The Effect of Support and Reduction Methods on Catalyst Performance in the Selective Oxidation of 1,2-Propanediol

Abstract

The oxidation of 1,2-propanediol (1,2-PDO) under alkaline heterogeneous catalysis can be optimized to produce lactic acid, a valuable commodity chemical. In this study, Pd nanoparticles supported on various metal oxides (CeO₂, CuO, ZrO₂, ZnO, SnO₂) were synthesized via a wet-chemistry method. Furthermore, CeO₂-supported Pd nanoparticle catalysts were prepared using different reduction methods. The catalytic performance for the selective oxidation of 1,2-PDO was evaluated using a range of characterization techniques. Under optimal conditions (120 °C, 1.0 MPa O₂ pressure, 2 h reaction time, and a NaOH/1,2-PDO molar ratio of 3.0), a high lactic acid yield of 62.7% was achieved. Single-factor experiments revealed that lactic acid selectivity decreased with prolonged reaction time. Conversely, increasing temperature, NaOH concentration, and O₂ pressure initially enhanced lactic acid selectivity, but further increases resulted in a decline. Physicochemical characterization revealed that different supports and reduction methods affect the basicity of the catalyst, which subsequently influences the selectivity of the target product, lactic acid.

1. Introduction

As the demand for fossil fuels continues to increase, the global energy crisis has intensified. Biomass energy, as a renewable resource, has garnered significant attention due to its environmental friendliness and sustainability [1,2,3]. 1,2-Propanediol (1,2-PDO), an important downstream product of biomass conversion, holds potential as a platform molecule [4]. Through appropriate catalytic processes and reaction conditions, 1,2-PDO can be efficiently converted into valuable C1–C3 carboxylic acids such as lactic acid (LA), formic acid (FA), acetic acid (AA), and pyruvic acid (PA). These carboxylic acids not only have significant industrial applications but also offer new possibilities for sustainable chemistry and green chemical processes [5,6]. Additionally, biomass-derived 1,2-PDO presents great potential as an alternative to traditional petrochemical feedstocks, contributing to the reduction in fossil fuel dependence and promoting the low-carbon transformation of the chemical industry [7].

Supported noble metal catalysts have become indispensable materials in the field of industrial catalysis due to their exceptional activity and selectivity in a wide range of chemical reactions [8,9,10,11,12,13,14]. The catalysts typically support noble metals, such as indium (In) [15], sliver (Ag) [16], platinum (Pt) [17], gold (Au) [18,19,20], and ruthenium [21], in the form of nanoparticles on high-surface-area supports, thereby significantly improving the utilization efficiency of the noble metals and enhancing the stability of the catalysts. However, considering the high cost of noble metals, the activity and stability of conventional supported noble metal catalysts are often not satisfactory [22,23,24,25,26]. An ideal approach to reducing costs is to enhance the dispersion of noble metals, especially achieving dispersion at the near-single-atomic-layer level, to maximize their utilization efficiency, thereby improving both activity and stability [27].

The supported noble metal catalyst exhibited good catalytic activity in the preparation of LA through 1,2-PDO oxidation. Dimitratos et al. found that during the oxidation of 1,2-PDO using a 1%Au/C catalyst at 60 °C, the conversion of 1,2-PDO reached 31%, and the selectivity for LA was 92%. To improve performance, a Au–Pd/TiO₂ catalyst was synthesized by sol immobilization, resulting in increased 1,2-PDO conversion and LA selectivity to 94% and 96%, respectively [28]. Ma et al. demonstrated that a Au/Mg(OH)₂ catalyst showed high catalytic activity, where larger gold particles exhibited lower catalytic activity but higher LA selectivity. Furthermore, the Au-to-1,2-PDO ratio greatly influenced selectivity, as excessive gold particles led to the formation of AA [29]. Ryabenkova et al. observed that Au-Pd supported on pretreated activated carbon exhibited better catalytic activity due to the smaller average particle size of nanoparticles, which increased the availability of active surface sites [30]. Wang et al. investigated the catalytic oxidation of 1,2-PDO over a mesoporous La-doped MCM-41 molecular sieve (LaM)-supported Pd-Bi catalyst. The Pd₄Bi₁/LaM10 catalyst achieved 99.8% 1,2-PDO conversion and 80.5% LA selectivity, attributed to the introduction of new basic sites via La doping [31]. These studies have demonstrated that noble metal catalysts, including gold, palladium, and platinum, exhibit excellent activity in catalyzing 1,2-PDO oxidation to LA. However, the structure–effect relationship between the metal and the support has not been deeply explored. Further systematic investigations are needed to understand the influence of metal–support interactions and different catalyst preparation methods on catalytic performance.

