Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO₂-based catalysts have attracted great attention in the field of elemental mercury (Hg⁰) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg⁰ oxidation by oxygen species (gas-phase O₂, chemisorbed oxygen, and lattice oxygen) on MnO₂ surface. The results indicate that Hg⁰ and HgO are chemically adsorbed on MnO₂ surface with the adsorption energies of ?69.50 and ?226.48?kJ/mol, respectively. The adsorption of O₂ on MnO₂ surface belongs to chemisorption. O₂ can decompose on MnO₂ surface with an energy barrier of 97.46?kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O₂ and dissociative adsorbed O₂ are more favorable for Hg⁰ catalytic oxidation than lattice oxygen, and perpendicular adsorbed O₂ is the most active oxygen for Hg⁰ oxidation. The reaction pathway of Hg⁰ oxidation by perpendicular adsorbed O₂ includes three reaction steps: Hg⁰?→?Hg(ads)?→?HgO(ads)?→?HgO. The third step (HgO(ads)?→?HgO) is endothermic by 168.17?kJ/mol with an energy barrier of 179.48?kJ/mol, and it is the rate-limiting step of the whole Hg⁰ oxidation reaction.     

Towards relativistic ECP / DFT description of chemical bonding in E112 compounds: spin-orbit and correlation effects in E112X versus HgX (X=H, Au)

The relativistic effective core potential (RECP) approach combined with the spin-orbit DFT electron correlation treatment was applied to the study of the bonding of eka-mercury (E112) and mercury with hydrogen and gold atoms. Highly accurate small-core shape-consistent RECPs derived from Hartree-Fock-Dirac-Breit atomic calculations with Fermi nuclear model were employed. The accuracy of the DFT correlation treatment was checked by comparing the results in the scalar-relativistic (spin-orbit-free) limit with those of high level scalar-relativistic correlation calculations within the same RECP model. E112H was predicted to be slightly more stable than its lighter homologue (HgH). The E112-Au bond energy is expected to be ca. 25–30 % weaker than that of Hg-Au. The role of correlations and magnetic (spin-dependent) interactions in E112-X and Hg-X (X=H, Au) bonding is discussed. The present computational procedure can be readily applied to much larger systems and seems to be a promising tool for simulating E112 adsorption on metal surfaces.     

Roles of γ-Fe2O3 in fly ash for mercury removal: Results of density functional theory study

First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe₂O₃ in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe₂O₃ is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe₂O₃ (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe₂O₃ (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe₂O₃ (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.     

密度泛函理论研究Hg与Auqn(n=1—6, q=0,+1,-1) 团簇的相互作用

采用密度泛函理论中的广义梯度近似对Hg与小团簇Au ^q_n (n=1—6, q=0, +1, -1)的相互作用进行了系统研究. 结果表明,除Au₅^+,-团簇外,前线分子轨道理论可以成功预测大部分Au _n Hg ^q 复合物的最低能量结构. Au_n团簇对Hg的吸附受团簇尺寸大小和团簇所携带电荷的影 关键词： 密度泛函理论 汞 金团簇 吸附能     

Adsorption of oxygen on the tungsten (110) plane at low temperatures; Spectroscopic measurements

The adsorption of oxygen on the W(110) plane was carried out at 26 K and investigated by means of ultraviolet and X-ray photoelectron spectroscopy. It was found that atomic oxygen is adsorbed first to essentially saturation coverage (O/W = 0.6) before adsorption of molecular oxygen occurs. The spectrum of the latter is very similar to that of gas phase O₂ but the shift to weaker binding energies is greater for the 1 s level than for the valence orbitals.     

A comparative study of nonrelativistic and relativistic 5d atoms as impurities in silicon

A comparison is made between calculations performed nonrelativistically and relativistically for W, Re, Os, Ir, Pt, Au and Hg as substitutional impurities in silicon. The calculations were carried out using the relativistic extended Hückel method. The direct and indirect relativistic effects upon the 5d-like levels and band-gap levels are analysed.     

