Laser spectroscopic monitoring of gas emission and measurements of the C/C isotope ratio in CO2 from a wood-based combustion

We report on the application of a compact and field-deployable instrument, based on a continuous-wave fiber-coupled Telecom external cavity diode laser, to measure the ¹³C/¹²C isotope ratio in CO₂ from a wood-based combustion. Carbon dioxide, the most important greenhouse gas, is a major product of combustion. The measurements of the ¹³C/¹²C isotopic ratio in CO₂ from combustion emission permit one to identify the CO₂ source and to study the temporal and spatial variations of pollution in the atmosphere. The average value of the ¹³CO₂/¹²CO₂ ratio is found to be (1.1011±0.0024)%. The corresponding δ-value relative to PDB standard is (−20.17±2.14)‰, which is in good agreement with the typical value of (−25±2)‰ for wood. Simultaneous monitoring of multiple species from gas emission has been performed using direct-absorption spectroscopy. The concentrations of C₂H₂, CO, CO₂ and H₂O were determined on the basis of integrated absorbance measured by least-squares fitting a Voigt lineshape to experimental absorption spectra.     

Acetic acid decomposition on a polycrystalline platinum surface

The decomposition of CH¹³₃COOH on a polycrystalline platinum surface has been examined at temperatures between 300 and 900 K during continuous exposure to CH¹³₃COOH at 7×10^-4 Torr. On an initially clean platinum surface ¹³CO, CO, ¹³CO₂, H₂ and adsorbed carbon-12 are the major reaction products. The adsorbed carbon eventually poisons completely the reactions that produce these products. For temperatures above approximately 800 K, the carbon overlayer that is formed is graphitic and saturates at a carbon adatom concentration of (2.6-3.5)×10¹⁵ cm^-2. The reaction quantities of ¹³CO and ¹³CO₂ that are produced depend both on the surface temperature and the carbon coverage. Lower temperatures and higher carbon coverages favor the production of ¹³CO. On the graphitized platinum surface, the catalytic dehydration of acetic acid to ketene and water proceeds at steady-state.     

Compact laser difference-frequency spectrometer for multicomponent trace gas detection

2 O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). Real-time detection of CO, N₂O, and CO₂ was performed in open air over a path length of 5 to 18 m. The feasibility of DFG spectroscopic measurement of the ¹³C/¹²C and ¹⁸O/¹⁷O/¹⁶O isotopic ratios in atmospheric carbon dioxide was also investigated. We report what to our knowledge is the first simultaneous spectroscopic measurement of all three isotopes of oxygen in ambient CO₂. Received: 14 August 1997/Revised version: 3 November 1997     

A Sensitive Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the ¹³CO₂/¹²CO₂-ratio in breath samples. A commercial NDIR spectrometer for CO₂ concentration measurements in industrial process control was modified using two separate optical channels for the ¹³CO₂ and ¹²CO₂ detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is ± 0.4‰ of the ¹³CO₂/¹²CO₂-ratio in a range of 2.5 to 5% of total CO₂. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operate and thus a true alternative to isotope ratio mass spectrometers (IRMS). Several biomedical applications with breath samples were demonstrated and were compared in very good agreement with IRMS.     

Tracer studies on the reaction path and kinetics of the Co oxidation over iridium

The reaction path and kinetics of the CO oxidation over polycrystalline indium were studied under low pressures by a transient isotope tracer method. Carbon monoxide is oxidized entirely through a Langmuir-Hinshelwood (LH) process, namely CO(a) + O(a) → CO₂. No evidence of an Eley-Rideal process, namely CO(g) + O(a) → CO₂, was observed. At constant temperatures the apparent rate constant of the LH process, i.e., the CO₂ formation rate divided by the product of the coverages of CO and oxygen, decreased rapidly with an increase in the total coverage. The kinetics was explained in terms of the boundary reaction model in which the oxidation took place predominantly outside or near the boundary of the CO(a) island.     

