Mechanisms of CH4 activation over oxygen-preadsorbed transition metals by ReaxFF and AIMD simulations.

The chemisorbed oxygen usually promotes the CH bond activation over less active metals like IB group metals but has no effect or even an inhibition effect over more active metals like Pd based on the static electronic structure study. However, the understanding in terms of dynamics knowledge is far from complete. In the present work, methane dissociation on the oxygen-preadsorbed transition metals including Au, Cu, Ni, Pt, and Pd is systemically studied by reactive force field (ReaxFF). The ReaxFF simulation results indicate that CH4 molecules mainly undergo the direct dissociation on Ni, Pt, and Pd surfaces, while undergo the oxygen-assisted dissociation on Au and Cu surfaces. Additionally, the ab initio molecular dynamics (AIMD) simulations with the umbrella sampling are employed to study the free-energy changes of CH4 dissociation, and the results further support the CH4 dissociation pathway during the ReaxFF simulations. The present results based on ReaxFF and AIMD will provide a deeper dynamic understanding of the effects of pre-adsorbed oxygen species on the CH bond activation compared to that of static DFT.

Investigation of the oxygen coverage effect on the direct epoxidation of propylene over copper through DFT calculations.

The direct epoxidation of propylene is one of the most important selective oxidation reactions in industry. The development of high-performance copper-based catalysts is the key to the selective oxidation technology and scientific research of propylene. The mechanism of propylene's partial oxidation catalyzed by Cu(111) under different oxygen coverage conditions was studied using density functional theory calculations and microkinetic modeling. We report here in detail two parallel reaction pathways: dehydrogenation and epoxidation. The transition states and energy distributions of the intermediates and products were calculated. The present results showed that propylene oxide (PO) selectivity was high under low oxygen coverage, and increasing the oxygen coverage would decrease the PO selectivity but increase the PO activity, and there was an inverse relationship between PO selectivity and activity. Increasing oxygen coverage would reduce the energy barrier for the C-O bond formation of C3H5O due to the weaker adsorption strength of C3H5, thus decreasing the PO formation selectivity. On the other hand, increasing oxygen coverage would reduce the energy barrier for the possible reaction steps of propylene epoxidation in general, and thus increasing the catalytic activity. It might be proposed that the active site for propylene epoxidation is the metallic copper or partially oxidized copper in terms of the change of PO formation selectivity with oxygen coverage.

M supported on Al-defective Al2-δO3 (M = Fe, Co, Ni, Cu, Ag, Au) as catalysts for acetylene semi-hydrogenation: a theoretical perspective.

Semi-hydrogenation of acetylene is of great importance for both industry and academia. High prices and limited supplements of noble metals leave room for developing base metal catalysts. Experiments revealed the atomically dispersed Cu supported by Al2O3 with excellent long-term stability and high ethylene selectivity, but the physical nature has rarely been investigated theoretically. DFT calculations and microkinetic modeling revealed that the surface OH species could stabilize Cu1/Al2-δO3 and enhance its catalytic performance. The selectivity of ethylene formation decreases with increasing copper clusters (e.g., Cu1/Al2-δO3> Cu4/Al2-δO3> Cu8/Al2-δO3), meaning that the atomically dispersed copper may be a potential candidate for acetylene semi-hydrogenation. The structures of a series of single site catalysts M1/Al2-δO3 (M = Fe, Co, Ni, Ag, Au) are similar to that of Cu1/Al2-δO3, but their performances in catalyzing acetylene semi-hydrogenation are different. M1/Al2-δO3 (M = Ag, Au) shows higher selectivity than Cu1/Al2-δO3, while M1/Al2-δO3 (M = Fe, Co, Ni) demonstrates a higher turnover frequency (TOF) of ethylene than Cu1/Al2-δO3. Moreover, our results indicate that the Ni1-Cu1/Al2-δO3 alloy shows both high activity and ethylene selectivity. The present results show a compensation between the reactivity and the selectivity, suggesting that alloys of VIIIB metals with IB metals like Ni1-Cu1/Al2-δO3 may be efficient candidate catalysts in acetylene selective hydrogenation.

