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UK ISSN 0032-1400
PLATINUM METALS REVIEW
A quarterly survey of research on the platinum metals and of
developments in their application in industry
VOL. 32 APRIL 1988
Contents
Flammable Gas Detection
Superconductivity in Platinum Compounds
Platinum Protects the Environment
The Largest Producer of Platinum Metals
Transition Metal Catalysed Synthesis of Oligo- and
Polysilazanes
Platinum Thermocouple Calibrations
Promoting Platinum Metals by &ria
Hydrogen in Amorphous Palladium Alloys
A Catalytic Reaction Guide
Frt5dCric Kuhlmann
Abstracts
New Patents
NO. 2
50
60
61
63
64
72
73
83
83
84
91
101
Communications should be addressed to The Editor, Platinum
Metals Review
Johnson Matthey Public Limited Company, Hatton Garden, London
ECl N 8EE
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Flammable Gas Detection THE ROLE OF THE PLATINUM METALS
By T. A. Jones and P. T. Walsh Health and Safety Executive,
Research and Laboratory Services Division, Sheffield
There is an obvious need to detect the presence offlammable
gases before their concentration levels approach explosive
proportions. The platinum metahfind application in the three main
types of detector developed for this purpose, and each is
considered here.
The presence of flammable gases in the atmosphere is a potential
hazard in many industrial, commercial and domestic en- vironments.
Following the introduction of natural gas as a primary fuel in the
United Kingdom during the 1970s~ methane is now the most prevalent
of the flammable gases in this country. Since it is a product of
decaying organic matter it is also found in dangerous quantities in
mines, sewers and waste tips. Hydrogen is still a major problem in
those countries where it is used as a fuel, and it also occurs as a
product evolved from lead-acid ac- cumulators. Liquid petroleum gas
(LPG) is us- ed as a portable fuel in a wide variety of
applications ranging from heavy industry to leisure caravans and
boats. Petrol is an obvious hazard in garages and enclosed car
parks. Final- ly a wide range of hydrocarbons is widely used in the
chemical and petrochemical industries.
The hazard posed by a flammable gas is well defined. The
concentration range over which the gas in air is flammable can be
experimental- ly determined and the two limits, the lower ex-
plosive limit (LEL) and the upper explosive limit are known for
most gases (I). It is com- mon practice to seek to maintain the
concentra- tion levels below about 20 per cent of the LEL, and the
alarm level is usually set at this figure. The LEL for most gases
lies between I per cent and 5 per cent vlv, therefore any sensor
used for monitoring the hazard has to be capable of indicating
concentrations in this range with a discrimination of better then
10 per cent of the LEL.
A number of sensors have been developed for
measuring flammable gas concentrations. This paper will deal
with the three main types, in all of which the platinum group
metals play a cen- tral role. The sensor which dominates the field
is the calorimetric or catalytic type, often known as the
“pellistor”; in this the flammable gas is oxidised on a catalytic
surface and the concentration determined from the quantity of heat
released in the reaction. This type of sen- sor is widely used for
quantitative measurement of the hazard, and since its introduction
(2) in the early 1960s it has had a major effect on working
practices and conditions in many in- dustries.
The second type of sensor is based on elec- trical conductivity
changes induced by gas ad- sorption on metal-oxide semiconductors.
This is used primarily as a qualitative indicator of the presence
of a gas, or of a change in its concen- tration. This type of
sensor is extensively used, particularly in situations where low
concentra- tions of a wide range of gases need to be in- dicated.
Platinum group metals are used in these devices as an additive to
the oxide; their catalytic properties can improve sensitivity and,
to some extent, selectivity.
The third type of sensor in which the gaseous analyte interacts
with a platinum group metal surface is the catalytic field effect
transistor (FET). Dissociation of hydrogen and some
hydrogen-containing gases on the catalytically active gate affects
the electrical characteristics of the FET by an amount proportional
to the gas concentration. As yet, these sensors have not made a
major impact in the market place, but they are likely to do so in
the future because
Platinum Metals Rev., 1988, 32, (2), 50-60 50
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they are silicon-based devices and thus lend themselves to
microfabrication techniques.
Calorimetric Gas Sensors The calorimetric device operates on the
prin-
ciple of detecting the heat evolved during the combustion of
flammable gas in ambient air. The total oxidation of methane, for
example, liberates 803 kJ/mol of heat. If, under the con- ditions
of measurement, the rate of reaction is dependent on the
concentration of the fuel then determination of the heat evolved
provides a means of measuring gas concentration. The evolved heat
can be measured as a temperature rise, and a catalyst is used in
order to achieve an adequate temperature rise (rate of reaction) at
a conveniently low temperature. Thus the basic constituents of a
catalytic calorimetric gas sensor are a temperature sensor, a
catalyst and a heater to maintain the catalyst at the operating
temperature.
The simplest form of sensor, used in many early instruments, is
a platinum coil which acts as sensor, catalyst and heater. However
bulk metallic platinum is a relatively poor catalyst for
hydrocarbon oxidation. Hence the element must operate at a high
temperature-about IOOOOC for methane-which reduces its lifetime
because of metal evaporation. The most signifi- cant improvement in
the lifetime of the sensor has resulted from the use of more active
catalysts, so allowing the operating temperature to be reduced
considerably. Separation of the catalyst and the heater allows
catalysts with a larger surface area, and hence greater activity,
to be employed.
The most active catalysts for oxidation reac- tions are
palladium, rhodium, platinum and iridium. This is because
conditions on their surfaces are at an optimum for the reaction
bet- ween fuel and oxygen. The heat of adsorption of the reactants
is low enough to reduce the ac- tivation energy for oxidation, yet
sufficiently high to ensure an adequate surface coverage (3, 4).
Gold is inactive as a catalyst because oxygen is adsorbed only very
weakly, thereby resulting in too low a coverage. Transition metals
adsorb oxygen too strongly, which results in a high ac-
1 d H
a- alumina support
P la t inum w i r e 1 PdlThO, c a t a l y s t
rl I 1 mm
( a 1
i Noble mdtal dispersed in f i n e p a r t i c u l a t e Y -a
lumina
TO5 transistor header post
P la i inum wire
‘ig. 1 In catalytic calorimetric gas sen- ora a platinum coil
serves both as a esistance heater and as a temperature ensor. The
heater maintains the surroun- ling catalyst at a temperature which
en- ures rapid combustion of any flammable ;as, the concentration
of which is then in- licated by a change in the resistance of the
rire, due to the temperature increase awed by combustion of the
gas
tivation energy for oxidation and in extreme cases they can form
bulk oxides. This em- phasises the suitability of platinum metal
catalysts at the elevated temperatures required for oxidation of
the flammable gas. Moreover palladium and rhodium are more active
than platinum for the oxidation of methane, catalys- ing the
reaction at around po°C. The form of the “pellistor” device is
shown in Figure I(a). It consists of a platinum coil encapsulated
in alumina, which is coated with catalyst (2). Platinum is used as
a resistance thermometer because of its high temperature
coefficient of resistance and coils can be easily made. The
coil
Platinum Metals Rev., 1988, 32, (2) 51
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Detect
I I I L E l e c t r i c a l
I I I I connector or
/ Ambient air Sinter
Fig. 2 In a typical calorimetric gas sensor head the
catalytically active sensor and a similar inert compensator form
two arms of a Wheatstone bridge circuit. Flammable gas in the
atmosphere diffuses through the sinter to the sensing element
also serves as a resistance heater. Typically a coil consists of
about 10 turns of 0.05 mm diameter wire forming a helix having a
length of I mm and a diameter of 0.5 mm.
In the commonest arrangement a catalytically active sensing
element and a similar but catalytically inert compensator element
form two arms of a Wheatstone bridge circuit (I). Power is supplied
to the circuit to heat the elements to their operating temperature;
the values of the fured resistors, arranged in parallel with the
elements, are chosen to balance the bridge in air. The out of
balance voltage, resulting from the presence of flammable gas, is
dependent on the change in temperature, which in turn is dependent
on the rate of reaction and the partial pressure of the flammable
gas. This and similar types of sensor are widely used in various
kinds of flammable gas detection in- struments, which may be hand
held or fmed in- stallations. G a s usually reaches the sensing
element by diffusion through a sinter exposed to the atmosphere in
a typical configuration shown in Figure 2. The effect of
temperature on the out of balance voltage and on the power drain of
a typical sensing element is shown in Figure 3. Also shown is the
current required to attain the temperature when the room
temperature resistances of the detecting and compensating elements
are about 1.38, each.
At high temperatures the rate (signal) is less dependent on
temperature and is limited by the mass transport of fuel across the
sinter to the sensing element. Under these conditions the rate is
independent of the chemical nature of the catalyst and only
geometric factors such as separation of the element from the sinter
in- fluence the signal (5). The mass-transport- limited (MTL) mode
of operation has the disadvantage of increasing the response time
to several seconds but confers several advantages:
Small variations in the catalytic activity of the elements do
not affect the signal.
Minor voltage (hence temperature) fluctua- tions do not
influence the signal.
The rate of diffusion and hence the response is linearly
dependent on the flammable gas con- centration.
A direct, approximate measure of flammabili- ty (per cent LEL)
is obtained which is largely independent of the gas or gases
present.
Thus, an instrument calibrated to read o to 100 per cent LEL in
a standard fuel will pro- vide an approximate estimate of the ex-
plosiveness of any vapour or mixture of vapours. If the composition
of the fuel-air mix- ture is known then simple correction
factors
CURRENT, mA
220 240 260 280 300 320
200 300 400 500 I
TEMPERATURE, 'C
500
z z
500 =- Y
n
300
200
J
Fig. 3 The effect of temperature and cur- rent on the response
and power drain of a typical calorimetric sensor is shown
Platinum Metals Rev., 1988, 32, (2) 52
-
0
2 20 Y
L a 0
3- 4 0 2
W ul z 0 ul n ; 60
80
I 5 10 15 20 25 30
EXPOSURE TIME, hours
0 L c ” k 5 10 ul- ul
W vl L 0
ul W Q
n
20
30 0
1.
