Transition metal nanoparticle catalysts in releasing hydrogen from the methanolysis of ammonia borane

Ammonia borane (H₃N·BH₃, AB) is one of the promising hydrogen storage materials due to high hydrogen storage capacity (19.6% wt), high stability in solid state as well as in solution and nontoxicity. The methanolysis of AB is an alternative way of releasing H₂ due to many advantages over the hydrolysis such as having high stability against self releasing hydrogen gas. Here we review the reports on using various noble or non-noble metal(0) catalysts for H₂ release from the methanolysis of AB. Ni(0), Pd(0), and Ru(0) nanoparticles (NPs), stabilized as colloidal dispersion in methanol, are highly active and long lived catalysts in the methanolysis of AB. The catalytic activity, lifetime and reusability of transition metal(0) NPs show significant improvement when supported on the surface of solid materials. The supported cobalt, nickel, copper, palladium, and ruthenium based catalysts are quite active in H₂ release from the methanolysis of AB. Rh(0) NPs are highly active catalysts in releasing H₂ from the methanolysis of AB when confined within the void spaces of zeolite or supported on oxide nanopowders such as nanosilica, nanohydroxyapatite, nanoalumina or nanoceria. The oxide supported Rh(0) NPs can provide high activity with turnover frequency values as high as 218 min⁻¹ and long lifetime with total turnover values up to 26,000 in generation of H₂ from the methanolysis of AB at 25 °C. When deposited on carbon the bimetallic AgPd alloy nanoparticles have the highest activity in releasing H₂ through the methanolysis of AB.     

Efficient hydrogen evolution from ammonia borane hydrolysis with Rh decorated on phosphorus-doped carbon

Development of supported ligand-free ultrafine Rh nanocatalysts for efficient catalytic hydrogen evolution from ammonia borane (AB) is of importance but remains a tremendous challenge. Here, ultrafine and ligand-free Rh nanoparticles (NPs) (2.19 nm in diameter) were in-situ decorated on porous phosphorus-functionalized carbon (PPC) prepared by pyrolyzing hyper-cross-linked networks of triphenylphosphine and benzene. The resultant Rh/PPC showed excellent hydrogen production activity from AB hydrolysis (Turnover frequency: 806 min⁻¹). Kinetic investigations indicated that AB hydrolysis using Rh/PPC exhibited first-order and zero-order reactions with Rh and AB concentrations, respectively. Activation energy (E_a) toward hydrogen generation from AB with Rh/PPC is as low as 22.7 kJ/mol. The Rh/PPC catalyst was recyclable and reusable for at least four times. The oxygen- and phosphorus-functional groups are beneficial for the affinity of Rh complex on the PPC surface, resulting in ultrafine and ligand-free Rh NPs with high dispersity and ability to supply abundant surface accessibility to catalytically active sites for AB hydrolysis. This study proposes a feasible approach for the synthesis of ultrafine and ligand-free metal NPs supported on heteroatom-doped carbon by using hyper-cross-linked networks.     

Preparation and characterization of amine-terminated delafossite type oxide,CuMnO2–NH2, supported Pd (0) nanoparticles for the H2 generation from the methanolysis of ammonia-borane

Among the cuprous metal oxides of the delafossite type, which are generally formulated as CuMO₂ (M = Al, Cr, Fe, Ga, Mn), the most promising is the CuMnO₂ structures. CuMnO₂ material, named crednerite, is a very interesting delafossite derivative with potential applications in many fields, mainly catalyst, photoelectrochemical cells and multiferroic tools. Herein, we report fabrication, characterization, and application of amine-terminated CuMnO₂ (CuMnO₂–NH₂) supported palladium nanoparticles (Pd/CuMnO₂–NH₂) as highly efficient and recyclable catalysts for the hydrogen production from the methanolysis of ammonia-borane (AB). The results of characterization using P-XRD, TEM, HRTEM, TEM-EDX, XPS, SEM, SEM-elemental mapping, and ICP-OES disclose that Pd (0) nanoparticles were well spread on the surface of CuMnO₂–NH₂ with a mean particle size of 3.91 ± 0.33 nm. Pd/CuMnO₂–NH₂ shows high catalytic activity in the methanolysis of AB with an initial turnover frequency of 146.68 min^?1 at 25 ± 0.1 °C which is one of the highest values ever reported for AB methanolysis in the literature. Besides, the extreme stability of Pd/CuMnO₂–NH₂ takes it a recyclable heterogeneous catalyst in this catalytic conversion.     

