Note: Descriptions are shown in the official language in which they were submitted.
P ~ /US9ltO8918
:~W O 92/09848
1 2096949
5PALLADIUM PARTIAL COMBUSTION CATALYSTS
AND A PROCESS FOR USING THEM
RELATED APPLICATIONS
This is a continuation-in-par~ of U.S. Ser. No. 07/617,974, to Dalla
Betta, Tsurumi, and Shoji, entitled " A GRA~ED PALLADIUM-CONTAINING
PARTIAL COMBUSTION CATALYST AND A PROCESS OF USING IT (PA-
0029)"; U.S. Ser. No. 07/617,975, to Dalla B~ta, Shoji, Tsurumi, and Ezawa,
~- 15 entitled " A PARTIAL COMBUSTION PROCESS AND A CATALYST FOR USE
IN THE PROCESS (PA-0006)"; U.S. Ser. No. 07/6t7,979, to Dalla Betta,
Tsurumi, Sho~i, and Garten, entitled " A PARTIAL COMBUSTION CATALYST
OF PALIADIUM ON A ZIRCONIA SUPPORT AND A PROCESS OF USING IT
(PA-0026)"; and U.S. Ser. No. 07/617,981, to Dalla Betta, Ezawa, Tsurumi,
20 Shoji, and Ribeiro entitled " A MIXED M~AL PARTIAL COMBUSTION
CATALYST CONTAINING PALLADIUM AND A PROCESS OF USING IT ~PA-
0025)". Each was filed on November 26, 1990 and the entirety of which are
incorporated by notice.
FIELD OF THE INVENTIQN
This invention is a catalyst comprising palladium on a support and a
partial combustion process in which fuel is partially combusted using that
catalyst. The palladium catalyst may also comprise palladium mixed with
30 m~tals selected from Group Vlll or IB, may be graded such as to have
higher actiYity in the forward edge of the catalyst, or may be placed on a
support comprising zirconia. The choice of catalysts and supports specified
each solves a variety of problems dealing with the long term stability of the
palladium as a partial combustion catalyst. The catalys~ structure is stable in
35 operation, has a comparatively low operating temperature, has a low
temperature at which oxidati~n begins, and yet is not susceptible to
temperature "runaway". The combustion gas produced by the catalytic
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WO 92/09848 PCrJUS91/08918
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process typically is at a temperature below the autocombustive temperature
and may be used a~ that temperature or it may be fed to other combustion
stages for further use in a gas turbine, furnace, boiler, or the like.
BACKGROUND OF THE INVENTION
With the advent of modern antipollution laws in the United States and
around the world, si~nificant and new methods of minimizing various
pollutants are being investigated. The burning of fuel --be the fuel wood,
10 coal, oil, or a natural gas-- likely causes a majority of the pollution problems
in existence today. Certain poilutants, such as SO2, which are created as
the result of the presence of a contaminant in the fuel source may be
removed either by treating the fuel to remove the contaminant or by treating
the exhaust gas eventually produced. Other pollutants such as carbon
15 monoxide, which are created as tha result of imperfect combustion, may be
~; removed by post-combustion oxidation or by improving the combustion
process. The other principal pollutant, NOX (an equilibrium mixture mostly of
NO, but also contaTning very minor amounts of NO2), may be dealt with
either by controlling the combustion procsss to minimizs NOX
20 production or by later removal. Removal of NOX, once produced, is a
difficult task because of its relative stability and its low concentrations in most
exhaust gases. One solution found in automobiles is the yse of carbon
monoxide chemically to reduce NOX to nitrogen while oxidizing the carbon
~' monoxide to carbon dioxide. However, in some combustion processes25 (such as gas turbines) ths carbon monoxide concentration is insufficient to
react with and to remove to the NOX~
It must be observed that unlike the situation with sulfur pollutants
where the sulfur contaminant may be removed from the fuel, removal of
nitrogen from the air fed to the combustion process is clearly impractical.
30 Unlike the situation with carbon monoxide, improvement of the combustion
reaction would likely increase the level of NOX produced due to the higher
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temperatures present in the combustion process.
Nevertheless, the challenge to reduce NOX in combustion processes
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WO 92/0984~ PCI /US91 /08918
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remains and several different rnethods have been suggested. The NOX
abatement process chosen must not substantially conflict with the goal for
which the combustion gas was created, i.e., the recovery of its heat value in
a turbine, boiler, or furnace.
Many recognize that a fruit~ul way of controlling NOX production in
combustion processes used for turbine feed gases is to limit the localized
and bulk temperatures in ~he combustion zone to something less than
1800C. See, for instance, U.S. Patent No. 4,731,~89 to Furuya et al. at
column 1, lines 52-59 and U.S. Patent No. 4,08~,135 to Hindin et ai. at
column 12.
There are a number of ways of controlling the temperature, such as
by dilution with excess air, controlled oxidation using one or more catalysts,
or staged combustion using variously lean or rich fuel mixtures.
Combinations of these methods are also known.
One widely attempted method is the use of multi-stage catalytic
combustion. Most of these disclosed processes utilize multi-section
catalysts of metal oxide on c0ramic catalyst carriers. Typica~ of such
disclosures are:
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It is, however, difficult to control intermediate, or between-stage,
temperatures in these processes. Since the object of each of the processes
is to produce a maximum amount of heat in a form which can be eff~ciently
used in some later process, the combustive steps are essentially adiabatic.
Consequently, a minor change in any of fuel rate, air rate, or operating
processes will cause significant changes in the inter-stage temperatures.
Very high temperatures place thermal strain on downstream catalytic
- elements.
This list also makes clear that platinum group mstals, including
palladium, are considered useful in catalytic combustion processes.
However, conventional catalytic cornbustion processes often mix the fuel and
air and then pass this rnixture over a catalyst with essentially complete
combustion in the catalyst bed. This results in extremely high temperatures,
typically 1100-C to 150û'C. For this reason, much of the catalyst
development work is directed at catalysts and supports that can withstand
those high temperatures and yet remain active. Some have relied on
process control schemes in which the flow rate of an intermediate stream of
air or fuel is introduced between catalyst stages and is controlled based
upon bulk gas temperature. Furuya ~., mentioned above, describes one
approach in circumventing the problems associated with a high catalyst
temperature through dilution of the fuel/air mixture with air fed to the catalyst
so that the resulting mixture has an adiabatic combustion temperature of
900 C to 1000CC. This mixture is passed through the catalyst and partial or
complete reaction gives a maximum catalyst temperature less than 1000C
and a ~as temperature less than 1000'C. Additional fuel is added after the
catalyst and homogeneous combustion of this mixture gives the required
temperature, 1200C to 1~00-C. This prc~cess, however, suffers from the
, . need to add fuel at two stages and the requirements to mix this additional
fuel with hot gases without obtaining a conventional high temperature
diffusion flame and the associated production of NC)X.
The process of our invention mixes air and fuel at the beginning of the
combustor in a ratio such that the final combustion temperature is, after
further combustion step or stcps, that required by somc later process or
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WO 92/09~,48 PC'r/US91/08918
2ag69d~9 6
device which recovers the heat from the combustion ~as, e.g., a ~as turbine.
A typical mixture might be methane and air at a volume fuel/volume air ratio
of 0.043. Such a nixture (after being preheated to 350 C) could produce a
combustion temperature of about 1300"C. This mixture passes over a
5 catalyst and is only partially combusted with the catalyst limiting the
maximum catalyst temperature to a level substantially less than the adiabatic
cornbustion temperature o~ the gas. The limiting effect is believed to be due
to the reac~ion:
:~' 10
, PdO------> Pd + 1~ 2
at the partial pressure of oxygen present during the reaction. The limiting
temperature has been found to be the temperature at which the
palladium/palladium oxide transition occurs in a thermogravimetric analysis
(TGA) procedure. As a rule of thumb, this transition temperature for pure
palladium is about 780''C to 800-C in air at one atm and 930 C to 950'C in
air at ten atm.
We have found that palladium catalysts can become unstable in
partial combustion operation: the reaction dies with time and the level of
preheat temperature required for stable operation increases. We have found
a number of solutions to this problem. For instance we have obse,ved that
use of the stable temperature self-controlling feature of this invention takes
place by employing one or more of the following:
a. Use of palladium (and optionally another Group
Vlll roble metal, such as platinum, osmium,
rhodium, ruthenium; preferably platinum; or a
Group jB metal, such as copper, gold, silver;
preferably silver) as the active catalytic metal,
. b. Use of a diffusion barrier applied over the catalyst
surface to limit the rate at which the h,~el diffuses
to the catalyst and, therefore, lim,ts the catalytic
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~ WO 92/09848 PCI'/~S91/08918
7 2~969~
; reaction rate and allows palladium to lim~ ~he
maximum temperature,
c. Use of a zirconia-containing support (preferably,
in turn, on a metal substrate) to support the
catalyst layer and provide a catalyst structure very
resistant to thermal shock, or
. .
d. placemen~ of the catalytic metals on the support
so that a leading portion of the catalyst structure
in the flowing gas stream contains catalytic
material of higher activity.