50 too 1 i O 2 EXPOSURE TIME, minutes
Fig. 4 The response of calorimetric sensors is affected by the
presence of poisons and inhibitors, here the improvement in
response that can be obtained with appropriate catalysts is shown
for 1 per cent methane in air in the presence of (a) 10 ppm hexa-
methyl disiloxane and (b) 100 ppm sulphur dioxide. The results from
a conventional pellistor are compared with those from poison
resistant (Pr) sensors
may be applied to produce more accurate measurements (I). The
use of platinum metal catalysts therefore permits operation in the
mass transport limited region, with its many advantages, at
comparatively low catalyst temperatures.
The major limitation of catalytic calorimetric gas sensors is
their loss of sensitivity on ex- posure to atmospheres containing
poisons (which have an irreversible effect) and in- hibitors (which
have a reversible effect). Com- mon examples are silicones, alkyl
lead compounds, phosphate esters, halogenated compounds and
sulphur-containing com- pounds. In the presence of these species a
low measure of the concentration of flammable gas is obtained. One
solution to the problem is to incorporate a filter, for example
activated char- coal, to remove the offending vapour. However
charcoal filters will also adsorb higher hydrocarbons and this
limits their use to methane, ethane, carbon monoxide and hydrogen.
This has led to the development of sensing elements which are much
more resis-
tant to poisons than the conventional devices. They generally
take the form of a platinum coil surrounded by a porous bead
comprising the platinum metal catalyst dispersed throughout a
support having a high surface area, such as y- alumina (6), as
shown in Figure I@). Improved poison resistance is achieved
through: [a] Dispersion of the platinum metal
throughout a porous support, which in- creases the effective
surface area available for reaction. Under MTL conditions in-
creasing the intrinsic activity of the catalyst - that is the
activity which would occur in the absence of mass transport con-
trol - effectively produces more “spare” surface, resulting in a
lower apparent rate of poisoning.
[blUtilising a catalyst having a high intrinsic resistance to
poisoning; for example in methane detection platinum is less
suscepti- ble to inhibition by hydrogen sulphide or sulphur dioxide
than either palladitun or rhodium.
Since platinum is intrinsically less active than
Platinum Metals Rev., 1988, 32, (2) 53
-
palladium or rhodium the relative resistance to poisoning will
be determined by the relative im- portance of factors [a] and [bl
above. The im- provement achieved using these catalysts is shown in
Figure 4 which illustrates the effect of hexamethyl disiloxane
(poison) and sulphur dioxide (inhibitor).
Poisons may deactivate different reactions at different rates.
On non-porous catalysts, site heterogeneity may result in some
reactions oc- curring on sites that are more easily poisoned than
others (7). For porous catalysts the poison- ing rate may vary with
the relative rates of ad- sorption and diffusion of the poison, and
reaction and diffusion of the reactants. It has been found that in
order to minimise inhibition of methane oxidation (slow reaction,
fast diffu- sion) by halogenated or sulphur-containing gases the
type of platinum metal is as important as the physical properties
of the catalyst, such as dispersion and porosity (6). For silicone,
lead and phosphorus poisons the increased disper- sion and porosity
of the catalyst, rather than the type of platinum metal, provide
the best means of increasing the poison resistance of the sens- ing
element. For butane oxidation (fast reac- tion, slow diffusion),
the differences in the behaviour of the platinum metals in
halogenated and sulphur-containing gases are less significant than
for methane oxidation. However for the poisons the behaviour in
butane follows a similar pattern to that in methane (6). It is
apparent from the above discussion that the design of
poison-resistant elements has strong parallels with catalyst design
in the automotive and petrochemical in- dustries. However, because
of the considerably smaller scale of use of platinum metals in gas
detection, the effectiveness of the catalyst is not as important,
therefore effort can be concen- trated on maximising
poison-resistance.
Another phenomenon, catalyst coking, can also adversely affect
the performance of catalytic elements. Here carbon is deposited
when the dehydrogenation rate of the fuel becomes appreciable at
very high concentra- tions, for example >20 per cent methane in
air. Surface carbon may poison the oxidation reac-
tion, and larger deposits change the physical size, morphology
and, to a lesser extent, emissivity of the element (8). All these
factors alter the power dissipated by the calorimeter and thus
change the sensor output. Even when carbon is subsequently burnt
off in air perma- nent damage to the element may occur, in- dicated
by a change in the zero level of the sensor. The incorporation of
thoria with palladium in the “pellistor” alleviates this pro- blem.
The role of thoria is to disperse the palladium which is present
only in metallic form after exposure to the reducing at- mosphere,
when methane concentrations are greater than 10 per cent (9). This
reduces the rate of sintering and thus the formation of large metal
particles on which coke would preferen- tially be formed. Thus
increased dispersion of platinum metals provides resistance to
coking as well as to catalyst poisoning and inhibition.
If the concentration of oxygen is too low for complete oxidation
to occur then the measured signal will be lower than that obtained
when the oxygen concentration is at normal levels (ap- proximately
21 per cent). Thus the sensor reading may be ambiguous at high
concentra- tions of gas in air, which are then falsely in- dicated
as being below the LEL; for methane this range is 40 to 100 per
cent (10). One method used to overcome this problem is to employ a
separate transducer, again calorimetric in nature, within the same
instru- ment, which utilises the difference in thermal conductivity
of flammable gas and air.
The rate of development in the field of calorimetric sensors has
slowed down in the last few years following the development of
poison- resistant elements. Currently attention is focus- ed on the
following areas, listed in approximate order of importance: [il
reducing the power drain of the sensor, [ii] increasing further the
long term stability
and poison-resistance of the sensor, [iiil providing advance
warning of poisoning, [ivl increasing the sensitivity, [vl reducing
the response time, [vil providing some degree of selectivity
bet-
ween different fuels.
Platinum Metals Rev., 1988, 32, (2) 54
-
,Contact lead
\ S i n t e r e d stannic oxide
Fig. 5 Semiconducting gas sensors consist of a porous
semiconducting metal oxide, a heating element, and two electrodes
to monitor electrical resistance. A typical ex- ample is shown
/ Ceramic t u b e
The successful operation of any catalytic calorimetric sensor
depends crucially on the performance of the catalyst. New catalysts
hav- ing increased activity, stability, poison resistance and
selectivity would influence areas [il, [iil, [ivl and [vil above.
There is however, little development work taking place in this
field. Attention is confiied to miniaturisation (area i), utilising
microprocessor technology for interrogation of the sensor at
different temperatures (areas iii and vi) and developing more
sensitive thermal sensors, for example pyroelectric materials (11)
(areas i and iv).
The catalytic calorimetric gas sensor, as demonstrated by its
widespread use throughout industry, has proved to be a reliable
means of measuring flammable gas concentration or ex- plosiveness
of a gas mixture in air. Poison- resistant devices have reduced the
problems of operation in poisonous environments thus ex- tending
the usefulness of these sensors.
Metal Oxide Semiconductor Gas Sensors
It has long been known that the electrical conductivity of many
semiconductors is chang- ed when some gases adsorb on the surface,
and that this effect can be reversible. This forms the basis of
some widely used gas sensors. A typical form of sensor is shown in
Figure 5 . General models have been developed which satisfactori-
ly explain the mechanisms underlying the ma-
jor effects (12, 13) but some of the more detail- ed behaviour
st i l l requires elucidation. It is however clear that, in most
instances, the in- teraction of gas with the solid surface is a
catalytic reaction involving adsorbed species. Thus metal oxides
are particularly suitable for donor gas detection since there is
always ionosorbed oxygen on the surface, associated with the
defects caused by non-stoichiometry in the solid. These defects are
also the source of free electrons contributing to the electrical
con- ductivity. Therefore interaction with the ad- sorbed oxygen
affects the conductivity and in an oxygen rich atmosphere (air) the
effects will be reversible.
A number of papers have reviewed studies related to this type of
gas sensor, for example
200
160 u c a u
u; 120
> t r t In 80 z Y In
40
0 200 400 600 8 00
TEMPE R A T W E , ' C
Fig. 6 The sensitivity of metal oxide semiconductor gas sensors
depends upon both the oxide and the gas. To improve the sensitivity
of stannic oxide sensors small amounts of a suitable platinum group
metal may be added. The data here. show the sen- sitivity to 100
ppm methane of stannic ox- ide: (a) without additives @) with 1 per
cent palladium (c) with 1 per cent platinum, and (d) with an
addition of 1 per cent iridium
Platinum Metals Rev., 1988, 32, (2) 55
-
see (14, IS). The major attraction of these devices is that they
are very sensitive; concen- trations below I ppm of some gases can
be detected (16). The major limitation is that they are almost
completely non-selective and, unlike calorimetric sensors, there is
no correlation between the indication obtained and the
explosibility of the gas. This is the major reason why these
devices have not been used to any signifcant extent for
quantitative measurement.
A number of approaches have been explored with a view to
improving the selectivity. Dif- ferent oxides show different
sensitivities to dif- ferent gases and although much of the
literature is concerned with stannic oxide and zinc oxide a wide
variety of both binary and, more recent- ly, ternary oxides have
been studied. The sen- sitivity of the sensors plotted against
temperature gives curves as shown in Figures 6 and 7. The
characteristic “volcano” shaped curve is almost universally
obtained. As the temperature of maximum sensitivity is different
for different gases, a degree of selectivity can be achieved by
judicious choice of operating temperature.