Pd nanoparticles anchoring to core-shell Fe3O4@SiO2-porous carbon catalysts for ammonia borane hydrolysis

A modified Stöber method is applied to synthesize the magnetic core-shell Fe₃O₄@SiO₂ particles, followed by compositing a series of porous glucose-derived carbon with ZnCl₂ as etchant. Then, ultrafine Pd nanoparticles (NPs) are successfully anchored to the resulting Fe₃O₄@SiO₂-PC composites with an in-situ reduction strategy. The particle sizes of Pd NPs are mainly centered in the range of 2.3–4.3 nm in the as-prepared Pd/Fe₃O₄@SiO₂-PC catalysts, owning a hierarchical porous structure with high specific surface area (S_BET = 626.0 m² g⁻¹) and large pore volume (V_p = 0.61 cm³ g⁻¹). Their catalytic behavior for the hydrogen generation from ammonia borane (AB) hydrolysis is investigated in details. The corresponding apparent activation energy is as low as 28.4 kJ mol⁻¹ and the reaction orders with AB and Pd concentrations are near zero and 1.10 under the present conditions, respectively. In addition, the magnetic catalysts, which could be easily separated out by a magnet, are still highly active even after nine runs, revealing their excellent reusability.     

Anchoring and space-confinement effects to synthesize ultrasmall Pd nanoparticles for efficient ammonia borane hydrolysis

Hydrogen production from ammonia borane (AB) hydrolysis catalyzed by efficient heterogenous catalysts is regarded as a compelling strategy to meet the increasing requirement for clean energy. Palladium (Pd) nanoparticle (NP)-based catalysts have stimulated intensive attention for AB hydrolysis, while their catalytic performances still need to be significantly improved. By exploiting sodium hydroxide and three-dimensional (3D) architecture of interconnected porous carbon nanosheets (IPCNs) as a NP carrier, a simple yet efficient strategy is developed to synthesize uniformly distributed ligand-free Pd NPs (2.17 nm in diameter) for hydrogen generation from AB hydrolysis. The particle size and spatial dispersion control of Pd NPs on the IPCNs surface supply an abundance of surface-active sites and thus remarkable improve the catalytic performance for AB hydrolysis to hydrogen evolution. Specifically, the achieved Pd/IPCNs reveals an extremely high catalytic activity with a turnover frequency (TOF) of 122.8 min⁻¹ toward AB hydrolysis, which is higher than that of many reported Pd-based catalysts. This simple, straightforward and efficient method is of significant importance for preparing metal NP catalysts with high catalytic activities for catalytic applications.     

Synergistic catalysis of Pd–Ni(OH)2 hybrid anchored on porous carbon for hydrogen evolution from the dehydrogenation of formic acid

The development of a facile yet efficient strategy to boost the catalytic performance of supported Pd nanoparticles (NPs) toward the dehydrogenation of formic acid (FA) is essential but remains challenging. Here, a novel hybrid nanocatalyst comprising Pd and Ni(OH)₂ supported on porous carbon (PC) is developed. The obtained Pd–Ni(OH)₂/PC nanocatalyst exhibits an excellent catalytic performance for FA dehydrogenation to produce hydrogen. The introduction of Ni(OH)₂ in PC support can significantly promote the catalytic activity of Pd NPs toward FA dehydrogenation. Additionally, the catalytic property of Pd–Ni(OH)₂/PC is correlated with the Pd/Ni ratio. The ₂Pd–₁Ni(OH)₂/PC with the optimum Pd/Ni ratio of 2/1 exhibits the maximum turnover frequency (TOF) of 3409 h⁻¹ at 60 °C for FA dehydrogenation. The highly dispersed ultrafine Pd–Ni(OH)₂ hybrid NPs with numerous accessible active sites and Ni(OH)₂−induced positive synergetic effects with Pd NPs considerably boost the catalytic performance for FA dehydrogenation.     

