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Ru(0001) Interface
C . Morterra, A. Zecchina and G . Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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MODEL STUDIES OF THE SMSI PHENOMENON AT THE TiOx/Ru(000 1) INTERFACE
JAS PAL S. BADYALI, ANDREW J. GELLMAN2, ROBERT W. JUDD3 and RICHARD M. LAMBERTl IDepartment of Chemistry, University of Cambridge, Cambridge, CB2 IEP, UK. 2Permanent address: School of Chemical Sciences, University of Illinois at UrbanaChampaign, 505 S. Mathews Avenue, Urbana, IL 61801, USA. 3British Gas plc, Research & Technology Division, London Research Station, Michael Road, London SW6 ZAD, TJK.
ABSTRACT Surface phenomena pertinent to SMSI behaviour exhibited by supported metal catalysts have been examined using a well characterised model system. LEED, AES, XPS and chemisorption of CO and H2 have been used to investigate the growth morphology, structure and chemical properties of TiOx films on Ru(OOO1) as a function of oxide loading and temperature. The properties of such films is critically dependent on the method of preparation. Blocking of metal adsorption sites by highly dispersed TiO, species is believed to occur in the case of submonolayer films deposited at room temperature. INTRODUCTION The existence of the 'Strong Metal-Support Interaction' (SMSI), between Group VIII metals and titania has been the subject of considerable interest since it was first reported that such catalysts can exhibit specific activities for CO hydrogenation which are substantially greater than those for silica- or alumina-supported metals, whilst their ability to adsorb H2 or CO is diminished (refs. 1-2). Recently, experimental evidence from EXAFS (ref. 3), Rutherford backscattering (ref. 4), TEM (ref. 5), Auger electron spectroscopy (ref. 6), HREELS (ref. 71,UPS and XPS (refs. 8,9), and SIMS (ref. 10) has suggested that support migration onto the metal particle occurs, resulting in both geometric and electronic perturbation of the active metal surface (refs. 11-12). The effects of decorating metal surfaces by TiO, moieties have been studied using metal foils and single crystals (refs. 13-15). However, the nature of the titanium species which migrates (metallic or oxidized) and the type of bonding which exists between this species and the group Vm metal remain as unresolved issues.
We report here on the modelling of such systems by investigating TiO, species adsorbed onto the basal plane of ruthenium, and the effect of these moieties on the chemisorptive behaviour of H2 and CO. The basal face of ruthenium has been chosen as the model system since ruthenium catalyst particles are reported to expose (0001) faces preferentially (ref. 16). EXPERIMENTAL PROCEDURE Measurements were made in two separate ultra high vacuum chambers of conventional design which were capable of routinely achieving base pressures of c: 2 x 10-11 Torr. The first system included a multiplexed mass spectrometer for temperature programmed desorption (TPD) measurements and a 3-grid retarding field analyser for LEED/Auger measurements. The mass spectrometer ioniser was enclosed in a shield which effectively discriminated against the scattered gas signal from the rear face of the specimen. The resulting TPD signals were therefore dominated by the line-of-sight flux from the crystal front face, and effects due to finite pumping speed were thereby also minimised. XPS measurements were made in the second chamber which was a VG ADES 400 system incorporating a twin-anode (Mg/Al) X-ray source and a rotatable hemispherical analyser. All XP spectra were referenced with respect to the Fermi level and calibrated against the Ru(3d5/2) feature at 279.9 eV (ref. 17). Gas exposures were carried out using a capillary array doser; a collimated, resistively heated evaporation source was employed for titanium dosing. The Ru(0001) specimen was prepared from a 99.99 +% pure ingot by standard methods and mounted onto a sample holder which could be cooled to 140 K and resistively heated using a programmable power supply. Cleaning was acheived (ref. 18)by heating to 1350 K in 10-7 Torr oxygen, followed by flashing to 1550 K in ultra high vacuum to remove traces of subsurface oxygen (ref. 19). Extreme care was taken to ensure that in all experiments the sample was free from dissolved titanium; this was achieved by leaching out bulk titanium using repeated cycles of heating in oxygen (1250 K/lO-7 Torr) and Ar+ etching until the Auger spectrum characteristic of clean ruthenium was obtained (ref. 20). In this connection, we have already reported in detail (ref. 20) on the assignment of certain significant features which appear in the Auger spectrum of Ru(0001). As explained in reference 20, these features may be variously assigned to Ru Auger transitions, diffraction features, or the presence of impurity titanium. The Auger and diffraction features were identified and it was shown that signals from impurity titanium were undetectable with a rigorously cleaned specimen. All gas exposures were corrected for ion gauge sensitivities (ref. 21). RESULTS AND ANALYSIS TiO,- Deposition on Ru(0001)
The method of preparation of the model surface is of great importance, and by using appropriate procedures (ref. 22) we ensured that no alloying occurred between
21
surface TiOx and the ruthenium single crystal substrate. A possible method of preparing adsorbed TiOx moieties is to deposit a certain amount of titanium and then expose it to oxygen. Fig. 1 shows the uptake of oxygen on a titanium monolayer at room temperature as monitored by the O(510 eV) AES signal. There are two regimes: the initial rapid uptake corresponds to oxygen chemisorption on the titanium film; this is followed by a slower process which involves migration of some oxygen to subsurface sites. XPS measurements show that under these conditions the titanium is not fully oxidized; the Ti(2p3/2) peak shifts from 455.2 eV to 456.6 eV on exposure of 4000L 0 2 to a IML predeposited titanium film, Fig. 2a-c, compared to the reported value of 458.8 If: 0.1 eV for bulk Ti02 (ref. 23).
8o
1
0
I
2
4
6
8
10
450 0 454 0 458 0 462 0 466.0
0, Exposure/Langmuirs
Fig. 1(left): O(510 eV) AES uptake of 02 on 1.OML Ti/Ru(0001) Fig. 2 (right): XP spectrum at room temperature of (a) clean Ru(OOO1); (b) 1.OML Ti/Ru(0001); (c) 4000L 0 2 exposure to (b); (d) 1.OML 'as-deposited TiOx/Ru(OOO1). Note that the large feature around 462 eV is mainly due to the Ru(3p312) level (with some contribution from Ti(2pl/2) in (b,c,d)).
