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Article

Effect of Oxygen-Containing Functional Groups on the Performance of Palladium/Carbon Catalysts for Electrocatalytic Oxidation of Methanol

by
Hanqiao Xu
1,†,
Hongwei Li
1,*,†,
Xin An
1,
Weiping Li
2,
Rong Liu
2,
Xinhong Zhao
1 and
Guixian Li
1
1
School of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, China
2
Gansu Research Institute of Chemical Industry Co., Ltd., Lanzhou 730030, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(8), 704; https://doi.org/10.3390/catal15080704
Submission received: 19 June 2025 / Revised: 16 July 2025 / Accepted: 18 July 2025 / Published: 24 July 2025
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Abstract

The methanol oxidation reaction (MOR) of direct methanol fuel cells (DMFCs) is limited by the slow kinetic process and high reaction energy barrier, significantly restricting the commercial application of DMFCs. Therefore, developing MOR catalysts with high activity and stability is very important. In this paper, oxygen-functionalised activated carbon (FAC) with controllable oxygen-containing functional groups was prepared by adjusting the volume ratio of H2SO3/HNO3 mixed acid, and Pd/AC and Pd/FAC catalysts were synthesised via the hydrazine hydrate reduction method. A series of characterisation techniques and electrochemical performance tests were used to study the catalyst. The results showed that when V(H2SO3):V(HNO3) = 2:3, more defects were generated on the surface of the AC, and more oxygen-containing functional groups represented by C=O and C–OH were attached to the surface of the support, which increased the anchor sites of Pd and improved the dispersion of Pd nanoparticles (Pd NPs) on the support. At the same time, the mass–specific activity of Pd/FAC for MOR was 2320 mA·mgPd, which is 1.5 times that of Pd/AC, and the stability was also improved to a certain extent. In situ infrared spectroscopy further confirmed that oxygen functionalisation treatment promoted the formation and transformation of *COOH intermediates, accelerated the transformation of COL into COB, reduced the poisoning of COads species adsorbed to the catalyst, optimised the reaction path and improved the catalytic kinetic performance.

Graphical Abstract

1. Introduction

As the energy demand grows, traditional fossil fuel consumption accelerates and global warming worsens. Based on this, China has put forward the critical “dual carbon” goals of “carbon neutrality” and “carbon peak”, and it is urgent to vigorously develop and promote clean energy [1]. As an organic liquid fuel, methanol is cost-effective and easy to handle, store, and transport. It can be industrially produced from carbon monoxide (CO) and hydrogen (H2), and these feedstocks can be derived from diverse resources, including natural gas, coal, and renewable energy sources. Furthermore, as an excellent carbon-neutral fuel, methanol offers notable advantages, such as easy handling, storage, and transportation, which underscore its practicality for widespread application [2]. Direct methanol fuel cells (DMFCs), which use methanol as a fuel, are electrochemical conversion devices that directly convert chemical energy into electrical energy. They have gained widespread attention due to their high portability and high energy density advantages [3]. However, the high costs, methanol permeation issues, and slow methanol oxidation reaction (MOR) kinetics remain the main obstacles to commercialisation. Therefore, designing highly efficient cathode catalysts with controllable composition, structure, and morphology is crucial for achieving high-performance DMFCs.
As an essential component of load-type catalysts, the carrier is a key factor that must be considered when designing anode catalysts. Choosing the right carrier can enhance the dispersion of metal nanoparticles (NPs) and improve the utilisation rate of active metals [4]. Carbon materials such as carbon black [5], activated carbon (AC) [6], mesoporous carbon [7], and nanocarbon materials [8] have high conductivity, large specific surface area, and good chemical stability under acidic and alkaline reaction conditions. They are commonly used as carriers for anode catalysts in DMFCs [9].
However, single carbon materials have surface inertia and lack metal “anchor points”, which hinders the dispersion of metal NPs. Therefore, the surface of carbon materials needs to be modified through chemical modification [10]. Industrially, oxidising agents (such as HNO3, HNO3/H2SO4, etc.) and potent oxidising agents (such as KMnO4/H2SO4, O3, H2O2, etc.) are commonly used to oxygen-functionalise carbon materials [11]. After being treated with oxidising agents, the surface of carbon materials generates hydroxyl, carboxyl, and quinone functional groups. The generated oxygen functional groups can promote the anchoring of metal NPs, and the confinement effect they produce can enable more metal NPs to be uniformly dispersed on the carrier surface. In addition, the oxygen functional groups on the carbon surface can regulate the electronic structure of metal NPs and enhance the interaction between metal and carrier, thereby enabling the catalyst to achieve better catalytic activity and durability [12].
Oxygen-functionalised carbon supports have been widely used to improve the catalytic performance of supported metal catalysts. For example, Chen et al. [13] treated carbon supports with H2O2 and HNO3 and investigated the effect of oxidising treatment on the stability of Pt/C catalysts. The FT-IR results showed that the surface of the oxidised carbon support was rich in oxygen functional groups. The oxidation treatment increased the interaction between metal and support, to some extent hindering the aggregation of metal NPs, thereby improving the electrochemical stability of Pt/C. Salgado et al. [14] proposed that Vulcan XC-72R treated with a mixed solution of H2SO4 and HNO3 exhibited stronger hydrophilicity and richer oxygen functional groups. Due to the synergistic effect between metal NPs and oxygen-functionalised carbon material surface oxygens, Pt catalysts prepared on well-dispersed oxygen-functionalised carbon materials showed significantly enhanced electrocatalytic activity for MOR.
However, potent oxidising agents such as KMnO4, H2O2, and O3 have high hazard coefficients. They are challenging to operate, and the carbon material surface produced by single acid treatment has fewer oxygen-containing functional groups. Therefore, the experimental conditions for oxygen-functionalising the carrier with a mixture of acid solutions are relatively mild and can build more defects on the carrier surface, attracting widespread attention. In this paper, the chemical modification of AC by H2SO4 and HNO3 mixed solution as an oxidising agent was carried out to produce hydroxyl and carboxyl oxygen-containing functional groups on the AC surface. The Pd catalyst was prepared by rapid reduction with hydrazine water, and the content of oxygen-containing functional groups on the AC surface was controlled by adjusting the volume ratio of the molar concentration-matched H2SO4 and HNO3 mixed acid solution. The influence of oxygen-containing functional groups on the dispersion mechanism of Pd and the structural–activity relationship between FAC and MOR were explored.

