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The formation of new oxygen adsorption states on Pt(100) by facetting induced by catalytic reaction
L701
Surface Science 204 (1988) Li’Ol-L707 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
THE FORMATION OF NEW OXYGEN ADSORPTION STATES ON Pt(ll0) BY FACE-ITING INDUCED BY CATALYTIC REACTION R. IMBIHL, Fritz-Haber-Institut
Received
M. SANDER
and G. ERTL
de-r Max-Planck-Gesellschaft,
10 June 1988; accepted
for publication
Faradayweg
4- 6, D-1000 Berlin 33, Germany
20 June 1988
Catalytic oxidation of CO on a Pt(ll0) surface may lead to the formation of microfacets as monitored by LEED as well as’ of atomic disorder type sites. Both structural elements are associated with an enhanced oxygen sticking coefficient and are hence responsible for the associated increase of catalytic activity. While the facets are stabilized by the presence of adsorbed oxygen, they are annealed during thermal desorption of 0, from the (110) terrace sites. The second type of sites is thermally more stable and manifests itself in two high-temperature 0, thermal desorption states.
It is well established that structural imperfections, such as monatomic steps, may sensitively affect the kinetics of dissociative oxygen adsorption on platinum surfaces [1,2,6]. In a recent investigation on the catalytic CO oxidation on Pt(ll0) [3] it was found that under suitable conditions the initially flat surface starts to form microfacets as monitored by LEED. This structural transformation is accompanied by an increase of the oxygen sticking coefficient and hence of the catalytic activity. The initial situation could, on the other hand, be restored by proper annealing. The present note will demonstrate that the altered adsorption kinetics is associated with the creation of new adsorption sites which at least in part may “survive” the thermal treatment necessary for annealing the long range order of the facet planes and show up as additional high-temperature peaks in thermal desorption spectroscopy (TDS). The experiments were performed with a standard UHV system equipped with Video-LEED with a thoriated Ir filament, a CMA Auger electron spectrometer, a quadrupole mass spectrometer for TDS and a self-compensating device for measuring work function changes (A+) by the vibrating condensator method. The single crystal sample was the same as used in previous work [3]. The concentrations of surface impurities was always kept below the Auger detection limit. Particular care was taken to ensure the absence of any measurable oxide formation even after prolonged oxygen treatments. 0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L702
R. lmbihl et al. / Catalytic oxidation of CO on Pt(ll0)
I
I
600
I
600
I
9000
I
12a
j’
T[Kl Fig. 1. Comparison of the 0, TD spectra for a facetted and non-facetted Pt(ll0)
surface.
Fig. 1 shows TDS data of 0, desorbing from both a flat and a facetted Pt(ll0) surface after 80 L exposure, corresponding to near saturation of the adlayer. The facetted surface had been prepared by keeping the sample at 460 K in a CO/O, atmosphere in the low4 Torr range, whereby the progress of facetting was monitored through the associated continuous splitting of integral order LEED beams. The data presented here are characteristic for surfaces with approximate (320) facet orientation [3]. After development of the facets the supply of the gas mixture was turned off, and after reaching a base pressure of about lop9 Torr the surface was exposed to 0, alone. During the subsequent TDS run the sample was heated up to about 1100 K at which temperature the original flat surface is restored. A second exposure-heating cycle then exhibits the TDS trace from the non-facetted, thermally annealed surface. The TDS data from the facetted surface exhibits two additional peaks (denoted & and 6;) at 860 and 945 K which have to be attributed to sites created in connection with the facet formation. The dominant peak &’ is also present with the flat surface and is attributed to desorption from the perfect (110) terraces. Integration of the TDS traces yields practically the same areas for both the facetted and the non-facetted surface, that means the amount of oxygen adsorbed at saturation is not noticeably affected by the formation of the microfacets - a conclusion which is in agreement with previous findings. However, the maximum A+ increase differed from about 0.5 eV for the
