PALLADIUM-CATALYZED CO OXIDATION only the weak dispersion forces acting between noncharged molecules and possibly weak hydrogen bonding. There is also a large negative change in the partial molal entropy of water on solution in the melt (Table 11). Since correction has been made for the entropy due to the change in concentration from gas to melt, AS,, this reduction of entropy must be attributed to changes in the translational or rotational motions of the molecule, the vibrational motions making negligible contribution to the entropy. The rotational entropy, S,, of the free water molecule, calculated by statistical mechanical methods,'g is given in Table 11. This term alone is of sufficient magnitude to account for the observed entropy of solution. The tight bonding of water molecule to cations suggested by the enthalpy of solution could lead to a loss of most of the rotational entropy of the gas molecule. At more elevated temperatures the increasing thermal energy would then cause increased libration about the lowest energy orientation so that less of the rotational entropy would be lost. The decrease in the ratio - z,)/S, with rise in temperature supports this theory. ~
(as
1787 The change in the partial molal heat capacity of water Alfrom vapor to solution is 6.9 cal deg-l mol-'. though E, probably varies with temperature, the solubility data are not of sufficient accuracy to reveal this. Since the heat capacity of water vaporlg is 8.0 cal deg-I mol-l, almost independent of temperature within the range considered, the partial molar heat capacity of water a t infinite dilution in the melt is 14.9 cal deg-' mol-'. This is significantly less than the heat capacity of liquid water20in the same temperature range, 18.6-22.0 cal deg-' mol-l. The difference may be attributed to the contribution of the hydrogen bond network to heat capacity in water as well as to a restriction of the rotational heat capacity of water bonded to cations in the melt.
(19) K. K. Kelley and E. G. King, U. 8. Bureau of Mines Bulletin 592, U. 8. Government Printing Office, Washington, D. C., 1961, p 49.
(20) N. 9. Osborne, M. F. Stimson, and E. SOC.Mech. Engrs., 5 2 , 191 (1930).
F,Fiock, Trans. Amer.
The Palladium-Catalyzed Carbon Monoxide Oxidation. Catalyst "Break-in" Phenomenon by Raymond F. Baddour, Michael Model], and Robert L. Goldsmith Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 09189 (Received August 21, 1969)
The break-in period for CO oxidation over pressed-disk catalysts of silica-supported palladium was investigated, Kinetic parameters and infrared spectra of adsorbed CO were monitored during this period. A t various stages of break-in, the reaction was interrupted and spectra of chemisorbed CO at 35' were determined in the absence of oxygen. Break-in did not commence until the catalyst was exposed to 02. For a stabilized catalyst, the apparent activation energy was 15 kcal/mol lower and the apparent preexponential a factor of lo7 lower than the values observed prior to break-in. Apparent discrepancies in literature values of peak frequencies were correlated with the state of the catalyst with respect to break-in. The break-in phenomenon was interpreted as arising from a redistribution of surface structures resulting from surface diffusion of palladium.
Introduction The activity of a freshly prepared catalyst almost invariably changes with time in use. Three regimes can usually be identified: a break-in period during which the change (increase or decrease) in activity is relatively rapid and sometimes erratic; a steady-state period during which the activity is relatively constant with time; and a deactivation period. The third
regime is sometimes associated with catalyst poisoning and/or aging and is well documented in the literature for many systems. Break-in is so commonly exhibited by catalysts that it is rarely reported in the literature. However, very few quantitative studies have been made of break-in and, hence, the phenomenon is not clearly understood. It may be the result of removal of a contaminant or Volume 74,Number 8 April 16, 1970
1788 foreign species introduced during preparation of the catalyst, or it may be the result of some modification of the true catalyst induced by the presence of the reactants and products and/or the occurrence of the reaction itself. The importance of understanding changes which oc-' cur during break-in has not been stressed adequately. Many catalytic and chemisorption studies are made with systems for which the history of the catalyst with respect to break-in is uncertain. For example, many studies of chemisorption are made without measurement of catalyst activity and, in general, are conducted in the absence of some reactants and products. Such results are commonly used in the interpretation of steady-state kinetics and mechanisms. Suchinterpretation may be very misleading if, for example, break-in occurs by modification of the catalyst as a result of the catalyzed reaction. I n a previous investigation in this laboratory of the oxidation of carbon monoxide over palladium wire catalysts, a relatively slow break-in period was 0bserved.l Over a period of five days, the apparent activation energy decreased from 45 to 28 kcal/mol. The activity was stable after this period, and the initial activation energy could not be regained by retreatment with hydrogen. Schwab and Gossner also reported a period of changing activity preceding a steady state for the same reaction over palladium foils.2 The present study was undertaken to determine if a similar break-in pattern could be observed for supported palladium catalysts suitable for study by infrared transmission spectroscopy. Changes occurring during break-in were followed by monitoring catalyst activity and the infrared spectrum of adsorbed carbon monoxide.
