HANSL. GRUBER
48
+
action Ce -t Ce3+ 3e-. However, Ce2+ ions could be assumed to contribute electronic conductance at a rate increasing with concentration due to overlap of their electronic orbitals, as discussed previouslylOJs for alkali metal atoms or molecule ions. An additional contribution might result from an exchange of electrons between two adjacent valence states, Ce2+ and Cea+. Recent results with Nd-NdCls solutions to be reported in
Vol. 66
detail elsewherel2 suggest the possibility of such an exchange. Acknowledgment.-The authors gratefully acknowledge the many helpful suggestions of Dr. M. J. Kelly of this Laboratory concerning the design of the electrode arrangement used in these measurements. We also wish to acknowledge the assistance of D. E. Lavalle of this Laboratory in preparing the anhydrous salts.
CHEMISORPTION STUDIES ON SUPPORTED PLATINUM BY HANSL. GRUBER Research and Development Department, The Atlantic Refining Company, Philadelphia, Penna. Receiaed June 19, 1061
The adsorption of hydrogen, oxygen and carbon monoxide on platinum-on-alumina re-forming catalysts and on the alumina base was investigated. By means of hydrogen adsorption the platinum surface area of freshly prepared samples was found to be 200 to 270 m.a/g. of platinum. The decrease of platinum dispersion due to heat treatment of catalysts is shown. Several catalysts were subjected to complete oxidation-reduction cycles and the amount of hydrogen and oxygen taken up in th; different steps of such a cycle was measured. A probable mechanism for the oxidation-reduction cycle was established. r h e mechanism of CO adsorption on supported platinum was found to depend not only on the nature of the support but also on the degree of dispersion of the metal. The fraction of CO adsorbed in the bridged structure decreases with decreasing metal dispersion.
Introduction With the increasing commercial importance of supported metal catalysts of low metal concentrsc. tion, t,he physical structure of such catalysts has become of considerable interest. In most cases the metal crystallite size is too small to be measured by X-ray techniques and therefore chemisorption seems to be the only approach to the measurement of metal dispersion. Chemisorption has been used in the past for surface area determinations. However, due to the experimental difficulties of obtaining clean surfaces and the theoretical difficulties of defining a monolayer, chemisorption was later completely displaced by the BET method. Only for samples of very small surface area (metal wires and films) and for multicomponent systems, where one wanted to discriminate between the surfaces of different components, was chemisorption still very useful. In their extensive studies of iron synthetic ammonia catdysts, Emmett and co-workers1p2 developed a low temperature chemisorption method to determine the surface concentration of the different components in the catalyst. The procedure later was successfully applied to FischerTropsch and other catalysts.a-5 In all these cases the metal concentration was high enough to represent a substantial part of the surface. For metal concentrations in the order of 1% or less the low temperature chemisorption procedure is not applicable. High temperature adsorption on supported metal catalysts was studied by several (1) P. H. Emmett and S. Brunauer, J . A m . Chem. Soc., 69, 310 (1937). (2) P. H.Emmett and N . Skau, ibid., 65, 1029 (1943). (3) R. B. Anderson, W. K. Hall and L. J. E. Hofer, ibid., 70, 2465 (1948). (4) P. Y. Butyagin and S. Y. Elovioh, Zhur. Fzz. Khim., 26, 692 (1952). (6) L. D’Or and A. Orzachowski, J . Chem. Phys., 81,467 (1954).
Their results gave mainly qualitative information, allowed comparisons of catalysts of similar nature but did not allow quantitative interpretations as to the surface area of the metals. Very recently, the chemisorption at room temperature or above has been employed successfully in several laborat o r i e ~ ~ -to ’ ~ study the metal dispersion of reforming catalysts. In the present work we have investigated the adsorption of hydrogen, oxygen and carbon monoxide on platinum-on-alumina catalysts and on the alumina base itself, in order to study metal dispersion, to correlate results obtained with different adsorbates, and to elucidate reductionoxidation mechanism of such catalysts. Procedure.-The basic principle of measuring the specific adsorption on a supported metal surface, as it was first, utilized by Boreskov,14 is to select adsorbate and adsorption parameters in such a way as to avoid or minimize adsorption on the support and to facilitate full monolayer coverage or maximize adsorption on the metal. If under the chosen optimum conditions adsorption on the support material still occurs to an extent which makes it advisable to correct for it, it is possible to do so by measuring adsorption on the nonmetallized support and by subtracting this value from the total adsor tion on the catalyst. This procedure gives what we will caly the “net adsorption” on the supported metal. This net adsorption (V), in cc. of gas adsorbed per g . of catalyst, ie by itself a measure of the metal dispersion for a given metal content. It is more convenient to express net (0) W. W. Russel and H.
