An international team led by researchers at the University of Melbourne has developed a new catalyst system for the efficient removal of CO2 from formic acid (HO2CH), resulting in the production of CO2 and H2 at a low temperature of 70 °C. Other methods for producing hydrogen from formic acid have required high temperatures, and also produce waste products.
The work, described in an open-access paper in Nature Communications marks a new frontier in catalyst design at the molecular level. Such catalysts are formulated to produce highly selective chemical reactions.
Nature uses a number of design principles to create different classes of enzyme catalysts capable of a wide range of chemical transformations of substrates. A metal ion or metal cluster often has a critical role as a co-factor. A key concept in enzyme catalysis is the preorganization of the reaction environment by the enzyme, directing the substrate to the reaction site, which provides a favorable geometry for the transition state required for bond activation. In essence, the enzyme steers the substrate along the required reaction coordinate to allow the desired transformation to product(s).
The concept of changing the environment at a metal centre to switch on reactivity has also been recently exploited in gold chemistry. … Here, we use gas-phase experiments and density functional theory (DFT) calculations to examine how the binuclear silver hydride cation, [Ag2(H)]+, can be structurally manipulated by the appropriate choice of phosphine ligands to switch on the protonation of the hydride by formic acid to liberate hydrogen, which is a key step in the selective, catalyzed decomposition of formic acid that does not occur in absence of ligands. We chose [Ag2(H)]+ since it has been spectroscopically characterized and ligated variants can readily be formed. Formic acid was chosen as a substrate since its decomposition is one of the most widely studied topics in chemistry, with a rich history spanning more than a century. Apart from the academic interest in establishing the mechanism(s) of decomposition, the selective, catalyzed decomposition of formic acid has potentially important applications in areas ranging from hydrogen storage through to the generation of in situ hydrogenation sources for reduction of organic substrates.—Zavras et al.
Professor Richard O’Hair, from the University of Melbourne’s School of Chemistry and Bio21 Institute, worked in collaboration with Professors Philippe Dugourd (from the University of Lyon), Philippe Maitre (University of Paris South), Bonačić-Koutecký (Humboldt-University Berlin) and Dr. Roger Mulder (CSIRO Manufacturing) for the study.
While the study successfully produces hydrogen and CO2, the ultimate aim of future research will be to ensure any derivative source of hydrogen produces zero emissions.
One of the grand challenges for chemists today is to develop perfect chemical reactions that proceed with 100% yield and 100% selectivity without forming any waste products. With formic acid, the aim was to transform it into hydrogen and carbon dioxide, which could really lend itself to the important practical applications of hydrogen energy in the transport sector.—Professor O’Hair
Using a suite of gas-phase techniques, the research team designed a series of silver complexes and examined their reactions with formic acid.
The team was able to identify and orchestrate the exact catalyst that would effectively manipulate a strict hydrogen/carbon dioxide-only production.
Two key concepts have emerged from this work: (i) that ligands can have a vital role in reshaping the scaffold of a metal cluster to activate its reactivity towards a substrate; and (ii) that fundamental gas-phase studies can be used to direct the search for new types of metal complexes that promote related reactivity in solution. Together these concepts have allowed us to achieve the selective extrusion of carbon dioxide from formic acid, an important process for applications in hydrogen storage.—Zavras et al.
One of the major challenges for hydrogen power is the lack of refueling infrastructure; the researchers suggest this obstacle could easily be overcome if one day the industry moves to using liquid formic acid.
Professor O’Hair notes that while the new catalyst design is an important step forward in addressing our hydrogen energy needs, there are still many barriers to overcome, such as the production of carbon dioxide and how it could potentially be recycled to regenerate formic acid.
The research was funded by the Australian Research Council.
Athanasios Zavras, George N. Khairallah, Marjan Krstić, Marion Girod, Steven Daly, Rodolphe Antoine, Philippe Maitre, Roger J. Mulder, Stefanie-Ann Alexander, Vlasta Bonačić-Koutecký, Philippe Dugourd & Richard A. J. O’Hair (2016) “Ligand-induced substrate steering and reshaping of [Ag2(H)]+ scaffold for selective CO2 extrusion from formic acid” Nature Communications 7, Article number: 11746 doi: 10.1038/ncomms11746