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 in situ, 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 (E_a = 42 kJ/mol) compared to E_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 not capped 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.