Changing the Mechanism for CO<sub>2</sub> Hydrogenation Using Solvent‐Dependent Thermodynamics

Burgess, Samantha A.; Appel, Aaron M.; Linehan, John C.; Wiedner, Eric S.

doi:10.1002/anie.201709319

Cited by 45 publications

(59 citation statements)

References 47 publications

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“…However, a notable trend is that hydricity values for transition metal hydrides in organic solvents tend to decline to a greater magnitude (become better donors) in water compared to HCO 2 − (ΔG HCO2− ). As a result, there are a few examples where CO 2 reduction to HCO 2 − is exergonic in water but endergonic in organic solvents (14,27,48). Additionally, detailed studies by Miller and coworkers (63) determined that aqueous hydricity can also be dependent on anions commonly found in aqueous buffers as well as hydroxide at higher pH values.…”

Section: Resultsmentioning

confidence: 99%

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Ceballos

Yang

2018

Proc. Natl. Acad. Sci. U.S.A.

119

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A critical challenge in electrocatalytic CO 2 reduction to renewable fuels is product selectivity. Desirable products of CO 2 reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H 2 . Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO 2 reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H + and CO 2 to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (ΔG H− ), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO 2 reduction is favorable and H + reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe) 2 ](PF 6 ) 2 as a potential catalyst, because the corresponding hydride [HPt(dmpe) 2 ] + has the correct hydricity to access the region where selective CO 2 reduction is possible. We validate our choice experimentally; [Pt(dmpe) 2 ](PF 6 ) 2 is a highly selective electrocatalyst for CO 2 reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO 2 reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery. electrocatalysis | CO 2 reduction | solar fuel | formate production | hydride T he emerging availability of inexpensive renewable electricity has motivated interest in using electrolytic methods to generate sustainable fuels. Electrocatalytic CO 2 reduction provides an entry to carbon-neutral fuels, but product selectivity remains a significant challenge (1, 2). Nearly all reductive reactions of interest involve protons as well as electrons, introducing the complication of direct proton reduction to H 2 under electrolytic conditions. Diversion of electron equivalents into proton reduction results in lower Faradaic efficiency for the desired CO 2 reduction reaction. Various strategies to inhibit or suppress H 2 evolution for heterogeneous (3-7) and homogeneous (8,9) catalysts have been explored.To understand the factors that determine selectivity between CO 2 and H + reduction, we have been investigating the reactivity of metal hydrides. Selectivity for formate production is particularly challenging because metal hydride intermediates are common to both reaction pathways (Fig. 1). As a result, very few heterogeneous (10-13) or homogeneous (14-16) catalysts have been reported with high (>90%) Faradaic efficiency for formate production. Understanding the reactivity of metal hydrides is key to controlling the bifurcating reaction pathways that ultimately determine selectivity. Most selective catalysts for CO 2 reduction utilize kinetic inhibition (or a high-...

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Section: Resultsmentioning

confidence: 99%

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Ceballos

Yang

2018

Proc. Natl. Acad. Sci. U.S.A.

119

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“…As a result, some metal hydrides that are insufficiently hydridic to reduce CO 2 in organic solvents will do so in water. 71,72,77…”

Section: H2/hco2– Formationmentioning

confidence: 99%

Thermodynamic Considerations for Optimizing Selective CO₂ Reduction by Molecular Catalysts

2019

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Energetically efficient electrocatalysts with high product selectivity are desirable targets for sustainable chemical fuel generation using renewable electricity. Recycling CO 2 by reduction to more energy dense products would support a carbon-neutral cycle that mitigates the intermittency of renewable energy sources. Conversion of CO 2 to more saturated products typically requires proton equivalents. Complications with product selectivity stem from competitive reactions between H + or CO 2 at shared intermediates. We describe generalized catalytic cycles for H 2 , CO, and HCO 2 – formation that are commonly proposed in inorganic molecular catalysts. Thermodynamic considerations and trends for the reactions of H + or CO 2 at key intermediates are outlined. A quantitative understanding of intermediate catalytic steps is key to designing systems that display high selectivity while promoting energetically efficient catalysis by minimizing the overall energy landscape. For CO 2 reduction to CO, we describe how an enzymatic active site motif facilitates efficient and selective catalysis and highlight relevant examples from synthetic systems.

