Ind. Eng. Chem. Res. 1993,32, 2500-2505
2500
Surface Composition of Iron Oxide Catalysts Used for Styrene Production: An Auger Electron Spectroscopy/Scanning Electron Microscopy Study Jorgen Lundin,t Leif Holmlid,’*+P. Govind Menon,$and Lars Nyborgs Reaction Dynamics Group, Department of Physical Chemistry, University of Gbteborg and Chalmers University of Technology, S-412 96 Gbteborg, Sweden, Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Gbteborg, Sweden, and Department of Engineering Metals, Chalmers University of Technology, S-412 96 Gbteborg, Sweden
The surface composition of two different types of potassium promoted iron oxide catalysts used for styrene production was studied with Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). Catalyst A was studied after use in ultrahigh vacuum mass spectrometry (MS)desorption experiments a t temperatures close to ordinary process temperatures of 860-890 K. The loss of surface reactivity observed in the MS experiments seems to be connected to formation of KOH rather than to extensive loss of K from the catalyst. Catalyst B had been used in the industrial styrene process prior t o the AES studies. The normal migration processes are confirmed. We also observe a low oxygen content, which indicates formation of K not bound as an oxide. Loss of oxygen should irreversibly decrease the amount of the active compound KFe02, and may thus be an important early step in the deactivation of the catalyst. 1. Introduction
2. Experimental Section
The alkali-metal-promoted iron oxide catalyst for the ethylbenzene-to-styrene process has been the subject of intensive study in recent years (Lee, 1973; Mross, 1983; Connell and Dumesic, 1985). In particular, the work of Hirano (1986a-d),Matsuietal. (l989,1991),andthegroups of Ertl (Muhler et al., 1989,1990,1992) and Geus (Stobbe, 1990; Stobbe et al., 1991) has revealed the details of the solid-state transformations of this catalyst during its precursor state, formation, activation, and deactivation under process conditions, and also identified the catalytically active surface phase and some of its properties and transformations. Our recent studies (Lundin et al., 1990; Engvall and Holmlid, 1992; Engvall et al., 1991; Holmlid et al., 1993; b a n and Holmlid, 1992, 1993) on this catalyst system were focused mainly on the loss of potassium from this catalyst at typical process temperatures and on the excited states of the alkali-metal species on emission. This has enabled us to derive the energetics of the various processes in the catalyst involving alkalimetal atoms on the basis of the model of the solid-state transformations proposed by Ertl’s group (Muhler et al., 1990). In this paper, we present the results of a surface analytical probe of both fresh and used styrene catalysts, employing a combination of Auger electron spectroscopy (AES) and scanning electron microscopy (SEMI. Some of the analyses were done on samples investigated in our earlier work (Lundin et ai., 1990; Engvall et al., 1991). Hence, the results obtained here also provide complementary and supporting evidence to the previous conclusions, besides giving some new insight into the changes which the catalyst undergoes during styrene process conditions.
The catalyst samples were of two commercial types called A and B (the same notation as used by Lundin et al. (1990),Engvallet al. (19911, and Holmlid et al. (1993)), used for the conversion of ethylbenzene to styrene at process temperatures of 590-620 “C (863-893 K). They were obtained in the form of cylindrical extrudates, with a diameter of 3 mm and a length of 8-12 mm. Catalyst A was examined in three forms: (a) fresh sample, curved surface of the extrudate, with no pretreatment except the vacuum in the AES apparatus, (b) fresh fractured sample, AES analysis of the fracture surface again with no pretreatment, and (c) a sample which was pretreated before the Auger spectroscopy studies in an ultrahigh vacuum (UHV) apparatus, by heating to temperatures up to 900 K for a few hours in the course of a mass spectrometric (MS) study (Lundin et al., 1990; Engvall et al., 1991). Prior to the MS experiments, the sample in (c) was cut to present a flat exposed surface for the MS study (here called the flat surface), and mounted in a Ta foil holder, with the flat surface exposed. During the MS experiments, the potassium and probably also the oxygen content of the catalyst sample were decreased by desorption and emission from it in the experiments. We estimate that approximately 50 % of the easily desorbable potassium content had been lost from the sample prior to the AES study. In the experiments several areas on the flat surface (exposed to the MS in the UHV desorption study), on the covered part, and on a cleaved surface inside the sample were analyzed. See Figure 1. On the flat surface, a few different locations were identified with the help of electron microscopy, as the locations indicated “block”, “plane”, and ”rough“ in Table I. The block had a size of 5 X 5 pm2, while the analysis region was 120 X 120 pm2. The B catalyst had been used in a commercial styrene reactor for 9 months before it was taken out. It was not pretreated but was studied as obtained. In one case, the sample was fractured in air, and the resulting surface was analyzed as well. The AESISEM study was performed by using a PHI (Perkin-Elmer) 660 scanning Auger microprobe. The
* To whom correspondence should be addressed. FAX: +4631-772 3107. t Department of Physical Chemistry. t Department of Engineering Chemistry. Department of Engineering Metals. 0888-588519312632-2500$04.00/0
0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32. No. 11, 1993 2601 between the mole fraction ci of an element i and the measured peak-to-peak intensity 1, is
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Composition-depth profiles were obtained by successive analvsis and ion etchine. The ion etchine was done with a diiferentially pumpei ion gun using Ggon at 5 5 O ion incidence. The ion beam voltage applied was 2 kV, and the ion beam was rastered 1 X 1 mm2,which produced an ion current density of 30 mA cm6 and an etch rate of 8.5 nm min-’ on Ta20S standard samples. 3. Results
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Figure I. Sample areas studied in the AES measurements.
