In Situ Synthesis of a Highly Active AuPd/NH₂-P-CNT Catalyst Using Citric Acid to Enhance Hydrogen Evolution from Formic Acid

Abstract

A novel citric acid-assisted in situ reduction method has been developed for the synthesis of bimetallic AuPd alloy nanoparticles supported on amine–phosphate-functionalized carbon nanotubes (AuPd/NH₂-P-CNTs). In this strategy, formic acid acts not only as the reducing agent for reducing metal precursors, but also as the hydrogen source for the subsequent catalytic dehydrogenation. The introduction of citric acid significantly accelerates the reduction kinetics and promotes the uniform formation of ultrafine AuPd nanoparticles (∼1.8 nm). As a result, the optimized Au_0.5Pd_0.5/NH₂-P-CNTs exhibit an extraordinary catalytic activity and 100% H₂ selectivity during hydrogen generation from FA with sodium formate as an additive, affording a remarkable initial turnover frequency of 5663.94 mol H₂ mol Pd⁻¹ h⁻¹ at 303 K. The experimental results reveal that the -NH₂ and -P functional groups on the support are crucial for stabilizing and uniformly dispersing the alloy nanoparticles. Furthermore, the enhanced reaction rate can be attributed to the strong metal–support interaction established between AuPd nanoparticles and -NH₂-P-CNT supports. This work provides a new perspective on the design of highly efficient Pd-based catalysts for hydrogen production from formic acid.

1. Introduction

The transition to sustainable energy sources is a critical aspect of modern energy policy, especially in the context of reducing carbon emissions and enhancing energy security [1,2,3,4]. Hydrogen (H₂), as a kind of clean and renewable energy, is critical for a wide range of applications [5]. In addition to decomposition-based routes, several sustainable hydrogen production technologies have been widely investigated, such as water electrolysis [6,7,8], photocatalytic hydrogen evolution [9,10,11,12,13], and ammonia decomposition [14,15,16]. Among the various hydrogen generation strategies, the selective dehydrogenation of formic acid (FA) has attracted considerable attention because of its simplicity, high efficiency, and potential for CO-free hydrogen generation [17,18,19].

Palladium (Pd) is considered as a promising active metal for the selective dehydrogenation of FA, and its corresponding bimetallic systems (AuPd in particular) are gaining widespread interest due to their enhanced catalytic capability compared to monometallic Pd catalysts [20,21]. For example, Szumełda’s team has developed a series of Au_xPd_100−x alloy nanoparticles (NPs) and found the best molar ratio of Pd to Au was about 9:1 [22]. Based on that, Ye’s team has further demonstrated that electrons would transfer from Au to Pd within the bimetallic AuPd nanoclusters, as confirmed by XPS and DRIFTS analyses. This electronic redistribution effectively optimized the d-band center of Pd, thereby weakening the adsorption strength of reaction intermediates and facilitating the cleavage of key bonds, which ultimately offered superior catalytic activity and excellent H₂ selectivity toward FA dehydrogenation [23]. Besides metal compositions, the carrier is equally critical in determining catalytic performance. Carbon nanotubes (CNTs) have garnered significant interest among different supports due to their high surface area, excellent electrical conductivity, tunable surface chemistry, and unique one-dimensional mesoporous structure. Su’s group highlighted that functionalized CNTs can serve as an ideal scaffold to immobilize metal NPs uniformly, thereby preventing aggregation and enhancing catalytic stability [24]. Moreover, Yan’s team demonstrated that the introduction of heteroatoms (such as N, B, or P) or surface grafting with electron-donating groups could effectively tune the electronic interaction at the metal–support interface, promoting charge redistribution and optimizing metal nucleation behavior, which ultimately improves catalytic efficiency [25]. Hence, designing a catalyst that integrates functionalized CNT support with bimetallic AuPd NPs constitutes a traditional and promising approach for enhancing FA dehydrogenation.

