The adsorption of CO, O2, and H2 on Pt

Surface Science 69 (1977) 85-113 0 North-Holland Publishing Company

THEADSORPTIONOFC0,02,ANDH20NPt I.Thermal desorption spectroscopy studies * D.M. COLLINS and W.E. SPICER Stanford Electronics Laboratories, Stanford, California 94305, USA Received 21 March 1977; manuscript received in final form 14 June 1977

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, 02, and Hz on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) X (100) and 6(111) X (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalbne foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative 02 adsorption at steps or defects than on terraces.

1. Introduction Since Pt is a catalyst for the oxidation of Hz and for the oxidation of CO, it has been the subject of a large number of recent studies. Many of these studies have been concerned with the overall kinetics of these reactions and have thus involved simultaneous exposure of the Pt to all of the gases participating in the reaction. Still other experiments, designed to investigate the more basic steps involved in these

* This research was supported primarily by the National Science Foundation through Grant DMR74-22230 and in part the Center for Materials Research at Stanford University Grant DMR72-03022 A05. 85

86

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

reactions, have focused their attention on the adsorption properties of a single gas. It is the latter approach which will be used here. In this paper, the results of a series of thermal desorption experiments for CO and O2 on both polyc~stalhne and single crystal Pt samples are presented. The results of thermal desorption experiments of Ha from the single crystal surfaces are also presented. There are numerous reports in the literature of thermal desorption experiments of CO, Hz, and 02 from various Pt surfaces. Thermal desorption of CO from polycrystalline Pt samples has been studied by Winterbottom [I 1, Collins et ai. [Z] , and Lambert and Comrie [3]. The results of Winterbottom and Collins et al. are in good agreement. Winterbottom [l] observed two distinct desorption states with peak temperatures (Z’n) of 480 and 580 K, while Collins et al. [2] observed desorption states with Tr, = 440 and 550 K. However, the results of Lambert and Comrie [3] exhibited only one desorption state, The peak temperature for this state was -570 K. Since polycrystalline Pt foils exhibit predo~nantIy (Ill) orientation in X-ray studies, a number of workers have argued that experiments on these foils give results representative of the (111) single crystal surface [3-51. The lack of agreement in results such as those mentioned above has raised doubts about the validity of these assumptions, and this question will be addressed here. Thermal desorption of CO from Pt single crystal surfaces has been studied by Bonzel and Ku {the (I 10) surface) (61, Kneringer and Netzer (the (100) surface) [7], and McCabe and Schmidt (the (110) surface) (81. The results of Bonzel and Ku [6] and McCabe and Schmidt [8] on the (110) surface are in good agreement. They show CO desorption states at Tn = 525 to 550 K and T = 425 to 450 K. 15 -1 Bonzel and Ku, by assuming a pre-exponential factor of 10 set , calculated activation energies for desorption of 31.4 and 25.1 kcal mol-‘, respectively, for these desorption states. The results of these single crystal studies indicate that multiple CO adsorption states do exist on single crystal Pt surfaces. However, to date it has not been possible to assign the different desorption states to specific adsorption sites. Indeed, it has not yet been established whether the multiple desorption states are due to different binding sites or to adsorbate-adsorbate interactions. Thermal desorption of Hz from Pt single crystal surfaces has been studied by LU and Rye (the (1 lo), (21 l), (I I l), and (100) surfaces) 191, Ch~stmann et al. (the (111) surface and the sputtered, “imperfect” (111) surface) [lo], and McCabe and Schmidt (the (110) surface) 181. These studies, other than those on the (111) surface, will not be discussed in detail here since the present studies were carried out on surfaces of predominantly (111) orientation. Rather, it will just be said that most of these surfaces give rise to two or more desorption states. This is indicative of either multiple adsorption sites or strong adsorbate-adsorbate interactions. The work of Christmann et al. [lo] has direct bearing on the work presented in this paper. Hence, the pertinent results of that work will be mentioned here. Moreover, many of the conclusions to be drawn in this paper may have qualitative bearing on the results of studies on the other Pt surfaces. Ch~stmann et al. [lo] studied thermal desorption of Ha from a well ordered Pt

D.M. Collins, W.E. Spicer / Adsorption

of CO, 02 and Hz on Pt. I

81

(111) single crystal and from the same crystal following a sputtering cycle which created imperfections on that surface. The ordered (111) surface exhibited a single dominant desorption state with Tn = 375 K at low coverages. As the coverage was increased, Tr, decreased to -3 15 K, and a second desorption state at Tp = 225 K, which was quite small at low coverages, became quite prominent. The lower temperature desorption state was attributed to adsorbateadsorbate interactions. On the sputtered surface, desorption states nearly identical to those found on the ordered surface were observed. In addition, there was a third desorption state observed with T, = 395 K which was attributed to desorption from defect sites created by the sputtering cycle. Thermal desorption of O2 has been studied by Alnot et al. (polycrystalline ribbon) [ 111, Collins et al. (polycrystalline foil) [2], Norton (polycrystalline foil) [ 121, and Kneringer and Netzer (( 100) single crystal) [7]. These studies all showed a broad, fairly symmetrical peak with Tp between -750 and -950 K which exhibited characteristics indicative of second order desorption kinetics. This implies dissociative adsorption of 02. In the work of Alnot et al. [ 111, Collins et al. [2] and Kneringer and Netzer [7], additional structure in the desorption spectra was observed at higher coverages. Whether this additional structure was due to different binding sites or to adsorbate-adsorbate interactions, is not well established.