electron beam voltage used was 3 kV. In order to avoid electrical charging of the oxide sample, the beam current was limited to approximately 20 nA. This low electron current also minimizes electron induced changes in the composition of the surface layer. Thesurface composition was determined from the peak-to-peak intensities in the differentiated spectra by means of elemental sensitivity factors Si given by Davis et al. (1976). The relationship
The AESspectraareevaluatedwithstandard sensitivity factors for 3-kV electron excitation. The analysis results for K, Fe, 0, and C are collected in Tables I and I1 as atomic percentages. The accuracy of the results obtained in this way is influenced by the validity of the tabulated sensitivity factors, and the relative error expected is of the order of *20%. Since elements in too small concentrations might have escaped analysis, the true fractions may thus be slightly lower than the values given in the table. It should also be observed that the AES signals are mainly derived from a few atomic layers in the surface, probably from the 10 topmost layers in the case of metal oxides. Thus, a full monolayer coverage will be observed only as an atomic percentage of approximately 10%. Three AES spectra for the sample of catalyst A [(I) the survey over the active surface (analysis no. 4 in Table I), (2) from a ‘block” on the flat (cut) surface (analysis no. 7),and (3) from the cleavage surface (analysis no. 9)] are shown in Figures 3-5. Several other analyses were also
B
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mre 2. Views of the surface of catalyst B. scanning electmn rnicrosc!ope(SEM) pictures (panels A and B) and Auger electron e p e c t r m p y (AES)mapa of Fe and K (panels C and D).In panel A three analysis Iegions are indicated in ares 1 in Table 11. The large one (snalysia no. 4) is called average, the bottom one (no. 3) is plane, and the inner om2 (no. 2) is rough. A portion mainly within analysis region 3 is shown in panel 9. the same region used for the AES maps.
2502 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 Table I. AES Results for Catalyst A. analysis no. [K], atom % [Fe], atom % 101,atom % [Cl, atom % [K1/101 [Fel/[Ol [KI/[Fel Ox 1 specification 18 14 44 0.41 0.32 1.29 0.63 2 fresh sample 38 8.3 45 0.84 0.18 4.58 0.67 32 25 43 0.74 0.58 1.28 1.15 3 cleaved area 1 4 survey (Figure 3) 44 0 37 14 1.19 0.00 0.59 Plane 39 11 33 6 1.18 0.33 3.55 1.04 5 rough 55 0 36 1.53 0.00 0.76 6 block (Figure 4) 20 19 41 0.49 0.46 1.05 0.86 7 8 after ion etching 43 7 43 1.00 0.16 6.14 0.72 area 2 9 cleaved (Figure 5) 32 10 42 11 0.76 0.24 3.20 0.70 area 3 covered part 50 0 37 1.35 0.00 0.68 10 0 Analyses 2 and 3 are for a fresh sample, while the lower rows are for the sample wed in an UHV ma88 spectrometric desorption experiment. See Figure 1 for the locations of the analysis areas. Ox is defined in eq 2. The relative error in the atom percent values is &20%. Table 11. AES Results for Catalyst Ba analysis no. [Kl, atom % [Fe], atom % [O], atom % [Cl, atom % [K1/[01 [Fel/[Ol [KI/[Fel Ox 1 specification 6 35 57 0.11 0.61 0.17 0.87 area 1 2 rough 32 19 31 19 1.03 0.61 1.68 1.33 3 plane 27 23 33 17 0.82 0.70 1.17 1.34 4 average 18 24 29 28 0.62 0.83 0.75 1.41 area 2 5 surface 30 23 38 10 0.79 0.61 1.30 1.20 6 at 100 nm 10 32 46 10 0.22 0.70 0.31 1.04 area 3 7 plane and rock 20 24 34 22 0.59 0.71 0.83 1.24 rough 26 29 33 12 0.79 0.88 0.90 1.57 cleaved area 4 9 light 62 5 19 15 3.26 0.26 12.40 1.98 10 dark 59 2 18 21 3.28 0.11 29.50 1.79 11 rough, dark 30 23 39 8 0.77 0.59 1.30 1.17 0 See Figure 1 for the positions of the analysis areas. Ox is defined in eq 2 . The relative error in the atom percent values is 120%. io
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performed as shown in Table I. One analysis was done after ion etching to a depth of approximately 100 nm, no. 8 in Table I. This was done to remove the surface layer directly exposed to air, to decrease the risk of erroneous results due to surface transformations in air before the analysis. The results found after ion etching do not differ
from the general behavior of the analyses, and it appears that the changes in surface layer composition in air are quite small. The analysis results from the used catalyst B are summarized in Table 11, in an analogous way to Table I. The values of the atomic percentages in Tables I and I1 are used to calculate the ratios [K1/[01 and [FeI/[Ol, which are given in the tables for the two catalysts. One further quantity is given in the tables, namely Ox = 0.5[K1/[01
