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Article

Preparation of Pt/xMnO2-CNTs Catalyst and Its Electrooxidation Performance in Methanol

by
Guang Chen
1,
Zhijun Teng
1,
Hanqiao Xu
2 and
Hongwei Li
2,*
1
Gansu Yinguang Juyin Chemical Industry Group Co., Ltd., Baiyin 730900, China
2
School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 864; https://doi.org/10.3390/catal15090864
Submission received: 29 July 2025 / Revised: 30 August 2025 / Accepted: 4 September 2025 / Published: 7 September 2025
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Abstract

In this study, MnO2-CNTs composite support was prepared by citric acid reduction method, and then, Pt nanoparticles were loaded on the surface by ethylene glycol reduction method to obtain a series of Pt/xMnO2-CNTs catalysts. Structural characterization (TEM, XRD, HRTEM) showed that Pt nanoparticles were uniformly dispersed on the surface of the catalyst with an average particle size of 3.6 nm. Electrochemical tests show that when the content of MnO2 is 20 wt.%, the Pt/20wt.%MnO2-CNTs catalyst has the best methanol oxidation performance, and its mass activity and long-term stability are 4.0 times and 5.41 times that of commercial Pt/C, respectively. The in situ FTIR results showed that MnO2 promoted the dissociation of water through synergistic effect, generated abundant OH species, accelerated the oxidation of CO intermediates, and inhibited the poisoning of Pt sites. In this study, it is clear that the excellent performance of Pt/xMnO2-CNTs is due to multiple synergistic effects. Modified carbon nanotubes facilitate proton conduction, Pt nanoparticles effectively activate methanol, and MnO2 modulates reaction intermediates via its bifunctional mechanism. This comprehensive mechanism understanding provides a theoretical basis for the design of high-performance catalysts for direct methanol fuel cells.

Graphical Abstract

1. Introduction

With the rapid development of society, the growing demand for energy in various countries has been accompanied by increasingly severe environmental issues. The development of novel, sustainable, and environmentally friendly energy conversion technologies and devices has become an imperative strategy [1,2,3,4,5]. Direct methanol fuel cells (DMFCs) are innovative green energy conversion devices that directly convert the chemical energy stored in methanol into electrical energy [6]. DMFCs have garnered significant attention in recent years due to their advantages of simple structure [7], high energy density, high efficiency [8], and environmental friendliness [9,10]. However, the anode methanol oxidation reaction (MOR) in DMFCs is constrained by slow kinetics and catalyst susceptibility to CO poisoning [11], which significantly reduce the energy conversion efficiency of DMFCs. These issues have impeded the large-scale commercialization of DMFCs, fundamentally due to the low catalytic activity, high cost, and poor CO tolerance of anode electrocatalysts. In summary, DMFCs present a promising research avenue with wide application potential. To achieve early commercialization, addressing the challenges associated with DMFCs anode catalysts is crucial. Therefore, designing and developing efficient and stable anode catalysts are essential for advancing the industrial application of DMFCs.
Supported metal nanoparticle catalysts are critically important in numerous industrial catalytic processes, including the production of chemicals and fuels, pollution control, and clean energy technologies such as fuel cells. The support, as an integral component of supported catalysts, plays a vital role in these reactions. High-quality supports can facilitate the adsorption of metal nanoparticles (NPs), regulate metal–support interactions, and enhance mass transfer of reactants. Carbon-based materials have been extensively utilized as catalyst supports in electrocatalysis [12,13]. Common electrocatalyst supports include carbon black (Vulcan XC-72, activated carbon) [14], mesoporous carbon (MC) [15], carbon nanotubes (CNTs) [16], carbon nanofibers (CNF) [17], and graphene [18]. However, single-carbon supports often exhibit surface inertness, insufficient anchoring of metal nanoparticles (NPs), and a lack of catalytic active sites. Therefore, modifying single-carbon supports is a common strategy to enhance the fixation of metal nanoparticles, improve atomic utilization, and increase the number of active reaction sites. Previously, researchers have focused on (1) constructing PtM alloys with specific morphologies (M = Fe, Co, Ni, Cu, Ru); (2) loading Pt onto functionalized carbon supports (C = CNTs, RGO, NC); and (3) designing Pt single-atom catalysts. For example, Li et al. [19] doped single-walled carbon nanotubes (SWCNTs) by exploiting the electronegativity differences between N and C atoms. Different annealing temperatures led to the formation of various nitrogen species, and the synergistic effects among these species improved the electronic structure of Pt NPs, enhancing MOR activity. However, issues such as high cost and poor CO tolerance, which result in slow kinetics, have not been adequately addressed. There is an urgent need to develop non-Pt-based MOR catalysts that are cost-effective, highly active, and stable. Additionally, catalyst supports can influence catalytic performance through strong metal–support interactions (SMSI) [20,21]. Generally, SMSI between metals and reducible oxide supports (e.g., TiO2 [22], CeO2 [23]) can alter the electronic structure of active metal components, potentially optimizing catalytic activity and selectivity. Alternatively, metal oxide supports can directly participate in certain catalytic reactions through synergy with metals, such as the dual catalytic sites observed at the Au/TiO2 interface [24]. Recently, carbon-transition metal oxide (C-MOx, where M = TiO2, CeO2, MnO2, etc.) composite supports, which combine the advantages of both carbon and oxides, have garnered considerable attention in electrocatalysis and heterogeneous catalysis [25,26,27]. In these composites, carbon facilitates electron transfer kinetics, while transition metal oxides, through SMSI with the carbon support, regulate the electronic structure of the active metals or directly participate in the electrocatalytic process. Back in 2009, Zhou et al. [28] reported a study. They loaded hydrated MnO2, Pt, and PtRu nanoparticles on the surface of CNTs, step by step, and carried out electrochemical tests in an acidic system of 1M CH3OH + 1M HClO4. It is found that MnO2 on the surface of CNTs can significantly enhance the electrocatalytic oxidation performance of methanol by improving the proton conduction efficiency of the catalyst. However, this work only focuses on the acidic environment, and neither the specific mechanism of MOR nor the alkaline system is involved. It is worth noting that alkaline conditions show unique advantages in the practical application of DMFCs [29]. Alkaline electrolyte is more friendly to the stability of metal catalysts, which can reduce metal dissolution [30,31]. These characteristics make it an important direction to break through the limitations of acidic systems. In view of this, systematically exploring the mechanism of action of MnO2 on MOR in alkaline environment and filling the gap of reaction path analysis can not only improve the structure–activity relationship of Pt-based catalysts, but also promote the practical development of DMFCs to alkaline systems, which has important scientific value and application significance.
In this study, we employed a simple citric acid reduction method to prepare MnO2-CNTs composite supports, followed by the deposition of Pt nanoparticles on the MnO2-CNTs surface using an ethylene glycol reduction method. A series of Pt/XMnO2-CNTs catalysts were synthesized. Characterization results revealed that the Pt nanoparticles were uniformly dispersed on the catalyst surface, with an average particle size of 3.6 nm. Electrochemical testing indicated that when the MnO2 content was 20 wt.%, the Pt/MnO2-CNTs catalyst exhibited optimal methanol oxidation performance, with activity and stability being 4.0 and 5.41 times that of commercial Pt/C catalysts, respectively. This work provides theoretical guidance for the preparation of high-performance Pt-based catalysts.

