oxidation chemistry of carbon disulfide (cs and its...
TRANSCRIPT
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Oxidation chemistry of carbon disulfide (CS2)
and its interaction with hydrocarbons in
combustion processes
Zhe Zeng, BEng
This thesis is presented for the degree of
Doctor of Philosophy
School of Engineering and Information Technology,
Murdoch University, Western Australia
2017
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i
Statement of originality
I declare that this thesis is my own account of my research and contains as its main content
work which has not previously been submitted for a degree at any tertiary education institution.
The thesis contains no material previously published or written by another person, except
where due reference has been made in text.
Zhe Zeng
June, 2017
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Supervisory statement
We, the undersigned, attest that Higher Research Degree candidate, Zhe Zeng, has devised and
synthesised the experimental program, conducted experiments, analysed data, performed
computational quantum-mechanical calculations and has written all papers included in this
thesis. Professor Bogdan Z. Dlugogorski and Dr Mohammednoor Altarawneh provided the
necessary advice on the experimental program, project direction and assisted with the editing
of the papers, consistent with normal supervisors-candidate relations.
Professor Bogdan Z. Dlugogorski
June, 2017
Dr Mohammednoor Altarawneh
June, 2017
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Acknowledgment
I would like to thank my supervisors Professor Bogdan Z. Dlugogorski and Dr Mohammednoor
Altarawneh for their unwavering and exceptional support. Their understanding, mentorship,
encouragement, dedication and magnanimity have provided a good fundamental driving force
for the present study. Thank you again, for your quick correspondence, corrections and
gestures of support.
I am grateful to Murdoch University for the award of a postgraduate research scholarship which
provided a valuable financial support for this research. This study has also been funded by the
Australian Research Council (ARC), with grants of computing time from the National
Computational Infrastructure (NCI) and the Pawsey Supercomputing Centre in Perth,
Australia.
I acknowledge my gratitude to my fellow student colleagues and staffs of the Fire Safety and
Combustion Kinetics Research Laboratory: Ibukun Oluwoye, Niveen Assaf, Nassim Zeinali,
Kamal Siddique, Arif Abdullah, Jomana Al-Nuairat, Sidra Jabeen, Dr Anam Saeed, Dr Juita
and Dr Jakub Skut. Courtesy of you all, I experienced an exciting and fun-filled years of
computation and experiments.
Special thanks to my wife Anqi Wang, who supported me through the most difficult period of
my research. This experience is the most memorable and valuable days in my life.
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Abstract
This thesis presents a series of scientific studies exploring the oxidation chemistry of carbon
disulfide (CS2) and its interaction with hydrocarbons in combustion systems. The results
illustrate the extreme flammability of CS2 even at low temperature (self-ignition temperature
at 363 K under ambient pressure). The thesis proposes a comprehensive oxidation mechanism
that works over a wide range of atmospheric and combustion conditions with and without
moisture. The thesis also provides experimental validation on the promotion of CS2 on the
ignition of methane.
Experiments involving CS2 oxidation in combustion processes have been conducted with
tubular-flow (TFR) and jet-stirred (JSR) reactors to provide experimental validation for the
proposed mechanism. Online Fourier transform infrared (FTIR) spectroscopy served to
identify and quantitate the product species and to obtain the detailed species conversion profiles
during the combustion process. Low ignition temperature of CS2 of 860 K in TFR and 710 K
JSR, with residence time at 0.3 s under ambient pressure, implies the extreme flammability of
CS2. The presence of moisture exhibits no effect on the oxidation of CS2 for ignition
temperature and species profile at below 1100 K, although moisture converts CO into CO2 at
higher temperature (> 1200 K). Co-oxidation experiments of CH4/CS2/O2 in JSR illustrate the
promotion effect of CS2 on the ignition of methane.
Quantum calculations afford the investigation of primary steps governing the OH-initiated
oxidation of CS2 in the atmosphere. We also propose a comprehensive oxidation mechanism
of CS2 in combustion systems. The thesis suggests the intersystem crossing (ISC) between
triplet and singlet pathways to explain the extreme flammability of CS2, in analogy to the
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controlling steps operating for other reduced sulfur species. DFT calculations examine the
interplay between the SH radical and C1 - C4 hydrocarbons to demonstrate the distinct
inhibition and promotion effects of H2S/SH on alkanes and alkenes/alkynes in pyrolysis
processes. The findings reported in this study apply to both atmospheric and combustion
systems. Especially, the developed mechanism provides improved understanding of the
oxidation of fossil fuels containing sulfur species.
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Table of contents
Statement of originality ............................................................................................................ i
Supervisory statement ............................................................................................................. ii
Acknowledgment .................................................................................................................... iii
Abstract .................................................................................................................................... iv
Table of contents ..................................................................................................................... vi
List of publications ................................................................................................................. xii
Chapter 1. Introduction .......................................................................................................... 1
1.1. Research Background ...................................................................................................... 2
1.2. Project objectives and thesis outline ............................................................................... 3
Reference ................................................................................................................................ 7
Chapter 2. Literature review ................................................................................................ 10
2.1. Introduction ................................................................................................................... 11
2.2. Oxidation of H2S ........................................................................................................... 14
2.2.1. Atmospheric oxidation of H2S ................................................................................ 14
2.2.2 Combustion mechanism of H2S ............................................................................... 20
2.3. Oxidation of CS2 ........................................................................................................... 27
2.3.1. Atmospheric oxidation of CS2 ................................................................................ 28
2.3.2. Combustion mechanism of CS2 .............................................................................. 32
2.3.3. Production of S atom in excited state in photolysis of reduced sulfur species ....... 40
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2.4. Interaction between reduced sulfur species and hydrocarbons ..................................... 43
2.4.1. Influence of H2S/SH on the combustion process of hydrocarbons ........................ 44
2.4.2. Impact of SO2 on ethylene pyrolysis ...................................................................... 46
2.5. Conclusion ..................................................................................................................... 48
Reference .............................................................................................................................. 52
Chapter 3. Methodology ........................................................................................................ 66
3.1. Experimental methodology ........................................................................................... 67
3.1.1. Gas feeding system ................................................................................................. 67
3.1.2. Tubular-flow reactor (TFR) .................................................................................... 68
3.1.3. Jet-stirred reactor (JSR) .......................................................................................... 69
3.1.4. Online analytical technique .................................................................................... 71
3.2. Computational methodology ......................................................................................... 72
3.2.1. Quantum chemistry calculation .............................................................................. 72
3.2.2. Kinetic modelling ................................................................................................... 77
Reference .............................................................................................................................. 79
Chapter 4. Atmospheric oxidation of carbon disulfide (CS2) ............................................ 80
Abstract ................................................................................................................................ 81
4.1. Introduction ................................................................................................................... 82
4.2. Computational Methodology ........................................................................................ 84
4.3. Results and Discussion .................................................................................................. 85
4.3.1. CS2 + OH ................................................................................................................ 85
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4.3.2. S-adduct SCS(OH) + O2 ......................................................................................... 92
4.3.3. C-adduct SC(OH)S + O2 ........................................................................................ 96
4.4. Conclusion ..................................................................................................................... 98
Acknowledgement ................................................................................................................ 98
Reference .............................................................................................................................. 99
Chapter 5. Inhibition and promotion of pyrolysis by hydrogen sulfide (H2S) and
sulfanyl radical (SH) ............................................................................................................ 102
Abstract .............................................................................................................................. 103
5.1. Introduction ................................................................................................................. 104
5.2. Computational Details ................................................................................................. 106
5.3. Results and discussion ................................................................................................. 107
5.3.1. Overview of labile H abstraction sites in C1-C4 hydrocarbons ............................ 107
5.3.2. H abstraction from alkanes ................................................................................... 111
5.3.3. H abstraction from alkenes and alkynes ............................................................... 118
5.3.4. Validation of CBS-QB3 calculations ................................................................... 121
5.3.5. Relationship between BDH of abstracted CH bond and activation enthalpy of SH
+ hydrocarbon reaction ................................................................................................... 123
5.3.6. Activity of SH radical compared with those of OH, NH2 and HO2 ..................... 125
5.4. Conclusion ................................................................................................................... 126
Appendix ............................................................................................................................ 127
Acknowledgement .............................................................................................................. 127
Reference ............................................................................................................................ 128
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Chapter 6. Flammability of CS2 and other reduced sulfur species ................................. 132
Abstract .............................................................................................................................. 133
6.1. Introduction ................................................................................................................. 134
6.2. Methodology ............................................................................................................... 137
6.2.1. Experiments with tubular flow reactor ................................................................. 137
6.2.2. Kinetic modelling ................................................................................................. 140
6.3. Results and discussion ................................................................................................. 141
6.3.1. Experimental results from tubular flow reactor .................................................... 141
6.3.2. Sensitivity analysis of CS2 consumption .............................................................. 146
6.3.3. CS2/O2 subset ........................................................................................................ 147
6.3.4. S/O2 subset ............................................................................................................ 153
6.3.5. Updated mechanism and modelling validation of our experimental results ........ 155
6.4. Conclusion ................................................................................................................... 156
Acknowledgement .............................................................................................................. 157
Reference ............................................................................................................................ 158
Chapter 7. Combustion chemistry of carbon disulfide (CS2) .......................................... 164
Abstract .............................................................................................................................. 165
7.1. Introduction ................................................................................................................. 166
7.2. Experimental ............................................................................................................... 168
7.3. Kinetic Modelling ....................................................................................................... 171
7.4. Results and Discussion ................................................................................................ 176
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7.4.1 Experimental results with JSR ............................................................................... 176
7.4.2. Kinetic modelling of dry oxidation of CS2 and revision of COS/O2 subset ......... 180
7.4.3. The influence of moisture on CS2 oxidation ........................................................ 187
7.5. Conclusion ................................................................................................................... 191
Acknowledgement .............................................................................................................. 192
Appendix ............................................................................................................................ 192
Reference:........................................................................................................................... 193
Chapter 8. Enhanced oxidation of CH4 in presence of CS2 ............................................. 198
Abstract .............................................................................................................................. 199
8.1 Introduction .................................................................................................................. 200
8.2 Methodology ................................................................................................................ 202
8.2.1 Experimental set-up ............................................................................................... 202
8.2.2 Kinetic modelling .................................................................................................. 204
8.3 Results and discussion .................................................................................................. 204
8.3.1 Sole oxidation of CS2/O2 and CH4/O2 ................................................................... 204
8.3.2 Co-oxidation of CS2/CH4/O2 ................................................................................. 206
8.3.3 Kinetic modelling of the co-oxidation of CS2/CH4/O2 .......................................... 209
8.4 Conclusion .................................................................................................................... 213
Reference ............................................................................................................................ 214
Chapter 9. Conclusion and recommendation .................................................................... 217
9.1. Conclusion ................................................................................................................... 218
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9.2 Recommendation .......................................................................................................... 220
Appendix A for chapter 5 .................................................................................................... 223
Appendix B for chapter 7 .................................................................................................... 243
Appendix C for risk assessment.......................................................................................... 274
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List of publications
Journal articles
1. Z. Zeng, M. Altarawneh, B. Z. Dlugogorski, Atmospheric oxidation of carbon
disulfide (CS2), Chem. Phys. Lett., 669 (2017) 43-48.