The third approach which has been widely studied is to introduce
additives or promoters into the oxide in order to change either the
elec- trical characteristics or, more importantly, the chemical
nature of the surface. The addition of promoters to catalysts in
order to increase their activity or selectivity is a well tried
technique in catalysis. The promoters achieve this by in- creasing
the coverage of reacting species or by easing the route for either
reaction or desorp- tion. The introduction of foreign cationic
species, of different valency to that of the parent cation, into
the lattice of a metal oxide can markedly affect the availability
of free elec- trons, and thus can have a large effect on the
conductivity. The indications from current understanding are that
this would have a general effect on all gases adsorbing on the sur-
face and would not necessarily achieve the desired effect of
enhancing the adsorption or the reaction of any particular gas. If,
on the other hand the additive is at the surface and af-
+ c - D v
L
g 1 2 0 - e a -
- 80- I >- I-
I- - L o . L w Lo
40 7
fects the catalytic activity of the surface, this could have the
effect of enhancing the oxidation reaction and the consequent
electronic effect of one gaseous species relative to another. Many
metallic species have been introduced into ox- ides in attempts to
do this. Most success has been achieved with the known oxidation
catalysts, in particular the metals of the platinum group, although
some success has also been reported with other metals such as
silver (I 7). Commercially available metal oxide flam- mable gas
sensors consist of stannic oxide with one or more of the platinum
group metals add-. ed at a concentration of around I per cent
w/w.
Although this approach is widely used the processes involved are
not well understood and the advances made have been achieved em-
pirically. One important process may, be preferential adsorption
onto the additive site at the surface and subsequent spillover onto
the oxide surface thus causing a conductivity change (12, IS). Very
strong adsorption onto the additive would, on this basis, inhibit
the reaction, although this is a simplification of what must be a
complex process. Evidence of
0 L 200 400 600 I
TEMPERATURE, ‘ C
Fig. 7 These characteristic volcano shaped curves show the
sensitivity of stan- nic oxide to 100 ppm carbon monoxide (a)
without additives (b) with 1 per cent palladium (c) with 1 per cent
platinum, and (d) with 1 per cent iridium
Platinum Metals Rev., 1988, 32, (2) 56
-
Dipole layer with voltage drop AV
l a )
I
VG
Pd Si 0,
p-Si
I I b )
with H., -
Fig. 8 The mechanisms by which the characterietice of a
palladium gate metal-oxide semiconductor field effect transistor
are changed by hydrogen are shown: (a) interac- tions at the gate
(b) effect on the transistor characteristics, and (c) effect on the
capacitor
I
ithout H.,
V O constant
PV H Si 0, I p - S i I
- wi thout H.,
V AV ( c l
complexity is the observation that known metallic catalyst
behaviour is not directly transferable to the oxide/additive
system; good catalysts for the oxidation of a particular gas do not
necessarily improve the sensitivity of an ox- ide to that gas when
they are used as additives. A more appropriate criterion is perhaps
the ad- sorption properties of the additive. It is well known that
the properties of catalysts are very dependent on the catalyst
support material,
hence the above observation is not altogether surprising.
Figures 6 and 7, show the effect of platinum group metal additives
on the sensitivi- ty of stannic oxide to methane and carbon
monoxide, respectively. Although palladium metal is an efficient
oxidation catalyst for both gases, its addition significantly
enhances sen- sitivity to methane but reduces sensitivity to carbon
monoxide. It is suggested that palladium produces a route for the
dissociative
Platinum Metals Rev. , 1988, 32, (2) 57
-
adsorption of methane, with subsequent spillover of adsorbed
species onto the stannic oxide. This increases the surface
concentration which is intrinsically low on the undoped oxide and
leads to greater sensitivity to the gas. On the other hand the high
coverage of carbon monoxide, which is more easily dissociated on
stannic oxide, is relatively unaffected by the ad- dition of
palladium. The effect of platinum is not as marked in terms of the
sensitivity to methane, but it has the effect of significantly
reducing the temperature of maximum sen- sitivity. However,
platinum reduces the sen- sitivity to carbon monoxide which may
imply that carbon monoxide is adsorbing more strongly on platinum
than on stannic oxide.
There is a wealth of catalysis literature relating to the
platinum group metals and metal oxides which might be tapped in the
search for selective detection materials. A fundamental difference,
however, between catalytic studies and sensor studies is that in
catalysis selectivity is defined in terms of the products obtained
whereas in sensor work selectivity relates to the adsorbents or
reactants. The full potential of this type of sensor is still far
from being realis- ed, perhaps more in terms of toxic rather than
flammable gas measurement. However the catalytic metals will play
an important role in combination with a range of metal oxides in
producing a number of sensors suitable for a variety of
applications.
Catalytic Gate Field Effect Transistor Gas Sensors
The first hydrogen sensitive metal-oxide semiconductor (MOS)
field effect transistor was described by Lundstrom in 1975 (19,
20). Since then this type of device has attracted con- siderable
attention but as yet the commercial impact has been slight. The
mechanism by which the presence of hydrogen in the at- mosphere
affects the characteristics of MOS devices has been explained by
Lundstrom and his co-workers in terms of dissociation of molecular
hydrogen to atomic hydrogen on the palladium surface (21). Atomic
hydrogen dif- fuses into the metal film and adsorbs on the in-
ner palladium surface, as shown in Figure 8(a). The adsorbed
atoms form dipoles at the metal- insulator interface, resulting in
an increase in the work function of the metal at the interface.
This in turn affects the threshold voltage (the voltage at which
the inversion layer starts to form) of the transistor, or the flat
band voltage if the device is in the form of a capacitor. The net
result of the dipole layer formation is to generate an extra
voltage in series with the ex- ternally applied voltage, thus
shifting the device characteristics as shown in Figure 8(b) and
8(c). The change in the work function is assumed to be proportional
to the interface con- centration of adsorbed hydrogen, so that the
maximum change will occur when every inter- face site is occupied
by a hydrogen atom. Measurable changes can be obtained for very low
concentrations (
-
as the oxidation of hydrogen by oxygen or chlorine, or the
oxidation of hydrogen sulphide by oxygen. The second role of the
metal is to allow transport of the hydrogen atoms through the metal
layer. Hydrogen is unique in this respect, because the hydrogen ion
is a bare pro- ton and thus much smaller than any other ion or
atom. Diffusion of hydrogen atoms is fast in most metals inchding
the platinum metals. The high solubility of hydrogen in palladium,
however, makes th is system unique. If the gate is fabricated of a
porous metal layer then any gas can diffuse directly through the
pores in the layer even without catalytic dissociation. The same
applies when the gate consists of a non- continuous metal film such
as a grid. However, some charge exchange must occur to provide the
dipole layer at the metal-semiconductor in- terface. This is the
third role of the metal layer which is to adsorb the diffused atoms
or molecules as dipoles at the metal-support (in- sulator,
semiconductor) interface. The dipole layer formed changes the
semiconductor sur- face field experienced by the semiconductor.
Thus a surface-field sensitive device such as a MOS capacitor, MOS
transistor or a Schottky diode generates a signal from this field
change.
In practice this type of device is used for hydrogen detection
in air and other en- vironments, particularly for hydrogen leak
detection. The operating temperature is usually in the region 60 to
15oOC; elevated temperatures being necessary to reduce the response
time and to avoid water adsorption, although for the
palladiumhydrogen system room temperature operation is possible.
The device can also be used for hydrogen sulphide and ammonia
detection where presumably both gases dissociate at the surface.
Hydrogen sulphide does not poison the catalyst in air but does so
in an inert atmosphere. Not all the devices utilising a palladium
gate respond to ammonia; the reasons for this are differences in
the structure of the metal films and the proper- ties of the
insulator surfaces. Limited success has been achieved using
different catalytic metals particularly platinum, iridium and lan-
thanum as the gate material. A number of dif-
ferent gate structures have also been in- vestigated. Porous
palladium films have been used in devices sensitive to carbon
monoxide, ethanol and some other hydrocarbons (21).
One of the problems with these devices is the temperature limit
determined by the silicon. A number of investigations have explored
the possibilities of the “floating gate”, that is a gate in grid
form which is physically separate from the silica layer (22).
Despite these different approaches this type of device is still
limited in application to hydrogen, hydrogen sulphide, ammonia and
a number of the more reactive hydrocarbons. The device cannot be
used as an explosimeter, as is the case for the calorimetric
sensor, or as a general indicator of almost any flammable gas, as
is the case for the metal oxide sensors, but because of the limited
interference it may be possible to use it to monitor for single
components in some atmospheres. Its high sen- sitivity in air and
inert backgrounds, ease of fabrication and compatibility with
microelec- tronic systems ensures that this type of device will
have a significant role to play in flammable gas detection.
Conclusions All three types of flammable gas sensor con-
sidered in this paper involve reactions between the analyte,
which is usually a reducing gas, and a solid surface. The reactions
which take place are governed by the nature of the surface, and
each of the three types of sensor described depend on the unique
adsorption properties of the platinum metals. This, however, is
manifested in different ways. The calorimetric sensor relies on the
catalytic activity of the metal to promote the oxidation of the
analyte on the sensor surface. The role of the metal in the
resistive semiconductor is to enhance the sur- face activity with
respect to certain gaseous species. In the field effect transistor
sensors the role of the metal is to dissociate the analyte at the
surface and to allow the formation of a dipole layer at the
metal/insulator interface. Thus, the platinum metals have been, and
will continue to be, central to the development of improved sensors
for flammable gas detection.
Platinum Metals Rev., 1988, 32, (2) 59
-
I
2
3
4
5
6
7
8
9
I0
I1
I2
References
J. G. Firth, A. Jones and T. A. Jones, Combust.
A. R. Baker, British Patent 892,530; 1962 G. C. Bond, “Catalysis
by Metals”, Academic Press, London, 1962 S. J . Gentry and T. A.
Jones, Sens. Actuators, 1986, 10, (I-z), 141 S. J. Gentry and P. T.
Walsh, Anal. Proc., 1986, 23, (2), 59 S. J . Gentry and P. T.
Walsh, Sens. Actuators, 1984, 5 9 (31, 239 S. J. Gentry and A.
Jones, 3. Appl. Chem. Bwtechnol., 1978, 28, ( I I ) , 727 J. G.
Firth and H. B. Holland, J . Appl. Chem. Bwtechnol., 1971, 21, (9,
139 S. J . Gentry and P. T. Walsh, Sens. Actuators,
A. R. Baker and J. G. Firth, Mining Eng. ( b n -
J. N. Zmel, B. Keramati, C. W. Spivak and A. D’Amico, Sens.
Actuators, 1981, I, (4), 427 S. R. Morrison, Sens. Actuators,
1982,2, (4), 329
Flame, 1973, 21, (3), 303
1984, 5 , (31, 229
h), 1969, (Jan-), 237
13 G. Heiland, Sens. Actuators, 1982, 2, (4), 343 14 D. E.
Williams, in “Solid State Gas Sensors”, ed.
