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Sulphur adsorption and sulphide growth on Rh(111)
640
Surface Science 164 (1995) 640 648 N o r i h - l t o l l a n d , A nlsterdam
S U L P H U R A D S O R P T I O N A N D S U L P H I D E G R O W T H ON R h ( l l 1) J.S. F O O R D and A.E. R E Y N O L D S Deparlmenl ~!f ('hetm~l*3', The ~ "nirersio', Soulhanll~lOt~ .S'09 5,,\'tt, UK
Received 14 April 1985: accepted for publication 6 Augu~,t 1985
The interaction of S2(g ) with Rh(111) has been studied, using LEED, AES, TPI) and -19 measurements. Efficient dissociativeadsorption hlto all electronegati\e overlayer takes place at 300 K and continuous compression of an initial (v,r3 ~
1. Introduction Sulphur p o i s o n i n g of transition metal catalysts is a major p r o b l e m which is likely to become of increasing importance, in view of the potential application for the catalytic conversion of synthesis gas of significant sulphur c o n t e n t to fuels and petrochemicals. The adsorption of sulphur can bring a b o u t marked changes in catalyst reactivity patterns [1] a n d a detailed knowledge of the surface chemistry of sulphur and its interaction with other adsorbates needs lo be gained in order to u n d e r s t a n d such effects. Here, we report results of a study of sulphur a d s o r p t i o n on R h ( l l I ) in c o l u u n c t i o n with a paper dealing with sulphur induced reactivity changes on Rh and Rh Cu bimetallic surfaces
[21. 2. Experimental All experiments were carried out in a stainless steel U H V chamber, equipped with LEED, AES and Ar ' ion etching facilities. The c h a m b e r also housed a line-of-sight QMS, used for thermal desorption studies, and base pressures of 10 s Pa were routinely o b t a i n e d d u r i n g the course of the work. S u l p h u r was generated in situ using a P t / A g / A g l / A g z S / P I electrolyte cell [3], which permitted a dosing flux of 1017 10 Is S~ m o l e c u l e s m 2 s i ) ( a s c o n t r o l l e d and m o n i t o r e d by m e a s u r e m e n t of the electrolysis current) to be m a i n t a i n e d at the 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 "' Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
J.S. Foord, A.E. Reynolds / Sulphur and sulphide on Rh(l l l)
641
sample whilst the background pressure remained below 5 x 10 s Pa, and quoted gas exposures refer to this S: flux. The R h ( I I I ) crystal had previously [4] been subject to an extensive cleaning procedure to free the crystal from bulk boron impurities which segregate to the surface at high temperatures. Consequently a contaminant-free, well ordered R h ( I I I ) surface was produced following system bakeout, after heating the sample in 10 4 Pa 02 at 1300 K for brief periods, Ar + ion etching and annealing in vacuo at 1400 K.
3. Results 3.1. Sulphur adsorption at 300 K
The adsorption of sulphur on the crystal at 300 K was followed by means of L E E D , AES and A~ observations. The intensity of the 151 eV S Auger transition was measured as a function of gas exposure and the resultant uptake curve is illustrated in fig. 1. A smooth curve is exhibited, which gradually levels off after an exposure of 2 × 1019 m 2 and a corresponding S: Rh (302 eV) Auger peak intensity ratio of 1.4. Work function measurements (fig. 1) show that sulphur adsorption produces a gradual increase in Jg, of 0.5 eV over the exposure range where the sulphur Auger signal was found to increase. A n u m b e r of L E E D patterns, as presented in fig. 2, were observed following sulphur uptake by the crystal. Little change in the diffraction pattern from the clean surface (fig. 2a) was apparent up to gas exposures of 4.0 × 10 la m =, at which point the L E E D pattern in fig. 2b, indexing as ( f 3 × ~/3 )R30 °, emerged. El
3
OA
1:>
0"> -o m
0
16 Sulphur
0 [3 sfafe
32
t+O
exposure (lO18m0tecules m-2)
Fig. 1. Uptake curves for sulphur adsorption on Rh(lll) at 300 K. as determined by AES (O), thermal desorption (O, O) and AO (zx)measurements.
642
.I.S. Fo~,'~L A. t-~. Re~'Ju~/~£ / Su/ptT,r apkt vu&hid~, r,1 R/I(I 1 Ij
(1
E
Fig. 2. LEEI) patterns observed for sulphur adsorbed on Rh(lll). (a) ('lean surface, g5 V: (b)
(v"3 ×,/3 )R30 °, 58 V: (c) "'split" (v/3 x~/3 )R30 °. 46 V: (d) "'split" c(2 ×4), 65 V: (e) c(2 >,"4). 100 V: (f) ( 7 x 7 ) , 50 V: (g) (4X4). 50 V.
