Relative Importance of Thermal and Chemical Deactivation of Noble Metal Automotive Oxidation Catalysts Ralph A. Dalla Betta,*
Robert C. McCune, and Joseph W. Sprys
Research Staff, Ford Motor Company, Dearborn, Michigan 48 72 1
A series of monolithic noble metal oxidation catalysts both in the fresh state and after operation on vehicles for 50 000 miles was studied. Electron microscope examination revealed that the noble metal articles grew from approximately 60 A to 1000 A. This latter value corresponds to a metal area of 9 X mFg-l. The available metal area, as measured by CO chemisorption, decreased to 3 X m2 g-l. Thermal sintering of the catalyst appeared to reach an asymptote that accounted for a 20-fold drop in the available noble metal area while poisoning decreased the area by another factor of 20.
Introduction In the development of automotive exhaust catalysts, the high operating temperatures to which these catalysts are subjected and the presence of large amounts of poisons, such as lead compounds, in the exhaust stream place severe demands on the catalyst. Supported noble metal oxidation catalysts, in which the noble metal surface is the active catalyst, deactivate by two main mechanisms. The first is the loss of metal surface area by sintering or metal particle size growth. Particle growth is mainly a function of the time-temperature history of the catalyst although other factors have a secondary effect. The second mechanism of activity loss is chemical poisoning where lead, phosphorus, or a number of other substances may coat or chemically react with the noble metal preventing its participation in the reaction. Several groups have studied aged oxidation catalysts and have observed Pt particle growth (Mooi et al., 1973;Briggs and Graham, 1973; Liederman et al., 1974). Mooi and coworkers (1973) report particles up to 500 8, in size as determined by x-ray diffraction Iine broadening. Liederman et al. (1974) also observe particle growth to 500 A and larger by x-ray techniques but scanning electron microscope observation of the monolithic catalyst surface revealed Pt particles up to 10 000 8, in size. These latter workers conclude that the thermal sintering of the noble metal is the major route of catalyst deactivation. A knowledge of the relative magnitude of activity loss by thermal sintering and chemical poisoning would be useful in the analysis of used catalyst samples and in understanding catalyst deterioration during use. Particle growth due to thermal treatment can be monitored from measurement of the actual metal particle size using transmission electron microscopy (TEM) while the total catalyst deactivation (thermal plus chemical) is a function of the total metal surface area loss as measured by selective CO chemisorption techniques. Experimental Section Catalysts. All catalysts studied consisted of noble metals supported on high surface area A1203 washcoated monolithic honeycomb supports and were manufactured by Engelhard Industries (ENG PTX-A and ENG PTX-IIB) and Matthey Bishop (MC-3C). The active metals were Pt, Pt P d and Pt, respectively, with metal concentration varying from 0.2 to 0.35 wt %. Several catalyst samples were examined in the fresh state, that is, as received from the manufacturer. In addition,
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Catalytica Associates, Inc., 2 Palo Alto Square, Palo Alto, Calif. 94304.
one of the fresh catalysts, PTX-IIB, was thermally sintered by heating in air containing 10% water vapor at 1000 OC for 5 h. The remaining samples were taken from vehicles operated under the AMA durability cycle in several Ford Motor Company fleet tests. The fuel used was a 1975 certification fuel and contained 0.02 to 0.04 g of Pb/gal, 0.002 to 0.005 g of P/gal, and 0.02 to 0.04 wt % S. After completion of the fleet test, the catalytic converters were removed from the vehicles and a 2.5 cm diameter by 3.8 cm long cylindrical core cut from the inlet portion of each converter. These samples were used for the activity measurements, surface area measurements and electron microscope examination. Apparatus. Carbon monoxide adsorption was measured in a flow system similar to that described by Gruber (1962). The specific metal surface areas obtained by this flow technique were compared with accepted techniques of static H2 and CO adsorption on Pt and P d catalysts, respectively. The static adsorption results were obtained after 400 OC H2 reduction and evacuation and were found to agree with the flow adsorption measurements. A room temperature reduction was used to minimize perturbation of the poisoned metal surface (Dalla Betta, 1973). In the flow experiment, a flowing stream of HZwas passed through a small volume injection valve, the catalyst sample, and a high sensitivity thermal conductivity detector. The hydrogen stream, at room temperature, reduced the noble metal leaving a hydrogen covered surface. A small dose of CO, or 10%CO in Hz, was injected into the hydrogen carrier gas stream. As the CO passed over the catalyst, the noble metal preferentially adsorbed the CO to monolayer coverage. Measurement of the remaining CO and comparison with the detector response upon injection of a similar dose over a sample containing no noble metals permitted calculation of the amount of adsorbed CO. Injection volumes could be varied from 0.0133 to 2.3 cm3 or 0.544 to 94 pmol of CO. The catalyst sample was generally 10 g. The total surface area including the high surface area washcoat was also measured using nitrogen adsorption a t -195 "C according to the BET method. Catalyst activity was measured by raising the catalyst temperature at a rate of 100 "C per minute and continuously monitoring the conversion of CO and hydrocarbon (HC). The CO was measured by a nondispersive infrared analyzer (Beckman, Inc.), the hydrocarbons by a flame ionization detector (Beckman, Inc.) and the oxygen by a fuel cell detector (Teledyne). The feed gas consisted of N2 70%, 0 2 4%, C02 12.5%, CO 2.5%, Hz 1%,H2O lo%, NO 1000 ppm, and C3Hs 1500 ppm. The space velocity was maintained a t 50 000 h-l. The temperature determined a t the catalyst inlet, at which 50% CO and 50% HC conversion was obtained. was taken as Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976
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Table 1. Adsorption and Electron Microscope Data on Monolithic Noble Metal Oxidation Catalysts 8
CO adsorption Sample 1. MB 3C
2. 3. 4. 5. 6. 7. 8. 9. 10.
