The deposition of a two-dimensional (2D) atomic nanosheet on a metal surface has been considered as a new route for tuning the molecule–metal interaction and surface reactivity in terms of the confinement effect. In this work, we use first-principles calculations to systematically explore a novel nanospace constructed by placing a 2D graphitic carbon nitride (g-C3N4) nanosheet over a Pt(111) surface. The confined catalytic activity in this nanospace is investigated using CO oxidation as a model reaction. With the inherent triangular pores in the g-C3N4 overlayer being taken advantage of, molecules such as CO and O2 can diffuse to adsorb on the Pt(111) surface underneath the g-C3N4 overlayer. Moreover, the mechanism of intercalation is also elucidated, and the results reveal that the energy barrier depends mainly on the properties of the molecule and the channel. Importantly, the molecule–catalyst interaction can be tuned by the g-C3N4 overlayer, considerably reducing the adsorption energy of CO on Pt(111) and leading to enhanced reactivity in CO oxidation. This work will provide important insight for constructing a promising nanoreactor in which the following is observed: The molecule intercalation is facile; the molecule–metal interaction is efficiently tuned; the metal-catalyzed reaction is promoted.Keywords: CO oxidation; confinement effect; first-principles; g-C3N4; molecule intercalation; Pt(111);

A catalyst composed of monolayer nonstoichiometric titanate nanosheets (denoted as TN) and Pd clusters is constructed for precise synthesis of cyclohexanone from phenol hydrogenation with high conversion (>99%) and selectivity (>99%) in aqueous media under light irradiation. Experimental and DFT calculation results reveal that the surface exposed acid and basic sites on TN could interact with phenol molecules in a nonplanar fashion via a hexahydroxy hydrogen-bonding ring to form a surface coordination species. This greatly facilitates the adsorption and activation of phenol molecules and suppresses the further hydrogenation of cyclohexanone. Moreover, the surface Pd clusters serve as the active sites for the adsorption and dissociation of hydrogen molecules to provide active H atoms. The synergistic effect of the surface coordination species, TN and Pd clusters remarkably facilitate the high yield of cyclohexanone in photocatalysis. Finally, the possible thermo/photocatalytic mechanisms on Pd/TN are proposed. This work not only highlights the great potential for monolayer nonstoichiometric composition nanosheets in the construction of catalysts for precise organic synthesis but also provides insight into the inherent catalytic behavior at a molecular level.Keywords: green photocatalysis; hydrogenation of phenol; monolayer titanate nanosheet; precision synthesis;

A catalyst composed of monolayer nonstoichiometric titanate nanosheets (denoted as TN) and Pd clusters is constructed for precise synthesis of cyclohexanone from phenol hydrogenation with high conversion (>99%) and selectivity (>99%) in aqueous media under light irradiation. Experimental and DFT calculation results reveal that the surface exposed acid and basic sites on TN could interact with phenol molecules in a nonplanar fashion via a hexahydroxy hydrogen-bonding ring to form a surface coordination species. This greatly facilitates the adsorption and activation of phenol molecules and suppresses the further hydrogenation of cyclohexanone. Moreover, the surface Pd clusters serve as the active sites for the adsorption and dissociation of hydrogen molecules to provide active H atoms. The synergistic effect of the surface coordination species, TN and Pd clusters remarkably facilitate the high yield of cyclohexanone in photocatalysis. Finally, the possible thermo/photocatalytic mechanisms on Pd/TN are proposed. This work not only highlights the great potential for monolayer nonstoichiometric composition nanosheets in the construction of catalysts for precise organic synthesis but also provides insight into the inherent catalytic behavior at a molecular level.Keywords: green photocatalysis; hydrogenation of phenol; monolayer titanate nanosheet; precision synthesis;

By means of first-principles calculations, the interaction of twelve different transition metal atoms (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) of groups VIII–XI with phosphomolybdic acid (H3PMo12O40, PMA), a newly emerging medium for trapping transition metal atoms, has been systematically investigated. The M–PMA systems have very high stability with the binding energies of transition metals higher than those on widely used metal oxide supports. The high diffusion barriers of these single metal atoms on the PMA surfaces suggest that they are sufficiently stable to prevent agglomeration. Based on the electronic structure analysis, the remarkable stability of single atoms is attributed to the strong mixing between the d orbitals of the metal atom and 2p orbitals of PMA oxygens, which results in electron transfer from the metal atoms to PMA, producing positively charged single metal atoms which can be used for catalytic applications. Finally, we test the activity of Pt–PMA as a low-cost, stable, and efficient catalysts for CO oxidation. This work is expected to provide useful insight to the development of new highly efficient heterogeneous single atom catalysts (SACs).

