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Molecular beam studies of alkali promotion: O2 sticking on Pt(111)+K
Volume 186, number 2,3
CHEMICAL PHYSICS LETTERS
8 November 1991
Molecular beam studies of alkali promotion: O2 sticking on Pt(lll)+K J.K. Brown and A.C. Luntz IBM Research, Almaden Research Center, San Jose, CL495120, USA Received 12 September 1991; in final form 18 September 1991
Molecular beam techniques have been used to study the promotion of O2 sticking on a Pt( 1I I ) surface due to co-adsorbed K The results show a complicated dependence of this promotion on K coverage, incident energy and surface temperature, and cannot be interpretedin terms of simple one-dimensional charge-transfer models often used to rationalize alkali promotion. There is evidence that molecular precursors, including that due to a physisorbed species,playa dominantrole in the sticking of O2 on the K-modified Pt ( 11I ) surface.
1.
Introduction
Small amounts of chemical modifiers can cause dramatic changes in both the rate and selectivity of many heterogeneous catalytic reactions, and this has profound implications for industrial catalytic processes. As a result, there has been an enormous effort over the past decade to understand the mechanism for this poisoning and promotion using well defined surface science studies on single crystal substrates. These have typically taken the form of co-adsorption studies using the whole alphabet of surface science techniques. One of the most actively studied of these is the role of alkali co-adsorption [ I] since effects can be quite dramatic and are key in promoting A$ dissociation in the Born-Haber synthesis of ammonia. The simplest picture that has emerged to understand many aspects of these co-adsorption experiments is based on charge-transfer arguments [ I] _ Electropositive additives such as the alkalis, which are largely ionized upon adsorption, donate electrons to the metal surface. This enhanced surface electron density can stabilize a mdlecularly adsorbed state through enhanced back donation into an adsorbate affinity resonance near the Fermi energy. Since such adsorbate resonances are often antibonding IC*resonances, this can also lower the barrier to dissociation of the adsorbed molecule, e.g.
“promote” dissociation. Although surface electron structure calculations have substantiated that these charge-transfer arguments can rationalize many aspects of the alkali co-adsorption experiments, considerably more complicated interactions have sometimes been invoked as well, i.e. complex formation, direct interactions, rehybridization, etc. [ 11. Static co-adsorption experiments such as vibrational or electronic spectroscopy tell us a great deal about the electronic interaction of alkalis with stable molecules adsorbed on the surface. They do not, however, directly probe the effect of the alkali on the dynamic properties, i.e. on barriers to adsorption or dissociation. As a result, conclusions regarding the dynamic properties are often only indirectly inferred from observed alkali perturbations of the stable molecular well region. One of the most direct ways to probe dynamic features of the potential energy surface (PES) is through molecular beam studies of sticking [ 2,3 1, especially when dissociative chemisorption is the end result of the sticking. One of the key advantages of molecular beam experiments is the possibility to probe dynamic pathways at energies far above the lowest thermally activated one. Such studies allow a good understanding of energy dissipation and nonreactive sticking, readily distinguish between precursor mediated and direct dissociation dynamics, probe the nature and dimensionality of “activation” barriers,
Om9-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
125
Volume 186, number 2,3
8 November 199I
CHEMICALPHYSICSLETTERS
and resolve many issues relating to the “location” of the activation barrier within the PES. With these generalities in mind, we report in this Letter specific molecular beam studies of the sticking of O2 on a Pt ( 111) surface, and how this is modified by co-adsorbed K. The aim is to directly probe the effects of K adsorption on sticking and dissociation dynamics, rather than inferring this indirectly from spectroscopic evidence, as has been done previously for this system [ 41. It is well known that O2 molecularly adsorbed on Pt ( 111) is in the form of Oy formed by partial charge transfer into a K* resonance, Hence, very large alkali perturbations are anticipated to barriers to sticking and dissociation from the enhanced back donation of the co-adsorbed K.
