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Decomposition of carbon monoxide on a (110) nickel surface
SURFACE
SCIENCE 35 (1973) 21 l-226 0 North-Holland
DECOMPOSITION
Publishing Co.
OF CARBON MONOXIDE
ON A (110) NICKEL SURFACE H. H, MADDEN* Department ofPhysics, Wayne State University, Detroit, Michigan 48202, U.S.A.
G. ERTL Institut fiir Physikalische Chemie, Technische Umiversitiit, Hannover, Germany New inv~ti~tions of the (110) nickeIl~rbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1 x 10-r torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.
1. Introduction The chemisorption of carbon monoxide on a (110) nickel surface has recently been investigated in our laboratory using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It became apparent during this project that the interaction of impinging electron beams with chemisorbed CO, and also the exposure of the heated surface to CO, lead to surface reactions other than the simple adsorption/desorption of CO. Because these spurious reactions led to the build up of surface impurities that could influence the CO chemisorption, an attempt was made to define these reactions and the conditions under which their rates become significant. The * Visiting Professor at the Technische through August 1972).
Universit~t,
211
Hannover,
Germany
(August 1971
212
H. H. MADDEN
AND G. ERTL
results were then used to formulate correct chemisorption of CO on (110) nickel.
procedures
for our study of the
LEED-beam induced changes in the chemisorbed CO phase on (111) nickel have been investigated by Edmonds and Pitkethlyr). They found that continuous monitoring of the surface during exposure to CO, with LEED beam energies between 40 and 100 eV, resulted in a different sequence of multiply-spaced LEED structures than were observed when they restricted the exposure of the CO-covered surface to the LEED beam to less than 17% of the total time of observation. Of the possible surface reactions (besides chemisorption) that can occur when CO interacts with the nickel surface at room temperature, they concluded that disproportionation leading to CO, and C adsorbed at separate sites on the surface was the reaction stimulated by the LEED beam. Their conclusions were based on their LEED investigations of the structures that form on (111) nickel due to exposures to CO and to COz, and also on arguments relating to the energetics of the disproportionnation reaction in comparison to two other possible reactions. The other reactions considered were carbonyl formation and dissociation. Our conclusion based on LEED, AES and CO-flash desorption measurements is that the dissociation reaction, leading to atomically dispersed carbon and oxygen on the surface, is the principal electron-beam stimulated surface reaction for CO chemisorbed on (110) nickel. The interaction of (100) nickel with CO has recently been investigated by Tracy 2, using both AES and LEED. He found that the LEED beam rapidly caused alterations in the LEED pattern and a build up of carbon, but no oxygen, on the surface when beam energies greater than 1.5 to 20 eV were used. Tracy found further that the heating of the sample in CO at temperatures greater than 475°K also resulted in a build up of carbon on the surface. The primary aim in our investigation of the (I 10 Ni/CO) system was a determination of the isosteric heat of adsorption using experimental techniques similar to those employed by Tracy. Thus, the effects of heating in CO as well as the interaction of the LEED and AES beams with adsorbed CO were expected to be important. Another consideration in our investigations was whether there is more than one binding state for CO on (110) nickel. Three chemisorbed states for CO on this surface have been reported from flash-desorption results3). Onchi and Farnsworth”) also found three CO desorption peaks for the (100) Ni/CO system and could correlate the three peaks with changes in their LEED patterns. Our present results indicate however that the higher temperature COdesorption peaks are intimately related to the presence of dispersed carbon and oxygen on the surface, and hence not simply related to the chemisorption of CO on (110) nickel.
DECOMPOSITIONOF
CO
ON(I
10)Ni
SURFACE
213
LEED-beam induced changes in a chemisorbed CO layer have also been reported for (100) platinum by Tuckers). For this same system Palmberg6) has found that impinging electron beams during LEED or AES measurements can result in decomposition of the adsorbed CO. It thus appears that some CO chemisorption studies performed without a means of monitoring the surface chemical composition may be in error because of these spurious reactions. A purpose in our work was to determine not only a “recipe” for minimizing the effects of these undesired reactions but also to determine actually what reactions do take place. 2. Apparatus and procedures Our investigations were carried out in a 4-grid Varian Model 120 LEED/ AES system. The base pressure for the system was about 2 x lo-” torr. The AES beam was supplied by an off-axis electron gun and the angle of incidence of the primary beam during AES measurements was 20” to the plane of the sample surface. An EAI Quad 150A residual gas analyzer (RGA) was mounted on the system so that a minimum distance of about 25 cm existed between the sample surface and the ionizer of the mass analyzer. Gases of spectral-grade purity could be admitted to the system from glass supply flasks through Varian variable leak valves. Changes in work function of the sample were measured using a self-compensating Kelvin-method vibratingcapacitor system. The reference electrode was an oxidized tantalum plate. The (110) nickel surface was prepared, after initial X-ray orientation, by polishing with finer and finer abrasives until a mirror-like finish was obtained with a 0.25 urn grain size. Following the polishing the crystal was washed in distilled water and acetone. The crystal was supported in the vacuum system by suspending it between two horizontal 0.3-mm diameter tungsten wires. These wires ran through slots cut in the upper and lower edges of the crystal by spark erosion. The crystal was heated indirectly by conduction from these tungsten wires through which an AC heating current was sent. The tungsten heating wires were under slight tension supplied by attachment at their ends to the more massive stainless steel part of the crystal holder. The temperature of the crystal was measured with a Pt/PtRh thermocouple that had been spot-welded to the back edge of the crystal before the final front-surface preparation. The cleaning of the crystal surface in vacuum, following initial outgassing at - 1300”K, was carried out by ion bombardment with 400-eV argon ions followed by annealing at 875°K. The major surface impurity present after the high temperature heating was sulfur but this was easily removed by the ion bombardment. After ion bombardment the major impurities were carbon
214
H. H. MADDEN
AND G. ERTL
and argon. The carbon and most of the argon were removed face during annealing. Attempts to remove all of the argon
from the surby heating to
higher temperatures during annealing resulted in the reappearance of sulfur. For this reason our “clean” surface had a small amount of argon embedded in it. Ion bombardment at 200 eV was sufficient to remove sulfur from the surface but did not appreciably affect the AES indications of argon for the “clean” surface. Carbon build up on the surface during investigations of the CO decomposition reaction could be removed by heating to 875°K. Repeated use of this technique for removing carbon from the surface however resulted in a high background in the “clean” surface LEED pattern although the AES spectrum sometimes did not indicate carbon on the surface. When such conditions were reached the crystal was ion bombarded again. The reactions that result in the decomposition of CO on the crystal surface were accelerated by impinging electron beams or by heating in CO. These reactions were therefore investigated as a function of the energy and time of exposure to the electron beams, the temperature and times for heating in CO, and the CO pressure. More details of these procedures will be given in the following sections. Flash desorption measurements were made using the RGA to monitor the CO pressure. The crystal face was turned toward the ionizer of the mass analyzer during flash desorption measurements and was about 25 cm away from the ionizer. Exposure of the surface to oxygen following a build up of carbon on the surface helped in the analysis of the flash-desorption measurements. 3. Results and discussion 3.1. CHEMISORPTIONOF CO ON A CLEAN (110) NICKEL SURFACE Although the primary concern of this report is the investigation of those reactions that take place in the adsorbed CO layer on a (110) nickel surface other than the simple chemisorption reaction, the clean (110) nickel surface and this surface covered only with chemisorbed CO molecules serve as reference for the investigation of the other reactions. For this reason a short review of our results for the chemisorption of CO on (110) nickel will be given here. A more detailed discussion of these results is planned for later publicationr4) where a comparison with the results of other LEED investigations of this system ‘9*) will be given. Following argon-ion bombardment and annealing the surface was checked for surface impurities by AES. The Auger electron spectrum for a “clean” (110) nickel surface is shown in three segments (A, B, and C) in fig. 1. The sensitivity of the AES detection system was 100 times less for segment A
DECOMPOSITIONOF CO ON (llO)Ni
215
SURFACE
than for the other spectrum segments in this figure. The small peaks around 200 and 235 eV are due to the residual argon embedded in the surface. There is also a very small carbon peak in this spectrum at N 270 eV. Carbon peaks of this size could be eliminated by argon-ion bombardment followed by annealing after the background pressure had fallen below 2 x lo-” torr. This involved long waiting times however and the carbon peak in this spectra is typical of the size that was used as a starting point for investigating the fuhrter build-up of carbon on the surface. The LEED pattern for this clean surface indicated a well-ordered (1 x 1) structure. Exposure of the clean surface to CO resulted first in streaks appearing in the LEED pattern. These streaks were seen only at low voltages (below -40 eV) and were parallel to the [lOO]-azimuthal direction. This streak pattern was not in registry with the (1 x 1) pattern from the nickel surface
I
,A x
I
I
(110)
I
NICKEL
0.01
200
400 ENERGY
600
800
(eV)
Fig. 1. Auger electron spectra for (1 IO) nickel. A, B and C are segments of the clean surface spectrum. Spectrum D was taken after the interaction of the AES beam with chemisorbed CO. Primary beam: 3 keV at 170 CIAinclined 20” to the surface.