2. Results and Discussion

2.1. Physical Characterisation Analysis

The carrier surface’s molecular structure and chemical composition were first analysed using FT-IR. Figure 1 depicts that AC and FAC-x exhibit two absorption peaks at approximately 1563.8 cm−1 and 3416.9 cm−1, corresponding mainly to the stretching vibration of aromatic groups and -OH [11]. The absorption peak observed at around 1203.9 cm−1 indicates the presence of C–O bonds, confirming the existence of endogenous structures, phenolic compounds, and carboxyl aromatic ethers on the carrier [15]. An additional absorption peak at approximately 1713.4 cm−1 is attributed to the stretching vibration of C=O [16].
Furthermore, compared to AC, the absorption peaks of the C=O stretching vibration, C–O stretching vibration, and aromatic stretching vibration in FAC are significantly intensified, indicating the successful introduction of oxygen-containing functional groups on the surface of AC after mixed acid treatment. It is also evident that FAC-D [V(H2SO4):V(HNO3) = 2:3] exhibits the strongest absorption peak, suggesting the highest concentration of oxygen-containing functional groups on the carrier’s surface at this stage. This is due to the significant impact of the NO2+ content of HNO3 on the extent of oxidation treatment (as demonstrated in Formulas (1) and (2)). The higher volume of HNO3 in the mixed acid solution treated with FAC-D results in an increased presence of NO2+, leading to a higher degree of oxidation for FAC-D, and consequently, an elevated content of oxygen-containing functional groups on its surface [17]. Although the HNO3 volume in the mixed acid solution treated with FAC-E is higher than that of FAC-D, the excessive oxidation degree will deteriorate the surface structure of the carrier, which is not conducive to introducing oxygen-containing groups [18]. Additionally, it has been demonstrated that a carboxyl group is a prerequisite for high dispersion of metal NPs. As the absorption peak intensity of FAC-D near 1203.9 cm−1 is high, it can be inferred that the Pd NPs are well dispersed on FAC-D [19].
2HNO3 = NO2+ + NO3− + H2O
HNO3 + 2H2SO4 = NO2+ + H3O+ + 2HSO4−
Raman spectroscopy was used to characterise the variation of the defect degree of the carbon carrier before and after oxygen functionalisation treatment, and the results are shown in Figure 2. Two diffraction peaks are observed at approximately 1350 cm−1 and 1590 cm−1, with the peak near 1350 cm−1 being characteristic of disordered sp2 carbon materials, known as the D band, which reflects lattice defects in carbon. The band around 1590 cm−1 is similar to the G band of an ordered graphite structure, indicating the material’s graphitisation level [20]. The intensity ratio of D peak to G peak (ID/IG) in the Raman spectrum can indicate the degree of defect and disorder on the carrier surface, with a higher ID/IG value corresponding to a higher degree of defect and disorder [21]. Compared to untreated AC, the ID/IG ratios of FAC-A, FAC-B, FAC-C, FAC-D, and FAC-E increased to 1.23, 1.33, 1.15, 1.30 and 1.26, respectively. This suggests that oxygen functionalisation can impact the surface structure of the carrier and enhance its surface structural defects, providing more active metal anchoring sites for catalyst preparation.
According to Figure 3 and Table 1, the specific surface area of AC was 1477 m2 ·g−1. After mixed acid treatment, the specific surface area of AC decreased significantly, the pore size increased, and the pore volume decreased. Among them, the specific surface area of FAC-D decreased the most, which indicates that when V(H2SO4):V(HNO3) = 2:3, the specific surface area of AC is more affected by mixed acids. This is mainly because during the treatment with the mixed acid solution, part of the carbon structure collapses and penetrates into the pore structure, resulting in the plugging of the carbon pore, thereby reducing the specific surface area, and more oxygen-containing functional groups on the surface of FAC-D result in a greater decrease in the particular surface area [22]. The results further proved that AC introduced additional oxygen-containing functional groups after mixed acid treatment, and the surface of FAC-D contained more oxygen-containing functional groups.
