Ligand-Mediated Tuning of Pd-Au Nanoalloys for Selective H₂O₂ Production in Direct Synthesis from H₂ and O₂

Abstract

Hydrogen peroxide (H₂O₂) is an important industrial chemical that is widely applied in many areas. The direct synthesis of H₂O₂ from H₂ and O₂ has proved to be a green and economic pathway. Pd-based bimetallic catalysts, due to their superior catalytic performances in this reaction, have attracted intensive attention. Herein, Tetrakis(hydroxymethyl)phosphonium chloride (THPC) was adopted as the protective ligand to immobilize Pd-Au alloy nanoparticles onto activated carbon (AC). The varied Pd/Au molar ratios demonstrated homogeneously distributed Pd-Au nanoalloys with average particle sizes ranging from 3.51 to 5.75 nm. The optimal ratio was observed over the Pd₃Au₁/AC-THPC catalyst with a maximum H₂O₂ productivity of 165 mol/(kg_Pd·h) and selectivity of 82.3% under ambient pressure. The relationship between the electronic structure and catalytic activity indicated Pd⁰ was the active site, while the presence of Au inhibited H₂O₂ degradation rate. This research could help in the design efficient bimetallic catalysts for the direct synthesis of H₂O₂.

1. Introduction

Hydrogen peroxide (H₂O₂) is a crucial industrial chemical with diverse applications, ranging from its use as a bleaching agent in the paper and textile industries to its role as a disinfectant, oxidizer, and in the production of chemicals such as peracetic acid [1,2,3,4]. Additionally, it has potential applications in green chemistry, environmental remediation, and energy storage [2,3,4]. The dominant method for industrial H₂O₂ production is the anthraquinone oxidation (AO) process, which relies on the hydrogenation of anthraquinone to form hydrogenated derivatives, followed by oxidation with oxygen to regenerate the anthraquinone. This process is generally carried out in large-scale facilities due to its complexity and the significant use of solvents [1,2,3,4,5,6].

In recent years, there has been a shift towards the direct synthesis of H₂O₂ from H₂ and O₂, which could theoretically eliminate the need for complex intermediates and toxic solvents [2,5,6,7]. However, this approach faces significant challenges due to the difficulty of selectively activating O₂ while minimizing side reactions, such as the production of water or other reactive oxygen species, which would lower the efficiency of H₂O₂ production [2]. This is where advances in catalysis become pivotal, as catalytic systems can lower activation energy and enable more controlled reactions. Over the past few decades, significant progress has been made in developing novel catalytic systems that facilitate this reaction under mild conditions, with minimal by-products. Various catalytic materials, including noble metals, metal oxides, and carbon-based materials, have been explored [5,8,9,10].

Pd-based catalysts have been widely used in the direct synthesis of H₂O₂ due to their unique catalytic properties [2,10,11]. They can adsorb and dissociate H₂ molecules while exhibiting a certain degree of selectivity for H₂O₂ formation, rather than fully oxidizing hydrogen to form water (H₂O), which is a common competing reaction [2,10]. However, achieving high selectivity for H₂O₂ generation remains challenging, as pure Pd catalysts tend to fully oxidize H₂ to H₂O. Therefore, studies on Pd-based catalysts often involve modifications or combinations with other elements or supports to enhance their selectivity, activity, and stability [12].

