Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
.
3 ~ ;~
Thïs invention relates to a process for preparing a
catalyst support composed of a low area refractory monolithic
,~ .,
~ structure coated with a film of a catalytically-active refr~c-
~s
tory metal oxide. More particularly) the invention relates
to a process for applying a coating that adheres tightly to
the refractory structure and is of substantially uniform
thic~ness.
Rigid, unitary or homogeneous skaletal structures,
which are often referred to as monolithic structures, have
~been widely used in the manufacture of catalysts for converting
waste gases containing combustible pollutants~ This is true
because o~ the compactness and ease of handling and replace-
ment of the monolithic structures. These structures generally
;,
have a plurality of channels or passageways for the waste gases
~; A thin film of high surface area, inorganic oxide is deposited
on the low area refractory structure. The coated monolithic
structure is usually impregnated with a catalytically active
metal such as a platinum group metalO The resul~ing catalyst
' I .
can then be inserted into a suitable canister for the conver-
1 20 sion of waste gases.
.!' Difficulties have been experienced in coating the
~i monolithic structures with the high area oxideO The main
; difficulty is in applying a coating of the high area oxide
;1 that is uniform and which does not plug any of the channels..",,, ;.
Non-uniformity of the film causes the channels or passageways
1~ to become plugged. This in turn causes a loss in ~he effecti-
i veness of the catalyst and an increase in the pressure drop
~ across the canis~er.
`~ With the present invention, there is provided a
process for preparing a catalyst support in which a unifonmly
distributed, smooth, tightly adhering coating of an acti~e ~,
metal oxide film is depos~ed on a l~w area monoLithic s~ruc- -
3 ~ 3
~ ture without the plugging problem~associated with the prior
- artO The process comprises the steps of:
(1) dipping the monolithic structure into
. a suspension of an ac~ive me~al oxide,
.: (2) removing the resulting coated struc~ure
- from the suspension9
(3) spinning the coa~ed structure until
essentially all of the excess suspension
. is removed to leave an active metal o~ide . :
~. 10 film of a substantially uniform thickness,~ and .
~ (4) calcining the coated structure to a temperature
.. in the range of 750 to 1800Fo
One of the monolithic structures contemplated for
`~ use in the process of the present invention comprises a sub-
,~;
stantially continuous thread of a ceramic material such as .
alpha alumina, sillimanite, petalite, cordierite, (2MgO-3A1203-
.: 5SiO2), mullite, (3Al20302SiO2), zircon, zirconmullite,
spodumene, magnesium silicates, alumino-silicates and ~he like
criss-crossed into a herringbone pattern, to form a cylinder .
having a longitudinal inner core therethrough and a system of
,j,, . , :~.
~1 random passagewaysO The diameter of each passageway mus~ be
.': ~l .
~: at least about 0.5 mm ~o enable the suspension to escape
dur~g the spinning stepO
~i~ The other type of monolithic structures contemplated
. for use in this invention comprises extrudates or laminates of
.f ceramic material having a plurality of ad~acent, parallel and .
u~idirectional passageway~s. These monoliths are ~ommonly
~;~. referred to as honeyeomb monoliths. Since the axes of the ;`
passageways in the ho~eycomb monoli~hs all extend in a longi-
~` tudinal directlon, they must be spu~ so ~ha~ the axis cf ~o-
.,; . :
: tatio~ is normal ~o the axis of the passag~ways.
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- The ~ypes o active metal oxides contemplated for
use in the present prooess include high surface area materials
such as alumina9 silica~ zirconia~ alumina-silica, alumina-
- zirconia and the likeO A partlcular preferred active metal
'oxide comprises alumina which is applied to the'monol'ithic
~' support in the form of a suspension and is then converted to
;~ gamma alumina during the calcining stepO
The alumina suspension is formed by digesting an ex-
cess of aluminum metal with acid selected from the group con-
sisting of HCl, HBr and HI Dr with an al~inum salt such as
" aluminum chloride, bromide or iodide, until hydrogen ceases
to evolve. The resulting suspension or hydrosol is water
whi~e and transparent in contrast to a colloidal suspension
of discrete particles of alumina. A detailed description of
the preparation of such a suspension is found in U.S. Patent
NoO 3,346,3360
; After the monolith is dipped in the suspension to
,.,. j .