The interconversion of palladium oxide and palladium at approximatsly
800'C has been described previously, for example, by Furuya et al. in U.S.
Patent No. 4,731,989. However, this patent describes this interconversion as
a disadvantage since the active palladium oxide species is converted to a
Iess active palladium species thus preventing the combustion reaction from
going to completion on the catalyst. The inventive process herein uses this
- 20 palladium oxide/palladium interconversion process stabilized or promoted
variously as specified herein to limit the catalyst temperature and thereby
permit the use of very high activlty and stable catalysts.,
By maintaining the catalyst temperature at a level substantiaily below
the adiabatic combustion temperature, problems associated with thermal
sintering of the catalyst, vaporization of the palladium, and thermal shock of
the support can b~ minimized or etiminated.
The use of metal cataiyst supports for platinum group metals has
been suggested in passing. For instance, see U.S. Patent No. 4,088,435 to
Hindin et aL, "platinum group metals" at column 4, lines 63 et seq.. and "th~
30 support may be metallic or ceramic..." at oolumn 6, line 45. Conversely, the
use of a platinum group alloy monolithic catalyst as a combustion c~talyst is
; suggested in U.S.Pat. No. 4,287,856 to Hindin et al. at column 1, line 65 et
- al. Other similar disclosures are found in the earlier U.S. Patent Nos.
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WO 92/09848 PCr/US91/08918 ~`
~ o ~ 9 8
`3,966,391; 3,956,188; 4,008,037; and 4,021,185 all to Hindin et ~I. Platinum
on a steel ("Fecralloy") support as a combustion catalyst for low heating
. value gas is suggested in U.S. Patent No. 4,366,668 to Madgavkar et al.
Other disclosures of metals and metal supports used mainly for
5 automotive catalytic converters include:
Country Document Patent~e
U.S. 3,920,583 Pugh
U.S. 3,969,082 Gairns et al.
U.S. 4,279,782 Chapman et al.
U.S. 4,318,828 Chapman et al.
U.S. 4,331,631 Chapman et al.
U.S. 4,414,023 Aggen et al.
U.S. 4,521,532 Cho
U.S. 4,601,999 Retallick ~!-
U.S. 4,673,663 Maqnier
U.S. 4,742,038 Matsumoto
U.S. 4,752,599 Nakamura et al.
U.S. 4,784,984 Yamanaka et al.
Graat Britain 1,52~,455 Cairns et al.
,~
As a group, these patents generally discuss ferritic catalyst supports
25 upon which alumina is found as micro-crystals, coatings, whiskers, etc.
Many disclose that platinum group metals are suitably placed on those
supports as catalysts. None suggest the catalysts comprising palladium nor
their abilities stably to limit the catalyst temperature.
Moreover, in a practical sense the use of metal substrates has
30 been limited to applications where the adiabatic combustion temperature is
~; below 1100-C or 1000 C whsre the complete combustion of the fueljair
mixture will result in a substrate temperature that would not damage the
metal. This limitation caps the final gas temperature that can be achieved or
requires the use of staged fuel or air addition further complicating the
35 combustor design. The use of the inventive process limits the metal
substrate temperature to less than 850'C at one atm pressura and to less
~han 950'C at 16 atm pressure even for fuel/air mixtures with adiabatic
combustion temperatures of 1300 to 1600 ' C.
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~ W O 92/0984~ PC~r/US91/08918
9 ` ' 2~9~9
Our inventive process for stably limiting the substrate temperature
also offers advantage for c0ramic substrates since limiting ~he substrate
temperature reduces thermal stress and failurs due to thermal shock during
start up and shutdown of the combustor. rhis protection is especially
5 important for fuel/air ratios corresponding to adiabatic combustion
temperatures of 1300-C to 1600 C. ~n summary, atthough the literature
suggests various unrelated portions of the inventive process and the catalyst
structure, none of these documents suggests that ~he disclosed palladium-
containing catalysts offer the disclosed advantages in addition to stably.
10 limiting the substrate temperature.
SUMMARY OF THE INVENTION
This invention is a combustion catalyst comprising palladium. The
combustion catalyst may optionally contain a Group IB or Vlll noble metal
15 and may be placed on a support comprising zirconium, Additionally, the
combustion catalyst may be graded, that is, may have a higher activity
portion at the leading edge of the catalyst structure. The invention includss
a partial combustion process in which the fuel is partially eombusted using
that catalyst. The choice of catalysts and supports solves a problem in the
20 art dealing with the long term stability of palladium as a partial combustioncatalyst. The catalyst structure is stable in operation, has~a comparatively
' low operating temperature, has a low "light of~' temperature, and yet is not
susceptible to temperature "runaway". The combustion gas produced by the
catalytic process may be at a temperature below the autocombustive
25 temperature, may be used at that temperature, or fed to other combustive
stages for further use in a gas turbine, furnac0, boiler, or the like.
.
BRIEF DESCRIPTION C)F THE DRAWIN~iS
. Figures 1 and 2'show schematic, cross section closeups of a number
30 of ca~alysts within the scope of the invention.
Figure 3 is a graph comparing the respective operating tempera~ures
of palladium or platinum catalysts at various of fuel/air ra~ios.
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w o 92/0984~ P ~ /US91/08918
209~349 10
Figure 4A is a graph of the TG A of palladium oxide/palladium at one
atm air.
Figure 4B is a graph of the TGA of palladiurn oxide/palladium at one
atm of pure 2-
Figure 5 is a graph of various process outlet temperatures as a
function of catalyst preheat temperature for a particular uncoated catalyst.
Figure 6 is a graph of various process outlet temperatures as afunction of catalyst preheat temp~rature for a particular catalyst having a
diffusion barrier.
Figures 7A and 7B are graphs of LOT and steady state operation
temperatures for a zirconia-coated cordierite monolith.
Figures 8A and 8B are graphs of LOT and steady state operation
temperatures for a zirconia-coated metal monolith.
Fi~ures 9A and 9B are temperature graphs showing operation of the
invention using a zirconia coated ceramic support.
Figures 10A and 10B are temperature graphs for comparison to
Figures 9A and 9B.
Figures 11A and 11B are temperature graphs showing operation of
the invention using a zirconia coated rnetal support.
Figures 12A and 12B are temperature graphs for comparison to
Figures 11A and 11B.
Figures 13A-13D and 14A-14D are graphs showing the effect of
varying amounts of platinum on the operation of a cordierite supported
palladium combustion catalyst.
Figures 15A 15B and 16A-16B ara graphs showing the effect of
platinum on the operation of a metal supported palladium combustion
catalyst.
Figures 17 and 1~ are temperature graphs showing the operation of
the inventive gradedand comparative catalyst properties.
DESCRIPTIQN OF THE INVENTION
This invention is a combustion catalyst comprising palladium and may
optionally contain a Group IB or Vlll noble metal and may be placed on a
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209~94~
suppor~ comprising zirconium. Additionally, the combustion catalyst may be
graded, that is, may have a higher activity portion at the leading edge of the
catalyst structure. The invention includes a partial combustion process in
which the fuel is partially combusted using that catalyst. The choice of
5 oatalysts and suppor~s solves a problem in the art dealing with the long term
stability of palladium as a partial combustion catalyst. The catalyst struc~ure
is stable in operation, has a comparatively low operating temperature, has a
low "light off" temperature, and yet is not susceptible to temperature
"runaway". The combustion gas produced by the catalytic process may be
at a temperature below the autocombustive temperature, may be used at
that temperature, or fed to other combustive stages for further use in a gas
turbine, furnace, boiler, or the like.
Catalyst and Catalytic Structures
The catalyst contains palladium and, optionally, one or more Group
Vlll noble metal (platinum, ruthenium, rhodium, osmium, or iridium;
preferably platinum) or Group IB metal (preferably silver) in an amount no
more than equimolar to palladium. Palladium is fairly active as an oxidation
catalyst at temperatures of 325C and lower and consequently is useful in a
partial combustion process as a "light off" catalyst. As was discussed
above, the catalytic activity of palladium as a fuel oxidation catalyst is
believed to be due to the presence of palladium oxide. Palladium metal
does not appear to be a very active as a catalyst except at fairly high
; temperatures, 8.g., above 750 C to 8W C. Palladium metal is readily
oxidized to palladium oxide in the presence of excess oxygen at
temperatures as low as 325C according to the equilibrium reaction:
1~2 2 + Pd <~ - > PdO .
