A novel two-dimensional Janus TiSiGeN4 monolayer with N vacancies for efficient photocatalytic nitrogen reduction.

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H2O and N2 into NH3 under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN4 structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN4 structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N2 through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN4 with the coexistence of both top and middle N vacancies (TiSiGeN4-VN-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.

A density functional theory study on the mechanism of toluene from dimethylcyclopentane catalyzed by the [GaH]2+ active site of Ga-ZSM-5.

The ONIOM (ωb97xd/6-31G(d,p):pm6) method was used to study the reaction mechanism of dimethylcyclopentane to toluene by the [GaH]2+ active site of Ga-ZSM-5. The results showed that the rate-determining step in the dimethylcyclopentane aromatization process is the ring expansion process. Compared to those of methylcyclopentane to benzene (D. D. Zhang, H. Y. Liu, L. X. Ling, H. R. Zhang, R. G. Zhang, P. Liu and B. J. Wang, Phys. Chem. Chem. Phys., 2021, 23, 10988-11003.), the free energy barriers of dimethylcyclopentane to toluene are significantly decreased, indicating that toluene is easier to produce than benzene, which confirmed the experimental results that a higher proportion of toluene than benzene is produced in the MTA process.

The effect of introducing Ga on the ZSM-5-catalyzed methanol to aromatics reaction: taking methylcyclopentane to benzene as an example.

Naphthenes are key intermediates in the formation of aromatic compounds during the methanol to aromatics (MTA) reaction, and the dehydrogenation process is more important than the hydrogen transfer process. Theoretical studies were performed to investigate the methylcyclopentane, which represents a naphthene, to benzene MTA process catalyzed by ZSM-5 before and after introducing Ga, showing that Ga-ZSM-5 was more favorable for carrying out the reaction than two H-type ZSM-5 (H-Z1 and H-Z2) models. H-Z1 and H-Z2 are favorable for the transfer of H during ring expansion reactions and the reformation of Brønsted acids, but the dehydrogenation reactions involving H-Z1 and H-Z2 require high free-energy barriers to be overcome. Although introducing Ga to ZSM-5 is not conducive to the transfer of H after dehydrogenation, it can reduce the extremely high dehydrogenation free-energy barrier compared with H-Z1 and H-Z2; this is mainly because Ga at dehydrogenation active centers, [GaH]2+, can accept electrons and donate them to the H atoms of [GaH]2+, giving H negative charge and making it easy to combine with positive B-acid H atoms that come from methylcyclopentane, cyclohexene, and cyclohexadiene to produce H2. Also, analysis of the transition state structures of all DH processes shows that Ga-ZSM-5 is more favorable for promoting the combination of H to produce H2 than H-Z1 and H-Z2.

Catalytic performance of Pdn (n = 1, 2, 3, 4 and 6) clusters supported on TiO2-V for the formation of dimethyl oxalate via the CO catalytic coupling reaction: a theoretical study.

The formation of dimethyl oxalate (DMO) via CO catalytic coupling on a series of catalysts including Pdn (n = 1, 2, 3, 4 and 6) clusters loaded on TiO2-V has been explored by density functional theory (DFT) calculation. The results show that different Pdn clusters have a remarkable influence on DMO formation. The Pd1/TiO2-V catalyst is not suitable for the CO catalytic coupling reaction since CO is easily bound to the O atom on the surface of TiO2-V leading to the formation of CO2. The activity of four catalysts complies with the following order of Pd4/TiO2-V > Pd6/TiO2-V > Pd2/TiO2-V > Pd3/TiO2-V by comparing the activation energy barriers of the rate-determining steps in the optimal paths. Charge analysis implies that less charge is transferred from the Pd4/TiO2-V and Pd6/TiO2-V catalysts to CO than on the other catalysts, which leads to the relatively weak adsorption of CO, and therefore CO has a greater tendency to react with other species on the surface. In addition, Pd6/TiO2-V also exhibits relatively higher selectivity toward DMO than the other three catalysts. Therefore, Pd6 is regarded as a suitable cluster, which is supported on TiO2-V demonstrating high catalytic activity and selectivity to DMO.

Understanding the Role of Surface Oxygen in Hg Removal on Un-Doped and Mn/Fe-Doped CeO2 (111).