In order to overcome limitations due to oxygen migration (ref. 24) an alternative method was developed in which titanium was deposited in a background pressure of 1x10-6 Torr of oxygen at room temperature; the oxygen doser was always turned on before each titanium dose and shut off after completion of metal evaporation. The formation of adsorbed TiOx must involve a surface reaction under these conditions (mean free path at 1x10-6 Torr is 45 m (ref. W)). At a pressure of 1x10-6 Torr the rate of adsorption is approximately one monolayer of oxygen every 1.5 seconds (ref. 261, whereas the deposition rate of titanium was approximately a monolayer every 15 minutes. Hence every titanium atom should adsorb in the vicinity of an oxygen atom and in addition should experience a sufficient flux of oxygen molecules to ensure complete oxidation, and indeed XPS measurements show this to be the case (Fig. 2(d)).
22
Growth Mode of TiO,- on Ru(0001)
Fig. 3 shows the AES signals of Ru(231 eV), Ti(387 eV) and O(510 eV) versus deposition time; care was taken to minimise the effects of electron irradiation on the intensity of the O(510 eV) AES signal. Breaks are evident at regular intervals, and this behaviour is characteristic of monolayer-by-monolayer growth (Frank-van der Merwe growth (ref. 27)). The ratio of the Ti(387 eV) AES signal to the Ru(231 eV) AES signal at the first breakpoint is 0.38 0.02, a value which is significantly larger than that for titanium metal deposition on Ru(0001) (0.31 0.03) (ref. 24), because of the additional attenuation of the substrate signal by oxygen atoms. A monolayer coverage of TiO, on
*
*
Ru(OOO1) exhibits a weak (1 x 1) LEED pattern which sharpens up on annealing to 400 K, consistent with the presence of an in-registry TiO, overlayer; this point will be addressed below. ~
~
~
~
_
_
_7 160 _
Fig. 3 (left): Growth mode of TiO, on Ru(0001) as determined by AES at 295 K. Fig. 4 (right): Excited H2 cleaning of 0.5ML O/Ru(0001) as followed by the O(510 eV) AES signal. Selective Removal of Oxveen from the Bare Ruthenium Surface In the past, one of the major difficulties which arises when attempting to model the SMSI effect using this type of approach has been the procedure used to selectively remove oxygen adsorbed on the bare metal patches, without disturbing the oxygen associated with the TiO, species (refs. 14,15). In the present work this problem was overcome by using a hot cathode ion gun operating in a background pressure of 2 x 10-6 Torr of hydrogen, and in line of sight with the specimen, the latter being held at 575 K (no alloying occurs under these conditions (ref. 28)). This arrangement provides a flux of hydrogen ions (and excited molecules) (ref. 29) which are very effective in removing oxygen chemisorbed on bare ruthenium. Fig. 4 shows the effect of this procedure on Ru(OOO1) saturated with oxygen; the exponential decay is characteristic of a first order rate process, as might be expected. On repeating this cleaning procedure for a deposited full monolayer of TiO,, no detectable change was observed in the O(510 eV), Ti(387 eV) or Ru(231 eV) AES signals. It is therefore apparent that this technique can be successfully employed to remove ( 0 ) a d species selectively from Ru metal
23
without significantly affecting the composition of the TiO, film. This cleaning procedure had the additional beneficial effect of allowing selective masking of the back face of the specimen by (O)& thereby suppressing H2 and CO desorption from the back face during TPD measurements. Structure and Stoichiometrv of the TiOx - Film
Under ultra high vacuum conditions, a saturation exposure of oxygen to the Ru(OOO1) surface leads to a (2 x 2) LEED pattern and an 0:Ru ratio of 0.5, (ref. 30). This information can be used in conjunction with the known properties of a range of titanium oxides to draw inferences about the structure and stoichiometry of the TiOx
film on Ru(OOO1). Titanium monoxide (TiO) has the NaCl structure, and Table 1 lists the lattice parameters for a range of defect oxides which exhibit non-stoichiometry due to either anionic or cationic vacancies. TABLE 1 Lattice parameters of non-stoichiometric titanium oxides (ref. 31) Oxide
Lattice Parameter a/A 4.1850 4.1780 4.1766 4.1733 4.1689 4.1661
The number of oxygen ions per unit area in the (111)planes of these oxides is plotted in Fig. 5 as a function of the non-stoichiometric parameter (x), and Fig. 6 shows the O(510 eV) AES signal observed before and after hydrogen-cleaning the surface as a function of TiOx coverage. Each point was taken for a freshly prepared TiOx film, thus ensuring ,that no electron beam damage occurred. By using Fig. 5 and cross-calibrating the amount of oxygen in a complete monolayer of the TiOx film (using Auger data for the Ru(0001)-(2x 2) 0 structure), the stoichiometry of the TiOx overlayer is found to be consistent with the presence of a structure composed of a (111) sheet of Ti interleaved between two (111)layers of oxygen atoms. The important point to emerge from this procedure is that the structure and stoichiometry of the TiOx film lie close to that of the bulk monoxide (TiO) rather than those of TiO2. In fact, in the monolayer regime, the Ti:O stoichiometry i s Ti02 since the structure corresponds to Ru-0-Ti-0. Thus in the monolayer regime Ti atoms are expected to be in a higher oxidation state than in the bulk oxide TiO; this in turn this will cause the
24
lattice parameter ofthe monolayer filmto shrink, facilitatingregistry with the Ru(OOO1) surface mesh in agreement with the LEED observations.
