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An XPS and ups study of chloride chemisorption on Ag(110)
Volume 53, number 3
CHEhIICAL PHYSICS LETTERS
AN XPS AND UPS STUDY OF CHLORINE CHEMISORPTION ON
-1 February 1978
Ag(l10)
D. BRIGGS * I.C.1. Limited, Corporate Laboratory, Runcom WA7 4QE, UK and R.A. MARBROW ** and R.M. LAMBERT Department ofPhysical Cknzist~.
University of Cambn‘dge, Cambridge CB2 IEP. UK
Received 19 October 1977
Chlorinechemisorbsrapidlyon Ag(l10) at 300 K to a saturation coverageof about one monolayer and with a sticking probability of order unity. The corresponding work function shift isi- 1.7 eV. XP and UP spectra are awsistent with the presence of a single kind of chemisorbed species. It appears that Ag behaves as a simple s-band solid and that the adsorbate 3p level is shifted so as to tie in the metal s-band; these observations are in agreement with the recent theoretical predictions of Anderson.
I_ Introduction
2. Experimental
Single crystal studies of chlorine chemisorption on a number of different silver crystal planes have recently been reported [I-S] , and at least part of the chemical interest in these systems arises from the ability of chlorine to selectively enhance the partial oxidation of ethylene over silver surfaces. Sachtler et al. [6] have suggested that this effect depends on the selective blocking of oxygen chemisorption sites by Cl adatoms. All the studies carried out to date have dealt with the surface crystaliography and kinetic properties of the Ag-Cl system, and have employed a combination of LEED, Auger spectroscopy, and thermal desorption measurements_ The purpose of this communication is to report on the XP and UP spectra of the Ag(l lO)Cl system with the object of providing comp1erBentar-y information about the number of chemically distinct Cl species present on the surface, and their electronic interaction with the Ag lattice.
Experiments were carried out with an AEi ES200B spectrometer equipped with an Mg Kcu(1253.6 eV) source and a helium resonance lamp. This system has been described in detail elsewhere [7] ; the base pressure was = 5 X lo-t1 torr rising to 2 X lo-r0 torr during operation of the He source. Details of the preparation, mounting and cleaning of the Ag(l10) specimen have also been given in an earlier publication [8]. After Ar+ etching the XP spectrum of the annealed surface showed only very small quantities of residual C and 0 (0 Is : Ag 3dsp = 0.1%; C 1s : Ag 3d5,2 = 0.5%) and the instrument was calibrated as before [7 ] to give Ag 3d5i2 binding energy (BE) = 368.0 + 0.2 eV relative to the Fermi energy (EF).
*
Present address: I.C.I. Limited. Plastics Division, Research _* Department, RL-129, Welwyn Garden City, UK. Present address: I.C.I. Limited, Corporate Laboratory, Runcorn WA7 4 QE, UK_
462
3. Results and discussion At 300 K, XP measurements showed that Cl3 adsorption was very rapid, the surface saturating after an exposure of = 5 L (1 L = lo4 torr s)_ If the saturation coverage is of the order of one monolayer (see below) this corresponds to an initial sticking probability of
Voltime 5 3, number 3
CHEMICAL
order unity. Such behaviour is entirely in line with the previously reported [4,5] behaviour of CL, on the smooth Ag(ll1) and stepped Ag(331) planes; the Ag(ll0) surface topography is intermediate between these two cases. The Cl XP signal showed partly resolved 2p1j2 and 2~~~~ components, and is consistent with the presence of a single kind of chemisorbed speties (fig. 1). After Cl, exposure the Ag 3d,,2 signal was found to have moved to higher BE by = 0.3 eV. However, this is not thought to be a chemical effect; it is most likely to be due to a change in “spectrometer work function” resulting from slight alteration of analyzer potentia!s by chlorine interaction. Heating the crystal to = 970 K resulted in slow removal of the adlayer, almost certainly by atomic Cl desorption [4,5] , as monitored by the Cl 2p signal. Ar+ etching was found to be the most efficient method for complete removal of Cl_ Fig. 2 shows the clean surface HeI UP spectrum, the UP spectrum of the chlorine