Oxygen Atoms Of CO Research Articles

Hydroxylation/carbonylation carbonaceous microspheres: A route without the need for an external functionalization to a “hunter” of lead(II) for electrochemical detection

Hydroxylation/carbonylation carbonaceous microspheres (H/C CMSs) are designed as a “hunter” for lead(II) (Pb2+) in electrochemical detection. Without the need for external functionalization, the carbonyl and hydroxyl groups were introduced to the surface of carbonaceous spheres during hydrothermal synthesis using glucose. The features of H/C CMSs before and after lead adsorption were studied using FTIR and XPS. CO and OH vibrations were affected by lead adsorption, which can be attributed to the coordination bonds between oxygen atoms of CO and OH and Pb2+. A H/C CMSs modified glassy carbon electrode (GCE) was electrochemically characterized using cyclic voltammetry (CV). Anodic stripping voltammetric performance of bound Pb2+ was evaluated using square wave anodic stripping voltammetry (SWASV) analysis. Linear increases in the stripping peak current were observed for concentrations from 0 to 30nM, with a sensitivity of 1.9067μA/nM. This was almost 260 times higher than the sensitivities to Cu2+ and Hg2+. A limit of detection was found to be 2nM based on the 3σ method. The successive addition of 250nM Zn2+, Cr3+, Cd2+, Cu2+, and Hg2+ to 25nM Pb2+ acetate buffer solution did not interfere with the adsorption of Pb2+ and stripping of Pb0.

Effect of Functionalized Linker on CO2 Binding in Zeolitic Imidazolate Frameworks: Density Functional Theory Study

Density functional theory (DFT) calculations are performed to investigate the effect of functionalized linkers on CO2 binding in zeolitic imidazolate frameworks (ZIFs). Three typical ZIFs with different functionalized linkers are considered: ZIF-68 with bIM linker (C6H6 group), ZIF-69 with cbIM linker (chlorine group), and ZIF-78 with nbIM linker (nitro group). Compared to bIM linker, the charge distributions of cbIM linker and nbIM linker have been significantly changed by the substitution of the functional groups and have been proved to have positive effects to enhance the binding capacity of CO2. The withdrawal of electronic charge from the linker leads to enhanced acidity for the hydrogen atoms on linker, which can form weak hydrogen bond interactions with the oxygen atoms of CO2. The binding capacity of nbIM linker for CO2 is much higher than those of bIM and cbIM linkers for CO2, which is due to the strong effect of nitro group on polarizing the CO2 molecule and thus enhances the interaction force between them. The binding energies of CO2 in the IM linkers are within the range of −1.64 to −18.54 kJ/mol, which suggests that the binding belongs to physisorption.

Oxygen permeation behavior of La 0.6Sr 0.4Co 0.8Fe 0.2O 3 hollow fibre membranes with highly concentrated CO 2 exposure

La 0.6Sr 0.4Co 0.8Fe 0.2O 3− α (LSCF) hollow fibre membranes with an asymmetric structure consisting of a central porous layer and two dense surface layers were prepared using a mixture of 50% ethanol + 50% NMP as the internal coagulant. Using the resultant hollow fibre, the influence of CO 2 as the sweep gas on the oxygen permeation behavior of the LSCF membranes has been investigated. The membranes before and after permeation test were characterized by XRD, SEM and EDX. The oxygen permeation fluxes of the LSCF membrane decreased with increasing the CO 2 concentration in sweep gas. The significant poisoning effect of CO 2 on the oxygen permeation through the LSCF perovskite membranes was mainly due to the chemical adsorption of CO 2 to the membrane surface with a result that the oxygen vacancy sites were occupied by the oxygen atoms of CO 2. The reaction between the LSCF membrane material and the adsorbed CO 2 to form carbonates caused Sr segregation and corrosion on the membrane surface during the permeation process. Operated in pure CO 2 as sweep gas for more than 100 h at 900 °C or above, the LSCF hollow fibre still displayed a stable performance. Although some areas of the membrane have been etched with a thickness about 1.4 μm, no sign of membrane collapsing or failure exists.

