Efficient Low-Temperature H<sub>2</sub> Production from HCOOH/HCOO<sup>–</sup> by [Pd<sup>0</sup>@SiO<sub>2</sub>‑Gallic Acid] Nanohybrids: Catalysis and the Underlying Thermodynamics and Mechanism

Hybrid Pd<sup>0</sup>-based nanoparticles have been synthesized in aqueous solution by two routes: (a) reduction of Pd ions by gallic acid (GA) producing Pd<sup>0</sup>-GA and (b) Pd<sup>0</sup> formed on SiO<sub>2</sub>-GA nanohybrids where GA was covalently grafted on SiO<sub>2</sub> nanoparticles (Pd<sup>0</sup>@­SiO<sub>2</sub>-GA). In both protocols, Pd<sup>0</sup> nanoparticles were formed <i>in situ,</i> under alkaline pH, via reduction of Pd<sup>2+</sup> ions by GA radicals formed by atmospheric O<sub>2</sub>. XRD and TEM data show that the Pd<sup>0</sup>@­SiO<sub>2</sub>-GA consists of 6.5 nm Pd<sup>0</sup> nanoparticles finely dispersed on the SiO<sub>2</sub>-GA nanosupport, whereas Pd<sup>0</sup>-GA consists of aggregated 12 nm Pd<sup>0</sup> nanoparticles. The two families of Pd<sup>0</sup> nanohybrids have been studied for catalytic H<sub>2</sub> production from formic acid/​sodium formate in aqueous solution at near ambient temperatures 40–80 °C. Pd<sup>0</sup>@­SiO<sub>2</sub>-GA achieves H<sub>2</sub> production from NaCOOH/​HCOOH at 19 mL/min per mg of Pd. This outperforms by a factor of 400% the H<sub>2</sub> production by (Pd<sup>0</sup>-GA) particles, as well as all Pd<sup>0</sup>-SiO<sub>2</sub> catalysts, so far reported in the literature. The Pd<sup>0</sup>@­SiO<sub>2</sub>-GA catalyst faces a significantly lower activation barrier (<i>E</i><sub>a</sub> = 42 kJ/mol) compared to <i>E</i><sub>a</sub> = 54 kJ/mol for Pd<sup>0</sup>-GA. A physicochemical mechanism is discussed which entails the involvement of CO<sub>2</sub>/​HCO<sub>3</sub><sup>–</sup>, as well as an active cocatalytic effect of gallic acid as proton shuttle. The results reveal that the SiO<sub>2</sub>-GA matrix plays a dual role: (i) GA moieties capped by Pd<sup>0</sup> nanoparticles impose a fine dispersion of the Pd<sup>0</sup> nanocatalysts on the surface, and (ii) surface-grafted GA moieties <i>not capped</i> by Pd<sup>0</sup> provide cocatalytic agents that promote the HCOOH deprotonation. From the engineering point of view, the superior H<sub>2</sub> production rate of the Pd<sup>0</sup>@­SiO<sub>2</sub>-GA system is due to two factors: (i) the lower thermodynamic barrier, which is due to the cocatalytic GA moieties not capped by Pd<sup>0</sup> particles, and (ii) fine dispersion of the Pd<sup>0</sup> nanoparticles on the SiO<sub>2</sub> surface optimizes the kinetics of the reaction.