Electronic Structure Modulation of Ag2 S by Vacancy Engineering for Efficient Bacterial Infection.

Vacancy engineering can modulate the electronic structure of the material and thus contribute to the formation of coordination unsaturated sites, which makes it easier to act on the substrate. Herein, Ag2 S and Ag2 S-100, which mainly have vacancy associates VAgS and VAgSAg , respectively, are prepared and characterized by positron annihilation spectroscopy. Both experimental and theoretical calculation results indicate that Ag2 S-100 exhibits excellent antibacterial activity due to its appropriate bandgap and stronger bacteria-binding ability, which endow it with a superior antibacterial activity compared to Ag2 S in the absence of light. The in vivo antibacterial experiment using a mouse wound-infection model further confirms that Ag2 S-100 has excellent antibacterial and wound-healing properties. This research provides clues for a deeper understanding of modulating electronic structures through vacancy engineering and develops a strategy for effective treatment of bacterial infections.

Metallic S-CoTe with Surface Reconstruction Activated by Electrochemical Oxidation for Oxygen Evolution Catalysis.

Developing highly active electrocatalysts toward oxygen evolution reaction (OER) is critical for the application of water splitting for hydrogen production and can further alleviate the energy crisis problem, but still remaining challenging. Especially, unlocking the catalytic site, in turn, helps design the available catalysts. Herein, the nanorod cobalt telluride with sulfur incorporation grown on a carbon cloth (S-CoTe/CC) as catalysts for OER, which displays extraordinary catalytic activity, is reported. Significantly, the in situ formed CoOOH species on the surface of S-CoTe merited from the structure evolution during the OER process serves as the active species. Furthermore, density functional theory calculations demonstrate that sulfur incorporation can tailor the electronic structure of active species and substantially optimize the free energy, accelerating the OER kinetics. This work provides an in-depth understanding of enhanced OER mechanism through foreign elements incorporating into precatalysts and is beneficial for the guiding design of more efficient catalysts.

First-principles theoretical study on dry reforming of methane over perfect and boron-vacancy-containing h-BN sheet-supported Ni catalysts.

The entire reaction mechanism of the dry reforming of methane (DRM) as well as the competition processes over perfect and boron-vacancy-containing h-BN sheet-supported Ni-catalysts (labeled Ni2/h-BN and Ni2/h-BN-B-D) was studied by density functional theory calculations in the present work. Our calculation results show that B-defected h-BN strongly binds to the Ni2 active sites (i.e., shows a strong metal-support interaction (SMSI) character) due to the better electron transfer between Ni2 sites and the support. It was found that CH4 is easier to activate than molecular CO2. The activation of CO2 occurs on the surface of Ni2/h-BN through a direct route, whereas it is prone to follow a hydrogen-assisted path for Ni2/h-BN-B-D via the COOH* intermediate, and the results show that the oxidant O* is easily formed on the surface of Ni2/h-BN-B-D. It was also found that O* is the main oxidant agent for CHx* intermediates through the CH3-O oxidation mechanism. The reaction kinetic analysis indicated that the reverse water gas shift reaction (RWGS) is much more favorable than DRM (1.30 vs. 1.72 eV) over the Ni2/h-BN system, whereas the RWGS and DRM are comparable on Ni2/h-BN-B-D (1.77 vs. 1.66 eV), suggesting a high DRM activity on Ni2/h-BN-B-D. Moreover, neither methane cracking nor a Boudouard reaction to form C* species is thermodynamically and kinetically unfavorable over Ni2/h-BN-B-D; hence, Ni2/h-BN-B-D has strong resistance to carbon deposition. Compared to Ni(111), both Ni2/h-BN-B-D and Ni2/h-BN show strong resistance to carbon deposition. Our results provide a further mechanistic understanding of the DRM over an Ni-based catalyst through the SMSI characteristic and the SMSI favors strong resistance to carbon deposition.