However, as the ternperature rises, the equilibrium shifts to the left, i.e., the
palladium oxide decomposes. This transition causes the reaction
temperature to be self-limiting. In one atm pressure air the combustion goes
readily up ~o a temperature of about 750C to 800C, the palladium oxide
WO 92/09848 2 ~ 9 ~ PCI/US91/1)8918 ~"~
` ` 12
becomes the lesser present species, and the reaction consequerltly slows.
Ths temperature at which palladium oxide converts to pa!ladium
depends in part on the oxygen partial pressure. That conversion
temperature appears to be readily determinable through thermogravimetric
5 analysis ("TGA" -- a procedure that measures the weight loss of a sample of
palladium oxide as it is subjected to a temperature increase). The PdO-Pd
transition point establishes the self-limiting substrate temperature for those
operating conditions. A palladium catalyst used as a combustion catalyst will
generally limit the substrate temperature to this TGA transition limiting
10 temperature.
We have ~ound however that the use of palladium on some
substrates, notably those containing alumina, results in a partial oxidation
catalyst having an unpredicta~le life. The reason for the deactivation is
unclear although the resulting decline in outlet temperature may be quite
15 pronounced. The effect is observed on both alumina coated metal supports
and with neat aluminas. The addition of a discrete amount of a Group Vlll
noble metal or Group IB metal (such as platinum or silver) to the palladium
catalyst provides long term stabliity to the catalyst composition and does not
substantially affect the desirable low "light off" temperatures found with
20 palladium catalysts. We have also found that the use of a catalyst support
comprising zirconium (preferably in the form of zirconia) also stabilizes the
steady-state operation of palladium-based partial oxidation catalysts.
The palladium metal is added in an amount sufficient to provide
catalytic activity. The specific amount added depends on a number of
25 requirements, e.g., the fuel used, economics, activ~y, life, contaminant
presence, etc. The theoretical maximum amount of metal is likely enough to
cover the maxirnum amount of support without causing undue metal
crystallite growth and concomitant loss of activity. These clearly are
cornpe~ing factors: maximum catalytic activity requires higher surface
30 coverage but higher surface coverage can promote growth between
adjacent crystallites. Furtherrnore, the form of the catalyst support must be
considered. If the support is used in a high space velocity environment, the
catalys~ loadings should be high to main~ain sufficient conversion svsn
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: WO 92/09848 PC'r/US91/08918
13 20~694~
though the residence time is low. Fconomics has, as its general goal, the
use of the smallest amount of cataly~ic rnetal which will do the required task.
Finally, the presence of contaminants in the fuel would mandate the use of
higher catalyst loadings to offset deterioration in the catalyst due to
5 deactivation.
The palladium metal content of this catalyst composite typically should
be from about 0.01% to about 25% by weight. The amount also is affected
by the make up of the feed.
The palladium may be incorporated onto the support in a variety of
10 different methods using palladium complexes, compounds, or dispersions of
the metal. The compounds or complexes may be water or hydrocarbon
soluble. The palladium metal may be precipitated from solution. The liquid
carrier generally needs only to be removable from the catalyst carrier by
- volatilization or decomposition while leaving the palladium in a dispersed
form on the support. Exarnples of the palladium complexes and compounds
suitable in producing the catalysts used in this invention are palladium
chloride, palladium diammine di-nitrite, palladium nitrate, palladium
tetrammine chloride, sodium palladium chloride1 palladium 2-ethylhexanoic
acid, and a variety of other palladium salts or complexes. Although the
chloride compounds produce catalysts which are typically quite active,
chlorides are not an excellent choice when the catalyst is used in a
combustor for a gas turbine. Chlorides, even in very small amounts, cause
significant turbine blade and bucket corrosion. Consequently, nitrogen-
containing palladium precursors are most desirable.
As mentioned above, the catalyst may contain an adjunct cataiyst
such as a Group IB metal (such as silver~ or a Group Vlll noble metal (such
as platinum) in an arnount up to slightly more than the molar amount of the
palladium found in the catalyst composition. Molar ratios of the palladium to
adjunct me~al between 0.95 and 25 are effective. Although the adjunct
metal may be added by inclusion in the liquid carrier containing the
palladium as a complex, compound, or metallic dispersion, the resulting
catalyst is more predictably stable i~ the adjunct metal is added in a
subsequent step. Examples of platinum complexes and compounds suitable
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WO 92/09848 PCI`/US91/08918 "~- `
2096949 14
in producing the optional catalysts of this invention are platinum chloride,
platinum diammine dinitrite, platinum nitrate, platinum tetrammine chloride,
sodium platinum chloride, and a variety of othsr platinum salts or complexes.
Similar salts and complsxes are known for the other Group Vlll noble metals
or Group IB metals.
Also as mentioned above, we have observed that us0 of a catalyst
structure in which the ~ront edge of the catalyst structure is higher in activity
than that of the trailing portions results in a structure which has the benem ofa low0r light-off temperature, no "hot-spotting" in th~ lattsr portion of the
catalyst structure, and an overall operationally stable catalyst.
The graded structure can be produced a number of different ways.
As shown in Figure 1, the substra~e metal or ceramic (102) can be coated
with three different catalysts (104, 106, and 108). Each catalyst has a
different activity: catalyst 104 having the highest activity, catalyst 106 having
an intermediate activity, and catalyst 108 having the lowest activity.
Catalysts 104, 106, and 108 can be obtained by varying the loading of the
activc catalytic material, for instance, catalyst ~04 would have 20% palladium,
catalyst 106 would have 10% palladium, and catalyst 108 would have 5%
palladium. Alternatively, the palladium dispersion could be varied with
catalyst 104 having the highest dispersion and catalyst 108 the lowest
dispersion.
Another approach to a grade~ catalyst would be to l~se a washcoat
containing both an oxidic material and a catalytic material but having a
constant activity and to vary the thickness along the catalyst. In Figure IB, a
constant activity catalytic washcoat is applied to the substrate ~110) in a
thick (of higher activity) layer at the inlet (112), thinner in the middle (114),
and very thin at the outlet (116).
A third approach is to use catalytic washcoats of diffcrent activity as in
Figure 1 ~a) but apply these washcoats in overlapping layers as in Figure 1 (c)
where the low activity catalytic washcoat (1~0) is applied over the entire
substrate (118) followed by the medium activity washcoat coverin~ a portion
(122) of the substrate and finally the high activity washcoat (124) at the inletonty. An additional structure is shown in Figure 1 ~d) where the high activity
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`' WO 92/098~8 PCT/US91/08918
2096~49
washcoat (126) is first applied at the inlet followed by the medium (1283 and
low activity washcoat (130).
The high, medium, and low activity washcoats can be produced by a
variety of approaches. The active component (such as palladlum) can be
5 varied from a high concentration to a low concentration. A~ernatively, the
dispersion of the active palladium can be varied by using different
preparation procedures or by heat treating the catalyst at different
temperatures. Another procedure would be to vary the surface upon which
the active component is depos-lted. For examplel a palladium/AI2O3 or
10palladium/ZrO2 catalyst using different surFace area Al203 or ZrO2 supports
would result in catalysts of different activlty, the higher surface area supportwould have a higher activity.
These graded catalyst structures can be manufactured by several
procedures. For example, the structure shown in Figure 1 (d) can be
15 prepared on a ceramic honeycomb monolith by partially dipping the monolith
in washcoat and blowing the excess out of the channels. Subsequently, the
process is repeated by dipping further into the washcoat sol. This same
general procedure can be used to prepare the structure found in Figure 1 (c).
The same procedures can be applied to metal monoliths by rolling the metal
20 foil into a spiral or folding the metal foil into the desired shape ~or dipping as
described for the ceramic monolith.
An alternative approach would be to apply the graded catalyst layers
to a suitably corrugated metal foil that is then rolled into a spiral structure to
form the final catalyst unit. The washcoat can be sprayed or painted onto
25 the metal foil surface or applied by other known obvious techniques such as
by chemical Yapor deposition, sputtering, etc. To achieve the desired
graded structure the foil may be partially masked to restrict the catalyst to
the desired region. Using this procedur~ structures such as seen in Figures
l(a), 1 (b), 1(c), and 1 (d) may be prepared by spraying or painting onto the
30 foil. The washcoat can be applied to one side only of the metal foil or to
both sides.
It should be noted that in these assembly procedures, the catalyst
can be applied as a combined mixture of active catalyst (such as palladium)
~' ' , , " ' . ' ` '.
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WO 92/0984X PCI`/US91/08918
20`969~9 16
and the high surface area support (Al203, ZrO2, and SiO2, etc.). These
would be prepared by imprsgnating the pallaclium onto the high surface area
oxide powder, calcining, then convertin~ to a colloidal sol. In a second
method, the high surface area washcoat may be applied firs~ to the monolith
or metal foil and fixed in place. Then the ~talyst, e.g. palladium, may be
applied by the same dipping or spraying procedure. The structure shown in
Figure 1 (d) may be prepared by using a single washcoat oxide sol and a
single catalyst (palladium) solution and repeating the process.
