The origin of the site preference of H adsorption on Pd(100)

Spin-polarized density functional theory(DFT)calculations are carried out to determine the site preference of H adsorption on Pd(100)surface and subsurface.We carefully scrutinize the energy difference between different patterns at=0.50 ML and confirm the LEED observation that surface adsorption can form c(2×2)ordering structure.On the contrary,we disclose that p(2×1)structure become more favorable than c(2×2)for subsurface adsorption.These site preferences are rationalized via an analysis of the layer and orbital resolved density of states.Furthermore,we propose that the interstitial charge as a key factor determining the preferred H adsorbed site.     

Density Functional Theory Study on the Adsorption of CN on Transition Metal M（100） （M = Ni,Pd, Pt,Cu, Ag and Au） Surfaces

The adsorption of cyanide on the top site of a series of transition metal M（100） （M = Cu, Ag, Au, Ni, Pd, Pt） surfaces via carbon and nitrogen atoms respectively, with the CN axis perpendicular to the surface, has been studied by means of density functional theory and cluster model. Geometry, adsorption energy and vibrational frequencies have been determined, and the present calculations show that the adsorption of CN through C-end on metal surface is more favorable than that via N-end for the same surface. The vibrational frequencies of CN for C-down configuration on surface are blue-shifted with respect to the free CN, which is contrary to the change of vibrational frequencies when CN is adsorbed by N-down structure. Furthermore, the charge transfer from surface to CN causes the increase of surface work function.     

Density Functional Study of the C Atom Adsorption on the α-Fe2O3 （001） Surface

The adsorption of C atoms on the α-Fe2O3(001) surface was studied based on density function theory(DFT) ,in which the exchange-correlation potential was chosen as the PBE(Perdew,Burke and Ernzerhof) generalized gradient approximation(GGA) with a plane wave basis set. Upon the optimization on different adsorption sites with coverage of 1/20 and 1/5 ML,it was found that the adsorption of C atoms on the α-Fe2O3(001) surface was chemical adsorption. The coverage can affect the adsorption behavior greatly. Under low coverage,the most stable adsorption geometry lied on the bridged site with the adsorption energy of about 3.22 eV;however,under high coverage,it located at the top site with the energy change of 8.79 eV. Strong chemical reaction has occurred between the C and O atoms at this site. The density of states and population analysis showed that the s,p orbitals of C and p orbital of O give the most contribution to the adsorption bonding. During the adsorption process,O atom shares the electrons with C,and C can only affect the outermost and subsurface layers of α-Fe2O3;the third layer can not be affected obviously.     

Study of H2O and HOCH2CH2OH Adsorption on the Relaxation Surface of β-Si3N4（0001） by Density Functional Theory

Density functional theory （DFT） B3LYP method is used to theoretically investigate the adsorption conformations of H2O and glycol on the relaxation surface of β-Si3N4（0001） with cluster models. For H2O, the most stable structure is that adsorbed through the H atom lying above a N（3） site of the relaxation surface of β-Si3N4（0001）; while for glycol, it is the one adsorbed via the H atom lying above the center of Si（2） and N（3） of the same relaxation surface. The adsorption energy, adsorption bond and transfer electrons of the two adsorbed substances prove that glycol is easy to be adsorbed on the relaxation surface of β-Si3N4（0001）.     

Adsorption of Co2B2 and Ni2B2 Clusters on the TiO2 (110) Surface: a Density Functional Study

Based on density functional theory and generalized gradient approximation calculations, the adsorption of Co2B2 and Ni2B2 clusters on the rutile TiO2 (110) surface has been investigated utilizing periodic supercell models. Unambiguously, the results demonstrate that the hollow site turns out to be preferable for Co2B2 cluster while Ti2 site is for Ni2B2 cluster to adsorb. Orbital population analysis indicates a strong interaction between Co2B2 and O atom of TiO2 surface, which can be attributed to the overlap of Co 3d and surface O 2p orbital. Similarly, for Ni2B2 , the bonding interaction occurs mostly through the interaction of Ni 3d/4s and O 2p orbitals. Note that, there is also an interaction within the Co2B2 clusters (Ni2B2) through B 2s/2p and Co 3d orbitals (Ni 3d/4s). Moreover, orbital analysis results shows that the strong bonding between Ni2B2 and Ti2 site is due to the overlap of HOMO of Ni2B2 and d-orbital of five-coordinated titanium atoms.     