,543
J.S. fbord. A.E. Reynolds / Sulphur and s'ulphide on Rh( l l l )
Rh (111) {I×I)
+S [ 11350K
;>
(q3 ×q3) R30 o desorb
+S
c (2.4)
1
[ ,,
L
. ,3star e ,
670K
o
~ ={7x7)
aJ
t
*S
desorb ~ sfafe
a i
600K disordered
h
~ {4×4)
i
i
_ _
500 900 1300 TEMPERATURE (K)
Fig. 3. Sumnlary of the observed changes in LEED, during sulphur adsorption and subsequent heating of the Rh(II1) specimen. Fig. 4. Thermal desorption spectra monitoring S2 desorption from Rh(IIl). Spectra (a) (j) refer, respectively, to initial sulphur exposures of 3.0. 4.1, 4.7. 5.3, 7.8, 10, 16, 56. 450. 1200~10 Is nlolecules m 2.
The visible fractional order beams split into triads, the cornponents of which moved towards adjacent integral order spots as more sulphur adsorbed on tile specimen (fig. 2c). Split spots also appeared at {n + ~, m + ~> positions (n, m integers) (fig. 2d) which sharpened up to produce a c(2 x 4) pattern after a gas dose of 8.0 x l0 ts m : (fig. 2e). Subsequent exposure of the surface to sulphur produced no further change in the diffraction pattern apart from a slow rise in background intensity and all diffraction features were found to have disappeared following sulphur exposures of 102o m : or greater. The sequence of appearance of ordered superstructures is summarised for convenience in fig. 3, where effects induced by crystal heating and described in the following section. are also included. 3. 2. T h e r m a l d e s o r p t i o n
Thermal desorption studies of the R h ( I I I ) / S system ',','ere carried out using a linear heating rate of 25 K s 1, desorption products being observed only at 32
644
.L S. f~ord. A.E. Re~'nold~ / Sulphur and ~ulphide on Rh( I 1 l
amu (S ~) and 64 amu ($2+) in the mass range 0 120 amu. The signal at the lower mass arises from the fragmentation of S~ within the ion source of the mass spectrometer so it can be concluded that desorption occurs predominantly as S~ and desorption spectra monitoring this species are shown in fig. 4. L E E D indicated that the (~/3 × ( 3 ) R 3 0 ° sulphur superstructure was thernqally stable up to 1350 K, the m a x i m u m temperature obtainable in the desorption sweeps. However, spectra monitored to higher initial sulphur coverages intermediate between those of the 0f33 × ~/3)R30 ° and c(2)< 4) adlayer configurations, reveal a single broad desorption feature (j~ state) in the temperature range 950 1350 K, a strong d o w n w a r d shift in desorption temperature occurring with increased adatom concentration. The p state saturates at an initial sulphur coverage corresponding to the formation of the c(2 × 4) adlayer, and further sulphur adsorption brings about the rapid growth of a second desorption peak (c~ state) at 900 K. The temperature maximum of the c~ peak shifts to higher temperatures with increased sulphur concentration. Additionally, the c~ peak undergoes continuous growth without any apparent limit as the sulphur exposure is increased: such behaviour is visible in fig. 1 where the desorption yield from the ~ and /~ states is plotted out as a function of sulphur exposure. As discussed below, the ~ state represents approximatel>, one quarter of a physical monolayer of sulphur atoms and on this basis the ~ state coverage increases smoothly with gas dose up to at least 4 adlayers. Effects brought about by crystal heating were also examined, using L E E I ) and A~ measurements. Although the (~/33 × ~/3 )R30 ° structure was stable to 1350 K. at which temperature the L E E D pattern slowly disappeared, the c(2 x 4) superstructure irreversibly converted to a (7 x 7) configuration (fig. 21"), upon annealing the specimen at 670 K. The ~ desorption peak can therefore be associated with desorption from this phase to leave behind a (f33 x ~/3 )R30 ° adlayer. Heating the disordered surface phase, formed at high sulphur coverages, to 600 K resulted in the appearance of an ordered (4 × 4) structure as witnessed by the L E E D pattern in fig. 2g, while further heating to 900 K caused a reversion to the (7 x 7) superstructure. The ~ desorption peak therefore arises through desorption from the (4 x 4) adlayer to leave behind the (7 x 7) structure. A s u m m a r y of these thermally driven processes is given in fig. 3. Work function measurements indicated that the c(2 x 4) ~ (7 x 7) phase transition was accompanied by a reduction in A~ from 0.30 -* 0.15 eV.
4. Results interpretation and discussion 4.1. O ~ T e r l a v e r . / 3 r m a t i o n a t 3 0 0 K
The formation of a ~/3 x ( 3 R30 ° L E E D pattern and the continuous spot movement which accompanies its conversion to c(2 × 4) as further sulphur
645
J.S. Foord, A.E. R
® ;1,2x,o . . . . . . . . . . . . . . . .