Figure 2 shows another series of catalysts where the catalytic coating
is made using any of the methods discussed above. In Figure 2(a) the
thicker (and hence, rnore active) catalytic layer (202) is placed upstream of
the thinner catalytic layer (206). Similarly, in Figure 2(b) a shorter thick layer
(208) is applied. Such a configuration might be applied (as opposed to that
of Figure 2(a)) where using a higher activity catalyst or using a higher level
of preheat. Figure 2(c) shows a stepped varied catalyst and Figure 2(d)
shows a constant variation.
The preferred supports for this catalyst are metallic. Metallic supports
in the form of honeycombs, spiral rolls of corrugated sheet (which may be
interspersed with flat separator sheets~, columnar (or ~handful of straws"), or
other configurations having longitudinal channels or passageways permitting
hi~h space velocities with a minimal pressure drop are desireable in this
service. They are malleable, can be mounted and attached to surrounding
structures more readilyt and offer lower flow resistance due to the thinner
walls than can be readily manufactured in ceramic supports.
Another practical benefit attributable to rnetallic supports is the ability
to survive thermal shock. Such thermal shocks occur in gas turbine
operations when the turbine is started and stopped and, in particular, when
the turbine must be rapidly shut down. In this latter case, the fuel is cut off
or the turbine is "tripped" because the physicai load on the turbine --e.g, a
generator set-- has been remoYed. Fuel to the turbine is immediately cut off
to prevent overspeeding. The temperature in the combustion chambers
; (where the inventive process takes place) quickly drops from the
temperature of combustion to the temperature of the compressed air. This
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WO ~2/09848 PCT/US91/08918
17` 209t~9DJ;9
drop could span more than 1000 C in less than one second. In any event,
the oatalyst is deposited (or otherwise placed) on the walls within the
channels or passageways of the metal support in the amounts specffled
above. Several types of support materials are satisfactory in this service:
alurninum, aluminum containing or aluminum-treated steels, and certain
stainless steels or any high temperature metal alloy, including cobalt or
nickel alloys where a catalyst layer can be deposited on the metal surface.
The preferred materials are aluminum-containing steels such as those
found in U.S. Patent Nos. 4,414,023 to Aggen et al., 4,331,~31 to Chapman
et al., and 3,969,082 to Cairns, et al. These steels, as well as others sold by
Kawasaki Steel Corporation (River Lite 20-5 SR), Vereinigte Deutchse
;~ Metallwerke AG (Alumchrom I RE,l, and Allegheny Ludlum Steel (Alfa-lV)
contain sufficient dissolved aiuminum so that, when oxidized, the aluminum
forms alumina whiskers or crystals on the steel's surface to provide a rough
15 and chemically reactive suRace for better adherence of the washcoat.
The steels (after alumina whiskers formation) are treated with a
zirconium-containing compound or, preferably, a suspension or sol of
zirconium oxide or hydrated zirconium oxide. The palladium compounds
and any other catalyst precursors typically are applied to the zirconia coating
20 although the zirconium coating may be formulated to incorporate the
palladium. The washcoat of zirconium may be applied using one or more
coats of zirconia sols or sols of mixed oxides containing silicon or titanium
and additives such as barium, cerium, lanthanum, chromium, or a variety of
other components. After application of the suspension, it may be dried and
25 calcined to form a high surface area adherent oxide layer on the metal
surface.
The washcoat may b0 applied in the same fashion one would apply
paint to a surface, e.g., by spraying, direct application, dipping the support
into the washcoat material, etc. An alternative process for adding the
30 catalyst layer to the support structure is first to add the catalytic metals to
: the inert oxide powder. The catalyst metal is fixed on the oxide by heat
treatment or by chemical treatment. The palladium/inert oxide mixture may
then be milled to form a colloidal sol. The sol is applied to the substrate by
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, WO 92/09848 PCI`/US91/08918 `~.
2~9:4~ 18
spraying, dipping, or the like.
Aluminum structures are also suitable for use in this invsntion and
may bs treated or coated in essentially the same manner. Aluminum alloys
are somewhat mora ductile and likeiy to deform or even to melt in the
temperature operating envelope of the process. Consequently, they are less
desirsable supports but may be used ~ the temperature criteria can be met.
Once the washcoat, palladium, an~ and any adjunct ~talytic metals
have been applied to ths metallic support and calcined, one or mora
coatings of a refractory oxide may then be applied as a diffusion barrier to
prevent the temperature "runaway" discussed above. This barrier layer rnay
be silica, zirconia, titania, or a variety of other oxides with a low catalytic
activity for oxidation of the fuel or mixed oxidss or oxides plus additives
similar to those described for the washcoat layer. Alumina likely is not
~5 desirable as the barrier layer but may be acceptable in some circumstances.
The barrier layer may range in thickness from 1% of the washcoat layer
thickness to a thickness substantially thicker than the washcoat layer,
preferably from 10% to 100% of the washcoat layer thickness. The preferred
thickness will depend on the operating conditions of the catalyst, including
the fuel type, the gas flow velocity, the preheat temperature, and the catalyticactivity of the washcoat layer. It has also been found that the application of
the diffusion barrier coating only to a downstream portion of the catalyst
structure, e.g., 30% to 70% o~ the length, can provide sufficient protection forthe catalyst under certain conditions. As with the washcoat, the barrier layer
:. 25 or layers may be applied using the same application techniques one would
:- ~ use in the application of paint.
The washcoat, catalyst, and diffusion or barrier coat may be applied
to all surFaces of a catalyst support such as described herein, or may be
applied only to a surface opposite a non-coated surface. For instance, the
spiral corrugated structure noted above may be coated on one side with the
washcoat, catalyst, and dfflusion barrier coat. The treated corrugated
structure may then ba rolled into a monolith. A separator sheet of similar
material may also be coated on one side with the catalytic material and
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19 2~9~;949
rolled along with the corrugated sheet into the spiral monolith. In any event,
the surface in the rnonolith having the catalyst placed thereon produces heat
during the combustion process. This heat may pass to the gas flowing by
or may be conducted through the catalyst structure to the adjacent non-
5 catalytic --and hence, cooler-- sur~ace. From there the heat would pass into
the non-combusted gas passin~ along that surface. This allows control of
the temperature of the catalytic surFace of the catalyst structure by an
~: integral heat exchange without resorting to Such m~asures as air dilution or
extraneous heat exchange struc~ures. Such a control might ba desireable
10 where, for instance, the preheat temperature of the inlet gas is quite high
and the gas flow rate is unstable.
This catalyst structure should be made in such a size and
configuration ~hat the average linear velocity of the gas through the
longitudinal channels in the catalyst structure is greater than about 0.2
15 m/second throughout the catalytic structure and no more than about 40
mlsecond. This lower limit is greater than the flame front speed for methane
and the upper limit is a practical one for the type of supports currently
commercially available. These average velocities may be somewhat different
for fuels other than methane.
The Process
This process may be used with a variety of fuels an,d at a broad range
of process conditions.
Although normally gaseous hydrocarbons, e.g.l methane, ethane, and
25 propane, are highly desireable as a source of fu~l for the process, most
carbonaceous fuels capable of being vaporized at the process temperatures
discussed below are suitable. For instance, the fuels may be liquid or
gaseous at room temperature and pressure. Examples include the low
molecular weight aliphatic hydrocarbons mentioned above as well as butane,
30 pentane, hexane, heptane, octane; gasoline; aromatic hydrocarbons such as
benzene, toluene, ethylbenzene, and xylene; naphthas; diesel fuel and
kerosene; jet fuels; other middle distillates; heavier fuels. (preferably
hydrotreated to remove nitrogenous and sulfurous compounds); oxygen-
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WO 92/1)9848 PCI/US91/08918 !'~,
~6;~9 20
containing fuels such as alcohols including methanol, ethanol, isopropanol,
butanol, or the like; and ethers such as diethlyether, ethyl phenyl ether,
MTBE, etc. Low-BTU gases such as town gas or syngas may also be used
as fuels.
The fuel is typically mixed into the combustion air in an amount to
produce a mixture having an adiabatic combustion temperature greater than
the temperature achieved by this inventive process. Preferably the adiabatie
combustion temperature is above 900 C, most preferably above 1000C.
Non-gaseous fuels should be at least partially vaporized prior to their
contacting the catalyst zone. The combustion air may be at atmospheric
pressure or lower
(~.25 atm) or may be cornpressed to a pressure of 35 atm or more.
Stationary gas turbines, which ultimately could use the gas produced by this
process, often operate at gauge pressures in the range of eight atm to 35
atm. Consequently, this process may operate at a pressure between -0.25
atm and 35 atm, preferably between zero atm and 17 atm.
