bsc(hons) - dissertation
TRANSCRIPT
Laser Copyrolysis of Chlorofluorocarbons with
Metal Systems in the Gas Phase
Grant Allen
A dissertation submitted in partial fulfilment of the requirements for the degree
of Bachelor of Science with Honours in Chemistry
University of Auckland
1996
ii
Abstract
The reaction mechanisms of the gas phase decomposition of Freon 12 (CF2Cl2), Freon
22 (CF2HCl) and dichloromethane (CH2Cl2), induced by Infrared Laser Powered
Homogeneous Pyrolysis (IR LPHP), are investigated and compared with
decomposition when a volatile transition metal carbonyl compound is also present.
The introduction of either Fe(CO)5 or W(CO)6 to each of the systems under study is
found to alter the mechanism of decomposition with respect to that of the substrate
alone. Halogen abstraction, (where the abstracting species, M(CO)x, is the product of
metal carbonyl decomposition) occurs in preference to that mechanism normally
associated with the decomposition of the selected compound.
Freon 12 is found to decompose to the major product CF3Cl. In the presence of either
Fe(CO)5 or W(CO)6, Freon 12 decomposes to give C2Cl2F4 and C2F4. The
decomposition of Freon 22 is found to involve the elimination of HCl. Subsequent
dimerisation of the resultant CF2 species yields C2F4. In the presence of either
Fe(CO)5 or W(CO)6, the decomposition of Freon 22 does not involve the formation of
either HCl or C2F4, suggesting an alternative mechanism. An initial Cl abstraction is
proposed. Similarly the decomposition of dichloromethane in the presence of either
Fe(CO)5 or W(CO)6, is found to yield products dissimilar to those obtained from the
decomposition of dichloromethane alone. An initial Cl abstraction is occurring in
preference to the elimination of HCl.
iii
Statement of Originality
The work presented herein contains no material that has been accepted for the award
of any other degree or diploma at any university, and to the best of my knowledge
contains no material previously published by another person except where due
reference is made in the text.
Grant Allen
iv
Acknowledgements
The author would like to express his gratitude to those people, without whose help,
this dissertation would not have been possible. In particular Professor Douglas
Russell, for his untiring assistance and enthusiasm, and to Dr Rebecca Berrigan for
her constructive insight. Many thanks are also extended to those fellow researchers,
namely Fergus Binnie, Janet Everett, Nathan Hore and to our technician Dr Noel
Renner.
v
Contents
Abstract ..………………………………………………………………………….. ii
Statement of Originality .......................................................................................... iii
Acknowledgements .................................................................................................. iv
Contents ................................................................................................................... v
List of Figures .......................................................................................................…vii
List of Tables ............................................................................................................ viii
Chapter 1. Introduction ............................................................................... 1
1.1 References and Notes for Chapter 1 ..............................................…… 5
Chapter 2. Experimental ............................................................................. 6
2.1 Introduction .............................................................................................. 6
2.2 Infrared Laser Powered Homogeneous Pyrolysis ..............................…6
2.3 Equipment ..............................................................................................…7
2.3.1 Vacuum Line
2.3.2 Pyrolysis Cell
2.3.3 Window Material
2.3.4 Photosensitiser
2.3.5 CO2 Laser
2.4 Chemicals.................................................................................................. 13
2.5 Experimental Procedure ......................................................................... 13
2.5.1 Introduction
2.5.2 Procedure for Sample Preparation
2.5.3 Pyrolysis Setup
2.6 Experimental Analysis .......................................................................….. 15
2.6.1 Introduction
2.6.2 Fourier Transform Infrared Spectroscopy
2.6.3 Matrix Isolation ESR Spectroscopy
2.6.4 X-ray Photoelectron Spectroscopy
2.6.5 Attenuated Total Reflectance
2.7 References and Notes for Chapter 2 ....................................................... 18
vi
Chapter 3. Pyrolysis Results and Discussion ............................................. 19
3.1 Introduction .............................................................................................. 19
3.2 IR LPHP of Freon 12................................................................................ 19
3.2.1 Literature
3.2.2 Experimental
3.3 IR LPHP of Freon 22 ............................................................................... 24
3.3.1 Literature
3.3.2 Experimental
3.4 IR LPHP of Dichloromethane ................................................................ 26
3.5 IR LPHP of Transition Metal Carbonyl Compounds .......................... 30
3.5.1 Introduction
3.5.2 IR LPHP of Fe(CO)5
3.5.3 IR LPHP of W(CO)6
3.6 References and Notes for Chapter 3 ....................................................... 33
Chapter 4. Copyrolysis Results and Discussion ........................................ 34
4.1 Introduction .............................................................................................. 34
4.2 Copyrolysis of Freon 12 with Fe(CO)5 ................................................... 34
4.3 Copyrolysis of Freon 12 with W(CO)6 ................................................... 36
4.4 Copyrolysis of Freon 22 with Fe(CO)5 ................................................... 44
4.5 Copyrolysis of Freon 22 with W(CO)6 ................................................... 45
4.6 Copyrolysis of Dichloromethane with Fe(CO)5 .................................... 47
4.7 Copyrolysis of Dichloromethane with W(CO)6 ..................................… 48
4.8 References and Notes for Chapter 4 ................................................….. 49
Chapter 5. Conclusions and Future Work ................................................ 50
vii
List of Figures
1.1 The Cl free-radical catalysis of O3 decomposition as proposed by Molina
and Rowland ................................................................................................…. 1
1.2 The role of polar stratospheric clouds in ozone depletion ............................... 3
1.3 Mechanism of reaction between atomic potassium and a halogenated
compound, RX ................................................................................................. 4
2.1 Schematic diagram of the conventional pyrolysis cell .................................… 8
2.2 Schematic diagram of the pyrolysis cell used for ATR analysis ..................... 9
2.3 Energy level diagram for the CO2 laser ........................................................... 11
2.4 Schematic diagram of the ATR setup .............................................................. 17
3.1 The decomposition scheme of CF2Cl2 as proposed by Zitter et al .................. 20
3.2 The decomposition scheme of CF2Cl2 as proposed by Hill et al ..................... 21
3.3 FTIR spectra of Freon 12 before and after IR LPHP ....................................... 22
3.4 The proposed high temperature decomposition scheme of CF2Cl2.................. 23
3.5 The decomposition scheme of CF2HCl ........................................................... 24
3.6 FTIR spectra of Freon 22 before and after IR LPHP ....................................... 25
3.7 The decomposition scheme of CH2Cl2 ............................................................ 27
3.8 FTIR spectra of CH2Cl2 before and after high temperature IR LPHP ............. 28
3.9 The decomposition scheme of CH2Cl2 in the presence of oxygen .................. 29
3.10 ATR/FTIR spectrum of a film deposited after W(CO)6 pyrolysis ................... 32
4.1 FTIR spectra of Freon 12 with W(CO)6 before and after IR LPHP ................. 38
4.2 The decomposition scheme of CF2Cl2 ............................................................. 39
4.3 XPS spectrum of a film deposited after Freon 12/W(CO)6 copyrolysis .......... 41
4.4 XPS spectrum of the W 4f photoelectrons ....................................................... 42
4.5 ATR/FTIR spectrum of a film deposited after Freon 12/W(CO)6
copyrolysis ....................................................................................................... 43
4.6 FTIR spectra of Freon 22 with W(CO)6 before and after IR LPHP ................ 46
viii
List of Tables
2.1 Aperture diameters ........................................................................................... 12
2.2 Chemicals used and source .............................................................................. 13
4.1 The dependence of CF2Cl2 decomposition on the ratio of Fe(CO)5 to
CF2Cl2 .............................................................................................................. 35
4.2 Diatomic bond energies ................................................................................... 37
1
Chapter 1. Introduction
The association of chlorofluorocarbons with ozone depletion has gained widespread
acceptance only within the last decade. After announcing in 1985, that ozone
depletion had been occurring over the Antarctic continent each austral spring since the
late 1970s, Farman and coworkers [1] proposed a mechanism that might account for
the observed ozone hole. Based in principle on the work performed in the mid 1970s
by Molina and Rowland [2], in which chlorine free-radicals were shown to catalytically
decompose O3 (refer to figure 1.1), Farman et al theorised that chlorine containing
compounds, and in particular chlorofluorocarbons, were responsible for the ozone
depletion observed.
Figure 1.1 The Cl free-radical catalysis of O3 decomposition as proposed by
Molina and Rowland [2]
Cl + O3 ClO + O2 ...1
ClO + O Cl + O2 ...2
Net: O3 + O O2 + O2 ...3
Once released to the atmosphere these chlorofluorocarbons, used in such diverse
applications as agents for producing insulating foam, coolants for air conditioners, and
solvents for cleaning circuit boards, could eventually reach the middle stratosphere
(approximately 30 km above ground level), whereupon ultraviolet radiation would
tear them apart. The resultant Cl could exist as free chlorine or, in a manner analogous
to that proposed by Molina and Rowland [2], react with O3 to form ClO and O2.
Farman et al however proposed that these two forms of Cl could react with either
methane (as is the case for free Cl) to form HCl or NO2 (as is the case for ClO) to
form ClONO2 [1]. As a result of their stability, both HCl and ClONO2 do not destroy
2
ozone and as such are labelled chlorine reservoirs. The extent of ozone depletion,
specifically above the Antarctic continent however, was observed to be greater than
that expected on account of that mechanism proposed by Farman and coworkers [1].