OK
0.33 t 0.22 0 -
0.11 v _
cn”c
0.07 A 0.05 l 0.035P 0.00
l
(a)T,= 100 K
0.00
’
0.00
1.00
I
0.25
I
I
I
I
I
I
I
0.50 0.75 1.00 Incident energy(eV) I
1.25
0,
I 1.50
I
2. Experimental The experimental apparatus and techniques have been described in detail previously [ 5-71 and will not be repeated here. Sticking measurements were performed using the method of King and Wells [ 8 ] using a triply differentially pumped seeded supersonic nozzle molecular beam of O2 to control incident translational energy and angle of incidence. K was dosed onto the surface using a well outgassed SAES getter source, with typical deposition rates of 2 ML/min. K coverages &G 0.33 were obtained by annealing to various T,,In any event, f3, was measured for each sticking experiment by measuring the ratio of K to Pt Auger peaks [ 71.
3. Results and discussion The results for various sticking experiments are given in fig. 1. Fig. la shows the initial O2 sticking coefficient (zero adsorbed 0 or 0,) S0 as a function of incident energy Ei for a variety of initial K coverages f&, and for normal incidence and a surface temperature T, = 100 K. Fig. lb shows equivalent measurements, but for a T,=300 K. The curves drawn through the points have no theoretical significance and are merely present to aid the eye. The results for &=O are in good agreement with those presented earlier [ 5 1. The overwhelming qualitative 126
0.75
G? 0.50 0.07 A 0.25
0.00 0.00
0.25
0.50 0.75 1.00 Incident energy(eV)
1.25
1.50
Fig. I. (a) Initial sticking coefficient S,, as a function of incident energy for O2on a K modified Pt( 111)surface at T,= I00K. 0, is the K coverage and is given by the various symbols.Normal incidence wasemployed.The lines through the points are only for clarity. (b) Sameas (a) but for TS=3C41 K.
feature apparent in fig. 1 is that there is a dramatic increase in sticking with 19,for all incident energies and T,. This “promotion” in sticking is in good qualitative agreement with prior experiments monitoring the adsorption of ambient O2 gas at T,=300 K on
Pt(ll1)
with &=0.33 [4].
3. I. Sticking on bare surface The adiabatic interaction of O2 with a bare Pt (111) surface can be constructed from three sep
Volume 186, number 2,3
CHEMICALPHYSICSLETTERS
arate diabatic interactions, as indicated schematically in fig. 2. It must be emphasized, however, that although these diabatic surfaces are drawn in fig. 2 as one dimensional, they are each in reality multidimensional. These three diabatic surfaces extrapolate asymptotically to the Pt+O,, Pt+O, and Pt t 20 limits. This PES supports three distinct adsorbed species corresponding to local minima in the three diabatic surfaces; physisorption, molecular chemisorption formed by charge transfer from the metal to 02, and the atomic well formed by dissociation [9,10]. Befitting such a complicated PES, the sticking behavior of O2 on Pt ( 111) is also rather complicated [ 5,9,lL 1. Depending upon experimental conditions, sticking can result in final occupation of any of the three PES minima, with prior minima possibly playing precursor roles. At low incident energies (Ei < 0.1 eV), and low surface temperatures T,<38 K, only the physisorbed species is produced in sticking. For 38 6 r, G 130 IS, the molecularly chemisorbed species is produced by thermal conversion from the physisorbed species. For T,> 130 K, the atomic state is then produced by thermal conversion from the molecularly chemisorbed species. Thus, at low Ei dissociative chemisorption is described in terms of sequential precursors; the physisorbed state is a precursor to molecular chemisorption and the molecularly chemisorbed species is a precursor to dissocia-
Fig. 2. Schematic one-dimensional representation of the PES for Oz dissociation on Pt( I1 I ). The dashed lines represent some of the anticipated changes in this PES with adsorbed K. Pt+02, Pt+O? and Pt+20 label the asymptote of the three diabatic surfaces used to construct the PES.