The fuel/air mixture supplied to the catalyst should be well mixed and
the gas inlet temperature may be varied depending on the fuel used. This
temperature may be achieved by preheating the gas through heat exchange
~0 or by adiabatic compression.
~` The process uses a catalytic amount of a palladium-containing
material on a catalyst, preferably metal, support with low resistance to gas
flow having a zirconium-containing coating. The bulk outlet temperature of
the partially combusted gas leaving the zone containing the catalyst and the
~5 wall temperature of th0 catalyst will be at temperatures significantly lower
than the adiabatic or autocombustive temperature of the gas. Generally,
neither temperature will be more than about 800C, preferably no greater
than 550ac to 650~C. In addition, the catalyst temperature should not
exceed 1000~C and preferably not exceed 950DC. These temper~ures will
30 depend on a variety of factors including the pressure of the system, the
partial pressure of ~he oxygen, the calorific value of the fuel, and the like.
Nevertheless the catalyst will partially oxidize the fuel but will limit the ultimate
temperature to a vaiue lower than the adiabatic combustion temperature.
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W O 92/09848 PC~r/US9~/~8918
20969'19
EXAMPLE~
These examples show the production of catalysts within the scope of
the invention and its use in the inventive process. Comparative catalysts
and processes are also shown.
The catalysts are tested for ~o parameters: stability during steady-
state operation and "light-off temperature" (LOT) .
The LOT is determined by placing the catalyst in the combustion
reactor and inltiating a flow of fuel and air to the catalyst. Although these
tssts were conducted at atmospheric pressure, others are conducted at
higher pressure. The temperature of the gas/air mixture is increased at a
constant rate. The temperature of the gas leaving the catalyst and the
temperatures at several points in the catalyst are monitored. If the catalyst isactive, at some point during this temperature increase, the catalyst will begin
to oxidize the fuel and an increase in both the outlet gas temperature and
the intermediate catalyst will be observed. For ease of comparison, the LOT
is the average temperature between the preheat temperature and tha self-
limiting temperature value.
Thé stability of the catalyst is determined by measuring the
temperatures at the same points in the catalyst and gas stream as in the
LOT determination but after an appropriate (and non-varying) preheat
temperature for the mixture of fuel and air is chosen.
,,
Example 1
, This example is in several parts and demonstrates the temperature
limiting capabilUies of the inventive palladium-based catalyst as compared to
a similar platinum-based combustion catalyst.
.;~,~,
Part A (Comparative Platinum Catalyst Preparation)
A platinum catalyst was prepared as follows: 250 g of a low
alkali gamma-alumina, 422 ml distilled water, and 42 ml concentrated
(70.3/O) nitric acid were placed in a half gallon polymer-lined ball mill.
The ball mill was filled half full with alpha-alumina grinding media.
~ ~ The mixture was ball milled for eight hours to produce a
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WO 92/~9848 PCr/~JS91/08918 ~; .
~ 2~96~49 22
colloidal alumina sol containing approximately 35% by weight Al203.
A 100 cell/inch2 (cpsi) cordierits monolith (two inch diameter by
~wo inch length) was dipped in this alumina sol and the excess blown
from th~ channels of the monolith with air. -rhis monoiith was then
dried at 100'C and calcined in a muffle furnace at 850C for ten
hours. The final monolith contained approximately ~0% by weight
alumina washcoat.
The alumina washcoated monolith was dipped in an H2P~CI6
solution containing approximataly 0.1~ 9 platinum/g solution. Ths
excess solution was blown out with air and ~he monolith dried and
calcined at 500C. The platinum impregnation was repeated twice
more. The final catalyst was calcined at 850'C for ten hours. The
final catalyst contained 4.5% by weight platinum.
Part B ~Palladium Catalyst Preparation)
A palladium catalyst was prepared. An alumina washcoated
cordierite monolith was prepared and calcined as described above. A
palladium solution was prepared by dissolving PdC12 in two
equivalents of hydrochloric acid and diluting to 0.042 g palladium/ml.
The washcoated monolith was dipped in this solution, the excess
~.
solution blown out wlth air, and the catalyst dried and calcined at
850~C for ten hours. The final catalyst contained approximately 0.5%
palladium by weight.
.1
~
In this Part each of the two catalysts rom Parts A and B were
installed in a combustion test reactor. The reactor had a two inch
inside diameter and allowed careful control of the preheat temperature
- of the CH4/air mixture prior to contact with ~he catalyst. The reactor
was also equipped with thermocouples to measure a variety of
different gas and catalyst wall temperatures.
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~ '1WO92/09848 PCl'/US91/08918
23 -2~969~
Platin~m Catalyst
The cornparative platinum catalyst made in Part A was
installed in the reactor. Air at 500 Standard Liters Per Minute
(SLPM) was passed over an electric heater, a static gas mixer,
S and through the catalyst. Natural gas containing approximately
93% methane was introduced into the air stream just upstream
of the gas mixer. Gas ternperatures were measured before
and a~ter the catalyst with ceramic covered thermocouples
suspended in the gas stream. The catalyst substrate
temperature was measured by a thermocouple positioned in
one of the channels of the ceramic monolithic catalyst near the
outlet of the catalyst.
Air was heated to 550~C and methane flow increased to
1.1 SLPM corresponding to a ~uel/air ratio of .0022. The
substrate temperature was monitored. The fuel/air ratio was
increased in steps to .002 and the substrate temperature
recorded for each fuel/air ratio. These data are presented in
Figure 3. At a fuel/air ratio of .010, the catalyst was sufficiently
active to raise the substrate tamperature to 740C. This value
: 20 approximated the calculated adiabatic combustion temperaturefor the mixture of 760~C. As the fuel/air ratio is increased, the
substrate temperature closely matched the a,diabatic
combustion temperature. This showed that the piatinum
catalyst was combusting all of the fuel at the catalyst surface.
:,'. ~Ç~
The palladium catalyst prepared above was then tes~ed
in a similar manner. Again, as the fuel/air ratio was increased
the substrate temperature rose and tracked the calculated
adiabatic combustion temperature. However, as shown in
Figure 3 (at fuel/air ratio between 0.013 and 0.020) the
substrate temperature remained at 800C.
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WO 92/09848 PCI`/US91/08~18
2~9~9~ 24
As displayed in ~igure 3, this example shows that a combustion
catalyst comprising palladium limits the temperature of ~he catalyst
composition to approximately 780~C. The temperature of the platinum
catalyst, on the other hand, clearly tracks the calculated adiabatic
combustion temperature.
Example 2
This example demonstrates measurement of the temperature at which
palladium oxide converts to palladium metal and, therefore, the temperature
limit of the catalyst substrate during methane combustion in excess air.
A sample of 21.9 mg of palladium oxide powder was loaded into a
~- TGA apparatus and the sample chamber purged with ~ry air flowing at 40
ml/minute. The temperature of the sample was increased at 10C/minute
and the sample weight monitored continuously to produce the TGA curve
15 shown in Figure 4A. At 795 C the palladium oxide decomposed into
` palladium and evolved oxygen resulting in a weight loss. The measured
weight loss of 2.74 mg resulting sample corresponded to 12.5% of the
A original palladium oxide sample weight. The theoretical weight loss ~or the
equation:
~, 20 PdO------> Pd ~ ()2
was t3.~%~ Repetition of t~e 'rGA exper~ment ~t a 5 C/minute heatin~ rate
also gave a palladiùm oxide to palladiùm transition point at 795C.
. The transition from palladium oxide to palladium in air at atmospheric
pressure measured by TGA occurs at approximately the same temperature
as th~ limiting substrate temperature (approximately equal to 780'C)
determined when using palladium as the catalyst as found in Example 1
above.
The TGA experiment was repeated with a new sample oF palladium
oxide but with the sample chamber purged with pure oxygen. As is shown
in Figure 4B, the measured palladium oxide to palladium transition
temperature was 880C. At higher oxygen partial pressure, the palladium
oxide to palladium transition point would occur at even higher temperatures.
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WO 92/09848 PCT/US91/08918
25 21~6~ll9
This example shows that the TGA for palladium oxide/platinurn is a
function of the particular oxygen partial pressure.
Example 3
This example is in ~vo parts. Part A shows the preparation of a steel
monolith using palladium but having no protective d-fflusion barrier layer
above the catalyst layer; Par~ B shows the use of the monolith and its
propensity for "runaway" even when used at low temperatures.
Part A
A 75.~ inch long sample of two inch wide Kawasaki River Lite 20-5SR
corrugated steel and a 73 inch long sample of two inch wide Kawasaki River
Ute 20-5SR flat steel strip were heat-treated in an oven in open air at 950C
for 16 hours. The heat treatment resulted in the growth of alumina whiskers
on the steel surface due to the aluminum contained in the steel.
t~ A primer coat was applied to both flat and corrugated strips on both
sides by spraying with a 5% by weight pseudo-boehmite colloidal aqueous
suspension to obtain a layer representing approximately 1% by weight of the
metal. The metal was dried at 90C.
