Pd(Cu)在MgO(100)表面吸附CO和O2的理论研究

采用固体镶嵌势能模型和DFT/B3LYP方法研究了在Pd/MgO和Cu/MgO表面吸附CO和O₂分子的电子性质. 计算结果表明, 在完美MgO(100)表面Pd原子对CO和O₂的吸附能分别为206.5和84.8 kJ/mol, 因此可知Pd原子更容易吸附CO分子; 而当Pd原子附着于有氧缺陷的MgO表面时, 它对两种分子的吸附都非常弱. 相反, 附着于MgO表面的Cu原子对O₂分子的吸附更为有利, 其吸附能在140～155 kJ/mol之间. 研究结果还表明, 对于双分子吸附体系, 即CO＋CO, CO＋O₂, O₂＋O₂体系, 双分子之间的结合力可减小完美MgO表面上Pd原子与被吸附分子的相互作用, 使吸附能减少了46～96 kJ/mol. 而对于在MgO表面上的Cu原子, 只有O₂＋O₂ 体系使吸附能减少了大约50～71 kJ/mol.     

Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ～32% (100), ～18% (111), ～34% (311), and ～15% (331)) at P _O2 ≤ 1.3 × 10^?5 Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O₂ adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O₂ adsorption rate is constant at ? ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an O_ads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ? ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (～800 K) O₂ desorption peak is observed for ? ≤ 0.25, which is due to O₂ desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E _des = 230 kJ/mol. A lower temperature (～700 K) O₂ desorption peak is observed for ? ≥ 0.25, which is due to O₂ released by the c(2 × 2) structure according to a first-order rate law with E _des = 150 kJ/mol. At ? ≥ 0.25, there are repulsive interactions between O_ads atoms on Pd(poly) (ε_aa = 5–10 kJ/mol).     

Density-functional theory calculation on the nitrogen oxide reduction reaction with carbon monoxide on a titanium dioxide nanocluster

The density functional theory was used for simulation of the NO reduction reaction with carbon monoxide on a reduced Ti₈O₁₅ nanocluster. The reaction proceeds on oxygen vacancies formed via the removal of terminal or bridging O atoms. In the case of adsorption of two NO molecules of such sites, a stable adsorption complex with the bidentate ligand ‥ONNO is produced. When a CO molecule approaches one of the O atoms of this cycle, the following exothermic reaction yielding N₂ and CO₂ adsorbed on the Ti₈O₁₆ cluster takes place: 2NO+ CO + Ti₈O₁₅ → N₂+ Ti₈O₁₆ · CO₂. The proposed model of the reaction agrees well with experimental data.     

An ONIOM and DFT study of water and ammonia adsorption on anatase TiO2 (001) cluster

Density functional theory (DFT) calculations at ONIOM DFT B3LYP/ 6‐31G**‐MD/UFF level are employed to study molecular and dissociative water and ammonia adsorption on anatase TiO₂ (001) surface represented by partially relaxed Ti₂₀O₃₅ ONIOM cluster. DFT calculations indicate that water molecule is dissociated on anatase TiO₂ (001) surface by a nonactivated process with an exothermic relative energy difference of 58.12 kcal/mol. Dissociation of ammonia molecule on the same surface is energetically more favorable than molecular adsorption of ammonia (?37.17 kcal/mol vs. ?23.28 kcal/mol). The vibration frequency values also are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO₂ (001) surface. The computed adsorption energy and vibration frequency values are comparable with the values reported in the literature. Finally, several thermodynamical properties (ΔH°, ΔS°, and ΔG°) are calculated for temperatures corresponding to the experimental studies. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Density Functional Theory Studies on the Adsorption of NH2NO2 on Al13 Cluster

Density functional theory method with full geometry optimization was used to study the adsorption of nitroamine (NH₂NO₂) on Al₁₃ cluster. Both dissociative and nondissociative adsorption structures were predicted with different NH₂NO₂ molecule orientations on Al₁₃ cluster surfaces. In dissociative chemisorption, the main decomposition products of NH₂NO₂ are O atom(s) and NH₂NO or NH₂N species. The O atoms being ruptured from the N?CO bond form strong Al?CO bonds with the neighboring Al around the adsorbed sites. In addition, the species obtained as a result of O atom elimination remains bonded to the surface. The largest adsorption energy is ?737.66?kJ/mol when the NH₂NO₂ molecule decomposes into two O atoms and a NH₂N fragment. For nondissociative adsorption, the seriously deformed nitroamine forms various N?CO?CAl bonding configurations with Al. The significant charge transfer occurs for all adsorption configurations. The most charge transfer is 2.068 e from the Al cluster surface to the fragments of the decomposed NH₂NO₂. The change of the electronic structures is obvious due to the adsorption or dissociation of NH₂NO₂ molecule. Nitroamine readily oxidizes the aluminum surface of the Al₁₃ cluster.     