216
H.H.MADDEN
AND G.ERTL
and was always weak and diffuse. AboveaCO pressure of 10W7tot-r a double spaced structure formed on the surface and the streak pattern could no longer be seen. This double-spaced structure was in registry with the nickel pattern and could be described as a (2 x 1) structure except for the fact that all fractional-order beams in the ill01 azimuth were missing. The (2 x 1) LEED pattern was more intense and less diffuse than the streak pattern. The diffraction spots were never as sharp as the spots in the clean surface pattern however. The (2 x 1) pattern could be observed at higher LEED energies than the streak pattern. Upon pump out of the CO the (2 x 1) structure dissappeared rapidly and the streak pattern reappeared. The streak pattern persisted for a longer period even after the pressure had fallen below 1 x 10m9 torr. The intensity of the streaks slowly decreased for pressures in the 10-lo torr range however until they were no longer visible. Provided the exposure of the crystal to CO had taken place near room temperature and the interaction of the LEEDbeam with energies >20 eV with the CO-covered surface was su~ciently limited in time, the CO could be completely removed from the surface by heating to 450°K after the pressure in the system had fallen below -4x 1O-10 torr. The Auger electron spectrum observed in such cases would not be significantly ahered from the spectrum taken before exposure to cu. An increase In work function greater than 1.3 eV occurred when the ctean surface was exposed to CO at room temperature. The change in work function was nearly complete for a CO pressure of 5 x 10e7 torr. When the crystal was exposed to CO at temperatures around 450°K or higher the changes in work function were less than 1.0 eV even for CO pressures as high as 10e4 torr. Under these conditions there was certainly a build-up of carbon on the surface, as will be discussed below, and the number of sites for the chemisorption of CO was probably reduced resulting in a smaller work function change. 3.2. INTERACTJON OFELECTRONBEAMS
WITH CHEMISORBED
CO
Since AES was used to monitor the amount of carbon and oxygen on the surface, the effects of the AES beam should be considered first. EIectrons in this beam had 3-keV energy. The beam current was - I SOpA and the energy delivered to the crystal by the AES beam caused an increase in the sample temperature of 15 to 20 Kelvin deg for a sample temperature near 300°K. Since the beam was focused to a small spot on the sample surface it might be argued that the temperature increase at the point of impact was sufficient to cause the decomposition effects observed. That this was not the complete explanation of the results, if indeed this local heating was significant, may be
DECOMPOSITIONOF
CO
ON
(llO)Ni
217
SURFACE
seen in the results obtained using the much less severe conditions provided by the LEED beam. Auger electron spectrum D in fig. 1 gives the results of measurements made after the clean surface was allowed to stand in UHV (with a total pressure <4x lo-” torr) for 120 min before the AES beam was directed onto the surface. The principal active gas in the background was CO. The actual Auger electron spectrum was recorded at a rate of 50 V/min. (The same sweep rate was used for all of our Auger electron spectra.) The AES beam was on the surface therefore for less than 20 min. The increase in the carbon peak, measured (in arbitrary units) as the distance from maximum to minimum of the differentiated AES peak, was 8 units while the increase in the oxygen peak was also 8 units. For a rough comparison, the peak height of the carbon peak in the “clean” spectrum (segment C of fig. 1) was 2 units while the large high-energy nickel peak at 850 eV was 170 units. (All AES data were taken under constant sensitivity settings for the AES measurement system.) The information contained in spectrum D of fig. 1 is summarized in table 1, row (a). The speed with which the carbon and oxygen peaks build up due to the action of the AES beam may be seen by comparing the results of row (b) with row (a). The only difference between these two sets of data was the fact that during the 120 min wait in UHV before taking the data of table 1 row (b), the AES beam was constantly striking the surface. Although the total time of interaction of the beam with the CO on the surface before the data of row (b) was recorded was many times that for row (a), the increase in the two peaks are essentially the same. In row (c) of table 1 the results of allowing the TABLE
Interaction Beam energy (eV)
g; :; g;
(g) 00 (9 (3
3000 3000 3000 3000 3000 3000 3000 40 40 80
1
of electron beams with chemisorbed Interaction time (minj \------,
PC0 04
UHV background < 20 UHV background 120 < 25 UHV background UHV background 360 1 x 10-T <5 After UHV flash off following (e) 1 x 10-e 10 1 x 10-G 20 1 x 10-B 60 1 x 10-e 20
CO Increase in AES peak size (arbitrary units) Carbon Oxygen
8
8
7 13 22 44 29 26 0 5 7
7 14 20 11 9 7 0 2 4
218
H. H. MADDEN
AND G. ERTL
sample to stand for a total period of 270 min in UHV are given. The total interaction time with the AES beam was less than 25 min in this case. The peak heights of C ond 0 had nearly doubled compared with the results of rows (a) and (b). These results indicate that the heights of the carbon and oxygen peaks observed in this situation are dependent primarily on the total amount of CO adsorbed on the surface from the background and are relatively independent of the time of interaction with the electron beam. Thus when the carbon- and oxygen-peak segments of spectrum D (fig. 1) were immediately repeated there was no change in the heights of these peaks. The effect of the AES beam on the chemisorbed CO is “instantaneous” when compared to the times involved in recording a spectrum. It might be supposed that the carbon and oxygen peaks observed in spectrum D of fig. I and the data from rows (a)-(d) of table 1, are actually an indication of the number of CO molecules chemisorbed on the surface. That this is not the case may be seen if one considers the results obtained when the crystal is heated to 450”K, and allowed to cool again to near room temperature, immediately before recording a spectrum such as in fig. 1D. (As noted in section (3.1) this treatment leads to complete desorption of undissociated adsorbed CO.) Provided the sample had been standing at room temperature in UHV without any interaction with the AES or LEED beams, the carbon and oxygen peaks would be unchanged after standing in UHV. There would be no carbon or oxygen peaks in the final Auger electron spectrum if the surface was initially clean before the start of exposure to the CO background. The same would be true if the clean surface were exposed at room temperature to a CO pressure between lo-’ and 10e4 torr (without impinging LEED or AES beams) and the flash-off at 450°K made after the background pressure has fallen below 4 x lo-” torr. In contrast to this the C- and O-peaks in fig. 1D would be unchanged after such a flash-off. An alternate supposition would be that the C- and O-peaks in fig. ID indicate CO molecules in a more tightly bound surface state and that the action of the AES beam was to promote the transition from the weakly bound state to a state of greater binding energy. That such a multiplicity of binding states for CO on (110) nickel is not the case should become evident in the discussion below. Because of the extreme sensitivity of small amounts of chemisorbed CO to the AES beam all except one of the Auger electron spectra to be discussed below were recorded after a UHV flash-off had been made. These data were taken during the investigation of electron-beam effects at CO pressures of heating in CO. above lo-’ torr and during the investigations As an illustration of the effects of exposing the CO-covered surface to the AES beam while CO was being admitted to the system consider rows (e)-(g) of table 1. For these data, the times of interaction were kept relatively
DECOMPOSITIONOF
CO
ON(~
10)Ni
SURFACE
219
short and for row (e) the spectrum was recorded while the CO pressure was at 1 x 10m7 torr. The data for row (f) were taken following those of row (e) but after a UHV flash-off as described above. The decrease in the size of the peaks in row (f) is not particularly significant because the effect of the interaction of the AES beam on the surface is highly localized. A very slight displacement of the beam from its initial point of impact can result in an appreciable change in peak size. There is an apparent discrepancy between the results for rows (f) and (g) since the net increases in the C- and O-peaks for row (f) are greater than for row (g) although the interaction time and pco were greater for row (g). This discrepancy can be explained, apart from a possible shift in beam position, by the fact that during the interaction time for row (g) the AES beam was defocused in an attempt to increase the effected area on the surface. The beam was refocused before the Auger electron spectrum was recorded so that only part of the effected area was sampled. At LEED beam energies below 100 eV and with the much smaller primary LEED beam currents (- 1 PA), the effects of this electron beam on the chemisorbed CO was much less noticeable. During observations of the (2 x 1) LEED pattern at energies between 60 and 90 eV however, one noticed a degradation of the LEED pattern in a matter of minutes. The LEED spots became weaker and the background, especially at reflectation angles close to the primary beam, increased. That the effects were localized, as with the AES beam, was seen by displacing the LEED beam slightly to an adjacent area of the surface. The LEED pattern was immediately restored to its original quality but then started to degrade again. That this was not simply a desorption or disordering of the CO could be seen by returning the LEED beam to an area of the surface where degradation of the pattern had already been observed. The pattern in this area would still be of poor quality. The build up of C- and O-peaks in the Auger electron spectrum could also be observed after interaction of the LEED beam with CO on the surface. The effects whenp,, was > 10m9torr were appreciably less than those causedbythe AES beam although the times of interaction were longer. Sample results are given in rows (h)-(j) of table 1. Direct comparison with the AES entries in table 1 would be misleading however because the impact area of the LEED beam was changed continuously during the interaction. The change was accomplished either by magnetic deflection of the primary bean or by moving the sample. This was done to ensure that the later AES probe, following UHV flash off,would intersect an area of the surface that had been effected by the LEED beam. TWO possible single-step reactions have been proposed in earlier publications to explain the effects of electron beams on CO adsorbed on nickel