Effects of surface-adsorbed O and lattice O for the CeO2 (111) surface on Hg removal has been researched. In this work, periodic calculations based on density functional theory (DFT) were performed with the on-site Coulomb interaction. Hg is oxidized to HgO via the surface-adsorbed O by overcoming a Gibbs free energy barrier of 114.1 kJ·mol-1 on the CeO2 (111) surface. Mn and Fe doping reduce the activation Gibbs free energy for the Hg oxidation, and energies of 70.7 and 49.6 kJ·mol-1 are needed on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. Additionally, lattice O also plays an important role in Hg removal. Hg cannot be oxidized leading to the formation of HgO on the un-doped CeO2 (111) surface owing to the inertness of lattice O, which can be easily oxidized to HgO on Ce0.96 Mn0.04 O2 (111) and Ce0.96 Fe0.04 O2 (111) surfaces. It can be seen that both surface-adsorbed O and lattice O play important roles in removing Hg. The present study will shed light on understanding and developing Hg removal technology on un-doped and Mn/Fe-doped CeO2 (111) catalysts. © 2019 Wiley Periodicals, Inc.

CO oxidative coupling to dimethyl oxalate over Pd-Me (Me = Cu, Al) catalysts: a combined DFT and kinetic study.

CO oxidative coupling to dimethyl oxalate (DMO) on Pd(111), Pd-Cu(111) and Pd-Al(111) surfaces was systematically investigated by means of density functional theory (DFT) together with periodic slab models and micro-kinetic modeling. The binding energy results show that Cu and Al can be fine substrates to stably support Pd. The favorable pathway for DMO synthesis on these catalysts starts from the formation of two COOCH3 intermediates, followed by the coupling to each other, and the catalytic activity follows the trend of Pd-Al(111) > Pd(111) > Pd-Cu(111). Additionally, the formation of DMO is far favorable than that of dimethyl carbonate (DMC) on these catalysts. The results were further demonstrated by micro-kinetic modeling. Therefore, Pd-Al bimetallic catalysts can be applied in practice to effectively enhance the catalytic performance and greatly reduce the cost. This study can help with fine-tuning and designing of high-efficient and low-cost Pd-based bimetallic catalysts.

Insights into the mechanism of the capture of CO2 by K2CO3 sorbent: a DFT study.

The adsorption and reactions of CO2 and H2O on both monoclinic and hexagonal crystal K2CO3 were investigated using the density functional theory (DFT) approach. The calculated adsorption energies showed that adsorption of H2O molecules was clearly substantially stronger on the K2CO3 surface than the adsorption of CO2, except on the (001)-1 surface of hexagonal K2CO3, where CO2 is competitively adsorbed with H2O. Carbonation reactions easily occur on pure K2CO3 and involve two parallel paths: one is where adsorbed H2O reacts with molecular CO2 in gas to form the bicarbonate, while the other is where H2O dissociates into OH and H before bicarbonate formation, and then OH reacts with gaseous CO2 to form a bicarbonate. Our results indicate that adding a support or promoter or using a special technique to expose more (001)-1 surfaces in hexagonal K2CO3 may improve the conversion of CO2 to the bicarbonate, which provides a theoretical direction for the experimental preparation of the K2CO3 sorbent to capture CO2.

Formation of C2 oxygenates and ethanol from syngas on an Fe-decorated Cu-based catalyst: insight into the role of Fe as a promoter.

In this study, the formation mechanism of C2 oxygenates and ethanol from syngas on Fe-decorated Cu bimetallic catalyst was investigated using density functional theory (DFT) calculations together with microkinetic modeling. The results showed that CH2 was the most favored monomer among all the CHx (x = 1-3) species over the FeCu bimetallic catalyst, which was more favorable than CH3OH formation. Namely, the FeCu catalyst exhibited a good selectivity toward CH2 formation instead of CH3OH formation in syngas conversion. Starting from the CH2 monomer, CH2CO and CH3CO via CO insertion into CH2 and CH2CO hydrogenation were the major products instead of C2 hydrocarbons or methane, CH3CO was successively hydrogenated to ethanol via CH3CHO and CH3CH2O intermediates. Moreover, the microkinetic modeling showed that the FeCu bimetallic catalyst had a high selectivity toward ethanol rather than methanol and methane. Further, the addition of Fe into the Cu catalyst promoted CHx formation by accelerating C-O bond cleavage, suppressed methanol formation, and facilitated C2 oxygenate formation rather than methane formation, suggesting that the synergetic effect between Fe and Cu played an important role in the formation of C2 oxygenates and ethanol. In addition, it is believed that the insights derived from this study can provide clues for the catalyst design of oxygenate synthesis and other bimetallic catalytic systems.