2. Results and Discussion

The phase structure of the catalyst was studied by X-ray diffraction (XRD), as shown in Figure S1. The diffraction peak at 2θ = 26.3° corresponds to the (002) plane of graphite structure in carbon nanotubes (JCPDS 41-1487) [29]. For Pt/xMnO2-CNTs catalyst, the diffraction peaks observed at 2θ = 39.8°, 46.3°, 67.7°, and 81.3° are attributed to the (111), (200), (220), and (311) crystal planes of Pt (PDF#04-0802) [32]. It can be observed from the figure that the corresponding peak position is obviously broadened, indicating that Pt exists in the form of nanocrystals. Meanwhile, the diffraction peaks at 2θ = 19.0° and 37.2° correspond to the (111) and (311) crystal planes of MnO2 (PDF#42-1169) [33]. However, the diffraction peak of standard MnO2 should be sharp, and the MnO2-related peak in the sample is also broadened and relatively weak, suggesting that MnO2 may be amorphous or semi-crystalline. In addition, the presence of Pt does not cause the shift in the diffraction peak of MnO2, which indicates that the introduction of Pt does not cause the lattice distortion of MnO2. The sharp peaks of CNTs prove that they have good crystallinity and provide structural support as carriers. XRD results confirmed the successful synthesis of Pt/xMnO2-CNTs catalyst.
According to the XRD pattern, the size of Pt and MnO2 was further determined using the Scherrer formula. The Scherrer formula is a classical formula for estimating the average crystallite size of nanocrystals [34]. Its core principle is based on the inverse relationship between the broadening degree of XRD peaks and the crystallite size [35]. The average particle size of Pt (111) and MnO2 (111) crystal plane direction can be calculated by the formula (Equation (1)), where K is the Scherrer constant, usually 0.89; λ is the wavelength of X-ray, λ= 0.15406 nm; β is the full width at half maximum of the diffraction peak; and θ is the Bragg angle of the diffraction peak.
D = K λ β c o s θ
The analysis of Pt/20wt.%MnO2-CNTs samples shows that the diffraction peaks of Pt(111) crystal plane are fitted by Gauss, and the full width at half maximum and Bragg angle parameters are obtained [35]. The average crystallite size in the crystal plane direction is calculated to be 3.7 nm. Similarly, the characteristic diffraction peaks of MnO2 (111) crystal plane are treated by the same method, and the average crystallite size is 5.6 nm.
In order to accurately quantify the actual loading of Pt and Mn, the quantitative analysis of elements was carried out by inductively coupled plasma optical emission spectroscopy (ICP-OES) [36]. As shown in Table 1, the metal effective loading rates in Pt/10wt.%MnO2-CNTs, Pt/15wt.%MnO2-CNTs, Pt/20wt.%MnO2-CNTs, Pt/25wt.%MnO2-CNTs, and Pt/30wt.%MnO2-CNTs are 76.63 wt.%, 84.94 wt.%, 85.68 wt.%, 84.27 wt.%, and 84.46 wt.%, respectively. The corresponding actual total metal loadings are 22.99 wt.%, 29.73 wt.%, 34.27 wt.%, 37.92 wt.%, and 42.23 wt.%. The data show that the actual load of Mn is highly consistent with the theoretical value, with a deviation mostly within 5%. This indicates that the deposition of Mn has good controllability and that the synthesis process is more stable for regulating the Mn load. At the same time, the theoretical load of Pt is fixed at 20 wt.%, but the actual load is 14–16 wt.%, which increases first and then decreases with the Mn content. When the theoretical load of Mn is 20 wt.%, the actual load of Pt reaches the peak (15.62 wt.%), and the effective load rate of the corresponding system is the highest (85.68%). Currently, the co-deposition efficiency of Pt and Mn is optimal. When the Mn content is further increased, Mn still maintains efficient deposition, but the actual load of Pt falls back. This is attributed to the fact that Mn preferentially occupies the active sites on the surface of the carrier at high Mn content, which inhibits the adsorption and reduction in Pt precursors, resulting in a slight decrease in Pt loading rate.
The specific surface area and pore size distribution of the catalysts were analyzed using N2 physical adsorption–desorption methods. As depicted in Figure S2, the N2 adsorption–desorption isotherms of the Pt/xMnO2-CNTs catalysts exhibit type IV curves, with hysteresis loops observed at P/P0 = 0.7~0.8, indicative of mesoporous structures. The BET characterization data are summarized in Table S1, showing that all catalysts possess average pore diameters within the mesoporous range (2–50 nm). With increasing MnO2 content, the specific surface area initially increases and then decreases, suggesting that a moderate amount of MnO2 enhances the specific surface area, while excessive MnO2 may block the pores, reducing the surface area. Pt/20wt.%MnO2-CNTs exhibited a moderate pore size and relatively large specific surface area, favorable for the loading and dispersion of metal particles, as well as the adsorption of reactants and desorption of reaction products [37].
Subsequently, the morphology and elemental distribution of Pt/xMnO2-CNTs were investigated using SEM and TEM. Figure 1 displays the morphological features and elemental mapping of the Pt/20wt.%MnO2-CNTs catalyst. As observed in Figure 1a,b, the morphology of metal particles is clearly visible, and the uniform distribution of metal particles on the carbon nanotube surface indicates that the oxygen-containing functional groups introduced by pretreatment effectively anchor the metal particles, significantly enhancing the utilization of Pt. Some flocculent structures, attributed to MnO2, were also observed. Statistical analysis revealed that the average particle size of the metal particles is 3.6 nm, which is conducive to optimal catalytic performance. The particle size distribution histogram, derived from the statistical analysis of 65 particles in the region shown in Figure 1b, is consistent with the calculation results obtained using the Scherrer formula. Figure 1c shows the presence of three lattice spacings, 0.20, 0.23, and 0.245 nm, corresponding to the Pt (200), Pt (111), and MnO2 (400) planes, respectively [38,39]. This is consistent with the earlier analysis, which indicates that Pt possesses a face-centered cubic structure, with the exposure of more Pt(111) planes being crucial for enhanced catalyst performance. Elemental mapping in Figure 1e through i demonstrates the presence of Pt, Mn, C, and O, with a uniform distribution and no significant agglomeration. The microscopy results further confirm the successful synthesis of the Pt/xMnO2-CNTs catalyst, and these results are consistent with those obtained from XRD.
