Introduction

Hydrogen (H2), a sustainable and environmentally attractive energy carrier, is now widely regarded to has the potential to play a major role in high-efficiency power generation systems, including polymer electrolyte membrane (PEM) fuel cell, in the future1,2. However, secure generation and storage of H2 remain as the insufficiently solved problems toward the H2 energy based economy3. Methods for H2 storage, such as high-pressure gas containers, cryogenic liquid/gas containers, solid metal hydrides, carbon nanotubes and metal–organic frameworks, are suffering from more or less obstacles in safety (high pressure or low temperature are needed) and efficiency (low volumetric and gravimetric densities of H2)1,4,5,6. Consequently, H2 stored in some stable chemical compounds is then desired7,8,9.

Recently, formic acid (FA, HCO2H), a major product of biomass process, has attracted a great of research interests in H2 storage due to its high energy density, excellent stability and non-toxicity at room temperature7,10,11,12. FA can be catalytically decomposed to H2 and carbon dioxide (CO2) via a dehydrogenation reaction13:

However, carbon monoxide (CO), which is a fatal poison to catalysts of fuel cell14,15, can also be generated through another undesirable dehydration pathway13:

The reaction pathway depends on the catalyst and the reaction condition including temperature and pH value of the reaction system7.

It is reported that FA can be selectively dehydrogenated without CO generation by homogeneous catalysis at near-ambient temperature4,16,17,18,19. Nevertheless, homogeneous catalysts unavoidably lead to the difficulties in separating, controlling and recycling the catalysts in the reaction systems20. Therefore, heterogeneous catalysis nowadays attracts tremendous research interests21,22,23,24,25,26,27,28,29,30. For H2 generation from FA system, Pd nanoparticles (NPs) are found to be more efficient than other monometallic NPs23,27, although their catalytic activities and selectivities are still very poor even at elevated temperature up to 365 K23,26. The current methods for improving the dehydrogenation performance of FA are mainly focused on application of bi/tri-metallic Pd-based composites, alloys or core-shell nanostructures as the catalysts, in which the incorporation of some other elements, such as Au, Ag, Dy, K, etc., can modify the catalytic surface of Pd NPs and then result in the higher activities and selectivities23,26,27,28,31,32. However, most of these catalysts have to work at high temperature23,26,28,31,32 and the kinetic property of FA dehydrogenation still needs to be promoted. Moreover, the structural complexities of these polymetallic NPs make their preparations much more complicated and subeconomic than those of monometallic NPs. For instance, multi-step synthetic process, long reaction time and strict reaction conditions including high temperature, inert gas protection and post-processing are generally required during syntheses of the polymetallic Pd-based catalysts23,26,27,28,29,30,31,32, which greatly hinders their large-scale practical applications.

During FA dehydrogenation, formate, an intermediate product in FA dehydrogenation24, is usually added to promote the activity of catalyst22,23,26,28,29. It is reported that sodium formate (SF, HCO2Na) can liberate H2 through a hydrolysis reaction with some homogenous catalysts33:

Therefore, if SF could also generate H2 in a heterogamous catalysis system of FA, the efficiency of H2 generation from FA/SF system can be further improved.

In light of the above reason, exploring a facile and low-cost strategy to endow the solid catalyst in a simple structure, such as monometallic catalyst, with excellent activity, selectivity and stability for H2 generation from both FA and SF in aqueous solution at room temperature without CO contamination is of great interest to meet the demand of on-board fuel cell applications.

Herein, we report the highly efficient H2 generation from FA/SF aqueous solution at 298 K catalyzed by in situ prepared monometallic Pd/C in the presence of citric acid. The key to obtain the excellent catalytic property of Pd/C is the modification of citric acid during the formation and growth of the Pd NPs on carbon, which results in the ultrafine and well dispersed Pd NPs with highly active sites on carbon. Moreover, SF in the reaction system plays the important roles as not only the co-catalyst but also the reducing agent and H2 source. This greatly simplifies the catalyst preparation, efficiently increases the activity of catalyst and successfully improves the efficiency of the H2 generation from FA/SF system.

