Catalyst Composition and Structure for a Diesel-Fueled Reformer Configured
in an Exhaust Stream
Field of the Invention
The present invention relates to diesel power generation systems with
exhaust aftertreatrnent.
Background
NOx emissions from diesel engines are an environmental problem.
Several countries, including the United States, have long had regulations pending
that will limit NOx emissions from trucks and other diesel-powered vehicles.
Manufacturers and researchers have put considerable effort toward meeting those
regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures,
three-way catalysts have been shown to control NOx emissions. In diesel-powered
vehicles, which use compression ignition, the exhaust is generally too oxygen-rich
for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from
diesel-powered vehicles. One set of approaches focuses on the engine.
Techniques such as exhaust gas recirculation and partially homogenizing fuel-air
mixtures are helpful, but these techniques alone will not eliminate NOx emissions.
Another set of approaches remove NOx from the vehicle exhaust. These include the
use of lean-burn NOx catalysts, selective catalytic reduction (SCR) catalysts, and
lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich
conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven
challenging to find a lean-burn NOx catalyst that has the required activity, durability,
and operating temperature range. A reductant such as diesel fuel must be steadily
supplied to the exhaust for lean NOx reduction, introducing a fuel economy penalty
of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx
catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia.
The reaction takes place even in an oxidizing environment. The NOx can be
temporarily stored in an adsorbent or ammonia can be fed continuously into the
exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage
in the lack of infrastructure for distributing ammonia or a suitable precursor. Another
concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOx under lean exhaust conditions and
reduce and release the adsorbed NOx under rich conditions. An LNT generally
includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline
earth compound, such as BaCO3 and the catalyst is typically a combination of
precious metals including Pt and Rh. In lean exhaust, the catalyst speeds oxidizing
reactions that lead to NOx adsorption. In a reducing environment, the catalyst
activates reactions by which hydrocarbon reductants are converted to more active
species, the water-gas shift reaction, which produces more active hydrogen from
less active CO, and reactions by which adsorbed NOx is reduced and desorbed. In
a typical operating protocol, a reducing environment will be created within the
exhaust from time-to-time to regenerate (denitrate) the LNT.
Regeneration to remove accumulated NOx may be referred to as
denitration in order to distinguish desulfation, which is carried out much less
frequently. The reducing environment for denitration can be created in several ways.
One approach uses the engine to create a rich exhaust-reductant mixture. For
example, the engine can inject extra fuel into the exhaust within one or more
cylinders prior to expelling the exhaust. A reducing environment can also be created
by injecting a reductant into lean exhaust downstream from the engine. In either
case, a portion of the reductant is generally expended to consume excess oxygen in
the exhaust. To lessen the amount of excess oxygen and reduce the amount of
reductant expended consuming excess oxygen, the engine may be throttled,
although such throttling may have an adverse effect on the performance of some
engines.
Reductant can consume excess oxygen by either combustion or reforming
reactions. Typically, the reactions take place upstream of the LNT over an oxidation
catalyst or in a fuel reformer. The reductant can also be oxidized directly in the LNT,
but this tends to result in faster thermal aging. U.S. Pat. Pub. No. 2004/0050037
(hereinafter "the '037 publication") describes an exhaust system with a fuel reformer
placed in an exhaust line upstream from an LNT. The reformer includes both
oxidation and reforming catalysts. The reformer both removes excess oxygen and
converts the diesel fuel reductant into more reactive reformate.
The oxidation and reforming catalysts of the '037 publication are subject to
harsh conditions. According the '037 publication, it is desirable to heat the fuel
reformer to steam reforming temperatures for each LNT regeneration and to pulse
the fuel injection during regeneration to prevent the fuel reformer from overheating.
Pulsing causes the catalyst to alternate between lean and rich conditions while at
high temperature. The catalyst itself tends to undergo chemical changes through
this cycling, which can lead to physical changes, especially sintering, which is the
growth of catalyst particles. As the particles grow, their surface area and number of
surface atoms decrease, resulting in a less active catalyst.
Numerous choices are available for the oxidation and reforming catalysts,
With regard to the oxidation catalyst, the '037 patent lists Pd, Pt, Ir, Rh, Cu, Co, Fe,
Ni, Ir, Cr, and Mo as possible choices, without limitation. The catalyst support is also
important. The '037 patent lists as examples, without limitation, cerium zirconium
oxide mixtures or solid solutions, silica alumina, Ca, Ba, Si, or La stabilized alumina.
Many other oxidation catalysts, supports, and stabilizers are known in the art.
Likewise, many examples or reforming catalysts are known. The '037 patent list Ni,
Rh, Pd, and Pt as possible reforming catalysts, without limitation. As with the
oxidation catalyst, a wide range of supports and stabilizers could be considered for
use.
In spite of advances, there continues to be a long felt need for an
affordable and reliable diesel exhaust aftertreatment system that is durable, has a
manageable operating cost (including fuel penalty), and reduces NOx emissions to a
satisfactory extent in the sense of meeting U.S. Environmental Protection Agency
(EPA) regulations effective in 2010 and other such regulations.
Summary
After considerable research, the inventors have developed oxidation and
reforming catalysts for use in diesel exhaust aftertreatment systems. The catalysts
are economical and superior in terms of durability under lean-rich cycling at high
temperatures. The catalysts comprise precious metals supported on La stabilized
refractory metal oxides. The La is distributed on the surface of the refractory metal
oxide support in an amount to form at least about a monolayer, preferably about 1-2
monolayers. Preferably, the La is substantially amorphous in the sense of having no
crystalline structure shown by X-ray diffraction. Nd and mixtures of La and Nd can
be used in place of La. The La is typically in an oxide form and the precious metal
may be either reduced or in oxide form.
