In situ UV-vis and EPR study on the formation of hydroperoxide species during direct gas phase propylene epoxidation over Au/Ti-SiO(2) catalyst

In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O(2) and H(2) over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O(2) and H(2). In order to obtain direct evidence for the different type of active oxygen species, in situ UV-vis and EPR measurements were carried out during the epoxidation of propylene with O(2) and H(2) over a Au/Ti-SiO(2) (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O(2)(-)) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H(2)O(2) produced from O(2) and H(2) adsorbed on the Au surfaces.     

Distinct reaction pathways followed upon reduction of oxy-heme oxygenase and oxy-myoglobin as characterized by Mössbauer spectroscopy

Activation of O(2) by heme-containing monooxygenases generally commences with the common initial steps of reduction to the ferrous heme and binding of O(2) followed by a one-electron reduction of the O(2)-bound heme. Subsequent steps that generate reactive oxygen intermediates diverge and reflect the effects of protein control on the reaction pathway. In this study, M?ssbauer and EPR spectroscopies were used to characterize the electronic states and reaction pathways of reactive oxygen intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of oxy-heme oxygenase (HO) and oxy-myoglobin (Mb). The results confirm that one-electron reduction of (Fe(II)-O(2))HO is accompanied by protonation of the bound O(2) to generate a low-spin (Fe(III)-O(2)H(-))HO that undergoes self-hydroxylation to form the alpha-meso-hydroxyhemin-HO product. In contrast, one-electron reduction of (Fe(II)-O(2))Mb yields a low-spin (Fe(III)-O(2)(2-))Mb. Protonation of this intermediate generates (Fe(III)-O(2)H(-))Mb, which then decays to a ferryl complex, (Fe(IV)=O(2-))Mb, that exhibits magnetic properties characteristic of the compound II species generated in the reactions of peroxide with heme peroxidases and with Mb. Generation of reactive high-valent states with ferryl species via hydroperoxo intermediates is believed to be the key oxygen-activation steps involved in the catalytic cycles of P450-type monooxygenases. The M?ssbauer data presented here provide direct spectroscopic evidence supporting the idea that ferric-hydroperoxo hemes are indeed the precursors of the reactive ferryl intermediates. The fact that a ferryl intermediate does not accumulate in HO underscores the determining role played by protein structure in controlling the reactivity of reaction intermediates.     

In situ FTIR studies of primary intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions

Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO(2) films in contact with aqueous solutions. UV irradiation in the presence of dissolved O(2) caused the appearance of new bands peaked at 943, 838, and 1250-1120 cm(-)(1) together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H(2)O --> D(2)O exchange on the spectral changes have revealed that the primary step of photocatalytic O(2) reduction is the formation of the surface peroxo species, Ti(O(2)), giving the 943 cm(-)(1) band, probably with the surface superoxo species, TiOO., as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250-1120 cm(-)(1) bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of primary intermediates for the photocatalytic O(2) reduction in aqueous solutions. On the basis of the assignment, a possible reaction scheme for various processes of the photocatalytic O(2) reduction is proposed, which is in harmony with other spectral changes induced by the UV irradiation.     

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.     

An EPR study of the C-O bond activation of 1,2-epoxybutane by ground-state Al atoms

Group 13 metal atoms react with ethers under matrix isolation conditions to give a number of interesting products. This work has been extended to include the reaction of Al atoms with 1,2-epoxybutane (CH(3)CH(2)H(2)) and its isotopomers, 1,2-epoxybutane-1,1-d(2) (CH(3)CH(2)D(2)) and 1,2-epoxybutane-2-d(1) (CH(3)CH(2)H(2)). The paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy. Two divalent Al insertion products were spontaneously formed. Species A, with the magnetic parameters a(Al) = 855 MHz, a(H)(1) = 28.8 MHz, a(H)(2) = 13.6 MHz, and g = 2.0014, is the C(1)-O insertion radical CH(3)CH(2). Species B, thought to result from the insertion of Al atoms into the C(2)-O bond, CH(3)CH(2), has the magnetic parameters g = 2.0003, a(Al) = 739 MHz, a(H)(1) = 15.1 MHz, a(H)(2) = 18.5 MHz, and a(H)(1) = 37.8 MHz. Support for these assignments was obtained by comparing the experimental values of the Al and H hyperfine interaction (hfi) with those calculated using a DFT method. At temperatures < 150 K, there is evidence for the formation of the alkyl radical CH(3)CH(2)CH(O(-))CH(2)* due to ring opening at the C(1)-O bond, while at higher temperatures a radical with magnetic parameters similar to those reported for 1-methallyl was detected.     