Sulfur-Deficient Bismuth Sulfide/Nitrogen-Doped Carbon Nanofibers as Advanced Free-Standing Electrode for Asymmetric Supercapacitors.

The use of free-standing carbon-based hybrids plays a crucial role to help fulfil ever-increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect-rich bismuth sulfides in combination with surface nitrogen-doped carbon nanofibers (dr-Bi2 S3 /S-NCNF) as flexible free-standing electrodes for asymmetric supercapacitors. The dr-Bi2 S3 /S-NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g-1 at a discharge current density of 1 A g-1 . The high performance of dr-Bi2 S3 /S-NCNF electrodes originates from its hierarchical structure of nitrogen-doped carbon nanofibers with well-anchored defect-rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr-Bi2 S3 /S-NCNF electrodes exhibit a reduced OH- adsorption energy of -3.15 eV, compared with that (-3.06 eV) of defect-free bismuth sulfides/surface nitrogen-doped carbon nanofiber (df-Bi2 S3 /S-NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr-Bi2 S3 /S-NCNF hybrid as the negative electrode and S-NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg-1 at a power density of 677.3 W kg-1 . This work demonstrates a feasible strategy to construct advanced metal sulfide-based free-standing electrodes by incorporating defect-rich structures using surface engineering principles.

Water dissociation on K2O-pre-adsorbed transition metals: a systematic theoretical study.

It is imperative to regulate O-H bond cleavage on metal surfaces with a pre-adsorbed K2O promoter in heterogeneous catalysis. Density functional theory (DFT) calculations have been performed to explore the adsorption and dissociation of water on K2O-pre-adsorbed transition metal surfaces (Au, Ag, Cu, Ni, Pt, Rh, Ir, Pd, Ru, Co and Fe) as compared with those on clean and K-pre-adsorbed metal surfaces. The calculation results indicate that the presence of K2O species significantly promotes water dissociation and the promoting effect depends on the adsorption strength of K2O, namely, the more strongly K2O binds to the metal surface, the less promoting effect it has on the water O-H bond cleavage. Based on geometrical and electronic analysis, the stronger promoting effect of K2O than K on water dissociation on the given metal surfaces can be attributed to stronger attractive electrostatic interactions between OH and the dissociating H of H2O at the TSs as well as between O of H2O and K of K2O at the ISs on K2O-pre-adsorbed surfaces compared with those on K-pre-covered surfaces. Moreover, the additional hydrogen bond interaction between H and Oad of K2O at the ISs on Cu/Ag/Au and Fe/Co/Ni metals would be responsible for the much greater promoting effect of K2O than K on these metal surfaces, while there is a slightly greater promoting effect of K2O on the remaining metal surfaces. From the above analysis, we expect our studies can provide profound understanding of the nature of the promoting effect of K2O on O-H bond scission.

Ethanol synthesis from syngas over Cu(Pd)-doped Fe(100): a systematic theoretical investigation.