Studies of the roles of Sn or Fe on γ-Al2O3-supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al₂O₃-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al₂O₃ catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N₂O₃ catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al₂O₃ catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N₂O₃. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al₂O₃ catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C₄ alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes. Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training Program Foundation for the Talents by The State Education Commission of China.     

First-principles study of O2 adsorption and dissociation on the CuCr2O4 (100) surface

The adsorption and dissociation of molecular oxygen on spinel CuCr₂O₄ (100) surface were carried out by first-principles calculations based on density functional theory (DFT). The calculated results indicate that the Cr site is most favorable for atomic oxygen adsorption, with an adsorption energy of 402.8 kJ/mol. For molecular oxygen adsorption, there are three types of favorable interaction modes: O₂ forms bonds with the Cu site or O₂ binds to two Cr sites or O₂ interacts with both Cu and Cr sites simultaneously. The lowest activation energy (E_a = 35.4 kJ/mol) was found through exploring possible reaction pathways for O₂ dissociation. The relationship between E_a and reaction enthalpy (ΔH) for O₂ dissociation adsorption reactions fits Brønsted-Evans-Polanyi (BEP) behavior.     

Pd/Cu(111)双金属表面催化糠醛脱碳及加氢的反应机理

采用密度泛函理论(DFT)研究糠醛在最稳定Pd/Cu(111)双金属表面上的吸附构型和糠醛脱碳及加氢的反应机理。结果表明,当糠醛初始吸附于O_3-Pd-top、O_7-Cu-hcp位时,吸附构型最稳定,其吸附能为73.4 kJ/mol。糠醛在Pd/Cu(111)双金属表面上更易发生脱碳反应。对于糠醛脱碳反应,所需活化能较低,各个基元反应均为放热反应,糠醛更易先失去支链上的H形成(C_4H_3O)CO,然后中间体脱碳加氢得到呋喃,其中,C_4H_3O加氢生成呋喃所需活化能(72.6 kJ/mol)最高,是反应的控速步骤。对于加氢反应,糠醛与首个氢原子的反应需要最大的活化能(290.4 kJ/mol),是反应的限速步骤。     

DFT quantum-chemical calculations of nitrogen oxide chemisorption and reactivity on the Cu(100) surface

A DFT quantum-chemical study of NO adsorption and reactivity on the Cu₂₀ and Cu₁₆ metal clusters showed that only the molecular form of NO is stabilized on the copper surface. The heat of monomolecular adsorption was calculated to be ΔH _m = ?49.9 kJ/mol, while dissociative adsorption of NO is energetically unfavorable, ΔH _d = + 15.7 kJ/mol, and dissociation demands a very high activation energy, E _a = + 125.4 kJ/mol. Because of the absence of NO dissociation on the copper surface, the formation mechanism of the reduction products, N₂ and N₂O, is debatable since the surface reaction ultimately leads to N-O bond cleavage. As the reaction occurs with a very low activation energy, E _a = 7.3 kJ/mol, interpretation of the NO direct reduction mechanism is both an important and intriguing problem because the binding energy in the NO molecule is high (630 kJ/mol) and the experimental studies revealed only physically adsorbed forms on the copper surface. It was found that the formation mechanism of the N₂ and N₂O reduction products involves formation (on the copper surface) of the (O_adN-NO_ad) dimer intermediate that is chemisorbed via the oxygen atoms and characterized by a stable N-N bond (r _N-N ～1.3 Å). The N-N binding between the adsorbed NO molecules occurs through electron-accepting interaction between the oxygen atoms in NO and the metal atoms on the “defective” copper surface. The electronic structure of the (O_adN-NO_ad) surface dimer is characterized by excess electron density (ON-NO)^δ? and high reactivity in N-O_ad bond dissociation. The calculated activation energy of the destruction of the chemisorbed intermediate (O_adN-NO_ad) is very low (E _a = 5–10 kJ/mol), which shows that it is kinetically unstable against the instantaneous release of the N₂ and N₂O reduction products into the gas phase and cannot be identified by modern experimental methods of metal surface studies. At the same time, on the MgO surface and in the individual (Ph₃P)₂Pt(O₂N₂) complex, a stable (O_adN-NO_ad) dimer was revealed experimentally.     