8 November1991
tion [ 91. For higher incident beam energies, sticking increases due to direct translational activation over some barrier. Although this was originally ascribed to a quasi-direct dissociation, i.e. to passage over the barrier between molecular chemisorption and dissociation [ 51, it has recently been shown that this is in fact due to the barrier to molecular chemisorption, so that dissociation at high Ei must be described in terms of an “activated” molecular precursor [II]. The results in fig. 1 for &=O are in accord with this general picture. At T, = 300 K, So measures only the dissociated fraction since this r, is above the desorption-conversion temperature of the molecular states. The increase in So with Ei is interpreted as “activated” adsorption directly into the molecularly chemisorbed state, which acts as a precursor to dissociation [ 111. At T,= LOCI K, both the molecularly chemisorbed and the atomic species are stable on the surface, and So cannot distinguish between these ultimate fates. The increase in So at low Ei for T,= 100 K has been interpreted as initial trapping into the physisorbed state followed by thermal conversion into the molecularly chemisorbed state [ 91. 3.2. Slicking on K-modified surface The dominant influence of co-adsorbed K on the dissociative PES for O2 on Pt ( 111) is anticipated to be that shown schematically in fig. 2 by the dashed curves. The major effect is stabilization of the molecularly chemisorbed state via enhanced charge transfer and a huge lowering of the asymptote for this channel. The latter occurs due to the dramatic alkaliinduced lowering of the work function [ 1 ] since this asymptote occurs at @-A, where 9 is the surface work function and A is the O2 electron affinity. This lowering of the asymptote will certainly cause the avoided crossing between the physisorbed diabatic surface at@ the molecularly chemisorbed one to occur further from the surface and thus lower the barrier. This could in principle completely destroy the stability of the physisorbecl species as a separate entity. In addition, since the enhanced charge transfer occurs into a n* O2 affinity level, we anticipate a lowering of the barrier to dissociation as well. In fact, it is certainly possible and reasonable that the barrier between the molecularly chemisorbed state and dis127
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CHEMICAL PHYSICS LETTERS
sociation is effectively removed. In addition to the effects of charge transfer on the molecularly chemisorbed state, the lowering of the work function should enhance Pauli repulsion in the physisorbed state by causing the Pt 5s electrons to tail off more slowly into the vacuum. In the absence of charge transfer stabilization, this effect can cause a significant increase in the barrier to dissociation or chemisorption, as has been recently demonstrated for H2 on Pt ( 111) t K [ 7 1. Effects of co-adsorbed K on the atomic state are neglected in this discussion since they should not dominate the sticking behavior. It must be emphasized again that fig. 2 should not be taken too literally as this is only a one-dimensional representation of a complicated multidimensional PES. The dramatic effect of the co-adsorbed alkali on the dissociative PES is anticipated to strongly affect the sticking via any of the many paths available. This could, for example, open up direct dissociation as a channel, as well as strongly affecting the precursor channels. For sticking with &=0.33, S, is quite large for all Ei at both T,= 100 and 300 K. At T,=300 K, electron spectroscopy studies [ 41 have demonstrated that the ultimate fate of sticking is a highly perturbed dissociated 0 species, but probably not KzO. At T,=100 K, we have no information as to whether the ultimate fate of the adsorbed O2 is dissociation or a perturbed molecularly chemisorbed species. Since S0 is so large and nearly independent of .Ei and T,at 19~~0.33,we assume that sticking is not limited by either trapping into the physisorbed species or by the barrier to molecular chemisorption. We suspect that at this high &, the O2simply dissociates directly upon impact since all barriers are obliterated in the PES. The sticking behavior for intermediate 0, is quite complicated. At Ei3 0.5eV, S,, increases with T,for large SK, but decreases with T,for small 19,. For Eic 0.1eV, there is a large and abrupt increase in So, especially at T,=100 K in the lower ranges of 0,. Because of the complex behavior of S, with 0, and T,, and limited experimental information, we will not attempt to develop any quantitative description. The results are qualitatively consistent, however, with a general description in terms of sticking behavior of a mixed two-phase region; one essentially describing initial interaction with the bare Pt ( 1I 1) surface and the other representing initial interaction with the 128
