Inter-site structural heterogeneity induction of single atom Fe catalysts for robust oxygen reduction.

Metal-nitrogen-carbon catalysts with hierarchically dispersed porosity are deemed as efficient geometry for oxygen reduction reaction (ORR). However, catalytic performance determined by individual and interacting sites originating from structural heterogeneity is particularly elusive and yet remains to be understood. Here, an efficient hierarchically porous Fe single atom catalyst (Fe SAs-HP) is prepared with Fe atoms densely resided at micropores and mesopores. Fe SAs-HP exhibits robust ORR performance with half-wave potential of 0.94 V and turnover frequency of 5.99 e-1s-1site-1 at 0.80 V. Theoretical simulations unravel a structural heterogeneity induced optimization, where mesoporous Fe-N4 acts as real active centers as a result of long-range electron regulation by adjacent microporous sites, facilitating O2 activation and desorption of key intermediate *OH. Multilevel operando characterization results identify active Fe sites undergo a dynamic evolution from basic Fe-N4 to active Fe-N3 under working conditions. Our findings reveal the structural origin of enhanced intrinsic activity for hierarchically porous Fe-N4 sites.

Understanding and controlling the formation of single-atom site from supported Cu10 cluster by tuning CeO2 reducibility: Theoretical insight into the Gd-doping effect on electronic metal-support interaction.

Controlling the formation of single-atom (SA) sites from supported metal clusters is an important and interesting issue to effectively improve the catalytic performance of heterogeneous catalysts. For extensively studied CO oxidation over metal/CeO2 systems, the SA formation and stabilization under reaction conditions is generally attributed to CO adsorption, however, the pivotal role played by the reducible CeO2 support and the underlying electronic metal-support interaction (EMSI) are not yet fully understood. Based on a ceria-supported Cu10 catalyst model, we performed density functional theory calculations to investigate the intrinsic SA formation mechanism and discussed the synergistic effect of Gd-doped CeO2 and CO adsorption on the SA formation. The CeO2 reducibility is tuned with doped Gd content ranging from 12.5 % ∼ 25 %. Based on ab initio thermodynamic and ab initio molecular dynamics, the critical condition for SA formation was identified as 21.875 % Gd-doped CeO2 with CO-saturated adsorption on Cu10. Electronic analysis revealed that the open-shell lattice Oδ- (δ < 2) generated by Gd doping facilitates the charge transfer from the bottom-corner Cu (Cubc) to CeO2. The CO-saturated adsorption further promotes this charge transfer process and enhances the EMSI between Cubc and CeO2, leading to the disintegration of Cubc from Cu10 and subsequent formation of the active SA site.

Density Functional Theory Study of Methanol Steam Reforming on Pt3Sn(111) and the Promotion Effect of a Surface Hydroxy Group.

Methanol steam reforming (MSR) is studied on a Pt3Sn surface using the density functional theory (DFT). An MSR network is mapped out, including several reaction pathways. The main pathway proposed is CH3OH + OH → CH3O → CH2O → CH2O + OH → CH2OOH → CHOOH → COOH → COOH + OH → CO2 + H2O. The adsorption strengths of CH3OH, CH2O, CHOOH, H2O and CO2 are relatively weak, while other intermediates are strongly adsorbed on Pt3Sn(111). H2O decomposition to OH is the rate-determining step on Pt3Sn(111). The promotion effect of the OH group is remarkable on the conversions of CH3OH, CH2O and trans-COOH. In particular, the activation barriers of the O-H bond cleavage (e.g., CH3OH → CH3O and trans-COOH → CO2) decrease substantially by ~1 eV because of the involvement of OH. Compared with the case of MSR on Pt(111), the generation of OH from H2O decomposition is more competitive on Pt3Sn(111), and the presence of abundant OH facilitates the combination of CO with OH to generate COOH, which accounts for the improved CO tolerance of the PtSn alloy over pure Pt.

Two-dimensional metal-organic frameworks as bifunctional electrocatalysts for the oxygen evolution reaction and oxygen reduction reaction (OER/ORR): a theoretical study.

