苯乙烯在Ag(111)和Ag(110)表面环氧化反应结构敏感性的理论研究

采用密度泛函理论(DFT)对苯乙烯在Ag(110)表面和Ag(111)表面的环氧化反应进行了计算研究. 经计算, 在Ag(110)表面预吸附氧原子更易吸附在3 重穴位(3h), 吸附能为-3.59 eV; 在Ag(111)表面预吸附氧原子的最稳定吸附位是fcc 位, 吸附能为-3.69 eV. 苯乙烯的环氧化反应过程首先经过一个金属中间体, 然后再进一步反应变为产物, 其中经过直链中间体较支链中间体更加有利. Ag(110)面的反应活化能一般大于Ag(111)面的, 并且微观动力学模拟结果表明, Ag(111)表面生成环氧苯乙烷的选择性要明显高于Ag(110)表面(0.38 与 0.003), 原因是Ag(111)面环氧化反应活化能小于苯乙醛及燃烧中间体的活化能, 而在Ag(110)上正相反.     

Engineering selectivity in heterogeneous catalysis: Ag nanowires as selective ethylene epoxidation catalysts

Controlling selectivity in heterogeneous catalysis is critical for the design of environmentally friendly catalytic processes that minimize the production of undesired byproducts and operate with high energy efficiency. We show that the Ag nanowire catalysts exhibit higher selectivity in the ethylene epoxidation reaction than conventional spherical particle catalysts. The higher selectivity of the nanowire catalysts was attributed to a higher concentration of the Ag(100) surface facets in the nanowire catalysts compared to the particle catalysts. Density functional theory calculations show that the transformation of the surface oxametallacycle intermediate to form the selective product, EO, is more favorable on the Ag(100) than on Ag(111). The studies show that recent advances in the controlled synthesis of uniform nanostructures with well-defined surface facets might provide an important platform for the design of highly selective heterogeneous catalysts.     

Synthesis, structure, and reactions of stable oxametallacycles from styrene oxide on Ag(111)

Styrene oxide undergoes an activated ring opening on Ag(111) at temperatures above 200 K. The product of this reaction is a stable oxametallacycle intermediate. The structure of this species has been obtained by density functional theory calculations and the computed vibrational spectrum is consistent with the experimental spectrum obtained using high-resolution electron energy loss spectroscopy. The oxametallacycle formed by ring-opening styrene oxide is structurally analogous to that previously observed for ring opening of epoxybutene on Ag(110) and represents the largest member of this adsorbate structure class yet isolated. In both cases, the epoxide ring opens at the carbon bearing the pendant unsaturated group, and the pendant group (phenyl in styrene oxide) is oriented nearly parallel to the surface plane. The oxametallacycle formed from styrene oxide reacts at 485 K to regenerate styrene oxide plus small amounts of phenylacetaldehyde. This peak temperature is similar to that previously reported for generation of styrene oxide from adsorbed styrene and oxygen atoms on Ag(111), suggesting that the epoxidation proceeds via the oxametallacycle intermediate isolated in the present work.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

Adsorption and reaction of 1-epoxy-3-butene on Pt(111): implications for heterogeneous catalysis of unsaturated oxygenates

High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations were used to study the adsorption and reaction of 1-epoxy-3-butene (EpB) on Pt(111). These investigations were conducted to help elucidate mechanisms for improving olefin hydrogenation selectivity in reactions of unsaturated oxygenates. EpB dosed to Pt(111) at 91 K adsorbs molecularly on the surface through the vinyl group with apparent rehybridization to a di-sigma-bound state. By 233 K, however, EpB undergoes epoxide ring opening to form an aldehyde intermediate, which further decomposes upon heating to yield gas phase products CO, H2, and propylene. Comparison of the HREELS and TPD data to experiments performed with 2-butenal (crotonaldehyde) shows that EpB and 2-butenal decompose through related pathways. However, the EpB-derived aldehyde intermediate clearly has a unique structure, features of which have been elucidated by DFT calculations. In conjunction with previous surface science studies of EpB chemistry, these results can help explain selectivity trends for reactions of EpB on Pt catalysts and bimetallic PtAg catalysts, with indications that the enhanced olefin hydrogenation selectivity of PtAg catalysts likely originates from a bifunctional effect.     