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which is equal to unity if the catalyst only contains K2O and FeaOr. If other oxides also exist in the catalyst, this number will be less than unity. Oxygen loss will increase the number, while potassium loss will decrease it. This number is chosen to simplify the discussions, since the composition of both types of catalyst studied seems to agree quite closely with this composition.
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2603 4. Discussion 4.1. Compositionof the Surface Layer. The general consensus from recent studies by several research groups (Hirano, 1986a-d;Matsui et al., 1989,1991;Muhler et al., 1989, 1990, 1992;Stobbe et al., 1991) is that the main active phase in the styrene catalyst is potassium ferrite, KFe02. It is also stated that this ferrite is formed only during the correct process conditions, with an atmosphere of the process reactants. Hence, it is of interest to search for this compound on the surface of the catalyst samples. In the case of catalyst A the surface layer is in general poor in Fe. Iron is not even found in all analyzed locations, as seen in Table I. Only the plane and the block positions on the active surface show an iron content, and in these cases in quite large concentrations. The block has a composition KF~.9602.o6 which is close to that for the active compound KFe02. It thus seems possible that this active compound can be formed in small quantities on the sample surface also in the MS desorption experiments in an UHV environment, at typical process temperatures of 590-620 O C (860-890 K). The catalyst A contains more potassium than catalyst B, as seen from the specifications in Tables I and 11.This is also seen in the analyses of the flat surface on the sample, which was the surface exposed in the MS desorption experiments. The topography of the surface is complicated, with large variations in the composition. The surface layer has a larger [K]/[OI ratio than the catalyst bulk material and the fresh catalyst, while the Fe content is small. This shows that the surface layer is not built up of iron oxides or mixed oxides. Instead, potassium compounds like KzO, KOH, and possibly KFeO2 tend to get enriched in the surface layers, thus accounting for the excess of K at the surface. Catalyst B was analyzed on a larger number of surface locations than type A. These locations were all on the cylindrical surface of the catalyst extrudate, as shown in Figure 1. The SEM pictures in Figure 2 show that the surface contains complicatedstructures and irregularities. The ratios given in Table I1indicate that the surface layer consists of Fe304, excess K, probably in the form of interstitial K in the iron oxide, and probably also KOH. The AES maps in Figure 2, panels C and D, show that K and Fe dominate in different parts of the surface, which can be expected if the surface layer consists primarily of iron oxide and potassium species and not of well-defined compounds like KFeOz or K2Fe22034. The possibility that FeO is formed on the catalyst surface (in the process) cannot be ruled out on the basis of our results: in fact, the value of the "Oxn parameter in Table I1 should range between 1.0 and 1.3 if the composition was mainly K2O and FeO. The possibility that the surface layers analyzed here have been changed by reactions with various components in the air before the AES studies should also be discussed. The surface concentration of oxygen is actually lower than in the bulk for both samples, so there is no indication of any formation of KOH or potassium carbonates. However, if the topmost surface layer consists of the hydroxylgroups in KOH, the oxygen in this compound will contribute approximately 10 atom % to the oxygen value. Such a contribution may certainly exist, but only if the oxygen content in the lower atomic layers contributing to the AES signal is still lower than that derived assuming a homogeneous surface slab. One more factor is also of interest in this connection, and that is the graphitic carbon present on many surfaces of the catalyst samples, especially in the case of catalyst B. These points are further treated below.