Results

H2 generation from FA/SF over Pd/C synthesized with citric acid

The strategy for in situ preparation of Pd/C with citric acid and H2 generation from FA/SF is shown in Fig. 1. Typically, 5.0 mL of aqueous solution containing FA (1.06 M), SF (0.84 M) and citric acid (0.03 M) is added into 10.0 mL of Na2PdCl4 (4.70×10−3 M) aqueous solution with 95.0 mg of Vulcan XC-72 carbon at 298 K in air. Pd2+ cations can be easily reduced to Pd NPs by SF (Supplementary Fig. S1)34 and well dispersed on carbon within 2 min. The rapid in situ formed Pd/C, noticeably, in the presence of citric acid, then serves as an efficient catalyst for H2 generation from FA/SF at 298 K.

Figure 1
figure 1

Experimental design.

Schematic illustration for H2 generation from FA/SF catalyzed by in situ prepared Pd/C with citric acid at 298 K.

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After the in situ formation of Pd/C with citric acid, continuous gas evolution is observed (Fig. 2a) and only mixture of H2 and CO2 but no CO has been detected by mass spectrometry (MS) and gas chromatography (GC) analyses (Supplementary Fig. S2–4). This means that the present Pd/C with citric acid has a good catalytic selectivity for FA dehydrogenation, which is very important for fuel cell applications35. More interestingly, 322 mL of gas, a large volume far exceeding the theoretical value from FA dehydrogenation (259 mL), can be generated within 160 min at 298 K. It is noteworthy that pure citric acid aqueous solution with catalyst of Pd/C generates no H2 gas (Supplementary Fig. S6). Thereby, the excess of gas is contributed to H2 generation from hydrolysis of SF in this system. Thus, SF, which is usually used as a catalyst promoter (with negligible H2 generation) for FA dehydrogenation, can also act as the H2 source in the present system. Namely, the in situ synthesized heterogeneous Pd/C with citric acid has a remarkably high activity to both FA dehydrogenation and hydrolysis of SF for H2 generation.

Figure 2
figure 2

Catalytic activities of different Pd/C.

Gas generation by decomposition of FA/SF (1.06 M/0.84 M, 5 mL) catalyzed by Pd/C synthesized (nPd/n(FA+SF) = 0.005) (a) with citric acid added initially, (b) without citric acid, (c) with citric acid added after 40 min of the reaction and (d) with citric acid after washing at 298 K. Inset: rates of gas generation from FA/SF (1.06 M/0.84 M, 5 mL) over Pd/C (nPd/n(FA+SF) = 0.005) synthesized in different concentrations of citric acid at 298 K.

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It is reported that adsorption of water on the surface of catalyst might lead to the deactivation of catalyst in FA dehydrogenation at 333 K13. In this sense, to check if such deactivation from water exists in the present Pd/C catalyst, the solvent (water) is substituted by tetrahydrofuran (THF). As a result, no gas has been generated even after reaction for 300 min (Supplementary Fig. S5). Thus, water in the present system does not inhibit but plays an important role in the highly efficient decomposition of FA/SF. On the other hand, it has been found that, at the elevated reaction temperature, catalysts for FA and formate decomposition always lose their initially good activity within a short period23,36. This may be explained by the facts that, at the elevated temperature, CO is easier to be generated to poison the nanocatalyst4,23 and the stability of the nanocatalyst in its size, distribution and structure is decreased as well37. Therefore, the present reaction temperature (298 K) may be one of the important reason accounting for the low-deactivation of the present Pd/C catalyst during 160 min (Fig. 2a).

Considering the gas generated from FA/SF, the theoretical molar ratio of H2:CO2 (Supplementary Eq. S3) should be 1.80∶1. However, the measured value by GC is 1.55∶1 (Supplementary Fig. S4). This may be resulted from the small outcoming of CO2 from NaHCO3, the product of reaction (3), in the present acid solution (with the initial pH value of 3.65)33. Based on the volume of H2, the total conversion (Supplementary Eq. S1) can reach 85% within 160 min. Furthermore, the turnover frequency (TOF) is calculated (Supplementary Eq. S2) to be 64 mol H2 mol−1 catalyst h−1 at 298 K. This excellent conversion and TOF values are the highest one ever reported in FA/SF system at room temperature and even comparable to those at the elevated temperature (Supplementary Table S1)23,26,27,28. It is noteworthy that, the molar ratio of FA to SF has an obvious effect to the performance of the resulted Pd/C catalyst. Whether increase or decrease the mole percent of FA from the present value of 56% to others (0%, 25%, 75% and 100%), the activity of Pd/C is decreased (Supplementary Fig. S7). Typically, pure FA generates no gas at all due to the non-formation of Pd NPs with only FA (Supplementary Fig. S1). On the other hand, pure SF liberates the pure H2 gas (Supplementary Fig. S8), which agrees well with the equation (3).