The elemental composition and chemical states of the Pt/xMnO2-CNTs catalysts were investigated using X-ray photoelectron spectroscopy (XPS). Figure 2 presents the XPS spectra of Pt/xMnO2-CNTs. The peaks at 74.5, 285.2, and 641.9 eV in Figure 2(a1–a5) correspond to Pt 4f, C 1s, and Mn 2p, respectively, where C primarily originates from the carbon nanotube support, and Pt and Mn serve as active components [40]. The valence states and content distribution of the main elements were analyzed by fitting the narrow scan spectra of the catalysts. In Figure 2(b1), the characteristic peaks at 71.63 and 74.99 eV correspond to Pt0, while those at 72.35 and 75.86 eV correspond to Pt2+, and the peaks at 77.17 and 78.34 eV correspond to Pt4+ [41]. The Mn spectral results in Figure 2(c1) are less ideal due to the low Mn content in the sample. In Figure 2(b2), the characteristic peaks at 71.65 and 74.97 eV correspond to Pt0, those at 72.28 and 75.72 eV to Pt2+ [42], and the peak at 78.25 eV to Pt4+. Similarly, in Figure 2(c2), the Mn spectral results are not ideal due to low Mn content. In Figure 2(b3), the characteristic peaks at 71.78 and 75.06 eV correspond to Pt0, while those at 72.96 and 76.10 eV correspond to Pt2+, and the peak at 77.97 eV corresponds to Pt4+. Figure 2(c3) shows the characteristic peaks at 652.66 eV for Mn3+, 641.62 and 654.32 eV for Mn4+, and 646.36 and 656.97 eV for Mn5+ [43]. In Figure 2(b4), the characteristic peaks at 71.76 and 75.06 eV correspond to Pt0, 72.57 and 75.98 eV to Pt2+, and 77.61 and 78.78 eV to Pt4+. In Figure 2(c4), the characteristic peaks at 642.66 and 653.61 eV correspond to Mn4+, and those at 646.03 and 657.22 eV correspond to Mn5+. In Figure 2(b5), the characteristic peaks at 71.74 and 75.09 eV correspond to Pt0, 72.78 and 76.08 eV to Pt2+, and the peak at 77.92 eV corresponds to Pt4+. In Figure 2(c5), the characteristic peaks at 653.54 eV correspond to Mn3+, 641.91 eV to Mn4+, and 646.64 and 655.45 eV to Mn5+. These results indicate the coexistence of Pt0, Pt2+, and Pt4+ in the catalysts. It can be seen from Table 2 that Pt mainly exists in the form of metallic Pt0, with Pt2+ and Pt4+ being less prevalent, indicating incomplete reduction. The metal Mn takes MnO2 as the main phase to ensure the structural stability, and the oxygen defects introduced by Mn3+ and Mn5+ promote Pt dispersion and electron transfer. There is uncertainty in the attribution of oxidation states here, and the relevant signals may be derived from surface oxidation or interface effects. Studies have shown that Pt0 is the catalytically active center in MOR, with a higher Pt0 content correlating with higher catalyst activity and Pt utilization efficiency. Among the five catalysts, Pt/20wt.%MnO2-CNTs has the highest Pt0 content, suggesting superior electrocatalytic performance. The presence of Pt2+ and Pt4+ indicates incomplete reduction in the Pt precursor during preparation. XPS results reveal that Mn exists primarily as Mn4+, i.e., MnO2, with minor amounts of Mn3+ and Mn5+. The XPS analysis confirms the successful preparation of the Pt/xMnO2-CNTs catalysts.
The electrocatalytic activity for methanol oxidation reaction (MOR) of the catalysts was evaluated using cyclic voltammetry (CV). The ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) serves as a key indicator for assessing the tolerance of Pt-based catalysts to poisoning during MOR [44,45]. The peak observed in the forward scan (If) corresponds to methanol oxidation, while the peak in the reverse scan (Ib) corresponds to the oxidation of carbonaceous intermediates. The current density of the forward scan peak is indicative of the electrocatalytic activity for MOR. A higher If/Ib ratio suggests better CO tolerance of the catalyst [46]. Figure 3a presents the CV curves of Pt/xMnO2-CNTs and Pt/C catalysts measured in N2-saturated 1 M KOH + 1 M CH3OH solution. As shown in Figure 3a, the MOR peak current density varies with the MnO2 content. At 10 wt.% with MnO2 doping, the MOR peak current density is 1137.92 mA/mgPt. Increasing the MnO2 content to 15 wt.% results in a peak current density of 1736.52 mA/mgPt, indicating an enhancement in catalyst activity due to MnO2 doping. The peak current density reaches a maximum of 2027.25 mA/mgPt at 20 wt.% MnO2 doping. However, further increasing the MnO2 content to 25 wt.% decreases the peak current density to 1302.81 mA/mgPt, suggesting that excessive MnO2 doping begins to block some of the catalyst’s pores, reducing activity. At 30 wt.% MnO2, the peak current density further decreases to 1067.70 mA/mgPt, indicating that excessive MnO2 severely clogs the pores, significantly diminishing the catalyst’s activity. Thus, the Pt/20wt.%MnO2-CNTs catalyst has the optimal MnO2 doping level among the five catalysts tested.
