Formic acid dehydrogenation reaction on high-performance Pd_xAu_1−x alloy nanoparticles prepared by the eco-friendly slow synthesis methodology

Abstract

Dehydrogenation of formic acid (FA) is considered to be an effective solution for efficient storage and transport of hydrogen. For decades, highly effective catalysts for this purpose have been widely investigated, but numerous challenges remain. Herein, the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloys over the whole composition range were successfully prepared and used to catalyze FA hydrogen production efficiently near room temperature. Small PdAu nanoparticles (5–10 nm) were well-dispersed and supported on the activated carbon to form PdAu solid solution alloys via the eco-friendly slow synthesis methodology. The physicochemical properties of the PdAu alloys were comprehensively studied by utilizing various measurement methods, such as X-ray diffraction (XRD), N₂ adsorption–desorption, high angle annular dark field-scanning transmission electron microscope (HAADF-STEM), X-ray photoelectrons spectroscopy (XPS). Notably, owing to the strong metal-support interaction (SMSI) and electron transfer between active metal Au and Pd, the Pd_0.5Au_0.5 obtained exhibits a turnover frequency (TOF) value of up to 1648 h⁻¹ (313 K, n_Pd+Au/n_FA = 0.01, n_HCOOH/n_HCOONa = 1:3) with a high activity, selectivity, and reusability in the FA dehydrogenation.

Graphical abstract

Keywords

FA dehydrogenation / face-centred cubic structures / PdAu solid solution alloy nanoparticles / slow synthesis methodology / SMSI effect

1 Introduction

Since the 21st century, the world faces two major challenges, energy crisis and environment deterioration. Finding and exploring safe, clean, and renewable energies has become a hot research topic. As an ideal carbon-zero fuel, hydrogen is viewed as the most promising alternatives for changing the traditional fossil energy system because of a variety of advantages (high-efficiency, recyclability, cleanness, etc.) [1,2]. Particularly, H₂ has been widely used in proton exchange membrane fuel cells because its product is only water [3]. Chemical hydrogen storage materials are widely preferred due to their high weight and volumetric hydrogen density compared to classical pressurization and cryogenic liquefaction technologies [4,5]. For decades, the chemical H₂ storage materials: formic acid (FA, HCOOH) [6,7], methanol (CH₃OH) [8,9], sodium borohydride (NaBH₄) [10,11], hydrazine (N₂H₄) [12,13], hydrous hydrazine (N₂H₄·H₂O) [14–16], and hydrazine borane (NH₃BH₃) [17,18] have been comprehensively investigated and developed for transportation and generation of hydrogen [19,20].

As one of the common liquid biomass products, formic acid (FA) possesses a relatively high hydrogen storage content, density, and volume. Therefore, it has received widespread attention in the application of portable hydrogen storage devices because of its low price, chemical stability, and easy storage and transportation [7,21,22]. Moreover, it is easily charged and discharged through sharing current infrastructures of liquid fuels, which greatly promotes its production and application. As the reverse reaction of CO₂ hydrogenation [23,24], it can be decomposed into H₂ and CO₂ by dehydrogenation (HCOOH → CO₂ + H₂, ΔG₂₉₈ = −48.4 kJ/mol) or by dehydration (HCOOH → CO + H₂O, ΔG₂₉₈ = −28.5 kJ/mol), but the last is undesirable because CO can poison the catalysts in fuel cells [25,26]. Therefore, rapid development of highly active catalysts accompanied with high selectivity and efficient H₂ generation is of paramount for FA-based hydrogen storage.