在预吸附氧原子的Ag(100)面上氯乙烯环氧化反应的密度泛函理论研究

采用密度泛函理论和周期性平板模型对氯乙烯在预吸附氧原子的Ag(100)面上选择性环氧化反应进行了模拟计算.结果表明,该反应首先由反应物生成中间体,再由中间体生成产物.由于氯乙烯是一种不对称的分子,所以该反应存在两种可能的途径.比较反应活化能可以发现,在一个反应通道中环氧氯乙烷要比氯乙醛的生成更容易;而在另一反应通道中,...     

Ethene epoxidation selectivity inhibited by twisted oxametallacycle: a DFT study on Ag surface-oxide

Competitive ethene oxidation pathways are presented for a p(4 x 4) surface-oxide phase on Ag(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceed from a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies on low coverage O adatom phase, although acetaldehyde (AcH) is favored by almost 2 kcal/mol. An even more striking difference with pure metal surface appears with the oxide regeneration pathways, which are found non-rate controlling. Furthermore, a kinetic model is developed on the basis of these DFT calculations and yields 96% selectivity in favor of AcH for a simulation in realistic catalytic conditions (600 K and respective partial pressures of 1 atm for ethene and oxygen reactants). As a key finding, this low ethene epoxide selectivity is proposed to be directly linked to the conformational barrier of the pivotal intermediate. In fact, the elasticity of the ultrathin oxide adlayer enables a twisted OMME structure as a true minimum, which agrees well with orbital prerequisite of the concerted H migration toward AcH. On the contrary, the desired selective ring closure forming ethene epoxide (EO) requires conformational inversion although the eclipsed form lies 2 kcal/mol above.     

Identifying Multinuclear Organometallic Intermediates in On‐Surface [2+2] Cycloaddition Reactions

We investigate the on‐surface [2+2] cycloaddition reaction of 2,3,6,7,10,11‐hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu‐dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo‐silver framework on Ag(111) and an organo‐copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo‐silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo‐copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo‐gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo‐copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.     

Interactions of oxygen and ethylene with submonolayer Ag films supported on Ni(111)

We investigate the oxidation of, and the reaction of ethylene with, Ni(111) with and without sub-monolayer Ag adlayers as a function of temperature. The addition of Ag to Ni(111) is shown to enhance the activity towards the ethylene epoxidation reaction, and increase the temperature at which ethylene oxide is stable on the surface. We present a systematic study of the formation of chemisorbed oxygen on the Ag-Ni(111) surfaces and correlate the presence and absence of O(1-) and O(2-) surface species with the reactivity towards ethylene. By characterizing the samples with low-energy electron microscopy (LEEM) in combination with X-ray photoelectron spectroscopy (XPS), we have identified specific growth of silver on step-edge sites and successfully increased the temperature at which the produced ethylene oxide remains stable, a trait which is desirable for catalysis.     

New insights into ethene epoxidation on two oxidized Ag[111] surfaces

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime.     

Mechanistic and spectroscopic identification of initial reaction intermediates for prenal decomposition on a platinum model catalyst

The prediction of a reaction mechanism and the identification of the corresponding chemical intermediates is a major challenge in surface science and heterogeneous catalysis, due to a complex network of elementary steps and surface species. Here we demonstrate how to overcome this difficulty by tracking the temperature dependent formation of the initial reaction intermediates and identifying the decomposition pathways in the case of prenal, an α,β-unsaturated aldehyde, on the Pt(111) model catalyst surface by combining vibrational spectroscopy, thermal reaction/desorption spectroscopy (TPRS) experiments and detailed theoretical analysis. TPRS characterization of this reaction up to 600 K shows a series of desorption states of H(2) (～280 K, 410 K and 473 K) and CO (～414 K), giving valuable insights into the sequence of elementary steps suggesting that the loss of hydrogen and the carbonyl functions are among the first elementary steps. HREELS experiments recorded after annealing to specific temperatures result in complex spectra, which can be assigned to several subsequently formed and transformed surface intermediates. Starting from stable prenal adsorption structures, complementary DFT calculations allow the determination of the most likely reaction pathway for the initial decomposition steps and the identification of the corresponding intermediates by comparison with HREELS. The decomposition occurs from the strongly bonded prenal adsorption structures via a dehydro-η(3)-triσ(CCC)-H1 intermediate to the highly stable η(1)-isobutylidyne species at high temperatures.     

