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N , = saturation concentration of poison, molecules/cm2 P = Pressure, torr Q = volumetric gas flow rate, cm3/s 7 = volume-averaged pore radius, cm or A R = pellet radius, cm s = surface area, cm2/g or m2/g t = time, s T = temperature, K V = pore volume, cm3/g V , = reactor volume, cm3 a = constant in eq 22 e = porosity 6 = fractional poison penetration depth into a catalyst pellet pa = solid density, g/cm3 pp = pellet density, g/cm3
Bird, R. B.; Stewart, W. E.; Llghtfoot, E. N. “Transpat phenomena”, Wlley: New York. 1960. BOX, M. J. Comput. J . 1965, 8 , 42. Carberry, J. J.; Gorring, R. L. J. Catal. 1966, 5, 529. Hegedus, L. L.; Summers, J. C., 4th North American Meeting of The Catalysis Society, Toronto, Ont., Canada, Feb 18, 1975. Hegedus, L. L.; Summers, J. C. J. Catal. 1977a, 48, 345. Hegedus, L. L.; Summers, J. C. US. Patent 4051 073, Sept 27. 1977b. Hegedus, L. L.; Baron, K. J. Catal. 1978, 54, 115. Hegedus, L. L.; Summers, J. C. U.S. Patent 4 119571, Oct 10, 1978a. Hegedus, L. L.; Summers, J. C. U.S. Patent 4 128 506, Dec 5, 1978b. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J. Catal. 1979, 56, 321. Kuester, J. L.; Mize, J. H. "Optimization Techniques wtth Fortran”, McGrewHiii: New York, 1973. Oh, S. H.; Baron, K.; Cavendish, J. C.; Hegedus, L. L. ACS Symp. Ser. 1978, 65, 461. Smith, J. M. “Chemical Engineering Kinetics”, M&aw-HIiI: New York, 1970. Summers, J. C.; Hegedus, L. L. J. Catal. 1978, 51, 185. Summers, J. C.; Hegedus, L. L. U.S. Patent 4153579, May 8 , 1979. Zemke, B. E.; Gumbieton, J. J. Society of Automotive Engineers, Paper No. 800398, Detroit, Mich., Feb 28, 1980.
Literature Cited Adomaitis, J. R.; Smith, J. E.; Achey, D. E., Society of Automotive Engineers, Paper No. 800084, Detroit, Mich., Feb 25, 1980. Bedford, R. E.; Berg, M. US. Patent 4051 072, Sept 27, 1977.
Received for review July 31, 1980 Accepted August 26, 1980
Chloriding of Pt-AI2O3 Catalysts. Studies by Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy Francls Delannay, Camille Defosse,’ and Bernard Delmon Groupe de Physico-Chlmle Minerale et de Catalyse, Universit6 Catholique de Louvaln, B- 1348 Louvain-la-Neuve, Belglum
P. Govlnd Menon and Gllbert F. Froment Laboratorlum voor Petrochemische Techniek, Rijksuniversiteit Gent, 8-9000 Gent, Belgium
The changes occurring to R-Al2O3 catalysts on chloriding them with CCI4 in H2 or dry HCI gas were studied by X-ray photoelectron spectroscopy (XPS or ESCA) and transmission electron microscopy (TEM). The TEM results show that R crystallites grow in size and the clusters of the crystallites are dispersed on chloriding with CCI4, but not with HCI. The XPS data support the observation on the crystallie size from TEM. They also indicate that coke laydown during the CCI4 treatment occurs preferentiallyon the surface of Pt crystallites. These results account for the decrease in H2 chemisorption capacity and the strong attenuation of the hydrogenolysis activity of the CC14-chloridedcatalyst. The coke content and CI content of the catalyst calculated from XPS data are of the same order as the values determined independently from direct combustion of the coke and electron-probe microanalysis of CI, respectively.
