Selective Formic Acid Decomposition for High‐Pressure Hydrogen Generation: A Mechanistic Study

Abstract

A homogenous catalytic system has been developed that efficiently and selectively decomposes formic acid into hydrogen and carbon dioxide. [Ru(H(2)O)(6)](2+), [Ru(H(2)O)(6)](3+) and RuCl(3) x xH(2)O are all excellent pre-catalysts in presence of TPPTS (TPPTS = meta-trisulfonated triphenylphosphine), the formic acid decomposition taking place in the aqueous phase, under mild conditions and over a large range of pressures. Optimisation of the reaction conditions is described together with a detailed mechanistic study leading to a tentative catalytic cycle. The performance of the catalytic system for continuous hydrogen generation is presented. Overall, the method proposed overcomes the limitations of other catalysts for the decomposition of formic acid making it a viable hydrogen-storage material.

Decomposition of Formic Acid Catalyzed by a Phosphine‐Free Ruthenium Complex in a Task‐Specific Ionic Liquid

“…[29] Water plays an important role in this catalytic reaction; the addition of 1 mL of water had a detrimental effect (Table 1, entry 3), in contrast to Laurenczy's system, which operates in water. [16] In our system, excess water gave lower conversions, and after 24 h, only 41 % of the FA was dehydrogenated at 80 8C, which corroborates a recent report using the same catalytic system in the presence of an extra base. [25] In this case, the excess of water may make difficult the formation of catalytic active species in the medium, probably as a result of solvation or coordination processes that have a negative effect on the reaction.…”

“…Although no transition species containing the ILÀamine as a ligand were detected by electrospray ionization mass spectrometry (ESI-MS) analysis (see later), the second (site-blocking) aspect is in agreement with previous observations, where excess ligands such as phosphine blocked the catalyst active site and drastically lowered the conversion. [16] Moreover, the addition of phosphine ligands to our system completely inhibited the catalytic decomposition of FA. In other words, the active sites of the ruthenium species are blocked by strongly coordinating phosphine ligands.…”

“…[1] In particular for portable applications, the use of liquid hydrogen has several disadvantages due to its continuous evaporation. Among various storage materials and methods currently under investigation, molecular hydrogen adsorption on materials of large surface area [5][6][7][8] and clathrate hydrates, [9] the use of bonded hydrogen atoms in hydrocarbons, [10] metal hydrides [11,12] or formic acid (FA) [13][14][15][16][17] show considerable promise.…”

“…[18][19][20][21][22] FA decomposition to hydrogen and carbon dioxide can easily be catalyzed by several homogeneous [14,16,[18][19][20][21][22][23] and heterogeneous catalysts. [24] In particular, Ru phosphine complexes in the presence of a base, such as triethylamine or sodium formate, in water are quite effective catalysts for this transformation.…”

Dehydrogenation of Formic Acid over a Homogeneous Ru-TPPTS Catalyst: Unwanted CO Production and Its Successful Removal by PROX

“…mTPPTS was chosen as a phosphine-ligand because of its high stability and water solubility of the resulting catalytically active complex [7]. Previously, it was reported that this catalyst was stable over more than one year of intermittent use, while being kept in air [27]. The turnover frequencies (TOF's) reached in a continuous mode were 210 h −1 and 670 h −1 at 100 • C and 120 • C, respectively [27].…”