Direct dynamics simulation of dioxetane formation and decomposition via the singlet [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical: Non-RRKM dynamics

Electronic structure calculations and direct chemical dynamics simulations are used to study the formation and decomposition of dioxetane on its ground state singlet potential energy surface. The stationary points for (1)O(2) + C(2)H(4), the singlet [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical, the transition state (TS) connecting this biradical with dioxetane, and the two transition states and gauche [middle dot]O-CH(2)-CH(2)-O[middle dot] biradical connecting dioxetane with the formaldehyde product molecules are investigated at different levels of electronic structure theory including UB3LYP, UMP2, MRMP2, and CASSCF and a range of basis sets. The UB3LYP∕6-31G? method was found to give representative energies for the reactive system and was used as a model for the simulations. UB3LYP∕6-31G? direct dynamics trajectories were initiated at the TS connecting the [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical and dioxetane by sampling the TS's vibrational energy levels, and rotational and reaction coordinate energies, with Boltzmann distributions at 300, 1000, and 1500 K. This corresponds to the transition state theory model for trajectories that pass the TS. The trajectories were directed randomly towards both the biradical and dioxetane. A small fraction of the trajectories directed towards the biradical recrossed the TS and formed dioxetane. The remainder formed (1)O(2) + C(2)H(4) and of these ～ 40% went directly from the TS to (1)O(2) + C(2)H(4) without getting trapped and forming an intermediate in the [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical potential energy minimum, a non-statistical result. The dioxetane molecules which are formed dissociate to two formaldehyde molecules with a rate constant two orders of magnitude smaller than that predicted by Rice-Ramsperger-Kassel-Marcus theory. The reaction dynamics from dioxetane to the formaldehyde molecules do not follow the intrinsic reaction coordinate or involve trapping in the gauche [middle dot]O-CH(2)-CH(2)-O[middle dot] biradical potential energy minimum. Important non-statistical dynamics are exhibited for this reactive system.     

Selective catalytic reduction of NO by propane over CoO(x)/Al2O3: an investigation of the surface reactions using in situ infrared spectroscopy

The partial oxidation of propane and the mechanism of the selective catalytic reduction (SCR) of NO by C3H8 over CoO(x)/Al2O3 catalysts were investigated using in situ infrared spectroscopy. Emphases are placed on the formation and reactivity of surface oxygenates during the SCR reaction. The SCR reaction starts with partial oxidation of propane to adsorbed acetate and formate. Impregnation of cobalt onto alumina greatly enhanced this reaction. The as-formed acetate acts as an efficient reductant for NO reduction. Surface nitrates (nitrites) are also reactive to propane and to oxygenates generated from C3H8 + O2 reaction. Surface -NCO species are formed over CoO(x)/Al2O3 catalysts. These nitrogen containing organic species are believed to be the direct intermediates in the final formation of N2. On the basis of these investigations, a proposed reaction mechanism explains the formation and roles of all intermediates detected by IR spectroscopy in this study.     

Ir/SiO2上甲烷部分氧化制合成气反应的原位时间分辨红外和原位Raman光谱表征

采用原位时间分辨红外光谱和原位显微Raman光谱技术对Ir/SiO₂上甲烷部分氧化(POM)制合成气反应的初级产物和反应条件下催化剂表面物种进行了跟踪考察,实验结果表明,在H₂预还原的新鲜Ir/SiO₂表面,CO是V(CH₄):V(O₂):V(Ar)=2:1:45混合气反应的初级产物,因而甲烷的直接氧化过程是CO生成的主要途径;而在稳态反应条件下,CO生成的途径可能主要来自CO₂和H₂O与催化剂表面积碳物种(CHx)和/或CH₄的反应.催化剂上生成的积碳可能是导致稳态条件下Ir/SiO₂上POM反应机理不同于H₂预还原的新鲜催化剂的主要原因.     

Reactivity of the Coordinated Hydroperoxo Ligand

The reactivity of the hydroperoxo complex [Co(CN)(5)OOH](3)(-) has been studied in aqueous solution. The complex undergoes acid-catalyzed aquation (k = 1.89(5) x 10(-)(2) s(-)(1), pK(a) = 5.21(4), T = 20 degrees C, I = 0.1 M). Assuming an I(d) mechanism, this allows the relative affinity for Co(III) to be deduced as H(2)O(2) < H(2)O < HO(2)(-) and implies H(2)O(2) to be a very weak ligand. At neutral pH the hydroperoxo complex effects efficient oxygen atom transfer to L-methionine to give an intermediate identified as [Co(CN)(5)(L-methionine S-oxide)](2)(-), which then dissociates to [Co(CN)(5)OH(2)](2)(-) and L-methionine S-oxide. The reaction is acid catalyzed and is proposed to take place via nucleophilic attack of sulfur on the proximal oxygen of the hydroperoxo ligand with concerted loss of water. The significance of these results for the interaction of hydrogen peroxide with labile metal ions is discussed.     