In one embodiment, the catalyst is a reforming catalyst comprising an
effective amount of Rh on a ZrO2 support. The catalyst preferably comprises from
about 0.5 to about 1.0 mg La per m2 refractory metal oxide surface distributed over
the surface. For a typical ZrO2 support that has a surface area of about 100 m2/g,
this gives from about 5 to about 10% La by weight refractory metal oxide. The
catalyst preferably also comprises from about 0.01 to about 0.1 mg Rh per m2
refractory metal oxide surface area. The Rh is distributed on the surface of the
refractory metal oxide particles along with or over the La. For the typical ZrO2
support, this loading gives from about 0.1 to about 1.0% Rh by weight refractory
metal oxide. The Rh is present in an amount effective to catalyze steam reforming of
diesel fuel at 650 oC. Preferably, the Rh has an average particle size of under 5 nm
and the catalyst is functional to maintain the Rh particle size under 5 nm through 400
25 minute lean/25 minute rich lean/rich cycles at 750 oC. Preferably, the Rh has a
dispersion of at least about 40% and the catalyst is functional to maintain a
dispersion of at least about 40% through 400 25 minute lean/25 minute rich lean/rich
cycles at 750 °C. Preferably, the catalyst comprises little or no platinum.
According to a further aspect of the invention, the Rh is provided in a
relatively low concentration: from about 0.01 to about 0.05 mg per m2 refractory
metal oxide surface area, which corresponds to about 0.1 to about 0.5% Rh by
weight refractory metal oxide for the typical ZrO2 support. The inventors have found
that if the Rh loading is kept sufficiently low, the Rh can be maintained in the form of
small particles (less than 5 nm, typically about 2 nm or less) while the catalyst
undergoes lean-rich cycling through an effect involving the La. The improvement in
stability is such that as the Rh loading is reduced from about 0.10 mg/m2 to about
0.05 mg/m2 or less, nearly the same or greater catalyst activity results after aging
than is achieved with the larger Rh loading.
In another embodiment, the catalyst is an oxidation catalyst comprising an
effective amount of Pd on an Al2O3 refractory metal oxide support. The catalyst
preferably comprises from about 0.5 to about 1.0 mg La per m2 refractory metal
oxide distributed over the surface of the refractory metal oxide particles. For a
typical AI2O3 refractory metal oxide support that has a surface area of about 200
m2/g, this corresponds to from about 10 to about 20% La by weight refractory metal
oxide. The catalyst preferably also comprises from about 0.25 to about 1.0 mg Pd
per m2 refractory metal oxide surface area, which corresponds to from about 5 to
about 20% Pd by weight refractory metal oxide for the typical AI2O3 refractory metal
oxide support. The Pd is present in an amount effective for the oxidation catalyst to
light off at 275 °C, more preferably at 240 °C. Preferably, the Pd has an average
particle size of under 10 nm and is functional to maintain a particle size under 10 nm
through 400 hours of 25 minute lean/25 minute rich lean/rich aging at 750 °C.
Preferably, the Pd has a dispersion of at least about 15% and the catalyst is
functional to maintain a dispersion of at least about 15% through 400 hours of aging
in a lean atmosphere comprising 10% steam at 750 °C.
A further aspect of the invention relates to a method of operating a power
generation system comprising operating a diesel engine to produce lean exhaust
and passing the exhaust through a fuel reformer and a lean NOx trap in that order,
whereby a portion of the NOx in the exhaust is absorbed by the lean NOx trap. From
time-to-time, a control signal to regenerate the lean NOx trap is produced. In
response to the control signal, diesel fuel is injected into the exhaust upstream from
the fuel reformer at a rate that makes the exhaust-fuel mixture overall lean, whereby
the injected fuel combusts within the fuel reformer raising the temperature of the fuel
reformer. After the fuel reformer has heated to at least about 500 °C, a rich phase is
initiated by increasing the fuel injection rate and/or lowering the exhaust oxygen flow
rate to cause the exhaust-injected fuel mixture to become overall rich, whereby the
fuel reformer produces reformate that regenerates the lean NOx trap. The fuel
reformer comprises oxidation and reforming catalysts. The reforming catalyst
comprises a catalyst washcoat comprising a ZrO2 refractory metal oxide support, a
LnxOY distributed on the surface of the refractory metal oxide in an amount at least
sufficient to form about a monolayer over the refractory metal oxide support, wherein
Ln is selected from the group consisting of La, Nd, and mixtures thereof, and Rh
distributed over the catalyst surface in an effective amount to catalyze steam
reforming at 650 °C. In one embodiment, the method further comprises
discontinuing the fuel injection to allow the fuel reformer to cool in a lean phase and
cycling repeatedly between the rich and lean phases to complete the regeneration of
the lean NOx trap. This pulsed operation creates harsh operating conditions to
which the claimed compositions are particularly well adapted.
The primary purpose of this summary has been to present certain of the
inventors' concepts in a simplified form to facilitate understanding of the more
detailed description that follows. This summary is not a comprehensive description
of every one of the inventors' concepts or every combination of the inventors'
concepts that can be considered "invention". Other concepts of the inventors will be
conveyed to one of ordinary skill in the art by the following detailed description
together with the drawings. The specifics disclosed herein may be generalized,
narrowed, and combined in various ways with the ultimate statement of what the
inventors claim as their invention being reserved for the claims that follow.
Brief Description of the Drawings
Fig. 1 is a schematic illustration of an exemplary exhaust aftertreatment
system that can embody various concepts described herein.
Fig 2 Surface Rh in micromoles per g ZrO2 on a 5% La/ZrO2 support as a
function of time under cyclic aging for various Rh loadings.