2. Experimental The experimental system used to obtain the data presented in this paper has been described previously [2]. It consisted of a stainless steel chamber pumped by a combination differential ion (Ultek 120 liter set-‘)-titanium sublimation pump. Torr, as measured with an ion gauge. The base pressure of the system was -lo-” The system had Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS) capabilities. Electron energy analysis for AES and UPS measurements was performed with a 120”, four grid, spherical retarding potential analyzer (Varian LEED optics) using the retarding potential-ac-modulation technique [ 131. The primary electron beam for AES was oriented perpendicular to the axis of the energy analyzer and struck the sample at near grazing incidence. The sample was rotated 15’ to 30’ toward the electron gun for these measurements. The energy of the primary electron beam was 2 keV. For TDS measurements, the samples were rotated such that they were in line-ofsight with a quadrupole mass spectrometer (UT1 100 C). The samples were heated resistively. The temperature was measured with a Pt-Pt/lO% Rh thermocouple spot-welded to the back of the sample. The data presented in this paper were taken with linear heating rates which are specified in the appropriate figure captions. Care was taken in sample mounting to insure that the sample supports did not get hot enough during the sample cleaning procedure that they also became clean. Since the

88

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

supports were not clean and they did not reach nearly as high a temperature as the sample, extraneous desorption from the supports was small enough that it was negligible. The single crystals were uniformly heated to within the accuracy measurable with an optical pyrometer. The pumping speed of the system was high enough to insure that readsorption of gases during the TDS measurements was negligible. These factors insure that the major features of the TDS results presented in this paper are not artifacts of the experimental setup, but that they reflect actual structural differences in the surfaces studied.

3. The surfaces studied Five different Pt surfaces were studied during the course of this work. Two of the surfaces were polycrystalline and the other three were single crystals. All of the samples were cleaned by heating in 0 a, and surface cleanlinesss was judged with Auger Electron Spectroscopy (AES). The polycrystalline surfaces were both formed on the same polycrystalline Pt foil. The first of these was prepared by heating the foil, a 1 cm X 2 cm X 0.013 cm Pt sheet of 99.9% purity, to temperatures not exceeding 1120 K both in O2 and in vacuum. Periods of heating in Oa, totaling many hours, were continued until AES demonstrated that heating in vacuum resulted in no further diffusion of bulk contaminants to the surface. The two contaminants observed were C and Ca. Once Ca was completely removed, it did return. However, exposure to the background gases of the vacuum system for extended periods resulted in the buildup of a carbonaceous layer at the surface. This layer was removed before every series of experiments by heating the foil at 1120 K for 3 to 5 min in 5 X lo-’ Torr of Oz. The Oa was then pumped away, while maintaining the sample temperature at 1120 K. After the background pressure returned to less than 2 X 10m9 Torr, the sample was cooled to room temperature. This procedure resulted in a clean surface as judged by AES (fig. 1). The surface prepared in this manner will henceforth be referred to as the “1120 K foil.” An X-ray diffraction scan of the 1120 K foil, using a rotating crystal diffractometer, indicated that the foil consisted predominantly of (111) oriented crystallites. Also, Laue back-reflection X-ray analysis yielded diffraction patterns with fairly well defined spots having three-fold symmetry. This was interpreted as being indicative of fairly good single crystal grains of (111) orientation on the 1120 K foil. Analysis using a scanning electron microscope (SEM) showed that the individual grains were 4.2 to 0.4 mm across (fig. 2). The grain boundaries were well defined but were not particularly sharp. The surfaces of the individual grams appeared smooth, up to the resolution of the microscope (-1000 A), but were not flat; i.e., shallow “hills” and “valleys” were apparent. A C/Pt replica was made from the 1120 K foil, and the transmission electron micrographs (TEM’s) of fig. 3 were obtained. These show thermal etch marks at the

D.M. Collins, W.E. Spicer / Adsorption I

I

I

of CO, 02 and Hz on Pt. 1

I

I

I

89

I

(b)

1120

0 AUGER

200 ELECTRON

ENERGY

400 (eV)

K FOIL

600

Fig. 1. AES spectrum of the clean 1120 K Pt foil. The letters (a) through (f) designate the chronological order in which the traces were taken. Time for the total spectrum was -1 h. Traces (a) through (e) were taken with an ac-modulation voltage of 10 V peak-to-peak. Curve (f) was taken with an ac-modulation voltage of 5 V peak-to-peak in order to resolve the Pt 150 and 158 eV Auger transitions.

grain boundaries which extend -0.5 pm onto the surface of the grains. At the highest magnification, surface roughness on the order of 50 A is resolved (fig. 3). After completion of the ultraviolet photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TDS) experiments on the 1120 K foil, the Pt was heated to 1720 K in 5 X lo-’ Torr of O2 for short periods. This 02 treatment was followed by thorough annealing in vacuum at the same temperature. This high temperature treatment resulted in an irreversible reconstruction of the foil. SEM analysis of the reconstructed foil (fig. 4) which will henceforth be referred to as the “1720 K foil”, showed crystalhtes of the same size (-0.2 to 0.3 mm) as were observed on the 1120 K foil (fig. 2). However, the grain boundaries were much more well defined on the 1720 K foil than on the 1120 K foil. Furthermore, the SEM’s of the 1720 K foil (fig. 4) show ridges -1 to 2 pm wide, extending completely across the surfaces of the individual grains. These features are also observed in the TEM’s of the 1720 K foil (fig. 5). An X-ray diffraction scan of the 1720 K foil again indicated that the foil consisted predominantly of (111) oriented crystallites. Also, Laue back-reflection analysis again yielded diffraction patterns characteristic of predominantly (111) oriented crystallites. The spots in the Laue back-reflection diffraction pattern were quite sharp, presumably indicative of good single crystal grains. The 1720 K foil was also cleaned prior to each set of experiments in a manner

90

Lkiw. Collins, W.E. Spicer / Adsorption

of CO, 02 and H2 on Pt. I

Fig. 2. Scanning electron micrographs for the 1120 K Pt foil.

similar to that for the 1120 K foil. The surface cleanliness was again judged by AES (fig. 6). The three single crystal surfaces studied were the flat (111) surface and the 6(lfl)X(lOO) and 5(111)X(111) * stepped surfaces. The (1 I I) surface is the close-packed plane of the fee Pt crystal. The 6(111) X (100) and 6( I f 1) X (I 11) surfaces are crystals which are cut NlOo off the (111) direction toward the (100) and (111) directions, respectively. Somorjai and co-workers have reported that these surfaces consist predominantly of (111) terraces six atoms wide separated by one atom high steps of (100) or (111) orientation, respectively [14-161. This means that approximately one out of every six surface atoms (-17%) resides at a * The authors are indebted to Gabor Somorjai and Carol Smith for the loan of the Pt 6(111) X (111) single crystal used in these studies and for helpful instruction on the preparation of Pt singIe crystals, incIudi~g the use of the excellent crystal preparafion facilities at the Inorganic Materials Research Division of the Lawrence Berkeley Laboratories.