One significant breakthrough in this field over the years has been the incorporation of Au into supported Pd-based catalysts, forming Pd-Au alloy catalysts [13]. In the direct synthesis of H₂O₂, the unique role of Pd-Au alloy catalysts is manifested in several ways [13,14,15]: (1) Enhanced selectivity: Pure Pd catalysts often over-oxidize H₂ to form H₂O, which reduces H₂O₂ productivity. The addition of Au can adjust the surface electronic properties of Pd, making its surface active sites more selective for H₂O₂, thereby reducing water formation. This selectivity enhancement is related to the electronic and geometric effects on the Pd-Au surface. (2) Improved catalytic activity: the formation of Pd-Au alloy catalysts increases the catalyst’s activity. The interaction between Au and Pd results in surface active sites with higher catalytic potential, helping to more effectively activate reactant molecules (H₂ and O₂) and thus promote H₂O₂ generation. (3) Improved stability: Compared with pure Pd catalysts, Pd-Au alloys exhibit higher stability in long-term reactions. They are less prone to deactivation and possess better antioxidant and anti-aggregation properties under reaction conditions, thus extending the catalyst’s lifespan. Professor Graham J. Hutchings and his research group at Cardiff University have made outstanding contributions to the field of direct H₂O₂ synthesis, particularly in the development and optimization of Pd-Au alloy catalysts [2,13]. They were among the first teams to discover that Pd-Au alloy catalysts exhibit significant selectivity and high activity for direct H₂O₂ synthesis [7,16]. Through the synergistic effect between Au and Pd, they demonstrated that Au can significantly inhibit the complete oxidation pathway of Pd catalysts, thereby increasing the selectivity for H₂O₂. Their research revealed the unique advantages of Pd-Au catalysts and explored the high selectivity and high activity of these catalysts in direct H₂O₂ synthesis [13].

Pd-Au alloy catalysts have demonstrated superior activity and selectivity in the direct synthesis of H₂O₂ from hydrogen and oxygen. Consequently, the preparation of Pd-Au alloy catalysts with uniform size and morphology becomes crucial. The conventional and simple impregnation method was initially used to prepare Pd-Au catalysts; however, this method often fails to achieve a uniform alloy phase, and the calcination process can easily cause nanoparticle (NP) aggregation, negatively impacting catalyst performances [17,18,19,20]. The sol-immobilization method, on the other hand, can produce uniformly distributed Pd-Au nanoalloys with a narrow particle size distribution (PSD), making it particularly suitable for studying catalytic reactions [11,21]. The most commonly applied protective agents in the sol-immobilization method are poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) [11,22,23,24]. However, the remaining PVA and PVP can have positive effects in the productivity, catalytic activity, and selectivity, thus hindering the investigation and interpretation of the catalytic active sites and catalytic nature [25,26]. Tetrakis(hydroxymethyl)phosphonium chloride (THPC) as a reducing and stabilizing agent has also been used in the synthesis of small metal NPs with a narrow PSD, especially for noble metals such as Au, Pd, and Pt [27,28]. Moreover, regarding the THPC method, it has not been reported that its presence over the metal NPs would make an impact on the catalytic performances.

In this research, the sol-immobilization method using the THPC agent was adopted for the preparation and investigation of Pd-Au alloy NPs over activated carbon (AC) material. The AC was selected as the support due to its high specific surface area (SSA) and abundant acidic sites. These acidic sites on the carbon support surface can inhibit the further decomposition of the generated H₂O₂, thus increasing its selectivity [15]. Further investigations were focused on the Pd/Au molar ratio effect on the structural and electronic modifications and how these modifications influence the catalytic performances in the direct synthesis of H₂O₂.

2. Results and Discussion

2.1. Structural Properties of Pd_xAu_y/AC-THPC

Four bimetallic Pd_xAu_y/AC-THPC catalysts were synthesized using the sol-immobilization method with varied Pd/Au ratios (1/1, 2/1, 3/1 and 4/1).

Figure 1 shows the transmission electron microscopy (TEM) images and the particle size distribution (PSD) of the metal NP sizes for Pd_xAu_y/AC-THPC catalysts with different Pd/Au ratios. From the TEM images, it can be observed that small metal NPs were uniformly distributed on the AC support with a narrow PSD. The average NP size of the Pd_xAu_y/AC-THPC catalysts increased gradually with the Pd/Au ratio, from 3.51 nm over Pd₁Au₁/AC-THPC to 5.75 nm over Pd₄Au₁/AC-THPC. This may be due to the insufficient amount of THPC added in the preparation of Pd-Au/AC-THPC catalysts with higher Pd/Au ratios, which results in less effective control over the PSD. Nevertheless, notably, the THPC-stabilized NPs supported on Pd-Au/AC-THPC catalysts demonstrated a significantly narrower PSD (±0.9 nm) compared to their ligand-free Pd-Au/Al₂O₃ counterparts (±2.4 nm), as quantified in Supplementary Figure S1. This disparity in monodispersity highlights the critical influence of THPC coordination chemistry in suppressing nanocristal agglomeration during NP synthesis. Interestingly, by measuring the lattice fringes of the nanoparticles, we can easily observe the Pd (111) lattice fringes with an interplanar spacing of 0.23 nm [29]. This indicates that in Pd-rich (Pd/Au ≥ 1) Pd-Au/AC-THPC catalysts, the Pd component is still more easily identified, while the elemental distribution of Au requires other methods for detection.