Furthermore, the electrochemical active surface area (ECSA) of the catalyst is another crucial parameter for evaluating the MOR activity [47]. Figure 3b shows the CV curves of Pt/xMnO2-CNTs and Pt/C catalysts measured in N2-saturated 1 M KOH solution. Standard hydrogen electrode model measured the ECSA of the catalyst. The ECSA value of the Pt/20wt.%MnO2-CNTs catalyst is 79.3 m2/g, while that of the commercial Pt/C catalyst is 34.2 m2/g. From the overall curve, the broad peak of Pt/C catalyst in the oxidation zone (0.8–1.2 V) corresponds to the formation of Pt(OH)2 or PtO2 on the surface of Pt [48], and the symmetrical peak in the reduction zone (0.4–0.8 V) corresponds to the reduction in Pt (OH)2 or PtO2 to Pt, and the current in the low potential zone (<0.4 V) is gentle, only reflecting the weak adsorption of OH− on the surface of Pt, without additional redox peaks. In contrast, the Pt/xMnO2-CNTs catalyst shows a pair of significant redox peaks near 0.2 V in addition to the characteristic peak of Pt. The peak shape and current change with Mn loading, reflecting the regulation of Mn on the electrochemical behavior of Pt surface. This is due to the reversible redox reaction of Mn-based species. In the alkaline environment, the Mn4+ on the surface of MnO2 undergoes a reversible valence state transition with OH−, which directly confirms that the Mn species participates in the low-potential electron transfer, which is the unique interface effect of Pt/xMnO2-CNTs in the alkaline system [49]. It is also the core source of 0.2 V peak. Combined with Table S2, it was found that the oxidation current of Pt/xMnO2-CNTs was significantly higher than that of Pt/C in the high potential region of >0.8 V, indicating that the introduction of Mn promoted the oxidation reaction of Pt surface. The reduction peak area of Pt/xMnO2-CNTs, especially the PtO reduction peak, is generally larger than that of Pt/C, suggesting that Mn increases the exposure of Pt active sites, which is attributed to the smaller average particle size of Pt nanoparticles in Pt/20wt.% MnO2-CNTs (3.6 nm), while the average particle size in Pt/C is about 4.1 nm [50]. In addition, the oxidation/reduction current of Pt/20wt.%MnO2-CNTs reached the peak value, which echoed the law of the highest Pt0 content in Table 2, and verified that 20 wt.% Mn was the optimal ratio of synergistic effect. Figure 3c is the LSV curve of the catalyst. It can be clearly found from the figure that the Pt/20wt.%MnO2-CNTs catalyst has the lowest initial potential (0.31 V), indicating that it has both stronger initial catalytic activity and lower oxidation ability in MOR, reflecting better reaction kinetics. As shown in Figure 3d, the Nyquist curve of Pt/20wt.% MnO2-CNTs catalyst shows the smallest semicircle diameter, corresponding to a lower charge mass transfer resistance [51], indicating that the electrode/electrolyte interface has the lowest charge transfer resistance, which is consistent with the highest mass activity and specific activity of the sample, which is consistent with the CV results.
The mass activity, ECSA, ECSACO, and area-specific activity, as well as the If/Ib data of the catalyst, were summarized in Table S2. Overall, the performance of the Pt/xMnO2-CNTs catalyst exhibits a volcano-like variation with Pt loading. When the Pt loading increases from 10 wt.% to 20 wt.%, the mass activity increases from 1137.92 mA/mgPt to 2027.25 mA/mgPt, which is due to the synchronous improvement of ECSA from 62.48 m2/g to 79.33 m2/g, reflecting the optimization of Pt dispersion and the increase in active sites, and the specific activity reaches 2.56 mA/cm2. It is suggested that the interface electron coupling effect between Pt and MnO2 is the strongest in this loading range, and the intrinsic catalytic ability of a single active site is the best. When the Pt loading exceeded 20 wt.% (e.g., 30 wt.%), although the anti-toxicity (If/Ib = 10.49) was significantly improved due to the decrease in low coordination atoms due to the agglomeration of Pt particles and the weakening of intermediate product adsorption, the ECSA (55.62 m2/g) and specific activity (1.92 mA/cm2) decreased, resulting in a sudden drop in mass activity. Compared with commercial Pt/C, all Pt/xMnO2-CNTs samples have achieved several times the mass activity, ECSA, and specific activity, which confirms the synergistic advantages of dispersion promotion and electronic regulation in the MnO2-CNT carrier. Among them, 20 wt.% Pt is the performance balance point. Subsequently, the anti-toxicity of low-load samples can be optimized by interface engineering, or the dispersion of high-load samples can be improved to break through the performance bottleneck. This durability was assessed through chronoamperometry (CA) tests to determine the long-term electrochemical stability of the catalysts during MOR [52]. Figure 3e shows the CA curves of Pt/xMnO2-CNTs and Pt/C catalysts measured in N2-saturated 1 M KOH + 1 M CH3OH solution. As shown in Figure 3e, the catalysts exhibited a high initial current at the beginning of the test, which then rapidly decreased and stabilized. The CA potential was chosen at 0.7 V vs. RHE, corresponding to the peak potential in the CV curve (Figure 3a), where the catalyst exhibits optimal MOR activity. The rapid decline in the initial stage of the polarization current is due to the formation of intermediates during the MOR. All catalysts exhibited a sharp decrease in current density within the first 100 s, followed by a gradual decline between 100 and 1000 s, and then stabilized with a slow decrease thereafter. Throughout the entire potentiostatic scan, the Pt/20wt.%MnO2-CNTs catalyst not only exhibited a higher methanol electrooxidation current but also demonstrated superior catalytic stability compared to other catalysts in the same group. After 4000 s of steady-state chronoamperometry testing, the methanol oxidation current density of the Pt/20wt.% MnO2-CNTs catalyst (54.24 mA/mgPt) was 5.41 times that of the Pt/C catalyst (10.02 mA/mgPt). Apart from Pt/20wt.%MnO2-CNTs, all other catalysts exhibited lower steady-state current densities than the Pt/C catalyst. The reasons for this observation can be attributed to two main factors: First, at lower MnO2 doping levels [51], the number of exposed active sites might be limited due to the relatively small specific surface area of the support, which hinders the dispersion of nanoparticles and thus partially negates the proton conductivity enhancement provided by MnO2 doping. Second, at higher MnO2 doping levels, the exposure of active sites might also be limited due to pore blockage caused by excessive MnO2, reducing the specific surface area and nanoparticle dispersion, which again offsets the benefits of MnO2 doping on proton conductivity. CO stripping voltammetry was employed to assess the catalysts’ resistance to CO poisoning. CO is the primary poisoning agent generated during methanol electrooxidation, readily occupying Pt active sites and thus reducing the catalyst’s activity and stability. Generally, a lower onset potential and peak potential for CO oxidation indicate a stronger resistance to CO poisoning.