P. T. Moseley and B. C. Tofield, Adam Hilger,
1 5 M. Egashira, Proc. Symp. on Chemical Sensors, Honolulu,
1987, ed. D. R. Turner, U.S. Elec- trochem. Soc., p. 39
16 W. Mokwa, D. Kohl and G. Heiland, Sens. Ac- tuators, 1985, 8,
(2), IOI
17 N. Yamazoe, Y. Korokawa and T. Seiyama, Sens. Actuators,
1983, 4, (2), 283
18 P. A. Sermon and G. C. Bond, Catal. Rev.-Sci.
19 I. Lundstrtim, M. S . Shivaraman, C. Svensson and L.
Lundkvist, Appl. Phys. Lett., I975,26, 5 5
20 I. Lundstdm, M. S. Shivaraman and C. Svensson, 3. Appl.
Phys., 1975, 46, (9), 3876
21 I. Lundstrtjm and C. Svensson, in “Solid State Chemical
Sensors”, ed. J. Janata and R. J. Huber, Academic Press, London,
1985
22 G. F. Blackburn, M. Levy and J. Janata, Appl. P b s . Lett.,
1983, 43, (71, 700
1987, P. 71
1973, 8, (21, 211
Superconductivity in Platinum Compounds A review summarising the
published data on
the superconductivity of platinum group metal compounds was
published here in 1984 (I), and since then efforts to understand
and develop superconducting materials have continued.
Last year some 390 scientists from twenty countries met in
Sendai, Japan, for the Yamada Conference XVIII on Superconductivity
in Highly Correlated F e d o n Systems, and the proceedings have
now been published (2).
Materials containing five of the platinum metals were
considered; the exception being osmium, although it is known that
high purity osmium and several osmium-containing com- pounds are
superconductors. Of the fifteen contributions that dealt, at least
in part, with the platinum metals, six were concerned with the
system UPt,. Recent developments were reviewed by H. R. Ott,
antiferromagnetic ordering has been achieved in UPt, by replac- ing
platinum with palladium or gold, or by substituting thorium for
uranium. These systems were included in an overview of work on
magnetic fluctuation and order, by G. Aep- pli. The specific heat
and the resistivity of (U,Th)Pt, were considered by K. Kadowaki.
The compound UPt was one of several whose normal ground state
properties were in- vestigated by B. Renker, while V. Miiller
reported the results of ultrasonic attenuation experiments on the
same material. Using
polarised light scattering S. L. Cooper examin- ed single
crystals of UPt, and URu,Si,. Nor- mal and superconducting
properties of the latter were reported by Y. Onuki, while a con-
tribution from H. Iwasaki considered super- conducting and
heavy-fedon behaviour in the (La,.,Ce,)Pd,Ge, system.
The three pseudoternary systems Ho(Rh, .x-
Cox), B, were the subjects of contributions by H. Adrian, H.
Iwasaki and H. C. Ku, respec- tively. In addition thermal expansion
measurements on the magnetic superconductor Er,,,Ho,,,Rh,B, were
given by R. Villar, while Y. Koike reported the effects of strain
on superconducting and ferromagnetic transitions of ErRh,B,. The
superconducting and magnetic properties of CeRh,B, and F’rRh,B,
were reported by K. Kumagai, and evidence for triplet
superconductivity in LuRu,B, was presented by A. Sulpice.
Regrettably, it is not possible to give here the names of the 63
people who co-authored the papers noted; readers are strongly
recommend- ed to refer to the published proceedings.
References I Ch. J. Raub,Platinum Metals Rev., 1984,28, (2), 63
2 “Proceedings of the Yamada Conference XVIII
on Superconductivity in Highly Correlated Fermion Systems”,
Physica B + C , 1987, 148,
Rux),B,, R(f i i&J ,B, and R(Rh1-x-
(1-313 1-54’’
Platinum MetaLr Rev., 1988, 32, (2) 60
-
Platinum Protects the Environment POLLUTION CONTROL FOR STAND-BY
GENERATORS
By J. E. Philpott Johnson Matthey, Catalytic Systems Division,
Royston
Although the electric power supplies provid- ed by the grid
systems of the Western World are generally reliable, there is still
a chance that these power supplies will fail for short periods,
possibly during severe storms. For many users of electricity a
break in the supply could be disastrous. Precautions are therefore
made to overcome any such interruption, the most com- mon being the
installation of a local stand-by electric power generator.
Often these generators are diesel driven and their exhaust
emissions can seriously impair the environment when the stand-by
power is need- ed. These engine fumes can cause irritation to the
eyes, nose and throat of people in the im- mediate vicinity, who
may even experience drowsiness and headaches. In addition there is
the risk that a product may be spoilt. To check that they are in
good working order stand-by generators must be run-up regularly,
often for a few hours each week; thus engine emissions can be a
very real problem. For this reason, ex-
Catalysts containing platinum group metals enable the gaseous
exhaust emissions from diesel fuelled stand-by electric generators
to be incinerated at normal engine combuetion temperatures. In
addition to maximising catalyst surface area, the honeycomb support
helps to reduce engine noise. The compact, flanged catalyst module
can be readily inetalled in the exhaust pipework of new or existing
plant. Once installed, little maintenance is required and the
catalyst will perform reliably when the generator is turned on,
even d e r long periods of inactivity
pensive and often unsightly chimneys are erected to disperse the
exhaust emissions into the atmosphere.
Growing public anxiety about the pollution of the environment by
emissions of poisonous hydrocarbons and toxic carbon monoxide has
brought into question the use of dispersion and dilution as an
acceptable method of exhaust emissions disposal, and favours
instead the destruction of the exhaust emissions at source.
The simplest and most effective way of achieving this is by
catalytic incineration which changes the polluting gases coming
from the engine into carbon dioxide and water vapour, both of which
are already present in the at- mosphere, naturally.
A Diesel Exhaust Purifier For stand-by applications, diesel
exhaust
purifiers are now available based on the same, very successful
Johnson Matthey catalyst technology that is used for cleaning the
exhaust
Plotinurn Metals Rev., 1988, 32, (2), 61-63 61
-
emissions from gasoline fuelled motor cars. These catalysts are
now a legal requirement on all United States and many European cars
and light vehicles, and well over 100 million are already in use
(I).
Many similar emission control catalysts are also in use on
mobile diesel engines working in enclosed spaces such as mines,
mineral caverns and warehouses, where the emission of diesel fumes
would otherwise make working condi- tions most unpleasant, and
could contaminate the product being handled. Other applications
include pumps and compressors driven by diesel engines, as well as
on specialised vehicles such as mechanical road sweepers and
construc- tion equipment (2).
The Platinum Metals Catalyst The diesel exhaust purifier
catalyst comprises
a ceramic honeycomb, coated with a porous washcoat within which
are dispersed very small amounts of finely divided platinum metals.
This arrangement causes minimum pressure drop as the exhaust gases
pass through the catalyst unit, and gives maximum surface area of
catalyst so promoting the incineration reac- tion at temperatures
lower than those at which this would normally take place. As the
engine combustion temperature is high enough to cause the reactions
to proceed, the catalyst unit is fitted directly into the exhaust
gas stream. The construction of the catalyst modules enables them
to be installed readily in both new and existing plants.
An additional benefit resulting from the use of a ceramic
honeycomb support is that it serves to reduce engine noise
significantly.
The platinum metals content of the catalyst averages 4og/cuft;
for a 900 brake horse power engine generating 500 kW of electrical
power, a typical exhaust purifier would contain ap- proximately IZO
grams of platinum metals.
The catalyst does not alter significantly the concentration of
either sulphur oxides or nitrogen oxides. However, in general,
diesel oil
Most hydrocarbon fuel combustion processes produce soot
particles. Some, but not all, of the soot produced in the diesel
exhaust is burnt off over the catalyst. However, because the un-
burnt hydrocarbons in the exhaust emission have been converted to
carbon dioxide, the soot particles are free of the sticky, tarry
coating normally present on diesel soot and do not form disfiguring
stains on adjacent structures.
Application on Stand-By Electricity Generators
A relatively new application for diesel ex- haust purifiers is
on the diesel engines used to power stand-by electric power
generator sets. Exhaust emissions from these static engines are
almost always released near to large buildings and it is obviously
desirable to avoid degrada- tion of the working conditions in these
areas when the generator is in use. This may be achieved by
building a high chimney, but this
The world famous Radcliffe Hospital, Oxford, is perhaps typical
of the type of location where catalytic incineration of diesel
exhaust emissions is superior to pollution dispersion through a
high chimney stack. The stand-by generator is situated adjacent to
areas where any form of
does not contain large quantities of sulphur;
minimum amounts of nitrogen oxides.
poflution would be WaccePtabk and where a chimney stack would
constitute a visual intru-
fence which serves as a bicycle store and ProprlY tuned engines
generate sion. Here the exhaust is surrounded by a
Platinum Metals Rev., 1988, 32, (2) 62
-
is usually an expensive solution and is not always possible. The
use of diesel exhaust purifiers obviates this need, and allows
regular stand-by generator testing to take place in nor- mal
working hours; since there is no risk of ex- haust emissions
polluting the immediate en- vironment. This is particularly
applicable for generators installed to provide emergency power to
hospital complexes, but is also impor-
tant for computer installations, department stores, hotels,
large banks, radio and television stations, railway stations,
telephone exchanges and other large buildings.
References I M. P. Walsh, Platinum Metals Rev. , 1986~30,
(3),
2 E. J. Sercombe, Platinum Metals Rev., 1975, 19, I 0 6
(11, 2
The Largest Producer of Platinum Metals Platinum in South
Africa, Special Publication No. 12 COMPILED BY E. M. EDWARDS AND M.