Methanolysis of ammonia borane catalyzed by magnetically isolable RHODIUM(0) nanoparticles

This article reports the preparation and employment of rhodium (0) nanoparticles (Rh⁰NPs) on the surface of magnetite nanospheres, denoted as Rh⁰@Fe₃O₄, as magnetically isolable nanocatalyst in the methanolysis of ammonia borane (MAB). The monodispersed Fe₃O₄ nanospheres are fabricated by a simple technique and used as nanosupport for Rh⁰NPs which are well stabilized and homogeneously distributed on the surface of nanospheres with a mean particle size of 2.8 ± 0.5 nm. The as-synthesized Rh⁰@Fe₃O₄ has a remarkable TOF value of 184 min⁻¹ in the MAB to produce H₂ gas in RT. Most of all, Rh⁰@Fe₃O₄ nanocatalyst can be reused, evolving 3.0 mol of H₂ gas for a mole of AB, keeping 100% of its initial activity even in the fourth reuse of MAB at 25 °C. Recovery of the Rh⁰@Fe₃O₄ nanocatalyst can be accomplished by simply approaching an external magnet, which eliminates many laborious catalyst removal steps in catalytic reactions. Reported are the outcomes of kinetic investigation, done by altering the concentration of substrate and catalyst together with temperature. Kinetic studies reveal that the catalytic MAB shows dependence on the concentration of reactants and temperature.     

TiO2–CdS supported CuNi nanoparticles as a highly efficient catalyst for hydrolysis of ammonia borane under visible-light irradiation

TiO₂–CdS nanotubes (NTs) were used for the first time as a support to load metal nanoparticles (NPs) for the hydrolysis of ammonia borane (AB) which is a new strategy. The TiO₂–CdS NTs support was first synthesized using a hydrothermal method, and then the CuNi NPs were loaded using a liquid-phase reduction method. The synthesized samples were characterized by XRD, SEM-EDS, TEM, XPS, ICP, UV–Vis, and PL analyses. The characterization results show that the CuNi NPs existed in the form of an alloy with a size of ~1.2 nm and uniformly dispersed on the support. Compared with their single metal counterparts, the bimetallic CuNi-supported catalysts showed a higher catalytic activity in the hydrolysis of AB under visible-light irradiation: Cu_0·45Ni_0·55/TiO₂–CdS catalyst had the fastest hydrogen evolution rate with a high conversion frequency (TOF) of 25.9 mol_H2·mol_cat⁻¹ min⁻¹ at 25 °C and low activation energy of 32.8 kJ mol⁻¹. Cu_0.45Ni_0.55/TiO₂–CdS catalyst showed good recycle performance, maintaining 99.3% and 85.6% of the original hydrogen evolution rate even after five and ten recycles, respectively. Strong absorption of visible light, improved electron–hole separation efficiency, and metal synergy between Cu and Ni elements played a crucial role in improving the catalytic hydrolysis performance of AB. The catalyst prepared in this study provides a new strategy for the application of photocatalysts.     

Monodisperse PtCu alloy nanoparticles as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Pt-M alloy nanoparticles (NPs) with well-defined size and compositions exhibit dramatically catalytic performance in chemical reactions. In this work, monodisperse PtCu NPs with controlled size and compositions were synthesized by the co-reduction method in the presence of oleylamine. These NPs have excellent catalytic activities in the hydrolytic dehydrogenation of ammonia borane (AB) and their activities were composition dependent. Among the different-composition PtCu NPs, the Cu₅₀Pt₅₀ NPs exhibit the highest catalytic activity with an initial turnover frequency of 102.5 mol_(hydrogen)·mol_(catalyst)^?1·min^?1 and an apparent activation energy of 36 kJ·mol^?1, which demonstrate the validity of partly replacing Pt by a first-row transition metal on constructing high performance heterogeneous nanocatalysts for the hydrolytic dehydrogenation of AB.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

