J. Deppe’s research while affiliated with University of Göttingen and other places

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Publications (13)


ChemInform Abstract: A Kinetic Study of the Reaction of NH2 with NO in the Temperature Range 1400-2800 K
  • Article

April 2010

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8 Reads

ChemInform

Joachim Deppe

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Heinz Gg. Wagner

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.


Nonequilibrium Processes During Fe(CO) 5 Pyrolysis in a Shock Wave

January 2008

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10 Reads

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1 Citation

Zeitschrift für Physikalische Chemie

J. Deppe

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[...]

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H. Gg. Wagner

In this work the processes of nonequilibrium radiation and ionization in the weak shock waves (2 < M < 4) in argon or helium, containing a small admixture (0. 1-2%) of Fe(CO)(5) are experimentally studied. The spectra- and time-resolved measurements, performed using a ICCD camera (StreakStar II, LaVision GmbH) have shown, that the unresolved radiation spectra are situated in the range 400-700nm. The maximum of the spectra lies approximately at 615 nm. The radiation appeared immediately at the propagating shock front and lasted about 8-12 mu s. The following time-resolved measurements, performed using photomultipliers (at 615 +/- 10 nm) and calibrated electric probes have shown intensive peaks of emission and electric current with duration of few lis, that correlates with the characteristic time of active growth of iron clusters. The analysis of the results obtained allowed to conclude that the real mechanism of generation of the observed peaks of radiation and ionization is the instant dissociation of Fe(CO)(5) causing an active condensation process of a supersaturated vapor of iron atoms, which results in the formation of excited and ionized iron clusters.


Formation of Carbon Nanoparticle in Carbon Suboxide Pyrolysis behind Shock Waves

January 2002

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28 Reads

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12 Citations

Zeitschrift für Physikalische Chemie

The process of carbon nanoparticles formation following the pyrolysis of carbon suboxide C3O2 in the wide temperature range 1700-3700K behind shock waves was investigated by measuring the extinction and emission profiles in the visible and IR ranges of spectra. The temperature dependence of the optical density shows two bell-shaped curves with the maxima at 1600K and 3200K. It is remarkable, that the ratio of extinction in the IR range (1.31 μ) to HeNe-laser (633 nm) as well as the ratio of the IR extinction to emissivity show the same temperature behavior. The possible correlation between the observed optical properties of the forming carbon particles and the change of their size and structure is discussed.



Nonequilibrium Excitation of C2 Radicals during the Thermal Decomposition of C3 O2 behind Shock Waves

January 2001

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17 Reads

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4 Citations

Zeitschrift für Physikalische Chemie

C2 concentrations and populations of the ground and the first excited vibrational levels of electronically excited C2(d3Π) radicals during the thermal decomposition of C3O2 behind shock waves were monitored by emission and absorption measurements in the (0-0) and the (1-0) Swan-bands. Experiments were carried out in mixtures of 1% C3O2 + Ar and 1% C3O2 + He at T = 2100-3200 K and p = 1.5-7.5 bar. In the whole range of conditions studied an essential nonequlilibrium of electronic and vibrational states of nascent C2 radicals was observed. For analyzing possible formation and consumption reactions of the excited C2radicals, a convenient kinetic scheme of the C3O2 decomposition was developed, which considers both the recombinative pumping process and exchange and quenching reactions of the C2(d3Π, v = 0, 1) states.


Detection of 1CH2 Radicals in Hydrocarbon Pyrolysis Behind Shock Waves Using FM Spectroscopy

January 2001

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4 Reads

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5 Citations

Zeitschrift für Physikalische Chemie

Singlet methylene radical ((CH2)-C-1) concentrations were measured for the first time in the pyrolysis of methane (CH4) and ethane (C2H6) behind shock waves. The very sensitive frequency modulation (FM) spectroscopy, already established for sensitve detection of amino radicals (NH,,) [ 1, 2], was used for that purpose. Applying computer simulations using a complex reaction mechanism the experimental (CH2)-C-1 signals were fitted and rate coefficients of different reaction channels were obtained. For the reaction channel (1a) CH4 + M --> CH3 + H + M an extented Arrhenius expression of k(1a) = 6.5 (.) 10(18) (.) (T/298)-(1.70) (.)exp [-366kJmol(-1)/RT] cm(3) mol(-1) s(-1) (+/- 50%), and futhermore rate coefficients for (7) (CH2)-C-3 + CH3 --> H+C2H4 with k(7) = 3.2 (.) 10(13) cm(3) mol(-1) s(-1) (+/- 40%), (11) H-3(2) + H --> CH + H-2 with k(11) = 7.9 (.) 10(13) cm(3) mol(-1) s(-1) (+/- 40%), and for the intersystem crossing via (6) (CH2)-C-1 +M --> (CH2)-C-3 + M with k(6) = 1.40 (.) 10(10) - (T/K)(0.9) cm(3) mol(-1) s(-1) (+/- 40%) were determined. The experimental conditions ranged from 1900 to 4000K with corresponding pressures between 0.23 to 0.54bar.


