A thermogravimetric method for the measurement of CO/CO2 ratio at the surface of carbon during combustion

This work presents a new method of measuring the CO/CO₂ ratio at the surface of carbon particles during combustion. This thermogravimetric method deduces the ratio of CO to CO₂ by comparing the rate of consumption of carbon with the rate of oxidation of an external reference material with fast oxidation kinetics, in this case Cu. The method is useful when combustion is controlled by external mass transfer, commonly encountered in large-scale processes. The viability of this method has been demonstrated experimentally with graphite and a lignite char. It was found that in an atmosphere of ～ 1% O₂, the graphite produced CO₂ between 700 and 900 °C whilst the lignite char produced a mixture of CO and CO₂ between 700 and 800 °C with the proportion of CO increasing with temperature, and above 850 °C, only CO was produced. It was also found that for this particular lignite char, the ratio of CO/CO₂ increased with decreasing pO₂ in the environment.     

Influence of one CO molecule on structural and electronic properties of monatomic Cu chain

By using first-principles calculations, we have systematically investigated the structural and electronic properties of an infinite linear monatomic Cu chain with an adsorbed CO molecule. We find that the bridge geometry is energeticabsally favored not only when the Cu–Cu bond below the molecule is unstretched, but also for a wide range of d_Cu–Cu up to about 4.20 Å, while the substitutional geometry is favored only in the hyperstretched situation d_Cu–Cu>4.80 Å. Charge density differences point out the electron transfer is from the Cu atoms to the adsorbed CO molecule. The binding mechanism of CO to Cu chain can be described by the Blyholder’s model, in terms of σ-donation of electron density from the nonbonding CO-5σ orbital into empty metal orbitals and π-backdonation from the occupied metal d orbitals to empty CO-2π^⁎ orbital. The donation/backdonation process leads to the formation of bonding/antibonding pairs, 5σ_b/5σ_a and 2π_b^⁎/2π_a^⁎, with the 5σ_a lying above E_f and the 2π_b^⁎ below E_f.     

An XPS and UPS study of the kinetics of carbon monoxide oxidation over Ag(111)

The oxidation of carbon monoxide over a Ag(111) catalyst has been studied by XPS and UPS. The kinetics have been determined over the temperature range of 180 to 400 K and found to be of the Langmuir-Hinshelwood type, although the Eley-Rideal mechanism is mimicked. A negative activation energy, ?1.7 kcal/mole, and a preexponential, 6 × 10^?18 cm², are found. The former corresponds to the difference in the activation energies for carbon monoxide desorption and for carbon monoxide oxidation (leading to CO₂ desorption). At 90 K, upon carbon monoxide exposure to the active oxygen precovered surface, the O ls and C ls spectral regions show the formation of CO₂-like and carbonate species; the latter is stable to at least room temperature. That is, at 90 K, the residence time and mobility of CO₂ formed at the surface permits a new surface reaction — the formation of stable surface carbonate. The identifications are based on C and O coverages and on line positions from the literature for Cu/CO₂ and several bulk carbonates. With UPS, the 1π_g, the unresolved doublet 1π_u and 3σ_g, and the 4σ_g molecular orbitals of adsorbed CO₂-like species are identified, as well as the unresolved triplet 1α′₂, 1e″ and 4e′ and the unresolved triplet 3e′, 1α″₂ and 4a′ molecular orbitals of the carbonate species. Surface CO₂-like species formed by surface oxidation of CO seem to be more strongly bound than reversibly adsorbed CO₂.     

Quenching of excited strontium atomic state measured in H2- and CO-flames

The efficiency of resonance fluorescence, Y, of the strontium resonance line (¹P₁→¹S₀ transition) at 4607.33 Å was measured in CO/N₂O, CO/O₂/Ar, and H₂/O₂/CO₂/N₂ flames at atmospheric pressure. From these data, the specific quenching cross sections, σ_qu, for CO₂ and CO were found to be (60 ± 10) Ų and  (300 ± 60) Ų, respectively. The experimental cross sections were confronted with the intermediate ionic-state curve-crossing model and chemical quenching model, respectively.     