The phase structures of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts were characterised using XRD. As depicted in Figure 4, the absence of a diffraction peak near 23.8° corresponds to the C (002) crystal plane. The position and width of the carbon peaks remained unchanged before and after oxygen functionalisation, indicating that acid treatment did not impact the structural integrity of AC. The diffraction peaks of the catalyst at approximately 23.8°, 40.1°, 46.7°, 68.1°, 82.1° and 86.5° correspond to the crystal faces of Pd (111), Pd (200), Pd (220), Pd (311) and Pd (222), respectively, demonstrating that the face-centred cubic crystal structure of Pd has been successfully reduced from its precursor [23].
Transmission electron microscopy (TEM) was employed to characterise the morphological features of the catalysts to elucidate the structure–performance correlation. As shown in Figure 5, both Pd/AC (Figure 5a) and the FAC-functionalised Pd/FAC-X series catalysts (Figure 5b–f) exhibit the basic morphology of Pd NPs supported on carbon carriers, but significant differences in the particle dispersion are observed: on the untreated AC support, the Pd NPs show slight agglomeration with a broad particle size distribution (average diameter d = 5.63 nm), whereas the Pd NPs in Pd/FAC-A to Pd/FAC-E demonstrate remarkably improved dispersion on the support surface with significantly reduced aggregation, e.g., the average particle size of Pd/FAC-A (Figure 5b) decreases to 4.85 nm, Pd/FAC-B (Figure 5c) further reduces to 4.75 nm, and Pd/FAC-D (Figure 5e) shows the smallest average diameter of 4.65 nm, which is the minimum among all the samples, indicating that FAC functionalisation effectively refines the Pd particle size and enhances the dispersion. This improvement in the dispersion can be attributed to the introduction of oxygen-containing groups on the FAC-X support surface, which strengthen the anchoring effect on the Pd precursors through strong interactions, thereby inhibiting the aggregation process. Notably, Pd/FAC-D (Figure 5e) shows almost no obvious aggregation of Pd NPs, achieving a highly uniform distribution on the support surface, which can be explained by the higher concentration of oxygen-containing groups in FAC-D reducing the hydrophobicity of the carbon material and promoting the uniform diffusion of Pd precursors on the support surface, thus realising superior particle dispersion [24]. In summary, regulating the density of the oxygen-containing groups on the support via FAC functionalisation can effectively optimise the dispersion and particle size of the Pd NPs, establishing a favourable structural foundation for the enhancement of the catalytic performance.
The HRTEM images in Figure 6a–c show different magnifications of Pd/FAC-D. It is evident from the figure that when V(H2SO4):V(HNO3) = 2:3, the dispersion of the Pd NPs on the carrier is good. Furthermore, Figure 6c depicts the crystal faces of Pd(111) and Pd(200), providing further confirmation of the successful loading of Pd NPs onto the carrier, consistent with the XRD results.
Comparing the pore structure parameters in Table 1 (support) and Table 2 (Pd-loaded catalyst), it can be seen that the loading of Pd significantly changed the specific surface area, average pore size and pore volume of the catalyst, and these changes were closely related to the density of oxygen-containing groups modified by FAC and the loading behaviour of the Pd particles. Specifically, in terms of the specific surface area, it is 1477 m2/g−1 before AC loading, and Pd/AC is reduced to 1358 m2/g−1 after Pd loading. However, the specific surface area of the FAC-x carrier (such as 681 m2/g−1 of FAC-D) is lower than that of AC, and the specific surface area of Pd/FAC-x is further attenuated after loading (Pd/FAC-D was only 493 m2/g−1), which not only reflects the general rule that Pd particles occupy the pore channels and active sites of the carrier but also reflects that FAC-D with high oxygen-containing group density enhances the anchoring effect of Pd and promotes the uniform dispersion of particles, resulting in more surface sites being occupied and a more significant decrease in the specific surface area. The average pore size of Pd/FAC-x is generally reduced after loading, which is due to the fact that Pd particles are mainly loaded on the surface of the carrier and in the larger pores, and the effect on the pores with a smaller pore size is limited. At the same time, the loading of Pd particles may block the opening of some pores, resulting in the effective pore size distribution shifting to a small size, so the average pore size shows a slight downward trend. The change trend of the pore volume is consistent with the specific surface area. After loading, the pore volume of Pd/FAC-x is further attenuated, reflecting the behaviour of Pd particles blocking the pores. FAC-D with high oxygen-containing group density has better dispersion of Pd, more obvious pore blockage and more serious loss of pore volume. On the whole, the loading of Pd leads to the general attenuation of the pore structure of the support. Although the pore volume and specific surface area of Pd/FAC-D are significantly lost, its high Pd0 indicates that the strong metal–support interaction enhances the reduction degree of Pd, which is expected to show higher intrinsic activity.