2. Z. Zeng, M. Altarawneh, I. Oluwoye, P. Glarborg, B. Z. Dlugogorski, Inhibition and
promotion of pyrolysis by hydrogen sulfide (H2S) and sulfanyl radical (SH), J. Phys.
Chem. A, 120 (2016) 8941-8948.
3. Z. Zeng, B. Z. Dlugogorski, M. Altarawneh, Flammability of CS2 and other reduced
sulfur species, Fire Saf. J., in press, DOI: https://doi.org/10.1016/j.firesaf.2017.03.073.
Conference papers
1. Z. Zeng, M. Altarawneh, B. Z. Dlugogorski, Reactions of SH radical with C1-C4
Hydrocarbons. Proceedings of the Australian Combustion Symposium, 2015,
Melbourne, Australia
2. Z. Zeng, B. Z. Dlugogorski, M. Altarawneh, Flammability of CS2 and Other
Reduced Sulfur Species. 12th International Symposium on Fire Safety Science (the
12th IAFSS Symposium), 2017, Lund, Sweden
3. Z. Zeng, M. Altarawneh, B. Z. Dlugogorski, Enhanced oxidation of methane by
CS2. Asia-Pacific Conference on Combustion, 2017, Sydney, Australia.
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Chapter 1. Introduction
1
Chapter 1
Introduction
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Chapter 1. Introduction
2
1.1. Research Background
Sulfur bearing compounds represent a group of major impurities in most organic fuels [1, 2].
In combustion, sulfur transforms primarily into oxide species, i.e., SOx, acting as the critical
source for a series of atmospheric air contaminants. The typical sulfur content in coal varies
from 0.4 - 4.0 wt. % [3]. In commercial diesel and gasoline, sulfur is limited to 500 ppm and
30 ppm, respectively [4]. Likewise, pipeline-quality natural gas usually contains sulfur at
levels of around 2,000 ppm [5]. In typical biomass, the content of sulfur varies from 0.02 - 0.3
wt. % [2]. The enormous consumption of fossil fuels in oxidation processes, e.g., in power
plants, engines and stoves, results in significant emission of sulfur oxides, especially in
countries that do not require or enforce the emissions of SOx, limiting the exploitation of fossil
and biomass fuels.
Sulfur dioxide (SO2) constitutes the most notorious sulfur compound emitted from combustion
of fossil fuels. However, under fuel rich combustion conditions or during thermal pyrolysis of
fuels, the product gases may also contain reduces sulfur species, such as hydrogen sulfide
(H2S), carbon disulfide (CS2) and carbonyl sulfide (COS). Gas processing plants and oil
purification and gasification processes produce H2S as a side product. While CS2 and COS
evolve as major by-products in Claus gas-desulfurising process [6, 7], which converts H2S into
elementary solid sulfur, volcanic eruptions and bushfires [8] also contribute to the total
atmospheric budget of CS2 and COS. CS2 exists at levels of parts per trillion (ppt) in the earths
troposphere; 15 to 30 ppt in the nonurban troposphere and 100 to 200 ppt in polluted urban
areas [9]. On the other hand, COS stands as the most abundant sulfur compound in the
atmosphere (> 400 ppt) with a relatively long life-time of up to years [10].
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Chapter 1. Introduction
3
Existing as a volatile liquid at room temperature (25 oC), CS2 represents a non-polar solvent
which has found numerous applications in laboratories and industry, such as extractive
metallurgy. Likewise, the production of synthetic fibres, rubbers and pesticides requires CS2
and COS as feedstocks. Unfortunately, CS2 displays extreme flammability and high explosion
propensity [11], properties responsible for fires in laboratories and chemical warehouses [12,
13].
Apart from H2S [14], the oxidation mechanism and combustion properties of CS2 and COS
remain poorly understood. Lack of appropriate reaction mechanisms prevent detailed
modelling of important industrial plants, such as the Claus process or separating sulfur
impurities in the feedstocks in oil refineries [15]. To gain insight into the extreme flammability
of CS2 and the role of sulfur compounds in oxidation of fuels, one needs not only the
mechanisms of oxidation of CS2 and COS, but also the mechanisms describing the interaction
between sulfur and hydrocarbon species.
1.2. Project objectives and thesis outline
The general objective of this study is to construct a comprehensive oxidation mechanism for
CS2 and to investigate the reactions between sulfur species and hydrocarbons. Specifically,
this thesis intends to:
1. Perform accurate quantum chemistry calculations to improve the existing mechanism
of atmospheric oxidation of CS2
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Chapter 1. Introduction
4
2. Collect precise measurements of CS2 oxidation under combustion condition using
tubular-flow (TFR) and jet-stirred (JSR) reactors
3. Investigate the extreme flammability and low ignition temperature of CS2 with quantum
chemistry calculations and propose further enhancements to the oxidation mechanism
for CS2
4. Validate the proposed mechanism against the experimental results from TFR and JSR
through kinetic modelling
5. Study the interaction between SH radical and C1 C4 hydrocarbons with density
function theory (DFT) calculations, and
6. Explore the effect of CS2 addition on the ignition of methane with careful JSR
experiments.
To address the above aims, the thesis has the following structure:
Chapter 2 reviews the literature pertinent to experiments and reaction kinetics of reduced sulfur
species. We highlight the occurrence of intersystem crossing (ISC) in H2S and S/SO oxidation
processes. ISC is a transition between two electronic states with different spin multiplicities
but same energy level. In the case of sulfur chemistry, the oxidation reaction will start with
triplet oxygen molecule and transit to singlet reaction pathway through the cross-over point on
the potential energy surfaces, offering an alternative reaction corridor with much lower
activation barriers. For CS2 oxidation, the high activation energy of triplet oxidation pathway
fails to explain the low ignition temperature for the experiments conducted with the tubular-
flow reactor, later prompting us to introduce the ISC to the CS2 combustion mechanism.
Photolysis of reduced sulfur species (H2S, CS2 and COS) also leads to the formation of singlet
sulfur atom (1S). This chapter also examines the interaction between sulfur species and
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Chapter 1. Introduction
5
hydrocarbons, summarising 1) influence of H2S/SH on hydrocarbons combustion process and
2) impact of SO2 on the pyrolysis of ethylene.
Chapter 3 covers the experimental and theoretical methodologies applied in this project. It
depicts the experimental set-up constructed in this work to study the oxidation of CS2 and its
interaction with hydrocarbons, including the reactant feeding system, details of TFR and JSR
and online analytical measurements. This chapter also comprises a description of the quantum
chemistry calculations and kinetic modelling, including the software used in this thesis:
Gaussian 09, MESMER 3.0, ChemRate, KiSTheIP and Chemkin Pro.
