Porous carbon supported Pd as catalysts for boosting formic acid dehydrogenation

Highlights

• The hybrid ZnCo-ZIF templated porous carbons support Pd nanoparticles.

• The as-prepared catalyst has highly efficient for dehydrogenation of formic acid.

• N and Co inherited from ZnCo-ZIF improve Pd catalytic efficiency.

• Dispersion of PdCo nanoparticles by adding Zn atom.

• Zn doping also increases N content and specific surface area of the catalysts.

Abstract

The design of efficient catalysts is the essential to realize formic acid (FA) as a hydrogen carrier. However, it remains a challenging task. Herein, the porous carbon was prepared using ZnCo-containing zeolitic imidazole frameworks (ZIFs) as a precursor, which supported Pd as an effective catalyst for FA dehydrogenation. Porous carbon containing Co and N was synthesized by one-step method, and the Co and N promoted the activity of Pd by modifying its electron state. The catalytic performance was further improved by doping Zn into the predesigned bimetallic ZnCo-ZIFs. The addition of Zn increased the dispersion of PdCo nanoparticles, N content and specific surface area of the catalysts. When Zn/Co molar ratio was 2, the prepared catalyst (Pd/Co@CN-2) with an average diameter of PdCo about 2.6 nm exhibited the best catalytic activity, showing an initial turnover frequency value (TOF) as high as 2302 h⁻¹ even at 30 °C.

Introduction

In future hydrogen economy schemes, the crucial processes will be based on fixity transformations towards mobility with applications mainly for fuel cell [1]. Finding optional routes for storing hydrogen energy is the key to achieve the transformations. Among the many possible strategies, storing hydrogen in liquid hydrogen carriers is one of the most promising solutions [2]. Formic acid (FA) is considered as one of the most promising candidates, due to its nontoxic, excellent stability and high H₂ content (4.4%). Taking the advantages of facilitate transportation, refuelling, and handling, FA is a suitable candidate for the application in portable fuel cells [3]. To utilize FA as an effective hydrogen storage material, one must follow the dehydrogenation pathway (1) and avoid the dehydration pathway (2).

$$HCOOH\left(1\right)\rightleftharpoons H_2\left(g\right)+C{O}_2\left(g\right)\mathrm{\Delta }{G}^{\Theta }\left(298K\right)=-33kJmo{l}^{-1}$$

$$HCOOH\left(1\right)\rightleftharpoons H_2\left(g\right)+CO\left(g\right)\mathrm{\Delta }{G}^{\Theta }\left(298K\right)=-12.9kJmo{l}^{-1}$$

In recent years, heterogeneous catalysts have attracted great interest for FA dehydrogenation because of separability, reusability and relatively low reaction temperature (<80 °C) [[4], [5], [6]]. Pd/C is the most common and effective catalyst for H₂ production from FA in aqueous solution. Unfortunately, the Pd/C catalysts still suffer from multiple competitive disadvantages, including aggregation of Pd particles, adsorption of poisonous intermediate, together with the high cost and scarcity of the noble Pd metal [7,8]. On this basis, the catalytic efficiency of Pd/C can be improved by adding another metal or modifying the support.

Porous carbon, which have chemical stability, a large specific surface area, adjustable pore structure, and low-cost, are widely used as catalyst supports [[9], [10], [11]]. A simple route to obtain porous carbon is to carbonize metal organic frameworks (MOFs) directly [12]. Xu et al. reported the prepare porous carbon using MOF as template, which had excellent electrochemical performance as electrode material of electrochemical double-layer capacitor [13]. The preparation of carbon composite membrane by carbonization of zirconium MOF had the advantages of high porosity, small particle size and easy to produce into membrane, which could be applied as water filtration carrier to extract toxic phenols from water [14]. Zeolitic imidazole frameworks (ZIFs) is a branch of MOFs, and it was viewed as a suitable precursor for thermal-decomposing synthesis of porous carbon composite material which was modified by transition-metal and N [15,16]. They derive porous carbon materials with uniform heteroatom decoration and high porosity, which modify the properties of active sites and afford high surface area, can promote the catalytic efficiency [17,18]. A Zn-containing ZIFs (ZIF-8), had been converted to porous carbon, which supported Pd nanoparticles for biofuel upgrade [19]. The prepared catalyst had excellent catalytic performance due to the well-dispersed Pd nanoparticles, large surface area and hierarchical pores. It is worth noting that structures and functions of ZIFs can be tailored for on-demand applications through the transition metal ions and organic linkers [20,21].

Co as an assistant can effectively improve the efficiency of Pd catalytic decomposition of FA [22,23], and doping N species into the skeleton of porous carbon is another way to improve Pd activity [24]. An efficient catalyst was synthesized by supporting PdCo on graphitic carbon nitride (g-C₃N₄). The TOF of PdCo/g-C₃N₄ (1/0.7) for dehydrogenation of FA was 1193 h⁻¹ at 75 °C. The results showed that the combination of alloying effect and nitrogen function on PdCo/g-C₃N₄ was helpful to form small and uniform nanoparticles [25]. Feng et al. utilized the N-rich porous covalent triazine frameworks (CTFs) as the support to load PdCo, the TOF value of prepared catalyst reached 2129 h⁻¹ toward H₂ generation from FA at 50 °C. It was mainly attributed to the synergistic effect of PdCo nanoparticles and N-rich CTF, as well as the well-dispersed PdCo [26].

The porous carbon obtained by carbonization Co-containing ZIFs (ZIF-67) can realize the above two strategies in one step. ZIF-67 can provide critical Co–N_x sites, whereas it tends to have a smaller specific surface area and lower N contents [27]. And, Co is easy to agglomerate in the process of high temperature carbonization [28,29]. As mentioned above, porous carbon from ZIF-8 has been demonstrated to provide high specific surface area porous carbon with high N content [19,30,31]. When the pyrolysis temperature is higher than the boiling point of Zn, the generated ZnO is reduced to Zn by carbon and subsequently evaporated [32]. Zn, which is homogeneous dopants in the ZnCo-ZIF, is beneficial for adjusting the coordination environment of Co. Besides, it can also prevent potential agglomeration of Co during carbonizing [12]. In addition, it is well accepted that narrowing down the metal particle size can significantly accelerate the H₂ production rate from FA [33,34], since small size metal particles with high surface-to-volume ratio and “clean” surface lead to the exposure of more active sites [4,35]. Hence, high dispersion of Co by doping Zn should be a feasible strategy to improve catalytic activity for dehydrogenation of FA.

In this work, a series of ZIFs were obtained by adjusting Zn/Co molar ratio (ZnCo-ZIF), and ZnCo-ZIF were pyrolyzed into porous carbon under argon atmosphere (Co@CN). Then Pd nanoparticles were loaded on Co@CN (Pd/Co@CN), which as catalyst for H₂ production from FA (Scheme 1). The obtained Pd/Co@CN exhibited preferred activity and stability for FA decomposition at 30 °C.