Effects of CO and CO2 on the desulfurization of H2S using a ZnO sorbent: a density functional theory study.

The density functional theory (DFT) method has been performed to study the effects of CO and CO2 on the desulfurization of H2S over a ZnO sorbent. It shows that COS is inevitably formed on the ZnO(101¯0) surface, which tends to be adsorbed onto the surface via a S-C bond binding with either a long or a short Zn-O bond. Potential energy profiles for the COS formation via reactions between H2S and CO, and H2S and CO2 on the ZnO(101¯0) surface have been constructed. In the presence of CO, the dissociated active S of H2S reacting with CO leads to the formation of COS, and the activation energy of the rate-determining step is 87.7 kJ mol(-1). When CO2 is present, the linear CO2 is first transferred to active CO2 in a triplet state, and then combines with active S to form COS with the highest energy barrier of 142.4 kJ mol(-1). Rate constants at different temperatures show that the formation of COS via the reaction of CO and H2S is easier than that of CO2 and H2S over the ZnO surface.

Ethane Dehydrogenation over the Core-Shell Pt-Based Alloy Catalysts: Driven by Engineering the Shell Composition and Thickness.

Pt-based catalysts as the commercial catalysts in ethane dehydrogenation (EDH) face one of the main challenges of realizing the balance between coke formation and catalytic activity. In this work, a strategy to drive the catalytic performance of EDH on Pt-Sn alloy catalysts is proposed by rationally engineering the shell surface structure and thickness of core-shell Pt@Pt3Sn and Pt3Sn@Pt catalysts from a theoretical perspective. Eight types of Pt@Pt3Sn and Pt3Sn@Pt catalysts with different Pt and Pt3Sn shell thicknesses are considered and compared with the industrially used Pt and Pt3Sn catalysts. Density functional theory (DFT) calculations completely describe the reaction network of EDH, including the side reactions of deep dehydrogenation and C-C bond cracking. Kinetic Monte Carlo (kMC) simulations reveal the influences of the catalyst surface structure, experimentally related temperatures, and reactant partial pressures. The results show that CHCH* is the main precursor for coke formation, and Pt@Pt3Sn catalysts generally have higher C2H4(g) activity and lower selectivity compared to those of Pt3Sn@Pt catalysts, which is attributed to the unique surface geometrical and electronic properties. 1Pt3Sn@4Pt and 1Pt@4Pt3Sn are screened out as catalysts exhibiting excellent performance; especially, the 1Pt3Sn@4Pt catalyst has much higher C2H4(g) activity and 100% C2H4(g) selectivity compared to those of 1Pt@4Pt3Sn and the widely used Pt and Pt3Sn catalysts. The two descriptors C2H5* adsorption energy and reaction energy of its dehydrogenation to C2H4* are proposed to qualitatively evaluate the C2H4(g) selectivity and activity, respectively. This work facilitates a valuable exploration for optimizing the catalytic performance of core-shell Pt-based catalysts in EDH and reveals the great importance of the fine control of the catalyst shell surface structure and thickness.

Engineering interfacial structures to accelerate hydrogen evolution efficiency of MoS2 over a wide pH range.

Developing low-cost electrocatalysts with outstanding electrochemical performance for water splitting over a wide pH range is urgently desired to meet the practical needs in different areas. Herein, a highly efficient hierarchical flower-like CoS2@MoS2 core-shell nanostructured electrocatalyst is fabricated by a two-step strategy, in which MoS2 nanosheets with a layered structure are grown on the CoS2 core supported on carbon paper (CP) and used as hydrogen evolution reaction (HER) electrocatalysts working in the whole pH range (0-14). Remarkably, benefiting from the interface-engineering in this 3D core-shell structure of the electrocatalyst, the optimum CoS2@MoS2/CP catalyst exhibits outstanding HER activity over a wide range of pH values and an overpotential of 69 mV in acidic solution, 145 mV in neutral solution and 82 mV in alkaline solution, respectively, to afford the standard current density of 10 mA cm-2. Furthermore, it demonstrates superior stability under different pH conditions for at least 48 h. Density functional theory (DFT) calculations are performed to gain further insight into the effect of CoS2@MoS2 interfaces, revealing that the strong interfacial interaction between CoS2 and MoS2 dramatically reduces the Gibbs free energy of hydrogen adsorption and the energy barrier for water dissociation, thus enhancing the electrochemical HER activity in the whole pH range (0-14).

