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Ti3C2O2MXene has been proposed as a promising electrode material for alkali-ion batteries owing to its tunable physical and chemical properties without sacrificing the excellent metallic conductivity. However, it still suffers from low specific capacity due to its limited interlayer spacing, especially for a larger ion like sodium (Na). Sulfur doping was suggested as a viable strategy to improve the electrode’s storage performance. Herein, first-principles calculations and kinetic Monte Carlo (kMC) simulations were carried out to study the role of S doping on Li/Na intercalation. Based on experimental findings, two different doping sites, C (SC) and O (SO), with various S concentrations were reported and therefore used as the models in this study. Computations reveal that S doping on both C and O sites improves the electronic conductivity of the MXenes as their densities of states at the Fermi level are increased. In addition, the doped MXenes reveal an expanded lattice parameter in the normal direction, which agrees with experimental observations. However, only the SO-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the SO-doped MXenes improves stabilities and transport kinetics of ion intercalation as indicated by their significantly lower insertion energies and diffusion barriers when compared with those of the pristine system. The kMC simulations were carried out to account for anisotropic diffusion in the SO-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the SO-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li(Na) are improved by 4(8) orders of magnitude upon SOdoping. A fundamental understanding of the role of S doping on the improved capacitive kinetics serves as a good guide for developing MXene-based electrode materials for Li- and Na-ion batteries.

Characterization of Mo species on Rh metal particles in Rh–MoOx/C catalyst for C–O hydrogenolysis reactions was carried out with in situambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations. The as-prepared Rh–MoOx/C sample with high oxidation states of Rh3+, Rh4+and Mo6+was reduced with 0.2 Torr of H2. All the Rh species and most Mo species were reduced to Rh0and Mo4+during the heating to 570 K. The thickness of the Mo layers after the reduction was calculated by a numerical analysis based on the XPS intensity ratio and a uniform adlayer stacking model. The calculated averaged thickness was about 0.3 nm, suggesting the formation of Mo species with a monolayer order on the metallic Rh particle surface. The O 1s XPS showed the presence of Mo–OH and Mo–O oxygen species with a ratio of 1.4 in reduced Rh–MoOx/C. Considering the presence of residual Mo6+species with a large number of Mo6+–O bonds, the ratio of Mo4+–OH to Mo4+–O is estimated to be 2, suggesting the formation of MoO(OH)2. The DFT calculations of various monomeric MoOxHyspecies on the Rh(111) surface were carried out, and the stability was examined by the ab initiothermodynamic approach. The relative stability is changed when the surface coverage of hydrogen atoms is taken into account. The MoO(OH)2species is calculated to be the most stable species in the presence of surface hydrogens over a wide range of conditions, including those for the catalytic reaction (ca. 400 K, 100–1MPa H2).

Recently, the Pd–Au alloy catalyst has been developed for the hydrosilylation of internal alkynes as well as α,β-unsaturated ketones under mild conditions. In this work, the mechanism of the hydrosilylation reaction of internal alkynes on the Pd–Au bimetallic cluster and the Pd–Au(111) alloy surface has been investigated by density functional theory calculations. The calculated energy profiles show that the reaction follows the Chalk–Harrod mechanism. The rate-determining step is the hydrometalation in the Pd–Au cluster while it is the Si–C reductive elimination in the Pd–Au(111) alloy surface. The Pd site acts as the adsorption site and the reactive center as observed in experiments. The surrounding Pd–Au bridge and Au sites are also relevant for the bond activation and accepting the substrates or intermediates during the reaction, which is characteristic in the Pd–Au alloy catalysts and not available in the homogeneous catalyst. The hydrosilylation reaction proceeds without too much high energy barriers and too stable intermediates in the Pd–Au alloy catalyst. The monometallic Au and Pd catalysts are not preferable because of the high energy barriers (Au) or too stable intermediates (Pd). The present picture of the relevance of the Pd atomic site and its surrounding Pd–Au bridge or Au sites will be useful for developing the alloy catalysts for the related catalytic reactions.

V2O5has been proposed as a potential candidate for cathode materials of Li-ion batteries due to its high theoretical capacity and cost effectiveness, but it still suffers from high capacity fading and slow charge/discharge kinetics. To improve its electrochemical performance, heterostructure engineering with the Ti3C2O2MXene was computationally studied in this work. Herein, we carried out density functional theory calculations to study such effects on the electronic conductivity and Li intercalation kinetics of the cathode. We find that the formation of V2O5/Ti3C2O2is energetically favorable where the interaction at the interface is characterized as a weak van der Waals force. When the heterostructure is formed, electrons are transferred from Ti3C2O2to the V2O5surface where the charge accumulation induces small lattice distortion of the inner V2O5layer. Such charge accumulation and distortions, in turn, reduce the polaron–lattice interaction, leading to less stable polaron formation energy when compared with that of bulk V2O5(−0.20 vs −0.35 eV). This weakened polaron–lattice interaction enhances the polaron hopping kinetics as it correlates with smaller polaron hopping barriers. The higher hopping rate constant of the polaron alleviates the Li intercalation kinetics where the ion-coupled polaron movement, used to have polaron hopping as rate-limiting, is now ion diffusion-limiting with somewhat smaller barriers. The calculated average diffusion rate constant is slightly higher at the heterostructure (4.03 × 108s–1) than that in bulk V2O5(2.17 × 108s–1). Overall, it is suggested by our computational study that the improved electronic conductivity and ion diffusion kinetics could have their origin from the enhanced rate constant of polaron hopping at the interface of the heterostructure.