S. Taylor, J . Phys. Cham., 29, 1325 (1925). (7) L. H. Reyerson and L. E. Swearingen, ibid., 31, 88 (1927). (8) R. Burshtein and A. Frumkin, Trans. Fmadoy Soc.. as, 273 (1932). (9) T.R. Hughes, R. J. Houston and R . P. Sieg, 135th Natl. Meeting, Am. Cham. SOC., Div. Petrol. Chem., Preprints 4, C-33 (Apr. 1959). (IO) L. Spenadel and M. Boudart. J . Phys. Chem., 64, 204 (1900). (11) 8. F. Adler and J. J. Keavney, dbid., 64, 208 (1980). (12) G. A, Mills, 8. Weller and E. B. Cornelius, Preprint No. 113, Seoond Intern. Congr. on Catalysis, Paris, 1900. (13) H. L. Gruber and J. H. Ramser, presented at 130th Natl. Meeting, Am. Chem. 800.. Atlantic City, Sept. 1959. (14) G. K. Boreskov and A. P. Kamaukhov, Zhur. Fie. Khim., 26, 1814 (1952).
Jan., 1962
CHEMISORPTION STUDIES ON SUPPORTED PLATISUM
adsorption per g. of mctal, independmt of metal roncenl ration, by rising thc ratio of numbcr of atoms or moleculrs of gas adsorbed per rnctnl atom ( G / M ) . Knowing the adsorption mcolinnisrn one can convcrt G / M into the ratio of the number of aocossible metal &COMB t o the total numbcr of metal atoms ( M * / M ) . M*/M o m vary from 0 to 1, corresponding to “zero dispersion” (bulk metal) and “complete dispersion” (monolayer or particles with no interior atoms), respectively. It also is possible t o calculate from this data surface area or average particle size of the metal, but not without making a few rather arbitrary assumptions. This point will be discussed in detail later. Selection of Adsorbate.-From the considerations outlined above, it follows that physisorption must be avoided as far as possible. The amount of physisorption on a given adsorbent will decrease with decreasing boiling point of the adsorbate, with decreasing pressure and with increasing temperature. In general, therefore, one should work at the highest possible temperature and the lowest possible pressure with a low boiling adsorbate which is chemisorbed readily by the metal with veIy little or no chemisorption occurring on the support. The gases most likely to meet these requirements are hydrogen, oxygen and carbon monoxide. Oxygen, which was used as an adsorbate by Webb16 and by Mills,’* is known to form surface oxide films which can be more than 1 layer thick and of undefined stoichiometry. We have used oxygen therefore only to study the oxidation mechanism but not to determine the metal dispersion. Carbon monoxide, which was used by Hughes9 and Mille,’* is physisorbed to about 1% coverage a t room temperature and 100 mm. I t therefore should be used only a t low pressures ((1 mm.). At higher pressures, physisorption would lead to sizable correction terms in a volumetric procedure and could cause substantial errors. We have used carbon monoxide primarily in a flow procedure, where by using a “slug technique”18 physisorption is avoided. Hydrogen, which was used as adsorbate by several workers, shows no measurable physisorptlon at room temperature or above. With some metals (e.g., Pd), absorption or solid solution might occur to an extent which would complicate or even prohibit its use. With platinum, absorption below 400’ is not mea~urable,’~ and extrapolation of high temperature data indicates absorption of less than 0.01 vol. yo a t 250”. ‘Ihis value, being several orders of magnitude smaller than the amount adsorbed, can be safely neglected and hydrogen thus appears very well suited for volumetric adsorption measurements. Selection of Adsorption Parameters.-The optimum temperature for the adsorption measurements with a given system of adsorbent and adsorbate is the one a t which the difference in adsorption on the two components (metal and support) is a maximum. Such a maximum can best he located from the corresponding adsorption isobars. As to the hydrogen isobar on platinum, quite a few data are available in the literature.17-1B At pressures >lo0 mm. and in the temperature range of interest, the data show a Flight and steady decrease of adsorption. For hydrogen adsorption on alumina, the few data available20.21 indicate poor reproducibility, even with identical samples. This is not surprising in view of the fact that the surface properties of alumina will not only depend on the specific type of alumina used and its purity, but also on the previous history of the sample (method of preparation, calcination time and temperature, degree of dehydration, etc.). We therefore have taken great care to detmmine adsorption isobars for hydrogen on our alumina at several pressures, in order to establish a reliable correction value for the catalyst support.
