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UK ISSN 0032-1400
P L A T I N U M METALS R E V I E W
A quarterly survey of research on the platinum metals and of
developments in their application in industry
VOL. 36 APRIL 1992 NO. 2
Contents
Platinum Metals as Components of Catalyst-Membrane Systems
Platinum-Iridium Carbon Monoxide Sensor
Solvated Atoms of Platinum, Palladium and Gold
Platinum Metals Catalyst Studies
Advances and Developments in Emissions Control
Platinum Group Metals in 1991
Platinum Improves Protective Coatings
The Large Scale Production of Hydrogen from Gas Mixtures
Platinum in High-Temperature Superconductors
Ruthenium Oxide Contacts
Temperature-Programmed Reduction of Platinum Group Metals
Catalysts
Iridium Protects Rocket Thrusters
Abstracts
New Patent
70
79
80
85
86
89
89
90
97
97
98
103
104
116
Communications should be addressed to The Editor, Platinum
Metals Review
Johnson Matthey Public Limited Company, Hatton Garden, London E
C l N 8EE
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Platinum Metals as Components of Catalyst-Membrane Systems By
ProfessorV. M. Gryaznov A. V. Topchiev Institute of Petrochemical
Synthesis, Russian Academy of Sciences, Moscow
Platinum group metals are extensively used as catalysts both in
dispersed form and as solids. During recent years we have been wit-
nessing the rapid and successful development of a new branch of
catalysis, namely the cre- ation of catalyst-membrane systems. The
system combines a catalyst and a membrane which has selective
permeability for one of the reagents. Platinum group metal
catalysts en- sure h igher target product yields a n d durability
than other catalysts.
In general, catalyst-membrane systems en- hance both reaction
rate and selectivity, due to directed transfer of energy and
reagents (1). Three functional variations of catalyst-mem- brane
systems have been investigated:
(a) One of the initial reactants, for example hydrogen, reaches
the catalyst through the membrane, which is permeable for this sub-
stance only. The second reactant comes from a gaseous or liquid
phase.
(b) One of the reaction products is selec- tively removed
through the membrane.
(c) The substance penetrating through the membrane is being
formed on the catalyst adjacent to one surface of the membrane,
this then diffuses through the membrane and reacts at its other
surface on the second cata- lyst with the substance being
introduced from the gaseous or liquid phase.
In the first case the catalyst-membrane sys- tem provides
independent control of the surface concentrations of the two
reagents in order to suppress their competing adsorption which is
harmful, but inevitable on conven- tional catalysts. For example
during phenol hydrogenation on palladium-ruthenium alloy foil, the
cyclohexanone yield decreases very rapidly with time if hydrogen is
fed in as a mixture with phenol vapour, see Figure l(a). However,
if hydrogen penetrates through the foil to the surface which is in
contact with
k
2 6 0 W
U
U
Pd-RU
l b l
TIME
Fig. 1 The time dependence of the eyclohexanone yield during
phenol hydrogenation on a palladium-ruthenium foil, supplied with:
(a) hy- drogen in a mixture with phenol vapour and (h) with
hydrogen passing through the membrane catalyst
Platinum Metals Rev., 1992,36, (2), 70-79 70
-
Fig. 2 Cyclohcxane conver- sion is shown as a function of the
flow rate of: (a) argon and (b) a mixture of argon with
1,3-pentadiene vapour along the other surface of the
palladium-ruthenium foil. The mole fractions of cyclohexane and
1,3-penta- diene in the initial mixtures with argon are equal to
0.17 and 0.12, respectively
Palladium-ruthenium -Membrane- Palladium-ruthenium
c Ar+ H
z (a) 0
10
2 8 05Fl = o s 10 20 30
+ cs H10
+ Ar
5 K) 20 30 P v,, RW RATE OF ARGON, (rmolesls V,,FLOW RATE OF
ARGON. FENTADIENE
Urnoles/s
phenol vapour, the product yield is much higher, see Figure
l(b). In this case the foil is acting as both the catalyst and the
membrane. Under optimal conditions 92 per cent of phe- nol is
hydrogenated on the catalyst in one step into cyclohexanone, which
is used for the pro- duction of nylon-6.
In the second case the reaction rate in- creases due to the
removal from the reaction zone of the obtained product as it passes
through the membrane. Thus the reaction rate of cyclohexane
dehydrogenation increases with the removal through the
palladium-ru- thenium foil of the hydrogen formed. The catalyst in
this reaction contains 0.4 per cent platinum and 0.4 per cent
rhenium supported on alumina. The catalyst pellets are repre-
sented by circles in Figure 2. On increasing the velocity of the
inert gas flow, V,, which washes the other surface of the membrane,
cyclohexane conversion rises dramatically, Figure 2(a) lower curve,
and attains the equili- brium degree of conversion, indicated by
the blue line, which is obtained under the same conditions but
without hydrogen withdrawal. I t also works for much lower V,
values when the cyclohexane feed rate is four times smaller, and
its conversion reaches 0.93, as indicated by the upper curve of
Figure 2(a). If, however, the palladium-ruthenium foil is replaced
by a
steel plate (non-permeable to hydrogen) which is covered by a
palladium-ruthenium alloy layer, then cyclohexane conversion is re-
duced to the values shown by the points on the ordinate axis of
Figure 2(a).
The two catalysts shown in Figure 2 permit the use of hydrogen,
which is removed through the palladium-ruthenium foil, for the
hydrogenation of 1,3-pentadiene (see right hand scheme). Thus
reaction coupling takes place on the catalyst-membrane system. This
illustrates the third type of catalyst-membrane system.
As compared with hydrogen removal by the inert gas, the degree
of cyclohexane dehy- drogenation increases when reactions are so
coupled; which can be seen in Figure 2(b), where the curves lie
higher than the respective curves of Figure 2(a). The dotted curve
shows pentadiene conversion into the products of selective
hydrogenation, which are pentenes, the selectivity of which attains
a maximum of 98 per cent. Under these conditions the con- versions
for both cyclohexane and pentadiene reach 0.99. Experimental
evidence of such re- action coupling was found independently in the
U.S.S.R. (2) and in the U.S.A. (3), some 25 years ago. Later,
coupled reactions were stu- died on hydrogen permeable palladium
alloys in the form of foils and thin-walled
Platinum Metals Rev., 1992,36, (2) 71
-
Fig. 3 A laboratory-scale spiral membrane catalyst supplied by
the A. V. Topchiev Institute of Petrochemical Synthesis, and a
pilot plant reactor containing two hundred similar spirals, are
shown
tubes (4-7). A system analogous to that shown in Figure 2 was
used for coupling butane de- hydrogenation and hydrogen oxidation
(6). The system consisted of a commercial alumi- n o c h r o m i u
m o x i d e c a t a l y s t a n d a palladium-ruthenium alloy foil.
The foil served both as the hydrogen permeable mem- brane and as
the catalyst for the oxidation of hydrogen by air, which was fed
along the other surface of the foil. A similar system, compris- ing
an alumino-platinum catalyst and a palladium tube, has been used
for cyclohexane dehydrogenation (8,9).
Types of Catalyst-Membrane Systems Monolithic Membrane
Catalysts
Some materials are both catalytically active and selectively
permeable. Palladium and its alloys have been extensively studied
for hy- drogen evolution or consumption (10-12), and silver has
been used for partial oxidation. A non-porous membrane catalyst
made of these materials may be called monolithic. Foils and tubes
made of palladium alloys are monolithic catalysts permeable only
for hy- drogen. Such monolithic membrane catalysts with smooth
surfaces are easily produced by foil rolling and seamless tube
drawing and are commercially available in Russia. Palladium foil
alloys can vary from 100 to 10 pm in thickness; tube has a wall
thickness of 50 pn
and an outer diameter of 0.6 mm. The foils are active both for
hydrogenation and dehydroge- nation reactions and also possess
higher hydrogen permeability than pure palladium and some other
industrially used alloys. A thin walled palladium alloy tube
twisted into the form of a spiral is shown in Figure 3. This tube
can be used as a liquid phase hydrogena- tion catalyst. Hydrogen is
fed into it when the tube is submerged into the liquid to be hy-
drogenated and heated to the required temperature. The effect of
the structures of the substituted ally1 alcohol and propargyl
alcohol on the hydrogenation rate (13) and on the kinetics of the
hydrogenation of phenylace- tylene to styrene have been studied on
such catalyst systems (14). The latter reaction is of interest
because phenylacetylene admixture in styrene complicates the
polymerisation. With hydrogen diffusing through the membrane
catalyst the phenylacetylene hydrogenation rate is ten times higher
than in experiments where hydrogen is bubbled into the hydroge-
nated substance, and selectivity to styrene reaches 0.92.
Methods for the hermetic connection of foil sheets or tube
bundles to other parts of the reactor system have been described (1
1).
The reactors, one of which is also shown in Figure 3, have
successfully withstood pilot- plant tests at temperatures up to 973
K and pressure drops up to 10 MPa. These reactors
Platinum Metals Rev., 1992,36, (2) 72
-
can also be used for hydrogen extraction from reforming gases,
the product gases of methane steam conversion and also the purge
gases of ammonia synthesis.
The merits of monolithic membrane cata- lysts are: durability
and reliability, stability at high temperatures and during
thermocycling in hydrogen; resistance to corrosion and mechanical
damage, so practically excluding precious metal losses; and the
reaction pro- ducts are readily separated, this being especially
important for creating flexible and ecologically pure
technologies.
A monolithic membrane catalyst, permeable only for hydrogen, is
shown in Figure 4(a), which represents a cross-section through pal-
ladium alloy foil sheet or tube wall.
The drawback of monolithic catalysts is the low ratio of their
surface area to noble metals volume, which can be increased by
roughen- ing one or both surfaces of the membrane catalyst, for
example by thermal diffusion of a chemically active metal into the
palladium alloy sheet and the subsequent removal of this metal by
an acid treatment. The porous layers,
Figure 4(b), formed in this way are strongly bound to the
palladium alloy sheet and are not dispersed during the reaction,
unlike Raney catalysts. A certain part of the chemically ac- tive
metal introduced into the palladium alloy remains after the acid
treatment. Thus the membrane catalyst can be modified in a differ-
ent way on both surfaces if a reaction coupling is to be
performed.