-
70
6s
5:
60
\ X
55
50
0.7
o . ~0 . 0
1.0
1.1
1.2
1.3 1 . 4
Oxide S t o i c h i o m a t r y . x
0
0.2
0.4
0.8
0.8
1.0
1.2
1.d
T i O , Covmragm/Wonolaycrm
Fig. 5 (left): Number of oxygen ions per unit area in the close packed planes of the defective titanium monoxides (Tiox) as a function of stoichimetry parameter. Fig.6 (right): Variation with TiO, coverage of the a 5 1 0 eV) AES s&al before and after hydrogen cleaning the surface.
For the 'as-deposited' Ti& film the Ti(Zp3/2) binding energy was 459.2eV (Fig.2d), slightly higher than that found for bulk T e i (ref. 231, and entirely consistent with the model proposed above for the monolayer TiOx film. Thus for the monolayer sheet of "Tiof lying on top of the ruthenium substrate, the presence of Ru atoms causes the Ti atoms to carry a somewhat larger net positive charge than they do in bulk T i@ (ref. 23). It is noteworthy that in this sandwich configuration the number density of oxygen atoms in the Ru contact layer is twice that observed for the Ru(OOOl)/oxygen interface under UHV conditions (ref. 30). This is in keeping with the observation that the adsorbed oxygen/surfaceRu ratio can be doubled at elevated oxygen pressures (ref. 32). CO Chemisomtion The effect of TiOx on neighbouring Ru sites was investigated using a near saturation dose of 44L of CO at 295 K and following the subsequent "PD behaviour of the chemisorbed CO. The change in "I'D behaviour as a function of TiO, coverage is shown in Figs. 7a and 7b from which it can be seen that no new desorption features
25
appear and that the TiO, exerts a simple site-blocking effect. A slight shift of the peaks towards lower temperature is seen with increasing TiO, coverage; this need not be an electronic effect, but could be ascribed to destabilisation of CO islands due to reduction in their size. Dosing at 140 K did not lead to any new features, and use of a 1:l mixture of 13C16O and 12C180 gave no evidence for any isotopic scrambling in the desorbing gas. This simple site-blocking effect is therefore consistent with a geometric effect with no apparent electronic effect arising from the presence of the TiOx moieties.
0.7
1
,
I
I
I
i
I
350 400 450 500 550 600 650 700 TEMPERATURE/K 0
0 2
0.4
0.6
0.8
1.0
1.2
1.4
TiO, C o v e r a g e / M o n o l a y e r s
Fig. 7a (left):TPD spectra of 44L CO doses at room temperature as a function of TiOx coverage. Fig. 7b (right): CO desorption yield per surface ruthenium atom, (8) as a function of TiOx coverage. H2 -Chemisorption
Thermal desorption data following a saturation dose of H2 on clean Ru(OOO1) at
140 K are in accord with our earlier work (ref. 24). Two poorly resolved peaks are evident, characteristic of two different types of chemisorbed atomic hydrogen species. Peak shifts towards lower temperature are seen with increasing TiO, coverage in the TPD spectra following H2 adsorption at 140 K (Fig. 8a); the presence of TiO, also leads to a remarkable decrease in the amount of H2 desorption (Fig. 8b). This behaviour can
also be simply understood in terms of a geometric effect if an initial ensemble of the
Ru atoms is required for the dissociative chemisorption of H2.
Fig. 8a (left): TPD spectra following 200L H2 doses at 140 K as a function of TiO, coverage. Fig. 8b (right): Hydrogen desorption yield (0) defined as hydrogen atoms per surface ruthenium atom as a function of TiO, coverage. Hz+CO Chemisorption
Peebles et al. (ref. 33) have suggested that coadsorption of CO and H2 on Ru(OOO1) leads to the formation of segregated islands of the two adsorbates. This important point was investigated further by the following method. A mixture of 6%CO and 94%H2 was used for competitive coadsorption studies, a high fraction of H2 being employed to compensate for its lower sticking probability, thus enabling approximately equal amounts of the two gases to adsorb. Dosing of this CO/H2 mixture onto clean
Ru(0001) gave identical CO TPD behaviour (Fig. 9a), as that reported for CO/Ru(OOOl), (ref. 24). This is to be expected given that H2 desorption has already occurred before the sample reaches temperatures characteristic of CO desorption. With increasing total coverage, the H2 peaks shift to lower temperatures (Fig. 9b); which would be consistent with repulsive (CO)ad-(H)ad interaction at the edges of the surface islands. Most importantly, it can be seen that for most of the total coverage range the fractional (C0)ad and (H)ad coverages show a linear relationship (Fig. 9c); this verifies the segregated islands model (ref. 33) for CO/H2 coadsorption on clean Ru(0001) (see below). At very low exposures, there is a relatively higher (H),d coverage than observed in the linear region; some enhancement of the relative H2 sticking probability is to be expected at low coverages because unfavourable effects due to limited precursor state lifetime and ensemble requirements are minimised. The independent behaviour of CO and H2 on coadsorption can be used to probe the effect of TiO, on CO and H2 chemisorption for submonolayer coverages of TiO,.
27
- -.._ .
+.-..1 OL
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'
200
5 OL
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.
400
300
l
0 1L
'
500
I
200
Temperature/K
'
l
300
~
400
l
-
,
'
500
Temoerature/K
Fig. 9a (top left):CO TPD spectra for 1OL exposure of a 6%cO/94%H2gas mixture at 140 K on clean Ru(OOOl), (note that the low CO content in this mixture leads to only the high temperature CO feature, which is characteristic of CO/Ru(OOOl)). Fig. 9b (top right): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K on clean Ru(OOO1). Fig. 9c (left): Relationship between (CO)ad and (H)ad per surface ruthenium atom in competitive adsorption.
0
0
0.1
0.2
Fractional
I
I
I
0.3
0.4
0 5
H,,
Coverage 8 ,
H2 TPD behaviour for the COIH2 mixture is similar to that observed for the H2/TiOx/Ru(0001) system, Fig. IOa-b. However, the amount of CO desorbing increases for low coverages of TiO,, Fig. 1Oc-d, this can be attributed to an associated decrease in H2 chemisorption; the eventual fall in CO uptake at high TiOx coverages is due to site blocking as found for the CO/TiOx/Ru(OOO1) system.