Unraveling the High Uptake and Selectivity of CO2 in the Zeolitic Imidazolate Frameworks ZIF-68 and ZIF-69

Understanding the CO2 adsorption mechanism of zeolitic imidazole frameworks (ZIFs) is the key to improving the adsorption ability of new metal–organic framework materials for gas capture applications and designing novel frameworks. In this work, the ONIOM2(B3LYP/6-31G**:UFF) method is used to investigate the interactions between CO2 and CO molecules with ZIF-68 and ZIF-69, respectively. Calculation results indicate that a strong Lewis acid–base interaction exists between the gas molecules and nitro groups of 2-nitroimidazole linkers, which thus prevents the entry of gas molecules into the small pores of ZIFs. The simultaneous interaction of both oxygen atoms of CO2 with the H atoms of the benzimidazole (5-chlorobenzimidazole) rings in the large pores results in the high CO2 selectivity observed for both ZIFs. The presence of electron-withdrawing groups in the phenylimidazole linker of ZIF increases the interaction between the oxygen atoms of CO2 and hydrogen atoms of the phenyl rings in its large pores. T...

Syntheses, structural characterization and antimicrobial activities of novel cobalt-pyrazine-2,3-dicarboxylate complexes with N-donor ligands

Abstract Novel cobalt-pyrazine-2,3-dicarboxylate complexes with 1,10-phenanthroline (phen), [Co(pzdca)(phen) 2 ] 2 ·11H 2 O (1) , N,N,N′,N′ -tetramethylethylenediamine (tmen), (H 2 tmen)[Co(pzdca) 2 (tmen)]·9H 2 O (2) and 2,2-dimethylpropane-1,3-diamine (dmpen), [Co(CO 3 )(dmpen) 2 ](pzdca) 0.5 ·H 2 O (3) have been synthesized and characterized by elemental and thermal analyses, spectroscopic (IR and UV–vis) and X-ray diffraction techniques. In 1 and 2 , pyrazine-2,3-dicarboxylate ligand coordinated to the Co(II) ions through one nitrogen atoms of pyrazine ring and oxygen atoms of carboxylate group as a bidentate ligand and distorted octahedral geometries of 1 and 2 are completed by phen and tmen ligands, respectively. In 2 , the tmen molecules exhibit chemically different functions; it coordinated to the Co(II) ion as a bidentate ligand and in the other form it protonated and acts as counter-ion. In complex 3 , Co(III) ion is coordinated by four nitrogen atoms of dmpen and two oxygen atoms of CO 3 ligand and the pzdca behaves as a counter-ion. Furthermore, structures of 1 and 2 contain extensive hydrogen bonding between crystal water molecules to form infinite 2D water layers and 1D water chains, respectively. In vitro antimicrobial activities of new complexes were tested against selected wild type and clinical microorganisms by MIC. Complexes exhibited antimicrobial activity at high concentrations against the bacteria, fungi and clinical isolate tested.

Detection of Carboxyl Ylide Evidenced by Laser Flash Photolysis of (Biphenyl-4-yl)chlorodiazirine in the Presence of Carbon Dioxide

Ylide formation of a singlet carbene with carbon dioxide (CO2 was studied using laser flash photolysis (LFP) of (biphenyl-4-yl)chlorodiazirine (BCD) in CO2 saturated acetonitrile (MeCN) and in CO2 dissolved in 2,2,4-trimethylpentanone (i-Oc) at high pressure (ca. 2 X 106 Pa). (Biphenyl-4-yl)chlorocarbene (BCC), produced by LFP, adducts with an oxygen atom of CO2 to form a carboxyl ylide (CY, lmax = ca. 510 nm). The apparent rate constant for the addition of BCC to CO2 was 5.2 x 107 dm3 mol-1 s-1 (by measuring the decay rate of BCC) and 6.2 X 107 dm3 mol-1 s-1 (the rate of CY formation) in MeCN. CY decays via unimolecular and bimolecular mechanisms. CY is a 1,3-dipole, so in the presence of dimethyl fumarate, it gives a γ-butyrolactone derivative. On adding the carbene scavenger 2,3-dimethyl-2-butene (TME), the decay rate of BCC and the formation and decay rates of CY increased with TME concentration, indicating that CY was in equilibrium with BCC. The equilibrium constant was estimated roughly to be ca. 2 dm3mol-1.