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l10)
L703
non-facetted to 0.7 eV for the facetted surface, indicating a substantially higher auerage dipole moment of the adsorbate complex in the latter case. Since even for the flat Pt(ll0) surface the A+ variation is not proportional to the coverage [4] no attempt will be made for a more quantitative analysis of these data. For similar reasons the recorded A+ versus exposure data cannot be transformed directly into sticking coefficient versus coverage relations. It could, however, be verified by independent Auger measurements that the increase in the initial slope of the A+ versus oxygen exposure curve which is observed upon facetting [3] does in fact represent an increase in the initial sticking coefficient for oxygen adsorption. Fig. 2 compares TDS data from the literature for various types of Pt surfaces [5-71 with the present data in an attempt to possibly identify the features of the facetted Pt(ll0) surface with well-defined structural elements. In all other cases desorption is There exists in fact very little similarity: terminated at about 900 K, with the exception of a & state with Pt(lOO) which was attributed to the presence of defects on this plane [6] and which extents at least over the same temperature range as the fi; and j?; states with the facetted Pt(ll0) plane. At a first glance this result is somewhat surprising in so far as the facets on Pt(ll0) belonging to the [OOl] zone (including the (320) facet) can be decomposed into structural elements of (110) and (100) planes [3]. The limiting case for successive arrays of these elements is offered by the (210) plane, whose 0, TDS data exhibit nevertheless no features resembling the results with the facetted Pt(ll0) plane. The solution of this apparent puzzle has to be sought in the thermal stability of the various surfaces under discussion: While the (210) surface belongs to the set of platinum planes which remain stable, either if clean or oxygen covered [5,8], the microfacets formed on the Pt(ll0) surface are removed by thermal treatment. Now the question arises, if this process may even take place in the presence of adsorbed oxygen, that means prior to 0, desorption in a TDS experiment. In order to answer this question, variations of the LEED pattern from the facetted surface were recorded by the Video technique during heating up the sample parallel to thermal desorption of oxygen. Fig. 3 shows the thermal desorption spectrum from an experiment in which the pi’ state was not fully saturated, but where - on the other hand - the fi,’ and /3; states were fully developed, together with the width of the beam profile of the (0,l) LEED spot along the indicated line in the diffraction pattern and the intensity at the (0, 1) beam position. The width of the beam profile is a measure for the degree of facetting [3]. The presented data demonstrate clearly that the LEED spot starts to sharpen already during desorption of the j3; state and the facets (as far as they can be monitored by LEED) are essentially removed before 0, comes off the surface from the p; and /3; states. Parallel to the shrinking of the half-width of the beam profile also the intensity at the (0, 1) beam position
L704
R. Imbihl et al. / Catalytic oxidation of CO on Pt(II0)
, Pl
@
non-facetted--li 69L
,
I
PtHtOl Tads =L25K
;I
p
I!
qSKls
Ptl2101
B
Tr,ds=300K p =lOKls 37L
y---y
3
Pt(1001
d n
T&=573K p =lOKk
no
@’
.
*
Pt(l121 Tads =9OK p .ZKls
Pl
P2
j-&
600
800
1000
T [Kl Fig. 2. Comparison of the 0, TD spectrum from a facetted Pt(ll0) surface in (a) with 0, TD spectra from other Pt single crystal surfaces which were taken from the literature: (b) Pt(210) [S], (c) Pt(100) [6], (d) Pt(112) [7]. Note that the difference in the TD spectrum of (a) with that of fig. 1 originates from a lower oxygen coverage in (a) which may result as a consequence of experimental uncertainties (residual gas pressure, etc.) even if the exposure is the same.
increases; its decrease at even higher temperature is due to the Debye-Waller factor. If the analogous experiment is carried out with a facetted surface which has not been exposed to oxygen the thermal removal of the facets takes already place at a significantly lower temperature as indicated by the dotted line in fig. 3. Evidently the facets are strongly stabilized by the presence of
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l IO)
L705
1
-&
0.2 2
- beam
-‘\:
clean surface
,,, ..
