Environmental Catalysis - American Chemical Society

Chapter 4

Deactivation of Cu-ZSM-5 Probable Irrelevance of Modest Zeolite Degradation Compared to Cupric Sintering to CuO 1

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Karl C. C. Kharas , Heinz J. Robota , andAbhayaDatye 1

AlliedSignal Research & Technology, Des Plaines,IL60017-5016 The University of New Mexico, Albuquerque,NM87131-1341

Downloaded by CORNELL UNIV on May 21, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch004

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Cu-zeolite catalysts enable NOx reduction in gas streams that model oxidizing emissions from advanced lean-burn engines but are not durable and deactivate after catalysis at 800 °C. Precisely determined variation of micropore volumes correlate well with catalyst deactivation. CuO can be observed in catalytically or hydrothermally deactivated catalysts but not in fresh catalysts by XRD. High resolution TEM reveals a loss of crystallinity of the zeolite in thin regions of the exterior surfaces of zeolite crystal grains. Two phenomenaaccompany,and probably cause, catalyst deactivation: 1. superficial amorphization of the crystalline zeolite and 2. sintering of CuO and resultant local destruction of zeolite crystallinity in the vicinity of sintering oxides of Cu.

Cu-ZSM-5 can reduce NO under net oxidizing conditions (1-14) but the performance is not durable (15). Catalytic deactivation of Cu-ZSM-5-387 caused more substantial declines in pore volume than simple hydrothermal aging in air at comparable temperatures (15). Catalysis induced sintering of the initially present copper species to CuO, as shown by EXAFS and XRD(15). We suggested that deactivation is caused primarily by this sintering process and that degradation of the crystalline zeolite was subsidiary in cause and secondary in effect (15). Here we report more extensive laboratory characterization of Cu-ZSM-5-207 deactivation. As was the case with Cu-ZSM-5-387, we observe sintering of copper species to CuO as deactivation proceeds. High resolution TEM reveals additional phenomena that appear consistent with our overall model of catalyst deactivation.

0097-6156/94/0552-0039$08.00/0 © 1994 American Chemical Society Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

ENVIRONMENTAL CATALYSIS

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Experimental The ZSM-5 was purchased from PQ Corporation (H-ZSM-5, CBV 8020, lot ZN 10, Si/Al = 30.5). Samples were suspended in deionized water and treated with sufficient cupric acetate solution to provide an overall Cu/Al atom ratio of about 1.0. The resulting material, Cu-ZSM-5-207, the primary catalyst studied here, has a nominal cupric ion exchange content is 207% and a Cu/Al mol ratio of 1.035. After stirring overnight, the addition of small amounts of concentrated ammonia raised the pH of the solution to 7.5. After holding the pH constant for 2 hr, the suspension was filtered and dried. Subsequently, the Cu-ZSM-5 was calcined typically at 400 °C, but sometimes at 500 °C, 600 °C, 700 °C, or 800 °C. Powder X-ray diffraction data were collected by standard methods with a Rigaku diffractometer equipped with a rotating anode X-ray source. Micropore volume determinations were obtained from Ar sorption data using Micromeritics Instrument Corporation ASAP 2000M porosimeter. TEM analysis was performed using a JEOL 2000 FX microscope. Catalytic treatments use simulated lean-burn gas mixtures found in the patent literature (6,7). Specifically, the model A/F 18 gas mix contained 3000 ppm CO, 1000 ppm H , 1600 ppm C^, 1200 ppm NO, 3.2% 0 , 10% C0 , 10% H 0, balance N . The chemical equivalence ratio (oxidants/reductants) for this mixture is 3.54. Generally, 1 gram of 20-40 mesh granules is used in catalytic evaluations where the volumetric flow rate is 5 liters/minute. A space velocity of about 120,000 hr" results. Hydrocarbon conversion was monitored with a FID (Rosemount Analytical Model 400A), NOx reduction with a NOx chemiluminescent analyzer (Rosemount Analytical 951A set to determine NOx), 0 with an Horiba Magnetopneumatic analyzer (MPA-21), CO and N 0 with nondispersive IR analyzers (Rosemount Analytical 880). The N 0 analyzer is equipped with filters to compensate for C 0 and, in particular, CO present in the gas stream. At high CO conversion and low N 0 concentrations, the analyzer registers "negative" amounts of N 0. 2