Chapter 4 investigates primary steps governing the OH-initiated atmospheric oxidation of CS2
with theoretical calculations. In the absence of surface effects, we find the overall reaction OH
+ CS2 COS + SH too slow to account for the formation of the reported experimental
products. The S-adduct represents the most plausible product formed by addition of OH to
CS2. The adduct then undergoes a bimolecular reaction with atmospheric O2 yielding COS,
OH and SO. The kinetic analysis developed in this study explains the atmospheric fate of
reduced sulfur species, an important group of compounds in the global S cycle.
Chapter 5 resolves the interaction of sulfanyl radical (SH) with aliphatic (C1 - C4)
hydrocarbons, using CBS-QB3 based quantum chemistry calculations. Our findings
demonstrate that, the documented inhibition effect of hydrogen sulfide (H2S) on pyrolysis of
alkanes does not apply to alkenes and alkynes. During interaction with hydrocarbons, the
inhibitive effect of H2S and promoting interaction of SH radical depend on the reversibility of
the H abstraction processes. In the case of methane, we conclude that, the reactivity of SH
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Chapter 1. Introduction
6
radicals towards abstracting H atoms exceeds that of HO2 but falls below those of OH and NH2
radicals.
Chapter 6 discusses the experimental results of CS2 oxidation using a tubular-flow reactor
under dry conditions for an oxygen-fuel equivalence ratio of 0.7, 1.0 and 1.2 and for a
temperature range of 700 to 1200 K. Compared with moist oxidation, the reaction forms CO
as a final product, as we detected no CO2 in the exhaust gases. A sensitivity analysis confirmed
a significant influence of the CS2/O2 and S/O2 sub-mechanisms on the oxidation process.
Subsequent theoretical calculations identify the intersystem crossing between triplet and singlet
pathways that lowers the activation energy of the key elementary reactions, as the reason for
the extreme flammability of CS2. This chapter also presents the analysis of the nature of the
intersystem crossing and its impact on the low ignition temperature of CS2, and updates the
CS2/O2 and S/O2 reaction subsets for the dry oxidation mechanism of CS2. The results of the
kinetic modelling and the experimental measurements demonstrate good agreement.
Chapter 7 introduces an experimental study of dry and moist oxidation of CS2 in JSR and
develops a comprehensive oxidation mechanism with updated COS/O2 subset. The dry and
moist oxidation of CS2 exhibit the same conversion profile for each species in experiments
performed in the temperature range of 650 1100 K under the atmospheric pressure. In
comparison, kinetic modelling with the moist oxidation mechanism proposed in this work
predicts the conversion of CO to CO2 at temperatures in excess of 1200 K. The fast conversion
of COS in experiments, in which this species arises as an intermediate, forces us to study in
detail the consumption channels for COS. We propose the intersystem crossing process to
occur in COS/O2 subset, in analogy to the ISC appearing for other species in the CS2/S/O2
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Chapter 1. Introduction
7
system, to reduce the reaction barrier. Good agreement emerges between measured and
modelled onset temperature and CS2 consumption, confirming the robustness of the model.
Chapter 8 explores the impact of added CS2 in the oxidation of CH4. Co-oxidation experiments
of CH4/CS2/O2 in JSR illustrate the promotion effect of CS2 on the ignition of methane in
natural gas. However, the presence of CH4 significantly elevates the oxidation temperature of
CS2, indicating the inhibition effect of CH4 on CS2 oxidation.
Chapter 9 presents the concluding remarks of the thesis and provides suggestions for future
studies on chemistry of reduced sulfur species.
Reference
[1] C.F. Cullis, M.F.R. Mulcahy, The kinetics of combustion of gaseous sulphur compounds,
Combust. Flame 18 (1972) 225-292.
[2] A. Williams, J.M. Jones, L. Ma, M. Pourkashanian, Pollutants from the combustion of solid
biomass fuels, Prog. Energy Combust. Sci. 38 (2012) 113-137.
[3] G.P. Huffman, S. Mitra, F.E. Huggins, N. Shah, S. Vaidya, F. Lu, Quantitative analysis of
all major forms of sulfur in coal by x-ray absorption fine structure spectroscopy, Energy &
Fuels 5 (1991) 574-581.
[4] Department of Environment and Energy, Fuel Standard Determination 2001, Government
of Australia, 2001.
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Chapter 1. Introduction
8
[5] P. Jaramillo, W.M. Griffin, H.S. Matthews, Comparative life-cycle air emissions of coal,
domestic natural gas, LNG, and SNG for electricity generation, Environ. Sci. Technol. 41
(2007) 6290-6296.
[6] K. Karan, A.K. Mehrotra, L.A. Behie, A high-temperature experimental and modeling
study of homogeneous gas-phase COS reactions applied to Claus plants, Chem. Eng. Sci. 54
(1999) 2999-3006.
[7] K. Karan, L.A. Behie, CS2 formation in the Claus reaction furnace: a kinetic study of
methanesulfur and methanehydrogen sulfide reactions, Ind. Eng. Chem. Res., 43 (2004)
3304-3313.
[8] P. Warneck, The biogeochemical cycling of sulfur and nitrogen in the remote atmosphere,
EOS, Trans. Am. Geophys. Union, 68 (1987) 240-240.
[9] V. Gardiner, Global Tropospheric ChemistryA Plan for Action, National Academy Press,
1985. 110-111.
[10] M.P. Barkley, P.I. Palmer, C.D. Boone, P.F. Bernath, P. Suntharalingam, Global
distributions of carbonyl sulfide in the upper troposphere and stratosphere, Geophys. Res. Lett.
35 (2008) L14810.
[11] F.R. Taylor, A.L. Myerson, First limit induction time studies of CS2O2 explosions, Symp.
(Int.) Combust. [Proc.] 7 (1958) 72-79.
[12] American Industrial Hygiene Association, Carbon Disulfide (CS2) Fire.
https://www.aiha.org/get-involved/VolunteerGroups/LabHSCommittee/Pages/Carbon-
Disulfide-Fire.aspx (accessed 05/05 2017).
[13] G.M. Bodner, Lecture demonstration accidents from which we can learn, J. Chem. Educ.
62 (1985) 1105.
[14] F.G. Cerru, A. Kronenburg, R.P. Lindstedt, Systematically reduced chemical mechanisms
for sulfur oxidation and pyrolysis, Combust. Flame 146 (2006) 437-455.
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Chapter 1. Introduction
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[15] F. Viteri, M. Abin, . Millera, R. Bilbao, M.U. Alzueta, EthyleneSO2 interaction under
sooting conditions: PAH formation, Fuel 184 (2016) 966-972.
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Chapter 2. Literature review
10
Chapter 2
Literature review
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Chapter 2. Literature review
11
2.1. Introduction
Sulfur stands as an essential impurity in fossil fuels, biofuels and municipal wastes [1, 2].
Taking up to several percentage by weight in fuels, sulfur gives rise to industrial problems on
a large scale, especially as an atmospheric pollutant [3]. In nature, sulfur occurs in the
pure element form of sulfide and sulfate in minerals. Once released into air through oxidation,
the mainly gaseous product - sulfur oxide (SOx) is in the form of sulfur dioxide (SO2) [4],
which is an important greenhouse gas as well as the source of acid rain. Anthropogenic
emissions of sulfur into the troposphere peaked during the year 1972 at about 131 million
tonnes [5]. By 2011, due to the world-wide efforts to control air pollution, the emission of
sulfur species has been reduced to 106 million tonnes [6]. More than 85 % of sulfur compounds
originate from fuel combustions in power plants or other devices [7]. As a result, sulfur
chemistry draws a lot of attention among researchers in both energy and environmental
engineering.
Most of the current research on sulfur combustion focuses on the industrial thermal conversion,
oxidation of H2S and emission of SO2 as air pollutant. H2S emerges as the principal sulfur
carrier, in the gasification process of coal [8]. In oil industries, sulfur compounds such as thiols,
thiophenes, sulphides and disulfides (0 3 wt. %), are converted to H2S, before they could
poison the cracking catalysts for hydrocarbons [9]. H2S is also commonly present in natural
gas [10]. To remove H2S, amine treatment is applied to crude oil [11] while cation containing
materials, such as limestone (Ca2+) and Magnesium oxide (Mg2+), are sprayed into coals [12].
The multi-step Claus process can convert sulfur into the solid state from gaseous H2S in natural
gas and other exhausts containing H2S, derived from refining crude oil and other industrial
processes [13]. The oxidation of H2S releases SO2, known as the most notorious sulfur
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Chapter 2. Literature review
12
compound emitted from fuel combustion [14]. However, a comprehensive oxidation
mechanism for H2S still needs further studies, with key elementary reactions plagued by
significant uncertainties [15].
Apart from H2S, other less-known reduced sulfur species carbon disulfide (CS2) and carbonyl
sulfide (COS) are also produced during combustion or thermal pyrolysis of fuels. H2S, SO2,
CS2 and COS are major sulfur-containing species produced during coal pyrolysis [16]. In the
Claus process, CS2 and COS contribute up to 20 % of the total sulfur emissions in exhaust [17,
18]. The formation of CS2 and COS is confirmed in the oxidation of methane seeded with H2S
[19, 20], which is prevalent in natural gas as an impurity. The presence of SO2 in ethylene
pyrolysis also leads to the formation of CS2 [21]. However, studies on CS2 and COS oxidation
are scarce, leading to uncertainties in modelling combustion processes in the real-wold
scenario.
