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The effects of high temperature lean exposure on the subsequent HC conversion of automotive catalysts
Catalysis Today 184 (2012) 262–270
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The effects of high temperature lean exposure on the subsequent HC conversion of automotive catalysts Joseph R. Theis ∗ , Robert W. McCabe Chemical Engineering Department, Ford Motor Company, 2101 Village Road, Dearborn, MI 48124, USA
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 27 January 2012 Accepted 31 January 2012 Available online 2 March 2012 Keywords: Catalyst Palladium Platinum Methane Propane Propylene Oxidation
a b s t r a c t Degreened samples of monolithic catalysts containing Pd-only, Pt-only, Pt/Rh (5/0/1), and Pt/Pd/Rh (1/13/1 and 1/4/1) were reduced in rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature was increased from test to test, in order to assess the effects of the recent thermal-chemical history on the conversion of CH4 , C3 H8 , and C3 H6 . A highly loaded Pt-only catalyst displayed relatively consistent HC conversion on the consecutive lean temperature ramps. However, the HC conversion of the other catalysts degraded from run to run as the maximum temperature on the previous test increased. For the Pd-only catalyst, this degradation was attributed to the increasing oxidation of metallic Pd to Pd oxide. While previous studies suggested that Pd oxide is more active than metallic Pd for CH4 and C3 H8 conversion, the continued oxidation increasingly depleted the catalyst of metallic Pd sites, which may be involved in the dissociative adsorption of the HC. Alternatively, the continued oxidation may have decreased the phase boundary between Pd metal and Pd oxide, which has been reported to enhance CH4 combustion. When the Pd-only sample was exposed to ca. 800 ◦ C under lean conditions, where Pd oxide is known to decompose, the CH4 conversion on the subsequent test improved dramatically and was similar to that after the rich treatment. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Future emission regulations for gasoline-powered vehicles in the US, Europe, and other countries will require extremely active, robust, and durable emission control systems to convert hydrocarbons (HC), carbon monoxide (CO), the oxides of nitrogen (NOx ), and particulate matter (PM) into non-toxic carbon dioxide (CO2 ), water (H2 O), and nitrogen (N2 ) over the useful life of the vehicle. Of the 3 gaseous pollutants, the control of HC is often the most challenging due to the low emission levels required (e.g., 0.010 g/mile on the Federal Test Procedure (FTP) after 150,000 miles for partial zero emission vehicles (PZEV)). Another challenge is the wide variety of hydrocarbon species in the exhaust, specifically alkanes, alkenes, and aromatics. Alkanes are generally more difficult to oxidize than alkenes and aromatics, and the ability to oxidize alkanes decreases as the number of carbon atoms decreases [1]. As a result, methane (CH4 ) is the most difficult hydrocarbon to oxidize [2,3]. CH4 oxidation is important for vehicles in Europe, where the emission standards are based on total hydrocarbons including CH4 . Now the hydrocarbon standards in the US today are based on non-methane
∗ Corresponding author. Tel.: +1 313 3376941. E-mail addresses: [email protected] (J.R. Theis), [email protected] (R.W. McCabe). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.01.015
hydrocarbons (NMHC). However, discussions are currently underway to establish limits for nitrous oxide (N2 O) and CH4 emissions due to their high greenhouse gas (GHG) potentials. So the catalyst system needs to be effective for oxidizing all of the hydrocarbons in the exhaust, including CH4 . In the short term, the catalyst system needs to be robust to the different environments to which it can be exposed (i.e., lean, stoichiometric, or reducing conditions at different temperatures). In the long term, the catalyst needs to be extremely durable so it can remain active for converting all types of hydrocarbons at high mileage conditions. Modern three-way catalysts (TWC) containing platinum (Pt), palladium (Pd), and/or rhodium (Rh) have been demonstrated to satisfy these requirements for activity, robustness, and durability when utilized on engines that are designed to operate at stoichiometry (i.e., = 1.0) the majority of the time. In addition to satisfying strict emission standards, a key customer-driven and government-mandated requirement for vehicles today and in the future is fuel economy. Lean-burn engines (e.g., lean port-fuel-injected (PFI) engines, stratified-charge directinjection (SCDI) engines, and diesel engines) can provide improved fuel economies relative to the stoichiometric PFI or DI engines in wide use today. However, it is well known that the current three-way catalyst (TWC) cannot reduce NOx under lean conditions. Therefore, the NOx emissions will need to be controlled
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
Nomenclature C3 H6 C3 H8 CH4 CO CO2 FID FTP GHG H2 O HC LNT N2 N2 O NDIR NMHC NO NOx NO2 O2 OSC PFI PM Pt Pd PZEV PGM Rh SCSI SCR TGA TWC
Propylene or propene Propane Methane Carbon monoxide Carbon dioxide Flame ionization detector Federal Test Procedure Greenhouse gas Water Hydrocarbon Lambda or A/F ratio/stoichiometric A/F ratio Lean NOx trap Nitrogen Nitrous oxide Non-dispersive infrared Non-methane hydrocarbons Nitric oxide Oxides of nitrogen (NO + NO2 ) Nitrogen dioxide Oxygen Oxygen storage capacity Port fuel injected Particulate matter Platinum Palladium Partial zero emission vehicle Platinum-group-metal Rhodium Stratified-charge direct injection Selective catalytic reduction Thermogravimetric analysis Three-way catalyst
with either a lean NOx trap (LNT), a selective catalytic reduction (SCR) catalyst with urea injection, or a combination of a LNT and a SCR catalyst. The control of HC will also be challenging on these lean-burn engines due to the reduced exhaust temperatures during lean operation and, for lean PFI and SCDI engines, the higher HC emissions that can result from incomplete combustion. Of the three precious metals (Pt, Pd, or Rh), Pd is known to be most effective for promoting CH4 oxidation [1,2,4,5], while platinum is more effective for small alkanes such as propane (C3 H8 ) [1,6] and butane (C4 H10 ) [1]. Pd is also known to be more thermally durable than Pt or Rh, making it a suitable catalyst for close-coupled converters [6]. However, the CH4 conversion of Pd is known to be sensitive to the temperature and A/F ratio, with the peak conversion occurring just rich of stoichiometry [2]. Also, the recent thermal history of the catalyst is known to affect the conversion activity of Pd [5]. Several researchers have suggested that the active form of Pd for CH4 oxidation is not metallic Pd but palladium oxide (i.e., PdOx , with x close to 1) [4,5]. Others have suggested that PdOx dispersed on metallic Pd crystallites (i.e., PdOx /Pd) is more active than PdOx on alumina (Al2 O3 ) for CH4 conversion [7]. Maillet et al. [6] suggested that PdOx is also more active than metallic Pd for C3 H8 oxidation. During extended lean operation, metallic Pd can be oxidized to PdOx . However, PdOx is known to decompose back to metallic Pd under lean conditions at approximately 800 ◦ C as shown by Farrauto et al. [4]. Those authors also showed that a significant hysteresis existed between this decomposition temperature and the temperature where the PdOx reformed, which was around
263
650 ◦ C. Between these two temperatures, there was no CH4 conversion over the Pd, suggesting that metallic Pd is inactive for CH4 conversion and PdOx is the active form for CH4 oxidation in that temperature range. They further suggested that PdOx is better able to chemisorb oxygen, which can then be used to oxidize the CH4 . Farrauto et al. [8] also showed that the nature of the support can affect these decomposition and reformation temperatures. Yao [1] reported that Pd was most active for CH4 and ethane (C2 H6 ) oxidation, while Pt was more active for C3 H8 and C4 H10 oxidation. She suggested that the rate limiting step in alkane oxidation was the dissociative chemisorption and H abstraction on the bare metal surface of the precious metal. The conversion increases with increasing length of molecule because the C H bond breaks more easily with increasing chain length. Cerium (Ce) degraded the alkane conversion of Pd and Pt but improved that of Rh. Oh et al. [2] also reported that Pd was more active than Pt or Rh for CH4 oxidation under stoichiometric and lean conditions. Consistent with Yao, cerium degraded the lean CH4 conversion of Pd and Pt but had little effect on Rh. Burch and Loader [9] reported that Pd was more active than Pt for CH4 conversion under lean conditions. Pd was also more active than Pt under stoichiometric or rich conditions at low conversion levels, but Pt was more effective than Pd at higher conversion levels. They suggested that multi-component catalysts (i.e., Pd + Pt) may be desirable for natural gas vehicles designed to operate near stoichiometry. In recent years, several papers have emerged [11–16] providing evidence of coexisting phases of Pd and PdOx under combustion conditions, and suggesting a cooperative effect between the two phases in methane or propane combustion. Kinnunen et al. [12] have recently carried out density functional calculations for methane dissociation (the presumed limiting step in methane combustion) on Pd and PdO surfaces. Their results suggest the importance of interface sites between Pd and PdO in promoting methane dissociation, with an OH group at the phase boundary serving as a stable intermediate and preventing the reverse reaction. Specchia et al. [14] suggested that a mixture of small metallic Pd particles and dispersed Pd oxide provides the highest CH4 activity at low temperatures: the CH4 may be activated on the metallic Pd, while the Pd oxide provides the oxygen for the oxidation. They further suggested that the oxidation of the small metallic Pd particles to PdOx and the coalescence of dispersed Pd oxide to completely oxidized PdOx (at least on the surface) decreased the low temperature conversion activity. Carstens et al. [15] suggested that the best low temperature conversion of CH4 is obtained with some metallic Pd on the surface of PdO. Yazawa et al. [16] found that a mixture of metallic Pd and Pd oxide provided the highest activity for C3 H8 conversion. In this study, degreened monolithic samples of automotive catalysts containing Pd, Pt, Pt/Rh, or Pt/Pd/Rh in two ratios were evaluated for their ability to oxidize CH4 , C3 H8 , and C3 H6 during lean temperature ramps. To assess the effect of the recent catalyst history (i.e., temperature and A/F ratio) on the conversion performance, the samples were initially pretreated with rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature increased from test to test. The objective of this study was to identify catalysts that are robust for hydrocarbon control after exposure to both rich and lean conditions. Although the fully formulated monolithic catalysts utilized in the study are not conducive to detailed characterization of the Pd oxidation state under reaction conditions, the results, nevertheless, appear consistent with the more fundamental surface chemistry and model supported catalyst studies that suggest the importance of having reduced Pd metal sites (even under lean conditions) to initiate the C H bond activation required for hydrocarbon combustion.