Fig. 3 shows the stability under steam aging of 10% Pd co-dispersed with
various amounts of La on a commercially available La-stabilized AI2O3 support.
Detailed Description
The catalysts of the present disclosure are adapted to a particular
application. Figure 1 is a schematic illustration of an exemplary power generation
system 100 embodying that application. The power generation system 100 is not the
only power generation system to which the inventors' concepts are applicable, but
the various concepts described herein were originally developed for systems like the
system 100 and the individual components of the system 100 pertain to preferred
embodiments. The power generation system 100 comprises a diesel engine 101
and an exhaust line 102 in which are configured components of an exhaust
aftertreatment system 103. The exhaust aftertreatment system 103 comprises a fuel
reformer 104, a lean NOx trap 105, and an ammonia-SCR catalyst 106. A fuel
injector 107 is configured to inject fuel into the exhaust line 102 upstream from the
fuel reformer 104. A controller 108 controls the fuel injection based on information
about the operating state of the engine 101, a temperature of the fuel reformer 104
measured by a temperature sensor 109, and a NOx concentration measurement
obtained by the NOx sensor 110 at a point in the exhaust line 102 downstream from
the lean NOx trap 105. A temperature sensor 111 is configured to measure the
temperature of the lean NOx trap 105, which is particularly important during
desulfation.
The diesel engine 101 is a compression ignition engine. A compression
ignition diesel engine normally produces exhaust having from about 4 to about 21%
O2. An overall rich exhaust-reductant mixture can be formed by injecting diesel fuel
into the exhaust during cylinder exhaust strokes, although it is preferred that any
reductant injection into the exhaust take place downstream from the engine 101.
The engine 101 is commonly provided with an exhaust gas recirculation (EGR)
system and may also be configured with an intake air throttle, either of which can be
used to reduce the exhaust oxygen concentration and lessen the amount of
reductant required to produce an overall rich exhaust-reductant mixture. A lean burn
gasoline engine or a homogeneous charge compression ignition engine can be used
in place of the engine 101. The engine 101 is operative to produce an exhaust that
comprises NOx, which is considered to consist of NO and NO2.
The engine 101 is generally a medium or heavy duty diesel engine. The
inventors' concepts are applicable to power generation systems comprising light duty
diesel and lean burn gasoline engines, but the performance demands of exhaust
aftertreatment systems are generally greater when the engine is a medium and
heavy duty diesel engine. Minimum exhaust temperatures from lean burn gasoline
engines are generally higher than minimum exhaust temperatures from light duty
diesel engines, which are generally higher than minimum exhaust temperatures from
medium duty diesel engines, which are generally higher than minimum exhaust
temperatures from heavy duty diesel engines. Lower exhaust temperatures make
NOx mitigation more difficult and place lower temperature light off requirements on
fuel reformers. A medium duty diesel engine is one with a displacement of at least
about 4 liters, typically about 7 liters. A heavy duty diesel engine is one with a
displacement of at least about 10 liters, typically from about 12 to about 15 liters.
A light-off temperature for the fuel reformer 104 is an exhaust temperature
at which when the fuel reformer 104 can be heated substantially above the exhaust
temperature by combusting within the fuel reformer 104 fuel injected into the exhaust
line 102 through the fuel injector 107. Typically, once the fuel reformer 104 has lit
off, the fuel reformer 104 will remain substantially above the exhaust temperature
even if the exhaust temperature is lowered somewhat below the light-off
temperature, provided the fuel injection continues.
The exhaust from the engine 101 is channeled by a manifold to the
exhaust line 102. The exhaust line 102 generally comprises a single channel, but
can be configured as several parallel channels. The exhaust line 102 is preferably
configured without exhaust valves or dampers. In particular, the exhaust line 102 is
preferably configured without valves or dampers that could be used to vary the
distribution of exhaust among a plurality of LNTs 105 in parallel exhaust channels.
Valves or dampers can be used to reduce the exhaust flow to a fuel processor or
LNT, allowing regeneration to be carried out efficiently even when exhaust conditions
are unfavorable. Nevertheless, it is preferred that the exhaust line 102 be configured
without valves or dampers because these moving parts are subject to failure and can
significantly decrease the durability and reliability of an exhaust aftertreatment
system.
Even when the exhaust line 102 is free from exhaust valves or dampers,
an exhaust line upstream from the exhaust line 102 may still contain an exhaust
valve, such as an exhaust gas recirculation (EGR) valve in an EGR line. Exhaust
valves are particularly problematic when they are configured within a main exhaust
line to divert a majority of the exhaust flow as compared to exhaust valves
configured to control the flow through a side branch off a main exhaust line. Exhaust
valves for larger conduits are more subject to failure than exhaust valves for smaller
conduits.
The exhaust line 102 is provided with an exhaust line fuel injector 107 to
create rich conditions for LNT regeneration. The inventors' concepts are applicable
to other method's of creating a reducing environment for regenerating the LNT 105,
including engine fuel injection of reductant and injection of reductants other than
diesel fuel. Nevertheless, it is preferred that the reductant is the same diesel fuel
used to power the engine 101. It is also preferred that the reductant be injected into
the exhaust line 102, rather than into the cylinders of engine 101, in order to avoid oil
dilution caused by fuel passing around piston rings and entering the oil gallery.
Additional disadvantages of cylinder reductant injection include having to alter the
operation of the engine 101 to support LNT regeneration, excessive dispersion of
pulses of reductant, forming deposits on any turbocharger configured between the
engine 101 and the exhaust line 102, and forming deposits on any EGR valves.