.
__........_...&.-..._...I
.................. -r.-___
200 . . . . . .
~
7 - 7 T 300 400
Sernoeratb-e/u
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3
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.:.. *... ......
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7
! OMLi
500
O : ]0 \ ,
0 2
TiO,
0 4
0 6
0 8
*
1 0
Coverage/Monolayers
Fig. 10a (left): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K for TiOx/Ru(OOO1). Fig. lob (right): Hydrogen desorption yield defined as hydrogen atoms per surface ruthenium atom, (0) as a function of TiO, coverage
TiO, C o v e r a g e / M o n o l a y e r s
Fig. 1Oc (left): CO TPD spectra for IOL exposure of a 6%C0/94%H2 gas mixture at 140 K for TiO,/Ru(0001). Fig. 1Od (right): CO desorption yield per surface ruthenium atom (0) as a function of TiO, coverage.
29
DISCUSSION The observed layer-by-layer growth of the TiO, film on Ru(0001) is consistent with TiO, moieties ’wetting’ the metal surface in a manner which resembles the behaviour of catalyst particles in the SMSI state. It would appear that by preparing a true model of the postulated decoration effect of metal catalysts in the SMSI state (i.e. no alloying between metal and TiO, species) we have duplicated the reported SMSI behaviour regarding CO and H2 chemisorption on supported catalysts. CO chemisorption is blocked in a simple linear fashion with respect to TiO, coverage, but a more dramatic effect is observed for H2 chemisorption; this can be attributed to the fact that an ensemble of surface ruthenium atoms may be prerequisite for dissociative H2 chemisorption. CO is believed to bond upright at all coverages with the carbon end attached to individual surface ruthenium atoms so that such ensemble effects are not expected to be of significance (refs. 34-36). The linear CO:H2 chemisorption behaviour for CO/H2 mixtures on clean Ru(0001) confirms the postulated island segregation model, since the formation of a mixed layer would selectively hinder H2 chemisorption through the operation of ensemble requirements. On this basis the very pronounced non-linear suppression of H2 chemisorption by TiO, can be attributed to a very hiehlv dispersed immobile TiO, species on the Ru(0001) surface. This contrasts with the hydrogen uptake behaviour exhibited by the CO+H2/Ru(0001) system in the absence of TiO, where CO mobility and concomitant island formation have much larger bare patches of Ru available for H2 chemisorption. The high dispersion of TiO, produced in this manner on the Ru(OOO1) surface at room temperature is consistent with an initial growth mode in which preadsorbed oxygen atoms on the Ru(0001) surface act as ‘anchors’ for the incident titanium atoms. CONCLUSIONS Coadsorption of titanium and oxygen under ultra high vacuum conditions yields a TiO, species of similar structure to bulk TiO, however at the metal-netal oxide interface a complete monolayer of this structure has in effect the stoichiometry of Ti02. It is possible selectively to deposit TiO, moieties which reside on the Ru surface rather than partially diffuse into the bulk. These TiO, moieties have a similar effect on H2 and CO chemisorption as reported for Ti02 supported catalysts in the SMSI state. CO+H2/Ru(0001) chemisorption experiments indicate that at submonolayer coverages these TiO, species are highly dispersed and that simple site blocking is responsible for their effect on CO chemisorption; however the loss of ensembles of surface ruthenium atoms hinders H2 adsorption in a more severe manner. ACKNOWLEDGEMENTS JPSB acknowledges financial support by the SERC and BP Research Company plc under CASE Studentship No. CB020. We are grateful to Johnson Matthey Ltd for a loan of precious metals.
30
REFERENCES M.A. Vannice, C.C. Twu and S.H. Moon, J. Cat., 79 (1983) 70-80. 1 2 G.C. Bond and R. Burch, Specialist Periodical Reports, Royal Society of Chemistry, Catalysis, 6 (1982)27-60. S. S&ellson, M. McMillan and G.L. Haller, J. Phys. Chem., 90 (1986) 1733-1736. 3 J.A. Cairns, J.E.E. Baglin, G.J. Clark and J.F. Ziegler, J. Cat., 83 (1983)301-314. 4 5 B.R. Powell and S.E. Wittington, J. Cat., 81 (1983) 382-393. D.N. Belton, Y.-M. Sun and J.M. White, J. Phys. Chem., 88 (1984) 5172-5176. 6 S. Takatani and Y.W. Chung, J. Cat., 90 (1984) 75-83. 7 H.R. Sadeghi and V.E. Henrich, J. Cat., 87 (1984) 279-282. 8 T. Huizinga, H.F.J. van't Blik, J.C. Vis and R. Prins, Surface Sci., 135 (1983) 5809 596. 10 D.N. Belton, Y.-M. Sun and J.M. White, J. Am. Chem. Soc., 106 (1984) 3059-3060. 11 D.E. Resasco and G.L. Haller, J. Cat., 82 (1983) 279-288. 12 J. Santos, J. Phillips and J.A. Dumesic, J. Cat., 81 (1983) 147-167. 13 Y.W. Chung, G. Xiong and C.C. Kao, J. Cat., 85 (1984) 237-243. 14 K.A. Demmin, C.S. KO and R.J. Gorte, J. Phys. Chem., 89 (1985) 1151-1154. 15 M.E. Levin, M. Salmeron, A.T. Bell and G.A. Somorjai, J. Chem. Zoc., Faraday Trans. I, 83 (1987) 2061-2069. 16 E. Guglielminotti, G. Spoto and A. Zecchina, Surface Sci., 161 (1985) 202-220. 17 J.C. Fuggle and N. Martensson, J. Electron Spectrosc., 21 (1980) 275-281. 18 G.E. Thomas and W.H. Weinberg, J. Chem. Phys., 70 (1979) 1437-1439. 19 G. Praline, B.E. Koel, H.-I. Lee and J.M. White, Appl. Surface Sci., 5 (1980) 296-312, 20 J.P.S. Badyal, A.J. Gellman and R.M. Lambert, Surface Sci., 188 (1987) 557-562. 21. L. Holland, W. Steckelmacher and J. Yarwood, Vacuum Manual p.52 (E. and F.N. Spon Ltd., 1974). 22 J.P.S. Badyal, A.J. Gellman, R.W. Judd and R.M. Lambert, Cat. Letts., 1 (1988) 23
24 25 26 27 28 29 30 31 32 33
31 35
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41-50.