saturated surface, and the resulting difference spectrum. The work function change at saturation coverage as measured by the shift in the low energy cutoff was A+ = + 1.70 * 0.05 eV. This is close to the corresponding values of + 1.8 eV and + 1.6 eV obtained for Ag(ll1) and Ag(33 1) respectively by the electron beam-retarding potentiai method [4,5]. Dosing of the chlorine saturated surface with 104 L 0, at 300 K led no detectable displacement of Cl; the XP spectra showed no change in the Cl 2p intensity and no take up of oxygen, while the UP spectrum and work function shift were also unchanged. This behaviour is very different from that of Ag(33 l)Cl [S] where O2 dosing leads to ready displacement of Cl and concomitant facetting to (100) and (110) pianes. It therefore seems likely that the Cl site-blocking mechanism [6] in the Ag-catalysed epoxidation
4 34,z
1 February 1978
PHYSICS LETTERS
CL2P,YZ
Binding Energy(eV) Fig. 1. Ag and Cl XPS signals from chlorine saturated surface.
\ C ,
I
‘L.--,
1 I
I
10
9
I
8
1
7
I
6-5
I
I
4
I
I
3
;
1
EF
Bimlmg Energy WI
I Fig. 2. He1 UP spectra. (a) Chlorine saturated surface; (b) clesn surface; (c) difference spectrum. reaction
can only occur on the low-index
planes.
The spectral changes resulting from chlorine adsorp-
tion were found to be very reproducible. In particular, as can be seen from fig_ 2, the principal chlorine-induced feature is a single rahthernarrow peak (half width = 1 eV) which occurs at 3.5 eV below EF_ The very simple difference spectrum suggests that there is relatively little alteration in the Ag 4d level upon chlorine chemisorption, and that there is but a single kind of chlorine species present on the surface. This is in agreement both with the XPS results and with the known [4,5] structural and desorption properties of chlorine overlayers on Ag. Furthermore, the Ag-Cl system appears to be markedly different from the W( lOO)-Cl system [9] _ In the latter case no less than fourchlorine-induced features are observed in the UP spectrum, and the authors consider that three distinct kinds of Cl adatom can be formed on the W(lO0) surface. By comparison with this, it seems that Ag behaves as though it were a simple %-band solid with essentially free-electron like proPerties. The feature at 35 eV below EF may therefore be identified as derived from the Cl 3p levels, which corresponds to a decrease in BE of 3.5-5.2 eV as compared with the valence shell ionization energy of the free Cl atom. (The difference of I.7 eV between these 463
Volume 53. number 3
CHEMICAL PHYSICS LETTERS
figures depends on whether the work function of the clean or the Cl-covered surface is used in estimating the shift [lO,l I] .) Chir conchrsions are very much in fine with the recent work of Anderson [i2] who has made use of cIuster calculations to anaiyse the experimental UP spectra [12] of Cu(LOO)-N. Ne suggests that a wfioie range of non-metallic adatoms [incfuding Cl) should adsorb on the group iB and IIB metah with Iarge shifts in adsorbate valence sheli ionisation energy which place the adsorbate p-levels in the s-band. This is exactly what seems to occur in the Ag( I IO)-Cl case. Furthermore, Anderson points out that in the Cu-N case the metal can reasonably be approx~~lated as an s-band solid, and that this simplified description should carry over to other systems - including Ag-Cl. The XPS intensities for Cl 2p and Ag 3ds,, may be used to make a rough estimate of the Cl coverage at saturation. Using an inelastic mean free path of * 15 I$
for the Ag 3dSfl electrons, taking account of the electron take-off angIe (73”) and assuming complete shadowing of second-layer Ag atoms, leads to a coverage estimate of =r I monolayer (relative to surf&e Ag atom density) when use is _tiade of the relative atomic sensitivity data of J&gensen and Wagner [13,14]. Although this procedure leads to an overestimate of the Cl coverage, it is not likely to be very seriously in error, and it agrees with the LEED studies of Ag(I ll>-Cl and Ag(331)-Cl (4,5] which show that Cl forms closepacked overlayers on both smooth and stepped Ag sur-
faces.