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and H2 on Pt. I

91

(b) Fig. 3. Transmission electron micrographs for the 1120 K Pt foil.

“step” site on the surface. These surfaces are all stable (i.e., they do not reconstruct) in their clean states or when they are covered with adsorbed 0, CO, or H under low pressure conditions [14-161. These particular crystal faces were chosen for three reasons. First, it was believed that it would be possible to correlate the data from these crystals with the data

92

D.M. Collins, W.E. Spieer /Adsorption

Fig. 4. Scanning

electron

micrographs

of CO, 02 and H2 on Pt. I

for the 1720 K Pt foil.

from the polycrystalline Pt foils since the foils did exhibit predominantly (111) orientation in X-ray diffraction studies. Second, the introduction of well-defined defects in this manner permits an investigation of the changes in electronic structure and chemisorption properties resulting from surface defects. Finally, it was essential to choose surfaces which were stable (i.e., did not reconstruct) under the conditions of these studies since no technique (such as Low Energy Electron Diffraction) was available in the vacuum system to monitor surface structure. All three single crystals were prepared in the same manner. They were oriented with Laue back-reflection X-ray techniques and cut by spark erosion. The crystals were then polished by standard metallographic techniques. The overall accuracy of the orientation method was estimated to be 21”. Both the (111) and 6(111)X (100) crystals were -0.5 mm thick and were polished on both sides. The 6(111) X (111) crystal was polished on only one side. The second side was ground flat with emery paper but was not polished to completion due to the thinness of the crystal (-0.2 mm).

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

(al

i

1

1000 i Fig. 5. Transmission electron micrographs for the 1720 K Pt foil.

Each crystal was initially cleaned by heating at -1200°C for two days in 10e6 Torr of 02. This resulted in the removal of the C impurity from the bulk of the samples to such an extent that it did not appear to out-diffuse during the course of all subsequent experiments. It also resulted in the segregation of the Ca impurity present in the samples to the surface. The Ca which had segregated to the surface

D.M. CoNins. W.E. Spicer 1 Adsorption

94

of CO, 02 and H2 on Pt. 1

fd) Xi

CLEAN Pt 1720 K FOIL

I

0

200

I

I

400

I

I

600

AUGER ELECTRON ENERGY teVf

Fig. 6. AES spectrum of the clean 1720 K Pt foil. The letters (a) through (f) designate the chronological order in which the traces were taken. Time for the total spectrum was -1 h. The small peak at -273 eV in trace (e) is due to a small buildup of C resulting from prolonged exposure to the primary electron beam.

appeared to be in the form of a calcium oxide layer. This calcium oxide layer was removed by flashing the crystal to -1600°C in vacuum. This presumably evaporated the calcium oxide from the surface, as after this procedure Ca was not again detected by AES *. Following the 1600°C flash to remove the calcium oxide from the 6(1 I 1) X (111) crystal, UPS data of CO and 02 adsorption were taken for hv = 10.2 eV. These data exhibited smeared out valence band structure. This was attributed to the disordered surface resulting from the evaporation of calcium oxide and was remedied by thoroughly annealing the crystal at -1200°C. The (111) and 6(111) X (100) crystals were also thoroughly annealed at -12OO”C, following the evaporation of the calcium oxide layer. As with the polycryst~line surfaces, the single crystals were cleaned before each set of experiments. The procedure followed was (1) heating the crystal at >lOOO”C for 3 to 5 min in 5 X lo-’ Torr 02, (2) pumping away the 02 until the background * The authors are indebted to Don Blakeley for suggesting this method for the removal of the Ca impurity.

D.M. Collins, W.E. Spicer /Adsorption

95

of CO, 02 and Hz on Pt. I

PI (Ill1

0

200 AUGER

ELECTRON

600

400 ENERGY

0

200 AUGER

IeV)

ELECTRON

600

400 ENERGY

WI

I

I

I

I’

0

1

200 AUGER ELECTRON

400 ENERGY

600

J

IeV)

AES spectra of the clean (ill), 6(111) X (loo), and 6(111) X (111) pt single crystals. The letters (a) through (e) designate the chronological order in which the traces were taken. Time for the total spectrum in each case was -1 h. The small C peak (-273 eV) present in several of the traces is due to a small buildup of C resulting from prolonged exposure to the primary electron beam. Fig.

7.

pressure was less than 2 X 10m9 Torr, and (3) cooling the sample to room temperature. Cleanliness was again checked by AES (fig. 7). The 6(111) X (111) crystal was smooth to within the resolution of the SEM (fig. 8). TEM’s of the 6(111) X (111) single crystal are shown in fig. 9. At the highest magnification, surface roughness on the order of 50 W is resolved.

4. Results and discussion In the present work, thermal desorption spectroscopy (TDS) has been used to study CO, 02, and H2 adsorption on various Pt surfaces. Thermal desorption experiments of CO and 02 are reported for both the 1120 and 1720 K Pt foils as well as the (11 l), 6(111) X (loo), and 6(111) X (111) Pt single crystals. Thermal desorption of H2 is reported for the (11 l), 6(111) X (loo), and 6(111) X (111) Pt single crystals.

96

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

Fig. 8. Scanning electron micrograp~s for the 61111)

x (I 11) Pt single

crystal.

4.1. Thermal desorption of CO Fig. 10 shows the TDS data of CO from the Pt (11 I), 6(111) X (1001, and 6{111) X (111) surfaces as a function of CO exposure in Langmuirs (1 Langmuir = 1 L = I@ Torr set). Only one dominant CO desorption state is observed for the Pt (111) surface (fig. lOa). The peak temperature is -450 K at low coverages and decreases with increasing coverage. As will be substantiated later, the small amount of desorption at T 2: 5.50 K is attributed to adsorption at a small density of defects on the otherwise smooth (111) surface, as well as to desorption from the edges of the crystal. The origin of the small structure at T 2 375 K has not been well established. The decrease in peak temperature with increasing coverage is attributed to a coverage-dependent heat of adsorption resulting from adsorbate-adsorbate interactions. Figs. 10b and 1Oc show the TDS data of CO from the 6(11 I) X (100) and 6(111) X (111) single crystal surfaces, respectively. For these surfaces, two desorp-