Metal oxides are widely used as supports for noble metal catalysts due to their stability, surface properties, and strong interaction with noble metal particles. In catalytic systems, while noble metal atoms serve as active sites, the properties of the metal oxide support also play a critical role in determining catalytic performance [32,33,34,35,36,37]. CeO₂ has been successfully utilized as a catalyst support in various oxidation reactions because of its low-temperature redox properties and high oxygen storage–release capacity during reactions [10]. In this study, palladium catalysts supported on different metal oxides (CeO₂, CuO, ZrO₂, ZnO, SnO₂) were prepared to evaluate their catalytic activities. Pd/CeO₂-NaBH₄, Pd/CeO₂-PVP, and Pd/CeO₂-H₂ catalysts were synthesized using distinct reduction methods. A range of physicochemical characterization techniques was employed to investigate the catalyst properties.

2. Results

2.1. XRD Analysis

The XRD patterns of different catalysts are shown in Figure 1a,b. The characteristic diffraction peaks of Pd/CuO, Pd/ZnO, Pd/ZrO₂, and Pd/SnO₂ corresponded to CuO (JCPDS 44-0706), ZnO (JCPDS 36-1451), ZrO₂ (JCPDS 37-1484), and SnO₂ (JCPDS 41-1445), respectively. For Pd/CeO₂-NaBH₄, Pd/CeO₂-H₂, and Pd/CeO₂-PVP catalysts, diffraction peaks observed at 2θ values of 28.6°, 33.1°, 47.5°, 56.3°, 59.1°, 79.4°, and 79.1° were assigned to the (111), (200), (220), (311), (222), (400), and (331) planes of the cubic fluorite structure of CeO₂ (JCPDS 34-0394), respectively. Among all samples, Pd/CeO₂ exhibited the most intense CeO₂ diffraction peaks, which could indicate higher crystallinity in the sample. Interestingly, no crystalline phase associated with Pd species (Pd or PdO) was detected. This suggested that Pd species was highly dispersed on the support, which prevented the formation of detectable crystal structures, or that the crystal size of Pd species was too small to be observed using XRD [38]. This conclusion regarding the high dispersion of Pd species was further supported by the TEM and XPS measurements.

2.2. CO₂-TPD Analysis

CO₂-TPD analysis of the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts was performed (Figure 1c). The results revealed CO₂ desorption peaks for all three catalysts, indicating the presence of basic sites on their surfaces. For the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts, the CO₂ desorption peaks were centered at 403 °C, 418 °C, and 161 °C, respectively. This suggested that the Pd/CeO₂-H₂ catalyst possessed only weak basic sites, while the Pd/CeO₂-PVP and Pd/CeO₂-NaBH₄ catalysts exhibited moderate basic sites. Furthermore, the CO₂ desorption peak area of the Pd/CeO₂-PVP catalyst was larger than those of the other catalysts, indicating a higher density of basic sites. The results suggest that the Pd/CeO₂-PVP catalyst sample had the largest base quantity, which could be beneficial for catalytic applications requiring basic sites [39,40]. In order to eliminate potential interference from the reducing agent on CO₂-TPD analysis, the catalyst was subjected to FTIR spectroscopy characterization (Figure S1). The absence of discernible characteristic peaks associated with the reducing agent in the FTIR spectra confirmed its effective removal within the catalyst.

2.3. CO-TPD Analysis

The chemisorption of CO on metallic Pd exhibited irreversible linear adsorption at single coordination sites. As the irreversible CO adsorption quantity directly correlates with the number of surface-exposed Pd atoms, both the metal dispersion and crystallite dimensions could be quantitatively determined [41], with the results of the CO-TPD analysis of the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts being shown in Figure S2 and

Table 1. Textural property of different catalysts.