Except for the modification of catalysts, a mild and efficient fabrication method is also crucial for practical application. Traditional liquid-phase reductions often rely on strong chemical reductants like NaBH₄, which pose safety hazards and complicate post-processing; therefore, the exploration of alternative synthetic routes that generate active metal species directly on the support has become an attractive pursuit. One common approach employs a reducing atmosphere, such as H₂. For example, Yu’s group fabricated ultrafine Pd clusters within zeolites via a hydrothermal treatment followed by H₂ reduction, and the as-prepared Pd/S-1-in-K catalyst exhibits excellent activity toward FA dehydrogenation under mild conditions [26]. Another more attractive approach involving the reducing capacity of the reaction system has been reported. Jiang’s group has reported an in situ reduction strategy in a formic acid/sodium formate (FA/SF) system, in which Pd precursors are directly reduced to form supported Pd NPs without the use of NaBH₄ [27]. The synthesized Pd/C catalyst enables efficient, CO-free hydrogen generation from FA/SF at room temperature, achieving a conversion of up to ~85% within 160 min and a turnover frequency of 64 mol H₂ mol Pd⁻¹ h⁻¹. These results demonstrate that Pd catalysts can be constructed via in situ reduction under mild conditions. However, the catalytic performance of such in situ-formed catalysts still requires further enhancement.

Given this background, the use of multifunctional organic acids as additives to control nanoparticle formation has emerged as an effective strategy [28]. Among them, citric acid (CA) has been widely reported to strongly bind to metal surfaces through coordination, thereby stabilizing nascent metal NPs and effectively suppressing aggregation during synthesis. Such a dispersing effect of CA has been demonstrated to significantly improve the uniformity and stability of metal NPs. Building on these insights, the development of an acid-assisted in situ synthesis of highly dispersed catalysts is of great importance for achieving efficient hydrogen production from formic acid.

Herein, we report a CA-assisted in situ reduction system for the fabrication of ultrafine AuPd NPs (~1.8 nm) on amine–phosphate-functionalized CNTs (NH₂-P-CNTs). The phosphate groups are essential in regulating the formation of particles, while the strong electronic interaction between AuPd and the NH₂-P-CNT support contributes significantly to the improved catalytic activity. As a result, the optimized Au_0.5Pd_0.5/NH₂-P-CNT catalyst delivers a high TOF_initial of 5663.94 mol H₂ mol Pd⁻¹ h⁻¹ at 303 K toward catalytic FA dehydrogenation.

2. Results and Discussion

2.1. Catalysts Characterization

The method used for the in situ synthesis of Au_0.5Pd_0.5/NH₂-P-CNTs is shown in Scheme 1 (see the experimental section for specifics).

Scheme 1. Schematic representation for the synthesis of catalysts.

TEM analysis is conducted to elucidate the morphology and dispersion of the Au_0.5Pd_0.5 NPs on the NH₂-P-CNT support. As shown in Figure 1a, Au_0.5Pd_0.5 NPs distribute uniformly along the NH₂-P-CNT surface, and the corresponding particle size histogram (Figure 1a inset) shows a narrow distribution at 1.80 ± 0.02 nm. No obvious aggregates or isolated particles appear within the observed region. High-resolution TEM (HRTEM), seen in Figure 1b, displays distinct lattice fringes with an interplanar spacing of 0.229 nm. This spacing lies between the (111) planes of Au (0.235 nm) [29] and Pd (0.224 nm) [30], thereby confirming the formation of AuPd alloy.

Figure 1. (a) TEM image of Au_0.5Pd_0.5/NH₂-P-CNTs (inset is the corresponding size distribution); (b) HRTEM and (c) SAED images of Au_0.5Pd_0.5/NH₂-P-CNTs; (d) HAADF-STEM and (e) the corresponding elemental distribution image of Au_0.5Pd_0.5/NH₂-P-CNTs; (f) TEM image of Au_0.5Pd_0.5/NH₂-CNTs (inset is the corresponding size distribution).

The selected-area electron diffraction (SAED) pattern in Figure 1c exhibits bright diffraction rings and clear diffraction spots, confirming the crystalline nature of the NPs. Figure 1d displays the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Au_0.5Pd_0.5/NH₂-P-CNTs, illustrating the uniform dispersion of AuPd nanoparticles on the NH₂-P-CNT substrate. The corresponding elemental mapping in Figure 1e further shows the uniform distribution of Au, Pd, N and P. Based on the ICP-AES analysis (Table S1), the molar ratio of Au:Pd is determined to be 0.48:0.52, which is consistent with the appointed value. In contrast, the sample supported on NH₂-CNTs without phosphate groups exhibits noticeable particle agglomeration, as shown in Figure 1f, with the average particle size increasing to 4.79 ± 0.05 nm (Figure 1f inset). This observation indicates that pure amine functionalization provides limited control over the nucleation and growth of AuPd NPs. The incorporation of phosphate groups introduces stronger interactions with Au and Pd, which helps confine AuPd alloy on the CNT surface and restrain particle migration and aggregation, thereby promoting the development of smaller and more evenly dispersed AuPd NPs.