XPS was utilised to characterise the valence distribution and electronic structure properties of the surface elements in various catalysts. Figure 7a,b present the XPS spectra of C and O, respectively. The C 1s characteristic peaks observed in all the catalysts primarily consist of three components: a symmetric peak attributed to aliphatic carbon (C–C), along with two symmetric peaks corresponding to oxygen-bonded carbon species, namely C–O and C=O [25]. The O 1s spectrum exhibits three symmetric peaks, explicitly associated with carboxyl/quinone oxygen (C=O), hydroxyl/ether oxygen (C–O–C), and carboxyl oxygen (C–OH) [26]. The atomic percentages of C, O, and various oxygen-containing functional groups are summarised in Table 3. The total carbon content of the Pd/FAC-x samples is observed to be lower than that of Pd/AC, with the lowest carbon content found in Pd/FAC-B (68.06 at%). In contrast, the oxygen content of the Pd/FAC-X samples is higher than that of Pd/AC, and the highest oxygen content is seen in Pd/FAC-D (24.28 at%). This confirms that the oxidation treatment using mixed acids introduces oxygen-containing functional groups on the AC surface, with a more significant introduction when V(H2SO4):V(HNO3) = 2:3, which is consistent with the FT-IR and BET characterisation results.
In addition, the relative atomic content and O/C ratio of different oxygen-containing functional groups were affected by the degree of oxygen functionalisation. As shown in Table 3, the C=O and C–OH contents and O/C ratio of Pd/FAC-X are higher than those of Pd/AC, and Pd/FAC-X also has specific differences due to the different degrees of oxidation. Among them, the C=O and C–OH contents and O/C of Pd/FAC-D are higher than those of the other catalysts. This confirms that oxygen functionalisation increases the content of oxygen-containing functional groups on the carrier surface. The content of different oxygen-containing species on the carrier surface can be regulated by changing V(H2SO4):V(HNO3).
Figure 8 shows the Pd 3d XPS spectra of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts. As can be seen from the figure, there are two doublet states of metal Pd0 and oxidised PdII in the Pd3d spectrum. Metal Pd0 mainly contains the peaks of Pd03d3/2 and Pd03d5/2, while the peaks of PdII3d3/2 and PdII3d5/2 are primarily oxidised PdII, and the existence of PdII can be attributed to the slight oxidation of Pd [27]. Quantitative analysis of the peak areas (Table 3) reveals the atomic percentages of Pd0 and Pd2+ in each catalyst. Notably, Pd/FAC-D shows the highest Pd0 fraction (85.36 at. %) and the lowest Pd2+ content (14.64 at. %), indicating that FAC functionalisation significantly promotes the reduction of the Pd precursors during synthesis. This trend confirms that oxygen-containing groups introduced by FAC oxidation act as anchoring sites, facilitating the dispersion and reduction of Pd species—thereby increasing the metallic Pd0 fraction. Specifically, the O/C ratio of Pd/FAC-D (0.356) is the highest among all the samples, suggesting that a higher density of oxygen-containing groups strengthens the interaction between the Pd precursors and the support, suppressing oxidation and enhancing Pd0 formation. It can be seen that most metal precursors have been successfully reduced and loaded onto the carrier [28]. In addition, Pd/AC has a positive shift compared with the Pd03d3/2 and Pd03d5/2 peaks of Pd/FAC-X, wherein the Pd03d3/2 and Pd03d5/2 peaks of Pd/FAC-D appear at 341.0 eV and 335.75 eV, respectively. Compared with Pd/AC, the positive shift is 0.2 eV and 0.32 eV, respectively, the most significant positive shift, which is a strong basis for the strong electronic interaction between Pd and oxygen-containing functional groups. It is inferred that the prepared Pd/FAC-D catalyst will exhibit excellent electrocatalytic activity.

