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Chemisorption of carbon monoxide on molybdenum
SURFACE
SCIENCE 7 (1967) 79-89 Q North-Holl~d
C~MISORPTION
OF CARBON
Publishing Co., Amsterdam
MONOXIDE
ON MOLYBDENUM*
A. D. CROWELL and LEE D. MATTHEWS Deparfment of Physics, University of Vermont, Burlington, Vermont, U.S.A.
Revised manuscript
received 30 January 1967
The chemisorption of carbon monoxide on polycrystalline molybdenum has been studied using a radiotracer technique. Following outgassing at temperatures of 1500”-2000 “K in an ultrahigh vacuum system, carbon monoxide tagged with 14C was admitted to pressures in the lo-* to 10e5 Torr range. Rates of adsorption were consistent with sticking coetLicients in the range of lO-3 to 10-l. Desorption at various temperatures was observed as a function of time, and the data are interpreted in terms of a set of several binding states corresponding to desorption energies of 3.24 & 0.03, 3.41 i 0.03, 3.61 & 0.02, 3.77 & 0.03 and 3.95 & 0.02 eV. The relative population of these states at saturated coverage was also determined, and the differential energy of desorption as a function of coverage and temperature has been calculated.
1. In~oduc~on
The desirability of using uniform surfaces of known structure and properties, i.e. surfaces of single crystals, for the study of the adsorption of gases on solids has been recognized for many years. In the case of bulk metals this condition is difficult to meet for surfaces with a total area of more than a few square centimeters. This factor, together with the necessity of employing ultra high vacuum, has been one of the major reasons for the popularity in recent years of flash filament methods, electron and ion emission techniques and low energy electron diffraction (LEED) for the study of adsorption of gases on metals. LEED investigations are possible on the most ideal surfaces attainable, namely single faces of single crystals, but they fail to yield direct information on the amount of gas on the surface. While flash filament methods provide this information, the surfaces are seldom of known and previously specified crystal structure. A number of years ago Beckl) suggested that radioactively tagged gases could be used to measure the amount of gas adsorbed on metal surfaces. That such data could in fact be obtained on small surfaces, including a face of a single meta crystal, was demonstrated by Crowell and Farns* Research sponsored No. 993-66.
by the U. S. Air Force Office of Scientific Research, under Grant
79
80
A. D. CROWELL
AND
L. D. MATTHEWS
worth2) with carbon dioxide tagged with 14C adsorbed on nickel. Using an improved system, Dillon and Farnsworths) studied the adsorption of carbon dioxide on the (110) and (100) faces of a nickel single crystal. These investigations involved either removing the sample from the vacuum system2) or the use of an internal counter 394) which was relatively difficult to operate. A method using an external counter and a thin window in the vacuum chamber wall was used in a preliminary study of the adsorption of carbon monoxide on polycrystalline nickels). A new feature of the measurements was the observation of the rate of desorption, which was found to be a first order reaction, making possible a calculation of the adsorption binding energy, a value of about 1 eV per molecule being reported. More recently a similar technique using more energetic radio sources not requiring a thin window has been employed by Godwin and Ltischer6) for studying the adsorption of gold and copper on tungsten, by von Goeler and Ltischer7) for studies of gold on molybdenum and by von Goeler and Peacock*) for investigations of silver on molybdenum and nickel. As in the case of carbon monoxide on nickel, the desorption data in each case were consistent with the assignment of a single binding energy to the absorbed atoms. The purpose of this paper is to report some investigations on the adsorption of carbon monoxide tagged with ‘4C on a sample of polycrystalline molybdenum with an area of about one square centimeter. 2. Apparatus and procedure
The metal specimen was cut from a piece of polycrystalline molybdenum, (H. Cross and Co., typical assay: <0.02% impurities) and, after chemical etching in hot concentrated sulphuric and nitric acid, was repeatedly rinsed in hot distilled water before mounting in an all-Pyrex vacuum system. A schematic diagram of this system is shown in fig. 1. The sample was mounted on a movable carriage and could be moved forward next to the thin mica window at one end of the vacuum chamber or withdrawn until it was near a bombarding filament (Fl) by means of an external ceramic ring magnet. The carriage itself consisted of a pair of nickel slugs which rode on a stainless steel center rail and which were attached to a light molybdenum rod carrying the sample. The mica window was 3.0 mg/cm* thick and was sealed to the system by the method of Wu et al .s). The system was evacuated using a conventional fore pump and an oil diffusion pump (CVC GF-25) mounted on the underside of a vacuum table the top of which constituted the bottom of a bake out oven, The diffusion pump was separated from the ball and socket valve (BSV) by a separately bakeable molecular sieve (Linde Type 13 x) trap. Additional pumping could be supplied by a molybdenum getter (G)
CHEMISORPTION
RING
81
OF CO ON MO
MAGNET
Fig. 1. Schematic diagram of Pyrex system used in studies of chemisorption and desorption of radioactively tagged carbon monoxide on polycrystalline molybdenum. In the diagram Fl refers to a filament for outgassing by electron bombardment, F2 to a filament for an ion source for argon ion bombardment (not used), BSV to an all glass ball-andsocket magnetically operated valve, IP to an ion-sputter pump, IG to a hot cathode ionization gauge, G to a molybdenum getter, V to an all metal bakeable vacuum valve, CO to a storage flask for carbon monoxide, and GM to a thin window Geiger counter. The cross hatched regions either side of GM represent a lead shield. and a one liter/set
ion getter
pump
(Varian
Model
No. 913-0009).