_-‘\
5
E = 66eV pr0f Ik
_i
0.3 -
5
0.1
I
= 0.L -
5
I
i
O.l-
01 LOO ’
+.....;-__k’ I
600
I ;
I-\\_ 1 )’
’ I
I’ 1
j
/
I
’ “0.0
10.1
I o . 0 1.0
I
I
1
800
loclu
1200
T IKI Fig. 3. Changes in the LEED pattern during the thermal desorption of oxygen from a facetted Pt(ll0) surface as measured by the variation of (0, 1) beam FWHM along the [OOl] direction. Included in fig. 3 is the change of the halfwidth if the same experiment is repeated with a bare facetted surface.
oxygen, since it is even necessary to desorb oxygen before the facets can be removed by thermal annealing. These results demonstrate clearly that the long-range order of the initial (110) surface is restored before the new states pi and pi are depopulated. Their desorption kinetics can therefore not be associated with the presence of facet planes as monitored by LEED, and hence it becomes also plausible why there is no similarity with the TDS data for Pt(210). These states have rather to be attributed to the existence of sites associated with atomic-scale disorder which persist the thermal annealing of the microfacets, but are finally removed upon heating to 1100 K. Since the more strongly held & and j3; oxygen species can now no longer be directly associated with the presence of microfacets, it becomes also a matter of dispute if the increase of the oxygen sticking coefficient during facetting has to be linked with the creation of facet planes or rather with the parallel formation of atomic disorder causing the pi and /3; states in TDS. In order to clarify this question the following experiment was carried out: Heating up of an oxygen-covered, facetted surface was stopped at 770 K prior
L706
R. lmbihl et al. / Catalytic oxidation of CO on Pt(l IO)
I
Pt(1101 0 r,,,_
t-3
T [Kl
Pt(110)/02 7 ,_lr,,
a3
Q-Exposure
1
[ Ll
Fig. 4. Change in the 0, adsorption kinetics after an incomplete annealing of the facetted Pt(ll0) surface has been carried out. After exposing the facetted surface to oxygen (full line in (b)) the heating ramp during the TD spectrum was stopped at 770 K (dashed line in (a)). Readsorption of oxygen (dashed curve in (b)) leads to the 0, TD spectrum shown by a full line in (a).
to desorption of the pi and /?; species, but after restoration of the LEED pattern of the non-facetted surface. After cooling down the sample to 425 K the oxygen left on the surface was reacted off by CO so that a clean surface was restored. This surface was then again exposed to 0, and the build-up of the adlayer followed through the work function change. It turned out that the initial slope of the A+ versus exposure curve was in this case just between the corresponding data for a sample exhibiting facets as discernible by LEED (fig. 4). Although quantitative evaluation of these data was not possible for the above stated reasons, the qualitative evidence is obvious: A surface with facets exhibits the highest oxygen sticking coefficient which is lowered if the facets are thermally removed. If this annealing does not proceed beyond 770 K, however, some atomic-scale disorder still persists and the oxygen sticking coefficient will still be higher than that with a flat Pt(ll0) surface which had been heated to 1100 K. For the latter a value of so = 0.3 had been reported [4]. On the other hand, the increase of the sticking coefficient upon facetting is obviously not solely due to the creation of the defects responsible for the p; and /3; desorption states, but is in part also linked to the formation of facet planes which manifest themselves in the LEED pattern and may be thermally removed at fairly low temperatures. The presence of adsorbed oxygen does not produce sufficient thermal stabilisation of these facets, so that no information
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l10)
L707
about the bond strengths of oxygen atoms coupled to these latter sites can be obtained from thermal desorption experiments. It has to be noted, on the other hand, that removal of the facets in the presence of adsorbed oxygen was observed only to take place after desorption of 0, from the (110) terraces (& state, cf. fig. 3), while with a clean surface this process takes place at substantially lower temperatures [3]. While adsorbed oxygen has thus clearly a stabilising effect on the facets, their formation cannot be initiated by heating a flat Pt(ll0) surface in low pressure 0, atmosphere alone. As demonstrated previously, this takes only place in a CO + 0, mixture under conditions of high CO coverage which lifts the 1 x 2 reconstruction of the surface [3]. This structural transformation is associated with substantial transport of matter which nucleates microfacets (as monitored by LEED) and also creates sites of atomic disorder. Both types of new structural elements exhibit increased oxygen sticking coefficients (in agreement with general experience) and are hence responsible for the enhancement of catalytic activity on CO oxidation. The latter sites are thermally quite stable and even “survive” oxygen desorption which manifests itself in two new high temperature desorption states, while the facets are annealed as soon as 0, desorption from the (110) terraces occurs so that these sites may not be directly identified in oxygen thermal desorption experiments. It should be added that the formation of additional high temperature adsorption states in 0, TDS from Pt(ll0) has recently also been reported by Vishnevski and Savchenko [9] after they held the sample for 15 h under oscillating conditions in a CO/O, atmosphere at 1O-4 Torr. These conditions are known to cause facetting of the surface [3], and it is therefore concluded that the same phenomena were observed in this work.