Introduction In bifunctional Pt-A1203-typecatalysts used for catalytic reforming of naphtha to produce higher-octane-number gasoline or aromatics for the petrochemical industry, a careful balance has to be maintained between the hydrogenation-dehydrogenation function of Pt and the acid function of A1203. If the metal function is too strong, excessive hydrogenolysis to C1-CI gases and dehydrogenation to polyolefinic coke precursors can occur; if it is too weak, then the catalyst also gets deactivated very soon due to excessive coking. If the acid function is too strong, it leads to excessive hydrocracking, coke lay-down on the catalyst, and consequent catalyst deactivation; if it is too weak, the rate-determing carbonium-ion reactions involved in dehydroisomerization and dehydrocyclization do not proceed fast enough, which in turn leads to an increase in
C1-CI gas production and a decrease in the yield of liquid reformate. Moisture in the naphtha feed and chlorine in the catalyst are the two main factors which control the acidity of the catalyst under actual reforming conditions. While water enhances the Bronsted acidity of A1203 at the reforming temperature of 490-520 OC, above 20 ppm levels it also strips off the chlorine from the catalyst, thereby lowering its acidity at the same time; the net result in such cases is sometimes the acute corrosion in downstream equipment due to the wet HC1 gas at high temperatures. More often, however, the chlorine stripped off from the catalyst, particularly during catalyst regeneration, has to be replenished by addition of an organic halide such as CCL, either in one lot at the start of the cycle after the regeneration or continuously at ppm levels in the feed. In the patent literature (for a review, see Birke et al., 1979) such additons of chloride to the catalyst have been claimed to enhance the stability of Pt crystallites on the alumina surface and
’ChargB de Recherches FNRS (Belgium). 0198-4321/80/1219-0537$01 .OO/O
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Table I. Characteristics of the Catalysts Used
BET surface area, m*/g Pt dispersion, % c1, wt % s, wt %
0.6%Pt-
2% Pt-
r-Al,O, 179 78
r-Al,O, 180 45
0.67 0.03
prevent their sintering, and even to cause a re-dispersion of Pt crystallites (rejuvenation). In the case of continuous chloriding via the naphtha feed, practiced in many reformer plants, the chloriding occurs in an atmosphere of Hz.In such cases, chloriding with CC4leads to coke deposition on the catalyst, this coke being primarily responsible for suppressing the high hydrogenolysis activity of a freshly reduced catalyst; chloriding the catalyst with dry HC1 gas (hence without any coke lay-down) affects the hydrogenolysis activity only marginally (Menon et al., 1979). Is this coke deposited on the Pt crystallites, or on A1203, or on both? The answer to this question could be of direct interest for stabilizing the activity and selectivity of reforming catalysts in industrial operations. Hence a comparative study of freshly reduced and chlorided catalysts was undertaken using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS or ESCA). Results of these studies are compared with the earlier data (Menon et al., 1979) for catalytic activity for reactions of n-pentane, chemisorption of Hz on the catalyst samples, and temperature programmed desorption of Hz from them.
Experimental Section (a) Catalyst Samples. Two catalysts were used in the present study: a commerical 0.6% Pt-A1203 reforming catalyst (CK 306 of Cyanamid-Ketjen, Amsterdam, The Netherlands) and a 2% Pt-Al2O3 catalyst prepared by impregnation of chloroplatinic acid on alumina extrudates (CK 300 of Ketjen, also used for preparing the commercial CK 306 catalyst). Some of the characteristics of these catalysts are given in Table I. The catalysts were crushed to 0.1-0.4 mm size, calcined at 500 "C in a stream of dry air for 1 h, and reduced at 400 "C in a stream of pure Hz for 3 h. The samples were chlorided by injecting slowly 20-100 pL/g of CC14 or 20-100 mL/g of dry HCl gas (Matheson) to the catalyst at 400 "C in a stream of HP, flushing with H2 for 10 min, at the same temperature, and then cooling to 20 "C. (b) Reactivity Studies. Details of the experimental setup used for pulse reactivity studies, H2 chemisorption measurements on the catalysts, and temperature programmed desorption of H2from them have been published elsewhere (Menon et al., 1979). (c) Coke on Catalyst. The 0.6% Pt and 2% Pt-A1203 catalyst samples (0.5 g) were taken in a small reactor and 5,10, and 30 pL of CC14were injected on to them at 400 OC in a stream of H2 at 1bar pressure. The coke deposited on the catalyst from CC14was burned off with air at 500 "C, and the C02formed from it was collected in a cold trap ( < E O "C) and subsequently expanded on to a gas chromatograph with a carbosphere column at 100 "C. From the C02 thus determined, the amount of coke on the catalyst was calculated, assuming it to consist of carbon only. (d) Transmission Electron Microscopy. The catalyst samples were finely ground, dispersed in ethanol with an ultrasonic vibrator, and deposited on a thin carbon film supported on a standard copper grid. Holey carbon films (microgrids) were used in some cases to improve the quality of the image. The TEM study was carried out on