R. F. BADDOUR, M. MODELL, AND R. L. GOLDSMITH
E
'I"
H
A
Figure 1. Reactor and catalyst mount: A, gas inlet; B, gas outlet; C , CaFz window; D, epoxy cement; E, Viton 0 ring; F, palladium rods; G, quartz ring; H, catalyst disk; I, palladium wire.
Experimental Section
Gas Mixtures. During adsorption experiments, catalysts were exposed to pure CO (Matheson CP, 99.5% minimum purity) or mixtures of CO and He (99.99% minimum purity). Oxygen-free mixtures of CO and He were prepared by passing mixed gases through a heated bed of copper. During kinetic experiments, mixtures of He, CO, and O2(illatheson CP, 99.5% minimum purity) were used. The gases were metered separately, mixed, and passed over beds of Drierite and Ascarite. The total reactor pressure was maintained at approximately 1atm. Reactor. Gas mixtures were contacted with a catalyst sample in a single pass using the reactor shown in Figure 1. Two calcium fluoride windows (30-mm diameter) were sealed onto the sides of the reactor with an epoxy-silicone rubber potting compound (Emerson and Cuming, Eccosil 4712). The catalyst disk was supported in the cell, facing the calcium fluoride windows, on two 3/32-in.palladium rods, which were passed through glass-to-Kovar feed-throughs at the top of the cell. The palladium rods were silver soldered The Journal of Physical Chemistry
to the Kovar feed-throughs to provide a vacuum-tight seal. The catalyst was replaced by removing the cell top, which was connected to the lower portion by an O-ring joint. The catalyst disk and cell were heated by irradiation from two 250-W heating lamps facing the cell windows. The voltage to the lamps was controlled by a potentiometer and, in this manner, the reaction temperature was varied. The disk temperature was controllable to within 0.5" with this arrangement. Catalyst Preparation and Characterization. A nonporous silica powder with a mean particle size of 80 A, Cab-0-Si1 HS5 (Cabot Corp.), was slurried in a 5% PdClz solution (Fisher Chemical Co.). The resulting paste was mixed in predetermined proportions to yield a final catalyst of 10 wt ToPd on silica. The paste was dried at 70" and ground to a fine powder. One catalyst (1) M. Modell, Sc.D. Thesis, Department of Chemical Engineering, M.I.T., Cambridge, Mass., 1964. (2) G. M. Schwab and K. Gossner, 2. Phys. Chem. (Frankfurt am Main), 16, 39 (1958).
