In the past decades, the mechanistic influence of water on formic acid dehydrogenation to carbon dioxide (and hydrogen) has garnered special attention owing to the putative link with
Introduction
9
water gas shift (WGS)[52–59] and in the context of sustainable hydrogen production and carbon dioxide valorization[60–66] using (aqueous) formic acid fuel cells. One of the first studies establishing the interrelation between WGS and formic acid decomposition was performed on zinc oxide and magnesium oxide.[67] In subsequent studies, de Jong and coworkers examined the kinetics of WGS and formic acid decomposition on Cu/ZnO and concluded that both the reactions proceeded via the same surface intermediate whose decomposition (formate dehydrogenation) limited the rates. This was deduced from the identical temperature dependence and the high carbon dioxide selectivity for formic acid dehydrogenation which correlated with the value (50) for the ratio between the rates of the forward and the reverse shift reactions.[68,69] Formates could be facilely formed upon flowing carbon monoxide over partially hydrated surfaces (magnesium oxide, γ-Alumina) or by flowing carbon dioxide and hydrogen.[68,70] Furthermore, the temperature and the density of formate formation from carbon monoxide were observed to decrease and increase, respectively, with increasing hydroxyl concentration on the surface.
Figure 1.3 Proposed mechanism for WGS on magnesium oxide. Adapted from Reference[54], Copyright (1990), with permission from Elsevier.
Following these works, Iwasawa and coworkers performed in-depth studies of the interrelationship between the two reactions and coined the phrase ‘reactant-promoted reaction mechanism’ in relation to WGS on bare metal oxides and metal oxide supported catalysts.[53,54,56] Using FT-IR, the hydroxyl groups on top of coordinatively unsaturated magnesium atoms were observed to react with carbon monoxide to produce surface formates that were uni-, bridge-bonded or bidentate in configuration.[54] The unidentate formates that were facilely produced at room temperatures transformed to bridge-bonded formates upon heating to higher temperatures (>450 K) and in the co-presence of adsorbed water. Furthermore, it was
revealed that the formate decomposition to carbon dioxide and water was initiated only in the presence of co-adsorbed water, while dry conditions triggered the reverse reaction to form carbon monoxide and hydroxyl. It was proposed that both electron-donation and withdrawal between water and a magnesium oxide pair facilitated the formate conversion from unidentate to bridge-bonded configuration, the decrease in the rate constant for the reverse reaction and promotion of the forward reaction (Figure 1.3).
The decomposition of the surface formate was explained from a stereochemical perspective as follows: (a) the delocalization of the electrons through the O-C-O bond in the formate at/near the transition state diminishes the bond order to ~one, (b) rotation and torsion around O(2), C-O(3) and C-H(1) bonds occur (c) interaction between H(1) (with δ– charge) and H(4) (with δ+ charge), and (d) finally, dissociation of C-H(1) bond upon tilting of bridge-bonded formate to form hydrogen (H(1)H(4)) accompanied with carbon dioxide and dissociation of adsorbed water to form Mg(1)-O(4)H and O(1)-H(2) bonds. Isotopic labelling studies using DCOOH/HCOOH determined that the RDS must involve C-H bond dissociation of the bridge-bonded formate in the transition state. However, fast hydrogen exchange between surface water and hydroxyls disallowed the identification of the origin of hydrogen deuteride (HD) which could arise from the deuterium of formate and hydrogen of hydroxyl or from the deuterium of formate and hydrogen of water. Along these lines, Davis and coworkers[58,71] reported that the formates arising from the adsorption of formic acid and D-formic acid (DCOOH) were identical to those arising from the adsorption of carbon monoxide to bridging OH and OD groups, respectively. Moreover, a normal kinetic isotope effect was observed upon switching from water to deuterium oxide (D2O) which was consistent with a link between the RDS of WGS and surface formate decomposition.[72,73]
Hence, with the existence of a common kinetically relevant intermediate (formate) between formic acid decomposition and the WGS, the activity of the catalysts for C-H bond cleavage became an important descriptor for the design of WGS catalysts.[74,75]
Another area of extensive formic acid decomposition research is founded on the promising potential of formic acid as a convenient ‘in situ’ source of hydrogen for fuel cells. Formic acid offers high energy density while being non-toxic and safe to be handled in aqueous solution.[76] It can be facilely stored in a disposable or recyclable cartridge that is readily available for on-demand release of hydrogen and easily replaced.[77] Moreover, a reversible cycle of hydrogen supply and storage based on formic acid decomposition and reverse hydrogenation of carbon dioxide is a highly attractive sustainable energy concept.[78–80] Homogeneous catalysts based on iridium or platinum phosphine complexes, dinuclear ruthenium complexes, etc, have been demonstrated to show high activity for formic acid decomposition at close to ambient conditions.[81–85] However, as with many homogeneous catalysts, their practical application is impeded by the difficulties in separation and the use of organic solvents, ligands and additives
Introduction
11
that complicate device fabrication. The development of heterogeneous (solid) catalysts is mainly centered on selective formic acid dehydrogenation to carbon dioxide and water while restricting carbon monoxide formation to negligible levels so as to prevent poisoning of fuel cell catalysts.
Further on, their low temperature (<50 °C) activity was crucial in view of the high volatility of formic acid/water mixtures and the complexity of heat management that forbids miniaturization.[86] Noble metal-based catalysts have been reported to show exciting potential in fulfilling these criteria as summarized in the extensive review by Grasemann and Laurenczy (Table 1.1). In line with earlier studies,[33,51] a (bidentate) formate intermediate formed on the large terrace sites of metal were identified to be the kinetically-relevant precursors for carbon dioxide formation (Figure 1.4) on palladium[87] and platinum[88]. On the other hand, the linear (monodentate formate) mode on surface-unsaturated metal sites (corners, steps, kinks) were predisposed to form carbon monoxide.
Figure 1.4 Dependence of formic acid decomposition selectivity on the surface structure of the metal particle.Adapted by permission from Macmillan Publishers Ltd: Nature Nanotechnology Reference[86], copyright (2011).
Table 1.1 Noble metal-based catalysts for the decomposition of formic acid. In part from Reference[77] with permission of The Royal Society of Chemistry.
Active
phase/support Solvent Performance Temperature Reference 0.61% Au/Al2O3 He/gas
1.3.2 Catalysis by gold: Relevance and importance in formic acid decomposition