a JEM lOOCX TEMSCAN instrument, equipped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPMA). The ultimate point-to-point resolving power of the microscope is 0.7 nm. TEM is a fairly well-established technique for the measurement of the size distribution of metallic particles on supports of various types. The minimum size above which such a distribution can be reliably measured is a function of the achievable contrast between the supported particle and the carrier. The contrast itself depends on the nature of the metal and of the carrier. A detailed investigation of this effect by Flynn et al. (1974) has indicated that, for Pt-on-A1203catalysts, particle-size distributions may become increasingly subject to error as the fraction of particles with sizes below 2.5 nm increases. Still, particle-size distributions below this limit continue to be reported in the literature quite often (e.g., Prestridge et al., 1977). (e) XPS. The samples for XPS were ground and slightly pressed in small troughs of 8 mm diameter X 1mm depth. The XPS measurements were taken at room temperature on a Vacuum Generators ESCA 3, equipped with a signal averager (Tracor Northern NS 560) for signalto-noise ratio improvement. The pressure in the analyzer chamber was lo* torr and the aluminum anode was powered at 14 kV and 20 mA. The sweep width was 17.6 eV for the Cls, Clzp3/z;l/z (noted ClZp)and Alzslines, and 91.6 eV for the Pta5p.3/2 doublet (noted Pta). The more intense Pt4f1/25/2doublet could not be used because of its complete masking by the A12p32;1/2 line. For each run, the sampling sequence was Cls, Cl,,, P t 4 d , and then again Alzsand Cis, allowing the peak of interest, P t 4 d , to be bracketed by the AlzS intensity reference line and the carbon contamination buildup to be monitored. In the present study no increase of superficial contamination was observed. The intensity is defined as the area under the peak after subtracting the background by linear interpolation. The P t 4 d region was accumulated overnight to obtain a signal-to-noise ratio of 101; for the C1, and Ala lines, this ratio was 101 and 60:1, respectively, or better. Owing to its extreme weakness, the Pta signal was practically impossible to monitor on the 0.6% Pt-Al2O3 catalyst; therefore the XPS studies were done on the 2% Pt-A1203 catalyst only. Two different samples (1and 2) were prepared by separate but identical treatments of the original batch of unreduced catalyst. Sample 1was run twice using different aliquots and in some cases the ClPP and/or P t 4 d levels were run twice on the same sample without re-exposure to air. The XPS intensity measurements can be used for the purpose of monitoring the dispersion of a metal on a supported catalyst. For a given surface area, the ratio of the XPS peak intensity of the metal to that of the support is directly related to the size of the metal crystallites (Sharpen, 1974; Kerkhof and Moulijn, 1979). For Pt-A1203 catalysts, this relation has been worked out on a theoretical basis by Fung (1979) quite recently. The XPS intensities will be reduced if the catalyst surface is covered by a carbon layer due to coke deposition. This reduction may be expressed by the relation Ic(X) = MX) exp[-t/X,(X)I (1) where Io(X) and Ic(X) are the peak intensities of line X for a clean and a carbon-covered surface, respectively, t is the thickness of the carbon layer, and Xc(X) is the electron mean free path in the carbonaceous layer. In the present case, X stands for Pta and Alzs. The mean free paths in carbon, Xc(Pta)and Ac(Alzs),may be estimated
hzs,
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3
Pt-M203and 2% Pt-A1203catalysts and the AZO3carrier (blank) on injecting varying amounts of CCI, on to them is shown in Figure 1. The lower coke deposition on the catalyst with higher Pt content is similar to the well-known behavior of Pt-Alz03catalysts under reforming conditions (cf. Mills et al., 1961). It is of interest now to examine whether the above results tally with the TEM and XPS
015
data.
C C k Injected t PI 19 catayst1
Figure 1. Coke deposited on the catalysts on injection of diffwent amounts of CCl. in a stream of H, a t 400 "C.
from Brundle's work (1975) to be 18 and 19 A, respectively, if it can be assumed that the carbonaceous layer is comparable to graphite.
Results (a) Pretreatments a n d Catalytic Activity. It has been shown in a previous study (Menon et al., 1979) that exposure of the CK 306 catalyst at 400 "C to comparable amounts of C1, charged either as CCl, in H2or as dry HCI gas, resulted in comparable surface loading of C1 on the catalyst (5-7 wt %), as seen from an electron-probe microanalysis of the catalyst samples. Chlorination with CCh in H, at 388 OC brought ahout a decrease of 18% in the chemisorption of H, a t 20 "C (used as a probe for the exposed Pt surface on the catalyst). The dosing of dry HC1 gas a t 20 "C caused a sharp decrease in the H, chemisorption value, but HCl gas at 400 "C hardly affected it. Similarly, the hydrogenolysis activity of the catalyst was considerably suppressed by CCl,, but only marginally by HC1 gas; in the former case, the formation of coke on the catalyst from CCl, could also be demonstrated. On regenerating the CC1,chlorided catalyst by air oxidation at 500 OC, followed by reduction at 400 "C, the hydrogenolysis activity of the catalyst as well as its Hz chemisorption capacity are fully restored. The coke formed on the 0.6%
(b) TEM. Some pretreatments of the catalyst do not seem to produce any significant change detectable by TEM imaging, e. g., chloriding of the reduced catalyst with HCI gas, or regeneration of the CCl,-chlorided sample. It was not possible to observe any coke on the catalyst after chloriding with CCL, presumably due to the very small amount of coke and the high transparency of carbon for electrons. Some of the major differences observed from TEM are shown in the electron micrographs of Figure 2 for the two reduced catalysts before and after chloriding with CCl, in H2 at 400 OC. The small-size Pt crystallites are fairly homogeneously dispersed on the 0.6% Pt-A1,03 catalyst, but they tend to cluster in the 2% Pt-Alz03sample. The dispersion of the crystallites is very conspicuously affected by the CC1, treatment: the mean crystallite size has increased for both samples and, for the 2% pt-A1203 catalyst, the clusters of Pt crystallites have been completely redispersed. The extent of these effects increases with increasing CC4 loading on the catalyst. On a heavily chlorided 0.6% Pt-A1203sample, Pt crystallites larger than even 10 nm have been observed. The mobility of the Pt crystallitee (in the cluster) and their tendency to grow when chlorided may be due to the volatility of Pt-Cl species being higher than that of reduced Pt (Adler and Keavney, 1960). The change of the Pt crystallite-size distribution on CCL-ehloridmgis shown in Figure 3 for the 2% Pt-Alz03 sample. Each of these distributions has been drawn from sampling of 150 particles. The mean size has a value of 2 nm for the reduced sample and 3.1 nm for the CChchlorided one. The relative standard deviation is 1.7 times larger for the chlorided sample than for the original reduced sample. This indicates that size increase and broadening of the distribution occur simultaneously.
Figure 2. Transmission electron micrographs of reduced 0.6% l'-A120sand 2% Pt-AI,O, catalysts (a and b) and after chloriding them with CCl, in H2 a t 400 "C (c and d, respectively).
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Table 11. XPS Intensity Ratios Observed after Pretreatments of the 2% Pt-AI,O, Catalyst intensity ratio Pt,d/A1as
pretreatment of catalyst
sample no.
1
reduced (H2 400 "C)
::
chlorided with CCl,/H,
chlorided with HCl/H, regenerated after chloriding with CC1, Average of two runs o n the same sample.
I
2 1 2 1 2
measd
normalized
0.094 0.107a 0.216 O.20aa 0.155 0.080
0.094 0.090 0.069 0.077 0.087 0.083
1.0 1.0 0.75 0.86 0.93 0.92
normalized av 1.0 0.80 0.93 0.92
Total number of runs was 3.