2.2. Electrochemical Performance Evaluation

Figure 9a is the CV diagram of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts in 1.0 M CH3OH + 1.0 M KOH solution. Each curve shows two typical oxidation peaks derived from the electrooxidation of methanol and carbonaceous intermediates [29]. Combined with Figure 9a,d, it can be seen that the order of the oxidation peak current density is Pd/FAC-D(2332 mA·mgPd−1) > Pd/FAC-B(2036 mA·mgPd−1) > Pd/FAC-A(1928 mA·mgPd−1) > Pd/FAC-E(1841 mA·mgPd−1) > Pd/FAC-C(1704 mA·mgPd−1) > Pd/AC (1545 mA·mgPd−1). The results show that the catalytic activity of Pd/FAC-X on methanol oxidation is significantly enhanced compared with Pd/AC. The peak current density of Pd/FAC-D is the highest, reaching 2320 mA·mgPd−1, 1.5 times that of Pd/AC. This is because a large number of defects on the surface of FAC-X make more oxygen-containing functional groups adhere to the surface of the carrier, increase the anchoring site of Pd NPs, improve the effective utilisation rate of Pd, and thus enhance the catalytic activity of the catalyst. Combined with the XPS results, it can be found that when the relative atomic content of the two oxygen-containing functional groups C=O and C–OH of the catalyst increases, the performance of the catalyst is enhanced, indicating that the dominant groups in the oxygen-containing functional groups are C=O and C–OH, and their content plays a decisive role in the catalytic activity of the catalyst.
Figure 9b shows the CA curves of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts in 1.0 M CH3OH + 1.0 M KOH solution at a constant potential of −0.2 V. As shown in the figure, the current density generated by all the catalysts during the MOR process decays rapidly in the first 100 s, which is due to the gradual deactivation of the catalyst due to the adsorption of the carbonaceous intermediates produced by the MOR on the catalytic surface. Secondly, the decay rate of the methanol oxidation current density differs for different catalysts. Compared with other catalysts, the current density of Pd/FAC-D decays slowly. After 1000 s of testing, the current density decays to 101.5 mA·mgPd−1, higher than the other catalysts. This shows that the oxygen-containing functional groups on the surface of the carrier can effectively remove the intermediate product CO, and the ability to remove CO is proportional to the number of oxygen-containing groups.
The CV curve of the catalyst in 1.0 M KOH solution is shown in Figure 9c. The figure shows that Pd/FAC-D has the highest current density compared with the other catalysts, consistent with the results obtained in Figure 9a. In addition, the oxidation peak of the Pd/FAC-D catalyst is also more apparent, which indicates that the content of oxygen-containing functional groups can indeed improve the activity of the MOR. ECSA is an essential index for catalyst evaluation. The integration area is obtained by integrating the reduction peak of Pd, and then the coulomb charge of the catalyst is further obtained. Finally, ECSA is obtained using Equation (3) [30].
ECSA = QH/(0.405 × MPd)
where QH is the coulomb charge of hydrogen adsorption–desorption (mC∙cm−2), MPd is the load of Pd (g∙cm−2), and 0.405 is a constant, representing the charge required for the oxidation of a single layer of hydrogen on the smooth Pd surface. The calculated ECSA of the catalyst is shown in Figure 9d. Among them, the Pd/FAC-D catalyst still had the highest ECSA (48.9 m2 g−1), which once again proves that the catalytic performance of the catalyst could be improved by increasing the content of oxygen-containing functional groups on the surface of the carrier.
Pd/FAC-D and Pd/AC were tested by in situ infrared spectroscopy to further explore the reaction path of methanol oxidation on the catalyst surface. As shown in Figure 10, in 1.0 M CH3OH + 1.0 M KOH solution, as the potential is negatively scanned from OCP to 0.2 V, both catalysts show prominent absorption peaks of *COOH adsorbed species at 1577 cm−1, 1380 cm−1 and 1350 cm−1 [31], which are attributed to the asymmetric stretching vibration of formate intermediates. It is worth noting that when the potential increases, the absorption intensity of *COOH gradually changes from an “upward generation trend” to a “downward consumption trend”, indicating that the potential significantly regulates its generation and consumption process. At the same time, the COOH peak intensity of Pd/FAC-D at 1577 cm−1 is substantially higher than that of Pd/AC. Its consumption rate is faster, indicating that the oxygen-enriched functionalised support (FAC) promotes the dehydrogenation of methanol to CH2O through the hydrogen bonding of C=O and C–OH groups on the surface. It then rapidly oxidises to COOH. The advantage of this pathway is that *COOH can be directly decomposed into CO2 and H2O, avoiding the generation of strongly adsorbed COads.
In summary, the oxygen-rich functional groups optimise the adsorption energy barrier of the reaction intermediate by enhancing the metal–support interaction, so that the methanol oxidation is more likely to be efficiently converted through the *COOH pathway, rather than generating toxic COads. This process not only accelerates the reaction kinetics but also improves the stability of the catalyst by reducing the adsorption of poisonous species, which is supported by the high activity and long durability results of Pd/FAC-D in the electrochemical tests.

3. Materials and Methods

3.1. Reagents and Chemicals

The palladium chloride (Macklin, Shanghai, China); activated carbon (XFNANO, INC); anhydrous ethanol (Macklin, Shanghai, China); concentrated hydrochloric acid (HCl, 37.0%); concentrated nitric acid (HNO3, 68.0%); concentrated sulfuric acid (H2SO4, 98.3%); ethylene glycol (EG, Macklin, Shanghai, China); hydrazine hydrate, purity > 85% (Beilian Reagent, Tianjin, China); potassium hydroxide (KOH, Macklin, Shanghai, China); methanol (Macklin, Shanghai, China); potassium ferrocyanide (K3[Fe(CN)6], Beilian Reagent, Tianjin, China); potassium chloride (KCI, Damao, Tianjin, China); deionised water obtained from a Milli-Q water purifier, and other reagents were of analytical grade and did not require further purification.

3.2. Preparation of Oxygen-Functionalised Carbon Carrier

First, 1.0 g of AC was added to 50 mL of a mixed acid solution consisting of 5.0 M H2SO4 and 5.0 M HNO3 [V(H2SO4):V(HNO3) = 1:1 and sonicated. The mixture was heated at 100 °C for 6 h and cooled. After that, 100 mL of deionised water was added, and the mixture was stirred at room temperature for one night. Finally, the sample was washed with deionised water to neutrality and dried at 80 °C, resulting in a sample denoted as FAC-A. By replacing the V(H2SO4):V(HNO3) in 1:2, 2:1, 2:3, and 3:2, respectively, the samples were prepared using the same experimental steps and denoted as FAC-B, FAC-C, FAC-D, and FAC-E.