Theoretical Study on the Adsorption and Decomposition of Methanol over the Pt-Mo（111）/C Surface

The density functional theory(DFT) and self-consistent periodic calculation were used to investigate the methanol adsorption on the Pt-Mo(111)/C surface.The adsorption energies,equilibrium geometries and vibration frequencies of CH3OH on nine types of sites on the Pt-Mo(111)/C surface were predicted and the favorite adsorption site for methanol is the top-Pt site.Both sites of valence and conduction bands of doped system have been broadened,which are favorable for electrons to transfer to the cavity.The possible decomposition pathway was investigated with transition state searching and the calculation results indicate that the O-H bond is first broken,and then the methanol decomposes into methoxy.The activation barrier of O-H bond breaking with Pt-Mo catalyst is only 104.8 kJ mol-1,showing that carbon supported Pt-Mo alloys have promoted the decomposition of methanol.Comparing with the adsorption energies of CH3OH on the Pt(111)/C surface and that of CO,the adsorption energies of CO are higher,and Pt(111)/C is liable to be oxidized and loses the activity,which suggests that the catalyst Pt-Mo(111)/C is in favor of decomposing methanol and has better anti-poisoning ability than Pt(111)/C.     

Adsorption of cyclohexene and its dehydrogenation intermediates on nAu/Pt(100)(n＝0,1,2) surfaces:A DFT study

Adsorption of cyclohexene and its dehydrogenation intermediates on the nAu/Pt(100) (n = 0, 1, 2 means clean Pt, one monolayer and two layers of Au covered Pt surfaces, respectively.) has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, i.e., hollow sites and bridge sites, and the adsorption energy at the hollow site is larger than that at the bridge site. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike at both the bridge site and the top site. The magnitude order of the adsorption energies is as follows: clean Pt > Au/Pt > 2Au/Pt. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C–M (M = Pt or Au, on the top layer of the slab) bonds are closely related to the corresponding adsorption energies.     

Comparison of Ethylation at External Surface and Internal Cavity of H-MCM-22 Zeolite from Theoretical Calculations

Summaryof main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis.Benzene ethylation is an industrially important process catalyzed by H-MCM-22 zeolite;while the active sites still remain elusive.In this work,density functional theory(DFT)calculations were employed to investigate the benzene ethylation at two different types of Br?nsted acid sites(BAS)in H-MCM-22 zeolite,namely the internal cavity(IC)acid site and the external surface(ES)acid site.Both the stepwise and concerted pathways were addressed.The compari-son of the calculated energetics of two pathways indicates that the benzene ethylation reaction primarily proceeds via the concerted pathway at both the IC and ES acid sites of H-MCM-22.The calculated overall Gibbs free energies at reaction condition(473 K and 3.5 MPa)on the IC and ES acid sites are 90 and 86 kJ/mol,with the rate constants of 1.20×10^3and 2.92×10^3s^-1,respectively.It indicates that benzene ethylation could occur both on the IC and ES acid sites,with the catalytic activity of IC acid site being slightly lower than that of ES acid site.Furthermore,we theoretically reveal that the acid strength at the ES site is slightly weaker than that at the IC site via the frequency shift after the adsorption of CO.The differences in dispersion interaction between ES and IC sites are also quantified by the adsorption of base molecules with different sizes.The calculated results in this work demonstrate that the acid sites at the external surface of H-MCM-22 zeolites are suitable for benzene ethylation to produce ethylbenzene,providing theoretical implications for tailoring the distribution of active sites in H-MCM-22 zeolite.     

First-principles Study on the Properties of CaO(100) Surface Adsorbing Carbon Dioxide

The increasing carbon dioxide emissions have a huge impact on the global environment. Carbonation reaction of CaO is regarded as a potential method to capture carbon dioxide. The density functional theory calculations have been performed to investigate the adsorption of CO_2 on CaO(100) surface. This paper systematically studied the adsorption of CO_2 at different adsorption sites on CaO(100) surface and the influence of adsorption angle on adsorption energy. Based on the studying of adsorption sites, adsorption energy and electronic structure of the CO_2/CaO(100) systems, chemical adsorption mainly happens when CO_2 molecules are absorbed on the CaO(100) surfaces, but physical adsorption may also happen. The research found that CO_2 molecules reacted with surface O atom through C, forming monodentate surface carbonate species and tridentate carbonate. Among them, low-coordinated monodentate ligands have a higher stability than tridentate ligands due to the shorter C–OS bond length of monodentate ligands.     

Theoretical Study of NO Dimer Adsorption and Dissociation on the CuCr2O4 （100） Surface

Theoretical simulation of the adsorption and dissociation of two NO molecules at the Cu^2＋, Cr^3＋ and bridge Cr^3＋ sites （b-Cr^3＋） on the normal spinel CuCr2O4 （100） surface has been carried out by density functional theory calculations. The results show that the formed N-down and O-down NO dimers are negatively charged. The formation of stable O-down dimers on the surface leads to the great elongation of N-O bond, which contributes to the NO reduction. The transition-state calculations indicate that the decomposition of O-down NO dimer at the b-Cr^3＋ site is most favorable and N2O is the major reduction product.     