A high surface area washcoat was applied by spraying with a 20% by
~, weight colloidal suspension of gamma-alumina, drying a~ 90C, and calcining
in air at 850~C for five hours. The final washcoat represented 20% of the
final catalyst weight.
A palladium-containing solution was prepared by dissolving
Pd(NH3)2(NO2)2 in nitric acid. This palladium solution was applied to the
washcoated foil strips by spraying to obtain a final catalyst loading of
approximately 2% by weight palladium metal. The strips were dried at 90~C
and calcined in air at 850 C for four hours.
The corrugated and flat strips were layered together and rolled to
-~ form a spiral monolith of approximately two inches diameter and withapproximately 300 channels per square inc~ of geometric area. The open
area of the monolith was approximately 2.36 inr,hr ~or approximately 770/~,
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W ~ 92/09848 P ~ /US91/08918 ~ j
; . 26
open). 2096949
P~t B
This Part shows ~he operation of the catalyst monolith fabricated in
5 Part A in "normal" inlet gas temperature ranges of betwsen 325C and
~- 400-C.
The catalyst structure was placed in the reactor system discussed
above. Two thermocouples were installed in the downstream end of the
monolith to measure monolith wall temperature. The bulk gas ternperature
at the outlet was also monitored.
A flow of 1500 SLPM of air and 70 SLPM of CH4 were introduced into
the monolith. The mixed gas was initially preheated to 300C. The preheat
temperature was slowly increased a~ a rate of approximately 20C/minute.
No substantial reaction was observed until the gas preheat
temperature reached 350C to 355 C. At that point the catalyst lit-off, that
is, the bulk gas temperature at the outlet increased to approximately 550 C.
The temperature at one of the monolith wall thermocouples increased quickly
to 1000C and then began to oscillate quickly between approximately 700C
and 1100C as shown in Figure 5.
This test sequence was then ~erminated. The catalyst was cooled.
A second test was performed at the monolith using the same test
sequence The catalyst lit-off between 325C and 335C. .The wall
temperature oscillation was again in evidence.
Consequentlyj even though the palladium component is considered in
the ar~ to limit the temperature incr0ase of the catalyst, the use of palladium
~; alone does not appear always adequate to limit the wall temperature.
'
Exam~e 4
This example shows the preparation of a steel monolith support using
palladium but having a barrier or diffusion barrier overcoat.
Part A
A 70.0 inch section of Kawasaki River Lite 20-5SR corrugated steel
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i "WO 92/09848 PCl tlJS91/08918
27 2~96~9
strip and a 70.0 inch section of Kawasaki River Lite 20-5SR flat steel strips
were heat treated in an oven in the open air at 950 C for 16 hours to cause
swrface alumina whisker growth.
Using the procedure of Example 3, the two metal strips were sprayed
5 with primer pseudo-boehmite, gamma-alumina washcoat, and palladium.
Ths various drying and calcining steps werc also done exactly as found in
Example 3.
,~ A diffusion barrier coating was then applied to the catalyst surface by
spraying a 30% gamma-alumina colloidal sol, drying at 90 C, and a calcining
10 at 850 C for five hours. The barrier coating was approximately 5% of the
total catalyst weight.
The two strips were then rolled together to make a spiral monolith of
approximately two inch diameter.
The free open area of the monolith was 2.36 inch2 (or approximately
15 78% open).
~art B
This Part shows the operation of the catalytic monolith fabricated in
Part A in the same temperature range and using the same temperature rate
20 increase as that use~ in Example 3.
The rolled monolith was inserted into the reactor system. The air rate
was 1500 SLPM and CH4 was the fuel at 60.5 SLPM. The, catalyst lit off at
approximately 365~C. The bulk gas temperature at the catalyst outlet
'quickly reached 600-C and stabilized. The wall temperature did not oscillate
25 as it had in Example 4.
This catalyst structure was then cooled and the test sequence
repeated four more times. The catalyst lit off in the 335-C to 345~C range
each tirne and the wall temperature did not oscillate in the normal preheat
range between 325C and 410-C.
ample5
-This example shows the temperature limiting effect of the inventive
catalyst with a single fuel/air ratio and a constantly increasing preheat
.,. . . . :
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wo 92t09848 Pcr/usgl/089l8 C."~.
?0969~9 28 i
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temperature. In spite of the increasing inlet temperature ancl the resulting
outlet partially combusted gas temperature, the wall of the catalyst structure
remains at approximately 800DC.
A high concentration palladium catalyst was prepared. A 50 mm
diameter by 50 mm length cordierite monolith with 100 cpsi was coated with
alumina washcoat as described above. The washcoated rnonolith was
calcined at 850 C for ten hours. A PdCI42- solution was prepared by
dissolving PdC12 in two equivalents of hydrochloric acid. The flnal solution
concentration was 0.081 g palladium/ml. The washcoated monolith was
dipped in this palladium solution and the excess solution blown out with air.
H2S gas was then passed through the monolith structure to entirely convert
the PdCI42 to PdS. The monolith was then calcined at ~OO~C in air. The
palladium impregnation procedure was repeated and the final calcination
performed at 850~C for ten hours.
This catalyst was placed in the test reactor described above.
Thermocouples were installed in a single channel at a distance from the inlet
of ten mm, 25 mrn, and 48 mm. This channel was sealed with ceramic
cement so that the thermocouples measured the substrate ceramic
temperature.
Air at 1000 SLPM and natural gas at 40 SLPM were passed through
the catalyst. ThiS feed gas mixture was heated to 300C and then increased
slowly to monitor catalyst activity as shown in Figure 6. At 360'C, the
catalyst lit-off and its temperature rose above the gas temperature. At
approximately 390C, the substrate temperature from ten mm to the outlet
(48 mm) was constant at approximately 800'C. As the inlet gas temperature
is further increased, the substrate temperature limited at approximately
800C.
At this fuel/air ratio and 400 C, the calculated adiabatic combus~ion
temperature was approximately ~2400C. The fact that this high activity
catalyst did not cause the substrate temperature to increase to 1240C is
due ~o ~he strong temperature limiting behavior of palladium.
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iS~ W O 92~09848 PC~r/US~1/08918
29 2 ~ 9
_ample 6
This example shows the LOT and steady state operation of a
palladium catalyst having a zirconia coated cordier-~e support.
The palladium/zirconia/cordierite catalyst was prepared by first
producing a zirconium sol. A 125 gm sampls of ZrO2 having a specfflc
surface area of 95 m2/gm was mixed with 211 ml water and 15 ml of
concentrated nitric acid in polymer lined ball mill containing ~rO2 grinding
media. The mixture was milled for eight hours.
A cordierite monolithic honeycomb having 1U0 cpsi was dipped into
the sol, dried, and calcined as described above. This process was repeated
until the monollth contained about 18% by weight of the ZrO2 washcoat.
A palladium solution was made by dissolving Pd(NO2)2(NH3)2 in
aqueous HNO3 and diluting with water until a concentration of 0.083 9
palladium/ ml was attained. The monolith was dipped into the palladium
solution, excess solution blown out with air, dried, and calcined at 850~C in
air. The process was repeated until the catalys~ composition contained 2.2%
palladium.
This catalyst composition was placed in an adiabatic combustion
reactor. An air flow of 1500 SLPM and a natural gas flow of 60 SLPM is
initiated through the catalyst. The mixed gas temperature ("preheat") is
increased at a constant rate. At 350C the catalyst becomes active. As is
shown in Figure 7A, at 370 C of preheat the catalyst outlel; becomes
constant at about 800 C. Further increases in the preheat temperature do
not cause the catalyst outlet temperature to increase. The palladium limits
the catalyst outlet temperature to that point.
The catalyst was additionally tested for steady statc operation at 1000
SLPM of air and 40 SLPM of fuel. The catalyst was operated at a constant
preheat of 400C. As is shown in Figure 7B, the ca~alyst was very stable
and maintained a catalyst outlet temperaturs of about 770C. No decline in
' 30 activitywas noted.
Example 7
This example is similar to Example 6 but shows instead the beneflcial
,~,
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wo 92/09848 PCr/US91/0~918 ,~
,~ 6 9 ~ 9 30
effect in our partial combustion catalyst of utilizing zirconia on a metal
support.
-~ A monolithic me~al-foil-based partial combustion catalyst having a
ZrO2 coating was prepared and tested for steady-state stabil~y using the
5 following procedure.
A ZrO2 colloidal sol was first produccd by hydrolyzing 66 gm of
zirconium isoperoxide with water and m~xing the resu~tant mixture with 100
gm of ZrO2 powder and an additional 100 gm of water. The zirconia powder
had a specific area of 100 m2/gm. This slurry was ball milled in a polymer
10 tined ball mill with cylindrical ZrO2 media for eight hours. Ths resultant sol
was diluted to a concentration of 15% ZrO2 with additional water.