€I. SILK, Mintek, Randberg, South Africa, 1987, 55 pages, ISBN
0-86999-830-7, U.S. $30
Within the geological formation known as the Bushveld Complex
lies, perhaps, eighty per cent of the world’s known reserves of
platinum. The currently exploitable reserves occur in the Merensky
Reef, the Platreef and the UG-2 chromitite layer, and from these
strata approx- imately fifty per cent of the world’s re- quirements
for platinum group metals are produced. As is well known to the
readers of Platinum Metals Review, the properties of the platinum
group metals ensure their use for a wide variety of industrial
applications. In addi- tion, platinum finds use in the manufacture
of jewellery and as a store of wealth, and over the past 35 years
the requirement for it has increas- ed enormously, but somewhat
erratically. One result of a recent increase in the demand for
platinum is that several new platinum pro- ducers are emerging.
This interesting publica- tion, which reviews the past and present
activities in platinum mining in South Africa before going on to
consider the future outlook, is therefore timely.
A summary is given of the discovery of platinum and the early
mining operations in the 1920s as many companies were established
to exploit the various Bushveld deposits. When expectations were
not realised, ambitious plans were abandoned and the industry
suffered a severe set-back. This led to rationalisation and the
emergence of Rustenburg Platinum Mines as the only significant
producer in South Africa.
After World War I1 there was an upsurge in demand for the
platinum group metals, especially by the chemical and oil
industries, and Rustenburg Platinum Mines embarked upon a programme
of expansion. By 1957 their production of platinum group metals was
at an
annual rate of 430,000 ounces, but by 1958 another set-back
resulted when the oil industry reduced its requirements
significantly. However, in response to the development of new
outlets for the metals, the 1970s showed a dramatic growth in South
African output, following the entry of three new major platinum
producers, but once again a period of severe market imbalance
resulted. By the end of the decade stability was restored and the
price of platinum increased substantially en- couraging existing
mines to expand their ac- tivities. Now several new concerns have
announced their intention to enter the industry. It should be
noted, however, that recent changes in the financial markets of the
world have occurred since this book was prepared, and may be
expected to influence future plans.
The authors consider briefly the present availability of
supplies from other established sources. The U.S.S.R., for many
years the leading producer of platinum and st i l l the se- cond
largest producer of platinum group metals, appears to be retaining
part of its out- put for growing applications within its sphere of
influence, while Canada, the U.S.A and Australia make a worthwhile
contribution to supplies.
As to the future, it is suggested that co- operation between
producers may be the best way to avoid the dramatic surges and
set-backs experienced by the industry in the past, but at present
this is regarded as unlikely. It is dif- ficult to disagree with
the opinion that the pro- ducer industry will be heavily dependent
on both the expansion of existing industrial uses and the
development of new applications, and that further research must be
vigorously pur- sued so that the latter will be achieved.
Platinum Metals Rev. , 1988, 32, ( 2 ) 63
-
Transition Metal Catalysed Synthesis of Oligo- and Polysilazanes
THEIR USE AS PRECURSORS TO SILICON NITRIDE CONTAINING CERAMIC
MATERIALS
By Richard M. Laine The Polymeric Materials Laboratory in the
Washington Technology Center and the Department of Materials
Science and Engineering, University of Washington, Seattle,
U.S.A.
Organometallic polymer research offers many potential academic
and industrial rewards because of the number of elemental
variations possi- ble. Unfortunately, there are no general
synthetic methods as found for carbon based polymers. Transition
metal catalysed dehydrocoupling reactions may prove to be generally
applicable to the synthesis of silicon based organometallic
polymers. We report here our eflorts to synthesise organometallic
polymers with a silicon-nitrogen backbone, polysilazanes, using the
dehydrocoupling reaction. We also describe the synthesis of
polysilazanes for use as precursors to silicon nitride.
The design and synthesis of organometallic polymers is currently
receiving considerable at- tention because of their potential
utility in a wide variety of applications ranging from precursors
to ceramic materials-for example for high T, superconductors-to
substrates for pharmaceutically active materials (I). Despite the
fact that general methods exist for the syn- thesis of an extensive
variety of carbon based polymers, no simple, general synthetic
methods exist for the preparation of organometallic polymers,
except for polysiloxanes and polyphosphazenes. Moreover, in
contrast to the industrially important carbon based polymerisation
methods, where transition metal catalysis plays a major role,
transition metal catalysed syntheses of organometallic polymers are
limited to the production of several specific types of polysiloxane
polymers.
Given the exceptional number of elements available for the
synthesis of organometallic polymers, it seems likely that many
more in- dustrially useful organometallic polymers will be
developed in the forthcoming years. The question is, “Can
transition metal catalysis play
a role in the development of general routes to organometallic
polymers?” It is the intention of this paper to illustrate a
potentially general method of synthesis, transition metal catalysed
dehydrocoupling, and show how it can be ap- plied to the
development of organometallic polymers, polysilazanes, that are
precursors to silicon nitride containing ceramics.
Polymer syntheses based on catalytic pro- cesses are attractive
because they can provide thermodynamic advantages, greater control
of product stereoregularity and, especially, im- proved control of
product selectivity/purity over non-catalytic processes. It is
assumed that catalyst removal is facile or not necessary, the
impurity level resulting from the presence of catalyst being
extremely low. Unfortunately, catalytic approaches to the synthesis
of organometallic polymers require the develop- ment of bond
forming reactions that are not likely to have analogies in organic
chemistry. The general types of catalytic reactions employed in the
synthesis of carbon based polymers often rely on the reactions of
un- saturated molecules such as in the production
Platinum Metals Rev., 1988, 32, (2), 64-71 64
-
Catalyst precursor
Fe(CO), Fe,(CO),, Rui (CO),z OS,(CO),Z
Rhn (CO),n CO,(CO).
(Ph, P), Pd
la1 Mole per cent of products formed Ibl Turnover frequency =
moles productlmoles catalyst precursorlhour
Turnover frequency1b' per cent
95 134 90 121 24 34 4 4
85 117 87 120
5 6
of polyethylene and polypropylene. Unsatura- tion in
organometallic molecules is relatively rare and cannot be expected
to provide access to general catalytic methods for the synthesis of
organometallic polymers.
General synthesis methods must take into ac- count the fact that
organometallic polymers are often air and moisture sensitive. Thus,
purifica- tion and characterisation in the presence of co- products
are extremely difficult. One approach that appears to be general in
nature and which should resolve the co-product problem requires
that the bond forming reactions which occur during polymerisation
lead to the loss of small, innocuous gaseous molecules.
Several catalytic reactions of this genre have proved to be
useful in the synthesis of organometallic polymers, as illustrated
below. The types of reactions include condensation [Reaction
i](& redistribution [Reaction iil(3), and dehydrocoupling
reactions, [Reactions iii and ivI(4,s).
xM(HNR) , > -lN(R)-Mlx- + xRNH, i 2(RO),SiH -> (RO),SiH, +
Si(OR), ii
PhCH, SiH , > -[PhCH,SiHI,- + xH, iii KH
R,SiH, + RNH, + R,SiH-NHR+H, iv Our own work in this area has
been directed
towards the synthesis of polysilazane,
catalyst
catalyst
Cp , TiMe ,
-[R,SiNR'l,- (R = R' = H, alkyl, aryl, etc), precursors to
silicon nitride and silicon car- bonitride using dehydrocoupling
reactions. We became interested in the possibility of using
transition metal catalysed reactions to prepare polysilazanes based
on our discovery (6) that transition metals will promote the
activation of the silicon-nitrogen bond, as illustrated by
Reactions v to viii:
PhNHSiMe, + CO, Ru,(CO),,/roo°C/zh PhNHCO , SiMe , v
Ru ,(CO), , / I ro°C/Ioh PhNHSiMe, + CO,
PhNHC( =O)NHPh + (SiMe ,), 0 vi R u , ( C O ) , , / I I ~ O C /
I ~ ~
NH(SiMe,), + CO, Me , SiNHCO, SiMe , vii
catalyst/IooOC
PhN=CHPh + Me,SiOH viii Reaction viii was used to survey a group
of
transition metal catalyst precursors to identify the most active
catalyst systems. This survey is shown in Table I. Surprisingly,
the Fe(CO), catalyst proved to be the most effective of all the
catalyst precursors examined.
However, as is shown below, the silylamina- tion catalyst
studies do not correlate with a similar survey designed to identify
the most active catalysts for activation of the silicon-nitrogen
bond during the production of oligosilazanes.
PhNHSiMe, + PhCHO >
Platinum Metals Rev., 1988, 32, (2) 65
Table I
Catalytic Promotion of Silylamination of Benzaldehyde Using
N-Silylaniline
Conversion
-
Given that we could promote the catalytic ac- tivation of Si-N
bonds as exemplified by Reac- tions v to viii and Table I, the
thought occurred to us that it might be possible to catalyse the
formation of polysilazanes using a ring opening polymerisation
reaction of the type shown in Reaction ix (7):
L[Me,SiNHI) + (Me,Si),NH catalyst Me , SiNH-[Me,SiNHl,-SiMe ,
+LIMe,SiNHIj!
y = 3-8 ix
The basic idea in Reaction ix is to catalytical- ly cleave a
Si-N bond in the cyclotetramer, L[Me , SiNHl , and couple this
ring-opened species with an already existing linear chain or with
another ring-opened species. To prevent ring closure, (Me Si) , NH
is added to cap one end of the Si-N bond, while permitting chain
extension to occur at the other end. In principle the relative
ratio of the chain capping agent to the cyclotetramer will control
the ultimate chain length of the resultant polysilazane.
In two sets of experiments, with Ru,(CO),, as catalyst, the
molar ratio of capping agent to cyclotetramer was first set at 2.5
: I and then at I : 2.5. In the first experiment, an envelope of
oligomers was observed with x = I to 6 together with cyclomeric
species containing from 3 to 6 silazane units (Me, Si-NH). The se-
cond experiment provided an envelope of oligomers with x = I to 12
and the correspon- ding cyclomers. Further decreases in the
relative amount of capping agent lead to in- creased production of
the various other cyclomers rather than to higher molecular weight
oligomers.
The flaw with the ring-opening polymerisa- tion reaction is that
the same catalyst species that promotes ring-opening via Si-N bond
cleavage will also catalyse Si-N bond cleavage in the linear
oligomers, to reform cyclomers and capping agent. However, a
catalyst survey car- ried out to identify the most active catalyst
for Reaction ix provided an escape from this dead-end.
As illustrated in Table 11, a variety of catalyst precursors
were found to promote Reaction ix. The most important finding of
this study was
the fact that small amounts of hydrogen greatly improve the rate
of oligomerisation and lower the reaction temperature required to
obtain ef- fective catalysis. Moreover, the use of metal hydride
containing precursors eliminates the need for hydrogen, which
indicates that metal hydrides are the true active catalysts
(7).
If metal hydrides are required to promote Si- N bond cleavage,
we can suggest a mechanism for cleavage that involves partial or
total hydrogenation of the Si-N bond:
MH, + %Si-NR',-> %Si-MH + NHR', -> M + %SiH + NHR', X
We can then suggest that the reverse reac- tion, Si-N bond
formation, must occur by the reaction of the amine N-H bond with
the %Si- MH species:
%Si-MH + NHR',- MH, + %Si-NR', xi If reaction xi is indeed
responsible for the for-
mation of Si-N bonds, and given that species similar to %Si-MH
are proposed to be in- termediates in hydrosilylation, Reaction
xii,
R$iH + M -> KSi-MH + R'CH=CH, h M + R'CH,CH,SiR, xii
then it should be possible to test the validity of Reaction xi
by direct reaction of a silane, for ex- ample Et,SiH,, with
ammonia. As shown in Reaction xiii, it is possible to form
oligosilazanes in this manner.
Ru , (CO) ,, /60°C/THF Et,SiH, + N H , ,
H, +LIEt,SiNHl: + H-[Et,SiNHl,-H xiii y = 3-5 x = 3-5
More important is the fact that in Reaction xiii, Si-N bonds are
formed under conditions ( 6 o O C ) where Si-N bond cleavage does
not occur. Thus, Si-N bond breaking reactions should not interfere
with the formation of hgher molecular weight polysilazanes.
Furthermore, the by-product, hydrogen, will not contaminate the
product polysilazanes.
In practice, it has not been possible to prepare high molecular
weight polysilazanes us- ing reactions analogous to xiii for a
variety of reasons (8,9). In particular, these reactions are
Platinum Metals Rev., 1988, 32, (2) 66
-
Table II .
Ring Opening Oligomerisation of Octamethyltetraeilazane in the
Presence of Hexamethyldisilazane
Temperature, OC
135 180 135 135 135
180 135 135 180 135
Time, hours
6 15
1 1 6
20 6 3
15 3
capping agent to catalyst is 250 : 84 : 1
extremely susceptible to the steric environment about both
silicon and nitrogen. We are cur- rently modelling the
dehydrocoupling reaction using Reaction xiv,
Ru (CO) iTHF/70°C Et,SiH + RNH, H, + Et,SiNHR ’ xiv
where R = n-Pr, n-Bu, s-Bu, and t-Bu (10). The kinetics and the
catalytic cycle(s) for this reaction turn out to be extremely
complex.
In the absence of amine, the silane reacts with the catalyst to
produce (Et Si) , Ru , (CO) 8 , Reaction xv, which can be isolated
and used as the catalyst in place of Ru (CO) I , .
I IOOC/IO min. 6Et,SiH + 2Ru,(CO),, -> 3Hz +
3(Et,Si),Ru,(Co), xv
Catalyst concentration studies demonstrate that the rate of
Reaction xiv is non-linearly and inversely dependent on both
[Ru3(CO) ],I and [(Et,Si),Ru,(CO),l. On a molar basis, the (Et Si)
, Ru , (CO) catalyst is more active than the trimeric carbonyl.
These results suggest that catalyst formation occurs as a result of
cluster fragmentation. Indeed, the true active species may be
monomeric.
With regard to the amines, we find from rate
Conversion of cyclotetramer,‘a’
per cent
22 80 80 77 80
80 75 78 70 78
Remarks
1 atm hydrogen
1 atm hydrogen catalyst
decomposes
1 atm hydrogen
1 atm hydrogen
versus [RNH,] studies that the steric bulk of R controls both
reaction rate and reaction mechanism. The simple primary amines n-
PrNH, and n-BuNH, show an inverse, non- linear rate dependence on
[RNH,], despite the fact that they are reactants; whereas, the
[s-BuNH, I studies reveal a non-linear positive dependence. On
moving to the most bulky amine, t-BuNH,, the rate shows almost no
dependence on either it-BuNH, 1 or [Et SiHl.
Our previous work on the reaction of Ru (CO) I , with amines
provides reasonable explanations for many of the above observa-
tions. We have already shown that simple primary and secondary
amines will react with Ru,(CO),, to form rather stable com- plexes
similar to the type shown in Reaction xvi (11) .
Ru,(CO),, + EtCH,NH, - (ji2-EtCH=NH)HR~J(C0),,, xvi
If these species are sufficiently stable to resist fragmentation
to the active catalyst species under the reaction conditions then
we would expect an inverse dependence on [n-RNH,], which we
observe. With s-BuNH, the steric bulk at the amine may reduce the
role that a
Platinum Metals Rev., 1988, 32, (2) 67
Catalyst
ial Molar ratio of cvclorner fa
-
reaction analogous to Reaction xvi plays in the global reaction
rate. With t-BuNH,, there is no
presence of the ruthenium catalyst, probably as in Reaction xx
(12).
alpha hydrogen and Reaction xvi need not be considered. However,
the apparent
Ru (CO) I, /paC/THF PhSiH , > SiH, + Ph,SiH,
AA simultaneous independence on [Et , SiHl is more difficult to
explain and suggests that The redistribution reaction illustrated
by Reac- catalyst activation becomes the slow step in the tion xx
represents an additional mechanism reaction mechanism. The relative
global rates whereby crosslinking could occur in of reaction are
qualitatively polysilazanes; although we have no evidence for
n-PrNH,>n-BuNH, > s-BuNH, 9 t-BuNH, The potential product,
(Et Si) NR, is never
observed. The severe steric effects observed in Reaction
xiv are also observed when the dehydrocoupl- ing reaction is
used to synthesise oligosilazanes (9). For example, in Reaction
xiii, we attemp- ted to prepare linear, high molecular weight
diethylpolysilazanes. The only products from this reaction are
mixtures of the cyclotrimer and cyclotetramer with low molecular
weight linear species (M,=~oo D). In contrast, the use of
monosubstituted silane precursors pro- vides access to true
oligosilazanes, as illustrated by Reactions xvii and xviii:
Ru,(CO) /60aC PhSiH, + NH, A>
H, + 4PhSiHNH1,- xvii
Ru (CO) /60aC M, = 800-1000 D
n-C,H,SiH, + N H , A > H, + -[n-C,H SiHNH1,- xviii
M, = 2700 D
At 6o°C both oligosilazanes are essentially linear. We find no
evidence for dehydrocoupl- ing at the tertiary Si-H bond nor do we
see any reaction at the internal N-H bonds. At 9ooC, we observe
activation of the internal Si-H bonds and crosslinking of the
polysilazanes is obtained as shown for the phenylpolysilazane in
Reaction xix.
its participation in Reaction xix. In contrast to PhSiH, and
n-hexylSiH,, EtSiH, reacts in- discriminately to give a crosslinked
polysilazane that is sufficiently intractable to resist effective
characterisation (I I).
Preceramic Polysilazanes Having established the basic mechanisms
in-
volved in catalytic dehydrocoupling and shown that they are
useful in the synthesis of oligo- and polysilazanes, we can now
consider their application to the synthesis of useful preceramic
polysilazanes. To be useful, a preceramic polymer must have several
proper- ties including tractability, latent reactivity and high
ceramic yield. The exact nature of the in- dividual properties is
developed in the follow- ing paragraphs.
Because the objective of using a preceramic polymer is to form a
ceramic shape that is dif- ficult or impossible to obtain by normal
ceramics processing techniques, a preceramic polymer must be
tractable (soluble, meltable or malleable). Moreover, because
different ap- plications, for example coatings and fibres, will
require different viscoelastic properties, some mechanism to
control viscoelasticity must be available. Linear or lightly
branched polymers such as produced in Reactions xviii and xix are
tractable.
Once the finished preceramic shape is obtain- Ru (CO) I, /9ooC
ed, the shape must be rendered infusible to
avoid further changes during pyrolytic transfor- mation to
ceramic product. In order to achieve this, the preceramic piece
must be susceptible to some form of manipulation that does not
distort its shape. It must have latent reactivity.
However, we have recently found that One common method of making
a tractable polymer intractable/infusible is to crosslink it.
H-[PhSiHNHI,-H + N H , i > H, + NH0.5
I -[PhSiHNHI,[PhSiNHl,-
solid, M, = 14wD xix
PhSiH, will disproportionate by itself in the
Platinum Metals Rev., 1988, 32, ( 2 ) 68
-
If crosslinking can be controlled, it also sors have been
synthesised by ammonolysis of represents a method of controlling
polymer H,SiCI, or MeSiHCl, as in Reaction Xu. viscoelasticity. For
example, Reactions i-iv and xix are useful forms of crosslinking
that can be used to control viscoelasticity in polysilazanes and to
render them infusible. Once crosslinking is sufficient to ensure
in- fusibility, the shaped piece can be pyrolysed.
Typical preceramic polymers will have den- sities that are as
little as 20 per cent of the densi- ty of the ceramic product.
Consequently, tremendous volume changes occur during pyrolysis. If
a portion of the precursor is volatilised during pyrolysis, then
the ceramic yield (on a weight per cent basis) will be diminished
and the volume changes will be magnified. For example, typical
polysilazanes have densities of the order of I. I g/cc, while that
for silicon nitride (Si,N,) is 3.2 g/cc. A 100 per cent ceramic
yield will result in a volume change-assuming a fully dense ceramic
is obtained-of approximately 70 per cent. A 50 per cent ceramic
yield results in a volume change of 85 per cent. Therefore, high
ceramic yields are extremely desirable. More important is the fact
that these volume changes limit applications for preceramics. It is
almost impossible to obtain a fully dense three dimen- sional
ceramic piece with near-net-shape using preceramic polymers.
However, preceramic polymers are still quite useful for binder,
coating and fibres applications where volume change is less
important.
With these directives in mind, we can discuss the design and
synthesis of a useful preceramic polysilazane. The tremendous
importance of ceramic yield limits the types of extraneous moieties
that can be added to the polymer chain to aid in tractability,
enhance stability or pro- vide latent reactivity. In polysilazane
synthesis the objective is to obtain pure silicon nitride following
pyrolysis; thus, -[H, SINHI,-, -[H2 SiNHNH1,-, -[MeHSiNHI,-, and
-[H, SiNMe1,- represent optimal silicon nitride precursors because
they only need to lose hydrogen and/or methane to form silicon
nitride upon pyrolysis.
Oligomers of most of these potential precur-
oOC/Et , 0 H,SiCI, + NH, ->
-[H,SiNHI,- +- xxi Unfortunately, these types of oligosilazanes
tend to be unstable or the ammonolysis product has too low a
molecular weight to be directly useful as a preceramic. For
example, -[H, SiNH1,- crosslinks rapidly, even at o°C, and
therefore cannot be easily handled. By comparison, -[H, SiNMe1,-,
synthesised in analogy to Reaction xxi, is stable for long periods
of time in the absence of oxygen or moisture but the ceramic yield
upon pyrolysis under nitrogen is only 38 to 40 per cent because of
the low molecular weight (x = 10,
A surprising problem associated with all reported methods of
synthesising polysilazanes (13) is their inability to provide
products with molecular weights much greater than M,, = 2000 D;
although oxygen analogs with molecular weights of millions can be
prepared readily.
In theory, the dehydrocoupling reaction could be used to make
any type of polysilazane, as illustrated by Reaction xxii.
M,, = 600 D) ( 5 ) .
catalyst MeSiH , + NH
H, + 4MeHSiNH1,- xxii
In practice, because of the dangers of handling MeSiH, or SiH,,
it is easier to prepare an oligomer via ammonolysis and modify it
catalytically to a useful ceramic precursor.
Given that the polymer -[H,SiNMel,- is ac- tually HNMe-[H,
SiNMe1,-H, and has both N-H bonds and Si-H bonds in the same
oligomer, we sought to increase the molecular weight, and thereby
the ceramic yield, of this polymer using the dehydrocoupling
reaction. We find that it is indeed possible to catalyse the
formation of high molecular weight species as typified by Reaction
xxiii.
Ru~(CO),,/~~-~O~C HNMe-[H, SiNMe1,-H polymers hi
x I 20, M, = 1200 D
The effects of reaction time on the
Platinum Metals Rev., 1988, 32, (2) 69
-
S5K
2.6K I
35.44
33.97
35.87
33.60
6 5
SO 60
30 ' 20
10
0
45.18
-
gelation permits us to control viscoelasticity. In this way, we
have tailored the N- methylpolysilazane (NMPS) precusor,
-IH,SiNMel,-, so that it can be used suc- cessfully for coating and
binder applications; and we have been able to draw fibres under ap-
propriate conditions.
As mentioned above, the use of the dehydrocoupling reaction
permits us to syn- thesise an organometallic polymer without the
formation of a contaminating by-product, hydrogen. It is important
to note that the catalyst concentration is at the hundred ppm level
and should not interfere with the utility of the ceramic product,
although this remains to be determined.
Pyrolysis Studies Perhaps the most important findings are
those of the pyrolyses tabulated in Table 111. The pyrolysis of
NMPS at temperatures of
800 to cp°C permits conversion to ceramic product (IS). We find
that there is a direct cor- relation between molecular weight and
ceramic yield. As noted previously, with molecular weights of the
order of 600 D, the ceramic yield is = 38 to 40 per cent. The M,, =
2300 D case
gives ceramic yields of 60 to 65 per cent. The theoretical
ceramic yield for NMPS should be just above 70 per cent, assuming
that the polymer is converted to silicon nitride alone.
The evidence shows that within the error limits of the method,
the chemical analyses of the ceramic products obtained by pyrolysis
of several NMPS derivatives are identical (I 6). This is despite
the signifcant differences in both molecular weight and viscosity.
We conclude that it is the chemical composition of the monomer
unit, -[H,SiNMel,-, that defines the type of ceramic product,
rather than the macromolecular properties.
If this conclusion is correct, and if it can be shown to be
general in nature, then it demonstrates the feasibility of
manufactur- ing ceramic materials by chemical means.
Acknowledgments We would like to thank many colleagues at
SRI
International for their input into the work described above. We
gratefully acknowledge sup- port for this research from the
Strategic Defense Sciences Office through the Office of Naval
Research Contracts Nooor4-84-C-0392 and N-14-85-C-0668.
References I See for example “Inorganic and Organometallic
Polymers”, Am. Chem. SOC. Symp. Ser., ed. M. ZeJdin, K. J. Wynne
and H. R. Allcock, 1988, Vol. 360
2 R. M. Laine and Y. D. Blum, to be submitted for
publication
3 M. D. Curtis and P. S. Epstein, Adw. Organornet. Chem., 1981,
19, 213
4 C. A. Aitken, J. F. H a r d and E. Samuel, 3. Am. Chem. Soc.,
1986, 108, 4059
5 D. Seyferth and G. H. Wiseman, “Ultrastructure Processing of
Ceramics, Glasses and Com- posites”, ed. L. L. Hench and D. R.
Ulrich, 1984, pp. 265-275, and references therein
6 M. T. Zoeckler and R. M. Laine, 3. Org. Chem.,
7 Y. D. Blum and R. M. Laine, Otgamtal l ics , 1986, 5, 2801
8 Y. D. Blum, R. M. Laine, K. B. Schwartz, D.J. Rowcliffe, R. C.
Bening and D. B. Cotts, “Better Ceramics Through Chemistry 11”,
Mat. Res. Syrnp. Proc., Vol. 73, ed. C. J. Brinker, D. E. Clark and
D. R. Ulrich, 1986, pp. 389-394
1983, 48, 2539
9 “Organometallic Polymers as Precursors to Ceramic Materials:
Silicon Nitride and Silicon Oxynitride”, R. M. Laine, Y. D. Blum,
R. D. W i n and A. Chow, “Ultrastructure Processing of Ceramics,
Glasses and Composites 11”, ed. D. J. Mackenzie and D. R. Ulrich,
1988, in press
10 C. Biran, Y. D. Blum, R. M. Laine, R. Glaser and D. S. Tse,
submitted for publication
X I A. Eisendtadt, C. Giandomenico, M. F. Fredericks and R. M.
Laine, organomerallics, 1985, 42 2033
12 R. M. Laine and G. Balavoine, unpublished results
13 R. M. Laine, Y. Blum, D. Tse and R. Glaser, op. cir., (Ref.
I), pp. 124-142
14 A. W. Chow, R. D. Hamlin, Y. Blum and R. M. Laine, Polym.
Sci., 1986, in press
15 K. B. Schwartz, D. J. Rowcliffe, Y. D. Blum, and R. M. Laine,
“Better Ceramics Through Chemistry 11”, Mat. Res. Symp. Proc., Vol.
73, ed. C. J. Brinker, D. E. Clark, and D. R. Ulrich,
16 R. M. Laine and Y. D. Blum, to be submitted for 1986, PP.
407-412
publication
Plotinurn Metals Rev., 1988, 32, (2) 71
-
Platinum Thermocouple Calibrations AN INTERCOMPARISON BY
EUROPEAN LABORATORIES
The importance of the accurate calibration of thermocouples
needs no reiteration here. It is generally accepted that
platinum-based thermo- couples provide the best accuracy and re-
producibility, and they are frequently used for assessing
calibration laboratories.
Between 1981 and 1984 an intercomparison of calibrations of
three types of platinum- rhodium thermocouples was carried out in
six European standards laboratories with support from the Community
Bureau of Reference of the European Economic Community, and the
results have now been reported (L. Crovini, R. Perissi, J. W.
Andrews, C. Brookes, W. Neubert, P. Bloembergen, J. Voyer, and I.
Wessel, High Temp.-Hzgh Pressures, 1987, 19,
The types of thermocouples examined were Type S (~o%Rh-Pt:Pt),
Type R (13YoRh- Pt : Pt) and Type B (30%Rh-Pt : 6YoRh-Pt). The
manufacturing tolerances for such thermo- couples are prescribed in
the International Electrotechnical Commission publication 584-2,
and for Types S and R are +IOC from 600 to IIOOOC and then
increasing linearly to +2.5OC at x6oo0C. For Type B, the tolerance
is +I .~OC from 600 to IIOOOC, and then increas- ing linearly to
-c4.ooC at 1600OC. However, in- accuracies in calibration of
metrological laboratories and calibration services should be at
least 10 times better than these figures.
Calibrations of thermocouples are generally made against the
freezing poiffts of pure metals in the range 600 to 1065OC, to
within +0.2OC; and against the melting points of gold and palladium
(usually in air) using the wire bridge technique in the range 1000
to 1600OC. These techniques give rise to some uncertainty, detail-
ed in the paper, which was the reason for set- ting up this
intercomparison study.
The measurements were made in two distinct circulations with
nine thermocouples in each, consisting of three thermocouples of
each type.
(21, 177-194).
The thermocouples were made of Class 2AEC wires, purchased from
Industrie Engelhard SpA (first circulation) and Johnson Matthey
(second circulation). Before each calibration, all thermocouple
wires were prepared and tested under standard conditions, and at
the end of each experiment the thermocouples were dis- mantled,
cleaned and coiled for transportation to the next laboratory. All
laboratories were supplied with high purity gold and palladium from
the same batch of material to eliminate the possibility of
differences due to varying purity. Full details of preparation and
experimental procedures are given in the paper.