A monolithic membrane catalyst can also be modified by
introducing ultra-dispersed par- ticles of catalytically active
metal or oxide into these porous layers, as is depicted schemati-
cally in Figure 4(c).
Porous Membrane Catalysts Porous membrane catalysts are
charac-
terised by higher, but less selective, gas permeability than
monolithic catalysts. Sheets obtained from metal powder can serve
as the matrix for such porous membrane catalysts. Some porous
metals, such as activated porous nickel, are catalysts for
hydrocarbon reac- tions. A porous membrane with catalyst particles
distributed throughout its volume is
I MONOLITHIC POROUS COMPOSITE I
(b)
(C)
Raney type catalyst layers Dispersed catalyst in porous
support
. . . . - ~ . . . ~ ~ . ~ ~ ~ . . ~ ~ ~ , ~
Metal a oxide particles
Pmtecting film Fallodium alloy film Intermediate !ayr
L&J
Intermediate layer CatdlySt 1
Catalyst (h) I- -, 11 Intermediate layer SYSTEMS OF GRANULAR AND
MEMBRANE CATALYSTS
(i )
Cdtdlyst 11
Fs. 4 A hvther explanation of these ten types of
catalyst-membrane systems is in the text
Plotinurn Metals Rev., 1992,36, (2) 73
-
presented in Figure 4(d). An example of such a system is a
catalyst prepared by inserting into a porous stainless steel sheet
1.5 weight per cent of palladium in the form of ultra- d i spersed
p o w d e r , p r o d u c e d b y t h e condensation of metal and
toluene vapours, followed by melting of the resultant glassy solid.
This catalyst gives a 25 times larger yield of linalool than the
same weight of palla- dium used in the form of a palladium-6 per
cent ruthenium foil, 50 pm thick, through which hydrogen diffuses
during dehydrolina- loo1 hydrogenation in to linalool, at a
temperature of 443 K and at atmospheric pressure. The monolithic
membrane catalyst has higher selectivity (0.99) than the porous
catalyst (0.90), although a selectivity of 0.95 can be achieved on
the porous membrane cata- lyst by a decrease in space velocity.
The porous metallic membrane catalyst dif- fers from the
conventional supported metal catalyst by having a higher mechanical
sta- bility and better heat conductivity than most supports used
for industrial catalysts. The porous metallic membrane also
provides cer- tain selectivity for mass transfer.
To save catalytically active metal its par- ticles need only be
introduced into the sub-surface layer of the porous membrane which
may be metallic, oxide, ceramic or polymeric form, see Figure
4(e).
The advantages of porous and monolithic membrane catalysts can
be happily combined in composite membrane catalysts.
Composite Membrane Catalysts Composites comprising a
mechanically
stable porous support and a thin but con- t i nuous pa l lad ium
alloy layer enable expenditure on platinum group metals to be
decreased and hydrogen permeability to be increased. Such
composites can be readily ob- tained, for example by diffusion
welding a palladium alloy foil, 10 pm thick, to a porous metal
sheet. An intermediate layer, Figure 4(f), is used to prevent the
transfer of compo- nents between the support and the alloy, and
also to augment adhesion.
A palladium film 20 pm thick, deposited by chemical reduction of
a palladium salt onto the outer surface of the porous glass
cylinder, with an average pore diameter of 300 nm (15), has been
used at 673 K to remove hydrogen from the products of the water gas
shift reac- tion over an iron-chromium oxide catalyst (16, 17) and
for steam conversion of methane on an industrial nickel catalyst
(18, 19).
A non-porous palladium layer of about 5 p m thick has been
obtained by chemical plating onto a silver disc, with a pore
diameter of 0.2 pm (20). This composite membrane was tested for
hydrogen permeability at tempera- tures up to 680 K and appeared to
be resistant to pressure drop as well as being stable to
thermocycling in hydrogen.
Composite membrane catalysts can also be assembled with
polymeric supports or inter- mediate layers (21-23). The use of
polyarilyde has been proposed by investigators at this In- stitute,
in order to widen the temperature range of their application (24).
Polyarilyde is resistant in air up to 623 K, and its hydrogen from
nitrogen separation factor is about 100.
Asymmetric membranes have been created at the A. V. Topchiev
Institute of Petrochemi- cal Synthesis and at the Chemical Machine
Building Institute. When such polyarilyde membranes are covered
with a 1 p.m palladium alloy layer they possess higher permeability
for hydrogen than non-metallised membranes at temperatures greater
than 373 K, and are not permeable for other gases.
At even higher temperatures composite membranes of porous
metals, oxides, ceramics and thin palladium or palladium alloy
layers are used. At the Patrice Lumumba Peoples’ Friendship
University a method of obtaining thin alloy films containing a low
melting com- ponent has been developed (25). T h i s component,
being liquid, coats the porous membrane, and after it has hardened
other components are introduced by magnetron sputtering. The whole
system is then annealed to make it homogeneous. A composite palla-
dium-indium-ruthenium alloy layer was prepared by liquid indium
distribution over a
Platinum Metals Rev., 1992,36, (2) 74
-
porous stainless steel sheet, or over a magnesia plate, with
subsequent palladium-ruthenium alloy magnetron sputtering. This 2
pm thick three-component alloy layer only allows hy- drogen to
permeate, at a speed of 10 m3/m2 per hour at 673 K and at a
pressure drop of 0.2 MPa. Such a composite has retained all the
above mentioned characteristics after 450 cycles of heating and
cooling in hydrogen.
A system comprising a porous support, an intermediate layer and
a film made of a selec- tively permeable and catalytically active
material, may be covered by a polymer, see Figure 4(g), which
allows the feedstock to per- meate but which keeps out catalyst
poisons. A porous metallic sheet covered by a polysilox- ane layer
was used to fix silica gel particles, on the surface of which
palladium complexes were immobilised (26). Such a system proved to
be active for the selective hydrogenation of cyclopentadiene into
cyclopentene.
A composite catalysts for reaction coupling should comprise five
layers, as shown in Fig- ure 4(h), consisting of a porous membrane
with intermediate layers and films of catalysts I and I1 on its
surfaces.
Granulated and Membrane Catalysts Systems
Of even more general purpose are the sys- tems containing a
conventional granulated catalyst and a membrane catalyst. Figure 4
shows two varieties of such a system: a pel- let catalyst with a
monolithic catalyst, 4(i), and with a composite membrane
catalyst,
A system, consisting of an industrial oxide aluminochromium
catalyst and a hydrogen permeable palladium alloy foil containing
20 weight per cent of silver, has been used for butane
dehydrogenation (27). Part of the hy- drogen formed was removed
through the membrane, with inert gas being fed along the other
membrane surface at 673 K and atmos- pheric pressure, and this
resulted in butene yields of 18 to 25 per cent and butadiene yields
of 0.8 to 1.4 per cent.
The influence of butane and its dehydroge-
4W.
nation products on the hydrogen permeability of a series of
binary palladium alloys has been investigated (6). Butane
conversion increased when an air flow was used instead of an inert
gas flow to remove the hydrogen which had penetrated through the
membrane. A similar result was obtained for butene-1 dehydrogena-
tion on an aluminochromium oxide catalyst (28), with hydrogen
withdrawn through a 25 pm thick palladium foil, on the other side
of which an argon mixture containing 10 per cent oxygen was fed.
For example, at 658 K and a butene feed rate of 5 ml/min, oxidation
of the hydrogen which had diffused through the membrane increased
the butene conver- sion rate by a factor of three, compared with
equilibrium conditions without hydrogen withdrawal (29).
Systems containing a pellet catalyst and a hydrogen permeable
composite membrane, Figure 4(i), have been used to intensify pro-
pane aromatisation (30) and steam conversion of methane (19). ZSM-5
zeolite, with gallium ions introduced by ion exchange, catalysed
propane aromatisation. The zeolite was ar- ranged in the central
part of a cylindrical reactor, which had a coaxial porous alumina
tube, the outer surface being coated with a palladium film 8.6 pm
thick. An equimolar propane-nitrogen mixture was fed into the space
between the tubes, while the inner tube volume was pumped. To
elucidate the func- tion of the membrane, the coated alumina tube
was replaced by a non-permeable pyrex tube. The yield of aromatic
hydrocarbons was 42 per cent with the pyrex tube, and rose to 76
per cent when about 90 per cent of the hydrogen formed was removed.
This phe- nomenon was explained by hydrocracking and by enhancing
propane dehydrogenation into propene, out of which a considerable
amount of naphthalene was formed, when a gallium-containing ZSM-5
catalyst was used (31).
Steam conversion of methane was studied in a similar reactor
over a nickel catalyst, with a tubular porous glass membrane
(average pore diameter 300 nm) coated with a 20 pm thick
Platinum Metak Rev., 1992,36, (2) 75
-
palladium layer. Methane conversion reached 78 per cent at 773 K
when the hydrogen dif- fusing through the membrane was removed by a
flow of nitrogen, while equilibrium methane conversion without
hydrogen withdrawal amounted to 44 per cent.
This classification of catalyst-membrane systems summarises and
generalises the ver- sions suggested earlier (32,33).
Catalytic Membrane Reactors Membrane reactors are subdivided
into ten
types according to the following criteria: cata- lyst
application in reaction side and in separation side; membrane
functioning only for separation or also as a catalyst; removal of
the gas passing through the membrane as it is or by the
introduction of a gas into the reac- tion (9).
A mathematical simulation of a catalytic membrane reactor and an
inert membrane re- actor with catalyst pellets in the feed side
showed a slight advantage of the former at not too small space
velocities (34) which is in good agreement with experimental data
for cyclo- hexane dehydrogenation into benzene. A porous glass
thimble, with 0.34 weight per cent platinum introduced into its
pores, served as the membrane catalyst. In this case the reaction
took place in the membrane cata- lyst itself, see Figures 4(d) or
4(e). The membrane made of the same porous glass without any
metallisation was catalytically inert, and had to be used with a
layer of pla- tinised porous glass particles on it.
A porous membrane catalyst of the type shown in Figure 4(e) was
obtained by intro- ducing 1 weight per cent platinum into the pores
of a multi-layered tubular alumina membrane (MembraloxR, Alcoa). It
provided a six-times greater yield of ethylene from ethane
dehydrogenation than equilibrium without hydrogen withdrawal (35).