ENG PTX-A ENG PTX-IIB ENG PTX-IIB ENG PTX-A MB A MB 3C MB 3C ENG PTX-IIB ENG PTX-A 11. ENG PTX-AC 12. ENG PTX-A
Mileage, Expt, miles pmolg-l 0 0 0 H.T. 50 000 50 000 50 000 19 000 35 930 50 000 50 000 50 000
1.3 1.7 5.0
0.37 0.11 0.11 0.045 0.014 0.007 0.006 0.007 0.038
S.A., m2 d , A g - l x 103 97 98 36 494 1500 1200 2 800 8 900 26 000 27 000 23 000 13 500
65.3 84.0 253 18.5 5.57 5.41
Electron microscope results
Activity
Tissue grinding Surface extraction
50% Conv. temp, OF
S.A., m2 d , A g-l X lo3
CO
42 217 39 630 304 730 440 2.28 480 0.71 0.354 1000 0.303 880 0.354 0.611
151 37.8 234 14.5 27.0 8.7 14.4 13.3 9.1 8.8
S.A., m2 d,A 64 290 61 1300 1260 1090 390 2200 4000 3800
g-l X lo3
98.8 28.0 149 7.01 6.51 5.81 16.3 2.88 2.3 2.0
HC
455 466 460 470 476 479 510 519 572 668 614 633 645 680 681 716 909 956 1000 1000 810 852 665 940
BET area, mzg-l 11.7 12.0 14.7
...
8.7 1.3 5.6 1.4 4.0 4.2 2.7 9.3
a Surface areas are expressed per gram of catalyst. Heat treated in air containing 10%water vapor at 1000 "C for 5 h. Sample cut from outlet of converter.
a measure of the catalyst activity for CO or hydrocarbon oxidation, respectively. * The noble metal particle sizes were obtained by electron microscopy using two different techniques of sample preparation (Sprys et al., 1975). In the first technique, a sample of the catalyst material (monolith substrate and washcoat layer) is crushed and ground in ethanol using a biological tissue grinder. The alcohol suspension is then sprayed onto a carbon support film for examination in the TEM. In the second technique, an extraction replica of the catalyst surface is obtained by placing a softened piece of cellulose acetate film in contact with the surface. Upon drying, this film is removed together with a portion of the high surface area washcoat layer containing the active noble metal. The replica is coated with carbon and the cellulose acetate dissolved by floating the carbon replica in acetone. The carbon film is supported on copper grids for TEM examination. This latter technique sampled only the outer portion of the washcoat and is not assumed to be representative of the entire catalyst. However, since only washcoat is included in the sample, the noble metal concentration is much higher and the measurement of particle size is simplified. The samples were examined using a Philips E M 300 electron microscope with a goniometer stage. An energy dispersive x-ray detector and pulse-height analyzer were used to identify the noble metal particles when these particles were either isolated from the support material or were larger than about 100 A. Particle sizes were measured either from a photographic plate or from an enlargement and averages were obtained for each sample. Noble metal concentrations were determined by x-ray fluorescence using standards of known Pt and P d concentration and correcting for interference by contaminants. Results and Discussions The CO adsorption results on each catalyst sample and the particle size as determined by T E M using both surface extraction and tissue grinding techniques are presented in Table I. From the CO adsorption result the particle diameter, d , can be calculated by the formula d = -5w uA where u is the metal density, A is the metal area per gram of catalyst as measured by CO chemisorption, and w is the grams 170
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976
of noble metal per gram of catalyst. It is assumed that CO adsorption occurs on a one to one basis with a surface metal atom, the metal particles are cubic with five sides exposed to the gas phase, and the average surface atom has an area of 8.4 A2 (Benson and Boudart, 1965). If, however, a portion of the metal surface is covered by contamination and is not accessible to measurement by CO adsorption, the calculated particle size will be larger than the actual particle size. This difference is a measure of the extent of catalyst poisoning. The actual particle size determined by transmission electron microscopy from specimens prepared by the tissue grinding technique are assumed to be representative of the entire catalyst sample and can be compared to an effective particle size obtained from adsorption data. A comparison of these two particle measurements is given in Table I. The fresh samples show reasonable agreement. The large particles observed by electron microscopy for the fresh ENG-PTX-A sample and the low particle size measured by adsorption may be due to the presence of a large fraction of small particles undetected by TEM. The adsorption and electron microscope particle size are similar on the heat-treated ENG PTX-IIB sample. The vehicle test samples, however, show metal particle sizes calculated from CO adsorption to be much larger than the actual particle size. The disparity varies from a factor of 1.6 to 30. The first ten samples listed in Table I are arranged in order of decreasing activity. I t should also be noted that this order parallels the order of increasing CO adsorption particle size. The actual particle size does not show a similar trend. However, it should be noted that the particle size