AbstractCeria (CeO2) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single-atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single-atom Pt/CeO2 catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO2, the single-atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO2-supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO2, making Pt–Sn/CeO2 a fully regenerable catalyst.

AbstractCeria (CeO2) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single-atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single-atom Pt/CeO2 catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO2, the single-atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO2-supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO2, making Pt–Sn/CeO2 a fully regenerable catalyst.

Defective hexagonal boron nitride nanosheets (h-BNNSs) supported by Ni(111) and Cu(111) surfaces have been systematically studied in this work by first-principles methods. The calculation results show that various defects play an important role in enhancing the stability of h-BNNS/metal heterostructure. Importantly, significant electron transfer through the interface between metal substrate and h-BNNS to the defect sites can make h-BNNS more catalytically active. Using the oxygen reduction reaction (ORR) as a probe, it is shown that the binding energies of O2*, OH*, OOH*, and O* on h-BNNS/Cu(111) with a boron vacancy (VB) are quite similar to those observed on the Pt(111) surface, suggesting inert h-BNNS materials with defects can be functionalized by metal surfaces to become catalytically active for the ORR process. On the other hand, the reaction mechanism of CO oxidation on Ni(111) and Cu(111) supported h-BNNS with VB is systematically investigated. The h-BN/Cu(111) catalyst with a VB precovered by a CO species exhibits catalytic capacity for CO oxidation with a lower energy barrier compared with that on h-BN/Cu(111) without any defect. While on Ni(111) supported h-BNNS with a N vacancy, the defect site turns to be dominated by O2 and the energy barrier is significantly increased, indicating its dependence on the type of defect. This work will provide information for designing h-BN-based catalysts in heterogeneous catalysis.Keywords: CO oxidation; defect sites; DFT with dispersion corrections; metal supported h-BN; oxygen reduction reaction

Surface oxygen vacancy has been investigated extensively due to its great influence on photocatalysis in recent years. Among these investigations, the location of surface defects is found to be a key factor during the photocatalytic procedure. Especially when a crystal facet is involved, the synergistic action between these two factors may greatly increase the photocatalytic efficiency. Location of defects would greatly help in understanding the mechanism of this coupling action. Without an available technique, however, it is very difficult to gain the position information directly. In this paper, we provide a low-cost and visualized method to solve this problem via in situ reduction of Au. TiO2 with surface oxygen vacancy on a specific area is synthesized via a photochemical reaction. Then the Au ion was reduced at the place where oxygen vacancy exists on the TiO2 surface. Hence, the localization of surface vacancy will be determined by examining the location of Au particles. Moreover, application of this method can be extended to other surface defects such as oxygen vacancy on oxide semiconductors, Ti3+ in TiO2, Zn vacancy on ZnO, etc. This technique is helpful for understanding how surface defects affect the properties of semiconductors.

ZnS is among the superior photocatalysts for H2 evolution, whereas the wide bandgap restricts its performance to only UV region. Herein, defect engineering and phase junction architecture from a controllable phase transformation enable ZnS to achieve the conflicting visible-light-driven activities for H2 evolution. On the basis of first-principle density functional theory calculations, electron spin resonance and photoluminescence results, etc., it is initially proposed that the regulated sulfur vacancies in wurtzite phase of ZnS play the key role of photosensitization units for charge generation in visible light and active sites for effective electron utilization. The symbiotic sphalerite-wurtzite phase junctions that dominate the charge-transfer kinetics for photoexciton separation are the indispensable configuration in the present systems. Neither ZnS samples without phase junction nor those without enough sulfur vacancies conduct visible-light photocatalytic H2 evolution, while the one with optimized phase junctions and maximum sulfur vacancies shows considerable photocatalytic activity. This work will not only contribute to the realization of visible light photocatalysis for wide-bandgap semiconductors but also broaden the vision on the design of highly efficient transition metal sulfide photocatalysts.Keywords: first principle DFT; phase junction; sulfur vacancy; visible light photocatalysis; ZnS;