Selectivity towards ethylene was more than 0.96. Ethane transfer
through the porous membrane catalyst oc- curred a t a rate
four-times slower than hydrogen transfer. A mathematical model of
the reactor with this catalyst has been worked
out, and it represents, very satisfactorily, the experimental
results when parameter adjust- ments have been made.
Mathematical models of a catalytic mem- brane reactor, an inert
membrane reactor with catalyst pellets, a plug flow reactor packed
with catalyst pellets, and also a mixed flow reactor have been
created (36). It was found that for liquid phase reactions without
any volume changes, the first two types of reactors are preferable
at high space velocities. It is also true for an inert membrane
reactor with catalyst pellets for reactions with a volume decrease,
but it does not work as well as a catalytic membrane reactor at a
low pressure drop on the membrane.
Investigations using a catalytic membrane reactor with a
palladium membrane showed that when the dehydrogenated substance
and the hydrogen carrier gas, coming through the membrane, flow in
opposite directions the de- gree of conversion is the highest at a
minimal reactor length, if compared with the regimes of co-current,
ideal mixing in the reaction chamber, hydrogen collection chamber
or in both of them (37). While these mathematical models were being
created it was assumed that hydrogen transfer through a palladium
mem- brane is described by the Sieverts law. It was proved
experimentally, however, that in the process of butane
dehydrogenation on palla- dium alloy foils the hydrogen
permeability decreases, not because of the butane but as a result
of the adsorption of butenes (6, 38). This effect is even more
pronounced for palladium-antimony alloy than for ruthe-
nium-palladium alloy. When the membrane ceased to be hydrogen
permeable, due to strong adsorption of butenes, these butenes are
then not subject to isomerisation or to hydrogenation in the
hydrogen flow from the gaseous phase. If, however, hydrogen
transfer through the membrane is only decreased by butene
adsorption then only isomerisation of butenes occurs, and not
hydrogenation. These results led to the assumption that in butene
isomerisation and hydrogenation reactions hydrogen atoms entering
the active centres of
Platinum Metals Rev., 1992,36, (2) 76
-
the membrane catalyst from its sub-surface layer are of great
importance.
A study of the influence of electron ac- ceptors (S, CO) and
electron donor (K) adsorption on the hydrogen permeability of
palladium-silver alloy concluded that the species passing through
the alloy is not H' (39). Hydrogen permeability decreased after
contact with the ethylene, but in the process of ethylene
hydrogenation by hydrogen, pas- sing through the same foil, a
10-fold increase in permeability was observed.
Another study has been performed to inves- tigate the effect of
the directions of flow of the hydrogenated substance and of the
hydrogen along adverse surfaces of the membrane cata- lyst on the
hydrogen permeability and also on the depth of cyclopentadiene
hydrogenation occurring on a palladium alloy containing 4 weight
per cent indium (40). The hydrogen transfer rate to the
hydrogenation chamber was higher with a counter flow of hydrogen-
nitrogen mixture and cyclopentadiene vapour-nitrogen mixture than
with co-current flow. Figure 5 shows the dependence of the degree
of cyclopentadiene (CPD) conversion on VH*:VCpD feed velocity
ratio, for counter- current (Curve 1) and co-current (Curve 2)
flows. The amount of hydrogen transferred through the membrane
catalyst during differ- ent flow directions was kept constant by
changes in the composition of the hydrogen- nitrogen mixture.
During the coupling reactions of terpene alcohol: borneol
dehydrogenation into cam- phor, on a copper catalyst, with
cyclopentadiene
; 6 1.0
5
In w
0.5 w 3
s w
:T hydrogenation on palladium-ruthenium alloy foil,
counter-current flow again proved to be more effective than
co-current flow (41). As Figure 6 depicts, the cyclopentene
concentra- t ion (Curve 1) in the cyclopentadiene hydrogenation
products is much higher for counter-current flow and decreases more
slowly during the course of the experiment than for co-current
flow, see Figure 6(b). The camphor yield from borneol was 93 per
cent. In the counter-current coupling of cyclohex- ane
dehydrogenation and 1,3-pentadiene hydrogenation, discussed earlier
and shown schematically in Figure 2, a 20 per cent in- crease in
selectivity for pentenes was obtained, compared with using
co-current conditions (42).
Structurally, a catalytic membrane reactor can resemble a block
made up of corrugated
Fig. 6 Time dependence of the concentrations of cy- clopentene
(Curve 1) and cyelopentane (Curve 2) are shown as products of cy-
clopentadiene hydrogenation by hydrogen coming through the
palladium-ruthenium during borneol dehydroge- nation on the other
surface of the foil: (a) counter-cur- rent and (b) co-current
flows
cyclopentadiene
(a )
cyclopentadoene introduction
20 2 -
20 4 0 60 TIME, minutes "I 20 40
Platinum Metak Rev., 1992,36, ( 2 ) 77
Fig. 5 The conversion of cyclopentadiene is dependent on the
ratio of hydrogen: cyclopentadiene in the feed velocities for both
counter-current (Curve 1) and co- current (Curve 2) flows
-
and plane sheets obtained by roasting mix- tures of inorganic
oxide powders, binders and plasticisers. Having parallel
orientation, each pair of corrugated sheets is separated by a plane
sheet and thus parallel channels are formed. Every second
corrugated sheet is turned through 90” with respect to the first,
thus two similar channel systems will be per- pendicular to each
other (43). Such blocks can be manufactured out of porous metal
sheets, as mentioned earlier. Blocks with perpendicular channel
systems are convenient for separating the flows along both sides of
the membrane catalyst.
The catalyst material may also be deposited on alumina
multi-channel membrane ele- men ts to produce catalytic membrane
reactors (44). Methods of depositing platinum group metals on solid
supports of different composi- tion, as well as on porous ones,
have been investigated at the Patrice Lumumba Peoples’ Friendship
University and at the A. V. Top- chiev Institute of Petrochemical
Synthesis of the Russian Academy of Sciences.
The palladium based monolithic membrane catalysts proposed by
the above named in- stitutions, in collaboration with A. A. Baikov
Ins t i tu te of Metallurgy of the Russian Academy of Sciences,
could be successfully combined with conventional catalysts for the
dehydrogenation or steam conversion of hy- drocarbons. The
apparatus for extracting pure hydrogen by diffusion (45), and
hydrogen generators incorporating catalytic cracking
and silver-palladium diffusion units (46), could also be
equipped with the above men- tioned palladium alloy membranes thus
giving high hydrogen permeability for the products of hydrocarbon
transformations.
Five years ago membrane reactor catalysts, along with two other
types of catalyst, were said to have high potential to make a major
impact on new catalyst technology for the fu- t u re (47). Clearly
the new data about catalyst-membrane systems fully justifies those
expectations.
Conclusions Combining catalysts and membranes has
become a most promising method for improv- ing existing
technologies. It seems likely that the suggested classification of
catalyst-mem- brane systems will be helpful in further
investigations.
Several types of catalytic membrane reactor have now been
studied both theoretically a n d experimentally. T h e results have
proved to be very encouraging, especially for thermodynamically
complicated reac- tions. Palladium alloys which maintain their
hydrogen permeability when in con- tact with dehydrogenation
products have been found. Such membranes are good for use in
combination with hydrocarbons dehy- drogenation catalysts and can
be used as catalysts for the oxidation of the removed hy- d r o g e
n , t h u s c o m p e n s a t i n g f o r t h e endothermicity of
dehydrogenation.
References 1 V. M. Gryaznov, Dokl. Akad. Nauk SSSR, 1969,
2 V. M. Gryaznov, U.S.S.R. Authors Certif. No.
3 W. C. Pfefferle, U.S. Patent 3,290,406; 1966 4 V. M. Gryaznuv,
V. S. Smirnov, L. K. Ivanova and
A. I! Mischenko, Dokl. Akad. Nauk SSSR, 1971, 190,144
5 V. S. Smirnov, V. M. Gryaznov, N. V. Orekhova, M. M. Ermilova
and A. I! Mischenko, Dokl. Akad. Nauk SSSR, 1975,224,391
6 N. V. Orekhova, and N. A. Makhota, in “Mem- brane Catalysts
Permeable for Hydrogen or Oxygen” (in Russian), Moscow, Nauka,
1985,49
7 N. N. Mikhalenko, E. V. Khrapova and V. M. Gry- aznov, Kinet.
Katal., 1986,27, 138
189,794
27,4092; 1969
8 N. Itoh, K. Miura, T. Shindo, K. Haraya, K. Obata and K.
Wakabayashi, Sekiyu Gakkaishi, 1989,32, (I), 47
9 N. Itoh, Sekiyu Gakkaishi, 1990,31, (3), 136 10 “Membrane
Catalysts Permeable for Hydrogen or
11 V. M. Gryaznov, Pkuinum Metah Rev., 1986,30,
12 J. N. Armor, Appl. Catal., 1989,49, (l), 1 13 A. N.
Karavanov, in Abstracts of the Papers of 25th
Scientific Conference of the Department of Physics-Mathematical
and Natural Sciences (in Russian), Moscow, Peoples’ Friendship
University Publishing, 1989,75
14 V. I. Lebedeva, ibid., 80
Oxygen” (in Russian), Moscow, Nauka, 1985
(2)s 68
Plaiinum Metals Rev., 1992,36, (2) 78
-
15 S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Mat- suda and E.
Kikuchi, c h . Len., 1988,1687
16 E. Kikuchi, S. Uemiya, N. Sato, H. Inoue, H. Ando andT.
Matsuda, Chem. Ltr., 1989,489
17 S. Uemiya, N. Sato, H. Ando and E. Kikuchi, Znd. Eng. Chem.
Res., 1991,30,585
18 S . Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi,
Sekiyu Gakkahhi, 1990,33,418
19 S . Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi,Appl.
Catal., 1991,67,223
20 K. Govind and D. Atnoor, Znd. Eng. Chem. Res.,
1991,30,391
21 V. M. Gryaznov, V. S. Smirnw, V. M. Vdovin, M. M. Ermilova,
L. D. Gogua, N. A. Pritula and I. A. Litvinov, BritishPutent
1,528,710; 1978
22 R. V. Bucur, and V. Mercea, Surf: Coating Technol.,
1986,28,387
23 E! Mercea, L. Murian, V. Mercea, and D. SilipasJ Membrane
Sci., 1988,35,19
24 M. M. Ermilova and S. I. Zavodchenko, in “Mem- brane
Catalysts Permeable for Hydrogen or Oxygen” (in Russian), MOSCOW,
Nauka, 1985,33
25 V. A. Nesmeyanm, S. I. Zavodchenko, 0. S. Sere- bryannikova,
Yu. M. Serov and V. M. Gryaznw, in Abstracts of the Papers of 25th
Scientific Con- ference of the w e n t of Physics-Mathematical and
Natural Sciences, (in Russian), Moscow, Peo- ples’ Friendship
University Publishing, 1989,77
26 V. M. Gryaznov, V. S. Smirnw, V M. Vdovin, M. M. Ermilova, L.
D. Gogua, N. A. Pritula and G. K. Fedorova, US. Patent 4,394,294;
1983
27 N. V. Orekhova and N. A. Makhota, in “Metals and Alloys as
Membrane Catalysts” (in Russian), Mos- cow, Nauka, 1981,168
28 R. Zhao, R. Govind and N. Itoh, Sep. Sci. Technol., 1990,25,
(13-15), 1473
29 R. Zhao, N. Itoh and R.Govind, in ”Novel Ma- terials in
Heterogeneous Catalysis”, Am. Chem. SOC. SR., 1990,Vol. 437, p.