A.F. Carley, P.R. Chalker, J.C. Riviere and M.W. Roberts, J. Chem. Soc., Faraday Trans. I, 83 (1987)351-370. J.P.S. Badyal, A.J. Gellman and R.M. Lambert, J. Cat., 111 (1988) 383-396. W.J. Moore, Physical Chemistry, 5th Edn., (1981) p.150, (Longman). G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry, 2nd Edn., (1985)p.3, (VCH). F.C. Frank and J.H. van der Merwe, Prof. Roy. Soc. (London) A198 (1949) 205-225; A200 (1950) 125-134. J.P.S. Badyal, A.J. Gellman and R.M. Lambert, In Preparation. S.C. Brown, Basic Data of Plasma Physics (MIT), (1966). S.L. Parrott, G. Praline, B.E. Koel, J.M. White and T.N. Taylor, J. Chem. Phys., 71 (1979) 3352-3354. J.D.H. Donnay, G. Donnay, E.G. Cox, 0. Kennard and M.V. King, Crystal Data Determinative Tables, 2nd Edition, (1963), American Crystallographic Society. C.H.F. Peden and D.W. Goodman, J. Phys. Chem., 90 (1986) 1360-1365. D.E. Peebles, J.A. Schreifels and J.M. White, Surface Sci. 116 (1982) 117-134. P. Hofmann, J. Gossler, A. Zartner, M. Glanz and D. Menzel, Surface Sci., 161 (1985)303-320. W. Riedl and D. Menzel Surface Sci., 163 (1985) 39-50, J. Rogzik and V. Dose, Surface Sci., 176 (1986) L847-L851.
19
MODEL STUDIES OF THE SMSI PHENOMENON AT THE TiOx/Ru(000 1) INTERFACE
JAS PAL S. BADYALI, ANDREW J. GELLMAN2, ROBERT W. JUDD3 and RICHARD M. LAMBERTl IDepartment of Chemistry, University of Cambridge, Cambridge, CB2 IEP, UK. 2Permanent address: School of Chemical Sciences, University of Illinois at UrbanaChampaign, 505 S. Mathews Avenue, Urbana, IL 61801, USA. 3British Gas plc, Research & Technology Division, London Research Station, Michael Road, London SW6 ZAD, TJK.
ABSTRACT Surface phenomena pertinent to SMSI behaviour exhibited by supported metal catalysts have been examined using a well characterised model system. LEED, AES, XPS and chemisorption of CO and H2 have been used to investigate the growth morphology, structure and chemical properties of TiOx films on Ru(OOO1) as a function of oxide loading and temperature. The properties of such films is critically dependent on the method of preparation. Blocking of metal adsorption sites by highly dispersed TiO, species is believed to occur in the case of submonolayer films deposited at room temperature. INTRODUCTION The existence of the 'Strong Metal-Support Interaction' (SMSI), between Group VIII metals and titania has been the subject of considerable interest since it was first reported that such catalysts can exhibit specific activities for CO hydrogenation which are substantially greater than those for silica- or alumina-supported metals, whilst their ability to adsorb H2 or CO is diminished (refs. 1-2). Recently, experimental evidence from EXAFS (ref. 3), Rutherford backscattering (ref. 4), TEM (ref. 5), Auger electron spectroscopy (ref. 6), HREELS (ref. 71,UPS and XPS (refs. 8,9), and SIMS (ref. 10) has suggested that support migration onto the metal particle occurs, resulting in both geometric and electronic perturbation of the active metal surface (refs. 11-12). The effects of decorating metal surfaces by TiO, moieties have been studied using metal foils and single crystals (refs. 13-15). However, the nature of the titanium species which migrates (metallic or oxidized) and the type of bonding which exists between this species and the group Vm metal remain as unresolved issues.