The diesel fuel is injected into the exhaust line 102 upstream from the fuel
reformer 104. The fuel reformer 104 comprises an effective amount of precious
metal catalysts to catalyze oxidation reactions at 275 °C and steam reforming
reactions at 650 °C. The fuel reformer 104 is designed with low thermal mass,
whereby it can be easily heated to steam reforming temperatures for each LNT
regeneration. Low thermal mass is typically achieved by constructing the fuel
reformer 104 using a thin metal substrate to form a monolith structure on which the
catalyst or catalysts are coated. A thin metal substrate has a thickness that is about
100 µm or less, preferably about 50 µm or less, and still more preferably about 30
µm or less.
[0036] Oxidation and reforming catalysts can be co-dispersed on the fuel
reformer 104, but preferably, they are applied separately. The oxidation catalyst
preferably forms a coating beginning proximate an inlet of the monolith and
continuing part way toward or entirely to an outlet of the monolith. The reforming
catalyst preferably forms a coating beginning proximate the outlet and continuing
part way or entirely toward the inlet. In one embodiment, the reforming catalyst does
not proceed entirely to the inlet, whereby injected fuel undergoes a substantial
degree of reaction over the oxidation catalyst prior to encountering the reforming
catalyst. The oxidation and reforming catalysts can occupy disjoint areas, abutting
areas, or overlapping areas.
If the catalyst areas do overlap, either catalyst can be uppermost. Making
the reforming catalyst uppermost has the advantage that it contact the reactants
after the least diffusion. This is the preferred configuration if the reforming catalyst
proceeds only partly toward the inlet. The reforming catalyst is more expensive than
the oxidation catalyst, and it is therefore desirable that it be utilized as efficiently as
possible. The oxidation catalyst, on the other hand, is least costly and can often be
provided in greater quantity when more oxidation catalyst activity is desired. An
advantage of applying the oxidation catalyst in a manner where the oxidation catalyst
extends into the region under the reforming catalyst is that additional oxidation
catalysis can be achieved in the same volume with essentially the same substrate
thermal mass at relatively little extra expense as compared to the case where the
oxidation catalyst terminates approximately where the reforming catalyst begins. On
the other hand, making the oxidation uppermost has the advantage of increasing the
extent of oxidation prior to contact with the reforming catalyst. This is the preferred
configuration of the reforming catalyst extends to the inlet.
Steam reforming temperatures are at least about 500 °C, which is
generally above diesel exhaust temperatures. Diesel exhaust temperatures
downstream from a turbocharger vary from about 110 to about 550 °C. Preferably,
the fuel reformer 104 can be warmed up and operated using diesel fuel from the
injector 107 stating from an initial temperature of 275 °C while the exhaust from the
engine 101 remains at 275 °C. More preferably, the fuel reformer 104 can be
warmed up and operated from initial exhaust and reformer temperatures of 240 °C,
and still more preferably from exhaust and reformer temperatures of 210 °C. These
properties are achieved by providing the fuel reformer 104 with effective amounts of
precious metals, such as Pd, for catalyzing oxidation of diesel fuel at the starting
temperatures. Low temperature start-up can also be improved by configuring a low
thermal mass precious metal oxidation catalyst upstream from the fuel reformer 104.
Preferably, the upstream catalyst combusts a portion of the fuel while vaporizing the
rest. A mixing zone between the upstream catalyst and the fuel reformer 104 is also
helpful.
The fuel reformer 104 is designed to light-off at relatively low temperature.
Light-off is the phenomena whereby the fuel reformer 104 heats to approach a
steady state temperature that is substantially above the exhaust temperature. Once
lit off, the fuel reformer 104 has a tendency to remain heated even when the
conditions bringing about light off are discontinued. Preferably, the fuel reformer 104
is adapted to light-off when the exhaust temperature is as low as about 275 °C, more
preferably when the exhaust temperature is as low as about 240 °C, still more
preferably when the exhaust temperature is as low as about 210 °C.
[0034]""" The fuel reformer 104 is design to warm up to and produce reformate at
steam reforming temperatures. Operation at steam reforming temperatures reduces
the total amount of precious metal catalyst required. Having the fuel reformer 104
operate at least partially through steam reforming reactions significantly increases
the reformate yield and reduces the amount of heat generation. In principal, if
reformate production proceeds through partial oxidation reforming as in the reaction:

1.925 moles of reformate (moles CO plus moles H2) could be obtained from each
mole of carbon atoms in the fuel. CH1.85 is used to represent diesel fuel having a
typical carbon to hydrogen ratio. If reformate production proceeds through steam
reforming as in the reaction:

2.925 moles of reformate (moles CO plus moles H2) could in principle be obtained
from each mole of carbon atoms in the fuel. In practice, yields are lower than
theoretical amounts due to the limited efficiency of conversion of fuel, the limited
selectivity for reforming reactions over complete combustion reactions, the necessity
of producing heat to drive steam reforming, and the loss of energy required to heat
the exhaust.
Preferably, the fuel reformer 104 comprises enough steam reforming
catalyst that at 650 °C, with an 8 mol % exhaust oxygen concentration from the
engine 101 and with sufficient diesel fuel to provide the exhaust with an overall fuel
to air ratio of 1.2:1, at least about 2 mol % reformate is generated by steam
reforming, more preferably at least about 4 mol %, and still more preferably at least
about 6 mol %. For purposes of this disclosure, functional descriptions involving
diesel fuel are tested on the basis of the No. 2 diesel fuel oil sold in the United
States, which is a typical diesel fuel..