Pd nanoparticles supported on MIL-101 as high-performance catalysts for catalytic hydrolysis of ammonia borane

Well dispersed ultrafine Pd NPs have been immobilized in the framework of MIL-101, and tested for the catalytic hydrolysis of ammonia borane. The powder XRD, N₂ adsorption–desorption, TEM, and ICP-AES were employed to characterize the Pd@MIL-101 catalyst. The as-synthesized Pd@MIL-101 exhibit the highest catalytic activity toward hydrolysis of AB among the Pd-based nano-catalysts ever reported, with the TOF value of 45 mol H₂ min⁻¹ (mol Pd)⁻¹.     

Hydrogen generation via the catalytic hydrolysis of morpholine-borane: A new,efficient and cost-effective hydrogen storage medium

Herein, for the first time, we introduce the morpholine-borane complex (MB) as a new, efficient, cost-effective and commercially available chemical hydrogen storage material for mobile applications. In this regard, hydrogen production from the hydrolysis of MB catalyzed by in situ generated water-soluble polymer stabilized Ag(0) and Pd(0) nanoparticles (NPs) is reported for the first time. In situ generated PSMA-stabilized Ag(0) and Pd(0) NPs showed remarkable activity in hydrogen production from the hydrolysis of MB at room temperature, providing initial TOFs of 16.1 min⁻¹ and 37.3 min⁻¹, respectively. A set of kinetic studies on the catalytic hydrolysis of MB were conducted by changing the catalyst/substrate amount and temperature, and the rate law expression and activation parameters were produced by collecting the kinetic data. The apparent activation energies for the in situ generated Ag(0) and Pd(0) NPs catalyzed MB hydrolysis were calculated to be 71.4 and 32.5 kJ mol⁻¹, respectively.     

Facile and eco-friendly synthesis of porous carbon nanosheets as ideal platform for stabilizing rhodium nanoparticles in efficient hydrolysis of ammonia borane

Carbon materials have been demonstrated as excellent carriers for preparing supported metal nanocatalysts in catalytic applications. However, numerous chemical activators including strong acids and bases were applied, leading to the entire process dangerous and hazardous. Eco-friendly, economic, and convenient synthesis of carbon materials with desired properties as supports for metal nanoparticle (NP) stabilization to boost performance is important but remains challenging. Here, we developed a facile and eco-friendly strategy to synthesize porous carbon nanosheets (PCNs) with ultrahigh specific surface area (2575.1 m²/g) via pyrolysis the mixture of potassium oxalate and glucose. The resultant PCNs can be used as ideal platform for in-situ distribution of small Rh NPs (Rh/PCNs) as efficient catalysts in hydrogen production from ammonia borane (AB) under ambient conditions. Specifically, Rh/PCNs displayed high activity for AB hydrolysis, with a turnover frequency (TOF) of 513.2 min⁻¹. Small and well-distributed Rh NPs on PCNs with large catalytically active surface atoms are contributed to the high catalytic property of Rh/PCNs for the reaction. Present study has demonstrated that the PCNs is a superior catalyst support for preparing a series of metal NPs in other catalytic applications beyond hydrolysis reaction.     

Synthesis of Fe0.3Co0.7/rGO nanoparticles as a high performance catalyst for the hydrolytic dehydrogenation of ammonia borane

Well-dispersed Fe_0.3Co_0.7/rGO nanocatalysts have been synthesized utilizing the two-step reduction method and successfully employed in the hydrolysis of ammonia borane (NH₃BH₃ AB) at room temperature. The mass percent of the supported Fe_0.3Co_0.7 nanoparticles (NPs) on graphene (rGO) sheets can reach to the maximum value of 50 wt%. The as-synthesized catalysts exerted satisfying activity and reusability for the hydrolytic dehydrogenation of AB at 298 K, especially for the specimen of 50 wt% Fe_0.3Co_0.7/rGO NPs. The catalytic hydrolysis reaction was rapidly completed within 1 min.     