O2, CO2, and H2O Chemisorption on UN(001) Surface: Density Functional Theory Study

We performed density functional theory calculations of O₂, CO₂, and H₂O chemisorption on the UN(001) surface using the generalized gradient approximation and PW91 exchange-correlation functional at non-spin polarized level with the periodic slab model. Chemisorp-tion energies vs. molecular distance from UN(001) surface were optimized for four sym-metrical chemisorption sites. The results showed that the bridge parallel, hollow parallel and bridge hydrogen-up adsorption sites were the most stable site for O₂, CO₂, and H₂O molecular with chemisorption energies of 14.48, 4.492, and 5.85 kJ/mol, respectively. From the point of adsorbent (the UN(001) surface), interaction of O₂ with the UN(001) surface was of the maximum magnitude, then CO₂ and H₂O, indicating that these interactions were associated with structures of the adsorbate. O2 chemisorption caused N atoms on the surface to migrate into the bulk, however CO₂ and H₂O had a moderate and negligible effect on the surface, respectively. Calculated electronic density of states demonstrated the electronic charge transfer between s, p orbital in chemisorption molecular and U6d, U5f orbital.     

Gas chemical adsorption characterization of lanthanide hexafluoroacetylacetonates

Newly-established adsorption enthalpy and entropy values of 12 lanthanide hexafluoroacetylacetonates, denoted Ln[hfac]₄, along with the experimental and theoretical methodology used to obtain these values, are presented for the first time. The results of this work can be used in conjunction with theoretical modeling techniques to optimize a large-scale gas-phase separation experiment using isothermal chromatography. The results to date indicate average adsorption enthalpy and entropy values of the 12 Ln[hfac]₄ complexes ranging from ?33 to ?139 kJ/mol K and ?299 to ?557 J/mol, respectively.     

Pt原子在γ-Al2O3(001)表面的吸附及迁移

采用密度泛函理论(DFT)中广义梯度近似(GGA)方法, 对Pt原子与γ-Al₂O₃(001)面的相互作用及迁移性能进行了研究. 分析了各种可能吸附位及吸附构型的松弛和变形现象, 吸附能和迁移能垒的计算结果表明: Pt团簇能够稳定吸附在该表面. Pt原子在表面O位的吸附能明显较高, 这主要是由Pt向基底O原子转移了电子所致. 电荷布居分析表明, Pt原子显电正性, Pt和Al原子之间存在排斥作用, 导致与Al原子产生较弱相互作用. 计算的平均吸附能大小依赖于Pt团簇的大小和形状, 总体趋势是随着Pt原子数增多, 吸附能降低. Pt原子在γ-Al₂O₃(001)表面迁移过程所需克服的迁移能垒最高值为0.51 eV. 随着吸附的Pt原子数增多,更倾向于形成Pt团簇. 因此, Pt原子在γ-Al₂O₃(001)表面的吸附演变不可能形成光滑、均匀平铺的吸附构型, 而在一定条件下容易出现团聚.     

Interaction of zinc oxide clusters with molecules related to the sulfur vulcanization of polyolefins ("rubber")

The vulcanization of rubber by sulfur is a large‐scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn₄O₄ (T_d) as a model species to study the thermodynamics of the initial interaction of various vulcanization‐related molecules with ZnO by DFT methods, mostly at the B3LYP/6‐31+G* level. The interaction energy of Lewis bases with Zn₄O₄ increases in the following order: CO62H₄3H₆2S₂<1,4‐C₅H₈2O2S3N?CH₃COO^?. The corresponding binding energies range from ?57 to ?262 kJ mol^?1. However, Brønsted acids react with the Zn₄O₄ cluster with proton transfer from the ligand molecule to one of the oxygen atoms of Zn₄O₄, and these reactions are all strongly exothermic [binding energies [kJ mol^?1] in parentheses: H₂O (?183), MeOH (?171), H₂S (?245), MeSH (?230), C₃H₆ (?121), and CH₃COOH (?255)]. The important vulcanization accelerator mercaptobenzothiazole (C₇H₅NS₂, MBT) containing several donor sites reacts with the Zn₄O₄ cluster with proton transfer from the NH group to one of the oxygen atoms of ZnO, and in addition the exocyclic thiono sulfur atom and the nitrogen atom coordinate to one and the same zinc atom, resulting in a binding energy of ?247 kJ mol^?1. A second isomer of [(MBT)Zn₄O₄] with a strong O? H???N hydrogen bond rather than a Zn? N bond is only slightly less stable (binding energy ?243 kJ mol^?1). The NH form of free MBT is 36 kJ mol^?1 more stable than the tautomeric SH form, while the sulfurized MBT derivative benzothiazolyl hydrodisulfide C₇H₅NS₃ (BtSSH) is most stable with the connectivity >CSSH.     