220
H. H. MADDEN
AND G. ERTL
surfaces. The disproportionation reaction proposed by Edmonds and Pitkethlyi) for the (111) surface would leave CO, and carbon on the surface. In that case both C- and O-peaks should be seen in the Auger electron spectra, as we have observed when the interaction takes place in a low CO background. If CO2 on (110) nickel has a binding energy equal to or, as is probably the case, less than that of CO the AES C-peak should decrease and the O-peak dissappear after heating such a CO&-covered surface in UHV at 450%. Such changes did not occur. Alternatively, if the COZ produced in such a reaction immediately desorbed there should never be oxygen left on the surface after the reaction. The disproportionation reaction may therefore be ruled out. Another type of dissociation reaction - the dissociative desorption of CO - has been suggested by Tracy and Palmbergs) to explain LEED beam induced decomposition of CO on (100) palladium; and that work was referred to by Tracy2) in that section on CO decomposition in his recent paper on (100) nickel. Since the oxygen is ejected from the surface during this reaction it is inconsistent with the O-peaks in our Auger electron spectra. 3.3. HEATINGIN CO Some results of heating the clean crystal in CO are summarized in table 2. As previously observed by Tracy2) for (100) nickel, this treatment results in the build up of carbon on the surface with little or no oxygen present. Our results indicate that the decomposition of CO can occur at temperatures less than 400°K. The most significant changes occur above this temperature however. Considering the results in tables 1 and 2 one sees that the build up of carbon on the surface is much greater than the build up of oxygen. The TABLE 2 Heating in carbon monoxide Heating temperature (“IQ
365 365 395 420 445 565 565 650
PC0 (torr)
1.5 x 10-T 1.5 1.5 1 2 2
x x x x x
10-S 10-b 10-d 10-B 10-s
2 x 10-B 2 x 10-B
Heating time (min)
200 20 20 20 20 200 20 200
Increase in AES peak size (arbitrary units) Carbon
4 4 6 9 24 10
25 50
Oxygen
DECOMPOSITIONOF
CO
ON
(llO)Ni
SURFACE
221
exceptions to this occur only when the CO pressure is low. In these cases the C- and O-peaks that appear in the Auger electron spectra as a result of the interaction of the electron beam with CO chemisorbed from the background are of roughly equal size. This result suggests that the primary reaction resulting from the interaction of the electron beam with chemisorbed CO, or from heating in CO, is dissociation. The resulting carbon and oxygen atoms remain individually bound to the substrate even above temperatures which lead to desorption of undissociated CO. The much smaller (or non-existent) O-peaks that result when this dissociation takes place in the presence of a CO-rich gas phase are the result of the combination of surface oxygen with CO from the gas phase. The CO, thus formed immediately desorbs. Since the second supposition would mean that CO, should appear in the gas phase when the crystal is heated in CO, an attempt was made to measure this COZ production. Monitoring both the CO and CO, peaks with the RGA, CO was introduced into the system at a constant pressure of low6 torr. With the crystal at room temperature the CO, pressure was 3.5 x 10-r’ torr. Most of this CO, was probably produced in the ionizer of the RGA. As the temperature of the sample was increased in steps to a maximum above 875 “K the CO, pressure increased as shown in fig. 2. Because the active surface area was small (N 0.3 cm’, and presumably becoming effectively smaller with the gradual build up of carbon on the surface) and its distance from the ionizer of the RCA relatively large, the sensitivity to CO2 production at the surface was not very great. A guide or channel for desorption reaction products, with which Onchi and Farnsworthd) were able to increase their system’s sensitivity by loo%, was not used in our system and therefore these COZ production results are complicated by the possible CO, production at the crystal supports. In a macroscopic study Gregg and Leachis) investi100
Sample
Fig, 2.
Temperature
CO2 production
[“K]
during heating in CO.
222
H.H.MADDEN
AND G.ERTL
gated the interaction between Ni sheets and CO and found that the main reaction at temperatures between 550 and 1000°K is the disproportionation 2co+C+co2. 3.4. CO
DESORPTION FROM (1 lo)
NICKEL
During CO desorption investigations the partial pressure of CO, measured with the RGA, was plotted on an XY-recorder against the thermocouple voltage. From published results for CO-desorption from nickela 4~Ii), three peaks were expected in the desorption spectra. The peak occurring at the lowest temperature is usually designated the cr peak while the two peaks that occur with increasing desorption temperatures are termed the /3i and p2 peaks, respectively. Although three such peaks were seen in our desorption spectra following room temperature exposure to CO, the l3 peaks were always much smaller than the c1peak. An example of such a spectrum is shown in plot A of fig. 3. It was further found that the b peaks were observed only after the interaction of electron beams with chemisorbed CO had caused a carbon and oxygen build up in the surface. When a clean surface was exposed at room temperature to CO and a subsequent CO
Fig. 3. CO desorption spectra. Average heating rates are given under the horizontal line segments indicating starting CO pressure levels. The steps at the very beginning of these spectra were due to desorption of CO from the heating wires which heated up quickly. The spectra have all been normalized so that the height of the largest peak in each is equal to an arbitrary set value.
DECOMPOSITIONOF
CO
ON (llO)Ni
SURFACE
223
desorption spectrum recorded, only the a peak appeared. The b peaks seemed to appear only when there was an indication of carbon and oxygen on the surface prior to CO exposure and desorption. For the system (110) nickel/CO, Degras3) has reported three binding states and has given a somewhat complicated scheme by which the kinetics of CO adsorption on this surface are explained. Exposing his sample at 300°K to a constant CO pressure of 2 x lo-* torr, Degras found significant reorganization of the three states occurring even after exposure times as long as five hours. In an attempt to see if such reorganization could be accelerated thermally thereby increasing the population of the l3 states from the a state, our clean (110) nickel sample was repeatedly exposed to CO and desorption spectra taken with average heating rates ranging from 8 to 68 (K”)/sec. Two such spectra are seen in plots B and C of fig. 3. In no case did b peaks appear in the spectra. Because the presence of oxygen and carbon on the surface seemed to be necessary for the appearence of the B peaks in the CO desorption spectra, the crystal was intentionally exposed to oxygen following a build up of carbon on the surface. Typical results of CO desorption following such treatment are seen in plots D and E of fig. 3. Before spectrum D was recorded the AES C-peak had first been built up by heating in CO to a height of 32 units based on the arbitrary scale used above. The crystal surface was next given an oxygen exposure of 12 Langmuir. The AES C- and O-peaks following this oxygen exposure were 23 and 40 units, respectively. After the background pressure had fallen to 5 x lo- lo torr, CO desorption spectrum D was recorded. Before spectrum E was recorded a similar O2 exposure was made. The AES C-peak was 16 units before the O,-exposure of 3 Langmuir was given. No AES data was taken between the 0, exposure and recording the desorption spectrum. Desorption spectra D and E clearly indicate the importance of oxygen to the appearance of the B peaks. (The small size of the a peak in spectra D is due to heating in UHV at 450°K shortly before this spectrum was recorded.) Two aspects of these spectra are especially noteworthy. First, the position of the b1 peak in spectrum D occurs at a lower temperature than the B1 peak of spectrum E, while the BZ peaks of both spectra occur roughly the same temperature. The pi peak seemed to occur at lower temperature, the more oxygen and carbon there was on the surface prior to the desorption. Second, the ratio of the height of the BZ peak to that of the B1 peak is greater for spectrum D than for E. Here the initial amount of carbon and its form on the surface is the crucial factor. Graphite is known to form on this surface12,ls). Following the heating in CO that resulted in the data given in the bottom row of table 2, a weak graphite ring LEED pattern was observed together