After
bake-
out the pressure was in the lo- 1o-1O-9 Torr range. Pressures were measured on a Bayard-Alpert type ionization gauge (IG) (Veeco RG-75). Prior to admitting the carbon monoxide, the sample was moved next to the filament Fl and submitted to electron bombardment on the back side. Electron currents were typically in the neighborhood of 20-30 mA accelerating potential of 2000-4000 V. Using an optical pyrometer meter Instrument Co., Mica Optical Pyrometer) the temperature sample was estimated at about 1600 “C at a bombarding power of
at an (Pyroof the about
45 W and 1750°C at 60 W. Electrons were prevented from striking the molybdenum carriage or the supporting framework by a deflector shown in the diagram. At times evaporation of metal from the sample was noted, and to prevent coating the mica window a magnetically operated shutter was installed. A second electron gun consisting of a filament F2 and an accelerating electrode was included to provide an ion source for cleaning the surface by argon ion bombardment. This procedure was not actually used in the experiments reported here.
82
A. D. CROWELL
AND
L. D. MATTHEWS
Following outgassing by electron bombardment the ball-and-socket (BSV) valve was closed, and carbon monoxide was admitted to the system thxough an all metal bakeable vacuum valve (v) (GranvilIe Phillips, type C) to pressures varying from IO-* to 10e5 Torr. The pressure could be accurately controlled by means of a low torque driver (Granville-Phillips, type C). The carbon monoxide (supplied by New England Nuclear Corp.) was obtained in a break seal b&b and had a specific activity of 5.0 ~lli~uries/mill~ole (me/mm). During exposure the sample couid be moved forward, and the activity on the surface was detected by an external thin window Geiger Counter (GM) (Atomic Laboratories, EWH-108, 1.4 mgjcm’) surrounded by a lead shield to reduce background. The background counting rate in these studies varied from about 45 to 55 cpm. Exposure was continued until the surface showed no further increase in activity with time. The valve (V) was then closed and the ball-and-socket valve was opened to evacuate the system. At room temperature the surface showed no decrease in activity with time over periods of several hours. In order to study desorption, the sample was heated by induction by means of an external radio frequency (300 kc/set) coil at temperatures in the neighborhood of 1000 “K to 1200°K. The sample was heated for a time interval and the temperature was held constant by continuously observing the surface with an optical pyrometer (Pyrometer Instrument Co., Mica Optical Pyrometer) through the mica window and manualIy adjusting the current in the R.F. coil. The temperature readings were corrected for emissivity of the molybdenum but not for adsorption in the mica window. After heating for various periods, the sample was moved in front of the counter and the activity was measured. Adjustments in the temperature and time interval of heating were made on the basis of the observations. 3. Data and results As described above, the rate of adsorption of carbon monoxide on the molybdenum surface was observed at room temperature at several pressures. A typical set of data is shown in fig. 2 in which the amount adsorbed on the surface is indicated in counts per minute on the ordinate as a function of time of exposure in minutes. In this particular case the indicated carbon monoxide pressure was 2 x 10T8 Torr. Assuming a linear rate of adsorption at the rate indicated. by the dashed line, and that a complete monolayer corresponds to 400 cpm, it can be seen that it would take about 30 minutes for the formation of a monolayer. With unit sticking coefficient at a pressure of 2 x lo-’ Torr, it would take about one minute (50 set) for a monolayer to form, implying an average sticking coefficient of about 3 x 10w2. Other data
CHEMISORPTION
ADSORPTION DATA OF 7/8/66 CARBON MO/VOX/DE PRESSURE : 2 X IO-~
TIME
OF CO ON MO
83
Tow
EXPOSED
TO
CO (#in)
Fig. 2. Plot of the activity in cpm observed on the surface of a molybdenum sample (area 0.8 cmz) as a function of time exposed to radioactively tagged carbon monoxide (activity: 5.0 millic/millimole) at a pressure of 2 x 10-s Torr. Vertical bars represent range of counting error. Dashed sloping line used to estimate sticking coefficient assuming linear rate of adsorption.
taken during similar runs corresponded to average sticking coefficients of 5 x 10e2 to as little as 4 x 10m3. Comparably low sticking coefficients were obtained in this laboratory for carbon monoxide adsorbing on polycrystalline molybdenum both in photoelectric work function investigationstO) and in studies of electrical resistance changes in evaporated film+). More significant results were obtained from the studies of the desorption of carbon monoxide from the molybdenum sample. When the residual activity on the surface was plotted semi-logarithmically as a function of time that the sample was held at a given temperature, the points fell, within counting error, on straight lines. A typical set of data is shown in fig. 3. A characteristic feature of these data was the abrupt changing of the slope of these curves. As the rate associated with later times decreased, the temperature of the surface was increased. As shown, in this case three different temperatures were used until the residual activity was too low to measure reliably over background. A linear logarithmic dependence of the amount of gas upon time implies an exponential dependence on time and a first order reaction. If R represents the counting rate t set after the rate was R,, then R = R, exp( - crt) where a is a constant given by rate reaction theory by the expression u = y exp( - E/kT) where y is a frequency factor, E is an activation energy associated with de-
84
A. D. CROWELL
DESORPTION
AND
DATA
L. D. MATTHEWS
Or
E/Z/66
h = 3.7seV
T/ME
R.F.