Catalyst	BET Surface Area a (m2·g−1)	Pore Volume a (cm3·g−1)	Pore Diameter a (nm)	CO Adsorption b (µmol g−1)	Dispersion of Pd b (%)	Pd Crystallite Size b (nm)	Base Quantity c (mmol·g−1)	Pd Loading d (wt, %)
CeO2	42.6	0.17	15.5	-	-	-	-	-
Pd/CeO2-H2	26.3	0.13	19.8	35.4	11.8	8.5	0.28	0.53
Pd/CeO2-NaBH4	26.7	0.14	20.3	40.9	15.6	6.4	1.84	0.61
Pd/CeO2-PVP	22.0	0.11	19.7	110.4	31.1	3.2	12.61	0.58

^a Detected via BET analysis. ^b Detected via CO-TPD. ^c Detected via CO₂-TPD. ^d Detected via ICP-OES.

. In the CO-TPD profiles below 500 °C, the Pd/CeO₂-PVP catalyst exhibited two distinct desorption peaks at 198 °C and 451 °C, while the Pd/CeO₂-NaBH₄ and Pd/CeO₂-H₂ displayed single desorption peaks at 199 °C and 209 °C, respectively. Comparative analysis revealed that the Pd/CeO₂-PVP demonstrated superior CO adsorption capacity and Pd dispersion relative to the Pd/CeO₂-NaBH₄ and Pd/CeO₂-H₂ catalysts. The Pd particles supported on the Pd/CeO₂-PVP catalyst were smaller in size compared to those on the Pd/CeO₂-NaBH₄ and Pd/CeO₂-H₂ catalyst. These results substantiate that Pd/CeO₂-PVP effectively stabilizes atomically dispersed Pd species on the CeO₂ support, achieving exceptional metallic dispersion [39].

2.4. BET Analysis

The N₂ adsorption isotherms and pore size distributions of CeO₂, Pd/CeO₂-H₂, Pd/CeO₂-NaBH₄, and Pd/CeO₂-PVP are presented in Figure 1d and Figure S3, with their corresponding BET surface area, pore volume, and pore diameter data summarized in Table 1. Based on the adsorption data, the calculated BET surface areas are 42.6, 26.3, 26.7, and 22.0 m²·g⁻¹, while the pore volumes are 0.17, 0.13, 0.14, and 0.11 cm³·g⁻¹, respectively. The corresponding pore diameters are 15.5, 19.8, 20.3, and 19.7 nm. The formation of aggregated Pd nanoparticles, which physically block the pores within the CeO₂ matrix, significantly reduces both the specific surface area and pore volume. Simultaneously, the metal–support interfacial encapsulation effect promotes localized pore coalescence, resulting in an increased average pore diameter [41]. These slight variations fall within the range of experimental error, suggesting that the reduction method has a negligible impact on the BET surface area, pore volume, and pore size. Thus, the overall surface structure of the catalyst remains largely unaffected regardless of the reduction method employed.

2.5. SEM Analysis

The surface morphology of the Pd/CeO₂-H₂, Pd/CeO₂-NaBH₄, and Pd/CeO₂-PVP catalysts, prepared using different reduction methods, was examined via SEM analysis, as shown in Figure 2a and Figures S4 and S5. The CeO₂ supports in the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts appear as spherical structures, and the overall morphology of the catalysts remains largely unchanged across different reduction methods. The Pd loading in the Pd/CeO₂-H₂, Pd/CeO₂-NaBH₄, and Pd/CeO₂-PVP catalysts was quantified using ICP-OES analysis (Table 1), confirming the successful incorporation of Pd into the synthesized catalysts.

2.6. TEM and HRTEM Analysis

The HRTEM and TEM images of the CeO₂, Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts are shown in Figure 2b and Figure S6. While discrete Pd nanoparticles were not observed, aggregated Pd clusters could be detected. Because of the high electron density of CeO₂, it was challenging to clearly distinguish Pd particles in the TEM images [42,43]. The Pd particles were identified using an interplanar spacing of d₁₁₁ = 0.23 nm in their lattice images obtained by HRTEM, which matched the characteristic spacing of metallic Pd and differentiates it from CeO₂ lattice fringes. This observation was consistent with the literature [43,44].