Ruthenium‐Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid in Alcohols

AbstractCatalytic hydrogenation of CO2 to formic acid with the (solvento)metal hydride complex, TpRu(PPh3)(CH3CN)H [Tp = hydrotris(pyrazolyl)borate], in various alcohols was studied. High‐pressure NMR monitoring of the catalytic reaction in non‐acidic methanol shows that the observable intermediate is a formate complex resulting from CO2 insertion into the Ru−H bond and is stabilized by the hydrogen‐bonding interaction between the formato ligand and a methanol molecule. However, in the case of the acidic alcohol, CF3CH2OH, the observable intermediates are [TpRu(PPh3)(CH3CN)2]+CF3CH2O− and the alkyl carbonate complex, TpRu(PPh3)(η2‐O2COCH2CF3), which are formed by the reaction of CO2 with the alkoxide species, TpRu(PPh3)(CH3CN)(OCH2CF3), generated by a very facile reaction between TpRu(PPh3)(CH3CN)H and CF3CH2OH. We propose that the productive catalytic cycles of the reactions conducted in a variety of alcohols are similar to the one we formulated for the catalytic hydrogenation of CO2 in hydrous THF. The formic acid is produced by the transfer of a hydride and a proton from the transient alcohol hydride intermediate, TpRu(PPh3)(ROH)H, to an approaching CO2 molecule. The activity of TpRu(PPh3)(CH3CN)H is higher in CF3CH2OH than in methanol and other non‐acidic alcohols and it is probably due to the enhanced electrophilicity of the carbon atom of CO2, which results from the strong interaction between the proton of the highly acidic alcohol in TpRu(PPh3)(CF3CH2OH)H and an oxygen atom of CO2. The electrophilic carbon atom of CO2 could in turn abstract the hydride from Ru−H in a more facile manner. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The interaction of formohydroxamic acid with carbon monoxide: FTIR matrix isolation and quantum chemistry studies

Abstract Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) and carbon monoxide have been recorded. The experimental results indicate formation of three isomeric complexes. In two complexes the NH or OH groups of formohydroxamic acid are attached to the carbon atom of the CO molecule and in the third complex the NH group interacts with the oxygen atom of CO. The formohydroxamic acid complex with two CO molecules is also trapped in the matrix. One of the two CO molecules interacts with the NH group and the second one with the OH group of HCONHOH, in both cases the site of interaction is the carbon atom of CO. Theoretical studies of the structure and spectral characteristics of the complexes were carried out on the DFT(B3LYP)/6-311++G(2d,2p) level. The calculated vibrational frequencies for the complexes present in the matrices are in good agreement with the experimental data. The calculations show also an additional potential energy minimum corresponding to the complex in which the OH group of formohydroxamic acid is attached to the oxygen atom of carbon monoxide.

Molecular Orbital Calculations on the Protonation of Hydrogen-Bonded Formamide Chains. Implications for Peptides

We report density functional studies of the protonation of H-bonded formamide chains containing up to 10 monomeric units. These chains contain H bonds that are similar to those in peptides and proteins. All structures considered were completely geometrically optimized at the B3LYP/D95** level of calculation. The proton affinities of the chains are greatest at the terminal CO oxygen atoms. They increase significantly with the length of the formamide chain from two to five formamides. Protonation of the terminal CO of chains containing five or more formamides results in the transfer of the NH proton of the terminal (protonated) monomer across the H bond to the adjacent formamide. Protonations at the NH2 termini of the formamide chains are generally unfavorable as they result in the rupture of the proximate H bond. The proton affinities at the CO's of formamide chains containing three or more monomers exceed that of the amino group of glycine, implying that peptides that contain such H-bonding chains might b...