Theoretical Studies on Adsorption Sites and States of O and N on Ni（311）

The adsorption of O and N atoms on the Ni(311) surface was investigated by the 5-parameter Morse potential(5-MP) method in detail. For the O-Ni(311) system, there are three surface adsorption states and thefcc-3-fold site is metastable; the frequency of 75 meV[67 meV in high resolution electron energy loss spectroscopy(HREELS) experiment] is attributed to the vibration at the hcp-3-fold site. For the N-Ni(311) system, however, there are only two surface adsorption states(no surface adsorption state was calculated at fcc-3-fold site). In addition, subsurface states were predicted and all critical characteris-tics were obtained for the two systems.     

H原子在W低指数面上的吸附位和吸附态

应用原子和表面簇合物相互作用的5参数Morse 势及由5参数Morse势组装推广的LEPS方法对H-W低指数表面吸附体系进行了研究, 并获得了全部临界点特性. 计算结果表明, 低覆盖度下, H原子优先吸附在W(100)面的内层吸附位二层桥位B', 获得156 meV的垂直振动频率, 随着覆盖度的增加, H原子稳定吸附在表层的五重洞位(二层顶位)、桥位及顶位. 内层吸附位的优先吸附, 对与其邻近的表面吸附位的临界点性质有一定影响. 在W(110)面上只存在三重洞位的稳定吸附态, 垂直振动频率为151 meV. 在W(111)面上存在三种稳定吸附态, 子表面吸附位H₁, 桥位B'和顶位T, 分别获得104, 200, 259 meV的垂直振动频率. 在低覆盖度下, H原子优先吸附在子表面吸附位H₁.     

Adsorption of CO and O2 on W2C(0001)

CO, O(2), and H(2) adsorption on a clean W(2)C(0001)√13×√13 R ± 13.9° reconstructed surface at room temperature (RT) were investigated using high-resolution electron energy loss spectroscopy (HREELS). The W(2)C(0001) adsorbs CO molecularly and adsorbs O(2) dissociatively, but does not adsorb H(2) at RT. In the CO adsorption system, two C-O stretching (antisymmetric CCO stretching) modes were found at 242.3 meV (1954 cm(-1)) and at 253.0 meV (2041 cm(-1)). The low-frequency site is occupied at first with subsequent conversion to the high-frequency site with increasing coverage. Additionally, a small peak was apparent at 104.5 meV (843 cm(-1)), and a middle peak at 50-51 meV (400-410 cm(-1)), which are assignable to a symmetric stretching mode and a hindered translational mode, respectively, of a CCO (ketenylidene) species. These observations are consistent with the CO adsorption model on top of the surface carbon. For oxygen adsorption, two adsorption states were found at 65.2-68.1 meV (526-549 cm(-1)) and 73.6 meV (594 cm(-1)): typical frequencies to oxygen adsorption on metal surfaces. Results suggest that atomic oxygen adsorption occurred on a threefold hollow site of the second W layer.     

DFT study of a weakly pi-bonded C2H4 on oxygen-covered Ag(100)

The coadsorption of ethylene, C2H4, and atomic oxygen on Ag(100) was studied using density-functional theory. As for the adsorption of oxygen alone, the on-surface hollow sites are predicted to be the most stable adsorption sites at low coverage (< or =1/2 ML). Above this coverage, mixed on-surface + subsurface oxygen configurations become more stable. The binding of ethylene to the clean Ag(100) is weak and little affected by oxygen when it is adsorbed on-surface. On the other hand, we find that the adsorption energy of C2H4 may increase considerably when oxygen is adsorbed into subsurface sites. Our results indicate that the increased reactivity of surface Ag atoms is because of their decreased coordination due to the push out effect of oxygen underneath, more than to their oxidation.     

First principle and ReaxFF molecular dynamics investigations of formaldehyde dissociation on Fe(100) surface

