Distribution of Contaminants in Used Automotive Emission Catalyst! John L. Bornback,* Mary Anne Wheeler, Jack Tabock, an
Scientific Research Staff, Ford Motor Co., Dearborn, Mich. 481
The electron probe analyzer and scanning electron microscope were used to determine concentrations and distributions of lead, zinc, iron, phosphorus, and sulfur in a series of used automotive emission oxidation catalysts containing platinum as the active element. Lead formed sulfate and phosphate crystals on the catalyst washcoat surface. These crystals coalesced to form a continuous layer which smothered the catalyst and/or formed branching treelike structures which grew up several hundred microns from the washcoat surface. Lead, sulfur, and phosphorus also penetrated into the washcoat micropore network and, in extreme cases, completely plugged it. Iron and zinc did not penetrate into the washcoat. These results suggest that lead, phosphorus, and sulfur are deposited from the gas phase, whereas zinc and iron are deposited as particulates. The formation of an impervious layer of lead sulfate and phosphate and the penetration into the washcoat micropore structure account for the poisoning effect which these elements have on catalytic activity.
Lead phosphorus, sulfur, and certain other elements contained in gasoline and oil are known to poison the activity of oxidation catalysts used to lessen carbon monoxide and hydrocarbon emissions from automobile exhausts (McConnell. R. J.. McDonnell. T. F.. SAE Prenrint No. r - - - ~ ~ 730597, 1973). However, the exact poisoning mechanisims a re not well understood. The morphology and spatial d ist:rihution of these contaminant elements in used cataly;sts ,-&An AFfirmniinn +hair --le in ,l,.,--0~4-~ n ntnl.r pA-,.-UCLLCLULa.6 ,,,,,tic a ctivity. Bulk analytical techniques, such as X-ray fluorescenlce, 9uantitatively determine the concentrations of contanni- 1 --..L . L . . ' ..__.I J . . . ~ ~ : . I I ~ ~ : _ 1:-/ UUL C ~ I U W L ue~errmnemeir spaual omrin --L a m ~NXIWLS hutions within the structural components of used catalyst systems. Electron optical instruments, such as the scanning electron microscope, transmission electron microscope, and the electron prohe analyzer, are able quantitatively to establish contaminant concentration, distrihution, and morphology with relatively high resolution. The contamination in a series of used platinum-type catalytic converters was examined by the above-mentioned electron optical techniques. Unused converters were also examined to provide a reference for changes which occurred with use. The converters studied were manufactured by Engelhard Industries. The active catalyst is a dispersion of fine platinum particles (
R
m m e comers or m e suosrraze (r'igure 21, ana some areas of the substrate are not completely covered (Figure 3). In regions of complete coverage, a crack network e:rtends through t he washicoat giving a "mud flat" appe;nrance (Figure 4). . . . I nree used converters were selected Used Caraiysrs. from a test fleet of cars and trucks after 50,000-mi service using low lead fuel (0.01-0.04 grams Ph/gal). The converters were all mounted on the front of the vehicles behind the exhaust manifold. The honeycomb structures were cylinders 5-7 in. in diameter and 3 in. long. Samples were taken within a half inch of either end and are referred to as inlet and outlet. Table I lists the vehicle type and degree of converter contamination as measured hy X-ray flu-
I I
Figure 2. Fresh converter honeycomb strucfure-washcoat centrates in corners and cracks appear
Con-
Volume9, Number2. February 1975
139
Table 1. Used Catalyst Description Converter number Vehicle. Position Of COnYertel Contaminant
Pb (inlet) Pb (outlet) Zn (inlet) Zn (outlet) Fe (iniet) Fe (outlet) P(inlet) P (outlet) S (inlet) S (outlet)
Figure 3. Fresh converter-region
1
F-100 truck
2 Galaxie
3 Galaxie
Front Left front Right front Concentrations, Wt. %, X-ray fluorescence
7.7 0.06 0.3 0.004 0.03 ND 0.38
ND 0.54 0.02
2.2
5.7 1.4 0.24 0.018 0.004 ND 0.23 ND
1.0 0.097 0.022 0.11 ND 0.13 ND 0.02 0.05
0.18 0.08
of partial washcoat coverage
11
,
-.
I
regon 01 comp ele washcoal covercnaracrertslic 01 coni n r o m wasncoat re-
byll.r.l-l.
age. Cram newor* g on:
IS
Inn...,.
