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
journal contribution
posted on 2016-09-09, 00:00 authored by Panagiota Stathi, Maria Louloudi, Yiannis DeligiannakisHybrid 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.
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Mechanism Hybrid Pd 0Pd 0 nanoparticlesgallic acidPd 0 nanocatalystsHCOOHPd 0 nanohybridsH 2 production rateTEM data showXRDSiO 2 catalystsH 2 productionaggregated 12 nm Pd 0 nanoparticlesPd 0surface-grafted GA moietiesSiO 2 surface optimizesHCOCOSiO 2 nanoparticlescocatalytic GA moietiesEfficient Low-Temperature H 2 Production6.5 nm Pd 0 nanoparticlesSiO 2Pd 0 particles
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