Si-doped hexagonal boron nitride nanosheets (Si-BNNS) and nanotubes (Si-BNNT) have been investigated by first-principle methods. The strong interaction between the silicon atom and the hexagonal boron nitride nanosheet or nanotube with a boron vacancy indicates that such nanocomposites should be very stable. The significant charge transfer from the Si–BNNS substrate to the O2 molecule, which could occupy the antibonding 2π* orbitals of O2, results in the activation of the adsorbed O2. The catalytic activity of the Si–BNNS for CO oxidation is explored and the calculated barrier (0.29 eV) of the reaction CO + O2 → CO2 + O is much lower than those on the traditional noble metals. This opens a new avenue to fabricate low cost and high activity boron nitride-based metal-free catalysts.

By using plane-wave density functional theory, the reaction mechanism of ethanol steam reforming (ESR) on the Co(0001) surface is investigated by systematically exploring the barriers and reaction energies of elementary steps. Our results suggest that ESR is initiated by decomposition of ethanol: CH3CH2OH* → CH3CH2O* → CH3CHO* → CH3CO* → CH3* + CO*. This is followed by the water–gas shift (CO* + OH* → COOH* → CO2* + H*) or direct oxidation (CO* + O* → CO2*) reaction to produce CO2. The reaction between CO* and OH*/O* is considered to be the key step in ESR. The proposed mechanism is consistent with most available experimental data and provides theoretical insight into the reaction pathways of the ESR process on cobalt catalysts.

We report a general synthetic route to well-crystallized metal nitrides through a high-pressure solid-state metathesis reaction (HPSSM) between boron nitride (BN) and ternary metal oxide AxMyOz (A = alkaline or alkaline-earth metal and M = main group or transition metal). On the basis of the synthetic metal nitrides (Fe3N, Re3N, VN, GaN, CrN, and WxN) and elemental products (graphite, rhenium, indium, and cobalt metals), the HPSSM reaction has been systematically investigated with regard to its general chemical equation, reaction scheme, and characteristics, and its thermodynamic considerations have been explored by density functional theory (DFT) calculations. Our results indicate that pressure plays an important role in the synthesis, which involves an ion-exchange process between boron and the metal ion, opening a new pathway for material synthesis.

The catalytic oxidation of CO toward CO2 on ruthenium-embedded hexagonal boron nitride nanosheet (h-BN) was studied by periodic first-principle methods. The calculation results indicate that this catalyst is extremely stable and the adsorbed oxygen species can be efficiently activated by the embedded metal atom. Two reaction pathways of the CO oxidation were considered in detail: the Langmuir–Hinshelwood (LH) and the Eley–Rideal (ER) pathways. As a result, the CO oxidation process would like to firstly take place following ER mechanism to produce CO2 plus an atomic O and then a second CO reacts with the remanent oxygen atom to form CO2 through LH pathway. The calculated energy barriers for these two reaction steps are as low as 0.42 and 0.37 eV, respectively, indicating its application at low temperatures. This study can be expected to provide useful information for the development of highly active catalyst for CO oxidation.Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► CO oxidation on Ru-embedded h-BN was studied by first-principle methods. ► Ru-embedded h-BN is extremely stable. ► The adsorbed oxygen can be efficiently activated by the embedded Ru atom. ► The obtained low barrier indicates Ru-embedded h-BN is a possible highly active catalyst.

By means of first-principles computation, metal (Cu, Ag, Au, Pt, Rh, Pd, Fe, Co, and Ir) doped hexagonal boron nitride nanosheets (h-BNNSs) have been systematically investigated. The strong interaction between the metal atoms and defect sites in h-BNNS, such as the boron vacancy and nitrogen edge, suggests that metal doped h-BN nanosheets (M-BNNSs) should be stable under high temperatures. The catalytic activity of Co doped h-BNNS is also investigated by using CO oxidation as a probe, and the calculated low barrier suggests that the Co-BNNS is a viable catalyst for CO oxidation. Based on electronic structure analysis, the catalytic capacity of Co-BNNS is attributed to the strong mixing between the cobalt 3d orbitals and oxygen 2p orbitals, which activates the adsorbed molecular or atomic oxygen.