_-
HEATED
(‘m/n
)
__
Fig. 3. Semilogarithmic plot of observed activity in cpm remaining on a molybdenum surface following exposure to radioactively tagged carbon monoxide as a function of time sample maintained at the indicated temperatures. Vertical bars represent counting errors.
sorption and T is the absolute temperature at which the process occurs. As the temperatures T were observed, for a given value of y values of E can be estimated from the measured values of a corresponding to the linear sections of plots of the type illustrated in fig. 3. The result is insensitive to the exact value of y and in these studies y was arbitrarily set equal to 1014. These calculations showed that the desorption of carbon monoxide from the polycrystalline sample were consistent with a set of activation energies E shown in table
1. The range of each value of E is the mean deviation
from
the mean value of E obtained in each case for the number of times shown. In most cases these energies were found at several temperatures, and in each case the slope used to determine CC,and hence E, was corrected for the residual gas on the more energetic sites. As shown in table 1, several observations were made corresponding to each energy found, although in some cases, as illustrated in fig. 3, only two or three points corresponding to a given energy might be noted during a single desorption run. The values represented in table 1 in no case rely only upon the slope of a single line running through only a few points for a single set of data, but rather upon repeated observations.
CHEMISORPTION
85
OF CO ON MO
TABLE
I
Values of activation energy E for the desorption of carbon monoxide from molybdenum and the initial relative populations corresponding to these energies Energy E (eV) 3.24 + 0.03 3.41 kO.03 3.61 + 0.02 3.77 i 0.03 3.95 f 0.02
Number of observations
7 5 17 17 8
Temperatures of observations (IK) 1073 1123 1073, 1123 1073, 1123, 1173 1123, 1173
Relative initial ‘A populations &O
55 4 18 13 10
By extrapolating the straight line curves corresponding to the different rates back to the time of initial measurement, the relative amounts of gas initially adsorbed at each energy level can be determined. The results are shown in the final column of tabIe 1. The individual values are averages given
to the nearest Q%. All the data reported have been in terms of the radioactive counting rates over background. The counting efficiency of the system was estimated using a secondary standard 14C source with a metal backing and was found to be approximately 4%, or 1 cpm corresponds to 25 disintegrations per minute (dpm). Our activity of 5 me/mm corresponds to about 54 x 10” carbon monoxide molecules per disintegration. The highest counting rates observed were approximately 550 cpm or 13 750 dpm or 79 x lOI carbon monoxide molecules. Whiie the surface of the molybdenum was polycrystalline with an unknown roughness factor, the approximate number of surface molybdenum atoms may be estimated. A (110) face (the most densely populated face on a (bee) crystal) of molybdenum has about 1.5 x 10” atoms/cm. The sample had an apparent area of about 0.8 cm2 or about 12 x lOi atoms in a surface layer. As this is of the same order as the maximum number of carbon monoxide molecules detected, it is estimated that the maximum rate did correspond to roughly one monolayer adsorbed.
4. Discussion
and conclusions
The fact that the plots of residual activity on the surface as a function of time consist of straight line segments which can in turn be associated with a set of energies is interpreted as meaning that the surface possesses adsorption sites for carbon monoxide with several different binding energies. This interpretation of the data assumes that certain other plausible processes do not play a significant role during the interaction between the molecules and the
86
A. D. CROWELL
AND L. D. MATTHEWS
surface,
or between
the molecules
surface.
Possibilities
which may be suggested
214C0 (adsorbed)+ and the exchange “C
themselves
in the neighborhood are the carburizing
14C02 (gas) + i4C (in solid solution)
of the reaction (6)
reaction
(solid solution)+ *“CO
14C0 (adsorbed)* (adsorbed)+r4C
(solid solution).
(7)
In the first place both of these reactions require the formation of a carbonmolybdenum solid solution in the neighborhood of the surface, and in no case was any residual surface activity noted after heating at temperatures of about 1200°K. It may also be pointed out that reaction (6) is bimolecular and therefore would be of second order in the time whereas all of our data can be resolved in terms of overlapping first order reactions. The possibility of exchange or isotopic mixing of ‘2CO-‘4C0 at a molybdenum surface has not been investigated, but these processes have been studied for carbon monoxide near a tungsten surface by Madey and collaboratorsia). Usbig mass spectrometric and flash filament techniques, these workers showed that although there are carbon and oxygen atom exchanges among the carbon monoxide molecules at the surface, there is no dissociation in the sense of residual carbon or oxygen either remaining on the surface indepmdently adsorbed or the formation of other carbon and oxygen compound: such as carbon dioxide or molecular oxygen. These conclusions are in al;reement with earlier field emission studies by Gomerla), who observed tha : desorption of carbon monoxide from a tungsten tip at 1300°K left no resillual carbon contamination, and the flash filament studies of Ehrlich14) w 10 interpreted his observations in terms of several binding states for carbon monoxide on tungsten with no evidence for any net dissociation of the I molecule. Although these investigations were for the carbon monoxide-tungsten system, the strong relationship between tungsten and molybdenum certainly suggest that similar behavior is to be expected for the carbon monoxidemolybdenum system. The different kinds of sites or patches attributed to the surface are probably to be associated with the various crystal faces exposed as well as grain boundaries and other surface defects. These individual patches undoubtedly also possess individual sticking coefficients which combine in some way to yield the adsorption vs. time curves such as that shown in fig. 2. No convenient method of separating the data for this purpose has been found, and the sticking coefficients calculated in the preceding section are regarded as overall averages designed to show orders of magnitude, and no detailed interpretation of the adsorption curves is attempted at this time.