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Results Figure 1 shows performance of fresh Cu-ZSM-5-207. As reported previously (1-5), Cu-ZSM-5 can reduce NO under net oxidizing conditions. If NO reduction under these conditions were durable, emissions could be controlled from lean-burn engines that offer increased fuel economy and correspondingly decreased C 0 emissions. Catalysts offering NO reduction under oxidizing conditions should not produce much N 0 since N 0 is itself believed to be a potent greenhouse gas. Reassuringly, this catalyst produces little N 0 in this fully formulated synthetic exhaust. This confirms the relevance of previous studies performed under more idealized conditions that reported high selectivity of NO reduction to N (1-5). Figure 2 shows performance of Cu-ZSM-5-207 both when fresh and after one hour of catalytic treatment at A/F 18 at various temperatures. Noticeable deactivation follows one hour of catalysis at 600 °C. However, a 4 hour calcination in static air at 600 °C results in fresh performance essentially indistinguishable from the fresh catalyst shown in Figure 2 which had been calcined to only 400 °C. 2

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Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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Deactivation of Cu-ZSM-5

-•-NO

- t - H C "A--N2O

-•-co

~o- 02

π ι ι rn—ι—ι—ι—ι—ι—ι—ι—r150

300 450 600 Inlet Temperature, °C

750

Figure 1. Conversion of NO, HC (propene), CO, 0 (in %) and N 0 formation (in ppmv) as a function of temperature. NO reduction over fresh CuZSM-5-207 bends over as temperature increases; little N 0 is formed. 2

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Fresh After 700°C catal — Δ - - After 800°C catal After 600°C catal -o— After 750°C catal x- - After 831°C ΗΤΑ Inlet Temperature, °C

500 600 100 200 300 Inlet Temperature, °C Figure 2. Conversion vs. temperature of fresh Cu-ZSM-5-207 and of Cu-ZSM-5207 after one hour of catalysis at A/F 18 at stipulated temperatures or after hydrothermal aging. Stepwise deactivation is observed after one hour catalytic treatment at increasingly high temperatures; hydrothermal treatment at 831 °C is worst of all. Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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Subjecting the material calcined at 600 °C to one hour of catalysis at 600 °C causes deactivation comparable to that shown in Figure 2 (15). Sequentially greater deactivation follows sequentially hotter catalytic treatments. Deactivation after one hour of hydrothermal treatment at 831 °C is a little more severe than one hour of catalysis at 800 °C. The exotherm across the catalyst bed during catalysis at 800 °C is about 50 °C, so the temperatures of the two treatments are similar. The time spent at high temperature may be a little longer for the hydrothermal aging since the cooling rate was slower. As detailed below, other differences are observable between the 800 °C catalytically aged and the hydrothermally aged catalysts. Figure 3 shows the evolution of micropore volume as a function of catalyst treatment. Micropore volumes shown here were calculated from the Ar sorption data using the Horvâth-Kawazoe method (16). Pore volume corresponding to pore diameters less than 9 Â is considered zeolitic micropore volume in these measurements. Both the Horvâth-Kawazoe and the T-plot methods provide similar pore volume loss results. Since this is true, the higher precision Horvâth-Kawazoe method was used. Analysis of differential pore volume vs. pore diameter reveals the fresh material to possess a step function pore size distribution centered at 5.4 5.5 Â, which conforms well with known crystallographic properties of ZSM-5 (Π Ι 9). The slit-pore assumption inherent in the Horvâth-Kawazoe method does not result in incorrect estimates of the diameter of the cylindrical pores of ZSM-5. The pore size distribution after catalysis or aging is no longer a simple step function. Several maxima in the differential pore volume appear between 5 - 6 Â and another small local maximum occurs at about 8.0 Â. Interestingly, less micropore volume is lost after hydrothermal aging than after catalysis at comparable temperatures (Fig. 3). Both atmospheres contained the same amount of steam, and, if anything, the hydrothermal aging may have been a little more severe since the catalyst was exposed to high temperatures for a longer time. Although increasingly hot catalysis results both in uniformly decreasing micropore volumes and uniformly declining performance, aged performance doesn't correlate precisely with micropore volume. The hydrothermally aged catalyst is more severely deactivated but has lost a smaller fraction of its original micropore volume. Figure 4 reveals formation of CuO, as detected by X-ray diffraction, in the aged but not in the fresh catalyst. Two reflections of CuO, centered at 35.5° and 38.7° 2Θ, grow in as severity of catalyst treatment increases. Features at 35.75°, 36.05°, and 37.20° 2Θ appear to arise from (080); (800) and (035); and (-562), (-713), (354), (534) reflections of H-ZSM-5, respectively (19). Crystallite sizes of CuO cannot be extracted from these data since both observed reflections have contributions from two crystallographic planes: (002) and (-111) for 35.5° 2Θ and (111) and (200) for 38.7° 2Θ, but the CuO crystallites must be at least 4 nm to be seen by XRD. The increasing magnitude of these two reflections relative to the zeolitic reflections suggests that as the severity of aging increases a larger fraction of total copper occurs as crystalline CuO. Unlike what was observed in Cu-ZSM-57387, no crystalline Cu 0 is observed (15). Figure 5 shows the formation of a halo due to amorphous material as the severity of catalytic treatment increases. A broad halo, centered at about 23° 2Θ but observable from 11° to 39.9° 2Θ, is essentially absent in the fresh catalyst but is pronounced in materials aged catalytically at 800 °C or hydrothermally at about 2