- An Fe/Cr/AI foil was corrugated in a herringbone pattern and oxidized
in air at 900C to form surface alumina whiskers. The foil was sprayed with
the sol using an air atomizer, dried, and calcined in air at 850C. The
15 resulting foil contained 2 mg ZrO2/cm2 of foil surface.
A solution containing 0.1 gm palladium/ml was formed by dissolving
palladium 2~ethylhexanoic acid in tolu0ne. This solution was sprayed onto
the coated metal foil. The foil was dried and calcined and contained about
0.5 mg palladium/cm2 of surface.
~, 20 The corrugated foil was rolled into a spiral structure having
longitudinal passageways throughout. The final structure was about h~o
inches in diameter and two inches in length. ,
The catalyst was tested for steady-state operation much in the same
-` way that the above catalysts were tested. Thermocouples were installed
within the catalyst at distances of 1, 2.5, and 4.8 cm from the entrance of the
catalyst structure. Other thermooouples measured the temperature at the
outlet of tha catalyst and in the ~as stream 15 cm after the catalyst.
An air flow of 1000 SLPM and a natural gas flow of 40 SLPM was
initiated through the catalyst. The mixed gas temperature ("preheat") was
increased at a constant rate . At 400 C the catalyst became active. As is
- shown in Figure 8A, at 440C of preheat the catalyst outlet bscomes
:
constant at about 770 C. Further increases in the preheat temperature did
; not cause ghe catalyst outlet temperature to increase. The palladium limits
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WO 92/09848 PCI`/US91/08918
3t . 2 a ~
the catalyst outlet temperature to that point.
The catalyst was additionally tested for steady state operation at 1000
SLPM of air and 40 SLPM of fuel. The catalyst was operated at a constant
preheat of 500-C. As is shown in Figure 8B, the catalyst was very stable
and maintained a catalyst ou~let temperature of about 7~0~C to 770C. No
decline in activity was noted.
Example 8
Part A
This Part A shows the production of a palladium catalyst on a zirconia
- coated cordierite support using a hydrazine reduction.
A polymer lined ball mill was loaded with 125 g of ZrO2 powder
(having a surface area of 95 m2/gm), 211 ml water, and 15 ml of
concentrated nitric acid. The mill was filled with ZrO2 media and the mixture
milled for eight hours.
Pd(NH3)2(N02)2 was dissolved in nitric acid to form a solution
containing 0.083 g palladium/ml. Forty-two ml of this palladium solution was
added to 50 g of the ZrO2 colloidal sol, the pH adjusted to 9.0, and 1.0 9 of
hydrazine added. This solution was stirred until the palladium was
;, 20 completely reduced. This sol contained 20% palladium by weight.
A cordierite, monolithic honeycomb 50 mm in diameter and 50 mm
tong with 100 square cells per square inch was dipped in the palladium/ZrO2
sol, the excess sol blown out of the honeycomb channels with air, dried, and
calcined at 850DC. This process was repeated until the monolith contained
12% palladium/ZrO2. The final catalyst contained 2.3% palladium by weight
and the washcoat had a surface area of 26 m2/gm.
The catalyst was placed in a two inch l.D. insulated section of a
;~ combustion reactor. Air at 1000 Standard Liters Per Minute (SL~M) waspassed through a heater, a static gas mixer, and then through the catalyst.
Natural gas was introduced into the air stream just upstream of the gas
mixer. Gas temperatures were measured before and af~er the catalyst by
thermocouples suspended in the gas stream. The catalyst substrate
ternperature was measured by use of thermocouples which had been placed
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WO 92/09848 PCr/US91/08918 ~.
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in channels at positions of 25 mm and 48 mm from the inlet and sealed with
ceramic cement.
Natural gas at 40 SLPM was introduced into the air stream and the air
temperature increased to 400 C. The catalys~ substrate temperature rose to
about 750~C and stabilized. The outlet gas temperature remained at 560'C.
These temperatures were very stable for the 3.5 hour duration of the test.
The LOT data and the steady state run data are shown in Figures 9A
and 9B, respectively.
- 10 .P~rt B
This comparative example shows the production of an alumina
supported catalyst.
A very high purity alumina containing less than 50 ppm total impurities
was ball milled as in Part A above. A Pd(HN3)2(NO2)2 solution was added
and reduced wi~h hydrazine (also as described in Part A) to form a
palladium/AI2O3 sol containing 20% palladium by weight. A cordierite
monolith was coated to produce a final catalyst containing 1 i%
palladium/AI2O3 and 2.2% palladium by weight. The final washcoat had a
surface area of 50 m2/gm.
The catalyst was tested as described in Part A. The catalyst was
tested both for LOT (Figure 10A) and for steady state performance (Figure
10B). The steady state perforrnance at a preheat of 400Ç (Figure 10B)
shows rapid deactivation of the catalyst with the substrate temperatures
dropping to 420-C at the center of the catalyst and the outlet gas
temperature dropping from 560Cc to 485'C in only three hours.
The catalysts tested in this Example had similar loadings and
. preparation procedures. The alumina supported catalyst tested in Part B
had a higher surface area and might be expected to be more active.
However, the palladium/AI2O3 catalyst deactivated very rapidly. This
suggests that Al2O3 is a less appropriate support for palladium in oatalytic
combustion catalysts; ZrO2 is much preferred.
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Example 9
P~rt A
This example shows the production of a corrugated matal foil
honeycomb substrate having a zirconia coating.
A ZrO2 colloidal sol was prepared as follows. Abou~ 66 g of zirconium
isopropoxide was hydrolyzed with 75 cc water. It was then mixed with 100 9
of ZrO2 powder having a surface area of 100 m2/gm and an additional 56 ml
water. This slurry was ball milled in a polymer lined ball mill using ZrO2
cylindrical grindin~ media for eight hours. This colloidal sol was diluted to a
concentration o~ 15% ZrO2 by weight with additional water.
An Fe/Cr/AI metal foil was corrugated in a herringbone pattern and
then oxidized at 900C in air to form alumina whiskers on the foil surface.
The ZrO2 sol was sprayed onto one sid~ of the corrugated foil. The coated
foil was then dried and calcined at 850'C. The final foil contained 2.0 mg
; 15 ZrO2/cm2 foil suRace~
Palladium 2-ethylhexanoic acid was dissolved in toluene to a
concentration of 0.1 g palladiumlrnl. This solution was sprayed onto the
ZrO2 coated metal foil and the foil dried and calcined at 850 C in air. The
final foil contained about 0.5 mg palladium/cm2 of foil surface.
The corrugated foil was rolled so that the corrugations did not mesh
to form a final metal structure of two inch diameter and two inch length with
Iongitudinal channels running axially through the structure,.
The catalyst was piaced in the combustion reactor described in
Exampla 8 and tested for LOT and steady state operation. The air flow rate
was 1000 SLPM, the methane rate was 40 SLPM, and the steady state
preheat temperature was 450C. The performace of this catalyst is shown in
Figure 11A and 11B.
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Part B
This catalyst was prepared to be similar to Part A of this Example but
using Al2O3 as the support.
An Al2O3 sol was prepared. A polymer lined ball mill was loaded with
250 g of gamma alumina, 422 ml water, and 21 ml of concentrat0d ntrlc
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WO 92/0~P~48 PCr/US91/08918
. 2096~ag ~ ~.
acid. Cylindricai zirconia media was added and the mixture milled for eight
hours.
This alumina sol was diiuted to about 15% solids with water, sprayed
onto a corrugated metal foil, and calcined at 850 C for ten hours. The final
. 5 Al2O3 loading was 2.2 mg/cm2.
A palladium 2-ethylhexanoic acid solution was sprayed onto the ZrO2
eoated metal foil and the foil dried and calcined at 850 C in air. The final foil
contained about 0.5 mg palladium/cm2 of foil surFace.
;~ The catalyst was placed in the reactor described above and tested.
The air flow rate was 1000 SLPM, the natural gas flow rate was 40 SLPM,
and the steady state preheat temperature was 450'C. The performance of
this catalyst is shown in Figures 12A and 12B. The palladium/ZrO2 shows
substantially improved stability over the catalyst containing alumina.
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Example 10
; This Example shows the production and use of a series of the
inventive catalysts containing varying amounts of platinum but a set amount
of palladium on a cordierite support. The catalysts were tested for two
parameters: "light off temperature" (LOT) and stability during steady-state
operation.
A monolithic honeycomb structure composed of cordierite ceramic
with 100 square cells per in2 was coated with a Al2O3 colloidal sol (Catapal
B) and the monolith calcined in air at 800~C. Tha resulting monolith
contained 23.7% Al203. The monolith was dipped in a solution of
; Pd(NO2)2(NH3)2 repeatedly, dried, and calcined in air at 850C for ten hours.