)g
,'"" ,11
~ ' ~ " (0
®
,,""a~ o / / " ''~,_j-,-1'2 x
c(2.z,)
(-! 2) 33
t
b 4a I Fig. 5. (a) Reciprocal lattice described by the matrix equation in section 4.1; (b) corresponding real space structure: (c) antiphase domain model for tile overlayer; (d) proposed model for thc cpilaxial sulphide layer. ()pen circles: Rh atoms, shaded circles: sulphur species, black circles: Rh ions. adsorbs on the crystal at 300 K, seems only consistent with the initial formation of ordered sulphur overlayers on a Rh surface of fcc(Ill) symmetry. We adopt the usual view [5] that the sulphur is adsorbed dissociatively and if it is assumed that increased a d a t o m coverages are taken up by compression of a uniformly spaced adlayer [6,7], the entire sequence of L E E D patterns spanning the (~3 × ~3 )R30 ° ~ c(2 × 4) conversion are described by the matrix equations:
h*
=7
1
:<+2
1,,*~ '
[h,]_ 2 x +1l [[_v+2 1-x J Ca,) l-x x+2 b2
a 2
"
T h e a and b vectors, respectively, represent the substrate and overlayer lattice vectors and the asterisk refers as usual to corresponding reciprocal lattice vectors, x is a variable parameter, lying in the range 0 0.25, and denotes the magnitude of the spot splitting of the { ~, - ~ } bealns associated with the original (~/3- × ~/3 )R30 ° structure. The reciprocal and real space lattices are illustrated in figs. 5a and 5b; all fractional order beams are accounted for provided multiple scattering and the occurrence of three s y m m e t r y related domains are included. Increases in sulphur coverage are a c c o m m o d a t e d by
646
.1. s. t'7*oreL /1,l 5. Revnr)lds / Sulphur and vulphi&' ,m Rhg I 11)
uniaxial c o m p r e s s i o n of the (~/3 X V.F3)R30 ° a d l a y e r along the close p e a k e d (11()) direction of the substrate. D u r i n g conversion from (~/~ × v.f3)R30 ° ( x = 0) to c(2 × 4) ( x = 0.25), the surface coverage increases from 5.3 x 10 ~'~ a t o m s m e ( 0 = 0 . 3 3 ) to 8 . 0 x : 1 0 is a t o m s m -" (0 0.5) and this is in good a g r e e m e n t with the relative sulphur concentrations, as d e d u c c d by AES, for the two structures. The nearest n e i g h b o u r a t o m s e p a r a t i o n in the c(2 x 4) configuration, which c o r r e s p o n d s to overlayer saturation, is 0.36 nm, close to the Van d e r W a a l s d i a m e t e r of sulphur (0.37 nm). A l t h o u g h we have put for~vard herc a modcl based on n o n - c o i n c i d e n t a d a t o m nets to explain the L E E I ) o b s e r v a t i o n s on overlaycr growth, it should be p o i n t e d out that m o d e l s involving the occurrence of o r d e r e d a n t i p h a s e d o m a i n s [8] can also be formulated. The structure of the c(2 x 4) a d l a y e r based on this scheme is shown in fig. 5c: in essence it consists of chains of sulphur a t o m s r u n n i n g along (112) and the (~/'3 x ~/3 )R30 ° ~ c(2 x 4) iuterconversion would be a c c o m m o d a t e d by an o r d e r e d variation in the c o n c e n t r a t i o n of v a c a n t rows along <112>. As is usual the scheme leads to e x t r e m e l y short a d a t o m s e p a r a t i o n s (0.27 nm), but nothing can further be usefully a d d e d to the extensive discussion in thc literature as to which of the different models is t o be favoured [9 12]. The essential conclusion that sulphur forms o r d e r e d overlayers on Rh(111 ) in the coverage range 0 = 0.33 ~ 0.5{) remains. 4.2. c(2 X 4) ~ ( 7 X 7) t h e r m a l c o m , e r . s i o n
As in the case of the N i ( l l l ) S a d s o r p t i o n system [13], the s a t u r a t e d s u l p h u r overlayer on R h ( l l l ) undergoes an irreversible phase transition upon raising the t e m p e r a t u r e of the crystal. In tile case of Ni. it has been suggested that the transition represents a structural reorganisation of the overlaycr [14] without p e r t u r b a t i o n of the u n d e r l y i n g substrate, arises from sulphide formation [15] or stems from a ( 1 1 1 ) ~ (100) " r e o r i c n t a t i o n " of the o u t e r m o s t layer of s u b s t r a t e a t o m s [13,16], c o n s i d e r a b l e interest being exhibited in this latter hypothesis in view of its possible catalytic i m p o r t a n c e . In the case of Rh(111), the c o n s i d e r a b l e changes in overlayer structure which take place at 300 K as the sulphur coverage rises, seem to indicate that the a d a t o m s tire in fact quite mobile within the overlayer at this tcmperature. Consequently, if the f o r m a t i o n of the (7 × 7) s u p e r s t r u c t u r e involves a structural r e o r g a n i s a t i o n within the overlayer, there seems to be no convincing e x p l a n a t i o n why the (7 x 7) a d l a y e r is not formed initially at r o o m temperature. The results therefore seem to favour a model where the phase transition also involves some structural a l t e r a t i o n to the underlying substrate. In the spirit of the " r e o r i e n t a t i o n ' " h y p o t h e s i s [13,16] a distorted Rh(100)-c(2 × 2) S a d l a y e r can be a c c o m m o d a t e d m the (7 x 7) coincidence lattice, allowing the possibility of reorientation to (100). Alternatively, the transition could merely reflect the i n c o r p o r a t i o n of Rh ions within the sulphur overlayer, the redtiction m work function a c c o m p a -
