 °C. This was attributed to its largest specific surface area, highest Oβ and Mn3+ content, greatest number of oxygen vacancies, optimal redox properties, and strongest CO adsorption capacity. Moreover, the catalyst’s chemical composition significantly affects its catalytic performance [33,34].
This study aims to modify the catalyst by adjusting the Mn/Cu molar ratios. By systematically varying the Mn/Cu molar ratios, we will investigate changes in catalyst performance and explore the mechanisms by which molar ratios influence Mn-Cu catalysts. Using systematic characterization techniques (e.g., XPS, XRD, HR-TEM, BET, H2-TPR, DRIFTS), we will examine the catalysts’ physicochemical properties and assess the impact of Mn/Cu molar ratios on their performance.

2. Results and Discussion

2.1. Effect of Mn/Cu Molar Ratio on Catalytic Activity

To investigate the effect of the Mn/Cu molar ratio on the catalytic performance of CO oxidation, this study evaluated the activity of CuO, MnO2, and a series of MnxCuy catalysts within a temperature range of 25–300 °C. As shown in Figure 1, at a weight hourly space velocity (WHSV) of 60,000 mL·g−1·h−1, the Mn4Cu1 catalyst exhibited the best performance, with a T90 (the temperature at which CO conversion reaches 90%) of 125 °C and complete conversion at 175 °C. Comparison of the data in Figure 1 and Figure 2 reveals that the Mn/Cu ratio significantly influences catalytic performance, with Mn4Cu1 demonstrating the highest low-temperature activity.
To further investigate the effect of space velocity on catalytic performance, the WHSV was increased to 90,000 mL·g−1·h−1 (Figure 3). The results show that increasing space velocity reduces the residence time of reactants, leading to a decline in the activity of all catalysts to varying degrees. However, Mn4Cu1 maintained the best performance under these conditions, demonstrating its excellent adaptability to different operational settings. Systematic studies indicate that, with a fixed Cu molar ratio, Mn doping enhances catalytic activity, which initially increases and then decreases as the Mn/Cu ratio changes. The optimal molar ratio is 4:1, with the activity order being Mn4Cu1 > Mn2Cu1 > Mn6Cu1 > CuO. Conversely, with a fixed Mn molar ratio, Cu doping also improves activity, with the optimal Mn/Cu ratio being 1:2 and the activity order being Mn1Cu2 > Mn1Cu4 > Mn1Cu6 > MnO2. These findings align with those reported by Chen et al. [35], confirming that MnCu bimetallic catalysts outperform single-component CuO and MnO2 catalysts in catalytic performance.

2.2. Effect of Mn/Cu Molar Ratio on Microstructure Characteristics

Figure 4 and Figure 5 present the N2 adsorption–desorption isotherms and corresponding Barrett–Joyner–Halenda (BJH) pore size distributions, respectively. Following IUPAC classification standards [36], the isotherm analysis reveals that Mn1Cu6 and CuO catalysts display Type II behavior, characteristic of non-porous or macroporous materials. In contrast, the other catalysts exhibit Type IV isotherms with H3-type hysteresis loops, indicating mesoporous structures with slit-shaped pores. These distinct pore architectures originate from gas adsorption–condensation phenomena in mesoporous systems. The Type IV materials demonstrate enhanced catalytic performance due to their optimized pore structure, which provides greater surface area and improved mass transport properties.
BET surface area measurements (p/p0 = 0.10–0.30, Table 1) show Mn4Cu1 possesses the highest specific surface area (45.54 m2·g−1) among the tested catalysts. This increased surface area facilitates reactant molecule adsorption and activation through greater exposure of active sites, thereby explaining the superior catalytic activity observed for Mn4Cu1. The combined effects of pore structure and surface area contribute significantly to the enhanced reaction rates.
Notably, all catalysts except Mn1Cu6 and CuO exhibit a significant adsorption surge within the relative pressure range of p/p0 = 0.80–0.95, accompanied by hysteresis loops. This phenomenon results from the capillary condensation effect: at specific relative pressures, gas molecules condense within mesopores to form liquid adsorbates, and the difference in condensation–evaporation pressures during adsorption–desorption leads to hysteresis loop formation. This provides strong evidence for the presence of mesoporous structures in the catalysts. As the relative pressure increases to p/p0 = 0.95–1.00, the continued rise in adsorption suggests the potential presence of macro-porous structures, as macro-pores can accommodate more gas molecules under high pressure.
Analysis of the pore size distribution curves reveals that the average pore size of all catalysts falls within 2–50 nm, consistent with the IUPAC definition of mesoporous materials. Mesoporous structures offer distinct advantages in heterogeneous catalysis: their moderate pore sizes provide a large specific surface area while ensuring effective diffusion of reactant and product molecules, avoiding the diffusion limitations typical of microporous materials [37]. Additionally, significant variations in pore volume and size are observed among catalysts with different Mn/Cu molar ratios, primarily due to differences in metal ion radii, coordination environments, and interaction forces, which collectively shape the catalyst’s microstructure. These differences in pore structure will directly affect the adsorption behavior of reactant molecules, mass transfer processes, and the selectivity and activity of catalytic reactions, providing important insights for optimizing catalyst performance.