The phase and surface functional groups of the Au_0.5Pd_0.5/NH₂-P-CNTs are characterized using XRD and FTIR spectroscopy, respectively. Figure 2a illustrates that all samples have a large diffraction peak at 26°, indicative of graphitic carbon C (002). In addition, Au_1.0/NH₂-P-CNTs and Pd_1.0/NH₂-P-CNTs show distinctive diffraction peaks of metallic Au (JCPDS No. 65-2870) and Pd (JCPDS No. 65-2867), respectively [31]. Notably, the main peak of Au_0.5Pd_0.5/NH₂-P-CNT appears at approximately 38.5°. This intermediate position between the characteristic peaks of pure Au and Pd confirms the formation of the AuPd alloy phase. Compared with Au_1.0/NH₂-P-CNTs, the diffraction peaks of Au_0.5Pd_0.5/NH₂-P-CNTs slightly shift toward higher angles, indicating that the Pd atoms with a smaller atomic radius have substituted into the Au crystal.

Figure 2. (a) XRD pattern of Au_0.5Pd_0.5/NH₂-P-CNTs, Au_1.0/NH₂-P-CNTs, and Pd_1.0/NH₂-P-CNTs; (b) FTIR spectra of Au_0.5Pd_0.5/CNTs, Au_0.5Pd_0.5/NH₂-CNTs, Au_0.5Pd_0.5/P-CNTs, and Au_0.5Pd_0.5/NH₂-P-CNTs.

Figure 2b shows the FTIR spectra of Au_0.5Pd_0.5/CNTs, Au_0.5Pd_0.5/NH₂-CNTs, Au_0.5Pd_0.5/P-CNTs, and Au_0.5Pd_0.5/NH₂-P-CNTs. All four samples exhibit several common characteristic bands. Herein, the broad adsorption peak located at 3500–3440 cm⁻¹ corresponds to -OH stretching from adsorbed H₂O [32], the peak at 2910 cm⁻¹ is assigned to C-H stretching [33], the peak near 1620 cm⁻¹ is interpreted as graphitic C=C stretching [34], and the weak band at ~560 cm⁻¹ is ascribed to C-C deformation vibrations [35]. Both Au_0.5Pd_0.5/NH₂-CNTs and Au_0.5Pd_0.5/NH₂-P-CNTs display a peak at 1382 cm⁻¹, corresponding to the N-H bending vibration of amine groups (-NH₂) resulting from APTS modification [36]. Meanwhile, the spectra of both Au_0.5Pd_0.5/NH₂-P-CNTs and Au_0.5Pd_0.5/P-CNTs exhibit a strong band at 920 cm⁻¹, which is characteristic of the P-O-C stretching vibration [37]. This result provides further convincing evidence that phosphate groups have been successfully grafted onto the CNT surfaces [38]. These FTIR results demonstrate the effective amine–phosphate functionalization of the CNT surfaces.

The surface composition and chemical state of the Au_0.5Pd_0.5/NH₂-P-CNT catalyst are analyzed by XPS. As presented in Figure 3a, the N 1s spectrum can be resolved into three distinct peaks, which are positioned at 398.7 eV, 399.8 eV, and 401.0 eV. These peaks are assigned to pyridinic N, amine-like N, and graphitic N, respectively [39,40]. This result confirms the coexistence of N-doped sites within the CNT framework and surface-grafted -NH₂ groups, which aligns well with the FTIR analysis. Figure 3b represents the P 2p spectrum of Au_0.5Pd_0.5/NH₂-P-CNTs. The two divided peaks are positioned at about 133.6 and 134.4 eV, representing P-C and P-O, respectively [41]. The P-C bonds indicate that phosphate groups are covalently anchored to the carbon framework, while the P-O bonds are primarily derived from phosphate groups grafted onto the CNT surface.