J.S. Foord, A.E. Revm)Ms" / Sulphur and .~ulphide on Rh(l 11)
647
nying the c(2 × 4)-+ (7 × 7) interconversion perhaps being most consistent with this latter process. 4. 3. Bulk sulphide growth
After saturation of the c(2 × 4) adlayer, LEED, AES and TPD all show that sulphur continues to adsorb on the surface. In view of the fact that growth of the a peak in the thermal desorption profiles takes place without limit, this continued sulphur adsorption must correspond to the formation of a bulk compound at the R h / S interface. The desorption energy from the a state is 230 kJ tool 1 (Arrhenius plot), considerably greater than the sublimation energy of S2(g) (130 kJ tool 1) [17]. The bulk compound must therefore be a rhodium sulphide rather than bulk sulphur, which would be expected to condense on the crystal at low temperatures. The shift of the a peak temperature to higher values with increased sulphur coverage is also consistent w,ith strong attractive interactions within the adlayer, expected in view of the long range bonding forces in ionic sulphides. Perhaps the most basic question concerns the identity of the rhodium sulphide which is produced since a number of bulk phases are known to exist [18]. Thermodynamic data [19] reveals that the desorption activation energy from the a¢ state (230 kJ tool ~) is in good agreement with the enthalpy change for the reaction 3Rh2S~(s).+4Rh(s)+S2(g),
A H = 2 4 3 kJ mol '.
and differs considerably from the decomposition energies of RhS (447 kJ mol 1) and RhS~ ( 3 0 9 k J m o l 1). The results therefore suggest that it is Rh:S~ which is produced on the surface, although attempts to fit the associated (4 x 4) coincidence lattice observed in LEED to a plane in the bulk structure of Rh2S~ [20] failed. However, in the case of bulk halide growth [7,21 24], it has been found that thin films generally adopt a structure based on anion close-packing, with the close-packed anion planes lying parallel to the metal surface, even when this is not the stable bulk structure of the compound [7,21,24]. If the same was true in the present case the ~-A120~ structure would be anticipated, close-packed anion planes once again lying parallel to the Rh(111 ) surface, with a hexagonal lattice parameter a = 0.59 nm, the value for CreS ) [25]. As is shown in fig. 5d. such a structure does indeed generate the required (4 × 4) coincidence lattice and we therefore propose it for the bulk sulphide film formed on Rh(ll 1).
Acknowledgements The authors are grateful to Dr. R.M. Lambert for loan of the R h ( l l l ) crystal, with which this work was carried out.
648
.I.S. Foord, A.E. R~!l,noht~ / Sulphur and sulphide on Rh( l 11)
References [1] [21 [3] [41 [5] [6] [7] [8] [9] [10] [11] [12] [131
[14] I15] [16] [17] [18] [19] [20] [21] [22] [23] [241 [25]
R.J. Madon and H. Shaw, Catalysis Rex. Sci. |:,ng. 15 (1977) 69. .I.S. Eoord and A.E. Reynolds, Surface Sci. 152/153 (1985) 426. C. Wagner. J. Chem. Phys. 21 (1953) 819. M.P. Cox and R.M. Lambert, Surface. Sci. 107 (1981) 547. C.tt. Bartholomc~ and P.K. Agrawal, Advan. Catalysis 31 (1982) 135. P.A. Dowben and R.G. Jones, Surface Sci. 84 (1979) 449. J.S. Foord and R.M. Lambert, Surface Sci. 115 (1982) 141. M. Huber and J. Oudar. Surface Sci. 47 (1975) 605. G. Rovida and F. PratesL Surface Sci. 67 (1977) 367. M. H u b e r a n d J. Oudar, SurfaceSci. 67(1977) 37/). J.P. Biberian. Surface Sci. 116 (1982) 1.223. R.G. J o n e s a n d P.A. Dowben. SurfaceSci. 116 (1982) L228. J.J. McCarroll, Surface Sci. 53 (1975) 297. W. I'.rley and H. Wagner. J. Catalysis 53 (1978) 287. M. Perdereaux and J. Oudar, Surface Sci. 20 (1970) 80. T. Edmonds, J.J. McCarroll and R.C. Pitkethly, J. Vacuum Sci. Technol. 8 (1971) GS. B. Meyer, Elemental Sulphur(Interscience, New York, 1960). Gmelin~ ttandbuch der Anorganischen Chemie (Verlag Chemic. Weinheim, 1962). J.E. Mcl)onald and J.W. Cobble. J. Phys. Chem. 66 (1962) 291. l'i. Parthe. D. H o h n k c a n d F. Hulliger, Acta Crvst. 23(1967) 832. D. Vigner, G. Dagoury. M.C. Paul and J. Rousseau, Vide 32 (1977) 64. J.S. goord, A.P.C. Reed and R.M. Lambert, Surface Sci. 129 (1983) 79. A.P.C. Reed, J.S. Foord and R.M. Lambert, Surface Sci. 134 (1983) 689. A.P.C. Reed, J.S. Foord and R.M. Lambert. Vacuum 33 (19831 707. A.F. Trotman-Dickenson, Ed., Comprehensive Inorganic Chemistry (Pergamon. Oxford, 1973).
