Efficient Low-Temperature H₂ Production from HCOOH/HCOO^– by [Pd⁰@SiO₂‑Gallic Acid] Nanohybrids: Catalysis and the Underlying Thermodynamics and Mechanism

Hybrid Pd⁰-based nanoparticles have been synthesized in aqueous solution by two routes: (a) reduction of Pd ions by gallic acid (GA) producing Pd⁰-GA and (b) Pd⁰ formed on SiO₂-GA nanohybrids where GA was covalently grafted on SiO₂ nanoparticles (Pd⁰@­SiO₂-GA). In both protocols, Pd⁰ nanoparticles were formed <i>in situ,</i> under alkaline pH, via reduction of Pd²⁺ ions by GA radicals formed by atmospheric O₂. XRD and TEM data show that the Pd⁰@­SiO₂-GA consists of 6.5 nm Pd⁰ nanoparticles finely dispersed on the SiO₂-GA nanosupport, whereas Pd⁰-GA consists of aggregated 12 nm Pd⁰ nanoparticles. The two families of Pd⁰ nanohybrids have been studied for catalytic H₂ production from formic acid/​sodium formate in aqueous solution at near ambient temperatures 40–80 °C. Pd⁰@­SiO₂-GA achieves H₂ production from NaCOOH/​HCOOH at 19 mL/min per mg of Pd. This outperforms by a factor of 400% the H₂ production by (Pd⁰-GA) particles, as well as all Pd⁰-SiO₂ catalysts, so far reported in the literature. The Pd⁰@­SiO₂-GA catalyst faces a significantly lower activation barrier (<i>E</i>_a = 42 kJ/mol) compared to <i>E</i>_a = 54 kJ/mol for Pd⁰-GA. A physicochemical mechanism is discussed which entails the involvement of CO₂/​HCO₃^–, as well as an active cocatalytic effect of gallic acid as proton shuttle. The results reveal that the SiO₂-GA matrix plays a dual role: (i) GA moieties capped by Pd⁰ nanoparticles impose a fine dispersion of the Pd⁰ nanocatalysts on the surface, and (ii) surface-grafted GA moieties <i>not capped</i> by Pd⁰ provide cocatalytic agents that promote the HCOOH deprotonation. From the engineering point of view, the superior H₂ production rate of the Pd⁰@­SiO₂-GA system is due to two factors: (i) the lower thermodynamic barrier, which is due to the cocatalytic GA moieties not capped by Pd⁰ particles, and (ii) fine dispersion of the Pd⁰ nanoparticles on the SiO₂ surface optimizes the kinetics of the reaction

Antioxidant and Antiradical SiO₂ Nanoparticles Covalently Functionalized with Gallic Acid

Gallic acid (GA) and its derivatives are natural polyphenolic substances widely used as antioxidants in nutrients, medicine and polymers. Here, nanoantioxidant materials are engineered by covalently grafting GA on SiO₂ nanoparticles (NPs). A proof-of-concept is provided herein, using four types of well-characterized SiO₂ NPs of specific surface area (SSA) 96–352 m²/g. All such hybrid SiO₂-GA NPs had the same surface density of GA molecules (∼1 GA per nm²). The radical-scavenging capacity (RSC) of the SiO₂-GA NPs was quantified in comparison with pure GA based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH^•) radical method, using electron paramagnetic resonance (EPR) and UV–vis spectroscopy. The scavenging of DPPH radicals by these nanoantioxidant SiO₂-GA NPs showed mixed-phase kinetics: An initial fast-phase (<i>t</i>_1/2 <1 min) corresponding to a H-Atom Transfer (HAT) mechanism, followed by a slow-phase attributed to secondary radical–radical reactions. The slow-reactions resulted in radical-induced NP agglomeration, that was more prominent for high-SSA NPs. After their interaction with DPPH radicals, the nanoantioxidant particles can be reused by simple washing with no impairment of their RSC

Proton-Promoted Iron Dissolution from Nanoparticles and the Influence by the Local Iron Environment

Nanostructured iron-containing compounds are promising for food fortification and supplementation to alleviate iron deficiency due to their fast dissolution in dilute acid and high dietary iron bioavailability. Furthermore, when such compounds are encapsulated in a nano-CaO matrix, their dissolution rate is increased. Here the relation between that rate and iron structure (amorphous/crystalline Fe₂O₃, crystalline Ca₂Fe₂O₅, or monomeric Fe³⁺ inside CaO) is investigated. We used X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy as complementary techniques to study the local iron environment in Ca/Fe oxides as a function of nanoparticle composition. Nanostructured mixed Ca/Fe oxide-containing powders were prepared by flame spray pyrolysis, and their dissolution over time in acidic solutions (pH 1 and 3) was monitored by EPR spectroscopy. Three types of Fe were distinguished in these as-prepared powders: monomeric Fe³⁺ and crystalline Ca₂Fe₂O₅ at low Fe content powders (Ca:Fe ≥ 3.6) and amorphous/crystalline Fe₂O₃ at Ca:Fe ≤ 0.7. During dissolution, monomeric Fe³⁺ and crystalline Ca₂Fe₂O₅ dissolved rapidly (<1 min), while crystalline Fe₂O₃ was more stable and only slowly released Fe³⁺ even at pH 1. The Fe release is discussed within a thermodynamic model based on the nanoparticle lattice energy for each of the nanocrystalline phases, revealing that Fe coordination and lattice dynamics play a more dominant role than particle size. Thus, we demonstrate that control of crystalline structure rather than “nanosizing” may be a prerequisite for rapid dissolution of ferric iron from nanoparticles

MOF-Derived Defective Co₃O₄ Nanosheets in Carbon Nitride Nanocomposites for CO₂ Photoreduction and H₂ Production

In photocatalysis, especially in CO2 reduction and H2 production, the development of multicomponent nanomaterials provides great opportunities to tune many critical parameters toward increased activity. This work reports the development of tunable organic/inorganic heterojunctions comprised of cobalt oxides (Co3O4) of varying morphology and modified carbon nitride (CN), targeting on optimizing their response under UV–visible irradiation. MOF structures were used as precursors for the synthesis of Co3O4. A facile solvothermal approach allowed the development of ultrathin two-dimensional (2D) Co3O4 nanosheets (Co3O4-NS). The optimized CN and Co3O4 structures were coupled forming heterojunctions, and the content of each part was optimized. Activity was significantly improved in the nanocomposites bearing Co3O4-NS compared with the corresponding bulk Co3O4/CN composites. Transient absorption spectroscopy revealed a 100-fold increase in charge carrier lifetime on Co3O4-NS sites in the composite compared with the bare Co3O4-NS. The improved photocatalytic activity in H2 production and CO2 reduction is linked with (a) the larger interface imposed from the matching 2D structure of Co3O4-NS and the planar surface of CN, (b) improvements in charge carrier lifetime, and (c) the enhanced CO2 adsorption. The study highlights the importance of MOF structures used as precursors in forming advanced materials and the stepwise functionalization of the individual parts in nanocomposites for the development of materials with superior activity

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil–water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil–water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil–water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil–water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach