Poisoning of Platinum-Rhodium Automotive Three-way Catalysts: Behavior of Single-Component Catalysts and Effects of Sulfur and Phosphorus W. Burton Williamson”, Henry K. Stepien, and Haren S. Gandhi Engineering and Research Staff, Research, Ford Motor Company, Dearborn, Mich. 481 21
T h e activit] and selectivity of platinum ( P t ) , rhodium ( R h ) , and Pt R h alumina-supported catalysts for nitric oxide (NO), carbon monoxide (CO), hydrocarbon, and methane conversions were investigated as a function of catalyst composition, feed-gas cpmposition, temperature, and sulfur (S)and phosphorus (P) fuel levels. T h e three-way conversions of P - R h catalysts during modulation conditions closely resembled the activity of pure R h catalysts. Steadystate activities of the individual components of three-way catalysts (TWCs) indicated that the net NO activity of Pt-Rh catalysts resultr from the high NO activity and selectivity of Rh. Pt-Rh catalysts exhibit good hydrocarbon oxidation activity since they behave similarly to R h catalysts in rich gas mixtures a n d similarly to Pt catalysts in lean regions. T h e Pt-Rh TWCs were severely poisoned by S b u t not by the presence of fuel P. Removal of S from the fuel increased net NO conversions and improved conversions of hydrocarbons and CO. T h e removal of fuel P substantially decreased NO reduction and hydrocarbon conversions.
+-
Three-way catalysts (TWCs) used to control nitrogen oxide
(NO,), carbon monoxide (CO), and hydrocarbon (HC) emissions from light-duty motor vehicles have been found to be very sensitive t o even the relatively low contaminant levels t h a t exist in currently marketed “unleaded” gasolines (1-3). Of the three major fuel contaminants [lead ( P b ) ,phosphorus ( P ) , and sulfur (S)],P b and S are probably the most important poisons in the deactivation of platinum-rhodium (Pt-Rh) TWCs, since they are considered the most important factors in the deactivation of NO reduction catalysts, such as copper-nickel or platinum-nickel catalysts ( 4 ) and rutheniumcontaining catalysts ( 5 ) .T h e poisoning effect of P b on Ptcontaining noble metal oxidation catalysts has received considerable study and is included in the review by Shelef e t al. ( 6 )for the poisoning of both noble- and base-metal automotive catalysts. Previous laboratory evaluations of monolithic TWCs ( I , 2, 3 , 7) indicated significant improvements in catalytic performance when trace P b levels in the fuel were lowered. T h e poison resistance of pelleted T W C s was improved by subsurface impregnation of R h below a n external shell of P t ; however, the better durability was accompanied by a loss in initial activity ( 8 ,9). T h e improvements in both cases were attributed to decreased poisoning of Rh, the selective NO, reduction component (10-12). Recent investigations of S poisoning of noble-metal TWCs attributed a simultaneous suppression of gross NO conversion formation in the presence of sulfur and ammonia (“3) dioxide ( S O z )to the poisoning of Pt sites on aged TWCs (13, 1 4 ) . IR results (13)indicated t h a t Pt catalysts had a much stronger affinity for SO2 in a reducing feed gas than R h catalysts. Pt also ha:j a considerably greater SO2 oxidation activity than Rh ( I ) . T h e poisoning of active metals such as Pt and Ni during reducing conditions may result from the formation of inactive surface sulfides (15). In addition to the possible catalyst poisoning derived from P-containing oil additives (16, a n d references therein), fuel P also exerted a poisoning effect on oxidation catalysts (6, 17, and included references). While the retention of P was substantial, only slight poisoning of Ru-containing reduction catalysts was observed ( 5 ) .In the evaluation of earlier TWCs 0013-936X/80/0914-0319$01,00/0 @ 1980 American Chemical Society
the presence of fuel P poisoned a particular T W C for NO reduction, b u t the HC activity was severely reduced upon removal of P from the fuel (7).T W C s having high R h and high noble-metal loadings were found insensitive to P from oil ( I , 18). Recent evaluations in this laboratory of currently formulated Pt-Rh T W C s showed no poisoning for NO nor H C conversions when the fuel P level was increased from 0.8 t o 8.0 mg of P/gal, but, instead, conversions were slightly improved (2),as observed previously (6, 19).T h e formation of stable, inert lead phosphate, Pb:j(P04)2, which would neutralize P b poisoning to some extent, has been previously identified on automotive catalysts (20-22). T h e present laboratory research extends the recent poisoning studies ( 2 , 3 )to further include the effects of fuel P and S on the performance of Pt-Rh TWCs. T h e detrimental effects of oil P (derived from zinc dialkyl dithiophosphate) on pelleted catalysts have been investigated elsewhere ( 1 6 ) ; however, the present study will be restricted to effects of fuel P (derived from cresyl diphenyl phosphate). In order to differentiate the poisoning effects on the individual T W C components, the activity and selectivity behaviors of P t , Rh, and Pt Rh alumina-supported catalysts for NO, CO, and HC conversions were investigated. In addition, methane oxidation activities of Pt-Rh catalysts were determined as a function of catalyst composition and fuel contaminants. In these laboratory studies, catalysts were durability tested under modulation conditions in pulse-flame reactors and followed by flow reactor steady-state activity measurements for comparison.