超临界CO2在金属铀表面吸附的理论研究

依据实验数据,假定CO2在金属铀表面吸附氧化初期形成的吸附中间体为UCO2.根据密度泛函理论(DFT)的Becke3lyp方法计算得UCO2(C2v构型)分子的5A1态能量最低,这与用原子分子反应静力学与群论确定UCO2的基电子状态为5A1的结果一致.计算表明,CO2在金属铀表面的吸附是放热反应,其吸附量随着温度的升高而不断减少,其吸附热在1 atm下为51.68 kJ.mol-1,该值大于40 kJ.mol-1,故CO2在金属铀表面的吸附是化学吸附.     

Adsorption and dissociation of ammonia on Au(1 1 1) surface: A density functional theory study

Periodic density functional theory (DFT) calculations using plane waves had been performed to systematically investigate the stable adsorption amine and its dehydrogenated reaction on Au(1 1 1) surface. The equilibrium configuration including on top, bridge, and hollow (fcc and hcp) sites had been determined by relaxation of the system. The adsorption both NH₃ on top site and NH₂ on bridge site is favorable on Au(1 1 1) surface, while the adsorption of NH on hollow (fcc) site is preferred. The adsorbates are adsorbed on the gold surface with the interaction between p orbital of adsorbate and the d orbital of gold atoms. The interaction between adsorbate and gold slab is more evident on the first layer than on any others. Furthermore, the dissociation reaction of NH₃ on clean gold surface, as well as on the pre-covered oxygen atom and pre-covered hydroxyl group surface had been investigated. The results show that the dehydrogenated reaction energy barrier on the pre-covered oxygen gold surface is lower. The adsorbed O can promote the dehydrogenation of amine. Additionally, OH as the product of the NH₃ dissociation reaction participates in continuous dehydrogenation reaction, and the reaction energy barrier is the lowest (22.77 kJ/mol). The results indicated that OH_ads play a key role in the dehydrogenated reaction on Au(1 1 1) surface.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

The energetics of mercury adsorption on Cu(100)

We have investigated the adsorption of mercury overlayers on Cu(100) by atom beam scattering, low energy electron diffraction and angle resolved photoemission. From our data we have calculated the isosteric heats in the adsorbed Hg layer on Cu(100) and compared these with results obtained for mercury on Fe(100), W(100) and Ni(100). We observe changes in the isosteric heat of adsorption that can be associated with the ordering of a c(2 × 2) Hg overlayer phase and the transition from a c(2 × 2) overlayer to a c(4 × 4) overlayer. The isosteric heat of adsorption is 0.50 ± 0.07 eV/atom (48 ± 7 kJ/mol) at zero coverage and reaches a maximum of 0.73 ± 0.04 eV/atom (70 ± 4 kJ/mol). From a combination of ABS and LEED, the structures of the two equilibrium ordered phases of Hg on Cu(100) have been identified, as well as the structures of several non-equilibrium phases.     

(TiO2)n(n=3—6)团簇吸附水分子的理论研究

给出了优化小分子在团簇表面吸附结构的遗传算法.结合经验势函数,搜寻了水分子在(TiO₂)_n(n=3—6)团簇上可能的吸附方式;利用B3LYP/6-31G^**方法对各种吸附结构进行了优化.结果表明水分子主要通过O原子以非解离方式吸附到团簇中配位数较低或位置比较凸出的Ti原子上.分子轨道分析表明,水分子与团簇之间的成键主要来自吸附位Ti原子3s3p轨道的贡献,水分子的轨道保持了气相水分子中的基本特征,但离域化程度增大 关键词： 2团簇')" href="#">TiO₂团簇 2O吸附')" href="#">H₂O吸附 遗传算法 DFT     