CHEMISORPTION
OF CO ON MO
87
The presence of a variety of adsorption sites on the face of the polycrystalline sample certainly provide a stimulus for investigations on the single faces of single crystals. Such observations might well be expected to differentiate between the contributions due to the exposed crystal faces and those due to the presence of grain boundaries and other defects, and therefore be relevant to a more detailed understanding of such phenomena as the catalytic activity of polycrystalline materials. This type of understanding requires measurements on both polycrystalline materials and single crystals, and it is hoped that the method used in this investigation will be particularly useful in this respect, since the same apparatus can be used for both types of surface. Although observations on single crystals have not yet been made, plans are underway, and the work reported here is regarded as one step in a more general investigation. Typical investigations of the energies of adsorption or desorption using calorimetric or isosteric techniques yield differential heats of adsorption or desorption as a function of coverage. From the shapes of the resulting curves, efforts are made to infer the nature of the adsorption process. Frequently surface heterogeneity is invoked to account for a decrease in energy with coverage and attempts are made to deduce the nature of the nonuniformity. In the present case, as for flash-filament studies, the data directly reveal the character of the surface. The binding energies associated with the various sites and the population on these sites are obtained directly. It may be of interest however to deduce a differential energy of desorption as a function of temperature and coverage for possible comparison with results obtained by more conventional chemical methods. The following argument is used to obtain such a quantity. If Bi represents the fractional coverage remaining on the sites associated with energy Ei, and @tois the fractional coverage on this type of site initially (i.e., at r = 0), then at any later time t the total coverage 8 will be Q=
c
Qi =
c
1014
e-mm.
0,
emair
(1)
where, as before cli =
(2)
At temperature T, desorption in a time interval dt will be accompanied by an energy change dE given by dE=CEid6i =C0iO(-~~i)Eie-‘~‘dt.
(3)
d@=:Ct)iO(-tlJe-“litdt.
(4)
On the other hand
A. D. CROWELL
88
A differential
AND L. D. MATTHEWS
heat or energy of desorption
q(U, T) can then be determined
as
(5) Using the values of E, and Bi, from table
1, q(Q, T) has been calculated
using an IBM 1620 computer as a function of 0 for several temperatures. The results are shown for 500 “K and 1250 “K in fig. 4. On the same graph horizontal lines at the various values of Ei and vertical dashed lines dividing the abscissa at coverages corresponding to the various Oi, are also shown. At the lower temperatures the desorption would proceed with a strongly sequential depopulation of the various sites, giving a markedly stepped curve. At the higher temperature the desorption from the various sites tends to overlap and the steps are rounded out. In terms of the graph, desorption proceeds from right to left, and the time required to reach any particular state is not shown. Finally, we would like to make a few comments about the technique. The use of radiotracers makes possible investigations on small surfaces yielding data from which a number of details concerning the adsorption and desorption processes can be inferred. In the present case considerable insight concerning the nature of a particular heterogeneous surface has been obtained. I‘ F c-a
D/FFERENT/AL
ENERGY
VS.
COVERAGE
I
differential energy of desorption, q = dE/dB, as functions of Fig. 4. Plots of calculated fractional surface coverage B at 500 “K and 1250 “K. Horizontal lines at energies corresponding to energies of observed binding states and dashed vertical bars at the observed initial fractional coverages associated with the various states are also shown.
CHEMISORPTION
In some ways the type of information
OF CO ON MO
89
is similar to that found in other radio-
tracer studies, but this appears to be the first example using this technique in which desorption associated with the presence of several binding energies has been reported. In some respects the radiotracer method yields data comparable with that obtained by flash filament methods. The presence of several adsorption states and their population on tungsten and molybdenum surfaces have been observed by this method, and curves similar to that shown in fig. 4 have been calculatedr5). On the other hand the radiotracer method has certain advantages over the flash filament method. In the first place it is readily applicable to surfaces prepared by cutting bulk single crystals with a desired face exposed. Only one face is examined at a time, and the presence of other absorbing surfaces, even on the sample itself, does not interfere with the observations on a particular small surface. Also, unless a mass spectrometer is used as the pressure indicator, the burst of gas observed in a flash filament experiment may not have the supposed composition. The radiotracer has some of the advantages of the mass spectrometer in that only the tagged substance is recorded. Adsorption of some residual background gas on vacated sites during the desorption studies would simply be ignored by the Geiger counter. Indeed, we have on occasion observed increases in pressure on the ion gauge during heating of the sample by induction which might erroneously have been attributed to the desorption of carbon monoxide had not the activity on the surface proven otherwise. It thus appears that the radiotracer method is capable of obtaining much useful information that cannot be found as directly or as conveniently by other techniques. The authors wish to express their thanks to Dr.D.W. Juenker for both helpful comments and for his assistance in programming the calculations leading to fig. 4 for the computer. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
C. K. Beck, Science 110 (1949) 371. A. D. Crowell and H. E. Farnsworth, J. Chem. Phys. 19 (1951) 1206. J. A. Dillon, Jr. and H. E. Farnsworth, J. Chem. Phys. 22 (1954) 1601. J. A. Dillon, Jr. and H. E. Farnsworth, Rev. Sci. Instr. 25 (1954) 96. A. D. Crowell, J. Chem. Phys. 32 (1960) 1576. R. P. Godwin and E. Liischer, Surface Sci. 3 (1965) 42. E. von Goeler and E. Ltischer, J. Phys. Chem. Solids 24 (1963) 1217. E. von Goeler and R. N. Peacock, J. Chem. Phys. 39 (1963) 169. Wu, Meaker and Glassford, Rev. Sci. Instr. 18 (1947) 693. J. D. Clewley, M. S. Thesis, University of Vermont, May (1966) (unpublished). T. H. Ansbacher, M. S. Thesis, University of Vermont, May (1965) (unpublished). T. E. Madey, J. T. Yates, Jr., and R. C. Stern, J. Chem. Phys. 42 (1965) 1372. R. Gomer, J. Chem. Phys. 28 (1958) 168. G. Ehrlich, J. Chem. Phys. 34 (1961) 39. G. Ehrlich, numerous publications, e.g. Annals New York Acad. Sci. 101 (1963) 722.