+
Experimental Catalysts. The catalysts identified in Table I were prepared on Corning EX-20 monolithic supports having a cell density of 49 square cells/cm2 with a 12-mil wall thickness. Catalysts t h a t were prepared in this laboratory contained 0.176% Pt, 0.022% Rh, 0.060% Rh, and 0.176%’P t 0.022% R h on a y-alumina washcoat. In addition, two fully formulated TWCs containing proprietary base-metal promoters had similar total noble-metal loadings ( P t R h = 0.2 wt %), but differing Pt/Rh ratios. Catalyst buttons 1.9 cm diameter X 1.3 cm long were used in these studies. Apparatus and Procedure. Pulse-Flame Reactor Durability Testing. T h e pulse-flame reactor technique (23) and the procedure for pulsator durability testing of TWCs (7) have been reported previously. T h e simulated mileage accumulation during catalyst aging included the following fuels: (a) contaminant-free isooctane; (b) simulated certification fuel consisting of 6 mg of Pb/gal, 0.8 mg of P/gal, and 0.03 wt 9% S in isooctane; (c) S-free simulated fuel (fuel b minus S); (d) P-free simulat,ed fuel (fuel b minus P);and (e) fuel containing 8 mg of P/gal (fuel b with 8 mg of P/gal). The source of P b was “ T E L Motor Mix” containing tetraethyllead, ethylene dichloride, and ethylene dibromide in a n atomic ratio of Pb: C1:Br = 1:2:1. T h e source of P was cresyl diphenyl phosphate, and diethyl sulfide was the source of S. T h e initial 2000 simulated miles in each case was accumulated using contaminant-free isooctane in order t o stabilize the catalyst before introduction of the aging fuel. In the laboratory catalyst aging procedure, the simulated mileages are based on a 30 m p h (5000 miles/week) steady-state vehicle operation a t a nominal 40 000 h-’ space velocity. Methane and total HC measurements were determined
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Volume 14, Number 3, March 1980
319
Table 1. Description of Catalysts a catalyst ident
noble metal, wt %
Pt Rh Rh Pt-Rh M-261Ab M-265B
BET area, m2/g fresh aged
PtlRh
0.176 0.022 0.060 0.198 0.2 0.2
6 5 5 7
a11 1911 511
Gross NO --
CO 0 A
E
0
a
0
A
"Cert"
Isooctane
16 17
a Supported on Corning EX-20 monolith, 49 square cellsicm2 base-metal oxide (promoter).
Contain
Table II. Steady-State Activity of Pt, Rh, and Pt/Rh Pulsator-Aged Catalysts a % conversion b R
0.022% Rh
-1.05
net NO NH3 gross NO
co
1
HC
net NO NH3 gross NO
1.8
co HC
0.176% Pt Rh
+ 0.022%
0.176% PI
90 0 90 92 67
90 0 90 96 84
22 0 22 100 92
93 7 100 34 54
66 19 82 38 54
0 100 10 33 19
MILES X I O + (
a Pretreatment: 10 500 pulsator miles using simulated certification fuel (6 mg of Pb 0.8 mg of P 0.03 wt % Sigal isooctane). Activity measurements in flow reactor: T = 550 OC; space velocity = 60 000 h-': simulated exhaust gas containing 20 ppm of SO2. As percent NO converted.