Cu/ZrO2催化剂中的氢和水逆溢流效应及其对甲醇分解反应的影响

应用漫反射红外和质谱在线技术对H₂, H₂O及甲醇在ZrO₂及Cu/ZrO₂上的程序升温脱附(TPD)及程序升温反应(TPSR)行为进行了研究. 结果表明, Cu/ZrO₂催化剂中铜锆组分间表现出显著的氢和水组分“逆溢流”效应. 对Cu/ZrO₂催化体系中ZrO₂表面线式及桥式羟基物种浓度随还原预处理温度变化的进一步分析表明, 由于氢和水“逆溢流效应”的存在, 使得Cu/ZrO₂在较低的还原温度下活化的同时, 在铜锆界面处形成较丰富的氧阴离子和氧空穴活性位, 而后者的形成与存在直接影响并决定了甲醇在Cu/ZrO₂催化剂上的低温催化分解行为.     

Impact of defects on the surface chemistry of ZnO(0001 macro)-O

Scanning tunneling microscopy and core level photoelectron spectroscopy measurements have been used to investigate the morphology of ZnO(0001 macro)-O, and its reactivity with carbon monoxide and carbon dioxide, as a function of surface preparation. Real space images of the surface indicate that increasing the substrate anneal temperature during preparation significantly reduces the surface step density. Surface defect concentration is also monitored by employing formic acid as a chemical probe, which is shown to adsorb dissociatively (HCOOH --> [HCOO](-) + H(+)) only on zinc cations at step edges. Carbon 1s X-ray photoelectron spectra show that carbon monoxide and carbon dioxide both react to form surface carbonate species. Spectra, recorded both as a function of surface preparation and following coadsorption, demonstrate that the carbonate formed from either reactant molecule is located at oxygen vacancies at step edges, evidencing the significant role that defects can play in the surface chemistry of ZnO(0001 macro)-O.     

Computational study of the reaction of P+ with acetylene: does spin-crossing play a significant role?

A computational study of the reaction of P(+)((3)P) with acetylene has been carried out. The only exothermic products correlating with the reactants are PCCH(+)((2)Π) + H((2)S). Two different pathways leading to these products that are apparently barrier-free have been found. Both pathways involve isomerization into open-chain intermediates followed by direct elimination of a hydrogen atom. The possibility of spin-crossing has been considered because the species on the singlet surface are considerably more stable than those on the triplet one. On the singlet surface, there are other possible channels for the reaction, namely, cyclic PC(2)H(+)((2)A') + H((2)S) and CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)). A computational kinetic study shows that, in agreement with the experimental evidence, the major products are PCCH(+)((2)Π) + H((2)S) at all temperatures. Only at very high temperatures is CCP(+)((1)Σ) + H(2) ((1)Σ(g)(+)) formed in non-negligible amounts. Therefore, only PCCH(+) should be formed in the interstellar medium.     

Axial ligand and spin-state influence on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Vinyl acetate formation by the reaction of ethylene with acetate species on oxygen-covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.     

The reaction of [FeII(tpa)] with H2O2 in acetonitrile and acetone--distinct intermediates and yet similar catalysis

The reaction of [FeII(tpa)(OTf)2] (tpa=tris(2-pyridylmethyl)amine) and its related 5-Me3-tpa complex with hydrogen peroxide affords spectroscopically distinct iron(III)-peroxo intermediates in CH3CN and acetone. The reaction in acetonitrile at -40 degrees C results in the formation of the previously reported Fe(III)-OOH intermediate, the end-on hydroperoxo coordination mode of which is established in this paper by detailed resonance Raman isotope-labeling experiments. On the other hand, the reaction in acetone below -40 degrees C leads to the observation of a different peroxo intermediate identified by resonance Raman spectroscopy to be an FeIII-OOC (CH3)2OH species; this represents the first example of an intermediate derived from the adduct of H2O2 and acetone. The peroxoacetone intermediate decays more rapidly than the corresponding FeIII-OOH species and converts to an FeIV=O species by O-O bond homolysis. This decay process is analogous to that observed for [FeIII(tpa)(OOtBu)]2+ and in fact exhibits a comparable enthalpy of activation of 54(3) kJ mol(-1). Thus, with respect to their physical properties at low temperature, the peroxoacetone intermediate resembles [FeIII(tpa)(OOtBu)]2+ more than the corresponding FeIII-OOH species. At room temperature, however, the behavior of the Fe(tpa)/H2O2 combination in acetone in catalytic hydrocarbon oxidations differs significantly from that of the Fe(tpa)/tBuOOH combination and more closely matches that of the Fe(tpa)/H2O2 combination in CH3CN. Like the latter, the Fe(tpa)/H2O2 combination in acetone catalyzes the hydroxylation of cis-1,2-dimethylcyclohexane to its tertiary alcohol with high stereoselectivity and carries out the epoxidation and cis-dihydroxylation of olefins. These results demonstrate the subtle complexity of the Fe(tpa)/H2O2 reaction surface.     