SCIENCE 7 (1967) 79-89 Q North-Holl~d
C~MISORPTION
OF CARBON
Publishing Co., Amsterdam
MONOXIDE
ON MOLYBDENUM*
A. D. CROWELL and LEE D. MATTHEWS Deparfment of Physics, University of Vermont, Burlington, Vermont, U.S.A.
Revised manuscript
received 30 January 1967
The chemisorption of carbon monoxide on polycrystalline molybdenum has been studied using a radiotracer technique. Following outgassing at temperatures of 1500”-2000 “K in an ultrahigh vacuum system, carbon monoxide tagged with 14C was admitted to pressures in the lo-* to 10e5 Torr range. Rates of adsorption were consistent with sticking coetLicients in the range of lO-3 to 10-l. Desorption at various temperatures was observed as a function of time, and the data are interpreted in terms of a set of several binding states corresponding to desorption energies of 3.24 & 0.03, 3.41 i 0.03, 3.61 & 0.02, 3.77 & 0.03 and 3.95 & 0.02 eV. The relative population of these states at saturated coverage was also determined, and the differential energy of desorption as a function of coverage and temperature has been calculated.
1. In~oduc~on
The desirability of using uniform surfaces of known structure and properties, i.e. surfaces of single crystals, for the study of the adsorption of gases on solids has been recognized for many years. In the case of bulk metals this condition is difficult to meet for surfaces with a total area of more than a few square centimeters. This factor, together with the necessity of employing ultra high vacuum, has been one of the major reasons for the popularity in recent years of flash filament methods, electron and ion emission techniques and low energy electron diffraction (LEED) for the study of adsorption of gases on metals. LEED investigations are possible on the most ideal surfaces attainable, namely single faces of single crystals, but they fail to yield direct information on the amount of gas on the surface. While flash filament methods provide this information, the surfaces are seldom of known and previously specified crystal structure. A number of years ago Beckl) suggested that radioactively tagged gases could be used to measure the amount of gas adsorbed on metal surfaces. That such data could in fact be obtained on small surfaces, including a face of a single meta crystal, was demonstrated by Crowell and Farns* Research sponsored No. 993-66.
by the U. S. Air Force Office of Scientific Research, under Grant
79
80
A. D. CROWELL
AND
L. D. MATTHEWS
worth2) with carbon dioxide tagged with 14C adsorbed on nickel. Using an improved system, Dillon and Farnsworths) studied the adsorption of carbon dioxide on the (110) and (100) faces of a nickel single crystal. These investigations involved either removing the sample from the vacuum system2) or the use of an internal counter 394) which was relatively difficult to operate. A method using an external counter and a thin window in the vacuum chamber wall was used in a preliminary study of the adsorption of carbon monoxide on polycrystalline nickels). A new feature of the measurements was the observation of the rate of desorption, which was found to be a first order reaction, making possible a calculation of the adsorption binding energy, a value of about 1 eV per molecule being reported. More recently a similar technique using more energetic radio sources not requiring a thin window has been employed by Godwin and Ltischer6) for studying the adsorption of gold and copper on tungsten, by von Goeler and Ltischer7) for studies of gold on molybdenum and by von Goeler and Peacock*) for investigations of silver on molybdenum and nickel. As in the case of carbon monoxide on nickel, the desorption data in each case were consistent with the assignment of a single binding energy to the absorbed atoms. The purpose of this paper is to report some investigations on the adsorption of carbon monoxide tagged with ‘4C on a sample of polycrystalline molybdenum with an area of about one square centimeter. 2. Apparatus and procedure
The metal specimen was cut from a piece of polycrystalline molybdenum, (H. Cross and Co., typical assay: <0.02% impurities) and, after chemical etching in hot concentrated sulphuric and nitric acid, was repeatedly rinsed in hot distilled water before mounting in an all-Pyrex vacuum system. A schematic diagram of this system is shown in fig. 1. The sample was mounted on a movable carriage and could be moved forward next to the thin mica window at one end of the vacuum chamber or withdrawn until it was near a bombarding filament (Fl) by means of an external ceramic ring magnet. The carriage itself consisted of a pair of nickel slugs which rode on a stainless steel center rail and which were attached to a light molybdenum rod carrying the sample. The mica window was 3.0 mg/cm* thick and was sealed to the system by the method of Wu et al .s). The system was evacuated using a conventional fore pump and an oil diffusion pump (CVC GF-25) mounted on the underside of a vacuum table the top of which constituted the bottom of a bake out oven, The diffusion pump was separated from the ball and socket valve (BSV) by a separately bakeable molecular sieve (Linde Type 13 x) trap. Additional pumping could be supplied by a molybdenum getter (G)
CHEMISORPTION
RING
81
OF CO ON MO
MAGNET
Fig. 1. Schematic diagram of Pyrex system used in studies of chemisorption and desorption of radioactively tagged carbon monoxide on polycrystalline molybdenum. In the diagram Fl refers to a filament for outgassing by electron bombardment, F2 to a filament for an ion source for argon ion bombardment (not used), BSV to an all glass ball-andsocket magnetically operated valve, IP to an ion-sputter pump, IG to a hot cathode ionization gauge, G to a molybdenum getter, V to an all metal bakeable vacuum valve, CO to a storage flask for carbon monoxide, and GM to a thin window Geiger counter. The cross hatched regions either side of GM represent a lead shield. and a one liter/set
ion getter
pump
(Varian
Model
No. 913-0009).