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Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
2604
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and also in the etching profile in Figure 7. Potassium is enriched at the surface, while Fe and 0 increase with depth into the bulk. The bulk composition in this case is close to FeaOd, with just a little KOH and excess K left. This agreeswith the results of Muhler et al. (1990). The amount of K on the surface is large, however, and the deactivation of the catalyst should not be directly coupled to the loss of K or the formation of KOH. 4.3. Desorption Processes in the MS Study. The analyzed sample of type A was not fully exhausted of desorbable potassium in the MS experiments. Since the catalyst initially is rich in KzO, it is likely that potassium is released thermally from the K20, as concluded by Lundin et al. (1990). No potassium oxides were observed in desorption, but oxygen and C02 were detected from the surface. It is thus likely that the oxygen content decreased during the MS experiments, as well as the potassium content. In the study by Engvall et al. (1991), catalyst type A was compared to catalyst type B with lower potassium content, and the desorption processes were analyzed. It was concluded from that study that the compounds KFeO2 and KzFe220~do not play an important role in the desorption in the type A catalyst. The present AES study confirms this. Since the catalyst sample was in contact with (covered by) a Ta foil in the sample holder during the MS experiments, it is interesting to analyze the covered part of the sample. The analysis shows no sign of any Ta, but the sensitivity factor is quite low for Ta. The covered part of the sample is found to have large concentrations of CaO and K20. This means that the oxide in the sample was not reduced by the Ta metal to any appreciable extent. 4.4. Carbonate o r Carbon. A carbon peak is found in several analyses for the catalyst A sample and in all cases for catalyst B. Two forms of carbon are likely, namely, carbonate or graphite-type carbon on the surface. Carbonate could be formed when the sample is kept in air, from potassium oxide or hydroxide in the topmost atomic layer in the surface layer. Graphite type carbon may be formed by pyrolysis of included hydrocarbons during the high-temperature experiments in the MS studies for the catalyst A sample, and could definitely be formed in the case of catalyst B during its 9-month use in the commercial styrene plant reactor. The part of the sample of catalyst A which was covered by Ta foil in the MS experiments does not show any carbon peak, even if the surface composition appears to be mainly K2O and CaO. Thus, not all surfaces with K2O will react to form detectable amounts of carbonate. The cleaved catalyst A shows a high carbon concentration, probably from freshly formed carbonates in air. Also the active surface contains appreciable amounts of carbon, while the ion etched part and the block show no carbon at all. On the two analysis locations showing no Fe content, on the
covered part and on the rough surface (no. 61,no carbide could be formed, and the results also show no carbon. The potassium content is still high, but no carbonate has been formed since no carbon is detected. (The survey over the active surface is not homogeneous enough to warrant any conclusion in this respect.) We thus conclude that the carbon observed on the catalyst A sample is likely to be in a graphitic or carbidic state. Only in the case of the cleaved surface where a fresh surface was exposed to air should carbonate exist on the surface. It is likely that the graphitic carbon resides in the topmost layer. This layer will be quite unreactive to air, and it will also diminish the rate of attack on the K-containing layers beneath. In the case of the sample of catalyst B it is unlikely that appreciable amounts of carbonate are formed. If carbonate was formed, the oxygen content should increase above the one corresponding to Ox = 1, i.e., only K2O and FeaOr in the catalyst, and as noted before, the oxygen content is too low even when carbon is not included in any oxygen balance. Thus, the carbon in this case is mainly graphitic, or possibly in the form of a carbide. The conclusion that the carbon observed is not in a carbonate form in any of the catalyst samples is in agreement with the results of Muhler et al. (1992). Carbon is found in all analyses, and as noted above, its graphitic form will probably prevent the layers beneath from reacting with air. This is the likely reason why thick carbonate layers are not formed on the surfaces of the catalyst. 