Figure 3. High-resolution XPS spectra of (a) N 1s; (b) P 2p for Au_0.5Pd_0.5/NH₂-P-CNTs; (c) Au 4f for Au_0.5Pd_0.5, Au_0.5Pd_0.5/NH₂-P-CNTs and Au_1.0/NH₂-P-CNTs; (d) Pd 3d for Au_0.5Pd_0.5, Au_0.5Pd_0.5/NH₂-P-CNTs and Pd_1.0/NH₂-P-CNTs.

Figure 3c,d show the XPS spectra of Au 4f and Pd 3d for the Au_0.5Pd_0.5/NH₂-P-CNTs. In the Au 4f spectrum, peaks located at 82.5 eV (Au 4f_7/2) and 86.2 eV (Au 4f_5/2) are observed [42], while the Pd 3d spectrum shows peaks at 335.3 eV (Pd 3d_5/2) and 340.6 eV (Pd 3d_3/2) [43], confirming that both Au and Pd primarily exist in their metallic states (Au⁰ and Pd⁰). A small contribution from Pd²⁺ species with binding energies at 335.9 and 341.2 eV is also detected, which can be attributed to slight surface oxidation during sample exposure to air [44]. Notably, compared with monometallic Au_1.0/NH₂-P-CNT and Pd_1.0/NH₂-P-CNT catalysts, the peaks of Au 4f and Pd 3d in Au_0.5Pd_0.5/NH₂-P-CNTs exhibit slight opposite shifts: the Au 4f_7/2 peak shifts negatively by approximately −0.30 eV, while the Pd 3d_5/2 peak shifts positively by +0.15 eV. This observation suggests electron transfer from Pd to Au within the AuPd alloy, leading to an electron-enriched Au surface and an electron-deficient Pd environment. This electron redistribution behavior aligns with the inherent electronegativity disparity between Au (2.4) and Pd (2.2) [45].

Furthermore, compared to the unsupported Au_0.5Pd_0.5, the Au 4f and Pd 3d peaks in the Au_0.5Pd_0.5/NH₂-P-CNT catalyst shift negatively by −0.50 eV and −0.30 eV, respectively, indicating that amine–phosphate functionalization further enhances interfacial electron redistribution between the CNT support and the AuPd alloy NPs. This interfacial electronic interaction induces an electron-enriched AuPd environment, thereby contributing to the stabilization of the electronic structure of the AuPd alloy NPs [46]. Collectively, these results reveal strong electronic coupling both within the AuPd alloy and at the NH₂-P-CNT interface, which is closely associated with the enhanced catalytic performance observed in formic acid dehydrogenation [47].

2.2. Catalytic Performance of Au_0.5Pd_0.5/NH₂-P-CNTs

The catalytic performance of the Au_0.5Pd_0.5/NH₂-P-CNT catalyst toward FA decomposition is evaluated at 303 K with sodium formate (SF) and compared with Au_0.5Pd_0.5/NH₂-CNTs and Au_0.5Pd_0.5/CNTs in Figure 4a,b. Among the tested catalysts, Au0.5Pd0.5/NH2-P-CNTs had the greatest catalytic activity of all the investigated catalysts (Figure 4a,b), producing 245 mL of gas in around 12 min and providing a TOF_initial of 5663.94 mol H₂ mol Pd⁻¹ h⁻¹. In comparison, Au_0.5Pd_0.5/NH₂-CNTs required roughly 23 min to reach complete conversion, while Au_0.5Pd_0.5/CNTs showed almost negligible activity. These variations in reaction rate and gas evolution profile indicate that performing amine–phosphate functionalization on CNTs is critical for constructing an efficient catalytic surface.

Figure 4. Gas generation from the decomposition of FA-SF (1.0 M, 5.0 mL) versus time over (a) Au_0.5Pd_0.5/NH₂-P-CNTs, Au_0.5Pd_0.5/NH₂-CNTs and Au_0.5Pd_0.5/CNTs; (b) Au_1-yPd_y/NH₂-P-CNTs at 303 K (y = 0.25, 0.5, 0.75 and 1, respectively); (c) Au_0.5Pd_0.5/NH₂-P-CNTs at different temperatures and (d) their related Arrhenius plot; (e) Au_0.5Pd_0.5/NH₂-CNTs at different temperatures and (f) their related Arrhenius plot.