PALLADIUM-CATALYZED CO OXIDATION batch was used for the preparation of all catalyst samples described in this paper. For the preparation of a catalyst disk, about 0.15 g of this powder was pressed at 3000 psig into a thin self-supporting disk enclosed in a quartz ring. The resulting disk had a diameter of 22 mm and a thickness of 0.2-0.3 mm. The catalyst temperature was followed by monitoring the resistance of a 2 mil in diameter palladium wire (99.9% purity, Engelhard Industries), which was embedded in the disk by placing it in the powder prior to compression. The ends of the wire, which extended beyond the quartz ring, were spot-welded to the palladium rods supporting the catalyst disk (see Figure 1). The resistance of the wire in the disk was much greater than that of the support rods, so that the total resistance could be related to a mean disk temperature. The area of the palladium wire and rods was negligible when compared to the area of the silica-supported metal. Pretreatment of catalyst disks began with a 12-hr evacuation at 25" and 5 X Torr, followed by a 6-hr evacuation at 275" and 10-5 Torr. During this period the disks turned from brown to black probably as a result of a reaction of PdCL with residual water to form palladium oxide. After a 4-hr reduction in 20 torr of flowing Hz at 150", the disks were evacuated a t 275" and 10-6 Torr for 24 hr. Samples which were reduced with hydrogen directly from PdClzat about 100" exhibited behavior similar to those reduced from palladium oxide. Recording of Spectral Data. A Perltin-Elmer Model 12 C spectrometer, described in detail previously,* was used. The instrument was modified with a continuous slit width drive in order to obtain greater accuracy over the spectral range studied. The region from 2200 to 1800 cm-1 was investigated. The background spectra of the catalyst disks were always measured for reduced samples in a vacuum or in helium, at room and reaction temperatures, prior to admitting CO or 02. The spectra of the reduced samples in a vacuum and in helium were identical. Little change was observed in background spectra over the temperature range 25-200". Since only the carbonyl frequency region was scanned, the spectra represent infrared absorption of chemisorbed carbonyl compounds. Very little gaseous CO infrared absorption wm observed because the path length of the cell was small (0.5 cm). Spectral data are reported as fractional infrared radiation transmitted ( I / I o ) vs. wave number, where I and Io are, respectively, the transmission of the catalyst disk with and without CO adsorbed. Recording of Kinetic Data. Product gas samples were analyzed with an on-line Fisher-Hamilton gas partitioner (Fisher Scientific Co., Model 29). The instrument was frequently calibrated with known composition mixtures of CO, COZ,and O2in helium. The accuracy of the analysis was about 1% above partial pressures of 0.02 Torr.
1789 Under the conditions studied here, it was shown previously that the rate of reaction is directly proportional to O2 partial pressure and inversely proportional to CO partial pressure for the infrared-transmitting silicasupported palladium c a t a l y ~ t . ~Conversion of both reactants was maintained below 5%, and since the reaction was not product inhibited, the reactor was treated kinetically as a differential reactor. The reaction rate and rate constant were calculated from
where R is the reaction rate (mol of COz/sec) ; F is the total flow rate (mol/sec) ; XcOz is the mole fraction of C02 in the product gas mixture; k is the reaction rate constant (mol of COz/sec); and PO!,and PCOare the partial pressures (Torr). The reaction temperature was varied from 75 to 175". Since the reaction mixture always contained more than 95% helium and since conversion of the reactants was maintained below 5%, the change in the flow rate due to reaction was neglected. On the basis of calculation of effectiveness factors8$4it was concluded that bulk and pore diffusion limitations were unimportant.
Results Variation of Rate. A freshly reduced catalyst sample (disk I) was exposed to a gas stream of constant composition (12 Torr of On, 17 Torr of CO) at 150" for 42 hr. The gas composition and infrared spectrum were recorded periodically. The calculated results are given in Figures 2 and 3. The spectra in Figure 3 were recorded at the times indicated for the corresponding data points in Figure 2. During the break-in period, the concentrations of the CO surface species increased with time, as indicated by the decreasing infrared transmission. Variation of Kinetic Parameters and Spectra. The variation of apparent activation energy during the break-in period was observed for a second catalyst sample (disk 11). The results, given in Figure 4,were obtained after disk I1 was exposed for prolonged periods to an oxygen-free mixture of CO and He. Points A1 through A15 were recorded in the order as numbered. Line I, drawn through points A1 to A5, was traced through data recorded during the 6-hr period following the introduction of O2 to the CO-He mixture. The data for line 11, points A8 to A12, were obtained at the end of the first day of the break-in period. Points A13, A14, and A15 were recorded on the fifth day of the break-in period. Data points B1 through B8 were recorded at a later time after the catalyst disk had undergone other studies. Immediately before record(3) R . F. Baddour, M. Modell, and U. K . Heusser, J. Phye. Chem., 72, 3621 (1968). (4) R. L. Goldsmith, P h D . Thesis, Department of Chemical Engineering, M.I.T., Cambridge, Mass., 1966.
Volume 74, Number 8 April 16, 1970
R. F. BADDOUR, M. MODELL, AND R. L. GOLDSMITH
1790
I
IO
40
30
j
TIME ( h r )
Figure 2. Variation of rate constant during break-in. Disk I, initially exposed to reactants at time zero. Conditions invariant a t 12 Torr of 0 2 , 17 Torr of GO, and 150".
t P-
I
02 21
22
23 24 IIT (OK-IX lo3)
25
26
Figure 4. Variation of apparent activation energy during break-in.
