The electrocatalytic carbon dioxide reduction reaction (CO 2 RR) addresses the need for storage of renewable energy in valuable carbon-based fuels and feedstocks, yet challenges remain in the improvement of electrosynthesis pathways for highly selective hydrocarbon production. To improve catalysis further, it is of increasing interest to lever synergies between heterogeneous and homogeneous approaches. Molecules adjacent to the active site provide additional binding interactions that may tune the stability of intermediates, improving catalytic performance by increasing Faradaic efficiency (product selectivity), as well as decreasing overpotential. We offer a forward-looking perspective on molecularly enhanced heterogeneous catalysis for CO 2 RR. We discuss four categories of molecularly enhanced strategies: molecular-additive-modified heterogeneous catalysts, immobilized organometallic complex catalysts, reticular catalysts and metal-free polymer catalysts. We introduce presentday challenges in molecular strategies and describe a vision for CO 2 RR electrocatalysis towards multi-carbon products. These strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of CO 2 RR.
Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but their roles in catalysis are currently not well understood. A series of high oxidation state tetranuclear-dioxido clusters comprised of three manganese centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Mn3M(μ4-O)(μ2-O) core that remains intact upon changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV, dependent upon the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between reduction potential and acidity with a slope of ca. 100 mV per pKa unit. The Sr2+ and Ca2+ compounds show similar potentials, an observation that correlates with the behavior of the OEC, which is active only in the presence of one of these two metals.
Electrocatalytic CO2 reduction
to generate multicarbon
products is of interest for applications in artificial photosynthetic
schemes. This is a particularly attractive goal for CO2 reduction by copper electrodes, where a broad range of hydrocarbon
products can be generated but where selectivity for C–C coupled
products relative to CH4 and H2 remains an impediment.
Herein we report a simple yet highly selective catalytic system for
CO2 reduction to C≥2 hydrocarbons on
a polycrystalline Cu electrode in bicarbonate aqueous solution that
uses N-substituted pyridinium additives. Selectivities of 70–80%
for C2 and C3 products with a hydrocarbon ratio
of C≥2/CH4 significantly greater than
100 have been observed with several additives. 13C-labeling
studies verify CO2 to be the sole carbon source in the
C≥2 hydrocarbons produced. Upon electroreduction,
the N-substituted pyridinium additives lead to film deposition on
the Cu electrode, identified in one case as the reductive coupling
product of N-arylpyridinium. Product selectivity
can also be tuned from C≥2 species to H2 (∼90%) while suppressing methane with certain N-heterocyclic
additives.
The oligomerization of ethylene typically leads to a broad range of R-olefins; catalytic systems that are selective for specific desirable alkenes would be of great interest. There have been several recent reports of trimerization of ethylene to 1-hexene with high selectivity. 1 While some ethylene trimerization catalysts are based on titanium and tantalum, 1j,k chromium-based systems generally display higher activity, selectivity, and thermal stability. 1b-i A recent communication describes a catalyst, generated in situ by treating Cr(III) salts with methylalumoxane in the presence of the diphosphine PNP OMe [1, PNP OMe ) (o-MeO-C 6 H 4 ) 2 PN(Me)P(o-MeO-C 6 H 4 ) 2 ], that trimerizes ethylene to 1-hexene with unprecedented selectivity and productivity. 1c,d Herein we report the synthesis and characterization of chromium complexes of 1, 2 along with some mechanistic investigations of the ethylene trimerization reaction they catalyze.Reactions of diphosphine 1 (Scheme 1) with suitable chromium-(III) etherate complexes afford compounds (PNP OMe )CrCl 3 (2) and (PNP OMe )CrPh 3 (3); 2 reacts with o,o′-biphenyldiyl Grignard to give (PNP OMe )Cr(o,o′-biphenyldiyl)Br (4). All three are hexacoordinate, displaying (P,P,O)-κ 3 coordination of the diphosphine, established by single-crystal X-ray diffraction as well as 2 H NMR. 3,4 The structures of 3 and 4 (Figure 1) reveal long Cr-O bonds (2.293(2) Å in 3 and 2.337(2) Å in 4) and significant differences between the two Cr-P bond lengths. The ether-chelated phosphine is closer to chromium by 0.16 Å (3) and 0.23 Å (4).
Understanding the effect of redox-inactive metals on the properties of biological and heterogeneous water oxidation catalysts is important both fundamentally and for improvement of future catalyst designs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.