LNT is a device that adsorbs NOx under lean conditions and reduces
NOx and releases NOx reduction products under rich conditions. An LNT generally
comprises a NOx adsorbent and a precious metal catalyst in intimate contact on an
inert support. Examples of NOx adsorbent materials include certain oxides,
carbonates, and hydroxides of alkali and alkaline earth metals such as Mg, Ca, Sr,
and Ba or alkali metals such as K or Cs. The precious metal typically consists of one
or more of Pt, Pd, and Rh. The support is typically a monolith, although other
support structures can be used. The monolith support is typically ceramic, although
other materials such as metal and SiC are also suitable for LNT supports. The LNT
105 may be provided as two or more separate bricks.
The ammonia-SCR catalyst 106 is functional to catalyze reactions
between NOx and NH3 to reduce NOx to N2 in lean exhaust. The ammonia-SCR
catalyst 106 adsorbs NH3 released from the LNT 105 during denization and
subsequently uses that NH3 to reduce NOx slipping from the LNT 105 under lean
conditions. Examples of ammonia-SCR catalysts include certain oxides of metals
such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Mo, W, and Ce and zeolites, such as
ZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co, Ag, or Zn.
Ammonia-SCR can be accomplished using certain precious metals, but preferably
the SCR catalyst 106 is substantially free of precious metals. Preferably, the
ammonia-SCR catalyst 106 is designed to tolerate temperatures required to
desulfate the LNT 105.
[0038] The exhaust aftertreatment system 100 can comprise other components,
such as a diesel particulate filter and a clean-up oxidation catalyst. A thermal mass
can be placed between the fuel reformer 104 and the LNT 105 to protect the LNT
105 from frequent exposure to high fuel reformer temperatures. A diesel particulate
filter can be used as the thermal mass.
During normal operation, the engine 101 produces an exhaust comprising
NOx, particulate matter, and excess oxygen. A portion of the NOx is adsorbed by
the LNT 105. The ammonia-SCR catalyst 106 may have ammonia stored from a
previous denization of the LNT 105. If the ammonia-SCR catalyst 106 contains
stored ammonia, an additional portion of the NOx is reduced over the ammonia-SCR
catalyst 106 by reaction with stored ammonia. The fuel injector 107 is generally
inactive over this period, although small fuel injections might be used to maintain the
fuel reformer 104 at a temperature from which it can be easily heated or to maintain
the lean NOx trap 105 at a temperature at which it effectively absorbs NOx.
From time-to-time, the LNT 105 must be regenerated to remove
accumulated NOx (denitrated). Denitration generally involves heating the fuel
reformer 104 to an operational temperature and then using the reformer 104 to
produce reformate. The reformer 104 is generally heated by injecting fuel into the
exhaust upstream from the fuel reformer 104 at a sub-stoichiometric rate, whereby
the exhaust-reductant mixture remains overall lean and most of the injected fuel
completely combusts in the reformer 104. This may be referred to as a lean warm-
up phase. Once combustion has heated the reformer 104, the fuel injection rate can
be increased and/or the exhaust oxygen flow rate reduced to make the exhaust-
reductant mixture overall rich, whereupon the reformer 104 consumes most of the
oxygen from the exhaust and produces reformate by partial oxidation and steam
reforming reactions. The reformate thus produced reduces NOx absorbed in the LNT
105. Some of the NOx may be reduced to NH3, which is absorbed and stored by the
ammonia-SCR catalyst 106.
From time to time, the LNT 105 must also be regenerated to remove
accumulated sulfur compounds (desulfated). Desulfation involves heating the fuel
reformer 104 to an operational temperature, heating the LNT 105 to a desulfating
temperature, and providing the heated LNT 105 with a rich atmosphere. Desulfating
temperatures vary, but are typically in the range from about 500 to about 800 °C,
with optimal temperatures typically in the range of about 650 to about 750 °C. Below
a minimum temperature, desulfation is very slow. Above a maximum temperature,
the LNT 105 may be damaged.
The LNT 105 is heated for desulfation in part by heat convection from the
reformer 104. To generate this heat, fuel can be supplied to the reformer 104 under
lean conditions, whereby the fuel combusts in the reformer 104. Once the reformer
104 is heated, the fuel injection rate can be controlled to maintain the temperature of
the reformer 104 while the LNT 105 heats. Heating of the LNT 105 can be
facilitated, and the temperature of the LNT 105 in part maintained, by frequently
switching between lean and rich phases, whereby some oxygen from the lean
phases reacts with some reductant from the rich phases within the LNT 105. The
contribution of this method to heating the LNT 105 can be regulated through the
frequency of switching between lean and rich phases.
During rich operation for either denitration or desulfation, the fuel reformer
104 tends to heat. Particularly when the exhaust oxygen concentration is at about
8% or higher, the heat produced removing oxygen from the exhaust tends to be
greater than the heat removed by endothermic steam reforming, regardless of the
fuel injection rate. Theoretically, increasing the fuel injection rate increases the
proportion of endothermic steam reforming, but in practice this is not always effective
to prevent the fuel reformer 104 from heating during rich operation. As a result, the
fuel reformer 104's temperature rises. To prevent overheating, fuel injection can be
stopped and the fuel reformer 104 can be allowed to cool for a period before
resuming rich regeneration. This results in alternating lean-rich condition within the
fuel reformer 104 at high temperature. Operation at high temperatures and cycling
between lean and rich conditions are detrimental to many catalysts.
The fuel reformer 104 preferably comprises both oxidation and reforming
catalysts. When the exhaust-fuel mixture supplied to the fuel reformer is overall
lean, the oxidation catalyst functions to combust nearly all the fuel and the reforming
catalyst has little or no excess fuel to reform. When the fuel reformer has been
heated sufficiently and the exhaust fuel mixture supplied to the fuel reformer is
overall rich, the oxidation catalyst functions to combust a portion of the fuel to
consume most of the oxygen in the exhaust and the reforming catalyst functions to
generate syn gas through endothermic steam reforming. Preferably, the oxidation
and reforming catalysts are in close proximity, whereby heat generated over the
oxidation catalyst maintains the temperature of the fuel reforming catalyst.