D.M. Collins, W.E. Spicer / Adsorption

(b)

of CO, 02 and Hz on Pt. I

91

I

I 1000 ii

Fig. 9. Transmission electron micrographs for the 6(111) X (111) Pt single crystal.

tion states are observed. The peak temperature of the higher temperature desorption state (Tn 5 550 K) appears to be independent of coverage. The lower temperature desorption state shows behavior similar to the desorption state observed on the (111) single crystal surface (fig. 1Oa) with Tp= 450 K at low coverages and decreasing as the coverage increases. The desorption state at Tp* 550K (figs. lob and

D.M. Collins, W.E. Spicer /Adsorption

98

!-----k I

I

I

500 T(K)

I

700

I

L

300

/

,

500 T(K)

of CO, 02 and Hz on Pt. I

I

I

700

I

/

300

I

1

500

,

700

T(K)

Fig. 10. TDS of CO from Pt as a function of CO exposure (1 L = 1 Langmuir = 10v6 Torr see). (a) Pt (111). Heating rate = 15 K set -r. The small amount of desorption for T > 500 K is attributed to desorption from a small density of defects present on the (111) surface, as well as to desorption from the edges of the crystal. (b) Pt 6(111) X (100). Heating rate = 12 K set-‘. (c) Pt 6(111)X (111). Heating rate = 14 K see -I. The desorption for T > 500 K in (b) and (c) is attributed to adsorption at steps while that at T < 500 K is attributed to adsorption on the terraces.

10~) is attributed to CO adsorbed at the steps. The low temperature desorption state (Tn 5 450 K), because of its similarity to the desorption state observed for the smooth (111) single crystal, is attributed to CO adsorbed on the (111) terraces (fig. 10). A question which arises when adsorbing CO on metal surfaces is whether or not some of the CO dissociates into adsorbed C and adsorbed 0. If so, C could be left on the surface while the 0 desorbs as Oa during a TDS experiment. It cannot be concluded de~nitively whether or not CO dissociated at any of the adsorption sites on the surfaces studied in the present work (e.g., at the step sites). However, it is quite certain that nearly all CU which might have dissociated upon adsorption recombined and desorbed as CO. The evidence for this is that CO TDS results could be reproduced almost indefinitely at all CO exposures, including low exposures which followed high exposures which, in turn, had followed previous low exposures. This would not be expected if extended CO TDS runs resulted in the buildup of an appreciable amount of C on the surface. The activation energies for desorption are estimated as 33 kcal mol-* for CO adsorbed at the steps and 24 to 27 kcal mol -’ (depending on coverage) for CO adsorbed on the terraces. This assumes a first order desorption with a preexponential factor of lOi sec.-’ for both adsorption sites. This a~umption may not be completely justified but, since it is the assumption usually made by other workers,

D.M. Collins, W.E. Spicer / Adsorption

of CO, 02 and Hz on Pt. I

99

it simplifies comparison with that work. Furthermore, the assumption does not affect the primary conclusions to be drawn here because changing the preexponential factor by a factor of 10” changes the activation energy for desorption by only +2 kcal mol-‘. The values obtained here compare quite well with those of Bonzel and Ku for a (110) single crystal [6]. Since the adsorption of CO on Pt is nonactivated, these activation energies for desorption correspond to the heats of adsorption of the two binding states [2]. In fig. 11, the TDS data for the stepped surfaces have been replotted for CO exposures of 10 L (corresponding to saturation coverage). In this figure, the “step” contributions and the “terrace” contributions have been separated. The algebraic sum of the two curves of fig. 1 la yields the TDS curve for the 10 L exposure shown in fig. lob. Similarly, the algebraic sum of the two curves in fig. 1 lb yields the TDS curve for the 10 L exposure shown in fig. 10~. The separation of the “step” and “terrace” contributions in this manner permits an estimate of the fraction of the total coverage which can be attributed to adsorption states associated with the steps. As indicated in figs. 1 la and 11 b, the fractional coverage of the step adsorption states relative to the total surface coverage is -0.3 for both the 6(111) surface. Since the fraction of surface Pt X(100) surface and the 6(111)X(111) atoms which reside at steps is l/6 = 0.17, we see that nearly twice this fraction of adsorbed CO species are influenced by the presence of steps. This can be understood by either one of two models. One possibility is that two

Pt 6(111)x(1111

(b)

(a)

-

& step” -

= 0.3

eTOTAL

CONTRIBUTION

*/

“TERRACE” CONTRIBUTION

CONTRIBUTION

CONTRIBUTION

I

I

400

I

500

I

600 T(K)

I

700

I

000

=0.3

II

I 400

1 500

I 600

I 700

I BOO

T(K)

Fig. 11. TDS for saturation coverage of CO (10 L) on the stepped Pt surfaces. The data has been separated into “step” and “terrace” contributions so that the step coverage to total coverage ratio can be estimated.

100

D.M. Collins, W.E. Spicer / Adsorption

of CO, 02 and H2 on Pt. I

types of adsorption states exist at steps: one in which an adsorbate resides on the terrace immediately above the step and one in which an adsorbate resides on the terrace immediately below the step. This interpretation has been suggested to explain the properties of hydrogen adsorption on stepped Pt crystals (I 71. A second possibility is that there is a single type of CO adsorption state at the steps, but that adsorption at step atoms saturates at a coverage of one CO molecule for each Pt atom at the step as opposed to a saturation coverage of one CO molecule for every two Pt atoms on the terrace (Le., the coverage at the steps is twice the coverage on the terraces). A saturation CO coverage of appro~mately one CO molecule for every two Pt atoms has been fairly well established for the Pt (111) surface [ 181. Thus, the second interpretation is favored here. A summary of the TDS studies of CO adsorption on the single crystal surfaces is useful at this point. TDS has been applied to three Pt samples with well defined step (or defect) densities. This has permitted the observation of the qualitative effects of surface irregularities on adsorption states for CO on Pt. In addition, it has allowed an estimate of the fraction of the adsorbates, at saturation coverage, whose chemisorption bonds are influenced by the presence of steps. The heats of adsorption for CO on these surfaces were estimated to be 33 kcal mol-’ at step sites and 24 to 27 kcal mol-’ at terrace sites.