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
2. Experimental
O2 2.1. Catalyst samples
S
2.2. Experimental procedure Each degreened sample was installed on the flow reactor shown in Fig. 1 and initially pretreated with rich exhaust at an elevated temperature for several minutes to thoroughly reduce the precious metals. The sample was cooled under neutral conditions to a selected starting temperature and then evaluated on consecutive lean temperature ramps with a total flow rate of 2.5 L/min (space velocity of 25,000 h−1 ) with either CH4 , C3 H8 , or C3 H6 . The initial temperature depended on the HC species and was low enough that the HC conversion was very low. The HC concentration during the ramps was maintained at 500 ppm on a C1 basis, and the oxygen (O2 ) concentration was 5%. The exhaust mixture also contained 10% H2 O and 10% CO2 with the balance N2 . The rate of the temperature increase during the ramps was 3.5–4.0 ◦ C/min. After the first temperature ramp, the temperature was decreased back to the same initial value while maintaining the lean environment. On the subsequent temperature ramps, the initial temperature remained the same, but the maximum temperature increased from run to run in
S
Manifold
H2O Feedgas (2.5 L/min)
TC Oven Flow
Most of the catalyst samples used in this work were extracted from full-size monolithic catalysts (400 cells per square inch). The Pt/Rh catalyst was a fully formulated three-way catalyst containing oxygen storage (OSC) materials and alumina (Al2 O3 ) as the principal washcoat materials. The catalyst was loaded with 40 grams per cubic foot (gpcf) or 1.41 grams/liter (g/L) of platinum group metal (PGM) in a 5/0/1 weight ratio of platinum/palladium/rhodium. The Pd-only catalyst was supported on an alumina washcoat, contained no cerium, and was loaded with 60 gpcf (2.12 g/L) of Pd. The trimetal (i.e., Pt/Pd/Rh) catalysts were fully formulated three-way catalysts containing OSC materials and Al2 O3 as the principal washcoat materials. The trimetal catalysts were loaded with 60 gpcf (2.12 g/L) of PGM in weight ratios of 1/4/1 and 1/13/1. All of these commercial catalysts contained small amounts of metal oxide promoter and stabilizer materials in addition to the principal OSC and alumina materials. In contrast, the Pt-only catalyst was a model catalyst containing only Al2 O3 without OSC material and was loaded with 100 gpcf (3.53 g/L) Pt, prepared by incipient wetness. In this paper, these five catalysts will be designated by the overall PGM loading (in gpcf) and the PGM type or ratio, specifically 40-5/0/1, 60-Pd, 60-1/4/1, 60-1/13/1, and 100-Pt. Table 1 summarizes the alumina and cerium loadings (in g/L), the PGM loading (in gpcf and g/L), and the PGM weight ratio for these 5 catalysts. Samples of each formulation were prepared that were 2.54 cm long and 1.75 cm in diameter. A small hole was drilled through the axial center of each sample so that a 1.6 mm type K thermocouple could be inserted into the sample for measuring the catalyst bed temperature (i.e., primarily the gas temperature within the sample). Prior to the evaluations, each sample was degreened for 2 h at 800 ◦ C on a flow reactor in neutral exhaust (i.e., 10% H2 O/10% CO2 /balance N2 ).
HC CO2 O2 N2
Timer CO/H2
Bypass
264
HC O22
Dilution N2 (22.5 L/min)
NOxx NO CO
Sample
CO22
TC Fig. 1. Experimental apparatus.
order to assess the effect of the recent thermal history on the HC conversion of the catalyst. The 2.5 L/min of exhaust exiting the reactor was diluted with 22.5 L/min N2 to provide enough flow for all of the analyzers and to reduce the H2 O concentration from 10% to 1%, in order to prevent condensation in unheated sections of tubing or the analyzers. The HC emissions were measured using a Beckman model 400A flame ionization detector (FID). The emissions bench also contained Beckman non-dispersive infrared (NDIR) analyzers for measuring CO2 (model 864) and CO (model 865), Beckman model 951 chemiluminescence analyzers for measuring NO and total NOx , and a SensorMedics Model OM-11EA paramagnetic analyzer for measuring O2 . After all of the temperature ramps for one of the HC species were completed, the sample was again reduced in rich exhaust, and similar testing was performed with the other HC species. A timer was used with 2 solenoid valves (labeled “S” in Fig. 1) to switch between the lean condition (5% O2 ) and the rich condition (3.0% CO + 1% H2 ). 3. Results 3.1. Pd-only catalyst A sample of the 60-Pd catalyst was reduced in rich exhaust gas, and a series of lean temperature ramps with CH4 was performed. The maximum temperatures on 7 consecutive ramps were approximately 500, 550, 600, 650, 700, 750, and 815 ◦ C; this last ramp was followed by an eighth and final test that was terminated at 550 ◦ C. Fig. 2 shows that the 50% CH4 conversion temperatures on the first 7 tests were approximately 477, 499, 535, 567, 603, 622, and 639 ◦ C. The CH4 conversion performance continued to degrade as the maximum temperature on the previous lean temperature ramp increased, with the 50% CH4 conversion temperature increasing by 162 ◦ C relative to that after the rich reduction. However, Fig. 2 also shows that, once the sample was exposed to lean conditions at 815 ◦ C, the conversion performance on the next test improved
Table 1 Physical characteristics of catalyst formulations. Catalyst designation
Al2 O3 loading (g/L)
40-5/0/1 60-Pd 60-1/13/1 60-1/4/1 100-Pt
195 122 195 195 183
Cerium loading (g/L) 12.8 0 12.2 12.2 0
PGM loading (gpcf, g/L)
PGM ratio (Pt/Pd/Rh)
40, 1.41 60, 2.12 60, 2.12 60, 2.12 100, 3.53
5/0/1 0/1/0 1/13/1 1/4/1 1/0/0
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
650 604
o
350
900
Fig. 2. CH4 conversion of 1st sample of 60-Pd catalyst on eight sequential lean temperature ramps after an initial rich reduction.
dramatically and was similar to the performance following the rich pretreatment, with a 50% CH4 conversion temperature near 485 ◦ C. A second sample of the 60-Pd formulation was evaluated in a similar manner. After the sample was reduced in rich exhaust at 650 ◦ C, it was evaluated on 7 consecutive lean temperature ramps with maximum temperatures of approximately 500, 550, 600, 650, 700, 750, and 805 ◦ C, and this was followed by an eighth ramp that was terminated at approximately 550 ◦ C. Fig. 3 compares the 50% CH4 conversion temperatures for both samples of the 60-Pd catalyst on the eight sequential lean temperature ramps after the samples had been reduced in rich exhaust (test 1) and as a function of the approximate maximum temperature during the previous lean temperature ramp (tests 2–8). After the rich reductions and the first several lean ramps, the 50% CH4 conversion temperature of the second sample exceeded that of the first sample by 15–44 ◦ C, attributable to sample-to-sample variability. However, after the samples were exposed to lean conditions at approximately 700 and 750 ◦ C, the 50% conversion temperatures were within 6 ◦ C of each other. After the exposures to 800 ◦ C under lean conditions, the 50% CH4 conversion temperatures dropped to 485 ◦ C and 516 ◦ C for samples 1 and 2, respectively, which were similar to the 50% conversion temperatures following the rich reductions. The second sample of the 60-Pd catalyst was reduced in rich exhaust, and similar testing was performed with C3 H8 . The maximum temperatures on seven consecutive lean temperature ramps
Fig. 3. 50% CH4 conversion temperatures on consecutive lean temperature ramps for both samples of 60-Pd catalyst after initial rich reduction (test 1) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–8).
1
#2 h
ric
0 le
an
84
2 78
an le
73 2
an
an le
le
1
w kd
63 an
0+
1 53
an
an
58 le
68
800
an
600 700 Bed Temp (C)
le
500
le
#1
400
38
h ric
0 300
2
300
10
le
after lean 759 C
450 412
9
20
481
48 1
after lean 708 C
30
400
42
after lean 650 C
40
(est.) 459 465
496
450
an
50
500
an
after lean 604 C
517
le
after rich
545
550
an
60
600
580 560
Undetermined
70
600
0
after lean 551 C
80 CH4 Conv (%)
after lean 504 C
le
after lean 815 C
le
90
81
50% C3H8 Lean Conv Temp ( C)
100
265
o
Approximate Max Temperature During Previous Ramp ( C) Fig. 4. 50% C3 H8 lean conversion temperatures for 2nd sample of 60-Pd catalyst after rich reductions (tests 1 and 13) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–12).
were approximately 380, 430, 480, 530, 580, 630, and 680 ◦ C. The sample was maintained in a lean environment at room temperature over a weekend. Four additional lean temperature ramps were performed with maximum temperatures of approximately 730, 780, 810, and 840 ◦ C. Following the exposure to 840 ◦ C, a 12th ramp was performed that was terminated at ca. 590 ◦ C. The sample was reduced in rich exhaust again, and this was followed by a 13th and final lean temperature ramp. Fig. 4 shows the 50% C3 H8 conversion temperatures during these tests. The first lean temperature ramp following the rich reduction was terminated at 380 ◦ C, where the C3 H8 conversion was only 10%, so it was not possible to estimate the 50% C3 H8 conversion temperature for this test. Even on the second test, which was terminated at 430 ◦ C, the maximum C3 H8 conversion achieved was only 30%. However, the 50% C3 H8 conversion temperature for this test was estimated at 459 ◦ C from a second-order polynomial fit to the data. Fig. 4 shows that the 50% C3 H8 conversion temperature continued to increase with continued lean exposure until it reached a maximum of 604 ◦ C following a lean pretreatment of 782 ◦ C. After the temperature reached 810 ◦ C under lean conditions, there was a small improvement in the performance at low conversion levels on the next test, although the 50% conversion temperature dropped only slightly to 600 ◦ C. However, the temperature on that test reached 841 ◦ C, and on the subsequent test the 50% conversion temperature dropped by 150 ◦ C to 450 ◦ C. After the final rich reduction, the 50% C3 H8 conversion temperature dropped further to 412 ◦ C. Relative to this lowest value, the 50% C3 H8 conversion temperature increased by as much as 192 ◦ C with continued exposure to lean conditions. Unlike the results for CH4 , where the conversion performance after exposure to lean conditions at ca. 800 ◦ C was similar to that after a rich reduction, the catalyst had to be exposed to lean conditions at the slightly higher temperature of 840 ◦ C for a significant recovery of the C3 H8 conversion, and a rich reduction was required to achieve the maximum C3 H8 conversion performance. The second sample of the 60-Pd catalyst was also evaluated with C3 H6 . After an initial rich treatment, the sample was evaluated on six successive lean temperature ramps with maximum temperatures ranging from approximately 280 to 740 ◦ C. The sample was held in a lean environment at room temperature overnight, and the next day five more lean temperature ramps were performed with maximum temperatures of 795, 825, 850, 890, and 920 ◦ C. Following the exposure to 920 ◦ C, a 12th ramp was performed that was terminated at 240 ◦ C. This was followed by a rich reduction at 240 ◦ C, after which a 13th and final lean temperature ramp was performed. Similar to the observations with CH4 and C3 H8 , the
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
250 214 200
187 188
199
230 225 222 228 229 215 212 212
146 Undetermined
150 100 50 0
R ic le h 1 an 2 le 82 an 3 le 76 an 4 le 79 an 5 le 82 an 6 le 92 an 7 le 40 an 7 le 95 an 8 le 24 an 8 le 54 an 8 le 91 an 92 1 R ic h 2
50% C3H6 Lean Conv Temp (oC)
266
Approximate Max Temp During Previous Ramp (oC) Fig. 5. 50% C3 H6 lean conversion temperatures for 2nd sample of 60-Pd catalyst after rich reductions (tests 1 and 13) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–12).