4.5. Deactivation Processes. The solid-state processes and the transformations leading to catalyst deactivation have been studied in several publications, including the recent comprehensive studies by Muhler et al. (1989,1990,1992). While the present results confirm the migration processes for K in the pellets (into the pellet) and within the material constituting the pellet (forced out to the surface from the bulk), the details in the catalyst deactivation processes still appear incompletely understood. There exist at least two possibilities for the deactivation, which is believed to correspond mainly to the disappearance of the active surface layer of KFe02: (1)loss of K from the surface and (2) loss of oxygen from the surface, both destroying the active surface layer. A discussion of some other possible reasons for the deactivation is given by Muhler et al. (19921, discarding the previous notions of blocking by carbonate or graphite layers. A major process in the final deactivation is the loss of K from the catalyst. This involves several loss processes, as neutral K atoms or as excited states K' and similar species, as observed in our previous studies (Lundin et al., 1990;Engvall and Holmlid, 1992;Engvall et al., 1991;Aman and Holmlid, 1992, 1993; Holmlid et al., 1993). In the present study which focuses on the remaining surface layer, there is no evidence that the loss processes of K are the most important ones for the deactivation of the catalyst of type B. For example, the [Fel/[Ol ratio is quite high for catalyst B which does not agree with loss of K from the material, leaving 0 behind. Loss of oxygen is however a likely deactivation process for catalyst B since the [Fe1/[0] ratio is high, maybe as oxygen molecules, in the form of metal hydroxyl or as water. This could be believed to be a major process also in the MS vacuum experiments with catalyst A, but it is not observed since the [Fel/[O] ratios still are low in this case. Even if hydroxyl species are likely to form on the catalyst surface, formation of KOH is not a major reaction for catalyst B in the process. This is not in complete agreement with Muhler et al. (1990,1992), who conclude
Ind. Eng.Chem. Res., Vol. 32, No. 11, 1993 2505
that the short-term deactivation is due to formation of OH bound to K on the surface, and also that the final form of K is KOH. It appears unlikely that KOH desorbs easily from the catalyst surface, and any KOH formed should thus remain in the catalyst, as also found for catalyst A. Since KOH will probably anyhow be formed by all K species leaving the catalyst surface by reaction with water in the reaction stream, KOH may still be found in large quantities in the reactor and on the samples even if most KOH is not formed on the catalyst surface. Thus, it appears likely that the deactivation for catalyst B is coupled to loss of oxygen in any form, presumably followed by loss of K. Loss of alkali-metal promoter is an important problem for the catalyst under process conditions. From the present study, however, one further driving force seems to exist, namely, loss of oxygen. This loss mechanism may even be an early step in the deactivation, leading also to loss of alkali-metal promoter. This is quite encouraging since it implies that it should be possible to find means to slow the rate of deactivationwithout at the same time decreasing the promoter activity. 5. Conclusions
Catalyst A was studied after use in UHV mass spectrometry-desorption experiments at the process temperature. A block observed on the catalyst surface has a composition close to KFe02. This indicates that this compound, which is the catalytically active compound according to earlier researchers, can form a separate phase on top of the catalyst surface in the vacuum experiments also. The catalyst bulk consists mainly of KOH and FesO4, which is believed to be the final form also for the catalyst in the industrial process. The surface is very rich in K in some locations, with little Fe in the surface layer, but the concentration of K does not increase toward the bulk in general. The loss of surface reactivity observed in the MS experiments with catalyst A seems to be connected to formation of KOH rather than to extensive loss of K from the catalyst. Catalyst B had been used in the industrial process prior to the AES studies. The normal migration processes for K toward the catalyst surface and to the interior of each catalyst pellet are confirmed. The surface layer is enriched in K. The bulk consists mainly of FeaO4 with some excess K and KOH. The oxygen content is low in catalyst B. This loss of oxygen should finally decrease the amount of the catalytically active compound KFe02 irreversibly. Thus, it may be an important early step in the deactivation of the catalyst, maybe preceding the loss of K. The carbon observed in both catalysts is mainly graphitic, which gives an unreactive surface in air at room temperature. Carbonates are probably only formed to an appreciable extent when the catalyst samples are cleaved in air. Acknowledgment We thank Klas Engvall for taking part in the catalyst preparation experiments.
