A review on platinum(0) nanocatalysts for hydrogen generation from the hydrolysis of ammonia borane

Exploring the Enhanced Catalytic Activity of Pt Nanoparticles Generated on the Red Phosphorus/Graphitic Carbon Nitride Binary Heterojunctions in the Photo-assisted Hydrolysis of Ammonia Borane

Ammonia borane (AB) holds great promise for chemical hydrogen storage, but its slow dehydrogenation kinetics under ambient conditions requires a suitable catalyst to facilitate hydrogen production from AB. Here, we fabricated binary red phosphorus/graphitic carbon nitride (RP/g-CN) heterojunctions decorated with Pt nanoparticles (NPs, denoted Pt/RP/g-CN) with a facile ultrasound-assisted two-step protocol as a photo-assisted catalyst for the hydrolysis of AB (HAB). The heterojunction established through intimate P-O-N bonds was proven to have improved photophysical properties such as a lower electron-hole recombination and enhanced visible light utilization compared to the pristine components. With the incorporation of Pt NPs, the optical properties of RP/g-CN heterojunctions were further improved through Schottky junction formation between semiconductors and Pt NPs, enabling a superb hydrogen gas (H2) generation rate of 142 mol H2·mol Pt-1·min-1 under visible light irradiation. Even though g-CN is a well-known host material for many metal NPs, here we discovered that the interaction of Pt NPs with RP in the ternary heterojunction structure is more favorable than that of g-CN, stressing the key role of RP as a support material in the designed ternary heterostructure. The band alignment of the ternary heterojunction catalyst along with the flow of charge carriers was also studied and shown to be a type-II heterojunction structure without hole migration, namely, a complex type-II heterojunction. Several scavenger experiments were also conducted to explain the mechanism of the photo-assisted HAB. To the best of our knowledge, this is the first example of a dual mechanism proposed for the visible light-assisted HAB. While the majority of the H2 was believed to be produced on the Pt NPs surface with the traditional B-N bond dissociation mechanism, the strong oxidizing action of OH• radicals formed by the heterojunction photocatalyst was also speculated to account for the 33% increase in the activity upon visible light irradiation through another mechanism.

Reducible tungsten(VI) oxide-supported ruthenium(0) nanoparticles: highly active catalyst for hydrolytic dehydrogenation of ammonia borane

Reducible WO3 powder with a mean diameter of 100 nm is used as support to stabilize ruthenium(0) nanoparticles. Ruthenium(0) nanoparticles are obtained by NaBH4 reduction of ruthenium(III) precursor on the surface of WO3 support at room temperature. Ruthenium(0) nanoparticles are uniformly dispersed on the surface of tungsten(VI) oxide. The obtained Ru0/WO3 nanoparticles are found to be active catalysts in hydrolytic dehydrogenation of ammonia borane. The turnover frequency (TOF) values of the Ru0/WO3 nanocatalysts with the metal loading of 1.0%, 2.0%, and 3.0% wt. Ru are 122, 106, and 83 min-1, respectively, in releasing hydrogen gas from the hydrolysis of ammonia borane at 25.0 °C. As the Ru0/WO3 (1.0% wt. Ru) nanocatalyst with an average particle size of 2.6 nm provides the highest activity among them, it is extensively investigated. Although the Ru0/WO3 (1.0% wt. Ru) nanocatalyst is not magnetically separable, it has extremely high reusability in the hydrolysis reaction as it preserves 100% of initial catalytic activity even after the 5th run of hydrolysis. The high activity and reusability of Ru0/WO3 (1.0% wt. Ru) nanocatalyst are attributed to the favorable metal-support interaction between the ruthenium(0) nanoparticles and the reducible tungsten(VI) oxide. The high catalytic activity and high stability of Ru0/WO3 nanoparticles increase the catalytic efficiency of precious ruthenium in hydrolytic dehydrogenation of ammonia borane.

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.

A review of the catalytic conversion of glycerol to lactic acid in the presence of aqueous base

Lactic acid is a high-value-added chemical with large production, which is used in many industries including the production of pyruvic and acrylic acids. Lactic acid is largely obtained from the oxidation of glycerol, which is a prevalent by-product of biodiesel production. However, the oxidation of glycerol to lactic acid requires harsh reaction conditions such as high temperature and pressure as well as the use of a hefty strong base. In the presence of suitable catalysts, the production of lactic acid from glycerol can be achieved under mild conditions with 1 equivalent base per mole of glycerol. Herein, we review the reports of the catalytic conversion of glycerol to lactic acid in an aqueous alkaline medium considering the reaction conditions, catalytic activity for glycerol conversion and selectivity for lactic acid. We start first with the reports on the use of homogeneous catalysts that have high catalytic activity but miserable recovery. Next, we discuss the employment of colloidal metal(0) nanoparticles as catalysts in glycerol oxidation. The papers on the use of supported metal(0) nanoparticles are reviewed according to the type of support. We then review the polymetallic and metal/metal oxide nanocatalysts used for the conversion of glycerol to lactic acid in an alkaline medium. The catalysts tested for glycerol conversion to lactic acid without any additional bases are also discussed to emphasize the importance of a strong base for catalytic performance. The proposed mechanisms of glycerol oxidation to lactic acid in the presence or absence of catalysts as well as for the formation of side products are discussed. The available experimental kinetics data are shown to fit the mechanism with the formation of glyceraldehyde from glycerol alkoxide as the rate-determining step.

Local charge transfer within a covalent organic framework and Pt nanoparticles promoting interfacial catalysis

A pyridine-functionalized covalent organic framework encapsulating Pt nanoparticles with local charge transfer was developed, which efficiently catalyzed H2 production from ammonia borane hydrolysis in water.

Metallosupramolecular polymers as precursors for platinum nanocomposites

Nanocomposites comprised of a polymer matrix and metallic nanoparticles (NPs) can merge the structural features of the matrix material with the functional characteristics of the NPs. While such materials are...

Reaction of H2 with polyoxometalate supported Rhodium(0) and Iridium(0) nanoparticles in aqueous suspensions: a kinetic study

The mechanism of the reaction between Rh0 and Ir0 NPs with H2 was measured in the absence of an electrical bias via monitoring the catalytic reduction of PW12O403− and it was compared to the previous results of Pt0 NPs.