Wenkai Chen

A DFT Study of CO Catalytic Oxidation by N2O or O2 on the Co3O4(110) Surface

The reaction mechanisms for CO catalytic oxidation by N2O or O2 on the Co3O4(110) surface were studied by DFT slab calculations. CO chemisorbs preferably at a surface Co3+ site. After the Co3+ site is completely covered, CO adsorbs at the neighboring twofold coordinated surface oxygen atom bonded to Co2+ and Co3+ cations, resulting in the formation of CO2 and an oxygen vacancy with a low energy barrier of 0.033 eV, which rationalizes the experimental observation that Co3O4‐based systems are active for CO oxidation at low temperatures. N2O or O2 interacts with the oxygen vacancy site to regenerate the surface, leaving N2 or the activated O2− species to be attacked by the second CO to yield CO2 to proceed with the catalytic cycle. The CO oxidation reaction follows the Mars– van Krevelen mechanism.

Reaction mechanism of CO oxidation on Cu2O(111): A density functional study

The possible reaction mechanisms for CO oxidation on the perfect Cu(2)O(111) surface have been investigated by performing periodic density functional theoretical calculations. We find that Cu(2)O(111) is able to facilitate the CO oxidation with different mechanisms. Four possible mechanisms are explored (denoted as M(ER1), M(ER2), M(LH1), and M(LH2), respectively): M(ER1) is CO((gas))+O(2(ads))→CO(2(gas)); M(ER2) is CO((gas))+O(2(ads))→CO(3(ads))→O((ads))+CO(2(gas)); M(LH1) refers to CO((ads))+O(2(ads))→O((ads))+CO(2(ads)); and M(LH2) refers to CO((ads))+O(2(ads))→OOCO((ads))→O((ads))+CO(2(ads)). Our transition state calculations clearly reveal that M(ER1) and M(LH2) are both viable; but M(ER1) mechanism preferentially operates, in which only a moderate energy barrier (60.22 kJ/mol) needs to be overcome. When CO oxidation takes place along M(ER2) path, it is facile for CO(3) formation, but is difficult for its decomposition, thereby CO(3) species can stably exist on Cu(2)O(111). Of course, the reaction of CO with lattice O of Cu(2)O(111) is also considered. However, the calculated barrier is 600.00 kJ/mol, which is too large to make the path feasible. So, we believe that on Cu(2)O(111), CO reacts with adsorbed O, rather than lattice O, to form CO(2). This is different from the usual Mars-van Krevene mechanism. The present results enrich our understanding of the catalytic oxidation of CO by copper-based and metal-oxide catalysts.

Coadsorption of CO and NO on the Cu2O(111) surface: A periodic density functional theory study

Coadsorption of carbon monoxide (CO) and nitric oxide (NO) on the Cu(2)O(111) surface was studied using periodic density functional theory calculations. It is interesting to find that CO+NO on Cu(2)O(111) could react to form adsorbed NCO surface species. Coadsorption of CO and NO could give rise to the formation of a O-C...N-O complex well bound to the Cu(2)O(111) surface, in which both the C-O and N-O bonds are greatly activated and the C-N bond is formed. Consequently, the reaction of CO with NO to form adsorbed NCO and CNO species may occur, for which it is disclosed that NCO formation is more possible than CNO formation both thermodynamically and kinetically. In addition, our calculations of searching transition states reveal that it is facile for NCO formation both kinetically and thermodynamically when CO+NO reaction takes place at Cu(CUS) site, and is impossible when this reaction takes places at O(vac) site. Moreover, CO(2) species cannot form when CO+NO reaction occurs at O(vac) site. Therefore, oxygen vacancy on Cu(2)O(111) does not play a positive role on CO+NO reaction to forming NCO, CNO, or CO(2) species.

The adsorption and dissociation of Cl2 on the MgO (001) surface with vacancies: Embedded cluster model study

The adsorption of Cl(2) at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F(+), or F(2+) center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption of Cl(2) at the edge and corner oxygen sites and the results show that Cl(2), energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl(2) on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl(2) approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl(2) adsorption at the oxygen vacancy site (F, F(+), or F(2+) center), both large adsorption energies and rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital of Cl(2). It suggests, at the oxygen vacancy site, that Cl(2) prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl(2) to atomic Cl is virtually barrierless.