2.7. XPS Analysis

The Pd 3d valence states of the fresh and spent catalysts were determined via XPS analysis (Figure 2c and Figure S7 and

Table 2. XPS analysis of fresh and spent catalysts.

Catalyst	Binding Energy (eV)		Ratio
	Pd 3d5/2	Pd 3d3/2	Pd0/Pd0 + Pd2+	Pd/Ce	Ce3+/Ce4+	Oads/Olatt
Fresh Pd/CeO2-PVP	335.29	340.58	0.18	0.048:1	0.36	0.42
Spent Pd/CeO2-PVP	335.88	341.18	0.17	0.061:1	0.32	0.49
Fresh Pd/CeO2-NaBH4	335.06	340.40	0.58	0.016:1	0.41	0.34
Spent Pd/CeO2-NaBH4	335.41	340.78	0.22	0.037:1	0.16	0.43
Fresh Pd/CeO2-H2	335.21	340.47	0.44	0.010:1	0.22	0.31
Spent Pd/CeO2-H2	335.85	340.96	0.22	0.052:1	0.17	0.48

). The binding energies of Pd 3d_5/2 and Pd 3d_3/2 were 335.29/340.58 eV and 337.18/342.47 eV for Pd/CeO₂-PVP, 335.06/340.40 eV and 337.52/342.81 eV for Pd/CeO₂-NaBH₄, and 335.21/340.47 eV and 337.56/342.83 eV for Pd/CeO₂-H₂. The standard binding energies for Pd⁰ and Pd²⁺ in Pd⁰ were 334.9/340.3 eV and 337.0/342.4 eV, respectively [45,46]. Pd⁰ and Pd²⁺ species coexist on the catalyst surface. This positive shift in the Pd²⁺ valence state can be attributed to electron redistribution within the Pd_xCe_1−xO_2−x−δ solid solution, formed due to strong metal–support interactions [40,47]. This shift suggests that oxygen vacancies migrate from adjacent CeO₂ nanoparticles to the Pd surface [48]. Furthermore, the positive shift in the binding energy of Pd⁰ compared to pure Pd⁰ (334.9 eV) indicates electron transfer from Pd to CeO₂, further emphasizing the strong metal–support interaction [47]. After the oxidation reaction, the relative percentage of Pd⁰ in the Pd/CeO₂-PVP, Pd/CeO₂-H₂, and Pd/CeO₂-NaBH₄ catalysts decreased from 0.18 to 0.17, 0.58 to 0.22, and 0.44 to 0.22, respectively. This change suggests that the electron transfer from Pd⁰ to CeO₂ leads to the partial oxidation of Pd⁰ to Pd²⁺ during the reaction [49]. Furthermore, the Pd/CeO₂ catalysts prepared by different reduction methods exhibited significant differences in basic strength, which may be closely related to the surface Pd²⁺ content of the catalyst, showing a strong positive correlation.

The Ce 3d XPS spectrum of the catalyst before and after the reaction is shown in Figure 2d and Figure S8. The Ce 3d peaks were fitted to eight peaks, among which the peaks between 882.0 and 898.3 eV were assigned to Ce 3d_3/2, marked as v, v₁, v₂, and v₃. The peaks between 899.3 and 917.8 eV belong to the characteristic peaks of Ce 3d_5/2, marked as u, u₁, u₂, and u₃. Among them, v, v₂, v₃, u, u₂, and u₃ were the characteristic peaks of Ce⁴⁺, while v₁ and u₁ correspond to Ce³⁺ [50,51]. Thus, the cerium species in the catalyst consists of Ce⁴⁺ and Ce³⁺. The Ce³⁺ content was calculated as Ce³⁺/(Ce³⁺ + Ce⁴⁺) [52], where Ce³⁺ represents the sum of the Ce³⁺ peak area and Ce⁴⁺ represents the sum of the Ce⁴⁺ peak area. The Ce³⁺ concentration was related to the concentration of oxygen vacancy on the CeO₂ surface, which can be used as an electron trap to promote oxygen chemical adsorption and molecular oxygen activation, benefiting the oxidized alcohols [47,53]. As shown in Table 2, the values of Ce³⁺/Ce⁴⁺ decreased after the reaction, indicating a significant decrease in the oxygen vacancy concentration on the catalyst after the reaction. Reduced Ce³⁺ tends to migrate into the subsurface or bulk regions of CeO₂ to stabilize oxygen vacancies [48]. This redistribution reduces the surface concentration of Ce, leading to an apparent increase in the Pd/Ce ratio. Our XPS Ce 3d spectra (Figure 2d) confirmed a decrease in surface Ce³⁺ content, consistent with this explanation.