We report here on the modelling of such systems by investigating TiO, species adsorbed onto the basal plane of ruthenium, and the effect of these moieties on the chemisorptive behaviour of H2 and CO. The basal face of ruthenium has been chosen as the model system since ruthenium catalyst particles are reported to expose (0001) faces preferentially (ref. 16). EXPERIMENTAL PROCEDURE Measurements were made in two separate ultra high vacuum chambers of conventional design which were capable of routinely achieving base pressures of c: 2 x 10-11 Torr. The first system included a multiplexed mass spectrometer for temperature programmed desorption (TPD) measurements and a 3-grid retarding field analyser for LEED/Auger measurements. The mass spectrometer ioniser was enclosed in a shield which effectively discriminated against the scattered gas signal from the rear face of the specimen. The resulting TPD signals were therefore dominated by the line-of-sight flux from the crystal front face, and effects due to finite pumping speed were thereby also minimised. XPS measurements were made in the second chamber which was a VG ADES 400 system incorporating a twin-anode (Mg/Al) X-ray source and a rotatable hemispherical analyser. All XP spectra were referenced with respect to the Fermi level and calibrated against the Ru(3d5/2) feature at 279.9 eV (ref. 17). Gas exposures were carried out using a capillary array doser; a collimated, resistively heated evaporation source was employed for titanium dosing. The Ru(0001) specimen was prepared from a 99.99 +% pure ingot by standard methods and mounted onto a sample holder which could be cooled to 140 K and resistively heated using a programmable power supply. Cleaning was acheived (ref. 18)by heating to 1350 K in 10-7 Torr oxygen, followed by flashing to 1550 K in ultra high vacuum to remove traces of subsurface oxygen (ref. 19). Extreme care was taken to ensure that in all experiments the sample was free from dissolved titanium; this was achieved by leaching out bulk titanium using repeated cycles of heating in oxygen (1250 K/lO-7 Torr) and Ar+ etching until the Auger spectrum characteristic of clean ruthenium was obtained (ref. 20). In this connection, we have already reported in detail (ref. 20) on the assignment of certain significant features which appear in the Auger spectrum of Ru(0001). As explained in reference 20, these features may be variously assigned to Ru Auger transitions, diffraction features, or the presence of impurity titanium. The Auger and diffraction features were identified and it was shown that signals from impurity titanium were undetectable with a rigorously cleaned specimen. All gas exposures were corrected for ion gauge sensitivities (ref. 21). RESULTS AND ANALYSIS TiO,- Deposition on Ru(0001)
The method of preparation of the model surface is of great importance, and by using appropriate procedures (ref. 22) we ensured that no alloying occurred between
21
surface TiOx and the ruthenium single crystal substrate. A possible method of preparing adsorbed TiOx moieties is to deposit a certain amount of titanium and then expose it to oxygen. Fig. 1 shows the uptake of oxygen on a titanium monolayer at room temperature as monitored by the O(510 eV) AES signal. There are two regimes: the initial rapid uptake corresponds to oxygen chemisorption on the titanium film; this is followed by a slower process which involves migration of some oxygen to subsurface sites. XPS measurements show that under these conditions the titanium is not fully oxidized; the Ti(2p3/2) peak shifts from 455.2 eV to 456.6 eV on exposure of 4000L 0 2 to a IML predeposited titanium film, Fig. 2a-c, compared to the reported value of 458.8 If: 0.1 eV for bulk Ti02 (ref. 23).
8o
1
0
I
2
4
6
8
10
450 0 454 0 458 0 462 0 466.0
0, Exposure/Langmuirs
Fig. 1(left): O(510 eV) AES uptake of 02 on 1.OML Ti/Ru(0001) Fig. 2 (right): XP spectrum at room temperature of (a) clean Ru(OOO1); (b) 1.OML Ti/Ru(0001); (c) 4000L 0 2 exposure to (b); (d) 1.OML 'as-deposited TiOx/Ru(OOO1). Note that the large feature around 462 eV is mainly due to the Ru(3p312) level (with some contribution from Ti(2pl/2) in (b,c,d)).
In order to overcome limitations due to oxygen migration (ref. 24) an alternative method was developed in which titanium was deposited in a background pressure of 1x10-6 Torr of oxygen at room temperature; the oxygen doser was always turned on before each titanium dose and shut off after completion of metal evaporation. The formation of adsorbed TiOx must involve a surface reaction under these conditions (mean free path at 1x10-6 Torr is 45 m (ref. W)). At a pressure of 1x10-6 Torr the rate of adsorption is approximately one monolayer of oxygen every 1.5 seconds (ref. 261, whereas the deposition rate of titanium was approximately a monolayer every 15 minutes. Hence every titanium atom should adsorb in the vicinity of an oxygen atom and in addition should experience a sufficient flux of oxygen molecules to ensure complete oxidation, and indeed XPS measurements show this to be the case (Fig. 2(d)).
22
Growth Mode of TiO,- on Ru(0001)
Fig. 3 shows the AES signals of Ru(231 eV), Ti(387 eV) and O(510 eV) versus deposition time; care was taken to minimise the effects of electron irradiation on the intensity of the O(510 eV) AES signal. Breaks are evident at regular intervals, and this behaviour is characteristic of monolayer-by-monolayer growth (Frank-van der Merwe growth (ref. 27)). The ratio of the Ti(387 eV) AES signal to the Ru(231 eV) AES signal at the first breakpoint is 0.38 0.02, a value which is significantly larger than that for titanium metal deposition on Ru(0001) (0.31 0.03) (ref. 24), because of the additional attenuation of the substrate signal by oxygen atoms. A monolayer coverage of TiO, on
*
*
Ru(OOO1) exhibits a weak (1 x 1) LEED pattern which sharpens up on annealing to 400 K, consistent with the presence of an in-registry TiO, overlayer; this point will be addressed below. ~
~
~
~
_
_
_7 160 _