Effective bifunctional catalysts are needed for the two main processes in metal-air batteries (oxygen evolution reaction and oxygen reduction reaction (OER/ORR)) to increase efficiency. Herein, we systematically investigate the stability, electronic structure, and catalytic performance of the OER/ORR of two-dimensional (2D) conducting metal-organic frameworks (MOFs) M3(C6Se6)2 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ir, and Pt) by first-principles calculations. The results show that Co3(C6Se6)2 has an overpotential of 0.51 V and 0.3 V for the OER and ORR, respectively, and Rh3(C6Se6)2 has an overpotential of 0.53 V and 0.29 V for the OER and ORR, respectively, which are very promising bifunctional catalysts. In addition, Ir3(C6Se6)2 is a very promising ORR catalyst with a low overpotential of 0.34 V. Volcano plots and contour maps of OER/ORR activity versus intermediate adsorption strength were established to describe the activity trend of M3(C6Se6)2 based on the relationship of adsorbed intermediates. Furthermore, the d-band center theory and crystal orbital Hamilton populations (COHPs) were used to relate the OER/ORR activity to the d-electrons of the central metal. Our study not only provides a novel bifunctional electrocatalyst but also provides some references for other 2D MOFs as electrocatalysts.

Atomic coordination structural dynamic evolution of single-atom Mo catalyst for promoting H2 activation in slurry phase hydrocracking.

Development of efficient catalysts with high atomic utilization and turnover frequency (TOF) for H2 activation in slurry phase hydrocracking (SPHC) is crucial for the conversion of vacuum residue (VR). Herein, for the first time, we reported a robust and stable single atoms (SAs) Mo catalyst through a polymerization-pyrolysis-in situ sulfurization strategy for activating H2 in SPHC of VR. An interesting atomic coordination structural dynamic evolution of Mo active sites was discovered. During hydrocracking of VR, the O atoms that coordinated with Mo were gradually replaced by S atoms, which led to the O/S exchange process. The coordination structure of the Mo SAs changed from pre-reaction Mo-O3S1 to post-reaction Mo-O1S3 coordination configurations, promoting the efficient homolytic cleavage activation of H2 into H radical species effectively. The evolved Mo SAs catalyst exhibited robust catalytic hydrogenation activity with the per pass conversion of VR of 65 wt%, product yield of liquid oils of 93 wt%, coke content of only 0.63 wt%, TOF calculated for total metals up to 0.35 s-1, and good cyclic stability. Theoretical calculation reveals that the significant variation of occupied Mo 4d states before and after H2 interaction has a direct bearing on the dynamic evolution of Mo SAs catalyst structure. The lower d-band center of Mo-O1S3 site indicates that atomic H diffusion is easy, which is conducive to catalytic hydrogenation. The finding of this study is of great significance to the development of high atom economy catalysts for the industrial application of heavy oil upgrading technology.

Synergy of Substrate Chemical Environments and Single-Atom Catalysts Promotes Catalytic Performance: Nitrogen Reduction on Chiral and Defected Carbon Nanotubes.

The catalytic activities of single-atom catalysts (SACs) are strongly influenced by the local chemical environments of their substrates, by which the electronic structures of the SACs can be effectively tuned. Together with the freedom of available reactive metallic centers, it would be feasible to maximize the catalytic performance by means of a synergetic optimization in the chemical space spanned by the features of both the substrate and the catalytic center. In this work, using first-principles calculations, we systematically assessed the synergetic effect between the substrate geometric/electronic structures and the catalytic centers on the electrocatalytic nitrogen reduction reaction (NRR). Carbon nanotubes with different chirality, defects, and chemical functionalization were used to support 15 transition metal atoms. Three SACs, TiN4CNT(3,3), TiN4CNT(5,5), and VN4CNT(3,3), simultaneously possess high NRR selectivities (w.r.t hydrogen evolution) and low overpotentials of 0.35, 0.35, and 0.37 V, respectively. Electronic structure analysis elucidated that larger metal atoms anchored on CNTs with higher curvature and doped by N atoms facilitate the rupture of the N-N bond in *NH2NH2 to lower the overpotentials. The synergy of substrate chemical environments and single atomic catalysis is a promising strategy to optimize the catalytic performance.

Enhancing the Photoinduced Interlayer Charge Transfer and Spatial Separation in Type-II Heterostructure of WS2 and Asymmetric Janus-MoSSe with Intrinsic Self-Build Electric Field.

