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Carbon monoxide is the key intermediate in the electrochemical CO2 reduction reaction and determines the overpotentials and selectivities for C1 and C2 products on copper electrodes. The kinetic mo...

A micro-kinetic model combining electrochemical rate theory and first-principles simulation is developed to study the influences of solution pH and electrode potential on reaction rate, reaction pathways, and product distribution of electrocatalytic CO2 conversion. Two critical issues involved in electrochemical reaction mechanism are investigated: 1) competing concerted and sequential proton-electron transfer pathways, 2) competing thermodynamics-controlled and kinetics-controlled pathways. Our results show that the electrochemical reduction of CO2 to CO and HCOOH adopts a thermodynamics-controlled CPET mechanism at low pH, while follows a kinetics-controlled SPET mechanism at high pH. The electrocatalytic activity and selectivity can be effectively modulated by manipulating of solution pH and electrode potential. It is demonstrated that HCOOH is the main product at low overpotential while CO becomes the main product at high overpotential. In addition, increasing pH is conducive to improving the Faradic efficiency of HCOOH production and suppressing the hydrogen evolution reaction.

The electrochemical reduction of CO2 is a promising route for converting intermittent renewable energy into storable fuels and useful chemical products. A theoretical investigation of the reaction mechanism and kinetics is beneficial for understanding the electrocatalytic activity and selectivity. In this report, a kinetic model based on Marcus theory is developed to compute the potential-dependent reaction barrier of the elementary concerted proton–electron transfer steps of electrochemical CO2 reduction reactions, different from the previous hydrogen atom transfer model. It is found that the onset potentials and rate-determining steps for CO and CH4 formation are determined by the first and third concerted proton–electron transfer steps C1 and C3. The influence of binding energy, electrode potential, and reorganization energy on the computed reaction barriers of the C1 and C3 reactions is discussed. In general, the calculated reaction barrier shows a quadratic relationship with the applied electrode potential. Specifically, the reaction barrier is merely determined by the reorganization energy at equilibrium potential. The present kinetic model is applied to compare the electrocatalytic activities in the electrochemical reduction of CO2 on various copper crystal surfaces. Among the four studied copper single-crystal surfaces, Cu(211) exhibits the best electrocatalytic activity for CO formation and CH4 formation due to its low onset potential and overpotential.

Electrochemical reduction of nitrogen to produce ammonia at moderate conditions in aqueous solutions holds great prospect but also faces huge challenges. Considering the high selectivity of Au-based materials to inhibit competitive hydrogen evolution reaction (HER) and high activity of transition metals such as Fe and Mo toward the nitrogen reduction reaction (NRR), it was proposed that Au-based alloy materials could act as efficient catalysts for N2 fixation based on density functional theory simulations. Only on Mo3 Au(111) surface the adsorption of N2 is stronger than H atom. Thermodynamics combined with kinetics studies were performed to investigate the influence of composition and ratio of Au-based alloys on NRR and HER. The binding energy and reorganization energy affected performance for the initial N2 activation and hydrogenation process. By considering the free-energy diagram, the computed potential-determining step was either the first or the fifth hydrogenation step on metal catalysts. The optimum catalytic activity could be achieved by adjusting atomic proportion in alloys to make all intermediate species exhibit moderate adsorption. Free-energy diagrams of N2 hydrogenation via Langmuir-Hinshelwood mechanism and hydrogen evolution via Tafel mechanism were compared to reveal that the Mo3 Au surface showed satisfactory catalytic performance by simultaneously promoting NRR and suppressing HER. Theoretical simulations demonstrated that Au-Mo alloy materials could be applied as high-performance electrocatalysts for NRR.

Organic carbonyl compounds are regarded as promising candidates for next-generation rechargeable batteries due to their low cost, environmentally benign nature, and high capacity. The carbonyl utilization is a key issue that limits the practical specific capacity of multi-carbonyl compounds. In this work, a combination of thermodynamic computation and electronic structure analysis is carried out to study the influence of carbonyl type and carbonyl number on the electrochemical performance of a series of multi-carbonyl compounds by using density functional theory (DFT) calculations. By comparing discharge profiles of six tetraone compounds with different carbonyl sites, it is demonstrated that pentacene-5,7,12,14-tetraone (PT) with para-dicarbonyl and pyrene-4,5,9,10-tetraone (PTO) with ortho-dicarbonyl undergo four-lithium transfer while the other four compounds with meta-dicarbonyl fragments show only two-lithium transfer during the discharge process. By further increasing the carbonyl number, the electrochemical performance of molecules with similar para-dicarbonyl sites to PT can not be strongly improved. Among all the studied multi-carbonyl compounds, triphenylene-2,3,6,7,10,11-hexaone (TPHA) and tribenzo[f,k,m]tetraphen-2,3,6,7,11,12,15,16-octaone (TTOA) with similar ortho-dicarbonyl sites to PTO exhibit the best electrochemical performance due to simultaneous high specific capacity and high discharge voltage. Our results offer evidence that conjugated multiple-carbonyl molecules with ortho-dicarbonyl sites are promising in developing high energy-density organic rechargeable batteries.