The Ce 3d XPS spectrum of the catalyst before and after the reaction is shown in Figure 2e and Figure S9. The binding energy at 528.5–530.5 eV was the characteristic peak of lattice oxygen O_latt (O²⁻), and the binding energy at 531.0–532.5 eV was the characteristic peak of surface adsorption oxygen O_ads (O₂²⁻ or O⁻) [38]. The greater the value of O_ads/O_latt, the higher the surface active oxygen content on the catalyst surface [54]. Pd/CeO₂-PVP catalysts had the highest value of O_ads/O_latt and a higher Ce³⁺ concentration (Table 2), thus showing higher catalytic activity. From the XPS results, NaBH₄ exhibited a reducing effect on both Ce⁴⁺ and Pd²⁺, while H₂ primarily reduced Pd, and PVP mainly reduced Ce⁴⁺. The differences in the reducing ability of the three reducing agents influenced the surface content of Pd⁰ and Ce³⁺ in the catalyst, thereby affecting its catalytic performance.

3. Catalytic Oxidation of 1,2–PDO

3.1. Catalyst Screening Experiments

Different metallic-oxide-supported Pd catalysts were prepared with NaBH₄ as the reducing agent to compare the catalytic activity of 1,2-PDO oxidation (

Table 3. Comparison of catalytic performance of 1,2-PDO conversion to LA over various catalysts.

Catalysts	NaOH/1,2-PDO	LA Yield (%)	1,2-PDO Conversion (%)	LA Selectivity (%)
1.7%Pd/CuO	2.0	48.8	73.5	66.4
1.7%Pd/SnO2	2.0	45.8	70.0	65.4
1.7%Pd/ZrO2	2.0	42.4	65.5	64.8
1.7%Pd/ZnO	2.0	38.9	65.3	59.6
1.7%Pd/CeO2	2.0	51.7	74.2	69.7
3%Pd/CeO2-NaBH4	3.0	59.8	90.9	65.8
3%Pd/CeO2-H2	3.0	57.3	77.6	73.9
3%Pd/CeO2-PVP	3.0	62.7	86.9	72.2

Reaction conditions: 1,2-PDO/total metal molar ratio = 500; substrate (50 mL, 0.05 M); stirring rate = 500 rpm; O₂ pressure = 1 MPa; temperature = 120 °C; reaction time = 2 h.

). The results showed that the yield order of LA was Pd/CeO₂ > Pd/CuO > Pd/SnO₂ > Pd/ZrO₂ > Pd/ZnO. The LA yield was highest over the Pd/CeO₂ catalyst. Furthermore, the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts were prepared with PVP, NaBH₄, and H₂ as reducing agents. Under the same reaction conditions, the Pd/CeO₂-PVP catalyst showed a higher LA yield and 1,2-PDO conversion. The yield of LA was 62.7%, the conversion of 1,2-PDO was 86.9%, and the selectivity of LA was 72.2%.