References [l] G. Pirug, G. Broden and H.P. Bonzel, Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, Vol. 2, Eds. R. Dobrozemsky et al. (Vienna, 1977) p. 907. [2] H. Hopster, H. Ibach and G. Comsa, J. Catalysis 46 (1977) 37. [3] S. Ladas, R. Imbihl and G. Ertl, Surface Sci. 197 (1988) 153; 198 (1988) 42. [4] N. Freyer, M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 166 (1986) 206. [5] M. Ehsasi, private communication. [6] K. Griffith, T.E. Jackman, J.A. Davies and P.R. Norton, Surface Sci. 138 (1984) 113, 125. [7] H.R. Siddiqui, A. Winkler, X. Guo, P. Hagans and J.T. Yates, Jr., Surface Sci. 193 (1988) L17. [8] D.W. BIakeIy and G.A. Somojai, Surface Sci. 112 (1981) 207. [9] L. Vishnevski and V.I. Savchenko, Kinet. Kataliz 28 (1987) 1516.
Surface Science 204 (1988) Li’Ol-L707 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
THE FORMATION OF NEW OXYGEN ADSORPTION STATES ON Pt(ll0) BY FACE-ITING INDUCED BY CATALYTIC REACTION R. IMBIHL, Fritz-Haber-Institut
Received
M. SANDER
and G. ERTL
de-r Max-Planck-Gesellschaft,
10 June 1988; accepted
for publication
Faradayweg
4- 6, D-1000 Berlin 33, Germany
20 June 1988
Catalytic oxidation of CO on a Pt(ll0) surface may lead to the formation of microfacets as monitored by LEED as well as’ of atomic disorder type sites. Both structural elements are associated with an enhanced oxygen sticking coefficient and are hence responsible for the associated increase of catalytic activity. While the facets are stabilized by the presence of adsorbed oxygen, they are annealed during thermal desorption of 0, from the (110) terrace sites. The second type of sites is thermally more stable and manifests itself in two high-temperature 0, thermal desorption states.
It is well established that structural imperfections, such as monatomic steps, may sensitively affect the kinetics of dissociative oxygen adsorption on platinum surfaces [1,2,6]. In a recent investigation on the catalytic CO oxidation on Pt(ll0) [3] it was found that under suitable conditions the initially flat surface starts to form microfacets as monitored by LEED. This structural transformation is accompanied by an increase of the oxygen sticking coefficient and hence of the catalytic activity. The initial situation could, on the other hand, be restored by proper annealing. The present note will demonstrate that the altered adsorption kinetics is associated with the creation of new adsorption sites which at least in part may “survive” the thermal treatment necessary for annealing the long range order of the facet planes and show up as additional high-temperature peaks in thermal desorption spectroscopy (TDS). The experiments were performed with a standard UHV system equipped with Video-LEED with a thoriated Ir filament, a CMA Auger electron spectrometer, a quadrupole mass spectrometer for TDS and a self-compensating device for measuring work function changes (A+) by the vibrating condensator method. The single crystal sample was the same as used in previous work [3]. The concentrations of surface impurities was always kept below the Auger detection limit. Particular care was taken to ensure the absence of any measurable oxide formation even after prolonged oxygen treatments. 0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L702
R. lmbihl et al. / Catalytic oxidation of CO on Pt(ll0)
I
I
600
I
600
I
9000
I
12a
j’
T[Kl Fig. 1. Comparison of the 0, TD spectra for a facetted and non-facetted Pt(ll0)
surface.