Widths and shifts of some plasma-broadened oxygen and carbon multiplets

Profile measurements for some multiplets of neutral oxygen and carbon have been carried out in a wall-stabilized d.c. cascade arc. A 3.4 m Ebert spectrograph with a dispersion of 1.1 mm/Å was adapted to simultaneous photoelectric scanning of two sources and was used. Different gas mixtures (O₂ + H₂O, O₂ + He, CO₂ + Ar + H₂) were used for the production of plasmas. Arc currents between 25 and 100 A yielded temperatures between 10,200 and 13,570 K and electron densities from 3.17 to 7.84 x 10¹⁶cm^-3. Using the nonlinear least-squares method, theoretical profiles of different types were fitted to sets of experimental points, and broadening parameters of multiplet components were obtained for one multiplet of CI and four multiplets of OI in the visible spectral region. Our electron-impact half-widths and shifts are compared with theoretical values and other experimental data. Reasonable agreement between theoretical and experimental results are found for OI 3947 Å, OI 4368 Å, OI 5436 Å, and CI 5798 Å. For the multiplet OI 5330 Å, the available theoretical values differ considerably and the difference between experimentally determined and calculated shift is too large. Griem's theoretical half-width is close to the experimental value.     

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni₂CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the NiNi distance fixed at the bulk value. The minimum energy is found at a NiC distance of 1.72 Å and a CO bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm^?1) shows reasonable agreement with EELS data (1810, 1870 cm^?1), whereas the (Ni₂)C frequency of 495 cm^?1 is remarkably higher than the experimental values (380, 400 cm^?1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a₁orbital manifold exhibits strong antibonding effects.     

Electronic structure of amino acids and ureas

The outer electronic levels of glycine, alanine, glycine ethyl ester, urea, and thiourea have been investigated by means of photoelectron spectroscopy using He I (584 Å) and He II (304 Å) radiation and CNDO/2 molecular orbital calculations. In the amino acids the molecular orbital (MO) ordering ha been found to be [σ core and carbonyl π > a″(O) a′(O) > a″(N)] with a first ionization potential of ～8.8 eV. Glycine and alanine are foun to exist as the undissociated amino carboxylic acids rather than zwitterions in the high temperature vapor. In urea the three lowest energy molecular o are near-degenerate [σ(4b₁) ～ π(1a₂) ～ π(2b₂)] while in thiourea only the two lowest energy MO's are near-degenerate [π(1a₂ σ(4b₁) ～ π(2b₂)]. The first ionization potentials of urea and thiourea are 9.7 and 7.9 eV respectively.     

Cross sections for the quenching of the excited strontium state (51P1) measured in CO/N2O flames

The quantum efficiency of fluorescence, Y, of the 4607.33 Å Sr line $\text{(}{}^1\text{P}{}_1\text{?}{}^1\text{S}{}_0\text{transition}\text{)}$ was measured in four pre-mixed, laminar, shielded CO/N₂O flames of about 2700 K, with different quantitative compositions at 1 atm. From these data, the specific quenching cross sections for O₂, CO₂, CO and N₂ were found to be (152±20 Ų), (30±5 Ų), (49±8 Ų) and (16±3 Ų), respectively.     

Counter anion effect on structural,opto-electronic and charge transport properties of fused π-conjugated imidazolium compound

The structure–activity relationship of fused π-conjugated imidazolium cation with three counter anion molecules, BF₄^?, CF₃SO₃^? and (CF₃SO₂)₂N^?, was studied using electronic structure calculations. The structural, opto-electronic and charge transport properties of these complexes were studied. The charge transfer from π-conjugated imidazolium(I) to counter anion was confirmed in all the studied complexes. Interaction energy varies significantly depending on the counter anion and the stability was found higher for I-BF₄ complex than both I–CF₃SO₃ and I–(CF₃SO₂)₂N complexes. The strong (C–H)⁺···F^? hydrogen bond of length 1.95 Å between fused π-conjugated imidazolium and BF^?₄ anion is the driving force for the strongest interaction energy in I–BF₄ complex. The energy decomposition analysis confirms that the interaction between imidazolium and counter anion is mainly driven by electrostatic and orbital interaction. It has been observed that the absorption spectra of the complex are independent of anion nature but the influence of anion character is observed on frontier molecular orbital pattern. The charge transport property of I–BF₄ complex was studied by using tight-binding Hamiltonian approach and found that the hole mobility in I–BF₄ is 1.13 × 10^?4 cm² V^?1 s^?1.     