The role of CS2 in CS2/NMP mixed solvent in weakening the hydrogen bond of OH-N in coal: a DFT investigation.

The interaction processes of trace amounts of N-methyl-2-pyrrolidinone (NMP), CS(2)/NMP (1:1 by volume) and pure NMP solvent with the hydrogen bond of OH⋯N in coal were constructed and simulated by density functional theory methods. The distances and bond orders between the main related atoms, and the hydrogen bond energy of OH⋯N were calculated. The calculated results show that pure NMP solvent does not weaken the hydrogen bond of OH⋯N in coal. However, trace amounts of NMP and CS(2)/NMP (1:1 by volume) have a strong capacity to weaken the hydrogen bond of OH⋯N in coal. The H2-N3 distances are elongated from 1.87 Å to 3.80 Å and 3.44 Å, the bond orders of H2-N3 all disappear, and the corresponding hydrogen bond energies of OH⋯N in coal decrease from 45.72 kJ mol(-1) to 7.06 and 11.24 kJ mol(-1), respectively. These results show that CS(2) added to pure NMP solvent plays an important role in releasing the original capacity of NMP to weaken the hydrogen bond of OH⋯N in coal, in agreement with experimental observations.

Density functional theory analysis of carbonyl sulfide hydrolysis: effect of solvation and nucleophile variation.

The detailed mechanisms of the hydrolysis of carbonyl sulfide (OCS) by nucleophilic water and hydroxide ion in both the gas phase and bulk water solvent have been investigated using density functional theory. Various reaction channels on the potential surface have been identified. The thermodynamic results demonstrate that the hydrolysis of OCS by nucleophilic water and hydroxide ion should proceed more favorably at low temperature. The hydrolysis of OCS by the hydroxide ion is the main reaction channel from thermodynamic and kinetic perspectives, and the bulk solvent can influence the rate-determining step in this channel. However, the solvent barely modifies the activation energy of the rate-determining step. For the hydrolysis of OCS by nucleophilic water, the solvent does not modify the rate-determining step, and the corresponding activation energy of the rate-determining step barely changes. This bulk solvent effect suggests that most of the contribution of the solvent is accounted for by considering one water molecule and a hydroxide ion.

A theoretical study on the hydrolysis mechanism of carbon disulfide.

The hydrolysis mechanism of CS(2) was studied using density functional theory. By analyzing the structures of the reactant, transition states, intermediates, and products, it can be concluded that the hydrolysis of CS(2) occurs via two mechanisms, one of which is a two-step mechanism (CS(2) first reacts with an H(2)O, leading to the formation of the intermediate COS, then COS reacts with another H(2)O, resulting in the formation of H(2)S and CO(2)). The other is a one-step mechanism, where CS(2) reacts with two H(2)O molecules continuously, leading to the formation of H(2)S and CO(2). By analyzing the thermodynamics and the change in the kinetic function, it can be concluded that the rate-determining step involves H and OH in H(2)O attacking S and C in CS(2), respectively, causing the C=S double bond to change into a single bond. The two mechanisms are competitive. When performing the hydrolysis of CS(2) with a catalyst, the optimal temperature is below 252°C.

Theoretical studies on reaction mechanism of H2 with COS.

The reaction mechanisms of H2 with OCS have been investigated theoretically by using density function theory method. Three possible pathways leading to major products CO and H2S, as well as two possible pathways leading to by-product CH4 have been proposed and discussed. For these reaction pathways, the structure parameters, vibrational frequencies and energies for each stationary point have been calculated, and the corresponding reaction mechanism has been given by the potential energy surface, which is drawn according to the relative energies. The calculated results show that the corresponding major products CO and H2S as well as by-product CH4 are in agreement with experimental findings, which provided a new illustration and guidance for the reaction of H2 with OCS.