464
_t February 1978
R.A.M. thanks the Science Research Council for-the award of a Research Studentship, and we are grateful to I.C.1,.Limited for additional fm&ncial support.
References [l] G. Rovida, F. Pratesi, M. Magbetta and E. Ferroni, Japan. J. Appl. Phys. Suppl. 2, Part 2 (1974) 117. [2] G. RovJda and F. Pratesi, Surface Sci. 51<1975) 2?0_ 131 E. Zanaazi, F, Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. Sk4 (1976) 432. [4] P-3. Goddard and R.M. Lambert, Surface Sci. (1977), to be published. 151 R-A. Marbrow and R.M. Lambert, Surface Sci. 71 (f978 1 107. [6] P-A. Kilty, NC. Rol and W.M.H. Sachtkr, in: Catalysis, ed. J.W. Hi&tower (North-Holland, Amsterdam, 1973) p- 929_ 17’1’D. Briggs, R.A. hfarbrow and R.M. Lambert, Surface Sci. 6.5 (1977) 314. [81 R.A. Marbrow and R.M. Lambert, Surface Sci. 61(1976) 329. f9j D-L. Perry and J-Q. Brougbton, Surface Sci. (l977), submitted For publication. [ 101 D.L. Perry and J-Q_ Broqhton. Surface Sci. (1977), to be published. fi 11 H-D. Ha&rum, Surface Sci. (19771, to be published. [121 A-B. Anderson,Chem. Phys. Letters49 (1977) 550. [ 131 C-K_ Jbrgensen and N, Berthou. Anal. Chem. 47 (1975) 482. [ 141 C-D. Wagner, Anal. Chem. 44 (1972)
1050.
CHEhIICAL PHYSICS LETTERS
AN XPS AND UPS STUDY OF CHLORINE CHEMISORPTION ON
-1 February 1978
Ag(l10)
D. BRIGGS * I.C.1. Limited, Corporate Laboratory, Runcom WA7 4QE, UK and R.A. MARBROW ** and R.M. LAMBERT Department ofPhysical Cknzist~.
University of Cambn‘dge, Cambridge CB2 IEP. UK
Received 19 October 1977
Chlorinechemisorbsrapidlyon Ag(l10) at 300 K to a saturation coverageof about one monolayer and with a sticking probability of order unity. The corresponding work function shift isi- 1.7 eV. XP and UP spectra are awsistent with the presence of a single kind of chemisorbed species. It appears that Ag behaves as a simple s-band solid and that the adsorbate 3p level is shifted so as to tie in the metal s-band; these observations are in agreement with the recent theoretical predictions of Anderson.
I_ Introduction
2. Experimental
Single crystal studies of chlorine chemisorption on a number of different silver crystal planes have recently been reported [I-S] , and at least part of the chemical interest in these systems arises from the ability of chlorine to selectively enhance the partial oxidation of ethylene over silver surfaces. Sachtler et al. [6] have suggested that this effect depends on the selective blocking of oxygen chemisorption sites by Cl adatoms. All the studies carried out to date have dealt with the surface crystaliography and kinetic properties of the Ag-Cl system, and have employed a combination of LEED, Auger spectroscopy, and thermal desorption measurements_ The purpose of this communication is to report on the XP and UP spectra of the Ag(l lO)Cl system with the object of providing comp1erBentar-y information about the number of chemically distinct Cl species present on the surface, and their electronic interaction with the Ag lattice.