2.3. Effect of Mn/Cu Molar Ratio on Phase Structure

To examine the effect of the Mn/Cu molar ratio on the phase structure of Mn-Cu catalysts, X-ray diffraction (XRD) was utilized for characterization, with the results presented in Figure 6. As depicted in Figure 6a, the CuO catalyst displayed only the characteristic diffraction peaks of the monoclinic fluorite CuO phase (PDF#48-1548). In contrast, Mn-doped catalysts (MnxCu1, where x = 2, 4, 6) exhibited mixed phases of pyrolusite MnO2 (PDF#24-0735) and spinel Cu1.4Mn1.6O4 (PDF#35-1030), with no monoclinic fluorite CuO peaks detected. This suggests that, at appropriate Mn/Cu molar ratios, Cu integrates into the tetragonal MnO2 lattice, driving the phase transformation into pyrolusite MnO2 and spinel Cu1.4Mn1.6O4. Further analysis revealed that increasing the Mn/Cu molar ratio led to a gradual decrease in the diffraction peak intensities of pyrolusite MnO2 and spinel Cu1.4Mn1.6O4, accompanied by a narrowing of peak widths. As is well known, these changes are closely associated with catalyst crystallinity. Reduced peak intensity and narrower peak widths indicate decreased crystallinity, likely due to the formation of additional lattice defects. Such defects can serve as active sites for catalytic reactions, significantly impacting catalytic performance. Notably, at a Mn/Cu molar ratio of 4:1, the mixed-phase ratio of α-MnO2 and Cu1.4Mn1.6O4 reached an optimal state, and the Mn4Cu1 catalyst demonstrated the highest catalytic activity. This underscores the importance of metal ratios in optimizing catalyst performance.
As illustrated in Figure 6b, the MnO2 catalyst exclusively exhibited the pyrolusite MnO2 phase (PDF#24-0735). However, upon Cu doping, the phase structure underwent significant alterations. All Mn1Cux (x = 2, 4, 6) catalysts were predominantly composed of monoclinic fluorite CuO (PDF#48-1548). Although the spinel Cu1.4Mn1.6O4 phase (PDF#35-1030) was also identified in the diffraction patterns, its peaks were relatively weak, and no pyrolusite MnO2 peaks were detected. This suggests that Cu doping substantially disrupted the MnO2 crystal structure, with some MnO2 participating in the formation of new phases. With increasing Cu content, the diffraction peak intensity of the monoclinic fluorite CuO phase significantly increased, accompanied by peak broadening, indicating enhanced crystallinity and a potential reduction in lattice defects. Catalytic activity tests revealed that Mn1Cu2 outperformed Mn1Cu4 and Mn1Cu6. This superior performance is likely due to an optimal number of lattice defects, which provide suitable active sites for catalytic reactions. Specifically, the moderate lattice defect density in Mn1Cu2 appears to be more favorable for catalytic activity.