Two-dimensional heterostructure manipulation is promising to overcome the high recombination rates and limited redox abilities of photogenerated electron-hole pairs in a single photocatalyst. The built-in electric field (Ehetero) in the type-II heterojunction is normally unfavorable for the desired charge transfer, which is an important but easily neglected issue that needs to be solved. Here, on the basis of the density functional theory (DFT) and the nonadiabatic molecular dynamics (NAMD) calculations, we obtain a type-II band alignment in Janus-MoSSe/WS2 heterostructure, which meets the band-edge position requirement for water splitting. Importantly, the intrinsic self-build electric field (Eself) of Janus-MoSSe can effectively weaken the hindrance effect of Ehetero for charge transfer by constructing a suitable Se-S stacking configuration, improving charge separation efficiency in the Janus-MoSSe/WS2 heterostructure. Our work provides a materials-by-design paradigm and interlayer charge-transfer dynamics understanding of heterostructure engineering against asymmetric structures lacking reflection symmetry.

Adsorption and dehydrogenation of C2-C6n-alkanes over a Pt catalyst: a theoretical study on the size effects of alkane molecules and Pt substrates.

Adsorption and dehydrogenation of C2-C6n-alkanes are investigated on a Pt substrate using density functional theory (DFT) calculations, and the size effects of alkane molecules and Pt substrates are discussed in detail. The Pt(111) surface and Pt55 cluster are chosen to represent large and small Pt nanoparticles, respectively. The C2-C6 straight-chain alkanes show no site preference on Pt(111) drifting over the surface, but prefer to locate along the edge sites of Pt55. Our results suggest that a linear relationship holds for the adsorption energies of n-alkanes against the chain length on Pt(111), in accordance with the experimental observations. Pt55 also exhibits a similar linear relationship for n-alkanes but with larger adsorption energies due to the low-coordinated Pt atoms at the edge site. For the two-step dehydrogenation from alkanes to alkenes, the first dehydrogenation reaction is the rate-determining step (RDS) on Pt(111), and a larger size of alkane molecule will lead to a lower dehydrogenation activity. While on Pt55, no RDS is present and the dehydrogenation activity oscillates slightly as the chain length of n-alkane increases. Generally, Pt55 involves lower energy barriers for most dehydrogenation steps compared to Pt(111), indicating that small Pt particles with more low-coordinated Pt atoms are more active towards alkane dehydrogenation. In addition, a clear BEP relationship is identified for all the dehydrogenation reactions of C2-C6n-alkanes on Pt substrates, and this linear relationship is independent of the particle size of the Pt substrate and the chain length of alkanes.

Identifying the functional groups effect on passivating perovskite solar cells.

Many organic molecules with various functional groups have been used to passivate the perovskite surface for improving the efficiency and stability of perovskite solar cell (PSCs). However, the intrinsic attributes of the passivation effect based on different chemical bonds are rarely studied. Here, we comparatively investigate the passivation effect among 12 types of functional groups on para-tert-butylbenzene for PSCs and find that the open circuit voltage (VOC) tends to increase with the chemical bonding strength between perovskite and these passivation additive molecules. Particularly, the para-tert-butylbenzoic acid (tB-COOH), with the extra intermolecular hydrogen bonding, can stabilize the surface passivation of perovskite films exceptionally well through formation of a crystalline interlayer with water-insoluble property and high melting point. As a result, the tB-COOH device achieves a champion power conversion efficiency (PCE) of 21.46%. More importantly, such devices, which were stored in ambient air with a relative humidity of ≃45%, can retain 88% of their initial performance after a testing period of more than 1 year (10,080 h). This work provides a case study to understand chemical bonding effects on passivation of perovskite.

General Nondestructive Passivation by 4-Fluoroaniline for Perovskite Solar Cells with Improved Performance and Stability.

Hybrid perovskite thin films are prone to producing surface vacancies during the film formation, which degrade the stability and photovoltaic performance. Passivation via post-treatment can heal these defects, but present methods are slightly destructive to the bulk of 3D perovskite due to the solvent effect, which hinders fabrication reproducibility. Herein, nondestructive surface/interface passivation using 4-fluoroaniline (FAL) is established. FAL is not only an effective antisolvent candidate for surface modification, but also a large dipole molecule (2.84 Debye) with directional field for charge separation. Density functional theory calculation reveals that the nondestructive properties are attributed to both the conjugated amine in aromatic ring and the para-fluoro-substituent. A hot vapor assisted colloidal process is employed for the post-treatment. The molecular passivation yields an ultrathin protection layer with a hydrophobic fluoro-substituent tail and thus enhances the stability and optoelectronic properties. FAL post-treated perovskite solar cell (PSC) delivers a 20.48% power conversion efficiency under ambient conditions. Micro-photoluminescence reveals that passivation activates the dark defective state at the surface and interface, delivering the impact picture of boundary on the local carriers. This work demonstrates a generic nondestructive chemical approach for improving the performance and stability of PSCs.

