Gas-phase CO Molecules Research Articles

Complex Organic Matter Synthesis on Siloxyl Radicals in the Presence of CO.

Heterogeneous phase astrochemistry plays an important role in the synthesis of complex organic matter (COM) as found on comets and rocky body surfaces like asteroids, planetoids, moons and planets. The proposed catalytic model is based on two assumptions: (a) siliceous rocks in both crystalline or amorphous states show surface-exposed defective centers such as siloxyl (Si-O•) radicals; (b) the second phase is represented by gas phase CO molecules, an abundant C1 building block found in space. By means of quantum chemistry; (DFT, PW6B95/def2-TZVPP); the surface of a siliceous rock in presence of CO is modeled by a simple POSS (polyhedral silsesquioxane) where a siloxyl (Si-O•) radical is present. Four CO molecules have been consecutively added to the Si-O• radical and to the nascent polymeric CO (pCO) chain. The first CO insertion shows no activation free energy with ΔG200K = −21.7 kcal/mol forming the SiO-CO• radical. The second and third CO insertions show Δ ≤ 10.5 kcal/mol. Ring closure of the SiO-CO-CO• (oxalic anhydride) moiety as well as of the SiO-CO-CO-CO• system (di-cheto form of oxetane) are thermodynamically disfavored. The last CO insertion shows no free energy of activation resulting in the stable five member pCO ring, precursor to 1,4-epoxy-1,2,3-butanone. Hydrogenation reactions of the pCO have been considered on the SiO oxygen or on the carbons and oxygens of the pCO chains. The formation of the reactive aldehyde SiO-CHO on the siliceous surface is possible. In principle, the complete hydrogenation of the (CO)1−4 series results in the formation of methanol and polyols. Furthermore, all the SiO-pCO intermediates and the lactone 1,4-epoxy-1,2,3-butanone product in its radical form can be important building blocks in further polymerization reactions and/or open ring reactions with H (aldehydes, polyols) or CN (chetonitriles), resulting in highly reactive multi-functional compounds contributing to COM synthesis.

Wolkenstein’s Model of Size Effects in CO Oxidation by Gold Nanoparticles

The Wolkenstein’s theory of catalysis and the d-band theory of formation chemical bonds between transition metal catalysts and adsorbates were used to develop the approach applied to the kinetics of CO oxidation by gold nanoparticles. In the model, within the framework of the mechanism of the reaction going through dissociative adsorption of oxygen molecules and reaction with gas-phase CO molecules, weak and strong chemisorption states of intermediates (O, CO2) were taken into account in the kinetic equations by introducing reversible electronic steps corresponding to electron transfers between the intermediates and the catalyst. As a result, we obtain the expression for the reaction rate, which exhibits a volcano-shape dependence upon the size of the gold nanoparticles at the conditions when the intermediates fractions are not small compared to the empty active sites of the catalyst. It is supposed that the approach can be also applied to the Langmuir-Hinshelwood mechanism.

Photoelectron recoil in CO in the x-ray region up to 7 keV

Carbon $1s$ photoelectron spectra of CO molecules in gas phase were recorded in the tender x-ray energy range, from 2.3 to 6.9 keV. The intensity ratios of individual peaks from $\ensuremath{\nu}=0$ to 3 within the vibrational progression of the C $1s$ photoelectron spectrum were determined at the various photon energies and are shown to be strongly affected by the photoelectron recoil effect. The experimental vibrational intensity ratios are compared with theoretical predictions at different levels of accuracy. Developments of the recoil model, using generalized Franck-Condon factors, rovibrational coupling, Morse potential energy curves, and accurate angular averaging are presented and applied to the analysis of the experimental results.

CO Oxidation on the Pd(111) Surface

Under technologically relevant oxygen-rich conditions, the reaction mechanism of CO oxidation over transition metals can be complicated by the formation of oxides. Questions of whether the active surface for CO oxidation is a pristine metal, a surface oxide, or a bulk oxide is still under active debate. In this study, density functional theory calculations are used to model CO oxidation on the Pd(111) surface. Our results show that a thin layer of Pd5O4 surface oxide is stable under catalytically relevant gas-phase conditions. Three-fold oxygen atoms in the surface are found to react with gas-phase CO molecules following an Eley–Rideal reaction mechanism. Such CO oxidation reduces the surface oxide, but the oxide can be replenished by O2 dissociation. Kinetic analysis shows that experimentally observed reaction conditions, that are uninhibited by CO and limited only by mass transfer, correspond to a surface oxide phase with CO oxidation occurring though the Eley–Rideal mechanism. Under steady-state operating conditions, the continuous formation and decomposition of the surface oxide is expected and is key to the high CO oxidation rate on Pd(111).