To investigate the primary factors contributing to the enhanced catalytic efficiency, the impacts of CA concentration, NaH₂PO₄ addition, APTS dosage, and the FA:SF molar ratio are systematically investigated in Figure S1. As summarized in Figure S1a–d, each parameter exhibits an optimal range. The optimal synthesis conditions are determined as follows: a CA concentration of 0.3 M, a NaH₂PO₄ concentration of 0.6 M, and an APTS dosage of 0.2 mL. Under these conditions, the catalyst achieved the highest reaction rate and most efficient gas evolution. Specifically, the introduction of CA markedly shortens the reaction time, while APTS is indispensable for achieving complete conversion. P incorporation further accelerates the dehydrogenation kinetics. A balanced FA:SF ratio of 1:1 achieves the highest reaction rate, which is consistent with the known promoting effect of SF in enhancing formate adsorption and facilitating dehydrogenation pathways [48]. Moreover, the influence of the CNT amount is also evaluated. Figure S2 illustrates that the response rate increased with the dose of CNTs, peaking at 25 mg, beyond which no further catalytic enhancement is observed, indicating the existence of an optimal CNT dosage. In addition, a comparative experiment of different organic reductants has been conducted. As shown in Figure S3, CA demonstrates superior efficacy in promoting FA decomposition compared to ascorbic acid and tannic acid. Moreover, the residual component on the catalyst surface exhibits almost no effect on the microstructure and activity of Au_0.5Pd_0.5/NH₂-P-CNTs (Figure S4 and Table S2).

To clarify the influence of different reduction pathways on catalytic performance, CA and NaBH₄ are examined as reductants during catalyst preparation. As can be observed from Figure S5, when NaBH₄ is employed, rapid chemical reduction occurs. However, the resulting catalyst exhibits slower gas evolution and requires a longer time to reach complete FA conversion. By contrast, the catalyst synthesized via the CA-FA-SF self-reduction route shows faster H₂ generation. This difference can be attributed to the mild and gradual reduction environment provided by CA, which favors the formation of smaller and more uniformly dispersed NPs with more suitable surface electronic characteristics [49].

To visualize the reduction behavior, precursor solutions containing different combinations of CA, FA, and SF are examined at room temperature without CNT support (Figure S6), and distinct color evolution patterns are observed. The solution containing only CA shows almost no color change, confirming that CA does not act as a primary reductant. In contrast, the only FA-containing solution gradually turned pale black after 60 min, demonstrating the inherent reducing ability of FA. The addition of either CA or SF to FA shortens the reduction time, with SF exhibiting a particularly pronounced accelerating effect. Most notably, in the combined FA-CA-SF system, the reduction is completed within just 15 min, and the resulting mixed solution appears more uniform and stable. These observations collectively indicate that CA and SF act as synergistic modulators, not only accelerating the reduction kinetics but also improving the dispersion of the forming metal species, thereby contributing to a more controlled in situ reduction process [50].

Temperature-dependent kinetic studies were conducted to evaluate the catalytic performance of Au_0.5Pd_0.5/NH₂-P-CNTs and Au_0.5Pd_0.5/NH₂-CNTs across a range of reaction temperatures. As shown in Figure 4c, the TOF_initial increases steadily from 5663.94 to 11,537.65 mol H₂ mol Pd⁻¹ h⁻¹ as the reaction temperature rises from 303 to 333 K. The Arrhenius plot in Figure 4d demonstrates an activation energy (E_a) of 20.06 kJ mol⁻¹, indicating a significantly lowered energy barrier for the reaction. For comparison, Figure 4e presents the performance of Au_0.5Pd_0.5/NH₂-CNTs under identical conditions. Although the catalytic activity similarly increases with temperature, the TOF values at all temperatures are notably lower than those of Au_0.5Pd_0.5/NH₂-P-CNTs. The Arrhenius fitting in Figure 4f gives a higher E_a of 22.46 kJ mol⁻¹, suggesting that the reaction over NH₂-CNTs requires overcoming a larger kinetic barrier. Overall, compared with Au_0.5Pd_0.5/NH₂-CNTs, the introduction of phosphate groups in Au_0.5Pd_0.5/NH₂-P-CNTs significantly strengthens the metal–support interactions and effectively lowers the kinetic barrier for formic acid dehydrogenation, thereby delivering higher catalytic performance [51,52,53]. Benefiting from the AuPd alloy and the strengthened interactions between the NH₂-P-CNT support and AuPd NPs, the Au_0.5Pd_0.5/NH₂-P-CNT catalyst shows higher catalytic activity than most reported Pd-based catalysts [48,54,55,56,57,58,59,60,61,62,63,64,65] (Table S3).