The reader is also recommended to refer to the paper for a
detailed analysis of the results. However, considerations of the
reproducibility of the melting points show that the mean inter-
laboratory differences were k0.25OC for gold and kO.35Oc for
palladium, under the best con- ditions, both melting points being
realised in air. The estimated repeatability within each laboratory
was better than ko.3OC for gold and +0.85OC for palladium. In the
second circula- tion, a larger drift was observed due to the longer
exposures to high temperatures.
With respect to IPTS-68, the comparisons carried out in the
second circulation at three laboratories gave an agreement of
melting point temperatures of better than 0.7OC at the gold point
and 1.5OC at the palladium point. The results indicated that the
temperature assigned to the melting point of palladium in air may
be too high by at least 0.5OC. All the thermo- couples changed
calibrations during the inter- comparisons, the drift generally
being due to chemical contamination.
In summary, it was concluded that the laboratories were able to
calibrate new thermo- couples to within -co.4OC at the gold melting
point and to within +o.g0C at the palladium melting point with
mutual agreement inside these figures. C.W.C.
Platinum Metals Rev., 1988, 32, (2), 72 72
-
Promoting Platinum Metals by Ceria METAL-SUPPORT INTERACTIONS IN
AUTOCATALYSTS
By B. Harrison, A. F. Diwell and C. Hallett Johnson Matthey
Technology Centre
Modem autocatalysts are complex, multi-component systems which,
when combined with vehicle fuel calibration systems, are able to
provide the activity, selectivity and durability required to meet
the exacting emissions standards now demanded. In addition to the
platinum group metals, one of the major components in current
three-way catalysts is ceria, whose main role was originally
thought to involve oxygen storage under tran- sient conditions. In
practice, the situation is more complex, with ceria contributing to
a number of catalytic functions and also interacting with the
active platinum group metals.
Catalytic converters have been the universally accepted method
of automobile emissions con- trol in the U.S.A. and Japan since the
mid 1 9 7 0 s ~ and more recently have been adopted in Australia.
Two types of catalyst have been used. Oxidation catalysts convert
unburnt hydrocarbons and carbon monoxide to carbon dioxide and
water, while three-way catalysts, in addition, convert oxides of
nitrogen to nitrogen (I). Catalysts can convert in excess of 90 per
cent of these pollutants and, if combined with appropriate engine
management systems, can meet any known existing or proposed
emissions standards. Oxidation and three-way catalysts differ in
formulation, the former usually con- taining palladium or platinum
+ palladium and the latter containing platinum + rhodium. Since the
introduction of catalytic converters, significant advances have
been made in their design with regard to activity, durability and
time of response to fluctuating exhaust condi- tions (2 , 3).
There are five major components in vehicle exhaust catalysts:
the substrate, the support, stabilisers, base metal promoters and
platinum group metals. The most commonly used substrate materials
are multicellular ceramic monoliths which have a high open area and
ex- ert little back pressure in the exhaust system. Metallic
monoliths are also used, especially in
situations where the greater open area and even lower back
pressure of these systems is an ad- vantage. The properties and
uses of ceramic and metallic monoliths are reviewed in depth
elsewhere (4-8) and will not be discussed fur- ther here. In order
to increase the surface area of the monolith, a coating of a highly
porous material, usually alumina, is applied. This is known as the
washcoat. Stabilisers are often added to the washcoat to maintain
the high sur- face area at the elevated temperatures which are
encountered under operating conditions (2, 3). Promoters are
included to improve the activity or selectivity of the catalyst and
can have a strong influence on performance. The most widely applied
promoters in three-way catalysts are nickel and cerium (9, 10). The
primary catalytic components of current car exhaust catalysts are
platinum group metals, which combine the benefits of high activity,
par- ticularly at low temperatures, with stability and resistance
to poisoning.
Cerium, usually as its oxide ceria, is used very widely in
present day three-way catalyst formulations. Initially, it was
thought that the main function of this component was as an oxygen
storage component (11), that is as a component which stores oxygen
under lean operating conditions-thus promoting conver- sion of
oxides of nitrogen-and releases it under
Plarinum Metals Rev., 1988, 32, (2), 73-83 73
-
1 0 0 -
c c QA " 2 8 0 - 0
4 Y lY U Y
6 0 - lY 3 VI
f w u 5 4 0 - I u
2 0 -
Fresh 7 5 0 1 0 0 0 1 2 0 0
TEMPERATURE, 'C Fig. 1 A number of oxides, including those of
the alkaline earths and rare earths can enhance the stability of
alumina. The addi- tion of cerium to alumina has a positive ef-
fect on washcoat stability, particularly around 1000°C, although
this is not the main reason for adding cerium to autocatalysts.
Barium additions provide relative stability to temperaturee well in
ex- cess of 1000°C
rich conditions by reaction with carbon monox- ide or
hydrocarbons. In practice, the role of cerium in promoting platinum
metals and in particular rhodium, is much more complex than this
and is the main subject of this paper.
Waehcoat Stabilieation An autocatalyst washcoat comprises
gamma-
alumina in combination with a mixture of pro- moters and
stabilisers. This provides a high surface area upon which to
maximise the dispersion of the active noble metals. Although
gamma-alumina is inherently thermally stable and only slowly
converts to the delta-, theta- and alpha-phases as the temperature
is raised, it is normal to add stabilisers to retard these
transitions and maintain a high surface area in situations where
exhaust gas temperatures can
exceed 10ooOC. A number of oxides, including those of the
alkaline earths and rare earths are able to enhance the stability
of alumina (12). This is illustrated in Figure I, where the
stabilising effect of barium and cerium is com- pared with that of
unstabilised alumina. Although washcoat stabilisation is not the
main reason for adding cerium to autocatalysts, it is seen to have
a positive effect, particularly in the region of 10ooOC. Barium,
however, provides relative stability to the washcoat to
temperatures well in excess of 10ooOC.
Enhancement of Rhodium Activity Rhodium is a crucial component
of three-way
catalysts, particularly with regard to carbon monoxide and
nitrogen oxides conversion at rich and stoichiometric air:fuel
ratios. Unfor- tunately, rhodium is extremely sensitive to
deactivation at high temperatures under the lean operating
conditions which can be en- countered during high speed cruising.
The deactivation is thought to be due to a strong rhodium-alumina
interaction, which fixes rhodium in a high oxidation state which is
dif- ficult to reduce ( I 3). This interaction can be blocked or at
least retarded by the incorpora- tion of ceria into the catalyst,
resulting in im- proved performance after high temperature ageing
under oxidising conditions, see Figure 2. Conversion of nitrogen
oxides shows im- provement particularly under rich conditions,
while carbon monoxide conversion is increased across the range of
equivalence ratios en- countered. The promotion of carbon monoxide
conversion in the rich region merits particular attention, since
this cannot be achieved by reac- tion with oxygen. Examination of a
series of aged platinum + rhodium three-way catalysts, containing
increasing quantities of ceria, in a simulated exhaust both with
and without water, reveals the nature of the reaction which is
being promoted. In the absence of water, an increase in ceria
loading has no effect upon the carbon monoxide conversion achieved
by the catalyst. When water is present, however, a dramatic ef-
fect is observed, with increasing ceria loading causing an increase
in conversion, see Table I.
Platinum Metah Rev., 1988, 32, (2) 74
-
Fig. 2 The interaction between rhodium and alumina can be
blocked or retarded by incor- porating ceria. This gives an im-
proved performance atter high temperature ageing under oxidis- ing
conditions. The conversion of nitrogen oxides is improved, par-
ticularly under rich conditions, while carbon monoxide conver- sion
is increased across the whole equivalence ratio range
+ El00 -
E ao - " L
z 0
LL >
60 - w
g 40- u
2 0 -
- Unpromoted --- Ceria-promoted c---- c o
\ / --\ \./* @
I
0 . 9 6 0 . 9 8 1.00 1 0 2 104 L A M B O A
4.00 -
3 .25 .
2 50-
1.75-
1.00 - 0.2 5 -
This leads to the conclusion that ceria is pro- moting the
water-gas shift reaction:
CO + H,O = CO, + H,
The Interaction of Platinum Group Metals with Ceria
If ceria is to be considered as an oxygen storage component
then, by definition, it should be capable of being reduced and re-
oxidised readily. Temperature programmed reduction (TPR) has been
used to explore the reduction of ceria in hydrogen, as shown in
Figure 3 where two reduction peaks are seen, this being in
agreement with previous workers (14). The low temperature peak
(500OC) is assigned to the reduction of a surface species;
the size of the peak appears to be dependent on preparation.
Other workers have attributed the peak to the reduction of surface
capping oxygen anions attached to a surface Ce' + ion (15). The
higher temperature peak (>8m°C) corresponds to the reduction of
bulk oxygen and the forma- tion of lower oxides of cerium. For
ceria to act effectively as an oxygen storage component, these
reduction processes must be readily rever- sible. Sequential
temperature programmed reductionloxidation has therefore been used
to establish the ease with which re-oxidation of ceria occurs. A
sample of ceria was reduced at 700OC in a TPR apparatus and then
heated in an oxygen-containing atmosphere to 98oOC. During the
latter process there was no oxygen
Fig. 3 The temperature pro- grammed reduction of ceria in
hydrogen shows two reduction peaks. One at 500OC is assigned to the
reduction of a surface -0.501 specier, while the higher peak at
around 8OOOC corresponds to - 1 ' 2 5 the reduction of bulk oxygen
and the formation of lower oxides of -2.00 cerium o loo 200 300 400
500 600 700 aoo 900 1000
T E M P E R A T U R E . 'C
Plotinurn Metals Rev., 1988, 32, (2) 75
-
F u r n a c e windings 0 0 0 0 0 0 0 0 0 0 0 0 0 0
O u t e r s i l i c a t u b e
Ceria loading
. T h e r m o c o u ~ l e Counter e l e c t r o d e l e a d ( P
t l R e f e r e n c e e lectrode l e a d ( P t 1
S t a i n l e s s s t e e l t u b e Z i r c o n i a d isc
Working e l e c t r o d e lead
CO conversion CO conversion with H,O, per cent without H,O, per
cent
u u u u u U ' U u u u u u u u