224
H. H.
MADDEN
AND G. ERTL
with a diffuse streak pattern. The streaks, which are presumably due to dispersed carbon atoms on the surface, were in registry with the LEED pattern from the nickel substrate. A similar LEED pattern had been observed as an intermediate stage during the decomposition of ethylene on Ni(ll0) prior to the appearance of the graphite ring-pattern”). After a short UHV tempering at 725 “K the streaks had dissappeared and the graphite ring pattern become stronger and sharper. That the carbon on the surface was not uniformly distributed could be seen from the large sensitivity of the AES C-peak to positioning of the AES beam. The surface was then exposed to oxygen for 3 Langmuir. The resulting LEED pattern consisted of both the graphite ring pattern and a p(2 x 1) pattern from the oxygen. This indicated that the graphite and oxygen existed on the surface in domains. In the CO desorption spectrum from this surface the & peak was more than twice the size of the p1 peak. The LEED pattern following the CO desorption was a sharp (1 x 1) pattern. These results are consistent with a scheme whereby the p peaks that we observe are not related to CO chemisorbed on the surface but are the result of the presence of carbon and oxygen atoms on the surface prior to desorption heating. The carbon on the surface can presumably exist as mobile atoms or in clusters. The p1 peak, which shifts in temperature position, is consistent with a bimolecular reaction between mobile carbon and oxygen atoms. The more atomic carbon and oxygen on the surface prior to heating, the lower should be the temperature at which the p1 peak occurs. The pZ peak results from the interaction of mobile oxygen atoms with graphite nuclei. Since this reaction would take place at the relatively fixed boundaries of the carbon clusters it would be first order and the & peak would not be expected to shift in temperature. 4. Summary At 300°K carbon monoxide adsorbs reversibly on a (110) nickel surface. The CO remaining on the surface after the gas phase pressure has fallen below 4 x 10-l’ torr can be removed by heating to 450°K. The interaction of electron beams with chemisorbed CO, and heating the crystal in CO, leads to the build up of a carbon impurity on the surface. The surface reactions that are important in this build up are :
(1) cod + Gd + O,, (dissociation), (2)
O,, + CO,,, -+ COZ, gas(surface reduction).
In CO flash desorption
spectra from the (110) nickel surface the high
CO ON (llO)Ni
DECOMPOSITIONOF
SURFACE
225
temperature p peaks are due to the interaction of mobile oxygen atoms on the surface with surface carbon either atomically dispersed or in graphite clusters. Only the a peak is related to molecular CO chemisorbed on the surface. The reactions leading to the desorption products in the three peaks are: a peak:
CO,, -+ CO,,, ,
PI peak:
Cad + Q,, -+ CO,,, ,
PZ peak:
Gti.
grapb.+ Oad -+ CO,,, .
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
T. Edmonds and R. C. Pitkethly, Surface Sci. 15 (1969) 137. J. C. Tracy, J. Chem. Phys. 56 (1972) 2736. D. A. Degras, Nuovo Cimento Suppl. 5 (1967) 408. M. Onchi and H. E. Farnsworth, Surface Sci. 11 (1968) 203. C. W. Tucker, Jr., Surface Sci. 2 (1964) 516. P. W. Palmberg, in: The Stru~t~e and Chemistry ofSolid Szafaces, EZd.G. A. Somorjai (Wiley, New York, 1969) p. 29-l. L. H. Germer and A. U. MacRae, Proc. Natl. Acad. Sci. (USA) 48 (1962) 997. R. L. Park and H. E. Farnsworth, J. Chem. Phys. 40 (1964) 2354. J. C. Tracy and P. W. Palmberg, J. Chem. Phys. 51 (1969) 4852. S. J. Gregg and H. F. Leach, J. Catalysis 6 (1966) 308. G. Ertl and J. Ktippers, Surface Sci. 24 (1971) 104. G. Ertl, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis et al. (McGrawHill, New York, 1969) pp. 147-165. E. N. Sickafus and H. P. Bonzel, in: Recent Progress in Surface Science, Eds. J. F. Danielli et al. {Academic Press, New York, 1970). H. H. Madden, 3. Ktlppers and G. Ertl, to be published in J. Chem. Phys.
Discussion D. A. KING (Univ. of East Anglia, Norwich) You have shown convincing evidence to indicate that an electron beam can interact with a CO adlayer on Ni(l10) to produce a strongly bound species, possibly dissociatively adsorbed, which only desorbs at relatively high temperatures. Further, you have shown that the C/O ratio on the surface increases with increasing exposure to the electron beam These two factors could readily be attributed to a single mechanism. A Frank-Condon transition from the ground state (a-CO) adsorbed molecule to excited or ionic states may be induced by the electron beam, but, as shown by Redhead and by Menzel and Gomer in 1964, re-neutralization of these states is an efficient process. The measured cross section for electron stimulated desorption is thus low compared with analogous gas phase processes. In your case, reneutrali~tion does not apparently always result in r~o~titution of the original a-CO molecule; the B-states are formed instead. If, further, we suggest that O+, and not C+ ions are desorbed during this process, a corollary of the electron beam interaction with the adlayer is an increasing C/O ratio on the surface. The escape of Of ions only is in accord with results on W surfaces, and with the generally accepted model in which the a-CO molecule is bound to the surface through the C, and not the 0, atom. Your explanation for the accumulation of C on the surface in terms of the interaction of CO gas phase molecules with 0 atoms is not in accord with results published by Hoogan and King for the interaction of CO with Oa on nickel films. The presence of gas phase
226
H. H. MADDENAND
G. ERTL
CO may be required simply to maintain a sufficiently high CO coverage on the surface for electron impact desorption effects to be appreciable. H. H. MADDEN The two aspects of our results that most led us to propose the two steps scheme for the build-up of carbon on the surface were: (1) The fact that the increase in the C/O ratio only occurred when the CO gas pressure was high (> 1O-g torr); and (2) the CO-flash desorption results that were obtained by intentionally adding oxygen to a surface with a large carbon contamination. The dissociative desorption reaction leading to the ejection of 0+ from the surface is not consistent with (1). The transition mechanism you suggest for the electron-beam induced changes in the adsorbed CO-layer does not seem to be substantiated or denied by our results. Whether the dissociated carbon and oxygen left on the surface alter the electron-beam interaction with chemisorbed CO (in the UHV-background conditions) may be regarded as a more tightly bound CO-state, is a matter of definition. Applying such a definition in the case where graphite and oxygen existing in separate domains on the surfaces give rise to the p-peaks would certainly be stretching it too far. A direct comparison of results obtained on tungsten with those obtained on nickel may be misleading. A comparison with results for the interaction of CO with 0~ on nickel may also be misleading, since in this case oxidation of the Ni crystal competes with the COZ formation14). B. GOLDSTEIN(R.C.A. Laboratories, Princeton N.J.) Did you note any electron-beam-enhanced desorption your experimental conditions.
from the surface under any of
H. H. MADDEN Direct observation of electron-beam-stimulated desorption products was not possible in our apparatus. The indirect observation that might bear on this question was that of the weakening of the LEED beam intensities when the (2 x I)-CO structure was examined using LEED beam energies >20-30 eV. This degradation of the LEED pattern was related, at least in part, to the build up of carbon on the surfaces and therefore a meaningful statement on accompanying desorption can not be made.