To further characterize the elemental distribution of Pd and Au, as well as the chemical composition of the metal NPs in the Pd-Au/AC-THPC catalyst, aberration-corrected scanning transmission electron microscopy (AC-STEM) and energy dispersive x-ray (EDX) spectroscopy were employed (Figure 2). The EDX mapping images (Figure 2(a₁,a₂,b₁,b₂)) and EDX spectra (Figure S2) reveal that the metal NPs were uniformly dispersed on the carbon support, with the elemental distributions of Pd and Au nearly overlapping and both elements being present on each individual NP. This observation directly demonstrates that the THPC method successfully produced uniformly distributed and randomly arranged Pd-Au alloy NPs. In addition, the C signal (Figure 2(a₃,b₃)) originated from the AC support, while the O signal (Figure S2) can be attributed to oxygen-containing species on the surface of the activated carbon (such as hydroxyl and carboxyl groups) as well as the oxidized state of the metal on the catalyst surface [30].

The XRD technique was adopted to investigate the crystalline structure of bimetallic Pd-Au/AC-THPC catalysts with the varied Pd/Au ratios, as shown in Figure 3. The XRD patterns of these catalysts exhibit three main characteristic diffraction peaks located at 2θ = 24.3°, 39°, and 43.3°. The first two peaks correspond to the characteristic reflections of C (002) and C (101), respectively [31]. The peak at 39° can be attributed to the mixed-metallic components of Pd and Au [32]. According to the literature, the Pd(111) and Au(111) planes are located at 40.1° and 38.3°, respectively. Furthermore, STEM-EDX images (Figure 2) confirm that Pd and Au coexist within the same NPs. Therefore, the characteristic diffraction peak at 2θ = 39° should be ascribed to the Pd-Au alloy phase, consistent with reported XRD peak positions in the literature [32,33]. Variations in the intensity of the Pd-Au alloy peak among different catalysts may be related to differences in the sample amount used during testing or the actual metal loading of the catalysts.

The results for the specific surface area (SSA), pore volume, and pore size distribution of the catalysts are presented in Figure 4 and

Table 1. Textual properties of Pd_xAu_y/AC-THPC catalysts with the variation of Pd/Au molar ratios.

Catalyst	SSA (m2/g)	Pore Volume (cm3/g)	Average Pore Size (nm)
Pd1Au1/AC-THPC	1087	0.589	3.18
Pd2Au1/AC-THPC	1088	0.588	3.17
Pd3Au1/AC-THPC	1053	0.573	3.13
Pd4Au1/AC-THPC	1027	0.561	3.15

. All the synthesized catalysts demonstrated SSAs in the range of 1000–1100 m²/g, suggesting that the Pd/Au ratio has no significant impact on the SSA. At low P/P₀ values, the N₂ adsorption increased rapidly with an initial rise in P/P₀ and then increased more gradually. At higher P/P₀ values, the N₂ adsorption stabilized, reaching a constant value. This behavior indicated that the nitrogen adsorption of the AC-supported Pd-Au bimetallic catalysts followed a microporous adsorption–desorption isotherm. The strong interactions between nitrogen molecules and the adsorbent in the low-pressure region led to substantial nitrogen adsorption. The pore volume and the average pore size of the catalyst samples exhibited minimal variation, remaining within the range of 0.56–0.58 cm³/g and the range of 3.13–3.18 nm.