The molecular basis of CO 2 interaction with polymers containing fluorinated groups: computational chemistry of model compounds and molecular simulation of poly[bis(2,2,2-trifluoroethoxy)phosphazene

Ab initio molecular orbital calculations of CO 2 and model compounds have been used to identify the nature of specific interactions between CO 2 and the fluorinated substituent groups of polymers such as poly(trifluoropropyl methyl siloxane) and poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) that exhibits high CO 2 permeability and permselectivity. Second-order Møller–Plesset (MP2) perturbation calculations (6-311++G ∗∗ basis set) were used to obtain energies, dipole and quadrupole moments, and polarizabilities of CO 2, three alkanes (CH 4, CH 3CH 3, and CH 3CH 2CH 3), and three fluoroalkanes (CF 4, CH 3CF 3, and CH 3CH 2CF 3). Results of energy calculations of three CO 2–alkane and three CO 2–fluoroalkane dimers indicate that CO 2 forms a favorable quadrupole–dipole interaction with the fluoroalkyl groups. The maximum quadrupole–dipole interaction energy obtained was −11.5 kJ mol −1 for CO 2–CF 3CH 2CH 3. This value is less than interaction energies typical for hydrogen bonding but greater than the London dispersion values reported for the interaction of CO 2 with the carbonyl group of poly(methyl methacrylate) (PMMA). Electrostatic potential distributions indicate a small redistribution of electron density to the fluorine atoms of the trifluoroalkanes and to the oxygen atoms of CO 2 in the CO 2–CF 3CH 3 and CO 2–CF 3CH 2CH 3 dimers. Sorption and molecular association of CO 2 with PTFEP has been investigated by molecular simulation of an amorphous cell using the COMPASS molecular mechanics force field. CO 2 sorption isotherms obtained by Grand Canonical Monte Carlo (GCMC) simulation indicate an upward deviation from the linear relationship between log S and the Lennard–Jones potential well depth parameter, ε/ k, in agreement with reported permeability data. Pair-correlation analysis obtained from molecular dynamics simulation show strong correlation of CO 2 with the trifluoromethyl group of PTFEP in agreement with the MP2 results showing an association of CO 2 with both CH 3CF 3 and CH 3CH 2CF 3.

Photosensitized oxidation of 13C,15N-labeled imidazole derivatives.

An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-labeled imidazole derivatives. A new product, CO(2), was observed in the photooxidation of 2-H,N1-H imidazoles, but not in 2-substitituted imidazoles. The carbon of CO(2) derives from the 2C of imidazole. As shown by 18O experiments, both oxygen atoms of CO(2) originate mainly from one molecule of oxygen. Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the isotope-labeled imidazoles. Quantitative analysis of the 13C NMR at different temperatures and times correlates the formation of one intermediate with the loss of another, thus allowing the complete decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8. The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to CO(2) and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two reactions share a similar reaction mechanism with singlet oxygen.

Formation of transient intermediates in low-temperature photosensitized oxidation of an 8-(13)C-guanosine derivative.

An 8-(13)C-labeled guanosine derivative, 2',3',5'-O-tert-butyldimethylsilyl-N-tert-butyldimethylsilyl-8-(13)C-guanosine, was synthesized and its photosensitized oxidation with singlet oxygen carried out below -100 degrees C. Two transient intermediates that decompose directly to the final major product 5 and CO(2) were detected by (13)C NMR between -100 and -43 degrees C. The two intermediates are carbamic acids based on (13)C NMR and 2D NMR (HMQC, HMBC) spectra and the formation of final product 5 and of 8-CO(2). No endoperoxide intermediate could be detected by low-temperature NMR spectroscopy even at -100 degrees C. A reaction mechanism is proposed involving initial [4 + 2] cycloaddition of singlet oxygen to the imidazole ring to form an unstable endoperoxide, subsequent rearrangement of the endoperoxide to a dioxirane, and decomposition of the dioxirane to the two observed intermediates. Both oxygen atoms of CO(2) are derived from a single oxygen molecule, which strongly supports a dioxirane structure for the precursor of the two observed intermediates. The distribution of products estimated by (13)C NMR accounts for all the (13)C-containing products in the reaction mixture.

Synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture in the presence of bifunctional catalyst

Ethylene carbonate was rapidly synthesized from supercritical carbon dioxide/ethylene oxide mixture by using as catalyst the system of tetradentate schiff-base aluminum complexes (designated as SalenAlX) coupling with a quaternary ammonium or phosphonium salt. The high rate of reaction was attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Various reaction periods present different formation rate of ethylene carbonate, mainly due to the existence of phase change during the reaction. The synergistic effect of the binary catalyst for ring-opening of ethylene oxide results from nucleophilicity of highly reactive anions of quaternary salts and the electrophilic interaction of SalenAlX with ethylene oxide. The activation of CO 2 was generally initiated by nucleophilic attack of the alcoholate(OCH 2CH 2BrNBu 4) at the carbon atom of CO 2, and weak interaction between the central metal ion of SalenAlX and the lone pairs of one oxygen atom of CO 2. It resulted in the insertion of CO 2 to AlO bond of Salen(X)AlOCH 2CH 2BrN( n-Bu) 4 or SalenAlOCH 2CH 2X to form linear carbonate which was transformed into ethylene carbonate by intramolecular substitution of halides. The experimental results demonstrate that supercritical carbon dioxide could be used as not only an environmentally benign solvent but also a carbon precursor in synthesis.

