Recent Advances and Perspectives on Supported Catalysts for Heterogeneous Hydrogen Production from Ammonia Borane

Ammonia borane (AB), a liquid hydrogen storage material, has attracted increasing attention for hydrogen utilization because of its high hydrogen content. However, the slow kinetics of AB hydrolysis and the indefinite catalytic mechanism remain significant problems for its large-scale practical application. Thus, the development of efficient AB hydrolysis catalysts and the determination of their catalytic mechanisms are significant and urgent. A summary of the preparation process and structural characteristics of various supported catalysts is presented in this paper, including graphite, metal-organic frameworks (MOFs), metal oxides, carbon nitride (CN), molybdenum carbide (MoC), carbon nanotubes (CNTs), boron nitride (h-BN), zeolites, carbon dots (CDs), and metal carbide and nitride (MXene). In addition, the relationship between the electronic structure and catalytic performance is discussed to ascertain the actual active sites in the catalytic process. The mechanism of AB hydrolysis catalysis is systematically discussed, and possible catalytic paths are summarized to provide theoretical considerations for the designing of efficient AB hydrolysis catalysts. Furthermore, three methods for stimulating AB from dehydrogenation by-products and the design of possible hydrogen product-regeneration systems are summarized. Finally, the remaining challenges and future research directions for the effective development of AB catalysts are discussed.

Modulating the Acidic Properties of Mesoporous Mox-Ni0.8Cu0.2O Nanowires for Enhanced Catalytic Performance toward the Methanolysis of Ammonia Borane for Hydrogen Production

Rational construction of inexpensive, highly efficient, and stable catalysts for ammonia borane (AB) methanolysis is in high demand but still remains a great challenge. In this work, we have successfully fabricated uniform Mo_x-Ni_0.8Cu_0.2O nanowires using a simple hydrothermal method followed by a post-calcination treatment and flexibly modulated the acidity of their surface by changing the amount of Mo introduced into Ni_0.8Cu_0.2O. The Mo_0.1-Ni_0.8Cu_0.2O catalyst displayed strong catalytic activity toward AB methanolysis with an ultrahigh turnover frequency of 46.9 mol_H2 mol_cat.^-1 min^-1, which is even higher than some noble metal catalysts. In this work, an equation regarding the relationship between the quantity of moderated acid sites of catalysts and its corresponding activity toward AB methanolysis was first determined. A plausible mechanism for AB methanolysis catalyzed by Mo_x-Ni_0.8Cu_0.2O was proposed, and the benefits of the introduction of MoO₃ to Ni_0.8Cu_0.2O for enhancing the catalytic performance were also discussed. These findings can form a basis for the rational construction of inexpensive catalysts with robust performance toward AB methanolysis for hydrogen production.

Metal-Support Synergistic Catalysis in Pt/MoO3-x Nanorods toward Ammonia Borane Hydrolysis with Efficient Hydrogen Generation

Ammonia borane (NH₃BH₃, AB) serves as a promising material for chemical storage of hydrogen owing to its high hydrogen density and superior stability, in which the development of highly efficient heterogeneous catalysts toward AB hydrolysis plays a crucial role. Herein, we report Pt atomic clusters supported on MoO_3-x nanorods using a two-step process: MoO_3-x nanorods were synthesized at various calcination temperatures, followed by a further deposition-precipitation approach to obtain Pt/MoO_3-x catalysts (denoted as Pt/MoO_3-x-T, T = 300, 400, 500, and 600 °C). The optimized Pt/MoO_3-x-500 catalyst exhibits a prominent catalytic performance toward hydrolytic dehydrogenation of AB for H₂ generation, with a turnover frequency value of 2268.6 min^-1, which stands at the top level among the reported catalysts. Moreover, the catalyst shows a remarkable stability with 90% activity remaining after five cycles. A combination investigation including HR-TEM, ac-HAADF-STEM, XPS, in situ CO-IR, XANES, and Bader charge analysis verifies the formation of Pt²⁺-O_v-Mo⁵⁺ (O_v represents oxygen vacancy), whose concentration is dependent on the strength of the metal-support interaction. Studies on the structure-property correlation based on an isotopic kinetic experiment, in situ FT-IR, and DFT calculations further reveal that the Mo⁵⁺-O_v sites accelerate the dissociation of H₂O molecules (rate-determining step), while the adjacent Pt²⁺ species facilitates the cleavage of the B-H bond in the AB molecule to produce H₂. This work provides a fundamental and systematic understanding on the metal-support synergistic catalysis toward robust H₂ production, which is constructive for hydrogen storage and energy catalysis.

Ammonia Borane: An Extensively Studied, Though Not Yet Implemented, Hydrogen Carrier

Ammonia borane H3N−BH3 (AB) was re-discovered, in the 2000s, to play an important role in the developing hydrogen economy, but it has seemingly failed; at best it has lagged behind. The present review aims at analyzing, in the context of more than 300 articles, the reasons why AB gives a sense that it has failed as an anodic fuel, a liquid-state hydrogen carrier and a solid hydrogen carrier. The key issues AB faces and the key challenges ahead it has to address (i.e., those hindering its technological deployment) have been identified and itemized. The reality is that preventable errors have been made. First, some critical issues have been underestimated and thereby understudied, whereas others have been disproportionally considered. Second, the potential of AB has been overestimated, and there has been an undoubted lack of realistic and practical vision of it. Third, the competition in the field is severe, with more promising and cheaper hydrides in front of AB. Fourth, AB has been confined to lab benches, and consequently its technological readiness level has remained low. This is discussed in detail herein.

Noble-metal-free nanocatalysts for hydrogen generation from boron- and nitrogen-based hydrides

We focus on the recent advances in non-noble metal catalyst design, synthesis and applications in dehydrogenation of chemical hydrides (e.g. NaBH₄, NH₃BH₃, NH₃, N₂H₄, N₂H₄BH₃) due to their high hydrogen contents and CO-free H₂ production.