+
+
routinely a t catalyst temperatures of 500 "C during pulsator measurements. T h e exhaust gases were passed through a small-volume (0.5 cm3) injection valve connected to a gas chromatograph equipped with a Beckman flame ionization detector. A portion of the sample was analyzed for methane by separation over a 6 ft X 0.25 in. stainless steel column of 5-A molecular sieve, and the remainder was passed directly to the FID for total HC measurement. T h e system was calibrated with known concentrations of methane in nitrogen. Steady-State Actioity a n d Selectivity. Following t h e pulsator aging, the catalytic steady-state activity was measured a t 550 "C and 60 000 h-' space velocity in a separate flow reactor system. A schematic diagram of t h e apparatus and synthetic gas mixture is described in ref 7. Propylene and propane represented fast-burning and slow-burning hydrocarbons, respectively, in a ratio of C ~ H C / C ~= H2. ~ T h e activity and three-way selectivity of the catalysts a r e reported as percent conversion of NO, CO, a n d HC as a function of the redox ratio, R , of the reacting gas mixture. (R is a ratio of the reducing to oxidizing components in the gas mixture and is determined as follows: R = [CO H:! 3nCnH2n + (3n + 1)CnH2,+2]/(N0 2 0 2 ) . Thus, R = 1 corresponds to a stoichiometric gas mixture, while R > 1 represents an overall reducing gas mixture.)
+
+
+
Results a n d Discussion
Three-way Activity and Selectivity of Pt, Rh, and Pt-Rh Catalysts. Two single-component catalysts, 0.176% Pt and 0.022% Rh, and a mixed catalyst, 0.176% Pt 0.022% Rh, were pulsator-aged for 10 000-15 000 simulated miles using contaminant-free isooctane and isooctane containing certification contaminant levels of 6 mg of Pb/gal, 0.8 mg of P/gal, and 0.03 wt 9" S.T h e three-way conversions during
+
320
Environmental Science & Technology
K m x 10-'/1.6)
Figure 1. Catalyst activities (500 OC, R = 1.15) during pulsator modulation using pure isooctane and isooctane containing 6 mg of Pblgal, 0.8 mg of Plgal, and 0.03 wt YO S: (a) 0.176% Pt; (b) 0.022% Rh; (c) 0.176% Pt 4- 0.022% Rh
pulsator modulation conditions are shown as a function of mileage accumulation for the Pt catalyst in Figure l a , for the Rh catalyst in Figure l b , and for the mixed catalyst in Figure IC.T h e effects of thermal aging on the three formulations are demonstrated by the catalysts' durability using contaminant-free isooctane. In the laboratory catalyst aging procedure, the catalysts are exposed to a low-temperature oxidizing cycle to simulate cold-start exhaust conditions, and to a high-temperature reducing cycle (maximum temperature of 730 "C for 6% of the time) to simulate high acceleration modes of operation. During thermal aging in t h e absence of potential poisons, the R h catalyst showed slightly higher NO conversion than the Pt catalyst, but similar CO and HC conversions. In the presence of P b , P , and S fuel contaminants, t h e chemical poisoning of the Pt was quite significant for NO and HC conversion. T h e three-way conversion of t h e mixed Pt-Rh catalyst during modulation conditions more closely resembled that of the pure R h catalyst. These results may be attributed t o insufficient oxygen transfer over the thermally and chemically deactivated Pt surface during high-frequency modulations. Also, R h may remain well dispersed and contribute significantly to the oxygen-transfer capability of the catalyst. T h e steady-state activities of the pure Pt and R h catalysts and of the mixed Pt-Rh catalyst were determined after about 10 500 simulated miles of aging using the simulated certification fuel. Three-way conversion activities are shown as a function of redox ratio, R, for the P t , Rh, and Pt-Rh catalysts in Figures 2a, 2b, and 2c, respectively. Summarized activities for the catalysts are given in Table I1 a t the redox ratio ( R N 1.05) corresponding to the maximum simultaneous three-way conversion of NO, CO, and HC (NO and HC conversion crossover) and for R = 1.8, corresponding to 2.8% rich of stoichiometry. T h e net NO activity of thermally and chemically aged Pt-Rh catalysts results from the high NO activity and selectivity of R h catalysts as demonstrated in Figure 2b for 0.022% Rh. Over the aged R h catalyst gross NO conversions are essentially 100%in the rich region ( R > 1). T h e selectivity of R h
40
08
I2 14 16 REDOX R A T I O , R
IO
18
200.8
IO
12 14 16 R E D O X RATIO, R
18
2008
12 14 16 REDOX RATI0,R
IO
I8
Figure 2. Effect of redox ratio on the steady-state gross NO, CO, and HC activity of catalysts pulsator aged for -10 500 simulated miles using simulated certification fuel: (a)0.176% Pt; (b) 0.022% Rh; (c) 0.176% Pt 0.022% Rh. Space velocity = 60 000 h-'; T = 550 "C; simulated exhaust with 20 pFim of SOz. Gases: (0)CO; ( A ) HC; (0) NO; ( 0 )"3, as ?YO NO converted
+
'I r,;
( a ) 0 176 k Pt
- 100
80 -
-
80
-
60
60 -
40 20
0
1
I
1
'
1
1
.