Experiments were carried out with an AEi ES200B spectrometer equipped with an Mg Kcu(1253.6 eV) source and a helium resonance lamp. This system has been described in detail elsewhere [7] ; the base pressure was = 5 X lo-t1 torr rising to 2 X lo-r0 torr during operation of the He source. Details of the preparation, mounting and cleaning of the Ag(l10) specimen have also been given in an earlier publication [8]. After Ar+ etching the XP spectrum of the annealed surface showed only very small quantities of residual C and 0 (0 Is : Ag 3dsp = 0.1%; C 1s : Ag 3d5,2 = 0.5%) and the instrument was calibrated as before [7 ] to give Ag 3d5i2 binding energy (BE) = 368.0 + 0.2 eV relative to the Fermi energy (EF).
*
Present address: I.C.I. Limited. Plastics Division, Research _* Department, RL-129, Welwyn Garden City, UK. Present address: I.C.I. Limited, Corporate Laboratory, Runcorn WA7 4 QE, UK_
462
3. Results and discussion At 300 K, XP measurements showed that Cl3 adsorption was very rapid, the surface saturating after an exposure of = 5 L (1 L = lo4 torr s)_ If the saturation coverage is of the order of one monolayer (see below) this corresponds to an initial sticking probability of
Voltime 5 3, number 3
CHEMICAL
order unity. Such behaviour is entirely in line with the previously reported [4,5] behaviour of CL, on the smooth Ag(ll1) and stepped Ag(331) planes; the Ag(ll0) surface topography is intermediate between these two cases. The Cl XP signal showed partly resolved 2p1j2 and 2~~~~ components, and is consistent with the presence of a single kind of chemisorbed speties (fig. 1). After Cl, exposure the Ag 3d,,2 signal was found to have moved to higher BE by = 0.3 eV. However, this is not thought to be a chemical effect; it is most likely to be due to a change in “spectrometer work function” resulting from slight alteration of analyzer potentia!s by chlorine interaction. Heating the crystal to = 970 K resulted in slow removal of the adlayer, almost certainly by atomic Cl desorption [4,5] , as monitored by the Cl 2p signal. Ar+ etching was found to be the most efficient method for complete removal of Cl_ Fig. 2 shows the clean surface HeI UP spectrum, the UP spectrum of the chlorine saturated surface, and the resulting difference spectrum. The work function change at saturation coverage as measured by the shift in the low energy cutoff was A+ = + 1.70 * 0.05 eV. This is close to the corresponding values of + 1.8 eV and + 1.6 eV obtained for Ag(ll1) and Ag(33 1) respectively by the electron beam-retarding potentiai method [4,5]. Dosing of the chlorine saturated surface with 104 L 0, at 300 K led no detectable displacement of Cl; the XP spectra showed no change in the Cl 2p intensity and no take up of oxygen, while the UP spectrum and work function shift were also unchanged. This behaviour is very different from that of Ag(33 l)Cl [S] where O2 dosing leads to ready displacement of Cl and concomitant facetting to (100) and (110) pianes. It therefore seems likely that the Cl site-blocking mechanism [6] in the Ag-catalysed epoxidation
4 34,z
1 February 1978
PHYSICS LETTERS
CL2P,YZ
Binding Energy(eV) Fig. 1. Ag and Cl XPS signals from chlorine saturated surface.
\ C ,
I
‘L.--,
1 I
I
10
9
I
8
1
7
I
6-5
I
I
4
I
I
3
;
1
EF
Bimlmg Energy WI
I Fig. 2. He1 UP spectra. (a) Chlorine saturated surface; (b) clesn surface; (c) difference spectrum. reaction
can only occur on the low-index
planes.
