Homogeneous Electrocatalytic Reduction of CO2 by a CrN3O Complex: Electronic Coupling with a Redox-Active Terpyridine Fragment Favors Selectivity for CO

Design of Cr-Based Molecular Electrocatalyst Systems for the CO2 Reduction Reaction

ConspectusHuman influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO2), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO2 through the use of homogeneous electrocatalysts generally follows a two-electron/two-proton pathway to produce either carbon monoxide (CO) with water (H2O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer-Tropsch process uses CO and H2 to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO2 to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO2 (CO2RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO2RR. However, metals early in the table have sufficient reducing power to mediate the CO2RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO2RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redox-active molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons─often accompanied by protons─to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO2 activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.

Highly efficient electrocatalytic CO2 reduction by a CrIII quaterpyridine complex

Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO2 reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl CrIII catalyst is found to be highly active for CO2 electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s-1) over the documented Cr-based catalysts (<10 s-1) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key CrII, CrI, Cr0, and CO-bound Cr0 intermediates as well as to verify the catalytic mechanism.

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.