5
'
I
'
I
,
1
7
40
200100
20
,
'
%a
- 40
-
( c ) 0176KPt + 0 0 2 2 % Rh
( b ) 0 0 2 2 KRh
i " " ' " " ' 100 -
0
Figure 3. Temperature effect on the steady-state conversions of net NO, CO, and HC over catalysts pulsator aged for -10 500 miles using simulated certification fuel: (a)0.176% Pt; (b) 0.022% Rh; (c)0.176% Pt 0.022% Rh. Space velocity = 60 000 h-'; R = 1.05;.simulatedexhaust with 20 ppm of SO2. Gases: (0)CO; ( A ) HC; (0) NO
+
for nitrogen is evident from t h e low ammonia formation. For example, NH3 formation even at R = 1.8 is only 7% of the converted NO. 13y contrast, the peak NO conversion for t h e Pt catalyst (Figure 2a) is 36%, with only 10% gross NO conversion a t R = 1.8. However, due to t h e poor selectivity of Pt for nitrogen formation in reducing conditions (10,24),all of leaving zero net NO converthe converted NO goes to "3, sion. For the Pt-Rh catalyst (Figure 2c), the gross N O conversion a t t h e peak R value and for R > 1 is considerably lower, and NH.3 formation is somewhat higher t h a n for t h e R h catalyst. As a result, the net N O conversion on the Pt-Rh catalyst (Table 11) a t R == 1.8 is 66%, compared t o 93% for the R h catalyst. The lower gross N O activity of the Pt-Rh catalyst (82%) compared to the R h catalyst (100%) indicates, perhaps, some Pt-Rh interaction t h a t reduces t h e R h area available for N O reduction. Decreased selectivity and increased NH3 formation of a Pt-Rh catalyst compared tu a R h catalyst also have been observed by Schlatter and Taylor ( 2 5 ) .I n our case it may result from a fourfold atomic excess of Pt t o R h in the Pt-Rh catalyst. T h e presence of Rh, however, substantially improves the nitrogen selectivity of t h e Pt-Rh catalysts as compared to the pure Pt catalyst. T h e effect of temperature a t the R value corresponding t o t h e peak three-way Conversion is shown for the Pt, Rh, and Pt-Rh catalysts in Figures 3a, 3b, and 3c, respectively. T h e R h and Pt-Rh catalysts have similar NO light-off characteristics, whereas t h e Pt catalyst required nearly 100 "C higher temperature to achieve the peak NO conversion when compared to the R h and Pt-Rh catalysts. T h e steady-state activities of t h e individual T W C components indicate that Pt-Rh catalysts exhibit good H C activity
(via oxidation and steam-reforming reactions) since they behave similarly t o R h catalysts in rich gas mixtures and similarly to Pt catalysts in lean regions. T h e pure Pt catalyst shows a significantly better H C activity in t h e lean region (Figure 2a, R < 1) compared to the R h catalyst (Figure 2b). For example, a t R = 0.8 the total HC conversion on the R h catalyst is 84%, compared to 98% for the Pt catalyst. T h e temperature for 50% conversion of CO and H C is about 50 "C higher for t h e P t catalyst (300 "C) compared to t h e R h or Pt-Rh catalysts (250 "C), as shown in Figure 3. However, the inability of R h to convert a saturated HC (propane in this case) is shown in Figure 3b. At temperatures near 550 "C t h e maximum total H C conversion achievable is 66%, which corresponds to the more easily oxidized olefinic fraction of the total HC, since t h e feed-gas mixture contained the ratio C ~ H ~ / C ~= H 2. SHence, due to t h e poor saturated H C activity of the R h catalyst and substantially less poisoning of the Pt surface by SO:! in the lean region (13,14),Pt exhibits the best H C activity in lean mixtures. However, in the rich region ( R > 1) SO:! poisoning of Pt sites becomes significant, probably due to t h e unavailability of excess oxygen for removal of surface sulfide species. T h e HC conversion becomes more severely suppressed for the Pt catalyst than for the R h catalyst, whose surface is not poisoned by SOz. T h e effects of pulsator modulation on the activities at R = 1.15 and 1.8 are shown in Table I11 for comparison with t h e steady-state results of Table 11. T h e steady-state data indicate substantial decreases in HC and CO conversions a t R > 1 over the three catalysts. However, during the modulation conditions of the pulsator the differences in activity between R = 1.15and 1.8 are much smaller, as indicated in Table 111. Since t h e activities in Tables I1 and I11 are from two different sysVolume 14, Number 3, March 1980 321
( 0 )
Table 111. Durability of Pulsator-Aged Pt, Rh, and Pt/Rh Catalysts
Gross NO CO ---
a
% conversion b R
gross NO
-1.15
60 58 56
co HC
gross NO
1.8
HC
27 52 42
58 44 50
32 41 37
a Catalyst aging: 10 500 pulsator miles using simulated certification fuel (6 mg of Pb 0.8 mg of P 0.03 w l % S/gal isooctane). Activity measurements: pulsator modulation of 0.5 Hz, i 1 A/F; T = 500 OC; nominal space velocity = 40 000 h-'.