In 1986 Soloman and coworkers [3] theorised that the difference between the level of
ozone loss expected and that observed, could be attributed to the presence of polar
stratospheric clouds in the stratosphere above the Antarctica. Polar stratospheric
clouds, that form in the Antarctic winter, may act as a medium for the decomposition
of those inert reservoir molecules, leading to the release of free molecular chlorine. As
the austral spring returns, the level of UV radiation increases, thereby promoting the
decomposition of Cl2. The resultant Cl radicals can then react in the manner proposed
by Farman and coworkers [1], thereby perpetuating the destruction of O3. Toon and
coworkers [4] along with Crutzen and Arnold [5] proposed that polar stratospheric
clouds could also prevent the formation of the inert reservoir, ClONO2, by removing
nitrogen from the atmosphere through the precipitation of nitric acid. In this way
ozone depletion was further promoted. The role of polar stratospheric clouds in ozone
depletion, shown schematically in figure 1.2, is extensively reviewed in an article by
Toon and Turco [6].
3
Figure 1.2 The role of polar stratospheric clouds in ozone depletion [6].
i)Without clouds
CFCs Cl
O3
CH4
HCl
ClONO2
ClONO 2
(Reservoirs)
UV radiation
ii) With clouds
HCl
ClONO 2
Cl2
Cl
Cl
O3
O3
ClO
ClO
Cl2O2
O2
O2
O2
HNO3
Polar
stratospheric cloud
UV radiation
UV
+
The discovery that chlorofluorocarbon compounds in the atmosphere could
catalytically decompose ozone [1,3,5] has prompted much research into the gas phase
chemistry of chlorofluorocarbons [7-12]. It was reported by Husain and Lee [13] that the
reactions of atomic potassium with the molecules CF3Cl, CF2Cl2, CFCl3, CF3Br and
SF6 were both rapid and of low activation energy. Kinetic studies involving atomic
sodium [14] have produced similar results. Husain and Lee proposed a mechanism
whereby atomic potassium or sodium would abstract a halogen from the
4
chlorofluorocarbon, as illustrated in figure 1.3. Consequently we have proposed that
the decomposition rate and/or mechanism of a selected chlorofluorocarbon may be
effected by the addition of volatile transition metal compound in the gas phase, to that
chlorofluorocarbon system. Assuming that the transition metal compound decomposes
at a temperature less than that required for the freon, it is hypothesised that the
resultant radical species may abstract a halogen atom from the chlorofluorocarbon,
and thus initiate freon decomposition.
Figure 1.3 Mechanism of reaction between atomic potassium and a halogenated
compound, RX [13]
K + RX KX + R
Of the several techniques available for the gas phase initiation of chlorofluorocarbon
decomposition, the two most notable are infrared photolysis and Infrared Laser
Powered Homogeneous Pyrolysis (IR LPHP). Infrared photolysis, while the most
popular is limited to those compounds that can absorb infrared radiation directly.
Conversely IR LPHP, which will be discussed further in section 2.2, involves the
introduction of an inert photosensitiser to the system under study, and can thus be
used to induce the pyrolytic decomposition of almost any chlorofluorocarbon having a
sufficient vapour pressure. Our research would utilise the technique of IR LPHP.
In an effort to establish the validity of our supposition the technique of IR LPHP will
be used to initiate the gas phase decomposition of those selected chlorofluorocarbons
both alone and in the presence of a specific volatile transition metal compound in the
gas phase. The compounds selected for study are the chlorofluorocarbons, Freon 12
(CF2Cl2) and Freon 22 (CF2HCl), both of which are, or have been used extensively in
industry, and the non fluorinated analogue, dichloromethane (CH2Cl2). The transition
metal compounds to be introduced into the chlorofluorocarbon system, so as to test
the proposed postulate, are iron pentacarbonyl, Fe(CO)5 and tungsten hexacarbonyl,
W(CO)6.
5
1.1 References and Notes for Chapter 1
[1] J. C. Farman, B. G. Gardiner, J. D. Shankin, Nature, 1985, 315, 207.
[2] M. J. Molina and F. S. Rowland, Nature, 1974, 249, 810.
[3] R. R. Garcia, F. S. Rowland, S. Soloman, D. J. Wuebbles, ibid., p. 755.
[4] P. Hamil, J. Pinto, O. B. Toon, R. P. Turco, Geophys. Res. Lett., 1986, 13, 1284.
[5] F. Arnold and P. J. Crutzen, Nature, 1986, 324, 651.
[6] O. B. Toon and R. P. Turko, Scientific American, June 1991, 40.
[7] R. A. Lau, K. S. Wills, R. N. Zitter, J. Phys. Chem., 1990, 94, 2374.
[8] S. H. Bauer and W. M. Shaub, Int. J. Chem. Kinet., 1975, 7, 509.
[9] D. F. Koster and R. N. Zitter, J. Am. Chem. Soc., 1976, 98, 1613; ibid., 1977, 99,
5491.
[10] J. Ludvik and J. Pola, J. Chem. Soc., Perkin Trans. 2, 1987, 1727.
[11] P. K. Choudhury, J. P. Mittal, J. Pola, K. V. S. Rama Rao, Chem. Phys. Lett.,
1987, 142, 252.
[12] J. Pola and J. Vitek, Collect. Czech. Chem. Commun., 1989, 54, 3083.
[13] D. Husain and Y. H. Lee, J. Chem. Soc., Faraday Trans. 2, 1987, 83, 2325.
[14] D. Husain and P. Marshall, J. Chem. Soc., Faraday Trans. 2, 1985, 81, 613.
6
Chapter 2. Experimental
2.1 Introduction
This chapter will briefly describe the equipment and experimental techniques used
specific to this research topic. Methods of experimental analysis including Fourier
Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS),
Electron Spin Resonance Spectroscopy (ESR), and Attenuated Total Reflectance
(ATR) will also be discussed.
2.2 Infrared Laser Powered Homogeneous Pyrolysis
The use of lasers in chemistry, particularly with regard to inducing chemical reactions
can be attributed to the considerable advantages laser light has over conventional light
sources. Laser radiation is:
coherent
linear
highly monochromatic in nature
Early studies involving laser induced chemistry was limited to those molecules that
could absorb infrared radiation directly [1,2]. It was found however, that this problem
could be overcome by introducing a chemically inert, infrared absorber to the system
[3]. Energy absorbed in a vibrational mode of the photosensitiser could be rapidly
converted to heat through a very efficient relaxation process. The resultant
translational energy could then be transferred to the reagent molecules via
intermolecular collisions, in much the same way as for conventional thermal pyrolysis.
Known as Infrared Laser Powered Homogeneous Pyrolysis (IR LPHP), this technique
provided many advantages over its conventional counterpart.
7
Unlike conventional ‘hot walled’ pyrolysis, IR LPHP, as the name implies is
homogeneous. Energy is conveyed directly into the gas phase at the centre of the cell.
The resultant inhomogeneous temperature profile has a twofold advantage:
Surface initiated reactions are eliminated.
The primary products of pyrolysis are initially ejected into the cooler regions of the
cell, thereby inhibiting their further reaction.
Consequently Infrared Laser Powered Homogeneous Pyrolysis has provided a
valuable tool for the study of gas phase reactions. IR LPHP has been extensively
reviewed in an article by Russell [4].
2.3 Equipment
2.3.1 Vacuum Line
Two vacuum lines were available for use. One vacuum line was used for the
manipulation of the selected gases, while the other was primarily for the matrix
isolation of free radicals for subsequent ESR analysis. This technique will be further
elaborated on, in section 2.6.3.
2.3.2 Pyrolysis Cell
With the exception of those experiments involving ATR and XPS analysis, the
pyrolysis cell, shown in figure 2.1, consisted of a pyrex cylinder approximately 100
mm in length and 38 mm in diameter. Protruding from the base of the cell was a small
well, used to retain liquids or chemicals of low vapour pressure. A zinc selenide
(ZnSe) window was fitted to each end of the cell. The windows were attached to the
cell using a quick setting epoxy resin.
8
Figure 2.1 Schematic diagram of the conventional pyrolysis cell
ATR analysis, required that the pyrolysis cell be modified such that part of the cell
wall consisted of a ZnSe prism. This is shown schematically in figure 2.2. In this way
any material, that during the course of a reaction, would deposit on the cell wall would
also accumulate on the prism plane, and thus be available for ATR analysis. Pyrolytic
experiments involving subsequent XPS analysis were performed within a specially
adapted cell [5].
9
Figure 2.2 Schematic diagram of the pyrolysis cell used for ATR analysis
2.3.3 Window Material
To each end of the pyrolysis cell was attached a zinc selenide window. An IR LPHP
pyrolysis cell window must be:
Transparent to the CO2 laser radiation.
Transparent to the infrared spectrometer radiation.
Thermally stable and strong.
Chemically inert.
10
Zinc selenide satisfies all of the above requirements, and unlike its alkali halide
counterparts, is non hygroscopic. This enables the study of moisture sensitive
organometallic compounds.
2.3.4 Photosensitiser
The addition of a photosensitiser to the pyrolysis cell allows for the IR LPHP of
compounds that will not directly absorb infrared radiation. The photosensitiser of
choice, sulfur hexafluoride (SF6), is ideal as it possesses the following characteristics:
Strong absorption of radiation at 10.6 m, which corresponds to that generated by
the CO2 laser.
A very efficient inter and intramolecular relaxation process, thus enabling it to
transfer energy to the reagent molecule.