Generation of isolated sub-40-as pulses from gas-phase CO molecules using an intense few-cycle chirped laser and a unipolar pulse

We theoretically investigate the high-order-harmonic generation (HHG) from the oriented gas-phase CO molecule exposed to the combination of an intense few-cycle chirped laser and a unipolar pulse. We show that the HHG is dramatically extended and an ultrabroad extreme ultraviolet supercontinuum spectra in the plateau is generated by adding a unipolar pulse to the few-cycle chirped laser at a proper time. Both the classical trajectory simulation and quantum time-frequency analysis show that a dominant short quantum path is selected to contribute the HHG spectra in the plateau. By superposing the supercontinuum harmonics centered at several different central frequencies, we can directly produce several isolated pulses in which the shortest one achieves 38 as.

A novel mechanism for spectator CO-mediated reaction with unique cis-(NO) 2 dimer on a Co 2+-dimer/γ-Al 2O 3(1 1 0) model catalyst: Density functional theory calculations

Density functional theory calculations for NO adsorption and NO–CO reaction on a Co 2+-dimer/γ-Al 2O 3(1 1 0) model catalyst were conducted to understand two new aspects of the increases in the amount and reduction rate of adsorbed NO by gas phase CO molecules which are undetectable at the Co 2+-ensemble/γ-Al 2O 3 catalyst surface. Three kinds of dinitrosyl adsorbates were found and assigned to normal gem-dinitrosyl species ( gem-dinitosyl I and II) and an unique cis-(NO) 2 dimeric species. The gem-dinitrosyl II with a reconstructed structure involving Co–O surf cleavage was considered as stable species experimentally observed, which is responsible for the spectator CO-promoted NO adsorption. We also found that the cis-(NO) 2 dimeric species specific for the Co 2+-ensemble structure possesses a much higher reactivity than the gem-dinitrosyl species, enabling the facile reaction with CO that is very weakly trapped at the surface. In the cis-(NO) 2 dimeric species two adsorbed NO molecules on two adjacent Co 2+ sites interact with each other due to the opposed orientation of the unoccupied d orbitals of the two Co 2+, resulting in easy formation of a N–N bond through the 2π*–2π* hybridization. The NO adsorbates modify and activate the surface (Co 2+ sites) to make pseudo-compounds (intermediates) with CO at the Co 2+ sites in such a way that CO is not detected at the surface. The potential energy surface for the NO–CO reaction is presented and the transition states and intermediates are discussed.

Surface Dynamics of Au/CeO2 Catalysts during CO Oxidation

The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/CeO2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2 through both the decomposition of the formate species and the formation of the [Au(CO)2]+ species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) chan...

Understanding of Novel Surface Events on a Supported Co2+-Ensemble Catalyst Promoted by Gas-Phase Molecules: Increases in the Amount and Reactivity of Adsorbed NO by CO Undetectable at the Catalyst Surface

The paper reports an understanding of the novel surface phenomenon that the amount and reactivity of adsorbed NO on a Co2+-ensemble/γ-Al2O3 catalyst are increased by gas-phase CO molecules undetectable at the catalyst surface. The nature of adsorbed NO species and the structures of divalent Co species were characterized by FT-IR, DR-UV/vis, XAFS, and DFT calculations. The Co2+-ensemble structure attached on the γ-Al2O3 surface was crucial for the increase in the NO adsorption equilibrium and the promotion of NO reactivity at the surface by gas-phase CO, while monomeric Co2+ sites did not bring out the promotional effects of CO. DFT calculations and FT-IR study revealed that the coordination structure of Co2+ sites was restructured to form a more stable geminal dinitrosyl species, showing Co−OAl (surface) bond breaking, which is an origin of the increase in NO adsorption. Direct interaction of the adsorbed dinitrosyl with gas-phase CO (Eley−Rideal mechanism) was conceived to be involved in the transition s...

