Authors: Brandon R. Galan*, Julia Shoffel#, John C. Linehan*, Candace Seu#, Aaron M. Appel*, John A. S. Roberts*, Monte L. Helm **, Uriah J. Kilgore*, Jenny Y. Yang*, Daniel L. Dubois*, and Clifford P. Kubiak#
Journal: Journal of the American Chemical Society
Affiliation: * Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA
** Chemistry Department, Fort Lewis College, 1000 Rim Drive, Durango, CO
#Department of Chemistry and Biochemistry, University of California, San Diego, CA
Formic acid is the simplest carboxylic acid and it may be synthesized from the addition of dihydrogen across a C=O bond in carbon dioxide. Formic acid fuel cells are fairly efficient and safe, making them strong candidates for sustainable power generation in the future, assuming dihydrogen can be produced cleanly from water. While there are a few molecular catalysts already able to reduce carbon dioxide to formic acid, researchers who study the opposite reaction, where formic acid is converted to CO2, 2 protons (H+) and 2 electrons (e–), would like to catalyze it without the use of precious metals such as platinum in order to lower the cost of aforementioned fuel cells. The work described in this paper outlines the use of a Nickel (Ni) complex that efficiently catalyzes the oxidation of formate with help from a supporting molecule, or ligand.
The ligand in question has been developed by the research group led by Dan DuBois at the Pacific Northwest National Lab, and has been shown to aid in the catalytic generation or oxidation of dihydrogen on group 10 transition metals. The authors attribute the ligands ability to increase catalytic turnover to the presence of the tertiary amines positioned above and below the transition metal which aid in acid/base catalysis. This strategy is also employed in enzymes, where amino acid residues participate in acid/base catalysis in the enzyme active site.
The primary method used to study this system is cyclic voltammetry (CV), where a changing potential difference is applied to a solution of catalyst and substrate (formate) and the resulting current, which is proportional to the rate of reaction, is measured. At very low (negative) potentials, the initial oxidation state of the Ni complex is 0, but upon reversible removal of 2 e-, a Ni(II) complex forms in the absence of formate. When formate was added to solution, however, addition of the second electron to the Ni complex resulted in a rise in current corresponding to the oxidation of formic acid. Analysis of the headspace of the reaction after bulk electrolysis found no dihydrogen, but plenty of CO2, suggesting that after being removed an amine, the proton formerly belonging to the formate molecule is lost to solution and not reduced to H2. As a control, a similar Ni complex lacking the pendant tertiary amines was studied, and irreversible production of a Ni-hydride (Ni-H) complex was observed.
The authors studied the effect that changing the phosphine and amine substituents on the ligand had on catalytic activity and found that very little change was observed across a range of phosphine substituents, but tuning the basicity of the amine had a marked effect on catalytic rate, with more basic amines displaying superior performance. The role of the amine was also shown in the scan-rate dependance of formate oxidation in the electrochemical experiments. Usually, if a catalytic intermediate sticks around for a while after accepting/losing an electron and before undergoing a chemical transformation, the amount of current observed in the returning scan (when the potential is scanned in the opposite direction, returning to the potential at which the electron transfer event took place) of the CV will vary with scan rate (a larger current will be observed on the return scan at higher scan rates). Even scanning at an astonishingly fast 20 V/s, no evidence was seen of a Ni(II)-formate species, suggesting that deprotonation of formate by the tertiary amine was incredibly fast (>50 s-1).
The authors have shown that it is possible to increase catalytic activity on a catalytically sluggish transition metal center by secondary coordination sphere engineering. A few attempts at catalyzing the reduction of CO2 with H2 using the same catalyst were ultimately unsuccessful, but one can be sure that the Kubiak and DuBois groups will continue to investigate the utilization of CO2, as they have made significant contributions to the field over the past few decades.