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“…[10] Since the pioneering work of Creutz,o ther metal hydride systems have been discoveredw here the simple act of moving from an organic solvent to water causes hydride transfer to CO 2 to become thermodynamically possible. To date, this solvent effect has been demonstrated for [HFe 4 (N)(CO)] À , [11] [HNi(diphosphine) 2 ] + , [12] and [(H) 2 Co(dmpe) 2 ] + , [13] as shown in Figure 1.…”

Section: Hydride Transfer To Comentioning

confidence: 91%

“…Hydride transfer from [HFe 4 (N)(CO) 12 ] − to CO 2 is exergonic (−9 kcal mol −1 ) in water, but endergonic (+5 kcal mol −1 ) in acetonitrile (Figure ). Wiedner and co‐workers recently reported that [(H) 2 Co(dmpe) 2 ] + is an active catalyst for aqueous hydrogenation of CO 2 to formate, and operates by a different mechanism in water than in organic solvent . In water, the hydricity of [(H) 2 Co(dmpe) 2 ] + is sufficient to react with CO 2 .…”

Section: Hydride Transfer To Co2mentioning

confidence: 99%

“…Wiedner and co-workersr ecently reported that [(H) 2 Co(dmpe) 2 ] + is an active catalystf or aqueous hydrogenation of CO 2 to formate, and operates by a different mechanism in water than in organic solvent. [13] In www.chemeurj.org water,t he hydricity of [(H) 2 Co(dmpe) 2 ] + is sufficient to react with CO 2 .I no rganic solvents, however,h ydride transfer from [(H) 2 Co(dmpe) 2 ] + to CO 2 is endergonic, and this intermediate must be deprotonated with av ery strong organic base to generate HCo(dmpe) 2 ,astrong hydride donor. [16] For both [HFe 4 (N)(CO) 12 ] À and [(H) 2 Co(dmpe) 2 ] + ,t he favorable impact of water on hydride transfer to CO 2 was criticalf or catalytic formate production.…”

Section: Hydride Transfer To Comentioning

confidence: 99%

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Making a Splash in Homogeneous CO₂ Hydrogenation: Elucidating the Impact of Solvent on Catalytic Mechanisms

Wiedner

Linehan

2018

Chemistry A European J

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Molecular catalysts for hydrogenation of CO are widely studied as a means of chemical hydrogen storage. Catalysts are traditionally designed from the perspective of controlling the ligands bound to the metal. In recent years, studies have shown that the solvent can also play a key role in the mechanism of CO hydrogenation. A prominent example is the impact of the solvent on the thermodynamic hydride donor ability, or hydricity, of metal hydride complexes relative to the hydride acceptor ability of CO . In some cases, simply changing from an organic solvent to water can reverse the direction of hydride transfer between a metal hydride and CO . Additionally, the solvent can impact catalysis by converting CO into carbonate species, as well as activate intermediate products for hydrogenation to more reduced products. By understanding the substrate and product speciation, as well as the reactivity of the catalyst towards the substrate, the solvent can be used as a central design component for the rational development of new catalytic systems.

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Changing the Mechanism for CO₂ Hydrogenation Using Solvent‐Dependent Thermodynamics

Cited by 45 publications

References 47 publications

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Thermodynamic Considerations for Optimizing Selective CO₂ Reduction by Molecular Catalysts

Making a Splash in Homogeneous CO₂ Hydrogenation: Elucidating the Impact of Solvent on Catalytic Mechanisms

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Changing the Mechanism for CO2 Hydrogenation Using Solvent‐Dependent Thermodynamics

Cited by 45 publications

References 47 publications

Directing the reactivity of metal hydrides for selective CO 2 reduction

Directing the reactivity of metal hydrides for selective CO 2 reduction

Thermodynamic Considerations for Optimizing Selective CO2 Reduction by Molecular Catalysts

Making a Splash in Homogeneous CO2 Hydrogenation: Elucidating the Impact of Solvent on Catalytic Mechanisms

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Changing the Mechanism for CO₂ Hydrogenation Using Solvent‐Dependent Thermodynamics

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Directing the reactivity of metal hydrides for selective CO ₂ reduction

Thermodynamic Considerations for Optimizing Selective CO₂ Reduction by Molecular Catalysts

Making a Splash in Homogeneous CO₂ Hydrogenation: Elucidating the Impact of Solvent on Catalytic Mechanisms