With the absence of citric acid in the system, Pd/C shows a very poor activity and only 223 mL of gas is finally obtained even after 440 minutes (Fig. 2b), corresponding to a conversion of about 59%. The TOF is only 16.1 mol H2 mol−1 catalyst h−1, which is exactly one-fourth of that catalyzed by Pd/C with citric acid. Based on the above results, it is reasonably to believe that citric acid is indispensable to the high catalytic performance of the in situ prepared Pd/C for H2 generation from FA/SF at room temperature.

To identify what kind of effect that citric acid plays on the dramatic enhancement in activity of Pd/C, the same catalytic reaction is applied by adding citric acid after the reaction begins for 40 min, which means that citric acid is nonexistent during the nucleation and growth of Pd NPs. As a consequence, no enhancement in terms of activity has been observed at all (Fig. 2c). Moreover, after Pd/C catalyst synthesized with citric acid is washed by water to remove the citric acid, it shows the similar high activity as that without washing (Fig. 2a, d). Therefore, citric acid has no strong interaction with Pd NPs to acts as a co-catalyst for decomposition of FA/SF but plays a key role in Pd NPs during their nucleation and growth processes, which thus leads to the obvious enhancement in catalytic activity of Pd/C at room temperature.

To further determine the influence of citric acid on the catalytic performance of Pd/C, the decomposition is also applied over Pd/C catalyst prepared in various concentrations of citric acid solution (Fig. 2 inset). Obviously, the activity of Pd/C increases with the increasing concentration of citric acid up to 0.03 M. However, further increase of the citric acid concentration brings out the decrease of the activity, which may be accounted for the over occupation of the active sites of Pd NPs by citric acid38.

Additionally, the temperature dependence of activity for the best Pd/C catalyst with citric acid has also been studied (Fig. 3). It can be seen clearly that the activity of Pd/C is enhanced greatly with the increasing temperature. The reaction time can be shortened from 160 to 49 min when the temperature is increased from 298 to 318 K.

Figure 3
figure 3

Activities at different temperatures.

Gas generation by decomposition of FA/SF (1.06 M/0.84 M, 5 mL) over Pd/C synthesized with citric acid (nPd/n(FA+SF) = 0.005) at different temperature.

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Characterizations of catalyst

The transmission electron microscopy (TEM) image of the in situ prepared catalyst with citric acid before reaction shows that the NPs are well-dispersed on carbon with an average particle size of 2.8 nm (Fig. 4a; Supplementary Fig. S9a). The high resolution TEM (HRTEM) image of a single particle (Fig. 4b) reveals the metallic lattice fringes of face-centered cubic Pd (111) plane with the space of 0.224 nm, which agrees well with its X-ray diffraction (XRD) result (Fig. 4d)39. In addition, the energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) results also support that Pd NPs have been successfully in situ synthesized (Fig. 4e, f). After the catalytic reaction, Pd/C shows the similar crystal structure and average particle size as that before the reaction (Supplementary Fig. S10). In addition, the BET surface areas for Pd/C before and after reaction are tested to be 169 and 178 m2/g, respectively. Based on the above results, it is reliable to believe that the present Pd/C has a good stability during the catalytic reaction.

Figure 4
figure 4

Catalyst characterizations.

(a) TEM image, (b) HRTEM image, (d) XRD pattern and (e) EDX pattern of Pd/C synthesized with citric acid. (c) TEM image of Pd/C synthesized without citric acid. (f) XPS spectra of Pd element in Pd/C synthesized with and without citric acid.