Although the reaction mechanism of syngas on Fe or Cu(Pd)-doped Fe has been studied extensively both experimentally and theoretically, the systematic prediction of the catalytic activity and selectivity for the formation of ethanol at the molecular level has not been reported to the best of our knowledge. In our present work, density functional theory calculations were performed to investigate the reaction mechanisms of the synthesis of ethanol, methanol, and methane from syngas over bimetallic Cu/Fe and Pd/Fe catalysts. Possible elementary steps involved in the formation of ethanol from syngas have been studied from thermodynamic and kinetic viewpoints. Our results show that an optimal route for the formation of ethanol on Cu/Fe and Pd/Fe catalysts starts with an initial process of the dissociation and hydrogenation of CO to produce CH3 species. Subsequently, the insertion of HCO groups into CH3 species leads to the formation of CH3CHO, followed by successive hydrogenation to form ethanol. The selectivity for ethanol is controlled by the formation of methyl species and the formation of C-C bonds between methyl species and CHO groups. Our kinetic model analysis shows that the selectivity for ethanol is highest on the Cu-Fe system, followed by Pd-Fe, and pure Fe (Cu) has the lowest selectivity. This is in close agreement with experimental findings in general. Possible reasons can be explained as follows: Fe sites favor the formation of CHx species, Cu and Pd sites are necessary to provide undissociated CO/HCO species, and Cu/Fe and Pd/Fe catalysts will provide dual active sites that are synergetic for chain propagation to generate precursors of C2 oxygenates by the insertion of CO/HCO groups into CHx species. The present results will further help the prediction of reaction activity and the design of efficient F-T catalysts for the formation of C2+ species to some extent.

An Efficient, Visible-Light-Driven, Hydrogen Evolution Catalyst NiS/Znx Cd1-x S Nanocrystal Derived from a Metal-Organic Framework.

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal-organic framework (MOF)-template strategy was developed to prepare non-noble metal co-catalyst/solid solution heterojunction NiS/Znx Cd1-x S with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine-tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn0.5 Cd0.5 S exhibits an optimal HER rate of 16.78 mmol g-1 h-1 and high stability and recyclability under visible-light irradiation (λ>420 nm). Detailed characterizations and in-depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.

Periodic density functional theory analysis of direct methane conversion into ethylene and aromatic hydrocarbons catalyzed by Mo4C2/ZSM-5.

Mo/ZSM-5-catalyzed methane conversion into aromatic hydrocarbons is an important reaction to produce ethylene and benzene, but the detailed reaction mechanism has not been investigated due to its high complexity. In the present study, density functional theory combined with a periodic model was used to investigate the reaction mechanism of direct methane conversion into aromatic hydrocarbons catalyzed by Mo/ZSM-5. The calculation results show that the active phase for Mo is Mo4C2 instead of MoOx. The whole reaction processes processed via the following steps: the C-H bond in methane was first activated by Mo4C2 with an energy barrier of 1.01 eV and then converted into ethylene species via the coupling of two CH3 species as well as two successive dehydrogenation steps (2CH3 → C2H6 → C2H4 + 2H). The rate-controlling step for the processes to form ethylene is the coupling of two methyl species with a barrier of 1.22 eV. The produced ethylene species then react with each other to produce C6H8via the reaction of 3C2H4 → C3H8 + 2H2, and molecular benzene is formed by successive dehydrogenation of C6H8. The rate-limiting step for benzene formation from ethylene is the cyclization step of chain C6H8 with an energy barrier of 1.21 eV. Additionally, molecular propane (C3H8) is formed by the reaction of C2H4 + CH4 → C3H8, and the controlling step C3H7 + H → C3H8 has a barrier of 1.46 eV. Molecular C10H12 is produced via coupling of C6H8 and C2H4, where the limiting step is the dehydrogenation step of C8H12 with an energy barrier of 1.44 eV. Our present calculation results indicate that the selectivity of benzene was the largest among the possible products, that is, C2H4, C3H6, C6H6 and C10H12, based on the corresponding controlling step barrier. Importantly, the rate-controlling step for the whole reaction process from methane to benzene is the dissociative adsorption of methane (CH4 → CH3 + H) with an energy barrier of 1.83 eV when considering entropy contribution. The present study may help people design a good catalyst for the formation of benzene from methane; in other words, the catalyst should have a good ability to activate the C-H bond of molecular methane.

Energy Transfer Dynamics of Formate Decomposition on Cu(110).