Surface Science 164 (1995) 640 648 N o r i h - l t o l l a n d , A nlsterdam
S U L P H U R A D S O R P T I O N A N D S U L P H I D E G R O W T H ON R h ( l l 1) J.S. F O O R D and A.E. R E Y N O L D S Deparlmenl ~!f ('hetm~l*3', The ~ "nirersio', Soulhanll~lOt~ .S'09 5,,\'tt, UK
Received 14 April 1985: accepted for publication 6 Augu~,t 1985
The interaction of S2(g ) with Rh(111) has been studied, using LEED, AES, TPI) and -19 measurements. Efficient dissociativeadsorption hlto all electronegati\e overlayer takes place at 300 K and continuous compression of an initial (v,r3 ~
1. Introduction Sulphur p o i s o n i n g of transition metal catalysts is a major p r o b l e m which is likely to become of increasing importance, in view of the potential application for the catalytic conversion of synthesis gas of significant sulphur c o n t e n t to fuels and petrochemicals. The adsorption of sulphur can bring a b o u t marked changes in catalyst reactivity patterns [1] a n d a detailed knowledge of the surface chemistry of sulphur and its interaction with other adsorbates needs lo be gained in order to u n d e r s t a n d such effects. Here, we report results of a study of sulphur a d s o r p t i o n on R h ( l l I ) in c o l u u n c t i o n with a paper dealing with sulphur induced reactivity changes on Rh and Rh Cu bimetallic surfaces
[21. 2. Experimental All experiments were carried out in a stainless steel U H V chamber, equipped with LEED, AES and Ar ' ion etching facilities. The c h a m b e r also housed a line-of-sight QMS, used for thermal desorption studies, and base pressures of 10 s Pa were routinely o b t a i n e d d u r i n g the course of the work. S u l p h u r was generated in situ using a P t / A g / A g l / A g z S / P I electrolyte cell [3], which permitted a dosing flux of 1017 10 Is S~ m o l e c u l e s m 2 s i ) ( a s c o n t r o l l e d and m o n i t o r e d by m e a s u r e m e n t of the electrolysis current) to be m a i n t a i n e d at the 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 "' Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
J.S. Foord, A.E. Reynolds / Sulphur and sulphide on Rh(l l l)
641
sample whilst the background pressure remained below 5 x 10 s Pa, and quoted gas exposures refer to this S: flux. The R h ( I I I ) crystal had previously [4] been subject to an extensive cleaning procedure to free the crystal from bulk boron impurities which segregate to the surface at high temperatures. Consequently a contaminant-free, well ordered R h ( I I I ) surface was produced following system bakeout, after heating the sample in 10 4 Pa 02 at 1300 K for brief periods, Ar + ion etching and annealing in vacuo at 1400 K.
3. Results 3.1. Sulphur adsorption at 300 K
The adsorption of sulphur on the crystal at 300 K was followed by means of L E E D , AES and A~ observations. The intensity of the 151 eV S Auger transition was measured as a function of gas exposure and the resultant uptake curve is illustrated in fig. 1. A smooth curve is exhibited, which gradually levels off after an exposure of 2 × 1019 m 2 and a corresponding S: Rh (302 eV) Auger peak intensity ratio of 1.4. Work function measurements (fig. 1) show that sulphur adsorption produces a gradual increase in Jg, of 0.5 eV over the exposure range where the sulphur Auger signal was found to increase. A n u m b e r of L E E D patterns, as presented in fig. 2, were observed following sulphur uptake by the crystal. Little change in the diffraction pattern from the clean surface (fig. 2a) was apparent up to gas exposures of 4.0 × 10 la m =, at which point the L E E D pattern in fig. 2b, indexing as ( f 3 × ~/3 )R30 °, emerged. El
3
OA
1:>
0"> -o m
0
16 Sulphur
0 [3 sfafe
32
t+O
exposure (lO18m0tecules m-2)
Fig. 1. Uptake curves for sulphur adsorption on Rh(lll) at 300 K. as determined by AES (O), thermal desorption (O, O) and AO (zx)measurements.