3.2. Effect of Reaction Conditions on 1,2-PDO Oxidation

The selective oxidation of 1,2-PDO was conducted using Pd/CeO₂-NaBH₄ with different temperatures, times, molar ratios of NaOH/1,2-PDO, and O₂ pressures as control variables. Thus, the effect of catalytic temperature was firstly investigated over the range of 80–140 °C on the substrate conversion and product selectivity. The results showed that, as the reaction temperature increased from 80 °C to 140 °C, the conversion of 1,2-PDO increased from 29.8% to 95.0% and the selectivity for LA decreased from 75.9% to 47.7%. The selectivity for FA, AA, and OA gradually increased, while the selectivity of PA tended to increase slightly. Higher temperatures would promote the oxidation of 1,2-PDO, but it would also cause LA to be further oxidized. The highest yield of LA was obtained when the reaction temperature was 120 °C (Figure 3a). When the reaction time increased from 1 h to 4 h, the conversion of 1,2-PDO increased significantly from 76.6% to 95.6%, and the selectivity of FA, AA, and OA increased, while the selectivity of LA and PA showed the opposite trend. Therefore, increasing the reaction time is beneficial to improve the catalytic activity, but the selectivity of the by-products also increases. The LA yield was highest when the reaction time was 2 h (Figure 3b). Figure 3c shows the effect of the molar ratio of NaOH/1,2-PDO on the 1,2-PDO oxidation reaction. When the molar ratio of NaOH/1,2-PDO increased from 1 to 4, the conversion rate of 1,2-PDO increased from 75.1% to 91.2%, and the selectivity of LA was first increased and then decreased. When the molar ratio of NaOH/1,2-PDO was increased to 3:1 and 4:1, the selectivity of FA and AA was greater than 4.0% and 20.0%, respectively. Meanwhile, the selectivity of LA was decreased. This suggested that increasing the NaOH/1,2-PDO molar ratio favors reactions involving the formation of products other than LA or the degradation of the previously formed LA. Thus, an appropriate molar ratio of NaOH/1,2-PDO favors the oxidation of 1,2-PDO. The LA yield was highest when the molar ratio of NaOH/1,2-PDO was 3:1. As the O₂ pressure increased from 0 MPa to 1.50 MPa, the conversion of 1,2-PDO increased accordingly (Figure 3d). The selectivity of LA decreased from 68.6% to 56.9%. The selectivity of PA had similar changes to LA, with the O₂ pressure increasing, and the selectivity of FA, AA, and OA increased. The results showed that higher O₂ pressure favors the oxidation of 1,2-PDO. However, the target product LA was more easily converted into by-products under higher O₂ pressure. The highest yield of LA was obtained when the O₂ pressure was 1 MPa.

3.3. Possible Reaction Mechanism

In this study, the XPS results confirm the presence of both Pd⁰/Pd²⁺ and Ce³⁺/Ce⁴⁺ in the catalyst. Based on these findings, we propose a reaction mechanism for the catalytic conversion of 1,2-PDO to LA over Pd-based catalysts. This process is governed by a synergistic mechanism involving metal–support interactions and oxygen vacancy. Pd nanoparticles serve as the primary active sites for the adsorption and sequential dehydrogenation of 1,2-PDO [55]. Pd⁰ preferentially interacts with the hydroxyl group of 1,2-PDO, initiating dehydrogenation to form reactive enol intermediates. Reactive oxygen species (O⁻, O₂⁻) associated with CeO₂ oxygen vacancies nucleophilically attack the hydroxyl group, facilitating its oxidation to a carboxylate [53,56,57]. For a more detailed discussion of the reaction mechanism, we will further explore this aspect in future studies.

Based on the above discussion, we propose a possible response pathway (Scheme 1). 1,2-PDO contains two hydroxyl groups, a primary hydroxyl group and a secondary hydroxyl group. There were two different reaction pathways in the catalytic oxidation process (Scheme 1). When the primary hydroxyl group is oxidized, it produces lactaldehyde, which is then rapidly oxidized to LA. When the secondary hydroxyl group is oxidized, it produces hydroxyacetone, which is then rapidly oxidized to pyruvaldehyde. Under alkaline conditions, there is tautomeric equilibrium between lactaldehyde and hydroxyacetone, and pyruvaldehyde can be converted to LA by the Cannizzaro reaction. As the reaction proceeds, products such as PA, LA, and FA are also produced. Experiments on the intermediates lactaldehyde and hydroxyacetone at 120 °C, 1.0 MPa O₂, 2 h reaction time, and NaOH/1,2-PDO molar ratio of 3 showed a decrease in the carbon mass balance after the reaction, possibly generating CO₂ out of equilibrium (Table S1), which we will demonstrate further experimentally.