Experimental Two different experimental procedures were uscd. Most of the CO-adsorption measurements were obtained by a new slug flow technique described in detail elsewhere.16 Adsorp(15) See ref. (8)in A. N. Webb and T. T. Mitchell, J . Phys. Chem., 6S, 1880 (1959). (16) H. L. Gruber, t o he published. (17) Grnelins IIandb. d. Anorg. Chernie, Syst. Eo. 68, “Platin,” Part C, Verlag Chernie, Weinheim, 1939,p. 7 ff. (18) A. F. Benton, J . A m . Chem. Soe., 48, 1850 (1926). (19) Takao Kwan, J . Research Inst. Cuatalysie, 1 , 81 (1949). (20) A. S. Russell and J. J. Stokes, Jr., J . A m . Chem. Soc., 69, 1316 (1947). al) H.W. Guenther, Dieaartalion Ab&., 18, 507 (1955).
49
,I5I-
3 I I
(I
L
I
\: \
.
/ -
2\
Fq
u v o
I
I
/
-\
-
.05
\*JO (0)
”
I
100
200
I
TEMPERATURE,
Fig, L-Hz
I
400
300 O
50mm.
(A)100 mm. ( 0 ) 2OOmm.
500
C.
adsorption isobars on q-alumina: 50, 100 and 200 mm.
tion of hydrogen and oxygen was measured by standard volumetric methods. The glass apparatus used was of conventional design.23 The sample size could be varied between 5 and 40 g. The gas buret and the constant volume manometer were temperature controlled to 0.1 The manometer (precision bore tube of 16 mm. i.d.) was read with tt cathetometer to 0.05 mm. Low pressures were measured by means of a McLeod gage or an ionization gauge. The system and the samples were evacuated by an oil diffusion pump, which was backed by a mechanical forepump. The samples were protected from oil and mercury vapors by means of cold traps at liquid nitrogen or Dry Ice temperatures. The prepurified and dried gases were introduced into the gas storage flasks through a purification train, consisting of a liquid nitrogen cold trap and of a silica gel trap at liquid nitrogen tem erature. All dead spacp calibrations were obtained with Eelium. BET surface areas were determined with nitrogen. The 11-alumina used as support material in this work was calcined a t 600” and had the properties BET surface area, rn.z/g. 210 Pore volume, cc./g. 0.55 Skeletal density, .g./cc. (determined with helium) 3.31 Impurities, p.p.m. : Alkali (NazO) 20 Alkaline earths 110 Silicon 33 Iron 55 The rntalyst samples were prepared by impregnation of the support with platinum salt solutions in conventional ways. The purity of the platinum salts used was better than 99.9%. The alumina blank for the adsorption measurements was treated identically to the catalyst preparations, except that the platinum salt solution was replaced by water of corresponding p H value. After calcination all samples were reduced in flowing hydrogen for 2 hr. a t 500” Evacuation of the samples overnight (16 hours) a t 500’ and a t a pressure of less than 10 -6 mm. was adopted as standard pretreatment. This resulted in good reproducibility of the adaorption mrasurements on the alumina, whereas less severe conditions (a temperatiire of 450” or lower, and a time of 6 hr. or less) were not satisfactory. The reproducibility of the hydrogen adsorption measurements was i1% using a 40-g. sample of the blank and a 5-g. sample of platinized catalyst.
.
I
Results and Discussion Adsorption of H on ~-A1200.-Hydrogen adsorption isotherms on the ?,-alumina support (without metal) were measured at several temperatures. At temperatures higher than 250” saturation occurred at a pressure of 50 to 100 mm. At lower temperatures no such saturation was found in the (22) G. L. Joyner, “Scientific Glass Blowing,” Instruments Publishing Co., Pittsburgh, Pa., 1949.