8 November I99 I
highly K perturbed part of the surface ( where S, k! 1) . From the increase in St, with f& at high Ei and T,=300 K, where long-lived precursors must play a minimal role, this perturbed area is estimated as roughly 3-5 Pt ( 111) surface unit cells per adsorbed K. Thus, O2 collisions with the K perturbed region should always produce sticking, while initial collision with the local “bare” surface region may produce either reflection back into the gas phase or trapping into the physisorbed state or molecularly chemisorbed state depending upon Eim Once in these “bare surface precursors”, the O2 can then subsequently either thermally desorb or transport to K perturbed regions and produce sticking. The sharp increase in S, at the lowest Ei observed at T,=100 K, even for modest &, is consistent with initial trapping into a “more or less” unperturbed bare physisorption state, followed by sticking at K perturbed regions. Since thermal desorption should compete with parallel transport of 02 on the surface to K “sinks”, & is also expected to be a strong function of T,for this precursor mechanism. Since desorption rates from the physisorbed state are extremely high at T,= 300 K, this precursor is anticipated to play only a limited role in sticking at this 7’,, even for modest 0,, This is evidently true since the sharp increase in S0 at low Ei is basically absent at T,=300K. The dominance in this sticking of a precursor based on the physisorbed stated is a somewhat surprising scenario since the naive expectation based on fig. 2 is that the K induced lowering of the work function should essentially destroy the physisorbed state as a separate entity. This expectation, however, is based on a one-dimensional picture and a homogeneous lowering of the work function. In reality, the adsorbed K induces a local electrostatic potential close to the surface, which only becomes a homogeneous work function change far from the surface. The interpretation of sticking in terms of a bare surface precursor then suggests that “local” aspects of the work function lowering are very important at distances to the surface characteristic of the physisorption well. In addition, the barrier to molecular chemisorption is in reality multidimensional. It must depend quite strongly on molecular orientation since charge transfer into a x* resonance should be most favorable when the molecule is parallel to the sur-
Volume186,number2,3
CHEMICAL PHYSICS LETTERS
face. Hence, even in the presence of adsorbed K, a considerable barrier may still exist to molecular chemisorption for some molecular orientations, and ultimately produce trapping into a physisorbed state. For high Ei the physisorbed state must play a minimal role in the sticking. In this case, initial sticking is either into the molecularly chemisorbed state for collision with the bare surface patches and (probably) directly dissociative for the collision with K perturbed regions. Since thermal desorption rates are not too fast at T,=300 K from the molecularly chemisorbed precursor, the difference between So at T,= 100 and 300 K is not extreme. There is, however, some difference and this once again points to the role of molecular precursors on “bare” surface patches in the alkali promotion of sticking.
4. Summaryand conclusions We have utilized molecular beam techniques to probe the effects of co-adsorbed K on O2 sticking on Pt ( 111). The dominant qualitative aspect is a significant promotion of the sticking with K coverage. The results show a complicated dependence of the promotion on both Ei and T,, and cannot be interpreted in terms of simple one-dimensional charge-
8 November1991
transfer models often used to rationalize alkali promotion. There is considerable evidence that molecular precursors, including that due to a physisorbed species, play a dominant role in the sticking on the K-modified surfaces. It also appears that the sticking on the K-modified surfaces is best interpreted in terms of O2 interaction with inhomogeneous mixed phase regions on the Pt( 111) t K. References 111H.P. Bonzel, Surface Sci. Rep. 8 (1987) 43. 121C.T. Rettner and D.J. Auerbach, Comments At. Mol. Phys. 20 (1987) 153. [3] A.C. Luntz, Physica Scripta 35 (1987) 193. [4] G. Pirug, H.P. Bonzel and G. Broden, Surface Sci. 122 (1982) I. [r;] A.C. Luntz, M.D. Williams and D.S. Bethune, J. Chem. Phys. 89 (1988) 4381. [6] A.C. Luntz, J.K. Brown and M.D. Williams, J. Chem. Phys. 93 (1990) 5240. [7] J.K. Brown, A.C. Luntz and P.A. Schultz, J. Chem. Phys., in press. [8] D.A. King and M.G. Wells, Proc. Roy. Sot. A 339 (1974) 245. [9] A.C. Luntz, J. Grimblot and D.E. Fowler, Phys. Rev. B 39 (1989) 12903. [IO] W. Wunh, J. Stohr, W. Jark, P. Stevens, J. Solomon and R.J. Madix, Phys. Rev. Letters 65 (1990) 2426. [ I I ] C.T. Rettner and C.B. Mullins, J. Chem. Phys. 94 ( I991 ) 1626.
129
CHEMICAL PHYSICS LETTERS
8 November 1991
Molecular beam studies of alkali promotion: O2 sticking on Pt(lll)+K J.K. Brown and A.C. Luntz IBM Research, Almaden Research Center, San Jose, CL495120, USA Received 12 September 1991; in final form 18 September 1991
Molecular beam techniques have been used to study the promotion of O2 sticking on a Pt( 1I I ) surface due to co-adsorbed K The results show a complicated dependence of this promotion on K coverage, incident energy and surface temperature, and cannot be interpretedin terms of simple one-dimensional charge-transfer models often used to rationalize alkali promotion. There is evidence that molecular precursors, including that due to a physisorbed species,playa dominantrole in the sticking of O2 on the K-modified Pt ( 11I ) surface.
1.
Introduction
Small amounts of chemical modifiers can cause dramatic changes in both the rate and selectivity of many heterogeneous catalytic reactions, and this has profound implications for industrial catalytic processes. As a result, there has been an enormous effort over the past decade to understand the mechanism for this poisoning and promotion using well defined surface science studies on single crystal substrates. These have typically taken the form of co-adsorption studies using the whole alphabet of surface science techniques. One of the most actively studied of these is the role of alkali co-adsorption [ I] since effects can be quite dramatic and are key in promoting A$ dissociation in the Born-Haber synthesis of ammonia. The simplest picture that has emerged to understand many aspects of these co-adsorption experiments is based on charge-transfer arguments [ I] _ Electropositive additives such as the alkalis, which are largely ionized upon adsorption, donate electrons to the metal surface. This enhanced surface electron density can stabilize a mdlecularly adsorbed state through enhanced back donation into an adsorbate affinity resonance near the Fermi energy. Since such adsorbate resonances are often antibonding IC*resonances, this can also lower the barrier to dissociation of the adsorbed molecule, e.g.
“promote” dissociation. Although surface electron structure calculations have substantiated that these charge-transfer arguments can rationalize many aspects of the alkali co-adsorption experiments, considerably more complicated interactions have sometimes been invoked as well, i.e. complex formation, direct interactions, rehybridization, etc. [ 11. Static co-adsorption experiments such as vibrational or electronic spectroscopy tell us a great deal about the electronic interaction of alkalis with stable molecules adsorbed on the surface. They do not, however, directly probe the effect of the alkali on the dynamic properties, i.e. on barriers to adsorption or dissociation. As a result, conclusions regarding the dynamic properties are often only indirectly inferred from observed alkali perturbations of the stable molecular well region. One of the most direct ways to probe dynamic features of the potential energy surface (PES) is through molecular beam studies of sticking [ 2,3 1, especially when dissociative chemisorption is the end result of the sticking. One of the key advantages of molecular beam experiments is the possibility to probe dynamic pathways at energies far above the lowest thermally activated one. Such studies allow a good understanding of energy dissipation and nonreactive sticking, readily distinguish between precursor mediated and direct dissociation dynamics, probe the nature and dimensionality of “activation” barriers,
Om9-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
125
Volume 186, number 2,3
8 November 199I
CHEMICALPHYSICSLETTERS
and resolve many issues relating to the “location” of the activation barrier within the PES. With these generalities in mind, we report in this Letter specific molecular beam studies of the sticking of O2 on a Pt ( 111) surface, and how this is modified by co-adsorbed K. The aim is to directly probe the effects of K adsorption on sticking and dissociation dynamics, rather than inferring this indirectly from spectroscopic evidence, as has been done previously for this system [ 41. It is well known that O2 molecularly adsorbed on Pt ( 111) is in the form of Oy formed by partial charge transfer into a K* resonance, Hence, very large alkali perturbations are anticipated to barriers to sticking and dissociation from the enhanced back donation of the co-adsorbed K.