Energy transfer dynamics of formate (HCOOa ) decomposition on a Cu(110) surface has been studied by measuring the angle-resolved intensity and translational energy distributions of CO2 emitted from the surface in a steady-state reaction of HCOOH and O2 . The angular distribution of CO2 shows a sharp collimation with the direction perpendicular to the surface, as represented by cosn θ (n=6). The mean translational energy of CO2 is measured to be as low as 100 meV and is independent of the surface temperature (Ts ). These results clearly indicate that the decomposition of formate is a thermal non-equilibrium process in which a large amount of energy released by the decomposition reaction of formate is transformed into the internal energies of CO2 molecules. The thermal non-equilibrium features observed in the dynamics of formate decomposition support the proposed Eley-Rideal (ER)-type mechanism for formate synthesis on copper catalysts.

Highly Active and Stable Catalysts of Phytic Acid-Derivative Transition Metal Phosphides for Full Water Splitting.

Application of transition metal phosphide (TMP) catalysts for full water splitting has great potential to help relieve the energy crisis. Various methods have been investigated to obtain high catalytic activity, but the use of electronic structure regulation by incorporation of different elements is of particular simplicity and significance for development of a universal TMP synthesis method. We herein describe a novel approach for fabricating a series of TMPs by pyrolyzing phytic acid (PA) cross-linked metal complexes. The introduction of oxygen atoms into TMPs not only enhanced their intrinsic electrical conductivity, facilitating electron transfer, but activated active sites via elongating the M-P bond, favoring the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER). MoP exhibited relative low HER overpotentials of 118 mV and 93 mV while supporting a current density of 20 mA·cm-2 in 0.5 M H2SO4 and 1 M KOH electrolytes, respectively. When CoP was applied as a catalyst for OER, only 280 mV overpotential was required to reach current density of 10 mA·cm-2. Additionally, PA-containing precursors enabled intimate embedding of TMPs onto a flexible substrate surface (carbon cloth), so that electron injection from substrate and transport to the active sites was facilitated. Remarkably, an alkaline electrolyzer was able to achieve a current density of 40 mA·cm-2 at the low voltage of 1.6 V, demonstrating its potential for practical overall water splitting without the use of noble metals.

Oxygen-assisted water partial dissociation on copper: a model study.

It is essential to understand and control the O-H bond cleavage on metal surfaces with pre-adsorbed oxygen atoms in heterogeneous catalytic processes. The adsorption and dissociation of water on clean and oxygen-pre-adsorbed copper surfaces, including Cu(111), Cu(110), Cu(100), Cu(210), Cu(211), Cu(310) and Cu(110)-(1 × 2), as well as Cu-ad-row and Cu-ad-atom, have been investigated by the density functional theory-generalized gradient approximation (DFT-GGA) method. The calculation results indicate that the presence of oxygen species significantly promotes the water dissociation. It is found that the promotion effect depends both on the adsorption energy of the pre-adsorbed oxygen and the distance between the pre-adsorbed oxygen and the stripped hydrogen in water: the more strongly the oxygen atom binds to the metal surface, the less the promotion effect it has on the water O-H bond cleavage; the shorter the distance between pre-adsorbed oxygen and hydrogen in water, the greater is the promotion effect. Based on electronic analysis, physical origin of the promotion effect can be attributed to the strong interaction of acid-base pair sites on oxygen-metal systems.

A density functional theory study of ethylene hydrogenation on MgO- and γ-Al2O3-supported carbon-containing Ir4 clusters.

Density functional theory was used to investigate the reaction mechanisms of ethylene hydrogenation on MgO(100)- and γ-Al2O3(110)-supported carbon-containing Ir4 clusters. The cluster supported on γ-Al2O3(110) is more active than that on MgO(100), which is consistent with experimental observations. The present calculations show that the binding energies of reactants on the carbon-containing Ir4 cluster are weaker on the γ-Al2O3 supported catalysts compared to the MgO supported Ir cluster. This relatively weak adsorption energy of ethylene on the γ-Al2O3 surface means that ethylene desorption is easier, hence a higher catalytic activity is achieved. To gain further understanding, the energy decomposition method and micro-kinetic analysis are also introduced.

