-
The effect of an electric field on thehydrogenation of ethylene
on zinc oxide
Item Type text; Thesis-Reproduction (electronic)
Authors Sikdar, Subhas K.
Publisher The University of Arizona.
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Download date 02/06/2021 03:19:31
Link to Item http://hdl.handle.net/10150/318141
http://hdl.handle.net/10150/318141
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THE EFFECT OF AN ELECTRIC FIELD ON
THE HYDROGENATION OF ETHYLENE ON ZINC OXIDE
by
Subhas Kumar Sikdar
A Thesis Submitted to the Faculty of the
DEPARTMENT OF CHEMICAL ENGINEERING
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
-
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfilment of
requirements for an advanced degree at The University of Arizona
and is deposited in the University Library to be made available to
borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special
permission, provided that accurate acknowledgment of source is
made. Requests for permission for extended quotation from or
reproduction of this manuscript in whole or in part may be granted
by the head of the major department or the Dean of the Graduate
College when in his judgment the proposed use of the material is in
the interests of scholarship. In all other instances, however,
permission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
Richard D. Williams /DateAsst. Prof. of Chemical Engineering
-
'ACKNOWLEDGMENTS
I express my gratitude to the Department of Chemical
Engineering for the financial support which enabled me to
pursue
higher education in this country. I am thankful to Professor R.
D.
Williams, my adviser, for his guidance and useful suggestions
during
the course of this research.
I appreciate the cooperation, obtained from the faculty
members and fellow .graduate students of this department.
Special
mention should be made of Mr. S. A. Shinde, who did the drawings
of
the reactor and its various parts, and of Dr. N. R„ Schott for
his
overall helpful attitudes. It is my pleasure to thank Mrs. I. A,
.
Shafiqulla for typing the final copy of this thesis.
Finally, I like to express my indebtedness to my parents
who from several thousand miles offered me a constant source
of
encouragement.
iii
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TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS ........ vi
LIST OF TABLES , . ............ ix" -A B S T R A C T
................. . . . . . . . , . . . . . . . . xi
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 1
THEORY AND-PREVIOUS WORK .............. 12
APPARATUS AND EXPERIMENTAL PROCEDURE.......... 29
A Flow Reactor . . . . . . . . . .Reactor.............. . . .A s
s e m b l y ........ ..Sand Bath ..................Chromatograph
‘........ . .The High Voltage Source . . .
Experimental Procedure . . . . . .Calibration of Flow Meters
Activation of the Catalyst Analysis by the Chromatograph Kinetic
Runs . . . . . . . .
RESULTS AND DISCUSSIONS . . . . . . . . . . . . . . . . . . .
50
Method of Preparation and Activation . . .......... 51Catalyst
Effectiveness .............. 53Kinetic Runs, No Field.......... .
53
Ethylene Dependence . ................. 56Hydrogen Dependence .
. . . . . . . . ......... 63
Effect of the F i e l d .......... 71Kinetic Runs, with
Field............. 75
Ethylene Dependence 75Hydrogen Dependence............ 75
Activation Energy .......... . . . . . .
83Mechanism............... '.......... 87
Temperatures greater than 150°C................. 88Temperatures
less than 150 C ............ 89
CONCLUSIONS AND RECOMMENDATIONS . . ......................
.91
2930 35 38 40 40 42 42 424445
iv
-
V
TABLE OF CONTENTS— Continued
Page
APPENDIX A: DERIVATION OF EQUATION 18. . . .
................92
APPENDIX B: CALIBRATION CURVES . . 93
APPENDIX C: KINETIC DATA WITHOUT THE F I E L D ................
98
APPENDIX D: KINETIC DATA IN PRESENCE OF A FIELD...........
109
APPENDIX E: ARRHENIUS ACTIVATION DATA . . . .
.............114
APPENDIX F: NOMENCLATURE ............ . .......... . . . 1 1
7
LIST OF REFERENCES ..............................118
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LIST OF ILLUSTRATIONS
Figure Page
.1 o Lattice Structure of Non-stoichiometric Zinc Oxide, . . .
6
. 2« Energy Band Diagram of Intrinsic Semiconductors ........
7
3. Energy Band Diagrams Showing Situations for (a) Donor: Type
and (b) Acceptor Type Semiconductors . . . . . . . 7
4. Effect of Temperature on the Fermi Level of
Semiconductors,(a) n-Type (b) p-Type . . . . . . ........ . . . . .
. . 10
5. Reactor Assembly ................ 32
6. Exploded View of the Reactor................ 33
7. Electrodes and Transite Rings . . . . . . . 34
8. Schematic of the Apparatus............... . 39
9. A Typical Chromatogram Showing Separation of (1) Hydrogen(2)
Ethylene and (3) Ethane in a Porapak Q Column at 50°C. 46
10. Order with Respect to Ethylene, 195°C, 20-28 MeshCatalyst .
* .......... 58
11. Ethylene Order at 1950C, Run la Replotted onCartesian
Coordinates ........................ . . . . . 59
12. Order with Respect to Ethylene, 146°C, 20-28 MeshCatalyst *
. ................................ 60
13. Order with Respect to Ethylene, 146°C, 35-48 MeshCatalyst
......... 61
14. Order with Respect to Ethylene, 100°C, 20-28 MeshCatalyst .
. . . . . . . . . . . . 61
15. Order with Respect to Ethylene, 56.5°C, 35-48 MeshCatalyst
.......... 62
16. Order with Respect to Hydrogen, 195°C, 20-28 MeshCatalyst *
» ............ 64 *
17. Hydrogen Order as Shown in Cartesian Plots * ..........
6.5
vi
-
vii
LIST OF ILLUSTRATIONS— Continued'
Figure Page
18o Order with Respect to Hydrogen, 146°C, 20-28
MeshCatalyst.......... .......................... 66
19. Order with Respect to Hydrogen, 146°C, 35-48. MeshCatalyst .
.............. . . . . . . . . . . . . . . 67
20. Order with Respect to Hydrogen, 100°C, 20-28 MeshCatalyst .
68
21. Order with Respect to Hydrogen, 56.5°C, 35-48 MeshCatalyst
................ . ........ . . . . . . . . 69
22. Hydrogen Order as Shown in Cartesian Plots 70
23. Ethylene Order in Presence of a Field of 3,800 Volts,146°C,
35-48 Mesh Catalyst . . . . . . . . 76
24. Ethylene Order in Presence of a Field as ShownIn Cartesian
Coordinates, 146°C, V = 3,800 . . . . . . 77
25. Ethylene Order in Presence of a Field of 4,180 Volts,86.5°C,
35-48 Mesh Catalyst, , ....................... 78
26. Hydrogen Order in Presence of a Field of 3,800 Volts,1460C,
35-48 Mesh Catalyst . . . . . . 79
27. Hydrogen Order in Presence of a Field as Shown in,Cartesian
Coordinates, 146°C, V = 3,800. . . . . . . . . 80
28. Hydrogen Order in Presence of a Field of 4,180 Volts,86c50C,
35-48 Mesh Catalyst , * ........................ 81
29. Hydrogen Order in Presence of a Field as Shown inCartesian
Coordinates, 86,5°C, V = 4,180 * ............. 82
30. Activation Energy Plots of Zinc Oxide Catalyst „ . . .
85
31. Activation Energy Plots of Zinc Oxide Catalyst,with and
without a Field . . . . 86
32. Calibration Curve for Hydrogen Flow Meter , . . . . . .
94
-
viii
LIST OF ILLUSTRATIONS™ Continued'
Figure Page
33. Calibration Curve for Ethylene Flow Meter . . . . . . 95
34. Calibration Curve for Helium Flow M e t e r ............
96
35. Calibration Curve for Ethylene Estimation 97
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LIST OF TABLES
Table ' Page
1.. Experiment on Catalyst Activity . 54I .
2 c Experiment on Catalytic Activity . . . . . . . . . . .
55
3. Observed Orders of Reaction „ . . . . . . . . . . • . 57
4. Effect of Field Strength on the Rate of Reaction . . . 72
5c Effect of Field Duration on the Rate of Reaction » . . 72
6, Effect of Field Strength on Reaction Rate atLow Temperature»
. 73
7* Effect of Field Duration on Rate . ........ . . . . . 73
8e Activation Energies Under Different Conditions » , ♦ . 84
9. Data of Ethylene Order Study at 195°C (Run lb) • . . . 99
10c Data of Hydrogen Order Study at 195°C.(Run la) » . « .
10U
11c Data of Hydrogen Order Study at 146°C (Run 2b) « . . .
101
12. Data of Ethylene Order Study at 146°C (Run 2a) . . . .
102
13. Data of Hydrogen Order Study at 146°C (Run 3b) . . . .
103
14. Data of Ethylene Order Study at 146°C (Run 3a) . . . .
104
15. Data of Hydrogen Order Study at 100°C (Run 4b) . . . .
105
16. Data of Ethylene Order Study at 100°C (Run 4a) ......
106
17. Data of Ethylene Order Study at 56.5°C (Run 5a) . . .
107
18. Data of Hydrogen Order Study at 56.5°C (Run 5b) . . .
108
19. Data of Ethylene Order Study with Field at 146°C~Run,15f(a)
. . , . ... ................ .. . . . . . 110
20. Data of Hydrogen Order Study with Field at 146°C-R m 155 (b)
............................. Ill
-
X
LIST' OF TABLES— Continued
Table Page
21. Data of ethylene order study with field at 86.5°C~Run 16f
(a). . . . '.............. 112
22. Data of Hydrogen Order Study with Field at 86-. 5°C-.Run
16.f (b ) * ’ . ............ . . .113
23. Data for Activation Energy Plots . . . . . . . . . . . .
115
24. Data for Activation Energy Plots with andwithout Field
........................................ 116
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ABSTRACT
Semiconducting oxide catalysts are characterized by non
stoichiometry in their lattice structures, A correlation between
the
catalytic activity and non-stoichiometry has been a major
question for
such oxidesc Since application of an electric field modifies the
semi
conductivity of an oxide catalyst, a study of a catalytic
reaction in
presence of a field would lead to a changed reaction pattern. A
study
of this kind has been made here on the hydrogenation of ethylene
on
zinc oxide (an n-type semiconductor) with an A.C. electric field
up to
about 5,000 volts. The field appeared to decelerate the reaction
rate
at a higher temperature, e.g., 150°C whereas at a"lower
temperature,
e.g., 85°C, it appeared to accelerate it. The reaction kinetics
have
been found to be too complicated to be explained by the existing
mecha
nistic models.
The reaction was studied in the temperature range of 560C to
195°C without the field and a mechanism has been suggested on
the basis
of the Langmuir-Hinshelwood model.
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CHAPTER I
INTRODUCTION
Catalytic hydrogenation of ethylene was discovered and first
studied by the French chemists9 Sabatier and Senderens (in 1)
who used
a finely divided Ni catalyst. Their discovery then triggered off
a
whole series of detailed studies of this reaction in particular
and
the hydrogenation of olefins and aromatics in general in the
following
years. Solid catalysts for hydrogenation of unsaturated
hydrocarbons
can be conveniently classified into three groups (2) : (a)
metallic
catalystss mostly studied are platinum, nickel, palladium,
cobalt etc.
(b) insulator oxides like alumina, silica and silica-alumina(c)
semi
conductor oxides like zinc oxide and nickel oxide. Among the
classes
mentioned, semiconductor oxide catalysts are the least studied
and the
least understood catalysts in relation to hydrogenation of
olefins.
However, some papers have recently reported kinetic studies of
the
hydrogenation of ethylene catalized by zinc oxide in batch
reactors.
In the present work, a study of the hydrogenation of
ethylene
on zinc oxide catalyst (which is an n-type semiconductor) in a
dif
ferential flow reactor was undertaken, and the effect of an
A.C.
electric field on the reaction rate, activation energy and
mechanism
was analyzed. The reason for choosing this reaction is that this
is
by far the simplest reaction known among olefin hydrogenations
and is
also relatively free,of undesirable side reactions. Despite the
large
-
volume of work which has been done on ethylene hydrogenation,
the
mechanism of the reaction is still debated (3) and apparently
depends
on the particular catalyst and the conditions used = An attempt
was
made in the present work to indicate a possible mechanism in
relation
to the particular catalyst used on the basis of the result
obtained
from the application of the electric field on the catalyst
particles.
The reason an effect of the field on the kinetics is expected
and how
it should related to a mechanism will be given shortly.
For an element or a compound to exhibit semiconductivity, it
must have a defect of some sort in its crystal lattice. Perfect
crys
tals seldom, if ever, exist (4). As a matter of fact, at any
tempera
ture greater than absolute zero, one should expect from
thermodynamics
to find seme form of lattic imperfections : the increased
disord^L
brings about an increased entropy and ̂ hence a decreased free
energy
F = U - TS
where F, U and S are, respectively, the Helmholtz free energy,
the
internal energy and the entropy, T being the absolute
temperature.
The semiconducting oxides important as catalysts have
molecular
non-stoichiometry as their defect in the lattice. That is, they
have
either metal or oxygen atoms in excess of a stoichiometric
proportion
in the lattice. Easy ionization of the metal or the oxygen atom
gives
rise to, respectively, free electrons, in which case the
substance is
called an n-type semiconductor, and free holes (or free positive
char
ges) in which case it is called a p-type semiconductor. The
holes, like
the electrons, are capable of conducting electric current.
Obviously,
this kind of behavior would not be expected from an insulating
oxide.
-
3
Attempts have been made in recent times to prove that the
catalytic property of semiconducting oxides is due to their
semi
conductivity but without much success. According to
electronic
theory of catalysis on semiconductors (5), since the free
electrons or
the holes of the semiconductor lattice can be accelerated by the
appli
cation of an external electric field on the substance, the rate
of a
catalytic reaction can be modified. Almost no work has been
initiated
in this direction.
Pure metallic zinc and pure stoichiometric zinc oxide have
been found to be inactive to hydrogen chemisorption (6). Zinc
oxide,
after a treatment in vacuo or hydrogen at an elevated
temperature,
however, shows activity towards hydrogen. This contrast in
behavior
suggests that the active species are neither zinc nor zinc
oxide, buu
probably non-stoichiometric zinc oxide. According to Wagner's
thermo
dynamic theory of defective oxides, at sufficiently high
temperature,
an equilibrium exists between solid zinc oxide and gaseous
oxygen
whereby excess of zinc atoms (Zn^, interstitial) can be
accomodated in
interstitial positions of the lattice :
Zn‘h2 + 0~2 = Zni + 1/2 Og
The interstitial Zn can then be thermally ionized :
Zn^ = Zn+ + e
Parravano and Boudart (6) have shown that a pure single crystal
of
-
4zinc oxide will become a non-stoichiometric semiconductor
through chemi-
sorption of hydrogen at high temperatures. Given the following
repre
sentation of ZnO crystal
0 2 - Zn+2 - 0-2 - Zn+2 - 0 2
chemisorption of hydrogen will at first lead to
— —— 0 2 — (ZnH)^" — (OH) — Zn+2 — 0 2 -----
The adsorption will not stop at this stage. The (ZnH)+ complex
may
dissociate, the proton moving to the next oxygen ion to form
another
stable hydroxyl ion
Zn — (OH) — Zn — (OH) — Zn~̂ 2 — 0 2
That gives an interstitial Zn atom in the lattice. The
ionization
energy of interstitial Zn in the crystal is much lower than for
an
isolated zinc atom, as a consequence of the high dielectric
constant,
k, of the crystal. For an isolated zinc atom, the ionization
energy
is 9.4 eV. In the crystal it is equal to 9.4/k^; k for zinc
oxide is
about 10, thus the ionization energy is only 0.1 eV. This
suggests
that at moderate temperatures the interstitial zinc atom will
ionize
freely, releasing one electron which can be accelerated by an
external
electric field. The lattice structure of non-stoichiometric zinc
oxide
is shown in Figure 1.
-
The band theory of solids can be utilized to explain the
development of semiconductivity in substances in general♦ Two
bands of
interest in this case are the so called valence band and the
conduction
band, which in all crystals are separated by what is called the
for
bidden energy gap (Figure 2). Figure 2(a) depicts the case of
the
semiconductor at absolute zero at which temperature no free
electrons
exist in the conduction band and no free holes in the valence
band. At
high temperatures, however9 as shown in Figure 2(b), thermal
ionization
will cause electrons to cross the energy barrier to get to the
conduc
tion band,
Figure 3 depicts the energetics of donor or acceptor type of
semiconductors (7). Here only the region between the top of
the
valence band and the bottom of the conduction band is of
interest.
Figure 3(a) would describe the energetics of, say, n-type ZnO
and
Figure 3(b) would describe that of, for example, p-type NiO*
The free electrons from interstitial zinc atoms lie in a
higher energy state than the normal valence electrons of the
ZnO
molecule. Hence a localized extra level of electrons must lie
above
the top of the filled valence band. But since there is some
binding
energy for an electron in this state to remain on the ZnO, the
level
must lie below the lowest free electron state in the conduction
band.
An analogous argument would explain why the acceptor levels,
arising
from NiO must lie as shown in Figure 3(b).
A great deal of chemisorption studies on semiconducting
oxides have been made to date. The result has been to throw
some
light on the electronic mechanism that is involved in such a
process.
-
6
Z n 2 o2 Z n 2 o2 Z n 2'
0 Z n 2-20 Zn2
-20
7 4- ® In
+ 2 Zn 0
+ 2 In 0
12n
042Zn
-20
-20
-20
-20
42Zn
Figure 1. Lattice Structure of Non-stoichiometric Zinc
Oxide.
-
7
conduction band
forbidden energy gap
valence band
energy
electrons
T production
recombination
+ “h f 4" 'h h + 4 -f-holes
(a) Low Temperature (b) High Temperature
Figure 2. Energy Band Diagram of Intrinsic Semiconductors.
conduction band conduction band
donor level energy
acceptor level -♦»
valence band
4- 4 4 4 4 4 4-
(a) (b)
Figure 3. Energy Band Diagrams Showing Situation for (a) Donor
Type and (b) Acceptor Type Semiconductors.
-
The theory that has evolved is called the boundary layer theory
of
chemisorption (8).
Chemisorption on the surface of a semiconductor is followed
by the rise or fall of the electrical conductivity, A fall in
the
conductivity of a p-type semiconductor signifies the transfer of
elec
trons from the chemisorbed molecules to the solid. As is
obvious3 this
is due to the neutralization of positive charge carriers (holes)
by the
electrons. Similarly3 a fall in the conductivity of an n-type
semi
conductor indicates an electron transfer from the conduction
band of
the semiconductor to the adsorbed species, ■ To bring the charge
carriers
from the donor or acceptor levels to the conduction band, one
has to
supply the necessary energy, thermal or otherwise. It is to be
noted
that in the above mentioned two cases, although the net result
is the
same, the mechanism of the depletion of carriers is the
opposite. In
contrast to the above phenomenon, known as depletive
chemisorption, the
other type, known as cumulative chemisorption, is characterised
by a
rise in conductivity in both n- and p-type semiconductors.
Clearly,
cumulative 'chemisorption involves cationic chemisorption on an
n-type
(e.g., H2 on ZnO) and anionic chemisorption on a p-type (e.g.,
on Nib) semiconductor. Both of these adsorption phenomena are
restricted
to the surface. The concentration of electrons in the bulk will
remain
unchanged. The direction of electron transfer depends on the
relative
position of the. Fermi potentials in the semiconductor and the
adsorbing
gas, and electron flow will take place until these are
identical.
Fermi potential is defined as the potential at which the
probability
of being occupied by electrons is equal to one half, that is, on
the
-
average-50% of available valence electrons have energy equal to
the
Fermi level. This flow of electrons will result in a space
charge
appearing between the surface of the semiconductor and its
interior (2).
Application of Fermi statistics shows that the number- of
particles in
a particular state depends upon the energy gap between the
acceptor or
donor level and the.Fermi level. This means that the adjustment
of the
Fermi level in a catalyst will alter the concentration of the
adsorbed
species in a system; in catalysis it can influence the
selectivity of
a reaction if more than one reaction route exists. These
predictions
have been experimentally verified, although a completely
satisfactory
explanation of what actually happens is yet to be found. One
method
of altering the Fermi level is to increase.the temperature.
Figure
4(a) shows the Fermi level of an n^type semiconductor being
depressed
with temperature as more electrons are being drawn into the
conduction
band. The reverse occurs with a p-type semiconductor. Another
method
of accomplishing the same thing is the application of an
electric
field normal to the surface where chemisorption is occuring,
thereby
changing the Fermi level of the crystal. One difficulty in the
inter
pretation must, for example, arise from the alteration of the
Fermi
level by the adsorbed species themselves. Such changes will be
a
function of the extent of the adsorption of the reactants and
the
products (2), An experiment on the chemisorption of oxygen on
ZnO (9)
coated on a pyrex plate across which a D.C. electric field was
applied
showed that the chemisorption could be hastened or decelerated
accord
ing to whether the positive electrode was on the top or at the
bottom
-
10
conduction band
T
valence band
conduction band
acceptors
valence band
(b)
Figure 4. Effect of Temperature on the Fermi Level of
Semiconductors, (a) n-Type (b) p-Type.
-
11
of the pyrex plate. Another very interesting aspect of the
modifi
cation of the Fermi level of a semiconductor by the application
of a
field of 229000 volts A.C. across NiO catalyst and the
subsequent
acceleration in the rate of carbon monoxide oxidation has
revealed
that the increase in rate is a function of the frequency of the
field
(10), This experiment showed that in this particular reaction,
other
things remaining constant, rate increased first with frequency,
reached
a maximum and then decreased with further increase in
frequency.
The present work was an attempt to examine the effect on the
hydrogenation of ethylene of applying 60 cycle A.C. fields up
to
5,000 volts across a bed of zinc oxide catalyst particles.
The
reactions were carried out in the temperature range of 70 to
250° C.
No attempt was made to examine the effect of frequency.
-
CHAPTER 2
THEORY AND PREVIOUS WORK
Being the simplest of all olefin hydrogenation reactions,
ethylene hydrogenation has caught the attention of a multitude
of
investigators. Most of the data has been obtained with nickel
and
platinum. Other noble and transition metals have also been
studied.
Earlier works were done on reduced powders, more recently they
were
done on wires activated by oxidation and reduction and on the
extremely
active deposits formed by evaporation of a film in vacuo
(11).
A bimolecular surface reaction like the hydrogenation of
ethylene may in general proceed by either of two mechanisms :
(a)
adsorption of the two reactants on adjacent active sites,
followed by
their interaction and desorption of the products or (b) the
inter
action between an adsorbed molecule of one species with a
molecule of
the other from the gas phase. The former is called a
Langmuir-
Hinshelwood mechanism and the latter a Rideal-Eley
mechanism.
The rate of reaction between two species (say A and B) in a
bimolecular surface catalyzed reaction, is proportional to the
probabi
lity that A and B are adsorbed on adjacent sites to effect a
reaction.
This probability in turn, is proportional to the product of
surface
coverages of A and B, and the rate expression, therefore, can
be
written as v^ = k 9^ 9^
where 9^ and 9g represent the surface coverages of A and B; k is
the
12
-
reaction rate constant. For the case where both A and B compete
for
the same surface adsorption sites and are not dissociated on
adsorp
tion :
bA p aeA =
( 1 + bA Pa + bB PB >and
bB pBeB -------------------------
( 1 + bA PA + bB PB ^where b^, bg are the adsorption
coefficients and p^, Pg, the partial
pressures of A and B respectively. Thus the rate is given by
k bA b B pA pBv h = : (i)
̂ i + da PA + uB PB )2
It can be inferred here, a fact that has been borne out by
experiments in some hydrogenation reactions, that if p^ or p^ is
kept
constant and the other varied, the rate passes through a maximum
when
p^b^ = Pgbg* The decrease in rate at high pressures may be
explained
by assuming that the more strongly adsorbed species displaces
the
other species.from the surface as its pressure is increased.
Further simplifications in specific cases can be made from
equation 1: ' 1
(a) If both A and B are weakly adsorbed giving a surface which
is
sparsely covered -
-
14The hydrogenation of ethylene over copper catalysts satisfies
this
second order rate equation under certain conditions.
(b) In the case where one of the reactants, say A, is weakly
adsorbed
so that
(c) When the reactant A is very weakly adsorbed and B is
adsorbed with
Low temperature hydrogenation of ethylene over copper follows
this
kind of behavior, i.e., the rate is proportional to hydrogen
partial
pressure but varies inversely as the pressure of ethylene. In
all
these cases it is assumed that the product of hydrogenation is
not
adsorbed nor does it inhibit the reaction.
acting species compete for the same kind of surface active
sites. If,
however, the species adsorb on different kinds of surface sites,
the
surface coverages will be given by the following
expressions:
bA PA « (1 + bB PB)» one gets
k b A bn Pa PA B A fBvH (3)
(1 + bB PB)2
Here the rate is proportional to p for constant pB, and, for
constantAp^, the rate passes through a maximum as p^ increases.
sufficient strength fui. b^ p^ >> 1 to be satisfied,
equation 3 simplifies
to
vH kpB
(4)
In the Langmuir-Hinshelwood models described above, the re-
-
and the rate of reaction will be
- — L V i J i l i -------< 1 ♦ b 1 P j ) ( i t b i p B )
Simplifications of equation 5 can be made assuming strong or
weak
adsorption of one species compared to the other as before.
When the molecules of one species, say A, dissociate on
adsorption, for example, into two atoms and the species compete
for
same surface sites, the surface coverages will assume the
following#
expressions:
(i+ hi pA2 + bB pb)and
^ —
(1 + b A PA + bB PB)
and the reaction rate in this case is
-
If, however, the species are adsorbed on different kinds of
sites, the
reaction rate will be given by
b% b pvH = k -----4... (7)
(1 + ) (1 + bB PB)
While a Langmuir-Hinshelwood mechanism requires the
adsorption
of both the reactants on the surface of the catalyst to effect
any
reaction, the Rideal-Eley treatment applies to the case where
one of
the gases reacts from the gas phase with the other component
adsorbed
on the surface. Thus in the case of B not adsorbed
VH = k eA PB
If B is not adsorbed at all
bA pA6a =
(1 + bA Pa )
and the rate expression becomes
v = k ^ Pa P3-- (8 )(1 + bA Pa)
If, however, B is adsorbed but adsorbed B does not enter into
reaction,
then
-
17
so that
(1 + h PA + b pB)
In contrast to a Langmuir-Hinshelwood mechanism, a Rideal-
Eley mechanism implies that the rate does not pass through a
maximum
as the pressure of either component is increased keeping the
other
constant. Until now very few hydrogenation reactions have been
found
to follow a Rideal-Eley mechanism. The only cases for which
there is
good evidence for such a mechanism are certain atom and
radical-recombi-
nation reactions (1).
Reference to the equilibrium constant as a function of
tempera
ture £itvwo ttie iulwetjlu reaction
H2 + C2H4 “ C2H6
is favored for ordinary pressures up to 600°C, at which
temperature at
1 atmosphere hydrogen pressure the yield of ethane is 97 per
cent. With
larger olefin concentration, however, the reverse reaction
becomes impor
tant at lower temperatures (11).
Nickel catalyzed hydrogenation of ethylene has been studied
by
many workers. It has been found that a Langmuir-Hinshelwood
mechanism
satisfies the data of most of the workers. It has been
postulated that
hydrogen and ethylene are reversibly adsorbed on two separate
parts of
the surface and interaction occurs at the borderline. Thus
equation 5
represents the rate equation. With A as hydrogen and B as
ethylene,
equation 5 can be rewritten
-
18
v.H k (5)a + bH p h) (i + bE p e)
In the case when hydrogen adsorption is weaker than ethylene
adsorption
so that
With excess ethylene and especially with a pretreatment of the
catalyst
with ethylene, the initial rate goes down. Schwab (12) has
pointed out
that this behavior is associated with poisoning of catalyst
surfaces by
'acetylenic complexes’ formed by ethylene.