Reduced
c
g
C12d-412S
Table 111. Cl/AI Atomic Ratio of the Catalysts from EPMA and XPS Intensity Measurements Cl/Al atomic ratio from
2% Pt/A1,0, catalyst
EPMA
XFJS
reduced sample chlorided with CCl, chlorided with HCl regenerated
0.024 0.062 0.047 0.012
0.027 0.056 0.041 0.021
10
t
n
I CCI&-ChlorideC
10
C r y s t a l l i t e s i z e (nrn)
Figure 3. Change in crystallite size on chloriding 2% Pt-A1203 catalyst.
A rough estimate shows that the Pt crystallites which are observed on the 0.6% Pt-Al20, sample have a mean size lower than 2 nm, which is in agreement for the Pt dispersion value for this commercial reforming catalyst, determined by the H a chemisorption technique. In such a case, particle-size distribution charts are likely to be skewed since a significant part of the Pt crystallites may be too small to be detected by TEM. As a consequence of the increase of the particle sizes, the number of detectable Pt crystallites is noticeably larger after chloriding by CCl, of the 0.6% Pt-A1203 catalysts. (c) XPS. In the XPS results, no shifts were observed for the binding energies of any of the lines. The intensities of the various lines are given in Table I1 as ratios with respect to the Alzsintensity. The last two columns give the Pta intensity ratios normalized with respect to the original reduced sample with no additional treatment. Data in Table I1 for the Clapline show that there is a small but significant amount of chlorine left over on the catalyst from the hexachloroplatinic acid used in the Pt impregnation step and this chlorine is not stripped off by calcination at 500 O C and reduction at 400 "C. A similar observation has been reported by Escard et al. (1976). Chloriding the catalyst with CCl, results in a net increase of the C12p/A12sintensity ratio; this also occurs on chloriding with HC1 gas. Regeneration in air at 500 OC followed by reduction at 400 "C brings down the C1 content back to the level of the original catalyst. This is quite in agreement with the complete restoration of strong hydrogenolysis activity in the regenerated catalyst, reported earlier (Menon et al., 1979). The average values of the normalized Pta/Al, intensity ratio show that chloriding with CC14 induces a 20% decrease of the Pt signal intensity. chlorination with HCl gas reduces the Pt intensity by only 7%. Though the
above differences in the XPS data are small,they are still significant and observed repeatedly in three sets of measurements. (a) EPMA. The chlorine content of the various samples has been determined by EPMA using the energy dispersive spectrometer fitted onto the microscope. The results are presented in Table I11 as Cl/Al atomic ratios. Discussion It has to be emphasized at the outset that collecting the TEM and XPS results presented above required the operation of these two instruments almost at the limit of their capabilities: ultimate resolution for TEM imaging of very small crystallites, and ultimate sensitivity for quantitative XPS measurement of low Pt concentration on a large-area alumina-supported catalyst in which the main Pt signal is completely masked by the Al signal. Hence one should expect only a limited precision while trying to correlate the data obtained from these two techniques. Three phenomena may be considered in order to interpret the XPS intensity ratio measurements, namely the change of the crystallite size distribution and the deposit of chlorine and coke on the catalyst surface. It is well known that chlorine interacts both with the surfaces of alumina and platinum. One calculates that the 6.2% Cl/Al atomic ratio measured by EPMA on the 2% Pt-A1203 sample after CC14 treatment (Table 111) corresponds to slightly less than one monolayer coverage of chlorine on the catalyst. Such a monolayer will bring about no effect on the PtU/Al% intensity ratio if it is uniformly deposited on both alumina and platinum surfaces. It is then justified to attempt to interpret the XPS measurements by only considering the crystallite size distribution and coke deposit. A. Crystallite Size of Pt. The change of the Pt