A new aspect of heterogeneous catalysis: Highly reactive cis-(NO) 2 dimer and Eley–Rideal mechanism for NO–CO reaction on a Co-dimer/γ-alumina catalyst

A catalytic aspect, ‘surface catalytic NO reactions assisted by gas phase CO molecules’ on a Co 2+-ensemble/γ-Al 2O 3 catalyst, was elucidated by successive density functional theory (DFT) calculations on a Co 2+-dimer/γ-Al 2O 3 model surface. Unique cis-(NO) 2 dimeric species formed due to a specific alignment of the unoccupied orbitals on the Co 2+-dimer, whose vibrational frequencies were almost identical to those for conventional gem-dinitrosyl species. The cis-(NO) 2 species showed much higher reactivity with CO than the gem-dinitrosyl species, through the 2π ∗–2π ∗ bonding between the two NO adsorbates. Detailed DFT calculations for the reaction pathways indicated an Eley–Rideal mechanism with an unusually large reactive cross-section.

Growth Energetics of Single-Wall Carbon Nanotubes with Carbon Monoxide

The microscopic growth energetics of single-wall carbon nanotubes (SWNTs) with gas-phase CO molecules is investigated. Our density functional calculations show that CO molecules can form carbon networks directly at an open edge of SWNT by adsorption and subsequent desorption procedure. CO molecules adsorb to the carbon atom in an open edge of the SWNT, not through the oxygen atom but through the carbon atom, because the frontier orbital of the CO molecule has more carbon character. Such adsorption results in the formation of a carbon-carbon bond. Formation of a hexagonal carbon ring is thermodynamically more favorable than that of a pentagonal carbon ring and becomes a driving force for the growth of SWNTs. A possible growth mechanism through formation of hexagonal ring structure is suggested from the results of this study. Our results of energy calculations suggest that growth in the direction of a zigzag wall is more favorable. The roles of a catalyst, which nucleates the growth of SWNT and stabilizes an open edge with dangling bonds, are further discussed.

First principles study of CO oxidation on TiO2(110): the role of surface oxygen vacancies.

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car-Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.

Cryogenic CO oxidation on TiO(2)-supported gold nanoclusters precovered with atomic oxygen.

Bulk gold has long been regarded as a noble metal, having very low chemical and catalytic activity. However, metal oxide-supported gold particles, particularly those that are less than 5 nm in diameter, have been found to have remarkable catalytic properties. In this study we show that impinging gas-phase CO molecules react readily with oxygen adatoms preadsorbed on Au/TiO(2)(110) to produce CO(2) even under conditions in which the sample is cryogenically cooled. Gold particle size seems to have little effect on the CO oxidation reaction when oxygen adatoms are preadsorbed. We also show that as the oxygen adatom coverage increases, the rate of CO oxidation decreases on Au/TiO(2) at cryogenic temperatures.

Multiple scattering theory for non-spherical potentials: application to photoelectron angular distributions from oriented diatomic molecules and the study of shape resonances

We use multiple scattering photoelectron diffraction theory to calculate the angular distribution of electrons photoemitted from core levels of fixed-in-space molecules. Non-spherical scattering potentials, needed for low photoelectron energies ( E <50 eV), are included in our formalism through non-diagonal scattering matrices; these fit directly into a new approach for multiple scattering theory originally based on spherical potentials. When the kinetic energy of the photoelectron is low its scattering cannot be adequately represented by spherically-symmetric potentials. In addition, we have considered the effect of the final-state hole, and found this to be important as well. As an example, we calculate the angular distribution of photoelectrons emitted from the K shells of C in oriented gas-phase CO molecules, as recently measured by several groups. Particularly important are the energies for which shape resonances appear in the continuum, with the angular distributions showing radical changes over such resonances.