Fig. 1 shows TDS data of 0, desorbing from both a flat and a facetted Pt(ll0) surface after 80 L exposure, corresponding to near saturation of the adlayer. The facetted surface had been prepared by keeping the sample at 460 K in a CO/O, atmosphere in the low4 Torr range, whereby the progress of facetting was monitored through the associated continuous splitting of integral order LEED beams. The data presented here are characteristic for surfaces with approximate (320) facet orientation [3]. After development of the facets the supply of the gas mixture was turned off, and after reaching a base pressure of about lop9 Torr the surface was exposed to 0, alone. During the subsequent TDS run the sample was heated up to about 1100 K at which temperature the original flat surface is restored. A second exposure-heating cycle then exhibits the TDS trace from the non-facetted, thermally annealed surface. The TDS data from the facetted surface exhibits two additional peaks (denoted & and 6;) at 860 and 945 K which have to be attributed to sites created in connection with the facet formation. The dominant peak &’ is also present with the flat surface and is attributed to desorption from the perfect (110) terraces. Integration of the TDS traces yields practically the same areas for both the facetted and the non-facetted surface, that means the amount of oxygen adsorbed at saturation is not noticeably affected by the formation of the microfacets - a conclusion which is in agreement with previous findings. However, the maximum A+ increase differed from about 0.5 eV for the
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l10)
L703
non-facetted to 0.7 eV for the facetted surface, indicating a substantially higher auerage dipole moment of the adsorbate complex in the latter case. Since even for the flat Pt(ll0) surface the A+ variation is not proportional to the coverage [4] no attempt will be made for a more quantitative analysis of these data. For similar reasons the recorded A+ versus exposure data cannot be transformed directly into sticking coefficient versus coverage relations. It could, however, be verified by independent Auger measurements that the increase in the initial slope of the A+ versus oxygen exposure curve which is observed upon facetting [3] does in fact represent an increase in the initial sticking coefficient for oxygen adsorption. Fig. 2 compares TDS data from the literature for various types of Pt surfaces [5-71 with the present data in an attempt to possibly identify the features of the facetted Pt(ll0) surface with well-defined structural elements. In all other cases desorption is There exists in fact very little similarity: terminated at about 900 K, with the exception of a & state with Pt(lOO) which was attributed to the presence of defects on this plane [6] and which extents at least over the same temperature range as the fi; and j?; states with the facetted Pt(ll0) plane. At a first glance this result is somewhat surprising in so far as the facets on Pt(ll0) belonging to the [OOl] zone (including the (320) facet) can be decomposed into structural elements of (110) and (100) planes [3]. The limiting case for successive arrays of these elements is offered by the (210) plane, whose 0, TDS data exhibit nevertheless no features resembling the results with the facetted Pt(ll0) plane. The solution of this apparent puzzle has to be sought in the thermal stability of the various surfaces under discussion: While the (210) surface belongs to the set of platinum planes which remain stable, either if clean or oxygen covered [5,8], the microfacets formed on the Pt(ll0) surface are removed by thermal treatment. Now the question arises, if this process may even take place in the presence of adsorbed oxygen, that means prior to 0, desorption in a TDS experiment. In order to answer this question, variations of the LEED pattern from the facetted surface were recorded by the Video technique during heating up the sample parallel to thermal desorption of oxygen. Fig. 3 shows the thermal desorption spectrum from an experiment in which the pi’ state was not fully saturated, but where - on the other hand - the fi,’ and /3; states were fully developed, together with the width of the beam profile of the (0,l) LEED spot along the indicated line in the diffraction pattern and the intensity at the (0, 1) beam position. The width of the beam profile is a measure for the degree of facetting [3]. The presented data demonstrate clearly that the LEED spot starts to sharpen already during desorption of the j3; state and the facets (as far as they can be monitored by LEED) are essentially removed before 0, comes off the surface from the p; and /3; states. Parallel to the shrinking of the half-width of the beam profile also the intensity at the (0, 1) beam position
L704
R. Imbihl et al. / Catalytic oxidation of CO on Pt(II0)
, Pl
@
non-facetted--li 69L
,
I
PtHtOl Tads =L25K
;I
p
I!
