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

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journal contribution
posted on 09.09.2016 by Panagiota Stathi, Maria Louloudi, Yiannis Deligiannakis
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 (Ea = 42 kJ/mol) compared to Ea = 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.