OK
0.33 t 0.22 0 -
0.11 v _
cn”c
0.07 A 0.05 l 0.035P 0.00
l
(a)T,= 100 K
0.00
’
0.00
1.00
I
0.25
I
I
I
I
I
I
I
0.50 0.75 1.00 Incident energy(eV) I
1.25
0,
I 1.50
I
2. Experimental The experimental apparatus and techniques have been described in detail previously [ 5-71 and will not be repeated here. Sticking measurements were performed using the method of King and Wells [ 8 ] using a triply differentially pumped seeded supersonic nozzle molecular beam of O2 to control incident translational energy and angle of incidence. K was dosed onto the surface using a well outgassed SAES getter source, with typical deposition rates of 2 ML/min. K coverages &G 0.33 were obtained by annealing to various T,,In any event, f3, was measured for each sticking experiment by measuring the ratio of K to Pt Auger peaks [ 71.
3. Results and discussion The results for various sticking experiments are given in fig. 1. Fig. la shows the initial O2 sticking coefficient (zero adsorbed 0 or 0,) S0 as a function of incident energy Ei for a variety of initial K coverages f&, and for normal incidence and a surface temperature T, = 100 K. Fig. lb shows equivalent measurements, but for a T,=300 K. The curves drawn through the points have no theoretical significance and are merely present to aid the eye. The results for &=O are in good agreement with those presented earlier [ 5 1. The overwhelming qualitative 126
0.75
G? 0.50 0.07 A 0.25
0.00 0.00
0.25
0.50 0.75 1.00 Incident energy(eV)
1.25
1.50
Fig. I. (a) Initial sticking coefficient S,, as a function of incident energy for O2on a K modified Pt( 111)surface at T,= I00K. 0, is the K coverage and is given by the various symbols.Normal incidence wasemployed.The lines through the points are only for clarity. (b) Sameas (a) but for TS=3C41 K.
feature apparent in fig. 1 is that there is a dramatic increase in sticking with 19,for all incident energies and T,. This “promotion” in sticking is in good qualitative agreement with prior experiments monitoring the adsorption of ambient O2 gas at T,=300 K on
Pt(ll1)
with &=0.33 [4].
3. I. Sticking on bare surface The adiabatic interaction of O2 with a bare Pt (111) surface can be constructed from three sep
Volume 186, number 2,3
CHEMICALPHYSICSLETTERS
arate diabatic interactions, as indicated schematically in fig. 2. It must be emphasized, however, that although these diabatic surfaces are drawn in fig. 2 as one dimensional, they are each in reality multidimensional. These three diabatic surfaces extrapolate asymptotically to the Pt+O,, Pt+O, and Pt t 20 limits. This PES supports three distinct adsorbed species corresponding to local minima in the three diabatic surfaces; physisorption, molecular chemisorption formed by charge transfer from the metal to 02, and the atomic well formed by dissociation [9,10]. Befitting such a complicated PES, the sticking behavior of O2 on Pt ( 111) is also rather complicated [ 5,9,lL 1. Depending upon experimental conditions, sticking can result in final occupation of any of the three PES minima, with prior minima possibly playing precursor roles. At low incident energies (Ei < 0.1 eV), and low surface temperatures T,<38 K, only the physisorbed species is produced in sticking. For 38 6 r, G 130 IS, the molecularly chemisorbed species is produced by thermal conversion from the physisorbed species. For T,> 130 K, the atomic state is then produced by thermal conversion from the molecularly chemisorbed species. Thus, at low Ei dissociative chemisorption is described in terms of sequential precursors; the physisorbed state is a precursor to molecular chemisorption and the molecularly chemisorbed species is a precursor to dissocia-
Fig. 2. Schematic one-dimensional representation of the PES for Oz dissociation on Pt( I1 I ). The dashed lines represent some of the anticipated changes in this PES with adsorbed K. Pt+02, Pt+O? and Pt+20 label the asymptote of the three diabatic surfaces used to construct the PES.