Perspective for the determination of chemical properties of element 112

Abstrac We present here results of thermochromatographic model studies with Rn and Hg, conducted in order to prepare future gas-adsorption chromatographic investigations of element 112. The adsorption properties of Rn on various transition metals were investigated by vacuum thermochromatography. From the results of these experiments, predictions have been deduced for the adsorption behavior of hypothetically noble-gas-like elements 112 and 114. Empirical predictions of the adsorption interaction of a noble metallic element 112 and its lighter homologues with transition metal surfaces are given in the literature. The results of these calculations are compared with experimental data obtained in thermochromatographic model experiments with Hg. The most efficient way to chemically identify element 112 is the use of a cryo-on-line detector (COLD)-like setup, which was already successfully applied in the chemical investigation of hassium. Modifications of this device needed for the on-line thermochromatographic investigation of element 112 are presented together with results of test experiments with short-lived isotopes of Rn and Hg. From Yadernaya Fizika, Vol. 66, No. 6, 2003, pp. 1182–1187. Original English Text Copyright ? 2003 by Eichler, Soverna. This article was submitted by the authors in English. The authors represent the a PSI-University Bern-GSI-TU Munich-FLNR-University Mainz-IMP Collaboration     

Unique geometric and electronic structure of CO adsorbed on Ge(100): A DFT study

CO adsorption on the Ge(100) surface has been investigated using a slab model with density functional theory implemented in SIESTA. CO was found to be exclusively adsorbed on the asymmetric dimer with C attaching on the lower Ge dimer atom. The adsorption process is barrierless. The calculated adsorption energy and vibration frequencies are comparable to previous experimental results. The crystal orbital Hamilton analysis showed that the bonding between Ge and CO is mainly attributable to the Ge 4p_z orbital overlapping with C 2 s, or with CO molecular orbitals 3σ and 4σ. The repulsive energy between adsorbed CO molecules is less than 1 kcal/mol. The diffusion barrier of CO on the Ge(100) surface is about 14 kcal/mol.     

Density functional study of the polymorphism of Cs2C2 and Rb2C2

By density functional theory-based calculations it is shown that in the athermal limit the orthorhombic polymorph of Cs₂C₂ is more stable by ≈7 kJ/mol with respect to the hexagonal modification, while the energy difference between the corresponding two Rb₂C₂-polymorphs is about 4 kJ/mol. The calculations do not corroborate the experimental finding of unusually long and short C-C bond lengths in Cs₂C₂ at low temperatures. This theoretical result is supported by calculations on monomeric LiCCH, where DFT calculations give all bond lengths within 1%.     

EPR properties of Au atoms adsorbed on various sites of the MgO(1 0 0) surface from relativistic DFT calculations

Using all electron fully relativistic DFT calculations we have computed the EPR properties of Au atoms bound to various sites of the MgO surface. Changes in g-tensor and hyperfine coupling constants provide a way to identify the gold adsorption site and to map the surface morphology by comparison of measured and calculated EPR spectra. We found a strong reduction of the isotropic hyperfine coupling constant, a_iso(Au), for adsorbed gold compared to the free atom; this reduction, which is about 45% for terrace sites, is more pronounced when Au interacts with low-coordinated sites like steps, edges and corners where it is about 60%. The reduction of a_iso(Au) is accompanied by a corresponding increase of the superhyperfine interaction with the surface oxygen sites, as measured by a_iso(¹⁷O). Large anisotropies in the g-tensor are computed for all sites.     

Adsorption of carbon monoxide Au/Pd(1 0 0) alloys in ultrahigh vacuum: Identification of adsorption sites

The adsorption of carbon monoxide is studied on Au/Pd(1 0 0) alloys by means of reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The alloy was formed by adsorbing a four-monolayer thick gold film on a Pd(1 0 0) substrate and by heating to various temperatures to form alloys with a range of palladium coverages. The alloy was characterized using X-ray photoelectron spectroscopy and the composition of the outermost layer measured using low-energy ion scattering spectroscopy. CO adsorbs on palladium bridge sites only for palladium coverages greater than 0.5 monolayers (ML) suggesting that next-nearest neighbor sites are preferentially populated by palladium atoms. CO adsorbs on atop palladium sites and desorbs at ∼350 K corresponding to a desorption activation energy of ∼117 kJ/mol. However, at lower palladium coverages, these sites are not occupied and CO desorption states are detected 170 and 112 K corresponding to desorption activation energies of ∼53 kJ/mol and ∼35 kJ/mol, respectively, for these states. It is suggested that these states are due to a restructuring of the surface to form low-coordination gold sites that obscure the atop palladium site.     