Geometric matching principle for adsorption selectivity of ionic liquids: a simple method into the fascinating world of shape-controlled chemistry.

Ionic liquids (ILs) possess effective functions in controlling the phase and morphology of nanomaterials. However, it is still unclear how ILs affect the morphology control and what the origin of adsorption selectivity of ILs is on different crystal facets. It is a challenge to develop a simple method to select the suitable kinds of ILs for achieving the controllable synthesis of nanomaterials with designable shape. Herein, density functional theory (DFT) calculations were combined with experiment to study the interaction mechanism between ILs and crystal facets. An important relationship is proposed, named as the geometric matching principle, in which the adsorption site of substrate should not only need to meet the space requirement for interionic stacking of ILs, but also needs to maximize the interaction between adsorbed ILs and substrate. This new finding is meaningful for prediction of the adsorption selectivity of ILs and clarification of their shape-controlled chemistry.

Insight into the general rule for the activation of the X-H bonds (X = C, N, O, S) induced by chemisorbed oxygen atoms.

Density functional theory calculations are presented for adsorption and dissociation of NH3, H2O, CH3OH, H2S and C2H4 on clean and oxygen atom pre-adsorbed metal surfaces (Cu, Ag, Au, Ni, Pd, Pt, Rh, Ru, Os and Ir). The calculation results indicated that the oxygen-promotion effect depends both on the metallic activity and the character of the X-H bond. On the one hand, for a given reaction on a series metals, a good linear correlation was found between the energy barrier difference of X-H bond breaking on clean and oxygen-covered metals and the binding strength of oxygen on metals, namely an oxygen-promotion effect was favorable to the less active metals but unfavorable to the more active metals. On the other hand, for a series of X-H bond breaking reactions on a given metal, it was found that the promotion effect follows the trend of O-H > N-H > C-H, that is, the O-H bond is most promoted by the oxygen atom. The possible reason is the O-H bond forms the strongest hydrogen bond in the transition state among the X-H bonds investigated in this work. Additionally, it was found that the oxygen coverage has little effect on the X-H bond scission.

A DFT + U study of acetylene selective hydrogenation over anatase supported PdaAgb (a + b = 4) cluster.

It is well known that the addition of Ag into Pd can promote the selectivity of acetylene hydrogenation to ethylene, and early theoretical studies focus on ideal single crystal model catalysts, so it is worth studying relatively realistic catalyst models, such as metal oxide supported PdAg systems. In this work, the reaction mechanisms for acetylene selective hydrogenation on the anatase TiO2(101) supported PdaAgb (a + b = 4) cluster are studied by density functional theory calculations with a Hubbard U correction. The results show that Ag addition to the Pd4 cluster reduces the interaction between the PdAg cluster and the support, and the possible reason is that the amount of electron transfer from the TiO2 support to the PdAg cluster decreases with increasing number of Ag atoms. Consequently the adsorption energies of acetylene and ethylene would become smaller on the anatase supported PdAg cluster as compared to that on the anatase supported Pd4 cluster, and this may help to enhance the selectivity of ethylene formation. Moreover, the reaction kinetics study of acetylene hydrogenation on anatase TiO2(101) supported PdAg cluster shows that the activation energy of the hydrogenation step is higher on the PdAg cluster than that on the pure Pd4 cluster, and thus reduces its catalytic activity. Importantly, the present calculation results suggested that the selectivity of ethylene formation, which is defined as the energy difference between the adsorption energy of ethylene and the barrier for its further hydrogenation, varies with the ratio of Pd/Ag in the PdAg cluster: the Pd3Ag system shows relatively low selectivity compared to that of the pure Pd4 cluster, whereas Pd2Ag2/PdAg3 displays higher selectivity than that of the pure Pd4 cluster. Furthermore, our present results demonstrated that the anatase support plays a key role in the acetylene hydrogenation processes: on one hand, it reduces the reaction activity of acetylene hydrogenation processes compared to the Pd2Ag2/Pd(111) and Pd2Ag2 clusters; on the other hand, it enhances the selectivity of ethylene due to its lower desorption energy. It was also found that the carbon species inside the Pd2Ag2 cluster has little effect on the catalytic selectivity towards ethylene formation, whereas the hydrogenation catalytic activity is enhanced significantly. Finally the role of the Pd2Ag2-anatase interface on the catalytic properties of acetylene hydrogenation was studied, and it was found that the interface can increase the activity of acetylene hydrogenation but the selectivity is not improved.