Many works have been made to investigate the homogeneous and heterogeneous catalysts in FA hydrogenation. For homogeneous catalysts, Schneider and its coworkers [27] reported that (^iPrPN^HP)Fe(CO)H-(COO) and Lewis acid cocatalyst obtain an particularly high turnover frequency (TOF) value of 196728 h⁻¹ at 353 K. However, homogeneous catalysts still face many problems such as special additives, separation, and recovery, which further limit the practical application of catalysts. To date, a large number of researchers have explored the heterogeneous catalysts for FA dehydrogenation. As the most competitive catalyst for FA hydrogenation, Pd-based catalysts have high activities and median adsorption energy values compared to other metal-based catalysts [4,28–30]. But Pd-based single metal catalyst exhibit an acceptable H₂ generation rate at a relatively high temperature (333–433 K) accompany with the addition of sodium and potassium salts. Additionally, the production of CO byproducts could cause the inactivation of mono-metal Pd-based catalyst [29,31–37]. Therefore, many Pd-based binary and ternary metal (PdAu [38–44], PdAg [45–47], PdCu [5], PdCo [48], PdAuCo [49], PdAgCo [50] and PdAu-MnO_x [51,52]) catalysts exhibit a higher reactivity as well as reusability compared with Pd mono-metal catalysts. Although Au alone is inactive for FA decomposition, PdAu [53,54] bimetal catalysts could greatly enhance the catalytic performance due to their synergistic effect. The charge transfer between Au and Pd not only plays an important role in regulating the chemical environment nearby the active center Pd, but also inhibits the generation of CO and prevents catalysts poisoning [55,56]. Moreover, many significant works are employed to increase the structure-reactivity relationship of PdAu bimetal catalysts including alloy structures [38,57], core shell structures [58], and contracture strong metal-support interaction (SMSI). For the abovementioned several structures, the uniformly mixing PdAu bimetals with smaller particle sizes poses a huge challenge [59,60], directly affecting the FA dehydrogenation activity.

Recently, new ultra-small solid-solution alloys have been synthesized, which aroused lots of attention due to their many innovative properties [61,62]. The greatest advantage of solid solution alloys is the control of their electronic structure at the atomic level in the bulk state and the influence of most physicochemical properties via altering the metals compositions in the solution [63]. By completely mixing of metals A and B, the AB alloy possesses not only the properties of A or B, but also a unique property because of the synergistic capacity of A and B. Although many immiscible alloys including PdAu [64], CuRu [65], RhNi [66], AuRu [63], etc. have been prepared, the synthesis of solid solution alloys nanoparticles (NPs) with large differences in reduction potential remains challenging. For noble metal alloy NPs, the most critical control step is the co-reduction rate of double metals, otherwise core shell or isolated single metal NPs can be obtained [67,68]. Hence, it is challenging to co-reduce bimetal ions with a large gap in reduction potential energy. The reduction potentials are shown in Table S1 in Electronic Supplementary Material. Difference in the reduction rates including Au and Pd that possess the large gap in noble metals (Pd²⁺ + 2e⁻→ Pd, E⁰ = 0.951 V vs. Au³⁺ + 3e⁻→ Au, E⁰ = 1.498 V) [63,69].

In this work, the slow synthesis methodology was carefully adopted to synthesize PdAu solid solution alloy so that all components are distributed randomly and homogeneously. Based on this idea, activated carbon (AC) with a developed porous structure was used as basic support to deposit ultrafine Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) solid solution alloy NPs. PdAu alloy NPs were successfully obtained and catalyzed FA dehydrogenation. By regulating various composition ratios, the optimal Pd_0.5Au_0.5 catalyst exhibits the highest activity (TOF = 1648 h⁻¹) for HCOOH/HCOONa (formic acid/formate (FA/SF)) dehydrogenation at 313 K. Furthermore, reaction temperature, FA/SF ratios, metal loading quantity and stability of catalysts were also tested.

2 Experiment

2.1 Chemicals

The Na₂PdCl₄ (99% purity), HAuCl₄·3H₂O (99% purity), NaBH₄ (98% purity), HCOOH (88% purity), and HCOONa (99.5% purity) were obtained from Aladdin Chemistry Co., Ltd., while the AC was obtained from Macklin Chemicals Co., Ltd. All the chemicals were not further purified.

2.2 Catalyst characterizations

The crystal structures of PdAu alloy NPs were detected via XRD using a SHIMADZU XRD-6000 with Cu Kα radiation (λ = 0.15406 nm). The 2θ data from 30° to 90° were 0.02°-intervals. The physical properties of catalysts including S_BET (m²/g), V_p (cm³/g), and D_p (nm) were determined using the nitrogen-sorption method on a constant volume adsorption equipment (Micrometrics ASAP 2020 Plus HD88, USA). The pore volume was analyzed by the Barrett-Joyner-Halenda (BJH). The microstructure and morphology were tested by the HAADF-STEM and EDX-mapping on the Thermo Scientific Talos-F200X. Thermo Scientific K-Alpha with monochromatic Al K-Alpha (1486.7 eV) was carried to record X-ray photoelectron spectroscopy. The atomic ratios of Pd and Au was recorded using an inductively coupled plasma mass spectroscopy (ICP-MS) on a NexION^TM 350D instrument (PerkinElmer, USA).