C3 H6 conversion performance degraded as the maximum temperature on the previous test increased. Fig. 5 shows that the 50% C3 H6 conversion temperature increased from 187 ◦ C after the initial rich treatment to 230 ◦ C after exposure to lean conditions at 795 ◦ C. It is noted that the increase in 50% conversion temperature for C3 H6 was significantly less than that of C3 H8 and CH4 . Following exposure to 824 ◦ C under lean conditions, the 50% C3 H6 conversion temperature decreased slightly to 225 ◦ C. After the sample was exposed to 854 ◦ C under lean conditions, the 50% conversion temperature dropped further to 215 ◦ C. However, the 50% conversion temperature dropped only slightly to 212 ◦ C after exposures to lean conditions at 891 ◦ C and 921 ◦ C. The sample was then reduced in rich exhaust at the relatively low temperature of 240 ◦ C, and the 50% C3 H6 conversion temperature dropped by approximately 65 ◦ C to 146 ◦ C. It is not known why the 50% C3 H6 conversion temperature following the initial rich reduction (187 ◦ C) was higher than the 50% conversion temperature after this final rich reduction (146 ◦ C). Unlike the results for CH4 and more consistent with the results for C3 H8 , the C3 H6 conversion of the 60-Pd catalyst recovered only partially after exposure to high temperature lean conditions, and a rich reduction was required to achieve the maximum C3 H6 conversion performance. Fig. 6 displays the best and worst conversion curves for CH4 , C3 H8 , and C3 H6 for the second sample of 60-Pd after the rich reductions and the lean oxidations, respectively. For all three HC 100 90 80 HC Conv (%)
70 60
C 3 H6 reduced
50
C 3H 6 oxidized
40
C 3H 8 oxidized
C3H8 reduced
CH4 reduced
CH4 oxidized
30 20
Fig. 7. Comparison of 50% CH4 lean conversion temperatures for 2nd sample of 60-Pd catalyst and 40-5/0/1 Pt/Rh catalyst after rich reduction (test 1) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–7).
species, the lean conversion performance after the oxidation was significantly worse than the performance after the rich reduction. The 50% conversion temperature for CH4 following the rich reduction was estimated at 512 ◦ C from a second-order polynomial fit to the data. Relative to the 50% conversion temperature following the rich reductions, the 50% conversion temperature for C3 H8 , CH4 , and C3 H6 increased by approximately 192, 121, and 84 ◦ C, respectively, following the lean oxidations. 3.2. Pt/Rh catalyst Similar testing was performed on the degreened sample of the 40-5/0/1 Pt/Rh catalyst. Following a rich reduction at 710 ◦ C, the sample was evaluated on a lean CH4 temperature ramp from 360 ◦ C to 640 ◦ C to determine the minimum 50% CH4 conversion temperature. After a second rich reduction, the catalyst was evaluated for CH4 conversion performance on multiple lean temperature ramps with maximum bed temperatures ranging from ca. 450 to 750 ◦ C. Fig. 7 compares the 50% CH4 conversion temperatures for the second sample of the 60-Pd catalyst and this 40-5/0/1 Pt/Rh catalyst sample after the rich reductions and after lean exposures from 500 ◦ C to 750 ◦ C. Following the rich reductions, the 50% CH4 conversion temperature for the 40-5/0/1 Pt/Rh sample was slightly higher than that of the 60-Pd sample (530 ◦ C vs. 512 ◦ C, respectively). However, there was much less degradation in the 50% CH4 conversion performance for the 40-5/0/1 Pt/Rh catalyst after the lean oxidations relative to that of the 60-Pd catalyst, with maximum increases of 56 ◦ C and 121 ◦ C, respectively. As a result, the 50% CH4 conversion temperature for the 40-5/0/1 Pt/Rh catalyst was approximately 50 ◦ C lower than that of the 60-Pd catalyst after the lean oxidations (583 ◦ C vs. 633 ◦ C, respectively). The Pt/Rh catalyst was similarly evaluated with C3 H8 and C3 H6 . Again, the Pt/Rh catalyst was more robust to the effects of the lean oxidations than the 60-Pd catalyst. While the 50% lightoff temperatures for C3 H6 and C3 H8 increased by 85 ◦ C and 192 ◦ C, respectively, for the 60-Pd catalyst following the lean oxidations, the corresponding increases for the 40-5/0/1 Pt/Rh catalyst were only 11 ◦ C and 49 ◦ C.
10 0 100
3.3. Pt-Only catalyst 200
300
400 500 Bed Temp (oC)
600
700
800
Fig. 6. Conversion of CH4 , C3 H6 , and C3 H8 during lean temperature ramps for 2nd sample of 60-Pd catalyst after rich reductions and after lean oxidations.
To assess the effects of a higher Pt loading, the degreened model catalyst containing 100 gpcf Pt was evaluated in a similar manner. Following the rich reductions, the 100-Pt catalyst had significantly lower 50% C3 H6 and C3 H8 conversion temperatures than
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
3.4. Pd/Pt/Rh catalysts The trimetal formulation 60-1/13/1 (with 4 gpcf Pt and Rh and 52 gpcf Pd) was similarly evaluated after the rich reductions and the lean oxidations. After an initial rich reduction, the catalyst was evaluated with CH4 on seven successive lean temperature ramps with maximum temperatures ranging from 500 to 823 ◦ C. This was followed by another rich reduction at 740 ◦ C and a final lean temperature ramp. Similar to the 60-Pd catalyst, the CH4 conversion performance of this catalyst continued to degrade as the temperature on the previous lean temperature ramp increased. However, while the performance of the 60-Pd catalyst did not improve until the temperature reached 800 ◦ C (ref. Fig. 3), Fig. 8 shows that the performance of the 60-1/13/1 catalyst improved significantly after the temperature on the previous test reached ca. 770 ◦ C. After exposure to lean conditions at 823 ◦ C, the CH4 conversion improved even further. There was no further improvement in performance after the catalyst was reduced in rich exhaust at 740 ◦ C. To assess the effects of a higher Pt loading in the trimetal formulation, the 60-1/4/1 catalyst (with 10 gpcf Pt and Rh and 40 gpcf Pd) was evaluated with CH4 . After a temperature of 763 ◦ C was reached under lean conditions, the performance improved dramatically on the next test (data not shown). Similar to the 60-1/13/1 catalyst, the CH4 conversion of 60-1/4/1 improved at a temperature approximately 40–50 ◦ C lower than that of the 60-Pd catalyst.
700
50% CH4 Lean Conv Temp (oC)
the 40-5/0/1 Pt/Rh catalyst (i.e., 165 ◦ C vs. 191 ◦ C and 321 ◦ C vs. 357 ◦ C, respectively), attributable to its higher Pt loading, while the 50% CH4 conversion temperature for the 100-Pt catalyst was only slightly lower (i.e., 523 ◦ C vs. 530 ◦ C). In addition, this highly loaded 100-Pt catalyst was even more robust to the effects of lean oxidation than the 40-5/0/1 Pt/Rh catalyst, as the maximum increases in the 50% conversion temperatures for C3 H6 , C3 H8 , and CH4 following the lean oxidations were only 9, 9, and 9 ◦ C, respectively, compared to the corresponding increases of 11, 49, and 56 ◦ C for the 40-5/0/1 catalyst. As a result, the maximum 50% conversion temperatures of the 100-Pt catalyst for C3 H6 , C3 H8 , and CH4 following the lean oxidations were approximately 28, 75, and 54 ◦ C lower than that of the 40-5/0/1 catalyst, respectively. Relative to the oxidized 60-Pd catalyst, the maximum 50% conversion temperatures for the oxidized 100-Pt catalyst were 57 and 101 ◦ C lower for C3 H6 and CH4 , respectively, and a remarkable 273 ◦ C lower for C3 H8 .
267
600 500
486
515
543
569
594
614 498
470
487
lean 823
rich 740
400 300 200 100 0 rich 675
lean 500
lean 550
lean 605
lean 667
lean 724
lean 769
Approximate Max Temp During Previous Ramp (oC) Fig. 8. CH4 conversion of 60-1/13/1 Pt/Pd/Rh catalyst after rich reductions (tests 1 and 9) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–8).
3.5. Effect of Pt and Pd loadings on HC conversion Fig. 9 summarizes the minimum and maximum 50% conversion temperatures for C3 H6 , C3 H8 , and CH4 for all five catalysts after the rich reductions and lean oxidations along with the increases in the 50% conversion temperature following the oxidations. For all three HC species, the 50% conversion temperatures of the two trimetal formulations (primarily Pd) were similar to that of the 60Pd catalyst following the rich reductions. However, after the lean oxidations, the 50% conversion temperatures of the two trimetal formulations were lower than that of the 60-Pd catalyst, particularly for C3 H8 (463 and 449 ◦ C vs. 604 ◦ C, respectively). Therefore, the two trimetal formulations were more robust to the effects of lean oxidation than the 60-Pd catalyst. The 100-Pt and 40-5/0/1 Pt/Rh catalysts provided the lowest 50% C3 H8 conversion temperatures after both the reductions and the oxidations. The conversion of the 100-Pt catalyst was the most robust to the lean oxidations, while the conversion of the 60-Pd catalyst was generally the least robust to the lean oxidations. An inspection of the data in Fig. 9 suggests that, after the rich reductions, the Pd-containing catalysts provided slightly lower
Fig. 9. Minimum and maximum 50% lean conversion temperatures for C3 H6 , C3 H8 , and CH4 for 60-Pd catalyst, 40-5/0/1 Pt/Rh catalyst, 100-Pt catalyst, 60-1/13/1 catalyst, and 60-1/4/1 catalyst after rich reductions and after lean oxidations. The increases in 50% conversion temperature following the oxidations are also shown.
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270 600
700
500
600
50% C3H8 Conv Temp (C)
o
50% Lean Conv T @ Reduction ( C)
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50% CH4 Conv T vs Pd loading 50% C3H8 Conv T vs Pt loading 50% C3H6 Conv T vs Pd loading
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Fig. 10. 50% lean conversion temperatures for C3 H6 and CH4 as a function of Pd loading and 50% lean conversion temperatures for C3 H8 as a function of Pt loading following rich reductions.
50% conversion temperatures for C3 H6 and CH4 than the non-Pd catalysts (i.e., the 40-5/0/1 and 100-Pt catalysts), while the 50% conversion temperatures for C3 H8 following the reductions were significantly lower for the catalysts with high levels of Pt. Fig. 10 displays the 50% conversion temperatures for C3 H6 and CH4 as a function of the Pd loading and the 50% conversion temperatures for C3 H8 as a function of the Pt loading after the rich reductions. There was a modest reduction in 50% conversion temperature for C3 H6 and CH4 as the Pd loading was increased from 0 to 60 gpcf. In contrast, the 50% C3 H8 conversion temperature was significantly more sensitive to the Pt loading, dropping by as much as 90 ◦ C as the Pt loading increased from 0 to 100 gpcf. Fig. 9 also suggests that the presence of platinum significantly improved the robustness of the catalyst toward the lean oxidations. For all three HC species, the increase in the 50% conversion temperature following the lean oxidations was significantly lower for the catalysts containing platinum than for the catalysts without platinum. Figs. 11–13 show the 50% conversion temperatures after the rich reductions, after the lean oxidations, and the increase in the 50% conversion temperature as a function of the Pt loading for C3 H6 , C3 H8 , and CH4 , respectively. Fig. 11 shows that, even though the 60-Pd catalyst (without Pt) had the lowest 50% conversion temperature for C3 H6 when reduced, it had the largest increase in 50% conversion temperature after the oxidation. As a result, it had the highest 50% C3 H6 conversion temperature of the 5 catalysts
100
120
when oxidized. The 40-5/0/1 catalyst had the highest 50% C3 H6 conversion temperature when reduced, but its C3 H6 conversion temperature increased by only 11 ◦ C following the oxidation. Fig. 11 shows that the increase in 50% conversion temperature for C3 H6 following the oxidations decreased with increasing Pt loading, and a loading of only 10 gpcf Pt significantly improved the robustness of the C3 H6 conversion to the lean oxidations. Fig. 12 indicates that the presence of Pt also improved the robustness of the C3 H8 conversion toward the lean oxidations. While the 50% C3 H8 conversion temperature of the 60-Pd catalyst increased by 192 ◦ C after the lean oxidation, the 50% C3 H8 conversion temperature of the 100-Pt catalyst increased by only 9 ◦ C after the oxidation. As a result, the 50% C3 H8 conversion temperature for the 100-Pt catalyst was 273 ◦ C lower than that of the 60-Pd catalyst following the oxidations. As observed with C3 H6 , a loading of only 10 gpcf Pt significantly improved the robustness of the C3 H8 conversion to the lean oxidations. Fig. 13 shows that the CH4 conversion of the Pd-based catalysts was slightly better than that of the Pt-based catalysts following the rich reductions. Consistent with the C3 H6 and C3 H8 results in Figs. 11 and 12, however, platinum significantly improved the CH4 conversion of the catalyst following the lean oxidations. In contrast to the C3 H6 and C3 H8 results, however, higher Pt levels were needed to significantly improve the robustness of the CH4 conversion to the oxidations. With 10 gpcf Pt, the 50% conversion temperatures
700 60-Pd
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60-Pd
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80
Fig. 12. 50% C3 H8 conversion temperatures after rich reductions and after lean oxidations and increase in 50% C3 H8 conversion temperature as a function of Pt loading in the catalysts.
