Note: Descriptions are shown in the official language in which they were submitted.
0 6 3087
This iZnvention relates to a process and apparatu~
for preparing a catalyst comprising a catalytic metal deposited
onto a low area refractory monolith coated with a film of a
high area oxide such as alumina. More.particularly, the -.
invention relates to a process and apparatus for impregnating
the coated monolith with a precisely controlled weight of
metal and for producing a monolithic cat-lyst that i8 useful
as an automotive emission control catalyst.
Rigid, unitary or homogeneous skeletal structureJ,
. . . . .
10 which are often referred to as monolithic st~uctures, have
been widely used in the manufacture of cataiysts for converting
waste gases containing combustible pollutants. This is true
. because of the compactness and ease of handling and replace-.
~i ment of the monolithic structures. These structures are
'I generally manuactured so as to have a plurality of channels
~ or passageways for the waste gases. .A thin film of high'sur- - '~
;î'i face area, inorganic o~ide is'deposited on the low area #-
:`~ . . .
.;Z fractory structure. The coated monolithic structure is usual-
:~ . ly imprçgnated with-a oatalytically active metal such as a
platinum group metal. The re~ulting cat~lyst can then be
' inserted into suitable canister for the conversion of waste
gases.
Prior eo the present invention, the only _ t W known
Z ~ for i~pr~gnating monoliths with a precisely controlled weight
:'. ` .~ ' :of platin~m group ~etals was~to soak the monolith in a bath
~: ~ of solution containing o~actly the weight of metal assignei
. ;~ to the monolith. This method does impregnate a precis-ly
oontrolled woight of.metal, but it is much too slow for com- .
.. ~ mercial production. The monol~th.must be soaked for.hour~
. '~ 30: or even days to ensure that all of the platinum group metal
: ia the ba~h has.been deposited on the monolith. An obvious
. method for impregn~ting a monolith i8 simply to dip it into.. :
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. 10 6 30 87
a solution, remove it from the solution, and ~pin it to remove
the excess solutionO This method i8 fast, but it does not
deposit a precisely-controlled weight of metal'on the monolith.
The water sorption capacity will vary among.coated monoliths,
even among monoliths from the same production runO The weight.
of metais impregnated by this method must vary likewise. When
the metals being impregnated are expensive platinum group '
metals, it is essential to deposit a precisely controlled
.
~ . weight of metal on each monolith.
;: 10 With the present invention,'there is provided a
; process for preparing an automotive emission control cataly~t
in which a catalytic'metal is rapidly ~mpregnated onto the
~i . . . .
' monolithLc structure coated with an.active oxide film. The . .. ... -
~ process comprises t~e steps of: '
,' - ' (1) spinning a catalyst support comp~ising a ..
. monolithic structure coated with an active
- Q 1~o~
' metal oxide, having tnt inner core extending
longitudinally therethrough, and having
radially extending channels,
,~ . , .
'' '20 (2) spraying a stream of a solution of a .
'~J, catalytic metal against the inne.r core
~;' 'while the support is being spun about
. .its longitudinal axls,
. (3) continuing step (2) until substantially .'
. the ent~re surace of the channels has
. ~ been impregnated with the'catalytic met~
. and '
~ (4) .calcining the impregnated support to
7 .. a temperature in the range of 400- to 1800-F.
r'i . ~ , ' . . .
Onc of the monolithic structures contemplated for
use ~n the proceis of tho present invention comprlses a
. . substan~lly continuous thread of a ceramlc mater~al such ..
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~063087
as alpha alumina, sillimanite, petalite, cordierite, (2MgO.
2A12O3.5SiO2), mullite, (3A2O3.2SiO2), zircon, zirconmullite,
spodemene, magnesium silicates, alumino-silicates and the like
criss-crossed into a herringbone pattern to form a cylinder
having a longitudinal inner core therethrough and a plurality
of random channels or passageways. A portion of the channels
extend radially from the inner core. The diameter of each
channel must be at least about 0.5 mm to enable the solution to
flow during the impregnation step.
The types of active metal oxides used to coat the
monolith include high surface area materials such as alumina,
silica, zirconia, alumina-silica, alumina-zirconia and the
,~:
like. A particular preferred active metal oxide comprises alumi-
na which is applied to the monolithic support in the form of a
~-' suspension or hydrosol and is then converted to gamma alumina
during the calcination step.
The alumina suspension is formed by digesting an excess
~ of aluminum metal with acid selected from the group consisting
~ of HCl, HBr and HI or with an aluminum salt such as aluminum
chloride, bromide or iodide until hydrogen ceases to evolve.
The resulting suspension or hydrosol is in contrast to the col-
loidal suspension of descrete particles of alumina. A detailed
description of the preparation of such a suspension is found
;~ in U.S. Patent 3,346,336.
~; The monolithic structure may be coated with the alumina
suspension by conventional techniques well known in the art or
it may be coated in accordance with the process disclosed and -
claimed in Applicant's copending Canadian Patent Application
Serial No. 231,579, filed July 16, 1975, and entitled "Process
for Coating a Refractory Support with an Active Metal Oxide Film".