Fig. 3 (left): Growth mode of TiO, on Ru(0001) as determined by AES at 295 K. Fig. 4 (right): Excited H2 cleaning of 0.5ML O/Ru(0001) as followed by the O(510 eV) AES signal. Selective Removal of Oxveen from the Bare Ruthenium Surface In the past, one of the major difficulties which arises when attempting to model the SMSI effect using this type of approach has been the procedure used to selectively remove oxygen adsorbed on the bare metal patches, without disturbing the oxygen associated with the TiO, species (refs. 14,15). In the present work this problem was overcome by using a hot cathode ion gun operating in a background pressure of 2 x 10-6 Torr of hydrogen, and in line of sight with the specimen, the latter being held at 575 K (no alloying occurs under these conditions (ref. 28)). This arrangement provides a flux of hydrogen ions (and excited molecules) (ref. 29) which are very effective in removing oxygen chemisorbed on bare ruthenium. Fig. 4 shows the effect of this procedure on Ru(OOO1) saturated with oxygen; the exponential decay is characteristic of a first order rate process, as might be expected. On repeating this cleaning procedure for a deposited full monolayer of TiO,, no detectable change was observed in the O(510 eV), Ti(387 eV) or Ru(231 eV) AES signals. It is therefore apparent that this technique can be successfully employed to remove ( 0 ) a d species selectively from Ru metal
23
without significantly affecting the composition of the TiO, film. This cleaning procedure had the additional beneficial effect of allowing selective masking of the back face of the specimen by (O)& thereby suppressing H2 and CO desorption from the back face during TPD measurements. Structure and Stoichiometrv of the TiOx - Film
Under ultra high vacuum conditions, a saturation exposure of oxygen to the Ru(OOO1) surface leads to a (2 x 2) LEED pattern and an 0:Ru ratio of 0.5, (ref. 30). This information can be used in conjunction with the known properties of a range of titanium oxides to draw inferences about the structure and stoichiometry of the TiOx
film on Ru(OOO1). Titanium monoxide (TiO) has the NaCl structure, and Table 1 lists the lattice parameters for a range of defect oxides which exhibit non-stoichiometry due to either anionic or cationic vacancies. TABLE 1 Lattice parameters of non-stoichiometric titanium oxides (ref. 31) Oxide
Lattice Parameter a/A 4.1850 4.1780 4.1766 4.1733 4.1689 4.1661
The number of oxygen ions per unit area in the (111)planes of these oxides is plotted in Fig. 5 as a function of the non-stoichiometric parameter (x), and Fig. 6 shows the O(510 eV) AES signal observed before and after hydrogen-cleaning the surface as a function of TiOx coverage. Each point was taken for a freshly prepared TiOx film, thus ensuring ,that no electron beam damage occurred. By using Fig. 5 and cross-calibrating the amount of oxygen in a complete monolayer of the TiOx film (using Auger data for the Ru(0001)-(2x 2) 0 structure), the stoichiometry of the TiOx overlayer is found to be consistent with the presence of a structure composed of a (111) sheet of Ti interleaved between two (111)layers of oxygen atoms. The important point to emerge from this procedure is that the structure and stoichiometry of the TiOx film lie close to that of the bulk monoxide (TiO) rather than those of TiO2. In fact, in the monolayer regime, the Ti:O stoichiometry i s Ti02 since the structure corresponds to Ru-0-Ti-0. Thus in the monolayer regime Ti atoms are expected to be in a higher oxidation state than in the bulk oxide TiO; this in turn this will cause the
24
lattice parameter ofthe monolayer filmto shrink, facilitatingregistry with the Ru(OOO1) surface mesh in agreement with the LEED observations.
-
70
6s
5:
60
\ X
55
50
0.7
o . ~0 . 0
1.0
1.1
1.2
1.3 1 . 4
Oxide S t o i c h i o m a t r y . x
0
0.2
0.4
0.8
0.8
1.0
1.2
1.d
T i O , Covmragm/Wonolaycrm
Fig. 5 (left): Number of oxygen ions per unit area in the close packed planes of the defective titanium monoxides (Tiox) as a function of stoichimetry parameter. Fig.6 (right): Variation with TiO, coverage of the a 5 1 0 eV) AES s&al before and after hydrogen cleaning the surface.
For the 'as-deposited' Ti& film the Ti(Zp3/2) binding energy was 459.2eV (Fig.2d), slightly higher than that found for bulk T e i (ref. 231, and entirely consistent with the model proposed above for the monolayer TiOx film. Thus for the monolayer sheet of "Tiof lying on top of the ruthenium substrate, the presence of Ru atoms causes the Ti atoms to carry a somewhat larger net positive charge than they do in bulk T i@ (ref. 23). It is noteworthy that in this sandwich configuration the number density of oxygen atoms in the Ru contact layer is twice that observed for the Ru(OOOl)/oxygen interface under UHV conditions (ref. 30). This is in keeping with the observation that the adsorbed oxygen/surfaceRu ratio can be doubled at elevated oxygen pressures (ref. 32). CO Chemisomtion The effect of TiOx on neighbouring Ru sites was investigated using a near saturation dose of 44L of CO at 295 K and following the subsequent "PD behaviour of the chemisorbed CO. The change in "I'D behaviour as a function of TiO, coverage is shown in Figs. 7a and 7b from which it can be seen that no new desorption features
25
appear and that the TiO, exerts a simple site-blocking effect. A slight shift of the peaks towards lower temperature is seen with increasing TiO, coverage; this need not be an electronic effect, but could be ascribed to destabilisation of CO islands due to reduction in their size. Dosing at 140 K did not lead to any new features, and use of a 1:l mixture of 13C16O and 12C180 gave no evidence for any isotopic scrambling in the desorbing gas. This simple site-blocking effect is therefore consistent with a geometric effect with no apparent electronic effect arising from the presence of the TiOx moieties.
0.7
1
,
I
I
I
i
I
350 400 450 500 550 600 650 700 TEMPERATURE/K 0
0 2
0.4
0.6
0.8
1.0
1.2
1.4
TiO, C o v e r a g e / M o n o l a y e r s
Fig. 7a (left):TPD spectra of 44L CO doses at room temperature as a function of TiOx coverage. Fig. 7b (right): CO desorption yield per surface ruthenium atom, (8) as a function of TiOx coverage. H2 -Chemisorption
Thermal desorption data following a saturation dose of H2 on clean Ru(OOO1) at
140 K are in accord with our earlier work (ref. 24). Two poorly resolved peaks are evident, characteristic of two different types of chemisorbed atomic hydrogen species. Peak shifts towards lower temperature are seen with increasing TiO, coverage in the TPD spectra following H2 adsorption at 140 K (Fig. 8a); the presence of TiO, also leads to a remarkable decrease in the amount of H2 desorption (Fig. 8b). This behaviour can
also be simply understood in terms of a geometric effect if an initial ensemble of the
Ru atoms is required for the dissociative chemisorption of H2.