The hydrogenation of CO on Pd can lead to methane via the Fischer–Tropsch process and methanol via oxygenate synthesis. Despite the fact that the former is thermodynamically favored, the catalysis is mostly selective to the latter. Given the importance of methanol synthesis in both industry applications and fundamental understanding of heterogeneous catalysis, it is highly desirable to understand the mechanism and selectivity of CO hydrogenation on Pd catalysts. In this work, this process is studied on Pd(111) using periodic plane-wave density functional theory and kinetic Monte Carlo simulations. The barriers and reaction energies for the methanol and methane formation are systematically explored. Our results suggest that methanol is formed via CO* → CHO* → HCOH* → CH2OH* → CH3OH*. The HCOH* and CH2OH* intermediates, which feature a C–O single bond, were found to possess the lowest barriers for C–O bond fission, but they are still higher than those in methanol formation, thus confirming the kinetic origin of the experimentally observed selectivity of the Pd catalysts toward methanol.

Methanol decomposition on noble metal surfaces is an important industrial process and prototype for understanding heterogeneous catalysis. Despite many advances, the role played by surface defects and structural sensitivity is still not fully understood. In this work, methanol decomposition on a stepped palladium surface, Pd(211), is investigated using periodic density functional theory (DFT). The activation barriers and thermochemistry for relevant elementary steps leading to the final decomposition products CO and H2 are obtained. Similar to the previous theoretical results on flat Pd surfaces, the initial C–H bond scission is preferred on Pd(211) because it has a lower barrier than those for the initial O–H and C–O scissions. It was also found that the barriers for the C–H or O–H bond scissions are lowered at the step sites. Finally, kinetic Monte Carlo simulations on a realistic Pd surface reproduce the temperature-programmed desorption spectrum for methanol decomposition but only when modified DFT data are used. These simulations show that most of the reaction occurs at under-coordinated sites.

Methanol steam reforming (MSR), catalyzed by the PdZn alloy, produces hydrogen gas and carbon dioxide with high selectivity. However, the mechanism for MSR has not been completely elucidated. It has been proposed that formate and methyl formate are possible intermediates in MSR. In this study, plane-wave density functional theory was used to investigate the role of methyl formate in MSR on PdZn. It is shown that methyl formate can indeed be formed by a reaction between formaldehyde and methoxyl. In the presence of surface OH species, methyl formate can further react to form formic acid, which can finally dehydrogenate to produce CO2. However, our calculations show that this hydrolysis process might have difficulties competing with desorption of methyl formate, which is weakly adsorbed on the PdZn surface. Our calculated results thus suggest a minor role for the methyl formate pathway in MSR. Interestingly, the methyl formate reaction pathway shares many similarities with the same process on copper, which is the traditional catalyst for MSR. The insights gained by studying the reaction mechanism on these two surfaces shed valuable light on designing future catalysts for the MSR process.Graphical abstractHighlights► Methyl formate can be formed between formaldehyde and methoxyl on PdZn(1 1 1). ► Methyl formate plays a minor role in methanol steam reforming (MSR) process. ► Methyl formate pathway shares many similarities with the same process on Cu(1 1 1). ► The calculated mechanism sheds valuable light on designing catalysts for MSR.

Si-doped hexagonal boron nitride nanosheets (Si-BNNS) and nanotubes (Si-BNNT) have been investigated by first-principle methods. The strong interaction between the silicon atom and the hexagonal boron nitride nanosheet or nanotube with a boron vacancy indicates that such nanocomposites should be very stable. The significant charge transfer from the Si–BNNS substrate to the O2 molecule, which could occupy the antibonding 2π* orbitals of O2, results in the activation of the adsorbed O2. The catalytic activity of the Si–BNNS for CO oxidation is explored and the calculated barrier (0.29 eV) of the reaction CO + O2 → CO2 + O is much lower than those on the traditional noble metals. This opens a new avenue to fabricate low cost and high activity boron nitride-based metal-free catalysts.