Search

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 Mo3Au(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 Mo3Au 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

The pH‐dependent surface‐enhanced Raman spectra of two typical surface sensor molecules p‐mercaptobenzoic acid (PMBA) and p‐mercaptopyridine (PMPY) were simulated by density functional theory calculations. First, the acid dissociation constants and individual surface Raman spectra of PMBA and PMPY were computed and compared with experimental results. It was found that acid dissociation constants calculated by the hybrid implicit‐explicit model and surface‐enhanced Raman scattering (SERS) spectra simulated by the explicit model most closely coincide with experimental results. Then, the pH‐dependent SERS spectra of PMBA and PMPY were obtained by overlaying the SERS of individual acid species and base species multiplied by their molar fractions, which were calculated from the acid dissociation constants. During the deprotonation process of PMBA, the Raman intensity from COOH stretching decreases and the Raman intensity from COO− stretching increases. During the protonation process of PMPY, the ν8a mode undergoes a significant blueshift and the relative Raman intensity of ν7a mode decreases. At last, pH calibration curves of PMBA and PMPY were obtained according to the simulated pH‐dependent Raman spectra, in which the logarithmic value of relative Raman intensity of characteristic peaks varies almost linearly with solution pH. [ABSTRACT FROM AUTHOR]

CO 2 can be electrochemically reduced to different products depending on the nature of catalysts. In this work, we report comprehensive kinetic studies on catalytic selectivity and product distribution of the CO 2 reduction reaction on various metal surfaces. The influences on reaction kinetics can be clearly analyzed from the variation of reaction driving force (binding energy difference) and reaction resistance (reorganization energy). Moreover, the CO 2 RR product distributions are further affected by external factors such as electrode potential and solution pH. A potential-mediated mechanism is found to determine the competing two-electron reduction products of CO 2 that shifts from thermodynamics-controlled product formic acid at less negative electrode potentials to kinetic-controlled product CO at more negative electrode potentials. Based on detailed kinetic simulations, a three-parameter descriptor is applied to identify the catalytic selectivity of CO, formate, hydrocarbons/alcohols, as well as side product H 2 . The present kinetic study not only well explains the catalytic selectivity and product distribution of experimental results but also provides a fast way for catalyst screening., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The electrochemical reduction of CO 2 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 CO 2 reduction reactions, different from the previous hydrogen atom transfer model. It is found that the onset potentials and rate-determining steps for CO and CH 4 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 CO 2 on various copper crystal surfaces. Among the four studied copper single-crystal surfaces, Cu(211) exhibits the best electrocatalytic activity for CO formation and CH 4 formation due to its low onset potential and overpotential.