After
bake-
out the pressure was in the lo- 1o-1O-9 Torr range. Pressures were measured on a Bayard-Alpert type ionization gauge (IG) (Veeco RG-75). Prior to admitting the carbon monoxide, the sample was moved next to the filament Fl and submitted to electron bombardment on the back side. Electron currents were typically in the neighborhood of 20-30 mA accelerating potential of 2000-4000 V. Using an optical pyrometer meter Instrument Co., Mica Optical Pyrometer) the temperature sample was estimated at about 1600 “C at a bombarding power of
at an (Pyroof the about
45 W and 1750°C at 60 W. Electrons were prevented from striking the molybdenum carriage or the supporting framework by a deflector shown in the diagram. At times evaporation of metal from the sample was noted, and to prevent coating the mica window a magnetically operated shutter was installed. A second electron gun consisting of a filament F2 and an accelerating electrode was included to provide an ion source for cleaning the surface by argon ion bombardment. This procedure was not actually used in the experiments reported here.
82
A. D. CROWELL
AND
L. D. MATTHEWS
Following outgassing by electron bombardment the ball-and-socket (BSV) valve was closed, and carbon monoxide was admitted to the system thxough an all metal bakeable vacuum valve (v) (GranvilIe Phillips, type C) to pressures varying from IO-* to 10e5 Torr. The pressure could be accurately controlled by means of a low torque driver (Granville-Phillips, type C). The carbon monoxide (supplied by New England Nuclear Corp.) was obtained in a break seal b&b and had a specific activity of 5.0 ~lli~uries/mill~ole (me/mm). During exposure the sample couid be moved forward, and the activity on the surface was detected by an external thin window Geiger Counter (GM) (Atomic Laboratories, EWH-108, 1.4 mgjcm’) surrounded by a lead shield to reduce background. The background counting rate in these studies varied from about 45 to 55 cpm. Exposure was continued until the surface showed no further increase in activity with time. The valve (V) was then closed and the ball-and-socket valve was opened to evacuate the system. At room temperature the surface showed no decrease in activity with time over periods of several hours. In order to study desorption, the sample was heated by induction by means of an external radio frequency (300 kc/set) coil at temperatures in the neighborhood of 1000 “K to 1200°K. The sample was heated for a time interval and the temperature was held constant by continuously observing the surface with an optical pyrometer (Pyrometer Instrument Co., Mica Optical Pyrometer) through the mica window and manualIy adjusting the current in the R.F. coil. The temperature readings were corrected for emissivity of the molybdenum but not for adsorption in the mica window. After heating for various periods, the sample was moved in front of the counter and the activity was measured. Adjustments in the temperature and time interval of heating were made on the basis of the observations. 3. Data and results As described above, the rate of adsorption of carbon monoxide on the molybdenum surface was observed at room temperature at several pressures. A typical set of data is shown in fig. 2 in which the amount adsorbed on the surface is indicated in counts per minute on the ordinate as a function of time of exposure in minutes. In this particular case the indicated carbon monoxide pressure was 2 x 10T8 Torr. Assuming a linear rate of adsorption at the rate indicated. by the dashed line, and that a complete monolayer corresponds to 400 cpm, it can be seen that it would take about 30 minutes for the formation of a monolayer. With unit sticking coefficient at a pressure of 2 x lo-’ Torr, it would take about one minute (50 set) for a monolayer to form, implying an average sticking coefficient of about 3 x 10w2. Other data
CHEMISORPTION
ADSORPTION DATA OF 7/8/66 CARBON MO/VOX/DE PRESSURE : 2 X IO-~
TIME
OF CO ON MO
83
Tow
EXPOSED
TO
CO (#in)
Fig. 2. Plot of the activity in cpm observed on the surface of a molybdenum sample (area 0.8 cmz) as a function of time exposed to radioactively tagged carbon monoxide (activity: 5.0 millic/millimole) at a pressure of 2 x 10-s Torr. Vertical bars represent range of counting error. Dashed sloping line used to estimate sticking coefficient assuming linear rate of adsorption.
taken during similar runs corresponded to average sticking coefficients of 5 x 10e2 to as little as 4 x 10m3. Comparably low sticking coefficients were obtained in this laboratory for carbon monoxide adsorbing on polycrystalline molybdenum both in photoelectric work function investigationstO) and in studies of electrical resistance changes in evaporated film+). More significant results were obtained from the studies of the desorption of carbon monoxide from the molybdenum sample. When the residual activity on the surface was plotted semi-logarithmically as a function of time that the sample was held at a given temperature, the points fell, within counting error, on straight lines. A typical set of data is shown in fig. 3. A characteristic feature of these data was the abrupt changing of the slope of these curves. As the rate associated with later times decreased, the temperature of the surface was increased. As shown, in this case three different temperatures were used until the residual activity was too low to measure reliably over background. A linear logarithmic dependence of the amount of gas upon time implies an exponential dependence on time and a first order reaction. If R represents the counting rate t set after the rate was R,, then R = R, exp( - crt) where a is a constant given by rate reaction theory by the expression u = y exp( - E/kT) where y is a frequency factor, E is an activation energy associated with de-
84
A. D. CROWELL
DESORPTION
AND
DATA
L. D. MATTHEWS
Or
E/Z/66
h = 3.7seV
T/ME
R.F.