High thermal stability, reportedly up to 1500 K [6].
Chemically inert.
Low thermal conductivity [7]; thus the heat generated by laser irradiation is
confined to the centre of the cell.
While the peaks attributed to SF6 were present when examining reactions by FTIR,
these rarely proved problematic when interpreting spectra. The product and reactant
bands infrequently overlapped with those of SF6 [8].
11
2.3.5 CO2 Laser
The CO2 laser is of immense importance in both industry and research. Its operation is
based on transitions between the vibrational levels of the CO2 molecule, as shown
schematically below in figure 2.3.
Figure 2.3 Energy level diagram for the CO2 laser
3000
2000
1000
Energy
cm-1
N2 CO2
Asymmetric stretch
Bending Symmetric stretch
(000) (000) (000)
(010)
(020)
(030)
(040)
(200)
(100)
(001)
Electric Discharge
10.6m
9.6m
Laser action is a product of a series of steps. An electrical discharge is passed through
a gaseous mixture of helium, nitrogen, and carbon dioxide, the proportions of which
are 80, 10 and 10 % respectively. Consequently the He atoms eject electrons, which in
turn excite the N2 molecules into the =1 vibrational level. Energy is transferred via
12
intermolecular collisions from this level into the antisymmetric stretching vibrational
level (001) of CO2. The resultant population inversion within the CO2 molecule is lost
as energy is transferred into the lower CO2 symmetric stretching mode (100) and CO2
bending mode (020). Laser emission at 10.6 m and 9.6 m respectively, accompanies
this process.
The IR LPHP experiments discussed herein were performed using an Electrox
Industrial ‘M-80’ free running carbon dioxide laser. This operated at a wavelength of
10.6 m, with a total power output range of between 60 and 80 W. The beam size, and
hence the power output, could be reduced by decreasing the aperture at the beam exit.
Aperture diameters are given below in table 2.1. To a first approximation, the power
output was taken to be proportional to the square of the diameter of the exiting beam.
Aperture 1 allowed the complete transmission of the exiting beam, thus the total
power output was not reduced. Conversely, the utilisation of aperture 18, having a
diameter approximately a tenth that of aperture 1, would reduce the total power output
by a factor of a hundred. It should be noted that the resultant temperature of the
pyrolysis cell is dependent on a number of variables (laser power, thermal
conductivity and chemical composition of the contents of the cell etc), and as such is
difficult to discern.
Table 2.1 Aperture Diameters
Aperture 1 2 3 4 5 6 7 8 9
Diameter (mm) 11.3 8.4 7.6 6.7 6.0 5.3 4.6 4.3 4.0
Aperture 10 11 12 13 14 15 16 17 18
Diameter (mm) 3.7 3.4 3.1 2.8 2.5 2.2 1.9 1.6 1.3
13
2.4 Chemicals
The chemicals selected for study were obtained as analytical grade. Table 2.2 lists
both the compounds used and their source.
Table 2.2 Chemicals used and source
Chemical Name Formula Source
Dichloromethane CH2Cl2 Aldrich
Freon 12 CF2Cl2 Matheson
Freon 22 CF2HCl Du Pont
Iron pentacarbonyl Fe(CO)5 Fluka Chemicals
Tungsten hexacarbonyl W(CO)6 Prosynth
2.5 Experimental Procedure
2.5.1 Introduction
Experiments involving the IR LPHP of specific molecules required prior sample
manipulation using a standard vacuum line. Those experiments involving subsequent
ESR analysis, namely the matrix isolation of free radicals were performed on a second
vacuum line. Both vacuum lines were set up when required according to instructions
detailed in the accompanying manual [9]. Once a pressure of 0.004 Torr [10] was
attained, sample preparation could proceed.
14
2.5.2 Procedure for Sample Preparation
The method of experimental analysis, that is FTIR, ATR or XPS, determined the type
of cell used. The appropriate pyrolysis cell was fitted to the vacuum line and
evacuated. Introduction into the cell of the selected compounds was achieved through
one of two ways. Samples with sufficient vapour pressure were stored in vacuum
tubes that could be fitted to the line. Once attached, the sample was subjected to
repeated freeze-pump-thaw cycles so as to remove any unwanted gases, a process
known as degassing. A quantity of vapour, of the order of 1 or 2 Torr, was then
transferred into the cell. Samples of low vapour pressure were placed directly into the
cell reservoir, where they were then outgassed. In all experiments, approximately 10
Torr of degassed SF6 was introduced into the cell as the photosensitiser. After the
required components had been added, the cell was removed from the line and, so as to
provide a means of reference, analysed using FTIR spectroscopy.
2.5.3 Pyrolysis Setup
For successful IR LPHP the cell had to be positioned such that the ZnSe window was
perpendicular to the beam. After selecting the power and aperture setting the cell was
appropriately placed approximately 20 mm from the laser. As a precautionary measure
a firebrick was placed behind the cell so as to absorb the emerging beam. The
pyrolysis cell was then irradiated for a predetermined length of time. When required,
the vapour pressure of those compounds contained within the cell reservoir could be
increased by gently warming the exterior of the cell with a hairdryer.
15
2.6 Experimental Analysis
2.6.1 Introduction
Whether by ATR, ESR, FTIR, or XPS the main purpose of analysis was to provide
information that could be used so as to elucidate the mechanism of substrate
decomposition. ATR and XPS provided a suitable avenue by which to examine any
pyrolytic deposits, while FTIR was invaluable in identifying products of a gaseous
nature. ESR spectroscopy can be a useful tool in determining the identity of short
lived intermediates.
2.6.2 Fourier Transform Infrared Spectroscopy
The primary means of monitoring the gas phase decomposition of those selected
chlorofluorocarbons was FTIR spectroscopy. FTIR provided a convenient, non-
invasive method by which the cell contents, both before and after pyrolysis could be
identified. All infrared spectra were obtained using a FTS-60 Bio-Rad Digilab
Division spectrometer. In order to inhibit the peaks attributed to atmospheric CO2 or
water a dry nitrogen purge was attached to the sample entry chamber of the
spectrometer.
2.6.3 Matrix Isolation ESR Spectroscopy
Experimental analysis involving ESR spectroscopy required the initial matrix
isolation of free radicals. This procedure was performed on a vacuum line by allowing
the reagent(s) to pass through a hot walled furnace, whereupon thermal pyrolysis
occurred. The pyrolysis products and any unreacted starting material would form a
matrix on a liquid nitrogen cooled ‘cold finger’ positioned directly above the furnace;
thereby trapping any radical intermediates present. Subsequent ESR analysis involved
placing the section of line containing the matrix into a Varian E-4 ESR spectrometer.
16
2.6.4 X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS) provided a suitable technique by which to
characterise the upper atomic layers of any material deposited as a result of substrate
decomposition. In order to perform XPS analysis, the pyrolytic experiment was
executed within a specially adapted cell, that following pyrolysis, could be transported
and fitted to the XPS spectrometer without exposing the sample to the atmosphere.
The details of this technique are given in the post doctoral report produced by Dr
Rebecca Berrigan [11].
XPS data acquisition and interpretation was performed by Dr Rebecca Berrigan. XPS
experiments were carried out in a Kratos XSAM 800 XPS/Auger spectrometer. The
base pressure in the analysis chamber of the spectrometer was 5.0 x 10-10 Torr.
Spectra were recorded using MgK X-rays with source settings of 12 mA and 14 kV.
Wide scans were collected using a pass energy of 65 eV and individual peaks
collected using a pass energy of 20 eV.
The spectrometer was controlled and data analysis performed using Vision software
operating on a Sun Sparc workstation. Peaks were referenced to the adventitious
carbon C 1s peak at 285.0 eV [12]. Quantitative analysis was performed using
empirically derived sensitivity factors [13]. The assignment of peak components was
made possible by comparison with literature values [12].
17
2.6.5 Attenuated Total Reflectance
In an effort to confirm those results obtained from XPS analysis, the technique of
Attenuated Total Reflectance (ATR) was utilised. This involved initial pyrolysis
within a modified pyrolysis cell. As described in section 2.3.2, the cell used had a
ZnSe prism affixed such that any material deposited would also accumulate on the
prism plane. The material deposited could be analysed using FTIR spectroscopy by
placing the cell in a specially adapted holder, such that the beam would be refracted
onto, and reflected from the prism surface. The beam’s path is diagrammatically
presented in figure 2.4. Unlike XPS, ATR characterises the interface between the
deposit and the ZnSe prism. Thus it is possible, when combining the results of the two
techniques, to characterise the film as a whole.
Figure 2.4 Schematic diagram of the ATR setup
Prism
100 mm
Mirror
IR Radiation
Cell holder
38 mm
18
2.7 References and Notes for Chapter 2
[1] C. Bordé, A. Henry, L. Henry,. Compt. Rend. Acad. Sci. Paris, Ser B, 1966, 262,
1389.
[2] C. Bordé, C. Cohen, L. Henry, Compt. Rend. Acad. Sci. Paris, Ser B, 1967, 265,
267.
[3] J. Tardieu de Maliesse, Compt. Rend. Acad. Sci. Paris, Ser. C, 1972, 275, 989.
[4] D. K. Russell, Chem. Soc. Rev., 1990, 19, 407.
[5] The pyrolysis cell used for subsequent XPS analysis was designed by Dr Rebecca
Berrigan.
[6] J. L. Lyman, J. Chem. Phys., 1977, 67, 1868.