Growth mechanism of palladium clusters on rutile TiO2(110) surface

Abstract Oxide-supported transition metal systems have been the subject of enormous interest due to the improvement of catalytic properties relative to the separate metal. Thus in this paper, we embark on a systematic study for Pdn (n = 1–5) clusters adsorbed on TiO2(110) surface based on DFT-GGA calculations utilizing periodic supercell models. A single Pd adatom on the defect-free surface prefers to adsorb at a hollow site bridging a protruded oxygen and a five-fold titanium atom along the [110] direction, while Pd dimer is located on the channels with the Pd-Pd bond parallel to the surface. According to the transition states (TSs) search, the adsorbed Pd trimer tends to triangular growth mode, rather than linear mode, while the Pd4 and Pd5 clusters prefer three-dimensional (3D) models. However, the oxygen vacancy has almost no influence on the promotion of Pdn cluster nucleation. Additionally, of particular significance is that the Pd-TiO2 interaction is the main driving force at the beginning of Pd nucleation, whereas the Pd-Pd interaction gets down to control the growth process of Pd cluster as the cluster gets larger. It is hoped that our theoretical study would shed light on further designing high-performance TiO2 supported Pd-based catalysts.

Deposition of (WO3)3 nanoclusters on the MgO(001) surface: A possible way to identify the charge states of the defect centers

Periodic density functional theory calculations have been performed to study the most stable structure of the (WO(3))(3) nanocluster deposited on the MgO(001) surface with three kinds of F(S) centers (F(S)(0), F(S)(+), and F(S)(2+)). Our results indicate that the configuration of (WO(3))(3) cluster, including the cyclic conformation and the heights of three W atoms, and the oxidation states are sensitive to the charge state of the F(S) center. It is interesting that the electron-riched F(S) (0) vacancy on the MgO(001) surface can act as a promoting site to enhance the W-W interaction and the W(3)O(3) cyclic conformation is maintained, while the skeleton of cluster becomes flexible when (WO(3))(3) is adsorbed on the electron-deficient vacancy (F(S)(+) and F(S)(2+)). Accordingly, three F(S)-centers exhibit different arrangements of X-ray photoelectron spectra, the scanning tunneling microscopy images, and the vibrational spectra after depositing (WO(3))(3) cluster. Present results reveal that the (WO(3))(3) cluster may be used as a probe to identify the different F(S) centers on the MgO(001) surface.

Effects of Ti doping at the reduced SnO 2 (110) surface with different oxygen vacancies: a first principles study

A series of Ti-doped SnO2(110) surfaces with different oxygen vacancies have been investigated by means of first principles DFT calculations combined with a slab model. Three kinds of defective SnO2(110) surfaces are considered, including the formations of bridging oxygen (Ob) vacancy, in-plane oxygen (Oi) vacancy, and the coexistence of Ob and Oi vacancies. Our results indicate that Ti dopant prefers the fivefold-coordinated Sn site on the top layer for the surface with Ob or Oi vacancy, while the replacement of sublayer Sn atom becomes the most energetically favorable structure if the Ob and Oi vacancies are presented simultaneously. Based on analyzing the band structure of the most stable configuration, the presence of Ti leads to the variation of the band gap state, which is different for three defective SnO2(110) surfaces. For the surface with Ob or Oi vacancy, the component of the defect state is modified, and the reaction activity of the corresponding surface is enhanced. Hence, the sensing performance of SnO2 may be improved after introducing Ti dopant. However, for the third kind of reduced surface with the coexistence of Ob and Oi vacancies, the sublayer doping has little influence on the defect state, and only in this case, the Ti doping state partly appears in the band gap of SnO2(110) surface.

