58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

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.

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.^[1] In carrying out this work, we have developed complexes of Ni and Pd supported by the Pyridyl-DiCarbene (PDC^R) 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. Shown in Figure 2 are the cyclic voltammetry traces recorded for [Pd(PDC^Me)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 E° ~ –1.0 V and –1.3 V (vs. Ag/AgCl), which correspond to Pd^II/I and Pd^I/0 couples, respectively. 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.

A number of heterogeneous cathode materials can also facilitate the electrocatalytic conversion of CO₂ to CO, however only noble metals such as Ag and Au can catalyze this reaction with Faradaic Efficiencies (FEs) that are in excess of 80% at ambient pressures.^[2] The production of CO using precious metal cathodes has been hampered by the exorbitant cost of these materials, which eliminates their practical use on the scale required for alternative fuel synthesis. Against this backdrop, our group recently developed a bismuth-based Carbon Monoxide Evolving Catalyst (Bi-CMEC), which can convert CO₂to CO at an appreciable rate (high current density; j_CO) with a FE over 90% at small overpotential (h = 165 mV).^[3]

We have shown that Bi-CMEC can be conveniently electrodeposited onto a glassy carbon electrode (GCE) by controlled potential electrolysis (CPE) of a quiescent solution of Bi³⁺ in aqueous acid.³ SEM revealed that this Bi containing material is composed of an array of striated clusters (Figure 3). Cyclic voltammetry (CV) experiments showed that the Bi material could electrocatalyze reduction of CO₂ in MeCN, and that the kinetics and efficiency of this process were greatly enhanced by addition of small amounts of imidazolium based ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF₄) to the catholyte solution (Figure 4).³ Conversion of CO₂ to CO using Bi-based cathodes represents an important development in the fields of CO₂ electrocatalysis and renewable energy conversion. Our development of Bi-CMEC and its significance to these fields was recently highlighted in a feature article in Chemical and Engineering News.^[4] As such this PRF award has had a significant effect on my career by allowing me to establish a research program that is making fundamental discoveries in the areas of CO₂ catalysis and renewable energy conversion. This award has also positively affected each of the students in my research lab (both graduate and undergraduate) by providing support and the means to develop our expertise in these areas.  

References:

[1].     Ariyananda, P. W. G.; Yap, G. P. A.; Rosenthal, J. “submitted.

[2].    Hori, Y., Wakebe, H., Tsukamoto, T., Koga, O. Electrochimica Acta 1994, 39, 1833–1839.

[3].    DiMeglio, J. L.; Rosenthal, J. “J. Am. Chem. Soc. 135, 8798–8801.

[4].    Jacoby, M. Chem. Eng. News, 2013, 91, 21–22.