F a
I
100pm Figure 5. Used converter No. I-fracture C ~ U S Js e c w r ~rwrrnal to honeycomb channels at inlet. Surface deposits grow outward from the washcoat surface 140
Environmental Science &Technology
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Figure 8. Used converter No. I-stereo pair of surface deposits at inlet. Structure has nucleated and grown on a protrusion in the underlying washcoat
orescence. A correlation was found between the deterioration of hydrocarbon and carbon monoxide conversion efficiencies and the lead contamination in this fleet of converters. Hydrocarbon emission was roughly proportional to the log of the concentration of lead deposited. Converter No. 1: Heavy Inlet Contamination. The inlet portion of this converter is covered with treelike deposits which appear to have grown from the vapor phase. A fracture cross section showing the substrate, washcoat, and surface deposits is seen in Figure 5 while Figures 6-8 are stereo pairs of the deposits and clearly show their hranching nature. (Stereo pairs are viewed with the right eye looking a t the right image and the left eye looking a t the left image. A card placed vertically between the images helps eliminate confusion. Alternatively, inexpensive viewers for stereoscopically viewing these images are available: Ernest F. Fullam, Inc., Schenectady, N.Y.; Huhhard Scientific, Northhrook, Ill.) The growth shown in Figure 8 has nucleated on a protrusion in the underlying suhstrate/washcoat structure. The contaminant does not completely cover the surface in this region as seen by the cracked washcoat. In other areas, a continuous layer completely smothers the underlying washcoat (Figure 9). This layer has a glazed appearance and results from the
sintering of neighboring particles as they grow from the vapor phase. Only occasional contaminant particles occur on the washcoat surface a t the outlet end. The major elemental constituents of these deposits are Ph, Fe, Zn, P, and S as determined by X-ray energy spectroscopy. X-ray powder diffraction confirmed that these contaminant deposits are mainly lead sulfate. Polished cross sections, normal to the honeycomb channels, were prepared for electron probe analysis. Elemental maps of the substrate, washcoat, and treelike deposits are seen in Figure 10. All elemental X-ray images (Figures 10, 11, and 14) were made using crystal spectrometem with appropriate crystals. An ADP crystal was used to separate the lead M-alpha line from the sulfur K-alpha and its strongest satelites. Peak-to-peak separation is approximately 0.08 whereas the peak full width a t half height is approximately 0.015 A. The washcoat is easily identified hy comparing the A1 and Si images. The outer surface of the washcoat coincides with the outer edge of the A1 image while the washcoat/suhstrate interface coincides with the outer edge of the Si image. The deposits are seen in the lower portions of these elemental maps-e.g., helow the continuous line of the zinc image. Lead and sulfur uniformly penetrate the washcoat up to
Figure 11. UseL --,,.-,.-,
..-.
,p
rnapsof polished cross section at inlet
._.--..-.. ._I.I _._...-... _.
Figure 9. Used washcoat N O . 1-surface deposits at inlet. Continuous laver is formed which completely smothers the washcoat
Figure 10. Used converter No. 1-electron maps of polished cross section at inlet
probe elemental
Figure 12. Used converter No. 2-surface deposits at inlet Volume 9, Number 2, February 1975 141
Table II. Washcoat Contaminant ConcentrationsElectron Probe Results (Wt. %) conrerter NO. 1
Converter No. 3
.~
Inlet
Outlet
Inlet
Outlet
Pb S P
22 1.6 1.0
N.D." 0.25 0.17
12 0.82 0.82
7.6 0.46 0.22
-
notoetecieo.
h.D.
Figure 14. Used converter No 2-electron maps of polished cross section at inlet
I 0
/P W I
2
K
W I
4
I
W
probe elemental
N
I
6 8 1 O I X-RAY ENERGY IKevI
I'igure 13. Used conierter No. 2-X-ray
2
1
4
energy spectrum of
jurface deposits at inlet
the substrate interface, Phosphorus penetrates to a lesser extent while zinc is contained in a continuous layer on the washcoat surface. This zinc rich layer extends throughout LL. . L : . :-ILL ---:-.. ^ P A L : - ---..--LernL^ &.,"&.*-.-: LLlt. arlrrle lllleli 'eg,"', "I LLIIS L"II"VLLC.I. 1 IPS p c u t 5 b L a u " ' J of lead, phosphorus, and sulfur into the washcoat suggests that these elements come from the gas phase whereas zinc and iron are deposited as particulates. The relative penetration of lead, zinc, phosphorus, and sulfur into the washcoat is seen in Figure 11 at a corner where the washcoat is relatively thick. Lead and sulfur penetrate over 100 fim, phosphorus between 30 and 50 p m , and zinc less than 2 pm. All of the elements except zinc penetrate down into the deep crack and laterallv into the washcoat. This supports the sup^iosition that zinc amvf !S a t the surface in particulates whi'le lead, phosphoks, and sulfur arrive in gas molecules. T'here is no contaminarI t ennnnr penetration into the cordierite-mr..Il;tn-N.rlnmi"r l.l -..,,,,.A structure in this catalyst. The contaminant concentrations in the washcoats were determined quantitatively using a defocused beam (-10 pm) in the electron probe. Standard correction procedures were employed. The results are listed in Table II for the inlet and outlet of converters No. 1 and No. 3. Expressed in terms of lead salts, the concentration at the inlet on converter No. 1 is approximately 16% PbSOa and 14% Pba(PO&. If the micropores in the washcoat at the inlet of converter No. 1 are completely plugged, the concentration implies a microporosity of 25%. This figure is reasonable in that it corresponds closely to the porosity of closepacked spheres. Since the washcoat represents approximately 10% of the weight of the entire structure, the probe results indicate that for this sample roughly one third of the contaminant reported hy X-ray fluorescence is in the washcoat and the remainder is on the surface.
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Environmental Science &Technology
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I
verter came from a vehicle equipped with an upstream thermal reactor and, therefore, reached a much higher temperature than converters No. 1 and No. 3. The inlet portion was severely overheated and had begun to collapse. The honeycomb structure was no longer distinguishable. A uniform distribution of fine contaminant particles was deposited over the entire inlet surface (Figure 12). This deposit contains the usual contaminant elements (Ph, S, P, and Zn) along with chromium, iron, anc1 nickel (Figure 13)I.The chromium, iron, and nickel ongi -&":-I---+...I +l...-...nl nate from oxide s..-I,:-pah~~~ UII t g&LL / L L ~D L ~ I I I I T U U OCZCI L I I T I I Y L . ~ I = actor housing. TIiere are no treelike growths on the surface. The electron p robe elemental maps (Figure 14) show .... . navmg that the washcoat. 1.s no longer aistmguisnaole. reacted with t h e substrate. Furthermore, lead , iron, an