Mechanism of efficient carbon monoxide oxidation at Ru(0001)

We performed density-functional theory calculations using the generalized gradient approximation for the exchange-correlation functional to investigate the unusual catalytic behavior of Ru under elevated gas pressure conditions for the carbon monoxide oxidation reaction, which includes a particularly high CO2 turnover. Our calculations indicate that a full monolayer of adsorbed oxygen actuates the high rate, enabling CO2 formation via both scattering of gas-phase CO molecules as well as by CO molecules adsorbed at oxygen vacancies in the adlayer, where the latter mechanism is expected to be very efficient due to the relatively weak adsorption energy of both CO and O, as well as the close proximity of these reactants. We analyze the bonding and electronic properties associated with the reaction pathway for CO2 production via the scattering reaction. We find that the identified “bent” transition state is due to electron transfer into the unoccupied 2π orbitals of the CO molecule which reduces the Pauli repulsion between the impinging CO and the O-covered surface. Bond formation to CO2 then proceeds by electron transfer back from the CO 2π orbitals into the bonding region between CO and the adsorbed O atom.

Study of CO oxidation over Ru(0001) at high gas pressures

Experiments performed at high gas partial pressures have demonstrated that the kinetics of the CO oxidation reaction over Ru(0001) are different and somewhat anomalous compared to those over other transition metal surfaces and, in particular, that the turnover rate is exceptionally high. In order to gain insight into the underlying reasons for this behavior, we performed density functional theory calculations using the generalized gradient approximation for the exchange-correlation functional. We find that the high rate is due to a weakly, but nevertheless well bound, (1 × 1) oxygen adsorbate layer which may form for high O 2 pressures but not under usual ultra-high vacuum conditions. The calculations indicate that reaction to CO 2 occurs both via scattering of gas-phase CO molecules and by CO molecules weakly adsorbing at vacancies in the oxygen adlayer, where the latter mechanism dominates the rate.

Stability of adsorbed states and site-conversion kinetics: CO on Ni(100).

The site conversion of adsorbed CO between the terminal site and the bridged site on Ni(100) was studied by means of infrared reflection absorption spectroscopy (IRAS). The temperature dependence of the relative occupation for two sites was measured from 80 to 266 K in detail, where the binding-energy difference was determined to be 11 meV. The driving force for the predominant occupation of the terminal site at higher temperature is ascribed to the vibrational entropy of the low-energy degenerate-hindered translational mode of the terminal CO. The kinetics of approaching the equilibrium was studied by time-resolved IRAS combined with a pulsed gas dose. Following a rapid dose, CO molecules are initially adsorbed at the terminal site and the bridged site with the a priori ratio of 1:2, indicating that gas-phase CO molecules are directly trapped by the potential minima initially, are thermalized, and migrate on the surface to approach the equilibrium occupation ratio. The microscopic hopping rate from the terminal site to the bridged site was estimated to be 0.02 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and that from the bridged site to the terminal site was estimated to be 0.005 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 83 K. A random-walk model assuming the microscopic hopping rates gives a self-diffusion coefficient of 3.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 83 K, which is in good agreement with the previously reported macroscopic results. Thus, the elementary step of surface diffusion is ascribed to the hopping between the terminal site and the bridged site. The difference between the estimated barrier by assuming a harmonic potential and the activation energy for diffusion suggests the presence of anharmonicity in the potential between the terminal site and the bridged site.

Spectroscopic and catalytic study on metal carbonyl clusters supported on Cab-O-Sil: I. Impregnation and decomposition of Fe 3(CO) 12

Impregnation of Fe 3(CO) 12 on Cab-O-Sil has been studied by ir spectroscopy, Mössbauer spectroscopy, and mass spectrometry. Isotope exchange between CO ligands of the impregnated sample and gas phase CO molecules was also investigated. On impregnation, two types of interaction can be distinguished: (i) interaction of the type FeCO…HOSi and CO…HOSi, shown by the shift to lower and higher ir frequencies for bridged and for terminal CO, respectively, and (ii) interaction between the metal framework and the support revealed in the oxidation of iron to form very small iron oxide particles. On impregnation a small amount of CO is evolved as a result of the interaction. CO exchange occurs faster with alumina-supported clusters than with silica-supported samples. On decomposition up to 370 K, the metal framework is retained and the cluster structure can be partly restored in a CO atmosphere. Above 420 K, Fe 3(CO) 12 is decomposed to form Fe 2+ oxide on the surface. A possible mechanism for impregnation is discussed in terms of electron donation from the support oxygen to the iron d-bands as a result of which the metal-carbon bond strength is influenced. On decomposition the metallic iron interacts with the support OH groups causing oxidation and Fe 2+ formation.