2120 2080 2040 2000 1960 1920 FREQUENCY (cm-')
1880
1840
Figure 3. Variation of spectrum during break-in. Data recorded at times corresponding to A, B, C, and D in Figure 2.
ing the data in the B series, disk I1 was retreated with 1 atm of hydrogen in a manner otherwise identical with the initial hydrogen reduction pretreatment. The apparent preexponential factors, kol corresponding to lines I, 11, and I11 in Figure 4 are 1.9 X 1016, 1.6 X 10'0, and 2.2 X logmol of COz/sec, respectively. Infrared spectra corresponding to several of the data points given in Figure 4 were recorded. The spectra above 2000 cm-l are given for 81-4 and A10-12 in Figures 5a and b, respectively. The transmission of disk I1 in the region below 2000 cm-l could not be measured quantitatively because this catalyst sample was relatively thick. The data in Figures 5a and b each cover the temperature range of 140-180". The results are in general agreement with those for disk I (Figure 3) in that the band intensity in the region above 2000 cm-l increased during the break-in period. The increase in intensity occurred preferentially on the high-frequency side of the asymmetric band. Variation of Spectra of Chemisorbed CO. Most of the previous work on infrared spectra of CO adsorbed on palladium was conducted in the range of 25-35' under an oxygen-free gas p h a ~ e . ~ -To ~ determine the effect of break-in on the low-temperature spectrum of chemisorbed GO, a fresh catalyst (disk 111)was prepared and CO chemisorption studies were conducted at various stages of break-in. The Journal of Physical Chemistry
$1 e
0.1
0.08~ I 2100
I
I
2060
2020
FREPUENCY
la1
(ern-')
0.08
j 2100
,
I 2060
,
/
'
2020
I 1980
FREPUENCY 1crn-l) I b)
Figure 5. Variation of spectrum above 2000 cm-l during break-in. Data points correspond to those given in Figure 4. Partial pressures invariant at 14 Torr of 0 2 and 19 Torr of CO.
Immediately after the normal in situ pretreatment, spectra were recorded for disk 111 in the absence of oxygen as a function of CO partial pressure at a constant temperature of 35". The results are illustrated in Figure 6. In the region below 2000 cm-l, bands appeared first at 1920,1885, and 1865 cm-' (spectrum A) With increasing coverage, two additional bands appeared at 1942 and 1967 cm-' (spectrum B). The most intense bands at high coverage were at 1965 and 1985 cm-'. In the region above 2000 cm-', there first appeared a broad shallow band with its center at about 2035 cm-1 (spectrum A). At pressures above I
(5) R.P. Eischens, S.A. Francis, and W. A. Pliskin, J . Phys. Chm., 60, 194 (1956). (6) N. N. Kavtaradze, E. Cr. Boreskova, and V. J. Lygin, Kinet, Catat., 2, 349 (1961). (7) C. W. Garland, R. C. Lord, and P. F. Troiano, J . Phus. Chem., 69, 1188 (1965).
1791
PALLADIUM-CATALYZED CO OXIDATION
I 2100
2050
2000
I950
1900
1850
FREQUENCY (cm-')
Figure 6. Spectra of CO chemisorbed on Pd prior to break-in. Temperature constant a t 35" (disk 111). CO partial pressures (Torr): A, 0,005; B, 0.068; C, 0.146; D, 0.34; E, 0.67; F, 1.81; G, 5.3; H, 15.7; I, 30.0.
--z
2
I .o 0.8
\
0.6
2
8
0.4
I v)
z a a
I-
0.2
_1
a z 9
IV
2
0.1
LA
.O 5
2120 2080 2040 FREQUENCY (cm-1)
2000
Figure 7. Spectra of linear species for CO chemisorbed on Pd prior to break-in. Temperature constant a t 3" (disk IV). CO partial pressures (Torr: A, 0.001; B, 0.03; C, 0.6; D, 8.2.
Torr, a band appeared at 2078 cm-l and became the dominant band in this region at higher pressures. These observations are somewhat more discernible in the spectra shown in Figure 7 , which were obtained for a somewhat thicker sample (disk IV).