Enhancement of epoxidation efficiencies for Ta-SBA15 catalysts. The influence of modification with -EMe3 (E = Si, Ge, Sn) groups

Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After thermal treatment under oxygen, the resulting Si-OH and Ta-OH sites of TaSBA15-O(2)were modified with a series of trimethyl group 14 species, Me(3)E-, by treatment with Me(3)E-NMe(2) (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me(3)E)(cap)TaSBA15 exhibit a significantly increased rate of cyclohexene epoxidation with H(2)O(2) as an oxidant, and provided a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O(2). The rate of nonproductive H(2)O(2) decomposition, as monitored via (1)H NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H(2)O(2)) were probed using UV-visible absorbance and diffuse-reflectance UV-visible spectroscopy. A Ta(V)(η(2)-O(2)) intermediate species is proposed for the TaSBA15-O(2), (Me(3)Si)(cap)TaSBA15, and (Me(3)Ge)(cap)TaSBA15 catalysts, while intermediate species for the (Me(3)Sn)(cap)TaSBA15 catalysts could not be characterized.     

Cu／ZnO催化剂上逆向水煤气变换反应反应机理的Monte Carlo模拟

采用Ｍｏｎｔｅ Ｃａｒｌｏ方法模拟了ＣＯ２在Ｕ／ＺｎＯ催化剂上的解离反应，发现在ＣＯ２的解离过程中只有ＣＯ生成；若用Ｈ２对生成的表面吸附氧进行还原，所生成的Ｈ２Ｏ量与前期生成的ＣＯ量相同，说明在Ｃｕ／ＺｎＯ上ＣＯ２和Ｈ２之间发生了化学计量的氧化还原反应，即逆向水煤气反应可能以表面氧化还原反应机理进行。     

O(2) Insertion into Group 9 Metal-Hydride Bonds: Evidence for Oxygen Activation through the Hydrogen-Atom-Abstraction Mechanism

A detailed density functional study was performed to examine the reaction of mixed-valence dirhodium and diiridium species [M(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)(Cl)(2) (1, tfepma = MeN[P(OCH(2)CF(3))(2)](2), CN(t)Bu = tert-butyl isocyaninde)] with HCl and oxygen with an interest in examining the pathways for oxygen insertion into the intermediate metal hydride to form hydroperoxo species. The O(2) hydrogen atom abstraction mechanism for both the Rh and Ir was found to be feasible. This is the first time this mechanism has been applied to a Rh system and only the second time it has been examined for a system other than Pd. The competing trans HCl reductive elimination pathway was also examined and found to be greatly dependent on the stereochemistry of the starting hydride primarily due to the intermediate formed upon the loss of Cl(-). As a result, the reductive elimination pathway was more favorable by 11.5 kcal/mol for the experimentally observed Ir stereoisomer, while the two pathways were isoenergetic for the other stereoisomer of the Rh complex. All findings are consistent with the kinetics study previously performed.     

Unexpected formation of a trinuclear complex containing a Ta(IV)-Ta(IV) bond in the reactions of Bu(t)N=Ta(NMe2)3 with silanes

A new trinuclear species containing a Ta(IV)-Ta(IV) bond, Ta(3)(μ-H)(μ-NMe(2))(μ=NBu(t))(2)(=NBu(t))(NMe(2))(5), has been formed by reductive elimination of H(2). Ta(2)H(2)(μ-NMe(2))(2)(NMe(2))(2)(=NBu(t))(2) has also been isolated. O(2) oxidizes the Ta(IV)-Ta(IV) bond to yield Ta(3)(μ(3)-O)(H)(μ=NBu(t))(μ-NMe(2))(2)(NMe(2))(4)(=NBu(t))(2) under ligand exchange. Delocalization of d electrons is discussed.