''~ wet all of the passageways, the monolith is spun at a centri ~'~
'l, fugal force of at least 2 G's for a period of time sufficient...1
";l 20 to remove all of the excess suspension and to for~ a unifonm,'''; smooth, tightly adhering coating of the suspensionO This ' period generally ranges from 1 to 30 seconds.
The calcining step comprises heating the coa~ed
monolith at temperatures in the range of 750 - 1800F~ pre- '~
ferably 1000 - 1600F or about 0.5 to 5 hours~ -
In the drawing:
Ir~
.,
'' Fig. 1 is an end view of one means for carrying out
~; the dipping step of the process of the present in~ention; and
; Figo 2 is a side view of one means for oarryi~g o~t
the spinning step of the process of the present in~ention.
.. .
;~ Referring now to Figs~ 1 and 2, there is provided
trough lO having no more than a por~ion of its vol~e occupied ;'
. , .
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:; ~Lff~ ,31;~
by alumina suspension llo Honeycomb-type monoli~hs 12 are
shown within baskets or buckets L4 having a plurali~y of per-
forations or apertures 15 therein~ The four bucke~s shown form
a cluster, each bucket of which is f;xedly attached to o~e end
of rod 160 The other end of rod 16 is affixed to mandrel 180
A plurality of clusters are laterally spaced (not ~hown) along
mandrel 180 Strap 20 is operably engaged within catch 22 o~
bucket 14 ~o firmly secure monolith 12 within the bucket dur-
ing ~he dipping and spinning steps. Strap 20 should be mounted
, . .
10 within catch 22 so that coated m~nolith 12 can be easily remov-
~; ed from bucket 14 after the spinning step and replaced by an-
:, . ., - .
~ Ot~er monolith to be coated. The only contact strap 20 has
s with monolith 12 is at its outer circumference so that suspen-
,;,l sion 11 can easily enter all of passageways 23 during the dip-
,;. I ,.
',' ping step andexcess suspension 11 can exit during the spinning
,' step.
. 1 ~
During the dipping step, mandrel 18 is sl~wly revolved
by drive means 26 so that each monolith 12 is immersed in sus~
pension 11 to thoroughly wet passageways 230 After all of the
~,~ 20 monoliths have been dipped, mandrel 18 is vertically raised
into the position show~ in Figo 2 by any suitable means (not
~, shown) well known in the artO Monoliths 12 are then spun at ;;
about 2-10 G's for approximately 5-30 seconds to remove excess
¦ suspension. The coated monoliths are removed from each o~ the
~;' buckets and calcined at temperatures of 1000-1500F ~o convert
., the alumina suspension to gamma alumina. This arrangement per
fl mits the optimum efficiency ln coating the monolith with a
`;, unifonm film of alf~mina sol, preferably in the range of about
0.004 to 09001 inches thick.
~ 30 The al~mina-coated monolithic structures of this
;'f;'` invention are particularly adapted or use as unitary cstalyst8
~ - in ~ehicular catalytic converters. Such catalysl:s compri3~
~. , .
: .
..... . . -
: . . . . . ... .. .
