Strategy for Efficient H2 Production from a Mixture of Formic Acid and Formate using Operando pH Measurements

Formic acid (FA) is a promising hydrogen carrier because it contains 4.3 wt % H-2 (53 g H-2/L) and releases hydrogen under mild conditions (<80 degrees C). Previous studies revealed that the coexistence of formate during FA dehydrogenation increases the evolved gas quantity and reaction rate. Most of these studies considered formate a promoter, notwithstanding that formate is a source of H-2 and can be dehydrogenated by reacting with water molecules under Pd nanoparticle catalysis. Moreover, formate is considered an intermediate species during FA dehydrogenation. Although the reaction pathways of the dehydrogenation of admixtures of FA and formate are diverse, the determinants thereof remain elusive. We demonstrate that the system proton concentration determines the hydrogen generation pathway. The system pH and composition of the accumulated gas were measured under operando conditions. We introduce the threshold point, defined by the specific pH values at which the dehydrogenation pathway utilizes formate instead of FA. The quantity and composition of the produced gas can be actively controlled by tuning the reaction system pH using the threshold point as a standard. Operando monitoring of the pH can help discern whether the gas evolution from the mixture originates from chemical equilibrium or catalyst deactivation.

Automated summary

This study reveals that the proton concentration in the system determines the pathway of hydrogen generation during formic acid dehydrogenation. The quantity and composition of the produced gas can be actively controlled by adjusting the pH of the reaction system. Operando monitoring of the pH can help determine the origin of gas evolution.