216
30 S. Uemiya, T. Matsuda and E. Kikuchi, Chem. Len.,
1990,1335
31 M. Shibata, H. Kitagawa, Y. Sendoda and Y. Ono, Stud. Surf:
Sci. Catal., 1986,28,717
32 V. M. Gryaznw, in “Metals and Alloys as Mem- brane Catalysts”
(in Russian), Moscow, Nauka, 1981,4
33 V. M. GryaznovJ D. Z. M e d b e v All-Union Chem. SOC. (in
Russian), 1989, (6), 604
34 Y. M. Sun and S. J. Khang, Znd. Eng. Chem. Res.,
1988,27,1136
35 A. M. Champagnie, T. T. Tsotsis, R. G. Minet and I. A.
Webster, Chem. Eng. Sci., 1990,45, (8), 2423
36 Y. M. Sun and S. J. Khang, Znd. Eng. Chem. Res.,
1990,29,232
37 N. Itoh, Y. Shindo and K. Haraya,J. Chem. Eng. Jpn., 1990,23,
(4), 420
38 V. M. Gryaznw, M. M. Ermilova, N. V. Orekhova and N. A.
Makhota, Proc. 5th Int. Symp. on Hete- rogeneous Catalysis, Varna,
Bulgaria, 1983, Vol. 1, 225
39 S. B. Ziemecki, in Abstracts 1990 Spring Meeting of Material
Research SOC., San Francisco, 207
40 N. N. Mikhalenko, E. V. Khrapova and V M. Gry- aznov, Russ.
J. Phys. C h . , 1986, (2), 5 1 1
41 V. M. Gryaznov, M. M. Ermilova, L. S. Morozova, N. V.
Orekhova, V. f! Polyakova, N. R. Roshan, E. M. Savitsky and N. I.
Parfenova, J. Lss-Common Met., 1983,89,529
42 N. V. Orekhova, M. M. Ermilova and V. M. Gryaz- nov, Dokl. A
M . Nauk SSSR, 1991,321, (l), 141
43 R. De Voss, V. Hatziantoniou and N. H. Schoon, Chem. Eng.
Sci., 1982,37, (ll), 1719
44 H. E! Hsieh, R. R. Bhave and H. L. Fleming,J. Membrane Sci.,
1988,39,221
45 J. E. Philpott,PhtinumMetaLFRev., 1985,29, (l), 12 46 J. E.
Philpott,PhtinumMetaIs Rev., 1989,33, (2), 58 47 J. E Roth, in
“Catalysis 1987”, Proc. of the 10th
North American Meeting of the Catalysis Soc., San Diego, U.S.A.
May 17-22, 1987, ed. J. W. Ward, Elsevier, Amsterdam, 1988, p.
925
Platinum-Iridium Carbon Monoxide Sensor Various heat of
oxidation and doped metal
oxides types of catalytic sensors have been used in gas
detectors, but in general they suffer from interference caused by
water vapour. These changes in humidity can produce spurious sig-
nals which have in the past been overcome by the use of high power
heaters.
In order to solve the humidity-effect problem that occurs with
catalytic carbon monoxide sen- sors and to eliminate the
requirement for heaters, researchers at the Chalk River Labora-
tories of AECL Research, in co-operation with Asahi Electronics
Inc., Ontario, Canada, have developed and tested several new
bimetallic platinum group metal catalysts, (K. Marcin- kowska, M.
€! McGauley and E. A. Symons,
Sens. Acrivafurs B, 1991,5, (1-4), 91-96). The optimised
catalyst contained a total of 10
weight per cent of platinum and iridium which was supported on
porous, inertly hydrophobic polystyrene-divinylbenzene granules
contained in nylon mesh thimbles.
This new carbon monoxide sensor was found to be independent of
humidity and even after testing for 10 months, no affect on the
carbon monoxide oxidation activity of the catalyst was detected
despite exposure to carbon monoxide concentrations of up to 250
ppm.
The sensor requires no heater as the catalyst is active at
ambient temperatures down to around -10°C. This has facilitated
production of a portable, battery-powered detector.
Platinum Metals Rev., 1992,36, (2) 79
-
Solvated Atoms of Platinum, Palladium and Gold PRECURSORS TO
COLLOIDS, FILMS AND CATALYSTS
By Professor Kenneth J. Klabunde Department of Chemistry, Kansas
State University, U.S.A.
Atoms of the noble and other metals can be trapped in cold
solvents, and solvated metal atom solutions can be prepared and
manipu- lated at low temperatures. Such solvated atoms have been
useful in: (a) the preparation of non-aqueous colloidal metal
solutions which, in turn, can be used to prepare metallic films of
platinum, palladium and gold; (b) preparing ultra-fine bimetallic
powders of gold-tin; (c) trapping platinum-tin fine par- ticles on
a lumina in order to prepare bimetallic PtoSno heterogeneous
catalysts. Ultra-fine particles of metals are usually pre- pared by
high temperature metal salt reduction methods (1-3). Under such
conditions, the approach to the most thermo-dynamically stable
state has often moved further than is desirable. Very small,
nano-scale particles and kinetically stable phases are often not
attain- able by such reduction techniques. This is particularly
true of bimetallic combinations; meta-stable bimetallic particles
either revert to the most thermodynamically stable state or may
phase-separate under high temperature reducing conditions. In order
to prepare meta-stable states or possibly new phases of nano-scale
metal particles, low temperature, kinetic growth methods should be
employed (4). Zero-valent atoms should be used rather than salts or
oxides since reduction steps can thus be avoided. Actually, in
recent years we have witnessed the development of several methods
for the low temperature, kinetically controlled growth of metal
clusters from free atoms. These include the gas phase “cluster
beam” approach (metal clusters, Buckyballs, etc.) and the
clustering of metal atoms in low temperature matrices. In order to
carry out
cluster syntheses on a large scale with rela- tively low
expense, we have studied such cluster growth in low temperature
organic sol- vents (5). I n fact this method has been practiced for
about 20 years and serves as a forerunner of other clustering
methods (6). Kinetic control of cluster growth can be re- alised,
and the unique structure/reactivity of such materials has been
demonstrated many times (4). Magnetic properties have also been
studied (7).
In this method, metal atoms are first trapped in frozen solvents
by codepositing the evaporated atoms with excess solvents at a
temperature of 77 K. Upon warming, a liquid of “solvated metal
atoms” is formed, often stable in the 180 to 250 K range. Further
warming leads to atom agglomeration to form particles 2 to 9 nm in
size, but further growth is precluded by particle solvation. Under
the right conditions, stable colloidal metal solutions are formed;
such as palladium particles in acetone (8).
In the presence of a catalyst support, metal atom nucleation and
cluster growth occurs on the surface of the support; an example
being very small platinum clusters on alumina. In this way
“solvated metal atom dispersed” (SMAD) catalysts have been prepared
(5 ) .
We describe recent findings of interest to the users of
platinum, palladium and gold.
Non-Aqueous Palladium and Gold Colloids
The codeposition of palladium or gold atoms with acetone
followed by slow warming leads to non-aqueous metal colloidal
solutions. In the case of palladium, black solutions result whereas
with gold, dark red-purple liquids are
Platinum Metals Rev., 1992,36, (2), 80-84 80
-
formed (8-1 1). The individual colloidal par- ticles are 4 to 9
nm in size and are indefinitely stable under the correct conditions
of concen- tration and solvent polarity.
Polar, organic solvents can be employed, such as acetone,
ethanol, isopropanol, di- methylsulphoxide and
dimethylformamide
Colloid stability is apparently feasible due to the
solvation/ligation effects of the solvent (steric effects),
combined with electronic ef- fects possible because of electron
scavenging to form negatively charged colloidal particles (8, l l)
, as shown by electrophoresis studies.
Perhaps the most remarkable feature of these colloidal particles
is their “living na- ture”. These particles will grow if their
solvating medium is perturbed. Solvent remo- val leads to metallic
films. Thus, these solutions can be “spray painted” onto sur-
faces. The solvent evaporates leaving films that look like
palladium or gold films. The films do retain some of the organic
solvent, however, which poorly affects their conducti- vities.
Heating tends to drive out the organic impurities, and the films
become smoother and better conductors (12, 13).
The best results have been obtained when surfaces that interact
with the metal particles are coated. For example palladium on
stain- less steel, gold on copper, gold on silver, or better still,
gold on polyphenylenesulphide polymer. In this case, the sulphur
containing surface “ligated” well with the depositing gold
particles, and upon heating a perfect, strongly adhering gold film
was formed (13).
(8-10).