Rh appears to be the most efficient steam reforming catalyst for the
conditions created by the system 100. The effectiveness of rhodium depends on its
dispersion. As an absolute number, dispersion is the number of surface-exposed
rhodium atoms per gram. As a percentage, dispersion is the fraction of Rh that can
be considered to be on the surface, in terms of its availability for reaction. The
fraction of surface Rh depends on the average particle size of the Rh metal or Rh
metal oxide. A catalyst with 1 wt% Rh loading and 100% dispersion (all surface Rh)
would provide 97.1 µmoles surface Rh/g. Rh dispersion can be measured by
chemisorption of H2. For the present application, not only is a high initial dispersion
of Rh desirable, but also a high dispersion after extensive lean operation and lean-
rich cycling at elevated temperatures.
The inventors have evaluated several refractory metal oxide supports for
Rh in the reforming catalyst. TiO2 was determined to have insufficient thermal
durability. Pure alumina is known to react with Rh. In an attempt to prevent such
reaction, the alumina was pre-coated with La2O3 (e.g., 10% by weight alumina). At
1% Rh loading, initial dispersions of Rh were good, e.g., 70% dispersion and 1-2 nm
rhodium particle size. After lean aging with 10% steam for 400 hours at 700 oC,
however, the rhodium dispersion was reduced to 10%. Using TGA, it was
determined that 50% of the rhodium was no longer in the form of Rh particles (metal
or oxide), suggesting it had dissolved in the La or alumina. Notably, such loss of
particulate rhodium did not occur over 1000 hours of lean rich aging 750 °C.
Lean rich aging, as the term is used herein, refers to the following
processing or equivalents thereof. In a lean portion of the cycle, the catalyst is
exposed to air with 10% steam for 25 minutes. In a rich portion of the cycle, the
catalyst is exposed to nitrogen having 3.8% hydrogen and mixed with 10% steam for
25 minutes. In between the lean and rich portions of the cycle, the catalyst is flushed
with nitrogen for 5 minutes. The absence of reduction in available rhodium after
1000 hours of lean rich aging 750 °C suggests that La in amounts sufficient to form
at least about a monolayer coating over the refractory metal oxide can redistribute
Rh under rich conditions.
Depositing 1 % Rh on a La-stabilized ZrO2 gave significantly better results
than La-stabilized alumina under lean aging. At a 2.5% La loading, the Rh
dispersion was 29% after steam aging for 400 hours at 700 °C. 2.5% La on a 100
m2/g refractory metal oxide, which is the approximate surface area of the ZrO2
support used in the experiments reported herein, corresponds to about a monolayer.
When the amount of La is increased to 5%, the dispersion was 42% after steam
aging for 400 hours at 700 °C. Accordingly, a preferred reforming catalyst includes a
ZrO2 refractory metal oxide component. Preferably, the refractory metal oxide
component consists essentially of ZrO2. The ZrO2 exists as subrnicron particles.
Typical ZrO2 surface areas are in the range from 70 to 130 m2/g.
Rh and La203 can be applied to the surfaces of the ZrO2 particles by any
suitable technique. Suitable techniques include precipitation and impregnation.
Impregnation of Rh begins by adding Rh salts or nitric acid solutions of Rh salts to
water. The water volume is adjusted to be slightly more (about 10% more) than the
pore volume of the refractory oxide support. Exemplary rhodium salts include
rhodium chloride and rhodium nitrate. After impregnation, the supports are dried at
150°C for 2-3 hours. The dried powder is then calcined at 450°C for two hours and
finally calcined at 600 -800°C for four hours. Deposition of Rh from rhodium nitrate
solution gives comparable dispersion to deposition of Rh from rhodium chloride, but
rhodium nitrate has the advantage of being less corrosive. The Rh and the La can
be incorporated in the same solution and impregnated onto the ZrO2 in a single step
or the Rh and La can be in separate solutions and incorporated onto the ZrO2 in
separate steps with a drying step in between each impregnation. Deposition of
La2O3 prior to deposition of Rh in a two step process appears to give higher stability
than deposition of La2O3 and Rh simultaneously in a one step process. The La and
the Rh can be applied to the ZrO2 either before or after the ZrO2 is applied to a
substrate such as a metal monolith.
Tables 1 and 2 show a series of results pertaining to the stability under
aging of 1 % Rh/ZrO2 catalyst having various amounts of La2O3. The La2O3 is
deposited on the surface of the ZrO2 particles together with the Rh. 1% La appears
to be insufficient to impart the desired stability under lean-rich cycling. 2.5% La
based on the weight of the refractory metal oxide, has a significant beneficial effect.
Further increasing the La loading to 5% or greater appears to provide a further
improvement. Additional La loading at least up to about 20% does not appear to
have any detrimental effect, but does not result in very significant further
improvements. There was some indication that thicker La2O3 coatings would result
in a separate La2O3 phase. Accordingly, it is preferred that the La loading be about
10% or less for the 100 m2/g refractory metal oxide. Preferably, the La2O3 is
amorphous. An amorphous layer, as the term is used herein, is one that has no
apparent crystalline structure shown by X-ray diffraction. A La2O3 particle with an
average particle size of about 1 nm or less would not show a crystalline X-ray
diffraction pattern.
Table 3 shows the effect of Rh loading for a 5%La/ZrO2 support.