~ 3
;~ the coated monolith composited with one or more catalytically
active me~allic components, preferably from ~he Group VIII
noble metals, eOgO platinum, palladium and rhodium. Ot~er
components include one or more of the oxides of copper9 iron,
nickel, cobalt, chromium, manganese, tin, vanadium, tungsten`,
molybdenum, silver, gold, germanium and the like~
;;~ The catalytic components are applied to the alumina-
coated monolith by conventional methods which generally include
immersing the monolith in an aqueous solution of a compound o~ ~ r
the desired metallic-component to impregnate on the high surface
areaalumina coatingO In the case of the noble metals, the sup-
. ~
,l port is impregnated with an aqueous solution of chloroplatinic
acid, platinum chloride, ammonium chloroplatinate, dinitrodi-
amino platinum and the likeO The impregnated support is then
l oxidized and/or reduced to yield the platinum metal in an oxi-
i dized or reduced state. The compounds of the catalytic metals
decompose upon calcination to fonm the oxide form.
The amount of the catalytic component varies de-
pending on the particular end-use appLicationO In general,
;;~ 20 the amount will be in the range of 0O05 to 10%, preferably in
the range of 0.1 to 1% based on the weight of the total sup- ;
; ported catalyst.
The Examples below illustrate the process of the
present invention for the preparation of catalyst supports
and their use as automotive emission control catalyst~.
i: ExamD,le 1 '
A monolith used in this Example was a ceramic alpha
alumina monolith that resembles a herringbone-wrapped ball of
.. J string in which the strands criss-cross over one another t~
form a woven cylindrically shaped nest having an axial hole
therethrough~ The wrapped strings form passageways~ some
of which are approximately normal to the longitudin~l ax~
~ ~ 5 ~
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. , . . ,. , .. , . . ~ . ~
of the monolith so that the ultimate flow of exhaust gas when
the finished catalyst is placed in a catalytic muffler is
i either from the axial hole radially outward through ~he nest
. . ~ . . ,
or from the exterior surface through the nest to the axial hole.
~ ubber stoppers were mounted in either end of the
axial opening of the monolith and an arbor of a stainless
steel rod was mounted through axial openings in each of the
stoppersO The arbor was placed in the chuck of an Arrow Engi-
neering air drive motor, Into a beaker was placed an alumina
hydrosol having a density of lo 32 g~ /cc prepared by digesting
1300 g. of alumina shavings with 1 liter of hydr~chloric acid
plus 3 liters of water under a reflux condenser for about 100
hours, The undissolved aluminum was advanced to the next
batch. The beaker was lifted around the arbor to completely
immerse the monolith for S to 10 seconds at room temperature,
and the beaker was ten 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
~he entire surface thereofO There was no reflective whiteness.
Al~ of the passageways in the monolith were open and free
alumina deposit. The coated monolith was then dried at a tem-
perature of 250F and calcined at 1100F in air in a muffle
furnace for a period of 2 hours. The foregoing procedure was
repeated six ~imed in order to deposit 0.11 grams of alumina
coating per gram of the bare monolith, i. e~ 10% by weight
alumina based on the finished catalyst support.
The 1100F calcination temperature of ~he coated
.... .
~ monolith after each dipping step was sufficient to set the ~.
~ . " .
~ 30 alumina sol on the monoLith and to drive off a sufficient
`~ ` amount of the hydrohloric acid so that the gamm~ alt~ina co~t-
ing wnuld not soften during the next dipping step. However,
... . i.
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- 6
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the temperature of the calcination after the last dipping step
' was increased to about 1450F in order ~:o reduce the chloride
ion content of the finished catalyst and thus m~imize its
activity.
'~ The ~inished support was subjected to an ultrasonic
;- bath test to determine the adherence of the active alumina
coating. The results are summarized in Table I below and
compared to a bare monolith as a controlO The ultrasonic bath
;~ used in the test was a Megason Ultrasonic Water Tank which
~; i0 commonly is used for cleaning surgi¢al instruments.
