Page 61 - Read Online

Page 2 of 14 Liu et al. Chem Synth 2023;3:24 https://dx.doi.org/10.20517/cs.2023.13

Keywords: Formic acid, catalysis, hydrogen production, Pd nanocluster, carbon functionalization

INTRODUCTION
Hydrogen storage technologies based on liquid chemical hydrogen carriers have attracted more and more
[1-4]
attention due to their safety, convenience, and high efficiency . Among them, formic acid (HCOOH, FA)
has the advantages of low dehydrogenation reaction enthalpy (30 kJ·mol ), high hydrogen gravimetric
−1
−1
density (4.4 wt %), and volumetric capacity (53.4 g·L ), a wide range of sources from biomass process and
CO reduction, non-toxicity and excellent stability in the liquid state, demonstrating great potential for
2
[5-8]
liquid chemical hydrogen storage . The decomposition of FA can take place through two pathways, one is
the dehydrogenation process into H and CO and another is the dehydration process into H O and CO [9,10] .
2
2
2
The CO generated from the dehydration pathway is the undesired product, which will seriously poison
noble metal catalysts and lead to a quick deactivation of catalytic performance [11,12] . Furthermore, in order to
effectively release H , the decomposition of FA requires higher temperature and extra additives (such as
2
HCOONa and HCOONH ), further complicating catalysis [13,14] . Therefore, it is greatly significant to develop
4
desired catalysts with high selectivity and efficiency for FA dehydrogenation under mild conditions.
Pd-based catalysts have displayed remarkable activity toward the dehydrogenation of FA [15-17] . Especially, the
heterogeneous catalysts with Pd nanoparticles dispersed on various supporting materials disclosed
[18]
outstanding activity . The use of supports is critical for facilitating the uniform dispersion of metal
catalysts with maximum utilization and preventing the metal active sites from agglomeration and loss, thus
leading to further improvements in catalytic activity and stability [19,20] . In addition, the unique surface
chemical and electronic properties of supports induce strong metal-support interactions, which has a great
effect on improving the catalytic performance of catalysts [21,22] . Therefore, surface modification of support
with selective functions is a more extensive strategy to obtain desired catalytic performance [23,24] . For
instance, heteroatom (N, B, P, S) doping in the carbon can provide anchoring sites to stabilize the metal
centers with uniform dispersion and introduce a strong metal-support interaction [25-29] . The introduction of
N in the carbon supports has been proven active for FA dehydrogenation, in which the N atoms can anchor
Pd nanoparticles on supports with good dispersion, causing an improved catalytic performance [30,31] .
Moreover, the doped electronegative N atoms contribute to the surface electronic modulation of Pd
nanoparticles via electron transfer, which also facilitate the efficient cleavage of C-H bonds [32,33] . Although
the modification of carbon supports with the N atoms can selectively catalyze the dehydrogenation of FA,
extra additives are still needed to obtain a satisfactory performance, which hinders their widespread use. In
addition, amine functional groups can also be used to decorate support materials for promoting FA
dehydrogenation [34-36] . It has been reported that the amine functional group has nucleophilicity and can
serve as a proton scavenger to facilitate the O-H bond cleavage, thus resulting in enhanced activity in FA
dehydrogenation [37,38] . Another important role of amine groups is to be used as a capping agent to protect
metal nanoparticles from aggregation and growth, while it is noteworthy that more defect points will
[3]
emerge as the particle size decreases, which would increase CO adsorption and cause catalyst poisoning .
Considering the advantages and disadvantages of the above two modifications discussed, which inspire us to
combine both functionalization groups on carbon supports to construct a heterogeneous catalyst, with the
synergistic promotion of Pd and nitrogen/amino groups for the FA dehydrogenation.

In this work, we propose the preparation of ultrafine Pd nanoclusters supported on nitrogen/amino co-
functionalized carbon toward FA dehydrogenation. The carbon supports with abundant nitrogen-dopant
atoms and amino groups enable the controllable growth of 1.42 ± 0.3 nm Pd nanoclusters with uniform
dispersion. Attributing to the multiple synergies of Pd nanoclusters, nitrogen-dopant, amino groups, and