PLATINUM METALS REVIEW · analyte interacts with a platinum group metal surface is the catalytic field effect transistor (FET). Dissociation of hydrogen and some hydrogen-containing

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

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 environments. 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 used 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 explosive limit (LEL) and the upper explosive limit are known for most gases (I). It is common practice to seek to maintain the concentration 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 central 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 sensor 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 industries.

The second type of sensor is based on electrical conductivity changes induced by gas adsorption on metal-oxide semiconductors. This is used primarily as a qualitative indicator of the presence of a gas, or of a change in its concentration. This type of sensor is extensively used, particularly in situations where low concentrations of a wide range of gases need to be indicated. 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

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 conditions 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 significant 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 reactions are palladium, rhodium, platinum and iridium. This is because conditions on their surfaces are at an optimum for the reaction between fuel and oxygen. The heat of adsorption of the reactants is low enough to reduce the activation 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

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‘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

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 instruments, which may be hand held or fmed installations. 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 influence 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 concentration.

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 provide an approximate estimate of the ex- plosiveness of any vapour or mixture of vapours. If the composition of the fuel-air mixture is known then simple correction factors

CURRENT, mA

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200 300 400 500 I

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200

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Fig. 3 The effect of temperature and current on the response and power drain of a typical calorimetric sensor is shown


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EXPOSURE TIME, hours

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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 hexamethyl 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 exposure to atmospheres containing poisons (which have an irreversible effect) and inhibitors (which have a reversible effect). Com- mon examples are silicones, alkyl lead compounds, phosphate esters, halogenated compounds and sulphur-containing compounds. 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 charcoal, 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 increases the effective surface area available for reaction. Under MTL conditions increasing the intrinsic activity of the catalyst - that is the activity which would occur in the absence of mass transport control - 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 susceptible to inhibition by hydrogen sulphide or sulphur dioxide than either palladitun or rhodium.

Since platinum is intrinsically less active than


palladium or rhodium the relative resistance to poisoning will be determined by the relative importance of factors [a] and [bl above. The improvement 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 poisoning rate may vary with the relative rates of adsorption 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 diffusion) 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 dispersion and porosity of the catalyst, rather than the type of platinum metal, provide the best means of increasing the poison resistance of the sensing element. For butane oxidation (fast reaction, 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 industries. 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 concentrations, 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, indicated by a change in the zero level of the sensor. The incorporation of thoria with palladium in the “pellistor” alleviates this problem. The role of thoria is to disperse the palladium which is present only in metallic form after exposure to the reducing atmosphere, 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 (approximately 21 per cent). Thus the sensor reading may be ambiguous at high concentrations of gas in air, which are then falsely indicated 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 instrument, 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.


,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 example is shown

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The successful operation of any catalytic calorimetric sensor depends crucially on the performance of the catalyst. New catalysts having 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 changed 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 detailed behaviour st i l l requires elucidation. It is however clear that, in most instances, the interaction 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 conductivity. Therefore interaction with the adsorbed 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

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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 sensitivity to 100 ppm methane of stannic oxide: (a) without additives @) with 1 per cent palladium (c) with 1 per cent platinum, and (d) with an addition of 1 per cent iridium


see (14, IS). The major attraction of these devices is that they are very sensitive; concentrations 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 different gases and although much of the literature is concerned with stannic oxide and zinc oxide a wide variety of both binary and, more recently, ternary oxides have been studied. The sensitivity 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 electrical 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 increasing 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 electrons, 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 surface 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-

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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 oxides 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 flammable 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 stannic 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


Dipole layer with voltage drop AV

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I

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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) interactions 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 oxide to that gas when they are used as additives. A more appropriate criterion is perhaps the adsorption 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 sensitivity of stannic oxide to methane and carbon monoxide, respectively. Although palladium metal is an efficient oxidation catalyst for both gases, its addition significantly enhances sensitivity 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 addition 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 sensitivity. However, platinum reduces the sensitivity 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 realised, 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 considerable attention but as yet the commercial impact has been slight. The mechanism by which the presence of hydrogen in the atmosphere 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 diffuses 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 concentration of adsorbed hydrogen, so that the maximum change will occur when every interface 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 interface. This is the third role of the metal layer which is to adsorb the diffused atoms or molecules as dipoles at the metal-support (insulator, semiconductor) interface. The dipole layer formed changes the semiconductor surface 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 environments, 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 properties 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 sensitivity 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 surface 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.