The Arrhenius plot of log versus 1/T starts to decrease
passes through a maxumum; the corresponding temperature is
called the
optimum temperature. Zur Strassen (13) postulated that a
saturated
layer of ethylene forms below the optimum temperature. Above
this
temperature! ethylene starts desorbing. He wrote the following
rela
tionships between the true and apparent activation energies with
res
pect to the optimum temperature:
and respectively, representing the heats of adsorption of
hydro
gen and ethylene on the catalyst.
with increasing temperatures around 100°C, and the reaction
velocity
T < ToptT > Topt
-
19
With other catalysts9 various different kinetic behaviors
were
obtained. Nd general pattern comes out of these conflicting
reports.
Thus a review of the literature reveals that the mechanism of
this
reaction depends on the type of the catalyst (i.e., whether
foil, wire
or particles), temperature range of study and the method of
preparation
and activation of the catalyst. A summary of the rate laws
applicable
to ethylene hydrogenation catalyzed by various catalysts under
various
conditions is tabulated in a review article by Laidler (14).
Many investigators have tried to formaulate a quantitative
expression for the rate of catalytic ethylene hydrogenation from
some
theoretical mechanistic models. Laidler (15) has tried the
Langmuir
kinetics and derived an expression for the maximum rate of
reaction in
terms of some parameters of the transition state theory. Beeck
has
taken the Rideal-Eley kinetics into consideration to explain his
experi
mental data on the hydrogenation of ethylene with a number of
different
metal catalysts. He postulated that the metal surface is
largely
covered, to a fraction 0, by adsorbed acetylenic complexes only
slowly
removed by reaction with chemisorbed hydrogen which is adsorbed
on the
surface not covered by ethylene. Hydrogenation occurs by
collision
between ethylene from the gas phase and chemisorbed hydrogen.
After
some simplifying assumptions, the rate expression is reducible
to one
which is dependent on the first power of hydrogen partial
pressure. It
is quite evident that no mechanism is entirely satisfactory,
although
some have done quite a remarkable job in shedding light on this
rather
obscure reaction.
-
20
The other scheme that invites attention now is the so called
half hydrogenated state, also known as associative theory. The
asso
ciative theory links hydrogenation and the hydrogen-deuterium
exchange
reaction via the half hydrogenated state, the ethylene being
adsorbed
by opening of the double bond as
H* + H* + *C2H^* t H* + *C2H5 J C2H6 (Horiuti-Polanyi)
or
H2 + *C2H4* ^ H* + *C H5 i C2H (Twigg-Rideal)
The dissociative theory of Farkas regards hydrogenation as not
related
in any way to the H-D exchange reaction. Hydrogenation is
effected by
simultaneous addition of 2H* to a (presumably physically)
adsorbed
ethylene.
C2H4 + 1)4 + H* 1 C2H6
This mechanism, however, has not attained much support from
experimental
results.
Oxide catalysts for hydrogenation of ethylene have attracted
attention rather recently and since much more has to be done to
eluci
date the behavior of these catalysts. As mentioned earlier,
oxide
catalysts are of two types, insulators and semiconductors.
Detailed
kinetic studies of ethylene hydrogenation on alumina in the
temperature
-
21
range of 120°C to 430oC and a suggested mechanism are available
. (16)«
These results have been obtained from a flow, reactor, Catalysts
such
as platinum on silica have also been studied (17).
Zinc oxide was reported to be a catalyst for hydrogenation
about thirty years ago but only recently have papers been
published
concerning kinetic and infra-red studies. All of these works
were
carried out in batch reactors and reaction was monitored by
noting the
pressure changes. Zinc oxide, being a semiconductor, is much
less
active than the metals as a hydrogenation catalyst. It is
therefore a
suitable catalyst for observaing the effect of an electric field
since
a small change in catalytic activity is detectable. The reason
why it
is a favored catalyst in hydrogenation of ethylene lies in the
fact
that its electronic structure determines its activity. Thus a
stoi
chiometric zinc oxide is inactive whereas non-stoichiometry is
a
necessary condition for its catalytic behavior. Despite this,
however,
many workers have expressed doubt as to whether there is any
correlation
.at all between the activity of the semiconducting oxides and
their elec
tronic structures (18, 19, 20). Aigueperse and Teichner (18)
noted that
doping with lithium or gallium does not change the activity of
zinc
oxide catalyst although they do modify its electronic .property
because
of their being altervalent ions.
To understand the'complex behavior of ethylene
hydrogenation,
conclusions have to be based on all of the kinetic, exchange,
chemi-
sorption and infra-red studies taken together. Adsorption
studies of
-
22
hydrogen and ethylene on pure zinc oxide and exchange reaction
of
deuterium have been extremely useful in giving some insight into
the
mechanisme
In studies on the hydrogenation of ethylene on zinc oxide
and
chromia using hydrogen-deuterium mixtures (21), the product at
low.
conversions consisted of a mixture of C^H^D, and the
relative amounts of which corresponded closely with the amounts
of
HD and in the reactant hydrogen. When pure deuterium is used
C^H^Dg only is obtained, From this experimental information it
was
proposed that the ethylene hydrogenation reaction occurs by
irreversible
two step addition to adsorbed ethylene molecules of hydrogen
atoms
adsorbed in pairs on isolated sites. The hydrogen involved in
the
hydrogenation of ethylene is identified primarily with hydrogen
respon
sible for the ZnH and OH bands obtained in the infra red
spectra.. The
kinetics of the reaction were observed to be half order with
respect to
the hydrogen pressure at room temperature (22). Dent and Kokes
(19)
have divided hydrogen adsorption into two types and have
experimentally
demonstrated the role of hydrogen in the hydrogenation reaction
on zinc
oxide. Thus type 1 hydrogen adsorption is rapid and reversible;
type
2 is irreversible and occurs rapidly initially but slowly in
the
latter stages. Type 1 hydrogen gives rise to ZnH and OH bands in
the
infra red spectra and is the one responsible for hydrogenation
of
ethylene. In contrast, type 2 hydrogen does not take part in
the
hydrogenation of ethylene at room temperature but modifies the
catalyst
and enhances its activity. In the presence of ethylene,
however,, type
2 hydrogen is reduced by a factor of 3. Type 1 hydrogen can be
totally
-
removed from the surface by evacuation but type 2 hydrogen
cannot be
so removed. On a fresh catalyst, the very first reaction
experiment
gave an unusually high rate of reaction suggesting that slow
chemi-
sorption modified the activity of the catalyst. The effect,
however,
did not last after the first run. This phenomenon has been
termed
1 hydrogen promotion’. The rather striking conclusion by these
authors
is that non-stoichiometry is not responsible for ethylene
hydrogena
tion. Instead, it is proposed that the strained sites, perhaps
formed
by dehydration, are the active sites. According to their model
chemi-
sorption of hydrogen can be represented by the following sets
of
equations :
H Hi I |
(g) + - Zn - 0 - ^ - Zn - 6 — (10)
H H H HI I , . I i I— Zn — 0 — 4* — 0 — -
-
24
CHo ~ CHoI I + I IH2C = CH2 + - Zn - 0 - % - Zn - 0 (14b)
H H H0C = CH.I I I I— Zn — 0 — + H^C — CH^ ~ Zn — 0 — (14c)
and hydrogenation is represented by
H H0C = CH0 CH0-CHqI I I 2 3— Zn — 0 — ->■ — Zn — 0 —
(15)I
CHo-CH-3 HI I I I I— Zn — 0 — + “ 0 — + — Zn — 0 — 4- — 0 —i 2
(16)
From these equations, they deduced a. i-aLc c^p^uaaiuu that
satisfied tiie half order hydrogen dependence
v H = k ’e' (H2 (g))^ (17)
where 9 1 represents the adsorption of ethylene on an oxide site
adjacent
to the exposed zinc. It is evident therefore, from the rate
equation,
that the ethylene dependence is similar to that found for
ethylene
chemisorption.
ZnO has been found by Aigueperse and Teichner (18) to be
excep
tionally prone to oxygen poisoning. For the catalyst activated
in a
hydrogen stream, if exposed to oxygen or air for a short time,
the acti
vity was found to be reduced to less than 1% of its initial
value.
Dent and Kokes (19,22) worked exhaustively on oxygen poisoning.
Their
-
25
finding is very interesting in the fact that they suggested that
oxy
gen by itself is not a poison. If there was even a minute amount
of
chemisorbed hydrogen, exposure to air drastically reduced the
activity
revealed in the rate of reaction. On the contrary, if the
catalyst
was preheated with dry oxygen at 400°C, it still had the same
activity
as a catalyst conventionally treated in a hydrogen atmosphere.
This
result led to the suggestion that water or its precursor rather
than
oxygen itself acted as the poison. An oxygen activated sample
had no
chemisorbed hydrogen on its surface and so no poisoning was
observed.
Oxygen treatment made zinc oxide more stoichiometric and since
it
still exhibited the same catalytic activity at room temperture,
the
non-stoichiometry as a reason of activity was ruled out by these
authors.
It is deeply suspected, though, that the correlation between
semi
conductivity and catalytic activity will eventually be
established at
high temperatures (23).
Teichner (24) showed that doping with altervalent ions
changed
the character of surface coverage, the behavior being also
dependent on
temperature. In other words, as a result of doping, the
activation
energy for a doped catalyst was different from the one with an
undoped
catalyst. However, this difference in activation energy and for
that
matter in the different coverage characteristics by the
reactants, may
result not only from the modification of the Fermi level but
also from
the fact that in a given temperature range the modification by
the dope
of the nature of the surface of the catalyst may be such that
two reac
tants are adsorbed in a different manner.