642
.I.S. Fo~,'~L A. t-~. Re~'Ju~/~£ / Su/ptT,r apkt vu&hid~, r,1 R/I(I 1 Ij
(1
E
Fig. 2. LEEI) patterns observed for sulphur adsorbed on Rh(lll). (a) ('lean surface, g5 V: (b)
(v"3 ×,/3 )R30 °, 58 V: (c) "'split" (v/3 x~/3 )R30 °. 46 V: (d) "'split" c(2 ×4), 65 V: (e) c(2 >,"4). 100 V: (f) ( 7 x 7 ) , 50 V: (g) (4X4). 50 V.
,543
J.S. fbord. A.E. Reynolds / Sulphur and s'ulphide on Rh( l l l )
Rh (111) {I×I)
+S [ 11350K
;>
(q3 ×q3) R30 o desorb
+S
c (2.4)
1
[ ,,
L
. ,3star e ,
670K
o
~ ={7x7)
aJ
t
*S
desorb ~ sfafe
a i
600K disordered
h
~ {4×4)
i
i
_ _
500 900 1300 TEMPERATURE (K)
Fig. 3. Sumnlary of the observed changes in LEED, during sulphur adsorption and subsequent heating of the Rh(II1) specimen. Fig. 4. Thermal desorption spectra monitoring S2 desorption from Rh(IIl). Spectra (a) (j) refer, respectively, to initial sulphur exposures of 3.0. 4.1, 4.7. 5.3, 7.8, 10, 16, 56. 450. 1200~10 Is nlolecules m 2.
The visible fractional order beams split into triads, the cornponents of which moved towards adjacent integral order spots as more sulphur adsorbed on tile specimen (fig. 2c). Split spots also appeared at {n + ~, m + ~> positions (n, m integers) (fig. 2d) which sharpened up to produce a c(2 x 4) pattern after a gas dose of 8.0 x l0 ts m : (fig. 2e). Subsequent exposure of the surface to sulphur produced no further change in the diffraction pattern apart from a slow rise in background intensity and all diffraction features were found to have disappeared following sulphur exposures of 102o m : or greater. The sequence of appearance of ordered superstructures is summarised for convenience in fig. 3, where effects induced by crystal heating and described in the following section. are also included. 3. 2. T h e r m a l d e s o r p t i o n
Thermal desorption studies of the R h ( I I I ) / S system ',','ere carried out using a linear heating rate of 25 K s 1, desorption products being observed only at 32
644
.L S. f~ord. A.E. Re~'nold~ / Sulphur and ~ulphide on Rh( I 1 l
amu (S ~) and 64 amu ($2+) in the mass range 0 120 amu. The signal at the lower mass arises from the fragmentation of S~ within the ion source of the mass spectrometer so it can be concluded that desorption occurs predominantly as S~ and desorption spectra monitoring this species are shown in fig. 4. L E E D indicated that the (~/3 × ( 3 ) R 3 0 ° sulphur superstructure was thernqally stable up to 1350 K, the m a x i m u m temperature obtainable in the desorption sweeps. However, spectra monitored to higher initial sulphur coverages intermediate between those of the 0f33 × ~/3)R30 ° and c(2)< 4) adlayer configurations, reveal a single broad desorption feature (j~ state) in the temperature range 950 1350 K, a strong d o w n w a r d shift in desorption temperature occurring with increased adatom concentration. The p state saturates at an initial sulphur coverage corresponding to the formation of the c(2 × 4) adlayer, and further sulphur adsorption brings about the rapid growth of a second desorption peak (c~ state) at 900 K. The temperature maximum of the c~ peak shifts to higher temperatures with increased sulphur concentration. Additionally, the c~ peak undergoes continuous growth without any apparent limit as the sulphur exposure is increased: such behaviour is visible in fig. 1 where the desorption yield from the ~ and /~ states is plotted out as a function of sulphur exposure. As discussed below, the ~ state represents approximatel>, one quarter of a physical monolayer of sulphur atoms and on this basis the ~ state coverage increases smoothly with gas dose up to at least 4 adlayers. Effects brought about by crystal heating were also examined, using L E E I ) and A~ measurements. Although the (~/33 × ~/3 )R30 ° structure was stable to 1350 K. at which temperature the L E E D pattern slowly disappeared, the c(2 x 4) superstructure irreversibly converted to a (7 x 7) configuration (fig. 21"), upon annealing the specimen at 670 K. The ~ desorption peak can therefore be associated with desorption from this phase to leave behind a (f33 x ~/3 )R30 ° adlayer. Heating the disordered surface phase, formed at high sulphur coverages, to 600 K resulted in the appearance of an ordered (4 × 4) structure as witnessed by the L E E D pattern in fig. 2g, while further heating to 900 K caused a reversion to the (7 x 7) superstructure. The ~ desorption peak therefore arises through desorption from the (4 x 4) adlayer to leave behind the (7 x 7) structure. A s u m m a r y of these thermally driven processes is given in fig. 3. Work function measurements indicated that the c(2 x 4) ~ (7 x 7) phase transition was accompanied by a reduction in A~ from 0.30 -* 0.15 eV.