[7] A. L. Horvath, Ed., Physical Properties of Inorganic Compounds, Arnold,
London (1975).
[8] E. A. Jones and R. T. Lagemann, J. Chem. Phys., 1951, 19, 534.
[9] R. Linney, Laser Pyrolysis Techniques; Experimental Methods (1994).
[10] 1 Torr = 133.332 Pa
[11] R. Berrigan, to be published.
[12] K. D. Bomben, J. F. Moulder, P. E. Sobol, W. F. Stickle, Perkin Elmer Handbook
of X-ray Photoelectron Spectroscopy (1992).
[13] D. Briggs and M. P. Seah, Practical Surface Analysis by Auger and X-ray
Photoelectron Spectroscopy, John Wiley & Sons (1983).
19
Chapter 3. Pyrolysis Results and Discussion
3.1 Introduction
The pyrolysis results presented herein were acquired so as to provide a reference for
later copyrolysis. The compounds selected for pyrolytic study were the
chlorofluorocarbons Freon 12 (CF2Cl2), Freon 22 (CF2HCl) and the non-fluorinated
analogue, dichloromethane (CH2Cl2). Subsequent copyrolytic studies would involve
the use of two transition metal organometallic compounds; thus each of these metal
carbonyl compounds was also the subject of IR LPHP.
3.2 IR LPHP of Freon 12
3.2.1 Literature
While the photolytic decomposition of CF2Cl2 is dissimilar from IR LPHP, in that a
photosensitiser is not present, much work has been done that would suggest the results
obtained from either technique are comparable [1]. Zitter and coworkers [2] have
proposed that the photolytic decomposition of CF2Cl2 occurs via an initial chlorine
abstraction, to give CF2Cl. Subsequent reaction of the CF2Cl radical is temperature
dependent, yielding the recombination product, C2Cl2F4 as the major product at low
temperature, and CF3Cl at high temperature. The postulated set of reactions is given in
figure 3.1.
20
Figure 3.1 The decomposition scheme of CF2Cl2 as proposed by Zitter et al [2]
CF2Cl2 CF2Cl• + Cl• ...1
CF2Cl• + CF2Cl• CF2ClCF2Cl ...2
Cl• + Cl• Cl2 ...3
CF2Cl• + CF2Cl2 CF3Cl + CFCl2• ...4
CFCl2• + CF2Cl2 CFCl3 + CF2Cl• ...5
CFCl2• + Cl• CFCl3 ...6
CF2Cl• + CFCl2• CF2ClCFCl2 ...7
CFCl2• + CFCl2
• CFCl2CFCl2 ...8
Cl• + CF2Cl2 Cl2 + CF2Cl• ...9
The IR laser photolysis of CF2Cl2 was also studied by Hill et al [3], the results of
which coincided with those of Zitter et al. Hill and coworkers, whose experiments
were performed at a static intermediate temperature, however, proposed an alternative
mechanism for CF3Cl and C2Cl2F4 formation. A singlet radical-radical reaction would
yield C2Cl2F4, while a triplet radical-radical reaction would give CF3Cl and CClF. The
reaction scheme is given in figure 3.2.
21
Figure 3.2 The decomposition scheme of CF2Cl2 as proposed by Hill et al [3]
CF2Cl2 CF2Cl• + Cl• ...1
2 CF2Cl• [singlet collision complex] CF2ClCF2Cl ...2
2 CF2Cl• [triplet collision complex] CF3Cl + :CClF (triplet) ...3
3.2.2 Experimental
In our studies it was found that the pyrolytic decomposition of CF2Cl2 commenced at
a temperature corresponding to a laser setting of 75 W at aperture 5. FTIR analysis
revealed product peaks attributed to CF3Cl, CF2O and one or more unidentifiable
compounds [4,5]. No peaks attributable to C2Cl2F4 were observed [6]. The results of
Freon 12 pyrolysis at a laser setting of 75 W at aperture 5 after 45 seconds are shown
in figure 3.3.
22
Figure 3.3 FTIR spectra of a gaseous mixture of Freon 12 and SF6 before (top)
and after (bottom) IR LPHP. Features identified are due to CF2Cl2()
and SF6()
0
.1
.2
.3
.4
.5
.6
.7
.8
.9
Ab
sorb
ance
2000 1800 1600 1400 1200 1000 800
Wavenumber (cm-1)
CF3Cl
CF2OCF2O
The apparent formation of CF3Cl at the expense of C2Cl2F4 would suggest that the
mechanism of freon decomposition concurs with that proposed by Zitter et al at high
temperature [2]. The temperatures obtained in our pyrolytic experiments, combined
with the statistically low chance of a triplet state radical recombination all but
eliminate decomposition by the method proposed by Hill et al [3]. Chowdhury and
coworkers [7] proposed that CF2O formation involved the reaction of CF2 with any
adventitious molecular oxygen. However the formation of CF2 from CF2Cl2 would
appear to require the loss of two chlorine atoms, an intuitively unlikely event.
Successive experiments involving the copyrolysis of Freon 12 with those selected
transition metal organometallic compounds, were found to provide significant results
pertaining to the standard pyrolytic decomposition of CF2Cl2. The results, given in
section 4.3 were employed so as to develop a modified scheme, as shown in figure
3.4, for the high temperature decomposition of CF2Cl2.
23
Figure 3.4 The proposed high temperature decomposition scheme of CF2Cl2
Cl
Cl
F2C CF2Cl
C2Cl2F4
Cl
CF2Cl2 + CF2
CF2
C2F4
+
CF2Cl
CF3Cl
CF2Cl2
CF3Cl CFCl 2+
?
O2CF2O
1
2
3
4
5
6
7
8
Note. Reaction 2 occurs more readily than reaction 3 at high temperature.
The exact mechanism of reaction 8 is as yet unclear.
It is possible that at high temperature the predominant path of CF2Cl2 decomposition
would be that of CF3Cl formation via the reaction of CF2Cl with CF2Cl2 [2]. The
alternate route, whereby two CF2Cl radicals recombine to form C2Cl2F4, would be less
likely on account of the lower effective concentration of the CF2Cl radical. The
absence of C2Cl2F4 in the IR would suggest that at such high temperatures, C2Cl2F4 is
highly unstable and rapidly decomposes. Zitter and Koster [8] proposed that the
decomposition of C2Cl2F4 would result in the formation of CF2Cl2 and CF2. The
CF2Cl2 produced would decompose as above, thus perpetuating the decomposition
cycle. The CF2 species could either react with any adventitious oxygen to form CF2O
[7] or dimerise to form C2F4 [9-12]. Subsequent studies involving the high temperature
24
pyrolysis of CF2HCl revealed that C2F4 decomposition was coupled with CF3Cl
formation. The exact mechanism of C2F4 decomposition however, is as yet unclear.
Subsequent experiments involving the pyrolysis of CF2Cl2 at a temperature
corresponding to a laser setting of aperture 4 or below were found to increase the rate
of freon decomposition. This was however, accompanied by the formation of CF4,
SiF4 and one or more unidentifiable gaseous compounds [13,14]. It was proposed that at
these high temperatures the photosensitiser, SF6, decomposes. The resultant fluorine
radicals combine with either the cell wall (SiO2) or other radical fragments to form the
products observed. In an effort to minimise the problems associated with secondary
reactions involving SF6 decomposition, further copyrolytic studies involving CF2Cl2
would be performed at the temperature at which decomposition was initiated.
3.3 IR LPHP of Freon 22
3.3.1 Literature
It has been postulated [9-12] that the decomposition of CF2HCl proceeds via an initial
-elimination step. Dimerisation of the resultant CF2 species follows, producing C2F4.
The proposed decomposition scheme of CF2HCl is given below in figure 3.5.
Figure 3.5 The proposed decomposition scheme of CF2HCl [9-12]
H
Cl
CF2 CF2 + HCl
CF22 C2F4
25
3.3.2 Experimental
Our results indicated that the decomposition of Freon 22 began at a temperature
corresponding to a laser setting of 75 W at aperture 15. The rate of decomposition,
illustrated in the IR spectrum by a decrease in peak intensity was, however, very slow.
It was observed that the pyrolysis of CF2HCl at aperture 12 afforded an increased rate
of Freon 22 decomposition. FTIR analysis also revealed peaks attributed to HCl and
C2F4 formation [15]. Lesser peaks indicative of CF2O were also present [5]. It was
proposed that the presence of CF2O could be attributed to the reaction of the CF2
species with any adventitious oxygen. Successive experiments involving the
copyrolysis of CF2HCl with excess oxygen showed preferential CF2O formation. The
results of Freon 22 pyrolysis at a laser setting of 75 W at aperture 12 are shown below
in figure 3.6.
Figure 3.6 FTIR spectra of a gaseous mixture of Freon 22 and SF6 before (top)
and after (bottom) IR LPHP. Features identified are due to CF2HCl
() and SF6 ()
0
.1
.2
.3
.4
.5
.6
.7
.8
Ab
so
rba
nce
3000 2500 2000 1500 1000
Wavenumber (cm-1)
HCl
C2F4
C2F4
CF2O CO2
CO2
Subsequent pyrolytic experiments involving higher temperatures (by utilising a larger
aperture size) revealed CF3Cl formation coupled with C2F4 decomposition. The
26
formation of SiF4, indicative of secondary reactions involving the pyrex cell wall was
observed at temperatures corresponding to apertures 4 and below. In an effort to
minimise the problems associated with secondary reactions, further copyrolytic
studies involving CF2HCl and either Fe(CO)5 or W(CO)6 would be performed at
aperture 12.