` ~xample 2
The procedure of Example 1 was followed except that
i~ an alumina sol havlng a density of 1.41 gJcc was employed
i during the dipping step. The procedure was repeated three
~,' times in order to deposit 0.1 grams of alumina per gram of bare
"' monolithO `
~, The ~inished support was also tested in an ultrasonic
.,, - -,i:bath and the results are summarized in Table I belowO `
~xample_3
The procedure of Example 1 was again followed except
that an alumina sol having a density of 1.43 gO/Cc was employed
~ during the dipping step and the procedure was repeated only
'~ twice to obtain the same alumina content on the monolith. `;
Table I also summarized the ultrasonic bath test
;-~ results for this monolithO
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The results in Table I show that the ac~ive alumina
;; is ~enaciously held onto the alpha alumina monolithO
; ~ 6
~ach ~f the inished supports described in Examples
3 was impregnated to load each nf the:~ with a total o 0.025
~ Troy ounce of Pt and Pd at a Pt/Pd weight ratio of 5/2. The
:.: results of testing cores cut from each of the fvregoing cata-
lysts in an engine laboratory at an exahust gas space velocity . :
of 15,000 GHSV are in Table II belowO After being tested,
.~~ 10 fresh, the cores were subjected to a standard aging process
; of 24 hours at 1800F and were then tested again. These re~
,~ 8ults are also in Table IIo
T~BLE II
., :
;., TESTING CATAL~STS I~ EM,INE L~B ~T 15,000 GT~SV : ~
"
: Exampie 4 5 6
, .1 -
!~onolith of Exa~rq?le 1 2 ~ ,~
Engine Lab ~cti~ity , Fresh ^ ~:
2 9 2 2 7 4 2 ? 4 . ~
; ~ T50}~C, F 306 281 279 ;
, . . .
~, 20 T~oco ~ F 300 277 278
, 8 0 ' 314 2 8 5 2 8 3 ,
After Ag1ng 24 Hr. at 1800F .;
i T50CO, F 335 375 380 .;
T50HC, F 385 382 386
~': l T8CO, F 400 385 . 395
.,.. , O . ,.
~i ~80 I~C, F 425 410 400
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In the preceding Table
T50 CO means the temperature in F. at which 50%
" of the CO in the feed to the emission control catalyst under
~,
test are converted to CO2;
`. T50HC means the temperature in F. at which 50%
of the unburned hydrocarbons in the feed to the emission con- .
,~......... trol catalyst under test are converted to oxidized hydro-
,~ carbons; ` ~
.~ T80CO means the temperature at which 80% of the CO `
,~ 10 is converted to CO2; and
T80HC means the temperature at which 80% of the un-
burned hydrocarbons are converted to oxidized hydrocarbons.
~'~, ..
Table II shows that the catalyst produced by the pre-
sent invention is effective in converting CO and HC to combustion .:
, ,. ~
,'; products which can be exhausted to the atmopshere without adverse
~' effect to the environment. .
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A cordierite ceramic honeycomb structure was used in
this Example as shown in Figso 1 and 20 It is commerncially
available from Corning Glass Works~ I~ had the dimensions of
4.5 inO ~11.5 cm) in diameter and 3 in. (706 cm) in length and
had 15 square passageways per inch. The honeycomb monolith
was i~mersed in a hydrosol having a density of 1~38 g./cc for
about 5 to 10 seconds to completely wet the passageways. The
monolith was removed from the hydrosol, taped to the arbor of
Example 1 so that the axes of the passageways were perpendi- `
cular ~o the shaf~ and spun at a centrifugal force of about
2 G's for about 10 to 20 seconds until ~he hydrosol coating
on the monolith was not tacky. The coated honeycomb was cal-
., .
'j cined at 1100F for about 2 hours. The above procedure was
repeated a second time to deposit 0O15 gram of alumina coating
per gram of bare monolithO Virtually, all of the approximately
, ~ 3600 square openings in the honeycomb were open and free of
pluggingO
~,,; The process of this invention has the particular
advantage of uniformly depositing a film of an alumina hydro- ;
801 having a density in the range of about lo 2 to 1.45 onto a
monolithic structure without plugging the passage~ays. The
process can be repeated a number of times to result in a !~'
. .
catalyst support having an active gamma alumina coating of
,, "
any desired amount. This amount is usually in the range of
- about 5 ~o 20% by weightO
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