Pure CS2 exists as a colourless liquid, with a low boiling point at 46.3C under atmospheric
pressure. As an excellent non-polar solvent, CS2 has found numerous applications in both
laboratories and industry. Additionally, the production of synthetic fibres, rubbers and
pesticides requires CS2 as feedstock. In industry, CS2 is manufactured with methane and solid
sulfur, in the presence of silica gel or alumina catalyst under 600 C [22]:
2 CH4 + S8 catalyst, 600C 2 CS2 + 4 H2S
However, CS2 is characterized by extreme flammability [23] and high explosion propensity
[24], which has led to several fire accidents in laboratories [25, 26] and chemical warehouses
[27].
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Chapter 2. Literature review
13
Natural production of CS2 and COS occurs from volcanic eruptions and bushfires [28], while
anthropogenic emission contributes to most of the total atmospheric content of CS2 and COS
[29]. CS2 exists at levels of parts per trillion (ppt) in the earths troposphere; 15 to 30 ppt in
the non-urban troposphere and 100 to 200 ppt in polluted urban areas [30]. On the other hand,
COS stands as the most abundant sulfur compound in the atmosphere (> 400 ppt) [31].
Despite the important role of reduced sulfur species in the environmental and energy fields,
studies on combustion chemistry of CS2 and COS are still limited. This study aims to review
the literature related to experiments and oxidation kinetics of reduced sulfur species (H2S, CS2
and COS), and their interaction with hydrocarbons in combustion processes. Firstly, we
examine the pre-established oxidation processes for H2S, which contains S/SO sub-mechanism
SH radical as the key intermediate. The occurrence of intersystem crossing (ISC) in S/SO and
H2S combustion processes is highlighted. Next, we survey the oxidation of CS2 and COS. In
the atmosphere, CS2 will convert itself to COS with the aid of OH radical. COS represents the
most stable and most abundant of sulfur carrier in the atmosphere. For CS2 oxidation, the high
activation energy of triplet oxidation pathway fails to explain the low ignition temperature for
the experiments conducted with the tubular-flow reactor. The ISC is highlighted to occur in
the CS2 combustion mechanism. COS is produced as an intermediate in CS2 oxidation process,
i.e., COS/O2 mechanism is involved in CS2 oxidation as a subset. We also discuss the
production of S atom in both ground state (3S) and excited state (1S) in photolysis of reduced
sulfur species. Finally, the interaction between sulfur species and hydrocarbons has been
investigated for 1) Influence of H2S/SH on combustion process of hydrocarbons and 2) impact
of SO2 on ethylene pyrolysis. Future research directions for sulfur chemistry will be suggested
in the conclusion section, for both experimental and theoretical aspects.
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Chapter 2. Literature review
14
2.2. Oxidation of H2S
As the most significant sulfur carrier in natural gas, oxidation and pyrolysis of H2S has been a
subject of many studies focusing on its formation in coal pyrolysis [32], conversion in Claus
process [13] and application as potential hydrogen source [33, 34]. As a flammable gas, the
oxidation of H2S has been investigated with batch reactor to reveal its explosion limit [35, 36],
and shock wave tube to decide its ignition delay time [37]. With the aid of a mass spectrometer,
SO, SO2 and SO3 are sampled from the flat flame of H2S [38].
The recent study by Zhou et al. proposes a comprehensive oxidation mechanism for H2S with
tubular flow reactor [14] and accurate theoretical calculations [39, 40]. Song et al. extended
both the experimental and kinetic modelling of H2S oxidation in plug flow reactor at elevated
pressure up to 100 bar [15]. Owing to the uncertainties in reactions related to SH radical,
further work is necessary to improve the mechanism of H2S oxidation.
In this section, H2S oxidation is reviewed under atmospheric and combustion processes. We
also make recommendations for future work on H2S oxidation in each part.
2.2.1. Atmospheric oxidation of H2S
Reduced sulfur species such as H2S, CS2 and COS represent major atmospheric S-carriers and
assume an important role in the global sulfur cycle. It is generally believed that the majority
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Chapter 2. Literature review
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of H2S is contributed by natural processes such as volcanic eruptions, bush fires, life-cycles in
marshes and sea [28, 41]. However, human activities also result in a significant emission of
H2S, especially in the energy industry [42]. In earths troposphere, H2S varies in the level of
20 60 ppt [43].
Direct interaction between H2S and O2 proceeds slowly under atmospheric thermal conditions.
While no direct experimental measurement has been found, a quantum chemistry calculation
was conducted by Montoya et al. [44] to demonstrate a high activation energy at 159.8 kJ/mol.
The reaction rate for R2-1 is expressed as k1 = 4.6 10-19 T2.76 exp (-159.8/RT)
[cm3/(molecules)] in Arrhenius equation fitted from 300 to 2000 K. As extrapolated in this
work, the rate constant for H2S + O2 corresponds to 2.7 10-40 cm3/(molecules), which is too
slow to account for the atmospheric conversion of H2S (even when the atmospheric oxygen
concentration is considered).
H2S + O2 HO2 + SH R2-1
With analytical techniques applied to atmospheric chemistry, researchers have investigated the
oxidation agent of H2S in the atmosphere among atomic oxygen O [45], ozone O3 [46], atomic
hydrogen H and hydroxyl radical OH. The reaction between H2S and O has been studied with
the aid of electron spin resonance (ESR) applied detect and measure O concentration in a flow
reactor [45]. UV-Vis absorption of O3 and resonance fluorescence of H facilitate researchers
in measuring the reaction rate of H2S + O3 [46] and H2S + H [47]. The reaction between OH
and H2S has been measured in discharge-flow reactor with laser induced fluorescence LIF, to
quantitate the concentration of OH radical [48].
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From Table 2.1, which contrasts the rate constant of H2S oxidized by O2, O, O3, H and OH in
the atmosphere, it can be observed that the reaction between OH radical and H2S contributes
to the most important consumption channel for atmospheric H2S. Moreover, the very low
concentrations of O and H compared to OH in the atmosphere [49] clearly weakens the
influence of R2-2 and R2-4.
Table 2.1. Atmospheric oxidation of H2S, reaction rate k (cm3/(molecules)) is under
atmospheric condition at 298 K, 1 atm.
Reaction Method k Source
R2-1 H2S + O2 HO2 + SH Theoretical
calculations
2.7 10-40 [44]
R2-2 H2S + O OH + SH Experimental
measurement
2.3 10-14 [45]
R2-3 H2S + O3 HO2 + SH Experimental
measurement
4.0 10-16 [46]
R2-4 H2S + H H2 + SH Experimental
measurement
9.6 10-13 [47]
R2-5 H2S + OH H2O + SH Experimental
measurement
3.6 10-12 [48]
By locating OH to be the most potent oxidisers for H2S in the atmosphere, the reaction pathway
of R2-5 has been studied intensively with quantum chemistry calculations by Mousavipour et
al. [50]. Potential energy surface is explored at the MP2/6-311++G(d,p) level of theory [51].
Rate constants were calculated using transition state theory (TST) [52] with one-dimensional
correction for tunneling effect [53], resulting in an activation energy at 4.2 kJ/mol fitted from
300 3000 K. The computed reaction rate amounts to 2.65 10-12 cm3/(molecules) at 298 K,
which is in good agreement with the experimental analogous value at 3.6 10-12
cm3/(molecules) [48], confirming the accuracy of the theoretical calculation.
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17
Figure 2.1 demonstrates the reaction pathway and structure of the transition state structure
calculated by Mousavipour et al. [50]. The highly reactive OH radical will extract one H from
H2S to form H2O and SH radical. This reaction can be understood as H exchange between SH
and OH radical. Since the bond dissociation enthalpy of H2O (498.8 kJ/mol) is much higher
than that of H2S (380.1 kJ/mol) at 298 K [54], the OH radical is far more competitive than the
SH radical, to acquire H.
Figure 2.1. Calculated reaction pathway and transition state structure for H2S + OH H2O +
SH, plotted from calculation by Mousavipour et al. [50]. All enthalpy values are with reference
to initial reactants H2S + OH, in kJ/mol at 0 K. All distance values (around chemical bonds)
are in = 10-10 m.
Further conversion of SH radicals, which involves O2, would proceed through R2-6: SH + O2
SO + OH. However, the reaction rate of R2-6 is measured to be less than 1.0 10-17
cm3/(molecules) [55] using resonance fluorescence technique, which is too slow to account
for the consumption of SH in the atmosphere. O3 and NO2 have been found to be the major
agents for SH conversion at a reasonable reaction rate, to produce HSO [56, 57] as products.