crystallite size distribution shown in Figure 3 should be related with the difference in the PtU/Ala intensity ratio observed between the original reduced sample and the sample regenerated after CCl, treatment (normalized averages 1.0 and 0.92, respectively; see Table 11). Assuming a hemispherical shape for the crystallites and using the data of Fung (1979),the theoretical PtU/Ala intensity ratio corresponding to the crystallite size distribution shown in Figure 3 can be calculated. Normalized ratios of 1.0 and 0.82, respectively, are then obtained. The decrease of the experimental X P S intensity ratios thus supports the TEM
Ind. Eng. Chern. Prod. Res. Dev., Vol. 19,
measurements. The agreement between the experimental values and the theoretical predictions may be considered as satisfactory, especially if it is borne in mind that these calculations do not take into account any possible effect on the XPS intensity ratio of the clustering of the Pt crystallites as seen in the reduced 2% Pt-A1203 catalyst (Figure 2). B. Coke Deposit, The 13% difference in XPS intensity ratio observed between the CC14-chloridedsample and the regenerated one (Table 11) should be attributed to the presence of coke on the catalyst. Two different situations could be imagined in such a case: (a) the coke is deposited on both the Pt-crystallite and alumina surfaces, or (b) the coke is preferentially deposited on the surface of the Pt crystallites only. Assuming a coke layer of uniform thickness and taking into account the mean free path of electrons with the energy of the P t M and Al,, lines in carbon, it can be calculated from eq 1that a coke layer of -5 nm would be necessary to cause a 13% difference on the basis of the first situation, while a coke layer of only -0.3 nm on the surface of the Pt crystallites suffices to bring about the same difference in the second situation. If the coke layer is assumed to have a uniform thickness of 5 nm on the alumina surface (BET surface area 180 m2/g), the volume of coke would be 0.9 cm3/g of fresh catalyst which is more than the porous volumepf the alumina support. TEM imaging would have obtained indications of such heavy coking if it has really occurred. In the second situation of coke from CC14 being uniformly deposited only on the Pt crystallite surface (2.3 m2/g of catalyst) to a thickness of 0.3 nm, the coke content of the catalyst will be 0.10 wt % if the density of coke is taken as 1.5 g/cm3, a value proposed quite recently by Beeckman (1979) and Beeckman and Froment (1980). If the density of coke is assumed to be that of graphite, 2.25 g/cm3 instead of 1.5 g/cm3 then the coke content of the catalyst will be 0.16%. The experimetnally determined values of coke on the two catalyst samples on chloriding with CC14 are given in Figure 1. The 2% Pt-A1203 catalyst treated of CC&/g of catalyst in H2 at 400 “C, prepared with 100 CLL under identical conditions as for the XPS study, had a coke content of 0.077 wt %, which is in reasonable agreement with the above value of 0.10 wt % from XPS. This suggests that the coke from CC14is deposited mainly, if not exclusively, on the surface of Pt crystallites in the catalyst. An additional proof for this is indeed the marked decrease in H2 chemisorption capacity and hydrogenolysis activity of the catalyst when chlorided with CC4,but only a very little change in these on treatment with dry HC1 gas at 400 “C, and the almost complete restoration of these two characteristics of the Pt in the catalyst once the coke on it is burned off in a regeneration step. C. Chlorine Content. For supports with surface area of the order of 200 m2/g, the atomic concentration ratio for two elements on the surface may be obtained as a first approximation from the XPS intensity ratio (Kerkhof and Moulijn, 1979; Defoss6 et al., 1978). Thus the chlorine content of the catalyst surface may be estimated from the relation
No. 4, 1980 541