2.3 Catalyst preparation

As shown in Fig.1, the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy NPs with an appropriate composition and a metal loading of 5% were prepared. Taking Pd_0.5Au_0.5 synthesis as an example, Na₂PdCl₄ (29 mg, 0.1 mmol) and HAuCl₄·3H₂O (0.1 mmol) were added in 60 mL H₂O and stirred for 30 min as metal precursor solutions. NaBH₄ (152 mg, 4 mmol) was added in 60 mL ethyleneglycol (EG). AC (580 mg) was dissolved in 100 mL H₂O. After that, the metal precursor solution and EG solution were added dropwise to the AC mixed solution at a rate of 1.2 mL/min [70]. After the dropping process, the mixed solution was stirred for 30 min. The sediment obtained was collected by centrifugation and washed over 5 times with H₂O. The sediment collected by centrifugation was washed with H₂O for 5 times. Finally, the product was dried at 60 °C under vacuum. The Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloy NPs and x% Pd_0.5Au_0.5 (x = 1, 5, 10) with different metal loadings were also obtained by altering the atomic ratios of Pd²⁺ and Au²⁺.

2.4 Catalyst evaluation

The synthesized catalyst was dispersed in deionized water in a round-bottomed flask and placed in a water bath at the preset temperature. Flask was connected to a reflux and a gas dropped where the gas produced from the FA dehydrogenation was collected. Subsequently, FA/SF in a certain molar ratio (1:3, 2:3, 1:8, 1:1, 3:2, and 3:1) was injected to the flask and started to react with vigorous stirring. Moreover, the activation energy (E_a) of reaction was calculated by conducting the catalytic reaction over a temperature range of 293–313 K. From the slope of each line, the rate constants K were obtained at preset temperatures. Finally, the TOF values were obtained using Eq. (1).

(1)\mathrm{T}\mathrm{O}\mathrm{F}=\frac{P_{\mathrm{a}\mathrm{t}\mathrm{m}}{V}_{\mathrm{g}\mathrm{a}\mathrm{s}}}{RT}/(n_{\mathrm{P}\mathrm{d}\mathrm{A}\mathrm{u}}\cdot t),

where P_atm is the atmospheric pressure (101325 Pa), V_gas is the gas volume produced from the FA dehydrogenation reaction which has reached 20%, R is the universal gas constant, T is preset temperature (313 K), n_PdAu is the total mole amount of PdAu atoms, and t is FA decomposition time (min). Based on ICP-MS analyses, the total mole amount of PdAu atoms on the AC are shown in Table S2.

3 Results and discussion

3.1 XRD

To investigate the structures of the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy NPs, the XRD patterns were obtained as shown in Fig.2. The lattice constants and crystal sizes of all the PdAu NPs were summarized in Tab.1. For Au NPs, apparent diffraction peaks appear at 38.3°, 44.3°, 64.7°, and 77.7° are assigned to the (111), (200), (220) and (331) planes of the fcc-Au (JCPDS No. 65-8601), showing the structural formation of Au NPs. Compared to the Au NPs, the diffraction peaks of Pd NPs are weaker, exhibiting that the particle size of Pd NPs should be smaller. With the increase in Au mixing content, the diffraction peaks of Pd_xAu_1−x alloy NPs shift to lower Bragg angles (Fig.2), but remain between the fcc-Au and fcc-Pd (JCPDS No. 05-0681) phases. According to the nominal composition, the Pd and Au atoms are randomly attributed to each lattice site. The lattice constant and crystal size of PdAu alloys decrease significantly with the mixing of palladium because of the smaller ionic radii of Pd compared to those of Au (Tab.1). These obvious characterizations, including lattice constant, diffraction peaks shift, and mean crystal size, strongly suggest the formations of the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloys in the whole composition range.

