Interface electron collaborative migration of Co–Co₃O₄/carbon dots: Boosting the hydrolytic dehydrogenation of ammonia borane

Abstract

Ammonia borane (AB) is an excellent candidate for the chemical storage of hydrogen. However, its practical utilization for hydrogen production is hindered by the need for expensive noble-metal-based catalysts. Herein, we report Co–Co₃O₄ nanoparticles (NPs) facilely deposited on carbon dots (CDs) as a highly efficient, robust, and noble-metal-free catalyst for the hydrolysis of AB. The incorporation of the multi-interfaces between Co, Co₃O₄ NPs, and CDs endows this hybrid material with excellent catalytic activity (r_B = 6816$\mathrm{m}{\mathrm{L}}_{{\mathrm{H}}_2}$ min^–1 g_Co^–1) exceeding that of previous non-noble-metal NP systems and even that of some noble-metal NP systems. A further mechanistic study suggests that these interfacial interactions can affect the electronic structures of interfacial atoms and provide abundant adsorption sites for AB and water molecules, resulting in a low energy barrier for the activation of reactive molecules and thus substantial improvement of the catalytic rate.

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 (r_B = 6816 $\mathrm{m}{\mathrm{L}}_{{\mathrm{H}}_2}$ min^-1 g_Co^-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.