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Formic acid (FA) is a promising hydrogen carrier because it contains 4.3 wt % H2 (53 g H2/L) and releases hydrogen under mild conditions (<80 °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 H2 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.

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