SCIENCE 35 (1973) 21 l-226 0 North-Holland
DECOMPOSITION
Publishing Co.
OF CARBON MONOXIDE
ON A (110) NICKEL SURFACE H. H, MADDEN* Department ofPhysics, Wayne State University, Detroit, Michigan 48202, U.S.A.
G. ERTL Institut fiir Physikalische Chemie, Technische Umiversitiit, Hannover, Germany New inv~ti~tions of the (110) nickeIl~rbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1 x 10-r torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.
1. Introduction The chemisorption of carbon monoxide on a (110) nickel surface has recently been investigated in our laboratory using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It became apparent during this project that the interaction of impinging electron beams with chemisorbed CO, and also the exposure of the heated surface to CO, lead to surface reactions other than the simple adsorption/desorption of CO. Because these spurious reactions led to the build up of surface impurities that could influence the CO chemisorption, an attempt was made to define these reactions and the conditions under which their rates become significant. The * Visiting Professor at the Technische through August 1972).
Universit~t,
211
Hannover,
Germany
(August 1971
212
H. H. MADDEN
AND G. ERTL
results were then used to formulate correct chemisorption of CO on (110) nickel.
procedures
for our study of the
LEED-beam induced changes in the chemisorbed CO phase on (111) nickel have been investigated by Edmonds and Pitkethlyr). They found that continuous monitoring of the surface during exposure to CO, with LEED beam energies between 40 and 100 eV, resulted in a different sequence of multiply-spaced LEED structures than were observed when they restricted the exposure of the CO-covered surface to the LEED beam to less than 17% of the total time of observation. Of the possible surface reactions (besides chemisorption) that can occur when CO interacts with the nickel surface at room temperature, they concluded that disproportionation leading to CO, and C adsorbed at separate sites on the surface was the reaction stimulated by the LEED beam. Their conclusions were based on their LEED investigations of the structures that form on (111) nickel due to exposures to CO and to COz, and also on arguments relating to the energetics of the disproportionnation reaction in comparison to two other possible reactions. The other reactions considered were carbonyl formation and dissociation. Our conclusion based on LEED, AES and CO-flash desorption measurements is that the dissociation reaction, leading to atomically dispersed carbon and oxygen on the surface, is the principal electron-beam stimulated surface reaction for CO chemisorbed on (110) nickel. The interaction of (100) nickel with CO has recently been investigated by Tracy 2, using both AES and LEED. He found that the LEED beam rapidly caused alterations in the LEED pattern and a build up of carbon, but no oxygen, on the surface when beam energies greater than 1.5 to 20 eV were used. Tracy found further that the heating of the sample in CO at temperatures greater than 475°K also resulted in a build up of carbon on the surface. The primary aim in our investigation of the (I 10 Ni/CO) system was a determination of the isosteric heat of adsorption using experimental techniques similar to those employed by Tracy. Thus, the effects of heating in CO as well as the interaction of the LEED and AES beams with adsorbed CO were expected to be important. Another consideration in our investigations was whether there is more than one binding state for CO on (110) nickel. Three chemisorbed states for CO on this surface have been reported from flash-desorption results3). Onchi and Farnsworth”) also found three CO desorption peaks for the (100) Ni/CO system and could correlate the three peaks with changes in their LEED patterns. Our present results indicate however that the higher temperature COdesorption peaks are intimately related to the presence of dispersed carbon and oxygen on the surface, and hence not simply related to the chemisorption of CO on (110) nickel.
DECOMPOSITIONOF
CO
ON(I
10)Ni
SURFACE
213
LEED-beam induced changes in a chemisorbed CO layer have also been reported for (100) platinum by Tuckers). For this same system Palmberg6) has found that impinging electron beams during LEED or AES measurements can result in decomposition of the adsorbed CO. It thus appears that some CO chemisorption studies performed without a means of monitoring the surface chemical composition may be in error because of these spurious reactions. A purpose in our work was to determine not only a “recipe” for minimizing the effects of these undesired reactions but also to determine actually what reactions do take place. 2. Apparatus and procedures Our investigations were carried out in a 4-grid Varian Model 120 LEED/ AES system. The base pressure for the system was about 2 x lo-” torr. The AES beam was supplied by an off-axis electron gun and the angle of incidence of the primary beam during AES measurements was 20” to the plane of the sample surface. An EAI Quad 150A residual gas analyzer (RGA) was mounted on the system so that a minimum distance of about 25 cm existed between the sample surface and the ionizer of the mass analyzer. Gases of spectral-grade purity could be admitted to the system from glass supply flasks through Varian variable leak valves. Changes in work function of the sample were measured using a self-compensating Kelvin-method vibratingcapacitor system. The reference electrode was an oxidized tantalum plate. The (110) nickel surface was prepared, after initial X-ray orientation, by polishing with finer and finer abrasives until a mirror-like finish was obtained with a 0.25 urn grain size. Following the polishing the crystal was washed in distilled water and acetone. The crystal was supported in the vacuum system by suspending it between two horizontal 0.3-mm diameter tungsten wires. These wires ran through slots cut in the upper and lower edges of the crystal by spark erosion. The crystal was heated indirectly by conduction from these tungsten wires through which an AC heating current was sent. The tungsten heating wires were under slight tension supplied by attachment at their ends to the more massive stainless steel part of the crystal holder. The temperature of the crystal was measured with a Pt/PtRh thermocouple that had been spot-welded to the back edge of the crystal before the final front-surface preparation. The cleaning of the crystal surface in vacuum, following initial outgassing at - 1300”K, was carried out by ion bombardment with 400-eV argon ions followed by annealing at 875°K. The major surface impurity present after the high temperature heating was sulfur but this was easily removed by the ion bombardment. After ion bombardment the major impurities were carbon
214
H. H. MADDEN
AND G. ERTL
and argon. The carbon and most of the argon were removed face during annealing. Attempts to remove all of the argon
from the surby heating to
higher temperatures during annealing resulted in the reappearance of sulfur. For this reason our “clean” surface had a small amount of argon embedded in it. Ion bombardment at 200 eV was sufficient to remove sulfur from the surface but did not appreciably affect the AES indications of argon for the “clean” surface. Carbon build up on the surface during investigations of the CO decomposition reaction could be removed by heating to 875°K. Repeated use of this technique for removing carbon from the surface however resulted in a high background in the “clean” surface LEED pattern although the AES spectrum sometimes did not indicate carbon on the surface. When such conditions were reached the crystal was ion bombarded again. The reactions that result in the decomposition of CO on the crystal surface were accelerated by impinging electron beams or by heating in CO. These reactions were therefore investigated as a function of the energy and time of exposure to the electron beams, the temperature and times for heating in CO, and the CO pressure. More details of these procedures will be given in the following sections. Flash desorption measurements were made using the RGA to monitor the CO pressure. The crystal face was turned toward the ionizer of the mass analyzer during flash desorption measurements and was about 25 cm away from the ionizer. Exposure of the surface to oxygen following a build up of carbon on the surface helped in the analysis of the flash-desorption measurements. 3. Results and discussion 3.1. CHEMISORPTIONOF CO ON A CLEAN (110) NICKEL SURFACE Although the primary concern of this report is the investigation of those reactions that take place in the adsorbed CO layer on a (110) nickel surface other than the simple chemisorption reaction, the clean (110) nickel surface and this surface covered only with chemisorbed CO molecules serve as reference for the investigation of the other reactions. For this reason a short review of our results for the chemisorption of CO on (110) nickel will be given here. A more detailed discussion of these results is planned for later publicationr4) where a comparison with the results of other LEED investigations of this system ‘9*) will be given. Following argon-ion bombardment and annealing the surface was checked for surface impurities by AES. The Auger electron spectrum for a “clean” (110) nickel surface is shown in three segments (A, B, and C) in fig. 1. The sensitivity of the AES detection system was 100 times less for segment A
DECOMPOSITIONOF CO ON (llO)Ni
215
SURFACE
than for the other spectrum segments in this figure. The small peaks around 200 and 235 eV are due to the residual argon embedded in the surface. There is also a very small carbon peak in this spectrum at N 270 eV. Carbon peaks of this size could be eliminated by argon-ion bombardment followed by annealing after the background pressure had fallen below 2 x lo-” torr. This involved long waiting times however and the carbon peak in this spectra is typical of the size that was used as a starting point for investigating the fuhrter build-up of carbon on the surface. The LEED pattern for this clean surface indicated a well-ordered (1 x 1) structure. Exposure of the clean surface to CO resulted first in streaks appearing in the LEED pattern. These streaks were seen only at low voltages (below -40 eV) and were parallel to the [lOO]-azimuthal direction. This streak pattern was not in registry with the (1 x 1) pattern from the nickel surface
I
,A x
I
I
(110)
I
NICKEL
0.01
200
400 ENERGY
600
800
(eV)
Fig. 1. Auger electron spectra for (1 IO) nickel. A, B and C are segments of the clean surface spectrum. Spectrum D was taken after the interaction of the AES beam with chemisorbed CO. Primary beam: 3 keV at 170 CIAinclined 20” to the surface.