The adsorption of carbon monoxide on Cu(110)-Ni surfaces prepared by dissociation of nickel carbonyl

Cu(110)-Ni surface alloys were prepared by dissociation of nickel carbonyl on clean Cu(110). The adsorption of CO is reversible in the temperature region of 22–200°C and the pressure range of 5 × 10^?8-0.7 Torr, as monitored with ellipsometry and AES. The amount of adsorbed CO depends on the amount of preadsorbed oxygen but not on the amount of carbon present at the surface. The isosteric heat of adsorption decreases from 31 ± 3kcal/mole to 18 ± 2 kcal/mole with increasing CO coverage (up to θ = 0.14θ_max) but is constant for higher coverages (up to θ = 0.4θ _max).     

Cl2 adsorption on MgO(001) surface supported sodium monolayers: a density functional theory study

The adsorption of Cl₂ Na monolayers supported on the MgO(001) surface has been studied by the density functional method using cluster models embedded in a large array of point charges (PCs). The value of PCs was determined by charge self-consistent technique. The results indicate that Na-promoted MgO(001) surface is an efficient catalyst toward Cl₂ adsorptive decomposition. Besides, it was found that the role of the MgO(001) surface is not passive, which is different from CO adsorption on MgO(001) surface supported Na metal monolayers. The analysis of band and projected density of states indicates that the electron transfer from the surface Mg 3s valence orbital and Na 3s valence orbital to the anti-bonding σ^∗ orbital of Cl₂ is the source of the Cl₂ bond weakening. This is also different from the CO adsorption on MgO(001) surface supported Na metal monolayers, where only the electrons from the Na valence orbital are transferred to the anti-bonding π^∗ orbital of adsorbed CO. Our study suggests that the essence of catalysis is different for CO and Cl₂ adsorption on Na metal monolayers supported an MgO(001) surface.     

A tilted precursor for CO dissociation on the Fe(100) surface

Near edge X-ray absorption fine structure (NEXAFS) has been used to study the molecular orientation of the α₃ state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45° ± 10° with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 ± 0.02 Å in the α₃ state, relative to the other molecularly adsorbed CO states on this surface.     

Studies of molecular oxygen adsorbed on Cu surfaces

Molecular oxygen adsorbed on (110) and polycrystalline Cu surfaces has been investigated by UPS, XPS, AES, HREELS and LEED. Molecularly adsorbed O₂ on the (110) surface shows the characteristic three-peak He II spectrum due to π_g, π_u and σ_g orbitals, accompanied by an O-O stretching frequency at 660 cm⁻¹. On the polycrystalline Cu surface, adsorbed O₂ shows the three peak He II spectra with a considerably smaller separation between the π_u and σ_g band and two O-O stretching bands at 610 and 880 cm⁻¹. O₂ adsorbed on the Cu(110) surface gives rise to a (1 × 1) LEED pattern and characteristic K π¹π¹ transition in the Auger spectrum.     

Spektren und Winkelverteilungen der Photoelektronen von Atomen und Molekülen

The photoelectron energy spectra produced by the impact of 584 Å photons on the gases Kr, H₂, N₂, O₂, CO, NO and several alkanes are reported. The angular distribution of photoelectrons corresponding to the simultaneous formation of specific electronic states of the ion have been measured in the range 30°—130°. A preference for electron ejection in the direction of light propagation is shown in the formation of the electronic ground states of NO⁺ and O ₂ ⁺ . Both involve the ejection of an electron from aπ _g orbital. The onset energies for the various electronic states have been obtained with a resolution of ca. 40 meV. The relative transition probabilities for formation of various electronic states in a given molecular ion, as well as the Franck-Condon factors within specific states have been obtained. Similar experiments with the alkanes (methane, ethane, propane, and n-butane) reveal directly the existence of widely separated electron states (or grouping of states) in the ion, as well as the probability of forming these states. This energy distribution function, of vital importance to the study of the unimolecular decay of ions, could only be inferred previously by use of a questionable assumption.     