Protein Intermediate Trapped by the Simultaneous Crystallization Process: CRYSTAL STRUCTURE OF AN IRON-SATURATED INTERMEDIATE IN THE Fe3+ BINDING PATHWAY OF CAMEL LACTOFERRIN AT 2.7 Å RESOLUTION

This is the first protein intermediate obtained in the crystalline state by the simultaneous process of Fe(3+) binding and crystal nucleation and is also the first structure of an intermediate of lactoferrin in the Fe(3+) binding pathway. Lactoferrin is an iron-binding 80-kDa glycoprotein. It binds Fe(3+) very tightly in a closed interdomain cleft in both lobes. The iron-free structure of lactoferrin, on the other hand, adopts an open conformation with domains moving widely apart. These studies imply that initial Fe(3+) binding must be in the open form. The protein intermediate was crystallized by the microdialysis method. The protein solution, with a concentration of 100 mg/ml in 10 mm Tris-HCl, pH 8.0, was loaded in a capillary and dialyzed against the same buffer containing 26% (v/v) ethanol placed in a reservoir. FeCl(3) and CO(3)(2-) in excess molar ratios to that of protein in its solution were added to the reservoir buffer. The crystals appeared after some hours and grew to the optimum size within 36 h. The structure was determined by molecular replacement method and refined to final R- and R-free factors of 0.187 and 0.255, respectively. The present structure showed that the protein molecule adopts an open conformation similar to that of camel apolactoferrin. The electron density map clearly indicated the presence of two iron atoms, one in each lobe with 4-fold coordinations: two by the protein ligands of Tyr-92(433) OH and Tyr-192(526) OH and two other coordination sites occupied by oxygen atoms of bidentate CO(3)(2-) ions leading to a tetrahedral intermediate. The CO(3)(2-) anion is stabilized through hydrogen bonds with the synergistic anion-binding site Arg-121(463) and with Ser-122 Ogamma in the N-lobe and Thr-464 Ogamma in C-lobe. The third oxygen atom of CO(3)(2-) interacts with a water molecule in both lobes.

Generalized oscillator strength profiles for inner shell excitation of CO2 derived from variable angle electron energy loss spectroscopy

GOS profiles for C 1s and O 1s excitation of CO2 have been derived from inner shell electron energy loss spectra of CO2 measured over a wide range of scattering angles (4–62°). The experimental profiles are compared to computed GOS profiles calculated within the first Born approximation, using ab-initio configuration interaction wave functions for the C 1s transitions and ab-initio generalized multi structural wave functions for the O 1s transitions. The theoretical and experimental results are in generally good agreement indicating that the first Born approximation holds to a surprisingly large momentum transfer for the core excitations studied. The computations predict there should be oscillations in the GOS for O 1s→nsσg (536.5, 539 eV) and npσu Rydberg states (∼539 eV) but not for π symmetry states. Measurements with a newly installed parallel detector provide experimental support for the existence of oscillations in the GOS profile for the O 1s excitation of CO2 at 539 eV. These oscillations may arise from interference between core excitations localized on the left or right oxygen atom of CO2.

