Complementary probes for the electrochemical interface.

The functions of electrochemical energy conversion and storage devices rely on the dynamic junction between a solid and a fluid: the electrochemical interface (EI). Many experimental techniques have been developed to probe the EI, but they provide only a partial picture. Building a full mechanistic understanding requires combining multiple probes, either successively or simultaneously. However, such combinations lead to important technical and theoretical challenges. In this Review, we focus on complementary optoelectronic probes and modelling to address the EI across different timescales and spatial scales - including mapping surface reconstruction, reactants and reaction modulators during operation. We discuss how combining these probes can facilitate a predictive design of the EI when closely integrated with theory.

A critical evaluation of the catalytic role of CO2 in propane dehydrogenation catalyzed by chromium oxide from a DFT-based microkinetic simulation.

Propane dehydrogenation under CO2 is an important catalytic route to obtain propene with a good balance between selectivity and stability. However, a precise description of the catalytic role of CO2 in propane dehydrogenation is still absent. In this work, we focus on the elucidation of the role of CO2 by using DFT-based microkinetic simulation. The influence of CO2 is categorized as direct and indirect effects. It was found that the chemisorbed CO2 can directly abstract hydrogen from propane and propyl with a comparable barrier to the counterpart at the surface oxygen site. On the other hand, the dissociation of CO2 yields active surface species of CO* and O* which are actively involved in the removal of surface hydroxyls. It is found that the TOFs of both propane conversion and propene formation are significantly increased with the presence of CO2, which is explained by the reduced apparent activation energy. The primary hydrogen abstraction is identified to be the most influential step from the DRC analysis. The main effects of CO2 are concluded to be removing hydrogen and restoring oxygen vacancies from reaction pathway analysis.

Revealing the role of HBr in propane dehydrogenation on CeO2(111) via DFT-based microkinetic simulation.

HBr, as a soft oxidant, has been demonstrated to have a good balance between stability and selectivity in catalytic propane dehydrogenation. However, the origin of enhancements induced by HBr (hydrobromic acid) remains elusive. In this study, DFT-based microkinetic simulations were performed to reveal the reaction pathway and performance of propane dehydrogenation catalyzed by CeO2 in the presence of HBr. Three scenarios were under the investigations, which are pristine, dissociated HBr, and Br assisted surface hydroxyl. The calculations indicated that HBr significantly enhanced the adsorption of propane and provided alternative pathways for propene formation. More significantly, the energy barrier of C-H bond activation in propane was reduced with the assistance of HBr. It was very interesting to find that the reactivity of surface hydroxyl remarkably increased for C-H bond activation in the presence of HBr. The positive role of HBr is clearly evident from the microkinetic simulation. The TOFs of both propane conversion and propene formation increased after the introduction of HBr, which correlates with the apparent decreased activation energy. The reaction rate has a first order dependence on C3H8 and zero order dependence on HBr. The current study lays out a solid basis for further optimization of the performance of propane dehydrogenation.

Machine Learning Derived Blueprint for Rational Design of the Effective Single-Atom Cathode Catalyst of the Lithium-Sulfur Battery.

The "shuttle effect" and sluggish kinetics at cathode significantly hinder the further improvements of the lithium-sulfur (Li-S) battery, a candidate of next generation energy storage technology. Herein, machine learning based on high-throughput density functional theory calculations is employed to establish the pattern of polysulfides adsorption and screen the supported single-atom catalyst (SAC). The adsorptions are classified as two categories which successfully distinguish S-S bond breaking from the others. Moreover, a general trend of polysulfides adsorption was established regarding of both kind of metal and the nitrogen configurations on support. The regression model has a mean absolute error of 0.14 eV which exhibited a faithful predictive ability. Based on adsorption energy of soluble polysulfides and overpotential, the most promising SAC was proposed, and a volcano curve was found. In the end, a reactivity map is supplied to guide SAC design of the Li-S battery.

Revealing the role of nitrogen dopants in tuning the electronic and optical properties of graphene quantum dots via a TD-DFT study.

Graphene quantum dots (GQDs) have been suggested to have a wide range of applications due to their unique electronic and optical properties. Moreover, heteroatom doping has become a viable way to fine-tune the properties of GQDs. However, the working principle of the doping strategy is still not conclusive. In this study, the effects of size, configuration of the nitrogen dopant, and N/C ratio on the electronic and optical properties of GQDs have been carefully examined. First, the variation of the adsorption wavelength of pristine GQDs was evaluated for which a linear relation is established against different diameters. Moreover, it is found that both the configuration and content of nitrogen dopants have a significant impact on the adsorption wavelength and band gap of GQDs. In particular, different nitrogen species could have exactly opposite effects on the adsorption behavior. The origin of the nitrogen doping effect is calibrated from orbital localization, charge analysis, natural transition orbitals, and atomic contribution towards excitation. It is noted that nitrogen doping can simultaneously reduce both light adsorption energy and emission energy compared with the pristine one. This study provides an insightful explanation for the electronic and optical properties of GQDs and consolidates the theory base of the doping strategy.

