3070
J. CARTER, P. LUCCHESI, P. CORNEIL, D. YATES,AND J. SINFELT
Exchange of Deuterium with the Hydroxyl Groups of Alumina
by J. L. Carter, P. J. Lucchesi, P. Corned, D. J. C. Yates, and J. H. Sinfelt Process Research Division, Esso Research and Engineering Company, Linden, New Jersey (Received March 20,1966)
The kinetics of the exchange of deuterium with the surface hydroxyl groups of alumina have been studied by infrared spectroscopy. It was observed that the reactivity of the different hydroxyl groups with deuterium varied. Of the three hydroxyl groups present (at 3785, 3740, and 3710 cm.-'), the high-frequency one was found to be the most reactive. The kinetics of the exchange obey a logarithmic rate law similar to that observed frequently for chemisorption processes. The kinetics may be a consequence of surface heterogeneity. It was observed that the presence of platinum on the surface of the alumina increased the rate of exchange. However, the effect depended on the method of impregnation of the platinum, probably due to the presence or absence of halogen in the platinum salt used for the impregnation. A study of the effect of platinum concentration for samples prepared by impregnation with chloroplatinic acid yielded complex results which may largely be a consequence of the presence of chlorine in the samples. Some evidence was obtained for a direct reaction (other than exchange) of deuterium with alumina at certain conditions.
The existence of hydroxyl groups on the surface of alumina has been shown by infrared spectroscopy.lJ? Estimates of the concentration of such hydroxyl groups have been made by Hall and Lutinski3 using a technique based on the exchange of the hydroxyl hydrogen with deuterium as the temperature is slowly and continuously increased. The method was shown to be useful for distinguishing forms of hydrogen which have markedly different reactivities with deuterium. The results of these workers indicated the presence of at least three different forms of hydrogen on alumina. This agrees with the infrared studies of Peri and Hannan2 and of workers in this lab~ratory,~ which have identified three different hydroxyl groups on alumina. In the work of Hall and Lutinski,a the exchange of deuterium with surface hydrogen was also studied with samples of alumina containing approximately 0.7% platinum. These samples were prepared by impregnation of alumina with chloroplatinic acid. It was observed that the presence of platinum did not materially lower the concentration of surface hydroxyl groups on the alumina. Furthermore, it was concluded that the presence of platinum on alumina did not enhance the rate of exchange of deuterium with hydroxyl groups. However, it was also observed that the presence of The Journal of Physical Chemistry
halogen (fluorine) on alumina markedly decreased the reactivity of the hydroxyl groups with deuterium. The platinum on alumina catalysts contained halogen by virtue of impregnation with chloroplatinic acid, and the authors pointed out that the presence of halogen very likely has a bearing on the comparison of the deuterium-exchange activity of their alumina and platinum on alumina samples. While the work of Hall and Lutinski has given a clear indication of the ease of exchange of deuterium with the hydroxyl groups of alumina and certain platinum on alumina catalysts, the kinetics of the exchange were not investigated extensively. In the work discussed in the present paper, the kinetics of the exchange of deuterium with the hydroxyl groups of alumina were studied in some detail. Additional information on the effect of platinum on the exchange was also obtained. The exchange reaction was followed by observing the change in the infrared spectrum (disappearance of OH and appearance of OD groups) as a (1) D. G . Rea and R. H. Lindquist, 136th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept. 1959. (2) J. B. Peri and R. B. Hannan, J . Phys. Chcm., 64, 1526 (1960). (3) W. K. Hall and F. E. Lutinski, J . Catalysis, 2, 518 (1963). (4) R. 0. Steiner, J. L. Carter, and D. J. C. Yates, unpublished results, Esso Research and Engineering Co., Linden, N. J., 1959.
3071
EXCHANGE OF DEUTERIUM WITH HYDROXYL GROUPS OF ALUMINA
function of the time of exposure to deuterium. This was found to be a convenient way to follow the kinetics. There is also the additional interest that the study gives information on the exchange by another method, different from that employed by Hall and Lutinski. The infrared method is an especially good way to study the reactivity of a particular hydroxyl group on the surface, as distinct from simply measuring the over-all rate of exchange of deuterium with surface hydroxyl groups.