THERMAL

II20

DESORPTION

K FOIL

I

443 TEMPERATURE

I

558

( K)

Fig. 12. TDS of CO from the 1120 K Pt foil. Heating rate = 5 K see-*

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

101

Fig. 12 shows CO TDS results for the 1120 K Pt foil. This data clearly shows two desorption states, one at Tr, = 550 K and one at Tr, = 440 K. The heats of adsorption which were estimated for these adsorption states are 32 and 25 kcal mol-‘, respectively [2], in good agreement with those found for the step and terrace sites, respectively, on the single crystals. The relative coverage of the state desorbing above -500 K is -0.6, suggesting a high fraction of defect adsorption sites on the 1120 K foil. The fact that the high temperature desorption state fills first (i.e., before the low temperature state), implies that it can not be simply attributed to adsorption at grain boundaries since sites physically isolated from one another, as the grain boundaries are from the surfaces of the grains, would be expected to fill simultaneously. The results of CO TDS from the 1720 K foil are shown in fig. 13. The most dominant feature in this data is the desorption state for 400 5 T 2 450 K. This state behaves similarly to the CO desorption observed for the smooth (111) single crystal surface and suggests that, upon annealing at the higher temperature, the foil reconstructed to expose primarily sites of (111) (close-packed) coordination. However, there is still evidence for a step-like desorption state at Tp = 550 K as well as an additional desorption state at T, = 650 K. The state at Tp u 650 K is most likely due to new sites created by the reconstruction (or faceting) which resulted from the 1720 K anneal. The broad, flat shape of this peak is suggestive of dissociative CO adsorption. However, a detailed analysis of this desorption state was not carried out. The activation energy for desorption for the state at 400 2 Tp 2 450 K ranges from 24 to 27 kcal mol-‘, depending on coverage. The activation energy for desorption for the state at Tp N 550 K is estimated to be 33 kcal mol-‘. The fraction of CO adsorbed in the states desorbing above -500 K (i.e., the states associated with surface defects) is -0.4. The values of the activation energies for desorption of CO on Pt are summarized in table 1. The peak temperatures (Tp) given in table 1 allow ready comparison between the number of desorption states observed on the various samples. The precise peak temperatures are expected to vary since the heating rates for the different

1720 K FOIL

2

‘E 3 ? e .; B c?

I 400

, I 500 600 TEMPERATURE

I 700

I 800

(K)

Fig. 13. TDS of CO from the 1720 K Pt foil. Heating

rate = 5 K WC-~.

D.M. Collins, W.E. Spicer / Adsorption

102

of CO, 02 and Hz on Pt. I

Table 1 Peak temperatures Surface

(Tp) and activation

energies

for desorption

(ED) for CO on various Pt surfaces References

TP (K)

ED (kcal mol-

400-450 400-450 550 400-450 550 440 550 400-450 550 650

24-27 24-21 33 24-27 33 25 32 24-21 33 _

480 580 570

25.1 31.7 _

(110)

410 510

_ -

(110)

425 525

25.1 31.4

]61

(110)

450 550 450

_ _ _

181 [81 171

(111) 6(111)

x (100)

6(111)x

(111)

1120Kfoil 1720 K foil

Foil Foil

(100)

550 125

_ _

l) Present Present

work work

Present

work

[2] and present Present

work

work

[II [31 131

a The pre-exponential

factor (v) was assumed to be 1013 see-l in calculating all values of ED except those from ref. [l] for which the values of LJwere calculated to be 5 X 10” set-’ (ED = 25.7 kcal mol-‘) and lot2 (ED = 31.7 kcal mol-‘).

samples varied between -5 and -40 K set-‘. However, this variation in heating rate would have only a second order effect on T,. A comparison of the single crystal data with the polycrystalline foil data suggests that the highly (111) oriented polycrystalline foils which have been previously studied by TDS [l-5,19] may have had high concentrations of surface defects which act in a manner similar to steps with regard to CO adsorption. Because of this, TDS may provide a means of estimating the relative density of defect sites on variously prepared metal surfaces. This is true, of course, only if one can assure that extraneous effects resulting from nonuniform heating or other sources are not important. That is clearly the case in these studies, as the present results were all obtained in the same experimental system with similarly mounted samples. A striking conclusion of this is that the precise nature of the defects ((100) steps, (111) steps, or certain types of surface defects present on the polycrystalline foils) does not seem to be as important as the fact that these defect sites differ from the sites on the (111) terraces (i.e., the smooth (111) surface). However, one type of defect site

D.M. Collins, W.E. Spieer /Adsorption of CO, 02 and Hz on Pt. I

103

on the 1720 K foil (i.e., at Tp= 650K) is clearly different from all of the other defect sites observed on the samples studied here. Furthermore, TDS results of other single crystal Pt surfaces (e.g., Pt(ll0)) [6], which exhibit more than one CO desorption state, may have more than one “type” of Pt atom on the surface; i.e., Pt atoms with different coordination with other Pt atoms within the surface layer. This would, for example, be expected of the (100)

a

0

2 4 6 6 CO EXPOSURE (Longmuirsl

IO

Fig. 14. (a) CO coverage versus CO exposure for the Pt singfe crystals. CO coverage was obtained from the areas under the CO TDS curves. (b) CO coverage versus CO exposure for the polycrystalline Pt surfaces. CO coverage was obtained from the areas under the CO TDS curves.

104

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

and (11O)Pt surfaces since they are known to reconstruct [20,2 11. It is also possible that adsorption-induced reconstruction could create new binding states. Of course, caution must be exercised in suggesting interpretations based on reconstruction of the metal surface since adsorbate-adsorbate interactions can also give rise to multiple adsorption states [lo]. The CO coverage versus CO exposure is shown in fig. 14a for the single crystals and in fig. 14b for the polycrystalline foils. The CO coverages as a function of CO exposure were obtained from the areas under the TDS curves. The straight line behavior of the coverage versus exposure curve for low coverage is indicative of CO adsorption via a precursor state [ 181. 4.2. Thermal desorption of Hz The results of H, TDS from the Pt single crystals are qualitatively similar to those for CO. In the data presented here, the Hz exposures were made at T % 200 K. For Pt (111) (fig. 15a), a broad structure, consisting of two desorption states, is observed for T < 400 K. This data agrees quite well with that obtained by Christmann et al. for Pt(ll1) [lo] as well as that obtained by Lu and Rye for Pt(ll1) [9]. In the work of Christmann et al. [lo], the low temperature shoulder in the

Pt(lll)

I

300

Pt 6 (Ill)x

I

1

400 T(K)

5(

(100)

Pt 6 (III)

I

400

300 T(K)

51

I 300

x (Ill)

I 400 T(K)

51

Fig. 15. TDS of Hz from the Pt single crystals. Heating rate 117 K set-l. Temperature of H2 exposure -200 K. The desorption for T > 400 K in (b) and (c) is attributed to adsorption at steps, while that at T < 400 K is attributed to adsorption on the terraces.