tained from the EPMA measurements. The agreement between the results obtained from these two methods confirms the theoretical prediction that, on a high surface area support, the concentration of the surface atom measured by XPS tends to be equal to the overall “bulk” concentration of this atom (Kerkhof and Moulijn, 1979; Defoss6 et al., 1978). Concluding Remarks Chlorination of Pt-A1203 catalysts by CC14in an inert or oxidizing atmosphere is generally believed to produce intermediates such as phosgene and volatile Pt-Cl species and ultimately result in an increase in the dispersion of Pt in the catalyst (for a recent review, see Birke et al., 1979). The present studies show that chlorination in a hydrogen atmosphere at 400 “C leads to a slight decrease in the Pt dispersion (growth of Pt crystallites) and a selective deposition of coke in a controlled way on the Pt crystallites, leading to a sharp attenuation of the hydrogenolysis activity of the catalyst. Clearly this method can be used to enhance the selectivity of the catalyst for isomerization (or aromatization): the effective attenuation of hydrogenolysis can then avoid any excessive coke laydown and hence ensure a fairly stabilized activity for the catalyst in a continuous-flow reactor, as demonstrated earlier (Menon et al., 1979; De Pauw, 1975; De Pauw and Froment, 1975) from bench-scale operations. Chloriding with dry HC1 gas at 400 “C has only a slight influence, if at all, on the catalyst. At lower temperatures, HC1 and C1 can be bound to the Pt surface as can be seen from chemisorption values of H2on the catalyst. The X P S results (Table 11)show a slight change in the PtM/Al% ratio on chloriding with HCl at 400 “C. If significant, this change can most likely be interpreted in terms of an uneven chlorine coverage of alumina and platinum. The present studies also show that satisfactory agreement can be found between measurements performed by TEM and XPS and those obtained from other independent techniques (exposed Pt surface from H2chemisorption, coke on catalyst by the combustion method, and C1 content by electron probe micro-analysis). Acknowledgment This work could be undertaken as a joint project of the two participating laboratories thanks to “Center of Excellence” grants awarded to them by the Belgian Ministry of Scientific Affairs within the framework of the “Actions Concert6es Interuniversitaires Catalyse”. Literature Cited Adler, S. F.; Keavney, J. J. J. Phys. Chem. 1060, 64, 206-212. Beeckman, J.; Froment, G. F. Chem. fng. Sci. 1060, 35, 805-815. Beeckman, J. Ph.D. Thesls, University of Ghent. 1979. Bike, P.; Engels, S.; Becker, K.; Neubauer, H. D. Chem. Tech. 1070, 31, 473. Brundle, C. R. Surf. Sci. 1075, 48, 99-136. Defoss6 C.; Canesson, P.; Rouxhet, P.; Delmon, 8. J. Catal. 1078, 51, 269-277. De Pauw, R. P. Ph.D. Thesis, University of Ghent, 1975. De Pauw, R. P.; Froment, G. F. Chem. Eng. Sci. 1075, 30, 789-796. Escard, J.; Pontvlanne, 6.; Chenebaux, M. T.; Casyns, J. Bull. Soc. Chim. Fr. 1076, 3, 349-354. Flynn, P. C., Wanke, S. E.; Turner, P. S. J. Catal. 1074, 33. 233-244. Fung, S . C. J. Catal. 1070, 58, 454-469. Kerkhof, F. P. J. M.; Moulljn, J. A. J. Phys. Chem. 1070, 83, 1812-1619. Lycourghiotis, A.; DefosS6, C.;Dehnnay, F.;Lemaitre, J.; Delmon, B. J. Chem. Soc., Faraday Trans. 7 1080, 76, 1677-1686. Menon. P. G.: de Pauw.. R. P.:. Froment. G. F. Ind. Em. Chem. Rod. Res. B v . 1070, 78; 110-116. Mills, G. A.; Weller, S.; Cornelius, E. B. Actes 2nd Int. Congr. Catel. Park 1061, 2221-2245. _ _ _ . _ _ . .. Nefedov, V. I.; Sergushin, N. P.; Band, I. M.; Trzahaskovskaya,M. 8. J. Elecbwr Spectrosc. 1073, 2, 383-403. Prestrldge, E. B., Vla, G. H., Sinfelt, J. H. J. Catal. 1077, 50, 115-123. Scofield, J. M. J. Electron Spectrosc. 1076, 8 , 129-137. Sharpen, L. H. J. Electron Spectrosc. 1074, 5. 369-376.
-
A value of k has been given by Nefedov et al. (1973) as 3.66; if it is calculated from the cross sections of Scofield (1976) using the method proposed by Lycourghiotis et al. (1980), it is found to be 3.91. The chlorine concentrations on the surface calculated using the mean of these two values have been presented in Table I11 together with the data ob-
Received for review February 20, 1980 Accepted July 25, 1980