The spectra of Figures 6 and 7 were invariant with time provided that the gas phase was maintained free of oxygen. The catalyst was then alternately exposed at room temperature to oxygen and carbon monoxide for three times. After a fourth oxidation, the sample was reduced again with HZ,evacuated, and the spectra of oxygen-free carbon monoxide were recorded again at 35" (see Figure 8). I n the region below 2000 cm-l, bands appeared again at 1920, 1885, and 1865 cm-I at low coverage (spectrum A), and were followed by bands appearing at 1942 and 1967 cm-I at moderate coverage (spectrum B) and at 1965 and 1865 cm-l at high coverage. In the region above 2000 cm-l, there first appeared a broad shallow band centered at about 2060 cm-l, and at higher coverage a band at 2087 cm-l predominated. These spectra differ from those in Figures 6 and 7 in that the dominant band above 2000 cm-1 is sharper and shifted slightly to higher frequency, and the 1985-cm-1 and is now somewhat more intense than the 1965-cm-' band at high coverage. After contacting the sample with a reacting mixture of CO and O2 at about 200" for 12 hr, the spectra of Figure 9 were recorded for carbon monoxide (oxygenfree) at 35". The bands above 2000 cm-l exhibited similar behavior to those of Figure 8. Below 2000 Volume 7 4 , Number 8 April 16, 1970
1792
R. F. BADDOUR, M. MODELL,AND R. L. GOLDSMITH
2100
2050
2000
1950
1900
1850
FREQUENCY (crn-')
Figure 8. Spectra of GO chemisorbed on Pd after partial break-in. Temperature constant at 35' (disk 111). GO partial pressures (Torr): A, 0.019; B, 0.035; C, 0.051; D, 0.23; E, 0.71; F,3.5; G,110.0.
2100
2050
2000
1950
1900
1850
FR E Q u E Nc Y ( c m") Figure 9. Spectra of GO chemisorbed on Pd after complete break-in. Temperature constant at 35" (disk 111). GO partial pressures (Torr): A, 0.008; B, 0.109; C,0.30; D,1.91; E, 110.0.
The JOUTWZ~ of Physical Chemistry
1793
PALLADIUM-CATALYZED CO OXIDATION
Table I : Summary of Reported Spectra of CO Chemisorbed on Palladium Frequency ( c m - ' )
,
Conditions
Pretreatment
Silica-supported; Hz redn, purged at 25' (ref 5 )
25'; 10-4-10-" Torr of co
Silica-supported; Hz redn, evacuated at 500' and Torr for 5 hr (ref 6 ) Vapor deposited under 5-20 Torr GO (ref 7 )
25'; 10-4-1.5 Torr of CO, with and without 8 Torr of 0 2
lA
in ear"
I
B r Idg ed 'I
I
I
I
II
I
l
I
I
I
I
l
I
I
25'; 5-20 Torr of
co
Silica-supported; Hz redn, evacuated at 200-280' and 10-6 Torr for 16 hr (ref 3)
20.5'; 0.04 Torr of CO, 0 . 2 Torr of 0 2 , and 780 Torr of He
Same as above (this work)
35"; l O + - l .O Torr of CO, prior to break-in 35'; 1.O-30 Torr of CO, prior to breakin; 0.1-110 Torr of CO, after break-in
Gaseous CO
! c
I
I
I
t I
cm-l, weak bands at low coverage were observed at 1920 and 1886 cm-' (spectrum A). Bands at 1985 and 1965 cm-' were again present, showing the same shift with coverage previously observed. However, the 1965-cm-' band appeared as a very weak shoulder on the much stronger 1985-cm-l band (spectra C through G). I n addition, the intensity of the 1985-cm-' band decreased noticeably from 1.9 to 110 Torr of CO (spectra D to E), while the linear band continued to increase. Additional spectra, obtained at 5.7 and 13.1 Torr fall between spectya D and E. These data were omitted from Figure 9 so as to illustrate more clearly the changes observed. The amount of CO adsorbed at high coverage was determined volumetrically for disk 111 at each of the three stages of break-in. No large variations were observed: all three values were within i 1 5 % of 6 X mol. Thermal Desorption of CO. The peak absorption intensities were monitored during thermal desorption of CO in the following manner. Disk I, after the end of the break-in period, was exposed to a flowing mixture of 20 Torr CO in He at 87". After equilibration, the flow was switched to a stream of pure He. I n Figure 10, the results are given in terms of the transmission at 2087 and 1985 cm-' a,s a function of time following the switching to pure He. The transmission at 2087
1
I I
ill00
0 IO Oo5I
1
ID00
2000
t: 1985 cm-l (bridgedl
2 3 4
5 6 7 8 9 IO II 12 TIME (min)
Figure 10. Peak frequency transmission during thermal desorption at 87' (disk I after break-in).
cm-I increased monotonically with time, but the transmission at 1985 cm-' passed through a minimum.