Bimetallic Fine Particles of AuSn as a Model of PdSn and
PtSn
In order to learn more about the atom-atom clustering process in
solvated metal atom media, two metals were simultaneously iso-
lated in excess cold solvents, and clustering was allowed to occur
on warming. The metal pair chosen was goldtin because these two
metals form several well characterised inter- metallic compounds
throughout their entire compositional range. If selectivity in
growth
Aubolv). Sn(s01v) - AuSn(sMv1 (I1 A d - 1 ~ ) . AuSnkolv) -
Au2Snl(solv) tni AulSn~(solv)~ AuSntsolv) - Au,Sn,(solv) trnl
AunSno(sorv). solv - AunSnntR&lsMv> ( IV)
5tilQh5ed toward further growth
Fig. 1 A generalisation of the rate pro- cesses for cluster
growth and cluster reac- tions with the host solvent, where:
solv=solvent molecule and Rzfragment of solvent that serves as a
&and. Mobility de- creases with size and therefore the rate of
reaction (111) decreases; mobility is low at high viscosities,
therefore reaction (IV) competes more effectively with (111)
was to be found, this bimetal system seemed to lend itself best
to these experiments.
Experimental parameters such as evapora- tion rate and
evaporation method, solvent polarity and viscosity, and warming
rate dur- i ng cluster formation were varied, and
cluster/crystallite sizes were monitored. Addi- t i ona l i n fo
rma t ion was g leaned f rom Mossbauer spectrometry, Differential
Scan- ning Calorimetry (DSC), X-Ray Powder Diffraction (XRD) and
X-Ray Photoelectron Spectroscopy (XPS) (9).
The results demonstrated that the cluster growth process was
somewhat selective toward the growth of clusters of AuSn, Au,Sn and
tin metal. Solvent viscosity had an effect only in cases where
large viscosity changes occurred in a narrow temperature range
during which cluster growth occurred (about 150-200 K). A more
sensitive parameter was solvent polarity - highly polar ethanol
allowed the crystallites to grow larger, to 27 nm. The most
sensitive parameter, however, was the rate of matrix warming. A
slow warm-up yielded smaller particles with higher surface areas,
results are given in the Table. These findings can be ex- plained
by considering a competition between cluster growth and the
reaction of the growing cluster with the host solvent, so that
ligand stabilisation (solvation) occurs and stops fur- ther growth.
In Figure l this is expressed as a competition between Reactions
(IV) and (111).
Overall these results are promising with re- gard to the
possibility of controlling the
Platinum Metals Re%, 1992,36, (2) 81
-
Solvent Properties Compared with Surface Areas and Crystallite
Sizes of Resultant AuSn Powders
Solvent
Pentane
Acetone
Toluene
Ethanol
Cyclo- hexane
Ether
Hexane
Melting point, “C
-1 30
-95
-95
-1 17
6.5
-1 16
-95
Dielectric constant, Ee
1.8
20.7
2.4
24.3
2.0
4.3 (20)
1.9 (20)
Viscosity”
0.289
0.399
0.772
1.733
1.02 (20)
0.284
0.401
a Viscosities are at 0°C unless indicated otherwise in
parentheses b Fast warm-up: -196°C to 25°C in -0.5 hours
Slow warm-up: -196°C to 25°C in 3-4 hours c BET method using
nitrogen adsorption d From XRD data using Scherrer equation
selectivity of cluster growth to certain bime- tallic
compositions while still maintaining a small particle size. This is
demonstrated most clearly by considering Figure 2, which shows
Differential Scanning Calorimetry spectra of AuSn clusters grown in
acetone by fast or slow warm-up. As shown, the endothermic (up-
ward) peaks are sharp, indicating the melting points of small
particles of specific composi- tions (tin -219”C, eutectic mix of
Au,Sn and AuSn -275”C, and AuSn -419°C). Note that more selective
growth to A u k , Au,Sn and tin is evident in the slow warm-up case
(b), which
Narm-upb
fast
slow
fast
slow
fast
slow
fast
slow
fast
slow
fast
slow
fast
slow
Surface area‘, m’/g
16.5
23.8
14.9
18.3
17.6
43.7
11.6
16.5
19.4
28.0
16.6
-
23.4
19.8
:rystal sized, nm
16.3
10.2
17.8
11.2
17.6
17.1
26.9
23.6
13.1
12.7
17.8
9.3
13.1
12.5
is good evidence for selective growth. In the fast warm-up case
(a), it is obvious that a “wilder” growth process took place
leading to more components (10).
PtSn/Al,O, SMAD Catalysts The commercial importance of the
Pt-
Sn/AI,O, catalyst systems, as well as the poor understanding of
the role of tin, led us to investigate this bimetallic system in
some de- tail. These solvated metal atom dispersed (SMAD) catalysts
were prepared in two ways. The “half S M A D process refers to
treating
Platinum Metals Rev., 1992,36, (2) 82
-
L
t w I
preformed, conventional Pt/Al,O, catalysts with solvated tin
atoms, thereby ensuring the deposition of metallic tin on the
platinum metal cluster. The "full SMAD" process refers to the
evaporation/trappingolvation of plati- num and tin atoms
simultaneously, followed by warming, bimetallic cluster growth and
trapping on high surface area alumina (5,14).
Two conventionally prepared Pt-SnO/Al,O, catalysts were also
prepared and compared with SMAD catalysts.
The unique feature of the SMAD catalysts is that Sn" is present,
and the study and com- p a r i s o n of t hese Pt'Sn" sys t ems w i
t h conventional PtoSnz+"+ systems should be of help in quelling
the debate about whether Sno plays a role in the commercial
catalysts.
Through the combined use of catalytic
,'
/' ,'
SCCOnd run \
I
probe reactions, IL9Sn Mossbauer, Extended X-Ray Absorption Fine
Structure, XPS and XRD it has been demonstrated that the half- SMAD
catalytic particles are made up of the expected platinum particles
with a partial thin Sn" coating. Interestingly, this type of
catalyst showed the highest activity for n-heptane re- forming to
benzene and toluene. On the other hand, the full-SMAD catalyst
particles were shown to be alloy-like, probably rich in Pt" on
the outer surface of the catalyst layer, Figure 3. This type of
catalyst showed lower activity but better selectivity to benzene
and toluene. Undesirable hydrogenolysis reac- tions were greatly
depressed, and this is perhaps due to an ensemble effect (Sn"
dilut- ing surface Pt") decreasing these unwanted, surface
sensitive reactions.
Platinum Metals Rev., 1992,36, ( 2 ) 83
Fv. 2 Differential scanning calorimetry spectra of AuSn
particles prepared in acetone by (a) fast warm-up and (b) slow
warm-up
-
n Sn
. S”(sol”)n (rero valent tin atom)
I I l l I I l l
Alumina
~t atom 0 sn atom
Fig. 3 Proposed sequence fo r the formation of full- SMAD Pt-Sn
bimetallic par- ticles on alumina (Sn:Pt atomic ratio of 2.5 is
illus- trated)
These results also demonstrate that zero- valent tin does affect
catalytic performance in beneficial ways. So, although zero-valent
tin is rarely detected in conventional Pt-Sn/Al,O, catalysts, small
amounts possibly formed on the platinum particles (by hydrogen
reduction of Sn2+) may be at least partially responsible for
beneficial changes in this important class of bimetallic
catalysts.
A comparison of catalytic properties for the SMAD and
conventional catalyst systems sug- gested some generalisations: (i)
the presence of SnO, and SnO in the con- ventional catalysts may
play a role in improving lifetimehtability by blocking plati- num
particle sintering; (ii) the presence of Sno in combination
with
Pto can affect catalyst activity and selectivity; (iii) the
presence of Pto-Sno alloy (rich in Sn’) can depress unwanted
hydrogenolysis, while activity for the desired dehydrocyclisation
is only lowered slightly (5, 14).
I n summary, the use of metal vapour methods, especially
solvated metal atoms, shows promise for producing ultra-fine mono-
metallic and bimetallic particles for many interesting
applications, including catalysts, magnetic materials, colloids and
films.
Acknowledgements The author would like to acknowledge the work
of
many fine students, in particular Dr. Y. X. Li, Yi Wang, Ellis
Zuckerman and Greg Youngers. The sup- port of the National Science
Foundation, for nearly 20 years now, is also deeply
appreciated.
References 1 V Haensel and R. Bunvell, Sci. Am., 1971,225,46 2
J. R. Anderson, “Structure of Metallic Catalysts”,
Academic Press, New York, 1975 3 R. D. Srivastava,
“Heterogeneous Catalytic
Science”, CRC Press, Boca Raton, Florida, 1988 4 K. J. Klabunde,
G. H. Jeong and A. W. Olsen, in
“Molecular Structures and Energetics”, ed. J. A. Davies, I! L.
Watson, J. E Liebman and A. Green- berg, VCH, NewYork, 1990, pp.
433463
5 K. J. Klabunde, Y.-X. Li and B.-J. Tan, Chem. Muter., 1991,3,
(l), 30
6 K. J. Klabunde, “Chemistry of Free Atoms and Particles,”
Academic Press, NewYork, 1980
7 C. E Kemizan, K. J. Klabunde, C. M. Sorensen and G. C.
Hadjipanayis, C k Muter., 1990,2, (l), 70
8 G. Cardenas-Trivino, K. J. Klabunde and B. E. Dale, Langmuir,
1987,3, (6), 986
9 S.-T Lin, M. T Franklin and K. J. Klabunde, Lnngmuir, 1986,2,
(2), 259
10 Y. Wang, Y.-X. Li and K. J. Klabunde, “Selectivity in
Catalysis: Clusters, Alloys, and Poisoning”, ACS Symp. Ser., ed. S.
Suib and M. Davis, in press, 1992
11 M. T. Franklin and K. J. Klabunde, “High Energy Methods in
Organometallic Chemistry”, ACS Symp. Ser. 333, ed. K. Suslick,
1987, pp. 246-259
12 G. Cardenas-Trivino, K. J. Klabunde and B. E. Dale, “Thin
Metallic Films from Solvated Metal Atoms”, SPIE Proc. (Opt. Eng.
paper 821-29 (8 pages), 1987, pp. 206-213
13 K. J. Klabunde, G. Youngers, E. Zuckerman, B.-J. Tan, S.
Antrim and I! M. A. Sherwood, invited paper for Ew.3 Solidstate
Inorg. Chem., submitted
14 Y.-X. Li and K. J. KlabundeJ Caral., 1990, Us, 173
Platinum Metals Rev., 1992,36, ( 2 ) 84
-
Platinum Metals Catalyst Studies Catalysis Volume 9: A
Specialist Periodical Report EDITED BY J. J. SPIVEY, Royal Society
of Chemistty, Cambridge, 1992,279 pages, ISBN 0-85186-604-2,
Ji97.50
Three of the five chapters that make up this report will help to
increase the understanding of catalyst deactivation and
regeneration mech- anisms in the industrially significant areas of
naphtha reforming and pollution control, where platinum metals
catalyst systems con- tinue to find important applications. All
catalysts become deactivated during use, and thus there is a
growing interest in gaining a much greater understanding of the
reactions involved.
