Research in the field of catalytic reduction of carbon dioxide to liquid fuels has grown rapidly in the past few decades. This is due to the increasing amount of carbon dioxide in the atmosphere and a steady climb in global fuel demand. This tutorial review will present much of the significant work that has been done in the field of electrocatalytic and homogeneous reduction of carbon dioxide over the past three decades. It will then extend the discussion to the important conclusions from previous work and recommendations for future directions to develop a catalytic system that will convert carbon dioxide to liquid fuels with high efficiencies.

Five Re(bipy)(CO)(3)Cl complexes were prepared and studied where bipy was 4,4'-dicarboxyl-2,2'-bipyridine (1), 2,2'-bipyridine (2), 4,4'-dimethyl-2,2'-bipyridine (3), 4,4'-di-tert-butyl-2,2'-bipyridine (4), and 4,4'-dimethoxy-2,2'-bipyridine (5). From this group, a significantly improved catalyst, Re(bipy-tBu)(CO)(3)Cl (4), for the reduction of carbon dioxide to carbon monoxide was found. The complex shows two one-electron reductions under argon, one reversible at -1445 mV (vs SCE), and one irreversible at -1830 mV. Under CO(2) the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich-Koutecky method (1.1 × 10(-5) cm(2)/s for the neutral complex and 8.1 × 10(-6) cm(2)/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO(2) of 1000 M(-1) s(-1) was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, η(CO) = 99 ± 2% in acetonitrile.

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(bpy-tBu)(CO)3Br and [Mn(bpy-tBu)(CO)3(MeCN)](OTf) are reported. Addition of Brönsted acids to CO2-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO2. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpy-tBu)(CO)3Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at -2.2 V vs SCE, a TOF of 340 s(-1) was calculated for Mn(bpy-tBu)(CO)3Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO2, with no observable production of H2. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn(bpy-tBu)(CO)3](-), shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn(bpy-tBu)(CO)3Br. These were of the parent Mn(bpy-tBu)(CO)3Br complex, the dimer [Mn(bpy-tBu)(CO)3]2, and the [Mn(bpy-tBu)(CO)3](-) anion.

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Reðbipy-tBuÞðCOÞ 3 ðLÞ catalyst for the reduction of CO 2 to CO. A remarkable selectivity for CO 2 over H þ was observed by stopped-flow UV-vis spectroscopy of ½Reðbipy-tBuÞðCOÞ 3 −1 . The reaction with CO 2 is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied d z 2 , which is believed to determine selectivity by favoring CO 2 (σ þ π) over H þ (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H∕D kinetic isotope effect, indicating that transfer of protons to Re-CO 2 is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system. carbon dioxide reduction | electrochemistry | kinetics | electrocatalyst T he development of artificial photosynthetic systems is of immediate concern in view of the world's dependence on fossil fuels and the increasing emissions of CO 2 . Rapid industrial growth in developing nations will significantly increase the global energy demand in coming years, and although known reserves of fuels such as natural gas and coal are sufficient for the near future, they are becoming increasingly costly to obtain. As the use of fossil fuels is fundamentally unsustainable and generates greenhouse gases and other pollutants, the development of environmentally benign energy sources is important. Solar energy is an abundant alternative but suffers from being a diffuse energy source, and its availability varies by location and time of day. If we can capture solar energy and use CO 2 as a C 1 feedstock for liquid fuels, we can envision converting our global energy economy into a nearly carbon-neutral system (1).Photosynthesis is one of the great triumphs of nature and is the cornerstone for advanced life on the planet. Mankind has yet to master nature's ability to store sunlight as chemical energy by splitting CO 2 and H 2 O to form C─C, C─H, and O─O bonds. The energy-dense liquid fuels formed by this process would have the advantage of conforming to the existing infrastructure. Photosynthesis can be divided into two parts: water splitting and reduction of carbon dioxide. Water oxidation has been reviewed extensively by others (2-4).Our laboratory is currently exploring the development of C...

Hydrogen-terminated p-type silicon was used as a photocathode for the selective photoreduction of CO 2 to CO in the presence of Re(bipy-Bu t )(CO) 3 Cl (bipy-Bu t ) 4,4′-di-tert-butyl-2,2′-bipyridine) as an electrocatalyst. The reduction of CO 2 to CO on p-type silicon was achieved at a potential more than 600 mV lower than that required with a Pt electrode. A Faradaic efficiency of 97 ( 3% and an overall efficiency of 9.3% and 10% for the conversion of monochromatic and polychromatic light, respectively, to electricity were observed for the CO 2 photoreduction process. A short-circuit quantum efficiency of 61% for light-to-chemical energy conversion was observed for the conversion of CO 2 to CO.