Insight into the mechanism for the methanol synthesis via the hydrogenation of CO2 over a Co-modified Cu(100) surface: A DFT study

A comprehensive density functional theory calculation was employed to investigate the reaction mechanism of methanol synthesis on a Co-modified Cu(100) surface via CO2 hydrogenation. The Cu(100) surface with embedded small Co clusters prepared experimentally was employed as a model system to explore the effects of Co dopant on the catalytic performance of Cu(100) surface towards CH3OH synthesis. The activation energy barriers and the reaction energies of 16 elementary surface reactions were determined. Our calculated results show that the most favorable reaction pathway for the hydrogenation of CO2 to CH3OH follows the sequence of CO2 → HCOO* →H2COO* →H2COOH* →H2CO* →H3CO* →H3COH*, and the OH* group hydrogenation to H2 O* is the rate-limiting step with an activation barrier of 112.3 kJ/mol. It is noted that, since the strength of Co-O bond is stronger than that of Cu-O bond, the introducing of Co dopant on the Cu surface can facilitate the formation of key intermediates for the CH3OH synthesis. Especially, the stability of the unstable dioxomethylene intermediate (H2COO*) found on the pure Cu(100) surface can be obviously enhanced on the Co-doped Cu(100) surface. As a result, with respect to the undoped surface, the productivity and selectivity towards CH3OH production on the Cu(100) surface will be improved after dispersing small Co clusters on the surface.

Effects of ligand functionalization on the photocatalytic properties of titanium-based MOF: A density functional theory study

The first principle calculations have been performed to investigate the geometries, band structures and optical absorptions of a series of MIL-125 MOFs, in which the 1,4-benzenedicarboxylate (BDC) linkers are modified by different types and amounts of chemical groups, including NH2, OH, and NO2. Our results indicate that new energy bands will appear in the band gap of pristine MIL-125 after introducing new group into BDC linker, but the components of these band gap states and the valence band edge position are sensitive to the type of functional group as well as the corresponding amount. Especially, only the incorporation of amino group can obviously decrease the band gap of MIL-125, and the further reduction of the band gap can be observed if the amount of NH2 is increased. Although MIL-125 functionalized by NH2 group exhibits relatively weak or no activity for the photocatalytic O2 evolution by splitting water, such ligand modification can effectively improve the efficiency in H2 production because now the optical absorption in the visible light region is significantly enhanced. Furthermore, the adsorption of water molecule becomes more favorable after introducing of amino group, which is also beneficial for the water-splitting reaction. The present study can provide theoretical insights to design new photocatalysts based on MIL-125.The first principle calculations have been performed to investigate the geometries, band structures and optical absorptions of a series of MIL-125 MOFs, in which the 1,4-benzenedicarboxylate (BDC) linkers are modified by different types and amounts of chemical groups, including NH2, OH, and NO2. Our results indicate that new energy bands will appear in the band gap of pristine MIL-125 after introducing new group into BDC linker, but the components of these band gap states and the valence band edge position are sensitive to the type of functional group as well as the corresponding amount. Especially, only the incorporation of amino group can obviously decrease the band gap of MIL-125, and the further reduction of the band gap can be observed if the amount of NH2 is increased. Although MIL-125 functionalized by NH2 group exhibits relatively weak or no activity for the photocatalytic O2 evolution by splitting water, such ligand modification can effectively improve the efficiency in H2 production because now ...

Why does F-doping enhance the photocatalytic water-splitting performance of mBiVO4? – a density functional theory study

By means of density functional theory (DFT) computations, we investigated the variations in the geometric structures and electronic properties, as well as the adsorption behavior of water on the (010) and (110) surfaces, introduced by an F dopant in a monoclinic BiVO4 system. For the bulk phase, F atoms are easier to substitute O atoms as they form a stable geometric structure. With F-doping, the band gap is narrowed and the separation efficiency of the photogenerated carriers is improved. On both (010) and (110) surfaces, F atoms prefer to substitute the two-coordinated O atoms at the outermost layer. Besides, F-doping on the surfaces can also reduce the band gap, which may enhance the visible light utilization. Due to hydrogen bonds between the F dopant and the O atom of water, the interactions between the F-doped surface and the absorbed water molecules are increased, which is favorable for water splitting under visible light.