2.4. Effect of Mn/Cu Molar Ratio on Surface Chemical Composition and Valence State Distribution

X-ray photoelectron spectroscopy (XPS) analysis was employed to systematically investigate the effect of Mn/Cu molar ratio on both surface chemical composition and valence state distribution of the catalysts. Figure 7 presents the Mn 2p, Cu 2p, and O 1s spectra for all catalysts. Quantitative analysis of Mn3+, Oβ, and Cu+ on the catalyst surfaces was conducted through peak deconvolution of the XPS spectra, with the results summarized in Table 2. Analysis of the Mn 2p spectra (Figure 7a,b) revealed two characteristic peaks at 642.3 eV and 654 eV, corresponding to the Mn 2p1/2 and Mn 2p3/2 spin–orbit peaks, respectively. A distinct satellite peak at 648.4 eV was observed across all catalysts, confirming the presence of Mn2+ species. To further elucidate the valence state distribution of Mn, the Mn 2p1/2 peak was deconvoluted into three components: 652.1 eV (Mn2+), 654.1 eV (Mn3+), and 656.1 eV (Mn4+). Similarly, the Mn 2p3/2 peak was resolved into three components: 640.5 eV (Mn2+), 642.1 eV (Mn3+), and 643.6 eV (Mn4+). These findings demonstrate that Mn exists in three valence states (Mn2+, Mn3+, and Mn4+) in all catalysts.
Quantitative analysis (Table 2) demonstrates that, under fixed Cu molar ratios, the Mn3+ content initially rises and then declines with increasing Mn doping. The Mn3+ proportions follow the order Mn4Cu1 (53.89%) > Mn2Cu1 (52.15%) > Mn6Cu1 (48.40%) > CuO, with the highest Mn3+ content observed at a molar ratio of 4:1. Similarly, under fixed Mn molar ratios, the Mn3+ content exhibits an initial increase followed by a decrease with increasing Cu doping, with proportions ordered as follows: Mn1Cu2 (49.01%) > Mn1Cu4 (44.20%) > Mn1Cu6 (39.49%) > MnO2 (36.28%), peaking at a molar ratio of 1:2. A significant positive correlation exists between Mn3+ content and catalyst activity, likely due to the role of Mn3+ in facilitating the formation of oxygen vacancies rich in active oxygen. Previous studies indicate that Mn3+ ions induce a pronounced Jahn–Teller effect, causing d-orbital splitting and their filling into lower energy levels [38]. This elongates the Mn-O bond length, reduces bond energy, and promotes the dissociation of surrounding oxygen atoms, fostering the generation of active oxygen species. Moreover, Cu doping enhances the reduction of Mn4+ to Mn3+, increasing Mn3+ content and promoting oxygen vacancy defects. Consequently, the Mn4Cu1 catalyst, with the highest oxygen vacancy concentration, creates favorable conditions for the redox transformation between Mn3+ and Mn4+, which likely underpins its superior catalytic performance.
The Cu 2p spectra of CuO, MnO2, and MnxCuy catalysts exhibit asymmetric peaks, as shown in Figure 7c,d. Deconvolution of these spectra revealed two spin–orbit components: 2p1/2 (949–958 eV) and 2p3/2 (929–937 eV). The Cu 2p1/2 peak was resolved into two distinct features at 950.4 eV (Cu+) and 954.3 eV (Cu2+), while the Cu 2p3/2 peak was separated into peaks at 930.9 eV (Cu+) and 940.3 eV (Cu2+). Shake-up satellite peaks were also identified near 961 eV and 942.1 eV, further confirming the presence of Cu2+ species. These findings demonstrate the coexistence of CuO and Cu2O on the sample surfaces, indicating the presence of both Cu2+ and Cu+. Previous studies suggest that CO molecules are preferentially adsorbed on Cu+ sites, where σ-bonding and π-back-bonding interactions facilitate their activation and subsequent reactions [39]. This CO oxidation process relies on the synergistic interplay