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Pt Loading (gpcf)
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Pt Loading (gpcf) Fig. 13. 50% CH4 conversion temperatures after rich reductions and after lean oxidations and increase in 50% CH4 conversion temperature as a function of Pt loading in the catalysts.
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for C3 H6 and C3 H8 increased by only ca. 20 and 45 ◦ C after the oxidations, respectively, compared to increases of 84 ◦ C and 192 ◦ C for the 60-Pd catalyst. However, the 50% CH4 conversion temperature increased by 93 ◦ C after the lean oxidations with 10 gpcf Pt, which was only marginally lower than the increase of 121 ◦ C for the 60Pd catalyst. A Pt loading of 33 gpcf dropped the increase in 50% CH4 conversion temperature following the oxidation to 56 ◦ C, and 100 gpcf Pt dropped the increase to only 9 ◦ C.
4. Discussion The dramatic increases in HC oxidation temperature observed for initially reduced Pd-containing catalysts following successively higher temperature oxidizing treatments are consistent with conversion of the Pd from a metallic state to a less active oxidized state. This is further supported by the observation that low-temperature light-off activity was restored when the oxidation temperature exceeded the decomposition temperature of PdO. Thus, either prereduction of the Pd-containing catalysts or decomposition of the oxide resulted in regeneration of the light-off performance back to the initially reduced catalyst state. For CH4 oxidation, in particular, the subject of the state of Pd under reaction conditions has been the subject of much study, with somewhat conflicting results [14, and references therein]. Several recent studies, however, point to the importance of both Pd metal and Pd oxide species for CH4 oxidation [11–15] and C3 H8 oxidation [16]. In particular, the results of this study parallel a recent study by Specchia et al. [14] of lowtemperature CH4 combustion on a 2% Pd/ceria-zirconia catalyst. Those authors utilized CO FTIR spectroscopy to show that successively more severe hydrothermal aging resulted in an evolution of the catalyst from a more active low-temperature form consisting primarily of a combination of small Pd metal particles and dispersed Pd oxide species to a less active form consisting of fully oxidized PdOx particles. The study of Specchia et al., as well as recent experimental [13] and theoretical [12] studies by others showing (or suggesting) both the co-existence of Pd metal and PdOx species under low-temperature combustion conditions, suggest that the CH4 and C3 H8 results of the present study are consistent with the need for both Pd metal and PdOx sites for optimal hydrocarbon reactivity. Indeed, Yao [1] suggested that the rate-limiting step in alkane oxidation was the dissociative adsorption of the alkane on the surface of the precious metal. If this adsorption occurs on the surface of the metallic Pd sites, then the opportunity for adsorption would decrease as more of the Pd metal was oxidized to Pd oxide. An alternative explanation is that the increasing oxidation decreases the phase boundary between metallic Pd and Pd oxide, which is claimed by Kinnunen et al. [12] to improve CH4 combustion by creating OH from the hydrogen formed from CH4 dissociation. Either way, it appears that a mixture of Pd metal and Pd oxide is beneficial for effective CH4 conversion, and the results of the present study – showing that the CH4 conversion of the 60-Pd catalyst and the two trimetal formulations were largely improved after the high temperature lean exposures – are consistent with a scenario in which some of the metallic Pd re-oxidized back to PdOx during the lean cool-downs resulting in a mixture of metallic Pd and PdOx that was very effective for CH4 conversion at the beginning of the next temperature ramp. The recovery in performance demonstrated by the 60-Pd catalyst and the two trimetal formulations after exposure to high temperature lean conditions is consistent with the decomposition of PdOx to Pd metal under lean conditions at ca. 800 ◦ C, as discussed by Farrauto et al. [4]. Indeed, the 60-Pd formulation recovered much of its conversion performance after exposure to a temperature very near 800 ◦ C. However, the two trimetal formulations demonstrated a large recovery in performance after exposure to lean conditions at
269
ca. 760 ◦ C, roughly 40 ◦ C lower than that of the 60-Pd formulation. One possible explanation is that the Pt in the trimetal formulations catalyzed the decomposition of the Pd oxide to metallic Pd, so the recovery occurred at a lower temperature. The results for C3 H6 in Fig. 5 indicated that only a small recovery in conversion was observed after exposure to lean conditions as high as 920 ◦ C. However, a large recovery was observed after exposure to rich conditions at the low temperature of 240 ◦ C. This could be due to the fact that multiple metallic sites are required for adsorption of the C3 H6 molecule [10]. After exposure to lean conditions at 800 ◦ C, some of the Pd oxide is reduced to metallic Pd. However, during the subsequent cooling, some of the metallic Pd is oxidized back to Pd oxide. If the metallic sites are the adsorption sites, this re-oxidation of the Pd limits the number of adjacent metallic sites that are required for adsorbing the C3 H6 , and therefore the C3 H6 conversion is limited. However, after a low temperature rich reduction, most of the Pd would be in the metallic state and therefore effective for adsorbing the C3 H6 . It can be speculated that some of the metallic Pd is oxidized back to Pd oxide in situ during the first lean temperature ramp following the reduction, producing the mixture of metallic Pd and Pd oxide that is effective for C3 H6 oxidation. Fig. 3 showed that a significant improvement in CH4 conversion was observed with the Pd-only catalyst after exposure to lean conditions at 800 ◦ C. However, Fig. 4 showed that only a small recovery in C3 H8 conversion occurred after exposure to lean conditions at 800 ◦ C, and exposure to lean conditions at 840 ◦ C was required to achieve a large recovery in C3 H8 conversion. Furthermore, a rich reduction was required to achieve the maximum C3 H8 conversion. These observations are consistent with the need to have multiple adjacent reduced sites for adsorbing the C3 H8 . Internal data at Ford (not shown) indicates that Pd oxide on alumina begins to decompose at approximately 800 ◦ C with 10% O2 and is still decomposing at 860 ◦ C. Therefore, the exposure to 840 ◦ C may have resulted in a more complete reduction of the Pd relative to the 800 ◦ C exposure. As a result, there may have been more adjacent reduced sites following the lean cool down after the 840 ◦ C exposure relative to that after the 800 ◦ C exposure. The rich reduction produced even more adjacent reduced sites, resulting in the maximum C3 H8 conversion. With the smaller CH4 molecule, there is less need for multiple adjacent reduced sites, so the maximum performance was achieved after exposure to lean conditions at 800 ◦ C. One of the goals for this project was to identify catalysts that could provide robust HC control after both high temperature lean operation and high temperature rich operation, since both conditions can be experienced by close-coupled three-way catalysts on vehicles. The 60-Pd catalyst provided good CH4 and C3 H6 conversion and reasonable C3 H8 conversion after a rich reduction, but the conversion performance for all three HC species decreased significantly after exposure to high temperature lean operation. Ptcontaining catalysts provided significantly better C3 H8 conversion than the Pd-only catalyst after the rich reductions, and a platinum loading as low as 10 gpcf significantly improved the robustness of the catalyst to high temperature lean exposure for C3 H6 and C3 H8 conversion. However, higher Pt loadings (e.g., 50 gpcf) were required to significantly improve the robustness of the CH4 conversion to the lean oxidations. Thus, catalysts containing both Pd and Pt provide appear to provide advantages for maintaining robust HC control under a wide range of air-fuel and temperature conditions. 5. Summary A laboratory study was performed to assess the effects of high temperature lean operation on the subsequent HC conversion of automotive catalysts containing different precious metals. Degreened samples containing Pd-only, Pt-only, Pt/Rh, or 2 ratios of
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Pt/Pd/Rh were reduced in rich exhaust and then evaluated for CH4 , C3 H8 , and C3 H6 conversion on multiple lean temperature ramps where the maximum temperature increased from test to test. The HC conversion of a model catalyst containing 100 gpcf Pt was relatively stable during the multiple lean temperature ramps. However, the lean HC conversion performance of the other catalyst samples degraded as the maximum temperature on the previous lean temperature ramp increased. For the Pd-containing catalysts, this was attributed to the increasing oxidation of Pd metal to PdOx . While other researchers have demonstrated that PdOx is more active catalytically than Pd metal, it is speculated that the Pd metal sites may serve the function of adsorbing the hydrocarbon. Alternatively, the continued oxidation may have decreased the phase boundary between metallic Pd and Pd oxide. So a balanced mixture of Pd metal and Pd oxide is needed for optimum HC control. The HC conversion of the Pt/Rh and trimetal formulations also degraded during the multiple lean temperature ramps, but the degradation was less pronounced than that of the Pd-only sample. This suggested that the Pt stabilized the performance of the catalyst during extended lean operation. Also, the presence of Pt significantly improved the conversion of C3 H8 relative to that of Pd-only, both after the rich reduction and particularly after the lean oxidations. However, Pt is less durable than Pd at very high temperatures, such as those encountered by close-coupled catalysts on gasoline-powered engines. Therefore, platinum-containing three-way catalysts may
be better utilized in the underbody location. For diesel applications, bimetallic Pt/Pd catalysts are recommended in the diesel oxidation catalyst for robust HC control. References [1] Y.Y. Yao, Industrial and Engineering Chemistry Product Research Development 19 (1980) 293–298. [2] S. Oh, P. Mitchell, R. Siewert, Journal of Catalysis 132 (1991) 287–301. [3] R. Hicks, H. Qi, M. Young, R. Lee, Journal of Catalysis 122 (1990) 280–294. [4] R. Farrauto, M. Hobson, T. Kennelly, E. Waterman, Applied Catalysis A: General 81 (1992) 227–237. [5] R. Burch, F. Urbano, Applied Catalysis A: General 124 (1995) 121–138. [6] T. Maillet, J. Barbier Jr., D. Duprez, Applied Catalysis B: Environmental 9 (1996) 251–266. [7] R. Hicks, H. Qi, M. Young, R. Lee, Journal of Catalysis 122 (1990) 295–306. [8] R. Farrauto, J. Lampert, M. Hobson, E. Waterman, Applied Catalysis B: Environmental 6 (1995) 263–270. [9] R. Burch, P. Loader, Applied Catalysis B: Environmental 5 (1994) 149–164. [10] Y.Y. Yao, Journal of Catalysis 87 (1984) 152–162. [11] S. Colussi, A. Trovarelli, E. Vessellil, A. Baraldi, G. Comelli, G. Groppi, J. Liorca, Applied Catalysis A: General 390 (2010) 1–10. [12] N. Kinnunen, J. Hirvi, M. Suvanto, T. Pakkanen, Journal of Physical Chemistry C 115 (2011) 19197–19202. [13] N. Kinnunen, J. Hirvi, T. Venalainen, M. Suvanto, T. Pakkanen, Applied Catalysis A: General 397 (2011) 54–61. [14] S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia, Journal of Catalysis 263 (2009) 134–145. [15] J. Carstens, S.C. Su, A.T. Bell, Journal of Catalysis 176 (1998) 136–142. [16] Y. Yazawa, H. Yoshida, N. Takagi, S.-I. Komai, A. Satsuma, T. Hattori, Applied Catalysis B: Environmental 19 (1998) 261–266.