+
8O
\]
k
O
i
r
+
% CH4 conversion a catalyst
Pt
003
60
z
+ 0.022%
Pt Rh Rh
50
CH4 as % total HC feed after
isooctane
cert fuel b
29
16
23
49
15 15
8 6 14
32 18 18
48 42 31
-
4o.k
Table IV. Methane Oxidation as a Function of Catalyst Composition
0.176% Rh 0.176% 0.022% 0.060%
A A
0.176% Pt
49 55 62
75 54 51
co
Wt %S -
HC
O
8
0.176% Pt Rh
+ 0.022%
0.022% Rh
( b ) CATALYST M - 2 6 5 8
CATALYST M - 2 6 1 A
5
0
15
IO
5 IO 15 Km x 10-3/i.6)
MILES X I O -
Figure 4. Effect of 0.03 wt % S fuel vs. S-free fuel (both containing 6 mg of Pb 0.8 mg of P/gal of isooctane) during pulsator modulation activity (500 O C , R = 1.60) of: (a) M-261A (Pt/Rh = 19); (b) M-265B (Pt/Rh = 5)
+
(0)
a Pulsator-aged 10 000 simulated miles; T = 500 O C , R N 1.3. Isooctane containing certification contaminant levels of 6 mg of Pb 4-0.8mg of P 0.03 wt % %gal.
GrossNO COHC O O A
8
0
A
Table V. Effect of Fuel Contaminants on Methane Oxidation % CH4 conversion a fuel b
isooctane Pb free
amgofp S free cert P free 60 mg of Pb
M-265B
20 12 16 15 12 7
40 30 28 27 15 19 14
60
Pulsator-aged 15 000 simulated miles; T = 500 " C . R 1.15. Isooctane containing certification contaminant levels of 6 mg of Pb 0.8 mg of P 0.03 wt % S/gal except for the noted changes.
+
+
8
tems during different experimental conditions, absolute comparisons of activities cannot be made; however, relative comparisons within each system are valid. T h e H C activity during steady-state conditions at R = 1.8 is reduced more severely over Pt than over the R h and Pt-Rh catalysts. T h e differences in activity near stoichiometry and a t R = 1.8are substantially less during the pulsator modulation conditions. Brief excursions into oxidizing regions thus appear beneficial by improving the oxidizing capability of P t , probably as a result of the removal of S species. T h e R h catalyst, however, maintains better HC and CO activity than t h e Pt catalyst at steady-state and during modulation conditions. During steady-state conditions the less demanding reaction of CO oxidation was quite similar over the P t , Rh, and Pt-Rh catalysts a t all R values. The R h catalyst exhibited somewhat higher CO activity during modulation at R = 1.8than the Pt catalyst. This implies that R h possesses better 0 2 storage and transfer processes than Pt, since it maintains better oxidation activities, even though at these loadings (Pt/Rh = 8) there is
M 5
40 5
322 Environmental Science & Technology
mgP/gol
e
M-261A
5
1:":-:4 ( b ) CATALYST M - 2 6 5 8
CATALYST M-261A
+
IO
15
MILES X
20
( Km
x IO-'/
IO
A L15A
20
16)
Figure 5. Effect of 8 mg of P/gal of fuel vs. P-free fuel (both containing 6 mg of Pb/gal 0.03 wt % S) during pulsator modulation activity (500 O C , R = 1.15) of: (a) M-261A (Pt/Rh = 19); (b) M-265B (Pt/Rh = 5)
+
a fourfold lower atomic concentration of R h to Pt. Methane Oxidation over Pt, Rh, and Pt-Rh Catalysts. Methane oxidation activity after pulsator aging was determined over single-component and mixed catalysts (Table IV) and over fully formulated TWCs, M-261A (Pt/Rh = 19) and M-265B (Pt/Rh = 5), as a function of catalyst composition and fuel contaminants (Table V). T h e typical feed-gas composition of the pulsator exhaust contains 67% unsaturated HC and 33% saturated HC as determined by Fourier transform IR analyses (26). However, the combustion of isooctane (2,2,4trimethylpentane) in the pulsator produces a relatively high methane content of 25% (23,26),as shown in Table IV, when compared t o typical concentrations of -5% methane usually found in automotive exhaust. Methane oxidation measure-
Table VI. Effect of S Poisoning and Rh Content on Performance of Pulsator-Aged TWC Catalysts a 06
R
catalyst:
wt % Slgal
-1.05
net gross
NO NH3 NO
co 1.8
net gross
HC NO NH3 NO
*
co HC a
550
C:
o
M-261A (PtlRh = 19) 0.03
90 0 90 98 88 70 21 88 52 59
90 0 84 95 86 56 26 76 44 51
conversion b 0
5) 0.03
89 0 89 99 89 69 27 95 42 46
86 0 86 100 86 66 28 92 44 50
M-2658 (PVRh
Pretreatment: -15 000 simulated miles of pulsator aging. Activity measurements in flow reactor: Simulated exhaust gas containing 20 ppm of SOn, T = O C ; space velocity = 60 000 h-’. Fuel containing 6 mg of Pb and 0.8 mg of P/gal of isooctane in addition to S. d A s percent NO converted.