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Figure 3. Micropore volume, determined by Ar sorption, as a function of deactivation severity: micropore volume decreases after catalytic or hydrothermal treatments do not correlate precisely with catalytic deactivation. *

CuO

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λ /ν 831°C ΗΤΑ Λ

/

/\ 800°C catalysis

Λ Λ 750°C catalysis \ A 700°C catalysis Λ 600°C catalysis Λ Fresh

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37 38 39 40 2Θ Figure 4. X-ray diffraction patterns of the various Cu-ZSM-5-207 samples reveal sintering of copper species to CuO correlates with deactivation.



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831 °C. The halo is observed due to the presence of crystallographically amorphous silica-alumina. Copper does not contribute to the halo. Initially, it is not detectable crystallographically and appears highly dispersed. The copper crystallizes to CuO after severe treatments. Zeolite crystallinity degrades somewhat with increasingly severe treatments. Of six reflections attributable to the zeolite examined in detail, three show substantial decreases in relative intensity as aging severity increases, two show moderate intensity declines, while one appears not to be affected. These data are gathered in Table I. Maximum intensities of the various reflections are tabulated relative to the intensities (defined as 100) of the corresponding fresh material. Relative zeolite crystallinity of a series of similar samples is generally measured by scaling the maximum intensity of a strong reflection to the maximum intensity of an arbitrarily chosen reference (20). We choose the fresh material as the reference. One could measure relative crystallinity using integrated intensities rather than maximum intensities; both methods provided equivalent trends for Cu-ZSM-5-387 (15). Since CuO is absent in the fresh material, the background intensities at angles where CuO reflections grow in are defined as 100. With the exception of data for the material treated catalytically at 800 °C, CuO relative intensities increase monotonically with severity of the aging. Intensities of the material that had been exposed to 800 °C catalysis appear systematically low for unknown reasons. For example, the zeolite reflection at 29.90° 2Θ is about constant for this series except for the 800 °C sample. If the 29.90° 2Θ reflection for this sample is scaled to 100, the CuO intensity at 35.55°2Θ is 143 and at 38.65°2θ is 146, which is similar to the 750 °C sample.

Table I. Relative intensities of selected reflections of ZSM-5 and CuO ZSM-5

Fresh 600°C 700°C 750°C 800°C 831°C A H T A

23.05° 100 108 97 93 72 88

23.90° 100 110 101 100 77 93

24.35° 100 96 87 87 75 85

29.90° 100 111 107 107 84 100

CuO 36.05° 100 101 96 97 87 94

37.20° 100 100 95 94 88 95

35.55° 100 123 138 143 120 151 e

38.65° 100" 108 137 145 123 165

•Background intensity defined as 100

If one scales the intensity of the major ZSM-5 reflections (23.05° through 24.35° 2Θ) to the 29.90° 2Θ zeolite reflection, the loss of crystallinity of ZSM-5 after catalysis at 800 °C appears comparable to the material that underwent hydrothermal aging. Zeolite crystallinity, as assessed by maximum diffraction intensities of several reflections, degrades modestly. However, the amount of amorphous material, as indicated by the intensity of the amorphous halo, increases with aging severity. An impervious shell of amorphous material forms on the exterior of crystals