HAYSL. GRUBER
30
Vol. 66 TABLE I Type B (descending branch of
Isobar)
!20i
Temp. range, "C. Rate Amt. adsorbed after 2 hr. ( % of equilibrium), % Time required for complete equilibrium, hr. E&imated heats of adsorption, kcal. ( 0 ) SPENDEL",
140-300 Slow 80
Type
4
(ascending branoh of
isobar)
300-500 Very slow
25
20-24
100-120
7-12
12-18
0.6Oe/eP1,250
0
>
suffice to draw final conclusions on the mechanism involved. The minimum in the isobar certainly suggests the temperature range of 250 to 300" as the optimum range for measuring specific adsorption on the alumina-supported platinum and therefore 250" was selected as the temperature for all hydrogen adsorption measurements on the platinumon-alumina catalyst. Adsorption of H on Pt-on-AlzOs.-An adsorption isotherm of hydrogen on a sample of 1.1% Pt on v-Al203 was measured a t 250" up to a pressure of 250 mm. The results, in terms of cc. (STP) of hydrogen adsorbed per g. of platinum, are shown in Fig. 2 in comparison with data obtained by Spenadel1° and by Adlerl' for catalysts containing 0.6 and 0.58% Pt, respectively. Plotting adsorption per gram of platinum does not imply that this adsorption is due to the platinum alone but, facilitates comparison of data for catalysts of different Pt content. Considering the difference in catalyst preparation as well as the fact that corrections for the support have not been applied, the agreement between the three isotherms, measured in different laboratories, is quite satisfactory. I n the pressure range which we studied, adsorption did not reach a true saturation value. However, a t pressures above 100 mm. the slope of the isotherm is very small and constant. For surface area determinations, the hydrogen adsorption therefore is measured between 100 and 200 mm. The rate of hydrogen adsorption on the catalyst was quite fast initially, about 90% of the total amount being taken up in the first 5 min., but fell off appreciably during the first hour. Complete equilibration a t 100 mm. and 250" took about 20 hr. Since as shown previously, adsorption on the support was slow, taking 20 to 24 hr. for equilibration a t 250" (see Table I) and since hydrogen adsorption on a clean platinum surface is very fast, it was thought that the slow uptake found with the catalyst might be due to the support only. A close comparison of the adsorption vs. time curves for a platinum catalyst and for the corresponding support (see Fig. 3, note break in ordinate scale), shows this to be true. If the adsorption on the support at any given time is subtracted from the adsorption on the catalyst, the net adsorption us. time curve is obtained. This curve levels out completely at 2 hr. proving that any gas uptake beyond this time is due to adsorption on the
2
OO
50
100
PRESSURE,
Fig. 2.--Hz
0
I50
200
250
mm.
adsorption isotherms on platinum-on-alumina.
60
120 TIME,
I
I
180 min.
240
1
300
Fig. 3.--Hz adsorption, 250°, 200 mm.
pressure range studied (up to 200 mm.). The data are plotted in the iorm of an isobar in Fig. 1. The minimum in the isobar indicates two types of hydrogen adsorption which we call Type A (high temperature) and Type B (low temperature). Type B cannot be ascribed to physisorption for two reasons: (a) the amount is too large to be accounted for by physical adsorption a t that temperature; (b) the rate is slow. Due to the appreciable pressure changes in the volumetric system during adsorption, we did not attempt any quantitative evaluation of the rate data accumu!ated in the course of the determination of the different isotherms. Qualitative information on rates and heats of adsorption is summarized in Table I. From this data one has to conclude that both Type A and Type B are chemisorption. Type B, having the smaller activation energy, may represent chemisorption on aluminum ions, A on oxygen i 0 n s . 2 ~ ~ 2However, ~ our data do not
(23) B. M. W. Trapnell, "Chemisorption," dcadeniic Press, New Pork, N. Y . . 1955, p 76. (24) E. Wiclce, Z. Elelctioehem., 63,279 (1949).