Carbon particle formation and decay in two-step pyrolysis of carbon suboxide behind shock waves

December 2000

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44 Reads

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21 Citations

Proceedings of the Combustion Institute

The formation and decay of carbon particles following the pyrolysis of C3O2 was investigated behind reflected shock waves toward high temperatures. It is known that in hydrocarbon pyrolysis the temperature range for soot formation extends from about 1300 to 2200 K. That holds also for C3O2. Here, it could be observed that particle formation starts again above 2300 K and increases toward a maximum at around 3000 K, falling off steeply above 3450 K. This maximum is nearly as high as that of the low-temperature particle yield curve at around 1600–1700 K. At temperatures above 3450 K, the process of particle disappearance behind the reflected shock wave was observed, which seems to depend on the history of their formation behind the incident shock wave.



A kinetic study of the reaction of NH2 with NO in the temperature range 1400 2800 K

February 1999

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3 Reads

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29 Citations

Physical Chemistry Chemical Physics

The reaction (1) NH2+NO→products was investigated behind incident and reflected shock waves in the temperature range 1400–2800 K and densities ranging from 1.4 to 20×10-6 mol cm-3. The source of NH2 radicals was the thermal dissociation of hydrazine (N2H4). Time histories of NH2 (597.365 nm), NH (336.100 nm) and OH (308.417 nm) were detected by narrow linewidth laser absorption. The H and O atom formations were followed by means of atom resonance absorption spectroscopy (ARAS) at 121.5 nm and 130.5 nm. In addition, NH2 radicals were detected at very low concentrations using very sensitive frequency modulated (FM) spectroscopy. For the overall rate constant k1 in the temperature range 1400–2200 K an Arrhenius expression of k1=1.3×1013×exp(-35 kJ mol-1/RT) cm3 mol-1 s-1 (±60%) was determined. In connection with previous literature data a minimum of the overall rate constant was found around 1200 K. According to several other investigations, the reaction channel (1a) NH2+NO→N2+H2O clearly dominates the channel (1d) NH2+NO→N2+OH+H at temperatures lower than 1000 K. At room temperature a branching ratio α(T)=k1d/(k1a+k1d) of around 10% is reported. Our investigations of the products H and OH of the channel (1d) and the determination of the branching ratio show that the contribution of channel (1d) increases from around 50% at 1500 K to 80% at 2800 K. The other reaction channels investigated, (1c) leading to N2O+H2 and (1g) to HNO+NH, are of minor importance.


The Thermal Decomposition of NH 2 and NH Radicals

October 1998

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18 Reads

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34 Citations

Berichte der Bunsengesellschaft für physikalische Chemie

The formation of NH 2 and NH radicals as well as H and N atoms during the thermal decomposition of hydrazine was investigated over a wide temperature range. For the first time frequency‐modulated spectroscopy was used for the detection of NH 2 radicals behind shock waves. In comparison with the dual beam laser absorption technique, the detection limit has been improved by one and a half orders of magnitude. For reaction (5a) NH 2 + M → NH + H + M the rate constants were obtained in the temperature range from 2200 to 4000 K k 5a = (1.2 ± 0.5)10 ¹⁵ · exp[‐(318±10) kJ mol ⁻¹ / RT ] cm ³ /mol s and for reaction (6) NH + M → N + H + M in the temperature range from 2500 to 3400 K k 6 = (1.8 ± 0.8)10 ¹⁴ · exp[‐(313 ± 15)kJ mol ⁻¹ / RT ] cm ³ /mol s. The experimental apparent activation energies were found to be in good agreement with theoretical calculations and recently recommended enthalpies of formation for NH 2 and NH. The competing NH 2 decomposition channel (5b) NH 2 + M → N + H 2 + M was shown to be of minor importance (⩽ 5%). The kinetic behavior of the unimolecular decomposition of NH 2 is compared with that of H 2 0 and ³ CH 2 .