Periodic Density Functional LDA and GGA Study of CO Adsorption at the (001) Surface of MgO

The deficiencies of the local density approximation (LDA) to describe weakly adsorbed species on ionic surfaces are exposed in the current study of CO on the (001) surface of MgO. The authors applied a periodic formulation of density functional theory with crystal orbitals and charge density expanded as linear combinations of atomic orbitals of Gaussian form. The binding was investigated for four orientations of the CO molecule; either the carbon or oxygen atom of CO down over either the surface magnesium cation or oxygen anion. The authors find that the carbon down over the magnesium cation is the only noticeably attractive configuration, when basis set superposition error is corrected for, and estimate the binding energy of CO to be 1.9 kcal/mol for the Perdew, Burke, and Enzerhof (PBE96) functional. The binding energy obtained in the LDA approximation is as large as 7.4 kcal/mol. For the C-down-over-Mg configuration, the CO stretching frequency shift is only 4 cm{sup {minus}1} for the PBE96 functional. This binding energy, vibrational shift, and analysis of density of states indicate that little chemical bonding accompanies the adsorption. The values of binding energy and vibrational shift support a suggestion by Nygren and Pettersson that the CO adsorption discussedmore » by He et al. actually takes place at defect sites on the (001) surface of MgO.« less

Analysis of thermal vibrations by temperature-dependent low energy electron diffraction: comparison of soft modes of pure and O-coadsorbed CO on Ru(0001)

Extensive new temperature-dependent LEED I/ V data of the well-known ( 3 × 3 )R30°-CO structure and of the (2×2)-(O+CO) coadsorbate structure on Ru(0001) have been obtained down to 27 K and analysed in terms of the thermal vibrations of the CO molecule, both by tensor low energy electron diffraction (LEED) with isotropic vibrations and by a scheme based on probability density functions including anisotropic vibrations. In both structures the CO molecule occupies the top site, as has been shown by previous LEED I/ V analyses. In the coadsorbate structure this top site is surrounded by three oxygen atoms on hcp sites and the molecule is tilted by about 13° in a direction away from one of the oxygen atoms. The different treatments agree on all aspects of geometry and vibrations, and also with earlier results where overlap exists. We show that in the coadsorbate system the thermal vibrational amplitudes are much smaller than in the single adsorbate system, and that the tilt of the CO molecule found there is mainly of static nature. Around this axis the static lateral displacement of the oxygen atom of CO increases from 0.23 Å at 27 K to 0.31 Å at 350 K and the vibration amplitudes increase from 0.08 Å to 0.11 Å. The motion at the lowest temperature corresponds to the zero point motion. For the pure CO system the corresponding mean vibrational displacement of the O, which are now symmetric around the normal direction, range from 0.16 Å at 27 K to over 0.4 Å at 350 K. We conclude that low temperature measurements improve the accuracy of geometry determination by LEED I/ V, and that temperature-dependent LEED can well be used for the determination of vibrational amplitudes. A corroboration of the reliability of LEED geometries is deduced from our extensive comparisons.

Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on a TiO2-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and As-Precipitated Titanium Hydroxide

The active oxygen species and mechanism for catalytic CO oxidation with O2 on a highly active TiO2-supported Au catalyst (denoted as Au/Ti(OH)*4), which was prepared by supporting a Au–phophine complex on as-precipitated wet titanium hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2 temperature-programmed desorption (O2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared spectroscopy (FT-IR). Surface lattice oxygen atoms on the Au/Ti(OH)*4 catalyst were inactive for oxygen exchange with O2 and CO and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2, probably via carbonates. O2 did not dissociate to atomic oxygen on the catalyst. The catalyst showed a paramagnetic signal at g=2.002 due to unpaired electrons trapped at oxygen vacancies mainly at the surface. O2 adsorbed on the oxygen vacancies to form superoxide O−2 with g1=2.020, g2=2.010, and g3=2.005, which are characteristic of O−2 with an angular arrangement. Upon CO exposure, all the adsorbed oxygen species disappeared. The adsorbed oxygen on Au/Ti(OH)*4 desorbed below 550 K. O−2 species were also observed on TiO*2 prepared by calcination of as-precipitated wet titanium hydroxide at 673 K, but were unreactive with CO. FT-IR spectra revealed that CO reversibly adsorbed on both Au particles and Ti4+ sites on the Au/Ti(OH)*4 surface. No band for adsorbed CO was observed on the TiO*2, which indicates that the presence of Au particles has a profound effect on the surface state of Ti oxide. No shifts of νCO peaks on Au/Ti(OH)*4 occurred upon O2 adsorption, suggesting that O2 was not directly bound to the Au particles on which CO adsorbed. Annealing of Au/Ti(OH)*4 under O2 atmosphere significantly suppressed the O2 adsorption and the CO oxidation due to a decrease in the amount of oxygen vacancies, while CO adsorption was not affected by annealing. From the systematic oxygen isotope exchange experiments along with O2-TPD, ESR, and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O−2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation. The mechanism for the catalytic CO oxidation on the active Au/Ti(OH)*4 catalyst is discussed in detail and compared with mechanisms reported previously.