Effect of oxygen on the production of abnormally high heats of interaction with hydrogen chemisorbed on gold

Abnormally high heats, exceeding 1600 kJ/mol (16 eV) per molecular oxygen, are generated by interaction of the oxygen with the hydrogen adsorbed on gold surfaces at 125 °C. The highest heats were observed during the interactions of fine gold particles supported on titanium oxide, approaching 1700 kJ/mol for three consecutive 100 nmol pulses of O₂ interacting with the adsorbed hydrogen atoms. The heats rapidly decrease after the hydrogen is consumed. It was also observed that the interactions of the gold particles with pure oxygen in the presence of noble gases, such as argon and helium, produced the heats markedly higher than those observed in the absence of noble gases. The abnormally high heats revealed by this work reach values from 3.5 to 6.1 times higher than the heats of formation of gaseous water from molecular hydrogen and oxygen.     

用扩散蒙特卡罗方法研究BH2, B(OH)2, BCl2和BCl的键离解能

采用扩散蒙特卡罗(DMC)方法计算了BH₂, B(OH)₂, BCl₂和BCl的HB-H和HOB-OH的键离解能, 同时也研究了轨道选择和Backflow变换对DMC计算结果的影响． 在Slater-Jastrow DMC(SJ-DMC)计算方法中,当采用B3PW91轨道时得到的HB-H和HOB-OH键离解能分别是359.1±0.12和98.2±0.12 kJ/mol;用B3LYP SJ-DMC计算键离解能得到了与用B3PW91 SJ-DMC方法类似的结果．通过BF-DMC(即在DMC中引入backflow修正)计算得到的HB?H键离解能为369.6±0.12 kJ/mol,也得到了更加接近实验值的HOB-OH键离解能为446.0±1.84 kJ/mol．由DMC的计算结果可以断定HB?H的键离解能的实验值为375.8 kJ/mol．另外还给出了BCl2和BCl的键离解能的计算结果.     

Surface atomic distribution and water adsorption on Pt-Co alloys

Density functional theory is used to study surface atomic distributions on slabs of PtCo and Pt₃Co overall compositions, as well as water molecule adsorption on PtCo(1 1 1) and Pt-skin structures. Pt-rich surfaces are energetically favored under vacuum in the PtCo and Pt₃Co alloys. The adsorption trend on the studied structures agrees with the d-band model, with stronger adsorption at higher surface Co composition. The most stable adsorption site for a water molecule on PtCo surfaces is on top of Co atoms, with the dipole vector parallel to the surface. This water/surface interaction is as strong as that of water molecule on Pt(1 1 1), whereas bonding to Pt-skin monolayers is found much weaker than that on Pt(1 1 1). It is found that water interacts mainly through its 1b₁ and 3a₁ orbitals with d orbitals of the Pt(1 1 1), PtCo(1 1 1) and Pt-skin surface atoms. Compared to the sum of the electron densities of the separated systems, the electron density of the water/surface gets depleted along O-Pt on Pt-skin surfaces while it becomes richer in the O-Co bonding region of PtCo.     

Reaction mechanism of ammonia oxidation over RuO2(1 1 0): A combined theory/experiment approach

Combining state-of-the-art density functional theory (DFT) calculations with high resolution core level shift spectroscopy experiments we explored the reaction mechanism of the ammonia oxidation reaction over RuO₂(1 1 0). The high catalytic activity of RuO₂(1 1 0) is traced to the low activation energies for the successive hydrogen abstractions of ammonia by on-top O (less than 73 kJ/mol) and the low activation barrier for the recombination of adsorbed O and N (77 kJ/mol) to form adsorbed NO. The NO desorption is activated by 121 kJ/mol and represents therefore the rate determining step in the ammonia oxidation reaction over RuO₂ (1 1 0).