-
26
Ethylene acts as a strong poison for hydrogenation of
ethylene
on zinc oxide (23). This kind of behavior has been reported
with
metals and alumina catalysts also. The retarding effect on the
rate
due to poisoning is very critical in an ethylene rich
hydrogenating
mixture. A pretreatment of the catalyst with ethylene before
introduc
tion of hydrogen also reduces the rate of reaction considerably.
Only
in mixtures rich in hydrogen is this poisoning effect
negligible. It
has been proposed that the poisoning effect of ethylene is due
to a
reaction involving dissociation of C-H bonds to form a hydrogen
defi
cient species similar to the 'acetylenic complex’ proposed by
Schwab,
Beeck and Rideal in studies of ethylene hydrogenation over
nickel and
other metals.
High temperature hydrogenation of ethylene was reported by
Bozon-Verduraz and Teichner (80°C- 400°C). Their kinetic data
showed
that the mechanism of the reaction was indeed strongly dependent
on
the range of temperature while the activation energy approached
zero
at higher temperatures from a value of 22 Kcals/g. mole at
lower
temperatures. Their rate expressions and the corresponding
tempera
ture ranges are as follows:
(a) 80° - 125°C vH = kpH
(b) 140° - 175°C vH = k (PH)°'3 (PE)°-7
for (P%)/(Pg) >> 1
-
27
(c) 210°C vR = k pE
(d) Above 210°C the overall reaction order was still unity but
the
initial rate and the rate constant were lower than those for
210°C.
In summary, it can be said that the hydrogenation of
ethylene
on semiconducting zinc oxide is an extremely complicated
reaction.
From the theoretical standpoint, it was widely believed that
non-stoi
chiometry is the reason for the catalytic activity of zinc
oxide. Dent
and Kokes, however, from their study at room temperature
concluded that
strained sites rather than non-stoichiometry gives rise to the
activity
of the catalyst. Bozon-Verduraz and Teichner came to the same
conclu
sion from their observation that doping with altervalent ions
did not
change rhe react j on rptf*, In opposirinn tr. this, Teichner 1
s earlier observation showed that the activation energy of
chemisorption was
changed due to doping. Whether this constancy in rate on doped
cata
lysts, despite the variation in the surface coverages and
activation
energy, is a manifestation of the compensation effect is yet to
be deter
mined. The question of a possible correlation between the
catalytic
activity and the non-stoichiometry is, however, far from
settled.
Bozon-Verduraz and Teichner have themselves suggested that a lot
more
work has to be done on the doping and that non-stoichiometry may
become
important at higher temperatures.
The present study is the first ethylene hydrogenation study
on
pure zinc oxide at temperatures higher than room temperature in
the
presence of an electric field. From theory it can be argued that
the
-
28
field may only bring about a change in the Fermi level and
cannot
influence the surface irregularities in any way and probably
also
cannot influence the way the gases are adsorbed on them. The
effect
of the field is, therefore, to be thought of as a consequence of
the
change in electronic properties of the catalyst.
-
CHAPTER 3
APPARATUS AND EXPERIMENTAL PROCEDURE
A Flow Reactor
In the present .studyj the hydrogenation of ethylene on zinc
oxide with and without the electric field was conducted in a
differen
tial flow reactor under isothermal conditions. In such a system
the
reactants are simply passed through a bed of catalyst at flow
rates
chosen to give the desired low conversion, A flow reactor has
the
following advantages over a static or batch reactor:
(a) Control of the temperature of the reactor as the reaction
progresses
is relatively easier,
(b) The change in catalytic activity can be easily followed.
At low conversion in the flow reactor9 the rate of reaction
can
be written as the product of the reactants flow rates and the
conversion
and this rate is representative of the initial rate of reaction
since
the composition of the gases do not change over the catalyst
appreciably
and heats of reaction or adsorption are sufficiently low that no
appre
ciable temperature variations occur. Thus rate of reaction based
on
mass of catalyst may be expressed by
,v = - . x (18)H m
29
-
30
where F represents the feed rate of ethylene to the reactor In
gram
moles per hour, m represents weight in grams of zinc oxide, and
x,
the fraction of ethylene converted to ethane. The reaction
rate
is then expressed as gram moles of ethylene hydrogenated per
hour
per gram of zinc oxide. The conversion per pass was in most
cases
kept below 10%, although in some cases it exceeded that limit.
The
derivation of equation 18 is given in Appendix A.
Sinfelt (25) has described a simple experimental method for
catalytic kinetic studies of this sort. His method was largely
followed
here, although special attention had to be given to the design
of the
reactor for the application of an electric field. In short,
the
metered gases (viz., hydrogen, ethylene and helium as a diluent)
passed
through the catalyst bed and the conversion was measured by a
gas
chromatograph. In the runs with the electric field, the field
was
applied for a short time, typically two minutes, across the bed
between
two electrodes insulated from the wall of the reactor while the
gases
went in and out through the electrodes which were perforated.
A
detailed description of the reactor will now be given, followed
by the
description of the whole system.
Reactor
Figure 5 shows the reactor assembly and Figure 6 and Figure
7,
its various parts with the pertinent dimensions. Essentially
the
reactor consisted of a cylindrical body of carbon steel, 2" long
and
3^n in outside diameter threaded at both ends to accommodate two
cover
-
31
nuts. The cover nuts are identical having two ports on top of
each.
One port was used for gas inlet or outlet and the other port
for
electrical connection. For the runs without the field, the ports
for
electrical connection were plugged.
The body is a cylindrical section with circular cut-out
depre
ssions at both ends about 5/8" deep. The internal diameter at
the
depressions is 2 3/16". The middle portion of the body, 3/4" in
length,
is of internal diameter 3/4" having a thermocouple well on the
wall.
An iron-constantan thermocouple in an 1/8" steel tube went
through the
well as far as the center of the body. The tubing was held at
the
outside wall by an 1/8" swagelok male connector. The other end
of the
steel tubing was sealed with silicone rubber cement so that the
reactant
gases could not leak out through the tubing. This thermocouple
was
connected to a calibrated temperature indicator (West Instrument
Co.)
which showed the temperature of the reactor.
Two identical transite rings, %" thick with outside diameter
the same as the inside diameter of the depressions, and whose
inter
nal diameter was the same as the internal diameter of the body
at the
central portion sat in the depressions, one at each end, thus
forming
a cylindrical section lh" in length and 13/4" in diameter.
This
cylindrical section was filled with the catalyst. An alundum
filter
disc of 2" in diameter, 3/32" in thickness and of porosity
40-60
microns was placed on top of the transite ring. A steel disc
with
perforations as shown in Figure 7 covered the alundum disc
completely.
The steel disc acted as one electrode with an 1/8" hole at the
center
-
32
/
3 / 3 2
Figure 5. Reactor Assembly
8 B OD Y MILD S T E E L 1
7 RING- T R A N S I T S 2
6 RING TRANSITS 25 F RI T TE D D/ SC ALUNDUM 2
4 COVER MILD S TE E L I
3 DISC MILD S T E E L 2
2 R I N G ' T R A N S I T S 2
1 C O V E R MILD S T E E L 1
NO. D E S C R I P T I O N M A T E R I A L OFF
-
33
15!G
z- PIPE THREAD
/ .8>
COVER
77 t :tX\o' PIPE _
° T H R 'P —
Figure 6. Exploded View of the Reactor
-
34
T
TRANSIT E. RING-T R A N S I T S RING
(fj -THROUGH ■ ™ - - T Y P / C A L , ON
° SQ. PITCH
ii
C O U N T E R S U N K
T R A N S I T S RI NG
Figure 7. Electrodes and Transite Rings
-
35
for attaching the electrical wire with a nut and screw
arrangement«
Another transite ring was placed between the protruding wall of
the .
body and the outer diameter of the steel disc, thus insulating
electri
cally the steel electrode from the wall of the reactor. On top
of
this another transite ring Sh11 in outside diameter and l%If in
inside
diameter, %n thick rested. Its purpose was to eliminate or
reduce the
chance of arcing across the space between the electrode and the
cover
nut. The nut was then screwed down on top of the body. The
tolerances
of these parts were such that in the assembled configuration no
move
ment of the internal pieces was possible. The bottom part of the
body
had exactly the same arrangement as the top. Thus in the
assembled
position gases entered the reactor through the top of the
vertically .
mounted reactor, passed through the perforated electrode, the
alundum
disc and the catalyst bed and through the disc the electrode on
the
other end out of the bottom of the reactor. The reactor had a
volume
of 49.1 cc. Inlet and outlet ports of the reactor were fitted
with
swagelok male connectors which held V ! O.D. steel tubing.
The
system was found to be leak proof by applying vacuum to the
outlet,
closing all other ports, and using a soap solution.
Assembly
Cylinders for the two reactant gases, hydrogen and ethylene,
and the diluent helium, fitted with pressure regulators, were
connected
in parallel to three flow meters. Hydrogen and helium were of
reactor
grade purity (99.998%) and ethylene was Matheson C.P. grade
(99.5%).
Hydrogen and ethylene each had a deoxo unit (Engelhard
Industries,
-
36
model D-'10-2500) in the line. These versatile
catalyticvpurifiers ope-
rate at room temperature and remove oxygen, carbon monxide,
carbon
dioxide, nitric oxide and nitrogen peroxide from hydrogen in the
form
of nonreactive water, methane, carbon dioxide and nitrogen by
the pro
cess of deoxidation, methanation, selective oxidation and
reduction*
Traces of acetylene, carbon monoxide and hydrogen in ethylene
are
removed by processes of selective hydrogenation and oxidation in
the
form of ethylene, ethane and carbon dioxide.