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of TPZ-3 after steaming at 676 °C (21). Although the abstract indicates the surface area of the steamed TPZ-3 (as determined by N BET measurements) was reduced to essentially that of the exterior of the crystallites (21), which is not what we observe for Cu-ZSM-5, we wanted to examine this possible mode of catalyst deactivation. Our efforts have concentrated on the fresh material, material exposed to catalytic conditions at 800 °C, and the hydrothermally aged material. The fresh zeolite appears crystalline right up to the edge of the crystal. "Postage stamp" perforations due to 1-dimensional pores along the 010 direction can be observed in Fig 6. Not all regions of the fresh catalyst exhibit such extremely high crystallinity out to the exterior surface. The lattice fringes of the fresh material shown in Fig. 7 appear to stop about 7 Â from the exterior surface. This smearing out of the fringes may be an artifact due to imaging through a portion of material insufficiently thick to have the periodicity required to produce lattice fringes. No evidence of sintered copper-containing materials is evident in the fresh material. The copper (presumably cupric) species appear to occur as small, highly dispersed species, unresolvable at the resolution (3 Â) of the microscope used. The catalytically "aged" material shows evidence of an amorphous film at the surface, sintering of copper to relatively large, dark (dense) objects (which XRD indicates should be CuO), and a pitted, "karst" topography suggestive of voids. (A referee suggests that the amorphous film might be carbon since the catalyst is expected to be black after reaction. Our catalysts, which are tested in the presence of steam, are not black but rather blue-gray and contain only 200-300 ppm C. Through discussion with other researchers in the area, we find that catalysts tested in the absence of steam are, indeed, black. Since both the catalytically used and hydrothermally aged catalysts show the film, film formation is not uniquely due to catalysis. The fresh materials were subjected to the same treatment in the microscope, but do not reveal films. The films in the aged, deactivated materials probably are not artifacts due to microscopy.) Fresnel contrast from the sample as one goes through focus is akin to that expected for voids. When they are apparent, fringes due to crystalline zeolite are mottled. This could be due to either amorphous material on the "top" and "bottom" of the materials or, more likely, due to beam damage. The used catalyst appears more susceptible to beam damage than the fresh material. Figure 8 exhibits all of these features. Numerous 30-40 Â copper particles appear in this image; three are pointed out. We have observed Cu particles as large as 400 Â in other photos. We have yet to observe any Cu particles on the exterior surface of a zeolite sample aged catalytically. The copper particles in catalytically aged materials tend to occur in regions that apparently contain voids. Two probable voids are pointed out. However, not all regions apparently containing voids are associated with copper particles. As copper sinters to CuO particles larger than the micropore diameter of ZSM-5 (about 5.5 Â) on the interior of Cu-ZSM-5 particles, local destruction of the initially crystalline zeolite should occur(22). However, when we hydrothermally aged H-ZSM-5 to dealuminate it and then made Cu-ZSM-5 in an otherwise normal fashion, agglomerates of CuO are clearly observable by TEM on the exterior surface of the zeolite. Catalytic properties of the pre-dealuminated Cu-ZSM-5 are consistent with CuO, as expected, and very little NO is reduced. We believe the sintered CuO tends to occur on the interior of the ZSM-5 particles that were aged catalytically. 2


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Figure 6. This TEM image of fresh Cu-ZSM-5-207 reveals "Postage stamp" perforations at the edge of the zeolite particle due to 010 channels showing the fresh catalyst to be crystalline out to exterior edges.



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Figure 7. This TEM image, together with others not shown, fail to reveal coppercontaining particles observable by TEM in the fresh catalyst while the zeolite appears crystalline to the exterior surface.

Figure 8. This TEM image of a material subjected to catalysis at 800 °C exhibits sintered copper, an amorphous film, and features that may be due to void regions inside the zeolite.