The spectral changes resulting from chlorine adsorp-
tion were found to be very reproducible. In particular, as can be seen from fig_ 2, the principal chlorine-induced feature is a single rahthernarrow peak (half width = 1 eV) which occurs at 3.5 eV below EF_ The very simple difference spectrum suggests that there is relatively little alteration in the Ag 4d level upon chlorine chemisorption, and that there is but a single kind of chlorine species present on the surface. This is in agreement both with the XPS results and with the known [4,5] structural and desorption properties of chlorine overlayers on Ag. Furthermore, the Ag-Cl system appears to be markedly different from the W( lOO)-Cl system [9] _ In the latter case no less than fourchlorine-induced features are observed in the UP spectrum, and the authors consider that three distinct kinds of Cl adatom can be formed on the W(lO0) surface. By comparison with this, it seems that Ag behaves as though it were a simple %-band solid with essentially free-electron like proPerties. The feature at 35 eV below EF may therefore be identified as derived from the Cl 3p levels, which corresponds to a decrease in BE of 3.5-5.2 eV as compared with the valence shell ionization energy of the free Cl atom. (The difference of I.7 eV between these 463
Volume 53. number 3
CHEMICAL PHYSICS LETTERS
figures depends on whether the work function of the clean or the Cl-covered surface is used in estimating the shift [lO,l I] .) Chir conchrsions are very much in fine with the recent work of Anderson [i2] who has made use of cIuster calculations to anaiyse the experimental UP spectra [12] of Cu(LOO)-N. Ne suggests that a wfioie range of non-metallic adatoms [incfuding Cl) should adsorb on the group iB and IIB metah with Iarge shifts in adsorbate valence sheli ionisation energy which place the adsorbate p-levels in the s-band. This is exactly what seems to occur in the Ag( I IO)-Cl case. Furthermore, Anderson points out that in the Cu-N case the metal can reasonably be approx~~lated as an s-band solid, and that this simplified description should carry over to other systems - including Ag-Cl. The XPS intensities for Cl 2p and Ag 3ds,, may be used to make a rough estimate of the Cl coverage at saturation. Using an inelastic mean free path of * 15 I$
for the Ag 3dSfl electrons, taking account of the electron take-off angIe (73”) and assuming complete shadowing of second-layer Ag atoms, leads to a coverage estimate of =r I monolayer (relative to surf&e Ag atom density) when use is _tiade of the relative atomic sensitivity data of J&gensen and Wagner [13,14]. Although this procedure leads to an overestimate of the Cl coverage, it is not likely to be very seriously in error, and it agrees with the LEED studies of Ag(I ll>-Cl and Ag(331)-Cl (4,5] which show that Cl forms closepacked overlayers on both smooth and stepped Ag sur-
faces.
464
_t February 1978
R.A.M. thanks the Science Research Council for-the award of a Research Studentship, and we are grateful to I.C.1,.Limited for additional fm&ncial support.
References [l] G. Rovida, F. Pratesi, M. Magbetta and E. Ferroni, Japan. J. Appl. Phys. Suppl. 2, Part 2 (1974) 117. [2] G. RovJda and F. Pratesi, Surface Sci. 51<1975) 2?0_ 131 E. Zanaazi, F, Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. Sk4 (1976) 432. [4] P-3. Goddard and R.M. Lambert, Surface Sci. (1977), to be published. 151 R-A. Marbrow and R.M. Lambert, Surface Sci. 71 (f978 1 107. [6] P-A. Kilty, NC. Rol and W.M.H. Sachtkr, in: Catalysis, ed. J.W. Hi&tower (North-Holland, Amsterdam, 1973) p- 929_ 17’1’D. Briggs, R.A. hfarbrow and R.M. Lambert, Surface Sci. 6.5 (1977) 314. [81 R.A. Marbrow and R.M. Lambert, Surface Sci. 61(1976) 329. f9j D-L. Perry and J-Q. Brougbton, Surface Sci. (l977), submitted For publication. [ 101 D.L. Perry and J-Q_ Broqhton. Surface Sci. (1977), to be published. fi 11 H-D. Ha&rum, Surface Sci. (19771, to be published. [121 A-B. Anderson,Chem. Phys. Letters49 (1977) 550. [ 131 C-K_ Jbrgensen and N, Berthou. Anal. Chem. 47 (1975) 482. [ 141 C-D. Wagner, Anal. Chem. 44 (1972)
1050.