Interface electron collaborative migration of Co–Co3O4/carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane
Graphical abstract
The Co–Co3O4/CDs was proved to be a highly efficient noble-metal-free catalyst for AB hydrolysis. The incorporation of the multi-interfaces between Co, Co3O4 NPs, and CDs endows the hybrid material with excellent catalytic activity.
Introduction
Nowadays, the excessive consumption of fossil fuels and growing environmental concerns have made the need to develop a hydrogen economy more urgent [1,2]. However, many issues must be overcome for the transition to hydrogen energy; in particular, the efficient generation of hydrogen at room temperature presents enormous challenges [3]. The main current hydrogen production method via the electrolysis of water involves sacrificing one energy source for another, which is not economically desirable [4]. Therefore, a safe and efficient hydrogen production method is necessary for sustainable hydrogen utilization. Hydrogen production from chemical hydrogen storage materials has thus attracted research attention [5]. Its high hydrogen gravimetric density (19.6 wt%) and low molecular weight (30.86 g mol-1) have led ammonia borane (NH3BH3, AB) to be regarded as competitive for chemical hydrogen storage [6,7]. Since the first AB synthesis reported by Shore and Parry in 1955, many methods with considerable yields have been developed [8].
Recently, the hydrolysis of AB has been widely investigated because of the high amount of released hydrogen. When appropriate catalysts are used, hydrogen can be released under ambient conditions via the following reaction: NH3BH3+ 2H2O → NH4+ + BO2- + 3H2. To date, fast hydrogen evolution from AB has been achieved using homogenous molecular systems and metallic nanoparticles (NPs), especially Pt- and Ru-based noble metals or their alloys, as the catalysts [9,10]. However, the high cost and limited reserves of noble metals have greatly limited their usage. Compared with noble-metal catalysts, non-noble-metal materials exhibit lower catalytic activity. Because of their relatively inert catalytic nature, non-noble metal catalysts require extensive morphological and interfacial engineering to be effective [11]. To date, many excellent non-noble-metal-based materials, using such as Mo [12], Ni [13], and Co [14], have been shown to be efficient catalysts for AB hydrolysis. Many efforts have been made to develop new structures (e.g., amorphous [15], NP [16,17], alloyed [18], core–shell bimetallic [19], and metal–metal oxide [14]) to enhance the catalytic activity in AB hydrolysis compared with that of equivalent crystalline or monometallic counterparts. For example, Xu and co-workers [20] reported that a novel Au@Co core–shell NP material synthesized via a one-step seeding–growth process exhibited higher catalytic activity for AB dehydrogenation than its pure metal counterpart. However, pure metals and their derivatives still face the problems of poor dispersity, low conductivity, and ease of agglomeration. To overcome these limitations, some functional carbon supports, such as metal–organic frameworks (MOFs) [21], carbon nanotubes [22], graphene [23], and organic polymers [24], have been applied. In particular, because of their microporous cages, MOFs can confine ultrafine metal particles, resulting in high turnover frequency (TOF) for the AB hydrolysis. However, these catalysts are usually prepared via complicated syntheses, typically involving toxic organic reagents, and only a small volume of products can be obtained. Therefore, the design and synthesis of efficient carbon-based non-noble-metal composites for AB hydrolysis in a simple and green way is critical to the popularization and application of chemical hydrogen storage materials.
Carbon dots (CDs), a new photoluminescent material, have attracted considerable attention because of their tunable photoluminescence (PL) and unique electron-transfer abilities as well as their low cost, nontoxicity, and good hydrophilicity [25], [26], [27], [28]. Recently, many excellent CDs have been prepared from inexpensive organic small molecules and biomass without the use of toxic organic reagents [29,30]. In addition, the strong complexing action between surface groups and many metal ions (such as Fe3+, Ru3+, Co2+, and Cu2+), combined with their large specific surface areas, have made CDs an excellent class of materials to support catalysts for photocatalysis and electrocatalysis [31,32]. To the best of our knowledge, there have been no reports on the use of CDs for AB hydrolysis. We anticipated that the excellent physicochemical properties of CDs would also endow the resultant catalysts with high catalytic performance for AB hydrolysis. In addition, the structures of active sites in these materials must also be clearly understood for the rational design of new efficient catalyst systems.
Herein, we first synthesized excellent PL CDs with a uniform size of approximately 2.54 nm via a simple hydrothermal process using inexpensive citric acid. Then, Co–Co3O4 hybrid NPs facilely deposited on CDs (simplified as Co–Co3O4/CDs) were constructed via direct calcination of the Co2+–CDs complexes and subsequent activation treatment in air. When used as a catalyst for AB hydrolysis, the material exhibited excellent catalytic activity (rB = 6816 min-1 gCo-1). In addition, even after five cycles, it retained very high catalytic activity. Further mechanistic study suggested that the high activity could be attributed to the interfacial interaction between the Co, Co3O4 NPs, and CDs, which provided abundant active sites for dehydrogenation of AB.
Section snippets
Chemicals
Cobalt nitrate hexahydrate (Co(NO3)2•6H2O, Aladdin Industrial Co., Ltd., AR), citric acid monohydrate (C6H8O7•H2O, Shanghai Macklin Biochemical Co., Ltd., GR), cobalt nanoparticles (Co-50 nm, Shanghai Macklin Biochemical Co., Ltd., 99.9%), sodium hydroxide (NaOH, Sino Pharm Chemical Reagent Co. Ltd., China, AR), ammonia borane (AB, Tianjin Chemical Reagent Co., Ltd., China, 97%) and deionized (DI) water are purchased from commercial suppliers. All of the chemicals are used as received without
Physicochemical properties of the as-synthesized CDs
A simple hydrothermal method was used to synthesize the CDs from dehydration and carbonization of citric acid molecules. The morphology of the as-prepared CDs was characterized using TEM. The TEM images (Fig. 1(a) and (b)) revealed a uniform dispersion of the as-prepared CDs with an average diameter of approximately 2.74 nm. The high-resolution TEM (HRTEM) image shown in the inset of Fig. 1(b) shows the high crystallinity of the CDs with an interlayer spacing of 2.05 Å, corresponding to the d-
Conclusions
In summary, we successfully synthesized Co–Co3O4 NPs facilely deposited on CDs as an efficient noble-metal-free catalyst for the hydrolysis of AB. Benefiting from the improved surface area, high dispersibility, good conductivity, and interfacial interactions between the Co, Co3O4 NPs, and CDs, the Co–Co3O4/CDs exhibited excellent catalytic activity (rB = 6816 min-1gCo–1) and good cyclic stability. Further mechanistic study suggested that the high interfacial synergy can provide abundant
Declaration of Competing Interest
This work is original, our own, and has not been published previously and they have no conflict of interest. We believe this work will be of wide general interest to readership, and await your views.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21774041 and 51433003) and the China Postdoctoral Science Foundation (2018M640681 and 2019T120632).
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