3.3. Preparation of Precursor Solution

First, 679 mg of PdCl2 was accurately weighed and added to a volumetric flask containing 626 μL of 12 M HCl. The mixture was ultrasonically dispersed until homogeneous, then deionised water was added to bring the volume to the mark. The resulting solution had a Pd concentration of 2 mg·mL−1, serving as the H2PdCl4 precursor.

3.4. Preparation of the Catalyst

Using hydrazine as a reducing agent, ethylene glycol as a dispersant, and a metal loading of 20 wt%, Pd/FAC-x (x = A, B, C, D, E) catalysts were prepared by the chemical reduction method, with the specific operation being as follows: 50 mg of FAC-x (x = A, B, C, D, E) was weighed and dispersed evenly in 100 mL of glycol using ultrasonic treatment. Then, 6.25 mL of H2PdCl4 (Pd: 2 mg·mL−1) was added to the above solution and stirred continuously for 14 h. Subsequently, 4 mL of hydrazine hydrate solution was added, followed by stirring and reduction for 3 h. Subsequently, samples Pd/FAC-A, Pd/FAC-B, Pd/FAC-C, Pd/FAC-D and Pd/FAC-E were obtained through filtration, washing and overnight vacuum drying at 60 °C. The preparation of Pd/AC followed the same steps.

3.5. Physical Property Characterisation

The samples’ elemental composition and molecular structure were analysed using Fourier infrared spectroscopy (FT-IR) (Thermo Scientific Nicolet iS50) with a resolution of 4 cm−1, 32 scanning times, and a test wave number range of 400~4000 cm−1. Raman spectroscopy (RM5C) was employed to acquire the Raman spectra of the sample in ambient air. The samples’ specific surface area, mean pore volume, and mean pore diameter were determined using N2 physical adsorption–desorption (Mike ASAP2420). The XPS spectrum of the sample was acquired using XPS (Thermo Scientific K-Alpha), and the catalyst was tested with non-monochromatic Al Kα rays (hv = 1486.6 eV) at 12 kV. During data collection, all the electron-binding energies were calibrated relative to the C 1s peak of 284.8 eV. The morphology, dispersion, and lattice spacing of the metal nanoparticles were examined using transmission electron microscopy (TEM, FEI Talos F200X) and high-resolution transmission electron microscopy (HRTEM, FEI-Talos F200S). The crystal structure of the catalyst metal nanoparticles was analysed using X-ray diffractometry (XRD, D/max-2400). The Cu Kα line (λ = 0.154 nm) was selected, and the scanning range of the X-ray diffractometer was set to 5~90°, with a scanning speed of 5°/min. The scanning test was performed in the range of 2θ.

3.6. Electrochemical Measurements

The experiment was conducted at room temperature using a three-electrode system on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A saturated calomel electrode (SCE) served as the reference electrode, a platinum wire electrode (Pt) as the counter electrode, and a glassy carbon electrode (GCE, Φ = 4 mm) as the working electrode.
The glassy carbon electrode was polished and calibrated before usage. Initially, the electrode was polished using three different mesh sizes of polishing powder on muntjac paper for 3 min, followed by ultrasonic treatment in water for 5 s, and this process was repeated three times. Subsequently, the glassy carbon electrode was activated with an Ag/AgCl reference electrode in 0.5 M H2SO4 solution, at a scan rate of 100 mV/s, cycled 4 times within the −1~1 V potential range. Then, in a solution containing 0.1~10 mM K3[Fe(CN)6] and 0.1 M KCl, cyclic voltammetry was performed with the Ag/AgCl electrode as the reference electrode, at a scan rate of 50 mV/s, within the 0.6~−0.1 V potential range, ensuring the peak potential difference was below 80 mV. After electrode calibration, the catalyst prepared with 2 mg was accurately weighed and dispersed in a mixed solution containing 50 μL of 5% Nafion membrane solution, 450 μL of anhydrous ethanol, and 500 μL of deionised water. The mixture was then ultrasonically dispersed for 30 min to form a uniform catalyst ink. Subsequently, a pipette gun was used to measure out 5 μL and drop it onto the surface of the glass carbon electrode until it dried naturally.
Cyclic voltammetry (CV) was conducted on the N2-saturated 1.0 M KOH solution and the 1.0 M KOH + 1.0 M CH3OH solution, respectively, within the potential range of −0.9 to 0.3 V. The catalytic activity and ECSA for methanol oxidation were evaluated at a 50 mV·s−1 scan rate. The stability of the catalyst in a 1.0 M KOH + 1.0 M CH3OH solution was assessed through chronoamperometry (CA) at a constant potential of −0.2 V for 3600 s.