qSKls
Ptl2101
B
Tr,ds=300K p =lOKls 37L
y---y
3
Pt(1001
d n
T&=573K p =lOKk
no
@’
.
*
Pt(l121 Tads =9OK p .ZKls
Pl
P2
j-&
600
800
1000
T [Kl Fig. 2. Comparison of the 0, TD spectrum from a facetted Pt(ll0) surface in (a) with 0, TD spectra from other Pt single crystal surfaces which were taken from the literature: (b) Pt(210) [S], (c) Pt(100) [6], (d) Pt(112) [7]. Note that the difference in the TD spectrum of (a) with that of fig. 1 originates from a lower oxygen coverage in (a) which may result as a consequence of experimental uncertainties (residual gas pressure, etc.) even if the exposure is the same.
increases; its decrease at even higher temperature is due to the Debye-Waller factor. If the analogous experiment is carried out with a facetted surface which has not been exposed to oxygen the thermal removal of the facets takes already place at a significantly lower temperature as indicated by the dotted line in fig. 3. Evidently the facets are strongly stabilized by the presence of
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l IO)
L705
1
-&
0.2 2
- beam
-‘\:
clean surface
,,, ..
_-‘\
5
E = 66eV pr0f Ik
_i
0.3 -
5
0.1
I
= 0.L -
5
I
i
O.l-
01 LOO ’
+.....;-__k’ I
600
I ;
I-\\_ 1 )’
’ I
I’ 1
j
/
I
’ “0.0
10.1
I o . 0 1.0
I
I
1
800
loclu
1200
T IKI Fig. 3. Changes in the LEED pattern during the thermal desorption of oxygen from a facetted Pt(ll0) surface as measured by the variation of (0, 1) beam FWHM along the [OOl] direction. Included in fig. 3 is the change of the halfwidth if the same experiment is repeated with a bare facetted surface.
oxygen, since it is even necessary to desorb oxygen before the facets can be removed by thermal annealing. These results demonstrate clearly that the long-range order of the initial (110) surface is restored before the new states pi and pi are depopulated. Their desorption kinetics can therefore not be associated with the presence of facet planes as monitored by LEED, and hence it becomes also plausible why there is no similarity with the TDS data for Pt(210). These states have rather to be attributed to the existence of sites associated with atomic-scale disorder which persist the thermal annealing of the microfacets, but are finally removed upon heating to 1100 K. Since the more strongly held & and j3; oxygen species can now no longer be directly associated with the presence of microfacets, it becomes also a matter of dispute if the increase of the oxygen sticking coefficient during facetting has to be linked with the creation of facet planes or rather with the parallel formation of atomic disorder causing the pi and /3; states in TDS. In order to clarify this question the following experiment was carried out: Heating up of an oxygen-covered, facetted surface was stopped at 770 K prior
L706
R. lmbihl et al. / Catalytic oxidation of CO on Pt(l IO)
I
Pt(1101 0 r,,,_
t-3
T [Kl
Pt(110)/02 7 ,_lr,,
a3
Q-Exposure
1
[ Ll
Fig. 4. Change in the 0, adsorption kinetics after an incomplete annealing of the facetted Pt(ll0) surface has been carried out. After exposing the facetted surface to oxygen (full line in (b)) the heating ramp during the TD spectrum was stopped at 770 K (dashed line in (a)). Readsorption of oxygen (dashed curve in (b)) leads to the 0, TD spectrum shown by a full line in (a).