Semiempirical computational study of oxygen vacancies in a decahedral anatase nanoparticle

Formation of oxygen vacancies (V_O) is an important step of many catalytic reactions following the Mars van Krevelen mechanism. High rate of oxidation is associated with low energy of V_O formation while high selectivity requires an optimal energy of V_O formation. In the present computational study, enthalpy of V_O formation (ΔH_OVF) is studied in a decahedral anatase nanoparticle (TiO₂)₁₂₁(H₂O)₆ using PM6 method. ΔH_OVF shows large variations for oxygen atoms in different locations on facets, edges and vertices. V_O are much more stable in the (101) facet compared to the (001) facet, while internal V_O are more stable for (101) but equally stable for (001) facet compared to surface vacancies on average. Comparison with literature DFT methods results reveals good consistency and high computational efficiency of the PM6 method for vacancies formation energy. Pm6 also correctly predicts admixture states of the Ti³⁺ within the band gap, but absolute values of electronic band gap and position of admixture states is overestimated and needs scaling factors.     

Formation enthalpies of rare earth titanate pyrochlores

High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure (Fd3m; Z=8) and energetics of a series of RE₂Ti₂O₇ (RE=Sm-Lu) compounds with the pyrochlore structure as well as La₂Ti₂O₇ with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu₂Ti₂O₇ was least stable in enthalpy (ΔHf-ox at 298 K=−56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm₂Ti₂O₇ with ΔHf-ox at 298 K=−113.4±2.7, −106.1±4.2, −115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, R_A/R_B, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE³⁺ cation vs. ΔHf-ox at 298 K is non-linear and approximately parallels the increasing “resistance” to ion-beam-induced amorphization as R_A/R_B decreases.     

The hydrolysis of phosphoranes in aqueous dioxane

The entropy of activation for the hydrolysis of pentaphenoxyphosphorane in 25% aqueous dioxane is ?188 J/mol deg. The enthalpy of activation is 22.5 kJ/mol, which is small for the relatively slow reaction. This suggests that the reaction is a multistep process having a preliminary equilibrium with a negative heat of reaction. This conclusion is supported by the results obtained for the hydrolysis of C₆H₅ [OCH(CF₃)₂]₂. A kinetic isotope effect k_H2O/k_D2O of 3.46 was found for the latter reaction. Orders in water were obtained, and a mechanism of hydrolysis is proposed.     

Proton affinity of SO3

Collision-induced dissociation (CID) of the radical cation H₂SO₄⁺ gives the product pairs H₂O⁺+SO₃ and HO+HSO₃⁺ with a 1:3 ratio that is essentially independent of collision energy. Statistical analysis of the two channels indicates that the proton affinity of HO is 3±4 kJ/mol lower than that of SO₃. This can be used to derive PA(SO₃)=591±4 kJ/mol at 0 K and 596±4 kJ/mol at 298 K. Previously, Munson and Smith bracketed the proton affinity as PA(HBr)=584 kJ/mol<PA(SO₃)<PA(CO)=594 kJ/mol. The threshold of 152±16 kJ/mol for formation of H₂O⁺+SO₃ indicates that the barrier to CID is small or nonexistent, in contrast to the substantial barriers to decomposition for H₃SO₄⁺ and H₂SO₄.     

Chemistry of O‐ and H‐Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO₂ (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H₂O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO₂ surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O₂ adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O₂. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.     

Thermodynamic stability and hydration enthalpy of strontium cerate doped with yttrium

The standard molar enthalpy of formation of SrY_0.05Ce_0.95O_2.975 has been derived by combining the enthalpy of solution in 1 M HCl + 0.1 KI with auxiliary literature data, Δ_fH° (SrY_0.05Ce_0.95O_2.975, s, 298.15 K) = −1720.4 ± 3.4 kJ/mol. The formation enthalpy of SrY_0.05Ce_0.95O_2.975 from the mixture of binary oxides is Δ_oxH° (298.15 K) = −45.9 ± 3.4 kJ/mol and the enthalpy of reaction of SrY_0.05Ce_0.95O_2.975 with water forming Sr(OH)₂, CeO₂, and Y₂O₃ is Δ_rH° (298.15 K) = −85.5 ± 3.4 kJ/mol. Our data and the entropies of different substances show that SrY_0.05Ce_0.95O_2.975 is thermodynamically stable with respect to a mixture of SrO, Y₂O₃, CeO₂ and that the reaction of SrY_0.05Ce_0.95O_2.975 with water is thermodynamically favourable.