4. Conclusions

In this paper, the catalysts Pd/AC and Pd/FAC-X were prepared using a mixed solution of H2SO4 and HNO3 with different volume ratios to oxygen-functionalised AC and the reduction method of hydrazine hydrate. The results of the electrochemical analysis showed that the catalytic activity and stability of Pd/FAC-X on the MOR in 1.0 M CH3OH + 1.0 M KOH solution were significantly higher than those of Pd/AC. When V(H2SO4):V(HNO3) = 2:3, the mass–specific activity of Pd/FAC-D and ECSA are superior to the other catalysts, reaching 2320 mA·mgPd−1 and 48.9 m2·g−1, respectively. Pd/FAC-D also showed good catalytic stability in the chronocurrent method test, and the current density only attenuated to 101.5 mA·mgPd−1 after 1000 s of testing. The results showed that mixed acid treatment introduced various oxygen-containing functional groups on the AC surface, increased the structural defects on the carrier surface, and improved the dispersion degree of the Pd NPs on the carrier surface. More importantly, precise regulation of oxygen-containing functional groups on the surface of the carrier can be achieved by adjusting the volume ratio of the mixed acid. When V(H2SO4):V(HNO3) = 2:3, the surface of FAC is rich in C=O and C–OH, and forming these two functional groups plays a decisive role in the catalyst activity. In situ infrared spectroscopy further confirmed that the oxygen-functionalised FAC-D surface promoted the generation and consumption of *COOH intermediates, accelerated the conversion of linear CO to bridge CO, optimised the methanol oxidation path to avoid COads poisoning, combined with the oxygen-containing functional groups introduced by mixed acid treatment to improve the structural defects of the carrier surface and the dispersion of Pd nanoparticles, and finally, gave Pd/FAC-D excellent methanol oxidation electrocatalytic activity and stability.

Author Contributions

Conceptualization, H.L.; methodology, H.L.; software, H.X.; validation, X.A.; formal analysis, X.A.; investigation, H.X., W.L. and R.L.; data curation, H.X.; writing—original draft preparation, H.X.; writing—review and editing, H.X. and H.L.; supervision, X.Z.; project administration, H.L. and G.L.; funding acquisition, H.L. and G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Hongliu outstanding young talent support program of Lanzhou University of Technology (No. 2023-HLYQ), project funded by the Science and Technology Support Plan Project of Lanzhou (No. 2024-3-6), project supported by the Basic Research Innovation Group Project of Gansu Province (No. 22JR5RA219), and project funded by the Provincial Talent Project of Gansu Province (No. 2024RCXM79). And The APC was funded by the enterprise commissioned technology development projects.

Data Availability Statement

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

Acknowledgments

Special thanks to Li Hongwei’s careful guidance throughout the research process, from experimental design to thesis revision, his rigorous academic attitude and profound professional knowledge have significantly improved the quality of this study.