between Cu+ and Cu2+ species [40]. Notably, the stable incorporation of Cu+ within the spinel structure underscores strong Mn-Cu interactions in these bimetallic oxides. While an optimal Cu+ content promotes efficient electron transfer between Mn and Cu, excessive Cu+ may disrupt the structural integrity of the composite oxides, compromising catalyst stability. As a result, the Mn4Cu1 catalyst, with its well-balanced Cu+ concentration, is expected to demonstrate superior catalytic performance. Further supporting this mechanism, the Mn 2p spectra analysis reveals an electron transfer process (Cu2+ + Mn3+ ↔ Mn4+ + Cu+) in the Mn-Cu system. The coexistence of Mn3+ and Cu+ not only enhances charge transfer between the oxides but also promotes oxygen defect formation, thereby accelerating the CO oxidation reaction.
Figure 7e,f demonstrate that the O 1s spectra of all catalysts, except CuO, could be deconvoluted into three distinct peaks. The peak at 528.7 eV corresponds to surface lattice oxygen (Oα), while the peak at 530.9 eV represents chemically adsorbed oxygen (Oβ), potentially comprising surface hydroxyl groups, oxygen vacancies, and/or carbonate species [41]. The peak at 533.4 eV is assigned to surface hydroxyl oxygen (Oλ). Notably, Oβ exhibits superior mobility compared to Oα, which significantly enhances the redox cycle and improves CO catalytic oxidation performance. Quantitative analysis of peak areas revealed the relative content of surface adsorbed oxygen (Oβ) for each catalyst, as detailed in Table 2. With a fixed Cu molar ratio, Mn introduction substantially increased Oβ content. The Oβ content initially increased then decreased with increasing Mn/Cu molar ratio, following the order Mn4Cu1 (32.26%) > Mn2Cu1 (29.98%) > Mn6Cu1 (27.30%) > CuO (25.30%), peaking at Mn/Cu = 4:1. Similarly, with a fixed Mn molar ratio, Cu doping enhanced Oβ content, which initially increased then decreased with decreasing the molar ratio, following the order Mn1Cu2 (30.98%) > Mn1Cu4 (29.87%) > Mn1Cu6 (28.12%) > MnO2 (27.03%), reaching a maximum at Mn/Cu = 1:2. Importantly, the Oβ content variation trend strongly correlated with catalyst activity order.
The Mn4Cu1 catalyst demonstrated the highest surface-adsorbed oxygen content, which aligns closely with its superior catalytic activity. This observation suggests that the Mn4Cu1 catalyst possesses a stronger oxygen adsorption capacity, as confirmed by activity test results. Comparative analysis of the binding energies of Oβ and Oα across different catalysts revealed that the Mn4Cu1 catalyst exhibited lower binding energy, indicating higher reactivity of its surface oxygen species. Lower binding energy facilitates the activation of oxygen species, enabling their participation in chemical reactions and providing a theoretical foundation for the catalyst’s high activity in CO oxidation reactions. In contrast to pure MnO2, the incorporation of Cu significantly enhanced the oxygen adsorption capacity of the Mn-Cu catalyst. This improvement is attributed to Cu doping, which induces the formation of additional surface lattice defects, thereby optimizing the material’s oxygen adsorption and storage performance. Moreover, as the Cu content increased, the binding energy of Oβ exhibited an upward trend. This suggests that Cu doping may partially substitute O atoms with Cu atoms in the Cu-O-Mn structure (O→Cu), increasing the instability of O and generating more reactive O atoms. These findings offer novel insights into the mechanistic role of Cu doping in enhancing the performance of Mn-based catalysts [42].