Meanwhile, gas chromatography (GC) analysis combined with NaOH trapping experiments (Figures S7 and S8, detailed in the Supplementary Materials) confirms that the generated gases consist exclusively of H₂ and CO₂, without the signal of CO, indicating a clean and CO-free FA dehydrogenation pathway over Au_0.5Pd_0.5/NH₂-P-CNTs under ambient conditions.

The cycling stability of Au_0.5Pd_0.5/NH₂-P-CNTs is further evaluated. As shown in Figure S9, although there is a minor drop in activity during the 4th cycle, the complete FA conversion is still achieved, indicating that Au_0.5Pd_0.5/NH₂-P-CNTs show a good recyclability. The ICP-AES results (Table S1) further provide quantitative evidence supporting the structural stability of the catalyst. For Au_0.5Pd_0.5/NH₂-P-CNTs, the total metal loading changes only marginally from 0.037 mmol (Au 0.0178 mmol; Pd 0.0193 mmol) to 0.036 mmol (Au 0.0170 mmol; Pd 0.0190 mmol) after four cycles. This negligible variation in absolute metal content, together with the nearly unchanged Au:Pd atomic ratio, confirms that no meaningful metal leaching occurs during the reaction. The slight performance decline is therefore attributed not to metal loss but to the structural evolution of the NPs. TEM analysis (Figure S10) shows that the average particle size increases from approximately 1.8 nm in the fresh catalyst to about 2.5 nm after four cycles, consistent with moderate nanoparticle aggregation. Furthermore, the XRD, FTIR and XPS spectra of the spent sample show a similar result to the original sample (Figure S11–S13), indicating that the small decrease in catalytic activity was mainly due to the increase in particle size [66].

2.3. Mechanism of Catalyst Formation and FA Dehydrogenation

Based on the experimental results, a plausible reaction mechanism involving both catalyst formation and FA dehydrogenation is proposed. FA serves as an active reductant during catalyst synthesis and reduces metal precursors through the reaction Pd²⁺ + HCOOH → Pd⁰ + H* + CO₂. Meanwhile, H* species are simultaneously generated during the dehydrogenation of FA (HCOOM* + * → CO₂ + H* + *). The adsorbed hydrogen atoms (H) can subsequently recombine to form H₂ (2H* ⇌ 2* + H₂). Therefore, the use of FA as a reducing agent does not suppress gas generation but instead directly participates in the in situ formation of metallic Au⁰ and Pd⁰ species. This observation is consistent with previous reports [67], confirming that FA-assisted in situ reduction provides a mild, green, and efficient route for synthesizing highly dispersed AuPd NPs.

Following catalyst formation, the FA dehydrogenation reaction proceeds via the surface-mediated pathway illustrated in Figure 5. The amine groups on the surface of CNTs function as proton scavengers, promoting the breaking of the O-H bond in FA molecules [68]. This results in the creation of a protonated amine species [H₂NH]⁺ and a metal–formate intermediate [69]. Subsequently, both intermediates undergo dehydrogenation to release H₂ and CO₂. The electron accumulation on the Pd surface facilitates the C-H bond breakage of formate species [20,70], while the active sites of the AuPd NPs are regenerated, allowing sustained catalytic turnover. Overall, this mechanism highlights the cooperative roles of FA as both a reductant and a hydrogen source, and the synergistic electronic modulation between Au, Pd, and the NH₂-P-CNT supports in achieving highly efficient CO-free hydrogen generation.

Figure 5. Schematic illustration for possible reaction mechanism for the dehydrogenation of FA over Au_0.5Pd_0.5/NH₂-P-CNTs.