Conflicts of Interest

The authors declare that this study received funding from GanSu Yinguang Juyin Chemical Industry group Co. Ltd. The funder was not in-volved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Catalysts 15 00704 g001
Figure 1. FT-IR spectra of AC and FAC-x (x = A, B, C, D, E).
Figure 1. FT-IR spectra of AC and FAC-x (x = A, B, C, D, E).
Catalysts 15 00704 g001
Catalysts 15 00704 g002
Figure 2. Raman spectra of AC and FAC-x (x = A, B, C, D, E).
Figure 2. Raman spectra of AC and FAC-x (x = A, B, C, D, E).
Catalysts 15 00704 g002
Catalysts 15 00704 g003
Figure 3. Isothermal adsorption–desorption curves and pore size distribution curves of (a) AC; (b) FAC-A; (c) FAC-B; (d) FAC-C; (e) FAC-D; and (f) FAC-E.
Figure 3. Isothermal adsorption–desorption curves and pore size distribution curves of (a) AC; (b) FAC-A; (c) FAC-B; (d) FAC-C; (e) FAC-D; and (f) FAC-E.
Catalysts 15 00704 g003
Catalysts 15 00704 g004
Figure 4. XRD spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts.
Figure 4. XRD spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts.
Catalysts 15 00704 g004
Catalysts 15 00704 g005
Figure 5. TEM images of (a) Pd/AC; (b) Pd/FAC-A; (c) Pd/FAC-B; (d) Pd/FAC-C; (e) Pd/FAC-D; and (f) Pd/FAC-E.
Figure 5. TEM images of (a) Pd/AC; (b) Pd/FAC-A; (c) Pd/FAC-B; (d) Pd/FAC-C; (e) Pd/FAC-D; and (f) Pd/FAC-E.
Catalysts 15 00704 g005
Catalysts 15 00704 g006
Figure 6. HRTEM images of Pd/FAC-D: (a) 20 nm; (b) 10 nm; and (c) 5 nm.
Figure 6. HRTEM images of Pd/FAC-D: (a) 20 nm; (b) 10 nm; and (c) 5 nm.
Catalysts 15 00704 g006
Catalysts 15 00704 g007
Figure 7. XPS spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts: (a) C 1s spectra; and (b) O 1s spectra.
Figure 7. XPS spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts: (a) C 1s spectra; and (b) O 1s spectra.
Catalysts 15 00704 g007
Catalysts 15 00704 g008
Figure 8. Pd3d XPS spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts.
Figure 8. Pd3d XPS spectra of Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts.
Catalysts 15 00704 g008
Catalysts 15 00704 g009
Figure 9. (a) CV curves of different catalysts tested in 1.0 M CH3OH + 1.0 M KOH solution at a sweep rate of 50 mV·s−1; (b) CA curves of different catalysts tested in 1.0 M CH3OH + 1.0 M KOH solution at −0.2 V vs. SCE; (c) CV curves of different catalysts tested in 1.0 M KOH solution at a sweep rate of 50 mV·s−1; and (d) mass activities and ECSA of different catalysts.
Figure 9. (a) CV curves of different catalysts tested in 1.0 M CH3OH + 1.0 M KOH solution at a sweep rate of 50 mV·s−1; (b) CA curves of different catalysts tested in 1.0 M CH3OH + 1.0 M KOH solution at −0.2 V vs. SCE; (c) CV curves of different catalysts tested in 1.0 M KOH solution at a sweep rate of 50 mV·s−1; and (d) mass activities and ECSA of different catalysts.
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Catalysts 15 00704 g010
Figure 10. In 1.0 M CH3OH + 1.0 M KOH solution, the potential sweeps from the OCP negative to the in situ FT-IR spectrum of 0.2 V: (A) Pd/AC; and (B) Pd/FAC-D.
Figure 10. In 1.0 M CH3OH + 1.0 M KOH solution, the potential sweeps from the OCP negative to the in situ FT-IR spectrum of 0.2 V: (A) Pd/AC; and (B) Pd/FAC-D.
Catalysts 15 00704 g010
Table 1. Specific surface area, average pore size, and average pore volume of AC and FAC-x (x = A, B, C, D, E).
Table 1. Specific surface area, average pore size, and average pore volume of AC and FAC-x (x = A, B, C, D, E).
SimpleSpecific Surface Area (m2·g−1)Mean Aperture (nm)Mean Pore Volume (cm3·g−1)
AC14772.2200.820
FAC-A8142.2520.458
FAC-B7582.2510.426
FAC-C8602.3020.495
FAC-D6812.2350.380
FAC-E7012.2660.391
Table 2. Specific surface area, average pore size, and average pore volume of Pd/AC and Pd/FAC-x (x = A, B, C, D, E).
Table 2. Specific surface area, average pore size, and average pore volume of Pd/AC and Pd/FAC-x (x = A, B, C, D, E).
SimpleSpecific Surface Area (m2·g−1)Mean Aperture (nm)Mean Pore Volume (cm3·g−1)
Pd/AC13582.1540.710
Pd/FAC-A7522.1310.560
Pd/FAC-B6392.2000.659
Pd/FAC-C7802.2170.452
Pd/FAC-D4932.1190.320
Pd/FAC-E6872.2100.398
Table 3. Correlation data of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts in the XPS spectra.
Table 3. Correlation data of the Pd/AC and Pd/FAC-x (x = A, B, C, D, E) catalysts in the XPS spectra.
SimpleC (at%)O (at%)O1s (at%)O/CPd
C=OC–O–CC–OPd0 (at%)PdII (at%)
Pd/AC79.1218.065.397.145.530.22856.8943.11
Pd/FAC-A73.1021.657.097.816.710.29679.7820.22
Pd/FAC-B68.0623.377.747.787.850.34382.2117.79
Pd/FAC-C74.5820.826.059.095.680.27963.5636.44
Pd/FAC-D68.2124.288.527.917.850.35685.3614.64
Pd/FAC-E73.6521.807.088.636.090.29673.9126.09
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Xu, H.; Li, H.; An, X.; Li, W.; Liu, R.; Zhao, X.; Li, G. Effect of Oxygen-Containing Functional Groups on the Performance of Palladium/Carbon Catalysts for Electrocatalytic Oxidation of Methanol. Catalysts 2025, 15, 704. https://doi.org/10.3390/catal15080704

AMA Style

Xu H, Li H, An X, Li W, Liu R, Zhao X, Li G. Effect of Oxygen-Containing Functional Groups on the Performance of Palladium/Carbon Catalysts for Electrocatalytic Oxidation of Methanol. Catalysts. 2025; 15(8):704. https://doi.org/10.3390/catal15080704

Chicago/Turabian Style

Xu, Hanqiao, Hongwei Li, Xin An, Weiping Li, Rong Liu, Xinhong Zhao, and Guixian Li. 2025. "Effect of Oxygen-Containing Functional Groups on the Performance of Palladium/Carbon Catalysts for Electrocatalytic Oxidation of Methanol" Catalysts 15, no. 8: 704. https://doi.org/10.3390/catal15080704

APA Style

Xu, H., Li, H., An, X., Li, W., Liu, R., Zhao, X., & Li, G. (2025). Effect of Oxygen-Containing Functional Groups on the Performance of Palladium/Carbon Catalysts for Electrocatalytic Oxidation of Methanol. Catalysts, 15(8), 704. https://doi.org/10.3390/catal15080704

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