The deactivation and regeneration of naph- tha reforming
catalysts is described by J. M. Parera and N. S. Figoli of INCAPE,
Santa Fey Argentina. The Universal Oil Products “plat- forming”
catalyst, introduced in 1949, based on a bimetal-acid catalyst, for
example, platinum- rhenium-sulphur/alumina, is more selective and
more stable than platinum/alumina, and is currently the most widely
used com- mercially. Catalyst deactivation is discussed in terms of
coke deposition, poisoning by sulphur and nitrogen compounds,
decrease of metallic and support areas and chloride concentration,
heavy metal deposits, and fines formation and deposition. All these
characteristics are revers- ible except the decrease in support
area and heavy metals deposition. With the most com- monly used
commercial, naphtha-reforming catalysts, the main reactions of the
process are controlled by the acid function of the catalyst, in
spite of the great initial deactivation of the metallic function.
These catalysts have plati- num contents of about 0.3 per cent and
the activity of the metal in the operating conditions used is
enough to produce all the olefins that can be isomerised or
dehydrocyclised on the acid function. The steps used for catalyst
regeneration are also indicated, including coke elimination by
controlled burning, oxy- chlorination to redisperse the metal
function and restore the acid function, reduction
with hydrogen, and passivation by sulphiding. In the chapter by
D. B. Dadyburjor of West
Virginia University, Morganstown, U.S.A., the effects of
deactivation in changing catalyst se- lectivity are examined in a
number of hydrocarbon reactions including those in- volving p la t
inum-rhenium-sulphur on alumina. The overall conclusions from this
chapter are that the various modes of deacti- vation including
coking, poisoning and sintering, all change the selectivities of
the different reactions.
The deactivation of stationary source air emission control
catalysts is reviewed by J. R. Kittrell, J. W. Eldridge and W. C.
Conner of the University of Massachusetts and KSE Inc., Amherst,
Massachusetts, U.S.A. Supported platinum catalysts feature strongly
among the favoured candidates both for the oxidation of volatile
organic compounds to carbon dioxide and water, and for the
reduction of nitrogen oxides to nitrogen and water, where ammonia
may be used as the reducing agent (selective catalytic reduction).
The authors review the morphological changes which take place
during catalyst deactivation, deposition of poisons on the active
surface, reactions between the feed and active catalyst sites, and
solid-state trans- formations of the catalyst to form inactive
solids. The mobility of platinum on oxide sup- ports is well known;
in reducing environments at higher temperatures the platinum
particles grow to raft-like structures and eventually to large
three dimensional particles. In oxidising environments, the
platinum is also mobile on the surface as PtO,, where x < 1, but
the inter- actions wi th the suppor t can be more favourable as the
PtO, wets the surface. Conse- quently a platinum on alumina
catalyst which has become deactivated in a reducing environ- ment
can be redispersed in an oxygen-rich environment. D.T.T.
Platinum Metals Rev., 1992,36, (2), 85 85
-
Advances and Developments in Emissions Control A REVIEW OF THE
1992 SAE INTERNATIONAL CONGRESS
The Society of Automotive Engineers Inter- national Congress,
held in Detroit, Michigan, U.S.A. from 24th to 27th February 1992,
con- tinues to be the worldwide forum for the presentation and
discussion of matters relating to vehicle emissions control.
Despite the re- cession the sessions were very well attended, and
papers were presented by speakers from around the globe.
Broadly speaking, the contributions can be divided into two main
categories, gasoline- and diesel-related. A recurrent theme
throughout was the emphasis upon the fact that noble metal
catalysts, particularly in gasoline applications, are very much
part of a control system involv- ing engine management strategies
and other engine components.
Catalyst Design for Diesel Applications
Noble metal catalysts for use on diesel en- gines were the
subject of several papers, with interest in the U.S.A. focused on
heavy duty engine applications.
The optimisation of noble metal formula- tions for diesel
catalysts which could exhibit good control of the volatile organic
fraction of the particulate matter, and also limit the forma- tion
of sulphates which can otherwise increase particulate emissions was
reviewed by M. G. Henk, W. B. Williamson and R. G. Silver of Allied
Signal Inc. (SAE 920368). Platinum, pal- ladium and rhodium can all
cause the formation of sulphate but it was demonstrated that
palladium systems can be optimised to limit this undesirable
effect.
The role of a noble metal flow-through oxi- dation catalyst as
part of a strategy for the development of a low emissions
specification heavy duty engine was considered by E Brear of
Perkins Technology Ltd., and S. Fredholm and
E. Anderson of Svenska Emissionsteknik AB (SAE 920367). The
problem of sulphate gener- ation over noble metal catalysts was
again highlighted. The authors conclude that while it is possible
to modify the platinum content and thus its interaction with other
catalyst compo- nents, in order to reduce sulphate emissions, the
solution may have to include a substantial reduction in the sulphur
content of diesel fuel. This conclusion was reinforced in a paper
by R. J. Farrauto and J. J. Mooney of Engelhard Cor- poration (SAE
920557).
Autocatalyst Systems Two papers in particular provoked
excite-
ment and debate. Both considered the application of noble metal
autocatalysts as part of advanced development systems designed to
speed-up activation of the catalyst when cold. It is generally
recognised that the major portion of both hydrocarbon and carbon
monoxide emissions are produced within the first two minutes of the
vehicle drive cycle, that is while the catalyst is cold. One
potential control strategy involves the use of electrically heated
catalysts and these have been the subject of papers at previous
Congresses.
A more radical approach to the cold-start emissions problem was
described by T. Ma of Ford Motor Co., N. Collings of Cambridge
University and T. Hands of Combustion Ltd., (SAE 920400).
Substantial reductions in cold- start emissions have been
demonstrated by a strategy which causes the engine to run rich,
together with air-injection and the use of electrodes - positioned
in front of the conven- tional noble metal catalyst - for the first
few seconds of vehicle operation.
L. S. Soucha, Jr. and D. E Thompson of Corning Inc., detailed
their investigations (SAE 920093). They concluded that future
Platinum Metals Rev., 1992,36, (2), 86-89 86
-
emissions standards legislation could be achieved with an
electrically heated platinum- rhodium-containing converter, using
an extruded metal substrate in conjunction with a conventional
noble metal catalyst.
A paper by W. A. Whittenberger and D. T. Sheller of Camet Co.,
and J. Walters of Gordon- Darby Inc., gave experiences of user
vehicles equipped with electrical heated catalysts (SAE 920722). It
indicated a number of areas where improved technology might make
such equip- ment more viable; these included battery and power
control technologies.
Catalyst Design for Gasoline Vehicles
It was reported by J. C. Dettling and Y. K. Lui of Engelhard
Corporation that platinum-palla- dium catalysts could be made to
perform more like rhodium-containing catalysts for the con- trol of
oxides of nitrogen, than had previously been shown (SAE 920094).
The platinum-palla- dium systems described are 85 per cent as
effective as a platinum-rhodium system for the control of nitrogen
oxides emissions on a ve- hicle, at reduced overall noble metal
loadings. The authors conclude that it may be possible to meet
forthcoming emission standards without increasing the usage of
rhodium per vehicle.
The utilisation of palladium, this time as a substitute for
platinum, was reviewed by C. N. Montreuil, S. C. Williams and A. A.
Adamczyk of Ford Motor Co., as part of a programme to generate an
experimental data base of catalyst conversion efficiency (SAE
920096). Platinum- rhodium and palladium-rhodium catalysts at
equivalent concentrations and ratios were examined in a tubular
flow reactor. Steady state conversion efficiencies for carbon
monoxide, nitric oxide, propane, propylene, hydrogen and oxygen
through the catalysts were determined for a variety of inlet
species, concentrations and inlet gas temperatures. The results of
these ex- periments show significant improvements in carbon
monoxide and nitric oxide conversion eficiencies for both of these
catalyst systems compared with previous generation catalyst for-
mulations, when the feed-gas stoichiometric
ratio was on the rich side. The conversion efi- ciencies
obtained with the platinum-rhodium formulation were similar to
those obtained with the palladium-rhodium formulation over a wide
range of conditions. Differences were noted, however, at low
temperatures or when a high concentration of lo^" burning hydro-
carbons (propane) was present.
The subject of noble metal cost optimisation was addressed by M.
A. H&konen and €! Tal- vitie of Kemira Oy (SAE 920395). Various
dual bed catalysts containing different platinum- rhodium,
palladium-rhodium, platinum and palladium loadings, and
combinations thereof, were subjected to oven and bench engine
ageing techniques, and then examined for perfor- mance. It was
found that catalyst performance is not necessarily proportional to
the cost of the noble metals used, and that although palladium and
palladium-rhodium catalysts have good thermal ageing resistance
they are more sensi- tive to the presence of poisons than either
platinum or platinum-rhodium. Also, the addi- tion of ceria to a
three-way catalyst is more beneficial for platinum-rhodium,
platinum and rhodium systems than for palladium-rhodium
catalysts.
A prominent topic emerging from the papers dealing with
three-way catalysts was catalyst deactivation modes, and their
effects on emissions. A number of these papers dealt with thermal
deactivation and the role of air to fuel ratio.
A paper by N. A. Hannington, R. J. Brisley and R. D. O’Sullivan
of Johnson Matthey de- scribed the effects of catalyst ageing under
different temperature and air to fuel ratio con- ditions, and
compared the emissions of these catalysts to those from catalysts
aged on ve- hicles driven under European conditions (SAE 920399).
It was concluded that bench engine ageing which gave catalyst inlet
temperatures in excess of 85OoC, with significant amounts of lean
running, most closely simulated “real life” European ageing on
vehicles and that these high temperature, lean ageing conditions
resulted in greater deterioration of platinum- rhodium
catalysts.