2.5. Effect of Mn/Cu Molar Ratio on Reducibility

To assess the reduction properties of the catalysts, H2-TPR technology was employed for sample characterization. Figure 8 displays the H2-TPR profiles of the catalysts, revealing that the reduction process of all samples can be deconvoluted into four distinct peaks. The characteristic temperatures and H2 consumption are detailed in Table 3. As illustrated in Figure 8a, the pure CuO catalyst exhibits two characteristic reduction peaks at 260 °C and 304 °C, corresponding to the sequential reduction processes of Cu2+→Cu+ and Cu+→Cu0, respectively. The introduction of Mn significantly alters the catalyst’s reduction behavior, with two new peaks emerging in the high-temperature region. These peaks correspond to the reduction processes of Mn4+→Mn3+ and Mn3+→Mn2+. Data from Table 3 indicate that Mn doping shifts the reduction peaks of both Cu and Mn species to lower temperatures, suggesting a reduction in activation energy and a notable enhancement in reduction performance. Notably, as the Mn/Cu molar ratio increases, the reduction peak positions initially decrease and then increase, with the Mn4Cu1 sample exhibiting the lowest reduction temperature. This trend is attributed to the synergistic effect between Mn and Cu at an optimal doping ratio, which modifies the catalyst’s electronic structure and surface properties, thereby facilitating the reduction reaction. However, excessive Mn content may disrupt this synergy, leading to an increase in the reduction temperature.
Figure 8b reveals that the pure MnO2 catalyst displays two distinct reduction peaks at 356 °C and 402 °C, attributed to the reduction processes of Mn4+→Mn3+ and Mn3+→Mn2+, respectively. The incorporation of Cu introduces two additional reduction peaks in the low-temperature region, corresponding to the sequential reduction of Cu2+→Cu+ and Cu+→Cu0. This observation demonstrates that Cu doping significantly modifies the reduction behavior of the MnO2 catalyst. Furthermore, as the Mn/Cu molar ratio increases, the reduction peak positions initially decrease and subsequently rise. Notably, the Mn1Cu2 sample exhibits the lowest reduction temperature, underscoring the pronounced influence of the Mn-Cu synergistic effect on the catalyst’s reduction performance.
A detailed analysis of Figure 8 and Table 3 shows that the molar ratio of 4:1 results in the lowest reduction temperatures for Cu and Mn in the catalyst, recorded at 209 °C, 265 °C, 303 °C, and 336 °C, respectively. These lower reduction temperatures suggest that the catalyst can facilitate redox reactions under mild conditions, highlighting its excellent redox performance. In CO catalytic oxidation reactions, superior redox performance is crucial for enhancing catalytic activity, as it enables rapid transitions between the oxidized and reduced states of the catalyst. This, in turn, promotes more effective adsorption and activation of CO molecules, accelerating the reaction between CO and oxygen. These findings align closely with the exceptional activity of the Mn4Cu1 catalyst in practical catalytic applications, further corroborating the results of the H2-TPR experiments. XPS analysis reveals that the reversible charge transfer process of Cu+ + Mn4+ ↔Cu2+ + Mn3+ significantly improves the material’s reduction performance, confirming that an optimal metal ratio enhances the metal–metal interaction between Mn and Cu, thereby reducing the reduction temperature. Moreover, the Mn4Cu1 sample exhibits the highest H2 consumption at 10.82 mmol·g−1, indicating greater reactivity and stronger reduction capability during the process. This further underscores the superior redox performance and catalytic activity of the Mn4Cu1 catalyst.

3. Catalyst Synthesis and Characterization

3.1. Catalyst Synthesis of CuO, MnO2 and MnxCuy

The following steps were taken to achieve catalyst synthesis: weigh 0.53 g of sodium hydroxide (NaOH) and dissolve it in 20 mL of a mixed solvent (ethanol and deionized water, 1:1 v/v) to prepare solution A. Next, dissolve 2.50 g of copper acetate (Cu(CH3COO)2) in 200 mL of deionized water to prepare solution B1. Using a peristaltic pump, slowly add solution A dropwise to solution B1 under continuous stirring for 2 h to obtain suspension C. Subsequently, transfer suspension C into a PTFE-lined high-pressure reactor and perform a hydrothermal reaction at 140 °C for 12 h. After the reaction, collect the precipitate via centrifugation and wash repeatedly with deionized water until neutral to remove residual ions. Finally, dry the washed product in an oven at 80 °C for 12 h, then calcine it in a muffle furnace at 450 °C for 2 h to obtain the CuO catalyst in black powder form.
Following a similar procedure, dissolve 2.11 g of potassium permanganate (KMnO4) in 200 mL of deionized water to prepare Solution B2, which serves as the precursor for MnO2 catalyst powder. MnxCuy catalysts with different Mn-to-Cu ratios can be synthesized by adjusting the amounts of KMnO4 and Cu(CH3COO)2 in solution B3. Specifically, adjust the Mn/Cu molar ratios to 6:1, 4:1, 2:1, 1:2, 1:4, and 1:6, corresponding to Mn6Cu1, Mn4Cu1, Mn2Cu1, Mn1Cu2, Mn1Cu4, and Mn1Cu6, respectively.