Dispersion on a percentage basis for aged samples improves as Rh loading
decreases to a very surprising extent. As the Rh loading is decreased from 1 % to
about 0.5%, the dispersion after 120 hours aging increases to such an extent that
the same or greater Rh activity (amount of surface Rh) is achieved with the smaller
amount of Rh. As the Rh loading is further decreased to 0.25%, Rh dispersion
continues to increase, whereby the Rh activity decreases only slightly as Rh loading
is reduced from 0.5% to 0.25%. It appears that the Rh sinters to a markedly greater
degree, forming particles that progressively grow, if Rh loading is about 0.75% or
greater, whereas the Rh is effectively stabilized by the La2O3 if the Rh loading is
about 0.5% or less. This result is further illustrated by Figure 2, which plots surface
Rh in micromoles per g as a function of time under cyclic aging for various Rh
loadings and shows the stability of the 0.50% and 0.25% loading samples after the
initial aging or "de-greening" period.

The values of Rh loading relate to concentrations of Rh on the surface of
the refractory metal oxide. For the material used in these tests, 0.5% Rh loading
corresponds to 0.005 g Rh per 100 m2 surface area. Thus, the Rh loading is
preferably about 5 x 10-5 g/m2 or less. Interpolation of the data suggests that an Rh
loading of 3.5 x 10-5 g/m2 or less is even more preferable.
The preferred loading of rhodium can also be characterized by the Rh
particles retaining at least about 40% dispersion, more preferably at least about 50%
dispersion, after 400 hours of lean-rich cyclic aging at 750°C. The phenomenon by
which Rh loses dispersion is sintering: the growth of Rh particles. According, yet
another way to characterize the preferred loading of rhodium is that Rh loading at
which the Rh average particle size remaining at about 2nm or less after 400 hours of
lean-rich cyclic aging at 750 °C through interaction with the La2O3 coating. Particle
size is defined as six times the particle volume divided by the particle surface area.
This equation can be converted to an approximately correct equation to calculate Rh
particle diameter in nm from Rh dispersion in percent: Rh particle diameter is about
100 nm divided by percent Rh dispersion. For example, the above case of a Rh
catalyst with a dispersion of 50% has a particle diameter of about 2 nm.
Another of the inventors' concepts is to use La2O3 in the same manner to
stabilize a precious metal oxidation. Pd is the precious metal. Tests were
conducted with Pt on a 14%La/AI2O3 catalyst. Even 1% Pt added to 10% Pd caused
a large degree of sintering. Accordingly, the precious metal of the oxidation catalysts
preferably consists essentially of Pd.
A preferred refractory metal oxide for the oxidation catalyst is Al2O3. ZrO2
and Si-Al2O3 also gave acceptable performance to the extent they were tested,
although higher dispersions were obtained with Al2O3then with ZrO2. AI2O3 had a
higher surface area than the ZrO2, the Al2O3 being approximately 200 m2/g (170-230
m2/g), which is an additional advantage over ZrO2. Dispersion of Pd on Al2O3 was
improved slightly by impregnating the Pd as Pd(NH3)4(NO3) solution as compared to
impregnating the Pd as palladium nitrate-nitric acid solution. Sintering occurred
much more rapidly when Pd chloride solutions were used.
[0856] Table 4 show the effect of La surface loading on the dispersion of 5%Pd
over ZrO2. 2.5% or more La significantly improved dispersion and dispersion stability
on aging. Initial dispersions when the refractory metal oxide was Pd were higher,
e.g., 22% for 5%Pd, 10% surface-deposited La, Al2O3. 5% La appears to be the
minimum amount of surface La for a 200 m2/g alumina.

A high concentration of active (surface) Pd is useful in promoting low
temperature light-off. The more active Pd/g, the lower the light-off temperature. The
amount of active Pd/g depends on the surface area of the catalyst, the Pd loading,
and the dispersion of the loaded Pd. 100% dispersion would give about 940 µmoles
Pd/g for a 10 wt% Pd loading.
Experiments showed that Pd dispersion on a molar basis increases
linearly with Pd loading up to about 15% for a 10% surface-deposited La/Al2O3
support, meaning that the dispersion remains constant on a percentage basis.
Accordingly, the Pd loading is preferably at least about 10%, more preferably from
about 15 to about 20%.
Figure 3 shows the stability of 10% Pd co-dispersed with various amounts
of La on a commercially available La-stabilized Al2O3 support. The commercial
product contained about 4% La, prior to impregnation with Pd and additional La. The
plot shows stability through 1000 hours of lean aging with 10% steam. Dispersion
improves with La loading up to about 10 or 15%. X-ray diffraction data showed no
separate La phase, even through 20% loading. Accordingly, the La loading is
preferably at least about a monolayer, more preferably at least about 10%, and still
more preferably from about 15% to about 25%. 10% La corresponds to about 0.5
mg La per m2 and 20% La to about 1.0 mg La per m2 distributed over the surface of
the refractory metal oxide particles.
A series of tests were conducted replacing all or part of the La with Nd.
Nd is chemically similar to La. Like La, Nd has a stable 3+ charge. The tests
showed that Nd is essentially fungible with La.
Other stabilizers were tested but did not show comparable advantages,
either not improving dispersion, not improving stability, or interfering with catalyst
activity. Sr in particular did not provided comparable performance to La. CeO2
formed a separate phase on aging, which is undesirable in terms of maintaining
dispersion. In addition, CeO2 has substantial oxygen storage capacity, which is
undesirable in this application.
The fuel reformer 104 typically has a metal or ceramic monolithic substrate
comprising longitudinal channels through which the exhaust gas is designed to flow.
The catalyst or catalysts can be applied as a washcoat layer on these channel walls.