Platinum Metals Rev., 1992,36, (2) 87
-
Information on the effect of oxygen concen- tration on the
ageing of three-way catalysts using inlet temperatures in excess of
850°C was presented by R. M. Heck, J. K. Hochmuth and J.C. Dettling
of Engelhard Corporation (SAE 920098). Higher catalyst temperatures
are seen as the converter is moved closer to the engine to reduce
cold start emissions. This, in combina- tion with lean conditions
resulting from a fuel cut-off strategy on deceleration, may give
the high temperature, lean conditions de- scribed in the paper. I t
was found that catalyst deactivation increased with higher
temperatures and with increasing oxygen concentration.
High catalyst temperatures leading to deacti- vation may also be
experienced during ignition-induced misfire. This topic was de- s c
r i b e d by C . D. Tyree o f t h e U.S. Environmental Protection
Agency (SAE 920298). Vehicles were run over the Federal Test
Procedure cycle with varying degrees of ignition-induced misfire.
It was found that hy- drocarbon emissions were increased the most
by this type of misfire, whereas much higher misfire rates were
required to increase carbon monoxide tailpipe levels to the same
extent. Nitrogen oxides emissions were reduced as the rate of
misfire increased. Catalyst temperatures could be increased by up
to 300°C while driving over the Highway Cycle with 20 per cent mis-
fire, al though dur ing the Federal Test Procedure cycle, with the
equivalent of one cylinder of a six cylinder engine permanently
misfiring, the catalyst temperature reached an average of 600°C and
a maximum of 851°C.
Catalysts may also become deactivated by poisons contained in
both fuel and lubricating oil. The effect of oil-derived phosphorus
and sulphated ash on catalyst deactivation was de- scribed by K.
Inoue, T. Kurahashi and T. Negishi of Nippon Oil Company and K.
Akiyama, K. Arimura and K. Tasaka of Toyota Motor Corporation (SAE
920654). They found that the catalyst surface phosphorus concentra-
t ion increased wi th increasing carbon monoxide and nitrogen
oxides emissions. The presence of sulphated ash reduced the
amount
of phosphorus on the catalyst but also had a negative effect on
catalytic activity. Catalyst deactivation was also much more
apparent at 800°C than at 720°C. They concluded that en- gine oils
low in phosphorus and sulphated ash can reduce catalyst
deactivation.
The effect of fuel sulphur and air to fuel ratio on catalyst
deactivation was described by J. C. Summers, J. E Skowran, W. B.
Williamson and K. I. Mitchell of Allied Signal Inc., (SAE 920558).
It was reported that relatively low con- centrations of fuel
sulphur resulted in loss of catalyst performance, but little
additional poi- soning effect is seen on increasing the sulphur
content of the fuel to comparatively high levels. This
deterioration in emissions is due to the diminished performance of
a catalyst at its operating temperature, rather than to an in-
crease in light-off temperature.
Catalyst selection for the reduction of formal- dehyde emissions
from methanol-fuelled vehicles was presented by M. S. Newkirk and
L. R. Smith from Southwest Research Institute, M. Ahuja, S. Albu
and S. Santoro from Califor- nia Air Resources Board, and J.
Leonard from South Coast Air Quality Management District (SAE
920092). The use of methanol as a fuel is seen as a viable approach
to reduce air pollu- tion; concern, however, has been expressed
about t he relatively high formaldehyde emissions from vehicles
using this fuel. This paper reported on investigations during which
the catalyst systems of several vehicles were modified in order to
reduce formaldehyde emissions. In general the strategy was to
reduce cold start emissions by using close coupled and electrically
heated catalyst systems. These tech- niques were successful, but
supplemental air injection above the catalyst was required when
using an electrically heated catalyst to optimise this system.
Conclusions The Society of Automotive Engineers Con-
gress is always a barometer to the current areas of work and
interest of the car companies and of the emissions control
industry. The need to meet forthcoming emissions legislation
clearly
Platinum Metals Rev., 1992,36, (2) 88
-
drives development activities. This was high- lighted by the
papers which considered improved cold-start performance and in the
attention given to catalyst deactivation mechanisms, by
The application of noble metal catalysts for diesel emissions
control continues to be a major part of the Congress, as are papers
considering the substitution of platinum or rhodium by
temperature and/or poisoning. palladium. C.J., R.D.O’S.
Platinum Group Metals in 1991 The Ayrton Metals Platinum
Yearbook 1992 BY B. H. NATHAN, Woodhead Publishing, Cambridge,
1992,195 pages, ISBN 1-855734847, E45.00
Following in the footsteps of the “Platinum Yearbook 1991”, this
new publication sets out to present a panorama of the many events
that influenced the market for platinum group me- tals during 1991.
These are covered in general terms, and in considerable detail in
Chapters 1 and 2, respectively, both being supported by statistical
data. A number of these events were industrial announcements or
scientific reports, but most were not, and the whole makes fasci-
nating reading, perhaps especially for those whose involvement with
the platinum group metals does not embrace metal dealing.
While the detailed review of 1991 occupies ninety-nine pages,
the prospects for the six platinum group metals in 1992 are
contained in some five pages. It is suggested that of these metals,
“platinum has the best prospects for a sound and widely-based
recovery in 1992”.
A chapter on the Tokyo Commodity Ex- change by Kazuhiko Noma is
complemented by two further chapters by Brian Nathan, in which the
dealing arrangements for the London Plati- num and Palladium Market
and the New York Mercantile Exchange are considered.
An interesting overview of fuel cells and their state of
development for stationary and trans- portation applications is
given by Jocelyn Cloete. Once again, it is concluded that proof of
phosphoric acid fuel cell technology will be provided in the
imminent future, from the re- sults of on-going field tests.
In the penultimate chapter Peter Gaylard out- lines the lengthy,
complex and expensive processes that are necessary to extract the
plati- num group metals from the limited number of major ore bodies
in which they occur, in minute quantities, and to then refine them
to the re- quired high purity levels. In response to the needs of
the market, and to a greater awareness of environmental
considerations, improved re- covery processes have been introduced
in recent years.
A brief chapter on the history of platinum and the platinum
group metals, based in part upon “A History of Platinum and its
Allied Metals” by D. McDonald and L. B. Hunt, con- cludes this
informative and interesting book which will serve as more than a
record of the events of the past year. I.E.C.
Platinum Improves Protective Coatings Gas turbine engines are
widely used for both
stationary and mobile applications, and the tur- bine blades,
which are highly stressed during service, are required to operate
at high tempera- tures in oxidising atmospheres which may be
contaminated with corrosive fuel residues and ingested salts. To
some extent nickel-based superalloy turbine components can be pro-
tected against both oxidation and hot corrosion by nickel aluminide
diffusion coatings, but in more severe environments the protective
coat- ing may break down, reducing service life.
The development of platinum-containing coating systems has been
reported here on sev- eral occasions over the past decade as
materials
scientists have sought both to improve the pro- tection given by
such coatings and to establish the precise role of the platinum in
the process.
A further contribution on the subject has been published
recently (H. M. Tawancy, N. M. Abbas and T. N. Rhys-Jones, Sut$
Coat. Tech-
Following an investigation of the microstruc- ture of
platinum-modified aluminide coatings on selected nickel-based
superalloys, the authors identify a number of ways by which the
platinum improves the protective ability of the coating. Oxidation
behaviour depends upon the composition of the superalloy substrate,
espe- cially on its rare earth content.
not?., 1991,49, (1-3), 1-7).
Platinum Metak Rev., 1992,36, (2) 89
-
The Large Scale Production of Hydrogen from Gas Mixtures A USE
FOR ULTRA THIN PALLADIUM ALLOY MEMBRANES
By V. Z. Mordkovich, Yu. K. Baichtock and M. H. Sosna State
Institute of Nitrogen Industry, Moscow, Russia
A method of pure hydrogen production using palladium alloy
membranes to separate hydrogen from hydrogen-rich gas mixtures has
been employed for many years in labora- tory and industrial
practices. Generally, palladium-silver membranes have been em-
ployed. Over the years a number of papers describing this method
have been published in this journal (1-4). There are many publica-
tions by British, Japanese, Russian, etc., producers describing
such units with produc- tive capacities ofup to 100 Nm’/h. These
units are intended for the purification of technical grade hydrogen
and operate in areas such as rare element metallurgy, the
electronic indus- try and general laboratory practice. Recently it
has been reported by Johnson Matthey (3, 4) that units capable of
separating up to 50 Nm’/h of hydrogen from methanol-water cracking
gas have been used as constituents of self-contained hydrogen
generators.
One may ask the obvious question: why not use palladium alloy
membranes for large-scale hydrogen production? It is easy for
anyone to obtain the answer that this method does not provide a
sufficient return on capital invest- ment, due to the high cost of
the noble metals involved. It is less evident that both the du-
rability and the hydrogen recovery level of the usually employed
membranes are also some- wha t de f i c i en t . Indeed , t h e ex
i s t ing membrane units are generally used only for relatively
small-scale purification of technical grade hydrogen, and not for
its separation from mixtures containing less than 95 per cent
hydrogen.
Nonetheless, in principal, membrane tech- nology promises
significant advantages in
comparison with conventional technologies. Palladium alloy
membrane units combine the compactness of polymer membrane units,
the high product purity of the pressure swing ad- sorption
technique, the recovery level of cryogenic separation and
furthermore the ability to utilise great pressure drops as the
directives for rapid permeation. There are, however, a number of
reasons which prevent the attainment of either the investment
return or the durability and recovery levels demanded in the case
of large-scale hydrogen-rich mixture separation technologies.
In this respect, five key problems may be emphasised, namely,
choice of membrane ma- terials, minimisation of membrane thickness,
design of the membrane holder, design of the membrane apparatus and
start-up/shut-down technology. All the listed advantages can be
realised and the disadvantages overcome by solving the key
problems.