Experimental Materials. The preparation of the Ab01 and Pt on AI203 samples used in the present work has been described previo~sly.~~6 The A1203, designated (A) M 2 0 3 in the previous publications, was prepared by heating p-alumina trihydrate for 4 hr. at 600". The B.E.T. surface area of the final material was 295 m."g. The Pt on AI2O3catalysts were prepared by impregnation of the alumina with aqueous chloroplatinic acid, followed by drying at 120" and subsequent calcination in air for 1hr. at 500". Several Pt-A1203 catalysts were prepared using an aqueous solution of Pt(NH3)r (NO& for the impregnation. Most of the experiments on the exchange of deuterium with the OH groups on alumina were made with a sample of (A) which was wetted with distilled water in an amount equivalent to the quantity of impregnating solution used in the preparation of the platinum catalysts and then recalcined for 1 hr. at 500". This was done to ensure that the A1203and PtA1203 catalysts were treated as nearly as possible in the same manner prior to the deuterium-exchange experiments, so that the effect of the platinum on the exchange could be more clearly defined. The sample of (A) Al203, wetted and subsequently recalcined in the manner described, is identified as (H) AI2O8in this paper. The deuterium used in the exchange studies was supplied by the Matheson Co. and had a stated purity of 99.5%. The hydrogen was obtained from the Linde Co. Both gases were dried by passing them through a liquid nitrogen trap prior to use. Apparatus. A detailed description of the apparatus has previously been reported6 in a study of the chemisorption of ethylene on alumina. The samples used were flakes 2 X 0.7 cm. in size with a '[thickness" of about 20 mg./cm.2. The flakes were prepared by pressing the powder at 15,000p s i . Procedure. The experiments reported in this paper were performed as follows. The samples were slowly heated to 600" while they were being evacuated. When the pressure reached 5 X mm., the heating
Figure 1. Typical infrared spectra of (H) AlzOs as a function of time of treatment with Dz at 250": A, 0; B, 1; C,10; D, 100; E,1130 min.
was discontinued, and the sample was cooled to room temperature under vacuum. Some samples were reduced in hydrogen at 500" prior to a final outgassing to 5 X mm. and were then cooled to room temperature under vacuum. After cooling to room temperature, the infrared spectrum was recorded. Following this, the sample was heated to the temperature to be used for the exchange experiment, and deuterium was admitted to the sample cell at a pressure of 15 em. After the desired time had elapsed, the gas was evacuated from the cell. The sample was then cooled to room temperature and the spectrum again recorded. The amount of deuterium present during exchange corresponded to a large excess, over 10 times the amount required to exchange all the surface hydroxyl groups.
Results The bulk of the data on deuterium exchange with the OH groups of alumina were obtained on the sample designated as (H) AlzO3. Typical infrared spectra obtained at room temperature, before and after exposure to deuterium for varying periods of time, are shown in Figure 1. In the original spectrum, before exposure to deuterium, three d8erent bands due to surface OH groups were observed. These bands occurred at wave numbers of 3785, 3740, and 3710 cm. -1. After exposure to deuterium, additional bands due to OD groups were observed at 2790, 2755, and (6) P. J. Lucchesi, J. L. Carter, and D. J. C . Yates, J . Phys. Chem., 66, 1451 (1962). (6) P. J. Lucchesi, J. L. Carter, and J. H. Sinfelt, J. A m . Chem. SOC., 86, 1494 (1964).
Volume 69, Number 9 September 1966
3072
J. CARTER,P. LUCCHESI, P. CORNEIL,D. YATES,AND J. SINFELT
2730 cm.-I. The high-frequency OH group (3785 cm.-I) exchanged more readily with deuterium than did the other OH groups. After exposure to deuterium for 10 min. at 150", the 3785-cm.-l band showed 30% exchange compared to 9% for the 3740- and 3710cm.-I bands. However, the difference in exchange activity appeared to decrease with increasing temperature. The remainder of the paper, including the data shown in Figures 2 through 5, is concerned with the high-frequency OH and OD groups. Since the absorbance scale used in recording the infrared spectra is a linear function of the amount of OH or OD present, the progress of the exchange reaction can be followed readily. Data showing the % exchange of the high-frequency OH group as a function of time over the (H) &Os at three different temperatures are given in Figure 2. The data show a good linear relationship between the per cent exchange and the logarithm of the time. Such a relation is frequently observed in chemisorption.' In the experiments on (H) A1203, the ratio of OD appearance (2790-cm.-l band) to OH disappearance (3785-em.-I band) was essentially unity (Figure 3). The ratio was calculated simply as the ratio of the increase in absorbance of the OD to the decrease in absorbance of the OH, assuming the extinction coefficients to be equal. The reaction with deuterium 1w
Figure 2. Exchange of Dz with the hydroxyl groups of (H) A120a:0, 75"; 0, 150"; A, 250".