The electrochemical reduction of N2 is a promising reaction candidate for the ammonia synthesis process. Density functional theory simulations are carried out to study the reaction thermodynamics and kinetics for a better understanding of the catalytic performance of Fe, Mo, Rh, and Ru electrodes. The distal pathway is the most likely reaction pathway for nitrogen reduction on transition metal surfaces according to the computed reaction free energies. The onset potential of nitrogen reduction on Fe(110) (-0.49 V) and Mo(110) (-0.52 V) is determined by the hydrogenation of NH to NH2, which is more positive than the onset potential on the Ru(0001) (-0.76 V) and Rh(111) (-0.98 V) surfaces attributed to the hydrogenation of N2 to NNH. In particular, the initial hydrogenation of N2 on Mo(111) is a spontaneous process due to the strong interaction of N2 and NNH with the Mo(110) surface. Electronic structure analyses including Bader charge analysis and projected crystal orbital Hamilton populations are performed to interpret the difference in adsorption energy of key intermediates on the four metal surfaces. It is found that both N2 and NNH species have the strongest interaction with Mo(110) leading to the initial activation of N2 on the Mo(110) surface being a spontaneous process. A kinetic model based on the Marcus theory is applied to calculate the potential-dependent reaction barrier of electrochemical hydrogenation steps of the N2 reduction reaction. The rate-determining step is the fifth hydrogenation step *NH → *NH2 on Fe(110) and Mo(110) surfaces, and the first hydrogenation step *N2 → *NNH on Rh(111) and Ru(0001) surfaces. The predicted electrocatalytic activity from the potential-dependent rate constant of the rate-determining step on the four metal electrodes decreases in sequence: Fe(110) > Mo(110) > Ru(0001) > Rh(111).

The electrochemical reduction of CO

The chemical enhancement due to ground-state charge transfer (GSCT) and photon-driven charge transfer (PDCT) in surface-enhanced Raman scattering (SERS) has been investigated by density functional theory. Para-substituted thiophenol derivatives adsorbed on silver and gold surfaces are selected as model systems to evaluate the chemical enhancement factor. By changing the functional groups on thiophenol, we are allowed to modulate the chemical interactions between the thiophenol and the metal cluster in both ground state and charge transfer excited state. Both off-resonance and pre-resonance SERS spectra are simulated to calculate the chemical enhancement factors. The GSCT enhancement factor, EFGSCT , shows a roughly linear relationship to (ωTP /ωM-TP )4 , where ωTP denotes the HOMO-LUMO gap of free molecule, and ωM-TP denotes the energy difference between the HOMO of the molecule and the LUMO of the metal. The PDCT enhancement factor, EFPDCT , is governed by the energy difference between the incident light energy and the excitation energy to the CT excited state. EFPDCT first increases and then decreases with the increase of incident light energy.

In comparison to very low density lipoprotein (VLDL), chylomicrons are cleared quickly from plasma. However, small changes in fasting plasma VLDL concentra- tion substantially delay postprandial chylomicron triglycer- ide clearance. We hypothesized that differential binding to lipoprotein lipase (LPL), the first step in the lipolytic path- way, might explain these otherwise paradoxical relation- ships. Competition binding assays of different lipoproteins were performed in a solid phase assay with purified bovine LPL at 4 8 C. The results showed that chylomicrons, VLDL, and low density lipoprotein (LDL) were able to inhibit spe- cific binding of 125 I-labeled VLDL to the same extent (85.1% 6 13.1, 100% 6 6.8, 90.7% 6 23.2% inhibition, P 5 NS), but with markedly different efficiencies. The rank or- der of inhibition ( K i ) was chylomicrons (0.27 6 0.02 n M apoB) . VLDL (12.6 6 3.11 n M apoB) . LDL (34.8 6 11.1 n M apoB). By contrast, neither triglyceride (TG) liposomes, high density lipoprotein (HDL), nor LDL from patients with familial hypercholesterolemia were efficient at displacing the specific binding of 125 I-labeled VLDL to LPL (30%, 39%, and no displacement, respectively). Importantly, smaller hydrolyzed chylomicrons had less affinity than the larger chylomicrons ( K i 5 2.34 6 0.85 n M vs. 0.27 6 0.02 n M apoB respectively, P , 0.01). This was also true for hydro- lyzed VLDL, although to a lesser extent. Chylomicrons from patients with LPL deficiency and VLDL from hypertriglyc- eridemic subjects were also studied. Taken together, our results indicate an inverse linear relationship between chylo- micron size and K i whereas none was present for VLDL. We hypothesize that the differences in binding affinity demon- strated in vitro when considered with the differences in par- ticle number observed in vivo may largely explain the para- doxes we set out to study. —Xiang, S-Q., K. Cianflone, D. Kalant, and A. D. Sniderman. Differential binding of triglyc- eride-rich lipoproteins to lipoprotein lipase. J. Lipid Res. 1999. 40: 1655-1662.