Key Problems of Palladium AUoy Membrane Technology
The choice of membrane material is the first highly significant
problem. The palladium alloy has to possess excellent hydrogen per-
meabili ty a n d be resistant to specific start-up/shut-down cyclic
stresses. It is well known that hydrogen-palladium alloy interac-
tions can induce the a-p phase transformation, which consequently
alter atom spacings in the metal lattice, causing dimensional
changes which are large enough to distort the mem- brane ( 5 , 6 )
. Thus conditions which favour the existence of p- and P-like
hydride phases are dangerous in regard to the possibilities of
membrane destruction. The task is therefore
Platinum Metals Rev. , 1992,36, (2), 90-97 90
-
Fig. 1 The variation of hy- drogen permeability, W, with
pressure drop for six differ- ent palladium alloys:
ure=O.l MPa
loo
T=50°C, output press- 80
E
E 60
0 X c
'- 4 0
20
-/
Pd-lOAg-55N
Pd-4 TI
e- Pd;5.5Ni
5 10 15 2 0 PRESSURE DROP, MPa
to develop an alloy which forms hydride phases only at
temperatures substantially dif- fe ren t from t h e opera t ing
conditions. Palladium membranes have usually been operated in the
temperature range 400 to 600°C, within which there i s good per-
meability and satisfactory durability.
The start-up/shut-down procedure has to be able to avoid hydride
phase formation during its operation. Additionally, it should be
em- phasised tha t internal stresses and the consequent cracking of
membranes are often induced by reasons other than p-phase forma-
tion. Certain palladium alloys seem unsuitable for use as membrane
material for large-scale technology as they have insufficient
resistance to specific start-up/shut-down stresses. For example,
the widely employed 25 palladium- 75 per cent silver alloy has low
cyclic resistance and often cracks even if apparently suitable
start-up/shut-down procedures are employed.
Different alloys also vary in hydrogen per- meability, and this
is illustrated by Figure 1. It may be noted that the permeability
of the best alloy is about five times as high as that of the worst.
In recent years a number of alloys with
large permeability and high cyclic resistance, such as the
Soviet B-X group of alloys, have been developed (7). Such alloys
may contain
(O-1)Pt-(O.O1-O.5)Al-(balance)Pd. Minimisation of membrane
thickness is the
second key problem. Clearly, a decrease in membrane thickness
while maintaining a con- stant membrane surface area, results in a
proportional reduction in the cost of equip- ment. Furthermore,
permeability through thin membranes is proportionally greater than
through thick membranes. When calculating the dependence of the
overall investment (namely, capital cost per unit of productive
capacity) on membrane thickness, the decisive significance of the
membrane thickness can be demonstrated convincingly. Such
dependence is shown in Figure 2 for the investment level of the
entire process of hydrogen production from natural gas where the
cost of the 0.1 mm thick membranes constitutes about half of the
total capital cost. It can be seen that 0.05 mm thick membranes
provide an investment level near that of conventional technologies,
while the use of 0.01 mm thick membranes would result in the most
inexpensive technology for
( ~O-~~)A~-(O.O~-~)AU-(O-~)Y-(O.O~-~)RU-
Platinum Metals Rev., 1992,36, (2) 91
-
005 0.10 0.15 0 2 0 MEMBRANE THICKNESS, mm
Fig. 2 Effect of membrane thickness on specific investment, L,
is shown for the en- t ire large-scale process of hydrogen pro-
duction from natural gae (output of about 10,000 Nm3/h of
hydrogen). This process principdy includes steam-& conversion
of methane followed by diffusion throqyh palladium alloy membranes.
The asterisk and consequent dotted line indicate the in- vestment
level of an alternate process where pressure swing adsorption is
used instead of membrane diffusion
the large-scale production of hydrogen from gas mixtures.
Such thin membranes could be destroyed, however, if there were
substantial increases of the operating pressure drop across the
mem- brane, and at present membranes for the separation of mixtures
are required to work at pressure drops of between 1 and 20 MPa.
Nevertheless, an increase of pressure drop would result in a
greater productive capacity and recovery level. To summarise, there
could be a difficult situation in trying to balance the problem of
minimisation of membrane thick- ness; on the one hand, a decrease
in the thickness can critically improve the entire technology,
while on the other hand very thin membranes may not be strong
enough to work at the conditions demanded.
This problematic situation can be partly re- solved by the use
of a suitably designed membrane holder, which constitutes the third
key problem area. A simple classification of
membrane holders is presented in Figure 3, the holders being
divided into “tube”, “plane- foil” and “folded-foil”. Three
examples of holder design are also shown in Figure 3, namely
“disc”, “capillary with external press- ure” and “capillary with
internal pressure”.
Each type of membrane holder may be fur- ther characterised by
the type of connection between the palladium alloy and the
adjoining material. Welding or soldering has been used, but it is
difficult to weld thin membranes and hence soldering often seems to
be the more attractive method. On the other hand strong erosion can
occur during the course of the soldering operation due to
dissolution of pal- ladium alloy in the liquid solder; and even if
this dissolution is not complete, the boundary alloy formed can be
easily damaged by hy- drogen and by the mutual diffusion of
components at high operating temperatures. The use of hard silver
or gold solder can be advantageous, although the operational life
of capillary holders with silver solder is usually not more than
one or two months. Different kinds of welding may be used, such as
arc welding, pressure welding, electron beam and laser welding.
Pressure tight welds can be pro- duced by each of these methods,
although mutual diffusion of palladium alloy and steel components
at the operating temperatures can still constitute a significant
problem which may result in the destruction of either the membrane
or the weld. Residual after-weld defects and stresses also have to
be taken into account, although multilayer welding tech- niques can
usually avoid these difficulties.
There can be a large variety of alternative membrane holders,
many of them having the same defects, such as low ranges of
practical pressure drop and short operational life. It seems that
the first of these limitations can be removed in the case of
“plane-foil” holders with membrane support, and also by the use of
“capillary” holders. In the present paper “tube” holders with tube
diameters of less that 1 mm and wall thicknesses of less than 0.1
mm are designated as “capillary” holders. “Plane- foil” holders
have an evident advantage of
Platinum Metals Rev., 1992,36, (2) 92
-
MEMBRANE HOLDERS
folded-toi I plane-foil
Fig. 3 Membrane holders can be classified as “folded-foil”,
“tube” and “plane-foil” and the latter two ca- tegories can be
further sub-divided
others disc %
lower cost, due to the lower cost of foil in comparison with
that of tubes. On the other hand, “tube” holders also have certain
par- ticular advantages especially in the case of “capillary”
holders, which allow thin mem- branes to be operated under large
pressure drop conditions without any membrane sup- port. Experience
has shown, however, that the membrane support can provoke corrosion
of the palladium alloy, and in addition the sup- port can create
difficulties for optimum gas flow and can reduce the purity of the
output hydrogen. The design of compact gas collec- tors is more
successful in the case of “tube” holders, but this last
consideration is close to another key problem, which is discussed
below.
The fourth key problem is the overall design of the membrane
assembly. As mentioned above, one aim is to create compact
equipment
and this is easy in the case of “tube” membrane holders but
“plane-foil” and “folded-foil” holders demand more complex
collectors for the purified hydrogen. Designs must also pro- vide
optimum rates of gas flow near the membrane surface in order to
attain maxi- mum levels of productive capacity and recovery.
Necessary temperature conditions near the membranes can be
achieved either by external heating of the apparatus or by
preliminary heating of the gas mixture. The later seems preferable
for large scale hydrogen produc- t ion, a n d i t should be noted t
h a t if preliminary heaters are used without heat ex- changers
then the expenditure on energy can constitute up to half the cost
price of the hydrogen produced. The membrane apparatus adopted
needs to be equipped with internal heaters for the gas mixture and
internal heat
Platinum Metals Rev., 1992,36, (2) 93
-
exchangers in order to conserve the heat, while the outlet gas
temperature should not be more than 200°C.
The development of optimum start-up/shut- down technology, which
is the fifth key problem, has been partially discussed above. I t
should again be noted that this technology requires specific
control of temperature and pressure, and the use of certain
alternative blow gases has also to be considered. For example,
heating to the operating temperature and cooling during the course
of shut-down can be usefully carried out in a nitrogen at- mosphere
in order to obtain a long operational life for the membranes.
GIAP Style Development The problems reviewed above are
common
key problems for any organisation which is going to attempt to
improve palladium alloy membrane technology. The solutions to these
problems and the relative significance of each depend, however, on
which branch of industry the proposed technology is principally in-
tended for.
The State Institute of Nitrogen Industry
(GIAP) began to investigate palladium alloy membrane technology
for the nitrogen indus- try more than twenty years ago. T h e
conditions and the needs of the nitrogen in- dustry may be
characterised as:
(a) high pressures of technological and waste hydrogen-rich gas,
from 3 to 32 MPa
(b) a wide range of hydrogen content in the mixtures, typically
40 to 75 volume per cent
(c) the presence of nitrogen, methane, am- monia, water, carbon
dioxide, and carbon monoxide as possible components of the mix-
ture and also the probable occurrence of small amounts of oil
(d) rapid growth in the demand for rela- tively low cost
hydrogen. With regard to the last item it should be emphasised that
the hydrogen-rich mixtures occurring in the ni- trogen industry can
provide a cheap source of pure hydrogen if appropriate technology
can be developed for its recovery.
Accordingly, certain directions of research were decided upon
and resulted in the devel- opment of an original GIAP style in
regard to metal membrane technology.
The use of the B-X series of alloys has been
Platinum Metals Rev., 1992,36, (2) 94
FK. 4 A variety of capillary membrane holders designed by GIAP
are displayed
-
one of the most important directions of im- provement. These
alloys possess good hydrogen permeability and the membranes can
provide operational lives of more than two years. The B-X alloys
are resistant to “poison- ing” by constituents present as
impurities in the gases from the nitrogen industry. These
conclusions were formed in the course of long term testing carried
out by GIAF!
Different types of membrane holders were also designed and
tested in GIAP, the most advanced being “capillary with internal
press- ure” and “capillary with external pressure”. These are shown
in Figure 4. B-X capillary tubes with wall thicknesses of from 0.05
to 0.1 mm are used and the free end of the tubes are sealed. The
hydrogen-rich mixture flows along the external surface of the tubes
and pure hydrogen is ejected from the internal tube space due to
the pressure difference be- tween the open and the sealed ends of
the tube. The “capillary with internal pressure” holders have two
heads, and both ends of the tubes are open. The mixture flows into
and around the internal space of the tubes and pure hydrogen is
removed from their external surfaces. Due to the successful design
of the holder head and the development of steel to B-X powder
welding technology (8, 9), the GIAP membrane holders in both
“internal’’ and “external” variants can operate at pressure drops
of up to 30 MPa and are characterised by ope