ments vs. redox ratio indicated maximum methane conversions were obtained slightly lean of stoichiometry a n d increased with temperature. However, the results reported here as percent conversions are for temperatures and redox ratios relevant t o the operation of TWCs. Of t h e methane oxidation observed over Pt-Rh catalysts, a considerable fraction occurred over Rh. While R h catalysts demonstrated poor saturated H C activity, the oxidation of CH4 over the 0.022% R h catalyst was comparable t o that over the 0.176% Pt catalyst. Increasing t h e R h content to 0.060% showed a proportional improvement in the CH4 conversion (Table IV)of the single-component catalyst. T h e increased R h content of the M-265B T W C significantly improved the CH4 (as well as total H C and NO) conversions for each of t h e fuel contaminant levels shown in Table V. T h e CH4 activities also were improved, in general, by the use of lower P b and S levels in the fuel. Effects of F u e l Sulfur. T h e poisoning of Pt-Rh TWCs by 300 pprn of fuel S is demonstrated in Figure 4 for the pulsator aging of the M-261A and M-265B TWCs and in Table VI for the steady-state activities following 15 000 simulated miles of aging. Removal of 800 ppm of S from the simulated fuel significantly improved the T W C performance, especially over the M-261A T W C containing the higher Pt content. After the initial 2000 simulated miles using isooctane fuel, t h e introduction of 0.03 wt % S fuel poisoned the T W C most severely during the first 10 000 miles of aging. This is shown most clearly by the sharp decrease in CO conversions in Figure 4, but the poisoning effect appears less pronounced at extended mileage accumulations. However, t h e steady-state results of Table VI indicate that the M-261A T W C with S-free fuel had 14% higher net NO and 8% better HC and CO conversions a t R = 1.8 than when aged with fuel containing 0.03 wt 9” S. T h e M-265B ‘TWC having the higher R h content was not poisoned for NO conversion, as shown in Figure 4b during aging and from the steady-state results in Table VI. Gross NO conversion over t h e higher Pt-containing M-261A T W C is thus decreased due t o S poisoning of Pt in t h e rich region ( R = 1.6), whereas the M-265B (containing a fourfold higher R h content) is not poisoned to t h e same extent under these conditions. In related studies, SO2 significantly suppressed gross NO over Pt catalysts (3, 1 3 , 1 4 ) ,b u t had no effect on t h e NO conversion over a R h catalyst (3, 1 4 ) . Higher CO and H C conversions were observed during pulsator modulation in the absence of S. E f f e c t s of F u e l Phosphorus. While Pt-Rh TWCs are severely poisoned by P b and S, our laboratory results indicate t h a t fuel P is not a poison a t t h e levels investigated here for P derived from cresyl diphenyl phosphate. T h e results shown in Figure 5 illustrate the effects of pulsator aging using P-free
fuel and 8 mg of P/gal levels (certification levels of P b and S kept constant) on the M-261A and M-265B TWCs. T h e TWCs are not poisoned at P levels 10-fold higher than the 0.8 mg of P/gal levels found in current certification fuels, b u t actually have improved significantly the three-way conversions over the TWCs aged in the P-free fuel, especially for the M-261A TWC. I n the absence of fuel P the NO and H C conversions after 15 000 pulsator miles were about 10%better over the M-265B compared to the M-261A TWC. While t h e lower Rh-containing T W C was more susceptible to poisoning by P b , the 10-fold atomic excess of P apparently neutralized any deactivation from P b by the formation of lead phosphates (20, 21 1. Having a larger R h reserve, t h e M-265B T W C was not poisoned by P b t o as great an extent in the absence of P. Thus, the observed neutralizing effects by P of P b poisoning appear greater over t h e M-26lA TWC.