Theoretical investigation on reaction pathways for ethylene epoxidation on Ti-decorated graphene

The activities of atomic Ti-decorated graphene (Ti/dG) for ethylene epoxidation and competitive paths for acetaldehyde (AA) formation are investigated by means of density functional theory together with the D3 dispersion correction (UM06-L-D3). Two reaction mechanisms for ethylene epoxidation, namely concerted and stepwise mechanisms, were considered. The computational results reveal that the electron transfer from graphene can effectively enhance the catalytic activity of Ti atom. Without graphene support, atomic Ti becomes an inert metal for this reaction. Strong adsorption and significant activation of the reactant O₂ molecule were observed on the Ti-decorated graphene material. Over the O₂-adsorbed Ti/dG, the direct attack of the olefin on an peroxo oxygen center is preferred. The activation for this step is 10.9 kcal mol^?1. After the reaction, an ethylene oxide is formed with one atomic oxygen on top of Ti. Consequently, a gaseous ethylene reacts with the remaining O atom of TiO moiety for the formation of the second ethylene oxide molecule. The formation of ethylene oxide over the TiO/dG involves a two-step process which is the formations of oxametallacycle intermediate and EO, respectively. The calculated barriers for these two steps are 9.9 and 18.9 kcal mol^?1, respectively. Furthermore, the Ti/dG showed a lower activation barrier toward EO formation than that of AA. Therefore, our theoretical study suggests that atomic Ti-decorated graphene could possess catalytic activity for ethylene epoxidation comparable to that of potential catalysts.     

Selective oxidation of styrene on an oxygen-adsorbed Au(111): A density functional theory study

The reaction mechanism for the styrene selective oxidation on the oxygen preadsorbed Au(111) surface has been studied by the density functional theory calculation with the periodic slab model. The calculated results showed that the process of reaction includes two steps: forming the oxametallacycle intermediate (OMME) and then producing the products. It was found that the second step, from OMME to product is the rate-controlling step, which is similar to ethylene selective oxidation on Ag. Importantly, the present density-functional-theory calculation results suggested that the mechanism via the OMME (2) (i.e. the preadsorbed atomic oxygen bound to the CH2 group involved in C6H5--CH=CH2) to produce styrene epoxide is kinetically favored than that of OMME (1).     

碘乙醇在Ni(100)表面的吸附和热分解-碳氢化合物氧化的中间产物：羟乙基和氧金属环

利用热脱附(TPD)实验和X射线光电子能谱(XPS)研究了碘乙醇在Ni(100)表面的吸附和热反应过程. 实验结果表明碘乙醇在100 K时以两种分子的形式吸附在Ni(100)的表面, 即: 以碘原子端吸附在表面或以碘原子端和羟基端同时吸附在表面. 由于两种吸附形式的分子的一致分解和吸附分子的不均匀性, 在140 K引起了较复杂的化学反应, 伴有少量的乙烯和水产生. 碘乙醇在150 K经过C—I键断裂, 有80%碘乙醇生成—O(H)CH2CH2—中间产物, 20%的碘乙醇生成羟乙基中间产物. 羟乙基在160 K的转化过程中包括两个互相竞争的化学反应: 与表面的氢原子进行还原反应生成乙醇, 或失去一个β-H原子生成表面乙烯醇. 另外, 在相同的温度下—O(H)CH2CH2—中间产物经过脱氢反应产生—OCH2CH2—氧金属环. 羟乙基和氧金属环都会发生异构, 分别在210 K和250 K生成乙醛, 这些乙醛一部分从表面脱出, 其余的部分发生分解反应产生氢气、水和一氧化碳. 在实验基础上, 进一步探讨了这种化学过程在催化中的作用和指导意义.     

Selectivity limitations in the heterogeneous epoxidation of olefins: branching reactions of the oxametallacycle intermediate in the partial oxidation of styrene

The partial oxidation of olefins on Ag surfaces has been a long standing problem in surface science and is of great commercial interest. We present a temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS) study of the partial oxidation of styrene on Ag(111). The reaction products are CO2, water, styrene oxide, benzene, and benzoic acid. XPS gives evidence for two different reaction intermediates that we assign to an oxametallacycle and to benzoate. The oxametallacycle seems to be the precursor for both the formation of styrene oxide and the formation of benzoate, and thus, in addition to leading to styrene oxide, also leads to undesired byproducts. The benzoate reacts further to form CO2, benzene, and benzoic acid.     