Figure 3f shows the CO stripping curves of Pt/xMnO2-CNTs and Pt/C catalysts in N2-saturated 1.0 M KOH solution. The CO onset oxidation potentials for the five catalysts are presented in Table S3. As indicated, the CO onset oxidation potential for Pt/20wt.%MnO2-CNTs is 0.165 V, lower than that of commercial Pt/C (0.272 V). Compared to the commercial Pt/C catalyst, the CO onset oxidation potentials for the other four catalysts are also reduced to varying degrees. This demonstrates that MnO2 doping can effectively enhance the catalyst’s resistance to CO poisoning. A lower onset potential suggests that CO adsorbed on the surface of Pt nanoparticles can be more easily removed. The appearance of the CO oxidation peak at a lower potential is due to the preferential oxidation and removal of CO adsorbed on Pt nanoparticles, indicating that Pt/20wt.%MnO2-CNTs possesses superior anti-poisoning capabilities. This enhancement can be attributed to the introduction of MnO2, which improves the catalyst’s proton conductivity, accelerates the oxidation of CO and other intermediates, thereby freeing up active Pt sites, reducing the poisoning rate, and enhancing the catalyst’s stability. At the same time, the ECSACO value was further calculated based on this diagram and compared with the previous ECSA of H ads/des [53]. When Mn is 10 wt.%, ECSACO (91.7 m2/g) is much higher than ECSA (62.5 m2/g), indicating that the oxygen vacancy/interface effect of MnO2 enhances the adsorption of CO on Pt. When Mn is 20 wt.%, the two indexes reach the peak synchronously, indicating that the dispersion promotion effect of Mn and the CO adsorption enhancement effect reach the optimal balance. MnO2 effectively inhibits Pt agglomeration, and at the same time, interfacial electron coupling occurs (the highest content of Pt in XPS is 62.38%), which optimizes the CO adsorption energy and maximizes the ECSACO. When Mn is highly loaded, ECSACO plummets, revealing the destruction of Pt active sites. High ECSACO means that more Pt sites can enhance the anti-poisoning ability through the bifunctional mechanism [54], so the cycle stability of 20 wt.% Mn sample should be the best, which also corresponds to the CA results. Overall, the improvement in catalyst activity and stability may be due to the synergistic effect between the unique structure of the modified carbon nanotubes and Pt.
To thoroughly analyze the methanol oxidation mechanism of the catalyst, we employed in situ Fourier-transform infrared spectroscopy to dynamically monitor the reaction process of the catalyst in an electrolyte system composed of 1M methanol and 1M KOH. We then combined the spectral characteristics and electrochemical analysis to derive the reaction path and mechanism( as shown in Figure 4).
In the infrared spectrum of Pt/C catalyst, the linear adsorption CO (COL) peak at 2155 cm−1 and the bridge adsorption CO(COB) peak near 1865 cm−1 are presented, which directly confirms the existence of CO adsorption intermediates in the process of methanol oxidation reaction, corresponding to the typical CO path. At the same time, the characteristic peak of formate at 1590 cm−1 continued to increase with the increase in potential, reflecting the second step reaction of methanol dehydrogenation, namely, the occurrence of the formate pathway. The vibration peak of the water molecule at 1644 cm−1 corresponds to the signal of water [55].
For the Pt/20wt.%MnO2-CNTs catalyst, the spectrum shows a key difference: the intensity of the linear adsorption CO peak is significantly reduced, and the bridge adsorption CO peak is relatively enhanced, indicating that the CO intermediate is transformed from the linear adsorption of the strong poisoning state to the bridge adsorption that is more easily oxidized. The increase in the CO peak value indicates that the intermediate conversion rate is accelerated, and the methanol oxidation process is more intense and efficient. In particular, a downward water molecular characteristic peak appears at 1687 cm−1, corresponding to the water consumption process, which directly confirms that MnO2 can accelerate the dissociation of water through surface oxygen defects and hydroxyl groups, and continuously supply highly active hydroxide species, which is consistent with the results of electrochemical active surface area analysis, that is, MnO2 enhances the bifunctional mechanism by increasing active sites and promoting water activation.
Based on in situ FT-IR characterization and electrochemical data, a synergistic reaction mechanism for methanol oxidation on Pt/XMnO2-CNTs can be constructed. Methanol molecules are adsorbed on Pt active sites and gradually dehydrogenated to form linear and bridge-type adsorbed CO intermediates, while some methanol is dehydrogenated to form formate. There is a competition between the two pathways [55]. MnO2 plays a bifunctional role. On the one hand, it accelerates the dissociation of water by surface oxygen defects and hydroxyl groups and continuously supplies hydroxide as an oxidant; on the other hand, it promotes the transformation of linear adsorption CO to more easily oxidized bridge adsorption, weakens the strong adsorption poisoning effect of CO at Pt sites, and shortens the reaction energy barrier. Finally, the bridged adsorbed CO and formate intermediates are rapidly oxidized to carbon dioxide with the participation of hydroxide ions. The CO pathway achieves efficient conversion due to the supply of hydroxide ions from MnO2, and the proportion of the formate pathway is relatively reduced.

