1 Introduction
Energy consumption around the world is increasing every day, which requires higher energy generation capacity, better energy management, and a shift away from non-renewable fossil fuels. The installation of a sustainable, secure and diversified energy supply chain is one of the greatest challenges to be addressed in this century.
Nowadays, industry and governments are increasing their attention on hydrogen as a candidate for clean energy due to the fact that its oxidation is highly exothermic and the only by-product is water [
1]. Despite the potential of the use of hydrogen, its widespread utilization is currently limited by the capacity limitations of hydrogen storage technologies, and by the safety issues related with its storage and transportation under mild conditions [
2].
Hydrogen gas is highly flammable in presence of oxygen and its traditional storage methods use compressed gas cylinders with pressure ranges between 200 and 350 bar. These high pressures require energy intensive processing and have safety risks, which makes public acceptance difficult, besides of the significant weight and volume requirements. During the last two decades, great scientific effort has been made in order to solve this problem, for instance, current research is exploring new methods to store or produce hydrogen under more secured and favourable conditions. These new methods can be classified depending on the interaction between hydrogen and the material, i.e. physisorbed or chemically incorporated in the structure. In the former method, hydrogen is adsorbed into a porous network such as zeolites [
3], MOFs [
4], clathrate hydrates [
5], various carbon materials [
3] and conventional organic polymers [
6]. In the latter, a hydrogen-rich material is subjected to a decomposition process, which can be potentially reversible. Examples of these structures include solid phase systems, such as metal and non-metal hydrides [
7], amines [
8], amides [
9], ammonia-like complexes [
10], and liquid carriers such as
N-ethylperhydrocarbazole [
11], alcohols [
12] or formic acid.
Catalytic hydrolysis of sodium and lithium borohydrides as well as ammonia borane have been widely studied because it provides a safe and low cost route to the production of hydrogen [
10‐
13]. Ammonia decomposition has been also studied at temperatures below 500 K showing a significant reduction in activation energy when using carbon nanotubes catalysts promoted with cesium [
14]. Metal hydrides such those of Mg–Al–Fe has been reported to achieve a maximum rate of 499.5 ml min
−1 g
−1 of hydrogen at 25 °C for Mg
60–Al
30–Fe
10 (wt%) in 0.6 mol l
−1 NaCl solution [
15]. One of the most promising solutions consists on the utilization of hydrous hydrazine as reagent [
16]. It presents the unique advantage that N
2 is the only by-product for the complete decomposition [
17].
Recently, formic acid, a major product formed during biomass processing, has been suggested and studied as a safe and convenient hydrogen storage material. It includes high volumetric hydrogen content (4.4 wt% of hydrogen) besides being liquid state at room temperature (volumetric capacity of 53.4 g l
−1 at standard temperature and pressure), highly stable, environmental benign and nontoxic [
18]. Furthermore, formic acid decomposition produces mainly gaseous products (H
2/CO
2) by decomposition. According to the U.S. Department of Energy, formic acid is one of the most promising hydrogen storage materials and its volumetric capacity surpasses that of most other storage materials today [
19]. Hence, an effective and controlled release of hydrogen via selective decomposition of formic acid to CO
2 and H
2 is a desirable approach. More importantly, if the production of formic acid can be carried out under mild conditions via biomass conversion, a carbon neutral hydrogen storage cycle can be completed [
20]. The proposed cycle can be closed when CO
2 evolved during dehydrogenation of formic acid is reduced with an external supply of low purity H
2 [
21]. Formic acid decomposition occurs by two different pathways: dehydrogenation (
1) and dehydration (
2). Selective dehydrogenation is indispensable for the production of ultrapure H
2 without undesirable dehydration, which also generates CO contaminants reducing the activity of Pd catalysts.