3.2 N₂ adsorption/desorption measurements

N₂ sorption isotherms and corresponding pore size distributions plot for PdAu alloys are shown in Fig.3. All PdAu alloy NPs exhibit typical IV isotherms with H1 hysteresis loops, which is characteristic of mesoporous structure. The values of S_BET, V_p, and D_p of all the PdAu alloy NPs are listed in Tab.1. The AC support used provides a large specific surface area for the anchorage of the PdAu alloy. The surface area (1166 m²/g) and total pore volume (0.622 cm³/g) of Pd_0.5Au_0.5 far outperform those of other PdAu alloy NPs. When the Pd/Au ions ratio is unequal, the total pore volume and specific surface area decrease significantly. Such a reduction may be related to the aggregation of larger PdAu alloy NPs blocking pores and/or causing some structural rearrangements [71]. In general, the large surface and high porosity of PdAu alloys facilitate the active point between catalyst and FA, thus promoting FA dehydrogenation.

3.3 High angle annular dark field-scanning transmission electron microscope (HAADF-STEM)

The synthesized PdAu alloy NPs were characterized by HAADF-STEM analysis. The mean diameter of the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy NPs were 10.9, 8.1, 7.3, 6.7, 6.5, 5.9, and 6.3 nm, respectively (Fig.4 and S1). These particle sizes were measured by averaging at least 100 nanoparticles. The mean particle sizes of Au NPs are relatively larger than that of other Pd_xAu_1−x (x = 0.2, 0.4, 0.5, 0.6, 0.8, 1) NPs. This may be related to the fast nucleating nature of Au [61]. From the ICP-MS analysis, the atomic ratios of Pd and Au in alloys, shown in Table S2, were in consistent with the original ratios.

To clarify the Pd and Au ions of the mixing state in the Pd_xAu_1−x alloy NPs, STEM EDX mappings of Pd-L and Au-L were performed. In Fig.4 and S2, the elemental maps of Pd and Au show that the two elements are randomly and homogeneously distributed in the whole area of NPs. The EDX line-scan profiles are shown in Fig.5 and S3, which indicate that the metal composition is transferred from Au-rich to Pd-rich. The Pd and Au ratios based on the EDX area-scan data are also listed in Table S2, which agree well with the theoretical Pd/Au atomic ratios in preparation. The EDX analysis suggests that the Pd/Au atomic ratios in the Pd_0.5Au_0.5 NPs is 49.3:50.7.

3.4 XPS measurements

As shown in Fig.6, the chemical states and surface electronic properties of the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy NPs were investigated via X-ray photoelectron spectra. The remarkable XPS peaks of C 1s for PdAu alloy NPs at 284.8 and 286.4 eV are shown in Fig. S4 and assigned to the C–C/C=C and C–O, respectively [4,72]. Meanwhile, Pd_0.2Au_0.8 (Pd⁰ 3d_3/2 at 341.3 eV and Pd⁰ 3d_5/2 at 336.0 eV) possesses a higher BEs of Pd⁰ 3d_3/2 (340.9 eV) and Pd⁰ 3d_5/2 (335.9 eV) for Pd_xAu_1–x (x = 0.4, 0.5, 0.6, 0.8, 1) alloy NPs. The deviation of BEs in the direction of high energy indicate that there exists an SMSI effect between PdAu and AC due to the introduction of Au [73]. In addition, the other two peaks at 343.3 and 337.5/338.2 eV is expected to Pd²⁺ 3d_3/2 and Pd²⁺ 3d_5/2. This may be caused by electronic interactions between the Pd d-orbitals and AC [74,75]. Au⁰ 4f binding energies for the Pd_xAu_1−x (x = 0.2, 0.4, 0.5, 0.6, 0.8) shift lower values compared to those of Au NPs (87.9 and 84.3 eV). The above XPS spectra of Pd 3d and electronegativity difference for Pd (2.2) and Au (2.4) also demonstrate that Pd loses a few electrons to Au in PdAu alloys. For the Pd_0.5Au_0.5, the BEs for Au⁰ 4f are located at 87.8 and 84.2 eV. The XPS results indicate that there exists an SMSI effect between PdAu alloy and AC. It is worth noting that Pd 3d and Au 4f binding energy shift positively as the Au content increasing. The BEs of Pd/Au ratio is related to the interactions between Pd and Au. Due to the formation of non-homogeneous junctions in the electronic structure of Pd and the SMSI effect [76], the adsorption of formate can be enhanced from electron-rich Pd. These phenomena remarkably enhance the catalytic FA dehydrogenation [38,72].