3. Materials and Methods

3.1. Chemicals

Formic acid (FA, HCOOH, Sigma Aldrich Chemistry Co., Ltd., Burlington, MA, USA, ≥96%), citric acid (CA, C₆H₈O₇, Aladdin Chemistry Co., Ltd., Riverside, CA, USA, ≥99.5%), carbon nanotubes (CNTs, Time Nano, Chengdu, China, OD 10–20 nm, Length 0.5–2 µm, 98%), palladium chloride (PdCl₂, Aladdin Chemistry Co., Ltd., Pd 59–60%), tetrachloroauric acid (HAuCl₄·4H₂O, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, Au ≥ 47.8%), sodium dihydrogen phosphate (NaH₂PO4, Beijing Chemical Works, Beijing, China, 99.8%), sodium chloride (NaCl, Aladdin Chemistry Co., Ltd., 99.5%), (3-Aminopropyl) triethoxysilane (APTS, C₉H₂₃NO₃Si, Aladdin Chemistry Co., Ltd., 99%), tannic acid (C₇₆H₅₂O₄₆, Aladdin Chemistry Co., Ltd., ≥95%), ascorbic acid (C₆H₈O₆, Aladdin Chemistry Co., Ltd., ≥99%), sodium formate (SF, HCOONa, Macklin Biochemical Co., Ltd., Shanghai, China, 99.99%), sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd., 96%), and deionized water with a specific resistance of 18.2 MΩ cm, produced through reverse osmosis, ion-exchange, and filtration.

3.2. Preparation of NH₂-P-CNTs

CNTs (25 mg) were first dispersed in 7.0 mL of distilled water using ultrasonication for 20 min to get a homogenous solution. Subsequently, APTS (0.2 mL), NaH₂PO₄ (1 mL, 0.3 M) and CA (0.3 mmol) were concurrently added into the dispersion, followed by an additional 5 min of ultrasonication to facilitate the formation of NH₂-P-CNTs, which was called solution A.

3.3. Preparation of Au_0.5Pd_0.5/NH₂-P-CNTs

Na₂PdCl₄ (0.02 M) aqueous solution was prepared previously by dissolving NaCl (0.8 mmol) and PdCl₂ (0.4 mmol) into 20.0 mL H₂O with magnetic stirring for 2 h. Typically, for preparation of Au_0.5Pd_0.5/NH₂-P-CNTs, aqueous solutions of HAuCl₄ (1 mL, 0.02 M) and Na₂PdCl₄ (1 mL, 0.02 M) were added sequentially to solution A while swirling magnetically for 5 min to achieve uniform mixing of the metal precursors. The reduction was conducted in a two-necked flask (50 mL) situated in a water bath at 303 K. A reducing solution comprising FA (5 mL, 1 M) and SF (5 mL, 1 M) was incrementally added to the mixture while maintaining continuous magnetic stirring, therefore enabling the in situ reduction of the metal precursors and the synthesis of the Au_0.5Pd_0.5/NH₂-P-CNT catalyst.

Au_0.5Pd_0.5/NH₂-P-CNTs were synthesized with varying dosages of CNTs, APTS, NaH₂PO₄, CA and SF by adjusting the quantities of CNTs (0 mg, 25 mg, 50 mg), CA (0 mmol, 0.15 mmol, 0.3 mmol, 0.45 mmol), 1 mL NaH₂PO₄ with different concentrations (0 M, 0.3 M, 0.6 M, 0.9 M), APTS (0 mL, 0.2 mL, 0.4 mL) and the FA:SF ratio (1:1, 3:1, 1:3, wherein n_FA is set at 5 mmol). For comparison, Au_0.5Pd_0.5/NH₂-CNTs and Au_0.5Pd_0.5/CNTs were synthesized using a similar technique with the incorporation of SF.

3.4. Characterizations

X-ray diffraction (XRD) measurements were conducted using a Bruker D8 ADVANCE X-ray generator (Bruker, Billerica, MA, USA) Cu Kα radiation. Transmission Electron Microscope (TEM) images were acquired using a FEI Tecnai G2 F30 microscope (FEI, Hillsboro, OR, USA) with accelerating voltage of 200 kV. TEM samples were generated by depositing two drops of dilute solutions onto copper-coated carbon grids and then drying them in an argon environment. The Fourier transform infrared (FTIR) spectra were obtained using a Bruker V80 spectrometer (Bruker, Billerica, MA, USA) using the standard KBr pellet method. Typically, approximately 20 mg of finely pulverized catalyst powder was uniformly blended with 2000 mg of spectroscopic-grade KBr in an agate mortar and then compressed into translucent pellets at a pressure of 10 MPa. Spectra were obtained in transmission mode over the wavenumber range of 400 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹. High-resolution X-ray photoelectron spectroscopy (XPS) spectra were acquired using an ESCALAB 250Xi (Vacuum Generators) spectrometer (Thermo Fisher, Waltham, MA, USA) employing Al Kα X-ray radiation as the excitation source. The binding energy scale of the spectra was calibrated by assigning the C 1s core level of adventitious carbon a value of 284.6 eV. Inductively coupled plasma–atomic emission spectroscopy (ICP-AES) measurement was conducted using a Thermo Jarrell Ash (TJA) Atomscan Advantage instrument (Thermo Jarrell Ash, Franklin, MA, USA). Comprehensive analyses for CO₂, H₂, and CO were conducted using the GC-7900 (Tianmei, Shanghai, China) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID)–Methanator (detection limit: ~10 ppm).