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As shown in Table 2.2, NO2 acts as the most powerful scavenger for SH in the atmosphere. In
spite of the important role of NO2 in atmospheric sulfur cycle, no quantum chemistry
calculation has been conducted to reveal the reaction pathway of R2-8.
Resende and Ornellas [58] applied theoretical calculations to explore the consumption of SH
in the atmosphere for reaction R2-7: SH + O3 HSO + O2. However, by conducting
calculations in ground state with 2SH and 3O3, the computed activation energy attains a value
of 26.5 kJ/mol. This energy barrier leads to a reaction rate for R2-7 at 2.3 10-17
cm3/(molecules) for atmospheric condition at 1 atm and 298 K, which is too slow compared
to experimental measurement at 4.3 10-12 cm3/(molecules). Since no spin forbidden channels
were considered in this work, we suggest re-investigation of the reaction, proceeding through
the excited state of reactants (1O3) or transition state. The intersystem crossing between triplet
and singlet pathway may account for the fast conversion of SH as measured in R2-7.
Table 2.2. Atmospheric conversion of SH, reaction rate k is under atmospheric condition at
298 K, 101 kPa.
Reaction Method k (cm3/(molecules)) Source
R2-6 SH + O2 SO + OH Experimental
measurement
< 1.0 10-17 [55]
R2-7 SH + O3 HSO + O2 Experimental
measurement
4.3 10-12 [56]
R2-8 SH + NO2 HSO + NO Experimental
measurement
6.5 10-11 [57]
The reaction rate of R2-9: HSO + O2 HO2 + SO is measured to be less than 2.0 10-17
cm3/(molecules) [59] under atmospheric condition. While the interaction between HSO and
O3 in the atmosphere (R2-10: HSO + O3 HSO2 + O2) is also too slow to afford the
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Chapter 2. Literature review
19
consumption of HSO in the atmosphere with a measured reaction rate at 5.0 10-14
cm3/(molecules) [60]. Reaction with NO2 (R2-11: HSO + NO2 HSO2 + NO) has been
found to be the main channel of HSO conversion, with a reaction rate measured to be 9.6
1012 cm3/(molecules) in the atmosphere [59]. To the best of our knowledge, literature
presents no account of the reaction between HSO and OH. In this regard, the OH radical would
extract the weakly bonded H atom from S-H bond in HSO to form H2O and SO.
Table 2.3. Atmospheric conversion of HSO and HSO2, reaction rate k is under atmospheric
condition at 298 K, 101 kPa.
Reaction Method k (cm3/(molecules)) Source
R2-9 HSO + O2 HO2 + SO Experimental
measurement
< 2.0 10-17 [59]
R2-10 HSO + O3 HSO2 + O2 Experimental
measurement
5.0 10-14 [60]
R2-11 HSO + NO2 HSO2 + NO Experimental
measurement
9.6 1012 [59]
R2-12 HSO2 + O2 SO2 + HO2 Experimental
measurement
3.0 1013 [59]
Subsequent conversion of HSO2 is measured by Lovejoy et al. [59] to proceed with O2 with a
reaction rate at 3.0 1013 cm3/(molecules). However, it must be noted that the methods used
by Lovejoy et al. to obtain the rate coefficient was indirect, which may lead to substantial error
due to the uncertainty of HO2 released from parallel reactions [61]. Table 2.3 lists the reaction
rate for HSO and HSO2 consumption in the atmosphere.
It is worthwhile mentioning that no theoretical studies on the atmospheric conversion of HSO
and HSO2 have been found. Here, we would recommend further studies on atmospheric
reaction involving HSO and HSO2 with computational chemistry, to reveal the detailed reaction
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Chapter 2. Literature review
20
pathways. The proposed reaction HSO + OH may open another corridor for HSO consumption,
which is operating independent of the NO2 species in the atmosphere.
Figure 2.2 presents a summary for H2S oxidation in literature, as well as a recommended
pathway, which is of potential importance. In a nutshell, NO2 is extremely important in the
consumption of H2S in the atmosphere. However, in the absence of NO2, an alternative
pathway is proposed here involving OH radical, which is relatively more prevalent in the
atmosphere. We also highlight the failure of ground state calculation on SH + 3O3 to reproduce
the experimental results. The intersystem crossing between triplet and singlet may assist in
reducing the activation energy for the reaction SH + 3O3. Finally, more theoretical work on
HSO and HSO2 is suggested to broaden our understanding on atmospheric oxidation of H2S.
Figure 2.2. Atmospheric oxidation pathway for H2S and suggested study on alternative
pathway.
2.2.2 Combustion mechanism of H2S
Early research on H2S oxidation focused on its ignition delay time [37], explosion limit [35,
36] as well as the flame structures [20, 38], and the reaction mechanism was based on measured
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Chapter 2. Literature review
21
product distributions. The first comprehensive review for the combustion of H2S was
conducted by Cullis and Mulcahy in 1972 [3], based on low-temperature photolysis
experiments and flame studies. The final product of H2S oxidation is identified as SO2 with
SO and S2O performing as important intermediates. The proposed consumption channels
involve intermediates such as SH, SO, SO3, S, S2, OH and HO2. However, due to the lack of
reliable kinetic parameters for reactions and thermodynamic data for these species, the
validation of the mechanism remains a very challenging task. Further study by Gargurevich
[62] summarized some early work on kinetics of sulfur chemistry and also estimated the rate
parameters for the missing steps, to resolve the role of H2S in Claus process. Further studies
by Hughes et al. [63] discussed the uncertainties caused by key steps such as SO + OH and SO2
+ H. By realizing the importance of SH, S and SO radical in the mechanism, Tsuchiya et al.
[64] measured the reaction rate for three key elementary reactions: SH + O2, S + O2 and SO +
O2 with shock wave tube at high temperature range (1000 1600 K). A systematic reduced
combustion mechanism for H2S has been proposed by Cerru et al [65, 66], extending the radical
pool to contain HSS, HSSH, HSO and HSO2. However, the author attributes the uncertainties
related to SH, HSS and HSSH reactions to insufficient studies, and highlights the paramount
importance of related reactions on the ignition of H2S oxidation.
Some recent studies by Zhou et al. [67, 68] and Gao et al. [39] have combined experimental
work and high level theoretical calculations to resolve the role of H2/S2 system (including SH,
HSS and HSSH) in H2S oxidation, in which they suggest a crossing process from triplet ground
state to singlet excited state for transition structure. The updated mechanism results in a
satisfactory agreement with further experimental validation, in a vertical tubular flow reactor
at 950 1150 K under atmospheric pressure [14]. Song et al. further expanded the experiments
with flow reactor under high pressure range between 30 and 99 atm (atmospheric pressure),
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Chapter 2. Literature review
22
and interpreted the results with the mechanism of Zhou et al. [14] with certain modification.
It is pointed out that the branch ratio of the reaction of SH + SH to give H2S + S (chain
propagation) or HSSH (chain termination) is the key step to control the oxidation of H2S.
We also intend to point out that the mechanism modelling with plug-flow reactor (PFR) in the
work of Zhou et al. work [14] may not be consistent with their experiments. For tubular flow
reactor, the flow condition can be approximately divided into the laminar [69] and the turbulent
[70] flow regime. As controlled by residence time, the flow condition of inlet gases wouldnt
reach a turbulent flow with a low flow velocity and Reynolds number. The species
composition and temperature distribution in laminar flow condition may result in disagreement
with plug flow modelling, in which species and temperature are homogeneous everywhere.
The modelling of PFR is only an approximation to tubular-flow reactor under atmospheric
pressure. This also explains the fact that Song et al. applied a high pressure tubular-flow reactor
to reach turbulent flow condition as modelled with PFR.
In this section, we review the key oxidation process of H2S based on the mechanism of Zhou
et al. [14]. The occurrence of intersystem crossing for selected elementary reactions, especially
in H/S system and S/O system, is the focal point of this part.
The chain initiation of H2S oxidation has been studied by Montoya et al. [44] through
theoretical calculation with G2 method [71]. However, direct H abstraction by O2 in ground
state is featured with a sizable activation enthalpy up to 168.8 kJ/mol at 0 K, as shown in Fig.
2.3. The reaction is also strongly endothermic by 169.5 kJ/mol, which is not favored in
combustion processes. The backward reaction is more prevalent due to a very shallow reverse
reaction barrier. This indicates that SH tends to abstract H from HO2.
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Chapter 2. Literature review
23
Figure 2.3. Potential enthalpy surface for H2S + O2 calculated with G2 method at 0 K by
Montoya et al. [44]. All enthalpy values are with reference to initial reactants H2S + 3O2, in
kJ/mol.
However, a crossing between singlet and triplet state is considered by the author to give us a
singlet HOOSH adduct, which is even stabler than the initial triplet reactants H2S + 3O2 by
15.8 kJ/mol. The intersystem crossing (ISC) from ground triplet state to singlet adduct opens
another corridor to form HSO + OH, with a lower activation barrier through the crossing point
(ISC point). However, the author has proposed the ISC process without locating the crossing
point. Here, we would recommend calculation of the detailed potential energy surface (PES)
for both triplet and singlet pathway separately. Then by overlapping the two PESs, we could
approximately fix the crossing point.