The gases out of the flow meters were led to a cross through
a
1/8" stainless tube and swagelok fittings. The flow meters for
hydro
gen and ethylene were of the thermal conductivity variety and
were
manufactured by Hastings & Raydists Corporation, Hampton,
Virginia.
An LF-300 monitored the flow of hydrogen and an LF-50,' the
ethylene.
An F-300 flow transducer was connected to the hydrogen flow
meter and
an F-50, to the ethylene flow meter. These consisted of a heated
tube
and an arrangement of thermocouples to measure the differential
cooling
caused by the passing gas. A direct current voltage proportional
to
the rate of mass flow through the tube was generated by a
thermoelectric
element. The transducer outputs were practically insensitive to
inlet
pressure and temperature changes since the operation depended
only on
the mass flow and specific heats of the gases. A Hoke no. 2231
micron
filter of type 316 stainless steel inserted before each
transducer
prevented any dust from entering the transducer and damaging
its
characteristics. No particle greater than 5 micron in size could
pass
through the filter. The helium flow meter was a Brooks rotameter
with
-
37
steel and glass balls with a tube size R-2-15AA. Each feed'line
had
a Whitey needle valve for control of flow through the flow
meters.
The fourth outlet of the cross brought the gaseous mixture out
■
to a horizontal drier which was a tube, one foot in length and
in
diameter which was filled with anhydrous calcium chloride,
guarded by
glass wool on both sides. Connected to the outlet of the drier
was a
preheat coil made out of a 12T long 1/8" O.D. stainless steel
tubing
immersed in a constant temperature sand bath. The sand bath was
heated
and controlled by a proportional controller which could work
both _
manually and automatically. For further details about this
instrument
reference is made to Cise (26). The reactor support equipment
with
flow meters and sand bath was designed according to Hall, et*al.
(27).
■ The preheated gaseous mixture entered the reactor through
a
tee fitted to the inlet port of the reactor. A pressure gauge
fixed
on the tee measured the gauge pressure in the reactor which was
found
to read zero for the kind of flow rates used (106 to 190
standard cc
per minute). The product gases were air cooled after emerging
from
the reactor and passed through a micron filter similar to the
ones used
for the transducers to prevent any catalyst fines that might
have been
carried in the stream to move further downstream. The outgoing
gases
were then dried further in a calcium chloride guard tube
immersed in an ■
ice bath before entering the sample loop of the chromatograph.
Except
for a small volume of sample taken for analysis, all effluent
product
gases went to the vent.
-
38
A tee was placed downstream of the drier and preceded the
reac
tor t° form a bypass that went directly to the air cooler via a
Whitey
needle valve» This bypass was used to measure the initial
concentra
tion of ethylene in the feed gas, When the bypass was used,
another
Whitey needle valve on the tubing that went to the preheat coil
in the
sand bath was closed so that no gas mixture could enter the
preheater
and for that matter in the reactor.
The reactor was placed on a transite board vertically. The
board had a hole in the middle sufficiently big to accommodate
the
outlet channel and the port for electrical cable. The board was
in
turn placed on a firm stand.
Sand Bath
The preheating sand bath (26) as shown in figure 8 was a bed
of 60 mesh sand contained in a 13" long section of 3%n O.D.
stainless
steel pipe. A 100 mesh stainless steel screen was fixed at the
bottom
to hold the sand and fluidizing air was blown through the sand
bath.
Heating of the sand bath was done by two sets of eight
electrical coils
wrapped around notched strips of transite boards„ One set of
coils was
connected to a 20 amp, 130 volt, type W 20 N variac (General
Radio, Con
cord, Mass). The other set of coils was connected to a Leeds and
Nor-
thrup model MA-800 magnetic amplifier that supplied a variable
A.C.
voltage. The regulation of the magnetic amplifier's output was
achieved
through a direct current signal not exceeding five milliamps
from a
Leeds and Northrup series 60 three mode control unit joined with
a model
-
Ho 2
0
Figure 8. Schematic of the Apparatus
6 © I
Thermocouple
Legends :
\ (1) Deoxo Purifier! (2) Flow meter
(3) CaCl2 drier
(4) Sand bath
(5) Preheating coil
(6) Pressure gauge
(7) Reactor
(8) Voltage source
(9) Micron filteru>
(10) Chromatograph ^
-
40
S speedomax ?H r recorder equipped with an adjustable zero and
range
package and an adjustable set point.
Two iron-constantan thermocouples placed in a lance were
immersed in the sand bath. One of these was connected to the
tempera
ture controller which measured the temperature with reference to
a
cold junction temperature of 0°C in an ice bath. The other
thermo
couple was connected to a calibrated temperature meter (West
Instru
ment Co.). The setting in the air flow rotameter was made by
visual
inspection of the bed. Depending on the temperature^ a higher
tempera
ture required less flow of air through the sand bath for
fluidization.
Chromatograph
A Perkin-Elmer model 154D vapor fractometer equipped with a
Speedomax type G recorder (Leeds and Northrup Co.) was used to
analyze
the product stream. A precision gas sampling valve of volume 5
cc was
used for sampling the product mixture. Helium was the carrier
gas and
the components were detected using a thermal conductivity cell.
The
column packing chosen was Porapak Q porous polymer beads which
have
been reported (28, 29) to give good resolution of light
hydrocarbon
gases in a single column. The column installed was a coil of 10T
copper
tubing, 3/16" in internal diameter filled with Porapak Q, 80-100
mesh.V ,
The High Voltage Source
The power supply used in this work has been described
previously
in full detail (30). The power supply.was primarily built to
supply
10,000 volts D.C. output although it could be changed to an A.C.
source
-
by changing several wire connections. In order to change from a
B.C.
supply to A.C., the wires connecting the' transformers to the
rectifier
circuit were removed. Then the high voltage connector was
wired
directly to the transformers. This arrangement eliminated the
recti
fying system from the circuit. The maximum A.C. voltage output
avail
able was 7 «> 980 volts. Since the voltmeter mounted on the
chassis of
the voltage source was a B.C. voltmeter, it was necessary to use
a
voltage divider in the ratio of 19:1 to measure the voltage with
a
Simpson V.O.H. connected across the output terminals.
For the runs with the electric field the cable used
connecting
the voltage to the reactor was rated for 10,000 volts. For the
portion
that went into the reactor from both ends of the reactor, the
rubber
sheath was peeled off and the bare strands were insulated with a
layer
of silicone rubber cement. The insulated cable was passed
through Zytel
male connectors fitted at the ports at each end and was
connected to
the electrodes. After the reactor was thus assembled, the Zytel
swage-
lok male connectors were sealed with silicone rubber cement and
kept at
room temperature for 48 hours. This cement could very easily
withstand
150°C indefinitely.
-
42
Experimental Procedure
Calibration of Flow Meters
The principle of flow meter calibration in all cases was to
measure the time taken by a soap film to travel up a. specified
length
of a calibrated burette. Thus the time was measured for a known
volume
of gas to flow up the burette. The temperature and pressure of
the
room were recorded and the volume was converted to 760 mm and
20°C.
The flow rate that was thus obtained was expressed as standard
cc per
minute. This method of flow meter calibration has been both
accurate
and easy to do (26). The flow meters calibrated were those for
hydro
gen s ehtylene, diluent helium and for the carrier gas in the
chromato
graph.
Flow rates were varied by use of the needle valves in each
gas
line. Five readings were taken at each setting and the mean of
the
readings were noted. The values of the flow rates in standard cc
per
minute were plotted against the flow meter settings. These
calibration
plots can be found in the Appendix B.
Activation of the Catalyst
Zinc oxide catalyst was obtained from the Harshaw Chemical
Co.
in the form of 3/16M extrudates (Zn-0401 E 3/16"). To avoid the
effect
of pore diffusion, these extrudates were crushed to two
different mesh
sizes, namely, 20-28 mesh and 35-48 mesh. Both of these sizes
were used
in the hydrogenation experiments without the field whereas only
the
latter was used when the effect of the field was being
examined.
-
The properties supplied by Ears haw on the.' extrudates' were
as
follows:
ZnO 100% pure
ABD '75 lbs/eft
SA 3 m2/g
Several different methods of activation of zinc oxide
.catalyst
have been cited in the literature (18, 19), but the method
followed in
the present work was different for reasons to be discussed later
(see
Results and Discussions).
The usual method of activation used in this study was to
give
the catalyst particles a pretreatment in a glass tube heated by
a
surrounding cylindrical oven at 450°C for one hour under vacuum
(28
inches). This treatment would remove adsorbed water and other
gaseous
impurities from the surface. After this treatment, the catalyst
was
brought to room temperature and was placed in the reactor.
Vacuum was
then applied to the reactor itself from the gas outlet channel
by
temporarily disengaging a reducing union that was connected to
the air
cooler♦ Tht reactor was then heated to 300°C for two hours. The
heat
ing was done by resistance heating band wrapped around the
reactor.
Power to this heater was adjusted by a variac. The reactor with
its
heating band was surrounded by cylindrical glass fibre
insulation. A
satisfactory temperature control was obtained by manual
operation (with
in ±1°F).
Heating in vacuum at a higher temperature (e.g., 450°C)
helps
in removing poisons, especially water, from the surface of the
catalyst
Strength 8 lb.