3. Experiment Section

3.1. Materials

Multi-walled carbon nanotubes (purity > 99.9%, OD 8–15 nm, length ~50 μm, SSA > 233 m2·g−1), KMnO4, 5% Nafion solution, and chloroplatinic acid hexahydrate (H2PtCl6·6·H2O) were obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Ethylene glycol (EG), sodium hydroxide (NaOH, AR), potassium hydroxide (KOH, AR), methanol (CH3OH, AR), and sodium borohydride (NaBH4) were all purchased from Sinopharm Chemical Reagent Company. Deionized (DI) water (18.2 MΩ/cm) used in all experiments was prepared by passing through an ultrapure purification system. Commercial Pt/C catalysts were purchased from Shanghai Macklin Co., Ltd., China, with Pt content of 20% and water content ≤ 60%.

3.2. Synthesis of Electrocatalysts

In the preparation of MnO2-CNTs composite support, a certain amount of KMnO4 solution was added to the round bottom flask, and then the pretreated CNTs and an appropriate amount of deionized water were added. The mixture was ultrasonically dispersed for 30 min to uniform; subsequently, 0.05 M citric acid solution was added dropwise at a rate of 5 mL/h by a liquid separation funnel at room temperature with magnetic stirring at 800 rpm. The total amount of citric acid should be controlled at a molar ratio of 1:1.2 to KMnO4 to ensure the complete reduction of MnO4−. Continuously stirring until the purple color of KMnO4 completely fades (indicating that MnO4− has been completely converted to MnO2), the flask is transferred to the oil bath and stirred at 600 rpm for 7 h at 80 °C to promote the growth and uniform deposition of MnO2 on the surface of CNTs. After the reaction, the filtrate was washed repeatedly with deionized water until it was neutral. The washed solid was transferred to an oven and dried at 110 °C for 12 h to remove residual moisture to obtain a black solid powder, denoted as MnO2-CNTs.
When synthesizing Pt/xMnO2-CNTs catalyst, 25 mL ethylene glycol (EG) was added to a 100 mL three-necked flask, and 1.06 mL 0.01931 M Pt precursor solution (H2PtCl6·6H2O/EG) and 20.0 mg MnO2-CNTs were added successively under 800 rpm magnetic stirring. The mixture was sonicated for 30 min to break up the aggregates and then stirred at 800 rpm for 30 min to ensure that the Pt precursor was evenly mixed with MnO2-CNTs; subsequently, 1.0 M NaOH solution was added to adjust the pH to 10, and the flask was equipped with a reflux condenser tube and placed in an oil bath. After reflux heating at 140 °C for 2 h, Pt4+ was reduced to Pt nanoparticles. After natural cooling to room temperature, it was washed with deionized water and anhydrous ethanol to remove residual EG, Na+, and Cl−. The washed solids were transferred to a vacuum drying oven, vacuum dried at 70 °C for 12 h, and then ground into fine powder with agate mortar for use. According to the theoretical MnO2 doping amount (10 wt.%, 15 wt.%, 20 wt.%, 25 wt.%, 30 wt.%), the catalysts with theoretical Pt loading of 20 wt.% were labeled as Pt/10wt.%MnO2-CNTs, Pt/15wt.%MnO2-CNTs, Pt/20wt.%MnO2-CNTs, Pt/25wt.%MnO2-CNTs, and Pt/30wt.%MnO2-CNTs, respectively.