The effect of Sn promoter on a Ni/γ-Al2O3catalyst towards the catalytic activity and catalyst stability viathe deoxygenation of stearic acid for the production of diesel-range hydrocarbons has been investigated herein. The decarboxylation (DCO2) and decarbonylation (DCO) of stearic acid using NiSn catalysts are proposed as the main pathways to produce n-heptadecane (n-C17H36), whereas hydrodeoxygenation (HDO) is a minor pathway, as confirmed by the high ratio of C17 to C18 alkanes. Moreover, the Sn promoter in Ni/γ-Al2O3significantly improves the catalyst reusability by enhancing the tolerance to the carbon-induced deactivation. This improvement was investigated by a density functional theory (DFT) calculation. The calculation results clearly explained that the Sn promoter can weaken the interaction between carbon and the catalyst surface, tending to prevent carbon segregation on the NiSn surface. This retardation of coke formation expands the life time of the catalyst, leading to a cost-effective approach for industrial processes.

Here, propane dehydrogenation (PDH) to propylene and side reactions, namely, cracking and deep dehydrogenation on Ni(111) surface, have been theoretically investigated by density functional theory calculation. On the basis of adsorption energies, propane is physisorbed on Ni(111) surface, whereas propylene exhibits chemisorption supported by electronic charge results. In the PDH reaction, possible pathways can occur via two possible intermediates, i.e., 1-propyl and 2-propyl. Our results suggest that PDH reaction through 1-propyl intermediate is both kinetically and thermodynamically more favorable than another pathway. The C–C bond cracking during PDH process is more difficult to occur than the C–H activation reaction because of higher energy barrier of the C–C bond cracking. However, deep dehydrogenation is the preferable process after PDH, owing to the strong adsorption of propylene on Ni(111) surface, resulting in low selectivity of propylene production. This work suggests that Ni(111) has superior activity toward PDH; however, the enhancement of propylene desorption is required to improve its selectivity. The understanding in molecular level from this work is useful for designing and developing better Ni-based catalysts in terms of activity and selectivity for propane conversion to propylene.

The complete reaction mechanism of H2S desulfurization on anatase TiO2(001) surface was elucidated using the plane-wave based density functional theory (DFT) method. The reaction starts from the dissociative adsorption of H2S on the TiO2surface. Subsequently, two competitive routes, H2O and H2formation, were investigated. The activation barriers for H2O formation range from 11 to 13 kcal mol−1, whereas those for H2formation are extremely high in the range of 67–87 kcal mol−1. On the basis of the activation energy barriers, the results indicate that the anatase TiO2(001) is very active for H2S desulfurization to produce H2O, resulting in S-substitution at the O2csites on the TiO2(001) surface. Electronic charge analyses indicate that S-doping onto the TiO2surface can enhance the photocatalytic activity of TiO2by reducing its band gap. In addition, by comparison with other metal oxide catalysts, such as TiO2(101), CeO2(111), CeO2(101), ZnO (1010) and α-Fe2O3(0001), we found that TiO2(001) is the most promising catalyst for H2S desulfurization.

Surface engineering of TiO2is significant in the SCR reaction and worth studying. In a previous study, we found that TiO2nanosheets (TiO2-NS) with the (001) plane exhibited a much better SCR performance than TiO2nanoparticles (TiO2-NP) with the (101) plane. In this work, in situDRIFTS and DFT calculations were applied to address the promotional effect of the (001) plane of TiO2-NS in the SCR reaction. The behavior of adsorption and desorption of NOxand NH3on the two surfaces was studied. It was found that NH3adsorbed on both TiO2-NS and TiO2-NP was in the form of NH3(g) on the Lewis acid sites. NOxon TiO2-NP were mainly trans-(NO)2and N2O4which were non-reactive. Differently, NO on TiO2-NS was mainly in the form of NO2which was probably due to the high energy and abundant active oxygen species of the (001) facet. The formed NO2could trigger the subsequent ‘fast SCR’ reaction thereafter promoting the activity. Therefore, TiO2-NS with more (001) facets had better SCR activity than TiO2-NP with the exposed (101) facet.

Vanadate-based compounds, in particular LiV3O8, are promising candidates for cathode materials of Li-ion batteries. Thanks to their open-layered structures and the various possible oxidation states of the V metal center, LiV3O8can effectively accommodate Li ions and store electron potential. To further improve the transport kinetics of the cathode, in this work, we used the first-principles method to explore the effects of oxygen vacancies on Li insertion, Li diffusion, and electronic conduction in the form of polaron hopping. We find that the polaron is mobile in the [010] direction with an effective hopping barrier, Ea,eff, of 0.11 eV and is sluggish in other directions with an Ea,effof 0.56 eV. Such anisotropic conduction is also observed in experiments. Interestingly, unlike other transition metal oxides, formation of a polaron negligibly affects Li insertion and diffusion, where the charge transport kinetics is solely limited by the ion movement with an Ea,effof 0.50 eV. The introduction of an oxygen vacancy, VO, creates two polarons at the two nearby V centers where at least one of them is relatively mobile (Ea,eff= 0.16 eV) contributing to electronic conductivity of the materials. At low VOconcentrations of up to 1%, the most stable VOexists far from the Li diffusion path and does not affect the ion transport kinetics. In contrast, if the VOconcentration increases to 2–3%, the second most stable VOstarts to form at the diffusion path, which greatly diminishes Li diffusion. Hence, it is suggested that controlling the low concentration of VOwithin 1% can enhance electronic conductivity by increasing the concentration of charge carriers while maintaining the ion diffusivity of the LiV3O8cathode.