3.4 IR LPHP of Dichloromethane
In much the same way that CF2HCl was shown to lose an HCl molecule, we proposed
that CH2Cl2 would lose two HCl molecules to give carbon, as shown in figure 3.7.
This in turn might polymerise to form specific polymeric compounds, in particular
fullerenes. In our studies it was found that dichloromethane showed signs of
decomposition at temperatures corresponding to a laser setting of 70 W at aperture 7.
FTIR analysis revealed a series of peaks centred at 2883 cm-1 indicative of HCl
formation. No other decomposition product peaks were observed.
27
Figure 3.7 The decomposition scheme of CH2Cl2
H
C
Cl
H
Cl
H
C
Cl
+ HCl
C + HCl
Cn
(Deposits on cell wall)
Subsequent experiments utilising a higher temperature regime (apertures 4 or below),
revealed the formation of CF3Cl, SiF4 and one or more unidentifiable compounds, as
evidenced by the peaks in the IR spectrum [4,14]. The presence of those peaks centred
at 750 or 805 cm-1 may be attributed to a C-Cl stretch, implying the existence of an as
yet unidentified chlorinated compound. The products observed were also seen in the
high temperature pyrolysis of CF2Cl2. It was proposed that in a manner analogous to
that for the high temperature pyrolysis of CF2Cl2 (refer to section 3.2.2), the products
formed were the result of secondary reactions involving SF6 decomposition. Further
copyrolytic studies involving CH2Cl2 will therefore be performed at temperatures
equal to or lower than that required for primary decomposition. The results of the high
temperature pyrolysis of CH2Cl2 are given in figure 3.8.
28
Figure 3.8 FTIR spectra of a gaseous mixture of dichloromethane and SF6 before
(top) and after (bottom) high temperature IR LPHP. Features
identified are due to CH2Cl2 () and SF6 ()
0
.05
.1
.15
.2
.25
.3
.35
Absorb
ance
1300 1200 1100 1000 900 800 700
Wavenumber (cm-1)
SiF4
CF3Cl
CF3Cl
CF3Cl
?
??
?
In an effort to confirm the hypothesised mechanism for CH2Cl2 decomposition at low
temperature (apertures 5 through 8) further experiments were attempted. The pyrolysis
of dichloromethane in the presence of excess oxygen was performed with the
intention of trapping the carbon as carbon dioxide. FTIR analysis revealed product
peaks attributed to CO, CO2 and surprisingly COCl2 [5]. While the decomposition
mechanism of CH2Cl2 to yield COCl2 has yet to be elucidated, the formation of CO
and CO2 can be illustrated by the proposed mechanisms given in figure 3.9.
29
Figure 3.9 The decomposition scheme of CH2Cl2 in the presence of oxygen
i) CO formation
H
C
Cl
H
Cl
H
C
Cl
+ HCl
O2
H
C
Cl
O
(unstable)
CO + HCl
ii) CO2 Formation
H
C
Cl
H
Cl
H
C
Cl
+ HCl
C + HCl
O2
CO2
30
If sufficient CH2Cl2 was subjected to IR LPHP, one might detect a deposit on the cell
wall. For previous experiments, a quantity of between 1 and 2 Torr of CH2Cl2 was
introduced into the cell. If any deposit did form as a result of pyrolysis, it would most
likely be in such minute amount so as to be unobservable. Thus a pyrolytic experiment
was performed in which 20 Torr of CH2Cl2 was decomposed at a temperature
corresponding to a laser setting of 75 W at aperture 5. Several events were observed as
the pyrolysis proceeded. Initially a red luminescence could be seen along the cell
center. Accompanying this was the accumulation of a black substance on the cell wall.
Subsequent FTIR analysis revealed a spectrum not unlike that seen after high
temperature pyrolysis (refer to figure 3.8). The red glow was most likely attributable
to the irradiation of carbon precipitating out of the gas phase. The irradiation of a solid
would be expected to yield a much higher temperature than that of a gas, thereby
possibly promoting secondary reactions involving SF6 decomposition.
Mass spectral analysis of the material deposited revealed a mixture of several
polycyclic aromatic hydrocarbons (ie. tar). While this result did not provide evidence
for fullerene formation, it was nonetheless significant in that it confirmed the presence
of high molecular weight polycyclic aromatic hydrocarbons.
3.5 IR LPHP of Transition Metal Carbonyl Compounds
3.5.1 Introduction
The copyrolytic study of a chlorofluorocarbon with a metal containing compound in
the gas phase required an initial understanding of the decomposition mechanism
and/or rate of both the freon and transition metal precursor. The addition of a metal
system in the gas phase could be achieved by introducing a volatile metal carbonyl
that would decompose at temperatures less than that required for the freon. The metal
carbonyls selected for study were iron pentacarbonyl, Fe(CO)5 and tungsten
hexacarbonyl, W(CO)6.
31
3.5.2 IR LPHP of Fe(CO)5
It was found that the decomposition of Fe(CO)5 commenced at a temperature
corresponding to a laser setting of 70 W at aperture 18. This was much lower than that
required for CF2Cl2, CF2HCl and CH2Cl2. Decomposition of Fe(CO)5 evolved
significant quantities of carbon monoxide as evidenced by the series of peaks centred
at 2140 cm-1 in the IR spectrum, and was accompanied by the accumulation of a black
deposit on the cell wall. No peaks attributable to the starting material were observed
after 45 seconds of Infrared Laser Powered Homogeneous Pyrolysis.
3.5.3 IR LPHP of W(CO)6
The relatively low vapour pressure of W(CO)6 warranted the addition of the solid
directly into the cell reservoir for subsequent pyrolysis. The vapour pressure was
increased during pyrolysis, by gently heating the cell exterior with a hairdryer.
Decomposition of the metal carbonyl commenced at a temperature corresponding to a
laser setting of 70 W at aperture 18; significantly lower than any of the selected
freons. Decomposition was evidenced by the appearance of a black deposit on the cell
wall, and by the presence in the IR spectrum of peaks attributed to CO.
Unlike those pyrolytic experiments involving a finite quantity of vapour, those
incorporating W(CO)6 could be sustained until no further solid remained. This
enabled the significant accumulation of a cell wall deposit. ATR analysis of the
deposit revealed several peaks, each corresponding to a carbonyl stretching vibration,
as shown in figure 3.10. An extensive literature search has failed to establish the exact
nature of the material deposited. It would however, seem reasonable to hypothesise
that the black deposit consists of a W(CO)x species (where x is less than 6), as
opposed to pure tungsten. Thus it could be implied that the abstracting species in
subsequent copyrolytic experiments is not gaseous tungsten, but a semi-carbonylated
tungsten species.
32
Figure 3.10 ATR/FTIR spectrum of a film deposited after W(CO)6 pyrolysis
(CO)
(CO)
-.001
0
.001
.002
.003
.004
.005
Absorb
ance
2100 2050 2000 1950 1900 1850
Wavenumber (cm-1)
33
3.6 References and Notes for Chapter 3
[1] S. P. Anderson, E. Grunwald, P. M. Keehn, K. J. Olszyna, Tetrahedron Lett.,
1977, 19, 1609.
[2] A. Cantoni, T. K. Choudhury, D. F. Koster, R. N. Zitter, J. Phys. Chem., 1990,
94, 2374.
[3] E. Grunwald, G. A. Hill, P. Keehn, J. Am. Chem. Soc., 1977, 99, 6521.
[4] H. W. Thompson and R. B. Temple, J. Chem. Soc., 1948, 1422.
[5] T. G. Burke, E. A. Jones, A. H. Nielsen, P. J. H. Woltz, J. Chem. Phys., 1952, 20,
596.
[6] E. K. Plyer and D. Simpson, J. Res. NBS., 1953, 50, 223.
[7] P. K. Choudhury, J. P. Mittal, J. Pola, K. V. S. Rama Rao, Chem. Phys. Lett.,
1987, 142, 252.
[8] D. F. Koster and R. N. Zitter, J. Am. Chem. Soc., 1976, 98, 1613.
[9] D. S. King and J. C. Stephenson, J. Chem. Phys., 1978, 69, 1485.
[10] Y. T. Lee, P. A. Schulz, Y. R. Shen, A. S. Sudbo, J. Chem. Phys., 1978, 69, 2312.
[11] J. T. Herron and R. I Martinez, Chem. Phys. Lett., 1981, 84, 180.
[12] J. H. Parks and R. C. Slater, Chem. Phys. Lett., 1979, 60, 275.
[13] A. C. Jeannotte II, D. Legler, J. Overend, Spectrochim. Acta A, 1973, 29, 1915.
[14] E. A. Jones, J. S. Kirby-Smith, A. H. Nielsen, P. J. H. Woltz, J. Chem. Phys.,
1951, 19, 242.
[15] H. H. Claassen and J. R. Nielsen, J. Chem. Phys., 1950, 18, 812.
34
Chapter 4. Copyrolysis Results and Discussion
4.1 Introduction
The copyrolysis of a chlorofluorocarbon with a volatile transition metal
organometallic compound may effect the decomposition rate and/or mechanism of
that chlorofluorocarbon. Assuming that a transition metal compound decomposes at a
temperature less than that required for the freon, it was proposed that the resultant
radical species may abstract a halogen atom from the chlorofluorocarbon, and thus
initiate freon decomposition. The transition metal compounds selected for study were
Fe(CO)5 and W(CO)6. These were both shown in earlier pyrolytic experiments to
decompose via a metal carbonyl bond homolysis, at temperatures much less than that
required for Freon 12, Freon 22 and dichloromethane decomposition.