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The lattice fringes appear to stop about 15-40 Â short of the exterior surface of the zeolite in the catalytically aged sample. Apparently, exposure to catalytic gases can cause some amorphization of the zeolite. Figure 9 shows: (a) two dark particles attributable to CuO, one of which is pointed out; (b) the zeolite to display a pitted morphology, as pointed out; and (c) lattice fringes that run to within 8-25 Â of the exterior edge. Other photos reveal regions where lattice fringes appear to stop as far as 80 Â from the nearest visible exterior edge. The amorphized surface layer is thin. The hydrothermally aged material is similar to the material treated catalytically at 800°C. Amorphous films, typically 40 Â thick and dark particles attributable to CuO are both observed. Figure 10 shows lattice fringes that stop about 25-35 Â from exterior edges while Figure 11 shows lattice fringes that stop as far as 80 Â from exterior edges. Figure 10 also shows regions apparently devoid of lattice fringes (and are therefore probably amorphous) and dark spots attributable to CuO about 30-35 Â across. Discussion Catalysis at 600 - 800 °C causes subsequent loss of performance and changes of several catalyst features generally associated with deactivation. Specifically, losses of microporosity, sintering of cupric species to cupric oxide, formation of an halo in the X-ray diffraction pattern due to amorphous material and a thin film on zeolite exteriors, and a pitted "karst" topography of the zeolite, consistent with voids and often associated with putative CuO particles, are all observed. It is interesting to assess which phenomena correlate best with catalyst deactivation. The thin amorphous films observed by TEM are apparently porous since micropore volumes remain considerable. The films forming in Cu-ZSM-5 after hydrothermal treatments or catalytic treatments (that are also hydrothermal since steam contents are comparable) apparently do not form an "impervious shell" as occurred in TPZ-3 (21). Films of comparable thickness occur in materials deactivated to different degrees. It would be instructive to show that this film does not contribute substantially to deactivation. Experiments concerning this point continue. Catalyst deactivation does not correlate exactly with pore volume loss. Catalysts subjected to catalytic treatments do show monotonically decreasing activity and pore volumes as treatment severity increases. However, the hydrothermally aged catalyst is least active but does not have the lowest pore volume. Pore volume measurements provide insight regarding zeolitic degradation. The degree of zeolitic degradation following hydrothermal aging is not greater than less highly deactivated, catalytically used materials. Hydrothermal aging does appear to result in somewhat more sintering of cupric species to CuO. Bulk CuO is relatively inactive for selective reduction of NO. Catalyst deactivation correlates most strongly overall with the degree of sintering to CuO. Sintering of copper species to CuO appears to be the dominant pathway toward deactivation of Cu-ZSM-5 as a catalyst for selective NO reduction. Degradation of ZSM-5, an unusually stable zeolite, appears to have subsidiary



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Figure 9. This TEM image of a material subjected to catalysis at 800 °C reveals 30-50 Â films formed after 800 °C catalysis, copper-containing particles, and "karst" regions consistent with voids.

Figure lO.This TEM image of the hydrothermally aged material reveals films of similar thicknesses to those formed during high temperature catalysis.



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Figure ll.This TEM image of the hydrothermally aged material reveals copper particles and an amorphous (presumably silica-alumina) film.


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Deactivation of Cu-ZSM- 5

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importance. One can envision the sintering process to consist of two important elementary processes. Ligand substitution at cupric ions in the zeolitic environment results in cupric ion migration. Exchange of water ligands in aqueous cupric solutions occurs near the diffusion control limit, and is 10 orders of magnitude faster than, for example, substitution of water ligands around [Al] (23). It is plausible that migration of extraframework cupric species may be faster than migration of framework Al. Previous nmr work (15) suggests that dealumination of Zn-ZSM-5 is largely suppressed during hydrothermal aging at 800 °C. Formation of larger copper-containing clusters from smaller ones should be thermodynamically favored (24). Dehydration of growing cupric clusters should be favorable at high temperatures, in spite of the hydrothermal environment. Slowing this process is a daunting task. Downloaded by CORNELL UNIV on May 21, 2017 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch004

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Conclusion

Cu-ZSM-5 is confirmed to reduce NO under oxidizing conditions. The catalyst appears selective toward production of N since only small amounts of N 0 are formed. The catalyst is unstable and deactivates considerably after brief periods of catalysis at elevated temperatures. Not much zeolitic degradation occurs. Increasingly severe catalyst treatments result in increasing degrees of sintering of copper species to CuO. The sintering of the catalytically active components, rather than degradation of the metastable support, probably dominates deactivation. 2