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Without citric acid, the obtained Pd NPs on carbon before reaction are severely aggregated with a larger average particle size of 11.2 nm (Fig. 4c; Supplementary Fig. S9b). Therefore, citric acid serves as an efficient dispersing agent during the formation of Pd NPs in the present system, which can be explained as follows: the three carboxyl anions of citric acid are strongly adsorbed on the surface of carbon at first40,41 and then attach Pd2+ by ion exchange or coordination reaction as the nucleation sites40; upon reduction by SF, due to the hydrophobic or coulombic effects on the Pd clusters42,43, citric acid can uniformly anchor the formed Pd NPs on carbon surface and control the sizes and distributions during the growth of NPs. However, citric acid, during the synthetic process, can not change the valence state of Pd NPs (Fig. 4f), which means that the interface of citric acid and Pd NPs has no strongly chemical interaction.

Recycle stability

Lifetime is very important for the practical application of nanocatalysts. In this sense, the recycle test of Pd/C synthesized with citric acid has been applied for the same decomposing reaction (Fig. 5). It can be seen clearly that there is only a slight deactivation has been observed in the 2nd run, which denotes the good stability of Pd/C during the initial two runs. Unfortunately, the catalytic activity losses seriously in the 4th run (Supplementary Fig. S11). Further investigations are needed to improve the durability of the present Pd/C catalyst.

Figure 5
figure 5

Recycle stability.

Recycle test of Pd/C synthesized with citric acid toward H2 generation from FA/SF at the 1st and 2nd runs.

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Discussion

The catalytic properties of the nanocatalysts usually depend on the sizes, morphologies, distributions and surface states of NPs44,45,46,47. Generally, the smaller the particle size, the greater its surface area, which may benefit its activity48,49,50. However, NPs with ultrafine sizes are easily aggregated due to their high surface energy and thereby induces low activites51,52. To avoid the aggregation, dispersing agents are usually employed to stabilize the NPs53,54,55,56. However, too strong affiliation of the agents to the surface of NPs may cause the severe deactivation of NPs55,57. In this sense, finding an appropriate dispersing agent for the target NPs is very important to reach the purpose of efficient reduction of the particle sizes, prevention of the aggregation and enhancement of the catalytic activity of the NPs. Citric acid, which is used here as a dispersing agent, is more appropriate than other ones for modification of Pd NPs to catalyze the decomposition of FA/SF (Fig. 6). Replace citric acid by L-ascorbic acid (C6H8O6), the resulted Pd NPs has a larger average particle size of 5.6 nm than that synthesized with citric acid (Supplementary Fig. S12) and exhibits a lower activity for the same reaction, where 250 mL of gas can be generated in 440 min (Fig. 6a). Replacing citric acid by polyvinylpyrrolidone (PVP, (C6H9NO)n), Pd NPs get the smallest particle size of 2.3 nm (Supplementary Fig. S13), but exhibit the lowest activity for the reaction (Fig. 6b, 25 mL, 440 min). The above results can be understand as follows: L-ascorbic acid, which has a similar molecule size as citric acid, has a weaker function to Pd2+ than citric acid39 and causes formation of the bigger Pd NPs and thus the lower activity of Pd NPs. However, PVP, a huge molecule resulting in the formation of the smallest particle size, occupies much of the active sites of Pd NPs and severely deactivates the catalyst. This has been confirmed by the fact that, after washing off the PVP, Pd/C shows a dramatic enhancement in its activity, which is even comparable to that synthesized with citric acid (Supplementary Fig. S14). Citric acid has a moderate binding to the surface of noble metal NPs39 and thereby efficiently leads to the formation of the small and well dispersed Pd NPs but does not block FA/SF adsorption and decomposition.

Figure 6
figure 6

Effect of different dispersing agents.

Gas generation by decomposition of FA/SF (1.06 M/0.84 M, 5 mL) catalyzed by in situ prepared Pd/C (nPd/n(FA+SF) = 0.005) in the presence of (a) L-ascorbic acid and (b) PVP at 298 K.

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In summary, a very facile strategy has been successfully applied for highly efficient H2 generation from FA/SF through a heterogeneous catalytic reaction over in situ synthesized Pd/C catalyst in the presence of citric acid. The present monometallic catalyst synthesized with citric acid exhibits the high catalytic activity, selectivity and durability for decomposition of FA/SF at room temperature. This simple but very effective monometallic catalyst is believed to strongly promote the practical applications of FA/SF as a CO-free H2 generation system for fuel cell applications.