The adsorption of CO,H2, H2, CO2 and H2O on carburized and graphitized Ni(110)

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

A mechanistic char oxidation model consistent with observed CO2/CO production ratios

Reliable prediction of char conversion, heat release, and particle temperature during heterogeneous char oxidation relies upon quantitative calculation of the CO₂/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of char combustion have been calibrated against the temperature-dependence of the CO₂/CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO₂/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO₂/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions characteristic of pressurized, oxy-fuel, and conventional high-temperature char combustion, for which essentially no experimental information on the CO₂/CO production ratio is available. The results suggest that the CO₂/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO₂/CO production ratio results, an analytical procedure for calculating the CO₂/CO production ratio is presented.     

Die kinetik der reaktion zwischen no und co an polykristallinem platin bei niedrigen drucken: I. Messmethode und experimenteller Befund

The reaction between NO and CO on a polycrystalline platinum ribbon under stationary conditions is followed by mass-spectrometry using isotopically labelled ¹³CO. At temperatures from 300 to 1100 K the ratio of the partial pressure of reactants varies between 0.1 < $\text{P}{}_{\mathrm{<mtext>NO</mtext>}}\text{P}{}_{\mathrm{<mtext>CO</mtext>}}$ < 10, the value of total pressure in the reactor being 1.75 × 10^?4 or 4.2 × 10^?3 Torr. The main reaction to N₂ and CO₂ is accompanied by the parallel reaction to N₂O and CO₂ which attains under certain conditions up to 50% of the total reaction. The pronounced temperature dependence of the reaction rate leads to maximum values between 500 and 650 K, the corresponding values of the reaction probabilities are as high as 0.3.     

Electromotive force of a COCO2 sensor in COCO2H2H2O atmospheres and simultaneous determination of partial pressures of CO and CO2

The electromotive force (EMF) of the COCO₂ sensor using Na₂CO₃ and NASICON (Na₃Zr₂Si₂PO₁₂) as solid electrolytes has been examined in COCO₂Ar atmospheres. The EMF is related to the partial pressures of CO and CO₂ and proportional to log(P²_CO2P⁻¹_CO). The simultaneous use of the oxygen sensor of stabilized zirconia gives the EMF proportional to log(P_CO2P⁻¹_CO). The EMF's of two sensors permit to determine individually partial pressures of CO and CO₂. The existence of H₂ with high concentration does not affect the EMF's. This fact proves the applicability of the two-sensor system to the monitoring and the controlling of reducing atmospheres in industrial processes.     

Determination of 13CO2/12CO2 Ratio by IRMS and NDIRS

Abstract

Breath tests using ¹³C-labelled substrates require the measurement of ¹³CO₂/¹²CO₂ ratio in breath gas samples. Next to isotope ratio mass spectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS) can be used to determine the ¹³CO₂/¹²CO₂ ratio in expired breath. In this study we compared NDIRS- with IRMS-results to investigate whether the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1-¹³C-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good correlation between these two methods for a small number of samples as required for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This indicates the superiority of IRMS concerning ¹³CO₂-kinetics over longer time periods.     

Development of a compact quantum cascade laser spectrometer for field measurements of CO2 isotopes

We report the development of a field-deployable, pulsed quantum cascade laser spectrometer. The instrument is designed to measure the ¹³C/¹²C isotopic ratio in the CO₂ released from volcanic vents. Specific ¹²CO₂ and ¹³CO₂ absorption lines were selected around 4.3 m, where the P-branch of ¹²CO₂ overlaps the R-branch of ¹³CO₂ of the 00⁰1–00⁰0 vibrational transition. This particular selection makes the instrument insensitive to temperature variations. A dual-channel cell balances the two absorption signals. We provide details of the instrument design and a preliminary demonstration of its performance based on laboratory measurements of ¹⁶O¹²C¹⁶O and ¹⁶O¹²C¹⁸O. PACS 42.55.Px; 42.62.Fi; 07.88.+y     

Biomedical Application of an Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

A newly developed isotope selective nondispersive infrared (NDIR) spectrometer for the measurement of ¹³CO₂ and ¹²CO₂ concentrations in breath samples was applied as a low cost and very simple to operate alternative to isotope ratio mass spectrometry (IRMS). We used this device for several biomedical applications ([¹³C]urea breath test, [¹³C]leucine metabolism, [¹³C]methacetin catabolism of rats) and found that the results agree very well with IRMS.     