3.2. Catalyst Characterization

To comprehensively assess catalyst performance, various characterization techniques were employed, including XRD, HR-TEM, SEM, BET, XPS, EPR, H2-TPR, CO-TPD, and in situ DRIFTS. The detailed methodologies for these characterizations are described in previous studies [43,44,45].

3.3. Evaluation of Catalyst Activity

CO oxidation activity was evaluated in a fixed-bed reactor. The gas mixture contained 1% CO, 16% O2, 50 ppm SO2 (when used), 10% H2O (when used), and N2. The weight hourly space velocity (WHSV) was 60,000 and 90,000 mL·g−1·h−1. Detailed information on catalytic activity testing is provided in previous studies [32].

4. Conclusions

In this study, Mn-Cu bimetallic catalysts with varying Mn/Cu molar ratios were synthesized via the redox precipitation method. The catalysts’ structural characteristics, phase composition, surface properties, and redox behavior were analyzed using techniques such as XRD, BET, etc. This study aimed to explore the effect of molar ratios on the CO catalytic performance of Mn-Cu bimetallic catalysts. The main findings are summarized as follows:
(1)
The Mn4Cu1 catalyst exhibited optimal low-temperature catalytic activity, achieving complete CO conversion (100%) at 175 °C. For the MnxCu1 catalysts, catalytic activity showed a volcano-shaped trend, initially increasing and then decreasing with higher x values. Similarly, the Mn1Cuy catalysts displayed an analogous trend as y decreased.
(2)
All synthesized catalysts possessed a well-defined mesoporous structure, with pore diameters distributed between 2 and 50 nm. It should be noted that the Mn4Cu1 catalyst exhibited the highest specific surface area of 45.54 m2/g, significantly surpassing the other catalysts.
(3)
The MnxCu1 catalysts exhibited mixed crystalline phases of pyrolusite MnO2 and spinel Cu1.4Mn1.6O4, whereas the Mn1Cuy catalysts mainly consisted of monoclinic fluorite CuO.
(4)
The Mn4Cu1 catalyst has the highest Mn3+ content, reaching 53.89%, which promotes the generation of activated oxygen species and significantly enhances the catalyst’s redox performance.

Author Contributions

Conceptualization, C.L.; methodology, Y.J.; software, Y.S.; validation, C.L.; investigation, Y.S. and P.L.; data curation, Y.S. and X.S.; writing—original draft preparation, C.L., Y.S. and Z.Y.; writing—review and editing, C.L. and Z.Y.; project administration, Z.Y.; funding acquisition, Y.J. and Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (52476143); Shandong Provincial Natural Science Foundation (ZR2024ME023); Qingdao Natural Science Foundation (24-4-4-zrjj-174-jch); Independent Innovation Research Program Projects of China University of Petroleum (21CX06012A); Development Plan for Youth Innovation Teams in Higher Education Institutions of Shandong Province (2023KJ064); Fundamental Research Funds for the Central Universities and the Innovation fund project for graduate student of China University of Petroleum (East China) (24CX04043A).