216
H.H.MADDEN
AND G.ERTL
and was always weak and diffuse. AboveaCO pressure of 10W7tot-r a double spaced structure formed on the surface and the streak pattern could no longer be seen. This double-spaced structure was in registry with the nickel pattern and could be described as a (2 x 1) structure except for the fact that all fractional-order beams in the ill01 azimuth were missing. The (2 x 1) LEED pattern was more intense and less diffuse than the streak pattern. The diffraction spots were never as sharp as the spots in the clean surface pattern however. The (2 x 1) pattern could be observed at higher LEED energies than the streak pattern. Upon pump out of the CO the (2 x 1) structure dissappeared rapidly and the streak pattern reappeared. The streak pattern persisted for a longer period even after the pressure had fallen below 1 x 10m9 torr. The intensity of the streaks slowly decreased for pressures in the 10-lo torr range however until they were no longer visible. Provided the exposure of the crystal to CO had taken place near room temperature and the interaction of the LEEDbeam with energies >20 eV with the CO-covered surface was su~ciently limited in time, the CO could be completely removed from the surface by heating to 450°K after the pressure in the system had fallen below -4x 1O-10 torr. The Auger electron spectrum observed in such cases would not be significantly ahered from the spectrum taken before exposure to cu. An increase In work function greater than 1.3 eV occurred when the ctean surface was exposed to CO at room temperature. The change in work function was nearly complete for a CO pressure of 5 x 10e7 torr. When the crystal was exposed to CO at temperatures around 450°K or higher the changes in work function were less than 1.0 eV even for CO pressures as high as 10e4 torr. Under these conditions there was certainly a build-up of carbon on the surface, as will be discussed below, and the number of sites for the chemisorption of CO was probably reduced resulting in a smaller work function change. 3.2. INTERACTJON OFELECTRONBEAMS
WITH CHEMISORBED
CO
Since AES was used to monitor the amount of carbon and oxygen on the surface, the effects of the AES beam should be considered first. EIectrons in this beam had 3-keV energy. The beam current was - I SOpA and the energy delivered to the crystal by the AES beam caused an increase in the sample temperature of 15 to 20 Kelvin deg for a sample temperature near 300°K. Since the beam was focused to a small spot on the sample surface it might be argued that the temperature increase at the point of impact was sufficient to cause the decomposition effects observed. That this was not the complete explanation of the results, if indeed this local heating was significant, may be
DECOMPOSITIONOF
CO
ON
(llO)Ni
217
SURFACE
seen in the results obtained using the much less severe conditions provided by the LEED beam. Auger electron spectrum D in fig. 1 gives the results of measurements made after the clean surface was allowed to stand in UHV (with a total pressure <4x lo-” torr) for 120 min before the AES beam was directed onto the surface. The principal active gas in the background was CO. The actual Auger electron spectrum was recorded at a rate of 50 V/min. (The same sweep rate was used for all of our Auger electron spectra.) The AES beam was on the surface therefore for less than 20 min. The increase in the carbon peak, measured (in arbitrary units) as the distance from maximum to minimum of the differentiated AES peak, was 8 units while the increase in the oxygen peak was also 8 units. For a rough comparison, the peak height of the carbon peak in the “clean” spectrum (segment C of fig. 1) was 2 units while the large high-energy nickel peak at 850 eV was 170 units. (All AES data were taken under constant sensitivity settings for the AES measurement system.) The information contained in spectrum D of fig. 1 is summarized in table 1, row (a). The speed with which the carbon and oxygen peaks build up due to the action of the AES beam may be seen by comparing the results of row (b) with row (a). The only difference between these two sets of data was the fact that during the 120 min wait in UHV before taking the data of table 1 row (b), the AES beam was constantly striking the surface. Although the total time of interaction of the beam with the CO on the surface before the data of row (b) was recorded was many times that for row (a), the increase in the two peaks are essentially the same. In row (c) of table 1 the results of allowing the TABLE
Interaction Beam energy (eV)
g; :; g;
(g) 00 (9 (3
3000 3000 3000 3000 3000 3000 3000 40 40 80
1
of electron beams with chemisorbed Interaction time (minj \------,
PC0 04
UHV background < 20 UHV background 120 < 25 UHV background UHV background 360 1 x 10-T <5 After UHV flash off following (e) 1 x 10-e 10 1 x 10-G 20 1 x 10-B 60 1 x 10-e 20
CO Increase in AES peak size (arbitrary units) Carbon Oxygen
8
8
7 13 22 44 29 26 0 5 7
7 14 20 11 9 7 0 2 4
218
H. H. MADDEN
AND G. ERTL
sample to stand for a total period of 270 min in UHV are given. The total interaction time with the AES beam was less than 25 min in this case. The peak heights of C ond 0 had nearly doubled compared with the results of rows (a) and (b). These results indicate that the heights of the carbon and oxygen peaks observed in this situation are dependent primarily on the total amount of CO adsorbed on the surface from the background and are relatively independent of the time of interaction with the electron beam. Thus when the carbon- and oxygen-peak segments of spectrum D (fig. 1) were immediately repeated there was no change in the heights of these peaks. The effect of the AES beam on the chemisorbed CO is “instantaneous” when compared to the times involved in recording a spectrum. It might be supposed that the carbon and oxygen peaks observed in spectrum D of fig. I and the data from rows (a)-(d) of table 1, are actually an indication of the number of CO molecules chemisorbed on the surface. That this is not the case may be seen if one considers the results obtained when the crystal is heated to 450”K, and allowed to cool again to near room temperature, immediately before recording a spectrum such as in fig. 1D. (As noted in section (3.1) this treatment leads to complete desorption of undissociated adsorbed CO.) Provided the sample had been standing at room temperature in UHV without any interaction with the AES or LEED beams, the carbon and oxygen peaks would be unchanged after standing in UHV. There would be no carbon or oxygen peaks in the final Auger electron spectrum if the surface was initially clean before the start of exposure to the CO background. The same would be true if the clean surface were exposed at room temperature to a CO pressure between lo-’ and 10e4 torr (without impinging LEED or AES beams) and the flash-off at 450°K made after the background pressure has fallen below 4 x lo-” torr. In contrast to this the C- and O-peaks in fig. 1D would be unchanged after such a flash-off. An alternate supposition would be that the C- and O-peaks in fig. ID indicate CO molecules in a more tightly bound surface state and that the action of the AES beam was to promote the transition from the weakly bound state to a state of greater binding energy. That such a multiplicity of binding states for CO on (110) nickel is not the case should become evident in the discussion below. Because of the extreme sensitivity of small amounts of chemisorbed CO to the AES beam all except one of the Auger electron spectra to be discussed below were recorded after a UHV flash-off had been made. These data were taken during the investigation of electron-beam effects at CO pressures of heating in CO. above lo-’ torr and during the investigations As an illustration of the effects of exposing the CO-covered surface to the AES beam while CO was being admitted to the system consider rows (e)-(g) of table 1. For these data, the times of interaction were kept relatively
DECOMPOSITIONOF
CO
ON(~
10)Ni
SURFACE
219
short and for row (e) the spectrum was recorded while the CO pressure was at 1 x 10m7 torr. The data for row (f) were taken following those of row (e) but after a UHV flash-off as described above. The decrease in the size of the peaks in row (f) is not particularly significant because the effect of the interaction of the AES beam on the surface is highly localized. A very slight displacement of the beam from its initial point of impact can result in an appreciable change in peak size. There is an apparent discrepancy between the results for rows (f) and (g) since the net increases in the C- and O-peaks for row (f) are greater than for row (g) although the interaction time and pco were greater for row (g). This discrepancy can be explained, apart from a possible shift in beam position, by the fact that during the interaction time for row (g) the AES beam was defocused in an attempt to increase the effected area on the surface. The beam was refocused before the Auger electron spectrum was recorded so that only part of the effected area was sampled. At LEED beam energies below 100 eV and with the much smaller primary LEED beam currents (- 1 PA), the effects of this electron beam on the chemisorbed CO was much less noticeable. During observations of the (2 x 1) LEED pattern at energies between 60 and 90 eV however, one noticed a degradation of the LEED pattern in a matter of minutes. The LEED spots became weaker and the background, especially at reflectation angles close to the primary beam, increased. That the effects were localized, as with the AES beam, was seen by displacing the LEED beam slightly to an adjacent area of the surface. The LEED pattern was immediately restored to its original quality but then started to degrade again. That this was not simply a desorption or disordering of the CO could be seen by returning the LEED beam to an area of the surface where degradation of the pattern had already been observed. The pattern in this area would still be of poor quality. The build up of C- and O-peaks in the Auger electron spectrum could also be observed after interaction of the LEED beam with CO on the surface. The effects whenp,, was > 10m9torr were appreciably less than those causedbythe AES beam although the times of interaction were longer. Sample results are given in rows (h)-(j) of table 1. Direct comparison with the AES entries in table 1 would be misleading however because the impact area of the LEED beam was changed continuously during the interaction. The change was accomplished either by magnetic deflection of the primary bean or by moving the sample. This was done to ensure that the later AES probe, following UHV flash off,would intersect an area of the surface that had been effected by the LEED beam. TWO possible single-step reactions have been proposed in earlier publications to explain the effects of electron beams on CO adsorbed on nickel