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
The effects of high temperature lean exposure on the subsequent HC conversion of automotive catalysts Joseph R. Theis ∗ , Robert W. McCabe Chemical Engineering Department, Ford Motor Company, 2101 Village Road, Dearborn, MI 48124, USA
a r t i c l e
i n f o
Article history: Received 30 August 2011 Received in revised form 27 January 2012 Accepted 31 January 2012 Available online 2 March 2012 Keywords: Catalyst Palladium Platinum Methane Propane Propylene Oxidation
a b s t r a c t Degreened samples of monolithic catalysts containing Pd-only, Pt-only, Pt/Rh (5/0/1), and Pt/Pd/Rh (1/13/1 and 1/4/1) were reduced in rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature was increased from test to test, in order to assess the effects of the recent thermal-chemical history on the conversion of CH4 , C3 H8 , and C3 H6 . A highly loaded Pt-only catalyst displayed relatively consistent HC conversion on the consecutive lean temperature ramps. However, the HC conversion of the other catalysts degraded from run to run as the maximum temperature on the previous test increased. For the Pd-only catalyst, this degradation was attributed to the increasing oxidation of metallic Pd to Pd oxide. While previous studies suggested that Pd oxide is more active than metallic Pd for CH4 and C3 H8 conversion, the continued oxidation increasingly depleted the catalyst of metallic Pd sites, which may be involved in the dissociative adsorption of the HC. Alternatively, the continued oxidation may have decreased the phase boundary between Pd metal and Pd oxide, which has been reported to enhance CH4 combustion. When the Pd-only sample was exposed to ca. 800 ◦ C under lean conditions, where Pd oxide is known to decompose, the CH4 conversion on the subsequent test improved dramatically and was similar to that after the rich treatment. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Future emission regulations for gasoline-powered vehicles in the US, Europe, and other countries will require extremely active, robust, and durable emission control systems to convert hydrocarbons (HC), carbon monoxide (CO), the oxides of nitrogen (NOx ), and particulate matter (PM) into non-toxic carbon dioxide (CO2 ), water (H2 O), and nitrogen (N2 ) over the useful life of the vehicle. Of the 3 gaseous pollutants, the control of HC is often the most challenging due to the low emission levels required (e.g., 0.010 g/mile on the Federal Test Procedure (FTP) after 150,000 miles for partial zero emission vehicles (PZEV)). Another challenge is the wide variety of hydrocarbon species in the exhaust, specifically alkanes, alkenes, and aromatics. Alkanes are generally more difficult to oxidize than alkenes and aromatics, and the ability to oxidize alkanes decreases as the number of carbon atoms decreases [1]. As a result, methane (CH4 ) is the most difficult hydrocarbon to oxidize [2,3]. CH4 oxidation is important for vehicles in Europe, where the emission standards are based on total hydrocarbons including CH4 . Now the hydrocarbon standards in the US today are based on non-methane
∗ Corresponding author. Tel.: +1 313 3376941. E-mail addresses: [email protected] (J.R. Theis), [email protected] (R.W. McCabe). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.01.015
hydrocarbons (NMHC). However, discussions are currently underway to establish limits for nitrous oxide (N2 O) and CH4 emissions due to their high greenhouse gas (GHG) potentials. So the catalyst system needs to be effective for oxidizing all of the hydrocarbons in the exhaust, including CH4 . In the short term, the catalyst system needs to be robust to the different environments to which it can be exposed (i.e., lean, stoichiometric, or reducing conditions at different temperatures). In the long term, the catalyst needs to be extremely durable so it can remain active for converting all types of hydrocarbons at high mileage conditions. Modern three-way catalysts (TWC) containing platinum (Pt), palladium (Pd), and/or rhodium (Rh) have been demonstrated to satisfy these requirements for activity, robustness, and durability when utilized on engines that are designed to operate at stoichiometry (i.e., = 1.0) the majority of the time. In addition to satisfying strict emission standards, a key customer-driven and government-mandated requirement for vehicles today and in the future is fuel economy. Lean-burn engines (e.g., lean port-fuel-injected (PFI) engines, stratified-charge directinjection (SCDI) engines, and diesel engines) can provide improved fuel economies relative to the stoichiometric PFI or DI engines in wide use today. However, it is well known that the current three-way catalyst (TWC) cannot reduce NOx under lean conditions. Therefore, the NOx emissions will need to be controlled
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
Nomenclature C3 H6 C3 H8 CH4 CO CO2 FID FTP GHG H2 O HC LNT N2 N2 O NDIR NMHC NO NOx NO2 O2 OSC PFI PM Pt Pd PZEV PGM Rh SCSI SCR TGA TWC
Propylene or propene Propane Methane Carbon monoxide Carbon dioxide Flame ionization detector Federal Test Procedure Greenhouse gas Water Hydrocarbon Lambda or A/F ratio/stoichiometric A/F ratio Lean NOx trap Nitrogen Nitrous oxide Non-dispersive infrared Non-methane hydrocarbons Nitric oxide Oxides of nitrogen (NO + NO2 ) Nitrogen dioxide Oxygen Oxygen storage capacity Port fuel injected Particulate matter Platinum Palladium Partial zero emission vehicle Platinum-group-metal Rhodium Stratified-charge direct injection Selective catalytic reduction Thermogravimetric analysis Three-way catalyst
with either a lean NOx trap (LNT), a selective catalytic reduction (SCR) catalyst with urea injection, or a combination of a LNT and a SCR catalyst. The control of HC will also be challenging on these lean-burn engines due to the reduced exhaust temperatures during lean operation and, for lean PFI and SCDI engines, the higher HC emissions that can result from incomplete combustion. Of the three precious metals (Pt, Pd, or Rh), Pd is known to be most effective for promoting CH4 oxidation [1,2,4,5], while platinum is more effective for small alkanes such as propane (C3 H8 ) [1,6] and butane (C4 H10 ) [1]. Pd is also known to be more thermally durable than Pt or Rh, making it a suitable catalyst for close-coupled converters [6]. However, the CH4 conversion of Pd is known to be sensitive to the temperature and A/F ratio, with the peak conversion occurring just rich of stoichiometry [2]. Also, the recent thermal history of the catalyst is known to affect the conversion activity of Pd [5]. Several researchers have suggested that the active form of Pd for CH4 oxidation is not metallic Pd but palladium oxide (i.e., PdOx , with x close to 1) [4,5]. Others have suggested that PdOx dispersed on metallic Pd crystallites (i.e., PdOx /Pd) is more active than PdOx on alumina (Al2 O3 ) for CH4 conversion [7]. Maillet et al. [6] suggested that PdOx is also more active than metallic Pd for C3 H8 oxidation. During extended lean operation, metallic Pd can be oxidized to PdOx . However, PdOx is known to decompose back to metallic Pd under lean conditions at approximately 800 ◦ C as shown by Farrauto et al. [4]. Those authors also showed that a significant hysteresis existed between this decomposition temperature and the temperature where the PdOx reformed, which was around
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650 ◦ C. Between these two temperatures, there was no CH4 conversion over the Pd, suggesting that metallic Pd is inactive for CH4 conversion and PdOx is the active form for CH4 oxidation in that temperature range. They further suggested that PdOx is better able to chemisorb oxygen, which can then be used to oxidize the CH4 . Farrauto et al. [8] also showed that the nature of the support can affect these decomposition and reformation temperatures. Yao [1] reported that Pd was most active for CH4 and ethane (C2 H6 ) oxidation, while Pt was more active for C3 H8 and C4 H10 oxidation. She suggested that the rate limiting step in alkane oxidation was the dissociative chemisorption and H abstraction on the bare metal surface of the precious metal. The conversion increases with increasing length of molecule because the C H bond breaks more easily with increasing chain length. Cerium (Ce) degraded the alkane conversion of Pd and Pt but improved that of Rh. Oh et al. [2] also reported that Pd was more active than Pt or Rh for CH4 oxidation under stoichiometric and lean conditions. Consistent with Yao, cerium degraded the lean CH4 conversion of Pd and Pt but had little effect on Rh. Burch and Loader [9] reported that Pd was more active than Pt for CH4 conversion under lean conditions. Pd was also more active than Pt under stoichiometric or rich conditions at low conversion levels, but Pt was more effective than Pd at higher conversion levels. They suggested that multi-component catalysts (i.e., Pd + Pt) may be desirable for natural gas vehicles designed to operate near stoichiometry. In recent years, several papers have emerged [11–16] providing evidence of coexisting phases of Pd and PdOx under combustion conditions, and suggesting a cooperative effect between the two phases in methane or propane combustion. Kinnunen et al. [12] have recently carried out density functional calculations for methane dissociation (the presumed limiting step in methane combustion) on Pd and PdO surfaces. Their results suggest the importance of interface sites between Pd and PdO in promoting methane dissociation, with an OH group at the phase boundary serving as a stable intermediate and preventing the reverse reaction. Specchia et al. [14] suggested that a mixture of small metallic Pd particles and dispersed Pd oxide provides the highest CH4 activity at low temperatures: the CH4 may be activated on the metallic Pd, while the Pd oxide provides the oxygen for the oxidation. They further suggested that the oxidation of the small metallic Pd particles to PdOx and the coalescence of dispersed Pd oxide to completely oxidized PdOx (at least on the surface) decreased the low temperature conversion activity. Carstens et al. [15] suggested that the best low temperature conversion of CH4 is obtained with some metallic Pd on the surface of PdO. Yazawa et al. [16] found that a mixture of metallic Pd and Pd oxide provided the highest activity for C3 H8 conversion. In this study, degreened monolithic samples of automotive catalysts containing Pd, Pt, Pt/Rh, or Pt/Pd/Rh in two ratios were evaluated for their ability to oxidize CH4 , C3 H8 , and C3 H6 during lean temperature ramps. To assess the effect of the recent catalyst history (i.e., temperature and A/F ratio) on the conversion performance, the samples were initially pretreated with rich exhaust and then evaluated on consecutive lean temperature ramps where the maximum temperature increased from test to test. The objective of this study was to identify catalysts that are robust for hydrocarbon control after exposure to both rich and lean conditions. Although the fully formulated monolithic catalysts utilized in the study are not conducive to detailed characterization of the Pd oxidation state under reaction conditions, the results, nevertheless, appear consistent with the more fundamental surface chemistry and model supported catalyst studies that suggest the importance of having reduced Pd metal sites (even under lean conditions) to initiate the C H bond activation required for hydrocarbon combustion.