1
b
Th4, HInr
.lop
loop
Figure 5. Effect of pt on the exchange of D2 with the hydroxyl groups of AIZOS. Pt impregnated on AlzOa with Pt(N€&)z(NOe)z solution: 0, (H) AlzOS at 75"; 0, (H) AlzOa , 0.6% Pt on A120a a t 150". a t 150"; 0,0.6% Pt on A l 2 0 a at 75"; .
on the (H) Also3therefore appears to be limited to the exchange, there being no evidence for the additional formation of OD groups beyond that due to exchange. However, the situation does not appear to be the same for the (A) A1203,which is different from the (H) M20ain that it was not wetted and recalcined. I n the case of the (A) A1203 there was evidence for a net formation of OD groups over and above that due to exchange. A comparison of the ratios of OD appearance (2790-cm.-l band) to OH disappearance (3785-cm.-l band), as well as the extents of OH exchange, for the (A) A1203 and (H) Azoa at 150" is given in the tabulation
2
10
The Journal of Physical Chemislru
IO
Figure 4. Effect of Pt on the exchange of Dz with the hydroxyl groups of A1203a t 150". Pt impregnated on &Oa with chloroplatinic acid solution: U, (H) Ah03; N,0.001% Pt OB Al2Oa; #, 0.1% Pt on AhO3; I. 0.6% P t on A1~03.
Time on DI, min.
Figure 3. Ratio of OD appearance to OH disappearance on (H) AlzOs: 0, 75"; 0, 150"; A, 250".
I
0
O H exchanged(HIAlrOr
7%
(A) AlrO:
21 29
19 29
-OD/(100 (A) AlnOr
1.3 1.4
- OH)(H)AlnOa
0.95
1.0
(7) B. M. W. Trapnell, "Chemisorption," Butterworth md Co. Ltd., London, 1955, p. 106.
3073
EXCWQEOF DEUTERIUM WITH HYDROXYL GROUPS OF ALUMINA
Thus, while the extent of OH exchanged is the same for (A) AlzOaand (H) AlzOa,there is an additional reaction of deuterium with the surface in the case of (A) AlzOa. Impregnation of alumina with platinum was found to increase the rate of exchange of deuterium with the surface OH groups, as shown in Figures 4 and 5 for the high-frequency OH group (3785 cm.-l). In the data presented in Figure 4 the platinum was impregnated from an aqueous chloroplatinic acid solution. An interesting dependence of the exchange on platinum concentration was found. On addition of 0.001% platinum, the exchange increased. However, further increase in the platinum concentration resulted in a decrease in the rate of exchange, until at 0.6% platinum the rate was not significantly higher than that observed with the alumina alone. In the data presented in Figure 5 the platinum was impregnated from an aqueous solution of Pt(NH&(NO&. Here the data indicate that the presence of platinum si&cantly increases the rate of exchange even at the 0.6% platinum level, whereas in the samples prepared from chloroplatinic acid the effect was significant only at much lower platinum levels. The results presented to this point are all for samples which were simply heated to 600" and outgassed prior to the exchange measurements. Data obtained on samples of Pt-Al~03 prepared from Pt (NH&(NO&, in which the platinum was reduced in Hz for 1 hr. at 500" prior to the final outgassing, showed little effect of the reduction on the subsequent rate of exchange of deuterium with the OH groups. Data on the extents of exchange after 1 hr. at 150" are shown below for a Pt-A1203 sample containing 2 wt. % platinum, and for a (H) MZO3sample for comparison -Pt.-AhOReduoed
% OH Exchanged
45
Not reduoed
(H)Altos
46
34
The difference shown between the reduced and nonreduced samples is probably within experimental error and not significant. Note that the extent of exchange of the 2% Pt sample is about the same as was observed on the 0.6% Pt sample (Figure 5) after 1 hr. at 150". Thus, increasing the platinum content above 0.6% appears to have little effect.
Discussion The present work has demonstrated the use of the method of infrared spectroscopy for studying the kinetics of a surface process involving the reaction of deuterium molecules from the gas phase with surface hydroxyl groups on alumina. Thus, infrared data are useful for characterking reactivity as well as the strutture of adsorbed species.616.8
The observation of the present work that the highfrequency OH group exchanged faster than the lowerfrequency groups appears to differ from the results of Pen and Hannana on alumina. These investigators reported that the lowest-frequency group frequently exchanged faster than the others. However, they used a y-alumina, whereas the present work was conducted with q-alumina, a crystallographically different form. The detailed surface environment of the OH groups on these aluminaa could be quite different and could play an important role in determining the exchange activity of the OH groups. Furthermore, the exchange data reported by Peri and Hannan were obtained in a higher temperature range, 250-500", compared to the range 75-250" used in the present work. The results of the present study indicate that the differences in the rates of exchange of the various groups decrease with increasing temperature, and it is possible that the order of exchange activity might change at a sufficiently high temperature. The reaction of deuterium with the surface hydroxyl groups of alumina is characterized by a strong deceleration of the rate as the reaction proceeds. The kinetics are best described by an exponential rate law, which is typical of many chemisorption proce~ses.~In the present case, if we let x equal the fraction of OH groups exchanged after exposure to deuterium for a time t, the rate law is
r =
dx
- =
dt
bexp(-ax)
where b and a are parameters which are functions of temperature. These parameters can be evaluated from a plot of In (dx/dt) us. x, which is a straight line with slope equal to -a and intercept equal to In b. The values of dx/dt for various values of x can conveniently be evaluated by noting that dx/dt = l/t(dx/d In t). The quantity (dx/d In t) is the slope of a plot of x vs. In t, which in the case of the (H) A1203 can be obtained from Figure 2. The values of the parameters b and a as a function of temperature are summarized in Table I. From the temperature deTable I: Effect of Temperature on the Parameters b and Temp., "C.