Discussion Interpretation of Spectra of CO Adsorbed on Palladium. Spectra of chemisorbed CO on palladium have been obtained by several g r o ~ p s . ~ ,As ~ -previously ~ noted,3 there are marked differences between groups in the (8) J. K. A. Clarke, G. Farren, and H. G. Rubaloava, J.Phys. Chem., 71, 2376 (1967). (9) C. P. Nash and R. P. DeSieno, ibid., 69, 2139 (1965).
Volume 74, Number 8 April 16, 1970
179.1: reported frequencies of the peak maxima. A summary of the reported values for those groups presenting quantitative information is given in Table I. The band positions reported by Eischens, et al. (row A), are very close to those observed in this study for a catalyst prior to break in (row E). Furthermore, the shapes of the spectra in these two cases are similar in that the two bands between 1800 and 1930 cm-l are of about equal intensity. The preparation and pretreatment procedures reported by Eischens, et al., are also consistent with the interpretation that their catalyst was not broken-in; there is no report of the catalyst having been exposed to oxygen. The band positions reported by Kavtaradze, et al. (row B), are similar to those for the post break-in period (row F). They observed no effect of oxygen on the spectra of chemisorbed CO. This observation is consistent with results obtained in this laboratory for catalysts which had undergone b r e a k - i r ~ . ~ ,In ~ one experiment reported by Kavtaradae, et al., the catalyst was exposed to 8 Torr of oxygen at 200” for 5 min. It is conceivable that the catalyst was exposed to oxygen for much longer periods prior to recording the reported spectra and, hence, that their catalyst had already been broken-in. The results of Garland, et al. (row C), obtained with vapor-deposited palladium under 5 to 20 Torr CO, are similar to those of Eischens, et al. (row A), except for a weak band which appeared at 1970 cm-l. Due to the radically different preparation procedure, a direct comparison with results for silica-supported palladium is not feasible. The results of Baddour, el al. (row D) , are very similar to those reported here for catalysts which had undergone break-in. The catalyst studied by Baddour, et al., was exposed to mixtures of carbon monoxide and oxygen at relatively high temperature prior to recording the reported spectra. It is clear from the above discussion that the apparent discrepancies among different groups for the reported band positions can be correlated in terms of the history of the catalyst with regard to break-in. Thus, it is essential to clearly relate the history of a catalyst when reporting infrared spectra of surface species. Some differences of opinion exist regarding the structures of carbon monoxide surface species corresponding to the infrared active bands. Eischens, et a1.,l0originally assigned the bands above 2000 cm-’ to “linear” CO species in which the carbon atom is bonded to a single metal atom, while the bands below 2000 cm-I were attributed to species in which the carbon is “bridged” between two adjacent metal atoms. These assignments were made by comparison with spectra of bulk metal carbonyls. Such a comparison is by no means conclusive and, in fact, the existence of the bridged species has been seriously questioned.” On the other hand, the assignment of the bands in the 2090-cm-’ The Journal of Physical Chemistry
R. F. BADDOUR, M. MODELL,AND R. L. GOLDSMITH region to a linear species is well accepted since this represents a relatively small displacement from the gaseous CO doublet. Additional evidence for the existence of bridged species can be obtained from the spectra of Figure 9. The decrease in intensity of the 1985-cm-’ band with increasing pressure (D to E) is difficult to explain on the basis of single-site adsorption. Hohvever, the explanation is straightforward if the 1985-cm-’ band is interpreted as due to multisite adsorption. Upon increasing pressure, one would predict from Le Chatelier’s principle that the following reaction should shift in favor of the linear species COB(a)
+ CO(g)
2COL(a)
where (a) and (9) refer to adsorbed and gaseous species, respectively, and B and L refer to bridged and linear forms, respectively. Thus, the linear species would eventually increase in concentration at the expense of bridged species. As observed in Figure 9, at high surface coverages all of the bridged species, except that corresponding to the 1965-cm-’ band, decreased in concentration with increasing pressure. This reaction should be important only at high coverage because the bridged form is preferred over the linear form when vacant neighboring sites are available.3 If the rate of conversion of bridged to linear species is a slow process, one mould observe a slow chemisorption in the high-pressure region. Such observations (i.e. , slow residual chemisorption) in which days or weeks have been required to reach equilibrium have been reported previously in many chemisorption studies. Since the spectra shown in Figure 9 were recorded at approximately l-hr intervals, it is conceivable that equilibrium was not attained. The assignment of the 1985-cm-I band to a bridged species is consistent with the thermal desorption results given in Figure 10. The linear species desorbs faster than the bridged species because it is less tightly bound to the surface. As vacant sites are created by desorption of linear species, some of the remaining linear species are converted to bridged species by the reaction: COL(a) * COB(a). Thus, the concentration of bridged species reaches a maximum at an early stage in the desorption process. E f e c t of Break-in o n Kinetics. It has been shown previouslySthat the observed rate expression
+
--f
is consistent with either of the following mechanisms: (a) adsorption of molecular oxygen rate-limiting, or (b) surface react’ionbetween chemisorbed carbon mon(10) R. P. Eischens, W. A. Pliskin, and s. A . Francis, J . Chem. Phys., 22, 1786 (1954). (11) G.Blyholder, J . Phys. Chem., 68, 2772 (1964).