_-
HEATED
(‘m/n
)
__
Fig. 3. Semilogarithmic plot of observed activity in cpm remaining on a molybdenum surface following exposure to radioactively tagged carbon monoxide as a function of time sample maintained at the indicated temperatures. Vertical bars represent counting errors.
sorption and T is the absolute temperature at which the process occurs. As the temperatures T were observed, for a given value of y values of E can be estimated from the measured values of a corresponding to the linear sections of plots of the type illustrated in fig. 3. The result is insensitive to the exact value of y and in these studies y was arbitrarily set equal to 1014. These calculations showed that the desorption of carbon monoxide from the polycrystalline sample were consistent with a set of activation energies E shown in table
1. The range of each value of E is the mean deviation
from
the mean value of E obtained in each case for the number of times shown. In most cases these energies were found at several temperatures, and in each case the slope used to determine CC,and hence E, was corrected for the residual gas on the more energetic sites. As shown in table 1, several observations were made corresponding to each energy found, although in some cases, as illustrated in fig. 3, only two or three points corresponding to a given energy might be noted during a single desorption run. The values represented in table 1 in no case rely only upon the slope of a single line running through only a few points for a single set of data, but rather upon repeated observations.
CHEMISORPTION
85
OF CO ON MO
TABLE
I
Values of activation energy E for the desorption of carbon monoxide from molybdenum and the initial relative populations corresponding to these energies Energy E (eV) 3.24 + 0.03 3.41 kO.03 3.61 + 0.02 3.77 i 0.03 3.95 f 0.02
Number of observations
7 5 17 17 8
Temperatures of observations (IK) 1073 1123 1073, 1123 1073, 1123, 1173 1123, 1173
Relative initial ‘A populations &O
55 4 18 13 10
By extrapolating the straight line curves corresponding to the different rates back to the time of initial measurement, the relative amounts of gas initially adsorbed at each energy level can be determined. The results are shown in the final column of tabIe 1. The individual values are averages given
to the nearest Q%. All the data reported have been in terms of the radioactive counting rates over background. The counting efficiency of the system was estimated using a secondary standard 14C source with a metal backing and was found to be approximately 4%, or 1 cpm corresponds to 25 disintegrations per minute (dpm). Our activity of 5 me/mm corresponds to about 54 x 10” carbon monoxide molecules per disintegration. The highest counting rates observed were approximately 550 cpm or 13 750 dpm or 79 x lOI carbon monoxide molecules. Whiie the surface of the molybdenum was polycrystalline with an unknown roughness factor, the approximate number of surface molybdenum atoms may be estimated. A (110) face (the most densely populated face on a (bee) crystal) of molybdenum has about 1.5 x 10” atoms/cm. The sample had an apparent area of about 0.8 cm2 or about 12 x lOi atoms in a surface layer. As this is of the same order as the maximum number of carbon monoxide molecules detected, it is estimated that the maximum rate did correspond to roughly one monolayer adsorbed.
4. Discussion
and conclusions
The fact that the plots of residual activity on the surface as a function of time consist of straight line segments which can in turn be associated with a set of energies is interpreted as meaning that the surface possesses adsorption sites for carbon monoxide with several different binding energies. This interpretation of the data assumes that certain other plausible processes do not play a significant role during the interaction between the molecules and the
86
A. D. CROWELL
AND L. D. MATTHEWS
surface,
or between
the molecules
surface.
Possibilities
which may be suggested
214C0 (adsorbed)+ and the exchange “C
themselves
in the neighborhood are the carburizing
14C02 (gas) + i4C (in solid solution)
of the reaction (6)
reaction
(solid solution)+ *“CO
14C0 (adsorbed)* (adsorbed)+r4C
(solid solution).
(7)
In the first place both of these reactions require the formation of a carbonmolybdenum solid solution in the neighborhood of the surface, and in no case was any residual surface activity noted after heating at temperatures of about 1200°K. It may also be pointed out that reaction (6) is bimolecular and therefore would be of second order in the time whereas all of our data can be resolved in terms of overlapping first order reactions. The possibility of exchange or isotopic mixing of ‘2CO-‘4C0 at a molybdenum surface has not been investigated, but these processes have been studied for carbon monoxide near a tungsten surface by Madey and collaboratorsia). Usbig mass spectrometric and flash filament techniques, these workers showed that although there are carbon and oxygen atom exchanges among the carbon monoxide molecules at the surface, there is no dissociation in the sense of residual carbon or oxygen either remaining on the surface indepmdently adsorbed or the formation of other carbon and oxygen compound: such as carbon dioxide or molecular oxygen. These conclusions are in al;reement with earlier field emission studies by Gomerla), who observed tha : desorption of carbon monoxide from a tungsten tip at 1300°K left no resillual carbon contamination, and the flash filament studies of Ehrlich14) w 10 interpreted his observations in terms of several binding states for carbon monoxide on tungsten with no evidence for any net dissociation of the I molecule. Although these investigations were for the carbon monoxide-tungsten system, the strong relationship between tungsten and molybdenum certainly suggest that similar behavior is to be expected for the carbon monoxidemolybdenum system. The different kinds of sites or patches attributed to the surface are probably to be associated with the various crystal faces exposed as well as grain boundaries and other surface defects. These individual patches undoubtedly also possess individual sticking coefficients which combine in some way to yield the adsorption vs. time curves such as that shown in fig. 2. No convenient method of separating the data for this purpose has been found, and the sticking coefficients calculated in the preceding section are regarded as overall averages designed to show orders of magnitude, and no detailed interpretation of the adsorption curves is attempted at this time.