Concluding R e m a r k s T h e present laboratory results provide some indications as to t h e poisoning behavior of t h e individual noble metal components present in fully formulated Pt-Rh TWCs. During simulated T W C aging conditions, the three-way activity of the Pt-Rh catalyst resembled that of the Rh catalyst. The NO and HC activities of t h e Pt catalyst a t these conditions were severely poisoned by the simultaneous presence of 6 mg of Pb, 0.8 mg of P , and 0.03 wt % S.T h e Pt-Rh catalyst exhibited a poorer net NO conversion than the pure R h catalyst, indicating that a separation of Rh from P t could improve net NO conversion (W). Pt-Rh catalysts behave similarly to R h in rich regions and similarly to P t in lean regions for HC conversions, resulting in good HC activity a t all redox ratios. Methane oxidation over Pt-Rh catalysts was favored by higher R h loadings and lower levels of P b and S in the fuel. While lower P b and S levels in the fuel also resulted in better, simultaneous three-way catalytic activity, the removal of fuel P resulted in poorer NO and H C activities. This indicates that, under the laboratory conditions, fuel P had no poisoning effect on Pt-Rh TWCs, but could effectively neutralize a portion of the P b poisoning mechanism. Acknowledgments We are grateful to A. G. Piken for BET surface area measurements and to W. Watkins and D. Lewis for operation of the pulse-flame reactors and methane oxidation activities.
Literature Cited (1) Gandhi, H. S.; P i k e n , A. G.: Stepien, H. K.; Shelef. M.; Delosh, R. G.; Heyde, M. E. SAE (Society of A u t o m o t i v e Engineers), 1977, Paper 770196. (2) W i l l i a m s o n , W. B.: Stepien, H. K.; W a t k i n s . W. L. H.; Gandhi, H. S. Enciron. Sei. Techno/. 1979,13, 1109. Volume 14, Number 3, March 1980
323
(3) Williamson, W. B.; Gandhi, H. S.; Heyde, M. E.; Zawacki, G. A. SAE (Society of Automotive Engineers), 1979, Paper 790942. (4) Jackson, H. R.; McArthur, D. P.; Simpson, H. D. SAE (Society of Automotive Engineers), 1973,Paper 730568. (5) Gandhi, H. S.; Stepien, H. K.; Shelef, M. SAE (Society of Automotive Engineers). 1975. Paaer 750177. (6) Shelef, M.;Otto, K.; Otto,”. C. A d u . Catal. 1978,27, 311. ( 7 ) Gandhi, H. S.; Piken, A. G.; Shelef,M.; Delosh,R. G. SAE (Society of Automotive Engineers), 1976, Paper 760201. (8) Hegedus, L. L.; Summers, J. C. J . Catal. 1977,48, 345. (9) Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J . Catal. 1979,56, 321. (10) Kobylinski, T. P.; Taylor, B. W. J . Catai. 1974,33, 376. (11) Ashmead, D. R.; Campbell, J. S.; Davies, P.; Farmery, K. SAE (Society of Automotive Engineers), 1974,Paper 74029. (12) Taylor, K. C. In “The Catalytic Chemistry of Nitrogen Oxides”; Klimisch, R. L., Larson, J. G., Eds.; Plenum: New York, 1975; p 173. (13) Gandhi, H. S.; Yao, H. C.; Stepien, H. K.; Shelef, M. SAE (Society of Automotive Engineers), 1978, Paper 780606. (14) Summers, J. C.; Baron, K. J . Catal. 1979,57, 380. (15) Katzer, J. R. In ref 12, p 133. (16) (a) Spearot, J. A,; Caracciolo, F. SAE (Society of Automotive Engineers), 1977, Paper 770637. (b) Caracciolo, F.; Spearot, J. A.