As shown in Figure 5a, the XPS spectrum of each PdAu/AC-THPC catalyst exhibits three characteristic peaks in the Pd 3d region. Taking Pd₁Au₁/AC-THPC as an example, these peaks are located at 337.9 eV, 335.9 eV and 335.4 eV, corresponding to Pd²⁺ 3d_5/2, Pd⁰ 3d_5/2, and Au 4d_5/2, respectively. Figure 5c demonstrated a composition-dependent modulation of palladium oxidation states, with the surface Pd²⁺ fraction decreasing linearly (R² = 0.94) as the Pd/Au molar ratio increases, while the Pd⁰ content exhibits a proportional rise (R² = 0.91). This inverse correlation directly correlates with Pd-Au alloy formation, wherein d-orbital hybridization between Pd and Au alters the electron density of Pd orbitals [17]. Correspondingly, the Au 4f XPS spectra reveal a systematic negative binding energy shift (Δ = 0.1–0.3 eV) in Au⁰ 4f_7/2 and Au⁰ 4f_5/2 peaks (Figure 5b), consistent with electron transfer from Pd to Au driven by Au’s higher electronegativity [33]. These parallel trends in Pd and Au electronic states mechanistically confirm alloy-induced interfacial charge redistribution.

2.2. Catalytic Performance of Pd_xAu_y/AC-THPC

The above characterization results demonstrated the formation of Pd-Au alloy over AC support, modifying the electronic structures of Au and Pd. The catalytic activity of Pd_xAu_y/AC-THPC catalysts was then evaluated in the direct synthesis of H₂O₂ from H₂ and O₂ and compared with reported Pd-based catalysts (

Table 2. Catalytic performances in the direct synthesis of H₂O₂ from H₂ and O₂.

Catalyst	Time (min)	Solvent	Temperature (°C)	Pressure (bar)	H2O2 Productivity mol/(kgPd·h)	H2O2 Selectivity (%)
Pd1Au1/AC-THPC	30	EtOH/H2SO4	10	1	18	3.8
Pd2Au1/AC-THPC	30	EtOH/H2SO4	10	1	14	5.3
Pd3Au1/AC-THPC	10	EtOH/H2SO4	10	1	165	82.3
Pd3Au1/AC-THPC	30	EtOH/H2SO4	10	1	71	35.9
Pd3Au1/AC-THPC	60	EtOH/H2SO4	10	1	42	6.0
Pd4Au1/AC-THPC	30	EtOH/H2SO4	10	1	56	19.5
Pd4Au1/AC-THPC	60	EtOH/H2SO4	10	1	45	5.7
Pd2.5Au0.5/TiO2 [35]	30	EtOH/H2SO4	10	1	2069	46.0
Pd0.5Au2.5/TiO2 [35]	30	EtOH/H2SO4	10	1	774	38.5
Pd/HAP [36]	30	EtOH/H2SO4	10	1	741	43
Pd40Ag1/AC [37]	15	MeOH/H2SO4	2	30	7022	71
Pd1Au1/CeZrOx [38]	180	MeOH/H2SO4	20	1	1270	61
Pd1Au1/AC [15]	30	MeOH/H2O	2	37	320	95
Pd3Au1/SiO2 [39]	150	MeOH/HCl	10	1	1770	63
Pd1Zn9/Al2O3 [40]	15	MeOH/H2SO4	2	30	25,431	78.5
Pd2Ga/TiO2 [41]	30	MeOH/H2O	2	29	4269	30

Reaction conditions: 60 mL solvent (ethanol/H₂SO₄), 60 mL/min (H₂/O₂/N₂ = 9/36/15), 50 mg, 1 bar, 10 °C, and stirring rate = 1000 r/min.

