Platinum-zeolite hybrid catalyst for the electrooxidation of formic acid

Formic acid oxidation reaction (FAOR) was studied using different platinum-zeolites hybrid catalysts. The catalysts were prepared by addition of two different forms of the ZSM-5 zeolite (ZSM-5-H+ and ZSM-5-NH4+) to an ink containing Vulcan carbon support and Pt nanoparticles (NPs). TEM images demonstrate a proper mixture of the components in the ink and a good dispersion of the Pt NPs mainly over the carbon. Both zeolites (ZSM-5NH(4)(+) and ZSM-5-H+) are stable under electrochemical conditions for FAOR, but ZSM-5-NH4+ undergoes fast conversion into ZSM-5-H+ due to the exchange between the compensator cation in the zeolite and protons in solution. A significant enhancement on the catalytic activity was found for both zeolite hybrid catalysts, being the ink containing ZSM-5-H+ the more stable over time. In particular, 7 and 10 times larger current densities than on 9 wt% Pt/C were achieved by the (7 wt% Pt/18.5 wt% ZSM-5-NH4+/C) and the (7 wt% Pt/18.5 wt% ZSM-5H(+)/C) catalysts in chronoamperometry, respectively. The results demonstrated the beneficial and synergetic effect of the addition of zeolites in order to improve the catalytic activity of Pt NPs. The addition of zeolite ZSM-5 within the electrocatalyst did not provoke any relevant modification in the type of features displayed in cyclic voltammetry, but provoked a significant enhancement of the peak current density associated with the CO stripping. We propose that the mechanism for this improvement lies in a more efficient distribution of Pt nanoparticles on the catalytic ink thanks to the presence of zeolites and in the preconcentration of the reactant (HCOOH) within the zeolite microporosity occurring in solution. Moreover, local pH modifications associated with the NH4+/H+ exchange could have some impact on the kinetics of the FAOR by the direct via.