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2. Experimental
O2 2.1. Catalyst samples
S
2.2. Experimental procedure Each degreened sample was installed on the flow reactor shown in Fig. 1 and initially pretreated with rich exhaust at an elevated temperature for several minutes to thoroughly reduce the precious metals. The sample was cooled under neutral conditions to a selected starting temperature and then evaluated on consecutive lean temperature ramps with a total flow rate of 2.5 L/min (space velocity of 25,000 h−1 ) with either CH4 , C3 H8 , or C3 H6 . The initial temperature depended on the HC species and was low enough that the HC conversion was very low. The HC concentration during the ramps was maintained at 500 ppm on a C1 basis, and the oxygen (O2 ) concentration was 5%. The exhaust mixture also contained 10% H2 O and 10% CO2 with the balance N2 . The rate of the temperature increase during the ramps was 3.5–4.0 ◦ C/min. After the first temperature ramp, the temperature was decreased back to the same initial value while maintaining the lean environment. On the subsequent temperature ramps, the initial temperature remained the same, but the maximum temperature increased from run to run in
S
Manifold
H2O Feedgas (2.5 L/min)
TC Oven Flow
Most of the catalyst samples used in this work were extracted from full-size monolithic catalysts (400 cells per square inch). The Pt/Rh catalyst was a fully formulated three-way catalyst containing oxygen storage (OSC) materials and alumina (Al2 O3 ) as the principal washcoat materials. The catalyst was loaded with 40 grams per cubic foot (gpcf) or 1.41 grams/liter (g/L) of platinum group metal (PGM) in a 5/0/1 weight ratio of platinum/palladium/rhodium. The Pd-only catalyst was supported on an alumina washcoat, contained no cerium, and was loaded with 60 gpcf (2.12 g/L) of Pd. The trimetal (i.e., Pt/Pd/Rh) catalysts were fully formulated three-way catalysts containing OSC materials and Al2 O3 as the principal washcoat materials. The trimetal catalysts were loaded with 60 gpcf (2.12 g/L) of PGM in weight ratios of 1/4/1 and 1/13/1. All of these commercial catalysts contained small amounts of metal oxide promoter and stabilizer materials in addition to the principal OSC and alumina materials. In contrast, the Pt-only catalyst was a model catalyst containing only Al2 O3 without OSC material and was loaded with 100 gpcf (3.53 g/L) Pt, prepared by incipient wetness. In this paper, these five catalysts will be designated by the overall PGM loading (in gpcf) and the PGM type or ratio, specifically 40-5/0/1, 60-Pd, 60-1/4/1, 60-1/13/1, and 100-Pt. Table 1 summarizes the alumina and cerium loadings (in g/L), the PGM loading (in gpcf and g/L), and the PGM weight ratio for these 5 catalysts. Samples of each formulation were prepared that were 2.54 cm long and 1.75 cm in diameter. A small hole was drilled through the axial center of each sample so that a 1.6 mm type K thermocouple could be inserted into the sample for measuring the catalyst bed temperature (i.e., primarily the gas temperature within the sample). Prior to the evaluations, each sample was degreened for 2 h at 800 ◦ C on a flow reactor in neutral exhaust (i.e., 10% H2 O/10% CO2 /balance N2 ).
HC CO2 O2 N2
Timer CO/H2
Bypass
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HC O22
Dilution N2 (22.5 L/min)
NOxx NO CO
Sample
CO22
TC Fig. 1. Experimental apparatus.
order to assess the effect of the recent thermal history on the HC conversion of the catalyst. The 2.5 L/min of exhaust exiting the reactor was diluted with 22.5 L/min N2 to provide enough flow for all of the analyzers and to reduce the H2 O concentration from 10% to 1%, in order to prevent condensation in unheated sections of tubing or the analyzers. The HC emissions were measured using a Beckman model 400A flame ionization detector (FID). The emissions bench also contained Beckman non-dispersive infrared (NDIR) analyzers for measuring CO2 (model 864) and CO (model 865), Beckman model 951 chemiluminescence analyzers for measuring NO and total NOx , and a SensorMedics Model OM-11EA paramagnetic analyzer for measuring O2 . After all of the temperature ramps for one of the HC species were completed, the sample was again reduced in rich exhaust, and similar testing was performed with the other HC species. A timer was used with 2 solenoid valves (labeled “S” in Fig. 1) to switch between the lean condition (5% O2 ) and the rich condition (3.0% CO + 1% H2 ). 3. Results 3.1. Pd-only catalyst A sample of the 60-Pd catalyst was reduced in rich exhaust gas, and a series of lean temperature ramps with CH4 was performed. The maximum temperatures on 7 consecutive ramps were approximately 500, 550, 600, 650, 700, 750, and 815 ◦ C; this last ramp was followed by an eighth and final test that was terminated at 550 ◦ C. Fig. 2 shows that the 50% CH4 conversion temperatures on the first 7 tests were approximately 477, 499, 535, 567, 603, 622, and 639 ◦ C. The CH4 conversion performance continued to degrade as the maximum temperature on the previous lean temperature ramp increased, with the 50% CH4 conversion temperature increasing by 162 ◦ C relative to that after the rich reduction. However, Fig. 2 also shows that, once the sample was exposed to lean conditions at 815 ◦ C, the conversion performance on the next test improved
Table 1 Physical characteristics of catalyst formulations. Catalyst designation
Al2 O3 loading (g/L)
40-5/0/1 60-Pd 60-1/13/1 60-1/4/1 100-Pt
195 122 195 195 183
Cerium loading (g/L) 12.8 0 12.2 12.2 0
PGM loading (gpcf, g/L)
PGM ratio (Pt/Pd/Rh)
40, 1.41 60, 2.12 60, 2.12 60, 2.12 100, 3.53
5/0/1 0/1/0 1/13/1 1/4/1 1/0/0
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
650 604
o
350
900
Fig. 2. CH4 conversion of 1st sample of 60-Pd catalyst on eight sequential lean temperature ramps after an initial rich reduction.
dramatically and was similar to the performance following the rich pretreatment, with a 50% CH4 conversion temperature near 485 ◦ C. A second sample of the 60-Pd formulation was evaluated in a similar manner. After the sample was reduced in rich exhaust at 650 ◦ C, it was evaluated on 7 consecutive lean temperature ramps with maximum temperatures of approximately 500, 550, 600, 650, 700, 750, and 805 ◦ C, and this was followed by an eighth ramp that was terminated at approximately 550 ◦ C. Fig. 3 compares the 50% CH4 conversion temperatures for both samples of the 60-Pd catalyst on the eight sequential lean temperature ramps after the samples had been reduced in rich exhaust (test 1) and as a function of the approximate maximum temperature during the previous lean temperature ramp (tests 2–8). After the rich reductions and the first several lean ramps, the 50% CH4 conversion temperature of the second sample exceeded that of the first sample by 15–44 ◦ C, attributable to sample-to-sample variability. However, after the samples were exposed to lean conditions at approximately 700 and 750 ◦ C, the 50% conversion temperatures were within 6 ◦ C of each other. After the exposures to 800 ◦ C under lean conditions, the 50% CH4 conversion temperatures dropped to 485 ◦ C and 516 ◦ C for samples 1 and 2, respectively, which were similar to the 50% conversion temperatures following the rich reductions. The second sample of the 60-Pd catalyst was reduced in rich exhaust, and similar testing was performed with C3 H8 . The maximum temperatures on seven consecutive lean temperature ramps
Fig. 3. 50% CH4 conversion temperatures on consecutive lean temperature ramps for both samples of 60-Pd catalyst after initial rich reduction (test 1) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–8).
1
#2 h
ric
0 le
an
84
2 78
an le
73 2
an
an le
le
1
w kd
63 an
0+
1 53
an
an
58 le
68
800
an
600 700 Bed Temp (C)
le
500
le
#1
400
38
h ric
0 300
2
300
10
le
after lean 759 C
450 412
9
20
481
48 1
after lean 708 C
30
400
42
after lean 650 C
40
(est.) 459 465
496
450
an
50
500
an
after lean 604 C
517
le
after rich
545
550
an
60
600
580 560
Undetermined
70
600
0
after lean 551 C
80 CH4 Conv (%)
after lean 504 C
le
after lean 815 C
le
90
81
50% C3H8 Lean Conv Temp ( C)
100
265
o
Approximate Max Temperature During Previous Ramp ( C) Fig. 4. 50% C3 H8 lean conversion temperatures for 2nd sample of 60-Pd catalyst after rich reductions (tests 1 and 13) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–12).
were approximately 380, 430, 480, 530, 580, 630, and 680 ◦ C. The sample was maintained in a lean environment at room temperature over a weekend. Four additional lean temperature ramps were performed with maximum temperatures of approximately 730, 780, 810, and 840 ◦ C. Following the exposure to 840 ◦ C, a 12th ramp was performed that was terminated at ca. 590 ◦ C. The sample was reduced in rich exhaust again, and this was followed by a 13th and final lean temperature ramp. Fig. 4 shows the 50% C3 H8 conversion temperatures during these tests. The first lean temperature ramp following the rich reduction was terminated at 380 ◦ C, where the C3 H8 conversion was only 10%, so it was not possible to estimate the 50% C3 H8 conversion temperature for this test. Even on the second test, which was terminated at 430 ◦ C, the maximum C3 H8 conversion achieved was only 30%. However, the 50% C3 H8 conversion temperature for this test was estimated at 459 ◦ C from a second-order polynomial fit to the data. Fig. 4 shows that the 50% C3 H8 conversion temperature continued to increase with continued lean exposure until it reached a maximum of 604 ◦ C following a lean pretreatment of 782 ◦ C. After the temperature reached 810 ◦ C under lean conditions, there was a small improvement in the performance at low conversion levels on the next test, although the 50% conversion temperature dropped only slightly to 600 ◦ C. However, the temperature on that test reached 841 ◦ C, and on the subsequent test the 50% conversion temperature dropped by 150 ◦ C to 450 ◦ C. After the final rich reduction, the 50% C3 H8 conversion temperature dropped further to 412 ◦ C. Relative to this lowest value, the 50% C3 H8 conversion temperature increased by as much as 192 ◦ C with continued exposure to lean conditions. Unlike the results for CH4 , where the conversion performance after exposure to lean conditions at ca. 800 ◦ C was similar to that after a rich reduction, the catalyst had to be exposed to lean conditions at the slightly higher temperature of 840 ◦ C for a significant recovery of the C3 H8 conversion, and a rich reduction was required to achieve the maximum C3 H8 conversion performance. The second sample of the 60-Pd catalyst was also evaluated with C3 H6 . After an initial rich treatment, the sample was evaluated on six successive lean temperature ramps with maximum temperatures ranging from approximately 280 to 740 ◦ C. The sample was held in a lean environment at room temperature overnight, and the next day five more lean temperature ramps were performed with maximum temperatures of 795, 825, 850, 890, and 920 ◦ C. Following the exposure to 920 ◦ C, a 12th ramp was performed that was terminated at 240 ◦ C. This was followed by a rich reduction at 240 ◦ C, after which a 13th and final lean temperature ramp was performed. Similar to the observations with CH4 and C3 H8 , the
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
250 214 200
187 188
199
230 225 222 228 229 215 212 212
146 Undetermined
150 100 50 0
R ic le h 1 an 2 le 82 an 3 le 76 an 4 le 79 an 5 le 82 an 6 le 92 an 7 le 40 an 7 le 95 an 8 le 24 an 8 le 54 an 8 le 91 an 92 1 R ic h 2
50% C3H6 Lean Conv Temp (oC)
266
Approximate Max Temp During Previous Ramp (oC) Fig. 5. 50% C3 H6 lean conversion temperatures for 2nd sample of 60-Pd catalyst after rich reductions (tests 1 and 13) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–12).