To apply the catalyst washcoat to the channel walls, a Pd-La-Al2O3 or Rh-La-ZrO2
catalyst powder such as described above can be mixed with water and other
components and milled or attrited to form a sol or colloidal dispersion of small
particles of the catalyst in water. This sol can then be coated onto the monolithic
structure and the monolithic structure dried and heat treated to form a catalyst unit
comprising the catalyst washcoat on the monolith walls. Many variations of this
process are available. The sol can be prepared by adding solutions of La and
precious metal to a slurry of refractory metal oxide powder in water that is then milled
or attrited to form the small particle sol that is then coated onto the monolith.
Alternatively, the La can be impregnated onto the refractory metal oxide, which is
then dried and calcined. The resulting material can then be mixed with water and a
precious metal solution and the slurry milled or attritted to form the final sol that is
coated onto the monolithic structure, followed by drying and heat treating to from the
final catalytic unit. To form a segmented catalyst with the oxidation catalyst coated
on the inlet section and the reforming catalyst on the outlet section of the reformer
104, the oxidation catalyst sol can be coated on an inlet section of monolith and the
reforming catalyst sol can be coated on an outlet section of the monolith.
[0063^* The invention as delineated by the following claims has been shown
and/or described in terms of certain concepts, components, and features. While a
particular component or feature may have been disclosed herein with respect to only
one of several concepts or examples or in both broad and narrow terms, the
components or features in their broad or narrow conceptions may be combined with
one or more other components or features in their broad or narrow conceptions
wherein such a combination would be recognized as logical by one of ordinary skill in
the art. Also, this one specification may describe more than one invention and the
following claims do not necessarily encompass every concept, aspect, embodiment,
or example described herein.
Industrial Applicability
The present invention is useful in controlling NOx emissions from diesel
and lean-burn gasoline engines
we claim:
1. A power generation system (100), comprising:
a diesel engine (101) operative to produce exhaust;
an exhaust aftertreatment system (103) comprising an exhaust line
(102) configured to receive at least a portion of the exhaust;
a fuel reformer (104) comprising an oxidation catalyst and a reforming
catalyst configured within the exhaust line (102);
a lean NOx trap (105) configured within the exhaust line (102)
downstream from the fuel reformer (104); and
a fuel injector (107) configured to inject fuel into the exhaust line (102)
upstream from the fuel reformer (104);
wherein the reforming catalyst comprises a catalyst washcoat
comprising:
a ZrO2 refractory metal oxide support;
LnXOY over the surface of the refractory metal oxide support in
an amount at least sufficient to form about a monolayer over the refractory
metal oxide support, wherein Ln is selected from the group consisting of La,
Nd, and mixtures thereof; and
Rh over the surface of the refractory metal oxide support in an
effective amount to catalyze steam reforming at 650 oC.
2. The power generation system (100) of claim 1, wherein the LnxOY has
no crystalline structure that is shown by X-ray diffraction.
3. The power generation system (100) of claim 1, wherein the catalyst
washcoat comprises from about 0.5 to about 1.0 mg La per m2 refractory metal oxide
surface distributed over the refractory metal oxide surface.
4. The power generation system (100) of claim 1, wherein the catalyst
washcoat comprises from about 2.5 to about 10% La by weight refractory metal
oxide.
5. The power generation system (100) of claim 1, wherein the oxidation
and reforming catalysts are in two separate washcoats over a single monolith
support, the oxidation catalyst washcoat extending from one end of the monolith
support and the reforming catalyst washcoat extending from the other end.
6. The power generation system (100) of 5, wherein the two washcoats
are not coextensive.
7. The power generation system (100) of claim 1, wherein the reforming
catalyst consists essentially of ZrO2, Rh, and Ln.
8. The power generation system (100) of claim 1, wherein the Rh is
present in an amount no greater than about 0.5 x 10-5 gram per meter squared
surface area of the refractory metal oxide support.
9. The power generation system (100) of claim 1, wherein the washcoat
comprises from about 0.2 to about 0.5% Rh by weight of the refractory metal oxide
support.
10. The power generation system (100) of claim 9, wherein the refractory
metal oxide support has a surface area from about 75 to about 125 m2/g.
11. The power generation system (100) of claim 1, wherein the Rh is
present in the form of particles from about 1 to about 2 nm in size.
12. The power generation system (100) of claim 1, wherein the oxidation
catalyst comprises a second catalyst washcoat comprising:
a second refractory metal oxide support;
LnxOY over the surface of the second refractory metal oxide
support in an amount at least sufficient to form about a monolayer over the
second refractory metal oxide support; and
Pd over the surface of the refractory metal oxide support in an
effective amount to catalyze oxidation of diesel fuel at 275 °C.
13. The power generation system (100) of claim 12, wherein the second
refractory metal oxide is Al2O3.
14. The power generation system (100) of claim 12, wherein the second
catalyst washcoat comprises from about 5 to about 20% Pd by weight of the second
refractory metal oxide.
15. The power generation system (100) of claim 12, wherein the oxidation
catalyst is substantially free of Pt.

The invention relates to catalysts comprising precious
metals supported on La stabilized refractory metal
oxides. The La is provided on the surface of the
refractory metal oxide in an amount to form at least
about a monolayer, preferably about 1 -2 mono-layers.
Preferably, the La has no crystalline structure shown
by X- ray diffraction. Nd and mixtures of La and Nd can
be used in place of La. In one embodiment, the catalyst
is a reforming catalyst comprising an effective amount
of Rh on a ZrO2 support. Preferably, the Rh is provided
in a relatively low concentration, typically about 0.5%
or less based on the weight of the ZrO2 support. The
inventors have found that if the Rh loading is kept
sufficiently low, the Rh can be maintained in the form
of small particles (less than 5 nm, typically about 2
nm or less) through lean rich cycling as a consequence
of an effect involving the La.