ESR study of pyrrhotite iron vacancies and the adsorption of CO and H2S

The ESR spectra were determined for pyrrhotite in vacuum, under CO, H₂S and a CO/H₂S mixture. The singlet signal observed in vacuum at room temperature is split into a doublet in H₂S and when heated in vacuum or CO. The g-values are dependent upon the nature of the adsorbing gas. The catalytic effect in H₂S-CO mixture is explained by a two step reaction: (i) H₂S⁺ and CO^- adsorption, (ii) interaction of chemisorbed H₂S⁺ with chemisorbed CO^- on the surface of catalyst to form active hydrogen.     

Relative line intensity measurement in absorption spectra using a tunable diode laser at 1.6 μm: application to the determination of 13CO2/12CO2 isotope ratio

The measurement of relative intensities in CO₂ combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 μm. The diode laser is stabilized with an external Fabry–Pérot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both ¹³CO₂ and ¹²CO₂ isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios are measured with an accuracy of about 0.5% in pure CO₂. Received: 9 June 2000 / Published online: 8 November 2000     

The effects of power ultrasound (24 kHz) on the electrochemical reduction of CO2 on polycrystalline copper electrodes

The electrochemical CO₂ reduction reaction (CO2RR) on polycrystalline copper (Cu) electrode was performed in a CO₂-saturated 0.10 M Na₂CO₃ aqueous solution at 278 K in the absence and presence of low-frequency high-power ultrasound (f = 24 kHz, P_T ~ 1.23 kW/dm³) in a specially and well-characterized sonoelectrochemical reactor. It was found that in the presence of ultrasound, the cathodic current (I_c) for CO₂ reduction increased significantly when compared to that in the absence of ultrasound (silent conditions). It was observed that ultrasound increased the faradaic efficiency of carbon monoxide (CO), methane (CH₄) and ethylene (C₂H₄) formation and decreased the faradaic efficiency of molecular hydrogen (H₂). Under ultrasonication, a ca. 40% increase in faradaic efficiency was obtained for methane formation through the CO2RR. In addition, and interestingly, water-soluble CO₂ reduction products such as formic acid and ethanol were found under ultrasonic conditions whereas under silent conditions, these expected electrochemical CO2RR products were absent. It was also found that power ultrasound increases the formation of smaller hydrocarbons through the CO2RR and may initiate new chemical reaction pathways through the sonolytic di-hydrogen splitting yielding other products, and simultaneously reducing the overall molecular hydrogen gas formation.     

Sensitive measurements of stable carbon isotopes of CO<Subscript>2</Subscript> with wavelength modulation spectroscopy near 2 μm

We performed highly sensitive measurements of stable carbon isotopes of CO₂ using wavelength modulation spectroscopy with a distributed feedback laser diode in the 2-μm wavelength range. Ro-vibrational transitions, which belong to the different combination bands, were selected to measure the ¹³CO₂/¹²CO₂ carbon isotope ratio. The δ ¹³C value was determined through the signals obtained in a Herriott-type multipass cell with an optical path length of 29.9 m. The limit of detection for CO₂ using our measurement system was 16±1 parts per billion by volume at the strongest absorption peak that is assigned to the 2ν ₁+ν ₃ R(16) line within the laser emitting frequency region. The 10-h long term precision was a δ ¹³C standard deviation of 0.24‰ (1σ) with the best suited line pairs of ¹²CO₂ and ¹³CO₂ and with careful temperature and pressure control in the cell. The 3-min response and high precision of this measurement allows for precise continuous measurements of stable carbon isotopes in ambient CO₂.     