C3 H6 conversion performance degraded as the maximum temperature on the previous test increased. Fig. 5 shows that the 50% C3 H6 conversion temperature increased from 187 ◦ C after the initial rich treatment to 230 ◦ C after exposure to lean conditions at 795 ◦ C. It is noted that the increase in 50% conversion temperature for C3 H6 was significantly less than that of C3 H8 and CH4 . Following exposure to 824 ◦ C under lean conditions, the 50% C3 H6 conversion temperature decreased slightly to 225 ◦ C. After the sample was exposed to 854 ◦ C under lean conditions, the 50% conversion temperature dropped further to 215 ◦ C. However, the 50% conversion temperature dropped only slightly to 212 ◦ C after exposures to lean conditions at 891 ◦ C and 921 ◦ C. The sample was then reduced in rich exhaust at the relatively low temperature of 240 ◦ C, and the 50% C3 H6 conversion temperature dropped by approximately 65 ◦ C to 146 ◦ C. It is not known why the 50% C3 H6 conversion temperature following the initial rich reduction (187 ◦ C) was higher than the 50% conversion temperature after this final rich reduction (146 ◦ C). Unlike the results for CH4 and more consistent with the results for C3 H8 , the C3 H6 conversion of the 60-Pd catalyst recovered only partially after exposure to high temperature lean conditions, and a rich reduction was required to achieve the maximum C3 H6 conversion performance. Fig. 6 displays the best and worst conversion curves for CH4 , C3 H8 , and C3 H6 for the second sample of 60-Pd after the rich reductions and the lean oxidations, respectively. For all three HC 100 90 80 HC Conv (%)
70 60
C 3 H6 reduced
50
C 3H 6 oxidized
40
C 3H 8 oxidized
C3H8 reduced
CH4 reduced
CH4 oxidized
30 20
Fig. 7. Comparison of 50% CH4 lean conversion temperatures for 2nd sample of 60-Pd catalyst and 40-5/0/1 Pt/Rh catalyst after rich reduction (test 1) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–7).
species, the lean conversion performance after the oxidation was significantly worse than the performance after the rich reduction. The 50% conversion temperature for CH4 following the rich reduction was estimated at 512 ◦ C from a second-order polynomial fit to the data. Relative to the 50% conversion temperature following the rich reductions, the 50% conversion temperature for C3 H8 , CH4 , and C3 H6 increased by approximately 192, 121, and 84 ◦ C, respectively, following the lean oxidations. 3.2. Pt/Rh catalyst Similar testing was performed on the degreened sample of the 40-5/0/1 Pt/Rh catalyst. Following a rich reduction at 710 ◦ C, the sample was evaluated on a lean CH4 temperature ramp from 360 ◦ C to 640 ◦ C to determine the minimum 50% CH4 conversion temperature. After a second rich reduction, the catalyst was evaluated for CH4 conversion performance on multiple lean temperature ramps with maximum bed temperatures ranging from ca. 450 to 750 ◦ C. Fig. 7 compares the 50% CH4 conversion temperatures for the second sample of the 60-Pd catalyst and this 40-5/0/1 Pt/Rh catalyst sample after the rich reductions and after lean exposures from 500 ◦ C to 750 ◦ C. Following the rich reductions, the 50% CH4 conversion temperature for the 40-5/0/1 Pt/Rh sample was slightly higher than that of the 60-Pd sample (530 ◦ C vs. 512 ◦ C, respectively). However, there was much less degradation in the 50% CH4 conversion performance for the 40-5/0/1 Pt/Rh catalyst after the lean oxidations relative to that of the 60-Pd catalyst, with maximum increases of 56 ◦ C and 121 ◦ C, respectively. As a result, the 50% CH4 conversion temperature for the 40-5/0/1 Pt/Rh catalyst was approximately 50 ◦ C lower than that of the 60-Pd catalyst after the lean oxidations (583 ◦ C vs. 633 ◦ C, respectively). The Pt/Rh catalyst was similarly evaluated with C3 H8 and C3 H6 . Again, the Pt/Rh catalyst was more robust to the effects of the lean oxidations than the 60-Pd catalyst. While the 50% lightoff temperatures for C3 H6 and C3 H8 increased by 85 ◦ C and 192 ◦ C, respectively, for the 60-Pd catalyst following the lean oxidations, the corresponding increases for the 40-5/0/1 Pt/Rh catalyst were only 11 ◦ C and 49 ◦ C.
10 0 100
3.3. Pt-Only catalyst 200
300
400 500 Bed Temp (oC)
600
700
800
Fig. 6. Conversion of CH4 , C3 H6 , and C3 H8 during lean temperature ramps for 2nd sample of 60-Pd catalyst after rich reductions and after lean oxidations.
To assess the effects of a higher Pt loading, the degreened model catalyst containing 100 gpcf Pt was evaluated in a similar manner. Following the rich reductions, the 100-Pt catalyst had significantly lower 50% C3 H6 and C3 H8 conversion temperatures than
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
3.4. Pd/Pt/Rh catalysts The trimetal formulation 60-1/13/1 (with 4 gpcf Pt and Rh and 52 gpcf Pd) was similarly evaluated after the rich reductions and the lean oxidations. After an initial rich reduction, the catalyst was evaluated with CH4 on seven successive lean temperature ramps with maximum temperatures ranging from 500 to 823 ◦ C. This was followed by another rich reduction at 740 ◦ C and a final lean temperature ramp. Similar to the 60-Pd catalyst, the CH4 conversion performance of this catalyst continued to degrade as the temperature on the previous lean temperature ramp increased. However, while the performance of the 60-Pd catalyst did not improve until the temperature reached 800 ◦ C (ref. Fig. 3), Fig. 8 shows that the performance of the 60-1/13/1 catalyst improved significantly after the temperature on the previous test reached ca. 770 ◦ C. After exposure to lean conditions at 823 ◦ C, the CH4 conversion improved even further. There was no further improvement in performance after the catalyst was reduced in rich exhaust at 740 ◦ C. To assess the effects of a higher Pt loading in the trimetal formulation, the 60-1/4/1 catalyst (with 10 gpcf Pt and Rh and 40 gpcf Pd) was evaluated with CH4 . After a temperature of 763 ◦ C was reached under lean conditions, the performance improved dramatically on the next test (data not shown). Similar to the 60-1/13/1 catalyst, the CH4 conversion of 60-1/4/1 improved at a temperature approximately 40–50 ◦ C lower than that of the 60-Pd catalyst.
700
50% CH4 Lean Conv Temp (oC)
the 40-5/0/1 Pt/Rh catalyst (i.e., 165 ◦ C vs. 191 ◦ C and 321 ◦ C vs. 357 ◦ C, respectively), attributable to its higher Pt loading, while the 50% CH4 conversion temperature for the 100-Pt catalyst was only slightly lower (i.e., 523 ◦ C vs. 530 ◦ C). In addition, this highly loaded 100-Pt catalyst was even more robust to the effects of lean oxidation than the 40-5/0/1 Pt/Rh catalyst, as the maximum increases in the 50% conversion temperatures for C3 H6 , C3 H8 , and CH4 following the lean oxidations were only 9, 9, and 9 ◦ C, respectively, compared to the corresponding increases of 11, 49, and 56 ◦ C for the 40-5/0/1 catalyst. As a result, the maximum 50% conversion temperatures of the 100-Pt catalyst for C3 H6 , C3 H8 , and CH4 following the lean oxidations were approximately 28, 75, and 54 ◦ C lower than that of the 40-5/0/1 catalyst, respectively. Relative to the oxidized 60-Pd catalyst, the maximum 50% conversion temperatures for the oxidized 100-Pt catalyst were 57 and 101 ◦ C lower for C3 H6 and CH4 , respectively, and a remarkable 273 ◦ C lower for C3 H8 .
267
600 500
486
515
543
569
594
614 498
470
487
lean 823
rich 740
400 300 200 100 0 rich 675
lean 500
lean 550
lean 605
lean 667
lean 724
lean 769
Approximate Max Temp During Previous Ramp (oC) Fig. 8. CH4 conversion of 60-1/13/1 Pt/Pd/Rh catalyst after rich reductions (tests 1 and 9) and as a function of approximate maximum temperature during previous lean temperature ramp (tests 2–8).
3.5. Effect of Pt and Pd loadings on HC conversion Fig. 9 summarizes the minimum and maximum 50% conversion temperatures for C3 H6 , C3 H8 , and CH4 for all five catalysts after the rich reductions and lean oxidations along with the increases in the 50% conversion temperature following the oxidations. For all three HC species, the 50% conversion temperatures of the two trimetal formulations (primarily Pd) were similar to that of the 60Pd catalyst following the rich reductions. However, after the lean oxidations, the 50% conversion temperatures of the two trimetal formulations were lower than that of the 60-Pd catalyst, particularly for C3 H8 (463 and 449 ◦ C vs. 604 ◦ C, respectively). Therefore, the two trimetal formulations were more robust to the effects of lean oxidation than the 60-Pd catalyst. The 100-Pt and 40-5/0/1 Pt/Rh catalysts provided the lowest 50% C3 H8 conversion temperatures after both the reductions and the oxidations. The conversion of the 100-Pt catalyst was the most robust to the lean oxidations, while the conversion of the 60-Pd catalyst was generally the least robust to the lean oxidations. An inspection of the data in Fig. 9 suggests that, after the rich reductions, the Pd-containing catalysts provided slightly lower
Fig. 9. Minimum and maximum 50% lean conversion temperatures for C3 H6 , C3 H8 , and CH4 for 60-Pd catalyst, 40-5/0/1 Pt/Rh catalyst, 100-Pt catalyst, 60-1/13/1 catalyst, and 60-1/4/1 catalyst after rich reductions and after lean oxidations. The increases in 50% conversion temperature following the oxidations are also shown.
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270 600
700
500
600
50% C3H8 Conv Temp (C)
o
50% Lean Conv T @ Reduction ( C)
268
400
300
200
50% CH4 Conv T vs Pd loading 50% C3H8 Conv T vs Pt loading 50% C3H6 Conv T vs Pd loading
100
20
40
60
80
100
After reduction
500
Increase @ oxidation
60-1/4/1 40-5/0/1
400
100-Pt 60-1/13/1
300 200 100 0
0 0
After oxidation
60-Pd
0
120
20
40
Fig. 10. 50% lean conversion temperatures for C3 H6 and CH4 as a function of Pd loading and 50% lean conversion temperatures for C3 H8 as a function of Pt loading following rich reductions.