Porous Carbon Materials Based on Graphdiyne Basis Units by the Incorporation of the Functional Groups and Li Atoms for Superior CO2 Capture and Sequestration.

The graphdiyne family has attracted a high degree of concern because of its intriguing and promising properties. However, graphdiyne materials reported to date represent only a tiny fraction of the possible combinations. In this work, we demonstrate a computational approach to generate a series of conceivable graphdiyne-based frameworks (GDY-Rs and Li@GDY-Rs) by introducing a variety of functional groups (R = -NH2, -OH, -COOH, and -F) and doping metal (Li) in the molecular building blocks of graphdiyne without restriction of experimental conditions and rapidly screen the best candidates for the application of CO2 capture and sequestration (CCS). The pore topology and morphology and CO2 adsorption and separation properties of these frameworks are systematically investigated by combining density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations. On the basis of our computer simulations, combining Li-doping and hydroxyl groups strategies offer an unexpected synergistic effect for efficient CO2 capture with an extremely CO2 uptake of 4.83 mmol/g at 298 K and 1 bar. Combined with its superior selectivity (13 at 298 K and 1 bar) for CO2 over CH4, Li@GDY-OH is verified to be one of the most promising materials for CO2 capture and separation.

Insight into thiophene hydrodesulfurization on clean and S-modified MoP(010): a periodic density functional theory study.

The hydrodesulfurization (HDS) of thiophene on clean and S-modified MoP(010) is investigated to understand the HDS mechanism as well as the surface sulfur (S) atom effect using periodic density functional theory (DFT). The results show that thiophene prefers strongly flat adsorption on both the clean and S-modified surfaces, in either the molecular state or the dissociative state breaking simultaneously one C-S bond, and the adsorption of thiophene can be slightly weakened by the surface S atom. Thermodynamic and kinetic analysis indicates that the HDS of thiophene in both the molecular and dissociative adsorption states prefers to take place along the direct desulfurization (DDS) pathway rather than hydrogenation on both the clean and S-modified MoP(010) surfaces. Surface S shows a promotion effect on the HDS catalytic activity of MoP(010), because the energy barrier/rate constant of the rate-determining step on the DDS pathway is decreased/enlarged under the S modification. Compared with the situation of MoP(001), MoP(010) should have relatively low HDS activity, since a higher energy barrier as well as weaker exothermicity is involved in the reaction on the latter surface.

Metallic Iron-Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media.

We report on the synthesis of iron-nickel sulfide (INS) ultrathin nanosheets by topotactic conversion from a hydroxide precursor. The INS nanosheets exhibit excellent activity and stability in strong acidic solutions as a hydrogen evolution reaction (HER) catalyst, lending an attractive alternative to the Pt catalyst. The metallic α-INS nanosheets show an even lower overpotential of 105 mV at 10 mA/cm(2) and a smaller Tafel slope of 40 mV/dec. With the help of DFT calculations, the high specific surface area, facile ion transport and charge transfer, abundant electrochemical active sites, suitable H(+) adsorption, and H2 formation kinetics and energetics are proposed to contribute to the high activity of the INS ultrathin nanosheets toward HER.

Ethanol decomposition on a Pd(110) surface: a density functional theory investigation.

Ethanol decomposition on Pd(110) is comprehensively investigated using self-consistent periodic density functional theory. Geometries and energies for all the intermediates involved are analyzed, and the decomposition network is mapped out to illustrate the reaction mechanism. On Pd(110), the most stable adsorption of the involved species tends to follow the gas-phase bond order rules, wherein C is tetravalent and O is divalent with the missing H atoms replaced by metal atoms. The most likely decomposition pathway of ethanol on Pd(110) is CH(3)CH(2)OH → CH(3)CH(2)O → CH(3)CHO → CH(3)CO → CH(3) + CO → CO + H + CH(4) + C, in which the initial dehydrogenation is the rate-limited step. No C-O scission pathway is identified. Comparing with ethanol decomposition on Pd(111) [Langmuir, 2010, 26, 1879-1888], Pd(110) characterizes relatively high activity and different selectivity. Two crucial factors controlling the variations of reactivity and selectivity from Pd(111) to Pd(110), i.e., the local electronic effect of the metals and the geometrical effect of the relevant transition states, are identified. Four distinct Brønsted-Evans-Polanyi (BEP) relations are identified for the three types of bond scission (C-H, C-O, and C-C) if we consider Pd(111) and Pd(110) as a whole, one for C-H bond scission, one for C-O bond scission, and two for C-C bond scission.