4.2 Copyrolysis of Freon 12 with Fe(CO)5
The decomposition of Freon 12 in the presence of an equivalent amount of Fe(CO)5
commenced at a temperature corresponding to a laser setting of 75 W at aperture 7.
This was somewhat less than that required for the pyrolysis of CF2Cl2 alone, where
decomposition commenced at a temperature corresponding to a laser setting of 75 W
at aperture 5.
After irradiating the pyrolysis cell for 30 seconds, FTIR analysis revealed a 30 %
reduction in the level of CF2Cl2, coupled with an absence of those peaks normally
observed after standard (CF2Cl2 without Fe(CO)5) pyrolysis. The product of Fe(CO)5
decomposition, namely CO was identified as a series of peaks in the IR centered at
2140 cm-1. Interestingly a peak at 1032 cm-1 was also observed, implying SiF4
formation [1]. SiF4 was not observed under standard pyrolytic conditions until a
temperature, corresponding to a laser setting of 75 W at aperture 4, had been reached.
In addition to those results obtained from FTIR, visual inspection of the pyrolysis cell
35
revealed the accumulation of a black/grey deposit. Surface analysis techniques have
not, as yet, been used to identify the material deposited, thus our understanding of the
copyrolytic decomposition mechanism of CF2Cl2 with Fe(CO)5 is limited. The results
obtained, however, would suggest a decomposition mechanism dissimilar to that
operating under standard pyrolytic conditions. It is believed that Fe(CO)x, where x is a
low integer (given the level of CO evolution), abstracts a halogen atom from the freon
to form a compound that is in turn deposited on the cell wall. While the subsequent
reactions involving the resultant freon radical in a Freon 12/Fe(CO)5 system are as yet
unconfirmed, it is apparent that those involving CF3Cl or CF2O formation are not
occurring. It may seem reasonable therefore to theorise that the freon radical is instead
losing additional halogen atoms and/or reacting with the cell wall (SiO2) to form the
SiF4 observed.
Subsequent experiments involving a different ratio of Fe(CO)5 to CF2Cl2, illustrated
that the level of CF2Cl2 decomposition, given by the decrease in peak intensity in the
IR spectrum, was greater for those experiments in which the ratio of metal carbonyl to
freon was higher. Given in table 4.1 are the results pertaining to the copyrolysis of
CF2Cl2 with varying amounts of Fe(CO)5. The product of pyrolysis in each case was a
black/grey deposit. No significant product peaks (excluding those attributed to CO)
were observed in the IR spectrum.
Table 4.1 The dependence of CF2Cl2 decomposition on the ratio of Fe(CO)5 to
CF2Cl2
Fe(CO)5 : CF2Cl2 Reduction in CF2Cl2 peak area (%)
1:1 approx. 30
6:1 approx. 40
9:1 approx. 60
36
4.3 Copyrolysis of Freon 12 with W(CO)6
It was found that the complete decomposition of CF2Cl2 in the presence of W(CO)6
took approximately 210 seconds (3 ½ minutes) at a temperature corresponding to a
laser setting of 75 W at aperture 12. A similar experiment involving CF2Cl2 alone,
resulted in no apparent decrease in the level of freon at that temperature. Therefore, in
a manner analogous to that described in section 4.2 (where a halogen abstracting
Fe(CO)x species (where x is a low integer) was thought to initiate decomposition), it
was proposed that a W(CO)x species (where 0<x<5) was promoting a halogen
abstraction at a lower temperature than that normally required for halogen loss.
The fact that W(CO)6 was shown to promote decomposition of Freon 12 at a
temperature lower than that of Fe(CO)5 (where decomposition commenced at aperture
7) may be attributed to the strength of the resultant M-X bond, where M is the
transition metal and X is the abstracted halogen. The strength of the M-X bond was
approximated by the diatomic bond energy of that bond. A stronger M-X bond would
be expected to form more readily than that of a weaker M-X bond. Assuming a Cl
abstraction (which was later verified), the results pertaining to the copyrolysis of
Freon 12 with either Fe(CO)5 or W(CO)6 appeared to support this proposition, in that
the W-Cl bond energy was larger than that of Fe-Cl [2]. The diatomic bond energy
however of Fe-Cl was in fact lower than that of C-Cl [2], suggesting that the
abstraction of Cl from CF2Cl2 using a Fe(CO)x species would not occur. The results
outlined in section 4.2 clearly show that this is not the case. While the diatomic bond
energy can give an indication, as to the outcome of Freon 12 copyrolysis, it is
fundamentally a thermodynamic quantity and as such, does not take into account the
kinetic stability of the diatomic bond. In some cases therefore the theory fails. It
should also be noted that the strength of the resultant M-X bond was approximated by
the bond energy of the diatomic M-X molecule, and as such may differ slightly to the
energy of the actual M-X bond. Given in table 4.2 are the relevant diatomic bond
energies along with the predicted and experimental results pertaining to Freon 12
decomposition.
37
Table 4.2 Diatomic bond energies
M-X Bond Diatomic Bond Predicted Rate of Experimental Rate of
Energy [2] Decomposition * Decomposition *
C-Cl 397 kJ mol-1 - -
C-F 536 - -
Fe-Cl 352 reduced increased
Fe-F# - - -
W-Cl 423 increased increased
W-F 548 increased increased
* With respect to the decomposition rate of Freon 12 alone.
# A literature value for the Fe-F bond strength could not be found.
With regard to the products formed, the results of CF2Cl2 decomposition in the
presence of W(CO)6 were significantly different to those obtained from the
decomposition of CF2Cl2 alone. FTIR analysis revealed the absence of those peaks
attributed to CF3Cl and CF2O (those products observed in the standard pyrolysis) and
the presence of peaks assigned to SiF4, C2Cl2F4, C2F4, and an as yet unidentified
compound [1,3-6]. The results of CF2Cl2/W(CO)6 copyrolysis at a laser setting of 75 W
at aperture 12 after 210 seconds are given in figure 4.1.
38
Figure 4.1 FTIR spectra of a gaseous mixture of Freon 12, W(CO)6 and SF6
before (top) and after (bottom) IR LPHP. Features identified are due
to CF2Cl2 (), SF6 () and C2Cl2F4 ().
-.3
-.2
-.1
0
.1
.2
Ab
so
rba
nce
2000 1800 1600 1400 1200 1000 800
Wavenumber (cm-1)
W(CO)6
W(CO)6
?C2F4
SiF 4
In an effort to establish the reasons for these differences, a mechanism was proposed
in which the products of CF2Cl2 decomposition were dependent on the temperature of
pyrolysis. The mechanism, given in figure 4.2, was derived from those results
obtained from both the high temperature (when W(CO)6 was not present) and the low
temperature (when W(CO)6 was present) pyrolysis.
39
Figure 4.2 The decomposition scheme of CF2Cl2
Cl
Cl
F2C CF2Cl
C2Cl2F4
Cl
CF2Cl2 + CF2
CF2
C2F4
+
CF2Cl
CF3Cl
CF2Cl2
CF3Cl CFCl 2+
?
SiF4
SiO2
O2CF2O
1
2
3
4
5
6
7
8
9
Note. Reactions 3 and 9 occur at high temperature only.
Reaction 2 occurs at low temperature only.
Reactions 1, 4, 5, 6, and 8 occur at both high and low temperature.
Reaction 1 involves a Cl abstraction at low temperature, where W(CO)x is the
abstracting species.
The exact mechanism of SiF4 formation (via reaction 2) is as yet unclear.
The absence of CF2O at low temperature suggests a lack of adventitious
oxygen.
The exact mechanism of reaction 9 is as yet unclear.
40
The decomposition of Freon 12 was shown by Zitter et al to commence with the loss
of a Cl to generate a CF2Cl radical [7]. Our studies, involving the matrix isolation ESR
spectroscopy of radical intermediates formed as a result of CF2Cl2 decomposition,
revealed the presence of a resonance signal with a g value of approximately 2. While
the characterisation of this signal is as yet not complete, it implies the presence of a
radical, containing a free spinning unpaired electron. In those studies involving both
CF2Cl2 and W(CO)6, it was proposed that the generation of a CF2Cl radical was
accompanied by the formation of a W(CO)xCl6-x species (the result of W(CO)x
abstracting a Cl from (6-x) CF2Cl2, assuming that only one Cl is abstracted from each
CF2Cl2 molecule).
Subsequent reactions involving CF2Cl were temperature dependent. Those reactions
occurring in the high temperature pyrolysis of CF2Cl2 have been described in section
3.2. At low temperature the CF2Cl radical may either react with the cell wall (SiO2) to
form SiF4, or recombine with another CF2Cl to form C2Cl2F4 [7,8]. In contrast to that at
high temperature, the reaction involving CF3Cl formation, whereby CF2Cl reacts with
CF2Cl2 [7,8] does not occur. The appearance of C2F4, given the proposed mechanism of
formation, would suggest that even at such low temperatures C2Cl2F4 decomposition
occurs. The absence of CF2O at low temperature may be attributed to the absence of
oxygen impurity in the pyrolysis cell. The CF2Cl2 produced as a result of C2Cl2F4
decomposition [9] would decompose as above, thus perpetuating the decomposition
cycle. Given that the subsequent decomposition of CF2Cl2 does not seem to involve
CF3Cl formation, it is apparent that at low temperature the likelihood of C2Cl2F4
formation is greater than for that at high temperature. FTIR analysis revealed that the
level of C2Cl2F4 remained static over time, suggesting that the rate of C2Cl2F4
decomposition is approximately equal to that of C2Cl2F4 formation [5]. Only after all
the CF2Cl2 had decomposed was a decrease in the level of C2Cl2F4 observed. This was
in marked contrast to that at high temperature, where the presence of C2Cl2F4 was not
detected. It was therefore postulated that at high temperature the rate of decomposition
of C2Cl2F4 was far greater than that of formation.