). Notably, at 10 min reaction, the Pd₃Au₁/AC-THPC catalyst exhibited the highest H₂O₂ productivity (165 mol/(kg_Pd·h)) and H₂O₂ selectivity (82.3%) in this research. However, these values dropped to 71 mol/(kg_Pd·h) and 35.9%, still outperforming the other Pd_xAu_y/AC-THPC catalysts. Among these catalysts, Pd_xAu_y/AC-THPC catalysts of Pd/Au = 3/1 and 4/1 seem to display higher activity than those of Pd/Au = 1/1 and 2/1. As shown in Figure 5, Pd₃Au₁/AC-THPC and Pd₄Au₁/AC-THPC catalysts possessed more Pd⁰ (82.8% and 84.6%, respectively) than the other two catalysts of 1/1 and 2/1 (68.4% and 73.7%, respectively), possibly suggesting Pd⁰ is the active valence state of Pd. With the increase in the Au molar fraction, the H₂O₂ degradation rate over Pd₁Au₁/AC-THPC (425 mol/(kg_Pd·h)) was observed to be lower than that over Pd₃Au₁/AC-THPC (723 mol/(kg_Pd·h)), which is consistent with the reported literature. These demonstrated that, for Pd-Au alloy catalysts prepared via the THPC method, the catalytic performance is optimal when the Pd/Au ratio is 3/1. Pd-Au alloy catalysts typically exhibit the best performance at an optimal Pd/Au ratio, which is attributed to the synergistic effect between Pd and Au. According to the literature, Pd exhibits strong adsorption, activation, and conversion abilities for H₂ and O₂, but these same properties make monometallic Pd catalysts more prone to overreaction, leading to the formation of H₂O rather than H₂O₂. The introduction of Au weakens Pd’s adsorption and desorption of intermediate species, optimizing the reaction pathway and favoring the formation of H₂O₂, thereby increasing selectivity [34]. Moreover, Flaherty et al. [14,34] demonstrated that this improvement is primarily due to the electronic structure optimization between Pd and Au. However, over time, the selectivity for H₂O₂ gradually decreases, likely due to the further decomposition of the produced H₂O₂ into water.

In order to further investigate the catalytic activity and stability of the Pd₃Au₁/AC-THPC catalyst, additional results were presented in Figure 6. In Figure 6a, a rapid accumulation of H₂O₂ was observed within the initial 10-min reaction period, demonstrating the catalyst’s superior activation kinetics. Maximum H₂O₂ selectivity and productivity were attained at 10 min (Table 2), corresponding to optimal surface active-site utilization before competitive degradation pathways became thermodynamically favorable. With the reaction time going, though H₂O₂ concentration was increasing over time, the H₂O₂ productivity and selectivity declined due to the high degradation rate, which can be supported by its high excellent reusability with three cycles of reaction running. The high catalyst stability validated the analysis and comparison of catalytic performances.

The catalytic performances of the reported Pd-based catalysts in the direct synthesis of H₂O₂ were presented in Table 2. It can be observed that our Pd₃Au₁/AC-THPC catalyst exhibits a selectivity comparable to that reported in the literature, with only the Pd₁Au₁/AC catalyst reported by Prof. Hutchings showing a higher selectivity of 95% under the conditions of 2 °C and 37 bar, which surpasses our 82.3% [15]. However, when comparing H₂O₂ productivity, it is evident that the catalysts reported in the literature achieve significantly higher productivity than ours. Under these optimized reaction conditions, the further decomposition of H₂O₂ can be largely suppressed, thus improving the selectivity and productivity [42]. In addition, pre-treatment using acid on the support would also dramatically enhance the H₂O₂ productivity. However, in this research, the reactions were conducted at 1 bar and 10 °C. The high degradation rate (723 mol/(kg_Pd·h)) of H₂O₂ vs. the maximum productivity (165 mol/(kg_Pd·h)) under these reaction conditions supports our hypothesis.

Therefore, to enhance the H₂O₂ productivity of Pd_xAu_y/AC-THPC catalysts, further optimization of the reaction conditions will be required in future work. Given that the aim of this study was to prepare uniform and stable Pd-Au alloy catalysts with THPC and investigate the effect of the Pd/Au ratio on the catalytic performance of Pd-Au alloy catalysts, our work would provide valuable insights for the design of efficient bimetallic catalysts for the direct synthesis of H₂O₂.