Effective hydrolysis of ammonia borane catalyzed by ruthenium nanoparticles immobilized on graphic carbon nitride

Graphic carbon nitride prepared by the thermal decomposition of urea was used a catalyst support for the in situ immobilization of Ru nanoparticles (NPs) (Ru/g-C₃N₄). The catalytic property of Ru/g-C₃N₄ was investigated in the hydrolysis of ammonia borane (AB) in an aqueous solution under mild conditions. Results show that the in situ generated Ru NPs are well dispersed on the surface of g-C₃N₄ with a mean particle size of 2.8 nm. The catalytic performance for AB hydrolysis indicates that 3.28 wt% Ru/g-C₃N₄ exhibits excellent catalytic activity with a high turnover frequency number of 313.0 mol H₂ (mol Ru·min)⁻¹ at room temperature. This strategy may provide an eco-friendly catalytic system for developing a sustainable catalytic route to hydrogen production.     

Metal organic framework derived nitrogen-doped carbon anchored palladium nanoparticles for ambient temperature formic acid decomposition

Well-dispersed palladium nanoparticles (NPs) anchored on a porous N-doped carbon is prepared by wet chemical method, using metal organic frameworks (ZIF-8) as a precursor to derive the porous N-doped carbon support. Benefitting from the N-doping and the porous structure of the carbon materials, the final Pd NPs are in high dispersion and exhibit reduced particle sizes, with electronic structure and chemical status tuned to favor the formic acid decomposition (FAD). The prepared Pd/C_ZIF-8-950 catalysts show enhanced catalytic performance and selectivity for FAD, the turnover of frequency (TOF) and the mass activity up to 1166 h⁻¹ and 11.01 mol H₂ g⁻¹ _pd h⁻¹ were obtained at 30 °C. This work provides an effective and easy way for synthesis the Pd-based catalyst, which has enormous application prospects for the next generation hydrogen energy preparation and storage.     

Bimetallic Pd–Bi nanocatalysts derived from NaBH4 reduction of BiOCl and Pd2+ and exhibit highly efficient hydrogen production over formaldehyde solution

Using liquid formaldehyde as a carrier to obtain clean hydrogen is a promising method. The development of inexpensive catalysts with high activity and stability is crucial for this reaction. Herein, bimetallic Pd–Bi nanocatalysts with different Pd to Bi ratios were prepared through one step in-situ reduction of BiOCl and Pd²⁺ ions by sodium borohydride (NaBH₄). The effect of Pd/Bi ratios and reaction parameters such as formaldehyde concentration and sodium hydroxide concentration on hydrogen production performance were systematically studied. By optimizing the Pd contents in Pd–Bi nanocatalysts under the optimized reaction conditions, an much higher hydrogen (H₂) production rate of 472.2 mL min^?1g^?1 over Pd/BiOCl-3% under 298.15 K can be achieved, which is 4.01 times that of pure Pd nanoparticles (NPs) and much higher than most reported metal-based catalysts.     

Poly(N-vinyl-2-pyrrolidone)-stabilized ruthenium supported on bamboo leaf-derived porous carbon for NH3BH3 hydrolysis

In this work, poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized ruthenium nanoparticles (NPs) supported on bamboo leaf-derived porous carbon (Ru/BC) has been synthesized via a one-step procedure. The structure and morphology of the as-synthesized samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM) and transmission electron microscope (TEM). As a catalyst for hydrogen generation from the hydrolysis of ammonia-borane (AB, NH₃BH₃) at room temperature, Ru/BC stabilized with 1 mg of PVP exhibited high activity (TOF = 718 mol_H2·mol_Ru⁻¹·min⁻¹) and low activation energy (E_a = 22.8 kJ mol⁻¹). In addition, the catalyst could be easily recovered and showed fairly good recyclability with 55.6% of the initial catalytic activity retained after ten experimental cycles, which confirmed that PVP could stabilize the Ru NPs and prevent their agglomeration on BC surface. Our results suggest that PVP-stabilized Ru/BC is a highly efficient catalyst for the hydrolysis of AB.     