CHEMISORPTION
OF CO ON MO
87
The presence of a variety of adsorption sites on the face of the polycrystalline sample certainly provide a stimulus for investigations on the single faces of single crystals. Such observations might well be expected to differentiate between the contributions due to the exposed crystal faces and those due to the presence of grain boundaries and other defects, and therefore be relevant to a more detailed understanding of such phenomena as the catalytic activity of polycrystalline materials. This type of understanding requires measurements on both polycrystalline materials and single crystals, and it is hoped that the method used in this investigation will be particularly useful in this respect, since the same apparatus can be used for both types of surface. Although observations on single crystals have not yet been made, plans are underway, and the work reported here is regarded as one step in a more general investigation. Typical investigations of the energies of adsorption or desorption using calorimetric or isosteric techniques yield differential heats of adsorption or desorption as a function of coverage. From the shapes of the resulting curves, efforts are made to infer the nature of the adsorption process. Frequently surface heterogeneity is invoked to account for a decrease in energy with coverage and attempts are made to deduce the nature of the nonuniformity. In the present case, as for flash-filament studies, the data directly reveal the character of the surface. The binding energies associated with the various sites and the population on these sites are obtained directly. It may be of interest however to deduce a differential energy of desorption as a function of temperature and coverage for possible comparison with results obtained by more conventional chemical methods. The following argument is used to obtain such a quantity. If Bi represents the fractional coverage remaining on the sites associated with energy Ei, and @tois the fractional coverage on this type of site initially (i.e., at r = 0), then at any later time t the total coverage 8 will be Q=
c
Qi =
c
1014
e-mm.
0,
emair
(1)
where, as before cli =
(2)
At temperature T, desorption in a time interval dt will be accompanied by an energy change dE given by dE=CEid6i =C0iO(-~~i)Eie-‘~‘dt.
(3)
d@=:Ct)iO(-tlJe-“litdt.
(4)
On the other hand
A. D. CROWELL
88
A differential
AND L. D. MATTHEWS
heat or energy of desorption
q(U, T) can then be determined
as
(5) Using the values of E, and Bi, from table
1, q(Q, T) has been calculated
using an IBM 1620 computer as a function of 0 for several temperatures. The results are shown for 500 “K and 1250 “K in fig. 4. On the same graph horizontal lines at the various values of Ei and vertical dashed lines dividing the abscissa at coverages corresponding to the various Oi, are also shown. At the lower temperatures the desorption would proceed with a strongly sequential depopulation of the various sites, giving a markedly stepped curve. At the higher temperature the desorption from the various sites tends to overlap and the steps are rounded out. In terms of the graph, desorption proceeds from right to left, and the time required to reach any particular state is not shown. Finally, we would like to make a few comments about the technique. The use of radiotracers makes possible investigations on small surfaces yielding data from which a number of details concerning the adsorption and desorption processes can be inferred. In the present case considerable insight concerning the nature of a particular heterogeneous surface has been obtained. I‘ F c-a
D/FFERENT/AL
ENERGY
VS.
COVERAGE
I
differential energy of desorption, q = dE/dB, as functions of Fig. 4. Plots of calculated fractional surface coverage B at 500 “K and 1250 “K. Horizontal lines at energies corresponding to energies of observed binding states and dashed vertical bars at the observed initial fractional coverages associated with the various states are also shown.
CHEMISORPTION
In some ways the type of information
OF CO ON MO
89
is similar to that found in other radio-
tracer studies, but this appears to be the first example using this technique in which desorption associated with the presence of several binding energies has been reported. In some respects the radiotracer method yields data comparable with that obtained by flash filament methods. The presence of several adsorption states and their population on tungsten and molybdenum surfaces have been observed by this method, and curves similar to that shown in fig. 4 have been calculatedr5). On the other hand the radiotracer method has certain advantages over the flash filament method. In the first place it is readily applicable to surfaces prepared by cutting bulk single crystals with a desired face exposed. Only one face is examined at a time, and the presence of other absorbing surfaces, even on the sample itself, does not interfere with the observations on a particular small surface. Also, unless a mass spectrometer is used as the pressure indicator, the burst of gas observed in a flash filament experiment may not have the supposed composition. The radiotracer has some of the advantages of the mass spectrometer in that only the tagged substance is recorded. Adsorption of some residual background gas on vacated sites during the desorption studies would simply be ignored by the Geiger counter. Indeed, we have on occasion observed increases in pressure on the ion gauge during heating of the sample by induction which might erroneously have been attributed to the desorption of carbon monoxide had not the activity on the surface proven otherwise. It thus appears that the radiotracer method is capable of obtaining much useful information that cannot be found as directly or as conveniently by other techniques. The authors wish to express their thanks to Dr.D.W. Juenker for both helpful comments and for his assistance in programming the calculations leading to fig. 4 for the computer. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
C. K. Beck, Science 110 (1949) 371. A. D. Crowell and H. E. Farnsworth, J. Chem. Phys. 19 (1951) 1206. J. A. Dillon, Jr. and H. E. Farnsworth, J. Chem. Phys. 22 (1954) 1601. J. A. Dillon, Jr. and H. E. Farnsworth, Rev. Sci. Instr. 25 (1954) 96. A. D. Crowell, J. Chem. Phys. 32 (1960) 1576. R. P. Godwin and E. Liischer, Surface Sci. 3 (1965) 42. E. von Goeler and E. Ltischer, J. Phys. Chem. Solids 24 (1963) 1217. E. von Goeler and R. N. Peacock, J. Chem. Phys. 39 (1963) 169. Wu, Meaker and Glassford, Rev. Sci. Instr. 18 (1947) 693. J. D. Clewley, M. S. Thesis, University of Vermont, May (1966) (unpublished). T. H. Ansbacher, M. S. Thesis, University of Vermont, May (1965) (unpublished). T. E. Madey, J. T. Yates, Jr., and R. C. Stern, J. Chem. Phys. 42 (1965) 1372. R. Gomer, J. Chem. Phys. 28 (1958) 168. G. Ehrlich, J. Chem. Phys. 34 (1961) 39. G. Ehrlich, numerous publications, e.g. Annals New York Acad. Sci. 101 (1963) 722.