3.3. Characteristic

The scanning electron microscope (SEM) used is the Zeiss Gemini 300 (Oberkochen, Germany), featuring a resolution of 1.0 nm at 15 kV. The acceleration voltage is 0.02~30 kV, the probe beam current is 3 pA~20 nA, and the stability is better than 0.2%/h. Magnification: 10×~1,000,000×; Objective lens: electromagnetic/electrostatic composite lens. Transmission electron microscopy (TEM) was performed using a Talos (New York, NY, USA) F200X transmission electron microscope equipped with a charge-coupled device (CCD) imaging system and an accelerating voltage of 100 kV. Nitrogen physisorption measurements were conducted using a Quantachrome (Boynton Beach, FL, USA) ASiQ analyzer at 77 K, with pore size distributions derived from the adsorption branch via the quenched solid density functional theory (QSDFT) model, which accounts for both microporous and mesoporous contributions. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker (Billerica, MA, USA) AXS D8 diffractometer using Cu Kα radiation (λ = 0.1542 nm), with 2θ scanned from 5° to 90° at 2°/min. The system features a 3 kW X-ray generator (Cu target ceramic tube, 2.2 kW output) and a 0.4 mm × 12 mm focal spot. Metal loadings in catalysts were quantified by PerkinElmer (Waltham, MA, USA) Optima 7300 DV inductively coupled plasma-optical emission spectrometry (ICP-OES). X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific (Waltham, MA, USA) K-Alpha+ instrument with an Al Kα source (hν = 1486.6 eV), operated at 150 W with a 400 μm spot size. Narrow scans utilized a 30 eV analyzer pass energy.

3.4. Electrochemical Measurements

The electrochemical performance evaluation in this study was conducted using a system comprising an electrochemical workstation and a rotating disk electrode (RDE). A classic three-electrode configuration was employed, with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode, and a glassy carbon electrode as the working electrode. All potentials were converted to the reference reversible hydrogen electrode (RHE) using the Nernst equation. The preparation of the working electrode is as follows: First, accurately weigh 2.0 mg of catalyst powder into a 2.0 mL centrifuge tube. Then, add 500 μL of deionised water, 450 μL of anhydrous ethanol, and 50 μL of Nafion solution. The mixture was sonicated for 30 min to obtain a uniformly dispersed suspension. Next, the working electrode was polished to a smooth finish using 1.0 μm and 0.3 μm alumina powder on polishing cloth, followed by ultrasonic cleaning with ethanol. The electrode was then activated in the three-electrode system. A micropipette was used to drop 5 μL of the catalyst suspension onto the polished glassy carbon electrode surface, which was then dried under an infrared lamp. Another 5.0 μL of the catalyst suspension was added and dried, resulting in a total of 10.0 μL per electrode. Two additional working electrodes were prepared in the same manner for each sample, with a minimum of three electrodes tested per sample to minimize errors. The activity of the catalysts was evaluated using cyclic voltammetry (CV) as follows: Nitrogen gas was bubbled through a 1.0 M KOH solution for 30 min to remove dissolved oxygen, followed by CV testing in this solution. The potential was scanned multiple times until a stable CV curve was obtained, with a scan rate of 50 mV/s over a potential range of 0 to 1.2 V. The electrochemical active surface area (ECSA) was calculated using the stable data. Subsequently, the same procedure was conducted in a 1.0 M KOH + 1.0 M CH3OH mixed solution to evaluate the methanol electrooxidation activity and stability of the catalysts. A higher peak current density of methanol oxidation indicates better catalyst activity. The stability of the catalysts was assessed using chronoamperometry (CA) in a 1.0 M KOH + 1.0 M CH3OH solution treated with nitrogen for 30 min. The test duration was 4000 s. The CO stripping method was utilized to evaluate the CO tolerance of the catalysts. The 1.0 M KOH solution was purged with nitrogen for 30 min to remove dissolved oxygen, followed by purging with high-purity CO for 30 min to achieve a saturated monolayer CO adsorption on the catalyst surface. The excess CO was then removed by purging with nitrogen for another 30 min. The CV test was immediately conducted without further purging, with a scan rate of 50 mV/s over a potential range of 0 to 1.2 V. The onset potential of CO oxidation determined the CO tolerance of the catalysts; a lower onset potential indicates better CO tolerance. In situ FTIR was used to determine the reaction intermediates of the catalyst by Nicolet iS50 spectrometer from Thermofisher.

4. Conclusions

This study prepared Pt/xMnO2-CNTs catalysts by loading Pt nanoparticles on MnO2-CNTs composite supports synthesized via citric acid reduction and ethylene glycol reduction methods. XRD analysis shows that MnO2 has an amorphous structure and Pt introduction does not cause its lattice distortion. TEM results reveal that the average particle size of metal particles in Pt/20wt.%MnO2-CNTs is 3.6 nm smaller than that in Pt/C. XPS analysis confirms that Mn mainly exists as Mn4+ with a small amount of Mn3+/Mn5+ contributing to catalytic activity. In situ FTIR results demonstrate that MnO2 can promote the conversion of highly toxic COL to easily oxidized COB, accelerate COads oxidation to reduce occupation of Pt active sites, and significantly enhance intermediate oxidation efficiency. Electrochemical tests indicate that Pt/20wt.%MnO2 exhibits the best performance with a MOR peak current of 2027.25 mA/mgPt and a steady-state current of 54.24 mA/mgPt which are 4.0 times and 5.41 times those of Pt/C, respectively. The excellent performance of this catalyst stems from the synergistic effect of modified CNTs optimizing mass transfer of Pt, activating methanol, with MnO2 exerting a bifunctional mechanism and proton conduction regulating intermediates. The core innovation of this finding lies in clarifying that transition metal oxides in noble metal catalyst systems do not merely function as supports but achieve performance enhancement through multi-dimensional synergy involving regulating the electronic structure of active sites via valence states promoting conversion of toxic intermediates and strengthening proton conduction. It holds significant scientific value for advancing the development of high-efficiency low-cost catalysts for fuel cells.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15090864/s1, Figure S1. XRD pattern of Pt/XMnO2-CNTs. (X = 10, 15, 20, 25, 30 wt.%). Figure S2. The N2 adsorption and desorption isotherms of Pt/XMnO2-CNTs. (X = 10, 15, 20, 25, 30 wt.%). Table S1. BET characterization data of catalysts. Table S2. Electrochemical test data of catalyst. Table S3. Electrochemical test data of catalyst.