75
150 250
b,
0ec.-1
2 . 8 X 10-6 6 . 1 x 10-4 2 . 9 X 10-8
CY
a
28.6
23.1 12.9
(8) J. L. Carter, p. J. LUG&&, J. H. ginfelt, and D. J. c. yates, Actes Congr. Intern. Catalyse, 9, Amsterdam, 1964,644 (1966).
vohn%69,Number 9 September 1966
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J. CARTER, P. LUCCHESI, P. CORNEIL, D. YATES,AND J. SINFELT
pendence of b the apparent activation energy Eo at the beginning of the exchange (Le., at x = 0) can be calculated. When this is done, it is found that Eo varies somewhat with temperature. In the range 75 to 150°,Eois 12 kcal./mole, decreasing to 7 kcal./mole in the range 150 to 250". The parameter CY characterizes the decrease in the exchange activity as the reaction proceeds and can be related t o an increase in activation energy during the course of the exchange. This, in turn, may be a consequence of surface heterogeneity. With regard to the detailed mechanism of the exchange, the information available is not sufficient to establish this with any certainty. Exchange reactions of the type discussed here may be more complex than is initially apparent. For example, as Peri points the exchange of deuterium with surface hydroxyl groups does not require substitution of deuterium on the same site from which the hydrogen was removed. It is conceivable that a surface OD group could form by interaction of deuterium with an oxide ion with subsequent removal of hydrogen from an adjacent hydroxyl site. Finally, surface migration processes could be involved; e.g., deuterium could dissociate at selected sites on the surface and then migrate to hydroxyl groups to exchange, similar to the mechanism of Porter and Tompkins'O for hydrogen chemisorption on metals. Turning now to the observed effect of platinum on the exchange, it should be noted that most of the data were obtained with nonreduced PtFAl~08 samples. The promotional effect of platinum on the exchange with such samples is similar to the results of Hall and Lutinski.3 These authors point out that reaction of deuterium with the oxygen on the platinum leads to formation of DzO, thus providing another mechanism of exchange in the system. They also indicated that the exchange was not enhanced if the platinum was reduced but was actually decreased. However, they obtained data showing that the presence of halogen markedly decreased the exchange activity. Since their Pt-A1203 samples all contained chlorine, the true
The Jmmd of Phyaica2 Chemistry
effect of the platinum was obscured, as the authors pointed out. In the present work, the effect of reducing the platinum was investigated using Pt-Ah03 samples prepared from a nonhalogen containing platinum salt to avoid the complications due to the halogen. While extensive data were not obtained on reduced samples, the available results indicate that the rate of exchange on a reduced platinum-containing sample is higher than that on alumina. Thus, when the alumina is impregnated with a nonhalogen-containing platinum salt, the rate of exchange is increased regardless of whether the platinum is reduced or not. These results are therefore taken as evidence of a cooperative effect of platinum in the exchange of deuterium with the hydroxyl groups of alumina. It may be that the effect involves transport of active species between platinum sites and sites on the alumina surface. It seems possible that deuterium would dissociate on platinum sites and subsequently migrate to hydroxyl groups on the alumina to exchange. The platinum sites would thus serve as a source of reactive deuterium atoms. The interfering effect of the halogen observed in the work of Hall and Lutinski may account for the unusual dependence of the rate of exchange on the platinum concentration in the ChIoroplatinic acid preparations used in the present study. The decrease in the rate of exchange with increasing platinum concentration above the 0.001% platinum level may be related to the increasing halogen content. It seems possible that halogen displaces some of the more reactive OH groups from the surface, with the remaining OH groups exhibiting a lower over-all reactivity than is observed with the original OH groups on alumina. Alternatively, the halogen may serve as a trap for reactive species (deuterium atoms) migrating between platinum sites and hydroxyl groups on the alumina. J. B.Peri, J. Phys. Chem., 69, 220 (1965). (10) A. 8. Porter and F. C . Tompkins, Proc. Roy. Soc. (London), A217, 529 (1953). (9)