4. Results interpretation and discussion 4.1. O ~ T e r l a v e r . / 3 r m a t i o n a t 3 0 0 K
The formation of a ~/3 x ( 3 R30 ° L E E D pattern and the continuous spot movement which accompanies its conversion to c(2 × 4) as further sulphur
645
J.S. Foord, A.E. R
® ;1,2x,o . . . . . . . . . . . . . . . .
)g
,'"" ,11
~ ' ~ " (0
®
,,""a~ o / / " ''~,_j-,-1'2 x
c(2.z,)
(-! 2) 33
t
b 4a I Fig. 5. (a) Reciprocal lattice described by the matrix equation in section 4.1; (b) corresponding real space structure: (c) antiphase domain model for tile overlayer; (d) proposed model for thc cpilaxial sulphide layer. ()pen circles: Rh atoms, shaded circles: sulphur species, black circles: Rh ions. adsorbs on the crystal at 300 K, seems only consistent with the initial formation of ordered sulphur overlayers on a Rh surface of fcc(Ill) symmetry. We adopt the usual view [5] that the sulphur is adsorbed dissociatively and if it is assumed that increased a d a t o m coverages are taken up by compression of a uniformly spaced adlayer [6,7], the entire sequence of L E E D patterns spanning the (~3 × ~3 )R30 ° ~ c(2 × 4) conversion are described by the matrix equations:
h*
=7
1
:<+2
1,,*~ '
[h,]_ 2 x +1l [[_v+2 1-x J Ca,) l-x x+2 b2
a 2
"
T h e a and b vectors, respectively, represent the substrate and overlayer lattice vectors and the asterisk refers as usual to corresponding reciprocal lattice vectors, x is a variable parameter, lying in the range 0 0.25, and denotes the magnitude of the spot splitting of the { ~, - ~ } bealns associated with the original (~/3- × ~/3 )R30 ° structure. The reciprocal and real space lattices are illustrated in figs. 5a and 5b; all fractional order beams are accounted for provided multiple scattering and the occurrence of three s y m m e t r y related domains are included. Increases in sulphur coverage are a c c o m m o d a t e d by
646
.1. s. t'7*oreL /1,l 5. Revnr)lds / Sulphur and vulphi&' ,m Rhg I 11)
uniaxial c o m p r e s s i o n of the (~/3 X V.F3)R30 ° a d l a y e r along the close p e a k e d (11()) direction of the substrate. D u r i n g conversion from (~/~ × v.f3)R30 ° ( x = 0) to c(2 × 4) ( x = 0.25), the surface coverage increases from 5.3 x 10 ~'~ a t o m s m e ( 0 = 0 . 3 3 ) to 8 . 0 x : 1 0 is a t o m s m -" (0 0.5) and this is in good a g r e e m e n t with the relative sulphur concentrations, as d e d u c c d by AES, for the two structures. The nearest n e i g h b o u r a t o m s e p a r a t i o n in the c(2 x 4) configuration, which c o r r e s p o n d s to overlayer saturation, is 0.36 nm, close to the Van d e r W a a l s d i a m e t e r of sulphur (0.37 nm). A l t h o u g h we have put for~vard herc a modcl based on n o n - c o i n c i d e n t a d a t o m nets to explain the L E E I ) o b s e r v a t i o n s on overlaycr growth, it should be p o i n t e d out that m o d e l s involving the occurrence of o r d e r e d a n t i p h a s e d o m a i n s [8] can also be formulated. The structure of the c(2 x 4) a d l a y e r based on this scheme is shown in fig. 5c: in essence it consists of chains of sulphur a t o m s r u n n i n g along (112) and the (~/'3 x ~/3 )R30 ° ~ c(2 x 4) iuterconversion would be a c c o m m o d a t e d by an o r d e r e d variation in the c o n c e n t r a t i o n of v a c a n t rows along <112>. As is usual the scheme leads to e x t r e m e l y short a d a t o m s e p a r a t i o n s (0.27 nm), but nothing can further be usefully a d d e d to the extensive discussion in thc literature as to which of the different models is t o be favoured [9 12]. The essential conclusion that sulphur forms o r d e r e d overlayers on Rh(111 ) in the coverage range 0 = 0.33 ~ 0.5{) remains. 4.2. c(2 X 4) ~ ( 7 X 7) t h e r m a l c o m , e r . s i o n