220
H. H. MADDEN
AND G. ERTL
surfaces. The disproportionation reaction proposed by Edmonds and Pitkethlyi) for the (111) surface would leave CO, and carbon on the surface. In that case both C- and O-peaks should be seen in the Auger electron spectra, as we have observed when the interaction takes place in a low CO background. If CO2 on (110) nickel has a binding energy equal to or, as is probably the case, less than that of CO the AES C-peak should decrease and the O-peak dissappear after heating such a CO&-covered surface in UHV at 450%. Such changes did not occur. Alternatively, if the COZ produced in such a reaction immediately desorbed there should never be oxygen left on the surface after the reaction. The disproportionation reaction may therefore be ruled out. Another type of dissociation reaction - the dissociative desorption of CO - has been suggested by Tracy and Palmbergs) to explain LEED beam induced decomposition of CO on (100) palladium; and that work was referred to by Tracy2) in that section on CO decomposition in his recent paper on (100) nickel. Since the oxygen is ejected from the surface during this reaction it is inconsistent with the O-peaks in our Auger electron spectra. 3.3. HEATINGIN CO Some results of heating the clean crystal in CO are summarized in table 2. As previously observed by Tracy2) for (100) nickel, this treatment results in the build up of carbon on the surface with little or no oxygen present. Our results indicate that the decomposition of CO can occur at temperatures less than 400°K. The most significant changes occur above this temperature however. Considering the results in tables 1 and 2 one sees that the build up of carbon on the surface is much greater than the build up of oxygen. The TABLE 2 Heating in carbon monoxide Heating temperature (“IQ
365 365 395 420 445 565 565 650
PC0 (torr)
1.5 x 10-T 1.5 1.5 1 2 2
x x x x x
10-S 10-b 10-d 10-B 10-s
2 x 10-B 2 x 10-B
Heating time (min)
200 20 20 20 20 200 20 200
Increase in AES peak size (arbitrary units) Carbon
4 4 6 9 24 10
25 50
Oxygen
DECOMPOSITIONOF
CO
ON
(llO)Ni
SURFACE
221
exceptions to this occur only when the CO pressure is low. In these cases the C- and O-peaks that appear in the Auger electron spectra as a result of the interaction of the electron beam with CO chemisorbed from the background are of roughly equal size. This result suggests that the primary reaction resulting from the interaction of the electron beam with chemisorbed CO, or from heating in CO, is dissociation. The resulting carbon and oxygen atoms remain individually bound to the substrate even above temperatures which lead to desorption of undissociated CO. The much smaller (or non-existent) O-peaks that result when this dissociation takes place in the presence of a CO-rich gas phase are the result of the combination of surface oxygen with CO from the gas phase. The CO, thus formed immediately desorbs. Since the second supposition would mean that CO, should appear in the gas phase when the crystal is heated in CO, an attempt was made to measure this COZ production. Monitoring both the CO and CO, peaks with the RGA, CO was introduced into the system at a constant pressure of low6 torr. With the crystal at room temperature the CO, pressure was 3.5 x 10-r’ torr. Most of this CO, was probably produced in the ionizer of the RGA. As the temperature of the sample was increased in steps to a maximum above 875 “K the CO, pressure increased as shown in fig. 2. Because the active surface area was small (N 0.3 cm’, and presumably becoming effectively smaller with the gradual build up of carbon on the surface) and its distance from the ionizer of the RCA relatively large, the sensitivity to CO2 production at the surface was not very great. A guide or channel for desorption reaction products, with which Onchi and Farnsworthd) were able to increase their system’s sensitivity by loo%, was not used in our system and therefore these COZ production results are complicated by the possible CO, production at the crystal supports. In a macroscopic study Gregg and Leachis) investi100
Sample
Fig, 2.
Temperature
CO2 production
[“K]
during heating in CO.
222
H.H.MADDEN
AND G.ERTL
gated the interaction between Ni sheets and CO and found that the main reaction at temperatures between 550 and 1000°K is the disproportionation 2co+C+co2. 3.4. CO
DESORPTION FROM (1 lo)
NICKEL
During CO desorption investigations the partial pressure of CO, measured with the RGA, was plotted on an XY-recorder against the thermocouple voltage. From published results for CO-desorption from nickela 4~Ii), three peaks were expected in the desorption spectra. The peak occurring at the lowest temperature is usually designated the cr peak while the two peaks that occur with increasing desorption temperatures are termed the /3i and p2 peaks, respectively. Although three such peaks were seen in our desorption spectra following room temperature exposure to CO, the l3 peaks were always much smaller than the c1peak. An example of such a spectrum is shown in plot A of fig. 3. It was further found that the b peaks were observed only after the interaction of electron beams with chemisorbed CO had caused a carbon and oxygen build up in the surface. When a clean surface was exposed at room temperature to CO and a subsequent CO
Fig. 3. CO desorption spectra. Average heating rates are given under the horizontal line segments indicating starting CO pressure levels. The steps at the very beginning of these spectra were due to desorption of CO from the heating wires which heated up quickly. The spectra have all been normalized so that the height of the largest peak in each is equal to an arbitrary set value.
DECOMPOSITIONOF
CO
ON (llO)Ni
SURFACE
223
desorption spectrum recorded, only the a peak appeared. The b peaks seemed to appear only when there was an indication of carbon and oxygen on the surface prior to CO exposure and desorption. For the system (110) nickel/CO, Degras3) has reported three binding states and has given a somewhat complicated scheme by which the kinetics of CO adsorption on this surface are explained. Exposing his sample at 300°K to a constant CO pressure of 2 x lo-* torr, Degras found significant reorganization of the three states occurring even after exposure times as long as five hours. In an attempt to see if such reorganization could be accelerated thermally thereby increasing the population of the l3 states from the a state, our clean (110) nickel sample was repeatedly exposed to CO and desorption spectra taken with average heating rates ranging from 8 to 68 (K”)/sec. Two such spectra are seen in plots B and C of fig. 3. In no case did b peaks appear in the spectra. Because the presence of oxygen and carbon on the surface seemed to be necessary for the appearence of the B peaks in the CO desorption spectra, the crystal was intentionally exposed to oxygen following a build up of carbon on the surface. Typical results of CO desorption following such treatment are seen in plots D and E of fig. 3. Before spectrum D was recorded the AES C-peak had first been built up by heating in CO to a height of 32 units based on the arbitrary scale used above. The crystal surface was next given an oxygen exposure of 12 Langmuir. The AES C- and O-peaks following this oxygen exposure were 23 and 40 units, respectively. After the background pressure had fallen to 5 x lo- lo torr, CO desorption spectrum D was recorded. Before spectrum E was recorded a similar O2 exposure was made. The AES C-peak was 16 units before the O,-exposure of 3 Langmuir was given. No AES data was taken between the 0, exposure and recording the desorption spectrum. Desorption spectra D and E clearly indicate the importance of oxygen to the appearance of the B peaks. (The small size of the a peak in spectra D is due to heating in UHV at 450°K shortly before this spectrum was recorded.) Two aspects of these spectra are especially noteworthy. First, the position of the b1 peak in spectrum D occurs at a lower temperature than the B1 peak of spectrum E, while the BZ peaks of both spectra occur roughly the same temperature. The pi peak seemed to occur at lower temperature, the more oxygen and carbon there was on the surface prior to the desorption. Second, the ratio of the height of the BZ peak to that of the B1 peak is greater for spectrum D than for E. Here the initial amount of carbon and its form on the surface is the crucial factor. Graphite is known to form on this surface12,ls). Following the heating in CO that resulted in the data given in the bottom row of table 2, a weak graphite ring LEED pattern was observed together