GC-R-CF-MS method to determine the 13C abundance of gaseous combustion products in cigarette smoke

To determine the ¹³C abundance of combustion and break down products formed in cigarette smoke, especially CO and CO₂, a simple and fast analytical method is needed. Taking into account the knowledge about the determination of the natural ¹³C abundance in air, an online method—based on gas chromatography-reaction-continuous flow mass spectrometry (GC-R-CF-MS)—has been developed, which enables the determination of the ¹³C abundance of CO and CO₂ in the vapour phase of cigarette smoke with a relative standard deviation of≤0.5% in one analytical run. Additionally, in a second step, the ¹³C abundance of total volatile carbon can be determined.     

On the reaction mechanism of CO2 reforming of methane over a bed of coal char

CO₂ reforming of methane was studied over a bed of coal char in a fixed bed reactor at temperatures between 1073 and 1223 K and atmospheric pressure with a feed composition of CH₄/CO₂/N₂ in the ratio of 1:1:8. Experimental results showed that the char was an effective catalyst for the production of syngas with a maximum H₂/CO ratio of one. It was also found that high H₂/CO ratios were favoured by low pressures and moderate to high temperatures. These results are supported by thermodynamic calculations. A mechanism of seven overall reactions was studied and three catalytic reactions of CH₄ decomposition, char gasification and the Boudouard reaction was identified as being of major importance. The first reaction produces carbon and H₂, the second consumes carbon, and the third (the Boudouard reaction) converts CO₂ to CO while consuming carbon. Equilibrium calculations and experimental results showed that any water present reacts to form H₂ and carbon oxides in the range of temperatures and pressures studied. Carbon deposition over the char bed is the major cause of deactivation. The rate of carbon formation depends on the kinetic balance between the surface reaction of the adsorbed hydrocarbons with oxygen containing species and the further dissociation of the hydrocarbon.     

Laser-induced reactions of NO2 in the visible region

The 488 nm laser-induced reaction of NO₂ with CO has been investigated and the results computer-modeled. Two reaction mechanisms were considered:1) the direct reaction of vibronically excited NO₂ (NO ₂ ^* ) with CO to form CO₂, and 2), the reaction of an intermediate NO₃ radical formed by the reaction of NO ₂ ^* with NO₂ followed by NO₃+CONO₂+CO₂. The modeling results strongly support the former as the dominant mechanism.Federal Junior Fellow (1980–1984)     

Diode laser absorption spectrometry for 13CO2/12CO2 isotope ratio analysis: Investigation on precision and accuracy levels

Near-infrared laser spectroscopy is used to measure the ¹³C/¹²C isotope abundance ratio in gas phase carbon dioxide. The spectrometer, developed expressly for field applications, is based on a 2 μm distributed feedback diode laser in combination with sensitive wavelength modulation detection. It is characterized by a simplified optical layout, in which a single detector and associated electronics are used to probe absorptions of a pair of ¹³CO₂ and ¹²CO₂ lines, simultaneously in a sample, as well as a reference gas. For a careful investigation of the achievable precision and accuracy levels, we carried out a variety of laboratory tests on CO₂ samples with different isotopic compositions, calibrated with respect to the international standard material by means of isotope ratio mass spectrometry. The 1-σ accuracy of the ¹³CO₂/¹²CO₂ determinations, reported in the so-called δ notation, is about 0.5‰ (including both statistical and systematic errors), for δ-values in the range from -30‰ to +20‰. We show that the major source of systematic errors is a consequence of the non-linearity of the Lambert–Beer absorption law, and can be corrected for to a very high degree of accuracy. PACS 42.62.Fi; 42.55.Px; 33.20.Ea