H2S + O2 HSO + OH R2-13
The consumption of HSO has not been investigated. The sole estimation from Zhou et al. [14]
suggests an activation energy at 27.6 kJ/mol to produce SO2 and OH. Here, we would
recommend a theoretical study on the formation of a four-membered ring of HOOSO, where
the O-O bond breaks to give OH and SO2. The produced OH radical extracts one H from H2S
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Chapter 2. Literature review
24
readily, through a trivial activation energy at 4.2 kJ/mol, as calculated by Mousavipour et al.
[50]. The quantum calculation of R2-2 has been discussed in the atmospheric part to give SH
as an important chain carrier.
HSO + O2 SO2 + OH R2-14
H2S + OH SH + H2O R2-2
Direct measurement of SH + O2 has been conducted by Tsuchiya et al. [64] with shock wave
tube. The activation energy is fitted at 75 kJ/mol in temperature range from 1400 1700 K.
However, the theoretical calculation by Zhou et al. [72] at ground state (triplet pathway)
predicted the formation of SO + OH and HSO + O via activation barriers at 81 kJ/mol and 89
kJ/mol at 0 K, respectively. The production of HSO + O is unlikely to proceed with an
endothermicity at 89 kJ/mol, which means the reverse reaction is featured with no activation
barrier. However, the theoretical values derived from this work underestimate the reaction rate
by a factor of around 1.5, compared to measured results from 1400 1700 K. A faster reaction
channel is needed to reproduce the experimental results. By considering an electronically
excited state HSO2 suggested by Freitas et al. [73], we recommend underpinning the singlet
reaction pathway and a potential crossing point to ground triplet reactions. This may explain
the relatively fast conversion between SH and O2 as measured in experiments.
SH + O2 OH + SO R2-15
SH + O2 HSO + O R2-16
Due to the low reactivity between SH and O2, the recombination of SH + SH is considered to
act as a major chain propagation step. Theoretical calculations were made by Zhou et al. [40,
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Chapter 2. Literature review
25
68] with consideration of ISC. As depicted in Fig. 2.4, the triplet reaction pathway is featured
with an activation barrier at 23.7 kJ/mol at 0 K. SH abstracts H from another SH to produce
H2S and S atom. An alternative path is provided through singlet pathway. Direct addition of
SH gives singlet 1HSSH, which further converts to singlet 1H2SS by H migration between S
atoms. Subsequent fission of S-S bond and intersystem crossing from singlet to triplet surface
result in 1H2S and 3S in ground state as products. Further experimental study by Gao et al. [39]
confirms the ISC channel affords the predominant reaction pathway under low temperature
range < 800 K, while triplet pathway would dominate the reaction above 1000 K.
Compared to R2-15, the recombination of SH proceeds through singlet pathway plays an
important role in chain propagating, to produce S atom in the system.
SH + SH H2S + S R2-17
Figure 2.4. Potential enthalpy surface for SH + SH at 0 K calculated by Zhou et al. [68]. All
enthalpy values are with reference to initial reactants 2SH + 2SH, in kJ/mol.
Further interaction between S and O2 produces SO and O, which exhibits a substantial influence
on the H2S oxidation, as the chain branching step. Lu et al. [74] reported results from combined
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Chapter 2. Literature review
26
experimental measurements and theoretical calculations. The experiments reveal that the S +
3O2 reaction demonstrates different temperature dependence for low (T < 1000 K, Ea < 3
kJ/mol) and high (T > 1000 K, Ea > 30 kJ/mol) temperature range, as illustrated in Fig. 2.5.
The behaviour of the S + 3O2 reaction is similar to that of the SH + SH reaction discussed
above. Computational work of the same researchers, performed at the G2M level of theory,
has also explored the possible reaction pathways. The reaction proceeding on a singlet surface
represents the only channel that can account for the low-temperature oxidation. At
temperatures higher than 1200 K, the triplet pathway dominates the overall S + 3O2 reaction.
S + O2 SO + O R2-18
Tsuchiya et al. [64] observed similar behaviour of SO + 3O2, concluding that the activation
energy of this reaction resides 34.0 kJ/mol above the separated reactants within a temperature
range of 1130 1640 K. However, for lower temperatures of 250 585 K, the activation
energy corresponds to 19.0 kJ/mol, as reported by Garland [75]. This significant discrepancy
likely originates from ISC phenomenon operating in the low temperature window.
SO + O2 SO2 + O R2-19
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Chapter 2. Literature review
27
Figure 2.5. Arrhenius plot for reaction S + 3O2 based on Lu et al.s [74] experiments. The
dashed lines represent the triplet (black) and singlet (red) pathways suggested in this work.
In Fig. 2.6, the key consumption steps for H2S oxidation reviewed in this study are
demonstrated. We highlight the ISC process in R2-13, R2-17 and R2-18, and highlight the
possible occurrence of ISC in R2-19. Further investigation on reaction pathway for R2-14 is
also recommended on both triplet and singlet surface.
Figure 2.6. Key oxidation step for H2S in combustion process.
2.3. Oxidation of CS2
In this section, the oxidation pathway of CS2 is reviewed under both atmospheric and
combustion processes. COS oxidation is also included as a sub-mechanism in CS2 combustion
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Chapter 2. Literature review
28
mechanism. We also reviewed the photolysis study of reduced sulfur species: H2S, CS2 and
COS, which produce S atom in its excited state (singlet 1S) instead of ground state (triplet 3S).
2.3.1. Atmospheric oxidation of CS2
Apart from H2S, reduced sulfur species of CS2 and COS represent major sulfur carriers in the
atmosphere. Both natural [28] and human activities result in CS2 emission into atmosphere.
However, the majority of atmospheric CS2 originates from industrial processes, such as
production of insecticides and man-made fibres. Moreover, CS2 and COS correspond to an
important by-product from the Claus gas de-sulfurising process [18], taking up nearly 20 % of
the total sulfur emission from the Claus process. CS2 exists at levels of parts per trillion in the
Earths troposphere; 15 to 30 ppt in the non-urban troposphere and 100 to 200 ppt in polluted
urban areas [30]. COS stands as the most abundant sulfur compound in the atmosphere (> 400
ppt) with a relatively long life-time of up to years [31].
Similar to H2S, direct interaction between CS2 and O2 is featured by a significant activation
energy, which renders the reaction less important in the atmosphere. With the very low
concentrations of O(3P) in the atmosphere [49] and the absence of any decomposition of CS2
near the UV-region [76], OH radicals constitute the sole initial atmospheric oxidiser. However,
experimental results on the reaction of CS2 and OH reaction indicated an extremely slow rate
constant:
CS2 + OH COS + SH R2-20
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Chapter 2. Literature review
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By producing OH with pulsed H2O photolysis technique, Kurylo [77] as well as Atkinson et
al. [78] reported the rate coefficient of R2-20 as 1.9 10-13 cm3/(molecules), while Atkinson
and Pitts measured an upper limit for R2-20 to be 7.0 10-14 cm3/(molecules) at 298.15 K.
However, photolysis of H2O at 165-185 nm may also dissociate the CS2 molecule, resulting in
an error in the reported rate constant of R2-20 due to loss of CS2. A CS2-filling cell was applied
by Wine et al. to filter radiation that dissociates CS2 and report an even lower rate constant for
R2-20, below 2.0 10-15 cm3/(molecules) at 298.15 K [79]. Similar result has been achieved
by Iyer and Rowland who produced OH radicals by a continuous wave (CW) photolysis of
H2O2 at 254 nm [80], avoiding the dissociation energy of C-S bond.
By introducing O2 into the system of CS2 and OH, the consumption of CS2 proceeds with a
much higher reaction rate. Jones et al. [81, 82] measured the rate constant of R2-20 in the
presence of O2 (i.e. CS2 + OH + O2) using photolysis of HONO, reporting a rather rapid rate
constant of 2.0 10-12 cm3/ (molecules) at 298.15 K. The promotion effect of O2 was also
confirmed by Barnes et al. using CW photolysis [83], who obtained a very similar rate constant.
Two important steps control the atmospheric oxidation of CS2, namely formation of a CS2OH
and reactions of O2 with this adduct:
CS2 + OH CS2OH R2-21
CS2OH + O2 products R2-22
Experiments by Murrells et al. [84] and Lovejoy et al. [85] tested a wide temperature range of
249 318 K to confirm the formation of OCS, and SO2 as the sole experimental products.
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Chapter 2. Literature review
30
Theoretically, calculations of Lunell et al. [86] at the HF/3-21G(d) level of theory predicted
two distinct isomers of CS2OH, viz., the S-adduct SCS(OH) and the C-adduct SC(OH)S.