Briefly, the process comprises: ~ -
' ~,1
~ C
.~ . .
- ` .` ` 10 6 3087
. (1) dipping the monolithLc structure into
a suspension of an active metal oxide,
(2) removing the'resulting coated structure
. from the s'uspension,
(3)- spinning the coated structure until essen-
' tially all of the excess suspension is
: removed to leave an active metal oxide
film o$ 4 substantially uniform thickness,
and
.~. 10 (4) calcining the coated structure to a
temperature in the range of 750 to 1800F.
~ . The spinning step of the present invention can'be
.'~'' carried out by clamping or otherwise securing the coated - : .
~. monolithic structure in a'device to impart sufficient-centri-
fugal force so tha~ the solution that is sprayed onto the
: ~nner core of the monolith impregnates the active`alumina
~'~ . coating substantially uniformly throughout the coated.mono-.'
.~ lith. It has been found that the force required.is in the
.~' range of about-2 to l5 G's. The t'ime required for the spinning
~ 20 and spraying steps is very short, on the order of 1 second to
. .
l 100 seconds, preferably 2 to 10 seconds~ ~ -
;~ Two alternate means for carrying out the spraying step.
'~' of this process are decr~bed below in connection with a detail-
, .
~; ed discussion of the drawing. In general, a vessel containing
.~ . '' 'an aqueou~ solution of one or more catalytically active metal
1i '
. ~ ~ ' is in fluid communication with the inner core of the coated ' ~.
. monolith. A spray of the ~olution i8 pumped from the vo9sel
~ ~ against the surface of the inner core of the spinning monolith.
.. ~ The active metal preferably i8 selected from a Group
-~ 30 VIII noble metal, e.g. plati~um, palladium, rhodium'and mi~-
`~ . tures thereof. The support is impregnated with an aqueous
~ solution'of chloroplatinic acid, platinum chloride, ammonium
.i`i; :
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1063087
chloroplatinate, dinitrodiami'no platinum or the like. The
impregnated support is then oxidized and/or reduced to yield
the platinum metal in an oxidized or reduced state.
The amount of the catalytic component varies depending
on the particular end-use applicationO In general, the amount
will be in the range of 0.05 to 10%, preferably in the range
of 0.1 to 1%, based on the weight of the total supported cata-
lyst. In the case of the noble metals, it is critical that
only that amount of metals required in the finished monolith
is sprayed against the inner core. This gives preci'se
control of'the weight of noble metal added to the monolith.
'~ ' Therefore, in one'embodiment of this invention, a syringe
contains an amount of the solution that is just suffic'ient
.
' to result in a finished catalyst having the desired amount '
of the meeal. ' -
` ' -' The calcining-step comprises heating the impreganted
monolith at temperatures in the range of 400-1800F, prefer-
ably 400-1100F in an oxidizing atmosphere such as air. Prior
to the calcining stëp, the impregnated monolith may be'dried
at temperatures in the range of 200 to'400F, although this
step is not necessary in the preparation of the catalyst.
In the drawings: '
~! . .
; Fig. 1 is a front view, paFtly in cross-section, ofone mean~ for carrying out the spinning and spraying steps of
^~ the process of the present invention;
' Fig. 2 is an isometric view of the monolith structure
;' employed in the process of the present invention; and
l Fig. 3 i8 front view of another means for carrying
'~ out the spraying step of the procéss of the present inven~ion.
Referring now to Figs. 1 and 2, coated monolith 10
having inner co~e 11 extending longitudinaLly therethrough
. ~8 secured in place by means of tie rods 12 mounted between
3 - 6 -
~ - , - , ,
1063087
upper clamp'14 snd lower cl~mp 16. Shaft 18 is fixedly sttach-
ed at one end to lower clamp 16 and is connected at the other
end to drive means 20 to spin the monolith 10. The solution
of catalytic metal.is contained within'syringe 26 between
plug 28 and pis~on 30-fixed in space. Syringe 26.is designed
so that the interior of its cylindrical sidewall is in slidable
engagement with piston 300 While monolith 10 is spinn ffig, .
syringe 26 is manually forced upward causing.the sidewall to
slide over piston 30 until plug 28 contac'ts piston 30. This
causes solution 24 to be expelled through tube 32 and nozzle
34 against the surface-of core llo The length of travel of
syringe 26 is ~ust equal to the length of core 11 that i8
being sprayed. The solution is uniformly'distributed along.'
the entire length of core 11, the uniformity of which iQ not ':
dependent on,the rate of the ascent of syringe 26. Therefore,
the means used for forcing the solut~on from the syringe is
not critical to the'present invention.. Centrifugal force makes .
the solution flow radially outward from core 11 so that the '
surface of the entire nest of channels is uniformly impregnated.