3.5. Catalytic Performance Tests

The catalytic dehydrogenation of FA was conducted immediately after the in situ reduction described in Section 3.3, without centrifuging or transferring the catalyst. Specifically, once the reduction reaction is finished, the decomposition of FA (1 M, 5 mL) is started spontaneously under magnetic stirring (600 r/min) in the two-necked flask (50 mL), which is catalyzed by the as-prepared Au_0.5Pd_0.5/NH₂-P-CNT (n_AuPd = 0.05 mmol) composite. The evolution of gas is monitored by using the gas burette, and the reaction is carried out at 303 K under ambient atmosphere.

After the H₂ generation is completed, the as-used Au_0.5Pd_0.5/NH₂-P-CNT catalyst is kept in the reaction solution, another equivalent of FA (1 M, 5 mL) is added to the reaction flask, and the released gas is monitored by the gas burette. The recycle experiments are repeated at 303 K under ambient atmosphere. The catalytic performance of other catalysts for the decomposition of FA are also applied as in the above method. The molar number of AuPd for all the catalytic reactions is kept as a constant of 0.05 (n_AuPd = 0.05 mmol). All experiments are repeated at least twice, and the results are repeatable.

3.6. Catalytic Performance Analysis

The catalytic performance is evaluated based on the conversion of formic acid and the corresponding kinetic parameters, which are calculated using the following equations.

Calculation methods:

$${\mathrm{x}}_{\mathrm{a}}={\displaystyle \frac{{\mathrm{P}}_{\mathrm{atm}}{{\mathrm{V}}^{\prime }}_{\mathrm{gas}}/\mathrm{RT}}{2{\mathrm{n}}_{\mathrm{FA}}}}$$

(1)

where x_a is the final conversion, P_atm is the atmospheric pressure, V′_gas is the total generated volume of gas (CO₂ + H₂), R is the universal gas constant, T is the room temperature, and n_FA is the molar number of FA.

$${\mathrm{TOF}}_{\mathrm{initial}}={\displaystyle \frac{{\mathrm{P}}_{\mathrm{atm}}{{\mathrm{V}}^{\prime }}_{\mathrm{gas}}/\mathrm{RT}}{2{\mathrm{n}}_{\mathrm{Pd}}\mathrm{t}}}$$

(2)

where TOF_initial is the initial turnover frequency when the conversion x_a reaches 20%, n_Pd refers to the content of active metal Pd in the catalyst, and t is the reaction time when x_a reaches 20%.

$$\ln \mathrm{TOF}=\mathrm{lnA}-E_a/\mathrm{RT}$$

(3)

where E_a is the activation energy, and A is the pre-exponential factor, which is a constant.

4. Conclusions

In summary, NH₂-P-CNTs supporting ultrafine AuPd NPs with high dispersion (approximately 1.8 nm) have been effectively synthesized using a CA-assisted in situ reduction method. Notably, the as-prepared Au_0.5Pd_0.5/NH₂-P-CNTs demonstrate exceptional catalytic activity, with a TOF_initial of 5663.94 mol H₂ mol Pd⁻¹ h⁻¹ and 100% H₂ selectivity toward catalytic formate dehydrogenation (FA-SF system) under mild conditions. The superior activity arises from the synergistic effects provided by the functionalized support, where the -NH₂ and phosphate groups collectively regulate nanoparticle nucleation and growth to ensure uniform dispersion, simultaneously strengthening the electronic interaction between the AuPd alloy and the support and optimizing the electron distribution of the AuPd surface. This work develops a facile and efficient synthesis approach for high-performance Pd-based catalysts and offers valuable insight into the design of multifunctional catalyst supports for advanced H₂ generation systems.