calculated from CO adsorption increases to 27 000 A while the actual particle size determined by TEM is never larger than approximately lo00 A. This latter value appears'to be a maximum a t least for the variety of catalysts and treatments studied here. A comparison of the various samples can be accomplished by plotting the extent of thermal deactivation (electron microscopy particle size) and total deactivation including thermal and chemical (CO adsorption particle size) vs. catalytic activity. However, particle size is not an adequate variable for this comparison since the activity is a function of metal surface area and the several types of catalysts vary in total metal content. For this reason metal surface area was used to compare all samples on an equal basis. The CO adsorption results are converted directly to surface area assuming the value of 8.4 A2 for the average area of a surface metal atom. However,
the calculation of the surface areas consistent with the average particle size determined by electron microscopy involved the assumptions mentioned earlier. Of course, the use of the average particle size rather than the total size distribution and the other assumptions introduce some error in this calculation. However, the error should be small compared to the magnitude of the effect observed here. The calculated areas are presented in Table I and a graph of metal surface areas vs. CO activity is given in Figure 1. The right-hand scale, particle size, is given for reference and is only approximate since the samples differ slightly in total metal content. The data in Figure 1 fall into two bands, one for the CO adsorption area and one for the area calculated from electron microscope data, using the samples prepared by the tissue grinding technique. The data from the extraction replication technique fall midway between these two bands. Since the tissue grinding technique is assumed to be most representative of the entire sample, only these data will be discussed. The band for the actual particle size starts at 30 to 50 A and, as the catalyst is aged, approaches a limiting value of 800 to 1000 A corresponding to an area of 9 X m2 g-l. These data show appreciable scatter but none of the average particle diameters fall above this limiting size. The CO adsorption area, however, m2 g-' corresponding to an decreases to a value of 3 x apparent particle size of 3 x IO4 A. The points representing CO adsorption area vs. activity fall into a relatively narrow band indicating a good correlation with the catalyst oxidation activity in spite of the variety between catalysts and noble metals (Pt or Pt Pd). A similar correlation was reported by Briggs and Graham (1973) and Johnson et al. (1974) for monolithic Pt oxidation catalysts. The metal surface area in this work correlated with a laboratory activity measurement as well as Federal CVS vehicle test results. Such a correlation is to be expected since the oxidation reaction occurs only on the noble metal surface and it is this area that is measured by CO chemisorption. The leveling off of the curve as the activity continues to decrease may be a result of diffusion limitations occurring within the catalyst during activity measurement. However, m2 g-l, is near the the lower limit of surface area, 3 X lower limit of detectability by the adsorption technique used here and the metal area may in reality continue to decrease. The very low surface areas observed for the least active catalyst samples, 3 X m2 g-l, represent a 30-fold decrease in available surface area from that estimated from the particle sizes observed by TEM. The difference in these two curves is a measure of the chemical deactivation of the noble metal surface. Thus, the thermal sintering of the active metal results in a surface area decrease by a factor of 20 from the fresh catalyst and the chemical poisoning of the catalyst surface represents another 20-fold drop in area for the least active samples. This is in disagreement with the results of Liederman et al. (1974) which attribute most of the catalyst deactivation to thermal effects. The use of scanning electron microscopy to observe Pt particles a t the catalyst surface does not give results representative of the entire catalyst sample. The approach of the actual metal particle size to a limiting value of 800 to 1000 A is quite surprising when one considers the diverse treatments the various samples have received. The condition under which the catalyst operates in a vehicle can vary widely in temperature, gas flow rate, and gas composition. With respect to the latter, when the CO or unreacted hydrocarbon concentration in the exhaust gas is extremely high, the catalyst temperature can be sufficient to melt the substrate. In fact, sample no. 10 showed substantial melting 2 to 3 cm from the catalyst inlet. However, the most severely heated catalyst samples would be no. 6 and no. 8 since these show the lowest BET surface area. At very high temperatures the high
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I OCO ADSORPTION
OELECTRON MICROSCOPE
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