50% conversion temperatures for C3 H6 and CH4 than the non-Pd catalysts (i.e., the 40-5/0/1 and 100-Pt catalysts), while the 50% conversion temperatures for C3 H8 following the reductions were significantly lower for the catalysts with high levels of Pt. Fig. 10 displays the 50% conversion temperatures for C3 H6 and CH4 as a function of the Pd loading and the 50% conversion temperatures for C3 H8 as a function of the Pt loading after the rich reductions. There was a modest reduction in 50% conversion temperature for C3 H6 and CH4 as the Pd loading was increased from 0 to 60 gpcf. In contrast, the 50% C3 H8 conversion temperature was significantly more sensitive to the Pt loading, dropping by as much as 90 ◦ C as the Pt loading increased from 0 to 100 gpcf. Fig. 9 also suggests that the presence of platinum significantly improved the robustness of the catalyst toward the lean oxidations. For all three HC species, the increase in the 50% conversion temperature following the lean oxidations was significantly lower for the catalysts containing platinum than for the catalysts without platinum. Figs. 11–13 show the 50% conversion temperatures after the rich reductions, after the lean oxidations, and the increase in the 50% conversion temperature as a function of the Pt loading for C3 H6 , C3 H8 , and CH4 , respectively. Fig. 11 shows that, even though the 60-Pd catalyst (without Pt) had the lowest 50% conversion temperature for C3 H6 when reduced, it had the largest increase in 50% conversion temperature after the oxidation. As a result, it had the highest 50% C3 H6 conversion temperature of the 5 catalysts
100
120
when oxidized. The 40-5/0/1 catalyst had the highest 50% C3 H6 conversion temperature when reduced, but its C3 H6 conversion temperature increased by only 11 ◦ C following the oxidation. Fig. 11 shows that the increase in 50% conversion temperature for C3 H6 following the oxidations decreased with increasing Pt loading, and a loading of only 10 gpcf Pt significantly improved the robustness of the C3 H6 conversion to the lean oxidations. Fig. 12 indicates that the presence of Pt also improved the robustness of the C3 H8 conversion toward the lean oxidations. While the 50% C3 H8 conversion temperature of the 60-Pd catalyst increased by 192 ◦ C after the lean oxidation, the 50% C3 H8 conversion temperature of the 100-Pt catalyst increased by only 9 ◦ C after the oxidation. As a result, the 50% C3 H8 conversion temperature for the 100-Pt catalyst was 273 ◦ C lower than that of the 60-Pd catalyst following the oxidations. As observed with C3 H6 , a loading of only 10 gpcf Pt significantly improved the robustness of the C3 H8 conversion to the lean oxidations. Fig. 13 shows that the CH4 conversion of the Pd-based catalysts was slightly better than that of the Pt-based catalysts following the rich reductions. Consistent with the C3 H6 and C3 H8 results in Figs. 11 and 12, however, platinum significantly improved the CH4 conversion of the catalyst following the lean oxidations. In contrast to the C3 H6 and C3 H8 results, however, higher Pt levels were needed to significantly improve the robustness of the CH4 conversion to the oxidations. With 10 gpcf Pt, the 50% conversion temperatures
700 60-Pd
40-5/0/1
60-1/13/1
200
50% CH4 Conv Temp (C)
60-Pd
o
80
Fig. 12. 50% C3 H8 conversion temperatures after rich reductions and after lean oxidations and increase in 50% C3 H8 conversion temperature as a function of Pt loading in the catalysts.
250
50% C3H6 Conv Temp ( C)
60
Pt Loading (gpcf)
Loading (gpcf)
100-Pt
150 60-1/4/1
After oxidation
100
After reduction Increase @ oxidation
50
60-1/4/1
40-5/0/1
600 500
100-Pt 60-1/13/1
400
After oxidation
300
After reduction Increase @ oxidation
200 100 0
0 0
20
40
60
80
100
120
Pt Loading (gpcf) Fig. 11. 50% C3 H6 conversion temperatures after rich reductions and after lean oxidations and increase in 50% C3 H6 conversion temperature as a function of Pt loading in the catalysts.
0
20
40
60
80
100
120
Pt Loading (gpcf) Fig. 13. 50% CH4 conversion temperatures after rich reductions and after lean oxidations and increase in 50% CH4 conversion temperature as a function of Pt loading in the catalysts.
J.R. Theis, R.W. McCabe / Catalysis Today 184 (2012) 262–270
for C3 H6 and C3 H8 increased by only ca. 20 and 45 ◦ C after the oxidations, respectively, compared to increases of 84 ◦ C and 192 ◦ C for the 60-Pd catalyst. However, the 50% CH4 conversion temperature increased by 93 ◦ C after the lean oxidations with 10 gpcf Pt, which was only marginally lower than the increase of 121 ◦ C for the 60Pd catalyst. A Pt loading of 33 gpcf dropped the increase in 50% CH4 conversion temperature following the oxidation to 56 ◦ C, and 100 gpcf Pt dropped the increase to only 9 ◦ C.
4. Discussion The dramatic increases in HC oxidation temperature observed for initially reduced Pd-containing catalysts following successively higher temperature oxidizing treatments are consistent with conversion of the Pd from a metallic state to a less active oxidized state. This is further supported by the observation that low-temperature light-off activity was restored when the oxidation temperature exceeded the decomposition temperature of PdO. Thus, either prereduction of the Pd-containing catalysts or decomposition of the oxide resulted in regeneration of the light-off performance back to the initially reduced catalyst state. For CH4 oxidation, in particular, the subject of the state of Pd under reaction conditions has been the subject of much study, with somewhat conflicting results [14, and references therein]. Several recent studies, however, point to the importance of both Pd metal and Pd oxide species for CH4 oxidation [11–15] and C3 H8 oxidation [16]. In particular, the results of this study parallel a recent study by Specchia et al. [14] of lowtemperature CH4 combustion on a 2% Pd/ceria-zirconia catalyst. Those authors utilized CO FTIR spectroscopy to show that successively more severe hydrothermal aging resulted in an evolution of the catalyst from a more active low-temperature form consisting primarily of a combination of small Pd metal particles and dispersed Pd oxide species to a less active form consisting of fully oxidized PdOx particles. The study of Specchia et al., as well as recent experimental [13] and theoretical [12] studies by others showing (or suggesting) both the co-existence of Pd metal and PdOx species under low-temperature combustion conditions, suggest that the CH4 and C3 H8 results of the present study are consistent with the need for both Pd metal and PdOx sites for optimal hydrocarbon reactivity. Indeed, Yao [1] suggested that the rate-limiting step in alkane oxidation was the dissociative adsorption of the alkane on the surface of the precious metal. If this adsorption occurs on the surface of the metallic Pd sites, then the opportunity for adsorption would decrease as more of the Pd metal was oxidized to Pd oxide. An alternative explanation is that the increasing oxidation decreases the phase boundary between metallic Pd and Pd oxide, which is claimed by Kinnunen et al. [12] to improve CH4 combustion by creating OH from the hydrogen formed from CH4 dissociation. Either way, it appears that a mixture of Pd metal and Pd oxide is beneficial for effective CH4 conversion, and the results of the present study – showing that the CH4 conversion of the 60-Pd catalyst and the two trimetal formulations were largely improved after the high temperature lean exposures – are consistent with a scenario in which some of the metallic Pd re-oxidized back to PdOx during the lean cool-downs resulting in a mixture of metallic Pd and PdOx that was very effective for CH4 conversion at the beginning of the next temperature ramp. The recovery in performance demonstrated by the 60-Pd catalyst and the two trimetal formulations after exposure to high temperature lean conditions is consistent with the decomposition of PdOx to Pd metal under lean conditions at ca. 800 ◦ C, as discussed by Farrauto et al. [4]. Indeed, the 60-Pd formulation recovered much of its conversion performance after exposure to a temperature very near 800 ◦ C. However, the two trimetal formulations demonstrated a large recovery in performance after exposure to lean conditions at
269
ca. 760 ◦ C, roughly 40 ◦ C lower than that of the 60-Pd formulation. One possible explanation is that the Pt in the trimetal formulations catalyzed the decomposition of the Pd oxide to metallic Pd, so the recovery occurred at a lower temperature. The results for C3 H6 in Fig. 5 indicated that only a small recovery in conversion was observed after exposure to lean conditions as high as 920 ◦ C. However, a large recovery was observed after exposure to rich conditions at the low temperature of 240 ◦ C. This could be due to the fact that multiple metallic sites are required for adsorption of the C3 H6 molecule [10]. After exposure to lean conditions at 800 ◦ C, some of the Pd oxide is reduced to metallic Pd. However, during the subsequent cooling, some of the metallic Pd is oxidized back to Pd oxide. If the metallic sites are the adsorption sites, this re-oxidation of the Pd limits the number of adjacent metallic sites that are required for adsorbing the C3 H6 , and therefore the C3 H6 conversion is limited. However, after a low temperature rich reduction, most of the Pd would be in the metallic state and therefore effective for adsorbing the C3 H6 . It can be speculated that some of the metallic Pd is oxidized back to Pd oxide in situ during the first lean temperature ramp following the reduction, producing the mixture of metallic Pd and Pd oxide that is effective for C3 H6 oxidation. Fig. 3 showed that a significant improvement in CH4 conversion was observed with the Pd-only catalyst after exposure to lean conditions at 800 ◦ C. However, Fig. 4 showed that only a small recovery in C3 H8 conversion occurred after exposure to lean conditions at 800 ◦ C, and exposure to lean conditions at 840 ◦ C was required to achieve a large recovery in C3 H8 conversion. Furthermore, a rich reduction was required to achieve the maximum C3 H8 conversion. These observations are consistent with the need to have multiple adjacent reduced sites for adsorbing the C3 H8 . Internal data at Ford (not shown) indicates that Pd oxide on alumina begins to decompose at approximately 800 ◦ C with 10% O2 and is still decomposing at 860 ◦ C. Therefore, the exposure to 840 ◦ C may have resulted in a more complete reduction of the Pd relative to the 800 ◦ C exposure. As a result, there may have been more adjacent reduced sites following the lean cool down after the 840 ◦ C exposure relative to that after the 800 ◦ C exposure. The rich reduction produced even more adjacent reduced sites, resulting in the maximum C3 H8 conversion. With the smaller CH4 molecule, there is less need for multiple adjacent reduced sites, so the maximum performance was achieved after exposure to lean conditions at 800 ◦ C. One of the goals for this project was to identify catalysts that could provide robust HC control after both high temperature lean operation and high temperature rich operation, since both conditions can be experienced by close-coupled three-way catalysts on vehicles. The 60-Pd catalyst provided good CH4 and C3 H6 conversion and reasonable C3 H8 conversion after a rich reduction, but the conversion performance for all three HC species decreased significantly after exposure to high temperature lean operation. Ptcontaining catalysts provided significantly better C3 H8 conversion than the Pd-only catalyst after the rich reductions, and a platinum loading as low as 10 gpcf significantly improved the robustness of the catalyst to high temperature lean exposure for C3 H6 and C3 H8 conversion. However, higher Pt loadings (e.g., 50 gpcf) were required to significantly improve the robustness of the CH4 conversion to the lean oxidations. Thus, catalysts containing both Pd and Pt provide appear to provide advantages for maintaining robust HC control under a wide range of air-fuel and temperature conditions. 5. Summary A laboratory study was performed to assess the effects of high temperature lean operation on the subsequent HC conversion of automotive catalysts containing different precious metals. Degreened samples containing Pd-only, Pt-only, Pt/Rh, or 2 ratios of
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Pt/Pd/Rh were reduced in rich exhaust and then evaluated for CH4 , C3 H8 , and C3 H6 conversion on multiple lean temperature ramps where the maximum temperature increased from test to test. The HC conversion of a model catalyst containing 100 gpcf Pt was relatively stable during the multiple lean temperature ramps. However, the lean HC conversion performance of the other catalyst samples degraded as the maximum temperature on the previous lean temperature ramp increased. For the Pd-containing catalysts, this was attributed to the increasing oxidation of Pd metal to PdOx . While other researchers have demonstrated that PdOx is more active catalytically than Pd metal, it is speculated that the Pd metal sites may serve the function of adsorbing the hydrocarbon. Alternatively, the continued oxidation may have decreased the phase boundary between metallic Pd and Pd oxide. So a balanced mixture of Pd metal and Pd oxide is needed for optimum HC control. The HC conversion of the Pt/Rh and trimetal formulations also degraded during the multiple lean temperature ramps, but the degradation was less pronounced than that of the Pd-only sample. This suggested that the Pt stabilized the performance of the catalyst during extended lean operation. Also, the presence of Pt significantly improved the conversion of C3 H8 relative to that of Pd-only, both after the rich reduction and particularly after the lean oxidations. However, Pt is less durable than Pd at very high temperatures, such as those encountered by close-coupled catalysts on gasoline-powered engines. Therefore, platinum-containing three-way catalysts may
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