224
H. H.
MADDEN
AND G. ERTL
with a diffuse streak pattern. The streaks, which are presumably due to dispersed carbon atoms on the surface, were in registry with the LEED pattern from the nickel substrate. A similar LEED pattern had been observed as an intermediate stage during the decomposition of ethylene on Ni(ll0) prior to the appearance of the graphite ring-pattern”). After a short UHV tempering at 725 “K the streaks had dissappeared and the graphite ring pattern become stronger and sharper. That the carbon on the surface was not uniformly distributed could be seen from the large sensitivity of the AES C-peak to positioning of the AES beam. The surface was then exposed to oxygen for 3 Langmuir. The resulting LEED pattern consisted of both the graphite ring pattern and a p(2 x 1) pattern from the oxygen. This indicated that the graphite and oxygen existed on the surface in domains. In the CO desorption spectrum from this surface the & peak was more than twice the size of the p1 peak. The LEED pattern following the CO desorption was a sharp (1 x 1) pattern. These results are consistent with a scheme whereby the p peaks that we observe are not related to CO chemisorbed on the surface but are the result of the presence of carbon and oxygen atoms on the surface prior to desorption heating. The carbon on the surface can presumably exist as mobile atoms or in clusters. The p1 peak, which shifts in temperature position, is consistent with a bimolecular reaction between mobile carbon and oxygen atoms. The more atomic carbon and oxygen on the surface prior to heating, the lower should be the temperature at which the p1 peak occurs. The pZ peak results from the interaction of mobile oxygen atoms with graphite nuclei. Since this reaction would take place at the relatively fixed boundaries of the carbon clusters it would be first order and the & peak would not be expected to shift in temperature. 4. Summary At 300°K carbon monoxide adsorbs reversibly on a (110) nickel surface. The CO remaining on the surface after the gas phase pressure has fallen below 4 x 10-l’ torr can be removed by heating to 450°K. The interaction of electron beams with chemisorbed CO, and heating the crystal in CO, leads to the build up of a carbon impurity on the surface. The surface reactions that are important in this build up are :
(1) cod + Gd + O,, (dissociation), (2)
O,, + CO,,, -+ COZ, gas(surface reduction).
In CO flash desorption
spectra from the (110) nickel surface the high
CO ON (llO)Ni
DECOMPOSITIONOF
SURFACE
225
temperature p peaks are due to the interaction of mobile oxygen atoms on the surface with surface carbon either atomically dispersed or in graphite clusters. Only the a peak is related to molecular CO chemisorbed on the surface. The reactions leading to the desorption products in the three peaks are: a peak:
CO,, -+ CO,,, ,
PI peak:
Cad + Q,, -+ CO,,, ,
PZ peak:
Gti.
grapb.+ Oad -+ CO,,, .
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
T. Edmonds and R. C. Pitkethly, Surface Sci. 15 (1969) 137. J. C. Tracy, J. Chem. Phys. 56 (1972) 2736. D. A. Degras, Nuovo Cimento Suppl. 5 (1967) 408. M. Onchi and H. E. Farnsworth, Surface Sci. 11 (1968) 203. C. W. Tucker, Jr., Surface Sci. 2 (1964) 516. P. W. Palmberg, in: The Stru~t~e and Chemistry ofSolid Szafaces, EZd.G. A. Somorjai (Wiley, New York, 1969) p. 29-l. L. H. Germer and A. U. MacRae, Proc. Natl. Acad. Sci. (USA) 48 (1962) 997. R. L. Park and H. E. Farnsworth, J. Chem. Phys. 40 (1964) 2354. J. C. Tracy and P. W. Palmberg, J. Chem. Phys. 51 (1969) 4852. S. J. Gregg and H. F. Leach, J. Catalysis 6 (1966) 308. G. Ertl and J. Ktippers, Surface Sci. 24 (1971) 104. G. Ertl, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis et al. (McGrawHill, New York, 1969) pp. 147-165. E. N. Sickafus and H. P. Bonzel, in: Recent Progress in Surface Science, Eds. J. F. Danielli et al. {Academic Press, New York, 1970). H. H. Madden, 3. Ktlppers and G. Ertl, to be published in J. Chem. Phys.
Discussion D. A. KING (Univ. of East Anglia, Norwich) You have shown convincing evidence to indicate that an electron beam can interact with a CO adlayer on Ni(l10) to produce a strongly bound species, possibly dissociatively adsorbed, which only desorbs at relatively high temperatures. Further, you have shown that the C/O ratio on the surface increases with increasing exposure to the electron beam These two factors could readily be attributed to a single mechanism. A Frank-Condon transition from the ground state (a-CO) adsorbed molecule to excited or ionic states may be induced by the electron beam, but, as shown by Redhead and by Menzel and Gomer in 1964, re-neutralization of these states is an efficient process. The measured cross section for electron stimulated desorption is thus low compared with analogous gas phase processes. In your case, reneutrali~tion does not apparently always result in r~o~titution of the original a-CO molecule; the B-states are formed instead. If, further, we suggest that O+, and not C+ ions are desorbed during this process, a corollary of the electron beam interaction with the adlayer is an increasing C/O ratio on the surface. The escape of Of ions only is in accord with results on W surfaces, and with the generally accepted model in which the a-CO molecule is bound to the surface through the C, and not the 0, atom. Your explanation for the accumulation of C on the surface in terms of the interaction of CO gas phase molecules with 0 atoms is not in accord with results published by Hoogan and King for the interaction of CO with Oa on nickel films. The presence of gas phase
226
H. H. MADDENAND
G. ERTL
CO may be required simply to maintain a sufficiently high CO coverage on the surface for electron impact desorption effects to be appreciable. H. H. MADDEN The two aspects of our results that most led us to propose the two steps scheme for the build-up of carbon on the surface were: (1) The fact that the increase in the C/O ratio only occurred when the CO gas pressure was high (> 1O-g torr); and (2) the CO-flash desorption results that were obtained by intentionally adding oxygen to a surface with a large carbon contamination. The dissociative desorption reaction leading to the ejection of 0+ from the surface is not consistent with (1). The transition mechanism you suggest for the electron-beam induced changes in the adsorbed CO-layer does not seem to be substantiated or denied by our results. Whether the dissociated carbon and oxygen left on the surface alter the electron-beam interaction with chemisorbed CO (in the UHV-background conditions) may be regarded as a more tightly bound CO-state, is a matter of definition. Applying such a definition in the case where graphite and oxygen existing in separate domains on the surfaces give rise to the p-peaks would certainly be stretching it too far. A direct comparison of results obtained on tungsten with those obtained on nickel may be misleading. A comparison with results for the interaction of CO with 0~ on nickel may also be misleading, since in this case oxidation of the Ni crystal competes with the COZ formation14). B. GOLDSTEIN(R.C.A. Laboratories, Princeton N.J.) Did you note any electron-beam-enhanced desorption your experimental conditions.
from the surface under any of
H. H. MADDEN Direct observation of electron-beam-stimulated desorption products was not possible in our apparatus. The indirect observation that might bear on this question was that of the weakening of the LEED beam intensities when the (2 x I)-CO structure was examined using LEED beam energies >20-30 eV. This degradation of the LEED pattern was related, at least in part, to the build up of carbon on the surfaces and therefore a meaningful statement on accompanying desorption can not be made.