D.&f. Collins, W.E. Spicer /Adsorption ofC0, 02 and Hz on Pt. I

105

desorption spectra was attributed to adsorbate-adsorbate interaction. This shoulder was not resolved in the data of Lu and Rye 191. For the stepped surfaces (figs. 1Sb and 15~1, desorption states similar to those found on the (111) surface are observed for T s 400 K. For the 6(11 I) X (100) crystal, this feature in the desorption spectra again appears as a broad structure with a barely perceptible low temperature shoulder, The low temperature shoulder is most apparent in the curve for the 4 L Ha exposure. However, for the 6(111) X (111) crystal, two distinct states are resolved. There are two possible explanations for the two distinct states observed for T < 400 K for the 6(I 1I) X (111) surface. One possibi~ty is that the temperature resolution was bettter for the 6(111) X (111) crystal as a result of more uniform heating of that crystal. This is likely since the 6(111) X (111) crystal was substantially thinner than the other two crystals (-0.2 mm compared to -0.5 mm). This would result in more uniform heating of the crystal under the conditions of these experiments. A second possibility is that the extra state arises from a second desorption state at the (111) steps, We prefer the first possib~ity because two states are known to exist for T s 400 R on the

Pt 6(lil)x(l~~)

Pt 6 (III) x (ill)

,

CONTRIBUTIOI

/ 300

I

400 T(K)

I

500

I

300

I

400 T(K)

I

500

Fig. 16. TDS for saturation H coverage on the stepped Pt surfaces. The data has been separated into “step” and “terrace” ca~tributions so that the step coverage to total coverage ratio can be estimated.

106

D.M. Collins, W.E. Spicer / Adsorption

ofCO, 0, nnd Hz on Pt. I

smooth (111) surface [lo]. Hence, we attribute the desorption for T 5 400 K on both stepped surfaces to adsorption on the (111) terraces. There is an additional desorption state observed on both stepped surfaces for T 2 400 K. This state, which is not observed on the smooth (111) surface, is attributed to adsorption at the steps. A state with increased desorption temperature associated with surface defects has been observed pre~ousIy for the case of Ha on

I

I

I

I

20

40

60

80

l-i, EXPOSURE

(L1

Fig. 17. H coverage versus H2 exposure for the single crystal surfaces. H coverage was obtained from the areas under the Hg TDS curves. Temperature of Hz exposure ~200 K.

DM, Collins, W.E. Spicer / Adsorption

of CO, 02 and Hz on Pt. I

107

Pt(ll1) by Christmann et al. [lo]. In that study, the defect sites were created by sputtering a smooth (111) surface. In fig. 16, the TDS data for the stepped surfaces has been replotted for H2 exposuresof40L(6(111)X(lOO),fig. 16a)and 12OL(6(111)X(111),fig. 16b).These exposures correspond to nearly saturation H coverage (for H2 exposures at -200 K), as can be seen in the H coverage versus H2 exposure data of fig. 17. In Ag. 16, the “step” and “terrace” contributions have been separated. The algebraic, sum of the two curves of fig, 16a yields the TDS curve for the 40 L H2 exposure shown in fig. 15b. Similarly, the algebraic sum of the two curves in fig. 16b yields the TDS curve for the 120 L H2 exposure shown in fig. 1SC. Just as in the case for CO, this separation of the “step” and “terrace” contributions permits an estimate of the fraction of the total coverage which can be attributed to adsorption states associated with the steps. As indicated in fig. 16, the fractional coverage of the step adsorption states relative to the total surface coverage is -0.2 for the 6(111) X (100) surface and -0.1 for the 6(111) X (111) surface. This suggests that (100) steps more strongly influence H adsorption on Pt than do (111) steps. However, it is clear that both types of steps have a strong influence on H adsorption on Pt. 4.3. Thermal desorption of O2 The results of O2 TDS for the three single crystal surfaces are shown in fig. 18. The desorption curves for these surfaces are ail qualitatively similar. They exhibit a broad peak which is nearly symmetric about TP, and T, decreases from ~950 to -750 K as the coverages are increased. This behavior is characteristic of second order desorption kinetics and compares quite well with previous 02 TDS studies by other workers [7,11,12]. For the 6(111) X (100) surface (Fig. 18b), two desorption states (at T = 800 K and T = 925 K) are observed for higher 0 coverages. Similar states have been observed on both the Pt (100) surface [7] and on certain polycrystalline Pt surfaces [l l] . The origin of the second desorption state is not well established; however, it is probably due to either adsorbate-adsorbate interaction or to a second adsorption site. There is one striking difference in the O2 TDS results for the single crystal surfaces. This difference is that the 0 coverage corresponding to saturation is substantially smaller for the smooth (111) surface than for the stepped surfaces. The absolute 0 coverages for these three single crystal surfaces were estimated by making two assumptions. The first assumption was that the saturation coverage of CO on all of these single crystal surfaces was 0.5 CO molecules per surface Pt atom at room temperature. This assumption is based on values of the saturation coverage of CO on Pt(ll1) found in the literature [18]. While it appears from the CO TDS results presented above that the saturation CO coverage on the stepped surfaces is slightly higher than on the smooth (111) surface, this difference is estimated to be less than 10% of a monolayer and has been neglected here. The second assumption was that the pumping speeds for CO and 02 are very nearly the same in

108

LAM. Collins, W.E. Spicer / Adsorption of CO, 02 and Hz on Pt. I

I

1

I

I

b)

I

I

Pt 6(11II x (100)

2OL

I

I

I

I

I

I

Pt 6(1111x(lll)