In addition to those gaseous products observed in the IR spectrum, visual inspection
of the pyrolysis cell revealed the accumulation of a black/grey deposit. In an attempt
41
to characterise this material, and as such confirm the proposed mechanism of
decomposition, the techniques of ATR and XPS were utilised.
XPS analysis revealed peaks attributed to tungsten, carbon, chlorine and oxygen as
shown in figure 4.3. A small peak assigned to fluorine was also present. Quantitative
analysis performed by a comparative integration of the Cl 2p and F 1s peaks revealed
an approximate Cl : F atomic ratio of 7 : 2. With regard to the proposed
decomposition mechanism of CF2Cl2, and in particular the halogen initially
abstracted, this may imply a preferential Cl abstraction (with reference to fluorine).
Figure 4.3 XPS spectrum of a film deposited after Freon 12/W(CO)6 copyrolysis
Cl LMM
C KLL
O KLL
F 1s
W 4s
O 1s
W 4p
C 1s
W 4f Cl 2p
Cl 2s
W 4d
Considering the wide variety of possible tungsten containing deposits from this
experiment (tungsten oxides, chlorides, fluorides, semi-carbonylated chlorides etc), an
unambiguous deconvolution of the W 4f photoelectron signal was very difficult.
Tentative deconvolution of the W 4f peak, as shown in figure 4.4, would indicate that
42
the tungsten in the deposit is oxidised, either present as an oxide, such as WO3, a
chloride, for example WCl6, or most likely a semi-carbonylated tungsten chloride
species such as W(CO)5Cl. The unambiguous assignment of each of those
components in the W 4f signal is not, at this stage, possible. Further experimentation
will be required, ensuring a rigorous exclusion of oxygen from the deposition
environment. This should inhibit the formation of those tungsten oxide species and
render the W 4f peak more distinctly interpreted.
Figure 4.4 XPS spectrum of the W 4f photoelectrons
While XPS analysis of the copyrolytic deposit illustrated that, for at least the upper
atomic layers, the predominant species present was W(CO)xCl6-x (where 1<x<6), the
technique provided no clues, as to the homogeneity of the deposit. ATR, used to
characterise the interface between the deposit and the ZnSe prism, was implemented
so as to ascertain whether the entire deposit, with regard to depth, was homogenous.
43
The results of ATR analysis, while not as comprehensive, concurred with those
obtained from XPS analysis. The appearance of several peaks each attributed to a
carbonyl stretching vibration, as shown in figure 4.5, suggested the presence of a
carbonylated species. While it is possible to characterise the exact nature of a carbonyl
containing compound by analysing the fine structure of the carbonyl peaks [10], this
was not possible in our studies, due to the low intensity and poor resolution of the
signals observed in the IR spectrum. The peaks were, however, only slightly shifted
with respect to those observed after W(CO)6 pyrolysis (refer to section 3.5.3),
indicating a similar species. Signals assigned to a W-Cl stretching or bending
vibration were not observed, probably as a result of lying below the transmission
cutoff for ZnSe (the window material).
Figure 4.5 ATR/FTIR spectrum of a film deposited after Freon 12/W(CO)6
copyrolysis
.08
.1
.12
.14
.16
.18
.2
.22
.24
Ab
so
rba
nce
2200 2150 2100 2050 2000 1950 1900 1850 1800
Wavenumber (cm-1)
(CO)
(CO)
While the results obtained from both XPS and ATR analysis do not provide a
comprehensive characterisation of the material deposited, it is apparent that a W(CO)x
species (1<x<5) is abstracting Cl preferentially from the freon. Interestingly this is
what was predicted on the basis of diatomic bond energies alone. The difference in
energy between a C-Cl bond and a W-Cl bond (26 kJ mol-1) [2] is larger than that
between a C-F and W-F bond (12 kJ mol-1) [2]. Thus by forming a W-Cl bond a more
stable bond is formed.
44
4.4 Copyrolysis of Freon 22 with Fe(CO)5
The decomposition of Freon 22 in the presence of an equivalent amount of Fe(CO)5
was found to commence at a temperature corresponding to a laser setting of 75 W at
aperture 15. While this is identical to that required for the initiation of CF2HCl
decomposition under standard pyrolytic conditions, the level of freon decomposition
was slightly greater when Fe(CO)5 was present.
With regard to the pyrolysis products, those formed in the decomposition of Freon 22
in the presence of Fe(CO)5 were significantly different to those detected after standard
(CF2HCl without Fe(CO)5) pyrolysis. FTIR analysis after 30 seconds revealed an
absence of peaks attributed to C2F4 and HCl, products normally seen in the
decomposition of CF2HCl [6]. FTIR analysis also revealed the presence of much CO,
the product of Fe(CO)5 decomposition. Visual inspection of the pyrolysis cell revealed
the accumulation of a black deposit.
The results obtained would indicate that the decomposition follows a mechanism
whereby a halogen is being abstracted in preference to HCl elimination. It is proposed
that the abstracted halogen is chlorine. Our copyrolytic studies involving Freon 12 and
W(CO)6 illustrated, through the use of X-ray Photoelectron Spectroscopy, that Cl was
abstracted preferentially. While the CF2HCl/Fe(CO)5 system has yet to be studied
using these techniques, it may seem reasonable to assume that a similar process is
occurring. It is postulated that Fe(CO)x, where x is a low integer (given the level of
CO evolution), is abstracting a Cl from the freon to form a compound that is in turn
deposited on the cell wall. Subsequent reactions involving the resultant CF2H radical
are as yet unclear, but seem likely, given the absence of any gaseous products
(excluding CO), to include the formation of a deposit.
Successive experiments involving a greater ratio of Fe(CO)5 to Freon 22 produced
results similar to those obtained from the copyrolysis of Freon 12 with varying
amounts of Fe(CO)5. It was found that by increasing the ratio of Fe(CO)5 to Freon 22,
a corresponding increase in the level of freon decomposition would be observed.
45
4.5 Copyrolysis of Freon 22 with W(CO)6
The decomposition of Freon 22 in the presence of W(CO)6 was initiated at a
temperature corresponding to a laser setting of 75 W at aperture 15. While this is
identical to that required for the initiation of CF2HCl decomposition under standard
pyrolytic conditions, the extent of freon decomposition (as given by the decrease in
peak intensity in the IR spectrum), was slightly greater when W(CO)6 was present. In
contrast to those results obtained from the copyrolysis of Freon 12 (where W(CO)6
was shown to promote freon decomposition at a greater rate than Fe(CO)5) the results
pertaining to Freon 22 copyrolysis showed no significant difference between the two
volatile transition metal compounds, with regard to the level of freon decomposition.
It would appear that these results do not concur with those predicted
thermodynamically; recall from section 4.3 that a W-Cl bond was expected to form
more readily than a Fe-Cl bond. Subsequent experiments involving the copyrolysis of
CH2Cl2 provided results that further illustrated the limitations of predicting
decomposition rates from diatomic bond energies.
The products formed from the decomposition of Freon 22 in the presence of W(CO)6,
as shown in figure 4.6, did not concur with those observed in the standard (CF2HCl
without W(CO)6) pyrolysis. FTIR analysis revealed an absence of peaks assigned to
C2F4 or HCl and surprisingly, after approximately 60 seconds the presence of peaks
attributed to SiF4, a product not normally seen at such a low temperature [1,6].
Unidentified peaks at 750 and 1150 cm-1 were also observed. Interestingly, the peak
centred at 750 cm-1 was recorded in the high temperature pyrolysis of
dichloromethane (refer to section 3.4) and may imply the existence of an as yet
unidentified chlorinated compound. Visual inspection of the pyrolysis cell revealed a
black/grey deposit.
46
Figure 4.6 FTIR spectra of a gaseous mixture of Freon 22, W(CO)6 and SF6
before (top) and after (bottom) IR LPHP. Features identified are due
to CF2HCl () and SF6 ()
0
.05
.1
.15
.2
.25
.3
.35
.4
Ab
sorb
ance
2200 2000 1800 1600 1400 1200 1000 800
Wavenumber (cm-1)
W(CO)6
W(CO)6
CO
SiF4
??
The results obtained would suggest a mechanism analogous to that proposed for the
decomposition of CF2HCl in the presence of Fe(CO)5, whereby Cl abstraction is
occurring in preference to HCl elimination. A W(CO)x species (0<x<5) may abstract
Cl from the freon to form a compound that is in turn deposited on the cell wall.