However, an incorrect structure of the SCS(OH) adduct is considered in Lunell et al.s work
[86], resulting in a strongly endothermic reaction by 104.2 kJ/mol, with respect to the reaction
of CS2 and OH in R2-21. Subsequent computations of McKee [87] at the modified G1 [88]
level of theory, at 298.15 K, yielded reaction enthalpies of -24.7 kJ/mol and -126.8 kJ/mol,
respectively, for the generation of the S- and C-adducts of CS2OH in R2-21. McKee and Wine
[89] further investigated R2-22 with optimised geometries obtained at the B3LYP/6-311+G(d)
level of theory and constructed a pathway for the formation of COS and HOSO. Figure 2.7
depicts the detailed reaction pathway for the S-adduct to interact with O2. The insertion of O2
on C atom of S-adduct results in an intermediate SCS (OH) O2 with a sizable activation barrier
at 28.6 kJ/mol. Subsequent fissions of C-S and O-O bond yield COS and HOSO as products.
However, the calculated well depth of S-adduct amounts to 26.9 kJ/mol, which underestimates
the stability of SCS(OH) as measured at 45.6 4.2 kJ/mol by Murrells et al. [84].
Figure 2.7. Potential enthalpy surface for CS2 + OH + O2 calculated at 0 K by McKee and
Wine [89]. All enthalpy values are with reference to initial reactants CS2 + OH + O2, in kJ/mol.
The HOSO + O2 reaction has a near zero activation barrier, as calculated by McKee and Wine
[89] The generation of HO2 + SO2 should proceed readily in the atmosphere. It explains the
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Chapter 2. Literature review
31
observation of SO2 in experimental measurements. R2-22 is updated below to show the
formation of HOSO, while R2-23 demonstrates its consumption channel in the atmosphere.
CS2OH + O2 COS + HOSO R2-22
HOSO + O2 HO2 + SO2 R2-23
While SO2 will be rapidly converted to acid rain by OH and O2 as oxidiser [90-93] in the
atmosphere, COS tends to accumulate as the most abundant sulfur carrier in the earths
troposphere (> 400 ppt), with a relatively long life-time of up to years [31].
Figure 2.8. Atmospheric oxidation pathway for CS2.
Figure 2.8 depicts the atmospheric conversion of CS2 with COS and SO2 as products. Although
a comprehensive quantum calculation has been carried out by McKee and Wine [89], the
relative low level of theory may lead to significant error, as compared with experiments.
Further calculation with an adequately high level of theory is desirable, to further improve the
atmospheric oxidation mechanism of CS2.
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Chapter 2. Literature review
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2.3.2. Combustion mechanism of CS2
Earlier work on the combustion of CS2 was reviewed by Cullis and Mulcahy [3] . Myerson
and Taylor extensively studied the ignition of CS2-O2 mixtures as an extremely flammable
chemical, as a representative case for branched-chain reactions [24, 94]. It was demonstrated
that a mixture containing as little as 0.03 % (by mol) CS2 in oxygen ignites at 80 oC and 0.05
atm, resulting in sustained cool flame propagation. Once ignited, a cool flame propagates
through the mixture at 55 oC, whereas the temperature rise in the flame is less than 15 oC. The
most important oxidation products were reported to be CO, SO2, CO2, CS and SO, as well as
COS. Intermediates such as SO, S and O appear much more reactive than COS and CS [95],
with relatively low accumulation.
Studies investigated the effect of various factors on the ignition behaviour of CS2, including
irradiation by ultraviolet light [96], the presence of inert gases [97] and the conditions of
surfaces [24]. Results from these investigations pointed to a profound distinction between cool
and hot flames of CS2 with reference to hydrocarbons [98, 99]. As evidence of the cool flame
combustion of CS2, it was found that the flammability of CS2 increases with the concentration
of oxygen in the combustible CS2 mixture [100]. This behaviour significantly differs from that
observed in combustion of hydrocarbons, where the second ignition limit arises due to a three-
body reaction H + O2 + M HO2 + M [101].
In CS2 oxidation, the two rapid steps R2-18 and R2-19 produce the reactive O radicals:
S + O2 SO + O R2-18
SO + O2 SO2 + O R2-19
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Chapter 2. Literature review
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Various chain inhibitors modify the duration of an induction period from a few seconds to
several minutes [24].
Hanst and Myerson et al. [102] were the first to study the kinetics of explosive combustion of
CS2. They used kinetic absorption spectroscopy to follow the variations in the concentrations
of radical and molecular species during the explosion of CS2. The CS2-O2 mixture, initially
heated to between 190 oC and 300 oC, exploded spontaneously. Strong and continuous
absorptions confirmed the formation of SO and CS with the appearance of SO2 in the early
stages of the explosions [96]. Spectroscopic techniques were also utilised to study explosions
of CS2 initiated by shock waves [103] and flash photolysis [104]. In both cases, CS2 was found
to be oxidized mainly into SO, SO2 and CO. However, experimental measurements on slow
combustion and explosive combustion of CS2 have not contributed much insight into
mechanisms governing its oxidation, as only final products at particular low temperature ranges
were determined.
Reaction pathways contributing to the explosion behaviour of CS2 are summarised in Fig. 2.9.
Central to these mechanisms is the slow build-up of CS via reaction of O atom with CS2 and
its further oxidation into SO/SO2. The fate of produced S atom was assumed to be controlled
by reactions with O atom and CS2, which performs as the initiation step for CS2 explosions. It
is worthwhile to note that the formation of an O atom in this mechanism was attributed to
unknown initiation reactions.
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Chapter 2. Literature review
34
Figure 2.9. Oxidation pathways of CS2 derived from explosion behaviour of CS2 at early stage
[96]. The initiation step is believed to be CS2 + O, while final products are CO and SO2.
Along the same line of enquiry, a low pressure flame of CS2 was studied by Azatyan et al.
[105] to address the high-temperature combustion mechanism, with the electron spin resonance
(ESR) technique to detect O, CS and SO as intermediates. By analysing the relationship
between O and SO in experiments, the authors proposed the formation of COS as a major
conversion channel between O and SO with the reaction: COS + O CO + SO.
To confirm the occurrence of these reactions, Azatyan et al. [105] added small concentrations
of COS to the CS2 flame at a temperature interval of 350 oC to 600 oC. A lower accumulation
of O and the maximum yield of SO were observed, validating the influence of COS in the
oxidation of CS2.
Homan et al. [106] deployed the isothermal flow reactor to study combustion of CS2 in an
excess of O2 at 927 oC and 0.4 atm. They confirmed the presence of COS in the oxidation
process of CS2. In order to describe their experimental product profiles, a reduced kinetic
model was constructed including COS as intermediate. The reaction CS2 + O COS + S
affords the production of COS.
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CS2 + O CS + SO R2-24
CS + O CO + S R2-25
CS + O2 CO + SO R2-26
COS + O CO + SO R2-27
CS2 + O COS + S R2-28
Glarborg and Marshall [107] have recently studied the oxidation mechanism of COS with
kinetic modelling. A detailed oxidation mechanism has been proposed, based on evaluation of
data from literature. For the consumption of COS, the author considered three pathways:
COS CO + S R2-29
COS + O CO + SO R2-30
COS + O2 CO + SO2 R2-31
For the thermal decomposition process (R2-29: COS CO + S), a high activation energy at
260 kJ/mol fitted from experimental work at high temperature (1900 3230 K) [108] makes
it too slow to account for the conversion of COS. The COS interaction with atomic oxygen
(R2-30: COS + O CO + SO) presents as the major consumption channel for COS. The
authors fitted the rate parameters from the experimental measurement by Homman et al. [16]
around 1270 K, indicating an activation energy at 21.8 kJ/mol. The dominant products are CO
+ SO, while CO2 + S could form at higher temperature. With the absence of atomic O for chain
initiation, the rate parameter of reaction between COS and O2, R2-31 (COS + O2 CO + SO2),
is estimated with an activation energy at 134.4 kJ/mol.
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Furthermore, to evaluate their model, the authors have compared it in the oxidation of COS
with analogous experimental data obtained in batch reactors [109], flow reactors [106], and
shock tubes [110]. Although the proposed COS oxidation mechanism captured well the
experimental profiles for major species measured with tubular flow reactor by Homann et al.
[111], a slight correction for the onset of reaction has to be made to match the experiment
initiation. The reaction COS + O has been recognized to be very sensitive to the overall
mechanism and intersystem crossing from triplet to singlet surface would occur with high
probability [112].
The first attempt to construct a detailed kinetic model for CS2 oxidation was undertaken by
Howgate and Barr [113] to model CS2-O2 flame. They include the direct interaction between
CS2 and O2 to account for the chain initiation with an activation energy at 179.5 kJ/mol, as
indicated: CS2 + O2 CS + SO2. Further investigation by Hardy and Gardiner [114] adopted
this chain initiation step in their mechanism and achieved satisfactory agreement with their
experimental measurement for the ignition delay time of CS2 with shock wave tube.
Subsequent measurement of Saito et al. [115] and Murakami et al. [116] revised the activation
energy of R2-32 to be 134.5 kJ/mol and 130.0 kJ/mol, respectively, to match their experimental
results from shock wave tube.
CS2 + O2 CS + SO2 R2-32
Very recently, Glarborg et al. [117] formulated a kinetic model for oxidation of CS2. Kinetic
data for key CS2/CS + O2 reactions have been investigated on the basis of ab initio calculations.