Jail., 1962
51
C H E N I S O R P T I O N S T U D I E S ON S U P P O R T E D P L A T I J S U U
support only. I n all the adsorption measurements for surface area determination a time of 2 hr. therefore was allowed for equilibration. The nearly instantaneous adsorption on the platinum of about 90% of the final amount also indicates that our reducing and pretreating conditions are satisfactory and result in a relatively clean surface. The slow rate of uptake of the remaining 10% which is adsorbed over the first two hours could be due to impurities as suggested by Adlerll or due to the high surface coverage on the platinum as suggested by Spenadel.1° Since in our work repeating the hydrogen adsorption up to 4 times on the same sample gave adsorption us. time curves identical within experimental error, and since several adsorption runs on a platinum black under similar conditions showed an instantaneous uptake of 98% of the total amount, neither one of the two above explanations seems satisfactory. We suggest that the slow adsorption on a fresh catalyst is due specifically to the supported nature of the metal in the following sense: Whereas, the bulk of the adsorbed hydrogen molecules will find two platinum sites on the same patch or crystallite of platinum, some of the H molecules adsorbed a t rather high coverage will find sing;le isolated platinum sites, which will adsorb one of the two H atoms only. Whether the second €1 atom then is adsorbed by adjacent oxygen ions of the support, or whether it migrates over the oxide surface to another patch or crystallite of platinum, in both cases we are dealing with a slow, activated process. This hypothesis is supported further by the experimental finding that the relative amount of slow adsorption decreases as the platinum dispersion decreases. Net Adsorption on Pt.-The difference between the total adsorption on the catalyst and the adsorption on the corresponding support under identical conditions is taken as the net adsorption of hydrogen on the metal. This procedure implies the assumption that the adsorption properties of the carrier toward hydrogen are not affected by the presence of up to 1% pla,tinum on the carrier surface. Since the BET area of the alumina is in the order of 200 m.2/g. and does not change due to the imprpgnation, calcination and reduction steps employed, the metal, even a t maximum dispersion, cannot cclver more than 1.5% of the support surface. Therefore, in the case where there is on the carrier surface an independent and random distribution of sites active in hydrogen adsorption and of sites active in platinum adsorption, the above assumption is correct. I1 is possible, however, that the same sites which are active in I3 adsorption preferentially adsorb platinum ions during impregnation. Since the total H adsorption on the carrier is far below a coverage of 1%, impregnation in this case might completely deactivate the carrier for hydrogen adsorption. Such an identity between H and Pt adsorption sites is very unlikely. There is strong experimental evidence for its non-existence in the fact that the impregnated catalyst shows exactly the same rate of slow Hz uptake over 20 hr. as found for the support. It appears, therefore,
that applying the correction for adsorption on the support is well justified. The same conclusion was reached previously by SpenadellO on the basis of a different argument, Metal Dispersion and Surface Area.-The net adsorption per gram of catalyst (see Table 11, col. 2) can be converted to a ratio of number of H atoms adsorbed per Pt atom present in the sample, as givei! in the 3rd column of Table 11. We consider this ratio as the physically most significant measure of metal dispersion, since it can be obtained without specific knowledge of surface coverage or adsorption mechanism and without any assumptions concerning these two points. The hydrogen-to-platinum ratio of most of the fresh catalysts studied was in the range of 0.8 to 1.0, in agreement with values reported by SpenadellO and by In no case did we find ratios higher than one after proper reduction of the catalyst, as was reported by Adler.ll The variation of metal dispersion in t,he range of 0.8 to 1.0 is attributed to minor variations in the preparation procedure. Sintered and deactivated catalysts showed substantially lower H/Pt ratios, in the order of 0.4 to 0.1. TABLE I1 HYDROGEN ADSORPTION AND METALDISPERSION Catalyst
Net ads eo./:. Pt
1.1%Pt on q-81208 TPH,a reduced 2 hi*. 120' and 2 hr. 510' CPA,b reduced 2 hr. 510' TPII, reduced i! hr. 510' TPH, deactivated by severe lob. regeneration 1.1% Pt (TPH) on q-Al108 Sintered in Ha, 500°, 0 hr. 72 hr. 200 hr. 1200 hr. 4% Pt on Alon-Cf Pt black, 99.0% Pt (J. Bishop & Co., Malvern, Pa.)
Ratio R =
SA.,
Q
m.z/g.
M
Pt
Par%. size,
A.
0.615 0.97 ,523 .83 .695 .85
268 229 233