Metal catalyzed ethylene epoxidation:A comparative density functional theory study

Ethylene epoxidation on Ag(111), Pt(111), Rh(111) and Mo(100) has been studied by density functional theory (DFT) calculations. The results show that the adsorption energies of possible adsorbed species involved in the ethylene epoxidation increase in the order: Ag相似文献   

DFT study on the selective oxidation of vinyl chloride on different metal surfaces

Selective epoxidation of vinyl chloride on Ag(111), Pt(111) and Rh(111) with pre-adsorbed atomic oxygen has been studied by density functional theory (DFT) calculation with the periodic slab model. The reaction energies and activation energies of the epoxidation reaction are determined. Because of the asymmetry of vinyl chloride, three competitive reaction pathways are investigated. The results indicate that the most possible reaction pathway is pathway III. Compared the activation energies of the epoxidation reaction on Ag(111), Pt(111) and Rh(111), it is obvious that the reaction via OMMC(3) on Ag(111) is the most possible process. However, the selectivity to the target product over Ag(111) is the lowest among the three metals. The results also indicate that the formation of chloroacetaldehyde is more favorable than that of chloroepoxide.     

High-pressure STM of the interaction of oxygen with Ag(111)

To identify surface phases that could play a role for the epoxidation of ethylene on Ag catalysts we have studied the interaction of Ag(111) with O(2) at elevated pressures. Experiments were performed using high-pressure scanning tunneling microscopy (STM) at temperatures between 450 and 480 K and O(2) pressures in the mbar range. Below p(O(2)) approximately 1 mbar the surface largely showed the structure of bare Ag(111). At p(O(2)) above approximately 1 mbar the (4 x 4)O structure and the closely related (4 x 5 radical 3)rect structure were observed. The findings confirm theoretical predictions that the (4 x 4)O structure is thermodynamically stable at the oxygen partial pressure of the industrial ethylene oxide synthesis. However, in other experiments only a rough, disordered structure was observed. The difference is caused by the chemical state of the STM cell that depends on the pretreatment and on previous experiments. The surface was further analyzed by X-ray photoelectron spectroscopy (XPS). Although these measurements were performed after sample transfer to ultra-high vacuum (UHV), so that the surface composition was modified, the two surface states could still be identified by the presence of carbonate or a carbonaceous species, and by the absence or presence of a high-binding energy oxygen species, respectively. It turns out that the (4 x 4)O structure only forms under extremely clean conditions, indicating that the (4 x 4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable. Chemical reactions at the inner surfaces of the STM cell also complicate the detection of the catalytic formation of ethylene oxide.     

HREELS, STM, and STS study of CH(3)-terminated Si(111)-(1x1) surface

An ideally (1x1)-CH(3)(methyl)-terminated Si(111) surface was composed by Grignard reaction of photochlorinated Si(111) and the surface structure was for the first time confirmed by Auger electron spectroscopy, low energy electron diffraction, high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). HREELS revealed the vibration modes associated to the CH(3)-group as well as the C-Si bond. STM discerned an adlattice with (1x1) periodicity on Si(111) composed of protrusions with internal features, covering all surface terraces. The surface structure was confirmed to be stable at temperatures below 600 K. STS showed that an occupied-state band exists at gap voltage of -1.57 eV, generated by the surface CH(3) adlattice. This CH(3):Si(111)-(1x1) adlayer with high stability and unique electronic property is prospective for applications such as nanoscale lithography and advanced electrochemistry.     

微量1,2—二氯乙烷在乙烯环氧化中的作用及其吸附态的表征

在乙烯环氧化的原料气中添加微量的1,2-二氯乙烷能较大地提高乙烯环氧化的选择性。1,2-二氯乙烷在反应条件下能分解出氯,并以强、弱两种吸附态存在于催化剂的表面上。它们一方面抑制催化剂表面起深度氧化作用的氧原子的吸附强度和浓度,一方面又排斥反应生成的环氧乙烷,使它更快地脱附,从而减少环氧乙烷异构化的机会,提高乙烯环氧化的选择性。同时吸附在催化剂表面上的氯又与乙烯反应生成1,2-二氯乙烷。这样有机氯化物在催化剂表面上的分解消耗和反应生成达成平衡,以维持催化反应的持续进行。