Methods

Chemicals

Formic acid (HCO2H, Sigma-Aldrich, 96%), sodium tetrachloropalladate (Na2PdCl4, Sinopharm Chemical Reagent Co., Ltd, 36.4%), sodium formate dehydrate (HCO2Na·2H2O, Sinopharm Chemical Reagent Co., Ltd, >99.5%), citric acid monohydrate (C6H8O7·H2O, Beijing Chemicals Works, >99.5%), L-ascorbic acid (C6H8O6, Sinopharm Chemical Reagent Co., Ltd, >99.7%), aqueous hydrochloric acid (HCl, Beijing Chemicals Works, >35.0%), polyvinylpyrrolidone K30 (PVP, (C6H9NO)n, Mw: av. 58000, Beijing Chemicals Works), acetic acid (CH3CO2H, 36~37%, Beijing Chemicals Works), tetrahydrofuran (C4H8O, THF, 99.9%, Sinopharm Chemical Reagent Co., Ltd,) and Vulcan XC-72 carbon (C, Sinopharm Chemical Reagent Co., Ltd) were used without further purification. De-ionized water with the specific resistance of 18.2 MΩ·cm was obtained by reversed osmosis followed by ion-exchange and filtration.

Physical characterization

Powder X-ray diffraction (XRD) was performed on a Rigaku RINT-2000 X-ray diffractometer with Cu Kα. Transmission electron microscope (TEM, Tecnai F20, Philips) and corresponding energy-dispersive X-ray (EDX) were applied for the detailed microstructure and composition analyses. Surface Area was determined by BET measurements using an Autosorb-1 Surface Area Analyzers (Quantachrome Instrument Corporation). X-ray photoelectron spectrometry (XPS) analysis was carried out on an ESCALABMKLL X-ray photoelectron spectrometer using an Al Kα source.

Mass spectrometry (MS) analysis of the generated gases was performed on an OmniStar GSD320 mass spectrometer. Detailed information for CO2, H2 and CO was performed on GC-7900 with thermal conductivity detector (TCD) and flame ionization detector (FID)-methanator (FID, detection limit: ~10 ppm for CO). UV-Vis absorption spectra were recorded on an Agilent Cary 50 spectrophotometer in the wavelength range of 350–600 nm.

H2 generation from FA/SF over Pd/C synthesized with citric acid

Typically, 10.0 mL of Na2PdCl4 (4.7×10−3 M) aqueous solution containing 95.0 mg of Vulcan XC-72 carbon was kept in a two-necked round-bottom flask. One neck was connected to a gas burette and the other was connected to a pressure-equalization funnel to introduce 5.0 mL of the mixed aqueous solution containing FA (1.06 M), SF (0.84 M) and citric acid (0.03 M). The reaction begins once the mixed aqueous solution was added into the Na2PdCl4 solution with magnetic stirring (600 r/min). The evolution of gas was monitored using a gas burette. The reactions were carried out under argon (Ar) and ambient atmosphere at different temperatures. Moreover, the similar reactions with different FA mole percents in FA/SF (0%, 25%, 56%, 75%, 100%) were also applied under ambient atmosphere at 298 K, where the total mole number of FA and SF was kept as 9.5 mmol and the molar ratio of Pd:(FA+SF) was 0.005.

For comparison, FA/SF decomposing reaction catalyzed by in situ prepared Pd/C without citric acid and with L-ascorbic acid, PVP have also been carried out under the similar experimental conditions. In addition, after Pd/C catalyst was formed in the presence of citric acid, it was separated and washed by water for several times. And then, the washed Pd/C was used for the same FA/SF decomposing reaction. Such washing and decomposing experiments were also carried on Pd/C synthesized in PVP. However, the washing solvent for PVP is acetic acid and water.

Moreover, the similar FA/SF decomposition has also been applied in pure THF over in situ prepared Pd/C with citric acid at 298K.

Recycle test of Pd/C synthesized with citric acid

After the decomposition of FA/SF was completed, the in situ synthesized Pd/C with citric acid was separated from the reaction solution by centrifugation and washed by water for several times. The washed Pd/C was then re-dispersed in 10 mL of water in a flask and the new aqueous solution (5.0 mL) containing FA (1.06 M) and SF (0.84 M) was added into the flask with magnetic stirring (600 r/min) to begin the recycle test of Pd/C. The evolution of gas was monitored using a gas burette. Such recycle tests were repeated for 4 times.