4. Materials and Methods

4.1. Materials

The chemicals, palladium chloride (PdCl₂, AR), sodium hydroxide (NaOH, AR), sodium borohydride (NaBH₄, AR), polyvinylpyrrolidone (K30) (PVP), 1,2-PDO (C₃H₈O₂) (CP), LA (C₃H₆O₃) (CP), formic acid (CH₂O₂) (CP), pyruvic acid (C₃H₄O₃) (CP), oxalic acid (C₂H₂O₄) (CP) and acetic acid (C₂H₄O₂) (CP), lactaldehyde (C₃H₆O₂) (CP), hydroxyacetone (C₃H₆O₂) (CP), and pyruvaldehyde (C₃H₄O₂) (CP) were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. The Fluronic F127, urea, cerium nitrate, zirconium nitrate, CeO₂, ZrO₂, CuO, ZnO, and SnO₂ were all purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification.

4.2. Catalyst Preparation

PdCl₂ (0.0289 g) was dissolved in 7 mL of 1% PVP aqueous solution, followed by the addition of 7 mL of 0.1 M NaBH₄ solution, forming a deep brown sol. The mixture was stirred at room temperature for 0.5 h before adding 1 g of CeO₂ (or other metal oxides: CuO, ZrO₂, ZnO, SnO₂). Stirring continued for an additional 2 h, after which the product was repeatedly washed with deionized water and methanol, collected by centrifugation, and dried under vacuum at 40 °C for 24 h to obtain 1.7% Pd/CeO₂. The 3% Pd/CeO₂ catalyst was synthesized using the same procedure, with the PdCl₂ mass adjusted accordingly.

PdCl₂ (0.0516 g) was dissolved in 10 mL of 1 M HCl to form a [PdCl₄]²⁻ solution. CeO₂ (1 g) was added, and the mixture was sonicated for 10 min, followed by stirring at room temperature for 4 h. The solid was collected by centrifugation, washed with ultrapure water until neutral, and dried under vacuum at 40 °C for 24 h. The dried sample was then heated to 300 °C at a ramp rate of 10 °C/min and reduced under a hydrogen atmosphere for 2 h, yielding 3% Pd/CeO₂-H₂.

PdCl₂ (0.0516 g) and NaCl (0.034 g) were dissolved in 10 mL of deionized water and sonicated for 10 min to form a [PdCl₄]²⁻ solution, followed by the addition of 1 g of KBr. The prepared solution and 10 mL of 75 mM PVP aqueous solution were simultaneously introduced into a three-neck flask and stirred at 100 °C for 1 h. CeO₂ (1 g) was then added, and stirring continued for another 0.5 h. The resulting product was repeatedly washed with deionized water and methanol, collected by centrifugation, and dried under vacuum at 40 °C for 24 h, yielding 3% Pd/CeO₂-PVP.