I

600

I

700

I

I

800

900

I

I

IO00

1100

T(K)

Fig. 18. TDS of 02 from the Pt single crystals. Heating rates: ill l), 40 K set-l; (loo), 20 K see-‘;6(111)

X (ill),

6011) X

20 K set-l.

the ion/Z pumped vacuum system. This second assumption is not critical as long as the pumping speeds for both CO and 02 are fast compared to the desorption rates of the gases from the Pt surface during the thermal desorption experiment. This was the case for the TDS results reported here. After making these two assumptions, the absolute 0 coverage was estimated by comparing the areas under the 02 TDS curves with the area under the CO TDS curve, for the same surface, which corresponded to saturation CO coverage. The dif-

D.M. Collins, W.E. Spicer / Adsorption

of CO, 02 and Hz on Ft. I

109

Table 2 Oxygen coverages in 0 atoms per surface Pt atom for the Pt surfaces studied in the present

work Surface

@o,sat

(111) 6(111)X (100) 6(111)X (1111 1120Kfoti 1720 K fait

0.04 0.19 0.24 0.20 0.36

ferences

in temperature

were all taken into account

scales,

heating

rates,

and mass spectrometer

sensitivities

in these calculations.

The 0 coverages calculated in this manner were -0.04 for the (111) surface, -0.19 for the 6(111) X (100) surface, and -0.24 for the 6(11 I) X (111) surface. These values are shown in fig. 18 and are summarized in table 2. Even though these coverages are of marginal absolute accuracy (estimated at t20%), a very important conclusion can be drawn from them. The saturation 0 coverages which were obtained under the experimental conditions of this study are clearly very close to the step densities of the three crystals. The values of 0.19 and 0.24 for the stepped crystals are remarkably close to the step density of 0.17 for those surfaces. The (111) surface, if cut -1” off the true (111) plane (which is the margin of error of the orientation method used), would have a step density of -0.02. The calculated 0 coverage of 0.04 is remarkably close to this, considering the accuracy of the coverage estimate, as well as the fact that adsorption on the edges of the crystal become important at such low step densities. A combination of two effects is probably responsible for the failure to adsorb 0 on terrace sites in the present work. First, the background gas in the vacuum system contained a small fraction of CO which increased during O2 exposures. At higher 02 exposures, the CO partial pressure sometimes rose as high as 0.01 times the 02 partial pressure. This background CO removed adsorbed 0 by reaction to form CO2. This phenomenon has been observed previously [22]. Therefore, if the dissociative sticking probablility of 02 on the terrace sites is less than -lo-’ (assuming a unity reaction probability of CO with 0 adsorbed on the terraces), 0 would be removed from the terraces as fast as it adsorbed. For dissociative O2 adsorption at step sites, the sticking probability is -0.1. Therefore, the removal of 0 adsorbed at step sites by CO is a less serious problem. Actually, at the lower O2 exposures required to saturate the step sites with adsorbed 0, the CO partial pressure remained small enough that the effect of CO removing 0 adsorbed at the step sites was negligible. A second factor which would favor 0 adsorption at step sites, rather than on terrace sites, is a difference in the reaction probability of CO with 0 adsorbed at the different sites. That is, if CO reacts more readily with 0 adsorbed

110

D.M. Collins, W.E. S’picer / Adsorption

ofCO, 02 and H2 on Pt. I

on the terraces than with 0 adsorbed at the steps, it would make it relatively more difficult to adsorb 0 on terrace sites than on step sites for a given background pressure of CO. A higher reaction probability for 0 adsorbed on terrace sites, as opposed to 0 adsorbed at step sites, has been observed by Hopster et al. [23]. The results of O2 TDS for the polycrystalline foils are shown in fig. 19. They are qua~tatively similar to the results obtained on the single crystal surfaces. It is interesting to note that two desorption states are observed on the 1120 K foil at high 0 coverages. This is similar to the behavior observed on the 6(1Il) X (100) stepped surface and may reflect a similarity between the defects present on the 1120 K foil and the (100) steps present on the 6(111) X (100) stepped surface. The saturation 0 coverages were calculated in the same manner as for the single crystals. As for the single crystals, a saturation CO coverage of 0.5 was assumed. The values obtained were -0.20 for the 1120 K foil and -0.36 for the 1720 K foil. The assumption that the saturation CO coverage is 0.5 for polycrystalline Pt surfaces is supported in the literature [S]. However, due to the variable nature of polycrystalline Pt surfaces, the 0 coverage estimates for these surfaces have more uncertainty than those for the singte crystal surfaces. The saturation 0 coverage of -0.36 for the 1720 K foil compares quite well with the relative coverage of defect sites (-0.4) suggested by the CO TDS results for that surface. This again suggests that 0 adsorption takes place primarily at

700 Fig. 19. TDS of 02 from 1720 K foil, 12 K see-l.

800 900 1000 II00 TEMPERATURE iK)

the polycrystalline

Pt foils. Heating

rates:

1120 K foil, 5 K set-t

;

D.M. Collins, W.E. Spicer /Adsorption

of CO, 02 and H2 on Pt. I

111

Pt 6 (111)x(100)

L

I IO

a

0,

I 20 EXPOSURE

1120

0

IO 0, EXPOSURE

b

I IO

I 20 OXYGEN

I

40

K FOIL

1

I 30 20 (Langmuirs)

1720

0

I 30 (L)

1 30 EXPOSURE

4(

K FOIL

I 40

I 50

I 60

(LANGMUIRS)

Fig. 20. (a) 0 coverage versus 02 exposure for the Pt stepped single crystals. 0 coverage was obtained from the areas under the 02 TDS curves. (b) 0 coverage versus 02 exposure for the polycrystalline Pt surfaces. 0 coverage was obtained from the areas under the 02 TDS curves.