to desorption of the pi and /?; species, but after restoration of the LEED pattern of the non-facetted surface. After cooling down the sample to 425 K the oxygen left on the surface was reacted off by CO so that a clean surface was restored. This surface was then again exposed to 0, and the build-up of the adlayer followed through the work function change. It turned out that the initial slope of the A+ versus exposure curve was in this case just between the corresponding data for a sample exhibiting facets as discernible by LEED (fig. 4). Although quantitative evaluation of these data was not possible for the above stated reasons, the qualitative evidence is obvious: A surface with facets exhibits the highest oxygen sticking coefficient which is lowered if the facets are thermally removed. If this annealing does not proceed beyond 770 K, however, some atomic-scale disorder still persists and the oxygen sticking coefficient will still be higher than that with a flat Pt(ll0) surface which had been heated to 1100 K. For the latter a value of so = 0.3 had been reported [4]. On the other hand, the increase of the sticking coefficient upon facetting is obviously not solely due to the creation of the defects responsible for the p; and /3; desorption states, but is in part also linked to the formation of facet planes which manifest themselves in the LEED pattern and may be thermally removed at fairly low temperatures. The presence of adsorbed oxygen does not produce sufficient thermal stabilisation of these facets, so that no information
R. Imbihl et al. / Catalytic oxidation of CO on Pt(l10)
L707
about the bond strengths of oxygen atoms coupled to these latter sites can be obtained from thermal desorption experiments. It has to be noted, on the other hand, that removal of the facets in the presence of adsorbed oxygen was observed only to take place after desorption of 0, from the (110) terraces (& state, cf. fig. 3), while with a clean surface this process takes place at substantially lower temperatures [3]. While adsorbed oxygen has thus clearly a stabilising effect on the facets, their formation cannot be initiated by heating a flat Pt(ll0) surface in low pressure 0, atmosphere alone. As demonstrated previously, this takes only place in a CO + 0, mixture under conditions of high CO coverage which lifts the 1 x 2 reconstruction of the surface [3]. This structural transformation is associated with substantial transport of matter which nucleates microfacets (as monitored by LEED) and also creates sites of atomic disorder. Both types of new structural elements exhibit increased oxygen sticking coefficients (in agreement with general experience) and are hence responsible for the enhancement of catalytic activity on CO oxidation. The latter sites are thermally quite stable and even “survive” oxygen desorption which manifests itself in two new high temperature desorption states, while the facets are annealed as soon as 0, desorption from the (110) terraces occurs so that these sites may not be directly identified in oxygen thermal desorption experiments. It should be added that the formation of additional high temperature adsorption states in 0, TDS from Pt(ll0) has recently also been reported by Vishnevski and Savchenko [9] after they held the sample for 15 h under oscillating conditions in a CO/O, atmosphere at 1O-4 Torr. These conditions are known to cause facetting of the surface [3], and it is therefore concluded that the same phenomena were observed in this work.
References [l] G. Pirug, G. Broden and H.P. Bonzel, Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, Vol. 2, Eds. R. Dobrozemsky et al. (Vienna, 1977) p. 907. [2] H. Hopster, H. Ibach and G. Comsa, J. Catalysis 46 (1977) 37. [3] S. Ladas, R. Imbihl and G. Ertl, Surface Sci. 197 (1988) 153; 198 (1988) 42. [4] N. Freyer, M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 166 (1986) 206. [5] M. Ehsasi, private communication. [6] K. Griffith, T.E. Jackman, J.A. Davies and P.R. Norton, Surface Sci. 138 (1984) 113, 125. [7] H.R. Siddiqui, A. Winkler, X. Guo, P. Hagans and J.T. Yates, Jr., Surface Sci. 193 (1988) L17. [8] D.W. BIakeIy and G.A. Somojai, Surface Sci. 112 (1981) 207. [9] L. Vishnevski and V.I. Savchenko, Kinet. Kataliz 28 (1987) 1516.