h a n , C.; Holmlid, L. Field ionization of Rydberg alkali statesoutside iron oxide catalyst surfaces: peaked angular distributions of ions. Appl. Surf. Sci. 1993,64,71-80. Connell, G.; Dumesic, J. A. Migration of Potassium on Iron and Alumina Surfaces as Studied by Auger Electron Spectroscopy. J. Catal. 1985,92,17-24. Davis,L. E.;McDonald, N. C.;Palmberg,P. W.;Riack,G.E.; Weber, R. E. Handbook of Auger Electron Spectroscopy; Physical Electronics Industries Inc.: Eden Prairie, MN, 1976. Engvall, K.; Holmlid, L. Field ionization of excited alkali atoms emitted from catalyst surfaces, Appl. Surf.Sci. 1992,55,303-308. Engvall, K.; Holmlid, L.; Menon, P. G. Comparative loss of alkali promotor by desorption from twocatalystsfor the dehydrogenation of ethylbenzene to styrene. Appl. Catal. 1991, 77,235-241. Hirano, T. Dehydrogenation of ethylbenzene on potassium-promoted iron-oxide catalyst containing magnesium-oxide.Bull. Chem. SOC. Jpn. 1986a,59,2672-2674. Hirano, T. Roles of potassium in potassium-promoted iron oxide catalyst for dehydrogenation of ethylbenzene. Appl. Catal. 1986b, 26,65-79. Hirano, T. Active phase in potassium-promoted iron oxide catalyst for dehydrogenation of ethylbenzene. Appl. Catal. 1986c,26,8190. Hirano, T. Dehydrogenation of ethylbenzene over potassiumpromoted iron-oxide containing cerium and molybdenum oxides. Appl. Catal. 1986d,28, 119-132. Holmlid, L.;Engvall, K.; b a n , C.; Menon, P. G. A new approach to loss of alkali promoter from industrial catalysts: importance of excited states of alkali. In New Frontiers in Catalysis, Proceedingsof the 1OthInternational Congresson Catalysis;Guczi, L., Solymosi, F., TBthyi, P., Eds.; Akadhmiai Kiad6: Budapest, 1993;pp 795-807. Lee, E. H. Iron oxide catalysts for dehydrogenation of ethylbenzene in the presence of steam. Catal. Rev. 1973,8,285-305. Lundin, J.; Engvall, K.; Holmlid, L.; Menon, P. G. Mechanism of potassium loss by desorption from an iron oxide catalyst for the styrene process. Catal. Lett. 1990,6,85-94. Mross, W.-D. Alkali Doping in Heterogenous Catalysis. Catal. Rev.Sci. Eng. 1983,25,591-637. Matsui, J.; Sodesawa, T.; Nozaki, F. Activity decay of potassiumpromoted iron-oxidecatalyst for dehydrogenation of ethylbenzene. Appl. Catal. 1989,51,203-211. Matsui, J.; Sodesawa, T.; Nozaki, F. Influence of carbon dioxide addition upon decay of activity of a potassium-promoted iron oxide catalyst for dehydrogenation of ethylbenzene. Appl. Catal. 1991, 67,179-188. Muhler, M.; Schlbgl, R.; Reller, A.; Ertl, G. The Nature of the Active Phase of the Fe/K-catalyst for Dehydrogenation of Ethylbenzene. Catal. Lett. 1989,2,201-210. Muhler, M.; Schutze, J.; Wesemann, M.; Rayment, T.; Dent, A.; Schlbgl,R.; Ertl, G. The Nature of the Iron Oxide-Based Catalyst for Dehydrogenation of Ethylbenzene to Styrene. 1. Solid-state Chemistry and Bulk Characterization. J. Catal. 1990,126,339360. Muhler, M.; Schlbgl, R.; Ertl, G. The Nature of the Iron Oxide-Based Catalyst for Dehydrogenation of Ethylbenzene to Styrene. 2. Surface Chemistry of the Active Phase. J . Catal. 1992,138,413444. Stobbe, D. E.On the Development of Supported Dehydrogenation Catalysts Based on Iron Oxide. Ph.D. Dissertation, University of Utrecht, The Netherlands, 1990. Stobbe, D. E.; van Buren, F. R.; Stobbe-Kreemers, A. W.; Schokker, J. J.; van Dillen, A. J.; Geus, J. W. Iron oxide dehydrogenation catalysts supported on magnesium-oxide. 1. Preparation and characterization, 2. Reduction behavior, 3. But-1-ene dehydrogenation activity. J. Chem. SOC.,Faraday Trans. 1991,87,16231629,1631-1637,1639-1647. Received for review February 3, 1993 Revised manuscript received June 14, 1993 Accepted July 2, 19936
Literature Cited Aman, C.; Holmlid, L.Desorption and emission of potassium Rydberg atoms and clusters from iron oxide catalyst surfaces. Appl. Surf. Sci. 1992, 62,201-208.
Abstract published in Advance ACS Abstracts, October 1, 1993. @