A pulsed EPR study of the catalytic oxidation of CO over Cu/SnO2

The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO₂ with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO₂ gel and (iii) impregnation onto calcined SnO₂. Two types of Cu(II) species were identified in these calcined Cu/SnO₂ materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the¹³C hyperfine coupling of enriched¹³CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of^{117, 119}Sn and¹³C and at twice these Larmor frequencies. Although these signals indicate that^{117, 119}Sn and¹³C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for^{117, 119}Sn an isotropic hyperfine constant,a _iso, of ±4.0 MHz and an anisotropic component,T _?, of ±2.0 MHz. Pulsed ENDOR spectra also showed^{117, 119}Sn signals which agree with the above values. The¹³C cross peaks yielda _iso=±1.0 MHz andT _?=±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO₂ after the adsorption of CO₂. Based on the same hyperfine couplings in the samples exposed to¹³CO and¹³CO₂ the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed.     

Accurate lifetime measurements for the noble gases by the electron beam alignment technique

Rotational rainbow scattering has been measured in the systems K- and Rb-CO₂ at collision energies 0.17≦E≦0.56 eV and CMS angles 110°≦?≦170°. The ellipsoidal hard shell anisotropy of the normal (repulsive) potentials is evaluated to be (c?a)=0.67Å in K?CO₂, (c?a)=0.63Å in Rb?CO₂, averaged over a slight energy dependence. Below an apparent activation thresholdE _a?0.23 eV, nearly equal in both systems, the scattering largely conforms to that of diatomic molecules, indicating linear, rigid rotor type behaviour as the dominant collisional response of triatomic CO₂. AboveE _a characteristic, eventually drastic distortions set in. They are probably caused by intermediary charge transfer which activates a complex, coupled vibrational, rotational motion inM ⁺-CO ₂ ^? , altering the outcome of the collisions altogether. The simple model of a partially absorptive, hard shell is used to find that some gross features of the distortions are consistent with distinct properties of the acceptingelectronic orbital 6a ₁ in CO ₂ ^? .     

Evaluation of Water Removal and Memory Effect in 13CO2 Breath Tests by Isotope Ratio Mass Spectrometry

The usefulness of different ways of water removal in off-line sample preparation of human breath samples for ¹³CO₂ breath tests was examined and compared. Cryogenic water trapping and water removal with common desiccants like silicagel blue, Mg(ClO₄)₂, and molecular sieves were checked for reliability and reproducibility. With silicagel blue and Mg(ClO₄)₂ memory effects for ¹³C content were observed. The use of molecular sieve 4 Å and 5 Å led to tremendous carbon isotope fractionation. Molecular sieve 3 Å was found to be an excellent alternative to the established use of Mg(ClO₄)₂ and of cryogenic water trapping.     

Thermo‐Raman spectroscopy of selected layered double hydroxides of formula Cu6Al2(OH)16CO3 and Zn6Al2(OH)16CO3

Raman spectroscopy using a hot stage was used to characterise layered double hydroxides (LDHs) of the formula (Cu,Zn)₆Al₂(OH)₁₆(CO₃)^·4H₂O. The spectra were used to assess the molecular assembly of the cations in the LDH structure. The sharp band at 1058 cm⁻¹for the Zn₆Al₂(OH)₁₆(CO₃)^·4H₂O is assigned to the ν₁CO₃²⁻ symmetric stretching mode. This band shifts to higher wavenumbers and is observed at 1103 cm⁻¹at 600 °C. It is proposed that metal carbonate species formed during the decomposition of the hydrotalcite structure is responsible for the increase in the band position. The Cu–Al hydrotalcite did not show the same trend. The symmetric stretching mode of carbonate is observed at around 1110 cm⁻¹, and at temperatures above 200 °C a shoulder appears at around 1210 cm⁻¹, suggested to be due to CuCO₃. Copyright © 2008 John Wiley & Sons, Ltd.     

The absorption cross sections of N2, O2, CO,NO, CO2, N2O,CH4, C2H4, C2H6 and C4H10 from 180 to 700Å

The absorption cross sections of N₂, O₂, CO, NO, CO₂, N₂O, CH₄, C₂H₄, C₂H₆, C₄H₁₀ have been measured photoelectrically in the 180–700 Å region using synchrotron radiation. The absorption cross sections in the region λ ≥ 500 Å was found to be structureless and to increase monotonically with wavelength for all gases. The positions of the structure observed in the 520–720 Å region for N₂, O₂, CO₂ and N₂O are consistent with the various Rydberg series reported by previous authors.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.