Solution-potential and solution-hydrides as key-parameters in H2 production via HCOOH-dehydrogenation by Fe- and Ru-molecular catalysts.

Formic acid dehydrogenation (FADH) by molecular catalysts is often reported to depend on cocatalysts, which, purportedly, are involved in hydride-catalyst association during the catalytic cycle. Herein we present a study on the effect of externally added hydrides, using NaBH 4 , on HCOOH dehydrogenation by two homologous molecular catalysts, a Fe/PP3/Lg, and a Ru/PP3/Lg catalyst, in propylene carbonate. A systematic mapping of the solution potential, E h , of the catalytic solution system, in tandem with the catalytic H 2 production via FADH, reveals two distinct trends for the Fe- and Ru-catalysts: (i) introduction of NaBH 4 promotes FADH by the [Ru/PP3/Lg] catalyst achieving a boosting of TOF from of 39 h-1 to 876 h-1 while the activation energy barrier drastically decreases from 21.2 to 9.8 kJ/mol 1H NMR confirms that NaBH 4 strongly promotes formation of Ru-H species in the catalytic solution which is the rate-limiting step. (ii) In contrary, introduction of NaBH 4 has no-beneficial effect on FADH catalyzed by the [Fe/PP3/Lg] in conjunction with the finding that solution-hydrides considerably increase the activation energy barrier from 28.4 to 42.1 kJ/mol. All these data are discussed within a novel comprehensive frame in relation to the distinct requirement of a co-catalyst by [Ru/PP3/Lg] catalyst vs. the ability of [Fe/PP3/Lg] catalyst to operate as co-catalyst free, in FADH. The present study indicates the monitoring of the solution potential Eh, is a useful tool to parametrize and to understand the performance of molecular systems in this type of FADH catalysis. [Display omitted] • Solution hydrides boost HCOOH dehydrogenation by [Ru/PP3/Lg] catalyst, lowering the Activation Energy (E a) by 240 %. • Solution hydrides have no beneficial effect on [Fe/PP3/Lg] catalyst, instead they increase the E a by 180 %. • Solution potential E h is a key physicochemical tool for FADH catalysis to parametrize the performance of molecular catalysts. [ABSTRACT FROM AUTHOR]