8 November1991
tion [ 91. For higher incident beam energies, sticking increases due to direct translational activation over some barrier. Although this was originally ascribed to a quasi-direct dissociation, i.e. to passage over the barrier between molecular chemisorption and dissociation [ 51, it has recently been shown that this is in fact due to the barrier to molecular chemisorption, so that dissociation at high Ei must be described in terms of an “activated” molecular precursor [II]. The results in fig. 1 for &=O are in accord with this general picture. At T, = 300 K, So measures only the dissociated fraction since this r, is above the desorption-conversion temperature of the molecular states. The increase in So with Ei is interpreted as “activated” adsorption directly into the molecularly chemisorbed state, which acts as a precursor to dissociation [ 111. At T,= LOCI K, both the molecularly chemisorbed and the atomic species are stable on the surface, and So cannot distinguish between these ultimate fates. The increase in So at low Ei for T,= 100 K has been interpreted as initial trapping into the physisorbed state followed by thermal conversion into the molecularly chemisorbed state [ 91. 3.2. Slicking on K-modified surface The dominant influence of co-adsorbed K on the dissociative PES for O2 on Pt ( 111) is anticipated to be that shown schematically in fig. 2 by the dashed curves. The major effect is stabilization of the molecularly chemisorbed state via enhanced charge transfer and a huge lowering of the asymptote for this channel. The latter occurs due to the dramatic alkaliinduced lowering of the work function [ 1 ] since this asymptote occurs at @-A, where 9 is the surface work function and A is the O2 electron affinity. This lowering of the asymptote will certainly cause the avoided crossing between the physisorbed diabatic surface at@ the molecularly chemisorbed one to occur further from the surface and thus lower the barrier. This could in principle completely destroy the stability of the physisorbecl species as a separate entity. In addition, since the enhanced charge transfer occurs into a n* O2 affinity level, we anticipate a lowering of the barrier to dissociation as well. In fact, it is certainly possible and reasonable that the barrier between the molecularly chemisorbed state and dis127
Volume 186, number 2,3
CHEMICAL PHYSICS LETTERS
sociation is effectively removed. In addition to the effects of charge transfer on the molecularly chemisorbed state, the lowering of the work function should enhance Pauli repulsion in the physisorbed state by causing the Pt 5s electrons to tail off more slowly into the vacuum. In the absence of charge transfer stabilization, this effect can cause a significant increase in the barrier to dissociation or chemisorption, as has been recently demonstrated for H2 on Pt ( 111) t K [ 7 1. Effects of co-adsorbed K on the atomic state are neglected in this discussion since they should not dominate the sticking behavior. It must be emphasized again that fig. 2 should not be taken too literally as this is only a one-dimensional representation of a complicated multidimensional PES. The dramatic effect of the co-adsorbed alkali on the dissociative PES is anticipated to strongly affect the sticking via any of the many paths available. This could, for example, open up direct dissociation as a channel, as well as strongly affecting the precursor channels. For sticking with &=0.33, S, is quite large for all Ei at both T,= 100 and 300 K. At T,=300 K, electron spectroscopy studies [ 41 have demonstrated that the ultimate fate of sticking is a highly perturbed dissociated 0 species, but probably not KzO. At T,=100 K, we have no information as to whether the ultimate fate of the adsorbed O2 is dissociation or a perturbed molecularly chemisorbed species. Since S0 is so large and nearly independent of .Ei and T,at 19~~0.33,we assume that sticking is not limited by either trapping into the physisorbed species or by the barrier to molecular chemisorption. We suspect that at this high &, the O2simply dissociates directly upon impact since all barriers are obliterated in the PES. The sticking behavior for intermediate 0, is quite complicated. At Ei3 0.5eV, S,, increases with T,for large SK, but decreases with T,for small 19,. For Eic 0.1eV, there is a large and abrupt increase in So, especially at T,=100 K in the lower ranges of 0,. Because of the complex behavior of S, with 0, and T,, and limited experimental information, we will not attempt to develop any quantitative description. The results are qualitatively consistent, however, with a general description in terms of sticking behavior of a mixed two-phase region; one essentially describing initial interaction with the bare Pt ( 1I 1) surface and the other representing initial interaction with the 128
8 November I99 I