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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.

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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 compounds 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 examined single crystals of UPt, and URu,Si,. Nor- mal and superconducting properties of the latter were reported by Y. Onuki, while a contribution from H. Iwasaki considered superconducting 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, respectively. 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 recommended 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 provided 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 common 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 immediate 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 atmosphere, 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 conditions 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 construction 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 reaction 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 approximately 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 unburnt 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 exhaust 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


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 normal working hours; since there is no risk of exhaust emissions polluting the immediate environment. 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 approximately fifty per cent of the world’s requirements 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 addition, 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 increased enormously, but somewhat erratically. One result of a recent increase in the demand for platinum is that several new platinum producers are emerging. This interesting publication, 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 activities. 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 second largest producer of platinum group metals, appears to be retaining part of its output 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 difficult to disagree with the opinion that the producer 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 possible. 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 attention 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 synthesis 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 industrially 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 applied to the development of organometallic polymers, polysilazanes, that are precursors to silicon nitride containing ceramics.

Polymer syntheses based on catalytic processes are attractive because they can provide thermodynamic advantages, greater control of product stereoregularity and, especially, improved 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 development 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 silylamination 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 >


Table I

Catalytic Promotion of Silylamination of Benzaldehyde Using N-Silylaniline

Conversion

Given that we could promote the catalytic activation 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 catalytically 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 second 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 increased production of the various other cyclomers rather than to higher molecular weight oligomers.

The flaw with the ring-opening polymerisation 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 carried 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 effective 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 reaction, 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 example 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 using reactions analogous to xiii for a variety of reasons (8,9). In particular, these reactions are


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 currently 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 complexes 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


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 dehydrocoupling 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 provides 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 dehydrocoupling 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 properties including tractability, latent reactivity and high ceramic yield. The exact nature of the in- dividual properties is developed in the following paragraphs.

Because the objective of using a preceramic polymer is to form a ceramic shape that is difficult or impossible to obtain by normal ceramics processing techniques, a preceramic polymer must be tractable (soluble, meltable or malleable). Moreover, because different applications, 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 densities 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 provide 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


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 appropriate conditions.

As mentioned above, the use of the dehydrocoupling reaction permits us to synthesise 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 correlation 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 manufacturing 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 support 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 thermocouples provide the best accuracy and reproducibility, 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 thermocouples 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 increasing 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, detailed 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 conditions, 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 circulation, 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 thermocouples 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 thermocouples 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 control 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 containing 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 conditions (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 further 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 surface 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, particularly 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 conversion 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 addition of cerium to alumina has a positive effect 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 excess of 1000°C

rich conditions by reaction with carbon monoxide 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 promoters 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 compared 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 encountered 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 difficult to reduce ( I 3). This interaction can be blocked or at least retarded by the incorporation of ceria into the catalyst, resulting in improved performance after high temperature ageing under oxidising conditions, see Figure 2. Conversion of nitrogen oxides shows improvement particularly under rich conditions, while carbon monoxide conversion is increased across the range of equivalence ratios encountered. The promotion of carbon monoxide conversion in the rich region merits particular attention, since this cannot be achieved by reaction 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 effect 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 improved performance atter high temperature ageing under oxidising conditions. The conversion of nitrogen oxides is improved, particularly under rich conditions, while carbon monoxide conversion is increased across the whole equivalence ratio range

+ El00 -

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- Unpromoted --- Ceria-promoted c---- c o

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This leads to the conclusion that ceria is promoting 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 formation of lower oxides of cerium. For ceria to act effectively as an oxygen storage component, these reduction processes must be readily reversible. 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 programmed 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

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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

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Documents

PLATINUM METALS REVIEW · analyte interacts with a platinum group metal surface is the catalytic field effect transistor (FET). Dissociation of hydrogen and some hydrogen-containing