SAE (Society of Automotive Engineers), 1979, Paper 790941. (17) Gagliardi, J. C.; Smith, C. S.; Weaver, E. E. A P I D i u . Refining Proc. 1972,52, 989. (18) Wotring, U’. T.; Meguerian, G.H.; Gandhi. H. S.; McCuiston. F. D.; Piken, A. G. SAE (Society of Automotive Engineers), 1978, Paper 780608. (19) Acres, G. J. K.: Cooper. B. .J.; Shutt, E.; Malerbi, B. W. A d c . C h e m . S e r . 1975, N o . 145. (20) Shelef, M.; Dalla Betta, R. A,; Larson, J. A.; Otto, K.; Yao, H. C. Presented at the 74th National Meeting of the AIChE, New Orleans, March 1973. (21) McArthur, D. P. In ref 12, p 263. (22) Williams, F. Id.;Baron, K. J . Catal. 1975, 40, 108. ( 2 3 ) Otto, K.; Dalla Betta, R. A,; Yao, H. C. APCA J . 1974, 24, 596. (24) Shelef, M.; Gandhi, H. S. I n d . E n g . C h e m . Prod. Res. Deu. 1972, 11, 393. (25) Schlatter, J. C.; Taylor, K. C. J . Catal. 1977,49, 42. (26) Williamson,W. B.; Gandhi, H. S. Ford Engineering and Research Staff, unpublished data, 1979. Received for reuiew June 25, 1979. Accepted December 26, 1979. Presented at t h e S i x t h N o r t h A m e r i c a n M e e t i n g of t h e Catalysis Society, Chicago, Ill., M a r c h 1979.
NO, (= NO -tNO2) Monitor Based on an H-Atom Direct Chemiluminescence Method Arthur Fontijn’”, Hermann N. Volltrauer, and William R. Frenchu AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, N.J. 08540
,A monitoring method has been developed for measurement of NO, based on the chemiluminescence reaction system of
monitoring. This direct chemiluminescence monitoring method is based On Reactions lL4 (’”’)’
NO and NO2 with H atoms. This method eliminates the need for, and errors connected with, tly NO2 to NO converters that are required when the NO/O3 chemiluminescence method is applied to NO, measurement. Feasibility studies have been performed and a first prototype constructed for measuring motor vehicle emissions. This unit has a linear response to NO, over a concentration range from 6 to a t least 4000 ppm (v/v), independent of the [NO]/[N02] ratio. Interferences by 0 2 and ethylene were encountered and experimental arrangements are described t h a t reduce these to acceptable levels in the prototype. T h e prototype performance characteristics represent a trade-off between sensitivity and interference levels; prospects for an ambient air NO, monitor are discussed.
H
H +NO
H
+
+ HNO-
H2
(1)
(2)
+hv
(3)
+ NO
(4)
The emission from Reaction 3 is a series of bands between 628 and 800 nm (9,12,13).Reaction 3 leads to the expression for the chemiluminescence light intensity:
I
+
Present address, Department of Chemistry, Queen Mary College, Mile End Road, London, E l 4NS, England. Environmental Science & Technology
+M
+ OH HNO + M
NO
H + NO-HNO
Following the initial development ( 1 ) of a NO analyzer, based on the N 0 / 0 3 chemiluminescent reaction, this method also gained rapid acceptance for NO, = NO NO2 monitors when Sigsby e t al. (2) demonstrated the practicality of converting NO2 to NO prior to passing pollutant gases through the reaction chamber. Both thermal/catalytic and chemical converters are in use ( 3 ) ,the latter only for ambient monitoring because of their low capacity. While such converters have generally performed well, errors have been observed in source monitoring due to reduction of NO, to N2 and oxidation of other N-containing compounds (“3, H C N ) to KO (4-6); problems have also been encountered in ambient monitoring (7, 8). To avoid such complications it is desirable to have a method t h a t does not require a separate converter. In the work described here such a method is established for (mobile) source
324
+ NOn+
a
lHl[NOl
(5)
Reaction 1 is a very fast process (14, I 5 ) , h 1 = 3 X 1O1O L mol-’ s-1, and hence conversion of NO2 to NO in the reactor is essentially instantaneous and the equation governing the monitoring method is thus:
I
a
[HI [NO,]
(6)
Much higher concentrations of NO, and other potentially interfering compounds are present in source monitoring than in ambient air. Thus, operating conditions of monitors usually represent a trade-off between sensitivity and levels of interference. Sensitivity, linearity, and interferences are the factors to investigate in a monitor feasibility and development study such as reported here. In the present case, negative interference due to consumption of H atoms could be calculated a priori because the rate coefficients for t h e pertinent H-atom reactions are available ( 1 5 ) . Such calculations for the most reactive ( 1 5 ) compounds present in engine exhausts (see below) showed ( 1 6 ) that it was desirable to use sample volume flow rates much lower than the reagent (H/H2) volume flow rates. For a 1 : l O O ratio, an operating pressure of 0.8 Torr ( a t 300 K), and a reaction time a t 5 X lop3s, the calculated fractional decrease in [HI and hence chemiluminescence intensity
0013-936X/80/0914-0324$01.00/0
@ 1980 American Chemical Society
