Efficient Low-Temperature H-2 Production from HCOOH/HCOO- by [Pd-0@SiO2-Gallic Acid] Nanohybrids: Catalysis and the Underlying Thermodynamics and Mechanism

摘要

Hybrid Pd-0-based nanoparticles have been synthesized in aqueous solution by two routes: (a) reduction of Pd ions by gallic acid (GA) producing Pd-0-GA and (b) Pd-0 formed on SiO2 GA nanohybrids where GA was covalently grafted on SiO2 nanoparticles (Pd-0@SiO2-GA). In both protocols, Pd-0 nanoparticles were formed in situ, under alkaline pH, via reduction of Pd2+ ions by GA radicals formed by atmospheric O-2. XRD and TEM data show that the Pd-0@SiO2-GA consists of 6.5 nm Pd-0 nanoparticles finely dispersed on the SiO2-GA nanosupport, whereas Pd-0-GA consists of aggregated 12 nm Pd-0 nanoparticles. The two families of Pd-0 nanohybrids have been studied for catalytic H-2 production from formic acid/sodium formate in aqueous solution at near ambient temperatures 40-80 degrees C. Pd-0@SiO(2)GA achieves H-2 production from NaCOOH/HCOOH at 19 mL/min per mg of Pd. This outperforms by a factor of 400% the H-2 production by (Pd-0-GA) particles, as well as all Pd-0-SiO2 catalysts, so far reported in the literature. The Pd-0@SiO2-GA catalyst faces a significantly lower activation barrier (E-a = 42 kJ/mol) compared to Ea = 54 kJ/mol for Pd-0-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 Pd-0 nanoparticles impose a fine dispersion of the Pd-0 nanocatalysts on the surface, and (ii) surface-grafted GA moieties not capped by Pd-0 provide cocatalytic agents that promote the HCOOH deprotonation. From the engineering point of view, the superior H-2 production rate of the Pd-0@SiO2-GA system is due to two factors: (i) the lower thermodynamic barrier, which is due to the cocatalytic GA moieties not capped by Pd-0 particles, and (ii) fine dispersion of the Pd-0 nanoparticles on the SiO2 surface optimizes the kinetics of the reaction.