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Acknowledgments

Transmission electron microscopy was performed at the Microbeam Analysis Laboratory within the Department of Geology at the University of New Mexico. We thank AlliedSignal Research & Technology colleagues Al van Til, Franz Reidinger, Anthony Eck, and Mark West for their contributions involving micropore analysis, X-ray diffraction analysis, catalyst preparation, and catalyst testing, respectively. References

1. For a review with leading references, see Iwamoto, M.; Hamada, H. Catal. Today. 1991, 10, 57-71. 2. Teraoka, Y.; Ogawa, H.; Furukawa, H.; Kagawa, S. Kagawa, Catal. Lett. 1992, 12,361-6. 3. Inui, T.; Iwamoto, S.; Kojo, S.; Yoshida, T. Catal. Lett. 1992, 13, 87-94. 4. Inui, T.; Kojo, S.; Shibata, M.; Yoshida, T.; Iwamoto, S. In Zeolite Chemistry and Catalysis, Jacobs, P.A.; Jaeger, N.I.; Kubelková, L.; Wichterlová, B., Eds.; Studies in Surface Science and Catalysis, Elsevier: Amsterdam, 1991, Vol. 69, 355364. 5. Iwamoto, M.; Mizuno, N.; Yahiro, H. Seikiyu Gakkaishi. 1991, 34, 375-90. 6. Muraki, H.; Kondo, S. Japanese Kokai Patent 127 044, 1989. 7. Ishibashi, K.; Matsumoto, N. Japanese Kokai Patent 310 742, 1989. 8. Kawaki, C.; Muraki, H.; Kondo, S.; Ishitani, A. Japanese Kokai Patent 265 249, 1990. Armor; Environmental Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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9. Takeshima, S. U.S. Patent 5 017 538, 1991. 10. Yoshihide, W.; Mareo, K.; Shinichi, M. European Patent Application 90108236.2, 1990. 11. Held, W.; König, Α.; Richter, T.; Puppe, L. Society of Automotive Engineers paper 900496, 1990. 12. Montrueil, C.N.;Shelef, M. Appl Catal B. Environmental. 1992, 1, L1-L8 13. Bennett, C.J.; Bennett, P.S.; Golunski, S.E.; Hayes, J.W.; Walker, A.P. Appl. Catal. A. General. 1992, 86, L1-L6. 14. K.C.C. Kharas, Appl. Catal. B. Environmental. 1993, 2, 207-24. 15. K.C.C. Kharas, D.J. Liu, H.J. Robota, Appl. Catal. B. Environmental. 1993, 2, 225-37. 16. Horváth, G.; Kawazoe, K. J. Chem. Eng. Japan. 1983, 16, 470-5. 17. Wu, EX.; Lawton, S.L.; Olson, D.H.; Rohrman, A.C. Jr.; Kokotailo, G.T. J. Phys. Chem. 1979, 83, 2777-81. 18. Olson, D.H.; Kokotailo, G.T.; Lawton, S.L.; Meier, W.M. J. Phys. Chem. 1981, 85, 2238-43. 19. van Konigsveld, H.; Jansen, J.C.; van Bekkum, H. Zeolites. 1990, 10, 235-42. 20. van Hooff, J.H.C.; Roelofsen, J.W. In Introduction to zeolite science and practice, van Bekkum, H.; Flanigen, E.M.; Hansen, J.C., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1991, Vol. 58, 241-283, in particular 246. 21. von Ballmoos, R.; Koermer, G.S.; Sung, S.; Allen, F.M. "Deactivation of Catalyst TPZ-3 by formation of impervious shell", Twelfth North American Meeting of the Catalysis Society, Talk D21, Lexington, Kentucky, May 5-9, 1991. 22. Schulz-Ekloff, G.; Jaeger, N. Catal. Today. 1988, 3, 459-66. 23. Purcell, K.F.; Kotz, J.C. Inorganic Chemistry; W.B. Saunders: Philadelphia, PA, 1977; pg. 717-718. 24. Kelvin, W.T. Phil. Mag. 1871, 42, 448-452. RECEIVED

September 29, 1993


Environmental Catalysis - American Chemical Society

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