Fig. 8a (left): TPD spectra following 200L H2 doses at 140 K as a function of TiO, coverage. Fig. 8b (right): Hydrogen desorption yield (0) defined as hydrogen atoms per surface ruthenium atom as a function of TiO, coverage. Hz+CO Chemisorption
Peebles et al. (ref. 33) have suggested that coadsorption of CO and H2 on Ru(OOO1) leads to the formation of segregated islands of the two adsorbates. This important point was investigated further by the following method. A mixture of 6%CO and 94%H2 was used for competitive coadsorption studies, a high fraction of H2 being employed to compensate for its lower sticking probability, thus enabling approximately equal amounts of the two gases to adsorb. Dosing of this CO/H2 mixture onto clean
Ru(0001) gave identical CO TPD behaviour (Fig. 9a), as that reported for CO/Ru(OOOl), (ref. 24). This is to be expected given that H2 desorption has already occurred before the sample reaches temperatures characteristic of CO desorption. With increasing total coverage, the H2 peaks shift to lower temperatures (Fig. 9b); which would be consistent with repulsive (CO)ad-(H)ad interaction at the edges of the surface islands. Most importantly, it can be seen that for most of the total coverage range the fractional (C0)ad and (H)ad coverages show a linear relationship (Fig. 9c); this verifies the segregated islands model (ref. 33) for CO/H2 coadsorption on clean Ru(0001) (see below). At very low exposures, there is a relatively higher (H),d coverage than observed in the linear region; some enhancement of the relative H2 sticking probability is to be expected at low coverages because unfavourable effects due to limited precursor state lifetime and ensemble requirements are minimised. The independent behaviour of CO and H2 on coadsorption can be used to probe the effect of TiO, on CO and H2 chemisorption for submonolayer coverages of TiO,.
27
- -.._ .
+.-..1 OL
.
..---.
__,
._ /
'
200
5 OL
'-.,
-i--u-.-
l
'
I
.
400
300
l
0 1L
'
500
I
200
Temperature/K
'
l
300
~
400
l
-
,
'
500
Temoerature/K
Fig. 9a (top left):CO TPD spectra for 1OL exposure of a 6%cO/94%H2gas mixture at 140 K on clean Ru(OOOl), (note that the low CO content in this mixture leads to only the high temperature CO feature, which is characteristic of CO/Ru(OOOl)). Fig. 9b (top right): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K on clean Ru(OOO1). Fig. 9c (left): Relationship between (CO)ad and (H)ad per surface ruthenium atom in competitive adsorption.
0
0
0.1
0.2
Fractional
I
I
I
0.3
0.4
0 5
H,,
Coverage 8 ,
H2 TPD behaviour for the COIH2 mixture is similar to that observed for the H2/TiOx/Ru(0001) system, Fig. IOa-b. However, the amount of CO desorbing increases for low coverages of TiO,, Fig. 1Oc-d, this can be attributed to an associated decrease in H2 chemisorption; the eventual fall in CO uptake at high TiOx coverages is due to site blocking as found for the CO/TiOx/Ru(OOO1) system.
.
__........_...&.-..._...I
.................. -r.-___
200 . . . . . .
~
7 - 7 T 300 400
Sernoeratb-e/u
_ _ _ ........
3
..... ".-
.:.. *... ......
'ijl'
7
! OMLi
500
O : ]0 \ ,
0 2
TiO,
0 4
0 6
0 8
*
1 0
Coverage/Monolayers
Fig. 10a (left): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K for TiOx/Ru(OOO1). Fig. lob (right): Hydrogen desorption yield defined as hydrogen atoms per surface ruthenium atom, (0) as a function of TiO, coverage
TiO, C o v e r a g e / M o n o l a y e r s
Fig. 1Oc (left): CO TPD spectra for IOL exposure of a 6%C0/94%H2 gas mixture at 140 K for TiO,/Ru(0001). Fig. 1Od (right): CO desorption yield per surface ruthenium atom (0) as a function of TiO, coverage.
29
DISCUSSION The observed layer-by-layer growth of the TiO, film on Ru(0001) is consistent with TiO, moieties ’wetting’ the metal surface in a manner which resembles the behaviour of catalyst particles in the SMSI state. It would appear that by preparing a true model of the postulated decoration effect of metal catalysts in the SMSI state (i.e. no alloying between metal and TiO, species) we have duplicated the reported SMSI behaviour regarding CO and H2 chemisorption on supported catalysts. CO chemisorption is blocked in a simple linear fashion with respect to TiO, coverage, but a more dramatic effect is observed for H2 chemisorption; this can be attributed to the fact that an ensemble of surface ruthenium atoms may be prerequisite for dissociative H2 chemisorption. CO is believed to bond upright at all coverages with the carbon end attached to individual surface ruthenium atoms so that such ensemble effects are not expected to be of significance (refs. 34-36). The linear CO:H2 chemisorption behaviour for CO/H2 mixtures on clean Ru(0001) confirms the postulated island segregation model, since the formation of a mixed layer would selectively hinder H2 chemisorption through the operation of ensemble requirements. On this basis the very pronounced non-linear suppression of H2 chemisorption by TiO, can be attributed to a very hiehlv dispersed immobile TiO, species on the Ru(0001) surface. This contrasts with the hydrogen uptake behaviour exhibited by the CO+H2/Ru(0001) system in the absence of TiO, where CO mobility and concomitant island formation have much larger bare patches of Ru available for H2 chemisorption. The high dispersion of TiO, produced in this manner on the Ru(OOO1) surface at room temperature is consistent with an initial growth mode in which preadsorbed oxygen atoms on the Ru(0001) surface act as ‘anchors’ for the incident titanium atoms. CONCLUSIONS Coadsorption of titanium and oxygen under ultra high vacuum conditions yields a TiO, species of similar structure to bulk TiO, however at the metal-netal oxide interface a complete monolayer of this structure has in effect the stoichiometry of Ti02. It is possible selectively to deposit TiO, moieties which reside on the Ru surface rather than partially diffuse into the bulk. These TiO, moieties have a similar effect on H2 and CO chemisorption as reported for Ti02 supported catalysts in the SMSI state. CO+H2/Ru(0001) chemisorption experiments indicate that at submonolayer coverages these TiO, species are highly dispersed and that simple site blocking is responsible for their effect on CO chemisorption; however the loss of ensembles of surface ruthenium atoms hinders H2 adsorption in a more severe manner. ACKNOWLEDGEMENTS JPSB acknowledges financial support by the SERC and BP Research Company plc under CASE Studentship No. CB020. We are grateful to Johnson Matthey Ltd for a loan of precious metals.
30
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