Dissociative adsorption of 2,3,7,8-TCDD on the surfaces of typical metal oxides: a first-principles density functional theory study.

The initial dissociative adsorption step of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) molecule on the surfaces of MgO, CaO, and CuO has been studied by density functional theory (DFT) using periodic slab models. It is found that the 2,3,7,8-TCDD molecule undergoes a similar dissociative adsorption step during the decomposition over the three metal oxide surfaces. The adsorption configuration of 2,3,7,8-TCDD first converts from a parallel mode into a vertical one, then a nucleophilic substitution process takes place, where the surface oxygen atom attacks the aromatic carbon to form a surface phenolate with the chlorine atom moving to the top of the nearest surface metal atom. The calculated apparent activation energy of the dissociation increases in the order of CuO < CaO < MgO. The reaction heat is -0.67 eV, -0.75 eV, and 0.45 eV for CuO, CaO, and MgO, respectively, suggesting the thermodynamic tendency of MgO < CuO < CaO, which parallels the trend of the nucleophilicity of surface oxygen atoms. This study suggests that metal oxides with more nucleophilic and less tightly-bonded surface oxygen atoms might be more promising for the decomposition of polychlorinated dibenzo-p-dioxins and dibenzofurans.

Dry Reforming of Methane on Ni/LaZrO2 Catalyst under External Electric Fields: A Combined First-Principles and Microkinetic Modeling Study.

Dry reforming of methane (DRM) reaction has great potential in reducing the greenhouse effect and solving energy problems. Herein, the DRM reaction mechanism and activity on Ni16/LaZrO2 catalyst under electric fields were comprehensively investigated by combining density functional theory calculations with microkinetic modeling. The results showed that La doping increases the interaction between Ni and ZrO2 by Ni cluster transfer of more electrons. The adsorption strength of species followed the order Ni16/ZrO2 > Ni16/LaZrO2, which is consistent with the results for the d-band center but opposite to the metal-support interaction. The best DRM reaction path on Ni16/LaZrO2 was the CH2-O pathway, which is different from the CH-O pathway on Ni(111) and Ni16/ZrO2. Both positive and negative electric fields of strong and weak metal-support interactions reduced the energy barrier of DRM reaction. Importantly, our results showed that the more dispersed and smaller Ni12/LaZrO2 model by considering the dispersing effect induced by La doping, which displayed very different results from that of Ni16/LaZrO2: reduced the energy barrier for methane decomposition, thereby promoting DRM reaction activity. Microkinetic results showed that the carbon deposition behavior of DRM becomes weaker on Ni16/LaZrO2 due to the suppression of methane decomposition in the presence of La doping compared to Ni16/ZrO2, but the opposite result is obtained on Ni12/LaZrO2. The order of DRM reactivity was Ni16/LaZrO2 < Ni16/ZrO2 < Ni12/LaZrO2, which is consistent with the experiment observations. The conversion of methane and CO2 was higher in positive electric fields than in negative electric fields at low temperatures, but the results were opposite at high temperature. Negative electric fields can improve the carbon deposition resistance of Ni-based catalysts compared to positive electric fields. The degree of rate control analysis showed that CHx* oxidation also plays an important role in the DRM reaction. We envision that this study could provide a deeper understanding for guiding the widespread application of electric field catalysis.