112

D&f. Collins, W.E. Spicer f Adsorption

of CO, 02 and Hz on Pt. I

defect sites under the conditions of this study. However, the saturation 0 coverage of -0.20 for the 1120 K foil is substantially lower than the relative coverage of defect sites (-0.6) suggested by the CO TDS results for that surface, This suggests that the Pt sites which give rise to the high temperature CO desorption state (i.e., the so-called “defect” state at Tr, c1(550 K) on the 1120 K foil are different, with respect to 0 adsorption, than the Pt sites associated with the high temperature (step or defect) desorption states of CO on the other Pt surfaces studied in this work. This is strong evidence that experimental results on polycrystalhne Pt foils cannot be assumed to be characteristic of good quality single crystals. Also, this suggests that the controversy in the literature regarding the dissociative sticking probability of O2 on Pt may be due, in large part, to the differences in the detailed structure of the surfaces studied [24]. The 0 coverage versus O2 exposure is shown in fig. 20a for the stepped single crystals and in fig. 20b for the polycry~t~ine foils. The 0 coverages as a function of O2 exposure were obtained from the areas under the TDS curves. 5. Summary and conclusions The primary results of this TDS study of CO adsorption on Pt can be summarized as follows: (I) The step sites on the Pt 6(111) X (100) and 6(11 I) X (Ill) single crystals lead to a CO desorption state with an activaiion energy for desorption of -33 kcal mol -’ ; (2) The activation energy for desorption of CO from the Pt (111) single crystal and the terrace sites on the 6(111) X (100) and 6(111) X (111) single crystals varied from -27 kcal mol-’ at low CO coverages to -24 kcal mol-’ at high CO coverages. (3) The polycrystalline foils exhibited desorption states with features similar to those observed on the stepped single crystals, suggesting a substantial fraction of defect sites on the foils. These results suggest that TDS can be used to estimate the density of defects on variously prepared Pt surfaces. This application of TDS could probably be extended to other metals as well. The key to the success of using TDS to estimate surface defect densities is to study samples with as well defined surface structure as possible (e.g., smooth and stepped single crystals). The results of measurements on well defined surfaces can then be compared with results of measurements on less well defined surfaces, such as polycrystalline foils, thereby providing an estimate of the defect density on the latter surfaces, While such an analysis may not give the precise structure of the defects, it does provide information about the density of defects on poiycrystalline samples that can not be obtained by other techniques designed to study surface structure (e.g., LEED). This is the case because TDS can examine adsorption states which reflect local structural differences, while LEED serves to examine the average periodic structure of surfaces. The results of Hz TDS on the Pt single crystals were qualitatively similar to those of CO TDS. H2 desorption states at T <400 K were observed for the (I 11) single crystal as well as for the 6(111) X (100) and 6flll) X (I I 1) single crystals. These

D.M. Colltns, W.E. Spicer /Adsorption

of CO, 02 and Hz on Pt. I

113

states were attributed to adsorption on terrace sites. An additional desorption state for T > 400 K was observed for the stepped surfaces which was not observed on the smooth (111) surface. This desorption state was attributed to H adsorbed at step sites. The O2 TDS studies reported here yielded estimates for the saturation 0 coverage for the various Pt surfaces studied. The estimated saturation 0 coverages were -0.19 for the 6(111) X (100) crystal, -13.24 for the 6(111) X (11 l), -0.04 for the (111) crystal, -0.36 for the 1720 K foil, and -0.20 for the 1120 K foil. These values, which are summarized in table 2, suggest that 0 adsorption takes place primarily at step or defect sites under the experimental conditions of this study. In conclusion, it is clear that step or defect sites have a definite influence on the adsorption of CO, Hz, and O2 on Pt. For CO and Hz, distinct desorption states are observed which can be attributed to adsorption at step or defect sites. For Os, under the experimental conditions of this study, adsorption takes place primarily at step or defect sites. Finally, it is clear that polycrystalline Pt foils give results that are different in detail to those obtained on Pt single crystals. Therefore, the results of studies on preferentially oriented polycrystalline Pt foils should not be assumed to be characteristic of good quality Pt single crystals. References [ 11 W.L. Winterbottom, Surface Sci. 37 (1973) 195. [2] D.M. Collins, J.B. Lee and WE. Spicer, Surface Sci. 5.5 (1976) 389. [3] R.M. Lambert and C.M. Comrie, Surface Sci. 46 (1974) 61. [4] J.M. Martinez and J.B. Hudson, J. Vacuum Sci. Technol. 10 (1973) 3.5. [S] R.A. Shigeishi and D.A. King, Surface Sci. 58 (1976) 379. [6] H.P. Bonzel and R. Ku, 3. Chem. Phys. 58 (1973) 4617. [7] G. Kneringer and F.P. Netzer, Surface Sci. 49 (1975) 12.5. [ 81 R.W. McCabe and L.D. Schmidt, Surface Sci. 60 (1976) 85. [9] K.E. Lu and R.R. Rye, Surface Sci. 45, (1974) 667. [lo] K. Christmann, G. Ertl and T. Pignet, Surface Sci. 54 (1976) 365. [ll] M. Alnot, A. Cassuto, J. Fusy and A. Pentenero, Japan. J. Appl. Phys., Suppl. 2, Part 2 (1974) 79. [ 121 P.R. Norton, Surface Sci. 47 (1975) 98. [13] C.N. Berglund and W.E. Spicer, Phys. Rev. 136 (1964) A1044; W.E. Spicer and C.N. Berglund, Rev. Sci. Instr. 35 (1964) 1665. [ 141 B. Lang, R.W. Joyner and G.A. Somorjai, Surface Sci. 30 (1972) 440. [ 151 B. Lang, R.W. Joyner and G.A. Somorjai, Surface Sci. 30 (1972) 454. [ 161 G.A. Somorjai, Surface Sci. 34 (1973) 156. [ 171 K. Christmann and G. Ertl, Surface Sci. 60 (1976) 365. 1181 W.H. Weinberg, CM. Comrie and R.M. Lambert, J. Catalysis 41 (1976) 489. [ 191 Y. Nishiyana and H. Wise, J. Catalysis 32 (1974) 50. [20] H.P. Bonzel, C.R. Helms and S. Keleman, Phys. Rev. Letters 3.5 (1975) 1237. [Zl] R.D. Ducros and R.P. Merrill, Surface Sci. 55 (1976) 227. [22] J. Joebstl, J. Vacuum Sci. Technol. 12 (1975) 347. [ 231 H. Hopster, H. Ibach and G. Comsa, J. Catalysis 46, (1977) 37. [24] H.P. Bonzel and R. Ku, Surface Sci. 40 (1973) 85; and references contained therein.