PV 0.26 cc/g
-
44
The reactor could not be heated to that high a temperature
because the
thread lubricant hardened at the threads of the cover nuts and
the
reactor could then be opened only with great difficulty. To
avoid damag
ing the threads and the reactor itself, a less rigorous in situ
activa
tion like the one described was carried out in the reactor
itsdlf pre
ceded by a pretreatment at 450°G in vacuum in the glass
tube.
After the two hour vacuum treatment at 300°C5 hydrogen was
passed through the catalyst at that temperature for half an
hour.oVacuum was again applied at 300 C for one hour. This
treatment was
directed at removing any oxygen from the surface in the form of
water
and result in a reactive surface. The catalyst was then cooled
to the
reaction temperature.
Analysis by the Chromatograph
The column was installed in the chromatograph and as.
prescribed
by the manufacturer was conditioned at 230°C for 2 hours. The
carrier
helium flow was 40 ml/min. For analysis of reaction runs, the
tempera
ture of the column was maintained at 50°C with a carrier flow
rate of
40 ml/min. The bridge voltage was 8.0 volts and the chart speed
was
0.75 in/min.
A calibration run of ethylene was made by varying the
ethylene
flow rate while keeping the total flow constant and measuring
the peak
area under the ethylene peak. Ethylene peak area was plotted
against the
concentration (mole fraction) of ethylene and a linear plot was
obtained
(See Appendix B). As a check, during an actual series of runs,
the by
pass was frequently used to measure the initial concentration of
ethylene.
-
, 45
An ethane lecture bottle (Matheson Gas) fitted with„a
precision
regulator was used in the feed line to "give a similar
-calibration curve
of ethane and another linear plot was obtained«, The ethane
calibration
matched the ethylene calibration pretty well so that in an
actual
conversion measurement the conversion was simply obtained by
dividing
the ethane peak area by the initial ethylene peak area.
With the 5 cc sample loop and the flow rates used, the
hydrogen peak was small and M-shaped. For this reason all
conversion
measurements were referred to peak areas of ethylene and ethane.
A
typical chromatogram is shown in Figure 9.
Kinetic Runs
Kinetic data were taken at three different temperatures,
194°C5
146°C and 100°C for 20-28 mesh catalyst particles. With 35-48
mesh
particles similar studies were made at 146°C and 56.5°C.
Hydrogen dependence of the reaction rate was determined in
the
following manner. Hydrogen flow rates were varied while ethylene
flow
rates were kept constant. The diluent helium flow rates were
adjusted
to keep the total flow rates constant for all readings in a
particular
run. Conversions were calculated by method mentioned before and
since
the ethylene flow rate and hence ethylene concentration was
constant,
the conversion was proportional to the rate reaction as can be
seen
from equation 18. Conversions corresponding to different
hydrogen
mole fractions were therefore plotted on a log-log graph, the
slope,of
which gave the order with respect to hydrogen. Reactant
concentrations
-
X32
X 3212.0 10.0
Figure 9. A Typical Chromatogram Showing Separation of (1)
Hydrogen (2) Ethylene and (3) Ethane in a Porapak Q Column at
50°C.
-
were taken as the initial values even though in some cases,
since
conversion was greater than 10%, the average concentrations
would be
somewhat less.
Ethylene dependence determinations were similarly carried
out
by keeping the hydrogen flow rate constant and varying the
ethylene
flow rate while adjusting the helium flow rate so that the total
flow
rate was constant for all runs in this series» In this case,
however,
as suggested by equation 18, the reaction rate is proportional
to the
product Fx, where F is the ethylene flow rate and x, the
fraction of
ethylene converted to ethane; Fx values were therefore plotted
versus
concentration of ethylene on a log-log graph, the slope of which
gave
the ethylene partial.order. This was done since all conversions
were
calculated from ethylene measurements, hydrogen being difficult
to
determine accurately.
A typical run could be described as followsThe reactor was
brought to the reaction temperature and the sand bath
temperature was
controlled by adjusting the current input from the temperature
control"
ler manually. Prior to the first run hydrogen was passed through
the
catalyst for half an hour. Then the needle valve on the inlet to
the
reactor was closed and the one on the bypass was opened.
Ethylene and
helium flows were adjusted by turning on the needle valves on
their feed
lines and then the inlet needle valve was opened, the one on the
bypass
was closed so that the gas mixture entered the reactor. The flow
was
continued for three minutes at the end of which sampling was
done in
the chromatograph and the ethylene flow was stopped but
hydrogen
-
48
and helium flows were continued. It took about 15-minutes'for
the
analysis to be done by the chromatograph. The next run was
conducted
by repeating the same procedure.
During the first few experiments3 attempts were made to
dupli
cate the initial run after every other different run to check
the
activity of the catalyst, as suggested by Sinfelt (25). This
was
discontinued as the activity did not change appreciably so as
to
vitiate the kinetics. •
Each day before taking runs, the catalyst was given
activation
treatment in situ. At the later stages of experiments, however,
either
helium was passed continuously overnight and runs were taken on
that
catalyst the next day or vacuum was applied overnight.
The runs for the Arrhenius activation plots were taken in a
similar manner except that in this case all flow rates were kept
cons
tant and the temperature was varied. The log of fractional
conversion
(being representative of the reaction rate) was plotted against
1000/T
to give the activation plot. The slope of the straight line
thus
obtained was equal to -E/R where E is the activation energy.
For the runs with the field, the catalyst was first given a
opretreatment at 450 C for one hour in vacuum in the glass tube,
tempera
ture was then brought down to 300°C and the catalyst was treated
with
hydrogen for % hour after which vacuum was applied again at
300°C for
one hour. After cooling the catalyst in vacuum to room
temperature it
was put into the reactor and the reactor was assembled and
sealed in
the manner described. The reactor.was grounded. The catalyst was
then
-
- 49.'
heated for 2 hours at 150°C in vacuum in the reactor and then
was cooled
to. the reaction temperature» The reactor was then ready for
operation.
Voltage was applied for two minutes typically, the last two
minutes of
the three minute reaction periodBetween two consecutive days of
data
taking the reactor was given vacuum treatment at 150°C
overnight.
-
CHAPTER 4
RESULTS AND DISCUSSION
Heterogeneous catalysis by nature is an extremely
complicated
phenomenon. Activity of the catalyst depends, among other
things, on
the method of preparation of the catalyst, method of its
activation and
in some cases also on the composition of the reactant gases. It
is
obvious therefore that classical kinetic studies alone in terms
of some
empirical mechanistic model is not a very satisfactory approach
to try
ing to elucidate the "facts" about any particular heterogeneous
chemical
reaction. More attention should therefore be paid to knowing
more about
the surface that effects the catalysis. There exists no general
approach
either to explain why a particular method of preparation and
activation
of a catalyst is more effective than another or to control it.
There
has been, nevertheless, an encouraging trend in applying infra
red
spectra to study the physical and chemical nature of the
adsorbed species
on the surface. The present work has had its obvious limitations
because
of the fact that no attempt was. made to follow the surface
condition as
hydrogenation occured on zinc oxide surface with or without an
electric
field. This study was made in large excess of hydrogen in order
to avoid
the frustrating reaction pattern at high ethylene concentration,
pre
sumably due to poisoning.
50
-
51
Method of Preparation and Activation-
Catalytic activity of zinc oxide depends to a considerable
extent on the mode of preparation of the catalyst. Thus a
catalyti-
cally active zinc oxide is obtained by careful thermal
decomposition
of zinc hydroxide or carbonate, but the oxide from the nitrate
has a
low activity. The lowere the temperature of preparation of the
cata
lyst from hydroxide or carbonate, the greater is the activity.
Vari
ous preparations lead to the same crystal structure, but differ
in the
extent of lattice imperfections (31). Bozon-Verduraz and
Teichner (23)
have shown that rate of reaction increases with the temperature
of
activation until about 500°C above which sintering brings about
a reduc
tion in rate* They have expressed the activity of zinc oxide in
terms
of the half life periods of ;the hydrogenation of ehtylene and
have
also shown that the surface area suffers a progressive
diminution with
the increasing temperature of activation.
As mentioned in Chapter 2, the activation procedure followed
in
the present work was designed to give a measurable conversion.
During
the earlier stage of experiments, activation was carried out in
the
glass tube at 450°C and the catalyst, while still warm, was
transferred
to the reactor. But no hydrogenation activity was noted.
Similar
results.were obtained even when the catalyst after activation
was brought
to room temperature while still under vacuum before transferring
to the
reactor. This presumably was due to reaction of oxygen with type
2
chemisorbed hydrogen (slow chemisorption) described by Dent and
Kokes
-
. 52
that could not be removed by vacuum, to form water which acted
as poi
son for the catalyst, Another attempt was made to activate the
catalyst
by heating in vacuum at 190°C for 48 hours*» This catalyst also
did not
work. It is probable that the temperature was too low to drive
out
moisture already adsorbed on the catalyst supplied by Harshaw
Chemical
Co. Oxygen activation as described by Dent and Kokes was also
tried
but again no conversion could be detected. In this case no
vacuum was
applied to the reactor itself before exposing the catalyst for
the first
time to hydrogenation. As is well known, zinc oxide being an
n-type
semiconductor adsorbs oxygen at room temperature with a
consequent fall
in its conductivity and, therefore, the adsorbed oxygen most
likely
reacted with hydrogen to form the poisoning wo ter-
In contrast, the preactivation at 450°C under vacuum given
to
the catalyst helped in driving out moisture and other impurities
from
the surface and when exposed to air at room temperature for a
short
while, only oxygen could have been adsorbed. But in situ
activation
which involved heating in vacuum at 300°C drove out the adsorbed
oxy
gen once again. Introduction of hydrogen presumably removed the
last
traces of oxygen in the form of water.vapor. During the time the
acti
vation and all following experimental runs were taken, the
catalyst
was never exposed to oxygen or air. During the first few days
of
experiments, the catalyst had to be given the same in situ
treatment
before taking data since the vent was connected to the reactor
through
the outlet tubing. This could be avoided by fixing a needle
valve on
the exit line. During the latter part, however, either helium
was •
-
53
passed through the bed overnight or the vacuum applied prior to
next
days experiment.