3. Materials and Methods

3.1. Chemicals and Materials

PdCl₂, Palladium Chloride (Sigma-Aldrich, China, ≥99%); AuCl₃·HCl·4H₂O, Chloroauric Acid (Damas-beta, China, 99%); THPC, Tetrakis(hydroxymethyl)phosphonium chloride (Aladdin, China, 0.5%); NaOH, Sodium Hydroxide (Sinopharm Chemicals, China, >96.0%); C₂H₅OH, EtOH (Sinopharm Chemicals, China, >99.7%); H₂SO₄, Sulfuric Acid (Sinopharm Chemicals, ≥95%); AC, activated carbon (Guangzhou Changyu Chemical, China, 99.999%); Molecular H₂ (Jining Xieli Special Gas, China, 99.999%), Molecular O₂ (Jining Xieli Special Gas, China, 99.999%); and Molecular N₂ (Jining Xieli Special Gas, China, 99.999%)

3.2. Catalyst Preparation

3.2.1. Pd-Au/AC-THPC Catalysts

Typically, 0.3 mL of 0.1 mol/L NaOH solution measure was added to 45 mL of deionized water in a beaker, followed by 1.2 mL of a 10 mg/mL THPC aqueous solution under continuous stirring. After 10 min, calculated quantities of Au and Pd precursor solution were added to the above beaker. During the reaction, the color of the solution changes from brownish-yellow to dark brown. After 15 min of reduction, 1 g of AC support was added and followed by 3–4 drops of H₂SO₄. The slurry was stirred for 2 h and then filtered and washed with 1.5 L of deionized water to remove residual ions. The filtrate should be colorless, indicating that the NPs are fully loaded onto the support. After washing, the resulting material was moved to the drying oven at 100 °C for 12 h to obtain the final catalyst sample. The catalysts were denoted as Pd_xAu_y/AC-THPC, where x/y represents the molar ratio of Pd/Au, e.g., Pd₂Au₁/AC-THPC.

3.2.2. Pd-Au/Al₂O₃ Catalyst

Typically, calculated amounts of Au and Pd precursor solution were added to 100 mL DI water under rigorous stirring. After 10 min, 1 g of Al₂O₃ powder was added and the slurry was kept stirring for another 1 h. Before transferred to the 100 °C drying oven, the paste was obtained through filtration and a washing process. The resulting catalyst was collected after calcination in air at 400 °C for 3 h.

3.3. Catalyst Testing

Direct hydrogen–oxygen synthesis of H₂O₂ is a typical gas–liquid–solid three-phase reaction. The gas is introduced from the bottom, dispersed through a sintered glass filter, and then participates in the reaction with the solvent. After the reaction, the gas passes through an online chromatographic detector. Due to the explosive nature of H₂ (4–75%), N₂ is used for dilution and protection (H₂/O₂/N₂ = 9/36/15 (vol)), which also serves as an internal standard for the quantitative analysis of H₂ and O₂ gas concentrations. The total flow rate is controlled at 60 mL/min. In order to further ensure the experimental safety, the flame-free operational regulation is implemented in the lab. Magnetic stirring is employed to enhance the dispersion of catalyst suspension particles. The reactor is equipped with a jacket for temperature control during the reaction.

The experiment involves the direct synthesis of H₂O₂ from H₂ and O₂. H₂O is used as the mobile phase, and a specifically designed high-temperature, high-pressure fixed-bed reactor system is set up to meet these conditions. Raw gases and H₂O are introduced from the bottom of the reactor to react with the catalyst to produce H₂O₂. As H₂O exits the reactor, H₂O₂ is carried out and separated via a gas–liquid separator. The exhaust gas then enters the gas chromatograph and gas mass spectrometer for the online detection of gas composition and concentration. The H₂O₂ concentration in the liquid phase is subsequently measured by a UV spectrophotometer.