The rational design of gCN/a-WOx/Pt heterostructured nanophotocatalysts for boosting the hydrogen generation from the hydrolysis of ammonia borane under visible light

In-situ generation of platinum nanoparticles (Pt NPs) supported on graphitic carbon nitride/amorphous tungsten oxide (gCN/a-WO_x) binary heterojunctions under white-light irradiation was performed during the hydrolysis of ammonia borane (HAB). The gCN/a-WO_x/Pt(IV) nanocomposites including different amount of W were prepared to study their comparative photocatalysis for the photocatalytic HAB. The yielded gCN/a-WO_x/Pt nanocatalysts provided a maximum turnover frequency (TOF) value of 419.2 mol H₂ mol Pt⁻¹ min⁻¹, which is higher than that of gCN/Pt nanocatalysts (287.7 mol H₂ mol Pt⁻¹ min⁻¹). Many advanced analytical techniques comprising ICP-MS, TEM, HAADF-STEM, XRD, XPS, EDX, and BET were used to determine the elemental composition, morphology, elemental distribution, crystal structure, chemical/oxidation state of the surface elements and the textural properties of the nanocatalysts. The characterization results support the formation of wrinkled paper-like amorphous phase WO_x (a-WO_x) materials in multiple oxidation states over the gCN nanosheets. The photophysical properties of gCN/a-WO_x nanocomposites were also analyzed by using UV–Vis DRS, PL, and TRES techniques to clarify the contribution of the heterojunction formation between gCN and a-WO_x semiconductors to the photocatalytic activity. Owing to the enhanced visible light absorption, suppressed charge recombination, and promoted charge carrier transfer, gCN/a-WO_x/Pt nanocatalysts boosted the hydrogen production from the HAB under white-light irradiation by providing 419.2 mol H₂ mol Pt⁻¹ min⁻¹ TOF, which is 4.8 times higher compared to the one obtained in dark. A plausible photocatalytic mechanism for the photocatalytic HAB reaction in the presence of gCN/a-WO_x/Pt nanocatalysts was suggested based on the results of performed scavenger experiments. The rate law and the activation parameters for the of gCN/a-WO_x/Pt catalyzed HAB were also reported along with kinetic studies. Additionally, a reusability test was performed to understand the stability of gCN/a-WO_x/Pt nanocatalysts in the HAB such that the significance of a-WO_x species in the enhancement of photocatalytic activity became more pronounced. This study reports for the first time that gCN/a-WO_x heterojunctions are favorable support materials for the in-situ generation of Pt NPs and promoting the photocatalytic activity of Pt NPs in the hydrogen generation from the HAB under white-light illumination.     

Poly(2-aminoethyl methacrylate) based microgels catalyst system to be used in hydrolysis and methanolysis of NaBH4 for H2 generation

The poly(2-aminoethyl methacrylate) (p(AEM)) microgels were synthesized by microemulsion polymerization technique and used for in situ metal nanoparticle preparation to render as p(AEM)-M (M: Co or Ni) microgel composites and were used in p(AEM) based poly ionic liquid (PIL) microgels. Next, these p(AEM)) based microgel materials were used as catalysts for hydrogen (H₂) production from both hydrolysis and methanolysis reactions of sodium borohydride (NaBH₄). It was found that the catalytic hydrolysis of the NaBH₄ reaction, catalyzed by p(AEM)-Co microgel composite was completed in 140 min, whereas the methanolysis of NaBH₄ methanolysis catalyzed by the PIL of p(AEM)⁺Cl⁻ microgels was completed in 5 min both with 250 ± 2 mL H₂ production. Furthermore, p(AEM)-Co microgel composite catalysts maintained 80% catalytic activity after 5 consecutive uses in NaBH₄ hydrolysis. On the other hand, p(AEM)⁺Cl⁻ microgels were found to afford more than 50% catalytic activity even after 20 repetitive use in NaBH₄ methanolysis due to superior regeneration ability. Moreover, activation energy values for p(AEM)-Co microgel composites catalyzed NaBH₄ hydrolysis reaction were calculated as 38.9 kJ/mol in comparison to 37.3 kJ/mol activation energy of p(AEM)⁺Cl⁻ microgel catalyzed methanolysis reaction.