Initial hydrogenations of pyridine on MoP(001): a density functional study.

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(ß),C(ß),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(ß) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.

Decomposition of methanthiol on Pt(111): a density functional investigation.

Decomposition of methanthiol on Pt(111) is systematically investigated using self-consistent periodic density functional theory (DFT), and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to form the sp(3) hybridized configuration of both C and S atoms, in which C is almost tetrahedral and S has the tendency to bond to three atoms. Spontaneous dissociation rather than desorption is preferred for adsorbed methanthiol. Based on the harmonic transition state theory calculations, the decomposition rate constants of the thiolmethoxy and thioformaldehyde intermediates are found to be much lower than those for their formation, leading to long lifetimes of the intermediates for observation. Under the ultrahigh vacuum (UHV) condition, the most possible decomposition pathway for methanthiol on Pt(111) is found as CH(3)SH --> CH(3)S --> CH(2)S --> CHS --> CH + S --> C + S, in which the C-S bond cleavage mainly occurs at the CHS species. However, the decomposition pathway is CH(3)SH --> CH(3)S --> CH(3) + S under the hydrogenation condition; the C-S bond scission mainly occurs at CH(3)S. The Brønsted-Evans-Polanyi relation holds for each of the S-H, C-H, and C-S bond scission reactions.

Theoretical survey of the potential energy surface of Ti+ + methanol reaction.

The gas-phase reaction of Ti(+) ((4)F and (2)F) with methanol is investigated using density functional theory. Geometries and energies of the reactants, intermediates, and products involved are calculated. The approach of Ti(+) toward methanol could form either a "classical" O- or a "nonclassical" eta(3)-methyl H-attached complex. The reaction products observed in the experiment (Guo, Kerns, Castleman J. Phys. Chem. 1992, 96, 4879) are produced via the classical association rather than the nonclassical complex. All possible pathways starting with C-O, C-H, and O-H activation are searched. Methane and methyl loss products (TiO(+) and TiOH(+)) are produced via the C-O activation; the O-H activation accounts for the H(2) and H elimination (producing TiOCH(2)(+) and TiOCH(3)(+)); and the C-H activation is unlikely to be important. Through the bond insertion (H shift) reductive elimination mechanism, the products of a closed-shell molecule (H(2) or methane) elimination could take place on both the quartet and doublet PESs owing to a spin inversion occurring in the course of initial bond insertion, whereas only the quartet products are produced adiabatically via the simple bond insertion-reductive elimination mechanism for the loss of a radical-type species (H or CH(3)). The computational results are in concert with the available experimental information and add new insight into the details of the individual elementary steps.

Theoretical investigation of the gas-phase Mn(+)- and Co(+)-catalyzed oxidation of benzene by N(2)O.

The gas-phase Mn(+)- and Co(+)-mediated oxidation of benzene by N(2)O has been theoretically investigated using density functional theory. The geometries and energies of all the stationary points involved are located. Two different oxidation mechanisms, i.e., mediated by M(+)(benzene) and MO(+), are taken into account. In the former catalytic cycle, benzene initially coordinates to the metal ion affording the M(+)(C(6)H(6)) adduct (M = Mn or Co), then N(2)O coordinates to the nascent benzene complex and gets activated by the metal to yield (C(6)H(6))M(+)O(N(2)). After releasing a molecular nitrogen, through the non-radical and/or O-insertion pathways, the system would be oxidized to phenol and regenerates the active catalyst M(+). This catalytic mechanism is energetically favourable, explaining the efficient Mn(+)- and Co(+)-catalyzed benzene hydroxylation observed in ion cyclotron resonance (ICR) experiments [J. Am. Chem. Soc., 1994, 116, 9565-9570]. For the alternative MO(+)-mediated oxidation mechanism, spin inversion as well as high energy barrier in the course of the N-O activation imply low reaction efficiency of the ground-state reactants, according with the ICR experiment finding that MO(+) was formed from exited M(+)*, thus both Mn(+) and Co(+) are unable to work as a catalyst in this case.