Catalyst Effectiveness
Catalytic activity was found to change from day to day.
Even,
for two different batches of fresh catalysts5 the activity
differed
somewhat. This is presumably because of the problems involved in
.
strictly controlling the activation procedures (temperature and
time
of heating). Nevertheless as shown in Table 1 and Table 2, the
cata
lyst activity remained reasonably constant over a period of five
hours
of data taking. The variation of activity, however, inhibited
any
formal determination of the effectiveness factor of the
catalyst.
Nevertheless activation energy plots in Figures 30 and 31 show
that
the logarithm of the fractional conversions versus 1/T was a
straight
line over a certain temperature range, changing in slope
drastically
at some transition temperature. This change in slope is expected
as
will be shown shortly and has been reported in the literature
for most
metal catalysts. Since diffusion is not an activated process, a
dif
fusion controlled process would not give rise to this kind of
behavior
with varying temperature. The straight line plot of Arrhenius
activa
tion is therefore a proof that the process was essentially
surface
reaction controlled and the effectiveness factor of the
catalysts was
essentially unity (2, 32).
Kinetic Runs, No Field
Table 3 summarizes the observed dependencies of the reaction
rate on the concentrations of hydrogen and ethylene under
five
-
54
Table 1
Experiment on Catalyst Activity
Conditions :Temperature 146°C
Hydrogen flow 68.0 std. cc/min
Ethylene flow 10.9 11 11
Helium flow 93.0 11 11
Total flow 171.9 11 It
Time from run 1 to run 8 = 5 hours
observations A C°2H4 frac. conv. (x)0.3995 V o V v/ ̂ V 0.107
.
2 ii M 0.1055
3 ir 0.1045
4 H 11 0.1005
.5 11 11 0.1040
6 11 11 0.1032
7 It 11 0.1075'
8 11 11 0.1000*
* After this series of runs5 helium was flown thrown through
thecatalyst bed overnight at room temperature prior to the next
series of similar runs the next'day (See Table 2),
-
55
Table 2
Experiment on Catalytic Activity
Conditions : Same as in Table 1
Time from run. 1 to run 10 - 6 hours
observationC2H4
frac. conv.(x).
1
2
3
4
5
6
7
8 9
10
0.3995 0.0630 • 0.1125
0.0985
0.0980
0.0912
0.0872
0.0935
0.0887
0.084
0.0815
0.0860
t
-
56
different temperature conditions» The effect of the field on
the
partial orders is also shown.
Ethylene Dependence
As stated earlier, (Fx)/m was plotted against the
concentration
of ethylene of log-log graph paper, the slope of which was the
ethylene
order. Hydrogen rich mixtures were always used. Ethylene
concentration
was varied from 0.04 to 0.35 mole fraction, while the total
pressure was
kept at 1 atmosphere.
At 195°C with 20-28 mesh catalyst, the ethylene order
appeared
to be one half (Figure 10). Figure 11 represents the same data
plotted
on cartesian coordinates to check that they satisfy the initial
condi
tion i.e., zero conversion at zero ethylene concentration. The
ethyl
ene order determination at other temperatures, e.g., 146°C,
100°C and 56.5°C, was characterized by marked scattering of the
data points.
Figures 12 and 13 show ethylene order at 1460C for 20-28 mesh
and 35-48
mesh catalysts, respectively. The apparent ethylene order at
146°C is
zero. The same type of behavior is noted at 100°C and 56.5°C
(Figures
14 and 15, respectively). Figures 13 through 15 would probably
suggest
a negative dependence on ethylene but this cannot be said with
certainty
because of scattering. Similarly Figure 12 might appear to
suggest a
positive dependence less than one half but since it differs from
Figure
13 only in mesh size, a suggested zero order would be
conceptually more
realisticb
-
Table 3 .
Observed Orders of Reaction
Reaction Temperature ( °C )
56.5 86.5 . 100 146 195
Voltage 0 4,180 0 0 3,800 0
order 1 % 1 1 . % ' %
C2H4order
. o* 0* . 0**0 ■ % 1
* uncertain due to scatter
Ln
-
001 X
58
10 T 1---1---1— | | TI T
1.0
Run la
0.5.01
.1 I ! 1 1 1 1.10
J L .40
Figure 10. Order with Respect to Ethylene, 1950C, 20-28 Mesh
Catalyst.
-
001 X
59
dE
3,0
Run la
1.0
00.20.10
'C2H4
Figure 11. Ethylene Order at 195°C, Run la Replotted, on
Cartesian Coordinates.
-
001 X
60
10
E
Run 2a
.01 .10 .40c2H4
Figure 12. Order with Respect to Ethylene, 146°C, 20-28 Mesh
Catalyst.
-
61
Run 3a
0.3
Figure 13. Order with Respect to Ethylene, 146°C, 35-48 Mesh
Catalyst.
8.0 O i— ] j f r T T
oo O O Ox O
O O O
Run 4a
i.O_______ I_I I___ L_J__I._L_l_l____________ I---------.02 0.1
0.3
cc2h4Figure 14. Order with Respect to Ethylene, 100°C, 20-28
Mesh
Catalyst.
-
X 100
62
1.0
Run 5a
— O'
.10.4.03 0.1
Figure 15. Order with Respect to Ethylene, 56.5°C, 35-48 Mesh
Catalyst.
-
63
Hydrogen Dependence
For hydrogen order determinations5 fractional conversion, x,
was plotted against the concentration of'hydrogen on log-log
graph
paper 9 the slope of the straight line being the order with
respect to
hydrogen. One half hydrogen order was observed at 195°C (Figure
16).
Figure 17 (Run lb) shows the same data on a cartesian plot
which
verifies the initial condition at zero hydrogen concentration.
Runs
2b and 3b were taken at 146°C for 20-28 mesh and 35-48 mesh
catalyst
respectively. Figures 18 and 19 show the approximate first
order
hydrogen dependence for these runs. These runs were replotted on
carte
sian coordinates (Figure 17) to check the validity of the
observed
hydrogen order at this temperature. The data for Run 2b and 3b
fall
on two different straight lines which can be explained on the
assump
tion that the activity (e.g., available surface area) of the two
diffe
rent batches of catalyst was different such that no effective
reproduci
bility could be achieved.
At 100°C and 56.5°C, the observed hydrogen order was one
(Figure
20, Run 4b for 100°C and Figure 21, Run 5b for 56.5°C). These
data
plotted on cartesian coordinates are shown in Figure 22.
-
0.2
Run
0.1.2 .5 1.0
Figure 16. Order with Respect to Hydrogen, 195 C, 20-28 Mesh
catalyst.
ON
-
65
0H
0.4 2 0.8
p o,1 6
Run lb195 C
XRun 3b, 146 C
.08
.04
Run 2b, 146 C
0.80.40
Figure 17. Hydrogen Order as Shown in Cartesian Blots
-
Run 2b
01
0051.00.1
Figure 18. Order with Respect to Hydrogen, 146° 20-28 Mesh
Catalyst.
-
67
0.3
.1
Run 3b
.0 21.00.1
Figure 19. Order with Respect to Hydrogen, 146°c, 35-48 Mesh
Catalyst.
-
68
03
Run 4b
0010.80.|
Figure 20. Order with Respect to Hydrogen, 100°C, 20-28 Mesh
Catalyst.
-
Run 5b
.0030.2 0.5 1-0
Figure 21. Order with Respect to Hydrogen, 56 35-48 Mesh
Catalyst.
-
70
.016
.012
Run 4b, 100 C.004Run 5b, 56.5 C
00 0.4 0.8 1.0
Figure 22. Hydrogen Order as Shown in Cartesian Plots.
-
Effect of the Field
Maximum voltage that could be applied’was 3,800 volts at
146°C
and 5,700 volts at 86,5°C. At still higher voltages the
indicator of
the Simpson V.O.M. was fluctuating wildly indicating some
instability
arising presumably out of the failure of the transite ring
insulators
in the reactor, There was a very definite reduction in rate in
the
presence of the field observed at 146°C whereas the rate was
accelera
ted at 86.5°C. The reduction was as much as three times greater
with
longer duration of the field and higher field strength. Table 4
shows
how with increasing voltages for the same field duration, the
conversion
under otherwise identical conditions was progressively reduced.
Table
5 shows that with larger duration of the field before sampling,
the rate
is decelerated, longer duration giving rise to higher reduction
in the
rate of reaction, •
In contrast to this deceleratory effect of the field on the
reaction rate, at lower temperatures like 86.5°C the field
enhanced the
reaction; a five hundred per cent increase over the zero field
rate was
observed with 5,700 volts. The effect of the field was exactly
opposite
that at higher temperatures, higher field strength progressively
accele
rated the rate. Longer duration of the field yielded higher
accelera