As in the case of the N i ( l l l ) S a d s o r p t i o n system [13], the s a t u r a t e d s u l p h u r overlayer on R h ( l l l ) undergoes an irreversible phase transition upon raising the t e m p e r a t u r e of the crystal. In tile case of Ni. it has been suggested that the transition represents a structural reorganisation of the overlaycr [14] without p e r t u r b a t i o n of the u n d e r l y i n g substrate, arises from sulphide formation [15] or stems from a ( 1 1 1 ) ~ (100) " r e o r i c n t a t i o n " of the o u t e r m o s t layer of s u b s t r a t e a t o m s [13,16], c o n s i d e r a b l e interest being exhibited in this latter hypothesis in view of its possible catalytic i m p o r t a n c e . In the case of Rh(111), the c o n s i d e r a b l e changes in overlayer structure which take place at 300 K as the sulphur coverage rises, seem to indicate that the a d a t o m s tire in fact quite mobile within the overlayer at this tcmperature. Consequently, if the f o r m a t i o n of the (7 × 7) s u p e r s t r u c t u r e involves a structural r e o r g a n i s a t i o n within the overlayer, there seems to be no convincing e x p l a n a t i o n why the (7 x 7) a d l a y e r is not formed initially at r o o m temperature. The results therefore seem to favour a model where the phase transition also involves some structural a l t e r a t i o n to the underlying substrate. In the spirit of the " r e o r i e n t a t i o n ' " h y p o t h e s i s [13,16] a distorted Rh(100)-c(2 × 2) S a d l a y e r can be a c c o m m o d a t e d m the (7 x 7) coincidence lattice, allowing the possibility of reorientation to (100). Alternatively, the transition could merely reflect the i n c o r p o r a t i o n of Rh ions within the sulphur overlayer, the redtiction m work function a c c o m p a -
J.S. Foord, A.E. Revm)Ms" / Sulphur and .~ulphide on Rh(l 11)
647
nying the c(2 × 4)-+ (7 × 7) interconversion perhaps being most consistent with this latter process. 4. 3. Bulk sulphide growth
After saturation of the c(2 × 4) adlayer, LEED, AES and TPD all show that sulphur continues to adsorb on the surface. In view of the fact that growth of the a peak in the thermal desorption profiles takes place without limit, this continued sulphur adsorption must correspond to the formation of a bulk compound at the R h / S interface. The desorption energy from the a state is 230 kJ tool 1 (Arrhenius plot), considerably greater than the sublimation energy of S2(g) (130 kJ tool 1) [17]. The bulk compound must therefore be a rhodium sulphide rather than bulk sulphur, which would be expected to condense on the crystal at low temperatures. The shift of the a peak temperature to higher values with increased sulphur coverage is also consistent w,ith strong attractive interactions within the adlayer, expected in view of the long range bonding forces in ionic sulphides. Perhaps the most basic question concerns the identity of the rhodium sulphide which is produced since a number of bulk phases are known to exist [18]. Thermodynamic data [19] reveals that the desorption activation energy from the a¢ state (230 kJ tool ~) is in good agreement with the enthalpy change for the reaction 3Rh2S~(s).+4Rh(s)+S2(g),
A H = 2 4 3 kJ mol '.
and differs considerably from the decomposition energies of RhS (447 kJ mol 1) and RhS~ ( 3 0 9 k J m o l 1). The results therefore suggest that it is Rh:S~ which is produced on the surface, although attempts to fit the associated (4 x 4) coincidence lattice observed in LEED to a plane in the bulk structure of Rh2S~ [20] failed. However, in the case of bulk halide growth [7,21 24], it has been found that thin films generally adopt a structure based on anion close-packing, with the close-packed anion planes lying parallel to the metal surface, even when this is not the stable bulk structure of the compound [7,21,24]. If the same was true in the present case the ~-A120~ structure would be anticipated, close-packed anion planes once again lying parallel to the Rh(111 ) surface, with a hexagonal lattice parameter a = 0.59 nm, the value for CreS ) [25]. As is shown in fig. 5d. such a structure does indeed generate the required (4 × 4) coincidence lattice and we therefore propose it for the bulk sulphide film formed on Rh(ll 1).
Acknowledgements The authors are grateful to Dr. R.M. Lambert for loan of the R h ( l l l ) crystal, with which this work was carried out.
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.I.S. Foord, A.E. R~!l,noht~ / Sulphur and sulphide on Rh( l 11)
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