H₂O₂ selectivity and H₂O₂ productivity were calculated according to the following equations:

$${\mathrm{H}}_2{\mathrm{O}}_2 \mathrm{selectivity} \left(\%\right)={\displaystyle \frac{mol of{ \mathrm{H}}_2{\mathrm{O}}_2}{mol of converted {\mathrm{H}}_2}}\times 100\%$$

(1)

$$r_{{\mathrm{H}}_2{\mathrm{O}}_2}={\displaystyle \frac{c_{{\mathrm{H}}_2{\mathrm{O}}_2}\times F_{{\mathrm{H}}_2\mathrm{O}}}{m_{cat}\times S_{Pd}\times Di{s}_{Pd}/M_{Pd}}}$$

(2)

where $c_{{\mathrm{H}}_2{\mathrm{O}}_2}$ is the concentration of H₂O₂, mol/L; $F_{{\mathrm{H}}_2\mathrm{O}}$ is flow rate of water, L/s; $m_{cat}$ is the catalyst mass, g; $S_{Pd}$ is the Pd loading, wt%; $Di{s}_{Pd}$ is the Pd dispersion; and $M_{Pd}$ is the molecular weight of Pd, g/mol.

3.4. Catalyst Characterisation

XRD measurements were performed using a Bruker D8 Advance instrument with the following conditions: voltage 40 kV, current 40 mA, Cu-Kα radiation source (λ = 1.54056 Å), diffraction range 10–80°, and a typical scanning speed of 0.4 °/min.

The SSAs and pore volumes of the catalysts were measured using the Micromeritics ASAP-2460 automatic rapid surface area analyzer (Micromeritics Instrument Corporation, Shanghai, China). The measurement conditions were as follows: the catalyst samples were placed in a vacuum oven and dried at 50 °C for 72 h. Prior to measurement, the samples were degassed with nitrogen at 100 °C for 12 h. The specific surface area of the catalysts was then determined at −196 °C.

AC-STEM was performed using a TECNAI G2 F20 transmission electron microscope manufactured by FEI (Hillsboro, OR, USA). The measurement conditions were as follows: working voltage of 200 kV. The samples were thoroughly ground and dispersed in anhydrous ethanol for 1 h of ultrasonication. Afterward, a droplet of the clear middle liquid was placed on a copper grid using a pipette, and the sample was left to dry as the ethanol evaporated before being placed in the sample chamber.

The average particle size (d) of the catalyst was calculated using Equation (3):

$$\mathrm{d}={\displaystyle \frac{\sum n_i{d}_i^3}{\sum n_i{d}_i^2}}$$

(3)

where $n_i$ represents the number of catalyst particles with a diameter $d_i$, and $d_i$ represents the particle size of the catalyst. i represents the total number of particles in the image.

XPS is used to determine the types and oxidation states of elements, primarily for studying the elemental composition and various oxidation states at the surface of materials. In this study, XPS analysis was performed using an ESCALAB250Xi system (Thermo Fisher Scientific, Shanghai, China) to analyze the electronic environment of the surface elements of the catalyst. The test used an Al-Kα radiation source (1486.6 eV, with a pass energy of 30.0 eV). Binding energies were calibrated based on the position of the C 1s peak at 284.8 eV, and the spectra were analyzed following references from the literature.

4. Conclusions

The direct synthesis of H₂O₂ from H₂ and O₂ is an atom-efficient approach compared to the industrial production process. Herein, THPC, the protective-ligand, stabilized Pd-Au alloy NPs were immobilized onto AC support for this reaction. STEM-EDX confirmed composition-tunable Pd-Au nanoalloys with uniform homogeneous distribution and controlled particle sizes via THPC stabilization. The strong interaction between Pd and Au within alloy NPs modified their electronic structures, exhibiting the optimal ratio at 3/1. The optimized Pd₃Au₁/AC-THPC catalyst displays the maximum H₂O₂ productivity (165 mol/(kg_Pd·h) and selectivity (82.3%) at 10 min. The reduced H₂O₂ productivity and selectivity were observed due to the high H₂O₂ degradation rate under the mild reaction conditions: 1 bar and 10 °C. Its high activity can be maintained for three cycles of reaction. This research would benefit the design of selective bimetallic catalysts.