Halogenation of graphene triggered by heteroatom doping.

Halogenation is one of the most important ways to tailor the properties of graphene. We demonstrate for the first time that boron and nitrogen doping can effectively tune the interactions between halogen diatomic molecules and graphene from first principles calculations. Boron and nitrogen doping disrupt the regular pi-electron pattern and create spin density and orbital polarization. More interesting, nitrogen and boron doping not only significantly increases the binding energies of Cl2, Br2, and I2 but also induces the spontaneous dissociation of F2. The tunable effects from nitrogen and boron doping can adjust the interactions in a wide range. Overall, it is suggested that doping can be a very promising method for the facile halogenation of graphene.

Oxidative dehydrogenation reaction of short alkanes on nanostructured carbon catalysts: a computational account.

Recent progress from first principles computational studies is presented for catalytic properties of nanostructured carbon catalysts in the oxidative dehydrogenation (ODH) reaction of short alkanes. Firstly, a brief introduction is given on the development of carbon catalysts in ODH since 1970. Oxygen functional groups have pivotal importance for ODH on nanostructured carbon catalysts. We discuss the oxidation process by HNO3 on pristine and defective carbon materials. The interactions between the oxygen molecule (oxidant) and the nanostructured carbon catalysts are quantitatively calibrated. Moreover the different nucleophilic abilities of oxygen functional groups are carefully compared and the strongest nucleophilic sites are proposed. The active sites and detailed reaction pathway are revealed from several computational studies. Diketone/quinone groups are generally considered to be the active centers in ODH. A reaction pathway via radical formation is considered as the favorable path. Furthermore, single ketone and carbon sites are verified to be active in ODH from the analysis of aromaticity. Heteroatom doping effects in ODH are examined. Nitrogen doping is found to be very reactive towards oxygen molecule activation. Other dopants such as boron, phosphorous and sulfur also have positive effects on the reactivity of ODH. Extensive calculations suggest that the BEP relation is applicable for the doped nanostructured carbon catalysts. In the end, an outlook for the future direction of the computational study is supplied.

The effect of defects on the catalytic activity of single Au atom supported carbon nanotubes and reaction mechanism for CO oxidation.

The mechanism of CO oxidation by O2 on a single Au atom supported on pristine, mono atom vacancy (m), di atom vacancy (di) and the Stone Wales defect (SW) on single walled carbon nanotube (SWCNT) surface is systematically investigated theoretically using density functional theory. We determine that single Au atoms can be trapped effectively by the defects on SWCNTs. The defects on SWCNTs can enhance both the binding strength and catalytic activity of the supported single Au atom. Fundamental aspects such as adsorption energy and charge transfer are elucidated to analyze the adsorption properties of CO and O2 and co-adsorption of CO and O2 molecules. It is found that CO binds stronger than O2 on Au supported SWCNT. We clearly demonstrate that the defected SWCNT surface promotes electron transfer from the supported single Au atom to O2 molecules. On the other hand, this effect is weaker for pristine SWCNTs. It is observed that the high density of spin-polarized states are localized in the region of the Fermi level due to the strong interactions between Au (5d orbital) and the adjacent carbon (2p orbital) atoms, which influence the catalytic performance. In addition, we elucidate both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms of CO oxidation by O2. For the LH pathway, the barriers of the rate-limiting step are calculated to be 0.02 eV and 0.05 eV for Au/m-SWCNT and Au/di-SWCNT, respectively. To regenerate the active sites, an ER-like reaction occurs to form a second CO2 molecule. The ER pathway is observed on Au/m-SWCNT, Au/SW-SWCNT and Au/SWCNT in which the Au/m-SWCNT has a smaller barrier. The comparison with a previous study (Lu et al., J. Phys. Chem. C, 2009, 113, 20156-20160.) indicates that the curvature effect of SWCNTs is important for the catalytic property of the supported single Au. Overall, Au/m-SWCNT is identified as the most active catalyst for CO oxidation compared to pristine SWCNT, SW-SWCNT and di-SWCNT. Our findings give a clear description on the relationship between the defects in the support and the catalytic properties of Au and open a new avenue to develop carbon nanomaterial-based single atom catalysts for application in environmental and energy related fields.