1795
PALLADIUM-CATALYZED CO OXIDATION oxide and chemisorbed molecular oxygen rate-limiting. I n either case, it is assumed that oxygen coverage is much smaller than carbon monoxide coverage. For cases (a) and (b), respectively, the derived rate expressions are given in eq 3 and 4
'*
i
i (4) where k, and k b are the rate constants for the ratelimiting steps, and KCOand KO,are adsorption equilibrium constants. Activation energies corresponding to these two rate equations may be expressed as
2L: l-
(I
2100
2050
2000
1950
FREO JE N C Y
- qco (EA)b = Eb + q0a - qC0 (EA)*= E ,
(5)
(6)
where E, and Eb are activation energies for the rate constants R, and Rb, and qco and qo, are the heats of adsorption for CO and 02,respectively. On the basis of present data, it is not possible to determine what changes in these energy terms are responsible for the observed decrease in the apparent activation energy during break-in. Interpretation of the Break-in Phenomenon. The break-in pattern observed here was very similar t o that observed with palladium wire cata1ysts.l Since these catalysts were prepared under very different conditions, it is unlikely that break-in mas the result of removal of a contaminant or foreign species. The initially high apparent activation energy (45 kcal/mol) could not be reproduced by H2 retreatment of the catalyst (B series data in Figure 4). Therefore, the break-in period could not result from gradual removal of hydrogen remaining after the initial catalyst reduction. The observed break-in period was not a purely thermal process since the catalyst samples were heat-treated for several hours at a temperature of 50" higher than the highest reaction temperature studied. It is also unlikely that break-in was associated with a significant change in total palladium surface area since the total amount of adsorbed CO at high coverage did not vary significantly. It is proposed that the break-in phenomenon results from a structural rearrangement of the palladium catalyst. This rearrangement occurs only in the presence of oxygen or 02-CO mixtures, since the spectrum of CO adsorbed on a freshly prepared catalyst from an oxygenfree gas phase is invariant with time. This interpretation is supported by the results of Manly and Ricel2 which indicated that crystal growth of palladium can occur well below the Tamman temperature (460") and that oxygen is an accelerator for this process. The structural rearrangement is most likely associated with a redistribution of exposed crystallographic planes
1900
1850
h-'l
Figure 11. Spectral changes during break-in: A, prior to break-in at 0.34 Torr of CO; B, partial break-in at 0.23 Torr of CO; C, after break-in a t 0.30 Torr of CO.
.I
2100
I
1
I
I
1
2050
2000
1950
1900
1850
F R E O U E N C Y icm-II
Figure 12. Spectral changes during break-in: A, prior to break-in a t 15.7 Torr of CO; B, partial break-in at 11.2 Torr of CO; C, after break-in at 13.1 Torr of CO.
resulting from surface diffusion of palladium atoms or palladium-oxygen complexes. The process most likely involves transformations from structures which bind CO tightly to those of lower binding energy. This result can be seen more clearly in Figures 11 and 12, in which spectra from Figures 6, 8, and 9 are crossplotted for two different regions of pressure. The overall result of break-in is a shift of absorption from low to high frequency. I n each of Figures 6, 8, and 9 upon increasing pressure the bridged bands appeared in order of increasing frequency. Eischens, et u Z . , ~ reported similar results and furthermore found that the bands were removed in reverse order of appearance upon evacuation. Since increasing frequency is characteristic of a stronger carbon-oxygen bond, which in turn (12) D.G.Manly and F.J. Rice, Jr., J. Phys. Chem., 68,4201 (1964).