Data Availability Statement

The original contributions presented in the study are included in the article and further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00353 g001
Figure 1. CO activity curves of CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, and N2 balance, WHSV = 60,000 mL·g−1·h−1).
Figure 1. CO activity curves of CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, and N2 balance, WHSV = 60,000 mL·g−1·h−1).
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Figure 2. Temperature of CO conversion reaches 90% for CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, N2 balance, WHSV = 60,000 mL·g−1·h−1).
Figure 2. Temperature of CO conversion reaches 90% for CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, N2 balance, WHSV = 60,000 mL·g−1·h−1).
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Figure 3. CO activity curves of CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, and N2 balance, WHSV = 90,000 mL·g−1·h−1).
Figure 3. CO activity curves of CuO, MnO2 and MnxCuy catalysts (1 vol% CO, 16 vol%O2, and N2 balance, WHSV = 90,000 mL·g−1·h−1).
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Figure 4. N2 uptake and desorption curves of CuO, MnO2 and MnxCuy.
Figure 4. N2 uptake and desorption curves of CuO, MnO2 and MnxCuy.
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Figure 5. Pore size distribution curves of CuO, MnO2 and MnxCuy.
Figure 5. Pore size distribution curves of CuO, MnO2 and MnxCuy.
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Figure 6. XRD patterns of CuO, MnO2 and MnxCuy.
Figure 6. XRD patterns of CuO, MnO2 and MnxCuy.
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Catalysts 15 00353 g007aCatalysts 15 00353 g007b
Figure 7. XPS spectra of CuO, MnO2 and MnxCuy catalysts. Analysis of the Mn 2p spectra (a,b), Cu 2p spectra (c,d), and O 1s spectra (e,f).
Figure 7. XPS spectra of CuO, MnO2 and MnxCuy catalysts. Analysis of the Mn 2p spectra (a,b), Cu 2p spectra (c,d), and O 1s spectra (e,f).
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Figure 8. H2-TPR curves of CuO, MnO2 and MnxCuy catalysts.
Figure 8. H2-TPR curves of CuO, MnO2 and MnxCuy catalysts.
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Table 1. Pore structure characteristics of CuO, MnO2 and MnxCuy catalysts.
Table 1. Pore structure characteristics of CuO, MnO2 and MnxCuy catalysts.
CatalystSpecific Surface Areas (m2·g−1)Pore Volume (cm3·g−1)Pore Size (nm)
CuO4.580.0124.52
Mn2Cu118.380.0414.28
Mn4Cu145.540.0911.14
Mn6Cu137.780.1823.15
MnO28.990.0315.39
Mn1Cu214.620.1026.60
Mn1Cu49.700.0631.53
Mn1Cu68.610.0322.37
Table 2. Surface elemental properties of CuO, MnO2 and MnxCuy catalysts.
Table 2. Surface elemental properties of CuO, MnO2 and MnxCuy catalysts.
CatalystsSurface Chemical Composition (%)Mn3+/Mn
(%)		Oβ/O
(%)		Cu+/Cu
(%)
MnOCu
MnO230.1569.85—36.2827.03/
Mn1Cu227.0167.195.8049.0130.9822.14
Mn1Cu422.9768.005.8144.2129.8710.25
Mn1Cu626.7365.028.2539.4928.128.48
CuO27.2663.679.07/25.3022.44
Mn2Cu111.2765.4523.2852.1529.9820.91
Mn4Cu18.1260.8631.0253.8932.2619.81
Mn6Cu17.0156.1936.848.84223.01
Table 3. Analysis data of H2-TPR profiles for the catalysts.
Table 3. Analysis data of H2-TPR profiles for the catalysts.
CatalystsCharacteristic TemperaturesH2 Consumption (mmol/g)
α (°C)β (°C)λ (°C)η (°C)
MnO2//35640210.75
Mn1Cu223427832936610.80
Mn1Cu423528233636910.66
Mn1Cu6277324//10.51
CuO260304//10.22
Mn2Cu122027432436310.55
Mn4Cu120926530333610.82
Mn6Cu122727933237110.27
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Liang, C.; Sun, Y.; Li, P.; Jiang, Y.; Sun, X.; Yang, Z. Effect of Mn/Cu Molar Ratios on CO Oxidation Activity of Mn-Cu Bimetallic Catalysts. Catalysts 2025, 15, 353. https://doi.org/10.3390/catal15040353

AMA Style

Liang C, Sun Y, Li P, Jiang Y, Sun X, Yang Z. Effect of Mn/Cu Molar Ratios on CO Oxidation Activity of Mn-Cu Bimetallic Catalysts. Catalysts. 2025; 15(4):353. https://doi.org/10.3390/catal15040353

Chicago/Turabian Style

Liang, Cong, Yingchun Sun, Peiyuan Li, Ye Jiang, Xin Sun, and Zhengda Yang. 2025. "Effect of Mn/Cu Molar Ratios on CO Oxidation Activity of Mn-Cu Bimetallic Catalysts" Catalysts 15, no. 4: 353. https://doi.org/10.3390/catal15040353

APA Style

Liang, C., Sun, Y., Li, P., Jiang, Y., Sun, X., & Yang, Z. (2025). Effect of Mn/Cu Molar Ratios on CO Oxidation Activity of Mn-Cu Bimetallic Catalysts. Catalysts, 15(4), 353. https://doi.org/10.3390/catal15040353

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