Active oxygen species and reaction mechanism for low-temperature CO oxidation on an Fe2O3-supported Au catalyst prepared from Au(PPh3)(NO3) and as-precipitated iron hydroxide

Active oxygen species and the reaction mechanism for catalytic CO oxidation with O2 on a highly active Fe2O3-supported Au catalyst (denoted as Au/Fe(OH)3*), which was prepared by supporting Au(PPh3)(NO3) on as-precipitated wet iron hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2-temperature programmed desorption (TPD) and FT-IR. Surface lattice oxygen atoms on the Au/Fe(OH)3* catalyst were inactive for oxygen exchange with O2 and CO, and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2 probably via carbonates. There is no evidence that O2 dissociates to atomic oxygen on the catalyst. TPD spectra following adsorption of 36O2 or a mixture of 32O2+36O2 showed no oxygen exchange, where the adsorbed oxygen on Au/Fe(OH)3* desorbed below 500 K. Upon CO exposure, all the adsorbed oxygen species disappeared. FT-IR spectra revealed that CO reversibly adsorbed on Au particles and irreversibly adsorbed on Fe3+ sites on the Au/Fe(OH)3* surface. Only CO molecules adsorbed on the Au particles were active for low-temperature CO oxidation. No band for adsorbed CO was observed on Fe2O3* prepared by calcination of the as-precipitated wet Fe(OH)3* at 673 K, which indicates that the presence of Au particles causes a profound effect on the surface state of Fe-oxide. Annealing of Au/Fe(OH)3* under an O2 atmosphere did not suppress the catalytic CO oxidation, unlike a remarkable suppression observed with Au/Ti(OH)4*. The presence of water vapor did not significantly decrease the CO oxidation rate due to the facile water gas shift reaction on Au/Fe(OH)3*, also unlike the case of Au/Ti(OH)4*. From the systematic oxygen isotope exchange experiments along with O2-TPD and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation on Au/Fe(OH)3*. The mechanism for the catalytic CO oxidation on the active Au/Fe(OH)3* catalyst is discussed in detail and compared with those reported previously.

Interconversion of ROC+ and RCO+ (R = H and CH3): Gas-Phase Catalysis by Argon and Dinitrogen

Molecular orbital calculations using density functional theory at the B3LYP/6-311++G(d,p) level have been used to optimize structures for ions COR+···M and M···RCO+ and also for the transition structures COR+···M(ts) for their interconversion (R = H, CH3 and M = Ar and N2). For the unsolvated ions and for ions COH+···M, M···HCO+, and COH+···M(ts) the optimized structures were used for single-point calculations at QCISD(T)(full)/6-311++G(2df,p). Critical points on the COH+ and ArCOH+ potential energy surfaces were also optimized at MP2(full)/6-311++G(3df,3pd). For the uncomplexed ions COR+, the barriers to 1,2-migration of R+ at B3LYP/6-311++G(d,p) are 35.4 kcal mol-1 for R = H and 14.2 kcal mol-1 for R = CH3. Inclusion of a dinitrogen molecule removes this barrier by permitting COR+ to deposit R+ on N2 followed by CO retrieving the R+ to produce the lower energy isomer, RCO+. Argon has a lower R+ affinity than the oxygen atom of CO and does not remove R+ from COR+. Preferential stabilization by argon of the transition structure for the 1,2-migration of R+ over stabilization of COR+ at the minimum results in a reduction in the barrier to rearrangement. The gas-phase rearrangements of ions COR+ via “solvated” transition structures COR+···Ar(ts) are calculated to have barriers of 8.3 kcal mol-1 for R = H and 5.7 kcal mol-1 for R = CH3, while for COH+···Ar at MP2(full)/6-311++G(3df,3pd) the barrier is only 2.0 kcal mol-1. These findings indicate noble gas atoms may catalyze the rearrangement of cations rather than simply cool them by collisions.