highly K perturbed part of the surface ( where S, k! 1) . From the increase in St, with f& at high Ei and T,=300 K, where long-lived precursors must play a minimal role, this perturbed area is estimated as roughly 3-5 Pt ( 111) surface unit cells per adsorbed K. Thus, O2 collisions with the K perturbed region should always produce sticking, while initial collision with the local “bare” surface region may produce either reflection back into the gas phase or trapping into the physisorbed state or molecularly chemisorbed state depending upon Eim Once in these “bare surface precursors”, the O2 can then subsequently either thermally desorb or transport to K perturbed regions and produce sticking. The sharp increase in S, at the lowest Ei observed at T,=100 K, even for modest &, is consistent with initial trapping into a “more or less” unperturbed bare physisorption state, followed by sticking at K perturbed regions. Since thermal desorption should compete with parallel transport of 02 on the surface to K “sinks”, & is also expected to be a strong function of T,for this precursor mechanism. Since desorption rates from the physisorbed state are extremely high at T,= 300 K, this precursor is anticipated to play only a limited role in sticking at this 7’,, even for modest 0,, This is evidently true since the sharp increase in S0 at low Ei is basically absent at T,=300K. The dominance in this sticking of a precursor based on the physisorbed stated is a somewhat surprising scenario since the naive expectation based on fig. 2 is that the K induced lowering of the work function should essentially destroy the physisorbed state as a separate entity. This expectation, however, is based on a one-dimensional picture and a homogeneous lowering of the work function. In reality, the adsorbed K induces a local electrostatic potential close to the surface, which only becomes a homogeneous work function change far from the surface. The interpretation of sticking in terms of a bare surface precursor then suggests that “local” aspects of the work function lowering are very important at distances to the surface characteristic of the physisorption well. In addition, the barrier to molecular chemisorption is in reality multidimensional. It must depend quite strongly on molecular orientation since charge transfer into a x* resonance should be most favorable when the molecule is parallel to the sur-
Volume186,number2,3
CHEMICAL PHYSICS LETTERS
face. Hence, even in the presence of adsorbed K, a considerable barrier may still exist to molecular chemisorption for some molecular orientations, and ultimately produce trapping into a physisorbed state. For high Ei the physisorbed state must play a minimal role in the sticking. In this case, initial sticking is either into the molecularly chemisorbed state for collision with the bare surface patches and (probably) directly dissociative for the collision with K perturbed regions. Since thermal desorption rates are not too fast at T,=300 K from the molecularly chemisorbed precursor, the difference between So at T,= 100 and 300 K is not extreme. There is, however, some difference and this once again points to the role of molecular precursors on “bare” surface patches in the alkali promotion of sticking.
4. Summaryand conclusions We have utilized molecular beam techniques to probe the effects of co-adsorbed K on O2 sticking on Pt ( 111). The dominant qualitative aspect is a significant promotion of the sticking with K coverage. The results show a complicated dependence of the promotion on both Ei and T,, and cannot be interpreted in terms of simple one-dimensional charge-
8 November1991
transfer models often used to rationalize alkali promotion. There is considerable evidence that molecular precursors, including that due to a physisorbed species, play a dominant role in the sticking on the K-modified surfaces. It also appears that the sticking on the K-modified surfaces is best interpreted in terms of O2 interaction with inhomogeneous mixed phase regions on the Pt( 111) t K. References 111H.P. Bonzel, Surface Sci. Rep. 8 (1987) 43. 121C.T. Rettner and D.J. Auerbach, Comments At. Mol. Phys. 20 (1987) 153. [3] A.C. Luntz, Physica Scripta 35 (1987) 193. [4] G. Pirug, H.P. Bonzel and G. Broden, Surface Sci. 122 (1982) I. [r;] A.C. Luntz, M.D. Williams and D.S. Bethune, J. Chem. Phys. 89 (1988) 4381. [6] A.C. Luntz, J.K. Brown and M.D. Williams, J. Chem. Phys. 93 (1990) 5240. [7] J.K. Brown, A.C. Luntz and P.A. Schultz, J. Chem. Phys., in press. [8] D.A. King and M.G. Wells, Proc. Roy. Sot. A 339 (1974) 245. [9] A.C. Luntz, J. Grimblot and D.E. Fowler, Phys. Rev. B 39 (1989) 12903. [IO] W. Wunh, J. Stohr, W. Jark, P. Stevens, J. Solomon and R.J. Madix, Phys. Rev. Letters 65 (1990) 2426. [ I I ] C.T. Rettner and C.B. Mullins, J. Chem. Phys. 94 ( I991 ) 1626.
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