.'~ '20 Collecting vessel 40 encircles monolith 10 and is '
provided t'o catch any e~cess solution that may penetrate - '~
through monolith 10 to the exterior surface.
The impregnated monolith is then caLcined at tempera~
tures of 400'-1100F to convert the metal to its activated,
oxide.,
It is contemplated in accordance with the process of '
this invention that a series of spinning and spraying lin ~
can be pDsitioned to all'feed into one common drier-calciner
for continuous or semi-continuous'operation.
., ~ 30 Referring now to Fig. 3, an slternate means for spra~-
ing the s'oiution'24 onto inner coro 11 is shown in whlch vo~
48 is in fluid communication by means of tube 50 containing a
_ 7 _
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10 6 ~087
plurality of evenly spaced nozzles 52. Pump 56 pumps the 901u-
tion from vessel 48 through tube 50 and nozzles 52 to core 11. ,
This alternative el'iminates the necessity for requiring a
syringe to move as the inner core is being-sprayed.
The Examples helow illustrate the process of the present
invention for the preparation of catalysts and their use a~
automotive emission control catalysts.'
. .
~ The four monoliths used in these Examples were ceramic '
" lO' alpha alumina monoliths that resemble a herringbone-wrapped
ball of string in which the strands cri~s-cross over one another
to form a woven'cylindrically shaped nest having an axial'hole
therethrough as shown in Figs. 1-2. The wrapped strings'form
;'~ ' channels, some of which are approximately normal to the longi- -
,, tudinal axis of the monolith so that the ultimate flow of
,l, exhaust gas when the finished catalyst is placed ~n catalytic
, muffler is either from the axial'hole radially outward through
, the nest or,from the exterior surface through the nest to the
,, axial hole. The dimensions of the monolithg were: height'
~, 20 15 cm., outside di,ameter lO cm. and core diameter 4.5 cm.
The bulk volume of the monolith was one liter.
. ~ . .
The monoliths were coated with active alumina by the
following method: '
, Rubber stoppers were mountet in e~ther end of the axial -~
opening cf the monolith and an'arbor of a stainless steel rod
was mounted through axial openings in each of the st,oppers.
' The arbor was placed in the chuck of an ,Arrow Engineering air
.~, ,
~' drive motor.' Into a beaker was placed an alumi~a hydrosol
having a density of 1.41 g./cc prepared by diiesting 1300 8.
of alumina shavings with 1 liter of hydrochloric acid plu~
' ~3'1iters of water under a reflux condenser for about 100 hours.
The undissolved alu~inum was advanced to the next batch, The
:~
, - 8 -
.. . .- , . . .
1063087
'beaker was lifted around the arbor to completely immerse the
monolith for S to 10 seconds at room temperature, and beaker
was then lowered. The monolith was rotated at a centrifugal
force of about 2 G's for 10 to 20 seconds until the coated
monolith was almost dry to the touch. The alumina hydrosol
that coated the monolith was uniformly dispersed over the
' entire surface thereofO There was no reflective whitenessO
'' All of the passageways in the m~nolith were open and free
of alumina deposit. The coated monolith was then dried at
' 10 a temperature of 250-F and calcined at 1100F in air ffi a
muffle furnace for a period of 2 hours. The foregoing pro-
cedure was repeate'd three times in order to deposit Ool gram8
' of alumina coating per gram of the bare monolith, i.eO lQ%
by weight alumina b~sed on the finished catalyst support.
The 1100F calcination temperature of the coated
mono~ith after each-dipping step was sufficient to set the
`~ alumina sol on the m'onolith and to drive off a sufficient
amount of the hydrochloric acid so that the gamms alumina
' coating would not soften during the next dipping step. How-
' 20 ever, the temperature of the calcination'after the last dipping '
step was increased to about 1450F in order to reduce the
1 chloride ion content of the finished catalyst and thus maximize
rl . its activity. The final calcining at 1450F was conducted for
`~J 2 hours.! . -
.~ . . . .
''~' Each of the four monoliths was impregnated with the
apparatus in Figo lo The weights of platinum and palladium
impregnated onto'each monolith we-re substantially the same,
1.2 and 0.24 gm. respectively. The volume of solution sprayed
' was almost constant at'74 to 76 cc. The platlnum was aided
30 ' to the solution as H2PtC16 and the palladium as PdC12. When
HCl was the'add~tive in the solution, the amount was about'0.5
-' cc., when the additive was acetic acid, the amount was about
.: _ g _
~ .
: . .
' . '
10 6 3o~37
10 cc. The time for spraying was 50 to 60 seconds. After
impregnating, the monoliths were dried at 150C. and calcined
in air at 600F. Two cores were cut from each of the mono-
liths and were .tested in an engine laboratory at 35,000 GH~V.
The purpose of cutt ffig two cores from each monolith was
simply to.provide a duplicate check.
After being tested fresh, the cores were aged for 24
. .
hours at 1800F and tested again. All of the test results ..
are set forth in Table I below. All four of the monoliths
showed good activity, both fresh and after aging.
.
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