Joel Rosenthal, PhD, University of Delaware
Fossil fuels account for roughly 80% of mankind's current energy portfolio with roughly one third coming from petroleum based sources. Petroleum resources are especially important to the transportation sector and are driving annual atmospheric emissions of 2.5 Gt of carbon in the form of CO2. Given the evidence linking anthropomorphic carbon emissions to climate change, the long term use of petroleum resources will be reliant on the development of systems capable of sequestering and recycling CO2. The reduction of CO2 to CO would allow for the mitigation of atmospheric CO2 levels while producing a versatile energy rich commodity chemical. This reaction is a proton-coupled multiple electron transfer process (2e– + 2H+). Since CO is an important chemical for the production of synthetic petroleum and hydrocarbons using Fischer-Tropsch methods our work seeks to uncover the fundamental science needed to drive the conversion of CO2 to liquid fuels.
For the reaction CO2 + 2e– + 2H+ → CO + H2O the standard potential is E0 = –0.53 V at pH = 7.0, however, most previously studied CO2 reduction catalysts display poor selectivities, limited stabilities and require large overpotentials to effect CO2 activation at an appreciable rate. These shortcomings can be overcome by judicious ligand and catalyst design. For example, the instability of some of the most successful CO2 reduction catalysts is due to the high lability of the ligand scaffolds used to support low-valent catalytic centers. The sluggish kinetics observed for such platforms can be attributed to the fact that these systems do not minimize the large nuclear reorganization energy associated with binding of CO2 to reduced metal centers. In confronting these issues, we have developed a new kinetically robust platform for CO2 conversion and are designing platforms with hydrogen-bonding functionalities to stabilize M–CO2– adducts and promote the controlled delivery of protons to drive C–O bond cleavage to efficiently generate CO and H2O. In pursuing these research opportunities, we have identified the following objectives.
The development of transition metal carbene complexes for CO2 activation has been a major area of focus in our lab. In carrying out this work, we have developed complexes of Ni and Pd supported by the Pyridyl-DiCarbene (PDCR) ligand scaffolds depicted in Figure 1A. The two NHC moieties on the ligand backbone support the electron-rich Ni and Pd metal centers needed for catalysis and are suitable for activating CO2. Moreover, the modular synthesis of these compounds allows for the steric and electronic properties of these architectures to be tuned with ease and fidelity. Crystal structures for a homologous set of these palladium CNC-pincer complexes are shown in Figure 1B. Inspection of these ORTEP diagrams clearly illustrates how the size and accessibility of the molecular cleft is attenuated by variation of the pincer R-group. Attenuation of the steric bulk about the pincer periphery significantly impacts the kinetics and efficiacy and catalysis at the metal center.
We have successfully shown that the palladium systems of Figure 1A are competent electrocatalysts for reduction of CO2. Our work in this area was recently recognized by Oak Ridge Associated Universities and highlighted by Nature Magazine. Shown in Figure 2 are the cyclic voltammetry traces recorded for [Pd(PDCMe)MeCN](PF6)2 in MeCN. The CV trace recorded under N2 (Figure 2, blue) reveals the redox activity of the Pd-pincer architecture with reduction waves centered at Eº ~ –1.0 V and –1.3 V (vs. Ag/AgCl), which correspond to PdII/I and PdI/0 couples, respectively. There is also a small peak at Eº ~ –1.7 V, which corresponds to reduction of an electrogenerated dimer. Notably, the size of this final reduction wave is small compared to that observed for related palladium complexes supported by phosphine ligands. Suppression of [PdI(PDCMe)]+ dimerization by our inert pincer ligands is an important development, since bimetallic palladium species are not competent catalysts for CO2 reduction. Accordingly, our systems offer improvement over previously studied complexes that display low TON for CO2 conversion due to this dimerization process.
Minimization of PdI—PdI formation is reflected by the efficacy with which [Pd(PDCMe)MeCN](PF6)2 activates CO2. CVs recorded for this complex under 1 atm of carbon dioxide result in a more pronounced PdI/0 wave at –1.3 V, consistent with reduction of CO2 by the electrogenerated Pd0 (Figure 2, red). Repetition of this experiment in the presence of a weak acid results in further enhancement of this redox wave. This observation is consistent with the need for proton transfer to complete the PCET conversion of CO2 to CO (Eq. 1) and is evidenced by the broad electrocatalytic wave observed between –1.35 and –1.9 V (Figure 2, dashed red).
The pincer constructs of Figure 1 proficiently catalyze the electrochemical conversion of CO2 to CO. Bulk electrolysis of solutions of these complexes in acidic MeCN under an atmosphere of CO2 has allowed for the products of the electrocatalytic reduction to be ascertained by gas chromatography. Our Pd(PDC) architectures display exquisite selectivity for CO production over other reduced carbon species including CH3OH, HCO2H and H2CO. Moreover, we have shown that H2 is not formed as an unwanted side product via direct reduction of protons by the Pd catalyst. Accordingly, our novel Pd(PDC) construct displays far higher stability, selectivity and efficiency compared to homologous Pd-phosphine complexes.
Figure 2. CV measurements recorded for [Pd(PDCMe)MeCN](PF6)2 under N2 (blue), CO2 (red) and CO2 with acid (dashed red).
Figure 1. (A) Basic framework of Pyridyl-DiCarbene complexes. (B) Thermal ellipsoid plots for a series of [Pd(PDCR)MeCN](PF6)2 complexes. All counteranions and solvent are omitted for clarity.
. Ariyananda, P. W. G.; Yap, G. P. A.; Rosenthal, J. "Reaction of carbon dioxide with a palladium–alkyl complex supported by a bis–NHC framework" submitted.
. "Turning Point: Joel Rosenthal" Nature 2011, 476 , 243.
. Dubois, D. L.; Miedaner, A.; Haltiwagner, R. C. "Electrochemical reduction of carbon dioxide catalyzed by [Pd(triphosphine)(solvent)](BF4)2 complexes: synthetic and mechanistic studies" J. Am. Chem. Soc. 1991, 113, 8753-8764.