Volume 7 4 , Number 8 April 16, 1970
W. D. GARRETT AND W. A. ZISMAN
1796 is probably associated with weaker metal-carbon bonds, it was concluded that the lower frequency bridged species are more tightly bound to the palladium. The type of structural rearrangement proposed here has been observed on palladium and other transition metal single crystals by low-energy electron diffract i ~ n . ’ ~ On - ~ ~the (100) face of palladium, several ordered surface structures were found, each of which was stable in well-defined temperature ranges. l4 Transitions from one structure to another were accomplished by temperature manipulation under high-vacuum conditions. Thus, it is conceivable that phenomenasimilar to break-in could be initiated bv controlled heatingofthe in the absence Of oxygen’ It be extremely interesting to see how the low-energy electron
diffraction patterns change when a crystal is exposed to the conditions which have resulted in break-in.
Acknowledgment. This work was supported in part by the National Science Foundation (Grant GP-607). The authors are grateful to the Atlantic Refining Co. for the use of the spectrometer. The authors are indebted to Drs. R. P. Eischens, C. W. Garland, and R. C. Lord for discussions and advice during the course of the investigation. (13) C. W. Tucker, Jr., J . Appl. Phys., 35, 1897 (1964). (14) A. M . Mattera, R. M. Goodman, and G. A. Somorjai, Surface Sei., 7 , 26 (1967). (15) H. B . Lyon and G. A. Somorjai, J . Chem. Phys., 46, 2539 (1967).
Damping of Capillary Waves on Water by Monomolecular Films of Linear Polyorganosiloxanes by W. D. Garrett and W. A. Zisman Naual Research Laboratory, Washington, D . C. 20900 (Received September 26, 1969)
We have investigated the wave damping effects caused by adsorbed insoluble monolayers of a variety of organic compounds with emphasis on the behavior of polyfunctional molecules, especially surface-active linear polymers. We report here the results obtained with various polyorganosiloxanes, some of which were of exceptional purity and freedom from homologs. Graphs are given of the wave damping coefficient ( k ) us. the area per adsorbed molecule ( A ) and of the film pressure ( F ) vs. A . The k vs. A graphs were both reproducible and reversible. Our results reveal the importance of measuring the wave damping behavior of each polymer over the entire film pressure range, because with some polymers as many as four maxima and three minima were found in the k us. A plot. We also found conditions where k had nearly the same value as that for water free of organic film for several values of F distributed over the film pressure range between the gaseous film state and that of complete film collapse. The results of this investigation teach also that the k us. A graph of an insoluble monomolecular film can be used as a sensitive surface-chemical tool having particular value for following and identifying changes in molecular configuration through intermolecular or intramolecular rearrangements.
Introduction Waves on a large body of water having wavelengths of less than 1.7 cm are known as “capillary and these are sometimes damped with remarkable effectiveness by an adsorbed film of surface-active material. Interesting early reviews of the history of sea slicks and wave calming oils have been given by Banting* and Davies3 and Davies and Rideal.4 Reyno l d ~theorized ~ that an elastic surface film dissipated wave energy due to an alternating tangential drag on the water as the surface film was expanded and contracted by the passing crests and troughs. This The Journal of Physical Chemistry
concept was later expressed mathematically by Lamb,e who essentially ignored any other properties of the adsorbed surface film. (1) Sir William Thomson, Phil. Mag., 42, 368 (1871). (2) J. D. Banting, “The Influence of Oil on Water,” Report, 3rd International Lifeboat Conference, Rotterdam, Holland, June 1932, pp 155-173. (3) J. T. Davies, Chem. Ind. (London), 906 (1962). (4) J. T. Davies and E. K. Rideal, “Interfacial Phenomena,” Academic Press, New York, N. Y., 1961, pp 266-274. ( 5 ) 0. Reynolds, Brit. Assoc. Advan. Sci. Rep., paper i409 (1880). (6) H. Lamb, “Hydrodynamics,” 6th ed, Cambridge University Press, London, 1932.