Citations (8)


... The NH 2 radical may dissociate at high temperature or react with other radicals. For the thermal dissociation (R3b), we rely on the data from Deppe et al. [34], which constitute the only direct measurements. Figure 5 compares the data, reversed through the equilibrium constant, with the rate constant proposed in the present work. ...

Reference:

Challenges in Kinetic Modeling of Ammonia Pyrolysis
The thermal decomposition of NH2 and NH radicals
  • Citing Article
  • January 1998

Berichte der Bunsengesellschaft für physikalische Chemie

... As we will show, the technique of code division multiple access (CDMA) partners naturally with FMS to add the capacity of multiplexing, yet the two had not been combined previously, to our knowledge. While FMS techniques have flourished in applications of diode-laser spectroscopy, the principles of CDMA multiplexing apply quite generally to FMS over its broad spectral practice, ranging from the microwave [3,4], the midand near-infrared [5][6][7][8], to visible transitions [9][10][11], and the ultraviolet [12][13][14][15][16]. Under CDMA, transitions, whether from different species, isotopes, or the same absorber, are assigned unique codes, and codes may arrive at the receiver at various lag times. ...

Detection of 1CH2 Radicals in Hydrocarbon Pyrolysis Behind Shock Waves Using FM Spectroscopy
  • Citing Article
  • January 2001

Zeitschrift für Physikalische Chemie

... The measured particle yield did not depend on the C 3 O 2 mole fraction and had a maximum at approximately 1600 K for all mixtures. Recently, a series of measurements have been carried out to study the particle formation at a higher temperature range (from 2000 K to 3700 K) [6][7][8][9]. It was found that at the measured time of 1 ms after the reflected shock wave, the temperature dependence of the particle yield showed double bell-shaped curves with two local maxima at 1600 K and 3200 K. ...

Formation of Carbon Nanoparticle in Carbon Suboxide Pyrolysis behind Shock Waves
  • Citing Article
  • January 2002

Zeitschrift für Physikalische Chemie

... The overexcitation of growing clusters is also a weakly known process. Probably the first observation of this effect was reported in [10] where strong overexcitation of the smallest carbon clusters C 2 during the decomposition of carbon carbonyl (carbon suboxide) C 3 O 2 behind the shock wave had been observed. ...

Nonequilibrium Excitation of C2 Radicals during the Thermal Decomposition of C3 O2 behind Shock Waves
  • Citing Article
  • January 2001

Zeitschrift für Physikalische Chemie

... The measured particle yield did not depend on the C 3 O 2 mole fraction and had a maximum at approximately 1600 K for all mixtures. Recently, a series of measurements have been carried out to study the particle formation at a higher temperature range (from 2000 K to 3700 K) [6][7][8][9]. It was found that at the measured time of 1 ms after the reflected shock wave, the temperature dependence of the particle yield showed double bell-shaped curves with two local maxima at 1600 K and 3200 K. ...

High-Temperature Carbon Particle Formation and Decay in Carbon Suboxide Pyrolysis behind Shock Waves
  • Citing Article
  • January 2000

Zeitschrift für Physikalische Chemie

... However, the first electronic state of CH is associated with the 1 ′ surface which explains the difference in the comparison. Finally, for NH, the comparison is done with shock tube experiments carried out by Deppe et al. [39], Fig. 7(c). The experiments are carried out in a premixed ammonia-oxygen flame which serves as a source for NH 2 and NH. ...

The Thermal Decomposition of NH 2 and NH Radicals
  • Citing Article
  • October 1998

Berichte der Bunsengesellschaft für physikalische Chemie

... The diverse applications of iron nanoparticles stimulate the development of various methods for their production. One of the prospective ways of iron nanoparticle synthesis is the condensation of supersaturated iron vapour formed during the thermal decomposition of iron carbonyl Fe(CO) 5 [1]. To study this process, shock wave investigations of Fe(CO) 5 pyrolysis are quite informative. ...

Nonequilibrium Processes During Fe(CO) 5 Pyrolysis in a Shock Wave
  • Citing Article
  • January 2008

Zeitschrift für Physikalische Chemie

... Comparisons of the branching ratio (k 3 /(k 3 +k 4 )) from different sources. Symbols and lines denote the measured results [22,[52][53][54][55][56][57][58][59][60][61] and the theoretical calculation results [41][42][43][44] in the previous investigations, respectively. ...

A kinetic study of the reaction of NH2 with NO in the temperature range 1400 2800 K
  • Citing Article
  • February 1999

Physical Chemistry Chemical Physics