$${\text{Dehydrogenation: HCOOH}} \to {\text{C}}{{\text{O}}_2}+{{\text{H}}_2}\,\Delta {\text{G}}= - 48.4\,{\text{kJ\,mol}}^{ - 1}$$
(1)
$${\text{Dehydration: HCOOH}} \to {\text{CO}}+{{\text{H}}_2}{\text{O}}\,\Delta {\text{G}}= - 28.5\,{\text{kJ\,mol}}^{ - 1}$$
(2)
Previous studies have reported the utilisation of homogeneous catalysts to decompose formic acid at ambient temperatures and pressures. They showed promising results in terms of catalysts stability and selectivity to H
2 and CO
2 while significantly improving the catalytic efficiency [
22‐
25]. However, the catalysts separation from the reaction mixture, moderate selectivity, their need for organic solvents/additives and, in several cases, harsh reaction conditions [
26,
27], prevent them from scaling-up for practical applications. An alternative and attractive approach is the utilisation of heterogeneous catalysts that can achieve high catalytic activity [high turnover frequency (TOF) and utilisation of high substrate to metal molar ratio] at low temperature and with high selectivity towards H
2 [
19,
28].
One of the first reported studies on the formic acid decomposition were published in 1957 using Pd–Au alloy wires as catalytic materials [
29] followed by studies using Pd/C. However, one of the main drawbacks was that the synthesised Pd/C catalysts deactivated quickly due to the poisoning intermediates resulting in its failing to applications [
30]. Recent research has shown how to overcome this challenge by using a solution of formic acid and sodium formate of 9:1 volumetric ratio respectively and Pd/C reaching a TOF of 228.3 h
−1 at 30 °C after 2 h [
31]. Using this solution on a combination of Ag–Pd nanoparticles deposited on a basic resin a higher TOF of 820 h
−1 at 75 °C was achieved with volumetric ratio of formic acid:sodium formate of 9:1 [
32].
It has been reported that bimetallic nanoparticles can enhance the catalytic activity and selectivity compared to monometallic species. Recent studies by Xing and co-workers have shown the development of Pd–Au and Pd–Ag alloys supported on carbon to overcome the poisoning and stability issued on monometallic Pd analogues. These bimetallic particles generated high purity hydrogen production from the decomposition of formic acid at low temperatures. The authors also reported that the activities of Pd–Au/C and Pd–Ag/C can be enhanced by co-deposition with CeO
2 [
33]. In recent years, Tedsree et al. [
34] developed a Ag–Pd core–shell catalyst supported on carbon based materials for dehydrogenation of formic acid resulting in a TOF of 626 h
−1 although the drawback of generating CO due to the high temperature could not be avoided. A very recent research has reported that a metal–organic framework loaded with Ag–Pd alloy resulted in 100% selectivity for hydrogen generation from formic acid solution with TOF of 848 h
−1 at 80 °C [
35]. A wide variety of bimetallic and trimetallic Pd-based catalysts have been recently reported, e.g. AuPd [
36,
37], PdNi [
38], PdCo [
39], PdCu [
40], AuPdAg [
41], CoAgPd [
42] and NiAuPd [
43] nanoparticles showing that the enhancement in the catalytic performance is mainly due to electronic and geometric effects.
The last years, research efforts led to improved experimental conditions, although for practical applications in portable electric devices, there are still limitations on component cost, catalyst deactivation, regeneration of by-products and control of the reaction kinetics, which current research tried to overcome.
Supported metal nanoparticles are important owing to their unique physical and chemical properties and various methods of preparation. Those methods have been investigated to synthesise metal nanoparticles with tailored size, shape and composition followed by their assembly and activation on support materials helping us to identify and minimise main drawbacks of traditional synthetic methodologies [
44‐
49].
In the present work, we report the catalytic performance of an efficient commercial 5 wt% Pd/C for the production of hydrogen from the catalytic aqueous additive-free formic acid decomposition. 5 wt% Pd/C has been selected as a commercial reference and starting point for future research and optimisation of reaction conditions. The characterisation of these catalysts series (fresh and used) was thoroughly investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) and Brunauer–Emmett–Teller (BET) surface area analysis. The performance of the catalyst toward aqueous formic acid decomposition was carried out systematically in a batch reactor by varying a set of reaction parameters, such as substrate/metal molar ratio, stirrer speed, temperature and concentration of formic acid. Kinetic isotope studies and reusability tests were as well performed. Finally, periodic density functional theory (DFT) calculations were employed to gain insights on the energetics of formic acid decomposition on Pd (111) surface as the most represented model.