For reactions CS2 + O2, the authors investigated the reaction pathway on both triplet and singlet
surface, and concluded the products to be COS + SO, instead of formation of CS + SO2.
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CS2 + O2 COS + SO R2-33
Figure 2.10. Potential energy surface for CS2 + O2 calculated by Glarborg et al. [117]. All
energy values are with reference to initial reactants CS2 + O2, in kJ/mol.
As demonstrated in Fig. 2.10, the triplet pathway of CS2 + O2 climbs through the CS2O2 adduct,
which dissociates into COS and SO, through an overall reaction barrier of 223 kJ/mol.
However, the singlet oxidation offers an optional pathway with a much lower activation barrier.
Compared to the triplet CS2O2 structure that features a strong O-O bond, the singlet SOOCS
adduct forms through the dissociative addition of oxygen atoms at C and S atom, with a much
lower energy level. By locating the cross-over between the energy surface of triplet and singlet
pathways, Glarborg et al. estimated the crossing point to reside 145 kJ/mol above the triplet
reactants CS2 + 3O2, which coincides with the experimentally interpreted value in the shock
wave tube system (130.0 179.5 kJ/mol) [114-116].
With the formation of SO through R2-33, its interaction with O2 gives O as the most important
chain carrier. The subsequent interactions between CS2 and O proceed through three reaction
channels, resulting in CS + SO (R2-24), COS + S (R2-28) and CO + S2 (R2-34) as products.
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CS2 + O CO + S2 R2-34
An experimental study using a fast flow reactor concluded that the formation of CS + SO
dominates the overall process, with only a small generation of COS + S and CO + S2 [118].
However, theoretical calculations on the triplet surface support only the formation of CS + SO,
with a small activation energy at 4.2 kJ/mol. To produce COS + S or CO + S2, the reactions
must overcome higher barriers of 41.3 kJ/mol and 33.2 kJ/mol, respectively. Thus, both
reactions should be negligible, when contrasted with the formation of CS + SO [119]. Here,
we would like to suggest the possible singlet reaction pathways to account for the formation of
COS + S and CO + S2 [120], as detected in experiments.
As calculated by Glarborg et al. [117], CS is consumed by O2 with an activation energy at
117.5 kJ/mol on triplet surface. From shock-tube experiments, Murakami et al. reported the
activation energy of R2-26 as 51.0 61.7 kJ/mol [116]. The significant uncertainty from
experiments and high activation barrier derived by triplet reaction pathway is to be noted; we
would like to suspect the occurrence of ISC for R2-26.
CS + O2 CO + SO R2-26
In general, the mechanism proposed by Glarborg et al. [117] displays good agreement with
experimental results pertinent to ignition delays and explosion limits of CS2. However, their
mechanism tends to over-predict the concentration of species under low temperature
conditions, especially with respect to measurements from flow reactors [111] and shockwave
tubes [115]. The uncertainty of the chain initiation reaction, CS2 + O2, and competition
between the chain branching reactions, CS + O2, accounted for the discrepancy.
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More recently, Abin et al. [121] studied the moist oxidation of CS2 in a flow reactor at 1 atm
between 127 1127 oC under different oxygen-fuel equivalence ratios () of 0.2, 0.7, 1.0, 2.0
and 20.0. They compared their experimental profiles of consumed CS2 with the corresponding
profiles calculated from the mechanism of Glarborg et al. [117] The modelling results from
the mechanism of Glarborg et al. [117] over-predict the onset of the oxidation temperature for
stoichiometry condition ( = 1.0) by around 260 K. By adjusting the activation energies of the
initiation reaction (CS2 + O2) and the chain propagation step (CS + O2), to lower values, Abin
et al. reproduced their experimentally-measured concentration of species, as illustrated in Fig.
2.11. However, it is worthwhile noting that the mechanism updated by Abin et al. [121] also
underestimates the ignition of CS2 for around 100 K. The introduction of moisture, lack of
control of residence time at different temperatures and plausible involvement of surface-
mediated reactions on the reactor walls in Abian et al.s experiments [121], make it difficult to
interpret their experimental measurements and to differentiate plausible initiation by catalytic
surfaces and water molecules. Further experiments on CS2 oxidation for kinetic modelling are
required to develop a more accurate mechanism of CS2 oxidation.
Figure 2.11. Comparison between experimental results for CS2 oxidation and the modelling
results derived from the mechanisms of Glarborg et al.[117] and Abin et al. [121].
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Figure 2.12. Oxidation pathways of CS2 with formation of COS. The initiation step is
considered to be CS2 + O2, while final products are CO and SO2.
To summarise, Fig. 2.12 demonstrates the key oxidation pathway for CS2, with formation of
COS as an important intermediate. From literature, we highlight the occurrence of ISC in R2-
33 as chain initiation step. More theoretical calculations are recommended on the key
elementary reactions such as R2-26, R2-30 and R2-31, with consideration of singlet reaction
surface. Experimental data for kinetic modelling is also desired to validate and update the
oxidation mechanism for CS2.
2.3.3. Production of S atom in excited state in photolysis of reduced sulfur species
With consideration of the ISC process predicted from theoretical calculation for oxidation of
reduced sulfur species, we review the direct experimental measurements of singlet/triplet
species produced in pertinent studies. In the work of Nan et al. [122], the Doppler-broadening
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fine spectrometry of S is measured by laser induced fluorescence (LIF) at 147 nm to identify
the ratio of 3S/1S. As photolysis of COS at 222 nm light proceeded in a glass cell, the integrated
intensity of 3S profile increased with time, due to the quenching of 1S produced in reaction.
The production of singlet 1S appears predominant in photolysis of COS, with the produced
3S/1S = 0.05/0.95. While further study on the photolysis of H2S gives combined 3S/1S =
0.87/0.13 as products [123]. The mixed sulfur atoms with distinct electron spin state yielded
from photolysis of reduced sulfur species reinforce the occurrence of ISC in sulfur-related
reactions.
Recently, by coupling mass spectrometry (MS) and photon-ionization techniques with
synchrotron vacuum ultra-violet light as photon source, McGivern et al. [124] and Qi et al.
[125] manage to distinguish the 3S and 1S atom with their ionization energy at 10.36 eV (3S)
and 7.61 (1S), respectively. Compared to the traditional electron ionization applied with MS,
photon ionization is featured with a high resolution at 0.1 eV, which is adequate to recognize
the difference in the energy level of 3S and 1S. A study on CS2 photolysis with 193 nm light
reveals that the combined formation of 3S/1S = 0.75/0.25 was conducted by McGivern et al.
[124]. Figure 2.13 illustrates the energy level of CS2 photolysis reactions.
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Figure 2.13. The energy level of CS2 photolysis reactions (in kJ/mol) with branch ratio
calculated in this work by Gaussian 09 at CBS-QB3 composite [126].
As demonstrated in Fig. 2.13, we observe that the ground state of CS2 is in singlet (1CS2), while
that of S is in triplet (3S). If the photolysis of CS2 proceeds from ground state to ground state,
an ISC point should exist to build a bridge to triplet S. As measured in experiments, 25 % of
CS2 proceeds through the ISC to form ground state 3S [126]. The difference of electron
quantum spin number of ground state 1CS2 and 3S has prompted consideration of the energy
level of relevant reduced sulfur species. It is revealed that the ground state of species constituted
by only S and O atoms are in triplet (3SO, 3S, 3O and 3O2), except for 1SO2, while the gourd
state for species such as 1CS2, 1COS, 1CS and 1SO2 are in singlet. If we consider all oxidation
processes to have proceeded on the ground state surface, we should have the reaction process
with two ISC points as shown below:
1CS2 3SO 1SO2
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Table 2.4 summarises the branching ratio of 3S/1S from photolysis of reduced sulfur species,
as measured in experiments. Although the parent compounds are in their singlet (ground) state,
the corresponding sulfurs atom arise in both singlet and triplet states. This has indicated that
the ISC is predominant in sulfur related species, especially when it couples with O2.
Table 2.4. The branching ratio of 3S/1S from photolysis of reduced sulfur species (H2S, COS
and CS2).
Species 3S/1S Wavelength (nm) Source
H2S 0.87/0.13 193 [122]
COS 0.95/0.05 222 [123]
CS2 0.75/0.25 193 [124]
However, the identification of electronic spin states of intermediates or products has not been
conducted to study the oxidation process of reduced sulfur species. We would recommend the
application of the photon-ionization technique on oxidation reactions of reduced sulfur species
such as H2S + O2, SH + O2, CS2 + O2, CS + O2, S + O2 and SO + O2, to identify any
singlet/triplet intermediates or products, as predicted in theoretical calculations. Any direct
measurement of these species will fortify the proposed ISC process, which might be universal
in sulfur oxidation reactions.
2.4. Interaction between reduced sulfur species and hydrocarbons
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Despite the significant presence of sulfur species in fossil fuels, the interactions between
reduced sulfur species and hydrocarbons are poorly under