Author Contributions

G.C.: Writing—original draft, Investigation, Data curation. Z.T.: Investigation. H.X.: Formal analysis, Validation, Software, Writing—review and editing. H.L.: Investigation, Methodology. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors sincerely thank Li for his incisive guidance throughout the research. Special appreciation is extended to Hanqiao Xu for her thought-provoking discussions on data interpretation. Although no dedicated funding supported this work, we acknowledge the foundational facilities provided by our research group, which enabled this study.

Conflicts of Interest

Guang Chen and Zhijun Teng were employed by GanSu Yinguang Juyin Chemical Industry group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Morphology and element distribution of Pt/20wt.%MnO2-CNTs catalyst, (a) TEM image; (b,c) HRTEM image; (d–i) HAADF-STEM image.
Figure 1. Morphology and element distribution of Pt/20wt.%MnO2-CNTs catalyst, (a) TEM image; (b,c) HRTEM image; (d–i) HAADF-STEM image.
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Figure 2. (a1–a5) XPS survey spectra, (b1–b5) high resolution Pt 4f spectra, (c1–c5) Mn 2p spectra of Pt/xMnO2-CNTs, X = 10, 15, 20, 25, 30, respectively.
Figure 2. (a1–a5) XPS survey spectra, (b1–b5) high resolution Pt 4f spectra, (c1–c5) Mn 2p spectra of Pt/xMnO2-CNTs, X = 10, 15, 20, 25, 30, respectively.
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Figure 3. Electrocatalytic performances of Pt/xMnO2-CNTs (X = 10, 15, 20, 25, 30 wt.%) electrocatalyst. (a) The mass-normalized CV in 1.0 M KOH +1.0 M methanol at 50 mV s−1; (b) the CV curves for MOR recorded in 1.0 M KOH at 50 mV·s−1; (c) LSV curve; (d) EIS energy quister diagram; (e) CA curves of the catalysts for MOR at a potential of 0.7 V vs. SCE; (f) CO-Stripping curves in 1.0 M KOH solution.
Figure 3. Electrocatalytic performances of Pt/xMnO2-CNTs (X = 10, 15, 20, 25, 30 wt.%) electrocatalyst. (a) The mass-normalized CV in 1.0 M KOH +1.0 M methanol at 50 mV s−1; (b) the CV curves for MOR recorded in 1.0 M KOH at 50 mV·s−1; (c) LSV curve; (d) EIS energy quister diagram; (e) CA curves of the catalysts for MOR at a potential of 0.7 V vs. SCE; (f) CO-Stripping curves in 1.0 M KOH solution.
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Figure 4. (a) Pt/C and (b) Pt/20wt.%MnO2-CNTs in situ infrared spectroscopy analysis.
Figure 4. (a) Pt/C and (b) Pt/20wt.%MnO2-CNTs in situ infrared spectroscopy analysis.
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Table 1. ICP-OES results of Pt/XMnO2-CNTs.
Table 1. ICP-OES results of Pt/XMnO2-CNTs.
SamplesTheoretical Metal Load (wt.%)Actual Metal Load (wt.%)Actual Load (wt.%)Effective Load Rate (%)
PtMnPtMn
Pt/10wt.%MnO2-CNTs201014.638.3622.9976.63
Pt/15wt.%MnO2-CNTs201515.1614.5729.7384.94
Pt/20wt.%MnO2-CNTs202015.6218.6534.2785.68
Pt/25wt.%MnO2-CNTs202514.3623.5637.9284.27
Pt/30wt.%MnO2-CNTs203015.1027.1342.2384.46
Table 2. The contents of Pt0, Pt2+, Pt4+, and Mn3+, Mn3+, Mn5+ in Pt/xMnO2-CNTs with different ratios.
Table 2. The contents of Pt0, Pt2+, Pt4+, and Mn3+, Mn3+, Mn5+ in Pt/xMnO2-CNTs with different ratios.
SamplesPtMn
Pt0 Content (%)Pt2+ Content (%)Pt4+ Content (%)Mn3+ Content (%)Mn4+ Content (%)Mn5+ Content (%)
Pt/10wt.%MnO2-CNTs51.2636.512.24///
Pt/15wt.%MnO2-CNTs56.1936.457.36///
Pt/20wt.%MnO2-CNTs62.3831.266.366.8770.3622.77
Pt/25wt.%MnO2-CNTs61.8930.547.57/62.9537.05
Pt/30wt.%MnO2-CNTs60.5928.2311.184.4560.6434.91
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Chen, G.; Teng, Z.; Xu, H.; Li, H. Preparation of Pt/xMnO2-CNTs Catalyst and Its Electrooxidation Performance in Methanol. Catalysts 2025, 15, 864. https://doi.org/10.3390/catal15090864

AMA Style

Chen G, Teng Z, Xu H, Li H. Preparation of Pt/xMnO2-CNTs Catalyst and Its Electrooxidation Performance in Methanol. Catalysts. 2025; 15(9):864. https://doi.org/10.3390/catal15090864

Chicago/Turabian Style

Chen, Guang, Zhijun Teng, Hanqiao Xu, and Hongwei Li. 2025. "Preparation of Pt/xMnO2-CNTs Catalyst and Its Electrooxidation Performance in Methanol" Catalysts 15, no. 9: 864. https://doi.org/10.3390/catal15090864

APA Style

Chen, G., Teng, Z., Xu, H., & Li, H. (2025). Preparation of Pt/xMnO2-CNTs Catalyst and Its Electrooxidation Performance in Methanol. Catalysts, 15(9), 864. https://doi.org/10.3390/catal15090864

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