4.3. Catalyst Characterization

The catalysts’ crystal structure was analyzed by X-ray diffraction (XRD) on a Bruker-AXS D8 Advance instrument (Bruker AXS, Karlsruhe, Germany) equipped with Cu Kα radiation (λ = 0.154 nm) and a Ni filter. Scans were performed over a 2θ range of 10–80° at a scanning rate of 7° min⁻¹. Morphological characterization was carried out via transmission electron microscopy (TEM) using a JEOL JEM-2100 microscope (Tokyo, Japan) operated at 200 kV, while high-resolution TEM (HRTEM) provided insights into the crystalline lattice structure. Surface elemental composition was probed by energy-dispersive X-ray spectroscopy (EDS). N₂ adsorption–desorption isotherms were recorded on a Micromeritics TriStar II 3020 analyzer (Norcross, GA, USA) to calculate the specific surface area (BET method) and pore size distribution. The samples were previously degassed at 120 °C for 2 h under vacuum. The Pd and Ce contents of the catalysts were quantified using inductively coupled plasma-atomic emission spectrometry (ICP-AES, model Optima 7300DV, PerkinElmer, Hopkinton, MA, USA). X-ray photoelectron spectroscopy (XPS, model Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was utilized to detect the binding energy and the surface chemical state of the catalysts. The intensity and number of basic sites in the catalyst were measured using Temperature-Programmed Desorption (CO₂-TPD, TP-5080-D Automatic Multi-Absorbent Analyzer, Tianjin Xianquan Industrial, Tianjin, China). The samples were preheated at 150 °C for 2 h to remove physically adsorbed water and organic contaminants. After preheating, the samples were cooled to 100 °C, followed by saturation adsorption in a pure CO₂ stream (30 mL/min) for 30 min. Physically adsorbed CO₂ was then desorbed at 100 °C for 30 min before performing the CO₂-TPD analysis. For the TPD measurement, the temperature was increased from 100 °C to 800 °C in a 30 mL/min stream at a heating rate of 10 °C/min. The Fourier-transform infrared (FTIR) spectra were acquired using a Nicolet Nexus 470 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) over the spectral range of 4000–400 cm⁻¹, with a spectral resolution of 4 cm⁻¹. Carbon Monoxide Temperature-Programmed Desorption (CO-TPD) was carried out on a Micromeritics Auto Chem II 2920 (Norcross, GA, USA). The sample (100 mg) was heated to 300 °C in a He stream with a volumetric flow rate of 30 mL min⁻¹ at a ramping rate of 10 °C min⁻¹ for 1 h. After cooling to room temperature, CO was supplied for 1 h. For the measurements, the samples were heated from 50 to 950 °C at a heating rate of 10 °C/min.

4.4. Product Analysis

The oxidation reaction of 1,2-PDO was studied in an autoclave reactor (YZQR-100, Yanzheng, Shanghai, China). The catalyst was placed in 50 mL of 1,2-PDO aqueous solution at a 1,2-PDO/Pd molar ratio of 500. The air was discharged from the reactor with N₂, and O₂ pressure was set to the required pressure. The autoclave was heated to the specified reaction temperature, regardless of the heating time, and then the stirring speed was set to 500 rpm.

At the end of the reaction, the autoclave was cooled to room temperature, and the reaction mixture was acidified with HCl (12M) to a pH value of ca.3. The concentration of the remaining 1,2-PDO in was analyzed on an Agilent 7890A gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with a DB-HeavyWax capillary column (0.25 mm × 30 m). Products were analyzed on an Agilent 1120 HPLC (Agilent Technologies, Santa Clara, CA, USA) system equipped with Carbomix H-NP column (7.8 mm × 300 mm) UV detector (λ = 210 nm) at 308 K. The mobile phase was a H₂SO₄ solution of pH = 2.3 with a flow rate of 0.6 mL min^–1. Product concentration was analyzed by the external standard method. The product concentration was analyzed by external standard method. The selectivity of product was calculated on the basis of carbon.

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} (\%)=1-{\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{1,2}\text{-}\mathrm{P}\mathrm{D}\mathrm{O}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{1,2}\text{-}\mathrm{P}\mathrm{D}\mathrm{O} \mathrm{a}\mathrm{d}\mathrm{d}\mathrm{e}\mathrm{d}}}\times 100$$

(1)

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} (\%)={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{1,2}\text{-}\mathrm{P}\mathrm{D}\mathrm{O} \mathrm{a}\mathrm{d}\mathrm{d}\mathrm{e}\mathrm{d}}}\times 100$$

(2)

5. Conclusions

The metal-oxide-supported palladium catalyst was prepared with NaBH₄ as the reducing agent and various oxides (CeO₂, CuO, ZrO₂, ZnO, SnO₂) as the support. The palladium-supported CeO₂ catalyst showed better catalytic activity. Based on this analysis, the Pd/CeO₂-PVP, Pd/CeO₂-NaBH₄, and Pd/CeO₂-H₂ catalysts were prepared using three reducing agents (PVP, NaBH₄, and H₂). The catalytic activity was investigated under the same conditions, and the results showed that the Pd/CeO₂-PVP catalyst had higher base quantities, the highest value of O_ads/O_latt, thus showing better catalytic activity in the oxidation reaction. Under the reaction conditions of 120 °C, 1.0 MPa O₂, a reaction time of 2 h, and a NaOH/1,2-PDO molar ratio of 3, the conversion of 1,2-PDO reached 86.9%, with LA selectivity of 72.2%.