Subsequent reactions involving the resultant CF2H radical are as yet unclear, but may
involve SiF4 formation. In a manner analogous to that described in section 4.2, where
a CF2Cl radical was thought to react with the cell wall (SiO2) to form SiF4, it is
proposed that at low temperature the CF2H radical produced as a result of CF2HCl
decomposition, can react with the cell wall to form SiF4. While SiF4 was detected in
the copyrolysis of Freon 22 with W(CO)6 it was, however, not observed in those
copyrolytic experiments involving CF2HCl and Fe(CO)5 (refer to section 4.4). It is
proposed that this difference is attributable to the dissimilar durations of pyrolysis.
Experiments involving the copyrolysis of Freon 22 with W(CO)6 were not limited in
duration by the loss of W(CO)6 vapour (the solid nature of W(CO)6 allowed for the
continual replenishment of a vapour pressure), thus pyrolysis could continue until all
the freon had decomposed. Conversely, those copyrolytic experiments involving
47
Fe(CO)5 were limited in duration to the time taken for complete Fe(CO)5
decomposition, at such temperatures, approximately 30 seconds. SiF4 was first
observed in the copyrolysis of Freon 22 with W(CO)6, after the cell had been
irradiated for 60 seconds. It is proposed therefore that reactions involving the
formation of SiF4 at low temperature, namely that of CF2H reacting with SiO2 (the cell
wall) are not initiated until some time after 30 seconds.
4.6 Copyrolysis of Dichloromethane with Fe(CO)5
The decomposition of CH2Cl2 in the presence of an equivalent amount of Fe(CO)5
was observed at a temperature corresponding to a laser setting of 75 W at aperture 15.
After irradiating the pyrolysis cell for 30 seconds it was found that the level of CH2Cl2
decomposition, given by the decrease in peak intensity in the IR spectrum, was
comparable to that observed after the pyrolysis of CH2Cl2 alone at aperture 7.
The primary product of CH2Cl2 pyrolysis, namely HCl, was not observed in the
copyrolytic studies involving CH2Cl2 and Fe(CO)5. FTIR analysis revealed the
presence of CO, the product of Fe(CO)5 decomposition. Visual inspection of the
pyrolysis cell revealed a black/grey deposit. The results obtained would indicate a
mechanism whereby Cl abstraction is occurring in preference to HCl elimination. In a
manner analogous to that proposed for CF2HCl decomposition in the presence of
Fe(CO)5 (refer to section 4.4), it is theorised that Fe(CO)x, where x (given the level of
CO evolution) is a low integer, is the abstracting species. The compound formed as a
result, would in turn accumulate on the cell wall. Subsequent reactions involving the
resultant CH2Cl radical are as yet unclear, but seem likely given the absence of any
gaseous products (excluding CO), to include the formation of a deposit.
48
4.7 Copyrolysis of Dichloromethane with W(CO)6
Our copyrolytic experiments involving Freon 22 with W(CO)6 were shown to provide
results dissimilar to those obtained in the standard (CF2HCl without W(CO)6)
pyrolysis. The absence of peaks in the IR spectrum assigned to C2F4 and HCl, in
combination with the appearance of peaks indicative of SiF4 formation, suggested an
alternative decomposition mechanism [1,6]. It was reasoned in section 4.5 that the
formation of SiF4 at such a low temperature could be attributed to the reaction of
CF2H, (the freon radical formed as a result of CF2HCl decomposition), with the cell
wall. Assuming a similar decomposition mechanism, the copyrolysis of CH2Cl2 (a non
fluorinated analogue) with W(CO)6 would not be expected to produce SiF4.
The copyrolytic decomposition of dichloromethane with W(CO)6 commenced at a
temperature corresponding to a laser setting of 75 W at aperture 15. While the level of
decomposition, as given by the decrease in peak intensity in the IR spectrum, was not
significant, it was greater than that for standard (CH2Cl2 without W(CO)6) pyrolysis,
where decomposition was not initiated until a temperature corresponding to aperture 7
had been reached. Unlike that predicted on the basis of diatomic bond energies, the
level of CH2Cl2 decomposition in the presence of W(CO)6 was in fact lower than that
when Fe(CO)5 was present. FTIR analysis revealed an absence of any product peaks,
including those assigned to HCl and, as was theorised SiF4 [1]. Visual inspection of the
pyrolysis cell revealed the accumulation of a black/grey material. The results obtained
suggest a mechanism similar to that proposed for CF2HCl/W(CO)6 decomposition,
whereby an initial chlorine abstraction is favoured over HCl elimination. Subsequent
reactions involving the resultant radical are as yet unclear, but seem likely to include
the formation of a deposit.
Further copyrolytic experiments involving a higher temperature (those utilising an
aperture greater in size than aperture 10) revealed peaks in the IR spectrum indicative
of HCl formation. It could therefore be possible that the decomposition mechanism of
CH2Cl2 in the presence of W(CO)6 exhibits two temperature regimes. At temperatures
corresponding to apertures greater in size than aperture 10, decomposition occurs via
the elimination of HCl, as shown in figure 3.7, while at lower temperatures (apertures
10 or above) a chlorine abstraction is favoured.
49
4.8 References and Notes for Chapter 4
[1] E. A. Jones, J. S. Kirby-Smith, A. H. Nielsen, P. J. H. Woltz, J. Chem. Phys.,
1951, 19, 242.
[2] R. C. Weast, Ed., CRC Handbook of Chemistry and Physics, 60th Edition, CRC
Press, Inc., Florida, 1979, p. F220.
[3] H. W. Thompson and R. B. Temple, J. Chem. Soc., 1948, 1422.
[4] T. G. Burke, E. A. Jones, A. H. Nielsen, P. J. H. Woltz, J. Chem. Phys., 1952, 20,
596.
[5] E. K. Plyer and D. Simpson, J. Res. NBS., 1953, 50, 223.
[6] H. H. Claassen and J. R. Nielsen, J. Chem. Phys., 1950, 18, 812.
[7] A. Cantoni, T. K. Choudhury, D. F. Koster, R. N. Zitter, J. Phys. Chem., 1990,
94, 2374.
[8] E. Grunwald, G. A. Hill, P. Keehn, J. Am. Chem. Soc., 1977, 99, 6521.
[9] D. F. Koster and R. N. Zitter, J. Am. Chem. Soc., 1976, 98, 1613.
[10] F. A. Cotton and G. Wilkenson, Advanced Inorganic Chemistry, Fifth Edition,
John Wiley & Sons, New York, 1988, p.1034.
50
Chapter 5. Conclusions and Future Work
The results presented herein demonstrate that the gas phase decomposition rate and/or
mechanism of those selected compounds, in the presence of a specific volatile
transition metal compound, can differ significantly to that when a volatile transition
metal compound is not present.
With regard to the rate of substrate decomposition, it was shown that while the
temperature at which decomposition commenced, did not always change with the
introduction of a transition metal carbonyl compound (as was the case for CF2HCl
copyrolysis), in each copyrolytic system studied, the level of substrate decomposition
was invariably enhanced. It was reasoned that a halogen abstraction, (where the
abstracting species, M(CO)x, is the product of metal carbonyl decomposition), was
occurring in preference to that mechanism normally associated with the
decomposition of the selected compound.
In all the systems studied copyrolytically, the products of substrate decomposition
differed to those observed when a volatile transition metal compound was not present.
It was proposed that this difference was largely due to the alternative mechanism of
decomposition. In those copyrolytic experiments involving Freon 22, the abstraction
of Cl in preference to the elimination of HCl, resulted in the formation of a CF2H
species. Subsequent reactions involving this freon radical were likely to involve the
formation of a deposit and/or the formation of SiF4 (via the reaction of CF2H with
SiO2 (the cell wall)). Those copyrolytic experiments involving CF2Cl2 showed that a
semi-carbonylated transition metal species could abstract Cl at a much lower
temperature than that required for Cl loss under standard pyrolytic conditions.
Consequently, the intermediate products of Freon 12 decomposition were detected,
and as a result, a modified decomposition mechanism was derived.
From the results pertaining to the copyrolysis of Freon 12 with each of the selected
volatile transition metal compounds, it was proposed that the diatomic bond energy of
the resultant M-X bond (where M is the metal and X is the abstracted halogen) could
51
be used to predict the outcome of freon copyrolysis. Successive experiments involving
CF2HCl and CH2Cl2 however, provided results that suggested such a proposition was
invalid.
While many questions have been resolved, with regard to the decomposition of those
selected compounds, the work presented herein has also raised a number of other
questions that may provide the basis for future work.
If a more thorough understanding of the mechanisms behind the decomposition of a
substrate in the presence of a volatile transition metal compound is to be realised, then
further experiments involving the characterisation of those non-gaseous products
formed, as a result of substrate decomposition, will be required. Characterisation of
those films produced as a result of Freon 22 or dichloromethane decomposition in the
presence of a volatile transition metal compound, has yet to be realised.
The results pertaining to those experiments involving CH2Cl2 decomposition, while
not providing evidence for fullerene formation, did indicate the presence of several
unidentifiable high molecular weight, polycyclic aromatic hydrocarbons. The
formation of such compounds from the Infrared Laser Powered Homogeneous
Pyrolysis of CH2Cl2 is in itself significant, and raises the obvious question, what is the
mechanism responsible? Further work utilising a more comprehensive array of
analytical techniques will be required if such a mechanism is to be discerned.
It is anticipated that those results acquired from the experiments performed herein
may contribute to the existing literature pertaining to freon decomposition. Moreover
it is believed that those results relating to the copyrolysis of a chlorofluorocarbon with
a volatile transition metal compound may provide an impetus for further studies,
ultimately with the intention of establishing optimum conditions for freon
decomposition.