The catalyst contained 1.0 % palladium based on the alumina washcoat.
Four 20 mm diameter cylinders ~25 mm in iength) were cut from the
monolith structure prepared above. Three of these sections were then
treated with varying amounts of a Pt(N02)2(NH3)2 solution using drip
impre~nation. Each was dried and calcined in air at 850'C for one or ten
hours. The resultin~ catalysts contained 0.15%. 0.30%, and 1.2% platinum
based on the alumina washcoat.
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WO 92/09848 PCI'/US91/08918
35 2~69~9
The catalysts were each placed in a combustion reactor. Three
thermocouples were placed in a single channel spacecl along the catalyst at
distances of one, 2.5, and 4.8 cm from the iniet. This channel was filled with
ceramic cement so that the thermocouples measurled the substrate
5 temperature (wall) and not the gas temperature.
Air at 1500 standard llters/minute lSLPM) was passed through an
electric heater, mixed with methane, and passed over the catalyst.
All four catalysts were then tested using the LOT and steady-state
`' activity procedures outlined above. Graphs of the data ratrieved are shown:
Catalvst Composition LOT Fia. Steadv State Fi~.
1% Pd 13A 14A
1% Pd, 0.15% Pt 13B 14B
.~ 1% Pd, 0.30% Pt 13C 14C
1% Pd, 1.20% Pt 13D 14D
It may be observed by comparing Figures 13A with Figures 13B, 13C,
and 13D that the LOT in each case rernained at about 350-360~C
irregardless of the increasing amounts of platinum. The self-limiting
20 temperature for the platinum containing catalysts were surprisingly lower
than that for the platinum-free catalyst and in no event did tha temperatures
reach the adiabatic combustion tcmperature. The theoretjcal adiabatic
combustion temperature for this gas mixture is about 1160C. The
temperature of the gas leaving the catalyst was constant for each of the
25 platinum containing catalysts showing overall stability even though some
deactivation was observed in the early regions of the catalysts containing the
lower arnounts of platinurn (Figures 13B and 13C).
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Example 1 1
This Example shows the beneficial effect of adding platinum metal to a
catalyst of palladium on a metal support.
Two catalysts were prepared. The firs~ catalyst was a "control" in
which palladium alone was used as the catalyst. The second catalyst was
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WO 92/09848 PCrtUS91/08918
' 36
one using both pl~ alladium.
Pd-on!y metal s~pported çat~!yst
One side of an aluminum-containing steel foil (CAMET), after
oxidation to form an alumina microcoating, was spray-ce~ated on one
side with two coats of an alumina sol (Catapal-B), dried, and calcined
at 850 C. The foil was sprayed with a solution of Pd(NH3)2(NO2)2,
dried, and calcined at 850CC for 10 hours. The resuiting toil was the
rolled into a monolith ha\/ing about 150 cells per square inch (CPSI).
The coating contained about 1.3 mg-Pd/cm2 based on the alumina
washcoat.
Pd/pt supported catalyst
One side of an aluminum-containing steel foil (CAMET), after
oxidation to form an alumina microcoating, was spray-coated on one
side with two coats of an alumina sol ~Catapal-B), dried, and calcined
at 850'C. The foil was sprayed with a solution o~ Pd(NH3)2(NO2)2,
dried, sprayed with a solution of Pt(NH3)2(NO2)2, and dried. Tha
dried foil was calcined at 850'C for ten hours. The resulting foil was
the rolled into a monolith having about 150 cells per square inch
(CPSI). The coating contained about 1.3 mg-Pd/cm2 and about 0.13
mg-Pt/cm2 based on the alumina washcoat. The ~d-Pt molar ratio
was about 15.
Each of the catalysts was tested using the steady-state test outlined
abov~. The air rata was 1000 SLPM and the fuel/air ratio was either of
0.019 or 0.040. The preheat level in each instance was 500'C. The
temperature of the gas leaving the catalyst and the preheat temperature
were measured in each case.
By comparing Figure 15A with Figure 15B, b~th runs using a fuel/air
ratio of 0.019, the post catalyst temperatura of the palladium catalyst
.. decreas0d to a level of 590C and at the conclusion of the three hour test
was still declining. The inventive palladium/platinum catalyst dernonstrated
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in Figure 15B, in comparison, increased to that Ievel and when the test was
terminated at 1.7 hours, was generally level.
The runs shown in Figures 16A and 16B show, at a fuel/air ratio
of 0.040, that the platinum-stabili~ed palladium catalyst is signficantly more
5 stable. In the Figure 16B run, tha catalyst is still operating at a stable level of
about 650C. In contrast, the stability o~ the unstablilized catalyst in the
Figure 16A run has caused the outlet temperature to degrade to 600C at
about three hours.
This Example demonstrates the difference between the catalyst based
10 on palladium alone and based both on palladium and platinum. The metal
support in this example was coated with alumina.
Example 12
This example shows the preparation of palladium/ZrO2-cordierite
15 catalyst with graded palladium concentration and a comparative non-graded
catalyst.
Part A
A 125 g amount of ZrO2 having a surface area of 45 m2/gm was
impregnated with 45 ml of a solution containing 0.0834 g palladium/ml. This
solution was prepared by dissolving Pd(HN3)2(NO2)2 in nitric acid. The
palladium/ZrO2 mixture was dried and calcined at 500C~
A polymer lined ball mill was loaded with 125 9 of the palladium/ZrO2
mixture, 230 ml of water, 2.0 ml 70% HN03 (nitric acid), and ZrO2 media.
A sample of 50 ml of the resulting sol was mixed with an additional 36
ml of 0.0834 g palladium/ml solution, th0 pH adjusted to 9.0 with NH40H,
and 0.64 g hydræine added with stirring. The palladium was reduced over a
period of sev~ral hours. This sol (after full calcination) produced a washcoat
containing 13.6% palladium on ZrO2.
A cordierite monolith with 100 cells/in2 (2 inches in diameter by 2
inches long) was dipped into the above palladium/ZrO2 sol, the excess sol
was blown from the channels with a stream of air, and the monolith was
dried and calcined at 850C. The resulting single coat had 7.69%
wo 92/09848 Pcr/us91/o89l8 ~
2096~9 38
palladium/ZrO2 loading and 1.03% palladium loading on the monolith.
This catalyst was again dipped in the palladium/ZrO2 sol but only to a
depth of 12 mm, the excess sol was blown from tha channels, and the
monolith was dried and calcined. The palladium concentration on this
monolith was 4.2% palladium on the 12 mm inlet section and 1.0% palladium
on the 39 mm outlet portion. The average palladium concentration was
1.8% palladium.
Part A
This example shows a non-graded panadium/ZrO2-cordierite ca~alyst
for comparison.
Another cordierite monolithic structure was dipped and blown out
using a process similar to that shown in Part A. The final catalyst was
calcined at 850C. The final catalyst had a relatively uniform palladium
concentration of 1.9% from inlet to outlet.
E~ample 13
This example cornpares the relative activities of the two catalysts of
Example 12. The two catalysts were subjected to a test in which the
methane and air were preheated. The preheat temperature was constantly
increased beginning at 325C and outl~t temperaturas for the partially
combusted gas and a variety of internal temperatures were measured and
recorded.
The catalysts were separately loaded into an adiabatic reactor. Air
was introduced at a rate of 800 standard liters per minute (SLPM) and
methane was introduced at 36 SLPM.
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Part A
As may be seen in Figure 17, at 350-C preheat the Example 12, Part
A ~raded catalyst lit-off and became a~ive. The monolith wall temperature
rose to 780 C and the outlet gas temperature stabilized at about 550~C. As
the preheat was continually increased, the catalyst outlet was visually
unHorm in temperature and appeared to be a dull red color. At a preheat
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. 39
temperature of 448C, a bright white spot appeared as a portion of the
catalyst increased in temperature to above 800C. The white region began
to grow in size and the fuel was cut off to prevent destruction of the catalyst.
.~ 5 This graded catalyst had an operating window between 3~0DC and
M8C or a 98~C span.
Part B
The comparison ca~alyst from Example 12 Part B was tested in a
similar fashion. As may be seen in Figure 18, this catalyst lit-off at
approximately 350~C and exhibited a hot spot ~>800~C) at a preheat
temperature of about 390~C. This catalyst had a window between 350'C
- and 390C or only a span of about 40C.
The graded catalyst had a much larger operating window even though
both catalysts had the same palladium loading,
; The invention has been disclosed both by description and by the use
of examples. The examples are only examples and must not be used to limit
the invention in any way. Furthermore one having ordinary skill in this art
would be able to determine equivalents to the invention described here but
outside the literal scope of the appended claims. We alsp consider those
equivalents to be part of our invention.
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