Detailed formaldehyde adsorption and dissociation reactions on Fe(100) surface were studied using first principle calculations and molecular dynamics (MD) simulations, and results were compared with available experimental data. The study includes formaldehyde, formyl radical (HCO), and CO adsorption and dissociation energy calculations on the surface, adsorbate vibrational frequency calculations, density of states analysis of clean and adsorbed surfaces, complete potential energy diagram construction from formaldehyde to atomic carbon (C), hydrogen (H), and oxygen (O), simulation of formaldehyde adsorption and dissociation reaction on the surface using reactive force field, ReaxFF MD, and reaction rate calculations of adsorbates using transition state theory (TST). Formaldehyde and HCO were adsorbed most strongly at the hollow (fourfold) site. Adsorption energies ranged from ?22.9 to ?33.9 kcal/mol for formaldehyde, and from ?44.3 to ?66.3 kcal/mol for HCO, depending on adsorption sites and molecular direction. The dissociation energies were investigated for the dissociation paths: formaldehyde → HCO + H, HCO → H + CO, and CO → C + O, and the calculated energies were 11.0, 4.1, and 26.3 kcal/mol, respectively. ReaxFF MD simulation results were compared with experimental surface analysis using high resolution electron energy loss spectrometry (HREELS) and TST based reaction rates. ReaxFF simulation showed less reactivity than HREELS observation at 310 and 523 K. ReaxFF simulation showed more reactivity than the TST based rate for formaldehyde dissociation and less reactivity than TST based rate for HCO dissociation at 523 K. TST‐based rates are consistent with HREELS observation. © 2013 Wiley Periodicals, Inc.     

氧原子在钨低指数表面及(211)高指数表面上的吸附和振动

 应用原子和表面簇合物相互作用的5参数Morse势方法研究了氧原子在W低指数表面及(211)高指数表面上的吸附和振动. 计算结果表明,低覆盖度下,在W(100)面氧原子吸附在四重洞位,随着覆盖度增加,(100)面发生缺行重构,膺式三重位为稳定吸附位; 在W(110),(111)及(211)面,氧原子均趋向于吸附在膺式三重位. 该计算结果和实验符合得很好.     

The Adsorption and Thermal Decomposition of CH2CO (CD2CO) on Si(111)-7 × 7

The adsorption and thermal decomposition of ketene on Si(l 11)-7 × 7 were investigated using various surface analysis techniques. When the surface was exposed to ketene at 120 K, two CO stretching modes at 220 and 273 meV appeared in HREELS, corresponding to two adsorbed ketene states. After the sample was annealed at ?250 K, the 273 and the 80 meV peaks vanished, indicating the disappearance of one of the adsorption states by partial desorption of the adsorbate. In a corresponding TPD measurement, a desorption peak for ketene species was noted at 220 K. Annealing the sample at 450 K caused the decomposition of the adsorbate, producing CH_x and O adspecies. Further annealing of the surface at higher temperatures resulted in the breaking of the CH bond, the desorption of H and O species and the formation of Si carbide. The desorption of H at 800 K was confirmed by the appearance of the D₂ (m/e = 4) TPD peak at that temperature when CD₂CO was used instead of CH₂CO.     

Spectroscopic evidence for the partial dissociation of H2O on ZnO(1010)

The interaction of water with the non-polar ZnO(1010) surface has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Adsorption of water at room temperature leads to the partial dissociation of water molecules giving rise to a well defined (2x1) superstructure. This observation was confirmed by the HREELS data which show the water-induced O-H stretching modes at 396 and 460 meV (3193 and 3709 cm-1) as well as the peak at 456 meV (3677 cm-1) arising from the OH species. The large red shift of the loss at 396 meV indicates unusually strong hydrogen bonding interactions of water to both neighbouring adsorbate molecules and the surface O atoms which are responsible for the partial dissociation of water molecules on the perfect ZnO(1010) surface.     

A first-principles potential energy surface and vibrational states for hydrogen on Cu(100)

Density-functional theory calculations based on plane-wave expansion and pseudopotential treatment were carried out for atomic hydrogen on a rigid Cu(100) surface. A global potential energy surface was then obtained by using a three-dimensional spline interpolation. It is found that the minimum of the potential is located at the fourfold hollow site with a diffusion barrier of 88 meV at the bridge site. The vibrational states of atomic hydrogen and deuterium on the Cu(100) surface were calculated on the potential surface. Our calculations show that the vibrational states A(1) (0), A(1) (1), E(1), and B(2) (1) of H/Cu(100) exhibit strong localized character and very narrow band widths, whereas other excited vibrational states have considerable delocalized character and broad band widths. The vibrational frequency of 71.2 (51.5) meV for H(D) in the perpendicular direction obtained in this study is in good agreement with the experimentally observed value of 70 (52) meV.     

Study of H2O and HOCH2CH2OH Adsorption on the Relaxation Surface of β-Si3N4(0001) by Density Functional Theory

Density functional theory (DFT) B3LYP method is used to theoretically investigate the adsorption conformations of H2O and glycol on the relaxation surface of β-Si3N4(0001) with cluster models. For H2O, the most stable structure is that adsorbed through the H atom lying above a N(3) site of the relaxation surface of β-Si3N4(0001); while for glycol, it is the one adsorbed via the H atom lying above the center of Si(2) and N(3) of the same relaxation surface. The adsorption energy, adsorption bond and transfer electrons of the two adsorbed substances prove that glycol is easy to be adsorbed on the relaxation surface of β-Si3N4(0001).