3.5 FA dehydrogenation in FA/SF solution

Catalytic performances over the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloys obtained toward the FA dehydrogenation with SF were measured in a burette system. The AC support and Au NPs showed a poor hydrogenation production (Fig.7(a)). As a synergistic promoter of metals, the contribution of Au to the decomposition of FA is assigned to the SMSI effect at the heterogeneous interfaces between the PdAu alloy and the AC support. As shown in Fig.7(a) and S5, the Pd_0.5Au_0.5 has the highest catalytic activity (TOF = 1648 h⁻¹) of all the Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) NPs. The excellent catalytic performance of Pd_0.5Au_0.5 due to the ultrafine and polydispersity PdAu NPs on the surface of AC and electrons transfer efficiently from Pd to Au over PdAu alloy NPs. Meanwhile, the large specific surface area of AC also improves the dispersion of active metal sites, allowing for a greater exposure to the FA dehydrogenation. Moreover, Au doping allows the Pd to be the electron-state, thus enhancing the catalytic performances for FA decomposition efficiently. In FA decomposition, SF is used as an accelerator to increase the reaction activity of hydrogen evolution reaction [19,77]. The generated gas compositions from the aqueous FA/SF solution over the Pd_0.5Au_0.5 are shown in Fig. S6. The optimal ratio of FA/SF for Pd_0.5Au_0.5 is 1:3 at 313 K and it showed a conversion of around 100% for FA decomposition near room temperature (Fig.7(c) and S6). Additionally, optimum metal loading of PdAu on the AC is 5 wt.% (mass fraction) in FA dehydrogenation (Fig.7(d)).

The Arrhenius plot and corresponding kinetic parameters over Pd_0.5Au_0.5 and Pd NPs catalysts were shown in Fig.7(b) and S7. Through the V_{{\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2} rate versus time, the Arrhenius plot shows a slope of −6.8144, indicating that FA decomposition is nearly a first order reaction relative to the reaction temperature. In addition, the apparent activation energy (E_a) for Pd NPs and Pd_0.5Au_0.5 alloy NPs of FA/SF is obtained from 293 to 313 K. The above analysis indicates that the rate of H₂ production increases with rising reaction temperature. According to the Arrhenius equation, the E_a of Pd_0.5Au_0.5 is calculated at 56.65 kJ/mol lower than that of Pd NPs at 64.66 kJ/mol. As displayed in Fig.8, the superior catalytic performance of the Pd_0.5Au_0.5 alloys can be ascribed to the well-distribution of bimetal active sites and synergy effect between Pd and Au that reduce activation energy obviously and boost FA decomposition.

3.6 Recyclability of optimized Pd_0.5Au_0.5 catalyst for FA dehydrogenation

As shown in Fig.9, the stability and recyclability of Pd_0.5Au_0.5 for FA decomposition at 313 K were also measured. Although the gas produced slightly decreased, the complete conversion of the Pd_0.5Au_0.5 catalyst was still maintained after 5 cycles, thus displaying a high stability of Pd_0.5Au_0.5 catalyst. The STEM EDX mappings of Pd and Au were conducted to show the catalysts recovered after 5 cycles (Fig.10). Before and after the catalytic decomposition of FA, there exists no significant change in size and morphology of Pd_0.5Au_0.5 alloy NPs, indicating the excellent structure stability of the catalyst.

4 Conclusions

In this study, Pd_xAu_1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy NPs over the whole composition range with various metal loading (x = 1, 5, 10 wt.%) were synthesized via a facile slow synthesis methodology. As support, the AC with large specific surface areas enlarge the polydispersity of PdAu alloys. In all PdAu alloy NPs, the as-synthesized Pd_0.5Au_0.5 showed the best activity, stability and recyclability for H₂ production in FA/SF mixed solution at 313 K, affording an excellent initial TOF value of up to 1648 h⁻¹ with a lower E_a (56.65 kJ/mol). The optimal activity of Pd_0.5Au_0.5 was mainly due to the ultra-small alloy NPs, particle size polydispersity, and uniform mixing states of Pd and Au. The catalyst was also confirmed to catalyze FA dehydrogenation near room temperature, which could encourage the wide utilization of FA as one of the prospective recyclable liquidus H₂ storage materials.

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