Abstract
The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.
Highlights
CO2 is currently under consideration as a viable means to produce useful chemicals while the atmosphere.[1−5] During the limiting the rise past few years, of CO2 levels in transition metal complexes have been extensively studied for the electro- and fpohromtoarteedu(cHtioCnOoOf C−)O.62−, 9maTinhley to carbon monoxide molecular nature (CO) and of these complexes allows specific fine-tuning of their structure using synthetic chemistry to rationally control the activity,[10] selectivity,[11] and stability of the catalysts via a profound understanding of the catalytic mechanisms involved.[12,13]
The integration of these molecular species onto electrodes in many cases gives an enhancement of the catalytic activity and provides new insights into the involved mechanisms.[33−36] Immobilization enables catalysts that would otherwise be insoluble to operate in water and overcomes limitations from diffusion-controlled electrocatalysis with catalysts in the bulk solution.[37−41] In this respect, carbon nanotubes (CNTs) are an established platform for the immobilization of molecular electrocatalysts.[42−45] Their high surface area and excellent conductivity allow grafting of large amounts of electrocatalytically active species while retaining good electron transfer properties.[46]
Product selectivity can be fine-tuned by controlling the surface loading of the catalyst on the CNT sidewalls
Summary
Low-cost, and scalable electrocatalytic reduction of CO2 is currently under consideration as a viable means to produce useful chemicals while the atmosphere.[1−5] During the limiting the rise past few years, of CO2 levels in transition metal complexes have been extensively studied for the electro- and fpohromtoarteedu(cHtioCnOoOf C−)O.62−, 9maTinhley to carbon monoxide molecular nature (CO) and of these complexes allows specific fine-tuning of their structure using synthetic chemistry to rationally control the activity,[10] selectivity,[11] and stability of the catalysts via a profound understanding of the catalytic mechanisms involved.[12,13] In order to replace expensive Re,[14−17] Ru, and Ir20 based catalysts, a range of different catalytic structures incorporating more abundant first-row transition metals such as Ni,[21−23] Co,− Fe,[28,29] and Mn30−32 have been described for catalytic. The appearance of the four ν(CO) bands upon applying reductive conditions is indicative of the formation of a Mn0 dimer species, as previously reported.[56,73] A fifth band (predicted at around 1963 cm−1 from previous measurements in THF73) is known to exhibit only weak IR absorption; it is most likely masked by the more intense adjacent 1968 cm−1 absorption.[56,73] A frequency downshift (up to −15 cm−1 compared to literature values in organic solvents listed in Table 1) is observed in this work, which may be attributed to decreased electron density at the CO bond resulting from hydrogen bonding to the carbonyl oxygen atoms in aqueous electrolyte solution.[82] The assignment of these bands to a Mn0 dimer is supported by the UV−vis SEC experiments above (Figure 2, inset). The presence of the two species could be directly linked to the electrocatalytic product selectivity (CO via the dimer and HCOO− and H2 via the Mnhydride)
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